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MANUAL OF ZOOLOGY 


BY 

RICHARD HERTWIG 

PROFESSOR OF ZOOLOGY IN THE UNIVERSITY AT MUNICH 


THIRD AMERICAN 
FROM THE NINTH GERMAN EDITION 


TRANSLATED AND EDITED BY 

J. S. KINGSLEY 

PROFESSOR OF ZOOLOGY IN TUFTS COLLEGE 


NEW YORK 

HENRY HOLT AND COMPANY 
1912 


COPYRIGHT, 1902 

BY 
HENRY HOLT AND CO. 


COPYRIGHT, 1912 

BY 
HENRY HOLT AND COMPANY 


THE- MAPLE . PRESS. YORK. PA 


PREFACE TO THE THIRD EDITION. 


THE favor with which the first and second American editions of 
Hertwig's Zoology have been received has led to a thorough revision 
of the whole with a close comparison with the latest German edition. 

In this there have been introduced many new features bringing 
the work up to date. These include a discussion of Mendelian inherit- 
ance, many modifications in the account of the theory of evolution, 
and a considerable enlargement of the Protozoa and especially of the 
pathogenic forms, making the volume of more value to the student of 
medicine. 

To have included these without changes elsewhere would have 
resulted in a much larger volume. But the demand in American colleges 
has been for a smaller work and so a reduction has been made in two 
ways. There has been a condensation by the elimination of unnecessary 
words and phrases and by the omission of considerable matter of minor 
importance. Then there has been the recognition of the fact that the 
book has two uses, one in the class room the other as a reference work. 
The two classes of matter have been distinguished by differences of type. 

No attempt has been made to bring the systematic names into accord 
with the latest vagaries of the systematists. No useful and could be 
served by changing or transferring the well-known names of Echidna, 
Coluber, Amia, Homarus, Unio, Holothuria, Amoeba, etc., while the 
confusion this would introduce would be enormous. 

It should be understood that while the revision is based upon the 
German edition of Professor Hertwig, he should not be held responsible 
for any changes introduced. The whole responsibility for these rests 
upon the American reviser. 

J. S. K. 
TUFTS COLLEGE, MASS., June, 1912. 


111 


PREFACE TO THE FIRST EDITIO 


ON account of its clearness and breadth of view, its comparatively 
simple character and moderate size, Professor Richard Hertwig's 
'Lehrbuch der Zoologie' has for ten years held the foremost place 
in German schools. The first or general part of the work was trans- 
lated in 1896 by Dr. George W. Field, and the cordial reception which 
this has had in America has led to the present reproduction of the whole. 

This American edition is not an exact translation. With the consent 
of the author the whole text has been edited and modified in places to 
accord with American usage. For these changes the translator alone 
can be held responsible. Some of the alterations are slight, but others 
are very considerable. Thus the group of Vermes of the original has 
been broken up and its members distributed among several phyla; the 
account of the Arthropoda has been largely rewritten and the classification 
materially altered; while the Tunicata and the Enteropneusti have been 
removed from their position as appendices to the Vermes and united 
with the Vertebrata to form the phylum Chordata. Other changes, like 
those in the classification of the Reptilia and the nephridial system of the 
vertebrates, are of less importance. 

A large number of illustrations have been added, either to make 
clearer points of structure or to aid in the identification of American 
forms. Except in the Protozoa, American genera have in most cases 
been indicated by an asterisk. Numerous genera have been mentioned 
so that the student may see the relationships of forms described in 
morphological literature. 

In the translation the word Anlage, meaning the embryonic material 
from which an organ or a part is developed, has been transferred directly. 
As our language is Germanic in its genius, there can be no valid objection 
to the adoption of the word. 

As this work is intended for beginners, no bibliography has been 
given. A list of literature to be of much value would have been so 
large as materially to increase the size of the volume. Experience has 
shown that beginners rarely go to the original sources. This omission 
it the less important since in all schools where the book is likely to be 
used other works containing good bibliographies are accessible. Refer- 


vi PREFACE TO THE FIRST EDITION 

ence might here be made to those in the Anatomies of Lang and Wieder- 
sheim, the Embryologies of Balfour, Korschelt and Heider, Minot, and 
Hertwig, and Wilson's work on The Cell. 

The editor must here return his thanks to Dr. George W. Field for 
his kindness in allowing the use of his translation of the first part of the 
book as the basis of the present edition. 

J. S. KlNGSLEY. 
TUFTS COLLEGE, MASS., Sept. 19, 1902 


TABLE OF CONTENTS. 


PAGE 

INTRODUCTION... i 

HISTORY OF ZOOLOGY 5 

DEVELOPMENT OF SYSTEMATIC ZOOLOGY . . 6 

DEVELOPMENT OF MORPHOLOGY 9 

HISTORY OF THE THEORY OF EVOLUTION 14 

THEORY OF THE ORIGIN OF SPECIES .... 18 

GENERAL MORPHOLOGY AND PHYSIOLOGY. .. 50 

GENERAL ANATOMY 50 

The Morphological Units of the Animal Body 51 

The Tissues of the Animal Body 63 

Epithelial Tissues 64 

Connective Tissues 74 

Muscular Tissues 80 

Nervous Tissues 83 

Summary 87 

The Combination of Tissues into Organs 

Vegetative Organs 91 

Organs of Assimilation 9 1 

Digestive Tract 93 

Respiratory Organs 96 

Circulatory Apparatus 99 

Excretory Organs 104 

Sexual Organs 107 

Animal Organs no 

Organs of Locomotion no 

Nervous System 111 

Sense Organs 114 

Summary . . 122 

Promorphology 123 

GENERAL EMBRYOLOGY.... 1-37 

Spontaneous Generation 127 

Generation by Parents 128 

Asexual Reproduction 128 

Sexual Reproduction 129 

Combined Methods of Reproduction 13 

General Phenomena of Sexual Reproduction. . . 132 

Maturation of the Egg ... 132 

Fertilization ' 134 

Cleavage Processes 

Heredity 

Mendel's Law 

Formation of the Germ Layers 145 

Different Forms of Sexual Development 149 

Summary 1 5 1 

vii 


viii TABLE OF CONTENTS 

PAGE 

RELATION OF ANIMALS TO ONE ANOTHER 153 

Relations between Individuals of the Same Species i ^3 

Relations between Individuals of Different Species 156 

ANIMAL AND PLANT 159 

GEOGRAPHICAL DISTRIBUTION OF ANIMALS... 160 

DISTRIBUTION OF ANIMALS IN TIME 164 

SPECIAL ZOOLOGY 166 

Phylum I. PROTOZOA .... 166 

Class I. Rhizopoda J.-JQ 

Order I. Monera .... 172 

Order II. Lobosa 172 

Order III. Heliozoa 173 

Order IV. Radiolaria .... 174 

Order V. Foraminifera .178 

Order VI. Mycetozoa . . iSo 

Class II. Flagellata . . . . 181 

Order I. Autoflagellata 181 

Order II. Dinoflagellata . . 184 

Order III. Cystoflagellata ... 184 

Class III. Sporozoa 185 

Order I. Gregarinida .... 186 

Order II. Coccidise . . . 188 

Order III. Haemosporida .... 188 

Order IV. Myxosporida . . 190 

Order V. Sarcosporida . . 191 

Class IV. Ciliata 191 

Order I. Holotricha 195 

Order II. Heterotricha 196 

Order III. Peritricha 196 

Order IV. Hypotricha 197 

Order V. Suctoria 197 

SUMMARY .... 198 

METAZOA 201 

Phylum II. PORIFERA 201 

SUMMARY 206 

Phylum III . CCELENTERATA 206 

Class I. Hydrozoa 208 

Order I. Hydraria 217 

Order II. Hydrocorallinse 217 

Order III. Tubulariae = Anthomedusae 217 

Order IV. Campanulariae = Lepto medusas 217 

Order V. Trachymedusse 218 

Order VI. Narcomedusae 218 

Order VII. Siphonophora 218 

Class II. Scyphozoa 220 

Order I. Discomedusae 223 

Order II. Stauromedusae 224 

Order III. Peromedusae 224 

Order IV. Cubomedusae 224 


TABLE OF CONTENTS ix 

PAGE 

Class III. Anthozoa ---- 224 

Order I. Tetracoralla ......... 230 

Order II. Octocoralla . 230 

Order III. Hexacoralla. . 230 

Class IV. Ctenophora. 232 

SUMMARY .................. 2 35 

VERMES .................. -237 

Phylum IV. PLATHELMINTHES . . . 238 

Class I. Turbellaria ............. . 240 

Class II. Trematoda .......... 242 

Order I. Polystomiae ...... 244 

Order II. Distorrme ....... - 245 

Class III. Cestoda ........ .247 

Class IV. Nemertini ............. 2 55 

SUMMARY .................... -258 

Phylum V. ROTIFERA .................. 259 

Phvllim VI. CCELHELMINTHES ............ ................................ 260 

Class I. Chastognalhi .......... ................. 262 

Class II. Nemathelminthes ........................... . 263 

Order I. Nematoda ................... .263 

Order II. Gordiacea ............... . 268 

Order III. Acanthocephala ......... . 268 

Class III. Annelida ......... .269 

Sub Class I. Chaetopoda. . . .270 

Order I. Polychastae .......... .276 

Order II. OligochaetEe ..... ........ 278 

Sub Class II. Gephyraa. . . .279 

Order I. Chaniferi ....... . 281 

Order II. Inermes ....... ........ 281 

Order III. Priapuloidea. . .... - 281 

Sub Class III. Hirudinei ..... .281 

Class IV. Polyzoa ............... .284 

Class V. Phoronida ................ - 287 

Class VI. Brachiopoda .............. . 287 

SUMMARY ........................ 2 9 

Phylum VII. ECHINODERMA ...................... 291 

Class I. Asteroidea ........... 295 

Class II. Ophiuroidea ................. 298 

Class III. Crinoidea ............ .299 

Sub Class I. Eucrinoidea ................ 3 2 

Sub Class II. Edrioasteroidea .......... . 3 2 

Sub Class III. Cystidea ............. 33 

Sub Class IV. Blastoidea .......... 33 

Class IV. Echinoidea ................ 33 

Class V. Holothuroidea .......... 3 6 

SUMMARY ................................ 


Phylum. VIII. MOLLUSCA ................. 3 IQ 

Class I. Amphineura .................. 3 T 5 

Class II. Acephala ................... 3 J 7 

Order I. Protoconchise .......................... 3 2 3 

Order II. Heteroconchia? ........................................ 3 2 4 


x TABLE OF CONTENTS 

PAGE 

Class III. Scaphopoda 325 

Class IV. Gasteropoda 325 

Order I. Prosobranchiata 333 

Order II. Opisthobranchiata 334 

Order III. Pulmonata 335 

Class V. Cephalopoda - 336 

Order I. Tetrabranchia 346 

Order II. Dibranchia 347 

SUMMARY 347 

Phylum IX. ARTHROPODA 349 

Class I. Crustacea 359 

Sub Class I. Trilobitas 363 

Sub Class II. Phyllopoda 365 

Order I. Branchiopoda 366 

Order II. Cladocera 366 

Sub Class III. Copepoda 366 

Order I. Eucopepoda 369 

Order II. Siphonostomata 369 

Sub Class IV. Ostracoda 371 

Sub Class V. Cirripedia 371 

Sub Class VI. Malacostraca 374 

Legion I. Leptostraca 375 

Legion II. Thoracostraca 375 

Order I. Schizopoda 375 

Order II. Stomatopoda 376 

Order III. Decapoda 377 

Order IV. Cumacea 382 

Order V. Syncarida 382 

Legion III. Arthrocostraca 383 

Order I. Amphipoda 383 

Order II. Isopoda 385 

Class II. Acerata 387 

Sub Class I. Gigantostraca 388 

Order I. Xiphosura 388 

Order II. Eurypterida 389 

Sub Class II. Arachnida 389 

Legion I. Arthrogastrida 391 

Order I. Scorpionida 391 

Order II. Phrynoidea 392 

Order III. Microthelyphonida 393 

Order IV. Solpugida 393 

Order V. Pseudoscorpii 394 

Order VI. Phalangida 394 

Legion II. Sphuaerogastrida * 395 

Order I. Araneina 395 

Order II. Acarina 396 

Order III. Linguatulida 397 

Tardigrada 398 

Pycnogonida 399 

Class III. Malacopoda 309 

Class IV. Insecta JQO 

Sub Class I. Chilopoda 402 


TABLE OF CONTENTS xi 

PAGE 

Sub Class II. Hexapoda 403 

Order I. Apterygota . 418 

Order II. Archiptera . . 419 

Order III. Orthoptera .421 

Order IV. Neuroptera . 42 1 

Order V. Strepsiptera .... 422 

Order VI. Coleoptera 423 

Order VII. Hymenoptera . . 424 

Order VIII. Rhynchota .. 427 

Order IX. Diptera . . 430 

Order X. Aphaniptera . . 431 

Order XI. Lepidoptera . . 432 

Class V. Diplopoda 433 

SUMMARY . . 434 

Phylum X. CHORDATA 438 

Sub Phylum I. Leptocardii 439 

Sub Phylum II. Tunicata 441 

Order I. Copelata; . . 443 

Order II. Tethyoidea 443 

Order III. Thaliacea 447 

Sub Phylum III. Enteropneusta 448 

Sub Phylum IV. Vertebrata 449 

Series I. Ichthyopsida 491 

Class I. Cyclostomata 491 

Sub Class I. Myzonles 493 

Sub Class II. Petromyzonlei 493 

Class II. Pisces 493 

Sub Class I. Elasmobranchii 504 

Order I. Selachii 506 

Order II. Holocephali 507 

Sub Class II. Ganoidei 507 

Sub Class III. Teleostei 508 

Order I. Physostomi 510 

Order II. Pharyngognalhi 510 

Order III. Acanthopteri 511 

Order IV. Anacanthini 511 

Order V. Lophobranchii 511 

Order VI. Plectognathi 512 

Sub Class IV. Dipnoi 512 

Class III. Amphibia 513 

Order I. Stegocephali 519 

Order II. Gymnophiona 519 

Order III. Urodela 519 

Order IV. Anura 520 

Series II. Amniota 520 

Class I. Reptilia x 521 

Order I. Theromorpha 526 

Order II. Plesiosauria 526 

Order III. Ichthyosauria 526 

Order IV. Chelonia 526 

Order V. Rhynchocephalia 527 

Order VI. Dinosauria 528 

Order VII. Squamata 529 


xii TABLE OF CONTENTS 

PAGE 

Order VIII. Crocodilia. . 531 

Order IX. Pterodactylia . . ... 532 

Class II. Aves 

Order I. Saururae... ... 

Order II. Odontornithes . . 541 

Order III. Ratitse. . . 54! 

Order IV. Carinatce 542 

Class III. Mammalia 545 

Sub Class I. Monotremata 558 

SubC lass II. Marsupialia -559 

Sub Class III. Placentalia. . . 561 

Order I. Edentata . 

Order II. Insectivora. ... 

Order III. Chiroptera . . 

Order I\". Carnivora. ... ... 565 

Order V. Rodentia 566 

Order VI. Ungulata . . 

Order VII. Proboscidia 

Otder VIII. Hyracoidea 57 

Order IX. Sirenia 57 

Order X. Cetacea ^-i 

Order XI. Prosimiie ^72 

Order XII. Primates.. .. 573 

SUMMARY 575 


GENERAL PRINCIPLES OF ZOOLOGY. 


INTRODUCTION. 

Man's Relation to Other Animals. The observant man sees him- 
self in the midst of a manifold variety of organisms, which in their struc- 
ture, and even more in their vital phenomena, exhibit a similarity to 
his own being. This similarity, with many of the mammals, especially 
the anthropoid apes, has the sharpness of a caricature. In the inverte- 
brate animals it is softened; yet even in the lowest organisms it is still 
to be found: although here the vital processes which have reached such 
complexity and perfection in ourselves can only be recognized in their 
simplest outlines. Man is part of a great whole, the Animal Kingdom, 
one form among the many thousand forms in which animal organiza- 
tion has found expression. 

Purpose of Zoological Study. If we would, therefore, fully under- 
stand the structure of man, we must, as it were, look at it upon the back- 
ground which is formed by the other animals, and for this purpose we 
must investigate their conditions. Apart from its relations to man, 
zoology has to explain the organization of animals and their relations to 
one another. This is a rich field for scientific activity; its enormous 
range is a consequence, on the one hand, of the well-nigh exhaustless 
variety of animal organization, and, on the other, of the different points of 
view from which the zoologist attacks his problem. 

In the first half of the last century the conception was prevalent that 
the aim of zoology is to furnish every animal with a name, to characterize 
it according to some easily recognizable features, and to classify it in a 
way to facilitate quick identification. By Natural History was under- 
stood the classification of animals or systematic zoology; that is to say, 
only one part of zoology, which can pretend to scientific value only when 
it is brought into relation with other problems (geographical distribution, 
variation, evolution). This conception has become more and more 
subordinated. The ambition to describe the largest possible number of 
new forms belongs to the past. In fact there is a tendency to undue 
neglect of classification. Morphology and Physiology to-day dominate 
the sphere of the zoologist's work. 

I 


2 GENERAL PRINCIPLES OF ZOOLOGY 

Morphology, or the study of form, has first to describe all which can 
be seen externally, as size, color, proportion of parts. But since the 
external appearance cannot be understood without knowledge of the 
internal organs which condition the external form, the morphologist 
must make use of dissection, of Anatomy, and describe their forms and 
methods of combination. In his investigation he only stops when he has 
arrived at the morphological elements of the animal body, the cells. 
Therefore we cannot contrast Morphology and Anatomy, and ascribe to 
the former the description of only the external, and to the latter of only 
the internal parts; the kind of knowledge and the mental processes are 
the same in both cases. The distinction, too, is unnatural, since in 
many instances organs which usually lie in the interior of the body, belong 
in other cases to the surface of the body, and are accessible for direct 
observation. 

Comparative Anatomy. For morphology, as for every science, 
the mere accumulation of facts is not sufficient to give the subject the 
character of a science; an additional mental elaboration of this material 
is necessary. Such a result is reached by comparison. The morphologist 
compares animals with each other according to their structure, in order 
to ascertain what parts of the organization recur everywhere, what only 
within narrow limits. He thus gains a double advantage: (i) an insight 
into the relationships of animals, and hence the foundation for a Natural 
System; (2) the evidence of the laws which govern organisms. Any 
organism is not a structure which has arisen independently and which is 
hence intelligible by itself: it stands in relation to the other members of 
the animal kingdom. We can only understand its structure when we 
compare it with the closely and the more distantly related animals, as 
when we compare man with the other vertebrates and with many lower 
invertebrates. We have to consider one of the most mysterious phenom- 
ena of the organic world, the path to the full explanation of which was 
first outlined by the Theory of Evolution, as will be shown in another 
chapter. 

Ontogeny. To morphology belongs, as an important integral part, 
Ontogeny or Embryology. Only a few animals are completely formed 
in all their parts at the beginning of their individual existence; most of 
them arise from the egg, a relatively simple body, and then step by step 
attain their permanent form by complicated changes. The morphologist 
must determine by observation the different stages, compare them with 
the mature animals, and with the structure and developmental stages of 
other animals. Then there is revealed to him the same conformity to law 
which dominates the mature animals, and a knowledge of this conformity 


INTRODUCTION 3 

is of fundamental importance as well for classification as for the causal 
explanation of the animal form. The developmental stages of man show 
definite regular agreements, not only with the structure of the adult human 
being, which in and of itself would be intelligible, but also with the struc- 
ture of lower vertebrates, and even with many of the still lower inverte- 
brate groups. 

Physiology. In the same way as the morphologist studies the 
structure, the physiologist studies the vital phenomena of animals and the 
functions of their organs. Formerly life was regarded as the expression 
of a special vital force peculiar to organisms, and any attempt at a logical 
explanation of the vital processes was thereby renounced. Most modern 
physiologists have abandoned this theory of a vital force; they have 
attempted to explain life as the summation of extremely complicated 
chemico-physical processes, and thus to apply to the organic world those 
principles which prevail in the inorganic realm. 

Developmental Physiology ("Entwicklungsmechanik"). Since 
each organism is the product of its development; since, further, the 
development represents the summation of most complicated vital processes, 
the explanation of the organic bodily form is, therefore, in ultimate anal- 
ysis a physiological problem; a problem whose solution lies still in the in- 
definitely distant future. It has to explain how the apparently simple 
fertilized egg is converted into the complicated adult organism with its 
many organs regularly arranged. The potentiality of the complexity of 
the adult must be contained in the egg. But it is still a matter of dispute 
as to how this potentiality is conditioned: whether as a mosaic of minute 
particles, each corresponding to a peculiarity of the adult organism, or as a 
substance of simpler structure, in which complexity only appears in the 
course of development. We can proceed experimentally in such a way 
that the conditions of development are artifically altered, and the results 
may be compared with the normal processes. It is also possible to study 
the modifications which one and the same developmental process under- 
goes in different species, modifications which are dependent upon the life 
conditions of the animals and of their young. Then, too, there are ex- 
periments of the same kind performed by nature and which have the same 
informing value as the artificially arranged conditions. Such researches 
have accomplished much in the last decade and have resulted in a 
deeper understanding of the developmental processes. 

The potentialities contained in the fertilized egg are the hereditary 
elements which are transmitted from the parent to the next generation 
and which result in the resemblance of the offspring to the parents. The 
study of these elements and the way in which they are transmitted from 


4 GENERAL PRINCIPLES OF ZOOLOGY 

one generation to another in other words, the laws of heredity has 
recently made a great advance by investigations in two directions: (i) 
through the biometric method or the statistics of variation, and (2) by 
the so-called 'Mendelian analysis' of the hereditary potentialities. Both 
lines of investigation (to be considered more in detail later) have opened 
up in an unexpected way the possibility of submitting to exact research 
the questions of variation and heredity, fundamentally important for the 
understanding of the living world. 

Biology. According as the relations of each organism to the external 
world are brought about through its vital phenomena, there belongs to 
physiology, or at least is connected with it, the study of the conditions of 
animal existence, (Ecology, often called Biology in the narrower sense, 
the broader meaning being the science of all living things, both animals 
and plants. This branch has of late attained considerable importance. 
How animals- are distributed over the globe, how climate and condi- 
tions influence their distribution, how by known factors the structure 
and the mode of life become changed, are questions which are to-day 
discussed more than ever before. 

Paleontology. Finally to the realm of zoology belongs also Paleozo- 
ology or Paleontology, the study of the extinct animals. For between the 
extinct and the living animals there exists a genetic connection: the former 
are the precursors of the latter, and their fossil remains are the most trust- 
worthy records of the history of the race, or Phylogeny. 


HISTORY OF ZOOLOGY. 

Methods of Zoological Study. In the history of zoology we can 
distinguish two great currents, which have been united in a few men, but 
which on the whole have developed independently, nay, more often in 
pronounced opposition to each other; these are on the one side the system- 
atic, on the other the morphologico-physiological mode of studying animals. 
In this brief historical summary they will be kept distinct from one another, 
although in the commencement of zoological investigation there was no 
opposition between the two points of view, and even later this has in many 
instances disappeared. 

Aristotle, the great Greek philosopher, has been called the Father of 
Natural History. Equipped with the literary aid of an extensive library and 
pecuniary means, he pursued the inductive method, the only one capable of 
furnishing secure foundations in the realm of natural science. There 
have been preserved only parts of his three most important zoological 
works, "Historia animalium," "De partibus," and "De generatione," 
works in which zoology is founded as a universal science, since anatomy 
and embryology, physiology and classification find equal consideration. 
How far Aristotle, notwithstanding many errors, had a correct knowledge 
of the structure and embryology of animals, is shown by the fact that many 
of his discoveries have been confirmed only within a cenlury. Thus 
Aristotle knew, though only lately rediscovered by Johannes Mtiller, that 
many sharks are not only viviparous, but that also the embryo becomes 
fixed to the maternal uterus and there is formed a contrivance for its 
nutrition resembling the mammalian placenta; he knew the diii'erence 
between male and female cephalopods, and that the young cuttlefish has 
a preoral yolk-sac. 

The position which Aristotle took in reference to the classification of 
animals is of great interest; he mentions in his writings about live 
hundred species. Since he does not mention very well-known forms, 
like the badger, dragon-fly, etc., we can assume that he knew many more, 
but did not regard it necessary to give a catalogue of all the forms known 
to him, and that he mentioned them only if it was necessary to refer to 
certain physiological or morphological conditions found in them. 

5 


6 GENERAL PRINCIPLES OF ZOOLOGY 

This neglect of the systematic side is further shown in the fact that the 
great philosopher is satisfied with two systematic categories, with etSos, 
species or kind, and yeVos or group. His eight yevrj //.eyio-ra would about 
correspond with the Classes of modern zoology; they have been the start- 
ing-point for all later attempts at classification, and may therefore be 
enumerated here: i. Mammals (OJOTOKOVVTO. ev auroTs) . 2. Birds (opvi0es). 
3. Oviparous quadrupeds (rerpaTroSa woroKoiWa) . 4. Fishes (t^ves) . 
^. Molluscs (/xaAaKia). 6. Crustaceans (/mAa/co'crT-para) . 7. Insects 
(evro/Aa). 8. Animals with shells (oo-TpaKoSep/iaTo). Aristotle also noticed 
the close connection of the first four groups, since he, without actually 
carrying out the division, contrasted the animals with blood, erat^o. (better, 
red blood), with the bloodless, avai/u.a (better, colorless blood or no 
blood at all). 

DEVELOPEMENT OF SYSTEMATIC ZOOLOGY. 

Pliny. It is a remarkable fact that after Aristotle, an exclusively 
systematic direction should have been taken. This is explicable only 
when we consider that the continuity of investigation was interrupted by 
the decline and ultimate complete collapse of ancient civilization, and by 
the triumphant advance of Christianity. The decay is seen in the writings 
of Pliny. Although this Roman scholar was long lauded as the foremost 
zoologist of antiquity, he is now given the place of a not even fortunate 
compiler, who collected from the writings of others the accurate and 
the fabulous indiscriminately, and replaced the natural classification 
according to structure by the unnatural division according to the place 
of abode (flying animals, land animals, water animals). 

Zoology of the Middle Ages. The rise of Christianity resulted in 
the complete annihilation of science and investigation. Then came a 
time when answers to questions capable of solution by the simplest obser- 
vation were sought by rummaging of the works of standard authors. 
How many teeth the horse has, was debated in many polemics, which 
would have led to bloodshed if one of the authors had not thought 
to look into a horse's mouth. Significant of this mental bias which 
prevailed throughout the entire Middle Ages is the 'Physiologus' or 
'Bestiarius,' from which the zoological authors of the Middle Ages 
drew much material. The book in its various editions names about 
seventy animals, among them many creatures of -fable: the dragon, 
the unicorn, the phoenix, etc. Most of the accounts given of 
various animals are fables intended to illustrate religious or ethical 
teachings. There are indeed, exceptions to this general characteristic of 


HISTORY OF ZOOLOGY 7 

the Middle Ages, notably in the writings of the Dominican, Albertus 
Magnus and the Augustinian, Thomas Cantipratensis. In so far as he 
had opportunity, Albertus Magnus, endeavored to support his statements 
by personal observation. But that this beginning of the scientific method 
raised hardly an echo only emphasizes the general spirit of the time. 

At the close of the Middle Ages, when the interest in science awoke 
anew, Aristotle's conceptions were taken up and elaborated by the Eng- 
lishman Wotton. In 1552 he published his work "De differentiis animal- 
ium," in which he essentially copied the system of Aristotle, except that 
he admitted the new group of plant-animals or zoophytes. However, the 
title, 'On the Distinguishing Characters of Animals,' shows that of 
Aristotelian knowledge the systematic side obtained the chief recognition, 
and thus Wotton's work inaugurated the period of systematic zoology, 
which in Ray, but even more in Linnasus, found its most brilliant 
exponents. 

Linnaeus, the son of a Swedish clergman, was born in Rashult in 1707. 
Pronounced by his teachers to be good for nothing at study, he was saved 
from learning the cobbler's trade through the influence of a physician, 
who recognized his fine abilities and turned him to medical studies. He 
studied at Lund and L T psala; at the age of twenty-eight he made ex- 
tended tours on the Continent, and at that time gained recognition from 
the foremost men in his profession. In 1741 he became professor of 
medicine in Upsala, some years later professor of natural history. He 
died in 1778. 

Improvement of Zoological Nomenclature by Linnaeus. Linnaeus's 
most important work is his "Systema Naturae," which, first appearing in 
1735, up to 1766-68 passed through twelve editions. This has become 
the foundation for systematic zoology, since it introduces for the first time 
(i) sharper divisions, (2) a definite scientific terminology, the binomial 
nomenclature, and (3) brief, comprehensive, clear diagnoses. Linnaeus 
divided the entire Animal Kindom into Classes, the Classes into Orders; 
these into Genera, the Genera into Species. The term Family was not 
employed. Still more important was the binomial nomenclature. Hitherto 
the common names were in use and led to much confusion; the same 
animals had different names, and different animals had the same names; 
in the naming of newly discovered animals there was no generally accepted 
principle. This inconvenience was entirely obviated by Linnaeus in the 
tenth edition of his Systema by the introduction of a scientific nomenclature. 
The first word, a noun, designates the genus to which the animal belongs, 
the following word, usually an adjectve, the species within the genus. The 
names Canis famttiaris, Canis lupus, Canis vulpes, indicate that the dog, 


8 GENERAL PRINCIPLES OF ZOOLOGY 

wolf, and fox are related to one another, since they belong to the same 
genus, the genus of doglike animals, of which they are different species. 
Linnaeus's method was particularly valuable in the description of new 
species, inasmuch as it at the ouset informed the reader of the relationships 
of the new species. 

In his characterization of the various groups Linnaeus broke with the 
prevailing custom. His predecessors (as Gessner, Aldrovandus) had 
given a verbose and detailed description of each animal, from which the 
beginner was scarcely able to see what was specially characteristic for that 
animal, a matter which should have been emphasized in the definition. 
Linmeus, on the other hand, introduced brief diagnoses, which in a few 
words, never in sentence form, gave only what was necessary for recog- 
nition, a matter of great importance, in view of the enormously increasing 
number of known animals. 

Influence -of the Linnean System. But in the great superiority 
of the Linnean System lay at the same time the germ of the one-sided 
development which zoology came to take. The perfecting of the system, 
which undoubtedly had become necessary, gave that a brilliant aspect, 
and hid the fact that classification is not the ultimate purpose of investi- 
gation, but only an important and indispensable aid to it. In the zeal for 
naming and classifying animals, the higher goal, knowledge of the nature 
of animals, was lost sight of, and the interest in anatomy, physiology, and 
embryology flagged. 

From these reproaches we can scarcely spare Linnaeus himself, for 
while in his "Systema Natura?" he treated of a much larger number of 
animals than any earlier zoologist, he brought about no deepening of our 
knowledge.' The manner in which he divided the animal kingdom is 
rather a retrogression than an advance. He recognized six classes: 
Mammalia, Aves, Amphibia, Pisces, Insecta, Vermes. The first four 
classes correspond to Aristotle's four groups of animals with blood. In 
the division of the invertebrated animals into Insecta and Vermes Linmcus 
stands undoubtedly below Aristotle, who set up a larger number of 
groups. 

But in his successors, we see the damage wrought by the systematic 
method. The diagnoses of Linnaeus were for the most part models, which, 
mutatis mutandis, could be employed for new species with little trouble. 
There was needed only some exchanging of adjectives to express the 
differences. With the hundreds of thousands of different species of 
animals there \vas no lack of material, and so the way was opened for 
that spiritless species-making which in the first half of the last century 
brought zoology into such discredit. 


HISTORY OF ZOOLOGY 9 

DEVELOPMENT OF MORPHOLOGY. 

Anatomists of Classic Antiquity. Comparative anatomy for this 
chiefly concerns us here for a long time owed its development to the 
students of human anatomy. The disciples of Hippocrates studied 
animal anatomy for the purpose of obtaining an idea of human organiza- 
tion from the structure of other mammals. The work of classical antiquity 
most prominent in this respect, the Human Anatomy of Galen (131-201 
A.D.), is based chiefly upon observations upon dogs, monkeys, etc.; for 
in ancient times, and even in the Middle Ages, there was repugnance to 
making the human cadaver a subject of study. 

Middle Ages. The first thousand years, in which Christianity ruled 
the mental life of the people, held to the writings of Galen and the works 
of his commentators, and seldom took occasion to prove their correctness 
by observations. With the ending of the Middle Ages the interest in 
scientific research first made its way. 

Vesal (1514-1564), the creator of modern anatomy, investigated the 
human cadaver and pointed out numerous errors in Galen's writings 
which had arisen through the extension to human anatomy of the dis- 
coveries made upon other animals. By his corrections of Galen, Yesal 
was drawn into controversy with his teacher, Sylvius, and with his 
renowned contemporary Eustachius, which did much for the development 
of comparative anatomy. At first animals were dissected only for the 
purpose of disclosing the cause of Galen's mistakes, but later through a 
zeal for facts. It was natural that vertebrates were first studied, since 
they stand next to man in structure. Thus there appeared in the same 
century with Yesal drawings of skeletons by Coiter; the zootomical 
writings of Fabricius ab Aquapendente, etc. 

Beginning of Zootomy. But later attention was turned to insects 
and molluscs, even to the echinoderms, ccelenterates, and Protozoa. 
Here three men who lived at the end of the seventeenth century deserve 
mention, the Italian Malpighi and the Dutchmen Swammerdam and 
Leeuwenhoek. The former's "Dissertatio de bombyce" was the pioneer 
for insect anatomy, since by the discovery of the vasa Malpighii, the 
heart, the nervous system, the trachea?, etc., an extraordinary extension 
of our knowledge was brought about. Of Swammerdam's writings 
attention should be called to "The Bible of Nature," a work to which no 
other of that time is comparable, since it contains discoveries of great 
accuracy on the structure of bees, Mayflies, snails, etc. Leeuwenhoek, 
finally, was most fortunate in the field of microscopic research. Besides 
other things he studied especially the minute inhabitants of the fresh 
waters, the 'infusion-animalcules.' 


10 GENERAL PRINICPLES OF ZOOLOGY 

The great service of the men named above consists chiefly in that they 
broke away from the thraldom of book-learning and, relying alone upon 
their own eyes and their own judgment, regained the blessing of inde- 
pendent and unbiased observation. They spread the interest in obser- 
vation of nature so that in the eighteenth century the number of natural- 
history writings had increased enormously. There were busy with the 
study of insect structure and development, de Geer in Sweden, Reaumur 
in France, Lyonet in Belgium, Rosel von Rosenhof in Germany; the 
latter besides wrote a monograph on the indigenous batrachia, which is 
still worth reading. The investigation of the infusoria formed a favorite 
occupation for Wrisberg, von Gleichen-Russwurm, Schiiffer, Eichhorn, 
and O. F. Miiller. As a criterion of the progress made, a mere glance 
at the illustrations is sufficient. Any one will at a glance recognize the 
difference between the shabby drawings of an Aldrovandus and the 
masterly figures of a Lyonet or a Rosel von Rr.senhof. 

Peroid oif Comparative Anatomy. Thus through the zeal of 
numerous men a store of anatomical facts was collected, which needed 
only a mental reworking; and this was brought about, or at least entered 
upon, by the great comparative anatomists who lived at the end of the 
eighteenth and the beginning of the nineteenth century. Among these 
the French zoologists Lamarck, Savigny, Geoffrey St. Hilaire, Cuvier, and 
the Germans Meckel and Goethe are especially to be named. 

Correlation of Parts. When the various animals were compared 
with one another with reference to their structure there was obtained a 
series of fundamental laws, particularly the law of the Correlation of 
Parts and the law of the Homology of Organs. The former showed that 
there exists a dependent relation between the organs of the same animal ; 
that local changes of one organ lead to corresponding changes at some 
distant part of the body, and that therefore from the structure of certain 
parts an inference can be drawn as to the constitution of another part of 
the body. Cuvier particularly made use of this principle in reconstructing 
extinct animals. 

Homology and Analogy. Still more important was the idea of the 
Homology of Organs. In the organs of animals a distinction was drawn 
between an anatomical and a physiological character; the anatomical 
character is the sum of form, structure, position, and mode of connection 
of organs; the physiological character is their function. Anatomically 
similar organs in closely related animals will usually have the same 
functions, as, for example, the liver of all vertebrates produces gall; here 
anatomical and physiological characteristics go hand in hand. But this 
need not be the case; very often it may happen that the same function 


HISTORY OF ZOOLOGY 11 

is possessed by organs anatomically different; as, for example, the res- 
piration is carried on in fishes by gills, in mammals by lungs. Con- 
versely, anatomically similar organs may have different functions, as 
the lungs of mammals and the swim-bladder of fishes; similar organs 
may also undergo a change of function from one group to another; the 
hydrostatic apparatus of fishes has come to be the seat of respiration in 
the mammals. Organs with like functions physiologically equivalent 
organs are called 'analogous'; organs of like anatomical constitution 
anatomically equivalent organs are called 'homologous.' It is the 
task of comparative anatomy to discover in the various parts of 
animals those which are homologous, and to follow the changes in 
them conditioned by a change of function. 

Cuvier. The foremost representative of comparative anatomy was 
Georges Dagobert Cuvier. His investigations extended to* the coelenter- 
ates, molluscs, arthropods, and vertebrates, living and fossil. He 
collected his extensive observations into his two chief works "Le regne 
animal distribue d'apres son organization" and "Lefons d'anatomie 
comparee." Of epoch-making importance was his little pamphlet "Sur 
un rapprochement a etablir entre les differentes classes des animaux," 
in which he founded his celebrated type theory, and which introduced a 
reform of classification. The Cuvierian division, the starting-point for 
all later classifications, differed from all the earlier systems in that the 
classes of mammals, birds, reptiles, and fishes were united in a higher 
grade under the name, introduced by Lamarck, of 'vertebrate animals'; 
and the so-called 'invertebrate animals' were divided into three similar 
grades, each equal to that of the vertebrate animals, viz., Mollusca, 
Articulata, and Radiata. Cuvier called these grades standing above the 
classes, provinces or chief branches (embranchements). But still more 
important are the differences which appear in the structural basis of 
the system. Instead of, like the earlier systematists, using a few ex- 
ternal characteristics for the division, Cuvier built upon the totality of 
internal organization, as expressed in the relative positions of the most 
important organs, especially the position of the nervous system, as 
determining the arrangement of the other organs. Thus for the first time 
comparative anatomy was employed in the formation of a natural system 
of animals. 

Cuvier found prevalent the theory that all animals formed a single 
connected series ascending from the lowest infusorian to man ; within this 
series the position of each animal was determined by the degree of its 
organization. On the other hand Cuvier taught that the animal kingdom 
consisted of several co-ordinated unities, the types, which exist inde- 


12 GENERAL PRINCIPLES OF ZOOLOGY 

pcndently side by side, within which again there are higher and lower 
forms. The position of an animal is determined by two factors: first, by 
its conformity to a type, by the structural plan which it represents; second, 
by its degree of organization, by the stage to which it attains within its type. 

Comparative Embryology. Evolution vs. Epigenesis. The 
same results which Cuvier reached by the way of comparative anatomy 
were attained two decades later by C. E. von Baer by the aid of embry- 
ology. Embryology is the youngest branch of zoology. The difficulties 
of observation, due to the delicacy and the minuteness of the develop- 
mental stages, were lessened by the invention of the microscope and 
microscopical technique. Further, there was no idea of developmental 
history in the present sense of the word ; each organism was thought to be 
laid down from the first complete in all its parts, and only needed growth 
to unfold its organs (evolutid) ; either the spermatozoon must be the young 
creature which found favorable conditions for growth in the store of food 
in the egg, or the egg represents the individual and was stimulated to the 
'evolutio' by the spermatozoon. This theory led to that of inclusion, 
which taught that in the ovary of Eve were included the germs of all 
human beings who have lived or ever will live. 

Caspar Friedrich Wolff combated this idea (1759) ; he sought to prove 
by observation that the hen's egg at the beginning is without organization, 
and that gradually the various organs appear in it. In the embryo there 
is a new formation of all parts, an epigenesis. This assault upon the 
evolutionist school was without result, chiefly because Albrecht von 
Haller, the most celebrated physiologist of the eighteenth century, sup- 
pressed the idea of epigenesis. 

Von Baer. Carl Ernst von Baer in his classic work, "Die Entwick- 
lung des Hlihnchens, Beobachtung und Reflexion" (1832), established 
embryology as an independent study. Baer confirmed Wolff's doctrine 
of the appearance of layerlike anlagen, from which the organs arose; and 
on account of the accuracy with which he proved this he is considered the 
founder of the germ-layer theory. Further, he came to the conclusion that 
each type had not only its peculiar structural plan, but also its peculiar 
course of development. Here we meet for the first time the idea that for 
the solution of the questions of relationship of animals, and therefore a 
basis for a natural classification, comparative embryology is indispensable; 
an idea which in recent years has proved exceedingly fruitful. 

Cell Theory. Of fundamental importance for the further growth of 
comparative anatomy and embryology was the proof that all organisms, 
as well as their embryonic forms, were composed of the same elements, 
the cells. This cell theory, was advanced by Schleiden and Schwann 


HISTORY OF ZOOLOGY 13 

during the third decade of the last century and three decades later was 
completely remodelled by the protoplasm theory of Max Schultze. In 
the cell theory a simple principle of organization was found for all living 
creatures, and the wide ream of histology was open for scientific treat- 
ment. But the theory was of the greatest importance for developmental 
physiology, for only with the recognition of egg, spermatozoon, and 
cleavage spheres as nucleated cells, was there a sound basis for the solution 
of the problems of fertilization, heredity, and embryonic differentiation 
and for the experimental proof of hypotheses. 

With the establishment and systematic use of comparative anatomy, 
the cell theory and histology, the ground was prepared for the series of 
researches which characterized the second half of the nineteenth century. 
Great advances were made in vertebrate anatomy by the classic researches 
of Owen, Johannes Miiller, Rathke, Huxley, Gegenbaur and others; 
our conceptions of organization have been completely altered by the work 
of Dujardin, Max Schultze, Haeckel, and others, who have proved the 
unicellularity of the lowest animals. The germ-layer theory was further 
developed by Remak and Kolliker; and applied to the invertebrates by 
Kowalewsky, Haeckel, and Huxley. It is beyond the limits of this brief 
historical summary to go into what has been accomplished in other 
branches of the animal kingdom; it must here suffice to mention the most 
important changes which the Cuvierian system has undergone under the 
influence of increasing knowledge. 

Changes in the System. Of the four types of Cuvier the branch 
Radiata was the one of which he had the least knowledge; it was 
also the least natural, since it comprised, besides the radially sym- 
metrical ccelenterates and echinoderms, other forms, which, like the 
worms, were bilaterally symmetrical, or, like many infusorians, were 
asymmetrical. C. Th. von Siebold introduced the first important change. 
He limited the type Radiata, or, as he termed them, the Zoophytes, to 
those animals with radially symmetrical structure (Echinoderms and the 
Plant-animals); separating all the others, he formed of the unicellular 
organisms the branch of 'primitive animals' or Protozoa; the higher 
organized animals he grouped together as worms or Vermes; at the same 
time he transferred a part of the Articulata, the annelids, to the worm 
group, and proposed for the other articulates, crabs, millipedes, spiders, 
and insects, the term Arthropoda. 

Leuckart, about the same time (1848), divided the Radiata into two 
branches differing greatly in structure. The lower forms, in which no 
separate body-cavity is present, the interior of the body consisting only 
of a system of cavities serving for digestion, he called the Ccelenterata 


14 GENERAL PRINCIPLES OF ZOOLOGY 

(essentially the Zoophyta of older zoologists) ; to the rest, in which the 
alimentary canal and the body-cavity occur as two separate cavities, he 
gave the name Echinoderma. 

Thus there resulted seven classes: Protozoa, Ccelentera, Echinoderma, 
Vermes, Arthropoda, Mollusca, and Vertebrata. Still this arrangement 
does not meet the requirements of a natural system and is more or less 
unsatisfactory. Upon anatomical and embryological characters the 
Brachiopoda, the Bryozoa, and the Tunicata have been separated from 
the Mollusca; they form the subject of diverse opinions. The relation- 
ships of the first two groups have not yet been settled: of the Tunicata 
we know indeed that they are related to the Vertebrata, but the differences 
are such that they cannot be included in that group. The only way out 
of the difficulty is to unite vertebrates, tunicates, and some other forms in 
a larger division, Chordata. The Vermes, too, must be divided, as will 
appear in the .second part of this volume. 

In the last decade of the nineteenth century and the beginning of the 
present, physiological investigation has taken a place beside morphology. 
Its most important tool is experiment. While experiments have long 
been invoked to settle biological problems, they are now used in the most 
extensive and systematic manner; especially are methodical breeding and 
crossing experiments employed to solve the problems of variation and 
heredity. There are also investigations into the laws which regulate the 
animal form, in which the separate stages of embryonic and post-embry- 
onic development are exposed to modifying influences (removal or trans- 
plantation of blastomeres or parts of the body, employment of different 
temperatures, chemical, mechanical, electrical stimuli), and the results 
are compared with those of normal conditions. An important aid in 
these studies is the mathematical statistical method by which the value 
of the results of observation and experiment is tested. The second half 
of the century just closed was especially characterized by the development 
of the theory of evolution, the history of which is given in a separate 
section. 

HISTORY OF THE THEORY OF EVOLUTION. 

The theory of evolution has developed into a question whose impor- 
tance might, on a superficial examination, be underrated, but which has 
grown into a problem completely dominating zoological research, and 
has occupied not only zoologists, but all interested in science generally. 
This is the question of the logical value of the conceptions species, genus, 
family, etc. 


HISTORY OF ZOOLOGY 15 

The Nature of Species. In nature we find only separate animals: 
how comes it that we classify them into larger and smaller groups? Are 
the single species, genera, and the other divisions fixed quantities, as it 
were fundamental conceptions of nature, or a Creator's thoughts, which 
find expression in the single forms? Or are they abstractions which man 
has introduced into nature for the purpose of making it comprehensible 
to his mental capabilities? Are the specific and generic names only 
expressions which have become necessary, from the nature of our mental 
capacity, for the expression of relationships in nature, which in and for 
themselves are not immutable, and hence can undergo a gradual change? 
Practically speaking, the problem reads: are species constant or change- 
able? What is true for species must necessarily be true for all other 
categories of the system, all of which in the ultimate analysis rest upon 
the conception of species. 

Ray's Conception of Species. One of the first to consider the con- 
ception of species was John Ray. In the attempt to define a species he 
encountered difficulties. In practice, animals which differ little in 
structure and appearance from one another are ascribed to the same 
species; this practical procedure cannot be carried out theoretically; for 
there are males and females of the same species which differ more from 
one another than do the representatives of different species. Ray reached 
the genetic definition when he said: for plants there is no more certain 
criterion of specific unity than their origin from the seeds of specifically 
or individually like plants; that is to say, generalized for all organisms: 
to one and the same species belong individuals which spring from similar 
ancestors. 

The 'Cataclysm Theory.' With Ray's definition an uncontrollable 
element was brought into the conception of species, since no systematist 
usually could know anything, as to whether the representatives of the 
species before him sprang from similar parents. It was therefore only 
natural that the conception of species put on a religious garb, since by 
resting upon theological ideas it found a firmer support. Linmeus said: 
"Tot sunt species quot ab initio creavit infinitum Ens." Linmeus's 
definition showed itself untenable, as soon as paleontology began to make 
accessible a vast quantity of extinct animals preserved as fossils. Cuvier 
proved that these fossils were the remains of animals of a previous time. 
Just as the formation of the earth's crust by successive layers made possible 
the recognition of different periods in the earth's history, so paleontology 
showed how to recognize different periods in the vegetable and animal 
world of life on our globe. Each geological age was characterized by a 
special world of animals; and these animal worlds differed the more from 


16 GENERAL PRINCIPLES OF ZOOLOGY 

the present, the older the period of the earth to which they belonged. All 
these generalizations led Cuvier to his cataclysm theory: that a great 
revolution brought each period of the earth's history to an end, destroy- 
ing all life, and that upon the newly formed virgin earth a new organic 
world of immutable species sprang up. As a believer in the immuta- 
bility of species, Cuvier was forced to combat the idea of any genetic 
connection between the living and the fossil forms. 

Cuvier's theory of cataclysms gave no scientific explanation of ihe 
origin of the successive populations of the earth. Such an explanation 
is only possible by the hypothesis that the later animal worlds have 
descended from the earlier. So it happened that the idea of the fixity of 
species was given up and replaced by that of mutability and descent the 
theory of evolution. 

Darwin's Predecessors. Even in Cuvier's time there prevailed a 
strong current in favor of this theory. It found expression in England 
in the writings of Erasmus Darwin (grandfather of Charles Darwin) ; 
in Germany in the works of Goethe, Oken, and the disciples of the 'natural 
philosophical' school; in France the genealogical theory was developed by 
Buffon, Geoffrey St. Hilaire, and Lamarck. Its clearest expression was 
found in Lamarck's "Philosophic zoologique" (1809); its arguments 
will be considered in the following paragraphs. 

Lamarck (born, 1744, died, 1829) taught that at first organisms of 
the simplest structure arose through spontaneous generation from non- 
living matter. From these simplest living creatures have developed, by 
gradual changes in the course of an immeasurably vast space of time, the 
present species of plants and animals, without any break in the continuity 
of life upon our globe; the terminal point of this series is man; the other 
animals are the descendants of those forms from which man has developed. 
Lamarck regarded the animal kingdom as a single series grading from 
the lowest animal up to man. Among the causes which may influence 
the change of organisms, Lamarck emphasized particularly use and 
dimse; the giraffe has obtained a long neck because he was compelled to 
stretch, in order to browse the leaves on high trees; conversely, the eyes of 
animals which live in the dark have degenerated from lack of use. The 
direct influence of the external world must be unimportant; the changes 
in the surroundings must for the most part act indirectly upon animals by 
altering the conditions for the use of organs. 

Evolution vs. Creation. Lamarck's work remained almost unno- 
ticed by his contemporaries. Later there arose a violent controversy 
between the defenders and the opponents of the evolution theory when 
[1830] Geoff roy St. Hilaire in a debate defended against Cuvier the thesis 


HISTORY OF ZOOLOGY 17 

of a near relationship of the vertebrates and the insects. The conflict 
ended in the complete overthrow of the theory of evolution; the defeat 
was so complete that the problem vanished for a long time, and the 
theory of the fixity of species again became dominant. This was occa- 
sioned by many causes. The theory of Geoff roy St. Hilaire and Lamarck 
was rather a clever conception than founded on abundant facts; besides, 
it had in it as a fundamental error the doctrine of the serial arrangement 
of the animal world. Opposed to this stood Cuvier's extensive knowledge, 
making it easy for him to show that the animal kingdom was made up of 
separate co-ordinated groups, the types. 

Lyell. In the same year in which Cuvier obtained his victory, his 
theory of the succession of numerous animal worlds upon the globe 
received its first blow. Cuvier's cataclysm theory had two sides, a geo- 
logical and a biological. Cuvier denied the continuity of the various 
terrestial periods, as well as the continuity of the fauna and flora belonging 
to them. In 1830-32 appeared the "Principles of Geology" by Lyell, 
which, in the realm of geology, completely set aside the cataclysm theory. 
Lyell proved that the supposition of violent revolutions was not necessary 
to explain the changes of the earth's surface and the superposition of its 
strata; that rather the constantly acting forces, elevations and depressions, 
the erosive action of water, are sufficient to furnish a complete explanation. 
Very gradually in the course of time the earth's surface has changed, and 
passed from one period into the next, and still at the present day constant 
change is going on. The continuity in the history of the earth, here 
postulated for the first time, has since then become one of the fundamental 
axioms of Geology; on the other hand the discontinuity of living creatures, 
was for a long time regarded as correct. 

Darwin. It is the great merit of Charles Darwin that he took up the 
theory of descent anew after it had rested a decade, and later brought it 
into general recognition. With this began the most important period in 
the history of zoology, a period in which the science not only made such 
an advance as never before, but also began to obtain a permanent influ- 
ence upon the general views of men. 

Charles Darwin was born in 1809. After studying at the universities 
of Edinburgh and Cambridge, he joined the English war-ship "Beagle," 
as naturalist. In its voyage from 1831 to 36 around the globe, Darwin 
recognized the peculiar character of island faunas, particularly of the 
Galapagos Islands, and the remarkable geological succession of edentates 
in South America; these facts formed the germ of his epoch-making 
theory. After his return to England Darwin lived, entirely devoted to 
scientific work, up to his death in 1882. He was incessantly busy in 
2 


18 GENERAL PRINCIPLES OF ZOOLOGY 

developing his conception of the orgin of species, the fundamental ideas 
of which he communicated to friends, but not until 1858 did Darwin 
decide to make them public. In this year he received an essay by Wallace 
which in its most important points coincided with his own views. At the 
same time with Wallace's manuscript an abstract of Darwin's theory was 
published. In the next year (1859) appeared the most important of his 
writings, "On the Origin of Species by means of Natural Selection," and 
in rapid succession a splendid series of works, the most important of 
which are: (i) " Upon the Variation of Plants and Animals under Domes- 
tication," (2) on "The Descent of Man." 

No scientific work of that century has attracted so much attention in 
the whole educated world, as "The Origin of Species." It was generally 
received as something entirely new, so completely had the scientific 
tradition been lost. In professional circles it was stoutly combated by one 
faction, with- another it found hesitating acceptance. Only a few men 
placed themselves from the beginning in a decided manner on the side 
of the great British investigator. There was a lively scientific battle, 
which ended in a brilliant victory for the theory of evolution. At the 
present time all our scientific thoughts are permeated with the idea of 
evolution. 

Post-Darwinian Writers. Among the men who have most influ- 
enced this rapid advance is to be mentioned, besides A. R. W T allace, the 
co-founder of Darwinism, above all Ernst Haeckel, who in his "General 
Morphology" and his "Natural History of Creation" has done much 
towards the extension of the theory. Among other energetic defenders 
of the theory in Germany should be mentioned Fritz Miiller, Carl Vogt, 
Weismann, Moritz Wagner, and Nageli. Among the English naturalists 
are to be named particularly Huxley, Hooker, and Lyell. In America 
Gray, Cope, Morse, and Hyatt were early supporters. Darwinism was 
long in obtaining an entrance into France. 

DARWIN'S THEORY OF THE ORIGIN OF SPECIES. 

Before Darwin wrote the idea of fixity of species prevailed. It was 
recognized that all the individuals of a species are not alike, and that 
more or less variability occurs, so that it was possible to distinguish races 
and varieties within the species, but it was believed that the variations 
never transcended specific bounds. 

Darwin begins with a criticism of the term species. Are the concep- 
tions of species on the one side and that of race and variety on the other 
something entirely different? Are there special criteria for determining 


HISTORY OF ZOOLOGY 


19 


beyond the possibility of a doubt whether in a definite case we have to 
do with a variety of a species or with a different species? or do the con- 
ceptions pass into one another in nature? Are species varieties which 
have become constant, and are varieties species in the process of 
formation? 

Morphological Characters. A. Distinction bet-ween Species and Variety. 
For the settlement of these fundamental questions morphological and 


FIG. i A. English carrier-pigeon (after Darwin). 


FIG. IB. English tumbler-pigeon (after Darwin). 

physiological characters can be considered. In the practice of the system- 
atists usually the morphological characters prevail exclusively; and 
hence will be here considered first. If, among a great number of forms 


20 


GENERAL PRINCIPLES OF ZOOLOGY 


similar to one another, two groups can be recognized which differ consider- 
ably from each other, if the differences between them be obliterated by 
no intermediate forms, and if in several successive generations they remain 
constant, then the systematist speaks of a 'good species;' on the other 
hand. he speaks of varieties of the same species when the differences are 
slight and inconstant, and when they lose their importance through the 
existence of intermediate forms. A definite application of this rule dis- 
closes great incongruities, many groups being regarded by one set of 
systematists as good species, by another only as 'sports,' i.e., as varieties 
of the same species. The differences between the 'races' of our domestic 
animals are often so considerable that formerly they were regarded not only 
as sufficient for the foundation of good species, but even of genera and 
families. In the fantail pigeon the number of tail-feathers, originally 
only 12-14, has increased to 30-42 (fig. ic) ; among the other races of 


FIG. ic. English fantail pigeon (after Darwin). 


pigeons enormous variations are found in the size of the beak and feet in 
comparison with the rest of the body (figs. IA, IB) ; even the skeleton itself 
participates in this variation, as is shown by the fact that the total number 
of vertebrae varies from 38 to 43, the number of sacral vertebrae from 
14 to n. 

B. Variation within the Species. Now in respect to the occurrence of 
transitional forms and the constancy of differences, there is within one 
and the same 'good species' the greatest conceivable difference. In 
many very variable species the extremes are united by many transitions; 


HISTORY OF ZOOLOdV 21 

in other cases sharply circumscribed groups of forms, or races, can he 
distinguished within the same species. In the race, the peculiar character- 
istics are inherited from generation to generation with the same constancy 
as in good species. This is shown in man, and in many pure races of 
domesticated animals. 

Physiological Characters. A. Crossing of Species and Varieties 
A critical examination leads to the conclusion that morphology is indeed 
useful for grouping animals into species and varieties, but it leaves us in 
the lurch when called upon to show the disinctions between a specie? and 
a variety. Therefore there remains open to the systematist only one 
resource, i.e., to summon physiology to his aid. This has disclosed 
considerable distinctions in reproduction. We should expect a priori 
that the individuals of different species would not reproduce with each 
other; on the other hand the individuals of one and the same species, even 
though of different varieties or races, should be entirely fertile. One 
must beware of arguing in a circle in proof of these two propositions; 
so the question must read: does physiological experiment lead to the 
same distinctions as does the common systematic experience? 

B. The Intercrossing of Species. This field is as yet far from being 
sufficiently investigated; yet some general propositions can be set up: 

(1) that not a few so-called 'good species' can be crossed with one another; 

(2) that in general the difficulty of crossing increases, the more distant 
the systematic relationship of the species used; (3) that these difficulties 
are by no means directly proportional to the systematic divergence of the 
species. Thus hybrids have been obtained from species which belong to 
quite different genera, while very often nearly-related species will not 
cross. Among fishes we know hybrids of A bramis brama and Blicca bjorkna, 
of Trutta salar (salmon) and Tmtta fario (trout); among sea-urchins the 
spermatozoa of Strongylocentrotus lividus fertilize with great readiness the 
eggs of Echinus microtiiberculatus, but only rarely the eggs of Sphcerechinus 
granularis, which is nearer in the system. It also happens that crossing 
in one direction (male of A and female of B) is easily accomplished, but in 
the other direction (male of B and female of A) it completely fails; as, 
for example, the sperm of Strongylocentrotus lividus fertilizes well the eggs 
of Echinus microtuberculatus, but, conversely, the sperm of E. microttiber- 
culatus does not fertilize the eggs of S. lividus; salmon eggs are fertilized 
by trout sperm but not trout eggs by salmon sperm. Eggs have been 
fertilized by sperm belonging to different families, orders, and possibly 
classes eggs of Pleuronectes platessa and Labrus rupestris by sperm of 
the cod, frogs' eggs by sperm of two species of Triton, eggs of a starfish 
(Aster ias forbesii) by milt from a sea-urchin, Arbacia pusttilosa. In 


22 GENERAL PRINCIPLES OF ZOOLOGY 

these extreme cases, it is true, the hydrids die during or at the close of 
segmentation, before the embryo is outlined. 

In the case of animals where copulation is necessary the difficulties 
of experimentation increase, since often there exists an aversion between 
males and females of different species which prevents any union of the 
sexes. Yet we know crosses of different species; e.g., between the horse 
and the ass; our domestic cattle and the zebu; ibex (or wild buck) and 
she-goat; sheep and goats; dog and jackal; dog and wolf; hare and rabbit 
(Lepus darwini); American bison and domestic cattle; etc.; among birds, 
between different species of finches and of grouse; mallard and the pintail 
duck; the European and the Chinese goose (Anser ferns and A. cygnoides). 
Among the insects, especially the Lepidoptera, the cases are many, but 
the resulting eggs usually produce larvae of slight vital force. 

C. Fertility of Hybrids and Mongrels. Since many hybrids, as the 
mule, have been known for thousands of years, the criterion is, as it were, 
pushed back one stage; if the infertility in cases of crosses in many species 
is not immediately noticeable, yet it may be apparent in the products 
of the cross. While the products of the crossing of varieties, the 'mongrels,' 
always have a normal, often an increased, fertility, the products of the 
crossing of species, the hybrids, should always be sterile. But even this is 
a rule, not a law. The mule (which only very rarely reproduces) and 
many other hybrids are indeed sterile, but there are not a few exceptions, 
although the number of experiments in reference to this point is very 
small. Hybrids of hares and rabbits have continued fruitful for genera- 
tions; the same is true of hybrids obtained from the wild buck and the 
domesticated she-goat; from Anser cygnoides and A . domesticus; f rom 
Salmo salvelinus and S. font mails; Cyprinits carpio and Carassius vulgar is; 
Bombyx cynthia and B. arrindia. 

Difficulties in Classification. The final result of all this discussion 
may be summed up as follows: up to the present time, neither by physio- 
logical nor by morphological evidence has there been found a criterion 
which can guide the systematist in deciding whether certain series of forms 
are to be regarded as good species or as varieties of a species. Zoologists 
are guided rather in practice by a certain tact for classification, which, 
however, in difficult cases leaves them in the lurch, and thus the opinions 
of various investigators vary. 

Change of Varieties into Species. The conditions above discussed 
find their natural explanation in the assumption that sharp distinctions 
between species and variety do not exist; that species are varieties which 
have become constant, and "varieties are incipient species. The meaning 
of the above can be made clear by a concrete case. Individuals of a 


HISTORY OF ZOOLOGY 23 

species vary, i.e., compared with one another they attain greater or less 
differences. So long as the extreme differences are bridged by transitional 
forms we speak of varieties of a species; if, on the other hand, the inter- 
mediate forms have died out, or were not present in the beginning, and the 
differences have in the course of time become fixed, and so intensified 
that a sexual union of the extreme forms results either in complete 
sterility or at least in a marked tendency towards sterility, then we speak 
of different species. 

Species may be Related to each other in Unequal Degrees. In 
favor of this view, that varieties will in longer time become species, is the 
great agreement which usually exists between the two. In genera which 
comprise a larger number of species, the species usually show also many 
varieties; the species are then usually grouped in sub-genera, i.e., they are 
related to each other in unequal degrees, since they form small groups 
arranged around certain species. With varieties the case is similar. In 
such genera the formation of species is in active progress; but each 
species formation presupposes a high degree of variability. 

Phylogeny. It is clear that what has here been worked out for the 
species must also apply to the other categories of the system. Just as by 
divergent development varieties become species, so must species by con- 
tinued divergence become so far removed from one another that we dis- 
tinguish them as genera. It is only a question of time when these differ- 
ences will become still greater, and give rise to orders, classes, and branches, 
just as the tender shoots of the young plant become in the tree the chief 
branches from which spring lateral branches and twigs. If we pursue this 
train of thought we reach the conception that all the animals and plants 
living at present have arisen by means of variation from a few primitive 
organisms. Inasmuch as thousands of years are required for the forma- 
tion of new species through the variability of one, there must have been 
necessary for this historical development of the animal and vegetable 
kingdoms a space of time greater than our mental capacity can grasp. 
Since for the individual development (embryology) of an animal the term 
ontogeny has been chosen, it has also proved convenient to apply to the 
historical development of animals the term History of the Race or 
Phylogeny. 

Spontaneous Generation. If we attempt to derive all living animals 
from a common ancestor, we must assume that this was extremely simple, 
that it was unicellular; for the simpler, the less specialized, the organiza- 
tion, so much the greater is its capacity for modification. Only from 
simple organisms can the unicellular organisms, the Protozoa, be derived. 
Finally, for the simple organisms alone can we conceive a natural origin. 


24 GENERAL PRINCIPLES OF ZOOLOGY 

Since there was undoubtedly a time upon our earth when temperatures 
prevailed which made life impossible, life must have arisen at some time, 
either through an act of creation or through spontaneous generation. If, 
in agreement with the spirit of natural science, we invoke for the explana- 
tion of natural facts only the forces of nature, we are driven to the hypothe- 
sis of spontaneous generation, namely, that by a peculiar combination 
of materials without life, the complicated mechanism which we call 'life' 
has arisen. 

Starting from a basis of facts, by generalization we reach a simple 
conception of the origin of the animal kingdom, but we have in equal 
measure departed from the results of direct observation. Observations 
only show that species are capable of modifications. That this capacity 
for variation is a principle which explains to us the origin of the animal 
world, needs further demonstration. 

Proofs of Phylogeny. The evolution of the existing animal world 
has taken place in the thousands of years long past, but is no longer acces- 
sible for direct observation, and therefore it can never be followed in the 
sense that we follow the individual development of an organism. In 
regard to the conception of the evolution of animals we can merely prove 
the probability; yet all our observations of facts not only agree with this 
conception, but find in it their only simple explanation. Such facts are 
furnished to us by the classification of animals, paleontology, geographical 
distribution, comparative anatomy, and comparative embryology. 

(1) Proofs from Classification. It has long been recognized, that if we 
wish to express graphically the relationships of animals, their classes, 
orders, genera, and species, simple co-ordination and subordination are 
not sufficient, but we must have a treelike arrangement, in which the 
principal divisions, more closely or distantly related to one another the 
branches, phyla, or types represent the main limbs, while the smaller 
branches and twigs correspond to the several classes, orders, etc. This 
is, in fact, the arrangement to which the theory of evolution necessarily 
leads. 

(2) Paleontological Demonstration approaches nearest to direct proof; 
for paleontology gives the only traces of existence which the predecessors 
of the present animal world have left. Even here a hypothetical element 
creeps in. We can only observe that various grades of forms of an animal 
group are found in successive strata; if we unite these into a develop- 
mental series, and regard the younger as derived from the older by varia- 
tion, we connect the single observations by a very probable hypothesis. 
But the value of paleontological evidence is weakened much more by its 
extreme incompleteness. In fossils only the hard parts are generally 


HISTORY OF ZOOLOGY 


25 


preserved; the soft parts, which alone are present, or at least make up the 
most important portions of many animals, are almost always lost. Only 
rarely are they (muscle of fishes, ink-bag of cephalopods, outlines of 
medusae) preserved in the rocks. Even the hard parts remain connected 
only under exceptionally favorable conditions. If further we take into 
consideration the fact that these treasures are buried in the earth, and are 
usually obtained only by accident, in quarrying and road-building, it 
becomes clear how little of the racial history is to be expected from 
paleontology. 


FIG. 2. Archvopteryx lUkographica (after Steinmann-Doderlein). d, clavicle; co, 
coracoid; h, humerus; r, radius; u, ulna; c, carpus; I-IV, digits; sc, scapula. 

Examples of Paleontological Proof. Yet paleontology has already 
furnished many important proofs of the theory of descent. It has shown 
that the lower forms appeared first, and the more highly organized later. 
Among animals in general the latest to appear were the vertebrates, and 


26 GENERAL PRINCIPLES OF ZOOLOGY 

of these the mammals; among the mammals man. For smaller groups 
genealogical material has fortunately been found. Transitional forms 
connect the single-toed horse of the present with the four-toed Eohippos 
of the eocene; for all the hoofed animals a common ancestral form has 
been found in the Condylarthra. Transitional forms have also been 
found between the greater divisions, as, e.g., between reptiles and birds, 
the remarkable toothed birds, and the Archeeopteryx (fig. 2), a bird with 
a long, feathered, lizard-like tail. 

(3) Morphological Proofs. When we employ comparative anatomy 
and embryology in support of evolution, we find that the two have so 
much in common that they can best be treated together. 

Cuvier and von Baer taught that the separate types of the animal 
kingdom are units, each with a special structure and plan of development 
peculiar to it; farther, that there are no similarities in structure or develop- 
ment forming a bridge from type to type. The first of these propositions 
is still regarded as correct, but the second, which alone is important for 
the theory of evolution, has become untenable. All animals have a 
common organic basis in the cell and are thereby brought close to one 
another; all multicellular animals agree in the principal points during the 
first stages of their development, during the fertilization, cleavage of the 
egg, and the formation of the first two germ-layers, and vary- from one 
another from this time on only in such differences as may occur within one 
and the same type. Also the peculiarities which distinguish each type 
in structure and in the mode of development are not without intermediate 
phases. In some representatives of each type the structure and the 
mode of development are simpler, thereby approaching to the conditions 
which obtain in the other types. The existence of such transitional forms 
is one of the most important proofs in favor of the theory of evolution. 

Fundamental Law of Biogenesis. A fact that weighs heavily 
in favor of the theory of evolution is that the structure and mode of develop- 
ment of animals is ruled by a law which at present can only be explained 
by the assumption of a common ancestry. Each animal during its develop- 
ment passes through essentially the stages which remain permanent in 
the case of lower or better, more primitive animals of the same branch, 
as the following examples show: (i) In the early stages of development 
the human embryo (figs. 3, 609) possesses remarkable resemblances to 
the lowest vertebrates, the fishes. Like these it has gill-slits, a simple 
heart with auricle and ventricle; instead of a separation of aorta and 
pulmonary arteries (body and lung arteries) a single arterial trunk going 
from the heart; and aortic arches connecting this trunk with the descend- 
ing aorta. All of these are structure adapted for branchiate respiration 


HISTORY OF ZOOLOGY 


27 


and are functionally intelligible in fishes hut they are not compatable 
with a lung-breathing mammal and must undergo fundamental changes 
to become of use. They are, like so many other structures in the human 
being, not to be understood from present functions, but must have an 
historical meaning. (2) Frogs in their tadpole stage have an organiza- 
tion similar to that which is permanent in certain Amphibia, the Per- 


3 


FIG. 3. FIG. 4. 

FIG. 3. Human embryo, 4.2 mm. long (after His). Pericardium and lateral 
body wall opened, yolk-sac and allantois cut away, course of blood-vessels shown; a, 
arterial trunk; a/, allantois stalk; c, precava, uniting with yolk and umbilical veins; d, 
intestine; do, yolk stalk; h, ear vesicle; A", ventricle of heart; o, upper jaw; r, olfac tory 
groove; s, tail; n, lower jaw; us, somites; V, atrium of heart; 1-5, the five arterial arches. 

FIG. 4. Tadpoles of Rana temporaria. in, mouth; g, upper jaw; z, lower jaw; 
s, sucking-disc; kb, external gills; ik, region of the internal gills; n, nose; a, eyes; 
o, auditory vesicle; h, cardiac region; d, operculum. 

ennibranchiata (fig. 5), which stand lower in the system; they have a 
swimming tail and tuft-like gills, which are lacking in the adult frog. 
(3) There are certain parasitic Crustacea, which live upon the gills of 
fishes, and seem not at all like their relatives. They are shapeless masses 
which were formerly regarded as parasitic worms. Their systematic 
position was only determined by their embryology (fig. 6). Here it is 


28 


GENERAL PRINCIPLES OF ZOOLOGY 


shown that they pass through a nauplius-stage (fig. 6a), characteristic 
of most Crustacea, and that they then assume the shape of small Crustacea 
(fig. 6, b), like Cyclops (fig. 7, A), so widely distributed in fresh waters. 
Very often the males make a halt in the cyclops-stage while the female 


FIG. 5. Siredon pisciformis (larva of Amblystoma tigrinum) (after Dumeril and 

Bibron). 

develops farther and assumes a shapeless form, so that there arises a very 
remarkable sexual dimorphism (fig. 8). All these examples, which can 
be multiplied by hundreds, can be explained in the same way. The higher 


FIG. 6. 


\ I 

Ichtheres pcrcarum. a, nauplius-, /;, cyclops-stage; c, adult female (after 

Claus). 


forms pass through the stage of the lower, because they spring from an- 
cestors which were more or less similar to the latter. Man in his em- 
bryological development passes through the fish stage, the frog the 
perennibranchiate stage, the parasitic crustacean first the nauplius- 


HISTORY OF ZOOLOGY 


29 


and then the cy clops-stage, because their ancestors were once fish-like, 
perennibranchiate-like, nauplius- and cyclops-like. Here is expressed 
a general phenomenon which Haeckel has stated under the name of 'the 
Fundamental Law of Biogenesis.' "The development history (ontogeny) 


m. 


A 


FlG. 7. C\'djps coronatus (.4) and also its nauplius in lateral (5) and in ventral 
view (C). 7, 'head; II-V, the five thoracic, and behind these the five abdominal 
segments; F, furca; i, the first, 2, the second, antenna;; 3, mandibles; 4, maxilla-; 
5, maxillipeds; 6-9, the first four pairs of biramous feet, while the rudimentary fifth 
pair are hidden; au, eye; o, upper Up; e, egg-sacs; d, gut; m, muscle. 

of an individual animal briefly recapitulates the history of the race (phylog- 
eny); i.e., the most important stages of organization which its ancestors 
have passed through appear again, even if somewhat modified, in the 
development of individual animals." 


30 


GENERAL PRINCIPLES OF ZOOLOGY 


FIG. 8. Philicthys xiphice. 
a, female (after Claus), X4; b, 
male (after Bergsoe), Xi3. 


The Nervous System. This law applies as well to single organs as to 
entire animals. The central nervous system of many lower animals 
(echinoderms, coelenterates, many worms) forms part of the skin; in its 

first appearance it belongs to the surface 
of the body, because it has to mediate the 
relations with the external world. In the 
case of higher animals, e.g., the vertebrates, 
the brain and spinal cord lie deeply imbed- 
ded in the interior of the body; but in the 
embryo they are likewise laid down as a 
part of the skin (medullary plate) which 
gradually through infolding and cutting off 
from this comes to lie internally (fig. 9) . 

The Skeletal System. The skeleton of 
vertebrates is a further example. In the 
lowest chordates, amphioxus and the cyclo- 
stomes, the vertebra are lacking, and in 
their place we find a cylindrical cord of 
tissue, the notochord. In the fishes and 
Amphibia the notochord usually persists; 
but it is partially reduced and constricted 

by the vertebrae, which in the lower forms consist of cartilage, and in 
the higher of bone. Mature birds and mammals finally have a com- 
pletely ossified vertebral column; their embryos, on the other hand, have 
in the early stages only the notochord (amphioxus stage) ; later this 
notochord becomes constricted by the vertebrae (fish-amphibian stage) 
and finally entirely replaced; the vertebral column is in the beginning 
cartilaginous, only later becoming ossified. Comparative anatomy and 
embryology thus give the same developmental stages of the axial skeleton: 
(i) notochord, (2) notochord and vertebral column, (3) vertebral column 
alone, the latter at first formed of cartilage, then of bone. 

We have spoken of a parallelism between the facts of comparative 
anatomy and of embryology. But we should expect a threefold parallel- 
ism; for according to the theory of evolution the systematic arrangement 
of animals is based upon a third factor phylogeny. The fossils, should 
give the same progressive series in the successive geological strata as the 
stages of forms found by comparative anatomy and embryology. We 
know instances of such threefold parallelisms. Comparative anatomy 
teaches that the lowest developed form of a fish's tail is the diphyceral 
(fig. 10, A); that from this the heterocercal (B), and from the hetero- 
cercal the homocercal form of tail-fin (C, D] can be derived. Embryo- 


HISTORY OF ZOOLOGY 
mf m f 


mk 2 

ik 


mf 


mp 


III. 


FIG. g. Cross-sections through the dorsal region of Triton embryos at different ages 
(from O. Hertwig). In / the medullary plate (anlage of spinal cord) mp is marked 
otf from the skin (epidermis, ep) by the medullary folds (mf). In // the medullary 
plate, by inrolling of the medullary folds, is converted into a groove. In III the groove 
has closed into a tube (), the spinal cord, which has separated from the rest of the 
ectoderm (epidermis), c, body cavity (ccelom); ch, notochord; rf>. cavity of primitive 
somite (myotome); r/z, yolk-cells; ik, entodenn; /<[, lumen of gut; ink 1 , ink-, somatic 
and splanchnic layers of mesoderm; n, spinal cord. 


32 


GENERAL PRINCIPLES OF ZOOLOGY 


logically the most highly developed fishes are first diphycercal, later 
heterocercal, and finally become homocercal. Last of all, paleonto- 
logically the oldest fishes are diphycercal or heterocercal, and only later 
do homocercal forms appear. 


FIG. 10. Tail-fins of various fishes (from Zittel). A, Diphycercal fin of 
Polypterus bichir . (Vertebral column and notochord divide the tail into symmetrical 
dorsal and ventral portions.) B, Heterocercal tail of the sturgeon. (As a result cf an 
upward bending of the notochord and vertebral column the fin has become asymmetrical, 
the ventral portion much larger than the dorsal.) C, D, Homocercal fins, C, of Amia 
calva; D, of Trutta salar. (By a still greater upward bending of the notochord and 
vertebral column the dorsal portion has almost entirely disappeared and the ventral 
portion almost alone forms the fin, externally apparently symmetrical, but in its internal 
structure very asymmetrical.) ch, chorda; a, b, c, cover-plates. 


What has here been referred to is only a small fraction of the proofs 
which morphology offers in favor of evolution; it can only serve to show 
how morphological observations can be employed. For the reflecting 
naturalist the facts of morphology are a great inductive proof in favor 
of the theory of evolution. 


HISTORY OF ZOOLOGY 33 

Distribution of Animals. From Animal Geography we learn that 
the present distribution of animals is the product of the past. It will 
therefore be possible from this to figure out many of the earlier conditions 
of things. 

If we assume that from the beginning all animal species were consti- 
tuted as they now are, they would then have been placed by the purposeful 
Creator in the regions best suited to them; their distribution would there- 
fore have been determined by favorable or unfavorable conditions of 
life prevailing in the various regions, as the climate, food-supply, etc. 
If, on the other hand, we assume that the animal species have arisen from 
one another through variation, then there must have been, as an influence 
determining the manner of distribution, besides the conditions of exis- 
tence, still a second factor, which we will call the geological. We know 
that the configuration of the earth's surface has changed in many respects 
in the course of the enormous time of the geological periods; that land 
areas which earlier were united, have become separated by the encroach- 
ments of the sea; that by the upheaval of mountains important barriers 
to the distribution of animals were also formed. On the other hand 
regions which were formerly separated have become connected; islands, for 
example, being united by the emergence of land from the sea. From the 
fact that these two changes the changes in the earth's surface and in the 
animal world established upon it have gone on hand in hand there 
follows necessarily the consequence that greater differences in the faunal 
character of two lands must result, the longer the inhabitants have been 
separated by impassable barriers. For the various groups the character 
of the barriers is different; terrestrial animals, which cannot fly, are 
hindered in their distribution by the sea; marine forms by land barriers; 
for terrestrial molluscs mountain ranges, which are dry and barren, or con- 
stantly snow-capped, are effectual. 

Instances of Proofs. Since attention has been called to these 
conditions, many facts favorable to the theory of evolution have been 
ascertained: (i) Of the various continents Australia has faunally an in- 
dependent character; when discovered it contained almost none of the 
higher (placental) mammals, except such as can fly (Chiroptera), or 
marine forms (Cetacea), or such as are easily transported by floating 
wood (small rodents), or such as could be introduced by man (dingo, the 
Australian dog) ; instead, it had remarkable birdlike animals (with beak 
and cloaca), and the marsupials, which have become extinct in the Old 
World and, opossums excepted, in America as well. The phenomenon 
is explained by the geological fact that in the earth's history Australia, 
with its surrounding islands, was certainly the earliest to lose its connec- 

3 


34 GENERAL PRINCIPLES OF ZOOLOGY 

tion with the other continents. While in the other parts of the earth the 
higher vertebrates, which were developed from the marsupials and their 
lower contemporaries, came, by way of the lands connecting the various 
continents, to have a wide distribution, in isolated Australia this process 
of evolution did not go on, and its ancient faunal character was preserved. 
(2) As Wallace has shown, the Malay Archipelago is divided faunally 
into an eastern and a western half. The fauna of the first has a thoroughly 
Australian character; that of the latter recalls Farther India and the 
Oriental Province. Differences in climate and vegetation cannot be the 
cause of this, since in both there are islands with dry and others with 
moist climates, with sparse and with luxuriant vegetation. The only ex- 
planation is that the eastern Malay Islands have developed geologically 
in connection with Australia, the western with India. Wallace tried 
to draw a sharp line ('Wallace's line') between the two regions, passing 
between the islands of Bali and Lombok. Later studies have not confirmed 
this, but have shown that between the two regions is a zone of islands 
(including Celebes) in which a mixture of faunas occurs. (3) Long 
before Darwin, the geologist von Buch, from the distribution of plants on 
the Canary Islands, came to the conclusion of a change of species into new 
species; viz., on islands peculiar species develop in secluded valleys, be- 
cause high mountain-chains isolate plants more effectually than do wide 
areas of water. Moritz Wagner has collected many instances which prove 
that localities inhabited by certain species of beetles and snails have been 
sharply divided by wide rivers or by mountain-chains, while in neighbor- 
ing regions related so-called 'vicarious species' are found. The peculiar 
character of the fauna and flora of isolated island groups also needs 
mention. The Hawaiian Islands have no less than 70 endemic birds out 
of a total of 116, the Galapagos 84 out of 108. 

Causal Foundation of the Theory of Evolution. The Darwinian 
theory, so far as the above exposition shows, is fundamentally like the 
theories of descent advocated at the beginning of the last century by 
Lamarck and other zoologists; it is distinguished from these only by its 
much more extensive foundation of facts, and further in that it abandoned 
the successional arrangement and replaced it by the branched, tree-like 
mode of arrangement the genealogical tree. But still more important 
are those advances which relate to the causal foundation of the descent 
theory. The doctrine of causes which has brought about the change of 
species forms the nucleus of the Darwinian theory, by which it is especially 
distinguished from Lamarckism. In order to substantiate causally the 
change of species, Darwin proposed his highly important principle of 
'Natural Selection by means of the Struggle for Existence.' 


HISTORY OF ZOOLOGY 35 

Artificial Selection. In the development of this principle Darwin 
started with the limited and hence easily comprehended subject, the arti- 
fical breeding of domesticated animals. Whether these be the descendants 
of a single species or have arisen from crosses of two or more species 
(authorities are not in agreement in all cases) they behave like repre- 
sentatives of a single species. How have the various races and sub- 
race of pigeons, horses, cattle, dogs, etc. arisen? Darwin finds the 
causes of these great differences in artificial selection, practised by man for 
thousands of years. The method is to choose from the stock individuals 
showing the tendency toward the desired ideal in even the slightest degree 
more than their fellows, and then pairing these. By repetitions of this 
selection and breeding, the desired goal is slowly reached. 

This artificial selection depends upon three factors: (i) Variabilitv; 
the descendants of one pair of parents have the capability of developing 
new characteristics, thereby differing from their parents. (2) Hered- 
Uability of newly-acquired characters; the tendency of the daughter- 
generation to transmit the newly-developed characteristic to the 
succeeding generation. (3) Artificial selection; man selects for breeding 
suitable individuals, and prevents a new character which has arisen 
through variation from disappearing by crossing with animals of the 
opposite variational tendencies. 

If we compare with the facts of domestication the conditions of animals 
living in the state of nature, we find again variability and heredity, as effici- 
ent forces, inherent in all organisms, though the former is not everywhere 
of the same intensity. There are many species which vary only slightly 
or not at all, and therefore have remained unchanged for thousands of 
years. But contrasted with these conservative species are in every group 
plastic species, which are in the process of rapid change, and these alone 
are of importance in causing the appearance of new forms. Since 
heredity is present in all organisms, there is only lacking a factor corre- 
sponding to artificial selection, and this Darwin discovered in the so-called 
'natural selection.' 

Natural Selection: Struggle for Existence. Natural selection 
finds its basis in the enormous number of descendants which every animal 
produces. There are animals (e.g., most fishes) which produce many 
thousands of young in the course of their lives; not to mention parasites, 
whose eggs are numbered by millions. For the development of this 
multitude of germs there is no room on the earth. In order to preserve 
the balance of nature great numbers of unfertilized and fertilized eggs, 
as well as young animals and many that are mature but have not yet 
attained their physiological destiny, must perish. Many individuals will 


36 GENERAL PRINCIPLES OF ZOOLOGY 

be blotted out by accidental causes; yet on the whole those individuals 
which are best protected will best withstand adverse conditions. Slight 
superiority in structure will be of importance in this struggle for existence, 
and the possessors of this will gain an advantage over their companions 
of the same species, just as in domestication each character which is 
useful to man is of advantage to the possessor. Among the numerous 
varieties that appear the fittest will survive, and in the course of many 
generations the fortunate variations will increase by summation, while 
destruction overtakes the unfit. Thus will arise new forms, which owe 
their existence to 'natural selection in the struggle for existence.' 

The 'Struggle for Existence.' The expression 'struggle for exis- 
tence' is figurative, for only rarely does a conscious struggle decide the 
question of an animal's existence; for example, in the case of the beasts 
of prey, that one which by means of his bodily strength is best able to 
struggle with Ids competitors for his prey is best provided in times of 
limited food-supply. Much more common is the unconscious struggle: 
each man who attains a more favorable position by special intelligence 
and energy, limits to an equal degree the conditions of life for many of his 
fellow men, however much he may interest himself in humanity. The 
prey, which by special craft' or swiftness escapes the pursuer, turns the 
enemy upon the less favored of its companions. It is noticeable that in 
severe epidemics certain men do not fall victims to the disease, because 
their organization better withstands infection. Here the term 'survival 
of the fittest,' which Spencer has adopted in preference to 'struggle for 
existence,' is better. 

Instances of the Struggle for Existence. Although the foregoing 

suffices to show that the struggle for existence plays a very prominent 

role, yet on account of the importance of this feature it will be illustrated 

by a few concrete examples. The brown rat (Mus decumanus) , which 

swarmed out from Asia at the beginning of the eighteenth century, has 

almost completely exterminated the black or house-rat (Mus rattus) in 

Europe, and has made existence impossible for it in other parts of the 

world. Several European species of thistle have increased so enormously 

in the La Plata states that they have in places completely crowded out the 

native plants. Another European plant (Hypochoeris radicata) has 

become a weed, overrunning everything in New Zealand. Certain races 

of men, like the Dravidian and Indian, die off to the same degree that 

other races of men, like the Caucasian, Mongolian, and Negro, spread. 

The more one attempts to explain that endlessly complicated web of the 

relations of animals to one another, the relations of animals to plants and 

to climatic conditions, as Darwin has done, so much the more does he 


HISTORY OF ZOOLOGY 37 

appreciate the methods and effects of the struggle for existence. Islands 
in the midst of the ocean have a disproportionately large number of species 
f wingless insects, because the flying forms are easily carried out to sea. 
For example, on the Kerguelen Islands, remarkably exposed to storms, 
the insects are wingless; among them one species of butterfly, several 
flies, and numerous beetles. 

Sympathetic Coloration. Very often, in regions which have a pre- 
vailing uniform color, the coat of the animals is distinguished by a similar 
hue; this phenomenon is called sympatlietic coloration. Inhabitants of 
regions of snow are white, desert animals have the pale yellow color of the 
desert, animals which live at the surface of the sea are transparent; 
representatives of the most diverse animal branches show the same phe- 
nomenon. The advantages connected therewith scarcely need an expla- 
nation. Every animal may have occasion to conceal himself from his 
pursuers; or it may be his lot to approach his prey by stealth: he is much 
better adapted for this the closer he resembles his surroundings. Natural 
selection fixes every advantage in either of these directions, and in the 
course of many generations these advantages increase. Among the most 
interesting are the cases of sympathetic coloration, of mimicry and of the 
development of secondary sexual characters as a result of sexual selection. 

Mimicry is referable to the same principle, except that the imitation 
is not here limited to the color, but also influences form and marking. 
Frequently parts of plants are imitated, sometimes leaves, sometimes 
stems. Certain butterflies with the upper surfaces of the wings beauti- 
fully colored escape their pursuers by the rapidity of their flight; if they 
alight to rest, they are protected by their great similarity to the leaves of 
the plants around which they chiefly fly. When the wings are folded over 
the back, the dark coloring of the under sides comes into sight and the 
color on the upper side is concealed. The parts are so. arranged that the 
whole takes on a leaf-like form, and certain markings heighten the imita- 
tion of the leaf (fig. n). Among the numerous species of leaf-butterflies 
there are different grades of completeness of mimicry; in many even the 
depredations of insects are imitated; in others the form and marking are 
still incompletely leaf-like, the marking being the first to come into exis- 
tence. Among the grasshoppers also there are imitations of leaves, like 
the 'walking-leaf,' PliyUium sice (folium, P. scythe, while other nearly 
related forms more or less completely approach the appearance of dried, 
sometimes of thorny twigs (fig. 12, a and b). 

Very often insects are copied by other animals. Certain butterflies, 
the Heliconias of warmer America, the Danaids of the Old World, fly 
heavily in large swarms, clumsy and yet are unmolested by birds, because 


38 


GENERAL PRINCIPLES OF ZOOLOGY 


they contain bad-tasting fat bodies. Another species of butterfly accom- 
panies them (Fieri dae), which does not taste bad, and yet is not eaten, 
because in flight, in cut, and marking of the wings it imitates the Helic- 
onix so closely that even a systematist might easily be confused (fig. 13). 
In a similar way bees and wasps, feared on account of their sting, are 
imitated by other insects. In Borneo there is a large black wasp, whose 


FIG. ii. Leaf-butterflies. A, Kallima paralecta, flying: a, at rest (after Wallace). 
5, Siderone strigosus, flying; b, at rest (after C. Sterne). 


wings have a broad white spot near the tip (Mygnimia aviculus). Its 
imitator is a heteromerous beetle (Coloborlwmbus fasciatipennis), which, 
contrary to the habit of beetles, keeps its hinder wings extended, showing 
the white spot at their tips, while the wing-covers have become small 
oval scales (fig. 14). With many species the mimicry occurs only in the 


HISTORY OF ZOOLOGY 


39 


a 


FIG. 12. Grasshopper mimicry, a, Acanthoderus wattacei + . b, Phyllium scytlie 


FIG. 13. Methona psidii, a bad-tasting Heliconiid, copied by theT?iend,LeptaliSGrise. 

(after Wallace.) 


40 


GENERAL PRINCIPLES OF ZOOLOGY 


females, since these are less numerous and have heavier bodies than the 
males. So there arises a sexual dimorphism. If the mimicing species 
have a wide distribution, different bad flavored species may be mimicked 
in different parts of the range. The females of Papilio merops mimic, 
in different regions, Danais chrysippus, Amanris eclieria and A. mavias, 
while the males have the same appearance throughout the whole habitat. 


FIG. 14. a, Mygnimia aviculus, a wasp imitated by a beetle, b, Coloborhombus'fascia- 
tipennis (after Wallace). Three-fourths natural size. 

Sexual Selection is a special phase of natural selection, chiefly 
observed in birds and hoofed animals. For the fulfilment of his sexual 
instincts the male seeks to drive his competitors from the field, either in 
battle or by impressing the female by his charms. With strong wings and 
with spurs the cock maintains possession of his flock, the stag by means 
of his antlers, the bull with his horns. The male birds of paradise win the 
favor of the females by means of beautiful coloring; most singing-birds, 
by means of song; many species of fowl, by peculiar love-dances. Since 
all these characters belong chiefly to the male, and since only exception- 
ally are they inherited by the female (and even then are less pronounced), 


HISTORY OF ZOOLOGY -11 

it is almost certain that in a great measure they have been acquired by the 
males through the struggle for the female. In the case of birds a second 
factor has undoubtedly co-operated to impress distinctly the often 
enormous difference between the feathers of the male and of the female 
as is shown, for example, in the case of the birds of paradise (fig. 15); 


\ 


FIG. i5A. Paradisea apoda, male (after Levaillant). 

for the nesting female inconspicuous colors and a close-lying coat of 
feathers are necessary in order that, undisturbed by enemies, she may 
devote herself to incubation. 

On the Efficiency of Natural Selection. In the course of the last 
twenty-five years there has been much controversy as to how far natural 
selection alone is a species-forming factor. A number of objectors dispute 
the possibility of fortuitous variations being utilized in the struggle for 


42 GENERAL PRINCIPLES OF ZOOLOGY 

existence and thus fixed as permanent characters. It is not easy to 
see how many characters, especially those used in classification, can be 
of use to their owners. It can only be said that they have developed in 
correlation with other important characters. But useful characters 
must be considerable in order to be seized upon by natural selection. 
Fortuitous variations with which Darwinism deals are too inconsiderable 
to be utilized by the organism and so to be of value in the struggle for 
existence. In most cases, too, alteration in one organ alone is not enough 
to be of value; usually a whole series of accessory structures must be 


Fig. 15!}. Paradisea apoda, female (after Levaillant). 

modified. In short, there must exist a harmonious co-adaptation of parts, 
which presupposes a progressive and well-regulated development extend- 
ing through a long space of time, during which the struggle for existence 
could have exerted no directing influence. Thus, the wing of a bird in 
order to be used for flight must have already reached a considerable size; 
the muscles for moving it, the supporting skeletal parts, the nerves 
running to it must have a definite formation and arrangement. Then 
there are difficulties in that most animals are bilaterally or radially sym- 
metrical, many in addition segmented. In all these cases the same organ 
is repeated two or more times. Organs which are repeated symmetric- 
ally and usually those which are segmental agree in general in structure. 
One must therefore admit that the alterations of chance must have oc- 
curred at at least two points simultaneously and in exactly the same way. 
A further objection is that the action of natural selection would, under 
ordinary conditions, be negatived by unhindred crossing of the varying 
forms. If, for example, we do not isolate fantails from other pigeons, 
they will cross with these, and their descendants will soon resume the 


HISTORY OF ZOOLOOY 43 

character of common pigeons. Finally, it has been claimed that for the 
formation of new species a simple variation of form is not sufficient; it 
must reach still farther: (i) a variation in different directions, a divergent 
development of the individual members of a species; (2) the disappearance 
of the transitional forms which unite the divergent forms. 

The objection that the struggle for existence cannot bring about the 
divergent development of individuals necessary for improvement is of 
least importance. Of the many variations appearing at the same time in a 
species two or more may be equally useful; then one set of individuals 
will seize upon one, another set upon the other advantage, and that in 
consequence of this both sets will develop in different directions. Conse- 
quently the intermediate forms which are not pronounced in the one or the 
other direction will be in an unfavorable position, and must carry on the 
struggle for existence with both groups of partially differentiated com- 
panions of their species, and, being less completely adapted, must fall. 

More important are the first two objections; they have led to the idea 
that the principle of selection alone is insufficient to explain the origin 
and adaptive modification of new forms. So new theories have been 
advanced, older ones revamped, sometimes to replace that of natural 
selection, sometimes to strengthen certain links of its chain. Limited 
space permits only an outline of the more prominent of these and that 
without any attempt at a decision as to how far they complete the Darwin- 
ian hypothesis, are compatible with it, or replace it. 

Germinal Selection. According to its author, Weismann's 'germinal 
selection ' is only a completion of natural selection, the 'individual selection.' 
It presupposes a detailed knowledge of modern investigations in the lines 
of fertilization and heredity (see chapter on Fertilization) and hence can 
only be outlined here. Weismann believes that all variations which are 
selected in the struggle for existence and are fixed in the successive 
generations must have their sources in the germ cells and since these arise 
from the fusion of male and female sex cells, they must, in the first in- 
stance, have been contained in these. Each germinal anlage consists of 
extremely numerous elements, the 'determinants' of the peculiarities 
of the organism. Accordingly as certain of these determinants develop 
at the expense of the others or are weakened or modified, the organism 
arising from the germ has special peculiarities or variations. If certain 
determinants tend constantly in a certain direction and these be present 
in large numbers, these will persist so that individual selection can have 
its influence. 

Mutations. The Mutation theory of de Vries is a considerable 
modification of the Darwinian theory. In rearing large numbers of the 


44 GENERAL PRINCIPLES OF ZOOLOGY 

evening primrose, CEnothera lamarckiana, besides plants of the true 
lamarckiana type there also arise a not inconsiderable number of others 
which are distinguished from the mother plant in noticeable ways and 
these may be arranged in sharply defined groups of forms which de Vries 
has named Oe. gigas, Oe. nanella, Oe. scintillans, etc. These groups of 
forms resemble 'small species' to the extent that, from the first, inter- 
mediate forms are lacking and, prevented from crossing, they produce only 
individuals with the same characteristics. These suddenly appearing 
and sharply marked and hereditary variations de Vries calls Mutations. 
They have long been known in English as 'sports' and are Darwin's 
'single variations.' While formerly these were regarded as merely special 
instances of general variation, de Vries regards them as different. The 
slight variations with which Darwinism had previously dealt, oscillate- 
like a pendulum about its point of rest around a central point of greatest 
frequency and.result in no permanent modifications. Even in domestica- 
tion it is not possible to advance by the continued selection of the slight 
differences of these 'fluctuating variations/ and to fix them as inheritances. 
On the other hand stable forms are produced by mutations and these per- 
sist if the mutants are better adapted to the conditions of life than is the 
parent stock. Just so far as the struggle for existence here plays a 
deciding role the mutation theory is a basis for the selection theory. It 
differs from Darwinism to the extent of being an 'explosive' method of 
species formation, by which several kinds, and these relatively constant, 
suddenly come into existence. For the proper valuation of the mutation 
theory it will be necessary in the future to separate two questions: (i) 
Whether the sharp distinctions postulated by de Vries between mutation 
and variation actually exist; (2) W 7 hether the mutation theory is able to 
explain the numerous adaptations of organisms to their -environment. 

The mutation theory has shown anew how necessary it is to subject the 
phenomena of heredity and variation to exact examination. Thus there has 
developed in the field of botany an experimental direction which has contributed 
much to the solution of the problems and promises rich results for the future. 
Here come the studies of the statistics of variation, established by the mathe- 
maticians Galton and Pearson upon the old foundation of Quetelet, which more 
recently had received considerable modifications at the hands of the botanist 
Johannsen. This strives to show by the statistical method whether the charac- 
ters arising by fluctuating variations can be made hereditary by selection. 

If a study be made of the modifications of a single character in a 'population' 
(that is a large number of men, animals or plants living under similar conditions, 
but springing from different ancestors), it is seen especially clearly if char- 
acters such as length, breadth, weight, number, capable of accurate quantitative 
statement be chosen that the majority of individuals assume a middle point 
with regard to the development of this character, and that the variations from 
this mean, on either the positive or negative side (plus and minus variants) are 


HISTORY OF ZOOLOGY 45 

the fewer, the farther they be removed from the middle value. From the 
figures of the statistics which express the relative frequency of each grade of 
peculiarity there can usually be constructed a curve (Gallon's curve) with regular 
ascending and descending limbs. 

If now the extreme plus or minus variants of such an animal or plant popula- 
tion be bred and the peculiarities under investigation be studied in the descend- 
ants, there is found a 'regression,' the descendants of the plus or minus variants 
approaching the mean fixed for the population. For example, very large parents 
have on the average, offspring smaller than themselves, diminutive ancestors 
children larger than they. But since the return to mean is not complete, there 
appears a possibility, by continued selection to establish the variation from the 
mean as a permanent character. 

Again there are the cases where 'pure strains' are employed in breeding, 
where the descendants from one and the same parents, or better, from a her- 
maphrodite plant, are used and the plus and minus variants are continually 
inbred. There then follows a complete reversion; the descendants of the plus 
variants give the same curve, with the same mean and the same limbs as the 
minus variants and the same as the pure strain. "Within the pure strain there is 
therefore a certain 'genotypic character,' revealed by Gallon's curve, equally ap- 
plicable, whether plus or minus variants or mean forms be employed for breeding; 
against which selection is powerless. From these results, drawn certainly from 
insufficient empiric material, many have concluded that the fluctuating variations, 
on which Darwin's theory lays such stress, cannot be taken into account in the 
origin of new species. 

The breeding of pure strains has given another important conclusion regard- 
ing the nature of variations: that mutations, that is pure-breeding, sharply cir- 
cumscribed variations, are much more common than had been thought. There 
are mutations, which on account of the inconspicuousness of the character are 
easily to be regarded as fluctuating variations, and very likely some of these were 
regarded by Darwin as such. If different field crops (wheat, oats, vetches, 
alfalfa) or many meadow grasses be cultivated in pure strains, there are found 
among the descendants of the same ancestors not a few varieties, which differ 
by such slight characters that the eye of the trained breeder is necessary to 
recognize them; yet, by prolonged pure cultures, they show themselves constant 
and furnish most favorable material for artificial and natural selection. 

Mendelism. In a third way these studies of pure strains have been remark- 
ably fruitful. These are the researches which have followed the experiments of 
the Abbe Mendel upon inheritance, by crossing varieties, races and species. 
Since the explanation of the complicated phenomena involved will be given in 
a later section, it need only be said here that if in certain cases races be crossed 
and inbred for several generations, there arises a multiplicity of forms which 
have the appearance of newly arisen varieties. But more accurate study shows 
that these are not new forms but are only the grouping of manifold characters 
which were indeed present in the parents, often in a latent state. These 'analytic 
varieties' called forth by crossing are in part constant and can furnish material 
for new varieties. 

The importance of all of these researches cannot be estimated too highly; 
not because they have already given a final decision on the significance ot the 
various kinds of variability but because they have brought the problem of 
species formation from the region of much sterile theoretical speculation into 
the clear light of exact experimental investigation. 

Migration Theory. To explain how characters formed by variation 
become fixed, and do not disappear again through crossing with differently 


46 GENERAL PRINCIPLES OF ZOOLOGY 

modified individuals, Moritz Wagner has proposed the Theory of Geo- 
graphical Isolation, or the Migration Theory. New species may arise if 
a part of the individuals of a species should wander to a new place, in which 
crossing with the others of their species who were left behind is not possible. 
The same might occur, if geological changes should divide the region 
inhabited by a species into two parts, between which interchange of 
forms would no longer be possible. The animals remaining under the 
old conditions would retain the original characteristics; the wanderers, on 
the other hand, would change into a new species. Direct observations 
support this. A litter of rabbits placed at the beginning of the fifteenth 
century on the island of Porto Santo has increased enormously and the 
descendants have taken on the characteristics of a new species. They 
have become smaller and fiercer, have acquired a uniformly reddish color, 
and no longer pair with the European rabbit. A further proof of the 
theory of geographical isolation is the peculiar faunal character of regions 
separated from adjacent lands by impassable barriers, broad rivers or 
straits, or high mountains (comp. p. 34) ; especially instructive is the pecul- 
iar faunal character of almost every island. The fauna of an island 
resembles in general the fauna of the mainland from which the island has 
become separated by geological changes; it usually has not only these but 
also 'vicarious species,' i.e., species which in certain characteristics closely 
resemble the species of the mainland. Such vicarious species have 
plainly arisen from the fact that isolated groups of individuals, scattered 
over the island, have taken on a development divergent from the form 
from which they started. With all due recognition of the migration 
theory, it will never be possible to explain the multiformity of the organic 
world by it alone. It must be assumed that formerly the earth's surface 
possessed an enormous capacity for change; but the more rescent investi- 
gations make it probable that the distribution of land and water has not 
varied to the degree that was formerly believed. The experience of 
botanists, too, teaches that several varieties can arise in the same locality 
and become constant. 

Lamarckism. \Vhile the migration theory agrees with Darwinism 
in this, that the new characters appearing through variation are to be 
regarded as the products of chance, yet it is just this part of the theory 
which has been subjected to searching criticism. Many zoologists have 
again adopted the causal foundation of the descent theory proposed by 
Lamarck and believe that the cause of species formation is to be found in 
part in the immediate influence of changing environment, in part in the 
varying use and disuse of organs, brought about by alterations in the con- 
ditions of life. Both principles, they say, are sufficient, even without the 


HISTORY OF ZOOLOGY 47 

help of the struggle for existence, to explain the phylogenesis of organ- 
isms. (Neo-Lamarckism.) 

Influence of Environment. To what extent can the environment 
directly bring about a permanent change in the structure of plants and 
animals? To decide this is no simple problem, on account of the com- 
plexity of the factors entering into the question. 

In cases where the food-supply is altered, organisms change in a very 
remarkable manner and within a short time; but these changes (Nageli's 
' Modifications through Nutrition') seem to have no permanence. Plants 
which, found in nature in poor soil, are transplanted into rich soil, or 
vice versa, soon acquire quite a different appearance, and preserve this 
through the following generations, so long as they remain in the rich soil; 
but the plant quickly returns to its former appearance when replaced in 
its previous surroundings. 

In general, a change seems to be the more permanent the more slowly 
it has developed. In researches upon the influence of environment, it is 
better to experiment with slowly-working factors, such as light and heat, 
dry or moist air, different intensities of gravitation, of stimuli, etc., which 
can be excluded from the environment of the organism. In this way 
positive results seem to have been attained. When pupre of Vanessa 
urtica and Arclia caja are reared in the cold (down to 8C.) the butterfly 
or moth which escapes is more or less conspicuously modified, the male 
the more. If these altered males and females be used for further breeding, 
a part of the male offspring will have the cold markings, even if reared in 
the normal conditions. It is however probable that in these cases the 
cold had a direct effect upon the germ cells from which the aberrant indi- 
viduals arose. The results of Tower in breeding potato-beetles are in the 
same direction and are even more conclusive as to the modification of the 
germ cells. 

Use and Disuse. Regarding the efficiency of use and disuse, there 
is no doubt that an animal is influenced to a great extent by the manner in 
which the organs are used. The organs which are much used become 
strong and those which are not used become weak. The only question is 
whether these, in the strict sense of the word, newly-acquired character- 
istics are transmitted to the offspring, or whether the descendants, in 
order to attain to the same condition, must repeat in the same way use and 
disuse. In the latter case the cumulation of characteristics, and with it 
the possibility that these may become permanent, is excluded. It is to be 
regretted that accurate results are still lacking on a point so well adapted 
for experimental treatment. At this time rudimentary organs strongly 
favor the Lamarckian principle; for we see that cave animals, which for 


48 GENERAL PRINCIPLES OF ZOOLOGY 

many generations have lived in darkness, are blind, either having no eyes, 
or only vestiges of them, incapable of function. This seems to justify the 
view that this condition is attributable to lack of use, since it has brought 
about a functional and anatomical incapacity, which has increased from 
generation to generation. Now we must believe that what is true for 
disuse must express itself in the reverse sense in the case of use. 

Nageli's Principle of Progression. In conclusion, there is still to 
be considered the change of species from internal causes, which Nageli 
has termed the 'perfecting principle,' or the 'principle of progression.' 
It cannot be denied that each species is compelled, by some peculiar 
internal cause, to develop into new forms, up to a certain degree inde- 
pendently of the environment and of the struggle for existence. In all 
branches of animals we see the progress from lower to higher going on, 
very often in a quite similar way, in spite of the fact that the plan of 
organization is- so different in the various phyla. We see how the nervous 
system, lying near the surface in the lower animals, becomes in the higher 
animals internal; how the eye, at first a simple pigment-spot, becomes in 
worms, arthropods, molluscs, and vertebrates, provided with accessory 
apparatus, as lens, vitreous body, iris, chorioid. Here we see an energy 
for perfection which, since it occurs everywhere, must be independent of 
the individual conditions of life, and must have its special explanation 
in the reaction of the living substance to light. 

It is by no means justifiable to call an assumption, as here expressed, 
teleological, and to reject it as unscientific; rather the organism seems to be 
just as mechanically conditioned as a billiard-ball, whose course is deter- 
mined not only by contact with the cushions of the billiard-table, but also 
in a large measure by its indwelling force, imparted to it by the stroke of 
the cue. An organism, too, is a store of energy which it must necessarily 
have developed from itself, but it is of more extraordinary complexity, 
and to an equal degree also is independent of the external world. A 
complete independence naturally never occurs. Instead there is always an 
'action' of the external world, a modifying influence which is carried on 
by the external conditions of existence, either directly or by the mediation 
of use and disuse. 

This outline of evolution has been given in a rather detailed way, 
because in the history of zoology it is the most important feature. No 
other theory has gained such a hold, none has propounded so many 
new problems and opened so many new fields for research. There is no 
other zoological theory which compares with it in value as a working 
hypothesis. To the objection that the theory is insufficiently grounded, 


HISTORY OF ZOOLOGY 49 

it can only be replied that it is the only theory which agrees with our ex- 
periences and explains these in a simple way and on a scientific basis. 
In this sentence is given the merit of the theory, and also a limitation of its 
applicability. For on the one side the statement attributes the merit in the 
applicability of the system to the necessity of the human mind for simple 
explanations of the facts of natural science, and on the other hand it makes 
the degree of correctness dependent upon the state, whatever it may be, 
of our knowledge. Many investigators see no necessity of reconciling 
paleontology and our knowledge of plants and animals. To such the 
Darwinian theory proves just as little as any opposing theory. Meanwhile 
thoughtful naturalists will keep in mind that our knowledge of nature is 
making considerable advances, and is visibly becoming wider and deeper. 
It is possible, even probable, that these advances will lead to many modi- 
fications of the theory. The conception of the way in which forms have 
developed from one another admits, as the mutation theory shows, 
of very different expressions. On the other hand, we can affirm with 
great certainty that the principle of descent, which first obtained cre- 
dence through Darwinism, will be a permanent landmark of zoological 
investigation. 


GENERAL MORPHOLOGY AND PHYSIOLOGY. 

General Zoology: Animal Morphology. In vital phenomena 
a certain degree of similarity can be followed through the animal kingdom; 
the way in which animals are nourished and reproduce their kind, move, 
and gain experience, is essentially the same in great groups. Correspond- 
ing to this, the apparatus concerned with the above-mentioned functions, 
the organs of nutrition and reproduction, of motion and sensation in their 
grosser and finer structure, and in their ontogeny, must be similar to one 
another and show evidence of some fundamental characters which always 
or frequently recur. 

(Ecology or Biology. When by means of anatomy and embryology 
we have learned the general character of the organism, we must then 
study its relations to the environment. In this study of the conditions of 
animal life, cecology or biology, we consider the geographical range of 
animals, their distribution over the surface of the earth and in the different 
depths of the sea; further, the reciprocal relations of animals and plants, 
and of beast to beast, as these find special expression in colony-building, 
symbiosis, parasitism, etc. 

General Anatomy. The synthesis of an organism, of which we can only 
gain an idea by general anatomy, actually takes place in nature during the 
development of every animal. Embryologically every organism is at 
some time a simple element, a cell; this divides and gives rise to tissues; 
from the tissues are formed organs, and from the organs the regularly 
membered whole of the animal body is composed. 

I. GENERAL ANATOMY. 

The Morphological Units. The expression 'constituent parts of the 
animal body' can be used in a double sense. We can speak of the chemical 
units, the chemical combinations, which form the tissues; these are the 
subject of animal chemistry, and may therefore be passed over here. 
But we may also speak of the units of form (morphological units) of the 
animal body; these are the cells. These and their transformation into 
tissues, organs, and entire animals are for us of vastly greater importance. 

50 


GENERAL ANATOMY 51 

I. THE MORPHOLOGICAL UNITS OF THE ANIMAL BODY. 

The Cell. The study of the morphological units of the organic 
body first found a firm foundation in the cell theory. Every scientific study 
of the anatomy of plants and animals must therefore take the cell as its 
starting-point. 

The Cell Theory. In order clearly to understand the conception of 
the cell and its name it is necessary to follow a little of the history of the 
theory of the cell. When the name was first given to the structures in 
plants it implied small chambers with firm walls and filled with air or 
fluid. Then came the discovery of a small body, the nucleus, inside the 
cell. Next Schleiden made the generalization that the cell was the ana- 
tomical and physiological unit from which all plants are formed, but he 
held the erroneous view that in the development of cells, the nucleus was 
formed in a sort of matrix, then around it a nuclear membrane arose 
by precipitation, and then a larger membrane, the cell wall, around the 
whole. Then Schwann extended the generalization to animals, thus 
giving it an extension to all organisms. 

In this Schleiden- Schwann cell theory the cell wall was all important, 
as through it diffusion currents must pass between the contents and the 
surrounding medium. Hence the wall and the contents must determine 
the character of the cell according to physical laws. Since the life of an 
organism is but the totality of the life of the cells of which it is composed, 
it was thought that the theory was a great advance in the problem of the 
physical explanation of the phenomena of life, and the origin of the cells 
themselves was as well explained as the formation of a crystal. 

Our conception of a cell has completely changed. We know that they 
do not arise like crystals, but from preexisting cells. The cell is not 
merely a part of an organism; it is a physiological whole, which shows us 
all the enigmas of life. The membrane and cell sap, so important in the 
Schleiden-Schwann theory, have but a subordinate place, but all impor- 
tant is the previously disregarded substance, protoplasm. Now a cell 
may be defined as a small mass of protoplasm with one or more nuclei. 
This change in the conception came so gradually, that the name cell has 
persisted, although it is an evident contradiction to call a solid lump 
without a membrane a cell. 

These changes were due to many different lines of investigation. Thus 
the early discovery that the chlorophyl bodies in plant cells move and, later, 
that the motion is due, not to the bodies, but to the substance in which 
they are imbedded. This substance, to which the name protoplasm was 
given, acquired prominence when it was found that in the simplest 


52 GENERAL PRINCIPLES OF ZOOLOGY 

algae, it, together with the chlorophyl, could leave the cell wall and swim 
freely in the water, eventually giving rise to a new plant, while the cell 
wall no longer showed signs of life. Then it was discovered that many 
animal cells had no cell membrane. These observations at once placed 
the cell wall in the background, while the protoplasm was recognized as 
all important. Here, too, should be mentioned the researches on the Pro- 
tozoa, by which it was recognised that these organisms had no true organs, 
but carried on all of the functions of life by means of a granular substance, 
at first called sarcode. 

Then followed the recognition of the identity of the protoplasm of 
plants, the sarcode of the protozoa and the cell substance of animal cells. 
Equally important was the new idea as to the modifications of cells 
and the differentiation of tissues. These are not so much modifications 
of form and the like, based on osmosis and other physical phenomena 
as upon chemical changes. By means of its formative potentiality the 
protoplasm gives rise to non-protoplasmic structural parts, as, for ex- 
ample, -connective-tissue fibrils, muscle fibrils, nerve fibres, etc. These 
give the various tissues their specific character and perform their 
functions. The tissues also retain as the source of life and formation 
the unemployed remnants of cells, the connective-tissue corpuscles, 
muscle corpuscles, etc. 

Nature of the Cell. The size of the animal cell varies; the smallest 
are the male sexual cells, the spermatozoa, whose bodies, in case of the 
mammals, may measure only 0.003 mm.; the largest, with the exception 
of the giant plasmodia of some Mycetozoa, are the egg cells. The yolk 
of the bird's egg, which alone forms the egg in the narrower sense, has for 
a time the value of a cell, and in the ostrich egg may reach a diameter 
of several inches. The form is likewise variable. Free cells, are usually 
spherical or oval as the egg cell shows; united into tissues, the cells, on the 
contrary, may be pressed together into polygonal or prismatic bodies, or 
may send out branching processes. 

Protoplasm. So there is left to characterize the cell only its sub- 
stance: the cell is a mass of protoplasm with one or more nuclei. It is 
not known whether protoplasm is a definite chemical body, capable of 
infinite variation, or is a varying mixture of different chemical substances. 
So, also, we are not certain whether or not these substances (as one is 
inclined to believe) belong to the proteids. We can only say that the con- 
stitution of protoplasm must, with a certain degree of homogeneity, have 
a very extraordinary diversity. For if from the egg of a dog there comes 
always and only a dog with all his individual peculiarities; that a sea- 
urchin's egg, under the most diverse conditions, produces always a sea- 


GENERAL ANATOMY 


urchin; that a species of amoeba always performs only the movements 
characteristic of that species, we must assume that the functioning part 
of this cell, the protoplasm, has in each case its peculiarities. We are 
driven to the assumption of an almost unlimited diversity of protoplasm, 
even if we concede an important share in the prominent differences to 
the nucleus, of which we shall speak later. 

General Properties of Protoplasm. The similarity of protoplasm 
expresses itself in its appearance and in its vital phenomena. Under slight 
magnification, protoplasm appears as a faintly gray substance (sometimes 
colored yellowish, reddish, etc., by pigments) in which numerous strongly- 
refracting granules are imbedded. The vital characteristics of this sub- 
stance are movement, irritability, power of assimilation and of reproduction. 

By using higher powers a finer structure can be seen in the 'homogeneous 
protoplasm' of earlier writers. It looks like a fine-meshed framework (filar 
substance, spongioplasm, cell reticuluin) the interstices of which are filled with 
other material (interfilar substance, enchylcma, ground substance}. The question 
whether this framework is formed of threads and trabeculae or whether the 
appearance is not formed by small cham- 
bers, bounded by fine partition-walls 
(foam structure of protoplasm), such as 
results when two fluids which do not mix 
(like olive oil and soda solution) are 
shaken together until a very fine froth 
is produced. This view that protoplasm 
has a foam structure explains how it can 
be a fluid aggregate with a fine structure. 
Regarding the fluid aggregate condition cf 
protoplasm (long called a 'solid-fluid') 
there has been much dispute. Exact re- 
searches regarding its physical condition 
show that it behaves like a fluid. 

Movement of Protoplasm. Move- 
ment expresses itself first in changes of 
form of the whole body amoeboid move- 
ment and secondly in the change of 
position of the small granules in the 
interior of the protoplasm streaming 
of granules. Examples of amceboid 
movement (fig. 16) are found in many 
Protozoa, and the colorless blood-cells (leucocytes) of multicellular ani- 
mals; here the protoplasmic body sends out coarser and finer processes, 
which may be again withdrawn, serving for locomotion and hence called 
pseudopodia. The streaming of granules can be observed in the interior 
of the cell-body, as well as in the pseudopodia extending from this. The 
pseudopodia may even be so fine as to be at the limits of visibility with our 


FIG. 16. Amccba proteus (after 
Leidy). ek, ectosarc; en, entosarc; 
cv, contractile vacuole; n, nucleus; 
N, food-vacuoles. 


54 


GENERAL PRINCIPLES OF ZOOLOGY 


NT 


FIG. 17.^ Gromia oi'iformis (from Lang, after M. Schultze). 


GENERAL ANATOMY 55 

microscopes (fig. 17), yet in them the granules wander hither and thither 
like people on a promenade, simultaneously centripetally and centrifugally, 
some with greater, others with less speed. The granules are moved by 
the protoplasm, for if we feed the creature with finely-pulverized car- 
mine, these granules show the same remarkable streaming. Indeed, 
nothing better illustrates the great complexity of protoplasm than these 
phenomena of motion in such narrow limits as pseudopodia in general. 
Irritability of Protoplasm. That amoeboid movements and 
streaming of granules can be induced, brought to a standstill, and modified 
by mechanical, chemical, and thermal stimuli, is a proof of the irritability 
of protoplasm. Most important are the thermal stimuli; if the surround- 
ing medium rise above the ordinary temperature, the movements at first 
become more rapid up to a maximum: from that point begins a slowing, 
finally coming to a standstill heat-rigor. If the high temperature 
continue much longer, or if it rise still higher, death results. The fatal 
temperature for most animals is between 40 and 50 C. (io4-i22 F.); 
its influence explains a part of the injurious effects which high- 
fever temperatures have upon the human organism. Like the heat- 
rigor, there is a cold-rigor, induced by a marked sinking of the tempera- 
ture below the normal. This is accompanied by a gradual diminution 
of mobility ; it results in death by freezing, which is, however, not so easily 
produced as death by heat. It is a remarkable fact that many animals, 
consequently their cells, may be frozen; and in this condition can endure 
still severer cold without dying. (For example: goldfish, a temperature 
of -- 8 to -- 15 C.; frogs, to - 28; 'blind worms', to -- 25). 

Nutrition and Reproduction. Irritability and power of motion 
are necessary for assimilation. Most animal cells, for example almost 
all the tissue cells, are not suitabe for studying assimilation, because they 
live upon liquid nourishment. But certain cells of higher animals, 
the leucocytes, and most unicellular animals can be fed with solid 
substances; they take the food-particles into the midst of the protoplasm 
by flowing around them with the pseudopodia. They extract all the 
assimilable and reject the indigestible portions (fig. 16). 

In the case of assimilation it is to be noted not only that the cells use 
the food which they have taken for their own growth and for replacing 
worn-out parts, but also that most of them have the power of producing 
substances other than protoplasm; for example, many Protozoa form 
shells or skeletons which are hardened with silica or lime. This formative 
power, the building of plasm ic products, is the starting-point for tissue- 
formation. 

Cell Nucleus. The reproduction of protoplasmic bodies is synony- 


50 


GENERAL PRINCIPLES OF ZOOLOGY 


mous with the division of the cell; but to understand this we must first 
consider the nucleus. This is a body enclosed in the protoplasm, whose 
form, though definite for each kind of cell, shows in general wide varia- 
tions. Usually it is spherical or oval, but it may be elongated or rod- 
shaped, bent into a horseshoe, with constrictions like a rosary, or even be 
branched or treelike (fig. 18) ; in many living cells it is but little different 


FIG. 18. Various forms of nuclei, a, horseshoe-shaped nucleus of an Acinete; b, 
branching nucleus from the Malpighian vessel of a Sphingid larva; c, rosary-shaped 
nucleus of Stentor cceruleus. 


in appearance from the protoplasm and can only be seen with care and by 
employment of a special technique based upon the microchemical reaction 
of the nuclear substance. 

The Nuclear Substance. The nuclear substance is distinguished 
from protoplasm, among other ways, by its greater coagulability in certain 
acids, e.g., acetic and chromic, which therefore are often used for demon- 
strating the nucleus. In its minute structure the nucleus affords a wonder- 
ful variety of pictures varying according to the objects chosen. Accord- 
ing to their reactions to stains two substances in particular are distin- 
guished: chromatin or nude in (fig. 19, ch), which is easily stained by certain 
staining-fluids (carmine, haematoxylon, saffranin), and the achromatin 
or linin, which stains only under special conditions. 

The achromatin forms a network or reticulum (according to another 
view a honeycomb structure) filled with a nuclear fluid, bounded exter- 
nally by a nuclear membrane. If little nuclear fluid be present, and the 
reticulum consequently be narrow-meshed, the nucleus seems compact. 


GENERAL ANATOMY 


57 


If the fluid be abundant, the nucleus appears vesicular. This is espe- 
cially the case when the lines of the framework are separated \)y consider- 
able amounts of nuclear fluid (fig. 19, 4). 

The chromatin enters into close relations with a less stainable element, 
also distinct from, achromatin, the plastin, (paranudein, p). In the 
protozoan nuclei plastin and chromatin are usually intimately united, 
the first forming a substratum in which the latter is imbedded (clip). 
The united substances are most frequently closely and regularly dis- 
tributed as fine granules on the reticulum, so that the entire nucleus 


ch p 

4 5 6 

FIG. iq. Vesicular nuclei with achromatic reticulum and different arrangements 
of the chromitin and nucleolar substance: p, plastin (nucleolar substance); ch, 
chromatin; chf, chromatin plus plastin. i and 2, nuclei of Actinosphcerium; 3, of 
Ceratium hintndclla (after Lauterborn); 4, germinal vesicle of Unio (after Flemming); 
5, nucleus with many chromatin nucleoli. 

appears uniformly chromatic (fig. 18). More rarely the mixture collects 
into one or more special bodies, the chromatic nucleoli (amphinucleoli, 
caryosomes, fig. 19, i, 2). The nucleolus is ordinarily a rounded body, 
more rarely branched (fig. 19, i). 

In the nuclei of the Metazoa there may occur the same intimate mix- 
ture of plastin and chromatin (6). As a rule, however, the plastin 
(apparently not the whole, but a surplus) is separate from the chromatin. 
Thus there occur in the nuclei of many eggs nucleoli which consist of two 
distinguishable parts, the one containing chromatin, the other chromatin 
free (4). Usually in tissue cells only the plastin has the form of 


58 


GENERAL PRINCIPLES OF ZOOLOGY 


nucleoli (true or chromatin-free nucleoli), while the chromatin is dis- 
tributed on the nuclear reticulum (chromatin reticulum). Much the 
same may occur in ihe Protozoa (fig. 19, 3). 

Beside and outside of the nucleus there occurs in many Protozoa a 'chromid- 
ial apparatus, ' a substance agreeing in its staining properties with the nuclear 
substance. Its pertinence to the nucleus is also shown by the fact that repeatedly 
it has been observed to arise from the nucleus (ActinosphcFriutn), as well as 
to be transformed into nuclei (Radiolaria, Monothalamia). The chromidial 
mass may surround the nucleus like a cortical layer (Euglypha, fig. 20, III, 
Radiolaria), or penetrate the protoplasm as a loose network (II), or form lumps 
or coiled threads. In this last shape the chromidial mass seems to be widely 
distributed in strongly functioning cells of Metazoa (I). Possibly the structures 
described as ' mitochondria' are identical with it. 


II. 


TIL 


I. 


ch 


FIG. 20. Cells with chromidial apparatus. I, muscle cell of Ascaris (after 
Goldschmidt). II, Arcella, with two nuclei and loose chromidial net. Ill, Euglypha 
with compact chromidial envelope of the nucleus, ch, chromidial mass;/, food body; 
m, mouth of shell; n, nucleus; r, reserve material for new shell. 

Function of the Nucleus. For a long time the function of the 
nucleus in the cell was shrouded in complete darkness, so that it was 
regarded, in comparison with the protoplasm, as of little importance. The 
evidence that the nucleus plays the most prominent role in fertilization 
has altered this conception. Then arose the view that the nucleus deter- 
mines the character of the cell ; that the potentiality of the protoplasm is 
influenced by the nucleus. If from the egg a definite kind of animal 
develop, if a cell in the animal's body assume a definite histological 
character, we are, at the present time, inclined to ascribe this to the nu- 
cleus. From this it follows that the nucleus is also the bearer of heredity; 
for the transmission of the parental characteristics to the children can 
only be accomplished through the sexual cells of the parents, the egg and 
sperm cells. Again, since the character of the sexual cells is determined 
by the nucleus, the transmission in its ultimate analysis is by the nucleus. 
This idea has a further support in experiments on Protozoa. If one of 


GENERAL ANATOMY 


59 


these animals be cut into nucleate and anucleate halves, the first lives and 
regenerates the lost parts; the anucleate portion moves about for a time, 
apparently as long as the stored energy lasts, but it cannot assimilate or 
reproduce the missing portions and so sooner or later it dies. 

The Centrosome. Besides the nucleus there frequently occurs a 
special body in the protoplasm, the centrosome, which, on account of its 
small size and a behavior similar to achromatin with reference to staining- 
fluids, was long overlooked. It is well distributed among the Metazoa, 
but is absent from most Protozoa, in many of which it appears only at 
certain times and then disappears. It is probable that it is a derivative of 
the nucleus, a part of the achromatin which has left the nucleus; in other 
cases possibly a second nucleus which by degeneration has lost the chro- 
matin and retained only the motor nuclear substance, the achromatin. 
In its function the centrosome is a specific organ of cell division which 
controls both the division of the nucleus and that of the cell itself. 

Multiplication of Cells. Increase in cells occurs exclusively by 
division or by budding (gemmation). Most common is binary division 
in which a circular furrow appears 
on the surface of the cell, deepens 
and cuts the cell into two equal 
parts. Mutiple division is more 
rare and can only occur in multi- 
nucleate cells. Here the cell 
divides simultaneously into as 
many (sometimes hundreds) 
daughter-cells as there were nuclei 
present. In all forms of division 
the similarity of the products is 
characteristic, while in budding 
the resulting parts are unequal, 
one or more smaller daughter- 
cells, the buds, being constricted from a large mother-cell (fig. 21). 

Direct Cell Division. Every cell division is accompanied by nuclear 
division or nuclear division has previously occurred. Direct and indirect 
division are recognized. Direct division is most common in Protozoa, 
especially in nuclei with abundant chromatin (figs. 21, 120, 150, 155). 
The nucleus elongates and is divided by constriction, in the same way 
that the cell itself constricts. Since the protoplasm has no special arrange- 
ment for dividing the nucleus (the latter besides protected by its mem- 
brane), we must conclude that the nucleus divides itself. The dividing 
force resides in the achromatic framework, which correspondingly often 


FIG. 21. Cell budding. Pcdjphrya geni- 
mipara with buds (a) which separate and 
form free young (b). N, nucleus. 


60 


GENERAL PRINCIPLES OF ZOOLOGY 


FIG. 22. Spindle formation and 
division of the centrosomes in As- 
caris megalocephala (after Brauer). 
c, centrosomes; ch, chromosomes. 


exhibits a certain arrangement, a fibrous structure in the direction of the 
elongating nucleus. 

Indirect Cell Division, Karyokinesis. Indirect cell division, 
karyokinesis or mitosis,' is most beautifully shown in cells, poor in chro- 
matin, which possess a centrosome. The process is introduced by a 
division of the centrosome (fig. 22). The daughter centrosomes migrate 
to two opposite poles of the nucleus, which now loses its membrane and 
becomes the nuclear spindle. The characteristics of the spindle are that 

it is drawn out into points at two poles 
which are indicated by the position of the 
centrosomes, while from these poles fine 
threads, the spindle-fibres, run to the 
centre or equator of the nucleus. These 
fibres are in many cases certainly derived 
from the achromatic nuclear reticulum, 
while in others a greater or less part in 
their formation is taken by the protoplasm 
(fig. 22.) A debated point is the rela- 
tions of the fibres in the equatorial plane 
of the spindle. Do all the fibres extend 
from pole to pole ? Do all of them end 
in the equatorial plane, so that the spindle consists of two cones of fibres 
separated at the equator? Or, lastly, are fibres of both kinds present 
in the same spindle ? It would appear that differences exist in these 
respects in different cells. 

All of the chromatin of the nucleus lies in the equator, united in the 
'equatorial plate,' but by this must not be understood a connected mass 
but a layer of separate bodies, the chromosomes (fig. 23, a). These 
develop at the beginning of nuclear division by the union of the chro- 
matin granules (which are distributed diffusely over the reticulum of the 
resting nucleus) to strongly staining bodies, which are rarely spherical 
or rodlike, but usually have the shape of U-shaped loops. It is of the 
greatest theoretical significance, that their number is identical in all the 
cells of all the tissues of one and the same species. 

The first step in the mitotic formation of the daughter nuclei is the 
division of the chromosomes, which is usually completed in the equatorial 
plate (division of the equatorial plate), but may be completed earlier. 
The division is an accurate halving (fig. 23, b). The two halves of a 
mother-chromosome, the daughter chromosomes, now travel, under the 
influence of the spindle-fibres, towards the poles of the spindle. In this 
way, by a splitting of the equatorial plate, the lateral plates arise, the 


GENERAL ANATOMY 


61 


elements of each uniting and producing the daughter nuclei. The centro- 
somes remain separate as division organs for the next nuclear division 
(fig. 23, c, d, e). 

What further distinguishes the indirect from the direct cell division 
is the active participation of the protoplasm. The centrosome is the 
centre of a marked radiation (aster) of the protoplasmic reticulum (fig. 22). 
When the centrosome divides a double radiation appears, the monaster 
becomes an amphiaster. Not only the spindle-fibres but the protoplasmic 


d e f 

FIG. 23. Cell division in the skin of SalamanJra maculosa (after Rabl). 

rays extend from the daughter chromosomes. Since the arrangement 
and degree of development of the protoplasmic radiations stand in definite 
relation to the different phases of cell division we must recognize in them 
the expression of the forces (apparently contractile) in the protoplasm 
which cause cell division. 

Between these two extremes of direct and indirect division are transitions 
which show how the mechanism of nuclear division has been completed step by 
step, first, by the fibrous arrangement of the nuclear reticulum (spindle structure : 
second, through the development of the centrosome by which the division ob- 
tains an influence on the protoplasm; and third, by the organization of the 
chromosomes. The irregular division of the chromatin mass in direct division 
is relatively crude in comparison with the complicated processes involved in the 
formation and division of the chromosomes. These become intelligible it we 
regard the chromatin as the controller of the cellular processes and the bearer 
of heredity (cf. fertilization, infra). The more highly organized the animal, 
the more its cells have to inherit and the more important it is that the physical 
basis of heredity should be accurately divided in amount and in quality be- 
tween the daughter cells. This is accomplished by mitosis. 


62 GENERAL PRINCIPLES OF ZOOLOGY 

Connected with this great functional importance of the chromosomes as the 
bearers of characteristics are two much disputed problems, (i) The 'individual- 
ity of the chromosomes.' This sees the persistent organization of the cell in 
the chromosomes, which persist between two cell divisions, but are not recog- 
nizable as such because their substance is vacuolated and distributed through 
greater space. Of course this view does not conflict with the fact that they, 
like all living substance, undergo a gradual renewal, in which effete parts are 
replaced by new and there is an increase of its substance without which a repro- 
duction of chromosomes by division would be impossible. (2) The theory of 
the functional diversity of the chromosomes. If the chromosomes carry the 
characteristics, it is more probable that each one does not contain the germs of 
all the peculiarities of the organism; rather there is a division of labor by which 
the separate peculiarities are distributed among the different chromosomes. 
This view is supported by the fact that there is, in numerous instances, a morpho- 
logical differentiation between them (differences in size, shape, staining qualities). 
That in the last analysis each category of characters consists of at least two 
lines (male and female) is shown by the fact that, as is ex- 
plained in the section on fertilization, half of each nucleus is 
derived from the father, half from the mother. 

Nuclear Fragmentation is to be distinguished from 
direct division; by it the nucleus becomes broken up into a 
few or numerous parts. Such nuclear fragmentation is not 
rare in the Infusoria but it occurs occasionally in Metazoa 
(giant cells of bone marrow fig. 24 osteoblasts, certain 
stages of the genital cells). It is explained as follows. There 
normally exists a certain size relation between nuclear mass 
and protoplasmic mass. With greater cell activity, as with 
Infusoria which have long been well fed, the nucleus grows 

24 Giant- at * ne ex P ense of the protoplasm until it reaches a size which 

cell with many makes further assimilation and increase impossible. This 
nuclei. kind of animal (or cell) can return to the normal vital 

activities if the nuclear mass be reduced. This is begun by 
the fragmentation and is completed by the resorption of the nuclear substance. 
Many cases of nuclear fragmentation, formerly regarded as amitoses are really 
functional conditions of actively functioning cells and have erroneously led to 
the view that amitosis is beginning cell degeneration. 

Multinuclearity, Multicellularity. Nuclear division and cell division 
commonly constitute a well-arranged mechanical process, the separate 
phases of which follow one another according to a definite law. The 
plane of division is perpendicular to the long axis uniting the two poles of 
the spindle; usually also each phase of division of the nucleus corresponds to 
a certain phase of the protoplasmic division. But the interrelation of 
cytoplasm and nucleus is by no means an unchangeable and indissoluble 
one, for very often nuclear division takes place without participation of 
the cytoplasm. If this process be repeated several times, there results 
a mass of protoplasm with many nuclei (fig. 24), which now may become 
many cells, if subsequently the protoplasm divide according to the number 
of nuclei. Hence multinucleated protoplasmic masses are transitional 
stages between the simple mononucleated cell and a collection of several 


GENERAL HISTOLOGY 


63 


mononucleated cells, and in consequence of this are sometimes regarded 
as the equivalent of one cell, sometimes as equivalent to many cells, and 
are called sometimes multinucleated giant-cells, sometimes syncytia. In 
the following pages a multinucleated mass of protoplasm will he consid- 
ered as a single cell, because a cell is a vital unit, has a physiological 
individuality, and in this respect a multinucleated mass of protoplasm 
behaves like a mononucleated. As the tissue cells and the Protozoa show, 
the plane of organization is not raised in the least by the multinuclearity. 
A change only begins at the moment when many particles of protoplasm 
are separated from one another, and many vital units are formed, i.e., 
when in place of multinuclearity a true multicellularity appears. 

II. THE TISSUES OF THE ANIMAL BODY. 

Definition of Tissue. In the formation of tissues two processes are 
operative: (i) the multiplication of cells into cell-complexes, and (2) 
the histological differentiation of cells. A tissue, 
therefore, can be denned as a complex of differen- 
tiated cells histologically similar. 

Histological Differentiation. The chief result 
of the histological differentiation is that the cells 
have a definite form and definite relations to neigh- 
boring cells. In addition, there almost always 
occurs, as a more important feature, the histological 
modification of the cell. The fact has already been 
mentioned that the cell uses its food-material, no', 
only for its own growth, for increase of its proto- 
plasm, but also for forming substances, plasmic 

products, either in its interior (internal plasmic procl- 

- , 11- FIG. 2v Forma- 

ucts), or more often on its surface (external plasmic tion of muscle t - lbri i s 

products). The histological differentiation is the in the frog (diagram). 

._ , . ,, a, formative cell; b, 

formation of specific plasmic products. A cell in f ormat } ve cell with 

becoming a muscle fibre (fig. 25), continually secretes 
upon its surface new fibrillae of specific muscle sub- 
stance until finally the remnant of the formative cell, 
the 'muscle corpuscle,' is contained in a mantle of 
muscle fibrillae. In the same way, each tissue, upon histological ex- 
amination, is seen to be composed of cells and plasmic products. The 
former control the formation, the renewal, and the sustenance of the 
tissue; the latter are the agents of its physiological function. The 
advantages of tissue formation are far-reaching, since in general they 
are connected with division of labor. So long as the cell unites in itself 


t\vo transversely stri- 
ated muscle fibrils; 
<-, formative cell with 
numerous muscle 

fibrils. 


64 GENERAL PRINCIPLES OF ZOOLOGY 

all the vital functions, these are incomplete because they mutually hinder 
each other in their free development; the plasmic product, on the other 
hand, has only the single function peculiar to it and can therefore per- 
form this with greater completeness. Muscle fibrilke, the characteristic 
elements formed by the muscle cells, have preserved of the various prop- 
erties of pro oplasm only the capability of contraction; but this contrac- 
tion is much more energetic and stronger than the mere movement of 
protoplasm. Nerve fibriliae only transmit stimuli, but in far more rapid 
and orderly manner than does simple protoplasm. 

Classification of Tissues. Since in every tissue its function interests 
us most, it is natural to classify tissues by the function and the intimate 
structure connected therewith. The tissues are arranged in four groups: 
i. Epithelial tissue; 2. Supporting tissue; 3. Muscular tissue; 4. Nervous 
tissue. Within these, however, certain parts of the animal body, to which 
indeed the term- 'tissue' is scarcely applicable, find no shelter; these are the 
sexual cells, the blood, and the lymph. The first may be spoken of in 
connection with the epithelium, the others with the supporting substances. 

i. Epithelial Tissues. 

Morphology of Epithelial Tissues. An epithelium is a layer of 
cells covering any free surface, external or internal, of the body. The 
epithelia must be considered first, because they are the oldest tissues; 
the first to appear in the animal kingdom, there being animals which 
consist only of epithelia. Further, every metazoan, during the first 
stages of embryonic life, consists only of epithelial layers, the germ- 
layers. With this is connected the fact that epithelial cells have under- 
gone the least degree of histological change, and that the formation of 
plasmic products is subordinated. 

Function of Epithelium. Epithelium forms a protecting and ex- 
cluding covering over surfaces, equally valuable whether the surfaces are 
external (surface of the body) or in cavities in the interior of the body 
(the body cavity, lumen of the gut and blood-vessels). The importance 
of the epithelia in this respect is shown by the fact that if the protecting 
layer be removed, inflammation arises and continues until the epithelium 
is regenerated. Only exceptionally do areas occur which are free from 
epithelium; the teeth of vertebrates, the antlers of stags, on account of 
their hardness, can exist, at least for a time, without epithelial covering. 

Glandular and Sensory Epithelia. By their position epithelia are 
suited for two other functions: all substances which ought to be removed 
from the body some because they have become useless, and consequently 


GENERAL HISTOLOGY 65 

injurious (excreta), and others, as, for example, the digestive fluids, because 
they have to perform important functions (seer eta, must pass the surface 
and are therefore exuded by the epithelia; these are the glandular epithelia. 
Further, all external influences chiefly impress the surface of the body, 
causing sensations; hence certain epithelia are of the greatest importance 
for the reception of sensory stimuli, and serve for hearing, seeing, smelling, 
tasting, and touching. Such areas of epithelium are called sensory 
epithelia. 

Covering Epithelium. The covering epithelium consists of cells 
which are united by a small quantity of cementing substance. We speak 
of simple or of stratified epithelia, according as we find, in sections running 
perpendicularly to the surface, one or several superimposed layers of cells 
(figs. 26, 27, 28). 

Simple Epithelium. Only one-layered epithelia are found in all in- 
vertebrated animals and in Amphioxus; in the vertebrates, on the other 
hand, they are limited to the internal cavities of the body, and even here 
are occasionally, as always in the skin, replaced by a many-layered 
epithelium. According to the shape of the cells we distinguish cuboidal 
or pavement, flat, and columnar epithelium. In pavement epithelium 
(fig. 26. b) the cells are developed about equally in all directions of space, 
and because they have become compressed by lateral pressure have the 
appearance of cubical blocks or paving-stones. In columnar epithelium the 
long axis, the distance from the deeper to the peripheral end of the cell, 
is especially great (fig. 26, c); finally, in flat or squamons epithelium this 
is greatly shortened (fig. 26, a) and the separate cells form thin plates. 

Flagellated and Ciliated Epithelia. Further differences in these 
three kinds of epithelium are caused by the presence or absence of pro- 
cesses (cilia, or flagella) on the peripheral end of the cells. These arc fine 
threads which arise from the body of the cell, extend above the surface 
and maintain an extremely lively motion. In flagellated epithelium 
(fig. 26, d) each cell has only one vibratile projection, but this is strongly 
developed; in ciliated epithelium (fig. 26, e), the surface of each cell is 
covered with a thick forest of shorter threads moving in unison. 

Cuticle. The majority of the one-layered epithelia are covered by a 
cuticle, a membrane secreted by the epithelial cells which hence very fre- 
quently shows the impression of the cells as polygonal markings. In 
many cases thin and inconspicuous, it may in other instances become 
thickened into a very considerable layer, much thicker than the matrix 
layer of epithelium which secretes it. The cuticle is composed of layers 
parallel with the surface, and forms a more effective protection for the 
body than does epithelium; it becomes a protective armor, as shown, 

5 


66 


GENERAL PRINCIPLES OF ZOOLOGY 


among other examples, by the calcareous shells of molluscs and the chit- 
inous integument of insects (fig. 26, f). 

Stratified Epithelia. The protection furnished by the cuticle in the 
case of simple epithelium, may in the stratified be obtained immediately 
through a change of a part of the cells themselves. In (he stratified 
epithelia the cells of the various layers can be distinguished by their form. 


FIG. 26. Various forms of epithelia. a, flattened epithelium oiSycandra raphanus, 
a' in cross-section, a" in surface view; b and c, cuboidal and columnar epithelium of a 
mollusc (Haliotis tuberculata); d, flagellated epithelium of an actinian (C alii act is 
parasitica); e, ciliated epithelium from the intestine of the fresh-water mussel;/, epithe- 
lium (e) with cuticle (c) of Cimbex coronjtus( a wasp). 

The deepest layer consists of cylindrical cells; the superficial, of more or 
less flattened elements; between lie several layers of transitional forms, 
so that starting from the cylindrical cells we gradually pass through cubical 
cells to the flat cells of the surface. As this arrangement shows, there 
exists a genetic connection between the layers of cells: the lower cylindrical 
cells are in a state of active multiplication; their descendants, with gradual 


GENERAL HISTOLOGY 


07 


changes of form, become the superficial layers, here to replace an equal 
quantity of worn-out cells (fig. 27). 

In the course of this change of position, the cells may undergo an altera- 
tion; in the reptiles, birds, and mammals (fig. 28) they became cornified, 
first the margins, then the inner part of the cell, changing into horn 
{^keratin 1 } . The nucleus remains for some time, but at length this vanishes, 


-Xo 


Co 


FIG. 27. Section through the skin of Petro- 
myzon planeri. Ep, the many-layered epithelium 
of the epidermis, including B, goblet cells; Kd, 
granular cells; Ko, club-cells; Co, corium (with 
blood-vessels, G), consisting of bundles of fibrils 
running horizontally (W) and vertically (5) (from 
Wiedersheim). 


FIG. 28. Stratified epithelium 
of man. 5.17, stratum Malpighi; 
sc, stratum corneum. 


FIG. 29. Single-layered epithe- 
lium of a snail, c, cuticle; d, 
goblet cells. 


and then the cell becomes completely changed into a dead, horny scale. 
In the skin of the higher vertebrates the zones of the living protoplasmic, 
and the cornified cells, are sharply marked off from one another. In 
cross-section they are readily distinguished as the stratum corneum (sc) 
and the stratum Malpighi (sM) of the skin (fig. 28). In the many- 
layered epithelia the cuticle has lost its importance, and it is either an in- 
conspicuous boundary line or is entirely absent. 


68 


GENERAL PRINCIPLES OF ZOOLOGY 


Glandular Epithelium. Glandular epithelium is distinguished 
physiologically from ordinary epithelium by the fact that it also produces 
secretions or excretions; anatomically it is recognizable by the presence 
of gland cells, which carry on the secretion and, to a greater or less extent, 
reveal their character by their structure. Characteristic glandular cells 
are, for example, the goblet cells; here the secretion, usually mucus, is 
collected as a clear mass in the interior of the cell, the cytoplasm being 
compressed into a thin external wall, reminding one of a goblet, contain- 
ing the nucleus at its base (fig. 27,5, 29, d). Other gland cells are granular 
cells, swollen bodies filled with secretory granules (fig. 27, A'o). Natu- 
rally all grades between pavement and glandular epithelium occur. Com- 
monly the latter name is only employed when the gland cells are especially 
numerous, thereby giving to the area a pre-eminently secretory character. 
This is especially the case with the structures which have the name of 
glands, among which we distinguish unicellular and multicellular glands. 
Unicellular Glands. Unicellular and multicellular glands increase 
the secretory surface by invagination. Invagination of a single cell 

produces the unicellular glands which are 
chiefly found among the invertebrate 
animals (fig. 30) ; a gland cell here be- 
comes so enormous that there is no room 
for it in the epithelium, but it is pushed 
into the deeper, the subepithelial layers, 
the nucleated cell body, distended by 
secretion, sending up a slender process, 
the duct, to the epithelial surface. 

Multicellular Glands. In the forma- 
tion of multicellular glands a considerable 
area of glandular epithelium grows as a 
tube or duct from the surface down into 
the deeper tissues; this seldom remains 
simple; it usually branches and forms 
the compound glands, which may consist 
of hundreds or thousands of glandular 


FIG. 30. Unicellular glands 
from edge of the mantle of Helix 
pomatia. e, epithelium; d, unicel- 
lular glands; p, pigment cells. 


sacs, all emptying into a common duct. Among the multicellular glands 
are to be distinguished tubular and acinmis (racemose) forms. In tubular 
glands (fig. 31) the simple or branched glandular pouches preserve the 
same tubular diameter from beginning to end; in the acinous glands 
(fig. 32), on the contrary, the blind end of the glandular pouch widens 
into a sac (acinus'), largely composed of secretory cells, and related to 
the duct, as grapes to their stem. To the tubular glands belong the 


GENERAL HISTOLOGY 


69 


liver, kidney and sweat glands of man; to the acinous belong the salivary 
glands, not only of vertebrates, but also of arthropods and molluscs. 


FIG. 31. Tubular glands (after Toldt). A, glands of Lieberkiihn from the 
human intestine; A', of the conjunctiva of the eye; B, of the cat's stomach; C, from the 
medullary pyramids of the dog's kidney; D, from the cortex of the rabbit's kidney. 

Sexual Epithelium. The sexual cells may be considered in connec- 
tion with glandular epithelium. As the secretion of some glands must 
be expelled from the body, so the sexual cells are elements which must 


FIG. 32. Acinous salivary gland of the aph'd Orthezia catapliractti ('after List). In 
some acini the nuclei and boundaries of the cells are shown. 

reach the exterior in order to perform their function. Just as the 
gland-cells are usually scattered among ordinary epithelial cells, so 
the sexual cells almost invariably lie imbedded in epithelium of the skin 


70 


GENERAL PRINCIPLES OF ZOOLOGY 


(fig. 33), of the gut, of the body cavity, or of parts cut off from this 
(fig. 34). This connection of the sexual cells with the epithelium 
has a deeper meaning since many organisms, particularly those of low 
structure, consist exclusively of epithelia and therefore must develop 


FIG. 33. Germinal epithelium of a medusa, ek, ectoderm; en, entoderm; o, egg; 

e, epithelium. 

their sexual products in epithelium. In other words, sexual and epithelial 
cells are the oldest elements of the animal body, and hence very early 
came into rela.tion with one another. 

Sexual epithelium (or germinal epithelium} like glandular epithelium 
has a tendency to grow into the subepithelial tissues in the form of 


FIG. 34. Section through the ovary of a new-born child (after Waldeyer). ge, 
germinal epithelium; pe, primitive egg in the germinal epithelium; p, egg-pouch; g, 
egg-nest constricted off from the pouchlike growth (p) ; /, single egg with follicle ; 
v, blood-vessel. 

isolated or branching tubes (figs. 34, />, 35), and thus in many groups 
of animals the sexual organs resemble branched glands; hence one speaks 
as often of sexual glands as of sexual organs (fig. 34). The male and 
female cells, the specific elements of the germinal epithelia and of the 


GENERAL HISTOLOGY 


71 


sexual glands, differ from each other in the fact that the eggs are generally 
the largest, the spermatozoa the smallest, cells of the animal body. 

Egg-cell. The egg-cell or oo'cyte (fig. 36) as it 
is formed in the ovary varies in size according to 
the animal group: in case of the microscopic 
Gastrotricha it is less than 0.04 mm., in man about 
0.2 mm., in the frog several millimetres, and in 
large birds often several inches; however, only the 
yolk of the bird's egg is the egg-cell, the white and 
the shell are structures formed outside of the 
ovary in the oviduct. These remarkable differ- 
ences in size are caused less by the quantity of 
the peculiar cell-substance, protoplasm (primary 
yolk), than by the accumulation of dcutoplasm 
(food or accessory yolk, also briefly called yolk). 

!f 'V5;j[ The deutoplasm is to nourish the embryo during 

development, and hence consists of substances 
rich in fat and proteid, arranged in oil-drops, or 
in fine granules or polygonal bodies, the yolk- 
granules. Its quantity, and therefore the size of 
the egg, is in part proportional to the length of 
time which the egg is cut off from any other 
supply of nourishment. In general we find the 
largest eggs in the case of the highly organized 
oviparous animals, where a long development 


uj 

FIG. 35. TIG. 36. 

p IG 35 Ovarian tube of an insect, T",;r.^-; wtira: a, formative cell; />, follinilnr 
epithelium; c, nutritive cells; d, egg-cells; /, fibrous covering extending out into the 
terminal fibres (?) (after Waldeyer). 

JT IG> 26. Immature egg-cell of Strongylocentrotus lividus. 

inside of the shell is necessary to lay the foundation of the manifold 
organs. Besides the protoplasm and deutoplasm, a cell nucleus or 


72 


GENERAL PRINCIPLES OF ZOOLOGY 


germinal vesicle always occurs in the egg. Its contents are mainly the 
nuclear fluid, through which is distributed an achromatic network, and 
in addition the nucleolus (germinal spot). 

The Spermatozoa, the morphological elements of the male reproductive 
product, are so small that their finer structure can be studied only with the 
strongest powers of the microscope (fig. 37, I and II). Easiest to rec- 


ognize is the head, which from its form spherical, oval, sickle-shaped, 
etc. often renders possible the specific determination of the spermatozoa. 
The head (2) is the closely compacted chromatic part of the nucleus, and 
hence colors very deeply in staining fluids. Often the head is continued 
in front as a sharp point, the perforatorium (i), which is apparently 
adapted to aid in the penetration of the egg in fertilization. Next comes 


GENERAL HISTOLOGY 


73 


an unstaining second part, the middle piece (4), and then the tail (5), 
a long flagellum, which causes the active motility of the ripe spermatozoon. 
Cytoplasm is present only in an extremely thin layer surrounding the 
nucleus. A centrosome (3) is nearly always present in the middle piece. 

With the exceptions of the Crustacea, nematodes and many myriapods, the 
spermatozoa are usually constructed after this type, often, with complicated 
modifications. In the groups just mentioned the spermatozoa are large and 
immobile and contain a homogeneous body (b) which is strongly refractive; 
its functions are uncertain. The spermatozoa of Ascaris (V) are shaped like a 
sugar loaf, the broad, rounded end containing the nucleus. The spermatozoa 
of the decapod Crustacea (III, IV) have three or more stiff processes arising 
from the periphery of the cake-like or cylindrical body which contains the 
refractile body, and in this again a rod (III, i), possibly to be compared to the 
perforatorium. In other Crustacea the spermatozoa are threads, often of extreme 
length (7 mm. in many ostracoda). It is noticeable that in some animals there 
are dimorphic spermatozoa. In Paludina vivipara (the same is true of other 
Prosobranchs), there arise in the same individual hair- formed spermatozoa with 
cork-screw shaped heads (Ila) and others, worm-like and with a bundle of 
flagella at the hinder end (lib). The first of these contains the normal chro- 
matin mass (eupyreme spermatozoa); the others have very little chromatin 
(oligopyreme spermatozoa). In many spiders where a similar dimorphism 
occurs, the second type of spermatozoa is chromatin free (apyremc}. The 
supposition that the dimorphism of the 
spermatozoa is connected with sex determi- 
nation receives support in the study of the 
spermatozoa of some Hemiptera. Here half 
of the spermatozoa have one chromosome 
('accessory chromosome') more than the 
other half. Eggs which are fertilized by the 
relatively oligopyreme spermatozoon proba- 
bly produce male animals. 


,1 


Sensory Epithelium. The last 
modification of epithelium is sensory 
epithelium, characterized by the connec- 
tion of certain of its cells, the sensory 
cells, with twigs of branching nerves 
which arise in the central nervous system. 
This connection may be of two kinds. 
In the first the cell is slender and filiform, 
the position of the nucleus being indicated 
by a swelling. The peripheral end is concerned with the reception of 
sensory stimuli, while the deeper end is continued directly into the nerve 
ends and correspondingly is branched into two or more extremely fine 
processes which take on the character of nerve fibrillae (fig. 38). In the 
second type the sensory nerve ends in a ganglion cell beneath the epithe- 
lium, sending processes into the latter, the ends of these being applied to 


FIG. 38. Sensory epithelium. . 1 , 
of an Actinian; B, from the olfactory 
epithelium of man; d, supporting 
cells; s, sensory cells. 


74 GENERAL PRINCIPLES OF ZOOLOGY 

the sensory cell, the connection being one of contact, not of continuity. In 
both the peripheral end of the cell bears appendages for sense perception; 
auditory and tactile hairs, stronger processes in olfactory and taste 
cells, conspicuous rods in visual cells. Almost without exception the 
sensory cells are part of the skin (ectoderm), or arise from it in develop- 
ment. This is true for sense organs like the eye and ear of vertebrates, 
which are separated from the skin by thick intermediate tissue, for in these 
the sensory epithelium (retina, crista acustica) is derived from the ecto_- 
derm. Recent studies seem to show that the taste organs in some forms 
are entodermal in origin. 

Supporting Cells. In sensory epithelium between the sensory cells 
are found still other epithelial cells, which are not connected with nerves, 
but have accessory functions: they serve as supports for the sensory cells; 
in the eyes they contain pigment; in the auditory organs they often bear 
the otoliths, etc. They have the general name of supporting cells. 

2. Supporting or Connective Tissues. 

From a histological point of view there is no greater difference than 
that between epithelium and connective tissue; the former belongs to the 
surface, the latter to the interior of the body; in the former the cells play 
the chief role, in the latter their importance is subordinate to the plasmic 
products, the intercellular substances which chiefly determine the character 
of the various kinds of connective tissue. 

In spite of this contrast the connective tissues are genetically connected 
with epithelium. In embryos, which at first consist only of epithelia, 
the connection can be directly seen. The epithelia secrete a gelatinous 
substance from their deeper surfaces into which separate cells migrate. 
Thus arises the embryonic connective tissue, the mesenchyme (fig. 108). 

Function of Connective Tissue. The primary function of con- 
nective tissue is to fill the spaces between the various organs in the 
interior of the body, thus connecting not only the single parts of the organs, 
but also the various organs themselves. In consequence of this the con- 
nective tissues contribute to the firmness of body, and are frequently 
employed in building up a skeleton. To accomplish this, substances which 
are usually firmer than protoplasm are formed on the surface of the cells, 
and, since they lie between the cells, these are called intercellular substances. 
In proportion as the intercellular substance increases in volume the cells 
themselves diminish and become inconspicuous connective-tissue cor- 
puscles, or, as seldom happens, entirely disappear. Since in connective 
tissues, the intercellular substances are most important, it follows that the 


GENERAL HISTOLOGY 


75 


distinctions between the various kinds rest chiefly upon the differences 
of this intercellular substance. The following forms are distinguished: 
(i) cellular connective tissue; (2) homogeneous connective tissue; (3) 
fibrous connective tissue; (4) cartilage; (5) bone. 

Cellular Connective Tissue (which, strictly speaking, does not 
belong here, since it does not arise from mesenchyme but directly from the 
metamorphosis of epithelium) shows the characteristics of the group least 
distinctly. It owes its name to the fact that the cells make up the chief 


FIG. 39. Cellular connective sub- 
stance. Cross-section through the 
notochord of a newly hatched trout. 


FIG. 40. Homogeneous connective sub- 
stance of Sycandra raphanus (after F. E. 
Schulze). 


mass, while the cell-products are inconsiderable. The cells are large, 
vesicular bodies which are closely pressed together and are consequently 
polygonal (fig. 39). They have between them a firm but thin layer of 
intercellular substance. 

Homogeneous Connective Tissue. In homogeneous connective 
substance the intercellular substance (or matrix] is usually present in 
considerable quantity as a transparent mass, sometimes soft like jelly, 
often firmer (fig. 40). The cells lying in it are either spherical or send 
branching processes into the matrix. Such processes may unite to form 
meshes which, like a pseudopodial network, unite cell to cell. Frequently 
the matrix contains, in addition, isolated firm fibres or threads, which, on 
account of their physical characteristics, are called elastic fibres, and 
consist of a substance (elastiri) exceedingly resistant to all reagent-. 
Finally, in the matrix there may develop the finer connective-tissue fibrils 
characteristic of the next group; they may become so increased in 
number as to determine the character of the tissue. 

Fibrous Connective Tissue is characterized by the rich supply of 
connective-tissue fibrillae; these are fibres of extraordinary fineness, lying 
in a homogeneous matrix, which is the less evident the richer it is in 


76 


GENERAL PRINCIPLES OF ZOOLOGY 


fibres. The fibres may either cross in all directions, or may run essen- 
tially parallel and in a definite direction. Between them are found the 
rounded, spindle-shaped or branched connective-tissue corpuscles (fig. 
41). It is characteristic of vertebrates that the fibres are grouped into 
bundles, each bundle generally surrounded by connective-tissue corpuscles, 


FIG. 41. Fibrous connective tissue 
of an Actinian. 


;? 


FIG. 42. Areolar fibrous connective 
tissue (after Gegenbaur). 


metamorphosed into flat cells. The bundles, loosely interwoven, run in 
all directions (areolar connective tissue, fig. 42), or they may be parallel, 
forming a compact mass of fibres (tendinous tissue fig. 43). The fibrils 
of the fibrous connective tissue cf the vertebrates have the peculiarity 
not met with elsewhere, they are composed of glut-in, and upon boiling 
become gelatine or glue. 


FIG. 43. Tendinous tissue (after 
Gegenbaur). 


i 


FIG. 44. Cartilage (after Gegen- 
baur). r, perichondrium; b, transition 
into typical cartilage (a). 


Elastic Tissue. In all fibroas connective tissue there may appear, 
as a further constituent, elastic fibres; they may indeed supplant the ordi- 
nary connective-tissue fibrils and become the predominant element of the 
tissue, which is then called elastic tissue. 


GENERAL HISTOLOGY 


77 


Cartilage. Cartilage and bone are likewise tissues which find their charac- 
teristic development only in the vertebrates. In appearance cartilage is similar 
to the homogeneous connective tissue of many invertebrates; the matrix is 
homogeneous and, at first glance, appears structureless (fig. 44), but, under the 
action of certain reagents, assumes a fibrous condition. This, as well as the 
fact that the cartilage grows through 
changes of the perichondrium a thin, 
fibrillar membrane covering its surface- 
makes it more evident that it is homo- 
geneously fibrillar; and it is thereby dis- 
tinguished from homogeneous connective 
tissue since it is not, like the latter, a 
lower but a higher stage of tissue forma- 
tion. The matrix of cartilage (chc.ndrin} 
by cooking produces a kind of glue which 
differs from true or glulin glue in that it 
is precipitated by acetic acid. The car- 
tilage cells lie in the matrix united in 
groups and nests, a mods of grouping 
pointing to their origin, since each group 
has arisen from a single mother-cell by 
successive divisions. In cartilage also, c 
elastic fibres are found; if present in great 
number, these change the bluish, shiny, 
hyaline cartilage into the yellow-colored 
elastic cartilage. 

The 'head cartilages' of the cepha- 
lopoda differ from vertebrate cartilage 
in that the cartilage corpuscles have e 
branched processes. 

Bone is the most complicated struc- 
ture in the series of connective tissues. 
It consists of a matrix (ossein), closely 
allied to glutin, so intimately combined 
with inorganic constituents that it appears 
under the microscope as a homogeneous 
mass. The proportion of organic and 
inorganic substances varies according to 
the age and species of animal: in man 
there is 65% inorganic to 35% organic 


<*,- rt^H '^J*M* "i, !*. 

'' -;*-- -^ '- **-* 


substance; in the turtle, 63% to 37' , 
Of the inorganic constituents, the most 
important is calcic phosphate, 84'^; in 
smaller quantities, combinations of 
fluorine, chlorine, carbonic acid and mag- 
nesia. In compact bone the matrix is 
composed of the bone lamella: (fig. 45), 
whose arrangement is determined by 


' 'v- f n_ '- J - '.S^*? ** J - -" - i".i-i'^i 

MS^&&'f^& 


FIG. 45. Cross-section through the 
human metacarpus (after Frey). </. 
surface of the periosteum; b, surface 
of the marrow-cavity; c, cross-sections 
of the Haversian canals and their 
system of lamella;; d, fundamental 
lamella?; c, bone corpuscles. 

In a hollow bone (like that of the 


the surfaces present in and on the bone. 

upper arm) there is an outer surface to which a fibrous membrane, the 
periosteum, is closely applied; the presence of the marrow-cavity necessitates a 
second surface. Finally, the mass of the bone is permeated by the Haversi< 
canals, which run chiefly in a longitudinal direction, united by cross or oblique 
canals, and serve for the passage of blood-vessels. Since the bone lamellae 


78 


GENERAL PRINCIPLES OF ZOOLOGY 


arrange themselves parallel to the surfaces mentioned, two systems may be 
distinguished in cross-section, the fundamental lamella; and the Haversian 
lamellcc. The former are arranged parallel to the surface of the periosteum 
and of the marrow-cavity and form a mantle of concentric layers around the 
marrow-cavity. Into this groundwork the Haversian canals with their lamellae 
enter, destroying and superseding the fundamental lamellae coming in their 
way. The Haversian lamellae are concentrically arranged around the lumen 
of the Haversian canals just as the fundamental lamellae are around the marrow- 
cavity and beneath the periosteum. 

Formation of Bone. The stratification of bone is caused by its mode of 
origin. Where the bone borders upon the Haversian canals, the marrow-cavity, 
and the periosteum, there is transiently or permanently an epithelial-like layer 
of cells, osteoblasts, which secrete the bone-substance on their surface, which, 
as with all similar conditions, gives the substance a stratification. Certain cells 
in the matrix participate in this secretion, and here give rise to the bone-cor- 
puscles, which are distinguished from the cartilage-cells by their numerous 
processes ramifying through the matrix. The processes of a bone-corpuscle 
branch and unite with the neighboring cells through fusion of the processes, an 
arrangement most beautifully seen in dried bone, because here the cavities and 
the canals of the matrix are filled with air. In spongy bone the structure is 
simplified since the Haversian canals with their lamellae, and often the stratifica- 
tion of the ground lamellae, are lacking. Special modifications of bone are 
found in the substance of fish scales and in the dentine or ivory of teeth. 


' o~6a 


Blood and Lymph, here treated in connection with the connective 
tissues, are in reality not tissues at all, but nutritive fluids. Two kinds 

of nutritive fluids occur in the verte- 
brates, red blood and the colorless, 
weakly opalescent, or cloudy white 
lymph. The blood of man and other 
vertebrates consists of a fluid and the 
organized constituents. The fluid or 
blood- plasma is specially rich in proteids; 
after the removal of the blood from the 
blood-vessels a part of these separate by 
coagulation and form the blood-clot, 
made up of fibrin, leaving a fluid poor 
in proteids, the blood-serum. The organ- 
ized constituents, the blood-cells, are dis- 
tinguished as red and white blood-cor- 
puscles. The latter, the leucocytes, are 
present in smaller numbers and have 

great similarity to the Amoebae; they are particles of protoplasm, con- 
tain a nucleus, devour foreign bodies (for example, carmine granules 
injected into the blood), and move in the 'amoeboid' manner by putting 
out pseudopodia (fig. 46). 


FIG. 46 White blood-corpus- 
cles a, of man; b, of crab (n, the 
nucleus). 


GENERAL HISTOLOGY 


79 


Red Blood-corpuscles. In the mature condition the red blood-corpuscles 

of vertebrates (fig. 47) are circular or oval discs, which by external influences 
(e.g., pressure) may temporarily be bent or otherwise modified in form, but 
cannot actively change their shape, because they no longer consist of protoplasm. 
Embryologically they arise from true, nucleated, protoplasmic cells; gradually 
the protoplasmic cell-body changes completely into a plasmic product, the 
stroma of the blood-corpuscle. If the nucleus be retained in this metamorphosis, 
there is a slight swelling in the centre of the disc (fig. 47, d'); if, however, the 
nucleus degenerate, the bilateral convexity is replaced by a shallow concavity. 
In the latter case, one has, in reality, no right to speak of blood-cells, since the 
characteristic constituents of the cell -nucleus and protoplasm have disap- 
peared. Systematically the red blood-corpuscles are of interest, since non- 
nucleate forms are found only in the mammals (fig. 47, a, b), nucleated ones in 


FIG. 47. Red blood-corpuscles, a, of man; ?>, of the came!; c, of the adder; </', of 
Proteus (seen from the edge); d", surface view; e, of a ray;/, of Petromyzon; n, nucleus 
(all the blood-corpuscles are magnified 700 times, except d, which is magnified 350 
times). 

all the other vertebrates (c, d}. The mammals also have circular, the other 
vertebrates oval, discs. To this, however, exceptions occur, since among the 
mammals the Tylopoda (camel, llama) have oval, the cyclostomes have circular 
blood-corpuscles. Recent investigations tend to show that the corpuscles, at 
least in mammals, have a hat shape while in the living blood-vessels and that 
they become disc-like when the normal conditions are interfered with. 

Haemoglobin. The red blood-corpuscles cause the color of the blood, and 
are the agents of one of its most important functions, the interchange of gases; 
both are connected with the fact that the stroma contains the coloring matter, 
hemoglobin, of the blood. Haemoglobin is one of the few crystallizable proteids 
and is remarkable for the presence of a small, though extremely important, 
quantity of iron, and also for its affinity for oxygen. Haemoglobin containing 
oxygen, oxy-h&moglobin, causes the carmine-like color of the so-called arterial 
blood; oxygen-free, 'reduced' haemoglobin causes the dark bluish-red color of 
venous blood. 

Lymph is distinguished from blood by the entire lack of red blood-corpuscles 
and the slight coagulability of its plasma. Lymph is accordingly a proteid- 
containing fluid with leucocytes, which are here called lymph-corpuscles. 

In the majority of invertebrates there is only one kind of nutritive fluid, and 
not even this in every class; the fluid is called blood, although it is usually color- 
less. Where color is present, it is generally, if not always, a yellowish-red or 
an intense red; this may, as in the vertebrates, be caused by haemoglobin (some 
molluscs, annelids, and insects). Often other coloring matter occurs instead of 
haemoglobin: in the cuttlefish, many snails, and in the lobster and Limulus, the 


80 GENERAL PRINCIPLES OF ZOOLOGY 

oxygen is taken up by the bluish hcemocyanin, which contains a trace of copper: 
in the Sipunculids by hcemoerythrin, etc. The blood-plasma, as a rule, is the 
seat of the color (Chironomtts, Hirudinea, earthworms, and most other annelids); 
only exceptionally do colored blood-corpuscles occur, as in the case of Area, 
Solcn and some other molluscs, and also mPhoroiiis. Colored elements contain- 
ing haemoglobin, identical with blood-corpuscles, are found besides in the 
ccelomic fluid of many annelids, and in the ambulacral vessels of some echino- 
derms. Most widely distributed in the invertebrates are the leucocytes, dis- 
tinguished by their active amoeboid movements; still even these may be absent, 
and then the blood is a fluid without any organized corpuscles. 

3. Muscular Tissue. 

Muscular Tissue is the agent of active movements in the animal body. 
Since active mobility occurs in protoplasm, it is important to notice the 
differences between the two kinds of movement. The distinctions lie in 
the direction and in the intensity of the movement. A mass of protoplasm 
has the capacity to move hither and thither in all directions, because in 
it there is a high degree of mobility between the smallest 
particles. Muscles and hence their separate elements, the 
muscle-fibres and muscle-fibrils, on the contrary, can shorten 
^Jj only by correspondingly increasing in diameter (fig. 48); 
they can therefore accomplish motion only in a definite 


^^J direction, that of the axis of the muscle. The muscle- 

" . SSMHEHB 

, H| substance consequently is more limited in its movement 

than is protoplasm, but on the other hand it has the 
uu* 
FIG g _ advantages of greater energy and greater rapidity. One 

Four striped is able to decide with considerable accuracy, from the 

muscle fibres intensity and rapidity, whether in a given case a movement 
in resting and " 

contracted has been brought about by the agency of protoplasm or by 

condition muscle-substance. 
(after Rollet). 

Structure of Muscle Substance. These physiological 

considerations show that protoplasm and the contractile substance are 
morphologically different, and that therefore one must distinguish sharply 
between formative cells, or muscle-corpuscles, and the product of these 
cells, the contractile substance, just as in the case of connective tissue, 
between the corpuscles and fibrils. There are recognized two kinds of 
muscle-substance, the homogeneous, or smooth, and the cross-striated. 
Since the former looks very similar to non-granular protoplasm, the 
boundary-line between it and the muscle-corpuscle is more difficult to 
recognize than in the case of the latter, which in its minute structure is 
quite different in appearance from protoplasm. In cross-striated muscles 
the contractile portion consists of two substances regularly alternating 
with one another in the direction of the contraction of the muscle, of which 
the one is doubly, the other singly, refractive (figs. 25, 48, 51). 


GENERAL HISTOLOGY 81 

Smooth and Cross-striated Muscle Fibres. The smooth muscle- 
substance represents a lower stage than the cross-striated, since it chiefly 
occurs in the less highly organized and more inactive animals. Interest- 
ing in this respect is the fact that in the two stages of development of the 
same animal the simple and inert polyp has smooth muscles, while the 
more highly organized and actively motile medusa has cross-striated 
muscles (fig. 49). The difference in their action has led in the verte- 
brates to a peculiar distribution of the muscle-substance, the smooth 
musculature being chiefly in internal organs, which are not under control 
of the will (Involuntary muscles] , while the musculature of the body, subject 


a. 


FIG. 49. Epithelial muscle-cells, a, of a medusa; b, of an actinian. 

i 

to the will and hence demanding more rapid action, is cross-striated 
(voluntary muscles}. We must not conclude that the difference between 
smooth and cross-striated musculature coincides with the distinction be- 
tween visceral and body musculature; it should be noticed that the body 
musculature of all molluscs is smooth, the visceral as well as the body 
muscles of many insects and Crustacea, and the muscles of the heart of 
vertebrates are cross-striated. 

It was pointed out above, in connection with epithelia and connective 
tissue, that these tissues differed fundamentally. This contrast has its 
bearing in dealing with the muscles, for both epithelial and mesenchy- 
matous cells may form contractile substances and therefore there are two 
genetically different kinds of muscles, the epithelial and the mesenchy- 
matous (contractile fibre-cell). Both kinds of muscle-cells can a priori 
form smooth as well as cross-striated muscle-substance; but the collection 
of connective (mesenchymatous) tissue around inner organs has caused 
most contractile fibre-cells to be smooth, while most of the epithelial 
muscle-cells are cross-striated. 

Epithelial muscle-cells are cells of which one end extends to the surface 
of the body or the surface of an internal cavity (body cavity, lumen of the 
gut, etc.), and may here have a cuticle, cilia, or flagella, while at the op- 
posite end it has secreted contractile substance in the form of muscle- 
fibrils (fig. 49). They combine the double function of epithelial and 

muscle cells. 
6 


82 


GENERAL PRINCIPLES OF ZOOLOGY 


Contractile fibre-cells, on the other hand, are connective-tissue cells, 
which usually have surrounded themselves with a layer of contractile 
substance; corresponding to their origin, they have the form of connective- 
tissue cells, and are spindle-formed or branched; the branches arising from 
the ends of the cells (fig. 50). The similarity of form renders the distinc- 
tion between ordinary connective-tissue cells and fibre-cells difficult; 
if the contractile layer on the surface be slightly developed, the distinction 


FIG. 50. FIG. 51. 

FIG. 50. Contractile fibre-cells, a, of man; b-e, of Beroe (a Ctenophore); b, young 
fibres; c, branched ends; d, middle portion of a fibre; e, cross-section. 

FIG. 51. Cross-striated primary bundle (after Gegenbaur). a, nuclei; s, a point 
where the sarcolemma is plainly shown on account of the tearing of the fibres. 

is impossible. To recognize the character of the elements, therefore, 
we must choose well-defined examples, in which the nucleated mass, the 
'axial substance,' is sharply marked off from the muscle-mass, the 'cortical 
layer' (fig. 50, c, d, e). The regular arrangement of the epithelial cells 
side by side, gives the muscle fibres arising from them a parallel arrange- 
ment, so that a layer is formed, which becomes folded when much muscle 


GENERAL HISTOLOGY 83 

substance is formed in a limited space (see figs. 193, 240, 241 and their 
legends). 

In vertebrates and arthropods the contractile fibre-cells occur in the 
vegetative organs as elements of the 'organic musculature'; on the other 
hand, we find here the epithelial musculature in the cross-striated primary 
bundles, separated from the epithelium, and only developmentally refer- 
able to the epithelium of the body cavity (fig. 51). A primary bundle is 
cylindrical, bounded externally by a structureless envelope, the sarco- 
lemma. Its contents consist of fine fibrils, which, closely parallel to one 
another and pressed closely together, run from one end of the mass to the 
other. Each fibril is formed of singly and doubly refractive parts, 
which alternate with one another in more or less complicated arrangement. 
Since now the doubly refracting parts of the fibrils within a bundle lie 
at about the same level, there is caused across-striation extending through 
the whole bundle (fig. 51). Finally, scattered here and there between the 
muscle-fibrils are the muscle-corpuscles, spindle-shaped protoplasmic 
bodies with a nucleus, the remnants of the cells which have formed the 
musculature. Although the primitive bundles retain no epithelial 
characters, their epithelial nature is shown in their origin from the epithe- 
lium of the primitive body cavity (from that part of its wall known as the 
protovertebrae). 

4. Nervous Tissue. 

Function of Nervous Tissue. As muscular tissue causes motion, 
so nervous tissue serves for the transmission of stimuli. It conveys the 
stimulations of the sense-organs at the periphery to the central nervous 
system, and here brings about perception (centripetal nerve tracts) ; further, 
it transmits the voluntary and reflex impulses to the periphery, particu- 
larly to the musculature and glands (centrifugal nerve tracts). By the 
nervous system, finally, the stimuli arising in various places are co-ordi- 
nated, thus furnishing the elements for that which we call independent 
psychic activity. 

Elements of Nervous Tissue. The agent of the transmission of 
stimuli is undoubtedly a specific nerve-substance different from proto- 
plasm. The prevailing view is as follows: The elements of the nervous 
system are divided into ganglion cells and nerve-fibres, but it must be re- 
membered that these are not independent of each other, but that the fibres 
are enormously elongated processes of the ganglion cells. In both gan- 
glion cells and nerve fibres there are extremely fine fibrilhe which are 
connected with each other and are to be compared to the wires of a 


84 


GENERAL PRINCIPLES OF ZOOLOGY 


a 


telegraph system. These are to be 
considered as the specific element 
of the nervous system. 

In the vertebrates the ganglion 
cells vary greatly in size; besides 
small elements there are large cells, 
only exceeded by the eggs in size, 
which correspondingly have large 
nuclei recalling the germinal vesi- 
cles. Unipolar, bipolar, and multi- 
polar ganglion cells are recognized, 
the differences depending upon the 
number of processes (nerve-fibres) 
which arise. In multipolar cells 
the number is large (fig. 52) and 
they are of two kinds, dendrites and 
axons or neurites. Dendrites are 
so called because they branch again 
and again, not far from their origin 
from the cell. The axons (of which 
there is usually but one to a gang- 
lion cell) can extend a long dis- 
tance without giving off branches, 

except here and there side twigs (collaterals) which arise at right angles 

to the main fibre; they often 

continue into peripheral 

nerves. They branch at their 

tips (telodendra) so the mor- 

phological distinction from 

dendrites lies in the greater 

distance of the region of 

branching from the body of 

the ganglion cell. In bipolar 

ganglion cells both processes 

appear as neurites, but if one 

is to be regarded as a dendrite 

with its branching at some 

distance from the cell, the 

definite physiological distinc- 

, , , 
tion has to be invoked that 

the neurite carries the im- 


FIG. 52. Multipolar ganglion cell of man 
(after Gegenbaur). a, axon. 


^- Motor ganglion cell from the thoracic 
the spinal cord of a dog (after Bethe). 

n, nucleus. 


FIG. 
region 


GENERAL HISTOLOGY 


pulse from the cell; the dendrite is the afferent tract. The unipolar 
nerve cell is also to be regarded as bipolar, its processes leaving the 
cell from a common point, running together a short distance and then 
branching like the letter ~]~. This conception is intelligible if the 
fibrilke be regarded as the carriers of stimuli, each process con- 
sisting of a varying number of fibriHa?. These enter the cell, where 
they cross similar fibrilke coming from 
other processes, and then are dis- 
tributed to the various neurites and 
dendrites. The branching of a fibre 
is thus a gradual distribution of the 
constituent fibrilke, and the cell is the 
place where the exchange of fibrillie 
between the different processes occurs. 
Thus it is immaterial whether two 
bundles of fibrillae leave the cell at 
different points or whether they are 
united for a distance like a cable. 

In the central nervous system of 
vertebrates the most minute elements 
are the nerve fibrillae, distinguished 
from muscle fibrilke by the absence of 
cross-striation; from connective-tissue 
fibrilke by the ease with which they 
are injured; in preserved material they 
frequently swell and show varicosities 
(fig. 54). Many fibrilke united in a 
bundle form a nerve-fibre (fig. 55, A) 
which is called a gray nerve-fibre in 
distinction from the 'white or medullated fibres. In the latter the fibre or 
axis-cylinder is surrounded by a medullary sheath (fig. 55, B) composed 
of myelin, a strongly refractive fat-like substance, which appears to act 
as an insulator. 

Both medullated and non-medullated fibres can be enclosed in a slicatli 
of Schwann. This is a feature of the peripheral nervous system and is 
lacking in brain and spinal cord. It is a delicate envelope with nuclei 
here and there (fig. 56). At times it has constrictions which cut through 
the medullary sheath to the axis-cylinder (constrictions oj Ranvier). 

Multipolar and bipolar ganglion cells also occur in the invertebrates, 
most commonly in the ccelenterates (fig. 57), more rarely in worms, 
arthropods, and molluscs, and then chiefly in the peripheral nervous 


1 

A BAB 

FIG. 54 FIG. 55. FIG. 56. 

FIG. 54. Nerve fibrillae with vari- 
cosities (from Hatschek). 

FIG. 55. A, Non-medullated; B, 
nerve-fibres. 

FIG. 56. A, Non-medullated; B, 
medullated nerve-fibres, with sheath 
of Schwann (from Hatschek). 


86 


GENERAL PRINCIPLES OF ZOOLOGY 


system. In the ganglia (the nervous centres of the last three groups) 
the ganglion cell usually gives rise to a single strong process, which, how- 
ever, is richly provided with lateral branches or dendrites (fig. 77). The 
medullary sheath and sheath of Schwann are usually absent in inverte- 
brates even in the peripheral nerves. On the other hand, the true con- 
ducting elements, the nerve fibrilloe, have been seen in invertebrate nerve- 
fibres, and have been followed into the ganglion cell in which the afferent 
and efferent fibrilke are united in a lattice-like manner. 


FIG. 57. Ganglion cells of an actinian. 

A ganglion cell with its dendrites and neurites (the latter in large animals 
may be several feet long, since they extend from the central nervous system to 
the muscles) form a physiological unit, called a neurotic. The advocates of the 
'neurone theory' maintain that the processes of two neurones do not anastomose 
and that there is no continuity between them; they approach each other so closely 
that the nervous impulse may jump from one to another like an electric spark. 
The opponents of the theory assert that there is real continuity as seems to be 
certainly shown with the giant ganglion cells of the nematodes. A second 
disputed point is that the neurone theory holds that all processes, even the long- 
est neurite, grows from the ganglion cell and throughout its length is a product 
of it. The opponents claim that the neurites which compose the peripheral 
nerve fibres are formed by special nerve-forming cells (neuroblasts) and these 
contribute to the length of the neurite starting from a ganglion cell. In many 
invertebrates (Hirudinei) such neuroblasts persist in the course of the peripheral 
nerves. Also the peripheral system of invertebrates can consist of a network of 
anastomosing neuroblasts (gang ionic plexus of medusae and chaetognaths). 
When we recall that the primitive ccelenterates (hydroid polyps) lack a central 
nervous organ and the whole nervous system is only a plexus of similar nerve 
cells, we may suppose that from this primitive condition the more highly devel- 
oped forms of nervous systems have developed through division of labor, cells in 
a suitable position giving rise to the central system, while the remaining cells of 
the plexus form the conducting tracts. Recent experiments in growing nerve 
cells in nutrient fluids seem to support the view that the axons are exclusively 
products of the cell. 


GENERAL HISTOLOGY 87 

SUMMARY OF HISTOLOGICAL FACTS. 

Cells. i. The most important morphological element of all tissues 
is the cell. 

2. The cell is a mass of protoplasm which contains one or several 
nuclei (uninucleated, multinucleated cells). 

3. The nucleus probably determines the specific character of the cell, 
since it influences its functions; accordingly it is also the bearer of heredity. 

4. Cells and nuclei increase exclusively by division or budding, the 
cells of the Metazoa nearly always by mitosis, those of the Protozoa 
frequently by direct division. 

Tissues. 5. Tissues are complexes of numerous similar histologically 
differentiated cells. 

6. Histc logical differentiation rests in part upon the fact that the cells 
assume a definite form and arrangement, in part upon the formation of 
plasmic products, which determine the character of the tissue (muscle-, 
nerve- and connective-tissue fibrils). 

Classification of Tissues. 7. According to function and structure 
(i) epithelia, (2) connective tissue, (3) muscular tissue, (4) nervous tissue 
are distinguished. 

8. The physiological character of epitlielia is due to the fact that they 
cover the surfaces of the body; morphologically they consist of closely 
appressed cells united by a cementing substance. 

9. According to their further functional character epithelia are divided 
into glandular epithelia (unicellular and multicellular glands), sensory, 
germinal, and protective epithelia. 

10. According to the structure are distinguished simple (cubical, 
cylindrical, squamous epithelia) and stratified epithelia, ciliated and 
flagellated epithelia, epithelia with or without cuticle. 

11. The physiological characteristic of the connective tissues is that 
they fill spaces between other tissues in the body. 

12. The morphological distinction depends upon the presence of the 
intercellular substance. 

13. According to the quantity and the structure of the intercellular 
substance the connective tissues are divided into (i) cellular (scanty 
intercellular substance); (2) homogeneous; (3) fibrous connective tissue; 
(4) cartilage; (5) bone. 

14. The physiological character of muscular tissue is its increased 
capacity for contraction. 

15. The morphological characteristic is the fact that the cells have 
secreted muscle-substance. 


88 GENERAL PRINCIPLES OF ZOOLOGY 

1 6. According to the nature of the muscle-substance are distinguished 
smooth and cross-striated muscle-fibres. 

17. According to the character and origin of the cells (muscle-cor- 
puscles) the muscles are divided into epithelial (epithelial muscle-cells, 
primary bundles) and connective-tissue muscle-cells (contractile fibre- 
cells). ' 

1 8. The physiological distinction of nervous tissue rests upon the 
transmission of sensory stimuli and voluntary impulses, and upon the 
co-ordination of these into unified psychic activity. 

19. The conduction takes place by means of nerve-fibres (non-medul- 
lated and medullated fibrils and bundles of fibrils); the co-ordination of 
stimuli by means of ganglion-cells (bipolar, multipolar ganglion-cells). 

20. Blood and lymph are proteid-containing fluids; rarely without cells, 
they may contain only colorless amoeboid cells (white blood-corpuscles, 
leucocytes), or in addition to these also red blood-corpuscles. 

21. Red blood-corpuscles occur in vertebrates and cause the redness 
of the blood; they are absent in most invertebrates. 

22. When invertebrates have colored blood (red, yellow, green), this 
is usually due to the color of the blood-plasma. 

23. The red blood-corpuscles are non-nucleated in mammals, nucle- 
ated in all the other vertebrates. 

III. THE COMBINATION OF TISSUES INTO ORGANS. 

An Organ Defined. Organs are formed from the tissues. An organ 
is a tissue complex, marked off from the other tissues, which lias taken on a 
definite form for carrying on a special function. Thus a single muscle is 
an organ which consists of a certain amount of muscular tissue; with 
scalpel and scissors it can be removed as a connected whole from its 
environment and can still accomplish a definite movement. 

Principal and Accessory Tissues. In each organ there is a tissue 
which determines- its function, and therefore its physiological character. 
This is called the principal tissue, for there may be accessory tissues 
present, which merely support or render possible the action of the prin- 
cipal tissue. In the muscle of the vertebrates we find, besides the muscle- 
fibres, connective tissue which unites the bundles of muscle; blood-vessels 
which provide nourishment; finally, nerves by which the muscles are 
aroused to action. In the human liver also, besides the functionally 
most important part (the liver-cells), blood-vessels, nervous and connective 
tissues occur. These ^accessory tissues are usually found only in the 
highly developed organs; in the case of the lower animals they may be 


GENERAL ORGANOLOGY 89 

absent; thus the digestive tract of coelenterates is only an epithelium; 
their nervous system consists merely of a plexus of nerve-fibres and 
ganglion-cells. 

Effect of Use and Disuse. It is of the greatest importance for the 
permanency of an organ that it be constantly in function. Living sub- 
stance is distinguished from the non-living by the fact that, if it be de- 
stroyed by use, it is immediately replaced, often by more than sufficient to 
make good the loss. Functioning tissues and organs under favorable 
conditions increase in volume; on the other hand, functionless parts 
undergo a gradual decrease, which finally leads to their disappearance. 
To the extent that functioning tissues grow at the expense of those not 
used there can be a 'struggle of the parts within the organism' (Roux). 
There is also the same struggle between the structural elements in one and 
the same tissue as is well shown in certain bones like the femur and tibia. 
These retain only those bony parts, the outer tube and the bony bars at 
the ends, which are necessary to support the body, all other parts being 
absorbed. These bony bars are in the position which can be mathe- 
matically demonstrated to be necessary to meet the strains. If the line of 
strain be altered, as when a leg is broken and badly set, the bars are altered 
in position to meet the new strains. 

Change of Function of Organs. The two factors mentioned, that 
the permanence of the tissues depends upon continued use, and that 
usually several tissues enter into the structure of an organ, are important 
for the understanding of the principle of change of function which plays 
a prominent role in the metamorphosis of animal form. It may happen 
that an organ is brought under changed conditions and no longer has an 
opportunity to function as before. In that case the functioning tissue, 
from lack of use, gradually degenerates, but the organ may persist by 
means of its accessory tissues if the new conditions make it possible for 
one of them to attain to functional activity, and to give the organ a new 
physiological character. 

A muscle, for example, may become functionless from many causes. 
Should the muscle-tissue disappear there are still left the accessory tissues, 
particularly connective tissue permeated by blood-vessels; this may remain 
intact and form a protecting band, a tendon, or fascia. We have then, 
morphologically, the same organ, changed in its physiological character; 
the muscle has undergone a change of function, and has become a liga- 
mentous band. The visceral arches of fishes primarily are supports for 
the gills; if now by the acquirement of terrestrial habits the gills be lost, 
the visceral arches become functionless and correspondingly undergo 
a partial degeneration; but a part persists by assuming a new function, 


90 GENERAL PRINCIPLES OF ZOOLOGY 

and forms the jaws, the hyoid bone, and the small bones of the ear, which 
in spite of their quite different functions, are morphologically the same 
structures as the gill-arches. 

Homology and Analogy. In the History of Zoology (page 10) 
it was shown that comparative anatomy has caused a discrimination be- 
tween homology or morphological equivalence, and analogy or physio- 
logical equivalence, i.e., between organs which appear in the same relative 
positions and relations, and organs which have the same function. What 
we have here learned of the structure of organs makes it evident that 
morphological and physiological characters do not necessarily coincide, 
that morphologically similar organs (lungs of mammals, air bladder of 
fishes) may have different functions, morphologically different organs 
(lungs of mammals, gills of fishes) the same functions. 

Systems of Organs. Organs identical, or, at least, functioning in an 
equivalent manner, may occur in considerable numbers in the same body. 
A man has many muscles, and many organs which carry on digestion. 
Hence we may group all organs which in the body have equivalent or 
similar functions into systems of organs. In all we recognize nine such 
systems: (i) skeletal, (2) digestive, (3) respiratory, (4) circulatory, (5) 
excretory, (6) genital, (7) muscular, (8) nervous, and (9) sensory systems. 
Not all are necessarily present; a skeleton, for instance, is frequently lack- 
ing. The various functions, which in man are divided among different 
complicated and specialized systems, may be performed in a lower animal 
by one and the same apparatus. Yet according to the fundamental 
functions the following groups of organs may be recognized: I. Organs 
of assimilation (2-5); II. Organs of reproduction (6); III. Organs of 
motion (7); IV. Organs of perception (8 and 9). 

Vegetative and Animal Organs. The organs of assimilation and of repro- 
duction (I and II) are grouped together as vegetative, the others (III and IV) 
as animal organs. The older zoologists used to say that assimilation and repro- 
duction are functions common to animals and plants; that, on the contrary, 
sensation and motion are lacking in plants, and are exclusively characteristic 
of animals. The atom of truth in the fundamental idea needs reconsideration 
in the light of our present knowledge. We have seen that the protoplasm of 
plants and animals has not only the power of assimilation and reproduction, 
but also power of motion and of irritability. The latter characteristics conse- 
quently cannot be entirely lacking in all the plants, for they are found in their 
most important constituent. In fact many plants, as the mimosas, compass- 
plants, insectivorous plants, show great irritability; the reproductive states of 
algae move quite as actively as, or even more actively than, many of the lower 
animals. On the other hand, many animals are fixed in position like plants. 
Many Protozoa and worms, most polyps, some echinoderms like the crinoids, 
even many Crustacea, the cirripedes (barnacles), can change their location only 
during the earlier stages of development, in later life being limited to movements 
of single parts of the body, the arms, tentacles, etc. In the sponges motions are 


GENERAL ORGANOLOGY 91 

so insignificant that they cannot be seen at all by the naked eye, and scarcely 
with the microscope. 

Nevertheless the two terms, animal and vegetative, must be retained. For 
although motion and sensation occur in the vegetable kingdom, still they reach 
no high development; they become more and more inconspicuous the higher 
the plants; in the animal kingdom, on the contrary, they are unfolded in extra- 
ordinary perfection and lie at the basis of its most characteristic features. 

SKIN AND SKELETON. 

With a few exceptions (cestodes, trematodes, nematodes) which are 
not fully explained, the outer surface of the Metazoan body is covered 
with a typical epithelium, commonly called epidermis and occasionally, 
from its origin, ectoderm as well. In invertebrates and in Amphioxus 
it is one layered, but in all true vertebrates it is stratified. To this im- 
portant part of the integument there is usually added the mesodermal 
part of the skin which is especially developed in vertebrates (corium or 
cutis, derma) and is derived from the connective tissue. The skin is 
frequently concerned in skeleton formation, as when in Coelenterates, 
molluscs and arthropods it secretes on the outer surface of the epidermis 
a cuticular armor, frequently hardened by lime. On the other hand there 
may be a calcification (echinoderms) or an ossification of the corium 
(scales of fishes, bony plates of reptiles and mammals). This dermal 
skeleton is in strong contrast to the axial skeleton of vertebrates which 
consists of cartilage and ossifications in the interior of the body. 

Vegetative Organs. 
A. Organs of Assimilation. 

Assimilation Defined. If the term assimilation be used in its 
widest sense, it includes all the contrivances in the animal body which 
render growth possible during the period of progressive development, 
and, during mature life, compensate for the loss of energy connected with 
each functional act, in order to preserve to the body its functional powers. 
With each functional act organic compounds are oxidized. Compounds 
which are especially rich in carbon and hydrogen (as well as some nitrogen 
and sulphur) and are poor in oxygen are changed by oxidation into carbon 
dioxide, water, and various nitrogenous products, like urea, uric acid, etc. 
A compensation takes place, for not only is the useless substance removed, 
but also compounds of oxygen and materials rich in carbon are furnished 
to the tissues to replace the material oxidized. An apparent exception 
is furnished by the 'anaerobic' organisms which live, move and per- 
form work without oxygen. Aside from some bacteria, which are outside 


92 GENERAL PRINCIPLES OF ZOOLOGY 

our scope, the entoparasites (cf. p. 157) take oxygen-containing com- 
pounds from their hosts, form from them carbohydrates and reduce them 
by a process analogous to fermentation into carbon dioxide and com- 
pounds poor in oxygen. 

Assimilation in Animals. In lowly organized animals all the pro- 
cesses connected with assimilative changes take place through the agency 
of one and the same organ, the digestive tract; but in the higher animals r 
through specialization, normal assimilation is a definite series of separate 
phenomena. Between the lower and the higher animals there are 
evidently intermediate conditions where specialization has halted at an 
earlier or a later stage. 

Different Organs of Assimilation. Assimilation begins with the 
presence of suitable food; the solid and liquid constituent parts of the 
body must digest and incorporate this, i.e., it must be altered so that it can 
be absorbed and distributed to the tissues. All this takes place through 
the agency of the digestive tract, which is provide! with accessory organs, 
the digestive glands; the digestive tract likewise removes all matter remain- 
ing undigested (the feces). The necessary oxygen, gaseous food, so to 
speak, is usually taken, however, by particular parts of the body, the 
respiratory organs, the gills or lungs. The oxygen and the digested 
(consequently liquefied) organic and inorganic compounds must further 
be distributed in the body to the organs and tissues according to their 
needs. Therefore there are usually blood-vascular or circulatory organs, 
which permeate the body. But the tissues need not only a means of 
obtaining new materials but also of getting rid of certain useless com- 
pounds. The accumulation of the oxidation products arising from 
functional activity is injurious to the organism; consequently they must 
be removed, and in a dissolved state they are taken up by the blood- 
vascular apparatus, and are brought to definite places for expulsion or 
excretion. Fluid wastes are expelled by the kidneys of vertebrates, the 
Malpighian vessels of insects, the water-vascular system of worms; these, 
together with their accessory apparatus, are embraced under the name 
excretory organs. Excreta are to be distinguished from J 'trees; excreta are 
substances which have been a. part of the tissues of the body itself, and, 
through oxidation, have become useless; while those substances which 
constitute the faeces were useless from the beginning, and have never 
belonged to the body, but have remained separated from the tissues by the 
epithelium of the digestive tract. The gaseous oxidation product of the 
animal body, carbon dioxide, is removed by the blood-vascular apparatus 
through the agency of the respiratory organs. Since in the respiratory 
organs there takes place an exchange of the useless carbon dioxide for the 


GENERAL ORGANOLOGY 


93 


oxygen necessary to life, these organs have a double function, being, at 
the same time, excretory organs for taking up food. 

I. The Digestive Tract. 

Archenteron or Primitive Digestive Tract. Since the taking in 
of food and its assimilation are functions most important for the well- 
being of the animal, it is to be expected that of all the organs the digestive 
tract should be formed first. The fact that many worms (cestodes) and 


FlG. 58. Fir,. 50. 

FIG. 58. Longitudinal section through the nutritive polyp of a siphonophore 
(after Haeckel). o, mouth-opening; en, entoderm; ek, ectodern. 

FIG. 59. Stenostoma leucops, in division, a, ectodermal fore-gut, at a' forming 
anew for the hinder animal; m, the blindly ending entodermal mid-gut, c, ectodermal 
ciliated epithelium; g, ganglion with ciliated pit; iv, water-vascular canal; g f , ganglion 
of the hinder animal. 

Crustacea (Rhizocephala) have no digestive tract does not alter this 
statement; for it can be definitely affirmed that, in adaptation to para- 
sitism, the digestive tract has degenerated. The simplest multicellular, 
free-living animals are merely simple or branched digestive pouches 


94 GENERAL PRINCIPLES OF ZOOLOGY 

which have only a single opening, functioning both as mouth and anus 
(fig. 58). Such an animal has necessarily at least two epithelial layers, 
one of which lines the digestive tract, the other covers the surface of the 
body. These two fundamental cell-layers are called entoderm and ecto- 
derm. In many ccelenterates they are the only layers of the body. In 
most animals they are separated by intermediate tissues, called collec- 
tively mesoderm. The higher the animal, the more differentiated is the 
mesodermal layer. The primitive digestive cavity lined by entoderm is 
called the archenteron. In the case of medusre and polyps (fig. 58) it 
forms the entire digestive tract, but in most animals this is not sufficient 
for the needs of digestion and the alimentary tract is increased by invag- 
inations of parts of the surface (ectoderm) of the body. 

Stomodaeum and Proctodaeum. Even in many ccelenterates and 
lower worms an imagination arises at the anterior end of the digestive 
tract, forming the ectodermal fore-gut or stomodffum (fig. 59). From the 
higher worms onwards, it is accompanied by a second imagination at the 
hinder end, the ectodermal hind-gut, or proctodceum (fig. 60) ; embryolog- 
ically, this is formed as a blind sac whose closed end unites with the like- 
wise closed posterior part of the archenteron (now called also mesenteron 
or mid-gut) until the separating wall disappears, whereupon mid- and 
end-gut communicate with each other, and the digestive tract becomes 
a canal extending through the entire body. 

The part which the archenteron takes, in comparison with the ectodermal 
proctodteum and stomodaeum in making up the completed digestive tract, is very 
different in the various groups. On one side the Crustacea, on the other side 
the vertebrates, offer the strongest contrast; the Crustacea have a very short 
mid-gut and consequently a long extent of fore- and hind-gut formed from the 
ectoderm; in vertebrates, on the contrary, the ectodermal portions are extremely 
short. 

Divisions and Appendages of the Digestive Tract. The width 
of the lumen varies in the course of the alimentary canal and renders 
possible the recognition of different divisions, which, so far as possible, 
have been provided with uniform names. Fig. 61, drawn from a domestic 
fowl, illustrates the usual terms. The mouth-opening leads into a wider 
cavity, which is usually divided into an anterior division, the buccal 
cavity, and a posterior one, the pharynx. The narrow tube leading from 
this is the oesophagus (a) ; here and there it may widen, or bear a pouchlike 
evagination, the crop or in glumes (b), for the temporary reception of food. 
From the oesophagus the food passes into a considerable enlargement, 
the stomach. Birds, like many other animals, have a double stomach, 
a thin-walled portion rich in glands, and a second part, the walls of which 


GENERAL ORGAXOLOGY 


9.5 


are remarkable for the thick masses of muscle; the former is the glandular 
stomach (c), the latter is the grinding stomach or gizzard (d), serving for 
comminution of the food. Behind the stomach the digestive tube narrows 
into the small intestine (//), following which is the hinder widened part, the 


- b 


I C 


o 


FIG. 60. FIG. 61. 

FIG. 60. Bee larva just after hatching: seen from the ventral surface. The diges- 
tive tract consists of three portions; a, fore-gut; m, mid-gut; e, hind-gut (not yet con- 
nected with the mid-gut); sg, limits of segments; st, stigma; t, trachea; H, ventral nerve- 
cord (after Butschli). 

FIG. 61. Digestive tract of the domestic fowl: a, oesophagus; b, crop; c, glandular 
stomach; d, gizzard; e, liver; f, gall-bladder; g, pancreas; h, /, small intestine; k, ca-ca; 
I, large intestine; m, ureters; n, oviduct; o, cloaca. 

large intestine (/), terminating in the anus. The limit of the small and 
large intestine is usually marked by blind pouches, the cccca (k). Con- 
nected with the anal gut also are the outlets of the kidneys (m) and of t bi- 
sexual apparatus (n) ; hence the terminal portion, serving as the outlet 
for the urine and faeces, and also for the sexual products, is called the 


96 GENERAL PRINCIPLES OF ZOOLOGY 

cloaca (0). In the more highly organized animals there are accessory 
structures connected with the alimentary canal. Into the mouth empty 
the salivary glands; into the first part of the small intestine, close behind 
the stomach, the liver (e) and the pancreas (g) (or a single glandular 
apparatus, whose secretion combines the characters of gall and of pan- 
creatic juice, the h e pat o pancreas}. Finally, in the hind-gut there sometimes 
occur glands which form a fetid secretion the anal glands. 

Digestive Functions. Besides the comminution of the food which is often 
necessary, the alimentary canal has (i) to digest the food; that is to convert it 
into a solution; and (2) to resorb the digested food, that is to forward it to the 
tissues by the blood and lymph vessels. Digestion is effected by fluid ferments 
(enzymes], substances which by their presence can produce definite chemical 
changes without, apparently, being altered themselves. Thus the pepsin from 
the gastric glands of the vertebrates, in the presence of hydrochloric acid, can 
convert the proteid of the food into the soluble peptone; the trypsin of the pan- 
creas has the same effect in alkaline media; the steapsin of the bile saponifies 
fats and makes them resorbable, while the ptyalin of the saliva converts starch 
into sugar. The resorbed substances are distributed to the tissues and are here 
assimilated; that is, so altered and appropriated that it becomes an integral part 
of the living, functioning structures muscles, nerves, cells, etc. 

In the vertebrates there is a division of labor, the glands functioning exclu- 
sively in furnishing the digestive fluids, the walls of the alimentary canal being 
chiefly resorbtive. In the invertebrates this distinction has not gone so far, so 
that the transfer of names from the higher group may lead to misconceptions 
as to the functions of the organs. When we speak of the liver of a crustacean, 
spider or mollusc, we must remember that this organ not only dissolves fat, and 
proteids and cellulose as well, but that it plays an important part in the resorb- 
tion of nourishment. In the protozoa there is a cellular digestion, food par- 
ticles being taken into the cell. A similar condition obtains in many coelenter- 
ates, the individual entodermal cells eating the food particles; but there is also 
a true digestion in the archenteron, the walls of which secrete digestive fluids. 

Even when the digestive tract has but little differentiation of its parts, it 
usually has a mesodermal layer added to its entodermal lining, the whole wall 
thus formed being called the splanchnopleure. The mesodermal additions take 
the form, not only of connective tissue but muscles as well (usually smooth, 
rarely of the striped variety). These muscles are important in effecting the 
(peristaltic) motion by which the contents of the canal are moved about inde- 
pendently of the body musculature. When these splanchnopleuric muscles are 
absent, the movement is caused by the contraction of the body muscles, or by 
the cilia which may cover the digestive epithelium. 

The length of the digestive tract is chiefly influenced by the kind of food. In 
many groups of animals there is found a difference between herbivores and 
carnivores, the former having a very long and consequently convoluted digestive 
tract. That of a carnivore is about four or five times the length of the body, 
while in an herbivorous ungulate, on the other hand, it is twenty to twenty- 
eight times. Similar, though not so great, are the differences between carnivor- 
ous and plant-eating beetles. 

II. Respiratory Organs. 

Sources of the Oxygen used in Breathing. The oxygen which each 
animal must obtain in exchange for the carbon dioxide formed in the 


GENERAL ORGANOLOGY 97 

tissues is derived either from the air or from the water, according as the 
animal is terrestrial or aquatic. Less frequently it is the case that water- 
dwellers breathe air, and hence are compelled, from time to time to 
rise to the surface of the water for air; this is true for the large marine mam- 
mals, and for many insects, spiders, and snails found in fresh water. Air- 
and water-breathing takes place exclusively through the skin, so long as 
this is delicate and readily permeable and no higher development of organ- 
ization necessitates a more active interchange. If the demand for oxygen 


cxs 


j V'r V^ 'X S 

~,jp& ; ,.'/- : : 

x -rf' *'* ^"PL T < *~ , ', i 

W^j* :^'-' 


FIG. 62. Left second foot of a crayfish with ('/)/-) attached gill (after Huxley). 
cxp, coxopodite; bp, basipodite; ip, ischiopodite; nip, meropodite; cp, carpopodite; pp t 
propodite; dp, dactylopodite; cxs, bristles of the coxopodite; e, lamina of the gill. 

be greater, special breathing-organs are found gills (branchiai) for water- 
breathing, lungs and trachea; for air-breathing, in addition to which the 
skin functions as an accessory organ of more or less importance. 

Gills are usually thin-walled, frequently ciliated areas of the skin 
which are abundantly supplied with blood-vessels, and where richly 
branched tuftlike projections or broad leaves have grown out, thus 
furnishing the largest possible surface for the interchange of gases; 
these occur in such a position as to be most exposed to fresh water; in the 
crayfish, for example, they are on the legs, where the motion drives fresh 

7 


93 


GENERAL PRINCIPLES OF ZOOLOGY 


water constantly through them (fig. 62); in the swimming worms, on the 
back; in the tube-dwelling worms, at the anterior end, projecting out of the 
tube (fig. 63) ; in most amphibians (fig. 4), on each side of the neck. More 
rarely the digestive tract functions for water-breathing; in the fishes, 
Enteropneusta, and tunicates gills have been formed in connection with 
the pharynx, its lateral walls being pierced by the gill-slits, which open to 
the exterior on the surface of the body. The water containing oxygen in 
solution passes out through the gill-slits, and bathes the gill-leaves which 


FIG. 63. Anterior end of Terebella nebidosa (after Milne Edwards), ph, pharynx; 
vd, dorsal, irv, ventral, blood-vessel; br, gills; t, tentacles. 

are richly provided with blood-vessels. The hind-gut also in many 
fishes, insects, and worms may become an accessory respiratory organ, 
being filled from time to time with fresh water. 

Aerial Respiration. In the air-breathing animals the respiratory 
apparatus is derived either from the digestive canal or from the skin. 
With the vertebrates the former is the case, since the lungs, either directly 
or by way of the trachea and bronchi, are in connection with the lumen of 
the digestive tract. On the contrary, in snails and spiders when the term 
'lung' is used, it refers always to an invagination or sac of the skin; the 
tracheae of insects are similar tubes containing air, beginning at the surface 
of the body with a hole, the spiracle or stigma, and branching internally 
(fig. 60, sf). 


GENERAL ORGANOLOGY 99 

Distinctions between the Respiratory Systems of Chordates 
and Invertebrates. In general, then, a distinction can be drawn be- 
tween the respiratory systems of vertebrates and invertebrates: in the 
former, the digestive tract, or derivatives from it, are respiratory; in the 
latter, on the contrary, it is the skin. Of the vertebrates the only excep- 
tions are most amphibians and a few fishes (Protopterus], in which the 
gills are tuftlike projections of the skin (figs. 4 and 5) ; while among the in- 
vertebrates some aquatic insects respire by the hinder end of the digestive 
tract. 

III. Circulatory Apparatus. 

In order that the oxygen, taken up by the respiratory organs, and the 
constituents of the food digested in the alimentary canal may reach the 
tissus, there is no need of special organs, so long as the body consists of 
only two thin epithelial layers, the ectoderm and entoderm. \Yhen, 
however, a third, a mesodermal, layer is interpolated between these, and 
the body consequently becomes more bulky, there is usually some appa- 
ratus for distributing the food. The simplest is when the digestive tract 
departs from the character of a straight tube and either gives off a few 
broad sacs (gastral pouches') or it branches, and by means of these 
branches extends into the various parts of the body. We speak then of a 
gastro-vascular system, because the alimentary canal itself takes on the 
function and the branching arrangement generally characteristic of the 
vessels or 'vascula' (fig. 64). 

Coelorn. The ccelom or enteroccele is apparently derived from a pair 
of gastric diverticula which have become completely cut off from the 
archenteron (compare development of mesoderm, infra). It is a right and 
left cavity pushed in between the intestinal tract and the body- wall, is 
lined by a special epithelium, the peritoneum, and encloses most of the 
vegetative organs. If the two halves of the coelom approach, without unit- 
ing, dorsal and ventral to the gut, the result is dorsal and ventral mem- 
branes, the mesenteries, which support the alimentary canal (fig. 241). In 
many invertebrates the ccelom plays an important role in nutrition since 
it contains a lymphoid fluid, rich in proteids and containing cellular 
corpuscles; it is also important for excretion since it may communicate 
with the nephridia (see p. 105) by the ciliated funnels. It loses this 
significance the more the blood system is developed, and in the vertebrates, 
so far as nutrition is concerned, it is a rudimentary organ. 

A sharp distinction should be drawn between the coelom and other cavities in 
the body. Not every 'body cavity' is a crelom, but frequently there occur 
large spaces which are entirely different in origin and in relations. Frequently, 


100 


GENERAL PRINCIPLES OF ZOOLOGY 


as in arthropods, these 'body cavities' contain blood and are in reality but ex- 
pansions of the vascular system. To such cavities the term hcemoca'le has been 
given. 

Heart, Arteries, Veins, Capillaries. The most complete method 
of food distribution is accomplished by the blood-vessels, which, therefore, 

belong generally to the higher animals, and 
function, whether a body cavity be present or 
not. Blood-vessels are tubes containing the 
blood, which transports the oxygen received 
through the respiratory organs, as well as the 
food absorbed from the digestive tract, and later 
gives these up to the tissues. Since such an 
interchange of substances presupposes that the 
blood circulates in the vessels, definite parts of 
the blood-vessels are contractile; they are covered 
by muscles which by contraction narrow the 
tube and push the fluid forwards. In the lower 
forms wide areas are contractile; in higher 


FIG. 64. FIG. 65. 

FIG. 64. Dendroccelum lacteum (after lijima). b, brain; d, digestive tract with 
CEecal branches; n, lateral nerve cords; p, pharynx with sheath and mouth. 

FIG. 65. Schema of circulation of the blood, a, arteries; c, capillaries; h, auricle; 
k, ventricle; kl, valves; p, pericardium; v, veins. 

animals a greater regularity of circulation is reached; a definite special- 
ized muscular part of the course, the heart, alone propels the blood. 

The Higher Development of the Heart. A free motion of the heart 
is only possible when it is separated from the contiguous tissues and en- 
closed in a special cavity (fig. 65). Hence the heart always lies either 


GENERAL ORGANOLOGY 101 

free in the body cavity or enclosed in a special pouch (/>), the pericardium. 
The division of the heart into a part which receives the blood, the atrium 
or auricle (//), and a part which drives the blood onward, the ventricle 
(k), is of less functional importance; hence is not carried out in all cases. 
There are also valves (kl), which, by closing, prevent the blood from 
flowing back when the walls relax at the end of the contraction. 

Blood-vessels. In order that the blood system may properly perform 
its function, in addition to circulation, it is necessary that the nutritive 
substances be readily taken up and given out again to the tissues. The 
part of the course of circulation concerned in this must have easily 
permeable walls, must be widely distributed in the body, and have a large 
superficial area. These demands are met by the capillaries (r), extremely 
fine, thin-walled and permeable epithelial tubes, which surround and 
penetrate all organs. Between the heart and the capillaries there exists, 
corresponding to their different functions, great differences in structure; 
they must therefore be united by special transitional vessels (i) vessels 
which begin large and thick-walled at the heart, and by branching, and 
thinning of their walls, pass gradually into the capillaries, the arteries (a) 
and (2) vessels (veins} which start from the capillaries and lead back to 
the heart, uniting to form larger and stronger vessels (v). 

Correlation of Respiratory Organs and Blood System. It is a 
law that in all animals the blood-vascular system has been influenced in 
its arrangement and structure more by respiration than by nutrition in the 
narrower sense; there exists a correlation between the organs of respira- 
tion and of circulation. A double capillary region must be distinguished; 
besides the body capillary system already mentioned there is the respiratory 
capillary region, whose exclusive office is to remove the carbon dioxide 
from the blood and to furnish oxygen to it (gill and lung capillaries). A 
twofold capillary region makes necessary also a twofold system of arteries 
and veins (systemic arteries and systemic veins, respiratory arteries and 
respiratory veins'). The accompanying diagram (fig. 66) of the blood 
circulation of fishes illustrates this. Veins lead from the capillary region 
of the tissues of the body to the auricle of the heart. The contraction or 
systole of the auricle drives the blood into the ventricle. While the auricle 
enlarges (diastole) and refills with blood from the veins, the systole of the 
ventricle forces the blood through the gill arteries to the gill capillaries. 
Since systole and diastole of a heart chamber alternate, the heart acts as a 
suction and force pump, and the systole of auricle and ventricle must 
alternate in time. From the gill capillaries the blood goes to the 'gill- 
veins' ^efferent gill arteries), which unite into a single large trunk: this 
again gives off lateral branches passing into the capillary region of the 


102 


GENERAL PRINCIPLES OF ZOOLOGY 


body. Since the branches of the main trunk formed by the 'gill-veins' 
lead again into a capillary region they must, like the main stem, be called 
arteries. 

Arterial and Venous Blood. During its course through the body 
the blood twice changes its chemical character and correspondingly its 
color. The blood which flows from the body capillary region has given up 


FIG. 66. Scheme of circulation in a fish. a', ascending (Ventral) oarta; a~, descend- 
ing (dorsal) aorta; c, carotid; da, intestinal arteries; dc, intestinal capillaries; dv, 
intestinal veins; h, auricle; k, ventricle; ka, afferent gill-arteries; kc, gill-capillaries; kv, 
efferent gill-arteries; Ic, liver-capillaries; sc, body-capillaries; vc, cardinal veins; i<h, 
hepatic vein, vj, jugular vein. 

its oxygen to the tissues, receiving in exchange carbon dioxide, and has be- 
come dark red. This character is maintained until, in the gill-capillaries, it 
again becomes oxygenated, giving up the carbon dioxide and becoming 
bright red. Since the different character of the blood was first known in 


GENERAL ORGANOLOGY 


103 


the arteries and veins of the systemic circulatory system; the dark blood 
containing carbon dioxide is called venous, and the bright red, containing 
oxygen arterial blood, since the former flows in the veins, the latter in the 
arteries. These terms are entirely unsuitable, as can readily be seen 
from the diagram (fig. 66), because they easily lead to the false assumption 
that veins must always conduct blood containing carbon dioxide, and 
arteries always oxygenated blood. On the contrary, the diagram shows 
that, in the respiratory circulation, the conditions must be the reverse of 
those in the systemic circulation, since here the arteries contain 'venous' 
while the veins contain 'arterial,' blood. 

Closed and Lacunar Blood-vascular Systems. Such a blood- 
vascular system as has here been described is called a closed one, because 
the blood always flows in special tubes provided 
with their own walls. Opposed to the closed stands 
the lacunar blood-vascular system; here the blood- 
vessels lose, after a time, the character of tubes and 
become wide cavities, or sinuses, which, without 
special walls, are enclosed between the intestines and 
other organs (hremoccele, supra). 

Example of Lacunar Blood-vascular System. 
The best example of a lacunar blood-vascular 
system is furnished by the insects and myriapods, 
which have only the heart and short arterial trunks; 
from the ends or the arteries the blood enters the 
haemoccele, and from this through lateral slits (ostia) 
again enters the heart (fig. 67). In the arthropods 
and molluscs are found all transitions between so 
extreme a case of a lacunar system and the almost 
completely closed one. Here appears again a close 
correlation of the circulatory and respiratory organs 
the latter determining the development of the 
former. If the respiration be diffusely distributed 
over or through the body, the circulatory apparatus 
is very simple; on the other hand, if the respiration 
be connected with definitely restricted areas, the 
apparatus is differentiated into heart, arteries, veins, 
and capillaries. Details may be found in the sections on crustaceans, 
spiders, and insects, infra. 

Lymph-vessels. A special part of the vascular system is the lymph 
system, known only in vertebrates. In the capillary region of the body 
proteids may pass into the tissues, but it is evident that an overflow 


o - 


FIG. 67.- Anterior 
end of the heart of 
Scolopcndra (after 
Newport), ac, cepha- 
lic artery; <;/>, artrri.il 
arch; al, lateral artery; 
/")(/, alary muscles (alae 
corclis); lik, chambers 
of heart; o, ostia. 


104 GENERAL PRINCIPLES OF ZOOLOGY 

cannot re-enter the blood-vessels in the same way, on account of the 
higher pressure in the capillaries. This overflow is conducted back to 
the veins by the lymph-vessels. These begin with lacunae in the tissues, 
and gradually pass into vessels with definite walls. The lymph-vessels 
of the digestive tract are particularly important since, during digestion, 
they become filled with the proteid and fatty constituents of the digested 
food; they are called chyle-vessels, because they contain the chyle, dis- 
tinguished from ordinary lymph by its milky color. 

Cold- and Warm-blooded Animals. In connection with the blood- 
vascular system, two expressions are much used but not generally correctly 
understood, viz., cold-blooded and warm-blooded or, more correctly, 
animals with variable and animals with constant temperatures. Under the 
head of animals with varying temperature (poikilothermal} or cold blood 
are placed forms whose temperature is largely dependent upon the tem- 
perature of the- environment, rising and falling with it, but usually a few 
degrees above it. In our climate, where the atmospheric temperature is 
considerably lower than the temperature of the human body , such animals, 
for example the frog, feel cold to our touch, since they have a much lower 
temperature than we. 

Such creatures as maintain about the same temperature, under 
any thermal condition are termed warm-blooded or constant temperatured, 
(idiothermal, homoiothermal) animals. Man in summer and winter 
under the equator and at the north pole, has approximately a temperature 
of 36 C. (98! F.), showing higher temperatures only in fever. In order 
to maintain a constant temperature during the varying conditions, the 
animal must have the power to regulate the warmth of its body, either 
by limiting the production of heat, or by controlling its loss. If the en- 
vironment be warmer than is suitable for the body temperature, then the 
production of heat must be limited to the smallest quantity compatible 
with the vital processes; but, if this does not suffice, the loss of heat must 
be increased by evaporation from the surface, usually accomplished by 
active perspiration. If, on the contrary, the environment be cold, then 
every unnecessary loss of heat must be avoided, while the production of 
heat must be increased. It is clear that idiothermy, since it requires 
complicated apparatus, can occur only in the highly organized animals. 

IV. Excretory Organs. 

The excretory organs are tubes or glandular canals which open to the 
exterior either directly or by way of an end-gut (cloaca), and conduct 
substances which have become useless to the body to the outside. The 
presence of a blood-vascular system or a ccelom or both exercises an 


GENERAL ORGANOLOGY 


105 


important influence on their structure. When neither are developed 

the excretory tubules, in order to remove the excreta from the tissues, 

must branch and penetrate the body in all directions like a drainage system, 

being frequently connected in a network recalling the blood-capillaries 

(protonephridia or water-vascular system of parenchymatous worms, fig. 68). 

The canals begin with closed tubes, which are 

provided internally at the end with a bundle of 

actively vibrating cilia, the 'flame' (fig. 70). 

These flame cells are replaced in many proto- 

nephridia ('head kidneys' of many annelid 

larvae) by solcnocytes (fig. 69), cells with a 

flagellum enclosed in a tube. One or more 

main trunks lead from the canal system to the 

exterior. A little before the external opening 

(excretory pore) there is frequently a contractile 

enlargement, the urinary bladder. 

With the appearance of a ccelom there is a 
central place for the collection of excreta. 
The nephridia or segmcntal organs are usually 
simple (rarely branched) tubes, open at both 
ends. One opening is external (fig. 71), the 
other communicates with the ccelom by means 
of a ciliated funnel, the ncphrostome, a wide 
mouth with active cilia which connects with 
the canal of the tube. Through this the excre- 
tion is carried to the outside. 

The excretory organs (kidneys) of verte- 
brates are derived from such nephridia. The 
fact that in the embryos (and frequently in the 
adults) these open into the ccelom by nephro- 
stomes makes it probable that also in the 
vertebrates the ccelom was once important in 
excretion (fig. 72). The increasing import- 
ance of the blood-vessels which envelop the nephridial canals and l>ring 
to them the waste matter taken from the tissues is probably the cause 
of the loss of connection of the kidneys with the ccelom by degeneration 
of the nephrostomata. The relation of the blood vessels to the 
nephridial tubes becomes specially close by the development of the 
glomendi (Malpighian corpuscles'), bundles of capillaries carrying the 
walls of the canal before them and so projecting into the lumen of the 
tube. Since the nephridial tubules of the vertebrates open into a com- 


Fi.; 68 Distomum hrp- 
aticum with water-vascular 
system (Jrom Hatschck). />, 
porous excre tori us; o, mouth. 


10G 


GENERAL PRINCIPLES OF ZOOLOGY 

n 


FIG. 69. FIG. 70. 

FIG. 69. Blind end of an annelid protonephridium with two connected soleno- 
cytes which open with their flagellate tubes into the excretory duct (after 
Goodrich) 

FIG. 70. Blind end of one of the finest water-vascular canals () of a Turbellarian 
(from Lang). , nucleus; /, processes of the terminal cell; u'f, 'flame' of the terminal 
cell; v, vacuole. 


n 


FIG. 71. FIG. 72. 

FlG. 71.- Segmental organ of an oligocha^te (from Lang), fz, ciliated funnel; 
dis, septum; ng l , non-glandular, ng-, glandular, part of the canal; eb, terminal vesicle; 
In, body-wall. 

FIG. 72. Scheme of a mesonephros of a vertebrate, h, nephridial tubules; 
m, Malpighian tubules with afferent and efferent blood vessels; n, nephrostomes; 
u, urinary duct. 


GENERAL ORGANOLOGY 


107 


mon canal leading to the exterior (ureter) they are commonly aggregated 
into a compact mass, the 'kidney.' 

B. Sexual Organs. 

Sexual Glands and Ducts. In the sexual apparatus are distinguished 
the areas where the germ cells are produced, the sexual glands or gonads, 
and the ducts for these. The former are present, temporarily or perma- 
nently, in all multicellular animals; the latter may he absent. If the 
sexual products arise in the skin or in the walls of the digestive tract, as is 
usual in the ccelenterates, then special outlets are superfluous, since the 
ripe elements can reach the exterior directly by rupture of their covering 
or by means of the digestive tract. 

Germinal Epithelium and Germinal Glands. Male and female 
sexual cells, as we have seen, originate from an undifferentiated incipient 


FlG. 73. Sexual organs of Lnmbricu<; agricala (from Lang, after Yogt and 
Yung). The seminal vesicles of the right side are removed, but, ventral nerve 
cord; bv and bl, ventral and lateral ro\vs of sets; st, recepUcula semi n is, ,s/>, seminal 
vesicles of the left side, connected with a median unpaired seminal capsule (v/>;<). 
Enclosed in the latter are the te?tes (/;), and the seminal funnels (/), which lead into 
the vas deferens (rd); o, ovaries; -w, ciliated funnels leading to oviducts with egg 
capsule ((); di, dissepiments; 8-15, eighth to fifteeeth segment-. 

organ, or anlage, which is called the germinal epithelium. Usually it forms 
a part of the epithelial lining of the body cavity, in many animals per- 
manently, in others only temporarily; in the former case it separates, 
usually by constriction, and forms gland-like bodies, the gonads or sexual 
glands. 

Gonochorisrn and Hermaphroditism. In n^ost animals the ger- 
minal epithelium produces either only female or only mak- sexual cells; 


108 GENERAL PRINCIPLES OF ZOOLOGY 

such animals are called separate-sexed, diivcimis or gonochoristic, in 
opposition to the hermaphroditic forms, in which both kinds of sexual 
glands are contained in one and the same individual. Different degrees 
of hermaphroditism can be distinguished; commonly testes and ovary 
are contained in the same animal, some distance apart, as in the earth- 
worm, in which two segments are male, while a third segment is female 
(fig. 73). More rarely there is a union of testes and ovary into a single 
hermaphroditic gland; land-snails have an hermaphroditic gland, which' 
produces spermatozoa and eggs in the same follicle. 

Occurrence of Hermaphroditism. Hermaphroditism is, in general, of 
more frequent occurrence in the lower than in the higher animals. Insects and 
vertebrates are, almost without exception, dioecious; only among the teleosts is 
hermaphroditism not rare. It also occurs in the myxinoids. More commonly 


FIG. 74. Lateral hermaphroditism of a gipsy moth (Ocn r ria dispar). Left female, 

right male (after Taschenberg). 

hermaphroditism occurs as an abnormality; a striking form is lateral her- 
maphroditism, in which one half of the animal has only male, the other half only 
female, gonads. If the males and females of a species be distinguishable by 
their appearance, then lateral hermaphroditism is expressed in their external 
form, since one half of the animal has the characteristic marks of the male, the 
other half those of the female (fig. 74). Still it must be noted that, in many 
instances where the external appearance pointed toward hermaphroditism 
anatomical investigation has disclosed either only male or only female sexual 
glands in a rudimentary condition (gynandromorphism). True hermaphroditism 
(the presence of both kinds of sexual glands in the same animal) is extremely 
rare in mammals and in man. What is described as hermaphroditism is usually 
gynandromorphism; rarely are both kinds of gonads present in the same indi- 
vidual, and then not in a functional condition. 

The wide distribution of hermaphroditism among the lower Metazoa has 
led to the erroneous view that this was the primitive condition, from which the 
gonochoristic condition has been evolved. Studies on nematodes, Crustacea and 
possibly molluscs have shown that, on the contrary, hermaphroditism has fol- 
lowed a dioecious condition, since with the disappearance of males, the female 
animals may develop male sexual cells before the ovaries are mature. Con- 
trasted to this ' protandry' 1 a ' protogyncccy' is rare. 

Genital Ducts. Very frequently the excretory apparatus furnishes 
outlets for the sexual products. In the annelids and vertebrates portions 


GENERAL ORGANOLOGY 


of the nephridial system, either exclusively or in addition to their excretory 
function, become genital ducts. Hence we speak of a urogenital system. 
This connection of genital and excretory organs has a double cause. 
Physiologically important is the fact that eggs and spermatozoa behave 
like excreta; substances which are no longer needed by the individual, but 
must reach the exterior in order to be- 
come of use. The morphological cause 
is the relation to the coelom. A urogen- 
ital system occurs only in animals in 
which the germinal epithelium arises 
from the epithelium of the ccelom, and 
in which the kidneys or their rudiments 
are in connection with the body cavity 
and thus form the natural outlet for its 
products. Whether the accessory sexual 
parts are portions of the excretory organs 
or are independent structures, they have 
in the animal series a definite arrange- 
ment adapted to their function (figs. 73 
and 75). Canals lead from the gonads 
to the exterior, oviducts in the female, 
vasa defercntia in the male (and the 
hermaphroditic duct from the hermaph- 
roditic gland). 

Accessory Sexual Apparatus. The 
terminal portion of the vas deferens is 
often very muscular and is called the 
ductus ejaculatorius; it may be evaginated 
or project permanently beyond the sur- 
face of the body as a penis or cirrus. 
The terminal portion of the oviduct is 
often widened so that two portions may 
be distinguished, the uterus, which har- 
bors the eggs during their development, and the vagina, which serves for 
copulation. In addition there may occur in both sexes other accessory 
glands of the most diverse character. 

Occasionally, in the animal kingdom, a part of the eggs degenerate and are 
used for the nourishment of the others. This degeneration may take place 
the uterus (Salamandra), in the egg cocoons (annelids), or in the ovary ( 
arthropoda, fig. 35, <). In some cases a definite part of the ovary produ 
these 'yolk cells,' a condition that explains the fact that in many animal.-, (Fla- 


P \e \- 


FiG. 75. Vortex rinlis (after 
Schultze and von GratT): b, brain 
with eyes; be, bursa copulatrix; </, 
digestive tract; g, genital pore; o, 
ovary with oviduct; pit, pharynx; 
pe, penis; r, receptaculum seminis; 
t, testis with vas deferens; it, uterus; 
i'a, vagina; i'i, vitellarium; r.v, vesi- 
culum seminalis. 


110 GENERAL PRINCIPLES OF ZOOLOGY 

todes) there are glands (vifellaria), distinct from the ovaries, which form the 
yolk cells (fig. 75). 

Secondary Sexual Characters. Often we can distinguish between the 
male and female of dioecious organisms only by the sexual products (medusas, 
polyps, sponges). In other cases the sexual ducts are also characteristic. In 
the higher animals these primary sexual characters are associated with those of 
a secondary nature so that it is possible to recognize male or female at a glance. 
These secondary sexual characters are exemplified in many birds and mammals 
by the voice, the hair or feathers, strength of muscles and skeleton, presence of 
offensive or defensive weapons, etc.; in insects by structure and markings of 
wings, form of antennae, etc. (fig. 74). This sexual dimorphism may become 
so marked that only careful study, especially of the development, shows that the 
male and female belong to the same species; dwarf males of Bonellia (fig. 268), 
Cirripedia, Copepoda (fig. 8). 

A part of these secondary sexual characters are developed at the approach of 
sexual maturity and can be restricted or even suppressed when the gonads are 
destroyed or removed (castration). This leads to the conclusion that the 
development of the secondary sexual characters is correlated with the matura- 
tion of the gonads and is influenced by it. As a causal factor it is thought that 
'internal secretions' (hormones) arise in the sex glands; these are passed into the 
circulation and cause the modification of distant parts like hair, larynx, mam- 
mary glands. 

Yet this explanation must not be carried too far. Many secondary sexual 
characters, like those connected with the genital ducts, develop independently 
of the gonads. The peculiar developmental direction taken by the dwarf males 
just alluded to is begun before the maturation of the testes and apparently 
would appear even if the anlage of the gonads were removed in the embryo. 
We are on firmer ground with the corresponding modifications in the Lepidop- 
tera. Here the secondary sexual characters clearly develop in the way laid 
down in the embryo, if the gonads be removed from the young larva; even if 
the testes are removed and replaced by ovaries taken from other individuals, or 
vice versa. The transplanted gonads become mature, while the rest of the 
sexual apparatus and the secondary sexual characters show the peculiarities of 
the original sex. All of these observations show that a correlation of gonads, 
genital ducts and secondary sexual characters exists to only a limited extent. 
The harmonious development of parts is rather regulated by a third factor, 
the peculiarities of the fertilized egg or its early developmental stages. It is 
these that prescribe, in a more or less striking manner, a certain developmental 
direction, not only for the gonads, but for the whole organism. 

Animal Organs. 
I. Organs of Locomotion. 

Voluntary Locomotion. The power to change their location volun- 
tarily is a peculiarity so prominent in animals that usually it is sufficient 
for deciding whether an organism belongs to the vegetable or to the 
animal kingdom. On this account it is necessary to call attention to the 
fact that numerous animals, freely mobile in the larval stages, lose the 
power of locomotion, becoming fixed to the ground, to plants, or to other 
animals, and only retain the power to move parts of the body, as in the 
corals the crown of tentacles, the barnacles their feathery feet; many 
attached molluscs can actively close the shell. 


GENERAL ORGAXOLOGY 


111 


Locomotion among Lower Animals. The lowest forms, the 
Protozoa, progress almost exclusively by processes of the cell: pseudopodia, 
cilia, or flageUa. In the metazoa this is rarely the case. Amoeboid 
movements of the epithelial cells, indeed, occur in the ccelenterates and 
in many worms, but do not suffice for change of position. More effective 
is the ciliated or flagellated epithelium, by which ctenophores, turbel- 
larians, and rotifers swim; this occurs, besides, in many larvae of animals 
which, in the mature state, are unable to change their location or do so 
only by the aid of muscles. Nearly all ccelenterates, echinoderms, 
molluscs, and the majority of the worms leave the egg-membranes as 
lame swimming by means of cilia. 

The musculature is alone adapted for energetic motions. The 
arrangement of this varies with and depends upon the constitution of the 
skeleton. Forms without a skeleton have commonly the dermo-muscular 
tunic, a sac of circular and longitudinal muscle fibres which is firmly 
united with the skin. If a skeleton be formed by the skin, as in the 
arthropods, where the epidermis secretes a 
firm cutlcular skeleton, then the sac breaks 
up into groups of muscles, which find 
points of attachment upon it; if, on the 
other hand, as in the vertebrates, an axial 
skeleton be formed, fixed points are fur- 
nished for muscular attachment, so that 
the musculature obtains a new character, 
in particular a deeper position. A unique 
locomotor apparatus is the ambulacra! 
system of the echinoderms, a system of 
delicate little tubes with protrusible por- 
tions which function as feet, described in 
connection with that group. 


ffff 


FIG. 76. Diagrammatic sec- 
tion of electrical apparatus (from 
Wiedcrsheim). The arrow points 
dorsally or anteriorly. BG, con- 
nective-tissue framework; 1:1', 
electrical plates; G, gelatinous 
tissue ; A r , nerves entering through 
the septa; NN, nerve termina- 


Electric Organs. In several fishes the 
muscles at certain places are modified into 
electric organs, which, in Malapterurus, Tor- 
pedo, Gymnotus and Astroscopus can give ener- 
getic discharges; in Raia and Mormyrus the 
discharges are weak and cannot be felt by man. Each organ is formed of 
columns of numerous superimposed plates separated by connective tissue. 
Each plate is a metamorphosed muscle fibre, the side to which the nerve is 
attached forming the negative pole. 

II. Nervous System. 

Scarcely a system of organs shows such a regular progression as the 
nervous system. The different stages may be termed the diffuse, the 
linear, the ganglionic, and the tubular types. 


112 


GENERAL PRINCIPLES OF ZOOLOGY 


Diffuse Nervous System. The diffuse type is certainly the most 
primitive; it shows the two elements, nerve fibres and ganglion cells, 
distributed through the whole body, or, at least, through certain layers 
of it. The skin of the body, the ectoderm, is one of the fundamental 
elements in the nervous system, since it is related to the external world, 
and hence receives the sensory impressions, so important for the develop- 
ment of nervous tissue. The corals and hydroid polyps are examples, 

C 


FIG. 77. Third abdominal ganglion of a crayfish (after Retzius). C, connective 
or longitudinal commissure; G, ganglion cell layer; g', ganglion cells whose neurites 
enter the connective; g 2 , ganglion cell whose neurites enter the peripheral nerve; 
L, granules, (Leydig's dotted substance) ; N, peripheral nerve. 

since in them the ectoderm is permeated in all directions by a subepi- 
thelial spider-weblike network of nerve fibres and ganglion cells, which 
encroach even upon the entoderm (fig. 57). 

Linear Nervous System. From the diffuse type the other chief 
types can be derived through concentration, which is chiefly conditioned 
by the fact that there are a few points which are most advantageously 
located for the reception of sensory stimuli, and hence for the development 


GENERAL ORGANOLOGY 


113 


of nervous elements. In the medusae such a place is the rim of the bell ; 
consequently a stronger nerve-cord much richer in ganglion cells is found 
here. This, as well as the nerve-ring and the live radial nerves of 
echinoderms, may be called a central system, thereby distinguishing the 
rest of the nervous network as the peripheral nervous system. 

Ganglionic Central Nervous System. Numerous transitional 
forms lead to the ganglionic central nervous system of the worms, molluscs, 
and arthropods (fig. 77). The central nervous 
system here consists of two or more ganglia; each 
ganglion being a bunch of regularly arranged 
nerve-fibres and ganglion-cells. The former con- 
stitute the centre of the mass, and, since they 
cross in all directions giving off branching den- 
drites, they appear like fine granulations. The 
ganglion-cells, on the other hand, collect in a 
thick layer around the granules. The peripheral 
nerves, and also the commissures, the cords con- 
necting similar ganglionic masses, extend out- 
wards from the ganglia. 

Supraoesophageal (or Cerebral) Ganglia. 
Since most animals are symmetrical, the ganglia 
occur in pairs; left and right ganglia correspond 
to one another and are connected simply by a 
cord of nerve-fibres, the transverse commissure. 
Of most constant occurrence are two ganglia, 
which lie dorsally above the pharynx, and hence 
are called the supra-ccsophageal or cerebral ganglia. 
If other ganglia occur, they lie ven trail v ar.d 
below the digestive tract (ventral nerve-cord}. 

In the Ladder Nervous System of annelids 
and arthropods (fig. 78), numerous pairs of 
ganglia (in the example before us, nine) lie in 
serial order on the ventral side of the animal, and 
are connected by longitudinal commissures (con- 
nectives'), and also by transverse commissures 
connecting the left and right ganglia. The first 
pair of the series is the infra-oesophageal ganglion, 

which sends out connectives right and left, surrounding the pharynx, 
to the supraoesophageal ganglion. The supra- and infra-cesophagral 
ganglia together with the cesophageal connectives form the cesophageal 
ring, a nerve-ring surrounding the oesophagus. 
8 


FIG. 78. Ladder 
nervous system of sowbug 
(Porcellio scaber) (after 
Leydig). A, brain: B, 
ventral cord, connected 
with the brain by the 
cesophageal commissures 
b, a cord formerly re- 
garded as syrapatheticus. 


114 


GENERAL PRINCIPLES OF ZOOLOGY 


The Tubular System is found only in the chordates (fig. 79). The 
vertebrate brain and spinal cord form a tube with greatly thickened walls. 
In the centre lies the extremely narrow central canal, which widens 
anteriorly into the ventricles of the brain. In a transverse section the 
nervous elements are seen grouped around the central canal in a manner 
almost the reverse of that of the ganglionic type. On the periphery lies 
a layer of nerve-fibres (the while mailer) ; next is a central portion formed 

of ganglion-cells and nerve-fibres 
(the gray matter), which is marked 
off from the central canal by a 
special epithelium (ependyma). In 
addition there are modified sup- 
porting cells which form a frame- 
work (glia, neuroglia) for the 
nervous parts. 

Relations of Nervous System 
and Skin. It has been ascer- 
tained in almost all animals that 
the nervous system arises from the 
ectoderm. Therefore, in many 
animals, the nerve-cords and the 


FIG. 79. Cross-section of the human 
spinal cord (from Wiedersheim). Black 
represents the gray, white the white sub- 
stance of the cord; Cc, central canal, sur- 


rounded by the anterior and posterior com- 

missures (C and CO; Sa ,Sp anterior and ganglionic masses H e permanently 

posterior fissures; VW, HM , anterior and f 

posterior nerve-roots; VH, HH, anterior in the skin; in others only during 

and posterior horns of gray matter; V, S, the development, later becoming 

H , anterior, lateral, and posterior columns 

of white matter. separated by splitting off or by 

infolding, and thus coming to lie 

in the deeper layers of the body (fig. 9). In the vertebrates and some 
other higher animals, besides the body nervous system, there is a sympa- 
thetic system for the control of the vegetative organs which are not 
influenced by the will. 

III. Sensory Organs. 

What we know of the external world is founded upon experiences 
gained through our sensory organs, controlled by the judgment. If 
things exist outside of ourselves which have no influence upon our senses, 
we can form no conception of them. It follows from this proposition that 
we can gain knowledge of the capacity of the sensor}' organs of animals 
only by analogy with our own experiences. Hence the distinction of five 
senses, touch, taste, smell, hearing, and sight, based upon human physiol- 
logy has been extended to the whole animal kingdom. A prior'', however, 
it cannot be denied that sensations may occur in animals which we do not 


GENERAL ORGAXOLOGY 


11.", 


experience; following out this course of thought has led to the idea of a 
'sixth sense,' a designation which is no longer correct since mankind has 
more than five senses. The former sense of 'touch' really includes, 
besides true touch, the senses of temperature and pain. In addition there 
are also the muscular and equilibrium senses. A still more important 
reason for our very fragmentary knowledge of animal sensations is the 
fact that, in regard to the function of the sensor}' apparatus, we can 
seldom depend upon experiments, and consequently must base our 
conclusions upon structure. But the anatomy of many sensory organs, 
like those of smell and taste, is by no means so characteristic that it alone 
is sufficient to determine the function. 


FIG. 80. FI G. Si. 

FIG. 80. Tactile hairs of a crab Cyrtomuia (after Doflein). 

FIG. 81. Vater-Pacinian corpuscle of the mesentery of a cat. <;. axis cylinder: /, 
fat; g, blood-vessel; z, inner bulb; k, capsule with nuclei; n, medullated nerve-fibre. 

Tactile Organs. The skin is tactile, usually over the whole an-a, 
although not everywhere with equal intensity. Prominent parts, like 
the tentacles of polyps and of many worms, the antennae of arthropods and 
snails, need only mention. Special epithelial cells with stiff hairs pro- 
jecting above the surface, the tactile bristles or tactile hairs, are tactile 
(fig. 80). Only in the vertebrates do the nerves of touch terminate in 
specially modified end organs (Vater-Pacinian corpuscles, corpuscles of 
Mcissner, etc., fig. Si); these usually lie under the epithelium. 

Organs of Smell and of Taste are accurately known only in verte- 
brates. The olfactory organ of fishes consists of two pits in the skin, 


116 


GENERAL PRINCIPLES OF ZOOLOGY 


above or in front of the mouth. In the air-breathing vertebrates this 
pair of pits, which here also arise from the skin, are taken into the dorsal 
wall of the two respiratory canals leading from the outside to the mouth 
or pharynx. Now since the olfactory cells distributed in these pits (fig 
38, B) are frequently characterized by bundles of olfactory hairs, while 
the surrounding epithelium is often ciliated, one is inclined to regard as 
organs of smell sensory organs of invertebrates, which have the form of 
ciliated pits or lie near the respiratory apparatus (e.g., the osphradiwn of 
molluscs). Yet there are exceptions. Experiments seem to show that in 
the arthropods the antennae serve for smelling. Here the sensory per- 
ception can be connected only with certain modified hairs, the olfactory 
tubules of the Crustacea and the olfactory cones of insects. In a similar 
way certain nerve end organs in the region of the mouth are considered 
as organs of taste, since the taste organs of vertebrates, the so-called taste 
buds, are abundant in the mouth cavity. 

Organs of Hearing and of Sight are called the higher sense-organs, 
because they are of much greater importance than the other organs, 


Ot 


N 


Wz 


FIG. 82. Auditory vesicle of a mollusc (Pterotrachea). N, auditory nerve; Us, audi- 
tory cells with the central cell; Cz, Wz, ciliated cells, Ot, otolith (after Claus). 

since they furnish sensations which are quantitatively and qualitatively 
much more definite. Ears and eyes have therefore a complicated and 
characteristic structure, which renders them easily recognizable by the 
almost invariable presence of certain structures accessory to their 
functions. 

The auditory organs of vertebrates and of most other animal groups 
can be traced back to a simple fundamental form, the auditory veside 
(fig. 82). This has an epithelial wall, a fluid contents, the endolymph, 
and an auditory ossicle or otolith, formed from one or from several 
lused concretions. In some instances the otoliths, to the number of 


GENERAL ORGANOLOGY 


117 


thousands, may remain separate. In a definite region of the epithelial 
Avail the sensory cells are developed into the crista acustica or the auditory 
ridge; they are in connection with the auditory nerve and bear the auditory 
hairs projecting into the endolymph. The otoliths are usually free in the 
centre of the vesicle, or are often held in place by bundles of cilia which 
project from the non-sensitive epithelial cells. 

Every auditory vesicle develops from a pitlike invagination of the 
skin, and consequently is for a time an auditory pit. Therefore it is not 
surprising that in many animals the organ has stopped at the lower stage 
of development; for example, the crayfish has an open auditory pit 
(fig. 378). On the other hand, the auditory vesicle may develop a com- 
plicated system of cavities as in mammals (fig. 83), where it is divided by 
a constriction into the sacculiis and the utriculus. The sacculus is pro- 


C 


-u 


FIG. 83. Diagram of the human labyrinth. U, utriculus with the semicircular 
canals; 5, sacculus connected with the cochlea (C) by the canalis reuniens; R, recessus 
labyrinthi; V, blind sac of the cochlea; A", apex of the cochlea. 

vided with a spirally-wound blind sac, the cochlea, the utriculus with the 
three semicircular canals, the whole being called the labyrinth. In addi- 
tion there is formed in most vertebrates, a sound-conducting apparatus, 
so that the auditory organ acquires a very complicated structure. 

Other Forms of Auditory Organs. Since there are animals without 
auditory vesicles which hear well, like the spiders and insects, we must 
assume that there are auditory organs of another type. Still we have 
no certain knowledge of these except in the case of the tympanal auditory 
organs of the grasshoppers (see p. 410). 

Function of the Semicircular Canals. Experiments upon repre- 
sentatives of the different classes of vertebrates have led to the conclusion 
that the three semicircular canals, standing at right angles to each other, 
are organs of equilibrium, while the cochlea of the mammals and the 
homologous lagena of the other vertebrates is the seat of hearing. Corre- 
sponding to the poor development of the lagena, hearing is so poorly 
developed in the fishes that it was long believed to be absent and the laby- 
rinth was regarded as a balancing organ, since when it was destroyed the 


118 


GENERAL PRINCIPLES OF ZOOLOGY 


animals stagger and lose their balance. Starting from this assumption, 
recent investigators have attempted to prove that the auditory vesicles of 
invertebrated animals are exclusively, or at least largely, organs of equili- 
bration. This would explain the otoliths, for these bodies, of relatively 
high specific gravity, would affect the crista in different ways according 
to the position of the body. Statoliths is thus a better name. 

Stimulation by Light is a phenomenon widely distributed among 
animals and plants; in its simplest form it is manifested by the organisms 


C. 


IV 


FIG. 84. Invertebrate eyes. I, Phyllodoce (an annelid, after Hesse); II, Nau- 
phanta (annelid, after Greef and Hesse); III, larva of a beetle, Acilius, after Grenacher; 
IV, a medusa, Lizzia; c, cuticle; d, gland cells which secrete the vitreous body; e, 
epidermis; C, vitreous body; L, lens; o, optic nerve; oc, ocellus; />, pigment; r, rhabdoms 
of the retina; s, visual cells. 

collecting in or shunning the lighted spot (positive and negative phototaxis). 
Phototaxis occurs, even when there are no special organs for the recog- 
nition of light (Infusoria, Hydra, many worms). It is increased when 
there are visual cells, that is light percipient spots connected with nerves. 
These may be on the surface, or deeper in position, if the overlying layers 
be translucent (earthworms, Amphioxus}. If numerous visual cells be 
united into a layer this is called a retina. In the lowest developmental 


GENERAL ORGANOLOGY 


119 


stages the visual cells are closely related to accumulations of pigment 
which occur either in or surrounding the cells. That this pigment is not 
absolutely essential for light perception is shown by the. visual powers of 
albinos which are free from pigment, but it clearly must increase the 
sensitivity of the cells, for pigmentation is so common that the simplest 
eyes may be defined as sharply denned pigment spots, to which there is 
frequently added a lens to concentrate the light (fig. 84, III). 

Eyes. From such beginnings, which are evidently only intended to 
recognize light and darkness, there are all transitions to the image-forming 
eyes of the vertebrates and apparently the cephalopods. The retina is 
rendered more efficient by the development of r/iabdoms on the peripheral 
ends of the visual cells, rod-like processes which aid in light perception, 
and in the vertebrates usually divided into rods and cones (fig. 85, 9). 


P E G 

FIG. 85. Human retina (after Gegenbaur). P, pigment- layer; E, layer of 
sensory cells; G, optic ganglion; i, limitans interna; 2, nerve-fibre layers; 3, ganglion 
cells; 4, inner reticular layer; 5, inner granular layer; 6, outer reticular layer; 7, outer 
granular layer; 8, limitans externa; 9, rods and cones; 10, tapetum nigrum; .17, Mtiller's 
fibres. 

In the vertebrates and manv invertebrates the retina contains a 

j 

reddish pigment, the 'visual purple,' which is quickly bleached in the 
light and as quickly regenerated in darkness, and which apparently plays 
an important part in vision. In the course of the optic nerve there are 
numerous ganglion cells which form an optic ganglion (figs. 85 and 356), 
lying outside the eye in the invertebrates, in die vertebrates forming a 
number of layers (G, fig. 85), inside the retina proper (K), which is 
formed of the visual cells (outer granular layer) with the fibres of the 
rods and cones and the rhabdoms themselves. 

Accessory Structures. If a sharp image is to be cast on the retina, 
the light rays coming from a point without the eye must be brought again 
to a point on the retina by refractive substances (lens, cornea) ; therefore 
there must be a space between the dioptric apparatus and the retina. The 


120 


GENERAL PRINCIPLES OF ZOOLOGY 


eye is therefore developed as a camera obscura, the space between retina 
and lens being filled by the vitreous body (transparent cells or jelly fig. 84). 
The amount of light is regulated by an iris, a pigmented membrane with 
circular opening, the pupil, the width of which is enlarged or contracted 
in accordance with the intensity of the light. Then nutrition is provided 
by a richly vascular coat, the chorioidea, and for protection there is a firm 
outer coat, the sclera. These accessory structures are developed and 
combined in the most diverse ways in the different classes of animals ;- 
eyes which are very similar in structure, like those of vertebrates and 
cephalopods (figs. 86, 349), have developed along entirely different 
ontogenetic and phylogenetlc lines. 


CNO VO 

FIG. 86. Horizontal section through the human eye (after Arlt, from Hatschek). 
E, epithelium of the cornea (conjunctiva); C, cornea; vA, anterior chamber of the 
eye; /, iris; hA, posterior chamber of the eye; Z, zonula Zinnii; Os, ora serrata; Sc, 
sclerotic coat; Ch, choroidea; R, retina; />, papilla of optic nerve; m, macula lu'tea' 
area of most distinct vision; VO, sheath of the optic nerve; NO, optic nerve; C, 
arteria centralis; Cc, corpus ciliare; L, lens; Cv, vitreous body. 

The Eye of the Vertebrates. The eye of the vertebrates usually is an 
approximately spherical body. Over the greater part of the circumference 
there is an opaque, fibrous or cartilaginous sclera, or sderotica, transparent only 
in the most anterior part, where it forms a projecting portion like a watch-glass, 


GENERAL ORGANOLOGY 


121 


the cornea. Internally to the sclera lies the chorioidea, which, at the junction 
of sclera and cornea, is changed into the iris. The iris, the seat of the color of 
the eye, is pierced by the pupil, which regulates the amount of light. Next 
internal to the chorioid follows a layer of black cells, the tapctum nigrum (pig- 
mented epithelium), and finally the retina itself, the expansion of the optic 
nerve which enters the eye at the hinder part. The tapetum nigrum and the 
retina arise together, and hence both end at the edge of the pupil, although the 
retina loses its nervous character at the or a serrata, some distance from the outer 
edge of the iris. 

The cavity of the eye is completely filled by the vitreous body, aqueous 
humor, and the lens. For vision the lens is the most important, since, next to 
the cornea, it influences most the course of the rays of light. It lies behind the 
iris, fixed to the anterior wall of the chorioid, which here is changed into the ciliary 
process. In front of it is a serous fluid, the aqueous humor, partly in the so-called 
posterior chamber of the eye, between the lens and iris, partly in the anterior 
chamber, between the iris and cornea. 
The single, larger cavity behind the lens 
is filled up by a jelly-like vitreous body. 
The image formed on the retina is in- 
verted. 

Shining of Eyes. In many verte- 
brates there is a tapctum lucidum inside 
the chorioid which causes the so-called 
shining of eyes (cats). This is a layer, 
which reflects light so strongly that only a 
little light from the outside is necessary to 
illumine the back of the eye. There is no 
real production of light. The tapctum 
nigrum must be free from pigment in 
order that the tapetum lucidum may act. 
In many insects and spiders light is sim- 
ilarly reflected from the back of the eye. 

Phosphorescent Organs. For a long 
time eye-like organs have been known, 
especially in animals from the deep seas 
(fishes, cephalopods, Crustacea). Many 
of these have been proved to be organs 
for the production of light, and the same 
is probably true of the others. Each is 
a spherical, eye-like body, arranged in a 
definite manner in the skin and of very 
varying structure. Many have a great 

resemblance to glands (fig. 87). The cells of the gland" follicle are apparently 
to secrete the phosphorescent substance, its light being made more effective 
by a lens-shaped body of transparent cells and by a reflector (not always 
present) consisting of strongly iridescent cells, and all surrounded by a 
pigment layer, the whole being so eye-like that they were at first taken for 
visual organs. We have in these to do with highly specialized structures 
differing from the phosphorescent apparatus so common in marine animals of 
all classes, where (Noctilnca, medusa;, corals, etc.), the phosphorescent sub- 
stance is widely distributed through the body. Perhaps the concentration of 
the phosphorescence in definite organs may serve to light the surroundings, to 
attract the prey and perhaps as an attraction between the sexes. In the hitler 
case there would be an analogy with the phosphorescent organs of insects, which 
are formed in a totally different way. 


FIG. 87. Phosphorescent organ of 
dus (after Brauer). c, cutis; /, 
lens; p, pigment layer; s, phosphorescent 
secretion cells; t, reflecting tajn-tuni. 


122 GENERAL PRINCIPLES OF ZOOLOGY 

SUMMARY OF THE MOST IMPORTANT POINTS OF ORGANOLOGY. 

1. Organs are tissue complexes, differentiated from the surrounding 
structures by a definite form and adapted to the performance of a peculiar 
function; consequently each organ can be classified morphologically 
(according to structure and relations) and physiologically (according to 
function). 

2. Organs of different animals may be physiologically equivalent, 
analogous organs (i.e., with similar functions). 

3. Organs of different animals may be morphologically equivalent, 
homologous (developing in similar relations). 

4. In the comparison of the organs of two animals three possibilities 
become evident, a. They may be at the same time homologous and 
analogous, b. They may be homologous, but not analogous (swim- 
bladder of fishes, lungs of mammals), r. They may be analogous, but 
not homologous (gills of fishes, lungs of mammals). 

5. Organs are divided into animal and vegetative according to function. 

6. Animal functions are those which are only slightly developed in 
plants ; in the animal kingdom, on the contrary, they undergo an increase 
and become characteristic. 

7. Vegetative functions are developed with equal completeness, 
though in a different manner, in plants and animals. 

8. Animal organs include the organs of motion and sensation, such 
as muscles, sense-organs, nervous system. 

9. To the vegetative organs belong the organs of nutrition and re- 
production. 

10. Under nutrition, in the widest sense, are included not only the 
taking in and digestion of food and drink, but also the taking in of oxygen 
(respiration), the distribution of food to the parts of the body, and the 
removal of matter which has become useless. 

11. With nutrition, therefore, are concerned not only the digestive 
tract and its accessory glands, but also the organs of respiration, the 
blood-vascular system, and the excretory organs (kidneys). 

12. The male and female sexual organs serve for reproduction. 

13. The male and female organs may occur in different individuals 
(dicecious], or both may be found in one and the same animal (hermapliro- 
ditic) . 

14. The highest degree of hermaphroditism is attained when one and 
the same gland (the hermaphroditic gland) gives rise to both eggs and 
spermatozoa. 

15. Very often the sexual organs and the ducts from the kidneys are 
closely united; we then speak of a urogenital system. 


PROMORPHOLOGY 123 

IV. Promorphology ; the Fundamental Forms. 

The structure of the individual animal depends uj on the definite arrange- 
ments of its organs, which are definite or only vary slightly in each group. 
Comparison shows that there are a few fundamental forms which play the 
same role in morphology as the crystal forms in mineralogy. But there is an 
important difference. A crystal is made up of similar parts, its form is the result 
of its physico-chemical composition. This condition cannot exist in animals 
as each organ is a complex of many chemical compounds. Nor, even where 
the symmetry is the most pronounced is there that mathematical accuracy found 
in crystals. 

The form of an animal depends upon its extension in space, and accordingly 
we may pass through it three axes at right angles to each other, these defining 
the position of three planes. According to the relations of the body to these we 
may define five fundamental forms. 


FIG. 88. Sponge, Lophocalyx philippensis, with buds (after F. E. Schulze). 

Asymmetrical (anaxial) animals (fig. 88) are such as have no definite arrange- 
ment of parts with regard to axes or planes, the body growing irregularly in any 
direction as in many sponges and protozoa. 

Spherical (homaxial) animals have the parts arranged around a central 
point, through which innumerable axes and planes may be passed, each plane 
dividing the whole symmetrically as in some spherical protozoa, chiefly radiolaria 
(fig. 89). 

In radial (monaxial) symmetry there is a main or longitudinal axis which lies 
in the direction of growth. It may be longer, shorter or of the same length of 
the other axes, but it may be distinguished by the fact that it passes through 
parts, as the mouth, which are not found in other axes. Around this main axis 
the parts of the body are symmetrically arranged, like the spokes of a wheel, 
so that any plane passing through the main axis will divide the body into 
symmetrical parts. Most ccelenterates and echinoderms are more or les: 
completely radially symmetrical (fig. 90). 


124 


GENERAL PRINCIPLES OF ZOOLOGY 


In the case of biradial symmetry (fig. 91) there is the main axis, as in the last, 
and two other unequal axes at right angles to this, the inequality consisting in 
that organs occur in the line of the one that are not found in the other. One 
of these is called the sagittal axis, the other the transverse. Planes passing 


FIG. 89. Haliomma erinaceus, a radiolarian. a, external, i, internal, latticed spherical 
skeleton; ck, central capsule; wk, extra-capsular soft parts; n, nucleus. 


FIG. 90. Young Chrysaora (after Claus). I, perradii; II, interradii; gf, gastral 

filaments; sk, sensory pedicels. 

through the main axis and either of the others will divide the animal symmet- 
rically. Corals, sea anemones and ctenophores belong here. 

Bilateral symmetry has the same three axes, the two ends of the main or 
longitudinal axis being dissimilar, as well as those of the sagittal axis. These 
axes define three planes which have received names. That passing through the 


PROMORPHOLOGY 


125 


main and the sagittal axis is the sagittal or median plane and it divides the 
animal into symmetrical halves. A frontal or horizontal plane passes through 
the longitudinal and transverse axis, separating dorsal and ventral halves. 


FIG. 91. Section of a young sea anemone (after Boveri). ss, sagittal plane; 
tt, transverse axis; I, II, III, septa of first, second and third orders; ek, ectoderm; en, 
entoderm;/, mesenterial filament; m, muscles; r, directive septa. 

D 


rn 


.. 

' 


FIG. 92 Cross- section of a fish passing through the fore limbs. DV, sagittal axis; 
RL, transverse axis; a, dorsal aorta; c, body cavity; d, gut; ch, notochord; g, shoulder- 
girdle; h, heart; m, muscles; n, anterior end of the kidneys; p, pericardium; oh, neural 
arch; lib, haemal arch; r, spinal cord. 

A transverse plane passes through transverse and sagittal axes, separating ante- 
rior and posterior parts of the body. The great majority of animals belong here 
(fig. 92). 


126 GENERAL PRINCIPLES OF ZOOLOGY 

Antimeres and Metameres. The symmetrical parts of an animal 
are called antimcrcs; each antimere has organs which occur likewise in its 
adjacent antimere. The right arm of man is the antimere of the left, 
the right eye of the left, etc. Frequently there is also a repetition of 
organs in the direction of the long axis. Thus the body is made up not 
only of symmetrical parts, the antimeres, but also of similar parts placed 
one behind the other, the metameres. 

Metamerism or segmentation is spoken of when the body consists of 
numerous segments or metameres (consult fig. 60). Very often it is 
recognizable externally when, for instance, the limits of the segments 
are marked on the surface by constrictions (arthropods and annelids). 
But this external metamerism may be entirely lacking, and the metamerism 
find expression only internally in the serial succession of organs. Man, 
for example, is segmented only internally; in his skeleton there are numer- 
ous similar parts, the vertebras, which follow one another in the long axis. 
In fishes the musculature also is made up of numerous muscle segments, 
as any one can readily see by examining a cooked fish. In the case of 
the externally segmented earthworm also, the ganglia of the nervous 
system, the vascular arches, the nephridia or segmental organs, the setae, 
and the septa of the body cavity are repeated metamerically. 

Homonomous and Heteronomous Metamerism. The examples 
mentioned are well adapted for illustrating homonomous and heteronomous 
metamerism. The earthworm is homonomously metameric, because the 
single segments are much alike in structure, and only slight differences 
exist between them. Man and all vertebrates, on the contrary, are hetero- 
nomously metameric, because the successive segments, in spite of many 
points of agreement, have become very unlike. The segments of the 
head have an importance, for the organism as a whole, quite different 
from those of the neck, the thorax, or the tail. A division of labor has 
taken place among the segments of an heteronomous animal. 

Heteronomy and Homonomy. The distinction between heteronomy 
and homonomy is of great physiological interest. 1 he more different the 
segments of an animal become the more dependent they are upon each other; 
so much has the whole become unified that the single parts can live only while 
the continuity is maintained. On the contrary, if the connection between the 
parts be less intimate, they are more similar, and the more able to exist after 
separation from one another. This is well shown in instances of mutilation. 
When many species of Lumbricicke are cut in two each part not only lives, but 
it even regenerates the part which is lacking; if, on the other hand, the same 
thing is done to a heteronomously segmented animal, either death immediately 
ensues, as in the case of the higher vertebrates, or the parts live for a short time a 
hopeless existence, as can be seen in the case of frogs, snakes, insects, etc. There 
is always a certain capacity for regeneration, which is the more restricted, 
the more complete the organization. While Crustacea, amphibia and reptiles 


GENERAL EMBRYOLOGY 127 

can, for instance, regenerate lost appendages, the mammals have the regenera- 
tive powers reduced to the healing of wounds. In metamerism a phenomenon 
is repeated which obtains widely in the animal kingdom, and contributes 
towards its higher development; first there is a reduplication of part (here the 
segments), then a division of labor, so that the final result is a whole comj>osed 
of many parts, but a singly organized whole. 

II. GENERAL EMBRYOLOGY. 

Origin of Organisms. Since the development of every individual 
begins with an act of generation, the ways by which new organisms may 
arise comes first. Admitting only that which has been actually observed, 
we must cling to the aphorism of Harvey, "Omne vivum ex ovo," 
altering it to Omne vivum e vivo: every living organism is derived from 
another living organism. We must limit ourselves to the mode of origin 
which has been termed tocogony, or generation by parents. The great 
importance which the question of generation without parents, or spon- 
taneous generation, has obtained through the evolution theory renders a 
consideration of this question necessary at this point. 

I. GENERATIO SPONTANEA (ARCHEGONY, ABIOGENESIS). 

The old zoologists, including Aristotle, believed that many animals, 
even frogs and insects arose spontaneously from the mud. This was not 
disproved until the seventeenth and eighteenth centuries, and even then 
the idea of spontaneous generation still held, especially for parasites, for 
in the history of each animal there was a time when it contained none of 
these, and they were supposed to arise from the superfluous plastic mate- 
rial of the host. Later it was found how the eggs obtained entrance, 
and then the idea of abiogenesis persisted only for microscopic organisms. 
Water, which contained no living thing, after standing a while, was found 
to contain organisms. Lastly it was discovered that these 'do not arise 
dc noi'o, but come from minute germs, carried by the winds, or distributed 
in other ways. If the fluids and the utensils are heated, and germs are 
prevented from entrance by proper means, no life will appear, even if the 
medium be kept for years. So it may be said, as the result of all recent 
experiment, that the present occurrence of spontaneous generation is not 
proved. 

First Origin of Life. If we adopt the view that our earth was at one time 
in a molten condition and has gradually cooled, we must assume that life has 
not existed on the earth from eternity, but at some time has had its beginning. 
If we wish to base our explanation, not upon a supernatural act of creation, 
nor upon hypotheses like that of the transference of living germs from other 
worlds by meteors, there is left only the hypothesis that compounds of carbon, 


128 GENERAL PRINCIPLES OE ZOOLOGY 

oxygen, hydrogen, nitrogen, and sulphur have been brought together to produce 
living substance. This process is called sptmtaneans generation. If the carbon, 
oxygen, nitrogen, etc., which are now combined in a stable manner in organisms 
were formerly unstable, the conditions for the origin of compounds, through 
whose wider combination life would be possible, may have been more favorable. 
Thus the hypothesis of the first origin of life through spontaneous generation is 
carried to a logical postulate. 

II. GENERATION BY PARENTS, OR TOCOGONY. 

We deal here only with those methods of reproduction which 
have actually been observed, i.e., generation by parents. These methods 
fall mainly into two great groups, asexual and sexual generation, monogony 
and amplngony, to which may be added a third group, a combination of 
the two. 

a. Asexual Reproduction. Monogony. 

Monogony Defined. The chief characteristic of asexual reproduction 
is the fact that only a single organism is necessary. But since, in certain 
modes of sexual reproduction (hermaphroditism, parthenogenesis), 
this also holds true, further explanation is necessary. Asexual reproduc- 
tion must be a result of the growth of the organism, which has the peculiar- 
ity that it is not growth for an existing individual, but leads to the forma- 
tion of new individuals. It is noteworthy in this connection that many 
animals can reproduce asexually before they have reached the normal 
size (budding in embryo and larval polyzoa and tunicates). This growth 
may be general and result in an equal growth of all parts; or it may be 
local and consequently lead to the formation of an outgrowth in the 
region of greatest increase. In the first case division takes place, in the 
latter budding. 

Division. In the case of division (cf. figs. 120, 123, 150) an animal 
separates into two or more equivalent parts, so that it is not possible to 
distinguish the mother and the daughter animal, for the original animal 
has completely disappeared in the young generation. The division 
is commonly a transverse one, the plane of division being perpendicular 
to the long axis of the animal; less common is longitudinal division, 
rarest is oblique. 

Budding. In budding (fig. 93), the products are unequal. One 
animal maintains the identity of the mother, while the bud, the out- 
growth caused by local increase, appears as a new formation, as the 
daughter individual. Yet the difference between division and budding is 
bridged by intermediate conditions. 


GENERAL EMBRYOLOGY 


129 


b. Sexual Reproduction: Amphigony. 

Amphigony. For sexual reproduction two animals are commonly 
necessary, a female and a male; the reproductive cells the eggs of one 
must be fertilized by the reproductive cells the spermatozoa of the 
other, and thus acquire the capacity of giving rise to a new organism. 
Now, since there are hermaphroditic animals and since with many of them 
the possibility of self-fertilization has been demonstrated, it becomes clear 


FIG. 93. .4 , Hydra grisea with a bud ; B, first stage of bud. en, entoderm ; ec, ectoderm ; 
s, supporting lamella; /, tentacle of mother and bud; m, stomach; o, mouth. 

that the emphasis in the definition of sexual reproduction must be laid, not 
upon the individual, but upon the sexual products. Consequently the 
essential point of sexual reproduction is to be sought in the union of male and 
female sexual cells. 

Parthenogenesis and Paedogenesis. This explanation is applicable 
to by far the greater majority of cases, namely, to all cases where the 
term sexual reproduction can be applied. Still, it has been demonstrated 
in many instances that two modes of reproduction formerly considered as 
monogony parthenogenesis and paedogenesis must be regarded as 
modifications of sexual reproduction, although the conditions mentioned 
above are not strictly satisfied. In both cases the eggs develop be- 
cause of some peculiar internal stimulus, without the occurrence of 
fertilization by spermatozoa. In case of padogenesis there is the addi- 

9 


130 GENERAL PRINCIPLES OF ZOOLOGY 

tional circumstance that reproduction is accomplished by animals which 
have not completed their normal development; for example, the larvae of 
certain flies reproduce before they have passed through the pupal stage. 
Paedogenesis consequently is parthenogenesis in an immature organism. 

Parthenogenesis and Typical Amphigony. There is no absolute 
distinction between parthenogenetic eggs and those needing fertilization. 
On the other hand, their equivalency is fully shown in cases as the bees, 
where the queen decides at the moment of oviposition whether the egg 
shall receive a spermatozoan or not, this decision determining further 
whether the egg shall develop into a female (fertilized) or a male (unfer- 
tilized). Parthenogenesis is, therefore, not an asexual reproduction 
which was antecedent to sexual reproduction, but rather one which must 
have been derived from the sexual; it is a sexual reproduction in ivJiicJi a 
degeneration of fertilization has taken place. It is, therefore, more in 
accord with the natural relations to contrast reproduction by sex-cells 
with vegetative or growth reproduction (division, budding) rather than 
asexual with sexual reproduction. 

Sexual and Somatic Cells. The distinction of sexual cells from the 
asexual reproductive bodies, the parts arising by division and budding, is 
shown by their relations to the vital processes of animals. The cells of a 
bud had a share in the vital processes of the animal before the beginning of 
reproduction; they were functional or somatic cells. In the fresh-water polyp 
(fig. 93), when a bud arises, the cellular material employed is that which was 
previously related to the mother animal in exactly the same manner as the other 
parts of the body wall. The sexual cells of an animal, on the contrary, are 
excluded from the vital processes, remaining in a resting condition, and con- 
serving their vital energies. Asexual reproduction is closely related to growth; 
sexual reproduction is not even a special form of it, but a complete renewal of the 
organism, a rejuvenescence of it. This explains the fact that asexual repro- 
duction is most common in the lower animals (coelenterates, worms), but is 
lacking from vertebrates, molluscs, and arthropods. The higher the organiza- 
tion of the animal the more the energies of its cells must be employed to meet the 
increasing demands upon their functional capacity, and so the more necessary 
is sexual reproduction. It is farther noteworthy that fission and budding occur 
most frequently in attached, sessile, or slightly moving animals (ccelenterates, 
polyzoa, ascidians, oligochaates), an indication that the distribution of asexual 
reproduction may be determined by the method of life. 

c. Combined Modes of Reproduction. 

Very often two modes of reproduction occur in the same species side by 
side. Many corals and worms have the power of multiplying by division 
or budding, and also of forming sex cells; other animals have no asexual 
reproduction, but their eggs develop according to circumstances, either 
parthenogenetically or after fertilization. The appearance of two kinds 
of reproduction is very often governed by the fact that individuals with 


GENERAL EMBRYOLOGY 


131 


different modes of reproduction alternate with each other. This is called 
alternation of generations in the wider sense, and of this two special forms 
are distinguished: metagenesis (progressive alternation of generations), 
and heterogony (regressive alternation of generations). 

Metagenesis. Alternation of generations in the narrower sense, or 
metagenesis, is the alternation of at least two generations, one reproducing 
only asexually, by division or budding, the other exclusively, or at least 
to a great extent sexually. The first generation is called the nurse, the 
second the sexual animal. The reproduction of hydromedusae furnishes 
the best examples (fig. 94). The nurses here are the polyps, which 


Fir,. 94. Bougainvillea ramosa (from Lang). /;, hydranths (nurse) which have 
given rise to medusa-buds (mk) ; m, separated medusa, Margelis ramosa (sexual 
animal). 

usually united into a colony, never produce sexual organs, but bud sexual 
animals, the medusce. The medusae are unlike the polyps, being much 
more highly organized, and freely motile; only very rarely do they repro- 
duce asexually; on the other hand, they develop eggs and spermatozoa, 
from which the non-motile nurses, the polyps, develop. This example 
shows that, in alternation of generations, there is not only a difference in 
the mode of reproduction, but usually in addition, a difference in form 


132 GENERAL PRINCIPLES OF ZOOLOGY 

and organization. Between polyp and medusa the difference is so great 
that for a long time these two, though stages of the same species, were 
referred to different classes of the animal kingdom. In many cases the 
alternation of generations may be still further complicated by two asexual 
generations following each other, before the return to the sexual genera- 
tion takes place. 

Heterogony is distinguished from metagenesis by the fact that the 
asexual generation is replaced by parthenogenesis. Consequently there 
alternate animals of sometimes quite different structure, one arising from 
fertilized, the other from unfertilized, eggs. Certain Crustacea, the 
Daphnidoe, show heterogony in a typical manner. During a large part 
of the year only females are found ; these increase parthenogenetically by 
'summer eggs'; then males appear for a short time; they fertilize the 
' winter eggs, ' which now are formed, from which again parthenogenetic 
generations arise. Very often heterogony has been insufficiently distin- 
guished from metagenesis, parthenogenetic reproduction being regarded 
as an asexual mode, as was the case in the trematodes. The sexually 
ripe Distomum produces very peculiar sporocysts; these again give rise 
parthenogenetically to the larvae of Distomum, the cercaria?. For a long 
time the erroneous view was held that the cells from which the cercariae 
arose were not eggs, but 'internal buds'. On the other hand there have 
been included under heterogony modes of reproduction in which no 
parthenogenesis whatever occurs, but in which only different forms and 
organization alternate. A hermaphroditic worm, formerly called 
Ascaris nigrovenosa, lives in the frog's lungs; it produces the separate- 
sexed Rhabdonema nigrovenosum living in mud, from whose eggs the 
ascarid of the frog is again produced. 

GENERAL PHENOMENA OF SEXUAL REPRODUCTION.. 

In sexual reproduction a series of developmental processes is observed 
which is repeated in an essentially similar manner in all multicellular 
animals. They are: (i) the maturation of the egg; (2) the process of 
fertilization; (3) the process of cleavage; (4) the formation of the germ- 
layers. 

i. Maturation. 

The egg (oocyte) with the large vesicular nucleus cannot yet be fertilized ; 
it must undergo a series of changes the process of maturation, which 
consists in the replacement of the germinal vesicle by a much smaller 
egg-nucleus, and the formation at one pole of the egg of the 'directive 
corpuscles' or 'polar bodies ' 


GENERAL EMBRYOLOGY 


133 


r 


B 


C 


Formation of the Polar Bodies. The germinal vesicle initiates 
these changes, its walls disappearing, its contents in 
part mingling with the cytoplasm of the egg, in part 
being employed in the formation of a nuclear spindle 
(directive spindle). The latter places itself with its 
axis in a radius of the egg so that one pole is turned 
towards the centre, the other being in the superficial 
layer of the egg (fig. 95, A). Now begins a regular 
cell-division, but the products of the division are of 
very unequal size; the larger part is the egg, the 
smaller quite insignificant part is the polar body (fig. 
95, C). The latter projects above the surface carry- 
ing with it one half of the spindle, and when the globule 
is cut off half of the spindle is included in it. 

The Second Polar Body. The part of the directive 
spindle remaining in the egg immediately forms a new 
spindle; the cell-budding is repeated (C, D) and leads 
to the formation of the second polar body. As a result 
two small cells (fig. 95, ) lie at one pole of the egg, 
in many cases even three, since during the formation 
of the second polar body the first may divide. The 
part of the directive spindle remaining after the second 
division becomes a vesicular resting nucleus, the egg- 
nucleus or female pronuclcns, the characteristic feature 
of the ripe egg capable of fertilization. In other words, 
by a double division there have been formed from the 
immature egg four (sometimes three) cells, of which 
one has retained by far the greatest part of the original 
mass of the cell and constitutes the ripe egg, while the 
others are small bodies like rudimentary eggs. The 
name directive corpuscles was given to them because 
in very many cases their position renders possible a 
definite orientation of the egg; i.e., a diameter, the 
main axis, can be passed through the egg, one end of 
which is marked by the directive corpuscles. With 
reference to later processes of development this end is 
called the animal pole of the egg, the opposite end the 
vegetative pole. 


D 


FIG. 95- 
Formation of polar 
globules in AM iirix 
megalocephala (dia- 
grammatic, after 
Boveri). .-1. first 
directive spindle; 
B, cutting off of 
first polar body; C 
and /', two stairs 
of the second spin- 
dle; E, separation 
of second polar 
body. 

Relation between Maturation and Fertilization. 

In many cases the maturation takes place before the entrance of the sperm, 
either in the ovary or at the beginning of the oviduct; in many animals, on 


134 GENERAL PRINCIPLES OF ZOOLOGY 

the contrary, there ensues a pause after the first polar body has been formed, or 
the egg may remain in the oocyte stage; the egg then requires the entrance of a 
spermatozoon in order to complete the further changes, i.e., the formation of the 
second polar body and reconstruction of the egg-nucleus. This dependence of 
the last phenomena of maturation upon the beginning of fertilization led for a 
long time to the error that the formation of the polar bodies was a part of the 
fertilization process itself. 

Spermatogenesis. The maturation of the egg has its counterpart in the 
formation (maturation) of the spermatozoa spermatogenesis. As the oocyte, 
by division, gives rise to four cells (the polar globules and the ripe egg), so the 
spermatocyte, a cell comparable to the oocyte, divides into four spennatids. 
Yet the two differ in that usually all four spermatids become spermatozoa. 
That three of the four sex-cells of the female remain rudimentary (polar globules) 
the fourth alone forming an egg, is explained by the need of the egg to con- 
tain all possible material for use in development. 

Reduction Division. In the maturation division of both male and female 
sex-cells agree in the chromosome reduction. This is due to the fact that the 
maturation spindles have but half the number of chromosomes characteristic 
of the species. Usually these chromosomes are distributed in four groups 
(tetrads] in the preparatory stages of the egg, the tetrads later being distributed 
among the four products of the divisions (fig. 95). The significance of this 
will be shown in connection with the phenomena of fertilization (p. 138). 

2. Fertilization. 

Copulation and Fecundation. The term fertilization refers to the 
internal processes which, after the meeting of the egg and spermatozoon, 
go on in the interior of the former and end with a complete fusion of the 
two sexual cells; on the other hand, special expressions are necessary 
for those preparatory processes whose purpose is to render fertilization 
possible. Very often, but not in all cases, there is an active transfer of the 
sperm from the male to the female, a copulation. In many marine animals 
(most fishes, echinoderms, ccelenterates) the eggs and the spermatozoa 
are discharged into the water, and the union of these (impregnation or 
fecundation) depends upon chance. 

Fertilization. The process of fertilization begins with the entrance 
of the spermatozoon into the egg. Usually the egg is surrounded by a 
gelatinous envelope, the chorion, to which the spermatozoa adhere, and 
through which they bore until they reach the surface of the egg (fig. 96). 
But since the chorion, particularly in eggs laid in the air, may be hard and 
resisting, there exists very often a special arrangement, the micro pylar 
apparatus, for the entrance of the spermatozoon; this may be a single 
canal extending through the chorion, as in the eggs of fishes, or a group 
of such canals, as in most insects. 

Monospermy and Polyspermy. Many spermatozoa may reach the 
egg but normally only one serves for fertilization. The spermatozoon 
which is in the slightest degree ahead of the others is met by a process of 


GENERAL EMBRYOLOGY 


135 


the protoplasm (fig. 96, A) by means of which it enters the egg. The 
egg is now impervious to all others. Only in pathological eggs can two or 
more spermatozoa enter and then multiple impregnation (di- or polyspermy) 
occurs. There are means of protection against this abnormal fertilization ; 
one, though not the only one, is the formation of the yolk-membrane, an 
impermeable envelope which is suddenly secreted from the surface of the 
egg, as soon as the spermatozoon has entered. Within the yolk-membrane 


FIG. 96. Egg of Asterias glacialis during fecundation (after Fol). A, entrance 
of the spermatozoon; B, the spermatozoon has entered; the yolk-membrane has 
formed. 

the body of the egg contracts by discharging some of the more fluid con- 
stituents, so that between the yolk-membrane and the surface of the egg 
a cavity is formed, easily recognized in smaller fertilized eggs (fig. 96, B). 

In the large yolk-laden eggs of many insects and vertebrates several sper- 
matozoa may normally enter; but only one fuses with the egg-nucleus, the others 
degenerating sooner or later. 

Essential Feature of Fertilization. After the spermatozoon has 
penetrated into the egg, the head and the middle piece containing the 
centrosome can still be recognized, as the chromatic and achromatic 
parts of the spermatozoon or sperm-nucleus (male proniicleus), while the 
tail and the slight amount of protoplasm disappear in the yolk. The centro- 
some of the sperm-nucleus gives rise to rays in the cytoplasm of the egg, 
like those observed during division. Preceded by these rays the sperm- 
nucleus travels towards the egg-nucleus until it reaches (fig. 97), ami 
fuses with it to form a single cleavage nucleus. The centrosome n<>\\- 
divides into two daughter centrosomes, which migrate to opposite poles 
of the cleavage nucleus and control its division. The cleavage nucleus 
changes to a cleavage spindle, which divides and thus initiates the em- 
bryonic development, the successive divisions being known as the cleavage 
or segmentation of the egg. Since not until this point is fertilisation 
complete, we arrive at the fundamentally important proposition that the 
essential feature of fertilization consists in the union of egg and sperm nuclei. 


136 


GENERAL PRINCIPLES OF ZOOLOGY 


Part Played by the Two Nuclei. In many cases an abbreviation of 
development may take place, the stage of the cleavage nucleus being omit- 
ted, and the egg and sperm nuclei, without uniting, pass directly into the 
cleavage spindle. This in no wise alters the above-mentioned proposition, 
but yet it is important, because it shows more plainly how the two nuclei 


D 


A 


FIG. 07. Four stages in the fertilization of Strongylocentrohts Uvidus (after 


L'n c v i (./(. c f 1 1 ' ( '( n .1 f i t (j {Co ^ ill IC1 

Kostanecki). .4, entrance of spermatozoon; B, turning cf sperm nucleus; C, approach 
and D, fusion of egg and sperm nuclei. (In A and B only a part of the egg is shown.) 

participate in the formation of the cleavage spindle. It shows that of 
the chromosomes which form the equatorial plate exactly one-half are fur- 
nished by the egg-nucleus, the other by the sperm-nucleus. For, even be- 
fore the spindle has been formed and the contour of the two nuclei has 
disappeared, the chromosomes destined for the spindle are completely 
developed in exactly the same number in each of these (fig. 98). 


A 


B 


FiG. q8. Fertilization of A scar is megnloccphala (after Boveri). A, the ends 
(centrosomes) of the spindle formed; B, the spindle completed; sp, sperm-nucleus with 
its chromosomes; ei, egg-nucleus; p, polar bodies. 

Heredity. Recent observations have furnished a certain basis for 
the theory of heredity, the transmission of parental characteristics to the 
offspring. This transmission, on the whole, takes place with equal effect 
from the father's and from the mother's side; if we take the average of 


GENERAL EMBRYOLOGY 137 

numerous cases, the child's peculiarities hold the mean between those of 
father and mother; or, in other words, male and female individuals on the 
average have an equal power of transmitting characteristics. 

Physical Basis of Heredity. Since in all animals with external 
fertilization a material connection between parents and offspring can exist 
only through the sexual cells, these latter must contain the substances which 
render heredity possible; further, the two hereditary substances, in cases of 
equal capacity for transmission, must be present in the egg and in the 
spermatozoon in equal quantity. By this course of reasoning, the chro- 
matin which forms the chromosomes has come to be regarded as the bearer 
of heredity; for we know that the egg contains a great quantity of cytoplasm, 
but the spermatozoon only the slightest trace of it; that, on the other hand, 
egg-nucleus and sperm-nucleus furnish equivalent substances, and espe- 
cially the same number of chromosomes, to the cleavage spindles; hence 
only the chromatin can be regarded as the hereditary substance (idio- 
plasm'). This supports the view expressed before (p. 58) that the nucleus 
is the bearer of hereditary qualities and determines the character of 
the cell. 

Theory of Determinants.- These facts of the maturation of the egg and 
spermatozoa and of fertilization have become the starting point for further 
investigations and associated theories, which in the last few years have acquired 
great significance. Their relations have also been shown to the extremely im- 
portant but long forgotten experiments on inheritance in plants by Mendel. 

If we accept the sexual nuclei or their chomosomes as the bearers of heredity, 
it follows that certain constituents of the chromosomes must contain the anlagen 
of the characteristics, partly male, partly female, which later develop in the 
offspring. The simplest setting forth of the connection between the anlagen 
and the developmental product is Weismann's 'theory of determinants.' This 
may be taken as a basis for the following discussion, although objections are 
brought against it. It represents an organism as a complex of innumerable 
peculiarities, as a sort of mosaic; and in a corresponding way, the anlagal sub- 
stance, the chromosome mass (idioplasm) as a similar mosaic of anlagal particles, 
the determinants. There is a determinant for every paternal or maternal char- 
acteristic in the offspring, be it prominent or be it latent. The chromosomes 
must therefore consist of orderly groupings of innumerable determinants. 

Merogony, Artificial Parthenogenesis. Parthenogenesis shows that since 
an egg may develop, without the entrance of a spermatozoon, into a complete 
organism, the chromosomes of the egg are sufficient to produce all of the features 
necessary for life. This is more clearly shown by artificial parthenogenesis. 
Many eggs, which normally need fertilization for development, may be stimu- 
lated to develop by number of chemicals. Similarly the male chromosomes ;ne 
sufficient for normal development, for if an egg from which the nucleus has 
been removed be fertilized by a single sperm, it likewise forms an organism with 
all the necessary features (merogony). A fertilized egg must therefore possess 
duplicate determinants, male and female, for each elementary character, a 
double assortment of chromosomes. Whether a certain characteristic shall 
appear purely maternal, or purely paternal in form, or in varying compromises 
between the two would depend upon the energy of the determinants concerned. 


138 GENERAL PRINCIPLES OF ZOOLOGY 

If a determinant be so strong that its fellow cannot come to expression it is 
spoken of as dominant, the one that succumbs is recessive. 

Mendel's Law. Long before the phenomena of fertilization were known, 
Mendel had shown by experiments on plants, that if two individuals, easily 
distinguished by some peculiarity like color, be crossed, the cross so produced 
was, in many cases, not a blend of the two, but resembled exclusively one of the 
two parents. On crossing red and white flowered peas, no matter in which 
direction, the crosses were invariably red. The determinant for red was so 
dominant that the recessive determinant for white is powerless. But that the 
white determinant was present in the cross (according to the morphological con- 
clusions regarding fertilization this must be so) was shown when the red-flowered 
crosses were self fertilized (fig. 99). Then one fourth of the offspring were 

o 


0=O 3=0 3 O 


} white, J red, red, 

breeding true, capable of splitting. breeding true. 

FIG. 99. Diagram of Mendelian inheritance. Black and white representing red and 

white flowering kinds. 

white and bred true; that is the white color, the recessive character, persisted 
in all the succeeding generations. Of the remaining three fourths, one fourth 
also bred true, but red, a sign that the recessive character was lost. Such pure 
breeding animals are called homozygous because the sygote, the fertilized cell 
from which they arise, contains, with reference to the peculiarity under investi- 
gation, only similar determinants. The remaining two fourths are heteroz\<gotes; 
that is they possess determinants of two kinds and are thus like the first crossed 
generation, the red color alone appearing, owing to the dominance of the red 
determinant over the recessive white. If these be 'split' by breeding, the next 
generation will consist of one fourth red, one fourth white,'each breeding true, 
and two fourths red with recessive white. If the process be continued, the 
percentage of homozygotes (pure-breeding forms) rapidly increases at the ex- 
pense of the heterozygote forms. 

Chromosome Reduction. The results of Mendel's experiments may now 
be brought into harmony with the phenomena of maturation and fertilization, 
transferring for this purpose the role of the chromosomes to the hypothetical 
determinants. It has been found in numerous cases that the material of the 
egg and sperm nuclei the two assortments of chromosomes do not fuse with 
each other but remain distinct in all divisions of the somatic cells (p. 130), a 
phenomenon apparently applicable to all organisms. Hence the similar 
determinants, the 'pairlings,' or as they have been called, the allelomorphs (in 
the example quoted, the determinants for red and white) remain separate from 
each other in the cleavage cell and in all of its descendants. They persist in the 
cells of the flower, where, according to their dominance, they determine the 
color of the blossom; they are also continued in the sex-cells, the oocytes and 
spermacytes, of the hybrid generation. Now the sex-cells, in the so-called 
growth period preceding the maturation divisions possess only half the number of 
chromosomes characteristic of the tissue cells. 


GENERAL EMBRYOLOGY 


139 


Conjugation of the Chromosomes. This reduction in the number of 
chromosomes is explained by the plausible supposition (and this is supported 
by many observations) that the previously separated paternal and maternal 
chromosomes, and chromosomes with the same significance, have united (con- 
jugation of chromosomes), and accordingly the allelomorphs, the equivalent male 
and female determinants unite (in our example the determinants for red and 
white flowers). If in this a complete fusion occur then there would result an 
inseparable compound of characters, but if it be but a juxtaposition, then in 
the further divisions of the sex-cells, the so-called maturation divisions, both 
kinds of determinants, paternal and maternal, would be separated again. The 
latter would occur in cases following Mendel's law. The white and red determi- 
nants, present but only juxtaposed in the male sex cells, would be separated again 
in one of the two maturation divisions (reduction division) and would be dis- 
tributed among the resulting cells so that two cells would contain only white, 
the other two only red determinants (fig. 100). The same would occur in the 


FIG. 100. Scheme of the maturation divisions of the sex cells to explain the splitting 
of hybrid characters. It is based on the idea that the splitting, the reduction of the 
chromosomes, occurs in the first maturation division, while many regard the second 
as the reduction division. I and II, the first, III and IV, the second maturation 
division. C, centrosomes; d l , d-, centrosomes derived from different parents, which 
are distributed among separate sex-cells. 

female sex cell, where at the end of maturation resulting in four cells three 
polar globules and the ripe egg two would contain only white, the other two 
only red determinants. Thus in the maturation divisions the sex cells lose their 
hybrid character, and we speak of 'purity of gametes.' Now if such material 
be used, as must be the case in close fertilization of crosses, there are four com- 
binations possible, which according to the law of probabilities (we know no 
reason favoring any particular combination) would occur in equal numbers; 
(a) one fourth red male with red female, producing a true-breeding, homozygote 
with red flowers, since no other determinant is present; (b) one fourth white 
male with white female, producing homozygotes with white flowers; (f) one 
fourth red males with white females; and (d) one fourth white males with red 
females. The last two fourths would be red-flowered like the first, but accord- 
ing to the composition of their cells, they are heterozygotes like their parents, 
and like them, can be 'split' according to the same law. 

Mendel's law, illustrated above by a single example of hybrid peas, has 
led to rich results, partly by Mendel himself, partly by his successors (Bateson, 


140 


GENERAL PRINCIPLES OF ZOOLOGY 


Correns, Cue not, Tschermak, Castle, Davenport, DeVries, and others). Some 
of these matters must be mentioned here. 

(1) Mendel's Law Applicable to Animals. First applied to plants the 
law is applicable to animals as an interesting case Illustrates. There are two 
races of the snail, Helix hortensis, which breed true. The shell of one has five 
parallel bands, the other shell is unicolor and lacks bands. On crossing, the 
offspring is entirely without bands; bands are therefore recessive. The de- 
scendants of the crosses conform to the law and consist of one fourth banded 
and three fourths of bandless individuals, a third of the latter (a fourth of the 
whole) being homozygote and breeding true, the other two fourths being hetero- 
zygotes, capable of being split in the succeeding generations. 

(2) It does not always occur that one character is dominant in crosses; 
intermediate conditions may arise. Crossing of white and dark red individuals 
of Mirabilis jalap pa produces light red offspring, their descendants being one 
fourth dark red, three fourths light red, capable of farther splitting. 

(3) Di- and Polyhybrids. The examples given so far deal with only a 
single character in crossing, and if the varieties used in crossing differ by only 
a single character, the result is called a monohybrid. There may be di- and 
polyhybrids where two or more'pairlings' are concerned. The latter condition 
is met in peas, which differ not only by color of flower, already mentioned, but 
by size, position of flowers, color of pod, color and form of peas. If all of these 
characters were indissolubly connected with each other, were only the external 
expressions of one and the same fundamental character, there would be, in 
polyhybrids, the simple relations found in monohybrids. The different char- 
acteristics would be inherited together and their relations to dominance would 
be the same. But usually this is not the case. More commonly each character 
of a polyhybrid is inherited independently of the others. In this lies the possi 
bility of taking a single character of a variety, even those which seem to be 
closely correlated, and by cross-breeding, to separate it and to cause it to be 
inherited in the most diverse combinations. This brings about very complicated 
relations in polyhybrids, which can only be made clear by extensive contrasts. 
Hence, to illustrate the principle, a case of dihybrid is given here, a crossing of 
two varieties of Indian corn (maize), one with only wrinkled (r), blue (b) 
kernels, those of the other being white (w) and smooth (s). Smooth and blue 
are the dominating characters. Taking up each character separately, there 
would follow in the second generation: 

For color, bb. \ bw=b, \ wb=b, J ww; a total f b, \ w. 
For form, J rr, J rs, \ sr= s, \ ss; a total f s, | r. 

If now the two characters be combined, we have the following exposition, in 
which the recessive characters are designated by Italics, the homozygotes by 
capitals: 


bb= B 


bw= b 


wb=b 


aw= W 


ss= S 


BS 


bS 


bS 


irs 


sr=s 


Bs 


bs 


bs 


Ws 


rs= s 
rr=R 


Bs 
BR 


bs 
bR 


bs 
bR 


Ws 
WR 


GENERAL EMBRYOLOGY 141 

In this scheme each character appears once from the father, once from the 
mother, the result being sixteen combinations, of which nine are blue and smooth, 
three are blue and wrinkled, three white and smooth, and one white and wrinkled. 
Of these four groups only the last (-^ ) is homozygote with reference to both 
pairlings. Of the other groups there is only one homozygote in each, BS, 
BR, and WS, iV of each. Of these four homozygote, true breeding forms, 
two, BR and WS are reversions to the beginning type; the other two, BS and 
WR, are new and constant associations. They are the 'analytic species' to 
which allusion was made on an earlier page, which have no new characters 
but only a new combination of characters. Of the other, not true breeding 
forms, two IFs (g of the whole) as to color, w, and two bR, as to shape, are 
homozygote. Among the blue forms there is the greatest variety: two bS are 
constant in shape, two Bs constant in color, four bs are heterozygotes in shape 
and color and are capable of splitting. 

It is evident that with every new character entering, the number of com- 
binations possible increases enormously, as does the number of new analytic 
species. Therefore in high polyhybrids many thousands of descendants are 
necessary to have all possible combinations actually realized. It is also apparent 
that species with a low rate of reproduction many combinations may not 
appear, others only in distant descendants (atavism). 

The numerical relations 9:7 (nine blue-smooth to seven of all others) is 
important, since it is characteristic of the second generation of dihybrids. Where 
it occurs it indicates that two pairlings are concerned, even when other facts seem 
to point to the presence of but one. Thus, in the crossing of white breeds of 
fowl, only colored animals appear in the second generation, but in the grand- 
children there are nine colored, seven white. So one can say, with certainty, 
that the color depends upon the mutual action of two substances, which must 
be present simultaneously and as dominants. If these conditions be not ful- 
filled, the color is lacking and is replaced by white. 

(4) Mendel's law is chiefly concerned with the crossing of varieties; it 
appears to have no significance for species, since with them there usually appear 
varying degrees of intermediate forms which remain constant in the m-xt 
generation. But this cannot be proved in most cases, since hybrids are 
usually infertile. Still it does not appear that a fundamental distinction exists 
between species and varieties. Farther researches are necessary. 

The great significance of the results of Mendel and his successors is apparent. 
There has been established a wide reaching harmony between the theoretical 
conclusions based upon the phenomena of maturation and fertilization and 
facts obtained by crucial experiments. Further, the extraordinarily complex 
phenomena of inheritance are now capable of expression in exact mathematical 
form. The knowledge of this regularity of inheritance has an enormous value 
for practical animal (and plant) breeding, since it affords the foundation for 
intelligent procedure. 

3. Cleavage Process. 

Arrangement of the Cleavage Planes. The fertilized egg-cell 
divides in rapid succession into 2, 4, 8, 16, etc., cells, which become con- 
tinually smaller, since the mass of the egg does not increase. The evils 
are called cleavage spheres, or blast omeres, the whole process the cleavage 
process, or segmentation, because, at each division, furrows arise on the 
surface which continue to penetrate more deeply until finally the cells are 


142 GENERAL PRINCIPLES OF ZOOLOGY 

cleft from each other (fig. 101). As a rule each new plane of cleavage is 
perpendicular to the preceding. Hence the first three cleavage planes, 
which cause the division into 2, 4, and 8 parts, are similarly arranged in 
most animals. Using the globe for comparison, one speaks of a first and 
second meridional furrow (I, II), and calls the third the equatorial furrow 
(III). The intersections of the two meridional furrows form the poles of 
the egg, the animal and the vegetative, so called because the material of the 
one is used chiefly for animal organs (nervous system), that of the other 
for vegetative organs (digestive tract). 

i I ni 


FIG. 101. The equal cleavage of Amphioxus lanceolatus (after Hatschek). 
I, division into two (formation of the first meridional furrow); II, division into four 
(second meridional furrow) forming four cleavage spheres (fourth is hidden) ; III, 
division into eight (equatorial furrow; the seventh and eighth cleavage spheres hid- 
den;) TV, blastula, in optical section; a single layer of cells surrounds the cleavage 
cavity. In I, II, III, a polar body is shown. 

Influence of the Yolk upon Segmentation. Different kinds of 
cleavage are distinguished, the peculiarties of which depend upon two 
factors: (i) upon the quantity of material (food-yolk) serving for nourish- 
ment of the egg; (2) upon its arrangement. The food-yolk hinders the 
division, since it is incapable of active movement, and is only passively 
divided through the activity of the protoplasm. The more this increases 
in proportion to the protoplasm, the more slowly does the cleavage process 
proceed. Finally there comes a point where the resistance of the yolk 
becomes so great that the protoplasm is no longer able to carry out the work 
completely; then only the protoplasmic part of the egg is divided, that 
with much yolk remaining undivided. In this case we have a partial 
cleavage in comparison with the more primitive mode, the total cleavage; 
further, eggs which show a partial cleavage are called meroblastic, because 
only the segmented part of the egg is directly employed in the formation of 
the embryo (^Xao-ro's), while the undivided main mass serves merely as 
food-material. Eggs with total cleavage are called Jioloblastic. 

The distribution of the yolk is connected with the position of the 
nucleus; either this is central and the yolk is concentric around it (cen- 
trolecithal eggs), or it is pushed, together with the greater part of the pro- 
toplasm, to one pole of the egg, while at the other pole the yolk predomi- 


GENERAL EMBRYOLOGY 


1 1:; 


nates (telolecitlml eggs). Since the nuclear pole always becomes the ani- 
mal pole, there can be distinguished in the egg an animal part rich in pro- 
toplasm and a vegetative part rich in yolk. In many telolecithal eggs the 
two regions pass gradually into one another, but in others a distinct 
boundary separates an almost purely protoplasmic animal portion from a 
yolk-containing vegetative portion. This is well shown in the bird's egg 
(fig. 102). Here only the yolk is to be regarded as an egg in the embryo- 
logical sense, while the white, egg-membrane, and calcareous shell are 
depositions upon the surface of the egg. The chief mass of the yolk is 


et.Z. 


FIG. 102. Diagrammatic longitudinal section through a bird's egg (after 
Balfour). (i) The egg: b.l., blastoderm; w.y., white yolk; v.v.. yellow yolk. 
Coverings of the egg: v.t., yolk membrane (vitelline membrane); x and ic., inner and 
outer layers of white; ch.l., chalazar, i.s.m. and s.ni., inner and outer shell-membrane; 
between them at the right end is the air-chamber (a.ch.) ; s, shell. 

deutoplasm, upon which rests a thin layer of protoplasm, the germinal 
disc, always uppermost in the bird's egg, whatever the position of the egg. 
The protoplasmic layer contains the egg-nucleus, and, after fertilization, 
by progressive development is separated (blastoderm) more and more 
sharply from the underlying yolk. 

Types of Cleavage. A brief explanation will now render the following 
figures of the various modes of cleavage intelligible. 

a. Holoblastic Eggs with Total Cleavage. 

i. Equal Cleavage, The yolk, present only in small quantity, is distributed 
equally through the egg; upon cleaving, the egg divides into parts of approxi- 
mately the same size and equally rich in yolk (alecithal eggs, fig. 101). 


144 


GENERAL PRINCIPLES OF ZOOLOGY 


2. Unequal Cleavage. The yolk is abundant, but not in such a quantity 
as to prevent complete cleavage; it lies especially at the vegetative pole of the 
egg, causing the cleavage in this region to progress more slowly; here larger 
cleavage spheres are formed, because richer in yolk; hence the embryo, from the 
first, is composed of smaller animal cells poor in yolk, and larger vegetative 
cells rich in yolk (tehlecithal, holoblastic eggs, figs. 103 and 104). 


B 


FIG. 103. Unequal cleavage of the egg of Petromyzon (after Shipley, from 
Hatschek). .1, stage of eight cleavage spheres; B, blastula in meridional section. 
The dissimilarity of the cleavage spheres begins with the equatorial furrow. 


FIG. 104. Unequal cleavage of a snail's egg, Nassa mutabilis (after Bobretzky). 
I, the first meridional furrow has divided the egg into unequal parts; II, the second 
meridional furrow has formed three smaller and one larger cleavage sphere (seen 
from the side); III, the equatorial furrow has formed four smaller animal and four 
larger but unequal vegetative cells (seen from the animal pole) . 


b. Meroblastic Eggs with Partial Cleavage. 

3. Discoidal Cleavage. The yolk is so abundant in the vegetative portion of 
the egg that it prevents cleavage; cleavage, therefore, is limited to the region 
around the animal pole and here forms a disc of small cells, the anlage of the 
embryo, or blastoderm (telolecithal, meroblastic eggs) (figs. 102, 105). 

4. Superficial Cleavage. The yolk is collected in the centre of the egg and 
prevents cleavage; consequently only the outer layer of the egg divides, forming 
a superficial layer of cells, enclosing the unsegmented central mass (centrolecithal 
eggs, fig. ^106). 

Distribution of the Types of Cleavage. Of the four types of cleavage 
mentioned the superficial occurs exclusively in the arthropods. The others 
are distributed as follows: the discoidal in the majority of the vertebrates and in 
the most highly organized molluscs, the cuttlefishes and a few arthropods and 
tunicates, while the equal and the unequal cleavage can be found in all groups of 
Metazoa. 


GENERAL EMBRYOLOGY 


145 


Blastula. Sometimes during the first stages of segmentation, some- 
times later, there is usually formed a cavity, the cleavage or segmentation 
cavity, (archiccele) between the cells, in the interior of the egg; with the 
progress of development this cavity becomes larger (figs. 101, IV; 10^). 
Around it the cells lie in the form of a one- or many-laye r ed epithelium 


A 


B 

' 

- 


FIG. 105. Discoidal cleavage of the egg of a cephalopod, Loligo pealii (after 

Watase). 


B 


FIG 106. Superficial cleavage of an insect egg, Pieris cratii^i al'a-r Bnlnvt/.ky). 
.1, division of the cleavage nucleus; B, movement of the nuclei to the periphery to 
form the blastoderm; C, formation of the blastoderm. 

and form the blastoderm; hence the name for this stage, blastodcrmic 
vesicle, or blastida. The more yolk present, the smaller is the cleavage 
cavity; in eggs with superficial cleavage it is entirely absent. 

4. Formation of the Germ-layers. 

Gastrula. Besides the blastula there is a second stage of development, 

the gastrula or the two-layered embryo, common to all the Meta/oa. 

This stage is understood easiest in eggs which have an equal cleavage 

(fig. 107, B) ; here it has the form of a double-walled cup with a wider or 

10 


146 


GENERAL PRINCIPLES OF ZOOLOGY 


narrower mouth. The cavity of the cup (the primitive digestive tract or 
arclienlerori) is the beginning of the most important part of the digestive 
system ; the opening is the primitive mouth or blastopore. Of the two layers 
of cells forming the wall of the cup and uniting at the blastopore, the 
external is the ectoderm or outer germ-layer, the internal the entoderm 

or inner germ-layer. In the gastrula we meet for 
the first time the formation of germ-layers, i.e., 
the formation of definite embryonic layers marked 
off from each other, from which organs arise by 
differentiation. 

Invagination. The gastrula is formed from 
the blastula by imagination (fig. 107, A). The 
result is the same as when one side of a hollow 
rubber ball is pressed into the other; the region of 
vegetative cells gradually sinks in and becomes sur- 
rounded by the cells of the animal pole (fig. 107, B). 
Thus there arises, in addition to the cleavage 
cavity, a new cavity, the anlage of the lumen of the 
digestive tract; this increases and finally obliterates 
the cleavage cavity, so that the invaginated part 
of the blastoderm, the entoderm, becomes pressed 
against that which remains external, the ectoderm. 


FIG 107 Gastrula- 
tion of Amphioxus 
(after Hatschek). The 
animal pole above, the 
vegetative below, in 
comparison with fig. 
1 01. In .4 the cells of 
the vegetative pole are 
beginning to sink in; 
B, the invagination 
completed, the cleav- 
age cavity reduced to 
a slit between the ento- 
derm (en) and the ecto- 
derm (e&) ; o, blastopore. 


Modified Modes of Gastrulation. In the case of 
eggs with much food-yolk the relation of the structure 
and of the mode of formation of the gastrula is more 
difficult to understand. Here, however, it is sufficient 
to say that the gastrula stage has been discovered in 
almost all eggs with a great quantity of food-yolk, and 
that the yolk-material finds lodgement principally in 
the entodermal cells. In most ccelenterata the gastrula 

is formed in another and apparently more primitive way. Isolated cells from 

the wall of the blastula migrate into the segmentation cavity and form a mass. 

In this a cavity is hollowed out and then a mouth breaks through, the result 

being structurally a gastrula. 

Epiblast and Hypoblast. For outer and inner germ-layer the terms epi- 

blast and hypoblast, upper and lower germ-layer, have been much used. Other 

terms for the two germ-layers are entoblast andectoblast. 

Formation of Mesoderm. The Mesenchyme. Many lower animals 
e.g., most ccelenterates, have in general only two germ-layers. When 
these are laid down there begins immediately the differentiation of muscle 
and nerve fibres and the other histological changes of the cells, as well as 
changes of form, by which the gastrula becomes the adult. In higher 
organisms, on the other hand, before further differentiation begins, there 


GENERAL EMBRYOLOGY 


1-17 


arises a third germ-layer, which owing to its position between the first 
two, is called the mesoderm, mcsohhist, or middle germ-layer; this naturally 
can come only from the cell material of the existing germ-layers, indeed 
only the entoderm seems to participate in it. Two methods can be 
distinguished in its formation. In one the space between ectoderm and 


FIG. 108. Larvae of Strongylocentrotus llvidus (after Boveri). Left a blastula with 
mesen chyme formation; right a gastrula with differentiated mesenchyme. 

entoderm becomes widened by the secretion of gelatinous substance, and 
from the entoderm isolated cells push into this jelly; thus there arises 
a middle layer, the mesenchyme (fig. 108), somewhat similar to gelatinous 
connective tissue, from which certain organs either wholly or in part 
take their origin. The mesenchyme can be formed before the beginning of 
gastrulation or after that process has been completed (fig. 108; cf. p. 74). 


FIG. 109. Formation of the mesothelium and cu'lom of Sagidti. A. From the 
bottom of the gastrula arise two folds, which divide the archenteron into the perm;uu nt 
digestive tract and the ccelomic diverticula. B. The separation is almo-t c<. m plrird 
by the pushing up of the folds, ak, outer, ink, middle, //,', inner germ-layer; >nk\ 
somatic layer; mk~, splanchnic layer; Ih, body-cavity. 

Mesothelium. In the second case the mesoderm may preserve the 
epithelial character' of the two primary germ-layers, and is called m< 
thdiitm. It is cut off from the entoderm, the mode of development being 
shown in the worm Sagitta (fig. 109). When the gastrula of Sii : ^itta 
has been formed two folds arise from the archenteric walls opposite the 


148 GENERAL PRINCIPLES OF ZOOLOGY 

blastopore (A), thus partially separating a pair of lateral chambers from 
the rest. The process continues; the blastopore closes, while the 
entodermal folds extend to the opposite side, where they fuse with 
the walls (B). In this way a pair of ccvlomic pouches are cut off 
from the rest of the archenteron which forms the lumen of the 
digestive tract and its derivatives, while the walls of the pouches 
form the mesothelium, that of the digestive region the secondary 
entoderm. In each ccelomic pouch two walls are recognizable, an 
inner or splanchnic layer which unites with the entoderm to form the 
wall of the digestive tract, the splanchnopleure, while the somatic layer 
unites similarly with ectoderm to form an outer body wall, the soma- 
topleure. From the foregoing it is evident that the mesothelium is strictly 
not a single layer, but consists of two layers which pass into each other, and 
that its origin is closely connected with the formation of the body cavity. 

Occurrence of Mesenchyme and Mesothelium. There are purely 
mesenchymatous animals, like the flat-worms, and purely mesothelial, 
like Sagitta, many annelids, and Amphioocus; there are also animals in 
which the mesoderm consists of mesenchyme and mesothelium: either the 
mesenchyme arises first and later the mesothelium, as in the echinoderms, 
or in the reverse order, as in most vertebrates. 

Histological and Organological Differentiation. All the organs of 
an animal arise from the three germ-layers. The details differ in the 
various groups; the following is the most general: from the ectoderm arise 
the skin with its glands and appendages, the nervous system, and the 
sensory epithelium; the entoderm gives rise to the most important part of 
the digestive tract with its glands; while muscles, blood, supporting and 
connective substances, excretory organs, in whole or in part, arise in the 
mesoderm; the sexual organs are also usually mesodermal. 

Relations of the Germ-layers in Budding. The question has been raised 
as to how far the germ-layer theory is applicable to the occurrences in asexual 
reproduction. At first one would expect that each organ of the daughter would 
arise from the corresponding organ of the maternal animal, or at least from a 
mass of tissue belonging to one of the same germ-layers. In many instances 
this is the case; in the budding of hydroids the entoderm and ectoderm of the 
bud arise from the corresponding layers of the maternarbody (fig. 93). But 
exceptions are known. In polyzoans and tunicates there are undifferentiated 
cells which are employed in cases of budding. In the regeneration of lost parts 
it is not necessary that the missing structure should be re-formed by the same 
layer from which it originally arose. The lens of Triton arises ontogenetically 
from the epithelium of the skin. If extirpated, it is regenerated from the pig- 
mented epithelium of the iris. 

Review of the Different Kinds of Reproduction. The foregoing outline 
of reproduction is in accordance with the prevailing ideas. Although these are 
justified theoretically, they do not correspond to the actual relations, since the 


GENERAL EMBRYOLOGY 149 

divisions of the Protozoa cell divisions, and those of the Metazoa divisions 
of cell complexes, are brought into the same category, in spite of their different 
morphological value. Further, they accept a causal connection with fertiliza- 
tion in a form that does not agree with the actual conditions. 

To make this clear we start with the fact that every reproduction depends 
upon an increase of cells. In unicellular organisms reproduction and multi- 
plication of cells are one and the same thing. In multicellular organisms, on 
the other hand, two kinds of cell division may be distinguished, which may be 
called somatic and propagating cell divisions. Both are continuations of the 
cleavage process. The first is concerned with the growth of the individual, 
since it increases the formative material for the functioning organs; the other 
renders reproduction possible, since it furnishes the sex cells from which the 
new individual develops. 

Fertilization enters in the course of the cell divisions which occur during life, 
in the Protozoa as the fusion of two individuals, in the Metazoa as the union <if 
two sex cells which contain the anlage of the new individual. Both have in 
common the union (amphimixis) of two organizations to a combined organ- 
ization. In the Protozoa the influence which fertilization exerts upon repro- 
duction is very different in the different classes. In the Infusoria the rate of in- 
crease is not altered to any appreciable extent; in the Rhizopoda it is brought to 
a standstill, in which a period of rest (encystmenf) ensues; in Sporozoa it leads 
to a special form of increase and only in this group is there a sexual reproduction. 
The two others show that fertilization and reproduction are not necessarily 
connected, that the most important result of fertilization is not that it renders 
increase possible, but that, in some way not yet clear but still fundamentally 
important, it modifies the organism and influences the vital processes. 

With these conditions an explanation is necessary of why, in the Metazoa 
in contrast to the Protozoa, there is such an intimate relation of fertilization to 
reproduction. It was held, indeed is the prevailing idea, that fertilization was 
introduced to render reproduction possible. The explanation lies in multi- 
cellularity. If it be necessary for the maintenance of the organism that the char- 
acters of two organizations be fused in one, the fusion can only take place when i ! it- 
characters of each or their anlagen are condensed into a single cell, as is the case 
with egg reproduction. But the union of two multicellular organisms to a 
single one is usually but a juxtaposition of parts; in only a few cases, not clearly 
understood (graft hybrids of many plants), is there a mingling of characters. 
Hence the inherited reproduction of the colonial Protozoa, which always 
arise from a single cell, is retained and combined with fertilization. On the 
other hand, the division and budding of the Metazoa are new acquisitions which 
have arisen independently in the different groups in adaptation to special con- 
ditions of life. They may be traced back to the phenomena of regeneration, to 
the capacity of parts of an organism to re-form, the whole. While typical re- 
generation results from a loss of a part from external causes, division and bud- 
ding are apparently spontaneous, the result of the growth of the organism. 
That asexual (better 'propagative') reproduction occurs chiefly in lower animals 
is not a sign of its primitive character, but only the- result of the fart that only 
in the simpler organisms is there the capacity of complete regeneration, 
explains why the 'striking regularity which prevails in sexual reproduction i 
lacking in regeneration, division and budding. 

5. The Different Forms of Sexual Development. 

Embryonic and Postembryonic Development. While the occur- 
rences described (fertilization and cleavage of the egg, formation of the 


150 GENERAL PRINCIPLES OF ZOOLOGY 

germ-layers) are going on the young animals are usually enclosed within a 
firm protective covering, or even in the maternal sexual apparatus (uterus), 
and are hence called embryos. Later stages, even the formation of the most 
important organs, may occur during embryonic life, as we see in many ver- 
tebrates, worms and crabs, which, at the end of their embryonic existence, 
are complete in all their parts, and need only the maturity of the sexual 
organs, and growth of the body as a whole, in order to reach the climax of 
their development. On the other hand, there are animals, chiefly aquatic, 
which, after leaving the egg, undergo important changes, like the ccelen- 
terates, echinoderms, insects, amphibians, etc. The ccelenterates, echino- 
derms, and many worms usually escape from the egg even before the forma- 
tion of the germ-layers, and, as free-swimming ciliated planula, form the 
germ-layers and organs. Since there is here a more or less extensive post- 
embryonic development, it is a misnomer to apply the term 'embryology ' to 
both stages; it is" necessary, rather, to limit the name to the developmental 
processes inside the egg, and, on the other hand, to speak generally of the 
history of the development of the individual, or ontogeny. As the unde- 
veloped animal within its membrane is called an embryo, so the name larva 
is applicable to the free-living but not mature animal. 

Direct and Indirect Development Metamorphosis. Larval de- 
velopment may be either direct or indirect. In direct development, as 
the term implies, the larva pursues a straight course towards the sexually 
mature animal, the lacking organs being outlined one after another; 
hence it is continually becoming more like the sexually mature animal. 
In indirect development, on the contrary, organs belonging only to the 
larval life, larval organs, are formed and later are destroyed. Therefore 
in the definition of indirect development, or metamorphosis, emphasis is 
laid upon the presence of larval organs. Thus caterpillars are distin- 
guished from butterflies not only by the absence of compound eyes and 
wings, but by the presence of anal feet and spinning-glands, and further 
by the different shape of the jaws, antennae, and legs, the different arrange- 
ment of the tracheae and nervous system, etc. Tadpoles are distinguished 
from frogs not only by the absence of lungs and extremities, but also by 
the presence of gills and tail. The more numerous the larval organs, the 
more pronounced will be the metamorphosis. 

Neoteny. Occasionally the gonads of an indirectly developing animal 
become mature before the metamorphosis is complete, and as a result develop- 
ment is brought to a standstill (sexually mature, gill- breathing Triton larvae, 
larvae of Miastor, etc.). This peculiarity in which the gonads restrict the devel- 
opment, is called 'neoteny'; and the attempt is made to regard a number of 
species as neotenic, that is sexually ripe in the larval stages. 


GENERAL EMBRYOLOGY 151 

Oviparous and Viviparous Animals. The time at which the egg 
leaves the mother's body is independent of that at which the embryo escapes 
from the egg membranes. Two extremes are known, the oviparous or 
egg-laying animals, and the viviparous or those which give birth to living 
young. Only those forms can be considered as strictly oviparous in which 
the egg at the time of laying is a single cell, as in the case of most fishes, 
sea-urchins, batrachians, insects, etc. In viviparous animals, on the con- 
trary, birth and the rupture of the egg membranes occur quite, or almost, 
at the same time, and from the mother there emerges an animal which has 
completed its development or, 'at least, has progressed so far that it is able 
to live without protective coverings. 

Ovo-viviparous Animals. Varying degrees of ovo-viviparous develop- 
ment connect these two extremes. What here appears at birth at first im- 
presses us, on account of its covering, as being an egg; but the first stages of 
development have already passed, so that, by artificial rupture of the ci;g 
membranes, an embryo more or less developed, but usually not yet capable of 
independent life, is exposed. Birds really belong in the category of ov<>- 
viviparous animals, for their eggs are fertilized before they are laid, and ha\c 
already completed the formation of the blastoderm. In the case of many 
snakes the egg-shell may contain, even at the time of laying, an animal all 
ready for hatching. Transitional forms of this kind show that no sharp liiu- 
can be drawn between 'egg-laying' and 'bearing living young' and one must 
guard against attributing too much importance to the apparent distinctions. 
In many divisions of animals oviparous as well as viviparous forms are found. 
The majority of sharks are viviparous, but some lay eggs; in most bony fishes 
the eggs are laid before fertilization. Exceptions are the viviparous surf perches 
of the Pacific and many Cryprinodonts of fresh water. Most of the Amphibia, 
reptiles, and insects are egg-layers, but not a few forms are viviparous and 
even in the same genus (Lacerta and Chamivlemi) there may be viviparous and 
ovo-viviparous species. Even among the mammals, for which for a long time 
the 'bearing young alive' was regarded as diagnostic, it has been discovered that 
the monotremes lay eggs. Finally, exceptions to the rule occur in one and the 
same species. Adders are ovo-viviparous, but under unfavorable conditions they 
retain the eggs inside their body until ready to hatch. 

SUMMARY OF THE FACTS OF ONTOGKXV. 

1. The development of an animal begins with an act of generation; 
spontaneous generation and generation by parents are to be distinguished. 

2. Spontaneous generation (abiogenesis) is the origin of living beings 
from lifeless matter (without pre-existing organisms). 

3. The present existence of spontaneous generation is neither known 
nor is it, on the whole, probable; yet it is a logical postulate, in order to 
explain the origin of life on our globe. 

4. Generation by parents, derivation of an animal from another of 
similar structure, can take place either by the sexual or the asexual mode. 

5. Asexual generation may be either by division or by budding. 


152 GENERAL PRINCIPLES OF ZOOLOGY 

6. In division an organism grows regularly in all its parts, and by 
constriction falls into two or more equivalent new pieces. 

7. According to the direction of the plane of division in reference to 
the long axis of the animal we speak of longitudinal, transverse, and 
oblique division. 

8. In case of budding a local growth occurs ; the local outgrowth, the bud, 
separates from the mother as a smaller, usually incompletely formed 
animal. 

9. According to the position and number of the buds we distinguish 
lateral, terminal, and multiple budding. 

10. Sexual reproduction occurs by means of special sexual cells, which 
have no part in the ordinary functions of the body. 

11. In sexual reproduction two kinds of cells unite, the female egg and 
the male spermatozoon (fertilization). 

12. In rare 'cases the egg develops without fertilization: partheno- 
genesis; this is a sexual reproduction with degenerated fertilization. 

13. P ado genes is is parthenogenetic reproduction by a young (i.e., 
incompletely developed) animal. 

14. Different modes of reproduction (asexual, sexual, parthenogenic, 
predogenic) may occur in the same species; then these often occur in a 
regular order, and in such a way that individuals with different modes 
of reproduction alternate with one another: alternation of generations in 
the wider sense. 

15. Alternation of generations in the strict sense (metagenesis} is the 
alternation of two generations, one reproducing by division or budding, 
the other sexually. The former is called the nurse, the latter the sexual 
animal. 

1 6. The alternation of parthenogenesis or pasdogenesis with pro- 
nounced sexual reproduction is called heterogony. 

17. Development which is inaugurated by sexual reproduction shows 
in nearly all multicellular animals a general agreement in the incipient 
stages: fertilization, cleavage, formation of germ-layers. 

18. The essential point of fertilization lies in the complete fusion of 
egg and spermatozoon, particularly in the fusion of the nuclei, egg and 
sperm nuclei, to form the cleavage nucleus. 

19. The cleavage of the egg is a cell division, a division of the fertilized 
egg into the cleavage spheres (blastomeres). The cleavage may be 
total (holoblastic egg) or partial (meroblastic egg) ; total cleavage is either 
equal or unequal, the partial either discoidal or superficial. 

20. By repeated divisions of the cleavage spheres, and by the forma- 
tion of a cleavage cavity, there arises a one-layered embryo, the blastula. 


(ECOLOGY 153 

21. By the invagination of the blastula the gastrula or two-layered 
embryo arises. 

22. The gastrula contains the primitive digestive tract or archenteron, 
opening to the exterior through the blastopore; it consists of two epithelial 
layers, the entodenn or the inner germ-layer, lining the archenteron, and 
the ectoderm or outer germ-layer. 

23. Between the inner and the outer germ-layer still a third, the 
middle germ-layer, mesoderm, may be formed. 

24. The middle germ-layer arises either by an infolding or a cutting 
off of a part of the entodermal epithelium: epithelial mesoderm, mesotlie- 
lium; or by the migration of separate cells to form a gelatinous tissue 
(mesenchyme). 

25. Many animals deposit their eggs before or shortly after fertilization 
(oviparous}; others lay eggs which have already been fertilized in the 
maternal body, and at the time of laying have passed through some of the 
stages of development (ovo-viviparous) . A third series of animals give 
birth to living young (viviparous}. 

26. The development of an animal is either direct or indirect (meta- 
morphosis). 

27. Indirect development or metamorphosis is where the young animal, 
as it comes from the egg, differs from the sexually mature animal in two 
points: (a) by the lack of certain organs which occur in the sexually 
mature animals; (b) by the appearance of organs, larva.! organs, which are 
lacking in the sexually mature animals. 

III. RELATION OF ANIMALS TO ONE ANOTHER. 

Just as between the organs of an animal there exists a regular relation 
which is termed correlation of parts, so also the different individual 
animals stand in intimate reciprocal relations to one another. The con- 
ditions of existence of many animal species are altered, if other forms 
appear or disappear, or undergo an extraordinary reduction or increase 
in number of individuals. Such reciprocal effects are usually of a special 
nature and can be understood only by individual study; a few are of wide 
occurrence and are hence suitable for a general consideration; to such 
belong colony and society formation, parasitism, and symbiosis. 

I. RELATIONS BETWEEN INDIVIDUALS OF THE SAME SPECIES 

Colony Formation. Colony and society formation are relation-; 
which exist between individuals of the same species An animal colon 
is a union of individual animals by an organic bodily connection; the 


154 


GENERAL PRINCIPLES OF ZOOLOGY 


latter may arise in. two ways: first, by animals, originally separate, par- 
tially fusing together; secondly, by individuals, formed by division and 
budding, remaining united with one another instead of separating. The 
first is extremely rare. 

Colony Formation by Fusion. Many Protozoa fuse with one 
another and form larger bodies in which the individual animals can still 
be recognized. Among the metazoa, Diplozoon paradoxum (fig. no) is 
the only case known where two animals (Diporpa?), arising from different" 
eggs, normally unite into a double animal, which recalls certain double 
monsters, as the Siamese twins. 


FIG. no. Development of Diplozoon paradoxum (from Boas), (i) Larva, from 
which comes (2) 'Diporpa.' (3) Two Diporpa; uniting. (4) The Diporpae have 
united into Diplozoon. m, mouth; d, digestive tract; h, posterior adhering apparatus; 
b, ventral sucking-disc, which serves for attachment to the dorsal cone, r. 

Colony Formation by Incomplete Division and Budding. In 

general it can be said that colony formation rests upon incomplete asexual 
reproduction, since the new generation does not separate from the parent. 
The colonies of marine hydroids and corals (figs. 94, 205) may consist 
of thousands of individuals which, by repeated incomplete budding or 
division, have sprung from a single sexually produced mother animal. 

Community of Functions. In the majority of cases the connection 
results in a considerable degree of community of functions. Stimuli 
which affect one individual are transmitted to the others of the colony; 
thus movements in common are rendered possible. In a similar way the 
food captured and digested by one animal serves for all. On account of 
the community of its functions, a colony appears like a unified whole, 
like an individual of a higher order; the same process which led to the 
formation of multicellular organisms is repeated. Just as there the 
elementary organisms, the cells (individuals of the first order) are united 
into a single animal (individual of the second order), so here the multi- 
cellular animals are united into a colony (individual of the third order). 

Polymorphism. When a whole is made up of numerous equivalent 


(ECOLOGY 


155 


parts, the conditions for division of labor are present. Instead of the 
functions of the entire organism being distributed equally to the in- 
dividual parts, many of the latter become employed more for this, others 
again more for that function, and acquire a corresponding structure. 
In such colonies one speaks of polymorphism. Polymorphism appears 
oftenest in connection with the vegetative functions, leading to a dis- 
tinction between sexual animals and nutritive animals, as in the case 
of most Hydrozoa, where often nutrition is accomplished by animals 

B A 


FIG. in. Praya diphyes (after Gegenbaur). .-1, the entire animal; B, a single 
group of individuals greatly magnified (Eitiloxid). i, covering scale; 2, nutritive 
polyp; 3, nettle-threads; 4, sexual bell. 

without sexual organs, and reproduction is carried on by animals without 
a mouth. But other functions may also become specialized. Siphon. >- 
phores are the classical examples of polymorphism (fig. in). Here 
united into a single body are locomotor animals, the swimming-bells, 
for locomotion only; covering scales, which serve only to protect the otli.-r- 
nutritive polyps, which alone take in and digest food; sexual animals and 
tactile polyps, concerned only with reproduction and with srn-ntum. 
In regard to the other functions each animal is related to its br.tlu-rs and 


156 GENERAL PRINCIPLES OF ZOOLOGY 

sisters; its very existence therefore has become dependent upon these; 
the single individual can live only while a part of a whole. Thus divi- 
sion of labor leads to greater centralization ; the more polymorphic a colony 
becomes, the more unified it is, the more it gives the impression of.being a 
single animal instead of an aggregation of single animals. 

In Social Animals the reciprocal dependence of the individuals 
is much less, since there is no organic connection, only a voluntary com- 
munal life. As asexual reproduction is of importance in colonies, so here 
the sexual plays a prominent role. Under the influence of the sexual 
impulse, many animals, even some of the lowest organisms, flock together, 
either permanently or periodically; sea-urchins, sea-cucumbers, many 
fishes, collect near the coast at the time of egg-laying; it draws together 
herds of deer, elephants, etc. The care of the young further leads to a 
closer organization, to a society. All insect societies are built up on this 
basis. Consequently, since the sexual life is the starting-point of social 
life, in the different groups of individuals forming the community, the 
sexual organs may be influenced in their development. Besides males 
and females (kings and queens) there are other animals with degenerated 
sexual organs incapable of function, the workers; the latter are either 
only rudimentary females (bees and ants) or rudimentary females and males 
(termites). While the kings and queens give rise to the next generation, 
the workers care for the young, look after the hive, provide food, pro- 
tection, and defence, if the latter be not delegated to a special class, the 
soldiers (termites). 

II. RELATIONS BETWEEN INDIVIDUALS OF DIFFERENT SPECIES. 

Where individuals of different species stand in close reciprocal rela- 
tions to each other the cause is in the advantages which the one derives 
from the other, or which both furnish reciprocally; the former condition 
is called parasitism, the latter symbiosis. 

Parasitism. Parasites are organisms which dwell upon or in 
another organism, the host, and obtain nourishment from it. They 
have consequently come into a dependent condition and have undergone 
a more or less extensive change in their organization. 

Degeneration Caused by Parasitism. The degree to which a 
parasite has become dependent upon its host is determined by the extent 
to which the parasite has adapted itself to the organization of its host. 
Therefore it is necessary, in speaking of parasitism, to consider the modi- 
fications which the parasitic life has caused in the structure. These 
concern most immediately the organs of locomotion and nutrition. Since 


(ECOLOGY 


157 


oe 


a parasite needs to fix itself as firmly as possible to the host, the locomotor 
apparatus more or less completely disappears and an apparatus for 
fixation becomes necessary; parasites of different groups are provided 
with hooks, sucking-discs, etc. The fluids of the host furnish nourish- 
ment to the parasite: these are substances in solution which scarcely need 
digestion. Usually, therefore, the digestive canal is simplified or dis- 
appears; among the parasites there are gutless worms as well as gutless 
Crustacea. Very frequently the intestinal parasites live without oxygen; 
they are anaerobic (p. 92). The mode of 
life of a parasite is also simplified, since it 
is no longer compelled to seek its food; the 
nervous system and sense-organs undergo 
great degeneration; the former becomes 
limited usually to the most indispensable 
portion; the latter, except those of touch, 
may entirely disappear. 

Modification of the Sexual Apparatus 
by Parasitism. The sexual apparatus, on 
the contrary, undergoes a strong develop- 
ment. While it becomes easier for the 
parasite to maintain itself, the existence of 
the species is more precarious. If a man 
die, then most of his parasites die with him, 
especially those in the interior of his body. 
In order that a parasitic species may not 
become extinct, it is necessary that the eggs 
be introduced into a new host. Since this 
is attended with difficulties, the parasites 
must produce an enormous number of eggs. 
The eggs, too, are distinguished by great 
resisting power and well-developed protec- 
tive organs, such as strong shells, etc.; the 
eggs of Ascarids continue to develop for 
some time in alcohol, being protected by 
their impermeable shell. 

Ectoparasites and Entoparasites. 
All the above-mentioned phenomena are 
more conspicuous in the case of parasites which live inside of 


112. 


Fir.. IT.}. 


FIG. 112. Ta-nia muni (after 
Leuckart). 

FlG. 113. Pentastonnim 
tirnioides female (after Li-u. - 
kart). //., hooks right ami left 
of mouth; or, unpaired ovary, 
I.:, i .rliing into two oviduct-, 
which unite into the unpaired 
vagina (;,;); the latter receives 
the outlets of two rcceptacula 
semiiiis (rs), and winds around 
the digestive tract (rf); n; ceso- 
phagus. 


other animals, entoparasites, than in the case of the dwellers upon the 
skin or other superficial organs, the ectoparasites. In case of ento- 
parasites the transforming influence of parasitism is so considerable 


158 


GENERAL PRINCIPLES OF ZOOLOGY 


that representatives of the most diverse groups take on a remarkable 
similarity of appearance and structure. Pcntastomum (fig. -113), 
for example, belongs in the same class with the spiders, the Arachnida, 
but in external appearance it is entirely unlike them, resembling 
the tape-worms (fig. 112). Hence for a long time all entoparasites, on 
account of their similarity, were united into a single systematic group 
under the name of 'Helminthes,' comprising members of the Crustacea, 
worms, and spiders. Only by embryology was the unnaturalness of 
this grouping recognized. Entoparasitism therefore is one of the best 
examples for illustrating convergent development, i.e., animals of different 
systematic position acquiring, under similar conditions of life, a great 
similarity of structure and appearance. 

Symbiosis. Less frequent than parasitism is symbiosis, or the association 
of animals for reciprocal advantages. Social animals frequently not only hold 
certain animals in bondage, but even seek to protect and serve them; as, for 
example, certain blind beetles, like Claviger or some species of plant-lice, 
(myrmecophiles) or even ants of other species and genera. But such cases of 
association correspond in part to the domestication of animals, or to slavery, 
as carried on by man. The ants keep the plant-lice in order to lick the sweet 
juice ('honey dew') which is secreted in their honey-tubes; they steal the pupas 
of other ants and rear them, to use them later as slaves. This state of things 
rests, consequently, not upon equal rights, since the one animal, in the present 
example the ant, brings about the association, while the other is passively led 
into it. Symphyly is close to true symbiosis. Besides Claviger, mentioned above, 

many other insects, mostly beetles, which are 
cared for by the ants, are found in colonies of 
ants and termites, since they have a sweet 
secretion on special bundles of hairs, which 
the ants lick off. Frequently the beetles eat 
the younger stages of the ants. 

An instance of most complete equal rights 
and true symbiosis is furnished us, however, 
by a hermit-crab and an actinian (fig. 114), 
Eupagiirus pubescens and Epizoantlius attieri- 
(-(iiiiis. Like all hermit-crabs this also inhabits 
a snail-shell from the opening of which only 
his legs and pincers are protruded. Upon 
this shell an Epizoantlius becomes attached and by budding soon covers it with 
a colony of polyps. The advantage which the actinian derives from this 
symbiosis is clear: it gains a share of the food which the crab obtains. It is less 
clear what the crab gains; however, the polyp is perhaps a protection to him by 
its nettle cells, while by growth it increases the size of the 'house' occupied by 
the hermit and thus saves him periodic changes of abode. 

Occurrence of Symbiosis. That animals rarely live symbiotically with 
one another rests largely because the conditions of life of all animals to a certain 
point are similar or identical. They take in compounds rich in carbon and 
nitrogen, decompose them into carbon dioxide, water, and oxidation products 
containing nitrogen. All animals consequently are competitors in the struggle 
for food. For the same reason, conversely, symbiosis between plants and 
animals is not so uncommon. There are certain lower algae, the Zooxanthellae, 


FIG. 114. A colony of Epizo- 
anthus americanus on the shell 
occupied by a hermit-crab (from 
Yerrill). 


(ECOLOGY 159 

which often live in animals. The radiolarians so constantly contain green- 
or yellow-colored cells that for a long time these were regarded as constituent 
parts of the animal. Similar yellow and green cells inhabit the stomach epithe- 
lium of many actinians, corals, and even of many worms. The ZooxanthelUe 
are nourished by the carbon dioxide formed by the animal tissues, and breathe 
out oxygen, which in turn is used by the animal; further, they form starch and 
other carbohydrates, and any surplus thus formed may be food for the animal. 
Thus there is on a small scale that cycle which exists on a grand scale in nature 
between the animal and vegetable kingdoms. By aid of chlorophyl and sunlight 
plants decompose water and carbon dioxide and form from them oxygen, which 
they respire, and compounds rich in carbon, which they store in their tissues: 
they are reducing organisms. On the contrary, animals give off carbon dioxide 
and water, but take their oxygen from the air, and carbon compounds in their 
food; they use oxygen to break down the chemical combinations, to oxidize: they 
are oxidizing organisms. This explains why the favorable influence of plants upon 
animals ceases immediately when they change the character of their metabolism. 
With the disappearance of their' chlorophyl fungi and bacteria have lost the 
power of reducing carbon dioxide; they derive their food from other organisms 
and decompose this into carbon dioxide, water, etc.; like animals, they are oxi- 
dizing organisms, and consequently dangerous competitors and are the cause 
of many serious ailments. 

IV. ANIMAL AXD PLAXT. 

Distinction between Animal and Plant. The consideration of symbiosis 
leads to the fact that a distinction exists between plants and animals in^the mode 
of metabolism, which may be expressed thus: plants usually take in carl urn 
dioxide and give off oxygen, while animals absorb oxygen and give out carbon 
dioxide. Hence it might be concluded that it is easy to discover universal 
distinctions between plants and animals. But the more one studies this ques- 
tion, the more difficult becomes its solution. The old 
zoologists believed that there are organisms which stand 
on the limits between the animal kingdom and the 
vegetable, and named these zoophytes or plant-ani- 
mals. Now we know that these are true animals with 
but a superficial similarity to plants; but, by means of 
the microscope, we have become acquainted with 
numerous lower organisms, and it is still doubtful in 
which of the two realms some of these, like the Myx<>- 
mycetes and many Flagellata, belong. 

' Physiological Distinctions. In the search for 
distinctions both physiological and morphological 
characters may be considered. Starting from the / 

physiological side Linnaeus ascribed to plants only the ^ ^_ _ Lff)ilSt]miti _ 
capacity of reproduction and nutrition, but to animals ''^fter Schmarda). 

the power of motion and sensation in addition. Now ^ car ina; /, tcrgum; .>-, 
we know that vegetable, like animal, protoplasm is s ' cutum . 
irritable and is capable of motion, as is shown by the 
active movements of the lower Algae, the great sensitiveness of the Mm, 
other plants; but further, we know that even many highly organized i 
e.g., Crustacea (fig. 115), lose the power of locomotion and become 
many fixed forms, e.g., the sponges (fig. 88), appear immovable and unatt 
by stimulation; thus the so-called animal functions cannot be re; 


affording accurate distinctions. 


160 


GENERAL PRINCIPLES OF ZOOLOGY 


Even the difference in metabolism is by no means sufficient. Every plant 
has a double exchange of material. In its movements and other vital functions 
the vegetable protoplasm produces carbon dioxide and consumes oxygen; at the 
same time there goes on, under the influence of sunlight and of chlorophyl, the 
reduction of carbon dioxide and the giving off of oxygen. In chlorophyl- 
containing plants the reducing process preponderates so considerably during the 
day that they give off a quantity of oxygen, and only at night, when the reducing 
process becomes interrupted on account of the lack of sunlight, does the pro- 
duction of carbonic-acid compounds become perceptible. But if the chlorophyl 
be absent the reducing processes disappear; chlorophylless fungi and bacteria, 
have, therefore, the same metabolism, so far as carbon dioxide is concerned, as 
animals. So also it is incorrect to say that only plants have the power to make 
cellulose, for cellulose is found in many lower animals, the rhizopods, in the 
highly organized group of tunicates and even among the arthropods. 

Morphological Distinctions. Turning to the morphological characters, 
multicellular animals and multicellular plants are readily distinguished, since 
the former in the germ-layers have a principle peculiar to them; with the appear- 
ance of the gastrula each organism is undoubtedly an animal. But in unicellular 
animals the arrangement of the cells is lacking, and only the constitution of the 
single cell can guide us. Now are there unmistakable morphological differences 
between the animal and the vegetable cell ? 

Plant-cells have a Cellulose Membrane. In the structure of plant and 
animal cells an important distinction is found in the fact that the former has a 

cellulose membrane, but the latter is usually membrane- 
less. To this distinction must be referred in the last 
analysis the widely different appearance of the two 
realms. Since the plant-cell is early surrounded with 
a firm coat, it loses a large part of its power of further 
changing its form; hence vegetable tissues and organs, 
in spite of the manifold' intracellular differentiations, 
like the chlorophyl granules, are uniform in comparison 
w^ith the inconceivable multiformity which animal 
structures disclose. The higher stages of organization 
which the animal kingdom reaches, even in its lower 
classes, is in great part the result of the fact that the 
cells of animals do not become encapsuled, but have 
preserved the capacity for more varied and higher de- 
velopment. But even here transitions are found be- 
tween the lower plants and animals. In the lower 
Algae the cells can leave their cellulose membrane, and 
swim about freely (fig. 116), before they enclose them- 
selves anew. On the other hand, most unicellular 
animals encyst; they pause in their ordinary functions, 
become spherical, and surround themselves with a firm 
membrane, sometimes even of cellulose. Since in both 
cases an alternation between the encapsuled and the 
free condition occurs, only the longer duration of the 
one or of the other can lead to a distinction. But here occurs the possibility 
that indifferent intermediate forms exist; their actual existence prevents, even 
yet, a sharp distinction between the animal and vegetable kingdoms. 

V. GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 

The Different Faunal Regions. Even a superficial knowledge cf the 
distribution of animals shows that the animal population, the fauna, in different 


FIG. 1 1 6 . CEdogo- 
nium in spore-formation 
(after Sachs) . A , a piece 
of the alga with escap- 
ing cell-contents: B, 
zoospore formed from 
the contents; C, zoospore 
fixed and germinating. 


DISTRIBUTION 161 

reckons of the earth has an essentially different character. In part this is the 
immediate result of climatic differences. The polar bear, arctic fox, eider- 
ducks, are restricted to the polar zones, because they cannot endure more than 
a certain degree of warmth; on the other hand, the larger species of cats, the 
apes, the humming-birds, etc., occur only in warmer regions, because they are 
not sufficiently protected against cooler weather. 

If climate were the sole factor determining distribution, the faunal character 
of two lands which have similar climatic conditions would be essentially the 
same; conversely, the separate regions of a continuous territory extending 
through several climatic zones must have different faunas, according as they are 
nearer the equator or the poles. But such is not the fact; two tropical countries 
may differ more widely in their fauna than the hot and cold regions of one and 
the same country. 

Factors in Distribution. Modern zoology endeavors to explain these 
conditions by regarding the present distribution of animals as the product of two 
factors: the gradual changes of the animal world, and the gradual changes of the 
earth's surface on which the animals are distributed (p. 33). The history of 
the earth as disclosed by geology shows two facts: (i) that the connections 
between parts of the earth have varied greatly; that, for example, at a time when 
the Mediterranean had not yet reached its present extent, Morocco, Algiers, 
Tunis, and Egypt were more closely united with the European coast of the 
Mediterranean than with the southern part of the African continent separated 
from them by the Sahara; (2) that considerable variations of climate have taken 
place: there prevailed in Europe in the tertiary period a subtropical climate 
which rendered possible the existence of animals which now occur in Algeria 
(lions). But later a glacial period began, which introduced over a large part 
of Europe arctic conditions, and consequently a fauna of northern animals 
(reindeer) which has left a few traces (Alpine hare) in the glacier regions. Hand 
in hand with the geological changes went changes in the animal world, the then 
existing species dying out under the change of conditions, or perhaps forming 
new species through gradual variations. Thus the distribution of animals 
constitutes an extremely complicated problem, for the solution of \\ InYh it must be 
known how the climates and the connections between the continents have 
changed, particularly in the later geological periods; further, not only how 
animals are distributed at present over the earth's surface but also how they were 
distributed in earlier times. Finally, we must have clear and detailed idea;- of 
the relationships and interrelationships of animals. 

It will be a long task to solve all the problems. What has been accomplished 
so far can only be regarded as showing that zoology, with its prevailing views 
of the changes of animals and of the earth, is on the right track. Two region-,, 
separated early in the earth's history and never again connected, must have 
greater differences in faunal characters than two lands still coniuvied or only 
recently separated. We travel in the northern hemisphere and find in widely 
separated regions strikingly similar fauna?, while under the equator or in the 
southern hemisphere, under the same conditions for each region, Mriking 
differences are seen. This is explained on the hypothesis that in all past periods 
as now the land masses of the northern hemisjxhere have hem closely connected, 
while the parts of the continents extending to the south have been separated 
through most of the earth's history. 

Students of distribution have' attempted to define the great faunal areas of 
the earth, the faunal provinces or regions, and within these the subregions 
These provinces have been based chiefly upon the distribution of mammals 
less upon that of birds and other animals; for the distribution of mammals 
chiefly determined by those changes of the earth's surface which are best known 
geologically and possess most interest. Elevation or depression of the earth'? 
ll 


162 GENERAL PRINCIPLES OF ZOOLOGY 

surface often opposes impassable barriers to most mammals: rising, if it lead to 
the formation of glaciered mountain-chains; sinking, when arms of the sea are 
formed, which interpose straits or broader bodies, impassable to most mammals, 
between two land areas. Birds and strong-flying insects are less affected by 
all such changes; the majority of them can fly over arms of the sea and moun- 
tain-chains; there are birds which can even cross the Atlantic. 

The Six Primary Regions. Of the systems of animal geography proposed 
that advocated by Sclater and Wallace finds most favor. They distinguish six 
primary regions: (i) the pakcarctic, comprising all Europe, northern Africa as 
far as the Sahara, and northern Asia to the Himalayas; (2) the Ethiopian, Africa 
south of the Sahara; (3) the oriental, including upper and farther India, southern 
China, and the western Malay Islands; (4) and (5) the nearctic and the neo- 
tropical regions, which make up the American continent and are divided at about 
the northern border of Mexico; (6) the Australian, with, besides Australia itself, 
the larger and smaller islands of the Pacific Ocean and the Malay Islands, east 
of Celebes and Lombok. 

(1) The Australian region is most sharply distinguished and by many is set 
apart as a distinct division called 'Notogaea.' Its isolated geographical position 
together with the fact that it has long been separated from other countries 
(apparently since the beginning of the tertiary) explains why only the oldest 
mammals, the monotremes and marsupials, have entered the region, while the 
placental mammals have not been able to follow. The marsupials, which in 
the secondary period also inhabited the northern hemisphere, and were replaced 
there in tertiary times by the placentals, were able to develop farther in the 
Australian region. Australia and the adjacent islands are thus the land of 
marsupials, which have persisted elsewhere only in South America, the opossum 
ranging north into the United States. On the other hand, at the time of dis- 
covery Australia lacked all terrestrial placental mammals except those (bats) 
which were not restricted by water and the Muridae, easily transported on floating 
wood. Two larger mammals, the wild dog (Canis dingo} and the pig of New 
Guinea (Sns papitanus), may have accompanied man, this being probable for the 
dingo, although his remains occur in the pleistocene along with those of the 
giant marsupials. Further peculiarities of the Australian region are the birds- 
of-paradise in New Guinea, the egg-laying mammals (monotremes), and the 
cassowaries and the Australian ostrich (DromcBus novcchollanditf}. 

It is easily understood that the island groups of the South Sea (Polynesia) 
have developed many faunistic peculiarities, as well as that an exchange of forms 
may have taken place between the islands of the oriental province and those 
faunally related to Australia, and that 'Wallace's Line' (p. 34) is not so sharp 
a boundary as was once thought (extension of marsupials into Celebes, of pla- 
centals into the Moluccas). On the other hand the distinctness of New Zealand 
needs mention. It is distinguished from Australia by a large number of peculiar 
birds (Apteryx and the extinct Dinornithidas), reptiles (the ancient Sphenodcni), 
and molluscs. If the bats and mice be excepted, New Zealand lacks all native 
mammals, even marsupials. 

(2) The Neotropical province (South and Central America) is, next to 
Australia, the most sharply characterized, and has also been set aside as a 
special division 'Neogaea,' partly in view of its geological history; during the 
cretaceous and early tertiary time it was separated from North America by the 
sea and had developed a peculiar fauna (e.g., gigantic edentates, no carnivores). 
These peculiarities disappeared towards the end of the tertiary by the entrance 
of carnivores and ungulates from the north and an extension of the edentates, 
marsupials, humming birds, etc., to the northern hemisphere. To Neogaea 
belong the platyrhine apes, the catarrhine to the Old World. Characteristic 
edentates are trie armadillos, sloths, and ant-eaters; the marsupials are repre- 


DISTRIBUTION 163 

sented by the opossums and Ccrnolestcs, nearly related to the Australian Di- 
protodonts; among birds the humming-birds, toucans, Cotingidae, Tanagrichc, 
Tinamous, Palamedidae, Rhea, etc. The almost entire absence of insect! vores 
and the considerable development of rodents (cavies, agoutis, chinchillas) are 
noteworthy. 

The four remaining provinces are still closely connected geographically and 
form a third great division, 'Arctoga-a,' characterized by the entire absence of 
platyrhine apes, monotremes, and, except the North American opossum, of 
marsupials. In the secondary and tertiary times the northern parts of these 
lands were connected and an interchange of faunas occurred, this being the 
easier on account of the extension of the warm climate to the far north. Hence 
many unite the palasarctic and nearctic provinces into a 'holarctic' province, but 
when existing conditions are concerned it is better to retain them as distinct. 

(3) The Nearctic region has three mammalian families, the prong-horned 
antelope, the opossums, and the Haplodontae, peculiar to it; of the group of 
Amphibia, the Sirenidae and Amphiumidae. The Nearctic is distinguished 
from the nearly related palasarctic region through the crowding in of neotropical 
forms like the raccoon, opossum, humming-birds, etc. The absence of the stag, 
badger, wild swine and all true mice is noticeable. 

(4) The Pahearctic region covers the greatest area and consequently touches 
several others; hence climate and great distances have caused important differ- 
ences between the local faunas, but its contact with other regions explains the 
fact that it has no peculiar families. Deer, cattle, sheep, and camels have 
reached a great development; especially conspicuous genera are the chamois, 
squirrel, badger, and marmot. 

(5) The Ethiopian region has many animals found only there; among li 
the hippopotamus, giraffe, the recently discovered Oeapia, the aardvark, and, if 
we include Madagascar, the lemurs are characteristic. To these are added a 
rich development of antelopes and zebras and the gorilla and chimpanzee. 
Equally noteworthy is the entire absence of striking families and genera, such 
as the bears, moles, deer, goats, tapirs, sheep, and the true swine. 

Within the region the island of Madagascar occupies a remarkable position. 
This is the land of lemurs and Insectivora, the majority of the genera of lemurs 
living exclusively in Madagascar. On the other hand, the large beasts of prey, 
all the true apes, antelopes, elephants, and the various species of rhinoccro- arc 
absent. Consequently, since Madagascar is conspicuously distinguished from 
Africa, many zoologists separate the island from the Ethiopian region as an 
independent Malagassy province. 

(6) The Oriental region contains, next to Madagascar, the most lemurs 
among which the Tarsidae and Galeopithecidae arc exclusively oriental. Re- 
markable inhabitants are the gibbons and orang-utans, musk-deer, numerous 
families and genera of birds. 

Arctic and Antarctic Provinces. Of late the view has gained ground 
that, besides these six, two other, circumpolar, provinces must he distinguished, 
the Arctic and the Antarctic. Both have a fauna consisting of few specie.- but 
numerous individuals, of which the auks, polar bear, reindeer, and arctic foxes 
are characteristic of the northern or arctic region, the penguins and the entire 
absence of land mammals of the antarctic. 

The Distribution of Aquatic Animals. Since most seas are connected, 
the faunal regions cannot be distinguished so sharply as in the case oi the land 
faunas; conspicuous differences are present only when two oceans are separated 
by continents extending far to the north and south; such, for example, i-xi-t 
between the Red Sea and the Mediterranean, between the east and west coa 
of North America, even where they are separated only by the isthmus of Panama. 


164 GENERAL PRINCIPLES OF ZOOLOGY 

Then, too, considerable differences may exist where currents of greatly different 
temperatures meet. 

Much more remarkable in the marine fauna are differences caused by the 
conditions of life in the different depths of the sea. A deep-sea fauna, a coast 
''niiia, and a pelagic fauna can be distinguished. The coast fauna embraces the 
animals which inhabit the plant-covered rocky or sandy shore to a depth of a 
few hundred feet. The deep-sea fauna swims, creeps, or is fixed at the bottom 
of the ocean at depths of 1000 to the greatest depth yet known, 9430 meters, 
5156 fathoms; it is distinguished from the coast fauna in part by its archaic 
character, for here very often genera and entire groups of animals exist, like the 
Hexactinellidas, crinoids, etc., which long were chiefly known through fossils 
from earlier geological ages. 

The Plankton. The pelagic fauna comprises all forms which swim freely 
in the water, the plankton; here belong many ccelenterates, medusae, and cteno- 
phores, entire groups of Protozoa, like the radiolarians, many Crustacea, the 
heteropods and pteropods. These animals live either at the surface of the sea 
itself or floating at greater or lesser depths, to 8000 meters or even more. Usually 
they are gelatinous and of glasslike transparency; this must be regarded as 
sympathetic coloring and adaptation to the transparency of the water. The 
plankton of the deep seas, extending up to about 800 meters, forms a special 
fauna characterized by the brownish-red color, which is also found in the bottom 
animals. 

Distribution of Fresh-water Animals. In fresh water two groups of 
animals must be distinguished, of which the one comprises mainly the more 
highly organized forms, the molluscs, fishes, and Crustacea, the other the lower 
animal world. The distribution of the former is mainly determined by the 
same factors which influence terrestrial forms; they are therefore of great impor- 
tance in matters of geographical distribution, yet it must be remarked that many 
fish at the breeding season ascend from the seas to the rivers (salmon, alewives, 
etc.) and on the other hand, others like the eels go from the rivers to the seas, so 
that the sea is not that sharp boundary for these animals that it is for land ani- 
mals. The distribution of the lower fresh-water animals, however, is cosmo- 
politan. The same infusorians and rhizopods, copepods, fresh-water sponges 
and polyps which occur in America seem to be distributed over nearly the entire 
earth. This is connected with the fact that all these animals have resting 
stages in which they endure desiccation. The resting stage, be it as a hard- 
shelled egg or as an encysted animal, may be borne about by the wind, or may 
be carried with the mud by aquatic birds, and upon again reaching the water 
resume its active state. 


VI. DISTRIBUTION OF ANIMALS IN TIME. 

It is the province of paleontology or paleozoology, to treat of animals in the 
earlier periods of the earth's history, but since it is necessary to draw upon 
paleontological facts to understand the living forms, and especially the verte- 
brates, an outline of the geological periods with the characteristic animals may 
be given here. 

I. Azoic OR ARCHEAN ERA. 

No organisms are certainly known from this age. The animal nature of 
Eozoon canadense of the Laurentian beds, once referred to the Foraminifera, is 
more than doubtful. 


DISTRIBUTION 105 

II. PALEOZOIC ERA. 

1. Cambrian. 4. Carboniferous. 

2. Silurian. 5. Permian. 

3. Devonian. 

The oldest paleozoic period, the Cambrian, contains only invertebrate 
fossils: silicious sponges, the problematical graptolites, medusa-, trilobites, 
gigantostraca, cystoids, holothurians, brachiopods, nautiloids, gasteropods, and 
a few lamellibranchs. Trilobites, cystoids, gigantostraca, and the blastoids 
and tetracoralla, which appear in the Silurian, reach their culmination and 
become extinct in the paleozoic. Fishes appear in the Silurian, and acquire a 
great development in the Devonian. The earliest Amphibia and reptiles come 
from the carboniferous. 

III. MESOZOIC ERA. 

i. Triassic. 2. Jurassic. 3. Cretaceous. 

The mesozoic era was the age of reptiles, which were represented by numer- 
ous forms, some of gigantic size; most of them becoming extinct in the creta- 
ceous. The first mammals appear in the triassic, the birds in the Jurassic. 
Among the invertebrates the ammonites, which appeared in the Devonian, 
reached their greatest development and became extinct in this era. 

IV. CENOZOIC ERA. 

(a) Tertiary. 

1. Eocene. 3. Miocene. 

2. Oligocene. 4- Pliocene. 

(b) Quaternary. 

5. Pleistocene (Ice Age, Diluvium). 6. Recent. 

In the tertiary all of the now living orders of mammals and birds appeared, 
among them probably man, whose remains have been traced with certainty to 
the pleistocene. 


SPECIAL ZOOLOGY. 

SINCE comparative anatomy and the theory of evolution have made 
their impress upon systematic zoology one recognizes in classification not 
only a means of arranging the species, but also the possibility of expressing 
the relations which the larger and smaller groups bear to each other. The 
solution of these problems demands an accurate knowledge of compara- 
tive anatomy and embryology and a complete knowledge of animal forms 
based upon them. We are yet far from such a knowledge, farther with 
regard to some groups than others, and as a consequence systematic 
problems are not all equally advanced towards solution. 

In general it may be said that certain natural groups are recognized: 
(i) Chordata; (2) Mollusca (after the elimination of the Brachiopoda); 
(3) Arthropoda; (4) Echinoderma; (5) Ccelenterata (after the separation 
of sponges) ; (6) Protozoa. On the other hand, it is yet uncertain exactly 
how to regard the worms, brachiopods, polyzoa, and a few other forms. 
The general tendency is to distribute the worms into at least three branches 
(flat worms, round worms, and annelids) and to unite the Polyzoa and 
Brachiopoda in a branch of Molluscoida. In this way groups poor in 
species and of little importance in a general account of the animal kindom 
are placed on the same basis as the large and exceedingly important 
groups of vertebrates, arthropods, and molluscs, and thus obtain, espe- 
cially in the eyes of the beginner, an importance which does not belong to 
them. It therefore seems better in an elementary work to pursue a rather 
conservative course. 

PHYLUM I PROTOZOA. 

All of the Protozoa are small ; some may be seen by a sharp eye as mere 
specks, but the majority are so minute as to be invisible except with a micro- 
scope. On the other hand, a few have a diameter to be measured by milli- 
meters, especially where hundreds of individuals are united in colonies. 
This small size is a result of the fact that the Protozoa are single-celled 
animals. Like all cells they consist of protoplasm, and they have the 
further cell attribute, one or more nuclei. Being unicellular, it follows 
that they lack true tissues and true organs; alimentary canal, nervous 
system, sexual organs, etc. The functions of nourishment, sensation, 
movement, and reproduction are performed more or less directly by the 
protoplasm. 

106 


PROTOZOA 167 

In nutrition, in so far as it is not produced by substances in solution, 
foreign particles pass into the protoplasm and are digested by it. They 
usually lie during digestion in special collections of fluid, the food vacuoles 
(figs. 121, 150, etc., na), more rarely in the protoplasm itself. All in- 
digestible portions are cast out after a time. This taking in and casting 
out of foreign matter can take place in the naked Protozoa at any point 
of the surface, while in the more highly organized species when the outer 
surface is hardened by a pellicle or a thin c-uticula, there are definite open- 
ings which according to analogy with many-celled animals are spoken of 
as mouth and anus, or more precisely, cytostome and cytopyge. The mouth 
may connect with a tube, the oesophagus or cytopharynx, which ends free 
in the protoplasm. 

Structures may occur within the protozoan cell which recall the organs 
of higher animals, and which are called cell organs. \Yhile motion is 
usually produced by the protoplasm and its processes pseudopodia, 
flagella, and cilia there are Protozoa, like Stcntor and the Yorticellida- 
which have muscular fibrilke. The sensitiveness to light is often increased 
by an eye spot, a small pigment body in which even a lens may occur. 
More constant of cell organs are the contractile vacuoles (tig. 117, etc., rr), 
rarely absent from fresh-water species, but commonly lacking from marine 
forms. These have a definite place in the cell; their number is approxi- 
mately constant in most species; they exhibit extremely constant phe- 
nomena. The walls contract and empty the fluid contents to the exterior, 
often through a special duct. When one empties it completely disap- 
pears and is formed again anew in a short time, and is filled with fluid 
from the surrounding protoplasm. It thus resembles the contractile 
vacuoles in the water vascular system (excretory organs) of the worms to 
be described later. Apparently the contractile vacuoles are for the 
elimination of injurious substances in solution produced by the vital pro- 
cesses, among them possibly carbon dioxide, like a respiratory organ. 

The occurrence of such diverse differentiations, recalling organs and tissues, 
gives such a complicated appearance and such a decree of spi-ciali/ation t<> tin- 
protozoan body, that it was questioned for a time whether all could belong t" a 
single cell. Yet it was a mistake to doubt the unicellularity of the I'roto/oa. lr 
according to our conceptions of the cell, there is the capacity to develop in 
many directions, to produce a kind of stomach, muscle fibres, sense apparatu-, 
skeleton and the like; although in the organization of the higher animals it 
produces only a specific product (muscle cells, contractile substance, gland cells, 
secretion). 

The vital phenomena of the Protozoa proceed from the protoplasm, 
but with a certain dependence upon the nucleus. If an infusorian or an 
Amoeba be cut into nucleate and anucleate portions, only the lirst can 


168 PROTOZOA 


live. The part without the nucleus loses the capacity for assimilation, 
for growth, and for regenerating the lost parts. For a time it can react to 
stimuli, move about. Sensibility and contractility persist only so long as 
the necessary elements, formed under the influence of the nucleus, are 
present. When they are used up the last manifestations of life are lost 
and death ensues. So it may be said that the chemism of the cell needs 
the participation of the nucleus. 

The nucleus is also concerned in reproduction, of which the most 
primitive type is binary division (figs. 120, 150, 151). Budding is rarer, 
its character being most evident when several buds are separated simul- 
taneously from the mother animal (fig. 21). The nuclear division occurs 
in different ways. Like the cell body, it may divide amitotically, but it 
can present the complicated phenomena of mitosis (formation of spindle 
and chromosomes). In not a few instances the specific organ of division, 
the centrosome, "appears, so that all transitions from direct to extremely 
complicated division are present in the phylum. 

Very frequently the nuclei multiply without a corresponding division 
of the protoplasm, so that large masses of protoplasm, with hundreds or 
even thousands of nuclei arise (multinucleate cells, syncytia); or both 
nucleus and protoplasm may grow, without division, to extraordinary 
size. In both instances, after an interval of time, there is a simultaneous 
division into hundreds or thousands of reproductive particles; the pro- 
toplasm, in the first case, dividing in accordance with the number of 
nuclei present ; in the other following the division of the mother nucleus 
into a multitude of daughter nuclei. Many Protozoa divide in the free 
state while swimming or creeping about; others first encyst, that is, assume 
a spherical shape and secrete a protective envelope. 

In the Protozoa may occur a fusion of individuals conjugation' 
which in many respects has much similarity to the process of fertilization 
in Metazoa and in plants. In some (conjugation of many Rhkcpods) 
this does not correspond to true fertilization, since only the protoplasm 
unites (plasmogamy), while the fusion of nuclei (car yoga my) necessary to 
fertilization does not occur. In others a fusion of nuclei takes place. In 
the cases which have been accurately studied there has been seen, before 
the fusion of the nuclei, a process comparable to the formation of the polar 
globules in the egg, to this extent, that in each of the conjugating individ- 
uals the nucleus divides twice and of the products of division only one, 
the nucleus intended for caryogamy, persists, while the others (polar 
globules) degenerate. 

These cases of true fertilization may differ greatly. The conjugating 
individuals may be equal in size, iso gametes (most Infusoria, many Rhizo- 


PROTOZOA 169 

poda), or there is a disparity in size (sexual dimorphism), in which 
smaller and consequently more mobile 'males' (microgamctes, zoosporcs) 
fertilize the larger fixed or slowly moving 'females' (macro gametes, oospores) 
as in Yorticellidce, most Sporozoa, and flagellates, forming with them 
a permanent zygote (copulation). The formation of a zygote can also 
occur by the permanent fusion of two isogametes, but usually the union 
of isogametes is transitory (conjugation) and lasts only long enough for 
cross fertilization, gamete A fertilizing B, and in turn bring fertilized by 
B, after which the two separate. A striking phenomenon is the not very 
rare 'autogamy' in which the mother animal divides into two daughter 
animals, which form polar globules and fuse to a zygote, an extreme case 
of inbreeding. 

Thirty years ago it could be laid down as a universal fact that the Protozoa 
in contrast to the Metazoa lacked sexuality. Since then observations have so 
increased that the conclusion is that fertilization occurs in all Protozoa, although 
the rarity of the process in many species renders the demonstration difficult. 
Perhaps also in many groups fertilization has been lost through degeneration 
(similar to the apogamy of plants). 

Still there remain certain interesting differences from the Metazoa. The 
Protozoa lack special sexual cells eggs and spermatozoa. On the contrary, 
the whole body functions as a sexual cell. Further, the relations of fertilization 
to reproduction are not the same as in the Metazoa. (i) Protozoa may increa e 
in the same way before and after fertilization, indeed somewhat more slowly 
after (Infusoria). (2) Sometimes fertilization brings nourishment and repro- 
duction to a standstill, in which case encystment appears (many rhizopods and 
flagellates). (3) A third case is where division follows fertilization, occurring 
more rapidly and having another character (sexual reproduction, better 'nutu- 
gamic division,' 'sporogamy') than the pre-fertilization divisions (asexual repro- 
duction, 'schizogony,' better, 'metagamic reproduction'). These alternating 
pro- and metagamic reproductions have been called alternations of generations 
(most Sporozoa, many rhizopods). 

Analysis of these phenomena leads to the conclusion that we may speak ef 
fertilization but not of sexual reproduction in the Protozoa. As was said pre- 
viously (p. 149) these facts have great importance in the explanation of the 
existence of fertilization, since they show that it has not always the purpose c f 
stimulating the reproductive processes and thus leading to the formation of a 
new individual. Fertilization has to accomplish other things for the organism; 
they must be of great importance, since they are so widely spread; but as yet 
their significance is not clea 1 ' 

The Protozoa, with small and soft protoplasmic bodies, are but slightly 
protected against drying up, and therefore they are aquatic. Nmie, 
like Amoeba terricola, are terrestrial, but these only occur in moist places. 
Salt and fresh water, of the latter stagnant pools rich in vegetation, are 
the favorite places. The fresh- water forms are cosmopolitan, so that the 
species in all lands are very similar. This depends upon certain peculiar- 
ities. The fresh-water Protozoa can become encysted and in the encysted 


170 PROTOZOA 

stage can endure unfavorable conditions such as lack of food, freezing, 
or complete evaporation of the water. When thus protected they may be 
blown about by the wind or carried far on the feet of birds. Hence one 
group bears the name Infusoria, for if dry earth or dry plants (e.g., hay) 
be soaked in water and this infusion allowed to stand for some time, 
a Protozoan fauna will develop in it. The encysted animals in the earth or 
on the plants are awakened by the moisture to new life and leave the cyst. 
Spontaneous generation, as was once believed, does not occur here, for 
if one sterilize the materials and prevent the entrance of germs the water 
will remain uninhabited. 

The protozoa are very important from the pathological standpoint. 
Each of the four classes includes numerous parasites, the Sporozoa being 
exclusively parasitic. Many cause severe infective diseases (malaria, 
relapsing typhus, sleeping sickness, etc.) especially in the warmer climates, 
while in the north, at least as far as man is concerned, the bacterial 
diseases predominate. Many protozoan diseases are 'inherited,' that is 
the egg cells are infected by the parasites. This is the case with the 
pebrine disease of silkworms, the Texas fever of cattle and others. 

Historical. On account of their invisibility the Protozoa were unknown 
until 1675; they were discovered in infusions by Leeuwenhoek, the discoverer of 
the microscope. Wrisberg called them Animalcula infusoria infusion animals, 
and Siebold gave them the name Protozoa. Ehrenberg maintained that the 
Protozoa, like all animals, possessed alimentary canal, nervous system, muscles, 
excretory and sexual organs. Dujardin denied all this and recognized in them 
only a single homogeneous substance as sufficient to produce all vital phe- 
nomena. Siebold discovered that the Protozoa were unicellular. The fact 
that there are unicellular animals without organs and yet capable of existence 
was an extremely valuable addition to knowledge, for it not only broadens our 
conception of animal life, but it furnishes for the theory of evolution from 
simple organisms the most important link, the first of the chain. 

The different appearances of Protozoa depend upon the degree of organ- 
ological and histological differentiation. Since these are most prominent in the 
nourishing and locomotor structures, these become important in subdividing 
the group. In accordance with the motion and taking of food by pseudopodia, 
flagella or cilia, there are three classes: Rhizopoda, Flagellata and Ciliata 
(Infusoria, s. str.). To these are added the Sporozoa, modified in motions and 
mode of feeding by parasitism. Undoubtedly Rhizopoda, Flagellata and Spo- 
rozoa are much closer to each other than are the Ciliata; hence they are grouped 
as Plasmodroma or Cytomorpha in contrast to Ciliomorpha or Cytoida. 

Class I. Rhizopoda. 

First of the Protozoa are those organisms which lack permanent struc- 
tures for locomotion and nourishment, the protoplasm of the body per- 
forming these functions. The term Rhizopoda refers to the fact that the 
protoplasm sends out root-like processes or pseudopodia for locomotion and 


I. RHIZOPODA 


171 


for taking nourishment. These differ from true appendages in that they 
are not constant, but are formed according to demand and again disappear. 
A pseudopodium arises when the protoplasm streams to one point of the 
body and extends as a process beyond the surface. Since the proces 
becomes attached and draws the body after it, or since the protoplasm of the 
body may flow into it, a slow change of place occurs. In either case the 
process disappears in the organism, and new pseudopodia are formed 
at other places which are retracted in turn. This type of locomotion is 
called amoeboid after the Ama'ba, in which it was first studied. When 
the Rhizopoda in their wanderings meet particles of nourishment, they 
enclose them with their protoplasm and take them into the interior of 
the body (fig. 117, A"). 


1 1 > 

,!l/f/ 


FIG. 117. 

FIG. 117. Aaiirlni proteus (after Leidy). 
ectosarc; n, nucleus; N, food-body. 

FIG. 118. Rotalia freyeri (from Lang, after M. Schultze). 


FiG. 118 
cv, contractile vacuole; en, entosarc; ek, 


The shape of the pseudopodia is approximately constant for each 
species, but it varies so with different forms that it may be used not only 
for separating species but groups. On the one hand, there are finger-liki- 
pseudopodia (fig. 117), on the other, those of such delicai -y that even 
under strong magnification they appear like fine threads (tig. i iS); and 
between these extremes many intermediate forms. Thread-like pseudo- 
podia usually branch, and when the branches meet they may fust' and 
form anastomoses, from which it follows that the pseudopodia are not 


172 


PROTOZOA 


covered by a membrane. The fine granules of the protoplasm usually 
enter the pseudopodia and produce here, as they move back and forth, 
the phenomenon of 'streaming.' Since foreign particles can participate in 
this streaming, it follows that the movements depend on the protoplasm 
itself. We have already used this fact (p. 55) to demonstrate the extraordi- 
nary complexity of protoplasm. 

When Rhizopoda increase by division, the division products frequently 
become flagellate spores or zoospores. The body becomes oval and 

develops, on the end which contains the nucleus, 
one or more rlagella, which move more ener- 
getically than pseudopodia, and are permanent 
as long as the zoospore stage persists (fig. 122). 
Since many Protozoa possess flagella along with 
pseudopodia, the boundary between Rhizopods 
and Flagellates is not distinct (fig. 119). 


The Rhizopoda form an ascending series in which 
the systematic characters become more and more 
pronounced; such are the assumption of a definite 
form, as in the Radiolaria and Heliozoa, the forma- 
tion of a skeleton of regular character, as in the 
Thalamophora, or the development of a peculiar re- 
production, as in the Mycetozoa. At the bottom 
FIG. 1 19. Mastigamceba stand the Monera and the Lobosa whose characters 
aspera (after F. E. Schulze). are mostly negative, for neither form, skeleton, nor 

reproduction affords systematic distinctions. 

Order I. Monera. 

The most important character of the Monera is the lack of a nucleus. As 
with other negative characters this is somewhat uncertain. In many cases, 
especially when the protoplasm is filled with pigment granules, the nucleus is 
recognized with difficulty, and hence animals have been described as anucleate 
in which the nucleus was overlooked. So it is possible that, in the few forms 
now remaining in the group, the nucleus has merely escaped observation; 
possibly it is functionally replaced by the chromidia (p. 58). There are several 
theoretical reasons favoring the idea of anucleate organisms. It is easier to 
suppose that with the appearance of life there were organisms consisting of but 
a single substance than that these organisms had nucleus and protoplasm al- 
ready differentiated. Several species of Protamccba are placed in the Monera. 

Order II. Lobosa (Amcebina). 

Lobosa are primitive Rhizopoda with one or several nuclei. The 
species of Anuvba, forms which owe their name to their constant change 
of shape, are typical (figs. 117, 120). This change of form is due to the 
formation and disappearance of a few finger-like (lobose) pseudopodia. 
Body and pseudopodia consist of two layers, a soft granular inner entosarc 
(en) and a firmer, clear, outer ectosarc (ek). In the entosarc is usually 


I. RHIZOPODA: HELIOZOA 


173 


a single (sometimes several) nucleus (n), which is vesicular, and contains 
cither one large or several smaller nucleoli. 
A contractile vacuole is usually present. 
Reproduction occurs by binary or multiple 
division (fig. 120), and encystment has been 
observed, the protoplasm dividing into many 
hundred small amoeba?, a phenomenon always 
connected with fertilization processes (au- 
togamy?). 

Most Lobosa occur in fresh water; A. terricola 
in moist earth. There are also parasites like A. 
coli, rare in colder climates, frequently observed 
i n the tropics. According to recent researches 
two forms have been included under the name 
.4. coll, one innocuous, Entamceba coli, and another 
(possibly several) pathogenic forms, including E. 
histolytica, which appears in enormous numbers 
in abscesses of the liver and ulcers of the colon of 
men ill with tropical dysentery For the first of 
these it is certain, for the other probable, that 
infection is caused by encysted forms, which arise 
as a consequence of fertilization and are passed 
out with the feces. 


FlG. 1 20. A mtrba poly podia 
in division (<ifter F. E. 
Schulze). CT, contractile 
vacuole; ek, ectosarc; en. en- 
tosarc; n, nucleus. 


Order III. Heliozoa, Sun Animalsules. 

-k- 

The Heliozoa owe their name to the shape of the body, with the pseudopodia 
arranged like rays. In each pseudopodium is a firm axial thread, forming a. 


x 

; . . ' 


..---- ::: -.r^fc7^' 


cv Na 


FlG. 121. Actwosphavium eicJihorni. M, nu-dullary substance with nuck-in 
cortical substance with contractile vacuoles (a-); Na, food-body. 

skeleton, and a thin coating of protoplasm. Branching and anuM 

the pseudopodia are rare. The axial threads frequently converge at 


174 


PROTOZOA 


B 


FIG. 122. Clathndina elegans. A, with extended pseudopodia; B, divided into two 
cysts; C, zoospore; n, nucleus; CT, contractile vacuole. 


Sl^Jllw 


FIG. 123. Encystment of Actinospharium, schematized to the extent that in III 
and IV, successive stages are shown in the same figure. I, just encysted animal; 
II, formation of the primary cysts, after resorption of most of the nuclei; III, i and 2, 
division of primary into secondary cysts; 3, 4, maturation of secondary cysts; 3, forma- 
tion of first, 4, of second polar globule, 5, fusion of secondary cysts (autogamy); 
IV, 1-4, stages of conjugated cysts; 5, escape of young Actinosphoerium. 


I. RHIZOPODA: RADIOLARIA 175 

centre of the body. Here lies the centrosome separated from the nucleus 
(cf. p. 59), which controls the division of nucleus and protoplasm. The body 
consists of cortical and medullary portions (fig. 121), distinguished by differences 
in the protoplasm, but not separated by membrane. In the cortex are the con- 
tractile vacuoles (cv); the medulla contains the usually single nucleus. Repro- 
duction takes place by division, and one or both moieties may become swarm 
spores, i.e., assume an oval form bearingatone poleoneortwoflagella(fig. 122, C). 
These become widely distributed by the llagella before they resume the spherical 
shape, lose their flagella and form pseudopvxlia. It frequently occurs that 
several Heliozoa of the same species become connected by protoplasmic bridges, 
and so form unions of from two to ten individuals (plasmogamy). True fertili- 
zation (karyogamy) has only been observed in Actlnophrys and Acthwsphcerium. 
Forms with skeleton and those without arc distinguished. Among those 
with a silicious skeleton are Clathndina elegans, with a spherical lattice- work 
skeleton supported on a stalk (fig. 122), and Acanthocystis lurfacca, skeleton of 
radial, branching needles. To the forms without a skeleton belongs Actino- 
sphccrium eichhorni (fig. 121), as large as a pinhead, milk-white, protoplasm 
foamy from the numerous vacuoles, the different sizes of which distinguish the 
cortical from the medullary parts. There are contractile vacuoles in the cortex, 
many nuclei in the medulla. In encystment the foamy appearance is lost and 
most of the nuclei are dissolved (fig. 123). Inside the gelatinous cyst the con- 
tents are divided into primary cysts, corresponding in number to the persistent 
nuclei, and these become enclosed in a silicious envelope (fig. 122, II). Each 
primary cyst divides mitotically into two daughter cysts (III, i, 2), which become 
mature by forming two polar globules (III, 3, 4) and then completely fuse with 
each other (III, 5). The germ cells arising in this autogamic manner enclose 
themselves in a resistent envelope (IV, 1-4), from which, after a considerable 
period of rest, the young Actinospluzria escape (IV, 5). 

Order IV. Radiolaria. 

The Radiolaria, the most beautiful and most highly organized of the Rhizo- 
poda, recall the Heliozoa. They are spherical, only rarely flattened into disks, 
or taking conical or lobular shapes. A second resemblance lies in the delicate 
pseudopodia, often with an axial filament. The most characteristic feature is 
the central capsule. This is the central portion of the body surrounded by a 
membrane, outside of which is the extracapsulnm. If the central capsule be 
removed from the extracapsulum it not only lives but regenerates the lost parts, 
while the extracapsular portions die. Since the protoplasm of both parts is 
identical, the difference in regenerative powers can only depend on the nuclei, 
which are confined to the central capsule. The capsular membrane is either 
perforated on all sides by numerous pore canals or by openings in certain pla< 
The protoplasm passes through these and spreads itself in the exiracapsulum. 
The chief part of this is a gelatinous mantle, through which the protoplasm 
extends as a fine network before it forms pseudopodia on the surface. In the 
larger Radiolaria it may be extensive, containing vacuoles (extracapsular <j/r<W/) 
developed in the protoplasmic net. 

The central capsule may be uni- or polynucleate. In the first case the 
nucleus (fig. 124), a vesicle of appreciable size, lies in the centre of the capsule, 
in the others the capsule is thronged by hundreds of small homogeneous nuclei. 
The fact that certain species are nearly always multinucieate, others uniini- 
cleate is due to the fact that in them the primitively single nucleus di\ide^ early, 
while in the other it becomes considerably enlarged and forms in its interior 
material for the secondary nuclei, which escape, the remainder of the nucleus 
degenerating. 


170 


PROTOZOA 


With few exceptions the Radiolaria possess skeletons of wonderful beauty; 
latticed spheres, single or one within another, and bound together with radial 
rods (fig. 89), frequently ornamented on the outer surface with spines, or latticed 
discs, helmet-like or cage-like structures (fig. 12-7). In other cases occur rings, 
tubes, spines, which meet in the central capsule (fig. 125), etc. In rare cases 
the skeleton is formed solely of organic substance (acanthiri) ; usually it is silicious 
and resistant. Skeletons of Radiolaria occur in rocks of various ages, as in 
Sicily, the Nicobars (both tertiary), and the Barbadoes. 

In reproduction there are numerous forms with a skeleton in which division 
begins with a cleavage of the central capsule and usually extends through the 
extracapsulum. If this latter does not divide a colony results, in which a jelly 


.\ \\ \, I 

-A\ V 

\S^ WM/W^ 

Vv ' sv ;-^^f\>-^ /v/ Crx 


FIG._ 124. Thalassicolla pelagica. In centre the nucleus with coiled nucleolus, 
around it central capsule with oil globules; still outside the extracapsulum with vacuoles 
(extracapsular alveoli), yellow cells (black) and pseudopodia. 

contains numerous central capsules, bound together by protoplasmic threads, 
which form the pseudopodia on the surface (fig. 128). A second type is repro- 
duction by swarm or zoospores, which begins when the nucleus has divided 
into hundreds or thousands of daughter nuclei. The contents of the central 
capsule then divides into as many portions as there are nuclei, these become oval 
and develop two flagella (fig. 126), which soon begin to vibrate so that the central 
capsule is filled with a tumultuous crowd. With the breaking of the capsular 
membrane these swarm spores escape; here our knowledge of this type ceases. 
Since in many species there are macrospores and microspores it is probable that 
a copulation is necessary. 


I. RHIZOPODA: RADIOLARIA 


FIG. 125. FIG. 126. 

FIG. 125. Acanthometra elastics, ck, central capsule; n, nuclei; />, pseudopodia; 
St, spines; Wk, extracapsulura. 

FIG. 126. Zoospores of Collozoum inerme. a, niicrospore; b, zoospore with fusi- 
form body; c, macrospore. 


- 

-:,,:- 

' 


d 

\ I. 

- b 

c 


FIG. 127. 


FIG. 128. 


FIG. 127. Eucvrtidium cranioides (after Haeckel). 

FIG. 128. Collozoum inerme. a, jelly; b, oil globules in the central capsule; c, d, 
yellow cells; e, vacuoles. 
12 


178 


PROTOZOA 


Common, if not constant, in the Radiolaria are the yellow cells, unicellular 
algae (Zooxanthellcr), which are also present in other animals. (Thalamophora, 
actinians, sponges, etc.). They afford an instance of symbiosis, or the living 
together of different organisms for mutual good. The Radiolaria are exclusively 
marine. In fair weather they float at the surface, but sink in times of storm. 
Certain species and even large groups (Phaeodaria) occur only at great depths 
(1500-4000 fathoms); several thousand species known. 

Order V. Thalamophora (Foraminifera, Reticularia). 

The Foraminifera, though not equalling the Radiolaria in beauty and 
variety of forms, exceed them in numbers of individuals, and have a great 
importance in the history of the earth. No other group of animals has 
had so great a part in the formation of beds of rock. 

The most prominent characteristic is afforded by the shell, which 
is closed at one pole, and usually open at the other, the pseudopodia passing 
through the aperture (fig. 129). Accordingly as the axis connecting 
these poles is altered, the shell becomes disc-like, spherical, flask formed 

or even coiled in a spiral. The interior of 
the shell is frequently divided by transverse 
partitions into numerous chambers (fig. 131). 
Such many-chambered shells (Polythalamia) 
are at first small, and consist of one or few 
chambers, but as the animal grows new 
chambers are added at the mouth of the 
shell. Openings (foramina') in the walls 
connect the adjacent chambers. The spiral 
shells with many chambers have a striking 
resemblance to the shells of the Nautilus 

(% 35 2 )- 

In the fresh-water forms the shell is 

built of an organic substance which may 
be strengthened by silica or the incorpora- 
tion of foreign particles. The more typical 
members, exclusively marine, have cal- 
careous shells with but the slighest trace of organic matter. The presence 
of minute pores in the shell is of systematic importance, the group of 
Perforata (fig. 118) being characterized by them. 

The animal portions form a cast of the inside of the shell (fig. 130), 
and consist of as many pieces as there are chambers in the shell, connected 
by plasma bridges passing through the foramina of the partitions. In 
the protoplasm there is a large nucleus (figs. 129, 130, ), which in some 
cases is early replaced by daughter nuclei. Contractile vacuoles usually 
occur only in the fresh-water forms. The pseudopodia project through 


FlG. 129. Quadrula sym- 
metric a (after F. E. Schulze). 
cv, contractile vacuole; n, nu- 
cleus; TV, food-body. 


I. RHIZOPODA: THALAMOPHORA 


179 


the chief opening of the shell and in the Perforata probably through the 
pores in the shell wall. They are rarely finger-like (fig. 129); usually 
they are thread-like, branched and anastomosing (tigs. 17, 118). 

Reproduction is generally by fission, but presents many variations. Only 
rarely (fresh water Monothalamia) do both animal and shell divide; frequently 
the protoplasm protrudes from the mouth of the shell, a new shell is formed on 
the outgrowth and the protoplasm then divides, one of the resulting individuals 
retaining the old shell. In the marine Polythalamia the following process is 
general: The polynucleate protoplasm divides into numerous uninucleate 
'embryos' which frequently, while still within the mother, develop a shell. 


FIG. 130. Protoplasm of Globigerina 
after solution of the shell, n. nucleus. 


FIG. 131. Young Milinla \vith 
several nuclei (from Lang). 


A second kind of reproduction leading to a fertilization process appears to be 
common. Many swarm spores arise in the shells of the Polythalamia. These 
fuse in pairs with each other. Both of these reproductions alternate with e;u h 
other, and with them is often connected a dimorphism of the individuals. The 
progamic generation, leading to the formation of gametes is distinguished by 
the long persistence of the chief nucleus and often by the structure of the shell 
(large central chamber, megasporic generation) of the metagametes arising 
from fertilization (polynucleate, microspheric gametes). A corresponding 
alternation has been observed in the Monothalamia. 

Sub Order I. MONOTHALAMIA. Mostly fresh-water. They have one- 
chambered shells of chitin or silica, often strengthened by foreign bodies. Con- 
tractile vacuole usually present. Pseudopodia lobose or filiform, branched or 
simple. A. Forms with finger-form pseudopodia: Arcclla,* (Jiiadrulu* (fig. 
129); Difflttgia* These forms are merely shelled Amoeba: and are frequently 
referred to the Lobosa. B. Forms with branching and anastomosing filiform 
pseudopodia. Euglypha,* Groinia (fig. 17). Sub Order II. POLYTHAL- 
AMIA. Exclusively marine; many-chambered shells. Thick beds of rock 
like the chalk, nummulitic limestone, and green-sand are largely foraminiferal 
in origin. The living species have an average diameter of about i mm. Rarely 
species have a diameter of several centimeters (Psammonyx i'ulf<iiiiciis, 5-6 cm.). 
The fossil nummulites reach a diameter of 10 cm. A. IMPERFORATA. Shell 


180 


PROTOZOA 


wall massive, the terminal opening the only communication with the exterior. 
Miliola (fig. 131). B. PERFORATA. Shell perforated by many pores; terminal 
opening may be lacking. Polystomella, Rotalia (fig. 118), bottom dwelling; 
Globigerina bulloides (fig. 130), pelagic. Among the fossils the Nummulites 
need mention. 

Possibly the XENOPHYOPHORA of the deep seas belong near the For 
aminifera. These are large biconcave discs or fan-like plates 2 to 7 cm. across, 
formed of delicate tubes, sometimes arranged in a network, sometimes branched 
like antlers, and between them a skeleton of foreign bodies, the 'xenophya.' 
The fine tubes (granellare) are filled with a polynucleate protoplasm; the larger 
tubes (stercomare} contain large balls, interpretated as fa?cal The animals 
have not been studied in the living condition. 

Order VI. Mycetozoa (Myxomycetes). 

The Mycetozoa or slime animals are regarded by some as animals, by others as 
plants (slime moulds). The first view is supported by the structure of the motile 
stage, the plasmodium, the second by the reproductive bodies, resembling those 
of many fungi. The plasmodia appear in damp weather as networks of bright- 
red, orange or yellow slime on decaying wood. They are giant Amoeba? of 
reticulate protoplasm several centimeters in extent, containing many nuclei and 


FIG. 152. FIG. 133. 

FIG. 132. Chondrioderma difforme (after Strasburger). a, dry spore; b, swollen 
in water; c, spore with escaping contents; d, zoospore; e, amceboid modification of 
zoospores which are uniting to form a plasmodium;/, part of a plasmodium; in d and e, 
nuclei and contractile vacuoles. 

FIG. 133. Spore-sacs of Arcyria incarnata (after de BaryV At the left the sporan- 
gium ruptured by the expanding capillitium, which has discharged the spores. 

much foreign matter taken as food. They creep slowly by means of pseu- 
dopodia (fig. 132, /). On drying, the plasmodium encysts in a peculiar manner, 
and forms the reproductive bodies, the sporangia (fig. 133) and the 'car pome.' 
These are firm-walled vesicles, frequently stalked, the stalk sometimes extending 
into the axis of the sporangium as a columella. The space between the wall cf 
the sporangium and the columella is filled with fine powdery spores and an 
exploding apparatus, either a network of fine filaments (capillitium) or many 


II. FLAGELLATA: AUTOFLAGELLATA 


181 


spirally coiled threads (elaters). When wet, as by rain, the elaters or capillitium 
expand, rupture the sporangium and scatter the spores. The spores germinate 
in water or on moist surfaces, and from each comes out a small amceba-like 
embryo, frequently furnished with a llagellum (fig. 132). Several of these 
embryos fuse to form a plasmodium: -Ethalium septicum, flowers of tan, plas- 
modium yellow, on spent tanbark; Arcyria (fig. 133), Plasmodiophora brassica;, 
par aside in cabbage. 

Class II. Flagellata (Mastigophora). 

In many Rhizopoda (p. 172) the pseudopodia disappear at the time 
of reproduction and are replaced by one or two flagella; others have, be- 
sides pseudopodia, permanent flagella. Such flagellate spores and flagel- 
late Rhizopods form the transition to the Mastigophora, which arc per- 
manently flagellate, the flagella serving as organs of locomotion and 


feeding. 


The three orders must be described separately. 


Order I. Autoflagellata. 

The Autoflagellata form a polymorphic group. The 
free forms are usually oval, with a nucleus in one end 


FIG. 134. FIG. 135. FIG. 136. FIG. 137. 

FIG. 134. Englena riridis (after Stein), c, contractile vacuole; ;/, nucleus; o, 
pigment (eye) spot. 

FIG. 135. Dinobryon sertularia (after Stein), a, a parasitic flagellate often found 
in the lorica; b, contractile vacuole; n, nucleus. 

FIG. 136. Conocladium umbelhitum (after Stein). 

FIG. 137. Chilimonas paranurcium (after Butschli). oe, cytostome; n, nucleus; v, 
contractile vacuole. 

and a contractile vacuole in the other. At the anterior end there is 
often a red or brown eye-spot (tig. 134) which gives these forms 
great sensitiveness to light. In the parasitic forms the eye-spot and 


182 


PROTOZOA 


contractile vacuole are usually lacking (figs. 138, 139, 140, 141). 
There are usually one or two flagella at the anterior end; when more 
occur (eight or more) they are distributed over the body. They differ 
in form and function so that chief, accessory and trailing flagella 
are distinguished. In the parasitic species there is usually a duality 
of the nucleus, recalling that of the Infusoria, there being, besides the 
principal nucleus, a second, the blep/iaroplast, at the base of the flagellum 
(fig. 138, r). The surf ace of the body is frequently naked, and then is often" 
capable of amoeboid motions. Again it may be covered with a more or less 

evident cuticle. Closed gelatinous en- 
velopes or open, goblet-shaped cups 
(loricce, fig. 135) are common. There 
may also be slender stalks, to which 
he animals are attached in small groups 

(% 13)- 

There are great differences in the 
feeding and in the organs connected 
therewith. Many feed like animals, 
taking solid food with pseudopodia like 
the Rhizopoda or with a mouth like the 

Infusoria. In the Choanoflagellata (fig. 
FIG. 138. Trvpanosoma leivisi 
(after Laveran et Mesnil). a, ag- I 3) there is a collar, a funnel-like 

glomerate; b, single one enlarged; process of the body protoplasm around 

g, flagellum; n, nucleus; c, blephar- , a .. J r , . 

oplast. the flagellum, to which foreign particles 

thrown by the flagellum adhere and are 

conveyed to the interior. Besides these animal forms are plant-like 
species which contain chlorophyl (Volvocinae, Euglenidze) or brown 
chromatophores (Chromomonadinese), aiding in assimilation and ena- 
bling the organism to produce paramylum or even starch. It is note- 
worthy that vegetable and animal methods of nourishment appear in 
forms closely related anatomically. Indeed, there are species which 
possess a cytostome without taking solid nourishment, assimilating by 
means of chlorophyl or living on fluid food (fig. 137). All this shows 
that the Flagellata have relations with Rhizopoda, Infusoria, and the 
lower plants (bacteria and alga?). 

Reproduction is nearly always by fission. Conjugation is known in many 
In the Volvocina two individuals fuse completely to form a resting 
spore. In the colonial Volvox globator the conjugating individuals are unequal, 
some animals of the colony growing to large immobile oospheres (macrogametes), 
while others by division form groups of minute active zoospores (microgametes). 
When fertilized by the zoospore the oospheres fall to the bottom, encyst, and 
enter a resting stage before they form a new colony. Young colonies reproduce 


II. FLAGELLATA: AUTOFLAGELLATA 


1S3 


asexually. While most of the individuals of a colony are concerned with nutri- 
tion and motion, others grow larger, and by rapid division, form new colonies. 
There is yet uncertainty about the fertilization of the parasitic Autoflagellates. 
Encystment with autogamy has been described for the intestinal parasites, and 
in the Trypanosomes, parasitic in blood, conditions similar to those of the 
Hasmosporida seem to occur. The transfer of the parasite from host to host is 

\uVi 

X-^ 


FIG. 139. Leucozytozoon ziemanni (after Schaudinn). a-c, growth and multi- 
plication of nuclei in ookinete, in intestine of mosquito; d, transformation of ripe ookinete 
into Spirochaete; e, division of Leucozytozoon. 

accomplished by blood-sucking animals; insects (mosquitos, flies, lice, etc.) for 
mammals and birds; leeches (Piscicola) for fishes. Fertilization very probably 
takes place in the intermediate host. The following account, doubtful in 
several points, is given for Trypanosoma (Leucozytozoon) ziemanni. After the 
parasite has multiplied rapidly for some time in the blood of the owl, some 


FIG. 140. FIG. 141. FIG. 142. 

FIG. 140. Lamblia intcstinalis (after Grassi). Front and side views, n, nucleus. 
FIG. 141. Trichomonas batrachorum (after Dobell). ax, axial rod; ex, cytostome; 
b, blepharoplast; m, undulating membrane; n, nucleus. 
FIG. 142. -Spirochccta pallida (after Schaudinn). 

become macrogametes and others form eight microgametes. Macro- and 
microgametes unite in the intestine of the mosquito, L'ldcx pi pirns, to a zygote 
(ookinete), which grows to a large multinucleate body (fig. 139, a-c). From 
this, together with the formation of a residual body (cf. Sporozoa) numerous 
young flagellates arise, which are transferred by the bite of the mosquito to a 
new host. 


184 


PROTOZOA 


Sub Order I. PHYTOFLAGELLATA. Plant-like chlorophyl-bearing 
flagellates, mostly with eye-specks. VOLVOCINA: Volvox glvbator* green 
sphere 0.2-0.7 mm - m diameter, consisting of thousands of individuals. ' EUGLE- 
NID/E: Euglena viridis* (fig. 134), solitary, coloring small pools by their im- 
mense numbers. CHRYSOMONADINA, plant-like in nourishment- Dinobryon * 
Sub Order II. CHOANOFLAGELLATA. With collars; mostly small 
colonial forms. Codosiga* Conocladium (fig. 136). Sub Order III. EUFLA- 
GELLATA. Animal flagellates, taking solid food by pseudopodia or by cyto- 
stome. Here belong, besides numerous free forms, several parasites of man: 
Laniblia intestinalis (fig. 140), also in rats and mice: Trichomonas hominis in- 
small intestine. T. batrachomm (fig. 141) is similar. Numerous flagellates are 
blood parasites, among them Trypanosoma brucei, which is introduced into 
horses and cattle in Africa by the tze-tze fly (Gloss ina morsitans), sometimes deci- 
mating the herds. Two other species, Tr. evansii and Tr. cquiperdum attack 
horses. Tr. gambiense (castellan i) occurs in the blood and cerebrospinal fluid of 
man in tropical Africa, causing the 'sleeping sickness,' especially fatal to negroes, 
intermediate host Glossina palpalis. Tr. Icu'isii (fig. 138) in rat's blood. 

Since the reproductive stages of Tr. ziemanni show many resemblances to 
iheSpiroc/ia-to', the latter, long regarded as Bacteria, may be flagellates. These 

extremely dangerous organisms multiply by longi- 
tudinal division (the Bacteria by transverse), have 
a very motile corkscrew body, prolonged at either 
end into a flagellum. Sp. pallida (fig. 142) cause 
of syphilis; Sp. recurrent, introduced by blood 
sucking arthropods (ticks, lice), causes recurrent 
typhus. Possibly Licshmania, which causes spleno- 

lf ^jjjj!& r rj^, megaly and the Aleppo evil in man in warm 

,?- "-/ .-'vYV/ -$&< climates, belongs near here. 

Order II. Dinoflagellata (Cilioflagellata). 

These forms, occurring in both fresh water and 
the sea, are placed near the plants because, with 
their brown chromatophores, their food relations 
are like those of plants, although the taking of 
solid food by a mouth opening has been observed. 
The armor formed of cellulose plates is also plant- 
like. This armor is divided by a groove into two 
parts which recall somewhat a box and its lid. 
There is also a longitudinal furrow which crosses 
the other. At the crossing are two flagella; the 
one in the transverse groove was regarded as a 
circle of cilia, whence the old name cilioflagellates 
given the order. Peridinium tabulatum and Cera- 
tium connttum, fresh water (fig. 143); Ceratium 
tripos* marine. 


aaJt 


FIG. 143. Ceratium cornu- 
tum (after Stein), apo, ante- 
rior horn with opening; aah, 
rsh, posterior and right horn; 
g, flagellum; gs, flagellar 
groove; //, longitudinal 
groove; r, rhomboidal plate; 
v, vacuole. 


Order III. Cystoflagellata. 

The cystoflagellates, characterized by a gelatinous body surrounded by a 
membrane, include three or four species, all marine, which differ markedly in 
external appearance. Noctiluca milians* (fig. 144), is remarkably phos- 
phorescent. These spherical forms, about i mm. large, sometimes occur in 
such numbers at night as to make the sea light at the slightest agitation. 
The phosphorescence is apparently caused by oxidation processes in the proto- 
plasm. The body, a gelatinous sphere, is covered by a membrane, interrupted 


III. SPOROZOA 


185 


by a pit at one point, the cytostome, near which is the nucleus surrounded by an 
aggregation of protoplasm which sends branching threads into the jelly of the 
body. At the entrance to the cytostome is a llagcllum, used in feeding but not 
in locomotion, and a band-like tentacle consisting of an outgrowth of the body 
membrane with a transversely banded muscular axis; it moves slowly with a 
swinging motion. Noctiluca reproduces by simple fission (progamic repro- 
duction) and by formation of swarm spores (? metagamic reproduction) (fig. 144, 


FIG. 144. Noctiluca midaris (in part aiter Cienkowski). .4, entire animal; /, 
flagellum; ;;, nucleus; o, cytostome, beside it the 'tooth' and 'lip'; t, tentacle; B, C, 
upper end with two stages in the formation of zoospores; D, zoospores. 

B, C, D}. In the latter two individuals lose tentacles, flagella, and cystos- 
tomes, and conjugate; after mutual nuclear fertilization ( ?) the animals separate, 
while the protoplasm in each collects in a disc which, bv successive divisions, is 
converted into numerous uninucleate oval germs which at first project from 
the sphere, then separate and form small flagellate spores whose history is not 
known. Leptodiscus medusoides of Europe looks and swims like a small medusa. 
Craspedotella.* 

Class III. Sporozoa. 

The Sporozoa are exclusively parasitic, and this has modified their 
feeding, movements and reproduction and in very similar ways. Many 
of them live as parasites in cells ('Cytosporida') as long as their size will 
permit; and after they have left the host cell, they live on fluid, not on solid 
food, hence they lack all arrangements for taking food, even in those cax> 
where the body is covered with a cuticle. As a rule, there is also a lack of 
locomotor structures; but the occasional presence of amoeboid motion or 
flagella indicates a near relationship with rhizopods and flagellates, so 
that it is difficult to draw sharp lines between the three classes. There are 
very close relations between the parasitic flagellates (Trypanosomes) 
and the Hrcmosporida, and the Sporozoa must be regarded as rhizopods 
or flagellates modified by parasitism. 

It is characteristic of the reproduction that the developmental stages before 
and after fertilization pro- and metagamic have different characters. The 


186 PROTOZOA 

progamic development (schizogony) leads as a rule to autoinfection, to increase of 
the parasite in the tissues of the host. The parasite increases in size and breaks 
up into numerous young (merozoites), which grow in turn and divide. This 
process may continue many times until a sexually ripe form appears. Only in 
the gregarines is this replaced by a single large growth, which, in the period of 
preparation for fertilization, is divided into many parts. Fertilization usually 
precedes encystment, only rarely occurring in the cyst. Occasionally there is a 
fusion of isogametes; usually there are non-motile' macrogametes fertilized by 
extremely active microgametes. In this the sexual dimorphism may be pre- 
pared long before and only be expressed in the generation (gametocytes) from 
which the macro- and microgametes arise. The metagamic development 
(sporogony) requires encystment and serves to introduce the germs into a new- 
host. Inside of the cyst spores are formed, which are rarely naked, usually 
surrounded with a firm envelope. In the spore the sporozoite arises, the starting 
point for the progamic development. In all of the divisions which precede 
fertilization or immediately follow it, a part of the protoplasm, containing degen- 
erating nuclei, remains behind as the residual body. From the development 
thus outlined the Myxosporida and Sarcosporida differ in some points, though 
they form spores .and sporozoites for new infections. 

Order I. Gregarina. 

The typical and longest known sporozoa are the Gregarines, parasites 
of oval or thread-like form (recalling round worms), usually somewhat 
flattened, which have only been found in the intestine or gonads, more 
rarely in the body cavity, of invertebrates. The protoplasm (fig. 145, A ) is 
separated sharply into a clear ectosarc (ek) and a granular entosarc (en). 
The ectosarc is covered by a cuticle (/), permeable by fluid food, for no 
cytostome exists. In many (perhaps all) there is a double striping of the 
body, a longitudinal recognizable by furrows on the surface and hence 
cuticular, and a transverse marking in the ectosarc, produced by circular 
or spiral muscle fibrilke. These muscles explain the peristaltic motion 
and the occasional bending of the body, but not the peculiar gliding 
motion by which locomotion is usually effected. It may be that the greg- 
arines secrete stiff gelatinous threads from the posterior end, and the 
elongation of these forces the body forward. 

In many gregarines (Polycystidae) the body is divided into a smaller anterior 
part^ the protomerite, and a larger deutomerite (fig. 145, A). Internally this 
division is marked by a bridge of ectosarc across the entosarc. The vesicular 
nucleus (there is but one in any gregarine) lies in the deutomerite. All gregarines 
are parasitic in youth wholly inside of cells or with the anterior end imbedded 
in the^host cell, which they leave in the developed stage. Many remain for a 
long time with a process of the protomerite in the cells. This process the 
epimerite is provided with threads or hooks for anchorage, and is usually lost 
when the animal gives up its connection with the host cell. Among the intestinal 
gregarines frequently occur 'associations' where two or more animals are fast- 
ened together head to tail in a row (fig. 145, A). Perhaps these associations 
are preparations for conjugation which occurs in development. 


III. SPOROZOA: GREGARIXA 


Reproduction typically occurs in an encysted condition (fig. 145, II). 
Usually two animals occur in a cyst. After each individual has become 
polynucleate by division of its nucleus, it divides at first superficially, 


T. 


FIG. 145. Different Gregarina. I-VII, development of Stylorhynckus; I, S. 
longicollis (after Schneider); II, encysted 5. oblongatus (two animals) beginning 
gamete formation; III, same in later stage, the sexually differentiated gametes in 
copulation; IV-VII, formation and development of the zygote of S. l/i^icollis more 
enlarged; IV, copulation of gametes; V, fusion; VI, beginning division; VII, 
8, sporozoites formed. A. Clepsidrina Uattarum. 1-4, Monocystis magna (after 
Cuenot). i, two individuals in copulo in the spermatheca of an earthworm, sur- 
rounded by its spermatozoa; 2, encysted; 3 and 4, parts of cysts, formation and con- 
jugation of the gametes, more enlarged (according to Brasil the gametes are slightly 
differentiated sexually), cu, cuticula; dm, deutomerite; ek, ectosarc; en, entosarc; g, 
gametes; g l , zoospores, g-, oospore; pm, protomerite; n, nucleus; r, residual body: x, 
sperm of earthworm; z, zygote. 

later internally into small spheres, the gametes (III) . The gametes fuse in 
pairs to bodies which take a spindle shape and become enclosed in a firm 
envelope, the spores, zygotes or 'pseudonavicellae' (fig. 145, 4, ^ ' 


188 PROTOZOA 

That always gametes of different origin fuse is shown by Stylorhynchus 
where the gametes of one animal are flagellate, those of another are station- 
ary. This dimorphism is so great in the Aggregatae that filiform spermato- 
zoids occur, as in the Coccidia. After the formation of the gametes the 
movements of the residual body bring about the expulsion of the pseudo- 
navicelke; and in many Gregarines sporoducts are present for their 
escape. With repeated formation of a residual body, the contents of the 
pseudonavicella divides into (usually eight) sporozoites or falciform spores, 
which must leave the spores and pass anew into the tissue cells in order to 
form gregarines. This escape of the sporozoites depends upon entrance 
into the proper host. Often the transformation of the contents of the 
cysts into pseudonavicellce takes place when the cysts have left the original 
host. 

Best known are the Monocystis tenax of the spermatheca of earthworms, and 
Clepsidrina blattarum of the cockroach. The American species have scarcely 
been touched. 

Order II. Coccidiae. 

Of all Sporozoa the gregarines are nearest the Coccidiae, which are also cell 
parasites with a single nucleus, but without either cell membrane or division into 
protomerite and deutomerite. Best known is Eimeria stieda; (also called 
Coccidium cuniculi and oviforme), parasitic in the liver and intestinal epithelium 
of mammals. In the progamic development the fully grown parasite (fig. 146, 
2) divides inside the infected cell into many cells (3, 4, 8); these separate, infect 
other cells and begin growth and division anew (autoinfection). After this is 
repeated several times fertilization appears (5), some parasites giving rise to 
macrogametes, others by division forming small, actively swimming micro- 
gametes with one or two flagella. The fertilized macrogamete or zygote (6, 7, 
9, 10) encysts, passes out and serves to infect a new animal. Beginning earlier 
or later, but only concluded in a new host, the contents of the cyst divide into 
several fin Eimeria, four) sporoblast-containing spores. Each spore (7, n) 
forms one or several (Eimeria, two) sporozoites, a residual body being left 
behind (r). Eimeria stieda produces cheesy granules in the liver of mammals. 
It is common in rabbits, rare in man. In cattle it is the cause of red dysentery. 

Order III. Haemosporida. 

The Haemosporida are very similar in structure and development to the 
Coccidia. They live in blood corpuscles, and on this account and from some 
analogies not sufficiently understood, they are regarded as related to the Try- 
panosomes. The Haemamcebal forms parasitic in man cause malaria, there 
being in these a progamic reproduction with autoinfection and a metagamic 
in which the disease is transferred to another host. The parasites in the blood 
corpuscles (fig. 147, 1-3) grow and divide (daisy form, 2), characterized by 
little accumulations of pigment derived from the haemoglobin of the corpuscle. 
These division products are set free by a breaking down of the corpuscle 
(period of chill) and infect other corpuscles. This autoinfection can continue 
a long time, until the Haemamoebae in the corpuscles grow, without dividing, to 
'half moons' (4); these either become round and form macrogametes (5) or divide 
into eight microgametes (6). The conjugation of these seems only to take place 


III. SPOROZOA: ILEMOSPIIORIDA 


189 


FIG. 146. 1-7. Developmentof Coccidium schubergi (after Schaudinn). i, entrance 
of sporozoites in cell; 2, its growth; 3, nuclear multiplication; 4, division into mero- 
zoites; 5, macro- and microgametes; 6, zygote divided into four sporozoiles. 8-u, 
Emeria stiedce (after Wasiele\vsky und Mctzner). 8, autoinfection (progamic increase) ; 
9, formation of sporobUsts, 10, change of spores into sporozoites; u, spore with two 
sporozoites, more enlarged; c, z, sporozoite; e, epithelial cell; k, n, nucleus; mi. micro- 
gamete; o, macrogamete; r residual body; sp, spore; sp', sporoblasts. 


FIG. 147. Development of Plasmodium pracox (pernicious malaria) (after 
Grassi). i, blood corpuscle with ncsvly t-nti-red Ha-mama-ba; 2, multiplication of 
parasite- 3 young before the breaking down of corpuscle; 4-5, formation of ma< i 
metes; 6 formation of microgametes; 7, fertilization; 8, fertili/.ed motile macro.ua m 
(ookinet)-o digestive tract of mosquito, in front salivary glands, stomach coverec 
encysted parasites; 10-12, division of cyst; 10, formation of sporoblast 
formation of sporozoites from sporoblast with the residual body; i.;. part 
of mosquito infected with sporozoites. All figures except 9 greatly enla 


I'.H) 


PROTOZOA 


when the gametes are taken into the digestive tract of a blood-sucking mosquito. 
The fertilized macrogamete, the ookinete, wanders into the intestinal wall, en- 
larges enormously, encysts and produces numerous naked sporoblasts. Each 
sporoblast gives rise to numerous sporozoites (u, 12) which wander into the 
salivary glands of the mosquito (13) and are transferred to the blood of man by 
the bite of the insect. For the transfer of human malaria apparently only 
mosquitos of the genus Anopheles will serve, not the more common Citlex. 
Since a temperature above 20 C. (68 F.) is best for the development of mos- 
quitos, and water is necessary for their development, the prevalence of malaria 
in warm climates is easily understood. The different kinds of malaria are caused 
by different parasites, the quartan fever being caused by Plasmodium (Hccma- 
mceba malaria, pernicious malaria by P. prcecox. Allied to the Haemamcebas 
and possibly also to the Trypanosomes, is Babesia (Piroplasma) bigemina, the 
cause of Texas fever in cattle. The tick, Boophilus bovis, serves as the interme- 
diate host, the parasites being passed by the eggs ('inherited') to the next genera- 
tion. Babesia bovis, intermediate host Ixodes rediwius, causes haemoglobinuria 
in cattle. 

Order IV. Myxosporida. 

The Myxosporida (fig. 148) are mostly large (sometimes visible to the naked 
eye) and occur especially in fish and arthropods. When they occur in hollow 

organs they are naked and have pseudopodia, 
but in parenchymatous organs like the heart, 
liver, brain, kidney, etc., they are usually en- 
closed in a membrane, and here they produce 
the greatest injury. At first binucleate, they 
soon become polynucleate, and apparently they 
can reproduce by fission. Even before the 
growth is ended they begin the process of sporu- 
lation, hence the name 'neosporida.' Repro- 
ductive bodies with one or two nuclei, the 
anlagen of the pansporoblasts, are differentiated 
in the protoplasm. In the best known forms 
each pansporoblast gives rise to two spores. 
By division there arise in all fourteen nuclei, 
two of which (fig. 148, r), with the surrounding 
protoplasm, form the envelope of the pansporo- 
blast. The others separate into two groups of 
six each. One pair in each group with their 
protoplasm form an 'amoeboid germ;' they ap- 
pear to be separated from each other early, but, 
though long separated, they at last unite (II, 
III, g), a case of caryogamy. Two other nuclei 
and protoplasm form the two-valved spore case, 
and the remaining pair furnish the 'pole cap- 
sules,' these being oval, and containing threads 
which under proper conditions, are protruded 
(III), the whole resembling a ccelenterate nettle 
cell. The threads are for attaching the 'psoro- 
sperms' (as the spores were formerly called). 


FIG. 148. Development of 
Myxobolus pfeifferi, schematized 
(after Keisselitz). 7, pansporo- 
blast with envelope and residual 
nuclei, r, divided into two sporo- 
blasts; II, sporoblast developing 
into spores; s, envelope cells; p, 
pole cells with pole capsule; g, 
amceboid germs with two nuclei ; 
III, developed spore with ex- 
truded threads of the pole cap- 
sule, both nuclei of the amceboid 
germs fused. 


The amceboid germs are set free, as experi- 
ments on fishes show, by the digestive fluids, 

when they crawl into the tissues of the host. The number of pole capsules and 

of spores differs with the species. 


IV. CILIATA 


191 


The Myxosporida cause serious epidemics among fishes. The pebrine of 
the silkworm (the eggs are also infected) is caused by Nosema (Glugea) bom- 
bycis. Khinosporidium hominis, a parasite of the nasal mucous membrane of 
man in the tropics, is nearly related to the Myxosporidia. 

Order V. Sarcosporidia. 

The Sarcosporida (fig. 149) also called Rainey's or Miescher's corpuscles- 
occur in the voluntary muscles of vertebrates, especially mammals. They are 
oval cysts lying in sarcolemma sacs between the fibrillae. They have a cyst, 
the wall of which is radially striped, and inside this, in the ripe condition, are 
snores, imbedded in a stroma, each spore containing numerous reniform or 
falciform sporozoites. Sarcocystis miescheriana in muscles of pig; S. nmris in 
the mouse; 5. lindemanni rare in human muscle. 

At the end of the Sporozoa may be mentioned some much disputed bodies 
of very minute size, which are found in several infectious diseases (variola, 
trachoma, hydrophobia, etc.) and are regarded as their cause. They have been 
united under the common head of CHLAMYDOZOA. 


.It 


1. 


,': -: nk 

' '. -|- k 
-o 

na 


FIG. 149. FIG. 150. 

FIG. 149. Sarcocystis miescheriana, from diaphragm of pig (after Butschli). 65, 
cyst; sp, spheres of spores. 

Fig. 150. Paramcecium aurcl la in division; 2, separation of cytostome of new indi- 
vidual from old cytostome, at an earlier stage ; 3, P. camlntum, flattened and schematic; 
cv, contractile vacuole, expanded and contracted; k, nucleus; na, nn' food vacuole and 
one forming; nk, micronucleus; o, cytostome; /, t', trichocysts, t', discharged. 

Class IV. Ciliata. 

The Ciliata rival the Rhizopoda in numbers and variety of form. 
They are so complicated in structure that they were long he'd as multicel- 
lular. The form is definite for the species; and in the 'ametabolous' 
forms is unalterable, the 'metabola' can be temporarily pressed out of shape 
in passing through a narrow space. This constancy of form is due to a cu- 


192 PROTOZOA 

tide on the outside of the body, which in the 'ametabola' is firm; in the 
others very flexible. The cuticle is covered with cilia small vibrating 
processes which move together, and serve not only as organs of loco- 
motion, but by creating vortices in the water bring food to the organism. 
They furnish the most important characteristic of the class (fig. 150). 

The presence of a cuticle necessitates a cytostome, except in the para- 
sitic species, since food particles cannot be taken in at every point. At 
the cytostome the cuticle with its cilia forms a funnel-like food tube (cyto- 
pharynx) into the protoplasm. At the bottom the cuticle is interrupted 
so that water and protoplasm are in contact. By the action of the cilia 
food particles are taken into the cytopharynx and pressed against the 
protoplasm, forming a small enlargement which finally sinks into the 
substance as a, food vacuole (na) which, by the streaming of the protoplasm, 
is carried about in the body. The digestible portions are absorbed, 
and those not -capable of digestion are cast out of the body at a fixed 
point (cytopyge) usually not recognizable at other times (fig. 150.3). 
Contractile vacuoles (cv) are lacking only in parasites and marine species. 
They are constant in number and position, and frequently have afferent 
ducts which empty into the vacuole, the vacuole in turn forcing the fluid 
to the exterior. 

Trichocysts, nettle bodies, and muscular fibrillas occur in some species. 
Trichocysts are minute rods vertical to the surface in the cortical layer, which 
under the influence of reagents (chromic acid) elongate into threads penetrating 
the cuticula. To these have been ascribed defensive functions; others regard 
them as tactile structures. They have no connection with the cilia. Nettle 
bodies are extremely rare. Muscle fibres lie between ectosarc and cuticle, and 
cause quick convulsive motions of the animal. 

There are two nuclei physiologically unlike. The larger of these 
(nucleus of older writers, macronucleus) is a large oval, rod-like, or spiral 
body, deeply staining with microscopic stains, and surrounded with a 
membrane. It controls all the common vital functions of the animal 
(motion, feeding, etc.). Beside it or in a depression in it is the much 
smaller micronudeus (nucleolus or paranucleus of older authors) which 
stains less deeply. In all sexual processes it comes to the front and can 
be called the sexual nucleus. 

Multiplication of Ciliata occurs by binary fission (fig. 150); more 
rarely, and then only in the encysted condition, by division into numerous 
parts. Budding is known in the Peritricha and Suctoria. In fission first 
the micronucleus divides mitotically, and then the macronucleus separates 
by elongation and constriction. The old cytostome persists in the an- 
terior offspring, but often an outgrowth from it (2, o') passes into the 
posterior half and develops into a new mouth. 


FIG. 151. Conjugation in Param&cium. k, macronucleus: nk, micronucleus; o, cyto- 

stomes. 

I. Changes of micronucleus; left sickle stage, right spindle stage. 

II. Second division of micronucleus; into primary spindles (i, 5) and secondary 
spindles (2, 3, 4; 6, 7, 8). 

III. Degeneration of secondary spindles (2, 3, 4; 6, 7, 8); division of primary spindle 
nto male (im, $m) and female spindles (rz#, yu-'). 

IV. Exchange of male spindles nearly complete (fertilization), one end still in the 
parent animal, the other united with the female spindle, im, with 5^- and $>n with i-ui 1 ; 
macronucleus broken up. 

V. The cleavage spindle t formed by male and female spindles dividing into the 
secondary cleavage spindles t', t". 

VI. VII. End of conjugation. The secondary cleavage spindle dividing into the 
anlage of the new micronucleus (nk'\ and that of the new micronucleus, pt (placenta). 
The fragments of the old macronucleus begin to degenerate. 

Since P. caudatum shows the earlier and P. aurclia the later stages better, thcx- 
forms have been used, P. caudatum for I-III, P. aurclia for the rest. The differences 
consist in the existence of one micronucleus in P. caudatum, two in P. aurclia and 
that in the latter the nuclear degeneration begins in I. 


13 


194 


PROTOZOA 


The periods of fission are interrupted from time to time by the sexual 
process of conjugation, which will be described as it occurs in Paramcecium 
(fig. 151). Two individuals touch by their whole ventral surfaces, so 
that their cytostomes come together. In the neighborhood of the latter a 
bridge of protoplasm connects the two animals. Later the individuals 
separate. While these easily observable external processes are occurring 
there is a complete modification of the nuclear apparatus in the interior. 
The macronucleus increases in size, and breaks into small portions which 
disappear within the first week after copulation (probably by absorption), 
and give place to a new nucleus derived from the micronucleus. At the 
beginning of copulation the micronucleus becomes spindle-shaped, 
divides .and repeats the process, the result being the formation of four 
spindles in each animal, three of which break down, thus recalling the 
polar globules in the maturation of the egg (p. 133). The fourth or 
principal spindle places itself in the neighborhood of the cytostome at 
right angles to the surface and divides into two nuclei, the superficial 
being called the wandering or male nucleus, the deeper, the stationary or 

female nucleus. The male nuclei 
of the two copulating animals are 
exchanged, traversing the proto- 
plasmic bridge in their course (III). 
Both male and female nuclei usually 
become spindle-shaped, and the im- 
migrant male spindle fuses with the 
female spindle, forming a single 
spindle of division. At last, after 
processes which differ in the various 
genera, the division spindle pro- 
duces (usually by indirect means) 
two nuclei, one of which becomes 
the new macronucleus, the other 
the new micronucleus. 

In a comparison of the fertiliza- 
tion of the Metazoa, the female 
nucleus corresponds to the egg 
nucleus, the male nucleus to that of the spermatozoa. As the fusion 
of egg and sperm nuclei forms a segmentation nucleus, so here the divi- 
sion nucleus is formed in a similar manner. As the egg cell through 
fertilization acquires the capacity not only to produce sex cells but 
somatic cells cells which carry on the common functions of the body 
the fertilized micronucleus forms not only the new micronucleus, but 


FIG. 152. Epistylis umbellaria (after 
Greeff). Part of a colony in 'bud-like' 
conjugation; r, microspores arising by 
division; k, microspore conjugating with 
a macrospore. 


IV. CILIATA: HOLOTRICHA 


19.5 


also the macronucleus which controls the body processes, and hence is 
the somatic nucleus. In other words, fertilization in the Ciliates leads to 
a complete new formation of the nucleus and thus to a new organization 
of the organism. 

In most Ciliata the conjugating individuals are similar, the fertiliza- 
tion is mutual, and the individuals separate later. In the Peritricha 
(mostly sessile forms, fig. 152), on the contrary, the resemblance to 
fertilization in the Metazoa is strengthened in that there is a sexual differ- 
entiation and a permanent fusion of the con- 
jugating individuals. Some animals the 
macrogametes retain their size and sessile 
habits; others by rapid division produce 
groups of markedly smaller microgametes. 
The latter separate and fuse completely 
with the macrogametes. The nuclear 
phenomena are much the same as' with 
Paramoscium, allowance being made for the 
permanence of the fusion. 


_, """ """ 


a... 


r. 


t 


FIG. 153. FIG. 154. 

FIG. 153. Stentor polvmorphus (after Stein), a, peristomial area; b, roof of 
hypostome; g, contractile vacuole; n, nucleus; o, cytostome; r, adoral ciliated spiral; /, 
hypostome (excavation for mouth). 

FIG. 154. Balantiaium coll (after Leuckart). 

Order I. Holotricha. 

The Holotricha are the most primitive Ciliates, since the cilia on all parts 
of the body are similar; being at most slightly stronger at one end of the body or 
inside of the cytostome. Best known are the species of Paramoecium* (\'\^. 150) 
occurring in stagnant water. Opalina ranarum* lives in the intestine of the frog. 
It lacks mouth, has numerous similar nuclei, no micronucleus and no conjuga- 
tion. The small encysted Opalina' pass out with the faeces, and are eaten by the 
tadpoles, which thus become infected. 


196 


PROTOZOA 


Order II. Heterotricha. 


Like the Holotricha the Heterotricha are everywhere ciliated, but they have 
a tract of stronger cilia, the adoral ciliated spiral, beginning at some distance 
from the cytostome and leading in a spiral course into the mouth. It consists 
of rows of cilia united into membranellce placed at right angles to the course of 
the spiral. In the Stentors* (fig. 153), the peristomial area, surrounded by 
the spiral, forms the broader end of the body, which tapers toward the other 
end, by which the animal may attach itself. Muscle fibres running lengthwise 
immediately under the cuticle produce energetic movements. Balantidiiim 
coli (fig. 154) appears in the large intestine of men ill with diarrhoea, it also occurs 
in swine without causing sickness. Other parasites of man are B. minutnm 
and Nyctotherus faba. 

Order III. Peritricha. 

The Peritricha have a broad peristome area around the cytostome; the oppo- 
site end has a corresponding pedal disc or is narrowed like a goblet and ends in a 
stalk (fig. 155). Only the adoral ciliated spiral is constant. It arises from the 
swollen margin T)f the peristomial area, and continues on the 'operculum,' a 


FIG. 155. Carehesium polypinum (after Butschli). Left, a single animal; right, 
three stages of division, cv, contractile vacuole; n, macronucleus; n', micronucleus ; 
Nv, food vacuoles; os, cytopharynx; per, peristome; 7-5, reservoir of contractile vacuole; 
urn, undulating membrane; vst~ vestibule; wk, ring on which a posterior circle of cilia 
may develop. 


disc which projects free from the peristomial area, but in contraction is 
close against it, the peristome lips folding over all. Besides, there may 


ciliated 

drawn close 

be a temporary or permanent circle of cilia near the hinder end. The nucleus 

is usually sausage-shaped, much bent, and with the small micronucleus in its 

hinder angle (fig. 155, n'). 

The VoRTiCELLiD.(figs. 152, 155), are attached by a long stalk which 
contains a slightly spiral muscle, dividing in the body into fine fibrillae which 
extend under the cuticle to the peristome. When the muscle in the stalk con- 
tracts it becomes coiled into a corkscrew spiral, drawing back the animal, and 


IV. CILIATA: HYPOTRICIIA, SUCTORIA 


197 


folding in the anterior end. Vorticrlla* is solitary; Carchesium* forms colonies 
with branched stalks; Zoothamnion,* colonies imbedded in a common jelly; 
Epistylis* (fig. 52), branched colonies with rigid stalks. 

The fantastic Ophryoscolex, Cycloposthinm, etc., are parasites in the stomach 
of ruminants. 

Order IV. Hypotricha. 

In this order the body is more or less flattened and ventral and dorsal sur- 
faces are differentiated. The back lacks cilia, but often bears spines and bristles. 
On the ventral side are several longitudinal rows of cilia, and also straight spines 
and hooked cirri composed of united cilia, of use in creeping. The cilia are 


$ 


w ~~ 


rif-/- , 


FIG. 156. FIG. 157. 

FIG. 156. Stylonychia wytilns (after Stein), a, anal hooks; b, ventral hooks; c, 
contractile vacuole; d, frontal ridge; g, canal leading to contractile vacuole; /, upper 
lip; n, nucleus with micronucleus; f>, adoral ciliated spiral; r, marginal cilia; s, caudal 
cilia; 5^, frontal spines; z, anus (cytopyge). 

FIG. 157. Division of Stylonychia invlilus (after Stein), c, c', contractile vacuoles 
of the two individuals; n, nucleus and micronucleus; />, />', adoral ciliated spiral; r, r', 
marginal cilia; w, w', ciliated ridges. 

used in locomotion and producing vortices which bring food. The macro- 
nucleus is often divided into two oval bodies connected by a thread; the micro- 
nuclei vary in number from 2 to 4 in the same species. These are the best forms 
for studying the micronuclei. Stylonychia* (figs. 156, 157). 

Order V. Suctoria (Acinetaria). 

The Suctoria differ from other Infusoria in the absence of cilia from the 
adult and consequently have no means of locomotion. They are fixed to some 
support either by the base or by a slender stalk. The body is usually spherical 


198 


PROTOZOA 


and is covered with a cuticle, which in Acineta is produced into a cup-like lorica. 
There is no mouth, but in its place tentacles, very fine tubes with contractile 
walls which begin in the protoplasm and protrude through the cuticle (fig. 158, 
F}. The Acinetaria kill other animals, especially infusoria, with their tentacles, 
and then suck the substance through these tubes. The contractile vacuole, 
rarely lacking, lies near the compact macronucleus; micronuclei are generally 
present. The ciliated young (fig. 158, E) are good swimmers. They arise 
either as buds from the surface of the mother (fig. 20) or as 'embryos' in her 
interior. This latter condition is only a modification of the other, part of the 


FIG. 158. Suctoria (after various writers). A, Dendrosoma; B, Rhyncheta; 
C, Ophryodendron; D, Tokophrya; E, ciliated young of Sphcerophrya; F, diagram of 
capitate and styliform tentacles arising from ectosarc and canals in entosarc. 

outer surface being pushed into the interior to form a brood cavity in which the 
embryos arise. After swimming for a while the young come to rest, lose the 
cilia, and develop the tentacles. Some species of Podophrya in fresh water, also 
Sphcerophrya, parasitic in Infusoria. Acineta and Podophrya gemmipara (fig. 
20) are marine. 

Summary of Important Facts. 

1. The Protozoa are unicellular organisms without true organs or 
true tissues. 

2. All vital processes are accomplished by the protoplasm, digestion 
directly by its substance, locomotion and the taking of food by means of 
protoplasmic processes (pseudopodia) or by appendages (cilia and 
flagella). 

3. Excretion takes place by special accumulations of fluid, the con- 
tractile vacuoles. 

4. Reproduction is by budding or by fission. At intervals there is a 
true fertilization (caryogamy) sharply distinct from mere fusion of plasma 
(plasmogamy). Fertilization may be accomplished by a permanent 
fusion (copulation) or a transitory union (conjugation) ; it may be isogamic, 
anisogamic or autogamic. 

5. Protozoa are aquatic, a few living in moist earth; they can only exist 

in dry air, surrounded by a capsule (encysted) which prevents desiccation. 

6. Since encysted Protozoa are easily carried by the wind, the occur- 


PROTOZOA: SUMMARY OF IMPORTANT FACTS 109 

rence of these animals in water which originally contained none is easily 
explained. 

7. The mode of locomotion serves for division of the Protozoa into 
the classes Rhizopoda, Flagellata, Ciliata, and Sporozoa. 

8. The RHIZOPODA have temporary protoplasmic processes, the 
pseudopodia. 

9. The Rhizopoda are subdivided into Monera, Lobosa, Heliozoa, 
Radiolaria, Foraminifera, and Mycetozoa. 

10. The Lobosa and Monera have no definite shape. The Lobosa 
have a nucleus, the Monera are anucleate. 

11. Heliozoa and Radiolaria are spherical and have fine radiating 
pseudopodia and frequently silicious skeletons. They are distinguished 
by a central capsule in the Radiolaria which is lacking in the Heliozoa. 

12. The Thalamophora (Foraminifera) have a shell, closed at one end, 
the other open for the extension of pseudopodia. The shell is chitinous 
or calcareous, one or several chambered, straight or spiral; the pseudo- 
podia are occasionally lobular, but usually filiform, branching and 
anastomosing. 

13. The Foraminifera are of great geological importance on account 
of their numbers and their shells, which have built and are still building 
extensive beds of rock (chalk, nummulitic limestone). The silicious 
skeletons of the Radiolaria are less important. 

14. Mycetozoa (Myxomycetes) are mostly enormous Amoebae with 
reticulate protoplasm (plasmodium). They form complex reproductive 
structures (sporangia), recalling those of the fungi. 

15. FLAGELLATA have one or a few long vibratile processes flagella 
which serve for locomotion and for the taking of food. 

16. The Autoflagellata have only flagella; they feed like plants by 
means of chlorophyl (Volvocinae) , or upon fluid food (parasites), or upon 
solid food, either by pseudopodia, by a mouth (cytostome), or by a collar. 

17. Several are parasitic in man (Trichomonas vaginalis, Lamblia 
intcsfinalis, and especially prominent Trypanosoma gambiense (cause of 
sleeping sickness). Perhaps Spirochccte pallida (cause of syphilis) 
belongs here. 

1 8. The Dinoflagellata have two kinds of flagella and usually an 
armor of cellulose. 

19. The Cystoflagellata have a gelatinous body enclosed in a firm 
membrane (Noctiluca). 

20. SPOROZOA are parasitic Protozoa, usually without organs of loco- 
motion or mouth. They take no solid food, but live by osmosis on tissue 
fluids. The encysted animals produce spores (beginning with fecunda- 


200 PROTOZOA 

tion and accompanied by a change of host). The spores divide into 
sporozoites. Multiplication without change of host (autoinfection) can 
occur. 

21. The Gregarinida are temporary or permanent parasites in cells. 
CoccidlcB, Htzmosporida (cause of malaria, parasitic in blood corpuscles). 

22. The Sarcosporida (Rainey's or Miescher's corpuscles of mamma- 
lian muscles) and Myxosporida (psorosperm capsules of fishes, psorosperm 
= spore) live in tissues or hollow organs. 

23. The CILIATA have numerous vibrating processes, the cilia, a 
cuticle, and hence fixed openings for the ingestion of food (cytostome) 
and for extrusion of indigestible matter (cytopyge). 

24. Of great interest is the occurrence of two kinds of nuclei, a func- 
tional macronucleus and a sexual micronucleus. 

25. In conjugation portions of the micronucleus are exchanged and 
accomplish impregnation. The macronucleus degenerates and is replaced 
by part of the fecundated micronucleus. 

26. The classification of the Ciliata is based on the structure and 
arrangement of the cilia. 

27. The Holotricha have similar cilia over the whole body. The 
Heterotricha have, besides the total ciliation, stronger cilia in the neigh- 
borhood of the mouth (adoral ciliary spiral). The Peritricha have only 
adoral ciliation. The Hypotricha have the ciliary spiral and rows of 
cilia and coalesced cilia on the ventral surface. The Suctoria have cilia 
only in the young, later they become attached and feed through suctorial 
tentacles. 

APPENDIX. 

According to the evolution theory one should expect forms between (he 
Protozoa and Metazoa. The CATALLACTA spheres of ciliated cells which in 
reproduction break up into single cells have been described as such. Other 
peculiar many-celled animals whose position in the system is difficult to decide 
a,re,Salinella salve, LohmaneUa catenula, the ORTHONECTIDA and the DICYEMIDA. 
The Orthonectida and Dicyemida have a many-celled ectoderm, enclosing a solid 
mass of cells in the Orthonectida, a single giant cell in the Dicyemida. Salwrlla 
and Lohmanella consist of a single layer of cells enclosing a central digestive 
space. Since the Dicyemida live as parasites in the nephridia of cephalopods, 
the Orthonectida in worms and echinoderms, it is possible that their low organi- 
zation is the result of degeneration. Trichoplax adhtrrens, formerly placed here, 
is discoid, consisting of two epithelial layers separated by gelatinous tissue. 
It has recently been shown to be the larva of a medusa, Eleutheria. 


PORIFKRA 201 

METAZOA. 

Excluding the Protozoa, all the phyla of the animal kingdom are 
included under the Metazoa, i.e., higher animals. The point of union is 
that they consist of numerous distinct cells, arranged in several layers. 
At least two layers are present; one the ectoderm bounding the 
body externally, and a second the entoderm lining the digestive tract. 
Between these two a third may occur, frequently separated by a body 
cavity into an outer or somatic layer forming part of the body wall, and an 
inner or splancJinic layer forming part of the intestinal wall. This middle 
layer is called mesoderm, no matter whether there be a body cavity or not. 

The multicellular condition allows a higher organization, which ap- 
pears in varying grades in the specialization of tissues and organs. Xo 
metazoan lacks a true sexual reproduction, that is one by sexual cells, but 
the possibility must not be overlooked that some species may have lost 
fertilization and may reproduce exclusively by unfertilized eggs in a par- 
thenogenetic manner. Many species, especially the lower worms and 
ccelenterates, also reproduce by budding and fission. 

The segmentation of the egg is characteristic of all Metazoa. The 
fecundated egg divides into numerous cells which, as blastomeres, remain 
united and form the germ. No Protozoan has a true segmentation, 
division producing new individuals which either separate completely or 
remain in slight connection as a colony. 

PHYLUM II. PORIFERA (SPONGIDA). 

The Porifera, or sponges, the most familiar representative of which is 
the bath sponge (Euspongia officinalis), are, with few exceptions, marine. 
In fresh water occur but a few species of Spongilla. The animals have 
no powers of locomotion, but are attached to stones or plants, along the 
shores or at depths up to 4000 fathoms. They form spherical masses, 
thin crusts, small cylinders, or upright branching forms. Frequently the 
shape varies so that there is no typical form. Striking motions are rare ; 
only with the microscope can one see the opening and closing of the pores 
and the currents of the gastrovascular system. 

The simplest sponges, the Ascons (fig. 159), are thin-walled sacs, 
fixed at one end, and with an opening, the osciilum (functional anus), 
at the other. The cavity of the sac, the 'stomach,' is a wide digestive 
cavity into which water bearing food enters through numerous small 
pores in the body wall. The basis of the body is a connective tissue 
permeated with branching cells (fig. 160) covered externally by a thin 


202 


PORIFERA 


layer of pavement epithelium which is easily destroyed. This epithelium 
(earlier called ectoderm) and the connective tissue (mesoderm) are now 
regarded as a common layer, 'mesectoderm,' since the pavement epithelium 
is often genetically only connective -tissue cells which have spread over 


FIG. 159. FIG. 160. 

FIG. 159. Ascon stage of Sycandra (after Maas). e, entoderm; m, mesectoderm; 
o, osculum; p, pores. 

FIG. 160. Section of wall of Sycandra raphanus (after Schulze). e, epithelium; 
en, collared flagellate cells; m, mesoderm with connective-tissue cells; o, eggs; st, 
calcareous spicules. 


<* d 


FIG. 161. Section of Plakiua (after F. E. Schulze). c, canals leading from ampullae 
to cloacal tubes; e, ampullae; d, afferent canals; o, osculum. 

the surface. On the other hand, there is a distinctly differentiated ento- 

j 

derm in the shape of a one-layered flagellate epithelium lining the stomach, 
the cells of which (en) recall the Choanoflagellata (p. 184), since they 
have collars surrounding the flagella. The taking of food is accom- 
plished by the collared cells, its distribution by the amoeboid cells. 


PORIFERA 


203 


Sponges of this simple ascon type are few. As a rule sponges are more 
massive and have a more complicated canal system (iigs. 161, 162). The 
first step towards complication is seen in the Sycon type, in which the gas- 
tral cavity consists of numerous radial chambers or ampulla: which alone 


FIG. 162. Section of cortex of Chondrillanucula, the skeleton omitted (after Schuize). 
c l , afferent canals; c 2 , efferent canals; g, ampullae; m, cloaca; o, osculum. 

contain the collared cells, while the central cavity, now called cloaca, is 
lined with pavement epithelium. By increase of mesoderm and corre- 
sponding thickening of the body wall the ampulla become separated from 
external and cloacal surfaces (Leucon type). They nevertheless retain 
their connection with both surfaces by means of cavities which may 


sSSfr 

A 


rf 


FIG. 163. FIG. 164. FIG. 165. 

FIG. 163. Surface view of dermal pores of Aplysina aerophoba (after Schuize). 
FIG. 164. Ascyssa acufera (after Haeckel). 
FIG. 165. Leucetta sagittata (after Haeckel). 

be lacunar (fig. 161) or consist of a system of canals. The canal system 
is double; one part is incurrent and leads from the dermal pores to the 
ampulke; the other or excurrent, from the ampulla- to the cloaca, the two 
being connected by the ampulke alone (fig. 162), the canals from the pores 


204 


PORIFERA 


uniting in trunks and these in turn branching to go to the ampullae. 
The excurrent canals also show a similar tree-like arrangement. Not 
infrequently extensive subdermal or subcloacal spaces occur. The 
relations may be more complicated by the development of several 

cloacae, or by the branching of the sponge (fig. 
164), while still further the branches may 
anastomose (fig. 165), giving rise to a netwoik. 
Sponges may reproduce asexually, small 
portions separating as buds and producing new 
animals (fig. 88). Usually sexual reproduction 
prevails. The eggs, which like the spermatozoa 
arise from mesoderm cells (fig. 160), undergo 
segmentation and leave the parent as flagellate 
larvae (fig. 166, A). At fixation a kind of gas- 
trulation takes place, the blastopore (B) closes, 
and the osculum, an entirely new formation, 
arises at the opposite pole. 


FIG. 166. Development 
of Sycandra raphainis (after 
Schulze). .4 blastula; B, 
gastrula at the moment of 
fixation; ek, ectomesoderm; 
en, entoderm. 


The sponges are frequently regarded as Coe- 
lenterata, but scarcely a single homology can l;e 
drawn between the two. The ccelenterate mouth 
is different from either pores or oscula. Indeed, 't 
is disputed whether the collared cells are entoderm. 
Most sponges possess a skeleton secreted by special 
mesoderm cells, and this skeleton affords the 
means, according as it is composed of calcic car- 
bonate or of silica, of dividing the sponges into two classes. Besides, there are 
two groups, Ceraospongiae and Myxospongiae, in which the skeleton is respec- 
tively of horny substance (spongin) or is lacking entirely. These seem to be 
descendants of the silicious forms. 

Order I. Calcispongiae. 

The calc sponges are exclusively marine and mostly live in shallow water. 
They are grayish or white in color, of small size, rarely exceeding an inch in 
length. The skeletal spicules usually project through the epithelium, forming 
silky crowns in the neighborhood of the osculum. One-, three-, and four-rayed 
spicules are recognized, these ground forms presenting by unequal development 
a great variety of shapes. 

Sub Order I. ASCONES. Thin porose walls and central 'stomach.' 
Leucosolenia* Sub Order II. SYCONES. Cloaca present surrounded by 
ampulL'e radially arranged. Grantia,* Sycon,* Sycandra * Sub Order III. 
LEUCONES. A complicated system of branching canals in thick walls 
connects the ampullae with outer surface and cloacal cavity. Leucetta, Leucortis. 

Order II. Silicispongiae. 

The siliceous sponges are richest in species and occur at all depths of the 
sea, being frequently noticeable from their size and bright colors. They are 
subdivided into Triaxonia and Tetraxonia. In the Triaxonia the spicules 
composing the skeleton appearing as if of spun glass (hence Hyalospongia, or 


SILICISPONGLE 


205 


glass sponges) have three crossed axes (six threads radiating from a common 
point) hence Hexactinellidte. The mesoderm is scanty and in consequence 
the canals are loose-meshed, lacunar spaces and the ampullae large and barrel- 
formed. In the Tetraxonia the mesoderm is usually abundant and the canal 
system well developed. The four-axial spicules of the Tetractinellidae must be 
regarded as the fundamental skeletal type. From this are derived the compact 
frameworks of the Lithistidas and the monaxial spicules of the Monactinellidae. 

In both groups the spicules may be united by secondary deposits of silica to 
an extensive framework; or the union is affected by spongin, which, if the spicules 
disappear, forms the whole skeleton (horny sponges); or, as in slime-sponges, 
the whole skeleton may be lost. 

Sub Order I. TRIAXONIA. HEXACTINELLID^;, chiefly deep teas; 
Eiiplectella aspergillum, Venus' flower-basket. Hyahmema. Sub Order II. 
TETRAXONIA. Typical are the largely extinct LITHISTID^E (some genera 
Discodcnnia persist in deep seas) and TETRACTINELLIDJS: Geodia.* Near 
here apparently belongs Oscar ella* without a skeleton (MYXOSPONGIA). MON- 
ACTINELLID.E, spicules united by spongin (Cornacuspongia); can even be 
entirely replaced by that substance. Numerous marine forms, and the fresh- 
water SPONGlLLlDjE (Spongilla* Ephydatia*), usually colored green by algae. 
They are distinguished by formation of gemmulce or statoblasts. At times the 


FIG. 167. Skeletal structures of sponges (after Schulze and Maas). i, Horn 
fibre of bath sponge with spongiob lasts; 2-7, spicules of, 2, Esfxriti; 3, 4, ( 'orlicum; 5, 
Mysilla; 6, Tethya; 7, Farrea. 

protoplasm divides into round bodies, as large as the head of a pin and these 
become surrounded by a firm membrane often strengthened by collar-button-like 
spicules, the amphidiscs. These statoblasts survive times of freezing or drought 
On return of good conditions the contents escape and form small Spongillce, often 
utilizing the old skeleton. The spicules entirely disappear and nothing but the 
spongin fibres remain in the horny sponges, CERAOSPONGLE. The skeleton 
consists of an organic substance, spongin, which differs chemically from true 
horn keratin. This spongin is laid down by peculiar cells, the spongioblasts 
(fig. 167, i), and it always consists of concentric layers. The fibres interlace, 
branch, and unite. Best known are the bath sponges; Eitspimgia ofjinnalis,'"'- 
occurring in the Mediterranean, West Indies, Florida, and other seas in many 
varieties. Best are the Levant sponges (var. inoUissiiim). Sponges of com- 
merce consist only of the skeleton, the animal parts being washed away.- Less 
valuable are Euspongia zimocca and Hippospongia cquina,* the horse-sponge. 


206 CCELENTERATA 

Summary of Important Facts. 

1. The sponge body is largely a mass of connective tissue covered 
externally with pavement epithelium (mesectoderm) and penetrated by 
canals. 

2. An entoderm of collared flagellate cells occurs only in the ampullae 
or flagellate chambers which are intercalated between incurrent and ex- 
current canals (in ascons in the central cavity). 

3. The animals receive food through fine pores in the body wall; 
indigestible matter is cast out through one or more oscula. 

4. Since nerves, muscles, and sense organs are lacking or very weakly 
developed, only inconspicuous movements occur. 

5. Sponges are divided into Calcispongue and Silicispongiae according 
to the character of the skeleton. 

f 

PHYLUM III. CCELENTERATA (CNIDARIA). 

The ccelenterates, formerly called Zoophyta (plant-animals), were 
united by Cuvier with the Echinoderma to form the type Radiata, a union 
which Leuckart, the father of the name Ccelenterata, set aside because 
separate intestinal and body cavities occur in the Echinoderma, while in 
the Ccelenterata' there is but a single cavity in the body. Each name 
indicates certain important characters of the group. 

(1) The name Zoophyta referred to the general appearance. Most 
ccelenterates, like plants, are fixed and by incomplete budding form bush- 
like or mossy colonies. This resemblance is but superficial, for there is 
not the slightest doubt of the animal nature of any ccelenterate. The 
name therefore does not imply that these are doubtful forms on the border 
between plants and animals. Besides, there are free-moving forms which 
swim with great ease. 

(2) Most Coelenterata are radially symmetrical. There is a main 
body axis, one end of which passes through the mouth and the other 
through the blind end of the digestive tract, and the organs of the body are 
radially arranged around this so that the body may be divided into 
symmetrical halves by numerous planes. In the higher Ccelenterata this 
may be replaced by a biradial symmetry or even by bilaterality (Cteno- 
phora, many Anthozoa). 

(3) The term Coelenterata is given because these animals contain a 
single continuous ca'lenteron or gastrovascular cavity. In its simplest 
form this is a wide-mouthed sac into which food passes for digestion. 
The single opening into it serves for both mouth and anus; the sac itself is 
the alimentary tract. Frequently lateral diverticula or branched canals 


CGELENTERATA 


20- 


are given off from the central sac which distribute the nourishment to the 
peripheral parts of the body, and thus functionally replace the vascular 
system of higher forms. Since this gastrovascular system is primarily for 
nourishment, it is not a body cavity and one cannot say that the ccelen- 
terates are stomachless. On the other hand, the term 'ccelentcron,' that 
is, a cavity at once gastric and ccelomic (p. 148), is perfectly defensible, 
since in many higher animals which possess a true body cavity (coelom) 
this arises in development as diverticula from the primitive stomach 
(enteron) . Since such diverticula occur in ccelenterates without becoming 
independent, one can say that the gastrovascular system consists not only 
of intestinal portions but, in potent; a, of the ccelom as well. 

To even a superficial observation the Ccelenterata are more clearly 
animals than are the sponges. The single animals, though often united 
in colonies and fixed to some support, are capable of quick and energetic 
motion. These movements are most striking in the tentacles long 
tactile processes in the neighborhood of the mouth, which feel for food, 
grasp it, and convey it to the mouth. The means of killing the prey are the 
cnidce (whence the name Cnidaria for the phylum), nematocysts, or nettle 
cells (fig. 1 68). These structures, of great systematic importance, are oval 
or elongate vesicles with fluid con- 
tents and firm membrane. Each is 
drawn out at one end into a long 
thread-like tube (hence an additional 
name, thread cells). In the resting 
stage the thread is spirally coiled 
inside the cell. On stimulation the 
thread is quickly extended ('explosion 
of cell') and produces a wound into 
which passes the irritating fluid con- 
tents. Some ccelenterates (e.g., 
Physalid) can produce in this way very painful nettling even in man. 
The nettle capsule arises as a plasma product inside a cell. When 
fully developed the nettle cell extends to the surface and ends with a 
tactile process (cnidocill) which, upon contact, stimulates the protoplasm 
and causes the explosion, the thread being everted like the finger of a 
glove. The cell itself is frequently enclosed by a muscular sheath or a 
network of muscle fibres. 

Among the coelenterates both sexual and asexual reproduction may 
occur, the latter usually by budding, more rarely by division. Sexual and 
asexual reproduction can be combined in the same species, producing an 
alternation of generations. 


FIG. 1 68. Nettle cells of Coelen- 
terata (after Hertwig, Lendenfcld, ami 
Hamann). 


208 CCELEXTERATA 

In comparison with the sponges the Ccelenterata may be called epi- 
thelial organisms. A mesoderm (mesogla-a) may be entirely lacking or 
may have but a subordinate development. The ectoderm and entoderm, 
on the other hand, are the important tissues producing muscles, nerves, 
sense organs, sexual products and cnidae. Hence the group is often called 
Diploblastica two-layered animals. 

Class I. Hydrozoa (Hydromedusae) . 

According to varying standpoints the Hydrozoa can be placed either 
higher or lower than the Anthozoa in the system, since in the former group 
two forms frequently occur in the life history, one agreeing well in struc- 
ture with the Anthozoa, the other standing on a higher grade. The first is 
the sessile and usually colonial polyp, the second the free-swimming medusa, 
well provided with sense organs. These are usually related to each other 
by an alternation of generations. The polyp is asexual and by budding 
produces medusae;. the medusa, on the other hand, is the sexual stage, and 
from its eggs polyps arise. 

The polyp of the Hydrozoa is the Jiydropolyp, forming an important 
archetype from which all other conditions medusas, scyphopolyp, and 
coral polyp may be derived. Our best example of this is the fresh-water 
Hydra. The body (fig. 169) is a sac, the closed end of which, the pedal 
disc, is used for attachment. The other end bears the mouth which leads 
to the gastrovascular (digestive) cavity. Around the mouth is a circle of 
tentacles used in capturing food. These are outgrowths of the body wall; 
the circle dividing the body into a peristome inside the circle and a column 
constituting the rest of the outer wall. 

Hydra has but two body layers (fig. 170), an entoderm of flagellate cells 
lining the gastrovascular space, and the ectoderm covering the outer sur- 
face. Between the two is the supporting layer (mesoglcea), a membrane 
without cells and hence not a body layer. Both layers consist of epithelial 
muscular cells (cf. p. Si), the basal ends of which are produced into 
smooth muscle fibres, those of the ectoderm running lengthwise, those of 
the entoderm around the body. The ectoderm further contains ganglion, 
nettle and sex cells. The nettle cells on the tentacles are crowded into 
small ridges or ' batteries.' The sex cells (at certain times) produce swell- 
ings on the column; a circle of male swellings close beneath the tentacles, 
the female cells farther down the column (fig. 169). Individuals reprodu- 
cing by budding are more common than the sexually mature (fig. 93). 
Small elevations appear on the column, enlarge, form tentacles, and at 
last a mouth, after which they may separate from the parent. 


HYDROZOA 


209 


In the sea are numerous hydroid polyps which, while agreeing in the 
main with Hydra, are distinguished from it in two important respects: 
(i) they do not directly produce sexual organs; (2) they reproduce asexu- 
ally, and by incomplete budding form persistent colonies. In this a series 
of parts have arisen which require special designations (fig. 171). The 
separate animals, liydrant/is, are connected by a system of tubes, the cocno- 
sarc, which, like the hydranths, consist of ectoderm, entoderm, and 
mesoglcea, and since the gastrovascular space continues in them, these 
distribute food throughout the colony. The ccenosarc may creep over 


en s ek c 

FIG. 169. FIG. 170. 

FIG. 169. -Hydra viridis* testes above; ovarian enlargement and escaping egg 
below. 

FIG. 170. Body layers of Hydra (after Schulze, from Hatschek). c, cuticula; en, 
nettle cells; ek, ectoderm; en, entoderm; s, supporting layer. 

some support (stone, alga, snail-shell, etc.) and form a network, the 
hydrorhiza, or it may stand erect and free, forming a Jiydrocaulus. Usually 
both hydrorhiza and hydrocaulus occur in the same colony. 

Usually the colony is strengthened and protected by the perisarc , a 
cuticular secretion of the ectoderm. In some (fig. 172) the perisarc stops 
at the base of the hydranth; in others (fig. 173) it expands distally into a 
wide-mouthed bell, the hydrot/ieca, into which the hydranth may retract. 
In rare cases this perisarc may be greatly increased and calcified, forming 
large coral-like masses with openings from which the hydranths may 
protrude (fig. 174). 

14 


210 


CCELENTERATA 


FIG. 171. Campanularia Johnston! (after Allman). a, hydranth with hydro- 
theca; b, retracted"; d, hydrocaulus; /, gonotheca, with blastostyle and medusa buds; 
g, free medusa. The hydrorhiza is shown as the creeping portion from which the 
hydrocauli and gonothecse arise. 


FIG. 172. Section of Eudendrium ramosum. ek, ectoderm; en, entoderm; p, perisarc; 

s, supporting layer. 


HYDROZOA 


I'll 


The lack of sexual organs, which distinguishes most marine species 
from Hydra, is due to the fact that sexual individuals of special form are 
produced from the colony by budding. These, the medusae, may separate 
early from the colony and swim freely. A medusa (figs. 175, 176) has the 
form of a dome-like or disc-like bell and consists chiefly of very watery 
jelly. The bell or umbrella of the medusa is covered on both its surfaces- 
the concave or subumbrclla, the convex or exumbrella with ectodermal 
epithelium. At the margin of the bell the ectoderm is produced into a 


FIG. 173. FIG. 174. 

FlG. 173. Campanularla geniculata. ek, ectoderm; en, entoderm; />, perisarr, ex- 
panded around hydranth to a hydrotheca; s, supporting layer. 

FIG. 174. A bit of Millepara alcicornis*, enlarged (after Agassiz). 

two-layered sheet with a central opening, the velum or craspedon (fig. 175, 
B, v) of systematic importance, since these medusa? are often called ("ras- 
pedota. Tentacles (usually 4, 8, or multiples in number) also arise from 
the edge of the bell just outside the velum. 

Comparable to the tongue of the bell or the handle of the umbrella 
is the manubrium, hanging from the highest point of the subumbrella and 
bearing the mouth at its tip. It contains the chief digestive space, front 
which radial canals run on the subumbrellar surface to a ring canal in 
the margin of the umbrella. The radial canals are usually four in nuinl >er, 
but in some species the number is increased during growth even to a 
hundred or more. Manubrium and canals are lined by entoderm, which 
also extends into the tentacles and forms their axes. 


212 


CCELENTERATA 


All other important organs arise from the ectoderm. Gonads arise 
in many species (fig. 176 from the ectoderm of the manubrium; in others 


A 


Fir,. 175. Rhopalonema velatum. c, ring canal; e, exumbrella; , gonads; 7z, oto- 
cysts; m, stomach; n, nerve ring; o, mouth; s, subumbrella; t r , /', tentacles of first and 
second order; v, velum. 

from the same layer covering the subumbrellar surface of the radial 
canals (fig. 175), forming in either case conspicuous, often orange or red, 
thickenings. Longitudinal ectodermal muscles move the tentacles in a 


HYDROZOA 


213 


snaky fashion, whence the name medusa. Circular striped muscles 
run on the subumbrellar side of bell and velum, causing the characteristic 
motion. By their contraction the bell becomes more arched and narrowed, 
while the velum (which hangs down when at rest fig. 175, A) contracts 
like a diaphragm across the mouth of the bell (B). Since water is thus 
forced out through the opening the medusa is forced forward by the reac- 
tion. The circular muscles of the umbrella and velum are separated 


FIG. 176. Tiara pilcata (after Haeckel, from Hatschek). 

by two nerve rings, one subumbrclhv, the other cxumbrellar in position 
(fig. 177, n 1 , ir), the first supplying the muscle plexus, the other the 
sensory organs eyes of the simplest type, red pigment spots with or with- 
out a lens; and open or closed statocysts ('ears'). Tactile hairs are 
abundant on the tentacles. 

The statocysts are of two types, both beginning as open organs and reaching 
their highest development as closed vesicles. One type, the tentacular organs, 
occurs in the Trachymedusae (fig. 177, 1-4) the other, or velar organ, in the 
Leptomedusaj (5-6).' The tentacular organs are modified tentacles, the ento- 


214 


CGELENTERATA 


dermal axis forming the statoliths and the ectodermal covering the sense cells. 
In the yEginidae (i and 3) the club-like tentacles, seated on an auditory cushion, 
project freely into the water; in the Trachynemidae (2) they are partially trans- 
formed into vesicles, and in the Geryonidse they are completely enclosed and 
are sunk in the jelly of the bell. The velar organs of the Leptomedusae are 
placed on the subumbrellar surface of the velum. They may be either simple 
pits, or the mouths of the pits may close. In these both sense cells and stato- 
liths are ectodermal. Eyes and statocysts occur in different forms, a fact which 
formerly led to a division of medusae into ocellate and vesiculate groups. 


1. 


ea.- 


m 


$06. 


m. - 


FIG. 177. Statocysts (ear vesicles) of medusae. 1-4, tentacular statocysts of 
Trachymedusse; 5, 6, velar of Leptomedusae; 7, marginal body of Acraspedia. i and 3, 
auditory clubs of Aeginopsis; 2, same of Rhopalonema, with beginning of ear vesicle; 4, 
statocyst of Gerycmia; 5, of Aequoria; 6, auditory pit of Mitrocoma annir; 7, marginal 
body of Aurelia. ek, ectoderm; en, entoderm; g, mesogloea; h, auditory hairs; m, cir- 
cular muscles cut across; l , n~, upper and lower nerve ring; r, ring canal; s, statolith. 

While polyps and medusae apparently differ so greatly from each 
other, the medusae are only highly modified polyps adapted to a swimming 
life. The long axis of the polyp has been greatly shortened (fig. 178) 
and the cylindrical body developed into a disc; the mesoglcea of column 
and disc thickened to a thick layer of jelly; while manubrial cavity, radial 
and ring canals are remnants of the large gastrovascular space of the 
polyp, obliterated in the other regions by the pressure of the mesoglcea. 
To the parts thus formed only the velum and sense organs are added. 

This comparison of medusa with polyp is important in understanding 


HYDROZOA 


215 


the development, which usually includes an alternation of generations. 
From an egg of a medusa a small ciliated embryo (planula) escapes, 
which becomes attached, develops mouth and tentacles, and, by budding, 
produces a hydroid colony. This colony lacks sexual organs. By budding 


tr 


FIG. 178. Diagram of sections of (A} a polyp and (B) a medusa, ek, ectoderm; 
ek', of exumbrella; ek-, of subumbrella; ek 3 , of manubrium; el, endoderm (cathamnal) 
layer arising from obliteration of digestive space; en, entoderm; r, ring canal; s, sub- 
umbrella; t, tentacles; v, velum; x, supporting layer (gelatinous in E). 

it produces sexual individuals (medusae) which separate and swim away. 
Since polyp and medusae are morphologically comparable, before the 
escape of the medusae the colony is polymorphic, consisting of individuals 
(hydranths) which reproduce only asexually and of others which have 
taken over the sexual reproduction (medusae). Hence alternation of 


FIG. 179. Comparison of a medusa and a sporosac forig.X .1, fully developed 
medusa; B, medusa with the manubrium closed, still attached to the blastostyle; < ', 
medusa reduced to a simple manubrium (sporosac); D, last stage, eggs being produced 
in the body wall (Hydra). 

generations has arisen here from a division of labor or polymorphism 
of individuals originally of equivalent value, in which some individuals 
(the sexual) have separated and acquired a peculiar structure. 

While alternation of generation has arisen from polymorphism, it ran 
again produce it. This occurs when the medusae, instead of separating, 


216 CCELENTERATA 

remain permanently attached to the colony. They then degenerate into 
'sporosacs,' which always lack mouth, tentacles, and velum (fig. 179), often 
also radial and ring canals, so that at last there remains only the manubrium 
(spadix) and the sexual organs, the latter enveloped by the rudiments 
of the umbrella. Since medusae and sporosac replace each other in closely 
allied species, a common name, gonophore, has been applied to both. 

This developmental history may be modified in two ways: either the 
polypoid or the medusan generation may be suppressed. In the first 
case we have polyps which reproduce both sexually and asexually, in the 
other medusae whose eggs develop directly into other medusae. (A few 
medusae may bud new medusa?.) Thus we can have four conditions: 
(i) Polyps which produce sometimes asexually, sometimes sexually, but 
always polyps; (2) Medusae which always produce medusae; (3) Polyps and 
medusae in alternating generations; (4) Polyps and sessile medusae (sporo- 
sacs) united in a" polymorphic colony. 


FIG. 180. American Trachy- and Narcomedusre. A, Liriope scutigera (after 
Fewkes). B, Cunocantha octon^ria (after Brooks). 

The Hydrozoa are almost exclusively marine. The colonial forms occur 
mostly on rocky coasts down to a depth of 100 fathoms, but have been found 
in water 4000 fathoms deep. The medusas belong to the pelagic fauna. For a 
long time the only fresh-water species known belonged to the cosmopolitan 
genus Hydra, but more recently both hydroid and medusan forms have been 
found in various parts of the world. 

The Hydrozoa may be classified according to characters, derived either 
from the hydroid or the medusan stage. The former gives four groups: (i) 
HYDRARIA. Polyps with asexual and sexual reproduction; no persistent colonies, 
no perisarc, no gonophores (fig. 169). (2) TUBULARLY. Mostly colonial, 
with perisarc but without hydrothecae. Reproduction by gonophores (medusas 
or sporosacs, figs. 94, 172). (3) CAMPANULARI^;. Colonial, with perisarc and 
hydrotheca. Reproduction by gonophores arising in special perisarcal en- 
velopes, the gonotheca (figs. 171, 173). (4) HYDROCORALLINA. Colonial, 
with massive, calcified perisarc, resembling coral. Reproduction by sporosacs 
or rudimentary short-lived medusas (fig. 174). 

The characters derived from the medusas give five groups: (i) ANTHOMEDUS^E 
(Ocellatas). Gonads on the manubrium; no statocysts; eyes usually present; 
polyp generation present. (2) LEPTOMEDUS.E. Gonads on radial canals; 
usually velar statocysts; polyp generation present. (3) TRACHYMEDUS^:. 


I. HYDROZOA: HYDRARIA, CAMPANULARL 217 

Gonads on the radial canals; tentacular statocysts; develop directly to medusae 
(rig. 180, A). (4) NARCOMEDUS^;. Gonads on the manubrium or gastral 
pouches; tentacular statocysts; no polypoid stage (fig. 180, B.) (5) SIPHONO- 
PHORA. Polymorphic, free-swimming colonies of Anthomedusas, no polyp 
generation. 

As there are medusae without polyp stages and polyps without medusae, 
a natural system must take into account both these features. When the life 
histories are traced it is seen that the Anthomedusa; and the Tubulariae are 
connected by an alternation of generations, as in Leptomedusas and Campanu- 
lariaa. There are three groups Trachymedusae, Narcomeduste, and Sipho- 
nophora without a hydroid stage, and two in which the polyp plays the chief 
role, the medusa being rudimentary _in the Hydrocorallinae, lacking in the Hy- 
draria. The hydroid polyps are usually but a few millimeters or fractions of 
a millimeter in size, but the huge Monocaulis iwperator, of the deep seas, two 
yards in length, forms an exception. The colonies are usually only a few inches 
in extent. The medusae have bells varying between a millimeter and a few 
inches in diameter (sEquoria forskalea sixteen inches). 

Order I. Hydraria. 

Until recently only the cosmopolitan species of Hydra were known. During 
most of the year they reproduce by budding (fig. 93), only occasionally develop- 
ing gonads (fig. 169). The eggs remain in connexion with the mother during 
segmentation, and later form an embryonal shell. In this 'encysted stage' they 
can be distributed by wind or water birds. These animals formed the basis 
of the celebrated researches of Trembley on regeneration. He showed that 
small portions w 7 hich included both body layers could regenerate the whole 
animal. His experiments upon turning the animals inside out have not been 
fully confirmed; for in such cases the layers resume their normal positions. 
Hydra grisea* (fusca), brown; H. viridis,* green, from the presence of symbiotic 
algae. Protohydra rydcri* without tentacles. 

Order II. Hydrocorallinae. 

Exclusively marine, forming colonies of thousands of polyps whose cal- 
careous skeletons so resemble true corals that they were associated with them 
until the animals were studied. Millepora alcicornis* (fig. 174), stag-horn coral, 
in Florida. The rosy Stylasters in tropical seas. 

Order III. Tubulariae =Anthomedusae (Gymnoblastea). 

As a rule these colonial forms with perisarc but without hydrotheca produce 
anthomedusse, but there are forms like Clava* and Hydractinia* which have 
sporosacs. Indeed, Coryuwrplia* and Monocaulis* differ only by medusa 1 in 
the former and sporosacs in the latter. The medusae have the gonads on the 
manubrium, lack statoliths, and usually have a high-arched umbrella, and 
frequently eye spots. In the forms with alternation of generations different 
names are applied to the hydroid and medusan stages. 

Amon'T hydroids are Pennaria,* Syncoryne,* Endendri,* Tubularia,* among 
medusae Sarsia,* Turritopsis* Margclis* Nemopsis.* 

Order IV. Campanulariae = Leptomedusae (Calyptoblastea). 

These forms differ from the last in that they are always colonial and possess 
hydrothecaa, the medusae always being flattened Leptomedusae (p. 216). A 
peculiarity is the existence of gonothecas, closed perisarcal envelopes, inside 


218 


CCELENTERATA 


which the gonophores arise from the blastostyle, a specialized polyp, without 
mouth or tentacles (fig. 171, /). The typical Campanulariae produce medusae, 
while some forms, like Thaumantia* and sEquoria* have no hydroid stage; on 
the other hand, Sertularia* and Plunmlaria* have no medusa stage. Other 
common genera, Clytia* Diphasia* and Aglaophenia* among hydroids; 

Obelia* Tima* Rhegmatodes* among me- 
dusae. Possibly the fossil GRAPTOLITES be- 
sb long near here. Only the perisarc is known; 

this has hydrothecae, in which it is supposed 
the hydranths occurred. 

Order V. Trachymedusae. 

These medusas, mostly from warmer seas, 
have no hydroid stage. The characters are 
given on p. 216, Trackynema,* Liriope* (fig. 
1 80), and Campanella* in our waters, 
Geryonia, etc., in Europe. 


.fl 


Order VI. Narcomedusae. 

In addition to the characters on p. 217 
may be added that the tentacles arise from 
the outside, above the rim of the bell. 
Cunocantha* (fig. 180), Cunina*, sEgina. 
The larvae frequently live as parasites on 
other medusae, and they may be able to re- 
produce asexually, forming sacs in which 
new medusae are budded. 

Order VII. Siphonophora. 


t 


FIG. 181. Diagram of Siphono- 
phore (from Lang). A-H, groups 
of different individuals; ds, cover- 
ing scales; go, gonophores; hy, feed- 
ing polyps; p, 'feelers' (digestive); 
sb, float; sg, swimming bell (necto- 
calyx) ; st, stalk. 


The Siphonophora are among the 
most beautiful of pelagic animals, some 
transparent, some brightly colored. 
Each (fig. 181) consists of a colony of 
individuals springing from a common 
cosnosarcal tube which is strongly mus- 
cular and contains a central canal, lined 
with entoderm, by which the members 
of the colony receive their nourishment. 
At one end the tube is usually closed by 
a float of invaginated ectoderm, filled 
with air, the pneumataphore, which keeps 


the colony vertical in the water. 
The individuals, springing from the ccenosarcal axis, perform different 
functions and hence differ in structure. Close behind the float commonly 
come several swimming bells (nectocalyces) which retain only those medusan 
structures (bell, velum) necessary for swimming and those (ring and 


I. HYDROZOA: SIPHONOPHORA 


219 


radial canals) for the distribution of nourishment received from the 
common tube. Then come, scattered through the colony, the covering 
scales, for protection, firm gelatinous plates which have lost the ring canal, 
the muscles, and the bell shape of the medusa.'. Food is taken by wide- 
mouthed feeding tubes (liy) which may be compared to polyps (fig. 58) or 
the manubrium of a medusa. They digest the food by means of large 
masses of glands ('liver bands,') and send it by the central tube to all 
the members of the colony. At the base are long muscular tentacles 
(/) from which small lateral threads depend, each ending in a brightly 
colored swelling, the nettle head composed of large, closely packed nettle 
cells. These are the cause of the nettling, which in many species is so 


FIG. 182. American siphonophores. A, Nanomia cara (after A. Agassiz). B, 
Velella meridionalis (.after Fewkes). C, Diph yes pray a (after Fewkes). 

severe as to be feared by man. The 'feelers' (/>) recall mouthless polyps 
and manubria; they are very sensitive and mobile and, while tactile, ap- 
parently in some cases are digestive organs. Latest to develop in the 
colony are the sexiial bells. They are usually brightly colored and re- 
semble small mouthless Anth.omed.usae without tentacles. They but 
rarely (Chrysomitra) separate from the colony, but usually persist as 
more or less reduced sporosacs. From this it follows that the Siphono- 
phora afford fine examples of division of labor and of the consequent 
polymorphism of individuals. This can indeed be carried so far that 
many convey the impression of being individuals with a multiplicity of 
organs. The Siphonophora are all marine, and occur most abundantly 
in tropical seas. 

Sub Order I. PHYSOPHOR/E (Physonectse). Float present, small; next a 
large series of swimming bells; then the other members of the colony, f'liy- 
sophora, Agalmia, Nanomia* (fig. 182). Sub Order II. CALYCOPHOR^E 
(Calyconectae) . Float lacking; one or two large swimming bells; the other in- 


220 


CCELENTERATA 


dividuals in groups which frequently separate before becoming mature, were once 
regarded as distinct animals. Praya, Diphyes* (fig. 182), in warmer seas. 
Sub Order III. CYSTONECT^E. Float greatly enlarged; the ccenosarcal 
tube reduced, the individuals (no covering scales nor swimming bells) attached 
to under side of the float. Physalia,* Portuguese man-of-war, stings severely. 
Sub Order IV. DISCONANTILE. Float a flattened disc; manubrium pro- 
jects from centre of lower surface. Par pita,* disc. Velella* (fig. 182). 

Class II. Scyphozoa (Scyphomedusae) . 

The Scyphozoa parallel the Hydrozoa in frequently having an alter- 
nation of generations; the asexual generation being the scyphopolyp or 
scyphostoma, the sexual an acraspedote medusa. In contrast to the 
Hydrozoa the asexual stage plays a subordinate role; it is closely similar 
in all species, and can even be lost (Pelagia), while the medusae are always 
well developed and present great variety of form. 

The scyphostoma (figs. 183, 184) recalls Hydra, but has a small 
perisarcal cup around the aboral end. Internally there are four longi- 
tudinal folds projecting into the gastral cavity and extending from the 


FIG. 18: 


FIG. 184. 


FIG. 183. Scyphostoma of Aurelia aurita (from Korschelt-Heider). k, perisarc 
cup; pb, proboscis; s, stalk; t, gastral folds; tr, ectodermal funnels. 

FIG. 184. Section of Scyphostoma (from Hatschek). gr, gastric pouches; s, 
gastric septa; sm, muscles. 

margin of the mouth to the opposite pole. These septa or taniola 
(fig. 184, s) appear in cross-section as small folds of entoderm supported 
by a process of the supporting layer containing a muscle band extending 
down from the peristome (fig. 184). They are important morpholog- 
ically, since in budding they produce the gastral tentacles (phaccllcz) of 
the medusa?. Further, they are the- first appearance of the septal system, 
so strongly developed in the Anthozoa. 

The medusas are large (four inches to four feet or more in diameter) 
with a slightly arched umbrella, often of almost cartilaginous consistency. 


II. SCYPHOZOA 


221 


A knowledge of the development is necessary in order to understand 
the medusa. The young medusa (Ephyra stage, fig. 185) is eight lobcd, 
each lobe with a sensory pedicel in a notch at the tip. These lobes 
indicate eight radii, the four passing through the angles of the mouth 
being the perradii, the others the interradii, the adradii being between 
the lobes (fig. 186). In most species the adradial regions increase with 
growth, and at last form a circular margin to the bell, divided by the 
notches of the original lobes (fig. 187), tentacles occurring only in the 
adradial regions. The medusae differ 
externally from those of the Hydrozoa 
in the absence of a velum (hence acra- 
spedote). 

Instead of a nerve ring there are 
eight nerve centres connected with the 
sensory pedicels. Each pedicel is a 
modified tentacle (fig. 177, 7) its en- 
todermal axis furnishing a statolith at 
the end, and usually a simple eyespot. 

The gastrovascular system begins 
with a quadrate or X-shaped mouth 
(fig. 1 86). The angles of the mouth 
are usually produced into long curtain- 
like oral tentacles of use in the capture 
of food. The 'stomach,' which begins just inside the mouth, gives off 
four interradial pouches, the gastro genital pockets, each containing a 
group of small gastral tentacles (phacelhc}, and the plaited folds of the 
gonads, these being, in contrast to the Hydrozoa, of entodermal origin. 
In this the Scyphomedusoe show relationships to the Anthozoa. From 
the central digestive sac arise in the Ephyra stage (fig. 185) eight 
radial canals to the sensory pedicels, and most adult medusa? have these 
same pouches and eight others, adradial in position. In some this 
primitive arrangement is complicated by a network of tubes (fig. 186). 

In the species with an alternation of generations the egg produces a 
ciliated larva, the planula (fig. iSS) which attaches itself and develops 
into a scyphostoma. This scyphostoma is capable of terminal, and often 
of lateral, budding. The lateral buds always produce new scyphostoma', 
the terminal, medusa?. In the latter the scyphostoma develops into a 
strobila, becoming divided by circular constrictions into a series of saucer- 
like discs, the young jelly-fish. As the successive discs become ready 
they separate from the pile and swim away as epliyrcc. At lirst the 
ephyra^ (fig. 185) have only four gastral tentacles, parts of the gastral 


FIG. 185. Ephyra of Cotylorhi-,i 
(after Claus). gt, gastral tentacles 
(phacelke); rk, marginal (sensory) 
body. 


222 


CCELENTERATA 

/ / II 


II 


FIG. 186. Ulmaris prototypits (from Hatschek). /, radii of first order (perradii); 
77, radii of second order (interradii) ; /, marginal lobes; o, oral lobes (cut away on right 
side) ; t, tentacles (adradial) ; the gonads (right side) are interradial. 


< 


FiG. 187. Polydoniafrondosa* and one of its branching oral lobes, showing the closed 

grooves (.<;) (after Agassiz). 


II. SCYPHOZOA: DISCOMEDUS/E 


223 


septa of the scyphostoma (p. 220). Since the ephync differ markedly 
from the adult medusas and only gradually change into the sexual form, 
the alternation of generations is complicated by a metamorphosis. This 
metamorphosis persists in some cases (Pelagia noctilucd) where the alter- 
nation of generations is suppressed; the egg develops directly into an 
ephyra, which transforms into the adult jelly-fish. 


FIG. 188. Development of Aurelia aurita (from Hatschek). First row, growth 
of planula to scyphostoma; below, strobilation (separation of ephyra?): left, oral view 
of scyphostoma; right, two ephyra:. 

Order I. Discomedusae. 

The foregoing account applies, as a whole to only the Discomedusa?, the 
widest distributed and most abundant of the Scyphomedusje. The order is 
divided into two suborders, I. SEM.*;OSTOME.E, mouth X-shaped with long fringed 
and very mobile arms at the corners of the mouth. Aurclid flavidula* and 
Cyanea arctica* common in north Atlantic waters, the latter large, exceptionally 
seven feet in diameter; Pelagia* Ulmaris (fig. 186). (2) RHIZOSTOMF/K, four 
oral arms which branch dichotomously; the mouth and grooves on the arms 
closed by union of their edges so that many small stomata remain through which 
food is taken. Stomolophus* Polydonia* (fig. 187). 

Certain Scyphomedusas are distinguished from the Discomedusae. Some 
of these are inhabitants of the deep seas and only recently known; others ditlrr 
so from the Discomedusae that the relationship was not seen at first. These 
have in common the rathannua, four partitions, homologous to the ta-niolir of 
the scyphostoma, which bear the phacellse and divide the peripheral part <>t the 
gastral cavity in such a way that the gonads are separated into eight groups. 
The marginal bodies vary in three ways. 


224 


CCELENTERATA 


Order II. Stauromedusae. 

Best known are the LUCERNARLE (fig. 189) which lack marginal bodies, but 
usually have four small tentacles in their place, while the adradial regions are 
drawn out into arms, bearing bundles of tentacles. The aboral surface of the 
bell is produced into a stalk by which the animals are attached. The TESSE- 
RUXE (unknown in America) are free-swimming. 

Order III. Peromedusae. 

Free-swimming, cup-shaped medusae, with four interradial sense bodies; 
mostly from the high seas. Pericolpa, Periphylla in Gulf Stream. 

Order IV. Cubomedusae. 


Differ in the four perradial sense bodies, 
ment unknown. Charybdea (fig. 190). 


Tropical and subtropical; develop- 


FIG. 189.- Halyclystus auricularia* 
(after Clark). 


FIG. rgo. Char \bdea marsiipialis 
(from Hatschek). 


Order V. Coronata. 

A coronal furrow on the exumbrella; four to sixteen marginal sense bodies as 
in Discomedusae, but eight gonads and presence of cathamma. Some of these 
formerly regarded as Discomedusae (under the name of Cannostomeas), because 
of eight sense bodies. Nausithoe albida arises by terminal budding from a 
scyphostoma (Stephanoscyphus mirabilis) parasitic in sponges. Atolla. 

Class III. Anthozoa (Actinozoa). 

The Actinozoa, including the sea anemones, sea pens, and corals, 
are exclusively marine. With few exceptions they are sessile and usually 
form colonies, often of enormous size. In this as in appearance (fig. 192) 


III. AXTHOZOA 


225 


they resemble the hydroid polyps. They have a pedal disc, column, 
tentacles, and peristome with central mouth. They are distinguished 
by their greater structural differentiation. The Anthozoan polyp has a 
well-developed mesoglcea, this being a layer of connective tissue with 
numerous cells, giving the animals a tough fleshy consistency. Still 
more important are the oesophagus and septa; bearing mesenterial 
filaments and gonads. 

The mouth, in the centre of the peristome, is usually oval or slit-like. 
Hence there is a biradial symmetry for there is a sagittal axis (fig. 191, s,s) 


- m 


P 


FIG. 191. FIG. 192. 

FIG. 191. Antheomorpha elegans. s, s, sagittal plane. 

FIG. 192. Sagartia parasilica split lengthwise, a, acontia; c, septal canal; f, 
mesenterial filaments; g, gonads; m, sphincter muscle; o, oesophagus; p, peristome; r, 
septa of different orders; s, siphonoglyphe; ic, cut wall of column. 

passing in the long axis of the mouth, and a transverse axis at right angles 
to it. From the mouth the oesophagus hangs down into the body as a 
flattened tube and opens at its lower end into the wide gastro vascular cavity. 
This oesophagus is an inflected part of the peristome and hence lino I 
with ectoderm, and its lower end alone can be compared with the mouth 
of the hydrozoan (fig. 192). It usually bears at either end a specialized 
groove, the siphonoglyphe (s). 

The oesophagus is held in position by radial partitions, the septa (r), 
which stretch from base, column, and peristome to the oesophagus, dividing 
the peripheral part of the gastral space into small pockets, the radial 
chambers, connected below the end of the oesophagus with the central 
part. Above, these chambers continue into the tentacles. The tentacles 
therefore are outgrowths from the radial chambers and usually equal 
them in number. Besides the complete or primary septa which reach 

15 


226 


CCELENTERATA 


the oesophagus, there may be others which do not reach the oesophagus 
and belonging to secondary, tertiary or other series (fig. 194). 

The septa support a number of important organs: the mesenterial 
filaments, gonads, and muscles. The mesenterial filaments are thick 
strands of epithelium, rich in glands and nettle cells, fastened like a hem 
on the edge of the septa. Since they are much longer than the peristomial- 

pedal length of the septa, they cause 
these latter to wrinkle and fold, thus 
strikingly resembling the mesenteries 
of the mammals. They envelope the 
food and press it in, thus aiding the 
succeeding intracellular digestion. 
Lower down, in some species, the 
filaments become free and form long 
threads, acontia, rich in nettle cells 
which are protruded for defence, 
either through the mouth or pores 
(cinclides) in the column. The gonads 
only exceptionally hermaphroditic 
lie beside the mesenterial threads 
as thickenings of the septum (fig. 
192, g). The germ cells arise from 
the entoderm, but early migrate into 
the mesoglcea of the septum (193, 0). 
The eggs, when ripe, escape into the 
gastrovascular cavity. The young 
leave the parent at various stages of 
development, sometimes as planulae 
(fig. 197, A), sometimes as young with 
tentacles. 

The muscles are very important, 
morphologically. Muscles and nerves 

occur in both ectoderm and entoderm; but while the nerves are best 
developed in the ectoderm, especially around the mouth, and extend 
into the mesgolcea, the muscles of the ectoderm are weakly developed 
and are mostly confined to the peristome and the tentacles. The ento- 
dermal musculature is much stronger. Just outside of the tentacles is 
usually a strong circular (sphincter) muscle (m) which can close in the 
top of the column over the peristome. The septa also bear muscles, 
transverse on one side, longitudinal on the other, the latter producing 
ridges on the septa (fig. 193). 


FIG. 193. Section of septum of 
Edwardsia tuberculata. ek, ectoderm; 
en, entoderm; me, supporting layer; 
mf, septal muscle; o, ovary; v, mesen- 
terial filament. 


III. ANTHOZOA 


227 


In the Hexacoralla (fig. 194) the septa are in pairs, with the muscle ridges 
facing each other, except at the ends of the sagittal axis, where they face out- 
wards. These are called the directives. Since the septa occur in' pairs, two 
kinds of radial chambers occur, those between the pairs being called inler- 
septal, those between the two of a pair being intrascptal. At first all Hexac- 
tinians have six pairs of septa two pairs of directives, and four of lateral 
septa. With growth, septa of a secondary order may appear between these, 
giving twelve in all, then tertiary septa, the number of tentacles increasing 
with the septal chambers. The rule is not invariable, for some have modified 
the plan of six to four or ten, without altering the primitive condition. 

A 


B 
FIG. 194. FIG. 195. 

FIG. 194. Transverse section of actinian (Adamsia diaphana) AB, plane of sym- 
metry, a second lies at right angles. I-IV, septa of four orders. 

FIG. 195. Transverse section of an Octocorallan (Alcyonium}. x, siphonoglyphe; 
1-4, septa of one side, with their muscles on one side, symmetrical with those of the 
other side. 


In the Octocoralla only eight septa are developed. These are disposed 
equally on either side of the oesophagus and may have (most octocorallans) all 
their muscles towards one end (fig. 195) or (Edwardsia, fig. 196, IV) have one 
pair reversed. It is to be noted that hexactinians pass through an Edwardsia 
stage. In Cerlanthus new septa are always added at one end of the sagittal 
axis (fig. 196, II), while in the extinct Tetracoralla (I), so far as one may judge 
from the hard parts, the septa have an arrangement with four as the basis. 

Most Anthozoa reproduce by division or budding as well as by eggs. 
Occasionally the buds separate; usually they remain connected with the 
mother, forming colonies of hundreds or thousands of individuals, con- 
nected by a cccnosarc, consisting largely of mesogloea with a covering 


228 


CCELENTERATA 


of ectoderm and penetrated by a network of entodermal canals. On 
disturbance the polyps retract into the coenosarc. 

Colonial Anthozoa, with few exceptions, have a skeleton (coral), 
secreted by the ectoderm, consisting of calcic carbonate or of an organic 


FIG. 196. Arrangement of septa in various Actinozoa. I, Tetracoralla; II, Cerlan- 

thus; III, Octocoralla; IV, Edwardsia. 

horny substance, the two sometimes occurring together. The skeleton may 
be internal (axial) secreted by the coenosarc, or external (cortical), and 
formed by the polyps, repeating to a large extent their complicated struc- 
ture (figs. 199, 200). \i\Fimgia (mushroom corals) the cortical skeleton 


FIG. 197. Corallium rubrum, red coral (after Lacaze Duthiers). A, ciliated 
young; B, young colony; C, part of colony with polyps in extension (a) and contraction 
(c); d, crenosarc; A, greatly, B, C, slightly enlarged. 

consists only of a base, with radiating ridges (sclerosepta} on the side to- 
wards the flesh. These alternate with the septa (sarcosepta) of the polyp. 
In most forms there is, in addition, a cup (theca) in the column of the 
polyp, the sclerosepta extending inward from this. 


III. ANTHOZOA 


229 


The theca arises by a fusion of sclerosepta. If this fusion takes place some 
distance inside the peripheral ends of the sclerosepta, the distal ends of these 
project on the outer surface as costce. Still outside these may be a second cup, 
the epitheca. In the centre may occur a large calcareous column or several 
smaller ones, the columella. As the polyps grow they build the thcca? higher and 
higher and consequently draw out from the deeper 
portions, which may become cut off by horizontal 
partitions, the tabulce. Such tabulae occur in some 
Madreporaria, Octocorallans, and Millepores (p. 
217) which were formerly united in a group Tabu- 
latas. 

It was once thought that the coral was a cal- 
cified portion of the soft parts and hence that 
sclerosepta were hardened sarcosepta, etc. This 
has been disproved. The sclerosepta are formed 
in the radial chambers between the sarcosepta, 
and the theca inside and at some distance from 
the column, the outer surface of which secretes 
only the inconstant epitheca (fig. 199). From 
the above it would appear that the sclerosepta 
correspond in number to the sarcosepta, but this is not always the case. Thus 
the Helioporidae, which on the grounds of the skeleton were regarded as Hexa- 
coralla, are shown by the soft parts to be undoubted Octocoralla. 

By means of their skeletons the Anthozoa produce the well-known coral 
reefs. When the reef reaches the surface it produces an island, the most note- 


FIG. icj8. Sderophyllia 
margariticula Rafter Klunz- 
inger). 


FIG. 199. FIG. 200. 

FIG. 199. Diagrammatic section of the flesh and coral of a hexacorallan; above the 
line the section passes through the oesophagus, s; below the line it is lower down; r, 
directives; coral black. 

FIG. 200. Diagram of relations of soft parts to coral (after Pfurtscheller). Shows 
beginning sclerosepta and theca. 


worthy form being the atoll, a ring-like structure with a central lagoon. The 
origin of these atolls, as well as that of fringing and barrier reefs, was for a long 
time explained by Darwin's and Dana's theory of coral reefs. Later invcsiiga- 
tions, notably those of Mr. Agassiz, afford another explanation. 


230 


CCELENTERATA 


Order I. Tetracoralla (Rugosa). 

Extinct forms from the paleozoic rocks with the parts arranged in fours (fig. 
196, I). The present tendency is to regard them as modified Hexacoralla. 

Order II. Octocoralla (Alcyonaria) . 

These forms, which have eight single septa, are recognizable by their eight 
feathered tentacles (fig. 197). They occur in all seas from near the shore to great 
depths. In development there is a planula (fig. 201) in which the oesophagus 
arises as a solid ingrowth which becomes perforated later. The eight septa 
arise simultaneously. Usually colonies are formed by budding and a poly- 
morphism may occur, some individuals which have reduced septa and lack 
tentacles, taking in water for the colony. Many are phosphorescent. 


B 


V 


FIG. 201. FIG. 202. 

FIG. 201. Three stages in development of Renilla reniformis (after Wilson). 
A, cleavage of egg; B, planula; C, development of oesophagus; ec, ectoderm; en, ento- 
derm; ;, mesoglcea; o, oesophagus. 

FIG. 202. American sea-anemones. A, Edward sieUa sipunculoides (after Stimp- 
son). B, Bicidium parasiticum (after Verrill) . C, Bunodes stella (after Verrill). 

ALCYOXIID^; (Alcyonium*') , axial skeleton is lacking, the flesh contains 
numerous calcareous particles (sclerodcrmitcs). The sea pens, PENNATULID^:, 
have the basal part buried in the mud, the rest, expanded like a disc or feather, 
bears the polyps. An axial skeleton usually occurs in the stalk. Peimatnla* 
Renilla*. The GORGONIID.E (sea fans, sea whips) have an axis of more firm- 
ness, which may be calcareous, and the colony may branch and the branches 
anastomose. Here belongs, besides many tropical genera whose names end in 
'gorgia,' the precious coral (Corallium rubrum, fig. 197), the fishing for which at 
Naples amounts yearly to half a million dollars. TUBIPORDXE, organ-pipe 
corals. The HELIOPORHXE were long regarded as Hexacoralla because of their 
massive skeletons with six sclerosepta. The paleozoic Syringopora belongs 
near Tubipora, while the FAVOSITID^E resemble the Alcyoniidae. 

Order III. Hexacoralla (Zoantharia). 

The simple tubular tentacles are highly characteristic of the Hexacoralla, 
as is the arrangement of the paired septa in sixes as described above. Yet there 
are exceptions to this rule. On the one hand is Edwardsia* with sixteen or 


III. ANTHOZOA: HEXACORALLA 


231 


more tentacles and only eight septa (fig. 202), but which exhibits a condition 
through which the young actinians pass; on the other hand, in the Zoantharia, 
Cerianthiae, and Antipatharia the rule of six has undergone extensive modifi- 
cation. 

Sub Order I. ACTINARIA (Malacoderma). The sea-anemones are mostly 
solitary, without skeleton; with numerous septa and tentacles. They occur in 


FlG. 203. Astrangia dance*', five polyps 
in various stages of expansion. 


FlG. 204. Cceloria arabica (after 
Klunzinger). 


all seas from tide marks to the greatest depth. A few are free, but most are 
sessile. Metridium,* Bunodes* Sagarlia,* Bicidium* (parasitic on Cyanea), 
Halcampa*. ZOANTHE^E have two kinds of alternating mesenteries, individuals 
of the colonies usually incrusted with foreign matter. Epizoanthus* lives symbi- 
otically with hermit crabs (fig. 114). 

Sub Order II. ANTIPATHARIA. Six 
pairs of septa and six (Antipathcs) or twenty- 
four (Gerardia) simple tentacles; colony with 
a black horny axis and no calcareous skele- 
ton. Simulate the Gorgonids. 

Sub Order III. MADREPORARIA 
(Scleroderma). This group, the richest in 
species of any, is characterized by the great 
development of the skeleton. Theca, septa, 
and usually columella are present, and fre- 
quently costse as well. Solitary forms are 
few. Usually they form colonies, frequently 
of thousands of individuals, bound together 
by a ccenosarc extending over the surface of 
the coral. A colony arises from a single 
animal by continued fission or budding. 
When the division is not complete the ani- 
mals may form long series with numerous 
mouths but with the other parts united, the 
result being that the surface of the coral is 
marked by long winding grooves incompletely separated theca with sclcro- 
septa, as in the brain corals (fig. 204). The fossil Tetracoralla (p. 230) are 
now regarded as modified Hexacorallans. (i) The APOROSA, a compact skel- 
eton, the gastral canals running outside of the skeleton. Some, like Sclerophylla 
(fig. 198), are solitary. Others, like Oculina* branch, and still others form 


FlG. 205. Fai'ia carernosa (after 
Klunzinger). 


232 CCELEXTERATA 

compact masses. Astrangia dance (fig. 203), only coral in New England; Astrcea; 
brain corals (Ccvloria, fig. 204, Mairicina); Favia (fig. 205). (2) FUNGIACEA, 
or mushroom corals, no theca. Some colonial, others (Fungia) solitary. A 
sort of strobilation in development. (3) POROSA, with skeleton porous like a 
fine sponge. Madrepora* deer's-horn coral (fig. 206), Poritcs, Astroides. 


FIG. 206. Madrepora erythrcca (after Klunzinger). 

Class IV. Ctenophora. 

The Ctenophores excel all animals, even the medusa?, in transparency 
and delicacy of tissues; many are so soft that a strong current tears them, 
and no attempts to preserve them have been successful. The body is 
biradially symmetrical; i.e., is divided by both sagittal and transverse 
planes into symmetrical halves. Since the longitudinal axis is usually 
longer than the others, which are generally equal, the body is usually 
oval or pear-shaped. In Cesium the sagittal axis is greatly longer, giving 
the animal the form of a band, whence the name 'Venus girdle.' 

The bulk of the animal is composed of a soft jelly with connective- 
tissue cells, penetrated in every direction by polynucleate muscle cells 
(fig. 50) branched at their ends and apparently innervated by special 
nerve cells. On the outer surface is a layer of ectoderm, while in the in- 
terior is a system of branched entodermal canals. 

At the bottom of a depression (fig. 207, s) at the aboral pole is a thick- 
ened patch of ectoderm, the sense body, a typical statocyst (fig. 208). 
The thick sensory epithelium forms a shallow groove, strong hairs which 
rise from the edge of the groove arch over it, enclosing a space to be com- 
pared to an incomplete vesicle. In the centre is a spherical mass of stato- 
liths, supported on four bundles of S-shaped agglutinate cilia. From 
these bundles of cillia eight bands of thickened epithelium, at first in pairs 
(fig. 209, ws), later diverging, pass to the oral pole (fig. 207, ;-). These 
meridional bands (so called from their course) consist in part of ciliated 
epithelium, in part of the characteristic 'combs' which are the locomotor 


IV. CTENOPHORA 


FIG. 207. FIG. 

FIG. 207. Diagram of Hormophora, cut in two. /, tentacle;/-- 3 , root and sheath 
of tentacle; g, main perradial vessel which divides twice dichotomously to form the 
meridional vessels; m, stomach; mg, paragastric canals; f 1 - 4 , rows of combs overlying 
meridional canals; t, t l , funnel and funnel vessels; s, sense body. 

FIG. 207 A. Swimming plate and epithelial cushion (after Chun). 

to 


ws 


ws 


L/t?>V -A 

\Myjgfi 


sk 


FIG. 208. FIG. 2oq. 

FIG. 208. Section of sense body of Callianira. A. through the centre; B, excentric; d, 

roof of sensory groove;/, support of statoliths, o; p, pigment cell; sc, sensory cells. 

FIG. 2oq. Aboral pole of Callianira (from Lang). /, supports of statoliths, o; />/>, 

pole plate; sk, sense body; to, openings of gastral funnels; ws, ciliated bands. 


234 


CCELENTERATA 


organs, and which must be regarded r.s transverse rows of long agglutinated 
cilia. The combs (tig. 2oyA) arise from thick epithelial ridges, transverse 
to the meridional bands, and are so far apart that the free edges of one 
comb overlap the base of the next like shingles. In consequence of their 
fibrous structure the combs are strongly iridescent and in motion cause a 
beautiful play of metallic red, blue, and green over the meridional bands 
These combs act like oars and row the body about. Since the combs 
begin some distance from the aboral pole, they are connected with it by 
means of ciliated grooves following the line of the meridional bands. 
Experiment shows that the sense body is an organ of equilibration and for 
correlating the action of the different rows of combs. 

The ectoderm gives origin to two other important organs, two pole fields 
and two tentacles. The pole fields (fig. 209, pp) are two epithelial patches 
extending a short distance in the sagittal axis from the sense body and 
possibly are olfactory or taste organs. The tentacles arise, in the trans- 
verse axis, from the bottom of deep tentacular sacs (fig. 207, f-) from which 
they project as long cords with numerous lateral branches, and into which 
they may be retracted. Tentacles and branches contain an axial muscle, 
while the ectodermal coating consists largely of adhesive cells. These are 
spherical bodies (fig. 210) covered with a very sticky granular secretion, 
and, like a Vorticella, supported on the end of a spiral 
stalk muscle. These are used in capturing prey, which 
adheres to them and is drawn inward by the muscles. 

The ectoderm also forms part of the gastrovascular 
system. It turns inward at the mouth situated at 
the lower end of the chief axis and lines the large 
space commonly called stomach (fig. 207, /), but which 
corresponds to the oesophagus of the Actinozoa. At 
the aboral end of this stomach begin the true ento- 
dermal portions, the so-called funnels, and from them 
run canals distributed through the jelly to the various 
organs. Two (rarely four) funnel canals run to the 
aboral pole and empty (fig. 209, to) near the sense body; 
a second pair, the paragastric canals (fig. 207, mg), 
which run parallel to the oesophagus, end blindly. The perradial canals 
(g) proceed outward from the funnel, and besides giving off a canal to the 
base of the tentacle, each divides dichotomously Twice, first into interradial 
and then into adradial canals, each of these last connecting with a meridio- 
nal vessel running just beneath a row of combs, nourishing them as well 
as the gonads. The gonads consist of two bands, one male, the other 
female, running in that wall of the meridional vessel nearest to the combs 


FIG. 210. Ad- 
hesive cells of 
Ctenophora (after 
Samassa). 


SUMMARY OF IMPORTANT FACTS 235 

These gonads are regular in distribution, those of two vessels which are 
nearest each other being of the same sex. The eggs and sperm pass out 
through the gastrovascular system. 

The few species are divided into TENTACULATA, with tentacles, and 
XUDA, without. To the first belong the CYDIPPID^C, with pear-shaped bodies 
(Pleurobrachia*), Hormiphora (fig. 207); the LOBAT^E (Mnemiopsis,* Bolina*), 
with lobes; and the band-like CESTID/E (Cestum, Venus girdle). The BKROHXE 
(Beroe, Idyia*), with wide mouth, belong to the Xuda. The small creeping 
Cceloplana and Ctenoplana, are supposed by some to form a transition to the 
Turbellaria. 

Summary of Important Facts. 

1. The CGELENTERATA and Echinoderma were formerly called 
Radiata because in most a radial structure is present; in the higher groups 
this is replaced by biradial or even bilateral symmetry. 

2. The Coelenterata are sometimes called Zoophyta (animal plants), 
from their appearance and their attachment. In many the resemblance 
is heightened by their formation of plant-like colonies by fission and 
budding. 

3. The name Coelenterata was chosen because they have but one 
cavity, a simple or ramified digestive sac, representing at once the ali- 
mentary tract and the as yet undifferentiated body cavity. 

4. This coelenteric cavity is called the gastrovascular system because 
its branches distribute nourishment to all parts and so perform the func- 
tion of blood vessels. 

5. The reproduction is either sexual or asexual, very frequently 
cyclical (alternation of generations). 

6. The animals are provided with nerves, muscles, and sense organs 
and possess marked sensibility and mobility. 

7. Especially characteristic are the tentacles and small nettling organs, 
the cnida?, in special cells. 

8. Nearly all histological differentiation proceeds from ectoderm or 
entoderm, since the mesoderm (mesoglcea) plays but a subordinate role 
and usually functions only as a support (Diploblastica). 

9. Four classes Hydrozoa, Scyphozoa, Anthozoa, and Ctenophora 
are recognized. 

10. In HYDROZOA and SCYPHOZOA there are usually two alternating 
generations, the sessile asexual polyp and the free-swimming sexual 
medusa. 

1 1 . The hydroid polyp and the craspedote medusa are characteristic of 
the HYDROZOA. 

12. The hydroid polyp is a two-layered sac of ectoderm and entoderm, 


236 CCELENTERATA 

a supporting layer and a circle of tentacles. In the colonial forms there is 
usually a cuticular envelope, the perisarc, secreted by the ectoderm. 

13. The craspedote medusa is bell-shaped, with smooth bell margin, its 
aperture partially closed by a diaphragm-like velum; the gonads are 
ectodermal. 

14. The medusa 1 arise by lateral budding from the hydroid. 

15. If the medusa remain attached to the parent as a sporosac, alterna- 
tion of generations is replaced by polymorphism; it can entirely disappear 
with the total loss of either hydroid or medusa. 

16. The scyphostoma and the acraspedote medusa are typical of the 
SCYPHOZOA. 

17. The scyphostoma differs markedly from the hydroid polyp in the 
presence of four longitudinal gastric folds or septa (tocnioke). 

18. The acraspedote medusa lacks a velum, has a lobed umbrella edge, 
gastral tentacles (phacelke), and entodermal gonads. 

19. The medusa arises from the polyp by terminal budding (strobila- 
tion). 

20. Alternation of generations rarely is lost, and then only by suppres- 
sion of the scyphostoma. 

21. The ANTHOZOA have only one form, the coral polyp; it is distin- 
guished from the hydroid polyp by the ectodermal oesophagus, the radial 
septa reaching the oesophagus; the well-developed mesoglcea and the 
gonads which, arising from the entoderm, early migrate into the meso- 
gloea. 

22. Most Anthozoa are colonial and produce a skeleton (coral) usu- 
ally of calcic carbonate, but sometimes of 'horny' substance. 

23. The skeleton may be either axial or it may be outside the indi- 
vidual polyps (cortical skeleton). 

24. The living Anthozoa are divided according to the number of septa 
into Octocoralla and Hexacoralla. With the latter the fossil Tetracoralla 
are allied. 

25. The Hexacoralla have numerous tubular tentacles and six, or a 
multiple of six, pairs of septa. 

26. The Octocoralla have eight single septa and eight feathered ten- 
tacles. 

27. The CTENOPHORA are always free-swimming and have a large 
mesoderm with numerous muscle cells. 

28. Nettle cells are absent, and are replaced by adhesive cells. 

29. Most characteristic are the eight meridional rows of 'combs' 
whose motions are controlled by a common organ, the sense body, a 
statocyst. 


SUMMARY OF IMPORTANT FACTS L>:;7 

30. The digestive tract consists of an ectodermal oesophagus (stomach) 
and a branching system of entodermal vessels. 

VERMES. 

A large number of forms above the Coelenterates are frequently grouped 
as Vermes, but there is little agreement as to what smaller divisions shall be 
included, some denying the existence of a natural group of worms, and separating 
the groups as phyla. Others include not only the flat-, round- and segmented 
worms, the Chaetognaths and rotifers, but also the brachiopods, Polyzoa and 
even the tunicates. Yet such is the variety of form, structure and development 
that, no matter what limitation be accepted, it is impossible to frame a definition 
which shall include all of the species commonly known as worms. In taking 
the step from the diploblastic Ccelenterates to even the lowest 'worms' such 
advances in structure are seen that a brief review of these is appropriate here. 

The worms are distinguished from the Ccelenterates by bilaterality, which 
is seen in the internal structure, in even those cases (round worms) where it is 
not visible on the exterior. There is also a higher degree of differentiation of 
organs -the development of a ganglionic nervous system, excretory organs, and 
frequently of a blood-vascular system. This advance is correlated with the 
appearance of a true mesoderm, the layer from which (the nervous system 
excepted) these organs and the muscles arise. Then there is the dermo-muscular 
sac, the cause of the familiar 'worm-like' motions. This consists of an intimate 
connexion of the skin with the underlying muscles (figs. 212, 240, 241). The 
skin, a one-layered epithelium, ciliated or covered with a cuticle, rests on either 
a structureless membrana propria or on a cellular connective tissue, to which 
the muscles are attached. Longitudinal muscles are always present, and fre- 
quently circular as well, while in the parenchymatous worms diagonal, crossed 
and dorsoventral fibres may occur. 

While certain worms (cestodes) lack an alimentary canal, or like many 
nematodes, may have a blind, functionless gut, these conditions are the result 
of parasitic habits. In the lower worms the digestive tract is like that of the 
Coelenterates, consisting of archenteron and stomodeum, proctodeum and anus 
being absent. In most worms the tract is 'complete,' it being a tube with mouth 
and vent. 

The digestive tract is either imbedded in the parenchyma and cannot be 
dissected out (fig. 212), or it is surrounded by the body cavity (ccelom) which 
separates it from the body wall (figs. 238, 241), the muscles of which are meso- 
thelial in origin, in contrast to the mesenchymatous muscles of the flat worms. 
In the flat worms the excretory organs are protonephridia, and similar organs 
with solenocytes (p. 105) are common in the larvas of the higher worms, being 
often replaced in the adult by true nephridia. 

Here, too, a blood-vascular system first appears. Where it is lacking (flat 
worms) its place may be taken by the branches of the digestive tract, or in higher 
forms by the ccelom with its albuminous fluids. The phylogenctic origin of 
the circulatory system may be considered here. Two explanations of the 
circulatory vessels have been advanced, (i) They are canals separated from 
the alimentary canal and developed farther independently; therefore they have 
an entodermal epithelium. (2) They are spaces filled with albuminous fluid, 
arising between entoderm and mesoderm (a modified form of this is that . 
are remnants of the segmentation cavity); hence at first they had no epithelium 
and obtained it later from the mesoderm. Lately the latter view is regarded as 
the more probable, since most observers deny the existence of an epithelium in 
the blood vessels of worms and other invertebrates. Then in many worms 


238 


PLATHELMINTHES 


a perigastric sinus, arising by a separation of the intestinal layers, forms a part 
of the circulatory system. 

The nervous system of the 'worms' has in common a pair of supra-cesopha- 
geal ganglia ('brain') which sends out two strong longitudinal cords (to which 
others may be added) which must be regarded as part of the central nervous 
system since they bear ganglion cells. These cords may be lateral or on either 
side of the mid-ventral line. In the latter case those of the two sides may be 
united at regular intervals, thus giving the ladder type (p. 113), the ventral chain 
being connected with the brain by cords on either side of the oesophagus. This 
nervous system, always ectodermal in origin, may be epithelial, forming part 
of the skin, or it may sink to different depths in the other tissues. 


Mi 


FIG. 211. Trochophore (Loven's larva) of Polygordius (from Hatschek). A. 
anus; dLM, dorsal muscles; ED, hind gut; J, stomach; J,, intestine; Mstr, mesodermal 
band; n, nerves; Neph, protonephridia; O, mouth; Oe, oesophagus; oeLM, oesophageal 
muscle; SP, apical plate; vLM, ventral muscle; i<LN, lateral nerve; Wkr,wkr, pre- and 
post-oral zones of cilia; WS, apical cilia; wz, adoral cilia. 

A peculiar type of larva appears in various groups of invertebrates, being 
recognizable in modified forms in echinoderms and molluscs as well as in worms. 
This is the trochophore (fig. 211) a gelatinous ball traversed by fore, mid and hind 
gut. At first the body is uniformly ciliated, but the cilia are later restricted 
to definite tracts. One of these ciliated bands is pretty constant, passing around 
the body in front of the mouth, thus marking off an apical region, in the centre 
of which is an epithelial thickening, the anlage of the brain, often marked by a 
bundle of cilia. Besides mesenchymatous muscle fibres, one or more pairs of 
protonephridia may be present. In the corresponding larva of many of the 
flat worms (the protrochula, fig. 216) hind gut and protonephridia are lacking. 

PHYLUM IV. PLATHELMINTHES (PLATODES, Flatworms). 

This group is well characterized by the names. With few exceptions 
(rhabdocceles, many trematodes) the nearly flat ventral surface and the 
slightly arched back pass with a more or less sharp margin into each 
other (fig. 212). In many cases the ventral surface is distinguished by 
its lighter color. In all the bilaterally symmetrical body, without ccelom, 


PLATHELMINTHES 


239 


is composed of a solid parenchyma, a mass of connective tissue traversed 
by muscle fibres, in which the various organs alimentary tract, nerves, 
excretory and reproductive organs are imbedded. The digestive 


FIG. 212. Transverse section (right half) of a Planarian. d, vitellaria; dv, dorso- 
ventral muscle fibres; e, ectodermal epithelium with cilia; g, gastric diverticula; h, 
testicular follicles; lin, longitudinal muscles (dots, in section); n, lateral nerve cord. 

system has various shapes; in the lower groups it resembles that of the 
ccelenterates in that there is but a single opening and this leads by an 
ectodermal oesophagus (stomodaeum) to the interior (fig. 59). In 

parasites the digestive tract may be lost. The 
skin is a one-layered epithelium, sometimes cili- 
ated, sometimes protected by a thick cuticula. 
Inside this comes a muscular layer (fig. 212) in 
which longitudinal muscles are always present, 
and in addition frequently circular and oblique 
muscles, as well as those passing from dorsal to 


FIG. 213. FIG. 214. 

FIG. 213. Excretory system of Cercaria (after Albert Lang), b, limb of bladder; 
b', same with urinary concretions; cc, collecting canal; cs, canals of ventral sucki-r; cr, 
collecting vacuole; e, eye; ep, excretory pore; /, lumen of tail; os, oral sucker; vs, ventral 
sucker. 

FIG. 214. Eggs of Distomum nodulosum (after Schauinsland). A, before de- 
velopment; B, later, the yolk broken down. </, yolk cells; ei, egg cell; ek, ectoderm; 
en, entoderm; p, pigment spot. 

ventral surfaces. The nervous system (fig. 215) consists of a pair of 
ganglia ('brain') in front of (i.e., above) the oesophagus, and longitudinal 
nerves leading backwards from it. The excretory organs (fig. 213) are 


240 


PLATHELMINTHES 


composed" of a series of tubes, the protonephridia or 'water-vascular 
system,' which branch and ramify the parenchyma, and open to the 
exterior by one or more openings variously arranged. Most flatworms 
are hermaphroditic and the reproductive organs take up considerable 
space. There is a small paired or unpaired ovary and vitellaria, usually 
paired and branched. The eggs arise in the ovary, and to these are 
added nourishment in the shape of cells (abortive ova, 'yolk cells') rich 
in yolk from the vitellaria. At the point where oviducts and yolk ducts 
join, a single egg cell and several yolk cells are united into an oval body 
the compound egg protected by a shell, formed by the 'yolk cells,' 
which also have to do with the nutrition of the embryo. Of the 'com- 
pound egg' only the egg cell takes a direct part in the formation of the 
embryo and is the true ovum. 

The Platodes are usually divided into four classes. Of these the 
Turbellaria are the most primitive, and the others have come from them. 
The Trematodes and Cestodes have been altered by parasitism, resulting 
in more or less degeneration. By some the Nemertini are not regarded 
as allied to the other flatworms, but as nearer the Annelids. 

Class I. Turbellaria. 

The Turbellaria are small, only a few being measured by inches, 
while many are almost microscopic in size. The name Turbellaria refers 
to the currents produced by the ciliated ectoderm which covers the body 
(fig. 59), and which serves at once for motion and for respiration. Most 


FIG. 215. Digestive and nervous systems of Synccelidium pellucidum* (after 
Wheeler), a, alimentary tract; b, brain; In, longitudinal (ventral) nerves; m, marginal 
nerve; pi, longitudinal nerve of pharynx; pr, ring nerve of pharynx; tn, transverse nerve; 


u, uterine ostium. 


species are aquatic (fresh water or marine), only a few land planarians 
living in moist earth. In the water they either creep slowly over stones 
or plants, or they swim freely. The larger species swim by undulations 
of the body, the smaller by means of the cilia. Several are ecto- or ento- 
parasites and often show adaptations to their life in the degeneration of 


I. TURBELLARIA 


241 


part of the digestive tract (Fecampia) or by the development of anchoring 
structures. Appearance, ciliation, etc., often give the smaller turbellaria 
a resemblance to the ciliate Protozoa, so that the beginner is often con- 
fused between them. 

The alimentary canal (figs. 59, 215) consists only of oesophagus 
(pharynx) and mesenteron, the latter terminating blindly since no anus 
is present. The mouth is on the lower surface, at some distance from 
the anterior end, being occasionally in the middle or even behind the 
middle of the body. It leads into the muscular oesophagus, which fre- 
quently can be protruded like a proboscis. The mesenteron, of ento- 
dermal origin, varies greatly in shape, its modifications being made the 
basis of division of the class into orders. In the Polycladidea there is a 
central portion from which numerous branched caeca are given off; in the 
Tricladidea (fig. 215) there are three main trunks, each with lateral ca^cal 
diverticula; while in the Rhabdocoelida the digestive tract is a simple 
rod-like sac, in some cases (Acoela) without internal cavity. The supra- 
cesophageal ganglia always lie at the anterior end of the body, which is 

- 

ffillfi 


w 


' ' 


- 

- 


- 


, 

^~*~^''-^ . " 


FIG. 216. Larva of Stylochus pilidium (from Korschelt-Heider, after (lotto). /', 
enteron; En, remains of entoderm cells; S, resophagus. 

most sensitive and may be produced into feeler-like processes. This 
region usually bears two or more simple eyes, and in a few a single 
statocyst. Most of the turbellaria have numerous rhabditcs and rJiani- 
ni/es, rod-like structures of varying shape and structure, in the skin. 
The nettle cells found in the skin of many species (Microstomidae) are 
derived from the hydroids eaten. 

The hermaphroditic sexual organs (fig. 75) and the excretory system 
vary considerably. The eggs are usually large and are fastened by a stalk 

16 


242 PLATHELMIXTHES 

to water plants. Many species form a sort of cocoon, containing a few 
eggs and numerous yolk cells. There are a few viviparous forms (summer 
generation of Mesostomicke). In the marine species a free-swimming 
larva (protrochula, fig. 216), with lobe-like processes may hatch from the 
egg. This larva, by a metamorphosis, is converted into the creeping adult. 
Not infrequently besides the sexual, asexual reproduction occurs. The 
Microstomidae and some Planar ice are capable of transverse division, and 
will form chains of individuals. For each posterior individual a new 
brain and a new oesophagus are formed (fig. 59). The Turbellaria 
possess a marked power of reproducing lost parts, making them favorites 
for regeneration experiments. 

In a few Turbellaria the pharynx connects with a solid protoplasmic mass, 
in which, as in the protoplasm of a protozoan, the food is digested. This ento- 
derm is hardly marked off from the mesoderm, but it is a question whether these 
'Accela' are primitive or degenerate, the latter being the more probable. 

Order I. Polycladidea. 

Marine species of considerable size, digestive caeca, springing from a central 
chamber. Leptopland* Stylochus;* Thysanozoon, Europe. 

Order II. Tricladidea. 

Alimentary canal with an anterior unpaired and a pair of posterior branches, 
arising from the pharynx, and bearing lateral caeca. Marine, Bdelloura* and 
Synccelidium* (fig. 215) (parasitic on Limulus), Gunda,* Polychcerus;* fresh- 
water, Dendroca>lum* (fig. 64), Planaria,* and Polyscelis;* 'Phagocata* with 
divided pharynx. The tropical land planarians (Bipalium,* 10 or 12 inches 
long) have been introduced into greenhouses. 

Order III. Rhabdoccelida. 

Small, even microscopic, recalling in habits and appearance the Infusoria; 
alimentary canal rod-like, without branches. Vortex* (fig. 75), fresh water; 
Mono ps* Monoscelis* marine. The fresh- water MiCROSTonnxE reproduce 
almost exclusively by fission. 

Class II. Trematoda. 

These are exclusively parasitic, some living on the skin or gills (ecto- 
parasites) or in the interior of other animals (entoparasites). In structure 
they are closest to the triclad Turbellaria, from which they differ by 
characters, the direct result of their parasitic life. Thus they have lost 
the cilia or have them only in the larva. On the other hand, they are 
covered with a cuticle often with spines and with suckers and hooks for 
adhesion to the host. The suckers are shallow pits of columnar epithelium 
lined with cuticle and furnished with a thick layer of radial and circular 
muscles, which by their contraction increase the lumen of the sucker, the 
edges of which are closely applied to the host. At least one such sucker 


II. TREMATODA 


243 


is present; if but one or two (entoparasites), one is at the anterior end 
(oral sucker) surrounding the mouth, while a second larger sucker may 
occur near the mouth (fig. 217), but may be (Ampliistonmm) at the ]><ir- 
rior end. In the ectoparasites there are a pair of anterior suckers near the 
mouth; at the posterior end a single sucker, or a number of suckers or 
hooks or both on a sucking disc (fig. 219). 

Other results of parasitism are the weak development of sense organs and 
brain and development of accessory ganglia near the adhesive organs. Eye 
spots (two to four) occur occasionally in the ectoparasitic species and in the 
larvae of the entoparasitic, rarely in their adult condition. The alimentary tract 
is forked (fig. 218) and occasionally (fig. 217) has dendritic blind sacs. 


-4-U 


-CJ 


FIG. 217. FIG. 218. 

FlG. 217. Distomum hepaticuni, liver fluke (from Boas), m, ca?ca of ta, limbs of 
digestive tract; s^.,, anterior and posterior suckers. 

FIG. 218. Distomum lanccolatum. c, cirrus, beneath it the opening of the oviduct; 
d, vitellaria, the ducts leading to the shell gland; g, ganglion; h, testes with ducts to 
cirrus; /, Laurer's canal; o, ovary, the shell gland behind it; 5', s", anterior and median 
suckers, the pharynx and the bifurcated digestive tract leading from s'; u, uterus; IL', 
terminal vesicle of water-vascular (excretory) system. 

To parasitism may also be attributed the great development of the sexual 
organs, which at maturity fill a great part of the body. Their features may be 
seen in fig. 218. Two vasa deferentia pass forward from the testes (//), unite 
and form a seminal vesicle. The terminal portion of the united ducts can be 
protruded as a penis or cirrus (c) armed with retrorse hooks. It is usually 
enclosed in a 'cirrus pouch.' The unpaired ovary (o) is very small and pro- 
duces small eggs, deficient in yolk; hence the paired vitellaria (</) arc well 
developed. The united ducts from these join the oviduct, producing the uterus 
(u), which receives the eggs, is much convoluted, and empties beside iin .some 


244 


PLATHELMINTHES 


species in a common antrum with) the male sexual opening. The first part of 
the uterus is called the ootype because here the eggs and yolk cells are formed 
into eggs (fig. 214) and enclosed in a shell with a lid or cover formed by the 
secretion of the yolk cells. A second duct Laurcr's canal goes from the 

ootype to the dorsal surface. In the Distomes the 
canal is rudimentary or lacking. It apparently cor- 
responds to vagina of the Polystomes, used in copu- 
lation. On the other hand, it may be homologous 
with the vitcllo-intestinal canal which connects in- 
tc-stine and vitelline duct. In the Distomes copula-^ 
tion occurs in the uterus, leading to self -impregna- 
tion. 

The Trematodes fall into two great groups, the 
Polystomeoe, largely ectoparasites, and theDistomeae, 
exclusively entoparasitic, the distinctions in para- 
sitism being correlated with differences in structure 
and development. 


sw 


FIG. 2ig. Polystomum 
integerrimum (after Zcller). 
Above two individuals in 
copulation; below a single 
animal enlarged, d, diges- 
tive tract, distended with 
blood; dg, yolk duct; dst, 
vitellarium; gp, genital pore; 
h, testicular vesicles; m, 
mouth; pit, pharynx, m', 
ovary; sw, openings of the 
paired vaginae; u, uterus; 7 r , 
vaginae; vd, vas deferens; x, 
vitello-intestinal canal. 


Order I. Polystomeae (Monogenea, Heterocotylea). 

Most Polystomes live on aquatic animals usu- 
ally fish, where they attach themselves to the gills. 
Since they are exposed to more dangers, their adhe- 
sive organs are stronger than in the entoparasites. 
So while the anterior suckers are weakly developed 
or absent, the hinder end bears sometimes only a 
single sucker, but usually a large adhesive disc 
armed with many suckers and hooks (fig. 219). 
The transfer of Polystomes from one host to another 
is easy and the life history is without complications. 
The stalked eggs are attached near the mother and 
produce larvae, which soon after hatching have the 
adult form (hence the name Monogenea). 

Gyrodactylus, parasitic on the gills of the carp, 
brings forth living young which, even before birth, 
produce a new generation in their interior, and 
these may contain a thnd generation. More striking 
is Diplozoon paradoxum, in which, at the time of 
sexual maturity, two individuals become fused like 
Siamese twins (fig. no). The young, called 
Diporpa, escape from the eggs and only unite later. 
Each has a ventral sucker and a dorsal papilla. 
Each of the pair seizes the papilla of the other with 
the sucker, and then the two grow together so that 
the male opening of one comes opposite the female 
opening of the other. Polystomum integer riir.v.m of 
the frog (fig. 219) affords a transition to en'oparasit- 
ism. At first it lives on gills of the tadpole, but at 
the time of metamorphosis it is forced to leave this 
place and to pass, by way of the alimentary caral, 
to the urinary bladder. The monogenetic ASPIDO- 

turtles, fishes 


COTYLID^E are internal parasites m 
and molluscs. The TEMNOCEPHALID^: of warmer regions are parasitic on 
Crustacea, molluscs, and turtles. American genera of Polystomeas are Epibdclla, 
Polystomum, Tristoma, Sphyranura, Microcetyle. 


II. TREMATODA: DISTOME.& 


J 1.1 


Order II. Distomeae (Digenea). 

The entoparasitic Trematodes usually occur as sexual animals in the 
digestive tract and its appendages; more rarely in blood-vessels, urogenital 

A 


FIG. 220. Development of Distonmm hepaticum (from Korschelt-Heider after 
Leuckart). .4, young larva; B sporocyst from the lung of Limmra; C, older sporocyst 
with rediffi; D, redia which has produced rediae internally; E, redia with cercaruL-; /', 
cercaria; G, encysted Distomum. A, eye spot; D, digestive tract; Dr, glands; A.v, 
ciliated lobules and main trunks of excretory system; G, birth opening; Kz, germ cells; 
N, nervous system. 

organs, and coelom of vertebrates. As inhabitants of the dark they have, 
with few exceptions, lost the eyes, which appear in larval life, and not 
always then. Since not exposed to danger of being pulled from the host, 


246 


PLATHELMINTHES 


they possess either the oral sucker alone (Monostomum) or this and a 
second ventral sucker, and only rarely other attaching apparatus. They 
are markedly separated from the Polystomes by their life history. The 
alternation of hosts necessitated by the endoparasitic life is complicated 
by an alternation of generations (heterogony, p. 132) with metamorphosis, 
hence 'Digenea.' To illustrate this the history of Distomum hepatic-urn, 
the liver fluke of the sheep is chosen (fig. 220). 

The eggs leave the maternal uterus at the beginning of embryonic 
development, pass down the bile ducts and thence by the intestine to the 
exterior. They must come into water and remain here a while before 
the ciliated larva (miracidium, A) escapes by lifting of the lid of the shell. 
This larva bores its way into a small snail (sp. of Limncca), where it 
grows into a sporocyst (B}. The sporocyst, a muscular sac with proto- 
nephridia but lacking all other organs, produces in its interior eggs which 
develop into a second reproductive sac, the redia (D). These are dis- 
tinguished from the sporocysts by the possession of pharynx and a tubular 

intestine as well as a birth-opening for the 
escape of the young produced inside. Ac- 
cording to the season these young are either 
cer carlo; (F), or another generation of rediae 
may follow before the cercariae appear. 
The cercariae are adapted for an aquatic 
life, since each has, besides the characteristic 
organs of a Distomum (genitalia excepted), 
a strongly vibratile tail. The cercariae 
escape from the snail, swim about in the 
water until the tail drops off, when they 
encyst on water plants. When these en- 
cysted young are eaten by sheep along with 
the vegetation, infection follows. 

In general it can only be said of the life 
history of other Trematoda that the miracidia 
must penetrate a mollusc, and that the different 
species have many modifications. Best known 
are the following: Distomum (Fasciolaria) 
hcpalicum, the liver fluke (fig. 217), about the 
size and shape of a pumpkin-seed, lives in the 
bile-ducts of sheep, cows, pigs, etc., and rarely 
of man. It causes a disease known as 'liver rot,' generally resulting in death. 
This history shows why shep pastured in moist places are subject to the 
disease, and why wet seasons are times of epidemics. Thus in the rainy year 
of 1830 about one and a half millions of sheep were killed in England; in 1812, 
300,000 in the neighborhood of Aries, France. It is frequently accompanied 
by D. lanceolatum, less than half an inch in length (fig. 218). 


FlG. 221. Billiarzia h/rma- 
tobia. Female in the gynaeco 
phoral canal (c) of the male; 
s', s", anterior and posterior 
suckers. 


III. CESTODA 


247 


Rilharzia ha-matobia is a human parasite, most common in hot climates, 
especially among the Eellahin of Egypt. The sexes are separate. The male, 
half an inch long, by inrolling of the ventral side (fig. 221) forms a groove in 
which the more slender female usually lies. These united worms occur in the 
portal vein and connected vessels, which they 
follow and lay their eggs in the mucous mem- 
brane of the ureters and urinary bladder, as 
well as in liver and intestine. Several other 
species occur in man, among them D. carnosum* 
and D. wester nianni*. Amphistomum is com- 
mon in the intestine of Ungulates, one species, 
.1. Inntiinis, occurring in man. With few ex- 
ceptions the adult Distomes occur in verte- 
brates, the larval stages in molluscs. Aquatic 
birds are very apt to be infested with them. 

Class III. Cestoda (Tapeworms). 

The majority of the cestodes, and 
especially those of the human intestine, are 
sharply distinguished from the entopara- 
sitic trematodes. But the boundaries dis- 
appear in forms like Archigetes, Caryophyl- 
hcus, and Amphilina, parasitic in lower 
vertebrates or invertebrates which are now 
assigned to the trematodes, now to the 
cestodes. The most important character 
of the cestodes is that as a result of their 
parasitic life they have lost the last traces 
of an alimentary canal, and are nourished 
by the juices or the partially digested food 
of the host, since the fluid nourishment is 
taken in through the skin. Two other 
characters are striking: (i) The differen- 
tiation of two developmental stages, the 
bladder worm, or cysticercus, living chiefly 
in solid organs (muscles, liver, brain), and 
the sexually mature animal, living in the 
alimentary tract; (2) the division of the 
body of the adult into the head or scole.v, 
and following this a series of joints or 
proglottids. Since this last feature holds for 
all human tapeworms, the best known species, the following description 
begins with these typical forms. 

The sexually mature tapeworm or strobila (fig. 222) consists of a 
scolex in front, and behind this follow in a row the proglottids. The 


FIG. 222. Tccnia saginata 
(from Boas, after Leuckart 1 ). 
Head with series of proglottids 
taken from various 
the strobila. 


regions of 


24S 


PLATHELMINTHES 


number of these varies from smaller forms (Tcenia echinococcus, fig. 232) 
with three or four to hundreds or even several thousands, a fact which 
speaks for the enormous size of some species. The proglottids arise 
from the hinder end of the scolex, by a kind of budding. This explains 
the well-known fact that one is not rid of the tapeworm, so long as the head 
remains in the host. It also explains the peculiar shape which is almost 
thread-like in front, increasing posteriorly to a broad band, whence the 
common name. At first the proglottids are small ; they increase with age 

to considerable size, and separate from the hinder 
end of the chain and live independently when a 
certain development is reached. For example, 
the young proglottids of the human tapeworm, 
Tccnia soli-urn, are 0.5 mm. broad and o.oi mm. 
long; the ripe proglottids are 5 mm. broad and 
12 mm. (half an inch) long. 

Head and proglottids have certain common 
characters. Their connective-tissue parenchyma 
consists of cortical and medullary substance. 
The first contains most of the muscles, the latter 
the other organs. Nerves and water-vascular 
system extend the whole length of the worm. In 
the head is the paired cerebral ganglion (fig. 223), 
sometimes fused to a single mass by the great 
development of the commissure. From the brain 
two principal nerves run backwards, usually near 
the edge of the proglottids (fig. 228, N}. The 
water-vascular system begins with a capillary 
network richly provided with flame cells. It ex- 
tends through head and proglottids; usually four 

main trunks are present, two being less developed and sometimes absent. 
The two chief trunks are frequently connected by a cross-trunk on the 
hinder margin of each proglottid (fig. 228). The system opens on the 
posterior edge of the last proglottid, but accessory mouths may occur 
on other proglottids. 

Scolex and proglottids differ in that the proglottids contain the sexual 
organs, while the scolex bears the anchoring apparatus, for the latter has, 
besides producing proglottids, to fasten the worm in the intestines. Most 
important of the adhesive organs are the suckers (acetalntla] ; less import- 
ant are the numerous hooks, either arranged in a circle or borne on 
protrusible and retractile probosces (figs. 224-226). 


FIG. 223. Nervous 
system of Monczia (after 
Tower). a, sucker.?; e, 
excretory tubes; g, cere- 
bral ganglia. Nerves 
black. 


III. CESTODA 


249 


When a circle of hooks is present it is on the anterior end and is moved by a 
epecial apparatus, the rostellum, a plug of complexly arranged muscles (fig. 226) 
which can arch and flatten the central area. Each hook has its point outwards 
and its base with two roots, one of which rests on the rostellum; the protrusion 
of the rostellum forces the points outwards into the mucous membrane of the 
intestine. In some Tcenice without hooks (T. saginata) the rostellum is replaced 
by a sucker-like depression. Since the rostellum arises in development from a 
similar cup, it may be a modified apical sucker, but it is doubtful how far com- 
parisons may be made with the oral sucker of the trematodes. 

The animals are almost always hermaphroditic and the gonads are 
equal in numbers to the proglottids, so that these were formerly regarded 
as sexual individuals of a colony, each with its own reproductive apparatus. 
Two types must be recognized. In the one the presence of vitcllaria and the 
separate openings of uterus and vagina recall the conditions in trema- 
todes, while in the second the uterus ends blindly and the vitellaria are 
modified into a small albumen gland. Since vagina and vas deferens 
almost always open together, self-impregnation is possible, but cross- 
fertilization of separate proglottids has been seen. The general features 
of the two types may be made out from figures 227 and 228. 


FIG. 224. FIG. 225. FIG. 226. 

FIG. 224. Apical view of head of Tcenia solium (from Hatschek). 

FIG. 225. Head of Tetrarhynchus viridis (after Wagner) . Dissected to show the 
internal parts of the proboscides (o) and the ganglion (g). 

FIG. 226. Schema of action of rostellum. On the right the hooks are exserted for 
adhesion, on the left retracted, r, rostellum; s, sheath; /, longitudinal muscles. 

In the Bothriocephalidae numerous testes (fig. 227, /;) are scattered through 
the parenchyma. , The small vasa deferentia unite repeatedly to a chief canal 
which opens in the middle line, near the anterior margin of the proglottid, the 
terminal portion, the cirrus being retractile into a cirrus pouch (cb). The ovary 
is two-lobed (ov) and is near the posterior margin of the proglottid. It forms 
eggs poor in yolk. The scattered vitcllaria are voluminous. The unpaired 
oviduct and the vitelline duct unite to form the shell gland, in which each egg 
unites with a number of 'yolk cells' to form a compound egg (see p. 240). The 
uterus (u) leads from the gland to the exterior. There is also a vagina (va) 
leading from the oviduct to the exterior. In distinction to the Trematodes, the 
vagina empties with the uterus apart from the male organs. The general 
differences in the Taeniidae may readily be made out from fig. 228, the chief 
points being the replacement of the vitellaria by an albumen gland, the blind 
uterus, and the termination of male and female ducts (vagina) at the side of the 
proglottid. 


250 


PLATHELMINTHES 


FIG. 227. Proglottid of Bofhrwcephalus latus (after Sommer). Right only vitella- 
rium, left only testes, shown, cb, cirrus sheath opening with the vagina; dg, vitelline 
duct; dt, vitellarium; h, testes; od, oviduct; n\ ovary; s:i, shell gland; u, uterus; va, 
vagina; i<d, vas deferens (dark-lined); w, excretory canal. 


tt 


FIG. 228. Proglottid of Tcenia saginata, near maturity (after Sommer). cb, 
cirrus sheath; dt, vitellarium; k, genital pore; A r , nerve cord; Neph, excretory canal; 
or, ovary; rs, receptaculum seminis; sdr, shell gland; /, testes; u, uterus; I'd, vas deferens. 


III. CESTODA 


251 


FIG. 229. Ciliated embryo 
of Bolhriocephalus latus en- 
closing the six-hooked larva. 


The differences in the sexual apparatus influence the peculiarities of 
the egg. In BotJiriocephalns it is large, has a tough shell with a lid, and 
encloses a small egg cell with numerous yolk cells. The eggs of Tccnia are 
small, with a layer of albumen and a delicate shell which is lost early. It 
is replaced by a radially striped envelope secreted by the embryo in a 
somewhat advanced stage. It is in this condition that the eggs escape. 
A further consequence is a difference in development. In most Bothrio- 
cephalidce, as in the Trematoda, the egg must 
enter the water for its further development. 
Here a ciliated oval larva escapes which con- 
tains a six-hooked larva (oncosp/ucra, fig. 229). 
The ciliated envelope is temporary and is cast 
off like the ciliated coat of the trematode larva. 
The six-hooked larva enters a fish, becomes 
encysted (pleurocercoid) in muscles or viscera, 
and changes directly into the head of a 
Bothriocephalus. This on being taken with 
food into the intestine of the proper host de- 
velops into the adult. 

The history of the Tcenias differs considerably. The distinctions are 
early recognizable, since the six-hooked larva lacks the ciliated coat but is 
enclosed in its homologue, the envelope already alluded to. Since this can- 
not open of itself, the young are set free by its digestion in the stomach. 
Thus the eggs of Tccnia solium must pass into the stomach of the pig (em- 
bryos in faecal matter get mixed in the food) and after being freed from 
their shell in the stomach, the larvas with their six hooks bore through 
the intestinal wall and migrate, using the blood-vessels in their course, 
into the muscles, or more rarely other organs. Here they develop into 
bladder worms (cysticerci), becoming oval and secreting a cyst to which 
the pig adds an envelope of connective tissue. The cysticercus grows 
by increase of cells, and by the infiltration of serous fluid, so that it 
becomes distended into a delicate translucent vesicle. In T. solium the 
microscopically small embryo can grow in three or four months to the size 
of a bean or pea; in some species as large as a hen's egg. By invagination 
the wall of the bladder produces the anlage of the scolex (fig. 230, c). 
This is at first sac-like but soon increases in length, its growth being con- 
fined by an envelope (</), so that it is bent. The scolex appears like a 
white swelling through the wall of the bladder. 

At the apex of this blind sac arises the characteristic armature of the 
scolex which makes it possible to say what tapeworm will come from the 
cysticercus. Thus in T. solium there are four suckers and a crown of 


252 


PLATHELMINTHES 


hooks. These parts are at first inverted and only come to their definitive 
position on the outside of the scolex when the latter is protruded as one 
would turn out the finger of a glove. Rarely is the scolex protruded at 
this time (cysticercus of the mouse liver) and begins the formation of 
proglottids. Even in this case they do not become sexually mature until 
in the intestine of the cat. 

The further development follows when the cysticercus is taken into the 
stomach of the new host When man, for instance, eats infected ('measly') 


FIG. 230. Structure and development of tli2 cysticerus (C. cellulosae of Tcenia 
solium). a, measly meat, natural size; below an escaped cysticercus; b, cysticercus, 
with exserted scolex, enlarged; ce, development of the scolex, more enlarged; c, young 
cysticercus with blastema of scolex (above) and water-vascular net; d, e, different stages 
of scolex in receptaculum, the cysticercal wall mostly removed. 

pork, the cysticerci are freed by action of the digestive juices and the scolex 
is everted. The embryo passes to the intestine, becomes attached and, 
surrounded by nourishment, begins to grow, the bladder remaining at- 
tached to the hinder end. Soon the formation of proglottids begins in the 
piece connecting the bladder with the scolex So rapid is the growth that 
in ten or twelve weeks Tcenia solium begins to set proglottids free. 

In cases where the bladder reaches a considerable size it may produce more 
than a single scolex. The bladder of Ccenurus cerebralis, which lives in the 
brain of sheep, produces hundreds of scolices. The number is even greater in 
Tcenia echinococcus , in which the bladder increases by budding, and by the 


III. CESTODA 


253 


formation of numerous daughter bladders produces marked tumors in the liver 
of man and domestic animals, before the formation of scolices begins. In the 
interior of each daughter vesicle appear a number of brood vesicles, each of 
,-hich produces numbers of scolices, so that from a single six-hooked embryo 


\v 


thousands of scolices can arise (fig. 232). In contrast to this extreme case are 
others which connect with the development of Botliriocephalus, in which the 
cvsticercus is replaced by a cysticercoid (fig. 231). Here there is no infiltration 
and the scolex is closely enclosed by an envelope comparable to the bladder wall. 
In several cysticercoids a caudal appendage recalls the cercaria. 

The development of a tapeworm was earlier believed to be a complicated 
alternation of generations; the bladder to be a stage which by budding produced 
scolices; the scolex, in turn, a stage which by terminal budding produced the 
sexual animals, the proglottids, and the tapeworm itodf a chain of individuals, 
a strobila. This view, so easy to learn, contains' two errors. The bladder is 
not an independent generation, but only the hinder end of the scolex. The 
tapeworm is not a colony, but a single animal; the proglottids are not individuals, 
but specialized parts of a whole. This view is confirmed by a comparison with 
other forms. The Caryophyllaeidae (fig. 233) are single bodies, the anterior end 


FIG. 231. FIG. 232. 

FIG. 231. Invaginated and extended cysticercoid from a slug (Arioii) (from 
Hatschek) . 

FIG. 2^2.T(rnia echinococcus (after Leuckart). Right sexually mature; left a part 
of an echinococcus with two brood capsules and their scolices. 

elongate and taking the place of the scolex, while the broader hinder part con- 
tains a single hermaphroditic apparatus. In the Ligulidae the body is still 
unjointed, but has increased in length and contains numerous sets of sexual 
organs. This duplication of the reproductive apparatus explains the appear- 
ance of proglottids. In this multiplication of the gonads, which is connected 
with the entoparasitic life and the necessity of increasing the fertility, lies the 
reason for the division into proglottids. 

Family i. CARYOPHYLL.EID.E. No suckers, simple sexual apparatus, scolex 
and proglottis not differentiated. Distinguished from trematodes by absence 
of digestive tract. Larval stages in invertebrates, adults nearly always in fishes. 
Caryophylheus (fig. 233); Arrhigetcs in annelids (Sa-mtris'). Family 2. LIGULID^C. 
No suckers; numerous sexual organs, but no proglottids. The immature 
stages in fishes, the adults in birds. Linn-la* Family 3. TETRARHYXCHID.*:. 
With scolex and proglottids, the head with four protrusible hooked probosces 


254 


PLATHELMINTHES 


(fig. 225). Immature and mature stages in fishes. Tetrarhymchus* Ryncho- 
bothrium* Family 4. TETRAPHYLLID.*:. Head with four very mobile suckers 
often armed with hooks. Echinobothrium* Acanthobothrium* 

Family 5 BOTHEIOCEPHALID^. Scolex and proglottids present; head 

spatulate, with two sucking grooves on the 
narrower sides. Bothriocephalus latus* (fig. 
234), the largest tapeworm in the human intes- 
tine (also dogs and cats), may reach a length 
of forty feet and consist of over four thousand 
proglottids. As said above, the pleurocercoid 
occurs in fishes, and man acquires the parasite 
by eating uncooked fish. It is abundant in 
Russia, Prussia, and Switzerland; rare in 
America. 

Family 6. TJENIADJE. With scolex and 
separable proglottids; the scolex always bears 
four suckers and in many a rostellum with a 
circle of hooks (fig. 234). In the proglottids 
the genital pore occurs usually laterally in the 
proglottids, alternating right and left, rarely 
only on one side. It is rarely doubled in a 
proglottid. Intermediate stage a cysticercus or 
cysticercoid. The human tapeworms are here 
subdivided accordingly as the sexual animal or 
the cysticercus has been found in man. 

A Tccnia sexually mature in the human 
intestine. Most noticeable are Tccnia sol him* 
and T. saginata* the differences between which 
are shown in fig. 234. In spite of the lack of 
hooks, the stronger suckers render T. saginata 
more difficult to expel. Tccnia solium is not 
rare in the cysticercus stage in man and occurs 
sometimes in places, like the brain and eyes, 
where it causes severe injury. These cases are 
in part explained by lack of cleanliness in the 
food, which may contain eggs, but possibly 
occur through internal infection; pieces of the 
worm passing the pylorus and entering the 
stomach, where they are digested, setting the 
233.Caryophyllteus embryos free. 

Many other Tanicc, which are common to 
other mammals, occur occasionally in the 
human intestine. In mice and rats occur T. 
(Hymenolepis) nana* and T. diminuta. The 
first has recently been very abundant in human 
intestines in Italy and is probably common with 
us (fig 112). The worm, an inch or two long, 


FIG. 

mutabilis (after M. 'Schultze). 
df, vas deferens; civ, vitelline 
duct; k, scolex; ov, ovaries; ps, 
penis; i<s, vagina with recepta- 
rulum se minis; t, testes; ut, 
uterus; vi, vitellarium; vs, vesi- 
cula seminalis. 


The connection 

of vagina with the crossing point - <-> >- ~ ^ 

of genital duct, vitelline duct, mav occur in thousands and cause severe injury 
and uterus is lacking in the figure. This species may develop without an interme- 
diate host; the eggs taken into the stomach pass 

the cysticercoid stage in its walls and then pass to the intestine to become 
adult. T. diminuta* which has insects for its intermediate host, has been 
described from man. 

B. Forms passing the cysticercus stage in man. Besides the cysticercus 


IV. NEMERTINA 


255 


cellulose of T. solium, found in man, more frequent and of more important e i- 
the cysticercus of Tcenia cchinococcus (fig. 232), which lives as an adult in the 
dog, and is easily overlooked on account of its size. It is at most inch long 
and consists of a scolex and three or four proglottids. When the eggs are taken 
into the human stomach, as may easily happen by stroking and kissing infected 
dogs, the embryos are set free and wander into liver, lungs, brain, or other organs 
and produce here tumors which, in the case of the liver, may weigh ten or even 
thirty pounds. This extraordinary size is explained by the formation of 
daughter bladders (echinococcus) described above. 


FIG. 234. Heads and proglottids of three tapeworms of man. Left, Ta-nia sagi- 
nala; middle, T. solium; right, Bothriocephalus latus, flat and side view of head. The 
heads enlarged about six times, the proglottids about ii (after Leuckart, Braun, 
and Schauinsland). 

Common Tee-nice of domestic animals are in the horse Anoplocephala plicata 
(4 to 30 inches), A. perfoliata (\ to 3 inches), A. mamillana (J to 2 inches); 
in ruminants, Moniezia,-* in the dog, T cent a marginata* (cysticercus in sheep 
and swine), T. serrata* (cysticercus in rabbits), T. echinococcus (above), T. 
ccenurus (cysticercus in brain of sheep, causing the disease called 'staggers'), 
Dipylidium cucumerina* (most common, larva in the flea and dog-louse); in the 
cat, Tcenia crassicollis* (cysticercus in mice). Several species occur in domestic 
birds, one (Drepanidotcenia infundibuliformis*), causing epidemics among 
chickens. Others in ducks and geese. 

Class IV. Nemertini. 

Most nemerteans are of appreciable size, some reaching a length of a 
yard or more (Linens longissimus 90 feet !), and yet they are so contractile 
that our Cerebratulus lacteus, which can extend itself to fifteen feet, can 
retract to two. Nemerteans are rare in fresh water or moist earth, but 
are most abundant in the sea, where they burrow through the mud or lie 
rolled up beneath stones. Many are noticeable for their bright colors. 
Their systematic position is a problem. 


256 


PLATHELMINTHES 


Like some flatworms they have a solid parenchyma bounded exter- 
nally by a ciliated ectoderm rich in mucus cells, and inside this at least two 
muscular layers, an outer circular and an inner longitudinal layer. They 
differ from all other Plath.elminth.es in having a complete alimentary 
tract, beginning with a ventral anterior mouth and continuing as a 
straight tube, with, usually, paired diverticula, to the vent at the posterior 
end of the body (fig. 235). 

Especially diagnostic is the proboscis, which lies dorsal to the alimen- 
tary tract and usually opens in front of the mouth. The proboscis is a 


p ps pm 


Iv di 


FIG. 235. Diagram of Nemertean forig.). b, brain; c, ciliated pit; d, dorsal nerve 
trunk; di, dorsal blood-vessel; g, gastric caeca; i, intestine; /, lateral nerve trunk; h>, 
lateral blood-vessel; p, proboscis retracted; pm, proboscis muscles; pr, protonephridial 
tube; po, its opening; ps, cavity of proboscis sheath. 

muscular tube closed at one end and at rest is infolded like the finger of 
a glove inside a closed sac, the proboscis sheath, which extends far back 
in the body. Its tip is bound to the posterior end of the sheath by a 
retractor muscle. By contraction of the sheath the proboscis is everted, 
while it may be retracted again by the muscle. Nettle cells are not un- 
common in the proboscis wall, while in some- forms (the older Enopla) 
the effectiveness of the organ is increased by the presence of a dart-like 
stylet at the tip (reserve stylets occur on either side, fig. 236), and at the 
base of the stylet is a poison sac. 

The blood-vascular system consists of a pair of lateral tubes connected 
by transverse loops, and in most forms a third tube is present lying between 
the intestine and the proboscis sheath. The blood is colorless; it rarely 
contains red or green corpuscles. 

The central nervous system (in some forms still in the ectoderm) con- 
sists of a supracesophageal brain of a pair of ganglia, from which nerves 
run to the proboscis, and two lateral cords united on the ventral side by 
numerous transverse commissures. Connected with the brain, either 
directly or by means of a short nerve, are the cerebral organs or ciliated 
grooves, pits on the sides of the head, formerly regarded as respiratory, 
are now considered sense organs. Tactile organs and simple eyes are 


IV. NEMERTINI 


257 


widely distributed; statocysts are very rare. The excretory system 
consists of two tubes lying beside the lateral blood-vessels and connecting 

with branches terminating in flame cells, while 
they open separately to the exterior by one or 
several openings. 

4 - l> As a rule the nemertines are dioecious, the 

gonads forming a row of lateral sacs, alternat- 
ing with the intestinal blind sacs and opening 
p n dorsally. The development is sometimes 
direct, but usually a metamorphosis occurs in 
which a larva, the pilidium (or a reduced 
/ form of it, Desor's larva), appears. The 
. pilidium is a helmet-shaped larva with, right 
and left below, a pair of lappets (fig. 237). 

P s The margins of lappets and helmet are ciliated, 

while at the top a bundle of longer cilia pro- 

- d ject from a thickened patch of ectoderm, the 


cs 


--- d 


ffi/C 


m 


FIG. 236. FIG. 237. 

FIG. 236. Amphiporus pidcher (after Burger), a, alimentary canal; h, brain; 
c, ciliated groove; d, dorsal blood-vessel; /, lateral blood-vessel; m, retractor of proboscis; 
n, lateral nerve cord; o, ovary; p, poison sac; pn, protonephros; pr, proboscis; /.?, pro- 
boscis sheath; r, rectum; 5, stylet of proboscis. 

FIG. 237. Pilidium larva (from Lang, after Salensky.) es, invaginations which 
later give rise to the nemerrine skin; m, oral lobes; md, archenteron; rn, ring nerve; sp, 
apical plate; st, oesophagus; ink, ciliated band. 

apical plate, which functions as a central nervous organ. Inside is the 
simple oecal archenteron, the mouth (blastopore) opening between 
the lappets. By a complicated process of growth and infolding this 
mesenteron becomes enclosed in a separate skin, produced from four 
inpushings (es) ; an anus is formed, and at the time of metamorphosis 

17 


258 PLATHELMINTHES 

the worm thus produced escapes from the rest of the pilidium, which 
quickly dies. 

Order I. Protonemertini. 

Nervous system outside the muscles; no stylets in the proboscis; mouth 
behind brain. Carinella.* 

Order II. Mesonemertini. 

Nervous system in the muscles; mouth behind brain; no stylets. Cephalothrix.* 

Order III. Metanemertini. 

Nervous system inside the muscles, mouth in front of brain; proboscis as a 
rule with stylets. Geonemcrtes* and some species of Tetrastemn.a* terrestrial. 
Am phi poms* (fig. 236, fresh water), Nectonemertes* Malacobdclla* leech- 
like, with posterior sucker, parasitic in lamellibranchs. 

Order IV. Heteronemertini. 

Several muscular layers; nervous system in the muscles; mouth behind 
brain; proboscis unarmed. Linens* Micrura,* Cerebratulus* Zygeupolia* 

Summary of Important Facts. 

1. The PLATHELMINTHES are flattened bilateral animals with- 
out coelom, whose nervous system consists of a supracesophageal ganglion 
and lateral nerve trunks; the excretory system of branched protonephridia. 

2. The TURBELLARIA are the most primitive; the Trematoda and 
Cestoda have descended from them. 

3. The Turbellaria are ciliated externally. They have no anus and 
no circulatory system. The digestive tract consists of ectodermal 
pharynx and entodermal stomach, the latter many-branched in the 
Polyclads, with three main branches in the Triclads, and rod-like in the 
Rhabdocccles. 

4. Polyclads and Triclads are often united under the name Dendro- 
coela. 

5. In the parasitic TREMATODA the cilia are entirely lost or confined 
to the larval stages. Hooks and suckers are present for attachment to the 
host; several in the ectoparasitic forms; only one or two suckers in the 
internal parasites. 

6. In the Distomitv there occur heterogony and alternation of hosts. 
From the egg arises a sporocyst, always parasitic in molluscs, from the 
parthenogenetic eggs of which develop cercarias which become encysted 
Distomiae in the second host, sexual Distomue in the third. 

7. Best known of the Distoma are D. hepaticurn and D. lanceolatum 
(rare in man, common in sheep) and D. hccmatobium in the portal vein of 
man in warm climates. 


ROTIFKRA 


259 


8. The CESTODA have no digestive tract; scolex and proglottids are 
usually developed. 

9. The scolex is the organ of attachment, and as such is provided with 
suckers and frequently with hooks. It also produces the proglottids 
by terminal budding. 

10. The proglottids contain an hermaphroditic sexual apparatus. 

11. The eggs produce a six-hooked embryo which must pass into an 
intermediate host, either by taking the eggs with the food, or the embryo 
must pass into the water, where it infects fishes. 

12. The embryo, in the intermediate host, becomes encysted and 
changes directly to a scolex (pleurocercoid) or into a bladder worm 
(cysticercus) which prcduces internally one or more scolices. 

13. The scolex is freed from its cyst when taken into the stomach of 
the proper host, and then can develop into a tapeworm. 

14. In man occur as cysticerci Tccnia echinococcus (adult in dog) and T. 
solium; as adults Tania solium (cysticercus in pigs), T. saginata (cysti- 
cercus in cattle), and Bothriocephalus latus (pleurocercoid in fish). 

15. The NEMERTINI have a complete alimentary canal with anus, 
blood-vessels and a proboscis dorsal to the digestive tract. 

PHYLUM V. ROTIFERA (ROTATORIA). 

The aquatic rotifers or wheel animalcules are among the smallest Metazoa, 
and can be distinguished from the Infusoria, which they resemble in habits, 
only by the microscope. The body is divisible into three regions, head, trunk, 
and tail. The trunk is covered by a tough cuticle into which head and tail can 


FIG. 238. Diagram of rotifer (after Delage et Herouard). b, brain; fc, flame cell; 
gg, gastric gland; i, intestine; m, mastax; ov, ovary; pg, pedal gland; pi', pulsating 
vesicle of excretory system; s, stomach. 

be retracted. The tail or 'foot' is often composed of rings which can be tele- 
scoped into each other. The last tail ring often bears a pair of pincer-like 
stylets by which, together with adhesive glands, the animal adheres to objects. 
The head is expanded in front to a trochal disc, an apparatus of varying shape, 
surrounded by a ring of cilia of use in swimming and in directing food to the 
ventral mouth. The alimentary canal consists of oesophagus, mastax (chewing 


260 


CCELHELMINTHES 


stomach), glandular stomach, and intestine; all except the mastax ciliated. 
The mastax bears two chitinous jaws (trophi), which in life are in constant 
motion and comminute the food. Above the oesophagus is the cerebral ganglion 
with which simple eyes and peculiar sense organs, the cervical tentacles, are 
frequently connected. The usually single ovary and the paired protonephridia 
empty into the posterior part of the alimentary canal, which thus becomes a 
cloaca. The males are much rarer and smaller and have a much simpler 
structure (fig. 239, 5). Usually the alimentary tract is reduced to a solid cord 
in which the testes are imbedded. 


PIG. 239. Brachionus urceolaris. A, female with four eggs in various stages; B, 
male; C, 'flame' from protonephridia, greatly enlarged; b, urinary bladder; c, cloacal 
opening; (/, gastric glands; g, ganglion, with eye; /;, testis; k, mastax; m, stomach; o, 
ovary; p, penis; /, tentacle; iv, protonephridia. 

The Rotifers have large winter eggs enclosed in a thick shell and smaller 
thin-shelled summer eggs. The latter develop parthenogenetically and by their 
numbers and rapid growth aid in the distribution of the species. The winter 
eggs require fertilization, and have a long resting period, thus serving to tide 
over periods of cold or drought. The adults can withstand a certain amount 
of desiccation; and often occur in damp moss or in eave troughs in a sort of 
sleep from which they are awakened by water. 

In structure the Rotifiers are much like the trochophore larva; of annelids 
and molluscs to be described later. They are primitive forms, connected with 
the ancestors of these groups, and also, as sho\vn by nervous system and excretory 
organs, with the flatworms as well. Most species are cosmopolitan and in- 
habitants of fresh water. Near the Rotifera may be placed the fresh-water 
GASTROTRICHA (Ichtliydiiim* Clurttmotits*) and the marine ECHINO- 
DERID^E, forms which are little understood. 

PHYLUM VI. CCELHELMINTHES. 

The Ccelhelminthes are distinguished from all the forms which have 
gone before by a body cavity, separating the outer body wall from the 


OELHELMINTHES 


201 


intestine; but whether this coelom be homologous in different groups, e.g., 
ncmatodes and annelids, is not settled. The body muscles are 
developed from the outer (parietal) epithelial wall of the coelom 
and hence are 'epithelial muscle cells' (figs. 240, 241). The pro- 


FIG. 240. -Section of A scaris lumbricoides through the pharyngeal bulb; beside it 
a bit of the body wall more enlarged, c, cuticle; d, dorsal line; h, hypodermis; ;, long- 
itudinal muscle; n, nucleus of muscle cell; p, muscle cell; 5, lateral line; v, ventral line: 
w, excretory canal. 


0... 


* -m : 

^tlllJPt 

5 


more 


FIG. 241. Transverse section of Sagitta bipuncta'.a and a bit of the body wall m^ic 
enlarged (after O. Hertwig). c, coelom; dd, entoderm; df, splanchnic mesothelium; 
e, epidermis; m, somatic mesoderm (muscles and epithelium); o, ovary. 

tonephridia of the larval stages are replaced by nephridia (fig. 71), 
connecting the body cavity with the outer world. Internally they begin 
with a ciliated funnel, the nephrostome, and continue as long coiled tubes, 
expanding just before the outer end to a kind of bladder. The gonads 


262 


CCELHELMINTHES 


(fig. 241, 0) are specialized parts of the coelomic epithelium and their prod- 
ucts are usually carried to the exterior by the nephridia (more rarely by 

special ducts), so that here, as in vertebrates, we can 
speak of a urogenital system. A closed blood system 
is now present, now absent. Little in general can 
be said of the nervous system; details will be given 
in connection with the separate classes. 


sc 


f 


\A 


ovd 


-fl 


-ov 


Class I. Chaetognathi. 

These marine forms, a half to two inches long, per- 
fectly transparent, live at the surface of the sea, preying 
on other animals, and from their shapes and rapid motions 
deserve the name Sagitta arrow given some forms. 
The animals swim by means of horizontal fins, one sur- 
rounding the tail and one or two pairs on the sides of the 
trunk (fig. 42). On either side of the mouth are strong 
bristles used in seizing prey (Chaetognath, bristle-jaw). 
Internally the body is separated into head, trunk, and 
tail, by transverse septa which divide the ccelom into 
corresponding chambers. Each segment of the coelom 
again is divided into right and left halves by a mesen- 
tery (fig. 241), supporting the straight intestine, running 
lengthwise through it and terminating at the anus at the 
end of the trunk segment. 

The nervous system is entirely ectodermal. In the 
head is a pair of fused cerebral ganglia (fig. 243), in the 
trunk segment a large ventral ganglion, and these are 
connected by long oesophageal commissures. Of inter- 
est, because characteristic of nematodes and many an- 
nelids, are the relations of the musculature, which 


FIG. 242. FIG. 243. 

FlG. 242. Sagitta hexaptera, ventral view (after O. Hertwig). a, anus; bg, 
ventral ganglion; d, intestine;//, fin; ho, testes; ;, mouth; oi 1 , ovary; m>d, oviduct; sb, 
seminal vesicle; sc, cesophageal commissure; sfl, tail fin; si, sperm; wo, female opening. 

FIG. 243. Head of Sagitta bipunctaia, dorsal view (after O. Hertwig). an, 
nerve to au, eye; g, brain; gh, bristles; rn, nerves to ro, olfactory organs; sc, cesophageal 
commissure. 

consists of longitudinal fibres alone. The body cavity is lined with epi- 
thelium (fig. 241), which, where it abuts against the alimentary tract, is 


II. NEMATHELMINTHES: NEMATODA 263 

called splanchnic mesoderm; that on the side of the coelom towards the ecto- 
derm is the somatic mesoderm. The muscles arise from the latter and are 
divided into four fields, right and left dorsal, right and left ventral. The sex 
cells also arise from the epithelium of the ccelom, the eggs in the trunk segment 
(fig. 241), the sperm in the tail. The eggs are carried to the exterior by special 
ducts. The sperm-forming cells early fall into the ccelom, where they develop 
the spermatozoa. These are carried out by canals which recall the nephridia 
of the annelids. 

The development of Sagitta is significant from two points of view. The 
archenteron (fig. 109) is divided by lateral folds into an unpaired middle portion 
and two paired lateral chambers; the first is the definitive digestive tract, the 
latter the anlagen of the coelomic diverticula. In other words, the ccelom is an 
outgrowth from the archenteron, i.e., is an enteroccele. Second: The gonads are 
derived from a pair of cells in the primitive entoderm, which later are carried 
into the coelomic walls. Here each divides into anterior and posterior cells, 
the anterior developing into the ovary, the posterior into testes. Hence here 
the male and female sex cells are beyond doubt descendants of a common 
mother cell. 

The few species are arranged in two or three genera, of which Sagitta, 
represented on our coasts by S. elegans,* is best known. Spadclla. 

Class II. Nemathelminthes. 

Like the flatworms, the roundworms are characterized by their shape, 
they being thread-like or cylindrical animals whose form is the result of 
the existence of a body cavity in which the viscera are so loosely held that 
on cutting through the muscular body wall they will fall out (fig. 244). 
Since the Nemathelminthes share this coelom with most annelids, the 
distinction between the two rests largely upon negative characters, the 
roundworms lacking the segmentation of the body cavity and the corre- 
sponding ringing or annulation of the body wall. The body cavity 
apparently is different since the splanchnic wall is lacking, the space 
lying between mesoderm and entoderm (pseudocccle). To the Nemathel- 
minthes belong three orders, much alike in habits and appearance but 
differing considerably in structure. Of these the most important are the 
nematodes. 

Order I. Nematoda. 

The nematoda contain numerous species of thread-shaped worms 
varying from o.ooi to i.o metre in length, many of which, through their 
wide distribution as parasites in plants, animals, and man, possess special 
interest. The outer surface is covered by a tough cuticle secreted by the 
subciiticiila, a fibrous ectodermal syncitium (fig. 240), which in cross- 
section shows four thickenings, the dorsal, ventral, and lateral lines. In 
the lateral lines run the excretory vessels, two longitudinal canals, united 
near the head by a transverse vessel opening on the ventral surface by an 


264 


CCELHELMINTHES 


unpaired pore to the exterior. They are related to the coelom by two 
giant cells on either side which send processes into the body cavity. 
These lateral and median lines divide the muscles (here only longitudinal) 
into four fields. These muscles are parts of the somatic epithelium, a 
layer of vesicular cells which by their size (fig. 240) so encroach upon the 
ccelom that scarce space is left for the alimentary canal and reproductive 
organs. 


\.n 


FIG. 244. FIG. 245. 

FIG. 244. Structure of young female A scar is (based on a drawing by Leuckart). 
d, intestine; o, ovary; p, pharynx; s, lateral line; v, ventral line; va, vagina. 

FIG. 245. Diagram of nervous system of a nematode (after Biitschli). c, com- 
missures; ii, dorsal nerve; i, infracesophageal, s, supracesophageal part of nerve ring; 
v, ventral nerve. 

The alimentary canal begins with a terminal mouth and ends with 
the ventral anus in front of the end of the body. The mouth connects 
with the muscular sucking oesophagus, which is expanded posteriorly to a 
pharyngeal bulb and is lined throughout with a cuticle. From this point to 
the anus the stomach-intestine is usually uniform (fig. 244). The oesoph- 
agus is surrounded by a nervous ring which sends forward and back a large 


II. NEMATHELMINTHES: NEMATODA :>J.> 

number of nerves, those in the mid-dorsal and ventral lines being strongest. 
At points on these nerves are collections of ganglion cells, but a formation 
of ganglia, as in the annelids, does not occur (rig. 245). The only sense 
organs are tactile papilla; near mouth and genital opening, and eyespots in 
a few free living forms. 

The sexual organs of these rarely hermaphroditic forms are very 
simple. Males and females are easily distinguished, not only by the 
copulatory organs, but by the openings of the genital ducts. These, in 
the male (fig. 246), are in the end of the alimentary canal, which hence is a 
cloaca. In the female (fig. 244) there is a special genital opening on the 
ventral surface between mouth and anus, the position varying with the 
species. In general the structure of the reproductive organs is alike in 
both sexes. These are long tubes coiled forward and back and ending in 
line threads which produce eggs or sperm (ovaries, testes), while the rest 
serves as seminal vesicle, or receptaculum seminis, and ducts. In the male 
the genital tube is always single; in the female it is usually double, the 
right and left halves uniting a little before the external opening (fig. 244, 
va). Most common of copulatory organs in the male are spicula, bent 
spines, which lie in a sheath behind the vent and can be protruded 
through tfie cloacal opening. Besides there may be valves to right and 
left to clasp the female, or, as in Trichina, the whole cloaca is protrusible. 

Since there is copulation, the eggs are fertilized in the uterus, after which 
they are either laid or retained for more or less of their development, many, like 
Trichina, being viviparous. The postembryonic development depends largely 
upon the mode of life. Free-living species grow by repeated molts without 
much change of form. In many Anguillulidae, which show how free life can be 
transformed into parasitic, there is an alternation of generations (heterogony) 
from a protandric hermaphroditic entoparasitic to a free dioecious generation. 
The occasional suppression of the free generation which occurs in many Anguil- 
lulids leads to the Strongylidas, where the offspring of the parasitic generation 
can live free for a time (rhabditis larva?), but must return to parasitism to undergo 
a metamorphosis and become sexually mature. The free life is shortened again 
in the Ascaridas, where the eggs must pass to the exterior for a longer or shorter 
time, but the embryos only escape when the eggs are taken into another host. 
Lastly, there are species like Trichina where the free life is entirely suppressed 
and transportation from host to host takes place in the encysted condition pas- 
sively by food. This purely parasitic condition leads to species in which the 
rhabditis larva developed in water, enter a second host for encystment as the 
larvce of Filaria medinensis in the Cyclopicke. 

Family i. ANGUILLULIDAE; small thread-like nematodes which live in mud, 
organic fluids or plants, rarely in animals; male with two spicula. Angitillitla 
aceti* vinegar eel, in vinegar and stale paste. RliabJitis (Rhabdoncnia) nigro- 
venosa, lives in mud and stands in heterogony with a second form which lives in 
the lung of frogs. Strongyloides intestinalis, which has recently appeared in 
southern Europe, has a somewhat similar history, the adult stage being reached 
in the human intestine. In the tropics one stage of this is passed in moist earth, 
but in colder climates the free-living generation drops out. Here belong 


286 


CCELHELMINTHES 


Tylenchus tritici and Hcterodera schachti, the first doing great damage to wheat, 
the second to turnips in Europe. T. devastatrix attacks rye and hyacinths. 

Family 2. ASCARID^E. Mouth with three lips; males with two spicules. Nu- 
merous species in lower vertebrates, Ascaris lumbricoides,* the round worm of 
man (fig. 246), inhabits the small intestine, often in enormous numbers. The 
females are. about 5-6 inches, the males 4 inches in length. A female contains 
about 60,000,000 eggs. Shortly after fertilization the eggs pass out with the 
faeces, but develop without intermediate host if, in the course of two or three 


FIG. 246. Dorsal, end, and ventral views of head and hinder end of male Ascaris 

lumbricoides (from Hatschek.) 

months, when . the embryo is formed, they are taken into the human 
intestine. The development of the pinworm, Oxyuris vermicular is*- is some- 
what similar except that the embryos are developed in the egg at the time of 
oviposition, and hence after a shorter stay outside the body are capable of in- 
fection. The white worm, not half an inch long, lives in the rectum, especially 
of children, and causes intolerable itching. Ascaris mystax* occurs in dogs 
and cats (occasionally in man). A. megalocephala* (a favorite animal for 
cytological researches) and Oxyuris equi in the horse, do little harm. Heterakis 
maculosa often destroys whole flocks of pigeons. Family 3. STRONGYLUXE. 


o 


FIG. 247. Anterior end of hook worm, Ankvlostoniii duodenale (after Looss). 
di, lower teeth; g, lateral gland; m, oral capsule; p, dorsal tooth; o, oesophagus; v, ven- 
tral teeth. 

These are readily recognized by the bursa of the male, a broadening of the 
hinder end of the body by two wing-like processes, which contains two spicula. 
Frequent but not constant is a widened capsule surrounded by papillae at the 
mouth. Strongyltts* in domestic animals. Syngamus frachealis,* half to three 
quarters of an inch in length, the male and female always in pairs, cause the disease 
known as 'gapes' in fowl. Ankylostomum (Dochmius) duodenale* (fig. 247), 
about two fifths of an inch in length, lives in the small intestine of man, causing 
severe loss of blood. The eggs develop in moist earth, and hence people who 


II. NEMATHELMINTHES: NEMATODA 


L'i',7 


drink muddy water (Fellahin of Egypt) or work much with clay (potters and 
brick-makers) are especially subject to infection. It was first known in Egypt; 
caused considerable trouble during the building of the St. Gotthard tunnel in 
Switzerland. More recently it has been recognized as frequent in our southern 
states, where it has become notorious under the common names of hook-worm 
and lazy worm. It has been thought that the Ankylostoma larvas obtain 
entrance to man through the skin, as in bathing, etc. 

Family 4. TRICHOTRACHELID^:. These are called 'hair necks' because 
that part of the body which contains the pharynx is hair-like and elongate. 
Trichocephalus dispar* of man (fig. 248, A), about an inch or an inch and a half 
in length, lives with its neck in the wall of the intestine near the caecum. Since 
it does not move, it causes little injury. 


,->Sj'.-^* r '/.-- 

ip%>^ 


A C 

FIG. 248. A, Trichocephalus ilispar, male with anterior end embedded in intestinal 
wall (from Leuckart). B, Trichina spiralis, male (.from Hatschek). cl, cloaca; 
t. testes. C, Trichina in muscle (from Boas). 

Trichina spiralis* (fig. 248, B, C), is much smaller, but much more dangerous. 
Two stages are to be distinguished, the encysted muscle Trichina and the sex- 
ually mature intestinal Trichina. The first was discovered in a human body 
in 1835; the latter was not known until much later. In the encysted stage it 
occurs in the muscles of pigs, rats, mice, man, rabbits, guinea pigs, dogs, etc. 
(never in birds), enclosed in an oval capsule about o. 4 to o. 6 mm. long and hence 
recognizable by a practised observer with the naked eye. Certainty in their 
recognition demands a low power of the microscope. Coiled up in the capsule 
is the worm, about i mm. long, which is not yet sexually mature. To attain 
this it must be transported into the intestine of another host. When, for instance, 
man eats trichinosed pork the worms are freed by the digestive fluids and, enter- 
ing the small intestine, become sexually mature in a few days. The female 
(3-4 mm. long, the male 1.5 mm.) penetrates the intestinal villi and in course 
of a month gives birth to 1500 (some say 10,000) living young, after which she 
dies. The young enter the lymph vessels, are carried by way of the thoracic 
duct into the blood-vessels, and wander into the muscles, especially those which 
are much worked, like the diaphragm, eye muscles, and muscles of the neck, 
and which consequently have a rich blood supply. They enter the sarcolemma 
of the muscle, destroy the muscle substance, and finally become enclosed by a 
capsule secreted by the host. The wandering takes place about the second 
or third week after infection, the encystment in about three months. A slight 
infection causes disagreeable symptoms; but where large numbers obtain 
entrance the cases are frequently fatal. The worst epidemic known was in 


268 


CCELHELMINTHES 


Emmersleben, Saxony, in 1884, where 57 died in four weeks from infection 
from one pig. 

Family 5. FILARIID.E. Extremely elongate, hair-like worms. Dracuncnliis 
medinensis, the guinea worm (the female about a yard long, and about as large 
as stout packing twine), produces abscesses beneath the skin in which the 
worm is coiled up. The embryos break through the wall of the mother and 
must enter the water and penetrate a small crustacean, Cyclops. It is appar- 
ently introduced into the human system by swallowing the Crustacea with drink- 
ing water. The worm occurs in tropical America. 

Filaria sanguinis hominis, 3 to 6 inches long, lives in the lymphatic glands of 
man, the young escaping into the blood, often in immense numbers. They 
often pass through the kidneys, where they produce serious disturbance. There 
is possibly a connection between them and elephantiasis. The intermediate 
host is apparently the mosquito. As yet they are known only in the tropics. 
Other species occur in man and other animals. 

Family 6. MERMITHID/E. Elongate nematodes in the body cavity of insects; 
they pass into damp earth, where they become sexually mature. They share 
with the Gordiacea the name 'hairworms.' Mermis* 

Order II. Gordiacea. 

The hairworms resemble the nematodes in general appearance, but differ 
greatly in structure. The body cavity has both splanchnic and somatic epithe- 
lium; the intestine is supported by mesenteries (fig. 249) ; there is an cesophageal 


FIG. 249. Transverse section of young Gordius (after von Linstow). a, hypo- 
clermis; b, muscular layer; c, cuticle; d, parenchyma; e, f, muscles and mesenteries; g, 
alimentary canal; h, nervous system. 

nerve ring and unpaired ventral nerve cord, and the female genitalia open into 
the cloaca. The adults live in water, where they lay their eggs; the larva? live 
in insects, there being in some cases an alternation of hosts. They are popularly 
believed to be horse hairs changed into worms. Gordius* Chordodes* Near 
the Gordiacea must be mentioned the marine Nectanema,* young stages appar- 
ently passed in the mosquito. 

Order III. Acanthocephala. 

The adult spine-headed worms live in the alimentary canal of vertebrates. 
They resemble the Ascaridae (p. 266), but are easily distinguished by the pro- 


III. AXXKLIDA 


260 


r*K, 


nt-4 


< 


ffi 


boscis, which may be retracted by muscles and exserted by contraction of 
the muscular body wall. This proboscis bores into the intestinal wall ami is 
held in place by numerous retrorse hooks (fig. 250). 1 he 
entire absence of an alimentary canal marks them off from 
Xematodes and Gordiacea, as also the peculiar structure 
of the reproductive organs and a closed vascular system in 
the body wall which extends into two sacs, the lemnisd, 
lying beside the proboscis sheath. The unpaired ganglion 
lies on the proboscis sheath between the lemnisci. An 
intermediate host occurs in development, the larva living 
in an arthropod. Thus the larva of Echinorhynchus gigas* 
of the pig lives in the larva of the 'June bug' (Melohntlui), 
that of E. proteus of European fresh-water fishes in Crusta- 
cea. E. hominis is extremely rare in man. 

The Acanthocephali are dioecious. The ovaries of the 
female early break up into groups of eggs which float in 
the body cavity. The ripe eggs have a peculiar method 
of escape from the body. There is a muscular uterus 
which connects by two narrow canals with the vagina and 
thus with the outer world. The uterus picks up immature 
and fertilized eggs indiscriminately by its wide mouth, but 
only those which are elongate, have a shell and contain 
embryos can pass the canals; the immature eggs are led 
through a ventral opening back to the ccelom. In E. 
gigas protonephridia open beside the genital opening. 


7, 


Class III. Annelida. 

The metamerism, which occurs in a slight degree 
in the Chaetognathi, reaches its highest development 
in the Annelids, where it appears both in the outer 
ringing of the body and in the arrangement of the 
most important systems excretory organs, nervous 
system, blood-vessels internal segmentation. To 
this is added an extraordinary increase in number 
of body segments (somites, metameres), which can 
far exceed a hundred. The epithelial longitudinal 
muscles are reinforced by an outer layer of mesen- 
chymatous circular fibres. We can thus define the 
Annelids as worms with ccelom and with external 
and internal segmentation. In the development 
there frequently occurs a type of larva, the trocho- 
phore (p. 338). 

The above account applies most closely to the 
Chsetopoda and Archianellida. In other forms one 
may be lacking in the Gephyrsea segmentation of 
Hirudinei most of the ccelom and the trochophore. 


-vet 


\--dr 


FIG. 250. Male 
Echinorhynchus angu- 

status (from Hatsch- 
ek). b, penis sac; 
de, seminal vesicle; 
dr, glands; , gan- 
glion; /, lemnisci; lig, 
ligament; >,;,,, re- 
tractors of proboscis 
and its sheath; />, 
pL-nis; r, proboscis; 
rs, proboscis sheath ; 
t, testes; ?</, vas def- 
erens. 

or more features 

the body; in the 

Yet these are so 


270 


CCELHELMINTHES 


closely related that they must be included under the common head; the 
missing characters have been lost during evolution. 


FIG. 251. Diagram of annelid somites (brig.), am, acicular muscles; c, ccelom; 
cm, circular muscles; cv, circular blood-vessels; d, dorsal blood-vessel; i, intestine; Im, 
longitudinal muscles; m, mesentery; n, nerve cord; na, nephridium; ne, no, neuro-'and 
notopodia, forming parapodium; 5, septum; so, somatopleure; sp, splanchnopleure; t, 
typhlosole. 

Sub Class I. Chcrtopoda. 

These, like the Nematoda, are cylindrical worms, but are at once 
distinguished by the segmentation. Deep circular constrictions (fig. 252) 

bound the somites externally. Inter- 
nally the ccelom is divided by the 
septa delicate double membranes 
which extend from the ectoderm to 
the alimentary canal into as many 
chambers as there are metameres, 
while a longitudinal mesentery, also 
double, separates the ccelomic pouches 
of the right side from those of the 
left (figs. 251, 253). The alimentary 
canal has a terminal anus, while the 
mouth is ventral and is overhung by 
the preoral segment, the prostomium. 

Nervous system, blood-vessels, and 
excretory organs are influenced by the 
segmentation. The nervous system 
is on the ladder plan (p. 113). It 


FIG. 252. Earthworm, side view 
and anterior end enlarged (after Vogt 
and Jung). i, first segment with 
mouth and prostomium; 15, male 
sexual opening; 33-37, clitellum. 


begins with a supracesophageal ganglion ('brain') in the prostomium, 
from which the cesophageal commissures pass around the oesophagus 


III. ANNELIDA: CfLTOPODA 


271 


to form the ventral chain, which consists of as many pairs of ganglia, 
united by longitudinal commissures, as there are somites present. These 
ganglia of the ventral chain are closely similar, since the segmentation 
of the body is homonomous. There is but the slightest division of labor 


FIG. 253. Anterior end of Nais elinguis. h, cerebrum, connected by commissure 
with ventral chain, ; dg, dorsal, vg, ventral blood-vessel; ;;/, muscular layer of skin; 
df, if, dorsal and ventral duetse; d, septa; k, prostomium; o, mouth. 

among the somites, and hence they differ but slightly among themselves. 
The prostomium always bears tactile organs and frequently eyes, which 
in many marine forms are highly developed, \vii.h lens, vitreous body, 


rm vt , bin tm 

FIG. 254. Schematic cross-section of an annelid (after Lang), ac, aciculum; 
b, choetae; bm, ventral nerve cord; dc, dorsal cirrus; dp, notopodium; k, gill; Im, longi- 
tudinal muscles; md, digestive tract; />, nephridium; 07', ovary; rm, circular muscles; 
tm, transverse muscles; tr, nephrostome; vc, ventral cirrus; vd, vv, dorsal and ventral 
blood-vessels; vp, neuropodium. 

and retina (fig. 84, 1, II) . Statocysts are rare, but occur in diverse groups. 
Ciliated pits (olfactory ?) occur on the head, goblet organs (taste) on head 
and trunk, and lastly, lateral organs, sensory structures of unknown 
function, may be metamerically arranged. 


272 


CCELHELMINTHES 


The blood-vessels usually are represented by two main trunks which fre- 
quently (as in earthworms) contain blood colored red by haemoglobin. One 
trunk is dorsal, the other ventral, to the intestine, the two being connected 
by vessels (figs. 251, 255) in each segment. The blood passes forwards 
in the dorsal vessel, backwards in the ventral. It is propelled by con- 
tractile portions of the vessels; usually the dorsal vessel pulsates, but as 
in the earthworms, certain of the circular vessels in the anterior part of the 
body may function as hearts (fig. 255, c ). Rarely, as in the Capitellida?, 
circulatory organs may be lacking. 

dg Ig a 


oe 


ph st gc 


I b 


CO 


pt i-g p b < 

FIG. 255. Anterior end of Pontodrilus marionis (after Perrier). a, vascular 
arches; b, ventral nerve chain; c, 'hearts'; co, oesophageal commissure; dg, dorsal 
blood-vessel; ds, septa; gc, cerebrum; I, retractors of pharynx; Ig, lateral blood-vessel; 
o, ovary; oe, oesophagus; p, receptacula seminis; ph, pharynx; pt, ciliated funnels of vas 
deferens; s, nephridia; 5/, pharyngeal ganglion; vd, vas deferens. 

The excretory organs (nephridia) were formerly known as 'segmental 
organs,' since they occur in pairs in each segment. Strictly speaking, 
each organ belongs to two segments. It usually begins in the anterior of 
the two with a ciliated funnel (nephrostome), passes through the septum, 
and, after convolutions, opens to the exterior in the second segment. 
The nephridial canal (usually lined with ciliated epithelium) often serves 
to carry off the sexual products, which in all chtetopods, arise in the 
ccelomic epithelium. 

The nephridia of Annelids seem to be derived from protonephridia (fig. 
256, I, II), which finally opened into the ccelom (III, IV). In many species 
they are simple or branched tubes, closed internally by solenocytes, large cells 
drawn out into a tube which empties into the blind end of the excretory tubule 
and has a flagellum in the interior (fig. 69, p. 106). With the development of 
the nephrostome the branched condition and the solenocytes are usually lost. 
The relations of the nephridia to the sexual products appear to be secondary, 
and are brought about by large ciliated grooves of the peritoneal epithelium. 
There are three possibilities, (i) The sexual products are emptied by dehis- 
cence of the body wall; the ciliated organs are phagocytic. (2) The ciliated 
grooves at the time of sexual maturity open directly to the exterior and carry 
off the eggs and sperm (I and III). (3) They connect with the excretory 
tubules, be these nephridial or protonephridial (II or IV), the segmental 


III. ANNELIDA: CtL-ETOPODA 


273 


organs thus becoming sexual ducts. The second of these conditions explains 
the coexistence of reproductive funnels and nephridia in the genital segments 
of many oligochastes. In the oligochaites there are other modifications: Opening 
of the nephridia into fore or hind gut, connection of the tubules into a network 
with several openings to the exterior in each segment (Megascolicida,-). 

In many marine annelids there occurs a metamorphosis in which 
pelagic larvae occur. These, in spite of many modifications, are com- 
parable with 'Loven's larva,' the trochophore already described (p. 238). 


II. 


FIG. 256. Different relations of nephridia and sexual ducts in chastopods (after 
Goodrich). I, hypothetical primitive condition; II, Phyllodoceids and Goniads; III, 
Dasybranchus; IV, Syllids, Spionids, etc. In I and III the ciliated grooves (sexual 
sacs, g) lead the sexual products (ei) direct to the exterior; in II and IV they empty 
into the nephridial canals, which are either protonephridia (I and II) covered with 
solenocytes, or (III and IV) are nephridia. 

The differences largely consist of modifications of the ciliary apparatus; 
the number of bands may be increased (polytroclie larvae), or a single band 
may occur at the middle (mesotroche) or at the end (telotroche) of the body. 
The larva becomes a segmented worm by the hinder end of the larva 
growing out and dividing into segments (fig. 257, B}. In this growth new 
mesoderm develops as a pair of bands (usually from a pair of cells at the 
hinder end, the teloblasts). This mesoderm divides, from in front back- 
ward, into the primitive segments. Each of these become hollow, forming 
a coelomic chamber. Since these, right and left, grow around the diges- 
tive tract, they give rise to the somatic and splanchnic mesothelium and 
form part of the digestive and body walls. Where they come in contact 
above and below the intestine they form the mesenteries which frequently 
is 


274 


CGELHELMINTHES 


disappear in the adult. In many worms the septa between the somites 
also breaks down and the coelomic cavities unite into one. The nephridia 
also arise independently of the protonephridial system, which is often called 
head kidney because the chief part of the trochophore forms the head of 
the adult. 

B 


kn 


mes 


mes 


FIG. 257. A, larva of Polygordius; B, same changing to segmented worm (after 
Hatschek). a, anus; kn, excretory organ; mes, segmented mesoderm. 

The land and fresh-water annelids develop directly, but the embryos pos- 
sess a reminiscence of a larva in that the head lobes are very apparent and 
contain protonephridia, which leads to the conclusion that these animals earlier 
had a metamorphosis. From the resemblance of the trochophore to the 
Rotifera the farther conclusion is drawn that the annelids have descended from 
rotifer-like ancestors, the body cavity, nephridia, blood-vessels, and ventral 
nerve chain being new formations. 

Besides sexual reproduction many fresh-water and marine species 
reproduce asexually, this being possible from the great homonomy of the 
segmentation. By rapid growth at the hinder end as well as at a more 
anterior budding zone numerous somites are formed, which separate in 
groups from the parent to form young worms. In some cases the forma- 
tion of new somites may take place more rapidly than the separation, the 
result being chains of worms (fig. 258) which in some instances branch. 

By a combination of sexual and asexual reproduction a typical alternation 
of generations occurs, the origin of which receives light from the following facts: 


III. ANNELIDA: CH^TOPODA 


275 


In many polycha^tcs which reproduce exclusively by the sexual process the srx- 
less, slowly-moving young (a take) becomes so altered at sexual maturity as to 
have been described under another name. It becomes very active in its move- 
ments, and the hinder somites, which contain the gonads, develop special bristles 
and parapodia (fig. 263, A). Thus many species of Nereis pass into the ' Hcter- 
onereis' stage. In other Polychrctes the sexual part (epitoke) separates from the 
sexless atoke portion and swims freely, while the atoke produces new epitokes. 
In Samoa Eunice viridis reproduces in this way, the epitokes coming to the sur- 
face at certain times in incredible numbers, forming the 'palolo worm,' 
a delicacy in the Samoan diet. In still other species the epitoke regenerates the 
head and thus becomes an independent generation. Syllis and Heterosyllis are 
thus related. The Autolytidas are most complicated. Here the atoke, by 


FIG. 258. 


259- 


FIG. 258. Budding in Myrianida (after Milne-Edwards). The sequence of letters 
shows the ages of the individuals. 

FIG. 259. Arrangement of a bristle in an Oligochaete (after Yejdowski). e, 
epithelium; rm, Im, circular and longitudinal muscles; m, muscle of the follicle; b l , 
chseta follicle, its chaeta in function; b 2 , follicle for replacement, the formative cell at its 
base. 

budding as in Myrianida (fig 258), forms chains of dimorphic individuals which 
later separate. The individuals of male chains (fig. 263) were formerly de- 
scribed as 'Polybostrichus,' the females as 'Sacconereis.' This same homonomy 
explains the regenerative powers of many worms. Thus if certain earthworms 
be cut in two, they will live and reproduce the lost parts. 

Another important character of the ChcTtopoda is the possession of 
bristles or c/nclcc. These arise in special follicles, singly or several in a 
group, there usually being four groups right and left, dorsal or lateral and 
ventral in each somite. Each follicle (fig. 259) is a sac of epithelium open- 
ing to the surface and having at the base a special cell for the development of 
each bristle. The developed chaetse project from the follicle and, moved by 
appropriate muscles, form small levers of use in locomotion. Their num- 
bers, shape, and support are of much systematic importance. 


276 


CCELHELMINTHES 


K 


Order I. Polychaetse. 

The Polychsctae owe their name to the fact that each group of bristles 
contains many chaetae; but more important is that the bristles of each side 

are supported by a fleshy outgrowth of the 
somite, the parapodium, in which two por- 
tions corresponding to the bunches of 
bristles dorsal, notopod'mm; ventral, neuro- 
podium- may be recognized (fig. 254). 
This is the first appearance of true appen- 
dages, but they differ from those of Arthro- 
poda in not being jointed to the body nor 
jointed in themselves. On the dorsal sur- 
face may occur diverse outgrowths, known, 
according to position or function, as cirri, 
elytra, gills, etc.; on the head, palpi and 
tentacles. The cirri are long processes on the parapodia, and like palpi 
are tactile (fig. 254). Elytra are thin lamelke which cover the back like 

shingles and thus protect the 
body (fig. 262). 

Nearly all Polychaetes are dioe- 
cious and undergo a more or less 
pronounced metamorphosis; with 
few exceptions (M anyimkia* from 
the Schuylkill, Nereis* in California) 
they are marine. They are usually 


FIG. 260. Head with pro- 
truded pharynx of Nereis 
versipedata (after Ehlers). c, 
cirri, k, jaws; I, head with eyes; 
p, palpi; t, tentacles. 


FIG. 261. 


FIG. 262. 


FIG. 261. Amphitrite oriiata* (from Verrill). 

FIG. 262. Head of Polynoe spinifera (after Ehlers). Back entirely covered with 
elytra; cirri and parapodia projecting at the sides. 

divided according to their habits into fixed (Sedentaria) and free forms (Er- 
rantia). The Sedentaria feed on vegetable matter, usually form leathery 


III. ANNELIDA: CrLTOPODA 


.1 1 


organic tubes in which foreign matter may be incorporated or which may be 
calcified. The worms project their anterior segments from the tubes. The 
Errantia often burrow, but from time to time swim about preying on other ani- 
mals. Correlated with habits are differences in structure. In the Errantia the 
head and trunk are not very different; the anterior part of the alimentary tract 
can be protruded as a proboscis, and, corresponding to their predaceous habits, 
is often armed with strong jaws (fig. 260). The Sedentaria have no such 
teeth, but there is a greater difference between anterior and posterior somites, 
In the latter the parapodia are weakly developed, and this part resembles the 
Oligocruetes in appearance. The head and beginning of the trunk (thorax) are 
richly provided with gills and tentacles for respiration and taking food (fig. 261). 


B 


FIG. 263. New England Annelids (from Emerton and Verrill). A, male Autolytus; 
B, Sternaspis fossor; C, Cistenides gouldii; D, Clymene torquata. 

Sub Order I. ERRANTIA. Predaceous annelids with strongly armed 
pharynx. The EUNICIDJE, mostly represented on our shores by small species, 
contains some species a yard in length. Diopatra,* Nothria.* ALCIOPID^E, 
transparent, pelagic, with large, highly developed eyes (fig. 84). The SYLLID/E 
usually have three long tentacles; Autolytus* (fig. 263), Myrianidd* (p. 275). 
The POLYNOID/E* (Lepidonotus,* Polvnoe?' (fig. 262), are bottom forms with 
elytra. NEREIDS; Nereis virens* clam worm of all northern seas. 

Sub Order II. SEDENTARIA (Tubicola). These cannot wander, but 
live in tubes. SABELLID/E, tube is membranous and there is a crown of gills; 
Myxicola,* Chone,* Manyunkia* SERPULID^;, tube calcified and closed by an 


278 


CCELHELMINTHES 


operculum on one of the gills. Hydroidcs;* Proiula* ARENICOLID^E,* burrow 
in sand, have gills on the sides of body. MALDANID^E (Clymene,*fLg. 263) have 
extremely long segments and build tubes of sand. TEREBELLID^E (Tcrebella* 
Amphitrite (fig. 261), numerous filiform tentacles and branched gills on the 
anterior end. 

The ARCHIANELLID/E, which lack bristles and parapodia, must be 
placed near the Polychaetae and are usually regarded as very primitive forms 
which in structure and development (fig. 257) are of importance in the phylo- 
genesis of the Annelids. Polygordius* 

Order II. Oligochaetae. 

The Oligochsetes are almost as preeminently fresh-water and terrestrial forms 
as the Polychuetes are marine. They are in most respects simpler than their 
marine relatives, apparently the result of degeneration. Eyes are rudimentary 
or lacking, there are no palpi, cirri, or tentacles; gills are rare, but most striking 


FIG. 264. Aulophonis vagus* in tube of Pectinatella statoblasts (after Leidy). 


FIG. 265. Sexual organs of Lunihrlcus agricola (from Lang, after Vogt and 
Yung). The seminal vesicles of the right side are removed, bm, ventral nerve 
cord; bv and bl, ventral and lateral rows of setae; st, receptacula seminis; sb, seminal 
vesicles of .the left side, connected with a median unpaired seminal capsule (sbii). 
Enclosed in the latter are the testes (/z), and the seminal funnels (t), which lead into 
the vas deferens (rd). o, ovaries; iv, ciliated funnels leading to oviducts with egg 
capsule (e); di, dissepiments; 8-15, eighth to fifteenth segments. 

is the absence of parapodia, the bristles projecting directly from the body wall 
(fig. 259). The chaetas may be regularly distributed around each somite (Pcri- 
chceta) or gathered on the sides (Megascolex) or arranged in four groups so that 
in the animal four longitudinal rows occur. The animals are hermaphroditic, 
testes and ovaries lying in different somites. Usually the skin near the sexual 
openirrgs is thickened by numerous glands, forming a clitellum (fig. 252), which 
secretes the egg cocoons. and also functions in copulation, secreting bands which 
hold the animals together so that the sperm from one passes into the receptac- 


III. ANNELIDA: GEPHYR.^A 


279 


ulum seminis of the other. After impregnation the eggs are usually enclosed in 

cocoons. 

Sub Order I. LIMICOLA (Microdrili). Mostly fresh-water. The 

TUBIFICID.E, in consequence of the red blood, when present in large numbers 

color the bottom red. They form tubes in the 
mud. Tub if ex,* Clitellio irroratus* common on 
seashore. NAIDID/E, transparent forms living on 
water plants, reproduce asexually. Dero* and 
Aulophorus* (fig. 264) have gills around the anus. 
ENCHYTR.'EIDJ; Pachydrilus. DISCODRILID/E 
(Myzobdclla), parasitic. Sub Order II. TERRI- 
COLA (Macrodrili). Terrestrial; the earthworms, 
our species of moderate size, in the tropics large 
(Megascolex australis four feet long). Lumbricus* 
Allobophora*; Diplocardia* with double dorsal 
blood-vessel; Pcricliccta* introduced from the 


a 


/ 


(I 


1 d 

r * 

( a 


' 


pr 
g 


\... ...nc 


V 


FIG. 266. FIG. 267. 

FIG. 266. Anatomy of Phascolosoma gonldi (orig.). a, anus; a, anterior retractors; 
d, digestive tract; g, gonads; m, mouth; n, nephridia; nc, ventral nerve cord; pr, posterior 
retractors. 

FIG. 267. Larva (trochophore) of Echiurus (after Hatschek). a, anus; d, 
intestine; hw, postoral cilia; kn, protonephridia; m, mouth; mes, mesoderm bands with 
indication of segments; n, ventral nerve cord; sc, cesophageal commissure; sp, apical 
plate; vw, preoral ciliated band. 

tropics. Most species burrow through the earth, swallowing the humus and 
casting the indigestible portions on the surface. They loosen the soil and con- 
tinually bring the deeper parts to the surface. Details of the reproductive 
organs of one species in fig. 265. These vary and are used in classification. 

Sub Class II. Gephyrcca. 

The exclusively marine Gephyraea are distinguished at the first glance 
from the Chietopoda by the absence of segmentation. The body is oval 


2SO ' 


CCELHELMINTHES 


or spindle-shaped, circular in section. The mouth, at the extreme 
anterior end, is either surrounded by a circle of tentacles (fig. 266), 
retracted together with the anterior end of the body by internal muscles, 
or is overhung by a dorsal preoral lobe or proboscis which may be several 
times the length of the body and forked at its tip (fig. 268). 

Internal segmentation is also lost, septa being entirely lacking. The 
nephridia are reduced in number, at most but three pairs being present, 
and in some but a single unpaired organ. They are the sexual ducts ; 
the duetiferi have special excretory organs (fig. 268, g) covered with 
branching canals opening to the body cavity by nephrostomes and 

,4 


B 


FIG. 268. Bonellia viridis. A, female (after Huxley); B, male (after Spengel). c, 
cloaca; d, rudimentary intestine; g, excretory organ ; i, intestine; m, muscles supporting 
intestine; 5, balls of spermatozoa in B, in A, proboscis (preoral lobe); u, single segmental 
organ, functioning as oviduct ;vd, nephridium with ciliated funnel serving as vas deferens. 


emptying into the intestine. These resemble somewhat the branchial 
trees of the holothurians (infra), and hence the gephyrasa were formerly 
supposed to bridge the gap between holothurians and annelids, whence the 
name (ye'^vpa, bridge). The vascular and nervous systems are more 
like those of other annelids. The vascular system consists of a sinus 
around the digestive tract and a dorsal and usually a ventral longitudinal 
trunk; the nervous system of a brain, cesophageal collar, and ventral cord, 


III. ANNELIDA: HIRUDINEI 281 

the latter without division into ganglia. The relations of the Gephyra?a 
to the Cha-topoda are shown by the development. In some (Ckctiferi) 
there is a trochophore (fig. 267) from which the worm arises, as in the 
Chastopoda, by growth at the hinder end; this at first has a segmented 
coelom and nervous system, the metamerism being lost later. 

Order I. Chaetiferi (Armata, Echiuroidea). 

With spatulate preoral lobe, often forked at the tip; at least a pair of ventral 
seUe; a trochophore in development. Echiurus* northern, T/ialassema* In 
Boncllia there is a marked sexual dimorphism (fig. 268); female, 2 to s inches, 
has a proboscis a yard long. The male, only i mm. long, totally different in 
form and color, lives parasitically in the oesophagus of the female (fig. 268, B). 

Order II. Inermes (Achaeta, Sipunculoidea). 

Distinguished by lack of chastas, the mouth surrounded by tentacles, and 
the dorsal and anterior position of the anus. The larva is a modified trocho- 
phore without preoral ciliated band and without segmentation; only two, 
sometimes but one, nephridia. The vascular ring around the mouth, with its 
dorsal, heart-like prolongation, is not circulatory. It is a special part of the 
coelom for the protrusion of the tentacles and has no connection with the in- 
testinal blood sinus. It is doubtful whether the Inermes are related to the 
Chuetopoda. Some unite them with Brachiopoda and Polyzoa in a group 
Prosopygii, so called in allusion to the dorsal position of the anus. Phascolosoina* 
(fig. 266). Phascolion* Sipiinculus.* 

Order III. Priapuloidea. 

No tentacles, mouth with teeth, terminal anus, two protonephridia united 
with sexual organs and opening either side of vent. Priapidus. 

Sub Class III. Himdinei (DiscopJiori). 

Three points of external structure distinguish the leeches from the 
chcetopods. First, the absence of bristles (except in Acant/iobdclla) and 
the presence of two suckers; the one on the posterior ventral surface is 
used only for attachment and locomotion, the other, sometimes scarcely 
differentiated, surrounds the mouth and is used in sucking the food. In 
locomotion anterior and posterior suckers are alternately attached, the 
body being looped up and extended like that of a 'span worm.' The 
animals can also swim by a snake-like motion of the whole body. 

A second point is the fine ringing of the body, there being usually many 
more rings than somites, the segments being divided by secondary con- 
strictions, there being three, five, or even eleven rings to a segment. The 
middle or one of the anterior rings often bears strongly developed 
sense organs. As in earthworms, certain of the somites may develop 
a clitellum which secretes the egg cocoons. 


282 


CCELHELMINTHES 


A third character is the marked dorsoventral flattening of the body 
(except in Ichthyobdellidie and a few other forms), the animals thus re- 
calling the flatworms. This may be the result of the very slight develop- 
ment of the ccelom. In most leeches there is a body parenchyma, traversed 
by muscles in which the organs are immediately im- 
bedded (fig. 269). 

The alimentary tract bears paired diverticula (fig. 
270), varying in number in different species. Between 
the last and largest pair of these is the intestine, which 
opens dorsal to the posterior sucker. The jawed and 
jawless leeches show considerable differences in the 
pharyngeal region. In the first there are three semi- 
circular jaws in the pharynx, the free edge of each 
armed with teeth (fig. 271). To these are attached two 
muscles, one to'retract them, while the other exserts and 
rotates them, causing a triradiate wound from which the 
blood flows. This bleeding is difficult to staunch, since 
a secretion of glands on the lips and between the jaws f/^ Xf 

jf - ^ 

SC 


* ; 

'-' 

T 

c.y - 


- v- 

m 


. : 

:f"5 


-a 

L<?i 


.#" 


vW 


: f 


'*& 


1m 


I 


w 

FIG. 269. FIG. 270. 

FIG. 269. Transverse section of Hirudo medicinalis (from Lang), dm, Im, rm, 
dorsoventral, longitudinal, and cirular muscles; vl, vd, vv, lateral, dorsal, and ventral 
blood-vessels, the latter surrounding the ventral nerve cord, ni; h, testes; vd, vas deferens; 
md, midgut; np, nephridial tubule; enp, urinary bladder. 

FIG. 270. Digestive tract of Hirudo medicinalis (after Hatschek). a, rectum 
and anus; b, last crccal pouches of intestine; d, intestine, opened at d l ; dg, dorsal, Ig, 
lateral blood vessel; o, oesophagus; sc, nephridia. 

hinders the coagulation of the blood. In the jawless leeches a sharp 
conical process arising from the pharynx serves for wounding and 
sucking. The vascular system usually contains red blood, and consists, 
in the Gnathobdellida?, of four longitudinal trunks, a dorsal, two lateral, 


III. ANNELIDA: HIRUDIXEI 


2.s:{ 


and a ventral, the latter surrounding the ventral nerve cord. These are 
connected by a complicated system of capillaries. 

The nervous system consists of brain and ventral cord, the latter con- 
taining frequently twenty-three ganglia (the first of five fused, the last of 
seven). Nerves from the brain go to the eyes. Right and left of the 
ventral cord are the hermaphroditic sexual organs. In Hlrudo medicina- 


FIG. 271. Hirudo medicinalis, medicinal leech (after Leuckart). a, anterior end 
with three jaws (k)\ h, a single jaw with its muscles. 

FIG. 272. Nervous system, blood-vessels, sexual organs, and nephridia of a leech, 
ventral view. /;, testes; hb, urinary bladder; Ig, lateral blood-vessel; n, ventral nerve 
cord; nh, epididymis; ov, ovary; p, penis; sc, nephridia; u, uterus and vagina; vd, vas 
deferens; vg, ventral blood-vessel. 

Us (fig. 272) there are nine pairs of testes (//), the ducts of which unite to 
form a vas deferens on either side (rd). These pass forward, form by 
coiling a so-called epididymis (nh) and empty into the median unpaired 
penis (/>). In the jawless leeches the penis is lacking and the sperm, in 
pointed sprematophores, is inserted in the tissues of the leech. In the 
space between the epididymis and the first pair of testes are the ovaries 
(ov) and oviducts and an unpaired vagina (). The nephridia (17 pairs 
in this species) are complicated canals and are provided with bladder- 
like expansions. 

That the Hirudinei are true annelids and not segmented plathelminthes 
is based upon the view that their ccelom is reduced by ingrowth of parenchyma 
and altered to canals connected with the vascular system. At any rate the ven- 
tral and lateral vessels are to be regarded as remnants of a ccelom. In Clepsinc, 
etc., there are the dorsal and ventral blood-vessels of the Chastopoda and besides 
four longitudinal coelomic sinuses connected by lacunar spaces. The dorsal 
sinus encloses the dorsal blood-vessel, the ventral many of the viscera, among 
them the ventral nerve cord. These sinuses also have flagellated funnels 
which lead into lymphoid capsules, not, as was formerly thought, into nephridia. 
In the jawed leeches (apparently by degeneration as is the case in many poly- 
chaetes) the true blood-vessels are replaced by a canal system derived from the 
coelomic sinuses, which in Nephilis has in part a lacunar character. For the 


284 


COELHELMINTHES 


origin of these vessels from the ccelom the following points are in favor, (i) 
The ventral cord is enclosed in the ventral blood sinus; (2) the flagellate funnels, 
just alluded to, lie in the blood lacunas, usually in ampullar spaces between the 
ventral and lateral blood sinuses. Further relations are shown by Acanthobdella 
pelcdina, parasitic on fishes. This has both blood-vessels of the oligochaetes, a 
body cavity divided by septa, and chaetae. On the other hand, it is leechlike 
in other features; two suckers and sexual apparatus on the Hirudinean pattern. 
Branchiobdella, parasitic on the gills of the crayfish, is a chstopod devoid of 
bristles and furnished with a sucker in correlation with its habits. 

Order I. Gnathobdellidse. 

The jawed leeches include Hirudo medicinalis, once extensively used for 
blood-letting, now little employed. Hivmadipsa includes land leeches of the 
tropics. Nephelis* soft jaws. Macrobdella* includes our largest species. 

Order II. Rhynchobdellidae. 

Without jaws. CLEPSPINID^; mostly feed on snails and fishes. Clepsine* 
Hcementaria ghiliani of South America is poisonous. ICHTHYOBDELLID^E,* 
cylindrical, occur in salt and fresh water, parasitic on fishes. Ichthyobdella,* 
Pontobdella,* marine; Piscicola, fresh water. 

Class IV. Polyzoa (Bryozoa). 

In external appearance the Polyzoa closely resemble the hydroids, 
so that the inexperienced have difficulty in distin- 
guishing them. Like them by budding they form 
colonies which are either incrusting sheets or assume 
a more bush -like character. Further they have a 
crown of ciliated tentacles which can be spread out 
or quickly retracted. In internal characters the two 
groups are greatly different. The Polyzoa have a 
complete alimentary canal, with its three divisions, 
which is bent upon itself so that the anus lies near 
the mouth. The central nervous system lies be- 
tween mouth and anus, and the single pair of nephri- 
dia empty by a common opening. Beyond this it is 
difficult to go, since the two groups Entoprocta 
and Ectoprocta differ widely. The Entoprocta 
have no ccelom, resembling in this respect the 
Rotifera; the Ectoprocta are true Ccelhelminthes 
and by way of Phoronis show resemblances to the 
Sipunculoida ('Prosopygii,' p. 281) and also to the 
Annelida. 

Sub Class I. Entoprocta. 

The single individuals of the Entoprocta (fig. 273) are shaped like a wine- 
glass and are placed on stalks which rise from (usually) creeping stolons. The 


FIG. 273. Loxosoum 
sinidare (after Nit- 
sche) in optical sec- 
tion. A, rectum; Ga, 
ganglion; /, intes- 
tine; T, tentacles; V, 
stomach. 


IV. POLYZOA: ECTOPROCTA 


285 


circle of tentacles, arising from the edge of the cup, enclose the peristomial area 
in which are both mouth and anus, and between these the excretory and re- 
productive organs open. The space between the horseshoe-shaped intestine 
and the body surface is filled by a parenchyma containing muscle cells, and 
correspondingly the excretory organs are protonephridia. Urnatella* fresh- 
water. Pedicellina,* Loxosowa, marine. 

Sub Class II. Ectoprocta. 

The Ectoprocta have a spacious, often ciliated, ccelom between the 
alimentary canal and skin, so that these are separated and have a certain 
amount of independence (fig. 274). On this account has arisen a pecu- 
liar method (morphologically indefensible) of treating them as two in- 
dividuals, polypid, the intestine and tentacles; cyst id the rest, especially 
the body wall and skeleton. 


-en, 


FIG. 274. Flustra membranacea (after Nitsche), a single animal, a, anus; ek, 
ectocyst; en, entocyst; /", funiculus; g, ganglion; k, collar which permits complete retrac- 
tion; m, stomach, also dermal muscular sac; o, cesophagus. A, avicularium; B, vibracu- 
larium of Bugula (after Claparede). 

The cystid is cup-shaped or saccular. It consists of an endocyst 
the body wall and an ectocyst a cuticular skeleton, often strongly 
calcified, secreted by the ectoderm. The ectocyst covers only the 
base and side walls of the endocyst, leaving the outer end soft, thus forming 
a sort of collar into which the tentacles and adjacent parts of the cystid 
can be retracted. The opening thus formed in the ectocyst in many species 
(Chilostomata) can be closed by a lid (operculum). The circle of tentacles 
. surrounds the mouth alone, while the anus is outside near the collar. The 


288 CCELHELMINTHES 

strongly bent alimentary canal extends into the ccelom and is bound 
at its hinder end by a cord, thefuniculus, to the base of the cystid. Gang- 
lion and nephridia lie between the mouth and anus. The gonads arise 
from the epithelium of the ccelom, the testes usually on the funiculus, 
the ovaries on the wall of the cystid. 

Hundreds and thousands of individuals form colonies (fig. 275) in 
which cystid abuts against cystid. The ccelom of adjacent cystids may 
be distinct or a wide communication may exist. The colonies grow by 
budding; in the Gymnokemata a part of a cystid becomes cut off as a 


FIG. 275. -Small colony oiLophopus crystallinus Rafter Kraepelin), with young and old, 
some extended, others more or less retracted, o, statoblasts. 

daughter cystid in which the polypid alimentary tract and tentacles- 
arises by new formation; or (Phylactolsemata) the bud anlage of the 
polypid arises before the first appearance of the cystid. 

Division of labor or polymorphism is common. Besides the animals 
already described, which are primarily for nourishment, three other indi- 
viduals may occur, ovicells, vibracularia, and avicularia. All three are 
cystids which have lost the polypid The ovicells are round capsules which 
serve as receptacles for the fertilized eggs. The vibracularia (fig. 274, B) 
are long tactile threads: the avicularia (fig. 274, A) are grasping structures 
which seize small animals and hold them until decay sets in ; the fragments 
serve as food for the polypids. The avicularia have the form of a bird's 
head, the movable lower jaw being a modified operculum. 

Under unfavorable conditions a polypid in a cystid may break down and be 
lacking for some time until better relations cause its new formation. Besides, 
in the depopulated cystids, there may appear statoblasts, internal buds en- 
veloped in a firm envelope which form a resting stage (fig. 275). Each stato- 
blast is surrounded by a girdle of chambers which by drying become filled with 


V. PHOROXIDEA. VI. BRACHIOPODA _>x; 

air, causing the statoblast to float when it again comes into water. From the 
statoblast a smaller polyzoon escapes which develops a new colony. The 
statoblasts are adaptations to the conditions of fresh-water life and occur only 
in the Phylactolaemata, where they arise as a sort of internal buds on the fu- 
niculus, just before the degeneration of the polypids. 

Order I. Gymnolaemata (Stelmatapoda). 

Tentacles in a ring around mouth. Numerous species, almost exclusively 
marine. CHILOSTOMATA, cystids can be closed by an operculum: Gemmel- 
laria* CelMaria* Bugida* Flustra* (fig. 274), Eschara* CYCLOSTOMATA, 
tubular cystids without operculum. Crisia* Tubulipora* Hornera* CTEN- 
OSTOMATA, cystid is more calcareous, closed by a folded membrane. Alcy- 
onidium,* Vcsicularia, Valkeria* marine; Paludicclla* (fresh-water). 

Order II. Phylactolaemata (Lophopoda). 

Tentacles borne on a horseshoe-shaped lophophore extending on either side of 
the mouth, the tentacles on its margins. All are fresh-water species. Pecti- 
natella,* Lophopus (fig. 275), Plumatdla.* 

Class V. Phoronidea. 

The single genus Phorcmis* was first called a chaetopod on account of its 
worm-like body situated in a chitinous tube like many sedentary annelids. 
Then it was placed in the Polyzoa, with which it is more nearly related. The 
young, described as Actinotrocha, is a modified trochophore with the mouth 
overhung by a large hood and with the postoral band of cilia drawn out into a 
series of fingers which become the tentacles of the adult; the anus is terminal. 
At the metamorphosis this larva becomes doubled on itself, so that the alimentary 
canal is U-shaped, the anus near the mouth, while the tentacles are borne on a 
horseshoe-shaped basis around the mouth. 

Class VI. Brachiopoda. 

On account of the bivalve calcareous shells the Brachiopoda were long 
regarded as molluscs, but later the fact that the valves are not paired 
as in the lamellibranchs, but are dorsal and ventral, that the nervous 
system, the excretory and reproductive organs, the body cavity, and the 
development resemble those of the annelids rather than those of the 
molluscs, led to their recognition as a distinct class allied to the former 
group. 

The body has a greatly shortened long axis (fig. 276) and in conse- 
quence a transversely oval visceral sac. In most a stalk (sf) for attach- 
ment arises from the posterior end. From the anterior side two folds, the 
mantle lobes (/>), extend forward, one ventral, the other dorsal, their free 
edges bearing bristles. Each mantle secretes a calcareous shell. In a 
few the dorsal and ventral shells are similar, but usually the ventral valve 
(in Crania attached directly without the intervention of a stalk) is more 


288 


CCELHELMINTHES 


strongly arched and has an opening at the posterior end for the passage 
of the stalk (figs. 277, 278). The flatter dorsal valve frequently bears a 
characteristic feature in the calcareous skeleton of the arms (fig. 278) which, 
when present, has very different forms. \\ hen closed the valves completely 


rf 


FIG. 276. Anatomy of Rhynchonella psittacea (after Hancock), a', left, a~, 
right arm; , opening into the cavity of the arm; d, intestine; e, blind end of the intestine; 
g, stomach with liver; m, adductors and divaricators of shell; o, cesophagus; />', /'-'. 
dorsal and ventral mantle lobes; st, stalk; i, 2, first and second septum, on the second a 
nephrostome. 

enclose the body. When they open the gape is anterior, the posterior 
parts remaining in contact. Here, except in the Ecardines, a hinge is 
developed just in front of the posterior margin, consisting of teeth in the 
ventral valve which fit into corresponding grooves in the dorsal. Opening 


D 


h 


FIG. 277. II aldheimia flavescens (from Zittel). Shell with arms and muscles, 
a, arm with fringed border (h); c, c', divaricators; d, adductors; D, hinge process (the 
vertical line shows position of hinge). 

and closing the valves are, contrary to what occurs in Lamellibranchs, 
active processes, accomplished by appropriate divaricator and adductor 
muscles (fig. 277). These produce scars on the shell, important in the 
study of fossil forms. 


VI. BRACHIOPODA 


The usually spirally coiled arms, which lie right and left of the mouth 
and which give the name to the class, fill most of the shell. On the outer 
side of each arm is a longitudinal groove, bounded by a row of small ten- 
tacles. By means of cilia on tentacles and groove food is brought to the 
mouth. These arms resemble the lophophore of a phylactoliemate Poly- 
zoan, which only needs extension and coiling to produce this condition. 
In development the arms of the Brachiopod pass through a lophophore 
stage. 

In the body there is a ciliated ccelom which extends into both arms and 
mantle folds. It encloses alimentary tract, gonads, and liver, and is 


FIG. 278. Waldheimia flavescens (from Zittel). A, dorsal, B, ventral valve; a, 
b, c, impressions of muscular insertions; a, adductors; b", adjusters (stalk muscles); 
r, r', divaricators; s, hinge groove of upper valve in which the tooth (/) of the lower 
valve passes; /, support of arms; d, deltidium;/, foramen for stalk. 

divided into right and left halves by dorsal and ventral mesenteries sup- 
porting the intestine. Each half in turn is divided by incomplete septa into 
anterior, middle, and posterior divisions recalling those oiSagitta (p. 252). 
The arrangement of the septa is not so clear as in that form, the result of 
the shortening of the long axis and the twisting of the alimentary tract. 
This latter consists of cesophagus, stomach, which receives the liver ducts, 
and intestine, which in some species terminates blindly. 

The gonads are chiefly in the mantle lobes. The sexual cells pass out 
through the nephridia, which begin in one ccelomic pouch with wide 
nephrostomata, perforate the septum, and open to the exterior in the next 
somite. Since usually there are two septa, two pairs of nephridia may 
occur, but one is usually degenerate. The nervous system consists of an. 
cesophageal ring with weak dorsal ganglion, extending into the arms, and a 
stronger ventral mass representing the ventral chain. The heart lies 
dorsal to the stomach. 

19 


290 


COELHELMINTHES 


In development the brachiopods recall both Sagitta and the Annelida. 
They resemble Sagitta in that in Argiopc the ccelom arises by outgrowths from 
the archenteron (fig. 279), divided later by septa into three pairs of pouches. 
They are annelid-like in the form of larva and in the presence of chaetse which 
are formed in separate follicles. Brachiopods were formerly so numerous that 
they are among the most important fossils in the determination of geologic 
horizons. Now there are but few species, some inhabitants of the greatest depths 
of the sea. 


FIG. 279. Development of brachiopod (after Kowalevsky). A, gastrula with 
early enteroccelic pouches; B, closure of blastopore; C, ccelom. separated, body annu- 
lated; D, cephalic disc and mantle developing, the latter with long setae; E, attached 
embryo, the mantle lobes folded over cephalic disc (setae omitted), c, cephalic disc; 
d, dorsal lobe of mantle; e, enteroccele; m, mantle; v, ventral mantle lobe. 


IG ' 


Order I. Ecardines. 

Hinge absent: valves similar, the stalk passing between them (Lingula*), 

or unequal, the ventral perforated by the stalk (Dis- 
cina), or the animal is directly attached by the shell 
(Crania). 

Order II. Testicardines. 


Hinge present, valves unequal, the ventral per- 
forated by the stalk; anus degenerate. Rhyn- 
chonclla* Terebratulina* in our colder waters. 
Spirifer, Orthis, Pentamenis, A try pa, important fossil 


genera. 


Summary of Important Facts. 

(1) The CCELHELMINTHES have a well-developed body cavity (ccelom). 

(2) The CEUETOGNATHI are hermaphroditic worms with three pairs of 
coelomic pouches, with fins, a'-d bristle-like jaws. 

(3) The NEMATODA are mostly dioecious, usually parasitic, elongate worms, 
with cylindrical unsegmented body, with nerve ring (no ganglia), paired excre- 
tory organs, and tubular gonads. 

(4) The most important species parasitic in man are Ascaris luvibricoides 
in the small intestine, O.vwm vcrmic-ularis in the large intestine, Ankylostoma 
duodenalis, and the notorious Trichina spiralis. In hot climates Filaria san- 
gulnis ho ni in is and Dracunculus medinensis. 

(5) The GORDIACEA have mesenteries and splanchnic mesoderm; they 
are parasitic in insects. 


ECHINODERMA 291 

(6) The ACANTHOCEPHALI lack alimentary tract, have a spiny proboscis 
and a very complicated reproductive apparatus. The adults are parasitic 
in vertebrates, the young in arthropods. 

(7) The CH/ETOPOD ANXELIDS have segmented bodies, the segmentation 
showing itself in ringing of the body wall and in the separation of the ccelom 
into a series of pouches by transverse septa and the metameric arrangements of 
blood-vessels, ganglia, and excretory organs. 

(8) The CH^TOPODA are distinguished from other annelids by the chaetae 
(usually four groups in a somite) arising in special follicles. The cruets are few 
in the hermaphroditic Oiigochaeta?, numerous and borne on special parapodia 
in the Polychaette. 

(9) The GEPHYR^A are related to the Chaetopoda. They are saccular, with 
tentacles or well-developed preoral lobe. They have largely or entirely lost 
the segmentation. Evidence of segmentation appears in some cases in develop- 
ment and in the ventral nerve cord and nephridia. 

(10) The HIRUDIXEI are hermaphroditic Annelida which lack chata-, but 
have sucking discs. Their flattened bodies, rudimentary ccelom, and rich 
body parenchyma give them a certain similarity to the Plathelminthes. 

(n) The Hirudinei have either a protrusible pharynx (Rhynchobdella) 
or three toothed jaws (Gnathobdella). To the latter belongs the medicinal 
leech (Hirudo medicinalis}. 

(12) The POLYZOA are like the Hydrozoa in being colonial and having a 
circumoral crown of tentacles. They are distinguished by the complete ali- 
mentary canal, the large ccelom, and the ganglionic nervous system. 

(13) The PHORONIDEA are closely like the Polyzoa. 

(14) The BRACHIOPODA have a bivalve shell, the valves being dorsal and 
ventral. 

(15) The body cavity is divided by two septa into three (paired) chambers, 
of which one, rarely two, are provided with nephridia. 

(16) Most brachiopods are attached by means of a stalk. They are divided 
into Ecardines, without a hinge and with anus, and Testicardines, with a hinge 
and no anus. 


PHYLUM V. ECHINODERMA. 

The Echinoderms differ from most other animals by their radial 
symmetry, but recall in this respect the Ccelenterata, a fact which led to 
their inclusion in the 'Radiata' (p. 206), a view of their relationships which 
was set aside on account of their different structure, especially the pres- 
ence of a ccelom. In fact the radial symmetry of the echinoderms has 
a different value, for while in the Ccelenterata the number four or six 
is fundamental, Echinoderma are, with few exceptions, five-radiate. 
Further, the radial symmetry of the Ccelenterata is primitive, the echino- 
derms have descended from bilateral, possibly worm-like, ancestors, as 
is shown by the bilateral larrae and many indications of bilaterality in 
structure, especially in the more primitive forms (crinoids). This 
primitive bilaterality is to be sharply distinguished from that resulting 
from modification of radially symmetrical organs like the sexual and 
ambulacral systems of highly differentiated echinoderms (bivium of sea 


292 


ECHINODERMA 


urchins, trivium of holothurians). One pole of the axis of radial sym- 
metry is marked by the mouth, which in the echinoids, starfish and brittle 
stars, is turned downward; the other is frequently indicated by the anus. 
The structure of the integument gives these animals a characteristic 
appearance. Calcareous plates arise in the mesoderm, under the epithe- 
lium, which form a body armor or test, and since these are usually produced 
into spines, they give the name Echinoderma, spine skin, to the group. 
This skeleton at times becomes degenerate, as in the Holothurians (ft 
rarely entirely disappears as in Pelagothuria), but even then shows itself 
as 'anchors' and 'wheels' of lime. The sp/iccridia and pedicellaria, com- 
mon in echinoids and asteroids, are characteristic appendages of the in- 
tegument. The first are sense organs; the latter are usually stalked 
forceps-like grasping structures with calcareous skeleton. In life they 

are active and apparently either clean 
the skin or are defensive. They are oc- 
casionally provided with poison organs. 

Certain plates possess a morphological 
interest since they appear early in many 
larvae, and in the adults of different classes 
can be recognized in similar positions. In 
the neighborhood of the anus are five 
basalia, interradial in position, farther five 
radialia ('apical skeleton') and five inter- 
radial 'oralia' around the mouth. 

Not less characteristic than the 
skeleton is the ambulacral (or water- 
vascular) system (fig. 281). This is a 
system of ciliated tubes which begins 


FIG. 281. Water- vascular system 
of starfish (orig.). a, ampullae; ab, 


\ O ' f IT i 

ambulacra; c, radial canal; OT, madre- usually at the surface, ordinarily by a 

porite; n, radial nerve; p, Polian vesi- , 

ele;r, ring canal, beneath it the nerve calcareous plate, the madreponte, per- 


nng; s 
vesicle. 


stone canal; t, racemose f orated with fine pores for the entrance 

of sea water. The water passes into a 
tube which, on account of its calcified walls in the starfish is called the 
stone canal, and leads orally to a ring canal around the mouth. The 
ring canal bears usually several (up to five pairs) of Polian vesicles, 
which, with Tiedemann's vesicles of the starfishes, are now regarded as 
appendages which, like lymph glands, produce leucocytes. From the 
ring canal radiate five radial canals which give off, right and left in 
pairs, the ambulacral canals. These in turn connect with the ambulacra 
and ampullae, the highly characteristic locomotor organs of the echino- 
derms. An ambulacrum is a muscular sac which can be distended and 
lengthened by forcing in fluid from the ambulacral vessels, and is retracted 


ECHINODERMA 293 

and shortened by its muscles. The ampulla is a reservoir connected with 
the ambulacrum and projecting into the body cavity. In locomotion the 
animal extends its ambulacra, anchors them by the sucking disc at the 
tips, and then pulls the body along by contraction of the ambulacral 
muscles. In the sessile crinoids and the ophiuroids (which move by their 
snake-like arms) the ambulacra lack suckers and ampulla?, and are not 
locomotor but tactile in function. So among the holothurians and sea 
urchins the ambulacra are often replaced by tentacles. Frequently each 
radial canal ends in an unpaired tentacle with olfactory functions. 

The arrangement of the ambulacral system influences that of other 
organs. Alongside the stone canal is an elongate organ formerly called the 'heart,' 
but now regarded as a lymphoid gland or a collection of excretory lymphoid cells 
(ovoid gland, paraxon gland, septal organ}. Ring and radial canals are accom- 
panied by corresponding blood canals, with which are often associated two 
vessels to the alimentary canal. The blood system is surrounded by a peri- 
haemal space, the ovoid gland and stone canal by the axial sinus, which in the 
starfishes and urchins passes into an ampulla close beneath the madreporite; 
this in turn connects with the lumen of the stone canal and also with the exterior 
through the pores of the madreporite. When the axial sinus is lacking (crinoids, 
holothurians) the stone canal may open into the body cavity, water entering this 
in the crinoids by pores in the theca. There is a nerve ring and radial nerve, 
frequently in the ectoderm, to which may be added an 'apical' nervous system. 

The courses of the radial vessels and nerves mark out five chief lines 
in the animal, the radii; between them come the inter radii. The stone 
canal, madreporite, and lymphoid gland are interradial in position, as are 
the gonads, usually five single or five pairs of racemose glands; in some 
cases but one is present. Echinoderms are rarely hermaphroditic. 
The gonads are supported in the spacious coelom by special bands, while 
mesenteries support the alimentary tract and its derivatives. 

The five gonads develop from a single anlage which is genetically con- 
nected with the lymphoid gland. Except in crinoids and holothurians this anlage 
becomes modified into a perianal ring (rhachis} from which the gonads bud. 
The so-called blood-vessels hardly deserve the name, since they are fibrous cords 
with lacunar spaces. The perihagmal space, like the axial sinus and the space, 
around the gonads and the genital rhachis, are derived from the coelom. 

Respiratory organs are represented by very various structures: branchicc, 
or thin-walled outpushings of the coelom, either around the mouth, as in Echin- 
oidea, or on the aboral surface, as in the Asteroidea, thebursce of the Ophiuroidea, 
the branchial trees of the Holothoroidea and the various parts of the ambulacral 
system. 

The Echinoderma are exclusively marine, occurring even in the deepest seas. 
Many groups, like the crinoids, are largely bathybial, others frequent rocky 
coasts. At the period of reproduction they pass their sexual cells into the sea, 
where fertilization occurs. In some, however, the young are carried about in 
brood cases until the earlier developmental stages are past. 


294 


ECHINODERMA 


Where there is no brood pouch the young escape from the egg as 
lame (fig. 282, I) which swim at the surface, and are distinguishable 
from the adults by their soft consistency, transparency, and bilateral 
symmetry. By the development of lobe-like processes and slender arms 
supported by calcareous rods the larvae assume the most different and bi- 
zarre shapes (plutei of echinoids and ophiuroids (VI), brack iolaria (VII) 
and bipinnaria (VI) of asteroids, auricularia (III) of holothurians), all 


FIG. 282. Echinoderm larvae (after J. Miiller). a, anus; m, mouth; the black 
line, the course of the ciliated bands. 7, form common to all; II, III, developmental 
stages of auricularia (Holothurian); IV, V, stages of the Asteroid bipinnaria; VI 
pluteus of a spatangoid; VII, larva (brachiolaria) of Asterias (orig.). m. mouth- v 
vent. 

of which can be referred back to a common type with tri-regional alimen- 
tary tract and a ciliated band around the mouth, strikingly resembling 
tornaria, the larva of Balanoglossus. The different appearances of the 
larva? are due to the drawing out of the ciliated band into lobes and arms, 
and also to its becoming broken into parts which unite themselves into 
complete rings (}'). 

The metamorphosis of the bilateral larva into the radial adult is very compli- 
cated. It begins early with the formation of outgrowths from the archenteron 
(fig- 283), which become separated and form the anlagen of the coelom and 
ambulacral system. It is difficult to give a short summary of the development, 
partly from the differences in the separate groups, partly from the contradictions 
of authors. The following seems to be the most common. A vasoperiloneal 
diverticulum (fig. 283, he) arises from the bottom of the archenteron; this soon 
Iivides into right and left vesicles, the left acquiring a connection with the ex- 
terior Jmadreporic opening). Each vesicle separates into anterior (h) and 
posterior (c) parts, the anterior forming the anlage of the water-vascular 
(hydrocKle) system, the others the coelom. The two coelomic sacs expand and 


I. ASTEROIDEA 


295 


form the roomy body cavity of the adult, the membranes separating them furnish- 
ing the mesenteries. The right hydroccele remains rudimentary; the left, which 
has the external opening, separates into (i) a smaller anterior portion, the anlage 
of the ampulla and axial sinus; (2) the connecting duct or stone canal; and (3) 
a posterior cavity, the hydroccele in the narrower sense. The latter surrounds 
the oesophagus (ring canal) and sends off five radial diverticula, the anlagen 
of the radial canals, which form the basis of the conversion of the bilateral 
larva into the radially symmetrical adult (fig. 284). It is a question as to 
which group of Echinoderma is the most primitive, but ease of treatment 
makes it best to begin with the Asteroidea. 


FIG. 283. FIG. 284. 

FIG. 283. Three stages in the development of the coelom and \vater-vascular system 
(after Bury and McBride). a, ampulla; b, stone canal; c 1 , c~, left and right coelom 
sacs; d, hind gut with anus; h l . Ir, left and right (rudimentary) hydroccele sac; he, 
common anlage of hydrocoele and coelom; m, stomach; s, stomocleum and mouth. 

FIG. 284. Formation of Ophiuran from the pluteus larva uifter Miiller, from 
Korshelt-Heider) . 

Class I. Asteroidea (Starfish). 

Two parts can be recognized in the body of a starfish, a central disc and 
the arms, usually five in number, which radiate from it (fig. 290). The 
relations in which these stand to each other vary between two extremes. 
In many starfish the arms play the chief role and the disc appears as only 
their united proximal ends (fig. 285). On the other hand, the disc may 
increase at the expense of the arms, so that they form merely the angles of 
a pentagonal disc (fig. 286). In both arms and disc two surfaces are 
recognized, oral and aboral, which pass into each other, usually without 
a sharp margin. In the normal position the oral side is downwards and 
has the mouth in the centre and radiating from it to the tips of the arms the 
five ambulacral grooves. Near the centre of the aboral surface is the 
anus (when not degenerate) and excentric from it in an interradius is the 
madreporite (in many-armed species two to sixteen interradii may have 
madreporites). 


296 


ECHINODERMA 


A line passing through the madreporite and the opposite arm divides the 
body into symmetrical halves. This arm is called anterior, since in the irregular 
sea urchins (Spatangoids) the homologous area is clearly anterior, while the 
madreporic interradius is posterior. This plane of symmetry does not corre- 
spond with that of the larva. The two rays on either side of the madreporite 
form the bivium, the three others the trivium. 


FIG. 285. FIG. 286. 

FIG. 285. Comet form of Linckia mult (flora (from Korschelt-Heider) One of the 
arms is producing'a new animal by budding. 

FIG. 286. Culcita pent angular is, aboral view (from Ludwig). a, madreporite; b, 
reflexed end of ambulacral grooves. 

The skin is everywhere protected by large and small plates jointed 
together. In life it is extremely flexible, the arms can be bent in any 
direction, and the animal can work its way through narrow openings. 
Of the skeletal pieces the ambulacral plates need special mention. These 


FIG. 287. A, cross-section of starfish arm (orig.). o, adambulacral plates; am, 
ambulacra; ap, ambulacral plates; b, branchiae; c, coelom; /?, hepatic caeca; i, inter- 
ambulacral plates; n, radial nerve; p, ampulla; r, radial canal; 7', radial blood vessel. 
B, ambulacral plates, ventral view, showing the ambulacral pores between. 

form the roofs of the ambulacral grooves, and between them are openings 
(fig. 287, B), the ambulacral pores, through which connection is made be- 
tween the ambulacra and ampullae. In each arm the pairs of ambulacral 
plates meet above the groove like the rafters of a roof. Laterally each 


I. ASTEROIDEA 


297 


ambulacral plate abuts against a small interambulacral plate, bearing 
usually movable spines. Beyond these come the less constant adambuln,- 
ral or marginal plates, and then those of the aboral surface. Each ambu- 
lacral area terminates at the tip of the arm with an unpaired ocular plate. 


FIG. 288. Asterhcus verruciihilus, aboral surface removed (after Gegenbaur). g, 
gonads; h, hepatic caeca; i, stomach with anus. 

The organs lie in part in the ccelom, in part in the ambulacral grooves. 
The alimentary tract is in the ccelom and extends straight upward from 
the mouth to the aboral surface, where it ends with an anus or is entirely 


FIG. 28g. Section through ray and opposite interradius of a starfish forig.). B 
branchiae; C, cardiac pouch of stomach; E, eye spot; G, gonad; H, 'liver'; .!/, mouth; 
N, radial nerve; P, pyloric part of stomach; RC, ring canal; RD, radial canal of water- 
vascular system; S, stone canal. 

closed (figs. 288, 289). By a constriction it is divided into a larger, lower 
cardiac portion and a smaller, upper pyloric division. From the latter 
arise five hepatic ducts which connect with five pairs of hepatic glands 
lying in the arms, while small ca-ca arise from the intestine in some 


29? 


ECHINODERMA 


forms. The cardiac division gives origin to five gastric pouches which 
can be protruded from the mouth or retracted by appropriate muscles. 
The gonads are five pairs of racemose glands lying in the basis of the arms 
and opening interradially between the arms. Lastly, in the ccelom is the 
stone canal (accompanied by the lymphoid glands, and with it enclosed 
in the axial sinus) extending from the aboral madreporite to the ring 
canal near the mouth. 

The radial nerve, canal and blood-vessel, which start from the cir- 
cumoral rings, lie in the roof of the ambulacral groove between the 

ambulacra. The nerve, lying in the ecto- 
derm, ends at the tip of the arm in a 
compound eye spot colored with red or 
orange pigment which experiment shows 
is sensitive to light. A second nerve has 
been described lying in the ccelom of the 
arm. The ambulacral system corre- 
sponds with the foregoing description (p. 
292), the ampulke as well as the five or 
more Polian and Tiedemann's (racemose} 
vesicles projecting into the ccelom. 

Since the arms contain nearly all im- 
portant organs, their physiological independ- 
ence is easily understood. Arms broken oQ. 
not only live, but regenerate first the disc and 
then new arms which appear at first like small 
buds (comet form, fig. 285). This separation 
of arms may occur through accident, or it not 
infrequently is produced by the animal itself. 
ASTERID.E, well developed arms, four 
rows of ambulacra; Asterias* Leptasterias* 

Heliaster* (numerous arms). SOLASTERID^E, two rows of ambulacra, arms 
sometimes numerous; Pythnnaster (fig. 290). ASTERINID.E, arms short or 
body is pentagonal, no large plates on the margins of the arms. Aster iscus 
(fig. 288). In other forms (Cukita* fig. 286, Hippasteria* Ctenodiscus*) 
the body is more or less pentangular, margin with large plates. 

Class II. Ophiuroidea (Brittle Stars). 

In these, as in the Asteroidea, there are disc and arms, the latter some- 
times branched, but the internal anatomy is different. The ambulacral 
plates have been drawn inside the arm and each pair fused to a large 
'vertebra' (fig. 291). As a result the ccelom of the arms is greatly re- 
duced, the hepatic caeca are lacking, and the alimentary canal, which 
lacks an anus, is confined to the disc. By the ingrowth of ventral plates 


FIG. 2QO. Pythonasler murrayi 
(after Sladen). Oral view show- 
ing ambulacral grooves. 


III. CRIXOIDEA 


299 


the ambulacral grooves are closed, and the ambulacra, which lack 
sucking discs, are tactile, locomotion being effected by the snake-like motion 
of the arms. The madreporite is on 
the ventral surface. Also on the ven- 
tral surface are five slits which con- 
nect with as many burscc, thin-walled 
respiratory sacs into which the sexua! 
organs open. The gonads are at- 
tached to a genital rhachis which coils 
through the disc. 

In many brittle stars, especially in 
young specimens, there is a kind of 
asexual generation (schizogony), the 
animal dividing through the disc, the 
halves regenerating the missing parts. 
OPHIURID.^, arms unbranched (Ophio- 

pholis* (fig. 292), Ophioglypha* Atnphhtra*); EURYALID.E, the arms branched 
(Astrophyton,* fig. 293). 


FIG. 291. Section of Gphiuroid 
arm (orig.). a, ambulacrum; b, blood- 
vessel; c, ccelom; m, muscles of arm; 
n, nerve; r, radial water tube; v, 'ver- 
tebra' (coalesced ambulacral plates). 


FIG. 292. Ophiopholis aculeata* 
(from Morse). 


FIG. 293. Astrophyton arborescens, 
basket fish (from Ludwig). 


Class III. Crinoidea (Pelmatozoa). 

The crinoids or sea lilies are on the road to extinction. In early times, 

j 

especially in the paleozoic, they were very abundant, but to-day there are 
but few species, these mostly restricted to the greater depths of the ocean, 
only the Comatulidse occurring near the shore. The crinoids are attached 
to the sea bottom by a long stalk (fig. 294), composed of cylindrical discs 
which often bear five rows of outgrowths, the cirri. The young Coma- 
tulida? (fig. 295) are similarly attached, having a Pcntacrhuis stage, but 
later they separate and live a free life, a proof that the attached condition 
was primitive. When the separation takes place, one joint of ilu- sulk 


ECHINODERMA 


with its cirri remains attached to the animal, as the centr odor sal united 
with the lowest cup plates, the infra ba sals. On the upper joint of the 
stalk is a cup-shaped body (theca) the edges of which bear five or ten 
(usually branched) arms. The walls of the theca are covered with poly- 
gonal calcareous plates. 


FIG. 294. FIG. 295. 

FIG. 294. Pent acr inns ma'-hayamts (after Wyviile Thompson), br, brachialia; 
c, cirri; d, distichalia; r, radialia; p, pinnuhf. 

FIG. 295. Different Pentacrinus stages (a, b, c) of Antedon rosacea. I, arms; 2, 
cirri; 3, stalk. 

Usually the stalk bears five plates, the basalia, and then come five radialia, 
alternating in order with the basalia (fig. 296). In some there is a circle of 
infrabasalia in a line with the radialia. Frequently the elements of the arm, 
the brachialia, are directly attached to the radials (fig. 296). But often the arm 
branches once or several times dichotomously, and the first branching takes 
place at the base, so that the arms seem to spring from the theca. In these 
cases the first brachialia are considered as part of the theca and are called 
radialia distichalia (fig. 294). From the arms arise, right and left, a row of 


III. CRIXOID1.A 


301 


pinnules, lancet-shaped processes supported by calcareous bodies in which the 
sexual products ripen until freed by dehiscence (fig. 298). 

The mouth opening, in the middle of the oral disc which closes the 
theca, is frequently surrounded by five interradial plates, the oralia (fig. 
205, B). The mouth, which in contrast to other echinoderms is directed 
upwards, connects with a spiral digestive tract in which oesophagus, 
stomach, and intestine can be distinguished. The anus is interradial 
and near the mouth (fig. 297). Five ambulacral grooves begin at the 


A B 

FIG. 296. Hyocrinus bethleyainis. A, upper end of stalk with cup, and the bases 
of the arms; b, basalia; br, brachialia; r, radialia. B, oral surface of cup with mouth, 
five oralia, and the bases of the arms. 

mouth and extend out on the arms, branching with them and extending 
to the tips of the pinnuke. In the ten-armed species (fig. 297) the grooves 
fork near the mouth. These are ciliated and serve as conduits to bring 
food to the oral opening. Nervous, ambulacral, and blood systems begin 
with a circumoral ring. As in the asteroids, they follow the ambulacral 
grooves to the pinnuke, but the ambulacra have no suckers nor ampullae 
and are merely tactile tentacles. 

A typical stone canal is also lacking; in its place are five or several hundred 
tubules leading from the ring canal to the ccelom. Opposite their coelomic 
mouths are fine pores in the theca through which water enters to pass through 
the tubules into the ambulacral system. The ambulacral nervous system is 
weakly developed or may be absent. The apical system, on the other hand, 
is well developed and forms the axial cord running through the brachialia and 


302 


ECHINODERMA 


radialia to unite in a complicated plexus in the centrodorsal. A problematical 
so-called dorsal organ also begins in the centrodorsal and extends up through 
the axis of the theca to the oral disc. It is apparently homologous with the 
'heart' of other echinoderms. Its upper end lies in a cell complex from which 
the reticulum of genital cords extends into the arms, swelling in the pinnuke 
to the gonads (fig. 298). The dorsal organ in the centrodorsal is enclosed in 
the chambered organ, a prolongation of the ccelom which extends into stalk ard 


cirri. 


Sub Class I. Eucrinoidea. 

The foregoing account applies entirely to the Eucrinoidea, which may be 
divided into two groups: 

Order I. TESSELLATA (Pateocrinoidea). Theca with its side walls 
composed of immovably united thin plates; the ambulacra! grooves usually 
completely covered by calcareous plates. Exclusively paleozoic. 


a 


Fio. 207. FIG. 298. 

FIG. 297. Oral area of crinoid (Antedon), showing by dotted lines the course of the 
intestine from the mouth (m) to the anus (a) ; g, ciliated grooves leading from the arms 
to the mouth (prig.). 

FIG. 298. Cross-section of pinnula of Antedon (after Ludwig). a, axial nerve 
cord; c, ciliated cups; cc, cceliac canal; g, gonad; s, sacculi; sc, subtentacular canal; t, 
tentacles. 

Order II. ARTTCULATA (Neocrinoidea). Ambulacra! grooves open, 
theca with compact, in part movably articulated, plates. This order left the 
other in the mesozoic age, and some families have persisted until now. Rhizo- 
crinus* (fig. 296) and Pentacrinus (fig. 294), deep seas; the COMATULID.*: of 
shallow water are fixed in the young, free in the adult. Antedon* (fig. 295). 

Sub Class II. Edrioasteroidea (Agelacrinoided). 

Theca of irregular plates; arms unbranched and lying on the theca. Possibly 
the ancestors of the noncrinoid echinoderms. Paleozoic. Agelacrinus. 


IV ECHINOIDEA 


303 


Sub Class III. Cystidea. 

Exclusively paleozoic; body spherical, composed of polygonal plates. Stalk 
and arm structures rudimentary, sometimes lacking. The AMPHORIDA are 
by some regarded as ancestral of all echinoderms. Holocystites, Echino- 
splucnles (fig. 299). 


V< 


FIG. 299. FIG. 300. 

FIG. 299. Ecliinosplnrrites aurantium (from Zittel). 

FIG. 300. Pentremites florealis (from Zittel). Lateral, oral, and aboral views. 

Sub Class IV. Bias to idea. 

Arms lacking; the mouth surrounded by five petal-like ambulacral areas. 
The group appears at end of Silurian and dies out with the carboniferous. 
Pentremites (fig. 300). 

Class IV. Echinoidea (Sea Urchins). 

The structure of the sea urchins is best understood in the spherical 
forms (figs. 301, 303). Mouth and anus lie at opposite poles of the main 
axis, each opening immediately sur- 
rounded by areas covered by calcare- 
ous plates, the arrangement of which 
varies with the family. Around the 
anus is the periproct, around the mouth 
the peristoine, the latter bearing sprue- 
ridia and in the Echinoids five pairs 
of interambulacral gills. Between 
peristome and periproct the body wall 
(coron'.i) is composed of calcareous 
plates, which, except in the Echinothu- 

ridce, are immovably united. Aside / IG .^.-C^oplcnrus ./?,/,/<,,* 

(after Agassiz). Aboral view, the 

from the extinct Pakechinoidea the spines removed to show the ambula- 

plates are arranged in ten double oral (a) and (6) interambulacral areas 

endme; respectively m the ocular and 

meridional TOWS, two rows being genital plates; in "the centre the four 

always intimately associated together. P lates of the periproct. 
Five of these double rows are ambulacral, the alternating five interam- 
bulacral. Both bear small hemispherical articular surfaces on which 


304 


ECHINODERMA 


are situated the spines, either long and pointed or swollen to spherical 
plates. These spines are moved by muscles so that they serve both as 
protecting and locomotor structures. The ambulacral plates are distin- 
guished from the interambulacral by the ambulacral pores by which the 
ambulacra on the surface are connected with the internal radial canals and 
ampulke. In most sea urchins the paired grouping of the pores results 
from the fact that a double canal extends from ampulla to ambulacrum. 


FIG. 


FIG. 303. 
Aboral view, showing the 


FIG. 302. Clypeaster subdepressus (after Agassiz). 
petaloid ends of the ambulacral areas. 

FIG. 303. Diagrammatic longitudinal section through a sea urchin. 

In the arrangement of the ambulacra two modifications, the band form 
and the petaloid, occur. In the first (Regularia) the ambulacra are equally 
developed from peristome to periproct (fig. 301). In the second oral and aboral 
regions may be distinguished (fig. 302). In the oral region alone are loco- 
motor feet always present, but these are irregularly arranged. In the aboral 
area the ambulacra are branchial or tentacular and are regularly arranged, their 
pores bounding five petal-like figures around the periproct, very distinct after 
removal of the spines (fig. 306). In the Regularia, the Cidaridae excepted, the 
interambulacral plates around the peristome show five pairs of notches for the 
gills, five pairs of thin -walled branching extensions of the body cavity. 

Ambulacral and interambulacral fields both end at the periproct with 
an unpaired plate, the five ambulacral plates (terminalia of morphology) 
being called ocular plates, since they often bear pigment spots formerly 
regarded as eyes. Each is perforated by the end of the radial canal and 
nerve. The five interambulacral plates (basalia*) are called genital 
plates, since they usually contain the openings of the genital ducts. 
One is often madreporite as well. 

Inside of the body is a spacious ccelom, to the walls of which the thin- 
walled alimentary tract is fastened by a mesentery. In the Clypeastroids 
this tract forms a simple spiral, but elsewhere it ascends from the mouth, 
turning once, and then, bending on itself, coils in the reverse direction to 


IV. ECHINOIDEA 


305 


the anus (fig. 304). Usually the first portion of the canal is accompanied 
by a siphon, an accessory tube opening into the main tube at either end. 
Except in the Spatangoids the mouth is surrounded by five sharp-pointed 


ed 


\ 


nd d 


A 


st 


FIG. 304. Sea urchin opened around the equator. A, ambulacral area; 7. intrr- 
ambulacral area; L, lantern; d, intestine; ed, anal end of intestine; g, gonads; nd, 
siphon; oe, oesophagus; p, p', ring canal and Polian vesicles; st, stone canal. 

calcareous teeth, which in the Regularia are supported by a complicated 
system of levers, fulcra, and muscles, the 'lantern of Aristotle' (fig. 305). 
The ring canal and the ring of the blood system lie on the lantern, 
the stone canal and septal organ ('heart') extending upwards from them 
(fig. 303). The blood-vascular ring gives off two 
blood-vessels which run along the alimentary 
canal, while from the ring canal arise five ambu- 
lacral or radial canals which run on the inner side 
of the test, accompanied by nerves which, enclosed 
in a tube of infolded ectoderm, radiate, from a 
nerve ring. The gonads are five (rarely four or 
two) unpaired organs in the aboral half of the test, 


FIG. 305. Aristo- 
tle's lantern of Stron- 
Ih'idus 


opening through the genital plates, that is, interra- ^^f ^aSli; I', 
dially as in the starfish. teeth. 

Order I. Palsechinoidea. 

Paleozoic forms with five ambulacral areas, the interambulacral areas con- 
taining more than two rows of plates. Melonites. 

Order II. Cidaridea (P^egularia). 

Ambuiacral areas band-like, body more or less spherical, mouth and anus 
polar. Common urchins; Toxopneustes* Strongylocentrotus* Arbacia* Ccelo- 

pleurus* (fig. 301). 
20 


306 


ECHINODERMA 
Order III. Clypeastroidea. 


Flattened echinoids with central mouth and teeth; anus in the posterior inter- 
radius, sometimes marginal; five petaloid ambulacral areas. Clypeastcr (fig. 
302), Echinarachnius* (sand dollar, fig. 306), Mellita,* with holes through the 
test. 

Order IV. Spatangoidea. 

Bilateral flattened forms more or less heart-shaped; mouth and anus ex- 
centric, no teeth; usually five petaloid ambulacral areas and four genital plates. 
From the forward position of the mouth it follows that only two ambulacral 
areas (bivium, p. 291) are upon the lower surface. Warmer seas. Spatangus* 
(fig. 307), Echinocardium, Brissus. 


a A 

FIG. 306. FIG. 307. 

FIG. 306. Oral (A) and aboral (B) surfaces of the sand dollar, Echinarachnius 
par ma. a, anus; g, genital pores; /, ambulacral areas; m, madreporite; o, mouth. 

FIG. 307. Young Spatangus purpureus (after Agassiz), the spines removed, 
oral surface. In front, the slit-like mouth; behind, the anus. The bivium without 
tubercles. 

Class V. Holothuroidea. 

The sea cucumbers are most removed of any group from the typical 
echinoderm appearance. At the first glance, except in Psolus, the skin 
appears naked and the characteristic plates absent. Yet these are im- 
bedded in the skin in the shape of plates, wheels, and anchors. The 
integument is leathery and muscular, with longitudinal and circular fibres. 
The saccular body gives these forms a worm-like appearance, strength- 
ened by its elongation in the main axis, and with the mouth and anus at 
the poles. Unlike other echinoderms these move with the main axis 
parallel to the ground, a condition which, to a greater or less extent, leads 
to a replacement of radial by bilateral symmetry. One surface (trivium) 
becomes ventral, the bivium dorsal, and in many the trivial ambulacra 
alone are locomotor, those of the bivium being tactile or wholly absent. 

The alimentary canal (fig. 308) (except in Synapta) is coiled in a 
uniform manner, although many minor convolutions may obscure this. 


V. HOLOTHUROIDKA 


It passes backwards in the median dorsal interradius, forward in the left 
ventral interradius, and then back in the right dorsal interradius to the 
anus. It is held in position by mesenteries (fig. 309), and near the anus 
by numerous muscular filaments. Into the terminal portion one or two 


FiG. 308. Anatomy of Caudina arenata (after Kingsley). a, anastomoses of 
dorsal blood-vessel; b, branchial tree; d, dorsal blood-vessel;/, mesenterial filaments; 
g, genital opening; /, alimentary canal; /, longitudinal muscles; t, mouth; <>, genital 
duct; p, pharyngeal ring; r, gonads, cut away on right side; /, ampulke of tentacles; 
v, ventral blood-vessel. 

branchial trees may empty. These are tubular sacs with small branched 
outgrowths which are filled with water. They are respiratory, and are 
periodically filled with fresh water. In many species 'L'ui'icrian organs' 
occur; these are morphologically specially modified portions of the 
branchial tree and are either connected with them or separately with the 


308 


ECHINODERMA 


cloaca. Many zoologists regard them as defensive structures because of 
their sticky nature and because they can be cast out through the anus. 

The oesophagus is usually surrounded by a ring of five radial and five inter- 
radial plates which serve as points of attachment for the longitudinal muscles. 
Just behind it lie the ring canal, ring nerve, and the ring of the blood system, 

each giving off five radial branches which run 
inside the muscular sac of the body. From 
the beginning of the radial canals (rarely, as in 
Synapta, from the ring canal) tubes extend out- 
ward to form the extremely sensitive retractile 
tentacles which surround the mouth, and which 
either branch (Dendrochirotas) or bear frilled 
shield-shaped extremities (Aspidochirota;). A 
single Polian vesicle is usually present, and 
the occasionally branched stone canal connects 
(except in the Elasipoda) with the ccelom. 


FIG. 309. Transverse section 
of Holutliuriatub'ulosa (after Lud- 
wig) . (/, digestive tract ; db, dorsal 
blood-vessel; g, gonad duct; h, 
skin; /;;/, longitudinal muscles; 
Iw, left branchial tree; m, mesen- 


Blood-vessels going from the vascular ring 


tenes; r l ,r 2 , ambulacra! complex 
of bivium (ambulacral vessel) and 
nerve; r 3 -r 3 , same of trivium; rw, 
right branchial tree. 


form rich anastomoses on the alimentary canal. 
Only a single gonad (or a pair of united 
gonads) occurs. This consists of numerous 
tubules which open interradially, usually near 
the mouth. 

The regenerative powers of these animals 
are of interest. In unfavorable conditions 
they void the whole viscera and yet may live 
and reproduce the lost parts. In certain 
species are found a few parasites. One or 

two harbor a small fish (Fierasfer) in their cloaca and branchial trees, while 
parasitic snails (Entoconcha, Entocolax, Entcroxcnus) live in several species and 
a mussel, Entovalva mirabilis, in another. 

Order I. Actinopoda. 

Radial canals present, sending branches to the tentacles and ambulacra 
when present. Divided into Pedata, with ambulacra, and Apoda without. 
The PEDATA include the HOLOTHURID^E with peltate tentacles, (Aspido- 
chirotae). Holothuria* in warmer waters, one species forming the trepang of 
Chinese markets. Of the forms with branched tentacles (Dendrochirota?) are 
the CUCUMARIID.E, Citcumaria* Psolus* Thyonc* The deep sea ELASIPODA 
have statocysts and peculiar dorsal ambulacral processes. The APODA are 
represented by Caitdina* and Molpadia* 

Order II. Paractinopoda. 

No radial canals nor ambulacra. Tentacular canals arising from ring 
canal. Myriotrochw* Synapta* with statocysts, Oligotrochus* In Pelago- 
thuria the anterior end is expanded to a disc with tentacular processes, used in 
swimming, like the bell of a medusa. 

Summary of Important Facts. 

i. The ECHINODERMA share the radiate structure with the 
Coelenterata, but differ from them (a) in the numerical basis of the 
symmetry (five) ; (b) in that, as embryology shows, they have descended 
from bilateral forms. 


SUMMARY OF IMPORTANT FACTS 


309 


2. Farther characters are the existence of a ccelom, the ambulacral 
system, and the mesodermal spiny skeleton, which has given the name 
to the phylum. 

3. The ambulacral system is locomotor and occurs nowhere else. 
It consists of a sieve-like madreporite (not always present), which passes 
water to the stone canal, and from this to the ring canal and the five 
radial canals to fill the ampulla and ambulacra. Lateral branches supply 
the tentacles and cause their extension. 


"%"' I- . aN^tfvxt* s ^^^MV M^-^-i - 


FIG. 310.- 


"ucumaria frondosa, sea cucumber (from Emerton). 


4. Blood-vessels and nerve cords run in the same radii as the radial 
canals of the ambulacral system; stone canal, madreporite, septal organ 
and genital ducts are interradial. 

5. The Echinoderma are divided into five classes: (i) Asteroidea, 
(2) Ophiuroidea, (3) Crinoidea, (4) Echinoidea, and (5) Holothuroidea. 

6. The ASTEROIDEA have a disc and (usually) five arms into which 
the gastric pouches and hepatic cssca extend. The ambulacral groove 
open. 


310 MOLLUSCA 

7. The OPHIUROIDEA also have disc and arms, but the ambulacral 
groove is closed and the hepatic caeca absent. 

8. The CRINOIDEA have a cup-shaped body bearing arms, usually 
branching, with pinnuke, and a stalk, usually with cirri by which they 
are attached, either permanently or in the larval stages. In these latter 
free forms only the centrodorsal persists as the remains of the stalk. The 
Crinoidea are 'subdivided into Eucrinoidea, Edrioasteroidea, Cystidea, 
and Blastoidea. 

9. The ECHINOIDEA are usually spherical or oval, armored with 
calcareous plates which extend as five pairs of ambulacral and five of inter- 
ambulacral meridional bands from peristome to periproct. 

10. The ambulacral plates end at the periproct with an unpaired ocular 
plate; the interambulacral with a similar genital plate. The madre- 
porite is fused with one of the genital plates. 

11. The regular sea urchins have the anus in the periproct, the 
mouth in the peristome; the ambulacral areas band-like. 

12. The Clypeastroidea have a central mouth, the anus outside the 
periproct in the posterior interradius; the ambulacral areas petaloid. 

13. The Spatangoidea are markedly bilateral, the mouth anterior, 
the anus posterior; ambulacral areas petaloid. 

14. The HOLOTHUROIDEA are elongate and worm-like; the skeletal 
system greatly reduced; they are more or less bilaterally symmetrical and 
have usually a single gonad and one or two branchial trees. They are 
divided into Actinopoda, with radial canals, and Paractinopoda, without. 

PHYLUM VI. MOLLUSCA. 

At the first glance the molluscs, like the leeches and flatworms, appear 
like parenchymatous animals. A spacious coelom is absent; what was 
formerly regarded as such is a system of blood sinuses surrounding the 
viscera, and is especially well developed in the snails. More recently 
it has been shown that the molluscs have descended from ccelomate 
animals, in which, by encroachment of connective tissue and muscles, 
the coelom has been reduced to inconspicuous remnants, the pericardium 
and the lumen of the gonads^ 

Where the molluscan features are well developed, as in the snails, 
four parts may be recognised (fig. 311,5). The visceral sac forms most 
of the body; it is less muscular'than the rest and contains the alimentary 
tract, liver, nephridia, and gonads. In front it is continuous with the 
head, often separated by a neck, which bears the mouth and the most 
important sense organs, eyes and tentacles. Below, the visceral sac passes 


MOLLUSCA 


311 


into the foot, a muscular mass, usually used for locomotion. The mantle 
or pallium, a dermal fold, extends downward from the body and encloses 
a part of the body. In the Acephala (C) it has two halves, but in the 
snails (B) and cephalopods (A) it is unpaired, and either extends down 
on all sides (Chiton, Patella), or, like a cowl, covers the anterior side 
(most gasteropods), or envelops the posterior part of the body (pteropods, 
cephalopods). The mantle is important in two ways: its outer surface 


B 


FIG. 311. Diagrams of three molluscan classes. A, a cephalopod (S?pia)\ B, a 
gasteropod (Helix); C, an acephal (Anodonta). a, anus; c, cerebral ganglion ;/M, foot; 
m, mantle chamber; sch, shell; t, siphon; v, visceral ganglion. Visceral sac dotted; 
mantle lined, shell black. 

is covered with epithelium which, like that of the adjacent surface, may 
secrete shell, a thick layer of organic matter (conchioliri) largely impregnated 
with calcic carbonate. The inner surface of the mantle, together with 
the outer surface of the body, bounds a space, the mantle cavity, which, 
from its most important function, is also called the respiratory chamber. 
Since most molluscs are aquatic, special vascular processes of the body, 
the gills or branchial, lie in this space; in the terrestrial forms it contains 
air and with a richly vascular dorsal wall, serves as a lung. 


312 MOLLUSCA 

From the foregoing it will be seen that the mantle must exert an in- 
fluence on the shell and on the respiratory organs. Paired mantle folds 
form two valves, right and left, to the shell; a right and left branchial 
chamber, and right and left gills. With an unpaired mantle the shell 
is always unpaired, while the gills may retain their primitive paired 
condition. 

The gills in the mantle cavity are called ctenidia, from their resemblance to 
combs with two rows of teeth. Each consists of an axis (back of the comb), con- 
taining the chief blood-vessels, and two rows of branchial leaves. The whole 
is united to the wall of the branchial cavity by the axis (fig. 351). Many aquatic 
forms lack ctenidia, and then the respiration is either by the skin or by accessory 
gills which differ from ctenidia in structure and position (usually outside the 
mantle cavity). 

Those parts of the surface which are not covered by the shell have a 
columnar epithelium which is frequently ciliated and which contains 
large unicellular mucus glands (fig. 29), especially abundant on the edge 
of the mantle. These give these animals the soft slippery skin which is 
implied in the name Mollusca (moll is, soft). Although head, foot, and 
mantle are very characteristic of the molluscs, they are not always present. 
In the Acephala there is no distinct head region; many gasteropods lack the 
mantle and hence the shell and mantle cavity; in the Cephalopoda the 
foot is converted into the siphon and arms. 

r r. 

f\r ^=-^_ i""-/ " * r u>\ 

jm w 


B 


FIG. 312. Nervous systems of Molluscs. A, most gasteropods; B, acephals; C, 
cephalopods and pulmonates. c, cerebral; pa, parietal, pe, pedal, pi, pleural, and 
v, visceral ganglia. 

The nervous system has some highly characteristic features. As a 
rule it consists of three pairs of ganglia associated with important sense 
organs and connected by nerve cords. One pair lies dorsal to the oesoph- 
agus and corresponds to the supracesophageal ganglion of the worms; it 
is the brain (cerebrum] and supplies the tentacles and eyes. A second 
pair lies ventral to the alimentary tract on the front part of the muscle 
mass of the foot ; these are the pedal ganglia with which the statocysts are 
connected. The third pair, the visceral ganglia, are also ventral, and near 
them are the third sense organs, widely distributed through the Mollusca, 
and from position and structure are regarded as organs of smell (osphra- 
dia). They are thickened patches of ciliated epithelium in the mantle 


MOLLUSC A 313 

cavity. Pedal and visceral ganglia are united to the cerebrum by nerve 
cords, the cerebropedal and cerebramsceral connectives respectively. Ac- 
cordingly as these connectives are long or short the ganglia are wide 
apart or united into a nerve mass around the oesophagus. 

Primitive Mollusca (Amphineura) have a simpler condition. The cerebral 
ganglia are connected by a ring around the oesophagus (rig. 315, B). From it 
are given off two pairs of longitudinal nerve tracts, the ventral or pedal cords, 
and lateral or pleural cords, the latter united by a loop dorsal to the anus. By a 
concentration of ganglion cells in the higher molluscs the pedal cords give rise 
to the pedal ganglia, and similarly the pleural cords form three pairs of ganglia, 
the plcural and the parietal, as well as the visceral already mentioned, of the cere- 
brovisceral cord (fig. 312, A}. The pleural ganglia are connected with the 
pedal by nerve cords; the parietal innervates the osphradium. When farther 
concentration takes place the pleural may unite with the cerebral, and the par- 
ietal with the visceral (B), or both may fuse with the visceral (C). In the latter 
case (pulmonates, cephalopods) the visceral ganglion (in the wider sense) is 
associated with the pedal by the pleuropedal connective; while in the other 
(lamellibranchs, scaphopods) the connective is fused with the cerebropedal. 
Although the statocyst receives its nerve from the pedal ganglion, the centre of 
innervation lies in the cerebrum. In the Nuculidae the statocysts retain their 
connection with the parent ectoderm by means of a canal, which though closed, 
remains in part in the Cephalopods. Besides accessory eyes in various places, 
there are cephalic eyes, in general structure like those of the annelids. They are 
pits in the skin, the bottom differentiated to a retina. Usually they close to a 
vesicle, but only in the cephalopods do they reach a high development (fig. 

349)- 

The heart, which lies dorsally, is arterial and consists of auriclap and 
ventricles. The ventricle is always unpaired; there are two auricles where 
two gills exist from which the blood flows to the heart, but with the loss of 
one gill one auricle may disappear. Distinct arteries and veins occur; 
capillaries are found only in the Cephalopoda, while in the lower molluscs 
(especially Acephala), the smaller arteries open into lacunar spaces which 
were formerly regarded as the body cavity. A completely closed vascular 
system does not exist even in the Cephalopoda. 

The heart is enclosed in a spacious sac or pericardium, which, with 
few exceptions, is connected with the nephridia by a ciliated canal 
(nephrostome), and in many molluscs (Cephalopoda, Solenogastres) 
is also related to the gonads. These facts support the view that the 
pericardium and the lumen of the gonads are the remnants of the ccelom; 
for here, as in the- annelids, the nephridia open by ciliated nephrostomes 
into the ccelom, and the sexual cells arise either from the ccelomic walls 
or from sacs cut off from them. 

Nephridia and sexual organs are primitively paired, but frequently 
are single by the degeneration of those of one side. The animals are 
either hermaphroditic or dioecious, but the gonads are always very large. 


314 


MOLLUSCA 


Even more room in the visceral sac is demanded by the digestive tract in 
which oesophagus, stomach, a coiled intestine, a voluminous liver, and 
usually salivary glands may be recognized. The liver is usually a paired 
tubular gland, emptying into the stomach. It not only digests fat and 
stores up glycogen, but forms an enzyme (cytase) which converts cellulose 
into sugar. The radula or lingual ribbon is also a characteristic organ, 
and its absence from the Acephala is probably the result of degeneration. 
It is a plate or band armed with teeth which lies on the floor of the pharynx- 
on a ventral ridge, the tongue, and is used for the comminution of food 

(figs- 334, 335)- 

Reproduction is exclusively sexual; budding, fission, or partheno- 
genesis being unknown. The eggs are usually united in large numbers, 


FIG. 313. Veliger larva (trochophore) of Teredo navalh (from Hatschek). A, 
anus; /, stomach; ./,, intestine; I,, liver; LM.d, LAI.v, dorsal and ventral longitudinal 
muscles; Mes, primitive mesoderm cells; MP, teloblast; NepJi, protonephros ; O, mouth; 
Oe, oesophagus; R, rectum; 5, shell; Schl, hinge; SM.h, SM .v, posterior and anterior 
adductors; Sp, apical plate; Wkr, wkr, pre- and postoral ciliated bands; ws, cilia of 
apical plate. 

in a jelly and are either rich in deutopksm or are enveloped in a nourish- 
ing albumen. A few molluscs (e.g., Paludina vivipara) are viviparous. 
A metamorphosis is of wide occurrence, in which a veliger larva escapes 
from the egg (fig. 313); in this can be recognized head, foot, and mantle, 
even when one or the other of these is lacking in the adult. This shows 
that the frequent absence of mantle, shell, or head is not a primitive 
condition, but can only be explained by degeneration. The name 


I. AMPIIINEURA 


315 


veliger arises from the velum, a strong circle of cilia, which surrounds a 
velar field in front of the mouth, and which serves as a locomotor organ 
for the larva. In some cases (fig. 314, B) it is lobed like the trochus of 
a Rotifer. The veliger recalls the annelid trochophore and serves for the 
distribution of the species; it is therefore of great importance for animals 

A B 


FIG. 314. Veliger stages, A, of a snail; B, of a Pteropod (from Gegenbaur). o, 
shell; op, operculum; p, foot; t, tentacle; v, velum. 

which, like most molluscs, are sedentary or slow-moving. In cases with- 
out metamorphosis (Cephalopoda, Pulmonata, etc.) the veliger stage 
is frequently indicated during embryonic development by a ridge of 
cells surrounding a preoral velar field 

Class I. Amphineura. 

These forms, some of which appear in the Silurian, are clearly the most 
primitive of molluscs, and are distinguished by a marked bilateral sym- 
metry. The nervous system already described (p. 313) consists of pleural 
and pedal cords with scattered ganglion cells and no ganglia, these cords 
being connected by numerous commissures (fig. 315, B) 

Sub Class I. Placophora (Chitonidce) . 

The chitons were formerly included among the gasteropods because 
of the presence of a creeping foot and a radula. They are at a glance 
distinguished from them by the rudimentary condition of the head (which 
lacks tentacles and eyes), and the peculiar shell. This last consists of 
eight transverse plates, overlapping like shingles which allows the animal 
to roll into a ball. The edge of the mantle extends beyond the shell and 
is covered with spines, while in the mantle cavity beneath are, right and 
left, a series of ctenidia. Nerves enter the shell and end with noticeable 
sense organs (aesthetes and, in some, eyes, fig. 316). There are no stato- 
cysts. The symmetry of the body is also expressed in the viscera. 


316 


MOLLUSCA 


The anus is medial, and right and left of it are the openings of the neph- 
ridia and sexual organs. The sexes are separate, the gonads unpaired, 
while, corresponding to the paired arrangement of the gills, there are two 
auricles to the heart. Trachydermon,* Amicula* Cryptochiton* 


B 


FIG. 315. Lhiton squamosus, dorsal view (after Haller). A, the entire animal 
B, after removal of shell and viscera; a, anus; C, brain; K, ctenidia; o, mouth; P, pedal 
nerve cord; pi. pleurovisceral nerve cord. 


a. .... 


FIG. 316. FIG. 317. 

FIG. 316. Eye and aesthetes of Acanthopleura spiniger (after Moseley). 0, 
macrassthete; b, micraesthete;/, calcareous cornea; g, \ens;h, iris; k, pigmented capsule; 
n, p, nerves; r, retina. 

FIG. 317. Neomenia carinata, ventral and side views (after Tulberg). a, anterior; 
b, posterior; c, ventral groove. 

Sub Class II. Solenogastres (Aplacophora). 

Wormlike forms without a shell (occurs in the larva of Dondersia) ; instead of 
a foot there is a longitudinal ventral ciliated groove; the radula may be lost; in 
Conchoderma it bears but a single tooth. The gills are either small or wanting. 
The usually hermaphrodite animals have the gonads emptying into the peri- 
cardium and thence by the paired nephridia ( ?). Marine, living in ooze or sand. 
Chatodcrma,* Neomenia, Dondersia. 


II. ACEPHALA 


317 


Class II. Acephala (Lamellibranchiata, Pelecypoda). 

These have, among the molluscs, the least powers of locomotion. 
Some are fixed, the majority burrow slowly through sand or mud; only 
a few spring by means of the foot or swim by strokes of the valves. 
Hence they need more protection than other species, and this is afforded 
by the strong shells in which the body can usually be completely enclosed. 
This shell recalls that of the brachiopod in that it consists of two halves or 
valves, but these valves are right and left and hence are usually similar 
in shape. Only when the animal rests permanently on one side is this 
symmetry lost (the extreme is reached in the fossil Ritdistes), and then 
the symmetry of the soft parts is affected. 


FIG. 318. 


a 


FIG. 319. 

FIG. 318. Left valve of Crassatella plumbea, inner and outer surfaces (from Zittel). 
The outer surface showing lines of growth; no pallial sinus. 

FIG. 319. Right valve of Mactra still tonim, with pallial sinus (from Ludwig- 
Leunis). Letters for both figures: a', anterior, a", posterior adductor scar; e, hinge; 
/, internal ligamental groove; in, pallial line; s, pallial sinus. 

The two lobes of the mantle which secrete the shell on their outer 
surface arise from the back of the animal and grow downwards, forwards, 
and backwards, so that they envelop the whole (fig. 322). Hence the 
oldest and thickest part of the shell, the wnbo, occurs near the back 
(fig. 318). Around this the lines of growth are arranged concentrically, 
lines which show how, by gradual growth of the mantle, the shell has in- 
creased in size. On the back the valves approach each other, and in the 
majority are movably connected by a hinge, which consists of projections 
or teeth on one valve fitting into depressions in the other. The valves 


318 


MOLLUSCA 


are opened by an elastic hinge ligament usually placed dorsal to and be- 
hind the hinge. The shell is closed by adductor muscles which extend 
through the body from shell to shell, leaving their impressions on the 
inner surface (fig. 319). Usually there occur an anterior and a posterior 
adductor equally well developed (Dimyaria) ; less frequently the anterior 
is rudimentary (Heteromyaria) or entirely disappears (Monomyaria). 
When the muscles are relaxed the elastic ligament opens the valves. 

The Jieterodont hinge is the typical form (fig. 319); each valve bears a group 
of teeth near the umbo, those of the left alternating with those of the right. 
Besides these cardinal teeth there are lateral teeth in front and behind, often pro- 
duced into ridges. The ligament lies behind the hinge and is usually visible 
from the outside (external ligament), but is occasionally transferred to the 
interior (internal ligament, fig. 318). The so-called schizodont and desmodont 
hinges are modifications of the heterodont. Then there are Acephala of appar- 
ently primitive character which either lack the hinge (dysodont), or have one 
composed of numerous teeth in a series symmetrical to the umbo (taxodont), 
or of two strong teeth likewise symmetrical to the umbo (isodont}. In these 
cases the ligament is developed in'front of as well as behind the umbo, and may 
be either external or internal. 


C 


FIG. 320. Ventral views of siphonate and asiphonate acephals. .4, An^donta 
nea; B, Isocardia cor; C, Lutrana elliptica. a, anal siphon; b, branchial siphon; 
/, foot; k', outer, k", inner gill lamella; m, mantle; s, shell. 

Since the secretion of shell takes place most rapidly at the edge of the 
mantle, both are closely united, the union being strengthened by small 
muscles. So the edge of the shell has a different appearance from the 
rest, this part being marked off by a pallial line parallel to the margin 
(fig. 318). In many species (the Sinupalliata) the line at the hinder end 
makes a large bay (pallial sinus} (fig. 319, s}. Since the mantle folds 
are membranes with free margins, it follows that when the shell is closed 
these edges are pressed together, which would prevent the free entrance 
and exit of water. To accommodate this each mantle has its margin 


II. ACEPHALA 


31 !> 


excavated at the posterior end, so that when brought together two open- 
ings, an upper and a lower, result (fig. 320, C). The lower of these is 
the branchial opening by which fresh water passes into the mantle cham- 
ber; it flows out after passing over the gills, along with the faeces, through 
the upper or cloacal opening. In many bivalves the free edges of the 
mantle grow together, leaving three openings (fig. 320, B), one for the 
protrusion of the foot, the others the two just described, now called the 
branchial and cloacal sip/ions. By further development the margins of 
these openings are drawn out into two long conjoined siphonal tubes (A), 
which for their retraction need special muscles; these are attached to the 
valves and thus cause the pallial sinus referred to above. 

In the shell there are three layers (fig. 321): on the outside a thin organic 
cuticula and below two layers largely of calcic carbonate. In many these two 
layers are distinguished as the prismatic layer and the nacreous laver, the first 
consisting of closely packed prisms; the nacreous layer of thin lamellae generally 


.-..- 


FIG. 321. Section of shell of Anodonta. c, cuticula; p, prismatic layer; /, nacreous 

layer. 

parallel to the surface. These produce diffraction spectra and so the iridescent 
appearance of the shell; the finer the lines thus formed the more beautiful the 
play of colors. This is especially noticeable in the mother-of-pearl shell J/Y/<u- 
grina and many Unionidae. When foreign substances get between mantle and 
shell they stimulate a greater secretion of nacreous substance and become 
surrounded by layers of it. Pearls are formed in this way. 

The gills lie between the mantle and the body and from their lamellar 
character have given rise to the name Lamellibranchiata (figs. 322, 323). 
Two gill-leaves occur on either side, although occasionally the outer or 
both may degenerate. Frequently the inner gills of the two sides unite 
behind the body and produce a partition which divides the mantle cavity 
into a small dorsal cloaca and the larger lower branchial chamber. The 
anus and the water which has passed the gills empty into the cloaca 
which connects with the excurrent siphon. The incurrent siphon leads 


320 


MOLLUSCA 


into the branchial chamber. In front of the gills are two pairs of leaf -like 
lobes, the labial palpi, between which is the mouth. 

The Nuculidae the most primitive of living Acephala have true ctenidia 
consisting of an axis grown to the body and an inner and an oncer row of gill 
leaves (fig. 326). From this the filibranch type is easily derived. The gill 
leaves grow out into long threads, each bent on itself so that it presents two limbs, 


pa 


ar 


- f 


ob ib b m g 

FIG. 322. Anatomy of Anodonta, mantle, gills and liver of right side removed, 
pericardium opened, a, auricle; aa, anterior adductor; ar, anterior retractor; b, base 
of right gills, cut away; bs, branchial siphon; c, cloaca; eg, cerebral ganglion; d, dorsal 
mantle opening;/, foot; g, gonad; /, intestine; ib, inner gill; /, liver; m, mantle; ', n'-, 
parts of nephridium; no, nephridial opening; ns, nephrostome; ob, outer gill; pa, posterior 
adductor; pg, pedal ganglion; pr, posterior retractor; v, ventricle; vg, visceral ganglion. 

a descending and an ascending. These branchial threads are so hooked to- 
gether that they give the impression of a continuous leaf. In the true lamellar 
gill the threads of the filibranch grow together at intervals, leaving openings, the 
gill slits. Since there is an ascending and a descending limb, it follows that 
each gill consists of an inner and an outer leaf (fig. 323), leaving a space between 
into which the gill slits open. This internal space in some serves to contain the 
young. 

The complete enclosure of the body in the mantle folds has led to a 
degeneration of the head (acephala) and its normal appendages (tentacles, 
usually the cephalic eyes, radula, and salivary glands). Hence there are 
only two divisions in the body, dorsally the visceral sac and ventrally the 
foot. The foot, degenerate in many, has a broad sole only in Pec- 
tuncidus and the Nuculids; usually it is hatchet-shaped (Pelecypoda), 
that is, compressed with a rounded ventral margin. It may be enor- 
mously expanded and contracted again. This expansion is accom- 


II. ACKFHALA 


321 


plished by forcing blood from other regions into the foot. While this 
makes the foot on organ of locomotion, it often serves as a means of 
attachment. Inside is a large byssiis gland which can secrete silky 
threads, the byss-ns (fig. 324), one end of which is fastened to foreign 
objects by means of a finger-like process of the foot, while the other end 
remains in connection with the foot. Molluscs with a byssal gland are 
found anchored by byssal threads to stones, etc. 

The heart, surrounded by a pericardium, usually occupies the most 
dorsal part of the visceral sac. It consists of a ventricle and a pair of 


FIG. 323. FIG. 324. 

FIG. 323. Projection of section through foot and heart of fig. 322. ?', b-, upper 
and lower limbs of nephridium; </, intestine; e, nephridiopore; fu, foot; g, gonad; 
h 1 , ventricle surrounding the intestine; lr, auricle; &', k-, inner and outer gill lamellae; 
I, hinge ligament; m, mantle; n, cerebro-visceral commissure; sp, nephrostome; r, 
venous sinus. 

FIG. 324. Mytihts edulis* (after Blanchard). a, edge of mantle; b, spinning 
finger of foot; c, byssus; </, e, retractors of foot;/, mouth; g, labial palpi; h, mantle; 
i, j, inner and outer gills. 

auricles (figs. 322, a, v, 323, Ii l , /r). The auricles receive the blood direct 
from the gills; the ventricle forces it out through anterior and posterior 
aortae (fig. 322), the latter lacking in many species. The excretory 
organs (organs of Bojanus) lie immediately below the pericardium (fig 
322, n}. Each consists in fresh water mussels of a dorsal smooth-walled 
chamber and a lower portion traversed by threads, both connected behind 
but separated elsewhere by a thin partition. The lower chamber is con- 
21 


322 MOLLUSCA 

nected in front with the pericardium by a ciliated canal, the nephrostome, 
while the upper opens to the outside by a short canal, the ureter, in the 
region of the inner cavity of the inner gill. In this way a connection is 
established from the pericardium to the exterior, the apparatus being 
apparently a true nephridium. In primitive forms it serves also as 
genital duct, but usually the genital and reproductive ducts are separate. 
The animals are usually dioecious, the gonads being acinose glands. 

The digestive tract (fig. 322) begins with a short oesophagus, widens 
out to a large stomach from which a slender intestine leads, with many 
convolutions, to the anus. In most Acephals the intestine passes through 
the pericardium and ventricle. The alimentary tract is enveloped by the 
gonads and the voluminous liver, the secretion of the latter emptying 
by two ducts into the stomach. Usually the stomach has a blind sac, 
in which lies the crystalline style, a rod-like structure of uncertain 
significance. 

The three typical molluscan ganglia (p. 312) are uncommonly wide 
apart. The two brain (cerebropleural) ganglia lie either side of the mouth 
at the base of the labial palpi. They are very small, since cephalic sense 
organs are lacking, and are united by a transverse supracesophageal 

commissure. The posterior ganglia 
(united parietal and visceral ganglia) 
lie near the anus, ventral to the poste- 
rior adductor. The pedal ganglia, 
rather far forward in the muscles of the 
foot, are closely approximate. Of the 
higher sense organs only the statocysts 
near the foot are constant. The labial 
FIG. 325. GlochidiumoiAnodonta palpi are also highly sensory, while two 
(from Balfour) ad, adductor; by, ^ osphradia occur at the basis of 

byssus; s, sense hairs; sn, shell. 

the gills. When eyes occur they are, as 

in the scallops (Pectinida?) , arranged in a row like pearls on the margin 
of the mantle. Their structure is different from that of the cephalic eyes 
of other molluscs. Small tentacles with sensory powers may occur on 
the margin of the mantle and the tip of the siphon. 

Veligers (fig. 313) are very common in development. When this stage 
is lacking the history may contain a metamorphosis as in the fresh-water mussels. 
The young, known as Glochidia, live in the maternal gills and differ from the 
adult by a byssus thread, by only a single adductor, and by a hook or tooth on 
the free margin of the shell (fig. 325). After escape from the gills they attach 
themselves by means of the hooks to passing fish, where they produce an ulcer 
in the skin in which they grow, and by developing the adductor muscles attain 
the definitive form. After this metamorphosis they fall to the bottom, to live 
henceforth half buried in the mud. 


II. ACEPHALA: PROTOCHONCHLE 
Order I. Protochonchiae. 

The primitive character of these forms is shown by the structure of the 
gills, which are either ctenidia (Protobranchiata) or filamentary (Fili- 
branchiata) , yet here and there, as in the scallops and oysters (Pseudo- 
lamellibranchiata), the fusion of gill filaments is already begun. Hinge 
and ligament are symmetrical with regard to the umbo, or vary little 
from symmetry. Hinge teeth may be lacking, and the ligament is 
wholly or in part internal. The mantle edges are free, and rarely is there 
the first trace of fusion to form a siphon. 


FIG. 326. Anatomy of Nueula (after Drew), aa, anterior adductor; b, byssal 
gland; c, cerebral ganglion; ct, ctenidium;/, foot; h, heart; /, labial palpus; o, statocyst; 
p, pedal ganglion; pa, posterior adductor; s, stomach; t, appendage of palpus; v, visceral 
ganglion. 

Sub Order I. DIMYARIA. Two equally developed adductors. The 
taxodont NUCULID^E have ctenidia, a broad foot, pleural and cerebral ganglia 
separate, and gonads emptying through the nephridia, all points which show them 
extremely primitive. Nueula,* Leda,* Yoldia* The ARCID^E are also taxodont, 
but filibranch. Scapliarca,* Argina.* SOLEMYID^E. 

Sub Order II. ANISOMYARIA. Anterior adductor rudimentary (Hetero- 
myaria) or wanting (Monomyaria). With the exception of SPONDYLID^E, all 
the families lack a hinge. To the Heteromyaria belong the MYI.ILID.S, or 
mussels, with strong byssus and shells pointed anteriorly. MoJiula,* Pinna,* 
Mytilns edulis;* eaten in Europe, but occasionally poisonous. Lifhodomus* 
bores into stone. The AVICULID^E includes the pearl oysters of the East and 
West Indies (Meleagrina) . OSTR^ID^ and PECTINID.*:, monomyarian. The 
Ostrasidas, or oysters, usually attached by the right valve. The Pectinidas, or 
scallops, are free-swimming and have highly developed green eyes on the edge of 
the mantle. 


324 


MOLLUSCA 


Order II. Heteroconchiae. 

Gills always lamellar, their outer surface frequently plaited. Hinge- 
in rare cases (Anodonta) lost by degeneration is heterodont or modified 
from a heterodont condition. The mantle edges but rarely free in their 
whole extent; siphons usually present, but in some so 
small (Integripalliata) as to cause no sinus in the pallial 
line; in others (Sinupalliata) large, the pallial line hav- 
ing a marked sinus. Anterior and posterior adductors 
equally developed. 

Sub Order I. INTEGRIPALLIATA. UNIONID*:, fresh- 
water mussels; hundreds of species in the Mississippi basin 
afford material for pearl buttons. In some pearls of value 
are occasionally found. Unio* Anodonta* The tropical 
TRIDACNHXE includes the largest acephalan, Tridacna gigas, 
the shell of which may be four feet long and weigh three 
hundred pounds. The heart shells (CARDIID/E Cardium,* 
Serripes*) and ASTARTID^, marine, and the fresh-water 
CYCLADID^E (Cyclas, Pisidi'um*), about the size of peas, 
belong here, as probably do the extinct RUDISTID^: of the 
cretaceous, in which the attached right valve is drawn out 
into a long cone, the left fitting like a lid over the small cavity. 

Sub Order II. SINUPALLIATA. The VENERID^: with 
swollen shells, Vcmts mcrcenaria* quahog, MACTRID.E, hen 


C 


FIG. 327. 


12 


FIG. 328. 


FIG. 327. A, Saxicava arctica; B, Astarte sulcata; C, Siliqua costata (from Binney- 
Gould). 

FIG. 328. Teredo navalis, ship-worm (after Grobben). The mantle opened back 
to the siphons, i, foot; 2, shell; 3, liver; 4, 5, ventricle and auricle of heart; 6, 
gonads; 7, gills; 8, cloaca; 9, gill chamber; 10, n, anal and branchial siphons; 12, 
pallets on siphons. 

clams, and the TELLINID^; (Tellina* Macoma*), have short siphons. In 
others the siphons are so large that they cannot be entirely retracted within the 
shell, as in the MYID^E, represented in all northern seas by the long clam, Myc 
arcnaria,* and in the razor clams (SOLENID^E; Solcn, Ensatella*). The allied 


III. SCAPHOPODA. IV. GASTEROPODA 


325 


SAXICAVID^E have burrowing species. These forms connect with others 
(Teredo*) in which the united siphons far exceed the rest of the body in length, 
giving the animal a worm-like appearance (fig. 328). Since the valves do not 
cover the whole body, they are supplemented by accessory shells, or the worm- 
like body secretes a tube in which the rudimentary valves are imbedded. 
The PHOLADID.E (phosphorescent), burrow in wood, clay, or stone. The 
shell is well developed. In the ship worms (TEREDID^;) the shells, on the 
other hand, are small, while some species line their burrows with lime. The 
species of Teredo* by their boring habits do much 
damage to wood in the sea, especially in the tropics. 

Lastly, there should be mentioned the little-known 
SEPTIBRANCHIATA, in which the gills have the 
shape of a septum perforated by gill slits separating the 
branchial and cloacal chambers. Silenia, Cuspidaria. 

Class III. Scaphopoda (Solenoconchae). 

The tooth shells resemble the Acephala in the 
paired liver and nephridia and in the nervous system 
(except that a buccal ganglion is present and the 
pleural ganglia are distinct from the cerebral). In 
Ci some points they are primitive (persistence of jaws 
and radula), but in others they are considerably 
modified. They lack gills, have unpaired dioecious 
gonads, rudimentary heart (no auricle), and have two 
bunches of thread-like tentacles either side of the 
mouth. The mantle lobes, paired in the larva, unite 
below, forming a sac open at either end, and this 
secretes a shell shaped like the tusk of an elephant, 
from the larger end of which protrudes the long three - 
lobed foot used for boring in the sand. Dentalium 
(fig. 329), Entails* 

Class IV. Gasteropoda. 

Although more highly organized than the Acephala, the snails are in 
some respects more primitive. The regions of the body foot, visceral 
sac, head, and mantle occur in all orders, although in each forms may 
occur in which one or another part is lost. As a rule, the foot is flattened 
ventrally to a creeping sole. In it may be distinguished anterior and 
posterior processes, the propodlum and metapodium, a sharp lateral margin, 
the parapodium, and, above these, appendages or ridges, the e pi pod 'in. 
Inside the foot is usually a pedal gland. 

The head bears (i) the tentacles, a pair of muscular lobes or hollow 
retractile processes; (2) a pair of primitive vesicular eyes, which usually 
lie at the basis of the tentacles, but may rise even to their tips. In many 
snails the eyes are on special stalks which (stylommatophorous Pulmon- 
ata) form a second pair of tentacles. The protrusion of the hollow 
tentacles is caused by an inflow of blood, their retraction by muscles 
attached to the tip which draw them in like a finger of a glove. 


FIG. 329. Dentalium 
elephantinum, tooth shell; 
left the animal, right the 
shell. /, foot; /, liver re- 
gion; o, hinder opening 
of mantle 


326 MOLLUSCA 

The mantle extends from the back forward over the body to near the 
beginning of the head. It covers the spacious mantle cavity, which 
in the water-breathing Prosobranchiata, etc., contains the gills (ctenidia) 
and opens outward by a large aperture under the margin of the mantle. 
The edge of the mantle may be produced into a long groove-like ciliated 
siphon, conveying water to and from the branchial chamber, and is of 
importance in determining the shape of the shell. In the terrestrial 
snails branchiate respiration is replaced by pulmonate (Pulmonata, 
Cyclostoma), which is retained in many forms (Basommatophora) which 
have returned to an aquatic life. In place of the mantle cavity there is a 
sac filled with air, with a rich network of blood-vessels in its dorsal wall, 
and with only a small opening, the spiracle, on the right side. This 
lung was formerly thought to be the mantle cavity in which the ctenidium 
had degenerated, but development shows it to be an evagination arising 
in the mantle groove. 

The visceral sac, by the great development of the gonads and liver, 
becomes very large. Since growth downwards is prevented by the 
muscular foot, the organs press towards the back, carrying before them 
the dorsal wall at the origin of the mantle folds, the line of least resistance. 
Some organs, like hind gut, nephridia and heart, may be pressed into the 
roof of the mantle cavity. When the visceral sac, as often occurs, 
becomes enormous, it does not stand directly upwards, but coils, usually 
from left to right, in a spiral. The older the animal the more the spiral 
coils and the larger the last or body whorl. 

From the foregoing the shape of the shell is easily understood. As a 
secretion of the mantle and the visceral sac it takes the form of the 
latter. With slight development of the visceral sac it forms a flattened 
cone (fig. 330, A), or is slightly coiled at the apex, as in the abalone (B~). 
When the visceral sac is greatly elongate the shell is correspondingly 
an elongate cone. It is rarely irregularly coiled (Vermetidae, D}. It is 
usually coiled like a watch spring in one plane, or like a spiral staircase; 
in the latter case the shell is more or less conical (C, E) and one can 
speak of its apex and base. In the middle of the base is usually a depres- 
sion, the umbilicus. Sometimes the coils do not touch in the axis connect- 
ing umbilicus and apex, but usually they fuse into a calcareous pillar, 
the columella, around which the whorls pass (E, r). 

The shell increases by additions from the mantle edge; and since this 
determines the aperture, the shell is marked with parallel lines of growth. 
The pigment is produced on the edge of the mantle, and passes into 
the shell as formed, causing its color pattern. When the siphon is pres- 
ent the shell shows a corresponding process. Thus are distinguished 


IV. GASTEROPODA 


32; 


holostome shells with entire mouths (C) and siphonostome shells, in 
which the anterior margin is drawn out in a groove (). 

A simple conical shell without further evidence is not proof of primitive 
structure. It may arise from the spiral form by degeneration, if the visceral sac 
be reduced. Thus the shells of Fissurella and Patella are to be explained, for 
the viscera here show the results of an earlier spiral twist. 

In most places the union between shell and soft parts is not very firm, but 
the connection at the aperture is more intimate, while a muscle is attached to 
the columella at about its middle, the other end being inserted in the foot. This 
retracts the animal within the shell, the head and anterior part of the foot 

A D 


B 


C 


. s 


FIG. 330. Various forms of shells (after Schmarda, Bronn, and Clessin). A 
Patella costata; B, Haliotis lubercidata; C,Lithogl\<phns naticoides; D, Yennctns denti- 
ferus; E, shell of Murex opened to show c, columella; s, siphon. 


going first and then the metapodium, with its dorsal surface towards the aper- 
ture. Hence in many species this surface secretes a door, or operculum, which 
closes the aperture when the body retracts. Since the aperture increases in size 
with growth, the operculum enlarges in a spiral manner (fig. 330, C), some 
times forming a spiral line on the outside, or on both surfaces. Land snails 
are usually without opercula, but in hibernation they can close the shell by a 
calcareous plate, the cpiphragm. In the spring this separates from the shell 
and is lost 

In most gasteropods the shell is coiled to the right, but in some species (tig. 
331) the whorls are constantly turned to the left, while reversed specimens 
occasionally occur in many species which are normally dextral. 

There are usually two layers in the shell, an inner lamellar layer (not always 
present), which sometimes is highly iridescent, and an outer opaque, porcellan- 


328 


MOLLUSCA 


ous layer, contains the pigment and may be covered by a horny cuticle. In rare 
cases mantle and shell are lacking, or the mantle is present but the shell is rudi- 
mentary and not visible externally because the mantle folds have grown over it. 
In these cases the visceral sac is not prominent. Since the shellless forms 
possess a mantle and shell in the young, the adult conditions are explained by 
degeneration. 

Only a few gasteropods are bilaterally symmetrical. Usually the spiral 
twist of the visceral sac has resulted in a torsion of other parts from left to 

right, in which alimentary tract, nephridia, gills, 
heart, and nervous system take part (fig. 332). The 
intestine is bent in this way, the anus opening into 
the mantle chamber on the right side (B) or the 
twisting may be continued so far as to double the 
intestine on itself, the anus being in the middle line 
in front, near the head (C). Nephridia, gills (with 
them the osphradia), and heart wander in company, 
^^^^__ so that the organs primitively belonging on the left 

side may be transferred to the right and vice versa. With this there is 
a tendency to asymmetry and the loss of the organs (usually of the 
primitively left side). When the nervous system takes part in the twist- 


FIG. 331. Sinis- 
tral shell of Lanistex 
carinatus (from 
Leunis). 


FIG. 332. Three diagrams illustrating the torsion of the body and the twisting of 
the nervous system in gasteropods (after Lang). A, bilateral; B, asymmetrical; 
C, streptoneurous condition. The reference letters are placed upon the organs of the 
primitive left side, a, anus; C, cerebral ganglion; g, ctenidium; /, auricle; m, mouth; 
, nephridial opening; o, osphradium; pa, parietal ganglion; pc, pedal ganglion; pi, 
pleural ganglion; v, ventricle. 

ing a crossing of the cerebrovisceral commissures takes place, known as 
streptoneury or cliiastoneitry (C). 

The alimentary canal begins with a muscular region which in some 
groups is developed into a large protrusible proboscis (fig. 333). The 


IV. GASTEROPODA 


pharynx, which follows, contains the tongue, a ventral ridge supported by 
one or more cartilages and covered by the lingual ribbon or radula (odon- 
dophore). The upper surface of the radula is armed with sharp, back- 
wardly directed teeth (fig. 334) arranged in transverse and longitudinal 
rows; these vary so in number, form, size, and arrangement that they 
are of value in classification. The radula is formed in the radula sac, 

which lies behind the tongue 
(fig- 334, rs). From this it 
grows forward like a nail over 
its bed as fast as it is worn out 
in front. It is opposed in eating 
by a single median or a pair of 
lateral jaws (lacking in carnivor- 
ous forms). 

The rest of the alimentary 
canal is convoluted, the anus 


p IG . 3 33 


FIG. 333. FIG. 334. 

. la tuba, male (after Souleyet). The mantle has been cut on the 

right side and turned to the left, reversing the pallial organs, a, anus; c, ctenidium; 
cm, columellar muscle;/, foot; h, heart in pericardium; /, intestine; /, liver; m, mantle; 
mf, floor of mantle cavity; n, nephridium; ns, opening of nephridium; o, osphradium; 
p, proboscis; pe, penis; /, testes; v, vas deferens cut in two. 

FIG. 334. Pharyngeal region of Helix pomatia. A, side view; B, section, m, 
muscle; oe, oesophagus; r, radula; rs, radula sac; sp, salivary duct, z, lingual 
cartilage. 

being usually on the right side in front, in or beside the mantle chamber 
(figs. 337, 338, 339). Rarely it empties in the middle line behind. 
(Esophagus, stomach, and intestine are slightly marked off from each 
other. The convolutions of the intestine are enveloped by the liver, 
which forms the chief part of the visceral sac. A pair of salivary 
glands empty into the pharynx, these in the Doliidae secreting a saliva 
containing 5 per cent, of free sulphuric acid. 

The nervous system usually differs from that of other molluscs in 


330 


MOLLUSCA 


that the pleural and parietal ganglia are separate. If the commissures 
be short, the ganglia are collected near the pharynx and, thus freed from 
the body torsion, are symmetrical (orthoneurous, fig. 336, //). If the 


FIG. T.T.Z,. Row of teeth from the radula of Trochus cincran'us (after Schmarda). 


1L 


FIG. 336. FIG. 337. 

FIG. 336. 7, streptoneurous nervous system of Paludina (after Hering, from 
Gegenbaur). II, orthoneurous system of Limna-a (after Lacaze Duthiers). A, 
visceral; 5, buccal; C, cerebral; p, pedal; PI, pleural; sb, sp, sub- and supraintestinal 
ganglia; n, olfactory nerve; o, otocyst. 

FIG. 337. Diagram of circulation in Doris (after Leuckart). a, auricle; 
c, gills around anus; /, tentacle; v, ventricle; x, vessels returning venous blood from 
the body. 

cerebrovisceral commissures be longer, the result is almost always strep- 
toneury. Pleural and visceral ganglia hold their place, but the right 
parietal ganglion crosses above the intestine to the left side (hence called 


IV. GASTEROPODA 


33] 


supraintestinal), while the left passes under the intestine to the right side 
(subintestinal), the cerebrovisceral commissures being twisted like the 
figure 8. The strong development of the pharynx is accompanied by 
buccal ganglia. 

Gills, heart, and nephridia are best treated together. Certain genera 
(H allot is, Fissurella, etc.) recall the Acephala in having the heart traversed 
by the intestine, the paired ctenidia, nephridia and nephridial ducts, and 
two auricles to the heart. As a rule the asymmetry induced by the torsion 
of the body has resulted in the loss of the ctenidium, osphradium, and 
nephridium of the primitively left side, and with the loss of one gill there 


FIG. 338. Anatomy of Cyprcca tigris (after Quoy et Gaimard). br, ctenidium ; 
c, heart; df, vas deferens, h, liver; in, stomach; N, cerebral ganglion; oc, eye; pe, penis; 
ph, pharynx, the radula drawn out; r, rectum; re, nephridium; t, testes. 

is usually a loss of the corresponding auricle. Prosobranchs and Opistho- 
branchs are recognized accordingly as the gills are on the anterior or 
posterior part of the body. In the Opisthobranchs (fig. 337) the ctenidia 
have been lost and are replaced by secondary gills on the back. Here the 
heart is in front of the gills; it receives blood from behind and forces it 
forward to the head by an aorta. In the Prosobranchs the heart has been 
twisted so that the auricle and the ctenidium are in front (fig. 338). 
while the aorta leads backwards. The nephridium, which communicates 
with the pericardium by a nephrostome, is usually saccular, its duct 
empties beside the anus. 


332 


MOLLUSCA 


The always unpaired sexual organs in some forms (Cyclobranchs and many 
Zygobranchs) empty into the nephridia and possibly the others utilize the rudi- 
ments of the right kidney. The sexual opening is almost always on the right 
side, beside the anus or in front of it on the head. Its position may be recog- 
nized in males and in hermaphroditic species by the grooved dermal fold, the 
penis (fig. 338, pe). Occasionally this is separated from the genital pore, 
but is connected with it by a ciliated groove. The sexual organs are very 
variable in structure. They show two extremes. On the one hand are com- 
pletely dioecious species, on the other there may be complete hermaphroditism 


FIG. 339. Anatomy of Helix pomatia, the roof of the pulmonary sac cut at the 
left side and turned to the right; the pericardium and visceral sac opened and the 
viscera separated, a, anus; c, columellar muscle; d, intestine; el, albumen gland; 
/, finger-form gland;//, flagellum; /w, foot; g, cerebral ganglion; h, heart; /, liver; lu, 
lung; m, stomach; , nephridium; n', its opening; p, penis; ps, dart sac; r, receptaculum 
seminis; 5, pharynx; sp, salivary gland; u, uterus; 7', vagina; vd, vas deferens;' z, her- 
maphrodite gonad. 

(many Tectibranchs, Pteropoda), in which the male and female organs are 
united throughout their extent. Intermediate stages occur, that of the pul- 
monates is shown in fig. 339. 

The terrestrial snails lay their large tough-shelled eggs in damp earth; in 
the aquatic forms the eggs are laid in masses, usually gelatinous, each egg with 
a layer of albumen and a firm shell. Occasionally there is a kind of nest, 
as is the case with lanlhina which carries the mass of eggs, attached to the foot, 
about with it. A few gasteropods are viviparous. 


IV. GASTEROPODA: PROSOBRAXCHIA 


333 


In the development the great constancy with which the veliger stage 
(figs. 313, 314) appears is noticeable. Most marine larvae swim at the 
surface by their velum before creeping at the bottom. But in those cases 
where the snail leaves the egg in the adult form the velum is usually 
developed in embryonic life, sometimes so strongly that the embryo 
rotates in the surrounding fluid. 

Order I. Prosobranchia. 

In the prosobranchs the twisting of the body has brought the ctenidia 
and consequently the auricles in front of the ventricle, and hence the aorta 
leads backwards. It also results in the twisting of the nervous system 
like the figure 8. The sexes are separate and the shell and mantle are 
usually well developed. Certain Prosobranchs are near the primitive 
Amphineura in the retention of both ctenidia, both auricles, and both 
nephridia, but in the great majority only one gill (the primitive right) is 
present and the corresponding auricle alone is well developed, although 
the other may exist in a rudimentary condition. 


FIG. 340. Fissurella patagonica, 
ventral view (from Bronn). br } the 
paired gills; p, foot. 


FIG. 341. Acmcea teshulintilis,* limpet 
(from Binney-Gould). 


Sub Order I. ASPIDOBRANCHIA (Diotocardia), Ctenidia bipectinate 
(rig. 340) or absent. Usually two auricles and two nephridia. DOCOGLOSSA 
(limpets), auricle single; intestine not passing through heart, shell conical. 
ACM^ID.E with ctenidium. Acmaa* (fig. 341). ' PATELLID^, ctenidia replaced 
by a ring-like mantle gill. ZYGOBRANCHIA. Two ctenidia (fig. 340), slu-11 
with marginal slit or with holes; auricles and usually nephridia paired; heart 
traversed by intestine. FISSURELLHXE, keyhole limpets; shell conical, with apical 
opening. HALIOTID.-E, abalones (fig. 330, B). AZYGOBRANCHIA. One 
ctenidium, two auricles. TROCHIDJE, Trorhu-s, Margarita* TURBIXID.K, top 
shells; Sub Order II. PECTINIBRANCHIA (Monotocardia). Ctenidium uni- 
pectinate, a single auricle, osphradium well differentiated (fig. 333), intestine 
not passing through the heart. Many groups based upon structure of lingual rib- 


33-i 


MOLLUSCA 


bon. RHACHIGLOSSA; siphonostomate, predatory. MuRicnxs (Murex, Pur- 
pura* Urosalpinx*) secrete a substance turning purple by exposure to air. Ty- 
rian purple was produced by Murex trunculus. Urosalpinx cinereus* drills into 
oysters. BUCCINID^;, whelks, VOLUTION, and OLIVID^: belong here. TOXI- 
GLOSSA; CONID.E, with cesophageal poison gland. T/ENIOGLOSSA; 
NATiciDyE, Neverita* Lunatia* LITTORINID.E; periwinkles. CvpR^iDyE, 
cowries; Cyprcra moneta used as money in Africa. AMPULLARID^E; amphibious, 
part of branchial cavity acting as lung, part containing ctenidium. PALUDIX- 
IDM, fresh water. 

HETEROPODA. In details of gills, genitalia, heart, and nervous system 
these are true Pectinibranchs, but from an exclusively pelagic life have acquired 
peculiar modifications. Like most pelagic animals they are transparent. The 
head is elongate. Most characteristic is the division of the foot into pro- and 


FIG. 342. Carinaria mediterranea (after Gegenbaur), shell removed. A, meta- 
podium; a, anus; ar, aorta; B, visceral sac; br, branchiae, the heart above; df, vas 
deferens; o, mouth; oc, eye with tentacle; oe, oesophagus; />, propodium; ps, penis; 
/, II, III, cerebral, pedal, and visceral ganglia. 

metapodium (fig. 342), the latter forming a tail-like elongation of the body. 
The propodium is vertically flattened and serves as a swimming organ. The 
Heteropoda are predaceous and extremely voracious. ATLANTIDVE and CARIN- 
ARIID^:, with shells; PTEROTRACHEID^;, without. 

Order II. Opisthobranchia. 

The Opisthobranchia have not varied from the primitive symmetry to such 
an extent as have Prosobranchs and Pulmonates. The anus is in or near the 
median line, although it may be far forwards. The nervous system is ortho- 
neurous, the twist being straightened (except in Actaeonida;). The heart also 
retains its primitive position, receiving blood from behind and forcing it forwards 
to the body through the aorta (fig. 337). In rare cases a (right) ctenidium, a 
poorly developed mantle, and a thin shell occur. Usually these have been lost 
and the place of the ctenidium is taken by accessor; gills of various forms or a 
dermal respiration occurs. The larvae have well-developed mantle and shell. 
Many of the Opisthobranchs afford fine examples, in form and coloration, of 
protective resemblance. All are hermaphroditic and marine. 

Sub Order I. TECTIBRANCHIA. Mantle and usually shell and cteni- 
dium present, Bulta* Philme* Aplysia. Sub Order II. PTEROPODA 
Transparent pelagic forms which in most points agree with the Tectibranchs. 


IV. GASTEROPODA: PULMONATA 


335 


The head and usually eyes and tentacles are lacking, while the fins 
(greatly developed parapodia) are highly characteristic, giving the name 
'wing-footed' to these forms. They have rarely a single ctcnidium. 
The THECASOMATA have shells, LIMACINID^E, HYALEID.*: The shells of 


br 


FIG. 343. FIG. 344. 

FIG. 343.- Hvaltfa complanata from above (after Gegenbaur). a, anus; br, gill; 
c, heart; g, gonad; h, liver; m, mantle; oe, oesophagus; re, nephridium; v, stomach; 
II, pedal ganglion and otocyst. 

FIG. 344. A, Clione papilionacea.* 

CAVOLINID^ make the 'pterpod ooze' of the deep seas. GYMNOSOMATA; 
shell lacking. Pneumodermon, Clime* Sub Order III. NUDIBRANCHIA. 
Shell, ctenidia, and osphradia lacking; most possessing accessory gills (ccrata) of 
varying form and distribution. DORIDIID^; (Fig. 345;. TRITONIID^;, 


FIG. 345. Doris bilamellata* 


FIG. 346. .Eolidia papillosa (from 
Lud \vig-Leunis). 


(Dendronolus*}; ELYSHD^E, cerata lacking. In ^olidae branches of the diges- 
tive tract enter the cerata, expand distally to small sacs filled with nettle cells 
(p. 207) used for defense,; they are derived from hydroids on which these 
animals feed 

Order III. Pulmonata. 

In several respects the Pulmonata are intermediate between the Proso- 
branchs and Opisthobranchs. Like the latter they are orthoneurous and her- 
maphroditic (fig. 339). On the other hand, the respiratory organ is far forwards 


336 


MOLLUSCA 


near the head, with the result that the auricle is forwards, the aorta behind. 
The Testacellidye have the lungs at the posterior end of the body. Occasionally 
streptoneurous conditions occur (Chilina). The lung, the most characteristic 
feature of the order, has already been mentioned (p. 326). 

Many pulmonates are aquatic, but since they have no gills they must occa- 
sionally come to the surface to fill the lung with air, but some, which live at 
great depths in the Swiss lakes and consequently cannot reach the surface, 
use the skin and to some extent the lung for water-breathing. Several genera 
(Planorbis, Pulmobranchia, Siphonaria) have formed secondary gills. 

Sub Order I. STYLOMMATOPHORA. Four retractile tentacles, eyes at 
the tips of the second pair. The HELICID^, a well-developed shell. Helix,* 
many hundred species. Pupa* Acliatina, Bulimus, many tropical species. 
LiMACiDyE. Shell reduced, completely concealed in the mantle. Li max,* 
Arion* Sub Order II. BASSOMATOPHORA. Only one pair of non- 
retractile tentacles, eyes at their base. LIMN^EHXE, pond snails. Limncea,* 
Planorbis.* 

Class V. Cephalopoda. 

The Cephalopoda are distinguished among the molluscs by their size 
and high organization. The majority measure, including the arms, from 


FIG. 347. FIG. 348. 

FIG. 347. Octopus tonganus from the side (after Hoyle). Funnel and mantle fold 
to the right; back and eyes on the left. 

FIG. 348. Loligo kobiensis, ventral view (after Hoyle). 

eight inches to three feet in length, a few are smaller (two to seven inches), 
while especially rare are the giants, some of which may be over forty feet 
long. For a long time these large species were only known from the tales 
of sailors. In the last half-century some of these forms, belonging to the 
genus Architeuthis, have been stranded on the coasts of Newfoundland 


V. CEPHALOPODA 

and Japan. One of the Newfoundland specimens had a body twenty 
feet long from head to tail, and one of the arms was thirty-five feet in 
length. 

The body of a cephalopod is divided by a constriction into head and 
trunk. At the extremity of the head is the mouth, and around this a 
circle of arms or tentacles. Each tentacle is tapering and bears on its 
oral surface rows of suckers (in some species altered to hooks). The 
Octopoda have eight of these arms, all equal in size (fig. 347), four on the 
right side, four on the left. The Decapoda (fig. 348) have in addition two 
longer arms between the third and fourth of the Octopoda, counting from 
the dorsal side. These Dear suckers only on the enlarged tips and can be 
retracted into special pouches. 


ae 


Tnf 


JV.Qp 


FIG. 349. FIG. 350. 

FIG. 349 Diagrammatic section of Cephalopod eye (after Gegenbaur). ae, 
argentea (chorioid); C, cornea; ci, ciliary process; go, optic ganglion; ik, iris; k, carti- 
lages; L, lens; p, pigment layer; Re, cellular layer of retina; Ri, rod layer of retina; 
u 1 , white body. 

FIG. 350. Schematic section of eye of Nautilus (from Balfour). .1, aperture <>f 
optic cup; hit, iris-like fold of integument, N op, optic nerve: R, retina. 

Behind the tentacles are the pair of large eyes which superficially 
closely resemble those of the vertebrates, since they have a transparent 
cornea and a large pupil surrounded by an iris. Internally the resem- 
blance is not less pronounced (fig. 349). Behind the iris is a lens and a 
vitreous body, the latter being bounded by the retina and this in turn by a 
pigmented silvery layer, the argentea or chorioid, which contains cartilages 


338 


MOLLUSCA 


recalling the sclerotic coat. Two striking peculiarities separate these 
eyes from those of the vertebrates and show that they have arisen inde- 
pendently and have an entirely different developmental history, (i) 
The cornea in most Decapoda has an opening by which water enters the 
anterior chamber; (2) the layer of rods in the retina abuts against the 
vitreous body and the ganglionic layer lies behind, while in the ver- 
tebrates the reverse is the case. 


FIG. 351. Sepia officinal is, the mantle and left nephridial sac opened to show the 
vena cava leading to the branchial heart, ti, anus; b, d, lock of siphon and mantle; 
g, genital opening; A", head; k, ctenidium; n, nephridial sac; ', nephridial opening; 
sp, nephrostome; /, ink sac; Tr, siphon. 

The foregoing description applies to but part of the Cephalopoda. The very 
different Nautilidas have, instead of tentacles with suckers, numerous shorter 
tentacles on lobular appendages, which are developed differently in the two 
sexes. The eyes are deep pits, opening to the exterior by a small aperture, the 
base of the pit being occupied by the retina, while lens, vitreous body, iris, and 
cornea are lacking (fig. 350). It is to be noticed that the other cephalopod eyes 
pass through a Nautilus stage. 

In the trunk anterior and posterior regions are distinguishable, the 
two passing into each other on the sides. The anterior (which corre- 
sponds only in part to the ventral side of other molluscs) is wholly covered 
by the mantle, a strong muscular fold, which takes its origin from the 
periphery of the body, often encroaching upon the back and always ter- 
minating with free margins at the head. On opening the mantle by a 


V. CEPHALOPODA 


339 


ventral incision (tig. 351) the two ctenidia (four in Nautihis) are seen on 
the sides. Between them, in the middle line, is the anus, and right and 
left of this and a little behind are the nephridial openings (four in A T au- 
tilns, which also has osphradia). More lateral are the sexual openings 
of which one (usually the right) is commonly absent. At the head the 
mantle opens by a transverse slit to the exterior, but it can be closed and 
fastened by various locking contrivances (in Sepia, Loligo, etc., by button- 
like projections (</) which lit into corresponding sockets (b) on the trunk). 
When thus closed the communication with the exterior is by a special 
conical muscular tube, the funnel or siphon (TV.), which is fastened to the 
body and opens widely to the mantle cavity. Since the cephalopods, by 
contraction of the mantle wall, can drive the water from the mantle cavity 
through the siphon with great force, they can swim very rapidly by the 


FIG. 352.- Female Nautilus, the shell bisected (from Ludwig-Leunis). i, 
mantle; 2, dorsal lobes; 3, tentacles; 4, head fold; =5, eye; 6, siphon; 7, position of 
nidamental gland; 8, shell muscle; 9, living chamber; 10, partitions between chambers; 
n, siphuncle. 

reaction. Nautilus is peculiar in that throughout life the siphon is com- 
posed of two overlapped folds, which is significant since in the embryos 
of other forms the siphon (tig. 361) arises as two separate folds which 
later unite to produce the definitive condition. A typical foot is lacking, 
but comparative morphology shows that the siphon is composed of a pair of 
epipodia, while many zoologists regard the arms as differentiations of the 
fused foot and head, since they are innervated from the pedal ganglia. 

Head and trunk are covered with a thin mucous skin, which has the power 
of changing color in a marked degree. Loligo will quickly pass from a dark 
red to a translucent white; Oct-ipus has an even greater gamut of color. These 
color changes are possible since in the corium there is a silvery layer over which 
are numerous different-colored pigment cells or chromatophores, in which radial 
muscle fibres are inserted. On contraction of these tlu- chromatophores are 


340 


MOLLUSC A 


flattened and thus influence the color; when the fibres relax the pigment cells 
contract to small spots. In deep-sea cephalopods phosphorescent organs have 
been observed. 

Notwithstanding the soft bodies a well-developed shell occurs in 
living cephalopods only in Nautilus and Argonauta (figs. 352, 363). 


FIG. 353. Spirula, with internal shell (after Owen). 


Externally the shell of the former, coiled in a plane, resembles that of 
certain snails like Planorbis; but on section it is seen to be divided by 
partitions into numerous chambers which increase in size towards the 
aperture. The animal occupies only the last chamber with its back 


FIG. 354. Diagram of shells, etc. of various cephalopods (after Lang). A, 
Sepia; B, Belosepia; C, Belemnites; D, O.ttracoteuthix; E, Omniastrephes. a, anterior; 
p, posterior; pit, phragmocone; pr, proostracum; r, rostrum; 5, siphon. 

towards the centre of the shell (exogastric position) ; the other cham- 
bers are filled with air. Each partition has a small opening, and 
through these runs a stand of tissue, the siphnndc. Among the fossil 
cephalopods many forms the Nautiloids and Ammonites have similar 


V. CEPHALOPODA 341 

chambered shells; but in other recent forms and in many extinct species 
the shell is more or less rudimentary. In Spirilla (the animals of which 
are extremely rare, the dead shells common) there is a similar chambered 
shell, buried in the mantle (fig. 363). Its position (ento gastric) is the 
reverse of that of Nautilus. 

In the Decapoda the equivalent of the shell is completely concealed in 
the back of the animal. In the Sepias it is a lamellar calcareous structure, 
the well-known cuttle bone; in the Loliginidae it forms a 'pen' of purely 
organic nature (fig. 311, A). Like true shell these dorsal structures are 
products of the external epithelium, but the epithelium, the shell gland 
which forms them, has become folded in and the walls have united over it. 

The shell of Argonauta (fig. 363) is different. It occurs only in the female, 
is thin as paper, spirally coiled at the tip, and is only in part a secretion of the 
body, for a part of it is formed by two tentacles which are expanded for this 
purpose. Internal partitions are lacking, and this shell serves as a nest for the 
eggs. Most Octopoda also lack a shell. A word or two may be added to 
correlate the recent and fossil shells of the Dibranchiata, which are always 
internal and more or less rudimentary. The fossil Belemnites (fig. 354, C) had 
a chambered shell (phragmocone) perforated for the siphuncle. In front this 
is prolonged ventrally into a thin broad plate, the proostracum, while behind it 
is inserted in a calcareous sheath, the guard or rostrum. From this, by compari- 
son with the fossil Bdosepia (B), it is seen that the cuttle bone of commerce (A) 
is the anterior part of the chambered shell, its laminae being the partitions, while 
in the animal the rostrum and siphuncle are in part retained. On the other 
hand, comparison with the fossil Ostracotcuthis (D) shows that in Ommastrcphes 
(E) we have but a remnant of the phragmocone, while the bulk of the pen is 
proostracum. In Loligo the phragmocone is entirely lacking. 

The mouth, situated in an oval buccal mass, lies between two horny 
jaws, like the beak of a parrot (fig. 355); then follows a pharynx with a 
radula, and in turn a long oesophagus, 
often with a crop-like dilatation The 
oesophagus opens into a wider stomach, 
with which is connected a blind sac, fre- 
quently coiled. Here the tract doubles 

on itself and goes straight, or with one or 

,.- FIG. 3"? 1 ?- Jaws of Sepia 

two convolutions, to the anus (fig. 356). officinalis. 

One or two salivary glands (upper and 

lower, the latter poisonous in Octopus) open into the oesophagus, and a 
pair of liver sacs (frequently fused) open by two bile ducts into the 
gastric blind sac. These ducts may bear racemose glands called the 
pancreas. Lastly, the ink sac opens into the intestine near the anus. 
This gland secretes a brownish or blackish pigment. When alarmed 
the animal ejects this secretion and clouds the water so that it can 
escape unseen. This organ is best developed in Sepia officinalis, and 


342 


MOLLUSCA 


its secretion forms the basis of the well-known color, sepia. Nautilus 
and some Octopoda have no ink sac. 

Just behind the buccal mass are the closely united chief ganglia of 
the nervous system (fig. 357) surrounding the oesophagus. 


A single 


FIG. 356. Anatomy of Octopus vulgar is. T, basis of tentacles; K, head; M, 
mantle split ventrally, opening visceral sac; liver and nephridia removed, venae cava? 
with appendages folded back; a, anus; ao, aorta; au, eye; cv, vena cava with nephridial 
appendage; d, intestine; e, pericardial sac with nephridial opening; go, optic ganglion; 
h, systemic heart; z, crop (cesophageal appendage); k, gills; kh, branchial heart; kn, 
cephalic cartilage; /, liver and /', gall duct (position of liver indicated by dotted line); 
m, stomach; o, ovary; od, oviduct; />, pedal ganglion; r, passage connecting with ovary; 
r', mouth of pericardial sac in nephridial sac; s, cesophagus with dorsal salivary gland; 
s/>, ventral salivary glands; st, stellate ganglion; sv, sympathetic ganglion; /, ink sac; 
v, visceral ganglion; rk, auricle of systemic heart; x, spiral blind sac. 

dorsal mass represents the cerebral ganglia; connected with this by 
broad commissures, the pedal and visceral (viscero-pleuro-parietal) 
ganglia lie close together ventrally. With these parts are associated 


V. CEPHALOPODA 


343 


upper and lower buccal ganglia. The large optic ganglia, in the optic 
nerve arising from the cerebrum and enclosed ventrally in the 'white- 
body,' a lymphoid mass, are especially characteristic, as are the gang- 
lia stellata, right and left at the anterior edge of the mantle (fig. 356, si), 
which owe their name to the radiation of fibres to innervate the 
mantle. An. unpaired sympathetic ganglion lies at the junction of 
stomach and intestine. Cerebral, pedal, visceral and optic ganglia are 
enclosed in the cephalic cartilage, which has the shape of a ring with 
wing-like processes. The complicated statocysts lie in the ventral arch 
of the ring. The sense of smell is highly developed. Apparently it 
resides in a pair of spots of skin between the eyes and the mantle which 
are richly supplied with nerves. In the Decapoda these are sunk in pits, 
in the Octopoda they form papillae. In Nautilus, which has also two 
pairs of osphradia, there is a papilla with a ciliated 
groove, beneath each eye, corresponding to the 
olfactory organ of the other groups. 

Most noticeable of the circulatory structures is 
the presence of two kinds of hearts (fig. 356). 
The systemic heart consists of two (four in Nau- 
tilus) auricles receiving arterial blood from the 
gills, and a median ventricle from which arise 
anterior and posterior aortae. Then there is a 
branchial heart at the base of each ctenidium 
which receives the blood from the vena cava and 
pumps it into the gill. Of venae cavae there are 
an anterior unpaired and two posterior paired 


op 


FIG. 357. Nervous 
system oiSepia ofiicinalis 


from the side. gbi, in- 

trunks, the former dividing and sending a branch fe , rior bucca l ganglion; 

gas, superior buccal 
to each branchial heart. These trunks are con- 


nected with the nephridia. The nephridial open- 


ganglion; gc, cerebral 
ganglion ; gp, pedal gan- 
glion; gv, visceral gan- 
ings (p. 339) lead to two spacious sacs through g ii on; n ,b i buccal mass; 

which the veins pass obliquely. This part of the oe > oesophagus; op, optic 

ganglion. 

blood vessels bears venous diverticula which pro- 
ject into the nephridial sac and are covered with an epithelium of 
excretory cells. Near its mouth each nephridial sac communicates by 
a nephrostome with the (usually large) coelom. 


In the Octopoda the coelom is reduced to the gonads and narrow canals 
leading from the nephrostome to the gonads and branchial hearts, but else- 
where there is a well-developed system of connected cavities, consisting of the 
pericardium around the systemic and branchial hearts and the thin-walled 
genital sac, one wall of which bears the genital ducts, while on the other the 
sexual cells arise or the ducts of a separate sexual gland open (fig. 358). 


344 


MOLLUSCA 


- n 


The gonads of the always dioecious Cephalopoda are unpaired and lie 
far back in the visceral sac. The ducts in the female Octopoda (rarely 
in the males) and in some Decapoda (Oigopsida) are paired. In Nautilus 

only the right duct is functional in either 
sex, although the left is well developed. 
Elsewhere there is only the left duct. The 
oviducts are saccular with glandular walls; 
independently of them two pairs of glands 
open to the exterior, the accessory glands 
and the large nidamental glands. The vas 
deferens (fig. 358) is more complicated. It 
has swellings known as seminal vesicle, 
prostate, and Needham's sac, in which the 
spermatophores are stored. These latter 
have such a complicated structure and 
show such motions when swollen with 

k 

water that they were long regarded as para- 
sitic worms (fig. 359). 

The spermatophores are conveyed to 
the female by means of more or less modi- 
fied (hectocotylised) tentacles of the male. 
In a few genera the whole tentacle becomes 
a 'Hectocotylus' (fig. 360). It swells at its 
base to a sac in which the peripheral end is 
enclosed. This part, which contains a 
canal for the spermatophores, cuts loose 
from the male, and can creep about for days in the mantle chamber 
of the female. Since it appears like an independent animal, it was first 
described as a parasitic worm under the name Hectocotylus. In others 
the hectocotylization is not carried so far. 


t 


FIG. 358. Male sexual organs 
of Sepia officinalis (after Grob- 
ben). a, ccelomic sac passing 
to the left and above into the 
pericardium; c, ccelomic canal 
to the vas deferens; d, vas def- 
erens; d', its opening to ccelom; 
/, portions of ccelom; n, Need- 
ham's pocket; n', its mouth; 
p', p 2 , prostates; t, testis; t', its 
opening to ccelom; t'5, seminal 
vesicle. 


FIG. 359. Spermatophore of Sepia (from Hatschek, after Milne Edwards), a, dis- 
charging apparatus; b, packet of spermatozoa; c, envelope. 

The eggs are either fastened singly to aquatic plants or are laid in large 
gelatinous masses. They are rich in yolk, and in consequence undergo partial 
discoidal segmentation (fig. 105). The blastoderm, on the end of the oval egg, 
forms the anlagen of the separate organs (eyes, arms, siphon, and shell gland) 
as flattened projections. Later the embryonic body becomes distinct from the 


V. CEPHALOPODA 


345 


yolk, which, enclosed in a cellular envelope, remains attached to the rest, near 
the mouth, until it is absorbed in the growth of the young and the animal is 
ready for hatching (fig. 361). 


FIG. 360. Male of Argonauta argo (after Muller, from Hatschek). 1-4, arms 
of right side; i.~4., arms of left side; 3, hectocotylised arm, at the left in its sac, at the 
right protruded. 


FIG. 361. Embryos of Loligo pealei (orig.). a, arms; e, eyes;/, fin; g, ctenidia; 
h, statocyst; m, mantle; s, siphonal folds and siphon; v, anus; y, yolk sac. 

The Cephalopoda are exclusively marine. Some inhabit rocky shores, 
others the high seas. All are carnivorous and in turn are preyed upon by fishes, 
etc. Classification is based upon the number of gills and number and character 
of the arms. 


346 


MOLLUSCA 
Order I. Tetrabranchia. 


With four gills, four auricles, and four nephridia; numerous tentacles without 
suckers, a well-developed chambered shell (fig. 352), siphon of two separate 
epipodia, and simple eyes (fig. 350). Four living species, all belonging to Nautilus. 


FIG. 362. Octopus bairdii* (from Verrill). A hectocotylised arm on the right side. 


363. Argonauta argo, paper sailor, female (after Rymer Jones). 

The animals, which live in the Malaysian regions, are rare, but their shells are 
abundant. In past time the tetrabranchs were very abundant; NAUTILID^, 
with straight (Orthoceras) or coiled shells (Goniatitcs, etc.), paleozoic. They 


V. MOLLUSCA: SUMMARY OF IMPORTANT FACTS 347 

had simple septa. AMMOXITID/E, folded septa, largely mesozoic. Their 
pertinence to the tetrabranchiates is assumed from the character of the shell. 

Order II. Dibranchia. 

With two nephridia, two gills, and two auricles; eight or ten arms with 
suckers; highly organized eyes; shell rudimentary or absent. 

Sub Order I. DECAPODA. Ten arms, body with lateral fins. Rudimen- 
tary shell usually present. SPIRULID^;, with internal chambered loose-coiled 
shell. Spirula (fig. 353). OIGOPSIDA, with perforated cornea (p. 338) and two 
oviducts. Ommastrephes*; Architcntkis,* the giant squid (p. 336). MYOPSIDA. 
Oviduct single (left); cornea unperforated. Loligo* common squid; Rossia*; 
Sepia, cuttle fish, furnishing the 'cuttle bone' fed to cage birds, and the pigment 
sepia. Sub Order II. OCTOPODA. Eight arms, webbed at their base; shell 
very rudimentary, sometimes fragmentary or wanting; oviducts paired. Ocro- 
PODID^;, Octopus'^ (fig. 362), Alloposus.* ARGONAUTID.^, female with boat- 
like shell (fig. 363), males much smaller and without shell. In Argonautidse 
and PHILONEXID.E the hectocotylus separates. 

Summary of Important Facts. 

1. The MOLLUSCA are parenchymatous animals with reduced 
coelom. They consist of head, visceral sac, mantle, and foot. 

2. The head bears eyes and tentacles. 

3. The foot is an unpaired muscular mass used in locomotion. 

4. The mantle bounds the mantle cavity which is connected with 
respiration; it either functions as a lung or covers the gills (ctenidia). 
It secretes the shell from its outer surface. 

5. Foot, head, mantle, and with the latter the shell, may be lost in 
many groups. 

6. The molluscs agree in the nervous system. Three pairs of ganglia, 
connected with three pairs of sense organs, occur: a, cerebral ganglia and 
eyes; b, pedal ganglia and statocysts; c, visceral ganglia and osphradia 
(olfactory). 

7. The heart is dorsal and arterial; it is enclosed in a pericardium 
(reduced ccelom) which connects with the nephridia by nephrostomes. 

8. There is always a single ventricle and, according to the number 
of respiratory organs, one, two, or four auricles. 

9. The alimentary canal is well developed; the liver large; salivary 
glands usually present. In most there is a pharynx or buccal mass with 
radula and jaws. 

10. A veliger stage is common in development. 

11. The Mollusca are divided according to the respiratory organs 
and appendages of the body into five classes: (i) Amphineura; (2) 
Acephala; (3) Scaphopoda; (4) Gasteropoda; (5) Cephalopoda. 

12. The AMPHINEURA have an extremely simple nervous system 
in which the ganglia are replaced by nerve tracts. 


34S MOLLUSCA 

13. The ACEPHALA lack head and cephalic appendages. 

14. They are bilaterally symmetrical and have paired organs: mantle 
folds, bivalve shell, gills, nephridia, and gonads. 

15. In many Acephala (Asiphoma) the mantle folds are completely 
separated ventrally. 

16. In theSiplionata the lower edges of the mantle are united, leaving 
three openings: (i) in front for the foot; (2) behind and below, the branchial 
siphon for the ingress of water and nourishment; (3) behind and above, 
the anal or excurrent siphon for the water used by the gills and the 
fa?ces. 

17. There are two pairs of gills (ctenidia), which maybe comb-like, 
filiform, or most commonly lamellar (lamellibranchs). 

1 8. Correspondingly the heart has two auricles; the unpaired ventricle 
is usually traversed by the rectum. 

19. The foot is a compressed muscular mass frequently provided 
with a byssus gland. 

20. The shell consists of cuticular, prismatic, and nacreous layers. It 
is closed by one or two adductors and opened by an elastic ligament. 

21. Some Acephals (Protoconc/ia) have primitive gills and hinge; 
others (Heteroconcha) are more highly developed. 

22. The SCAPHOPODA are primitive forms with tubular shells. 

23. The GASTEROPODA (Cephalophora, or snails) have a distinct 
head bearing eyes and tentacles; a creeping foot, an unpaired mantle 
(occasionally absent), and a univalve shell or none. 

24. The mantle cavity contains one or less frequently two ctenidia, 
or these may be degenerate and a lung may occur. 

25. Nephridia and auricles are rarely paired (with paired gills); 
the gonads, always unpaired, are hermaphroditic or dioecious. 

26. The shell is always unpaired; it is usually coiled in a (right-hand) 
spiral, and is frequently closed by an operculum. 

27. According to character of nervous system, sexual organs, heart, 
and respiratory organs the Gasteropods are divided in to (i)Prosobranchia; 
(2) Opisthobranchia; and (3) Pulmonata. 

28. The Opisthobranchia are hermaphroditic; orthoneurous; have 
secondary gills (or none), and have the auricle always behind the ventricle; 
shell and mantle reduced or absent. 

29. The Pteropoda are pelagic Opisthobranchs with wing-like pro- 
cesses of the foot and frequently reduced shell or none. 

30. The Prosobranchia have the gills (ctenidia occasionally paired) 
far in front, and in consequence the auricle in front of the ventricle ; they 
are streptoneurous and dioecious; the mantle and shell well developed. 


ARTHROPODA 349 

31. The Heteropoda are pelagic Prosobranchia with foot divided into 
fin and tail, shell rudimentary or absent. 

32. The Pidmonata are in some respects (orthoneurous and her- 
maphroditic) Opisthobranch-like; in other respects as in position of 
heart, development of shell and mantle like the Prosobranchs; the 
mantle cavity connects with a lung. 

33. The CEPHALOPODA have no true foot; but its homologues are to be 
found in the siphon and in the tentacles, usually provided with suckers, 
on the head ; they have an unpaired mantle and mantle cavity and a single 
shell or none. 

34. The mantle cavity contains one or two pairs of ctenidia. Water 
is forced from the mantle cavity through the siphon. 

35. The number of auricles and nephridia corresponds with the number 
of ctenidia; besides the systemic heart there are one or two pairs of branch- 
ial hearts, elsewhere unknown in molluscs. 

36. The sexes are separate. 

37. The ink sac is peculiar to Cephalopoda. 

38. The eye is (usually) highly developed (with retina, chorioid, 
iris, cornea, vitreous body, and lens), as is the nervous system, which has, 
in addition to the usual centres, optic, sympathetic, and stellate ganglia. 

39. The eggs have a discoidal segmentation. 

40. The Cephalopoda are divided into Tetrabranchia and Dibranchia. 

41. The Tetrabranchia (extinct save for Nautilus) have four gills, 
a chambered shell, primitive eyes, and finger-like cephalic lobes in place 
of tentacles. 

42. The Dibranchia have two gills, eight or ten tentacles with suckers, 
and the shell is reduced or absent. 

PHYLUM VII. ARTHROPODA. 

Under the term Arthropoda are included the crabs, spiders, insects, 
and myriapods, which, together with the annelids, were united by Cuvirr 
in his sub-kingdom Articulata. Annelids and arthropods agree in many 
features. They are, as the term articulates implies, segmented animals, 
and they differ from the vertebrates, which are also segmented, in the 
extension of the segmentation, the ringing of the body, to the external 
surface. The boundaries between the successive segments, which cannot 
be recognized in the skin of the vertebrate, are marked in the articulates 
by constrictions of the body wall, whence the old names evro/xa and 
Insecta, applied to these forms. The articulates are further characterized 
by a ladder-like nervous system (fig. 78), in which the brain is supple- 


350 


ARTHROPODA 


merited by a ventral chain composed of ganglia metamerically arranged. 
The most evident distinctions between the annelids and the arthropods 
are (i) the character of the segmentation and (2) the presence of jointed 
appendages. 

In superficial appearance the lines between the segments are con- 
stricted more deeply in the arthropods than in the annelids. The cause 
of this lies in the character of the integument (fig. 26, f), 
which is developed as a hard armor, in which two 
layers are recognizable, the epidermis ('hypodermis') 
and the chitinous layer. The epidermis is a thin 
epithelium, while the chitinous layer is of greater 
thickness and, since it is secreted by the epidermis, is 
stratified parallel to the surface. Its firmness is due 
to chitin, which is unlike most organic substances in 
its resistance to acids and alkalis; only under the 
action of sulphuric acid and heat is it broken up into 
sugar and ammonia. Frequently (Myriapods, Crus- 
tacea) the chitinous armor is strengthened by the 
deposition of calcium carbonate and phosphate. A 
firm coat would render the animal incapable of motion 
were there not joints between the parts. While the 
segments themselves are heavily armored, the cuticle 
between them is reduced to a delicate articular skin, 
and this is so protected by a kind of telescoping of the 
segments that injury in these softer regions is nearly impossible (fig. 364). 

Since the ringing of the body is connected with this armoring, it disappears 
with the need for such protection. The hermit crabs (fig. 406) illustrate this. 
These animals live with the abdomen inserted in a snail shell. That part of 
the body which projects from the shell is armored, while the abdomen is soft- 
skinned and without traces of external ringing. The hardened cuticula causes 
the periodic molting (ccdysis or exuviation } . When once hardened it is incapable 
of distention and so would prevent farther growth. Hence when the body has 
completely filled the shell, the latter splits in definite places and the animal crawls 
out of the old 'skin' (exui'ia) and rapidly increases in size, while the new cuticula 
is yet soft and distensible. Another result of the cuticula is seen in the peculiar 
relations of both ordinary and sense hairs. These are cuticular structures, each 
usually secreted by a single epidermal cell and renewed after each molt. Each 
hair has a ball-like base situate in a socket in the surrounding chitin, and hence 
is movable; it is traversed by a canal in which is a process of the underlying 
matrix cell. In the case of sensory hairs these structures are connected with a 
nerve (fig. 80). The sense cell has. two processes; one peripheral, which enters 
the axis of the hair, the other central, which runs as a nerve fibre to the central 
nervous system. The cell itself may be in the epithelium or situated deeper and 
interpolated as a ganglion cell in the sensory nerve. 

The muscles which are inserted on the integument are sesrmental in character 


FIG. 364. Dia- 
gram of Arthropod 
jointing; .4, in ex- 
panded, B, in con- 
tracted condition; 
1-4, rings with con- 
necting membranes, 
the muscles indi- 
cated by dotted 
lines, (after 
Graber). 


ARTHROPODA 


) l 


and are arranged in metameric muscle groups. Frequently they are inserted 
on the chitin by special tendons, portions of the chitin drawn inwards. Through 
such infoldings there arises in many arthropods an 'entoskeleton.' 

Another important character is the heteronomous segmentation, which, 
in the lowest forms (Peripatns and myriapods), is little pronounced, hut 
elsewhere leads to a marked inequality of the regions of the body. A 
few segments at the anterior end always fuse and form a head (fig. 365, C) ; 
behind this there is usually a second segment complex, the thorax 
(per don} (T), and then a third, the abdomen (pleori) (A). An apparent 


FIG. 365. FIG. 366. 

FIG. 365. Campodea staphylinus (from Huxley). A, abdomen; C, head; T, thorax. 
FIG. 366. Euscorpius italicus. a, abdomen; c, cephalothorax; />, post- abdomen; 
5, sting; i, chelicera;; 2, pedipalpi; 3-6, walking legs. Below chelicera enlarged. 

reduction of regions occurs when the head and thorax unite (fig. 367, C/) 
to form a cephalothorax; or the number of regions may be increased (fig. 
366) by a division of the abdomen into abdomen proper (a) and post- 
abdomen (/>). Finally, in many arthropods (e.g., the mites or acarina, 
fig. 368) it is impossible to recognize regions or somites because internal 
fusion of parts has obliterated the external evidences of segmentation. 
In order clearly to understand what is meant by head, thorax, etc., 
requires a consideration of the second character distinguishing the arthro- 
pods from the annelids, the jointed appendages, which give the name to 
the former group. The arthropodan appendages are highly developed 


352 


ARTHROPODA 


parapodia, differing in being jointed to the body, in consisting of a series of 
joints themselves, and in having their intrinsic musculature. There is but 
a pair of appendages to a somite, and this belongs to the ventral surface. 
Hence it follows (Savigny's law) that although a region may show no 
external signs of segmentation, if it bear more than one pair of appendages, 
we conclude that it is a complex of at least as many somites as there are 
pairs of appendages. Thus the unsegmented head of an insect consists 
of four somites, the cephalothorax of a lobster of thirteen, for the one bears 
four, the other thirteen, pairs of appendages. Ontogeny supports this, for 
in the embryo the somites are clearly visible. This statement is not ex- 
actly correct, for in certain insects and in the lobster there is one more 


FIG. 367. Pal(pmon serratus (from Lud \vig-Leunis). A, abdomen; Ct, cephalo- 
thorax. 

FIG. 368. Gamasus coleoptratorum (from Taschenberg). 

somite which is entirely lost in the adult. It is not necessary that each 
somite in the adult should bear appendages, since these may disappear 
in growth without leaving a trace. 

While originally all were locomotor, the appendages subserve many 
functions (fig. 369). Locomotor appendages (pereiopoda, feet or legs) 
are long and consist of a number of joints which may form flattened oars 
or may be provided with claws for creeping (8). Besides locomotor ap- 
pendages there are tactile appendages or antenna (i), chewing appendages 
(jaws, mandibles, maxilla, 2-4), false feet or pleopoda (9) of varying func- 
tions, and forms maxillipeds (5-7) transitional between jaws and legs. 

Aside from being tactile, antennae are characterized by position and in- 
nervation. They are always in front of the mouth and receive their nerve 
supply from the supracesophageal ganglion, while all other appendages 


ARTHROPODA 


353 


are innervated from the ventral chain. In their elongate shape antennae 
are not unlike legs, but they lack the terminal claws. 

The form of the jaws is strikingly modified. One or two basal joints 
serve for the comminution of food, and these parts are strong and are 
covered, especially on the medial side with a 
hard, toothed chitin (figs. 369, 2 ; 374, ///, I'). 
The other joints may entirely disappear, or 
may form a more or less leg-like appendage, 
the palpus. Since several appendages may be 
modified into jaws, the first are called mandi- 
bles, the next maxilla?, and second maxilla? 
may follow. The maxillipeds may have more 
the appearance of jaws, at other times are 
more leg-like (tig. 369, 5-7). The false feet 
(pleopoda) are small and inconspicuous ap- 
pendages which have various functions: they 
may serve as gills or supports for gills, places 
for the attachment of eggs, organs for the 
transfer of sperm, or as swimming or creep- 


ing organs. 


FIG. 369. Appendages of 
the crayfish, i, first antenna; 
2, mandible; 3, 4, first and 
second maxillce; 5, 6, 7, maxil- 
lipeds; 8, walking leg; 9, 
pleopod. 


These appendages have constant positions 
in the body. First on the head come the an- 
tenna? and then, in the region of the mouth, 
the jaws and, so far as they are present, the 
maxillipeds. Third come the true feet, and 
lastly, when they exist, the false feet. Those 
somites which bear antennae or jaws belong to 
the head, those bearing walking feet to the 

thorax, while the somites of the abdomen bear either false feet or lack 
appendages. As a sequence the cephalothorax is that region of the 
body which bears, besides antenna? and jaws, legs as well. 

The somites of Arthropoda have given rise to various disputes. Many zoolo- 
gists speak of a pre-antennal somite and a pre-antennal appendage, referring to 
the eye stalk of some Crustacea, which, however, differs markedly in its develop- 
ment from the true appendages. Those who accept an ocular somite must u<l<l 
one to the number of somites as stated in this volume. A second theory regards 
the antennas as ventral appendages innervated from the ventral chain which 
secondarily become dorsal and receive their nerves from the brain. This view 
is firmly grounded for the second antennas of the Crustacea. Other questions 
are as to the possible loss of both segments and appendages. 

The union of somites to body regions has had an influence upon the 
internal structure and especially upon the nervous system (fig. 370). A 

23 


3.34 


ARTHROPODA 


ladder-like nervous system consists, as was pointed out (p. 113), of a 
dorsal brain (supraoesophageal ganglia) and a ventral chain of ganglia, 
all connected by longitudinal nerve cords, the brain being connected with 
the rest by cords or commissures passing on either side of the oesophagus. 
The ventral chain should contain as many pairs of ganglia as there are 
somites, but this is not the case except in the embryo. The tendency is 
rather towards a fusion of ganglia, especially of those somites which unite 
or fuse. This fusion of ganglia occurs to a varying extent in different 
species, the extreme being reached in the spiders and crabs (fig. 402) } 


B 


C 


D 


FIG. 370. Different degrees of concentration of the ventral cord of Arthropods 
(from Gegenbaur). A, Termite (after Lespes). B, water beetle (after Blanchard). 
C, fly (after Blanchard). D, Thelyphonid (after _ Blanchard). a, abdomen; g s ,_ g 3 , 
ganglia of ventral cord; gi, infracesophageal ganglion; gs, supracesophageal ganglion; 
o, eye; p'-p", walking feet; ir, lung books; i, chelicerai; 2, pedipalpus. 

where the whole ventral chain forms a single ganglionic mass. In all 
cases, however, the brain remains distinct from the rest, its position dorsal 
to the oesophagus precluding its fusion with the ventral chain. 

Two types of eyes are recognized, the simple (ocellus, stemma) and 
the compound (faceted). The ocelli are small. In their highest develop- 
ment, as in spiders (fig. 371), they are composed of lens, vitreous body, 
and retina. The lens is formed by the cuticula, the rest from the epider- 
mis. The lens differs from the rest of the cuticle in being transparent, 
and is usually thickened to a biconcave body (i) which converges the 
light upon the retina. Only exceptionally (larvae of Ephemerida) is the 


ARTHROPODA 


355 


lens cellular, being formed by a thickening of the epidermis. Beneath 
the lens comes a layer of transparent cells, the vitreous body (2), and 


456 7 

FIG. 371. Diagrammatic section through anterior (A} and posterior (B) eyes of 
Epeira diadema (after Grenacher). The hinder eye shows the inverted retina; i, 
lens; 2, vitreous body; 3, epidermis, outside this, chitinous layer; 4, rhabdomes; 5, 
retinal cells; 6, capsule of eye; 7, rhabdomes of inverted eye. 


1234 5 

FIG. 372. Section of compound eye of Forficula (after Carriere, from Ha tschek). 
i, cuticula, producing the cornea of many lenses over the eye; 2, epidermis, which in 
the eye forms the ommatidia; 3, basal membrane; 4, reentrant chitinous fold ('sclerotic') ; 
5, rudimentary larval eye. 

behind this, in turn, the retina, consisting of visual cells which, at the 
one end, bear 'rhabdomes' (4 and 7), at the other pass into nerve fibres. 
The retina and vitreous body, surrounded by pigment, form a spherical 


ARTHROPODA 


thickening sharply marked off from the rest of the epithelium, 
eyes, like those of vertebrates, form inverted images. 


These 


Spiders have two kinds of ocelli (fig. 371), the chief eyes and the accessory. 
In the first (A) the rhabdomes are next the vitreous body and in front of the 
nuclei of the cells. In the other (B) the relations are reversed, the structure 
recalling the inverted eyes of vertebrates. Apparently the first are for distant 
vision, the others for near objects. 


The compound 
eyes to the fact that 


FIG. 373. A single 
om.matid.him (with 
sections) of a com- 
pound eye. k, crys- 
talline cone; kz, cone 
cells; /, lens with 
hypodermis; r, rhab- 
domes; rz, retinular 
cell. 


eyes are much larger. They owe their name faceted 
the cuticle over them is divided into polygonal (usually 
hexagonal) areas or facets. Each facet corresponds 
to a small chitinous lens (the number of which varies, 
in different species, between a dozen and several 
thousand), and bounds the eye externally, whence 
tliis layer is called the cornea (fig. 372). The part 
of the eye beneath the cornea consists of radially 
arranged pyramidal parts or ommatidia which corre- 
spond in number and position to the facets, their 
broader ends being placed beneath the facets, their 
narrower internal ends connecting with fibres of the 
optic nerve which go to the brain. Each omma- 
tidium (fig. 373) has essentially the structure of an 
ocellus: (i) the lens (/) with its epithelium; (2) the 
virtreous body (kz); (3) the retinula (rz). The 
vitreous body is usually composed of four cells 
which, in the so-called enconous eyes, surround a 
transparent body, the crystalline cone (k), secreted 
by these cells. The retinular cells are almost 
always seven in number, each bearing on its inner 
surface a rhabdome (;), the seven rhabdomes fre- 
quently fusing into a common mass. Each omma- 
tidium is surrounded by a pigment sheath, isolating 
it optically from its fellows. 


There are really two pigment zones, an anterior which corresponds function- 
ally to the vertebrate iris, and a posterior comparable to tapetum and chorioid. 
The iris pigment lies in special cells and is most abundant at the line between 
the vitreous body and retinula. The posterior pigment is in the retinular cells. 

From this it appears that the compound eye may be regarded as a 
complex of ocelli. This anatomical conception must not, however, ob- 
scure the physiological. The whole compound eye forms but a single 
erect picture composed of separate images of small area formed by the 


ATHROPODA :;:,7 

separate ommatidia. This 'mosaic theory' has completely replaced the 
view that each ommatidium formed a complete inverted picture. 

While the number of ocelli varies, the compound eyes are almost invariably 
two in number, but can be constricted into four or eight. The single eye of 
Daphnids is the result of fusion. There is always a large optic ganglion in the 
optic nerve of each compound eye. As in vertebrates both ocelli and com- 
pound eyes can shine in the dark. This is the result of the 'tapetum lucidum,' 
formed in the Crustacea and arachnids of modified pigment cells, in the insects 
by a rich tracheal network. 

The tactile organs, consisting of tactile hairs (fig. So), are uniform 
in structure. On the other hand the senses of hearing, taste, and smell 
are subserved by varying organs. We know but little of these senses in 
arthropods, although they are frequently well developed. The sense of 
smell resides chiefly in the antenna: and in the palpi of the jaws. The 
organs are olfactory tubules or cones (modi tied hairs) which frequently 
lie in pits in the skin. Similar organs in the mouth are probably con- 
nected with taste. As organs of hearing ( ? equilibration) besides the 
statocysts of the Podophthalmata and the tympanal organs of the Orthop- 
tera, the widely distributed 'chordotonal' nerve ends of insects are to be 
mentioned. 

The larger proportion of the alimentary canal is formed of ectodermal 
stomodeum and proctodeum, while the entodermal portion (mesenteron) 
usually forms but one-third of the total length. At ecdysis the chitinous 
lining of the ectodermal parts, including the large chewing stomach, is 
cast with the rest of the integument. The entire absence of ciliated 
epithelium is noteworthy. Ciliated cells have never been found in 
arthropods. 

The most constant portion of the circulatory system is the heart, 
which lies immediately beneath the back, enclosed in a more or less 
distinct sac which, although called pericardium, is not a part of the 
ccelom. From the pericardium blood passes into the heart by openings 
(ostia) right and left. Since the margins of the ostia project far into the 
lumen of the heart and so form folds functioning as valves, the heart 
itself may be divided into a series of chambers (fig. 67). The chambers 
disappear when, the heart is reduced to a sac. In small arthropods the 
heart together with the whole vascular system may be lost. Since the 
Annelida have a well-developed circulatory system, the loss in the arthro- 
pods is secondary rather than primitive, and is explained by the tact that 
with reduction in size the organization is simplified. The heart can thus 
be lost in small Crustacea (many ostracods, copepods, and barnacles), 
in the smaller arachnids (mites), and in insects (many aphids) , while it is 
present in allied forms. 


358 ARTHROPODA 

The blood may pass from the large arteries either directly into the 
large blood sinuses of the body (hiemocoele, p. no), erroneously called 
the body cavity, or by a more complicated course through capillaries and 
veins as well as through the respiratory organs. There are, on this 
account, great differences in the development of the vascular system, 
but even in the highest forms the system is not entirely closed, the blood 
passing to the haemocoele and thence to the pericardium (probably 
arising from the coalescence of veins), from which it is sucked through 
the ostia into the heart. The variations in the circulation depend 
chiefly upon the modifications of the respiratory organs, which can be 
described adequately only in connexion with the various groups. In 
general it can be said that the more respiration is localized in regions and 
organs the more nearly complete is the circulation, while with respiration 
diffused over or through the whole body, the vascular system, including 
even the heart, may be reduced. 

Anatomy and embryology show that the coelom is greatly reduced in the 
arthropods and is represented by the cavities of the reproductive and nephridial 
organs, and in the decapods by a pair of thin-walled sacs connected with the 
green glands. Sinus-like enlargements of the blood-vessels like the haemocoele 
occur in several annelids (Magelona). 

The excretory organs are of two different types. The segmental 
organs of Peripatus, the shell and green glands of Crustacea, the coxal 
glands of Acerata, and the head glands of insects are modified nephridia 
in which the nephrostomes have been converted into small sacs. The 
other type includes the Malpighian tubules of insects, diverticula of the 
alimentary canal, opening at the junction of intestine and rectum. 

The sexual organs, which empty through ducts which are apparently 
modified nephridia, are rarely hermaphroditic. In the bisexual species 
one can usually distinguish males and females by external characters, 
such as size, coloration, or form of appendages, especially those used in 
copulation. The eggs are usually large and rich in yolk, and conse- 
quently but rarely undergo total segmentation. Instead there is a super- 
ficial segmentation (fig. 106), in which the surface of the egg divides into 
the cells which form the blastoderm; the central yolk long remains un- 
divided a condition of systematic interest since it is not known to occur 
outside the Arthropoda. The cases of discoidal and unequal segmenta- 
tion are apparently derived from the superficial. 

In accordance with the high organization, reproduction by fission or 
budding never occurs, but parthenogenesis and pa?dogenesis do. In 
some parthenogenesis has a certain relationship to the life history. In 
lower Crustacea and in Aphides (plant lice) it allows the species to spread 


I. CRUSTACEA 359 

rapidly in large numbers over suitable feeding grounds. Among the bees 
parthenogenesis has a relation to the sexes, since males are only produced 
from unfertilized eggs. Along with parthenogenesis there may be 
rare exceptions sexual reproduction occurs, so that asexual alternates 
with sexual generation (heterogony), though not in such a typical manner 
as in the worms. 

Latreille divided the Arthropods into four classes: Crustacea, Myriapoda, 
Arachnida, and Insecta. Later the discovery that Peripatus possesses trachea; 
led to the creation of a new class, Protracheata, and the grouping of all arthro- 
pods into Branchiata and Tracheata, the branchiates including the Crustacea 
alone. Later researches have shown that these divisions are not natural and 
that tracheae have had different origins, the spiders being nearer to the Crustacea 
than to the insects, and that Crustacea and insecta have come from the annelids 
through different lines. 

Class I. Crustacea. 

The Crustacea are so called because their chitinous cuticle is usually 
rendered hard and firm by deposits of carbonate and phosphate of lime 
and, in contrast to that of other arthropods, has become 'crusty.' A 
carapace, recalling the mantle of the molluscs, is widely distributed in the 
Crustacea. It arises as a fold from the head, which may extend back- 
wards as a shield, completely covering some or all of the thoracic seg- 
ments (fig. 376), or it extends right and left on the sides of the body (fig. 
389) and produces two valves strikingly like those of a lamellibranch, the 
resemblance being strengthened in the cirripeds and ostracodes by the 
extensive calcification. Another important characteristic is the habitat 
of the group; the Crustacea are typically aquatic and hence breathe by 
means of gills. This branchial respiration persists, as in the case of 
crayfish, when the animals are taken from the water, for they retain water 
in the gill chamber and hence for a long time the gills are wet by this 
fluid. There are but few exceptions to this rule, as some land crabs and 
the sow bugs; these breathe air, either by means of the gills or by special 
structures in the gill chamber to be mentioned later. 

The branchiae or gills occur where a rapid change of water is possible. 
The appendages afford such a position, and hence one finds the gills as 
thin-skinned vascular plumes or plates (figs. 62, 398) either on the appen- 
dages or on the body near by; or the whole appendage may take a leaf- 
like, thin-skinned shape and thus serve as a gill (fig. 375). Again, the 
whole body surface may be respiratory and in small forms may entirely 
replace the gills, so that these organs become rudimentary or entirely 
disappear, there being a diffuse respiration with corresponding effects in 
the circulatory system. With a localized respiration heart, arteries, 


360 


ARTHROPODA 


capillaries, and veins are well developed, but with the diffuse respiration 
only the heart persists as a reduced structure, or with its disappearance 
the last traces of a circulatory system are lost. 

Locomotion is also related to the aquatic life and these animals usually 
possess appendages of the biramous or schizopodal type, which at once 


FIG. 374. Copepod appendages. I IV, Diaptomus castor; I, a pair of schizopodal 
feet; II, second right antenna: 777, right mandible; IV, right maxilla; V, right mandible 
of Cyclops coronatus. i, 2, joints of basiopodite; /, endopodite; a, exopodite. 

differentiates these forms from all other arthropods. While in the latter, 
as every insect shows, the joints of the limb are in a single row, the crus- 
tacean appendage has a two-jointed base (basiopodite}, followed by two 
many-jointed branches (fig. 374, 7), an fnner or endopodite and an 
outer or exopodite. 


FIG. 375-- 


-7 and 77 first and sixth legs of Bronchi pus grubei (after Gerstacker). 
a, flabellum; b, basis; /, axon and its endites; k, gills. 


The schizopodal appendage occurs only when the limb is used for swimming; 
when it is used for walking, as in crayfish and crabs, the exopodite is lost and 
only the endopodite persists as the functional limb, which then closely resembles 
the appendages in the insects. This loss rarely occurs on all the appendages; 


I. CRUSTACEA 


36] 


usually the abdominal feet and the mouth-parts retain the two-branched condi- 
tion. Embryology further shows that even in the crabs all the feet are at first 
schizopodal and that the walking legs lose the exopoditc during growth. There 
is evidence to show that the schizopodal foot is not the primitive type. This 
is furnished by the phyllopod foot (fig. 375, //), which consists of a median axis, 
b, bearing on the inside six cnditcs, i, and on the outer side two exiles, a 
flaliclln-m or epipodite, a, and a gill, k. This furnishes the schizopodal form by a 
loss of the four basal endites (those nearest i) and the development of the two 
terminal endites into exopodite and endopodite. Still the schizopodal condition 
is so nearly universal among Crustacea that it must be accorded great weight in 
classification. 

The appendages furnish a further diagnostic character in that two 
pairs of antennae are present in the Crustacea (see, however, Trilobitze). 
Antenna?, it will be remembered, are preoral appendages innervated from 
the brain. In some cases, as many Entomostraca, the second pair may 
lose their sensory functions and become mere swimming organs. 


llr 


FIG. 376. 


FIG. 377. 

FlG. 376. A pus equal-is* (after Packard). 
FIG. 377. Antennal gland of Mysis (after Grobben). 
external opening; h, bladder; re, canal; s, internal vesicle. 

FIG. 378. Otocyst of crayfish, as, crista acustica; n, nerve. 


FIG. 378. 
blr, blood lacuna-; ea, 


Concerning the internal organs but few general remarks can be made. 
Salivary glands are wholly absent; on the other hand, the stomodeum is 
usually widened into a strong chewing 'stomach,' and behind this empty 
the ducts of the so-called liver. This varies between the two simple blind 
sacs of the Daphnidse (fig. 383) and the enormous livers of the Decapoda 
(fig. 400, .1). It is not exclusively glandular (hepato-pancreas), but is an 
expansion of the digestive tract, and like it partakes in the resorption of 
nourishment. Excretory organs are represented by so-called green glands 


362 ARTHROPODA 

(antennal glands) and sJicll glands. The latter (so called from the erroneous 
idea that they produced the shell) open to the exterior on either side at the 
bases of the maxillae (fig. 383, s). The green gland opens similarly on the 
bases of the second antenrue. Both have essentially the same structure 
(fig. 377) ; they begin with a terminal vesicle, which passes into a slender, 
greatly coiled tube. Both occur together only in the larva?; in the adult 
one or the other is suppressed. In some amphipods there are excretory 
diverticula developed from the intestine (fig. 409), which resemble the 
Malpighian tubes of insects, but differ from them in being of entodermal 
origin. In some decapods caeca occur in the same region, but nothing is 
known of their function. 

Besides compound eyes there may be a 'nauplius eye' situated on the 
brain and consisting of an X-shaped pigment mass in which are placed 
three lens-formed groups of visual cells, connected with nerves. These 
are distinct from ocelli, but recall the eyes of Platodes. Both kinds of 
eyes may coexist in the same species. 

Auditory (equilibration) organs (otocysts) occur only in the Malacos- 
traca, either in the base of the first antennae or (Schizopoda) in the en- 
dopodite of the last abdominal feet (fig. 396, o) with a large statolith 
of calcium fluoride. The antennal ears of Decapoda (fig. 378) are sacs, 
the opening to the exterior protected by strong hairs, and each internally 
with a row of chitinous sense hairs, the crista aciistica, connected below 
with an auditory nerve, while their free ends extend between a cluster of 
statoliths. 

At ecdysis these otocysts with their sensory hairs and statoliths are cast off. 
If a crayfish which has just molted be placed in perfectly clean water, the otocyst 
will remain without statoliths; but if some easily recognizable substance, like 
uric acid crystals, be placed in the water, some of these will soon be found in 
the sac, thus proving that the statoliths are introduced from the outside. 
Experiment shows that these organs are statocysts, but apparently they are 
also auditory, since some forms which have well developed otocysts have no 
statoliths. 

Crustacea are only exceptionally hermaphroditic. The spermatozoa are 
noticeable for their great size, in many ostracodes equalling the body in length 
(Pontocypris paradoxa, 8 times as long as the body 5-7 mm.). Except in the 
Cirripedia the spermatozoa lack a flagellum and (Ostracods excepted) are 
immobile. Their round or elongate body is covered with rigid processes (fig. 
37, III, IV). They are frequently enclosed in spermatophores (fig. 385). 

The typical development of a crustacean includes a metamorphosis, 
and where direct development occurs the metamorphosis is either sup- 
pressed or the corresponding stages are passed in the egg. Two of the 
larval stages are especially important, the nauplius and the zoea. The 
nauplius (figs. 7, 393) consists of three segments covered by a dorsal 


I. CRUSTACEA 


363 


shield and bearing three pairs of appendages. The first pair, developing 
later to the first antenna?, are simple; the others, corresponding to 
the second antennae and mandibles, are schizopodal. Internally 
there is a three-regional alimentary tract, a supraoesophageal 
ganglion on which is an unpaired eye, and a ventral chain. The nauplius 
is almost universal among the lower Crustacea, and some believe that it 
represents an ancestral form from which the crusta- 
cea have descended, a view open to much objection. 

The zoea consists (fig. 379) of cephalothorax and 
abdomen, the latter without appendages, the former 
with several pairs of schizopodal swimming feet. 
There are two large compound eyes and, dorsal to 
the intestine, a heart. Frequently the carapace is 
armed with very long 
spines projecting from 
front, back, and sides, 
which are intended as pro- 
tection from enemies. 

Both nauplius and zoea 
rarely appear in the life 
cycle of one individual. 
The nauplius is charac- 
teristic of the lower crusta- 
cea the 'Entomostraca,' 
and appears in only a few 
Malacostraca, like . the 
schizopods and Peneus, 
and there precedes the 
zoea stage. The zoea, on 
the other hand, has never 
been noticed in the Ento- 
mostraca, but occurs in many Malacostraca. It must 
not be forgotten that many forms among both Entomostraca and 
Malacostraca have no zoeal or nauplius stage. Frequently the lower 
Crustacea are united under the name Entomostraca, but, aside from the 
nauplius stage and the possession of a shell gland, the only characters 
of the group are negative. 

* 

Sub Class 1. Trilobita:. 

The most important crustacean fossils are the Trilobites which 
appeared in the Cambrian and died out in the Permian, being extremely 


FIG. 379. Zoea of Carcinus 
mcrnas (after Faxon), h, heart; 
/, intestine, I-VII, cephalic ap- 
pendages. 


364 


ARTHROPODA 


abundant in the Silurian. The body (fig. 380) consists of head and trunk, 
the latter segmented. In the young the segments are very few, but increase 
in number (10-29, according to the species) with age. The hinder seg- 
ments frequently differ from the rest and form an abdomen or pygidium. 
Dorsally the animal is divided by two grooves into three lobes, marking 
off in the head a glabella and two gcncr; in the trunk rhachis and two 
pleurcc. On the head there are usually a pair of compound eyes, which 
in the young were frequently ventral, but are brought to the dorsal surface 
with growth. For years little was known of the ventral side, but lately 
specimens of Triartlims bccki (fig. 381) from the Utica slate have revealed 


FIG. 380. FIG. 381. 

FIG. 380. Paradoxides bohemicus (from Zittel). 

FIG. 381. Triarthnis becki, ventral surface, restored (after Beecher). The head 
bears one pair of antennae and four pairs of biramous feet, the basal joints serving as 
maxillae. Trunk with biramous feet. 

* 

the appendages. On the head are a pair of simple antennae, and four 
pairs of schizopodal feet, the bases of which acted as jaws. It is a 
question whether the first pair of jaw feet correspond to the second 
antennae or whether these have been lost in the group. The trunk 
segments bear biramous feet. In some respects the trilobites resemble 
the Xiphosura (infra), but the possession of antenna and biramous feet 
place them among the Crustacea. Here their position is very uncertain. 
We have little knowledge of but one species, and this with its single 
pair of antennas differs from all recent Crustacea. 


I. CRUSTACEA: PHYLLOP'i>\ 

Sub Class II. Phyllopoda. 

The Phyllopoda are the most primitive Crustacea. The name is 
derived from the leaf -like feet (fig. 375), which occur upon the thoracic 
region. The anterior appendages are schizopodal, the second pair of 
antennae often being efficient swimming organs. The number of somites 
varies between wide limits, there being less than a dozen in the Cladocera, 
while, if Savigny's law (p. 352) hold true, there are over sixty in seme 
Apodidae. Most forms (the Branchipodidae excepted) have a carapace. 
This forms a broad oval shell covering most of the body in the Apodidae 
(fig. 376); in the Estheriidae and Cladocera it is divided into right and 
left halves hinged together in the mid-dorsal line, thus giving these 
animals the appearance of bivalve molluscs. 

These forms have, besides the nauplius eye, a pair of compound eyes 
which in the compressed forms are frequently fused, although distinct 
in the young and retaining the double optic nerve throughout life. The 
liver is present in the shape of simple caeca; the heart, elongate, chambered, 
and with many ostia in the Branchiopoda, a short sac with only a pair 
of ostia in the Cladocera (fig. 383, //), lies dorsal to the intestine. The 
shell gland is well developed. 

In development summer and winter eggs are distinguished. The summer 
eggs form a single polar globule and develop parthenogenetically. The winter 
eggs form two polar globules and require fertilization. The thin-shelled summer 
eggs are carried by the mother in a brood pouch and soon hatch. The thick- 


FIG. 382. Branchipus vernal is,* fairy shrimp (after Packard). 

shelled winter eggs fall to the bottom, where they require a long time for develop- 
ment. They may be dried or frozen without injury, and in some cases drying is 
necessary for development. This explains the appearance in early spring oi 
large numbers of Branchipus and Esther ia in pools which are dry in summer. 
The phyllopods are largely inhabitants of fresh water. The winter ri^s ; 
serve the species through times of drought and cold; the summer eggs are for the 
rapid increase of the species during the wet season. The same relations also 
explain the fact that males are rare and only appear at intervals, indeed are not 
known in some species. 


366 ARTHROPODA 

Order I. Branchiopoda. 

The Branchiopoda are relatively large with numerous segments, leaf-like 
appendages, long, chambered heart, and lack swimming antennae. With few 
exceptions they are inhabitants of fresh water. According to the development 
of the carapace they are subdivided into three families. 

i. APODID.. Body depressed, with large oval undivided carapace. Eggs 
carried in brood capsules formed by a pair of appendages. A pus* (fig. 376), 
Lcpidiirns* Protocaris of the Cambrian is apparently an Apodid. 2. BRAN- 
CHIPID.E. Body without carapace, the second antennae of the male large and 
modified for clasping the female. The female carries the summer eggs in a 
wide 'uterus' in the abdomen. Branchipus* (fig. 382), fresh water; Artemia* 
in brine; one has been transformed into the other by changing the water from 
fresh to salt or the reverse. 3. ESTHERIID/E. Body laterally compressed and 
enclosed in a bivalve shell, compound eyes fused; males very rare. Estheria* 
Litnnadia,* fresh water. 

Order II. Cladocera. 

Like the estheriids the small Cladocera have the body enclosed in a bivalve 
carapace, which in some is small and reaches back only over the first trunk seg- 
ments, in others is large, enclosing the body, with a notch for the protrusion of 
the head, while behind it terminates in a sharp spine. The head bears a pair of 
large swimming antennae and a much smaller first pair bearing olfactory bristles 
and, in the male, hooks for clasping the female. The body consists of few seg- 
ments, the heart is a simple sac, and the fused faceted eyes are capable of motion 
in a special optic capsule. The young eggs in the sexual organs always occur 
in groups of four (fig. 383). Of these but one grows into an egg, the others 
serving this as nourishment. Larger eggs with more yolk occur when several 
groups fuse to form a single egg. The summer eggs arise from a single group, 
the winter eggs from several groups of primordial ova. The space between the 
back of the animal and the shell serves as a brood pouch. The larger winter 
eggs one or two in number frequently remain for a while in the brood 
chamber and are there enveloped in a peculiar shell, the ephippium, consisting 
of two chitinous plates, like watch crystals, their edges closely appressed. 

DAPHNHXE. Shell well developed; Daphnia* (fig. 383), Bosmina* POLY- 
PHEMID.*:. Shell small, only functioning as a brood case; head with an enor- 
mous eye and large swimming antenna; no phyllopodous feet; marine and lacus- 
trine. Leptodora hyalina* appears at night, sometimes in great numbers, in 
some of our lakes. Evadne* (fig. 384), marine. 

Sub Class III. Copepoda. 

A general description of the copepods can only apply to the non- 
parasitic forms, since many of the parasites are so degenerate (figs. 6, 388) 
as to be recognized even as arthropods only by a knowledge of the develop- 
ment. The sixteen somites of the body are nearly equally divided among 
the three regions head (6), thorax (5), and abdomen (5) of the animal. 
(In Cyclops the first thoracic segment is fused with the head, the first two 
abdominal segments are fused fig. 7.) The last abdominal segment 
is two-forked, forming ihefurca. While the abdomen lacks appendages, 
the thorax bears typical biramous appendages, consisting of a two-jointed 


I. CRUSTACEA: COPEPODA 


307 


FIG. 383. Daphnia pulex. b, brood chambers with embryos; g, brain with nauplius 
eye; go, optic ganglion; h, heart; o, ovary; s, shell gland. The eggs arise at k, and 
separate, forming in groups of four, as at e, of which one becomes the egg, while the 
others abort (o) and form food. The egg then passes to the brood chamber, i, 2, 
first and second antennae; 3, mandible (maxilla rudimentary and invisible); 5-9, legs. 
Alimentary canal cross-lined. 

basiopodite, the basiopodites of a pair being frequently united for com- 
mon motion (fig. 374, /). Exopodite and endopodite, usually three-jointed, 
are fringed with bristles. Usually the fifth pair of thoracic appendages 


368 


ARTHROPODA 


is not so well developed, and in some cases is represented by two 
bunches of bristles. 


FIG. 384. Evadne (orig.), showing the brood pouch filled with eggs and young. 
a 2 , second antenna; ao, adhesive organ; b, brain;/, furca; h, heart; i, intestine; I, liver; 
s, shell gland. 


FIG. 385. Dioptonnts castor, b, ventral nerve cord; g, brain with nauplius eye; /;, 
heart, beneath it the ovary and digestive tract; sf>, spermatophores ; i, 2, first and 
second antenna-; 3, mandibles; 4, maxilla;; 5, maxilliped; 6-10, swimming feet. 

The first pair of antennae in the males may be hooked near the base for 
clasping; the second are sometimes biramous (fig. 374, //). The mandible 
(fig. 374, 777, V) is instructive, since a study of several species shows that it is 
derived from a schizopodal condition and that the first basal joint alone is used 
for chewing, the rest being reduced to a palpus of varying development. Both 


I. CRUSTACEA: COPEPODA 369 

basal joints of the maxillae (fig. 374, 71') can be used in eating. Two maxillipeds 
(formerly regarded as the separated branches of an appendage) mark the 
termination of the head (fig. 385, 5). 

The internal anatomy is simple. There is no liver, and the straight 
intestine (fig. 385) runs without marked changes in size to the anus 
between the branches of the furca. The visual organ is the unpaired 
nauplius eye (which has given the name to one genus, Cyclops). It lies 
directly on the brain. The ventral chain has its ganglia irregularly dis- 
tributed. Gills are always absent, as are usually the heart and blood- 
vessels. The gonads are unpaired, but the sexual ducts, which open at 
the base of the abdomen, are paired. The females possess a receptaculum 
seminis distinct from the oviducts, to which the male attaches spermato- 
phores packed with sperm (fig. 385, sp). As the eggs leave the oviduct 
they are fertilized by the sperm issuing from the spermatophores, and 
are enclosed in a gelatinous substance, thus producing the so-called egg- 
sacs, attached to the abdomen, by which one can easily recognize the 
females (fig. 7). A nauplius hatches from the egg, and by budding seg- 
ments and appendages at the hinder end, and by a change of the nauplius 
appendages into antennas and mandibles, passes through a ' cyclops-stage ' 
into the adult. The Copepoda have descended from some phyllopod- 
like form. The poorly developed ventral chain, the loss, partial or com- 
plete, of a circulatory system, and the absence of gills are all against the 
view which would consider them primitive. 

Order I. Eucopepoda. 

The forms to which the foregoing description will apply are the Eucopepoda, 
and include many species, which often occur in enormous numbers in both fresh 
and salt water, forming the larger proportion of the plankton. They thus 
furnish the most important food supply not only for fishes but for the giant 
baleen whales. Celochilus septentrionalis occurs at times in such myriads that 
the sea for long distances is colored red. 

The CvcxopiDyE, no heart and paired egg sacs, fresh-water; Cyclops* (fig. 
7). CALANID/E, fresh water and marine; heart present, single egg-sac. Diap- 
tomus* fresh water (fig. 385); Cetochilus* Pontilla* marine. HARPACTID^E, 
creeping forms, mostly marine; Canthocamptus* fresh water. The CORYC.^ID.E, 
half parasitic, include the wonderfully iridescent Sappliirhia* and the XOTODEL- 
PHID.E, parasitic in ascidians, form a transition to the next order. 

Order II. Siphonostomata (Parasita). 

There are aiso Copepoda to which the account in large type will not apply, 
animals of such strange appearance that many of them were long regarded as 
parasitic worms (figs. 6, 386, 388). Their mandibles are altered to piercing 
bristles and enclosed in a piercing proboscis formed of upper and lower lips. 
With this sucking organ they bore into the skin or gills of fishes. They have 
cylindrical forms or bodies of the most bizarre shapes, in which frequently no 
24 


370 


ARTHROPODA 


segmentation is visible, while the appendages are rudimentary or even entirely 
lost. Indeed one would not recognize them as arthropods save for the following 
features: 

(i) Most of them have the typical Copepod egg-sacs attached to the hinder 
end. (2) A complete series of intermediate forms allows one to trace, step by 
step, the alterations of form from the free-living species to the most modified 
parasites. (3) Ontogeny is convincing. Most parasitic Copepoda leave the 
egg as a nauplius and pass through a Cyclops-stage before attaching themselves 


a. 


FIG. 386. FIG. 387. FIG. 388. 

FIG. 386. Female Lernccocera esocina (from Lang, after Claus). A, armlike 
processes of anterior end; d, digestive tract; es, egg-sacs; od, oviduct; / r / 4 , rudimentary 
thoracic appendages. 

FlG. 387. Argulus foliaceus (from Ludwig-Leunis). a, sting; a', antenna; b, 
mouth; c, intestine with liver; d, abdomen; pm 1 , pnr, first and second maxillipeds; 
p l -p*, biramous feet of thorax. 

FIG. 388. Lerncea branchialis* (orig.). 

to fishes and becoming the highly degenerate parasites. These parasites are 
always females. The males scarcely pass the Cyclops-stage, copulate with the 
females and then die, or if they pass through the metamorphosis, they remain 
small and different in appearance (fig. 8). They occur attached to the female 
near the genital openings. There is thus here a marked sexual dimorphism. 

ARGULID^; (sometimes made a distinct order, Branchiura), fresh- water forms 
with compound eyes, liver lobes, and second maxillipeds metamorphosed into 
suckers. A r gains* (fig. 387). CALIGID.E (C aligns*), marine and brackish- 
water. LERN^OPODID^E. Fish parasites with maxillae united into an adhesive 
organ. Achthercs* (fig. 6), perch. LERN^HXE; worm-like parasites. Lerna'a* (fig. 
388) ; Lenieeocera* (fig. 386) ; Pcnclla* 


I. CRUSTACEA: CIRRIPEDIA 371 

Sub Class IV. Ostracoda. 

Like the Cladocera and the Estheriidae the Ostracoda are enclosed 
in a bivalve shell, which, when closed, includes not only the body but the 
head and appendages as well, these being protruded when the shell is 
opened. The valves are closed by an adductor muscle, opened by a 
hinge ligament like that of lamellibranchs. This resemblance to the 
molluscs is heightened by lines of growth upon the shell. The antennae, 


FIG. 389. Cyprisfasciatiis, adult female (after Claus). I-IV, appendages; c, furca; 
e, eye; /, liver; m, adductor muscle of shell; o, ovary; 5, shell gland. 

the first simple, the second frequently two-branched, are used for swim- 
ming and creeping. The mandibles, maxillae, and three pairs of legs 
vary greatly from genus to genus. The internal structure is also variable. 
The Ostracoda are bottom forms and live in fresh and brackish water 
as well as in the sea. 


First two pairs of legs maxillary in character, the last de- 
veloped into a hook for cleansing the shell; heart present; marine; Cypridimi.* 
CYPRIDID.-K. First pair of legs maxillary in character; heart lacking; fresh 
water. Cypris,* Candona.* 

Sub Class V. Cirri pedia. 

The barnacles differ from all other Crustacea in that they have lost 
their locomotor powers and live attached to rocks, floating timber, and 
the like. In some cases they attach themselves to other animals, as crabs 
and molluscs, or, as in the case of Coronula and Tubiclnclla, to whales. 
This leads in Anelasma and the Rhizocephala to a true parasitism, the 
barnacle not only attaching itself to an animal but sucking its juices as 
food. 

The attachment is by the dorsal surface in the neighborhood of the head , 
and is initiated by the first antennae, in which is a cement gland secreting 
a rapidly hardening cement. The Hat region of fixation in the Balanidae 


372 


ARTHROPODA 


(fig. 390) is drawn out in the Lepadida? into a long muscular stalk (fig. 115). 
To this attached life are related all the peculiarities of structure. A fixed 
animal has greater need of protection than one which can flee from its 
enemies, therefore we find right and left mantle folds capable of com- 
plete closure, like those of an ostracode, each with two calcified plates 
the scuta and terga (figs. 115, 390, fy 
s, /), the first cephalic, the other pos- 
terior, in position. Between the 
pairs of these is the gap through cr 
which the feet are protruded. 

Besides there are other calcified por- 
tions, one of which, the carina (r), 
corresponds to the hinge-line of the 
ostracode and in some Lepads is sup- 
plemented by a farther unpaired piece, 


a -. 


t . 


.- p 


- c 


FIG. 390. FIG. 391. 

FIG. 390. Balanits hameri* acorn barnacle (from Lang, after Darwin). Formed of 
rostrum, lateralia, and carina, the operculum of scuta (s) and terga (/). 

FIG. 391. Anatomy of Lepas (after Claus). a, adductor muscle; c, carina; 
cr, cirri (feet); g, cement gland; I, liver; o, o', ovary and oviduct; p, penis; t, testis; 
tr, tergum; v, vas deferens. 

the rostrum. In the Balanidae the rostrum and carina are much stronger, 
while between them other paired pieces, the lateralia, are intercalated. Later- 
alia, rostrum, and carina arise from a base (usually calcareous) and form a 
capsule, closed above by a double valve formed of the paired scuta and terga, 
between which, when open, the animal can be seen (fig. 390). 

The body in both lepads and balanids has essentially the same struc- 
ture. It is flexed ventrally, so that mouth and vent are near each other, 
and bears six pairs of feathered feet, or cirri, which, when extended, 
become widely separated and form a most efficient means of straining 
small organisms from the water and conveying them to the mouth. These 
feet are biramous, with their branches ringed and thickly haired. Behind 
them is a rudimentary abdomen and an elongate penis; while the mouth 
is surrounded by a pair of mandibles and two pairs of maxillae. 

In internal structure the most noticeable feature is that the animals 


I. CRUSTACEA: CIRRIPEDIA 


373 


with few exceptions, in contrast to most other arthropods, are hermaphro- 
ditic, a condition possibly correlated with their sedentary life and the con- 
sequent need of self-impregnation. The testes lie in the sides of the body; 
the ovaries in the Lepadids are in the stalk, in the Balanids in the basal 
plate. In cases of several solitary hemaphrodite species complementary 
dwarf males occur. These are very small, purely male forms, with ex- 
tremely simple structure (fig. 392), which live inside the mantle cavity near 
the genital openings. The unsegmented body is enclosed in a sac (a 


B 


FIG. 392. FIG. 393. 

FIG. 392. Male of ALcippe lampas. an, antenna; /, mantle lobes, m, muscles; oc, 
ocellus; p, penis; t, testis; vs, seminal vesicle. 

FIG. 393. Nauplius (.4) and Cypris (B) stages of Sacculina carcini. (after Delage). 
i, 2, antennae; 3, mandible; /, cirrous feet; m, muscles; oc, nauplius eye; ov, anlage 
of ovary. 

soft-skinned shell), and anchored by the antenna?. The long penis pro- 
trudes from the mantle. In the genus Scalpellum there are purely her- 
maphroditic species, hermaphroditic species with complemental males, 
and purely dioecious species. 

Since the hard shells of the barnacles resemble those of the molluscs, it is 
not to be wondered that these forms were long regarded as belonging to that 
group. It was not until the development (fig. 393) was studied that the error 
was corrected. A large nauplius comes from the egg and later is metamorphosed 
into a second larval stage with bivalve shell which, from its appearance, is 
called the cypris-stage. This becomes fixed and develops into the adult, losing 
the compound eyes and retaining the nauplius eye. 

Order I. Lepadidae. 

Stalked cirripeds, with shell largely formed of scuta, terga, and carina; 
other parts may be added. Lepas anatifcra* (fig. 115), the goose barnacle, o\\rs 
its common name to a mediaeval myth which claimed that the Irish (or bernicle) 
goose developed from these animals. Anelasma squalicola, thin-skinned, 
parasitic on sharks, forms a transition to the Rhizocephala. 


374 ARTHROPODA 

Order II. Balanidae. 

Sessile cirripeds with calcareous shell formed of carina, rostrum, and la'er- 
alia; scuta and terga forming the valves (fig. 390). Balanus* Coronula, attached 
to whales. 

Order III. Rhizocephala. 

These differ greatly from other cirripeds. They are parasitic on the abdo- 
mens of decapod crabs and consist of a stalk which penetrates the body of the 
host and a body which remains outside (fig. 394). The stalk branches in a 
root-like manner, penetrates the cephalothorax and absorbs its juices. Since 
the stalk furnishes the food, an alimentary canal is absent. The body lacks 
all appendages, is enclosed by a soft-skinned mantle, and is almost entirely 


"" "d 

FIG. 394. Sacculina carcini attached to Carcinus mcenas, whose abdomen (d~) is ex- 
tended. /, sex opening of Sacculina; r, network of roots ramifying the crab; 5, stalk. 

filled with the gonads. Since the adult parasites lack all arthropodan features, 
their position is only settled by their development (fig. 393), which is like that 
of other cirripeds. These forms are rare on the American coast. Sacculina, 
Peltogaster* 

Two more orders, ABDOMINALIA and APODA, parasitic in the mantle 
and shells of molluscs and other cirripeds, scarcely need mention. 

Sub Class V. Malacostraca. 

The Malacostraca are sharply marked off from the other Crustacea by 
having a body which consists of twenty segments, of which seven are 
abdominal (Nebalia has twenty-one, eight abdominal). The excretory 
organs are represented by the antennal glands, and shell glands are lacking 
except in the larvae and some Isopoda. The male genital ducts open on 
the thirteenth, the female on the eleventh, segment. 


I. CRUSTACEA: SCHIZOPODA 


Legion I. Leptoslraca. 


'.'> , ."> 


The Leptostraca connect the Phyllopoda with the higher groups. Their 
twenty-one somites (eight abdominal, eight thoracic, and five cephalic), and the 
openings of the genital ducts ally them to the Malacostraca. On the other 
hand, the bivalve carapace covering the cephalothorax and part of the abdomen, 
and the leaf-like thoracic feet, are phyllopoclan. They have an antennal gland 
and a rudimentary shell gland; an elongate heart which extends through 
cephalothorax and abdomen; and stalked compound eyes. The few species 
are all marine and belong to the genus Nebalia* (fig. 395). 


FIG. 395. Nebalia bipes* (after Sars). /;, heart; 7, intestine; o, ovary: a, adductor 

of carapace; b, brain; r, rostrum. 

Legion II. Thoracostraca (Podophthalmia) . 

The names given this division have reference, first, to the fact that the 
head and some of the thoracic segments are immovably united and covered 
by a firm carapace; second, that the compound eyes (except in Cumacea) 
are placed on movable eye stalks. The first five appendages are always 
two pairs of antennae, a pair of mandibles, and two pairs of maxilla 1 . 
The remaining pairs vary greatly and from one to three may be modified 
into maxillipeds, while the abdominal somites except the last (telson) 
usually bear appendages, at least in the female. There is usually a 
metamorphosis in which a nauplius stage may appear, most frequently 
in the lower forms (schizopods), but even in the decapods (Peneus). 

Order I. Schizopoda. 

These are small forms (fig. 396), mostly marine, in which the cephalothorax 
is covered by a carapace with which some or all of the thoracic somites are 
firmly united. The eight thoracic feet are biramous throughout life and are 
used in swimming. The posterior pair of abdominal feet together with the 
telson form a caudal 'fin' by means of which the animal can swim backwards. 
The delicate skin permits of diffuse respiration, and gills are frequently lacking. 
In some genera plates from the legs of the female enclose a brood case beneath 
the cephalothorax, thus giving these forms the common name of opossum 
shrimps. 


376 


ARTHROPODA 


The MYSIDID/E are widely distributed, several species of Mysis (fig. 396) 
occurring on our coasts and one in the Great Lakes. In these the endopodite 
of the sixth abdominal appendage contains a otocyst, with a calcic fluoride 
statolith. Other families are the EUPHAUSIID.E and LOPHOGASTRID^; of the 


high seas. 


FIG. 396. Mysis elongata (from Gerstacker). , J3, first and second antennae; a, 
expedite; au, eye; z, endopodite; o, otocyst; 1-7, abdominal somites. 

Order II. Stomatopoda. 

In structure of the cephalothorax these forms, known as mantis shrimps 
(from a resemblance to the insect, the praying mantis), are lower than the 
schizopods, since the last three or four thoracic somites are free and are not 
covered by the carapace. The appendages, however, are more developed, 
since only the three posterior of the thoracic feet are biramous and natatory. 
The four in front of these are prehensile and bear a pincer formed of the last 
two joints, the last being slender and usually toothed and closing in a groove of 
the penult joint like a knife blade in the handle. The first of" these raptorial 


sac. 


FIG. 39j.Squilla mantis, at, at', first and second antenna?;/, sixth abdominal feet; 
k, gills; p, schizopodal thoracic feet; pr, pr', raptorial feet; />\, pleopoda; sa, 
telson. 

feet are the largest and are used in capturing fishes, etc. Since the thoracic feet 
are of little service for locomotion, the abdomen is long and stout, especially the 
caudal fin. The five anterior abdominal feet bear the gills, and correspondingly 
the elongate heart with many ostia extends into the abdomen. The transparent 
pelagic larvae were formerly regarded as adults and described as Alima and 
Enchthus. Squill a* Gonodactylus* They are burrowing animals and deposit 
their eggs in their holes. 


I. CRUSTACEA: DECAPODA :577 

Order III. Decapoda. 

The Decapoda is the most important group of Crustacea, since it 
contains the shrimps, lobsters, crayfish, and crabs. It agrees with the 
Schizopoda in having a cephalothorax composed of thirteen fused somites, 
but differs in the structure and function of the thoracic extremities. Only 
the last five pairs (whence the name Decapoda) are locomotor. These 
lose the exopodite during development (Peneidai excepted) and become 
strong walking legs, terminated either with claws or pincers (chela). 
Usually the first pair is distinguished from the others by its size and by 
being chelate, and is a grasping organ. In the development of a chela 
the penult joint sends out a strong process, the 'thumb,' which extends 
as far as the last joint (the 'finger'), which closes against it. 

The mouth parts a pair of mandibles, two pairs of maxillae, and three pairs 
of maxillipeds (fig. 369) lie in front of the first pair of legs. The maxillipeds 
(7, 6, 5) show a biramous condition, while the maxillae (4, 3) retain considerable 
of the phyllopod character. In the mandibles (2) there is always a strong basal 
joint, which serves as a jaw, while this may bear additional joints, the palpus. 
Behind the mouth are a pair of scales, the paragnaths or metasfoma, which are 
not appendages. The antennae are usually distinguished as antennae (second 
pair) and aiitcnmila (first pair, fig. 369). They have large basal portions, 
which in the antennulae bear two many-jointed flagella, while the antennae 
proper have but a single though usually much larger flagellum. On the basal 
joint of the antennulae is the otocyst (p. 362), while the green gland opens on 
the basal joint of the antennae (fig. 400, gd). 

When the abdomen is not rudimentary (as in the crabs) the appendages of 
the sixth abdominal segment together with the telson form a strong caudal fin 
(fig. 400) ; the other appendages (fig. 369, /) are small, biramous organs to 
which, in the female, the eggs are attached. In the female the first pair is 
reduced, but in the male (except in Palinuridae) this pair is well developed, 
curiously modified, and serves as a copulatory (intromittent) organ. The shape 
of these appendages and the openings of the genital ducts on the base of the 
third walking foot of the female, the fifth in the male serve at once to distin- 
guish the sexes. Frequently also the males have the larger pincers. 

The thickness of the integument prevents diffuse respiration and 
accounts for the numerous gills (fig. 398) which are attached to the bases 
of the maxillipeds and walking feet or to the sides of the body near them. 
(In the Thalassinidas the gills are on the abdominal appendages). These 
gills are not visible externally, for the carapace extends down on the 
sides of the body as a fold (branch iostcgitc) over them, thus enclosing them 
in a branchial chamber. A process of the second maxilla? the scaphog- 
natliite plays in this branchial chamber and pumps the water over the 
gills, the water flowing out near the mouth. All decapods can live some 
time out of water; they retain some water in the gill chamber, which keeps 
the gills in a moist condition. In some tropical crabs which live almost 
exclusively on land there is a true aerial respiration, the lining of the 


378 ARTHROPODA 

gill chamber being modified into a kind of lung traversed by numerous 
blood-vessels. In the palm crab (fig. 399) the gill chamber is divided 
into two portions, the upper part being pulmonary, the lower containing 
the reduced gills. 

Correlated to this localized respiration is the nearly closed circulatory 
system (fig. 400, A, B). The heart (//), a compact pentagonal organ, 
receives its blood from the pericardial sinus (pc) through three pairs of 
ostia, and forces it out through five arteries to the capillary regions of the 
body. The venous blood collects in a large venous sinus at the base 
of the gills (r), passes thence through gills, and is returned by several 
branchial veins (vbr) to the pericardium. 


FIG. 398. Gills of Astacus exposed by cutting away the branchiostegite. pdb, plb, 
podo- and pleurobranchia of the corresponding segments; r, rostrum; i, stalked eyes; 
2, 3, antennae; 4-6, mandibles and maxillae; 7-9, maxillipeds; 10, 14, bases of thoracic 
eet; 15, first pleopod. 

The alimentary canal is straight and has only one conspicuous enlarge- 
ment, the so-called stomach (fig. 400, A, m), divided into two portions, 
an anterior (cardiac) sac, lined with chitinous folds and teeth and serving 
to chew the food and bearing in its walls the 'crab-stones,' masses of 
calcic carbonate stored up to harden the armor rapidly after the molt. 
The second (pyloric) portion of the stomach is guarded by hairs and 
serves as a strainer, allowing only food sufficiently comminuted to pass. 
The two liver lobes voluminous masses of branched glandular tubes 
(/) open just behind the stomach. 

The two antennal glands (C, gd), each provided with a large urinary 
bladder (&/), are dirty green in color, whence the name green glands often 
given them. The gonads (fig. 401) lie close beneath the heart, those of 
the two sides being united behind, while their ducts remain separate. 
The structure of the nervous system is in part dependent upon that of the 
abdomen. In the Macrura (fig. 400, C) the ventral chain consists of 
six ganglia in the thorax, six in the abdomen, but in the Brachyura 


I. CRUSTACEA: DECAPODA 


379 


,cf, a. 


FIG. 399. Diagrammatic section through Birgus Intro, showing lungs (from 
Lang, after Semper), a,, a 4 , afferent blood-vessels; a/z, pulmonary chamber; e&, e/, el', 
efferent blood-vessels; h, heart; k, gills; M, branchiostegite; />. pericardium. 


FIG. 400. Anatomy of Crayfish (Antaeus']. A, dorsal surface removed; B, scheme 
of circulation; C, viscera removed, showing green gland and nervous system, a, anus; 
aa, hepatic artery; ae, antenna; ai, antennula, also sternal artery; am, muscles of 
stomach; ao, ophthalmic artery; ap, abdominal artery; av, ventral artery; bl, urinary 
bladder; br, gill arteries; c, cesophageal commissures; gd, green gland; gn', brain; 
gn 2 - 13 , ganglia of ventral chain; h, heart; hd, intestine; k, mandibular muscles; /, /', liver 
and its duct; m, stomach; o, otocyst; oes, cesophagus; on, oj>tic nerve; pc, pericardium; 
sgn, sympathetic nerve; t, t', unpaired and paired portions of testes; v, ventral blood 
sinus; vd, vas deferens; i'l>r, veins from gills to heart. 


380 


ARTHROPODA 


(fig. 402) these all flow together in a common mass, connected with the 
brain by two long oesophageal commissures. 

The development of most decapods is interesting from the number of larval 
forms. As a rule a zoea (fig. 379) is hatched from the egg; this passes next into 
a Mysis stage (fig. 403) in which head, thorax, and abdomen are distinct, the 
thorax bearing biramous feet like those of schizopods a proof of the origin of 
the simple feet from the biramous type. In the crabs (Brachyura) the Mysis 
stage is replaced by a Megalops (fig. 404), in which the abdomen is well de- 
veloped, but the feet have lost their biramous character. In some prawns^ 


FIG. 401. FIG. 402. 

FIG. 401. Reproductive organs of (.4) female and (B) male crayfish (from 
Huxley), od, oviduct; od', its opening on nth appendage; <n<, ovary; t, testes; vd, 
vas deferens; vd', its opening on 13 th appendage. 

FIG. 402. Nervous system of crab, Carcinus (from Gegenbaur). a, antennal 
nerves; c, cesophageal commissures; gi, fused ventral chain perforated for sternal 
artery; gs, brain; o, optic nerve. 

(Pencils) the series is rendered more complete by the appearance of a nauplius 
and a metanauplius with many appendages, before the zoeal stage. In the 
crayfish and many land crabs the metamorphosis has been lost, but the lobster 
leaves the egg in the Mysis stage. Differences may occur even in the same 
species; thus in the European Palcemonetes rarians the embryo, in the sea, leaves 
the egg as a zoea; in fresh water in the Mysis-stage. 

Sub Order I. MACRURA. Abdomen large; antennae long; ventral nerve 
chain elongate; no megalops in development. CARIDEA. Body compressed; 
no sutures on carapace; feet weak, external maxillipeds pediform; a large scale 
on second antennae. PENEID/E, weak exopodites. Peneus* Sicyonia* PAL- 
#:MONID/E, mandibles bifid at tip. Palcem-.m, Alpheus* Hippolyte* Pandalus* 
CRANGONID.E, mandible simple. Crangon* Sabinea* ASTACOIDEA. Car- 
apace crossed by a transverse groove. ASTACID^E have well-developed chelae. 
Cambants* includes the crayfish of the eastern states; those of the Pacific coast 
and Europe belong to Astacus* Ho mams,* lobsters. PALINTJRID.E (Loricata), 
no chelae, body with heavy armor; larvae leaf -like (fig. 403). Palinitrus* spiny 
lobster. PAGURIDEA, hermit crabs; abdomen reduced, soft-skinned, and 
hidden in a snail shell which the animal carries about, which has resulted in a 
spiral abdomen. Some hermits carry sea anemones or hydroids on their shell, 


I. CRUSTACEA: DECAPODA 


381 


cases of symbiosis (p. 158) Eiipagurus* Clibanarius* Allied is Birgtis, palm 
crab of the islands of the Indian Ocean; its respiratory organs referred to on 
p. 378. Sub Order II. BRACHYURA. Body depressed; abdomen rudi- 


sr 


FIG. 403. FIG. 404. 

FlG. 403. Phyllosoma larva (Mysis-stage) of Pali minis (after Gerslacker). 
A, abdomen; C, head; T, thorax; a and /, exopodites and endopodites of thoracic 
feet. 

FIG. 404. Megalops larva of Portunus (from Lang, after Claus). 2, anterrw; IV- 
VIII, thoracic appendages; a 2 - 6 , abdominal somites (a 6 is the seventh). 


FIG. 405. Pandulus montagui* 


FIG. 406. Eiipagurus bcmhtirilus, hermit 
crab (from Emerton). 


mentary and folded in a groove under the cephalothorax; antenna? short; never 
more than one pair of feet chelate; ventral nerve cord concentrated ((\^. 402). 
Some inconspicuous groups like the PORCELLANID.E, the HIPPID.E, and the 


382 


ARTHROPODA 


LITHODIDJF, are united as SCHIZOSOMI because the last thoracic segment is 
free from the carapace and its appendages are rudimentary. LEUCOSOIDEA 
(Oxystomata). Body oval or triangular, area of mouth parts triangular. 
Calappa, Hepatus* OXYRHYNCHA (Maioidea). Cephalothorax tri- 
angular, narrowed in front; mouth area (as in the following tribes) quadrilateral. 


FIG. 407. B, Libinia emarginata* spider crab (from Emerton). 

Mostly tropical. Hyas* Libinia* Pugctiia* spider crabs. CYCLOME- 
TOPA. Body broader than long, arcuate in front. CANCRID^E, with last pair of 
feet pointed. Cancer* shore crab; Panopeus,* mud crab. PORTUNID^E, with 
last pair of feet flattened paddles. Neplunus hastatus* when thin-skinned after 
molting, is 'soft-shell crab.' CATOMETOPA. Front of carapace nearly 
straight; body from above nearly quadrilateral; Gelasinms,* fiddler crabs, 
Pinnotheres ostreum* common in oysters; GECARCINID^E (Uca, etc.), land crabs 
of the tropics, which only go to the sea at the reproductive season to lay their 
eggs. 

Order IV. Cumacea. 

Small marine forms with sessile eyes, three or four free thoracic somites; 
appendages biramous; a brood sac beneath the cephalothorax. Of interest 
because combining arthrostracan and thoracostracan features. Diastylis.* 


FIG. 408. Diastylis quadrispinosus.* 

Order V. Syncarida. 

Especial interest also centres in Anaspides tasmanicr (and a few other forms) 

from lakes in Tasmania, which unite schizopod and amphipod characters. 

They have the stalked eyes, caudal fin, and biramous feet of a schizopod; 

otocysts in the antennulae like a decapod; but agree with the amphipods in shape 


CRUSTACEA: AMPIIIPODA 

of body and in free thoracic segments. The epipodial plates are paralleled 
elsewhere only in carboniferous species, with which these forms apparently are 
closely allied. 

Legion III. Arthrostraca (Edriopht/ialmala, Tetradecapoda). 

Although the Arthrostracan head consists of six segments, it is very short. 
It bears six pairs of appendages, one of the normal thoracic pair being 
added to it as maxillipeds. Eyes, when present, are clusters of ocelli on 
the sides of the head. There are seven thoracic segments, the appendages 
of which are walking feet without exopodites. The abdominal appendages, 
when present, are always biramous; the telson never bears appendages, 
and in the Amphipods is greatly reduced, sometimes being split nearly its 
whole length. The nervous system (figs. 78, 409) is of the ladder type. 
The alimentary canal is straight and has an anterior enlargement, the 
chewing stomach, behind which empty one or more pairs of long liver 


FIG. 409. Male Orchestia cavimana (after Nebeski). a', a-, antenna?; ao, aop, 
anterior and posterior aortae; c, heart; d, digestive tract; g, brain and eye: /;, testes; 
k, gills; kf, maxilliped; I, liver; m, excretory organ and mandible; n, ventral nerve cord; 
o, rudimentary ovary; vd, vas deferens; I-VII, thoracic feet; 1-3, anterior, 4-6, pos- 
terior abdominal feet. 

tubes, while in a few Amphipods a pair of excretory tubes (m), the so-called 
Malpighian tubules, empty into the intestine near its end. Respiratory 
and circulatory systems vary so that they are best described in connec- 
tion with two orders. 

Order I. Amphipoda. 

The Amphipods are almost exclusively aquatic, a few species living 
on the shore near high-tide mark. A few live in fresh water ((iaintnaru^,'-'- 
Allorchestes*), the majority being marine. On land thry move by a 


384 


ARTHROPODA 


leaping motion, whence the common name, beach fleas. In swimming 
the abdomen is alternately bent against the breast and then forcibly 
straightened. 

The body is usually strongly compressed from side to side. The 

anterior thoracic feet generally bear large 
epimeral plates (fig. 433), which extend 
the sides of the body downwards, while 
on the inner side delicate gills or gill sacs 
(fig. 410, br) arise from their bases. In 
the female brood lamella? (brl) are added 
which enclose a brood chamber beneath 
the body in which eggs or young are car- 
ried. The three anterior pairs of ab- 
dominal feet are two-branched, richly 
haired, and serve to create currents of 
water which pass over the gills. The 
remaining abdominal feet, through bira- 
mous, are shorter and stout and form 
springing organs. The position of the 
gills explains why the abdominal part of the heart is degenerate and only 
the anterior thoracic portion with three pairs of ostia persists. 

Sub Order I. HYPERINA. Large head and eyes; strong prehensile feet. 
Live attached to other pelagic animals on which they feed; Hyperia* Phronima* 
Sub Order II. GAMMARINA. Head much smaller; abdomen well developed; 
mostly free swimmers. Numerous species in the sea. Gammarus* in shallow 
water, some fluviatile; Orchestia* above tide marks. Chelura terebrans* 


FIG. 410. Cross-section of 
kmphipodCorophium, (from Lang, 
after Delage). bf, thoracic leg; 
<bm, ventral nerve cord; br, 
branchiae; brl, brood lamella; d, 
intestine; h, heart; /, liver; ov, eggs 
in brood chamber. 


FIG. 411. Gammarus ornatus* (from Smith) 

destroys submerged wood. Sub Order III. LffiMODIPODA. Parasitic or 
semi-parasitic; second thoracic somite is fused to head; appendages lacking 
from some thoracic segments, abdomen reduced. Capretta* on hydroids. 
Cyamus ceti, parasitic on whales. 


I. CRUSTACEA: ISOPODA 


385 


Order II. Isopoda. 

The Isopoda are readily distinguished from the Amphipoda by their 
depressed (horizontally flattened) bodies. The feet are adapted for 
creeping, and a brood pouch is formed as in the Amphipoda; gills are 
lacking on the thorax. In the abdomen, the somites of which exhibit a 


FIG. 412. -Asellus aquaticus (from Luchvig-Leunis). a 1 , a 2 , antennae; br, brood 
pouch; k, pleopoda modified to gills; md, mandibles; p'-p" 1 , thoracic feet; pa l -pa e , 
abdominal feet (pleopoda); I-VI, head; YII-XIII, thoracic segments; XIV-XX, 
abdominal segments partly fused. 

great tendency to fusion, the sixth somite bears, in the walking forms, long 
forked appendages (fig. 412); in the swimming species (414, C) they are 
flattened and, with the telson, make a swimming organ. The five anterior 
pairs of pleopoda are modified for respiration (Fig. 412, k), the endop- 


FIG. 413. Entomscus porcdlana> ("from Gerstackcr, after Muller). .1. male; 7?, 
female; C, heart; he, liver; la. brood lamella:; <;r, ovary. 

odites being thin-walled plates, while the exopodites and the whole first 
pair serve as opercula or gill covers. As a result of this position of the 
gills the heart (usually with two pairs of ostia) is abdominal in position. 

25 


386 


ARTHROPODA 


In the terrestrial species the gills are adapted for breathing damp air. In 
Porcellio and Annadillidiim the first or first and second opercula are permeated 
with a system of air tubes, which physiologically, though not morphologically, 
are comparable to the tracheae of insects. 

In the Isopoda the tendency to parasitism is greater than in the Amphipoda. 
Many swimming forms attach themselves to fishes and feed by boring with 
their modified mouth parts into the skin. The Bopyridae live in the branchial 
chamber of shrimps. Cr \ptoniscus is a shapeless sac which attaches itself to the 
stalk oiSacculina (p. 374), and, after causing the death of this parasite, uses its 
network of 'roots' for its own nourishment. The Entoniscidae (fig. 413) 
attack Dccapoda and, pressing the skin before them, penetrate the inte- 
rior. Their strange shape is largely due to the lobe-like brood lamellae. 
They are usually hermaphroditic, but have besides complemental dwarf males 
(fig. 413* A )- 


A 


FIG. 414. .1, Idotea irrorata*; B, Limnoria lignorum*; C, &ga psora* ('salve bug'); 

D, Lcptochela algicola * (after Harger). 


Sub Order I. ANISOPODA. Six free thoracic segments; heart thoracic; 
first thoracic foot (on head) chelate; abdomen with swimming feet; intermediate 
between Amphipoda and other Isopoda. Tanais* Leptochela* (fig. 414). Sub 
Order II. EUISOPODA. Seven free thoracic segments. OxisciDyE; terres- 
trial, 'sow bugs'; Porcellio* Oniscus* Armadittidum,* 'pill bug.' ASELLID^E 
(fig. 412), fresh water. SPH^ROMID^E, head broad, body rounded and convex; 
Sphceroma* Limnoria lignorum* (fig. 414), gribble, destructive to submerged 
wood. IDOTEID^;, free-living, marine, with usually elongate bodies; Idotea* 
Ccecidotea* BOPYRID/E, parasitic on Caridea; body of female disc-like, asym- 
metrical, without eyes; Bopyrus* CYMOTHOID^E, parasitic on fishes or in their 
mouths. Cymoilioa* Mga* Cirolana* Sub Order III. ENTONISCIDA, 
general features are described above. Entmiscus. 


II. ACERATA 


:i,s7 


Class II. Acerata. 

The animals comprising this group were formerly divided among the 
tracheates (p. 359) and the Crustacea, but although differing widely in 
respiration, the forms included are closely allied in structure and develop- 
ment and present many differences from both Crustacea and Insecta. 
The former views were based upon the view that trachea wherever found 
were homologous structures. 

In the Acerata the body is usually divided into cephalothorax and 
abdomen, though in some cases (mites) the two regions become fused. 

The cephalothorax consists of six somites which 
always bear appendages, arranged in a circle 
around the mouth, the basal joints of one or 
more pairs frequently serving as jaws. None 
of these appendages are like antenna: (whence 


a a 

FIG. 416. 

FIG. 415. Digestive tract of Ctenida c&mentaria (from Lang, after Duges). a, 
abdomen; an, anus; da, dt, diverticula ('liver') of midgut; g, brain; i<b, rectal bladder 
(stercoral pocket) ; vm. excretory tubules. 

FIG. 416. Schematic long section of lung of spider (after Mac Cleod). a, air space; 
c, chitin layer; II, lung leaves; p, posterior wall of lung sac. 

the name of the group). The abdomen consists of a varying number of 
somites, all of which may be free, or may be fused. These abdominal 
somites bear appendages in the embryo, but in the adults (except the 
Xiphosura) these are usually lost or so modified that their existence is 
only recognized by a study of development. 

The alimentary canal is straight, without marked enlargements, and 
lacks a chewing stomach. The liver is large and opens into the intestine 
by two or more pairs of ducts. The nervous system has some or all of its 
ventral ganglia arranged in a ring around the oesophagus, and in many 


388 


ARTHROPODA 


forms is enclosed in the ventral artery. Excretory organs, in the shape of 
nephridia, are frequently present and open to the exterior at the base of the 
second or the fifth pair of appendages. Malpighian tubes may occur, but 
these, unlike those of other tracheates, are entodermal in origin and hence 
not homologous with them 

The respiratory organs are either gills, lungs or tracheae. The gills 
are borne on some of the abdominal appendages. The lungs are sacs on 
the anterior abdominal somites opening by narrow slits (fig. 421) to the. 
exterior. The anterior wall of each lung sac is made up of thin plates ar- 
ranged like the leaves of a book (fig. 416), and embryology shows that these 
lung books are gill books drawn into the ventral surface of the abdomen. 
The tracheae replace one pair of the lungs and are apparently homologous 
with them. They penetrate all parts of the body. The reproductive 
openings are on the basal somite of the abdomen. The spermatozoa 
are motile. The development is direct, there being no metamorphosis. 

Sub Class I. Gigantosiraca. 

Marine forms with gills en the 2-6 abdominal appendages; bases of 
five pairs of cephalothoraciac feet masticatory ; a pair of median ocelli and 
a pair of compound eyes on the cephalothorax. 


.. 


FIG. 417. FIG. 418. 

FIG. 417. Limulus polyphemus* horseshoe crab (orig.). 

FIG. 418. Ventral surface of Limulus moluccanus (from Ludwig-Leunis). i, 
chelicen-e; 2-5, walking feet; 6, pushing foot; 6, flabellum; 7, genital operculum; 
8, gills (there should be five) ; 9, base of telson. 

Order I. Xiphosura. 

Cephalothorax large; abdomen with joints fused, terminated by a long 
spiniform telson. Limulus polyphemus* king crab or horseshoe crab. Other 
species on eastern shore of eastern continent. 


II. ACERATA: ARACHXIDA 389 

Order II. Eurypterida. 

Extinct Silurian and Devonian forms with small cephalothorax and large 
twelve-jointed abdomen; intermediate between xiphosures and scorpions. 
Eurypterus; Pterygotus, some species seven feet long. 

Sub Class II. Arachmda. 

Under this name are included a number of orders of greater or less 
extent which can be arranged around the spiders, or Aranea, as a centre. 
There is considerable modification of form, and the following account 
applies only to the more typical groups. In these the cephalothorax and 
abdomen are separated by a distinct line, and since the abdominal ap- 
pendages almost entirely disappear in the adult, the number of abdominal 
somites can only be ascertained where their boundaries are evident. 
The number varies between six in the phalangids and thirteen in the 
scorpions. 

The cephalothorax is, except in the Solpugidas, a single piece which 
bears six pairs of appendages; the four posterior pairs, each typically 
seven jointed, are locomotor, so that eight 
legs are as characteristic for an arachnid as 
ten for a decapod or six for a hexapod. The 
first pair of appendages, the chelicercz (fig. 
419), are preoral, the second, or pedipalpi, 
beside the mouth. The chelicene are short 
and consist of two or three joints, the terminal 
joint either folding back upon the other or, 
pincer-like, meeting an opposable thumb. In 
the spiders the last joint or claw is forced FIG. 419. Mouth parts of 

into the prey, introducing poison from a sac Epeira. i, chelicene; 2. pedi- 
, i .-TV' ,. , . palpi ;p, palpus;/, basal plate, 

in the basal joint. The pedjpalpi are elon- 
gate, leg-like, their basal joints often forming a lip, the other joints form- 
ing the palpus, which may end with a claw or a pincer. 

The question has often been discussed as to whether the chelicera? are the 
homologues of the antennas of other arthropods. The embryological evidence 
is in favor of their equivalence to the second antenna of the Crustacea, and to the 
mandibles of insects. In the cephalothorax is a fibrous entostcrnite to which 
most of the muscles are attached, a structure paralleled in Liiunlns. 

Since the Arachnida usually suck their food, the oesophagus is fre- 
quently widened to a sucking stomach, behind which comes the true 
stomach, with which, as well as with the intestine, a number of so-cullrd 
liver tubes may arise (fig. 415, da, dt). These may be restricted to ilic 
abdomen, as in the scorpions. The hinder part of the intestine is often 


390 


ARTHROPODA 


enlarged into a stercoral pocket, just in front of which the excretory tubules 
empty. These resemble the Malpighian tubes of insects in function, but 
differ in being entodermal in origin. Besides there are also coxal glands 
(modified nephridia), of which only one pair comes to development, 
and this may lose its external opening on the base of the first or third 

leg. 

The cesophagus is always closely surrounded by a nerve nng composed 
of brain above and of part of the ventral chain on the sides and below,, 
the thoracic and more or fewer of the abdominal ganglia entering 
into its composition (fig. 370, >). Of sense organs, besides tactile hairs, 
only the eyes (fig. 371), 2-12 in number, are well known. The large 
number of rods in the retina makes it probable that these eyes see well. 
Hearing is well developed, but it is uncertain whether certain hairs on the 
legs and palpi are the auditory organs. The function of the lyriform 
organs, which occur in the skin of body and legs in several groups, is 

unknown. 

The respiratory organs already alluded to (p. 388) have their spiracles, 
always few in number, on the anterior ventral part of the abdomen and, 

it is stated, sometimes on the cephalothorax. The 
internal organs are the lungs and the tracheae. A 
lung is a rounded sac just inside the spiracle and 
consists of numerous leaves on the anterior wall 
of the lung sac. Each leaf contains a blood space 
in its interior, while between the leaves are flat- 
tened spaces into which the air enters (fig. 416). 
The tracheae are branched tubes arising from the 
abdominal spiracles and penetrating the abdomen 
(fig. 420). These are lined with chitin, and to 
strengthen them without undue thickness this lining 
is thrown into folds, usually arranged in a spiral. 
In the scorpions and tetrapneumonous Araneina 
only lungs occur. In other spiders one pair of lungs 
is replaced by tracheae, while in most other arachnids 
only tracheae occur. (The smaller mites and par- 
asites lack specialized respiratory organs and circulatory organsas well.) 
These facts show that lungs and tracheae are morphologically equivalent. 
The localization of respiration in the abdomen has resulted in having the 
heart in the same region. It is noticeable that, as the tracheae are de- 
veloped, the circulatory vessels are reduced. In the scorpions, which 
have only lungs, the circulation is most nearly complete. 


FIG. 420. Beginning 
of paired trachea: of 
Anyphccna accentual a 
(after Bertkau). st, 
unpaired spiracle. 


II. ACERATA: SCORPIONIDA 


391 


In development the arachnidan tracheae arise in connection with the ab- 
dominal appendages, as do the lungs. (In the Solpugida; and some mites 
cephalothoracic trachea? occur.) This shows that the arachnidan tracheae 
are entirely different in origin from the trachea? of insects. 

The gonads (only the Tardigrades are hermaphroditic) are abdominal 
in position and open by paired ducts (sometimes with a single mouth) on 
the first abdominal somite. In most cases the animals are oviparous, 
but the scorpions and many mites bear living young. In many instances 
the mothers care for their eggs and young, the scorpions carrying their 
families on their bodies. Only rarely is there a metamorphosis, and 
then in the aberrant forms like the Linguatulida and Acarina, where the 
young have but two or three pairs of appendages, acquiring the others 
later. 

Legion I. Arthrogastrida. 
Arachnida in which the abdominal somites are distinct. 

Order I. Scorpionida. 

The scorpions bear a superficial resemblance to crayfish and for a long 
time were associated with them, since (figs. 366, 421) they have four pairs 
of walking feet (3-6), while the pedipalpi (2) are large and bear pincers. 


FIG. 421. Under surface of scorpion, showing the combs and the outlines of the 

lung sacs with their spiracles (orig.). 

The chelicera3 are also chelate. The pedipalpi and the two anterior 
pairs of legs have the basal joint expanded for chewing. The peculiarities 
of the abdomen mark the group off from all other arachnids. It consists 
of seven broader somites attached by their whole width to the cephalo- 
thorax and six narrower somites behind, forming a postabdomen. The 
last somite is produced into a sharp spine and contains two large poison 


392 


ARTHROPODA 


glands. It is the 'sting' of the animal which causes painful wounds in 
man, and in the large tropical species is, perhaps, fatal. Usually scorpions 
feed upon insects, which they seize with the pincers and kill with the sting. 
On the ventral surface of the second abdominal somite (fig. 421) are a 
pair of appendages, the combs or pectines, rods with teeth on one side, of 
uncertain function. They are clearly appendages with modified gill 
leaves, and from their nearness to the sexual opening and their rich nerve 
supply are supposed to be stimulating organs in copulation. The next 
four segments bear spiracles which lead to four pairs of lung sacs. The 


FIG. 422. Thclyphomts caudatus. i, chelicera; 2, pedipalpi; 3, flagellate third leg; 
4-6, walking feet. Below, chelicera enlarged. 

heart is abdominal and the liver diverticula are confined to the same 
region. The large number of abdominal ganglia distinct from the 
cesophageal ring is also characteristic. From three to six pairs of eyes 
occur. 

The scorpions are inhabitants of warm regions, ranging north with us to the 
Carolinas and Nebraska. Buthus,* Centrums* 

Order II. Phrynoidea (Pedipalpi, Thelyphonida). 

The thoracic segments are fused, and of the appendages only the last three 
are walking feet, the third pair having the last joint (tarsus) developed into a 
long many-jointed tactile flagellum. The chelicerae are strong and spined, but 
end in a pincer in some species. The chelicerae are also clawed and are possibly 
poison organs, since the bite of these animals is feared. The abdomen consist 


II. ACERATA: SOLPUGIDA 


of eleven or twelve somites and contains two pairs of lungs. There are eight 
eyes two large ones in the middle of the cephalothorax, and three small ones 
on either side. The species are tropical. Phrynus, simple abdomen; Thely- 
phonus* (fig. 422), short postabdomen which bears a long, many-jointed thread. 

Order III. Microthelyphonida. 

Small animals only known from Texas, Sicily, Paraguay, and Siam. They 
have a general resemblance to a scorpion; the chelicene are three-jointed and 
chelate, the pedipalpi simple; neither these nor any of the legs having chewing 


FIG. 423. Kirnenia whteleri (from Wheeler). 

lamella?. The head is distinct from two 'thoracic segments,' the abdomen is 
eleven-jointed and is terminated by a long many-jointed caudal flugdlum. 
Lung sacs, which are true appendages without lung leaves, occur on abdominal 
segments four to six, and are eversible. The ovary is unpaired, the testes paired. 
There is a circumoesophageal nerve ring and a single abdominal ganglion. No 
Malpighian tubes occur. Kirnenia* 

Order IV. Solpugida (Solifugae). 

In these the cephalothorax is broken up into a head bearing the chelirrnr, 
pedipalpi, and the first pair of legs; and three posterior free somites, each bear- 


394 


ARTHROPODA 


ing a pair of legs, thus giving these forms a certain resemblance to the Hexapoda 
(infra). The chelicerae are strong and chelate, the pedipalpi are simple and are 
used in walking, while the first pair of legs are tactile. Respiration occurs by 
four pairs of tracheae, the first of which opens between the first and second 
'thoracic' somites, a condition which deserves embryological investigation. 
The abdomen consists of nine or ten somites, and the head bears two ocelli. As 
the name implies, the Solpugidae are nocturnal, living by day in holes in the sand 
and searching for their prey at night. In the Old World they are reputed as 
poisonous, but no poison glands occur. Solpuga,* Galeodes,* Datamcs* (fig. 424). 


FIG. 424. 

FIG. 424. Datames formidibilis* (after Putnam). 
FIG. 425. Chelifer braraisi (from Schmarda). 


FIG. 425. 
i, chelicerar, 2, pedipalpi. 


Order V. Pseudoscorpii. 

These small flattened forms resemble the true scorpions in the chelate 
chelicerae and pedipalpi (fig. 425), and in the abdomen joined by its whole 
breadth to the thorax. They differ in the lack of postabdomen and sting. 
They breathe by tracheae; have from two to four ocelli, and spinning glands 
opening on the second abdominal somite. These animals, 2-3 mm. long, live 
in moss, etc., and among dusty books, feeding on mites and minute insects. 
Chelifer * Obisiuin* Chernes.* 

Order VI. Phalangida. 

The abdomen in the harvestman, or 'daddy long legs,' is less evidently 
segmented than in the forms already mentioned, nor is it sharply distinct from 
the cephalothorax. The small body bears four pairs of exceedingly long legs; 
the chelicerae are drawn out in long horny processes; the pedipalpi are tactile 
organs as in the true spiders. The males possess a long penis, and the females 
a long ovipositor. They have two or four ocelli and breathe by tracheae. These 
largely nocturnal animals are predaceous, feeding upon small mites. In struc- 
ture they form in some ways an approach to the Acarina. Phalangium* 
Liobunum* 


II ACERATA: ARANEIXA 


3 Do 


Legion II. Sphtxrogaslrida. 

Arachnida with the abdominal somites fused so that no traces of seg- 
mentation remain. 

Order I. Araneina. 

In the spiders the soft-skinned body is divided by a deep constriction 
into cephalothorax and abdomen (fig. 426). The four pairs of legs are 
adapted for springing or for walking, 
the hinder pair being also accessory to 
the spinning. It bears a comb-like claw 
with which several threads are combined 
into a stronger cable. The chelicera 
bears a sharp claw (fig. 419), traversed 
by the duct of the poison gland with 
which the prey is killed, although but 
few (species of Latrodecte s, fig. 427, the 
tarantula, and the bird spiders, Myga- 
lidae) can injure man. The pedipalpi 
are used as feeling organs and with the 
basal maxillary process to comminute 
the food. In the male the pedipalpi have 
the terminal joint swollen to a pear- 
shaped structure (fig. 428) by which 
the sexes are easily distinguished. This is used to convey the spermat- 
ozoa to the female, a rather dangerous process, as the male is apt to be 
killed by the much stronger mate. 


FIG. 426. Epeira insular is* round- 
web spider (after Emertcn). 


FIG. 427. FIG. 428. FIG. 429. 

FIG. 427. Latrodectes mactans* poison spider (after Marx). 
FIG. 428. Pedipalp of Pardosa uncata (after Emerton). 

FIG. 429. Spinnerets of Epeira diadema (after Wurburton). i, 2, 3, first, second, 
and third spinnerets;/, threads. 

At the hinder end of the abdomen, just in front of the anus, are the 
spinnerets, which are reduced appendages, as is shown by their paired ar- 


39G ARTHROPODA 

rangement and their jointing (fig. 429), as well as by development. They 
are truncate and have at the tip a 'spinning field' from which numerous 
minute, two-jointed spinning tubes, resembling hairs, arise, each of which 
is the end of a duct of a silk gland. Different kinds of glands, producing 
silk for different purposes, occur. The number of spinnerets varies 
between two and three pairs, and in front of these may be an unpaired 
spinning region, the cribrelliim, so that hundreds or even thousands 
(Epeiridae) of glands may be present. 

The secretion of the glands hardens in contact with the air, and the single 
threads are united by the combs of the hinder feet into a larger cord which can 
be regulated in size according to the number of glands which are active. Yet 
the largest cord is finer than the finest silkworm silk, hence it is often used for 
the cross-hairs of telescopes. The spider silk has many uses; it is used to line 
the nests, to form cocoons for the eggs, as a means of descent from high places, 
and to form the well-known webs. 

The nervous system consists of a brain and a circumcesophageal ring, and, 
in the Mygalidae, a single abdominal ganglion. The arrangement of the six or 
eight ocelli and the relative lengths of the legs are matters of systematic impor- 
tance. Two pairs of respiratory organs occur. In the Tetrapneumones there 
are two pairs of lungs, but in the Dipneumones the hinder pair are replaced by 
tracheae, which may open by separate spiracles (Tetrasticta) or by a common 
opening (Tristicta, fig. 420). Rarely both lungs are replaced by trachea. 

Sub Order I. TETRAPNEUMONES. Four lungs, four spinnents and 
eight eyes in two rows. MYGALID^;, large forms which spring upon their prey, 
capturing even small birds and mice. To Mygale* belong the spiders (errone- 
ously called tarantulas) which occur in banana bunches, and the trapdoor 
spiders, Cteniza* of the southwest, which excavate burrows in the soil, line them 
with silk, and close them with a hinged lid. Atypus.* Sub Order II. 
DIPNEUMONES. One pair of lungs, one of trachea; at most six spinnerets. 
Here belong most of the native and numerous tropical species. Some (VAGA- 
BUND.E) use their webs only to line the nests and enclose the eggs, which are 
either hidden away or carried about attached to the body; they spring upon cr 
chase their prey. SEDENTARIA are the web builders, their webs varying 
widely in structure. Of the first group the SALTIGRADA include forms which 
jump upon their prey (Attus* Phidipt-us* Habrocentrum*), and the CITIGRADA 
(Lycosa,* Dolomedes* Trochosa*), which run their prey down. Among these is 
the true Tarantula, T. apulice of Italy, whose bite was once believed to cause a 
frenzy only to be cured by peculiar music ('Tarantella'). The Sedentaria are 
divided according to the web-building habits. The ORBITELARM or orb 
weavers (Epeira* Ar slope*) form vertical webs which in many instances are 
complete circles. The RETITELARI^: (Theridiitm* Erigone*) build irregular 
webs. Latrodectes,* reputed poisonous to man (fig. 427). The TUBITELARI.E 
build horizontal webs with a tube to the margin in which they lay in wait for 
insects. 

Order II. Acarina. 

The mites, partly from parasitism, partly from other conditions of life, have 
become, in some instances, considerably modified. With the fusion of cephalo- 
thorax and abdomen the last traces of segmentation in the body are lost. Yet 
they retain the six pairs of appendages four pairs of legs, which at once distin- 
guish them from the parasitic hexapods; and two pairs of mouth parts, modified 


II. ACERATA: LINGUATULIDA 


397 


into a sucking beak. This consists of a tube formed by the basal joints of the 
pedipalpi, in which the chelicerae, either chelate, clawed, or stylet-like, play 
Since the mites are small and half or wholly parasitic, they are much simplified 
in structure. Frequently heart and trachea 1 are lacking. The larva as it 
escapes from the egg lacks the last pair of legs and then closely resembles certain 
imperfectly segmented parasitic insects like the lice. 

The red mites (TROMBIDIID/E) and water mites, HYDRACHXID/E (Hydrachna* 
A tax*}, are free-living as adults, but parasitic as young. The IXODID.F. or ticks 
(Ixodes*), attack man and other in immals, burrowing beneath the skin, sucking 


FIG. 430. FIG 431. 

FIG. 4^0. Sarcoptes scabei, female itch mite (after Leuckart). 
FIG. 431. Demodex folliculorum, follicle mite (from Ludwig-Leunis). 

the blood. Their relation to disease is referred to on p. 190. The much smaller 
males are attached to the females and take no food. Argas persicus, of eastern 
lands, with habits like a bedbug, is poisonous. GAMASID.E, parasitic, Gamasus* 
on beetles, Dermanysyus* on bats. The ACARID,*: include permanent parasites 
like Sarcoptes scabei* (fig. 430), the cause of the 'itch,' and closely allied cheese 
mite The follicle mite, Demodex folliculorum* lives in the sebaceous glands 
of various mammals, including man (fig. 431). 

Order III. Linguatulida. 

Elongate mites like Demodex lead to the Linguatulida, which as adults live 
in the frontal sinuses of carnivorous mammals, as encysted young in the liver of 
herbivorous forms, especially rodents. The body is long, flattened and ringed, 
and somewhat tapeworm-like (fig. 113). The adults have the mouth at the base 
of a chitinous capsule, and on either side are two hooks regarded as the claws of 
the first and second legs. Inside the body is a spacious cavity traversed by the ali- 
mentary canal which is without appendages. The nervous system is largely a cir- 
cumoesophageal ring; the sexual organs are very complicated, the males having 
the openings in front, the females at the hinder end. The presence of these para- 
sites causes a profuse catarrh, and the eggs pass out with the mucus. Falling on 
vegetation, these are liable to be eaten by various animals. The larvie (fig. 43 2 ) 
have a boring apparatus in front and two pairs of legs, the latter lost in the 
metamorphosis except for the hooks. It is by no means certain that these arc 
degenerate arachnids. Pentastonntm. 

Usually associated with the Arachnida are two other groups of very doubtful 
position, which until more definite knowledge is obtained, may remain near them. 


398 


ARTHROPODA 


Tardigrada. 

These are minute fresh-water forms, known to microscopists as 'water bears' 
(fig. 433), which owe their name to their slow motions. They have four pairs of 
short, hooked legs, their sole Arachnidan character. The genital ducts empty 


FIG. 432. FIG. 433. 

FIG. 432. Larva of Pentastomum proboscideum (after Stiles), d, stomach; e, gland 
cells; m, mouth; st, stylet; y, posterior larval hooks; i, 2, legs 

FIG. 433. Macrobiotics hufelandi, water bear (after drawings by G reef and Plate). 
I-IV, legs; d, accessory glands; m, stomach; mk, mouth capsule; ov, ovary; sp, salivary 
glands; st, stylets; vm, excretory tubules; blood cells in the body. 


FIG. 434. Nymphon strcemii* (orig.). c, cheliceras; o, ovigerous legs; p, pedipalpi; 

r, rostrum. 

into the rectum; the nervous system has four ventral ganglia; heart and respira- 
tory organs are lacking. In development they are remarkable for the large 
ccelomic pouches. In the feet are glands recalling nephridia in their history. 
It is possible that these animals are to be placed among the Ccelhelminthes. 
Macrobiotus.* 


III. AIALACOPODA 399 

Pycnogonida (Panlopoda). 

These marine animals have a cylindrical body, with a tubular proboscis in 
front and an abdominal appendage behind, and four pairs of very long legs. In 
front of the legs is a pair of small chelate appendages and usually a pair more 
like pedipalpi. In the male there is an additional pair of 'ovigerous legs' to 
which the eggs are attached after being deposited by the female, thus giving a 
total of seven appendages, a number not reached in any arachnid. Diverticula 
of the stomach extend into the legs; a heart is present, but respiratory organs are 
lacking. The Pycnogonids, which creep slowly over seaweeds and hydroids, 
may be (i) a distinct group of arthropoda, or (2) modified arachnids, or (3), 
and less probable, Crustacea. Nymphon,* Phoxichilidium* Colossendeis.* 

Class III. Malacopoda (Protracheata). 

These forms, including only a single family PERIPATID^., show a 
strange mixture of annelid and arthropodan (or 'tracheate') characters, 
so that they are usually regarded as representatives of the stock, early 
separated from the annelids, from which the Insecta have descended. 


FlG. 435. Peripatus capensis (from Balfour, after Moseley). 

They recall the annelids by the nephridia, which begin by a closed 
vesicle (reduced ccelom), pursue a short course, and expand into a urinary 
bladder before opening at the bases of the legs (fig. 436, so). On the 
other hand, they possess trachea?, long unbranched tubes which arise in 
numbers from the spiracles, which are irregularly distributed in each 
somite (tr). 

Each segment of the soft-skinned body, which shows no external 
ringing, bears legs, each terminated by claws. These legs resemble the 
annelidan parapodia in not being jointed and not sharply separated from 
the trunk. The head is provided with three pairs of appendages: a pair 
of ringed antennas, a pair of mandibles, which lie in the oral cavity, and 
a pair of mouth papillae, at the tips of which are the openings of the slime 
glands, the sticky secretion of which is squirted out and serves to capture 
insects (sd) . 

The nervous system consists of a pair of cerebral ganglia (og), sup- 
plying the antennae and a pair of very primitive eyes; and a pair of ventral 
cords (bm), swollen slightly in each segment, which connect dorsal to the 
anus and are connected in the trunk by numerous non-segmental com- 
missures. The muscles are of the smooth variety. 


400 


ARTHROPODA 


The description may be completed by saying that the straight alimentary 
canal (p and d) bears only salivary glands (sp) ; that it is accompanied throughout 
by a dorsal heart; that the gonads (the sexes are separate) open just in front of 
the anus (go), their ducts being modified nephridia. The animals are vivipar- 
ous, live in decaying wood, hide by day and hunt their prey at night. The 
several species have a wide but discontinuous distribution (South America, Cape 
of Good Hope, New Zealand, etc.), an indication of great antiquity. Recently 
the forms have been divided into several genera, Peripatns, Peripatopsis, 
Opisthopatus, etc. 


Fir,. 436. Anatomy of female Peripatus opened dorsally (from figures of Moseley 
and Balfour). a, anus; at, antenna;; bm, ventral nerve cords; d, digestive tract; go, 
genital opening; o, ovary; og, brain; p, pharynx; sd, slime gland; so, nephridia; sp, 
salivary gland; tr, tracheae; u, uterus. 


Class IV. Insecta. 

The Insecta is a distinct group marked off from all other arthropods 
by several important characters. The appendages show no signs of a 
schizopodal condition. The head is always a distinct region, bearing a 
single pair of antennae, a pair of mandibles, and two pairs of maxillae, the 
posterior pair often being fused into a lower lip or labium. 


IV. IXSECTA 


401 


The respiratory organs are trachea: (figs. 437, 438) which resemble 
the trachea of man only in that they are tubes filled with air, and kept 
from collapse by firm walls. They open to the exterior by openings 
(spiracles, stigmata) on the sides of the body. They are inpushings of 
the skin and consequently have the same structure, an epithelium and an 

outer chitinous layer. The latter lines 
the lumen of the tubes, and since it must 
be thin to permit the passage of gases 
(oxygen, carbon dioxide), and at the 
same time firm, to keep the tubes open, 
it is thrown into folds which usually 
pursue a spiral course. The turns of the 
spiral are so close that it gives the tubes a 


FIG. 437. FIG. 438. 

FIG. 437. Tracheal system of Machilis (f rom Lang, after Oudemans). k, head; 
I-III, thoracic somites; s, spiracles; i-io, abdominal somites. 

FIG. 438. Portion of trachea of caterpillar (from Gegenbaur). .-1, main trunk; 
B, C, D, branches; a, epithelium with nuclei, b; d, air in tracheal tube. 

ringed appearance (fig. 438). Inside the spiracles the trachea* branch 
repeatedly until they end in fine tracheal capillaries in the tissues. 
In general it may be said that each segment has a pair of spiracles 
and corresponding tracheal systems (tig. 60), but this scheme is 
complete in no known species, for there are always some segments 
(especially in the head) which lack these organs and are supplied from 
adjacent segments (fig. 437). Again, the trachea? may be connected by 

26 


402 ARTHROPODA 

longitudinal trunks (fig. 452, Ib), so that spiracles occur in only a part of 
the segments, these supplying the whole body. Although the trachea? 
are for aerial respiration, there are aquatic insects, but these also breathe 
air, which they carry about with them entangled among the hairs surround- 
ing the spiracles. Then aquatic larva? often have tracheal gills, thin- 
walled processes of the integument which project into the water and are 
penetrated by numerous tracheal twigs (fig. 453). 

The alimentary tract always has excretory organs, the Malpighian 
tubules, connected wdth it. These vary in number between wide limits, 
but are always placed at the junction of the rectum with the rest of the 
tract. They differ from the physiologically similar tubes of the Arachnida 
in being of ectodermal origin, so that no homology can be traced between 
them. The gonads are always paired and placed dorsal to the intestine, 
while the ducts (at least in some cases modified nephridia) open ventrally 
at the hinder end of the body. The spermatozoa are motile. 

Tn the subdivision of the 'tracheate' arthropods a group of Myriapoda is 
usually recognized, containing the centipedes and 'galley worms.' These two 
types are in reality very different. The centipedes (Chilopoda) show in all 
structural features close relationships to the Hexapoda, while the Diplopoda 
differ in almost every respect, except the presence of numerous walking legs, 
from the Chilopoda. Hence, since the object of classification is to show resem- 
blances and differences, the Myriapoda has been dismembered, the Chilopoda 
being considered here, the Diplopoda as a distinct class at the end of the group 
of Arthropoda. 

Sub Class I. Chilopoda. 

The most striking characteristic of the chilopods is their long, flattened 
bodies, each of the numerous similar somites bearing a pair of six- or 
seven-jointed limbs. The head bears a pair of long antennae and usually 


FIG. 439. Diagram of transverse section of a centipede (orig.). d, digestive tract; 
g, gonad; n, nerve cord; s, spiracle and tracheae. 

numerous ocelli, which only mScutigcra show a tendency to become com- 
pound. The mouth parts (fig. 440) are a pair of mandibles and two pairs 
of maxilke, both united in the median line, the first pair forming a ' gnatlio- 
cliilarium,' the second the lower lip. Besides, the first pair of legs (5), 
with their fused bases, extend forward beneath the head and form the 


IV. INSECTA: IIEXAPODA 


403 


poison claws. Their terminal joints are sharp and contain the ducts of 
poison glands. The spiracles (at least a pair to every other somite 
except those of the head) are lateral in position in the soft integument 
between the dorsal and ventral plates (tergum and scutum) (fig. 439). 
The heart is elongate, with chambers in each somite (fig. 67); there are 
two large Malpighian tubes, and the nervous system is elongate, with 
ganglia in each somite. The unpaired gonads are dorsal to the intestine, 
while the single duct opens ventrally in the preanal somite. 


FIG. 440. FIG. 441. 

FIG. 440. Mouth parts of Scolopendra niorsitans. i, antenna-; 2, mandibles; 3, 
maxilla- (gnathochilarium) ; 4, second maxilke (labium) ; 5, poison feet. 
FIG. 441. Scolopendra morsitans, centipede (after Schmarda). 

LITHOBIID^:, 15 leg-bearing somites; certain dorsal plates enlarged and 
overlapping the succeeding somites; Lithobius* etc. SCOLOPEXDRID,*:, centi- 
pedes; at least 17 legs and 5 ocelli; Scolopendra* (fi^- 44 1 )- GEOPHILID.*:, not 
less than 30 pairs of legs, spiracles 2 less than legs. Geopktlus.* Scr i 
legs very long, 15 leg-bearing segments, but only 8 dorsal plates. 

Sub Class II. Hexapoda. 

The Hexapoda is by far the largest division of the Arthropods, 
containing at least ten times as many known species as all the rest. The 


404 


ARTHROPODA 


number is so large that it cannot be given with accuracy; an estimate is 
250,000. Since the tropics, which have not been exhaustively studied, 
are very rich in insects, it is conceivable that there are at least a million 
different species in the world. On the other hand, great uniformity of 
structure exists, all adhering with great fidelity to plan, regional divisions, 
and number of appendages, so that the difference between the most extreme 
forms is far less than that in Crustacea or Arachnida. But while hexa- 
pods thus lose in morphological interest, they gain in their life relations,^ 
in the way that they are injurious or beneficial to man, in their breeding 
habits, and in their intellectual and social relations. From the evolution- 
ary standpoint they show marked adaptations to environment, and the 
large number of species is only possible by taking advantage of every 
opportunity in nature. 


FIG. 442. Schematic section of a hexapod through the thorax (orig.). ex, coxa; 
d, digestive tract; f, femur; h, heart; n, notum; pi, pleuron; st, sternum; t, tibia; ta, 
tarsus; tr, trochanter. 

Of systematic importance are the regional division of the body and 
the number and character of the appendages. In the body three regions 
are distinguished, often separated by marked constrictions: head, thorax, 
and abdomen. The number of abdominal somites varies with the order 
and even with the family, ranging between eleven (in some larvas and 
embryos twelve) in the Orthoptera and five in many Diptera. Each 
cuticular abdominal segment consists of two plates, tergite (dorsal) and 
sternitc (ventral), united on the sides by a softer membrane which con- 
tains the spiracles. Head and thorax, on the other hand, have a constant 
number of somites. (See, however, Hymenoptera.) The thorax is 
plainly divided into three segments, pro-, me so- and metathorax, each com- 
posed of three elements, an unpaired dorsal portion, notum; a pair of 
lateral plates, pleura; and an unpaired ventral sternum (fig. 442). For 
simplicity one speaks of pronotum, mesosternum, etc., to indicate the por- 


IV. IXSECTA: HEXAPODA 


105 


tions of the separate segments. The head (fig. 443) is a continuous capsule 
in which the following parts are recognized: in front and dorsal clypens 
and frons; dorsal and posterior a vertex and an occiput; laterally gcna. 
ventrally a gula. The appendages show that the head is composed of at 
least four somites. 

The view that the head consists of six somites is based on the existence of 
two more segments without appendages in the embryo, a preantennal and a 
postantennal (premandibular), as well as the fact that the brain consists of three 
pairs of ganglia (proto-, deuto-, and trito-cerebrum). 

The three thoracic segments bear three pairs of legs, whence the name 
Hexapoda. The legs (fig. 442) are inserted between pleura and sterna 
and begin with a short coxa (r), followed by a trachanter (/;), also short. 
The two following joints are long, the first, 
the femur (/<?), being large and containing 
the muscles; the next, tibia (/), being more 
slender; the foot, or tarsus (to), is composed 
of several joints, the last bearing a pair of 
claws. 

The first of the cephalic appendages, the 
antennae, are the most leg-like. They spring 
from the frons above the mouth and are 
innervated from the brain. The number 
and shape of the antennal joints vary with 
the group, often with the sex, and according 
as the single joints are lengthened or short- 
ened, narrowed or expanded, or provided 
with appendages, etc., different kinds of 
anteniKE knobbed, club-shaped, toothed, 
feathered, etc. are recognized, distinctions of great value in classification. 

The morphology of the three pairs of mouth parts, the mandibles (md), 
maxilla (mx), and second maxilte, or labium (la, figs. 443447), is more 
interesting. The labium, formed of united right and left appendages, 
lies behind the mouth and forms the lower lip, and is in contrast to the 
upper lip, or labrum (/;), which, however, is not appendicular in char- 
acter. Both labium and labrum may bear unpaired processes on their 
oral surfaces, an epipliarynx above, a hypopharynx below the mouth, 
neither of them true appendages. 

The different kinds of food necessitate differences in the character of the 
mouth parts chewing, licking, sucking, or piercing all referable back to the 
chewing kind, and these in turn are modified legs. In the description of the 
chewing type it is well to begin with the nnixillir (fig. 444), because of their easy 
comparison with the other mouth parts and with the legs as well. These begin 


FIG ^^j^ of a grass _ 
hopper, c, clypt-us, /", frons; 
*,;_ 

ble; mp, maxillary palpi; w. 
"axilla; , occiput; z;, vertex 


406 


ARTHROPODA 


\vith a triangular joint, the cardo (c), which is followed by a larger stipes (sf). 
The stipes in turn supports two chewing lobes, the inner, or lacinia (It), and an 
outer, or galea (le). The galea may either form a sheath for the lacinia, or, as 
in many beetles (fig. 470), it may be tactile and jointed again. The stipes also 
bears the maxillary palpus (pm), consisting of from three to six joints, and is 
the most leg-like part of the appendage. The labhim arises as a pair of ap- 
pendages which early approach each other and fuse behind the mouth. All the 
parts of the maxilla may be recognized, only it must be remembered that the 
basal parts of the two sides are fused. The united cardines form an under chin, 
the submentum, the stipites a chin or mentum, cleft in Orthoptera, a result of 


FIG. 444. FIG. 445. 

FIG. 444. Chewing mouth parts of cockroach (Peri planet a orient alls). The lettering 

is the same in figs. 444-447. C, cardo; gl, glossa; h y, hypopharynx; /, lobe; le, li, external 

and internal lobes of maxilla; Ir, labrum; m, mentum; md, mandible; mx, maxilla; 

p, pm, maxillary palpus; pg, paraglossa; pi, labial palpus; sm, submentum; 5/, stipes. 

FIG. 445. Licking mouth parts of bumble bee (Bombus terrestris). 

incomplete fusion. This may bear inner and outer processes, the glossce (gl) 
and the paraglossce (pg) respectively, and the labial palpus. The mandible con- 
sists of merely the basal joint, altered for biting, while the rest of the appendage, 
common in Crustacea as the mandibular palpus, is lacking. 

The licking mouth parts, like those of the bees (fig. 445), stand next to those 
already described, there being many transitional stages. Labrum and mandi- 
bles retain their primitive condition, while maxilla? and labium are greatly 
elongate, are connected at the bases, and can be folded away beneath the head 
or extended at will. The small submentum is followed by an elongate mentum 
which bears the unpaired tongue or glossa (gl), which corresponds to the fused 
glossa; (or to the hypopharynx ?) of the first type and which is used for sucking 


IV. INSECTA: HEXAPODA 


407 


honey and hence has the form of a nearly closed tube. Beside it lie the rudi- 
mentary paraglossae (pg) and the well-developed palpi. Similarly the maxillae 
have small cardines and palpi, while the stipites and the undivided lobe (/) are 
long and well developed. 

The piercing mouth parts of the flies (Diptera) and bugs (Rhynchota) can 
be compared with those of the bees in so far as the labium forms the groundwork 
of the whole (fig. 446). The beak (rostrum, haustellum) of these animals cor- 
responds to the labium; it is a grooved structure, either fleshy and flexible, or 


FIG. 446. FIG. 447. 

FIG. 446. Sucking mouth parts of mosquito, Culex pipiens (after Muhr). The 
groove of labium opened by removing labrum; the stylets separated. 

FIG. 447. Sucking mouth parts of a butterfly (after Savigny). nix', nix", shows 
how right and left maxilke unite into a tube; right labial palpus \J)D with hairs 
removed. 

stiff and jointed. The edges of the groove are inrolled so that there remains a 
narrow dorsal slit, which can be closed by the slender upper lip (Ir). The 
tube formed of these parts contains four stylets, toothed or with rctrorse hooks 
at the tip. These are the mandibles and maxilke, and a fifth stylet, the hypo- 
pharynx (hy) can be present. The palpi, which only occur in the Diptera, belong 
to the maxillae (/>). Reduction in number of stylets to four or three, or their 
complete absence (some flies), is brought about by fusion or by degeneration. 
The haustellum serves as a case for the sucking tube, which in the Rhynchota is 
formed by the united maxillae, in the Diptera by labrum and h\popharynx. 

The proboscis, or haustellum (the so-called tongue), of the Lepidoptera 


408 ARTHROPODA 

(fig. 447) is a long tube coiled like a watch spring beneath the head. It con- 
sists of two long grooved maxillary galea firmly united by their edges. The 
maxillary palpi are well developed in the moths; elsewhere they show all stages 
of reduction to complete disappearance. Labium and labrum are reduced to 
small triangular plates at the base of the proboscis, the labium bearing a pair of 
hairy palpi (pi). The mandibles are represented by small plates or bunches of 
hair. These conditions gain in interest when we remember that in the larva 
the mandibles are strong biting organs, while the maxillae are small hooks, and 
the labium is better developed only in those parts connected with the silk glands, 
a beautiful example of relations of structure to life conditions. 

In contrast to the other regions, the abdomen lacks appendages in the adults. 
Only in the Thysanura are small lobes present, behind and in the same line with 
the thoracic fee't, which may be abdominal feet. Apparently, too, the append- 
ages of the last segment, the stylets and cerci, are modified limbs, but the parts 
(qonapophyses) used in copulation and oviposition are different in character. 
False feet, or pro-legs, occur on the abdomen of the larvas of the Lepidoptera and 
the Tenthredinidas, but since these are fleshy un jointed processes, it is doubtful 
whether these are true abdominal limbs, or are structures independently 
acquired. 

Besides ventral appendages the insects usually have two pairs of dorsal 
outgrowths upon the meso- and metathorax, the wings. They are lateral 
folds of the chitinous coat of the notum and contain on their interior exten- 
sions of the blood sinuses and of the tracheae, which are protected by thick- 
enings of the chitin, causing the network of 'veins' or 'nervures' in the wing. 
Both wings may be elastic, flexible, and adapted for flight, or the hinder 
pair may alone partake of this character (true wings or alec}, while the 
first pair may be thick and parchment-like wing covers, or elytra, under 
which the true wings are concealed when at rest. When only the base of 
the wing is thus thickened hemelytra result. Between the bases of the 
anterior wings is frequently a chitinous plate, the scuteUum, between the 
hinder wings a similar postscutellwn. In many insects one pair of wings 
is lacking, the anterior pair being retained in the Diptera (fig. 486), the 
posterior in the Strepsiptera (fig. 469). The entire absence of wings may 
occur from two causes; wings have apparently never been developed in 
some (primary lack of wings of the Apterygota), while there are others in 
which wings once' present have been lost, because nearly related forms 
bugs, male cockroaches, sexual ants and termites are winged (figs. 
464, 482, 483). The prothorax of all recent insects is wingless, but some 
Archiptera of the coal period had wing rudiments on this somite. 

As a result of differences in food the alimentary canal (figs. 448, 449) 
varies greatly. The ectodermal stomodaeum begins with a pharynx, which 
in the sucking insects is a sucking apparatus with radial muscles. The 
oesophagus, which follows, may be widened to a crop (ingluvies), or it may 
have a cascal outgrowth which in the butterflies and flies may take the 
shape of a stalked vesicle (falsely 'sucking stomach'). Also ectodermal 


IV. INSECTA: HEXAPODA 


40!) 


is the gizzard (km, pv),or proventriculus,the chitinous lining of which is 
toothed for grinding the food. The true stomach, of entodermal origin 
(m, cd), frequently bears blind sacs or gastric cceca (ap); in general it is 
short and its junction with the hinder ectodermal portion, the proctodeum, 
is marked by the entrance of the Malpighian tubules (vin). The latter, ex- 
cretory in function, arise from the proctodeal region. The proctodeum 
is usually differentiated into a small intestine and a two-regional (colon 
and rectum) large intestine. The rectum may have enlargements called 
rectal glands. True glands, however, 
occur only at the beginning and end of 
the alimentary tract; from two to four 
salivary glands (sp) empty into the 
mouth; at the anus are defensive anal 
glands with malodorous secretions of a 
protective character. The alimentary 
tract with the other viscera is enveloped 
in the fat body, a soft mass which con- 
tains, besides fat cells and connective 
tissue, concretions of uric acid. 

The nervous system (fig. 570) has 
the ventral cord, especially in primitive 
forms (Apterygota, Archiptera, Ortho- 
ptera, fig. 449), and nearly all larvae (fig. 
60), long and composed of numerous 
separate pairs of ganglia. In beetles, 
moths, bees (fig. 452), and flies the cord 
is shortened and the ganglia are in part 
fused. The brain, arises by the fusion of 
three pairs of ganglia (proto-, deuto-, 
and tritocerebrum) , and is, especially in\ 
colonial species, very complex. It is 
connected on either side with a large 
optic ganglion, the size of which is cor- 
related to that of the eyes. In the adult 
condition the Hexapoda have a single 
pair of highly developed compound eyes (fig. 372), (each occasionally 
divided into two), which not infrequently occupy nearly the whole of 
the top of the head. Between and in front of these, small and simple 
ocelli, usually three in number, frequently occur, especially in insects 
which are strong fliers. These are different from the more numerous 
simple eyes of the larvas of holometabolous insects (e.g., butterflies and 


FIG. 448. Alimentary tract of 
Carabus awatus (from Lang, after 
Dufour). av, anal vesicle; ad, anal 
gland; cd, stomach with ca?ca; <</, 
hind gut; in, ingluvies (crop); k, 
head; oe, cesophagus; pi', proven- 
triculus (gizzard); r, rectum; mi, 
Malpighian tubules. 


410 


ARTHROPODA 


beetles) in the position of the later compound eyes. Of other sense organs 
only the tactile hairs of the skin are known with certainty, while similar 
hairs on the antenna.' and about the mouth are supposed to be organs of 
smell and taste, since these senses are known to be well developed. The 
tvmpanal organs of the Orthoptera are the only structures which can be 


ik 6g. ilCf. I tcjf. 


dig 


FIG. 440. Viscera of male cockroach (Peri planet a orientalist (partly after Huxley). 
I III, segments of thorax and corresponding legs; i-io, abdominal segments; a, anus 
ag, ventral ganglia; ap, gastric oeca; at, antenna; bl, salivary bladder; g, sexual opening 
h, heart; kr, crop; km, gizzard; I, labial palpus; m, stomach (the arrow shows the con 
nection between m and km), also maxillary palpus; mg, male genitalia; oe, oesophagus 
og, brain; r, rectum; sp, salivary gland; tg, thoracic ganglia, ug, infracesophagea 
ganglion; vm, Malpighian tubules. 

with much probability connected with hearing. These are thin drum-like 
parts of the chitin, framed in thicker portions (figs. 450, 451), beneath 
which is a tracheal vesicle, with a nerve ending in a 'crista acustica.' End 
organs similar to those of the criste acusticae occur elsewhere than in the 
tympana! organs and are regarded as auditory (' chordotonal' sense organs). 


FIG. 450. FIG. 451. 

FIG. 450. Side view of grasshopper, s, spiracles; I, tympanal organ. 
FIG. 451. Anterior tibia of a Locustid with tympanum, /; (from Hatschek, after 
Fischer). 

The power of producing sound is widely distributed and often highly 
developed, the organs for this purpose varying widely in character. 
Stridulating organs are formed by ridges on wings and legs, which are 
rubbed against each other or against similar ridges on the body. Hum- 
ming is produced by the action of the wings or by the passage of air 


IV. INSECTA: HEXAPODA 


411 


through the spiracles, which are often provided with vibrating membranes 
which also serve to close these openings. 

The trachea? (figs. 437, 452) are usually united, just inside the spiracles, 
by longitudinal trunks from which fine branches extend, enveloping and 
penetrating all the organs with delicate silvery threads. This connection 
of trachea renders it possible for the spiracles of some segments to dis- 
appear, leaving but a single pair in the aquatic larva) of some Diptera. 

Jr 10 


nut 


cm 


ed. 


FIG. 452. FIG. 453- 

FIG 4-52. Anatomy of honey bee (from Lang, after Leuckart). a, antennae; 
au eve; b, legs; cm, chyle stomach; ed, rectum; km, honey stomach (proventaculus) 
rd, rectal glands; st, spiracles; tb, tracheal chambers with trachea-; vm, Malpij 

tubules. 

FIG. 453. Abdomen of Ephemera larva (from Gegenbaur) with tracheal | 
tracheal trunks; b, intestine; d, caudal bristles (cerci). 

The spiracles of the abdomen are the most constant, usually occurring in 
or near the soft membrane between the sternites and tergitrs; tin- thorax 
at most has but two pairs, the head none. In insects with good power, 
of flight many of the tracheal trunks are expanded to large air sacs, which 
may be of value as reservoirs of air, so that the ordinary respiratory 
motions are less necessary during flight. 


412 


ARTHROPODA 


An interesting adaptation of the tracheal system to acquatic life occurs in 
the larvae of many Archiptera (dragonflies and Mayflies) and Neuroptera, and 
even among Lepidoptera (Paraponyx) and Coleoptera (Gyrinidae). The spira- 
cles here are usually closed, and oxygen is taken either through the skin or by 
so-called tracheal gills bushy or leaf-like appendages of the surface or the 
rectum, richly permeated by tracheal branches (fig. 453). In such cases the 
tracheal system has two portions, one which receives oxygen from and gives 
off carbon dioxide to the water; the other which supplies the tissues with oxy- 
gen and receives carbon dioxide. 

Since the trachea?, with their fine branches, supply the tissues directly 
with oxygen, the blood-vascular system is rudimentary. Directly under 
the back lies the elongate tubular heart in a special pericardial sinus. 
This is a part of the ha?moccele cut off from the gastric portion of this space 


A. 


B. 


FIG. 454. A, Male genitalia of Melolontha (from Gegenbaur, after Fab re), gl, 
accessory glands; /, testes; vd, vas deferens; vs, seminal vesicles. B, genitalia of female 
Hydrobius (from Gegenbaur, after Stein), be, bursa copulatrix; gl, tubular glands; 
o, ovarial tubes; ov, oviduct with glands; rs, receptaculum seminis; v, vagina. 

by an incomplete partition in which, right and left, are the lateral muscles 
(alee cordis) of the heart. Since folds from the margins of the ostia ex- 
tend into the cavity of the heart, and in the systole, which proceeds from 
behind forward, not only close the ostia, but prevent any back flow of blood 
into the posterior part of the heart, there is an appearance of a chamber- 
ing of the heart. The blood passes forward through an anterior aorta 
into the ruemoccele and from this back to the pericardial sinus, the alary 
muscles aiding by moving the viscera, and enlarging the sinus. The 
arrangement of the viscera, fat bodies, and muscles gives a certain regular- 
ity to the circulation, especially in the appendages. Accessory pulsating 
ampullae in the bases of the antennae (Orthoptera) help in the flow of the 
blood. Many beetles (Meloidas and Coccinellidse) squirt blood contain- 


IV. IXSECTA: HEXAPODA 


413 


ing an irritating substance (cantharidin) through the jointing membranes 
of the legs as a means of defense. 

The Hexapoda are dioecious. The paired gonads consist of a few or 
many ovarial or testicular tubules (fig. 454), in the abdomen. Their 
paired ducts (oviducts, vasa deferentia) open separately in the Ephemerida 
and young Apterygota, but all other Hexapoda have a single ventral un- 
paired sexual opening just in front of the anus. This arises as a median 
invagination of the ectoderm (hence lined with 
chitin), which extends inwards and meets the 
genital ducts (modified nephridia). The re- 
ceptaculum seminis, a sac connected with the 
female genitalia, has a special biological in- 
terest. In insects which copulate but once 
during life it retains the spermatozoa for a 
long time (four years in bees) in a living con- 
dition. As the eggs are laid they may be 
impregnated by spermatozoa from it. Since 
a firm shell or chorion is developed around the 
egg in the ovary, entrance of spermatozoa is 
only possible by a micropylar apparahts, a 
system of tubes penetrating the chorion at one 


end of the egg. 


FIG. 455. Ventral view 
of sting of bee (after Packard 
and Cheshire). bl, poison 
sac; d, poison gland; g, its 
duct; ga, terminal ganglion, 
beside it accessory gland; 
i, stylet; 2, groove of sting 
(black); 3, sheath of sting; 7, 
angle piece; 77, quadrate 
plate for attachment of 
muscles. 


Oviposition occurs in many insects by means of 
an ovipositor which may be developed in two ways. 
In beetles, flies and butterflies the last somites of 
the body are small, and are normally retracted into 
the body but can be protruded as a long tube for 
oviposition. In Hymenoptera, Hemiptera, Orthop- 
tera and dragonflies the ovipositor (tcrcbra) is 
formed by special appendages, the gonapophyses, 
four to six in number, which arise from the ventral 
side of the eighth and ninth abdominal segments. 

In the Orthoptera two pairs of gonapophyses of the eighth and ninth somites 
form a sheath in which two other parts, the egg-guide, also derived from the 
ninth, are enclosed. In the Hymenoptera (fig. 455) the latter are fused to a 
tube in which both parts of the eighth somite play as a piercing stylet, while 
two other parts lie at the sides as the sheath. In the wasps and bees these parts 
can be withdrawn into the body, and have frequently been converted into a 
sting (aculeus) provided with a poison gland, which is confined to the female. 
In the males there is usually a protrusible penis which is frequently composed 
of the same parts as the ovipositor; in others of metamorphosed somites. 
Further sexual differences lie in the form of the antennae, shape and color of 
the wings, modifications of the eyes, etc. (fig. 74). 

In many insects the eggs may develop parthenogenetically. Plant lice and 
scale insects reproduce for generations asexually, and parthenogenesis is widely 
distributed among Hymenoptera, Lcpidoptera, and Neuroptera. The condi- 


414 


ARTHROPODA 


tions among the bees are especially interesting, since here the determination of 
sex rests with the existence or non-existence of fertilization (pp. 130, 134). 
Much rarer than the ordinary parthenogenesis is paedogenesis (p. 129), which 
occurs only in certain Diptera like Miastor. In the female Miastor larva (fig, 
456) the eggs develop before the appearance of the ducts, so that the young can 
only escape by rupture of the mother. After several paedogenetic generations 
there appear at last larvae which pupate and produce adult male and female flies. 


FIG. 456. Larva of a Cecidomyid with pcTclogenetic daughter larvae (from Hatschek 

after Pagenstecher). 

With the exception of these paedogenetic forms, the Pupipara, many Aphidae 
and a few other viviparous species, the Hexapoda are oviparous. The develop- 
ment begins, after oviposition, by a superficial segmentation of the egg. Later 
there appear two embryonic structures, the yolk sac and the amnion; the first, in 
contrast to the vertebrate structure with the same name, is dorsal. The amnion 
is a thin layer of cells which covers the ventral surface and arises in a manner 
similar to the vertebrate amnion; folds arising from the blastoderm in front 
and behind, right and left of the embryo, fuse with one another and produce a 
double envelope, an inner amnion, an outer serosa, enclosing the germinal area. 

The postembryonic development presents two important features, 
i . As in other arthropods growth is possible only by periodic ecdyses so that 

the life cycle consists of several periods 
separated by molts of the cuticle. 2. No 
insect, as it escapes from the egg, has 
wings. If present in the adult they must 
arise during the larval stages. This 
postembryonic development of the wings 
is the starting-point of the metamor- 
phosis, and forms the basis of a division 
of the development into ametabolous (no 
metamorphosis), hemimetabolous (in- 
complete metamorphosis) and holometab- 
olous (complete metamorphosis). An 
ametabolous development is possible only 
in wingless insects, the postembryonic 
development consisting only of periodic molts. Some wingless forms 
(fleas, wingless moths, ants, etc.), have a metamorphosis, because they have 
inherited it from winged ancestors and have not lost it with the wings. 
Hemimetabolous development is marked by a gradual change from 
the newly hatched animal, the larva, to the sexually mature adult or 
imago (fig. 457). There often appears with the second molt, the 


FIG. 457. Hemimetabolous de- 
velopment of Perla nigra (from 
Huxley). .4, wingless larva; B, 
larva with wing pads, i, 2; C, adult; 
I, II, III, thoracic segments. 


IV. INSECTA: HEXAPODA 


415 


anlagen of the wings as small folds in the chitinous coat of the meso- and 
metathorax; these increase with each successive molt, until, with the last, 
they become functional wings in size, form and motion. Inside of each 
wing pad (B, i and 2) there is the anlage of the wing of the next stage. 
Since the larvae, from lack of wings, are forced to live under different 
conditions from the adults, conditions which demand special structures, 
the differences between the larva? and the adults are emphasized by the 
presence of specific larval organs. Thus the aquatic larva: of dragon- 
flies and Mayflies, are distinguished, not only by the absence of wings, 
but by different form, different shaped mouth parts, and especially by 
the tracheal gills (fig. 452), usually lost at the last molt. 

Increase in larval characters leads to complete metamorphosis. In 
order to profit as much as possible by its adaptation to its environment 
the larva retains its shape as long as possible; the gradual change to the 
adult is suppressed and the alteration in form is postponed until the end 
of the larval life, to the period between the last two molts. In this inter- 
val there is such an energetic transformation of the organism that ordi- 
nary vital functions, especially motion and feeding, are interfered with or 
rendered impossible. This last stage therefore becomes a period of rest 


FIG. 458. Larva and free pupa of May beetle, a', a", fore and hind wings; an, anus; 
at, antenna;; o, eyes; p'-p'", legs; st, spiracles. 

the pupal stage, the existence of which is important in the definition of com- 
plete metamorphosis. The more complete the condition of rest the more 
pronounced is the holometabolous development. From this point of view dif- 
ferent types of pupa? are distinguished: pupa? liberae, pupa? obtecfce, and 
pupa? coarctata?. In a free pupa (pupa liber a) the appendages stand out 
from the body (fig. 458), so that not only the segmentation of the body but 
the antenna?, legs, wings, and often the mouth parts of the imago are visible. 
Such pupa? have a certain power of motion, as, for instance, the pupa? of 
many Neuroptera and mosquitos, the latter rising and falling in the water. 
The covered pupa? (pupa: obtectff) at the moment of pupation have free 
appendages which with the hardening of the chitin become closely 


416 


ARTHROPODA 


appressed to the body, so that only indistinct contours can be seen (fig. 
459). Motion is confined to bending of the whole body, as is familiar 
in the pupie of moths and butterflies. The pupa coarctata are without 
motion because here the pupa (in structure a pupa libera) is enclosed in a 
larger coat, the last larval skin (some flies). 

The variations among larva} are even greater than with pupa}. Here 
structure is so completely under the influence of environment that with 
similar or different conditions larvae widely remote, from the systematic 
standpoint, may closely resemble each other, while those of closely related 
species may differ extremely. The leaf-feeding larva} of Lepidoptera 
(tig. 460) and Tenthreds are brightly colored, the thoracic appendages 
remaining small and reinforced by the fleshy ventral prolegs. The pre- 


1 2 


st 


FIG. 459. FIG. 460. FIG. 461. 

FIG. 459. Pupa of Sphinx ligustri (after Lud \vig-Leunis). i, eye; 2, head; 3, 
antenna;; 4-6, thoracic somites; 7, hind, 8, fore wing; 9, legs; 10, proboscis; n, abdomi- 
nal somites; 12, spiracles. 

Fn;. 460. Larva of Sphinx ligustri (after Ludwig-Leunis). n, caudal disc; p, 
thoracic feet; ps, prolegs. 

FIG. 461. Larva (maggot) of blowfly, Musca vomit or ia (after Leuckart). 


daceous larvae of many beetles and Neuroptera have long thoracic legs, 
strong mandibles, and no prolegs. Other beetle larvae, which burrow in 
wood or live in the earth, often have the legs rudimentary or wholly lack- 
ing. These lead to the maggot-like larvae, in which the mouth parts are 
inconspicuous and the distinction between head and thorax may vanish. 
Such soft-skinned annulated sacs occur in the bees (fig. 59) and other 
Hymenoptera, as well as in many flies (fig. 461); that is, larvae which live 
in an abundance of food either because of parasitism or because the 
mother has provided plenty. 


IV. INSKCTA: HEXAPODA 


41' 


From the outer appearance one would conclude that these holometabolous 
lame not only lacked the wings, but that the appendages of the imago were 
entirely absent or had an entirely different form; farther, that wings, and 
frequently antennas, legs, and mouth parts, come into existence at the moment 
of pupation, and then in remarkable size and completeness. In fact, the anlagcn 
of all these structures are formed long before pupation, often at the first molt. 
The wings of a butterfly are present in the caterpillar as small folds or processes 
of the surface which increase in size with each molt. They are not visible 
externally because they are pushed into the body and enclosed in sacs opening 
to the exterior. Such anlagen are called imaginal discs (fig. 462); with their 
recognition the distinctions between complete and incomplete metamorphosis in 


FIG. 462. Diagram of development of wings and legs from the imaginal discs of a 
fly during metamorphosis (after Lang), h, larval hypodermis; /, imaginal hypoder- 
mis; /, ', imaginal discs and legs and wings formed from them; ,v, connection of discs 
with hypodermis; .v, chitinous larval skin. 

part disappear, since in the first the structures of the imago, even if in a modified 
shape, are outlined very early. Still there remains much to be remodelled 
during the pupal rest. The muscles must be adapted to the new locom<>t<>r 
organs, the digestive tract to the altered food, the nervous system re-formed. 
Since a great part of the larval structures must be broken down to afford material 
for the reconstruction of the organs, the pulpy nature of the inside of the pupa is 
easily understood. With a rapid degeneration of the tissues this material ij so 
homogeneous that it was formerly thought that the pupa returned to the indiffer- 
ent condition of the egg. 

With the sexual life is connected, in termites and Hymenoptera (bees, 
wasps, ants), a community formation or social state, consisting in the association 
of the sexual animals ('kings' or 'drones' and 'queens'), which have only 
reproduction as their function, with 'workers' which care for and protect the 
young and form the complicated nests, but which, on account of their unde- 
veloped reproductive organs, can take no part in the perpetuation of the species. 
In the Hymenoptera these 'neuters' are rudimentary females; among the termites 
there are rudimentary males as well (fig. 464). With the termites, wasps 
and bees, this rudimentary character of the sexual organs appears to be the 
result of insufficient food in the larval stages. Some think this true of the ants 
as well; others deny it. With the ants the distinction between sexual animals 
and workers is frequently obliterated by the presence of intermediate forms 
('ergatoids' and 'ergatomorphs'). But even where the distinction is the sharp- 
est, as in bees, the workers, by proper feeding, can be made to produce eggs. 
Not infrequently there is a polymorphism among the neuters, the most irequent 
condition being" small-headed workers and large-headed 'soldiers' (termites, 
ants) . 

27 


418 


ARTHROPODA 


The communities of termites and ants are complicated farther by the intro- 
duction of other insects of various kinds (mostly beetles) as 'guests' or 
'symphila.' These, together with their young, are fed and cared for by the 
ants on account of the sweet fluid which they secrete. The ant communities are 
farther enlarged by other species kept as slaves. In the nests of many tropical 
termites and ants there are 'mushroom gardens' in the nests, cultures of fungi 
upon a layer of organic material formed of leaves chewed by the ants. 

The highly developed capacities of these communal insects, their ability 
to recognize the members of their own colony from the same species of another 
colony, the working together for the common good, for a long time led to the 
idea of a high grade of moral and intellectual development, especially in the 
bees and ants. This view was erroneous. Since the behavior of the animals 
is much the same when they are separated from their fellows in the pupal stages 
and are reared so that they have no chance to learn how to work from others, 
it follows that their acts are innate complex reflexes and are not the result of a 
conscious education. Yet they have a certain ability to learn, an ability to 
modify their acts under strange conditions. It is noteworthy that the strikingly 
similar communities of ants and termites have developed independently of one 
another, and that the same is true of wasps and bees is shown by the existence 
of both solitary and social species in both families. 

In the classification four points are of special importance: (i) The seg- 
mentation of the body; whether the segments of thorax and abdomen follow 
without change of form, or whether the thorax is sharply marked off from both 
head and abdomen. (2) The character of the wings, which in the lower forms 
are either lacking or are delicate chitinous structures, with numerous veins, 
the wings of the two pairs similar. In the higher forms a degeneration of the 
wing veins or a leathery consistence of the membrane, together with a divergent 
development, partial reduction of anterior and posterior wings may occur. 
(3) The structure of the mouth parts, and (4) the type of 
development, both described above. With these characters 
it is easy to differentiate six orders: Lepidoptera, Diptera, 
Aphaniptera, Rhynchota, Hymenoptera, and Coleoptera. 
The remaining species were formerly divided among the 
Orthoptera and Neuroptera, but these groups are not natural 
and they have been divided into more or fewer groups. Here 
the Pseudoneuroptera or Archiptera are separated from the 
Neuroptera, the wingless forms or Apterygota from the 
Orthoptera. 

Order I. Apterygota. 

At the bottom of the Hexapoda come ametabolous forms 
which lack wings and which show no evidence of having 
descended from winged ancestors. They are regarded as 
slightly modified descendants of the ancestral Hexapod. 
Besides the lack of wings they show many primitive charac- 
ters; compound eyes are poorly developed or lacking; the 
tracheal system degenerate in Collembola consists of 
isolated tracheal bushes, rarely connected by longitudinal 
trunks (fig. 437); the mouth parts are biting or piercing, 
though frequently rudimentary. Especially to be noted is the existence of 
abdominal appendages, possibly indicating a relationship to certain 'myriapods.' 
Thus in Scolopendrella (p. 434) there are small stylets on the feet and beside 
them protrusible sacs. These reappear in the Thysanura and Campodea, and 
Camapodea has in addition a rudimentary pair of appendages on the first abdom- 


FIG. 463. Lep- 
isma saccharina* 
silver fish (after 
Packard) . 


IV. INSECTA: HEXAPODA, ARCHIPTERA 


419 


inal somite; two or three additional pairs appear in other genera. The cerci 
or large 'bristles' are also regarded as abdominal appendages. 

Sub Order I. THYSANURA (Bristle-tails). Body elongate, with long 
bristles (cerci) at the hinder end. Lepisma saccharina* silver fish, common 
among old books and papers, does considerable damage. Campodca* (fig. 365). 
^racl^lis* lapyx* with caudal forceps. Sub Order II. COLLEMBOLA 
(Spring-tails). Compressed forms in which two jointed appendages bent under 
the body serve as a spring, throwing the animals (one to three mm. long) for- 
wards. Podura*; Anurida, maritima,* in tide pools; Achor elites nivalis* snow 
flea. 

The recently discovered group of PROTURA may be mentioned here. 
They lack antennae, have the first leg directed forwards and tactile, twelve 
abdominal somites, appendages on first three, two thoracic spiracles. Europe 
and India. Acerentonioti. 

Order II. Archiptera (Pseudoneuroptera). 

These represent the primitive forms of winged insects. The elongate body 
usually bears the cerci of the Thysanura. The wings are delicate and trans- 
parent, supported by a close network of nervures, both pairs being very closely 
alike. The mouth parts are of the typical biting kind, the labium frequently 
deeply cleft. These points of primitive structure are correlated with a primi- 
tive, usually hemimetabolous development. The distinction between larva 
and imago is largely one of presence or absence of wings, although larval organs 
like gills (Amphibiotica) may occur. Frequently the development is direct 
when the adults, as in some Termites and the Psocicke, are wingless. 


FIG. 464. Termes ftavipes* white ant (from Riley). a, larva; b, winged male; c, 
worker; d, soldier; e, queen;/, pupa. 

Sub Order I. CORRODENTIA. Larvae distinguished from the imagines 
by difference in size and, in the winged forms, by lack of wings. Best known 
are the TERMITID.^; (Isoptera), or white ants, which must not be confused with 
the true ants (Hymenoptera). Like the true ants, they have a well-developed 
social state, their communities resembling each other in many details as the 
'guests' (termitophiles), the mushroom gardens, etc. They differ from the ants 
in the similar segments of the body, the character of the mouth parts, the hemi- 
metabolous development and by the fact that the workers include both males 


420 


ARTHROPODA 


and females. A colony of termites, consisting usually of thousands of individuals, 
forms a nest with numerous chambers and passages. They are nocturnal, and 
they burrow, without coming to the surface, through old wood (timbers of houses, 
furniture, picture frames, dead wood in the forest, etc.). They line these 
chambers with a cement-like substance composed of refuse which has passed 
through the alimentary canal. Many species build dome-like nests, ten or 
fifteen feet high, of chewed earth. In a colony are winged and wingless indi- 
viduals, the latter with ametabolous development (fig. 464). The wingless 
forms have the sexual organs rudimentary, but, in contrast to ants and bees, 
may belong to either sex. They are frequently blind, have strong mandibles, 

and are of two kinds, the workers (c) and the large- 
headed soldiers (d). The winged forms are sexually 
functional (b). Shortly after the metamorphosis they 
swarm, and then the wings are bitten off at the base and 
'king' and 'queen' either form a new colony or enter one 
already in existence. After copulation the abdomen of 
the queen, by the formation of numerous eggs, swells to 
an enormous size (e). Since the swarming individuals 
form the prey of birds and other animals, it often hap- 
pens that a colony is left without a royal couple. In 
such cases the line is perpetuated by reserve males and 


FIG. 465. FIG. 466. 

FIG. 465. Larva of jEschna grandis (after Rosel von Rosenhof). a 1 , a'-, wing pads; 
m, mask; st, spiracles. 

FIG. 466. Ephemera vulgata (from Schmarda). The caudal bristles incomplete. 

females, sexual animals which have not completed the metamorphosis, but are 
in the wing-pad stage. The termites are able, by quantity and quality of 
food, to modify the development of the larvae and to determine which type of 
individual shall be produced. 

Allied to the Termites are the often wingless PSOCHXE, or book lice, Trades.* 
Other species are winged. Near here belong the MALLOPHAGA, bird lice, which 
live upon mammals and especially on birds. Like true lice they are wingless, 
but have biting mouth parts. Trichodectes,* dog, ox, etc; Goniodes* Nirmus* 
etc., on birds. Sub Order II. AMPHIBIOTICA. The three families differ 
in structure, but agree in having aquatic larvae with tracheal gills (fig. 453). 
All of these larvae are predaceous; Odonate larvae have a peculiar apparatus for 
capture of prey. The mentum and submentum of the labium are greatly 
elongate and when folded bring the tip like a mask beneath the mouth. 
The structure can be suddenly extended (fig. 465) and grasps the food. PER- 
LID.E (Plecoptera) ; hind wings larger. Perla,* Pteronarcys* EPHEMERID.E, 
fore wings large, hinder small or absent; May flies. Ephemera* (fig. 466), 
liii-lisca* ODONATA (Libellulidae) , wings nearly equal, hinder slightly larger; 
dragon flies, veritable insect hawks destroying numberless mosquitos. Libel- 
//</,* Eschmi* Agrion*. Sub Order III. PHYSOPODA (Thysanoptera). 
\Yings slender, fringed with hairs; tarsi bladder-like at tip; mouth parts bristle- 
like, probably used for sucking. Position uncertain. Thrips* 


IV. INSECTA: HEXAPODA, NEUROPTERA 
Order III. Orthoptera. 


42! 


Like the Archiptera these are hemimetabolous (a few ametabolous) 
and the mouth parts (fig. 444) are fitted for biting, the mentum cleft. 
On the other hand, the wings have lost the delicate membranous character 
and have become more parchment-like, the fore wings being smaller and 
serving as covers for the larger, softer, and folded hind wings, which are 
the organs of flight; the condition in these respects recalls somewhat the 
Coleoptera. The abdomen bears cerci and frequently stylets. In 
internal anatomy the large number of Malpighian tubules is noticeable 

(fig. 449)- 

Sub Order I. CURSORIA. With rather long legs fitted for rapid running. 
Only cockroaches (BLATTHXE) belong here. Wings may be absent, according 
to species, in either sex; more frequently in females. Blatta* Periplaneta* 
Sub Order II. DERMATOPTERA (Euplexoptera). Front wings short 
elytra; hind wings folded crosswise and packed beneath them, or rudimentary; 
cerci developed to a forceps-like structure; Forficula* earwigs. Sub Order 
III. GRESSORIA. Legs long, slender, adapted to walking. MAXTID*:, 
long prothorax bears a pair of long raptorial feet; praying Mantes. Phasmo- 
wantis* PHASMID.E, with short prothorax, almost exclusively tropical, Diaphero- 
mcra* walking stick. This family is noted for mimicry of twigs and leaves 
(fig. 12). Sub Order IV. SALTATORIA. Hinder legs long, strong, and for 
jumping; other pairs much smaller. Hinder femora large and muscular, 
tibke elongate and spined. Wings usually functional. Males produce sound 
(stridulate) by rubbing the anterior wings together (Locustidas, Gryllidae) <>r 
against the legs (Acridiidae). Tympanal apparatus (p. 410) on the anterior 
tibiae (Locustidae, fig. 451, many Gryllidae) or on first somite of abdomen 
(fig. 450). Females readily recognized by the ovipositor. ACRIDIID.E; antenrue 
and ovipositor short; tympani abdominal. Acridium*; Mdanoplus*, (Edipoda* 
LocusxiDyE; antennae long; tympani on first tibiae; ovipositor long, flattened. 
Hademrcus* Anabrus* wingless; Conocephalus*; Cyrtophilus* and Micro- 
centrum* katydids, GRYLLID.E, Crickets: antennae long; ovipositor long, 
cylindrical; tympani on first tibia. Grylhts*; (Ecantlius* tree crickets; Gryllo- 
talpa,* mole crickets. 


FiG. 467. Myrmeleo formicarius (from Schmarda). I, imago; 2, larva; ;,, pupa in 

its cocoon. 

Order IV. Neuroptera. 

The Neuroptera and Archiptera were formerly united, since they have the 
same wing structure and show in general appearance great similarities 


422 


ARTHROPODA 


the ant lions (fig. 467) recall the dragon flies; the Chrysopinae, the Perlidae. The 
Neuroptera, however, are holometabolous and have a resting stage, although 
the pupae (pupae liberae) are capable of some motion. 

Sub Order I. PLAXIPENNIA. Biting mouth parts. SIALID^E, wings 
well developed, larvae aquatic. Corydalis* hellgrammite (fig. 468); Sialis.* 
HEMEROBIID.-E, lace wings; wings well developed; larvae with sucking mouth 
parts, predaceous. Chrysopa* feeds on plant lice; ^fyn}leleo* ant lions (fig. 
467); larvae dig pits and capture ants, etc., which fall into them. PAXORPID^E 


FIG. 468. Corydalis cornutus,* hellgrammite, male (from Riley). 

(Mecoptera) ; mouth prolonged into a rostrum; Bittacns* Sub Order II. TRI- 
CHOPTERA (caddis flies). Wings usually large; mouth parts rudimentary, 
forming a short sucking tube which, with the wings covered with hair-like scales 
recalls the Lepidoptera; larvae aquatic with trachea! gills; build cases of foreign 
matter, stones, sticks, etc., in which they live like a hermit crab Phrvsauca * 
Hydropsyche* 

Order V. Strepsiptera. 

SIYI.OPID/E are parasitic on Hymenoptera. The six-legged larvae (fig. 469, 
3) press in between the ventral abdominal plates of bees or wasps and pupate 


IV. INSECTA: HEXAPODA, COLEOPTERA 


423 


there. The quickly flying male (2) escapes from the pupal skin; it recalls 
somewhat a beetle; has rudimentary fore wings and large hinder ones. The 
wingless, legless female (i) remains in the pupal skin and is fertilized there; she 
is viviparous. Insects infested with these parasites are 'stylopized.' The 
affinities of the order are doubtful; they are frequently included with the beetles. 
Slylops,* Xenos.* 


FIG. 469. Xenos rossi (after Boas), i, female; 2, male; 3, larva; I-1II, thoracic 
somites; a 1 , rudimentary fore wing; a'-, hind wing. 

Order VI. Coleoptera. 

The beetles are the highest Hexapoda with biting mouth parts. They 
are closest to the Orthoptera, as is shown by the structure of mouth parts 
and wings. The mandibles are strong; the maxillae (fig. 470) have 
lacinia and galea; the labium consists of a submentum (often called 
men turn), behind which the rudimentary mentum with its palpi, para- 


FIG. 470. FIG. 471. FIG. 47 _\ 

FIG. 470. Maxilla of Procrustes coriaceus. c, cardo: le, galea; li, lacinia; pm, 
palpus; st, stipes. 

FIG. 4ji.Calosonia sycophanta (after Lud \vig-Leunis). 

FIG. 472. a, pentamerous tarsus of Dytiscus; b, cryptopentamerous tarsus of 
Coccinella; t, tibia; *, reduced tarsal joint. 

glossa?, and glossae (the latter frequently fused) are retracted. The group 
is distinguished from the Orthoptera by the holometabolous development 
with pupae liberae, while the larvae (fig. 458) show many modifications 
corresponding to the mode of life. Another character is afforded by the 


424 ARTHROPODA 

wings. The anterior pair, separated at the base by a scutellum, are hard 
elvlra not fitted for flight, and from these comes the name Coleoptera, 
sheath wings. Under the elytra are protected the delicate much folded 
hinder wings, the organs of flight. Since the second and third thoracic 
rings and those of the abdomen are covered by the elytra, these are soft 
above. Externally the relations of the elytra cause a regional division 
peculiar to the beetles (fig. 471): head, prothorax, and a third division 
composed of meso- and metathorax plus abdomen covered by the elytra.- 

The numerous species of beetles over 100,000 described are divided into 
normal forms and Rhynchophora, the normal forms being subdivided upon 
characters derived from the tarsi as follows: 

Sub Order I. PENTAMERA. Tarsus five-jointed, the last club-shaped 
and bearing the claws; the other four are short and somewhat heart-shaped 
(fig. 472, a). This largest sub order contains the tiger beetles (CICINDELID.E), 
the predaceous CARABID.E (fig. 471); water beetles, HYDROPHILHX*: and 
DYTISCID.E; LAMELLICORNIA or SCARABEID^:, represented by the 'June bugs,' 
Melolontha*; fire flies, LAMPYRID.E; rove beetles, STAPHYLINID.E, etc. Sub 
Order II. HETEROMERA. First and second legs pentam- 
crous, third apparently four-jointed; few species; 'oil bottles' 
(MELOUXE) and the blister beetles, CANTHARID.E, both con- 
taining a peculiar substance, cantharidin, which renders the 
'Spanish flies,' an important ingredient of blistering plasters. 

TENEBRIONIDyE. 

Sub _ Order III. TETRAMERA (Cryptopentamera). 
Tarsi with the penult joint rudimentary, giving the impres- 
sion of four joints (fig. 472, b). The families numerous in 
species, are injurious to vegetation. The larvae of CERAM- 
BYCID.E bore in wood. The CHRYSOMELID^E (Colorado 
FIG. 473. Ba- potato beetle, Doryphora*} feed on leaves. Sub Order IV. 
laninus nasicus,* TRIMERA; tarsi with penult and antipenult joints rucli- 
hazel-nut weevil, mentary, so that they appear three-jointed. COCCINELLID.-E, 
lady birds, whose larvae, because of feeding on plant lice, 
etc., are of value to man. Sub Order V. RHYNCHOPHORA, snout beetles; 
head produced into a long snout with mouth parts at apex. Here belong 
weevils, which damage grain, nuts, timber, etc. Curculio* Conotrachdus* 
Calandra,* Balaninus* (fig. 473). 

Order VII. Hymenoptera. 

The Hymenoptera, of which bees, wasps, and ants are well-known 
representatives, have biting mandibles, while the other mouth parts are 
elongate and in a minority of the group converted into a sucking organ 
(p. 406). Since mouth parts vary, the wings and body segmentation 
have great value in defining the order. The wings are membranous and 
are supported by few nervures (fig. 474), and in flight they act as one 
pair, since the two are usually connected by hooked bristles on the hind 
wing, which engage in a groove on the hinder margin of the front wing. 
The fore wings are the larger and, correspondingly, the mesothorax 


IV. INSECTA: HEXAPODA, HYMENOPTERA 


125 


exceeds the ether thoracic somites, so that these, especially the prothorax, 
seem but parts of the strong mesothorax. Besides, the first abdominal 
ring unites to the thorax so intimately in the Entophaga and Aculeata as to 
seem part of it. The constriction which then separates thorax and abdo- 
men comes between the first and second 
abdominal somites, and when the second 
(petiole) is elongate the stalked abdomen, 
familiar in the wasps, results. 

The sexes are distinguished by the 
genital armature. The female is pro- 
vided with the ovipositor already de- 
scribed (p. 413), which when used for 
this purpose only (terebra) permanently 
projects from the hinder end of the body 
(fig. 474), but when used as a sting 
(aculeus), can be drawn into the body 
when at rest. The sting, naturally lack- 
ing in the male, is connected with a 


FIG. 474. Sirex gigas, saw fly 
(after Taschenberg). 


poison gland, the secretion of which owes its effect not, as once believed, 
to formic acid, but to a little known basic substance, possibly secreted in 
smaller accessory glands. 

The distinction between terebra and aculeus affords characters of system- 
atic importance; others are furnished by the development, which is always 
holometabolous. The pupae, in all important points, are similar (pupae liberae), 
but two kinds of larvae are distinguished. Some have well-developed legs. 
Others have footless larvae (fig. 60). The first occur where the larva must shift 

for itself, the second where it is surrounded by an abun- 
dance of food, either provided by the parents or by the 
host in which it is parasitic. 

Sub Order I. TEREBRAXTIA. Terebra present; 
larvae with feet at least on the thorax; eggs laid on leaves 
or in wood, usually without gall formation; the larvae 
therefore must move in order to feed. TI:XTI:RKDIMI> i , 
saw flies, feed on leaves, larvae caterpillar-like. Ciinbc.v,* 
Nematus.* SIRIOID.E (Vroceridie, fig. 474), horn tails, 
larvae bore in wood. Sub Order II. ENTOPHAGA. 
Terebra present; larvrc legless, parasitic in galls or in 
animals. Some use the ovipositor to lay their eggs in 
plants. Galls are then produced, diseased structures by 
which the larvae are nourished. Others lay their eggs on 
or in other insects. The young feed on the host and at 

last cause its death, often before the completion of the metamorphosis. Gall- 
producing forms are the CYNIPUX-E; some afford examples of heterogony 
(p. 132), the alternating generations distinguished bv different structure, by 
sexual and parthenogenetic reproduction, and by different kinds of galls, and 
have frequently been described as different genera. The un/niliiifs lay their 
eggs in the galls of other species. The insect parasites are divided among 


FIG. 475. Chalcis 
ftavipes* (after 
Howard). 


426 ARTHROPODA 

several families, the more prominent being the ICHNEUJIOXID.E, BRACONID.E, 
and CHALCIDID.* (fig. 475), those of the first being large, the others small or 
minute. These are of immense value to agriculture, as they keep down in- 
jurious forms as no economic entomologists or insecticides can. 

Sub Order III. ACULEATA. Females with stings, larvae footless, maggot- 
like. The digger wasps (FOSSORES) excavate tubes in earth or wood which 
they store with insects paralyzed by the sting, to serve as food for the larvae. 
Some true wasps have similar habits. Most wasps (VESPARI^E) and bees 
(APIARI/E) build wonderful homes of chewed w^ood or leaves, earth, etc., or of 
wax which the animals (bees) secrete between the joints of the abdomen. The 
nests for the young, are either small tubes or hexagonal cells which are united 
to 'combs;' the food is either honey, pollen, or chewed fruits. The fact that the 


FIG. 476. Heads of Apis mellifica (after Boas), a, queen; b, worker; c, drone with 

the compound eyes meeting above. 

offspring are better protected when numerous individuals guard them has 
apparently led to different grades of social states. The honey bees (Apis 
mellifica*), which live in a colony, consist of three kinds of individuals distin- 
guished by structure of the head (fig. 476) and other features: a single queen, 
some hundred drones, and ten to thirty thousand, just before swarming even 
sixty thousand, workers. These last are females and hence have stings, but 
have rudimentary functionless sexual organs; their work being to build the home, 
to protect the young, and provide food for the winter, and for the young brood 
honey and pollen. The queen copulates but once, at the beginning of her reign, 
when she and a drone take a wedding flight. For the four years of her life the 
sperm retains its vitality in the receptaculum seminis. In laying the eggs she 
can permit entrance or not of the spermatozoa at will and thus produce males or 
females. A queen who has not been fertilized can only lay drone eggs. The 
further fate of the eggs depends upon the food of the larvae; with a small amount 
of bee bread (pollen) workers are produced, but the same larva placed in a 
larger cell and fed with the 'royal jelly' will develop into a sexually mature 
queen. Seven or eight days before the escape of a new queen from the royal 
cell, the existing queen with a part of the hive, swarms to found a new colony. 
This operation may be repeated once or twice, but if there be danger of depleting 
the hive the remaining queen larvae are killed. Wasp and bumble-bee colonies 
last but a year with us and are reformed by a fertilized female which has lived 
through the winter. In the tropics there are perennial colonies, like those of 
the bees. 

The ants (FORMICARLE) have gone beyond the bees in the social organi- 
zation. They have also departed most from the other Hymenoptera in that the 
workers, sometimes the sexual individuals, are wingless and the sting is rudi- 
mentary or entirely lacking. Only the Poneridaa and Myrmicidas sting like 
bees and wasps; the others bite and squirt the secretion of the persistent poison 
gland (formic acid) into the wound. The homes of the ants are less wonderful 


IV. INSECTA: HEXAPODA, RHVXCIIOTA 


127 


than those of the bees, but their social organization is frequently more compli- 
cated. In the colony occur the wingless workers (rudimentary females with 
wing pads in larval life which are lost in pupation), and of these frequently 
there are different kinds, large-headed soldiers and small-headed workers; 
'honey sacs' in Myrmccocystus (fig. 477, A); and the sexual animals, queens and 
drones, which copulate in a marriage flight. The queens, after the flight found 
new colonies, in which, after biting off the wings, they enclose themselves in a 
royal chamber. They may (Dorylitue), swell like the termite queens so enor- 
mously that they were once regarded as different genera. There may be a 
number of queens in a colony, and as swarming is not a necessity, a colony may 


FIG. 477. A, M \rmecocystus melliger,* honey-sac ant. (orig.) B, Plant of 
Hydnophyton (after Forbes) showing the bulb occupied by ants. 

be enormous. It may send out other colonies which may retain relations with 
the mother colony, or may found a distinct state. Frequently other insects, 
like the Aphides, are kept for the honey dew they produce. Many ants steal 
the pupas of others and, when the adults emerge, keep them as slaves. In 
Polyergus rufescens this has gone so far that the masters cannot care for them- 
selves and must be fed by the slaves. The ants possess extreme interest on 
account of their carefully planned wars (Ecitons); on account of their relations 
to plants, some species making nests in the growing plant (fig. 477, B) and pro- 
tecting it by their bites; the leaf-cutting ants carry leaves into their underground 
nests for the cultivation of fungi on which they feed, the agricultural ants from 
their plantations and stores of grain, and the honey ants from the fact that certain 
workers (fig. 477, A) act as reservoirs of honey, these 'honey sacs' swelling up to 
enormous size. 

Order VIII. Rhynchota. 

The Rhynchota, or bugs, in external appearance are nearest to the 
Archiptera and Orthoptera. The head, thorax, and abdomen are 
joined in the same way; the development is hemimetabolous (in the 
wingless species ametabolous). Confusion with the Orthoptera has 
led to the Cicadas with their membranous wings being called locusts, 
on the other hand, the delicate-winged Aphides resemble the Archiptera. 


4-JS 


ARTHROPODA 


Yet all Rhynchota may he recognized by the sucking proboscis (fig. 478), 
consisting of the grooved labium in which the needle-like mandibles 

and maxillae play. The wing structures 
afford the basis of division into three sub 
orders. 

Sub Order I. HEMIPTERA (Heterop- 
tera). Anterior wings hemelytra, i.e., leath- 
ery at base, soft and elastic at tip (fig. 479);- 
between the hemelytra is a conspicuous 
triangular scutcllum (s) covering more or 
less of the dorsal surface. Hemelytra and 
scutellum occasionally disappear. A further 
characteristic is the presence of stink glands, 
which open in adults vcntrally on the meta- 
thorax; in larvae dorsally on the abdomen. 
According to habits families may be grouped 
into the aquatic HYDROCORES and the 
terrestrial GEOCORES Of the first the 
BELOSTOMHXE are noticeable from their size, 
Bclostoma americana* being nearly 2\ inches 
long. Other families are NEPID^E (Ratiatra* 
water scorpion), NOTONECTHXE, HYDRO- 
etc. Of the Geocores the REDU- 


FIG. 478. Head of Cicada s.-p. VIZM ' which fced on other insects; ACAN- 
tendecim, the mouth parts separated THIID - (Acanthia lecluaria* bed bug); 
(orig.). a, antenna; <>, compound LYG.ElDy, chinch bug, Blissus leucopterus,* 
eye; /, labium; md, mandible; mx, injurious to grain; and PENTATOMHXE, stink 
maxilla. bugs, may be mentioned. Sub Order II. 

HOMOPTERA. Winjjs, when not degen- 

erate, similar in texture throughout, although often differing in size. They 
are either parchment-like or delicate membranes. Frequently wax-like sub- 
stances are secreted from dermal glands and cover the surface like a down. 
The CICADID^E, Cicada,* are noticeable from their shrill notes, ' produced 
by a drum on abdomen vibrated by muscles. CERCOPID/E, the spittle bug 
(Aprophora*) causes drops of foam on grass. The leaf hoppers, or JASSHXE, 
contain some injurious forms, Erythronura vitis* damaging the grape; the 


FIG. 4-9. Pcntatoma rufipes (from Hajek). s, scutellum. 

tree hoppers, MEMBRACID.E (fig. 481), are scarcely less injurious. None 
are such serious pests as plant lice and scale insects. In the COCCHXE, or 
scale insects, the wingless female dies after laying the eggs and covers them 
with her dead scale-like body. Here belong the cochineal insects, Coccus 
cacti* which furnish carmine, the lac insects, and a host of injurious forms, 


IV. INSECTA: HEXAPODA, RHYNCIIOTA 


42'J 


FIG. 480. 


481. 


FIG. 480. Cicada septendecim* seventeen year locust (from Riley). a, pupa; b, 
)a case from which the imago, c, has escaped; d, twig bored for oviposition. 
FIG. 481. Ceresa bubalus* buffalo leaf hopper (after Marlatt). 


FIG. 482. FIG. 483. 

FIG. 482. Phylloxera vastatrix (from Ludwig-Leunis). i, winged generation; 2, 
grape root, with nodules (a) caused by Phylloxera ; 3, wirigk-ss root-generation. 

FIG. 483. Phthirius inguinalis, crab louse (after Luuckart). 


430 ARTHROPODA 

like the orange scale, Aspidotus aurantii* and the worse San Jose scale, A. 
pcniiciosus* recently spread through this country. The APHID.E, or plant lice, 
are soft-skinned and with their honey-containing excrement form a substratum 
fur the growth of injurious fungi. They reproduce largely by parthenogenesis, 
but their spread is not rapid, since the usually viviparous females are wingless. 
At times winged females appear and spread the pests. Winged males appear 
in the autumn; the fertilized eggs endure the winter. Xone is more injurious 
than Phylloxera vastatrix* of the grape, which with us does slight damage, but 
in Europe has destroyed whole vineyards. Sub Order III. APTERA. Wing- 
less bugs with direct development, commonly known as lice; three species attack 
man, one living in the hair (Pediculus eaptlis*}, the others (P. vestimentorum* 
and Phthinus inguinalis**) upon the body. Other species on other mammals. 

Order IX. Diptera. 

Like the Rhynchota, the Diptera, or flies, are sucking insects, but 
the haustellum is different, consisting of a tube, formed of labium and 
labrum, containing stylets which include, besides mandibles and maxillae 
(often rudimentary), the hypopharynx (fig. 446). Only the anterior 
win^s (hence Diptera) are developed, the hinder wings being replaced by 

the halteres or balancers, small drumstick-like struc- 
tures richly supplied with nerves and functioning as 
organs of equilibration (fig. 486). The thorax is, 
as in the Hymenoptera, sharply marked off from head 
a,nd abdomen, its somites frequently fused. The de- 
velopment is holometabolous, two kinds of larvae and 
pupae occurring. The larvae are always apodal, but 
have either a distinct head with biting mouth parts 
or ^y are headless and have a rudimentary sucking 
apparatus (fig. 484). The pupae are correspond- 
FIG. 484. Larva ingly either free with powers of motion, or are pupae 

of .4 nthom via canicn- - , , 

/ora(afterLeuckart). coarc tatae (p. 416). Development thus affords charac- 
ters of systematic importance, and these are supple- 
mented by differences in length of legs, antennae, haustellum, and in 
body form. In number of species the Diptera stand next to the 
Coleoptera; in number of individuals they far exceed them. 

Sub Order I. NEMOCERA. Elongate with long, many-jointed antennae, 
long proboscis, long legs. The larvae live in damp places or in water, where, 
lacking legs, they swim by movements of the body, capturing their prey with their 
strong mouth parts. The pupae also can swim well. The aquatic larvae have 
two respiratory tubes at the end of the abdomen; in the pupae they are on the 
back. Best known are the innocuous crane flies (TiPULiD^;) and the mosquitos 
(CuLiciD^:) with their numerous species affecting man, among them, Stegomyia* 
which carry yellow fever, and Anopheles* which distribute malaria. The CECI- 
DOMYID.-E include the injurious Hessian fly, Cecidomyia destructor* and the paedo- 
genetic Miastor (fig. 456). Sub Order II. TANYSTOMA. Resemble the 
Muscariae in the short stout bodies, short antennae and legs. Th^y resemble 


IV. IXSECTA: HEXAPODA, APHAXIPTERA 


431 


Nemocera in their long proboscis and in development. The larvae and pupae 
live in damp places or in water and move rapidly, the larvae having biting mouth 
parts. Black flies, SIMULIHXE, excel the mosquitos in viciousness; the horse 
flies, TABANIDJE. Sub Order III. MUSCARI/E (Brachycera). Body short, 
stout; antennae three-jointed with a bristle (arista) (fig. 485); legs short, ending 
in an adhesive organ (pulvUlus); larvae headless, living in decaying substances 
or parasitic in other animals. MUSCID^:; house flies (Musca domestica* and 


FIG. 485. FIG. 486. 

FIG. 485. Left, Erax bastardi, robber fly; right, antenna of Muscid showing 
arista at a. 

FIG. 486. Gastrophilus equi* hot fly (from. Hajek). h, halteres. 

other species), blow fly (Calliphora vomitoria*), the flesh fly (Sarcopliaga car- 
naria,* viviparous). Many Muscidae convey disease, the house fly carrying the 
germs of typhoid fever on its feet, while the tropical species of Glossina are 
responsible for the nagana disease of cattle and horses and for the sleeping 
sickness of man (p. 184). ASILID^;, robber flies (fig. 485) prey on other insects, 
as do some SYRPHID^;: Eristalis* CEsTRiD^:, bot flies; the larvae always 
parasitic; those of the sheep bot (CEstms ovis*) in the frontal sinuses of the 
sheep, those of the ox warble (Hypodcrma lincatti*) just beneath the skin of 
cattle; those of Gastrophilus equi* (fig. 486), in the stomach of horse. In the 
tropics Dermatobia noxialis lives as a larva in the human skin. Sub Order IV. 
PUPIPARA. Very active, often wingless forms living as parasites on mammals 
and insects; larval development inside the mother; pupation occurring soon 
after birth. Mehphagus ovinns* sheep tick; Braula c&ca,* bee louse. 

Order X. Aphaniptera (Siphonaptera) . 

In spite of the lack of wings the fleas are closely related to the Dip- 
tera, since they have doubtless descended from winged forms, as they 
have a holometabolous development. The larvae, 
long and footless, live in decaying wood or dust 
in cracks in the floor, etc., and give rise to pupae, 
both without traces of wings. Yet fleas and flies 
differ in that fleas have similar body somites and 
lack the haustellum, the sucking tube being formed 
of labrum and mandibles, while the sharp maxilla- 
puncture the skin. Besides Pulex irritans* the 
flea that attacks man, many other species occur 
on other animals. Fleas are now known to carry diseases, among them 
the bubonic plague. In warm countries the jigger or chigoe, Sarcopsylla 


FIG. 487. Pulex irri- 
tant* flea (from Blan- 
chard). 


432 


ARTHROPODA 


penetrans* attacks man, the female boring into the skin and there 
laying the eggs. 

Order XL Lepidoptera. 

This group of butterflies and moths is the most sharply limited of any 
order of Hexapods. The wings, both pairs of which are well developed 
(rarely lacking, as in many female Psychida? and some Geometridae) , are 
covered with scales (flattened hairs), and to these are due the frequently, 
brilliant color patterns. Frequentl} the fore and hind wings are united 
by hooks. The mesothorax is large and the smaller pro- and metathorax 
are closely united to it, giving the region a distinctness from head or abdo- 
men. The mouth parts are peculiar (fig. 447), although foreshadowed in 
the Phryganids, and not fully developed in the Microlepidoptera. The 
mandibles are rudimentary or absent, while the fused maxilla?, greatly 
elongate, form -the proboscis. The development is holometabolous; the 


FiG. 488. Leucaniii nnipunctata, army-worm and moth (from Riley). 

larva?, frequently called caterpillars (fig. 460), have biting mouth parts, 
the mandibles very strong; and also a pair of silk glands (sericteria), which 
open on the labium and produce a secretion hardening to silk ; besides the 
thoracic legs, prolegs (two to five pairs) are present. The pupa? are 
usually pupa; obtecta?, sometimes ornamented with golden spots, whence 
the name chrysalides often applied to them. 

Sub Order I. MICROLEPIDOPTERA. Small, inconspicuous; at rest 
holding the wings horizontally over the back. TINEID.E; the larva? form a tube 
of the food material which they carry around with them. Tinea pellianella,* 
the clothes moth. TORTRICID^E; the larva? roll leaves into a tube. Carpocapsa 
pnmoni'11,1* the codlin moth of apples. Sub Order II. GEOMETRINA. 
Moths slender, wings in pattern and shape recalling those of butterflies, but 
held horizontally when at rest; 'tongue' (proboscis) small; larva? with two, rarely 


V. DIPLOPODA 


433 


three, prolegs, known as measuring worms from their gait. Species numerous. 
Canker worms (Paleacrita vernata,* Alosophila pometaria* females wingless). 

Sub Order III. NOCTUINA. Owlet moths; with short bodies; fore wings 
usually gray and ornamented by two spots and zigzag lines which at rest cover 
the frequently (as in Catocala*) brightly colored hind wings; 1800 species in 
U. S. Hvpcna liumitli* hop worm: Aletia argillacea* cotton worm; Leucania 
unipunctata* army worm; cut worms. Sub Order IV. BOMBYCIXA, silk 
worms. Body large, woolly, usually broad dull-colored wings; occasionally 
lacking in females; proboscis frequently rudimentary; antennae long, pectinate; 
larvae with well-developed spinning powers. Most important is the silk worm 
(Bombyx won'*), native of China; others, like Tdea polyphemus,* furnish silk of 


FlG. 489. Everyx myrun (from Riley). 

value. Many damage forest trees, among them the tent caterpillars (Clisio- 
campa*) and the imported gipsy moth Ocneria dispar (fig. 72). Sub Order V. 
SPHINGINA. Hawk moths (fig. 489), body long, stout; fore wings long, slen- 
der, hind wings shorter; proboscis very long; antenna? short; larvae naked, with a 
caudal spine (fig. 459). Phlegeihontius* tomato and tobacco worms. SESIID^E, 
'clear wings,' resemble bees and wasps. 

Sub Order VI. RHOPALOCERA, butterflies. Body slender; wings held 
vertically when at rest, proboscis long; antennae clubbed at the tip; larva' usually 
spiny; puoae hung by a thread, never a cocoon. Species numerous. Vanessa 
anthpa*- lives over winter; Pieris* attack cabbages, etc.; Papilio* swallow tails. 

Class V. Diplopoda (Chilognatha). 

The Diplopoda are usually united with the Chilopoda in a group of 
Myriapoda; but while they agree in having a head followed by numerous 
foot-bearing segments, they differ so greatly that no union is possible. 
The body is nearly cylindrical, although in Polydesmids lateral outgrowths 
give it a flattened appearance; the legs are close together on the ventral 
surface, with the tracheal openings near them, while on the sides of the 
body are other openings of defensive glands, the foramina repugnatoria 
(fig. 490). Each segment of the body except the first four or five bears 
two pairs of appendages, which, with a similar duplicity in chambers of 
the heart, tracheae, ganglia, etc., shows that a fusion has occurred. The 
anterior somites bear at most but a single pair of legs; both legs and 
antennae are short. The head bears, besides the antennae, but two pairs 

28 


434 


ARTHROPOD A 


of appendages, a pair of several-jointed mandibles (fig. 491), and a pair of 
rudimentary maxilla- fused to a single plate, the gnathochilarium. 

The gonads lie ventral to the intestine far back in the body, those of the right 
and left sides enclosed in a single sac; the ducts open separately on the second 


FIG. 490. FIG. 491. 

FIG. 490. Schematic section of Diplopod (compare with fig. 439). </, digestive 
tract; g, gonad; /;, heart; r, repugnatorial gland; s, spiracle and trachea?. 

FIG. 491. Mouth parts of lulus (after Latzel). 2, mandibles of 7. molybdinus; 
3, gnathochilarium (fused maxilla;) of 7. luridus. 

somite of the trunk. The spermatozoa are not motile. The legs of the seventh 
segment of the male are used in copulation. The young escape from the egg 
with three pairs of legs, a point once thought to show resemblances to the 
Hexapoda, but which does not, for these legs are on the fourth, sixth, and 
seventh somites of the body. IULID^E, elongate cylindrical bodies; Spirobolus.* 


FIG. 492. lulus maximus (after Schmarda). 

GLOMERID.E short, capable of rolling into a ball; POLYDESMID.E. PAUROPODA: 
minute; body with twelve segments, tending to fuse to six. Pauropiis* More 
uncertain in position are the SYMPHYLA (Scolopendrella*) ; from the position of 
the genital opening they are placed here. 

Summary of Important Facts. 

1. The ARTHROPODA are animals with evident internal and 
external segmentation (metamerism) . 

2. The metamerism is expressed internally in the muscles, in the lad- 
der-like nervous system, in the structure of the heart, and in the arrange- 
ment of segmental organs and tracheae so far as these are present. 


V. ARTHROPODE, SUMMARY OF IMPORTANT FACTS IM". 

3. The outer segmentation is expressed in the rings of the chitin- 
ous coat of the body as well us in the metameric arrangement of the 
appendages. 

4. From the similarly metameric Annelida the Arthropoda are dis- 
tinguished by the presence of jointed appendages, at most a pair to a 
somite. The appendages may be divided according to function into 
antenme, jaws, accessory jaws, feet, and swimmerets. 

5. A further distinction is the grouping of the somites into regions 
of which usually head, thorax, and abdomen are recognized. 

6. The head bears the tactile and eating appendages; the thorax those 
used in locomotion (pereiopoda) , the abdomen the swimmerets (pleopoda), 
or it lacks appendages. 

7. By fusion of head and thorax a cephalothorax is produced; a 
postabdomen may be separated from the abdomen. 

8. The eyes are either ocelli or compound eyes. 

9. Hermaphroditism is rare; reproduction is by eggs; frequently there 
is parthenogenesis, rarely pa-dogenesis. The eggs usually have a super- 
ficial segmentation. 

10. The Arthropoda are divided into Crustacea, Acerata, Malacopoda, 
Insecta, and Diplopoda. 

n. The CRUSTACEA respire by gills; they usually have two pairs of 
antennae, and usually biramous feet; the reproductive ducts open near the 
middle of the body. 

12. The Crustacea are divided into Trilobitae, Phyllopoda, Copepoda, 
Ostracoda, Cirripedia, and Malacostraca. 

13. Phyllopoda, Copepoda, Ostracoda, and Cirripedia are frequently 
called Entomostraca; they have a shell gland and the nauplius as a larval 
stage. 

14. The Trilobitce are extinct forms with one pair of antenna?, and the 
body divided by longitudinal grooves into three regions. 

15. The Phyllopoda have variable segments and primitive leaflike 
feet recalling the parapodia of the annelids. 

16. The Copepoda are without shells and have biramous feet. 

17. The Ostracoda have reduced bodies enclosed in a bivalve shell. 

18. The Cirripedia are usually hermaphroditic and are sessile. 

19. The Malacostraca have 20 (21) segments, of which 7 (8) are 
abdominal; the male sexual openings are on the i3th, the female on the 
nth, segment; the excretory organ is the antennal gland; the larva is a 
zoea, rarely a nauplius. 

20. The Malacostraca are divided into Leptostraca, Thoracostraca, 
and Arthrostraca. 


436 ARTHROPODA 

21. The Leplostraca have twenty-one somites; they are closely related 
to the Phyllopoda. 

22. The Thoracostraca or Podophthalmia (Schizopoda, Stomatopoda, 
Decapoda) have stalked eyes and some or all of the thoracic somites 
fused with the head to a cephalothorax. 

23 . The A rthrostraca or Edriop/i l/ni hu la have sessile eyes and have seven 
free thoracic segments. They are divided into Amphipoda and Isopoda. 

24. The ACERATA lack antennas ; the body is divided into cephalothorax 
and abdomen; the cephalothorax bears six pairs of appendages; the genital 
ducts open on the seventh somite; the respiratory organs gills, lungs, 
or trachea develop, in connection with the abdominal appendages. 

25. The Acerata are divided into Gigantostraca and Arachnida. 

26. The Gigantostraca are large, and breathe by gills. The only 
living forms are Xiphosures. 

27. The Arachnida breathe by lungs or by tracheae derived from lungs, 
the openings to which are on the abdomen; they have a pair of chelicerse, 
a pair of pedipalpi, and four pairs of legs; they have in addition several 
pairs of highly developed ocelli. 

28. The Arachnida are divided into nine orders: Scorpionida, Phry- 
noidea, Microthelyphonida, Solpugida, Pseudoscorpii: Phalangida, 
Araneina, Acarina, and Linguatulida. 

29. The Scorpionida have chelate pedipalpi and a postabdomen 
terminated by a sting. 

30. The Phrynoidea have the first pair of legs tactile and not used in 
walking, and a continuous cephalothorax. 

31. The Microthelyphonida and the Solpugida have three 'thoracic' 
segments free. The Microthelyphonida have a long, jointed postab- 
domen, lacking in the Solpugida. 

32. The Pseudoscorpil resemble the Scorpionida, but lack the post- 
abdomen and sting. 

33. The Phalangida have very long legs and spider-like bodies. 

34. The Araneina have an unsegmented abdomen, bearing four or 
six spinnerets and numerous spinning glands. They &re divided into 
Tetrapneumones, with four lungs, and Dipneumones, with two lungs and 
two trachea-. 

35. The Acarina have cephalothorax and abdomen fused and the 
mouth parts for sucking. Several species are parasitic on man. 

36. The LinguafiiUda are complete parasites, ribbon-like and with- 
out legs; the young live in the lungs and liver. 

37. The Tardigrada and Pycnogonida agree with the Arachnida in the 
number of walking legs. Their position is very uncertain. 


VII. ARTHROPODA, SUMMARY OF IMPORTANT FACTS 437 

38. The ]\IALACOPODA are intermediate between Annelida and Insecta. 
They have indistinctly segmented bodies with parapodia-like feet, 
segmental organs, and tracheae. 

39. The INSECTA breathe by trachea.-; the head bears four pairs of 
appendages: antennae, mandibles, maxilke, labium; since tracheae are 
present the circulatory system is reduced; the reproductive organs open 
at the hind end of the body. 

40. The Insecta are divided into Chilopoda and Hexapoda. 

41. The C/iilopoda have numerous body segments with a pair of legs 
on each; behind the head are a pair of poison feet. 

42. The Hexapoda .have the body divided into head, thorax, and 
abdomen. 

43. The abdomen consists of a varying number of somites and lacks 
evident appendages. 

44. The thorax consists of three segments, pro-, meso-, and metathorax, 
each bearing a pair of legs, and meso- and metathorax usually a pair of 
wings each. 

45. The head bears, besides the mouth parts and antennae, an un- 
paired upper lip (labrum) ; two compound eyes, and usually one to 
three ocelli. 

46. The structure of the mouth parts varies with the food; they are 
either biting, licking and sucking, or piercing in function. 

47. Wingless insects usually have a direct (ametabolous) develop- 
ment with numerous ecdyses. 

48. Winged insects (and many without wings have descended from 
winged forms) have a metamorphosis in which the larva differs more or 
less from the imago (metabolous insects) ; the larva never has wings. 

49. An incomplete metamorphosis (hemimetabolous development) 
occurs when the larva with each molt becomes more like the adult, the 
wing pads becoming larger with each ecdysis. 

50. In complete metamorphosis (holometabolous development) 
the changes occur in the last molting stage, which is a stage of rest, the 
pupa. 

51. Classification of Hexapoda is based upon structure of mouth 
parts and wings as well as upon regional relations and development. 

52. The Apterygota are wingless, ametabolous Hexapoda with biting 
mouth parts. 

53. The Archiptera have biting mouth parts with incompletely fused 
labium, net-veined wings, and incomplete metamorphosis. 

54. The Orthoptera resemble the Archiptera in mouth parts and devel- 
opment, but have parchment-like wings. 


438 ARTHROPODA 

55. The Neuroptera have net-veined wings and a holometabolous 
development; the mouth parts are modified. 

56. The Coleoptera are biting insects with the fore wings changed 
to elytra; they differ from the somewhat similar Orthoptera by the com- 
plete metamorphosis. 

57. The Strepsiptera are parasitic forms allied to the Coleoptera. 

58. The Hymenoptera have partly biting, partly licking mouth 
parts; membranous wings with few nervures and holometabolous 
development. 

59. The Rhynchota are hemimetabolous or ametabolous, with 
piercing mouth parts; the bed bugs and the Pediculina are parasitic. 

60. The Diptera are holometabolous, with piercing mouth parts and 
not more than one pair of wings. The larvae of the CEstridoe are parasitic. 

61. The Aphaniptera are holometabolous, wingless, parasitic, with 
sucking mouth parts. 

62. The Lepidoptera have the wings covered with scales; labium and 
labrum rudimentary, the maxilke altered to a sucking tube; the develop- 
ment holometabolous. 

63. The DIPLOPODA have a head with three pairs of appendages; 
the trunk with double segments, each bearing two pairs of legs, the genital 
openings anterior. 

64. The term Myriapoda is frequently used to include Chilopoda and 
Diplopoda. 

PHYLUM VIII. CHORDATA. 

Within recent >ears it has been realized that a number of animals, 
formerly distributed among various groups, possess structural features of 
great importance which ally them to the vertebrates; but they lack the ver- 
tebrae and many other features characteristic of that group, so that the name 
cannot be extended to embrace them. Yet since all possess, as a temporary 
or a permanent feature, a structure known as the chorda dorsalis or noto- 
chord, the term Chordata has been adopted to include them. The notoclwrd 
is an elastic rod arising from the entoderm and coming to lie between the 
digestive tract and the nervous system (fig. 9). 

In all Chordates the anterior (pharyngeal) portion of the alimentary 
canal develops several pairs of pockets which grow outwards and fuse 
with the ectoderm. The fused portion then breaks through, and the 
pockets become converted into gill slits (branchial clefts), which, in the 
lower forms, allow the passage of water over the gills which line them. 

The central nervous system lies on one side of the alimentary canal,, 


I. LEPTOCARDII 


439 


there being no such nervous ring (Enteropneusta excepted) around the 
oesophagus, as is common in the invertebrata. This nervous system arises 
as a medullary plate on the dorsal side of the embryo around the blasto- 
pore. The edges of this plate are rolled inwards, converting it into a 
tube with nervous walls and a central canal. From this, as will readily 
be seen, when the blastopore remains open behind (fig. 500, ne), a tem- 
porary communication, the neur enteric canal, exists between the neural 
and alimentary canals. 

On the other hand the chordates share with the annelids and arthropods 
a segmentation of the body which, however, is internal and only exception- 
ally is visible from the surface. 

The Chordates include the Leptocardii, the Tunicata, doubtfully a 
group of Enteropneusta, and the Yertebrata. 

SUB PHYLUM I. LEPTOCARDII (CEPHALOCHORDIA, ACRANIA). 

The Leptocardii contains a few very similar forms. One of these, 
originally described as a mollusc, is comparatively simple in structure. 
The fish-like body, pointed at both ends (whence Amphioxus} lacks 
paired appendages but has a median fold, developed into a fin at the 
caudal end (fig. 493), and there is a ventral longitudinal fold on either 


a 


FIG. 493. Amphioxus lanceolatus (diagram after Boveri). a, anus; an, eye; b, 
peribranchial space;c, notochord;g, gonads; /, liver; m, muscles; n, nephridia; o, mouth; 
p, atrial opening; r, spinal cord; sp, gill slits. 

side of the peribranchial chamber (fig. 494). The epidermis is but a 
single cell in thickness, paralleled only in invertebrates, and through it 
the underlying muscle segments can be seen. Amphioxus differs from the 
fishes in the lack of skull (Acrania), vertebrae, brain, heart, and kidneys, 
although excretory organs and the rudiments of a brain are present. Con- 
nective tissue is scanty; the body is largely a much folded epithelium, 
separated by thin gelatinous layers, into which cells wander, the begin- 
nings of a mesenchyme. 

The notochord, which extends the length of the body (fig. 493, c) is 
the axial skeleton. Above it is a tubular spinal cord, expanded in front 


440 


CHORDATA 


to a rudimentary cerebral vesicle, with a pigment spot, the rudimentary 
eye (an), but other places in the spinal cord are sensitive to light. The 
neural canal long remains open in front, the opening (neuropore) being at 
the bottom of a pit, once regarded as an olfactory organ. 

The alimentary canal begins with the oval mouth (0) , surrounded by 
cirri; then comes a fold (velum], followed by the pharynx, perforated by 
numerous gill clefts, which extend over a third of the alimentary canal. 
Between the clefts are elastic gill arches (fig. 494, kb) to support the walls. 
In the young the clefts open directly to the exterior; then a fold grows 


-sn 


S22 


-I 


FIG. 494. Section of the gill region of Amphioxus (after Lankester and Boveri). 
a, aorta descendens; b, peribranchial space; c, notochord; co, ccelom (branchial body 
cavity); e, hypobranchial groove, beneath it the aorta ascendens; g, gonad; kb, gill 
arches; kd, pharynx; /, liver; m, muscles; n, nephridia, on the left with an arrow; r, 
spinal cord; sn, spinal nerve; sp, gill slit. 

down on either side, the two folds uniting below to enclose a peribranchial 
chamber (b) from which the water escapes by an opening, atriopore (493,), 
behind the middle of the body. On the floor of the pharynx is the hypo- 
branchial groove, a ciliated tract which conducts food to the digestive part 
of the canal. This ends at the anus (a) , on the left side of the body and has 
a liver connected with it in front, extending into the peribranchial chamber 
(figs. 493, 494, b). The vascular system, with colorless blood, consists of 
a dorsal arterial (a) and a ventral venous trunk connected by lateral loops 


II. TUNICATA 


441 


or arches. The ventral trunk begins as a subintestinal vein under the 
intestine, branches as a portal vein over the liver and, reuniting again in 
a ventral vessel, and joined by the paired veins (jugulars, cardinals and 
Cuvierian ducts) continues forward, as the aorta ascendens, below the 
gills. From this the gill arteries pass up between the gill slits and form 
the dorsal vessel, the aorta de- 
scendens. A true heart is lacking, 
but various parts of the vessels a 
part of the ventral trunk and the 
bases of the gill arteries are con- 
tractile, whence the name Lepto- 
cardii. 

The digestive portion of the 
tract lies in a true ccelom, which 
extends forward (fig. 494, co) into 
the gill-walls (branchial ccelom) and 
into the outer walls of the peri- 
branchial chamber (peribranchial 
ccelom). In a distinct part of the 
peribranchial ccelom are the gonads 
(g), a series of pouch-like cell folli- 
cles which allow their products to escape into the peribranchial chamber. 
Into this chamber also empty the excretory organs (n), a series, on right 
and left sides, of ciliated canals. Each canal begins with at least one 
ciliated nephrostome in the ccelom and opens separately and has the 
characteristic solenocytes like an annelid nephridium (fig. 495). 

Like the structure, the development is comparatively simple. The following 
points deserve special mention: (i) The eggs have a nearly equal segmentation 
(fig. 101). (2) A typical invaginate gastrula (fig. 107) occurs. (3) The meso- 
derm arises as a series of pouches, right and left, from the mesenteron, which 
later separate and form the primitive segments. Hence these are clearly 
mesothelial in nature. From the cavities of these arises the body cavity, which 
is consequently an enterocoele. (4) The dorsal surface of the entoderm between 
these coelomic pouches separates from the rest and forms the notochord, which 
lies between the digestive tract and the nervous system. (=;) The nervous 
system arises from a longitudinal groove which becomes folded into a tube and 
is connected for a while with the digestive tract by a neurenteric canal. 

Amphioxus* Asymmetron,* Hcteroplrnron. The animals bury themselves 
in the sand, with only the mouth above the surface. 

SUB PHYLUM II. TUNICATA (UROCHORDA) . 

The adult Tunicata, or sea-squirts, bear some resemblance to the 
clams in the possession of a mantle and incurrent and excurrent orifices, 
usually close together. Hence they were long called molluscs; later they 


FIG. 495. Excretory tubules of Am- 
phioxus (after Boveri and Goodrich), 
i, a whole canal with several nephro- 
stomes and the connected bunches of 
solenocytes. A", upper end of gill cleft; 
P, opening of canal into peribranchial 
chamber. 2, a bit of canal wall with 
several solenocytes. 


442 


CHORD ATA 


were grouped with the worms, but their development shows them to be 
more nearly related to the vertebrates. 

The name is due to the tunic or mantle lacking in the Copelatae 
an envelope (fig. 496, /) formed, like a cuticle, by the epithelium of the skin, 
but distinguished from ordinary cuticula by its structure. It resembles 
connective tissue in that cells from the mesoderm wander into the ground 
substance, which is sometimes fibrous, sometimes homogeneous, and has 
an interesting chemical nature. It has the same chemical composition 


FIG. 496. Diagram of a tunicate (orig.). a, atr'.um; />, nervous ganglion; e, endo- 
style; /, intestine; m, mouth; H, subneural gland; s, stomach; /, tunic. In the centre the 
branchial basket \vith the gill slits communicating with the peribranchial space, and 
this in turn with the atrium. 

(C 6 H 10 O 3 ) as cellulose and agrees with this substance, so characteristic 
of plants, in its reactions. No other animals have so much cellulose. 

The anterior part of the digestive tract is modified into a pharynx 
or branchial chamber, the walls of which are perforated with a varying 
number of gill slits, these leading either directly to the exterior or, more 
usually, into a peribranchial chamber, and from this to a cloaca or atrium 
(a), before reaching the outside world. While the respiratory water 
passes through the gill slits the food particles which it contains are re- 


II. TUNICATA: COPELAT.E 443 

ceived by a ring-shaped ciliated band (peripharyngeal band) and, envel- 
oped by mucus, are led to the oesophagus. This mucus is formed by 
a ciliated glandular groove, the endostyle (e) or hypobranchial groove, 
on the ventral surface of the pharynx. 

Between the end of the endostyle and the stomach lies the ventral 
heart enclosed in a pericardium. It has the peculiarity, met nowhere else, 
of changing the direction of its contractions at frequent intervals ; after the 
heart has driven the blood for a time to the gills it stops and then forces 
the blood in the opposite direction, pumping it from the gills towards the 
stomach. If we add to the foregoing that a dorsal ganglion and a usually 
hermaphroditic gonad are present, the striking features of the group are 
enumerated. The extreme forms, the Copelate and the Thaliacea, are 
rather remote, but they are connected by intermediate forms, the Ascidke 
and Pyrosomas. 

Order I. Copelatge. 

These small forms one or a few centimeters in length are pelagic; they 
have the anterior end inserted in a gelatinous envelope or 'house' which replaces 
the lacking tunic and which they may leave without injury. They swim like a 
tadpole by means of a tail which arises from the hinder end of the trunk. The 
alimentary canal (fig. 497) is bent on itself, and both it and the two large gill 
slits, in contrast to all other tunicates, open directly to the exterior. The nervous 
system consists of a cerebral ganglion, with beside it a very simple statocyst and 
a ciliated groove, and farther a chain of ganglia extending into the tail. The 
gelatinous notochord, enclosed by a sheath of cells, forms the skeletal axis of the 
tail ventral to the nerve cord and gives attachment to muscles. Oikopleura,* 
Appendicularia* 

Order II. Tethyoidea (Ascidiaeformes). 

With the exception of the pelagic Pyrosomidae all of the ascidians are 
attached to rocks, etc., in the sea. The necessity for protection caused 
by this sedentary life has resulted in a great development of the cellulose 
tunic or test, which gives these animals a swollen, somewhat shapeless 
appearance. Two openings, mouth and atriai opening, lead into the 
interior, and the water which issues from these, when the animals are 
taken from the ocean, has given them the common name of 'sea-squirts.' 

On removing the tunic, which is but slightly attached to the other 
parts except at mouth and atriai opening, a muscular sac is seen (fig. 498), 
the fibres running circularly and longitudinally. Inside this sac are the 
viscera, the pharyngeal region by far the most conspicuous. The mouth 
leads to a short tube with tentacles (/), and then to the pharynx, a wide 
sac which lies in a large peribranchial chamber, the walls of the pharynx 
and the enclosing space uniting on the ventral side (fig. 496). The 
pharyngeal walls are perforated like a net by small ciliated gill slits, 


444 


CHORDATA 


A 


B 


9 


FIG. 497. Oikoplcura cophocerca (after Fol.). A, the whole animal, removed 
from its 'house,' dorsal view; B, bodv, side view with base of tail, a, anus; c, noto- 
chord; a', branchial region; d", stomach; en, endostyle;/, ciliated peripharyngeal bands; 
g, g', brain and first ganglion of tail; h, testis; m, mouth; o, ov, ovary; s, gill slits. 


B 


FIG. 498. dona intestinalis. A , from the left side, the cellulose tunic and dermal 
muscular sac removed; B, from the right side, the tunic entirely removed, pharynx 
opened from the mouth, a, anus; c, cellulose tunic, below with adhesive processes; 
cl, cloaca; d, rectum; e, atrial opening; en, endostyle ending above in the peripharyngeal 
band; g, ganglion; h, mouth of the 'hypophysis'; he, heart, with pericardium; ho, 
branched testes; /, oral opening; .k, gill sac; m, muscular sac; oe, oesophagus; od, ovi- 
duct, the black line beside it the vas deferens; m>, ovary; s, partition between atrium 
and body cavity; st, stomach; t, crown of tentacles. 


II. TUNICATA: TETHYOIDEA 


445 


FIG. 499. dona intesti- 
nalis, a bit of the wall of the 
gill sac enlarged to show the 
gill slits. 


arranged in longitudinal and transverse rows (fig. 499), through which 
the water received from the mouth passes into the peribranchiai chamber, 
thence to the atrium, and so to the external world. 

While the respiratory water thus passes out in a nearly direct course, 
the food particles which it contains pass into the digestive tract. By 
means of the peripharyngeal band just inside of the tentacles and sur- 
rounded by mucus secreted by the endostyle the food is carried back to the 
oesophagus (oe) at the base of the gill chamber, 
and thence to the stomach (usually provided 
with liver glands), and on to the intestine. The 
anus is at the base of the special portion of the 
peribranchiai chamber, which also receives the 
genital ducts and hence is known as the cloaca 
or atrium. 

In the body cavity, which is greatly reduced ", 
in the species with compact bodies, occur the |j 
digestive tract, the sexual organs, and the 
heart; the latter (lie) frequently S-shaped, ex- 
tends between the stomach and the endostyle. 
Opposite to the endostyle is the ganglion (g) 
in the dorsal wall between oral and atrial openings. Near it is a 
branched subneural gland which has been compared to the vertebrate 
hypophysis. In many there exist special excretory organs, numerous 
blind vesicles filled with excreta. 

From the eggs are hatched small swimming tadpole-like lame (fig. 500), 
resembling Appcndicularia and, like it, consisting of trunk and tail, in which 
the chordate features are strongly marked. The digestive tract is confined to 
the trunk; dorsal to it lies the tubular nervous system in which can be recognized 
a vesicular brain with a simple eye and a statocyst imbedded in its walls; farther 
back a narrower portion ('medulla oblongata'); lastly, a spinal cord extending 
into the tail. In the axis of the tail is a notochord which extends forward a short 
distance into the trunk between digestive tract and nervous system. In the 
metamorphosis of the free larvae into the sessile ascidians four processes are 
important: (i) The larvae attach themselves by means of three ventral anterior 
papillae; (2) The tail is retracted and absorbed; (3) The body becomes more 
or less spherical by development of the tunic; (4) Two dorsal imaginations 
are formed, these envelop the pharyngeal region, fuse and form the atrium 
and peribranchiai chamber. It is to be noted that this arises from the dorsal 
surface and extends ventrally, while the peribranchiai chamber of Amphioxns 
arises by folds which grow ventrally over the pharynx. Besides sexual reproduc- 
tion many ascidians reproduce by budding. This results in the formation of 
colonies, a matter of systematic importance. 

Sub Order I. MONASCIDLE. Simple ascidians of considerable size. 
The CLAVELLINID^E produce small colonies by basal budding, each individual 
with ils own test; Perophora* CYNTHIID.E, oral and atrial openings four-lobed; 


44;) 


CHORDATA 


Cviitliia* MOLGULID-, oral opening, six-lobed, atrial four-lobed. Molgula* 
Kni;\ra* Sub Order II. SYNASCIDL3E. Compound ascidians consisting 
of numerous small individuals imbedded in a common tunic. Usually (fig. 502) 
the individuals of a colony are divided into small groups, the oral openings of a 
group forming a rosette around a common central atrium. Distaplia,* Lepto- 


FiG. 500. Development of an Ascidian (after Kupffer and Kowalevsky). i, 
larva, just hatched; 2, cross-section through the tail of a slightly younger larva; 3, much 
younger stage, formation of notochord and nervous system; 4, anterior end of a larva 
just before attachment. (2, Phallusia mentula; 3, 4, Ph. mammillata.) au, eye; c, 
notochord; cl, tunic; d, digestive tract; d', its nutritive, d", its respiratory division; e, 
atrial vesicle; ek, ectoderm; en, entoderm; h, brain; i, oral invagination; m, muscles of 
tail; n, neural tube; ne, neurenteric canal; o, otocyst; p, adhesive processes. 


.4 


B 


FIG. 501. FIG. 502. 

FIG. 501. A. Molgula manhattensis*; B, Eugyra pillularis* (from Verrill). 
FIG. 502. Botryllus violaceux (after Carpenter). .4, small colony of eighteen 
individual groups; B, two inaividual groups somewhat enlarged. 

clinum* Polyclinum* Amaroudum* Botryllus* Sub Order III. LUCI/E. 
Free-swimming pelagic synascidians, having the form of a hollow cylinder closed 
at one end; the animals vertical to the axis of the cylinder, oral apertures on 
the outside, atrial in the central cavity. Pyrosoma, very phosphorescent, 
tropical, some species four feet long. 


II. TUX1CATA: TIIALIACI \ 


44; 


Order III. Thaliacea (Salpaeformes). 

These, like the Lucias and Copelatse, are pelagic, and play an important par- 
in the plankton. In form a Salpa may be compared to a barrel consisting extert 


B 


FIG. 503. .4, Z?, Salpa democrat ica with stolon, ventral and lateral views; C, Salpa 
miuronata, part of a young chain not yet separated, a, anus; c, tunic; d, digestive tract; 
e, atrial opening; en, endostyle;/, peripharyngeal groove: g, ganglion _with horseshoe- 
shaped eye, and near it the tentacle and hypophysial groove; h, testis; i, mouth; k, gill; 
m, muscle hoops; st, stolo prolifer. 

nally of a cellulose tunic, lined internally with six or eight circular muscles, not 
always closed rings, li.ce hoops. By their contraction 
the water is expelled through the posterior or atrial end 
of the body, while on their relaxation fresh water enters 
the other or oral aperture. By the reaction the ani- 
mals swim through the water with the oral end for- 
ward. The cavity of the barrel corresponds to pharyn- 
geal and peribranchial chambers of the ascidian. In 
the Dolioliidae the two chambers are separated by a 
partition perforated by gill slits (fig. 504) ; in Salpa the 
partition is reduced to a bar with transverse rows of 
cilia so that branchial and peribranchial chambers are 
not distinct; yet the endostyle and the peripharyngeal 
band are retained. 

The viscera lie in the muscular sac, where the bran- 
chial bar and the endostyle meet and are usually com- 
pacted into a mass, the 'nucleus' (intestine, liver, gon- 
ads, heart). The ganglion is distinct and lies dorsally 
opposite the endostyle, just in front of the branchial 
bar. Associated with it is a horseshoe-shaped eye. 

For a long time two kinds of Salpa! have been known, one solitary, the other 
consisting of numerous individuals connected together like a chain or a rosette 


m 


FIG. 504. Doliohim 
denticulatwn. (For ex- 
planation of letters see 

% 53-) 


1 IS 


CHORDATA 


(fig. 503, C). At the beginning of the last century the poet Chamisso discovered 
that the chain salps were produced by the solitary individuals, and that these in 
turn came from the chain form, the first instance of alternation of generations. 
The solitary salp is asexual; gonads are lacking, but near the hinder end is a 
budding cone or stolo prolifcr from which colonies of salps bud one after another. 
When the first is separated a second matures and a third begins. These colonial 
forms, the chain salps, are sexual, and each produces a single egg from which a 
solitary individual is formed. 

Since both the solitary and the chain forms have received names, the species 
oiSalpa* now have double names likeSalpa dcmocratica-ir.ucronata, democrat ua 

being the asexual, m-ucronala the sex- 
ual, individual, etc. From the true 
Snip? Doliolum* is distinguished 1 y 
the better developed gills, the complete 
muscular bands, and a more compli- 
cated alternation of generations. 

SUB PHYLUM III. ENTEROPNEUSTA 
(HEMICHORDIA) 

The few marine forms here in- 
cluded are decidedly worm-like, and, 
like many worms, they burrow in the 
mud. The body consists of three 
parts proboscis, collar, and trunk 
(fig. 506). The proboscis, which sits 
in the collar like an acorn in its cup, 
whence Balanoglossus, contains a 
cavity opening to the exterior by a 
dorsal pore, while two similar cavities 
in the collar open separately. These 
can be filled with water, and by alter- 
nately enlarging and contracting these 
parts the animal is able to burrow. 
The mouth (fig 505) lies ventral and 

front of the collar and leads into a 


L h 


cc 


CO 


n 


n 


FIG. 505. Sagittal section of (Jlossj- 
balanus mimitus (diagram after Spengel). 
c, proboscis coelom; cc, colar ccelom; co, 
collar; h, so-called heart; /?, long muscles; 
', '-, dorsal and ventral nerve cords; o, 
oesophagus; p, proboscis; nc, so-called 
notochonl; , gill slits; v 1 , v 2 , dorsal and 
ventral blood-vessels; m, mouth. 


in 


digestive tract, which in its anterior 
part is perforated by numerous paired 
gill slits, whence the name Entero- 
pneusta, while the part behind it is 
covered with hepatic caeca. The in- 
testine is supported in the coelom by 
dorsal and ventral mesenteries, and is 
accompanied by a dorsal and a ventral blood-vessel, to which are added 
lateral canals and numerous anastomoses. A contractile vesicle on the dorsal 
vessel in the proboscis is called the heart. The nervous system is very peculiar. 
There is a dorsal portion lying in the collar region, which is produced by 
inrolling, as is the central nervous system in the Chordates, and a ventral part, 
as yet lying in the ectoderm, the two being connected by nerves in the collar. 
The gonads are numerous follicles lying between gill and liver regions and open- 
ing to the exterior. 

The systematic position of the Enteropneusta is uncertain. In the possession 
of gill slits and in the formation of the dorsal nervous system it closely resembles 
the other chordates, and the resemblance is strengthened by similarities in details 
of structure of the gills. The advocates of this view recognize the notochord in 


IV. VERTEBRATA 


449 


a blind tube, surrounded by tough membrane and thickened beneath, which 
extends from the pharynx into the proboscis. Embryology throws little light on 
the problem. Some species have a direct development (fig. 507, B, C), while 
others have a larva (Tornaria, A) which so resembles the larvae of certain echino- 
derms that it was long held to belong to that phylum. The chief resemblances 
are in the relations of the ciliated bands to the alimentary tract and in the pres- 
ence of the proboscis cavity, which, like the ambulacral system, opens to the 

exterior. The old genus Balano- 
glossus* has recently been sub- 
divided. Two deep-sea forms, 
Cephalodiscus and Rhabdoplciira, 
have the same type of 'notochord,' 
and the first has a pair of gill slits; 
in other respects these are strikingly 
Polyzoan in appearance. 

SUB PHYLUM IV. VERTEBRATA. 

In the vertebrates only the 
internal segmentation occurs. 
This is shown most clearly in 
the lower Vertebrata, in the 
muscles (myotomes, myomeres), 
the myosepta which separate 
them, and the protovertebra? from 


FIG. 506. 


FIG. 507. 


FIG. 506. Balanoglossus kowalewskii* (from Korschelt-Heider, after A. Agassiz). 
db, dorsal blood-vessel; e, proboscis; g, sexual region; k, gill region; kr, collar; vb, 
ventral blood-vessel. 

FIG. 507. A, Tornaria larva of Balanoglossus (after Morgan), a, apical plate; 
ac, preoral part of ciliated band; be 1 , be'-, be*, ccelomic pouches; m, mouth; p, postoral 
part of ciliated band B, C, two stages of Balanoglossus with direct development 
(after Bateson). a, anus,, be, branchial clefts; c, collar; dc, digestive part of alimentary 
canal; in, intestine; nc, 'notochord'; p, proboscis. 

which they arise; in the nerves (neurotomes), the skeleton (sclerotomes), 
the blood-vessels, and in the excretory organs (nephrotomes). In the 
higher vertebrates this metamerism is best seen in the embryonic stages. 
In part the absence of external segmentation has its cause in the heter- 
onomy (p. 126) of the body and the obliteration of segmental boundaries, 
consequent upon the union of somites into regions, of which at least three 

29 


4.30 CHORDATA 

head, trunk, and tail at most six head, neck (cervical), thorax, 
lumbar, pelvic (sacral), and tail (caudal) occur. Not less important in 
this respect is the character of the skeleton. The cuticular skeleton, the 
cause of the annulation of the arthropod, is entirely lacking. The skin 
remains soft, or contributes to a subordinate degree, more for protection 
than for support, to the formation of hard parts (dermal skeleton of fishes, 
alligators, turtles). The firmer tissue is formed in the axis of the body, 
which, in the lowest vertebrates and the embryos of the higher, appears as 
the notochord already mentioned, but in the higher is supplemented by the 
vertebral column and skull. 

The skin of the vertebrates is distinguished from that of all invertebrates 
by (figs. 27, 28) the many-layered epidermis, and the thickness of the cor- 
ium. The epidermis is rarely covered by a delicate cuticle (fishes, fig. 
27, cs); usually such a protection is unnecessary since, especially in the 
land forms, the superficial layers become cornified and hence furnish the 
necessary resistance without a cuticle. There are two epidermal layers, 
the deeper stratum Malpighii and the superficial stratum corneum (fig. 
27, sM and sc; ). 

The second constituent of the integument, the corium (cutis, derma), 
arises from the mesenchyme. It consists of many layers of close connect- 
ive tissue, and is usually separated from the underlying structures, especi- 
ally the muscles, by a loose tissue rich in lymph spaces, the subcutaneous 
tissue. Both of these constituents of the skin, aside from their own firm- 
ness, can give rise to protective structures. The horny layer of the epider- 
mis in places becomes greatly developed and thus forms the tortoise shell 
of the turtles, the scales and scutes of the snakes and lizards, the feathers 
of the birds, the hair and horns of the mammals. Other epidermal prod- 
ucts are the claws, nails, and hoofs of the terrestrial vertebrates. The 
corium is often the seat of ossifications which, in contrast to the deeper 
bones, are called the dermal skeleton. 

The firmness of the vertebrate skin may be increased in three ways: I. Bony 
scales develop in the corium which project into the epidermis and receive from it 
a horny outer coat, the horny scale (fig. 509, //). 2. The bony scales are lacking 
but the horny scales are formed (fig. 509, /). 3. The bony scales are developed 
but the epidermis remains soft, no horny scales being formed. 

Hoofs, claws and nails (fig. 508) are epidermal structures to be traced back 
to horny scales, one on the upper, the other on the lower side, enclosing the end 
of the digit. The first, the claw plate (/>) is the more important. In the mam- 
mals it grows back more and more into a pocket (w) the root of the plate, from 
which it extends distally over the upper side of the digit, the claw bed. In 
claws (imbues) the claw plate is curved in both directions, longitudinally and 
transversely (fig. 473, ///) reducing the lower claw sole (s). In the hoof (unguld) 
the claw plate is curved transversely (7), the claw sole (s) being reduced to a 


IV. VERTEBRATA 


.-. I 


band, following the contour of the plate. The nail (lanma) is nearly flat, and 
since the sole is reduced, the nail appears as a purely dorsal structure (II). 

Of great importance in understanding the dermal ossifications is the 
fact that all scales of fishes are derived from the placoid scale of the selach- 
ians. These are rhombic plates, bearing in the middle pointed spines, 
called dermal teeth from similarity in structure and development to the 
teeth of the mouth cavity (fig. 510). They consist of dentine (d) and 
have a large pulp cavity (p), with numerous blood-vessels in the interior. 


FIG. 509. FIG. 510. 

FIG. 508. Long sections through the toes of 7, horse; //, ape; II, dog. b, ball of 
toe; p, claw plate; s, sole plate; w, root of claw; 2, 3, second and third phalanges. 

FIG. 509. Sections (schematic) of scales of reptiles. /, snake; II, 'blind worm.' 
I, corneum; 2, Malpighii; 3, corium; 4, bony scale. 

FIG. 510. Sagittal section of a scale of Scyllium stellare (after Hofer). b, basal 
plate; d, dentine; p, pulp cavity; sch, enamel. 

Whether the thin layer (sch) covering the tip can be called enamel is dis- 
puted. Dermal teeth and true teeth are identical structures which, from 
different position and consequent difference of function, have developed 
differently. 

The scales of fishes have a further anatomical interest, since from them 
have arisen, besides the bony plates which form the armor of the turtles, 
alligators, and many mammals (armadillos), important parts of the axial 
skeleton, the secondary or membrane bones. A membrane bone is a bony 


4.VJ 


CHORDATA 


plate which has arisen from a fusion of dermal ossifications, becomes trans- 
ferred to a deeper position, and contributes to the completion of the axial 
skeleton. From what was said above about the relations of dermal and 
true teeth it is readily seen that the lining of the mouth cavity is a 
source of membrane bones. 

In describing the axial skeleton, the notochord comes first. This has 
already been mentioned in connexion with lower Chordates. It persists 
in the cyclostomes, but from them upwards it is gradually replaced by the 
vertebrae arising around it. It is of entodermal origin (fig. 9), arising as a 

longitudinal band of the epithelium of the 
archenteron (/, cli), and, becoming cut off, 
comes to lie in the long axis of the body be- 
tween digestive tract and nervous system (77, 
777) ; here it forms a cyclindrical rod consisting 
of a connective tissue which, as already said, 
resembles plant tissues because of the vesicular 
nature of its cells (fig. 39). 

In transverse section (fig. 511) the noto- 
chord is surrounded by three layers, internally 
by a fibrous notochordal sJieat/i, then an elastic 
layer (not always present), the elastica externa, 
so called because an elastica interna sometimes 
occurs inside the noto'chordal sheath; and 
lastly a skeletogenous layer (SS). This last 
is a connective-tissue layer and is therefore 
connected with the other connective tissues 
which surround muscles, nerves, etc. It de- 
serves special mention because in it the cartilages 
and bones arise from which the vertebra? and 
skull are formed. Cells from it can penetrate 
the notochordal sheath, converting it into 
fibrous cartilage, thus enabling it to participate 
in the formation of the vertebrae. 

Since the notochord and its envelopes are elastic and give under the 
strain of the muscles, they are unsegmented. The segmentation of the 
axial skeleton begins with the appearance of firmer tissue as carti- 
lage and bone. Then there is a separation of successive parts, and with 
this the gradual formation of vertebral column and skull. In both 
there is a connected series of modifications, whether studied onto- 
genetically or comparatively, from the lower to the higher forms. 

The first parts of the vertebral column to appear are the upper 


FIG. 511. Transverse 
section of axial skeleton of 
Petromyzon (from Wieders- 
heim). C, notochord; Cs, 
notochordal sheath; Ee, 
elastica externa; F, fatty 
tissue; M, spinal cord; P, 
its meninges; Ob, upper 
process of skeletogenous 
tissue; 55, skeletogenous 
tissue; Ub, lower process of 
(skeletogenous tissue.) 


IV. VERTEBRATA 


453 


(Cyclostome) and lower (figs. 511, 512), or neural and Iiamal arches. 
These consist of paired parts in the skeletogenous layer which abut 
against the notochord, and which are usually a pair to the somite, although 
occasionally two or more pairs, the arches proper and the intercalaria, 
may occur. The neural arches enclose a spinal canal surrounding the 


FIG. 512. Yertebnc of sturgeon. cJi, notochord;/, exit of nerve; /, dorsal and 
ventral intercalaria; H, neural canal; ob, neural arch; s, chordal sheath; r, ril>; ul>, ha>mal 
arch. Bone white, cartilage dotted. 


A 


PsPf 


FIG. 513. FIG. 514. 

FIG. 513. Caudal vertebras of a carp, section (.1) and nearly side view (7>). ch, 
space rilled by notochord; h, haemal arch; n, neural arch; ob, neural spine; ul>, ha-nuil 
spine. 

FIG. 514.- Thoracic vertebra, ribs, and sternum of a mammal (from \Yiedrrs- 
heim). Ca, capitular head of rib; Co, neck of rib; ( '/>, bony rib; A'w, cartilaginous lib; 
Ps, spinous process; Pt, transverse process (diapophysis) ; 5/, sternum; Tb, tulx-nular 
head of rib; \VK, vertebral centre. 

spinal cord, the parts of the arch, neurapophyses, uniting above the cord 
to form the spinous process (frequently independent). In the caudal 
region, in the same way, lice mat arches may be formed of hamapophyses 
and lucrnal spine, the arches surrounding the blood-vessels of tb" tail 
(fig. 513). In the trunk region the ventral arch behaves diffrruuly. 


454 CHORDATA 

Since the large body cavity with its viscera is here, the haemapophyses 
extend outwards and downwards and are divided into two parts, a basal 
apophysis and a lower movable portion, the rib (fig. 512). Also the union 
of htemapophyses with haemal spine does not occur; the ribs are either 
free (fishes) or are (at least in part) connected ventrally by a breast bone 
or sternum (amniotes, fig. 514). The sternum is a derivative of the ribs. 
In development the ventral ends of the ribs of a side fuse and then these 
fused tracts of the two sides unite to form the sternum. 

The hzemal arches lie medial to the longitudinal muscles of the body, and in 
the trunk region they lie in the same position just beneath the peritoneum. 
These are hcemal ribs and are found only in teleosts and ganoids. The ribs of 
all other vertebrates are morphologically different and are called lateral ribs, 
They develop in a horizontal connective-tissue septum which extends out through 
the longitudinal muscles from the axial skeleton to the skin, dividing the muscu- 
lature into dorsal (epaxiat) and ventral (hypaxial) portions (fig. 92). In the 
elasmobranchs these lateral ribs are attached to the haemapophyses, in the others 
to diapuphyses, which arise from the neuropophyses, and parapophyses, which 
arise from the vertebral centres. In the caudal region, often also in the cervical, 
lumbar, and sacral regions, the lateral ribs and dia- and parapophyses fuse to 
form transverse processes. These occur together with haemal arches in the 
tails of many Amphibia and reptiles and some mammals, the haemal arches 
forming the chevron bones which, as in fishes, enclose the caudal blood-vessels. 
The presence of intercalaria in cyclostomes, sharks, and ganoids indicates that 
primitively a double vertebra arose in each somite. Paleontological and 
embryological researches on reptiles support this view. 

In most vertebrates either the basal ends of the arches broaden out 
around the notochord and fuse with one another, or perichordal cartilages 
arise independently, furnishing in either case firm supports, the vertebral 
bodies, or centra, for the system of arches. These increase in size at the 
expense of the notochord on the inside, sometimes leading to its almost 
complete obliteration, as in the mammals; in others, as the fishes, the 
reduction is less complete. The fishes have amphiccele vertebrae (fig. 513), 
that is, the centra are hollow at either end. In these cups the notochord 
exists even in the adult, and when small connecting portions extend 
through the centra the notochord takes the form of a. rosary with alter- 
nating enlargements and contractions. 

Histologically the vertebral column may be either cartilage or bone; 
usually it is first formed in cartilage, which later may be replaced by bone. 
If the ossification do not occur, the column remains cartilaginous; if 
incomplete, cartilage and bone appear together. Since these histological 
differences are combined with varying degrees of persistence of the noto- 
chord and with modifications in the form of the vertebrae and their pro- 
cesses, there results an extraordinary variety in the appearance of the 
vertebral column. 


IV. VERTEBRATA -loo 

In Amphibia, reptiles and birds intervertebral cartilages develop between the 
centra, also constricting the chorda. In order that the column shall have the 
necessary flexibility, joints arise in the intervertebral cartilage in different man- 
ners: (a) Opisthoccele vertebrae have a socket on the hinder surface which 
receives the convex anterior end of the succeeding centrum, forming a ball-and- 
socket joint, (b) Procffloiis vertebrae have these relations reversed, the socket 
being in front, (c) The vertebrae may articulate by a 'saddle joint' (birds). 
If the intervertebral cartilage become rudimentary, the amphicoelous condition 
reappears, (d) Between two successive vertebrae an elastic intervertebral 
ligament may occur (mammals). The neurapophyses may bear, in addition 
to the transverse processes, anterior and posterior articular processes (zygapo- 
physes} connecting the separate vertebrae. 

The skull, the anterior continuation of the axial skeleton, occurs in all 
vertebrates; it appears before the vertebrae, for it is found in the cyclos- 
tomes, which lack these. It surrounds the brain as the vertebrae do the 
spinal cord; and, like them, its first stages are formed in the skeletogenous 
layer surrounding the anterior end of the notochord. It is so related to 
the surrounding parts that it may in general be said to be equivalent or 
homodynamous with the vertebrae, although we cannot agree with Oken 
and Goethe, the founders of the 'vertebrate theory of the skull,' that it 
has arisen by the fusion of vertebrae. On the other hand skull and verte- 
brae are parts arising in the common basis of the skeletogenous layer, but 
which have developed in different directions. The vertebral column is 
metameric since the segmental muscles attached to it would otherwise be 
ineffective. The cranium is a continuous capsule, because the most 
important sense organs are on the head and they prevent the development 
of locomotor muscles. Many facts of anatomy and development, especi- 
ally the relations of the nerves (p. 472), tend to show that one part of the 
skull, the paltfocraniitm, has no relation to vertebrae. This alone is 
found in cyclostomes. In other vertebrates this is joined by the occipital 
region (neocranium) of vertebrae secondarily fused with the palaeocranium. 

Three stages are recognized in the development of the skull: the 
membranous, the cartilaginous cranium (chondrocranium), and the bony 
skull. The first, which consists of connective tissue, occurs only in the 
early embryonic stages, scarcely a trace of it persisting in the adults. 
It is early replaced by the cartilaginous skull, which may persist unaltered 
throughout life in the lower fishes (elasmobranchs, sturgeon). In most 
vertebrates, however, ossification sets in, embracing a part (iishes, am- 
phibians) or practically the whole of the cartilage (birds, mammals), con- 
verting it in the latter case into a bony capsule. In the bony skull two 
kinds of bone, primary and secondary, are recognized, these varying in 
their origin. The primary or cartilage bones develop from the cartilage 
itself. The secondary or membrane bones are, in their origin, foreign to the 


456 


CHORDATA 


axial skeleton and arise from the ossifications in the skin (scales) or in the 
mouth (teeth), already referred to (p. 451). They sink into the deeper 
portions and apply themselves to the axial skeleton, especially to those 
parts where, from lack of cartilage, no primary bones can be formed. 
It is not settled how far these distinctions are valid. According to Gegen- 
baur all ossification arose primarily in the skin or mucous membranes, 
and primary bones are merely membrane bones which have entered the 


FIG. 515.- Chondrocranium of Amphinma. anp, antorbital process; ap, ascending 
process of quadrate; c, cornu trabecula?; c, ethmoid plate; ef, endolymph foramen; j, 
jugular foramen; /, lamina cribrosa; 111, Meckcl's cartilage; n, notochord; oc, oculomotor 
foramen; ocp, occipital process; of, optic foramen; p, parachordal; pal, palatine foramen; 
ff, perilymphatic foramen; q, quadrate; s, stapes; sp, stapedial process; t, trabecula; trc, 
crest of trabecula; V, VII, VIII, foramina for V, VII, VIII nerves. 

cartilages and replaced them. Accordingly it is conceivable that the same 
bone in one animal may arise as a membrane bone and in another as a 
primary bone, a point of importance in deciding the homologies and 
nomenclature of many bones. It is but just to say that this view is not 
universally accepted. 

The chondrocranlum is most complete beside and beneath the brain 
(fig- 5 T S)- This basal portion is a direct continuation of the vertebral 
column, and a part of it (the parachordals} embraces the anterior end 
of the notochord, while parts (the trcbeculcc} extend in front of the 


IV. VERTEBRATA 


157 


end of the notochord. The side walls of the skull are increased by the 
cartilaginous envelopes of the two sense organs, the nasal and otic capsules, 
around the nose in front and ear behind. Between these is a hollow for the 
eye which, althoughits capsule (sclera) may be cartilaginous or even ossified 
in part, needs to be movable and hence it contributes nothing to the skull. 
In only a few forms is the chondrocranium completely closed; usually 
<;aps (fontanellcs) occur in its roof, and frequently in its tloor. The 
higher the animal intellectually and the larger its brain the more the con- 
nective tissue (primordial cranium') is called upon to roof in the chondro- 
cranium. Hence it is that in the reptiles, birds, and mammals, where it is 
also confined to embryonic life, the chondrocranium is relatively the 
smallest. Since it only closes above in the occipital (hinder) region, 
while it gaps widely in front, it follows that the membrane bones play 
an i