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S a
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 91
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 51
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 .................. 235
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 ............. • 255
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 ........................ • 29°
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 ............. 3°3
Sub Class IV. Blastoidea .......... • 3°3
Class IV. Echinoidea ................ • 3°3
Class V. Holothuroidea .......... • 3°6
SUMMARY ................................ •
Phylum. VIII. MOLLUSCA ................. • 3IQ
Class I. Amphineura .................. • 3T5
Class II. Acephala ................... 3J7
Order I. Protoconchise .......................... • 323
Order II. Heteroconchia? ........................................ 324
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 LTpsala; 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. WTallace, 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 mf
mk2
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; ink1, 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) W7hether 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 — 8°C.) 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 exPense 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-lbriis
products). The histological differentiation is the in the frog (diagram).
._ , . ,, • a, formative cell; b,
formation of specific plasmic products. A cell in format}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
{^keratin1} . 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.
pIG 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).
JTIG> 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-fn_ '- J •-• '.S^*? • •** J - -" - i".i-i'^i
MS^&£&'f^&
FIG. 45. — Cross-section through the
human metacarpus (after Frey). , 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 animalsr
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; gf, 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; ppt
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, 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; ngl, 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 (Ocnrria 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 ; Ar, 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; g2, 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 He 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. FIG. 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\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 -
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 ^ carina; /, 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. l»r
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 clea1'
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 (-^/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
(%• 352)-
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
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; gl, 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-ui1;
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).
cl, afferent canals; c2, 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; tr, /', 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, n1, 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; ek3, 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 w7hich 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 medusa1 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; f1-4, rows of combs overlying
meridional canals; t, tl, 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 medusa1 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; ilum* (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; 7r,
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 she«p 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 Leuckart1).
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 —>- ~ ^
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
pn 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).
• Ps 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; oi1, 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: CfL£TOPODA
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; bl,
chseta follicle, its chaeta in function; b2, 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: CrL£TOPODA
.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). 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
sin«idare (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; c1, c~, left and right coelom
sacs; d, hind gut with anus; hl. 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. Plates 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; rl,r2, ambulacra! complex
of bivium (ambulacral vessel) and
nerve; r3-r3, 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.
rr.
f\r — ^=-^_ i""-/ "• *ru>\
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/, 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;
u1, 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 ATau-
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 buccal 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 giion; n,bi 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', p2, 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\
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.
a2, 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; pm1, pnr, first and second maxillipeds;
pl-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 maxilla1.
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,, a4, 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;
gn2-13, 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; ("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). 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 trachea1 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. 432)
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^- 441)- 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. 443—447), 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; 0, 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.
Jr10
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). a1, 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; a1, 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). coarctatae (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.
(C6H10O3) 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
% 5°3-)
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; v1, v2, 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; be1, 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- 5TS)- 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 important part in the completion of the skull.
/? pfo
ocs.
we.
FIG. 516. — Skull of carp, the visceral skeleton removed. (A) Cartilage bones: ocb,
od, ocs, basi-, ex-, and supraoccipitals; epo, epiotic; pto, pterotic; spho, sphenptic; pro,
prootic; as, alisphenoid; os, orbitosphenpid; me, mesethmoid; ee, ectethmoid.
Ventral membrane bones: ps, parasphenoid: vo, vomer. (C) Dorsal membrane bones
P, parietal; fr, frontal; 1-4, exits of nerves.
The bony skull presents great difficulties from the standpoint of
comparative anatomy, in part from its varying appearance in the different
groups, in part on account of the number and complicated arrangement
of the constituent bones. It may be said that in general the same bone
reappears in the different classes, and that the difficulties are connected
with the fact that certa'n bones may fail to develop (Amphibia), or they
may fuse to larger elements (mammals). A further complication results
from the intimate union of bones of the visceral arches with the cranium,
which, strictly speaking, do not belong to it.
458
CHORDATA
The cartilage bones can be divided according to the cranial regions
into four groups: (i) bones of the hinder part of the head — occipitalia;
(2) bones of the ear region — otica; (3) bones near the eye — sphenoidalia;
and 4) of the nasal capsule — etlimoidalia. The occipital a — four in
number (figs. 516-518) — united in the higher mammals to a single occip-
ital bone, surround the foramen magnum, the opening through which the
spinal cord passes to connect with the brain. They are a pair of ex-
occipitals, right and left, a supraoccipilal above and a basioccipital below.
The otica depend in their development upon the extent of the otic region.
In ;he fishes (fig. 516) where this part is large, several bones may pre-
sent: cpiotic, pterotic, s phenolic, prootic, and often opisthotic. In the
mammals, on the other hand, these are fused to a single petrosal bone
(figs. 517-519) of small size.
01
Yi
A
4«f jSgaJT
rimal, Mw,_ maxillary; vVo, nasal; Oft, basioccipital; O/, exoccipital- Ors
no,d; Pa, parietal; Pa/ palatine; Pe, petrosal; Pw, paramastoid process; Pv pre-
f .„, sinus Jn frontal bone;5/>/>, '
The line of the basioccipital is continued forward by the unpaired
lements of the sphenoidalia, a basisphenoid behind and a presphenoU
front. With each is connected, right and left, a pair of bones; with the
basisphenoid the alisphenoids, with the presphenoid the orbitosphenoids
s the exoccipitals flank the basioccipital. In the region of the nasal
ule there is an unpaired mesethmoid with a pair of ectethmoids. Hence
:ranmm of primary bones may be described as a chain of four median
IV. VERTEBRATA
459
basal bones, (tig. 519), basioccipital, basisphenoid, presphenoid, and
mesethmoid ; right and left of this a row of exoccipital, alisphenoid,
orbitosphenoid, and ectethmoid. The position of the otic capsule results
in the sum of the otic bones, the petrosal, being wedged in between
the exoccipitals and the alisphenoid. Only behind is there a dorsal
element, the supraoccipital.
The skull is roofed in by membrane bones, and of these three pairs are
almost constantly present. These are, from behind forwards, a pair of
parietals, a pair of Jrontals, and a pair of nasals, the latter covering the
FIG. 518. FIG. 5*9-
FIG. 518. — Sagittal section of hinder part of goat skull (from Gegenbaur). For
lettering see fig. 517.
FIG. 510. — Diagram of floor of skull of infant. 7, mesethmoid; i, ectethmoid ; 7 /,
presphenoid; 2, orhitosphenoid; 777, basisphenoid; 3, alisphenoid; IV, basioccipital;
4, exoccipital. Other letters as in fig. 517.
nasal capsules. The lower vertebrates have a large membrane bone on
the floor of the skull, the parasphenoid, which reaches from the basioccip-
ital to the mesethmoid.
The scheme of the cranium thus outlined undergoes the most modifications
in the sphenoidal region. Parasphenoid, on the one hand, and hasi- ami
presphenoid, on the other, may be substituted for one another, so that when the
parasphenoid is present (fishes, Amphibia) the others are small or absent and
vice versa (mammals). In the mammals, besides, the alisphenoids fuse with
the basisphenoid (greater win.i^s), the orbitosphenoids with the presphenoid
(lesser wings), so there arise here an anterior and a posterior sphenoid. I used in
man and some other mammals to a single sphenoid hone. Mesethmoid and
ectethmoids likewise fuse in the mammals to an ethmoid bone. Th<- existence
of these four complexes was the basis of the view (p. 455) that the skull is
composed of vertebrae.
The brain case, or cranium, is developed into the complete skull by the
addition of the visceral skeleton, a series of arches which embrace the begin-
400 CHORD ATA
rung of the alimentary tract and are related to the cranium, much as are
ribs to the vertebrae. These must be considered as parts of the skull,
although in part they are shoved backwards and lie under the anterior end
of the vertebral column. As the ribs arise in alternation with the muscu-
lature (myomcric), so the visceral arches are similarly related to the gill
formation (branchiomeric) . Like the cranium the visceral skeleton has a
cartilaginous and a bony stage. The visceral skeleton is entirely carti-
laginous only in Elasmobranchs, and here it is so loosely connected with
the cranium as to be easily separated from it. It consists in these forms
usually of seven (rarely nine) arches (fig. 546) ; these are, from in front
backwards, the large mandibular arch, the hyoid arch, and five (rarely
seven) gill or branchial arches. The mandibular arch consists, on either
side, of two pieces which bear teeth and oppose each other in biting; the
upper half, attached to the skull in front and behind, is the pterygoquadraie
(is not the upper jaw of higher forms) . The lower part, which is hinged to
the other, is the mandibular or Meckel's cartilage. In the same way the
hyoid arch is divided into an upper, or hyomandibular, alid a lower
hyoid proper on either side, the hyomandibular being fastened to the otic
capsule. The hyoids are united below by an unpaired piece, the copula.
A copula also exists between the halves of the branchial arches, each of
which consists of four parts on either side. Hyoid and gill arches bear
gills. Certain features (existence of rudimentary gills and a rudimentary
gill cleft, the spiracle) indicate also that the mandibular arch was once
gill -bearing and that it lost its original function upon being converted into
an organ of mastication.
In front of the mandibular arch, in the Elasmobranchs, are two or three
labial cartilages, but it is doubtful if they are visceral arches. Recently they
have been regarded as remnants of a support for tentacles around the mouth
like those of Amphioxus and Myxine, which reappear in the barbels of bony
fishes.
By ossification, the visceral arches of the higher fishes and all higher
vertebrates produce a great modification of the skull, this being increased
by a progressive change of function of the arches, which depart more and
more from their relations to the respiratory apparatus. From this stand-
point they may be divided into two groups, an anterior, consisting of labial
cartilages, mandibular arch, and the hyomandibular; and a posterior, of
the hyoid, the gill arches, and the copula-. The hinder arches are well
developed as long as branchial respiration persists. With the loss of gills
they largely disappear, but what remains forms the hyoid or tongue bone
(not to be confused with the hyoid proper), its body being composed of the
copula, its anterior horns of the hyoid, and its posterior horns of the rem-
IV. VERTEBRATA Hil
nants of a gill arch. Other gill arches contribute to laryngeal cartilages,
the epiglottis and the cartilages of the auditory meatus.
The anterior members of the visceral skeleton (pterygoquadrate,
Meckelian, and hyomandibular) become developed further, but lose more
and more their individuality and unite with the cranium; in the mammals
forming the 'bones of the face.' It is therefore a source of additional bones
which are difficult to follow from class to class, since they change in their
functions and consequently in shape and relative size.
All vertebrates with bony visceral skeleton (figs. 517, 547) have two
pairs of membrane bones, right and left, in front of the pterygoquadrates,
the premaxillaries (intermaxillaries) and maxillaries. They usually bear,
in toothed vertebrates, the marginal row of teeth, which are distinguished
from the palatopterygoid teeth in that they are opposed by the teeth of
the lower jaw. The pterygoquadrates are thus forced backwards and
form a second series of bones, parallel to the maxillary series, which like-
wise may bear teeth. This row of bones consists of an anterior palatine
portion and a posterior quadrate part. The cartilages of the palat im-
part largely disappear and are replaced, in front, by a pair of comers fol-
lowed by a pair of palatines, while farther back are a pair of ptcrygoids.
The quadrate portion ossifies into the quadrate bone, which, except in mam-
mals, afford the articulation for the lower jaw. The ossifications for the
lower jaw occur in a similar way; in front a series of membrane bones, of
which the dentary is most important, surrounding Meckel's cartilage,
while the hinder part of the Meckelian ossifies into the artimlare, so called
because it articulates with the quadrate. The hyomandibular forms
only one constantly present bone known by the same name.
All vertebrates with terrestrial habits have a sound-conducting ap-
paratus in connection with the ear. This is composed of elements which,
in the fishes, lie in the neighborhood of the otic capsule, the hyomandibu-
lar, the quadrate, and the articulare, to which is added another element,
the stapes, which occupies the fenestra ovalis (p. 479) and is derived from
the otic capsule itself. In Anura, reptiles, and birds the hyomandibular
apparently gives rise to an element, the colmnclln, which abuts against the
stapes. In the mammals stapes and columella are possibly fused, while
quadrate and articulare undergo a change of function, losing their position
in connexion with the articulation of the jaws and being converted into
part of the sound-conducting apparatus, the quadrate forming the inms,
and the articulare the 'malleus (figs. 533, 534). Since the lower jaw in this
way loses its articulation, a new one is formed by a process from the mem-
brane bones which articulates with a membrane bone, the squamosal,
to be mentioned below.
462 CHORDATA
According to this view the lower jaw of a mammal is not exactly equivalent
to the lower jaw of a bird, since in the latter the hinge is furnished by the quad-
rate-articulare joint. It should be said that there is another view, though not
so well supported, which considers the ear bones as exactly homologous through-
out terrestrial vertebrates.
In conclusion three other bones, widely distributed, must be mentioned
—the squamosal, the tympanic, and the jugal. The squamosal is a mem-
brane bone arising at the boundary of quadrate and otic capsule (petrosal),
and hence with relations to both these bones. It increases in size as the
quadrate diminishes in changing to the incus, and in the mammals fuses
with the petrosal to form the temporal bone. In common with the tympanic,
which in mammals also fuses with the petrosal, it forms a frame for the
attachment of the tympanic membrane of the ear. The jugal (malar),
belongs to the maxillary series. In many vertebrates the maxillary bone
is articulated only in front, its posterior end terminating freely in the soft
parts, but when the jugal occurs it forms a jugal or zygomatic arch which
bridges the gap between the maxillary and the quadrate region of the skull.
When the quadrate becomes modified to the incus, the jugal articulates
with its companion, the squamosal, which extends a zygomatic process
forward for this purpose.
Difficulties in ascertaining the morphological relations of bones arise where
the visceral and cranial parts join and where primary and secondary bones
touch. Thus the pterotic, sphenotic, and ectethmoid of fishes are often replaced
by secondary bones in the Amniotes; the pterotic by the squamosal; the sphenotic
and ectethmoid by two membrane bones in front of and behind the frontals, the
prefrontals and postfrontals of reptiles and other forms.
Just as skull and vertebral column form a firm axis for the body, the
appendages are supported by axial skeletal structures. Two kinds of
appendages are recognized, paired and unpaired, which generally occur
together only in fishes. The unpaired consist of a fold of the skin beginning
in the sagittal plane behind the head, running back around the tail and
forward on the ventral surface to the anal region. This continuous
fold is nearly always divided into three parts, a dorsal fin (often subdivided
into smaller fins), a caudal fin, and an anal fin. In a similar way, appar-
ently, the paired appendages — an anterior or thoracic and a posterior or
pelvic (abdominal or ventral) pair — have arisen from a pair of continuous
folds, by development of the appendages themselves and suppression of
the intermediate regions. Of these the unpaired are possibly the oldest,
since they occur not only in the cyclostomes, but in AmpJiiovus and the
tumcates as well, where paired appendages are lacking; on the other hand
they disappear in the higher forms. Since they are of service only in an
aquatic life, they are lost in Amphibia, in which a continuous fin, unsup-
IV. VERTEBRATA
463
ported by skeletal elements, occurs only in larval life. On the other hand
the paired appendages (arms and legs) gain in importance with terrestrial
habits.
In the fins of fishes two kinds of skeletal elements occur which, in the
Elasmobranchs (fig. 520), are distinguished by their histological structure,
since the one, the fin supports (basalia and radialia) , consist of cartilage, the
others (actinotrichia, dermal skeleton) are
of horny consistency. Since in the teleosts
both kinds of supports may ossify, the dis-
tinction is here less striking, yet the basalia
and radialia arise from cartilage and lie in
the basal part of the fin, while the others
FIG. 520. FIG. 521.
FIG. 520. — Pectoral girdle and left fin of Heptanchus (after Wiedersheim). a,
principal row of radialia; h, actinotrichia cut across at h'; nl, foramen for nerve; r,
radialia; 5, s', scapula; u, coracoid portion of girdle; i, 2, 3, basalia; i, propterygium; 2,
mesopterygium; 3, metapterygium.
FIG. 521. — Right half of shoulder girdles of (.4) frog, (B) turtle, (C) lizard (after
Gegenbaur, slightly modified), r/, clavicle; co, coracoid; e, episternum; 5, scapula; s\
suprascapula; st, sternum, in C with bases of ribs.
are never cartilaginous and occur in the distal portion. These dis-
tinctions are of importance, since the actinotrichial portions play no
part in the development of the extremities of the higher groups. These
arise from the cartilage supports of pectoral and pelvic fins, which there-
fore alone need further mention.
The skeleton of the paired appendages, preformed in cartilage, con-
sists of two parts, the girdles lying in the lateral walls of the trunk, and
the skeleton of the limbs themselves. A girdle — a shoulder or pectoral
girdle in front, a pelvic girdle for the hind limbs — is in its simplest form
an arch with ri^ht and left halves, each half with an articular surface for
464 CHORDATA
the limb, dividing it into dorsal and ventral portions (fig. 520). The
dorsal portion is the scapula (shoulder blade) in the pectoral, ilium in the
pelvic girdle. With the development of bone the lower portion is usually
split into anterior and posterior parts (fig. 521). The anterior of these is
the clavicle in the pectoral girdle, pubic bone in the pelvis; the hinder part
is the coracoid or the iscliium in the two girdles respectively. These parts
are most constant in the pelvic girdle. In the pectoral girdle either
coracoid or clavicle may be lacking, at times both are absent; but no verte-
brate with fore limbs lacks a scapula. In the clavicle there is frequently
an element, preformed in cartilage, the procoracoid, to be distinguished
from a membrane bone, the clavicle in the strict sense.
In the fishes the girdles are largely or entirely held in position by
muscles; in most terrestrial vertebrates there is a more intimate connexion
with the vertebral column. In the case of the pelvic girdle the connexion
is direct, since the ilium is articulated with one or more sacral vertebra
(in reality not with the vertebras themselves, but by the intervention of
sacral ribs). The connexion of the pectoral girdle is less direct and is
looser. This is effected by clavicle and coracoid. The latter connects
with the sternum, which in turn is connected to the vertebral column by
the ribs; the clavicle articulates with a bone, the epi sternum, which rests
upon the breast bone, the morphological relation of which is doubtful,
since under this term have been included different structures.
Since only the free portions of the appendages are concerned directly
in locomotion, and since the various modes of motion — swimming, flight,
running, leaping, climbing— demand special modifications, the skeleton of
the limbs shows great variety. It is usually believed that all these forms
are to be traced back to an ancestral type, the archipterygium. In this
(fig. 520) are numerous skeletal parts which vary little in size and form
and are arranged in many closely appressed rows. One of the rows has
acquired prominence and is called the principal row; it begins with a
larger piece, the metapterygium, which articulates with the girdle and
bears either on both sides (archipterygium biseriale} or only on one
(archipterygium uniseriale] the lateral rows of skeletal elements. Usually
most of the lateral rows are not attached to the principal row, but arise
independently from the girdle, and may begin with larger parts, the
propterygium and mesoptcr \gium.
From this archipterygium can be derived a primary form, the penta-
dactyle appendage, which serves for all terrestrial vertebrates from the
Amphibia onwards (fig. 522). In tracing this from the archipterygium
following modifications must be supposed. First a reduction in the
number of rows to five, a principal row and four accessory rows. The
IV. VERT KBR ATA
4G5
terminal portions of the principal row produce the bones of the fifth, the ac-
cessory rows those of the other fingers or digits. Then there is an unequal
growth of parts; the metapterygium, already in Elasmobranchs a consid-
erable element, increases in size and forms in the fore limb the humerus,
in the hind limb the femur. In like manner the
second clement of the principal row and the first of
the first accessory row increase and form respectively
•ulna and radius in front, fibula and tibia behind.
Then follow parts which remain small and some-
what cubical, carpal bones in the fore limb, tarsals
in the hinder extremity; they bear in turn slender
bones, the metacarpals or metatarsals, and these at
last the phalanges. (For the nomenclature of carpals
and tarsals see the explanation of fig. 522.) A process
on the upper end of the ulna is the olecranow; in the
hind limb there is an analogous 'knee-pan' or patella
in the tendon passing over the knee.
The third and most important modification is
brought about by the development of joints. So
long as the appendage serves as an oar it must act as
a single plate with its parts firmly held. On the other
hand, when it must act as a system of levers to support
and move the body, as is necessary in a terrestrial
animal, it must be divided into sections, jointed to
each other. By this there are developed two joints
of importance in both fore and hind limbs; the elbow
(knee) joint between humerus (femur) on the one
hand and radius and ulna (tibia and fibula) on the
other; and the wrist joint (ankle) between the bones
of the fore arm (shank) and the carpals (tarsals).
Less important are the joints of the fingers and toes.
If the limbs of terrestrial vertebrates be compared
with this primary form, variations are seen in two
directions. Rarely are there more bones than in the
schema; then there occur remnants of a sixth or even a seventh row or
finger. More frequently there is a reduction in the number of parts,
either by fusion or by absolute loss. Fusion accounts for the fact
that with complete pentadactyly the number of carpalia is usually less
than the ten, to be expected from the schema. Degeneration and loss
explain the existence of animals with four, three, two, or even one digit,
and one can say with certainty that the missing parts are in most cases
30
FIG. 522. — Schema
of a pentadactyle
appendage (after Ge-
genbaur). The dot-
ted lines indicate the
lateral rays; the
names for the hinder
extremities in paren-
theses. H, humerus
(femur); U, ulna (Tib-
ulah/i, radius (tibia).
Carpus (tarsus) con-
sisting of two rows
and two central por-
tions: Row I: r,
radiale ( tibiale) ; /,
intermedium; «, ul-
na re (tibularc:) r,
CfiHralia. Row II:
1-5, carpalia (tar-
salia); the metacar-
pals (metatarsals)
and phalanges not
lettered.
466
CHORD ATA
lost, though a fusion of digits is not unknown. Paleontology, for example,
teaches that. the one-toed horse has descended, by gradual reduction,
from five-toed ancestors.
The completeness and character of the skeleton thus outlined has a
great influence on the rest of the organism. As already pointed out, the
external appearance of vertebrates has been altered by it, since the skin
is no longer a supporting structure and has consequently lost its segmen-
tation. More immediate is its influence upon the arrangement of the
FIG. 523. Fie. 524.
FIG. 523. — Horizontal section through the anterior trunk region of a young Rhodeus
amarus at the level of the ventral arches, c, notochord; h, skin; li, myosepta; m,
longitudinal muscles; r, rib end of the cartilaginous ventral arch; v, osseous centrum.
FIG. 524. — Horizontal section of an embryo of Triton (from O. Hertwig). ch,
notochord; uh, myoccele; z*s,primary muscle segment (myotome).
musculature. The development of an internal skeleton renders it neces-
sary that the point of resistance of the muscles be transferred from the
skin to the interior. A dermal musculature occurs only as an incon-
spicuous remnant; it is replaced by a body musculature. This latter
consists primarily of a system of longitudinal muscle fibres on either side
of the vertebral column (fig. 523), which are divided by connective-tissue
partitions, the myosepta, into successive segments, the myotomes. Thus
when the connective tissue of a fish is dissolved by cooking, the muscles
fall into disk-like parts. The myosepta extend from skin to axial skeleton.
Since they run obliquely backwards from the skeleton to the skin, they
serve to render the skeleton a point of resistance for the action of the
muscles.
A segmented trunk musculature occurs in the Myxinoids (and in
Amphioscus), in which the axial skeleton consists only of notochord and is
consequently un jointed. The segmentation of the muscles is therefore older
than that of the skeleton and, as we can further say, is the cause of it.
IV. VERTEBRATA 407
The action of the muscles prevents the formation of a cartilaginous or
bony vertebral continuum such as the notochord and skeletogenous
layer are. It produces at intervals joints or flexible parts separating the
cartilaginous or bony column into vertebrae. Naturally these flexible
portions cannot coincide with the boundaries of the muscles, but must
lie between them; in other words, muscle segments and skeletal segments
— myotomes and sclerotomes — must alternate. Segmentation is lacking in
the cranium, since the myotomes here have no locomotor significance,
are reduced, and only small remnants of them persist.
In the mammals only a little of this segmental arrangement of muscles
is recognizable, a result of the development of the appendages; and the
more these gain in importance as the locomotor structures, the more the
muscles are modified and grouped for the service of the limbs, so that only
the intercostals and a part of the muscular system to the sides of the
vertebral column show clearly the primitive metamerism. Yet in all
vertebrate embryos the muscles appear at first strictly segmental, in the
form of the primitive somites (fig. 524), formerly called protovertebrcc.
Parts, like buds, separate from the ventral sides of the protovertebrae
and furnish the musculature of the limbs.
Another important point in the musculature lies in the fact that it is dorsal in
origin and therefore in fishes is largely dorsal in position throughout life. The
muscles which are ventral have largely been transferred from the back, and the
cause of the migration is to be recognized to a large extent in the progressive
development of the appendages. The dorsal position of the muscles is only a
part of a general fact, that the skeletal axis divides the body into a dorsal zone,
containing only animal organs, and a ventral zone, chiefly vegetal in character,
Besides the muscles, the central nervous system, and the most important sense
organs — eyes, nose, ears — belong to the dorsal zone.
The central nervous system of vertebrates consists of brain and
spinal cord. Like that of all chordates it is distinguished from that of
other segmented animals — annelids, arthropods, in which there is a dorsal
brain and a ventral nerve chain — in its purely dorsal position. It is
further distinguished from that of all non-chordates by its tubular character,
that is, by the presence of a central canal in the axis of cord and brain (fig.
79), lined by a special epithelium, the ependyma, and containing a fluid, the
liquor ccrebrospinalis. This central canal is the result of the mode of devel-
opment, the nervous system arising by an inrolling of the ectoderm (Jig. 9).
A median longitudinal medullary groove arises early in the dorsal ectoderm
of the embryo. The floor of this groove, the medullary plate, gradually
rolls into a tube, the edges curving upwards and meeting above. In this
process there is developed, as in the tunicates (fig. 500), a neurenteric
canal connecting the hinder end of the neural canal with the digestive
His CHORDATA
tract. Besides the neurenteric canal there long persists at the anterior
rnd an opening to the exterior, the neuropore.
In all vertebrates, in contradistinction to the lower chordates, the brain
is large and sharply marked off from the spinal cord (medulla spinalis).
The spinal cord is a cylindrical structure (flattened in Cyclostomes,
fig. 511) which is marked in the middle line above and below by two
longitudinal grooves, the dorsal and ventral fissures of the cord (sp, sa,
fig. 79). The central canal (CV) is greatly narrowed by the growth of the-
IUTVOUS tissue, in which two layers are distinguished, one containing almost
solely nerve fibres, the other both fibres and nerve or ganglion cells.
The arrangement of these layers is contrasted with that of the inverte-
brates in that the ganglion-cell layer — the gray matter — lies in the centre,
the fibrous layer — white matter (W) — on the periphery, a reversed position
consequent upon the development by infolding. The distinction in color
depends upon the fact that white medullated fibres run in the cortex,
while in the gray matter gray non-medullated fibres are present between
the nerve cells. The color distinctions fail in the cyclostomes (and
Amphioocus), which have no medullated fibres, although the same general
structure occurs.
The gray matter surrounds the central canal, but extends on either
side dorsally and ventrally into the white matter, so that in section it
resembles somewhat the letter H, with its dorsal (fig. 79, HH) and
ventral horns (VH}. These horns and the dorsal and ventral nerve
roots arising from them, divide the white matter on either side into three
tracts, the dorsal (H), ventral (s), and lateral (S) columns of the cord.
Corresponding to each muscle segment two nerve roots arise from the
cord, a dorsal root, with a ganglon (spinal ganglion) at some distance
from the cord, and a ventral root, without a ganglion. The dorsal root
contains mostly sensor}' fibres — i.e., those carrying nervous impulses to the
cord — and is afferent, while the ventral roots are efferent and contain only
motor elements (Bell's Law). These roots unite into a mixed root, which
then divides into dorsal and ventral branches.
The brain of vertebrates in general corresponds in its fundamental
plan (figs. 525, 526), best seen in development, with the brain of man.
At first, in the embryos of a few lower vertebrates there is a stage with
two divisions, an anterior arcliencephalon and a posterior metenceplialon
which passes into the spinal cord. This condition is transitory and
gives place to a brain with three regions by the division of the archen-
cephalon into a. fore brain (prosencephalori) and amid brain (mesencephalon),
the metencephalon forming the hind brain. Usually this stage is reached
before the closure of the medullary folds. Formerly it was stated that a
IV. VERTEBRATA
-Hi'.)
condition with five vesicles followed upon this with three, the mid brain
remaining undivided, while the hind brain divides into cerebellum (melen-
cephalon, cb) and medulla obi on gala (myelencephalon, m); the fore brain
into cerebrum and 'twixt brain. This is not exact so far as the hind
brain is concerned, for cerebellum and medulla are roof and floor of the
same cavity (fig. 526). The fore brain becomes divided, by lateral
outgrowths 011 either side in front, into three
parts, an unpaired portion, the 'twixt brain,
and paired anterior parts, the cerebrum.
Introducing the terms of human anatomy
for the separate parts of the brain, the first
vesicle consists of the two cerebral hemispheres
(telencephalon) whose dorsal and lateral walls
rz?
FIG. 525. FIG. 526.
FIG. 525. — Diagram of a vertebrate brain (from Wiedersheim). Aq, aqueduct;
Cc, central canal; FM, foramen of Monro (connection of lateral ventricles with each
other and with the third) ; H H, cerebellum; MH, corpora bigemina (optic lobes) ; A" // ,
medulla oblongata; R, spinal cord; SV, lateral ventricles; VH, cerebrum; ZH, optic
thalami ('twixt brain); ///, IV, third and fourth ventricles.
FIG. 526. — Scheme of brain in sagittal section, c, cerebrum; cb, cerebellum; cc,
canal of spinal cord; ch, notochord; cs, corpus striatum; h, hypophysis; i, infundibulum;
m, medullary region; o, optic chiasma; of, olfactory lobe; ol, optic lobes; p, parietal eye.
are called the pallium, while in the floor of each hemisphere is an enlarge-
ment, the corpus striatum (cs). The spaces in the hemispheres are the
first and second ventricles (SV) . From the front of each hemisphere arises
a distinct region, the olfactory lobe or rliinencephalon (of), which gives
origin to the olfactory nerve. Since the organ of smell is frequently at
some distance from the brain, the olfactory nerve must elongate, as in
Amphibia (fig. 568), or the olfactory lobe must lengthen, as in many
Elasmobranchs (fig. 549). In the latter case the swollen end of the
lobe, called the bulbus olfactorius, is close to the olfactory epithelium and
is connected with the brain by a long stalk, the tractus. Both, as parts
of the brain, must be distinguished from the olfactory nerve.
Only the lateral walls of the second vesicle become thickened, produc-
ing the optic thalami or diencephalon, directly adjoining the corpora
470 CHORDATA
striata; the roof of this vesicle develops no nervous substance, but remains
a thin layer of epithelium closing in the third ventricle above (///).
The floor is also thin-walled between the thalami and is pushed down-
wards, forming a funnel-like pocket, the infundibulum (•/). The mesen-
cephalon is divided on its dorsal side by a longitudinal groove into a pair
of optic lobes, divided in the mammals by a transverse groove into four
corpora quadrigemini. In the same group the ventricle of the mid brain is
reduced to a narrow canal, the aqueduct, so that the fourth ventricle
is the cavity of the hind brain.
This last region is called the medulla obhngata (myelencephalon);
it is a prolongation of the spinal cord, and in many respects shows a
similar structure. It is distinguished from the cord externally in that it
gradually increases in size in front, while its roof is reduced to a thin
epithelium (chorioid plexus), often torn away in dissection, leaving an
opening, the fossa rhomboidalh, into the ventricle. In front of this fossa
is the cerebellum, often a thin transverse nervous lamella, but usually is
a considerable part of the brain, composed of a median vermis and two
lateral cerebellar hemispheres.
Although these five parts are present in all vertebrates, the appearance
of the brain in the various classes is very different, because their relative
size and form vary greatly. In the lower vertebrates optic lobes and
medulla oblongata are disproportionately large, while the cerebrum, and
often the cerebellum, are insignificant in size. In the higher vertebrates,
on the other hand, the cerebrum and cerebellum far surpass the other
parts, the increase in size of the cerebrum being proportional to the in-
crease in intelligence. The cerebral hemispheres grow backwards, in
man and the apes covering the other parts, while a similar growth for-
wards carries the olfactory lobes to the lower surface. Since the capacity
of the skull is limited, the cortex of the cerebrum, the seat of intelligence,
is increased in amount by the development of folds (gyri), separated by
grooves (sulci). Somewhat similar conditions exist in the cerebellum,
which in mammals and birds is, next to the cerebrum, the largest part of
the brain.
Connected with the 'twixt brain are two problematical organs, one, the
epipliyxis (pinealis), being dorsal; the other, the hypophysis (pituitary body),
ventral. The hypophysis arises like a gland by an outgrowth from the embry-
onic mouth. This pocket cuts off from its source, increases by budding, and
fuses with parts derived from the end of the infundibulum to a single two-lobed
burly. It has been compared with the subneural gland of the Tunicata (p. 445)-
The epiphysis is an outgrowth from the roof of the brain, from which develops
in many vertebrates the parietal or^an. In cyclostomes and many reptiles this
has the structure of an eye (parietal eye), and in these, separated from the brain,
IV. VERTEBRATA
471
but connected with it by a nerve, it lies in a special cavity in the parietal bones,
which occurs not only in recent but in fossil forms. Above this eye the skin
may be transparent.
Most of the nerves which come from the brain (cranial nerves} arise
between the mid brain and the spinal cord, especially from the medulla
oblongata. The olfactory and optic nerves are an exception, the one
arising from the cerebrum, the other from the 'twixt brain, but both, and
especially the optic, differ so much from the peripheral nerves that they
can hardly be classed with them. Development shows that the optic
nerve is a part of the brain. Following custom, however, and including
these two, the pairs of cranial nerves may be enumerated in the terms of
FIG. 527.- — Diagram of cranial nerves (shark), a, alveolaris; b, buccalis; c, cere-
brum; cb, cerebellum; ct, chorda tympani; e, ear; er, external rectus muscle;/, inferior
rectus muscle; g, Gasserian ganglion; h, hyoid cartilage; hm, hyomandibular; /, internal
rectus muscle; io, inferior oblique muscle;?', Jacobson's commissure; /, lateralis of vagus;
m, mouth; me, Meckel's cartilage; md, mandibularis; mx, maxillaris superior; n, nose;
o, optic lobes; op, ophthalmicus profundus; os, ophthalmicus superficialis; p, pinealis;
pi, palatine; po, posttrematic branches; pr, pretrematic branches; pit, pneumogastric
(intestinal) of vagus; ptg, pterygoquadrate; s, spiracle; so, superior oblique muscle; sr,
superior rectus muscle; t, 'twixt brain; /-A", cranial nerves; 1-5, gill clefts.
human anatomy as follows: I, Ar. olfaclorhts; II, .V. opticus; III, A .
oculomotor ins; IV, N. trochlearis (patheticus); V, .V. trigcminns; \\,
N. abducens; VII, N. facialis; VIII, Ar. acustints; IX, -V. glossopliaryii-
geus; X, N. vagus (pneumogastricus); XI, Ar. acrcstoriiix; XII, A7, hy-
poglossus. To these must be added the recently discovered A', lenninalis,
first found in fishes and recently in man. It arises from the cerebrum and
supplies the wall of the nasal organ. The accessorius in fishes and am-
phibia is a part of the vagus; the hypoglossus, strictly speaking, belongs
to the spinal nerves and only secondarily is associated with the cranial
nerves, which explains its course, outside the skull, in cyclostomes and
amphibia.
472 CHORDATA
The relations of these nerves are the most primitive in the Ichthyopsida
(Hi,'. 527). Of these, three (I, II, and VIII) are purely sensory, going to the
olfactory organ, eye and ear respectively. The third, fourth and sixth are as
purely motor, supplying the eye muscles. The remainder are mixed in char-
acter. The glossopharyngeal nerve is typical. This leaves the medulla and,
at the top of the first gill cleft, divides into two branches, one of which (pre-
trr.mnlic) goes in front of the opening; the other (post-trematic) passing behind
it, both innervating the muscles of the region and supplying a sense organ above
the gill opening. The vagus, in the same wav, has pre- and post-trematic
branches for the posterior gill clefts, a condition which leads to the view-
that this is a compound nerve. In addition the vagus sends a branch to the
Fttitnach and heart. In the Amniotes, with the development of lungs, this
branch also supplies the lungs, hence the name pneumogastric. The vagus
als:> gives off a nerve which supplies the sense organs of the lateral line of
the trunk.
The facial nerve divides over the spiracle in the same way as the ninth over
the first gill cleft. It also supplies the lateral line organs of the head. The fifth
nerve also divides at the angle of the mouth, a branch going into each of the
jaws, a third going towards the tip of the head. With the loss of gills and
lateral line organs in the terrestrial vertebrates the seventh and tenth nerves lose
the corresponding branches. The facial nerve, which in the Ichthyopsida is
largely sensory, becomes almost exclusively motor in the mammals, as a result
of the great development of the muscles of the face, which are practically lacking
in the lower forms.
Since the head undoubtedly consists of several coalesced body segments (at
least as many as there are visceral arches, and apparently more), the question
arises whether the cranial nerves are as evidently segmental as are those of the
trunk. To this is allied the further question whether Bell's Law that a mixed
nerve consists of dorsal sensory and ventral motor components is applicable
here. In answering this question the olfactory and optic nerves, arising from
the prosencephalon, must be excepted. The latter, especially, is not a peripheral
nerve, buHs a tract connecting two parts of the brain, since the retina, as we
shall see, is but an evagination of a part of the brain towards the periphery.
With this cerebral character is connected the peculiarity of the crossing or
cliiasma of the optic nerve. In the teleosts the whole right nerve goes to the left
eye, ^the whole of the left to the right eye, the nerves either simply crossing or
passing through networks in each other. In most vertebrates only a part of
the fibres cross outside of the brain. If all of the fibres cross inside the brain
(as is the rule for true cranial nerves) the result is the appearance as in thecyclos-
tomes, that each eye is supplied by the nerve from its own side.
)f>the remaining nerves which spring from the hind brain and, with the
exception of the trochlearis, from its side and base, the oculomotor and abducens
(possibly also the trochlearis) develop like ventral roots. The eye muscles
vhich they supply are the remains of the somatic musculature arising from the
myotomes, which has largely degenerated. All of the other cranial nerves (the
hypoglossus excepted) arise like dorsal roots, are sensory, and are provided with
the equivalents of spinal ganglia (genirulate ganglion of the facial, Gasserian
anghon of the trigeminal, vagus and glossopharyngeal ganglia), but they also
ontam numerous motor fibres. It is important to note that the muscles in-
rvated by these motor fibres do not arise from the myotomes and hence do not
belong to the somatic musculature, but are a part of the 'visceral muscles.'
since the dorsal roots in Amphioxus contain motor fibres which supply
:eral muscles, and visceral motor nerves occur in the dorsal roots of the
es, it is necessary to modify Bell's Law and to say that the ventral roots
IV. VERTEBRATA 473
innervate the somatic musculature, the dorsal roots supply both the sensory
apparatus and the visceral muscles.
One of the most striking facts in the question of the segmental nature of the
cranial nerves is the relation of these to the visceral arches, the trigeminal in front
of the mandibular, the facial in front of the hyoid arch, and the glossopharyngeal
and vagus with similar relations to the gill arches.
Besides the nervous system of the body already outlined, the vertebrates have
a special nervous system supplying the viscera, the sympathetic system, and in
this a special central organ consisting of right and left cords beneath the vertebral
column, in which ganglia are incorporated. The last of these ganglia lies at the
base of the caudal vertebrae, the most anterior at the beginning of the neck.
From the latter nerve cords extend into the head and are connected with ganglia
(otic, sphcnopalitinc). This system sends out nerves in the form of delicate
networks (f>It\\*its sympathctici) which usually accompany the blood-vessels to
the vegetative organs (intestine, sexual apparatus, etc.). It is also connected
with the spinal nerves.
The space between the central nervous system and the surrounding
skeleton is large in fishes and is filled with a loose fatty tissue which, close
to the nervous mass, is thickened to a richly vascular envelope (meninx)
of the brain and spinal cord. In the groups from amphibia to birds
this primitive meninx is divided by the development of a subdural lymph
space into two layers, the dura mater and the pia mater. The pia, which
in mammals differentiates a delicate arachnoid membrane by the formation
of numerous lymph spaces, is next to the nervous system and is the
vascular layer, while the dura is a tougher membrane which approaches
and fuses with the periosteum lining the skull in proportion to the extent
that the brain fills the cranial cavity. In the ver-
tebral column the dura and the periosteum remain
distinct.
Regarding the sense organs of the vertebrates
we stand on firmer ground than with, the inverte-
brates, since their great similarity to those of man
supports the ideas of their functions derived from
studies of their structure. Of the sense organs of
the skin, the tactile organs of mammals consist of f^ ^g —Tactile
nerve fibres which extend into the epidermis, lose corpuscle from Innl's
the medullary sheath, and branch among the epi- J^j^^^^y"
thelial cells where they may terminate in special tac- nerve; .v, partition,.
tile discs (Elmer's organs of snout of pig and mole).
Besides these interepithelial nerve endings are others in the subepithelial
mesoderm (tactile corpuscles} (fig. 528), which are specially modified and
receive special names (Kraiise's and Crandry"s corpuscles}. They consist
of a few or many cells, probably of epithelial origin, enclosed in a con-
nective tissue envelope. The nerve fibres branch within the organ, the
171
CHORDATA.
branches ending in tactile discs. Though long known among terrestrial
vertebrates, these have recently been found in fishes.
The club-shaped corpuscles (also known as Vater-Pacinian corpuscles,
fig. 81) are allied to the tactile corpuscles by form and mesodermal
position, although they differ materially in structure and are not easily
to be regarded as ectodermal in origin, since they occur not only in the
skin but in deeper structures like the mesentery of the cat. The latter
position renders their function problematical.
While in the terrestrial vertebrates typical sensory epithelium is lack-
ing, it attains a high development in the skin of fishes. The dermal
nerves pass into the epidermis and end in oval corpuscles, which, while
imbedded in a stratified epithelium, consist of a single layer of sense and
supporting cells. According to structure, nerve hillocks (neuromasts} and
nerve-end buds are distinguished. The first are the specific organs of the
? I
feflk^'
V •"-"
J-VW '.--'*>
S
R,»
%
3I«
•'^
7.V
&1^'
Rj
• •. •
FIG. 529.
FIG. 530.
FIG. 529. — Section of olfactory epithelium of a fish Belone (from O. Hertwig, after
Blaue). e, epithelium; k, olfactory buds; n, nerves.
FIG. 530. — Diagram of nose of lizard (after Wiedersheim). AN, outer nasal
cavity: C, olfactory sac; Ca, Stenson's duct; Ch, choana; IN, inner nasal cavity; MS,
roof of mouth; P, Jacobson's organ; |, connexion between nasal cavities.
lateral line, to be mentioned later, of fishes and branchiate amphibians
and amphibian larva-, and therefore appear to subserve special and im-
portant sensations connected with aquatic life. The end buds are
especially collected in the neighborhood of the mouth, on the lips and bar-
bels. Since they also occur in the mucous membrane of the mouth,
especially in the palatal regions, they connect with the taste organs. The
taste buds have the same structure as the end buds of fishes. They occur
in all classes of vertebrates, and are most abundant in man in the walls of
tin- circumvallate papilla? at the base of the tongue; in rodents on the large
foliate papilkc, etc.
Whether the end buds are related to the olfactory organs is questionable.
IV. VERTEBRATA. 475
The olfactory epithelium of many rishcs and amphibia is a stratified
epithelium with closely arranged end buds (fig. 529;. It is conceivable
that by disappearance of the isolating parts of ordinary epithelium the end
buds would form a continuous sensory epithelium, which is the rule in
most vertebrates, yet this view meets difficulties in the finer structure of
the olfactory cells.
The olfactory organ, the nose, lined with its sensory epithelium, ac-
quires a special interest both from its grade of development and from the
important systematic distinctions it affords. Except the cyclostomes,
which have an unpaired nasal sac, all vertebrates have paired olfactory
organs. In adult fishes and in the embryos of higher forms there are two
pits which lie in front of or dorsal to the mouth; they are either distinct
from it or only connected with it by an oronasal groove in the skin (fig.
556). If the animal be terrestrial and replace branchial by pulmonary
respiration, a respiratory canal is developed in connection with the nose.
The oronasal groove closes to a tube which begins with a nostril (nans) on
the surface and ends with a second opening (choana) in the mouth cavity.
The olfactory sac proper is included in the wall of this tube, usually on its
dorsal surface (fig. 530). In Amphibia, lizards, snakes, and birds the
choana is far forward, behind the upper jaw; in alligators, turtles, and
mammals it is far back, in crocodiles and some mammals (edentates)
nearly to the vertebral column. This is brought about by the develop-
ment of the hard palate, a parting wall which divides the primitive mouth
cavity into two portions, a lower, the persistent or secondary mouth
cavity, and an upper, which, as a secondary nasal cavity, continues the
passage backwards. The bones of the maxillary and palatine series con-
tribute to the hard palate, sending out horizontal processes which meet
in the middle line. In mammals and crocodiles this partition is con-
tinued back by the soft palate.
A further increase in the nasal cavity is brought about by complicated folds
in the walls supported by special skeletal parts, the tnrbinal bones, and also by
the outgrowth of chambers, lined with mucous membrane which extend into the
neighboring bones. Again, a part of the primitive chamber lined with olfactory
epithelium can be cut off from the rest and form an accessory nose, Jacobson's
organ (fig. 530, P), which opens into the mouth behind the premaxillaries by
Stensori's duct. This organ is best developed in lizards, monotremcs and un-
gulates, but often occurs in a reduced condition in other terrestrial vertebrates.
In all vertebrates, except Myxlne and a few forms living in the dark,
the eyes are composed of all the principal constituents which occur in the
human eye and which have already been briefly described (p. 1 19, fig. 86).
In most vertebrates it is nearly spherical, the optic nerve entering it from
behind, its interior occupied by transparent, refractive substances (lens.
170 CHORDATA.
vitreous body) and a fluid, the aqueous humor, and its walls of three con-
centric layers. The outer of these is the tough protecting sclera (sclerotic),
a usually fibrous, but in many fishes a cartilaginous, layer, which in front
is transparent and strongly curved, forming the cornea. The second layer,
the chorioid coat, is richly vascular and pigmented; at the boundary be-
tween sclerotic and cornea it is changed .to the iris. The inner layer is the
retina, the structure and arrangement of which are characteristic of the
vertebrates.
From the developmental standpoint the retina (fig. 85) consists of two parts,
the retina proper and the tapclnm nigrum (pigmented epithelium), formerly
regarded as part of the chorioid. In the retina the following layers are dis-
tinguished: (i) the limitans interna; (2) nerve-fibre layer; (3) ganglionic layer;
(4) inner molecular layer; (5) inner granular layer; (6) outer molecular layer;
(7) outer granular layer; (8) limitans externa; and (9) layer of rods and cones.
The limitans externa is the bounding membrane of the embryonic retina, which
is later penetrated by the rods and cones. Between the two limiting membranes
Mailer's fibres ('m) extend, large supporting cells, the nuclei of which lie in the
inner granular layer, and which are aided in their supporting function by the
fine horny framework of both molecular layers. The nervous elements which
are imbedded in this support are best understood by beginning with the optic
nerve. This spreads out in the nerve-fibre layer, and on its way to the end
apparatus comes twice into relation with ganglion cells; first in the ganglionic
layer, second in the inner granular layer, the 'granules' being largely the nuclei
of bipolar ganglion cells. Thus a great part of the retina (layers i to 6) are to
be considered as an optic ganglion, such as occurs in molluscs and arthropods,
but which there lies outside the sensory apparatus. The sensory epithelium
(the retina in the sense this term is used in invertebrates) consists of but two
layers, the outer granular layer and the rods and cones. The outer granules are
the nuclei of the extremely slender epithelial cells which bear the rhabdomes
(rods and cones) on their peripheral ends. The pigment so necessary for the
visual function is supplied by the tapetum nigrum already mentioned. This is
a layer of hexagonal epithelial cells which lies on the tips of the rhabdomes and
sends pseudopodia-like processes between them, and since it is rich in black
pigment granules, the rods and cones are enveloped in a close pig merit mantle.
<
Although in this relation of pigment and in the union of the optic
ganglion with the sensory cells important differences are to be noted from
the eyes of the invertebrates, even from the closely similar cephalopod eye
(P- 337)j the most striking difference remains to be mentioned. The
retina abuts with its limitans interna and nerve-fibre layer against the
vitreous body; with its rhabdomes and tapetum against the chorioid.
Hence the incoming light must traverse the optic ganglion and pass
through the layer of sense cells before reaching the end organs, the rhab-
domes. In nearly all invertebrates, for example the Cephalopoda (fig.
349), the light falls directly on the peripheral end of the rhabdome. The
rhabdomes in cephalopods, as in most invertebrates, are turned towards
the light, in the vertebrates away from it.
IV. VERTEBRATA.
•177
This peculiar and functionally purposeless inversion of the vertebrate retina
is explained by the development of the eye. This can be divided, according to
origin, into two parts, a cerebral (optic nerve, retina, tapetum) and a peripheral
(all other parts). As the eye in tunicates and Amphioxus is permanently a part
of the brain, so is the retina of vertebrates genetically, coming from the first
cerebral vesicle. An outgrowth occurs on either side (fig. 531, B) of the 'twixt
brain and becomes expanded distally to an optic vesicle which is connected with
the brain by an optic stalk. 'The vesicle extends out to the periphery and, coin-
cidentally with the development of the lens, is folded into a double-walled optic
cup with outer or ta petal, inner or retinal layers. If the position of the epithelial
cells be followed (fig. 531), it will be seen that the peripheral ends rest upon the
tapetum, and when these ends develop the rhabdomes, these must grow into the
tapetal layer.
FIG. 531. — Diagram showing the inversion of layers in the formation of the retina
(orig.). The nuclei are placed in the (morphologically) deeper ends of the cells. In .1
the brain (b) has been closed in; in B the optic vesicle (v] has reached the lens (/) and
on the right is being converted into the double-walled optic cup with, as shown in (_',
an outer tapetal (e) and an inner retinal layer (r).
In contrast to the retina, the lens develops as an invagination from the epi-
thelium of the body (/) ; sclera, cornea and vitreous body from connective tissur.
Thus the important part of the eye arises from the brain and is later provided with
accessory apparatus which arise from peripheral parts The invertebrate eye,
on the other hand, with all its parts arises from the skin.
The vertebrate eye is furnished with secondary structures: with muscles
which move it, with lids which protect the cornea from injury and drying. 'I he-
lids are dermal folds which extend over the eyeball from above and below. The
lids enclose a conjunctival sac filled with lacrimal fluid, and bounded by the
epithelium (conjunctiva) which covers the cornea. To these a third lid, the
nictitating membrane, may be added. It arises as a conjunctival fold from the
inner angle of the eye, and can extend over the cornea beneath the upper and
lower lids. A special lacrimal gland, which occurs at the outer angle of the eye,
provides the fluid to moisten the cornea, while a second or Harder' 's gland occurs
at the inner angle when a nictitating membrane is present. Both arc lacking
in the fishes; there is a single gland in the Amphibia.
The ear, at the level of the medulla oblongata, has one point in com-
mon with the invertebrate otocyst — it arises as an ectodermal pit which
is usually completely cut off from its parent layer, and only in elasmo-
branchs (fig. 532, 77) remains connected with the exterior by a tube, tin-
elsewhere closed endolymphatic diict. In the cyclostomes (fig. 532, 7) it
consists of a single vesicle with a single patch of sensory cells, the macula
ac.Hstica; from the fishes upwards the vesicle becomes constricted into an
CHORDATA.
upper utriculus and a lower sacculus, the connecting utriculo-saccular duct
being narrow in the mammals. Both utriculus and sacculus receive a
part of the macula acustica. Outgrowths from the vesicle occur, giving
the whole the name of labyrinth. From the utriculus arise three semi-
circular canals, connected at either end with this cavity, each swollen at
one end to an ampulla, containing a special nerve termination, the crista
acustica. These canals stand at right angles to each other in the three
dimensions of space and without doubt subserve the function of equili-
S32- FlG- 533-
FIG. 532. — Membranous labyrinths of (7) Myxine; (II) Chimara (after Retzius).
a, ampulla;; c, undivided vesicle with endolymph duct; dp, endolymph duct;/, anlage of
lagena; n, macula neglecta; 5, sacculus; su, sinus utriculi; U, utriculus; i, anterior, 2,
posterior, 3, outer semicircular canals. Auditory nerve and its branches dark.
FIG. 533.- — Human ear (based on Schwalbe). a, incus and stapes; ae, external
meatus; c, cartilage; de, endolymph duct; //, malleus; K', temporal bone; K/, bony
labyrinth; Na, nerve; o, base of conch; ,<;, sacculus; T, tympanic membrane; /, fenestra
rotunda; te, Eustachian tube; u, utriculus.
bration (p. 117). They are an outer horizontal, an anterior vertical
(nearly sagittal), and a posterior vertical (nearly transverse). The non-
ampullar ends of the two vertical canals unite, a condition which is under-
stood when it is recalled that in cyclostomes these canals alone are present,
and in Myxine (fig. 532, 7) form a single canal with two ampulke. A late
formation is an outgrowth from the sacculus, which appears even in the
fishes as a small pocket, the lagena, containing a part of the macula
acustica; in the reptiles and birds the lagena becomes much larger, and
in the mammals is spirally coiled and is known as the cochlea. A part of
the macula acustica of the lagena develops into a special nerve-end appa-
ratus, the organ ofCorti.
IV. VERTEBRATA. 479
This membranous labyrinth is partially or entirely enclosed in the otic capsule
of the skull, which may ossify to the otic or petrosal bones (p. 458). In the
birds and mammals the enclosure is such that tin- structure is duplicated in bone,
so that the membranous labyrinth lies in a bony labyrinth, the two being sepa-
rated by lymph spaces. These spaces are developed in the cochlea into two
tubes, the scala tympani and scala vestibuli, the two connecting only at the tip,
being separated elsewhere in part by the membranous cochlea (the diictus
cochlearis or scala media). The spaces of the bony labyrinth are filled by two
different fluids: inside the membranous labyrinth an cndolymph, and between
this and the walls of the bony labyrinth a peril 'ym ph.
Accessory structures may be added to this auditory apparatus proper,
their purpose being to bring sound waves to it. Such structures are
rarely present in fishes (it is not certain that all hear), since the sound
waves are easily carried by the water to the tissues and thence directly
to the ears. On the other hand, with the change to terrestrial life a
sound-conducting apparatus is necessary on account of the differing den-
sities of the air and the tissues. So we find from Amphibia onwards an
air space closed by a vibrating tympanic membrane, which receives the
sound vibrations from the air and carries them to a chain of ear bones
(ossicula auditus), which in turn transmits them to the inner ear or laby-
rinth. These structures are not always functional (cetacea), and they
may be wholly or in part rudimentary (urodeles, snakes, Amphisba?nids) .
To understand this apparatus it must be recalled that the ear lies
between the hyoid and mandibular arches in the neighborhood of a canal
which leads from the surface to the pharynx. In many fishes this canal is
the spiracle, a reduced gill cleft. In the Anura and amniotes it consists
of an air chamber closed externally by the tympanic membrane, stretched
on a tympanic annuhis, while the opening to the pharynx is retained. The
part next the membrane becomes expanded into the tympanic cavity,
this with the membrane forming the tympanum or drum. The part
connecting with the pharynx is usually narrowed and is called the Eusta-
chian tube. The membranous labyrinth touches the wall of the tympanic
cavity at one or two places where the bony auditory capsule is interrupted.
One of these openings (foramen ovalc) is always present and is occupied
by the inner end of the ear bones.
As the mandibular arch lies just in front of the spiracle, and the hyoid close
behind it, it is readily understood how parts of these arches can enter the tym-
panum and produce the ear bones. In Anura, reptiles, and birds a coluniflla
has one end attached to the stapedial plate, which lies in the fenestra ovale, while
the other is inserted in the drum membrane, the whole conveying the waves
across the tympanum to the labyrinth. In the mammals the structure is differ-
ent, since the columella is replaced by two bones, the malleus, which is attached
to the drum membrane, and the incus, which articulates with the slapcx (fig. 534).
Most students believe incus and malleus to be parts (quadrate and articulare)
480
CHORDATA.
of the mandibular arch — a view which has its opponents, who believe these to
be a divided columella
The tympanic membrane is usually flush with the surrounding skin or only
slightly below its level. In the mammals it is protected by being placed at the
bottom of a deep tube, the external auditory meatus. The ear conch, a fold of
skin supported by cartilage, is also confined to the mammals.
The more important vegetative organs of the body are enclosed in a
large body cavity or adorn beneath the vertebral column. The ccelom
has degenerated in the
head and tail region, and
in the amniotes in the
neck as well. The cce-
lom is, as development
shows, an outgrowth from
the primitive digestive
tract, an enter oavle (pp.
147 and 263), lined with
epithelium. Since it
arises, as in other ccelo-
mate animals, by paired
IT
FIG. 534. _Fio. 535.^
FIG. 534. — Ear bones of man (from Wiedersheim). A, incus; H, malleus, S,
stapes.
FIG. 535. — Section of vertebrate in abdominal region, a, dorsal aorta; c, ccelom;
g, gonad; g/, glomerulus; /, digestive tract; /, liver; m, mesentery; mu, muscular part of
mytoomes; mv, its coelom (myoccele); o, omentum; s, spinal cord; so, sp, somatic and
splanchnic epithelia; /, nephridial tubule; vm, ventral mesentery, w, Wolifian duct.
outgrowths from the archenteron, it follows that at firsi the two cavities
must be separated by a partition which also encloses the intestinal tract
(fig. 535). This partition furnishes the mesentery which usually supports
the intestine dorsally in its whole length from the vertebral column, but
ventral of the digestive tract only reaches as far back as the liver, so that
right and left cceloms unite behind. Some other organs are also sus-
pended in the body cavity by membranes: the testes by the mesorc/i him,
the ovary by the mesovarlum. In most fishes and in many reptiles the
IV. VERTEBRATA. 48i
coelom is connected with the exterior by one or two openings (pori ab-
dominales) beside or behind the anus.
The body cavity is frequently called the pleuroperitoneal cavity, since
in mammals it is divided by a partition, the diaphragm, into an anterior
or pleural and a posterior or peritoneal (abdominal) cavity. The lining
membranes of these cavities are called pleura and peritoneum respectively.
The pericardial cavity is also a derivative of the coelom, and the lining,
the pericardium, but a part of the pleuroperitoneal membrane. Hence it is
that in many fishes (sharks, sturgeon) a communication persists between
the pericardial and the other coelom.
Of the vegetative organs, the alimentary tract possesses the greatest
systematic interest for it not only is concerned with digestion, but fur-
nishes, as in all chordates, the respiratory organs (gills and lungs) as well,
these arising in the non-chordates from the ectoderm. It begins with the
anterior ventral mouth and ends ventrally with the anus, some distance
in front of the tip of the tail; it is almost wholly entodermal-in origin,
there being but slight ectodermal portions at either end, which are lined
with the soft 'mucous membrane' much the same in character as the
entodermal part of the canal.
The first division is spacious and consists of the ectodermal mouth
cavity and the entodermal pharynx, two spaces which, in most vertebrates,
are not sharply marked off, but in alligators and mammals are separated
by the soft palate. Then, begins the narrow oesophagus, widening behind
to the stomach. From the hinder or pyloric end of the stomach begins the
small intestine, which enlarges into the large intestine, separated from the
small intestine in the higher vertebrates by a valve and one or two oeca.
The terminal portion in most vertebrates is called the cloaca because it
receives the urogenital ducts. The liver is the only gland constantly
present; it is a large compact organ, generally provided with a gall bladder.
Usually a smaller gland, the pancreas, occurs. The ducts of the liver
(bile duct, due/us choledoc/nis) and pancreas empty into the small intestine
near the pylorus. The mouth cavity may have salivary glands connected
with it, while the rectal region occasionally has blind sacs and glands.
A striking vertebrate characteristic occurs in the dentition. In the
cyclostomes there are horny teeth — strongly cornified epithelial products
totally distinct from the true teeth of dentine and enamel of the higher
groups, which occur in places where the underlying skeleton affords them
a firm support, especially on the upper or lower jaws, but they may occur
on other bones of the mouth and pharyngeal cavities (roof of the mouth,
gill arches). They have apparently arisen from a diffuse dentition, re-
calling the scales of the skin, since many elasmobranchs possess, besides
31
482 CHORDATA.
the ordinary teeth, rudimentary teeth in mouth and pharynx. Where
teeth are lacking (birds, turtles, baleen whales) they have been lost.
The respiratory organs arise from the pharynx. In the fishes and
some Amphibia its walls, right and left, are perforated by gill clefts, each
of which lies between two successive visceral arches (fig. 527). These are
canals which open internally into the pharynx, while the outer gill openings
are on the outer surface. The anterior and posterior walls of the clefts
bear delicate vascular folds of mucous membrane, the gill filaments.-
These are the internal gills, in contrast to the external gills of larvie of
Amphibia and several fishes which are dendritic external ectodermal
growths arising above and between the gill slits (figs. 4, 5). It is impor-
tant for the phylogeny of the vertebrates to note that reptiles, birds, and
mammals, which never breathe by gills, have gill clefts outlined and later
lost with the exception of the Eustachian cleft (fig. 3).
Two problematical organs, the thymus and the lateral lobes of the thyreoid
gland, develop from the epithelium of the gill clefts. The middle unpaired part
of the thyreoid has been regarded as a modification of the endostyle of the
Tunicata (p. 443). The thyreoid, which produces iodine compounds, is
doubtless very important; disease or extirpation of it causes serious nervous
disturbances.
The lungs also arise as two sacs (one occasionally remaining rudimen-
tary), which grow downwards and backwards from the pharynx. They
retain their opening into it either directly or by means of a trachea or
windpipe, which just before its entrance into the lungs usually divides
into two tubes (bronchi) (figs. 573, 589). At the opening into the pharynx
(glottis) the supporting cartilages (remnants of the visceral skeleton, p.
460) are strong and form the larynx, which in mammals may be closed
from the pharynx by a valve, the epiglottis. The lungs and trachea have
their counterparts in the fishes in the swim bladder (a hydrostatic appa-
ratus) and its duct.
The swim bladder of fishes and the lungs of most amphibia are smooth-
walled sacs, but in some have greater respiratory surface since folds extend into
the central space. In this way the bladder may become respiratory (Lepidostens,
Amia, Dipnoi). There is also a small increase of respiratory surface in the
Amphibia (fig. 536, i). In the reptiles the peripheral folding increases at the
expense of the single air chamber and grows inwards. The more the central
chamber is divided and restricted, the more it takes on the character of a canal,
lengthening the bronchus (fig. 536, 2; turtles and crocodiles). In the mammals
(3) there is no longer a central space, since the bronchus divides again and
again, to fine bronchioles, which communicate with inf und^ntl a or 'air cells' (4, e)
by means of an alveolar duct which is lined with vesicles (alveoli) like those of the
infundibula. This comparative conception of the lung needs modification
since development shows that the higher stages are not formed by the division
of an air chamber by the ingrowth of walls, but by lateral outgrowths, as in a
IV. VERTEBRATA.
is:;
gland, from a growing r.ir tube (primary bronchus) If the outpushings remain
rudimentary, and the primary bronchus widens, the result is the amphibian
lung. If the bronchus remain a canal and continue to develop new outgrowths
of the second and third order, the complicated lung forms (2, 3) result.
The circulatory apparatus is easily derived from that of annelids, and,
like it, is completely closed. In the annelids (p. 272) there is a longitudinal
blood-vessel and another below the digestive tract, these being connected
in each somite by loops which pass around the intestine. The vertebrate
scheme varies in the development of a heart in the ventral trunk (dorsal
of the annelid). In the fishes (figs. 66, 554), the heart lies close behind
the gills and sends to them the blood which it receives from the body.
Hence, like the whole ventral trunk, it carries venous blood. Since the
FIG. 536. — Diagrammatic long section of lungs (i) of a young salamander; (?} turtle;
(3) man; (4), a bronchiole (b), giving off several alveolar ducts (g), lined with alveoli (a),
and connecting with infundibula (e).
anterior loops, the gill arteries, pass through the gills, the dorsal trunk,
which collects from these, must contain oxygenated blood, which is sent
by the carotid arteries to the head, and by the dorsal aorta and the vascular
loops to the body. It there becomes venous and flows back into the
ventral trunk and so to the heart.
This scheme of circulation in fishes needs further description. The
heart, a strong muscular organ enclosed in the pericardium, consists of
two parts, auricle (atrium) and ventricle, separated by valves. The
trunk (ventral aorta) arising from the ventricle is arterial and corresponds
to the ascending aorta and pulmonary artery of man. The arterial arches
of the gill region which arise from it pass directly into the dorsal vessel
only in young fishes (fig. 554) ; later they furnish the branchial circulation
of gill arteries, gill capillaries, and gill veins (fig. 66). The dorsal trunk
is the dorsal aorta (aorta descendens) ; the posterior ventral trunk, which only
occurs in the embryo, is the subintestinal vein, from which the portal
vein, going to the liver, arises. To this are added a system of paired
484
CHORDATA.
veins, consisting of Cuvierian ducts and jugular and cardinal veins, the
latter with growth encroaching more and more into the territory of the
subintestinal vein.
The circulation of the fish type undergoes a great modification with the
loss of gills and the appearance of pulmonary respiration in the higher
vertebrates. Gills and gill capillaries disappear, and the branchial
circulation is reduced to arterial arches leading direct from the ventral to
the dorsal aorta. The swim bladder received its blood from the body
(systemic) circulation, but with the functioning of the lungs pulmonary
arteries and veins come into existence, while the arterial arches in part
7 77 777 IV
pIG- 5^7. — Diagram of modification of arterial arches in various vertebrate classes.
White, vessels which degenerate; cross-lined, vessels containing arterial blood; black,
vessels containing venous blood. 7, Dipnoi; 77, Urodeles with pulmonary respiration;
777, Reptiles; IV, Birds (in mammals the left instead of the right aortic arch persists).
aol ' venous aorta of reptiles; ao2, arterial aorta; ast, arterial trunk; a, b, arches which
usually disappear; ad, dorsal aorta; d.B, ductus Botalli; k, gill capillaries; pu, pulmo-
nary artery; 1-4, persistent arterial arches.
disappear, in part are divided between the pulmonary and systemic
circulations (fig. 537). Of the six arches which usually appear in the
embryo, the first and second, and the fifth in animals with lungs, usually
degenerate. The last arch (4), which even in the Dipnoi supplies the
swim bladder, becomes a pulmonary artery, the other arches (i and 2)
furnish the systemic portions — the dorsal aorta (2) and the carotids supply-
ing the head (i). Since special pulmonary veins, distinct from the systemic
circulation, carry the blood from the lungs to the heart, the heart be-
comes divided by a septum which separates it into right and left halves.
The right half retains the venous character of the fish heart; since the
right auricle receives the systemic veins, the right ventricle gives off the
pulmonary artery. The left half is purely arterial, receiving arterial
blood by the left auricle from the lungs and sending it out through
the aorta ascendens to the body. This complete separation of pul-
monary and systemic circulation, and the corresponding division of
IV. VERTEBRATA.
485
the heart, occurs only in birds and mammals. Reptiles and
amphibia show how the modification has been accomplished. In
these the separation begins in the venous system and extends to the
auricle, in the reptiles the septum arises in the ventricle. In the arterial
E1
FlG. 538. — Diagram of mammalian circulation. Hear: : ra, la, right and left auricles;
rr, Iv, right and left ventricles;/., capillary system of lungs; K, capillary system of head ;
£', E3, of anterior and posterior extremities; D, of intestine; P, of liver (portal system).
Arteries: ap, pulmonary arterv; a, ascending aorta; ad, descending aorta; c, carotid: s,
subclavian; cm, visceral (coeliac, mesenteric); n, renal; /, iliac; .v, sarnd < c lal).
Veins: vp, pulmonary; cs,'ci, pre- and postcavse (precava paired in most vcrtebr.ites);
i, jugular; s, subclavian; vh, hepatic; p, portal; n, renal; i, iliac; s, sacral.
system remnants may persist, such as a connection (ductus Botalli) of the
pulmonary with the aorta (//, <1.B), or one aortic arch may arise with
the pulmonalis from the right side of the heart (///. ao).
As a result of this in the pulmonate vertebrates systemic blood enters the
right auricle by the pre- and postcavcc, then passes through the riidit ventricle
1st; CHORD ATA.
and pulmonary artery to the lungs. Becoming arterial there, it flows by the
pulmonary vein into the left auricle and ventricle, and thence by the aorta into
the systemic circulation. For details see fig. 538.
llVsides blood-vessels, lymph vessels occur as complements of the venous
svsti-m. The fluids which collect in the spaces of the connective tissue are taken
by them and carried (thoracic duct) into the large venous trunks. Usually the
action of the heart and the movements of the body are sufficient to cause the
flow of this lymph, but special lymph hearts may occur. The lymph vessels
distributed to the digestive tract play an important role, since they serve in the
resorption of digested food. They are called chyle ducts because their contents,
the chyle, rendered white by oil globules at the time of digestion, distinguishes
them from other lymphatics. The most important features of lymph and blood
have already been noticed (p. 78). In special places small bodies, the lymph
glands, are inserted in the course of the lymph vessels, in which lymph corpuscles
arise. Among these from its structure is to be enumerated the spleen, colored
bright red by its rich blood supply.
The sexual and excretory organs are so closely associated that they are
generally united as the uro genital system. The sexual products are formed
in the embryo in a special region of the peritoneal epithelium on either
side of the vertebral column. These primordial cells arise from the ento-
derm, migrate into the epithelium and then early leave this position, and
sink into the underlying connective tissue (fig. 34), forming in the male
glandular tubes, in the female cords which break up into numbers of
round follicles, each containing a single larger cell, the ovum. In the
male the gonads thus formed are compact and frequently oval testes; in
the female they are looser and follicular ovaries.
In many fishes the sexual cells pass out by way of the body cavity and
the abdominal pores, and in this case a part of the ccelom may be cut off as
a special vas defer ens or oviduct. In mo.-t vertebrates the ducts are formed
from a part of the nephriclial system. The urogenital system thus formed
recalls that of the annelids and in both there is the same origin of gonads
and nephridia from the ccelomic epithelium. As the lower vertebrates
(Amphibia and Elasmobranchs) show, the first stage of the excretory
system is furnished by segmentally arranged canals (ncphridial tubules}
which open into the body cavity by ciliated funnels, the nephrostomes.
In many forms the nephrostomes persist throughout life, in others they
degenerate. In the higher vertebrates they are usually not formed, a
fact connected with the loss of the excretory function of the coelom and
its assumption by the blood system. This has entered into connexion
in a characteristic way with the nephridial tubules, and especially by the
formation of ^lomcmli, networks of blood capillaries which, pushing the
wall of the tubules before them, produce the Malpighian bodies. Rarely
there is a rich blood vascular development in the wall of the body cavity,
IV. VERTEBRATA. 487
in close proximity to the nephrostome. This may consist of several net
works or they may be united to a single large glomus, situated in a special
capsule of the ccelom.
The excretory system of the vertebrates is farther distinguished from
that of the annelids in that (i) the nephridial tubules do not open separately
to the exterior but by a common canal on either side ; (2) the segmental
arrangement is largely obliterated by the development of secondary,
tertiary, etc., tubules which produce a compact glandular body. The
accurate explanation of these relations requires a knowledge of the
different kinds of excretory organs in the vertebrates, for which we are
indebted to comparative embryology. Of these there are three kinds:
(i) pronephros or head kidney, (2) mesonepliros or Wolffian body, (3)
metancpliros, or true kidney, the relative positions of which along the verte-
bral column are indicated by their Greek names.
The pronephros is greatly reduced in all vertebrates with the possible
exception of some cyclostomes; it is functional but rarely in the larva (am-
phibians) and is confined to the anterior segments. Whether it formerly
extended farther back, as has been suggested, is questionable. Its vas-
cular system is a glomus, belonging to the ccelom and its short (pro-
nephridial) tubules unite distally to form the pronepliric duct, which grows
back from its point of origin to the cloaca, into which it opens. In the
elasmobranchs a so-called Mailer's duct splits from the pronephric duct,
retaining its connection with the pronephros. In the other vertebrates
the Mullerian duct develops from the pronephros as a canal growing
from in front backwards, and opening in front into the ccelom by large
ciliated funnel, the ostium tuba- abdominale, behind into the cloaca, tlence
Muller's duct belongs to the pronephric system. It develops in the female
into the oviduct and uterus, degenerates in the male. Whether the pro-
nephros is an individual excretory organ, or a part of the same system as
the meso- and metanephroi is disputed.
The mesonephros is a considerable organ in all vertebrates, function-
ing in amniotes only in f octal life, and later being replaced function; lly
by the metanephros and degenerating to a few inconsiderable remains
in the male sexual organs. Its tubules bear Malpighian bodies and unite
secondarily with the pronephric duct, which now is called the II olfinui
duct. The organ has a compact character because additional tubules, are
added to those first formed, these being more numerous farther back.
These later tubules do not empty directly into the duct but into the primary
tubules, the terminal section of which thus becomes a collecting tubule.
In the oecilians there is a modification in that the secondary canals
connect with branches from outpushings from the Woltlian duct, this
488
CHORDATA.
type occurring most abundantly in the posterior part of the mesonephros,
and leading some to regard this portion as a metanephros.
The metanephros, in its typical form as it appears in the amniotes,
consists of two anlagen. A canal grows out from the Wolffian duct,
extends cranialward, and branches richly at its anterior end. The canal
is the ureter, the tubules arising from it are the so-called straight tubules
B
777 q
\s
Ho-
Ov.—Z
FIG. 539. — Scheme of urodele urogemtal system based on Triton (from Wieders-
heim, after Spengel). A, male; B, female, a, excretory ducts; gn, sexual part of meso-
nephros; Ho, testis; Ig, Leydig's duct (ureter); nig, Miillerian duct (oviduct); ing', its
vestigial end in male; N, functional part of mesonephros; Ov, ovary; Ot, ostium tubte;
Ve, vasa efferentia; *, collecting duct of vasa efferentia (rudimentary in B).
of the kidney. The other part of the organ consists of tubules, which
arise from 'nephrogenic tissue,' have no nephrostomes, but include a
Malpighian body at the end. The other end connects with the straight
tubules. They form the convoluted tubules of human histology.
IV. VERTEBRATA. 489
The metanephros has the farther peculiarity of appearing late. The meso-
nephros functions during foetal life and degenerates accordingly as its work is
taken by the metanephros. At last only a part persists, and that in the male,
as a few tubules which have united with the testis and now serve as sperm
conduits (epididymis); farther the Wolfiian duct, which forms the vas deferens,
carrying the sperm; and a rudimentary structure, the paradidymis, remnants of
nephridial tubules which do not connect with the testis. In the female the
whole Wolfiian body disappears with the exception of cpoophoron and paro-
ophoron, corresponding to the epi- and paradidymis, and remains of the Wolfhan
duct (Gaertner's canal).
In close connection with the kidneys, usually in contact with them, are the
problematic organs, the suprar&nals. They consist of a cortical portion, derived
from the peritoneal epithelium, and a central portion, developmentally connected
with the sympathetic system. In the sharks the medullary portion is widely
separated from the other.
The ducts of the urogenital system open behind the anus in most
fishes on a urogenital papilla; in the elasmobranchs, amphibians, birds,
and most reptiles dorsally into the hinder part of the digestive tract, which
thus becomes a cloaca. In turtles and mammals the urogenital canal
opens into the urinary bladder, a ventral diverticulum of the rectum
which first appears in the Amphibia. Urinary and sexual ducts then
either open into the urogenital sinus, the lowest part of the bladder leading
to the cloaca (turtles, monotremes), or this part receives only the genital
ducts, while the ureters enter the base of the bladder. The urogenital
sinus remains in connection with the cloaca in the turtles and monotremes;
in the other mammals a cloaca occurs only in embryonic life. Later, by
formation of the perineum, the cloaca is divided into a hinder digestive and
an anterior urogenital canal.
Asexual and parthenogenetic reproduction are unknown in the vertebrates.
The impregnation of the eggs in the lower groups is usually external and occurs
during oviposition; in the higher internal copulation is effected by apposition
of the genital orifices or by the development of an intromittent organ, the penis.
The fertilized egg may undergo a part or the whole of its development in special-
ized parts of the oviduct (uterus). Accordingly viviparous and oviparous forms
are distinguished, and between these extremes those that are ovoviviparous
(cf. p. 151). Most elasmobranchs are viviparous, but many are oviparous. In
the teleosts oviparous forms predominate, but there are viviparous exceptions.
So, too, among the reptiles and Amphibia there are some viviparous species
among the egg-laying majority. The birds and mammals are most constant,
the first being exclusively ovoviviparous, while all the mammals with the excep-
tion of the ovoviviparous monotremes bring forth living young.
Three embryonal appendages may occur in the development, the yolk
sac, the amnion, and the allantois. The yolk sac is small in those verte-
brates which have some yolk, but not enough to cause meroblastic seg-
mentation (Amphibia), yet it is everwhere present and is best developed
in those groups (fishes, fig. 540; reptiles and birds (fig. 541, do) with
490
CHORDATA.
discoidal segmentation, and is the result of the accumulation of the mate-
rial in the digestive tract, which forces out its ventral wall like a hernia.
Its presence in the mammals, which have small eggs lacking in yolk, is an
indication that these have descended from large-yolked forms, such as
the monotremes yet are. The embryo either lies directly on the yolk or is
connected with it by a yolk stalk.
While the yolk sac is widely distributed, the amnion and allantois are
restricted to reptiles, birds, and mammals, which are consequently spoken
of as Amniota or Allantoidea, in contrast to the fishes and Amphibia,
which are frequently caled Anamnia or Anallantoidea, from the absence
of these structures. The amnion (fig. 541:, am] is a sac which envelops
FIG. 540. FIG. 541.
FIG. 540. — Shark embryo (from Boas), y, part of yolk sac; g, external gills in front
of pectoral fins.
FlG. 541. — Embryonic envelopes of chick (schematized, after Duval). a/, allan-
tois; am, amnion; an eye; c, extraembryonic ccelom; d, digestive tract; do, yolk sac; k,
gill clefts; m, mid-brain; i, 2, hinder and anterior extremities.
the whole embryo and is connected with the rest only at the umbilicus,
that is, the point where the yolk sac projects from the ventral wall. In
this sac is an albuminous amniotic fluid. The amnion is genetically a
part of the ventral surface ; it develops ventrally as a circular fold — lateral,
anterior, and posterior — which grows up over the back on all sides and
unites above the embryo.
The allantois (al) is an enlargement of the urinary bladder. This
grows out from the body cavity at the umbilicus and extends between
yolk sac and amnion and then grows in all directions until its folds meet
above the back carrying with it blood-vessels and connective tissue. The
blood-vessels are the most important, for the allantois forms the respiratory
IV. VERTEBRATA: CYCLO3TOMATA. I'.M
apparatus of the embryo, and in the mammals it develops the placenta,
by which nourishment as well is conveyed to the young. Yolk sac, am-
nion, and allantois are enveloped in a common coat, the serosa.
Aristotle recognized four divisions of vertebrates, and these were retained
by Linne and Cuvier under the names Pisces, Reptilia or Amphibia, Aves, and
Mammalia. Blainville divided the second of these into two classes, retaining
the name Reptilia for one, Amphibia for the other. Milne Edwards showed
that this division corresponded with one between the higher and lower groups,
the amniote and the anamniote divisions. Later Haeckel separated the Cyclo-
stomes from the other fishes as a distinct class, while Huxley pointed out the
close resemblances between reptiles and birds, uniting them as Sauropsida.
Another convenient division contrasts the fishes with all other forms, the Tetra-
poda, so called because they have legs rather than fins.
SERIES I. ICHTHYOPSIDA (ANAMNIA, ANALLANTOIDA).
Vertebrates respiring for a time or throughout life by means of gills;
neither amnion nor allantois present in the embryo.
Class I. Cyclostomata (Marsipobranchii, Agnatha).
The class of Cyclostomes contains but few species of lamprey eels and
slime or hag fishes. In shape they are eel-like. They are distinctly
vertebrate in the possession of large liver and nephridia; of a muscular
heart with auricle and ventricle, lying in a pericardium; olfactory lobes,
epiphysis and hypophysis, and the higher sense organs. In the brain,
cerebrum and cerebellum are not so prominent as are the optic lobes and
medulla. The inner ear (fig. 532, 7) is not divided into utriculus and
sacculus, and it has but one or two semicircular canals, but always two
ampullae. The skin (fig. 27) consists of corium and a stratified epidermis.
The cyclostomes are distinguished from the true fishes by the lack of
a vertebral column. The axial skeleton of the trunk consists either of
the notochord alone or of it and small cartilaginous arcs, representing
neural arches and intercalaria. A cranium and a basket-like gill skeleton
are present, but so different are these from those of other vertebrates that
homologies are difficult. The absence of paired fins is important. The
median fins are supported by horny threads alone, and the cartilaginous
appendicular skeleton — alone of importance — is entirely wanting. Then
the skin lacks scales, and the mouth true dentine teeth, for the pointed
teeth, arranged in circles in the mouth of the lamprey (fig. 542), and the
fewer teeth of the myxinoids, are purely epidermal products and cannot
be compared with the teeth of other vertebrates.
The name Cyclostomata refers to the circular mouth, which rests on
the important fact that the jaws are absent (Agnatha) or extremely
192
CHORDATA.
rudimentary, and do not close on each other as do the jaws of other verte-
brates. This cyclostome condition is of value to the animals, as it aids
them in sucking on to other animals. At the base of the dome-like
mouth cavity is the so-called tongue, which is the sucking apparatus,
since it can be drawn backwards like a piston.
The name Marsipobranchs refers to the form of the gills, usually six
or seven in number, but in Bdellostoma may be twelve or fourteen on either
side. Kach gill cleft consists of three parts, the gill sac (marsupium)^
FIG. 542. FIG. 543.
FIG. S42- — Mouth of Pciromyzon murimis with horny teeth and tongue (from
' ;enbaur).
In;. 543.— Gill apparatus of Myxine ^/utitiosa (after J. Miiller). a, atrium; ab,
"ill artery and gill arch; br, gill sac (the lines show the gills); br', efferent canal; c,
cesophageo-cutaneus duct; d, skin turned away; i, afferent gill canal; «, oesophagus; 5,
mouth of atrium; v, ventricle of heart.
which contains gills, and the afferent and efferent ducts (fig. 543). These
ducts develop separately, and may continue so (Bdellostoma), but in
J'ftnnnyzon the afferent ducts unite to a single tube which opens ventrally
iii the pharynx. In Myxine (fig. 543) the conditions are reversed, the
efferent canals of a side uniting to empty through a single external
opening.
A third name, Monorhina, has been given, since these forms, in con-
trast to all other vertebrates, have an unpaired olfactory organ. The
single nostril, lying in the mid line of the head, opens into a nasal sac, from
the bottom of which a canal descends towards the roof of the mouth, end-
ing blindly in Petromyzontes (Hyperoartia), or penetrating the palate in
the My/ontes (Hyperotretia), so that an inner nasal opening (choana) into
the pharynx occurs. A paired olfactory nerve supplies the organ.
IV. VERTEBRATA: PISCES.
493
Sub Class I. Myzontes (Hyperotretia}.
Semiparasitic cyclostomes with cirri around the mouth, very primitive
nephridia, eyes rudimentary. From the large amount of mucus they are known
as slime eels. They bore into fishes and eat the flesh. Myxine,* east coast;
Bdellostoma* west coast.
Sub Class. II. Petromyzontes (Hyperoartia).
The lampreys (Petromyzm*) have well-developed dorsal fins, and seven
gill openings. They occur in salt and fresh water, some marine species ascend-
FIG. 544.- — Petromyzon marinus* sea lamprey (after Goode).
ing streams to lay their eggs. The young have a larval (Ammocoetes) stage
with rudimentary eyes and slit-like mouth. Many species live on mucus and
blood which they rasp from fishes.
Here may be mentioned a group of paleozoic fossils, the OSTRACO-
DERMI, of uncertain position. They have fish-like bodies, but no skeleton or
jaws are known. Ptcraspis, Cephalaspis, Ptcrichthys.
Class II. Pisces (Fishes).
The term fish is used in a wider and a narrower sense. In the first is
includes any aquatic vertebrate swimming by means of fins and breathing
by gills; in the strict sense, as used here, it means aquatic branchiate forms
with vertebral column, cranium, and well-developed visceral skeleton;
with paired and unpaired fins, supported by a cartilaginous or bony
skeleton; with double nasal pits; with a skin and oral mucous membrane
which can produce ossifications, the scales and teeth. The fishes are the
best adapted of all vertebrates for an aquatic life, and their whole organ-
ization must therefore be considered from this standpoint.
The epidermis consists of numerous layers of cells writh an extremely
thin cuticle. Cornifications of this epidermis are lacking under ordinary
conditions, except a thin portion of the external subcuticular layer. At
the time of sexual maturity cornifications increase greatly in most
Cyprinoids and many Salmonids, producing hard bodies in the skin, the
pearl organs. Enormous numbers of large slime cells give the fishes their
well-known slippery skins. All protective structures arise from the
corium, which is composed of many layers of dense connective tissue and
494
CHORDATA.
furnishes the characteristic dermal skeleton, the scales. These lie at the
boundary of epidermis and corium, commonly imbedded in pockets of the
latter, and are, on account of their different structure, of systematic value,
although the classification based entirely upon them is no longer retained.
The placoid scales (figs. 510, 545, 4) have already been mentioned,
because they form the starting point for dermal ossifications and teeth
(p. 451). They are rhombic bony plates, usually close together like a
mosaic, but not overlapping. In the centre of each is a spine, directed
backwards, in which is a pulp cavity,
while the tip of the spine is covered
with a cap of hard substance, vari-
ously called enamel or vitrodentine.
The ganoid scales (3) are usually
rhomboid and arranged like par-
quetry. In the early stages they may
bear teeth, but these are lost in the
adult. The outer surface is always
covered with a thick layer of ganoin,
which gives, even in fossils, an iri-
descent effect, a most characteristic
feature. The ganoin is no longer
regarded as enamel, but the most
superficial layer of dentine (vitro-
dentine) .
Cycloid and ctenoid scales are closely related. They are always
more loosely placed in the pockets, from which they are easily withdrawn
as in 'scaling' a fish. They are arranged in oblique, transverse, and
longitudinal rows, and overlap like shingles, one scale covering the parts
of two scales behind. The cycloid scales (i) arc approximately circular,
marked on one side by concentric lines, while on the other numerous lines
radiate to the periphery. The ctenoid scale (2) has the radial and con-
centric lines of the cycloid, but has the hinder edge truncate and the free
portion bearing small spines or teeth, processes of the concentric ridges.
Besides these types of scales many fishes bear considerable spines
(strongly developed single scales) and larger bony plates, these last usually
resulting from the fusion of numerous scales.
The coloration of fishes may have three sources. The silvery lustre is due
to crystals of guanin which occur in the skin and in the peritoneum and peri-
cardial walls. In SOUK- fishes (Alburnits lucidus) from their iridescence these
crystals become of commercial value, forming the important part of essence of
pearl, used in making artificial pearls. The other colors of fishes are due in
part to the numerous strongly pigmented fat celis, in part to chromatophores in
FIG. 545. — Scales of fishes, i, cy-
cloid; 2, ctenoid; 3, ganoid; 4, placoid.
IV. VERTEBRATA: PISCES. 495
the corium, which, under control of the nervous system, can alter their form and
extent and thus produce color changes in the fish, thus adapting them to their
surroundings. Destruction of the eyes results in loss of power to change color.
Many deep-sea fishes possess phosphorescent organs (p. 121, fig. 87), possibly
of use in the assemblage of the sexes.
The axial skeleton shows many conditions unknown outside the class,
and varies in character from group to group, the most important differ-
ences consisting in its cartilaginous or bony character. The vertebrae are
nearly always amphiccelous, the notochord persisting in the cavities be-
tween the successive centra (fig. 513). Neural and haemal arches, com-
pleted by the unpaired spinous processes occur. The neural arches
extend throughout the column; the haemal are complete only in the tail;
in the trunk the hasmal spines are absent and the ha'mal processes,
divided into basal processes and ribs, surround the viscera. When
ossification is lacking or is incomplete, two pairs of arches may occur in
each segement, the anterior being the stronger and alone persisting in
fishes with ossified vertebrae; the second is much smaller, so that its
elements are not called arches, but inter calaria (figs. 512, 546). No fish
has a sternum.
The great number of visceral arches, and their independence from the
cranium, are characteristic of fishes. After removal of these the cranium
in all cartilaginous fishes is very simple (fig. 546), but in the teleosts, with
the appearance of ossification, it becomes very complicated, since the bones
are numerous. There are also great differences between the different
families of fishes, some having bones which are lacking in others (figs.
5l6> 547)-
The large membrane bones of the cranial roof (parietals, p, frontah, f, and
nasals, na) and the large ventral parasplienoid (ps) are especially constant. The
Tomer (vo), in front of the parasphenoid, is unpaired, while in all other verte-
brates it is paired. Most constant of the cartilage bones are the ethmoids (the
paired cctetlnnoids, ec, and the sometimes paired mesethmoid) , and the four
occipitals. On the other hand the otic and optic regions vary considerably; the
otic region usually has five bones (fig. 547) : ptcrotic, pto, often called squamosal;
splieiintic, spho, frequently called postfrontal; epiotic, cpo; prootic, pro; and
opisthotic, oo, the last sometimes lacking. In the region of the eye the cartilagi-
nous sphenoids are rarely well developed, the large parasphenoid taking their
place. The same is true of the all- and orbitosphenoids, these sometimes forming
an interorbital septum (fig. 516) or leaving a more or less wide interorbital
fenestra (fig. 547).
The character of the visceral skeleton is related to the aquatic life. All
fishes have numerous gill arches (five to seven, mostly five), which, since
their function — gill supporting — is similar, are similar in structure. So
far as they are not degenerate they consist each of four parts and are
connected below by unpaired copula-, these often being fused. The
196
CHORDATA.
upper ends are frequently toothed and, in chewing are opposed by the
rudimentary last arch, on which account these are spoken of as the
superior and inferior p/iaryngeal bones. The anterior visceral arches are
greatly different in cartilaginous and bony fishes. In the former (fig. 546)
the pterygoquadrate (pq) and the Meckelian cartilage (Md) bear teeth
and oppose each other in biting. In the bony fishes (fig. 547) the
teeth of the lower jaw oppose the tooth-bearing elements, premaxillary
and •nuL\illar\, of the maxillary series, while the pterygoquadrate ele-
ments— the palatine and the series of ptery golds — are the antagonists of
the hyoid. In the elasmobranch type the two halves of the pterygo-
ob ic
17. /i ff jto Tr
Jt.
FIG. 546. — Cranium, visceral arches, and part of vertebral column of Mustelus
vulqaris. ao, antorbital process; co, copula; gp, foramen for glossopharyngeal ; H,
otic capsule and hyoid; Hm, hyomandibular; ic, intercalare; Md, mandible (Meckel s
cartilage); N, nasal capsule; o, optic foramen; ob, neural arch; po, postorbital process;
Pq, pterygoquadrate; ps, spinous process; R, rostrum; r, ribs; tr, trigeminus foramen;
v, vagus foramen; 1-8, visceral arches; i, labial; 2, mandibular; 3, hyoid; 4-8, gill arches.
quadrate meet by symphysis in the middle line; in the others they are
separated by the floor of the skull.
A second characteristic of the bony fishes is already outlined in the
cartilaginous fishes: the modification of the hyomandibular to a suspensor
of the jaws. In the elasmobranchs (especially the skates) the parallelism
of hyoid and mandibular arches is lost, the hyomandibular separating
from the hyoid and attaching itself to the hinge of the jaws. In the
teleosts the hyomandibular is thus brought in connexion with the quad-
rate, and lies between it and the cranium, the joint being thus indirectly
supported from the cranium, a bone, the symplectic (known only in fishes)
helping out the suspensor, while another bone, the interhyal, connects
this with the hyoid, which itself divides into two, so that the hyoid arch,
like a gill arch, consists of four elements.
IV. VERTEBRATA: PISCES.
497
The opcrcnlar apparatus does not occur in all iishes. It consists of
a number of bony plates and processes which arise from the hyoid arch
and extend backwards over the gills, protecting them. .It arises in
part (opercular bones — O, Pro, So, lo, fig. 547) from the hyomandibular,
in part \branchostegal rays) from the hyoid bone. The significance of this
apparatus will be spoken of in connection with the gills; it gives the fish
skull a definite character, but hides its structure, on. which account it,
like the infraorbital ring, is shown in red in the figure.
cc
-me.
na>
FIG. 547. — Skull of haddock. Infraorbital ring and operculum outlined in red. a,
angulare; ar, articulare; as, alisphenoid ; de, dentary; ee, ectethmoid; ekt, ectopterygoid ;
eng, os entoglossum; ent, entopterygoid ; epo, epiotic;_/V, frontal; /;'-// :1, hyoid elements ;
hni, hyomandibular; ///, interhyal; ma, maxilla; me, mesethmoid; mt, metapterygoid ; na ,
nasal; ocb, od, ocs, basi-, ex-, and supraoccipital; oo, opisthotic; p, parietal; pa, palatine ;
pnn, premaxillary; pro, prootic; ps, parasphenoid; pio, pterotic; », quadrate; rbr ,
branchiostegals; spho, sphenotic; sy, symplectic; vo, vomer; i«:', vertebra. Bones out-
lined in rel; inf, infraorbital; Jo, interoperculum; O, operculum; Pro, preoperculum ;
So, suboperculum; i, 2, 3, axes of labial, mandibular, and hyoid arches.
The appendages are also influenced by the aquatic life. In contrast
to the cyclostomes, there are two pairs of paired fins, the tlwracic or pectoral
and the pelvic, ventral, or abdominal fins; in contrast with Amphibia, rep-
tiles and mammals, which occasionally have fin-like structures, the fishes
have three unpaired fins, dorsal, caudal, and anal fins. Only rarely, as in
the eels, the ventral fins are lacking; more rarely (Munenid;e) the pectorals
498 CHORDATA.
are lost. The function of the fins in swimming and in balancing makes
it necessary that they be broad and well-supported plates. Hence numer-
ous skeletal parts are present; besides those preformed in cartilage, numer-
ous horny or bony rays; further, that all parts should be similar and closely,
even if flexibly, bound to each other. Joints are unnecessary except at the
base where the fins join the supports and move upon the body. The
supports of the paired fins are the girdles, pectoral and pelvic, arched
skeletal parts, which in the sharks are held only by muscles, a statement
which is true for the pelvic girdle of all fishes. This is why the ventral
fins so readily change their place. Their primitive position is at the
hinder end of the body cavity (Pisces abdominales, fig. 559). From
this point they can move forward to beneath the pectorals (Pisces thoracici,
fig. 560), or may even come to lie in front of them (Pisces jugulares) in
the throat region (fig. 561). The pectoral arch is united to the verte-
bral column in the skates; in the teleosts it is covered by a large mem-
brane bone (clavicle or deithrirm), and connected to the epiotic region of
the skull by a chain of small bones.
The dorsal and anal fins are supported by elements, preformed in
cartilage, which rest upon the neural or ha?mal spines and in turn support
the fin rays. In the caudal fin the rays rest directly upon the spinous
processes. Three types of caudal fin are recognized (fig. 10) — distinctions
of great importance. The primitive type is the diphy cereal, in which
the vertebral column extends directly into the middle of the fin, dividing
it into symmetrical halves. In the hctcrocercal type the vertebral axis
binds upwards at the base of the fin, so that the dorsal part is reduced,
the ventral greatly enlarged, the result being extremely asymmetrical,
as seen from the exterior. The homocercal fin is symmetrical externally,
but in reality is extremely asymmetrical. The end of the vertebral
column is bent abruptly upwards, and hence the fin is almost entirely
formed of the ventral portion, which is usually divided by a terminal notch
into upper and lower halves. The homocercal fin begins with a diphy-
cercal and passes through a heterocercal stage in development.
Tn correspondence with the simple motions the musculature is simple and
consists largely of longitudinal muscles divided into myotomes, which are conical
\vith the apex in front, and are so inserted in each other that a cross-section gives
concentric circles. In a section there are at least two such systems, the muscles
bang divided by a lateral partition into dorsal and ventral halves. There are
also smaller groups of muscles related to fins, gill arches, jaws, etc., but of much
smaller size, derivatives from the larger mass. Reference has already been
made (|) 1 1 1) to the modification of muscles into electric organs.
The brain shows the low position of the class in the slight development of
the cerebrum. This is especially true of the teleosts (fig. 548), in which, in
place of a cortex, there is only a thin epithelial layer (Pall), what was formerly
IV. VERTEBRATA- PISCES.
499
called cerebrum being only the corpora striata (BG}. The independent ol-
factory lobes lie either close to the cerebrum (most teleosts, Lol] or are separated
from it by an olfactory tract (fig. 549, Lol}. The optic thalami are small (d),
but below them are enlargements characteristic of fishes, the lobi inferiores, and
between them the sacculus vasculosus. Both optic lobes and cerebellum are
greatly developed.
The nose consists of two preoral pits, the opening being divided by a bridge
of skin into anterior and posterior nostrils. In many selachians the nostrils
are connected with the mouth by a oronasal groove covered by a fold of skin,
and in the Dipnoi there is a choana. The eye has several peculiarities. The
FIG. 548. FIG. 549.
FIG. 548. — Brain of trout (after Wiedersheim). BG, corpus ?triatum; CP, epiphy-
sis; HH, cerebellum ;Lol, olfactory lobes; MH, optic lobes; NH, medulla oblongata;
Pall, pallium, in part cut away; VH, cerebrum; I-XII, nerves. (See p. 471).
FIG. 549. — Brain and nasal capsules of Scyllhun catulus (from \Yiedersheim). _/>,
fossa rhomboidalis; tr, olfactory tract.
lens is very convex, almost conical, and the eye is very short-sighted because
light is so absorbed by water that objects forty feet away are invisible. With
this is connected the campanula Hallcri. The processus falciformis, a sickle-
shaped outgrowth of the chorioid, extends from the entrance of the optic nerve
into the vitreous body as far as the lens, swelling out into the campanula; this
contains a muscle which draws back the lens and so is an apparatus of accom-
modation. Chondrifications and ossifications of the sclera are common. Lids
are weakly developed or absent, and only some elasmobranchs have a nictitating
membrane.
The ear has a relative size found in no other vertebrates, the labyrinth (fig.
532) has the sacculus and utriculus separated, the sacculus with a divrrticulum,
the lagena, the beginning of a cochlea. In the labyrinth of the teleosts there are
two large otoliths. Experiments show that the ear is especially an organ of
equilibration, and in some an organ of hearing to a limited extent. A lc\v
species have the power of making a noise, usually by the rubbing of parts on
each other.
Of all sense organs of the skin, the most noticeable are those of the lat, pyloric aeca; c, rectum; d, bile duct; dp,
duct of air bladder; /, intestine; oe, oesophagus; p, pylorus; v, stomach; -vs, spiral gland;
x, rectal gland.
are not sharply marked off from each other. Mouth and pharynx fre-
quently bear teeth. In the teleosts the bones of the roof of the mouth and
the visceral arches may be covered with coalesced heckel-like teeth. In
the elasmobranchs the lining of the mouth, like the external skin, is
frequently covered with small dermal scales; the large chewing teeth are
confined to the edges of the Meckelian and pterygoquadrate arches,
where they are implanted in several rows behind one another. Since the
teeth are held only by membrane and are easily torn out, they may be
replaced indefinitely. Liver and spleen are always present ; pancreas and
gall bladder usually occur. In many fishes blind sacs, the pylorit cccca,
occur at the junction of stomach and intestine (fig. 550, B}\ others have
a spiral valve (A), a. fold of mucous membrane, which extends like a spiral
IV. VERTEBRATA: PISCES.
501
stairway into the lumen of the intestine, increasing the digestive surface.
Caeca and spiral valve rarely occur in the same fish (La-mar gus, ganoids).
Gills of two types occur (fig. 551). In both the gill clefts, which lie
between successive branchial arches, begin by openings in the pharynx,
but differ in their external openings. In the elasmobranch type (A) the
FIG. 551 — Pharynges of (A) Elasmobranch (Zygrena) and (B) Teleost (Gadus), the
skull removed and on the left the gill slits cut across, a, attacnment of upper jaw to
cranium; as, outer gill slit; b, gill arch; bl1, bl'2, anterior and posterior gills (demibranchs) ;
h, dermal projection; hm, hyomandibular; is inner gill cleft; m, mouth; ma, maxillary ;
o, cesophagus; op, operculum; ops, opercular opening; pa, palatine; phi, inferior pharyn-
geal bones; pq, pterygoquadrate; pnn, premaxilla; s, shoulder girdle; uk, lower jaw; c
tongue.
external openings are a series of slits separated by broad dermal bridges
which cover the gills (fig. 555). The gills are vascular folds of
mucous membrane with secondary folds distributed on anterior and
posterior sides of the cleft. Each arch except the last, as the sections
(figs. 551, 552) show, bears two rows of gill folds (demibranchs) which
belong to different clefts and are separated from each other by tissue
containing the cartilaginous gill rays.
In the second type (B), which occurs in all other fishes, the dermal
bridges are lacking, and the septum between the demibranchs has more
502
CHORDATA.
or less completely disappeared, so that the demibranchs of one arch
become connected, their free ends projecting into the water like the teeth
of a double comb. Here, on account of their very delicate structure, they
would be exposed to serious injury were they not protected by the oper-
culum (op) or gill cover. This is a fold of skin arising from the hyoid arch
and extending back over the gill region. It is supported by two groups
of bones, the opercular bones proper (fig. 547, O, So, lo, Pro), attached to
the hyomandibular, and the branchiostegals (rbr) from the hyoid, these
latter supporting the branc/iiostegal membrane. Between the free edges of
the operculum and the branchiostegal membrane and the skin of the body
behind is the opercular cleft (fig. 551, ops), which is
obviously not identical with a gill cleft, but leads into
an atrium into which the gill clefts empty.
In many elasmobranchs and ganoids there is a
rudimentary cleft, the spiracle, between the pterygoquad-
rate and hyomandibular, in which a rudimentary gill,
or pseudobranch, may occur, this often persisting when
the spiracle is closed. The gills proper develop from
that part of the cleft derived from the skin, they are
therefore ectodermal in origin, agreeing with the exter-
nal gills of the amphibian larvae, a matter in which
the two groups were thought to differ. This explains
the existence of external gills in Protopterns and the
larvae oiLepidosircn and Polypterus.
L^~-^
of
Besides gills, fishes, with the exception of
FIG. 552. — Sections elasmobranchs and some teleosts, have a swim
of gill arches of Gadus ..
(left) and Zygana (right), bladder, usually regarded as the homologue of the
slightly enlarged, a, lungs. It is often shaped like an hour glass, filled
artery; b, gill arch; W, . . °
bl*, demibranchs ;Mer- Wlth air> and ma7 °Pen mto the oesophagus by a
mal projection; r, card- pneumatic duct (Physostomi), or this, appearing in
lageray;^, vein; s, tooth. .
development, may be lost in the adult (Physochsti).
In the physoclisti there is a spot, the 'oval,1 in the region of which is a richly
vascular network. Apparently this is for the resorption of the gases of the
bladder when the pressure is reduced in going to a higher level, a matter accom-
plished in the physostomes by passage of the gases through the duct. When a
fish is rapidly brought to the surface from great depths, neither process is suffi-
cient, and in order to accommodate the expansion of the bladder, the viscera are
frequently forced from the mouth. As the fish can resorb the gases from the
swim bladder, it can reform them, they being secreted from the blood the 'red
spots' or 'gas ,t,'/;/\,' spots rich in blood-vessels and covered with a special
epithelium. The possibility of this gas exchange shows how the swim bladder
can function as a respiratory organ, not only as long known in the Dipnoi, but in
other forms like Lcpidosteus and Amia.
Regarding the chief functions of the swim bladder there are two views which
arc not incompatible, (i) The swim bladder is a hydrostatic apparatus, since
the ability to regulate the amount of the contents makes it possible to compensate
IV. VERTEBRATA: PISCES.
for the differences in pressure due to different depths, so that the fish may
remain without muscular action at any desired level. (2) The bladder is a
hydrostatic sense organ for the recognition of water pressure and thus the depth
and, by reflex, for the regulation of the muscular action and muscle tonus.
Support for this view is found in the existence in many fishes of structures
adapted for conveying the variations in pressure to the ear, which is an
organ for equilibration. This may be accomplished, as in the Clupeidae, by
means of processes of the bladder which extend into the region of the ear,
or by the \Vcbcrian apparatus, a system of levers formed by appendages of
the vertebra? and extending from bladder to ear.
The heart, enclosed in the pericardium, lies immediately behind the
gill region, and is protected by the shoulder girdle. It always consists of
auricle and ventricle (fig. 553), separated by a pair of valves to prevent
back-flow of the blood; it sends the blood to the gills by the arterial trunk
(ventral aorta), and receives it from the body through a thin-walled sac,
FIG. 1553. — Forms of hearts of fishes in schematic long section (after Boas). A,
selachian and most ganoids; B, Amia; C. teleost. a, auricle; b, bulhus arteriosus; c,
conus arteriosus; k, valves; s, sinus venosus; t, truncus aorta;; v, ventricle.
the venous sinus, in which the hepatic veins and the Cuvierian ducts
(formed by union of jugular and cardinal veins) empty (figs. 66, 554)-
The most important differences lie in the development of conus and
bulbus arteriosus. These are muscular accessory organs, the first arising
from the heart, the other from the arterial trunk; and correspondingly the
conus has striped, the bulbus smooth muscle fibres. The anterior end of
the heart contains 'semiltmar' valrcs, which prevent the back-flow of the
blood. When, by increase in the number of valves, this part becomes
elongate, a conus arteriosus (fig. 553, A) is formed. The bulbus (C') is a
muscular swelling in front of the valves in the arterial trunk.
The connexion of ventral and dorsal aorta? is effected in young fishes
(fig. 554) by the gill arteries directly; later by means of the complicate 1
504
CHORD ATA.
loops of the gill circulation. When these are developed, afferent branchial
arteries, gill capillaries, and efferent arteries can be recognized, the latter
uniting to form the dorsal aorta and also giving off the arteries (carotids),
which go to the head.
The ncphridia are a pair of large reddish-brown organs lying outside
the body cavity to the right and left of the vertebral column, usually ex-
tending from heart to anus. Their ducts empty behind the anus or in the
dorsal wall of the intestine and are often provided with enlargements
called, from their functions, urinary bladders, although totally different
FIG. 554. — TTcad of embryo teleost (diagram from Gegenbaur). a, auricle; abr,
ventral aorta \vith arterial arches; ad, dorsal aorta; c, carotid; dc, Cuvicrian duct, formed
by union of jugular and posterior cardinal veins; n, nostril; s, gill clefts; sv, sinus veno-
sus; v, ventricle.
morphologically from the urinary bladder of the higher vertebrates. The
gonads, suspended by mesorchia or mesovaria, are large and project into
the body cavity. They are rarely unpaired. In the elasmobranchs and
most ganoids their products pass out by the urogenital system (p. 487),
in other forms by the pori abdominales or by special ducts. With
the exception of the Dipnoi and ganoids which have an unequal total
cleavage, the eggs of fishes have a discoidal segmentation.
Sub Class I. Elasmobrancliii (Plagiostomi, Cliondropterygii}.
The elasmobranchs, the shark-like fishes, are almost wholly marine'
varying in length from a foot and a half to sixty feet, living largely
on other vertebrates, and noted for their voracity. Sometimes slender
and cylindrical (sharks, fig. 555), sometimes flattened dorsoventrally
(skates, fig. 556), they agree in that the head is prolonged into a snout, sup-
ported by a cartilaginous prolongation of the cranium, the rostrum (fig.
546, R). The mouth is ventral, at some distance from the anterior
end, and is transverse, (Plagiostomi — transverse mouth). This makes
it necessary that a shark approaching its prey from below must turn on its
back before biting. The tail is heterocercal or is drawn out in a long
IV. VERTEBRATA: PISCES 505
filament. The skin is covered with placoid scales, usually close together,
these being so small in some_cases that the skin (shagreen) is used instead
of sandpaper. More rarely the scales are larger, and the spines, which
project from the skin, justify in size and form the term dermal teeth.
Such strong spines occur especially at the front of the dorsal fins (ichlhy-
odolnritcs of paleontologists). The skeleton is cartilaginous, frequently
calcified on the outside. The calcification can also extend into the
vertebra?, producing star-like figures (Asterospondyli) or concentric circles
FIG. ^q. — AcniitJiiaa rvlraris* dogfish ("from Claus). B, ventral fin; Br, pectoral
fin; Ks, gill clefts; n, nostril; R1, R-, dorsal fins; 5, heterocercal caudal fm;Spl, spiracle.
(Cyclospondyli). Since bone is lacking, the sharks have no upper jaws,
but bite with the pterygoquadrate. The amphicoelous vertebrae (lack-
ing in Holocephali and several extinct groups,) have neural arches, small
ribs, and intercalaria. The number of gill arches and clefts varies
between five and seven, the first cleft lying between the hyoid and the
first branchial arch. Besides, most elasmobranchs have a spiracle and
pseudobranch (fig. 555, Spl). Except in the Holocephali the gill clefts
open separately, the hyoid arch being without an operculum.
In the visceral anatomy these points are of importance as distinguish-
ing elasmobranchs from Teleostomes. (i) The heart has a large conus,
with several rows of valves (fig. 553, A), but lacks a bullms. (2) The
alimentary tract (fig 550, A) has a spiral valve, but lacks swim bladder
and pyloric ca^ca. (3) The sexual products are carried to the exterior
by the urogenital ducts except in Lccmargus which has abdominal pores.
The eggs escape from the follicles of the ovary (occasionally unpaired)
by dehiscence into the body cavity, and from thence by the unpaired
ostium tubas and the paired Mtillerian ducts to the exterior. The
spermatozoa traverse the anterior part of the Wolffian body ('kidney').
Sexual and urinary ducts open dorsally into the cloaca.
Male elasmobranchs are distinguished by the presence of a copulatory
structure (mixipterygium) developed by enlargement of some radii of the ventral
fin (fig- 55°) c). The large eggs, rich in yolk, are fertilized in the oviducts and
usually develop in uterine enlargements of the ducts. The embryos (fig. 540) ,
with long gill filaments protruding from the gill slits, are nourished by the yolk
506
CHORDATA.
in the yolk sac. In Ifiistelus and Carcharias, as Aristotle knew, there is the
formation of a placenta, which differs from that of the mammals in that the
embryonic blood supply arises from the blood-vessels of the yolk sac and are not
allantoic. There are oviparous elasmobranchs, and in these the egg is sur-
nnmclfd by albumen and a shell, but these eggs differ from those of birds in that
the shell is horny and is usually drawn out at the four corners, sometimes with
threads for attaching the egg to plants, etc.
Order I. Selachii.
With the notochord more or less completely replaced by vertebral centra.
Sub Order I. DIPLOSPONDYLI. Gill slits lateral, six or seven in num-
ber, a single dorsal fin. Chlamydoselaclnis with terminal mouth. Hexanchus*
mouth normal, six gill slits; Heptanchus, seven gill slits.
Sub Order II. SQUALI (Euselachii). Normal sharks, with cylindrical
bodies, free thoracic fins, heterocercal tail, lateral gill slits. Most are fast
—n
Firj- 556— Rai" balis, male, ventral view (after Mobius and Heincke). B
tral, Br, pectoral im; R, rostrum; a, anus; c, copulatory part of ventral; ks gill clefts-
in, mouth; n, nostril; between them the oronasal groove.
wimmers and rapacious, the teeth usually pointed, but in some the teeth are
Numerous families, distinguished by vertebral characters,
imber of dorsal fins, presence of nictitating membrane, etc. GALEIDE
nig membrane present; besides the dog-sharks (Miistelus* and Calais*)
largest sharks, Carckarinus* some with man-eating reputations The
ler heads (Zygana*) are closely allied. The mackerel sharks (Lamna*)
vhite 'man-eater,' Carcharodon* lack nictitating membranes.
IV. VERTEBRATA: PISCES, GAXOIDEI. 507
The foregoing are asterospondylous. The dog-fishes (Acanthias vnlgaris,*
fig. 555) are cyclospondylous; spine in front of each dorsal fin.
Sub Order III. RAI/E, skates; body flattened horizontally (fig. 556); the
pectoral fins, also flattened, united to sides of body, the union usually extending
to tip of snout, and frequently back to the pelvis, giving the body a rhombic
appearance from above. The animals swim by undulating motions of these
fins. The union of fins to the side has resulted in transfer of the gill slits to the
lower surface, the spiracles to the upper. The teeth are usually pavement-like.
PRISTINE, sawfishes, snout prolonged, the edges with teeth. Pristis* RAUDJE;
the typical members of the group; Raia* TRYGONHXE, sting rays, whip-like
tail with one or two spines at base; Dasyatis.* TORPEDINID.^E, smooth skins,
electrical organs between gill arches and pectoral skeleton. Torpedo,*
Order II. Holocephali.
These forms, which have no common names, differ from the selachii in
having the pterygoquadrate arch, which bears a few large chisel teeth, fused
with the cranium; in having a dermal fold (operculum) which covers the gill
-Chim&ra monstrosa (from Kingsley).
slits; and corresponding with this, the gills more on the teleost type (p. 501).
Lastly, the vertebral centra are not developed. Chim&ra.* Fossils appear in
the Devonian.
CLADOSELACHII, ICHTHYOTOMI (Pleur acanthus), and ACANTHODID^E, paleo-
zoic; vertebral centra lacking.
Sub Class II. Ganoid ci.
The ganoids form a transition group in which elasmobranch and
teleost characters are mingled. They have the spiral valve of sharks,
the swim bladder of teleosts; the heart with conus is selachian, the respira-
tory structures — the comb-like gills and the operculum — are as distinctly
teleostean. With the development of the operculum the hyoid arch has
not entirely lost its respiratory function, since in garpike and sturgeon
it bears an opercular gill, and often there is a pseudobranch in the spiracle.
The skeleton is always ossified in certain parts; large membrane bones
lie on the shoulder girdle, on the roof and floor of the skull (parasphenoid) ;
the horny threads of the fins are bony rays. In general the skeleton ranges
between two extremes — an extremely primitive cartilaginous condition
with persistent notochord, and one with considerable ossification, Lepidos-
508 CHORD ATA.
leus having opisthoccele vertebra.'. There are no characters in all ganoids
which occur only in the group. The ganoid scales are often absent, since
the sturgeon has bony plates free from ganoin, while the paddle bill
(Polyodon) has almost no dermal skeleton, and A mi a has cycloid scales.
Most recent and fossil forms possess fulcra, bony plates with forked ends
lying shingle-like in front of the fins (rig. io,5), but these are frequently
absent, e.g., in Amia and Polyptems (fig. 10, A and C). The group is
largely American. The few recent ganoids fall into three distinct groups.
Order I. Crossopterygii.
Largely extinct, two genera persisting. Tails diphycerca! or heterocercal;
pectoral fins scaled at base; broad gular plates beneath jaws in place of branchin-
g-trails; skeleton well ossified. Polyptems and Calamoichthys, Africa. Probably
ancestral to the Amphibia.
Order II. Chondrostei.
These resemble sharks externally in heterocercal tail, spiracle, ventral
position of mouth; internally in cartilaginous skull and (except Polyodon) in
pterygoquadrate serving as upper jaw. Vertebral column primitive, centra
lacking, neural and haemal arches and intercalaria resting on notochordal sheath
FIG. 558. — Acipenser sturij* common sturgeon (after Goode).
(fig. 512). ACIPENSERID.E, large bony dermal plates. Acipenser,* sturgeon.
POLYODONTHXS:, naked skin and long paddle-like snout, toothed maxillaries
present. Polyodon,* paddle fish.
Order III. Holostei.
Skull is ossified as in teleosts; maxillary and premaxillary bones present,
pterygoquadrates reduced and not meeting in front, mouth terminal. Body
with ganoid or cycloid scales. The living forms (the group appears in the trias)
have no spiracle and diphy- or homocercal tails. LEPIDOSTEID^. Scales
rhomboid, branchiostegal rays present, vertebrae opisthocoele. Lepidosteus,*
fjarpiki-. AMIID.E, distinctly teleostean in appearance with cycloid scales,
amphiccelous vertebrae, and heart with reduced conus (fig. 553, B). Amia,*
bow fin.
Sub Class III. Telcostci.
The teleosts owe their name to the extensive ossification of the skeleton,
which consists, in the trunk, of amphiccelous vertebras bearing large
ribs (p. 454), and in front a skull with numerous primary and secondary
bones, already enumerated (p. 495, fig. 547). Maxillaries and
premaxillaries are present, but these are frequently without
teeth, since other bones of the mouth (vomers, palatines, para-
IV. VERTEBRATA: PISCES, TELEOSTEI.
509
sphenoid, hyoid, gill arches, superior pharyngeals — the latter alone in
Cyprinoids) may bear teeth. Frequently there are present small bones,
epiplcurals, usually forked, lying in the intermuscular septa above the
ribs, which are not preformed in cartilage. In the fins both cartilage and
dermal rays are ossified, the former remaining small, the rays forming most
of the support. These rays may either be soft and flexible (Malacopteri)
or hard and spine-like (Acanthopteri), a matter of classificatory value.
In the first case they consist of numerous small threads (fig. 559, Br,
N
jr, / .•,)-.,) r3^*^ \> v N V N W V T
m
K B
FIG. 559. — Perca flitriiitilis (from Ludwig-Leunis). .1, anal fin; B, ventral tin;
Br, pectoral fin; A", operculum; N, nostrils; R1} R2, spinous and soft dorsal fins; S,
caudal fan; SI, lateral line.
A, B, R.,), in the other the parts of a ray are fused to a spine which, some-
times connected with poison glands (Scorpccna, etc.), are good defensive
weapons. The tail is usually homocercal ; the diphycercy of eels and other
fishes is secondary. The dermal skeleton consists of ctenoid or cycloid
scales, sometimes of spines or bony plates, these with true dermal teeth
in the armored Siluroids. In rare instances the skin lacks scales.
The hyoid arch always bears an operculum and branchiostegal
membrane; there is no spiracle and the opercular gill is rudimentary or
absent. The gills of the comb-like type are confined to the four anterior
gill arches, but they may be reduced to two and one-half pairs of demi-
branchs. Instead of a conus (present in Butrinus), the bulbus arteriosus
is well developed; a spiral valve is lacking, but pyloric appendages are
common. A swim bladder is usually present, but its duct is frequently
closed.
The teleosts are distinguished from all vertebrates except the cyclostomes
and perhaps some ganoids in that the nephridial system docs not lorm part of
the sexual ducts. The eggs and milt are deposited through the abdominal pores
or by special canals developed from the body cavity. Copulation occurs in only
a few viviparous forms (Embiotocidas, Gambusia, etc.). The rule is that males
.10
CliORDATA
and females deposit their reproductive products in the water at the same time.
Many species have brighter colors or develop pearl organs (p. 493), at the time
of oviposition. In rare instances the males care for the young (sticklebacks);
more noticeable are the conditions in the lophobranchs (sea horses and pipe
fish), where the males receive the eggs in a brood pouch on the ventral surface.
A metamorphosis is known only in the eel-like fishes, the larvae of which (Lepto-
cep/ialiis) are flat, transparent, with colorless blood, enormous tails, and extremely
small trunk. These larvas normally occur in the sea at the depth of some
hundred fathoms. The fresh-water eels go to the ocean for propagation. On
the other hand, many salt-water fish go to fresh water for reproduction.
The classification of the fishes is yet in an unsettled state. Most European
writers recognize tlu six divisions below. American authorities separate the
Ostariophysi from the Physostomi, the Pediculati and Hemibranchii from the
Acanthopteri, and unite the Anacanthini and some of the Pharyngognathi with
the Acanthopteri and make a distinct group, Synentognathi, of the'others. The
characters on which these divisions are based are less convenient for the tyro
than those adopted here.
Order I. Physostomi.
The character to which this name refers is not readily seen without dissection,
the persistence of the duct of the swim bladder. This is, however, correlated
with the soft character of the fin rays (few exceptions) and the abdominal position
of the ventral fins. The Ostariophysi are remarkable in having a chain of bones
(Weberian apparatus, p. 503) connecting the swim bladder with the ear. More
than a third of the food fishes and nearly all of the fresh-water fishes belong here.
The Ostariophysial families are SILURID.-E (1000 species), or cat-fish;
CYPRINID^E, or carp (1000 species); the suckers, CATOSTOMID^E. GYMNOTI',
electric eel of South America. The other families are true Physostomes. The
SALMONID^, trout and salmon (Salmo*) Oswerus* smelt; Coregonus* white
fish; CtuPEiD.*:, herring, shad; ANGUILLID.E, eels, ESOCID^:, pike and pickerel.
AiiBLYOPsnxE, blind fish of Mammoth Cave.
Order II. Pharyngognathi.
The inferior pharyngeal bones (i.e., the last rudimentary gill arch) fuse to
form a single bone. Some have spiny fins; LABRID.E, Ctenolabrus* the cunners
FIG. 560. — Ctenolabrus ccenilcus,* cunner (after Goode).
'hese arc placed among the Acanthopteri by American authors. Others have
These are the Synentognathi and include the EXOCCETHXE,
some flying hshes.
IV. VERTEBRATA: PISCES, TELEOSTEI.
511
Order III. Acanthopteri (Acanthopterygii).
The largest group of fishes, its members usually having the ventral tins
thoracic in position and more than three rays spiny in dorsal, anal, and ventral
fins. Sticklebacks (GASTEROSTEID.E) and some other forms have the pharyn-
geal bones reduced, the ventral fins farther back, and form the group Hemi-
branchii. Caster osteus* The perch (PzRcnxE), and the marine SERRAKID.E
have ctenoid scales. SCOMBRID^E, mackerel, XIPHIID.*:, sword fishes; snout
prolonged into a long sword; LORICATI, sculpins (Coitus,* Hemitrtpterus*). The
EMBIOTOCID^:, surf perches of the Pacific, viviparous. The suck fishes, Remora,*
first dorsal modified to a sucker on top of head.
Order IV. Anacanthini.
Soft-finned fishes with ventral fins in front of pectorals; descended from
Acanthopteran forms. With few exceptions (Lota,* burbot), marine. GADID.E,
FIG. 561. — Gadus morrhua* cod (after Storer).
cod and haddock; PLEURONECTID^E (flat fishes), halibut, flounders, sole. The
Pleuronectidae, from their asymmetry, need a word. The young are perfectly
symmetrical, but the animals turn on one side, the lower becoming white. The
eye of this side shifts to the upper side, twisting the bones of the skull in its
progress.
/w4-U
?*>\> OSW«W
•;\ vte /.";• --• ' \f water; it burrows in the mud at the dry season, and builds a cocoon
lined with mucus in which it remains quiescent until the wet season. The
nose is respiratory, with choanae opening into the mouth cavity. The
last gill vessels give off pulmonary arteries, and there are veins carrying
the blood back to the heart, pulmonary and systemic circulations being
differentiated. The heart shows the beginning of division into arterial
and venous halves, especially in the regions of the conus and auricle.
The few species now living have a wide and discontinuous distribution, and
are the remnants of a much richer group which appeared in the paleozoic.
IV. VERTEBRATA: AMPHIBIA.
513
MONOPNEUMONIA, with one swim bladder: Ceratodns, Australia. DIPNEU-
MONIA, with two bladders: Protopterus, Africa; Lepidosiren, South America.
The larvse of both have four pairs of gills, three of which are retained in Protop-
terus. Possibly the paleozoic ARTHRODIRA, some of gigantic size (Dinichthys),
belong here.
Class III. Amphibia.
There are two views as to the origin of the Amphibia. According to
one they have descended from Crossopterygian
ganoids; the other is that they have come from
the Dipnoi. The class is distinguished at once
from the fishes by the absence of fins. There is,
it is true, a median fin in larval life, and this
may persist (Perennibranchs, Triton}, but it is
never divided into dorsal, caudal, and anal,
and it lacks any skeletal support (figs. 4, 5).
The paired fins are replaced by toed feet (p.
464). These are often webbed and are used
for swimming, for creeping and leaping, and are
consequently jointed between the separate skele-
tal elements (fig. 564). Besides the shoulder and
hip joints, which alone occur in fishes, there occur
also elbow (knee), wrist (ankle), and finger
joints. The number of digits is not always five,
for a reduction to four, three, or even two occurs.
The connexion of the girdles with parts of the
axialeskeleton (lacking in most fishes) is of impor-
tance. The pelvic girdle is connected with the
vertebral column by means of the ilium, which
arti ulates either directly or by a sacral rib with
the single sacral vertebra. Ventrally the two
halves of the girdle fuse, and usually the limits
of ischium and pubis cannot be traced.
The attachment of the pectoral girdle is less
firm (fig. 521, A). The dorsal portion, the scap-
ula, ends free in the muscles; the ventral, differ-
entiated into coracoid and clavicle, either meets the opposite side in a
symphysis (finnisternmis Anura) or the two sides overlap (arciferous Anura
and urodeles). A connexion with the vertebral column does not exist
since the sternum (lacking in urodeles) is not connected with the ribs.
The sternum extends forward to the clavicle and coracoid, and in the
firmisterna is continued forward by an c pi sternum (fig. 521, .1).
33
FIG. 564. — Skeleton of
hind leg of Salamandra
maculosa larva (from
Gegenbaur). c, centrale;
F, fibula; f, fibulare: /•>,
femur; /, intermedium; T,
tibia; t, tibiale; 1-5, car-
palia and corresponding
metacarpals and digits.
514
CHORDATA
The vertebral column often (Perennibranchs, Derotremes, Cascilians,
and many Stegocephali) resembles that of fishes in amphiccelous centra
and persistence of notochord. The notochord may disappear, there then
occurring opisthoccelous (Salamandrina) or precocious centra (most
Anura). There is also an articulation of skull with vertebral column,
rare in fishes but characteristic of land animals, by which the first
vertebra (atlas) becomes distinct from the rest.
The skull is remarkable for the extent to which the chondrocranium
is retained and the consequent small number of primary bones (figs. 565,
566). The bones of the orbital region are represented by a pair each of
ali- and orbitosphenoids in the urodeles, by a ring of bone, ihesphenet/nnoid,
in the anura. The auditory region usually contains only prootics, the
TYL
C'o'cc
FIG. 565. — Frog skull from below (from Wiedersheim). For letters see fig. 566.
occipital only exoccipitals. The absence of other occipitals is often of
value in distinguishing between amphibian and reptilian skulls, since in
the former the articulation with the atlas is consequently by double
oa. i filial condyles. Of secondary cranial bones are to be mentioned the
nasals, f rentals (in many prefrontals also), and parietals, the latter two
fused in anura to Jronto parietals; ventrally the large parasphenoid.
The cranium is increased by the addition of the large quadrate cartilage,
which becomes applied to the otic capsule and (Anura) fuses with it, while the
rest of its arch (pterygoid) extends forward in a more or less complete condition,
reaching the nasal capsule in the Anura. The quadrate cartilage is covered
externally by the squamosal and supports the lower jaw, composed of Meckel's
cartilage surrounded by membrane bones (dentary, splenial, angulare, etc.); its
IV. VERTEBRATA: AMPHIBIA.
515
articular portion, like the quadrate, being rarely incompletely ossified. Vomers,
palatines, and pterygoids appear in the base of the skull, all three forming a
continuous arch in the Anura; in front of them lie the premaxillaries, and in
most cases maxillaries. Between the hinder end of the maxillaries and the
quadrate there may be a gap or it may be bridged by a jugal. By the modifica-
fo. as.
an.
FIG. 566. — Lateral and hinder views of frog skull (after Parker). Letters for
this and 565: an, angulare; As, alisphenoid cartilage; co (Cocc), occipital condyles;
col, columella; d, dentary; E (e), sphenethmoid;/b, foramen magnum; FP, frontp-
parietal; Gk, otic capsule; k', h", hyoid and copula; /<*, jugal; M (>»}, maxillary (in
lower jaw mento-Meckelian); mk, Meckel's cartilage; N, Nl, nasal capsule; na, nasal;
ob, os, cartilages from which basi- and supraoccipitals arise elsewhere; ol (Olat), exoc-
cipital; pf, frontoparietal; Pal, palatine; p (PP), palatine arch; Pmx, premaxillary; Pro,
prootic; 'Ps, parasphenoid ; Pi, pterygoid; Qu, quadrate; Qjg, jugal; sq, squamosal; Vo,
vomer. Cartilages dotted.
tion of the quadrate into a suspensor the hyomandibular loses its function, and
if represented at all, it is as part of the columella. The character of the remain-
ing visceral skeleton depends upon the respiration (fig. 567). Where gills
occur, the copula and hyoids — representing body and cornua — as well as four
gill arches are present, but with pulmonary respiration the hyoid apparatus is
reduced to a hyoid with anterior and poste-
rior cornua, the gill arches being contained
in the posterior horns. To understand the
conditions in the higher vertebrates it must
be remembered that the reduced fifth arch
becomes a support of the larynx and that in
many anura the hyoid extends to the skull
and fuses with the otic capsule.
Writh the assumption of a terrestrial
life changes occur in the sense organs. FIG. 567.— Hinder visceral skele-
ton of (.4) larva of a salamander;
The organs of the lateral line, which (5) Of toad (from Gegenbaur). a,
occur in all larvae and are persistent in body of hyoid: ft, anterior horn
(hyoid) ; c, rest of branchial skeleton,
the aquatic perenmbranchs, and the
nerves which supply them, disappear; the eyes in the Salamandrina have
upper and lower lids; in the frogs an under lid (really nictitating mem-
brane). The nose becomes respiratory and is provided with choarue
opening into the mouth. Especially noteworthy is the auditory appa-
ratus. This, in the urodeles and ca^cilians, is very primitive, even the
tympanum being absent, but in the Anura a sound-conducting appa-
516
CHORDATA
ratus appears. The spiracular cleft persists as a canal, opening into the
pharynx by the Eustachian tube, its outer end expanded into the tym-
panum, closed externally by the tympanic membrane, supported by the
tympanic annulus (dotted circle in fig. 566). The connexion of the
labyrinth with the tympanum is by an opening in the otic capsule, the
fenestra ovalis, in which is the stapes (? part of capsule), the columella
extending from this to the tympanic membrane
and carrying the sound waves across to the
inner ear. The brain (fig. 568) has advanced
above that of the fishes in the stronger develop-
ment of the cerebrum, but remains behind in
the small size of the cerebellum, which is but
a thin lamella.
The respiratory organs afford important
characters, since both gills and lungs occur.
The larva? of the anura, as a rule, have inter-
nal gills, but the dorsal part of these, as in
many fishes, develop early and very strongly
and extend out through the clefts to the skin
above them as three pairs of external gills.
These, however, are absorbed with the develop-
ment of the other gill leaves, which become
enclosed in a special chamber (atrium} by the
backward growth of an opercular fold from
each hyoid arch. The atrium usually opens
to the exterior by a spiracle on the left side,
but occasionally there are a pair of spiracles.
The external gills are strongly developed in
lobes; VH, cerebrum; Z, epi-
physis; ZH, 'twixt brain.
FIG. 568. — Brain of frog.
/, line between olfactory lobes
and cerebrum; Frh, fossa
rhomboidalis; HH, cerebel-
lum; /, olfactory nerve; Lol,
olfactory lobes; MH, optic the larval urodeles and caecihans, and the in-
ternal gills are correspondingly undeveloped.
The paired lungs open into the hinder part
of the pharynx, either directly through the glottis or more rarely by a
short trachea. Cartilages, the remnants of gill arches, may support
trachea and glottis, and on the latter support vocal cords (larynx).
Breathing is accomplished by a kind of swallowing, the air being
forced into the lungs by the muscles of the floor of the mouth and the
pharynx. Persistent gills and lungs are found together only in the Peren-
nibranchs. Usually the young breathe by gills, the adults by lungs, the
origin of the metamorphosis to be described below.
Besides gills and lungs the skin is an important respiratory organ,
as are pharynx and mouth cavity, in which the air must remain for
IV. VERTEBRATA: AMPHIBIA.
517
some time on account of the respiratory mechanism. This renders
intelligible the fact that many Salamandrina (Spclcrpcs, Desmognathus,
Plet/iodon, Gyrinop/iilus, etc.) have neither gills nor lungs, but have only
pharyngeal and cutaneous respiration. The capillary network in these
parts is greatly developed and may extend into the epithelium. Thus,
also, it happens that in the Anura the skin receives as large an artery
as the lungs (fig. 570, en).
The skin is thin and slimy from the numerous mucous glands, which
not infrequently produce a poisonous secretion (so called parotid gland in
the ear region). The epithelium bears a thin horny layer which at in-
tervals is molted as a continuous sheet. The corium in the Anura is
FIG. 569. FIG. 570.
FIG. 569. — Heart and arterial arches of salamander larva (after Boas), a1 a2
right and left auricles; aa, arterial trunk; ad, dorsal aorta; as, left aortic arch; b, direct
connection between afferent and efferent arteries ;c, carotid;/, afferent artery ;/>, pulmon-
ary artery; v, ventricle; 1-4, afferent arteries; I'-T,', gills.
' FIG. 570. — Heart and arches of frog (diagram), a, aa, right and left auricles; aa,
ventral aorta; ad, as, right and left aortic arches (radices aorta?) ; c, carotid ; r«, cutaneus;
/, lingualis; p, pulmonary artery; ss, subclavian; v, ventricle; ve, vertebralis; i, 2, 4,
persisting arches.
undermined by large lymph spaces, the presence of which makes the
skinning of a frog such an easy matter. Ossifications in the skin — enor-
mously developed in the fossil Stegocephali — occur but rarely (Gymno-
phiona) in recent Amphibia. On the other hand, the abundance of
chromatophores is noticeable, these, under the influence of the nerves,
changing their shape and thus producing color changes in many Amphibia.
Theheart (figs. 569, 5 70) has two auricles, distinctly separated in Anura,
a right with venous blood, a left which with pulmonary respiration re-
ceives arterial blood. There is, however, but a single ventricle, and the
518 CHORD ATA
arterial trunk is, at least externally, single. The arterial arches show
different relations and have different fates. With branchial respiration
the first three afferent and efferent arteries are connected in two ways,
the one by the capillaries of the gills, the other direct (fig. 569, b). In the
fourth arch there is no gill system, but on the other hand this arch gives
off the pulmonary arteries (/>) to the lungs (compare also fig. 537,77).
With the loss of gills (fig. 570) the third arch frequently disappears
entirely (Anura), as well as the gill circulation of the others, while the~
direct circulation persists, at least in part. The first arch gives rise to the
carotids (r) supplying the head, the second unites with its fellow of the
opposite side to form the dorsal aorta; the fourth forms the pulmonary
artery, and in the Anura, gives off a cutaneous artery (cu) to the skin. A
longitudinal fold inside the arterial trunk is so arranged that the venous
blood from the body coming to the heart through the right auricle is
mostly sent out through the fourth arch to the lungs and the skin, while
the blood returned from the lungs by the pulmonary vein passes through
the left auricle and then through the first and second arches (carotid and
aortic arches). So there is a separation of pulmonary and systemic cir-
culations, although the blood all passes through a common ventricle. In the
urodeles the primitive part of the last arch, connecting the pulmonary
artery with the dorsal aorta, may persist as a duct us Botalli.
The sexual organs (fig. 539) are similar to those of selachians. The
eggs pass from the ovary to the oviducts (Muller's duct), and in this are
enveloped with a gelatinous layer. The spermatozoa, on the other hand,
pass through the anterior part of the Wolffian body ('kidney') and thence
out through the ureter. The distinction from the selachians lies in the
fact that a urinary bladder, lying ventrally to the rectum, is present, at
some distance from the urinary ducts, which open dorsally into the
cloaca. Besides sexual organs fat bodies frequently occur, lobed and
often brightly colored structures, best developed between the repro-
ductive seasons.
A sort of copulation occurs, and internal impregnation is effected in many
urodeles and in the Gymnophiona, but not in the Anura. The Anura and most
other forms arc oviparous, but occasionally, a,s Salamandra maculosa and 5. atra
of Kurnpc, viviparous species occur. The male of Alytes obstetr leans wraps
the cords of eggs about his legs and crawls into a hole until the young are hatched,
while the females of Ampliiuma and Ichtliyophis watch over the eggs. The
male of Rhinoderma dnricinii has a large sac arising from the pharynx in which
the ei^s and young are cared for until the completion of the metamorphosis.
In Pi pa americana the male places the eggs on its back, the skin thickening
around them so that each lies in a separate pocket, from which the young escape
at length in nearly the adult form. In Nototrema and Notodelphys there are
dermal sacs upon the back for the reception of the eggs.
IV. VERTEBRATA: AMPHIBIA.
519
The development of the Amphibia possesses special interest, since it
affords the only easily observable instances of a metamorphosis among the
vertebrates. This metamorphosis is the more marked the more the adults
differ from the fishes. In the Anura a larva, the tadpole (fig. 4) escapes
from the egg. It lacks lungs, but has three pairs of external gills, no legs,
but a swimming tail with a fin-like fold. In the metamorphosis the ex-
ternal gills and tail — larval organs — are absorbed, while lungs and legs are
formed. A complication is introduced into the metamorphosis in that,
for a time after the loss of the external gills, internal branchiae, lying
in gill slits, occur (p. 516). In the tailed forms the metamorphosis
is simplified, usually consisting in the loss of the external gills and
sometimes in the change of form of the tail, which may lose its fin
fold and become cylindrical. The last traces of a metamorphosis disap-
pear in the perennibranchs, where lungs occur and the gills persist (Siren
is said to lose the external gills and then re-form them) . In the Anura the
metamorphosis is lost when, as in Hylodes
murtinifcnsis, Pi pa and several other forms,
the whole development occurs in the egg, the
young hatching in the adult form.
Order I. Stegocephali.
Extinct forms with well-developed tail, numer-
ous membrane bones in the skull, and frequently
a bony armor, at least on the ventral surface.
Some were of gigantic size, and some from the
folded condition of the enamel of the teeth are
known as Labyrinthodonta. The group ap-
peared in the carboniferous (footprints in the
Devonian), and died out in the trias.
Order II. Gymnophiona (Caeciliae, Apoda).
These are the nearest of living forms to the
Stegocephali, but fossils are entirely unknown.
The group is exclusively tropical, occurring in
Ceylon, African islands, and America, a discon-
tinuous distribution indicative of great age.
They are burrowing animals and feed on small invertebrates. As a result
of this subterranean life the eyes are small and concealed under the skin,
the legs are entirely lost, so that the animals are snake-like in appearance. In
the skin there are usually small bony scales; the drum of the ear is lacking; the
vertebrae are amphicoelous. Inside the egg many species have three pairs of
feathered gills (fig. 571), a proof of their pertinence to the Amphibia. Later,
there is an external gill opening which finally closes. Iclilliyophis, Ceylon;
Hypogeophis, Seychelles; Concilia, America.
Order III. Urodela (Gradientia).
Of recent Amphibia the urodeles are the most fish-like. The vertebral
column consists of numerous vertebrae, and of these a large part belong to the
FIG. 571. — Larva i.f Irlithy-
nphis glulinosus (from Boas,
after Sarasins).
520 CHORDATA
tail Ribs are present, but so short that they do not reach the sternum, which
is weakly developed or is entirely absent. Tympanum and Eustachian tube
are entirely lacking, as are the vocal cords and the production of sound.
Sub Order I. PERENNIBRANCfflATA. Two or three gill slits, three
bushy gills, and a swimming tail persist throughout life. Necturus* mud
puppy, with legs and two gill slits. Siren,* three gill slits, hind legs lacking;
Typhlomolge* Typhlotriton* both blind, from caves and underground waters of
V . S. Sub Order II. DEROTREMA. External gills lost, but an opening in
the neck leading to the gill slits. Menopoma* (Cryptobranchus) , hellbender,
legs strong; A mphiuma* legs rudimentary. Sub Order III. SALAMANDRINA. _
A'Fter the loss of gills the gill slits close. Amblystoma* (fig. 5), A. tigrinum
and the Mexican axolotl breed in the larval stage. This and other facts leads
to the view that possibly the Perennibranchs are all neotenic (p. 150). Pletho-
tl.ni* Spelerpes* Salamandra.
Order IV. Anura.
The anura have the compact bodies familiar in frogs and toads, with a
small number (7-9) of trunk vertebras and complete absence of tail, the caudal
vertebra being represented by a long bone, the urostyle. Ribs are sometimes
distinct, sometimes fused to the transverse processes; the limbs are larger than
in other Amphibia, and are frequently used for leaping and climbing. Ear
drum and tympanic membrane are lacking only in the Pelobatidae; their presence
is correlated with the existence of vocal cords and the production of sound.
The metamorphosis includes a tadpole stage.
Sub Order I. AGLOSSA. Toad-like anura with degenerate tongue and
unpaired opening of the Eustachian tube. Pipa (p. 518), South America;
Dactylethra, Africa. Sub Order II. ARCIFERA. Tongue present, Eustachian
tubes widely separate, coracoids of the twTo sides overlapping. BUFONITXE,
toads, toothless; Bufo,* the dermal glands poisonous. PELOBATID^E, with teeth.
Scaphiopus,* burrowing toad, with tympanum. HYLID.E, tree toads, toothed;
tips of toes with sucking discs; Hyla* Acris* Sub Order III. FIRMISTER-
XIA. Tongue present, Eustachian tubes distinct, coracoids firmly united in
the middle line. RAXUXE, frogs. Rana*
SERIES II. AMNIOTA.
Vertebrates with amnion and allantois (p. 490) in embryonic life;
•with the pro- and mesonephros functional only in the embryos, and re-
placed in the later stages by the true kidney (metanephros) ; ducts of the
embryonic excretory system retained only so far as they have genital func-
tions; gill slits appearing as transitory structures, but without gills and
never functional. There are two great divisions of the Amniotes, the
Sauropsida and the Mammalia. The Sauropsida include the
Reptilia and the Aves, which agree with each other and differ from the
mammals in having a single occipital condyle, the quadrate acting as
suspensor of the jaws; ankle joint between the first and second rows of
tarsals; the presence of epidermal scales, nucleated red blood corpuscles,
and a cloaca.
IV. VERTEBRATA: REPTILIA.
521
Class I. Reptilia.
On account of similarity of form, the reptiles and Amphibia were long
untied. They form parallel groups: urodeles and lizards, frogs and
turtles, caxilians and snakes. Hence the points of distinction must be
emphasized. The most important are two: the reptiles belong to the
Amniota and, as such, have the embryonal features of the group; second,
although often aquatic, they are, in the entire absence of branchial respira-
tion, in character of skin and skeleton, in
their entire structure, like the true land
animals.
The skin, the better to withstand desic-
cation by the air, is strongly cornified, so
that in the epidermis a many-layered
stratum corneum and a many-layered
stratum Malpighii can be distinguished.
At the tips of the toes the stratum corneum
develops strong claws on the dorsal side,
the claw plate being better developed than
the ventral claw sole. Further protection
is afforded by the thick corium in which
not infrequently bony plates occur.
Dermal glands are very rare, the femoral
pores of lizards (fig. 577, b), which appear
like the ducts of glands, being produced
by the ends of cornified epithelial cones.
Here must be mentioned the dorsal, anal
and musk glands of the crocodiles.
The axial skeleton, both skull and
vertebral column, is nearly always ossified;
only exceptionally (Sphenodon and the am-
phiccele Ascalabota?) are considerable parts
of the notochord retained. The vertebra?
are usually proccelous. In the skull of
reptiles (as in the allied birds) are many
characters which they share with Amphibia and which distinguish them
from mammals. This is especially the case with the visceral skeleton.
As in the Amphibia, the hinder end of the pterygoquadrate is attached
to the otic capsule; the quadrate is ossified and affords the articulation
for the lower jaw, which is composed of many bones. The squamosal lies
at the base of the quadrate and, in the Squamata, is intercalated between
FIG. 572. — Ventral view of
skull of Tropidonotus (from
Wiedersheim). Bp, basioccipital;
Bs, basisphenoid (in front also
parasphenoid); Ch, choana; Cocc,
occipital condyles; Iilh, ethmoid
cartilage; F, frontal; Fo, fenestra
ovalis; M, maxillary; ol, exoc-
cipital; P, parietal; Pf, pref rental;
PI, palatine; Pmx, premaxillary;
Pt, pterygoid; Qu, quadrate; Sgit,
squamosal; Ts, transversum; Vo,
vomer; //, optic foramen.
;,2_> CHORDATA
it and the cranium. Behind it is the columella, its inner end inserted in
the fenestra ovalis. From the quadrate the palatine series of bones—
pterygoid, palatine, vomer — extends forward, these being frequently
toothed; and in front of and parallel to it the premaxillaries and maxil-
laries. Extremely characteristic of the reptiles, the turtles excepted, is an
os transversum, which appears in no other vertebrates. It extends from
the hinder end of the maxillary to the pterygoid (figs. 572, 578, 579, 581,
Ts, //•). A jugal is also frequently present. Of the other visceral
arches, since gills are lacking, only the hyoid bone and laryngeal carti-
lages persist.
In the cranium the complete ossification of the occipital region is
noticeable, the four occipital bones being present. The basioccipital forms
the larger part of the single occipital condyle, in which parts of the ex-
occipitals participate, the single condyle being the sharpest distinction
between the reptilian and amphibian skull. The basisphenoid, which
lies in front of the basioccipital, has an anterior process or rostrum, rep-
resenting the rudimentary parasphenoid (possibly presphenoid). Above,
the skull is roofed in with membrane bones: parietals (frequently fused
and perforated by the parietal foramen for the parietal eye), f rentals,
nasals, as well as pre- and postfrontals and postorbitals, and usually
lacrimals as well.
The ethmoidal region is largely cartilaginous; ali- and orbitosphenoids are
small and variable. Only the prootic is constant in the otic region; epiotic
and opisthotic usually fusing with the occipitals, the opisthotic being large and
distinct only in the turtles. The zygomatic arch (lost in snakes) is formed of
jugal and quadratojugal, while above it may be a second arch formed of post-
orbital and squamosal. The zygomatic arch bounds a gap in the skull, the
orbitotemporal fossa; the other arch a supratemporal fossa. The first fossa
may be subdivided by a process from the zygomatic to the postorbital or post-
frontal bones, thus giving separate orbital and temporal fossae.
The convex occipital condyle forms, with the concave surface of the
first vertebra (atlas), an articulation for motion in the vertical plane and
lateral motions, while a twisting around the long axis of the body is per-
mitted by the joint between the atlas and the second vertebra, the axis
or epistropheus. The atlas is a bony ring, its centrum having separated
and united with the body of the axis, forming a pivot around which the
atlas turns. There are two sacral vertebrae, and the vertebra? of the trunk
are divided into thoracic and lumbar, the former bearing long ribs which
reach to the sternum, while the shorter ribs of the neck end freely.
Limbs are lacking in snakes and some lizards. When present the
number of digits varies between three and five. In the pelvis ischium and
pubis are separated by an obturator foramen and are united with the cor-
IV. VERTEBRATA: REPTILIA.
responding bones of the opposite side by a double symphysis. In the
shoulder girdle scapula and coracoid alone are constant, a clavicle occur-
7.H
FlG. 573. — Viscera of Alligator (from Wiedershcim). ED, rectum; 77, heart; L,
liver; Lg, lung; M, stomach; MD, intestine; Oe, oesophagus; P, pylorus; 7>, trachea;
ZB, body of hyoid; ZH, its cornua; *, perforations of hyoid.
ring in turtles and lizards, in the latter an episternum (fig. 521) as well.
Of considerable systematic importance is the position of the ankle joint.
524
CHORDATA
This is inleriarsal, occurring between the first and second rows of tarsal
bones (fig. 587, C).
Since reptiles lack even transitory gills, the gill slits are completely
degenerate before the young escapes from the egg. Dermal respiration is
far less important than with the Amphibia, lungs, as in birds and mammals,
being the respiratory organs, and in these
a progressive development may be fol-
lowed. The larynx is followed by a
trachea with cartilage supports in its
wall, and this either opens directly into
the two lungs or divides into two bronchi,
which, in Varanus, may divide again
inside the lungs. The lungs in the more
primitive forms are subdivided only per-
ipherally, but in the higher groups the
whole is chambered, partitions extending
inwards to the intrapulmonary bronchus
(fig- 536).
Since the respiration is entirely
pulmonary, the heart is divided into a
left arterial and a right venous half, and
a corresponding separation of systemic
and pulmonary blood-vessels occurs (fig.
574). The two auricles are completely
separated, while a septum extends into
the ventricle, complete in the crocodiles,
but not in turtles, lizards, and snakes.
Yet r™ in the crocodiles a mixin« of
/>, pulmonary artery; s, subclavians; arterial and venous blood occurs since
^t^liiSi'S1^ in p<.ix,* woodcock; Charadrius,* plover.
Sub Order V. SCANSORES. Climbing birds; zygodactyle feet (fig. 594, g),
toes 2 and 3 directed forwards, i and 4 backwards. The forms differ much in
structure and their association does not rest on blood-relationship. Section I.
CUCULIFORMES. The PSITTACI, parrots, brightly colored tropical birds with
short, compressed, and strongly bent beak and fleshy tongue. But one species
(Conurus carolin&nsis*} in the U. S. Cacatua, Plictolophits, cockatoos; Mclo-
psittacus, Psittacus, parrots. CUCULI, bill slightly arched or straight; outer toe
usually versatile; Coccygus,* cuckoos. Section II. PICARI.E, woodpeckers;
long, straight, conical beak and long, protrusible tongue ;Picus.* Nearly allied
are the toucans (Rhamphastos) .
Sub Order VI. PASSE RES. By far the richest in species of the groups of
birds. They are altrices of moderate size, with slender feathered tarsi and
strong, horny beak without cere. Of the three anterior toes the two outer are
either united or'separated to the base (fig. 594, b), the hind toe is at a level with
the rest. In some, noticeable for the powers of song of the males, there are
special muscles to the syrinx which are lacking in other birds. These are
called Oscines, in contrast to the crying birds, or Clamatores. These groups
are further distinguished by a large, freely movable hind toe in the Oscines, while
in the Clamatores it is restricted in its motions.
Section I. OSCINES. All our song birds belong here: FRINGILLHXE, finches;
Passer domesticus* English sparrow; Loxia* crossbills; ICTERID.E; Icterus*
orioles; Dolichonyx,* bobolink; ALAUDID.E, Alauda,* sky-lark; SYLVICOLID^;,
Dendrceca,* Helminthophaga,* warblers; TURDID/E, Turdns,* thrushes;
Siala,* bluebirds; HIRUXDIXID.E, Hirundo* swallows; TROGLODYTID^;, wrens;
CORVIOE, Corvus* crows; Cyanocitta* jays. The PARADISEID.E, or birds of
paradise (fig. 15), with marked sexual dimorphism, are closely related to the
crows. Section II. CLAIIATORES. Here are frequently included a few groups
(COTINGID/E, TYRANNTD.E) best developed in South America and the lyre birds
(MENURID.E) or Australia. Earlier other forms were regarded as allied, but
now are separated as Cypselomorphae, or Coraciformes, and united with the
owls and Picariae. CYPSELED.E; Chatura* chimney 'swallow,' with adherent
feet (fig. 594, c). TROCHILID.E, humming birds, best developed in tropical
America; TrochUus* CAPRIMULGID.E, night hawks; Antrostomus rocifcrus*
whippoorwill. ALCEDINID.E, kingfishers, Ceryle* BUCERONTID.E, horn bills,
tropical.
Sub Order VII. RAPTORES, birds of prey; strong birds of considerable
size; tarso-metatarsus feathered and four strongly clawed toes of the raptatorial
type (fig. 594, d). The beak is strong, the upper half, strongly hooked at the
tip, extending over the lower. The two groups probably are not closely related.
Section I. ^ FALCONIFORMES. Slender birds with close plumage and extra-
ordinary sight; related structurally to the herons. CATHARTHXE, buzzards;
Cathartesaura* turkey buzzard. PANDIONID.E, Pandion halieztus* fish hawk;
FALCONID/E: Aquila* Hali(i'tns* eagles; Bnteo* buzzards; Falco* falcons;
Accipiter* hawks. Section II. STRIGES, owls; compact birds with loose,
fluffy plumage, large eyes in a circle of feathers; related structurally to the
Caprimulgidae. Bubo* horned owls; Scops* screech owls; Strix* gray and
brown owls; Spent vln* burrowing owls.
IV. VERTEBRATA: MAMMALIA
Class III. Mammalia.
545
The mammals occupy the highest place among the vertebrates, and
consequently in the animal kingdom; they also possess a special interest
for us, for man, in structure and development, belongs to the group,
although separated in intelligence from the most highly organized of the
members by a wide gap.
The most striking characteristics of the mammals are furnished by the
skin. In fact one may, with Oken, call them hair-animals, since hair
is as diagnostic as feathers are for birds. The hairs (fig. 596, H) are
cuticular structures which are seated on papillae of the corium, and
are nourished by blood-vessels in these. The lower end, the root of the
hair, lies in a pit in the epidermis, the hair follicle, and is surrounded by a
FIG. 596. — Section of skin of man (from Wiedersheim). Co, corium (derma);
D, oil gland; F, fat; G, blood-vessels; GP, vascular papilla; H, hair; AT, nerves; A>,
nerve papilla; Sc, stratum corneum; SD, SD', sweat gland and duct; SM, stratum
Malpighii.
double envelope, the epithelial root sheath, formed by an inpushing of
the epidermis and an outer connective-tissue follicular sheath. Small
muscles attached to the base of the larger hairs serve for their erection.
Since side branches are lacking, the structure of the hair is more simple
than that of feathers, and the forms fewer. Wool is characterized by its
spiral turns; then there is straight hair which, by increase in size, forms the
'whiskers' (vibrissa) on the upper lip of many mammals, bristles (swine),
and lastly the spines of hedgehogs and porcupines. In the pelts of many
animals two kinds of hair may occur, wool below and straight hair outside.
Histologically hair consists of cornified cells, often arranged in medullary
35
51;) CKORDATA
and cortical layers. On the outside there may be another layer recalling
the pseudocuticula of reptiles. In most mammals there is a periodic
shedding and renewal of the hair, the new hair arising from the old follicle
(Pfrom the old papilla). Ordinarily this occurs only in spring. Besides
hair some mammals have true scales. Constant horny structures are the
armatures of the tips of the digits, which, according to form, are divided
into claws (ungues), hoofs (ungulce), and nails (lamncc). See p. 450.
The old view that the hair, like feathers, corresponds to the scales of reptiles
has recently found both defenders and opponents, the latter thinking it probable
that the hair has arisen from the neuromasts of aquatic vertebrates, which in
the transition to a terrestrial life, became functionless and cornified. Where
scales are present, the hair is regularly grouped around them. The same sort of
grouping occurs in the scaleless mammals, often only in the embryo, a fact
that leads to the view that the scales are an inheritance from the ancestral
mammal, which possessed them in addition to the hair, a view that is opposed
to any homology between hair and scale.
The skin of mammals is further characterized by its richness in glands,
of which there are two kinds, sebaceous and sweat glands. The first are
acinous, and usually open in the hair follicles, giving the hair the required
oiliness (fig. 596, D}. The sweat glands, except in the monotremes, are
entirely independent of the hairs, and are simple tubes, coiled at their
deeper ends (SD), secreting a fluid, sweat, which is of great value in the
preservation of a constant temperature, its evaporation cooling the body.
Under the influence of sexuality the glands in certain regions, and especi-
ally the sebaceous glands, develop great activity and form considerable
glandular pouches or pockets: caudal and anal glands of many carniv.ores,
hoof and suborbital glands of ruminants, musk and castor glands of musk
deer and beaver (fig. 603, a). More important than these are the modi-
fications of dermal glands into mammary or milk glands, which indeed are
so characteristic that they have given rise to the name mammalia. In
the monotremes the milk glands are very large branched sweat glands; and
the same is true for the other mammals, in which they were formerly
regarded as modified sebaceous glands, on account of the somewhat
vesicular ends. In Ornitlwrhyndms the glands open on a circumscribed
area of the skin, which in Echidna is sharply set off from the surroundings
(areola mamma?). In the higher mammals this area may be elevated
directly to a papilla, the true nipple (most mammals, fig. 597, A), or
the skin around the gland openings may be elevated in a tubular form
as in the teats of the cow (B). The mammae are always symmetrically
arranged upon the ventral surface, sometimes in the breast region, but
more frequently in the inguinal region. There are at least two, usually
more (22 in Centetes). In general the number corresponds to the maxi-
IV. VERTEBRATA: MAMMALIA :, 17
mal number of young at a birth. Although the mammas are present in
both sexes, they are functional only in the female, and here only after the
birth of the young.
A dermal skeleton occurs in few species (e.g., the farm bony plates of
the armadillos) ; on the other hand the axial skeleton shows many features
not occurring elsewhere. In the skull many of the bones already referred
to are evident only as centres of ossification, fusing early with their neigh-
bors to form larger bones. As the temporal bone shows, parts of diverse
origin may fuse — parts of the visceral skeleton and parts of the cranium;
membrane and cartilage bones — so that a sharp line between cranial and
A B
FIG. 597. — A, true; B, false nipple (after Gegenbaur).
facial portions cannot be drawn. So it becomes necessary in describing
the skull not to follow exactly the model adopted so far, but to take that
of human anatomy.
In the hinder region of the skull is a large occipital bone (figs. 517-
^19), jointed to the atlas by double occipital condyles, and arising by the
fusion of the four occipitalia, p. 458. Besides it includes usually a mem-
brane bone, the inter parietal, which occurs only in mammals. This is,
strictly speaking, a paired bone, arising in the angle between the parietal s
and the supraoccipital and fusing with the latter. In front of it lie, in the
roof of the cranium, as in other vertebrates, the parietals (fused with the
interparietals in many ruminants and rodents), the f rentals and nasals,
the lacrimals being always associated with them. In the floor of the
cranium the sphenoid, bone lies in front of the basioccipital portion of the
occipital. In many mammals this consists of an anterior and a posterior
portion throughout life; in man this condition occurs at least in the em-
bryo. Each of these parts in development consists of three elements, the
posterior with the basisphenoid as the body, and the paired alisphenoids
(greater -wings) ; the anterior is similarly composed of the presphenoid
and the paired orbitosphenoids (lesser wings) (fig. 518, Spb, Ps, Als,
Ors) . In front of the sphenoid lies the ethmoid, Eth, likewise formed from
three parts, the mesethmoid, which forms a partition between the two
548
CHORDATA
nasal cavities, and the paired ectethmoids, which form part of the lateral
walls of the nasal cavities. These last have complicated folds on their
inner surface, the superior and middle turbinal bones, which support the
olfactory membrane, thus greatly increasing its surface. With these is
associated the os turbinale, a distinct bone, the inferior turHnal bone of
human anatomy.
The temporal bone, which is intercalated between the roof and floor
of the skull, can only be understood by its embryonic relations and its
connexion with the first and second visceral arches (fig. 598). Its centre
la:
FIG. 598. — Skull of embryo Tatusia (after Parker, from Wiedersheim). Cartil-
age dotted, membrane and membrane bones lined, a, incus (quadrate); de, dentary;
fr, frontal; h, (above) membrane over anterior fontanelle, (below) hyoid bones; -im,
premaxillary; ju, jugal (malar); kb, remnants of gill arch; la, lacrimal; mk, Meckel's
cartilage; mx, maxillary; n, malleus (articulare); na, nasal; o, occipital cartilage; os,
supraoccipital; pa, parietal; pe, petrosal; sq, squamosal; st, stapes; ty, tympanic.
is formed by the petrosal (pe}, developed in the walls of the otic capsule,
to which, as elsewhere in the vertebrates, are attached: (i) the cartilag-
inous jaw arches, the quadrate (a), and mandibular (n and mk)', (2) the
cartilaginous hyoid arch, the stapes (in part equalling the hyomandibular,
st), and the hyoid proper (//) (compare with the visceral skeleton of the
selachian, fig. 546). To these are added the membrane bones, the squamo-
sal (sq), at the base of the quadrate, which increases as the latter loses
in size, and below the squamosal the tympanic (ty). With ossification
of the cartilaginous parts several centres form the petrosal, which fuses
with the squamosal, and frequently with the tympanic, which in some
forms enlarges to a conspicuous auditory bulla. Petrosal and squamosal
on the one side, tympanic on the other, enclose a space, the tympanic
cavity, into which the upper parts of both visceral arches extend, ossify-
IV. VERTEBRATA: MAMMALIA 549
ing into the ear bones, the quadrate to the incus, the hyomandibular pos-
sibly to stapes (fig. 534).
The tympanic in uniting with the squamosal encroaches on the mandib-
ular cartilage so that the upper end (n), which is homologous with the
articulare of other vertebrates, is enclosed in the tympanic cavity and,
along with a second bone, the goniale, ossifies to form the malleus,
while the lower portion, Meckel's cartilage proper (mk), is pinched off, to
form the axis of the lower jaw. Meckel's cartilage gradually disappears;
on the other hand the surrounding membrane bone, the dentary (de) in-
creases and alone forms the lower jaw, which now forms a new articulation
with the squamosal. It will be noticed that the old articulation was
between cartilage bones, the new between membrane bones developing
around the cartilages. (There is, however, some evidence to show that
the mammalian lower jaw consists of several bones, some of them pre-
formed in cartilage, and that one of these forms the articulation with the
squamosal.)
The transformation of the articulare and quadrate of the lower vertebrates
into the malleus and incus and the consequent view that the articulation of the
jaws is not homologous is strongly opposed by some, who see difficulty in the
transfer of the hinge. It must be remembered that the transfer is necessary
upon any hypothesis, and loses some of its difficulties, if we suppose a time
when both articulations were functional.
The lower part of the hyoid arch, the hyoid, remains outside the ear
and often fuses with the petrosal. Its upper end (styloid process) may
then become entirely separate from the lower, which becomes attached to
the copula (body of hyoid) as the anterior horn, the connecting cartilage
being reduced to a styloliyoid ligament. In the hyoid of mammals there is
also included a remnant of a gill arch as the posterior horn.
As the quadrate, by its modification into the incus, becomes strikingly
reduced, the rest of the arch — vomer, palatine, and pterygoid — is poorly
developed in contrast to the large maxillary bones. Premaxillaries and
maxillaries (fused in man to a single bone on either side) form an im-
portant element in the face, and send backwards and inwards palatine
processes into the roof of the mouth. These encroach upon the bones of
the palatal series; the vomers of the two sides are pressed together to a
single bone lying vertically entirely within the nasal partition; the palatine
and pterygoid are forced backwards. The palatines contribute to the
hard palate, the pterygoids only exceptionally (Cetacea, many edeniat-
the latter usually lose their independence and fuse with the nearest bone
of the base of the cranium, the basisphenoid (more accurately with a
process of the basisphenoid, the lamina externa of the pterygoid process,
550
CHORDATA
G
the pterygoid forming the lamina interna). Thus the hinder sphenoid,
like the temporal, contains cranial and visceral elements.
The vertebrae are completely ossified and are connected by interver-
Idn-al ligaments, discs of fibrous cartilage. The cervical and rib-bearing
thoracic vertebrae are always distinct, and except in the cetacea, lumbar,
sacral and caudal vertebrae as well. These regions are little variable in
extent. The cervicals, including atlas and axis (epistropheus) number
seven except in Brady pus tridactylus (9), Clwhvpus hofmanni and Manafus
((>). In the formation of the sacral region two steps are distinguishable.
There is the union of the true sacral vertebrae with the os ilium and then
the fusion of these and with them certain of the caudal vertebrae (pseudo-
sacral vertebra1) to form the os sacrum. The number of true sacral
vertebrae is frequently (Marsupials, ungulates, rodents, lemurs) one, as in
the amphibia, two (carnivores, pri-
mates) or rarely more (Echidna, many
edentates). The pseudosacral verte-
brae are rarely lacking entirely; usually
their number varies between one and
three.
Of the appendicular skeleton the
girdles are most interesting. The
coracoid, which in monotremes
reaches the sternum, is reduced in
all other mammals to a small coracoid
process of the scapula. More rarely
the clavicle is lacking (rapid runners) ;
in the monotremes it extends to the
episternum (fig. 599, Cl, Ep); else-
where it appears to articulate with the
sternum, in reality by the intervention
of interarticular cartilages (once regarded as a rudimentary episternum,
now called predavia). In the pelvis all three elements are fused to a
single os innominatmn; pubis and ischium unite ventrally with each other,
enclosing between them the obturator foramen (fig. 606). The pubes of
the two sides unite by a symphysis which can extend back to the ischia.
The great differentiation of the appendages is also highly characteristic
he mammals. In the climbing species the thumb and great toe become oppos-
, resulting m a grasping hand or foot; but usually they are used to support
body on the ground. Occasionally the whole hand or foot rests on the
ound (plantigrade) but usually the basal joints of the feet contribute to the
v;th ot the leg, the phalanges alone forming the 'sole' (di^iti^rade) or (unguli-
ide) the weight being supported on the tips of the toes.
599. — Sternum and shoulder
girlie of Ornithorhyn-hus paradoxus
(from Wiedersheim). Cl, clavicle; Co,
Co', coracoid; Ep, episternum; G,
glenoid fossa forhumerus; 5, scapula;
St, manubrium sterni (anterior element
of sternum).
IV. VERTEBRATA: MAMMALIA
Since the mammals in general are distinguished from other vertebrates
by their intelligence, the brain is characterized by the size of cerebrum and
cerebellum (fig. 600). In contrast to birds and fishes, the cerebellum (7F)
is differentiated into a median vcrmis and lateral cerebellar hemispheres.
In the cerebrum the frontal lobes of the pallium grow forwards over the
olfactory lobes, which consequently lie farther and farther back on the
lower surface. The temporal lobes extend right and left over the optic
lobes and down to the floor of the cranium; the occipital lobes cover
successively the mid brain, cerebellum, and medulla oblongata. Since
there is a great range of intelligence in the mammals, the cerebra may
be arranged in an ascending series. In the monotremes, marsupials,
insectivora, and rodents (fig. 600, .4) the olfactory lobes are visible in
B
C
L
FIG. 600. — A, brain of rabbit (after Gegenbaur). B, of fish otter. C, of
pavian monkey (after Leuret and Gratiolet). 7, cerebrum; ///, optic lobes; IV,
cerebellum; V, medulla oblongata; lo, olf actor}' lobes.
front, usually the mid brain behind (///). In the lemurs, carnivores
(B), and ungulates the olfactory lobes are nearly, the cerebellum partly,
covered. In man and the anthropoid apes (C), on removing the roof
of the skull, only the two cerebral hemispheres are visible.
Further, it is to be noted that in the first group the surface of the cere-
brum is smooth, while in the others the cortex is increased by infolding and
the formation of convolutions (gyri and s-ulci) which reach their greatest
complication in the anthropoid apes and in man. A consequence of the
increase in size of the brain is the great development of the connecting
nerve tracts, which become more and more prominent as parts of the
brain. Thus the two halves of the cerebrum are connected by two
transverse tracts, the corpus callosum and the fornix; two solid cords, the
crura cerebri, run back from the cerebrum to the other parts, while a
552 CHORDATA
transverse commissure, the pans Varolii, passes below, connecting the
two sides of the cerebellum. These connexions in the other vertebrates
are small, and even in the lower mammals, like monotremes and marsu-
pials, are but slightly developed.
The increase of cerebrum and cerebellum, which occurs chiefly in the
dorsal portion, has resulted in flexures in the axis of the brain, already indi-
cated in the reptiles, increased in the birds, and reaching their maximum
in the mammals. Instead of continuing in the direction of the spinal cord,
the axis of the brain bends ventrally in the medullary region (cervical flexure),
then in the region of the pons again dorsally (pontal flexure), and at the level of
the optic lobes again ventrally (cephalic flexure) . By its increase in size the
brain has influenced the skull in an interesting way; for, while even in birds the
brain is almost entirely confined to the region behind the eyes, in the higher
mammals it has extended forward to the olfactory region. Thus there comes
an increase of the cranium at the expense of the face.
Of the sense organs the nose is characterized by three features. An
outer nose, supported by cartilage and often extended as a proboscis,
has been formed. Its cavity has been increased, since by the formation
of hard and soft palate a part of the primitive mouth cavity has been
included in it. Its upper portion, the olfactory region, has been compli-
cated by the formation of olfactory folds, supported by the turbinal bones
already referred to (p. 548). The eye has the upper and lower lids,
besides the nictitating membrane in a more or less reduced condition.
The ear, except in monotremes, cetacea, sirenia, and some seals, has a
conch supported by cartilage, while the external auditory meatus is always
present. Internally the ear is much modified, since the three bones,
malleus, incus, and stapes (p. 479), occur nowhere else, while the lagena
has been greatly lengthened, coiled into a spiral with two to four turns
(figs. 83, 533), inside of which is the wonderful organ of Corti.
Of digestive structures, the teeth, restricted to maxillary, premaxillary,
and dentary bones, need special mention, because of the distinctions they
afford from all other vertebrates, and because of their importance in
differentiating the various orders. If we omit the monotremes, eden-
tates, and whales, in which there is marked degeneration in the dentition,
there are four particulars which show the dentition of mammals more
developed than that of other vertebrates, (i) The number of teeth is
constant for the species, usually for the genus, and often for the family.
As man normally has thirty-two teeth, so the dog has forty-two, the
anthropoid apes thirty-two, the platyrhine apes thirty-six, etc. (2) The
teeth are more firmly fixed. The body of dentine is divided, by a slight
constriction, into a crown covered with enamel, and a root enveloped in
cement (bony tissue). The roots are placed in separate sockets (alveoli)
IV. VERTEBRATA: MAMMALIA
in the jaws, and in those cases where continuous growth is necessary the
pulp persists and the teeth (incisors of rodents, tusks of elephants and pigs)
grow indefinitely. (3) In consequence of their greater hardness the
teeth are not used up so fast and do not require rapid replacement. There
occurs at most only one change, in which the dentition present at birth
or developed soon after — the milk, or lacteal, dentition or, better, first
dentition — is replaced by the second or permanent dentition (diphyodont
mammals). In some cases (monophyodont mammals) there is no change,
the first dentition being permanently retained (marsupials, perhaps
toothed whales), or the first dentition is more or less rudimentary
(edentates, many rodents, bats, seals, some insectivores) . Besides the
two typical dentitions traces of a third or even a fourth may occur. A
prdacteal dentition of calcified germs which are never functional is best
seen in marsupials, and is rare in placental mammals. A post-permanent
dentition following the permanent one is outlined in many placentalia,
and some of its teeth may exceptionally come into function. (4) Among
the teeth a division of labor has brought
about change of form (heterodont dentition').
The teeth of the premaxillaries and their
antagonists in the lower jaw are single-
rooted and usually have more or less a
chisel shape, hence they are called incisors
even when, as in insectivores, the crowns
are needle-like and there are two roots
(some insectivores, marsupials, lemurs)
(fig. 612). Behind the incisors (in the
maxillary bone in the upper jaw) is the
canine tootli (fig. 601, c), which is single-
rooted and has usually a conical crown
(probably a modified premolar). Follow-
ing the canine come the molars, broad teeth
mostly with two roots and tubercular crowns. Only the anterior ones
appear in the milk dentition, while the others appear only in the perma-
nent dentition and are not replaced. On this developmental basis the
molars are divided into premolar s (bicuspids of dentists), which appear
in both dentitions, and the true molars, which occur only in the last.
As stated above, every species of mammal has a characteristic denti-
tion; its features may be expressed by a short formula. It is only neces-
sary to place the number of each of the four kinds of teeth mentioned in
their regular order, those of the upper jaw separated from those of the
lower by a horizontal line, to express this. Since the two sides of the body
Fir,. Coi. — Permanent and
milk dentitions of the cat (from
Boas), c, canines; p'-p4, pre-
molars; in', molar (the milk
dentition darker and each letter
preceded by d).
55 I CHORD ATA
are symmetrical, only those of one side need be enumerated, and in case
that one kind be absent the deficiency is indicated by a zero. Thus the
dental formula of man is fHf ;°f the reindeer, in which in the upper jaw
incisors and canines are absent, |?ff . The different formulae, by com-
parison, give us a fundamental formula from which they have been
derived by reduction. This was probably fiff .
The molars undergo, according to the food, the greatest modification of
form. As a starting point the bunodont tooth of omnivorous mammals may-be
taken, which has the crown with several blunt projections or cones. With
animal food (figs. 601, 608) the cones are sharper and cutting (secodcmt dentition
of carnivores and insectivores), and when the cutting angle becomes very sharp,
with a special prominence on the inner side, it is spoken of as a flesh or car-
nasial tooth. In vegetable feeders the cones become connected by crests (lophs) or
are half-moon-shaped (lophodontor sclenodont) . Since the cones and lophs become
in part worn away and the grooves between them are filled with cement, there
arise broad grinding surfaces strengthened by the harder and more resistant
enamel of the cones and lophs; this extends inwards as folds from the outer
enamel wall of" the tooth; the folds may become cut off and form islands of enamel
on the grinding surface (denies comlicati of ungulates). When the folds extend
in regular order from the outside and imide and meet in the middle they form
numerous successive plates bound together by cement (compound teeth of
elephants, fig. 618, and many rodents).
Paleontological investigation, with which the more recent embryological
results are in accord, has shown that a great regularity prevails in the formation
of the cones of the molars. Triconodont and tritiibcrcular teeth are recognized, in
which the three cones are either arranged in a line or in a triangle, as well as
multituberciilar teeth with more numerous cones irregularly arranged. The
triconodont type develops farther by the formation of secondary cones. The
development of these occurs in different ways in molars and premolars. Since
the latter are the more simple, their distinction from the molars does not rest
alone upon the existence of a milk dentition, but upon structure as well. This
is important, because it happens that there are premolars which are not replaced
(marsupials, many insectivores and rodents) and, on the other hand, beneath
the molars the anlagen of replacing teeth may be found. The latter fact shows
that the molars, strictly speaking, belong not to the permanent but to the milk
dentition. They are late in formation and are therefore parts of the first
dentition carried over into the second.
The lips, bounding the opening of the mouth, differ from those of all other
vertebrates by their soft character and the mobility caused by special muscles.
The mouth cavity, which contains tongue and teeth, is separated from the next
division of the alimentary tract, the pharynx, by the uvula. The pharynx
narrows behind into the oesophagus, the entrance of which into the stomach is
marked by a constricting cardia. At its other end the stomach has a similar
constrictor, the pylorus, separating it from the intestine. In the latter small
and large intestines (the latter consisting of colon and rectum) are differentiated
by the diameter of the lumen. The small intestine opens laterally into the
colon and at the junction arises a blind diverticulum, the cacum, which is small
in mammals with animal food, but in herbivores (especially rodents) is always
large and forms a conspicuous part of the alimentary tract. The -vermiform
appendix (primates, rodents) is a narrower part of the caecum with numerous
lymph follicles. Three pairs of salivary glands empty into the mouth, the liver
and pancreas into the small intestine (duodenum}.
IV. VKRTEBRATA: MAMMALIA 555
Most important of respiratory peculiarities is the diaphragm, which
separates the body cavity into thoracic and abdominal cavities. This
occurs only in its beginnings in other vertebrates (perhaps even in Am-
phibia). In the thoracic cavity are the oesophagus, heart with its pericar-
dium, and especially the trachea, bronchi, and lungs; the remaining
vegetative organs are in the abdominal cavity. The diaphragm is a
muscular dome, its convex side towards the thoracic cavity; by contraction
it flattens and increases the size of the cavity, in consequence of which air is
drawn into the lungs (inspiration) . On relaxation the lungs contract from
their own elasticity and force out a part of the air (expiration). The in-
tercostal muscles, which raise and lower the framework of the chest, also
play a part, as in birds. The respiratory ducts begin with the larynx
(with vocal cords), supported by the thyreoid cartilage occurring only in
mammals, which can be closed from the pharynx by the epiglottis; this is
followed by the trachea, which divides into right and left bronchi. Each
bronchus divides again and again, and the finest of these divisions, the
bronchioles, are continued as alveolar ducts to small chambers, the infiind ib-
id a, both these and the alveolar ducts being lined with small respiratory
pockets, the alveoli. (See p. 482.)
The heart, with two auricles and two ventricles, is completely separated
into systemic and pulmonary halves. In early embryonic life the arterial
trunk, which at first is simple, is divided into a pulmonary artery, arising
from the right half of the heart and carrying venous blood, and an aorta
ascendens, with arterial blood, connected with the left half. In contrast
with reptiles and birds, the right aortic arch is lost, the left persisting.
The urogenital system is of great importance in the separation of the
group into smaller divisions. In both sexes this consists of practically the
same parts in early embryonic life (fig. 602). These are the early formed
Wolffian body (IF); the permanent kidneys, which appear later and are
not shown in the diagram; the urinary bladder (4), a part of the allantois
which extends (5) into the f octal appendages; the three ducts, the Mtil-
lerian (m), the Wolffian (TV), an(l the ureter (3). These ducts no longer
empty into the intestine, but into the allantoic structures, the ureters, ex-
cept in monotremes, into the base of the urinary bladder, the Wolffian and
Miillerian ducts into the urogenital sinus (iig), the lower continuation of
the bladder. In the anterior wall of the urogenital sinus is a mass of
highly vascular tissue (cp), from which and a surrounding fold the exter-
nal genitalia are developed. Since the urogenital sinus opens from in
front into the intestine, there is always a cloaca (cl) in the embryonic
stages. The entodermal section of the cloaca is separated in all mammals
into rectum and urogenital sinus. In the monotremes there is added a deep
556
CHORDATA
ectodermal cloacal pit, which is less distinct in the female marsupials,
many insectivores, rodents and edentates. In all other mammals anus
and urogenital openings are separated by the formation of a partition, the
perineum.
From this indifferent condition the male apparatus is derived, the
structures being closely similar in most species (fig. 603). The Miillerian
FIG. 602. FIG. 603.
FIG 602.— Diagram of embryonic mammalian urogenital system (from Balfour,
after Thompson), d, cloaca; cp, genital process; go, genital cord; i, rectum; Is, ridge
*°r ,°ymatlon of labia or scrotum; m, Mullerian duct; of, gonad; u%, urogenital sinus;
W, VVolffian body; u<, Wolffian duct; 3, ureter; 4, urinary bladder; 5, continuation
of latter to allantois furachus).
FIG. 603.— Urogenital system of male beaver (from Blanchard). a, castoreum
sacs; 6, openings of their ducts into preputial canal; c, tip of penis; d, preputial opening;
e anal glands;/, their ducts; g, anus; h, base of tail; i, corpora cavernosa; k, Cowper's
glands; I, seminal vesicles; m, vasa deferentia; n, testes; o, urinary bladder with ureters.
duct vanishes, while the Wolffian duct becomes the vas deferens and its
accessories, serving as a canal for the genital products, while the external
genitals arise from the other parts mentioned, these forming an intro-
mittent organ (penis) . In the female the Wolffian body and duct degener-
ate, the Mullerian ducts become the reproductive canals. The modifi-
cations of these become of great systematic importance. In the ir.ono-
tremes both ducts open separately into the urogenital sinus and become
differentiated into two parts (fig. 604, A), anterior oviducts (od) with wide
IV. VERTEBRATA: MAMMALIA
557
openings into the body cavity (/) and the uterus («). The ureters open
into the sinus between the uterine openings. In the marsupials (B and C)
there are three divisions, oviduct, uterus, and vagina; besides, the two
Miillerian ducts may approach, near the uterus (B), or fuse in this region
(C) in some species, forming an unpaired blind sac (vb), which may even
9" «
*A'i'
FIG. 604. — Female genitalia of (.4) Echidna aculeata; (B) of Didelphys dorsigera;
(C) Phascolomys -wombat (B and C, after Wiedersheim). d, cloaca; d, rectum;
h, urinary bladder; n, kidney; o, ovary; od, oviduct; pu, mouth of ureters; su, urogenital
sinus; t, ostium abdominale tubs; u, uterus; u', opening into vagina; ur, ureter; v,
vagina; vb, vaginal blind sac.
open into the urogenital sinus as a third vagina. This partial fusion of
the vaginae of the marsupials is completed in the placental mammals, the
single vagina and the sinus forming a single canal (rig. 605). Here the
uterine portions may remain distinct (uterus duplex or rodents, A), or they
may fuse partially (uterus bicornis of insectivores, whales, ungulates, and
FIG. 605. — .4, uterus duplex; B, uterus bicornis; C, uterus simplex (from Gegen-
baur). od, oviduct; u, uterus; v, vagina.
carnivores, B}, or they may be completely fused (uterus simplex of apes
and man, C).
Thus there are three different types of the female genitalia, in which
the vagina is not differentiated (Ornithodelphia), or is double (Marsupi-
alia), or is single and unpaired (Monodelphia). To these correspond
three types of development. The Ornithodelphia are oviparous, the
558
CHORDATA
others viviparous, but are distinguished by the duration of pregnancy.
The eggs of the viviparous species are so small (about .01 inch) that they
have a total, nearly equal segmentation. Such eggs require nourishment
from the mother in order to produce an animal with the complicated
structure of a mammal. Since in the Didelphia the uterine nourishment
is usually very incomplete, the period of pregnancy is very short, in com-
parison with the Monodelphia, in which a placenta, a complicated apparatus
for the nourishment of the young, appears; hence the marsupials, with
their small imperfectly formed young, are of en called Aplacentalia; the
Monodelphia, Placentalia.
.Ml mammals care for the young, this being chiefly or wholly done by the
mother, who not only supplies them with milk but protects them in nests.
Most mammals are monogamous, some polygamous, while in others there is no
permanent association of the sexes. The body temperature is constant and
ranges from 36°_to 41° C. (98° to 106° F.); in Echidna it is only 26° to 34° C.
(79° to 83° F.). In most, continual feeding is necessary for existence; from this
rule there are a few exceptions, like the bears, marmots, badgers, etc., which
hibernate during the winter, taking no food. At this time there is a fall in
the temperature (in the marmots nearly to freezing) due to the diminished
metabolism.
Sub Class I. Monotremata (Ornithodelphia, Prototheria).
A few mammals, confined to Australia and New Guinea, are the only
living representatives of the group. They are distinguished from all
other mammals by laying eggs about
half an inch long, rich in yolk and with
soft shells. These undergo in the uterus
a discoidal (meroblastic) segmentation
and are then incubated by Ornithorhyn-
chns in a nest, by Echidna in a temporary
pouch (marsupium) on the ventral sur-
face of the body. On hatching the
young are nourished by the secretion of
--& enormously enlarged sweat glands, which
form two large masses to the right and
left of the mid-ventral surface. Each
FIG. 606.— Pelvis (left side) of opens on a special region of the ventral
8S3J233"1 ftSSSj™ ^^ Whkh is ^^ in Ornithorhyn-
amen; //, ilium; is, ischiunv Om c^lus, a flattened pocket in the others.
>one; P, os pubis. Other distinctions from other mam-
mals, which are also points of resem-
>lance to reptiles and birds, are the strong development of the epi-
sternum and the extension of the coracoid to the sternum (fig. 599), the
IV. VERTEBRATA: MAMMALIA, MARSUPIALIA 559
termination of the ureters in the urogenital sinus (fig. 604), the existence
of a cloaca in both sexes, and the specifically bird-like character of the
female sexual organs, in which the large left ovary is alone functional,
and uterus and vagina are not differentiated. But with all this it
must not be forgotten that the monotremes have the hair, the skull
(ear bones, articulation of jaw), the urogenital sinus of true mammals;
and in the presence of marsupial bones connected with the pel.vis (fig.
606, Om) show a close relationship with the marsupials. The jaws are
toothless and enclosed in horny sheaths, yet in the young of Ornithor-
hync/ius there are in each jaw three pairs of multitubercular molars,
which are later replaced by four broad horny plates. In the embryo
Echidna tooth germs are formed and also a dentine egg-tooth for opening
the shell.
ECHIDNID/E. The spiny ant-eaters have the body covered with spines,
snout with a worm-shaped tongue used in catching insects; Echidna, Australia,
feet five-toed, with digging claws; Proechidna, New Guinea, three-toed. ORNITH-
ORHYNCHID.E. The duckbills are toothless, close-haired animals with horny
jaws which resemble those of a duck; the five-toed feet with a swimming web
FIG. 607. — Ornithorhynchus paradoxus, duckbill (from Schmarda).
especially well developed on the fore feet. Ornithorhynchus paradoxes of
Australia; male with a spine and gland on hind feet which fits in a corresponding
pit on the thigh of the female and apparently plays a role in copulation.
The oldest known fossil mammals are possibly related to the monotremes.
They appear in the trias and form an imperfectly known group, MULTITU-
BERCULATA (Tritylodon, Microlcstes, Plagiaulax) . Their multitubercular
teeth resemble the temporary ones of Ornithorhynchus, while there are indications
that the coracoid existed as a distinct bone. Less certain are the PROTODONTA
(Dromatherium, Microconodon} of the American Jurassic, of which only the
lower jaws are known.
Sub Class II. Marsupialia (Diddphia) .
These, like the remaining mammals, are viviparous. They have
small eggs which undergo a total segmentation in most species, and de-
velop in the maternal uterus, being nourished by a secretion from its walls.
Only in Perameles is there a placenta in which the allantois is so intimately
connected with the uterine wall that an exchange of fluids between the two
.-)(l(, CHORD ATA
is possible (to a less extent in Phascolarctos and Halmaturus). In Dasyu-
nts there is a similar connection of the yolk sac. Yet the vessels do not
extend with the villi into the uterine tissues as is the case with the placenta
of all Placentalia. In all there is insufficient nourishment and the young
an- \rry immature when born. They are therefore carried a long time by
the mother in the marsupium, a pouch formed by a fold of skin on the
posterior ventral surface, into which the nipples open. The ventral surface
is supported by the marsupial bones, slender rods articulated, right and left,
at the pubic symphysis. Other characteristics of the marsupial skeleton
are the inflected posterior angle of the lower jaw (fig. 608, a) and the rudi-
mentary replacement of teeth, only premolar 3 being replaced. In
front of the functional teeth is a row of dental anlagen, which never de-
velop. These are usually regarded as indicating a prelacteal dentition,
and the functional dentition as the milk dentition; but they may be the
milk dentition and the functioning teeth the permanent dentition. The
cloacal and sexual apparatus has already been described (p. 557).
Marsupials occur in the Jurassic and tertiary of Europe and America.
They were apparently then spread over the earth, but were crowded out by the
placental mammals and persisted only as remnants (C&nolestes and the opossums)
in America, but as a richly developed fauna in Australia; yet no fossils are known
FIG. 608 — Lower jaw of Thylacinus cynocephahis (from Flower), showing (a) the
inflected angle characteristic of marsupials; cd, articular surface.
there earlier than the pleistocene. In the latter region they continued because,
on account of the early separation of this continent from the rest of the world,
no development of Placentalia occurred (p. 162). In Australia, in adaptation to
similar conditions they have undergone a development analogous to that of the
placentals in other parts of the earth, so that they present groups parallel with the
carnivores, rodents, insectivores, and ungulates.
Order I. Polyprotodonta (Zoophaga).
Many marsupials, among them the oldest, have a dentition adapted to
animal food. They have numerous incisors (up to five in each upper half-jaw),
strong canines, and sharp-pointed molars (fig. 608). Some, in teeth and in
body form, resemble the Insectivora, others the carnivores. DASYURHX*:,
carnivorous: Dasyurus; Sarcophilus, the Tasmanian 'devil,' dangerous to larger
mammals; Thylacinus, pouched wolf. PERAMELOXE, insectivorous; Perameles,
bandicoot. DIDELPHYIDJE, opossums, confined to America (chiefly South) are
more carnivorous in dentition and recall the apes with their oppos'able thumb.
*
IV. VERTEBRATA: MAMMALIA, PLACEXTALI A
Order II. Diprotodonta (Phytophaga).
561
The herbivorous habits are correlated with the degeneration of canines,
usually lacking in the lower jaw and at least very small in the upper. There
are two large incisors in the lower jaw, while the middle two of the upper are
much larger than the one or two lateral which may be present. The fact that
young phalangers and kangaroos are polyprotodont makes it probable that the
Diprotodonts are descended from the Polyprotodontia. The PHASCOLOMYID^E
are the rodents of the marsupials with one chisel-like incisor in each half of each
jaw. Phascaloniys, wombat. The MACROPODID.£, or kangaroos, resemble
ungulates in their large herds on the grassy places. The fore legs being very
small, the animals leap with the strong hind legs and tail. Ufacropus. The
PHALANGISTID.*: have very variable teeth. They resemble in habits the
squirrels, Petaurus having the same parachute folds as does our flying squirrel.
Many fossil Diprotodonta in Australia, a few in South America. Some Aus-
tralian fossils were large, Diprotodon australis larger than a rhinoceros.
Sub Class III. Placentalia (Monodelphia).
The first reason for associating the mammals of the Old World and
most of those of the New together as Placentalia is an embryological one,
the presence of a placenta. When serosa, amnion, and allantois (p. 490)
FIG. 609. — Rabbit embryo with envelopes (after Van Beneden and JulinV al,
allantois; am, amnion; an, eye; d, digestive tract; h, heart; k. gill clefts; m, mid brain;
u, protovertebra.-; external black, chorion with villL from which UK- placenta develops.
have developed in the embryo, the vessels of the allantois spread out
beneath the serosa and form with this the chorion, which sends small pro-
cesses or villi into the now highly vascular mucous membrane of the
uterus in order to obtain nourishment somewhat as a tree obtains food by
its roots. These villi (sometimes poorly developed, as in the pig) may be
distributed over the greater part of the surface (fig. 609), producing the
36
562
CHORDATA
cliorion frondosum, or diffuse placenta, which occurs in whales, perisso-
dactyles and many artiodactyles. On the other hand, the villi may be
restricted to certain places, becoming very strong there. This gives rise
to cotyledonary, discoidal, or zonary placentae. To these correspond por-
tions of the uterine lining which are distinguished from the rest by be-
coming extremely vascular (uterine placenta). The cotyledonary placenta
(fig. 610) consists of many small placentar patches, the cotyledons (most
C1
C1
Ch
FIG. 6 10. — Cotyledonary placenta and embryo of cow (from Balfour, after
Colin). C1, cotyledons of uterine; C2, of foetal placenta; Ch, chorion; U, uterus;
V, vagina.
ruminants). In the zonary placenta the villous area takes the shape of a
girdle or barrel (carnivores, Sirenia), while the discoidal (other mammals)
is, as its name indicates, disc-like. By the restriction of the nutrient
structures to certain regions of the placenta, the corresponding part of the
uterus (the zonary or discoid uterine placenta) is more modified than is
the case with the diffuse or the cotyledonary type. While in the latter
the placental villi are withdrawn from the uterine walls without injury to
tlii'in (Indeciduata), in the case of the zonary or discoidal placenta the
superficial part of the mucous lining of the uterus, the dccidua, separates
(Deciduata), leaving the uterine placenta as a bleeding wound, the
hemorrhage being stopped by the energetic muscular contractions of the
uterus.
Besides the placental structures the higher mammals are characterized
by the disappearance of the cloaca, the unpaired vagina, and absence of
IV. VERTEBRATA: MAMMALIA, EDENTATA
563
marsupial bones and inflected angle of the jaw. The dentition, on the
other hand, has undergone a progressive, divergent development, so that
the distinctions are much more pronounced than in the marsupials, and
hence of importance in differentiating the orders.
Order I. Edentata.
A few families, poor in species, are united as Edentata because teeth are
absent or, more usually are markedly degenerate. Persistent functional in-
cisors are lacking, canines but rarely occur (Brady pus) ; molars may be present,
sometimes in great numbers (Priodon gigas, the large armadillo, has about a
hundred molars), but they are poorly rooted, prismatic, without enamel, and
usually monophyodont. Since the aardvark (Orycteropus) and Tatusia have
a heterodont milk dentition in embryonic life in which incisors occur, and
fossil edentates (Entelops) with complete dentition are known, the absence of a
replacement of the teeth is to be explained by degeneration. The great number
of sacral vertebrae is striking, being as many as thirteen in some armadillos.
The order is essentially tropical, but one species entering the United States.
The oldest fossils occur in Patagonia (eocene or oligocene).
Sub Order I. NOMARTHRA. Old World. FODIENTIA. Animals
with strong digging claws, long tail, and long, sticky tongue used in catching
insects. Orycteropus, aardvark, long snout, sparse bristly hair. SQUAMATA.
Toothless, body covered with overlapping scales. Man is, pangolins of Asia and
FIG. 611. — Manis longicaudata, pangolin (from Monteiro).
Africa (fig. 6n). Sub Order II. XEXARTHRA. Xew World. VERMIL-
INGUIA, ant eaters. Resemble manids in toothless jaws, tongues, and claws,
but are hairy and lack scales. Mynnecophaga. TARDIGRADA, sloths.
Hairy, rudimentary tail, and few teeth, long strong claws by which they hang
back downwards from limbs of trees. Brady pus, nine cervical vertebrae; Clu'lu'pits,
six cervicals. Fossils allied are Megatherium, as large as an elephant, .!/ \7 ,><»;,
Megalonyx, extending to Pennsylvania. Grvpnthcrinm, as lar^e as a cow, has
not long been extinct. LORICATA, armadillos. Body with armor of bony
plates, insectivorous. In the extinct GLYPTODONTID/E of South America the
plates fused to a continuous armor. One species twelve feet long. DASY-
PODID/E; dermal armor in movable transverse plates; nocturnal. Genera based
upon the number of bands; Dasypns; Taiusianovenicincta*.
564
CHORD AT A
Order II. Insectivora.
Of all living mammals the inscctivores stand nearest to the primitive type,
the order dating back to the eocene. The sharp-pointed teeth (sectorial),
adapted for insect food vary greatly in number in the different families (moles,
^iiil; shrews, Iil;|), as also does the replace-
ment, since many forms retain the milk denti-
tion for a long time or never lose it, while the
shrews lose it very early. They have the process
at the angle of the lower jaw, a primitive brain,
a uterus bicornis or one divided throughout.
, .„. 6l, —skull of Sorex (from There are frequently scales on the tail among
Ludwig-Leunis). the hairs; the clavicle is present; usually five
clawed toes. . The animals which as a rule are
small with soft hair, usually have a proboscis-like, tactile snout.
The ERINACIDVE, Old World hedgehogs, are spined; the SORICID.E, or shrews
(Sorc\ * Blarina*), are mouse-like, as are the allied TALPID^:, or moles (Scalops*
FIG. 613. — Skeleton of bat (after Brehm).
( 'imdylnra* star-nosed mole), which burrow in the earth and have the eyes more
or less rudimentary. Some authors place here Galcopilliecits of the East Indies,
which has a similar membrane and sailing powers as the flying squirrels. It
also resembles the bats and lemurs.
Order III. Chiroptera.
The bats are the only mammals which actually fly, and this at once charac-
tcri/.es them. The flying membrane (pata^iiun), a thin fold of skin, begins at
the tail, includes the lower extremities to the foot, and extends thence to the
fingers, leaving the thumb free. Fingers 2-5 are enormously elongated and
support the membrane. Since flight requires strong muscles, the sternum
develops a small keel for the attachment of the large pectoral muscle. In
connexion with the Hying powers the clavicle is strong. The patagium is very
sensitive; blinded bats ran fly among all kinds of obstacles without disturbing
them. The enormous ear conchs and a noticeable nose leaf, widely distributed
through the group, also have marked tactile powers. In the pectoral position
IV. VERTEBRATA: MAMMALIA, CARXIVORA 565
of the mammary glands and in the discoidal placenta these animals resemble
the primates. The dentition is variable, often ilaf. Fossils occur in the
eocene.
Sub Order I. MICRO CHIROPTERA, insectivorous dentition, only the
thumb clawed. YESPERTILIONID^E, tail long, no nose leaf; Vesperugo,* At/il-
apha.* PHYLLOSTOMID.E, with nose leaf, tropical America; Desmodus, blond-
sucking or vampyre bat. Sub Order II. MACHROCHIROPTERA (Frugi-
vora), smooth-crowned molars, claws on thumb and first two fingers. Includt s
the flying foxes, Ptcropus, of the East Indies.
Order IV. Carnivora.
The carnivores live chiefly on the flesh and blood of other vertebrates, which
they catch by craft, by coursing, or by pouncing upon them. With tin's mode
of life correspond the high development of the brain (fig. 600, B] and sense
organs, as well as structure of teeth and claws. Especially striking is the
shining of the eyes, especially in the cats (p. 121). Since the predacemis
character increases from the bears to the cats, and again tends to disappear in
the aquatic species, there is a great variation in structure. For greater mobility
the clavicle is reduced or lost, ulna and radius well developed. In the structure
of the feet there is a gradual transition from the plantigrade bears, in which the
whole sole rests upon the ground, to the digitigrade cats, which tread on the
toes. In the latter the claws, which occur in all carnivores, are kept from
injury by being retracted into pockets on the penult joint, from which they are
extended by strong muscles. In dentition (fig. 601) the striking features are
the almost constant three incisors, and the great size of the canines; the molars,
on the other hand, vary with the different families, the cusps assuming more of
the shearing character (secodont teeth). The last premolar of the upper jaw
and the first molar of the lower jaw become carnassial teeth (sector ial teeth),
and acquire a dominating position, while the others become smaller and tend
to disappear at either end of the series. Further characters are the possession
of a penis bone, the abdominal position of the milk glands and the uterus bi-
cornis; the placenta is zonary. Anal glands, furnishing a strong, even offensive
smelling secretion, are common.
Sub Order I. FISSIPEDIA. Terrestrial animals with well-developed toes
usually cleft to the base. Frequently five digits on all feet, but often reduced to
four on the hind feet (Felidae, Canidae), rarely on the fore feet (Hyaenidae); but
in these cases (domestic dog) the reduced toe may bear a claw. URSID.F,;
Ursus* bears; Procyon lotor,* raccoon. MUSTELID^E; many species of Muslild*
and Putorius* (minks, martens, sable, ermines, and weasels) are valuable for
their fur; Lu-tra,:{: otter; Enhydris,* sea otter; Mephitis,* skunk; anal glands
common, in this family. Fossils (Arctotlicrium, etc.) connect the bears and the
CANID.E five toes in front, four behind, claws not retractile; Cants* dogs, foxes and
wolves. The FELID^E have retractile claws. Fells domestica, our domestic cat.
F. leo, lion; F. tigris, tiger; F. concolor,* puma or cougar. HY^ENID/E, all feet
four-toed; Hyccna of Africa. VIVERRID^E; Vii'crra, civets; Herpestes, ichneu-
mons. Sub "Order II. PINNIPEDIA. Aquatic carnivores with the limbs
flattened to broad flippers, the five toes long and webbed, nails frequently
rudimentary; the dentition differs from that of the terrestrial forms in tin-
similarity of molars and premolars (absence of carnassial); the milk dentition
degenerates, without being functional. PHOCID^E, seals, no external cars;
PJioca I'itulina* harbor seal. OTARIID/E, with external ears; Otaria* sea lions;
Callorlihuts ur sinus, fur seal of Alaska. TRICHKCHID^;; incisors reduced, ujij er
canines developed into large tusks; Trichcchus, walrus. The first carnivores
566
CHORDATA
appear in the eocene in the order CREODONTA, plantigrade forms with
slightly differentiated dentition; they resemble marsupials, insectivores, as well
as the Condylarthra. True carnivores appear in the upper eocene.
Order V. Rodentia.
The rodents unite great similarity in appearance with a characteristic
dentition. The canines are absent, the molars separated by a large gap (dias-
ti- m,i) from the incisors (fig. 614). The latter are strong, chisel-like, have per-
sistent pulps and grow at the root as they are worn away at the cutting edge.
Since only the front surface has enamel, wear keeps them constantly sharp.
Usually there is but a single incisor; only the Duplicidentata have a second in
the upper jaw. The molars are cuspidate or have enamel folds and frequently
continue to grow throughout life. Their number is frequently reduced, the
formula varying between -f-Jj g § and |S J! 5 . Many species have an inflected angle
of the jaw like that of marsupials. 1 he
infraorbital canal is a striking feature in
Muridas and Hystricidae (fig. 614, 0), a
large opening in front of the orbit for a
part of the masseter muscle.
The rodents are distinguished from
the ungulates by the smaller size, the
possession of claws, five toes (sometimes
reduced to three), the occurrence usually
of a clavicle, and a discoid placenta.
The mammae are inguinal in position
and are very numerous. The occur-
rence of glands with a strong-smelling
secretion, which open near the anus, is
common (fig. 603). Over one thousand
living species are known, occurring in all
regions except the Australian. The order
appears in the eocene.
Sub Order I. DUPLICIDENTATA
(Lagomorpha), two upper incisors, in-
cludes hares, Lcpus* and picas, Lagoniys* Sub Order II. SCIUROMOR-
PHA. The squirrels, SCIURIOE, have soft fur and bushy tail. Sciurus*
squirrels; Cynomys* prairie dogs;Sciuropterus* flying squirrels. The CASTOR-
ID, v: have soft fur and scaly tail. Castor fiber* beaver. Sub Order III. MYO-
MOKIMIA, rats and mice. Mus nmsculus* common mouse; Mus ratt-us*
house rat, once abundant but now replaced by the gray rat, M. decumanns*
an immigrant from Asia. White rats are albinos of M. rattus. Fiber zibethicus,*
musk rat; An'imla* field mice. Sub Order III. HYSTRICOMORPHA.
Tin- porcupines ( HYSTRICID^E) have spines; the Old World forms, Hystrix, are
terrestrial, ours (Rrcthyzoii) arboreal. The CAVIID.E, South America, hoof-
like claws. Ciivia cobaya, guinea pig. Hydrochcerus, capybara, largest
existing rodent.
Order VI. Ungulata.
The Ungulata, or hoofed animals, include two groups of living animals in
which the body is supported on hoofs on the tips of the toes, and which are
sharply marked off from other forms. If, however, the fossils be included, the
limits of the group must be extended so that it includes the elephants and conies
of the existing fauna as well as several extinct forms, for these so intergrade
that sharp lines cannot be drawn.
Kir,. 614. — Skull of porcupine (from
Schmarda). /, frontal; im, premaxil-
lary; k, temporal fossa continuous in
front with orbit; o, infraorbital for-
amen, enormous on account of the
portion of the masseter muscle which
passes through it.
IV. VERTEBRATA: MAMMALIA, UXGULATA
5G7
The ungulates, which arise from the Condylarthra of the eocene (Phcna-
codon), are preeminently herbivorous; the canines are rarely well developed, the
molars numerous and adapted to grinding the food, more or less flattened and
frequently with folded enamel. The mammae are inguinal, the uterus bicornu-
ate, and the placenta either diffuse or (most ruminants) cotylcdonary (fig. 610).
The legs are exclusively locomotor structures and the clavicles are absent.
Since the metacarpals and metatarsals are greatly elongate, the wrist and ankle
are raised from the ground so that they are frequently confounded with elbow
and knee. With this exclusively supporting character of the limbs there is a
tendency to reduction and fusion of bones. There is a constant increase in the
development of radius and tibia to the chief supports of the body, the fibula
becoming rudimentary, the ulna being developed sometimes throughout its
whole extent, sometimes only in its upper part, and is more or less fused with the
radius. The same tendency to simplification prevails in the feet, but is ex-
C
D
•J
FIG. 615. — Fore feet of ungulates (after Flower). A-C, perissodactyle; D-F,
artiodactyle. A, tapir; B, rhinoceros; C, horse; D, pig; E, deer; F, camel, c, trique-
trum (ulnare); I, lunatum (intermedium); m, capitatum; -m2-m5, rudiments of meta-
carpals II and V; p, pisiforme; R, radius; .?, scaphoid (radiale); til, trapezoid; tin,
trapezium; U, ulna; u, hamatum; II-Y, digits.
pressed differently in the odd-toed (perissodactyle') and even-toed (artiodactyle)
forms. In the Perissodactyla the axis of pressure passes through the middle
toe (fig. 61=;, A-C, III), while the other toes disappear symmetrically around
this. Since the first toe is early lost, toe V is next to disappear (B), and then
toes II and IV (C), so that at last there remain only the skeleton of the middle
toe (horse), the rudiments of toes II and IV persisting as the small splint bum's.
In the Artiodactyla the axis of pressure falls between toes III and IV (D),
which support the body, are equally developed and frequently fuse, at least so
far as the metacarpals are concerned (E, F). The figures D-F show how the
other digits disappear, digit I being lost still earlier. Since the weight of the
body rests more upon the hind legs than upon the front ones, the former arc-
the first to become modified. Since we are able, with the fossils, to follow in
detail the lines of descent of both artiodaclvK-s and perissodactyles, it is certain
that these form diverging series, distinct from the beginning. In each series
CHORDATA
most of the common characters enumerated above have been independently
acquired so that the uniformity in appearance is in great part the result of
convergence.
Sub Order I. PERISSODACTYLA. Molars and premolars (with more
or less pronounced enamel folds) of equal size; predominant development of the
middle toe, the others reduced to different degrees. TAPIRID^;, fore feet four-
toed, hind feet three-toed; teeth '•}, \ ;j ;S ; nose elongate into a proboscis. Tapirus,
tapirs, tropical America and India. RHINOCEROTUXE, three toes on all feet,
teeth I1,1 ,':;; one or two horns on the nasal bones; skin thick, hairless (hence
these- were formerly united with elephants as Pachydermata). Rhinoceros,-
single horn, India; Ci'mturliiiiits (Asia), Atdodus (Africa), two horns. EQUID^,
a single functional toe (fig. 615, C); teeth |i-|-|; Equus caballus,* horse, a native
of Asia; E. asinus, ass; E. Zebra.
Sub Order II. ARTIODACTYLA. Besides the features of the feet, these
have the three or four premolars, smaller than the molars. Species much more
numerous than of perissodactyles. Section I, NON-RUMINANTIA (Buno-
dontia); omnivorous and have a bunodont dentition, r^ll!;], the canines fre-
quently developed into tusks; stomach usually simple, but occasionally divided
into three chambers (Dicotyles, Hippopotamus}, although rumination does not
occur. The leg skeleton is little modified (fig. 615, D], ulna and fibula not
reduced, metacarpals and metatarsals separate. HIPPOPOTAMID.E; all four toes
reach the ground; skin thick ('pachyderm'), body heavy; African. Hippopota-
mus. SI-ID.E; two functional toes, skin with bristles, snout proboscis-like.
Sus scrofa, swine; Dicotyles,* peccaric3.
FIG. 6r6.--Stomach of sheep (after Carus and Otto), a, abomasum (true stomach) ;
o, omasum (manyplies) ; re, reticulum (honeycomb); ru, rumen (paunch).
Section II. RUMINANTIA (Pecora); teeth and stomach adapted to the
exclusively herbivorous diet. The stomach (fig. 616) is divided into two per-
ms, each again subdivided. The first of these, the rumen, or paunch (nt),
eceives the food as it is eaten; then at a time of quiet it is regurgitated into the
i and -round by the molars ('chewing the cud'). It then passes back,
ime into the second division, the honeycomb, or retindum (re), thence to
urn-pin (psalterium) or omasus (o), and lastly to the abomasus, or true
Usually canines and incisors of upper jaw degenerate; incisors
.ower jaw strong and the canines have form and position of incisors
IV. VERTLBRATA; MAMMALIA, UXC.l'LATA
5 Hi
The molars arc selcnodont. With few r.xrcptions Ruminants are of large size
and many bear horns on the frontal bones, larger in the males and may occur
exclusively in that sex. In the giraffes these are cones free from the frontals and
covered with skin. In others (Cavicornia) the horn cores fuse secondarily with
the frontals and are covered with a sheath of horn. Lastly, the horns are out-
growths of the frontal bone, in which usually the outer coats of skin and hair
(velvet) are soon lost and only the bone projects freely (antlers). These are
shed yearly, the new antler which takes its place being larger and consisting of
a larger number of branches or tines, thus constituting an index of age (Cervi-
cornia). CAMELOPARDALID.E (Devexa), giraffes, long-legged forms from
Africa with persistent horns; teeth frit, Gira/a, Okapia. CERVID.*:, deer,
deciduous horns in the male. Cervus,* common deer; Alces,* moose; Rangift i , ::
reindeer; MOSCHID.E, horns lacking, males with enlarged upper canines and a
musk gland; MoscJnis, central Asia. TRAGULID^E, primitive, Asia and Africa.
The CAVICORNIA include many species, some of great economic importance;
teeth -3-^33. BOVID.E: Bos taunts, domestic cattle, probably descended from
three distinct stocks (B. primi genius, aurochs, B. Lm^ifrons and B. frimlosus);
Bison,* including B. euro pens, bison proper, and B. americanus* our 'buffalo.'
Bubalus, the true buffalo of the Old World. OYID.E: Ovis arics, sheep; O.
•in mtana,* big horn; Capra Jiircus, goat; Ovibos moschatus* musk ox. AXTI-
LOPID.'E: including a host of Old World forms (Antilope, Gazclla, Rupicapra
trains, the chamois, etc.) and Antilocapra americana* prong horn, which sheds
its horns, and Hoploceras montanus* Rocky mountain sheep.
Section III. TYLOPODA, stomach without manyplies, no frontal horns,
diJuse placenta. Camel us, Old World camels; Auchenia lama, A. alpaca of
South America.
Paleontology of the Ungulata.
Abundant paleontological material, especially from the tertiary of our
western states, has cleared up many lines of ungulate descent and has shown
it probable that the CONDYLARTHRA of the eocene, with five-toed planti-
grade feet, well-developed ulna and fibula, and omnivorous dentition, formed
the stock from which the artiodactyles and perissodactyles descended, and
FIG. 617. — Evolution of fore foot of horse (from Wiedersheim). i, Orohippus
(eocene); 2, Mesohipfrus (lower miocene); 3. Miohippus (miocene) ; 4, Protohippus
(upper pliocene); 5, Pliohippus (pleistocene); 6, Eqitux.
possibly carnivores and primates as well, the ungulate line extending through
the Amblypoda. From one group of these (PHEXACODONTIDJC) the lines of
rhinoceros and tapir have come, and in an almost complete scries we know the
ancestry of the horse. Hyracotherium (Eohippus] and Oroluppus of the eocene
had the fore feet four-toed (fig. 617, i); Pahcotherium and Mcsahippus (2) of the
lower miocene and Miohifip-ns of the later miocene were three-toed, while .\frr\-
hippus and Hipparion (Pliohippus, 4) of the pliocene were near the horse in
570
CHORDATA
tooth structure. The single-toed horses appeared in the pleistocene with
Pliohippus (5) and then Equus itself (6). It is a peculiar fact that the horse
entirely died out in America, although the chief part of its history was enacted
The AAIBLYPODA, mentioned above, were semi-plantigrade penta-
dactyle forms, appearing in the lowest eocene, and reaching, in U intatherium
(Dinocerus) an elephantine size. The TOXODONTIA of the South American
tertiaries combined perissodactyle, rodent, hyracoid, and proboscidian features,
while the TILLODOXTIA of the eocene recall both carnivores and rodents.
Order VII. Proboscidia.
The elephants and their allies, with their hoofs and herbivorous dentition
and lack of clavicle, are closely related to the ungulates. They are characterized
by their thick skin ('pachyderm'), the large, massive, five-toed legs, and espe-
cially by the nose drawn out into a long proboscis, lastly by the dentition.
Canines are lacking, but the incisors of the
upper jaw continue to grow throughout
life, forming the well-known tusks. In
the living elephaaits there are but a single
pair of tusks, but in some extinct Mas-
todons there were a second smaller pair in
the lower jaw, while Dinotherium had
only the lower incisors, these projecting
downwards. The molars (in Mastodon
and Dinothcriu-m with normal replace:
ment and cusps) consist of numerous
plates of enamel and dentine united by
cement, and undergo a horizontal dis-
placement. Of. the three large molars
and premolars only one at a time is functional (fig. 618, i); when worn out the
next one behind (2) takes its place. Further features are a uterus bicornis, a
zonary placenta, and two pectoral mammae.
ELEPHANTID^E: Elephas indicus, small ears; E. africanus, large ear?. E.
primigenius, mammoth, pleistocene; specimens found frozen in ice in Siberia
have close woolly hair, in some places three feet long. Mastodon, with tuber-
culate teeth, miocene and pliocene. DINOTHERID-E, Dinotherium, Old World
I i<;. 618. — Inside of left lower jaw
of Elephas indicus, the alveoli opened
(after Owen), i, functional molar; 2,
its successor.
miocene.
Order VIII. Hyracoidea.
The single genus Hyrax, including species from western Asia and Africa,
with four-toed front feet, hind feet with three toes, the digits with nails, the
placenta zonary, and the dentition |U -j§, forms this group, no fossils being known.
Hyrax syriacns is supposed to be the 'coney' of the Bible.
Order IX. Sirenia.
This order consists of a few aquatic whale-like mammals, with the fore
limbs fin-like, hind legs lacking, and a horizontal caudal fin. They live in
shallow seas or in the mouths of rivers, where they feed on the vegetation, which
they chew with jaws covered with horny plates.' The teeth (in the fossil Pro-
rostomus i! \ J }) are reduced or entirely lacking. The fore legs are pentadactyle,
often have rudimentary nails and always a flexible elbow. The two pectoral
m a in 11132 are possibly the germ of truth in the mermaid myth. Manatus atneri-
canus* maiiaicc, six cervical vertebrae, eight to ten molars; Halicore dugong,
[ndo-Pacific; Khytinn strllcri, northern Pacific, exterminated in 1768.
IV. VERTEBRATA: MAMMALIA, CETACEA
Order X. Cetaceao
571
In form the whales resemble the sirenians, a result of an aquatic life, but the
resemblance ends here. The whales are so fish-like that every one speaks of
the whale fishery. Head and trunk are scarcely distinguished, the cervical
vertebras being very short and more or less completely fused. The hind limbs
are absent, of the pelvic girdle only a small ilium remains, and no sacral vertebras
are developed. The caudal fin differs from that of a fish in being horizontally
flattened; the skin is thick and is sparsely haired or completely naked, some
lacking hair even in the embryo. Most of the species inhabit the high seas,
Inia boliviensis and Platanista gangetica occur in rivers.
The fore limbs are modified into flippers, the bones of which are of nearly
equal size and are jointed only at the shoulder. A dorsal fin ('fin backs') occurs
in some. The lack of hair is compensated by the thick layer of subcutaneous
fat (blubber). In order that the animals may breathe while feeding, the larynx
is prolonged into a tube which extends up through the pharynx to the choanae,
from which the nostrils extend directly upwards to the single (Denticetes) or
paired (Mysticetes) external opening. Since the air expelled contains much
moisture and this is condensed on contact with the cooler external air, the im-
pression was natural that the animals in 'blowing' spouted water. As the
olfactory membrane is degenerate and the olfactory lobes are reduced, the nose
is an organ of respiration only.
FIG. 619. — Section through jaws of whalebone whale (after Delage). r, septum
of nose; m, mouth cavity; mx, maxillary bone; p, premaxillary (hinder end); v, vomer;
iv baleen.
The eyes are small, external ears are lacking, the mammae are close to the
sexual opening. The teeth are either present in large numbers, similar and
conical, and, since the second dentition is rudimentary, are monophyodont
(Denticetae) or they are outlined early and then resorbed and replaced by plates
of baleen (Mysticetae), composed of large horny plates (whalebone) in large
animals a dozen feet long (fig. 619, w), of which hundreds are arranged in close
succession extending inward to the tongue. They correspond to the transverse
palatal folds which occur in other mammals. As they are fringed on the inner
edges they form a strainer which retains the small animals on which these
whales feed. The origin of the whales is one of the unsolved problems. 'I hat
they had a terrestrial, quadrupedal ancestry is beyond question, and the little
572
CHORD ATA
evidence would scorn to point to the ungulates, creodonts, or carnivores. The
toothed and whalebone whales may have had different ancestries, their resem-
l/hinces being the result of convergence.
Sub Order I. ZEUGLODONTA. Extinct (eocene) forms with heterodont
dentition, the posterior teeth two-rooted. Sub Order II. DENTICET/E,
toothed whales, carnivorous, some having but two teeth. Delphinus* dolphins;
Globiocephalus,* black fish; Mwwdoii, narwal, male with a long maxillary tusk.
Physctcr macrocephalus, sperm whale, pursued for the spermaceti, an oily mass
situated in the 'chair' between the cranium and the snout, as well as for amber-
gris, formed in the intestines. Sub Order III. MYSTACETI, whalebone
whales, with baleen. Baltznoptera* rorquals and fin backs. B. sibbaldi* the
largest whale, eighty-five feet long. Balccna, right whale.
Order XI. Prosimiae (Lemuroida) .
Linne united with the true apes a small group of animals known as lemurs
because of similarity in body form and climbing habits, because they had
grasping hands and feet (opposable thumb and great toe), and frequently nails
on some of the toes. To-day many set them aside as a separate order on account
of their lower organization. They have a less-developed cerebrum, uterus
bicornis, and a diffuse placenta. Further peculiarities are the peculiar and
FIG. 62o.—Stenops gracilis, slender loris (from Brehm).
variable dentition (Chiromys (-g-Jf, Lemur Sj^). Nocturnal habits have re-
eyes, which give these animals a most striking appearance. A
trom the primates is the connexion of orbital and temporal cavities
J osseous postorbital ring. Usually there are a pair of pectoral
ae to which are added in many species a pair in the abdominal or inguinal
on, the latter alone occurring in Cliiromvs. All come from the Indian and
-Malagassy regions.
CniKmiYin ,!•:, digits long, all except the great toe with claws; Chiromys,
lARSinxE, second and third hind toes clawed. Tarsius, East Indies,
closed orbits and discoidal placenta. LEMURID/E, second hind toe
IV. VERTEBRATA: MAMMALIA, PKIM.VI ES
573
alone clawed. Lemur; Stciwps, loris. The tertiary PACHYLEMURID;E and Ax-
APTOMORPHID.E are close to the most primitive mammals, creodonts, and
insectivores. The GALEOPITHECID^: (p. 564) are often placed here.
Order XII. Primates.
The most highly organized mammals, the monkeys, apes, and man, are
united in a single order because of their great agreement in features of clas-
sificatory value. If we here, as elsewhere, ignore grades of intelligence and
regard alone greater or lesser anatomical resemblances, we are forced to the
conclusion that the anthropoid apes are much closer to man than to the lower
monkeys. The primates have, in common, nails on all the fingers and toes
(except Hapalidrc), orbits separated from the temporal fossae by a bony wall,
and a cerebrum which covers the other parts of the brain (fig. 600, c}. . They
CO,
B
FIG. 621. — Hand and foot of gorilla, r, capitatum; ca, calcaneus; en, cuboid;
h, hamatum; /, lunatum; me, metacarpals; n:t, metatarsals; n, naviculare; />, pisiforme;
ph, phalanges; s, scaphoid; t, triquetrum; ta, talus; td, trapezoid; tr, trapezium; /-I',
digits; 1-3, cuneiformia.
single pair of pectoral mamma?, uterus simplex, and discoidal placenta,
itition is essentially the same throughout; in the Platyrrhime '-, \ Iff, in the
have a si
The dentition
Hapalidae I \ \ ;, in the Catarrhinae and in man |J, \\. Yet there is a tendency to
variation, since in the chimpanzee and in man the third molar (wisdom tooth)
is in process of degeneration, while in the orang a fourth molar often occurs.
The molars are bunodont.
As in the lemurs and opossums, the thumb and great toe can be opposed to
the other digits, so that an ape can grasp objects with either hand or foot. In
man this opposability of the thumb is increased, but that of the great toe, in
consequence of the upright position, is only retained to a slight degree by chil-
dren and primitive people. On this peculiarity rest the names Bimana, tor
man, and Quadrumana, for the apes and monkeys. But it must be emphasized
that the apes do not have a hand, but rather a grasping foot, on the hinder
574 CHORDATA
extremities. The grasping foot (fig. 621) has the same bones, similarly ar-
ranged and of about the same shape as in the foot of man, while the musculature
i> essentially the same. On the other hand, the same distinctions between hand
and foot (.1 and B) occur as are found in the hand and foot of man. The
.separation of Quadrumana and Bimana is without anatomical basis; it rests
solely upon functional peculiarities and egotism.
Sub Order I. PLATYRRHIN/E, New World monkeys. Nostrils separated
by a wide septum so that they open laterally; tympanum not extended by an
outer bony meatus. CEBHX*:, tail frequently prehensile, long. Cebus, sapajous;
A teles, spider monkeys. The HAPALID/E, or marmosets, are aberrant with
teeth '-',:;T and claws on all digits except the relatively small great toe, thumbs
not opposable. Hapale, Midas. Sub Order II. CATARRHIN^, Old
World apes; internasal septum small, the nostrils directed forwards and down-
wards; since the large canines interlock in the opposite ro\v of teeth, there is a
more or less evident diastema in each jaw; tympanum prolonged as in man into
a bony meatus. Section I. CYNOMORPJLE, with naked places on the
buttocks (ischial callosities), usually a long tail and hairy face, and usually only
two sacral vertebrae. Cvnocephalus, baboons, mandrils; Macacus, macaques;
\I . ,r ntdatus, entering Europe at Gibraltar. Section II. ANTHROPOID.-E
(Similize), man-like apes, usually without ischial callosities, face, fingers and
toes without hair, no tail, five sacral vertebrae (three in Hylobates) fused to an
os sacrum. Hyl abates, gibbons, very long arms; Simla satyrus, Sumatra and
Borneo, orang-utan; Gorilla engena; Troglodytes niger, chimpanzee, Africa.
Sub Order III. ANTHROPIN.E, man. Degeneration of hair on most
parts of the body; upright position and as a result slight mobility of great toe
(non-opposable) ; development of articulate speech; high intelligence; large
cerebrum and consequent increase of the cranium at the expense of the face, are
the most prominent characters of mankind. Dentition as in the Catarrhinas,
canines smaller, no diastema. It was long a question whether there was a
single species of man (Homo sapiens) with several races or whether there were
several species. Since crosses between the different races are fertile, the first
view receives general acceptance, although the differences are constant and
point to the second alternative. The answer to these questions, which in the
light of evolution have lost most of their significance, and the characterization
of the various races, belong to a special branch of science, anthropology.
Since an arboreal life was unfavorable for fossilization, the paleontological
material for the history of the primates is so far very scanty. Of these the great-
est weight has been laid on a 'find' in the pleistocene of Java. This consisted of
a top of a skull, a femur, and two molar teeth which were found near each other.
These fragments were regarded on one side as a connecting link, Pithecanthropus
creclus, between apes and man, on another as belonging to a true ape, and from
the third as true man. The latter is now to be regarded as out of the question.
Most probably it was an extinct gibbon-like animal of extraordinary size, an
enormous cranial capacity (about 800 ccm.) and correspondingly a very large
brain. In these respects no Anthropoid now living could compare with Pithecan-
thropus. Not less interesting than Pithecanthropus are the numerous
remains of man, mostly found in the caves of Europe, dating from the diluvium.
Most of these are portions of skulls, which differ so from the living European
that the first one discovered at Neanderthal was regarded as pathologic (micro-
cephalic). As the number of these increased, they were regarded Ss a distinct
species, Horn:) primigenius, but this has difficulties, since many low races, and
especially the Australians, show marked resemblances to them. It is certain
that diluvial man of Europe was very low. This is shown by the slight arch of the
skull, the prominence of the face, and the massive lower jaw which lacked a chin.
CHORD ATA. SUMMARY OF IMPORTANT FACTS 575
Summary of Important Facts.
1. The CHORDATA possess an axial skeleton, the notochord lying
between the nervous system and the alimentary tract; a central nervous
system entirely on one side of the digestive canal, and gill slits extending
from the pharynx to the exterior.
2. The Chordata are subdivided into Leptocardii, Tunicata, Entero-
pneusta, and Vertebrata.
3. The LEPTOCARDII are fish-like in form, have a notochord extend-
ing the length of the body, but lack heart, auditory organs, skull and verte-
bral column; the brain is rudimentary, the gill slits numerous.
4. The TUNICATA have a notochord only in the caudal region.
The young is tadpole-like, but in most forms there is a metamorphosis in
which tail and notochord are lost.
5. The body is usually enclosed in a tunic or mantle containing cellu-
lose ; gill slits and an endostyle are present in the pharynx, the heart changes
in the direction of the flow of blood. The nervous system in its develop-
ment is tubular. In the Salpidae there is a typical alternation of genera-
tions between a solitary asexual and a sexual chain form.
6. The ENTEROPNEUSTA are worm-like, with collar and proboscis ;
a diverticulum of the digestive tract is compared to the notochord; gill slits
occur in the pharynx; some undergo a metamorphosis in development,
the larva resembling those of Echinoderms. The pertinence of the
Enteropneusta to the Chordata is not certain.
7. The VERTEBRATA are segmented animals without external
ringing of the body, but with metameric arrangement of internal parts
(myotomes, neurotomes, sclerotomes).
8. A cuticular skeleton is absent, but there may be cornifi cations of the
epithelium or ossifications in the derma (scales of fishes, etc.).
9. An axial skeleton is present, consisting of skull and vertebral
column, which more or less completely replace the notochord.
10. There are two kinds of appendages supported by an axial skeleton:
the unpaired fins, occurring only in fishes and Amphibia, and the paired
appendages (anterior and posterior), which are usually present.
11. The central nervous system (brain and spinal cord) are tubular and
dorsal in position. The brain consists of five parts — cerebrum, 'twixt
brain, optic lobes, cerebellum, and medulla oblongata.
12. Eyes and ears are the most highly developed sense organs.
13. The respiratory organs arise from the entoderm (pharynx); gill
slits are present at least in the embryo, extending from the pharynx to the
exterior. In all terrestrial groups these are later replaced by lungs,
developed from the hinder end of the pharynx.
CHORDATA
14. The heart, consisting of auricle and ventricle, lies ventrally in a
pericardium. In gill-breathing species it contains only venous blood, but
with pulmonary respiration it is divided into venous and arterial halves.
The circulation is closed.
15. The sexes are usually separate. In most species the excretory
(nephridial) system forms the ducts for the reproductive products (urogen-
iial system).
1 6. The reproduction is strictly sexual.
17. In the CYCLOSTOMATA there is a primitive skull; but vertebra?,
paired fins, true scales, and teeth are lacking. The gills are saccular and
the nose is unpaired. There is no skeleton to the mouth (no jaws).
18. The true fishes (PISCES) have jaws (Gnathostomata). The fishes
are further distinguished from the Cyclostomes by the vertebral column
(usually amphiccele vertebrae), by paired pectoral and ventral fins, scales,
and paired nostrils. They breathe by gills, and have a venous heart with
auricle and ventricle.
19. The fishes are divided into Elasmobranchii, Ganoidei, Teleostei,
and Dipnoi.
20. The Elasmobranchii have a cartilaginous skeleton, usually a hetero-
cercal tail, placoid scales, usually a ventral mouth, gills covered, heart
with arterial cone, spiral valve in the intestine, no swim bladder.
21. They are divided into Selachii (subdivided into Squali, sharks,
and Raia.% skates) and Holocephali.
22. The Teleostei have bony skeleton, usually a homocercal tail, usu-
ally cycloid or ctenoid scales, comb-like gills and operculum, bulbus
arteriosus, usually pyloric appendages, and a swim bladder; no spiral
valve.
23. They are subdivided into Physostomi, Pharyngognathi, Acan-
thopteri, Anacanthini, Lophobranchii, and Plectognathii.
24. The Ganoidei form a connecting group; they resemble the elas-
mobranchs in the presence of a conus arteriosus and spiral valve, and usu-
ally in the heterocercal tail; they are like the teleosts in operculum and
comb-formed gills, swim bladder, and pyloric appendages. They usually
have fu'.cra and ganoid scales.
25. The ganoids are divided into Chondrostei, with cartilaginous, and
Crossopterygii and Holostei, with bony skeletons.
26. The Dipnoi have gills; occasionally the swim bladder serves as
lungs; heart with beginning division; nose with choana.
27. The AMPHIBIA, in contrast to the fishes, have pentadactyle
appendages; in contrast to the reptiles, double occipital condyles. They
liave bushy external gills and lungs, either persisting together or succeeding
CHORDATA. SUMMARY OF IMPORTANT FACTS .'.77
each other, the young (larva-) breathing by gills, the adult by lungs
(metamorphosis!). The heart consists of two auricles and one ventricle.
28. The Amphibia are subdivided into Gymnophiona, Urodela, and
Anura; to these are added the extinct Stegocephali.
29. The Gvmnophiona are blind and have lost the limbs.
30. The Urodeles have many vertebra? and a well-developed tail.
They retain the gills permanently (Perennibranchia), or at least a gill slit
(Derotrema), or they lose the branchial apparatus completely in develop-
ment (Salamandrina) ; the metamorphosis is not pronounced.
31. The Anura have few vertebrae, no tail nor gills in the adult, and a
marked metamorphosis (the larva.*, tadpoles, are furnished at first with
external, then with internal, gills, and with swimming tail, but at first
lack appendages and lungs).
32. Cyclostomes, fishes, and Amphibia are grouped as Anamnia
because of the lack of amnion and allantois; they are also called Ichthyop-
sida, because of their branchiae and aquatic habit. They are poikilo-
thermal (cold-blooded).
33. The reptiles, birds, and mammals are called Amniota on account
of the embryonal organs, the amnion and allantois. They never respire
by gills (although gill clefts occur in the embryo), and the appendages are
based on the pentadactyle type.
34. Reptiles and birds are united as Sauropsida from the similar
scales and single occipital condyle.
35. The REPTILIA are poikilothermous, have a strongly ossified
skeleton, with unpaired occipital condyle and usually an os transversum
in the skull ; a strongly cornified skin, two auricles, and usually two incom-
pletely separated ventricles in the heart.
36. Recent reptiles are divided among the Chelonia, Rhynchocephalia,
Squamata (including Lacertilia and Ophidia), and Crocodilia. To these
are added the extinct groups Theromorpha, Plesiosauria, Ichthyosauria,
Dinosauria, and Pterodactylia.
37. The Chelonia are compact, have a skeletal capsule (carapace +
plastron) composed of bone and horny plates, an immovable quadrate
and hard palate, no os transversum or teeth, but horny plates in the place
of the latter ; the cloacal opening elongate.
38. The Squamata have horny scales periodically renewed, a trans-
verse cloacal opening, with behind it paired penes, and a movable quadrate.
39. The Lacertilia have the ventral surface scaled, usually movable
eyelids, tympanic membrane, four appendages or their rudiments, and all
but invariably a sternum.
40. The Ophidia lack appendages, sternum, and tympanum; the
37
578 CHORDATA
eyelids are fused; the mouth is usually extensible; poison fangs are fre-
quently present. The ventral surface has scutes.
41. The Rhynchocephalia resemble the Lacertilia in form, but differ
in having a fixed quadrate.
42. The Crocodilia are elongate, have bony plates in the skin, elongate
cloacal opening, fixed quadrate, teeth placed in separate alveoli, and a
long swimming tail.
43. The AVES (birds) are distinguished by feathers, and by the heart
completely divided into right and left halves.
44. Other characters are homoiothermy (warm-blooded), pneuma-
ticity of bones, fusion of bones of manus, formation of tibio- tarsus and
tarso-metatarsus (intertarsal joint).
45. The birds are divided into Ratitcr, which lack a furcula and a keel
to the sternum, and the Carinalce, in which the sternum is keeled and the
clavicles are united to a furcula. To these are added the extinct Saururcs
and Odontornithes, with teeth.
46. The MAMMALIA have a double occipital condyle, hairy skin, and
milk glands in the female for the nourishment of the young. Other
characters are homoiothermy, the complete separation of the heart, the
modification of parts of the visceral arches into the ear bones, high develop-
ment of the dentition (formation of roots, usually heterodont and
diphyodont).
47. The mammals are divided into Monotremata, Marsupialia, and
Placentalia.
48. The Monotremata are egg-laying mammals with persistent cloaca;
they have a distinct coracoid and an episternum.
49. The Marsupialia are viviparous, but the young, on account of
imperfect nourishment (usually no placenta), are born early and usually
carried in a marsupium (marsupial bones).
50. In the skeleton the inflected angle of the lower jaw is characteristic;
uterus and vagina are double.
51. The Placentalia have well-developed young, nourished in the
uterus by a placenta; they have no marsupium nor marsupial bones.
The vagina is single (Monodelphia), the uterus simple or paired.
52. The clawed Edentata and the Cetacea and Sirenia, which have
flippers, have a degenerate dentition (teeth monophyodont or lacking).
53. The hoofed Ungulata (Perissodactyla and Artiodactyla), the
Proboscidia, and the clawed Rodentia are herbivorous.
54. The Chiroptera, which have a flying membrane (patagium), are
partly herbivorous, partly insectivorous.
55. The small Insectivora (with small canines and no carnassial)
CHORDATA. SUMMARY OF IMPORTANT FACTS 579
and the Carnivora (with strong canine and carnassial molar) are carniv-
orous. The Carnivora are subdivided into the terrestrial Fissipedia and
the aquatic Pinnipedia.
56. The Prosimias and Primates have a more or less indifferent denti-
tion. They have partly or entirely replaced claws by nails, and are
largely provided with grasping hands and feet. The Prosimia1 are lower,
the Primates more highly organized.
57. The Primates are subdivided, according to the position of the
nostrils, the development of the tail, and the dentition and the feet, into
the Platyrrhina?, or New World monkeys, the Catarrhinos, or Old World
apes, and the Anthropince, or man.
INDEX.
Aardvark, 563
Abalone, 326, 333
Abdominal cavity, 555
Abdominal fin, 462
Abdominalia, 374
Abdominal pores, 481
Abducens nerve, 471
Abiogenesis, 127
Abomasus, 568
Acanthia, 428
Acanthias, 507
Acanthin, 176
Acanthobothrium, 254
Acanthocephala, 268
Acanthocystis, 175
Acanthoderus, 39
Acanthodidse, 507
Acanthometra, 177
Acanthopleura, 315
Acanthopteri, 510
Acanthopterygii, 511
Acaridte, 397
Acarina, 396
Accessory chromosome,
Accessory nerve, 471
Accipiter, 544
Acephala, 317
Acerata, 387
Acerentomon, 419
Acetabulum, 248
Achasta, 281
Achatina, 336
Athoreutes, 419
Achromatin, 56
Achtheres, 28, 370
Acineta, 198
Acinetaria, 197
Acinous glands, 63
Acipenser, 50°)
Acmsea, 333
Acontia, 226
Acrania, 439
Acridium, 421
Acris, 520
Acrodont, 529
A( tinaria, 231
Actinophrvs, 175
Actinopoda, 308
Actinospruerium, 173
Actinotrichia, 463
Actinotrocha, 287
Actinozoa, 224
Ai uleata, 426
Aculeus, 413, 425
Acustic nerve, 471
Adambulacral plates, 296
Adamsia, 227
Adductor muscle, 228, 318
/^ga, 386
/Egina, 218
/£ginopsis, 214
/Solids, 335
yEolidia, 335
/Epiornis, 542
y^Equoria, 214, 218
^Eschna, 420
Esthetes, 315
/Ethalium, 181
73 After-shaft, 534
Agalmia, 219
Agamidae, 529
Agelacrinoidea, 302
Agelacrinus, 303
Agkistrodon, 531
Aglaophenia, 218
Aglossa, 520
Aglypha, 531
Agrion, 420
Air bladder, 502
cells, 482
pipes, 538
sacs of birds, 538
Ahe, 408
Alse cordis, 412
Alauda, 544
Albatross, 543
Albumen gland, 249
Alca, 543
Alcedinidffi, 544
Alces, 569
581
Alciopida?, 277
Alcyonaria, 230
Alcyonidse, 230
Alcyonidium, 287
Alcyonium, 227, 230
Alecithal, 143
Aleppo evil, 184
Aletia 433
Alima, 376
Alisphenoid, 458
Allantois, 490
Allelomorphs, 138
Alligator, 532
Alligator turtle, 527
Allobophora, 279
Alloposus, 347
Alosophila, 433
Alpaca, 569
Alpheus, 380
Alternation of generations,
Altrices, 541
Alula, 534
Alveolar duct, 482, 555
Alveoli, 482, 532
Amaroucium, 446
Ambergris, 572
Amblyopsidie, 510
Amblypoda. 570
Amblystoma, 27, 520
Ambulacral system, in,
292, 296
Ameiva, 528
Ametabolous, 414
Amia, 32, 508
Amicula, 316
Ammocoetes, 493
Ammonitidae, ,u(l
Anr.ion, 414, 4';<)
Amniota, 520
Amniotic fluid, 490
Amoeba, 53, 171, 172
Amcebina, 172
Amoeboid motion, 53, in
Amphiaster, 60
5X2
INDEX
Amphibia, 513
Amphibiotica, 420
Amphicoele, 454
Amphiccelias, 528
Amphidiscs, 205
Amphigony, 129
Amphilina, 247
Amphimixis, 140
Amphineura, 315
Amphinuclei, 57
Amphioxus, 439
Amphipoda, 383
Amphiporus, 258
Amphisbama, 530
Amphistomum, 247
Amphitrite, 276, 278
Amphiuma, 520
Amphiura, 299
Amphoridae, 303
Ampullae, 203, 292, 478
of Lorenzini, 500
Ampullaridse, 3^4
Amyda, 527
Anabrus, 421
Anacanthini, 511
Anaconda, 531
Anaerobic organisms, 91
Anal fin, 462, 497
Anallantoidea, 490
Analogy, 10, 90
Analytic species, 141
Anamnia, 490
Anaptomorphidae, 573
Anas, 543
Anaspides, 382
Anaxial animals, 123
Anelasma, 373
Angiostoma, 531
Anguillidue, 510
Anguillula, 265
Anguis, 530
Angulare, 549
Animalcula, 170
Animal organs, 90, no
pole, 133, 142
Animals and Plants, 159
Anisomyaria, 323
Anisopoda, 386
Ankylostoma, 266
Anlage, iii (preface)
Annelida, 269
Annulata, 530
Anodonta, 324
Anolis, 529
Anomodontia, 526
Anopheles, 190, 430
Anoplocephala, 255
Anser, 543
Anseriform.es, 543
Anteater, 563
3piny> 559
Antedon, 302
Antennae, 352
Antennulse, 377
Antheomorpha, 225
Anthomedusse, 217
Anthozoa, 224
Anthropinae, 574
Anthropoidea, 574
Antilocapra, 569
Anlilope, 569
Antimere, 126
Antipathes, 231
Antlers, 569
Ant-lion, 422
Ants, 417, 426
white, 419
Antrostomus, 544
Anura, 520
Anurida, 419
Aorta, 483
Aphaniptera, 431
APes, 573
Aphidae, 430
Apiariae, 426
Apis, 426
Aplacentalia, 558
Aplacophora, 316
Aplysia, .334
Aplysina, 203
Apoda, 308, 374, 519
Apodidae, 366
Apophysis, 454
Aporosa, 231
Appendicularia, 443
Appendix, vermiform, 5^4
Aprophora, 428
Aptenodytes, 543
Aptera, 430
Apteria, 534
Apterygota, 418
Apteryx, 542
Apus, 366
Apyreme, 73
Aqueduct, 470
Aquila, 544
Arachnida, 389
Arachnoid membrane, 473
Araneina, 395
Arbacia, 305
Arcella, 179
Archaeopteryx, 25, 541
Archegony, 127
Archencephalon, 468
Archenteron, 94, 146
Archianellida, 278
Archiccele, 145
Archigetes, 247, 253
Archiptera, 419
Archipterygium, 464
Architeuthis, 347
Arcidae, 323
Arcifera, 520
Arciferous, 513
Arctogaea, 163
Arctotherium, 565
Arcyria, 181
Ardea, 544
Arenicolidae, 278
Areolae mammae, 540
Areolar connective tissue,
76
Argas, 397
Argina, 323
Argiope, 291, 396
Argonautidae, 347
Argulus, 370
Arion, 336
Arista, 431
Aristotle's lantern, 305
Armadillidium, 386
Armadillos, 563
Armata, 281
Army worm, 433
Artemia, 366
Arterial arches, 484
Arterial blood, 102
Arteries, 101
Arthrodira, 513
Arthrogastrida, 391
Arthropoda, 349
Arthrostraca, 383
Articulare, 461
Articular process, 455
Articulata, 302, 349
Artificial parthenogenesis,
*37
selection, 35
Artiodactyla, 568
Arvicola, 566
Ascalabotae, 529
Ascaridas, 266
Ascaris, 264, 266
Ascidiaeformes, 443
Ascidians, 443
INDEX
Ascones, 204
Ascons, 201
Ascyssa, 203
Asellidas, 386
Asiliidas, 431
Asiphonate, 319
Asp, 531
Aspidobranchia, 333
Aspidochirotse, 308
Aspidocotylidse, 244
Aspidonectes, 527
Aspidotus, 430
Assimilation, .91
Astacoidea, 380
Astarte, 324
Aster, 60
Asterias, 298
Asteridae, 298
Asteriscus, 298
Asterinidfe, 298
Asteroidea, 295
As trophy ton, 299
Asterospondyli, 506
Astrsea, 232
Astrangia, 231
Astroides, 232
Ass, 568
Asymmetrical animals, 123
Asymmetron, 441
Atalapha, 565
Atavism, 141
Atax, 397
Ateles, 574
Atelodus, 568
Atheca, 527
Atlantidas, 334
Atlas, 514, 522
Atoke, 275
Atolla, 224
Atriopore, 440
Atrium, 445, 516
of heart, 101, 483
Atrypa, 290
Attus, 396
Atypus, 396
Auchenia, 569
Auditory bulla, 548
organs, 116
ossicles, 116, 461, 479
Auks, 543
Aulophorus, 278
Aurelia, 214, 223
Auricle, 101, 483
Auricularia, 294
Aurochs, 569
Autoflagellata, 181
Autogamy, 174
Autoinfection, 186
Autolytus, 277
Aves, 532
Avicularia, 286
Aviculidffi, 323
Axial skeleton, 91
Axis, 522
Axis cylinder, 85
Axolotl, 520
Axon, 84
Aye-Aye, 572
Azygobranchia, 333
Babesia, 190
Baboons, 574
Bietisca, 420
Bakena, 572
Batenoptera, 572
Balancers, 430
Balaninus, 424
Balanoglossus, 448
Balantidium, 196
Balanus, 374
Baleen, 571
Bandicoot, 560
Barnacles, 371
Basalia, 292, 300, 463
Bascanion, 531
Basioccipital, 458
Basipodite, 97, 360
Basisphenoid, 458
Bassomatophora, 336
Bats, 564
Bdellostoma, 493
Bdellura, 242
Beach fleas, 384
Bears, 565
Beaver, 566
Bedbug, 428
Bee louse, 431
Bees, 426
Beetles, 423
Belemnites, 341
Bell's law, 468, 472
Belosepia, 341
Belostoma, 428
Bicidium, 230
Bilateral symmetry, 124
Bile duct, 481
Bilharzia, 247
Binomial nomenclature, 7
Biogenesis, law of, 26
Biology, 50
Biometric method, 4
Bipalium, 242
Bipinnaria, 294
Bipolar cells, 84
Biradial symmetry, 124
Bird lice, 420
Birds, 532
Birds of Paradise, 41, 544
Birgus, 381
Bison, 569
Bittacus, 422
Bivium, 291, 296
Black fish, 572
Black flies, 430
Black snakes, 531
Bladder worm, 247
Blarina, 564
Blastoderm, 143
Blastodermic vesicle, 145
Blastoidea, 303
Blastomeres, 141
Blastopore, 146
Blastostyle, 218
Blastula, 145
Blatta, 421
Blepharoplast, 182
Blind fish, 510
Blissus, 428
Blister beetles, 424
Blood, 78
arterial, 103
Blood- vascular system, 103
Blood, venous, 103
vessels, 100
Blow flies, 431
Bluebirds, 544
Boa, 531
Bobolink, 544
Body cavity, 99
Bojanus, organ of, 321
Bolina, 235
Bombycina, 433
Bombyx, 433
Bone, 77
Bones, membrane, 451
Bonasa, 543
Bonellia, 281
Book lice, 420
Bopyrirlac, 386
Bos, 569
Bosmina, 366
Botall's duct, 485
Bot flies, 431
Bothriocephalus, 249, 254
Botryllus, 446
584
INDEX
Bougainvillea, 151
Bovidae, 569
Bow fin, 508
Brachialia, 300
Brachiolaria, 294
Brachionus, 260
Brachiopoda, 287
Brachycera, 431
Braconidae, 426
Bradypus, 563
Brain coral, 232
Brain of vertebrates, 468
Branchiae, 97
Branchial arches, 460
chamber, 442
clefts, 438
heart, 343
tree, 307
Branchiata, 359.
Branchiopoda, 366
Branchiomeric, 460
Branchiostegite, 377
Branchiostegal membrane,
502
rays, 497
Branchipus, 365, 366
Branchiura, 370
Braula, 431
Brevilinguia, 530
Brine shrimp, 366
Brissus, 306
Bristle tails, 419
Brittle stars, 298
Bronchioles, 482, 555
Bronchus, 482, 555
Bryozoa, 284.
Bubalus, 569
Bubo, 544
Bubonic plague, 431
Buccal cavity, 94
Buccinidae, 334
Bucerontidas, 544
Budding, 128
of cells, 59
and germ layers, 148
Buffalo, 569
Bufo, 520
Bugs, 427
bed, 428
June, 424
pill, 386
salve, 386
sow, 386
stink, 428
Bugu!a, 287
Bulbus arteriosus, 503
olfactorius, 469
Bulimus, 336
Bulk, 334
Bunodes, 230
Bunodontia, 568
Burbot, 511
Bursa, 266, 293, 299
Bustard, 544
Buteo, 544
Buthus, 392
Butterflies, 432, 433
Buzzards, 544
Byssus gland, 321
Cabbage worm, 433
Cacatua, 544
Caddis flies, 422
Caeca, pyloric, 500
Caecidotea, 386
Caecilia, 519
Caecum, 95, 554
Caenosarc, 227
Calamoichthys, 508
Calandra, 424
Calamus, 533
Calanidas, 369
Calappa, 382
Calcispongiae, 204
Caligus, 370
Callianira, 233
Calliphora, 431
Callorhinus, 565
Calosoma, 424
Calyconectas, 219
Calycophorse, 219
Calyptoblastea, 217
Camarasaurus, 528
Cambarus, 380
Camelopardidas, 569
Camelus, 569
Campanella, 218
Campanula Halleri, 499
Campanularia, 210
Campanulariae, 217
Campodea, 419
Cancer, 382
Cancridze, 382
Candona, 371
Canis, 565
Canker worms, 433
Cantharidas, 424
Canthocamptus, 369
Capillaries, 101
Capillitium, 180
Capra, 569
Caprella, 384
Caprimulgida?, 544
Capybara, 566
Carabidas, 424
Carapace, 359, 527
Carcharinus, 506
Carcharodon, 506
Carchesium, 196
Cardia, 554
Cardinal vein, 484
Cardium, 324
Cardo, 406
Caridea, 380
Carina, 372, 535
Carinariidae, 334
Carinatae, 542
Carinella, 258
Carmine, 428
Carnassial tooth, 554
Carnivora, 565
Carp, 510
Carotid arteries, 483
Carpal bones, 465
Carpocapsa, 432
Carpome, 180
Carpopodite, 97
Carpus, 465
Cartilage, 77
bones, 455
Caryogamy, 168
Caryosomes, 57
Caryophvllaeus, 247
Cassowary, 542
Castor, 566
Castration, no
Casiiarina, 542
Cataclysm theory, 15
Catallacta, 200
Catarrhinae, 574
Caterpillars, 432
Catfishes, 510
Cathamma, 223
Cathartes, 544
Catocala, 433
Catometopa, 382
Catostomidae, 510
Cats, 565
Caudal fin, 462, 498
Caudina, 308
Cavicornia, 569
Cavolinidas, 335
Cavy, 566
Cebus, 574
Cecidomya, 430
INDEX
585
Cell, 51
division, 59
nucleus, 55
reticulum, 53
theory, 12, 51
Cells, blood, 78
ganglion, 83
gland, 68
goblet, 68
sensory, 73
sexual, 130
sizes of, 52
somatic, 130
supporting, 74
Cellular connective tissue,
75
Cellulose, 442
Centipedes, 402
Centralia, 465
Central canal, 467
Central nervous system, 113
Centrodorsal, 300
Centrolecithal, 142
Centrosome, 59
Centrum of vertebra, 454
Centrurus, 392
Cephalaspis, 493
Cephalochordia, 439
Cephalodiscus, 449
Cephalopoda, 336
Cephalothorax, 351
Cephalolhrix, 258
Cerambycidae, 424
Ceraospongia?, 205
Ceratium, 184
Ceratodus, 513
Ceratorhinus, 568
Cercaria, 246
Cerci, 408
Cere, 537
Cerebellum, 469
Cerebellar hemispheres, 470
Cerebral ganglia, 113, 312
Cerebratulus, 258
Cerebrum, 469
Ceresa, 428
Cervicornia, 569
Cervidae, 569
Cervus, 569
Ceryle, 544
Cestoda, 247
Cestum, 235
Cetacea, 571
Cetochilus, 369
Chaetae, 275
Cha-tiferi, 281
Cha'toderma, 316
Ctuetognatha, 262
Chaetonotus, 260
Ctuetopoda, 270
Cruetura, 544
Chalcididre, 426
Chamois, 569
Chameleons, 530
Change of function, S()
Charadrius, 544
Charybdea, 224
Cheese mites, 397
Chela, 377
Chelicerae, 389
Chelifer, 394
Chelone, 527
Chelonia, 526
Chelura, 384
Chelydra, 527
Chernes, 394
Chevron bones, 454
Chiasma, 472
Chiastoneury, 328
Chigoe, 431
Chilina, 336
Chilognatha, 433
Chilomonas, 181
Chilomycterus, 512
Chilopoda, 402
Chilostomata, 287
Chhruera, 507
Chimpanzee, 574
Chinch bug, 428
Chiromys, 572
Chiroptera, 564
Chirotes, 530
Chitin, 350
Chitonidas, 315
Chlamydosaurus, 529
Chlamydoselachus, 506
Chlamydozoa, 191
Choana, 475
Choanoflagellata, 184
Choloepus, 563
Chondrilla, 203
Chondrin, 77
Chondrioderma, 180
Chondrocranium, 4=55
Chondropterygii, 504
Chondrostei, 508
Chone, 277
Chorda clorsalis, 438
Chordata, 438
Chordodes, 268
Chordotonal sense organs,
410
Chorioidea, 120
Chorioid plexus, 470
Chorioid coat, 476
Chorion, 134, 413, 561
Chromatin, c;6
Chromatophores, 339
Chromidial apparatus, 58
Chromosome, accessory, 73
reduction, 134, 138
Chromosomes, 60
conjugation of, 139
function of, 137
Chrysalis, 432
Chrysaora, 124
Chrysomelida1, 424
Chrysomitra, 219
Chryso monad ina, 184
Chyle, 104, 486
Chyle vessels, 104
Cicada, 428
Cicindelidae, 424
Ciconia, 544
Cidaridea, 305
Cilia, 65, in, 192
Ciliary process, 121
Ciliata, 191
Ciliated epithelium, 65
Cilioflagellata, 184
Cimbex, 425
Cinclides, 226
Ciona, 444
Circulatory apparatus, 99
Circumvallate papillae, 474
Cirolana, 386
Cirri, 276, 299
Cirripedia, 371
Cirrus, 109, 243
Cistenides, 277
Cistudo, 527
Citigrada, 396
Civets, 565
Cladocera, 366
Cladoselachii, 507
Clamatores, 544
Clathrulina, 174
Clava, 217
Clavellinida-, 445
Clavicle, 464
Claws, 450, 545
Cleavage cavity, 145
of egg, 141-145
nucleus, 135
Cleithrum, 498
536
INDEX
Ck'psidrina, 187
Clrpsine, 284
Cliliunurius, 381
Clidastcs, 530
Clione, 335
Clisiocampa, 433
Clitellio, 279
Clitellum, 278
Cloaca, 96, 203, 445, 481
Closed blood system, 103
Clothes moth, 432
Clupeidas, 510
Clymene, 278
Clypeaster, 304
Clypeus, 405
Clytia, 218
Cnemidophorus, 530
Cnirlie, 207
Cnidaria, 206
Cnidocill, 207 '
Cobra, 531
Coccida;, 428
Coccidiae, 188
Coccidium, iSS
Coccinellidffi, 424
Coccus, 428
Coccygus, 544
Cochlea, 117, 478
Cochineal, 428
Cockatoos, 544
Cockroaches, 421
Cod, 511
Codlin moth, 432
Codosiga, 184
Ccelenterata, 206
Coelenteron, 206
Ccelhelminthes, 260
Ccelom, 99
of vertebrates, 480
Ccelomic pouches, 148
Cceloplana, 235
Coeloria, 231
Coelopleurus, 303, 305
Coenosarc, 209
Coenurus, 252
Cold blood, 104
Cold rigor, 55
Coleoptera, 423
Collaterals, 84
Cnllembola, 419
Collozoum, 177
Colon, 409
Colony formation, 153
Coloborhombus, 40
Colorado beetle, 424
Coloration, sympathetic, 37
Colossendeis, 399
Colossochelys, 527
Colubriformia, 531
Columba, 543
Columella, 229, 326, 461,
529
of mycetozoa, 180
Columns of cord, 468
Colymbus, 543
Comatulidas, 302
Commissures, 113
Comple mental males, 373
Compound eyes, 354
Conchiolin, 311
Conchoderma, 316
Condylarthra, 567, 569
Condylura, 564
Coney, 570
Conidae, 334
Conjugation, 169
of ciliata, 193
Conjunctiva, 121, 477
Connectives, 113
Connective tissues, 74
Conocephalus, 421
Conocladium, 184
Conotrachelus, 424
Contractile fibre cells, 82
substance, So
vacuoles, 167
Conurus, 544
Conus arteriosus, 503
Convergent development,
158
Copelatse, 443
Copepoda, 366
Copperhead, 531
Copula, 460
Copulation, 134, 169
Coral, 228
Corallium, 230
Coral reefs, 229
Coral snake, 531
Coraciformes, 544
Coracoid, 464
Coracoid process, 550
Coregonus, 510
Corium, 91, 450
Cormorant, 543
Cornacuspongia, 205
Cornea, 119, 476
Corona, 303
Coronata, 224
Coronula, 374
Corpora quadrigemini, 470
Corpus collosum, 551'
Corpus ciliare, 120
striatum, 469
Corpuscles, blood, 78
connective tissue, 74
directive, 132
Grandry's, 473
Krause's, 473
lymph, 79
Malphigian, 105
Meissner's, 115
Miescher's, 191
muscle, So
Rainey's, 191
tactile, 473
Vater-Pacinian, 115, 474
Corrodentia, 419
Corticum, 205
Cords' organ, 552
Corvus, 544
Corycaaidas, 369
Corydalis, 422
Corymorpha, 217
Costae, 229
Cotingidse, 544
Cotton worm, 433
Cottus, 511
Coturnix, 543
Cougar, 565
Covering scales, 219
Coverts, 534
Cowries, 334
Coxa, 405
Coxopodite, 97
Crabs, 382
Crampton's muscle, 540
Crane flies, 430
Cranes, 544
Crangon, 380
Crania, 290
Cranial nerves, 471
Cranium, 455
Craspedon, 211
Craspedotella, 185
Crassilinguia, 529
Crayfish, 380
Creodonta, 566
Cribrellum, 396
Crickets, 421
Crinoidea, 299
Crisia, 287
Crista acustica, 117, 362,
478
Crocodilia, 531
INDEX
587
Crop, 94, 408
Crossbills, 544
Crossing of species, 21
Crossopterygii, 508
Cross-striated muscle, Ft
Crotalus, 531
Crows, 544
Crura cerebri, 551
Crustacea, 359
Cryptobranchus, 520
Cryptochiton, 316
Cryptodira, 527
Cryptoniscus, 386
Cryptotetramera, 424
Crystalline style, 322
Ctenida, 387
Ctenidia, 312
Cteniza, 396
Ctenodiscus, 298
Ctenoid scales, 494
Ctenolabrus, 510
Ctenophora, 232
Ctenoplana, 235
Ctenostomata, 287
Cubical epithelium, 65
Cubo medusae, 224
Cuculi, 544
Cucumaria, 308
Culcita, 296, 298
Culicidae, 430
Cumacea, 382
Cunina, 218
Cunner, 510
Cunocantha, 216, 218
Curculio, 424
Cursoria, 421
Cuspidaria, 325
Cutaneous artery, 517
Cuticle, 65
Cuticula, 167
Cuticular skeleton, 1 1 1
Cutis, 91, 450
Cuttle bone, 341, 347
Cuttlefish, 347
Cuvierian ducts, 484, 503
organs, 307
Cyamus, 384
Cyanea, 223
Cyanocitta, 544
Cyclas, 324
Cycloid scales, 494
Cycloposthium, 197
Cyclops, 29, 369
stage, 29
Cyclospondyli, 506
Cyclostomata, 287, 491
Cydippidaj, 235
Cygnus, 543
Cymothoa, 386
Cynipidae, 425
Cynocephalus, 574
Cynomorphoe, 574
Cynomys, 566
Cynthia, 445
Cypraaa, 330
Cypridina, 371
Cyprinicke, 510
Cypris, 371
Cypselidae, 544
Cyrtophilus, 421
Cysticercoid, 253
Cysticercus, 247
Cystid, 285
Cystidea, 303
Cystoflagellata, 184
Cystoneciae, 220
Cytopharynx, 167, 192
Cytopyge, 167, 192
Cytosporida, 185
Cytostome, 167, 192
Dactylethra, 520
Dactylopodite, 97
Daddy long-legs, 394
Daphnia, 366
Dasypus, 563
Dasyurus, 560
Datames, 394
Decapoda, 347, 377
Decidua, 562
Deer, 569
Degeneration, 156
Delphinus, 572
Demibranchs, 501
Demodex, 397
Dendrites, 84
Dendrochirota?, 308
Dendrocoelum, 242
Dendrceca, 544
Dendronotus, 335
Dendrosoma, 198
Dentary bone, 461
Dental formula, 554
Dentalium, 325
Dentes complicate, 554
Denticeta;, 572
Dentine, 451
Dentition, 481, 553
Derma, 91, 450
Dermal skeleton, 91
Dermanyssus, 397
Dermatobia, 431
Dermatoptera, 42 1
Dermochelys, 527
Dermo-muscular tunic, 1 1 1,
237
Dero, 279
Derotrema, 520
Desmodont, 318
Desmodus, 565
Desor's larva, 257
Determinants, 43, 137
Deutomerite, 186
Deutocerebrum, 405
Deutoplasm, 71
Diapheromera, 421
Diaphragm, 481, 555
Diapophysis, 454
Diarrhoea, 196
Diastole, 101
Diastylis, 382
Diaptomus, 369
Dibranchia, 347
Dicotyles, 568
Dicyemida, 200
Didelphia, 559
Didelphys, 560
Didus, 543
Diencephalon, 469
Difflugia, 179
Diffuse nervous system, 112
Digenea, 245
Digestive tract, 03
Digitigrade, 550
Dihybrids, 140
Dimorphism, sexual, 40, 169
Dimorphodon, 532
Dimyaria, 323
Dinichthys, 513
Dinobryon, 184
Dinoceras, 570
Dinoflagellata, 184
Dinornithida?, 542
Dinosauria, 528
Dinotherium. 570
Dioecious, 108
Diomedea, 543
Diopatra, 277
Diotocardia, 333
Diphasia, 218
Diphycercal, 32, 498
Diphyes, 220
Diphyodont, 553
Diplocardia, 279
Diplopoda, 433
588
INDEX
Diplospondylii, 506
I liplozoon, 154, 244
Dipneumonia, 513
] (ipnnunones, 396
Dipneusti, 512
1 )ipnoi, 512
Diporpa, 154, 244
Diprotodon, 561
1 Hprndonta, 561
Diptera, 430
Dipylidium, 2-5
Directive corpuscles, 131
Directives, 227
Discina, 290
Discodermia, 204
Discodrilidae, 271;
Disco medusae, 223
Disconanthse, 220
Discophori, 281 _
Dispermy, 135
Distaplia, 446
Distomeze, 245
Distomum, 105, 243, 246-
Distribution of animals, 33
Disuse of parts, 47, 89
Division of labor, 63, 155
Division, reproduction by,
128
Divaricator muscles, 2.SS
Dochmius, 266
Docoglossa, 333
Doctrine of causes, 34
Dodo, 543
Dogfish, 506
Dogs, 565
Dolichonyx, 544
Doliolum, 448
Dolomedes, 396
Dolphins, 572
Dominant, 138
Dondersia, 316
Doris, 335
Dorsal aorta, 483
fin, 462, 497
root, 468
Dorylinae, 427
Doryphora, 424
Double animal, 154
Draco, 529
Dranculus, 26$
Drepanidotaenia, 255
I )rom;i'Us, 542
Dromatherium, 559
1 )roni>s, 417
Duckbill, 559
Ducks, 543
Ducts of cuvier, 484
genital, 108
Botallii, 485, 518
cochlearis, 479
choledochus, 481
ejaculatorius, 109
Dugong, 570
Duodenum, 554
Duplicidentata, 566
Dura mater, 473
Dysodont, 318
Dytiscidae, 424
Eagles, 544
Ear bones, 461, 479
Ear conch, 480
Ear drum, 479
Ears, 116
vertebrate, 477
Earth worms, 279
Earwig, 421
Ecardines, 290
Echidna, 559
Echinarachnius, 306
Echinobothrium, 254
Echinocardium, 306
Echinoderida.% 260
Echinoderma, 291
Echinoidea, ;o;
7 \J \J
Echinorhynchus, 269
Echinospha?riles, 303
Echiuroidea, 281
Echiurus, 281
Ecdysis, 350
Eciton, 427
Ectoblast, 146
Ectobronchi, 538
Ectocyst, 285
Ectoderm, 91, 94, 146
Ectethmoid, 458
Ectoparasites, 157, 242
Ectoprocta, 285
Ectosarc, 172
Edentata, 563
Edrioasteroidea, 302
Edriophthalmata, 383
Edwardsia, 226, 230
Edwardsiella, 230
Eels, 510
Efferent branchial arteries,
504
Egg cell, 71
cleavage of, 141
fertilization of, 134
Egg, maturation of, 132
nucleus, 132
sacs, 369
segmentation of, 141
tooth, 525
Eimeria, 188
Eimer's organs, 473
Elaps, 531
Elasipoda, 308
Elasmobranchs, 504
Elastic cartilage, 77
Elastic tissue, 76
Elastica externa, 452
Elastin, 75
Elaters, 181
Electric organs, in
Elephant, 570
Elephantiasis, 268
Elephas, 570
Eleutheria, 200
Elysiidae, 335
Elytra, 276, 408
Embiotocidas, 511
Embryo, 150
Embryology, 127
Emu, 542
Enamel, 451, 494
Enchytrseidas, 279
Endites, 361
Enrlocyst, 285
Endolymph, 116, 479
duct, 477
Endopodite, 360
Endostyle, 443
English sparrow, 544
Enhydris, 565
Ensatella, 324
Entalis, 325
Entamoeba, 173
Entelops, 563
Enterocoele, 99, 480
Enteropneusta, 448
Enteroxenus, 308
Entoblast, 146
Entobronchi, 538
Entocolax, 308
Entoconcha, 308
Entoderm, 94, 146
Entomostraca, 363
Entoniscus, 385, 386
Entoparasites, 157, 242
Entophaga, 425
Entoplastron, 527
Entoprocta, 284
Entosarc, 172
INDEX
589
Entosternite,
Entovalva, 308
Entwicklungsmei hanik, 3
Environment, influent L- of,
47
Enzymes, 96
Eohippus, 569
Eozoon, 164
Epaxial muscles, 454
Epeira, 396
Ependyma, 114, 467
Ephemera, 420
Ephippium, 366
Ephydatia, 205
Ephyra, 221
Epibdella, 244
Epiblast, 146
Epidermis, 91, 450
Epididymis, 283, 489
Epigenesis, 12
Epiglottis, 482, 555
Epitoke, 275
Epimeral plates, 384
Epimerite, 186
Epiotic, 458
Epipharynx, 405
Epiphragm, 327
Epiphysis, 470
Epipleurals, 509
Epipodite, 361
Epipodium, 325
Epipterygoid, 529
Episternum, 464, 513
Epistropheus, 522
Epistylis, 197
Epitheca, 229
Epithelial muscle, 81
Epithelium, 64
germinal, 70, 107
Epizoanthus, 158, 231
Equatorial plate, 60
Equus, 568
Erax, 431
Erethizon, 566
Ergatoids, 417
Ergatomorphs, 417
Erinacidae, 564
Erichthus, 376
Erigone, 396
Ermine, 565
Errantia, 277
Erythroneura, 428
Eschara, 287
Esocidas, 510
Esperia, 205
Essence of pearl, 41)4
Estheria, 366
Ethmoidalia, 458
Ethmoid bone, 4=; l6°
Geological distribution, 164
Geometrina, 432
Geonemertes, 258
Geophilus, 403
Gephyraea, 279
Gerardia, 231
Germ layers, 145
and budding, 148
Germinal disc, 143
Germinal epithelium, 70,
107
glands, 107
selection, 43
spot, 72
vesicle, 72
Geryonia, 214, 218
Giant cells, 62
Gibbon, 574
Gila monster, 530
Gill arches, 460
arteries, 101
books, 387
clefts, 98, 438, 482
veins, 101
Gills, 97
tracheal, 402, 411
of vertebrates, 482
Gigantostraca, 387
Gipsy moth, 433
Giraffa, 569
Girdles, 463
Gizzard, 95, 409
Glabella, 364
Gland cells, 68
Glands, 68
anal, 96, 565
gas, 502
germinal, 107
green, 358, 361
Harder's, 477
lacrimal, 477
lymph, 486
lymphoid, 293
mammary, 546
milk, 546
nidamental, 344
ovoid, 293
paraxon, 293
repugnatorial, 434
salivary, 96
sebaceous, 546
sexual, 107
shell, 358, 362
INDEX
591
Glands, sweat, 546
subneural, 445
thymus, 4X2
thyreoid, 482
yolk, no
Glandular epithelium, <> ;
Glass snake, 530
Glia, 114
Globigerina, 171)
Globiocephalus, 572
Glochidia, 322
Glomeridae, 434
Glomeruli, 105, 486
Glomus, 487
Glossa, 406
Glossina, 431
Glossobalanus, 449
Glossopharyngeal nerve,
47 1
Glottis, 482
Glue, 76
Glugea, 191
Glutin, 76
Glyptodontidas, 563
Gnathobdellidae, 284
Gnathochilarium, 402, 434
Goats, 569
Goblet cells, 68
Gonads, 107
Gonapophyses, 408, 413
Goniale, 549
Goniatites, 346
Goniodes, 420
Gonochorism, 107
Gonodactylus, 376
Gonophore, 216
Goose, 543
Goose barnacle, 373
Gordiaceae, 268
Gordius, 268
Gorgoniidae, 230
Gorilla, 574
Gradientia, 519
Grandry's corpuscle, 473
Grallatores, 543
Grantia, 204
Graptolites, 218
Gray matter, 114, 468
Gray nerve fibres, 85
Grebes, 543
Green gland, 358, 361
Gregarina, 186
Gressoria, 421
Gribble, 386
Gromia, 54
Grouse, 543
Grus, 544
Gryllotalpa, 421
Gryllus, 421
Grypotherium, 563
Guanin, 494
Guinea pig, 566
Guinea worm, 268
Gula, 405
Gular plates, 508
Gulls, 543
Gunda, 242
Gymnoblastea, 217
Gymnodonta, 512
Gymnokemata, 287
Gymnophiona, 519
Gymnosomata, 335
Gynandromorphism, 108
Gyri, 470
Gyrodactylus, 244
Habrocentrum, 396
Haddock, 511
Hadenoecus, 421
Htemadipsa, 284
Hajmal arch, 453
ribs, 454
spine, 453
Hasmamoeba, iSS
Haemapophysis, 453
Hrcmentaria, 284
Haemoccele, 100, 103
Ha^mocyanin, 80
Ha?moerythrin, 80
Haemoglobin, 78
Ha?moglobinuria, 190
Haemosporida, 188
Hag fishes, 491
Hair, 545
Hair-neck worms, 267
Hair worms, 268
Halcampa, 231
Haliaetus, 544
Halibut, 511
Halicore, 570
Haliomma, 124
Haliotis, 327, 333
Halteres, 430
Halyclystus, 224
Hammer head, 506
Hapale, 574
Haploceras, 569
Hard«r's gland, 477
Hares, 566
Harpactidse, 369
Hurvestmen, 394
Haustellum, 407
Haversian canals, 77
Hawk moths, 433
Hawks, 544
Hazel weevil, 424
Head kidneys, 105, 487
Heart, 100
Heart urchins, 306
Heart of vertebrates, 483
Heat rigor, 55
Hectocotylus, 344
Hedgehogs, 564
Heliaster, 298
Helicidaa, 336
Heliconiids, 39
Helioporidas, 230
Hsliozoa, 173
Helix, 336
Hellbender, 520
Hellgrammite, 422
Helminthophaga, 544
Heloderma, 530
Hemerobiidas, 422
Hemibranchii, 511
Hemichordia, 448
Hemimetabolous, 414
Hemiptera, 428
Hemitripterus, 511
Hen clam, 324
Hens, 543
Hepato-pancreas, 96, 361
Hepatus, 382
Heptanchus, 506
Heredity, 136
basis of, 58, 137
Hermaphroditism, 107
Hermit crabs, 380
Herons, 544
Herpestes, 565
Herring, 510
Hesperornis, 541
Hessian fly, 430
Heterakis, 266
Heterocercal, 32, 498
Heteroconchae, 324
Heterocotylea, 244
Heterodera, 266
Heterodont hinge, 318
Heterogony, 132
Heteromera, 424
Heteromyaria, 323
Heteronemertini, 258
Heteronereis, 275
592
INDEX
Heteronomous segmenta-
tion, 126
Heteropleuron, 441
Heteropoda, 334
Heteroptera, 428
Heterosyllis, 275
Heterotricha, 196
Heterozygote, 138
Hexacoralla, 230
Hexactinellida?, 205
Hexanchus, 506
Hexapoda, 403
Hind brain, 468
Hind-gut, 94
Hinge ligament, 318
Hipparion, 569
Hippasteria, 298
Hippidas, 381
Hippocampus, 511
Hippolyte, 380
Hippopotamus, 568
Hippospongia, 205
Hirundo, 544
Hirudo, 284
Hirudinei, 281
Histology, 63
Holoblastic, 142
Holocephali, 507
Holocystites, 303
Holometabolous, 415
Holostei, 508
Holostome, 327
Holothuria, 308
Holothurioidea, 306
Holotricha, 195
Homarus, 380
Homaxial animals, 123
Homo, 574
Homocercal, 32, 498
Homodont, 553
Homogeneous connective
tissue, 75
Homology, 10, 90
Homonomous segmenta-
tion, 126
Homoptera, 428
Homothermal, 104
Homozygote, 138
Hoofs, 450, 545
Hookworm, 267
Hopworm, 433
Hormones, no
Hormophora, 233, 235
Horn bills, 544.
Horned toads, 529
Hornera, 287
Horntails, 425
Horns of cord, 468
Horse, 568
ancestry of, 569
flies, 430
sponge, 205
House flies, 431
Humerus, 465
Humming birds, 544
Hyaena, 565
Hyaleida, 335
Hyaline cartilage, 77
Hyalonema, 204
Hyalospongia, 204
Hyas, 382
Hybrids, 21
fertility of, 22
Hydrophyton, 427
Hydra, 129, 208, 217
Hydrachna, 397
Hydractinia, 217
Hydranth, 209
Hydraria, 216, 217
Hydrobatidse, 428
Hydrocaulus, 209
Hydrochoerus, 566
Hydroccele system, 294
Hydrocorallina?, 217
Hydrocores, 428
Hydroid, 209
Hydroides, 278
Hydromedusae, 208
Hydrophidffi, 531
Hydrophilidae, 424
Hydrophobia, 191
Hydropolyp, 207
Hydro psyche, 422
Hydrorhiza, 209
Hyclrotheca, 209
Hydrozoa, 208
Hyla, 520
Hylobates, 574
Hylodes, 519
Hymenolepis, 254
Hymenoptera, 424
Hyocrinus, 300
Hyoid arch, 460
Hyomandibular, 460
Hypaena, 433
Hypaxial muscles, 454
Hyperia, 384
Hyperina, 384
Hyperoartia, 492
Hyperotretia, 492
Hypoblast, 146
Hypobranchial groove, 440
Hypoderma, 431
Hypodermis, 350
Hypogeophis, 519
Hypoglossal nerve, 471
Hypopharynx, 405
Hypophysis, 470
Hyporachis, 534
Hypotricha, 197
Hyracoidea, 570
Hyracotherium, 569
Hyrax, 570
Hystrix, 566
Hystricomorphia, 566
Ibis, 544
Ichneumonidae, 426
Ichneumons, 565
Icterus, 544
Ichthydium, 260
Ichthyobdella, 284
Ichthyodolurites, 505
Ichthyophis, 519
Ichthyopsida, 491
Ichthyornis, 541
Ichthyosauria, 526
Ichthyotomi, 507
Idioplasm, 137
Idiothermal, 104
Idotea, 386
Iguanidas, 529
Ilium, 464
Imaginal discs, 417
Imago, 414
Impennes, 543
Imperforata, 179
Incus, 461
Indirect development, 150
Inermes, 281
Infrabasal, 300
Infraorbital ring, 497
Infundibula, 482, 555
Infunclibulum, 470
Ingluvies, 94, 408
Inheritance, Mendelian,
138
Inia, 571
Inquilines, 425
Insecta, 400
Insectivora, 564
Integripalliata, 324
Interambulacral plates, 296
Intercalaria, 453, 495
Intercellular substance, 74
INDEX
593
Interfilar substance, 53
Interhyal bone, 496
Intermaxillaries, 461
Intermedium, 465
Internal secretions, no
Interparietal bone, 546
Interradii, 124, 293
Intcrse-'tal space, 227
Interve. tebral cartilage, 455
ligaments, 550
Intestine, 95
Intraseptal space, 227
Intratarsal joint, 524
Involuntary muscles, Si
Iris, 120, 476
Ischiopodite, 97
Ischium, 464
Isodont, 318
Isogametes, 168
Isopoda, 385
Isoptera, 419
Itch, 397
Ixodes, 397
Jacobson's organ, 475
Japyx, 419
Jassidse, 428
Jays, 544
Jigger flea, 431
Jugal bone, 462
Jugular vein, 484
Julus, 434
June bugs, 424
Kallima, 38
Kangaroos, 561
Karyokinesis, 60
Katydid, 421
Keratin, 67, 205
Keyhole limpet, 333
Kidneys, 105, 488
Kingfishers, 544
Kinosternon, 527
Knee pan, 465
Kcenenia, 393
Krause's corpuscle, 473
Labial cartilages, 460
palpi, 320
Labium, 405
Labor, division of, 63, 126
Labrida?, 510
Labrum, 40=;
Labyrinth, 117, 478, 479
Labyrinthodonta, 519
38
Lacerta, 530
Lacertilia, 529
Lace wings, 422
Lucinia, 406
Lacrimal gland, 477
Lacunar blood system, 103
Ladder nervous system, 1 13
Lady birds, 424
Lacmodipoda, 384
Lagena, 117, 478
Lagomorpha, 566
Lagomys, 566
Lamarckism, 46
Lamblia, 184
Lamellibranchiata, 317
Lamellicornia, 424
Lamellirostres, 543
Lamna, 451, 546
Lamprey eels, 491
Lampyridae, 424
Land crabs, 382
Lantern of Aristotle, 305
Larus, 543
Larva, 150, 414
Larval organs, 1 50
Larynx, 482, 516
Laryngeal cartilages, 461
Lateralia, 372
Lateral line organs, 499
Lateral organs, 271
Latrodectes, 396
Laurer's canal, 344
Lazy worm, 267
Leaf hoppers, 428
Leather turtles, 527
Leda, 323
Leeches, 281
Lemnisci, 269
Lemurs, 572
Lens, 119, 477
Lepas, 159, 373
Lepidurus, 366
Lepidonotus, 277
Lepidoptera, 432
Lepidosauria, 528
Lepidosiren, 513
Lepidosteus, 508
Lepisma, 418
Leptasterias, 298
Leptalis, 39
Leptocardii, 439
Leptocephalus, 510
Leptochela, 386
Leptoclinum, 446
Leptodiscus, 185
Leptodora, 366
Leptomedusa-, 2 1 7
Leptoplana, 242
Leptostraca, 375
Lepus, 566
Lernasa, 370
Lernasocera, 370
Lernasopodida?, 370
Leucetta, 203
Leucocytes, 78
Leucon, 203
Leucones, 204
Leucortis, 204
Leucocytozoon, 183
Leucosoidea. 382
Leucosolenia, 204
Libellula, 420
Libinia, 382
Lice, bee, 43 1
bird, 420
book, 420
human, 430
plant, 430
Lieberkiihn's glands, 69
Lieshmania, 184
Life, origin of, 127
Ligulida?, 253
Limacidse, 336
Limacinida?, 335
Limax, 336
Limbs, skelton of, 462
Limicola, 279
Limnadia, 366
Limnaea, 336
Limnoria, 386
Limpets, 333
Limulus, 387
Linckia, 296
Linear nervous system, 112
Lines of growth, 317
Lineus, 258
Lingual ribbon, 314. 329
Linguatulida, 397
Lingula, 290
Linin, 56
Linnean system, 7
Liobunum, 31)4
Lion, 565
Lips, 554
Liquor cerebrospinalis, 467
Liriope, 216. 218
Lithistida?, 204
Lithobius, 403
Lithodidos, ,}8j
Lithodomus, 323
594
INDEX
Lilhoglyphus, 327
Littorinid;v, 334
Liver, 96, 481
Liver fluke, 243, 246
Lizards, 529
Llama, 569
Lobatac, 235
Lobi inferiores, 499
Lobosa, 172
Lobster, 380
Loco motor organs, no
Locustidae, 421
Locusts, 427
Loggerhead, 527
Lohmanella, 200
Loligo, 336, 347
Longipennes, 543
Loons, 543
Lophobranrhii, 511
Lophocalyx, 123
Lophodont, 554
Lophogastridae, 376
Lophophore, 287
Lophopoda, 287
Lophopus, 287
Lorenzini, ampulla;
500
Lorica, 182
Loricata, 380, 531, 563
Loricati, 511
Loris, 572
Lota, 511
Loven's larva, 238
Loxia, 544
Loxosoma, 285
Lucernarise, 224
Luciae, 446
Lumbricus, 279
sexual organs, 107
Lunatia, 334
Lung fishes, 512
pipes, 538
sacs, 387
Lungs, 98, 482
of birds, 538
Lutra, 565
Lycosa, 396
Lygaeidae, 428
Lymph, 78
glands, 486
hearts, 486
vessels, 103,
Lymphoid gland,
Lyre birds, 544
Lyriform organs, 390
Macacus, 574
Machilis, 419
Mackerel, 511
Macoma, 324
Macrobdella, 284
Macrobiotus, 398
Macrochelys, 527
Macrochiroptera, 565
Macrodrili, 279
Macrogametes, 169
Macronucleus, 192
Macropus, 561
Macrura, 380
Mactra, 324
Macula acustica, 477
lutea, 120
Madrepora, 231
Madreporaria, 231
Madreporite, 292
Maioidea, 382
Malaclemmys, 527
Malacobdella, 258
Malacoderma, 231
Malacopoda, 399
Malacopteri, 510
of, Malacostraca, 374
Malar bone, 462
Malaria, 188, 430
Maldanidae, 278
Male pronucleus, 135
Malleus, 461
Mallophaga, 420
Malpighian bodies, 486
corpuscles, 105
tubes, 358, 383, 387, 402
Mammalia, 545
Mammary glands, 546
Mammoth, 570
Man, 574
embryo of, 27
Manatee, 570
Manatus, 570
Mandibles, 352, 406
Mandibula, 460
Mandibular arch, 460
Mandrils, 574
Manicina, 232
Manis, 563
Mantis, praying, 421
shrimp, 376
Mantle, 311, 338, 442
cavity, 311
Manubrium, 211
Manyunkia, 276
Margarita, 333
Margelis, 131, 217
Marginal plates, 297
Marmosets, 574
Marsipobranchii, 491
Marsupial bones, 560
Marsupialia, 559
Marsupium, 558, 560
Martin, 565
Mas tax, 259
Mastigamceba, 172
Mastigophora, 181
Mastodon, 570
Matrix, 75
Maturation of egg, 132
Maturation divisions, 139
Maxillae, 352, 405
Maxillaries, 461
Maxillary palpus, 406
Maxillipeds, 352
May flies, 420
Measuring worms, 431
Meatus, auditory, 480
Meckel's cartilage, 460
Mecoptera, 422
Medulla oblongata, 469
Medullary plate, 467
sheath, 85
Medullated nerve fibre, 85
Medusa, 208
Megalonyx, ^63
Megalops, 380
Megapodes, 543
Megascolex, 279
Megatherium, 563
Meissner's corpuscles, 115
Melanoplus, 421
Meleagrina, 323
Meleagris, 543
Mellita, 306
Meloidas, 424
Melolontha, 424
Melonites, 305
Melophagus, 431
Melopsittacus, 544
Membracidae, 428
Membrana propria, 237
Membrane bone, 451, 455
Membranellae, 196
Mendelism, 45
Mendel's Law, 138
Meninx, 473
Menopoma, 520
Mentum, 406
Menuridae, 544
Mephitis, 565
Mermis, 268
Meroblastic, 142
Merogamy, 137
]\Ieropodite, 97
Merozoites, 186
Meryhippus, 569
Mesectoderm, 202
Mesencephalon, 468
Mesenchyme, 74, 147
Mesenterial filaments, 226
Mesenteries, 99, 480
Mesenteron, 94
Mesethmoid, 458
Mesoblast, 147
Mesoderm, 94, 146
Mesoglcea, 208
Mesohippus, 569
Mesonemertini, 258
Mesonephros, 106, 487
Mesonotum, 404
Mesopleuron, 404
Mesopterygium, 464
Mesorchium, 480
Mesosternum, 404
Mesothelium, 147
Mesothorax, 404
Mesotroche, 273
Mesovarium, 480
Metabola, 191
Metacarpals, 465
Metagenesis, 131
Metamere, 126, 269
Metamerism, 126
Metamorphosis, 150, 414
Metanemertini, 258
Metanephros, 488
Metanotum, 404
Metapleuron, 404
Metapodium, 325, 53:5
Metapterygium, 464
Metasternum, 404
Metastoma, 377
Metatarsals, 465
Metathorax, 404
Metuzoa, 201
Metencephalon, 468
Methona, 39
Metridium, 231
Miastor, 414, 430
Mice, 566
Microcentrum, 421
Microchiroptera, 565
Microconodon, 559
Microcotyle, 244
Microdrili, 279
INDEX
Microgametes, 169
Microlepidoptera, 432
Microlestes, 559
Micronucleus, 192
Micropylar apparatus, 134,
A T •>
Microthelyphonidse, 393
MiiTiira, 258
Midas, 574
Midbrain, 468
Mid-gut, 94
Miescher's corpuscles, igi
Migration theory, 45
Miliola, 179, 180
Milk glands, 546
teeth, 553
Millepora, 211, 217
Mimicry, 37
Mink, 565
Miohippus, 569
Miracidium, 246
Mites, 396
Mitochondria, ^8
Mitosis, 60
Mitrocoma, 214
Mixipterygium, 505
Mnemiopsis, 235
Moccasin, 531
Mole cricket, 421
Moles, 564
Molgula, 446
Molpadia, 308
Molting, 350
Mollusca, 310
Molluscoida, 166
Monactinellidse, 205
Monascidia;, 445
Monaster, 60
Monaxial symmetry, 123
Monera, 172
Monezia, 248
Monitor, 530
Monkeys, 573
Monocaulis, 217
Monocystis, 187
Monodelphia, 561
Monodon, 572
Monogenea, 244
Monogony, 128
Monohybrid, 140
Monomyaria, 323
Monophyodont, 553
Monopneumonia, 513
Monops, 242
Monoscelis, 242
595
Monospermy, 134
Monothalamia, ijcj
Monotocardia, 333
Monotremata, 55S
Moose, 569
Morphological limits, 50
Morphology, animal. 50
Mosaic theory of vision,
357
Mosasaurus, 530
Moschidas, 569
Mosquito, 430
Mother-of-pearl, 319
Moths, 432
Motor roots, 468
Mud crab, 382
Mudpuppy, 520
M tiller's duct, 487
Multicellularity, 62
Multinuclearity, 62
Multipolar cells, 84
Multituberculata, 559
Murex, 327
Muricidas, 334
Mus, 566
Muscarias, 431
Muscidas, 431
Muscle corpuscles, So
cross-striated, Si
epaxial, 454
fibres, 80
fibrils, 63, So
hypaxial, 454
smooth, 8 1
visceral, 472
of vertebrates, 466
Muscular tissue, So
Mushroom coral, 232
Musk deer, 569
Musk ox, 569
Musk rat, 566
Mussels, 323, 324 »
Mustela, 565
Mustelus, 506
Mutations, 43
Mya, 324
Mycetozoa, 180
Myelencephalon, 469
Mygale, 396
Mygnimia, 40
Mylodon, 563
Myocoele, 466
Myomeric, 460
Myomorpha, 566
Myopsidae, 347
596
INDEX
Myosepta, 466
Myotomes, 466
Alvrianida, 275, 277
Myriapoda, 402, 433
Myriotrochus, 308
Myrmecophaga, 563
Myrmicida-, 426
Myrmecocystus, 427
Mysilla, 205
Mysis, 376
stage, 380
Mystaceti, 572
Mytilidae, 323
Myxicola, 277
Myxine, 493
Myxobolus, 190
Myxomycetes, 180
Myxospongia, 205
Myxosporida, 190
Myzobdella, 279
Myzontes, 493
Nagana disease, 43 1
Nageli's principle of pro-
gression, 48
Naididre, 279
Nails, 450, 545
Nais, 271
Naja, 531
Nanomia, 219
Narcomedusa?, 218
Naris, 475
Nanval, 572
Nasal bones, 459
capsule, 457
Natatores, 543
Naticida;, 334
Natural selection, 35
Nauplius, 28, 362
eye, 362
Nausithoe, 224
Nautilus, 346
Ncbalia, 375
Nectocalyces, 218
Nt-ctonema, 268
Ncctonemertes, 2 58
Necturus, 520
Needham's sac, 344
Nematocyst, 207
Ncmatoda, 263
Nemathelminthes, 263
Nematus, 425
Nemerteans, 255
Nemertini, 255
Nernocera, 430
Nemopsis, 217
Neocranium, 455
Neocrinoida, 302
Nejgam, 162
Neo-Lamarckism, 47
Neomenia, 316
Neosporida, 190
Neoteny, 150
Nephelis, 284
Nephridia, 105, 272
Ncphridial tubules, 486
Nephrostome, 105, 242, 4^6
Nepidse, 428
Neptunus, 382
Nereis, 276
Neverita, 334
Nerve-cord, ventral, 113
Nerve-end buds, 474
Nerve fibres, 85
hillocks, 474
Nerves cranial, 471
eye-muscle, 472
spinal, 468
sympathetic, 473
Nervous system, in
development of, 30
of vertebrates, 467
Nervous tissue, 83
Neural arch, 453
spine, 453
Neurapophysis, 4^3
Neurenteric canal, 439, 467
Neurites, 84
Neuroblasts, 86
Neuroglia, 114
Neuromasts, 474
Neurone, 86
Neuropodium, 276
Neuropore, 440, 468
Neuroptera, 421
Nettle bodies, 192
cells, 207, 241
Nictitating membrane, 477
Nidamental gland, 344
Night-hawks, 544
Nirmus, 420
Noctiluca, 184
Noctuina, 433
Nomarthra, 563
Nosema, 191
Nothria, 277
Notochord, 438, 452
Notochordal sheath, 452
Notodelphidas, 369
Notogffia, 162
Notonectidse, 428'
Notopodium, 276
Notum, 404
Nuclear fragmentation, 62
spindle, 60
substance, 56
Nuclein, 56
Nucleoli, chromatic, 57
Nucleolus, 57
Nucleus, 55
cleavage, 135
function of, 58
Nucula, 323
Nuda, 235
Nudibranchia, 335
Nummulites, 180
Nurse, 131
Nutrition, 55
Nyctotherus, 196
Nymphon, 399
Obelia, 218
Obisium, 394
Obturator foramen, 522,
55°
Occipital bone, 458
condyles, 514
Occipitalia, 458
Occiput, 405
Ocellatae, 216
Ocellus, 354
Ocneria, 433
Octocoralla, 230
Octopoda, 347
Octopus, 336, 347
Ocular plate, 297, 304
Oculina, 231
Oculomotor nerve, 471
Odonata, 420
Odontoholca?, 541
Odontophore, 329
Odontormse, 541
Odontornithes, 541
CEcanthus, 421
(Ecology, 50
CKdipoda, 421
Q-Mogonium, 160
(Knothera, 44
Qvsophageal nerve ring, 113
(Esophagus, 94
Oigopsida, 347
Oikopleura, 443, 444
Oil bottles, 424
Okapia, 569
Olecranon, 465
INDEX
597
Olfactory lobe, 469
nerve, 471
organs, 115
Oligochseice, 278
Oligopyreme, 73
Oligosoma, 530
Oligotrochus, 308
Olividae, 334
Ommastrephes, 347
Omasus, 568
Ommalidia, 356
Oncosphasra, 251
Oniscus, 386
Ontogeny, 2
Oocyte, 71, 132
Ookinete, 183, 190
Oospores, 169
Ootype, 244
Opalina, 195
Opercular cleft, 502
Operculum, 285, 327
of fishes, 497
Ophidia, 530
Ophioglypha, 299
Ophiopholis, 299
Ophisaurus, 530
Ophiurioidea, 298
Ophryodendron, 198
Ophryoscolex, 197
Opisthobranchia, 334
Opisthocoele, 455
Opisthopatus, 400
Opisthotic, 458
Opossum, 560
shrimp, 376
Opoterodonta, 531
Optic cup, 477
ganglion, 119, 476
lobes, 470
nerve, 471
stalk, 477
thalami, 469
vesicle, 477
Orange scale, 430
Orang-utan, 574
Ora serrata, 121
Oralia, 292, 301
Orbitelaria, 396
Orbitosphenoid, 458
Orbitotemporal fossa, 522
Orb weavers, 396
Orchestia, 384
Organ-pipe coral, 230
Organs, 88
animal, 90, no
Organs, assimilation, 91
auditory, 116
of Bojanus, 321
circulator)', 99
of Corti, 478, 552
electric, in
excretory, 104
Jacobson's, 475
larval, 150
of locomotion, no
lyriform, 390
olf actor}', 115, 474
phosphorescent, 121
respirator}', 96
segmental, 105
sensory, 114
sexual, 107
of smell, 115, 474
systems of, 90
tactile, 115
of taste, 115, 474
tympanal, 117
vegetative, 90
visual, nS
urogenital, 109
Orioles, 544
Origin of species, 18
Ornithodelphia, 558
Ornithorhynchus, 559
Orohippus, 569
Oronasal groove, 475
Orthis, 290
Orthoceras, 346
Orthonectida, 200
Orthoneurous, 330
Orthoptera, 421
Orthopoda, 528
Orycteropus, 563
Oscaxella, 205
Oscines, 544
Osculum, 202
Os innominatum, 550
Osmerus, 510
Osphradium, 116, 312
Ossein, 77
Ossicula auditus, 479
Ostariophysi, 510
Osteoblasts, 78
Ostia of heart, 103, 357
Ostium tubas, 487
Ostracoda, 371
Ostracodermi, 493, 512
Ostracoteuthis, 341
Ostrasida;, 323
Os transversum, ^22
Ostriches, 542
Otaria, 565
Otica, 458
Otic capsule, 457
ganglion, 473
Otis, 544
Otolith, 116
Otter, 565
Ovaries, 486
Ovary, histology of, 70
Ovibos, 569
Ovicells, 286
Oviducts, 109, 486
Oviparous, 151
Ovipositor, 413
Ovis, 569
Ovoid gland, 293
Ovo viviparous, 151
Ovum, 486
Owlet moths, 433
Owls, 544
Oyster crab, 382
Oysters, 323
.Qx warble, 431
Oxy-haemoglobin, 79
Oxyrhyncha, 382 •
Oxystomata, 382
Oxyuris, 266
Pachyderm, 570
Pachydrilus, 279
Pachylemurida;, 573
Paddle fish, 508
Pasdogenesis, 129. 414
Paguridea, 380
Patechinoidea, 305
Palaemon, 380
Palaemonetes, 380
Pakeocranium, 455
Pakeocrinoidea, 302
Palaeotherium, 569
Palate, 475
Palatine bone, 461
Paleacrita, 433
Palinurus, 380
Pallial line, 318
sinus, 318
Pallium, 311, 469
Palolo worm, 275
Palpus, 353, 377, 406
Paludicella, 287
Paludinidas, 334
Pancreas, 96, 481
Pandalus, 380
Par.dicn, 544
598
Pangolin, 563
Panopeus, 382
I'anorpi
Protonemertini, 258
Protonephridia, 105
Protoplasm, 51, 52
INDEX
Protopterus, 513
Prototheria, 558
Protovertebrse, 467
Protozoa, 166
Protracheata, 399
Protrochula, 238
Protula, 278
Protura, 419
Proventriculus, 409
Psalterium, 568
Psammonyx, 179
Pseudobranch, 502
Pseudocoele, 263
Pseudocuticula, 528
Pseudonavicellae, 187
Pseudoneuroptera, 419
Pseudopodia, 53, m, 170
Pseudoscorpii, 394
Pseudosuchia, 532
Psittaci, 544
Psocidas, 420
Psolus, 308
Psorosperms, 190
Pteranodon, 532
Pteraspis, 493
Pterichthys, 493
Pterodactylia, 532
Pteronarcys, 420
Pteropoda, 334
Pteropus, 565
Pterosauria, 532
Pterotic, 458
Pterotrachea, 334
Pterygoid, 461
Pterygoquadrate, 460
Pterygotus, 388
Pterylse, 534
Ptyalin, 96
Pubic bone, 464
Pugettia, 382
Pulex, 431
Pulmonary artery, 101, 484
veins, 101, 484
Pulmonata, 335
Pulp cavity, 451
Pulvillus, 431
Puma, 565
Pupa, 336
Pupa coarctata, 416
libera, 415
obtecta, 415
Pupil of eye, 120
Pupipara, 431
Purpura, 334
Putorius, 565
Pycnogonida, 399
Pygidium, 364
Pygostyle, 537
Pyloric caeca, 500
Pylorus, 481, 554
Pyrosoma, 446
Python, 531
Pythonaster, 298
Pythonomorpha, 530
Quadrate, 461
Quadrula, 179
Quahog, 324
Quail, 543
Quill, 533
Raccoon, 565
Racemose glands, 68
vesicles, 298
Races, 20
Rachis, 533
Radial canals, 211, 292
chambers, 225
symmetry, 123
Radiale, 465
Radialia, 292, 300, 463
Radiata, 206, 291
Radii, 293
Radiolaria, 175
Radius, 465
Radula, 314, 329
Raia, 507
Rails, 544
Rainey's corpuscles, 191
Rallus, 544
Rana, 520
Ranatra, 428
Rangifer, 569
Ranvier, constrictions of, 85
Raptores, 544
Ratitae, 541
Rats, 566
Rattlesnake, 531
Rays, 507
Razor clam, 324
Receptaculum seminis, 265,
413
Recessive, 138
Rectal glands, 409
Rectrices, 534
Rectum, 409
Recurrent typhus, 184
Red coral, 230
Red mites, 397
Redia, 246
INDEX
601
Reduction division, 134, 138
Reduviidae, 428
Regeneration, 149
Regions, faunal, 160
Regression, 45
Regularia, 305
Reindeer, 569
Remiges, 534
Remora, 511
Renilla, 230
Reproduction, 55, 128
and fertilization, 149
asexual, 128
sexual, 129
Reptilia, 521
Residual body, 183, 186
Respiratory canal, 475
organs, 96
Reticularia, 178
Reticulum, 568
Retina, 118, 476
Retetelariae, 396
Rhabdites, 241, 265
Rhabdocoelida, 242
Rhabdom, 119
Rhabdopleura, 449
Rhachiglossa, 334
Rhachis, 293, 364
Rhamnites, 241
Rhamphastos, 544
Rhea, 542
Rhegmatodes, 218
Rhinencephalon, 469
Rhinoceros, 568
Rhinosporidium, 191
Rhizocephala, 374
Rhizocrinus, 302
Rhizopoda, 170
Rhizostomeae, 223
Rhopalonema, 212, 214
Rhopalocera, 433
Rhynchobdellida?, 284
Rhynchobothrium, 254
Rhynchocephalia, 527
Rhynchoeta, 198
Rhynchonella, 288, 290
Rhynchota, 427
Rhynchophora, 424
Rhytina,57O
Ribs, 454
Right whale, 572
Rigor, 55
Ring canal, 211, 292
Roaches, 421
Robber flies, 431
Rocky Mountain sheep, 569
Rodentia, 566
Rods and cones, 119, 47(1
Root barnacles, 374
Rorqual, 572
Rossia, 347
Rostellum, 249
Rostrum, 341, 372, 407, 504,
537
Rotalia, 171, 180
Rotatoria, 259
Rotifera, 259
Round worms, 263
Rove beetles, 424
Rudistidae, 324
Rugosa, 230
Rumen, 568
Ruminantia, 568
Rupricapra, 569
Sabellidas, 277
Sabinea, 380
Sable, 565
Sicconereis, 275
Sacculina, 374
Sacculus, 117, 478
Sacculus vasculosus, 499
Sagartia, 231
Sagitta, 262
Sagittal axis, 124
Salinella, 200
Salivary glands, 96
Salamanders, lungless, 517
Salamandra, 520
Salamandrina, 520
Salmon, 510
Salpa, 447
Salpseformes, 447
Saltigrada, 396
Salve bug, 386
Saltatoria, 421
Sand dollar, 306
San Jose scale, 430
Sapphirina, 369
Sapajou, 574
Sarcode, 52
Sarcocystis, 191
Sarcolemma, 83
Sarcoptes, 397
Sarcosepta, 228
Sarcophilus, 560
Sarcopsylla, 431
Sarcosporida, 191
Sarsia, 217
Saurii, 529
Sauropsida, 520
Saururas, 541
Savigny's law, 352
Savis' vesicles, 500
Saw fly, 425
Sawfishes, 507
Saxicava, 324
Scake of ear, 479
Scale insects, 428
Scales of fishes, 494
Scallops, 323
Scalops, 564
Scansores, 544
Scape, 533
Scapharca, 323
Scaphiopus, 520
Scaphognathite, 377
Scaphopoda, 325
Scapula, 464
Scarabseidse, 424
Sceleporus, 529
Schizodont, 318
Schizogony, 169, 186, 299
Schizopoda, 375
Schizopodal foot, 360
Schizosomi, 382
Schwann's sheath, 85
Scincidas, 530
Sciuromorpha, 566
Sciuropterus, 566
Sciurus, 566
Sclera, 120, 457, 476
Scleroderma, 231
Sclerodermites, 230
Sclerophylla, 231
Sclerosepta, 228
Sclerotica, 120, 476
Sclerotic bones, 525
Sclerotomes, 467
Scolex, 247
Scolopax, 544
Scolopendra, 403
heart of, 103
Scolopendrella, 434
Scombridas, 511
Scops, 544
Scorpions, 391
false, 394
water, 428
Scorpionida, 391
Sculpins, 511
Scutellum, 408
Scutigera, 403
Scutum, 372
Scyphomedusaa, 220
602
INDEX
Scyphopolyp, 220
Si \ phozoa, 220
Scvphostoma, 220
Sea anemones, 224, 231
( ucumbi.-rs, 306
fans, 230
horses, 511
lilies, 299
lions, 565
otter, 565
pens, 230
snakes, 531
squirts, 441, 443
urchins, 303
whips, 230
Seals, 565
Secodont, 554, 565
Secondaries, 534
Sectorial, 565
Secreta, 65
Sedentaria, 277, 396
Segmental organs, 105, 272
Segmentation cavity, 145
of body, 126
of egg, 141
Selachii, 506
Selection, artificial, 35
germinal, 43
natural, 35
sexual, 40
Selenodont, 554
Semaeostomeffi, 223
Semicircular canals, 117
478
Seminal vesicles, 107
Semilunar valves, 503
Sense organs of vertebrates,
473
Sensory cells, 73
organs, 114
roots, 468
Sepia, 341, 347
Septa, 225, 270
Scpial organ, 293
Septibranchiata, 325
Sericteria, 432
Serosa, 414, 491
Serpulkku, 277
Serripes, 324
Sertularia, 218
Smim, blood, 78
Sesiidse, 433
Srrranidae, 511
Sexual cells, 130
characters, secondary, 1 10
Sexual gland;, 107
dimorphisn, 40, 169
epithelium, 69
organs, 107
organs of vertebrates, 486
reproduction, 129
selection, 40
Shad, 510
Shaft, 533
Shagreen, 505
Sharks, 504
Sheath of Schwann, 85
Sheep, 569
Sheep bot, 431
tick, 431
Shell gland, 249, 358, 362
Ship worms, 325
Shoulder blade, 464
Shoulder girdle, 464
Shrews, 564
Shrimp, 380
brine, 366
fairy, 366
mantis, 376
opossum, 376
Siala, 544
Sicyonia, 380
Siderone, 38
Silicispongia?, 204
Silenia, 325
Siliqua, 324
Silk glands, 432
Silk worm, 433
Siluritke, 510
Silver fish, 419
Simla, 574
Simuliido?, 430
Sinupalliata, 324
Sinuses, blood, 103
Siphon, 305, 310, 339
Siphonaptera, 431
Siphonate, 310)
Siphonoglyphe, 225
Siphonophora, 218
Siphonostomata, 369
Siphonostome, 327
Siphuncle, 340
Sipunculoidea, 281
Sipunculus, 281
Siredon, 27
Siren, 520
Sirenia, 570
Sirex, 425
Siricidffi, 425
Sixth sense, 115
Skates, 507
Skeleton, 91
axial, 91
cuticular, in
dermal, 91
Skin, 91
of vertebrates, 67, 450
Skull, 455
Skunk, 565
Sky-lark, 544
Sleeping sickness, 184, 431
Slime fishes, 491
Slime moulds, 180
Sloths, 563
Smell, 115
Smelt, 510
Smooth muscles, 81
Snails, 325
Snakes, 530
Snapping turtle, 527
Snout beetles, 424
Social animals, 156
Social insects, 416
Soft-shelled crab, 382
Soft -shell turtles, 527
Solasteridse, 298
Sole, 511
Solemyidae, 323
Solen, 324
Sqlenoconchse, 325
Solenocytes, 105, 272
Solenogastres, 316
Solenoglypha, 531
Solifugae, 393
Solpuga, 393
Somatic cells, 130
layer, 148, 201
Somatopleure, 148
Somites, 269
Sorex, 564
Sow bugs, 386
Spadella, 263
Spadix, 216
Spanish flies, 424
Sparrows, 544
Spatangoidea, 306
Spatangus, 306
Species, analytic, 141
crossing of, 2 1
nature of, 15
and varieties, 18
Spelerpes, 520
Speotyto, 544
Spermaceti, 572
Spermatid, 134
INDEX
Spermatoryte, 134
Spermatogenesis, 134
Sptrmatophore, 344
Spermatozoa, 72, 134
Sperm nucleus, 135
Sperm whale, 572
Sphreridia, 292
Sphitrogastrida, 395
Spha?rophrya, 198
Sphenethmoid, 514
Sphenodon, 527
Sphenoidalia. 458
Sphenoid bone, 450, 546
Sphenopalatine ganglion,
473
Sphenotic, 458
Sphingina, 433
Sphyranura, 244
Spicules, sponge, 205
Spider crabs, 382
Spider monkeys, 574
Spiders, 395
Spinal canal, 453
cord, 468
nerves, 468
Spine-headed worms, 263
Spindle fibres, 60
nuclear, 60
Spinnerets, 395
Spinous process, 453
Spiny ant-eater, 559
Spittle insect, 428
Spiracle, 98, 326, 401. 4:1,
502> 5l6
Spiral valve, 500
Spirochaeta, 184
Spirifer, 290
Spirobolus, 434
Spirula, 347
Sphasroma, 386
Splanchnic layer, 148, 201
Splanchnopleure, 96, I4-S
Spleen, 486
Splenomegaly, 184
Sponges, 201
Spongida, 201
Spongilla, 205
Spongin, 204
Spongioblasts, 205
Spongioplasm, 53
Spontaneous generation. 23,
127
Sporangia, 180
Sporocyst, 246
Sporogony, 169, 186
Sporosac, 2 if>
Sporozoa, 185
Sporozoites, iSS
Sports, 20
Spring-tails, 419
Squali, 506
Squamata, 528, 563
Squamosal bone, 462
Squamous epithelium, 65
Squid, 347
Squilla, 376
Squirrels, 566
Stapes, 461
Starfish, 295
Star-nosed mole, 564
Statoblasts, 205, 286
Statocysts of medusae, 214
molluscan, 312
Statolith, 118
Stauromedusa?, 224
Steapsin, 96
Steganopocles, 543
Stegocephali, 519
Stegomya, 430
Stegosaurs, 528
Stellate ganglia, 343
Stelmatopoda, 287
Stemma, 354
Stenops, 572
Stenostoma, 93
Stenson's duct, 475
Stentor, 196
Stephanocyphus, 224
Stercoral pocket, 390
Sterna, 543
Sternaspis, 277
Sternite, 404
Sternum, 404, 454
Sticklebacks, 511
Stigma, 98
Stigmata, 401
Sting, 413
Sting rays, 507
Stink bugs, 428
Stipes, 406
Stolo prolifer, 448
Stomach, 1,14
Stomatopoda, 376
Stomoda?um, 94
Stomolophus, 223
Stone canal, 292
Storks, 544
Stratified epithelium, 66
Stratum corneum, 67, 450
Malpighii, 67, 450
Strepsiptcra, 422
Streptoneury, 328
Stridulating organs, 410
Strix, 544
Strobila, 221, 247
Strobilation, 221
Strongylidae, 266
Strongylocentrotus, 305
Strongyloides, 265
Strongylus, 266
Struggle for existence, 35, 36
Struthiones, 542
Stylaster, 217
Stylets, 408
Stylochus, 242
Stylohyoid ligament, 54*;
Styloid process, 549
Stylommatophora, 336
Stylonychia, 197
Stylopida;, 422
Stylopized, 423
Stylorhynchus, 187
Sturgeon, 508
Subcuticula, 263
Subcutaneous tissue, 450
Subintestinal vein, 483
Submentum, 406
Subneural gland, 445
Subumbrella, 211
Suckers, 510
Suck fishes, 511
Sucking stomach, 408
Suctoria, iq7
Suidse, 568
Sulci, 470
Summer eggs, 132, 365
Sun animalcules, 173
Supporting cells, 74
tissues, 74
Supraoccipital, 458
Supracesophageal ganglia,
"3> 354
Suprarenals, 489
Supratemporal fossa, 522
Surf perch, 511
Sus, 568
Swallows, 544
Swallow tails, 433
Swans, 543
Sweat glands, 546
Swell fishes, 512
Swim bladder, 482, 502
Swimming birds, 543
Swine, 568
Sword fishes, 511
604
INDEX
Sycandra, 202, 2
Syi on, 204
Syllis, 275
S\ IV'K olidu_-, 544
Symbiosis, 158
Symmetry, 123
Sympathetic coloration, 37
nervous system, 114
plexus, 473
system, 473
Symplectic bone, 496
Symphila, 417, 434
Synapta, 308
Synascidio;, 446
Syncarida, 382
Syncitia, 62
Synccelidium, 240, 242
Syncoryne, 217
Synentognathi, 510
Syngamus, 266
Syngnathus, 511
Syphilis, 184
Syringopora, 230
syrinx, S38
Syrphidae, 431
Systemic arteries, 101
heart, 343
veins, 101
Systole, 101
Tabanidas, 430
Tabula;, 229
Tabulate, 229
Tactile corpuscles, 473
organs, 115
Tadpoles, 27, 519
Taenia, 157, 247, 254
Taenioglossa, 334
TamiokE, 220
Tails of fishes, 32, 498
Talpid;e, 564
Tanais, 386
Tanystoma, 430
Tapetum, 121
nigrum, 476
Tapeworms, 247
Tapirus, 568
Tarantella, 396
Tarantula, 396
Tardigrada, 398, 563
Tarsal bones, 46 ^
Tarsius, 572
Tarso-metatarsus, 536
Tarsus, 405, 465
Tasmanian devil, 560
Taste, 115
Taste organs, 474
Tatusia, 563
Taxodont, 318
Tectibranchia, 334, 346
Tectrices, 534
Teeth, 481
mammalian, 552
Tejus, 530
Telea, 433
Telencephalon, 469
Teleostei, 508
Tellina, 324
Teloblasts, 273
Telodendron, 84
Telolecithal eggs, 143
Telo troche, 273
Temnocephalida?, 244
Temporal bone, 462, 548
Tendinous tissue, 76
Tenebrionidas, 424
Tentaculata, 235
Tent caterpillars, 433
Tenthredinidaa, 425
Terebella, 98, 278
Terebra, 413, 425
Terebrantia, 425
Terebratula, 290
Teredo, 325
Tergite, 404
Tergum, 372
Termes, 419
Terminalia, 304
Terminals nerve, 471
Termites, 417, 419
Termitophiles, 419
Terns, 543
Terrapins, 527
Terricola, 279
Tessellata, 302
Tesseridas, 224
Test, 443
Testes, 486
Testicardines, 290
Testudinata, 526
Testudo, 527
Tethya, 205
Tethyoidea, 443
Tetracoralla, 230
Tetractinellidae, 204
Tetradecapoda, 383
Tetrads, 134
Tetramera, 424
Tetraonidae, 543
Tetraphyllidtc, 254
Tetrapneumones, 396
Tetrapoda, 491
Tetrarhynchus, 249
Tetrastemma, 258
Tetrasticta, 396
Tetraxonia, 204
Texas fever, 190
Thalamophora, 178
Thalassema, 281
Thalassicola, 176
Thalassochelys, 527
Thaliacea, 447
Thaumantia, 218
Theca, 228, 300
Thecasomata, 335
Thecodont, 532
Thelyphonida, 392
Thelyphonus, 393
Theridium, 396
Theriodontia, 526
Theromorpha, 526
Thoracic cavity, 555
duct, 486
fin, 462, 497
Thoracostraca, 375
Threadworms, 263
Thrips, 420
Thrushes, 544
Thylacinus, 560
Thymus gland, 482
Thyone, 308
Thyreoid gland, 482
Thysanoptera, 420
Thysanozoon, 242
Thysanura, 419
Tiara, 213
Tioia, 405, 465
Tibiale, 465
Tibio-tarsus, 536
Ticks, 397
Tick, sheep, 431
Tiedemann's vesicles, 292
Tiger, 565
Tiger beetles, 424
Tillodontia, 570
Tima, 218
Tineidaa, 432
Tipulidae, 430
Tissues, 63
connective, 74
elastic, 76
epithelial, 64
muscular, 80
subcutaneous, 450
supporting, 74
INDEX
i in:,
Toads, 520
Tobacco worm, 433
Tocogony, 128
Tokophrya, 198
Tomato worm, 433
Tongue bone, 460
Tooth-shells, 325
Top-shells, 333
Tornaria, 449
Torpedo, 507
Tortoise, 527
Tortricida?, 432
Toucans, 544
Toxiglossa, 334
Toxodontia, 570
Toxopneustes, 305
Trabecuke, 456
Trachea, 482
Tracheae, 98, 387, 401, 411
Tracheal gills, 402, 411
Tracheata, 359
Trachoma, 191
Trachydermon, 316
Trachymedusae, 218
Trachynema, 218
Tractus olfactorius, 469
Trap-door spider, 396
Tree cricket, 421
Tree hoppers, 428
Tree toads, 520
Trematoda, 242
Trepang, 308
Triarthrus, 364
Triaxonia, 205
Trichechus, 565
Trichina, 267
Trichocephalus, 267
Trichocysts, 192
Trichodectes, 420
Trichomonas, 184
Trichoplax, 200
Trichoptera, 422
Trichotrachelida?, 267
Tricladidea, 242
Triconodont, 554
Tridacna, 324
Trigeminal nerve, 471
Trilobita;, 363
Trimera, 424
Trionychia, 527
Tristicta, 396
Tristoma, 244
Tritocerebrum, 405
Triton embryo, 31
Tritoniidoe, 335
Tritubercular, 554
Tritylodon, 559
Trivium, 292, 296
Trochal disc, _> y»
Trochanter, 405
Trochilus, 544
Trochlearis nerve, 471
Trochophore, 238, 273
Trochosa, 396
Trochus, 333
Troctes, 420
Troglodytes, 574
Trombidiida?, 397
Trophi, 260
Tropic birds, 543
Tropidonotus, 521, 531
Trout, 510
Trygonidae, 507
Trypanosoma, 184
Trypsin, 96
Tubicola, 277
Tubicinella, 371
Tubificidae, 279
Tubinares, 543
Tubiporidas, 230
Tubitelariae, 396
Tubular glands, 68
nervous system, 114
Tubularia, 217
Tubulipora, 287
Tunic, 442
Tunicata, 441
Turbellaria, 240
Turbinal bones, 475, 548
Turbinidaa, 333
Turdus, 544
Turkey buzzard, 544
Turkeys, 543
Turritopsis, 217
Turtles, 526
'Twixt brain, 469
Tylenchus, 266
Tylopoda, 569
Tympanal organs, 117,
410
Tympanum, 479
Tympanic annulus, 479
bone, 462, 548
membrane, 471;
Typhline, 530
Typhlotriton, 520
Typhlomolge, 520
Typhlops, 531
Typhoid, 431
Typhus, recurrent, 184
Tyrian purple, 334
Tyrannidiu, 544
Uca, 382
Uintatherium, 570
Ulmaris, 223
Ulna, 465
Ulnare, 465
Umbilicus, 326, 490
Umbo, 317
Umbrella, 211
Uncinate processes, 535
Unguis, 450, 546
Ungula, 450, 546
Ungulata, 566
Unguligrade, 550
Unio, 324
Unipolar cells, 84
Ureter, 107, 488
Urinator, 543
Urinary bladder, 105, 490,
5°4
Urnatella, 285
Urochorda, 441
Urodela, 519
Urogenital sinus, 489, 555
system, 109
Urosalpinx, 334
Ursus, 565
Use and disuse, 16, 47
effects of, 89
Uterus, 109, 557
Utriculus, 117, 478
Uvula, 554
Vagabundae, 396
Vagina, 557
Vagus nerve, 471
Valkeria, 287
Vampyre, 565
Vanessa, 433
Varanus, 524, 530
Variations, fluctuating, 44
Variola, 191
Vasa deferentia, 109, 486
Vascula, 99
Vater-Pacinian corpuscles,
"5. 474
Vegetative organs, 90, 91
pole, 133, 142
Veins, 101
Velella, 220
Veliger, 3I4
Velum, 211, 314, 440
Veneridos, 324
606
Venous blood, 102
Vmtral aorta, 483
fin, 462
nerve-cord, 113
nerve-root, 468
Ventricles of brain, 469
of heart, 101, 483
Yenus, 324
Yenus girdle, 235
Yenus' flower basket, 204
Yermes, 237
Yermiform appendix, 554
Yermilinguia, 530, 563
Vermis, 470
Vermetus, 327
Vertebral column, 452
Yertebrata, 449
circulation of, 483
digestive organs, 481
ear, 477
eye, 120, 475
respiratory organs, 482
skeleton, 453
skin, 450
sense organs, 473
urogenital system, 486
Vertex, 405
Vesicularia, 287
Yespariae, 426
Yespertilionidae, 565
Vesperugo, 565
Vestibule, 538
Vibracularia, 286
Vibrissse, 545
Vicarious species, 46
Vinegar eel, 265
Viperidae, 531
Visceral arches, 460
ganglia, 312
muscles, 472
sac, 310
skeleton, 459
Vision, 118
Visual cells, 118
purple, 119
Vital force, 3
Yitellaria, no, 240
Vitreous body, 120
Yitrodentine, 494
Yiverra, 565
Viviparous, 151
v Voluntary muscles, 81
Yolutida.-, 334
Yolvocina, 184
Volvox, 182, 184
INDEX
Yomer, 461
Vortex, 109, 242
Yorticella, 197
Wading birds, 543
Waklheimia, 288
Walking leaf, 37
Walking stick, 421
Wallace's Line, 34, 162
W'alrus, 565
Warblers, 544
Warm blood, 104
Wasps, 426
Water beetles, 424
bears, 398
mites, 397
scorpion, 428
snakes, 531
Water-vascular system, 105,
292
Weasel, 565
Weberian apparatus, 503
Weevils, 424
Whalebone, 571
Whales, 571
Wheel animalcules, 259
Whelk, 334
Whippoorwill, 544
W'hite ants, 419
White-fish, 510
White matter, 114, 468
White nerve fibres, 85
Wind pipe, 482
Wings of insects, 408
Winter eggs, 132, 365
Wish bone, 535
WolfSan body, 487
duct, 487
Wolves, 565
Wombat, 561
W'oodcock, 544
Woodpeckers, 544
Wool, 545
Worms, 237
army, 433
bladder, 247
cabbage, 433
canker, 433
cotton, 433
earth, 279
flat, 238
galley, 402
guinea, 268
hair, 268
hair-neck, 267
Worms, hook, 267
hop, 433
lazy, 267
measuring, 433
nemertean, 255
palolo, 275
pin, 266
round, 263
ship, 325
silk, 433
spine-headed, 268
tape, 247
thread, 263
tobacco, 433
tomato, 433
tube, 277
Wrens, 544
Xenarthra, 563
Xenophyophora, 180
Xenos, 423
Xerobates, 527
Xiphiidoe, 511
Xiphosura, 387
Yellow fever, 430
Yoldia, 323
Yolk, 71
cells, 109
and cleavage, 142
glands, no
membrane, 135
sac, 414, 489
Zebra, 568
Zeuglodontia, 572
Zoantharia, 230
Zoantheae, 231
Zoea, 362
Zonula zinnii, 120
Zonurida?, 530
Zoology, general, 50
history of, 5
Zoophaga, 560
Zoophytes, 13, 206
Zoospores, 169, 172
Zoothamnion, 197
Zooxanthelke, 158, 178
Zygapophysis, 455
Zygeupolia, 258
Zygobranchia, 333
Zygodactyle, 544
Zygomatic process, 462
Zygote, 138, 139