2ool:

BIOLOGY
LIBRARY

G

GENERAL ZOOLOGY

PRACTICAL, SYSTEMATIC
AND COMPARATIVE

BEING A REVISION AND REARRANGEMENT OF

ORTON'S COMPARATIVE ZOOLOGY
M

BY

CHARLES WRIGHT DODGE, M.S.

PROFESSOR OF BIOLOGY IN THE UNIVERSITY OF ROCHESTER

NEW YORK • : • CINCINNATI - : . CHICAGO

AMERICAN BOOK COMPANY

07

BIOLOGY
UBRARY

G

COPYRIGHT, 1876, 1883, 1894, BY HARPER & BROTHERS.

COPYRIGHT, 1903, BY AMERICAN BOOK COMPANY.

ENTERED AT STATIONERS' HALL, LONDON.

GEN. ZOOLOGY.
W. P I

PREFACE

IN preparing this text-book of General Zoology an
attempt has been made to meet the wants of teachers
who desire a treatment of the subject somewhat dif-
ferent from that contained in earlier editions of Pro-
fessor Orton's work, and to furnish a course of study
suited to the needs of the general student who wishes
to learn the principal facts and theories of zoology and
thus to obtain a fairly comprehensive idea of the sci-
ence. To this end it has seemed desirable so to
arrange a course of study that the student may gain
by ' personal observation concrete knowledge of the
structure and activities of animals, and, by so doing,
acquire some familiarity, slight perhaps, but neverthe-
less valuable, with the method of zoological investi-
gation ; that he may obtain also a knowledge of the
relationships of animals as expressed in an accepted
scheme of classification ; that he may, further, broaden
this knowledge by a comparison of animals in their
structural and physiological relationships ; and that,
finally, he be placed in position to understand the sig-
nificance of the more important theories of the science.
With these aims in view the text of Professor Orton's
" Comparative Zoology " has been revised and rear-
ranged as described below.

3

326085

4 PREFACE

The pedagogical importance of laboratory and field
study has led to the introduction of a series of exercises
upon the structure, physiology, and habits of represen-
tative animals. These exercises suggest the more im-
portant topics for study rather than give an inflexible
outline to be followed in detail. The teacher is thus
left free to adapt and modify the laboratory course to
suit the peculiar needs of his classes and his equip-
ment. The exercises lead to the study of Systematic
Zoology, to which they serve as the natural introduc-
tion, the classification of animals being based upon
their structural relationships. With the anatomy of
the typical forms examined in the practical exercises
in mind, the student ought to have no trouble in under-
standing the structural modifications mentioned in the
descriptions of the principal classes and orders. Hav-
ing thus enlarged his view of the animal kingdom, he
is in position to appreciate the elementary facts of
Comparative Zoology and to understand the main fea-
tures of the current zoological theories. Believing this
to be a logical sequence of study, the book has been
arranged in accordance therewith. With the exception
of slight changes, the laboratory exercises are the same
as those recommended by the New York State Science
Teachers' Association. The system of classification
adopted is that given by Parker and Haswell in their
" Text-book of Zoology," a work which will long be a
standard of reference for teachers in secondary schools.
Part I and Part II of Professor Orton's book have been
transposed so as to place classification before the dis-

PREFACE 5

cussion of Comparative Zoology. The addition of a
chapter on " The Origin of Animal Species " will, it is
hoped, enable the student to understand the most im-
portant, at least, of zoological theories. A number of
new figures have been incorporated. An asterisk at
the head of a chapter indicates that the subject-matter
of the chapter may be illustrated by practical work,
for which directions will be found in the Appendix.

Acknowledgments are due to Messrs. D. Appleton
and Company for permission to reproduce Figures
39(11) and 367 from Thomson's "Outlines of Zoology " ;
to the J. B. Lippincott Company for permission to
reproduce Figures 207, 208, 212, and 256 from Piersol's
" Normal Histology," and Figure 368 from Smith's
" Economic Entomology " ; to Messrs. W. B. Saunders
and Company for permission to reproduce Figure 203
from G. C. Huber's edition of Bohm and Davidoff's
"Text-book of Histology," and to Professor Alfred
Schaper of the University of Breslau, for permission to
reproduce Figure 211 from his edition of Stohr's "Text-
book of Histology."

CHARLES WRIGHT DODGE.
UNIVERSITY OF ROCHESTER.

The first thing to be determined about a new specimen is not its name
but its most prominent character. Until you know an animal, care not for
its name. — AGASSIZ.

The great benefit which a scientific education bestows, whether as
training or as knowledge, is dependent upon the extent to which the mind
of the student is brought into immediate contact with facts — upon the
degree to which he learns the 'habit of appealing directly to Nature.—
HUXLEY.

CONTENTS

INTRODUCTION

PAGE

Definition of Zoology, and its Place among the Sciences . . . 1 1
Historical Sketch i c

PART I

STRUCTURAL AND SYSTEMATIC ZOOLOGY

CHAPTER I

PRACTICAL ZOOLOGY 21

- CHAPTER II

THE CLASSIFICATION OF ANIMALS 47

Protozoa 54

Porifera . 65

Ccelenterata 68

Platyhelminthes 82

Nemathelminthes 84

Trochelminthes 85

Molluscoida . . . . . . . . -85

Echinodermata 87

Annulata . . -95

Arthropoda 97

Mollusca 124

Chordata 137

CHAPTER III

SYSTEMATIC ARRANGEMENT OF REPRESENTATIVE FORMS . . 202

7

8 CONTENTS

PART II

COMPARATIVE ZOOLOGY
CHAPTER IV

PAGE

MINERALS AND ORGANIZED BODIES DISTINGUISHED . . .215

CHAPTER V
PLANTS AND ANIMALS DISTINGUISHED . ... . .217

CHAPTER VI
RELATION BETWEEN MINERALS, PLANTS, AND ANIMALS ' . , . 224

CHAPTER VII
LIFE . ,'.--,;.. . ... . .225

CHAPTER VIII

ORGANIZATION . . . . r . ^ . . . . . . 227

1. Cells . . . . . . . . , .228

2. Tissues . . . . 229

3. Organs and their Functions ... . . . . 240

CHAPTER IX
NUTRITION . . . . . . . . . . . 244

CHAPTER X
THE FOOD OF ANIMALS . 247

CHAPTER XI

How ANIMALS EAT 250

1. The Prehension of Food 250

2. The Mouths of Animals . 256

3. The Teeth of Animals 265

4. Deglutition, or How Animals Swallow . . . . . 274

CONTENTS 9

CHAPTER XII

PAGE

THE ALIMENTARY CANAL 276

CHAPTER XIII
How ANIMALS DIGEST 294

CHAPTER XIV
THE ABSORBENT SYSTEM 297

CHAPTER XV
THE BLOOD OF ANIMALS . . 301

CHAPTER XVI
THE CIRCULATION OF THE BLOOD 308

CHAPTER XVII
How ANIMALS BREATHE 318

A

CHAPTER XVIII
SECRETION AND EXCRETION 329

CHAPTER XIX
THE SKIN AND SKELETON 335

CHAPTER XX

How ANIMALS MOVE .......... 363

1. Muscle 364

2. Locomotion 366

CHAPTER XXI

THE NERVOUS SYSTEM 376

1. The Senses 386

2. Instinct and Intelligence . . . . . . . 395

3. The Voices of Animals 399

10 CONTENTS

CHAPTER XXII

PAGE

REPRODUCTION 402

CHAPTER XXIII

DEVELOPMENT . .... . . - . . . . 409

1. Metamorphosis . . . .... . . 419

2. Alternate Generation • . . . ... ... . . 424

3. Growth and Repair . . 425

4. Likeness and Variation •. . . _ . . - . 427

5. Homology, Analogy, and Correlation . . ' . . . 429

6. Individuality . . - . • '.. . . . . 432

7. Relations of Number, Size, Form, and Rank . . . 433

8. The Struggle for Life 438

CHAFFER XXIV

THE DISTRIBUTION OF ANIMALS . . .... . . . 440

CHAPTER XXV

THE ORIGIN OF ANIMAL SPECIES .''..'... . . . . 450

NOTES ... . . . '';• 467

THE NATURALIST'S LIBRARY . . . . . .483

APPENDIX,. . . . . . . . . . .485

INDEX . . . • • . r 497

INTRODUCTION

i. Definition of Zoology, and its Place among the
Sciences. — The province of Natural History is to de-
scribe, compare, and classify natural objects. These
objects have been divided into the "organic" and -the
" inorganic," or those which are, and those which are
not, the products of life. Biology is the science of the
former, and Mineralogy the science of the latter.
Biology again separates into Botany, or the Natural
History of Plants, and Zoology, or the Natural History
of Animals ; while Mineralogy divides into Mineralogy
proper, the science of mineral species, and Lithology,
the science of mineral aggregates or rocks. Geology is
that comprehensive knowledge of the earth's structure
and development which rests on the whole doctrine of
Natural History.

If we examine a piece of chalk, and determine its
physical and chemical characters, its mode of' occurrence
and its uses, so as to distinguish it from all other forms
of matter, we have its Mineralogy. But chalk occurs
in vast natural beds ; the examination of these masses
— their origin, structure, position, and relation to other
rocks — is the work of the Lithologist. Further, we
observe that while chalk and marble are chemically
alike, they widely differ in another respect. Grinding
a piece of chalk so thin that we can see through it, and
putting it under a microscope, we find imbedded in it
innumerable bodies, about the hundredth of an inch in
diameter, having a well-defined, symmetrical shape, and
chambered like a nautilus. We cannot say these are

ii

12 INTRODUCTION

accidental aggregations, nor are they crystals ; if the
oyster shell is formed by an oyster, these also must be
the products of life. Indeed, the dredge brings up simi-
lar microscopic skeletons from the bottom of the Atlantic.
So we conclude that chalk is but the dried mud of an
ancient sea, the cemetery of countless animals that lived
and died long ago. The consideration of their fossil
remains belongs to Paleontology, or that part of Biology
which describes the relics of extinct forms of life. To
study the stratigraphical position of the chalk bed, and
by the aid of its Paleontology to determine its age and
part in the world's history, is the business of Geology.

Of all the sciences, Zoology is the most extensive. Its
field is a world of varied forms — hundreds of thousands
in number. To determine their origin and development,
their structure, habits, distribution, and mutual relations
is the work of the Zoologist. But so many and far-
reaching are the aspects under which the animal creation
may be contemplated, that the general science is beyond
the grasp of any single person. Special departments
have, therefore, arisen ; and Zoology, in its comprehen-
sive sense, is the combined result of the labors of many
workers, each in his own line of research.

Structural Zoology treats of the organization of animals.
There are two main branches : Anatomy, which con-
siders the constitution and construction of the animal
frame ; and Physiology, which is the study of the appara-
tus in action. The former is separated into Embryology,
or an account of the successive modifications through
which an animal passes in its development from the
egg to the adult state ; and Morphology, which includes
all inquiries concerning the form of mature animals, or
the form and arrangement of their organs. The micro-
scopical examination of any part, especially the tissues,
belongs to Histology. Comparative Zoology is the com-

INTRODUCTION 13

parison of the anatomy and physiology of all animals,
existing and extinct, to discover the fundamental like-
ness underneath the superficial differences, and to trace
the adaptation of organs to the habits and spheres of
life. It is this comparative science which has led to
such grand generalizations as the unity of structure
amidst the diversity of form in the animal creation,
and by revealing the degrees of affinity between species
has enabled us to classify them in natural groups, and
thus laid the foundation of Systematic Zoology. When
the study of structure is limited to a particular class or
species of animals, or to a particular organ or part,
monographic sciences are created, as Ornithotomy, or
anatomy of birds ; Osteology, or the science of bones ;
and Odontography, or the natural history of teeth.

Systematic Zoology is the classification or grouping of
animals according to their structural and developmental
relations. The systematic knowledge of the several
classes, as Insects, Reptiles, and Birds, has given rise
to subordinate sciences, like Entomology, Herpetology, or
Ornithology -,1 *

Distributive Zoology is the knowledge of the successive
appearance of animals in the order of time (Paleontology
in part), and of the geographical and physical distribu-
tion of animals, living or extinct, over the surface of the
earth.

Theoretical Zoology includes those provisional modes of
grouping facts and interpreting them, which still stand
waiting at the gate of science. They may be true, but
we can not say that they are true. The evidence is
incomplete. Such are the theories which attempt to
explain the origin of life and the origin of species.

Suppose we wish to understand all about the horse.
Our first object is to study its structure. The whole

* See Notes at the end of the volume.

14 INTRODUCTION

body is inclosed within a hide, a skin covered with
hair ; and if this hide be taken off, we find a great mass
of flesh or muscle, the substance which, by its power of
contraction, enables the animal to move. On removing
this, we have a series of bones, bound together with
ligaments, and forming the skeleton. Pursuing our
researches, we find within this framework two main cavi-
ties : one, beginning in the skull and running through
the spine, containing the brain and spinal marrow ; the
other, commencing with the mouth, contains the gullet,
stomach, intestines, and the rest of the apparatus for
digestion, and also the heart and lungs. Examinations
of this character would give us the Anatomy of the
horse, or, more precisely, Hippotomy. The study of the
bones alone would be its Osteology ; the knowledge of
the nerves would belong to Neurology. If we examined,
under the microscope, the minute structure of the hair,
skin, flesh, blood, and bone, we should learn its Histology.
The consideration of the manifold changes undergone
in developing from the egg to the full-grown animal,
would be the Embryology of the horse ; and its Mor-
phology, the special study of the form of the adult ani-
mal and of its internal organs.

Thus far we have been looking, as it were, at a steam
engine, with the fires out, and nothing in the boiler;
but the body of the living horse is a beautifully formed,
active machine, and every part has its different work to
do in the working of that machine, which is what we
call its life. The science of such operations as the
grinding of the food in the complex mill of the mouth ;
its digestion in the laboratory of the stomach ; the pump-
ing of the blood through a vast system of pipes over
the whole body ; its purification in the lungs ; the pro-
cess of growth, waste, and repair; and that wondrous
telegraph, the brain, receiving impressions, sending

INTRODUCTION 1 5

messages to the muscles, by which the animal is en-
dowed with voluntary locomotion — this is Physiology.
But horses are not the only living creatures in the world;
and if we compare the structures of various animals, as
the horse, zebra., dog, monkey, eagle, and codfish, we
shall find more or fewer resemblances and differences,
enough to enable us to classify them, and give to each
a description which will distinguish it from all others.
This is the work of Systematic Zoology. Moreover, the
horses now living are not the only kinds that have ever
lived; for the examination of the earth's crust — the
great burial ground of past ages — reveals the bones of
numerous horselike animals : the study of this preadam-
ite race belongs to Paleontology. The chronological
and geographical distribution of species is the depart-
ment of Distributive Zoology. Speculations about the
origin of the modern horse, whether by special creation,
or by development from some allied form now extinct,
are kept- aloof from demonstrative science, under the
head of Theoretical Zoology.

2. History. — The Greek philosopher Aristotle (384-
322 B.C.) is called the " Father of Zoology." Certainly,
he is its only great representative in ancient times,
though his frequent allusions to familiar works on anat-
omy show that something had been done before him.
His " History of Animals," in nine books, displays a
wonderful knowledge of external and internal structure,
habits, instincts, and uses. His descriptions are incom-
plete, but generally exact so far as they go. Alexander,
it is said, gave him nine hundred talents to collect mate-
rials, and put at his disposal several thousand men, for
hunting specimens and procuring information.

The Romans accomplished little in natural science,
though their military expeditions furnished unrivaled
opportunities. Nearly three centuries and a half after

1 6 INTRODUCTION

Aristotle, Pliny (23-79 A.D.) wrote his " Natural His-
tory." He was a voluminous compiler, not an observer;
he added hardly one new fact. He states that his work
was extracted from over two thousand volumes, most of
which are now lost.

During the Middle Ages, Natural History was studied
in the books of the ancients ; and at the close of the fif-
teenth century it was found where Pliny had left it, with
the addition of many vague hypotheses and silly fancies.
Albertus Magnus, of the thirteenth century, and Con-
rad Gesner and Aldrovandus, of the sixteenth, were
voluminous writers, not naturalists. In the latter half
of the sixteenth century men began to observe nature
for themselves. The earliest noteworthy researches
were made on Fishes, by Rondelet (1507-1556) and
Belon (1517-1564) of France, and Salviani (1514-1572)
of Italy. They were followed by valuable observations
upon Insects, by Redi (1626-1698) of Italy, and Swam-
merdam (1637-1680) of Holland; and toward the end
of the same century, the Dutch naturalist, Leeuwen-
hoeck (1632-1723), opened a new world of life by the
use of the microscope.

But there was no real advance of Systematic Zoology
till the advent of the illustrious John Ray (1628-1705)
of England. His "Synopsis," published in 1693, con-
tained the first attempt to classify animals according to
structure. Ray was the forerunner of "the immortal
Swede," Linnaeus (1707-1778), "the great framer of
precise and definite ideas of natural objects, and terse
teacher of the briefest and clearest expressions of their
differences." His chief merit was in defining generic
groups, and inventing specific names.2 Scarcely less
important, however, was the impulse which he gave to
the pursuit of Natural History. The spirit of inquiry,
which his genius infused among the great, led to voy-

INTRODUCTION 17

ages of research, which resulted in the formation of
national museums. The first expedition was sent forth
by George III. of England, in 1765. Reaumur (1683-
1757) made the earliest zoological collection in France;
and the West Indian collections of Sir Hans Sloane
(1660-1752) were the nucleus of the British Museum.
The accumulation of specimens suggested comparisons,
which eventually resulted in the highest advance of the
science.

The brilliant style of Buffon (1707-1788) made Zool-
ogy popular, not only in France, but throughout Europe.
While the genius of Linnaeus led to classification, that
of Buffon lay in description. He was the first to call
attention to the subject of Distribution. Lamarck
(1745-1829) of Paris was the next great light. The
publication of his " Animaux sans Vertebres," in 1801,
was an epoch in the history of the lower animals. He
was also the first prominent advocate of .the transmuta-
tion of species.

But the brightest luminary in Zoology was George
Cuvier (1769-1832), a German, born on French soil.
Before his time " there was no great principle of classi-
fication. Facts were accumulated, and more or less sys-
tematized, but they were not yet arranged according to
law ; the principle was still wanting by which to gen-
eralize them and give meaning and vitality to the
whole." It was Cuvier who found the key. He was
the first so to interpret structure as to be able from the
inspection of one bone to reconstruct the entire animal,
and assign its position. His anatomical investigations
revealed the natural affinities of animals, and led to
the grand generalization, that the most comprehensive
groups in the kingdom were based, not on special char-
acters, but on different plans of structure. Palissy had
long ago (1580) asserted that petrified shells were of
DODGE'S GEN. ZOOL. — 2

1 8 INTRODUCTION

animal origin ; but the publication of Cuvier's " Memoir
on Fossil Elephants," in 1800, was the beginning of
those profound researches on the remains of ancient
life which created Paleontology. The discovery of the
true relation between all animals, living and extinct,
opened a boundless field of inquiry ; and from that day
the advance of Zoology has been unparalleled. Special
studies of particular parts or classes of animals have so
rapidly developed, that the history of Zoology during
the last fifty years is the history of many sciences.3

But to Charles Darwin more than to any other inves-
tigator is due the credit for the great mass of researches
which has been accumulated during the last half cen-
tury. The publication of the " Origin of Species," in
1859, marks the starting-point of modern zoological re-
search. Darwin's statement of the facts of evolution
and his theory of the causes which produce species of
organisms, both plant and animal, attracted the atten-
tion of all biologists, and now practically all investiga-
tion in the sciences of zoology and botany is carried on
in the light of the great principle of evolution.

PART I

STRUCTURAL AND SYSTEMATIC
ZOOLOGY

Facts are stupid things until brought into connection with some general
law. — AGASSIZ.

No man becomes a proficient in any science who does not transcend
system, and gather up new truth for himself in the boundless field of
research. — DR. A. P. PEABODY.

Never ask a question if you can help it; and never let a thing go un-
known for the lack of asking a question if you can't help it. — BEECHER.

He is a thoroughly good naturalist who knows his own parish thor-
oughly.— CHARLES KINGSLEY.

STRUCTURAL AND SYSTEMATIC
ZOOLOGY

CHAPTER I

PRACTICAL ZOOLOGY

IT is very desirable that the student should get as
much as possible of his knowledge of zoology from
a study of the animals themselves rather than from
descriptions. It is of course impracticable as well as
undesirable to depend entirely upon this source of infor-
mation. Nevertheless, the student should be taught
how to study specimens, both living and dead. For this
reason the following exercises in the practical examina-
tion of animal forms have been prepared. They consist
mainly of mere suggestions of topics for study, the
details being left to the teacher, for it is recognized that
if a definite outline to be followed rigidly were offered,
it would probably be too elaborate for those schools
where only a few weeks can be devoted to the subject,
and too meager for the schools in which a longer course
is given.

The exercises provide for a study of the activities
and habits of the living, as well as an examination of-
the structure of the dead specimen. Every important
branch of the animal kingdom is represented by at least
one common and easily obtained example. It is sug-
gested that the example be studied before a text lesson
is assigned on the group which it represents. In this
way the student will have a certain amount of original

22 STRUCTURAL AND SYSTEMATIC ZOOLOGY

information which will enable him more clearly to com-
prehend the description of related forms mentioned in
the text.

In every case careful drawings should be made of
the specimen, and full notes on its habits and structure
prepared.

The appliances needed are a scalpel and a pair of
forceps, both of medium size ; a magnifying glass ; a
compound microscope, if protozoa and other minute
forms are to be studied ; and a small board on which
larger specimens may be laid for, the study of the struc-
ture. If alcoholic specimens are to be studied they
may be placed for examination in vegetable dishes con-
taining equal parts of alcohol and water to prevent
drying of the parts. There should be enough of the
mixture to cover the specimen. Specimens which
have been preserved in formalin may be examined in
water. For more particular descriptions of specimens
and methods of work reference may be made to the
laboratory manuals and text-books mentioned in the
Appendix.

INVERTEBRATES
Protozoa

Amoeba

Material. — More or less uncertainty usually attends
every attempt to provide at a given time a supply of
amoebas for a laboratory class. Nevertheless, the study
of this organism should not on any account be omitted,
for from no other one is so much to be learned regarding
the fundamental properties of living things. A thor-
ough study of the amoeba forms the basis of all sound
biological training.

Specimens of amoeba are often to be found in the

PRACTICAL ZOOLOGY 23

following places : in the slime on the under side of lily
pads and along the stem ; in the superficial layer of mud
in ponds and slowly flowing streams; in damp moss
from sphagnum swamps ; in the deposit on the sides of
water barrels in greenhouses ; in aquaria which have
been standing for some time and which contain no crusta-
ceans like DapJmia, Cypris, Cyclops, etc. In case no
specimens are obtained from ordinary sources, amoeboid
cells may be used instead. These may be found by
tearing to pieces the gills of a clam, or a mussel, or by
killing a frog, cutting through the skin of the abdomen
or leg, and removing a drop of the colorless fluid (lymph).
To study the specimen, collect with a pipette a drop
of the water supposed to contain amoebas, or a drop of
lymph, place it on a glass slide, put on the cover glass,
and examine with a low power, f , J, or \ inch objective.
Be sure to have some sediment or a hair under the cover
glass in order that the weight of the latter may not
crush the specimen.

Topics for Study. — The shape, an irregular outline,
changing as the animal moves along (sketch the outline
at intervals of one or two minutes, and compare the
successive sketches) ; the motion, note its rate and direc-
tion; the change of shape is due very largely to the pro-
trusion of portions of the body substance in the form of
blunt processes called pseudopodia (singular, pseudopo-
dium) (Fig. I, page 57).

With a higher power (^ or \ inch objective), examine
the animal's structure, noting that it is composed mainly
of a clear, semifluid substance, — protoplasm, — in which
are embedded numerous granular bodies of various
sizes and colors, some recognizable as fragments of vege-
table substance, together with, probably, one or more
diatoms or other minute organisms. In some part of

24 STRUCTURAL AND SYSTEMATIC ZOOLOGY

the body there will usually be seen a large, clear, appar-
ently empty circle, which from time to time suddenly
contracts and disappears from view. This is the con-
tractile vacuole, and is supposed to be an organ of excre-
tion, since uric acid, one of the forms in which nitrog-
enous waste material leaves the body in higher animals,
has been found in the vacuoles of certain animals closely
related to the amoeba. The vacuole, though apparently
disk-shaped, is really spherical. Closer examination will
show a small round mass, usually slightly darker than the
rest of the body and often distinctly and evenly dotted
with fine points, and surrounded by a plainly defined
line. This is the nucleus surrounded by its membrane.
Its shape is constant, except when the amoeba is in the
process of division. Still more careful study will show
that the body substance can be rather sharply divided
into two regions, an outer, clear, quite homogeneous
portion, the ectoplasm, and an inner, granular region,
the endoplasm.

If the animal be watched for a short time, it will proba-
bly be seen to ingest food particles, or, possibly, capture
another animalcule. In either case, the mode of pro-
cedure should be watched and the fate of the captured
particle followed. Some of the ingested material will
be disgorged, while certain pieces will be seen slowly to
disintegrate and to disappear as they dissolve in the
droplet of water in which the animal swallowed them.
From the behavior of these particles and from the
changes seen to take place in substances which have
been given the amoeba for experimental purposes, it is
believed that the animal produces in its body substances
analogous to the digestive juices of higher forms. The
disintegration of the food particles, then, indicates that
they are being digested. When they have reached the
stage of solution, they can, of course, no longer be seen.

PRACTICAL ZOOLOGY 25

The process of respiration cannot be followed in this
organism, there being no definite organs analogous to
the gills and lungs of higher forms devoted to this
function ; but the interchange of oxygen and carbon
dioxide, which is the essential part of the process, is
believed to take place through the superficial part of
the body.

The nervous properties of the animal are well shown
when it comes in contact with a foreign body, evidence
for the possession of the sense of touch being easily
obtained while the movements are being watched.

The contractions of the body substance show that it
is muscular.

In exceptional circumstances, an amoeba in the
process of division, or fission, may be found, the body
separating into two parts connected at first by a thread
of protoplasm .which eventually breaks, two distinct
organisms thus being formed. This process may be
more easily studied, however, in Paramecium^ the " slip-
per animalcule." Preceding the division of the single
cell composing the body, there is a division of its
nucleus. Some of the "shelled" forms, like Arcella
and Difflugia, may be used for comparison.

Euglena

Material. — Some of the green scum which forms on
the inside of aquaria is likely to yield abundant speci-
mens. If not, they may usually be raised by allowing
some pieces of green-coated bark, or a portion of a
flowerpot covered with the green film which forms on
the sides of damp pots, to stand covered with water in
a dish in a sunny place for several days. Scrape off
some of the green film in the bottom of the dish, and
examine according to the directions given for amoeba.
The animal may be recognized by its green color,

26 STRUCTURAL AND SYSTEMATIC ZOOLOGY

amcebalike changes in the shape of the body, and by
the presence of a whiplashlike organ (flagellum) by
means of which it propels itself through the water.

Topics for Study. — The elongated, highly flexible
body, its motions and color; the position and move-
ments of the flagellum; the red stigma, or "eye spot,"
and the contractile vacuole, both near the mouth

(Fig. 4).

Paramecium

Material. — The slipper animalcule is more readily
obtained than almost any other of the protozoa. There
are various ways of raising it in abundance for labora-
tory purposes. Hay or marsh grass cut into pieces a
few inches long may be placed in a convenient dish,
covered with water, and set in a warm room for one or
two weeks, at the end of which time there will probably
have formed a pellicle on the surface. This will con-
sist largely of rod-shaped or threadlike bacteria, and
feeding upon them will be seen many kinds of infusoria,
among the latter being Stylonychia, which may be recog-
nized by its large bristlelike cilia and its springing
motions, and Paramecium, the "slipper animal," covered
everywhere with fine cilia and having a more smooth,
gliding movement. Another satisfactory method of
procuring specimens is to place a handful of water
plants, like Anacharis (waterweed), Utricidaria (bladder-
wort), or Potamogeton (pond weed) in just enough water
to cover the plants, and let the mass stand in a warm,
dark place until decay begins, at which time the water
will probably be found to be swarming with animalcules.

In preparing the specimens for microscopic examina-
tion, follow the directions given for amoeba. It is
always well to put under the cover glass a few frag-
ments of the scum consisting of bacteria, for the ani-
malcules will gather around these masses and remain,

PRACTICAL ZOOLOGY 2/

feeding quietly. Otherwise their motions are likely to
be so rapid that study of the specimen may be quite
impossible. Or,, a few fibers of absorbent cotton may
be placed in the drop of water containing the animals,
thus forming meshes to entangle them.

Aquarium Study. — In the culture note the swarms
of animalcules, their position with reference to the sur-
face of the water, the sides of the dish, the direction of
the brightest light ; their size, color, and movements. -

Microscope Study. — With the low power study the
movements, their direction and rate; the flexibility of
the body, as seen when the animal passes through
narrow openings or around corners; the definite shape
of the body (compare with amoeba) ; the nervous prop-
erties, especially the sense of touch exhibited when the
animal comes into contact with a foreign body ; the
tendency to collect around food masses and air bub-
bles, or near the margin of the cover glass, the latter
tendency best seen if the water is very foul; animals
in the process of fission (resembling a single speci-
men more or less constricted in the middle of the
body, Fig. 10), or in conjugation (two individuals
attached together by their ventral sides).

With the high power the structure of individual ani-
mals and the functions of various parts of their bodies
may be studied. Note the arrangement, shape, size,
and movements of the cilia (their motions may be
stopped by the application of a drop of iodine solution) ;
the presence of the cuticle (cell wall) ; the mouth open-
ing leading to the gullet, the latter lined by short cilia
whose motions cause a current of water bearing food
particles to pas£ down into the body, where droplets
(food vacuoles) form and, after reaching a fairly uniform
size, are separated from the end of the gullet and carried
around through the body by the flow of the body sub-

28 STRUCTURAL AND SYSTEMATIC ZOOLOGY

stance (protoplasm) ; the position and movements of the
contractile vacuoles, one near each -end of the body, by
means of which the waste water is removed from the
body ; the movements of the cilia near the mouth open-
ing which produce currents in the water, thus bringing
food particles within reach ; the trichocysts, thought to
be organs of defense, lying parallel to one another just
under the cuticle (their discharge may be produced by
running a drop of acetic acid under the cover glass) ;
the large nucleus may be seen lying near the center of
the body after the application of a drop of acetic methyl
green or of methylene blue (Fig. 9).

Evidence of the possession of nervous properties will
be seen in the animal's extreme sensitiveness to contact
with foreign bodies, in its selection of food, which con-
sists almost entirely of bacteria, in its tendency to
collect on the lighter side of the aquarium jar when
the latter stands remote from the window.

Attention should be called to the possible means of
dispersal of the animal, to its value as a scavenger, and
particularly to the "physiological division of labor"
among the various portions of its body, which, though a
single cell, has its parts very plainly adapted to perform
many different functions.

Vorticella

Material. — Specimens are usually to be found attached
to the sides of the aquarium in which Paramecium has
been raised, or to the fragments of hay, jwater plants,
etc., therein. Prepare the specimens according to the
directions for Amoeba.

Topics for Study. — With the low power study the
shape of the animal, consisting of the body portion and
the flexible stalk ; the movements of each part, especially
the coiling of the stalk ; the position and movements of

PRACTICAL ZOOLOGy 2Q

the cilia, note that the latter are confined to the margin
of the bell-shaped body ; the miniature vortex into which
food particles are drawn by the action of the cilia.

With the high power, study the position and shape
of the large crooked nucleus, the motions of the single
contractile vacuole, the structure of the stalk, and the
ingestion of food particles (Fig. 1 1, b\ Some specimens
will probably be found in the state . of fission. The
early stages may be identified by the broadening of the
body transversely, the absence of cilia, and by a vertical
groove indicating the direction of division. Later the
two parts become more or less completely separated,
one having a circle of cilia around its lower portion. A
few minutes after the cilia are formed the animalcule
breaks away from its companion, leaving the latter in
possession of the stalk, and swims away by means of its
temporary locomotor cilia to select a site for attaching
itself and developing its own stalk (Fig. 1 1, a).

Conjugation may sometimes be observed, and may be
recognized by the fact that a large stalked individual
has attached to the lower portion of its body a much
smaller nonstalked individual, which gradually merges
into the body of the larger animal and disappears. The
nuclei of the two individuals fuse together and fertiliza-
tion is thus accomplished, but the changes which take
place within the two cells during the process of conjuga-
tion can be demonstrated only upon specimens especially
prepared for this purpose, the method of preparation
being too intricate for beginners.

Metazoa

Porifera

Material. — Simple marine sponges maybe obtained of
dealers in laboratory supplies. ,Fresh-water sponges,
Spongilla (green) and Myenia (brown) are to be looked

30 STRUCTURAL AND SYSTEMATIC ZOOLOGY

for in clear water attached to submerged branches, logs,
and rocks, and especially on the timbers of dams and
mill races. In purchasing toilet sponges for specimens,
care should be taken to select some which show single,
others numerous, openings and canals, while still others
should have particles of sand and of shells embedded in
the lower part.

Using Grantia as a type of the simple sponge, study
its shape, color, and mode of attachment; the large
opening (osculum) at the upper end surrounded with a
row of spicules. Note the small openings, inhalant pores,
on the surface. Cut the sponge open longitudinally and
note the pores opening into the central cavity. These
pores will be seen to be the ends of canals which run
horizontally outward. They do not, however, open to
the outer surface. These are the radial canals. Lying
between two adjacent radial canals will be found an
incurrent canal, the outer opening of which is on the
outer surface of the sponge. This canal has no opening
directly into the central cavity. Water carrying food
particles is drawn into the incurrent canals and passes
into the radial canals through pores in the walls of
tissue between the two canals. It then passes out of the
pores at the inner ends of the radial canals, into the larger
central cavity, and out through the osculum. The flow
of water is produced by the action of ciliated cells which
line the radial canals (Figs. 13, 14).

Microscopic sections will show the arrangement of the
canals and of the spicules of lime in the tissue of the
sponge, as well as the arrangement of the cellular parts
of the body. Large amoeboid cells (ova) are frequently
found in the walls between the canals. The spicules
may be obtained free from adhering tissue by boiling a
fragment of Grantia in caustic potash in a test tube.
As the spicules do not dissolve, the fluid may be drained

PRACTICAL ZOOLOGY 31

off and a drop of the sediment in the tube placed under
the microscope for examination. If a fragment of
Grantia be placed in 'weak acetic acid, an abundant
effervescence will take place, giving evidence that the
spicules are composed of carbonate of lime, the rest of
the sponge body remaining undissolved.

In studying the toilet sponge, note its color, shape,
weight, and elasticity ; study the position and arrange-
ment of the large and small canals ; note the embedded
sand particles, shells, etc. ; the texture of various speci-
mens; put a fragment under the low power of the
microscope and note how the fibers are arranged ; soak
a sponge in water and measure the amount held in the
meshes by squeezing it out into a graduate.

Fresh-water sponges are not easily kept alive in the
laboratory nor is their structure very plain. Their mode
of growth, branching, color, and friable texture may be
studied. With a magnifying glass numerous pores will
be seen on Spongilla, while the oscula of Myenia are
plainly visible. Microscopic sections will show the
double-pointed, flinty spicules traversing the tissues in
all directions. Small spherical, seedlike gemmules may
be obtained in the older part of the sponge in the fall,
and will " germinate " in a few days if kept undisturbed
in a dis4i of water. Only very little growth is likely to
take place.

Coelenterata
Hydra

Material. — Either the green or the brown species
may be used ; the latter, being much the larger, is
preferable. It will be found attached to the stems of
water plants which may be kept in aquarium jars.
The animals will often migrate to the sides of the
jar, where they can be studied with or without a lens.

32 STRUCTURAL AND SYSTEMATIC ZOOLOGY

The green species is common on species of Vaucheria
or greenfelt, which grow in rapidly flowing creeks.
Mats of the plant may be put into white earthenware
dishes. After a few minutes the hydras will expand
and be easily seen against the background formed
by the dish.

Aquarium Study. — With the naked eye or with a
lens study the hydra in situ, noting its color, shape,
size, the body and the tentacles, the number and
extensibility of the latter; touch the body or the ten-
tacles with a bristle and note the sensitiveness of the
animal. Look for individuals bearing buds ; the num-
ber and position of the latter; the radial symmetry
of the body. Note how well the shape and color
of the animal adapt it to its surroundings (Fig. 17).
' Microscope Study. — Transfer a fragment of the
plant bearing a hydra to a watch glass (or, if the
animal is fastened to the side of the jar, detach it
with a pipette), and examine under a lens or under
a low power. Note again the movements of the
body and tentacles. . Put a minute fragment of fresh
meat within reach of the tentacles and endeavor to
see the hydra catch and swallow it. Study the move-
ments of the mouth.

Mount a specimen under a cover glass arM study
the structure of the body, its walls consisting of two
layers of cells ; the central cavity with one exterior
opening ; the color of the inner cell layer and its
cause ; the structure of the tentacles, the groups of
nettle cells; cause the discharge of the latter by run-
ning a drop of weak acetic acid under the cover glass.
On individuals bearing buds study the structure and
actions of the latter ; their mode of attachment to
the parent ; the small " colony " formed by this mode
of reproduction (Fig. 18).

PRACTICAL ZOOLOGY 33

Note the effect of light on hydras by putting several
in a jar of water and covering it with an opaque paper
through which on one side a hole one inch in diameter
has been made, the jar then being placed near a win-
dow and the hole being directed toward the light.

For comparison use the sea anemone, Metridium
(Fig. 236). ,

Campanularian Hydroid

Material. — Specimens of Eucope or Obelia are found
attached to seaweed or submerged timbers, below low
tide mark, in the sea. The colonies are usually grayish
in color, much branched, and have a noticeably plant-
like aspect. If living specimens are available for
study, they may be placed in small dishes of fresh
sea water and examined with a magnifying glass.
The various motions of the polyps may be studied,
their protrusion from and withdrawal into the pro-
tective cup which surrounds each one, the rapid ex-
tension and twisting of the tentacles, the protrusion
of the funnel-like mouth to ingulf particles of food
(minute scraps of fresh meat are suitable), the sensi-
tiveness of the various members of the colony to
jarring, touching with a bristle point, agitation of the
water, and so on (Fig. 20).

It will be noticed that the colony consists of two
forms of zooid, one bearing tentacles, the nutritive
zooids ; the other being without tentacles, elongated
in shape, and containing a number of rounded bodies.
This form is the reproductive zooid, and the contained
bodies are the medusa buds or medusoids, which, when
mature, are liberated and produce the eggs from which
new, branched colonies arise. The phenomenon of
"alternation of generations" is here very marked.

The microscopic structure is best seen in mounted
specimens, which may be obtained from dealers.
DODGE'S GEN. ZOOL. — 3

34 STRUCTURAL AND SYSTEMATIC ZOOLOGY

Platyhelminthes and Nemathelminthes
Tapeworm, or Trichina

Material. — Alcoholic specimens of the former and
microscopic preparations of the latter may be studied
and attention called to their complicated developmental
history, and their pathologic significance (Figs. 37, 38, 39).

Echinodermata

Starfish

Aquarium Study- — If live specimens can be obtained,
study their mode of locomotion ; the flexibility of the
rays and the body; the movement of the spines along
the grooves, around the mouth opening, and at the
tip of the ray where the eye is located ; note the sensi-
tiveness of the various parts, particularly of the tube
feet* and of the branchiae ; note also that the numerous
tube feet move as though regulated or coordinated by
some governing power, their movements being thus
directed toward the attainment of some definite end
instead of being at random. Examine a number of
specimens and look for variations in the size of the
rays. These will show the power of regenerating lost
parts, which the starfish possesses to a high degree.

Structure. — Study the position and arrangement of
all external parts, the spines, tube feet, eyes, branchiae,
madreporic body, peristome, the radial nerve in the
roof of each groove. Remove the upper half of the
outer " shell " and note the internal organs : the digestive
system consisting of the stomach and digestive glands ;
the internal parts of the water-vascular system, the water
sacs and the " stone canal" ; the reproductive glands;
note the radial plan of structure (Figs. 46, 323, 330).

Exhibit a series of eggs, showing different stages
of segmentation, also the larval forms of the starfish.

PRACTICAL ZOOLOGY 35

Sea Urchin

Aquarium Study. — Note its mode of locomotion, its
sensitiveness, its movements in righting itself after hav-
ing been turned bottom side up.

Structure. — Trace all the resemblances you can be-
tween its external structure and that of the starfish.

>

Note that in spite of their difference in shape their
likeness in structure is very marked. Study the diges-
tive system, the teeth, and the long intestine. Note the
radial symmetry (Figs. 48, 226, 237, 293, 294).

Exhibit the larval stages.

The cake urchin (Eckinarachnius) and the holothurian
(Holothuria> Thyone, or Synaptd) may be used for com-
parison (Fig. 49). No other group of animals shows so
well as the echinoderms that the same plan of structure
may be associated with the greatest diversity of external
form.

Annulata
Earthworm

Field or Vivarium Study. — Study living specimens
out of doors, note their castings along paths, the amount
of earth brought up ; the diameter of the burrows ; trace
the latter down into the soil. Place a live worm on the
surface of the soil and note its mode of locomotion ; its
method of burrowing ; the protection from enemies
afforded by its color; draw one out of its burrow and
note the resistance ; touch a worm with a bristle and
note its sensitiveness ; note the effect of plugging the
mouth of the burrow with bits of straw, leaves, etc.

Watch the pulsation of the dorsal blood vessel.

Structure. — Note the shape of the body, the rings
composing it, the girdle, the mouth, the anus, the open-
ings of the reproductive glands, the bristles.

Cut an alcoholic or formalin specimen open along the

36 STRUCTURAL AND SYSTEMATIC ZOOLOGY

middle of the back and examine the muscular wall of
the body; the tough, transparent cuticle; the alimen-
tary tube within with the blood vessel and digestive
gland on its upper side ; the partitions connecting the
digestive tube with the body wall ; the long series of
cavities nearly separated from one another by these par-
titions, the whole forming the body cavity ; the continu-
ous digestive canal opening at each end to the exterior ;
the pair of excretory organs in each separate cavity.
Study the digestive tube, consisting of pharynx, esopha-
gus, crop, gizzard, and intestine ; note the structure
of the wall of the tube in each of these regions ; the
supra-esophageal ganglion or "brain" lying above the
pharynx ; the nerve cord below the alimentary canal ;
the reproductive glands along the anterior part of the
canal ; note the bilateral arrangement of all organs ; also
that the principal parts of the circulatory system lie
above, and of the nervous system below, the digestive
system (Fig. 52).

Draw attention to the economic and geologic impor-
tance of the earthworm in overturning the soil as it
feeds and constructs its burrows. If cocoons (egg cap-
sules) can be found (often attached to straws around
manure heaps) examine the various stages of develop-
ment of the earthworm.

The leech and Nereis (Fig. 215) or Arenicola (Fig. 274)
may be used for comparison.

Arthropoda

i. CRUSTACEA

Crayfish or Lobster

Material. — Live specimens of the former may be
kept indefinitely in aquarium jars containing algae and
supplied at intervals with a few crumbs or fragments of
beef or fish.

PRACTICAL ZOOLOGY 37

Aquarium Study. — Watch their movements when
walking and swimming ; the various motions of which
the legs are capable ; the movements of the antennae,
eyes, and swimmerets ; the position of the abdomen ;
the manner in which food (a scrap of fresh beef) is held
and pieces put into the mouth ; the movements of the
jaws ; of the breathing organs ; the position of the eggs,
if a female "in berry" can be obtained; the means of
offense and defense.

Structure. — With a dead specimen, preferably alco-
holic, note the hard covering of the body ; the two
regions (cephalothorax and abdomen); the rings or
segments of which the latter is composed ; the membra-
nous parts between adjacent rings ; the indications of
segmentation seen on the under side of the cephalo-
thorax; the number and structure of jointed appendages
on the abdomen ; the use of each kind ; the number,
structure, and use of the locomotor appendages on the
cephalothorax; the specialization of each pair for par-
ticular functions ; the relation between legs and gills ;
the arrangement, structure, and use of the various mouth
parts ; the structure of the eyes and " feelers ' ; the ear ;
the protection of the gills ; endeavor to make out the
fundamental plan of structure which underlies the great
diversity of form shown by the various appendages

(Fig. 54).

Cut through the shell along each side and remove the
upper part, thus exposing the internal organs. Note
the large' muscles in the abdomen ; the pericardium
and heart with the large artery running backward along
the middle line of the large abdominal muscle ; the
stomach with the bonelike parts in its walls ; the intes-
tine ; the digestive glands ; the esophagus ; the repro-
ductive glands ; the "green glands" (in the crayfish);
the nerve cord lying below the digestive system ; the

38 STRUCTURAL AND SYSTEMATIC ZOOLOGY

" brain " ; note the tendency of ganglia to fuse into larger
nerve centers (Figs. 55, 267).

Exhibit a series of larval crayfishes or lobsters, show-
ing the changes undergone at each molting.

Try to get specimens of the lobster, showing the re-
generation of lost parts, specially the pincers.

Call attention to resemblances in structure between
earthworm and crayfish ; note in the latter the tendency
to collect into definite regions the organs devoted to
definite uses ; also that every segment bears a pair of
jointed appendages.

Use the crab (Callinectes} for comparison.

2. INSECTA
Grasshopper

Living specimens may be kept in boxes or jars cov-
ered with gauze or netting and kept supplied with plenty
of fresh grass or wheat.

Field and Vivarium Study, — Note how the insect
walks, leaps, and flies ; the length of a single leap ; how
the food is held and eaten ; the use of the various sense
organs, as eyes and feelers; the mode of breathing;
the protective coloration of the body.

Structure. — Note the three regions of the body (head,
thorax, and abdomen), comparing with crayfish and
spider ; study the structure of each region ; the appen-
dages borne by each region ; their use ; the structure of
each kind of appendage and its adaptation to tits special
function ; the spiracles ; the ovipositor on the female ;
the ear (Figs. 64, 219, 276, 295, 344, 352).

Cut open a specimen lengthwise and note the parts
of the digestive system ; the muscles ; the reproductive
organs ; the structure and arrangement of the nervous
system. Examine tracheal tubes with the microscope
(Figs. 239, 277, 278).

PRACTICAL ZOOLOGY 39

Compare the plan of structure of the grasshopper
with that of the earthworm, crayfish, and spider.

Try to get young grasshoppers and note the changes
which are shown at the successive molts.

Butterfly

Field and Vivarium Study. — Note its wavering flight,
the way it walks, the position of the wings when at
rest, the use of the proboscis when gathering nectar,
the species of plants visited, the position of the pro-
boscis when not in use.

Structure. — Note the similarity of structure to the
grasshopper ; the differences between the wings of the
insects ; the greater uniformity in the structure of the legs
of the butterfly ; the position of the eyes ; the shape
of the antennae ; the structure of the proboscis ; the
microscopic appearance of the scales on the surface of
the wings (Figs. 70, 71, 221, 238, 241).

Examine the larva, noting its shape and color ; its mode
of locomotion ; locomotor organs ; food and method of
feeding (Fig. 73). Put mature larvae (of Mourning Cloak
butterfly, for example) in a glass-covered box and watch
them as they change to the pupa stage.

Study the chrysalis and eggs, if obtainable (Fig. 368).

The bee, fly, and beetle may be used for comparison.

3. ARACHNIDA
Spider

Field and Vivarium Study. — Study the mode of loco-
motion; the position, arrangement, and captured con-
tents of a web ; the position of the spider in the web.
Put a spider into a large pasteboard or wooden box,
cover with a sheet of glass, and note how the silk is
spun and the web is constructed. Look for cocoons
and study their structures and contents.

40 STRUCTURAL AND SYSTEMATIC ZOOLOGY

Structure. — Note the same regions as in the crayfish
(cephalothorax and abdomen) ; study the points of re-
semblance and of difference; indications of segmenta-
tion shown by each ; compare the number, position, and
structure of the legs ; study the spinnerets, their posi-
tion, number, and form ; the mouth parts ; the eyes ;
the structure and appendages of the skin ; the openings
of the breathing chambers (Figs. 81, 82, 216, 223).

Dissect alcoholic specimens in a dish of weak alcohol
or of water, and note the arrangement of the digestive
and nervous systems.

Mollusca

Mussel

The river mussels may be kept in aquariums having
two to four inches of sand or mud on the bottom.

Aquarium Study. — Study the movements of the ani-
mal ; the opening and closing of the shell ; the position
and use of the foot ; of the siphons ; the sensitiveness
of the siphonal tentacles ; the incurrent and excurrent
streams of water (Fig. 86).

Structure. — Examine a shell, noting the two similar
valves ; the hinge and hinge ligament ; the hinge teeth
(if present); the "epidermis," lines of growth, and
nacre ; the scars left by the muscles (Fig. 296).

Examine the soft part of the body, the mantle lobes ;
the gills; the body and the foot; the palpi and the
mouth ; the anal opening ; the adductor, protractor, and
retractor muscles.

Cut through the body lengthwise and trace the course
of the alimentary canal. Endeavor to trace the parts
of the nervous system ; the cerebral and the visceral
ganglia; the heart; the digestive gland (Figs. 244, 332).
Make three or four cross sections from a specimen

PRACTICAL ZOOLOGY 41

hardened in formalin or alcohol and study the supra-
branchial canal, gills, etc. (Fig. 275).
Examine the gills for eggs and young.

Land or Water Snail

Field or Aquarium Study. — -Study its movements ; its
mode of respiration; its feeding; the movements of the
" rasp " ; the use of the " feelers " ; the manner in which
the body is protruded from the shell and retracted.

Structure. — Compare its shell with that of the
mussel. Note the whorls ; the lines of growth ; the
attachment of the body to the shell (Fig. 297).

Remove the soft parts and study their structure,
the digestive system (Fig. 227); the large liver; the
heart, the brain, and nervous system (Figs. 243, 331,
351). Note that the bodies of both these mollusks
are unsegmented and are without appendages.

Snails kept in aquariums frequently attach their eggs
to the sides of the jar or to water plants. The seg-
mentation of the egg and the development of the larva
may be studied with a low power or with a hand lens.

VERTEBRATES

Vertebrata

Fish

Aquarium Study. — Study living specimens in the
aquarium, their movements of locomotion and of the
various fins ; mouth, gill covers, and gills ; the eyes ;
the method of feeding and of respiration ; the distribu-
tion of colored spots on the body ; the adaptation of
shape to locomotion in the water ; test the use of each
fin by binding them separately to the body by means of
rubber bands slipped on over the fish's head.

Structure. — On a dead specimen note the bilateral

42 STRUCTURAL AND SYSTEMATIC ZOOLOGY

symmetry ; the head ; the absence of a neck ; the body
and scales ; the position and structure of the fins ; the
shape and structure of the mouth ; the teeth ; nostrils ;
tongue ; gills ; the lateral line ; microscopic structure of
scales and gills (Figs. 119, 320).

Dissect away the skin ' from one side and study the
arrangement of the muscles.

Cut open a specimen and study the position and
relation of the internal organs ; the peritoneum ; the
digestive (Fig. 246), circulatory, and reproductive systems
(Figs. 268, 269, 272); the structure of the heart; open
the skull and examine the brain and the principal nerves
arising from it (Fig. 336). Remove, by boiling, the flesh
from the skeleton, and study the structure of the latter
(Fig. 309).

Make a microscopic examination of the blood
(Fig. 262).

Obtain (from one of the state hatcheries, if necessary)
a series of living eggs and embryos, and study the de-
velopment of the fish.

If opportunity offers, a fish market may be visited
and an examination made of the various kinds of food
fishes.

Frog

Vivarium and Aquarium Study. — Keep live speci-
mens in aquarium jars or in boxes containing damp
moss. Study the manner in which the frog creeps,
leaps, swims, breathes, moves, and closes its eyes,
catches flies ; the position of the body at rest ; with a
thermometer try to get the natural temperature of the
body.

Structure. — In a recently killed specimen note the
color and structure of the skin, the position of eyes,
ears, nostrils, lips, the position and arrangement of the
lips and the teeth, the shape and mode of attachment of

PRACTICAL ZOOLOGY ' 43

the tongue ; the sticky saliva and its use ; the absence
of a neck.

Dissect away the skin and study the shape and
attachments of the underlying muscles. Open the
abdomen and study the arrangement of the internal
organs, the digestive, circulatory, respiratory, excretory,
and reproductive systems (Figs. 273, 282); the structure
of the heart. Open the skull and examine the brain
(Fig. 337) ; trace the course of the principal nerves.
Study the principal parts of the skeletal system and
compare with that of the fish (Fig. 284). Examine
the circulation of the blood as seen in the web of the
foot (Fig. 263). Study the corpuscles in a drop of fresh
blood (Figs. 260, 261).

Collect the eggs of frogs or toads in the spring, keep
them in an aquarium, and watch the development of the
tadpole. Have a series of tadpoles showing the gradual
metamorphosis into the adult stage.

Draw attention to the changes of structural adapta-
tion necessitated by the change from the aquatic to the
aerial mode of life.

Turtle

Water or land turtles may be used, and may be kept
alive indefinitely in a damp box in the laboratory.

Field, Vivarium, and Aquarium Study. — On some of
the field excursions look for turtles in their native haunts,
and learn as much as possible of their habits. In the
laboratory note how the turtle walks, its clumsy motions,
rate of speed ; the motions of and positions taken by its
head, legs, tail ; movements of the eyelids, nostrils ; the
respiratory movements. Put the turtle into water and
watch its movements when swimming and diving.

Structure. — Study the external covering, its structure,
color, and modifications on the body, head, legs, and tail ;

44 STRUCTURAL AND SYSTEMATIC ZOOLOGY

compare with the fish and the frog ; the head, its shape
and various parts composing it, the jaws, eyes, nostrils ;
note the absence of teeth ; the tongue.

Remove the lower half of the shell and study the
internal organs composing the digestive, circulatory,
reproductive, and excretory systems; compare the
structure of the heart with that of the fish and the
frog (Fig. 273).

On a skeleton note the various parts which are
attached to the shell ; the skull and neck ; the hyoid
apparatus, the structure of the limbs and tail ; compare
the hyoid apparatus and the ribs with those of the frog
(Fig. 312).

If eggs can be obtained, note the shape, structure of
the shell, and the stages of development of the young.

Bird

Sparrows or pigeons may be used.

Field Study. — Note its general mode of life, whether
solitary or gregarious ; relations to other birds and to
man ; its manner of flight and of walking ; feeding
habits ; size, shape, and coloration of the body ; varia-
tions in coloration at different seasons of the year;
position and structure of the nest, number, shape, size,
and color of eggs, number of broods each year, season
when broods are produced, and number of young in
each brood ; enemies ; song ; if a living specimen can
be obtained, test the body temperature with a ther-
mometer.

Structure. — With a recently killed specimen, study
the shape of the body, the direction of its axis ; the
position and mobility of the head, wings, legs, and tail ;
the distribution of the various feathers, their structure
(Figs. 139, 302). Remove the latter and note the feather
tracts and the skin. Study the shape and structure of

PRACTICAL ZOOLOGY 45

the head, the beak, eyes, nostrils, and ears; compare
with the turtle. Examine the wings and legs, noting
the direction and movements of the various segments ;
the structure and movements of the parts of the foot ; the
position of the principal muscles, their uses.

Open the body and study the digestive, circulatory,
respiratory, and reproductive systems (Figs. 248, 273);
the air spaces among the muscles ; the structure of the
heart and brain as compared with the vertebrates pre-
viously studied (Fig. 338); the microscopic appearance
of the blood corpuscles (Fig. 262).

Prepare or purchase a skeleton and study the arrange-
ment of its various parts and the structure of the dif-
ferent bones, comparing with the fish, frog, and turtle
(Fig. 3i3>

Study the structure of the egg (Fig. 358), and the
development of the young (a convenient and satisfac-
tory substitute is the hen's egg) (Figs. 365, 366).

Draw attention to the economic value of the bird
studied.

Mammal

The cat or rabbit may be used.

Laboratory Study. — Study the motions of the animal as
it walks, runs, leaps, its position when at rest ; food and
mode of feeding ; respiratory movements ; motions of
head, legs, tail, ears, eyes ; mode of cleaning its fur ;
body temperature ; protective coloration.

Structure. — On a recently killed specimen note the
general shape of the body and direction of its axis;
the position and mode of attachment of the append-
ages ; the hairy covering, the groups of specialized
hairs in certain positions, the microscopic appearance
of hair.; the mobility of the skin ; its firmer attachment
in certain places, compare with the external covering of
fish, frog, turtle, and bird. Study the shape and structure

46 STRUCTURAL AND SYSTEMATIC ZOOLOGY

of the head, the position and structure of ears, eyes, nos-
trils, and mouth.

Remove the skin from the body and study the position
and attachment of the more important muscles, their uses.
Open the body and examine the organs composing the
digestive, circulatory, respiratory (Fig. 283), excretory,
and reproductive systems (Fig. 250). Note the posi-
tion and structure of the teeth and their fitness to mas-
ticate the special kind of food the animal eats ; the
surface of the tongue and its adaptation as an organ
for cleaning the fur; compare the heart and brain
(Fig. 339), and the microscopic appearance of the blood
corpuscles with those of other vertebrates examined
(Fig. 259).

Trace the course of some of the principal blood vessels
and nerves.

Examine a skeleton and compare with that of the
other vertebrates studied (Fig. 303).

A series of preparations of fetal kittens or rabbits may
be examined.

CHAPTER II

THE CLASSIFICATION OF ANIMALS

THE Kingdom of Nature is a literal Kingdom. Order
and beauty, law and dependence, are seen everywhere.
Amidst the great diversity of the forms of life, there is
unity ; and this suggests that there is one general plan,
but carried out in a variety of ways.

Naturalists have ceased to believe that each animal
or group is a distinct, circumscribed idea. " Every ani-
mal has a something in common with all its fellows :
much with many of them ; more with a few ; and,
usually, so much with several, that it differs but little
from them." The object of classification is to bring
together the like, and to separate the unlike. But how
shall this be done ? To a-rrange a library in alphabeti-
cal order, or according to size, binding, date, or lan-
guage, would be unsatisfactory. We must be guided by
some essential character. We must decide whether a
book is poetry or prose; if poetry, whether dramatic,
epic, lyric, or satiric ; if prose, whether history, philoso-
phy, theology, philology, science, fiction, or essay. The
more we subdivide? these groups, the more difficult the
analysis.

A classification of animals, founded on external re-
semblances — as size, color, or adaptation to similar
habits of life — would be worthless. It would bring
together fishes and whales, birds and bats, worms and
eels. Nor should it be based on any one character, as
the quality of the blood, structure of the heart, develop-
ment of the brain, embryo life, etc. ; for no character is

47

48 STRUCTURAL AND SYSTEMATIC ZOOLOGY

of the same value in every tribe. A natural classification
must rest on those prevailing characters which are the
most constant.^ And such a classification can not be
linear. It is impossible to arrange all animal forms
from the sponge to man in a single line, like the steps
of a ladder, according to rank. Nature passes in so
many ways from one type to another, and so multiplied
are the relations between animals, that one series is out
of the question. There is a number of series, and
series within series, sometimes proceeding in parallel
lines, but more often divergent. The animals arrange
themselves in radiating groups, each group being con-
nected, not with two groups merely, one above and the
other, below, but with several. Life has been likened to
a great tree with countless branches spreading widely
from a common trunk, and deriving their origin from a
common root ; branches bearing all manner of flowers,
every fashion of leaves, and all kinds of fruit, and these
for every use.

The groups into which we are able to cast the various
forms of animal development are very unequal and dis-
similar. We must remember that a genus, order, or
class is not of the same value throughout the kingdom.
Moreover, each division is allied to others in different
degrees — the distance between any two being the
measure of that affinity. The lines between some are
sharp and clear, between others indefinite. Like the
islands of an archipelago, some groups merge into one
another through connecting reefs, others are sharply
separated by unfathomable seas, yet all have one com-
mon basis. Links have been found revealing a relation-
ship, near or distant, even between animals whose forms
are very unlike. There are fishes {Dipnoi) with some
amphibian characters, and fishlike amphibians (Ajcolotl).
The extinct ichthyosaurus was a lizard with fish charac-

THE CLASSIFICATION OF ANIMALS 49

teristics. Birds seem isolated, but they are closely con-
nected with reptiles by fossil forms. Even the great
gap in the animal kingdom — that separating verte-
brates and invertebrates — is partially bridged on the
one side by amphioxus, and on the other by balano-
glossus (a wormlike animal) and the tunicates.

We have, then, groups subordinate to groups, and
interlocking, but not representing so many successive
degrees of organization. For, as already intimated,
complication of structure does not rise in continuous
gradation from one group to another. Every type starts
at a lower point than that at which the preceding class
closes ; so that the lines overlap. While one class, as a
whole, is higher than another, some members of the
higher class may be inferior to some members of the
lower one. Thus, certain starfishes are structurally
more complex than certain mollusks ; and the nautilus
is above the worm. The groups coalesce by their in-
ferior or less specialized members; e.g., the fishes do not
graduate into amphibians through their highest forms,
but the two come closest together low down in the
scale. Among the craniate animals the lines of descent
of the various classes may be represented as diverging
and ascending from a point occupied by a fishlike
ancestor.

A number of animals may, therefore, have the same
grade of development, but conform to entirely different
types. While a fundamental unity underlies the whole
animal kingdom, suggesting a common starting point,
we recognize several distinct plans of structure.5 Ani-
mals like the amoeba, with no cellular tissues and no
true eggs, form the branch Protozoa. Animals like the
sponge, with independent cells, one excurrent and many
incurrent openings, form the branch Porifera. Animals
like the coral, unlike all others, have an alimentary canal
DODGE'S GEN. ZOOL. — 4

50 STRUCTURAL AND SYSTEMATIC ZOOLOGY

but no body cavity, have no separate nervous and vascu-
lar regions, and the parts of the body radiate from a
center. Such form a branch called Ccelenterata. Ani-
mals like the starfish, having also a radiating body, but
a closed alimentary canal, and a distinct symmetrical
nervous system, constitute the branch Echinodermata*
Animals like the angleworm, bilaterally symmetrical,
one-jointed, or composed of joints following each other
from front to rear, with no jointed limbs, constitute the
branch Annulata. Animals like the snail, with a soft,
unjointed body, a mantle, a foot, a two or three cham-
bered heart, and a nervous system in the form of a ring
around the gullet, constitute the branch Mollusca. Ani-
mals like the bee, with a jointed body and jointed limbs,
form the branch Arthropoda. Anifnals like the ox, hav-
ing a double nervous system, one (the sympathetic)
lying on the upper side of the alimentary canal, the
other and main part (spinal) lying along the back, and
completely shut off from the other organs by a partition
of bone or gristle, known as the "vertebral column," and
having limbs, never more than four, always on the side
opposite the great nervous cord, constitute the branch
Vertebrata.

Comparing these great divisions, we see that the ver-
tebrates differ from all the others chiefly in having a
double body cavity and a double nervous system, the
latter lying above the alimentary canal ; while inverte-
brates have one cavity and one nervous system, the
latter being placed mainly below the alimentary canal.

But there are types within types. Thus, there are
five modifications of the vertebrate type — fish, amphib-
ian, reptile, bird, and mammal ; and these are again
divided and subdivided, for mammals, e.g., differ among
themselves. So that in the end we have a constellation
of groups within groups, founded on peculiar characters

THE CLASSIFICATION OF ANIMALS 51

of less and less importance, as we descend from the
general to the special.

Individuals are the units of the Animal Creation.
An animal existence, complete in all its parts, is an
individual, whether separate, as man, or living in a com-
munity, as the coral.7

Species is the smallest group of individuals which can
be defined by distinct characteristics, and which is
separated by a gap from all other like groups. A well-
marked subdivision of a species is called a variety.
Crosses between species are called hybrids, as the
mule.

Genus is a group of species having the same essential
structure. Thus, the closely allied species cat, tiger,
and lion belong to one genus.

Family, or Tribe, is a group of genera having a simi-
lar form. Thus, the dogs and foxes belong to different
genera, but betray a family likeness.

Order is a group of families, or genera, related to one
another by a common structure. Cats,' dogs, hyenas,
and bears are linked together by important anatomical
features ; their teeth, stomachs, and claws show carniv-
orous habits.

Class is a still larger group, comprising all animals
which agree simply in a special modification of the type
to which they belong. Thus, fishes, amphibians, rep-
tiles, birds, and mammals are so many aspects of the
vertebrate type.

Branch is a primary division of the animal kingdom,
which includes all animals formed upon one of the
various types of structure ; as vertebrate.

The branches are grouped into two great Series (Pro-
tozoa and Metazoa), according to their histological
structure and mode of development.8

These terms were invented by Linnaeus, except

52 STRUCTURAL AND SYSTEMATIC ZOOLOGY

Family, Branch, and Series. To Linnaeus we are also
indebted for a scientific method of naming animals.
Thus, the dog, in Zoology, is called Cam's familiaris,
which is the union of a generic and a specific name,
corresponding to the surname and the Christian name
in George Washington, only the specific name comes
last. It will be understood that these are abstract
terms, expressing simply the relations of resemblance ;
there is no such thing as genus or species.

Classification is a process of comparison. He is the
best naturalist who most readily and correctly recog-
nizes likeness founded on structural characters. As
it is easier to detect differences than resemblances, it
is much easier to distinguish the class to which an ani-
mal belongs than the genus, and the genus than the
species. In passing from species to classes, the charac-
ters of agreement become fewer and fewer, while the
distinctions are more and more manifest; so that ani-
mals of the same class are more like than unlike, while
members of distinct classes are more unlike than like.

To illustrate the method of zoological analysis by
searching for affinities and differences, we will take
an example suggested by Professor Agassiz. Suppose
we see together a dog, a cat, a bear, a horse, a cow, and
a deer. The first feature which strikes us as common
to any two of them is the horn in the cow and the deer.
But how shall we associate either of the others with
these ? We examine the teeth, and find those of the
dog, the cat, and the bear sharp and cutting ; while
those of the cow, the deer, and the horse have flat sur-
faces, adapted to grinding and chewing, rather than to
cutting and tearing. We compare these features of
their structure with the habits of these animals, and
find that the first are carnivorous — that they seize and
tear their prey; while the others are herbivorous, or

THE CLASSIFICATION OF ANIMALS 53

grazing, animals, living only on vegetable substances,
which they chew and grind. We compare, further, the
horse and cow, and find that the horse has front teeth
both in the upper and the lower jaw, while the cow has
them only in the lower ; and going still farther, and
comparing the internal with the external features, we
find this arrangement of the teeth in direct relation to
the different structure of the stomach in the two ani-
mals— the cow having a stomach with four pouches,
while the horse has a simple stomach. Comparing the
cow and deer, we find the digestive apparatus the same
in both ; but though both have horns, those of the cow
are hollow, and last through life ; while those of the
deer are solid, and are shed every year. Looking at
the feet, we see that the herbivorous animals are hoofed ;
the carnivorous, clawed. The cow and deer have cloven
feet, and are ruminants; the horse has a single hoof,
and does not chew the cud. The dog and cat walk on
the tips of their fingers and toes (digitigrade) ; the bear
treads on the palms and soles (plantigrade). The claws of
the cat are retractile ; those of the dog and bear are fixed.
In this way we determine the exact place of each ani-
mal. The dog belongs to the kingdom Animalia, branch
Vertebrata, class Mammalia, order Carnivora, family Ca-
nida, genus Cants, species familiaris, variety hound (it
may be), and its individual name, perhaps, is " Rover."
The cat differs in belonging to the family Felida, genus
Felis, species domestica. The bear belongs to the family
Ursidce, genus Ursus, and species ~horribilis, if the grizzly
is meant. The horse, cow, and hog belong to the order
Ungulata ; but the horse is of the family Equidcz, genus
Equns, species caballus ; the cow is of the family Bovi-
da, genus Bos, species tanrus ; the pig is of the family
Suidcz, genus Sns, species scrofa, if the domestic pig is
meant.

54 STRUCTURAL AND SYSTEMATIC ZOOLOGY

The diagram on the opposite page roughly represents
(for the relations of animals can not be expressed on a
plane surface) the relative positions of the branches and
classes according to affinity and rank.*

SERIES I. — PROTOZOA

Animals whose bodies consist of a single cell, the
process of reproduction being by division or by budding,
but never by means of true eggs.

Branch I. — PROTOZOA

In structure the Protozoa are the simplest of animals,
consisting of only a single cell. They are microscopic
in size and aquatic in habit, though in certain stages
of their lives (encystation) many of them may endure
dryness for weeks or months. Their bodies consist
mainly or wholly of protoplasm, which may or may not
be covered by a cuticle or by a shell-like excretion of
lime, chitin, or flint, or inclose spicules of the latter
substance. The various individuals may live separately
as single, independent organisms, or they may be or-
ganically joined together in clusters called colonies.
They exhibit all the essential functions of life — nutri-
tion, growth, nervous properties, and reproduction.
They feed upon minute algae, bacteria, vegetable de-
bris, and upon other microscopic animals. Some forms
are parasitic. It has been shown by experiment that
many species are sensitive to changes in the amount of
illumination to which their bodies are exposed and to
various colors of light; that they are attracted or re-

* The student should master the distinctions between the great groups,
or classes, before proceeding to a minuter classification. "The essential
matter, in the first place," says Huxley, " is to be quite clear about the
different classes, and to have a distinct knowledge of all the sharply de-
finable modifications of animal structure which are discernible in the Ani-
mal Kingdom."

THE CLASSIFICATION OF ANIMALS

55

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56 STRUCTURAL AND SYSTEMATIC ZOOLOGY

pelled by the presence of different chemical substances
in the water ; and that they are sensitive to contact with
foreign bodies and with one another. Thus it is proved
that these simple organisms possess the rudiments of
the nervous .properties seen in the higher animals. Cer-
tain species contain a green coloring substance (haema-
tochrome) which is chemically allied to the chlorophyll
of plants. Others, again, pass through amoeboid stages
resembling similar phases in the development of some
of the lowest plants. Because of these resemblances
some of the Protozoa are almost indistinguishable from
the lowest members of the plant kingdom (Protophyta).
On account of the apparent simplicity of their struc-
ture, it is difficult to select features by means of which
the animals in this group may be classified. The diffi-
culty is further increased by the fact that in the course
of their development some forms pass through stages in
which they resemble other species in the same branch.
In every case, however, it is found that certain phases
of their development predominate, and these well-
marked phases permit of dividing the Protozoa into
five classes.

CLASS i . — Rhizopoda

These are Protozoa which are predominantly amoe-
boid in shape and which move by means of pseudopodia,
as the slow-moving protrusions of the protoplasmic body
substance are called (Figs, i, 213). The body usually
contains a nucleus and a contractile vacuole. The com-
mon amoeba or proteus animalcule belongs in this class
(Fig. i). Some of the Rhizopods secrete shells of
chitin (Arcella), or construct a covering made of parti-
cles of sand (Diff.ugia). Both of these organisms are
found in fresh water in America. The most primitive
representative of the group is Protamceba, in which

PROTOZOA

57

neither nucleus nor contractile vacuole has been dis-
covered. Pelomyxa, a fresh-water form, may reach the
size of eight millimeters (.3 of an inch) in diameter.

An amoeba is a naked fresh-water Rhizopod, contain-
ing a nucleus and a contractile vacuole, the body sub-

FIG. i. — Amoeba, showing the structure of the body and the changes which take place
during division. The dark body in each figure is the nucleus ; the transparent circle,
the contractile vacuole ; the protrusions of the body substance, pseudopodia ; the
outer, clear portion of the body, the ectosarc ; the granular portion, the endosarc ;
the granular masses, food vacuoles. Much magnified.

stance consisting of two rather distinct layers, the outer
being quite clear and transparent, while the inner is usu-
ally filled with granules and ingested particles. During
movement the shape of the body is constantly changing,
owing to the protrusion and withdrawal of the pseudo-

58 STRUCTURAL AND SYSTEMATIC ZOOLOGY

podia. Food is taken into the body at any point, there
being no mouth.

A Foraminifer differs from an amoeba in having an
apparently simpler body, the protoplasm being without
layers or cavity ; its pseudopodia are long and threadlike,
and may unite where they touch each other. It has the
property of secreting an envelope, usually of carbonate
of lime. The shell thus formed is sometimes of extraor-
dinary complexity and singular beauty. In addition to
the terminal aperture, it is generally perforated by

a

FIG. 2. — Rhizopods : a, shell of a monothalamous, or single-chambered, Foraminifer
(Lagena. striata) ; b, shell of a polythalamous, or many-chambered, Foraminifer
{Polystomella. crispa), with pseudopodia extended ; c, shell of a Radiolarian, one
of the Polycystines (Podocyrtzs schomburgkii).

innumerable minute orifices (foramina) through which
the animal protrudes its myriad of glairy, threadlike
arms. The majority are compound, resembling cham-
bered cells, formed by a process of budding, the new
cells being added so as to make a straight series, a
spiral, or a flat coil. As a rule, the many-chambered
species have calcareous, perforated shells ; and the one-
chambered have an imperforated membranous, porce-
laneous, or arenaceous envelope. The former are marine.
There are few parts of the ocean where these micro-
scopic shells do not occur, and in astounding numbers.

PROTOZOA 59

A single ounce of sand from the Antilles was calculated
to contain over three millions. The bottom of the ocean,
up to about 50° on each side of the Equator, and at
depths not greater than 2400 fathoms, is covered with
the skeletons of these animals, which are constantly
falling upon it (Globigerina ooze). Their remains consti-
tute a great proportion of the so-called sand banks
which block up many harbors. Yet they are descend-
ants of an ancestry still more prolific, for the Foraminif era
are among the most important rock-building animals.
The chalk cliffs of England, the building stone of Paris,
and the blocks in the Pyramids of Egypt are largely
composed of extinct Foraminifers. Foraminifera are
both marine and fresh-water, chiefly marine.

The sun animalcule (Actinophrys sol\ one of the Heli-
ozoa, is common in the slime on the sides of aquaria.
Its spherical body, composed of frothy protoplasm,
bears numerous stiff radiating processes by means of
which the animal moves about and captures its food.

A Radiolarian differs from a Foraminifer in secreting
a siliceous, instead of a calcareous, shell, studded with
radiating spines ; and in the central part of the body is
a perforated membranous sac containing a nucleus or,
sometimes, several nuclei. The most of the protoplasm
of the body lies outside the sac. Radiolarians are more
minute than Foraminifera, but as widely diffused. They
enter largely into the formation of some strata of the
earth's crust, and abound especially in the rocks of Bar-
badoes and at Richmond, Va. The living forms are
marine.

CLASS 2. — Mycetozoa

These organisms are frequently classified among the
plants under the name of "slime molds." They consist
of masses of protoplasm of various sizes and colors, and

60 STRUCTURAL AND SYSTEMATIC ZOOLOGY

are terrestrial in habit, being often found in sum-
mer slowly crawling upon
stumps, logs, and leaves.
In their nonmotile stage
of development they re-
semble the spore-bearing
organs and spores of cer-
tain Fungi, but in their
locomotor phase they ex-
hibit the structure and
physiology of amoeboid

FIG. 3. — Germination of a spore of a Myceto- , n 11 4.

zoan (Trfchia), showing the development and flagellate
of the amoeboid stage. Much magnified. SOTTlPtimPS

forming large, multinucleated masses of
'protoplasm which crawl about and ingest
solid food (Fig. 3).

CLASS 3. — Mastigophora

The distinctive character of the ani-
mals in this group is the presence of
one or more flagella, long, whiplashlike
threads of protoplasm used for locomo-
tion and for obtaining food. Some kinds,
like Euglena (Fig. 4), live as independent
organisms, while others, as Volvox and
Dinobryon, form colonies (Fig. 5). The
latter two are of some sanitary impor-
tance, since either one, when present
in large numbers in a water supply, is
likely to cause unpleasant tastes and

. FIG. 4. — Euglena: /,

odors. JVoctiluca, a marine form, is one flageiium; g, guiiet;

r , i r i i • A!_ Pst pigment spot or

of the causes of phosphorescence in the «eye"; cv, contrac-
sea. Some of the higher kinds, e.g., tiievacuoie: r.reser-

voir ; /, paramylum

Codosiga (Fig;. 6\ are interesting; for bodies; r, chlorophyll

bodies. Much mag-

the reason that they bear a peculiar struc- nified.

PROTOZOA

6l

ture, the so-called " collar," which is found practically
nowhere else except on certain cells in sponges (Figs. 6,

FIG. 5. — Dinobryotti portion
of the motile colony show-
ing zooids, each in its own
lorica. Much magnified.

FIG. 6. — Codosiga: f, fla-
gellum ; c, " collar" ; bt
body ; n, nucleus ; cz>,
contractile vacuole ; nv,
nutritive vacuole. Much
magnified.

'CLASS 4. — Sporozoa
>

The Sporozoa are all parasitic, and are found in va-
rious parts of the bodies of fishes, frogs, turtles, insects,
crustaceans, worms, and so on, some living in the
digestive organs, others in glands, while still others
penetrate into the muscle fibers of the infested animal
(Fig. 7). They have no organs of locomotion, but move
by wormlike contortions of the body. The protoplasmic
body substance is covered by a cuticle, and contains a
nucleus (Fig. 8). Liquid food is absorbed through the
cuticle (Fig. 7).

62 STRUCTURAL AND SYSTEMATIC ZOOLOGY

Probably the parasite which causes malaria in man
belongs in this group.

FIG. 7. — Sporozoa: a, embedded in the striated muscle fiber of the ox; b, Gregarina
in the intestine of the beetle ; c, cyst and young germs of Klossia from the snail.
All much magnified.

Reproduction is by means of spores. The individ-
uals become surrounded by a thickened covering or cyst
secreted from their bodies. Within this cyst division
into numerous smaller masses takes place. Each mass

FIG. 8. — Gregarina gigantea, highly magnified : a, nucleus.

then secretes a thickened coat and becomes a spore.
When mature, the protoplasmic mass within breaks
through the wall of the cyst and enters the organ or
animal in which the parasite reaches its adult condition
(Fig. 7>

PROTOZOA

CLASS 5. — Infusoria

The name of this group is derived from the fact that
the animals composing it are almost always found in
infusions of vegetable substance.
The characteristic feature of the
group is the presence of fine, hair-
like protrusions of the body sub-
stance, which more or less com-
pletely cover the animal, and which
are called cilia. These are perma-
nent structures in some forms
(Ciliata), but are found only in the
young condition of others (Tenta-
culiferd), the adults developing -ten-
tacles (Fig. 12). Their bodies show
a great variety of shapes, spherical,
flattened, oval, cylindrical, conical,
and so on. Some live as indepen-
dent organisms, as Paramecium
(Fig. 9); others
are sedentary,
being attached
by a stalk, as

Vorticella (Fig. n); the trumpet
animal (Stentor) can attach itself at
will. Epistylis forms branching col-
onies. In some colonies the mem-
bers are all alike in structure and
function (Carchesium), while in
others (Zoothamnium) the members
which capture the food for the
FIG. ™.- Paramecium in colony are plainly different in shape,
the process of fission: m, sjze an(^ structure from those which

mouth ; cv, contractile

vacuoie;«,macronucieus; produce the new colonies, the latter

»', micronucleus. Much , 111 i 11

magnified. being mouthless, larger, and capable

FIG. 9. — Paramecium, c,
cilia ; g, gullet ; f, food
vacuole ; t, trichocysts ;
cv, contractile vacuole ;
m, macronucleus ; n, mi-
cronucleus. Much mag-
nified.

64 STRUCTURAL AND SYSTEMATIC ZOOLOGY

of freeing themselves from their stalk and swimming
away to another place where the new colony is to be

started. Thus there is
shown among these sim-
ple organisms the differ-
entiation of parts which
is one of the character-
istic features of the higher
forms of animal life.

The cilia are used for
locomotion and for ob-
taining food, which, ex-
cept in the parasitic
/ species, is in the form
of solid particles, consist-
ing of bacteria and micro-
scopic plants and animals,
or of minute fragments of
animal and vegetable ma-
terial. All of the ciliata
which ingest solid food have a permanent mouth open-
ing. Tentaculifera
suck through their
tentacles the soft ma-
terial composing the
bodies of their prey.
The cilia are uni-
formly arranged over
the body, as in Para-
mecium, or are re-
stricted to definite
regions, as in Vorti-
cella. In either case
there may be variations in their form, size, and func-
tion. Reproduction is by division and by budding,

FIG. ii. — Vortice lla : a, showing stages of
fission, and b, internal structure; d, cili-
ated disk ; g, gullet ; «, nucleus ; c, con-
tractile vacuole ; f, food vacuole. Much
magnified.

FIG. 12. — Actneta, animal in its lorica, /, showing
suctorial tentacles and nucleus, n, with contractile
vacuole. Only a small part of the stalk is
shown. Magnified,

PORIFERA 65

spore formation being exceptional. It has been esti-
mated that by self-division a Parameciunt may give rise
to 1,364,000 in forty-two days (Figs. 10, n).

SERIES II. — METAZOA

The Metazoa include all those animals whose bodies
are multicellular, which reproduce by true eggs and
spermatozoa. This series includes eleven of the
branches of the animal kingdom.

Branch II. — PORIFERA

The position of the sponges has been much dis-
puted. At first they were thought to be on the border
line between animals and plants, and were assigned by
some to the animals and by others to the vegetables.
Later, and up to very recent years, they were assigned
to the Protozoa. The discovery 0

of their mode of reproduction
and development has determined
that they belong to the Metazoa.

Simple sponges, like Grantia
(Fig. 13), are somewhat vase-
shaped in outline, and have a
single central cavity communi-
cating with the outside through
an opening called the osculum.
The wall of the body is pierced
by numerous fine canals which
communicate more or less di- FlG- X3- -Diagram of a simple

sponge: t, inhalant opening;

rectly with the central cavity on o, exhaiant opening or oscu-
the one hand and with the exte-
rior on the other. There is no body cavity. The
body wall is composed of the skeleton, together with
the cellular elements forming the "flesh." The sur-
DODGE'S GEN. ZOOL. — 5

66 STRUCTURAL AND SYSTEMATIC ZOOLOGY

face of the body is covered by a single layer of flat-
tened cells forming the ectoderm. The canals are more
or less completely lined with a layer of cells, each
of which is provided with & flag e Hum, by means of which
water is propelled through the canals toward the central
cavity. Between these two layers is a mass of amoeboid
and other cells which compose the mesoderm, and in
which the skeleton, or framework, of the sponge is de-
veloped. The skeleton may be composed of flexible
fibers of spongin, as in the toilet sponge ;
or of spongin fibers together with spic-
ules of calcareous matter, as in Grantia ;
or of siliceous spicules alone, as in
the fresh-water sponge (Spongilla and
Myenia) and Venus's flower basket
(Euplectelld) ', or of spicules of carbo-
nate of lime and so on, while a few
have no skeleton at all.

The flagellate cells are peculiar, in
that they have an upgrowth on their
free end, formed by a delicate expansion
of the cell substance, and having the
shape of a broad collar surrounding the
base of the flagellum, whence their
name of "collar cell" (Fig. 14).

The water flowing in through the
canals bears with it the small particles
of organic material upon which the sponge feeds, the
particles being captured apparently by the collar cells
as well as by the amoeboid cells. The same currents
of water serve also for respiration. Reproduction is by
means of eggs and by budding, the latter process giving
rise to a group of connected sponges. The young larval
sponge which develops from the fertilized egg is pro-
vided with cilia, by means of which it can swim around

FIG. 14. — "Collar
cell " from Grantia,
showing flagellum,
f; " collar," c ; nu-
cleus, n ; contractile
vacuole, cv . Much
magnified.

PORIFERA

67

for a time. Later, it comes to rest, attaches itself to
some support, and develops into the adult form which is

d

h

FIG. 15. — Hypothetical Section of a Sponge: a, superficial layer ; b, inhalant pores;
c, ciliated chambers ; d, exhalant aperture, or osculum ; e, deeper substance of the
Sponge.

never capable of locomotion. The fresh-water sponges
also multiply by means of gemmules, which are small,

FIG. 16. — Skeleton of a Horny Sponge.

seedlike bodies to be found in the sponge in the fall.
Each consists of a hard coating surrounding a mass of

68 STRUCTURAL AND SYSTEMATIC ZOOLOGY

cells and food substance. These gemmules survive the
winter, and.in the spring the cellular contents come out
and develop into a sponge.

The sponge individual contains one exhalant orifice
(osculum), with the channels leading into it. An ordi-
nary bathing sponge constitutes a colony of such indi-
viduals, which are not definitely marked off from each
other. Some other sponges have only one osculum, and
such are a single individual, e.g. Grantia.

Excepting a few small fresh-water species (as Spon-
gilla\ sponges are marine. In the former, the cellular
part is greenish, containing chlorophyll ; in the latter, it
is brown, red, or purple. In preparing the sponge of
commerce, this is rotted by exposure, and washed out.
The best fishing grounds are the eastern end of the
Mediterranean and around the Bahama Islands.

Branch III. — CCELENTERATA

In the animals comprising this group, the body
cavity is not distinctly separated from the digestive
cavity. The ccelenterates are almost wholly marine
forms, — hydroids, corals, sea anemones and jellyfishes, —
but there are a few which, like Hydra, live in fresh
water. The body is usually radially symmetrical and
shows three more or less definite cell layers, the ecto-
derm on the outer surface, the endoderm lining the inner
cavities, with the mesoderm, or middle layer, between
the others. In hydra and the hydroids the mesoderm
is reduced to a mere film, but in the jellyfishes and
sea anemones it forms a large part of the body. A
characteristic feature is the presence of the stinging
cells, or nematocysts, which are almost invariably to be
found except in one group, — Ctenophora, — where they
are replaced by adhesive cells.

CCELENTERATA 69

This branch consists of two rather divergent forms,
represented on the one hand by the sedentary type
(hydroid), and on the other by the free-swimming type
(jellyfish). These two forms may occur during the
course of development of one individual, thus illustrat-
ing the phenomenon of "alternation of generations."
Many of the members of the group are soft-bodied,
while others secrete calcareous material forming coral.

All of the coelenterates multiply by means of eggs
and sperm cells, and all, except the Ctenophora, by
budding as well, the latter method resulting in the
formation of colonies in which the various members
often differ greatly in form and in function.

The animals in this group are carnivorous, feeding
mainly on small organisms, although the sea anemone
can ingulf masses of considerable size.

There are four classes : —

Class i. Hydrozoa, represented by hydra and the

hydroids.
Class 2. Scyphozoa, containing the large jellyfishes,

for example, Aurelia.
Class 3. Actinozoa, including the sea anemone and the

corals.
Class 4. Ctenophora, including the jellyfishes which

have comblike swimming organs.

CLASS i . — Hydrozoa

In these coelenterates the body is a simple tube, or
cavity, in which there is a single aperture, the mouth.
The nervous system is slightly developed. Such are
fresh-water hydras and the oceanic hydroids (Eucope).

The body of the hydra is tubular, soft, and sensitive,
of a greenish or brownish color, and seldom over half
an inch long. It is found spontaneously attached by

70 STRUCTURAL AND SYSTEMATIC ZOOLOGY

one end to submerged plants, while the free end con-
tains the orifice, or mouth, crowned with tentacles, by
which the creature feeds and creeps. The body wall
consists of two cellular layers — ectoderm and endo-
derm. These surround a central cavity with one
opening. The animal may be compared to a bag with

a two-layered wall, and
with tentacles around
the opening. It buds,
and also reproduces by
eggs. The buds, when
adult, become detached
from the parent (Figs.
17, 1 8).

In most of the other
Hydrozoa the colony
is permanent, and sup-
ported by a horny
skeleton. There are
two kinds of Polyps in
each colony, one for
feeding and the other

FIG. 17. — Hydra: 2, with tentacles fully extended; for reproduction (Fig.
3, creeping ; 4, with ingested prey : 5, budding. ~ . -

20). Sometimes the

reproductive Polyps are separated from the stock
in the form of little jellyfishes, and are then called
medusae (Figs. 20 m, 21). Belonging to this class
are Hydractinia, found on the shells inhabited by the
hermit crab ; the elk-horn coral (Millepora) ; and the
beautiful Portuguese man-of-war, consisting of a bladder-
like float from the bottom of which depend tentacles
many feet in length and several kinds of polyps, the
tentacles being covered with stinging cells, which aid
in capturing the prey and in defending the colony.

CGELENTERATA

FIG. 18. — Hydra: longitudinal section of ani- FIG. 20. — Campanularian hydroid:

mal showing mt mouth; /, tentacle; d, portion of colony, showing nutritive

digestive cavity; b, bud; s, spermary ; zooids, _/; reproductive zooid, r ;

o, ovary ; ec, ectoderm ; en, endoderm. young zooid, y ; and medusa, m

Magnified. Magnified.

FIG. 19. — Hydroid (Sertularia) growing on a shell.

FIG. 21. — A Medusa, seen in
profile and from below,
showing central manu-
brium, radiating and mar-
ginal canals and tentacles.

72 STRUCTURAL AND SYSTEMATIC ZOOLOGY

CLASS 2. — Scyphozoa

These are jellyfishes which are characterized mainly
by having reproductive glands which discharge their
contents into the stomach whence the reproductive cells
make their way out through the mouth, by having the
gastric cavity much branched and ramifying through
the gelatinous body, and by the presence of filaments
projecting into the gastric cavity.

FIG. 22 — Jellyfish (Pelagia noctiluca). Mediter-
ra nean.

FIG. 23. — Portuguese Man-
of-war (Physalia), \ nat-
ural size. Tropical Atlantic.

The jellyfish has a soft, gelatinous, semitransparent,
umbrella-shaped body, with tubes radiating from the
central digestive cavity to the circumference, and with
the margin fringed with tentacles, which are furnished

CCELENTERATA

73

with stinging thread cells. The radiating parts are in
multiples of four. Around the rim are minute colored
spots, the "eye specks." In fine weather, these "sea
blubbers" are seen floating on the sea, mouth down-
ward, moving about by flapping their sides, like the
opening and shutting of an umbrella, with great
regularity. They are frequently phosphorescent when
disturbed. Some are quite small, resembling little glass
bells ; the common Aurelia is over a foot in diameter
when full-grown ; while the Cyanea, the giant among
jellyfishes, sometimes measures eight feet in diameter,
with tentacles more than one hundred feet long.
The tissues are so watery that, when dried, nothing
is left but a film of membrane weighing only a few
grains.

The two common types are Lucernaria and Aurelia.
The former is the Umbrella-acaleph and has a short
pedicel on the back for
voluntary attachment ;
tentacles disposed in
eight groups around
the margin, the eight
points alternating with

the four partitions Of FIG. z^.—Lucernaria auricula attached to a

piece of seaweed; natural size. The one on
the body CaVlty and the the right is abnormal, having a ninth tuft of

four corners of ' the tentacles'

mouth; not less than eight radiating canals, and no
membranous veil. The common species on the Atlantic
shore, generally found attached to eelgrass, is an inch
in diameter, of a green color. Aurelia, the ordinary
jellyfish, is free and oceanic. It differs from the Lucer-
naria in its usually larger size and solid disk, and in
Having four radiating canals, which ramify and open
into a circular vessel which runs around the margin of
the disk.9

74 STRUCTURAL AND SYSTEMATIC ZOOLOGY

FlG. 25. — Horizontal Section of Actinia
through the stomach, showing septa
or mesenteries, and compartments.

CLASS 3. — Actinozoa -

These marine animals, which by their gay tentacles

convert the bed of the ocean into a flower garden, or by

their secretions build up coral
islands, have a body like
a cylindrical gelatinous bag.
One end, the base, is usually
attached; the other has the
mouth in the center, sur-
rounded by numerous hollow
tentacles, which are covered
with nettling lasso cells. This
upper edge is turned in so as
to form a sac within a sac,
like the neck of a bottle

turned outside in. The inner sac, which is the digestive

cavity, does not reach the bottom, but opens into the

general body cavity

(Fig. 236).10 The

space between these

two concentric tubes

is divided by a series

of vertical partitions

or mesenteries, some

of which extend from

the body wall to the

digestive sac, but oth-
ers fall short of it.

Instead, therefore, of

the radiating tubes

of the Scyphozoan,

there are radiating

spaces. No members of this class are microscopic. All

are long-lived compared with the Hydrozoa, living for

FIG. 26. — Actinia expanded, seen from above,
showing mouth.

CCELENTERATA 75

several years. One kept in an aquarium in England
lived to be more than sixty years old.
There are two subclasses : —

1. Zoantharia, including the sea anemones and the
stony corals, and,

2. Alcyonaria, to which belong the organ-pipe coral
(Tubipord), sea fan (Gorgonia\ the precious red coral
(Corallium), and the sea pen (Pennatula).

Zoantharia usually have numerous tentacles, generally
arranged in multiples of five or six, the tentacles being
unbranched and hollow, while in the Alcyonaria the
tentacles are finely branched and are always eight in
number.

Zoantharia. — The best-known representative of this
group is the Metridium, or sea anemone. It usually
leads a solitary life, though frequently several are found
together, some of which have arisen as buds from the
others. It is capable of a slow locomotion. Muscular
fibers run around the body, and others* cross these at
right angles. The tentacles, which often number over
two hundred, and the partitions, which are in reality
double, are in multiples of six. At night, or when
alarmed, the tentacles are drawn in, and the aperture
firmly closed, so that the animal looks like a rounded
lump of fleshy substance plastered on the rock. It
feeds on crabs and mollusks. It abounds on every
shore, especially of tropical seas. The size varies from
one eighth of an inch to a foot in diameter (Fig. 236).

Alcyonaria. — The most of the animals in this group
grow in branching colonies, the axis consisting of a
horny substance covered with flesh in which spicules
of lime are found. The polyps are usually small.
The sea pen (Pennatula} grows with one end em-
bedded in the mud and sand of the sea bottom. In
Gorgonia, the sea fan, the branches arise in the same

76 STRUCTURAL AND SYSTEMATIC ZOOLOGY

vertical plane and unite to form a beautiful network

(Fig. 35).

Coral. — The majority of Actinozoa secrete a calca-
reous or horny framework called " coral." With few
exceptions, they are fixed and composite, living in colo-
nies formed by a continuous
process of budding. Their struc-
tures take a variety of shapes ;
often domelike, but often resem-
bling shrubbery and clusters of
leaves. The members of a coral
community are organically con-
nected; each feeds himself, yet
is not independent of the rest.
The compound mass is " like a
living sheet of animal matter,
fed and nourished by numerous
mouths and as many stomachs."
•- Life and death go on together,
the old polyps dying below as
new ones are developed above. The living part of an
Astrcea is only half an inch thick. The growth of the
branching Madrepora is about three inches a year. The
colors of the coral polyps are brilliant and varied, being
green, purple, pink, or brown. The organ-pipe coral has
green polyps and crimson skeleton, while the precious
£,sx2\(Corallium) has white polyps and a bright red axis.
Another kind is bright blue. The usual size varies
from that of a pin's head to half an inch, but the
mushroom coral (which is a single individual) may be
a foot in diameter.

Corals are of two kinds : those deposited within the
tissues of the animal (sclerodermic\ and those secreted
by the outer surface at the foot of the polyp (sclero-
basic). The polyps producing the former are actinoid,

FIG. 27. — Organ-pipe coral (Tu

pora ntusica). Indian Ocean.

CCELENTERATA 77

resembling the Actinia in structure.11 The skeleton of
a single polyp (called corallite, Fig. 292) is a copy of
the animal, except the stomach and tentacles, the earthy
matter being secreted within the outer wall and between
each pair of partitions. So that a corallite is a short
tube with vertical septa radiating toward the center.12

FIG. 28. — Madrepora aspera, living and expanded ; natural size. Pacific.

«

A sclerobasic coral is a true exoskeleton, and is dis-
tinguished by being smooth and solid. The polyps,
having eight fringed tenacles, are situated on the out-
side of this as a common axis, and are connected to-
gether by the fleshy ccenosarc covering the coral.

(i) Sclerodermic Corals. — ' Astrcea is a hemispherical mass
covered with large cells. Meandrina, or "brain coral,"

78 STRUCTURAL AND SYSTEMATIC ZOOLOGY

is also globular ; but the mouths of the polyps open into
each other, forming furrows. Fungia, or " mushroom
coral," is disk-shaped, and differs from other kinds in

FIG. 29. — Ctenactis ecktnata, or " Mushroom Coral " : one fourth natural size. Pacific.

being the secretion of a single gigantic polyp, and in
not being fixed. Madrepora is neatly branched, with

FIG. 30. — A strcea pallida, living colony; natural size. Fejee Islands.

pointed extremities, each ending in a small cell about
a line in diameter. Porites, or "sponge coral," is also
branching, but the ends are blunt, and the surface com-

CCELENTERATA

79

paratively smooth. Tubipora, or " organ-pipe coral,"
consists of smooth red tubes connected at intervals by
cross plates. The Astrcea, Meandrina, Madrepom, and
Porites are the chief reef-forming corals. They will
not live in waters whose mean temperature in the
coldest month is below 68° Fahr., nor at greater depth
than about twenty fathoms. The most luxuriant reefs

FIG. 31. — Diploria cerebriformis, or " Brain Coral " ; one half natural size. Bermudas.

are in the central and western Pacific and around the
West Indies.

A coral reef is formed by many corals growing to-
gether. It is to the single coral stock as a forest is to
a tree. The main kinds of reefs are fringing, where
the reef is close to the shore ; barrier, where there is a
channel between reef and shore; encircling, where there
is a small island inside of a large reef ; and coral islands,

80 STRUCTURAL AND SYSTEiMATIC ZOOLOGY

or atolls, where there is simply a reef with no land in-
side of it. The Great Barrier Reef off the east coast of

FIG. 32. — Astrcza rotulosa. West Indies.

-'

FIG. 33. — Cell of Madrepore Coral,
magnified. The cuplike depres-
sion at the top of a coral skele-
ton is called calicle.

FIG. 34. — Fragment of Red Coral (Coral-
Hum rubrutii) , showing living cortex
and expanded polyps. Mediterranean.

Australia is 1250 miles in length. All reefs begin as
fringing reefs, and are gradually changed into the other

CCELENTERATA

8l

forms by the slow sinking of the bottom of the ocean,
or by the death, decay, and disintegration of the corals
on the landward side of the reef, where the food supply
is necessarily restricted.

FIG. 35. — Sea Fan {Gorgonia} and Sea Pen (Pennatula}.

(2) Sclerobasic Corals. — Corallium rubrum, the precious
coral of commerce, is shrublike, about a foot high, solid
throughout, taking a high polish,
finely grooved on the surface, and
of a crimson or rose-red color.
In the living state the branches
are covered with a red ccenosarc
studded with white polyps (Fig.
34).

CLASS 4. — Ctenophora

The CtenopJiora (as the Pleuro-
brachia, Cestum, and Beroe) are
transparent and gelatinous, swim-
ming on the ocean by means of FIG. 36. — A ctenophore

, .. robrachia pileus); natural

eight comblike, ciliated bands, size.
DODGE'S GEN. ZOOL. — 6

82 STRUCTURAL AND SYSTEMATIC ZOOLOGY

which work like paddles. The body is not contractile,
as in the jellyfishes. They are considered the highest of
coelenterates, having a complex nutritive apparatus and
a definite nervous system. There is no trace of a polyp
stage in their development, and they do not form colo-
nies. They are found in all regions of the ocean, from
the arctics to the tropics.

Branch IV. — PLATYHELMINTHES

The group formerly called Vermes or worms was
composed of animals so very different in form and
structure that it has now been subdivided into several
branches, viz. : Platyhelminthes, or flat worms ; Ne-
mathelminthes, or round worms ; Trochelminthes , or
rotifers ; Molluscoida, including the Polyzoa and Bra-
chiopoda ; and Annulata, or segmented worms. All
these forms agree in being distinctly bilaterally sym-
metrical animals, as contrasted with the apparently
radial arrangement of parts seen in the C&lenterata
and EC kino derma ta, and in having the three body layers
— ectoderm, mesoderm, and endoderm — well developed,
the mesoderm or middle layer being relatively of more
importance than in any preceding group.

The Platyhelminthes, or flat worms, include some free
forms, as Planaria, which is common in fresh water,
and the tapeworms and flukes among the parasites.
As a group, they are soft-bodied, flattened animals,
without skeletal parts of any kind. There is no distinct
body cavity nor blood-vascular system nor anal opening.
The digestive system may be entirely absent, as in the
tapeworm, or it may be much branched and highly
complicated in structure, as in the planarians.

The tapeworm ( Tcenia) consists of the so-called head
and the body segments, which are really reproductive

PLATYHELMINTHES

joints. It develops from the egg in the digestive canal
of the pig, burrows into the muscular tissue of the
animal, and there becomes encased. Pork containing

these cysts is called
"measly pork." If
the pork be eaten
by man, in an un-
cooked condition,
this case is dis-
solved by the gas-
tric juice, and the
embryo thus re-
leased attaches it-
self to the intes-

FIG. 37. — Tapeworm (Ttettia soliunt) : a, bead; b, c, d,
segments of the body.

FIG. 38. — Planarian
Worm.

tine by its " head," and develops into the tapeworm by
budding off the reproductive segments, or proglottides.
As these become ripe and filled with fertilized eggs,
they are detached, and pass off with the excrement.

The disease called " rot," in sheep, is produced by the
fluke (Distoma), which grows in the bile ducts of the
sheep.

The flat worms are the most widely distributed of all
animals above the Protozoa. They are found on land,
at various depths in bodies of fresh water, and in the
sea. They also occur as parasites in animals in almost
every class of the Metazoa.

84 STRUCTURAL AND SYSTEMATIC ZOOLOGY

Branch V. — NEMATHELMINTHES

The round, or thread, worms include free forms, as
the vinegar eel ; parasitic forms, as the pin worm
(Ascaris) and trichina ; and forms free when adult, and
parasitic when young, as the hair worm ( Gordius). The
body is usually elongated and cylindrical in shape,
whence the name. In most forms there are
plainly marked digestive and nervous systems.
The trichina is usually derived by man from
11 the flesh of the pig. It

exists in the muscles,
inclosed in micro-
scopic cases or cysts,
composed of calcareous
matter. If the meat be
eaten uncooked or par-
tially cooked, the cases
are dissolved, and the
trichinae become sexu-
ally mature in the in-
testines. The young

FIG. •&.- Trichina spiralis (much enlarged): are pro(Juced and bur-
I, male; a, mouth; c, intestine; II, capsules,
with trichinae in muscle. TOW their Way into the

muscles, usually of the back and limbs, where they be-
come encysted in the muscle fibers. In burrowing they
cause great pain and fever, and sometimes death. The
adult trichina is about -^ of an inch long.

The " jiorse-hair snake," a hair worm {Gordius),
passes the early part of its existence in larval or adult
insects, e.g., the cricket. When mature the worms leave
the body of the insect and lay their eggs in damp places.
The eggs or the immature worms are then taken into
the bodies of other insects in which the parasites later
reach their full development.

MOLLUSCOIDA

Branch VI. • — TROCHELMINTHES

The wheel animalcules, or rotifers, mostly found in
fresh water, are composed of a few ill-defined segments,
and have on the anterior end a disk
which is ciliated on the edge, the
motion of the cilia causing the ap-
pearance of a rotating wheel, whence
the name. They are from 200 to WQ
of an inch long. They have a well-
developed digestive system, the food
consisting of minute organisms, and
a rudimentary nervous system. Roti-
fers have been kept for several years
in a dried condition and have after-
ward been revived (Fig. 40).

Branch VII.

FIG. 40. -r Rotifer, or
'' Wheel animalcule "
(Hydatina}, highly
magnified.

MOLLUSCOIDA

These ani-
mals have gen-
erally a body
cavity, in which lies the alimen-
tary canal, bent in such a man-
ner that the mouth and the anal
opening are close together. Near
the mouth is a curved ridge,
the lophophore, bearing tentacles.
There is a very rudimentary
nervous system (Fig. 41).

The PolyZOa rCSCmble polyps

FiG.4i.-DiagramofaPoly2oan:

Aiophophore bearing tentacles, jn appearance, living in clusters,

// m, mouth; a, digestive cav- • ...

ity; i, intestine: a, anus; e, each individual inhabiting a dell-

excretory organ; b, "brain." ,, , -. ,

Much magnified. cat e cell, or tube, and having a

86 STRUCTURAL AND SYSTEMATIC ZOOLOGY

simple mouth surrounded with ciliated tentacles. The
colony often takes a plantlike form ; sometimes spreads,
like fairy chains or lacework, over other bodies ; or
covers rocks and seaweeds in patches with a delicate
film. The majority secrete carbonate of lime. A poly-
zoan shows its superiority to the coral, which it resem-
bles, in possessing a distinct alimentary canal and a
nervous system. The cells of a group are never con-

FIG. 42. — Polyzoans : i. Hornera lichenoides, natural size. 2. Branch of the same;
magnified. 3. Discopora Skenei, greatly enlarged.

nected by a common tube, as in coelenterates. There
are both marine and fresh-water species.

The Brachiopoda or "lamp shells" have a bivalve
shell, the valves being applied to the dorsal and ventral
sides of the body. The valves are unequal, the ventral
being usually larger, and more convex ; but they are
symmetrical, i.e., a vertical line let fall from the hinge
divides the shell into two equal parts. The ventral
valve has, in the great majority, a prominent beak, per-
forated by a foramen, or hole, through which a fleshy
stalk protrudes to attach the animal to submarine rocks.

ECHINODERMATA 87

The valves are opened and shut by means of muscles,
and in most cases they are hinged, having teeth and
sockets near the beak. The mouth faces the middle of
the margin opposite the
beak ; and on either side of
it is a long fringed " arm,"
generally coiled up, and
supported by a calcareous
framework. The animal,

FIG. 43. — A Brachiopod
(Terebratulina septen-
trionalis} . Atlantic coast.

FIG. 44. — Dorsal Valve of a Brachiopod
(Terebratula) , showing, in descending
order, cardinal process, dental sockets,
hinge plate, septum, and loop supporting
the ciliated arms.

having no gills, respires by the arms and the mantle.
Brachiopods were once very abundant, over two thou-
sand extinct species having been described; but only
about a hundred species are now living.13 These are all
marine, and fixed. The animals in this group are
related to the mollusca.

Branch VIII. — ECHINODERMATA

The echinoderms, as starfishes and sea urchins, are
characterized by the possession of a distinct nervous
system (a ring around the mouth with radiating branches);
an alimentary canal, completely shut off from the body
cavity, having both oral and anal apertures ; a water-
vascular system of circular and radiating canals, con-
nected with the outside water by means of the madre-
poric tubercle, and a symmetrical arrangement of all the

88 STRUCTURAL AND SYSTEMATIC ZOOLOGY

parts of the txody around a central axis in multiples of
five,14 this radial arrangement, however, concealing a
definite bilateral symmetry. They are, thus, much
more highly organized than the coelenterates, with
which group they have very little in common except

FIG. 45. — Forms of Echinoderms, from radiate to annulose type: a, Crinoids: b, Ophi-
urans; c, Starfish; d, Echini; e, Holothurians.

their apparent radial symmetry. In the course of
development in echinoderms metamorphosis occurs, the
larval forms bearing no resemblance to the adults.

There are five principal classes, all exclusively marine
and solitary, and all having the power of secreting more
or less calcareous matter to form the skeleton.

CLASS i. — Asteroidea

Ordinary starfishes consist of a flat central disk, with
five or more arms, or lobes, radiating from it, and con-
taining branches of the viscera. The skeleton is leathery,
hardened by small calcareous plates (twelve thousand by
calculation), but somewhat flexible. The mouth is below ;
and the rays are furrowed underneath, and pierced with

ECHINODERMATA 89

numerous holes, through which pass the suckerlike tenta-
cles— the organs of locomotion and prehension. The red
spots at the ends of the rays are eyes. The usual color
of starfishes is yellow, orange, or red. They abound
on every shore, and are often seen at low tide half

FIG. 46. — Under surface of Starfish (Gom'aster reticulatus) , showing ambulacral
grooves and protruded suckers.

buried in the sand, or slowly gliding over the rocks.
Cold fresh water quickly kills them. They have to a
high degree the power of casting off their rays and of
reproducing the lost parts. They are carnivorous, very
voracious,, and are the worst enemies of the oyster.

90 STRUCTURAL AND SYSTEMATIC ZOOLOGY

About two hundred and fifty species are known. The
common starfish (Asterias) has four rows of feet in each
ray. Solaster, the "sun star," has numerous rays with
two rows of feet in each. Goniaster is somewhat pen-
tagonal in form with feet arranged as in the " sun star."
Asteroidea are found as fossils.

CLASS 2. — Ophiuroidea

These are star-shaped echinoderms with a central
disk and five flexible, jointed arms distinctly marked off

FIG. 47. — Ophiocoma russet, an Ophiuran; natural size. West Indies.

from the disk, the latter containing all the visceral parts.
There is no anal opening and the madreporite is on the

ECHINODERMATA

same side as the mouth. Ambulacral grooves are lack-
ing and the tube feet are rudimentary, locomotion being
effected by movements of the very flexible and muscular
arms.

The brittle Sfor^Qfikiurd) is common along the Atlan-
tic coast. Astrophyton, the " basket fish," has rays which
are very much branched.

CLASS 3. — Echinoidea

These are free echinoderms with a globular or disk-
shaped shell composed of closely-joined calcareous plates.
There are no ambulacral grooves, the tube feet projecting
through openings in the plates arranged either along
meridional lines or in the form of a star-shaped rosette.

The sea urchin is encased in a thin, hollow, spherical
shell covered with spines.15 The mouth is underneath,
and contains a
dental apparatus
more complicated
than that of any
other creature. It
leads to a diges-
tive tube, which
extends spirally
to the summit of
the body. The
spines are for bur-
rowing and loco-
motion, and are
moved by small

mncr>1^o <=» \\ K
JSC ieS, C

ing articulated by

ball-and-socket j'oint to a distinct tubercle. When stripped
of its spines, the shell (or "test") is seen to be formed
of a multitude of pentagonal plates, fitted together like a

4^' ~ Under surface of a Sea Urchin (Echinus escu-
lentus) , showing the mouth, tips of the teeth, and rows
of suckers among the spines. British seas.

92 STRUCTURAL AND SYSTEMATIC ZOOLOGY

mosaic.16 Five double rows of plates, passing from pole
to pole, like the ribs of a melon, alternate with five other
double rows. In one set, called the ambulacra, the plates
are perforated for the protrusion of tubular feet, or
suckers, as in the starfish. So that altogether there are
twenty series of plates — ten ambulacral, and ten inter-
ambulacral. The shell is not cast, but grows by the
enlargement of each individual plate, and the addition
of new ones around the mouth and the opposite pole.
Echini live near the shore, in rocky holes or under sea-
weed. They are less active than the starfishes ; and feed
almost entirely upon seaweed. They reproduce by eggs.
Regular Echini,-as the common Arbacia or purple sea
urchin, and the green sea urchin, are nearly globular,
and the oral and anal openings are opposite. Irregular
Echini, as the Clypeaster, are flat, and the anal orifice is
near the margin, as in the "sand dollar" or "cake
urchin " (Echinarachnius).

CLASS 4. — Holothuroidea

These wormlike " sea slugs," as they are called, have
a soft, elongated body, with a tough, contractile skin

FIG. 49. -Sea Slugs {Holothitria).

containing small calcareous plates. One end is abruptly
terminated, and has a simple aperture for a mouth, en-

ECHINODERMATA 93

circled with feathery tentacles. There are usually five
longitudinal rows of ambulacral suckers, but only three
are used for locomotion, of which one is more developed
than the rest. The mouth opens into a pharnyx lead-
ing to a long intestinal canal extending through the
body. Holothurians have the singular power of eject-
ing most of their internal organs, surviving for some
time the loss of these essential parts, and afterward
reproducing them. They occur on nearly every coast,
especially in tropical waters, where they sometimes
attain the length of three or four feet. As found dn the
beach after a storm, or when the tide is out, they are
leathery lumps, of a reddish, brownish, or yellowish
color. They may be likened to a sea urchin devoid of a
shell, and long drawn out, with the axis horizontal,
instead of vertical. They feed on small animals which
they catch with their tentacles, and upon organic par-
ticles from the sand.

CLASS 5. — Crinoidea

The crinoids, or. "sea lilies," are fixed to the sea
bottom, temporarily or permanently, by means of a hol-
low, jointed, flexible stem. On the top of the stem is
the body proper, resembling a bud or expanded flower,
containing the digestive apparatus, and bearing the
branched arms. The mouth looks upward. There is a
complete skeleton for strength and support, the entire
animal — body, arms, and stem — consisting of thou-
sands of pieces embedded in the tissue of the body.
Crinoids were very abundant in the old geologic seas,
and many limestone strata were formed out of their
remains. They are now nearly extinct : dredging in the
deep parts of the oceans has brought to light a few living
representatives. Pentacrinus is permanently attached,
but the rosy, or feather star, is free during its adult life.

94 STRUCTURAL AND SYSTEMATIC ZOOLOGY

FIG. 50. — A living Crinoid (Pentacrinus asteria) one fourth natural size.
West Indian Seas.

ANNULATA

95

Branch IX. — ANNULATA

The Annelidas include the highest and most special-
ized Worms. They have many segments, spines, or
suckers for locomotion, a supra-esophageal brain, a
ventral chain of ganglia, and usually a closed blood sys-
tem.

There are two principal classes : Chcetopoda, or bristle-
footed worms ; and Hirudinea, or leeches. The former

FIG. 51. — Marine Worm (Cirratulus grandis}, with extended cirri. Atlantic.

class includes the earthworms {Liimbricus and Allolobo-
phora), the sandworm (Nereis), and the lobworm (Areni-
cola).

The earthworm develops from eggs laid, several in a
capsule, in the earth or near refuse heaps, under boards

Q6 STRUCTURAL AND SYSTEMATIC ZOOLOGY

and straw. ' Its cylindrical body consists of numerous
segments, the wall being very muscular and covered
by a tough, smooth, transparent cuticle. The body
cavity is subdivided by numerous transverse membra-
nous partitions. The digestive system extends through-
out the body and there are well-developed nervous and
circulatory systems. The former lies
mainly below the digestive organs and
consists of a nerve cord running the
length of the body. This cord is
composed of pairs of ganglia con-
nected by longitudinal and transverse
branches. Above the mouth opening
is a pair of ganglia forming the
" brain." The larger blood vessels
surround the esophagus and one, the
dorsal vein, lies above the digestive
system and may be seen through the
FIG. 52. - Earthworm, in- skin on the back. There are no well-

ternal anatomy of the ir-i tii«i

anterior region. The body defined sense organs, although, judg-
has been opened along the mg from experiments, the earthworm

dorsal line and the inter-
nal organs turned to the is sensitive to touch, is affected by

left; ph, pharynx; h, ... . ,.

"heart"; /, muscular changes in the intensity of light, and
partitions; «,, waii of ^^ evidence of having the sense of

body; r, reproductive or-
gans (in part) ; dv, dorsal
blood vessel ; i, intestine ;
g, " brain " ; vn, ventral
nerve cord; si, subintes-
tinal blood vessel; sn,
subneural blood vessel.

taste. Respiration is carried on by
the vascular skin, there being no
lungs or gills. Earthworms feed
upon decaying vegetable matter and
upon organic particles contained in the earth, swallowed
in the process of making the burrow, or for the sake of
the contained food. The refuse from the body is piled
up around the mouth of the burrow in the form of pel-
lets. The amount of earth annually brought up from
the deeper layers of the soil is sufficient to be of consid-
erable geological and economic importance.

ARTHROPODA 97

The earthworm belongs to the subclass Oligochceta,
the members of which have but few bristles on each
segment.

Nereis lives in the sea, under rocks and among sea-
weeds. Like the earthworm, it has a distinctly seg-
mented body. There is a well denned head, bearing
sense organs, as eyes and tentacles. The throat is
provided with two protrusible jaws, by means of which
the worm seizes its food, often living prey (Fig. 215).
Each segment bears a pair qf flattened, paddlelike par-
apodia, which enable the worm to swim rapidly. The
arrangement of the digestive, nervous, and circulatory
systems is much like that seen in the earthworm.

Nereis is a member of the subclass Polychceta, which
is characterized by the presence of numerous bristles on
each segment.

The leeches are externally segmented, usually flat-
tened, and have a sucking disk at each end of the body.
The mouth is in the anterior disk and is provided with
three semi-circular, saw-toothed jaws, by means of which
the leech makes the incision through which it sucks the
blood of its prey. The disks are also used for locomo-
tion. The digestive system is very capacious, and some
leeches can live even if not fed more often than once in
two or three months. Leeches are generally fresh-
water animals, though some kinds are found in the sea
and others live on land.

Branch X. — ARTHROPODA

This is larger than all the other branches put together,
as it includes the animals with jointed legs, such as
crabs and insects. These differ widely from the mol-
luscan type in having numerous segments, and in show-
ing a repetition of similar parts ; and from the worms
DODGE'S GEN. ZOOL. — 7

98 STRUCTURAL AND SYSTEMATIC ZOOLOGY

in having jointed appendages and a definite number of
segments.

The skeleton is outside, and consists of articulated
segments or rings. The limbs, when present, are like-
wise jointed and hollow. The jaws move from side to
side. The nervous system consists mainly of a double
chain of ganglia, running along the ventral surface of
d the body under the

alimentary canal.
The brain is con-
nected to the ventral

FIG. 53. — Diagram of the structure of an Arthropod o-Qncrlip hv p rincr

(after Schmeil) : a, antenna ; c, circulatory system; o""^o^ "" ®J ** ' ***§

d, alimentary canal; n, nerve cord; g, ganglion; encircling" the °"ul-
s, skeleton.

let. The alimentary

canal and the circulatory apparatus are nearly straight
tubes lying lengthwise — the one through the center,
and the other along the back. The skeleton is com-
posed of a horny substance (chitin), or of this substance
with carbonate of lime. All the muscles are nearly
always striated.

There are five classes, of which the first almost exclu-
sively is water breathing, having gills, and the others
principally air breathing, being provided with tracheae.

CLASS i. — Crustacea

The Crustacea, with few exceptions, are water breath-
ing Arthropoda, usually with two pairs of antennae.17
Among them are the largest, strongest, and most vora-
cious of the branch, armed with powerful claws and
a hard cuirass, bristling with spines. Although con-
structed on a common type, crustaceans exhibit a won-
derful diversity of external form : contrast, for example,
a barnacle and a crab. We will select the lobster as
illustrative of the entire group.

ARTHROPODA

99

A typical crustacean consists of twenty segments, of
which five belong to the head, eight to the thorax, and
seven to the abdomen.18 In the lobster, however, as in
all the higher forms, the joints of the head and thorax
are welded together into a single piece, called the
c^pkato thorax. On the front of this shield is a pointed
process or rostrum; and attached to the last joint of
the abdomen (the so-called " tail ") is the sole repre-
sentative of a tail
— the telson. The
skeleton is a mix-
ture of chitin and
calcareous matter.19
On the under side
of the body we find
numerous appen-
dages, feelers, jaws,
claws, and legs be-
neath the cephalo-
thorax, and flat
swimmerets under
the abdomen. In
fact, every segment
except the last, car-
ries a pair of mov-
able appendages,
consisting typically

Of a Stalk Or protO- FIG. 54. - Under side of the Crayfish, or Fresh-water

. ' Lobster (Astacus ftnviatilis} : a, first pair of an-

Podlte, bearing tWO tennse; b, second pair; c, eyes; d, opening of

v i ,1 kidney; e, foot jaws; f, g, first and fifth pair of

branches, the eXOpO- thoracic legs; A, swimmerets; i, anus; k, caudal

dite and the endopo- fin-

dite. The five segments of the head are compressed
into a very small space, yet have the following mem-
bers : 2° the short and the long antennae ; the mandibles,
or jaws, between which the mouth opens; and the

k

100 STRUCTURAL AND SYSTEMATIC ZOOLOGY

two pairs of maxillae. The thorax carries three pairs
of modified limbs, called "foot jaws," and five pairs
of legs. The foremost legs, "the great claws," are
extraordinarily developed, and terminated by strong
pincers (chelce). Of the four slender pairs succeeding,
two are furnished with claws, and two are pointed. The
last pair of swimmerets, together with the telson, form
the caudal fin — the main instrument of locomotion ;
the others (called " swimmerets ") are used by the
female for carrying her eggs. The eyes are raised on
stalks, so as to be movable (since the head is fixed to
the thorax), and are compound, made up of about two

FIG. 55. — Internal anatomy of the Crayfish: em, extensor muscle of abdomen; Jm,
flexor muscle of abdomen; m, mouth; cs, cardiac portion of stomach; ps, pyloric
portion of stomach; dg, digestive gland; kt heart; r, reproductive gland; i, intes-
tine; g, " brain " ; vc, ventral nerve cord.

thousand five hundred square facets. On the base of
each small antenna is a minute sac, whose mouth is
guarded by hairs : this is the organ of hearing. The
gills, twenty on a side, are situated at the bases of the
legs and inclosed in two chambers, into which water is
freely admitted, in fact, drawn, by means of a curious
attachment to one of the maxillae, which works like
a paddle or scoop. The heart is a single oval cavity,
and drives arterial blood — a milky fluid full of corpus-
cles. The alimentary canal consists of a short gullet,

ARTHROPGbA, ; : ;••; .";,-,

a gizzardlike stomach containing teeth, and a straight
intestine.

Crustaceans pass through a series of strange metamor-
phoses before reaching their adult form. They also
periodically cast the shell, or molt, every part of the
integument together with the lining of the gullet and
stomach being renewed ; and another remarkable endow-
ment is the spontaneous rejection of limbs and their
complete- 'restoration. Many species are found in
fresh water, but the class is essentially marine and
carnivorous.

Of the numerous orders of this great class we will
mention only the following : —

1 . Phyllopoda ; small, almost microscopic, aquatic
Crustacea, with the appendages showing very little

differentiation, no gastric teeth, the
body distinctly segmented and cov-
ered by a cephalic shield. The
a appendages posterior to the head
are leaflike, hence the name of the

FIG. 56. - Water Fleas: a,

Cyclops (after Vosseier) ; order. Included here are the brine

b, Daphnia (after Vos- . . f . . N ' .

seier); c, Cypris (after shrimp (Artemia), and the fresh-
water forms Branchipus and Daph-
nia, the bivalve shell of the latter giving it the appear-
ance of a mollusk (Fig. 56).

2. Ostracoda; minute Crustacea with an unsegmented
body inclosed in a bivalve carapace or shell. This
order is represented in fresh water by Cypris (Fig. 56).

3. Copepoda ; . mostly of small size, with an elongated
and, usually, a distinctly segmented body without dorsal
shell. In this order belong the fish lice, and the water
flea (Cyclops} of fresh water, the female of which is
often seen darting about in aquarium jars bearing its
two egg masses attached to the abdomen (Fig. 56).

4. Cirripedia ; marine Crustacea, imperfectly seg-

2: .STWGTURA-L AND SYSTEMATIC ZOOLOGY

mented, and fixed or parasitic in adult life, growing
head downward in their shell. The feathery, branched,

FlG. 57. — Barnacles, or Pedunculate Cirripedes (Lepas anatijera).

thoracic feet are protruded through the opening of the
shell to grasp particles of food. Lepas, the ship
barnacle, grows attached to floating timber and the
bottoms of ships by a long, leathery
stalk (Fig. 57). The acorn shells
(Balamis} grow on rocks between
tide marks, their white, conical
" shells " forming an incrusting
layer on the rock (Fig. 58).

5. Decapoda ; large, highly or-
ganized crustaceans, having usually
a thorax of eight and an abdomen
of seven segments, the anterior re-
gions of the carapace united to form a cephalothorax ;
the eyes are borne on stalks and the gills are thoracic.

FIG. 58. — Acorn Shells (Ba-
lanus} on the shell of a whelk
(Bucctnum),

ARTHROPODA

103

There are ten legs. Here belong the lobster (Homarus)
(Fig. 59), and "crayfishes (Astacus and Cambarus) (Fig.

FIG. 59. — Lobster (Homarus -vulgaris)

FIG. 60. — Swimming Crab (Platyonychus).

54), prawn (Palczmon), shrimp (Crangori), hermit crab
(Pagurus), and crab (Platyonychus) (Fig. 60).

IO4 STRUCTURAL AND SYSTEMATIC ZOOLOGY

Crabs differ from lobsters chiefly in being formed
for creeping at the bottom of the sea instead of swim-
ming, and in the reduction of the
abdomen or " tail " to a rudiment,
which folds into a groove under
the enormous thorax. They are
the highest and largest of living
Crustacea : they have been found
at Japan measuring twenty feet
between the tips of the claws.

6. Arthrostraca ; with the tho-
rax reduced to six or seven seg-
ments owing to the fusion of one
or two of the anterior thoracic
segments with the head ; eyes
usually sessile. This order in-
eludes the wood louse or sow

U. S. coast. .

bug (Omsciis) found in damp

places, the slaters (Idotea) (Fig. 61), and the sand
fleas (Gammarus).

FIG. 62. — Antpkithoe maculata : a sand flea.

CLASS 2. — Onycophora

This class includes only a single genus, Peripatus,
the species of which are all terrestrial, living in damp
places, and are confined mainly to the Southern Hemi-
sphere. Peripatus is a cylindrical, soft-bodied animal
resembling a caterpillar, though the body is not seg-
mented. There is a plainly marked head bearing a
pair each of eyes, antennae, and jaws. The body is

ARTHROPODA 105

supported on many pairs (fourteen to forty-two, accord-
ing to the species) of short, fleshy appendages which
are not jointed. These animals are chiefly of interest
because of the fact that in certain features of structure,
as the size of the brain, the presence of tracheae, the

FIG. 63. — Perifatus ; natural size.

arrangement of the circulatory system, and the clawed
appendages, and in their mode of development, they
resemble the Arthropods, while in other respects, espe-
cially as regards the excretory and nervous systems
they approach the Annulata and the Flatworms, Thus,
the class serves to connect the Arthropods and the
" Worms."

CLASS 3. — Myriapoda

Myriapods are air-breathing Arthropods having the
body divided into similar segments, so that thorax and
abdomen are scarcely distinguishable. They resemble
worms in form and in the simplicity of their nervous
and circulatory systems ; but the skin is stiffened with
chitin, and the legs (indefinite in number) are articu-
lated. The legs resemble those of insects, and the
head appendages follow each other in the same order
as in insects — eyes, antennae, mandibles, maxillae, and
labium. They breathe by tracheae, and have two
antennae and a pair of eyes.

There are two important orders : —

i. Chilopoda, characterized by haying a flattened body
composed of about twenty segments, each carrying one
pair of legs, of which the hindermost is converted into

106 STRUCTURAL AND SYSTEMATIC ZOOLOGY

spines. They have longer antennae than the preceding,
and the mouth is armed with two formidable fangs con-
nected with poison glands. They*are carnivorous and
active. Such is the Centipede (Scolopendra, Fig. 80).

2. Diplopoda, having a cylindrical body, each segment,
except the anterior, being furnished with two pairs of
legs. They are slow of locomotion, harmless, and vege-
tarian. The thousand-legged worm (Julus) is a common
representative.

CLASS 4. — Insecta

Insects are distinguished by having head, thorax, and
abdomen distinct, three pairs of jointed legs, one pair of
antennae, and generally two pairs of wings. The number
of segments in the body never exceeds twenty. The
head, apparently one, is formed by the union of four
segments. The thorax consists of three, — the prothorax,
mesothorax, and metathorax, — each bearing a pair of
legs; the wings, if present, are carried by the last two
segments (Fig. 295). The abdomen is usually composed
of ten segments, more or less movable upon one another.
The skin is hardened with chitin, and to it, as in all
Arthropods, the muscles are attached. All the append-
ages are hollow.

The antennae are inserted between or in front of the
eyes. There is a great variety of forms, but all are
tubular and jointed. They are supposed to be organs
of touch, and seem also to be sensitive to sound and odor
(Fig. 344). The eyes are usually compound, composed
of a large number of hexagonal corneae, or facets (from
fifty in the ant to many thousands in the winged insects)
(Fig. 352). They are never placed on movable stalks, as
the lobster's. Besides these, there are three simple eyes,
called ocelli. The mouth may be fitted for biting (masti-
catory), as in beetles, or for sucking (suctorial), as in

ARTHROPODA

ID/

butterflies. The masticatory type, which is the more
complete, and of
which the other is ~

but a modification,
consists of four
horny jaws (mandi-
bles and maxilla)
and an upper and
an under lip (lab mm
and labium). Sensi-
tive palpi (maxillary
and labial} are de-
veloped from the
lower jaw and lower
lip. The labium is
also prolonged into
a ligula, or tongue
(Figs. 219,220,221).
The legs are in-
variably six in the
adult, the fore legs
directed forward
and the hinder pairs
backward. Each
consists of a hip,
thigh, shank, and
foot.21 Some larvae
have also " false
legs," without joints,
on the abdomen,
upon which they
chiefly rely in loco-
motion (Fig. 73).
The wings are ex-

— Under surface of a Beetle (Harpalus caligi-
nosus)'. a, ligula; b, paraglossae; c, supports of
labial palpi; d, labial palpus; ef mentum;/", inner
lobe of maxilla; .g, outer lobe; h, maxillary palpus;
/, mandible; k, buccal opening; /, gula, or throat;
MI, buccal sutures; «, gular suture; o, prosternum;
/, episternum of prothorax; /', epimeron; q, g' , g",
coxae; r, r, r, trochanters; s, s' , s", femora, or
thighs; t, t,' t", tibiae; v, ventral abdominal seg-
ments; w, episterna of mesothorax; x, mesoster-
num; y, episterna of metathorax; y', epimeron; z,
metasternum.

pansions of the crust, stretched over a network of horny

108 STRUCTURAL AND SYSTEMATIC ZOOLOGY

tubes (Fig. 278). The venation, or arrangement of
these tubes (called veins and veinlets), particularly in
the fore wings, is peculiar in each genus. In many
insects, the abdomen of the female ends in a tube which
is the sheath of a sting, as in .the bee, or of an ovipositor,
or " borer," as in the ichneumon, by means of which the
eggs are deposited in suitable places.

Cephalization is carried to its maximum in this class,
and we have animals of the highest instincts under the
articulate type. The "brain " is formed of several gan-
glia massed together, and lies across the upper side
of the throat, just above the mouth. The main nerve
cord lies along the ventral side of the body, and bears
several large ganglia ; besides this, there is a visceral
nerve representing, in function, the sympathetic system
of vertebrates. The digestive apparatus consists of a
pharynx, gullet (to which a crop is added in the fly,
butterfly, and bee tribes), gizzard, stomach, and intestine
(Figs. 239, 240, 241). There are no absorbent vessels,
the chyme simply transuding through the walls of the
canal. The blood, usually a colorless liquid, is driven
by a chain of hearts along the back, i.e., by a pulsating
tube divided into valvular sacs, ordinarily eight, which
allow the current to flow only toward the head. As it
leaves this main pipe, it escapes into the cavities of the
body, and thus bathes all the organs. Although the
blood does not circulate in a closed system of blood
vessels, as in vertebrates, yet it always takes one set of
channels in going from the heart, and another in return-
ing. Respiration is carried on by tracheae, a system of
tubes opening at the surface by a row of apertures
{spiracles), generally nine on each side of the body (Figs.
276, 277, 278).

The sexes are distinct, and the larvae are hatched from
eggs. As a rule, an insect, after reaching the adult, or

ARTHROPOD A

109

imago state, lives from a few hours to several years, and
dies after the process of reproduction. Growth takes
place only during larval life;- and all metamorphoses
occur then. Among the social tribes, as bees and ants,
the majority (called "workers ") do not develop either sex.

Insects (the six-footed arthropods) comprise about
one half of the whole animal kingdom as known, more
than two hundred and fifty thousand species having been
described. They may be grouped into seven principal
orders : —

i. Orthoptera have four wings : the front pair some-
what thickened, narrow, and overlapping along the back ;

FIG. 65. — Metamorphosis of a Cricket (Gryllus).

the hind pair broad, net veined, and folding up like a fan
upon the abdomen. The hind legs are usually large, and
fitted for leaping, all the species being terrestrial, although
some fly as well as leap. The eyes are small, the mouth
remarkably developed for cutting and grinding. The lar-

1 10 STRUCTURAL AND SYSTEMATIC ZOOLOGY

vae and pupae are active and resemble the imago. They
are nearly all vegetarian. Each family produces char-
acteristic sounds (stridulation). About ten thousand
species have been described. The representative forms
are crickets (Gryllus), locusts (Melanoplus\ grasshoppers
(Orchelimum), walking sticks {Diapheromera\ and cock-
roaches {Periplaneta).

2. Neuroptera have a comparatively long, slender body,
and four large, transparent wings, nearly equal in size,

FIG. 66. — Metamorphosis of an Hemipter, Water Boatman (Notonecta).

membranous and lacelike. The mouth parts are adapted
for biting. Among them are the brilliant dragon flies,
or devil's darning needles (Libelhila), well known by the
enormous head and thorax, large, prominent eyes (each
furnished with twenty-eight thousand polished lenses),
and scorpionlike abdomen ; the delicate and short-lived
May flies {Ephemera) ; caddis flies (Pkryganea), whose
larvae live in a tubular case made of minute stones,
shells, or bits of wood ; the horned corydalis (C&rydalis^
of which the male has formidable mandibles twice as
long as the head; and the white ants (Termes) of the
tropics.

ARTHROPODA

III

FIG. 67. — Seventeen-year Cicada (Cicada septendecim): a, pupa; b, the same, after
the imago, c, has escaped through a rent in the back; d, holes in a twig, where the
eggs, e, are inserted.

FIG. 68. — Dragon Fly (Ltiellula).

112 STRUCTURAL AND SYSTEMATIC ZOOLOGY

3. Hemiptera, or "bugs," are chiefly characterized by
a suctorial mouth, which is produced into a long, hard
beak, in which mandibles and maxillae are modified into
bristles and inclosed by the labium. The four wings are
irregularly and sparsely veined, sometimes wanting. The
body is flat above, and the legs slender. The larva differs
from the imago in wanting wings. In some species the
fore wings are opaque at the base and transparent at the
apex, whence the name of the order. Some feed on
the juices of animals, others on plants. Here belong the
wingless bed bug (Cimex) and louse (Pediculus), the
squash bug (Anasd), water boatman (Notonectd), seven-
teen-year locust (Cicada), cochineal (Coccus), and plant
louse (Aphis). More than twenty thousand species are
known.

4. Dipt era, or " flies," are characterized by the rudi-
mentary state of the hinder pair of wings. Although
having, therefore, but one available pair, they are gifted
with the power of very rapid flight. While a bee moves
its wings one hundred and ninety times a second, and a

FIG. 69. — Metamorphosis of the Flesh Fly (Sarcophaga carnaria) : a, eggs; b, young
maggots just hatched: c, d, full-grown maggots; e, pupa; f, imago.

butterfly nine times, the house fly makes three hundred
and thirty strokes. A few species are wingless. The
eyes are large, with numerous facets. In some forms, as
the house fly, all the mouth parts, except the labium, are
rudimentary ; and the labium has an expanded tip, by
means of which the fly licks up its food. In other forms,
as the mosquito, the other mouth parts are present as
bristles or lancets, fitted for piercing; the thorax is
globular, and the legs slender. The larvae are footless

ARTHROPODA 113

grubs. The Diptera number about forty thousand.
Among them are the mosquitoes (Culex)\ Hessian fly
(Cecidomyid), so destructive to wheat; daddy longlegs
or crane fly (Tipula), resembling a gigantic mosquito;
the wingless flea (Pulex) ; besides the immense families
represented by the house fly (Mused) and bot fly (CEstrus).
5. Lepidoptera, or " butterflies " and "moths," are
known chiefly by their four large wings, which are
thickly covered on both sides by minute, overlapping
scales. The scales are of different colors, and are often
arranged in patterns of exquisite beauty. They are in
reality modified hairs, and every family has its particular

FIG. 70. — Scales from the Wings of various FIG. 71. — Part of the Wing of a Moth

Lepidoptera. (Santia), magnified to show the

arrangement of scales.

form of scale. The head is small, and the body cylin-
drical. The legs are of but little use for locomotion.
All the mouth parts are nearly obsolete except the maxil-
lae, which are fashioned into a " proboscis " for pumping
up the nectar of flowers. The larvae, called "cater-
pillars," have a wormlike form, and from one to five
pairs of abdominal legs, or " false legs,",, in addition to
the three on the thorax. The mouth is formed for mas-
tication, and (except in the larvae of butterflies) the lip
has a spinneret connected with silk glands (Fig. 75).

There are two groups : the gay butterflies, having
knobbed or hooked antennae, and flying in the day only,
forming one group ; and the moths, which generally
DODGE'S GEN. ZOOL. — 8

114 STRUCTURAL AND SYSTEMATIC ZOOLOGY
prefer the night, and whose antennae are threadlike and

FIG. 72. — Vanessa polychloros, or " Tortoise-shell Butterfly."

often feathery, composing the second group. To this
belong the dull-colored Sphinges or " hawk moths,"

FlG. 73. — Moth and Larva of Attacus pavonia-major .

which have antennae thickened in the middle, and which

ARTHROPODA

fly at twilight. Generally, when at rest, the butterflies
keep their wings raised vertically, while the others hold

FIG. 74. — Fruit Moth (Carpocapsa pomonella) : b, larva; a, chrysalis; c, imago.

theirs horizontally. The pupa of the former is unpro-
tected, and is usually suspended by a bit of silk ; the
pupa of the moths is in-
closed in a cocoon.

From twenty-two thou-
sand to twenty-five thou-
sand lepidopterous species
have been identified. Some
of the most common but-
terflies are the swallow-tail
Papilio, the white Pieris,
the sulphur-yellow Colias ;
the Argynnis, with silver
spots on the under side of
the hind wings; the Va-
nessa, with notched wings.

The Sphinges exhibit little FlG 75-- Head of a Caterpillar, from be-

0 neath: a, antennae; b, horny Jiws; c,

Variety. They have

thread of silk from the conical fusulus,
,. , , on either side of which are rudimentary

row, powerful wings, and palpi. Magnified.

Il6 STRUCTURAL AND SYSTEMATIC ZOOLOGY

are sometimes mistaken for humming-birds. The
"potato worm" is the caterpillar of a sphinx. The
most conspicuous moths are the large and beautiful
Telea, distinguished by a triangular, transparent spot
in the center of the wing ; the white Bombyx, or " silk-
worm ; " the reddish-brown Clisiocampa, whose larva,
" the American tent caterpillar," spreads its web in many
an apple and cherry tree ; the pale, delicate Geometrids ;
and the small but destructive Tineids, represented by
the clothes moth.

6. Coleoptera, or "beetles." This is the largest of
the orders, the species numbering about ninety thousand.

FIG. 76. — a, imago, and b, larva, of the Goldsmith Beetle (Cotalpa lanigera) ; c, pupa
of June Bug (Lachnosterna/usca).

They are easily recognized by the elytra, or thickened,
horny fore wings, which are not used for flight, but
serve to cover the hind pair. When in repose, these
elytra are always united by a straight edge along the
whole length. The hind wings, when not in use, are
folded transversely. The mandibles are well developed,
and the integument generally is hard. The legs are
strong, for the beetles are among the most powerful
running insects. The larvae are wormlike, and the pupa
is motionless. The highest tribes are carnivorous. The
most prominent forms are the savage but beautiful tiger

ARTHROPODA

117

beetles (Cicindela)\ the common ground beetles (Har-
palus\ whose elytra bear parallel ridges; the diving
beetles (Dytiscus), with boat-shaped body, and hind
legs changed into oars; the carrion beetles (Silpha\

FIG. 77. — Sexton Beetles (Necrophorus vespillo), with larva and nymph. They are
burying a mouse, preparatory to laying their eggs in it.

distinguished by their black, flat bodies and club-shaped
antennae ; the goliath beetles (Goliatkus), the giants of
the order ; the click beetles (Alans) ; the lightning
bugs (Pyrophorus); the spotted lady-birds (Coccinella);
the showy, long-horned beetles (Cerambycidce}\ and

Il8 STRUCTURAL AND SYSTEMATIC ZOOLOGY

the destructive weevils (furculionida), with pointed
snouts.

FIG. 78. — Metamorphosis of the Mosquito {Cnlex pipiens).

7. Hymenoptera, comprising at least thirty-five thou-
sand species, include the highest, most social, and, we

ARTHROPODA 1 19

may add (if we except the silkworm), the most useful, of
insects. They have a large head, with compound eyes
and three ocelli, mouth fitted both for biting and lapping,
legs formed for locomotion as well as support, and four
wings equally transparent, and interlocking by small
hooks during flight. The females are usually provided
with a sting, or borer. The larvae are footless, helpless
grubs, and generally nurtured in cells, or nests. Such
are the honey bees (Apis], humble bees (Bombus\
wasps ( Vespa\ ants {Formica), ichneumon flies, and gall
flies. Those living in" societies exhibit three castes :
females, or " queens " ; males, or " drones " ; and neu-
ters, or sexless " workers." • There is but one queen in a

a

FIG. 79. — Honey bee (Apis mellifica) : a, female; b, worker; c, male.

hive, and she is treated with the greatest distinction, even
when dead. She dwells in a large, pear-shaped cell,
opening downward. She lays three kinds of eggs : from
the first come forth workers, the second produces males,
and the last females. The drones, of which there are
about eight hundred in an ordinary hive, are marked by
their great size, their large eyes meeting on the top of
the head, and by being stingless. The workers, which
number twenty to one drone, are small and active, and
provided with stings, and hollow pits on the thighs,
called " baskets," in which they carry pollen. Their
honey is nectar elaborated in the crop by an unknown
process ; while the wax is secreted from the sides of the
abdomen and mixed with saliva. There is a subdivision

120 STRUCTURAL AND SYSTEMATIC ZOOLOGY

of extra labor : thus there are wax workers, masons, and
nurses. Ants (except the Saiiba) have but two classes
of workers. While ants live in hollow trees or subterra-
nean chambers (called formicarium\ honey bees and
wasps construct hexagonal cells. The comb of the bee
is hung vertically, that of the wasp is horizontal.

CLASS 5. — Arachnida

The arachnids are closely related to the crustaceans,
having the body divided into a cephalothorax and abdo-
men.22 To the former are attached eight legs of seven
joints each ; the latter has no locomotive appendages.
The head carries two, six, or eight eyes, smooth and ses-
sile (i.e., not faceted and stalked, as in the lobster), and
approaching the eye of the vertebrates in the complete-
ness and perfection of their apparatus. There are no
antennae, the first pair of appendages on the cephalo-
thorax being modified into grasping organs. They are
all air breathers, having spiracles which open either into
air sacs or tracheae. The young of the higher forms un-
dergo no metamorphosis after leaving the egg.

Arachnids number nearly five thousand species. The
typical forms may be divided into three groups : —

i. Scorpionida, or scorpions, characterized by very
large maxillary palpi ending in forceps, and a prolonged,
jointed post-abdomen. The nervous and circulatory sys-
tems are more highly organized than those of spiders ;
but the long, tail-like post-abdomen and the abnormal
jaws place them in a lower rank. The abdomen consists
of twelve segments : the anterior half is as large as the
thorax, with no well-marked division between ; the other
part is comparatively slender, and ends in a hooked sting,
which is perforated by a tube leading to a poison sac.
The mandibles are transformed into small, nipping claws,
and the eyes generally number six. Respiration is car-

ARTHROPODA

121

ried on by four pairs of pulmonary sacs which open on
the under surface of the abdomen. The heart is a
strong artery, extending along the middle of the back,
and divided into eight separate chambers. Scorpions
are confined to the warm-temperate and tropical regions,
usually lurking in dark, damp places.

FIG. 80. — Scorpion (under surface) and Centipede.

2. Phalangida, the harvest men, or " granddaddy
longlegs " (P/ialangium), frequently seen about our
houses, belong to this order. They have a short, thick,
unsegmented body, extremely long legs, and no spinning
glands.

3. Araneida, or spiders. They are distinguished by
their soft, unjointed abdomen, connected to the thorax
by a narrow neck, and provided at the posterior end
with two or three pairs of appendages, called "spinner-
ets," which are homologous with legs. The office of
the spinnerets is to reel out the silk from the silk glands,

122 STRUCTURAL AND SYSTEMATIC ZOOLOGY

the tip being perforated by a myriad of little tubes,
through which the silk escapes in excessively fine threads.
An ordinary thread, just visible to the naked eye, is the
union of a thousand or more of these delicate streams of
a fluid which, like collodion, hardens on exposure to the
air.23

The mandibles are vertical, and end in a powerful
hook, in the end of which opens a duct from a poison

FIG. 81. — A, female Spider; B, male of same species; C, arrangement of the eyes.

gland in the head (Fig. 216). The maxillae, or " palpi,"
which in scorpions are changed to formidable claws, in
spiders resemble the thoracic feet, and are often mis-
taken for a fifth pair. The brain is of larger size, and
the whole nervous system more concentrated than in the
preceding order. There are generally eight simple
eyes, rarely six. They breathe both by tracheae and

ARTHROPODA 123

lunglike sacs, from two to four in number, situated under
the abdomen. All the species are carnivorous.

The instincts of spiders are of a high order. They
are, perhaps, the most wily of arthropods. They display
remarkable skill and industry in the
construction of their webs ; and some
species (called "mason spiders") even
excavate a subterranean pit, line it with
their silken tapestry, and close the en-
trance with a lid which moves upon a

, . n\ ritf. 02. — spinnerets

hinge/* of the spider, b,c; a,

4. Acarida, represented by the mites palpiform organs'
and ticks. They have an ov-al or rounded body, without
any marked articulations, the head, thorax, and abdomen
being apparently merged into one. They have no brain ;

only a single ganglion
lodged in the abdomen.
They breathe by tracheae
FIG. 83. -A Mite (DemodexfoincJtio- ™ through the skin. The

rum), one of the lowest Arachnids; a niOUth is formed for SUC-
parasite in human hair sacs ; X 125.

tion, and they are generally

parasitic. The mites (Sarcoptes) are among the lowest
of articulates. The body is soft and minute. The ticks
(Ixodes) have a leathery skin, and are sometimes half
an inch long. The mouth is furnished with a beak
for piercing the animal it infests.

5. Xiphosura, Arachnida with a broad carapace cov-
ering the cephalothorax, an abdomen consisting of seven
firmly united segments ending with a long slender tail
of one piece, five pairs of legs on the cephalothorax;
the abdomen with five pairs of platelike respiratory
organs covered anteriorly by an operculum. The king
crab or horseshoe crab (Limtilus), found on muddy
bottoms along the coast, belongs in this order, which is
interesting as containing the only living representatives

124 STRUCTURAL AND SYSTEMATIC ZOOLOGY

of the extinct trilobites. Limulus was formerly classified
among the Crustacea, but is now considered to have its
closest affinities among the Arachnida.

Branch XI. — MOLLUSC A

A mollusk is a soft-bodied animal, without internal
skeleton, and without segmentation of body or of parts,
covered with a moist, sensitive, contractile skin, which,
like a mantle, loosely envelops the creature. In some
cases the skin is naked, but generally it is protected by
a calcareous covering (shell). The length of the body
is less in proportion to its bulk than in other animals.
The lowest class has no distinct head. The nervous
system consists of three well-developed pairs of ganglia,
which are principally concentrated around the entrance
to the alimentary canal, forming a ring around the
throat. The other ganglia are, in most cases, scattered
irregularly through the body, and in such the body is
unsymmetrical (Figs. 331, 332). The digestive system
is greatly developed, especially the " liver," as in many
aquatic animals (Figs. 242, 243). Except in the cepha-
lopods, the muscles are attached to the skin, or shell.
There is a heart of two chambers (auricle and ventricle)
or three (two auricles and ventricle). As in all inverte-
brates, the heart is arterial. In mollusks, with rare
exceptions, we find no repetition of parts along the
antero-posterior axis. They are best regarded as
"worms" of few segments, which are fused together
and much developed. The total number of living species
probably exceeds twenty thousand. The great majority
are water breathers, and marine ; some are fluviatile or
lacustrine, and a few are terrestrial air breathers. All
bivalves, and nearly all univalves, are aquatic. Each
zone of depth in the sea has its particular species. The
most important classes are now to be described.

MOLLUSCA

125

CLASS i. — Pelecypoda

These mollusks, formerly called lamellibranchs, are
all ordinary bivalves, as the oyster and clam. The
shells differ from those of brachiopods in being placed
on the right and left sides of the body,
so that the hinge is on the back of the
animal, and in being unequilateral and
equivalved.25 The umbo, or beak, is
the point from which the growth of the
valve commences. Both brachiopods
and pelecypods are headless; but in FIG. 84. -Pearl Oyster

. , . (Meleagrina mar gar i-

tne latter the mouth points the same u/eray, one fourth nat-
way as the umbo, i.e., toward the uralsize" Ceylon'
anterior part. The length of the shell is measured
from its anterior to its posterior margin, and its breadth
from the dorsal side, where the hinge is, to the opposite,
or ventral, edge. The valves are united to the animal
by one muscle (as in the oyster), or two (as in the clam),
and to each other by a hinge. In some species, as some
fresh-water mussels, the hinge is simply an elastic liga-
ment, passing on the outside from one
valve to the other just behind the beak,
so that it is stretched when the valves
are closed. Another is placed between
the edges of the valves, so that it is
squeezed as they shut, like the spring
in a watch case. Such bivalves are
said to be edentulous. But in the
majority, as the clam and the fresh-
water Unio, the valves also articulate
by interlock-parts called teeth. The
valves are, therefore, opened by the
ligaments, and closed by the muscles.
The shell is secreted by the mantle.

FIG. 85. — Salt - water
Mussel (Mytilus pel-
lucidus). Atlantic
coasts.

126 STRUCTURAL AND SYSTEMATIC ZOOLOGY

Lamellibranchs breathe by four hollow, platelike gills
(whence the name), two on each side underneath the
mantle (Fig. 275), the water being drawn into the cavi-
ties in the gills by the action of ciliated cells. In the
higher forms, the margin of the mantle is rolled up into
two tubes, or siphons, for the inhalation and exhalation
of water. They feed on microscopic organisms gathered
from the water by the ciliated inner surface of the mantle,
the cilia producing a flow of particles toward the mouth.

CL

FIG. 86. — Lamellibranch (Mactra): a, foot; b, c, siphons.

A few are fixed ; the oyster, e.g. habitually lying on its
left valve, and the salt-water mussel hanging to the rocks
by a cord' of threads called " byssus " ; but most have a
" foot," by which they creep about. Unlike the oyster,
also, the majority live in an erect position, resting on the
edges of their shells. About five thousand living species
are known. These are fresh-water and marine, and range
from the shore to a depth of a thousand feet.

The chief characters for distinguishing lamellibranchs
are the muscular impressions,26 whether one or two ; the
presence of a pallial sinus, which indicates the possession
of siphons ; the structure of the gills, and the symmetry
of the valves (Fig. 296).

The following are the more important orders, classi-
fied according to gill structure : — -

MOLLUSCA 127

1. Filibranchia, with two pairs of platelike gills, the
filaments being V-shaped, usually two adductor muscles
of which the anterior is often the smaller or may even
be absent, sea mussel (Mytilns) (Fig. 85).

2. Pseudo-lame llibranchia, with gills showing vertical
folds, a single, large (posterior) adductor muscle, the
shell frequently inequivalve, oyster (Ostrea) (Fig. 242),
scallop (Pec fen), pearl oyster (Meleagrina) (Fig. 84).

3. Eulamellibranchia, with gills smooth or vertically
plaited and with two adductor muscles of equal size,
fresh-water mussel (Unio and Anodontd), cockle (Car-
dium) (Fig. 87), quahog (Venus), shipworm (Teredo),
and common clam (Mya).27

CLASS 2. — :Amphineura

The animals in this class were formerly placed among
the Gastropoda, but are now considered to be sufficiently
distinct to be grouped by them-
selves. They are bilaterally
symmetrical, elongated mollusks,
with a shell consisting of eight
separate pieces, or else entirely
lacking. The mantle is not di-
vided into paired lobes as in the
bivalves. Chiton, a sluggish ani-
mal with the habit of the limpet,

FIG. 87. — Cockle (Cardznm cos-
1S One Of the beSt-knOWn forms tatuni) \ one third natural size.

(Fig. ioo). The shell-less mem- Chinaseas-

bers of the class are the lowest in organization of all

of the mollusks.

CLASS 3. — Gastropoda

The snails are, with rare exceptions, all univalves.28
The body is coiled up in a conical shell, which is usually
spiral, the whorls passing obliquely (and generally from

128 STRUCTURAL AND SYSTEMATIC ZOOLOGY

right to left),29 around a central axis, or " columella "
(Fig. 297). When the columella is hollow (perforated),
the opening in the end is called the " umbilicus." When
the whorls are coiled around the axis in the same plane,
we have a discoidal shell, as the Planorbis. The mouth,
or " aperture," of the shell is " entire " in most vegetable-
feeding snails, and notched or produced into a canal for

FIG. 88. — Whelk {Buccinutri) , showing operculum, o, and siphon, s.

the siphons in the carnivorous species. The former are
generally land and fresh-water forms, and the latter all
marine. In some gastropods, as the river snails and
most sea snails, a horny or calcareous plate (operculum}
is secreted on the foot, which closes the aperture when
the animal withdraws into its shell. In locomotion, the
shell is carried with the apex directed backward.

The body of most gastropods is unsymmetrical, the
organs not being in pairs, but single, and on one side,
instead of central. The mantle is continuous around
the body, not bilobed, as in lamellibranchs. A few, as
the common garden snail, have a lung; but the vast
majority breathe by gills. The head is more or less
distinct, and provided with two tentacles, with auditory

MOLLUSCA I2Q

sacs at their bases ; two eyes, which are often on stalks ;
and a . straplike tongue (odontophore), covered with
minute teeth (Fig. 227). The heart is situated, in the
majority, on the right side of the back, and consists of
an auricle and a ventricle (Fig. 243). The nervous
ganglia are united into an esophageal ring or collar
(Fig. 351). All, except the pteropods, move by means
of a ventral disk or foot.

Gastropods are now the reigning mollusks, comprising
three fourths of all the living species, and are the types
of the branch. They have an extraordinary range in
latitude, altitude, and depth.

Omitting a few rare and aberrant forms, we may sepa-
rate the class into the following orders : —

1. Aspidobranchia, gastropods having a somewhat
diffuse nervous system, the cerebral ganglia being wide
apart, two auricles in the heart, gills plumelike, limpet
(Patella, Fig. 105), well known to every seaside visitor,
and the beautiful ear-shell (Haliotis, Fig. 95), frequently
used for ornaments and inlaid work, the pyramidal
Trochus, and the pearly Turbo (Fig. 102).

2. Pectinibranchia, gastropods with a somewhat con-
centrated nervous system, heart with a single auricle,
gill bearing a single row of lamellae and attached to the
wall of the mantle. This order includes many of the
most beautiful of the sea shells, the cowry (Cyprcza)
(Fig. 94), cones (Fig. 99), whelk (Buccinum) (Fig. 88),
trumpet shell (Triton), volute (Fig. 101), olive, harp,
cameo shell (Cassis) (Fig. 97), rock skell(Mupex), spindle
shell (Fus2is) (Fig. 96), and wing shell (Strombus)(¥ig.
103). All of these are marine. Many of them are
carnivorous and have the margin of the shell notched.

3. Opisthobranchia. The pteropods are small, marine,
floating mollusks, whose main organs of motion resem-
ble a pair of wings or fins, coming out of the neck,

DODGE'S GEN. ZOOL. — 9

130 STRUCTURAL AND SYSTEMATIC ZOOLOGY

whence the common name, " sea butterflies." Many
have a delicate, transparent shell. The head has six
appendages, armed with several
hundred thousand microscopic suck-
ers — a prehensile apparatus un-
equalled in complication. Ptero-
pods occur in every latitude, but
generally in mid-ocean, and in the
arctic regions are the food of whales

FIG. 89. - A Pteropod (Hya- an(J gea birds.
laa tridentata) . Atlantic.

The sea hare (Aplysia), which

discharges a purple fluid, and the" bubble shell (Build)
belong here.

The nudibranchs or
sea slugs are, for the
most part, naked mol-
lusks, only a few hav-
ing a Shell. They are FIG. 9o.-A Tritonian (Dendronotus arbores-

found in all seas, from

the arctic to the torrid, generally on rocky coasts.
When disturbed, most of them draw themselves up into
a lump of jelly or tough skin. Ex-
amples : sea lemon (Doris), the beau-
tiful Tritonia, and the painted sEolis.
4. Pulmonata. — These air breath-
ing gastropods, represented by the
familiar snail, have the simplest
form of lung — a cavity lined with
a delicate network of blood vessels,
which opens externally on the right side of the neck.
This is the mantle cavity. The entrance may be closed
to shut out the water in the aquatic tribes, and the hot,
dry air of summer days in the land species. They are
all fond of moisture, and are more or less slimy. Their
shells are lighter (being thinner, and containing less

FIG. 91. — Bulla ampulla,
or "bubble shell"; three
fourths natural size. In-
dian Ocean.

MOLLUSCA

earthy matter) than those of marine mollusks, having
to be carried on the back without the support of the

FIG. 92.— A, Land Suail (Helix)', B, C, D, Slugs (Limax}; E, F, G, Pond Snails
(Lzmnaa, Paludina, and Planorbis).

water. Their eggs are laid singly, while the eggs of
other orders are laid in chains.

They are found in all zones, but
are most numerous where lime and
moisture abound. All feed on vege-
table matter. A few are naked, as
the slug ; some are terrestrial ; others
live in fresh water. The land snails,
represented by the common Helix, the
gigantic Bidimus (Strcphocheilus), and
the slug (Limax), are distinguished by
their four "horns," the short front pair FIG. 93. — £«&>«
being the true tentacles, and the long
hinder pair being the eye stalks. They
have a sawlike upper jaw for biting leaves, and a short
tongue covered with minute teeth. The pond snails.

Guiana-

132 STRUCTURAL AND SYSTEMATIC ZOOLOGY

FIG. 94. — Cowry (Cyprcea capensis) ; two
thirds natural size. South Africa.

FIG. 95. —Haliotis, or " Pearly Ear Shell.
Pacific coasts.

FIG. 96. — Spindle Shell
(Fusus colus); one
half natural size.
Ceylon.

FIG. 97. — Cassis rufa, or
" Helmet Shell; " one fourth
natural size. Indian Ocean.

FIG. 98. — Auger Shell
( Terebra maculata) ;
one half natural size.
China seas.

FlG. 99. — Cone Shell (Conns
marmoreus} ; two thirds
natural size. China seas.

FIG. 100. — Chiton squa-
mosus; one half natural
size. West Indies.

FIG. 101. — Volute (Valuta
mnsica) ; one half natu-
ral size. West Indies.

MOLLUSCA

133

FIG. 102. — Top Shell (Turbo marnto-
ratus) ; one fourth natural size.
Australia.

FIG. 103. — Strombus gigas, or ''Wing
Shell"; one fifth natural size. West

FIG. 104. — Paludina, a fresh-water
snail.

FIG. 105. — Key-hole Limpet (Fissurella
listen"} . West Indies.

FIG. 106. — Ear Shell (//. tuberculata} , and Dog Whelk (Nassa reticulata) . England.

134 STRUCTURAL AND SYSTEMATIC ZOOLOGY

as Limncea and Planorbis, differ in having no eye stalks,
the eyes being at the base of the tentacles. They are
obliged to come frequently to the surface of the water
to breathe.

CLASS 4. — - Cephalopoda

The cephalopods stand at the head of the branch.
The head is set off from the body by a slight constric-
tion, and furnished with a pair of large, staring eyes, a
mouth armed with a rasping tongue and a parrotlike
beak, and eight or more tentacles or arms. The body
is symmetrical, and wrapped in a muscular mantle. The
shell, if present, may be internal or external (Fig. 245).
The nervous system is more concentrated than in other

invertebrates ; the cerebral
ganglia are partly inclosed
in a cartilaginous cranium.
All the five senses are
present. The class is en-
tirely marine (breathing by
plumelike gills on the sides
of the body), and carnivo-
rous. The naked species
are found in every sea.
Those with chambered shells
(as Nautilus, Ammonites, and
Orthoceras) were once very
abundant; more than two
thousand fossil species are
known, butonly three species
have been found living.

I . Dibranchs. — These are
the most active of mollusks,
FIG. 107.— Cuttlefish (Sepia officinaiis}-, and the tyrants of the lower

one fifth natural size. Atlantic coasts. , .-i « ,-1 .1

tribes. Among them are the
largest of invertebrate animals. They are naked, having

MOLLUSCA

135

no external shell covering the body, but usually a horny or
calcareous part within. They have a distinct head, promi-
nent eyes, horny mandibles, eight or ten arms furnished
with suckers, two gills, a complete tubular funnel, and an
ink bag containing a peculiar fluid (sepia}, of intense
blackness, with which the water is darkened to facilitate
escape. They have the power of
changing color, like the chame-
leon. They crawl with their arms
on the bottom of the sea, head
downward, and also swim back-
ward or forward, usually with the
back downward, by means of fins,
or squirt themselves backward by
forcing water forward through
their breathing funnels. ; -

The paper nautilus (Argonautd)
and the poulpe (Octopus) have eight
arms. The female argonaut se-
cretes a thin, unchambered shell
for carrying its eggs. The squid
(Loligo) and cuttlefish (Sepia) have FIG. 108.

1 -. -. . . , ,

ten arms, the additional pair be-
ing much longer than the others.
Their eyes are movable, while those
of the argonaut and poulpe are
fixed. The squid, so much used
for bait for cod, has an internal
horny "pen," and the cuttle has a spongy, calcareous
" bone." The extinct Belemnites had a similar structure.
Squid have been found with a body eleven feet and
arms thirty-nine feet long, and parts of others still
larger — as much as seventy feet in total length.

2. Tetrabranchs. — This group is characterized by the
possession of four gills, forty or more short tentacles,

yeatti) with
the mantle cut open : b, bran-
chial heart ; e, eye ; f, fin ; g,
gill; /', intestine; ib, ink bag;
m, cut edge of mantle; ma,
mantle artery; me, mantle
cavity; met, mantle carti-
lages; pvc, posterior vena
cava; s, siphon; t, tentacles
with sucking disks; us, visce-
ral sac.

136 STRUCTURAL AND SYSTEMATIC ZOOLOGY

and an external, chambered shell. The partitions, or
septa, of the shell are united by a tube called " siphun-

FIG. 109. — Female paper nautilus (A rgonauta argo) : i, swimming toward a by ejecting
water from funnel, b ; 2, crawling on the bottom; 3, coiled within its shell, which is
one fourth natural size. Mediterranean.

cle," and the animal lives in the last and largest
chamber.33 The living nautilus has a smooth, pearly

FIG. no. — Pearly nautilus, with shell bisected; one half natural size. Indian Ocean.

shell, a head retractile within the mantle or "hood,"
and calcareous mandibles, well fitted for masticating

CHORDATA

137

crabs, on which it feeds. The pearly nautilus dwells in
the Indian Ocean, crawling on the bottom at moderate
depths; and, while the shell is well known, only a few
specimens, comparatively, of the animal have ever been
obtained.

Branch XII. — CHORDATA

This grand division includes the most perfect animals,
or such as have the most varied functions and the most
perfect and complex organs. Besides the unnumbered
host of extinct forms,
there are about twen-
ty-five thousand liv-
ing species, widely
differing among them-
selves in shape and
habits, yet closely al-
lied in the grand
features of their or-
ganization, the gen-
eral type being end-
lessly modified.

The fundamental
distinctive character
of typical chordates

is the Separation Of FIG. in. - Ideal plans of the branches. V. trans-

, . ,. , verse section of vertebrate type; v, the same

tne mam maSS OI tne inverted. M, transverse section of molluscous

nprvniic; <;v<;t^m from type; and A/i/, of molluscoid. A and Ad, trans-

sysieni yerse sections of articulate type> high and low.

the general Cavity Of C> longitudinal section of ccelenterate type; a,

alimentary canal ; c, body cavity. In the other

the body. A tranS- figures, the alimentary canal is shaded, the heart

, . £ , i_ is black, and the nervous cords are open rings.

verse section of the

body exhibits two cavities, or tubes — the dorsal, con-
taining the cerebrospinal nervous system ; the ventral,
inclosing the alimentary canal, heart, lungs, and a
double chain of ganglia, or sympathetic system. This

STRUCTURAL AND SYSTEMATIC ZOOLOGY

ventral, or hemal, cavity cor-
responds to the whole body
of an invertebrate; while
the dorsal, or neural, is
mainly additional.

Vertebrates are also dis-
tinguished by an internal,
jointed skeleton, endowed
with vitality, and capable of
growth and repair. During
embryo life it is represented
by the notochord ; but in the
higher forms this is after-
ward replaced by a more
highly developed vertebral
column of cartilage or bone.
The column and cranium
are never absent in the Cra-
niata; other parts may be
wanting, as the ribs in frogs,
limbs in snakes, etc. The
limbs are never more than
four, and are always articu-
lated to the hemal side of the
body, while the legs of inver-
tebrates are developed from
the neural side. The mus-
cles moving the limbs are at-
tached to the endo-skeleton.

The circulation of the
blood is complete, the arte-
ries being joined to the veins

FIG. 112. — Diagram of circulation in the , .,, , ,

higher vertebrates: i, heart; 2, lungs; by CaplllariCS, SO that the

J\»TS^Ti±iS blood never escaPes into

tremities; 8, liver. (From Dalton's the visceral Cavity as in the
" Physiology.")

CHORDATA 139

invertebrates. All have a portal vein, carrying blood
through the liver; all have lacteals and lymphatics.
The blood is red, and contains both kinds of corpuscles.
The teeth are developed from the dermis, never from
the cuticle, as in mollusks and arthropods; the jaws
move vertically, and are never modified limbs. Except
in the lowest forms the liver and kidneys are always
present. The respiratory organs are either gills or
lungs, or both. Vertebrates are the only animals which
breathe through the mouth cavity.

The nervous system has two marked divisions : the
cerebrospinal, presiding over the functions of animal
life (sensation and locomotion); and the sympathetic,
which partially controls the organic functions (digestion,
respiration, and circulation). . In no case does the gullet
pass through the nervous system, as in invertebrates,
and the mouth opens on the side opposite to the brain.
Except in the lowest members of this group probably
none of the five senses is ever altogether absent. The
form of the brain is modified by the relative develop-
ment of the various lobes. In the lower vertebrates,
the cerebral hemispheres are small — in certain fishes
they are actually smaller than the optic lobes — in the
higher, they nearly or quite overlap both olfactories
and cerebellum. The brain may be smooth, as in most
of the cold-blooded animals, or richly convoluted, as in
man.

There is no skull in Amphioxus. In the Cyclosto-
mata and Elasmobranchii it is cartilaginous. In other
fishes it is cartilage overlaid with bone. In amphibians
and reptiles, it is mingled bone and cartilage. In birds
and mammals, it is mainly or wholly bony. The human
skull contains fewer bones than the skull of most
animals, excepting birds. The skull of all vertebrates
is divisible into two regions : the cranium, or brain case,

140 STRUCTURAL AND SYSTEMATIC ZOOLOGY

and the face. The size of the cranial capacity, com-
pared with the area of the face, is generally the ratio
of intelligence. In the lower orders, the facial "part is
enormously predominant, the eye orbits are directed
outward, and the occipital condyles are nearly on a line
with the axis of the body. In the higher orders, the
face becomes subordinate to the cranium, the sensual
to the mental, the eyes look forward, and the condyles
approach the base of the cranium. Compare the
"snouty" skull of the crocodile, and the almost vertical
profile of civilized man. A straight line drawn from
the middle of the ear to the base of the nose, and
another from the forehead to the most prominent part
of the upper jaw, will include what is called \he, facial
angle, which roughly gives the relation between the two
regions, and the intellectual rank of the animal.31 In
the cold-blooded vertebrates the brain does not fill the
cranium ; while in birds and mammals a cast of the
cranial cavity well exhibits the general features of
the cerebral surface.32

All higher vertebrates are single and free. Mammals
bring forth their young alive, the young before birth
deriving their nourishment directly from the mother
(viviparous). In almost all the others the nourishment
is stored up in the egg, which is laid before hatching
(oviparous), or is retained in the mother until hatched
(ovoviviparons\ as in some reptiles and fishes.

Of the branch Chordata there are' three subbranches :
Adelochorda, Urochorda, and Vertebrata. The first in-
cludes Balanoglossus, a wormlike creature regarded by
some zoologists as being related to the backboned
animals, together with two other forms (Rhabdopleura
and Cephalodiscus) whose affinities are less plain. The
second includes the tunicates, while the great mass of the
Chordata belong in the third subdivision of the branch.

ADELOCHORDA

141

The group Vertebrata consists of two divisions, the first,
Acrania, including the skull-less forms, e.g., the lancelet
(Amphioxus), while the second, and much larger divi-
sion, Craniata, consists of six great classes, Cyclostomata,
Pisces, Amphibia, Reptilia, Aves, and Mammalia. The
first four are "cold-blooded," the other two are " warm-
blooded." Cyclostomes, fishes, and amphibians have
gills during the whole or a part of their lives, while the
rest never have gills. Fishes and amphibians in embryo
have neither amnion nor allantois, while the animals in
the last three classes are provided with both.

The skull bearing vertebrates may be grouped into
three provinces.

Cyclostomes, fishes, and amphibians agree in having
gills or gill pouches, in wanting amnion and allantois,
and in possessing nucleated red blood corpuscles
(Ichthyopsida ).

Birds and reptiles agree in having
no gills, but both amnion and allan-
tois, in the articulation of the skull
with the spine by a single condyle,
in the development from the skin of
feathers or scales, and in having
oval, nucleated, red corpuscles (Sau-
ropsi'dd).

Mammals differ from birds and
reptiles in having two occipital con-
dyles, and their red blood corpuscles
are not nucleated 33 (Mammalia).

SUBBRANCH AND CLASS i . — Adelochorda

FIG. 113. — Balanoglossus,

The principal representative of this A proboscis; c," collar";

i • D / / r.-UJ-j **, gill slits. Enlarged.

class is Balanoglossus, a soft-bodied,

wormlike animal, one inch to six inches long, which lives

in the sand and mud, along the Mediterranean coast,

142 STRUCTURAL AND SYSTEMATIC ZOOLOGY

and is also found in the English Channel and Chesa-
peake Bay. It is placed among the Chordata because
it is regarded by some zoologists as having a notochord,
gill slits, and a dorsal nerve cord. All of these are so
rudimentary that the position of the animal in the
scheme of classification is not yet definitely determined.
Other structural and developmental features ally Bala-
noglossus to the annelids, and the echinoderms, for
which reason the animal may be looked upon as an inter-
mediate form between these groups and the Chordata.

SUBBRANCH AND CLASS 2. — Urochorda

The tunicates form a small and singular group of
animals now regarded as being the degenerate descen-
dants of primitive Chordata.

FIG. 114. — An ascidian.

FIG. 115. — Diagram of a tunicate, i,
inhalent opening; bs, branchial sac;
t, " tunic"; p, peribranchial cavity;
ce, esophagus; s. stomach; a, anus;
c, cloaca; h, heart; r, reproductive
organs; «, nerve ganglion.

They occur both as fixed
and as free swimming forms,
and as single individuals as
well as chains or groups of individuals. The most
common forms (the solitary Ascidians) are inclosed in a
leathery, elastic bag, one end of which is fastened to the
rocks, while the other has two orifices, for the inlet and
exit of a current of water for nutrition and respiration.

UROCHORDA

143

They are without head, feet, arms, or shell. Indeed,
few animals seem more helpless and apathetic than these
apparently shapeless beings. The tubular heart exhibits
the curious phenomenon of reversing its action at brief
intervals, so that the blood oscillates backward and for-
ward in the same vessels.
Another peculiarity is the
presence of cellulose in the
skin. The water is drawn
by cilia into a branchial sac,
an enlargement of the first
part of the intestine, whence
it escapes through openings
in the sides, to the excurrent
orifice, while the particles of
food drawn in with the water
are retained and passed into
the intestine. The larva is
active for a few hours, swim-
ming by means of a long tail.
It looks like a minute tad-
pole, and has a notochord and
a nervous system closely resembling those of a verte-
brate. Afterward it attaches itself by the head, the tail
is absorbed, and the nervous system is reduced to a
single small ganglion. Thus the animal, whose larval
structure is that of a vertebrate (since it possesses a
dorsal nerve cord, a notochord in the dorsal region,
and gill slits opening to the exterior), degenerates in
its adult stage into an invertebrate.

Besides developing from fertilized eggs, the tunicates
also multiply by the process of budding. In Salpa
and some other kinds, alternation of generations
takes place. All species are marine and some form
colonies.

FIG. 116. — Larval stage of a tunicate,
showing the notochord, « ; the spinal
cord, c ', and the sucking disks, d, by
which the larva becomes attached
previous to changing to the adult
condition. Much magnified.

144 STRUCTURAL AND SYSTEMATIC ZOOLOGY

SUBBRANCH 3. — Vertebrata

DIVISION A. — Acrania
Vertebrates without a skull.

CLASS. — Pharyngobranchii

The Acrania are represented by the singular animal
Amphioxus or lancelet. It is about two inches long,
semitransparent, without skull, limbs, brain, heart, or red
corpuscles. It has for a skeleton a notochord only. It
breathes by very numerous gill arches, without fringes,

FIG. 117. — Lancelet (Amphioxus). Notochord, c ; spinal cord, sc ; oral tentacles,/;
gills, g ; ovary, ov ; liver, / ; anus, a ; pore of branchial chamber, p ; muscle plates,
m ; tail fin,/. Natural size.

and the water is drawn in by cilia, which line the gill
slits. The embryo develops into a gastrula closely
resembling that of the invertebrates. The animal lives
in the sandy bottom of shallow parts of the ocean, and
has been found in the Mediterranean Sea, in the Indian
Ocean, and on the coasts of North America and South
America.

DIVISION B. — Craniata

Vertebrates with a distinct skull.

*

CLASS I. — Cyclostomata

The lampreys
and hagfish have
a persistent noto-
chord, a cartilagi-
nous skull, no lower
jaw, a round, suc-

Fic. 118. - Lamprey (Petromyzon marinus) . Atlantic. tOlial niOUth, homy

VERTEBRATA 145

teeth, one nasal organ, no scales, limbs, or gill arches.
The gills are in pouches which open separately. They
are found both in salt water and in fresh water.

CLASS II. — Pisces

Fishes fall far behind the rest of the typical craniates
in strength, intelligence, and sensibility. The eyes,
though large, are almost immovable, bathed by no tears,
and protected by no lids. Dwelling in the realm of
silence, ears are little needed, and such as they have are
without external parts, the sound being obliged to pass
through the cranium. Taste and smell are blunted, and
touch is nearly confined to the lips.

The class yields to no other in the number and variety
of its forms. It includes nearly one half of all the ver-
tebrated species. So great is the range of variation, it
is difficult to frame a definition which will characterize
all the finny tribes. It may be said, however, that fishes
are the only backboned animals having median fins (as
dorsal and anal) supported by fin rays, and whose limbs
(pectoral and ventral fins) do not exhibit that threefold
division (as thigh, leg, and foot) found in most other
craniates.

The form of fishes is admirably adapted to the element
in which they live and move. Indeed, Nature nowhere
presents in one class such elegance of proportions with
such variety of form and beauty of color. The head is
disproportionately large, but pointed to meet the resist-
ance of the water. The neck is wanting, the head be-
ing a prolongation of the trunk (Fig. 320). The viscera
are closely packed near the head, and the long, tapering
trunk is left free for the development of muscles which
are to move the tail — the instrument of locomotion
(Fig. 321). The biconcave vertebrae, with intervening
DODGE'S GEN. ZOOL. — 10

146 STRUCTURAL AND SYSTEMATIC ZOOLOGY

cavities filled with elastic gelatin, are designed for
rapid and versatile movements. The body is either
naked, as in the bullhead (Ameiurus), or covered with
polished, overlapping scales, as in the perch. Rarely,
as in the sturgeon, it is defended by bony plates, or by

ABC V^r D

FlG. 119. — Scales of fishes: A, cycloid scale (Salmon); B, ctenoid scale (Perch); C,
placoid scale (Ray) ; D, ganoid scales (Amblypterus} — a, upper surface; b, under
surface, showing articulating processes.

minute, hard spines, as in the shark. Scales with smooth,
circular outline are called cycloid ; those with notched or
spiny margins are ctenoid. Enameled scales are ganoid,
and those with a sharp spine, like those of the shark,
are placoid.

The vertical fins (dorsal, anal, and caudal) are peculiar
to fishes. The dorsal vary in number, from one, as in

FIG. 120. — Bluefish (Pomatomus saltatrix)

the herring, to three, as in the cod ; and the first dorsal
may be soft, as in the trout, or spiny, as in the perch.
If the dorsals are cut off, the fish reels to and fro. The

VERTEBRATA 147

caudal may be homocercal, as in ordinary species ; or
heterocercal, as in sharks. In ancient heterocercal
fishes, the tail was frequently vertebrated. The pectoral
and ventral fins stand for the fore and hind limbs of
other vertebrates. As the specific gravity of the body
is greater than that of the water, most fishes are pro-
vided with an air bladder, which is an outgrowth from
the esophagus. This is absent in such as grovel at the
bottom, as the rays, and in those, like the sharks, en-
dowed with compensating muscular power.

Fishes have no prehensile organ besides the mouth.
Both jaws are movable. The teeth are numerous, and

FIG. 121. — Salmon (Salmo salar). Both hemispheres.

may be recurved spines, as in the pike ; flat and triangu-
lar, with serrated edges, in the shark ; or flat and tessel-
lated in the ray (Fig. 230). They feed principally on
animal matter. The digestive tract is relatively shorter
than in other vertebrates. The blood is red, and the
heart has rarely more than two cavities, an auricle and
a ventricle, both on the venous side. Ordinary fishes
have four gills, which are covered by the operculum, and
the water escapes from an opening behind this. In
sharks there is no operculum, and each gill pouch opens
separately. The brain consists of several ganglia placed
one behind the other, and occupies but a small part of
the cranial cavity (Fig. 336). Its average weight to the

148 STRUCTURAL AND SYSTEMATIC ZOOLOGY

rest of the body may be as low as I to 3000. The eggs
of bony fishes are naked and multitudinous, sometimes
numbering millions in a single spawn ; those of the
sharks are few, and protected by a horny shell (Fig. 360).
There are about thirteen thousand species of fishes,
of which over two thirds are Teleostomi. There are
three principal subclasses of Pisces.

SUBCLASS I. — Elasmobranchii

These have a cartilaginous skeleton, and a skin naked
or with placoid scales. The gill openings are uncov-
ered ; and the mouth is generally under the head. The

FIG. 122. — Shark (Carcharias vulgar is). Atlantic.

ventral fins are placed far back ; the pectorals are large,
in the rays enormously developed ; and the tail is heter-
ocercal. Such are the sharks, rays, and dogfishes.
They are all marine. The largest shark found, and
therefore the largest fish, measured forty feet in length.

SUBCLASS II. — Teleostomi

This subclass includes all of the common fishes having
a bony endoskeleton and a scaly exoskeleton. The
skull is extremely complicated ; the upper and lower

VERTEBRATA 149

jaws are complete, and the gills are comblike or tufted,
and covered by an operculum. The tail is homocercal
except in the "ganoids," as the sturgeon and garpike,
in which it is heterocercal or unevenly lobed ; the other
fins are variable in number and position. In the soft-
finned fishes, the ventrals are absent, as in the eels ; or
attached to the abdomen, as in the salmons, herrings,

FIG. 123. — Thornback (Rafa clavata). European seas.

pikes, and carps; or placed under the throat, as in the
cod, haddock, and flounder. In the spiny-finned fishes,
the ventrals are generally under or in front of the pec-
torals, and the scales ctenoid, as in the perches, mullets,
and mackerels.

The so-called "ganoids" have the body covered with
enameled scales or bony plates.

150 STRUCTURAL AND SYSTEMATIC ZOOLOGY

FIG. 124. — Garpike (Lepidosteus osseus). Lake Ontario.

FIG. 125. — Sturgeon (Acipenser sturio). Atlantic coast.

FIG. 126. — Catfish, or Horned Pout (Ameiurus nebulosus). American rivers.

FIG. 127. — Cod (Gadus callarias}. Atlantic coast.

SUBCLASS III. — Dipnoi

These fishes connect the class with the Amphibia.
They have an eel-like body sometimes four or five feet
long, covered with cycloid scales ; an embryonic noto-
chord for a backbone; long, ribbonlike pectoral and
ventral fins, set far apart ; two incompletely separated
auricles, and one ventricle ; and, besides gills, a cellular
air bladder, which is used as a lung.

They live in muddy or stagnant water in which there
is little oxygen for respiration, not enough to be obtained

VERTEBRATA 151

by the gills alone, so these fishes occasionally come to
the surface and take air into the lungs. Lungfishes feed
upon the small animals captured among the water plants.

FIG. 128. — Protopterus annectens ; one fourth natural size. African rivers.

The representatives are Ceratodus from Australia, Pro-
topterus from Africa, and Lepidosiren from Brazil.

CLASS III. — Amphibia

These cold-blooded vertebrates are distinguished by
having gills when young, and usually true lungs when
adult. They have no fin rays, and the limbs, when
present, have the same divisions as those of higher ani-
mals. The skin is soft, and generally naked, and the
skeleton is ossified. The skull is flat, and articulates
with the spinal column by two condyles. There is no
distinct neck ; and the ribs are usually small or wanting
(Fig. 284). The heart consists of two auricles and one
ventricle (Fig. 273). In the course of development
nearly all undergo metamorphosis upon leaving the egg,
passing through the "tadpole " state (Fig. 370). They
commence as water-breathing larvae, when they resemble
fishes in their respiration, circulation, and locomotion.
In the lowest forms, the gills are retained through life ;
but all others have, when mature, lungs only (Fig. 282),
the gills disappearing. The cuticle is frequently shed,
the mode varying with the habits of the species.34 The
common frog, the type of this class, stands intermediate

152 STRUCTURAL AND SYSTEMATIC ZOOLOGY

FIG. 129. —Head and gills of Necturus. Cayuga
Lake. Copyright, 1901, by N. Y. Zoological
Society.

between the two extremes of the vertebrate series; no
fundamental part is excessively developed.

There are about seven hundred living species, grouped
in three orders : —

1 . Urodela, characterized by retaining the tail through-
out life, and in usually having two pairs of limbs approx-
imately equal in size.
In this group are
the Proteus of Aus-
tria and Necturus of
the Eastern United
States, both of which
retain their gills ;
Amphiuma of North
America, and the
salamanders and

newts, in which the gills are lost in the adult, though
the former retains a gill slit as an evidence of their
presence in the larval stage.35

2. Anura include all the well-known amphibians
which are tailless in the adult stage, as frogs and toads.
They have a moist,

naked skin, ten ver-
tebrae, and no ribs.
They breath by
swallowing the air.
They have four
limbs — the hinder
the longer, and the first developed. They have four
fingers and five toes. The tongue is long, and, fixed
by its anterior end, it can be rapidly thrown out as an
organ of prehension.36 The eggs are laid in the water
enveloped in a glairy mass ; and the tadpoles resemble
the urodelans till both gills and tail are absorbed, no
gill slit persisting. Frogs (Rand) have teeth in the

FIG. 130. — Red Salamander ( Spelerpes ruber).
United States.

VERTEBRATA

153

upper jaw, and webbed feet ; toads (Biifo) are higher in
rank, and have neither teeth nor fully webbed feet.

FIG. 131. — Bullfrog (Rana). North America.

The former have been known to live sixteen years, and
the latter thirty-six.

FIG. 132. — Proteus anguinus. Europe.

3. Gymnophiona have neither tail nor limbs nor gill
slit, a snakelike form, minute scales in the skin, and well-

154 STRUCTURAL AND SYSTEMATIC ZOOLOGY

developed ribs. They are confined to the tropics, and
are subterranean in habit.

CLASS IV. — Reptilia

These air-breathing, cold-blooded vertebrates are dis-
tinguished from all fishes and amphibians by never hav-
ing gills, and from birds by being covered with horny
scales or bony plates. The skeleton is never cartilag-
inous (Fig. 310); and the skull has one occipital condyle.
The vertebrae are ordinarily concave in front; and the
ribs are well developed. With few exceptions, all are
carnivorous ; and teeth are generally present (Figs. 231,
235), except in the turtles, where a horny sheath covers
the jaws. The teeth are never fastened in sockets, ex-
cept in crocodiles (Fig. 224). The jaws are usually very
wide. The heart has three chambers (Fig. 273), save in
crocodiles, where the ventricle is partially partitioned.
But in all cases a mixture of arterial and venous blood is
circulated. The lungs are large, and coarsely cellular
(Fig. 281). The limbs, when present, are provided with
three or more fingers as well as toes.

There are about three thousand species of living rep-
tiles, and of these there are three main orders : the first
has horny scales, the others have bony plates combined
with scales.

i. Squamata, including the lizards and the snakes.
The lizards (Lacertilia) may be likened to snakes provided
with fourlimbs, each having five digits.37 The body is cov-
ered with horny scales. All have teeth, which are simple
in structure; and the halves of the lower jaw are firmly
united in front, while those of snakes are loosely tied
together by ligaments. Nearly all have a breastbone,
and the eyes (save in the gecko) are furnished with
movable lids. In the common lizards and chameleon,
the tongue is extensile. The tail is usually long, and in

VERTEBRATA

155

some cases each caudal vertebra has a division in the
middle, so that the tail, when grasped, breaks off at one
of these divisions. The chameleon has a prehensile tail.
The iguana is distinguished by a dewlap on the throat
and a crest on the back. Except some of the monitors
of the Old World, all the lizards are terrestrial.

f FIG. 133. — Lizard (Lacerta).

The snakes (Ophidia) are characterized by the absence
of visible limbs ; 38 by the great number of vertebrae,
amounting to over four hundred in the great serpents ;
by a corresponding number of ribs, but no sternum ; and
no true eyelids, the eyes being covered with a transparent
skin. The tongue differs from that of nearly all other
reptiles in being bifid and extensile. The mouth is very
dilatable. The skin is frequently shed, and always by
reversing it. Snakes make their way on land or in water
with equal facility.

As a rule, the venomous snakes, as vipers and rattle-
snakes, are distinguished by a triangular head covered

156 STRUCTURAL AND SYSTEMATIC ZOOLOGY

with small scales ; a constriction behind the head ; two
or more fangs, and few teeth ; small eyes, with vertical

FIG. 134. — Adder, or viper ( Vipera berus}. England.

pupil; and short, thick tail. In the harmless snakes,
the head gradually blends with the neck, and is cov-
ered with plates ;. the teeth are comparatively numerous

FIG. 135. — a, Head of a harmless snake (upper view"); b, heads of various venomous

snakes.

in both jaws ; the pupil is round, and the tail tapering.
This rule, however, has many exceptions.

VERTEBRATA

157

2. Chelonia, or tortoises and turtles, are of anomalous
structure. The skeleton is external, so as to include not
only all the viscera, but also the whole muscular system,
which is attached internally ; and even the limbs are

FIG. 136. — Hawkbill turtle (Chelone imbricata}. Tropical Atlantic.

inside, instead of outside, the thorax. The exoskeleton
unites with the endoskeleton, forming the carapace, or
case, in which the body is inclosed. The exoskeleton
consists of horny plates, known as " tortoise shell " (in

the soft tortoises,
Aspidonectes, this
is wanting), and
of dermal bones,
united to the ex-
panded spines of
the vertebrae and
to the ribs, making
the walls of the
carapace(Fig.3i2).
The ventral pieces form the plastron.2® All are tooth-
less. There are always four stout legs ; and the order
furnishes the only examples of vertebrates lower than
birds that really walk, for lizards and crocodiles wriggle,

FIG. 137. — Box tortoise (Terrapene Carolina}.
United States.

158 STRUCTURAL AND SYSTEMATIC ZOOLOGY

and drag the body along. There are no teeth, but a
horny beak. The eggs are covered with a calcareous
shell.

The sea turtles, as the edible green turtle and the
hawkbill turtle, which furnish the "tortoise shell" of
commerce, have the limbs converted into paddles. The
fresh-water forms, represented by the snapping turtle
(Chelydra), are amphibious, and have palmated feet.
Land tortoises (Testudo) have short, clumsy limbs,
fitted for slow motion on the land ; the plastron is very
broad, and the carapace is arched (while it is flattened
in the aquatic species), and head, legs, and tail can be
drawn within it. The land and marine species are
vegetable feeders ; the others, carnivorous.

3. Crocodilia, the highest and largest of reptiles, have
also two exoskeletons — one of horny scales (epidermal),

FIG. 138. — Alligator (A. misszssz^zensz's) . Southern States.

and another of bony plates (dermal). The bones of the
skull are firmly united, and furnished with numerous
teeth, implanted in distinct sockets. The lower jaw
extends back of the cranium. The heart has four
cavities, but the pulmonary artery and aorta commu-
nicate with each other, so that there is a mixture of
venous and arterial blood. They have external ear
openings, closed by a flap of the skin, and eyes with
movable lids ; a muscular gizzard ; a long, compressed

VERTEBRATA 159

tail ; and four legs, with feet more or less webbed, and
having five toes in front and four behind. The existing
species are confined to tropical rivers, and are carnivo-
rous. The eggs are covered with a hard shell.

There are three representative forms: the gavial of
the Ganges, remarkable for its long snout and uniform
teeth ; the crocodiles, mainly of the Old World, whose
teeth are unequal, and the lower canines fit into a notch
in the edge of the upper jaw, so that it is visible when
the mouth is closed ; and the alligators of the New
World, whose canines, in shutting the mouth, are
concealed in a pit in the upper jaw. The toes of the
gavials and crocodiles are webbed to the tip ; those of
the alligators are not more than half webbed.

In the mediaeval ages of geological history, the class
of reptiles was far more abundantly represented than
now. Among the many forms which geologists have
unearthed are numerous gigantic saurians, which can-
not be classified with any of the four living orders.
Such are the Ichthyosaurus, Plesiosaurus, Pterodactylus,
Megalosaurus, and Iguanodon.

CLASS V. — Aves

Birds form the most clearly defined class in the whole
animal kingdom, and in some respects are the most
highly specialized of the craniata. The eagle and hum-
mer, the ostrich and duck, widely as they seem to be
separated by size, form, and habits, still exhibit one
common type of structure. On the whole, birds are
more closely allied to reptiles than to mammals. In
number, they approach the fishes, ornithologists having
determined eight thousand species, or more.

A bird is an air-breathing, egg-laying, warm-blooded,
feathered vertebrate, with two limbs (legs) for perching,
walking, or swimming, and two limbs (wings) for flying

160 STRUCTURAL AND SYSTEMATIC ZOOLOGY

or swimming. Organized for flight, it is gifted with a
light skeleton, very contractile muscular fibers, and a
respiratory system of the highest development.

The skeleton is more compact than those of reptiles
and mammals, at the same time that it is lighter, and
the bones are harder and whiter. It contains fewer
bones than usual, many parts being anchylosed together,
as the skull bones, the dorsal vertebrae, and bones of the
tarsus and metatarsus. The lumbar vertebrae are united
to the ilia. The neck is remarkably long (containing
from nine to twenty-four vertebrae), and flexible, ena-
bling the head to be a most perfect prehensile organ.
The ribs are generally jointed in the middle, as well as
with the backbone and sternum. The last, where the
muscles of flight originate, is highly developed. The
skull articulates with the spinal column by a single
condyle, and with the lower jaw, not directly, as in
mammals, but through the intervention of a separate
bone, as in reptiles (Fig. 313).

All birds have four limbs, while every other verte-
brate class shows exceptions. The fore limbs are fitted
for flight. They ordinarily consist of nine separate
bones, and from the hand, fore arm, and humerus are
developed the primary, secondary, and tertiary feathers
of the wing. The hind limbs are formed for progres-
sion— walking, hopping, running, paddling, and also
for perching and grasping. The modifications are more
numerous and important than those of the bill, wing, or
tail. There are twenty bones ordinarily, of which the
tibia is the principal ; but the most characteristic is the
tarsometatarsus, which is a fusion of the lower part of
the tarsus with the metatarsus. The rest of the tarsus
is fused with the tibia. The thigh is so short that the
knee is never seen outside of the plumage; the first
joint visible is the heel.40 Most birds have four toes

VERTEBRATA

161

(the external or " little " toe is always wanting) ; many
have three, the hallux, or "big " toe, being absent ; while
the ostrich has but two, answering to the third and
fourth. The normal number of phalanges, reckoning
from the hallux, is 2, 3, 4, 5. The toes always end in
claws.

Birds have neither lips nor teeth, epiglottis nor dia-
phragm. The teeth are wanting, because a heavy mas-
ticating apparatus in
the head would be
unsuitable for flight.
The beak, crop, and
gizzard vary with the
food. It is a pecul-
iarity of all birds,
though not confined
to them, that the gen-
erative products and
the refuse of digestion
are all discharged
through one common
outlet.

The sole organs of
prehension are the
beak and feet. The
circulation is double,
as in mammals, start-
ing from a four-
chambered heart (Fig. 273). Respiration is more com-
plete than in other vertebrates. The lungs are fixed, and
communicate with air sacs in various parts of the body, as
along the vertebral column, and also with the interior of
many bones, as the humerus and femur, which are usually
hollow and marrowless.41 Both brain and cord are much
larger relatively than in reptiles (Fig. 338); the cranium
DODGE'S GEN. ZOOL. — ,11

deb

FIG. 139. — Principal parts of a bird: a, primaries;
6, secondaries; c, spurious wing; d, wing coverts;
e, tertiaries; _/, throat, or jugulum; g, chin; k, bill;
the meeting line between the two mandibles is the
commissure; the ridge on the upper mandible is
called culmen; that of the lower, gonys; the space
between the base of the upper mandible and the
eye is the lore; /*, forehead; k, crown; /, scapular
feathers; m, back; n, metatarsus, often called tarsus
or tarsometatarsus; o, abdomen; p, rump; q, upper
tail coverts; r, lower tail coverts.

162 STRUCTURAL AND SYSTEMATIC ZOOLOGY

is larger in proportion to the face ; and the parts of the
brain are not situated in one plane, one behind the other.
The cerebrum is round and smooth, and the cerebellum
single-lobed. The ears resemble those of crocodiles ; but
the eyes are well developed, and protected by three lids.
They are placed on the sides of the head, and the pupil
is always round. The sexes generally differ greatly in
plumage, in some cases more widely than two, distinct
species, but the coloration of either sex of any one
species is very constant.

There are two divisions of living birds.

DIVISION A. — Ratitae (jCursores)

This small and singular group is characterized by
having no keel on the breastbone, rudimentary wings,

feathers with discon-
nected barbs, and
stout legs. The Af-
rican ostrich has two
toes, the cassowary
three, and the apte-
ryx four.

Its representatives
are the ostrich(5/r//-
thid) of Africa and
Arabia, South Amer-
ican ostrich (Rhea\
cassowary ( Casua-
rins) of the East
Indian Archipelago
and Australia, emu
(Drom&us) of Aus-
tralia, and Apteryx,

FIG. 140. — African Ostrich (Struthio camelus).

or kiwikiwi of

Zealand. Besides these, there are extinct gigantic forms

VERTEBRATA 163

from Madagascar (sEpyornis) and from New Zealand,
the moa (Dinornis). This singular geographical distri-
bution, like that of the Dipnoi and marsupials, shows
that the group was once widely spread over the earth,
but is now greatly restricted in area.

DIVISION B. — Carinatae

Birds which, with rare exceptions, e.g., the Penguins,
have a keeled sternum, and developed functional wings.

Of the birds composing this division, some live mainly
in the water, others on the land, while still others spend
a considerable- part of their lives on the wing. Their
bodily structure is, consequently, modified to suit their
mode of life. Hence, they may be broadly grouped
into aquatic, terrestrial, and aerial birds.

A. AQUATIC BIRDS. — Specially organized for swim-
ming ; the body flattened, and covered with water-proof
clothing — feathers and down ; the legs short (the knees
being wholly withdrawn within the skin of the body),
and set far apart and far back ; the feet webbed, and
hind toe elevated or absent. The legs are always feath-
ered to the heel at least. They are the only birds whose
neck is sometimes longer than the legs.

Examples, penguins, ducks, petrels, and gulls.

B. TERRESTRIAL BIRDS. — This group exhibits great
diversity of structure ; but all agree in being especially
terrestrial in habit, spending most of the time on the
ground, not on trees or the water, although many of
them fly and swim well. The legs are long or strong,
and the knee is free from the body. The hind toe, when
present, is small and elevated. Such birds are the storks,
plovers, and turkeys.

C. AERIAL BIRDS. — This highest and largest group
includes all those birds whose toes are fitted for grasping
or perching, the hind toe being on a level with the rest.

164 STRUCTURAL AND SYSTEMATIC ZOOLOGY

The knee is free from the body, and the leg is generally
feathered to the heel. The wings are adapted for rapid
or long flight, and they hop, rather than walk, on the

FIG. 141. — Loon (Urinator imber). North America.

ground.42 They always live in pairs, and the young
are hatched helpless. In this group may be placed the

pigeons, birds of prey, par-
rots, and the song birds.

The more important orders
of birds are the following : —
i. Pygopodes, or divers.
These lowest of the feath-
ered tribe have very short
wings and tail, and the legs
are placed so far back that
they are obliged, when on
land, to stand nearly bolt
upright. They are better
fitted for diving than for
flight, or even swimming.

FIG. 142.— Penguin (Aptenodytes pen- They belong tO the high
nantii). Falkland Islands. , . , ... _ .

latitudes, living on fishes
mainly, and are represented by the loons and grebes.

VERTEBRATA

I65

2. Impennes, or penguins. These birds, found only
in the southern hemisphere, have many of the structural
features of those in the preceding order, but their wings
are so rudimentary that flight is impossible (Fig. 142).

3. Turbinares, the albatrosses and petrels (largest and
smallest of web-footed birds), having a hawklike, hooked
bill and the nostrils opening through tubes. The wan-
dering ^albatross inhabits the southern seas but some-
times comes as far north as Florida. It measures twelve
to fourteen feet from tip to tip of its wings. Wilson's
petrel (Oceanites\ one of the smallest of the many
species, is known to the sailor as " Mother Carey's
chicken." The birds

in this order are noted
for their powers of
flight.

4. Stega no p odes,
characterized by a
long bill, generally
hooked ; wings rather
long ; and toes long,
and all four joined to-
gether by broad webs.
Throat generally na-
ked, and furnished
with a sac. The ma-
jority are large sea-
birds, and feed On FIG- 143- — Cormorant (Phalacrocorax). Copy-

right, 1901, by N. Y. Zoological Society.

fishes, mollusks, and

insects. Examples are the cormorants, pelicans, gan-

nets, and frigate bird (Fig. 143).

5. Herodiones. The herons, bitterns, storks, ibises,
spoonbills and flamingoes are included in this order.
(Fig. 144). They are readily distinguished by their
long and bare legs. Generally, also, the toes, neck, and

IP* •

166 STRUCTURAL AND SYSTEMATIC ZOOLOGY

bill are of proportionate length,- and the tail short.
They feed on small animals, and, with a few exceptions,

FIG. 144. — Heron (Ardea).

frequent the banks of rivers. In flying, their legs are
stretched out behind, while in most other birds they
are folded under the body. •

6. Anseres have a heavy body, moderate wings, short
tail, flattened bill, covered by a soft skin, with ridges
along the edges. Diet more commonly vegetarian than
animal. The majority inhabit fresh water — as the
ducks, geese, and swans. •"'-•'. !~

7. Accipitres, including the diurnal birds of prey.
They have a strongly hooked beak with a waxy mem-
brane (cere) at the base of the upper mandible, three
toes in front and one behind. The toes are armed with
long, strong, crooked talons; the legs are robust, the
tarsus and toes usually without feathers being covered
by scales ; and the wings are of considerable size,

VERTEBRATA

I67

FlG. 145. — Wild duck (Anas boschas}. North America.

FIG. 146. — Wild geese (Branta canadensis). United States. Copyright, 1901, by
N.Y. Zoological Society.

1 68 STRUCTURAL AND SYSTEMATIC ZOOLOGY

adapted for rapid and powerful flight. The bill is stout
and sharp, and usually toothed. The eyes are on the sides

FIG. 147. — Fishhawk (Pandion haliaetus carolinensis}. United States.

of the head, the wings pointed, and the plumage firm and
close. All are carnivorous. The female is larger than the

male, except the con-
dor. - Examples are the
eagles, hawks,f alcons,
kites, and vultures.

8. Gallince. As a
rule, this order, so
valuable to man, is
characterized by a
short, arched bill ;
short and concave
wings, unfitted for
protracted fl i g h t ;
stout legs, of medium
length; and four toes,

FlG. 148. — Golden eagle (Aquila chrysaetos).

North America and Europe. Copyright, 1901 ,
by N.Y. Zoological Society.

the three in
being united

front
by a

VERTEBRATA

169

short web, and terminating in blunt claws. The legs

are usually feathered to the heel, sometimes (as in

grouse) to the toes.

The feathers of the

body are large and

coarse. The males

generally have gay

plumage, and some

appendage to the

head. The nostrils

are covered by a scale

or valve. Their main

food is grain. Such

are the partridges,

turkeys, pheasants,

and poultry.

9. Gralla. The rails

and cranes are long-
legged, marsh birds

with four toes, of which the hinder one is usually small

and higher up than
the front ones. The
feet are adapted for
wading, for standing
upon floating vegeta-
tion, or walking over
soft mud, having long
spreading toes which
aid in distributing
the weight of the
body over much sur-
face. Cranes eat

FIG. 149. — Prairie chicken (Cupidonia cupido),
Western prairies.

snakes as well as veg-
etable food, while rails are fond of mollusks and worms.

I/O STRUCTURAL AND SYSTEMATIC ZOOLOGY

10. Gavice, distinguished by their long, pointed
wings, usually long tail, and by great powers of flight.
They are all carnivorous. Such are the gulls and terns,
which frequent the seacoast, lakes, and rivers; and the

FIG. 151.— Tern (Sterna),

auk which is found in 'northern seas. The great auk

was flightless and became extinct as a result of unre-
stricted killing by
fishermen and oth-
ers who regularly
visited the nesting
grounds of this bird
on the islands and
along the coast of
the North Atlantic
for the purpose
of collecting the
eggs, and killing
the birds for their
feathers and oil.

The last living specimen of the great auk was seen in 1 842.
ii. Limicolce, or shore birds, include the snipes,

plovers, woodcock, sandpipers, phalaropes, stilts, avocets,

FIG. 152. — Sandpiper ( T.

hypoleuca) . England.

VERTEBRATA

171

-Ve

and jacanas. The toes are three or four in number,

with the hind one when present elevated above the

others, the legs are long,

slender, and bare be-
low. The phalaropes have

webbed toes and can swim.

They feed mainly upon

worms and crustaceans

which they dig out of the

mud or from under stones

with their long bills.

12. Columbce, or pigeons

and doves, have wings for

prolonged flight, and slen-
der legs, fitted rather for an

arboreal life, with toes not

united, and the hind toe on FIG. i53. — Ringdove

a level with the rest. Their

mode of drinking is peculiar, the head not being raised

when the water
is swallowed.
The passenger
pigeon, which
was formerly
very abundant
in some sections
of the country,
seems to be
approaching ex-
tinction. Wilson,
the ornitholo-
gist, saw a flock

FIG. 154. — Foot of parrot and woodpecker. jj^ I 808 in JCen-

tucky in which he estimated there were 2,230,272,000
individuals. At present the bird is seen only occa-

1/2

STRUCTURAL AND SYSTEMATIC ZOOLOGY

sionally. The extinct,
flightless dodo (Didus),
a native of the island
of Mauritius, belonged
'in this order.

13. Ps it tad, or par-
rots. These birds have
a strong, arched upper

FIG. 155. — Barn owl (Stn'x pratin-

cola). Both hemispheres. Copy- Jr^i

right, 1901, by N. Y. Zoological "\'^-<m

Society. f*«S

bill, with a cere at the
base, a fleshy, thick, and
movable tongue, and
paired toes. They have,
usually, brilliant plu-
mage. They live in trees
and feed on fruits. Such'
are the parrots, paro-
quets, cockatoos, and
macaws.

FIG. 156. — Trogon elegans. Central America.

VERTEBRATA 173

14. Striges, or owls, have many of the characters of
the Accipitres, but may be distinguished from them by
having their large eyes directed forward and surrounded
by radiating feathers, a feathered tarsus, and soft plu-
mage (Fig. 155).

15. Picari&. This heterogeneous group is sometimes
subdivided into several orders since its members are
too unlike, structurally, to be classed together.43 The
toes are usually paired, two in front and two behind.

FIG. 157. —Goatsucker {Caprimulgus).

As here given the order includes swifts, goatsuckers
(Fig. 157), humming birds, cuckoos, kingfishers (Fig.
158), trogons (Fig. 156), toucans, hornbills, hoopoes,
and woodpeckers. These birds are not musical, and only
ordinary fliers. The most of them feed on insects or fruit.
The majority make nests in the hollows of old trees;1
but the cuckoos often lay in the nests of other birds.
In climbing, the woodpeckers are assisted by their stiff
tail.

1/4 STRUCTURAL AND SYSTEMATIC ZOOLOGY

FIG. 158. — Kingfisher (Ceryle).

FIG. 159. — Head of a flycatcher ( Tyrannus).

VERTEBRATA 175

1 6. Passeres. This order is the most numerous and
varied in the whole class. It comprehends all those

FIG. 160. — White-throated sparrow (Zonotrichia albicollis). United States.

tribes which live habitually among trees, excepting the
rapacious and climbing birds, whose toes — three in

FIG. 161. — Redstart (Setophaga ruticitta). United States.

front, and one behind — are eminently fitted for perch-
ing only. The legs are slender, and seldom used for
locomotion.

They are divisible into two sections : a. Clamatores,
having the tarsus usually enveloped in a row of plates
meeting behind in a groove, and the bill broad, and bent

1/6 STRUCTURAL AND SYSTEMATIC ZOOLOGY

down abruptly at the tip. Representatives are the
tyrant flycatchers (Fig. 159). b. Oscines, or songsters,

FIG. 162. — White eyed vireo (Fz>v0 noveboracensis). United States.

all of whom have a vocal apparatus, though not all sing.
The anterior face of the tarsus is one continuous plate,
or divided transversely into large scales ; and the plates
on the sides meet behind in a ridge. The toes, always
three in front and one behind, are on the same level.
The eggs are usually colored. Here belong the
crows, jays, birds of paradise, blackbirds, orioles, larks,

FIG. 163. — Swallow (Hirundo).

sparrows (Fig. 160), tanagers, finches, waxwings, swal-
lows (Fig. 163), wrens, warblers (Figs. 161, 162),
thrushes, etc.

VERTEBRATA

177

CLASS VI. — Mammalia

Mammals are distinguished from all other vertebrates
by any one of the following characters : they suckle
their young; the thorax and ab-
domen are separated by a perfect
diaphragm ; the red corpuscles
of the blood have no nucleus,
and are therefore double concave
(Fig. 259), and either a part or
the whole of the body is hairy at
some time in the life of the ani-
mal (Fig. 291, 30 1 ).44

They are all warm-blooded ver-
tebrates, breathing only by lungs,
which are suspended freely in
the thoracic cavity ; the heart is
four-chambered, and the circula-
tion is double, as in birds (Fig.
273); the aorta is single, and
bends over the left bronchial
tube ; the large veins are fur-
nished with valves ; the red cor-
puscles differ from those of all
other vertebrates in having no
nucleus and in being circular (ex-
cept in the camel) ; the entrance
to the windpipe is always guarded
by an epiglottis ; the cerebrum is
more highly developed than in
any other class, containing a
greater amount of gray matter
and (in the higher orders) more convolutions ; the cere-
bellum has lateral lobes, a mammalian peculiarity, and
there is a corpus callosum and a pons varolii (Fig. 335,
DODGE'S GEN. ZOOL. — 12

FIG. 164. — Longitudinal section
of human body (theoretical) : a,
cerebro-spinal nervous system;
b, cavity of nose; c, cavity of
mouth; d, alimentary canal;
e, chain of sympathetic ganglia;
/, heart; g, diaphragm.

1 78 STRUCTURAL AND SYSTEMATIC ZOOLOGY

339, 342), the cranial bones are united by sutures, and
they are fewer than in cold-blooded vertebrates ; the
skull has two occipital condyles, a feature shared by
the amphibians ; the lower jaw consists of two pieces
only (often united), and articulates directly with the
cranium ; with four exceptions there are always seven
cervical vertebrae : 45 the dorsal vertebrae, and therefore
the ribs, vary from ten to twenty-four; the lumbar
vertebrae number from two to nine; the sacral from
three to nine, and the caudal from two to forty-six ; the

articulating surfaces of the ver-
tebrae are generally flat; the
fore limbs are never wanting,
and the hind limbs only in a
few aquatic forms; excepting
the whales, each digit carries
a nail, claw, or hoof ; the teeth
(always present, save in certain

bro-spinal nervous axis contained ]ow tribes) are USUally in tWO

in neural tube; e, chain of sympa- ' J

thetic ganglia; </, alimentary canal; Sets, a milk Or deClduOUS Set,

and a permanent set, and are

planted in sockets ; the mouth is closed by flexible
lips ; an external ear is rarely absent ; 46 the eyes are
always present though rudimentary in some burrow-
ing animals ; they are viviparous ; and, finally, and
perhaps above all, while in all other animals the
embryo is developed from the nourishment laid up in
the egg itself, in mammals it draws its support, almost
from the beginning, directly from the parent, and,
after birth, it is sustained for a time by the milk se-
creted by the mammary glands (Fig. 367). From
the first, therefore, till it can care for itself, the young
mammal is in vital connection with the parent.

About twenty-one hundred species are known, inhabit-
ing the land, the water, and the air.

VERTEBRATA 179

SUBCLASS I. — Prototheria

These mammals have but one outlet for the intestine,
urinary and reproductive organs, as in birds. They are
implacental, and the mammary glands are rudimentary.
There is but one order.

i. Monotremata, This order includes two singular
forms, the duck mole (Ornithorhynchus) and spiny
ant-eater (Echidna), both confined to the Australian

FIG. 166. — Ornithorhynchus.

continent and New Guinea. The former has a cover-
ing of fur, a bill like that of a duck, and webbed feet.
The latter is covered with spines, has a long, toothless
snout, like the ant-eater's, and the feet are not webbed.
Both burrow, and feed upon insects. The brain is
smooth in the ornithorhynchus, and folded in the
echidna. In both, the cerebral hemispheres are loosely
united by transverse fibers, and do not cover the cere-
bellum and olfactory lobes,47 Both lay eggs which
resemble those of birds and from which the young are
hatched.

ISO STRUCTURAL AND SYSTEMATIC ZOOLOGY

SUBCLASS II. — Theria

In these mammals the mammary glands are typically
developed and there is no cloaca.
There are two sections : —

SECTION A. — Metatheria

In this group are the marsupials, opossums, bandi-
coots, wombats, and kangaroos.

Marsupialia are distinguished by the fact that the
young, always born premature, are transferred by the
mother to a pouch on the abdomen, where they are
attached to the nipples, and the milk is forced into

FIG. 167. — Virginia opossum (Didelphys virginianci).

their mouths by special muscles.48 They have " mar-
supial bones" projecting from the pelvis, which may
serve to support the pouch ; though as the monotremes
have the same bones, but no pouch, they doubtless
have some other function. These bones are peculiar
to animals having no placenta, namely, to monotremes
and marsupials. The brains of marsupials resemble
those of the monotremes, except that the cerebrum of

VERTEBRATA l8l

the kangaroo covers the olfactory lobes. All have the
four kinds of teeth, and all are covered with fur, never
with spines or scales. Except the opossums of Amer-
ica, all are restricted to Australia and adjacent islands.
The marsupials are almost the only mammals of
Australia, there being very few species of placental
mammals. The marsupials have here developed into
forms corresponding in their habits to the orders of
placental mammals in the rest of the world. The
kangaroos take the place of the large herbivores —
the ungulates. The thylacinus and dasyurus are the
marsupial carnivora. Other forms are squirrel-like in
shape and habits, and still others are insectivorous.

SECTION B. — Eutheria

In these mammals the young are connected with the
mother by means of a vascular structure, the placenta,
by which they are nourished. They are born in a rela-
tively perfect condition. There is no marsupial pouch.
The following orders are included : —

i. Edentata. — This strange order contains very di-
verse forms, as the leaf-eating sloths and the insectivo-
rous ant - eaters and
armadillos of South
America, and the pan-
golin and orycteropus

Of the Old World. The FIG 168. — Skull of the great ant-eater (Myrrne-
o-io-antiV fr»ccilc rn^cro cophaga jubata} : 15, nasal ; n, frontal; 7, parietal;
gig dUlll l US, i ic^d- 3> superoccipital; 2, occipital condyles; 28, tym-
therium and P"lvDtodon panic; 73, lachrymal; 32, lower mandible. Teeth
. wanting.

belong to this group.

The sloths and ant-eaters are covered with coarse hair ;
the armadillos and pangolins, with an armor of plates
or scales (Fig. 298). The ant-eaters and pangolins
are strictly edentate, or toothless ; the rest have molars,
wanting, however, enamel and roots. In general, it

1 82 STRUCTURAL AND SYSTEMATIC ZOOLOGY

may be said that the order includes all quadrupeds
having separate, clawed toes and no incisors. The
sloths are arboreal ; the others burrow. The brain is
generally smooth ; but that of the ant-eater is convoluted,
and has a large corpus callosum ; but in all the cerebel-
lum and part of the olfactory lobes are exposed.

FIG. 169. — Armadillo (Dasypus). Copyright, 1901, by N.Y. Zoological Society.

2. Cetacea, or whales, have the form and life of fishes,
yet they possess a higher organization than the preceding
orders. They have a broad brain, with many and deep
foldings ; the foramen magnum of the skull is entirely
posterior ; the whole head is disproportionately large, and
the jaws greatly prolonged. The body is covered with a
thick, smooth skin, with a layer of fat (" blubber ") under-
neath ; there are no clavicles ; the hind limbs are want-
ing, and the front pair changed to paddles (Fig. 171);
the tail expands into a powerful, horizontal fin ; neck
and external ears are wanting ; the eyes small, with only
two lids ; the nostrils (blowholes) — double in the whale,

VERTEBRATA

183

single in the porpoise — are on the top of the head. All
are carnivorous, and essentially marine, a few dolphins
only being found in the great'rivers. In the whalebone

FIG. 170. — Outline of the sperm whale (Physeter). The blowhole is seen at the extreme
tip of the head. In this region the spermaceti is found. Maximum length, eighty-
five feet. South Atlantic.

whales, the teeth are absorbed, and disappear before
birth, and their place is supplied by horny "baleen"
plates (Fig. 228). " The whale feeds by putting this

FlG. 171. — Greenland whale (Baleena mystzcetns} . North Atlantic.

gigantic strainer into operation, as it swims through the
shoals of minute mollusks, crustaceans, and fishes, which
are constantly found at the surface of the sea. Open-

1 84 STRUCTURAL AND SYSTEMATIC ZOOLOGY

ing its capacious mouth, and allowing the sea water,
with its multitudinous tenants, to fill the oral cavity, the
whale shuts the lower jaw upon the baleen plates, and
straining out the water through them, swallows the prey
stranded upon its vast tongue." In the other cetaceans
teeth are developed, especially in dolphins and porpoises ;
but the sperm whale has them only in the lower jaw,
and the narwhal can show but a single tusk. In the
toothed cetaceans the organ of smell is very rudimentary
or even absent (dolphins).

3. Sirenia resemble the cetaceans in shape, but are
closely allied to the hoofed animals in organization.

FIG. 172. — Troop of dolphins, with manatee in the distance.

They have the limbs of the whales, and are aquatic ; but
they are herbivorous, and frequent great rivers and
estuaries. They have two sets of teeth, the cetaceans
having but one. They have a narrow brain ; bristles
scantily covering the body ; and nostrils placed on the

VERTEBRATA 185

snout, which is large and fleshy. The living representa-
tives are the manatee, of both sides of the tropical Atlantic
Ocean, and the dugong of the East Indies (Fig. 270).

4. Ungulata, or hoofed quadrupeds. This large
order, comprehending many animals most useful to
man, is distinguished by four well-developed limbs, each
toe being generally encased in a hoof (Fig. 300). The
leg, therefore, has no prehensile power ; it is only for
support and locomotion. Clavicles are wanting ; and
the radius and ulna are so united as to prevent rotation

FIG. 173. — Indian rhinoceros (/?. indicus).

(Figs. 314, 316). There are always two sets of teeth,
i.e., milk teeth are succeeded by a permanent set. The
grinders have broad crowns (Figs. 234, 308). As a rule
all are herbivorous. The brain is always convoluted,
but the cerebellum is largely uncovered (Fig. 335).

Ungulates are divided into two groups: those in
which the feet are always digitigrade, with never more
than four functional digits, as the horse, ox, and rhinoce-
ros; and those in which the feet may be plantigrade with
four or five digits, as the cony (Hyrax\ of Syria, and
the elephant. The dental formula of the horse is —

1 86 STRUCTURAL AND SYSTEMATIC ZOOLOGY

The canines are often wanting in the mare. The horse49
walks on the third finger arid toe. The metacarpals and
metatarsals are greatly elongated, so that the wrist and
heel are raised to the middle of the leg (Fig. 314).
The rhinoceros and tapir50 each have three toes. The
first is distinguished by its very thick skin, the absence
of canines, and one or two horns on the nose. The
tapir has the four kinds of teeth, and a short proboscis.
The Even-toed Ungulates — hog, hippopotamus, and
ruminants — have two or four toes. The hog and hip-
popotamus have the four kinds of teeth (Fig. 232); and,
in the wild state, are vegetarian. The ruminants have
two toes on each foot, enveloped in hoofs which face
each other by a flat side, so that they appear to be a
single hoof split or "cloven." Usually there are also
two supplementary hoofs behind, but they do not ordi-
narily touch the ground. All chew the cud, and have a
complicated stomach (Fig. 254). They have incisors in
the lower jaw only, and these are apparently eight; but
the two outer ones are canines.51 The molars are flat,
typical grinders. The dental formula of the ox is —

o — o o — o

"

3

With few exceptions, as the camel, all ruminants have
horns, which are always in pairs. Those of the deer
are solid, bony, and deciduous ; those of the giraffe and
antelope are solid, horny, and permanent; in the goat,
sheep, and ox, they are hollow, horny, and permanent.

The elephant, now nearly extinct, is characterized by
two upper incisors in the form of tusks, mainly com-
posed of dentine (ivory). In the extinct dinotherium
the tusks projected from the lower jaw ; and in the
mastodon, from both jaws. . Canines are wanting. The
molars are few and large, with transverse ridges (ele-
phant) or tubercles (mastodon). The cerebrum is large

VERTEBRATA 187

and convoluted, but does not cover the cerebellum. The
skull is enormous, the size arising in great measure from
the development of air cavities between the inner and
outer plates. The nose is prolonged into a flexible
trunk, which is a strong and delicate organ of prehen-
sion. There are four massive limbs, each with five toes
incased in broad, shallow hoofs, and also with a thick,

FIG. 174. —Stag, or red deer (Cervus elaphus). Europe. Copyright, 1901, by
N.Y. Zoological Society.

tegumentary pad. The knee is below and free from the
body, as in monkeys and men. Clavicles are wanting
(Fig. 316). The body of the elephant is nearly naked ;
but the mammoth, an extinct species, had a covering of
long woolly hair. Elephants live in large herds, and
subsist on foliage and grass. There are but two living
species : the Asiatic, with long head, concave forehead,
small ears, and short tusks ; and the African, with round
head, convex forehead, large ears, and long tusks.52

1 88 STRUCTURAL AND SYSTEMATIC ZOOLOGY

5. Carnivora, or beasts of prey, may be recognized by
their four long, curved, acute, canine teeth, the gap be-
tween the incisors
and canines in the
upper jaw for the
reception of the
lower canine, and
molars graduating
from a tuberculate
to a trenchant form,
in proportion as
the diet deviates
from a miscella-
neous kind to one
strictly of flesh
(Figs. 303, 307).
The incisors, ex-
cept in the pinni-
grades, number six
in each jaw. There
are always two
sets. The skull is
comparatively

FIG. 176. — Wolf (Lupus occidentalis}. United States.
Copyright, 1901, by N.Y. Zoological Society.

FIG. 175. — Raccoon (Procyon lotor). United States.

the jaWS are

shorter and deeper
than in ungulates,
and there are nu-
merous bony ridges
on the inside and
outside of the cra-
nium — the high

FIG. 177. — Ermine weasel (Putorius noveboracensis) . QCCiDital CTCSt be-
United States.

ing specially char-
acteristic. The cerebral hemispheres are joined by
a large corpus callosum, but the cerebellum is never

VERTEBRATA

I89

completely covered (Fig. 339). Both pairs of limbs are
well developed, the front being prehensile ; but the
clavicles are rudimentary. The humerus and femur
are mainly inclosed
in the body. The
digits, never less
than four, always
have sharp and
pointed claws.53
The body is cov-
ered with abundant
hair.

Carnivores may
be divided accord-
ing to the modifica- FIG. 178. — Kz&foxtVulpespennsylvanicus). United

tions of the limbs : States" Co^^ht' ^OI> b^ N- Y- Zo°logical Society
a. Pinnigrades, having short feet expanded into webbed
paddles for swimming, the hinder ones being bound in
with the skin of the tail. -Such are the seals, walrus,

FIG. 179. — Southern sea lion (Otan'a jubata) . Antarctic Ocean.

and eared seals, or sea lions, b. Plantigrades, in which
the whole, or nearly the whole, of the hind foot forms
a sole, and rests on the ground. The claws are not

190 STRUCTURAL AND SYSTEMATIC ZOOLOGY

retractile ; the ears are small, and tail short. Bears, bad-
gers, and raccoons are well-known examples, c. Digiti-
grades keep the heel raised above the ground, walking
on the toes. The majority have long tails. Such are
the weasels, otters, civets, hyenas, foxes, jackals, wolves,
dogs, cats, panthers, leopards, tigers, and lions. . The
last five differ from all others in having retractile
claws, and the radius rotating freely on the ulna. The
cats have thirty teeth ; the dogs, forty-two, or twelve
more molars. In the former, the tongue is prickly ; in
the latter, smooth.

6. Rodentia, or gnawers, are characterized by two
long, curved incisors in each jaw, enameled in front,

FIG. 180. — Skull of a rodent (Capybara) : 22, premaxillary; 21, maxillary; 26, molar;
27, squamosal; 73, lachrymal; 15, nasal; n, frontal; 4, occipital processes, un-
usually developed; z, incisors; a, angle of lower jaw.

and perpetually growing ; they are specially formed for
nibbling. Separated from them by a wide space (for
canines are wanting), are the flat molars, admirably
fitted for grinding. The lower jaw has longitudinal
condyles, which work freely backward and forward in
longitudinal furrows. Nearly all have clavicles, and
the toes are clawed. The cerebrum is nearly or quite
smooth, and covers but a small part of the cerebellum.
All are vegetarian.

More than one half of all known mammals are rodents.

VERTEBRATA

They range from the equator to the poles, over every
continent, over mountains and plains, deserts and woods.

FIG. 181. — Incisor teeth of the hare.

The more important representatives are the porcupines,
capybaras, guinea-pigs, hares, mice, rats, squirrels, and

FIG. 182. — Beaver {Castor canadensis). North America. Copyright, 1900, by
A. Radcliffe Dugmore.

beavers. The capybara and beaver are the giants of
the race.

192 STRUCTURAL AND SYSTEMATIC ZOOLOGY

7. Insectivora are diminutive, insect-eating animals,
some, as the shrew, being the smallest of mammals.

They have small, smooth brains,
which, as in the preceding or-
ders, leave uncovered the cere-
bellum and olfactory lobes. The
molar teeth bristle with sharp,
pointed cusps, and are asso-
FIG. ,83.- shrew mouse (Sor**). c^^ with canines and incisors.

They have a long muzzle, short legs, and clavicles.
The feet are formed for walking or grasping, and are
plantigrade, five-toed, and clawed. The shrew, hedge-
hog, and mole are examples.

8. Cheiroptera, or bats, repeat the chief characters of
the Insectivores ; but some (as the flying fox) are fruit-
eaters, and have corresponding modifications of the

FIG. 184. — Bat (.Vespertilid).

teeth. They are distinguished by their very long fore
limbs, which are adapted for flight, the fingers being im-
mensely lengthened, and united by a membranous web.

VERTEBRATA

193

The toes, and one or two of the fingers, are armed with
hooked nails. The clavicles are remarkably long, and
the sternum is of great strength ; but the whole skele-
ton is extremely light, though not filled with air, as in

\ ,.

FIG. 185. — Skeleton of a bat.

birds. The eyes are small, the ears large, and the sense
of touch is very acute. The favorite attitude of a bat
when at rest is that of suspension by the claws, with
head downward. They are all nocturnal.

9. Primates, the head of the kingdom, are character-
ized by the possession of two hands and two feet. The
thigh is free from the body, and all the digits are fur-
nished with nails, the first on the foot enlarged to a
"great toe." Throughout the order, the hand is emi-
nently or wholly prehensile, and the foot, however
prehensile it may be, is always a locomotor organ (Fig.
IQ2).54 The clavicles are perfect (Fig. 317). The eyes
are situated in a complete bony cavity, and look for-
ward. There are two sets of teeth, all enameled ; and
the incisors number four in each jaw (Fig. 233). They
include the lemurs, monkeys and apes, and man.

The lemurs are covered with soft fur, have usually

a long tail, pointed ears, foxlike muzzle, and curved

nostrils. They walk on all fours, and the thumb and

great toe are generally opposable to the digits: The

DODGE'S GEN. ZOOL. — 13

194 STRUCTURAL AND SYSTEMATIC ZOOLOGY

second toe has a long, pointed claw instead of a nail.

The cerebrum is relatively small, and flattened, and

does not cover the
cerebellum and olfac-
tory lobes.55 They
are found mainly in
Madagascar.

The monkeys of
tropical America
have, generally, a
long, prehensile tail;
the nostrils are placed

FIG. 186. — Lemur (L. ruber). Madagascar. far apart, SO that the
Copyright, 1901, by N. Y. Zoological Society.

nose is wide and flat ;
the thumbs and great «toes are fitted for grasping, but are

1 FIG. 187. — White-throated sapajou (Cebus hypoleucus). Central America.

not opposable to the other digits ; and they have four
molars more than the apes or man — that is, thirty-six

VERTEBRATA

195

teeth in all. In the apes of the Old World the tail is never
prehensile, and is sometimes wanting; the nostrils are

FIG. 188. — Skull of orang-outang (Simia FIG. 189. — Skull of chimpanzee (Anthro~
satyrus}. popithecus troglodytes).

close together ; both thumbs and great toes are opposa-
ble ; and the teeth, though numbering the same as man's,

FIG. 190. — Female orang-outang (from photograph). Borneo.

are uneven (the incisors being prominent, and the canines
large), and the series is interrupted by a gap on one side

196 STRUCTURAL AND SYSTEMATIC ZOOLOGY

or other of the canines. Their average size is much
greater than that of the monkeys, and they are not
so strictly arboreal. In both monkeys and apes the
cerebrum covers the cerebellum (Fig. 34O).56 While
in the monkeys the skull is rounded and smooth, that
of the apes, especially those coming nearest to man —
the anthropoid, or long-armed, apes, as gorilla, chim-
panzee, orang, and gibbon — is characterized by strong
crests. Monkeys take a horizontal position ; but the
apes assume a semierect attitude, the legs being shorter
than the arms. In all the primates but man, the body
is clothed with hair, which is generally longest on the
back. Several monkeys and apes have a beard, as the
howler and orang.

The orang is the least human of all the anthropoid

FIG. 191. — Skeletons of man, chimpanzee, and orang.

apes as regards the skeleton, but comes nearest to man
in the form of the brain. The chimpanzee approaches
man more closely in the character of its cranium and
teeth, and the proportional length of the arms. The

VERTEBRATA 197

gorilla is most manlike in bulk (sometimes reaching
the height of five feet six inches), in the proportions
of the leg to the body and of the foot to the hand, in
the size of the heel, the form of the pelvis and shoulder
blade, and volume of brain.67

FIG. 192 — Gorilla.

Man differs from the apes in being an erect biped.
In him, the vertebrate type, which began in the hori-
zontal fish, finally became vertical. No other animal
habitually stands erect ; in no other are the fore limbs
used exclusively for prehensile purposes, and the hind
pair solely for locomotion.

His limbs are naturally parallel to the axis of his
body, not perpendicular. They have a near equality
of length, but the arms are always somewhat shorter
than the legs. In all the great apes the arms reach
below the knee, and the legs of the chimpanzee and
gorilla are relatively shorter than man's.

Only man has a finished hand, most perfect as an

198 STRUCTURAL AND SYSTEMATIC ZOOLOGY

organ of touch, and most versatile. Both hand and
foot are relatively shorter than in the apes. The foot

a b

FIG. 193. — Foot (a) and hand (b) of the gorilla.

is plantigrade ; the leg bears vertically upon it ; the
heel and great toe are longer than in other primates ;
and the great toe is not opposable, but is used only
as a fulcrum in locomotion. The gorilla has both an

FIG. 194. — Australian savage.

inferior hand and inferior foot. The hand is clumsier,
and with a shorter thumb than man's; and the

VERTEBRATA

I99

foot is prehensile, and is not applied flat to the
ground.58

The scapular and pelvic bones are extremely broad,
and the neck of the femur remarkably long. Man is
also singular in the double curve of the spine : the
baboon comes nearest to man in this respect.

The human skull has a smooth, rounded outline, ele-
vated in front, and devoid of crests. The cranium
greatly predominates over the face, being four to one ; 59
and no other animal (except the siamang gibbon) has a
chin.

Man stands alone in the peculiarity of his dentition :

FIG. 195. — Skull of European.

FIG. 196. — Skull of negro.

his teeth are vertical, of nearly uniform height, and close
together. In every other animal the incisors and canines
are more or less inclined, the canines project, and there
are vacant spaces.60

Man has a longer lobule to his ear than any ape, and
no muzzle. The bridge of his nose is decidedly convex ;
in the apes generally it is flat.

Man has been called the only naked terrestrial mam-
mal. His hair is most abundant on the scalp ; never on
the back, as in the apes.

Man has a more pliable constitution than the apes, as

200 STRUCTURAL AND SYSTEMATIC ZOOLOGY

shown by his world-wide distribution The animals near-
est him soon perish when removed from their native
places.

Though man is excelled by some animals in the acute-
ness of some senses, there is no other animal in which
all the senses are capable of equal development. He
alone has the power of expressing his thoughts by
articulate speech, and the power of forming abstract
ideas.

Man differs from the apes in the absolute size of the
brain, and in the greater complexity and less symmet-
rical disposition of its convolutions. The cerebrum is
larger in proportion to the cerebellum (being as 8|- to i),
and the former not only covers the latter, but projects
beyond it. The brain of the gorilla scarcely amounts
to one third in volume or one half in weight of that of
man. Yet, so far as cerebral structure goes, man differs
less from the apes than they do from the monkeys and
lemurs.

The view held by evolutionists that man and the man-
like apes are descendants of a common ancestor is based
upon arguments drawn from structural and physiologi-
cal features. In his anatomy man resembles apes more
closely than any other group of animals. He differs
from them mainly in having a much larger brain. In
his skeletal, muscular, nervous, and other systems he
possesses about seventy-five vestigial structures, i.e.,
anatomical parts which are more perfectly developed
and more useful in apes and lower animals. Physiologi-
cally, man resembles the apes in having a similar bodily
life, in performing many actions in the same manner,
in being subject to the same diseases, in making
similar gestures, facial expressions, etc. The great
gulf between man and the brute is not physical, but
psychical,61

THE CLASSIFICATION OF ANIMALS

2O I

Meso:zo^>vJ''

^fcdS^^Hiw

PfroVoZ ^

4 ^-

j. V

X^|V ^<X

FIG. 197. — Diagrammatic expression of classification in a genealogical tree. B indicates
possible position of Balanoglossus, D of Dipnoi, S of Sphenodon or Hatteria.

202 STRUCTURAL AND SYSTEMATIC ZOOLOGY

•

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g

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— Animals with cellular

- Metazoa, with numerou
keleton, independent cell

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Order 2. FORAMINII

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Order 4. RADIOLARI

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Order -2. TENTACUL

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ARRANGEMENT OF REPRESENTATIVE FORMS 203

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sclerobasic, smooth coral: Astrcea.
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204 STRUCTURAL AND SYSTEMATIC ZOOLOGY

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206 STRUCTURAL AND SYSTEMATIC ZOOLOGY

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: 7«/wj.

:t ; chitinous crust ; six thoracic legs ; winged ; two antennae;

lickened, narrow and overlapping, hind pair transparent, broad, and folded;
•ge, transparent wings; biters: Libellula,

is : Cimex.
( transparent: Cicada.
r forewings opaque at base : A nasa.
; suctorial: Musca.
f antennae feathery : Telea.

. , . r antennae spindle-shaped:
ot locomotive; spiral proboscis for suction; \ .
Sphinx.

[ antennae knobbed : Papilio.
ting by straight edge; biters: Harpalus.
\ fitted for both biting and suction: Apis.

rax.

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icheal respiration ; suctorial: Ixodes.
long spinelike tail ; marine: Limulus.

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208 STRUCTURAL AND SYSTEMATIC ZOOLOGY

:on ; notochord or backbone ; nervous chord dorsal

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( soft-finned : Salmo.
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210 STRUCTURAL AND SYSTEMATIC ZOOLOGY

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PART II
COMPARATIVE ZOOLOGY

CHAPTER IV

MINERALS AND ORGANIZED BODIES DISTINGUISHED

Nature may be separated into two great kingdoms, —
that of mere dead matter, and that of matter under the
influence of life.62 These differ in the following points : —

(i) Composition. — While most of the chemical elements
are found in different living beings, by far* the greater
part of their substance is composed of three or four, —
carbon, oxygen, and hydrogen ; or these three with the
addition of nitrogen. Next to these elements, sulphur
and phosphorus are most widely distributed, though
always found in very small quantities. The organic com-
pounds belong to the carbon series, and contain three,
four, or five elements. The former class, comprising
starch, sugar, fat, etc., are relatively stable. The latter,
possessing the three elements named, with nitrogen and
sulphur or phosphorus, are very complex, containing a
very large number of atoms to the molecule, and are
usually unstable. Here belong albumen, myosin, chon-
drin, etc., the constituents of the living tissues. The
formula for albumen is said to be C72H112N18SO22, or
some multiple of this formula. These compounds also
contain more or less water, and usually exist in a jelly-
like condition, neither solid nor fluid. All organic com-
pounds are formed through the chemical activities of
protoplasm, which is the only living substance. Inor-
ganic matter may, under its influence, be changed to
organic, and vice versa ; dead matter which enters the
body of organized beings in the form of nutriment is

215

2l6 COMPARATIVE ZOOLOGY

changed into living substance, which, after serving its
purpose, passes again as waste to the inorganic world.

(2) Structure. — Minerals are homogeneous, while organ-
ized bodies are usually heterogeneous, i.e., composed of
different parts, called tissues and organs, having peculiar
uses and definite relations to one another. The tissues
and organs, again, are heterogeneous, consisting mainly
of microscopic cells, structures developed only by vital
action. All the parts of an organism are mutually
dependent, and reciprocally means and ends, while
each part of a mineral exists for itself. The smallest
fragment of marble is as much marble as a mountain
mass ; but the fragment of a plant or animal is not an
individual.

(3) Shape and Size. — Living bodies gradually acquire
determinate dimensions ; so do minerals in their per-
fect or crystal condition. But uncrystallized, inorganic
bodies have an indefinite bulk. Most minerals are
amorphous ; crystals have regular forms, bounded, as a
rule, by plane surfaces and straight lines ; plants and
animals are circumscribed by curved surfaces, and
rarely assume accurate geometrical forms. 63

(4) Phenomena. — Minerals remain internally at rest, and
increase by external additions, if they grow at all. Liv-
ing beings are constantly changing the matter of which
they are composed, and grow by taking new matter into
themselves and placing it among the particles already
present. Organized bodies, moreover, pass through a
cycle of changes, — growth, development, reproduction,
and death. These phenomena are characteristic of liv-
ing as opposed to inorganic bodies. All living bodies
grow from within, constantly give up old matter and
replace it by new, reproduce their kind, and die ; and
no inorganic body shows any of these phenomena.

CHAPTER V*

PLANTS AND ANIMALS DISTINGUISHED

IT may seem an easy matter to draw a line between
plants and animals. Who cannot tell a cow from a
cabbage ? Who would confound a coral with a mush-
room ? Yet it is impossible to assign any absolute, dis-
tinctive character which will divide the one form of life
from the other. The difficulty of defining an animal
increases with our knowledge of its nature. Linnaeus
denned it in three words ;f a century later, Owen declared
that a definition of plants which would exclude all
animals, or of animals which would not let in a single
plant, was impossible. Each different character used
in drawing the boundary will bisect the debatable ground
in a different latitude of the organic world. Between
the higher animals and higher plants the difference is
apparent ; but when we reflect how many characters the
two have in common, and especially when we descend
to the lower and minuter forms, we discover that the
two " kingdoms " touch, and even dissolve into, each
other. This border land has been as hotly contested
among naturalists as many a disputed frontier between
adjacent nations. Its inhabitants have been taken and
retaken several times by botanists and zoologists ; for
they have characters that lead on the one side to
plants, and on the other to animals. To solve the
difficulty, some eminent naturalists, as Haeckel and

* See Appendix.

t " Minerals grow ; plants grow and live ; animals grow, live, and
feel."

217

2l8 COMPARATIVE ZOOLOGY

Owen, propose a fourth " kingdom," that of the
Protista, to receive those living beings which are
organic, but not distinctly vegetable or animal. But a
greater difficulty arises in attempting to fix its precise
limits.

The drift of modern research points to this : that
there are but two kingdoms of nature, the mineral and
the organized, and these closely linked together ; that
the latter must be taken as one whole, from which two
great branches rise and diverge. " There is at bottom
but one life, which is the whole life of some creatures
and the common basis of the life of all ; a life of sim-
plest moving and feeling, of feeding and breathing, of
producing its kind and lasting its day : a life which, so
far as we at present know, has no need of such parts
as we call organs. Upon this general foundation are
built up the manifold special characters of animal and
vegetable existence ; but the tendency, the endeavor, so
to speak, of the plant is one, of the animal is another,
and the unlikeness between them widens the higher the
building is carried up. As we pass along the series of
either [branch] from low to high, the plant becomes
more vegetative, the animal more animal."64

Defining animals and plants by their prominent char-
acteristics, we may say that a living being which has
cell walls of cellulose, and by deoxidation and synthesis
of its simple food stuffs produces the complicated or-
ganic substances, is a plant ; while a living being which
has albuminous tissues, and by oxidation and analysis
reduces its complicated food stuffs to a simpler form,
is an animal. But both definitions are defective, includ-
ing too many forms, and excluding forms that properly
belong to the respective kingdoms. No definition is
possible which shall include all animals and exclude all
plants, or vice versa.

PLANTS AND ANIMALS DISTINGUISHED 219

(1) Origin. — Both branches of the tree of life start
alike : the lowest of plants and animals consist of a
single cell. In fact, the cycle of life in all living beings
begins in a small, round particle of matter, a cell — in
the higher plants called an ovule, in the higher animals
an ovum. This cell consists mainly of a semifluid sub-
stance called protoplasm. In the very simplest forms
the protoplasm is not inclosed by a membrane or cell
wall. In most plants the cell wall is present, and con-
sists of cellulose, a substance akin to starch; in animals,
with few exceptions, the wall is a pellicle of firmer pro-
toplasm, i.e., albuminous.

(2) Composition. — Modern research has broken down
the partition between plants and animals, so far as
chemical nature is concerned. The vegetable fabric
and secretions may be ternary or binary compounds ;
but the essential living parts of plants, as of animals,
are quaternary, consisting of four elements, — carbon,
hydrogen, oxygen, and nitrogen. Cellulose (woody
fiber), starch, and chlorophyl (green coloring matter)
are eminently vegetable products, but not distinctive ;
for cellulose is wanting in some plants, as some fungi,
and present in some animals, as tunicates ; starch, under
the name of glycogen, is found in the liver and brains
of mammals, and chlorophyl gives color to the fresh-
water polyp. Still, it holds good, generally, that plants
consist mainly of cellulose, dextrin, and starch ; while
animals are mainly made up of albumen, fibrin, and
gelatin ; that nitrogen is more abundant in animal tis-
sues, while in plants carbon is predominant.

(3) Form. — No outline can be drawn which shall be
common to all animals or all plants. The lowest mem-
bers of each group have no fixed shape. The spores
of Confervae can hardly be distinguished from animal-
cules ; the compound and fixed animals, sea mat and

220 COMPARATIVE ZOOLOGY

sea moss (Polyzoa), and corals, often resemble vegetable
forms, although in structure widely removed from
plants. Similar conditions of life are here accompanied
by an external likeness. In free-living animals this
resemblance is not found.

(4) Structure. — A plant is the multiplication of the
unit — a cell with a cellulose wall. Some simple ani-
mals have a similar simple cellular structure ; and all
animal tissues, while forming, are cellular. But this
character, which is permanent in plants, is generally
transitory in animals. In the more highly organized
tissues the cells are so united as partly or wholly to lose
their individuality, and the characteristic part of the
tissue is the intercellular substance, while the cells
themselves are small and unimportant, or else the cells
are fused together and their dividing walls become in-
distinct, as in glandular tissue. Excepting the lowest
forms, animals are more composite than plants, i.e.,
their organs are more complex and numerous, and
more specially devoted to particular purposes. Repe-
tition of similar parts is a characteristic of plants ; and
when found in animals, as the angleworm, is called vege-
tative repetition. Differentiation and specialization are
characteristic of animals. Most animals, moreover,
have fore-and-aft polarity ; in contrast, plants are up-
and-down structures, though in this respect they are
imitated by radiate animals, like the starfish. Plants
are continually receiving additional members ; most
animals soon become perfect.

(5) Physiology. — In their modes of nutrition, plants
and animals stand widest apart. A plant in the seed
and an animal in the egg exist in similar conditions : in
both cases a mass of organic matter accompanies the
germ. When this supply of food is exhausted, both
seek nourishment from without. But here analogy

PLANTS AND ANIMALS DISTINGUISHED 221

ends: the green plant feeds on mineral matter, the
animal on organic. Some plants have the power to
form chlorophyl, the green coloring matter of leaves,
which uses the energy of the sunlight to form starch
out of the inorganic substances, — carbon dioxide and
water. They are able also to form albuminoid matter
out of inorganic substances. A very few animals which
have a substance identical with or allied to chlorophyl
have the same power, but in general animals are de-
pendent for their food on the compounds put together in
plants. Colorless plants, as fungi, possessing no chloro-
phyl, feed, like animals, on organic compounds. No
living being is able to combine the simple elements —
carbon, oxygen, hydrogen, and nitrogen — into organic
compounds.

The food of plants is gaseous (carbon dioxide and
ammonia) or liquid (water containing substances in so-
lution), that of animals usually more or less solid, though
solid substances must be changed to liquids before being
capable of absorption into the tissues. The plant, then,
absorbs these foods through its outer surface, while the
animal takes its nourishment in larger or smaller masses,
and digests it in a special cavity. A few exceptions,
however, occur, since certain animals, as the tapeworm,
have no digestive tract but absorb liquid food through
the surface of the body.

Plants are ordinarily fixed, their food is brought to
them, and a large share of their work, the formation of
organic compounds, is done by the energy of the sun-
light; while animals are usually locomotive, must seek
their food, and are unable to utilize the general forces
of nature as the plant does. The plant is thus able to
grow much more than the animal, as very little of the
nourishment received is used to repair waste, while in
most animals the time soon comes when waste and re-

222 COMPARATIVE ZOOLOGY

pair are approximately equal. But in both all work
done is paid for by waste of substance already formed.

In combining carbon dioxide and water to form starch
the plant sets oxygen free (6(CO2) + 5(H2O) = C6H10O5
+ 6(O2)) : in oxidizing starch or other food the animal
uses oxygen and sets carbon dioxide free. The green
plant in the sunlight, then, gives off oxygen and uses
carbon dioxide, while plants, which have no chlorophyl,
at all times, and all plants in the darkness, use oxygen
and give off carbon dioxide, like an animal. Every
plant begins life like an animal — a consumer, not a
producer : not till the young shoot rises above the soil,
and unfolds itself to the light of the sun, at the touch
of whose mystic rays chlorophyl is developed, does real,
constructive vegetation begin ; then its mode of life is,
in a sense, reversed; since more carbon is combined
than liberated, and more oxygen set free than main-
tained.

Most plants, and many animals, multiply by budding
and division ; on both we practice grafting ; in both the
cycle of life comes round again to the ovule or ovum.
Do annuals flower but to die ? Insects lay their eggs
in their old age.

Both animals and plants have sensibility. This is one
of the fundamental physiological properties of proto-
plasm. But in plants the protoplasm is scattered and
buried in rigid structures : feeling is, therefore, dull.
In animals irritability is a highly developed property of
certain organs, and so feeling, like electricity rammed
into Ley den jars, goes off with a flash.65 Plants prob-
ably never possess consciousness or volition, as the
higher animals do.

The self-motion of animals and the rooted state of
plants is a very general distinction ; but it fails where
we need it most. It is a characteristic of living things

PLANTS AND ANIMALS DISTINGUISHED 223

to move. The protoplasm of all organisms is unceas-
ingly active.66 Besides this internal movement, myriads
of plants, as well as animals, are locomotive. Rambling
diatoms, writhing oscillaria, and the agile spores of cryp-
togams crowd our waters, their organs of motion (cilia
and pseudopodia) being of the very same character as
in microscopic animals ; while sponges, corals, oysters,
and barnacles are stationary. A contractile vesicle is
not exclusively an animal property, for the several fresh-
water algae, as Gonium, have it. The muscular contrac-
tions of the highest animals and the sensible motions of
plants are both due to changes in the protoplasm in
their cells. The ciliary movements of animals and of
microscopic plants are precisely similar, and in neither
case necessarily indicate consciousness or self-determin-
ing power.

Plants, as well as animals, need a season of repose.
Both have their epidemics. On both, narcotic and
acrid poisons produce analogous results. Are some
animals warm-blooded ? In germination and flowering,
plants evolve heat — the stamens of the arum, e.g.,
showing a rise of 20° F. In a sense, an oak has just
as much heat as an elephant, only the miserly tree locks
up the sunlight in solid carbon.

At present, any boundary of the animal kingdom is ar-
bitrary. " We cannot distinguish the vegetable from the
animal kingdom by any complete and precise definition.
Although ordinary observation of their usual representa-
tives may discern little that is common to the two, yet
there are many simple forms of life which hardly rise
high enough in the scale of being to rank distinctively
either as plant or animal; there are undoubted plants
possessing faculties which are generally deemed charac-
teristic of animals ; and some plants of the highest grade
share in these endowments."67

CHAPTER VI

RELATION BETWEEN MINERALS, PLANTS, AND
ANIMALS

THERE are no independent members of creation : all
things touch upon one another. The matter of the liv-
ing world is identical with that of the inorganic. The
plant, feeding on the minerals, carbon dioxide, water,
and ammonia, builds them up into complex organic com-
pounds, as starch, sugar, gum, cellulose, albumen, and
gluten. When the plant is eaten by the animal, these
substances are used for building up tissues, supplying
energy, repairing waste, laid up in reserve as glycogen
and fat, or oxidized in the tissues to produce heat. The
albuminoids are essential for the formation of tissues,
like muscle, nerve, cartilage ; the ternary compounds
help in repairing waste, while both produce heat. When
oxidized, whether for work or warmth, these complex
compounds break up into the simple compounds, —
water, carbon dioxide, and (ultimately) ammonia, and as
such are returned to earth and air from the animal.
Both plant and animal end their life by going back
to the mineral world : and thus the circle is complete
— from dust to dust. Plants compress the forces of
inorganic nature into chemical compounds ; animals
liberate them. Plants produce; animals consume. The
work of plants is synthesis, a building-up ; the work
of animals is analysis, or destruction. Without plants,
animals would perish ; without animals, plants had no
need to be.

224

CHAPTER VII*

LIFE

ALL forces are known by the phenomena which they
cause. So long as the animal and plant were supposed
to exist in opposition to ordinary physical forces or inde-
pendently of them, a vital force or principle was postu-
lated by which the work of the body was performed.
It is now known that most, if not all, of the phenomena
manifested by a living body are due to one or more of
the ordinary physical forces, — heat, chemical affinity,
electricity, etc. There is no work done which demands
a vital force.

The common modern view is that vitality is simply a
collective name for the sum of the phenomena displayed
by living beings. It is neither a force nor a thing at
all, but is an abstraction, like goodness or sweetness ;
or, to use Huxley's expression, to speak of vitality is as
if one should speak of the horologity of a clock, mean-
ing its time-keeping properties.

A third theory is still possible. The combination of
elements into organic cells, the arrangement of these
cells into tissues, the grouping of these tissues into
organs, and the marshaling of these organs into plans
of structure, call for some further shaping, controlling
power to effect such wonderful coordination. More-
over, the manifestation of feeling and consciousness is a
mystery which no physical hypothesis has cleared up.
The simplest vital phenomenon has in it something over

* See Appendix.
DODGE'S GEN. ZOOL. — 15 225

226 COMPARATIVE ZOOLOGY

and above the known forces of the laboratory.68 If the
vital machine is given, it works by physical forces ; but
to produce it and keep it in order needs, so far as we
now know, more than mere physical force. To this
controlling power we may apply the name vitality.

Life is exhibited only under certain conditions. One
condition is the presence of a physical basis called proto-
plasm. This substance is found in all living bodies,
and, so far as we know, is similar in all — a viscid,
transparent, homogeneous, or minutely granular, albu-
minoid matter. Life is inseparable from this proto-
plasm ; but it is dormant unless excited by some
external stimulants, such as heat, light, electricity, food,
water, and oxygen. Thus, a certain temperature is
essential to growth and motion; taste is induced by
chemical action, and sight by luminous vibrations.

The essential manifestations of animal life may be
reduced to four : contractility ; irritability, or the power
of receiving and transmitting impressions ; the power of
assimilating food ; the power of reproduction. All these
powers are possessed by protoplasm, and so by all ani-
mals : all move, feel, grow, and multiply. But some of
the lowest forms are without any other trace of organs
than is found in a simple cell ; they seem to be almost as
homogeneous and structureless as a drop of jelly. They
could not be more simple. They are devoid of muscles,
nerves, and stomach ; yet they have all the fundamental
attributes of life, — moving, feeling, eating, and propa-
gating their kind. The animal series, therefore, begins
with forms that feel without nerves, move without mus-
cles, and digest without a stomach, protoplasm itself
having all these properties : in other words, life is the
cause of organization, not the result of it. Animals do
not live because they are organized, but are organized
because they are alive.

CHAPTER VIII*
ORGANIZATION

WE have seen that the simplest living thing is a
formless speck of protoplasm, without distinctions of
structure, and therefore without distinctions of function,
all parts serving all purposes — mouth, stomach, limb,
and lung — indiscriminately. There is no separate
digestive cavity, no separate respiratory, muscular, or
nervous systems. Every part will successively feed,
feel, move, and breathe. Just as in the earliest state of
society all do everything, each does all. Every man is
his own tailor, architect, and lawyer. But in the prog-
ress of social development the principle of the division
of labor emerges. First comes a distinction between
the governing and governed classes; then follow and
multiply the various civil, military, ecclesiastical, and
industrial occupations.

In like manner, as we advance in the animal series,
we find the body more and more heterogeneous and
complex by a process of differentiation, i.e., setting
apart certain portions of the body for special duty. In
the lowest forms, the work of life is carried on by very
simple apparatus. But in the higher organisms every
function is performed by a special organ. For example,
contractility, at first the property of the entire animal,
becomes centered in muscular tissue ; respiration, which
in simple beings is effected by the whole surface, is
specialized in lungs or gills ; sensibility, from j

* See Appendix.
227

228

COMPARATIVE ZOOLOGY

common to the whole organism, is handed over to the
nerves. An animal, then, whose body, instead of being
uniform throughout, is made up of different parts for the
performance of particular functions, is said to be organ-
ized. And the term is as applicable to the slightly dif-
ferentiated cell as to complex man. Organization is
expressed by single cells, or by their combination into
tissues and organs.

i. Cells. — A cell is the simplest form of organized
life. In general, it is a microscopic globule, consisting

FIG. igB. — A, diagram of a cell; TV, cell wall, with inclosed cytoplasm; «, nucleus,
consisting of nuclear membrane inclosing granular substance, in which are seen a
spherical nucleolus and several irregular masses of chromatin ; a, attraction sphere
containing a centrosome. B-F, changes which take place in the cell during fission.

of a delicate membrane inclosing a minute portion of
protoplasm. The very simplest kinds are without gran-
ules or signs of circulation ; but usually the protoplasm
is granular, and contains a defined separate mass called
the nucleus, within which are sometimes seen one or
two, rarely more, dark, round specks, named nucleoli.
The enveloping membrane is extremely thin, transpar-
ent, and structureless ; it is only an excretion of dead
matter acting as a boundary to the cell contents.69 The
nucleus generally lies near the center of the protoplasm,
and is the center of activity,

ORGANIZATION 22Q

Cells vary greatly in size, but are generally invisible
to the naked eye, ranging from ^J7 to IQTFO °f an mcn
in diameter. About 4000 of the smallest would be
necessary to cover the dot of this letter i. The natural
form of isolated cells is spherical ; but when they crowd
each other, as seen in bone, cartilage, and muscle, their
outlines become angular, either hexagonal or irregular.

Within the narrow boundary of a simple sphere, the
cell membrane, are exhibited all the essential phenomena
of life, — nutrition, sensation, development, and repro-
duction. The physiology of these minute organisms is
of peculiar interest, since all animal structure is but the
multiplication of the cell as a unit, and the whole life of
an animal is that of the cells which compose it : in them
and by them all its vital processes are carried on.70

The structure of an animal cell can be seen in blood
corpuscles, by diluting with a weak (.6 per cent) solu-
tion of salt a drop of blood from a frog, and placing it
under the microscope. (See Fig. 260.) With this may
be compared vegetable cells as seen in a drop of fluid
yeast or a drop of water into which pollen grains from
some flower have been dusted.

2. Tissues. — There are organisms of the lowest
grade (as Paramecium, Fig. 9) which consist of a single
cell, living for and by itself. In this case, the animal
and cell are identical : the Paramecium is as truly an
individual as the elephant. But all animajs, save these
unicellular beings, are mainly aggregations of cells ; for
the various parts of a body are not only separable by
the knife into bones, muscles, nerves, etc., but these are
susceptible of a finer analysis by the microscope, which
shows that they arise from the development and union
of cells. These cellular fabrics, called tissties, differ
fro/n one another both chemically and structurally, but
agree in being permeable to liquids — a property which

230 COMPARATIVE ZOOLOGY

secures the flexibility of the organs so essential to ani-
mal life. Every part of the human body, for example,
is moist ; even the hairs, nails, and cuticle contain water.
The contents as well as the shape of the cells are usually
modified according to the tissue which they form : thus,
we find cells containing earthy matter, iron, fat, mucus,
etc.

In plants, the cell generally retains its characters well
defined; but in animals (after the embryonic period)
the cell usually undergoes such modifications that its
structural features become altered. The cells are con-
nected together or enveloped by an intercellular sub-
stance (matrix), which may be watery, soft, and
gelatinous, firmer and tenacious, still more solid and
hyaline, or hard and opaque. In the fluids of the body,
as the blood, the cells are separate ; i.e., the matrix is
fluid. But in the solid tissues they are held together
by intercellular substance.

In the lowest forms of life, and in all the higher
animals in their earliest embryonic state, the cells of
which they are composed are not transformed into
differentiated tissues : definite tissues make their first
appearance in the sponges, and they differ from one
another more and more widely as we ascend the scale
of being. In other words, the bodies of the lower and
the immature animals are more uniform in composition
than the higher or adult forms. In the vertebrates only
are all the following tissues found represented : —

(i) Epithelial Tissue. — This is the simplest form of cel-
lular structure. It covers all the free surfaces of the
body, internal and external, so that an animal maybe
said to be contained between the walls of a double bag.
That which is internal, lining the mouth, windpipe,
lungs, blood vessels, gullet, stomach, intestines — in fact,
every cavity and canal — is called epithelium. It is a

ORGANIZATION

231

very delicate skin, formed of flat or cylindrical cells,
and in some parts (as in the windpipe of air-breathing
animals, and along the gills of the oyster) is covered
with cilia, or minute hairlike portions of protoplasm,
about -$-^0-$ of an inch long, which are incessantly mov-
ing. Continuous with this inner lining of the body (as
seen on the lip), and covering the outside, is the epi-
dermis or ctiticle. It is the outer layer of the " skin,"
which we can remove by a blister, and in man varies in
thickness from -g~J7 of an inch on the cheek to •£$ on
the sole of the foot.
It is constantly
wearing off at the
surface, and as con-
stantly being re-
plenished from the
deeper portion; and
in the process of
growth and pas-
sage OUtward, the FlG I99>_ Various kinds of Epithelium Cells magnified;
Cells change from a' c°lumnar» from small intestine; 3, a single cell,

the spherical form
to dead horny
scales (seen in scurf
and dandruff). In the lower layer of the cuticle we find
the pigment cells, characteristic of colored races. Nei-
ther the epidermis nor the corresponding internal tissue
(epithelium) has any blood vessels or nerves. The epi-
thelial tissue, then, is simply a superficial covering,
bloodless and insensible, protecting the more delicate
parts underneath, or, as in the alimentary canal, pro-
ducing mucus and digestive juices. Hairs, horns, hoofs,
nails, claws, corns, beaks, scales, tortoise shell, the wings
of insects, etc., are modifications of the epidermis.

The next three sorts of tissue are .characterized by a

showing nucleus; b, ciliated, from one of the small air-
tubes ; d, the same, from the windpipe, with single cell
magnified, about 200 times; c, squamous, from eyelid
of a calf, showing changes of form, from the deep to
superficial cells, i being the scurf.

232

COMPARATIVE ZOOLOGY

great development of the intercellular substance, while
the cells themselves are very slightly modified.

(2) Connective Tissue. — This is the most extensive tissue
in animals, as it is the great connecting medium by

which the differ-
ent parts are held
together. Could
it be taken out
entire, it would
be a complete
mold of all the
organs. It sur-
rounds the bones,
muscles, blood
vessels, nerves,
of the ligaments

FIG. 200. — Connective Tissue, showing areolar structure.

and glands, and is the substance

and tendons, and forms a large portion of "true skin,"

FIG. 201. — Connective Tissue from human peritoneum; highly magnified; a, blood vessel

ORGANIZATION

233

mucous membrane, etc. It varies in character, being
soft, tender, and elastic, or dense, tough, and generally
unyielding. In the former state, it consists of innu-
merable fine white and yellow fibers, which interlace
in all directions, leaving irregular spaces, and forming a
loose, spongy, moist web. In the latter the fibers are
condensed into sheets or parallel cords, having a wavy,
glistening appearance. Such structures are the fasciae
and tendons. Connective tissue is not very sensitive.
It contains gelatin — the matter which tans when hide
is made into leather. In this tissue the intercellular

substances take the

form of fibers. The

white fibers are ine-

lastic, and from

to °f an

diameter.

FIG. 202. -Hyaline Car-
tilage, Diagram : a, car-

tilage cell; t>, ceil about dons.

to divide; c, cell divided _. ,

into two; d, into four fibers are elastic, very

They are

^Q ten.

The yellow

long, and from „$„

to "V of an inch

highly magnified. in diameter, and
branched. Connec-
tive tissue appears areolar, i.e., shows
interspaces, only under the microscope.

(3) Cartilaginous Tissue. — This tissue, FIG. 203. — Longitudinal

. . section through area of

known also as " gristle, is composed ossification from long
of cells embedded in a granular or bone of human embryo.
hyaline substance, which is dense, elastic, bluish white,
and translucent. It is found where strength, elasticity,
and insensibility are wanted, as at the joints. It also
takes the place of the long bones in the embryo. When
cartilage is mixed with connective tissue, as in the ear,
it is called fibre-cartilage.

234

COMPARATIVE ZOOLOGY

FIG. 204. — Transverse section of a Bone (Human
Femur), x 50, showing Haversian canals.

(4) Osseous Tissue. — This hard, opaque tissue, called
bone, differs from the former two in having the inter-
cellular spaces or
meshes filled with
phosphate of lime
and other earths, in-
stead of a hyaline or
fibrous substance. It
may be called pet-
rified tissue — the
quantity of earthy
matter, and therefore
the brittleness of the
bone, increasing with
the age of the animal.
If a chicken bone be
left in dilute muriatic acid several days, it may be tied
into a knot, since the acid has dissolved the lime, leav-
ing nothing but cartilage and connective tissue. If a
bone be burned, it be-
comes light, porous,
and brittle, the lime
alone remaining.71

Bone is a very vas-
cular tissue; that is,
it is traversed by
minute blood vessels
and nerves, which
pass through a net-
work of tubes, called
Haversian canals.

The Canals average FjG 2Q5 _ Frontai Bone of Human Skull under the
__.!_._ Q£ g^ inch be- microscrope, showing lacunae and canaliculi.

ing finest near the surface of the bone, and larger
farther in, where they form a cancellated or spongy

ORGANIZATION

235

structure, and finally merge (in the long bones) into the
central cavity containing the marrow. Under the micro-
scope, each canal appears to be the center of a multitude
of lamince, or plates, arranged around it. Lying between
these plates are little cavities, called lacunce, which are
connected by exceedingly fine tubes, or canaliculi. The
two represent the spaces occupied by the original cells
of the bone, and differ in shape and size in different
animals.

True bone is found only in vertebrates, or backboned
animals.

(5) Dental Tissue. — Like bone, a tooth is a combination
of earthy and animal matter. It may be called petrified
skin. In the higher animals, it consists of three parts :

FIG. 206. — Highly magnified section of Dentine and Cement, from the fang of a Human
Molar: a, b, marks of the original dentinal pulp; d, dentinal tubes, terminating in
the* very sensitive, modified layer, g; hy cement.

dentine, forming the body of the tooth, and always pres-
ent; enamel, capping the crown; and cement, covering
the fangs (Fig. 229). The last is true bone, or osseous
tissue. Dentine resembles bone, but differs in having
neither lacunae nor (save in shark's teeth) canaliculi. It
shows, in place of the former, innumerable parallel tubes,
reaching from the outside to the pulp cavity within.
The " ivory " of elephants consists of dentine. Enamel
is the hardest substance in the body, and is composed
of minute six-sided fibers, set closely together. It is

236 COMPARATIVE ZOOLOGY

wanting in the teeth of most fishes, snakes, sloths,
armadillos, sperm whales, etc.

True dental tissue is confined to vertebrates.

(6) Adipose Tissue. — Certain cells become greatly en-
larged and filled with fat, so that the original protoplasm
occupies a very small part of the space within the cell
membrane. These cells are united into masses by con-
nective tissue, in the skin (as in the " blubber " of whales),
between the muscles (as in " streaky " meat), or in

FIG. 207. — Fat Cells embedded in Subcutaneous Areolar Tissue. _/, fat cells; «. nucleus;
c, connective-tissue corpuscles; w, migratory cells; e, elastic fibers; b, capillary
blood vessel.

the abdominal cavity, in the omentum, mesentery, or
about the kidneys. The marrow of bones is an example.
Globules of fat occur in many mollusks and insects ; but
true adipose tissue is found only in backboned animals,
particularly in the herbivorous. In the average man,
it constitutes about £$ part of his weight, and a single
whale has yielded 120 tons of oil. The fat of animals
has the different names of oil, lard, tallow, suet, sperma-
ceti, etc. It is a reserve of nutriment in excess of con-
sumption, serving also as a packing material, and as a
protection against cold.

(7) Muscular Tissue. — If we examine a piece of lean
meat, we find it is made up of a number of fasciculi, or
bundles of fibers, placed side by side, and bound together

ORGANIZATION

237

by connective tissue. The microscope informs us that

each fiber is itself a bundle of smaller fibers; and when

one of these is more closely exam-

ined, it is found to consist of a

delicate, smooth tube, called the sar-

co lemma, which is filled with very

minute, parallel fibrils, averaging

lowo °f an m°h m diameter, the

whole having a striated aspect and

containing numerous nuclei. Tissue

of this description constitutes all

ordinary muscle, or "lean meat,"

and is marked by regular cross FlG

lines, or s trice.

Besides this striated

muscular tissue, there

exist, in the coats of

the stomach, intestines,

blood vessels, and some

other parts of verte-

brates, smooth muscular fibers, which show

a single nucleus under the microscope, and

do not break up into fibrils (Fig. 319).

The gizzards of fowls exhibit this form.
All muscle has the property of shorten-

ing itself when excited; but the contraction

of the striated kind is under the control of

the will, while the movement of the smooth
™ - fibers is involuntary.72 Muscles are well

Man, divided by *

transverse septa supplied with arteries, veins, and nerves ;

into separate nu- , , . . . '

portions; but the color is due to a peculiar pigment,

208. — Voluntary
Muscle, portions of two
fibers showing the char-
acteristic transverse mark-
ings; the lighter band is
divided by the row of
minute beads constituting
the intermediate disk:
a, termination of muscular
substance and attachment
of adjoining fibrous tissue;
«, nuclei of muscle fibers.

much magnified.

Muscular tissue is found in all animals from the coral
to man.

(8) Nervous Tissue. — Nervous Tissue consists of large,

238

COMPARATIVE ZOOLOGY

nucleated cells, which give off one to several processes,
the latter serving as paths of communication between
the cells themselves or between the cells and the various
motor, sensory, and other organs with which they are
connected. Such threads of nerve tissue are called

R

[R

FIG. 210. — Portion
of two Nerve
Fibers: a, medul-
lary sheath; c, nu-
cleus of neuri-
lemma; R, annu-
lar constriction.

Dendrites

Nerve process
Collateral branch

Axis cylinder

Neurilemma

Terminal branches

FIG. 211. —Diagram of a Neuron: a, nerve pro-
cess; b, neurilemma; c, medullary sheath;
d, neurilemma and medullary sheath, combined.

nerve fibers, and each consists essentially of a prolonga-
tion of the protoplasrnic substance of which the cell
body is composed. The cells vary from 35*5-0 to %^Q of
an inch in diameter, and are found in the nerve centers
(Fig. 329), the gray portion of the brain, spinal cord,
and other ganglia. The fibers vary in structure. In
the lowest Metazoa they are merely naked threads of

ORGANIZATION 239

protoplasm. In the higher animals each thread, known
as the axis cylinder, 1s surrounded by a delicate, trans-
parent covering called the neurilemma, analogous to the
sarcolemma of muscle tissue. In the vertebrates, the
protoplasmic threads found in many parts of the nervous
system have an additional covering made of fatty
material, which lies between the axis cylinder and the
neurilemma, and is known as the medullary sheath.
These are called medullated nerve fibers, as distinguished

FIG. 212. — Spinal Ganglion, in longitudinal section, from Cat; the groups of nerve cells
lie embedded among the bundles of the nerve fibers.

from the nonmedullated or those which lack the medul-
lary sheath. Fibers of the former kind are found in the
white substance of the brain and spinal cord, and run
to the muscles and organs of sense. Nonmedullated
fibers are found in the gray substance of the nervous
system. The axis cylinders are destitute of a sheath in
the neighborhood of the cell body. Scattered along the
fibers nuclei are found. The large nerve fibers may be
T2Vo of an inch in diameter, and some are supposed to
extend from cell bodies situated in the lower part of the
spinal cord down the leg to the foot.

A bundle of nerve fibers surrounded by connective
tissue constitutes a nerve in the anatomical sense.

240 COMPARATIVE ZOOLOGY

3. Organs and their Functions. — Animals, like plants,
grow, feel, and move ; these three are the capital facts
of every organism. Besides these there may be some
peculiar phenomena, as motion and will.

Life is manifested in certain special operations,
called functions, performed by certain special parts,
called organs. Thus, the stomach is an organ, whose
function is digestion. A single organ may manifest
vitality, but it does not (save in the very lowest forms)
show forth the whole life of the animal. For, in being
set apart for a special purpose, an organ takes upon
itself, so to speak, to do something for the benefit of
the whole animal, in return for which it is absolved
from doing many things. The stomach is not called
upon to circulate or purify the blood.

There may be functions without special organs, as
the amoeba digests, respires, moves, and reproduces by
its general mass. But, as we ascend the scale of animal
life, we pass from the simple to the complex : groups
of cells or tissues, instead of being repetitions of each
other, take on a difference, and become distinguished
as special parts with specific duties. The higher the
rank of the animal, the more complicated the organs.
The more elaborated the structure, the more compli-
cated the functions. But in all animals, the functions
are performed under conditions essentially the same.
Thus, respiration in the sponge, the fish, and in man
has one object and one means, though the methods
differ. A function, therefore, is a group of similar
phenomena effected by analogous structures.

The life of an animal consists in the accumulation
and expenditure of force. The tissues are storehouses
of power, which, as waste goes on, is given off in various
forms. Thus, the nervous tissue generates nerve force;
the muscles, motion. If we contemplate the phenomena

ORGANIZATION 241

presented by a dog, the most obvious fact is his power
of moving from place to place, a power produced by
the interplay of muscles and bones. We observe, also,
that his motions are neither mechanical nor irregular ;
there is method in his movement. He has the power
of willing, seeing, hearing, feeling, etc. ; and these
functions are accomplished by a delicate apparatus of
nerves.

But the dog does not exhibit perpetual motion.
Sooner or later he becomes exhausted, and rest is
necessary. Sleep gives only temporary relief. In
every exercise of the muscles and nerves there is a
consumption or waste of their substance. The blood
restores the organs, but in time the blood itself needs
renewal. If not renewed, the animal becomes emaci-
ated, for the whale body is laid under contribution to
furnish a supply. Hence the feelings of hunger and
thirst, impelling the creature to seek food. Only this
will maintain the balance between waste and repair.
We notice, therefore, an entirely different set of func-
tions, involving, however, the use of motion and will.
The dog seizes a piece of meat, grinds it between his
teeth and swallows it. It passes into the stomach,
'where it is digested, and then into the intestine,
where it is further changed; there the nourishing
part is absorbed, and carried to the heart, which
propels it through tubes, called blood vessels, all
over the body. In this process of digestion, certain
fluids are required, as saliva, gastric juice, and bile :
these are secreted by special organs, called glands.
Moreover, since not all the food eaten is fitted to make
blood, and as the blood itself, in going around the body,
acts like a scavenger, picking up worn-out particles, we
have another function, that of excretion, or removal of
useless matter from the system^ The kidneys and lungs
DODGE'S GEN. ZOOL. — 16

242 COMPARATIVE ZOOLOGY

do much of this ; but the lungs do something else.
They expose the blood to the air, and introduce oxygen,
which, we shall find, is essential to the life of every
animal.

These centripetal and centrifugal movements in the
body — throwing in and throwing out — are so related
and involved, especially in the lower forms, that they
can not be sharply defined and classified. It has been
said that every dog has two lives, — a vegetative -and an
animal. The former includes the processes of digestion,
circulation, respiration, secretion, etc., which are com-
mon to all life ; while the functions included by the latter,
as motion, sensation, and will, are characteristic of animals.
The heart is the center of the vegetative life, and the brain
is the center of the animal life. The aim of the vegeta-
tive organs is to nourish the individual, and reproduce
its kind ; the organs of locomotion and sense establish
relations between the individual and the world without.
The former maintain life ; the others express it. The
former develop, and afterward sustain, the latter. The
vegetative organs, however, are not independent of the
animal; for without muscles and nerves we could not
procure, masticate, and digest food. The closer the
connection and dependence between these two sets of
organs, the higher the rank.73

All the apparatus and phenomena of life may be in-
cluded under the heads of -

NUTRITION,
MOTION,
SENSATION,
REPRODUCTION.

These four are possessed by all animals, but in a
variety of ways. No two species have exactly the same
mechanism and method of life. We must learn to dis-

ORGANIZATION 243

tinguish between what is necessary and what is only ac-
cessory. That only is essential to life which is common
to all forms of life. Our brains, stomachs, livers, hands,
and feet are luxuries. They are necessary to make us
human, but not living, beings. Half of our body is
taken up with a complicated system of digestion ; but
the amoeba has neither mouth nor stomach. We have
an elaborate apparatus of motion ; the adult oyster can
not stir an inch.

Nutrition, Motion, and Sensation indicate three steps
up the grade of life. Thus, the first is the prominent
function in the coral, which simply "vegetates," the
powers of moving and feeling being very feeble. In
the higher insect, as the bee, there is great activity with
simple organs of nutrition. In the still higher mam-
mal, as man, there is less power of locomotion, though
the most perfect nutritive system ; but both functions
are subordinate to sensation, which is the crowning
development.

In studying the comparative anatomy and physiology
of the animal kingdom, our plan will be to trace the
various organs and functions, from their simplest expres-
sion upward to the highest complexity. Thus Nutrition
will begin with absorption, which is the simplest method
of taking food; going higher, we find digestion, but in
no particular spot in the body ; next, we see it confined
to a tube ; then to a tube with a sac, or stomach ; and,
finally, we reach the complex arrangement of the higher
animals.

CHAPTER IX
NUTRITION

Nutrition is the earliest and most constant of vital
operations. So prominent is the nutritive apparatus,
that an animal has been likened to a moving sac, organ-
ized to convert foreign matter into its own likeness, to
which the complex organs of animal life are but auxil-
iaries. Thus, the bones and muscles are levers and cords
to carry the body about, while the nervous system directs
its motions in quest of food.

The objects of nutrition are growth, repair, propaga-
tion, and supplying energy to perform the work, or
functions, of the body. The first object of life is to
grow, for no animal is born finished. Some animals,
like plants, grow as long as they live; but the ma-
jority soon attain a fixed size. In all animals, how-
ever, without exception, food is wanted for another
purpose than growth, namely, to repair the waste
which is constantly going on. For every exercise of
the muscles and nerves involves the death and decay
of those tissues, as shown by the excretions. The
amount of matter expelled from the body, and the
amount of nourishment needed to make good the loss,
increase with the activity of the animal. The supply
must equal the demand, in order to maintain the life of
the individual ; and as an animal can not make food,
it must seek it from without. Not only the muscles and
nerves are wasted by use, but every organ in the body ;
so that the whole structure needs constant renewal. An

244

NUTRITION 245

animal begins to die the moment it begins to live. The
function of nutrition, therefore, is constructive, while
motion and sensation are destructive.

Another source of demand for food is the production
of germs, to propagate the race, and the nourishment
of such offspring in the egg and infantile state. This
reproduction and development of parts which can main-
tain an independent existence is a vegetative phenome-
non (for plants have it), and is a part of the general
process of nutrition. But it will be more convenient
to consider it hereafter (Chapters XXIL, XXIII.). Still
another necessity for aliment among the higher animals
is the maintenance of bodily heat. This will be treated
under the head of Respiration.

For the present, we will study nutrition, as mani-
fested in maintaining the life of an adult individual.

In all animals, this process essentially consists in the
introduction of food, its conversion into tissue, its oxida-
tion, and the removal of worn-out material.

1. The food must be procured, and swallowed. (In-
gestion.)

2. The food must be dissolved. (Digestion.)

3. The nutritive fluid must be taken up, and then dis-
tributed all over the body. (Absorption and circulation.)

4. The tissues must repair their parts wasted by use,
by transforming a portion of the blood into living mat-
ter like themselves. (Assimilation.)

5. Certain matters must be eliminated from the blood,
some to serve a purpose, others to be cast out of the
system. (Secretion and excretion.)

6. In order to produce work and heat, the food must
be oxidized, either in the blood or in the tissues, after
assimilation. The necessary oxygen is obtained through
exposure of the blood to the air in the lungs. (Respira-
tion in part.)

246 COMPARATIVE ZOOLOGY

7. The waste products of this oxidation taken up by
the blood must be got rid of ; some from the lungs (car-
bon dioxide, water), some from the kidneys (water, urea,
mainly), some from the skin (water, salines). (Respira-
tion in part, excretion.)

The mechanism to accomplish all this in the lowest
forms of life is exceedingly simple, a single cavity and
surface performing all the functions. But in the major-
ity of animals the apparatus is very complicated : there
is a set of organs for the prehension of food ; another
for digestion ; a third, for absorption ; a fourth, for dis-
tribution ; and a fifth, for purification.

CHAPTER X

THE FOOD OF ANIMALS

THE term food includes all substances which con-
tribute to nutrition and furnish energy, whether they
simply assist in the process, or are actually appropriated,
and become tissue. With the food is usually combined
more or less indigestible matter, which is separated in
digestion.

Food is derived from the mineral, vegetable, and ani-
mal kingdoms. Water and salt, for example, are inor-
ganic. The former is the most abundant, and a very
essential article of food. Most of the lower forms of
aquatic life seem to live by drinking : their real nourish-
ment, however, is present in the water in the form of
fine particles. The earthworm, some beetles, and cer-
tain savage tribes of men swallow earth ; but this, like-
wise, is for the organic matter which the earth contains.
As no animal is produced immediately from inorganic
matter, so no animal can be sustained by it.

Nutritious or tissue-forming food comes from the
organic world, and is albuminous, as the lean meat of
animals and the gluten of wheat ; oleaginous, as animal
fat and vegetable oil ; or saccharine, as starch and sugar.
The first is the essential food stuff ; no substance can
serve permanently for food — that is, can permanently
prevent loss of weight in the body — unless it contains
albuminous matter. As stated before, all the living
tissues are albuminous, and therefore albuminous food
is required to supply their waste. Albumen contains

247

248 COMPARATIVE ZOOLOGY

nitrogen, which is necessary to the formation of tissue ;
fats and sugars are rich in carbon, and therefore serve
to maintain the heat of the body, and to repair part of
the waste of tissues. Many warm-blooded animals feed
largely on farinaceous or starchy substances, which in
digestion are converted into sugar. But any animal,
of the higher orders certainly, whether herbivorous or
carnivorous, would starve if fed on pure albumen, oil,
or sugar. Nature insists upon a mixed diet ; and so we
find in all the staple articles of food, as mil^, meat, and
bread, at least two of these principles present. As a
rule, the nutritive principles in vegetables are less abun-
dant than in animal food, and the indigestible residue
is consequently greater. The nutriment in flesh in-
creases as we ascend the animal scale ; thus, oysters
are less nourishing than fish ; fish, less than fowl ; and
fowl, less than the flesh of quadrupeds.

Many animals, as most insects and mammals, live
solely on vegetable food, and some species are restricted
to particular plants, as the silkworm to the white mul-
berry. But the majority of animals feed on one an-
other; such are hosts of the microscopic forms, and
nearly all the radiated species, marine mollusks, crusta-
ceans, beetles, flies, spiders, fishes, amphibians, reptiles,
birds, and clawed quadrupeds.

A few, as man himself, are omnivorous, i.e., are main-
tained on a mixture of animal and vegetable food. The
use of fire in the preparation of food is peculiar to man,
who has been called " the cooking animal." A few of
the strictly herbivorous and carnivorous animals have
shown a capacity for changing their diet. Thus, the
horse and cow may be brought to eat fish and flesh ; the
sea birds can be habituated to grain ; cats are fond of
alligator-pears ; and dogs take naturally to the plantain.
Certain animals, in passing from the young to the

THE FOOD OF ANIMALS 249

mature state, make a remarkable change of food. Thus,
the tadpole feeds upon vegetable matter ; but the adult
frog is carnivorous, living on insects, worms, and
crustaceans.

Many tribes, especially of reptiles and insects, are
able to go without food for months, or even years. In-
sects in the larval, or caterpillar, state are very vora-
cious ; but upon reaching the perfect, or winged, state,
they eat little — some species taking no food at all, the
mouth being actually closed. The males of some roti-
fers and other tribes take no food from the time of leav-
ing the egg until death.

In general, the greater the facility with which an ani-
mal obtains its food, the more dependent is it upon a
constant supply. Thus, carnivores endure abstinence
better than herbivores, and wild animals than domesti-
cated ones.

CHAPTER XI

HOW ANIMALS EAT

i

i. The Prehension of Food. — (i) Liquids. — The sim-
plest method of taking nourishment, though not the
method of the simplest animals, is by absorption through
the skin. The tapeworm, for example, living in the
intestine of its host, has neither mouth nor stomach,
but absorbs the digested food with which its body is
bathed (Fig. 37). Many other animals, especially in-
sects, live upon liquid food, but obtain it by suction
through a special orifice or tube. Thus, we find a
mouth, or sucker, furnished with teeth for lancing the
skin of animals, as in the leech ; a bristlelike tube fitted
for piercing, as in the mosquito ; a sharp sucker armed
with barbs, to fix it securely during the act of sucking,
as in the louse ; and a long, flexible proboscis, as in the
butterfly (Fig. 221). Bees have a hairy, channeled
tongue (Fig. 220), and flies have one terminating in a
large, fleshy knob, with or without little " knives " at
the base for cutting the skin (Fig. 222); both lap,
rather than suck, their food.

Most animals drink by suction, as the ox ; and a few
by lapping, as the dog ; the elephant pumps the water
up with its trunk, and then pours it into its throat ; and
birds (excepting doves) fill the beak, and then, raising
the head, allow the water to run down.

Many aquatic animals, whose food consists of small
particles diffused through the water, have an apparatus
for creating currents, so as to bring such particles within

250-

HOW ANIMALS EAT 251

their reach. This is particularly true of low, fixed forms,
which are unable to go in search of their food. Thus,
the sponge draws nourishment from the water, which is
made to circulate through the system of canals travers-
ing its body by the vibration of flagella, lining parts of the
canals (Figs. 14, 15). The microscopic infusoria have
cilia surrounding the mouth, with which they draw or
drive into the body little currents containing nutritious
particles (Figs. 9, n). Bivalve mollusks, as the oyster
and clam, are likewise dependent upon this method of
procuring food, the gills and inner surface of the
mantle being covered with cilia. So the singular fish,
amphioxus (the only example among vertebrates), em-
ploys ciliary action to obtain the minute organisms on
which it feeds (Fig. 117). The Greenland whale has a
mode of ingestion somewhat unique, gulping great vol-
umes of water into its mouth, and then straining out,
through its whalebone sieve, the small animals which
the water may contain (Fig. 171).

(2) Solids. — When the food is in solid masses, whether
floating in water or not, the animal is usually provided
with prehensile appendages
for taking hold of it. The
jellylike amoeba (Fig. i) has
neither mouth nor stomach,
but extemporizes them, seiz-
ing its food by means of its
soft body. The food then
passes through the denser,

, , FIG. 213. — A Foraminifer (Rotalia
OUter portion OI the body Veneta~), with pseudopodia extended,

into the softer interior, where

it is digested. The waste particles are passed out in
the reverse direction. In the foraminifers, threadlike
projections (pseudopodia) of the body are thrown out
which adhere to the prey. The soft jellylike substance

252 COMPARATIVE ZOOLOGY

of the body then flows toward and collects about the
food, and digests it (Fig. 213).

A higher type is seen in polyps and jellyfishes, which
have hollow tentacles around the entrance to the stomach
(Figs. 1 8, 20, 26). These tentacles are contractile, and
some, moreover, are covered with an immense number
of minute sacs, in each of which a highly elastic filament
is coiled up spirally (lasso cells, nettle cells). When
the tentacles are touched by a passing animal, they
seize it, and at the same mome~nt throw out their myriad
filaments, like so many lassos, which penetrate the skin
of the victim, and probably also emit a fluid, which
paralyzes it ; the mouth, meanwhile, expands to an ex-
traordinary size, and the creature is soon ingulfed in
the digestive bag.

In the next stage, we find no tentacles, but the food
is brought to the mouth by the flexible lobes of the
body commonly called arms, which are covered with
hundreds of minute suckers ; and if the prey, as an
oyster, is too large to be swallowed, the stomach pro-
trudes, like a proboscis, and sucks it out of its shell.
This is seen in the starfish (Fig. 323).

A great advance is shown by the sea urchin, whose
mouth is provided with five sharp teeth, set in as many
jaws, and capable of being projected so as to grasp,
as well as to masticate, its food (Figs. 48, 226).

In mollusks having a single shell, as the snail, the
chief organ of prehension is a straplike tongue, covered
with minute recurved teeth, or spines, with which the
animal rasps its food, while the upper lip is armed with
a sharp, horny plate (Fig. 227). In many marine species,
as the whelk, the tongue is situated at the end of a
retractile proboscis, or muscular tube. In the cuttlefish,
we see the sudden development of an elaborate system
of prehensile organs. Besides a spinous tongue, it has

HOW ANIMALS EAT

253

a pair of hard mandibles, resembling the beak of a
parrot, and working vertically ; and around the mouth
are eight or ten powerful arms
furnished with numerous cup-
like suckers. So perfect is the
adhesion of these suckers, that
it is easier to tear away a limb
than to detach it from its hold.
The earthworm swallows
earth containing particles of
decaying vegetable matter,
which it secures with its lips,
the upper one being prolonged.
Other worms (as Nereis) are
so constructed that the gullet,
which is frequently armed with
teeth and forceps, can be pro-

FIG. 214. — Suckers on the Tentacles

truded to form a proboscis for Of a Cuttlefish: «, hoiiow axis of
seizing prey. the arm' containing nerve and ar'

The Arthropoda exhibit a
great variety of means for pro-
curing nourishment, in addi-
tion to the suctorial contrivances already mentioned, the
innumerable modifications of the mouth corresponding
to the diversity of food. Mille-
pedes, caterpillars, and grubs
have a pair of horny jaws moving
horizontally. The centipede has
a second pair of jaws, which are
really modified feet, terminated
by curved fangs containing a
, poison duct. The horseshoe crab

FIG. 215. — Nereis — head, with *

extended proboscis: ?, jaws; US6S its feet for prehension, aild
T, tentacles; H, head ; £, eyes. - , . , . , . . . .

the thighs, or basal joints of its
legs, to masticate the food and force it into the stomach.

tery; c, cellular tissue; d, radiat-
ing fibers; h, raised margin of the
disk around the aperture f, g, which
contains a retractile membrane, or
" piston," i.

254 COMPARATIVE ZOOLOGY

The first six pairs of legs in the lobster and crab are
likewise appropriated to conveying food into the mouth,
the sixth being enormously developed, and furnished
with powerful pincers. Scorpions have a similar pair

of claws for prehension, and
also a pair of small forceps
for holding the food in con-
tact with the mouth. In
their relatives, the spiders,
the claws* are wanting, and
. the forceps end in a fang,

* IG. 216. — One of the Fangs, or Perforated r °'

Mandibles, of the Spider, much magni- Or hook, which is perforated
fied. 7A

to convey venom.'4

The biting insects, as beetles and locusts, have two
pairs of horny jaws, which open sidewise, one above and
the other below the oral orifice. The upper pair are
called mandibles ; the lower, maxillae. The former are
armed with sharp teeth, or with cutting edges, and
sometimes are fitted, like the molars of quadrupeds, to
grind the food. The maxillae are usually composed of
several parts, some of which serve to hold the food,
or to help in dividing it, while others (palpi) are both
sensory and prehensile. There, is generally present a
third pair of jaws — the labium — which are united in
the middle line, and serve as a lower lip. They also
bear palpi. The mantis seizes its prey with its long
fore legs, crushes it between its thighs, which are armed
with spines, and then delivers it up to the jaws for
mastication. All arthropods move their jaws horizon-
tally.

The backboned animals generally apprehend food by
means of their jaws, of which there are two, moving
vertically. The toothless sturgeon draws in its prey by
powerful suction. The hagfish has a single tooth, which
it plunges into the sides of its victim, and, thus securing

HOW ANIMALS EAT

255

a firm hold, bores its way into the flesh by means of its
sawlike tongue. But fishes are usually well provided
with teeth, which, being sharp and curving inward, are
strictly prehensile. The fins and tongue are not prehen-
sile. A mouth with horny jaws, as in the turtles, or
bristling with teeth, as in the crocodile, is the only
means possessed by nearly all amphibians and reptiles
for securing food. The toad, frog, and chameleon cap-
ture insects by darting out the tongue, which is tipped
with glutinous saliva. The constricting serpents (boas)
crush their prey in their coils before swallowing ; and
the venomous snakes have poison fangs. No reptile
has prehensile lips. All birds use their toothless beaks
in procuring food, but birds of
prey also seize with their tal-
ons, and woodpeckers, hum-
mers, and parrots with their
tongues. The beak varies
greatly in shape, being a hook
in the eagle, a probe in the
woodpecker, and a shovel in
the duck.

Among the quadrupeds we
find a few special contrivances,
as the trunk of the elephant,
and the long tongues of the
giraffe and ant-eater; but, as
a rule, the teeth are the chief
organs of prehension, 'always FIG. 2i7.— A™ of the
aided more or less by the lips. Monkey (Ateles}'

Ruminants, like the ox, having hoofs on their feet,
and no upper front teeth, employ the lips and tongue.
Such as can stand erect on the hind legs, as the
squirrel, bear, and kangaroo, use the front limbs for
holding the food and bringing it to the mouth, but

256 COMPARATIVE ZOOLOGY

never one limb alone. The clawed animals, like the
cat and lion, make use of their feet in securing prey, all
four limbs being furnished with curved retractile claws ;
but the food is conveyed into the mouth by the move-
ment of the head and jaws. Man and the monkeys em-
ploy the hand in bringing food to the mouth, and the
lips and tongue in taking it into the cavity. The thumb
on the human hand is longer and more perfect than
that of the apes and monkeys ; but the foot of the latter
is also prehensile.

2. The Mouths of Animals. — In the parasites, as the
tapeworm, which absorb nourishment through the skin,
and insects, as the May fly and botfly, which do all their
eating in the larval state, the mouth is either wanting or
rudimentary. The amoeba, also, has no mouth proper,
its food passing through the firmer outside part of the
bit of protoplasm which constitutes its body. Mouth
and anus are thus extemporized, the opening closing as
soon as the food or excrement has passed through.

In the infusoria the " mouth " is a round or oval open-
ing leading through the cuticle and outer layer of proto-
plasm to the interior of the single cell which makes
their body. It is usually bordered with cilia, and situ-
ated on the side or at one end of the animal (Figs. 9, 1 1).

An ellipticah or quadrangular orifice, surrounded with
tentacles, and leading directly to the stomach, is the
ordinary mouth of the polyps and jellyfishes. In those
which are fixed, as the actinia, coral, and hydra, the
mouth looks upward or downward, according to the
position in which the animal is attached (Figs. 17, 34,
236); in those which freely move about, as the jellyfish,
it is generally underneath, the position of the animal
being reversed (Fig. 22). In some, the margin, or lip,
is protruded like a proboscis; and in all it is exceed-
ingly dilatable.

HOW ANIMALS EAT 257

The mouth of the starfish and sea urchin is a simple
round aperture, followed by a very short throat. In the
starfish, it is inclosed by a ring of hard spines and a
membrane. In the sea urchin it is surrounded by a
muscular membrane and minute tentacles, and is armed
with five sharp teeth, set in as many jaws, resembling
little conical wedges (Fig. 226).

Among the headless mollusks, the oral apparatus is
very simple, being inferior to that of some of the radiate
animals. In the oyster and bivalves generally, the mouth
is an unarmed slit — a mere inlet to the esophagus, situT
ated in a kind of hood formed by the union of the gills
at their origin, and between two pairs of delicate flaps,
or palpi. These palpi make a furrow, along which pass
the particles of food drawn in by the cilia, borne by
cells which cover the surface of the flaps.

Of the higher mollusks, the little clio (one of the
pteropods) has a triangular mouth, with two jaws armed
with sharp horny teeth, and a tongue covered with spiny
booklets all directed backward. Some univalves have a
simple fleshy tube or siphon.
Others, as the whelk, have
an extensible proboscis,
which unfolds itself, like the V y \
finder of a glove, and carries FIG. 218.- jaw of the
within it a rasplike tongue, (Helix albolabr^ P~tiy magnified.
which can bore into the hardest shells. Such as feed
on vegetable matter, as the snail, have no proboscis, but
on the roof of the mouth a curved horny plate fitted to
cut leaves, etc., which are pressed against it by the lips,
and on the floor of the mouth a small tongue covered
with delicate teeth. As fast as the tongue is worn off
by use, it grows out from the root.

The mouth of the cuttlefish is the most elevated type
below that of the fishes. A broad circular lip nearly
DODGE'S GEN. ZOOL. — 17

258 COMPARATIVE ZOOLOGY

conceals a pair of strong horny mandibles, not unlike
the beak of a parrot, but reversed, the upper mandible
being the shorter of the two, and the jaws, which are
cartilaginous, are imbedded in a mass of muscles, and
move vertically. Between them is a fleshy tongue
covered with teeth.

The parasitic worms, living within or on the outside
of other animals, generally have a sucker at one end or
underneath, serving simply for attachment, and another
which is perforated. The latter is a true suctorial mouth,
being the sole inlet of food. It is often surrounded with
booklets or teeth, which serve both to scarify the victim
and secure a firm hold. In the leech, the mouth is a
triangular opening with* thick lips, the upper one pro-
longed, and with three jaws. In many worms it is a
fleshy tube, which can be drawn in or extended, like the
eye stalks of the snail, and contains a dental apparatus
inside (Fig. 215).

Millepedes and centipedes have two lateral jaws and a
four-lobed lip.

In lobsters and crabs the mouth is situated underneath
the head, and consists of a soft upper lip, then a pair of
upper jaws provided with a short feeler, below which is a
thin bifid lower lip ; then follow two pairs of membranous
under jaws, which are lobed and hairy ; and next, three
pairs of foot jaws (Fig. 54). The horseshoe crab has
no special jaws, the thighs answering the purpose. The
barnacle has a prominent mouth, with three pairs of
rudimentary jaws.

With few exceptions, the mouths of insects in the lar-
val state are fitted only for biting, the two jaws being
horny shears. But in the winged, or perfect, state,
insects may be divided into the masticating (as the
beetle) and the suctorial (as the butterfly). In the for-
mer group, the oral apparatus consists of two pairs of

HOW ANIMALS EAT

259

horny jaws (mandibles and inaxillce\ which work hori-
zontally between an upper (labrum} and an under (la-
bium} lip. The maxillae and under lip carry sensitive
jointed feelers (palpi). The front edge of the labium is

FIG. 219. — Mouth of a Locust, dissected and much magnified: i, labrum, or upper lip;
2, mandibles; 3, jaws; 4, labium, or lower lip; 5, tongue. The appendages to the
maxillae and lower lip are palpi.

commonly known as the tongue (ligtdd)^ In such a
mouth, the mandibles are the most important parts ;
but in passing to the suctorial insects, we find that the
mandibles are secondary to the maxillae and labium,
which are the only means of taking food. In the bee

260

COMPARATIVE ZOOLOGY

tribe, we have a transition between the biting and the
sucking insects — the mandibles "supply the place of
trowels, spades, pickaxes, saws, scissors, and knives,"
while the maxillae are developed into a sheath to inclose
the long, slender, hairy tongue which laps up the sweets
of flowers. In the suctorial butterfly, the lips, mandi-
bles, and palpi are reduced to rudiments, while the

maxillae are excessively
lengthened into a proboscis,
their edges locking by means

FIG. 220. — Head of a Wild Bee (An-
thophora retusa), magnified, front
view: a, compound eyes; b,
clypeus; c, three simple eyes; d,
antennae; <?, labrum; f, mandibles;
z, maxillae; h, maxillary palpi; /,
palpifer; /, labial palpi; m, para-
glossae; k, ligula.

FIG. 221. — Proboscis of a Butterfly, magnified.

of minute teeth, so as to form
a central canal, through which
the liquid food is pumped up
into the mouth. Seen under
the microscope, the proboscis is made up of innumerable
rings interlaced with spiral muscular fibers. The probos-
cis of the fly is a modified lower lip ; that of the bugs and
mosquitoes, fitted both for piercing and suction, is formed
by the union of four bristles, which are the mandibles
and maxillae strangely altered, and encased in the labium
when not in use.

HOW ANIMALS EAT

26l

As most of the arachnids live by suction, the jaws
are seldom used for mastication. In the scorpion, the
apparent representatives of the mandibles of an insect
are transformed into a pair of small forceps, and the
palpi, so small in insects, are developed into formidable
claws : both of these or-
gans are prehensile. In
spiders, the so-called man-
dibles, which move more
or less vertically, end in

FIG. 222. — Mouth of the Horsefly ( Taba-
nus lineola), magnified: a, antennae;
nt, mandibles; tnx, maxillae; mp,
maxillary palpi ; Ib, labrum; /, labium,
or " tongue."

a fang; and the clublike
palpi, often resembling
legs, have nothing to do
with ingestion or locomo-
tion. Both scorpions and

spiders have a soft upper FlG- 223' -Under Surface of Male sPider'
lip, and a groove within
the mouth, which serves
as a canal while sucking
their prey. The tongue is external, and situated be-
tween a pair of diminutive maxillae.

In the ascidians the first part of the alimentary canal
is enormously enlarged and modified to serve as a gill
sac. At the bottom of this sac, and far removed from

enlarged: a, c, poison fang; b, teeth on in-
terior margin of mandible, e ; f, labium ; g,
thorax; h, limbs; i, abdomen; /.spinnerets;
m, maxillary palpus; d, dilated terminal
joint.

262 COMPARATIVE ZOOLOGY

its external opening, lies the entrance to the digestive
tract proper. Into it the particles of food entering with
the water are conveyed (Fig. 115).

The mouth of vertebrates is a cavity with a fixed roof
(the hard palate) and a movable floor (the tongue and
lower jaw), having a transverse opening in front,76 and
a narrow outlet behind, leading to the gullet. Save in
birds and some others, the cavity is closed in front with
lips, and the margins of the jaws are set with teeth.

In fishes the mouth is the common entry to both the
digestive and respiratory organs ; it is, therefore, large,
and complicated by a mechanism for regulating the
transit of the food to the stomach and the aerated water
to the gills. The slits leading to the gills are provided
with rows of processes which, like a sieve, prevent the
entrance of food, and with valves to keep the water,
after it has entered the gills, from returning to the
mouth. So that the mouths of fishes may be said to
be armed at both ends with teeth-bearing jaws. A few
fishes, as the sturgeon, are toothless ; but, as a class,
they have an extraordinary dental apparatus — not only
the upper and lower jaws, but even the palate, tongue,
and throat being sometimes studded with teeth. Every
part of the mouth is evidently designed for prehension
and mastication. Lips are usually present; but the
tongue is often absent, or very small, and as often aids
respiration as ingestion.

Amphibians and reptiles have a wide mouth ; even
the insect-feeding toads and the serpents can stretch
theirs enormously. True fleshy lips are wanting ; hence
the savage aspect of the grinning crocodile. With some
exceptions, as toads and turtles, the jaws are armed
with teeth. Turtles are provided with horny beaks.
The tongue is rarely absent, but is generally too thick
and short to be of much use. In the toad and frog it

HOW ANIMALS EAT 263

is singularly extensile ; rooted in front and free behind,
it is shot from the mouth with such rapidity that the
insect is seized and swallowed more quickly than the
eye can follow. The chameleon's tongue is also exten-
sile. Snakes have a slender forked tongue, consisting
of a pair of muscular cylinders, which is solely an in-
strument of touch.

FlG. 224. — Mouth of the Crocodile: d, tongue; e, glands; _/", inferior, and g, superior,
valve, separating the cavity of the mouth from the throat, h.

Birds are without lips or teeth, the jaws being cov-
ered with horn forming a beak. This varies greatly
in shape, being extremely wide in the whip-poor-will,
remarkably long in the pelican, stout in the eagle, and
slender in the hummer. It is hardest in those that tear
or bruise their food, and softest in water birds. The
tongue is also covered with a horny sheath, and is gen-
erally spinous, its chief function being to secure the
food when in the mouth. It is proportionally largest
and most fleshy in the parrots.

The main characteristics of the mammalian mouth
are flesh lips and mobile cheeks.77 In the duck-billed

264

COMPARATIVE ZOOLOGY

monotremes lips are wanting, and in the porpoises they
are barely represented. But in the herbivorous quadru-
peds they, with the tongue, are the chief organs of pre-
hension ; in the carnivorous tribes they are thin and
retractile ; while in the whale the upper lip falls down
like a curtain, overlapping the lower jaw several feet.
As a rule, the mouth is terminal ; but in the elephant,
tapir, hog, and shrew, the upper
lip blends with the nose to
form a proboscis, or snout. The
mouth is comparatively small in
the elephant and in gnawing
animals like the squirrel, wide
in the carnivores, short in the
sloth, and long in the ant-eater.
Teeth are usually present, but
vary in form and number with
the habits of the animal. The
ant-eater is toothless, and the
Greenland whale has a sieve

FIG. 225. — Human Tongue and ad- made of homy platCS. The
jacent parts: a, lingual papillae;

b, papiiise forming V-shaped tongue conforms in size and

lines; d, fungiform papillae; e, , .,•• ,-, -, . j

filiform papillae; g, epiglottis; shape with the lower jaw, and

-., uvula or conical process, '^ a muscular sensitive Organ,

hanging from the soft palate,

n; o, hard palate; r, palatine which SCrVCS many

glands, the mucous membrane .

being removed; v, section of the assisting in the prehension,

tication, arid swallowing of food,

besides being an organ of taste, touch, and speech. Its
surface is covered with minute prominences, called
papilla, which are arranged in lines with mathematical
precision. In the cats, these are developed into recurved
spines, which the animal uses in cleaning bones and
combing its fur. Similar papillae occur on the roof and
sides of the mouth of the ox and other ruminants. In
some animals, as the hamster and gopher, the cheeks

HOW ANIMALS EAT

265

are developed into pouches in which the food may be
carried. These may be lined with hair. The tongue is
remarkably long in the ant-eater and giraffe, and almost
immovable in the gnawers, elephants, and whales.

3. The Teeth of Animals. — Nearly all animals have
certain hard parts within the mouth for the prehension
or trituration of
solid food. If
these are want-
ing, the legs are
of ten armed with
spines, or pin-
cers, to serve
the same pur-
pose, as in the
horseshoe crab;
or the stomach
is lined with
"gastric teeth," as in some marine snails; or the defi-
ciency is supplied by a muscular gizzard, as in birds,

ant - eaters, and
some insects.
Even the lobster
and crab, in ad-
dition to their
complicated oral

FIG. 226. — Sea Urchin bisected, showing masticating
apparatus.

B

FIG. 227. -Teeth and Masticatory Apparatus of Gastro- Organs, haVC the

pods, enlarged: A, portion of odontophore, or " tongue," stomach fUT-
of Velutina, enlarged; B, portion of odontophore of

Whelk (Buccinum undatunt), magnified — the entire nished With 3.

tongue has too rows of teeth; C, head and odontophore r i' f

ofUmpettPattttavufeata); D, portion of same, greatly pOWeriUl. SCt OI
magnified, to show the transverse rows of siliceous teeth.

The sea urchin is one of the lowest animals which
exibits anything like a dental apparatus. Five calcare-
ous teeth, having a wedge-shaped apex, each set in a
triangular pyramid, or "jaw," are moved upon each

266

COMPARATIVE ZOOLOGY

other by a complex arrangement of levers and muscles.
Instead of moving up and down, as in vertebrates, or
from right to left, as in arthropods, they converge
toward the center, and the food passes between ten
grinding surfaces.

The rotifers have a curious pair of horny jaws. That
which answers to the lower jaw is fixed, and called the

" anvil." The upper jaw
consists of two pieces called
"hammers," which are
sharply notched, and beat
upon the " anvil " between
them (Fig. 40).

The horny-toothed mandi-
bles of insects, already men-
tioned, are prehensile, and
also serve to divide the food.
The three little white
ridges in the mouth of the
leech are the convex edges
of horny semicircles, each
bordered by a row of nearly
a hundred hard, sharp teeth.
When the mouth, or sucker,
is applied to the skin, a saw-
ing movement is given to
the horny ridges, so that
the " bite " of the leech is
really a saw cut.

The dentition of the uni-
valve mollusks, or the snails,
is generally lingual, i.-e., it
consists of microscopic teeth, usually siliceous and
amber-colored, planted in rows on the tongue. The
teeth are, in fact, the serrated edges of minute plates.

FIG. 228. — Section of one half of the Up-
per Jaw of a Whale (Balasnoptera),
showing baleen plates: a, superior
maxillary bone; 6, ligamentous gum
attaching the horny body of the baleen
plate, c; d, fringe of bristles ; e, smaller
plates.

HOW ANIMALS EAT

267

The number of these plates varies greatly ; the garden
slug has 1 60 rows, with 180 teeth in each row.

All living birds, and some other vertebrates, as ant-
eaters,78 turtles, tortoises, toads, and sturgeons, are with-
out teeth. Their place is often supplied by a horny
beak, a muscular gizzard, or both structures.

In a few vertebrates, horny plates take the place of
teeth, as the duck mole (Ornithorhynchus) and whalebone
whale. In the former, the plates consist of closely set
vertical hollow tubes ; in the latter, the baleen, or whale-
bone, plates, triangular in shape, and fringed on the
inner side, hang in rows from the gums of the upper
jaw. In some whales there are about 300 plates on each
side.79

True teeth, consisting mainly of a hard, calcareous
substance called dentine, are found only in backboned
animals. They are distinct from
the skeleton, and differ from
bone in containing more mineral
matter, and in not showing, under
the microscope, any minute cav-
ities, called lacuna. A typical
tooth, as found in man, consists
of a central mass of dentine,
capped with enamel, and sur-
rounded on the fang with cement.
The first tissue is always pres-
ent, while the others may be

T T , • • , f •

absent. It is a mixture of am-

mal and mineral matter disposed

in the form of extremely fine

tubes and cells, so minute as to prevent the admission

of the red particles of blood. One modification of it is

ivory, seen in 'the tusks of elephants. Enamel is the

hardest tissue of the body, and contains not more than

FIG. 229. — Section of Human MO-

lar, enlarged: k, crown; n,

nec'k. /( ££. ,; enamel. ^
dentine; c> cement: >» PUIP

cavity.

268

COMPARATIVE ZOOLOGY

two per cent of animal matter. It consists of six-sided
fibers set side by side, at right angles to the surfaces
of the dentine. Cement closely resembles bone, and is
present in the teeth of only the higher animals.

Teeth are usually confined to the jaws ; but the num-
ber, size, form, structure, position, and mode of attach-
ment vary with the food and habits of the animal. As
a rule, animals developing large numbers of teeth in the
back part of the mouth are inferior to those having
fewer teeth, and those nearer the lips. The teeth of
only mammals have fangs.

The teeth of fishes present the greatest variety. In
number, they range from zero to hundreds. The hag-
fish {Myxine) has a single
tooth on the roof of the
mouth, and two serrated
plates on the tongue; while
the mouth of the pike is
crowded with teeth. In
some we find teeth short and
blunt, in the shape of cubes,

FIG. 230.— jaws and Pavement teeth of a or prisms, arranged like mo-

saic work. Such pavement

teeth (seen in some rays) are fitted for grinding seaweed
and crushing shellfish. But the cone is the most common
form : sometimes so slender and close as to resemble
plush, as in the perch ; or of large size, and flattened like
a spearhead with serrated edges, as in the shark ; but
more often like the canines of mammals, curved inward
to fit them for grappling. In the shark, the teeth are con-
fined to the fore part of the mouth ; in the carp, they
are all situated on the bones of the throat ; in the parrot
fish, they occupy both back and front ; but in most fishes
the teeth are developed also on the roof,' or palate, and,
in fact, on nearly every bone in the mouth. They seldom

. HOW ANIMALS EAT 269

appear (as in the salmon) on the upper maxillary. As
to mode of attachment, the teeth are generally anchy-
losed (fastened by bony matter) to the bones which sup-
port them, or simply bound by ligaments, as in the shark.
In a few fishes, the teeth consist of flexible cartilage ;
but almost invariably they are composed of some kind
of dentine, enamel and cement being absent.

Of amphibians and reptiles, toads, turtles, and tor-
toises are toothless ; frogs have teeth in the upper jaw
only; snakes have a more
complete set; but saurians
possess the most perfect
dentition. The number is
not fixed even in the same
species; in the alligator it
varies from 72 to 88. The

teeth are limited tO the FIG. 231. -Poison Apparatus of the Rattle-

snake: g, gland, with duct, leading to the

jawbones in the higher fang,// *«, elevator muscles of the jaw,
r , • \ i • which, in contracting, compress the gland;

lOrniS (Saurians); but in s, salivary glands on the edge of the jaws;

others, as the serpents, w>nostril-
they are planted also in the roof of the mouth. With
few exceptions, they are conical and curved (Fig. 224).
In the serpents they are longest and sharpest; and
the venomous species have two or more fangs in the
upper jaw. These fangs contain a canal, through which
the poison is forced by muscles which compress the
gland. The bones to which they are attached are mov-
able, and the fangs ordinarily lie flat upon the gums, but
are brought into a vertical position in the act of striking.
As a rule, the teeth of reptiles are simply soldered to
the bone which supports them, or lodged in a groove ;
but those of crocodiles are set in sockets. Reptilian
teeth are made of dentine and a thin layer of cement,
to which is added in most saurians a coat of enamel on
the crown.

2/0 COMPARATIVE ZOOLOGY,

In the majority of mammals, the teeth are limited in
number and definite in their forms. The number ranges
from i in the narwhal (but the longest tooth in the
animal kingdom) to 220 in the dolphin. The average is
32, occurring in ruminants, apes, and man ; but 44 (as
in the hog and mole) is called the typical or normal
nutrfber, and this number is exceeded only in the lower

FIG. 232. — Skull of the Babirusa, or Malayan Hog, showing growth and curvature of the

canines.

groups. When very numerous, the teeth are of the
reptilian type, small, pointed, and of nearly equal size,
as in the porpoise. In the higher mammals, the teeth
are comparatively few, and differ so much in size, shape,
and use, that they can be classed into incisors, canines,
premolars, and molars. Such a dental series exhibits
a double purpose, prehension and mastication. The

HOW ANIMALS EAT 2/1

chisel-shaped front teeth are fitted for cutting the food,
and hence called incisors. These vary in number : the
lion has six in each jaw ; the squirrel has two in each
jaw, but remarkably developed ; the ox has none in the
upper jaw, and the elephant none in the lower ; while
the sloth has none at all.80 The canines, so called
because so prominent in the dog, are conical, and,
except in man, longer than the other teeth. They are
designed for seizing and tearing ; and they are the most
formidable weapons of the wild carnivores. There are
never more than four. They are wanting in all rodents,
and in nearly all herbivorous quadrupeds. The molars,
or grinders, vary greatly in shape, but closely correspond
with the structure and habits of the animal, so that a
single tooth is sufficient to indicate the mode of life
and sometimes to identify the species.81 In the rumi-
nants, rodents, horses, and elephants, the summits of
the molars are flat, like millstones, with transverse or
curving ridges of enamel. In the cats and dogs, they
are narrow and sharp, passing by each other like the
blades of scissors, and therefore cutting, rather than
grinding, the food. The more purely carnivorous the
species, and the more it feeds upon living prey, the fewer
the molars. In animals living on mixed diet, as the hog
and man, the crowns have blunt tubercles. Premolars,
or bicuspids, are those which were preceded by milk
teeth ; the true, or back, molars had no predecessors.

The dentition of mammals is expressed by a formula,
which is a combination of initial letters and figures in
fractional form, to show the number and kind of teeth
on each side of both jaws. Thus, the formula for man

The teeth of mammals are always restricted to the
margins of the jaws, and form a single row in each.

2/2 COMPARATIVE ZOOLOGY

But they rarely form an unbroken series.82 The teeth
implanted in the premaxillary bone, and in the corre-
sponding part of the lower jaw, whatever their number,
are incisors. The first tooth bjehind the premaxillary, if
sharp and projecting, is a canine.

nts

FIG. 233. — Teeth of the right lower jaw of adult male Chimpanzee (Anthropopithecus
troglodytes), natural size. The molar series does not form a curve, as in Man.

Each tooth has its particular bony socket.83 The
molars may be still further strengthened by having two
or more diverging fangs, or roots, a feature peculiar to
this class. The incisors and canines have but one fang ;
and those that are perpetually growing, as the incisors
of rodents and elephants, have none at all. The teeth
of flesh-eating mammals usually consist of hard dentine,
surrounded on the root with cement and capped with
enamel. In the herbivorous tribes, they are very com-
plex, the enamel and cement being inflected into the
dentine, forming folds, as in the molar of the ox, or
plates, as in the compound tooth of the elephant. This
arrangement of these tissues, which differ in hardness,
secures a surface with prominent ridges, well adapted
for grinding. The cutting teeth of the rodents consist
of dentine, with a plate of enamel on the anterior sur-
face, and the unequal wear preserves a chisel-like edge.

HOW ANIMALS EAT 273

Enamel is sometimes wanting, as in the molars of the
sloth and the tusks of the elephant.

In fishes and reptiles, there is an almost unlimited
succession of teeth ; but mammalian teeth are cast and
renewed but once in life.

Vertebrates use their teeth for the prehension of food,
as weapons of offense or defense, as aids in locomotion,
and as instruments for uprooting or cutting down trees.
But in the higher class they are principally adapted for
dividing or grinding the food.84 While in nearly all

FlG. 234. — Upper Molar Tooth of Indian Elephant (Elephas indicus}, showing trans-
verse arrangement of dentine, d, with festooned border of enamel plates, e; c,
cement; one-third natural size.

other vertebrates the food is bolted entire, mammals
masticate it before swallowing. Mastication is more
essential in the digestion of vegetable than of animal
food ; and hence we find the dental apparatus most effi-
cient in the herbivorous quadrupeds. The food is most
perfectly reduced by the rodents.

Teeth, as we shall see, are appendages of the skin,
not of the skeleton, and, like other superficial organs,
are especially liable to be modified in accordance with
the habits of the creature. They are, therefore, of great
zoological value ; for such is the harmony between them
and their uses, the naturalist can predict the food and
general structure of an animal from a sight of the teeth
alone. For the same reason, they form important
DODGE'S GEN. ZOOL. — 18

2/4 COMPARATIVE ZOOLOGY

guides in the classification of animals ; while their
durability renders them available to the paleontologist
in the determination of the nature and affinities of ex-
tinct species, of which they are often the sole remains.
Even the structure is so peculiar that a fragment will
sometimes suffice.

4. Deglutition, or how Animals Swallow. — In the
lowest forms of life, the mouth is but an aperture open-
ing immediately into the body substance, and the food is
drawn in by ciliary currents (Figs. 9, n). But in the
majority of animals, a muscular tube, called the gullet, or
esophagus, intervenes between the mouth and stomach,
the circular fibers of which contract, in a wavelike man-
ner, from above downward, propelling the morsel into the
stomach.85 In the higher mollusks, arthropods, and ver-
tebrates, deglutition is generally assisted by the tongue,
which presses the food backward, and by a glairy juice,
called saliva, which facilitates its passage through the
gullet.86 Vertebrates have a cavity behind the mouth,
called the throat, or pharynx, which may be considered
as a funnel to the esophagus.87 In air breathers, it has
openings leading to the windpipe, nose, and ears. In
man, as in mammals generally, the process of deglutition
is in this wise : the food, masticated by the teeth and
lubricated by the saliva, is forced by the tongue and
cheeks into the pharynx, the soft palate keeping it out
of the nasal aperture, and the valvelike epiglottis falling
down to form a bridge over the' opening to the wind-
pipe. The moment the pharynx receives the food, it is
firmly grasped, and, the muscular fibers contracting
above it and left lax below it, it is rapidly thrust into
the esophagus. Here, a similar movement (the peristal-
tic) strips the food into the stomach.88 The rapidity of
these contractions transmitted along the esophagus may
be observed in the neck of a horse while drinking.

HOW ANIMALS EAT 275

Deglutition in the serpents is painfully slow, and
somewhat peculiar. For how is an animal, without
limbs or molars, to swallow its prey, which is often
much larger than its own body ? The boa constrictor,
e.g., seizes the head of its victim with its sharp, recurv-
ing teeth, and crushes the, body with its overlapping
coils. Then, slowly uncoiling, and covering the carcass

FIG. 235. — Skull of Boa Constrictor: i, frontal; 2, prefrontal; 4, postfrontal; 5, basi-
occipital; 6, sphenoid; 7, parietal; 12, squamosal; 13, prob'tic; 17, premaxillary;
18, maxillary; 20, nasal; 24, transverse; 25, internal pterygoid; 34, dentary, lower
jaw; 35, angular; 36, articular; a, quadrate; s, prenasal; v, petrosal.

with a slimy mucus, it thrusts the head into its mouth
by main force, the mouth stretching marvelously, the
skull being loosely put together. One jaw is then un-
fixed, and the teeth withdrawn by being pushed forward,
when they are again fastened farther back upon the
animal. The other jaw is then protruded and refas-
tened ; and thus, by successive movements, the prey is
slowly and spirally drawn into the wide gullet.

CHAPTER XII

THE ALIMENTARY CANAL

The Alimentary Canal is the great route by which
nutritive matter reaches the interior of the body. It is
the most universal organ in the animal kingdom, and
the rest are secondary or subservient to it. In the
higher animals, it consists of a mouth, pharynx, gullet,
stomach, and intestine.

It is a general law, that food can be introduced into
the living system only in a fluid state. While plants
send forth their roots to seek nourishment from without,
animals, which may be likened to plants turned outside
in, have their roots (called absorbents) directed inward
along the walls of a central tube or cavity. This cavity
is for the reception and preparation of the food, so that
animals may be said to carry their soil about with them.
The necessity for such a cavity arises not only from the
fact that the food, which is usually solid, must be dis-
solved, so as to make its way through the delicate walls
of the cavity into the system, but also from the occur-
rence of intervals between the periods of eating, and
the consequent need of a reservoir. For animals, unlike
plants, are thrown upon their own wits to procure food.

The Protozoa, as the amoeba and Infusoria, can not be
said to have a digestive canal. The animal is here
composed of a single cell, in which the food is digested.
The jelly like amoeba passes the food through the firmer
outer layer (ectosarc) into the more fluid inner part
(endosarc), where it is digested (Fig. i). The Infusoria,

276

THE ALIMENTARY CANAL

277

which have a cuticle, and so a more definite form, pos-
sess a mouth, or opening, into the interior of their cell
body, and at least a definite place where the excrement
is passed out (Figs, 9, u). But we can not call this
cell cavity a digestive tract.

In the higher animals, the alimentary canal is a con-
tinuation of the skin, which is reflected inward, as we
turn the finger of a glove.89 We find every grade of
this reflection, from the sac of the hydra to the long in-
testinal tube of the ox. So that food in the stomach is
still outside of the true body.

The simplest form of such a digestive tract is seen in
the hydra (Fig. 18). Here the body is a simple bag,
whose walls are
composed of two
layers of cells (ecto-
derm and endo-
denri). A mouth
leads into the cav-
ity, and serves as
well for the out-
let of matter not
wanted. The en-
dodermal cells fur-
nish the juices by
which the food is
digested and ab-
sorb the nutritious
portions of it. The
polyps have also
but one external opening ; but from this hangs down a
short tube, open at both ends, reaching about halfway to
the bottom of the body cavity. Such an arrangement
would be represented by a bottle with its neck turned
inward. In this suspended sac, which is somewhat con-

FIG. 236. —Dissected Actinia: a, the thick opaque skin
consisting of ectoderm, lined with muscular fibers;
c , the tubular tentacles communicating with the inter-
spaces, h, between the membranous vertical folds;
gi S ' > orifices in the walls allowing passage of respira-
tory water from one compartment to another ; d, mouth
leading to gastric cavity, e.

2/8 COMPARATIVE ZOOLOGY

stricted at the extremities, digestion takes place ; but the
product passes freely into all the surrounding chambers,
along with the water for respiration (Fig. 236). The
Medusae, or jelly fishes, preserve the same type of a diges-
tive apparatus ; but the sac is cut off from the general
cavity, and numerous canals radiate from it to a circular
canal near the margin of the disk (Fig. 21). In the star-
fishes (Fig. 323), we find a great advance. The saclike
stomach sends off two glandular branches to each arm,
which doubtless furnish a fluid to aid in digestion (so-
called hepatic coeca). There is also an anus present in
some forms, but it hardly serves to pass off the waste
matter.

Thus far we have seen but one opening to the diges-
tive cavity, rejected portions returning by the same
road by which they enter. But a true alimentary canal
should have an anal aperture distinct from the oral.
The simplest form of such a canal is exhibited by the
sponge, in its system of absorbent pores for the entrance
of liquid, and of several main channels for its discharge.
The apparatus, however, is not marked off from the
general cavity of the body, and digestion is not distinct
from circulation.90

The sea urchin presents us with an important advance
— one cavity with two orifices ; and the complicated
apparatus of higher animals is but the development of
this type. This alimentary canal begins in a mouth
well provided with teeth and muscles, and extends
spirally to its outlet, which generally opens » on the
upper, or opposite, surface. Moreover, while in some
of the worms the canal is a simple tube running through
the axis of the cylindrical body from oral orifice to anal
aperture, the canal of the sea urchin shows a distinction
of parts, foreshadowing the pharynx, gullet, stomach,
and intestine. Both mouth and vent have muscles for

THE ALIMENTARY CANAL

2/9

constriction and expansion ; and, as the vent is on the

summit of the shell, and the latter is covered with

spines, the ejected

particles are seized by

delicate forks (pedicel-

larice\ and passed on

from one to the other

down the side of the

body, till they are

dropped off into the

water.91

The worms present
us with a great range

Of Structure in the

digestive tract. It is
sometimes almost as

Simple aS that Of the

a a mere SaC.

f^rt-h wnrm hv<z x

n nas a

tube running straight
through the body, di-
vided into pharynx, esophagus, crop, gizzard, and saccu-
lated intestine (Fig. 52). The leech has large sacs on
each side of the intestine. The sea worms, like Nereis,
have the pharynx armed with teeth, and some have
glandular coeca attached to the intestine. The plan is
that of a straight tube extending from mouth to anus.
In myriapods and larvae of insects, the same general
plan is continued, the canal passing in a straight line
from one extremity to the other, but showing a division
into gullet, stomach, and intestine.92 Crustacea, like
the lobster, have a short gullet leading to a large
cavity, situated in the front of the animal, which is a
gizzard, rather than stomach, as it has thick muscular
walls armed with teeth. A well-marked constriction

FIG. 237. — Diagrammatic Section of a Sea Urchin
(Echinus): a, mouth; b, esophagus; c, stom-
ach; d, intestine; ft madreporiform tubercle;
g, stone canal; h, ambulacral ring; k, Polian
vesicles, which are probably reservoirs of fluid;
m, ambulacral tube; o, anus; p, ambulacra, with
their contractile vesicles; r, nervous ring around
the gullet; s, two nervous trunks, the right termi-
nating, at anal pole, in an eye; t, blood-vascular
rings connected by v, the intestinal blood vessel;
iv, two arterial trunks radiating from the anal
ring; x, an ovary opening at the anal pole in a
genital plate, y; z, spines, with their tubercles.

280

COMPARATIVE ZOOLOGY

separates this organ from the intestine. The " liver,"
really a pancreas, is highly developed ; instead of

numerous folli-
cles, there is a
large bilaterally
symmetrical or-
gan, divided into
three lobes on
each side, pour-
ing its secretion
into the upper
part of the intes-
tine, which is the
true stomach.

Among insects,
there is great
variation in the
form and length
of the canal. The
following parts
can generally be
distinguished :
gullet, crop, giz-
zard, stomach,
and large and
small intestines,
with many gland-
ular appendages.
The crop, gizzard,

, iQrfr_ inf^c
dn Idrge

tmg are SOITl€-

timCS absent,
. ,,

especially in the
carnivorous species. In bees, the crop is called the
"honey-bag.'-' The gizzard is found in insects having

FIG. 238. — Anatomy of a caterpillar : g, h, esophagus; h,
/.stomach; k, hepatic vessels; /, m, intestine; q, r,
salivary glands; /, salivary duct; a, b, c, longitudinal
tracheal trunks; d, e, air tubes distributed to the vis-
cera; f, fat mass; v, x,y, silk secretors; z, their excre-
tory ducts, terminating in t, the spinneret, or fusulus.

THE ALIMENTARY CANAL

28l

mandibles, and is frequently lined with rows of horny
teeth, which are specially developed in grasshoppers,
crickets, and locusts. The intestines are remarkable

: a\

FIG. 239. — Alimentary canal of a Beetle: FIG. 240. — Alimentary Canal of the Bee

a, pharynx; b, gullet, leading to crop, (Apis mellifica}\ a, gullet; b, crop; c, d,

c, gizzard, d, and stomach, e; f, deli- stomach; e, small intestine; f, large in-

cate urinary tubes; gs intestine; h, testine; g, anal orifice; /*, urinary vessels;

other secreting organs. /, auxiliary glands.

for their convolutions. Insects have no true liver ; but
its functions are performed by little cell masses on the
inside of the stomach.93

FIG. 241. — Anatomy of a Sphinx Moth; «, nervous cord; «', brain sending ofT nerves
to the legs, /', I" , /'", and for the wings at «"; h, dorsal vessel, or heart; c, crop;
s, stomach; it intestines; o, reproductive organs; o' , oviduct; 8-20, segments.

The alimentary canal of spiders is short and straight,
the pharynx and gullet being very minute. The stom-

282

COMPARATIVE ZOOLOGY

ach is characterized by sending out tubular prolonga-
tions, and the intestine ends in a large bladderlike
expansion. Scorpions have no stomachal cavity — a
straight intestine passes directly through the body.

In bivalve mollusks, like the clam, the mouth opens
into a short esophagus which leads into the stomach,

which lies em-
bedded in a large
liver, and the in-
testine, describ-
ing a few turns,
passes directly
through the
heart.94 In the
univalve mol-
lusks, like the
snail, the gullet
is long and fre-
quently expands
into a crop ; the
stomach is often
double, the ante-
rior being a giz-
zard provided
with teeth for
mastication ; the intestine passes through the liver, and
ends in the fore part of the body, usually on the right
side.

The highest mollusks, as the cuttlefish and nautilus,
exhibit a marked advance. A mouth with powerful
mandibles leads to a long gullet, which ends in a strong
muscular gizzard resembling that of a fowl.95 Below
this is a cavity, which is either a stomach or duodenum ;
it receives the secretion from a large digestive gland or
pancreas. The intestine is a tube of uniform size, which,

FIG. 242. — Alimentary Canal of the Oyster: a, stomach
laid open; d, liver; b, c, d,f, convolutions of the intes-
tine; g, anal aperture; n, o, auricle and ventricle; /, m,
adductor muscle; h, k, lobes of mouth divided to show
the venous canals at the base of the gills.

THE ALIMENTARY CANAL

283

FIG. 243. — Anatomy of a Gastropod (Snail): a, mouth; b, foot; c, anus; d, lung; e,
stomach, covered above by the salivary glands; f, intestine; g, "liver"; h, heart;
f, aorta; j, gastric artery; k, artery of the foot; /, hepatic artery: mt abdominal
cavity, supplying the place of a venous sinus; «, irregular canal communicating
with the abdominal cavity, and carrying the blood to the lung; o, Vessel carrying
blood from the lung to the heart.

q

FIG. 244. — Anatomy of a Lamellibranch (Mactra): a, shell; b, mantle; c, tentacles, or
lips; d, mouth; e, nerves; f, muscles; g, anterior, and n, posterior ganglion; k,
"liver"; z', heart; k, stomach; /, intestine passing through the heart; m, kidney;
0, anal end of the intestine; /, exhalent, and q, inhalent respiratory tubes, or
siphons; r, gills; s, foot.

284

COMPARATIVE ZOOLOGY

after one or two slight curves, bends up, and opens into
the " funnel " near the mouth.

Fishes have a simple, short, and wide alimentary
canal. The stomach is separated from the intestine by
a narrow " pyloric " orifice, or valve, but is not so clearly
distinguished from the gullet, so that regurgitation is

easy.96 Indeed, it is common
for fishes, to disgorge the in-
digestible parts of their food,
and some, as the carp, send
the food back to the pharynx
to be masticated. The stom-
ach is usually bent, like a
siphon ; but the intestine is
nearly straight, and without
any marked distinction into
small and large. Its appen-
dages are a large liver and a
rudimentary pancreas.

In the amphibians, as the

FIG. 245. — Anatomy of a Cephalopod IT.

(diagram): a, tentacles; 6, masti- frOgS, the dlgCStlVC apparatus

Z?jSS*?~S?. ^i is vefy sirailar to that of fishes ;

/, esophagus; g, internal shell, or ^ut the tWO portions of the
cuttle-bone ; h, stomach; z, in- x

testine; k, anus; /, funnel; m, intestine can be more readily

ink bag; n, ovary; o, oviduct; /, T .• • -, •> ^r-i ^-i

"liver"; r, gill contained in the distinguished. The reptiles

™s;£n« "! generally have a long, wide

gullet, which passes insensibly

into the stomach, and a short intestine (about twice the
length of the body) very distinctly divided into small
and large by a constriction.97 The vegetable-feeding
tortoises have a comparatively long intestinal tube ; and
the serpents have a slender stomach, but little wider
than the rest of the alimentary canal.

The stomach of the crocodile (Fig. 247) is more com-
plex than any hitherto mentioned. " It resembles that of

THE ALIMENTARY CANAL

285

the cuttlefish, but offers a still more striking analogy to
the gizzard of a bird, having very thick walls, and the
muscular fibers radiating precisely in the same manner,

FIG. 246. — Anatomy of the Carp: br, branchiae, or gills; c, heart; ft liver; v.n, v.n'.
swimming bladder; c.i, intestinal canal; 0, ovarium; u, ureter; a, anus; o' ', gen-
ital opening; «', opening of ureter. The side view shows the disposition of the
muscles in vertical flakes.

286

COMPARATIVE ZOOLOGY

so that, in this respect, the crocodile may be considered
to be intermediate between reptiles and birds.98 In

crocodiles also
the duodenum,
with which the
intestine begins,
is first distinctly
defined. Into
this part of the
intestine the
liver and pan-
creas, or sweet-
bread, pour
their secretions.
Furthermore, in
the lower ani-
mals, the intes-
tines lie more
or less loose in

the abdomen ;
-i . • f-L. „ r^-r^r*

odile, and like-

wise in birds and mammals, they are supported by a
membrane called mesentery.

In birds, the length of the alimentary canal varies
with their diet, being greatest in those living on grain
and fruit. The gullet corresponds in length with the
neck, which is longest in the long-legged tribes, and in
width with the food. In those that swallow large fish
entire, the gullet is dilatable, as in snakes. In nearly
all birds, the food is delayed in some cavity before
digestion : thus, the pelican has a bag under the lower
jaw, and the cormorant has a capacious gullet, where it
stores up fishes ; while those that gorge themselves at
intervals, as the vulture, or feed on seeds and grains,

FIG. 247. — Stomach of the Crocodile: a, muscular fibers ra-
dialing from a central tendon, b; d, commencement of
duodenum; c, esophagus; /, intestine.

THE ALIMENTARY CANAL

287

as the turkey, have
near the lower end
of the gullet" The
ostrich, goose, swan,
most of the waders,
and the fruit or
insect-eating birds,
which find their
food in tolerable
abundance, and take
it in small quanti-
ties, have no such
reservoir. Pigeons
have a double crop.
In all birds, the
food passes from
the gullet into the
proventriculus, or
stomach proper,
where it is mixed
with a " gastric
juice" secreted
from glands on the
surface. Thence it
goes into the giz-
zard, an oval sac
of highly muscular
texture, and lined
with a tough, horny
skin.100 The giz-
zard is most highly
developed, and of a
deep-red color,, in
the scratchers and
flat-billed swimmers

a pouch, called the crop, developed

FIG. 248. — Digestive Apparatus of the Fowl: i,
tongue; 2, pharynx; 3, 5, esophagus; 4, crop;
6, proventriculus; 7, gizzard; 8, 9, 10, duodenum;
n, 12, small intestine; 13, two caeca (analogue of
the colon of mammals') ; 14, their insertion into the
intestinal tube; 15, rectum; 16, cloaca; 17, anus;
18, mesentery; 19, 20, left and right lobes of liver;
21, gall bladder; 22, insertion of pancreatic and
biliary ducts; 23, pancreas; 24, lung; 25, ovary;
26, oviduct.

(as fowls and swans) ; but compara-

288

COMPARATIVE ZOOLOGY

tively thin and feeble
in birds of prey (as the
eagle). The gizzard is
followed by the intes-
tines, which are longer
than those of reptiles :
the small intestine be-
gins with a loop (the
duodenum), and is
folded several times
upon itself ; the large
intestine is short and
straight, terminating in
the sole outlet of the
body, the cloaca. A
liver and pancreas are
always attached to the
upper part of the small
intestine.

The alimentary canal
in mammals is clearly
separated into four dis-
tinct cavities : the
pharynx, or throat; the
esophagus, or gullet ;
the stomach ; and the
intestines.

The pharynx is more
complicated than in
birds. It is a funnel-

FlG. 249. — Digestive Apparatus of
Man (diagram): i, tongue; 2, phar-
ynx; 3, esophagus; 4, soft palate; 5, larynx; 6, palate; 7, epiglottis; 8, thyroid cartilage;
9, beginning of spinal marrow; 10, n, 12, vertebrae, with spinous processes; 13, cardiac
orifice of stomach; 14, left end of stomach; 18, pyloric valve; 19, 20, 21, duodenum;
22, gall bladder; 27, duct from pancreas; 28, 29, jejunum of intestine; 30, ileum; 34, cce-
cum; 36, 37, 38, colon, or large intestine; 40, rectum.

THE ALIMENTARY CANAL 289

shaped bag, having seven openings leading into it : two
from the nostrils, and two from the ears ; one from the
windpipe, guarded by the epiglottis ; one from the mouth,
with a fleshy curtain called the soft palate ; and one from
the esophagus. It is the natural passage for food be-
tween the mouth and the esophagus, and of air between
the nostrils and windpipe. Like the mouth, it is lined
with a soft mucous membrane.

The esophagus is a long and narrow tube, formed of
two muscular layers : in the outer layer, the fibers run
lengthwise ; in the other, they are circular. It is lined

FIG. 250. — Ideal Section of a Mammalian Vertebrate: A, pectoral, or fore limb; B,
pelvic, or hind limb; a, mouth; b, cerebrum; c, cerebellum; d, nose; e, eye; ft
ear; g, esophagus; h, stomach; i, intestine; /, diaphragm, or midriff; k, rectum,
or termination of intestine; /, anus; m, liver; «, spleen; o, kidney; /, sympathetic
system of nerves; q, pancreas; r, urinary bladder; s, spinal cord; u, ureter; v,
vertebral column; -w, heart; x, lung; y, trachea, or windpipe; z, epiglottis.

with mucous membrane. While in all fishes, reptiles,
and birds the body cavity is one, in mammals it is
divided, by a partition called the diaphragm, into two
cavities, — the thorax, containing the heart, lungs, etc. ;
and the abdomen, containing the stomach, intestines, etc.
The esophagus passes through a slit in the diaphragm,
and almost immediately expands into the stomach.

In the majority of mammals, the stomach is a muscu-
lar bag of an irregular oval shape, lying obliquely across
the abdomen. In the flesh eaters, whose food is easy
DODGE'S GEN. ZOOL. — 19

2QO

COMPARATIVE ZOOLOGY

of solution, the stomach is usually simple, and lies nearly
in the course of the alimentary canal ; but in proportion
as the food departs more widely in its composition from
the body itself, and is therefore more difficult to digest,
we find the stomach increasing in size and complexity,

FIG. 251. — Section of Horse's Stomach; A, FlG. 252. — Stomach of the

left sac; B, right sac; C, duodenum. Porpoise: c, cardiac open-

ing; /, pyloric opening.

and turned aside from the general course of the canal,
so as to retain the food a longer time. The inlet from
the esophagus is called cardiac opening; the outlet
leading into the intestines is called pyloric opening.

In the carnivores,
apes, and most odd-
toed quadrupeds, the
stomach resembles
that of man. That
of the toothless ant-
eater has the lower
part turned into a

FIG. 253.— Stomach of the Lion: c, cardiac orifice, or kind of gizzard for
f esophagus; A pyloric orifice. crushing its fo^

The elephant's is subdivided by numerous folds. In the
horse, it is constricted in the middle ; and in the rodents,
porpoises, and kangaroos, the constriction is carried so

THE ALIMENTARY CANAL 291

far as to make two or three sections. But animals
that chew the cud (ruminants) have the most complex
stomach. It is divided into four peculiar chambers :
First, the paunch (rumen), the largest of all, receives
the half-masticated food when it is first swallowed.
The inner surface is covered with papillae, except in
the camel, which has large cells for storing up water.
From this, the food passes into the honeycomb stomach
(reticulum), so named from its structure. Liquids swal-
lowed usually go directly to this cavity, without passing
through the paunch, and hence it is sometimes called
the water bag. Here the food is made into little balls,

FIG. 254. — Complex Stomach of a Ruminant: a, gullet; b, rumen, or paunch; c, reticu-
lum ; d, psalterium, or manyplies ; e, abomasus ; _/", pylorus leading to duodenum.

and returned to the mouth to undergo a thorough mas-
tication. When finally swallowed, it is directed, by a
groove from the esophagus, to the third, and smallest,
cavity, the manyplies (psalterium), named from its
numerous folds, which form a strainer to keep back
any undivided food ; and thence it passes into the true
stomach (abomasus), from which, in the calf, the rennet is
procured for curdling milk in the manufacture of cheese.
This fourth cavity is like the human stomach in form
and function, and is the only part which secretes gastric
juice. The rumen and reticulum are rather dilatations
of the esophagus than parts of the stomach itself ; while
the latter is divided by constriction into two chambers,
the psalterium and abomasus, as in many other animals.

292

COMPARATIVE ZOOLOGY

nun.

In structure the stomach resembles the esophagus.
The smooth outside coat {peritoneum) is a reflection of
the membrane which lines the whole abdomen. The
middle, or muscular, coat consists of three layers of
fibers, running lengthwise, around and obliquely. The
successive contraction and relaxing
of these fibers produce the worm-
like motion of- the stomach, called
peristaltic. The innermost, or mu-
cous, membrane, is soft, velvety, of
a reddish gray color in man, and
filled with multitudes of glands,
which secrete the gastric juice.
The human stomach, when dis-
tended, will hold about five pints ;
that of the kangaroo is as long as
its body.

The intestinal canal in mammals
begins at the pyloric end of the stom-
ach, where there is a kind of valve or
circular muscle. Like the stomach,
FIG 255 -vertical section ft varies greatly, according to the

of the Coats of the Stomach : & J '

d, surface of mucous mem- nature of the food. It is generally

brane, and mouths of gastric , , . ,, - it r i i

follicles; m, gastric tubuii, longest m the vegetable feeders, and

or follicles; mm, dense con- shortest in the flesh feeders. The
nective tissue; sm, sub-mu-
cous tissue; cm, transverse greater length in the former is due

muscular fiber; lmt longi-
tudinal muscular fibers; to the fact that vegetable food re-

s, fibrous, or serous, coat. ^.^ ^ ^^ ^ f()r digestion>

and that a greater bulk of such food is required to
obtain a given quantity of nutriment. The intestines
measure 150 feet in a full-grown ox, while they are
but three times the length of the body in the lion,
and six times in man. Save' in some lower forms, as
the whales, there are two main divisions, the "small"
and "large" intestines, at the junction of which is a

THE ALIMENTARY CANAL 293

valve. The former is the longer of the two, and in it
digestion is completed, and from it the most of absorp-
tion takes place. The large intestine is mainly a tem-
porary lodging place for the useless part of the food,
until it is expelled from the body. The beginning of
the small intestine is called the duodenum, into which
the ducts from the liver and pancreas open. The in-
testinal canal has the same structure as the stomach,
and by a peristaltic motion its contents are propelled
downward. The inside of the small intestine is covered
with a host of threadlike processes (villi\ resembling
the pile of velvet.

In taking this general survey of the succession of
forms which the digestive apparatus presents among the
principal groups of animals, we cannot fail to trace a
gradual specialization. First, a simple sac, one orifice
serving as inlet for food and outlet for indigestible
matter; next, a short tube, with walls of its own sus-
pended in the body cavity ; then a canal passing through
the body, and, therefore, having both mouth and vent ;
next, an apparatus for mastication, and a swelling of
the central part of the canal irito a stomach, having the
special endowment of secreting gastric juice ; then a
distinction between the small and large intestine, the
former thickly set with villi, and receiving the secretions
of large glands. We also notice that food, the means
of obtaining it, the instruments for mastication, and
the size and complexity of the alimentary canal, are
closely related.

CHAPTER XIII*
HOW ANIMALS DIGEST

The Object of the Digestive Process is the reduction
of food into such a state that it can be absorbed into the
system. For this purpose, if solid, it is dissolved ; for
fluidity is a primary condition, but not the only one.
Many soluble substances have to undergo a chemical
change before they can form parts of the living body.
If albumen or sugar be injected into the veins, it will
not be assimilated, but be cast out unaltered.

To produce these two essential changes, solution and
transmutation, two agencies are used — one mechanical,
the other chemical. The former is not always needed,
for many animals find their food already dissolved, as
the butterfly ; but solid substances, to facilitate their
solution, are ground or torn into pieces by teeth, as in
man ; by jaws, as in the lobster; or by a gizzard, as in
the turkey.

The chemical preparation of food is indispensable.101
It is accomplished by one or more solvent fluids secreted
in the alimentary canal. The most important, and one
always present, is the gastric juice, the secretion of
which is restricted to the stomach, when that cavity
exists. In the higher animals, numerous glands pour
additional fluids into the digestive tube, as saliva into
the upper part or mouth, and bile and pancreatic juice
into the upper part of the intestine. In fact, the mucous

* See Appendix.
294

HOW ANIMALS DIGEST 295

membrane, which lines the alimentary canal throughout,
abounds with secreting glands or cells.

The Digestive Process is substantially the same in- all
animals, but it is carried farther in the more highly de-
veloped forms. In the Infusoria, the food is acted upon
by some secretion from the protoplasm of the body, the
exact nature of which is unknown. In the starfish and
sea urchin, we find two solvents — a gastric juice, and an-
other resembling pancreatic juice; but the two appear
to mingle in the stomach. Mollusks and arthropods
show a clear distinction between the stomach and intes-
tine, and the contents of the pancreas are poured into
the latter. There are, therefore, two stages in the digest-
ive act : first, the food is dissolved by the gastric juice in
the stomach, forming chyme ; secondly, the chyme, upon
entering the intestine, is changed into chyle by the action
of the pancreatic secretion, and is then ready to be
absorbed into the system.

In vertebrates, a third solvent is added, the bile, which
aids the pancreatic juice in completing digestion. But
mammals and insects have a still more perfect and elab-
orate process ; for in them the saliva of the mouth acts
chemically upon the food ; while the saliva in many other
animals has no other office, so far as we know, than to
moisten the food for swallowing.

Taking man as an example, let us note the main facts
in the process. During mastication, by which the rela-
tive surface is increased, the food is mixed with saliva,
which moistens it,102 and turns a small part of the
s(tarch into grape sugar. Passed into the stomach, the
food meets the gastric juice. This is acid, and, first,
stops the action of the saliva ; secondly, by means of
the pepsin which it contains, and the acid, it dissolves
the albumen, fibrin, and other such constituents of the
food. This solution of albuminoids is called -a peptone,

296 COMPARATIVE ZOOLOGY

and is especially distinguished from other such solutions
by its diff usibility — i.e., the ease with which it passes
through a membrane. Some of these peptones, with
the sugars of the food, whether original or the product
of the action of the saliva, are absorbed from the stomach.
The food, while in the stomach, is kept in continual
motion, and, after a time, is discharged in gushes into
the intestine. The name chyme is given to the pulpy
mass of food in the stomach.103 In the intestine the
chyme meets three fluids — bile, pancreatic juice, and
intestinal juice. All of these are alkaline, and at once
give the acid chyme an alkaline reaction. This change
permits the action of the saliva to recommence, which
is aided by the pancreatic and intestinal juices. The
pancreatic juice has much more important functions.
It changes albuminoid food into peptones, and probably
breaks up the fats into very small particles, which are
suspended in the fluid chyle. This forms an emulsion,
like milk, and causes the chyle to appear whitish. The
bile has important functions, but little understood. It
emulsifies and saponifies part of the fats, so that they
are dissolved, and perhaps aids in preventing the food
from decomposing during the process of digestion and
absorption. The chyle is slowly driven through the
small intestine by the creeping, peristaltic motion of its
walls,104 the nutritious portion being taken up by the
absorbents, as described in the next chapter, while the
undigested part remaining is discharged from the large
intestine.105 *

CHAPTER XIV

THE ABSORBENT SYSTEM

THE nutritive matter (chyle), prepared by the digestive
process, is still outside of the organism. How shall it
enter the living tissue ?

In animals, like the
Infusoria and polyps,
whose digestive de-
partment is not sepa-
rated from the body
cavity, the food, as
soon as dissolved, min-
gles freely with the
parts it has to nourish.
In the higher inverte-
brates having an ali-
mentary canal, the
chyle passes,by simple
transudation, through
the walls of the canal
directly into the soft
tissues, as in insects, or
is absorbed from the
canal by veins in con-
tact with it, as in sea
urchins, mollusks.
worms, and crusta-
ceans, and then dis-
tributed through the
body.

FIG. 256. — Section of Injected Small Intestine of Cat:
a, b, mucosa; g, villi; i, their absorbent vessels;
h, simple follicles; c, muscularis mucosae; d, sub-
mucosa; e, e' , circular and longitudinal layers of
muscle; f, fibrous coat. All the dark lines represent
blood vessels filled with an injection mass.

297

298

COMPARATIVE ZOOLOGY

In vertebrates only do we find a special absorbent sys-
tem. Three sets of vessels are concerned in the general
process by which fresh material is taken up and added
to the blood : Capillaries, Lacteals, and Lymphatics.
Only the two former draw material from the alimentary
canal.

The food probably is absorbed almost as fast as
it is dissolved, and, therefore, there is a constant loss in
the passage down the canal. In the mouth and esoph-
agus, the absorption is slight ; but much of that which
has yielded to the gastric juice, with most of the water,
is greedily absorbed by the capillaries of the stomach,
and made to join the current of blood which is rushing
to the liver. Absorption by the capillaries also takes
place from the skin and lungs. Medicinal or poisonous
gases and liquids are readily introduced into the system
by these channels.

We have seen that the oily part of the food passes
unchanged from the stomach into the small intestine,

where, acted upon by the
pancreatic juice, it is cut
up into extremely minute
particles, and that the un-
digested albuminoids and
starches are digested in
the intestine. Two kinds
of absorbents are present
in the intestine, lacteals
and blood capillaries. Both
the lymphatic and blood
systems send vessels into
the velvety villim with
which the - intestine is
lined. The blood capillaries lie toward the outside of
the villus and the lacteal in the center. The albumi-

FIG. 257. — Lacteal System of Mammal: a,
descending aorta, or principal artery; b,
thoracic duct; c, origin of lacteal vessels,
g, in the walls of the intestine, d; e,
mesentery, or membrane attaching the
intestine to walls of the body; /, lacteal,
or mesenteric, glands.

THE ABSORBENT SYSTEM

299

noids and sugars are chiefly absorbed by the blood vessels
and go to the liver. The fats pass on into the lacteals,
which receive their name from the milky appearance
of the chyle. These lacteals unite into larger trunks,
which lie in the mesentery
(or membrane which sus-
pends the intestine from
the back wall of the ab-
domen), and these pour
their contents into one
large vessel, the thoracic
duct, lying along the
backbone, and joining the
jugular vein in the neck.

The lacteals are only a
special part of the great
lymphatic system, which
absorbs and carries to the
thoracic duct matter from
all parts of the body.107
The lymph is a transpar-
ent fluid having many
white blood corpuscles.
It is, in fact, blood, minus
the red corpuscles, while
chyle is the same fluid ren-
dered milky by numerous

fat globules. During the FlG. 258._Prindpal Lymphatics of the Hu-

intervals of digestion, the man Body: *» union of left Jusular and

subclavian veins; b, thoracic duct; c, re-
laCteals Carry Ordinary ceptaculum chyli. The oval bodies are

lymph. This fluid is the

overflow of the blood — the plasma and white corpuscles
which escape from the blood capillaries, and carry nutri-
ment to, and waste from, those parts of the various tis-
sues which are not in contact with the blood capillaries.

300 COMPARATIVE ZOOLOGY

This surplus overflow is returned to the blood by the
lymphatics. The current is kept up by the movements
of the body, and in many vertebrates, as frogs and
fishes, by lymph hearts.

Like the roots of plants, the absorbent vessels do not
commence with open mouths ; but the fluid which enters
them must traverse the membrane which covers their
minute extremities. This membrane is, however, porous,
and the fluids pass through it by the processes of filtra-
tion and diffusion, or dialysis. How the fat gets into
the lacteals is not yet well understood, but the lacteals
are themselves rhythmically contractile, and force the
absorbed chyle toward the heart. The valves of the
lymphatics prevent its return.

CHAPTER XV*

THE BLOOD OF ANIMALS

The Blood is that fluid which carries to the living
tissues the materials necessary to their growth and
repair, and removes their waste and worn-out material.
The great bulk of the body is occupied with apparatus
for the preparation and circulation of this vital fluid.

The blood of the lower animals (invertebrates) differs
so widely from that of man and other vertebrates, that
the former were long supposed to be without blood. In
them the blood is commonly colorless ; but it has a bluish
cast in crustaceans ; reddish, yellowish, or greenish, in
worms ; and reddish, greenish, or brownish, in jelly-
fishes. The red liquid which appears when the head of
a fly is crushed is not blood, but comes from the eyes.
In vertebrates, the blood is red, excepting the white-
blooded, fishlike lancelet Amphioxus.m

As a rule, the more simple the fabric of the body, the
more simple the nutritive fluid. In unicellular animals
(as Protozoa), in those whose cells are comparatively
independent (as sponges), and in small and lowly organ-
ized animals (like hydra), there is no special circulating
fluid. Each cell feeds itself either directly from parti-
cles of food, or from the products of digestion. In
polyps and jellyfishes, the blood is scarcely different
from the products of digestion, although a few blood
corpuscles are present. But in the more highly organ-
ized invertebrates the blood is a distinct tissue, coagu-

* See Appendix.
301

302

COMPARATIVE ZOOLOGY

lating, and containing white corpuscles. The blood of
the vertebrates, apparently a clear, homogeneous liquid,
really consists of minute grains, or globules, of organic
mattep floating in a fluid. If the blood of a frog be
poured on a filter of blotting paper, a transparent fluid
(called plasma} will pass through, leaving red particles,
resembling sand, on the upper surface. Under the mi-
croscope, these particles prove to be cells, or flattened
disks (called corpuscles^ containing a nucleus ; some are
colorless, and others red. In mammals the red disks

E

FIG. 259. — Blood Corpuscles: A, red corpuscles in rouleaux, a, a, colorless corpuscles,
magnified about 400 times; B, red corpuscles in focus; C, view of edge; D, three-
quarters view; E, red corpuscle swollen with water; F, G, H, distorted red corpuscles.

have a tendency to collect together into piles ; the color-
less ones remain single. Meanwhile, the plasma sepa-
rates into two parts by coagulating ; that is, minute
fibers form, consisting si fibrin, leaving a pale yellowish
fluid, called serum.m Had the blood not been filtered,
the corpuscles and fibrin would have mingled, forming
a jelly like mass, known as clot. Further, the serum will
coagulate if heated, dividing into hardened albumen and
a watery fluid, called serosity, which contains the soluble
salts of the blood.

These several parts may be expressed thus : —

Blood

Corpuscles

Plasma

colored

fibrin
serum

( albumen.

\ serosity = water and salts.

THE BLOOD OF ANIMALS

303

FIG. 260. — Nucleated Blood Cells of a Frog, x
250: a, colorless corpuscles.

If now we examine the nutritive fluid pi the simplest
animals, we find only a watery fluid containing granules.
In radiates and the
worms and mollusks,
there is a similar fluid,
with the addition of
a few colorless corpus-
cles. But there is little
fibrin, and, therefore,
it coagulates feebly
or not at all. In the
arthropods and higher
mollusks, the circulat-
ing fluid contains col-
orless nucleated cells,
and coagulates.110 In
vertebrates, there are,

in addition to the plasma and colorless corpuscles of
invertebrates, red corpuscles, to which their blood owes
its peculiar hue. In fishes, amphibians, reptiles, and
birds, i.e., all oviparous vertebrates, these red corpuscles
are nucleated ; but in those of
mammals, no nucleus has been
discovered.111

All blood corpuscles are micro-
scopic. The colorless are more
uniform in size than the red ; and
generally smaller (except in mam-
mals), being about ^rVo °f an mcn
in diameter. The red corpuscles
are largest in amphibians (those of
Proteus being the extreme, or ^o"
of an inch), next in fishes, then birds and mammals.
The smallest known are those of the musk deer. In
mammals, the size agrees with the size of the animal

FIG. 261. — Elliptical Corpuscle
of the Frog, showing the nu-
cleus as a prominence in the
center. Greatly magnified.

304

COMPARATIVE ZOOLOGY

only within a natural order ; but in birds the correspon-
dence holds good throughout the class, the largest being
found in the ostrich, and the smallest in the humming bird.
In man, they measure ^Vo of an inch, so that it would
take 40,000 to cover the head of a pin.

As to shape, the colorless corpuscles are ordinarily
globular, in all animals ; but they are constantly chang-

FIG. 262. — Comparative Size and Shape of the red Corpuscles of various Animals.

ing. The form of the red disks is more permanent,
although they are soft and elastic, so that they squeeze
through very narrow passages. They are oval, circular,
or angular, in fishes ; oval in reptiles, birds, and the
camel tribe ; and circular in the rest of mammals. They
are double convex when nucleated, and double concave
when circular and not nucleated.

Blood is always heavier than water ; but is thinner in
cold-blooded than in warm-blooded animals, in herbi-

THE BLOOD OF ANIMALS 305

vores than in carnivores. The blood of birds, which is
the hottest known, being 104° F. which is 2°-i4° F.
higher than mammals', is richest in red corpuscles. In
man, they constitute about one half the mass of blood.
The white globules are far less numerous than the red ;
they are relatively more abundant in venous than arte-

FIG. 263. — Capillary Circulation in the Web of a Frog's Foot, X 100: a, b, small veins;
d, capillaries in which the oval corpuscles are seen to follow one another in single
series; c, pigment cells in the skin.

rial blood, in the sickly and ill-fed than in the healthy
and vigorous, in the lower vertebrates than in birds
and mammals. Their number is subject to great vari-
ations, increasing rapidly after a meal, and falling as
rapidly.

There is less blood in cold-blooded than in warm-
blooded animals ; and the larger the animal, the greater
is the proportion of blood to the body. Man has about
DODGE'S GEN. ZOOL. — 20

306 COMPARATIVE ZOOLOGY

a gallon and a half, equal to one-thirteenth of his
weight. The heart of the Greenland whale is a yard
in diameter.

The main Office of the Blood is to supply nourishment
to, and take away waste matters from, all parts of the
body. It is at once purveyor and scavenger. In its
circulation, it passes, while in the capillaries, within an
infinitesimal distance of the various tissue cells. Some of
the plasma, carrying the nutritive matter needed, exudes
through the walls of the capillary tubes ; the tissue as-
similates or makes like to itself whatever is suitable for
its growth and repair ; and the lymphatics take up the
transuded fluid, and return it to the blood vessels. At
the same time, the waste products of the tissues are col-
lected and brought through the venous capillaries, veins,
and lymphatics to the excretory organs. The special
function of the several constituents of the blood is not
wholly known. The corpuscles in the red marrow of
the bones of some vertebrates are supposed to be the
source of the red disks. The latter are the carriers of
oxygen which is taken up by their red matter (hemo-
globin) in the lungs and given up to the tissues. The
same office is performed by the blue coloring matter
(haemocyanin) in the blood of certain invertebrates, as
the squid and lobster. The carbon dioxide is taken up
mainly by the plasma.

Like the solid tissues, the blood, which is in reality a
liquid tissue, is subject to waste and renewal, to growth
and decay. The loss is repaired from the products of
digestion, carried to the blood by the lacteals, or ab-
sorbed directly by the capillaries of the digestive tract.
The white corpuscles probably are prepared in many
parts of the body, especially the liver, spleen, and lym-
phatic glands. In the lower organisms, the nutritive
food is prepared by contact with the tissues, without

THE BLOOD OF ANIMALS 307

passing through special organs. Lymph differs from
blood chiefly in containing less albumen and fibrin, and
no red disks. Chyle is lymph loaded with fat globules,
and is found in the lacteals and vessels connected with
them during the absorption of food containing fat.

CHAPTER XVI*

THE CIRCULATION OF THE BLOOD

The Blood is kept in continual motion
nourish and purify the body and itself.

means
, . brings

in order to
For as life
work, and work
waste, there is
constant need of fresh
material to make good
the loss throughout the
system, and of the re-
moval of matter which
is no longer fit for use.

In the very lowest ani-
mals, where all parts of
the structure are equally
capable of absorbing the
digested food and are in
contact with it, there is
no occasion for any cir-
culation, although even
in them the digested
food is not allowed to
stagnate. But in propor-
tion as the power of ab-
sorption is confined to
certain parts, the more is
the need and the greater the complexity of an apparatus
for conveying the nutritive fluid to the various tissues.

* See Appendix.

FIG. 264. — Venous Valves. They usually oc-
cur in pairs, as represented.

THE CIRCULATION OF THE BLOOD

309

In nearly all animals, the nutritive fluid is conveyed
to the various parts of the body by a system of tubes,
called blood vessels. The higher forms have two sets —
arteries and veins, in which the blood moves in opposite
directions, the former carrying blood from a central
reservoir or heart, the latter taking it to the heart. In
the vertebrates the walls of these
tubes are made of three coats,
or layers, of tissue, the arteries
being elastic, like rubber, and
many of . the veins being fur-
nished with valves.112 The great
artery coming out of the heart
is called aorta, and the grand
venous trunk, entering the heart
on the opposite side, is called
vena cava. Both sets divide and
subdivide until their branches
are finer than hairs; and joining
these finest arteries and finest
veins are intermediate micro-
scopic tubes, called capillaries
(in man about 3^0 of an inch in
diameter).113 In these only, so
thin and delicate are their walls,
does the blood come in contact
with the tissues or the air.

In those vertebrates which have lungs there are two
sets of capillaries, since there are two circulations — the
systemic, from the heart around the system to the heart
again, and the pulmonary, from the heart through the
respiratory organ back to the heart. This double
course may be illustrated by the figure 8. In gill-
bearing animals there are capillaries in the gills, but
not a double circulation.

FIG. 265. — Relation of artery, a,
vein, 6, and capillaries, c, as seen
in the muscles of a Dog.

COMPARATIVE ZOOLOGY

There is no true system of blood vessels below the
echinoderms. The simplest provision for the distribu-
tion of the products of digestion is shown by the jelly-
fish, whose stomach sends off radiating tubes (Fig. 21),
through which the digested food passes directly to the
various parts of the body instead of being carried by
the agency of a circulating medium — viz., the blood.

The First Approach to a Circulatory System is made by
the starfish and the sea urchin. A vein runs along the
whole length of the alimentary tube, to
absorb the chyle, and forms a circle
around each end of the tube. These
circular vessels send off branches to
various parts of the body ; but as
they are not connected by a network
of capillaries, there can be no circuit
(Fig. 237).

A higher type is exhibited by the
insects. If we examine the back of
any thin-skinned caterpillar, a long
pulsating tube is seen running beneath
the skin from one end of the body to
the other. This dorsal vessel, or heart,
as it is called, is open at both ends, and
divided by valves into compartments,
permitting the blood to go forward,
but not backward. Each compartment
communicates by a pair of slits, guarded
by valves, with the body cavity, so that
fluids may enter, but cannot escape.
" Circulation " is very simple. We have
seen that the chyle exudes through the
walls of the alimentary canal directly
into the cavity of the abdomen, where it mingles with
the blood already there. This mixed fluid is drawn

FIG. 266. — Part of the
Dorsal Vessel, or
Heart, of a Cock-
chafer bisected : a, b,
muscular walls; rf,
valves between the
compartments; c,
valve defending one
of the orifices com-
municating with the
general cavity of the
abdomen.

THE CIRCULATION OF THE BLOOD 311

into the dorsal tube through the valvular openings as it
expands ; and upon its contraction, all the side valves
are closed, and the fluid is forced toward the head.
Passing out at the front opening, it is again diffused
among and between the tissues of the body. The
blood, therefore, does not describe a circle in definite
channels so as to return constantly to its point of
departure.

Many worms (as the earthworm) have a pulsating
tube extending from tail to head above the alimentary
canal, a similar tube on the ventral side through which

FIG. 267. —Circulation in a Lobster: a, heart; 3, artery for the eyes; c, artery for an-
tennae; d, hepatic artery; e, superior abdominal artery: f, sternal artery; g, venous
sinuses transmitting blood from the body to the branchiae, h, whence it returns to the
heart by the branchio-cardiac vessels, i.

the blood returns, and cross tubes in every segment
(Fig. 52). In the lobster and crab, spider and scorpion,
the dorsal tube sends off a system of arteries (not found
in insects) ; but the blood, as it leaves these tubes,
escapes into the general cavity, as in t>ther Arthropoda.
The lobster and crab, however, show a great advance
in the concentration of the propelling power into a
short muscular sac.

A third development of the circulatory system is
furnished by the mollusks. Comparatively sluggish,
they need a powerful force pump in the form of a com-
pact heart. In the oyster and snail (Figs. 242, 243),
we find such an organ having two cavities — an auricle

312

COMPARATIVE ZOOLOGY

and a ventricle, one for receiving, and the other for
distributing, the blood. The auricle injects the blood
into the ventricle, which propels it by the arteries to
the various organs. Thence it passes not immediately

to the veins, as in higher ani-
mals, but into the spaces around
the alimentary canal. A part
of this is carried by vessels to
the gills or lung, and then re-
turned with the unpurified por-
tion to the auricle. The whole
of the blood, therefore, does
not make a complete circuit.
The clam has a similar heart,
but with two auricles.

A still higher form is seen
in the cuttlefish, the highest of
the invertebrates. This animal
has a central heart, with a
ventricle and two auricles,
and, in addition, the veins
which collect the blood from
the system to send it back to
the heart by the way of the
gills are furnished with two
branchial hearts, which accel-
erate the circulation through
those organs. Many of the
arteries and veins are joined
by capillaries, but not all ;
so that in no invertebrate

animai is the bi°°d returned

ricle; e, venous sinus; /, portal to ^Q heart by a COntinUOUS

vein; g, intestine; h, vena cava; J

f, branchial vessels; k, dorsal ar- closed System of blood VCS-

tery, or aorta; I, kidneys; m, •>

dorsal artery. S61S.

h~

FIG 268.— circulating Apparatus in

THE CIRCULATION OF THE BLOOD 313

As a rule, in all animals having any circulation at all,
the current always takes one direction. This is gener-
ally necessitated by valves. But a curious exception is
presented by the ascidians, whose tubular heart is valve-
less, and the contractions occur alternately at one end
and then the other ; so that the blood oscillates to and
fro, and a given vessel is at one time a vein and at an-
other an artery. In this respect it resembles the foetal
heart of higher animals (Fig. 364).

In vertebrates only is the circulating current strictly
confined to the blood vessels ; in no case does it escape
into the general cavity
of the body. In other
respects, there is no great
advance in the apparatus
of the lowest vertebrates
over that of the highest
mollusks. A fish's heart
has, like that of an oyster,
two cavities, but its posi-
tion is reversed. Instead
of driving arterial blood
over the body, it receives
the returning, or venous, FlG 2&9 "_ DiagraVof a Singie Heart! <f,

blood, and Sends it tO the auricle' '> venficle; c, veins leading to

auricle; a, aorta, or mam artery.

gills. Recollected from the

gills, the blood is passed into a large artery, or aorta,
along the back, which distributes it by a complex sys-
tem of capillaries among the tissues. These capillaries
unite with the ends of the veins which pass the blood
into the auricle114 (Figs. 268, 272).

In amphibians and in reptiles generally (as frogs,
snakes, lizards, and turtles), the heart has three cavities
— two auricles and one ventricle. The venous blood
from the body is received into the right auricle, and the

314

COMPARATIVE ZOOLOGY

purified blood from the lungs into the left. Both throw
their contents into the ventricle, which pumps the mixed
blood in two directions — partly to the lungs, and partly
around the system (Fig. 273). Circulation is, therefore,
incomplete, since the whole current does not pass
through the lungs, and three kinds of blood are found
in the body — arterial, venous, and mixed. In many
animals, however, arrangements exist which nearly
separate the venous from the arterial blood.

The ventricle of reptiles is partially divided by a par-
tition. In the crocodile, the division is complete, so that
there are really four cavities — two auricles, and two
ventricles. But both ventricles send off aortas which
cross one another, and at that point a small aperture
brings the two into communication. The venous and
arterial currents are, therefore, mixed, but not within
the heart, as in other reptiles, nor so extensively. In

the structure of the heart,
as well as in that of the
gizzard, crocodiles approach
the birds.

The Highest Form of the
Circulating System is pos-
sessed by the warm-blooded
vertebrates, birds and
mammals. Not a drop of
blood can make the circuit
of the body without passing

throush the lunss' the dr-

culation tO and from thoSC

Organs being aS perfect aS

, ,. ., f . ,

the distribution of arterial
blood. The heart consists of four cavities — a right
auricle and ventricle, and a left auricle and ventricle.
In other words, it is a hollow muscle divided internally

separated than in higher animals: E,
right ventricle; L, left ventricle; D,
right auricle; F, pulmonary artery;
K, left auricle; A, aorta.

THE CIRCULATION OF THE BLOOD

315

by a vertical partition into two distinct chambers, each
of which is again divided by a valve into an auricle
and a ventricle. The work of the right auricle and
ventricle is to receive the blood from the veins, and
send it to the lungs ; while the other two receive
the blood from the lungs, and propel it over the body.
The left ventricle has more work to do than any of

/ g

FIG. 271. — Theoretical Section of the
Human Heart: a, right ventricle;
b, inferior vena cava; c, tricuspid
valve; d, right auricle; e, pulmonary
veins; _/, superior vena cava; g, pul-
monary arteries; h, aorta; k, left
auricle; /, mitral valve; m, left ven-
tricle; n, septum.

FIG. 272. — Plan of Circula-
tion in Fishes: a, auricle;
b, ventricle; c, branchial
artery; e, branchial veins,
bringing blood from the
gills, d, and uniting in the
aorta,y/ g, vena cava.

the other parts of the heart. The two auricles con-
tract at the same instant; so also do the ventricles.
The course of the current in birds and mammals is as
follows : the venous blood brought from the system is
discharged by two or three large trunks 115 into the right
auricle, which immediately forces it past a valve 116 into
the right ventricle. The ventricle then contracts, and
the blood is forced through the pulmonary artery past
its semilunar valves into the lungs, where it is changed

COMPARATIVE ZOOLOGY

from venous to arterial, returning by the pulmonary
veins to the left auricle. This sends it past the mitral
valves into the left ventricle, which drives it past the
semilunar valves into the aorta, and thence, by its rami-
fying arteries and capillaries, into all parts of the body
except the lungs. From the systemic capillaries, the
blood, now changed from arterial to venous, is gathered
by the veins, and conveyed back to the heart.

The Rate of the Blood Current generally increases with
the activity of the animal, being most rapid in birds.117

In insects, however,
it is comparatively
slow ; but this is
because the air is
taken to the blood
— the whole body
being bathed in air,
so that the blood has
no need to hasten
to a special organ.
However, activity
nearly doubles the
rate of pulsation in
a bee. The motion
in the arteries is
several times faster
than in the veins,
but diminishes as
the distance from the heart increases. In the carotid
of the horse, the blood moves 12^ inches per second ; in
that of man, 16 ; in the capillaries of man, I to 2 inches
per minute ; in those of a frog, i.

The Cause of the Blood Current may be cilia, or the
contractions of the body, or pulsating tubes or hearts.
In the higher animals, the impulse of the heart is not the

FIG 273. —A, Plan of Circulation in Amphibia and
Reptiles; B, Plan of Circulation in Birds and
Mammals: a, right auricle receiving venous blood
from the system; b, left auricle receiving arterial
blood from the lungs; c, c', ventricles; d, e,ft
systemic artery, vein, and capillaries; g, pulmon-
ary artery; h, k, vein and capillaries.

THE CIRCULATION OF THE BLOOD 317

sole means : it is aided by the contractions of the elastic
walls of the arteries themselves, the movements of the
chest in respiration, and the attraction of the tissues for
the arterial blood in the capillaries. In the chick, the
blood mores before the heart begins to beat ; and if the
heart of 'an animal be suddenly taken out, the motion in
the capillaries will continue as before. It has been
estimated that the force which the human heart expends
in twenty-four hours is about equivalent to lifting 217
tons one foot.

CHAPTER XVII*

HOW ANIMALS BREATHE

Arterial Blood, in passing through the system, both
loses and gains certain substances. It loses constructive
material and oxygen to the tissues. These losses are
made good from the digestive tract and breathing organ.
It gains also certain waste materials from the tissues,
which must be got rid of. Of these waste products, one,
carbon dioxide, is gaseous, and is passed off from the
same organ as that where the oxygen is taken in. This
exchange of gases between the animal and its surround-
ings is called respiration.

The First Object of Respiration is to convert venous
into arterial blood. It is done by bringing it to the sur-
face, so that carbon dioxide may be exhaled and oxygen
absorbed. The apparatus for this purpose is analogous
to the one used for circulation. In the lowest animals,
the two are combined. But in the highest, each is
essentially a pump, distributing a fluid (in one case air,
in the other blood) through a series of tubes to a system
of cells or capillaries. They are also closely related to
each other : the more perfect the circulation, the more
careful the provision made for respiration,

Respiration is performed either in air or in water. So
that all animals may be classed as air breathers or water
breathers. The latter are, of course, aquatic, and seek
the air which is, dissolved in the water. Land snails,
myriapods, spiders, insects, reptiles, birds, and mam-

* See Appendix.

HOW ANIMALS BREATHE

319

mals breathe air directly ; the rest, with few exceptions,
receive it through the medium of water. In the former
case, the organ is internal ; in the latter, it is more or less
on the outside. But however varied the organs — tubes,
gills, or lungs — they are all constructed
on the same principle — a thin membrane
separating the blood from the atmosphere.

(i) Protozoa, Sponges, and Polyps have no
separate respiratory apparatus, but absorb
air, as well as food, from the currents of
water passing through them or bathing
the surface of their bodies.

In the starfish, sea urchin, and the
like, we find the first distinct respiratory
organs, although none are exclusively
devoted to respiration. There are two
sets of canals — one carrying the nutrient
fluid, and the other, radiating from a ring
around the mouth, distributing aerated
water, used for locomotion as well as
respiration. This may be called the
"water-pipe system." Besides this, there
are sometimes numerous gill-like fringes,
which cover the surface of the body and FIG. 274.— Lobworm

v 1.1 ' i • '4.'

probably aid in respiration.

Freshwater worms, like the leech and
earthworm, breathe by the skin. The
body is -always covered by a viscid fluid,
which has the property of absorbing air.
The air is therefore brought into immediate contact
with the soft skin, underneath which lies a dense net-
work of blood vessels.

But most water-breathing animals have gills. The
simplest form is seen in marine worms : delicate veins
projecting through the skin make a series of arborescent

(A renicola piscato-
rum)t a dorsibran-
chiate, showing the
tufts of capillaries,
or external gills.
The large head is
without eyes or
jaws.

320

COMPARATIVE ZOOLOGY

tufts along the side of the body ; as these float in the
water, the blood is purified.118 Bivalve mollusks have
four flat gills, consisting of delicate membranes filled
with blood vessels and covered with cilia. In the
oyster, these ribbonlike folds are exposed to the water
when the shell opens ; but in the clam, the mantle
incloses them, forming a tube, called siphon, through
which the water is driven by the cilia. The aquatic

gastropods (univalves) have
either tufts, like the worms,
or comblike ciliated gills in
a cavity behind the head, to
which the water is admitted
through an opening. In
others the breathing organ
is the vascular lining of this
cavity. The cuttlefish has
flat gills covered by the man-
tle; but the water is drawn
in by muscular contractions
of the mantle instead of by
cilia. The end of the siphon
through which it is ejected is
called the funnel. The gills
of lobsters and crabs are

placed in cavities covered by the sides of the shell
(carapace) ; and the water is brought in from behind by
the action of a scoop-shaped process attached to one of
the jaws, which constantly bails the water out at the
front.

The perfection of apparatus for aquatic respiration is
seen in fishes. The gills are comblike fringes supported
on four or five bony or cartilaginous arches, and contain
myriads of microscopic capillaries, the object being to
expose the venous blood in a state of minute subdivision

FIG 275. — Diagrammatic Section of a
Lamellibranch (Anodonta): a, lobes
of mantle; b, gills, showing transverse
partitions; c, ventricle of heart; d,
auricles; e, pericardium; f, g, kid-
neys; h, venous sinus; k, foot; A,
branchial, or pallial, chamber; B,
epibranchial chamber.

HOW ANIMALS BREATHE

321

FIG. 276! — Spiracle of an Insect, x 75.

to streams of water. The gills are always covered. In

bony fishes they are attached to the hinder side of bony

arches, all covered by

a flap of the skin,

supported by bones

(the gill cover, or

operculum\ and the

water escapes from

the opening left at

its hinder edge. In

sharks, the gills are

placed in pouches

which open separately

(Figs. 122,360). The

act of " breathing

water" resembles

swallowing, only the water passes over the surface of

the gills instead of entering the gullet.

(2) Air Breathers have trachea, or lungs. The former

consist of two principal tubes, which pass from one end

of the body to the other, opening on the surface by
apertures, called spiracles, resembling a
row of buttonholes along the sides of
the thorax and abdomen, and ramifying
through the smallest and most delicate
organs, so that the air rriay follow the
blood wherever it circulates.. To keep
the pipes ever open, and at the same
. time leave them flexible, they are pro-

FG. 277. — Tracheal

inside with an elastic spiral thread,
a droplight.

Respiration is performed by the move-
ments of the abdomen, as may be seen in the bee when
at rest. This " air-pipe system," as it may be termed,
is best developed in insects.

Tube of an Insect,
highly magnified,

showing elastic like the rubber tube of

spiral thread.

DODGE'S GEN. ZOOL. — 21

322

COMPARATIVE ZOOLOGY

The "nerves" of an insect's wing consist of a tube
within a tube, the inner one is a trachea carrying air,
and the outer one, sheathing it, is a blood vessel. So

FIG. 278. —Ideal Section of a Bee: a, alimentary canal; //, dorsal vessel; t, trachea:
n, nervous cord.

perfect is the aeration of the whole body, from brain to
feet, the blood is oxygenated at the moment when, and
on the spot where, it is carbonized ; only one kind of
fluid is, therefore, circulating — arterial. It is difficult
to drown an insect, as the water
can not enter the pores ; but if a
drop of oil be applied to the abdo-
men, it falls dead at once, being
suffocated. The largest spiracle is
usually found on the thorax, as
under the wing of a moth ; such
may be strangled by pinching the
thorax.

In millepedes and centipedes, the
spiracles open into little sacs con-
nected together by tubes; in spiders and scorpions, the

FIG. 279. — Section of in-
jected Lung (highly mag-
nified): a, a, free edges
of alveoli; c, c, partitions
between neighboring al-
veoli; b, small arterial
branch giving off capil-
laries to the alveoli.

HOW ANIMALS BREATHE

323

spiracles, usually four in number, are the mouths of
sacs without the tubes, and the interior of the sac is
gathered into folds. Land snails have one spiracle,
or aperture, on the right side of the neck, leading to
a large cavity, or sac, lined with fine blood vessels.
These sacs represent the primitive idea of a lung, which
is but an infolding of the skin, divided up into cells, and
covered with capillary veins.119

FIG. 280. — Part of a Transverse Section of a Pig's Bronchial Twig, x 240: a, outer
fibrous layer; bt muscular layer; c, inner fibrous layer; d, epithelial layer with cilia;
f, one of the neighboring alveoli.

Like the alimentary canal, the lungs of an animal are
really an inflected portion of the outer surface ; so that
breathing by the skin and breathing by lungs are one in
principle. Indeed, in many animals, especially frogs,
respiration is carried on by both lungs and skin.

In the course of embryonic development the lungs
of vertebrates are derived from the front part of
the alimentary canal. In some fishes, air is swal-
lowed, which passes the whole length of the diges-
tive tract, and is expelled from the anus. Here the
whole canal serves for respiration. In reptiles, birds,

324

COMPARATIVE ZOOLOGY

and mammals the hinder part of the intestine develops
an outgrowth (the allantois} during embryo life which
serves as the embryo's breathing organ
(Figs. 365, 366).

All vertebrates have two kinds of re-
spiratory organs in the course of their
life. Fishes have gills ; their lung (the air
bladder) rarely serves as a functional re-
spiratory organ, and is sometimes wanting.
Amphibians have gills while in the larval
state. Some keep them throughout life ;
but all develop functional lungs, and also
breathe by means of the skin.

In the remaining vertebrates, the allan-
tois is the breathing organ of the embryo,
and the lung is the breathing organ of
the adult. The skin is of small or no
importance in respiration.

The lungs of vertebrates are elastic,
membranous sacs, divided more or less
into cavities (the air cells] to increase the
surface. Upon the walls of the air cells
are spread the capillary blood vessels.
The smaller the cells, the greater the
extent of surface upon which the blood
lS a8s'nake"n^ *s exposed to the influence of the air, and,
trachea; t, its therefore, the more active the respiration

bifurcation; c,

pulmonary ar- and the purer the blood. The lungs are
naj veinTThe relatively largest in reptiles, and smallest

lung, B, is rudi-
mentary.

in mammals. But in the cold-blooded
amphibians and reptiles, the air cells are
few and large ; in the warm-blooded birds and mammals,
they are exceedingly numerous and minute.120 Respira-
tion is most perfect in birds ; they require, relatively
to their weights, more air than mammals or reptiles, and

HOW ANIMALS BREATHE

325

most quickly die for lack of it. In birds, respiration is
not confined to the lungs ; but, as in insects, extends
through a great part of the body. Air sacs connected
with the lungs exist in the abdomen and under the
skin of the neck, wings, and legs. Even the bones
are hollow for this purpose ; so that if the windpipe be

FIG. 282. — Lungs of a Frog; a,
hyoid apparatus; b, cartilaginous
ring at root of the lungs; c, pul-
monary vessels; d, pulmonary
sacs, having this peculiarity com-
mon to all cold-blooded air breath-
ers, that the trachea does not
divide into bronchial branches, but
terminates abruptly by orifices
which open at once into the gen-
eral cavity. A cartilaginous net-
work divides the space into little
sacs, on the walls of which the
capillaries are spread.

FIG. 283. — Distribution of Air Tubes in Mam-
malian Lungs: a, larynx; bt trachea; c, d, left
and right bronchial tubes: e,f. g, the ramifica-
tions. In Man the subdivision continues until
the ultimate tubes are one twenty-fifth of an
inch in diameter. Each lobule represents in
miniature the structure of the entire lung of a
Frog.

tied, and an opening be made in the wing bone, the bird
will continue to respire. The right lung is usually the
larger ; in some snakes, the left is wanting entirely.
In most vertebrates, lungs are freely suspended ; in
birds, they are fastened to the back.

The lungs communicate with the atmosphere by
means of the trachea, or windpipe, formed of a series of
cartilaginous rings, which keep it constantly open. It

326

COMPARATIVE ZOOLOGY

begins in the back part of the mouth, opening into the
pharynx by a slit, called the glottis, which, in mammals,
is protected by the valvelike epiglottis. The trachea
passes along the neck in front of the esophagus, and
divides into two branches, or bronchi, one for each lu-ng.
In birds and mammals, the bronchial tubes, after enter-
ing the lungs, subdivide again into minute ramifications.
Vertebrates are the only animals that breathe through
the mouth or nostrils. Frogs, having no ribs, and tur-
tles, whose ribs are soldered together into a shield,

are compelled to
swallow the air.
Snakes, lizards,
and crocodiles
draw it into the
lungs by the play
of the ribs.121
Birds, unlike other
animals, do not
inhale the air by
an active effort ;
for that is done by
the springing back
of the breastbone and ribs to their natural position.
To expel the air, the breastbone is drawn down toward
the backbone by muscles, which movement compresses
the lungs.

Mammals alone have a perfect thorax — i.e., a closed
cavity for the heart and lungs, with movable walls
(breastbone and ribs) and the diaphragm, or muscular
partition, separating it from the abdomen.122 Inspira-
tion (or filling the lungs) and expiration (or emptying
the lungs) are both accomplished by muscular exertion ;
the former, by raising the ribs and lowering the di-
aphragm, thus enlarging the capacity of the chest, in

FIG. 284. —Skeleton of a Frog.

HOW ANIMALS BREATHE

327

consequence of which the air rushes in to prevent a
vacuum ; the latter, by the ascent of the diaphragm and
the descent of the ribs.

As a rule, the more active and more muscular an
animal, the greater the demand for oxygen. Thus,
warm-blooded animals live fast, and their rapidly decay-
ing tissues call for rapid
respiration ; while in the
cold-blooded creatures the
waste is comparatively
slow. Respiration is most
active in birds, and least in
water-breathing animals.
The sluggish toad respires
more slowly than the busy
bee, the mollusk more
slowly than the fish.
But respirations, like
beats of the heart, are
fewer in large mammals
than in small ones. An
average man inhales about
300-400 cubic feet of air
per day of rest, and much

*
more When at WOrk.

. , i r

Another reSUlt OI reS-

piration, besides the puri-
fication of the blood, is

the production of heat. The chemical combination of
the oxygen in the air with the carbon in the tissues
is a true combustion ; and, therefore, the more active
the animal and its breathing, the higher its tempera-
ture. Birds and mammals have a constant temperature,
which is usually higher than that of the atmosphere
(108° and 100° F. respectively). They are therefore

/

FIG. 285. — Human Thorax: a, vertebral col-
umn; b, b' , ribs, the lower ones false; c,
clavicle; e, intercostal muscles, removed on
the left side to show the diaphragm, d ; f,
pillars of the diaphragm attached to the lum-

328 COMPARATIVE ZOOLOGY

called constant temperatured or warm blooded. Other
animals do not vary greatly in temperature from that
of their surroundings, and are called changeable tem-
peratured or cold blooded. Stilt, their temperature does
not agree exactly with that of the air or water. The
bee is from 3° to 10°, and the earthworm and snail
from i y to 2°, higher than the air. The mean temper-
ature of the carp and toad is 51° ; of man, 98.5° ; dog,
99°; cat, 101°; squirrel, 105°; swallow, 111°, all ac-
cording to the Fahrenheit scale.

CHAPTER XVIII*
SECRETION AND EXCRETION

IN the circulation of the blood, not only are the
nutrient materials taken around through the body to be
used in the construction of various tissues, but certain
special fluids are taken up and conveyed to the external
or internal surfaces in the body, where, in glandular
structures, further elaboration takes place. The result-
ing products are of two kinds : some, like saliva, gastric
juice, bile, milk, etc., are for useful purposes ; others,
like sweat and urine, are expelled from the system as
useless or injurious. The separation of the former is
called secretion ; the removal of the latter is excretion.
Both processes are substantially alike.

In the lower forms, there are no special organs, but
secretion and excretion take place from the general
surface. The simplest form of a secreting organ closely
resembles that of a respiratory organ, a thin membrane
separating the blood from the cavity into which the
secretion is to be poured. Usually, however, the cells
of the membrane manufacture the secretion from ma-
terials furnished by the blood. Even in the higher
animals, there are such secreting membranes. The
membranes lining the nose and alimentary canal and
inclosing the lungs, heart, and joints, secrete lubricating
fluids.

The infolding of such a membrane into little sacs or
short tubes (follicles), each having its own outlet, is the

* See Appendix.
329

330

COMPARATIVE ZOOLOGY

type of all secreting and excreting organs. The lower
animals have nothing more complex, and the apparatus
for preparing the gastric fluid attains no further devel-
opment even in man. When a cluster of these follicles,
or sacs, discharge their contents by one common duct,

we have a gland. But
whether membrane, folli-
cle, or gland, the organ
is covered with a network
of blood vessels, and lined
with epithelial cells, which
are the real agents in the
process.

The Chief Secreting Or-
gans are the salivary
glands, gastric follicles,
pancreas, and liver, all
situated along the diges-
tive tract.

i. The salivary glands,

FIG. 286. — Three plans of secreting Mem- which Open into the
. branes. The heavy line represents the

areolar-vascular layer; the next line is the mOUth, SCCrCte Saliva.
basement, or limiting membrane; and the Thpv pvici- in n<=»arl T all
dotted line the epithelial layer: a, shows ney eXlbt ] nearly ail
increase of surface by simple plaited or vertebrates, hig her mol-
fringed projections; b, five modes of in-
crease by recesses, forming simple glands, lusks, and inSCCtS, and are
or follicles: c, two forms of compound . , , . , .

glands. most largely developed in

such as live on vegetable

food. The saliva serves to lubricate or dissolve the
food for swallowing, and, in some mammals, aids also
in digestion of starch.123

2. The gastric follicles are minute tubes in the walls
of the stomach secreting gastric juice. They are found
in all vertebrates, and in the higher mollusks and arthro-
pods. In the lower forms, a simple membrane lined
with cells serves the same purpose. Under the micro-

SECRETION AND EXCRETION

331

scope, the soft mucous membrane of the human stomach
presents a honeycomb appearance, caused by numerous
depressions or cells. At the bottom
of these depressions are clusters of
spots, which are the orifices of the
tubular follicles. The follicles are
about 2^0 °f an mcn m diameter,
and number millions.

3. The pancreas, or "sweetbread,"
so important in the process of di-
gestion, exists in all but the lowest
animals. In its structure it closely
resembles the salivary glands. In the
cuttlefish, it is represented by a sac ;
in fishes, by a group of follicles. It is
proportionally largest in birds whose
salivary glands are deficient. The
pancreatic juice enters the duodenum.

4. A so-called " liver " is found in
all animals having a distinct diges-
tive cavity. In the lower animals its function has
been shown to be that of a pancreas. Thus, in polyps

it is represented by yellowish cells lining the
stomach ; in insects, by cells in the wall of
the stomach ; in mollusks, by a cluster of sacs,
or follicles, forming a loose compound gland.
In vertebrates, a true liver, the larg-
est gland in the body, is well defined,
and composed of a mul-
titude of lobules (which
give it a granular ap-
pearance) arranged on
the capillary veins, like

FIG. 288. -Pancreas of Man; o, pancreas; gt grapCS On a Stem, and
gall bladder; s. cystic duct; c, duct from the . i J

liver; A pyloric valve; e,f, duodenum. COn tainin g H UC leat ed

FIG. 287. — Follicles from the
Stomach of a Dog, X 150;
near the mouth, a, there
is a lining of columnar
epithelium.

332

COMPARATIVE ZOOLOGY

secreting cells. It is of variable shape, but usually two,
three, or five lobed, and is centrally situated — in mam-
mals, just below the diaphragm. In most vertebrates,
there is an appendage to the liver called the gall bladder,
which is simply a reservoir for the bile.

VP4

FIG. 289. — Liver of the Dog: F, F, liver; D, duodenum and intestines; P, pancreas; r>
spleen; e, stomach, f, rectum; R, right kidney; B, gall bladder; ch, cystic duct;
F', lobe of liver dissected to show distribution of portal vein, VP, and hepatic vein,
vh ; */, diaphragm; VC, venacava; C, heart.

The so-called liver of invertebrates is more like the
pancreas of vertebrates in function, as its secretion
digests starches and albuminoids. The liver of verte-
brates is both a secretory and an excretory organ. The
bile performs an important, although ill-understood, func-
tion in digestion, and also contains some waste products.

SECRETION AND EXCRETION 333

The gland also serves to form sugar (glycogen) from
part of the digested food, and may well be called a
chemical workshop for the body. In animals of slow
respiration, as crustaceans, mollusks, fishes, and reptiles,
fat accumulates in the liver. "Cod-liver oil" is an
example.

The Great Excreting Organs are the lungs, the kidneys,
and the skin ; and the substances which they remove from
the system — carbonic acid, water, and urea — are the
products of decomposition, or organic matter on its way
back to the mineral kingdom.124 Different as these
organs appear, they are constructed upon the same
principle : each consisting of a very thin sheet of tissue
separating the blood to be purified from the atmosphere,
and straining out, as it were, the noxious matters. All,
moreover, excrete the same substances, but in very dif-
ferent proportions : the lungs exhale carbon dioxide and
water, with a trace of urea ; the kidneys expel water,
urea', and a little carbon dioxide ; while the skin par-
takes of the nature of both, for it is not only respiratory,
especially among the lower animals, but it performs part
of the work of the kidneys in case they are diseased.

1. The lungs (and likewise gills) are mainly excretory
organs. The oxygen they impart sweeps with the blood
through every part of the body, and unites with the tis-
sues and with some elements of the blood. Thus are
produced heat and work, whether muscular, nervous,
secretory, etc. As a result of this oxidation, carbon
dioxide, water, and urea, or a similar substance, are
poured into the blood. The carbon dioxide and part of
the water are passed off from the respiratory organs.
This process is more immediately necessary to life than
any other ; the arrest of respiration is fatal.

2. While the lungs (and skin also, to a slight degree)
are sources of gain as well as loss to the blood, the kid-

334

COMPARATIVE ZOOLOGY

neys are purely excretory organs. Their main function
is to eliminate the solid products of decay which can not
pass out by the lungs. In mam-
mals, they are discharged in solu-
tion ; but from other animals
which drink little the excretion
is more or less solid. In insects,
the kidneys are groups of tubes
(Figs. 239, 240); in the higher
mollusks, they are represented
by spongy masses of follicles
(Fig. 244); in vertebrates, they
are well-developed glands, two
in -number, and consist of closely
packed tubes.

3. The skin of the soft-skinned
animals, particularly of amphibi-
ans and mammals, is covered with
minute pores, which are the ends of as many delicate
tubes that lie coiled up into a knot within the true skin.
These are the sweat glands, which excrete water, and
with it certain salts and gases.

Besides these secretions and excretions, there are
others, confined to particular animals, and designed for
special purposes : such are the oily matters secreted
from the skin of quadrupeds for lubricating the hair and
keeping the skin flexible; the tears of reptiles, birds,
and mammals; the milk of mammals'; the ink of the
cuttlefish ; the poison of jelly fishes, insects, and snakes ;
and the silk of spiders and caterpillars.

FIG. 290. — Section of Human
Kidney, showing the tubular
portion, 3, grouped into cones;
7, the ureter, or outlet for the
secretion.

CHAPTER XIX*

THE SKIN AND SKELETON

The Skin, or Integument, is that layer of tissue which
covers the outer surface of the body. The term Skele-
ton is applied to the hard parts of the body, whether
external or internal, which serve as a framework or
protection to the softer organs, and afford points of
attachment to muscles. If external, as the crust of the
lobster, it is called exoskeleton ; if internal, as the bones
of man, it is called endoskeleton. The former is a
modification of the skin ; the latter, a hardening of the
deeper tissues.

i. The Skin. — In the lowest forms of life, as amoeba,
there is no skin. The protoplasm of which they are
composed is firmer outside than inside, but no mem-
brane is present. In Infusoria, there is a very thin
"cuticle" covering the animal (Fig. 9). They have
thus a definite form, while the amcebas continually
change. Sponges and hydras also have no true skin.
But in polyps, the outside layer of the animal is sepa-
rated into two portions — ectoderm and endoderm 125 —
which may be regarded as partly equivalent to epider-
mis and dermis in the higher animals. These two layers
are, then, generally present. The outer is cellular, the
latter fibrous, and may contain muscular fibers, blood
vessels, nerves, touch organs, and glands. It thus be-
comes very complicated in some animals.

* See Appendix.
335

336 COMPARATIVE ZOOLOGY

In worms and arthropods, the cellular layer, here
called hypodermis, excretes a structureless cuticle, which
may become very thick, as in the tail of the horseshoe
crab, or may be hardened by deposition of lime salts,
as in many Crustacea. The loose skin, called the
mantle, which envelops the -body of the mollusk, corre-
sponds to the true skin of higher animals. The border
of the mantle is surrounded with a delicate fringe, and,
moreover, contains minute glands, which secrete the
shell and the coloring matter by which it is adorned.
The tunicates have a leathery epidermis, remarkable
for containing vegetable cellulose instead of lime.

In mammals, whose skin is most fully developed, the
dermis is a sheet of tough elastic tissue, consisting of
interlacing fibers, and containing blood, vessels, lym-
phatics, sweat glands, and nerves. It is the part con-
verted into leather when hides are tanned, and attains
the extreme thickness of three inches in the rhinoceros.
The upper surface in parts of the body is covered with
a vast number of minute projections, called papilla,
each containing the termination of a, nerve; these are
the essential agents in the sense of touch126 (Fig. 345).
They are best seen on the tongue of an ox or cat, arid
on the human fingers, where they are arranged in rows.

Covering this sensitive layer, and accurately molded
to all its furrows and ridges, lies the bloodless and
nerveless epidermis. It is that part of the skin which
is raised in a blister. It is thickest where there is most
pressure or hard usage ; on the back of the camel it
attains unusual thickness. The lower portion of the
epidermis (called rete mucosnm) is comparatively soft,
and consists of nucleated cells containing pigment gran-
ules, on which the color of the animal depends. Toward
the surface the cells become flattened, and finally, on
the outside, are changed to horny scales (Fig. 199, c).

THE SKIN AND SKELETON

337

These scales in the higher animals are constantly
wearing off in the form of scurf, and as constantly being
renewed from below. In lizards and serpents, the old
epidermis is cast entire, being stripped off from the
head to the tail ; in the toad it comes off in two pieces ;
in the frog, in shreds ; in fishes and some mollusks, in
the form of slime. However modified the epidermis,
or whatever its appendages, the like process of removal

FIG. 291. — Section of Skin from Horse's Nostril (magnified) : E, epidermis; D, dermis;
i, horny layer of epidermis; 2, rete mucosum; 3, papillary layer of dermis; 4, ex-
cretory duct of a sudoriparous, or sweat, gland; 5, glomerule, or convoluted tube of
the same; 6, hair follicle; 7, sebaceous gland; 8, internal sheath of the hair follicle;
9, bulb of the hair; 10, mass of adipose tissue.

goes on. Mammals shed their hair ; birds, their feathers;
and crabs, their shells. When the loss is periodical, it
is termed molting.

2. The Skeletons. — (i) The Exoskeleton is developed by
the hardening of the skin, and, with very few exceptions,
is the only kind of skeleton possessed by invertebrate
animals. The usual forms are coral, shells, crusts,
scales, plates, hairs, and feathers. It is horny or cal-
careous ; while the endoskeleton is generally a deposit
DODGE'S GEN. ZOOL. — 22

338

COMPARATIVE ZOOLOGY

of earthy material within the body, and is nearly con-
fined to the vertebrates. The exoskeleton may be of
two kinds — dermal and epidermal.

Some of the Protozoa, as Radiolaria and Foraminif-
era, possess siliceous and calcareous shells of the most
beautiful patterns (Fig. 2). The toilet sponge has a
skeleton of horny fibers, which is the sponge of com-
merce. Coral is the solid framework of certain polyps.
There are two kinds : one represented by the common

FIG. 292. — i, Vertical Section, and, 2, Transverse Section, of a sclerodermic Corallite:
a, mouth; b, tentacles; c, stomach; d, intermesenteric chamber; e, mesentery;
_/", septum; g, endoderm; k, epitheca; k, theca, or outer wall; in, columella; «,
short partitions; /, tabula, or transverse partition; r, sclerobase; s, ccenenchyma,
or common substance connecting neighboring corallites; /, ectoderm; x, pali, or im-
perfect partitions.

white coral, which is a calcareous secretion within the
body of the polyp, in the form of a cylinder, with par-
titions radiating toward a center (scleroderm)\ the
other, represented by the solid red coral of jewelry, is
a central axis deposited by a group of polyps on the
outside (sclerobase}.

The skeleton of the starfish is a leathery skin in
which are embedded calcareous particles and plates.
The sea urchin is covered with an inflexible shell of
elaborate and beautiful construction. The shell is really
a calcified skin, being a network of fibrous tissue and

THE SKIN AND SKELETON

339

FIG. 293. — Shell of Sea Urchin {Cidaris') without its spines.

earthy plates. It varies in shape from a sphere to

a disk, and consists of hundreds of angular pieces

accurately fitted

together, like

mosaic work.

These form ten

zones, like the

ribs of a melon,

five broad ones

alternating with

five narrower

ones. The

former (called

inter- ambulacra)

i . ,

are covered with
tubercles bearing movable spines. The narrow zones
(called ambulacra, as they are likened to walks through

a forest) are
pierced with
small holes,
through
which pro-
ject fleshy
sucker feet.

The skin
of the lobster
is hardened
by calcareous
deposit into

FIG. 294. — Structure of Sea Urchins' Spines (magnified) : i, a, a " CRISt," Or
spine of Cidaris cut longitudinally; /, s, ball-and-socket joint; , .. *~. ,
/, pedicellariae ; 2, 3, transverse sections of spines of Cidaris Snell J ' DUt,

instead of

forming one piece, it is divided into a series of seg-
ments, which move on each other. The number of these
segments, or rings, is usually twenty — five in the head,

340

COMPARATIVE ZOOLOGY

eight in the thorax, and seven in the abdomen. In the
adult, however, the rings of the head and thorax are
often soldered together into one shield, called cephalo-
thorax ; and in the horseshoe crab the abdominal rings
are also united. The shell of crustaceans is periodically

cast off, for the
animals continue
to grow even after
they have reached
their mature form.
This molting is a
very remarkable
operation. How
the lobster can
draw its legs
from their cases
without unjoint-
ing or splitting
them was long a
puzzle. The flesh
becomes soft, and
is drawn through

FIG. 295. — Diagram of an Insect: A, head bearing the

eyes and antennae; B, prothorax, carrying the first pair the joints, th.6
of legs, G; C, mesothorax, carrying the second pair of , .

legs, H, and first pair of wings, K; D, metathorax, carry- WOUndS tilUS
ing the third pair of legs, I, and second pair of wings, L; caused QUicklv
E, abdomen, with ovipositor, F; i, coxa, or hip; 2, tro- J

chanter; 3, femur, or thigh; 4, tibia, or shank; 5, tarsus, healing. The

or foot; 6, claw.

cast-off skeleton

is a perfect copy of the animal, retaining in their places
the delicate coverings of the eyes and antennae, and even
the lining membrane of the stomach with its teeth.

The horny crust of insects differs from that of crus-
taceans in consisting mainly of a horny substance called
chitin and in containing no lime. The head, thorax, and
abdomen are distinct, and usually consist of fourteen
visible segments — one for the head, three for the

THE SKIN AND SKELETON 341

thorax (called prothorax, mesothorax, metathorax), and
ten for the abdomen. The antennae, or feelers, legs,
and wings, as well as hairs, spines, and scales, are
appendages of the skeleton. As insects grow only
during the larval, or caterpillar, state, molting is con-
fined to that period. These skeletons are epidermal,
deposited in successive layers, from the inside, and are,
therefore, capable of but slight enlargement when once
formed.

The shells of mollusks are well-known examples of
exoskeletons. The mantle, or loose skin, of these ani-
mals secretes calcareous earth in successive layers, con-
verting the epidermis into a " shell." 128 So various and
characteristic is the microscopic character of shells,
that a fragment is sometimes sufficient to determine the
group to which it belongs. Many shells resemble that
of the fresh-water mussel ( Unio), which is composed of
three parts : the external brown epidermis, of horny
texture ; then the prismatic portion, consisting of minute
columns set perpendicularly to the surface; and the
internal nacreous layer, or " mother-of-pearl," made up
of exceedingly thin plates. The pearly luster of the
last is due to light falling upon the outcropping edges
of wavy laminae.129 In many cases,' the prismatic and
nacreous layers are traversed by minute tubes. Another
typical shell structure is seen in the common cone, a
section of which shows three layers, besides the epi-
dermis, consisting of minute plates set at different
angles. The nautilus shell is composed of two distinct
layers : the outer one having the fracture of broken
china ; the inner one, nacreous.

Most living shells are made of one piece, as the snail;
these are called " univalves." Others, as the clam, con-
sist of two parts, and are called " bivalves." In either
case, a valve may be regarded as a hollow cone, grow-

342

COMPARATIVE ZOOLOGY

ing in a spiral form. The ribs, ridges, or spines on the
outside of a shell mark the successive periods of growth,
and, therefore, correspond to the age of the animal.
Figures 296 and 297 show the principal parts of the
ordinary bivalves and univalves.
The valves of a bivalve are gen-
erally equal, and the umbones, or
beaks, a little in front of the cen- x
ter. The valves are bound to-

FlG. 296. — Left Valve of a Bivalve Mollusk (Cy- FIG. 297. — Section of a Spiral

therea chione}: h, hinge ligament; «, umbo; Univalve ( Triton corrugatus} :

/, lunule; c, cardinal, and /,' t' ', lateral teeth; a, apex; b, spire; c, suture;

a, a', impressions of the anterior and posterior d, posterior canal; e, outer lip
adductor muscles; /, pallial impression;, s, sinus,
occupied by the retractor of the siphons.

of the aperture ; f, anterior
canal.

gether by a ligature near the umbones, and often, also,
by means of a " hinge " formed by the " teeth " of one
valve interlocking into cavities in the other. The aper-
ture of a univalve is frequently closed by a horny or
calcareous plate, called "operculum," which the animal
carries on the back of the hinder portion of its foot,
and which is a part of the exoskeleton. The shells of
mollusks are epidermal, and are, therefore, dead and
incapable of true repair. When broken, they can be
mended only by the animal pouring out lime to cement

THE SKIN AND SKELETON

343

the parts together. They can not grow together, like a
broken bone.

Embedded in the back of the cuttlefish is a very light
spongy "bone," which, as already observed, is a secre-
tion from the skin, and therefere belongs to the exo-
skeleton. It has no resemblance to true bone, but is
formed, like shells, of a number of calcareous plates.
Nevertheless, the cuttlefish does exhibit traces of an
endoskeleton : these are plates of cartilage, one of which
surrounds the brain, and hence may be called a skull.
To this cartilage, not to the "cuttlebone," the muscles
are attached.

In vertebrates, the exoskeleton is subordinate to the
endoskeleton, and is feebly developed in comparison.
It is represented by
a great variety of ap-
pendages to the skin,
which are mainly or-
gans for protection,
not for support.
Some are horny
outgrowths of the epi-
dermis, such as hairs,
feathers, nails, claws,
hoofs, horns, and the
scales of reptiles ;

others arise from the hardening of the dermis by cal-
careous matter, as the scales of fishes, the bony plates
of crocodiles and turtles, and the shield of the armadillo.

The scales of fishes (and likewise the spines of their
vertical fins) lie embedded in the overlapping folds of
the skin, and are covered with a thin, slimy epidermis.
The scales of the bony fishes (perch, salmon, etc.) con-
sist of two layers, slightly calcareous, and marked by
concentric and radiating lines. Those of the shark

FIG. 298. — Skeletal Architecture in the Armadillo,
showing the relation of the carapax to the verte-
bral column.

344

COMPARATIVE ZOOLOGY

have the structure of teeth, while the scutes, or plates, of
the crocodiles, turtles, and armadillos are of true bone.

The scales of snakes and lizards are horny epidermal
plates covering the overlapping folds of the true skin.

FIG. 299. — Diagrammatic Section of the Skin of a Fish (Carp) : a, derm, showing lam-
inated structure with vertical fibers, b; c, gristly layer; e, laminated layer, with
calcareous granules; d, superficial portion developing into scales ;./, scale pit.

In some turtles these plates are of great size, and are
called " tortoise shell " ; they cover the scutes. The
scales on the legs of birds, and on the tail of the beaver
and rat, have the same structure. Nails are flattened

horny plates de-
veloped from the
upper surface of
the fingers and
toes. Claws are
sharp conical
nails, being de-
veloped from the
sides as well as
upper surface;
and hoofs are
blunt cylindrical
claws. Hollow

FIG. 300. —Vertical Section of the Forefoot of the Horse homS, aS of the

(middle digit): i, 2, 4, proximal, middle, and distal, , ,., _.

or ungual, phalanges; 3, sesamoid, or nut bone; 5, 6, 7, OX, may DC llKCned

tendons ; 9 elastic tissue ; 8, 10, internal and external ^Q daws sheathing
floor ol the hoof; n, 12, internal and external walls.

a bony core. The

horn of the rhinoceros is a solid mass of epidermal
fibers. " Whalebone," the rattle of the rattlesnake, and
the beaks of turtles and birds, are likewise epidermal.

THE SKIN AND SKELETON

345

Hairs, the characteristic clothing of mammals, are
elongated horny cones, composed of "pith" and "crust."
The latter is an outer layer of
minute overlapping scales, which
are directed toward the point, so
that rubbing a human hair or fiber
of wool between the thumb and
finger pushes the root end away.
The root is bulbous, and is con-
tained in a minute depression, or
sac, formed by an infolding of the

FIG. 301. — Section of the Root and part
of the Shaft of a Human Hair, highly
magnified : it is covered with epidermic
scales, b, the inner layer, c, forming the
outer covering of the shaft, e, being im-
bricated; the root consists of angular
cells loaded with pigment ; d, bulb.

FIG. 302. — Parts of a Feather:
a, quill, or barrel; b, shaft; c,
vane, or beard; d, accessory
plume, or down ; e, f, lower
and upper umbilicus, or orifice,
leading to the interior of the
quill.

skin. Hairs are usually set obliquely into the skin.
Porcupine's quills and hedgehog's spines make an easy
transition to feathers, which differ from hairs only in
splitting up into numerous laminae. They are the most

346 COMPARATIVE ZOOLOGY

complicated of all the modifications of the epidermis.
They consist of a " quill " (answering to the bulb of a
hair), and a "shaft," supporting the "vane," which is
made up of "barbs," "barbules," and interlocking
"processes." The quill alone is hollow, and has an ori-
fice at each end. The feather is molded on a papilla,
the shaft lying in a groove on one side of it, and the
vane wrapped around it. When the feather emerges
from the skin, it unfolds itself. Thus shaft and vanes
together resemble the quill split down one side and
spread out.

The teeth of mollusks, worms, and arthropods are
also epidermal structures. Those of vertebrates are
mixed in their origin, the dentine being derived from
the dermis and the enamel from the epidermis. In all
cases teeth belong to the exoskeleton.

(2) The Endoskeleton, as we have seen, is represented in
the cuttlefish. With this and some other exceptions, it
is peculiar to vertebrates. In the cuttlefish, and some
fishes, as the sturgeon and shark, it consists of cartilage ;
but in all others (when adult) it is bone or osseous
tissue. Yet there is a diversity in the composition of
bony skeletons ; that of fresh-water fishes contains the
least earthy matter, and that of birds the most. Hence
the density and ivory-whiteness of the bones of the
latter. Unlike the shells of mollusks and the crust of
the lobster, which grow by the addition of layers to
their borders, bones are moist, living parts, penetrated
by blood vessels and nerves, and covered with a tough
membrane, called periosteum, for the attachrhent of
muscles.

The surface of bones is compact; but the interior
may be solid or spongy (as the bones of fishes, turtles,
sloths, and whales), or hollow (as the long bones of
birds and the active quadrupeds). There are also cavi-

THE SKIN AND SKELETON 347

ties (called sinuses) between the inner and outer walls
of the skull, as is remarkably shown by the elephant.
The cavities in the long bones of quadrupeds are filled
with marrow; those in the long bones of most birds
and in skulls contain air.

The number of bones not only differs in different
animals, but varies with the age of an individual. In
very early life there are no bones at all ; and ossifica-
tion, or the conversion of cartilage into bone, is not
completed until maturity. This process begins at a
multitude of points, and theoretically there are as
many bones in a skeleton as centers of ossification.
But the actual number is usually much less — a result
of the tendency of these centers to coalesce. Thus, the
thigh bone in youth is composed of five distinct por-
tions, which gradually unite. So in the lower verte-
brates many parts remain distinct which in the higher
are joined into one. The occiput or bone at the base
of man's skull is the union of four bones, which are
seen separate in the skull of the fish, or of a baby.

A complete skeleton, made up of all the pieces which
might enter into its composition, does not exist. Every
vertebrate has some deficiency. All, except amphioxus,
have a skull and backbone ; but in the development of
the various parts, and especially of the appendages,
there is endless variety. Fishes possess a great number
of skull bones, but have no fingers and toes. The snake
has plenty of ribs and tail, but no breastbone ; the frog
has a breastbone, but neither tail nor ribs. As the skele-
ton of a fish is too complicated for the primary student,
we will select for illustration the skeleton of a lion —
the type of quadrupeds. It should be remembered,
however, that all vertebrates are formed on one plan.

In the lowest vertebrate, amphioxus, the only skele-
ton is a cartilaginous rod running from head to tail.

348

COMPARATIVE ZOOLOGY

There is no skull, nor ribs, nor limbs. In the carti-
laginous fishes, the backbone is only partially ossified.
But usually it consists of a number of separate bones,

called vertebra, arranged along the axis of the body.
They range in number from 10 in the frog to 305 in
the boa constrictor. The skull, with its appendages,
and the vertebrae, with the ribs and sternum, make

THE SKIN AND SKELETON 349

•

up the axial skeleton. The shoulder and pelvic girdles
and the skeleton of the limbs constitute the appendicular
skeleton.

A typical vertebra consists of a number of bony
pieces so arranged as to form two arches, or hoops,
connected by a central bone, or centmm™ The upper
hoop is called the neural arch, because it encircles the
spinal cord; the lower hoop is called the hemal arck,
because it incloses the heart and the great central blood
vessels. An actual vertebra, however, is subject to so

FIG. 304. — Vertebrae — A, cervical; B, dorsal; 2, centrum; 4, transverse process, con-
taining foramen, a, for artery; 5, articular process; 3, spinous process, or neural
spine; i, neural canal; 6, facets for head of rib, the tubercle of the rib fitting in a
facet on the process, 4; b, laminae, or neurapophyses.

many modifications, that it deviates more or less from
this ideal type. Selecting one from the middle of the
back for an example, we see that the centrum sends^off
from its dorsal side two branches, or processes, called
neurapophyses. These meet to form the neural arch,
under which is the neural canal, and above which is a
process called the neural spine. On the anterior and
posterior edges of the arch are smooth surfaces, or
zygapophyses, which in the natural state are covered
with cartilage, and come in contact with the correspond-
ing surfaces of the preceding and succeeding vertebrae.
The bases of the arch are notched in front and behind,
so that when two vertebrae are put together a round

FIG. 307.

THE SKIN AND SKELETON

351

BONES OF THE MAMMALIAN SKULL*

BRAIN CASE

NASAL.

NOSE.

ETHMOID.

FRONTAL. PARIETAL. SUPRAOCCIPITAL.

LAC HRYMAL. SQUAMOSAL.

ORBITOSPHENOID. EYE. ALISPHEXOID. PERI- EAR. OTIC. EXOCCIPITAL.

MALAR. TYMPANIC. .

PRESPHENOID. BASISPHENOID. BASIOCCIPITAL.

VOMER.

PREMAXILLA. MAXILLA. PALATINE. PTERYGO1D.
LOWER JAW, OR MANDIBLE.

HYOID ARCH.

THE SKULL OF THE DOG

FIG. 305. —Under surface. FIG. 306. — Upper surface. FIG. 307. — Longitudinal ver-
tical section; one half natural size. S%)f supraoccmital ; .fi'.j^^exoccipital; BO,
basioccipital; IP, interparietal; Pa, parietal; FV\ frontal; Sff* squamosal ; Ma,
malar; L, lachrymal; Afcrf/maxi\\a.; PMx, premaxilla; Na, nasal; MT, maxillo-
turbinal; ET, ethmoturbinal; ME, ossified portion of the mesethmpid; CE, cri-
briform, or sievelike, plate of the ethmoturbinal; l?£), vomer; &S, presphenoid;
OS, orbitosphenoid ; ;4J>j"Sfisphenoid ; Eg, basisphenoid; PI, palatine; PPj ptery-
goid; Per, periotic; Ty, tympanic bulla; an, anterior narial aperture; ap, or apf,
anterior palatine foramen; ppf, posterior palatine foramen; io, infraorbital foramen;
J0/f'postorb\ta.\ process of frontal bone ; ty, optic foramen; sf, sphenoidal fissure;
fr, foramen rotundum, and anterior opening of alisphenoid canal; ^/"posterior
opening of alisphenoid canal; fa, foramen ovale; flm, foramen lacertim medium;
gf, glenoid fossa; jtf', postglenoid process; Plgf, postglenoid foramen; earn, external
auditory meatus; sm, stylomastoid foramen; flp, foramen lacerum posterius; cf,
condylar foramen; pj/, paroccipital process; otf, occipital condyle; Jfrrt, foramen
magnum; a, angular process; s, symphysis of the mandible where it unites with the
left ramus; id, inferior dental canal; cd, condyle; cp, coronoid procass ; z indi-
cates the part of the cranium to which the condyle is articulated when the mandible
is in place; the upper border in which the teeth are implanted is called alveolar; sh,
eh, ch, bh, th, hyoidean apparatus, or os lingua, supporting the tongue. In the
skulls of old animals, there are three ridges: occipital, behind: sagittal, median, on
the upper surface; and superorbital, across the frontal, in the region of the eye-
brows. The last is highly developed in the gorilla and other apes.

* In this diagram, modified from Huxley's, the italicized bones are single; the rest
are double. Those in the line of the Ethmoid form the Cranio-facial Axis; these, with
the other sphenoids and occipitals, are developed in cartilage; the rest are membrane
bones. In the human skull, the four occipitals coalesce into one

352 COMPARATIVE ZOOLOGY

opening (intervertebral foramen} appears between the
pair, giving passage to the nerves issuing from the
spinal cord. From the sides of the arch, blunt trans-
verse processes project outward and backward, called
diapophyses. Such are the main elements in a repre-
sentative vertebra. The hemal arch is not formed by
any part of the vertebra, but by the ribs and breastbone.
Theoretically, however, the ribs are considered as elon-
gated processes from the centrum (pleurapophyses), and
in a few cases a hemal spine is developed corresponding
to the neural spine.

The vertebrae are united together by ligaments, but
chiefly by a very tough, dense, and elastic substance be-
tween the centra. The neural arches form a continuous
canal which contains and protects the spinal cord ; hence
the vertebral column is called the neuroskeleton. The
column is always more or less curved ; but the beautiful
sigmoid curvature is peculiar to man. The vertebrae
gradually increase in size from the head toward the
end of the trunk, and then diminish to the end of the
tail. The neural arch and centrum are seldom wanting ;
the first vertebra in the neck has no centrum, and the
last in the tail is all centrum. The vertebrae of the ex-
tremities (head and tail) depart most widely from the
typical form.

The vertebral column in fishes and snakes is divisible
into three regions — head, trunk, and tail. In the higher
animals there are six divisions of the vertebral column,
the skull, and cervical, dorsal, lumbar, sacral, and caudal
vertebra.

The skullul is formed of bones whose shape varies
greatly from that of typical vertebrae. The number of
distinct bones composing the skull is greatest in fishes,
and least in birds ; this arises partly from the fact that
the bones remain separate in the former case, while

THE SKIN AND SKELETON

353

those of the chick become united together (ancliylosed)
in the full-grown bird ; but many bones are present in
the fish which have no representatives in the bird. The
skull consists of the brain case and the face. The prin-
cipal parts of the skull, as shown in the dog's, are :
i. The occipital bone behind, inclosing a large hole, or
foramen magnum, on each side of which are rounded
prominences, called condyles, by which the skull articu-

FIG. 308. — Skull of the Horse: i, premaxillary bone; 2, upper incisors; 3, upper
canines; 4, superior maxillary; 5, infraorbital foramen; 6, superior maxillary spine;
7, nasal bones; 8, lachrymal; 9, orbital cavity; 10, lachrymal fossa; n, malar;
12, upper molars; 13, frontal: 15, zygomatic arch; 16, parietal; 17, occipital protu-
berance; 18, occipital crest; 19, occipital condyles; 20, styloid processes; 21, petrous
bone; 22, basilar process; 23, condyle of inferior maxillary; 24, parietal crest; 25, in-
ferior maxillary ; 26, lower molars; 27, anterior maxillary foramen; 28, lower canines;
29, lower incisors.

lates with the first cervical vertebra. 2. The two
parietal bones. 3. The two frontal bones. These five
form the main walls of the skull. 4. The sphenoid, on
the floor of the skull in front of the occipital, and Con-
sisting of six pieces. 5. The two temporal bones, in
which are situated the ears. In man each temporal is
a single bone ; but in most animals there are three or
more — the periotic, tympanic, and squamosal. 6. The
malars, or " cheek bones," each of which sends' back a
• DODGE'S GEN. zooi . — 23

354 COMPARATIVE ZOOLOGY

process to meet one from the squamosal, forming the
zygomatic arch. 7. The two nasals, forming the roof
of the nose. 8. The two maxillce, that part of the
upper jaw in which the canines, premolars, and molars
are lodged. 9. The two premaxillce, in which the
upper incisors are situated. 10. The two palatines,
which, with the maxillary bones, form the roof of the
mouth. There are two appendages to the skull : the
mandible, or lower jaw, whose condyles, or rounded
extremities, fit into a cavity (the glenoid) in the tem-
poral bone ; and the hyoid bone, situated at the root of
the tongue.

The simplest form of the skull is a cartilaginous box,
as in sharks, inclosing the brain and supporting the car-
tilaginous jaws and gill arches. In higher fishes this
box is overlaid with bony plates and partly ossified. In
frogs the skull is mainly bony, although a good deal of
the cartilage remains inside the bones. In higher ver-
tebrates the cartilage never makes an entire box, and
early disappears.

The cervical vertebra, or bones of the neck, are pecul-
iar in having an orifice on each side of the centrum for
the passage of an artery. The first, called atlas, because
it supports the head, has no centrum, and turns on the
second, called axis, around a blunt peglike projection,
called the odontoid process. The centra are usually
wider than deep, and the neural spines very short, ex-
cept on the last one. The number of cervical vertebrae
ranges from I in the frog to 25 in the swan.

The dorsal vertebra are such as bear ribs, which, unit-
ing with the breastbone, or sternum, form a bony arch
over the heart and lungs, called the thorax. The ster-
num may be wanting, as in fishes and snakes, or greatly
developed, as in birds. When present, the first verte-
bra whose ribs are connected with it is the first dorsal.

THE SKIN AND SKELETON 355

The neural spines of the dorsal series are generally long,
pointing backward.

The lumbar vertebra are the massive vertebrae lying
in the loins between the dorsals and the hip bones.

The sacral vertebra lie between the hip bones, and are
generally consolidated into one complex bone, called
sacrum.

The caudal vertebra are placed behind the sacrum,
and form the tail. They diminish in size, losing pro-
cesses and neural arch, till finally nothing is left but the
centrum. They number from 3 or 4 in man to 270 in
the shark.

Besides the lower jaw, hyoid, and ribs, vertebrates
have other appendages to the spinal column — two pairs
of limbs ^ The fore limb is divided into the pectoral
arch (or shoulder girdle), the arm, and the hand. The
arch is fastened to the ribs and vertebrae by powerful
muscles, and" consists of three bones, the scapula, or
shoulder blade, the coracoid, and the clavicle, or collar
bone. The scapula and coracoid are generally united
in mammals, the latter being a process of the former ;
and the clavicles are frequently wanting, as in the hoofed
animals. The hjimerus, radius, and ulna are the bones
of the arm, the first articulating by ball and socket joint
with the scapula, and by a hinge joint with the radius
and ulna. The humerus and radius are always present,
but the ulna may be absent. The bones of the hand
are divided into those of the carpus, or wrist ; the meta-
carpus, or palm ; and the phalanges, or fingers. The
fingers, or " digits," range in number from i to 5.

The hind limb is composed of the pelvic arch (or hip
bones), the leg, and the foot. These parts correspond
closely with the skeleton of the fore limb. Like the
shoulder, the pelvic arch, or os innominatum, consists of
three bones — ilium, ischium, and ptibis. The three are

356 COMPARATIVE ZOOLOGY

distinct in amphibians, reptiles, and in the young of
higher animals ; but in adult birds and mammals they
become united together, and are also (except in whales)
solidly attached to the sacrum. The two pelvic arches
and the sacrum thus soldered into one make the pelvis.
The leg bones consists of \hzfemur, or thigh ; the tibia,
or shin bone ; and the fibula or splint bone. The
rounded head of the femur fits into a cavity (acetabulum)
in the pelvic arch, while the lower end articulates with
the tibia, and sometimes (as in birds) with the fibula
also. An extra bone, the patella, or kneepan, is hung
in a tendon in front of the joint between the femur and
tibia of the higher animals. The foot is made up of the
tarsus, or ankle ; the metatarsus, or lower instep ; and
faz phalanges > or toes. The toes number from i in the
horse to 5 in man.

Certain parts of the skeleton, as of the skull, are
firmly joined together by zigzag edges or by overlapping ;
in either case the joint is called a suture. But the great
majority of the bones are intended to move one upon
another. The vertebrae are locked together by their
processes, and also by a tough fibrous substance between
the centra, so that a slight motion only is allowed. The
limbs furnish the best examples of movable articulations,
as the ball and socket joint at the shoulder, and the
hinge joint at the elbow. The bones are held together
by ligaments, and to prevent friction, the extremities are
covered with cartilage, which is constantly lubricated
with an unctuous fluid called synovia.

A chemical analysis of bone shows it to consist mainly
of phosphate and carbonate of lime and phosphate of
magnesia mingled with glutin, chondrin, and oil, the
amount of each varying in different animals.

358

COMPARATIVE ZOOLOGY

THE SKIN AND SKELETON

359

FIG. 312. — Skeleton of the Tortoise (plastron removed) : a, cervical vertebrae; c, dorsal
vertebrae; d, ribs; e, marginal bones of the carapace; /, scapula; k, precoracoid;
b, coracoid; f, pelvis; /, femur; gt tibia; h, fibula.

FIG. 313. — Skeleton of a Vulture: i, cranium — the parts of which are separable only in
the chick; 2, cervical vertebrae; 3, dorsal; 4, coccygeal, or caudal; the lumbar and
sacral are consolidated ; 5, ribs ; 6, sternum, or breastbone, extraordinarily developed ;
7, furculum, clavicle, or "wishbone"; 8, coracoid; 9, scapula; 10, humerus; u,
ulna, with rudimentary radius; 12, metacarpals; 13, phalanges of the great digit of
the wing; 19, thumb; 14, pelvis; 15, femur; 16, tibiatarsus and fibula, or cms ; 17,
tarsometatarsus ; 18, internal digit, or toe, formed of three phalanges; the middle toe
has four phalanges ; the outer, five ; and the back toe, or thumb, two.

360

COMPARATIVE ZOOLOGY

FIG. 314. — Skeleton of the Horse (Equus caballus): 22, premaxillary; 12, foramen in
the maxillary; 15, nasal; 9, orbit; 19, coronoid process of lower jaw; 17, surface of
implantation for the masseter muscle ; there are seven cervical vertebrae, nineteen
dorsal, D — D; five lumbar, a-e ; five sacral,/^// and seventeen caudal, />-?•/ 51,
scapula, or shoulder blade ; /, spine, or crest; //; coracoid process (acromion wanting) ;
i > first" pair of ribs (clavicle wanting, as in all Ungulates); e, sternum; a, shaft of
humerus; b, deltoid ridge; g head fitting in the glenoid cavity of the scapula — near
it is a great tuberosity for the attachment of a powerful muscle; k, condyles; 54,
radius, to which is firmly anchylosed a rudimentary ulna, 55, the olecranon; 56, the
seven bones of the carpus, or wrist; 57, large metacarpal, or " cannon bone," with
two "splint bones"; 58, fetlock joint; 59, phalanges of the developed digit, corre-
sponding to the third finger in man; 62, pelvis; 63, the great trochanter, or prom-
inence on the femur, 65; 66, tibia; 67, rudimentary fibula; 68, hock, or heel, falsely
called knee; 69, metatarsals.

THE SKIN AND SKELETON 361

FIG. 315. — Skeleton of the Cow (Bos ta-urus).

FIG. 316. — Skeleton of an Elephant {Elephas indicus).

362

COMPARATIVE ZOOLOGY

FIG. 317. — Skeleton of the Chimpanzee (Anthropopithecus troglodytes).

CHAPTER XX*

HOW ANIMALS MOVE

i. The power of animal motion is vested in proto-
plasm, cilia, and muscles. The power of contractility
is one of the fundamental physiological properties of
protoplasm, like sensibility and the power of assimila-
tion. Protoplasmic animals, like the amoeba and Rhiz-
opoda (Figs. I, 213), move by the contractility of their
protoplasm, as also may the embryos of higher animals
upon the yolk of the egg. Protoplasm may be extended
into projections called pseudopodia, by whose contrac-
tion the animal may move.

Infusoria, and nearly all higher animals, possess cilia
(Figs. 9, 1 1 ). These are short microscopic threads of
protoplasm which have the power of bending into a
sickle shape and straightening out. As they bend much
faster than they straighten, and as they all work together,
they can cause motion of the animal, or may serve to
produce currents in the water, the animal remaining at
rest. They are seen on the outside of Infusoria, and
of embryos of very many higher animals, serving as
paddles for locomotion ; they line the channels in the
gills of the oyster, creating currents for respiration ;
and they cover the walls of the passages to our lungs to
expel the mucus. Flagella (Figs. 4, 5, 6) are a sort of
long cilia, which are thrown into several curves when
active, resembling a whiplash, whence their name. Both
cilia and flagella seem to be wanting in arthropods.

* See Appendix.
363

364

COMPARATIVE ZOOLOGY

:Dytiscus.

The cause of ciliary motion is unknown. One-sided
contraction is their property, as straight contraction
is distinctive of the muscle fiber. No structure can,
however, be seen in them with the micro-
scope. No nerves go to them, yet they
work in concert, waves of motion passing
over a surface covered with cilia, as over
a field of grain moved by the wind.

i. Muscle. — Muscular tissue is the great
FIG i -waves motor agent, and exists in all animals from
of Contraction the coral to man.133 The power of con-
tractility, which in the amoeba is diffused
throughout the body, is here confined to
bundles of highly elastic fibers, called muscles. When a
muscle contracts, it tends to bring its two ends together,
thus shortening itself, at the same time increasing in
thickness. This shrinking property is excited by exter-
nal stimulants, such as electricity, acids, alkalies, sudden
heat or cold, and even a sharp blow ; but
the ordinary cause of contraction is an
influence from the brain conveyed by a
nerve. The property, however, is inde-
pendent of the nervous system, for the
muscle may be directly stimulated. The
amount of force with which a muscle
contracts depends on the number of its
fibers; and the amount of shortening, on
their length.

As a rule, muscles are white in cold-
blooded animals, and red in the warm- FIG.
blooded. They are white in all the
invertebrates, fishes, batrachians, and rep-
tiles, except salmon, sturgeon, and shark ;
and red in birds and mammals, except in the breast
of the common fowl, and the like.134

319. — Un-
striped Muscu-
lar Fiber, much
enlarged; «, nu-
cleus.

HOW ANIMALS MOVE 365

It is also a rule, with some exceptions, that the volun-
tary muscles of vertebrates, and all the muscles of the
lobster, spider, and insect tribes, are striated ; while the
involuntary muscles of vertebrates, and all the muscles
of radiates, worms, and mollusks, are smooth. All mus-
cles attached to internal bones, or to a jointed external
skeleton, are striated. The voluntary muscles of verte-
brates are generally solid, and the involuntary surround
cavities.135

This leads to another classification of muscles : into
those which are attached to solid parts within the body ;
those which are attached to the skin or its modifications;
and those having no attachments, being complete in
themselves. The last are hollow or circular muscles,
inclosing a cavity or space, which they reduce by con-
traction. Examples of such are seen in the heart, blood
vessels, stomach, iris of the eye, and around the mouth.
In the lower invertebrates, the muscular system is a net-
work of longitudinal, transverse, and oblique fibers inti-
mately blended with the skin, and not divisible into sep-
arate muscles. As in the walls of the human stomach,
the fibers are usually in distinct layers. This ar-
rangement is exhibited by soft-bodied animals, like
the sea anemone, the snail, and the earthworm. Four
thousand muscles have been counted in a caterpillar.
There are also " skin muscles " in the higher animals,
as those by which the horse produces a twitching of the
skin to shake off insects, and those by which the hairs
of the head and the feathers of birds are made to stand
on end. Invertebrates whose skin is hardened into a
shell or crust have muscles attached to the inside of such
a skeleton. Thus, the oyster has a mass of parallel
fibers connecting its two valves ; while in the lobster
and bee fibers go from ring to ring, both longitudinally
and spirally. The muscles of all invertebrates are

366 COMPARATIVE ZOOLOGY

straight parallel fibers, not in bundles, but distinct, and
usually flat, thin, and soft.

The great majority of the muscles of vertebrates are
attached to the bones, and such are voluntary. The
fibers, which are coarsest in fishes (most of all in the
rays), and finest in birds, are bound into bundles by
connective tissue ; and the muscles thus made up are
arranged in layers around the skeleton. Sometimes
their extremities are attached to the bones (or rather to
the periosteum) directly ; but generally by means of
white inelastic cords, called tendons. In fishes, the
chief masses of muscle are disposed along the sides
of the body, apparently in longitudinal bands, reaching
from head to tail, but really in a series of vertical flakes,
one for each vertebra. In proportion as limbs are de-
veloped, we find the muscles concentrated about the
shoulders and hips, as in quadrupeds. The bones of
the limbs are used as levers in locomotion, the fulcrum
being the end of a bone with which the moving one is
articulated. Thus, in raising the arm, the humerus is a
lever working upon the scapula as a fulcrum. The
most important muscles are called extensors and flexors.
The latter are such as bring a bone into an angle with
its fulcrum — as in bending the arm — while the former
straighten the limb. Abductors draw a limb away from
the middle line of the body, or a finger or toe away
from the axis of the limb, while adductors bring them
back.

2. Locomotion. — All animals have the power of vol-
untary motion, and all, at one time or another, have the
means of moving themselves from place to place. Some
are free in the embryo life, and fixed when adult, as
the sponge, coral, crinoid, and oyster. There may be
no regular, well-defined means of progression, as in the
amoeba, which extemporizes arms to creep over the sur-

HOW ANIMALS MOVE 367

face ; or movement may be accomplished by the con-
traction of the whole body, as in the jellyfish, which,
pulsating about fifteen times in a minute, propels itself
through the water. So the worms and snakes swim by
the undulations of the body.

But as a rule, animals are provided with special
organs for locomotion. These become reduced in num-
ber, and progressively perfected, as we advance in the
scale of rank. Thus, the infusorian is covered with
thousands of hairlike cilia; the starfish has hundreds
of soft, unjointed, tubular suckers ; the centipede has
from 30 to 40 jointed hollow legs; the lobster, 10; the
spider, 8 ; and the insect, 6 ; the quadruped has 4 solid
limbs for locomotion ; and man, only 2.

(i) Locomotion in Water. — As only the lower forms of
life are aquatic, and as the weight of the body is partly
sustained by the element, we must expect to find the or-

FIG. 320. —The Fins of a Fish (Pike Perch}.

gans of progression simple and feeble. The Infusoria
swim with great rapidity by the incessant vibrations of
the delicate filaments, or cilia, on their bodies. The
common squid on our coast admits water into the inte-
rior of the body, and then suddenly forces it out through
a funnel, and thus moves backward, or forward, or
around, according as the funnel is turned — toward the
head, or tail, or to one side. The lobster has a fin at

368

COMPARATIVE ZOOLOGY

the end of its tail, and propels itself backward by
a quick downward and forward stroke of the ab-
domen.

But fishes, whose bodies offer the least resistance to
progression through water, are the most perfect swim-
mers. Thus, the salmon can go twenty miles an hour,
and even ascend cataracts. They have fins of two
kinds : those set obliquely to the body, and in pairs ;
and those which are vertical, and single. The former,
called pectoral and ventral fins, rep-
resent the fore and hind limbs of
quadrupeds. The vertical fins, which
are only expansions of the skin, vary
in number ; but in most fishes there
are at least three : the caudal, or
tail fin ; the dorsal, or back fin ; and
the anal, situated on the abdomen,
near the tail The chief locomotive
agent is the tail, which sculls like a
stern oar ; the other fins are mainly
used to balance and raise the body.
When the two lobes of the tail
are equal, and the vertebral column
stops near its base, as in the trout,
it is said to be homo cereal. If the
vertebrae extend into the upper
lobe, making it longer than the lower one, as in the
shark, the tail is called heterocercal. The latter is the
more effective for varying the course ; the shark, e.g.,
will accompany and gambol around a ship in full sail
across the Atlantic. The whale swims by striking the
water up and down, instead of laterally, with a finlike
horizontal tail. Many air-breathing animals swim with
facility on the surface, as the water birds, having webbed
toes, and most of the reptiles and quadrupeds.

FIG. 321. — Diagram illustrat-
ing the locomotion of a
Fish. The tail describes
the arc of an ellipse; the
resultant of the two im-
pulses is the straight line
in front.

HOW ANIMALS MOVE

369

(2) Locomotion in Air. — The power of flight requires a
special modification of structure and an extraordinary
muscular effort, for air is 800 times lighter than water.
Nevertheless, the velocity attainable by certain birds is
greater than that of any fish or quadruped ; the hawk
being able to go at the rate of 1 50 miles an hour. The
bodies of insects and birds are made as light as possible
by the distribution of air sacs or air cavities.136

The wings of insects are generally four in number ;
sometimes only two, as in the fly. . They are moved by
muscles lying inside the thorax. They are simple ex-
pansions of the skin, or crust, being composed of two
delicate films of the epidermis stretched upon a network
of tubes. There are three main varieties : thin and
transparent, as in the dragon fly ; opaque, and covered
with minute colored scales, which are in reality flattened
hairs, as in the butterfly ; and hard and opaque, as the
first pair (called elytra) of the beetle.

The wings of birds, on the other hand, are modified
fore limbs, consisting of three sets of feathers (called

FIG. 322. — Flamingoes taking Wing.

primary, secondary, and tertiary), inserted on the hand,
forearm, and arm. The muscles which give the down-
ward stroke of the wing are fastened to the breastbone ;
DODGE'S GEN. ZOO'L. — 24

370

COMPARATIVE ZOOLOGY

arid their power, in proportion to the weight of the bird,
is very great. Yet the insect is even superior in vigor
and velocity of flight.137 In ascending, the bird slightly
rotates the wing, striking downward and a little back-
ward ; while the tail acts as a rudder. A short, rounded,
concave wing, as in the common fowl, is not so well
fitted for high and prolonged flight as the long, broad,
pointed, and flat wing of the eagle. The wing is folded
by means of an elastic skin and muscle connecting the
shoulder and wrist. Besides insects and birds, a few
other animals have the power of flight, as bats, by
means of long webbed fingers ; flying fishes, by large
pectoral fins. Flying reptiles, flying squirrels, and the
like, have a membrane stretched on the long ribs, or
connecting the fore and hind limbs, which they use as a
parachute, enabling them to take very long leaps.

(3) Locomotion on Solids. — This requires less muscular
effort than swimming or flying. The more unyielding
the basis of support, the greater the amount of power
left to move the animal along. The simplest method is

FIG. 323. — Diagrammatic Section of the Disk and one Ray of Starfish: a, mouth; b,
stomach; c, hepatic caecum; d, dorsal or aboral surface; e, ambulacral plates; f>
ovary; gt tubular feet; k, internal sacs for distending the feet.

the suctorial, the animal attaching itself to some fixed
object, and then, by contraction, dragging the body on-
ward. But the higher and more common method is by
the use of bones, or other hard parts, as levers.

HOW ANIMALS MOVE 3/1

The starfish creeps by the working of hundreds of
tubular suckers, which are extended by being filled with
fluid forced into them by little sacs. The clam moves
by fixing and contracting a muscular appendage, called
a "foot." The snail has innumerable short muscles on
the under side of its body, which, by successive contrac-
tions, resembling minute undulations, enable the animal
to glide forward apparently without effort. The leech
has a sucker at each end ; fixing itself by the one on
its tail, and then stretching the body, by contracting the
muscular fibers which run around it, the creature fastens
its mouth by suction, and draws forward the hinder
parts by the contraction of longitudinal muscles. The
earthworm lengthens and shortens itself in the same
way as the leech, but instead of suckers for holding its
position, it has numerous minute spines which may be
pointed backward or forward ; while the caterpillar has
short legs for the same purpose. The legless serpent
moves by means of the scutes, or large scales on the
under side of the body, acted upon by the ribs. In a
straight line, locomotion is slow ; but by curving the
body, laterally or vertically, it can glide or Jeap with
great rapidity.

Most animals have movable jointed limbs, acted upon
as levers by numerous muscles. The centipede has
forty-two legs, each with five joints and a claw. The
crab has five pairs of six-jointed legs ; but the front
pair is modified into pincers for prehension. With the
rest, which end in a sharp claw, the crab moves back-
ward, forward, or sideways. The spider has eight legs,
usually seven-jointed, and terminating in two claws
toothed like a comb, and a third which acts like a
thumb. In running, it moves the first right leg, then
the fourth left ; next, the first left, and then the fourth
right; then the third right and second left together;

372

COMPARATIVE ZOOLOGY

and lastly, the third left and second right together.
The front and hind pairs are, therefore, moved like
those of a quadruped. The insect has six legs, each
of five parts: the coxa ; trochanter ; femur; tibia, or

shank ; and tarsus.
The last is sub-
divided usually into
five joints and a
pair of claws.
Such as can walk
upside down, as
the fly, have, in
addition, two or
three pads between
the claws.138 These
pads bear hairs
which secrete a
sticky fluid by
means of which
the fly adheres to
the surface. While
the leg bones of
vertebrates are
covered by the muscles which move them, the limbs
of insects are hollow, and the muscles inside. The fore
legs are directed forward, and the two hinder pairs
backward. In motion, the fore and hind feet on one
side, and the middle one on the other, are moved simul-
taneously, and then the remaining three.

The four-legged animals have essentially the same
apparatus and method of motion. The crocodile has
an awkward gait, owing to the fact that the limbs are
short, and placed far apart, so that the muscles act at
a mechanical disadvantage. The tortoise is proverbially
slow, for a similar reason. Both swim better than they

FIG. 324. — Feet of Insects, magnified: A, Bibiofebrilis;
B, House Fly (Musca domestic a) ; C, Water Beetle
(Dytiscus).

HOW ANIMALS MOVE

373

walk. Lizards are light and agile, but progression is
aided by a wriggling of the body.

The locomotive organs of the mammalian quadrupeds
are much more highly organized. The bones are more
compact ; the vertebral column is arched and yet elastic,
between the shoulder and hip, and the limbs are placed
vertically underneath the body. The bones of the fore
limb are nearly in a line ; but those of the hind limb,
which is mainly used to project the body forward, are
more or less inclined to one another, the angle being

^ji^

FIG. 325. — Feet of Carnivores : A, Plantigrade (Bear); B, Pinnigrade (Seal); C, Digiti-
grade (Lion).

most marked in animals of great speed, as the horse.
Some walk on hoofs, as the ox (ungulate) ; some on the
toes, as the cat (digitigrade); others on the sole, touch-
ing the ground with the heel, as the bear (plantigrade).
In the pinnigrade seal, half of the fore limb is buried
under the skin, and the hind limbs are turned backward
to form a fin with the tail. The normal number of toes
is five ; but some may be wanting, so that we have one-
toed animals (as horse), two-toed (as ox), three-toed (as
rhinoceros), four-toed (as hippopotamus), and five-toed

374

COMPARATIVE ZOOLOGY

(as the elephant). The horse steps on what corre-
sponds to the nail of the middle finger ; and its swiftness
is conditioned on the solidity of the extremities of the

FIG. 326. — Feet of Hoofed Mammals: A, Elephant; B, Hippopotamus; C, Rhinoceros;
Z>, Ox; E, Horse, a, astragalus; cl, calcaneum, or heel; s, naviculare; bt cu-
boides; ce, ci, cm, cuneiform bones; the numbers indicate the digits in use.

limbs. Horses of the greatest speed have the shoulder
joints directed at a considerable angle with the arm.

The order in which the legs of quadrupeds succeed'
each other determines the various modes of progression,
called the walk, trot, gallop, and leap. Many, as the
horse, have all these movements ; while some only leap,
as the frog and kangaroo. In leaping animals, the hind
limbs are extraordinarily developed. In many mammals,
like the squirrel, cat, and dog, the fore legs are used for
prehension as well as locomotion. Monkeys use all four,
and also the tail, for locomotion and prehension, keeping
a horizontal attitude ; while the apes, half erect, as if
they were half quadruped, half biped, go shambling
along, touching the ground with the knuckles of one
hand and then of the other. In descending the scale,
from the most anthropoid ape to the true quadruped, we
find the center of gravity placed increasingly higher up

HOW ANIMALS MOVE

375

— that is, farther forward. Birds and men are the only
true bipeds, the former standing on their toes, the latter

FIG. 327. — Muscles of the Human Leg:
sartorzus, or " tailor's muscle," the
longest muscle in the body, flexes the
leg upon the thigh: rectus femoris
and i>astus externus and internus ex-
tend the leg, maintaining an erect
posture; gastrocnemius, or "calf,"
used chiefly in walking, for raising the
heel. Another layer underlies these
superficial muscles.

FIG. 328. — Muscles of an Insect's Leg
(Melolontha vulgaris): a, flexor, and
b, extensor, of tibia; c, flexor of foot;
</, accessory muscle ; e , extensor of
claw; f, extensor of tarsus. The
joints are restricted to movements
in one plane; and therefore the mus-
cles are simply flexors and extensors.
All the muscles are within the skele-

on the soles of the feet. Terrestial birds walk and run ;
while birds of flight usually hop. The ostrich can for a
time outrun the Arabian horse ; and the speed of the
cassowary exceeds that of the swiftest greyhound.

CHAPTER XXI*

THE NERVOUS SYSTEM

Nerve Tissue exists in the form of cells and fibers,
the latter being prolongations of the bodies of the former.
Where the cells predominate, nerve tissue is grayish.
Such accumulations are called ganglia, or nerve centers,
and these alone originate nervous force ; the fibers
are generally white, and arranged in bundles, called

FIG. 329. — Nerve cell
from cerebral cortex :
«, nerve process.

FIG. 330. — Diagram of Nervous Sys-
tem of Starfish: r, nervous ring
around mouth; n, radial nerves to
each arm, ending in the eye.

nerves, which serve only as conductors. Most nerves
contain two kinds of fibers, like in structure, but
each having its distinct office : one carries impres-
sions received from the external world to the gray
centers, and hence is called an afferent, or sensory,
nerve ; the other conducts an influence generated in
the center to the muscles, in obedience to which they

* See Appendix.
376

THE NERVOUS SYSTEM

377

contract, and hence it is called an efferent, or motor,

nerve. Thus, when the finger is pricked with a pin,

afferent nerve fibers convey the impression to the center,

the spinal cord, which immediately transmits an order

by efferent fibers to the muscles of the hand to contract.

If the former are cut, sensation is lost, but voluntary

motion remains ; if the latter are cut, the animal loses

all control over the muscles,

although sensibility is perfect;

if both are cut, the animal is

said to be paralyzed in the parts

which these nerves control.

The nerve fibers are connected

with nerve cells in the central

organs, and at the outer ends

are connected with the muscular

fibers, or with various sensory

end organs in the skin or other

parts of the body. The nature

of nerve force is not known.

AS tO the Velocity Of a nerVOUS FlG- 33^- Nervous System of a

•* Mollusk (the Gastropod Aplysta) :

impulse, We knOW it is far leSS a, anterior ganglion; c, cephalic;

,1 ,1 £ i , . ., 1. i , /, lateral ; g, abdominal.

than that or electricity or light,

and that it is more rapid in warm-blooded than in
cold-blooded animals, being faster in man than in the
frog. In the latter it averages about 85 feet per second,
the former over 100 feet.

The very lowest animals, like the amoeba and In-
fusoria, have no nerves, although their protoplasm has
a general sensibility. The hydra has certain cells which
are, perhaps, partly nervous and partly muscular in
function. The jellyfish has a nervous system, consist-
ing of a network of threads and ganglia scattered all
over its disk. We should look for a definite system of
ganglia and nerves only in those animals which possess

378

COMPARATIVE ZOOLOGY

a definite muscular structure, and show definitely coordi-
nated muscular movements. In the starfish we detect
the first clear specimen of such a sys-
tem. It consists of a ring around the
mouth, made of five ganglia of equal
size, with radiating nerves. The mol-
lusks are distinguished by an irregu-
larly scattered nervous system. The
clam has three main pairs of connected
/ ganglia — one near the
mouth, one in the foot, and
the third in the posterior
region, near the syphons.
In the snail, these are
united into a ring around
the gullet, and there are

FIG. 332. — Nervous Sys- - ,. ,

tem of ciam; , , cere- other ganglia scattered

bral ganglion ; /, pedal thrOUgh the body. The
ganglia ; ps, pane- J

tosplanchnic ganglia ;
c' , cerebral commis-
sure ; p', commissure
from cerebral to pedal
ganglia ; ps' , commis-
sure from cerebral to
parietosplanchnic
ganglia ; es, esopha-
gus.

same is true of the cuttle-
fish, where the brain is
partly inclosed in a carti-
laginous box (Fig. 348).
In the simpler worms
there is but a single
ganglion or a single pair. The earthworm
has a pair of brain ganglia lying above the
gullet, and connected by two cords with a
ventral chain of ganglia — one pair, appar-
ently a single ganglion, for each segment,
In the lower arthropods, such as Crustacea,
centipedes, and larval insects, the arrange-
ment is substantially the same. In higher
insects and Crustacea, many of the ganglia are fused to-
gether in the head and thorax, indicating a concentration
of organs for sensation and locomotion.

pillar (Sphinx li-
gustrt): the first
is the cephalic, or
head, ganglion.

THE NERVOUS SYSTEM

379

In vertebrates, the nervous
developed, more complex, and
more concentrated than in the
lower forms. In fact, there are
some parts, as the brain, to
which we find nothing homolo-
gous in the invertebrates ; and
while the actions of the latter
are mainly, if not wholly, auto-
matic, those of backboned ani-
mals are largely voluntary. Its
position, moreover, is peculiar,
the great mass of the nervous
matter being accumulated on
the dorsal side, and inclosed
by the neural arches of the
skeleton.

The brain and spinal cord
lie in the cavity of the skull
and spinal column, wrapped in
three membranes. Each con-
sists of gray and white nervous
matter; but in the brain the
gray is on the outside, and the
white within ; while the white
of the spinal cord is external,
and the gray internal. Both
are double, a deep fissure run-
ning from the forehead back-
ward, dividing the brain into
two hemispheres, and the spinal
cord resembling two columns
welded together ; even the
nerves come forth in pairs to
the right and left. The brain is

system is more highly

FIG. 334. — Human Brain and Spinal
Cord, seen from below, about one-
tenth natural size ; «, right hemi-
sphere of cerebrum; b, anterior
lobe; c, middle lobe; d, medulla
oblongata ; e, cerebellum ; f, first
spinal nerve ; g, brachial plexus
.of nerves supplying the arms ;
h, dorsal nerves ; i, lumbar nerves ;
k, sacral plexus of nerves for the
limbs ; /, cauda equina: the figures
indicate the twelve pairs of cranial
nerves, of which i is olfactory,
2 optic, and 8 auditory.

380 COMPARATIVE ZOOLOGY

the organ of sensation and voluntary motion ; the spinal
cord is the organ of involuntary life and motion. The
brain, above the medulla oblongata, may be removed,
and yet the animal, though it cannot feel, will live for a
time, showing that it is not absolutely essential to life ;
in fact, the brain does nothing in apoplexy and deep
sleep. All of the cord, except that part containing the
centers for respiration and circulation, may also be
destroyed, without causing immediate death.

The Brain is that part of the nervous system con-
tained in the skull.139 It increases in size and com-
plexity as we pass from the fishes, by the amphibians,
reptiles, and birds, to mammals. Thus the body of the
cod is 5000 times heavier than its brain — in fact, the
brain weighs less than the spinal cord ; while in man,
the brain, compared with the body, is as I to 36, and is
40 times heavier than the spinal cord. The brains of
the cat weigh only i oz. ; of the dog, 6 oz. 5^ dr. ; and
of the horse, 22 oz. 15 dr. The only animals whose
brains outweigh man's are the elephant and whale —
the maximum weight of the elephant's being 10 Ibs.,
and of the whale's 5 Ibs. ; while the human does not
exceed 4 Ibs. Yet the human brain is heavier in pro-
portion to the body. But quality must be considered
as well as quantity, else the donkey will outrank the
horse, and the canary bird, man ; for their brains are
relatively heavier.

The main parts of the brain are the cerebrum, cerebel-
lum, and medulla oblongata.

The cerebrum is a mass of white fibrous matter cov-
ered by a layer of gray cellular matter. In the lower
vertebrates, the exterior is smooth ; but in most of the
mammals it is convoluted, or folded, to increase the
amount of the gray surface. The convolutions multiply
and deepen as we ascend the scale of size and intelli-

THE NERVOUS SYSTEM

381

gence, being very complex in the elephant and whale,
monkey and man. As a rule, they are proportioned to
the intelligence of the animal; yet the brains of the
dog and horse are smoother than those of the sheep
and donkey. Evi-
dently the quality of
the gray matter must
be taken into account.
Save in the bony
fishes, the cerebrum
is the largest portion
of the brain ; in man
it is over eight times
heavier than the cere-
bellum.

The cerebellum, or
"little brain," lies be-
hind the cerebrum,
and, like it, presents
an external gray layer,
with a white interior.
In mammals, it is like-
wise finely convoluted,
consisting of gray and
white laminae, and is
divided into two lobes,
or hemispheres. In
the rest of the verte-
brates, the cerebellum
is nearly or quite
smooth ; and in the lowest fishes it is merely a thin
plate of nervous matter. In many vertebrates, how-
ever, it is larger compared with the cerebrum, than
in man, since in man the cerebrum is extraordinarily
developed.

FIG. 335. — Brain of the Horse — upper view, one
fourth natural size ; a, medulla oblongata ;
b, lateral and middle lobes of cerebellum; c, inter-
lobular fissure ; d, cerebral hemispheres; e, olfac-
tory lobes.

382

COMPARATIVE ZOOLOGY

The medulla oblongata is the connecting link between
the cerebrum and cerebellum and the spinal cord. In
structure, it resembles the spinal cord — the white
matter being external and the gray internal. The
former lies beneath or behind the brain, passing through
the foramen magnum of the skull, and merging im-
perceptibly into the cord. The latter is a continuous
tract of gray matter inclosed within strands of white
fibers. It usually ends in the lumbar region of the
vertebral column, but in fishes it reaches to the end of
the tail. In fishes, amphibians, and reptiles, the cord
outweighs the brain ; in birds and mammals, the brain
is heavier than the cord. In man, the cord weighs
about an ounce and a half.

Besides these parts,
there are also the olfac-
tory and the optic lobes,
which give rise respec-
tively to the nerves of
smell and sight.

The parts of the brain
are always in pairs ; but
in relative development
^^ position they differ
widely in the several
classes of vertebrates.

FIG. 336. — Brain of
the Perch, upper
view : a, cerebel-

b, optic

c, cere-

lum ;
lobes

Skr

brum; i, olfactory In fishes and TCptileS,
lobes' p- medulla ^.r. j •

theY are arranged in a olfactory lobes. Hc
horizontal line; in birds

FIG. 337. — Brain of the
Frog, upper view; X 4:
r, olfactory nerves : Lol,

cDerebral hemispheres;

Pn, pineal gland; Fho

and mammals, the axis of the spinal and shr, third and

. . , fourth ventricles ; Lop,

cord bends to nearly a right angle in optic lobes -, c cerebei-
passing through the brain, so that the J0unmgatf '*• medulla ob'
lobes no longer lie in a straight line.
In man, the fore brain is so developed that it covers all
the other lobes. In looking down upon the brain of a

THE NERVOUS SYSTEM

383

perch, we see in front a pair of olfactory lobes (which
send forth the nerves of smell), behind them the small
cerebral hemispheres, then the large optic lobes (near
which originate the nerves of sight), and, last of all, the
cerebellum. Not until we reach man and the apes do
we find the cerebrum so highly developed as to overlap
both the olfactory lobes in front and the cerebellum
behind.

Functions of the Brain. — The cerebrum is the seat of
intelligence and will. It has no direct communication
with the outside world, receiving its consciousness of
external objects and events through the spinal cord and
the nerves of special sense.140

The cerebellum seems to preside over the coordina-
tion of the muscular movements. When the cerebellum

FIG. 338. — A, C, upper and side views of the Brain of a Lizard ; B, D, upper and side
views of the Brain of a Turkey: Olf, olfactory lobes ; Hmp, cerebral hemispheres ;
/>«, pineal gland ; Mb, optic lobes of the middle brain ; Cb, cerebellum; MO, me-
dulla oblongata ; it, optic nerves; iv and vi, nerves for the muscles of the eye ;
Py, pituitary body.

is removed, the animal desires to execute the mandates
of the will, but can not ; its motions are irregular, and it
acts as if intoxicated. It is usually largest in animals
capable of the most complicated movements, being

COMPARATIVE ZOOLOGY

larger in the ape than in the lion, in the lion than in
the ox, in birds than in reptiles. The cerebellum of the

FIG. 339. — Brain of the Cat (Felis do-
mestica) : a, medulla oblongata;
b, cerebellum ; c, cerebrum.

FIG. 340. — Brain of the Orang-outang
upper surface ; one third natural
size.

frog is, however, smaller than that of fishes (Figs. 336,
337). The olfactory and optic lobes receive the mes-
sages from their respective nerves. The medulla
oblongata is not only the me-
dium of communication between
the brain and the spinal cord,

FIG. 341. — Human Brain, side view:
i, medulla oblongata ; 3, cerebellum ;
5, frontal convolutions of cerebrum.

FIG. 342. — Human Brain, upper view,
one fourth natural size : i, anterior
lobes ; 2, posterior; 3, great median
fissure.

but it is itself a nervous center : the brain above and
the cord below may be removed without death to the

THE NERVOUS SYSTEM

385

animal, but the destruction of the medulla is fatal. Of
the twelve pairs of nerves issuing from the contents of
the skull (encepJialoii), ten come from the medulla oblon-
gata. Among these are the nerves of hearing and taste,
and those that control the lungs and heart. Respiration
ceases immediately when the medulla is injured.

The spinal cord is a center for originating involuntary
actions, and is also a conductor — transmitting through
its cells and fibers to
the brain the impres-
sions received by the
sensory organs, and
taking back to the
motor organs the im-
pulses of the brain.
In man, thirty-one
pairs of nerves arise
from the cord to sup-
ply the whole body,
except the head.
Each nerve has an
anterior and a poste-
rior root. The fibers
of the former go to the
muscles, and carry
the impulses which
cause muscular con-
traction (hence called
motor fiber!) ; those
of the posterior root
convey sensations
from the exterior to

the central organs (sensory). The fibers leading from
the brain to the cord cross one another in the medulla
oblongata, so that if the right cerebral hemisphere be
DODGE'S GEN. ZOOL. — 25

Fig. 343. — Relation of the Sympathetic and Spinal
Nerves : c, fissure of spinal cord; «, anterior root
of a dorsal spinal nerve ; /, posterior root, with
its ganglion ; a ' , anterior branch ; /', posterior
branch ; s, sympathetic ; f-, its double junction by
nerve filaments.

386 COMPARATIVE ZOOLOGY

diseased, the left side of the body loses the power of
voluntary motion.'

The sympathetic nervous system is a double chain of
ganglia, lying along the sides of the vertebral column in
the ventral cavity. From these ganglia nerves are
given off, which, instead of going to the skin and mus-
cles, like the spinal nerves, form networks about those
internal organs over which the will has no control, as
the heart, stomach, and intestines. Apparently their
office is to stimulate these organs to constant activity,
but is little understood.

I. The Senses

Sensation is the consciousness of impressions on the
sensory organs. These impressions produce some
change in the brain ; but what that change is, is a dark-
ness on which no hypothesis throws light. Obviously,
we feel only the condition of our nervous system, not
the objects which excite that condition.141

All animals possess a general sensibility diffused over
the greater part of the body.142 This sensibility, like as-
similation and contractility, is one of the primary phys-
iological properties of protoplasm. But, besides this
(save in the very lowest forms), they are endowed with
special nerves for receiving the impressions of light,
sound, etc. These nerves of sense, as they are called,
although structurally alike, transmit different sensations:
thus, the ear can not recognize light, and the eye can not
distinguish sounds. In the vertebrates, the organs of
sight, hearing, and smell are situated in pairs on each
side of tire head ; that of taste, in the mucous membrane
covering the tongue ; while the sense of touch and that
of temperature are diffused over the skin, including the
mucous membrane of the mouth, throat, and nose.

THE NERVOUS SYSTEM

387

Sight and hearing are stimulated, each by one agent
only ; while touch, taste, and smell may be excited by
various substances. The agents awakening sight, hear-
ing, touch, and the sense of temperature are physical ;
those causing taste and smell are chemical. Animals
differ widely in the numbers and keenness of their
senses. But there is no sense in any one which does
not exist in some other.

Touch is the simplest and the most general sense ; no
animal is without it, at least in the form of general sen-
sibility. It is likewise the
most positive and certain
of the senses. In the sea
anemone, snail, and in-
sect, it is most acute in
the " feelers" (tentacles,
horns, and antennae) ; M3
in the oyster, the edge
of the mantle is most
sensitive ; in fishes, the
lips ; in snakes, the tongue ;
in birds, the beak and
under side of the toes ; in quadrupeds, the lips and
tongue ; and in monkeys and man, the lips and the tips
of the tongue and fingers. In the most sensitive parts
of birds and mammals, the true skin is raised up into
multitudes of minute elevations,
called papilla, containing loops of
capillaries and nerve filaments. At
the ends of the latter are the es-
sential organs of touch, the tactile
corpuscles and the touch cells.
There is a correspondence between
the * delicacy of touch and the development of intelli-
gence. The cat and dog are more sagacious than

FIG. 344 — Antennae of various Insects
(magnified).

FIG. 345. — Papillae of Human
Palm, x 35, the cuticle being
removed.

388

COMPARATIVE ZOOLOGY

hoofed animals. The elephant and parrot are remark-
ably intelligent, and are as celebrated for their tactual
power.

Taste is more refined than touch, since it gives a
knowledge of properties which can not be felt. It is
always placed at the entrance to the digestive canal,
as its chief purpose is to guide animals in their choice
of food. Special organs of taste have been detected in
only a few of the invertebrates, though all seem to
exercise a faculty in selecting their food. Even in fishes,
amphibians, reptiles, and birds this sense is very obtuse,
for they bolt their food. But the higher vertebrates
have it well developed. If is confined to the tongue,
and is most delicate at the root.144 A state of solution
and an actual contact of the fluid are necessary condi-
tions.

Smell is the perception of odors, i.e., certain substances
in the gaseous or volatile state. Many invertebrates have
this sense : snails, e.g., seem to be guided to their food
by its scent, and flies soon find a
piece of meat. In the latter the
organ is probably located on the
antennae. In vertebrates, it is placed
at the entrance to the respiratory
tube, in the upper region of the nose.
There the olfactory nerves/which
cavity- issue from the olfactory lobe of the

brain, and pass through the ethmoid bone, or roof of the
nasal cavity, are distributed over a moist mucous mem-
brane. The odorous substance, in a gaseous or finely
divided state, is dissolved in the mucus covering this
membrane. In fishes and reptiles generally, this organ
is feebly developed ; sharks, however, gather from a
great distance around a carcass. In the porpoises< and
whales it is nearly or entirely wanting. Among birds,

THE NERVOUS SYSTEM

389

The simplest
fluid, in which

waders have the largest olfactory nerves. It is most
acute in the carnivorous quadrupeds, and in some wild
herbivores, as the deer. In man it is less delicate, but
has a wider range than in any brute.

Hearing is the perception of sound,
form of the organ is a sac filled with
float the soft and delicate ends of
the auditory nerve. Usually the vibra-
tions of the fluid are strengthened by
the presence of minute hard granules,
called otoliths. Most invertebrates _

FIG. 347. — Ear of a Mol-

have no more complicated apparatus lusk (Cycias\ greatly

- . , ., . , , , ., enlarged, showing the

than this ; and it is probable that they
can distinguish one noise from an-
other, but neither pitch nor intensity.
The organ is generally double, but not always located in
the head. In the clam, it is found at the base of the
foot ; some grasshoppers have it in the fore legs ; and
in many insects it is on the wing. Lobsters and crabs
have the auditory sacs at the base of the antennas.145

otolith in the center of a
cavity which is filled with
fluid, and whose walls are
lined by ciliated cells.

FIG. 348. — Brain and Auditory Apparatus of the Cuttlefish: a, b, brain; c, auditory
apparatus; d, the cavity in which it is lodged ; e,f,g, eyes ; i, 2, 3, otoliths.

A complex organ of hearing, located in the head,
exists in all vertebrates, save the very lowest fishes.

390

COMPARATIVE ZOOLOGY

As complete in man, it consists of the following parts :
i. The external ear (which is peculiar to mammals146);
the auditory canal, about an inch long, lined with hairs
and a waxy secretion, and closed at the bottom by a
membrane, called tympanum, or " drum of the ear."

2. The middle ear, con-
taining three little bones
(the smallest in the
body), malleus, incus,
and stapes, articulated
together. The cavity
communicates with the
external air by means
of the Eustachian tube,

FIG. 349- - Section of Human Ear : a, external which Opens at the back

part of the mouth.

3. The internal ear, or
labyrinth, an irregular
cavity in the solid part

of the temporal bone, and separated from the middle
ear by a bony partition, which is perforated by two
small holes. - The labyrinth consists of the vestibule, or
entrance ; the semicircular canals or tubes ; and the
cochlea, or spiral canal. While the other parts are full
of air, the labyrinth is filled with a liquid, and in this
are the ends of the auditory nerve. The vibrations of
the air, collected by the external ear, are concentrated
upon the tympanum, and thence transmitted through
the chain of little bones to the fluid in the labyrinth.

The essential organ of hearing is the labyrinth, which
is, substantially, a bag filled with fluid and nerve fila-
ments. Fishes generally have but little more. In
amphibians and reptiles there are added a tympanum, a
single bone, connecting this with the internal ear, the
cochlea, and the Eustachian tube, the tympanum being

ear, with auditory canal ; b, tympanic cavity
containing the three bones; c, hammer, and its
three muscles, d, e,f; g, tympanic membrane,
or head of the drum ; h, Eustachian tube lead-
ing to the pharynx ; z, labyrinth, with semi-
circular canals and cochlea visible.

THE NERVOUS SYSTEM

391

external. Birds have, besides, an auditory passage, open-
ing on a level with the surface of the head, and sur-
rounded by a circle of feathers. Most mammals have
an external ear.

Sight is the perception of light.147 In all animals it
depends upon the peculiar sensitiveness of the optic
organ to the luminous vibrations. In vertebrates the
optic nerve comes from the middle mass of the brain,
in invertebrates it is derived from a ganglion. Many
animals are utterly destitute of visual organs, as the
Protozoa, and the lower radiates and mollusks, besides
intestinal worms and the blind fishes and many cave-
animals ; but the protozoan Euglena has a red pigment
spot which is probably affected by light waves in a
manner different from that in which the rest of the body
is influenced. The eyes of the starfish are at the tips
of its arms or rays. Those of the sea urchin form a ring
at the dorsal pole of the body. Around the margin of
the jellyfish are colored spots, supposed to be rudimen-
tary eyes ; but, as a lens is wanting, there is no image ;
so that the creature can merely distinguish light from
darkness and color without form. Such an eye is
nothing but a collection of pigment granules on the
expansion of a nervous thread, and the
perception of light is probably the sensa-
tion of warmth, the pigment absorbing
the rays and converting them into heat.

Going higher, we find a lens introduced,
forming a distinct image. The snail, for FIG. 35o. — Eye of

, , . , 11 i Pecten, much en-

example, has two simple eyes, called iarged: m> mantle;
ocelli, mounted on the tip of its long !JensjJjf ™
tentacles, each consisting of a globular nerve-
lens,148 with a transparent skin (cornea) in front, and
a colored membrane (choroid) and a nervous network
(retina) behind. The scallop (Pecten) has such eyes in

392

COMPARATIVE ZOOLOGY

the edge of its mantle (Fig. 350). Such organs are the
only eyes possessed by myriapods, spiders, scorpions,

and caterpillars. Adult
insects usually have three
ocelli on the top of the
head. But the proper
visual organs of lobsters,
crabs, and insects are two
compound eyes, perched
on pedestals, or fixed on
the sides of the head.
They consist of an im-
mense number of ocelli
pressed together so that
they take an angular

FIG. 351. — Head of a Snail bisected, showing form foUT-Sided in

structure of tentacles: a, right inferior tentacle . . , , .

retracted within the body; b, right superior CrUStaCCa, SIX-Slded in

tentacle fully protruded ; c, left superior ten- • .-j-%1 r

tacle partially inverted; rf, left inferior ten- IHSCCtS. They form tWO

tacle;/, optic nerve ; g retractor muscle; roim(Je(J protuberances
ft, optic nerve in loose folds; /, retractor muscle

of head ; k, nerve and muscle of left inferior Variously Colored

tentacle; /, m, nervous collar. . .

white, yellow, red, green,

purple, brown, or black. Under the microscope, the
surface is seen to be divided into a host of facets,149
each being an ocellus complete in itself. Each cornea
is convex on one side, and either convex or flat on the
other, so that it produces a focus like
a lens. Behind the cornea, or lens, is
the pigment, having a minute aperture
or " pupil." Next is a conical tube
— one for each facet — with sides and
bottom lined with pigment. These
tubes converge to the optic ganglion,
the fibers of which pass through the FIG. 352. -Head of the Bee.

_ showing compound eyes,

tubes tO the COrnea.150 Vision by the three ocelli, or stem-
, , . mata, and the antennae

such a compound eye is not a mosaic ; (magnified).

THE NERVOUS SYSTEM

393

but each ocellus gives
a complete image,
although a different
perspective from its
neighbor. The mul-
tiplied images are re-
duced to one mental
stereoscopic picture,
on the principle of
single vision in our-
selves.

The eyes of the
cuttlefish are the
largest and the most

perfect among mver- FlG 353._EyeofaBeetle(A/*&/o«*A«): A,-section;
TheV re- a' °P^C ganglion ; b, secondary nerves ; c, retina;
d, pigment layer; e, proper optic nerves; B, group

the eyeS Of of ocelli (magnified); f, bulb of optic nerve; g, layer
•t • , • i • of pigment; k, vitreous humor; /, cornea.

higher animals in

having a crystalline lens with a chamber in front (open,
however, to the sea water), and a chamber behind it

filled with " vitre-
ous humor."

The eye of ver-
tebrates is formed
by the infolding of
the skin to create
a lens, and an out-
growth of the brain
to make a sensitive
layer ; both inclosed

FIG. 354. — Section of Human Eye : a. and b, upper and . , .

lower lid; c, conjunctiva, or mucous membrane, lining in a WJllte
the inner surface ; d, external membrane ; e, sheath of
optic nerve ; f, g, muscles for rolling the eye up or
down; ht sclerotic; i, transparent cornea; j, choroid; of tOUffh tiSSUC with
k, I, ciliary muscle for adjusting the eye for distance;

m, iris and pupil; «, canal; 0,retina; s, vitreous humor; a transparent front,
f, crystalline lens ; v, anterior chamber ; x. posterior , , , ,

chan;ber. called the cornea.

394

COMPARATIVE ZOOLOGY

This case is kept in shape by two fluids — the thin
aqueous humor filling the cavity just behind the cornea,

and the jelly like
vitreous humor oc-
cupying the larger
posterior chamber.
Between the two
humors lies the dou-
ble-convex crystal-
line lens. On the
front face of the
lens is a contractile
circularcurtain(mV),
with a hole in the
center (pupil} ; and
lining the sclerotic
coat is the choroid
membrane, covered
with dark • pigment.
The optic nerve, en-
tering at the back of
the eye through the
sclerotic and choroid
coats, expands into
the transparent re-
tina, which consists
of several layers —
fibrous, cellular, and

FIG. 355. — Section of the Human Retina, x 400

i, internal limiting membrane ; 2, optic-nerve fibers granular. The mOSt

3, ganglion cells; 4, internal molecular layer Q~nQ1V:vp nqri- ,'<, f-Up

5, internal granules ; 6, external molecular layer benblUVC part Ib LUC

7, external granules ; 8, external limiting membrane surf aCC IvinST next tO
9, layer of rods and cones; 10, pigment layer. "Jo

the black pigment.

And here is a peculiarity of the vertebrate eye : the
nerve fibers, entering from behind, turn back and look
toward the bottom of the eye, so that vision is directed

THE NERVOUS SYSTEM

395

backward ; while invertebrate vision is directly forward.
In vertebrates only, the optic nerves cross each other
(decussate) in passing from the brain to the eyes ; so that
the right side of the brain, e.g., receives the impressions
of objects on the left side of the body.151

Generally, the eyes of vertebrates are on opposite
sides of the head ; but in the flatfishes both are on the
same side. Usually, both eyes see the same object at
once; but in most fishes the eyes are set so far back,
the fields of vision are distinct. The cornea may be
flat, and the lens globular, as in fishes ; or the cornea
very convex, and the lens flattened, as in owls. Purely
aquatic animals have neither eyelids nor tears, but nearly
all others (especially birds) have three lids.152 The pupil
is usually round; but it may be rhomb-shaped, as in
frogs ; vertically oval, as in crocodiles and cats ; or
transversely oval, as in geese, doves, horses, and rumi-
nants. Many quadrupeds, as the cat, have a membrane
(tapetum) lining the bottom of the eyeball, with a brilliant
metallic luster, usually green or pearly ; it is this which
makes the eyes of such animals luminous in the dark.

/

2. Instinct and Intelligence

The simplest form of nervous excitement is mere
sensation. Above this we have sensation awakening
consciousness, out of which come those voluntary activi-
ties grouped together under the name of Instinct ; and,
finally, Intelligence.

The lowest forms of life are completely mechanical,
for their movements seem to be due solely to their
organization. They are automatons, or creatures of
necessity. In the higher animals certain actions are
automatic, as breathing, the beating of the heart, the
contractions of the iris, and all the first movements of

396

COMPARATIVE ZOOLOGY

an infant.153 But, generally, the actions of animals are
not the result of mere bodily organization.

The inferior orders are under the control of Instinct,
i.e., an apparently untaught ability to perform actions
which are useful to the animal.154 They seem to be
born with a measure of knowledge and skill (as man
is said to have innate ideas), acquired neither by reason
nor experiment. For what could have led bees to
imagine that by feeding a worker larva with royal jelly,
instead of beebread, it would turn out a queen instead
of a neuter? In this case, neither the habit nor the
experience could be inherited, for the worker bees are
sterile. We can only guess that the discovery has been
communicated by the survivors of an older swarm.
Uniformity is another characteristic feature of instinct.
Different individuals of the same species execute pre-
cisely the same movements under like circumstances.
The career of one bee is the career of another. We do
not find one clever and another stupid. Honeycombs
are built now as they were before the Christian era.
The creatures of pure instinct appear to be tied down,
by the constitution of their nervous system, to one line
of action, from which they can not spontaneously depart.
The actions vary only as the structure changes.155 There
is a wonderful fitness in what they do, but there is no
intentional adaptation of means to ends.

All animals, from the starfish to man, are guided more
or less by instinct ; but the best examples are furnished
by the insect world, especially by the social hymenop-
ters (ants, bees, and wasps). The butterfly carefully
provides for its young, which it is destined never to
see ; many insects feed on particular species of plants,
which they select with wonderful sagacity ; and monkeys
avoid poisonous berries ; bees and squirrels store up
food for the future ; bees, wasps, and spiders construct

THE NERVOUS SYSTEM

with marvelous precision ; and the subterranean cham-
bers of ants and the dikes of the beaver show engineer-
ing skill ; while salmon go from the ocean up the rivers
to spawn ; and birds of the temperate zones migrate
with great regularity.

But in the midst of this automatism there are the
glimmerings of intelligence and free will. We see some
evidence of choice and of designed adaptation. Pure
instinct should be infallible. Yet we notice mistakes
that remind us of mental aberrations. Bees are not so
economical as has been generally supposed. A mathe-
matician can make five cells with less wax than the bee
uses for four; while the bumblebee uses three times
as much material as the hive bee. An exact hexagonal
cell does not exist in nature. Flies lay eggs on the car-
rion plant because it happens to have the odor of putrid
meat. The domesticated beaver will build a dam across
its apartment. Birds frequently make mistakes in the
construction and location of their nests. In fact, the
process of cheating animals relies on the imperfection
of instinct. Nor are the actions of the brute creation
always perfectly uniform; and, so far as animals con-
form to circumstances, they act from intelligence, not
instinct. There is proof that some animals profit by
experience. Birds do learn to make their nests ; and
the older ones build the best. Trappers know well that
young animals are more easily caught than old ones.
Birds brought up from the egg, in cages, do not make
the characteristic nests of their species ; nor do they
have the same song peculiar to their species, if they
have not heard it. Chimney swallows certainly built
their nests differently in America three hundred years
ago. A bee can make cells of another shape, for it some-
times does ; its actions, therefore, being elective and con-
ditional, are in a measure the result of calculation.

398 COMPARATIVE ZOOLOGY

The mistakes and variations of instinct are indications
that animals have something more — a limited range of
that principle of Intelligence so luminous in man. No
precise line can be drawn between instinctive and intel-
ligent acts ; all we can say is, there is more freedom of
choice in the latter than the former ; and that some ani-
mals are most instinctive, others most intelligent. Thus,
we speak of the instinct of the ant, bee, and beaver, and
the intelligence of the elephant, dog, and monkey. In-
stinct loses its peculiar character as intelligence becomes
developed. Ascending from the worm and oyster to
the bee, we see the movements become more complex in
character and more special in their objects ; but instinct
is supreme. Still ascending, we observe a gradual fad-
ing away of the instincts, till they become subordinate to
higher faculties — will and reason. We can predict with
considerable certainty the actions of animals guided by
pure instinct; but in proportion as they possess the
power of adapting means to ends, the more variable their
actions. Thus, the architecture of birds is not so uniform
as that of insects.156

We must credit brutes with a certain amount of obser-
vation and imitation, curiosity and cunning, memory and
reason. Animals have been seen to pause, deliberate,
or experiment and resolve. The elephant and horse,
dog and monkey, particularly, participate in the rational
nature of man, up to a certain point. Thinking begins
wherever there is an intentional adaptation of means to
ends ; for that involves the comparison and combination
of ideas. Animals interchange ideas : the whine of a
dog at the door on a cold night certainly implies that he
wants to be let in. Bees and ants, it is well known, con-
fer by touching together their antennae. All the higher
animals, too, have similar emotions : — as joy, fear, love,
and anger.

THE NERVOUS SYSTEM

399

While instinct culminates in insects, the highest devel-
opment of intelligence is presented in man.167 In man
only does instinct cease to be the controlling power.
He stands alone in having the whole of his organization
conformed to the demands of his brain; and his intelli-
gent acts are characterized by the capacity for unlimited
progress. The brutes can be improved by domestica-
tion ; but, left to themselves, they soon relapse into their
original wildness. Civilized man also goes back to
savagery ; yet man (though not all men) has the ambi-
tion to exalt his mental and moral nature. He has a
soul, or conscious relation to the infinite, which leads
him to aspire after a lofty ideal. Only he can form
abstract ideas. And, finally, he is a completely self-
determining agent, with a prominent will and conscience
— the highest attribute of the animal creation. In all
this, man differs profoundly from the lower forms of
life.

» 3. The Voices of Animals

Most aquatic animals are mute. Some crabs make
noises by rubbing their fore legs against their carapace ;
and many fishes produce noises in various ways, mostly
by means of the swim bladder. Insects are the inverte-
brates which make the most noise. Their organs are
usually external, while those of vertebrates are internal.
Insects of rapid flight generally make the most noise.
In some the noise is produced by friction (stridulation) ;
in others, by the passage of air through the spiracles
(humming). The shrill notes of crickets and grasshop-
pers are produced by rubbing the wings against each
other, or against the thighs ; but the cicada, or harvest
fly, has a special apparatus — a tense membrane on the
abdomen, acted upon by muscles. The buzzing of flies
and humming of bees are caused, in part, by the vibra-

400

COMPARATIVE ZOOLOGY

tions of the wings ; but the true voice of these insects
comes from the spiracles of the thorax.

Snakes and lizards have no vocal cords, and can only
hiss. Frogs croak 158 and crocodiles roar, and the huge
tortoise of the Galapagos Islands utters a hoarse, bellow-
ing noise.

The vocal apparatus in birds is situated at the lower
end of the trachea, where it divides into the two bronchi.159
It consists mainly of a bony drum, with a cross bone,
having a vertical membrane attached to its upper edge.
The membrane is put in motion by currents of air pass-
ing on either side of it. Five pairs of muscles (in the
songsters) adjust the length of the windpipe to the
pitch of the glottis. The various notes are produced
by differences in the blast of air, as well as by changes
in the tension of the membrane. The range of notes
is commonly within an octave. Birds of the same
family have a similar voice. All the parrots have a
harsh utterance; geese and ducks quack ; crows, magpies,
and jays caw ; while the warblers differ in the quality,
rather than the kind, of note.160 The parrot and mock-
ing bird use the tongue in imitating human sounds.
Some species possess great compass of voice. The bell-
bird can be heard nearly three miles ; and Livingstone
said he could distinguish the voices of the ostrich and
the lion only by knowing that the former roars by day,
and the latter by night.

The vocal organ of mammals, unlike that of birds, is
in the upper part of the larynx. It consists of four
cartilages, of which the largest (the thyroid) produces the
prominence in the human throat known as " Adam's
apple," and two elastic bands, called " vocal cords,"
just below the glottis, or upper opening of the wind-
pipe. The various tones are determined by the tension
of these cords, which is effected by the raising or lower-

THE NERVOUS SYSTEM

401

ing of the thyroid cartilage, to which one end of the
cords is attached. The will cannot influence the con-
traction of the vocalizing muscles, except in the very
act of vocalization'. The vocal sounds produced by
mammals may be distinguished into the ordinary voice,
the cry, and the song. The second is the sound made
by brutes. The whale, porpoise, armadillo, ant-eater,
porcupine, and giraffe are generally silent. The bat's
voice is probably the shrillest sound audible
to human ears. There is little modulation
in brute utterance. The opossum purrs,
the sloth and kangaroo moan, the hog
grunts or squeals, the tapir whistles, the
stag bellows, and the elephant gives a
hoarse trumpet sound from its trunk and
a deep groan from its throat. All sheep FIG. 356. -Human
have a guttural voice ; all the ox family ^J™/ ™a£
low, from the bison to the musk ox; all °f the hy°id

. -Hi bone ; e, trachea ;

the horses and donkeys neigh ; all the cats /, esophagus; s,
miau, from the domestic animal to the lion ; ^s10"15-
all the bears growl ; and all the canine" family — fox,
wolf, and dog — bark and howl. The howling monkeys
and gorillas have a large cavity, or sac, in the throat for
resonance, enabling them to utter a powerful voice ; and
one of the gibbon apes has the remarkable power of emit-
ting a complete octave of musical notes. The human
voice, taking the male and female together, has a range
of nearly four octaves. Man's power of speech, or the
utterance of articulate sounds, is due to his intellectual
development rather than to any structural difference
between him and the apes. Song is produced by the
vocal cords, speech by the mouth.

DODGE'S GEN. ZOOL. — 26

CHAPTER XXII

REPRODUCTION

IT is a fundamental truth that every living organism
has had its origin in some preexisting organism. The
doctrine of " spontaneous generation," or the supposed
origination of organized structures out of inorganic par-
ticles, or out of dead organic matter, has not yet been
sustained by facts.

Reproduction is of two kinds — sexual and asexual.
All animals, probably, have the first method, while a
very great number of the lower forms of life have the
latter also.

Of asexual reproduction there are two kinds — Self-
division (Fission} and Budding.

Self -division, the simplest mode possible, is a natural
breaking-up of the body into distinct surviving parts.
This process is sometimes extraordinarily rapid, the
increase of one animalcule (Paramecium) being com-
puted at 268 millions in a month. It may be either
transverse or longitudinal. Of the first sort, Fig. 10 is
an example ; of the latter, Fig. 1 1 , a. This form of re-
production is, naturally, confined to animals whose tis-
sues and organs are simple, and so can easily bear
division, or whose parts are so arranged as to be easily
separable without serious injury. The process is most
common in Protozoa, worms, and polyps.

Budding is separated by no sharp line from self-
division. While in the latter a part of the organs of
the parent go to the offspring, in the former one or

402

REPRODUCTION

403

more cells of the original animal begin to develop and
multiply so as to grow into a new animal like the
parent. The process in animals is quite akin to the
same operation in plants. The buds may remain per-
manently attached to the parent stock, thus making
a colony, as in corals and Bryozoa {continuous budding),
or they may be detached at some stage of growth (dis-
continuous budding). This separation may occur when
the bud is grown up, as in hydra (Fig. 18), or as in plant
lice, daphnias (Fig. 56), and among other animals the
buds may be internal, becoming detached when entirely
undeveloped and externally resembling an egg. They
differ, however, entirely from a true egg in developing
directly, without fertilization.

Sexual Reproduction requires cells of two kinds,
usually from different animals. These are the germ
cell or egg, and the sperm cell. The embryo is devel-
oped from the cells which are formed by the repeated
divisions of the ovum which take place as a result of its
union with the sperm cell.161

The egg consists essentially of three parts, the
germinal vesicle, the yolk, and the vitelline membrane,
which surrounds both the first. It is ordinarily globular
in shape. Of the three parts, the primary one is the
germinal vesicle — a particle of protoplasm. The yolk
serves as food for this, and the membrane protects both.
When a great mass of yolk is present it is divisible into
two parts — formative and food yolk. The latter is of
a more oily nature than the former, and is usually not
segmented with the egg. The structure of the hen's
egg is more complicated. The outside shell consists of
earthy matter (lime) deposited in a network of animal
matter. It is minutely porous, to allow the passage to
and fro of vapor and air. Lining the shell is a double
membrane (membrana putaminis) resembling delicate

404

COMPARATIVE ZOOLOGY

tissue paper. At the larger end it separates to inclose
a bubble of air for the use of the chick. Next comes
the albumen, or " white," in spirally ar-
ranged layers, within which floats the
yolk. The yolk is prevented from mov-
ing toward either end of the egg by
two twisted cords of albumen, called
chalazce ; yet is allowed to rise toward
with inclosed cyto- one side, the yolk being lighter than the

plasm; n, nucleus, J

consisting of nuclear albumen. 1 he yolk is composed or oily
gra^ui^substa'nceln granules (about gio" of an inch in diam-
which are seen a Qfcr\ and js inclosed in a sac, called

spherical nucleolus '

and several irregular the vitelline membrane, and disposed in

masses of chro- . . ... .

matin; a, attraction concentric layers, like a set of vases

centroesomrining * PlaCed OIle wlthm the °ther- That Part

of the yolk which extends from the
center to a white spot (cicatricula) on the outside can not
be hardened, even with the most prolonged boiling.
The cicatricula, or embryo spot, is a thin disk of cellular
structure, in which the new life first appears. This was
originally a simple cell, but development has gone some

FIG. 358. — Longitudinal section of .Hen's Egg before incubation: « , yolk, showing con-
centric layers; a , its semifluid center, consisting of a white granular substance —
the whole yolk is inclosed in the vitelline membrane ; b, inner dense part of the
albumen; b' , outer, thinner part; c, the chalaza, or albumen, twisted by the revolu-
tions of the yolk ; d, double shell membrane, split at the large end to form the
chamber,// e, the shell; h, the white spot, or cicatricula.

REPRODUCTION

405

way before the egg is laid. It is always on that side
which naturally turns uppermost, for the yolk can turn
upon its axis ; it is, therefore, always nearest to the
external air and to the hen's body — two conditions
necessary for its development. There is another reason
for this polarity of the egg : the lighter and more deli-
cate part of the yolk is collected in its upper region,
while the heavy, oily portion remains beneath.

In most eggs the shell and albumen are wanting.
When the albumen is present, it is commonly covered
by a membrane only. In sharks the envelope is horny ;
and in crocodiles is calcareous, as in
birds.

The egg of the sponge has no true
vitelline membrane, and is not unlike
an ordinary amoeboid cell. An egg
is, in fact, little more than a very large
cell, of which the germinal vesicle is
the nucleus.

The size of an egg depends mainly FIG. 359. - Egg of Sponge
upon the quantity of yolk it contains ; (magnifi< i: "• nucleus'
and to this is proportioned the grade of development
which the embryo attains when it leaves the egg.162 In
the eggs of the starfishes, worms, insects, mollusks (ex-
cept the cuttlefishes), many amphibians, and mammals,
the yolk is very minute and formative, i.e., it is con-
verted into the parts of the future embryo. In the
eggs of lobsters, crabs, spiders, cephalopods, fishes,
reptiles, and birds, the yolk is large and colored, and
consists of two parts — the formative, or germ yolk,
immediately surrounding the germinal vesicle ; and the
nutritive, or food yolk, constituting the greater part of
the mass, by which the young animal in its egg life is
nourished. In the latter case, the young come forth
more mature than when the food yolk is wanting.

406 COMPARATIVE ZOOLOGY

As to form, eggs are oval or elliptical, as in birds and
crocodiles ; spherical, as in turtles and wasps ; cylindri-
cal, as in bees and flies ; or shaped like a handbarrow,
with tendrils on the corners, as in the shark. The eggs
of some very low forms are sculptured or covered with
hairs or prickles.

The number of eggs varies greatly in different ani-
mals, as it is in proportion to the risks during develop-
ment Thus, the eggs of aquatic tribes, being unprotected
by the parent, and being largely consumed by many

FIG. 360. — Egg of a Shark (the external gills of the embryo are not represented).

animals, are numerous to prevent extinction. The spawn
of a single cod contains millions of eggs ; that of the
oyster, 6,000,000. A queen bee, during the five years
of her existence, lays about a million eggs.

Eggs are laid one by one, as by birds ; or in clusters,
as by frogs, fishes, and most invertebrates. The spawn
of the sea snails consists of vast numbers of eggs
adhering together in masses, or in sacs, forming long
strings.

As a rule, the higher the rank, the more care animals
take of their eggs and their young, and the higher the
temperature needed for egg development. In the major-
ity of cases, eggs are left to themselves. The fresh-
water mussel (Unid) carries them within its gills, and

REPRODUCTION

the lobster under its tail. The eggs of many spiders
are enveloped in a silken cocoon, which the mother
guards with jealous care. Insects, as flies and moths,
deposit their eggs where the larva, as soon as born, can
procure its own food. Most fishes allow their spawn,
or roe, to float in the water ; but a few build a kind of
flat nest in the sand or mud, hovering over the eggs
until they are hatched ; while the Acara of the Amazon
carries them in its mouth. The amphibians, generally,
envelop their eggs in a gelatinous mass, which they
leave to the elements ; but the female of the Surinam
toad carries hers on her back, where they are placed by
the male. The great Amazon turtles lay their eggs in
holes two feet deep, in the sand ; while the alligators
simply cover theirs with a few leaves and sticks.
Nearly all birds build nests, those of the perchers being
most elaborate, as their chicks are dependent for a
time on the parent.163 The young of marsupials, as the
kangaroo, which are born in an extremely immature
state, are nourished in a pouch outside of the body.
But the embryo of all other mammals is developed
within the parent to a more perfect condition, by means
of a special organ, the placenta. It is a general law,
that animals receiving in the embryo states the longest
and most constant parental care ultimately attain the
highest grade of development.

The Protozoa, which have no true eggs, have a sort
of reproduction called conjugation. In this process two
individuals unite into one mass, surround themselves
with a case, in which they divide into several parts,
each portion becoming a new individual, or the process
may be followed by repeated divisions of the two
individuals which separate as soon as the process is
finished, as in Paramecitim, or remain fused together, as
in Vorticella.

408 COMPARATIVE ZOOLOGY

The sperm cells differ from the egg in being very
small, commonly motile, and in that a large number are
usually produced from a single primary reproductive
cell, while the egg represents the entire primary cell.
The union of the sperm cell with the germinal vesicle
{fertilization) is the first step in development, and with-
out it the egg will not develop normally.

CHAPTER XXIII*

DEVELOPMENT

Development is the evolution of a germ into a com-
plete organism. The study of the changes in the
developing embryo constitutes the science of Embry-
ology ; the transformations after the egg life are called
metamorphoses, and include growth and repair.

The process of development is a passage from the
general to the special, from the simple to the complex,
from the homogeneous to the heterogeneous, by a series
of differentiations. It brings out
first the profounder distinctions,
and afterward those more external. a * t> mo c

„, , . i FIG. 361. — Fertilization and

That IS, the mOSt essential partS segmentation of mammalian

appear first. And not Only does ovum: ™, spermatozoon;

J n, nucleus ; nu, nucleo-

development tend to make the sev- lus ; *, z°n* nuiiata ; cit

. . segmenting cell.

eral organs of an individual more
distinct from one another, but also the individual itself
more distinguished from other individuals and from the
medium in which it lives. With advancing develop-
ment, the animal, as a rule, acquires a more specific,
definite form, and increases in weight and locomotive
power. Life is a tendency to individuality.

The first step in development, after fertilization, is the
segmentation of the egg, by a process of self-division.
In the simplest form, the whole yolk divides into two
parts ; these again divide repeatedly, making four, eight,
sixteen, etc., parts, until the whole yolk is subdivided

* See Appendix.
409

4io

COMPARATIVE ZOOLOGY

into very small portions (cells) surrounding a central
cavity. This stage is known as the " mulberry mass,"
or blastula (Fig. 361, c). If the yolk is larger, relatively
to the germinal vesicle, the process of division may go
on more slowly in one of the two parts of the egg, first
formed ; or in very large eggs, like those of birds and
cuttlefishes, only a small part of the yolk subdivides.

In some form, the process of segmentation is found in
the eggs of all animals, as is also the following stage.
This step is the differentiation of the single layer of
cells into two parts, one for the body wall, the other for
the wall of the digestive tract. In
the typical examples, this ' is ac-
complished by one part of the
wall of the blastula turning in so
far as to convert the blastula into
a sort of double-walled cup, the
gastrula (Fig. 362). One half of

FIG. 362. - Diagram of Gastrula \ f * '

of a worm ($«£*#«):«, prim- the wall of the blastula is now the
outer wall of the germ, the other

body cavity ; en, endoderm ; foalf that Qf ^Q digestive Cavity I
ec, ectoderm.

the original blastula cavity is now

the body cavity, the new cavity formed by the infold-
ing is the stomach and its opening is both mouth and
vent (Fig. 362). Some adult animals are little more
than such a sac. Hydra (Fig. 18), for instance, is
little different from a gastrula with tentacles, and one
of its relatives wants even these additions.

Ordinarily, however, development goes much further.
From the two original layers arises, in various ways, a
third between them, making the three primitive germ
layers — epiblast, mesoblast, and hypoblast. This new
layer is necessarily in the primitive body cavity, which
it may fill up ; or usually a new body cavity is formed,
in different ways in different groups. In by far. the

DEVELOPMENT 411

great majority of animals the digestive tract gets a new
opening, which usually becomes the mouth ; and the old
mouth may close, or serve only the functions of the
vent. From this point the development of each group
must be traced in detail.

Development of a Hen's Egg. —After the segmentation,
the germinal disk divides into two layers, between which
a third is soon formed. The upper layer (epiblast) gives
rise to the epidermis, brain, spinal cord, retina, crystalline
lens, and internal ear. From the lower layer (hypoblast)
is formed the epithelium of the digestive canal. From
the middle layer (mesoblast) come all the other organs
— muscles, bones, blood vessels, etc. The mesoblast

FIG. 363. — Transverse vertical sections of an egg, showing progressive stages of develop-
ment: a, notochord ; b, medullary furrow, becoming a closed canal in the last.

thickens so as to form two parallel ridges running length-
wise of the germ, and leaving a groove between them
(medullary furrow and ridges)™ The ridges gradually
rise, carrying with them the epiblast, incline toward each
other, and at last unite along the back. So that we
have a tube of epiblast surrounded by mesoblast, which
is itself covered by epiblast. This tube becomes the
brain and spinal cord, whose central canal, enlarging
into the ventricles of the brain, tells the story of its
original formation. Beneath the furrow, a delicate
cartilaginous thread appears (called notochord} — the
predecessor of the backbone. Meanwhile the mesoblast
has divided into two layers, except in the middle of the
animal, beneath the spinal cord, and in the head. One
of these layers remains attached to the epiblast, and

4I2 COMPARATIVE ZOOLOGY

with it forms the body wall; the other bends rapidly
downward, carrying the hypoblast with it, and forms
the wall of the intestine. The space thus left between
the layers of the mesoblast is the body cavity. At the
same time, the margin of the germ extends farther and
farther over the yolk, till it completely incloses it. So
that now we see two cavities — a small one, containing
the nervous system ; and a larger one below, for the
digestive organs. Presently, numerous rows of cor-
puscles are seen on the middle layer, which are subse-
quently inclosed, forming a network of capillaries, called
the vascular area. A dark spot indicates the situation
of the heart, which is the first distinctly bounded cavity

of the circulatory
system. It is a

short tube ^ing
lengthwise just be-

FIG. 364. — Rudimentary Hearts, human: i, venous hind the head, with
trunks; a, auricle; 3, ventricle ; 4, bulbus arteripsus.

causing the blood to flow backward and forward.
The tube is gradually bent together, until it forms a
double cavity, resembling the heart of a fish. On the
fourth day of incubation partitions begin to grow, divid-
ing the cavities into the right and left auricles and
ventricles. The septum between the auricles is the last
to be finished, being closed the moment respiration
begins. The blood vessels ramify in all directions
over the yolk, absorbing its substance, and all perform-
ing the same office ; it is not till the fourth or fifth day
that arteries can be distinguished from veins, by being
thicker, and by carrying blood only from the heart.165

The embryo lies with its face, or ventral surface,
toward the yolk, the head and tail curving toward
each -other. Around the embryo on all sides the epi-
blast and upper layer of the mesoblast rise like a hood

DEVELOPMENT

413

over the back of the embryo till they form a closed sac,
called the amnion. It is filled with a thin liquid, which
serves to protect the embryo. Meanwhile, another im-
portant organ is forming on the other side. From the

FlG. 365. — Embryo in a Hen's Egg during the first five days, longitudinal view :
A, hypoblast ; B, lower layer of mesoblast; C, upper layer of mesoblast and epiblast
united, in the last figures forming the amniotic sac ; 'D, vitelline membrane ;
e, thickened blastoderm, the first rudiment of the dorsal part (in the last figure it
marks the place of the lungs); h, heart; a, b, its two chambers ; c, aortic arches ;
m, aorta; /, liver; /, allantois.

COMPARATIVE ZOOLOGY

FIG. 366. — The hen's egg
at the end of the ninth

up in order to show its
shape more clearly :
an, inner or true am-
nion ; hm, hyomandibu-

hinder portion of the alimentary canal an outgrowth is
formed which extends beyond the wall of the embryo
— ta hm proper into the cavity of the amnion,

and spreads out over the whole inner
surface of the shell, so that it partly
surrounds both embryo and inner
layer of the amnion (amnion proper).
This is the .allantois. It is full of
blood vessels, and it serves as the
embryo naturally lies with respiratorv organ until the chick

its left side on the yolk J &

sac, but has been lifted picks the shell and breathes by its
lungs.166 The chorion is the outer-
most part of the allantois, and the

iar cleft; sv, air chamber; placenta of mammals is the shaggy,

ta, allantois; -wa, white

or albumen ; ys, yolk VaSCular edge of the chOROn.

The alimentary canal is at first a

straight tube closed at both ends, the middle being
connected with the yolk sac. As it grows faster than
the body, it is thrown into a spiral coil ; and at several
points it dilates, to form the crop, stomach, gizzard, etc.
The mouth is developed from an infolding of the skin.
The liver is an outgrowth from
the digestive tube, at first a clus-
ter of cells, then of follicles, and
finally a true gland. The lungs
are developed on the third day
as a minute bud from the upper
part of the alimentary canal, or
pharynx. As they grow in size, FlG- 367- - view of embryo with

1 its foetal membrane : am, am-

they paSS from a SmOOth tO a nion proper ; d, dwindled yolk

, , | , . . sac ; al, allantois ; al*, subzonal

Cellular Condition. membrane; z, villi ; outside the

The skeleton at the beginning S^^nStok^h^
consists, like the notochord^ of a blast, z'.
cellular material, which gradually turns to cartilage.
Then minute canals containing blood vessels arise, and

DEVELOPMENT

415

earthy matter (chiefly phosphate of lime) is deposited
between the cells. The primary bone thus formed is
compact : true osseous tissue, with canaliculi, laminae,
and Haversian canals, is the result of subsequent ab-
sorption.167 Certain bones, as those of the face and
cranium, are not preceded by cartilage, but by connec-
tive tissue ; these are called membrane bones. Ossi-
fication, or bone making, begins at numerous distinct
points, called centers of ossification ; and, theoretically,
every center stands for a bone, so that there are as many
bones in a skeleton as centers of ossification. But the
actual number in the adult animal is much smaller, as
many of the centers coalesce.168 The development of
the backbone is not from the head or from the tail, but
from a central point midway between ; there the first
vertebrae appear, and from there they multiply forward
and backward.

The limbs appear as buds on the sides of the body ;
these lengthen and expand so as to resemble paddles —
the wings and legs looking precisely alike ; and, finally,
they are divided each into three segments, the last one
subdividing into digits. The feathers are developed
from the outside cells of the epidermis : first, a horny
cone is formed, which elongates and spreads out into a
vane, and this splits up into barbs and barbules.

The muscle fibers are formed either by the growth in
length of a single cell, or by the coalescence of a row of
cells ; the cell wall thus produces a long tube — the sar-
colemma of a fiber — and the granular contents arrange
themselves into linear series, to make fibrillae.

Nervous tissue is derived from the multiplication and
union of embryo cells. The white fibers at first resem-
ble the gray. The brain and spinal marrow are devel-
oped from the epiblastic lining of the medullary furrow.
Soon the brain, by two constrictions, divides into fore

4I6 COMPARATIVE ZOOLOGY

brain, mid brain, and hind brains The fore brain throws
out two lateral hemispheres (cerebrum), and from these
protrude forward the two olfactory lobes. From the
mid brain grow the optic lobes ; and the hind brain is
separated into cerebellum and medulla oblongata. The
essential parts of the eye, retina and crystalline lens,
are developed, the former as a cuplike outgrowth from
the fore brain, the latter as an ingrowth of the epider-
mis. An infolding of the epidermis gives rise to the
essential parts of the inner ear, and from the same layer
come the olfactory rods of the nose and the taste buds
of the tongue. So that the central nervous system and
the essential parts of most of the sense organs have a
common origin.

Modes of Development. — The structure and embryol-
ogy of a hen's egg exhibit many facts which are common
to all animals. But every grand division of the animal
kingdom has its characteristic method of developing.

Protozoans differ from all higher forms in having no
true eggs.

The egg of the hydroid, after segmentation, becomes
a hollow, pear-shaped body, covered with cilia. Soon
one end is indented ; then the indentation deepens until
it reaches the interior and forms the mouth. The ani-
mal fastens itself by the other end, and the tentacles
appear as buds. In the sea anemone, the stomach is
turned in, and the partitions appear in pairs.

In the oyster, the egg segments into two unequal
parts, one of which gives rise to the digestive tract and
its derivatives, while from the smaller part originate the
skin, gills, and shell. It is soon covered with cilia, by
whose help it swims about.

The embryo of an insect shows from the first a right
and left side ; but the first indication that it is an articu-
late is the development of a series of indentations divid-

DEVELOPMENT

ing the body into successive rings, or joints. Next, we
observe that the back lies near the center of the egg, the
ventral side looking outward, i.e., the embryo is doubled
upon itself backward. And, finally, the appearance of
three pairs of legs proves that it will be an insect, rather
than a worm, crustacean, or spider.

The vertebrate embryo lies with its stomach toward
the yolk, reversing the position of the articulate ; but
the grand characteristic is the medullary groove, which
does not exist in the egg of any invertebrate. This
feature is connected with another, the setting apart of
two distinct regions — the nervous and nutritive. There
are three modifications of vertebrate development : that
of fishes and amphibians, that of true reptiles and
birds, and that of mammals. The amnion and allantois
are wanting in the first group ; while the placenta (which
is the allantois vitally connected with the parent) is pe-
culiar to mammals. In mammals, the whole yolk is seg-
mented ; in birds, segmentation is confined to the small
white speck (blastoderm] seen in opening the shell.

At the outset, all animals, from the sponge to man,
are structurally alike. All, moreover, undergo segmen-
tation, and most have one form or other of the gastrula
stage. But while vertebrates and invertebrates can
travel together on the same road up to this point, here
they diverge — never to meet again. For every grand
group early shows that it has a peculiar type of con-
struction. Every egg is from the first impressed with
the power of developing in one direction only, and never
does it lose its fundamental characters. The germ of
the bee is divided into segments, showing that it belongs
to the articulates ; the germ of the lion has the medul-
lary furrow — the mark of the coming vertebrate. The
blastodermic layer of the vertebrate egg rolls up into
two tubes — one to hold the viscera, the other to con-
DODGE'S GEN. ZOOL. — 27

4I8 COMPARATIVE ZOOLOGY

tain the nervous cord : while that of the invertebrate
egg forms only one such tubular division. The features
which determine the branch to which an animal belongs
are first developed, then the characters revealing its
class.

There are differences also in grade of development as
well as type. For a time there is no essential difference
between a fish and a mammal; they have similar ner-
vous, circulatory, and digestive systems. There are
many such cases, in which the embryo of an animal
represents the permanent adult condition of some lower
form. In other words, the higher species, in the course
of their development, offer likenesses, or analogies, to
finished lower species. The human germ at first re-
sembles that of all other metazoa in that it is a single
cell. In the course of its development, the appearance
of a medullary furrow excludes it at once from all inver-
tebrates. It afterward has, for a time, structures found
as permanent organs in the lower classes and orders of
vertebrates. For a time, indeed, the human embryo so
closely resembles that of the lower forms as to be indis-
tinguishable from them ; but certain structures belong-
ing to those forms are kept long after the embryo is
clearly human. For instance, the embryos of birds and
mammals at an early stage have gill slits, like fishes.
Not all the members of a group reach the same degree
of perfection, some remaining in what corresponds to
the immature stages of the higher animals. Such may
be called permanently embryonic forms.

Sometimes an embryo develops an organ in a rudimen-
tary condition, which is lost or useless in the adult.
Thus, the Greenland whale, when grown up, has not a
tooth in its head, while in the embryo life it has teeth in
both jaws ; unborn calves have canines and upper in-
cisors ; and the female dugong has tusks which never

DEVELOPMENT

cut the gum. The " splint bones " in the horse's foot
are undeveloped metatarsals.

Animals differ widely in the degree of development
reached at ovulation and at birth. The eggs of frogs
are laid when they can hardly be said to have become
fully formed as eggs, since they undergo still further
change in the water. The eggs of birds are laid when
segmentation is far advanced, while the eggs of mam-
mals are retained by the parent till after the egg stage
is passed.169 Ruminants and terrestrial birds are born
with the power of sight and locomotion. Most carni-
vores, rodents, and perching birds come into the world
blind and helpless ; while the human infant is depen-
dent for a much longer time.

I . Metamorphosis

Few animals come forth from the egg in perfect con-
dition. The vast majority pass through a great variety
of forms before reaching maturity. These metamor-
phoses (which are merely periods of growth) are not
peculiar to insects, though more apparent in them.
Man himself is developed on the same general principles
as the butterfly, but the transformations take place
gradually. The coral, when hatched, has six pairs of
partitions ; afterward, the spaces are divided by six
more pairs; then twelve intermediate pairs are intro-
duced ; next, twenty-four, and so on. The embryonic
starfish has a long body, with six arms on a side, in one
end of which the young starfish is developed. Soon the
twelve-armed body is absorbed, and the young animal is
perfectly formed. Worms are continually growing by
the addition of new segments. Nearly all insects
undergo complete metamorphosis, i.e., exhibit four dis-
tinct stages of existence — egg, larva, pupa, and imago.

420

COMPARATIVE ZOOLOGY

The wormlike larva 17° may be called a locomotive egg.

It has little resemblance to the parent in structure or

habits, eating and
growing rapidly.
Then it enters the
pupa state, wrapping
itself in a cocoon, or
case, and remaining
apparently dead till
new organs are devel-
oped, when it escapes
a perfect winged in-
sect, or imago.171
Wings never exist ex-
ternally in the larva ;
and some insects

i 10.368. — Milkweed butterfly: A, head of young larva;
B, larva; C, pupa; D, imago; E, egg (magnified), which Undergo no

apparent metamorphosis, as lice, are wingless. The

3

FIG. 369. — Metamorphosis of the Mosquito (Culex pipiens}'. A, boat of eggs ; B, some
of the eggs highly magnified ; d, with lid open for the escape of the larva, C ;
D, pupa; E, larva magnified, showing respiratory tube, e, anal fins, f, antennae, £•;
F, imago; a, antennae; b, beak.

DEVELOPMENT

421

grasshopper develops from the young larva to the
winged adult without changing its mode of life. In
the development of the common crab, so different is
the outward form of the newly hatched embryo from
that of the adult, that the former has been described as
a distinct species.

The most remarkable example of metamorphosis
among vertebrates is furnished by the amphibians. A

FIG. 370. — Metamorphosis of the Newt.

tadpole — the larva of a frog- — has a tail, but no legs;
gills, instead of lungs ; a heart precisely like that of the
fish ; a horny beak for eating vegetable food, and a
spiral intestine to digest it. As it matures, the hinder
legs show themselves, then the front pair; the beak
falls off ; the tail and gills waste away ; lungs are
formed ; the digestive apparatus is changed to suit an
animal diet; the heart is altered to the reptilian type
by the addition of another auricle ; in fact, skin, mus-
cles, nerves, bones, and blood vessels vanish, being ab-

422

COMPARATIVE ZOOLOGY

sorbed atom by atom, and a new set is substituted.
Molting, or the periodical renewal of epidermal parts,
as the shell of the lobster, the skin of the toad, the
scales of snakes, the feathers of birds, and the hair of
mammals, may be termed a metamorphosis. The change
from milk teeth to a permanent set is another example.
An animal rises in organization as development ad-
vances. Thus, a caterpillar's life has nothing nobler
about it than the ability to eat, while the butterfly ex-
pends the power garnered up by the larva in a gay and
busy life. But there are seeming reversals of this law.
Some mature animals appear lower in the scale than
their young. The larval cirripede has a pair of mag-
nificent compound eyes and complex antennae ; when
adult, the antennae are gone, and the eyes are reduced
to a single, simple, minute eye spot. The germs of the
sedentary sponge and oyster are free and active. The
adult animal, however, is superior in alone possessing
the power of reproduction. Such a change from an
active to a fixed condition is known as retrograde meta-
morphosis.

There are certain larval forms so characteristic of
the great groups of the animal kingdom as to demand
notice. Most worms leave the egg as a larva, called
the trochosphere (Fig. 371), an
oval larva, having mouth and
anus, and a circle of cilia an-
terior to the mouth. This

f FiG.372.-Veliger

sphere of Worm, larval Stage IS COmmon tO Of Snail, magni-
magnified : nt, • i i i« fied • i> velum •

mouth; <z,anus; worms with the most diverse '

c, circle of cilia. ^^ forms and habjts> ft

is also found in many of the mollusks. The mollusks
usually pass through a later stage called the veliger
(Fig. 372), in which a circle of cil-ia homologous to
that of the trochosphere is borne by a lobed expansion

DEVELOPMENT

423

on the head, called the velum, or sail. The Crustacea,
which exhibit so great a range of form in the adult
state, all pass through a stage in which they are sub-
stantially alike. Forms as different in appearance as
barnacles, entomostracans, and prawns hatch out as
Nauplii, little oval animals, with a straight intestine,
three pairs of legs, and a simple eye (Fig. 373). See

FlG. 373. — Nauplius of Entomostracan (Canthocamptus). See Fig. 57. A, first
antenna; An, second antenna; a, anus; L, labrum; O, ocellus; S, stomach. (From
Brooks, after Hoek.) Magnified.

Figures 56, 57, 58, 59. Figure 59 represents the lobster,
which does not hatch as a Nauplius, but is not very
unlike the prawn. These larval forms are of great
interest, because they disclose the relationships of the
adult forms, as the gastrula stage hints at the common
relationships of all animals above Protozoa.

424 COMPARATIVE ZOOLOGY

« 2. Alternate Generation

Sometimes a metamorphosis extending over several
generations is required to evolve the perfect animal;
" in other words, the parent may find no resemblance to
himself in any of his progeny, until he comes down to
the great-grandson." Thus, the jellyfish, or medusa,
lays eggs which are hatched into larvae resembling
Infusoria — little transparent oval bodies covered with
cilia, by which they swim about for a time till they find
a resting place. One of them, for example, becoming
fixed, develops rapidly ; it elongates and spreads at the
upper end ; a mouth is formed, opening into a digestive
cavity ; and tentacles multiply till the mouth is sur-
rounded by them. At this stage it resembles a hydra.
Then slight wrinkles appear along the body, which
grow deeper and deeper, till the animal looks like " a
pine cone surmounted by a tuft of tentacles"; and then
like a pile of saucers (about a dozen in number) with
scalloped edges. Next, the pile breaks up into separate
segments, which are, in fact, so many distinct animals ;
and each turning over as it is set free, so as to bring the
mouth below, develops into an adult medusa, becoming
more and more convex, and furnished with tentacles,
circular canals, and other organs exactly like those of
the progenitor which laid the original egg (Figs. 20,

374)-

Here we see a medusa producing eggs which develop
into stationary forms resembling hydras. The hydras
then produce not only medusae by budding in the man-
ner described, but also other hydras like themselves by
budding. All these intermediate forms are transient
states of the jellyfish, but the metamorphoses can not be
said to occur in the same individual. While a cater-
pillar becomes a butterfly, this hydralike individual pro-

DEVELOPMENT

425

duces a number of medusae. Alternate generation is,
then, an alternation of asexual and sexual methods of
reproduction, one or more generations produced from
buds being followed by a single generation produced

FIG. 374. — Alternate generation : a, b, c, ova of an Acaleph (Ckrysaora); d,e,f, Hydras;
g, ht Hydras with constrictions; z, Hydra undergoing fission; k, one of the separated
segments, a free medusa.

from eggs. Often, as in the fresh-water hydra, the two
kinds of generations are alike in appearance. The
process is as widespread as asexual reproduction, being
found mostly in sponges, ccelenterates, and worms. It
is also found in certain Crustacea and insects. The
name is sometimes limited to cases where the two kinds
of generations differ in form.

3. Growth and Repair

Growth is increase of bulk, as Development is increase
of structure. It occurs whenever the process of repair
exceeds that of waste, or when new material is added
faster than the tissues are destroyed. There is a specific
limit of growth for all animals, although many of the
low cold-blooded forms, as the trout and anaconda, seem
to grow as long as they live. After the body has at-
tained its maturity, i.e., has fully developed, the tissues
cease to grow ; and nutrition is concerned solely in sup-

426 COMPARATIVE ZOOLOGY

plying the constant waste, in order to preserve the size
and shape of the organs. A child eats to grow and
repair; the adult eats only to repair.172 Birds develop
rapidly, and so spend most of their life full-fledged;
while insects generally, fishes, amphibians, reptiles, and
mammals mature at a comparatively greater age. The
perfect insect rarely changes its size, and takes but
little food ; eating and growing are almost confined to
larval life. The crust of the sea urchin, which is never
shed, grows by the addition of matter to the margins of
the plates. The shell of the oyster is enlarged by the
deposition of new laminae, each extending beyond the
other. At every enlargement, the interior is lined with
a new nacreous layer ; so that the number of such layers
in the oldest part of the shell indicates the number of
enlargements. When the shell has reached its full size,
new layers are added to the inner surface only, which
increases the thickness. It is the margin of the mantle
which provides for the increase in length and breadth,
while the thickness is derived from the whole surface.
The edges of the concentric laminae are the " lines of
growth." The oyster is full-grown in about five years.
The bones of fishes and reptiles are continually grow-
ing ; the long bones of higher animals increase in length
so long as the ends (epiphyses) are separate from the
shaft. The limbs of man, after birth, grow more rapidly
than the trunk.

The power of regenerating lost parts is greatest where
the organization is lowest, and while the animal is in the
young or larval state. It is really a process of budding.
The upper part of the hydra, if separated, will repro-
duce the rest of the body; if the lower part is cut off,
it will add the rest. Certain worms may be cut into
several pieces, and each part will regain what is needed
to complete the mangled organism. The starfish can

DEVELOPMENT 427

reproduce its arms ; the holothurian, its stomach ; the
snail, its tentacles ; the lobster, its claws ; the spider, its
legs ; the fish, its fins ; and the lizard, its tail. Nature
makes no mistake by putting on a leg where a tail be-
longs, or joining an immature limb to an adult animal.173
In birds and mammals, the power is limited to the repro-
duction of certain tissues, as shown in the healing of
wounds. Very rarely an entire human bone, removed
by disease or surgery, has been restored. The nails and
hair continue to grow in extreme old age.

4. Likeness and Variation

It is a great law of reproduction that all animals tend
to resemble their parents. A member of one class never
produces a member of another class. The likeness is
very accurate as to general structure and form. But it
does not descend to every individual feature and trait.
In other words, the tendency to repetition is qualified
by a tendency to variation. Like produces like, but not
exactly. The similarity never amounts to identity. So
that we have two opposing tendencies — the hereditary
tendency to copy the original stock, and a distinct ten-
dency to deviate from it.

This is one of the most universal facts in nature.
Every development ends in diversity. All know that
no two individuals of a family, human or brute, are
absolutely alike. There are always individual differ-
ences by which they can be distinguished. Evidently a
parent does not project precisely the same line of influ-
ences upon each of its offspring.

This variability makes possible an indefinite modifica-
tion of the forms of life. For the variation extends to
the whole being, even to every organ and mental char-
acteristic as well as to form and color. It is very slight

428 COMPARATIVE ZOOLOGY

from generation to generation ; but it can be accumu-
lated by choosing from a large number of individuals
those which possess any given variation in a marked
degree, and breeding from these. Nature does this by
the very gradual process of " natural selection " ; man
hastens it, so to -speak, by selecting extreme varieties.
Hence we have in our day remarkable specimens of
poultry, cattle, and dogs, differing widely from the wild
races.

Sometimes we notice that children resemble, not their
parents, but their grandparents or remoter ancestors.
This tendency to revert to an ancestral type is called
atavism. Occasionally stripes appear on the legs and
shoulders of the horse, in imitation of the aboriginal
horse, which was striped like the zebra. Sheep have a
tendency to revert to dark colors.

The laws governing inheritance are unknown. No
one can say why one peculiarity is transmitted from
father to son, and not another ; or why it appears in one
member of the,family, and not in all. Among the many
causes which tend to modify animals after birth are the
quality and quantity of food, amount of temperature
and light, pressure of the atmosphere, nature of the soil
or water, habits of fellow animals, etc.

Occasionally animals occur, widely different in struc-
ture, having a very close external resemblance. Barna-
cles were long mistaken for mollusks, polyzoans for
polyps, and lamprey eels for worms. Such forms are
termed homomorpJiic.

Members of one group often put on the outward ap-
pearance of allied species in the same locality ; this is
called mimicry. " They appear like actors or masquer-
aders dressed up and painted for amusement, or like
swindlers endeavoring to pass themselves off for well-
known and respectable members of society." Thus,

DEVELOPMENT 429

certain butterflies on the Amazon have such a strong
odor that the birds let them alone ; and butterflies of an-
other family in the same region have assumed for pro-
tection the same form and color of wing, but lack the
odor. So we have beelike moths, beetlelike crickets,
wasplike flies, and antlike spiders ; harmless and venom-
ous snakes copying each other, and orioles departing
from their usual gay coloring to imitate the plumage,
flight, and voice of quite another kind of birds. The
species which are imitated are much more abundant than
those which mimic them. There is also a general har-
mony between the colors of an animal and those of its
habitation {protective resemblance). We have the white
polar bear, the sand-colored camel, and the dusky twilight
moths. There are birds and reptiles so tinted and mottled
as exactly to match the rock, or ground, or bark of a
tree they frequent ; and there are insects rightly named
"walking sticks" and "walking leaves." These coin-
cidences are often beneficial to the imitating species.
Generally, they wear the livery of those they live on,
or resemble the forms more favored than themselves.

Again, some animals which have a nauseous taste or
odor, as certain caterpillars, insects, salamanders, etc.,
advertise the fact by being brilliantly colored and spotted
(warning coloration), and are thus protected against other
animals which would prey upon them.

5. Homo logy, Analogy, and Correlation

The tendency to repetition in the development of
animals leads to some remarkable affinities. Parts or
organs, having a like origin and development, and
therefore the same essential structure, whatever their
form or function, are said to be homologous ; while parts
or organs corresponding in use are called analogous.

430

COMPARATIVE ZOOLOGY

By serial homology is meant the homology existing be-
tween successive parts of one animal.

The following are examples of homology : the arms
of man, the fore legs of a horse, the paddles of a whale,
the wings of a bird, the front flippers of a turtle, and the
pectoral fins of a fish ; the proboscis of a moth, and the
jaws of a beetle; the shell of a snail, and both valves
of a clam. The wings of the bird, flying squirrel, and
bat are hardly homologous, since the wing of the first is
developed from the fore limb only ; that of the squirrel
is an extension of the skin between the fore and hind
limbs ; while in the bat the skin stretches between the
fingers, and then down the side to the tail. Examples
of serial homology : the arms and legs of man ; the
upper and lower set of teeth ; the parts of the vertebral
column, however modified; the scapular and pelvic
arches; the humerus and femur; carpus and tarsus;
the right and left sides of most animals ; the dorsal and
anal fins of fishes. The legs of a lobster and lizard,
the wings of a butterfly and bird, the gills of a fish and
the lungs of other vertebrates, are analogous. The air
bladder of a fish is homologous with a lung, and analo-
gous to the air chambers of the nautilus.

In the midst of the great variety of form and structure
in the animal world, a certain harmony reigns. Not
only are different species so related as to suggest a
descent from the same ancestor, but the parts of any
one organism are so closely connected and mutually
dependent that the character of one must receive its
stamp from the character of all the rest. Thus, from a
single tooth it may be inferred that the animal had a
skeleton and spinal cord, and that it was a carnivorous,
hot-blooded mammal. Certain structures always coexist.
Animals with two occipital condyles, and non-nucleated
blood corpuscles, suckle their young, i.e., they are mam-

DEVELOPMENT

431

mals. All ruminant hoofed beasts have horns and
cloven feet. If the hoofs are even, the horns are even,
as in the ox ; if odd, as in the rhinoceros, the horns are

FIG. 377.

FIG. 378,

HOMOLOGIES OF LIMBS

FIG. 375. — Arm and Leg of Man, as they are when he gets down on all fours.
FIG. 376. — Fore and Hind Legs of Tapir. FIG. 377. — Fore Leg of Seal and Hind
Leg of Alligator. FIG. 378. — Wing of the Bat. S, scapula ; I, ilium, or rim-bone
of pelvis ; H, humerus ; F, femur ; O, olecranon, or tip of the elbow ; P, patella ;
U, ulna ; T, tibia ; R, radius ; Fi, Fibula; Po, pollex, or thumb ; Ha, hallux, or great
toe. Compare the fore, and hind limbs of the same animal, and the fore or hind limbs
of different animals. Note the directions of the homologous segments.

432 COMPARATIVE ZOOLOGY

odd, i.e., single, or two placed one behind- the other.
Recent creatures with feathers always have beaks.
Pigeons with short beaks have small feet; and those
with long beaks, large feet. The long limbs of the
hound are associated with a long head. A white spot
in the forehead of a horse generally goes with white
feet. Hairless dogs are deficient in teeth. Long wings
usually accompany long tail feathers. White cats with
blue eyes are usually deaf. A sheep with numerous
horns is likely to have long, coarse wool. Homologous
parts tend to vary in the same manner ; if one is diseased,
another is more likely to sympathize with it than one
not homologous. This association of parts is called
correlation of growth.

6. Individuality

It seems at first sight very easy to define an individ-
ual animal. A single fish, or cow, or snail, or lobster is
plainly an individual ; and the half of one such animal
is plainly not one. But when we consider animals in
colonies, like corals, it is not so easy to say whether the
individual is the colony or the single polyp. Is the tree
the individual, or the bud ? If we say the former — the
colony — what shall we say to the free buds of a hydroid
colony, living independent lives, and scattered over
square miles of ocean ? Are they parts of one individ-
ual ? If we choose the latter as our standard, we are in
equal difficulty ; for we must then call an individual the
bud of the Portuguese man-of-war, reduced to a mere
bladder or feeler, and incapable of leading an indepen-
dent life. We thus find it necessary to distinguish at
least two kinds of individuals — -physiological individ-
uals, applying that name to any animal form capable
of leading an independent life ; and morphological indi-
viduals, one of which is the total product of an egg.

DEVELOPMENT

433

Such an individual may be a single physiological indi-
vidual, as the fish ; or many united, as the coral stock ;
or many separate physiological individuals, as in the
hydroids or plant lice. The single members of such a
compound morphological individual are called zooids, or
persona, and are found wherever asexual reproduction
takes place.

7. Relations of Number, Size, Form, and Rank

The animal kingdom has been likened to a pyramid,
the species diminishing in number as they ascend in the
scale of complexity. This is not strictly true. The
number of living species known is at least 300,000, of
which more than nine tenths are invertebrates. A late
enumeration gives the following figures for the number
of described species : —

Protozoa 4,000

Coslenterata 3>5°°

Vermes 5»5^o

Arthropoda 250,000

Echinodermata . . . 2,300

Mollusca 21,000

Vertebrata 25,200

These figures are lower than those usually given. Of
vertebrates, fishes are most abundant ; then follow birds,
mammals, reptiles, and amphibians. There are usually
said to be about 200,000 species of insects and it is esti-
mated that there are about 500,000 living species in the
animal kingdom ; about 40,000 extinct species have been
described.

The largest species usually belong to the higher
classes. The aquatic members of a group are generally
larger than the terrestrial, the marine than the fresh-
water, and the land than the aerial. The extremes of
size are an Infusorium, ^Jo^ of an inch in diameter,
and the whale, eighty-five feet long, respectively the
smallest and the largest animal ever measured. The
DODGE'S GEN. ZOOL. — 28

434

COMPARATIVE ZOOLOGY

female is sometimes larger than the male, as of the nau-
tilus, spider, and eagle. The higher the class, the more
uniform the size. Of all groups of animals, insects and
birds are the most constant in their dimensions.

Every organism has its own special law of growth : a
fish and an oyster, though born in the same locality, de-
velop into very different forms. Yet a symmetry of
plan underlies the structure of all animals. In the
embryo, this syrrimetry of the two ends, as well as the
two sides, is nearly perfect ; but it is subsequently inter-
fered with to adapt the animal to its special conditions
of life. It is a law that an animal grows equally in
those directions in which the incident forces are equal.
The polyp, rooted to the rocks, is subjected to like con-
ditions on all sides, and, therefore, it has no right and
left, or fore and hind parts. The lower forms, generally,
are more or less geometrical figures : spheroidal, as the
sea urchin ; radiate, as the starfish ; and spiral, as many
foraminifers. The higher animals are subjected to a
greater variety of conditions. Thus, a fish, always go-
ing through the water head foremost, must show con-
siderable difference between the head and the hinder
end ; or a turtle, moving over the ground with the same
surface always down, must have distinct dorsal and
ventral sides.

Nevertheless, there is a striking likeness between the
two halves or any two organs situated on opposite sides
of an axis. And, first, a bilateral symmetry is most com-
mon. It is best exhibited by the arthropods and verte-
brates, but nearly all animals can be clearly divided into
right and left sides — in other words, they appear to be
double. A vertical plane would divide into two equal
parts our brain, spinal cord, vertebral column, organs of
sight, hearing, and smell ; our teeth, jaws, limbs, lungs,
etc. In fact, the two halves of every egg are identical.

DEVELOPMENT

435

There are many exceptions : the heart and liver of the
higher vertebrates are eccentric ; the nervous system of
mollusks is scattered; the hemispheres of the human
brain are sometimes unequal ; the corresponding bones
in the right and left arms are not precisely the same
length and weight ; the narwhal has an immense tusk on
the left side, with none to speak of on the other ; the
rattlesnake has but one lung, the second remaining in a
rudimentary condition ; both eyes of the adult flounder
and halibut are on the same side ; the claws of the lob-
ster differ; and the valves of an oyster are unequal.
But all these animals and their organs are perfectly sym-
metrical in the embryo state.

Again, animals exhibit a certain correspondence be-
tween the fore and hind parts.17* Thus, the two ends
of the centipede repeat each other. Indeed, in some
worms, the eyes are developed in the last segment as
well as the first. In the embryo of quadrupeds, the
four limbs are closely alike. But in the adult, the fore
and hind limbs differ more than the right and left limbs,
because the functions are more dissimilar. An extreme
want of symmetry is seen in birds, which combine aerial
and land locomotion.

Every animal is perfect in its kind and in its place.
Yet we recognize a gradation of life. Some animals are
manifestly superior to some others. But it is not so
easy to say precisely what shall guide us in assorting
living forma into high and low. Shall we make structure
the criterion of rank? Plainly the simple jellyfish is
beneath complicated man. The intricate and finished
build of the horse elevates him immeasurably above the
stupid snail. The repetition of similar parts, as in the
worm, is a sign of low life. So also a prolonged poste-
rior is a mark of inferiority, as the lobsters are lower
than the crabs, snakes than lizards, monkeys than apes.

436 COMPARATIVE ZOOLOGY

The possession of a head distinct from the region behind
it is a sign of power. And in proportion as the fore
limbs are used independently of the hind limbs, the
animal ascends the scale : compare the whale, horse,
cat, monkey, and man.

But shall the fish, never rising above the " monotony
of its daily swim," be allowed to outrank the skillful bee ?
Shall the brainless, sightless, almost heartless amphioxus,
a vertebrate, be allowed to stand nearer to man than the
ant ? What is the possession of a backbone to intelli-
gence ? No good reason can be given why we might not
be just as intelligent beings if we carried, like the insect,
our hearts in our backs and our spinal cords in our
breasts. So far as its activity is concerned, the brain
may be as effective if spread out like a map as packed
into its present shape. Even animals of the same type,
as vertebrates, can not be ranked according to complex-
ity. For while mammals, on the whole, are superior to
birds, birds to reptiles, and reptiles to fishes, they are
not so in every respect. Man himself is not altogether
at the head of creation. We carry about in our bodies
embryonic structures. That structural affinity and vital
dignity are not always parallel may be seen by compar-
ing an Australian aborigine and an Englishman.175

Function is the test of worth. Not mere work, how-
ever ; for we must consider its quality and scope. An
animal may be said to be more perfect in proportion as
its relations to the external world are more varied, pre-
cise, and fitting. Complexity of organization, variety,
and amount of power are secondary to the degree in
which the whole organism is adapted to the circum-
stances which surround it, and to the work which it has
to do. Ascent in the animal scale is not a passage from
animals with simple organs to animals with complex
organs, but from simple individuals with organs of

DEVELOPMENT 437

complex function to complex individuals with organs
of simple function : the addition as we ascend being not
function, but parts to discharge those functions ; and the
advantage gained, not another thing done, but the same
thing done better. Advance in rank is exhibited, not
by the possession of more life (for some animalcules are
ten times more lively than the busiest man), but by the
setting apart of more organs for special purposes.
The higher the animal, the greater the number of parts
combining to perform each function. The power is
increased by this division of labor. The most impor-
tant feature in this specialization is the tendency to
concentrate the nervous energy toward the head (ceph-
alizatiori). It increases as we pass from lower to higher,
animals.

As a rule, fixed species are inferior to the free, water
species to land species, fresh-water animals to marine,
arctic forms to tropical, and the herbivorous to the car-
nivorous. Precocity is a sign of inferiority : compare
the chicks of the hen and the robin, a colt with a kitten,
the comparatively well-developed caterpillar with the
footless grub of the bee. Among invertebrates, the male
is frequently inferior, not only in size, but also in grade
of organization. Animals having a wide range as to cli-
mate, altitude, or depth are commonly inferior to those
more restricted ; man is a notable exception.

There is some relation between the duration of life and
the size, structure, and rank of animals. Vertebrates
not only grow to a greater size, but also live longer than
invertebrates. Whales and elephants are the longest-
lived ; and falcons, ravens, parrots, and geese, alligators
and turtles, and sharks and pikes are said to live a cen-
tury. The life of quadrupeds generally reaches its limit
when the molar teeth are worn down : those of the sheep
last about 1 5 years ; of the ox, 20 ; of the horse, 40 ; of

438

COMPARATIVE ZOOLOGY

the elephant, 100. Many inferior species die as soon as
they have laid their eggs, just as herbs perish as soon as
they have flowered.

8. The Struggle for Life

Every species of animal is striving to increase in a
geometrical ratio. But each lives, if at all, by a struggle
at some period of its life. The meekest creatures must
fight, or die.

" There is no exception to the rule that every organic
being naturally increases at so high a rate, that, if not
destroyed, the earth would soon be covered by the prog-
eny of a single pair." If the increase of the human race
were not checked, there would not be standing room
for the descendants of Adam and Eve. A pair of ele-
phants, the slowest breeder of all known animals, would
become the progenitors, in seven and one-half centuries,
of 19,000,000 of elephants, if death did not interfere.
Evidently a vast number of young animals must perish
while immature, and a far greater host of eggs fail to
mature. A single cod, laying millions of eggs, if al-
lowed to have its own way, would soon pack the ocean.

Yet, so nicely balanced are the forces of nature, the
average number of each kind remains about the same.
The total extinction of any one species is exceedingly
rare. The number of any given species is not deter-
mined by the number of eggs produced, but by its sur-
rounding conditions.176 Aquatic birds outnumber the
land birds, because their food never fails, not because
they are more prolific. The fulmer petrel lays but one
egg, yet it is believed to be the most numerous bird in
the world.

The main checks to the high rate of increase are:
climate (temperature and moisture), acting directly or
indirectly by reducing food ; and other animals, either

DEVELOPMENT 439

rivals requiring the same food and locality, or enemies,
for the vast majority of animals are carnivorous. Off-
spring are continually varying from their parents, for
better or worse. If feebly adapted to the conditions of
existence, they will finally go to the wall. But those
forms having the slightest advantage over others in-
habiting the same region, being hardier or stronger,
more agile or sagacious, will survive. Should this ad-
vantageous variation become hereditary and intensified,
the new variety will gradually extirpate or replace other
kinds. This is what Mr. Darwin means by Natural
Selection, and Herbert Spencer by the Survival of the
Fittest.

CHAPTER XXIV

THE DISTRIBUTION OF ANIMALS

LIFE is everywhere. In the air above, the earth be-
neath, and the waters under the earth, we are surrounded
with life. Nature lives : every death is only a new birth,
every grave a cradle. The air swarms with birds, insects,
and invisible animalcules. The waters are peopled with
innumerable forms, from the protozoan, millions of which
would not weigh a grain, to the whale, so large that it
seems an island as it sleeps upon the waves. The bed
of the sea is alive with crabs, mollusks, polyps, star-
fishes, and Foraminifera. Life everywhere — on the
earth, in the earth, crawling, creeping, burrowing, bor-
ing, leaping, running.

Nor does the vast procession end here. The earth
we tread is largely formed of the debris of life. The
quarry of limestone, the flints which struck the fire of
the old Revolutionary muskets, are the remains of count-
less skeletons. The major part of the Alps, the Rocky
Mountains, and the chalk cliffs of England are the mon-
umental relics of bygone generations. From the ruins
of this living architecture we build our Parthenons and
Pyramids, our St. Peters and Louvres. So generation
follows generation. But we have not yet exhausted the
survey. Life cradles within life. The bodies of ani-
mals are little worlds having their own fauna and flora.
In the fluids and tissues, in the eye, liver, stomach,
brain, and muscles, parasites are found ; and these
parasites often have their parasites living on them.

44°

THE DISTRIBUTION OF ANIMALS 441

Even the unicellular forms, Stylonychia, for example,
have been found to be infested with parasitic protozoans.

Thus the ocean of life is inexhaustible. It spreads
in every direction, into time past and present, flowing
everywhere, eagerly surging into every nook and corner
of creation. On the mountain top, in the abysses of
the Atlantic, in the deepest crevice of the earth's crust,
we find traces of animal life. Nature is prodigal of
space, but economical in filling.it.177

Animals are distributed over the globe according to
definite laws, and with remarkable regularity.

Each of the three great provinces, Earth, Air, and
Water, as also every continent, contains representatives
of all the classes ; but the various classes are unequally
represented. Every great climatal region contains some
species not found elsewhere, to the exclusion of some
other forms. Every grand division of the globe,
whether of land or sea, each zone of climate and alti-
tude, has its own fauna. In traveling over the earth
and settling in new regions man has been accompanied
by many animals which have established themselves
and thriven in the land of their adoption. For example,
the house, or " English," sparrow has been brought to
America, and the sparrow and rabbit to New Zealand.
Hence, it is necessary to distinguish between the native
or indigenous fauna, and the introduced fauna, the latter
depending upon human agency. In spite of the many
causes tending to disperse animals beyond their natural
limits, each country preserves its peculiar zoological
physiognomy.

The space occupied by the different groups of ani-
mals is often inversely as the size of the individuals.
Compare the coral and elephant.

The fauna now occupying a separate area is closely
allied to the fauna which existed in former geologic

442 COMPARATIVE ZOOLOGY

times. Thus, Australia has always been the home of
marsupials, and South America of edentates.

It is a general rule that groups of distinct species are
circumscribed within definite, and often narrow, limits.
Man is the only cosmopolitan ; yet even he comprises
several marked races, whose distribution corresponds
with the great zoological regions. The natives of Aus-
tralia are as grotesque as the animals. Certain brutes
likewise have a great range : thus, the puma ranges
from Canada to Patagonia; the muskrat, from the
Arctic Ocean to Florida; the ermine, from Bering
Strait to the Himalayas ; and the hippopotamus, from
the Nile and Niger to the Orange River.178

Frequently, species of the same genus, living side
by side, are widely different, while there is a close re-
semblance between forms which are antipodes. The
mud eel of South Carolina and menobranchus of the
Northern States have their relatives in Japan and Aus-
tria. The American tapir has its mate in Sumatra, the
llama is related to the camel, and the opossum to the
kangaroo.

The chief causes modifying distribution are tempera-
ture, topography, ocean and wind currents, humidity,
and light. To these may be added the fact that ani-
mals are ever intruding on each other's spheres of exist-
ence. High mountain ranges, wide deserts, and cold
currents in the ocean are impassable barriers to the
migration of most species. Thus, river fish on opposite
sides of the Andes differ widely, and the cold Peruvian
current prevents the growth of coral at the Galapagos
Islands. So a broad river, like the Amazon, or a deep,
narrow channel in the sea, is an effectual barrier to
some tribes. Thus, Borneo belongs to the Indian region,
while Celebes, though but a few miles distant, is Aus-
tralian in its life. The faunae of North America, on

THE DISTRIBUTION OF ANIMALS 443

the east coast, west coast, and the open plains between,
are very different.

Animals dwelling at high elevations resemble those
of colder latitudes. The same species of insects are
found on Mount Washington, and in Labrador and
Greenland.

The range does not depend upon the powers, of loco-
motion. The oyster extends from Halifax to Charles-
ton, and the snapping turtle from Canada to the
equator; while many quadrupeds and birds have nar-
row habitats.

The distribution of any group is qualified by the
nature of the food. Carnivores have a wider range
than herbivores.

Life diminishes as we depart from the equator north
or south, and likewise as we descend or ascend from
the level of the sea.

The zones of geography have been divided by zoolo-
gists into narrower provinces. Three regions in the
sea are recognized : the Pelagic or surface region ; the
Littoral, between tide marks strictly but often inter-
preted to conclude depths to forty fathoms ; and the
Abyssal, extending from the Littoral to the greatest
depths of the ocean. Every marine species has its own
limits of depth. It would be quite as difficult, said
Agassiz, for a fish or a mollusk to cross from the coast
of Europe to the coast of America as for a reindeer to
pass from the arctic to the antarctic regions across the
torrid zone. Marine animals congregate mainly along
the coasts of continents and on soundings. The meet-
ing place of two maritime currents of different tem-
peratures, as on the Banks of Newfoundland, favors
the development of a great diversity of fishes.

Every great province of the ocean contains some
representatives of all the subkingdoms. Deep-sea life

444 COMPARATIVE ZOOLOGY

is diversified, though comparatively sparse. Examples
of all the five invertebrate divisions were found in the
Bay of Biscay, at the depth of 2435 fathoms.179

Distribution in the sea is influenced by the tempera-
ture and composition of the. water and the character of
the bottom. The depth acts indirectly by modifying
the temperature. Northern animals approach nearer
to the equator in the sea than on the land, on account
of cold currents. The heavy aquatic mammals, as
whales, walruses, seals, and porpoises, are mainly
polar.

The land consists of the following somewhat distinct
areas : the Neotropic, comprising South America, the
West Indies, and most of Mexico ; the Nearctic, includ-
ing the rest of America; the Palearctic, composed of
the eastern continent north of the Tropic of Cancer,
and the Himalayas ; the Ethiopian, or Africa south of
the Tropic of Cancer ; the Oriental, or India, the south-
ern part of China, the Malay Peninsula, and the islands
as far east as Java, Borneo, and the Philippine Islands ;
and the Australian, or the eastern half of the Malay
Islands and Australia. These are the regions of Sclater
and Wallace. Other writers unite the northern parts of
both hemispheres into one region, and the Oriental with
the Ethiopian regions.

Life in the polar regions is characterized by great
uniformity, the species being few in number, though
the number of individuals is immense. The same ani-
mals inhabit the arctic portions of the three continents ;
while the antarctic ends of the continents, Australia,
Cape of Good Hope, and Cape Horn, exhibit strong
contrasts. Those three continental peninsulas are, zoo-
logically, separate worlds. In fact, the whole southern
hemisphere is peculiar. Its fauna is antique. Aus-
tralia possesses a strange mixture of the old and new.

THE DISTRIBUTION OF ANIMALS 445

South America, with newer mammals, has older reptiles;
while Africa has a rich vertebrate life, with a striking
uniformity in its distribution. Groups, old geologically
and now nearly extinct, are apt to have a peculiar dis-
tribution ; as the Edentata in South America, Africa,
and India ; the marsupials in Australia and America ;
the Ratitae in South America, Africa, Australia, and
New Zealand.

In the tropics, diversity is the law. Life is more
varied and crowded than elsewhere, and attains its
highest development.

The New World fauna is old-fashioned, and inferior
in rank and size, compared with that of the eastern
continents.

As a rule, the more isolated a region the greater the
variety. Oceanic islands have comparatively few species,
but a large proportion of endemic or peculiar forms.
Batrachians are absent, and there are no indigenous
terrestrial mammals. The productions are related to
those of the nearest continent. When an island, as
Britain, is separated from the mainland by a shallow
channel, the mammalian life is the same on both sides.

Protozoans, ccelenterates, and echinoderms are limited
to the waters, and nearly all are marine. Sponges are
mostly obtained from the Grecian Archipelago and
Bahamas, but species not commercially valuable abound
in all seas. Coral reefs abound throughout the Indian
Ocean and Polynesia, east coast of Africa, Red Sea,
and Persian Gulf, West Indies, and around Florida;
and corals which do not form reefs are much more
widely distributed, being found as far north as Long
Island Sound and England. Crinoids have been found,
usually in deep sea, in very widely separated parts of
the world — off the coast of Norway, Scotland, and
Portugal, and near the East and West Indies. The

446

THE DISTRIBUTION OF ANIMALS 447

other echinoderms abound in almost every sea; the
starfishes chiefly along the shore, the sea urchins in
the Littoral zone, and the sea slugs around coral reefs.
Worms are found in all parts of the world, in sea, fresh
water, and earth. They are most plentiful in the muddy
or sandy bottoms of shallow seas. Living brachiopods,
though few in number, occur in tropical, temperate, and
arctic seas, and from the shore to great depths. Poly-
zoa have both salt and fresh water forms, and annelids
include land forms, as the earthworm and some leeches.

Mollusks have a world-wide distribution over land and
sea. The land forms are restricted by climate and food,
the marine by shallows or depths, by cold currents, by
a sandy, gravelly, or muddy bottom. The bivalves are
also found on every coast and in every climate, as well
as in rivers and lakes, but do not flourish at the depth
of much more than two hundred fathoms. The fresh-
water mussels are more numerous in the United States
than in Europe, and west of the Alleghanies than east.
The seashells along the Pacific coast of America are
unlike those of the Atlantic, and are arranged in five
distinct groups : Aleutian, Californian, Panamic, Peru-
vian, and Magellanic. On the Atlantic coast, Cape Cod
and Cape Hatteras separate distinct provinces. Of
land snails, Helix has an almost universal range, but is
characteristic of North America, as Bulimus is of South
America, and Achalina of Africa. The Old World and
America have no species in common, except a few in
the extreme north.

The limits of insects are determined by temperature
and vegetation, by oceans and mountains. There is an
insect fauna for each continent, and zone, and altitude.
The insects near the snow line on the sides of mountains
in the temperate region are similar to those in polar
lands. The insects on our Pacific slope resemble those

448 COMPARATIVE ZOOLOGY

of Europe, while those near the Atlantic coast are more
like those of Asia. Less than a score of insects are
known to live in the sea.

The distribution of fishes 'is bounded by narrower
limits than that of other animals. A few tribes may be
called cosmopolitan, as the sharks and herrings ; but
the species are local. Size does not appear to bear any
relation to latitude. The marine forms are three times
as numerous as the fresh-water. The migratory fishes of
the northern hemisphere pass to a more southern region
in the spring, while birds migrate in the autumn.

Living reptiles form but a fragment of the immense
number which prevailed in the Middle Ages of geology.
Being less under the influence of man, they have not
been forced from their original habitats. None are
arctic. America is the most favored spot for frogs and
salamanders, and India for snakes. Australia has few
batrachians, and two thirds of its snakes are venomous.
In the United States, only about one eighth of the species
are venomous. Frogs, snakes, and lizards occur at ele-
vations of over fifteen thousand feet. Crocodiles, and
most lizards and turtles, are tropical.

Swimming birds, which constitute about one four-
teenth of the entire class, form one half of the whole
number in Greenland. As we approach the tropics, the
variety and number of land birds increase. Those of
the torrid zone are noted for their brilliant plumage,
and the temperate forms for their more sober hues, but
sweeter voices. India and South America are the
richest regions. Hummers, tanagers, orioles, and tou-
cans are restricted to the New World. Parrots are
found in every continent except Europe; and wood-
peckers occur in every region, save in Australia.

The vast majority of mammals are terrestrial; but
cetaceans and seals belong to the sea, otters and beavers

THE DISTRIBUTION OF ANIMALS 449

delight in lakes and rivers, and moles are subterranean.
As of birds, the aquatic species abound in the polar
regions. Marsupials inhabit two widely separated areas
— America and Australia. Tn the latter continent they
constitute two thirds of the fauna, while nearly all placen-
tal mammals, except bats and a few rats and squirrels,
are wanting. Excepting a few species in South Africa
and South Asia, edentates are confined to tropical South
America. The equine family is indigenous to South
and East Africa and Southern Asia, while their fossil
remains are abundant in both North and South
Ameripa. In North America, rodents form about one
half the number of mammals ; there are very few
species in Madagascar. Ruminants are sparingly
represented in America. Carnivores flourish in every
zone and continent. The prehensile-tailed monkeys
are strictly South American ; while the anthropoid apes
belong to the west coast of Africa, and to Borneo and
Sumatra. Both monkeys and apes are most abundant
near the equator ; in fact, their range is limited by the
distribution of palms.

CHAPTER XXV
THE ORIGIN OF ANIMAL SPECIES

THE origin of the immense number of species of plants
and animals inhabiting the earth has been a matter of
speculation among naturalists and philosophers for many
centuries. One theory has held that each species was
created separately, while the other, known as the Theory
of Evolution, maintains that living forms are derived by
natural processes of descent from species that inhabited
the earth in earlier times ; that is, the ancestral forms
became extinct owing to changing conditions of climate,
food supply, enemies, and other factors, and their de-
scendants in the course of many generations have become
modified in bodily structure and function, these changes
leading to the development, or evolution, of the numer-
ous species now living. The evidence in favor of the
latter theory is so strong that it is now accepted by
scientific men as the true explanation of the mode of
origin of all known organisms.

Although the idea of evolution has been more or less
definitely held by various naturalists since the time of
Aristotle (384-322 B.C.), others, even as recently as Lin-
naeus (1707- 1778) and Cuvier (1768-1832), have insisted
that species are immutable, or unchanging in character-
istics. Bonnet (1720-1793) was the first among later
zoologists to suggest that variations of climate, nourish-
ment, and other features of the environment might pro-
duce new species, and to use the term evolution in its
modern sense ; but he adduced no important facts to

450

THE ORIGIN OF ANIMAL SPECIES 451

support his theory, and it failed to meet with the ap-
proval of his contemporaries. Lamarck (1744-1829)
afterward adopted this view, collected many facts in its
favor, and also advanced the hypothesis, in 1801, that
the use and disuse of organs would cause structural
modifications in them, producing either increased devel-
opment or atrophy of parts. These modifications, being
inherited by successive generations, would eventually
become characteristic of new species thus evolved from
the older ones. Lamarck's theory was opposed by
Cuvier, the greatest comparative anatomist and paleon-
tologist of his time, who insisted that, if the theory were
true, there ought to be among fossils transition forms
connecting the extinct with the living species, but that
no such forms were known, nor could a process be sug-
gested by which transition could take place. Under
Cuvier's leadership the belief became current among
geologists that the earth has passed through a series of
catastrophes or cataclysms which destroyed all living
things, and that it has successively been repeopled with
new forms quite unlike those which had perished. The
Lamarckian theory passed into obscurity, and was not
seriously considered again until it was brought forth for
comparison with Darwin's theory of natural selection.
The opinions of geologists regarding cataclysms under-
went a change after Hutton (1726-1797) urged that in
order to understand how the present condition of the
earth came about, the changes now taking place must be
studied. This view was later vigorously upheld and ex-
tended by Lyell (1797-1875), who contended that cata-
clysms have never occurred, but that the earth has
gradually reached its present state through the action
of natural forces which are still in operation. Thus the
way was prepared for the appearance of the theory
which, elaborated and maintained by numerous observa-

452 COMPARATIVE ZOOLOGY

tions, was propounded by Charles Darwin (1809-1882)
in his " Origin of Species by Means of Natural Selec-
tion, or the Preservation of Favored Races in the Strug-
gle for Life," published in 1859. Darwin had served as
naturalist on the British exploring ship Beagle on a five
years' cruise (1832-1837) around the world, and "was
much struck with certain facts in the distribution of the
organic beings inhabiting South America, and in the
geological relations of the present to the past inhabit-
ants of that continent." After his return home, twenty
additional years were spent in collecting facts, making
further observations and experiments, and in pondering
the theory before he ventured to publish his results and
to state what he regarded as the factors concerned in
the process of evolution. A similar conclusion had been
reached, simultaneously and independently, by Alfred
Russell Wallace (1823- ), who had travelled exten-
sively in South America and the Malay Archipelago,
and, like Darwin, had become convinced of the certainty
of evolution, and sought for its.explanation.

^As held by Darwin, the theory of evolution, together
with the causes of the process, may be briefly stated as
follows : —

(i) Organisms tend to produce a great many more off-
spring than can stirvive. Linnaeus showed that the
number of living descendants of an annual plant which
produced only two seeds each year would, at the end of
twenty years, be over a million. There is, however, no
plant known to be so unproductive. With reference to
the elephant, regarded as the slowest of breeders, pro-
ducing at the age of thirty a pair of young, and a pair
every thirty years thereafter, and living to be one hundred
years old, Darwin computed that at the end of 750 years
there would be about nineteen million living elephants
all descended from the first pair. Individual insects lay

THE ORIGIN OF ANIMAL SPECIES 453

hundreds, and fishes millions, of eggs. If all the young
were to survive, the earth would soon be unable to sup-
ply sufficient food and standing-room.

(2) In spite of this tendency to increase inordinately, the
number of animals remains, on the whole, stationary.
Even though there may be an enormous temporary in-
crease in the number of certain animals, as in " plagues
of grasshoppers," normal conditions are soon restored by
natural agencies. Eggs and young are devoured by older
animals. Disease, old age, parasites, enemies, storms,
floods, cold, heat, drought, and famine are responsible
for the death of so many individuals that comparatively
few young animals of any species live to maturity.

(3) There results, consequently, severe competition for
the necessaries of life, a veritable struggle for existence.
In order to thrive, animals need food, shelter from the
elements, protection from enemies, and freedom from
molestation while rearing their young. Deprivation of
any of these is likely to be followed by serious results for
the animals as individuals and for the race as a whole.
The introduction of sheep has made it impossible for
cattle to live on some of the Western ranges, because the
sheep crop the grass so closely that there is not enough
left to feed the cattle. The " English," or house, spar-
row appropriates the best protected nesting places, raises
several broods each season, eats whatever food is avail-
able, and remains the year round without migrating. By
reason of these habits it has been victorious in the con-
test for the places formerly occupied by native birds.
The struggle for existence is most keen between closely
related forms, since each will naturally want what the
other desires. Until about two centuries ago the black
rat was the common rat of Europe. Since then it has
been driven out by the brown rat, a larger and stronger
species.

454 COMPARATIVE ZOOLOGY

(4) There is more or less variation even between closely
related animals. Two individuals from the same litter,
for instance, always differ somewhat from each other, as
well as from all their relatives, in shape, size, vigor, in-
telligence, and other qualities. Human beings, domes-
ticated or wild animals, birds, shells, or insects are never
so much alike that differences between individuals of the
same kind cannot be detected. Wings and tails of birds
of the same species have been found in some individuals
to be twenty per cent longer, and in others as much
shorter, than the average. Variations, then, are by no
means necessarily minute, but may be considerable in
amount. Nor is variation confined to structure alone, for
it may also affect habits. Chimney swifts built their
nests in hollow trees before the country was settled. A
New Zealand parrot which, before the occupation of the
island by Europeans, lived on honey, insects, and fruits,
began to pick at meat and skins hung up to dry by the
settlers, and thus acquired a taste for flesh. During the
past fifty years its carnivorous propensities have increased
to such an extent as to lead the bird to attack living sheep.
So destructive has it become that stringent measures have
been taken for its extermination. In animals under do-
mestication variation is the rule. The numerous breeds
of cattle, horses, swine, fowls, pigeons, dogs, cats, rabbits,
canary birds, and in fact of all domesticated animals, have
been derived from a few ancestral forms. Breeders have
taken advantage of peculiarities arising by variation ; and
by a process of selecting and breeding only from those
individuals which show the peculiarity, have finally suc-
ceeded in fixing it more or less permanently, so that the
young of these animals may possess it in an even more
exaggerated form than their parents. In nature varia-
tions occur to such an extent, particularly in large and
dominant genera, as to give rise to many doubtful species,

THE ORIGIN OF ANIMAL SPECIES 455

or forms which are intermediate between typical
species.

The causes of variability are at present very imper-
fectly understood, but it is probable that climate, nourish-
ment, and physiological activity, as well as other factors,
have an influence on the process.

(5) Even though animals may be inclined to vary,
there is a marked tendency to inherit the characteristics
of their parents. Every animal bears a close re-
semblance to others of its kind. It is due to this
tendency that structural and physiological character-
istics once originated are perpetuated. The breeder
depends upon it to keep his varieties " true " to the
original stock. Whether or not characters acquired
during the lifetime of an individual are transmitted by
heredity to the offspring is still an unsettled problem.
Denial of the probability of such inheritance is a fun-
damental theory in the Weismannian school of evolu-
tionists. There seems to be no doubt, however, that
congenital characters are inherited. Should variations
appear, they are likely to be preserved to the race by
heredity. The essential nature of the process, as in
variation, is not known. To explain the phenomena of
heredity, Darwin proposed the theory of pangenesis,
which holds that particles or gemmules from all the
different parts of the body are collected into the repro-
ductive cells and through these are transmitted to the
offspring and help to modify the characteristics of the
latter. This theory has never been generally accepted.

(6) The preservation or survival of those individuals
inheriting the variations which are most advantageous in
the struggle for existence is due to natural selection.
Among all the variations appearing in the individuals
of a race some are likely to be advantageous, others the
opposite. An antelope with slightly longer legs or with

456 COMPARATIVE ZOOLOGY

more agility than other members of the herd would be
better able to escape his enemies, and consequent^ to
live longer and leave more offspring than his less
fortunate companions. Of his progeny some would
probably inherit the peculiarity, and it would thus be
transmitted from generation to generation to the evi-
dent advantage of the race. In time the variation
would become a definitely fixed and constant character,
serving to distinguish all the individuals possessing it as
a species. It is by a process of artificial selection that
breeders choose among domestic animals those indi-
viduals which possess a character which it is desired to
perpetuate, as long wool in sheep, speed in race horses,
strength in draught horses, peculiarly shaped jaws in
bull dogs, vocal powers in canary birds, and so on. By
breeding only from those individuals which show the
desired character, the latter may not only be perpetu-
ated but also intensified in degree. This is shown by
all domesticated animals and cultivated plants. To the
process by which favorable variations are selected and
perpetuated among wild animals, Darwin gave the name
of natural selection. By his hypothesis the phenomenon
of the evolution of organic forms is due to the natural
selection of favorable variations and their preservation
by heredity.

The test of the validity of a theory lies in its ability
to interpret and coordinate observed facts, and when
Darwin's hypothesis was applied to the elucidation of
the observations collected by the students of morphol-
ogy, paleontology, embryology, and other aspects of
the study of animal life, it brought order out of chaos,
and each of these sciences was seen to contain a mass
of evidence in favor of the theory of descent with
modification.

(i) Evidence from Classification. — As has been shown

THE ORIGIN OF ANIMAL SPECIES 457

in Part I, animals are divided according to their struc-
tural resemblances into groups of varying degrees of
affinity, as branch, class, order, genus, and species. A
pictorial representation of the scheme of classification
would have the form of a genealogical tree (Fig. 197),
the relative positions of whose branches would indicate
the degree of relationship among the different groups.
This scheme implies that there is actual genetic relation-
ship and community of descent of all animal forms, the
Metazoa from the Protozoa, the air-breathing vertebrates
from fishlike ancestors, the birds and mammals from
reptilian prototypes. There is thus evolution of the
more complex from forms of simpler structure. The
underlying principle of classification is heredity, or
community of descent, as indicated by family likeness.
After trying many other structural features as means of
classification systematic zoologists found that the surest
guides in determining relationship are frequently organs
of little or no assignable physiological importance.
There was no explanation of this seeming paradox
until it was seen that, according to Darwin's theory,
such organs are not likely to undergo change, since they
are apparently not of vital importance to the possessor
and are handed down through successive generations
little, if at all, modified, however much the rest of the
body may have changed in becoming adapted to its
environment.

(2) Evidence from Morphology. — The comparative
anatomy of animals furnishes some of the strongest
evidence in favor of the theory of evolution. Note, for
instance, the increasing complexity of form and function
as the various branches are passed in review — the single-
celled Protozoa, showing colony formation in the higher
orders with differentiation in form and function among the
members, as Zoothamnium ; the loosely cellular sponges,

458 COMPARATIVE ZOOLOGY

the lowest of the Metazoa, long considered to be col-
onies of Protozoa, so ill defined are their layers of tissue;
the two-layered coelenterates whose bodies contain but
a single cavity with one opening serving for ingestion
and egestion; the "worms," with a body consisting,
except in some of the parasitic and degenerate forms,
of three layers of tissue (ectoderm, mesoderm, and
endoderm), with an alimentary canal having both inlet
and outlet, a well-defined nervous system, and, in the
higher orders, a segmented body ; the arthropods, with
their segments showing a tendency to become grouped
into distinct regions, with jointed appendages for per-
forming functions, and with respiratory organs, in the
higher groups, for breathing air directly ; the verte-
brates, beginning with forms which have the merest
trace of a notochord, and progressing through the lower
fishes with cartilaginous skeletons, small brains, and
two-chambered hearts, to the amphibia and reptiles, with
bony skeletons, larger brains, and three-chambered
hearts, and finally to the warm-blooded birds and mam-
mals, with four-chambered hearts, and with brains and
nervous systems far superior in size, structure, and
function to those of all other groups. This hasty
review does not, by any means, take cognizance of all
the structural features that might be mentioned, but
only draws attention to some of the most obvious char-
acters which show progressive change from lower to
higher forms.

The metameric arrangement of the bodies of the
annulata, in which each segment is a more or less per-
fect repetition of the preceding and succeeding seg-
ments, is again recognizable, though less plain, in certain
structures in the bodies of crustaceans and insects, and
is very obvious in the chordates from fishes to man, as
the vertebrae and pairs of ribs, the muscle plates and

THE ORIGIN OF ANIMAL SPECIES 459

pairs of muscles, the spinal nerves and ganglia, the
intercostal arteries and veins.

A comparison of such dissimilar organs as the wing
of the bat and the bird, the flipper of the seal, the pec-
toral fin of the fish, the hoof of the horse, and the hand
of man shows evidence of genetic relationship in that all
are constructed on one fundamental plan, which has
been modified to meet the needs of different environ-
ments.

The testimony of rudimentary organs is also in favor
of the theory of descent with modification. The embryo
of the whalebone whale has teeth, but they never cut
the gum. Their presence is explainable only on the
hypothesis that this animal is a descendant of some form
that had functional teeth. Nearly half of the beetles
inhabiting the wind-swept island of Madeira have such
rudimentary wings that flight is impossible, though there
is no doubt that these insects were once capable of fly-
ing, since the nearest related species which live on the
mainland have fully developed wings. In this case,
inability to fly is a distinct advantage, because it renders
the insect less likely to be blown out to sea and drowned.
The presence of rudimentary and functionless eyes in
cave-inhabiting animals indicates descent from ancestors
having perfect visual organs.

(3) Evidence from Embryology. — Of the many im-
portant facts which this branch of science offers in
support of the theory of evolution, only a few can be
mentioned here. It has been learned that higher ani-
mals in the course of their embryonic development pass
through stages which are permanent conditions in lower
forms. Thus the bird and the mammal, though they
never possess gills in their adult life, have at an early
stage of existence a series of openings, gill slits, in the
side of the neck, corresponding to the gill openings of

460 COMPARATIVE ZOOLOGY

the fish, with a system of blood vessels similar to those
in the fish's gills. These openings afterward close and
disappear, and the most of the blood vessels waste away.
Thus, a condition which is permanent in the fish is only
transitory in the higher vertebrates, and it can be ex-
plained only on the supposition that during their devel-
opment these forms repeat the phases through which
their ancestors passed in the course of their evolution.
Embryology thus corroborates paleontology in showing
that the earliest vertebrates were fishlike. All of the
vertebrates possess at a certain period of their embryonic
life a notochord or rod of cartilage, which is later re-
placed by a vertebral column in all forms except Amphi-
oxus, in which the organ persists. This indicates that
vertebrates are descended from an amphioxus-like ances-
tor. Until their embryological history was learned,
Ascidians were considered to be mollusks. The dis-
covery of a rudimentary notochord at an early stage of
their development showed that they are closely allied to
the vertebrates. The adult halibut, turbot, sole, and
other "flat fish " have both eyes on the same side of the
head. In the embryonic stage the eyes are placed as
in other fishes, showing that in their ancestors the eyes
were in the usual position. A West Indian frog, Hylo-
des, which lays eggs on land, passes through its tadpole
stage in the egg. It has a large tail, like the ordinary
tadpole, and gill slits in place of functional gills. The
tail wastes away almost entirely, and lungs are devel-
oped before hatching occurs, the young animal thus
entering upon its terrestrial mode of life unhampered by
organs suited only to an aquatic existence. Its develop-
ment shows that it is descended from ancestral forms
which had both tail and gills.

The tendency of animals to. pass through stages of
development in which they temporarily exhibit features

THE ORIGIN OF ANIMAL SPECIES 461

which are permanent characteristics of forms lower than
themselves, is the basis of the Recapitulation Theory,
which holds that each animal bears the marks of its own
ancestry and reveals its parentage in its own develop-
ment.

(4) Evidence from Paleontology. — Although only a
very small part of the earth's crust has been examined,
the fossil animal remains already found furnish the pri-
mary and most direct evidence in favor of the theory of
descent with modification, and they show that the pro-
cess began as far back in geological history as they can
be traced. The conclusions reached from the study of
fossils may be stated in the words of the great paleon-
tologist Zittel : (i) All stratified sedimentary rocks
(with the exception of metamorphic rocks) inclose, more
or less richly, fossils, and thus prove that the earth, for
an immeasurable length of time before the appearance
of man, was inhabited by organisms. (2) Fossils of the
oldest and deepest strata represent extinct species, and
for the most part extinct genera; only in the more
recent strata are found forms which are identical with
those now living. The deeper down we penetrate in
the series of strata, the more divergent are the fossils
from the forms now living ; and, on the contrary, rising
from the earliest to the more recent formations there
is a continuously increasing resemblance to the present
creation. (3) The different fossil faunas and floras
follow each other the world over in the same regular
sequence ; the formations stratigraphically nearer to
each other contain the most similar fossils, and those
most separated in age present the greater differences.
(4) Constant change characterizes the evolution of the
organic creation. Species of one geological formation
are either completely or partly replaced by other species
in the next superimposed strata. (5) Each species, like

462 COMPARATIVE ZOOLOGY

the individual, has a certain shorter or longer life period,
after which it perishes, never to reappear.*

The genealogical history, or line of descent, of several
animals has been very completely established since
fossils came to be studied in the light of the theory
of evolution. The ancestry of the horse has been
traced through forms which follow one another in
linear series from remote geological periods. The
earliest form was about as large as a sheep and had
five toes and short molar teeth, ' with a comparatively
simple arrangement of the ridges on their crowns.
This was succeeded by four-toed, three-toed, and eventu-
ally the single-toed horse of modern times. The dimi-
nution in the number of the digits was accompanied by
gradually increasing stature and growing complexity of
the crown patterns of the teeth. An even more com-
plete series of remains from the bed of an ancient
lake establishes the genealogy of the fresh-water snail,
Planorbis. " In passing from the lowest to the highest
strata the species change greatly and many times, the
extreme forms being so different that, were it not for
the intermediate forms, they would be called not only
different species, but different genera. And yet the
gradations are so insensible that the whole series is
nothing' less than a demonstration, in this case at least,
of origin of species by derivation with modifications. "f
Other series show the evolution of the horn-bearing
ruminants from hornless ancestors. Casts of the brain
cavities of the early mammals show that their brains
were much smaller than those of living species and had
fewer, if any, convolutions.

Of the " missing links " none is more instructive than

* Quoted from Williams's " Geological Biology," pp. 82-83.
t Quoted from Le Conte's "Evolution and its Relations to Religious
Thought," pp. 254-255.

THE ORIGIN OF ANIMAL SPECIES 463

Archceopteryx, which occupies a position between the
reptiles and the birds. Its fossil remains show its
reptilian characters in the separate digits on the fore limb,
the elongated tail consisting of many vertebrae, and the
well-developed teeth in each jaw. Its most prominent
avian features were its wings and covering of feathers.
The discovery of extinct toothed birds served to make
the connection between birds and reptiles complete.

(5) Evidence from Geographical Distribution. — When
the faunas of the different continents are compared,
they are found to be very unlike. Even in those re-
gions which have much the same climate and other
physical conditions, as South Africa, South America,
and Australia, the faunas are not correspondingly simi-
lar. On the other hand, when the animals inhabiting
the northern part of South America are compared with
those living in the southern portion, there are found to
be closer resemblances than in the instances just noted,
even though the climatal differences are much greater.
A similar statement could be made regarding other
great continental areas. There is no native species
of mammal common to Europe, America, and Australia,
though introduced species thrive. Rabbits, for example,
taken from Europe to Australia have multiplied to such
an extent as to have become veritable pests. Evidently
differences of climate do not alone account for the pres-
ent geographical distribution of animals. Great barriers,
as oceans, lofty mountain ranges, and deserts, separate
faunas, though the differences are not so great as in the
case of distinct continents. Again, while it is noted that
different regions of a continent are inhabited by distinct
species, it is found that these species are more nearly
related among themselves than to the species of other
continents. For instance, the humming birds, near rela-
tives of the sunbirds of Africa and Asia, number about

464 COMPARATIVE ZOOLOGY

four hundred species, and are all confined to the Western
Hemisphere. The explanation of this fact is that they
originated in this part of the world, and are too small
and weak to make the long flight necessary to reach
other regions.

Islands are usually populated by forms brought from
the nearest mainland, unless the ocean currents are such
as to bring animals from places more remote. Such
islands as are separated from a neighboring continent
by deep channels have a fauna more archaic and primi-
tive than that found on the mainland. Australia and New
Zealand are thought to have been separated from the
nearest larger bodies of land for long geological periods,
and possibly since the time of their formation. Their
faunas are of a very primitive type, including the mar-
supials, one of the oldest and least highly developed
orders of mammals; the monotremes, Ornithorhynchus
and Echidna, the lowest representatives of the same
class, and Apteryx, the lowest of living birds. Isolated
on their island continents and free from the competition
of higher forms, and especially from the attacks of car-
nivores, these lowly organized and almost defenseless
species have retained to a marked degree the character-
istics of their remote ancestors. Where the separation
of island and continent has taken place more recently,
or where the channel is shallow or narrow, there is
greater resemblance between their faunas. Thus, wild
animals of Great Britain are quite the same as those
of western Europe. The number of species found on
islands is usually small as compared with those inhabit-
ing an equal continental area, because the number of
ancestral forms which have been carried to the island by
currents, wind, and man is likely to be small.

The animals found on high mountain peaks and ranges
are distinctly allied to arctic forms. On the northward

THE ORIGIN OF ANIMAL SPECIES 465

retreat of the great ice sheet which, during the last gla-
cial period, covered much of Europe, Asia, and North
America, these boreal species were left stranded in, and
have since been confined to, regions having arctic char-
acteristics. Species which are closely related to one
another are known to inhabit mountain peaks, separated
by long stretches of lowland, on which animals of the
arctic type could not possibly exist. Their presence
can be explained only on the supposition that they are
descended from forms which inhabited the entire region
during the glacial period and that when the climate
became warmer these animals retreated to the cold
mountain tops. Many oceanic islands are destitute of
batrachians and terrestrial mammals, these animals not
having had an independent evolution in these localities,
nor being able to make their way out from the mainland.
On the other hand, aerial mammals, as bats, are of
nearly universal distribution.

It is generally admitted that each species originated in
one locality and, by migration, spread into neighboring
regions, becoming modified as dispersal brought it into
different environments, thus giving rise to variations
which ultimately resulted in the development of a num-
ber of more or less closely related species.

Such are the main features of the theory of evolution
and of Darwin's explanation of the process through va-
riation, heredity, and natural selection. As a subordinate
factor should be mentioned his theory of sexual selection,
by which is meant that the choice of mates, among the
higher animals at least, is largely determined by such
physical characteristics as strength, beauty of form,
coloration, and vocal powers. Those individuals, for
instance, which possessed any of these pleasing char-
acteristics in a higher degree than their companions
would be more likely to find mates and to leave de-
DODGE'S GEN. ZOOL. — 30

466 COMPARATIVE ZOOLOGY

scendants. On this theory Darwin accounted for the
development of antlers, the beautiful colors of birds,
fishes, and insects, and the calls of various animals.

While evolution has come to be regarded as a fact of
as much certainty as gravitation, and natural selection,
with variation and heredity, to be accepted by many
naturalists as the process by which evolution is brought
about, not all are agreed as fo the importance to be
attributed to the Darwinian factor, i.e. natural selection.
Darwin himself regarded it as "the main but not the
exclusive means of modification." Search for additional
means has been made and is still being prosecuted. Thus
consciousness is claimed to be a controlling agent in the
use and disuse of organs, in the adoption of new habits,
in the selection of environment, in the choice of mates,
and so on. Isolation due to the geographical separation
of individuals or of species, or to the inability of forms
to interbreed {physiological isolation) has been suggested
as another cause of modification. Still another factor,
organic selection, has been proposed. It is claimed for
this that the adoption of a new habit by an animal will
lead to the development of structures adapted to the
habit, and thus produce changes in specific differences.

The most important addition to the philosophy of
organic evolution made since Darwin is Weismann's
theory of the continuity of the germ plasm, which
maintains, supported by facts of observation, that the
essential germinal substance is transmitted from gen-
eration to generation through the reproductive cells.
Whether or not this material — the bearer of heredity
— may be influenced by structural and physiological
changes occurring in the species, and thus be trans-
mitted to descendants, is a question which has not yet
been definitely answered.

NOTES

1 The complete and elaborate natural history of a single species or lim-
ited group is called a Monograph, as Darwin's " Monograph of the Cirri-
pedia." A Memoir is not so formal or exhaustive, giving mainly original
investigations of a special subject, as Owen's " Memoir on the Gorilla."

2 Before the time of Linnaeus, the ladybug, e.g., was called "the Cocci-
nella with red coleopters having seven black spots." He called it Cocci-
nella septem-punctata.

8 Mondino (1315) and Berenger (1518) of Bologna, and Vesalius of
Brussels (1543), were the first anatomists. Circulation of the blood dis-
covered by Harvey, 1616. The lacteals discovered by Asellius, 1622, and
the lymphatics by Rudbek, 1650. Willis made the first minute anatomy
of the brain and nerves, 1659. The red blood corpuscles were discovered
by Swammerdam, 1658. Infusoria first observed by Leeuwenhoek, 1675;
the name given by M tiller, 1786. Swammerdam was the founder of
Entomology, 1675. Comparative anatomy was first cultivated by Perrault,
Pecquet, Duverney, and Mery, of the Academy of Paris, the latter part
of the seventeenth century. Malpighi, the founder of structural anatomy,
was the first to demonstrate the structure of the lungs and skin, 1661.
About the same time, Ray and Willoughby first classified fishes on struc-
tural grounds. Foraminifers were seen by Beccarius one hundred and
fifty years ago; but their true structure was not demonstrated till
r835> by Dujardin. Peyssonel published the first elaborate treatise on
Corals, 1727. Haller was the first to distinguish between contractility
and sensibility, 1739. White blood corpuscles discovered by Hewson in
1775. Spallanzani was the first to demonstrate the true nature of the
digestive process, 1777. Cuvier and Geoffrey, in 1797, proposed the first
natural classification of animals. Before that, all invertebrates were di-
vided into insects and worms. Lamarck was the first to study mollusks,
1800; before him, attention was confined to the shell. He separated
spiders from insects in 1812. The law of correlation enunciated by Cuvier,
1826. Von Baer was the founder of Embryology, establishing the doctrine
omnia ex ovo, 1827; but the first researches in Reproduction were made
by Fabricius about 1600, and by Harvey in 1651. Wolff, in the i8th cen-
tury, was the pioneer in observing the phenomena of Development. Sars
first observed alternate generation, 1833. Dumeril is considered the

467

468 NOTES

father of Herpetology, and Owen of Odontology. Schleiden and Schwann
published their celebrated researches in cell structure, 1841; but Bichat,
who died 1802, was the founder of Histology. Protoplasm was discovered
by Dujardin in 1835, anc^ called Sarcode. The name Protoplasma was
formally given to the slimy contents of vegetable cells by the German bot-
anist, Hugo von Mohl, in 1846. The essential identity of the protoplasm
of plants and of animals was first claimed by Max Schulze in 1861, who
thus made one of the most important generalizations in science.

4 According to Mr. Darwin, the characters which naturalists consider
as showing true affinity between two or more species are those which have
been inherited from a common parent ; and, in so far, all true classification
is genealogical, i.e., it is not a mere grouping of like with like, but it in-
cludes, like descent, the cause of similarity. In the existing state of science
a perfect classification is impossible, for it involves a perfect knowledge of
all animal structure and life history. As it is, it is only a provisional at-
tempt to express the real order of nature, and it comes as near to it as our
laws do in explaining phenomena. It simply states what we now know
about comparative anatomy and physiology. As science grows, its lan-
guage will become more precise and its classification more natural.

5 The term type is also used to signify that form which presents all the
characters of the group most completely. Each genus has its typical
species, each order its typical genus, etc. The word is also applied to
the specimen on which a new species is founded. A persistent type is one
which has continued with very little change through a great range of time.
The family of oysters has existed through many geological ages.

6 The Coelenterata and Echinodermata together make up the Radiata,
the old subkingdom of Cuvier. Echinoderma is probably more correct -
than Echinodermata^ but we retain the old orthography.

7 Strictly speaking, no individual is independent. Such is the division
of labor in a hive, that a single bee, removed from the community, will
soon die, for its life is bound up with the whole. An individual repeats
the type of its kingdom, branch, class, order, family, genus, and species,
through its whole line of descent.

8 These definitions of the various groups are mainly taken from Agassiz.
They are not practically very useful, as they are not used by working natu-
ralists. The kind and degree of difference entitling a group to a particular
rank varies greatly with the naturalist, and the part of the animal kingdom
where the group is found. Some families of insects are separated by gaps
less than those which divide genera of mammals.

9 The millepore coral, so abundant in the West Indian Sea, is the work
of hydroids. The surface is nearly smooth, with minute punctures. Ge-
genbaur, Haeckel, and others hold that the acalephs have no body cavity
at all, the internal system of canals being homologous with the intestinal
cavity of other animals.

NOTES 469

10 This digestive cavity is really homologous to the proboscis of the
jellyfish, turned in. It is lined with ectoderm. The "body cavity" is
not really such, but is homologous to the digestive sac of the hydra.

11 Among the exceptions are Tubipora, which have eight tentacles and
no septa, and the extinct Cyathophylla, whose septa are eight or more.

12 The longer septa (called primary) are the older; the shorter, sec-
ondary ones are developed afterward. As a rule, sclerodermic corals are
calcareous, and a section is starlike; the sclerobasic are horny and solid.
The latter are higher in rank.

13 The most important genera are Terebratula, Rhynchonella, Discina,
Lingula, Orthis, Spirifera, and Productus. The first four have represen-
tatives in existing seas. Most naturalists now admit their affinity to the
worms, some still keep them in the branch Mollusca, while others include
them in the separate branch Molluscoida.

14 Some starfishes {Solaster} have twelve rays. In all echinoderms,
probably, sea water is freely admitted into the body cavity around the
viscera.

15 The shell is not strictly external, like the crust of a lobster, but is
covered by the external skin.

16 Six hundred pieces have been counted in the shell alone, and twelve
hundred spines. The feet number about eighteen hundred. They can
be protruded beyond the longest spines.

. 17 Certain crabs live on dry land, but they manage to keep their gills
wet.

18 The student should remember that this threefold division is not
equivalent to the like division of a vertebrate body.

19 Each ring (called somite) is divisible into two arcs, a dorsal and
ventral.

20 The eye stalks were formerly considered to be appendages, but are
no longer so regarded.

21 These parts do not correspond to the parts so named in human
anatomy. See also pp. 371, 372.

22 The four pairs of legs in arachnids answer to the third pair of max-
illge and the three pairs of maxillipedes in the lobster. The great claws of
scorpions and the pedipalpi of spiders correspond to the first maxillae of
the lobster.

23 Compare the single thread of the silkworm and other caterpillars.

24 The common spider, Epeira, which constructs with almost geometri-
cal precision its net of spirals and radiating threads, will finish one in forty
minutes, and just as regularly if confined in a perfectly dark place.

25 There are some exceptions : the oyster is unequivalved, and the pecten
equilateral.

26 The chief impressions left on the shell are those made by the muscles
— the dark spots called "eyes" by oystermen ; the pallial line made by

4/0 NOTES

the margin of the mantle ; and the bend in the pallia! line, called pallial
sinus, which exists in those shells having retractile siphons, as the clam.

27 The clam is the highest of lamellibranchs, and the oyster one of the
lowest. The Mya arenaria, or " soft clam," has its shell always open a
little; while Venus mercenaria, or "hard clam," keeps its shell closed
when disturbed.

28 The slug has no shell to speak of. It may be remembered, as a rule,
that all univalve shells in and around the United States are gastropods,
and that all bivalves in our rivers and lakes, and along our seacoasts (save
a few brachiopods), are pelecypods (lamellibranchs).

29 Hold the shell with the apex up and the mouth toward the observer.
If the mouth is on his right, the shell is right-handed or dextral, if on his
left, sinistral. In other words, a right handed shell is like a right-handed
screw.

30 Instead of a strong breathing tube with a valve, answering for a force-
pump and propeller, as in the cuttlefish, it has only an open gutter made
by a fold in the mantle, like the siphons of the gastropods. The back
chambers are filled with gas.

The common poulpe has two thousand suckers, each a wonderful little
pump, under the control of the animal's will.

81 The facial angle becomes of less and less importance as we go away
from man, and for two reasons. Where the brain does not fill the brain
case the angle is obviously of little value, and if the jaws are largely de-
veloped the angle is reduced, although intelligence may not be altered.

32 Oblong human skulls, whose diameter from the frontal to the occipi-
tal greatly exceeds the transverse diameter, are called dolichocephalic ; and
such are usually prognathous, i.e., have projecting jaws, as the negro's.
Round skulls, whose extreme length does not exceed the extreme breadth
by a greater proportion than 100 to 80, are brachycephalic ; and such are
generally ortkognathous, or straight-jawed.

33 The classes are variously grouped into the Hematocrya, or cold-
blooded, and the Hematotherma, or warm-blooded ; into the Branchiata
and Abranchiata ; into the Allantoidea and Anallantoidea.

34 Amphibians with a moist skin are also remarkable for their cutaneous
respiration. They will live many days after the lungs are removed. Their
vertebrae vary in form : in the lowest they are biconcave, like those of
fishes ; in salamanders they are opisthocoelous : in the frogs and toads
they are usually proccelous.

85 Salamanders are often taken for lizards, but differ in having gills in
early life and a naked skin. The proteus and siren resemble a tadpole
arrested in its development.

86 The Surinam toad has no tongue.

37 There are some notable exceptions. The slow worm is legless, and
the chameleon has a soft skin, with minute scales.

NOTES

471

38 The posterior pair of limbs is sometimes represented by a pair of
small bones; and the boas and pythons show traces of external hind
limbs.

39 The plastron is formed partly of dermal and partly of endoskeletal
pieces.

40 Knees always bend, forward, and heels always bend backward.

41 We cannot claim that this airy skeleton is necessary for flight. The
bones of the bat are free from air, yet it is able to keep longer on the
wing than the sparrow. The common fowl has a hollow humerus ; while
some birds of long flight, as the snipe and curlew, have airless bones.

42 Hopping is characteristic of and confined to the perchers ; but many
of them, as the meadow lark, blackbird, and crow, walk.

43 This order is artificial. But it is better to retain it until ornitholo-
gists agree upon some natural arrangement.

44 The whales are hairy during foetal life only.

45 The manatee has 6 ; Hoffmann's sloth 6 ; and two species of three-
toed sloth have respectively 8 and 9.

46 As in the whale, porpoise, seal, and mole. Teeth are wanting in the
whalebone whales, ant-eaters, manis, and echidna.

47 The monotremes resemble marsupials in having marsupial bones, but
have no pouch. They differ from all other mammals in having no distinct
nipples.

48 The pouch is wanting in some opossums and the dasyurus.

49 The extinct horse {Hipparion) had three toes, two small hoofs dan-
gling behind. The foot of the horse is of wonderful structure. The bones
are constructed and placed with a view to speed, lightness, and strength,
and bound together by ligaments of marvelous tenacity. There are elastic
pads and cartilages to prevent jarring ; and all the parts are covered by a
living membrane which is exquisitely sensitive, and endows the foot with
the sense of touch, without which the animal could not be sure-footed.
The hoof itself is made of parallel fibers, each a tube composed of thousands
of minute cells, the tubular form giving strength. There are three parts,
"wall," "sole," and "frog" — the triangular, elastic piece in the middle,
which acts as a cushion to prevent concussion and also slipping.

60 The fore feet of the tapir have four toes, but one does not touch the
ground.

51 The camel and llama are exceptional, having two upper incisors and
canines, are not strictly cloven-footed, having pads rather than hoofs, and
are hornless.

52 For the best account of the elephant, see Tennant's " Ceylon " or
Brehm's "Thierleben."

53 The hyena alone of the carnivores has only four toes on all the limbs,
and the dog has four hind toes.

64 The old term Quadrumana is rejected, because 'it misleads, for apes,

472 NOTES

as well as men, have two feet and two hands. There is as much anatomi-
cal difference between the feet and hands of an ape as between the feet
and hands of man. Owen, however, with Cuvier, considers the apes truly
" four-handed."

55 The eye orbits of the lemurs are open behind. The flying lemur
{Galeopithecus) is considered an insectivore,

56 It fails to cover in the howling monkey and siamang gibbon ; but in
the squirrel monkey it more than covers, overlapping more than in man.
As to the convolutions, there is every grade, from the almost smooth brain
of the marmoset to that of the chimpanzee or orang, which falls but little
below man's.

57 The tailed apes of the Old World have Iqnger legs than arms, and
generally have " cheek pouches," which serve as pockets for the temporary
stowage of food.

58 In the human infant, the sole naturally turns inward; and the arms
of the embryo are longer than the legs.

59 The aye-aye, one of the lowest of the lemurs, is remarkable for the
large proportion of the cranium to the face.

60 This feature was shared by the extinct Anoplotherium, and now to
some extent by one of the lemurs ( Tarsius).

61 We have treated man zoologically only. His place in nature is a
wider question than his position in Zoology ; but it involves metaphysical
and psychological considerations which do not belong here.

62 This twofold division is arbitrary. No essential distinction, founded
on the nature of the elements concerned, or the laws of their combination,
can be made ; and so many so-called organic substances, as urea, am-
monia, alcohol, tartaric and oxalic acids, alizarine, and glucose, have been
prepared by inorganic methods, that the boundary line is daily becoming
fainter, and may in time vanish altogether. We would here utter our pro-
test against the introduction of any more terms like inorganic, invertebrate,
acephalous, etc., which express no qualities.

63 Even the works of nearly all animals, as nests and burrows, are bounded
by curved lines.

64 London Quarterly Review, January, 1869, p. 142. It is true of any
.great primary group of animals, as of a tree, that it is much more easy to
define the summit than the base.

65 "There are certain phenomena, even among the higher plants, con-
nected with the habits of climbing plants and with the functions of fertiliza-
tion, which it is very difficult to explain without admitting some low form
of a general harmonizing and regulating function, comparable to such an
obscure manifestation of reflex nervous action as we have in sponges and
in other animals in which a distinct nervous system is absent." — Pro-
fessor WYVILLE THOMSON'S Introductory Lecture at Edinburgh.

66 " If nature had endowed us with microscopic powers of vision, and the

NOTES 473

integuments of plants had been rendered perfectly transparent to our eyes,
the vegetable world would present a very different aspect from the apparent
immobility and repose in which it is now manifested to our senses." —
HUMBOLDT'S Cosmos, i., 341.

67 See Gray's " Structural Botany," 6th ed., Introduction ; also Rolles-
ton's " Forms of Animal Life," Introduction.

68 " Life has been called the vital force, and it has been suggested that
it may be found to belong to the same category as the convertible forces,
heat and light. Life seems, however, to be more a property of matter in a
certain state of combination than a force. It does no work, in the ordinary
sense." — Professor WYVILLE THOMSON.

69 The vegetable cell usually consists of a cell wall surrounding the pri-
mordial utricle or protoplasmic sac. In animal cells the former, though
often present, is usually not easily seen. As a general fact, animal cells
are smaller than vegetable cells.

70 Cells are not the sources of life, as once thought, but are the products
of protoplasm. " They are no more the producers of vital phenomena than
the shells scattered in orderly lines along the sea beach are the instruments
by which the gravitation force of the moon acts upon the ocean. Like
these, the cells mark only where the vital tides have been and how they
have acted." — Professor HUXLEY.

71 Many of the bones of the skull are preceded by membrane — hence
called membrane bones,

72 In the heart, the muscular fibers are striated, yet involuntary ; but the
sarcolemma is wanting.

73 We may, however, infer that the animal functions are not absolutely
essential to the vegetative, from the facts that plants digest without muscles
or nerves, and that nutrition takes place in the embryo long before the
nerves have been developed.

74 Scorpions and spiders properly feed upon the juices of their victims
after lacerating them with their jaws, but fragments of insects have been
found in their stomachs.

75 The real tongue forms the floor of the mouth, and is found as a distinct
part in a few insects, as the crickets.

76 In the cyclostomata, it is circular or oval.

77 The mouth of the whale is exceptional, the walls not being dilatable.
The act of sucking is characteristic of all young mammals, hence the need
of lips.

78 The ant-eater has two callous ridges in the mouth, against which the
insects are crushed by the action of the tongue.

79 The baleen plates do not represent teeth ; for in the embryo of the
whale we find minute calcareous teeth in both jaws, which never cut the gum.
The whalebone is a peculiar development of hair in the palate, and under
the microscope it is seen to be made up of fibers which are hollow tubes.

474 NOTES

80 The " tusks " of the elephant are prolonged incisors ; those of the
walrus, wild boar, and narwhal are canines.

81 " I was one day talking with Professor Owen in the Hunterian Mu-
seum, when a gentleman approached, with a request to be informed respect-
ing the nature of a curious fossil which had been dug up by one of his
workmen. As he drew the fossil from a small bag, and was about to
hand it for examination, Owen quietly remarked, ' That is the third molar
of the under jaw of an extinct species of rhinoceros.' " — LEWES'S Studies
in Animal Life.

82 This gap or interspace, so characteristic of the inferior mammals, is
called diastema. It is wanting in the extinct anoplotherium, is hardly
perceptible in one of the lemurs, and is not found in man.

83 In the spermaceti whale, the teeth are fixed to the gum.

84 The iguana among reptiles, and fishes with pavement teeth, approach
the mammal in this respect.

85 This movement is called peristaltic or vermicular, and characterizes
all the successive movements of the alimentary canal.

86 Fishes and amphibians have no saliva, but a short gullet. Birds are
aided by a sudden upward jerk of the head.

87 Fishes and reptiles have no pharynx proper, the nostrils and glottis
opening into the mouth.

88 This movement of the pharynx and esophagus is wholly involuntary.
Liquids are swallowed in exactly the same way as solids.

89 The few animals in which the digestive cavity is wanting are called
agastric, and agree in having a very simple structure. Such are some
Entozoa (as tapeworm) and unicellular Protozoa (as Gregarina}. They
absorb the juices, already prepared, by the physical process of endosmose.
There are other minute organisms (bacteria) which seem to be able to ex-
tract the necessary elements, C H O N, from the medium in which they live.

90 The cavity of a sponge is perhaps homologous with the digestive
cavity, but is not functionally such. Each cell lining it does its own
digestion, taking the food from the water circulating in the cavity.

91 " Nothing is more curious and entertaining than to watch the neat-
ness and accuracy with which this process is performed. One may see the
rejected bits of food passing rapidly along the lines upon which these
pedicellariae occur in greatest number, as if they were so many little roads
for the conveying away of the refuse matters ; nor do the forks cease from
their labor till the surface of the animal is completely clean and free from
any foreign substance." — AGASSIZ'S Seaside Studies.

92 In the larva of the bee, the anal orifice is wanting.

93 The length of the canal in insects is not so indicative of the habits as
in mammals. Thus, in the carnivorous beetle the canal is nearly as long
as, and more complicated than, it is in the nectar-sipping butterflies.

94 The object of this is unknown. It does not occur in the oyster.

NOTES 475

95 In the nautilus, this is preceded by a capacious crop.

96 In the shark, this is impossible, owing to a great number of fringes in
the gullet hanging down toward the stomach.

97 At the beginning of the large intestine in the lizards (and in many
vertebrates above them, especially the vegetarian orders), there is a blind
sac, called cacmn.

98 The crocodile is said to swallow stones sometimes, like birds, to aid
the gastric mill.

99 In the crop of the common fowl, vegetable food is detained sixteen
hours, or twice as long as animal food. The dormouse, among mammals,
has an approach to a crop.

100 In invertebrates, the gizzard, when present, is situated between the
crop and the true stomach ; in birds, it comes after the stomach.

101 The tapeworm has no digestive apparatus, but absorbs the already
digested food of its host. This is no exception to the rule. The chemical
preparation of the food has preceded its absorption.

102 \ye finc[ tne most abundant saliva in those mammals that feed on
herbs and grain, but its action on starch is extremely feeble.

103 The acid in the gastric juice has an important function in killing or
preventing the growth of bacteria which are taken in with the food. The
gastric juice also dissolves the albuminous walls of fat cells, thus permitting
the contained fats to escape. The drops of fat fuse together into larger
masses, which are later broken up into droplets or emulsified by the pan-
creatic juice.

104 It is probablq that the digestive part of the alimentary canal in all
animals manifests a similar mechanical movement. It is most remarkable
in the gizzard of a fowl, which corresponds to the pyloric end of the human
stomach. This muscular organ, supplying the want of a masticatory appa-
ratus in the head, is powerful enough to pulverize not only grain, but even
pieces of glass and metal. This is done by two hard muscles moving
obliquely upon each other, aided by gravel purposely swallowed by the
bird. The grinding may be heard by means of the stethoscope.

105 Chyle is opaque in carnivores ; more or less transparent in all other
vertebrates, as in birds, since the food does not contain fatty matter.

106 In fishes, the villi are few or wanting. In man, they number about
10,000 to the square inch.

107 Except, perhaps, the tendons, ligaments, epidermis, etc.

108 The blood is colorless also in the muscular part of fishes. That of
birds is of the deepest red. The coloring matter of the red blood in worms
is not in the corpuscles, but in the plasma.

109 Coagulation may be artificially arrested for a brief time by common
salt. Arterial blood coagulates more rapidly than venous. The disposi-
tion of the red corpuscles in chains, or rouleaux, does not occur within the
blood vessels. The cause has not been discovered.

476 NOTES

110 The corpuscles of invertebrates are usually colorless, even when the
blood is tinged.

111 Except during the foetal life. The corpuscles of the camel are non-
nucleated, as in other mammals. If the transparent fluid from a boil be
examined with a microscope, it will be seen to be almost entirely composed
of colorless corpuscles.

112 There are no valves in the veins of fishes, reptiles, and whales, and
few in birds.

113 Capillaries are wanting in the epidermis, nails, hair, teeth, and carti-
lages. Hence, the epidermis, for example, when worn out by use, is not
removed by the blood, like other tissues, but is shed.

114 A part of the blood, however, in going from the capillaries of the
digestive organs to the heart, is turned aside and made to pass through the
liver and kidneys for purification. This is called the portal circulation,
and exists in all vertebrates, except that in birds and mammals it is con-
fined to the liver.

115 Two in the higher mammals, three in the lower mammals, birds, and
reptiles. They are called vena cava.

116 Tricuspid in mammals, triangular in birds.

117 The pulse of a hen is 140 ; of a cat, 1 10 to 120 ; of a dog, 90 to 100 ;
and of an ox, 25 to 42.

118 The bivalve brachiopods breathe by delicate fringed arms about the
mouth, and by the " mantle. "

119 The air bladder, found in most fishes, is another rudiment of a lung,
although it is used, not for respiration, but for altering the specific gravity
of the fish. In the gar pike of our Northern lakes it very closely resem-
bles a lung, having a cellular structure, a tracheal tube, and a glottis. It is
here functional. The gills represent lungs only in function ; they are totally
distinct parts of the organism.

12) In the human lungs they number 600,000,000, each about TJ7 of an
inch in diameter, with an aggregate area of 132 square feet. The thickness
of the membrane between the blood and the air is ^Q^ of an inch. The
lungs of carnivores are more highly developed than those of herbivores.
In the manatee, they are not confined to the thorax, but extend down
nearly to the tail.

121 Crocodiles are the only reptiles whose nostrils open in the throat be-
hind the palate, instead of directly into the mouth cavity. This enables
the crocodile to drown its victim without drowning itself ; for, by keeping
its snout above water, it can breathe while its mouth is wide open.

122 A rudimentary diaphragm is seen in the crocodile and ostrich.

123 The poison glands of venomous serpents and the silk vessels of cater-
pillars are considered to be modified salivary glands. Birds, snakes, and
cartilaginous fishes have no urinary bladder.

124 Since the weight of a full-grown animal remains nearly uniform, it

NOTES 477

must lose as much as it receives ; that is, the excretions, including the
solid residuum ejected from the intestinal canal, equal the food and drink.

125 Other names for derm are, cutis, corium, enderon, and true skin ;
and for epidermis, cuticle, ecderon, and scarfskin. The derm is often so
intimately blended with the muscles that its existence as a distinct layer is
not easily made out.

126 Papillae are scarcely visible in the skin of reptiles and birds.

127 The animal basis of this structure is chitin, a peculiar hornlike sub-
stance found in the hard parts of all the articulated animals.

128 The shell is always an epidermal structure, even when apparently
internal. The horny "pen" of the squid, the "bone" of the cuttlefish,
and the calcareous spot on the back of'the slug are only concealed under
a fold of the mantle. So the shell of the common unio, or fresh-water
clam, is covered with a brownish or greenish membrane, which is the
outer layer of the epidermis. Where the mantle covers the lips of a shell,
as in most of the large sea snails, or where its folds cover the whole ex-
terior, as in the polished cowry, the epidermis is wanting, or covered up
by an additional layer.

129 The pearls of commerce, found in the mantle of some mollusks, are
similar in structure to the shell ; but what is the innermost layer in the
shell is placed on the outside in the pearl, and is much finer and more
compact. The pearl is formed around some nucleus, as an organic particle,
or grain of sand.

130 \vhen the centrum is concave on both sides, as in fishes, it is said to
be amphiccelous ; when concave in front and convex behind, as in croco-
diles, it is called proccelous ; when concave behind and convex in front, as
in the neck-vertebrae of the ox, it is opisthoccelous. In the last two cases,
the vertebras unite by ball-and-socket joints.

131 Whether the skull represents any definite number of vertebrae was long
under discussion. We cannot speak of " cranial vertebrae " in the same
sense as " cervical vertebrae." The most that can be said is that in a gen-
eral way the skull is homologous to part of the vertebral column.

132 A few have but one pair, the whale and siren wanting the hind pair ;
while some have none at all, as the snakes and lowest vertebrates. In
land animals, the posterior limbs are generally most developed ; in aquatic
animals, the anterior. Dr. Wyman contends that the limbs are tegumen-
tary organs, and attached to the vertebral column in the same sense that
the teeth are attached to the jaws. Other theories are that they originate
from gill arches (Gegenbaur) or that they are remains of a once continu-
ous lateral fin (Thacher).

133 The first trace of muscular tissue is found' in the stem of vorticella —
an infusorian. In hydra we find neuro-muscular cells, and the jellyfishes
have muscular tissue.

134 The muscles of some invertebrates, as spiders, are yellow.

478 NOTES

185 The muscles of the heart and gullet are striped. In the lower ani-
mals these distinctions of voluntary and involuntary, striated and smooth,
solid and hollow, muscles can seldom be made.

136 The skeleton of the carrion crow, for example, weighs, when dry,
only twenty-three grains.

137 The dragon fly can outstrip the swallow ; nay, it can do in the air
more than any bird — it can fly backward and sidelong, to right or left, as
well as forward, and alter its course on the instant without turning. It
makes twenty-eight beats per second with its wings, while the bee makes
one hundred and ninety, and the house fly three hundred and thirty. The
swiftest race horse can run at double the rate of the salmon. So that
insect, bird, quadruped, and fish, would be the order according to velocity
of movement.

138 The theory that flies adhere by atmospheric pressure is now aban-
doned.

189 More precisely, the term brain applies only to the cerebrum, while
the total contents of the cranium are called encephalon.

140 The exact functions of the cerebrum are not yet clearly understood.
If we remove it from fishes, or even birds, their voluntary movements are
little affected, while the Amphioxus, the lowest of fishes, has no brain at
all, but its life is regulated by the spinal cord. Such mutilated animals,
however, make no intelligent efforts. The substance of the cerebrum, as
also the cerebellum, is insensible, and may be cut away without pain to
the animal; and when both are thus removed, the animal still retains
sensation, but not consciousness.

141 It is very difficult to define sensation, or sensibility. The power is
possessed by animals which have neither nervous system nor consciousness.
These low manifestations of sensibility are called irritability — the power-
by which an animal is capable of definitely responding to a stimulus from
without. The response is not called out by the direct action of the stimu-
lus, but is determined mainly by the internal structure and condition of
the animal.

142 Parts destitute of blood vessels, as hair, teeth, nails, cartilage, etc.,
are not sensitive.

143 « Tentacles " and " horns " are more or less retractile, while antennae
are not, but are hollow. Antennae alone are jointed.

144 In man, the soft palate and tonsils also have the power of tasting.

145 No organ of hearing has been discovered with certainty in the
radiates and spiders. The " ear " of many lower animals is probably an
organ for perceiving the animal's position rather than sound — an " equi-
librium organ."

146 It is wanting in the aquatic mammals. Crocodiles have the first
representative of an outside ear in the form of two folds of skin.

147 This, like the definition of smell and hearing, is loose language.

NOTES 479

There is no such thing as sound till the vibrations strike the tympanum,
nor even then, for it is the work of the brain, not of the auditory nerve.
Sound is the sensation produced by the wave movement of the air. If thus
defined in terms of sensation, light is nothing ; without eyes the world
would be wrapped in darkness. Some Protozoa, as Euglena, have a pigment
spot as an eye.

148 In invertebrates and aquatic vertebrates, the crystalline lens is globu-
lar; or, in other words, it is round in short-sighted animals, and flattish in
the long-sighted. The lens of the invertebrate is not exactly the same as
the lens of the vertebrate eye, though it performs the same function; it is
really a part of the cornea.

149 The ant has 50 in each eye, the house fly 400x5, the dragon fly 28,000.

150 The pigment, therefore, while apparently in front of the retina, is
really behind it, as in vertebrates. The layer beneath the cornea, serving
as an " iris," is wanting in nocturnal insects, since they need every ray of
light. The optic nerve alone is insensible to the strongest light.

151 It should be noticed that this corresponds with another peculiar fact
already mentioned, that either hemisphere of the brain controls the muscles
on the opposite side of the body. In invertebrates, the motor apparatus is
governed on its own side.

152 Sharks have eyelids, while snakes have none. The third eyelid (called
nictitating membrane} is rudimentary in many mammals. It may be seen
at the inner angle of the eye.

153 An infant would doubtless learn to walk if brought up by a wild
beast, since it was made to walk, just as an Infusorium moves its cilia, not
because it has any object, but because it can move them. Newborn
puppies, deprived of brains, have suckled; and decapitated centipedes run
rapidly. Such physical instincts exist without mind, and may be termed
" blind impulses."

154 \Ye say " apparently," because it may be a fixed habit, first learned
by experience, transmitted from generation to generation. A duckling may
go to the water, and a hound may follow game in some sense, as Sir John
Herschel devoted himself to astronomy, inheriting a taste from his father.
Breeders take advantage of this power of inheritance.

155 \ye may divide the apparently voluntary actions of animals into three
classes. First, organic, in which consciousness plays no part, and which are
due wholly to the animal machine. Second, instinctive, in which conscious-
ness may be present, but which are not controlled by intelligence. Third,
associative, in which the animals act under conscious combination of dis-
tinct, single ideas, or past impressions. To these we may add rational 'acts,
in which the mental process takes place under the laws of thought.

156 "Thus, while the human organism may be likened to a keyed instru-
ment, from which any music it is capable of producing can be called forth
at the will of the performer, we may compare a bee, or any other insect, to

480 NOTES

a barrel organ, which plays with the greatest exactness a certain number of
tunes that are set upon it, but can do nothing else." — .CARPENTER'S Mental
Physiology, p. 61. This constancy may be largely due to the uniformity
of conditions under which insects live.

157 We may say, as a rule, that the proportion of instinct and intelligence
in an animal corresponds to the relative development of the spinal cord
and cerebrum. As a rule, also, the addition of the power to reason comes
in with the addition of a cerebrum, and is proportioned to its development.
Between the lowest vertebrate and man, therefore, we observe successive
types of intelligence. Intelligence, however, is not according to the size
of the brain (else whales and elephants would be wisest), but rather to the
amount of gray matter in it. A honeycomb and an oriole's nest are con-
structed with more care and art than the hut of the savage. It is true,
this is no test of the capability of the animal in any other direction ; but
when they are fashioned to suit circumstances, there is proof of intelligence
in one direction.

158 An exception.to the general rule that the smaller animals have more
acute voices.

159 It is wanting in a few, as the storks.

160 The nightingale and crow have vocal organs similarly constructed,
yet one sings and the other croaks.

lei Egg cells and sperm cells are detached portions of the parental or-
ganisms. Generally, these two kinds of cells are produced by separate
sexes; but in some cases, as the snail, they originate in the same individual.
Such an animal, in which the two sexes are combined, is called an her-
maphrodite

162 The eggs of mammals are of nearly uniform size; those of birds,
insects, and most other animals are proportioned to the size and habits of
the adult. Thus, the egg of the gepyornis, the great extinct bird' of Mada-
gascar, has the capacity of fifty thousand humming-birds' eggs.

163 As a general rule, when both sexes are of gay and conspicuous
colors, the nest is such as to conceal the sitting bird; while, whenever
there is a striking contrast of colors, the male being gay and the female
dull, the nest is open. Such as form no nest are many of the waders,
swimmers, scratchers, and goatsuckers.

164 This lies at first transversely to the long axis of the egg. As the
chick develops, it turns upon its side.

165 The blood appears before the true blood vessels, in intercellular
spaces. It is at first colorless, or yellowish.

166 Exactly as the blood in the capillaries of the lungs is aerated by the
external air.

167 Thus, the hollow wing bone was at first solid, then a marrow bone,
and finally a thin-walled pneumatic bone. The solid bones of penguins
are examples of arrested development.

NOTES 481

168 The thigh bone ossifies from five centers. The bone eventually
unites to one piece.

169 jror this reason, mammals are called viviparous ; but, strictly
speaking, they are as oviparous as birds. The process of reproduction is
the same, whether the egg is hatched within the parent or without. The
eggs of birds contain whatever is wanted for the development of the em-
bryo, except heat, which must come from without. Mammals, having no
food yolk, obtain their nutrition from the blood of the parent, and after
birth from milk.

170 The larvae of butterflies and moths are called caterpillars ; those of
beetles, grubs ; those of flies, maggots ; those of mosquitoes, -wigglers.
The terms larva, pupa, and imago are relative only ; for, while the grub
and caterpillar are quite different from the pupa, the bee state is reached
by a very gradual change of form, so that it is difficult to say where the
pupa ends and the imago begins. In fact, a large number of insects reach
maturity through an indefinite number of slight changes. The bumblebee
moults at least ten times before arriving at the winged state.

171 Every tissue of the larva disappears before the development of the
new tissues of the imago is commenced. The organs do not change from
one into the other, but the new set is developed out of formless matter.
The pupa of the moth is protected by a silken cocoon, the spinning of
which was the last act of the larva ; that of the butterfly is simply inclosed
in the dried skin of the larva, which is called chrysalis because of the
golden spots with which it is sometimes marked. The pupa of the honey-
bee is called nymph ; it is kept in a wax cell lined with silk, which the
larva spins. The time required to pass from the egg to the imago varies
greatly ; the bee consumes less than twenty days, while the cicada requires
seventeen years.

172 Compare the amount of food required in proportion to the bulk of
the body, and also with the amount of work done, in youth, manhood, and
old age.

173 Excepting, perhaps, that the new tail of a lizard is cartilaginous.

174 The patella, or kneepan, has no representative in the adult fore
limb.

no « The structure of the highest plants is more complex than is that of
the lowest animals ; but, for all that, powers are possessed by jellyfishes of
which oaks and cedars are devoid." — MIVART.

176 It is, however, true that the number of eggs laid is proportioned to
the risk in development.

177 See Lewes's charming " Studies of Animal Life." Doubtless an
examination of all the strata of the earth's crust would disclose forms
immensely outnumbering all those at present known. And even had we
every fossil, we should have but a fraction of the whole, for many deposits
have been so altered by heat that all traces have been wiped out. Animal
DODGE'S GEN. ZOOL. — 3 i

482 NOTES

life is much more diversified now than it was in the old geologic ages ; for
several new types have come into existence, and few have dropped out.

178 Among the types characteristic of America are the gar pike, snapping
turtle, hummers, sloths, and muskrat. Many of our most common animals
are importations from the Old World, and therefore are not reckoned with
the American fauna; such as the horse, ox, dog, and sheep, rats and mice,
honeybee, house fly, weevil, currant worm, meal worm, cheese maggot,
cockroach, croton bug, carpet moth, and fur moth. Distribution is com-
plicated by the voluntary migration of some animals, as well as by man's
intervention. Besides birds, the bison and seal, some rats, certain fishes,
as salmon and herring, and locusts and dragonflies among insects, are
migratory.

179 when the cable between France and Algiers was taken up from a
depth of eighteen hundred fathoms, there came with it an oyster, cockle
shells, annelid tubes, polyzoa, and sea fans. Ooze brought up from the
Atlantic plateau (two thousand fathoms) consisted of ninety-seven per
cent of foraminifers.

THE NATURALIST'S LIBRARY

THE following works of reference, accessible to the American student,
are recommended : —

General Works and Text-books

PARKER and HASWELL, Text-book of Zool-
ogy-

CAMBRIDGE Natural History.

KINGSLEY, Elements of Comparative Zool-
ogy.

DAVENPORT, Introduction to Zoology.

JORDAN and KELLOGG, Animals.

AGASSIZ, Methods of Study in Natural His-
tory.

AGASSIZ and GOULD, Principles of Zoology.

ROLLESTON, Forms of Animal Life.

LEWES, Studies of Animal Life.

HUXLEY and MARTIN, Elementary Practi-
cal Biology.

OWEN, Comparative Anatomy of Inverte-
brates and Vertebrates.

PARKER and PARKER, Practical Zoology.

MORSE, First Book of Zoology.

PACKARD, Zoology.

GEGENBAUR, Comparative Anatomy.

PARKER, Zootomy.

PARKER, Elementary Biology.

KINGSLEY, The Riverside Natural History.

THOMSON, Outlines of Zoology.

CLAUS and SEDGWICK, Text-book of Zool-
ogy.

THOMSON, The Study of Animal Life.

LANKESTER, Zoological Articles.

MARSHALL and HURST, Junior Course in
Practical Zoology.

LANG, Comparative Anatomy.

SCHMEIL, Introduction to Zoology.

Invertebrates

HUXLEY, Anatomy of Invertebrated Ani-
mals.

MACALLISTER, Introduction to Animal
Morphology.

BROOKS, Handbook of Invertebrate Zool-
ogy.

SIEBOLD, Anatomy of Invertebrates.

SHIPLEY, Zoology of the Invertebrata.

McMuRRiCH, Invertebrate Zoology.

PACKARD, Text-book of Entomology.

Vertebrates

HUXLEY, Anatomy of Vertebrated Animals.

HUXLEY and HAWKINS, Atlas of Compara-
tive Osteology.

FLOWER, Osteology of Mammalia.

CHAUVEAU, Comparative Anatomy of Do-
mesticated Animals.

MIVART, Lessons in Elementary Anatomy.

WIEDERSHEIM, Comparative Anatomy of
Vertebrates.

MIVART, The Cat.

GRAY, Anatomy, Descriptive and Surgical.

QUAIN, Human Anatomy.

REIGHARD and JENNINGS, The Cat.

Embryology

BALFOUR, Comparative Embryology.

FOSTER and BALFOUR, Elements of Em-
bryology.

PACKARD, Life Histories of Animals.

MINOT, Human Embryology.

MARSHALL, Vertebrate Embryology.

HERTWIG, Text-book of Embryology : Man
and Mammals.

KORSCHELT and HEIDER, Invertebrate Em-
bryology.

Physiology

HUXLEY, Lessons in Elementary Physiol-
ogy.

CARPENTER, Comparative Physiology.

FOSTER, Text-book of Physiology.

MARTIN, The Human Body.

GRIFFITHS, Physiology of the Inverte-
brates.

LANDOIS and STIRLING, Human Physi-
ology.

BINET, Psychic Life of Micro-organisms.

MORGAN, Animal Life and Intelligence.

Geographical Distribution
WALLACE, Geographical Distribution of

Animals.
MURRAY, Geographical Distribution of

Mammals.
BEDDARD, Zoogeography.

483

THE NATURALIST'S LIBRARY

Microscopy

CARPENTER, The Microscope and its Reve-
lations.

GRIFFITHS and HENFREY, The Micro-
graphic Dictionary.

Evolution

SCHMIDT, Descent and Darwinism.
HAECKEL, History of Creation.
DARWIN, Origin of Species.
HUXLEY, Lay Sermons, etc.
MIVART, Lessons from Nature.
ROMANES, Darwin and after Darwin: I.

The Darwinian Theory.
ROMANES, The Scientific Evidences of

Organic Evolution.
MARSHALL, Lectures on the Darwinian

Theory.
WEISMANN, Essays on Heredity.

Special Works

CLARK, Mind in Nature.

AGASSIZ, Seaside Studies in Natural His-
tory.

TAYLOR, Half-hours at the Seaside.

KENT, Manual of the Infusoria.

GREENE, Manuals of Sponges and Ccelen-
terata.

DANA, Corals and Coral Islands.

DARWIN, Vegetable Mould and Earth-
worms.

VERRILL and SMITH, Invertebrates of Vine-
yard Sound.

GOULD and BINNEY, Invertebrata of Mas-
sachusetts.

WOODWARD, Manual of Mollusca.

HYATT, Insecta.

PACKARD, Guide to the Study of Insects.

COMSTOCK, Manual for the Study of Insects.

HOLLAND, The Butterfly Book.

HOWARD, The Insect Book.

SMITH, Economic Entomology.

DUNCAN, Transformation of Insects.

JORDAN, Manual of the Vertebrates of the
Northern United States.

COUES, Key to North American Birds.

CHAPMAN, Handbook of Birds of Eastern
North America.

BAIRD, BREWER, and RIDGWAY, Birds of
North America.

BAIRD, Mammals of North America.

ALLEN, Mammalia of Massachusetts.

FLOWER and LYDEKKER, Mammals, Living
and Extinct.

SCAMMON, Marine Mammals of North
Pacific.

HARTMANN, Anthropoid Apes.

PESCHEL, The Races of Man.

MARSH, Man and Nature.

TYLOR, Primitive Culture.

NICHOLSON, Palaeontology.

POULTON, The'Colors of Animals.

Of serial publications, the student should have access to the American
Naturalist, Science, American Journal of Science, Popular Science
Monthly, Smithsonian Contributions, and Miscellaneous Collections, Bul-
letins and Proceedings of the various societies, Annals and Magazine of
Natural History, and Nature,

The following works are recommended as having no English equiva-
lents : —

VOGT ET VUNG, Traite d'anatomie com-
pare'e pratique.

Also the periodicals : —

Zoologischer Anzeiger.

BRONN, Classen und Ordnungen des Thier-
reichs (unfinished and expensive, but
indispensable to the working zoologist).

Biologisches Centralblatt.

APPENDIX

THE following directions for experiments are given for the
purpose of enabling the teacher and pupil to make further
direct observation of the structure and functions of animals, and
are supplementary to those given under the head of " Practical
Zoology."

The experiments and dissections are purposely chosen* with
a view to their simplicity, and to the ease with which they may
be performed. Constant reference is made to figures which
will both guide and illustrate the dissections. More extended
studies may be carried out with the aid of the various works
mentioned on pages 483, 484.

CHAPTER V

The difficulty of distinguishing by ocular observation alone
the lower animals from the lower plants may be illustrated by
making a microscopic examination of drops of sediment from
the bottom of a stagnant ditch. The water will probably be
teeming with unicellular organisms, both animal and vegetable,
which cannot be differentiated by characters of form, size, color,
motion, etc., alone.

CHAPTER VII

It is especially important that the student become as familiar
as possible with protoplasm by a personal study of its structure
and physiology. For this purpose the most favorable objects
are the Protozoa, which are readily obtained and easily pre-
pared for examination. Directions are given on page 23.
Compare with these the protoplasm seen in the cells of the

485

486 APPENDIX

water plants, as Nitella, Chara (end cells of leaves, and in the
colorless rhizoids), and Anacharis; in the stamen hairs of
Tradescantia ; in Spirogyra ; in the cells of the bulb scales of
the onion, etc.

CHAFFER VIII

In studying protoplasm, many kinds of cell will probably be
seen. Those mentioned are especially large, and in them the
protoplasm is likely to be in quite active motion. To illustrate
cell structure use not only the lowest organisms, but also iso-
lated cells from higher animals and plants — for example, blood
cells from the frog and from the human body. Frog's blood
may be obtained by killing the animal in a box in which has
been placed a small wad of cotton saturated with chloroform;
as soon as the frog is dead cut into its skin to make the blood
flow, then on a glass slide mix a drop of the blood with a drop
of a .75 per cent solution of salt in water, put on a cover glass,
and examine under a one- fourth to one-sixth inch objective
(Figs. 260, 261). Human blood maybe obtained by pricking the
finger and mounting the drop in the same manner (Fig. 259).
Study also the cells seen in a drop of saliva. Some of these,
the salivary corpuscles, are small and usually spherical in shape ;
others, the epithelium cells, come mainly from the lining mem-
brane of the mouth, are polygonal in outline, have a large nu-
cleus, and are frequently found in groups consisting of several
cells. Ciliated cells are easily obtained by placing in a drop of
water on a slide a small portion of the gill of a live oyster or
clam, and picking it to pieces with dissecting needles (ordinary
cambric needles fixed by the eye end into wooden penholders) .
Examine under a one-fourth or one-fifth inch objective. Some
of the pieces will probably be seen swimming about by means of
their cilia (Fig. 199, £). With these animal cells compare such
vegetable cells as pollen grains, spores of fungi, the cells com-
posing the bodies of some of the fresh-water algae, etc.

As the satisfactory preparation of the tissues requires skill
obtained only by long training in manipulation and in the use
of hardening fluids, stains, etc., in many cases it will be prefer-

APPENDIX 487

able to buy prepared specimens. These may be obtained at
slight expense from dealers in microscopic supplies. Such
specimens, as well as sections of various organs, are very neces-
sary, as it is only by a clear comprehension of the structure of
the different tissues and of the organs which they compose that
the student can understand the functions of the various parts.

CHAPTER XIII

The principal chemical changes taking place during digestion
in the higher animals may be illustrated with very simple appa-
ratus, and at the cost of but little time. It is not necessary that
the student possess any knowledge of chemistry. The object
of digestion, viz., the changing of substances which are in-
capable of absorption into substances which may be absorbed,
can be made plain even to the youngest student. The chemi-
cals needed may be obtained of any druggist.

The" following experiments deal with the three principal di-
gestive fluids, viz., saliva, gastric juice, and pancreatic juice ;
and with the main kinds of foods, i.e., starchy, albuminous,
and fatty substances.

SALIVARY DIGESTION

(i) The microscopical appearance of undigested starch and its
reaction with iodine

Into a test tube about one fourth full of water put a pinch of
corn starch and shake the tube. Notice that the starch does
not dissolve. Examine a drop of the mixture under a micro-
scope and note the starch grains floating about in the water.
Add a drop or two of dilute iodine solution to the mixture in
the tube and note that it turns a deep blue. Examine a drop
of this mixture under the microscope and note that each starch
grain has turned blue.

Prepare another test tube with water and starch, and boil the
mixture in the flame of an alcohol lamp or of a Bunsen burner,
keeping the tube agitated all the time in order to prevent the

488 APPENDIX

starch from sticking to the inside of the tube. Note that the
starch swells up and forms a paste, but does not actually dissolve.
Cool the paste by holding the test tube in cold water. When
sufficiently cool add a drop or two of iodine and note that the
starch turns blue. This change of color serves as a test for
starch whether uncooked or cooked. Hence we see that undi-
gested starch is in the form of granules which do not dissolve
in water, but which turn blue when treated with iodine.

(2) The chemical test for digested starch, i.e., grape sugar

Into a test tube about one fourth full of water put a pinch of
grape sugar, shake the tube, and npte that the grape sugar dis-
solves. Test the solution with iodine and note that the blue
color does not appear.

Prepare another solution and to it add about one fifth its
volume of a strong solution of sodium hydrate, then to this
mixture add a drop or so of a one-per-cent solution of cupric
sulphate. Shake the tube to mix the contents thoroughly.
Note the light blue color. Boil the contents of the tube and
the color changes, varying from lignt yellow to orange or brick
red. Hence it is seen that digested starch (grape sugar) dis-
solves in water, does not turn blue with iodine, but turns yellow
or reddish when boiled with a mixture of sodium hydrate and
cupric sulphate.

(3) The digestion of starch by saliva

Collect about a third of a test tube full of saliva, the flow of
which may be promoted by chewing a piece of rubber or a
button. Dip a piece of red litmus paper into the saliva and
note that the paper becomes faintly blue, indicating that the
saliva is slightly alkaline in its chemical reaction. In another
test tube make a mixture of about equal parts of saliva and
water, and to this add a few drops of cool starch paste. Hold
the tube containing this mixture in the hand for five, or ten
minutes in order to keep it at the temperature of the body.
After a few minutes pour a portion of the mixture in another
tube and test with iodine, which will probably give the blue

APPENDIX 489

color indicating the presence of starch. Pour a second portion
into another tube, add sodium hydrate and copper sulphate,
and boil. If the yellow color appears it indicates that some
of the starch has already been digested by the saliva, i.e., has
been changed to grape sugar, which remains dissolved in the
fluid in the test tube. If the yellow color does not appear on
the first trial, make another after an interval of a few minutes.

(4) To show that digested starch is capable of absorption,
while undigested starch is not

Prepare two dialyzers. The parchment, or parchment paper,
which in each dialyzer separates the contents of the inner from
the contents of the outer jar, may be considered to represent
roughly the membrane lining the alimentary canal, through
which membrane substances are absorbed into the system.
Into the inner jar of one dialyzer put a solution of grape sugar ;
into the inner jar of the other put some thin starch paste.
After an hour or two test the water in the outer jar of the first
dialyzer for the presence of grape sugar : that in the outer jar
of the other dialyzer for starch. It will be found that grape
sugar — i.e., digested starch — dialyzes, while undigested starch
does not. In other words, undigested starch cannot be ab-
sorbed. The experiment may be varied by putting both grape
sugar and starch paste into the same dialyzer. Or, a mixture
of starch paste and saliva may be put into the one, while starch
paste alone is put into the other dialyzer.

GASTRIC DIGESTION

(i) Some of the chemical reactions of undigested albuminous
substances (proteids)

Into a bowl or beaker break the white of an egg, cut it to
pieces with a pair of scissors, add fifteen or twenty times its
bulk of water, mix thoroughly by stirring, but do not beat it,
then strain through muslin to remove the fine flakes of coagu-
lated matter.

490 APPENDIX

(a) Fill a test tube one fourth full of the mixture and boil.
The albumen coagulates.

(<£) Prepare another tube and add a few drops of nitric acid.
The albumen coagulates. Boil. The coagulated mass turns
yellow. Cool the tube and add ammonia. The color deepens
to orange.

(V) Prepare another tube and add a few drops of Millon's
reagent. The albumen is coagulated, and, on boiling, turns red-
dish. If only a little proteid is present no coagulation will occur,
but the mixture will redden when boiled.

(d) Make the contents of another tube strongly acid with
acetic acid, then add a few drops of potassium ferrocyanide,
and a white precipitate will form.

(2) Some of the chemical reactions of digested pro teids
{peptones}

Make a peptone solution by dissolving some of Merck's pep-
tone in water. Repeat the experiments given for proteids.
Results similar to those in (b} and (c) will be obtained, but
the peptone does, not coagulate on boiling, nor does it give the
white precipitate with acetic acid and potassium ferrocyanide.

(3) To show that peptones are diffusible through membranes,
while proteids are not

Prepare the two dialyzers as for the experiments with starch
and grape sugar. Into the inner jar of one dialyzer put some
of the white-of-egg mixture, and into the other some peptone
solution. After a few hours test the water in the outer jar of
each dialyzer. It will be found that the peptone passes through
the membrane, while the proteid does not.

(4) To show that the gastric juice digests proteids, i.e.,
changes them to peptones

Prepare some artificial gastric juice as follows : Make some .2
per cent hydrochloric acid by mixing 5.5 cubic centimeters
of hydrochloric acid (sp. gr. 1.16) with enough distilled water

APPENDIX 491

to make one liter. In 100 cc. of this acidulated water put 100
milligrammes of a 6000 pepsin, or 150 mg. of a 4000, or 300
of a 2000 pepsin. Any commercial pepsin maybe used. Pre-
pare the proteid by boiling an egg, and then cutting the white
into small cubes or shreds. In place of the boiled egg some
of Merck's prepared fibrin may be used.

With litmus paper test the reaction of the artificial gastric
juice. It will turn blue litmus paper red, thus showing that its
reaction is acid.

Fill a test tube about one fourth full of the artificial gastric
juice, and add a few pieces of coagulated white of egg or of
fibrin ; then set the tube in a warm place, as in a water bath
regulated to about 37° C., or near a stove. Examine the tube
from time to time. The cubes of egg will be seen to be disinte-
grating and dissolving.

A quantity of digested white of egg may be prepared in a
cup or bowl and emptied into the inner jar of a dialyzer. After
a time the water in the outer jar will give the peptone tests,
showing that the digested albumen is diffusible.

PANCREATIC DIGESTION

Procure some of the commercial pancreatic preparations and
make an artificial pancreatic juice according to the directions
furnished with each preparation. Test the reaction with litmus
paper. It will be found to be alkaline. Try the effect of the
artificial preparation on starchy and on albuminous substances
in the manner given above for each. The pancreatic juice will
be, found to change starch to grape sugar and proteids to pep-
tones. Try its effect also on oil by adding a few drops of olive
oil to some pancreatic juice in a test tube. At first the oil will
float on the surface of the liquid. Shake the tube vigorously
to mix the two substances. The oil will be broken up into fine
droplets, giving the contents of the tube a milky appearance.
On standing for a time it will be seen that the oil does not
separate from the digestive juice and collect at the surface as
it would if shaken up with water, but the two fluids remain
intimately mixed, forming an emulsion. Under a microscope

492 APPENDIX

examine a drop of the emulsion. It will be seen to consist of
innumerable fine drops of oil, which remain separate from one
another. If oil be shaken up with saliva or with artificial gastric
juice no emulsion will be formed, the oil soon separating.

CHAPTER XV

Directions for obtaining and studying blood corpuscles are
given in the notes on Chapter VIII. Sufficient blood to show
the phenomena of clotting may be obtained by chloroforming
a rabbit or a fowl, cutting one of the veins in the neck, and
catching the blood in small tumblers or beakers.

CHAPTER XVI

The beat of the heart is very conveniently studied in the
frog. Put a live frog into a glass bowl with a piece of cotton
batting or of cloth saturated with chloroform, and cover the
bowl. In a few minutes the animal will have become motion-
less and insensible. Remove it from the bowl ; with a sharp
knife divide the skin and cartilage at the base of the skull, thus
making an opening into the brain cavity ; into the latter thrust
a wire, and by twisting it about destroy the brain. The frog
will probably struggle, but its motions are reflex, and it has no
consciousness of pain. The heart may now be exposed by
making an incision through the skin and muscles of the upper
part of the abdomen and removing the cartilaginous part of the
breastbone. The heart will be seen beating inside the pericar-
dium. The latter may be removed and the heart freely exposed.
After studying the movements of the organ it may be removed
from the body by cutting the blood vessels close to their junc-
tion with the heart, and placed on a plate of glass or in a watch
glass containing .75 per cent salt solution. Its movements will
continue a long time after its removal from the body. The
organ may afterward be opened and the relation of its ventricle,
auricles, and the connecting veins and arteries studied (Fig. 273).

APPENDIX 493

•

The heart of the pig, sheep, or calf may be used to show the
structure of the mammalian heart. It is best to procure at the
meat shop several "plucks," i.e., heart, lungs, and trachea all
attached together. Instructions should be given the butcher
that the parts are to be left intact, otherwise they will be found
to be punctured with knife cuts. Dissect out the blood vessels
for some little distance from the heart in order to get their re-
lations. Open some of the hearts lengthwise, others crosswise,
to show the internal structure (Fig. 271). Pour water into the
cavities to show the action of the valves. The flow of blood
through the heart may be illustrated by connecting the aorta
with the venae cavae by means of rubber or glass tubing to
represent the systemic circulation, and the pulmonary artery
with the pulmonary veins to represent the pulmonary circula-
tioivthen rilling the heart with water or a colored fluid and
compressing the organ with the hand (Fig. 273).

The circulation may be studied in the web of the frog's hind
foot. Procure a thin board large enough to lay the frog upon ;
in one end make a hole about a half-inch in diameter, over
which the web may be stretched; anaesthetize the frog with
ether or chloroform ; as soon as the animal becomes insensible
lay it on the board, with its body covered with a moist cloth ;
over the larger toes of the foot to be examined slip nooses of
thread, and fasten these in slits around the edge of the board
in such positions as to spread the web between two of the toes
over the hole in the board. Put a drop of water on the web, lay
on the cover glass, place the board on the microscope, and ex-
amine with a one fifth or a one sixth objective. The anaesthetic
must be renewed from time to time, otherwise the struggles of
the animal will interfere with observation (Fig. 263).

CHAPTER XVII

The gross structure of the frog's lung may be studied in
specimens which have been removed from the body, inflated
with air blown through a small glass tube inserted through the
glottis, and placed in alcohol a few hours to harden. When

494 APPENDIX

cut open the lung will be seen to be a hollow sac with corru-
gated walls (Fig. 282).

" Plucks " obtained from a butcher will illustrate the struc-
ture of the mammalian larynx, trachea, bronchial tubes, etc. If
fresh and not punctured with the knife they may be inflated.
To work well they should be kept moistened (Fig. 283).

The presence of carbon dioxide in the air exhaled from the
lungs may be shown by using limewater or baryta water, with
either of which carbon dioxide forms an insoluble precipitate,
which at first floats as a delicate white film on the surface of
the liquid. Pour some of the fluid into a saucer or watch glass,
then breathe heavily upon it a few times through the mouth,
and the film will be formed.

CHAPTER XVIII

The structure of the kidneys is well illustrated by the kidney
of the sheep. Several of these should be procured and opened
in various directions to show the structure (Fig. 290).

CHAPTER XIX

With little trouble skeletons of frogs, birds, and mammals
with bones connected by flexible attachments may be prepared.
Carefully cut away all of the muscles and other soft parts, leav-
ing only the ligaments connecting the bones. Then place the
roughly prepared specimen for one or two weeks in Wicker-
sheimer's fluid, which is prepared as follows : In three liters of
boiling water dissolve 100 grams of alum, 60 grams of caustic
potash, 25 grams of salt, 12 grams of saltpeter, and 10 grams
of arsenic. Cool and filter the liquid. Then to each liter of
the fluid add 400 cubic .centimeters of glycerine and 100 cubic
centimeters of alcohol. The ligaments of skeletons soaked in
this fluid will remain flexible during many months of exposure
to the air. Should the ligaments become stiffened, their flexi-
bility may be restored by a few hours' immersion in the fluid.

APPENDIX 495

CHAPTER XX

Muscle fibers for microscopic examination may be obtained
from the leg of a frog, or even from the body of a recently
killed animal at the meat shop. Lay a small piece of muscle
in a drop of .75 per cent salt solution on a glass slide, and
with a pair of dissecting needles carefully pick the muscle to
pieces. Some of the smallest shreds, upon examination with a
one-fourth or a one-sixth inch objective, will be seen to be
single or grouped muscle fibers, which will show the striations
and the sarcolemma (Fig. 208).

CHAPTER XXI

Nerve fibers are readily obtained from the sciatic nerve in
the frog. This nerve may be found by removing the skin from
the back of a frog's thigh and carefully separating the under-
lying muscles. Among them will be seen the sciatic nerve,
covered in places with dark gray or black pigment spots.
Remove a quarter to a half inch of the nerve, being careful to
stretch it as little as possible ; lay it on the glass slide in a few
drops of .75 per cent salt solution ; cautiously tear it to pieces
in the direction of its length with dissecting needles ; then put
on a cover glass and examine under a high power. The nerve
will be found to consist of a number of nerve fibers, some of
which will show the primitive sheath (neurilemma), medullary
sheath, and axis cylinder (Figs. 210, 211).

The relation between the stimulation of a nerve and the con-
traction of the muscle to which the nerve runs may be shown
as follows : Expose the sciatic nerve as directed above ; then
with the quick stroke of a sharp scalpel sever the upper end of
the nerve as near the body as possible. At the moment of do-
ing this the muscles of the leg and foot will probably contract.
Allow the nerve to rest for a few minutes ; then pinch its upper
end with a pair of forceps. Again the muscles will contract.
The stimulation may be repeated at intervals if the nerve be
allowed to rest for a few minutes between successive stimula-

496 APPENDIX

tions. Try also the effect of touching the nerve with a hot
wire and with a drop of dilute acid or alkali. During experi-
mentation the nerve preparation must be kept moistened with
the salt solution.

CHAPTER XXIII

The structure of the egg may be studied in the starfish or
sea urchin, frog or fowl. Starfish eggs preserved in various
stages of segmentation may be purchased from the Department
of Laboratory Supply of the Marine Biological Laboratory at
Wood's Hole, Mass. Frogs' eggs may be found in ponds and
ditches in early spring. If transferred to the laboratory and
kept supplied with fresh water they may be watched through
their various stages of segmentation to the formation of the
tadpole, its liberation from the egg, and its later development.
Compare with Fig. 370. To watch the development of a chick,
eggs may be incubated by a hen or in an artificial incubator, one
egg being removed each day, and opened by breaking away a cir-
cular piece of the shell on the upper side. If kept submerged in
a dish of .75 per cent salt solution, warmed to the temperature
of the body, the embryo chick may be kept alive for several
hours to show the beating of the heart, etc. (Figs. 365, 366).

INDEX

In the Index the numbers in Roman type (289) refer to pages ; those in bold-faced
type (254) refer to cuts.

ABOMASUS 291, 354

Absorption '297, 256-258

Acaleph 73, 374

Acarida 123, 83

Accipitres 166

Acetabulum 356

Acineta 12

Acipenser 125

Acorn shell 102, 58

Acrania 141, 144. 117

Actinia 256, 25, 26, 236

Actinophrys sol 59

Actinozoa , "... . lilt

Adder . 134

Adelochorda 140, 141

Adipose tissue ...... 236, 207

Molis, 130

^Epyornis ....,"> 163

Alaus . . , . . 117

Albatross 165

Albumen «• . 215

Alcyonaria 75, 27, 34, 35

Alimentary canal 276, 414, 53, 164, 165

Allantoidea 470

Allantois 324, 414, 365-367

Alligator 159, 138, 377

Allolobophora 95

Alternate generation . . . 33, 424, 374

Amblypterus, scale of 119

Ambulacra 92, 339

Ameiurus 146, 126

Ammonites 134

Amnion 413, 414, 366

Amoeba . . 256, 276, 335, 364, 366, 377, 1
Amphibia . . 141, 151, 405, 407, 421, 426

Amphiccelous 477

Amphioxus . . 141, 144, 251, 301, 117

Amphithoe maculata 62

Anallantoidea 470

Analogy 429

Anasa 112

Anas boschas 145

Anatomy . 12

Anchylosed 353

Animal life . . 242

Animalcule, see Protozoa.

Annelides 95

Annulata 50, 82, 95

Anodonta 127, 275

see Clam.

Anseres 166

Ant 110, 119, 120, 398

Ant-eater . . . 179, 181, 290, 401, 168

Antennae 387, 344

Anthophora retusa 22O

Anthropopithecus troglodytes

189, 233, 317

Anura 152

Aorta 309,313

Ape 193, 270, 290, 383

Aphis 112

Apis 119, 79, 24O

Aplysia 130, 331

Aptenodytes pennantii ..... 142

Apteryx 162

Aquila chrysaetos 148

Arachnida 120, 261

see Centipede, Scorpion, Spider.

Araneida 121

see Spider.

Arbacia 92

Arcella . 56

Archseopteryx 463

Ardea 144

Arenicola 95, 274

Areolar tissue 233, 2O7

Argonauta 135, 109

Argynnis 115

Armadillo . . . 181, 344, 401, 169, 298

Artemia 101

Artery 309, 265

Arthropoda, 50, 97, 253, 274, 336, 346, 53

digestive process of 295

nervous system of 378

number of 433

see Crab, Insecta, Lobster, Myri-
apoda, Spider.

Arthrostraca 104

Ascidian 142, 313, 114

Aspidobranchia 129

497

498

INDEX

Aspidonectes

Assimilation

Astacus fluviatilis ......

Asterias

Asteroidea

see Starfish.

Astraea . 76, 77,

Astrophyton

Atavism

Ateles

Atlas

Attacus pavonia-major

Auger shell

Auk

Aurelia

Aves .m

Avocet

Axis .

Axolotl .

. 157
. 245
. 54

90

3O, 32

. 91
. 428
. 317
. 354
. 73
. 98
. 170
. 73
141, 159
. 170
. 354
48

BABIRUSA v

Baboon

Badger

Balaena

Balaenoptera

Balanoglossus .

232

199

190

171,311

228
140, 141, 142, 113

Balanus ......... 102, 58

Bandicoot .......... 180

Barb ............ 346

Barbule ........... 346

Barnacle ..... 258, 423, 428, 57

see Cirripedia.
Barn owl .......... 155

Basket fish .......... 91

Bat ... 192, 370, 401, 184, 185, 378

Batrachian, see Anura, Frog, Toad.

Bear . . ..... 190, 373, 401, 325

Beaver .... 191, 344, 397, 398, 182

Bedbug ........... 112

Bee, 119, 250, 280, 328, 399, 406, 417, 278
alimentary canal of ...... 24O

head of ........ 22O, 352

instinct of ....... 397,398

muscle in ......... 365

respiration in ...... 321, 327

see Hymenoptera, Insecta.
Beetle ..... 116, 117, 254, 369, 64

alimentary canal of ...... 239

eye of ........... 353

see Coleoptera, Insecta.
Belemnites .......... 135

Bellbird ........... 400

Beroe ............ 81

Bibio febrilis ......... 324

Bile ............ 295

Biology ........... 11

Bird, 159, 163, 250, 354, 356, 405, 406, 407,

410, 422, 426, 427, 429

alimentary canal of ...... 286

auditory passage of ...... 391

BIRD

beak of 344

brain of 380, 381, 384

blood of 303

bones of 346

circulation in .... 314, 316, 273

eggs of 419

eyes of 395

feathers of 337

flight of 370

instinct of 397

intelligence of 398

jaws of 263

locomotion of 375

lung of 279

muscle in 364, 366

parts of 139

pelvic arch of 356

respiration in 323-327

scales of 344

skeleton of 346

temperature of 327

vocal organs of ." 400

wings of 369

Bittern 165

Bivalves 256, 341, 296

see Clam, Lamellibranch, Oyster.

Blackbird 176

Blastoderm 417

Blastula 410, 361 c

Blood . . 301, 302, 306, 308, 309, 316, 318

corpuscles 259

of fishes 147

of frog 26O-262

Blubber 182

Bluefish 12O

Boa constrictor . . . 255, 275, 348, 235

Bombus 119

Bombyx . 116

Bones .... 234, 346, 356, 2O3, 204

of human skull 2O5

of the mammalian skull .... 351

Bos taurus 315

Bot fly 1 13, 256

Brachiopoda ..... 82, 86, 43, 44

Brachycephalic 470

Brain 147, 380, 335-342, 348

Brain coral 31

Branchipus 101

Branta canadensis ........ 146

Brittle star 91

Bronchi 326

Bubble shell 180, 91

Buccinum 129,58,88,227

Budding 402

Bufo 153

see Toad.

Bugs 112

see Hemiptera.
Bulimus 118, 93

INDEX

499

Bulla ....

Bullfrog

Bullhead

Butterfly . .
milkweed
proboscis of
tortoise-shell

Byssus

CADDIS FLY
Cake urchin
Cambarus
Camel
Cameo shell

130, 91
131

146

118, 250, 369, 396, 429
368

72
.. 126

110
92

103

186,291
129

Campanularian hydroid ..... 2O

Canaliculi .......... 235

Canines ......... 271, 232

Canthocamptus ........ 373

Capillaries ..... 298, 309, 263, 265

Caprimulgus ......... 157

Capybara ........ 191, 180

Carapace ........ 157, 312

Carcharias vulgaris . . ..... 122

Carchesium ......... 63

Cardium costatum ..... 127, 87

Carinatae .......... 163

Carnivora ..... 188,290,419,325

Carp . . . 149, 268, 284, 328, 246, 299
Carpocapsa pomonella ...... 74

Carpus ........... 355

Carrion beetles ...... . . 117

Cartilage, hyaline ....... 202

Cassis ......... 129, 97

Cassowary .......... 162

Castor canadensis ....... 182

Casuarius .......... 162

Cat ..... 190,256,328,336,387

brain of ........ 380, 339

eyes of .......... 395

intestine of ........ 256

molars of ......... 271

Caterpillar, 113, 334, 365, 371, 392, 422, 429
anatomy of ........ 238

head of .......... 75

nervous system of ...... 333

Catfish ........... 126

Cebus hypoleucus ....... 187

Cecidomyia ......... 113

Cell .... 228, 198, 199, 2O7, 357

Cement ....... 235, 267, 206

Centipede 106, 258, 322, 367, 371, 378, 8O
Centrum ......... 349, 304

Cephalizatlon ......... 437

Cephalodiscus ........ 140

Cephalopoda ..... 134, 405, 245

see Cuttlefish. Squid.
Cephalothorax ....... 99, 340

Cerambycidse ......... 117

Ceratodus .......... 151

Cere ... .166

Cerebellum 380, 334-342

Cerebrum 380, 314-342

Cervus elaphus 174

Ceryle 158

Cetacea 182

see Whale.

Chyetopoda 95

Chalaza 404, 358

Chameleon 155, 263

Cheiroptera 192

Chelae loo

Chelonia .' 157, 136

see Turtle.

Chelydra 158

Chilopoda 105

Chimpanzee . 196, 189, 191, 233, 317

Chitin 340

Chiton 127, 1OO

Chorion 414

Choroid membrane . .... 391,394

Chordata .137

Chrysalis 481, 74

Chrysaora 374

Chyle 295, 307

Chyme 295,296

Cicada 112, 399, 67

Cicatricula '. . 404, 358

Cicindela 117

Cidaris, spines of 293, 294

Cilia 63, 281, 363

Ciliata . . . , 68

Cimex 112

Circulation .... 245, 308, 112, 263

Cirratulus grandis 51

Cirripedia 101, 57

see Barnacle.

Civet 190

Clam, 125, 127, 251, 282, 312, 371, 389, 332

see Lamellibranch, Mollusca, Oyster.

Clamatores 175

Classification . . 47, 51, 55, 201, 457, 197

Claws 344

Click beetles 117

Clio, mouth of 257

Clisiopampa 116

Cloaca 288

Clothes moth 116

Clypeaster 92

Coccinella 117

Coccus 112

Cochineal 112

Cochlea . . . . ; 390

Cockatoo . 172

Cockchafer, heart of 266

Cockle 127,87

Cockroach HO

Cod 146, 149, 380, 406, 127

Codosiga 60, 6

Cceca 278, 279

Coecum . ... ... 249

5oo

INDEX

Ccelenterata 50, 68, 483

Coenosarc . 77

Coleoptera 116

see Beetle.

Colias 115

Columbae 171, 153

Columella 128

Condyle 353

Cone shell 129,99

Confervas 219

Conjugation 407

Connective tissue . . . 232, 200, 301

Cony 185

Copepoda 101

Coracoid 355

Coral ... 70, 74, 220, 337, 366, 27-35

Corallite 77, 992

Corallium 75, 76, 34, 81

Cormorant 165, 286, 143

Cornea 391, 354

Corpus callosum 177

Corpuscles, blood 302, 259

see Blood.

Correlation 432

Corydalis ........... 110

Cotalpa lanigera 76

Cow, skeleton of 315

Cowry 129,94

Coxa 372

Crab 254, 389, 399, 405

circulation in 311

legs of 371

mouth of 258

respiration in . 320

shells of. 337

swimming 60

teeth of 265

see Lobster.

Crane 169

Crane fly 113

Crangon 103

Craniata 138, 141, 144

Crayfish 54, 55

Cricket 110, 281, 429, 65

Crinoidea 93, 366, 45, 5O

Crocodilia . . 158,159,269,344,372,406

mouth of 224

respiration of 326

skeleton of 31O

stomach of 284, 247

see Keptilia.

Crop 280, 239-241, 248

Crow 176, 400

Crustacea 98

absorbent system of 297

eyes of 392

cuticle of 336

gullet of 279

nervous system of 378

shell of . . . 340

CRUSTACEA

see Crab, Lobster.

Ctenactis echinata

Ctenophora (

Cuckoo

Culex 113,

Cupidonia cupido

Curculionidae

Cursores

Cuticle

Cuttlefish, 135, 252, 343, 346, 405,

brain of .... *i ...

circulation in

eyes of

gizzard of

ink of

mouth of

nervous system of ....

pancreas of . . ...

respiration in

tentacles of

see Cephalopoda, Sepia, Squid.

Cyanea .

Cyclas

Cyclops

Cyclostoruata

Cyprsea

Cypris

Cytherea chione

9, 81, 36
. . 173
78, 369
. . 149

. 118

. . 231
410, 107
. . 348

. 312

334
256
378

. 214

. 73
. 347
. 56

141, 144

129, 94

. 56

. 296

DADDY LONGLEGS 113

Daphnia 101, 56

Dasypus 169

Dasyurus 181

Decapoda 102

Decussate 395

Deer 186, 174

Deglutition .274

Demodex folliculorum 83

Dendronotus arborescens . . . . 9O

Dental formulas 185, 186, 271

Dental tissue 235

Dentine 186, 235, 267, 206

Development 409, 411, 416

Devil's darning needle 110

Diapheromera 110

Diaphragm 289, 326, 285

Diapophyses 352

Dibranchs 134

Didelphys virginiana 167

Didus 172

Differentiation 227

Difflugia 56

Digestion .... 245, 294, 248, 249

Digitigrade 190, 373, 325

Dinobryon 60, 5

Dinornis 163

Dinotherium 186

Diplopoda 106

Diploria cerebriformis 31

INDEX

501

Dipnoi 150

Diptera 112

see Fly, Mosquito.

Discopora skenei 43

Distoma S3

Distribution of animals . . . 440 379

Divers . 164

Diving beetles 117

Dodo 172

Dog 190, 250, 374, 387, 398

artery of 365

brain of 380, 381

liver of 389

molars of 271

follicles from stomach of .... 387
skull of ...... 853,305-307

Dogfish . 148

Dog whelk 106

Dolicocephalic 470

Dolphin 183, 270, 173

Donkey 381, 401

Doris 130

Dove 171, 250, 153

Dragon fly . . 369, 68

Dromseus 162

Duck, wild 145

Duck mole 179, 267

see Ornithorhynchus.

Dugong 185, 370

Duodenum 293

Dytiscus 117, 318, 334

EAGLE 168, 148

Ear 390, 347, 349

Ear shell 129, 1O6

Earthworm, 95, 253, 279, 319, 328, 371, 378

Echidna 179

Echinarachnius 92

Echinodermata .... 50, 87, 433, 45

Echinoidea 91

Echinus 294, 45, 48, 337

see Sea urchin.

Ectoderm 66, 68, 277, 363

Ectosarc 276

Edentata 181

Eel 149

Egg 148, 403, 406, 416

embryo in 365

fertilization of 408, 361

of hen 358

of shark 360

of sponge 405, 359

section of 363, 366

segmentation of 361

Elasmobranchii 148

see Bay, Shark.
Elephant, 185, 250, 264, 272, 273, 388, 398, 401

brain of 380

feet of 336

molars of . . 271

ELEPHANT

sinuses of

skelefon of
Elk-horn coral
Elytra
Embryo
Embryology
Emu
Enamel
Encephalon
Endoderm
Endopodite
Endosarc
Endoskeleton
Entomostracan
Ephemera
Epiblast
Epidermis
Epiglottis ..
Epistylis
Epithelium
Equus caballus
Ermine weasel
Esophagus . .
Eucope
Euglena
Euplectella
Eulamellibranchia
Eustachian tube
Eutheria
Evolution
Excretion
Exopodite
Exoskeleton
Eye .... 391

288

347
316

..'.... 70
116,369
412, 365, 367
12, 14, 409
162

235, 267, 339
385

68, 277, 335, 363
.... 99
276

335, 337, 346
423, 373
110

410, 411, 365
231

326, 335, 349, 356
63

230, 199
314
177

, 347-350, 353, 356
69

60, 391, 4
66
127
39O
181
450

245,329
99

335, 337
, 416, 35O, 353, 354

FACIAL ANGLE ........ 140

Falcon ........... 168

Family ........... 51

Fasciculi .......... 236

Fat .......... 236, 3O7

Feather .... 337, 345, 369, 415, 3O3

Feather star ......... 93

Feet ........ 334, 335, 336

Felis domestica ........ 339

Felisleo ........... 303

Femur ......... 356, 372

Fertilization ....... 408,409

Fiber ........ .... 233

muscular ......... 309

nerve „ ....... 238, 31O

Fibrin ....... ...... 302

Filibranchia ......... 127

Finch ........... 176

Fish ............ 145

alimentary canal of ..... 284

blood of ......... 147,303

circulation in .... 313, 368, 373

eggs of .......... 148

epidermis of ........ 33T

502

INDEX

FISH

eyes of
fins of

gills of

heart of

locomotion of

mouth of

muscles of

pancreas of

respiration in . . . 320,

scales of

skeleton of

skin of

teeth of

vertebral column of . .

Fishhawk

Fission

Fissurella lister!

Flagella

Flamingo .......

Flatfish, eyes of

Flea, sand

water

Flight

Flounder

Fluke

Fly 112, 324,

metamorphosis of ...

mouth of

see Diptera, Mosquito.

Flycatcher

Flying fox

Follicles

Food . . .

Foramen

Foramina

Foraminifera

Formica

Formicarium

Forms of animals . . . .

Fowl

Fox

Frigate bird

Frog,

151, 323, 326, 337, 347, 348

blood of

brain of

circulation in

corpuscle of

lungs of

metamorphosis of . . .

skeleton of

teeth of

Fruit moth

Function

Fungia

Fusus -. ,

... 395
146, 368, 330
... 370
... 320
... 313
. 368, 331
... 262
. 364, 366
... 381
, 323, 324, 327
. 343, 119
. 346, 347
... 399
147, 268, 273
... 352
... 147
... 402
. . . 1O5

165, 333
. . 395
. . 63
. . 56

. . 369
. 149

), 397, 399
. . 69
. . 333

. 176, 159
... 192
. 329, 387
... 245
352, 353, 382
... 58
. 338, 313
. . . 119
... 120
... 434
. 364, 348
. 190,178
. 165

374, 377, 419
. 302, 36O
. 384, 337
. 313, 363
. . . 361
. . .383
... 421
... 384
. 152, 269
... 74
... 240
... 78
. 129, 96

GADUS CALLARIAS

137

Gall bladder 332, 389

Gallfly 119

Gallinae 168

Ganglion 876, 313, 343

Gannet 165

Ganoid 149

Garpike 149, 134

Gastric, follicles 330, 387

juice 287

teeth 265

Gastropoda 127, 320

anatomy of 243

teeth of .337

see Snail.

Gastrula 410, 363

Gavise 170

Gavial 159

Gecko 154

Gelatin 233

Gemmules 68

Genus 51

Geometrids 116

Germinal vesicle 403

Gills 147, 319, 346

Giraffe 401

Gizzard 280

Gland 330, 386

Glenoid cavity 354

Globigerina ooze 59

Glottis 326

Glyptodon 181

Gnawers 190

Goatsucker 173, 157

Goldsmith beetle 76

Goliath beetle 117

Goniaster 90, 46

Goose 287, 146

Gordius 84

Gorgonia 75, 35

Gorilla 196, 193, 193

Grallse 169

Granddaddy longlegs 121

Grantia 65, 66, 68, 14

Grasshopper 110, 281, 389

Gregarina 7, 8

Ground beetles 117

Grouse 169

Growth 425

Gryllus HO, 65

Guinea-pig 191

Guii no

Gymnophiona 153

HADDOCK .1*9

Haemocyanin 306

Hagflsh 144,268

Hair 337, 345, 3O1

Hairworm 84

Haliotis 129, 95, 106

Hallux 161

INDEX

503

Hare 191, 181

Harpalus . 107,64

Harp shell 129

Harvest fly . . 399

Harvest man 121

Haversian canals 284, 304

Hawk 168

Hawk moths 114

Hawkbill turtle 158, 136

Hearing 889

Heart 151,809,271

of cockchafer 266

ofdugong 27O

rudimentary 364

single 369

Hedgehog 192,345

Heliozoa 59

Helix 131, 92

Helix albolabris 318

Helmet shell 97

Hemal spine 352

Hematocrya 470

Hematotherma 470

Hemiptera 112, 66

Hemoglobin 306

Hen's egg 358

Heredity 427, 455

Hermit crab 108

Herodiones 165

Heron 165, 144

Herring 146, 149

Hessian fly 113

Heterocercal 368

Hippopotamus .... 186, 373, 326

Hirudinea 95

Hirundo 163

Histology 12, 14

Hog 186, 264, 271, 333

Holothuria 45, 49

Holothuroidea 92

Homarus vulgaris 59

Homocercal 368

Homology 429, 375-378

Homomorphic 428

Honey-bag 280

Honey bee 119, 79

Hoofed mammals, foot of .... 326

Hoopoe 173

Hornbill 173

Horned pout 126

Hornera lichenoides 42

Horns 344

Horny sponge, skeleton of .... 16

Horse 185, 419, 427

brain of 380,381,335

circulation in 316

foot of 326

forefoot of 3OO

intelligence>f 398

locomotion of . , 373

HORSE

* molars of 271

nostril, skin of 291

skeleton of 314

skin muscles of 365

skull of 3O8

speed of 374

stomach of 290, 251

teeth of 185

toes of . . . 356

Horsefly, mouth of 222

Horse-hair snake 84

Horseshoe crab 123, 258, 336

Housefly ......'.. 113,824

Humble bees 119

Humming bird 173

Hyalaea tridentata 89

Hyaline cartilage 2O2

Hydatina 4O

Hydra, 277, 335, 377, 410, 425, 17, 18, 374
Hydractinia ......... 70

Hydroid , . 416, 19, 20

Hydrozoa 69

Hyena 190

Hylodes 460

Hymenoptera 118

....... 354

. . . 410,411,365
. 185

Hyoid bone
Hypoblast .
Hyrax . .

IBIS 165

Ichneumon fly 119

Ichthyopsida 141

Ichthyosaurus 48, 159

Idotea robusta . . . .-. . . 104, 61

Iguana 155

Iguanodon 159

Ilium 855

Imago .... 69, 74, 76, 368, 369

Impennes . . , 165

Incisors 271

Incus 890

Individuality 432

Infusoria 63.251,276,377

absorbent system of 297

cuticle of 335

digestive process of 295

locomotion of 367

mouth of 256, 277

Ingestion 245

Insecta . 106, 405, 407, 416, 426, 429, 295

absorbent system of 297

antennae of 344

biting 254

circulation in 816

eyes of 392

feet of •. . . . 324

flight of . 370

horny crust of 840

504

INDEX

INSECTA

instinct of

legs of 372,328

liver of 331

locomotion of 367

metamorphosis of 419

mouth of 258

muscle of 365

poison of 334

skeletons of ........ 341

spiracle of 276

touch of .' . . . . . 387

tracheal tube of 277

wings of 369

Insectivora 192

Instinct . 395, 396

Integument ... * 335

Intelligence 395,398

Inter-ambulacra 339

Intervertebral foramen 852

Intestine, see Alimentary canal.

Iris 394, 354

Ischium 355

Ivory 186, 235, 267

Ixodes 123

JACANA 171

Jackal 190

Jay 176

Jellyfish 252, 424, 22

blood of. 301

circulation in 310

digestive sac of 278

locomotion of 367

mouth of 256

nervous system of 377

poison of 334

see Acaleph.

Julus 106

June bug 76

KANGAROO . . 180, 290, 292, 374, 401, 407

Key -hole limpet 1O5

Kidney . . . 333, 112, 244, 25O, 290

King crab 123, 258, 836

Kingfisher 173, 158

Kite 168

Kiwikiwi 162

Klossia . . . 7

LABIUM

Labrum

Labyrinth

Lacerta

Lacertilia

Lachnosterna fusca

254, 259, 319

259, 319, 22O

390

133

154

76

Lacteal .... ..... 298,267

Lacuna? ....... 235, 267, 2O5

Lady-bird .......... 117

Lagena striata ........ 2

Lamellibranch . 125, 126, 86, 244, 275
see Clam.

Laminae 235

Lamprey 144, 428, 118

Lamp shell 86

Lancelet 141, 144, 117

Land snail 93

Lark 176

Larva 76, 77, 368, 369

Larynx 356

Lasso cells 252

Leech . . 95, 97, 250, 258, 279, 319, 371

Lemur 193, 186

Leopard 190

Lepas anatifera 57

Lepidoptera ........ 113, 7O

see Butterfly.

Lepidosiren 151

Lepidosteus osseus 134

Libellula 110, 68

Life 225, 242

duration of 437

origin of 219

struggle for ........ 438

Lightning bug 117

Ligula 107, 259, 64, 33O

Likeness 427

Limax 131, 93

Limbs 355, 375-378

Limicolae 170

Limnaea 134, 93

Limpet 105, 337

Limulus 123,124,258,336

Lion 190, 256, 400, 417

brain of 384

feet of 335

intestines of 292

skeleton of 347, 303

stomach of 353

Liver ... . 280, 330, 331, 113, 117, 389
Lizard, 154, 155, 313, 326, 337, 373, 133, 338

see Lacertilia.
Lobster,

103, 254, 340, 405, 407, 422, 423, 59

auditory sacs of 389

circulation in 311, 367

eyes of 392

gullet of 279

locomotion of 367

mouth of 258

muscle of 365

respiration in 320

shell of 339

teeth of 265

Lobworm 95, 374

Locomotion 366, 331

Locust 110, 254, 281, 319

Loligo 135, 1O8

see Squid.
Long-horned beetle 117

INDEX

505

Loon 141

Lophophore ........ 86, 41

Louse 104, 112, 250, 420

Lucernaria 73, 24

Lumbricus 95

see Earthworm.

Lungfish 151

Lungs 321, 333

Lupus occidentalis 176

Lymph 299

Lymphatics 298, 258

MACAW 172

Mackerel 149

Mactra 86, 244

Madrepora 76, 78, 28, 33

Magpie 400

Malars 353

Malleus 390

Mammalia 177

brain of 380, 882

circulation in 314, 873

dentition of 271

ear of 391

eggs of 419

hair of 337, 345

lacteal system of* 257

locomotion of . • . . . . . .373

lungs of 283

milk of 334

mouth of 263

muscle of . . 364

pelvic arch of 356

respiration in 324, 326

skin of 336

stomach of 289

temperature of 327

teeth of 270

touch of 387

Mammalian vertebrate, section of . . 25O

Man 193,256

arm and leg of 375

brain of . 382, 383, 385

digestive apparatus of ..... 249

intelligence of 399

intestines of 292

locomotion of 367, 375

molars of 271

metamorphosis of 419

nervDus system of 377

organ of hearing of 390

pancreas of 288

skeleton of. ." 191

teeth of 199,270

temperature of 328

toes of 356

tongue of 225

touch of 387

voice of 401

Manatee . .172

Mandible, 254, 259, 354, 216, 22O, 222, 223

Mantis 254

Mantle 336, 244

Manyplies 291, 254

Marine worm 51

Marsh hen 150

Marsupialia ' 180, 407

Mason spiders 123

Mastigophora 60

Mastodon 186

Matrix 230

Maxillae 254,259,819

May fly 110, 256

Meandrina 77

Medulla oblongata, 380, 382, 384, 334-341

Medullary, furrows 411

sheath 239, 21O, 211

Medusa 70, 278, 424, 21

see Jellyfish.

Megalosaurus 159

Megatherium 181

Melanoplus 110

Meleagrina 127, 84

Melolontha 328, 353

Membrana putaminis 403

Mentum 64

Mesentery , 74, 286, 25

Mesoblast 410, 411, 365

Mesoderm . . . 66, 68

Mesothorax 341

Metacarpus 855

Metamorphosis . . 419, 421, 369, 37O

Metatarsus 356

Metatheria 180

Metathorax 341

Metazoa 51, 65

Metridium 75

see Sea anemone.

Milkweed butterfly 368

Millepora 70

Millipede 258,322

see Myriapoda.

Mimicry 428

Minerals and organized bodies . . . 215

Mite 123, 83

Moa 163

Mockingbird 400

Molar . . . 271, 206, 229, 233, 234

Mole 192

Mollusca,

50, 124, 251, 303, 391, 405, 428, 296

absorbent system of 297

circulation in 311

deglutition in 274

digestive process of 295

ear of 347

epidermis of 337

liver of 831

mantle of 336

mouth of 257

5o6

INDEX

MOLLCSCA

nervous system of 331

number of 433

respiration in 820, 327

shells of 341,342,

stomach of 282

teeth of 346

see Clam, Cuttlefish, Snail, Squid.

Molluscoida 82

Molting 337,422

Monkey . . . 193, 256, 374, 381, 387, 398

thumbless 217

Monotremata 179, 180

Morphology 12, 14

Mosquito 113, 250, 78, 369

Moth .... 113, 407, 429, 71, 74, 241
see Lepidoptera. *

Mother Carey's chicken 165

Mother-of-pearl 341

Motion 242,363

Mouse 191

Mouths of animals 256

Mucous membrane 255

Mulberry mass 410

Mullet 149

Murex 129

Musca 113, 324

see Diptera, Fly, House fly.

Muscle 364, 208, 327, 328

Muscular fiber .... 2O9, 318, 319

Muscular tissue 236

Mushroom coral 78, 29

Mussel 125, 126, 127, 341, 85

Mya . . . . 127

Mycetozoa . 59, 3

Myenia 66

Myriapoda 106,392

see Centipede.

Myrmecophaga jubata 168

Mytilus pellucidus 85

Myxine 144, 268

NAILS 344

Narwhal 270

Nassa reticulata 106

Natural selection 439, 455

Nauplius of Entomostracan .... 373
Nautilus . . 134-137, 282, 341, 109, HO

Necrophorus vespillo 77

Necturus 152, 129

Nemathelminthes 82, 84

Nematocyst 68

Nereis 95,97,253,279,215

Nerve, 238, 239, 376, 888, 210, 329, 346
Nervous system . 139, 376, 386, 33O-333

Nervous tissue 237

Neurapophyses 349

Neurilemma 239, 210, 211

Neurology 14

Neuron . . 211

Neuroptera 110

see Dragon fly.

Neuroskeleton 352

Newt, metamorphosis of 370

Noctiluca 60

Notochord 411, 117, 363

Notonecta 112, 66

Nucleolus 228, 198

Nucleus 228, 198

Nudibranch 130

Nutrition 242, 244

Nymph 77

OCEANITE8 165

Ocelli 106, 891, 352

Octopus 135

Odontoid process 354

Odontophore 129

(Estrus 113

Olfactory lobes .... 382, 336-338

Olfactory nerves 388, 346

Oligochaeta . 97

Olive shell 129

Oniscus 104

Onycophora 104

Operculum, 128, 147, 169, 321, 342, 88, 3O9
Ophidia 155

see Snake.

Ophiocoma russei 47

Ophiura . 91, 45, 47

Ophiuroidea 90

Opisthobranchia 129

Opisthocselous 477

Opossum 180, 167

Optic lobes 382, 394, 336-338

Orang-outang, 196, 340, 188, 190, 191

Orchelimum 110

Order 51

Organ 240

Organization 227

Organ-pipe coral 27

Origin, of life 219

of species 450

Oriole 176,429

Ornithorhynchus ... 179, 267, 166

Orthoceras 134

Orthognathous 470

Orthoptera 109

see Grasshopper.

Orycteropus 181

Oscines . .- 176

Osculum 65, 68, 13, 15

Os innominatum 355

Osseous tissue 234

see Bone.

Ossification 415

Ostracoda 101

Ostrea 127

see Oyster.
Ostrich 162, 287, 375, 14O

INDEX

SO/

Otariajubata 179

Otoliths ....... 389, 347, 348

Otter 190

Oviparous 140

Ovipositor 108, 295

Owl 173, 395, 155

Ox 185, 250, 256

brain of 384

foot of 326

horns of 344

intestines of 292

locomotion of 373

papilla? of 336

teeth of 186

see Ungulata.

Oyster, 125, 126, 12T, 251, 406, 416, 422, 426
alimentary canal of ...... 242

circulation in 311

locomotion of 366

mouth of 25T

muscle in 365

pearl 84

respiration in 320

see Clam, Lamellibranch.

PAGTTBUS .
Palaemon
Palatines .
Pallial sinus
Palpiform organs

. . 103

. . 354

126, 296

. 82

Palpus .... 259, 46, 75, 230, 223

Paludina 92, 1O4

Pancreas ...... 330, 331, 288

Pandion haliaetus 147

Pangenesis 455

Pangolin 181

Panther 190

Paper nautilus 135, 109

Papilio 115

Papillae .... 264, 336, 38T, 225, 345

Paraglossse 64, 220

Paramecium . 63, 64, 65, 229, 407, 9, 1O
see Infusoria.

Paramylum bodies 4

Paroquet 172

Parrot .... 164, 172, 388, 400, 154

Parrot fish 268

Partridge 169

Passenger pigeon . 171

Passeres 175

Patella ... 129, 356, 105, 227, 303

Pavament teeth 230

Pearl oyster 127, 84

Pearly ear shell 95

Pearly nautilus 11O

Pecten 127, 391, 35O

Pectinibranchia 129

Pedicellari* 279, 294

Pediculus 112

Pelagia noctiluca . 22

Pelecypoda 125

Pelican 165, 286

Pelomyxa 57

Pelvic arch 355

Pelvis 856

Penguin 163, 165, 142

Pennatula 75, 35

Pentacrinus 93, 50

Pepsin 295

Peptone 295

Perca fluviatilis 3O9

Perch . . . 146, 149, 343, 119, 3O9, 336

Periostium 346,355,366

Periotic bone 358

Peripatus 104, 63

Periplaneta 110

Peristaltic motion 292

Peritoneum 292

Petrel 165

Petrified tissue 234

Petromyzon marinus 118

Phalacrocorax 143

Phalanges 355,356

Phalangida 121

Phalangium 121

Phalarope 170

Pharyngobranchii 144

Pharynx 274, 288, 52, 239

Pheasant 169

Phryganea .......... 110

Phyllopoda 101

Physalia 23

Physeter 17O

see Whale.

Physiology 12,220

Picarise 173

Pieris 115

Pig, bronchial twig of 28O

Pigeon 164, 171, 287

Pike 147,149,268

Pike perch 320

Pinnigrade 189, 373, 325

Pisces 141, 145

see Fish.

Placenta 407, 414

Planaria 82, 38

Planorbis 128, 134, 462, 92

Plantigrade 189, 373, 325

Plant louse 112

Plants and animals 217

Plasma ' 302

Plastron 157

Platyhelminthes 82, 250, 37

Platyonychus 6O

Pleisiosaurus 159

Pleurapophyses 352

Pleurobrachia pileus 36

Plover 170

Podocyrtis schomburgkii . . -. . 2
Poison apparatus 231

5o8

INDEX

Polyp 262, 335, 428

absorbent system of 297

blood of . . 301

coral of 338

liver of 331

mouth of 256

respiration in 319

Polystomella crispa 3

Polyzoa 82, 220, 428, 41, 43

Pomatomus saltatrix 13O

Pond snails 92

Pons varolii 177

Porcupine 191,345,401

Porifera .49, 65

see Sponge.

Porites 78

Porpoise . . 183, 2W, 290, 388, 401, 353

Portal circulation 476

Portuguese man-of-war . . . . 70, 33

Potato worm 116

Poulpe 135

Poultry 169

Prairie chicken 149

Prawn 103,423

Prehension of food 250

Premaxillae 354

Primates 193

Proboscis, of butterfly 331

of elephant 260,264

Procaelous 477

Procyon lotor 175

Prognathous 470

Protamceba 56

Protective resemblance 429

Proteus 152, 303

Proteus anguinus 133

Prothorax 341

Protista . . . . 218

Protophyta 56

Protoplasm .... 23, 215, 219, 226

Protopodite 99

Protopterus 151, 138

Prototheria 179

Protozoa,

49, 51, 54, 276, 301, 391, 407, 416, 423

number of 433

respiration in 319

shells of 338

see Amoeba, Infusoria.

Proventriculus 287

Psalterium 291, 354

Pseudo-lamellibranchia 127

Pseudopodia .... 56, 251, 363, 313

Psittaci 172

see Parrot.

Pterodactylus 159

Pteropod 89

Pubis . 355

Pulex 113

Pulmonary circulation 309

Pulmonata

Pulse

Pupa

Pupil

Putorius noveboracensis

476
76

394

177

Pygopodes 164

Pyloric opening 284, 290

Pyrophorus 117

QUADRUMANA 471

see Monkey.

Quahog 127

Quill 346

EACCOON 190, 175

Eadiates 808

Eadiolaria 59, 338, 3

Eadius 355

Eaia clavata 133

Eail .... * 169, 15O

Eallus elegans 15O

Eana 131

see Frog.

Eank of animals 435

Eat . . ' 191, 344

Eatitae 162

Eattlesnake 155, 844, 331

Eay . . . 147, 148, 268, 866, 119, 33O

Eedstart '. . . 161

Eepair 425

Eeproduction 242, 402, 438

Eeptilia 141, 154, 269, 405, 426

blood of 303

brain of 380, 382, 384

circulation in 313, 373

gullet of 284

mouth of 262

muscle in 364

pelvic arch of 356

respiration in 323, 324

scales of 343

teeth of . 273

see Crocodile, Lizard, Snake, Turtle.

Eespiration 245, 246, 318

Eete mucosum 336

Eeticulum 291

Eetina . . 391, 35O, 355

Ehabdopleura 140

Ehea 162

Ehinoceros . . 185, 344, 373, 173, 336

Ehizopoda 56, 3

Eingdove 153

Eock shell 129

Eodentia . . . 190, 271, 290, 419, 180

Eotifers 82, 85, 26G, 4O

Eudimentary organs 418

Eumen 291, 354

Euminants 186, 255

feet of 186

molars of 271

INDEX

509

RUMINANTS

stomach of ...
teeth of ....
see Ox, Ungulata.

291, 254
. 270

SACRUM ... 855

Salamander 429, 130

Saliva 295

Salivary glands 330

Salmon . . . 149, 269, 364, 368, 397, 121

scale of 343, 119

Salmo salar 121

Salpa 143

Samia, wing of 71

Sand dollar 92

Sand flea .62

Sandpiper 152

Sandworm 95

Sapajou, white-throated 187

Sarcolemma 237, 415

Sarcophaga earn aria 69

Sarcoptes 123

Saurian, teeth of 269

Sauropsida 141

Scales . . . 146, 337, 343, 7O, 71, 119

Scallop 127, 391

Scapula 355

Sclerobase 76, 81,338

Scleroderm 76, 77, 338

Sclerotic 393, 354

Scolopendra 106

Scorpion 120, 311, 322, 392, 8O

jaws of 261

respiration in 322

Scorpionida 120

Scutes 344

Scyphozoa 69, 72

Sea anemone 75, 387

see Polyp.

Sea blubber 73

Sea butterfly 130

75, 35

130

Seal ...... 189, 378, 325, 377

Sea lemon 130

Sea lily . <»:>

Sea lion 189, 179

Sea mat 219

Sea moss 220

Sea pen 75,35

Sea slug 92, 130, 49

Sea urchin,

35, 91, 252, 257, 48, 226, 237

absorbent system of 297

circulation in 310

digestive cavity of 278

digestive process of 295

eyes of 391

growth of 426

respiration in 319

SEA URCHIN

shell of . .338, 293

spines of 294

teeth of 265

Sea worm 97, 279, 51

Secreting membranes 286

Secreting organs 330

Secretion 245,329

Segmentation 409, 361

Selection 455, 456, 465, 466

Self-division 402

Semicircular canals 390

Sensation 242,386

Sense organs 416

Senses 386

Sensibility 222

Sensory nerve 376

Sepia 135, 1O7"

Septum 25, 44

Series 51

Serosity 302

Serpent 269, 275, 284, 337, 371

see Snake.

Sertularia 19

Serum 302

Setophaga ruticilla 161

Seventeen-year cicada 67

Sexton beetles 77

Sexual reproduction .... 402, 403

Shaft 846

Shark . . . 147, 148, 268, 388, 406, 122

egg of 36O

endoskeleton of 846

muscle of 364

scales of 146,343

Sheath, medullary 239

Sheep 881,401,427

Shells 337

Shipworm 127

Shrew 192,264

Shrew mouse 183

Shrimp 103

Sight 391

Silk secretors 238

Silkworm 116

Silpha 117

Simia satyrus 188

Sinus 347

Siphon 320, 86

Siphuncle 136

Sirenia 184

Size of animals 433

Skeleton 335,337

of arthropod 53

of vertebrates. .191,303,309-317

Skin 333,335

from horse's nostril 291

muscles 365

offish .- 299

Skull, bone of 205

5io

INDEX

SKULL

formation of . . 352

of ant-eater 168

of babirusa 232

of boa constrictor 235

of chimpanzee 189

of dog 353, 305-3O7

of European 195

of horse 308

of negro 196

of orang-outang 188

of rodent 18O

Sloth 181, 273, 346

Slug 131, 267, 92

Smell 388

Snail 127, 128, 388, 92, 218

anatomy of 243

circulation in 311

eyes of 391

fresh-water 1O4

gullet of 282

head of 351

jaw of 218

locomotion of 371

nervous system of 378

teeth of 265, 266

temperature of 328

touch of 387

veligerof 372

see Gastropoda.
Snake .... 154, 155, 354, 422, 429

circulation in 313

heads of 135

locomotion of . 367

lungs of 281

poison of 334

respiration in 326

teeth of 156, 269

tongue of 263

touch of 387

vertebral column of 352

Snipe 170

Solaster 90

Somite 469

Songsters 176

Sorex 183

Sow bug 104

Sparrow 176, 160

Specialization 293

Species 51,433

Spelerpes ruber 130

Sperm cell 408

Sperm whale 17O

Sphenoid bone 353

Sphinx ligustri 333

Sphinx moth, anatomy of. . . 114,241
Spider, 121, 122, 123, 405, 407, 429, 81 , 223

alimentary canal of 281

circulation in 311

eyes of 892

SPIDER

fang of 216

legs of 371

locomotion of 867

muscle of 365

respiration in 322

silk of 334

Spinal cord, of am phiox us 117

of tunicate 116

Spindle shell 129, 96

Spinnerets 121,82,223,238

Spiracles 108, 321, 276

Splint bone 356, 419

Sponge, 301, 319, 335, 366, 422, 13, 15, 16

egg of 405,359

Spongilla 66

Spongin 66

Spoonbill 165

Sporozoa 61, 7

Squamata 154

Squamosal bones 353

Squash bug 112

Squid 135,367

see Cuttlefish.

Squirrel 191, 328, 374

flying 370

Stag 174

Stapes 390

Starfish,

220, 252, 405, 419, 426, 45, 46, 323

circulation in 310

digestive process of 295

eyes of 391

locomotion of 367, 371

mouth of 257

nervous system of .... 378, 33O

respiration in 319

skeleton of 338

stomach of 278

see Echinodermata.

Steganopodes 165

Stentor 68

Sternum 354, 151

Stilt 170

Stomach, coats of 255

of horse 251

oflion 253

of mammals 288

of porpoise 252

of ruminant 254

of tunicate 115

Stork 165

Stri* 237

Striated muscular fibers .... 2O9

Stridula 399

Striges 173

Strix pratincola . 155

Strombus 129, 1O3

Strophocheilus 131, 93

Struggle for existence .... 438, 453

INDEX

Struthio 162, 14O

Sturgeon . . 146, 149, 254, 346, 364, 125

Stylonychia 441

Sun star 90

Surinam toad 407

Survival of the fittest . . . . . .439

Suture 356

Swallow 176, 328, 397

instinct of 397

temperature of 328

Swan 287

Sweetbread 331

Swift 173

Swimmerets 100, 54

Sympathetic and spinal nerves . . 343

Synovia 356

Systematic circulation 309

TABANUS LINEOLA 282

Tactile corpuscles 887

Tamia 82, 250, 37

Tanager 176

Tapetum 395

Tapeworm 82, 250, 37

Tapir 186, 264, 376

Tarsus 356

Taste 388

Teeth, of animals 265

of chimpanzee 233

of elephant 234

offish 147

offrog 152

ofhorse 185

ofman 199

of ox 186

of snake 156

Telea 116

Teleostomi 148

Telson 99

Temperature 327

Temporal bones ........ 353

Tendon 366

Tentacles 387

Tentaculifera 63, 64

Tent caterpillar 116

Terebra maculata 98

Terebratulina septentrionalis . . 43, 44

Teredo 127

Termes 110

Tern 170, 151

Terrapene Carolina 137

Testudo 158

Tetrabranch 135

Theria 180

Thoracic duct 299, 258

Thorax 289, 354, 285

Thornback - ... 149

Thousand-legged worm 106

Thrush 176

Thumbless monkey 217

Thylacinus 181

Thyone 35

Thyroid cartilage 400

Tibia 372, 356

Tick 123

Tiger 190

Tiger beetle 117

Tineids 116

Tipula 113

Tissue 229, 20O, 2O1, 2O7

Toad .... 153,269,327,328,337,422

Toes 356,373

Tongue, human 225

Top shell 1O2

Tortoise . . 157, 269, 284, 372, 137, 312
see Turtle.

Tortoise shell 344

Tortoise-shell butterfly ..... 72

Toucan 173

Touch 387

Trachea 325, 278, 283, 356

Trachefe 321

Tribe 51

Trichia 3

Trichina spiralis .84,39

Tringa hypoleuca 152

Tritonia 129, 130, 9O, 297

Trochanter 372

Trochelminthes 82, 85

Trochosphere 422, 371

Trochus 129

Trogon 173, 156

Trout 146

Trumpet animal 63

Trumpet shell 129

Tubipora 75, 79, 27

Tunicates . . . 140, 142, 336, 115, 116

Turbinares 165

Turbo 129, 1O2

Turkey 169, 287, 338

Turtle . 157, 158, 262, 269, 406, 407, 136

beak of .344

bones of 346

circulation in 313

plates of 844

respiration in . . 326

see Chelonia.

Tusks 186

Tympanic bones 353

Tympanum 390, 349

Types 49

Tyrannus 159

ULNA 355

Umbo 125

Umbrella-acaleph 73

Ungulata 185,873

Unio 125,127,341,406

Univalve 320, 341, 297

Urinator imber - 141

512

INDEX

Urochorda 140,142

see Ascidian, Tunicates.
Urodela 152

VANE 346

Vanessa 115, 78

Variation 427,454

Vegetative life 242

Vegetative repetition 220

Veins 309, 865

Veliger of snail 422, 378

Velum 423

Velutina, odontophore of 287

Vena cava 309

Venous valves 864

Venus . . . * 127

Vermes ,. . 82, 433

see Earthworm, Worm.

Vertebra 848, 352, 304

Vertebrata .... 50, 138, 140, 144, 417

absorbent system of 298

blood of 301

brain of 380

circulation in 813, 118

deglutition in 274

exoskeleton of 343

eye of 393

liver of 331

mouth of 262

muscle of 365, 366

nervous system of .... 139, 379

number of 433

optic nerve of 391, 395

organ of hearing of 389

respiration in 324, 325, 326

teeth of 346

see Bird, Crocodile, Fish, Frog, Mam-
malia, Reptilia.

Vespa 119

Vespertilio 184

Vestibule 390

Villi 293,298

Vinegar eel 84

Viper 155, 134

Vireo . 168

Vitality 225

Vitelline membrane .... 403,404

Vitreous humor 393, 394

Viviparous 140

Voices of animals 399

Volute 129, 1O1

Volvox 60

Vorticella 63, 407, 11'

Vulpes pennsylvanicus 178

Vulture 168, 286, 313

WALKING STICK
Walrus . . .
Warbler

Warning coloration . ... 429

Wasp 119, 406

Water beetle 384

Water boatman 112, 66

Water flea 56

Waxwing 176

Weasel 190, 177

Weevil 118

Whale .... 182, 183, 251, 388, 401

bones of 346

brain of 380

Greenland 418, 171

intestines of 292

mouth of 264

sperm ......... 17O

upper jaw of 838

whalebone . . ' . . . 267, 344, 311
Wheel animalcule ...'.. 85, 4O
Whelk . . . 129, 252, 257, 58, 88, 887
see Snail.

White ants 110

Wild bee 220

Wilson's petrel 165

Windpipe 325

Wingless flea 113

Wings 369, 378

Wing shell 129, 1O3

Wolf 190, 176

Wombat 180

Woodcock 170

Wood louse 104

Woodpecker 173, 154

Worm .... 303,405,422,426,428

absorbent system of 297

bristle-footed 95

circulation in 311

cuticle of - 336

gastrula of 36SJ

locomotion of 367

mouth of . . 258

nervous system of 878

planarian 38

teeth of 346

see Earthworm, Leech, Nereis.

Wren . 176

Wrist 355

XIPHOSURA 123

YOLK 403, 404

ZEBRA 427

Zoantharia 75

Zonotrichia albicollis 16O

Zooid . . . 433, 8O

Zoology 11

Zoothamnium ' . 63

Zygapophyses 849

Zygmotic arch 354

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