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TWENTIETH CENTURY TEXT-BOOKS

EDITED BY

A. F. NIGHTINGALE, PH.D., LL.D.

SUPERINTENDENT OF SCHOOLS, COOK COUNTY, ILLINOIS

-

TWENTIETH CENTURY TEXT-BOOKS

ANIMALS

A TEXT-BOOK OF ZOOLOGY

BY

DAVID S. JORDAN, M. S., M. D., PH. D., LL. D.

PRESIDENT OF LELAND STANFORD JUNIOR UNIVERSITY

VERNON L. KELLOGG, M. S.

PROFESSOR OF ENTOMOLOGY IN LELAND STANFORD JUNIOR UNIVERSITY

AND

HAROLD HEATH, PH.D.

PROFESSOR OF ZOOLOGY IN LELAND STANFORD JUNIOR UNIVERSITY

OF THE

UNIVERSITY

OF

NEW YORK

D. APPLETON AND COMPANY
1907

BIOLOQY

LIBRARY

G

CoPtRIGHT, 1902

BY D. APPLETON AND COMPANY

Published May, J902

ANIMALS

A TEXT-BOOK OF ZOOLOGY

PUBLISHERS' NOTE

THIS volume comprises Animal Life and Animal Forms,
each of which is designed for schools that assign but a half
year to Zoology. The two are issued in this form to meet
more satisfactorily the requirements of normal schools and
all higher schools that give a full year to the subject. It
is believed that it will be found a very desirable text, also,
for an elementary course in colleges.

The prefaces of the two sections of the book explain
the plan, scope, and purpose of each. If preferred, the
second part of this volume, Animal Forms, treating the
morphological side of the subject, can be first taken up ; or
portions of each may be chosen and arranged in any order
that may best suit the teacher's purposes. The first part,
Animal Life, gives the ecological features of the subject
special prominence.

Full indexes will be found at the end of each section.

TWENTIETH CENTURY TEXT-BOOKS

ANIMAL LIFE

A FIRST BOOK OF ZOOLOGY

BY

DAVID STARR JORDAN, PH. D., LL. D.

PRESIDENT OF LELAND STANFORD JUNIOR UNIVERSITY
AND

VERNON L. KELLOGG, M.S.

PROFESSOR IN LELAND STANFORD JUNIOR UNIVERSITY

NEW YORK

D. APPLETON AND COMPANY
1907

fat

COPYRIGHT, 1900
Br D. APPLETON AND COMPANY

PREFACE

THE authors present this book as an elementary ac-
count of animal ecology — that is, of the relations of ani-
mals to their surroundings and their responsive adaptation
to these surroundings. The book takes the observer's point
of view, who is especially concerned with the reasons for
the varied structure and habits of animals. To understand
how naturally and inevitably all animal form, habit, and
life are adapted to the varied circumstances and conditions
of animal existence should be the motive of the beginner in
this fascinating study. The greatest facts of life, except
that of life itself, are seen in the marvelously perfect meth-
ods which Nature has adopted in the structure and habits
of animals. The keen observation of a fact should lead
the student to inquire into the significance of that fact.
The veriest beginner can be, and ought to be, an independ-
ent observer and thinker. In the study of zoology that
phase which treats of the why and how of animal form and
habit not only absorbs the attention of the most advanced
modern scholars of biology, but should also appeal most
strongly to the beginner. The beginner and the most
enlightened thinker in zoology should each have the same
point of view. With this belief in mind the authors have
tried to put into simple form the principal facts and
approved hypotheses upon which the modern conceptions
of animal life are based.

It is unnecessary to say that this book depends for its

V

167418

Vl ANIMAL LIFE

best use on a basis of personal observational work by the
student in laboratory and field. Without independent
personal work of the student little can be learned about
animals and their life that will remain fixed. But present-
day teachers of biology are too well informed to make a
discussion of the methods of their work necessary here.
As a matter of fact, the methods of the teacher depend so
absolutely on his training and individual initiative that it
is not worth while for the authors to point out the place
of this book in elementary zoological teaching. That the
phase of study it attempts to represent should have a place
in such teaching is, of course, their firm belief.

The obligations of the authors for the use of certain
illustrations are acknowledged in proper place. Where no
credit is otherwise given, the drawings have been made by
Miss Mary H. Wellman or by Mr. James Carter Beard, and
the photographs have been made by the authors or under
their direction.

DAVID STARR JORDAN,
VERNON LYMAN KELLOGG.

NOTE. — After the pages of the book were cast, it was thought that
a transposition of Chapters III and IV would present a more logical
arrangement, and teachers are advised to omit in their study scheme
Chapter III until Chapter IV is completed. D. S. J.

V. L. K.

CONTENTS

CHATTER PAGE

I. — THE LIFE OF THE SIMPLEST ANIMALS 1

The simplest animals, or Protozoa, 1.— The animal cell, 2. —
What the primitive cell can do, 5. — Amoeba, 5. — Paramoecium, 9.
— Vorticella, 12.— Marine Protozoa, 15.— Globigeriuffi and Kadio-
laria, 16. — Antiquity of the Protozoa, 20. — The primitive form,
20.— The primitive but successful life, 21.

II.— THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS ... 24

Colonial Protozoa, 24. — Gonium, 25. — Pandorina, 26. — Eudo-
rina, 27. — Volvox, 28. — Steps toward complexity, 30. — Individual
or colony, 31. — Sponges, 32. — Polyps, corals, and jelly-fishes, 37.
— Hydra, 37. — Differentiation of the body cells, 41. — Medusje or
jelly-fishes, 41. — Corals, 43. — Colonial jelly-fishes, 45. — Increase
in the degree of complexity, 48.

III. — THE MULTIPLICATION OF ANIMALS AND SEX ... 50

All life from life, 50. — Spontaneous generation, 51. — The
simplest method of multiplication, 53. — Slightly complex methods
of multiplication, 54. — Differentiation of the reproductive cells, 55.
— Sex, or male and female, 57. — The object of sex, 57.— Sex di-
morphism, 58. — The number of young, 61 .

IV. — FUNCTION AND STRUCTURE 63

Organs and functions, 63.— Differentiation of structure, 64.—
Anatomy and physiology, 64. — The animal body a machine, 65.
— The specialization of organs, 66. — The alimentary canal, 66. —
Stable and variable characteristics of an organ, 73. — Stable and
variable characteristics of the alimentary canal, 73. — The mutual
relation of function and structure, 77.

V.— THE LIFE CYCLE 78

Birth, growth and development, and death, 78. — Life cycle of
simplest animals, 78. — The egg, 79. — Embryonic and post-em-
bryonic development, 80. — Continuity of development, 83. — De-
velopment after the gastrula stage, 84.— Divergence of develop-

vii

Vlll ANIMAL LIFE

CHAPTER PAGE

ment, 84. — The laws or general facts of development, 86. — The
significance of the facts of development, 89. — Metamorphosis,
90. — Metamorphosis among insects, 90. — Metamorphosis of the
toad, 94. — Metamorphosis among other animals, 96. — Duration of
life, 101.— Death, 103.

VI. — THE PRIMARY CONDITIONS OF ANIMAL LIFE .... 106

Primary conditions and special conditions, 106. — Food, 106. —
Oxygen, 107. — Temperature, pressure, and other conditions, 108.
— Difference between animals and plants, 111. — Living organic
matter and inorganic matter, 112.

VII. — THE CROWD OF ANIMALS AND THE STRUGGLE FOR EXIST-
ENCE . 114

The crowd of animals, 114. — The struggle for existence, 116.
— Selection by Nature, 117. — Adjustment to surroundings a re-
sult of natural selection, 120. — Artificial selection, 120. — Depend-
ence of species on species, 121.

VIII.— ADAPTATIONS 123

Origin of adaptations, 123. — Classification of adaptations, 123.
— Adaptations for securing food, 125. — Adaptations for self-de-
fense, 128. — Adaptations for rivalry, 135. — Adaptations for the
defense of the young, 137. — Adaptations concerned with sur-
roundings in life, 143.— Degree of structural change in adapta-
tions, 146. — Vestigial organs, 147.

IX. — ANIMAL COMMUNITIES AND SOCIAL LIFE . . . 149

Man not the only social animal, 149. — The honey-bee, 149. —
The ants, 155.— Other communal insects, 158. — Gregariousness
and mutual aid, 163.— Division of labor and basis of communal
life, 168. — Advantages of communal life, 170.

X.— COMMENSALISM AND SYMBIOSIS . ... * .172

Association between animals of different species, 172. — Com-
mensalism, 173. — Symbiosis, 175.

XI. — PARASITISM AND DEGENERATION 179

Relation of parasite and host, 179. — Kinds of parasitism, 180.
—The simple structure of parasites, 181.— Gregarina, 182.— The
tape-worm and other flat-worms, 183.— Trichina and other round-
worms, 184.— SaccuUna, 187.— Parasitic insects, 188. —Parasitic
vertebrates, 193. — Degeneration through quiescence, 193. — De-
generation through other causes, 197. — Immediate causes of de-
generation, 198. — Advantages and disadvantages of parasitism
and degeneration, 198. — Human degeneration, 200.

CONTENTS ix

CHAPTER PAGE

XII. — PROTECTIVE RESEMBLANCES AND MIMICRY .... 201

Protective resemblance defined, 201. — General protective or
aggressive resemblance, 202. — Special protective resemblance,
207. — Warning colors and terrifying appearances, 212. — Alluring
coloration, 216. — Mimicry, 218. — Protective resemblances and
mimicry most common among insects, 221. — No volition in mim-
icry, 222.— Color : its utility and beauty, 222.

XIII.— THE SPECIAL SENSES 224

Importance of the special senses, 224 — Difficulty of the study
of the special senses, 224. — Special senses of the simplest ani-
mals, 225.— The sense of touch, 226.— The sense of taste, 228.—
The sense of smell, 229.— The sense of hearing, 232.— Sound-mak-
ing, 235 —The sense of sight, 237.

XIV. — INSTINCT AND REASON 240

Irritability, 240.— Nerve cells and fibers, 240.— The brain or
sensorium, 241.— Reflex action, 241.— Instinct, 242.— Classifica-
tion of instincts, 243.— Feeding, 244.— Self-defense, 245.— Play,
247.— Climate, 248.— Environment, 248.— Courtship, 248.— Repro-
duction, 249. — Care of the young, 250. — Variability of instincts,
251.— Reason, 251.— Mind, 255.

XV. — HOMES AND DOMESTIC HABITS 257

Importance of care of the young, 257.— Care of the young and
communal life, 257. — The invertebrates (except spiders and in-
sects), 258.— Spiders, 259.— Insects, 262.— The vertebrates, 264.

XVI. — GEOGRAPHICAL DISTRIBUTION OF ANIMALS .... 272

Geographical distribution, 272. — Laws of distribution, 274. —
Species debarred by barriers, 274. — Species debarred by inability
to maintain their ground, 275. — Species altered by adaptation to
new conditions, 276.— Effect of barriers, 283.— Relation of species
to habitat, 283. — Character of barriers to distribution, 288.— Bar-
riers affecting fresh-water animals, 294. — Modes of distribution,
296.— Fauna and faunal areas, 296.— Realms of animal life, 297.—
Subordinate realms or provinces, 303.— Faunal areas of the sea,
304.

CLASSIFICATION OF ANIMALS 307

GLOSSARY 313

INDEX . . 319

ANIMAL LIFE

CHAPTEE I

THE LIFE OF THE SIMPLEST ANIMALS

1. The simplest animals, or Protozoa. — The simplest ani-
mals are those whose bodies are simplest in structure and
which do the things done by all living animals, such as
eating, breathing, moving, feeling, 'and reproducing in the
most primitive way. The body of a horse, made up of
various organs and tissues, is complexly formed, and the
various organs of the body perform the various kinds of
work for which they are fitted in a complex way. The
simplest animals are all very small, and almost all live in
the water ; some kinds in fresh water and many kinds in
the ocean. Some live in damp sand or moss, and still others
are parasites in the bodies of other animals. They are not
familiarly known to us; we can not see them with the
unaided eye, and yet there are thousands of different kinds
of them, and they may be found wherever there is water.

In a glass of water taken from a stagnant pool there
is a host of animals. There may be a few water beetles
or water bugs swimming violently about, animals half an
inch long, with head and eyes and oar-like legs ; or there
may be a little fish, or some tadpoles and wrigglers. These
are evidently not the simplest animals. There will be
many very small active animals barely visible to the un-
aided eyes. These, too, are animals of considerable com-
plexity. But if a single drop of the water be placed
2 1

2 ANIMAL LIFE

on a glass slip or in a watch glass and examined with a
compound microscope, there will be seen a number of ex-
tremely small creatures which swim about in the water-drop
by means of fine hairs, or crawl slowly on the surface of the
glass. These are among our simplest animals. There are,
as already said, many kinds of these " simplest animals,"
although, perhaps strictly speaking, only one kind can be
called simplest. Some of these kinds are spherical in
shape, some elliptical or football-shaped, some cortical, some
flattened. Some have many fine, minute hairs projecting
from the surface ; some have a few longer, stronger hairs
that lash back and forth in the water, and some have no
hairs at all. There are many kinds and they differ in size,
shape, body covering, manner of movement, and habifc of
food-getting. And some are truly simpler than others.
But all agree in one thing — which is a very important
thing — and that is in being composed in the simplest way
possible among animals.

2. The animal cell — The whole body of any one of the
simplest animals or Protozoa is composed for the animal's
whole lifetime of but a single cell. The bodies of all other
animals are composed of many cells. The cell may be
called the unit of animal (or plant) structure. The body
of a horse is complexly composed of organs and tissues.
Each of these organs and tissues is in turn composed of a
large number of these structural units called cells. These
cells are of great variety in shape and size and general
character. The cells which compose muscular tissue are
very different from the cells which compose the brain.
And both of these kinds of cells are very different from
the simple primitive, undifferentiated kind of cell seen in
the body of a protozoan, or in the earliest embryonic
stages of a many-celled animal.

The animal cell is rarely typically cellular in character
— that is, it is rarely in the condition of a tiny sac or box
of symmetrical shape. Plant cells are often of this char-

THE LIFE OF THE SIMPLEST ANIMALS 3

acter. The primitive animal cell (Fig. 1) consists of a
small mass of a viscid, nearly colorless, substance called
protoplasm. This protoplasm is differentiated to form two
parts or regions of the cell, an inner denser mass called the
nucleus, and an outer, clearer, inclosing mass called the
cytoplasm. There may be more than
one nucleus in a cell. Sometimes
the cell is inclosed by a cell wall
which may be simply a tougher outer
layer of the cytoplasm, or may be a
thin membrane secreted by the pro-
toplasm. In addition to the proto-'
plasm, which is the fundamental and
essential cell substance, the cell may Flo. i._Biood ceil of a crab
contain certain so-called cell prod- <after HAKCKEL). show-
ncta, substances produced by the life 'S^ST^^

processes Of the protoplasm. The circular spot) and gran-

cell may thus contain water, oils,
resin, starch grains, pigment gran-
ules, or other substances. These substances are held in
the protoplasm as liquid drops or solid particles.

The protoplasm itself of the cell shows an obvious
division into parts, so that certain parts of it, especially
parts in the nucleus, have received names. The nucleus
usually has a thin protoplasmic membrane surrounding it,
which is called the nuclear membrane. There appear to be
fine threads or rods in the nucleus which are evidently
different from the rest of the nuclear protoplasm. These
rods are called chromosomes. The cell is, indeed, not so
simple as the words " structural unit " might imply, but
science has not yet so well analyzed its parts as to warrant
the transfer of the name structural unit to any single part
of the cell — that is, to any lesser or simpler part of the
animal body than the cell as a whole.

The protoplasm, which is the essential substance of the
cell and hence of the whole animal body, is a substance

4. ANIMAL LIFE

of a very complex chemical and physical constitution. Its
chemical structure is so complex that no chemist has yet
been able to analyze it, and as the further the attempts at
analysis reach the more complex and baffling the substance
is found to be, it is not improbable that it may never be
analyzed. It is a compound of numerous substances, some
of these composing substances being themselves extremely
complex. The most important thing we know about the
chemical constitution of protoplasm is that there are al-
ways present in it certain complex albuminous substances
which are never found in inorganic bodies. It is on the
presence of these albuminous substances that the power of
performing the processes of life depends. Protoplasm is the
primitive basic life substance, but it is the presence of these
complex albuminous compounds that makes protoplasm the
life substance. A student of protoplasm and the funda-
mental life processes, Dr. Davenport, has said, "Just as
the geologist is forced by the facts to assume a vast but
not infinite time for earth building, so the biologist has to
recognize an almost unlimited complexity in the constitu-
tion of the protoplasm." *

* The physical structure of protoplasm has been much studied,
but even with the improved microscopes and other instruments neces-
sary for the study of minute structure, naturalists are still very fat
from understanding the physical constitution of this substance. While
the appearance of protoplasm under the microscope is pretty generally
agreed on among naturalists, the interpretation of the kind of structure
which is indicated by this appearance is not at all well agreed on.
Protoplasm appears as a mesh work composed of fine granules sus-
pended in a clearer substance, the spaces of the mesh work being com-
posed of a third still clearer substance. Some naturalists believe, from
this appearance, that protoplasm is composed of a clear viscous sub-
tance, in which are imbedded many fine granules of denser substance,
and numerous large globules of a clearer, more liquid substance. Other
naturalists believe that the fine spots which appear to be granules are
simply cross sections of fine threads of dense protoplasm which lie
coiled and tangled in the thinner, clearer protoplasm. And, finally,

THE LIFE OF THE SIMPLEST ANIMALS 5

3. What the primitive cell can do. — The body of one of
the minute animals in the water-drop is a single cell. The
body is not composed of organs of different parts, as in the
body of the horse. There is no heart, no stomach ; there
are no muscles, no nerves. And yet the protozoan is a liv-
ing animal as truly as is the horse, and it breathes and eats
and moves and feels and produces young as truly as does
the horse. It performs all the processes necessary for the
life of an animal. The single cell, the single minute speck
of protoplasm, has the power of doing, in a very simple and
primitive way, all those things which are necessary for
life, and which are done in the case of other animals by
the various organs of the body.

4. Amoeba. — The simple and primitive life of these
Protozoa can be best understood by the observation of
living individuals. In the slime and sediment at the
bottom of stagnant pools lives a certain specially interest-
ing kind of protozoan, the Amceba (Fig. 2). Of all the
simplest animals this is as simple or primitive as any. The
minute viscous particle of protoplasm which forms its
body is irregular in outline, and its outline or shape slowly
but constantly changes. It may contract into a tiny ball ;
it may become almost star-shaped ; it may become elongate
or flattened ; short, blunt, finger-like projections called
pseudopods extend from the central body mass, and these
projections are constantly changing, slowly pushing out or

others believe that protoplasm exists as a foam work ; that it is a vis-
cous liquid containing many fine globules (the granule-appearing spots)
of a liquid of different density and numerous larger globules of a liquid
of still other density. It is a foam in which the bubbles are not filled
with air, but with liquids of different density. This last theory of the
structure of protoplasm is the one accepted by a majority of modern
naturalists, although the other theories have numerous believers. But
just as with what little we know of the chemical constitution of proto-
plasm, the little we know of its physical structure throws almost no
light on the remarkable properties of this fundamental life substance.

6

ANIMAL LIFE

drawing in. The single protoplasmic cell which makes up
the body of the Amoeba has no fixed outline ; it is a cell
without a wall. The substance of the cell or body is proto-
plasm, semiliquid and colorless. The changes in form of
the body are the moving of the Amoeba. By close watching
it may be seen that the Amoeba changes its position on the
glass slip. Although provided with no legs or wings or

FIG. 2. — An Amoeba, showing different shapes assumed by it when crawling.
—After VEBWOBN.

scales or hooks — that is, with no special organs of locomo-
tion— the Amoeba moves. There are no muscles in this tiny
body; muscles are composed of many contractile cells
massed together, and the Amoeba is but one cell. But it is
a contractile cell ; it can do what the muscles of the com-
plex animals do.

If one of the finger-like projections of the Amoeba^ or,
indeed, if any part of its body comes in contact with some
other microscopic animal or plant or some small fragment
of a larger form, the soft body of the Amoeba will be seen

THE LIFE OF THE SIMPLEST ANIMALS

to press against it, and soon the plant or animal or organic
particle becomes sunken in the protoplasm of the formless
body and entirely inclosed in it (Fig. 3). The absorbed
particle soon wholly or partly disappears. This is the
manner in which the Amoeba eats. It has no mouth or

FIG. 3.— Am&ba eating a microscopic one-celled plant.— After VERWOKN.

stomach. Any part of its body mass can take in and digest
food. The viscous, membraneless body simply flows about
the food and absorbs it. Such of the food particles as can
not be digested are thrust out of the body.

The Amoeba breathes. Though we can not readily ob-
serve this act of respiration, it is true that the Amoeba takes
into its body through any part of its surface oxygen from
the air which is mixed with water, and it gives off from any
part of its body carbonic-acid gas. Although the Amoeba
has no lungs or gills or other special organs of respiration,
it breathes in oxygen and gives out carbonic-acid gas, which
is just what the horse does with its elaborately developed
organs of respiration.

If the Amoeba, in moving slowly about, comes into con-
tact with a sand grain or other foreign particle not suitable
for food, the soft body slowly recoils and flows — for the
movement is really a flowing of the thickly fluid protoplasm
— so as to leave the sand grain at one side. The Amoeba
feels. It shows the effects of stimulation. Its movements
can be changed, stopped, or induced by mechanical or
chemical stimuli or by changes in temperature. The

8

ANIMAL LIFE

Amoeba is irritable ; it possesses irritability, which is sensa-
tion in its simplest degree.

If food is abundant the Amoeba soon increases in size.
The bulk of its body is bound to increase if new substance

FIG. 4.— Amoeba polypodia in six successive stages of division. The dark, white-
margined spot in the interior is the nucleus.— After P. E. SCHULZE.

is constantly assimilated and added to it. The Amoeba
grows. But there seem to be some fixed limits to the
extent of this increase in size. No Amoeba becomes large.
A remarkable phenomenon always oocurs to prevent this,

THE LIFE OF THE SIMPLEST ANIMALS 9

An Amoeba which has grown for some time contracts all
its finger-like processes, and its body becomes constricted.
This constriction or fissure increases inward, so that the
body is soon divided fairly in two (Fig. 4). The body,
being an animal cell, possesses a nucleus imbedded in the
body protoplasm or cytoplasm. When the body begins to
divide, the nucleus begins to divide also, and becomes en-
tirely divided before the fission of the cytoplasm is com-
plete. There are now two Amcebce, each half the size of
the original one ; each, indeed, being actually one half of
the original one. This splitting of the body of the Amoeba,
which is called fission, is the process of reproduction. The
original Amoeba is the parent ; the two halves of the parent
are the young. Each of the young possesses all of the
characteristics and powers of the parent ; each can move,
eat, feel, grow, and reproduce by fission. It is very evident
that this is so, for any part of the body or the whole body
was used in performing these functions, and the young are
simply two parts of the parent's body. But if there be any
doubt about the matter, observation of the behavior of the
young or new Amcebce will soon remove it. Each puts out
pseudopods, moves, ingests food particles, avoids sand
grains, contracts if the water is heated, grows, and finally
divides in two.

5. Paramcecimn. — Another protozoan which is common
in stagnant pools and can be readily obtained and observed
is Paramcecium (Fig. 5). The body of the Paramcecium is
much larger than that of the Amoeba, being nearly one fourth
of a millimeter in length, and is of fixed shape. It is elon-
gate, elliptical, and flattened, and when examined under the
microscope seems to be a very complexly formed little mass.
The body of the Paramcecium is indeed less primitive than
that of the Amoeba, and yet it is still but a single cell.
The protoplasm of the body is very soft within and dense
on the outside, and it is covered externally by a thin mem-
brane. The body is covered with short fine hairs or cilia,

10

ANIMAL LIFE

which are fine processes of the dense protoplasm of the
surface. There is on one side an oblique shallow groove
that leads to a small, funnel-shaped depression in the body
which serves as a primitive sort of mouth
or opening for the ingress of food.
The ParamcBcium swims about in the
water by vibrating the cilia which cov-
er the body, and brings food to the
mouth opening by producing tiny cur-
rents in the water by means of the
cilia in the oblique groove. The food,
which consists of other living Proto-
zoa, is taken into the body mass only
through the funnel-shaped opening, and
that part of it which is undigested is
thrust out always through a particular
part of the body surface. (The taking
in and ejecting of foreign particles can
be seen by putting a little powdered
carmine in the water.) Within the
body there are two nuclei and two so-
called pulsating vacuoles. These pul-
sating vacuoles (Amceba has one) seem
to aid in discharging waste products
When the Paramce-
cium touches some foreign substance or
is otherwise irritated it swims away,
and it shoots out from the surface of its body some fine
long threads which when at rest are probably coiled up in
little sacs on the surface of the body. When the Parar
mcecium has taken in enough food and grown so that it
has reached the limit of its size, it divides transversely into
halves as the Amoeba does. Both nuclei divide first, and
then the cytoplasm constricts and divides (Fig. 6). Thus
two new ParamoBcia are formed. One of them has to de-
velop a new mouth opening and groove, so that there is in

FIG. S.—Paramaicium au-
relia (after VKRWOKN).
At each end there is a
contractile vacuole, and from the body,
in the center is one of
the nuclei.

THE LIFE OF THE SIMPLEST ANIMALS

11

the case of the reproduction of Paramcecium the beginnings
of developmental changes during the course of the growth
of the young. The young Amcebce have only to add sub-
stance to their bodies, to grow larger, in order to be exactly
like their parent.

The new Paramcecia attain full size and then divide,
each into two. And so on for many generations. But it
has been discovered that this simplest kind of reproduction
can not go on indefinitely. After a number of generations
the Paramcecia, instead of simply dividing in two, come

together in pairs, and a part of
one of the nuclei of each mem-
ber of a pair passes into the
body of and fuses with a part

FIG. 6. — Paramcecium putorinum
dividing. The two nuclei be-
come very elongate before di-
viding.— After BiJTscHLi.

PIG. 7. — Paramcecium caudatum ,* two indi-
viduals separating after conjugation.

of one of the nuclei of the other member of the pair. In
the meantime the second nucleus in each Paramcecium has
broken up into small pieces and disappeared. The new
nucleus composed of parts of the nuclei from two animals
divides, giving each animal two nuclei just as it had before
this extraordinary process, which is called conjugation,
began (Fig. 7). Each Paramcecium, with its nuclei com-
posed of parts of the nuclei from two distinct individuals,

12

ANIMAL LIFE

now simply divides in two, and a large number of genera-
tions by simple fission follow.

Paramcecium in the character of its body and in the
manner of the performance of its life processes is distinctly
less simple than the Amoeba, but its body is composed of a
single structural unit, a single cell, and it is truly one of
the " simplest animals."

6. Vorticella, — Another interesting and readily found
protozoan is Vorticella (Fig. 8). While the Amoeba can crawl
and Paramcecium swim, Vorticella, except when very young,

Pie. 8— Vorticella microstoma (after STEIN). A, small, free-swimming individuals
conjugating with a large, stalked individual ; B, a stalked individual dividing
longitudinally ; C, after division is completed one part severs itself from the
other, forms a ring of cilia, and swimfl away.

is attached by tiny stems to dead leaves or sticks in the
water, and can change its position only to a limited extent.

THE LIFE OF THE SIMPLEST ANIMALS 13

The body is pear-shaped or bell-shaped, with a mouth
opening at the broad end, and a delicate stem at the
narrow end. This stem is either hard and stiff, or is
flexible and capable of being suddenly contracted in a
close spiral. In the body mass there is one pulsating
vacuole and one nucleus. Usually many Vorticellce are
found together on a common stalk, thus forming a proto-
zoan colony.

The life processes of Vorticella are of the simple kind
already observed in Amoeba and Paramoecium. Vorticella
shows, however, some modifications of the process of repro-
duction which are interesting. The plane of division of
Vorticella is parallel to the long axis of the pear-shaped
body, so that when fission is complete there are two Vorti-
cellce on a single stalk. One of the two becomes detached,
and by means of a circle of fine hairs or cilia which appear
around its basal end leads a free swimming life for a short
time. Finally it settles down and develops a stalk. Vorti-
cella shows two kinds of fission — one the usual division
into equal parts, and another division into unequal parts.
In this latter kind, called reproduction or multiplication
by budding, a small part of the parent body separates,
develops a basal circle of cilia, and swims away. The pro-
cess of conjugation also takes place among the Vorti-
cella^ but they are never two equal forms which conju-
gate, but always one of the ordinary stalked forms and
one of the small free - swimming forms produced by
budding.

Here, then, in the life of Vorticella, are new modifica-
tions of the life processes ; but, after all, these life processes
are very simply performed, and the body is like the body of
the Amceba, a single cell. Vorticella is plainly one of " the
simplest animals."

7. Gregarina. — A fourth kind of protozoan to which we
can profitably give some special attention is Gregarina
(Fig. 9), the various species of which live in the alimentary

ANIMAL LIFE

canal* of crayfishes and centipeds and certain insects.
Gregarina is a parasite, living at the expense of the host
in whose body it lies. It has no need to swim about quickly,

FIG. 9.— Gregarinidae. A, a Gregarinid (Actinocephalus oligacanthus) from the intes-
tine of an insect (after STEIN) ; B and C, spore forming by a Gregarinid (Coc-
cidium oviforme) from the liver of a guinea-pig (after LEUCKART) ; D, E, and
P, successive stages in the conjugation and spore forming of Gregarina poly-
morpha (after KOBLLIKER).

and hence has no swimming cilia like Paramcecium and
the young Vorticella. It does need to cling to the inner
wall of the alimentary canal of its host, and the body of
some species is provided with hooks for that purpose. The

* Specimens of Oregarina can be abundantly found in the alimen-
tary canal of meal worms, the larvae of the black beetle (Tenebrio moli-
tor), common in granaries, mills, and brans. "Snip off with small
scissors both ends of a larva, seize the protruding (white) intestine with
forceps, draw it out, and tease a portion in normal salt solution (water
will do) on a slide. Cover, find with the low power (minute, oblong,
transparent bodies), and study with any higher objective to suit."—

MURBACH.

THE LIFE OF THE SIMPLEST ANIMALS 15

food of Gregarina is the liquid food of the host as it exists
in the intestine, and which is simply absorbed anywhere
through the surface of the body of the parasite. There is
no mouth opening nor fixed point of ejection of waste
material, nor is there any contractile vacuole in the body.

In the method of multiplication or reproduction Gre-
garina shows an interesting difference from Amceba and
Paramcecium and Vorticella. When the Gregarina is
ready to multiply, its body, which in most species is rather
elongate and flattened, contracts into a ball-shaped mass
and becomes encysted — that is, becomes inclosed in a tough,
membranous coat. This may in turn be covered externally
by a jelly-like substance. The nucleus and the protoplasm
of the body inside of the coat now divide into many small
parts called spores, each spore consisting of a bit of the
cytoplasm inclosing a small part of the original nucleus.
Later the tough outer wall of the cyst breaks and the
spores fall out, each to grow and develop into a new Gre-
garina. In some species there are fine ducts or canals
leading from the center of the cyst through the wall to the
outside, and through these canals the spores issue. Some-
times two GregarincB come together before encystation and
become inclosed in a common wall, the two thus forming a
single cyst. This is a kind of conjugation. In some spe-
cies each of the young or new Gregarince coming from the
spores immediately divides by fission to form two indi-
viduals.

8. Marine Protozoa. — If called upon to name the char-
acteristic animals of the ocean, we answer readily with the
names of the better-known ocean fishes, like the herring and
cod, which we know to live there in enormous numbers ; the
seals and sea lions, the whales and porpoises, those fish-like
animals which are really more like land animals than like
the true fishes ; and the jelly-fishes and corals and star-fishes
which abound along the ocean's edge. But in naming only
these we should be omitting certain animals which in point

16 ANIMAL LIFE

of abundance of individuals vastly outnumber all other
animals, and which in point of importance in helping main-
tain the complex and varied life of the ocean distinctly out-
class all other marine forms. These animals are the marine
Protozoa, those of the " simplest animals " which live in the
ocean.

Although the water at the surface of the ocean appears
clear, and on superficial examination devoid of life, yet a
drop of this water taken from certain ocean regions exam-
ined under the microscope reveals the fact that this water
is inhabited by Protozoa. Not only is the water at the
very surface of the ocean the home of the simplest animals,
but they can be found in all the water from the surface to
a great depth beneath it. In a pint of this ocean water
from the surface or near it there may be millions of these
animals. In the oceans of the world the number of them
is inconceivable. Dr. W. K. Brooks says that the " basis
of all the life in the modern ocean is found in the micro-
organisms of the surface." By micro-organisms he moans
the one-celled animals and the one-celled plants. For
the simplest plants are, like the simplest animals, one-
celled. " Modern microscopical research," he says, " has
shown that these simple plants, and the Globigerinae and
Eadiolaria [kinds of Protozoa] which feed upon them, are
so abundant and prolific that they meet all demands and
supply the food for all the animals of the ocean."

9. The Globigerinse and Radiolaria.— The Globigerinse
(Fig. 10) and Radiolaria (Fig. 11) are among the most in-
teresting of all the simplest animals. Their simple one-
celled body is surrounded by a microscopic shell, which
among the Globigerinae is usually made of lime (calcium
carbonate), in the case of Eadiolaria of silica. These minute
shells present a great variety of shape and pattern, many
being of the most exquisite symmetry and beauty. The
shells are usually perforated by many small holes, through
which project long, delicate, protoplasmic threads. These

THE LIFE OF THE SIMPLEST ANIMALS

17

fine threads interlace when they touch each other, thus
forming a sort of protoplasmic network outside of the shell.
In some cases there is a complete layer of protoplasm —
part of the body protoplasm of the protozoan — surround-

FIG. lO.—Polystomella strigillata, one of the Globigerinae. — After MAX SCHTTLTZE.

ing the cell externally. The Kadiolaria, whose shells are
made of silica, possess also a perforated membranous sac
called the central capsule, which lies imbedded in the
protoplasm, dividing it into two portions, one within and
3

18 ANIMAL LIFE .

one outside of the capsule. In the protoplasm inside of
the capsule lies the nucleus or nuclei ; and from the proto-
plasm outside of the capsule rise the numerous fine, thread-
like pseudopods which project through the apertures in the
shell, and enable the animal to swim and to get food.

Most of the myriads of the simplest animals which
swarm in the surface waters of the ocean belong to a few
kinds of these shell-bearing Globigerinae and Radiolaria.
Large areas of the bottom of the Atlantic Ocean are cov-
ered with a slimy gray mud, often of great thickness, which
is called globigerina-ooze, because it is made up chiefly of
the microscopic shells of Globigerinae. As death comes to
the minute protoplasmic animals their hard shells sink
slowly to the bottom, and accumulate in such vast quanti-
ties as to form a thick layer on the ocean floor. Nor is it
only in present times and in the oceans we know that the
Globigerinae have flourished. All over the world there are
thick rock strata which are composed chiefly of the fos-
silized shells of these simplest animals. Where the strata
are made up exclusively of these shells the rock is chalk.
Thus are composed the great chalk cliffs of Kent, which
gave to England the early name of Albion, and the chalk
beds of France and Spain and Greece. The existence of
these chalk strata means that where now is land, in earlier
geologic times were oceans, and that in the oceans Globi-
gerinae lived in countless numbers. Dying, their shells
accumulated to form thick layers on the sea bottom. In
later geologic ages this sea bottom has been uplifted and
is now land, far perhaps from any ocean. The chalk strata
of the plains of the United States, like those in Kansas, are
more than a thousand miles from the sea, and yet they are
mainly composed of the fossilized shells of marine Pro-
tozoa. Indeed, we are acquainted with more than twice as
many fossil species of Globigerinae as species living at the
present time. The ancestors of these Globigerinae, from
which the present Globigerinae differ but little, can be

THE LIFE OF THE SIMPLEST ANIMALS

19

traced far back in the geologic history of the world. It is
an ancient type of animal structure.

The Radiolaria, too, which live abundantly in the pres-
ent oceans, especially in the marine waters of the tropical
and temperate zones, are found as fossils in the rocks from
the time of the coal age on. The siliceous shells of the

FIG. ll.—Heliosphcera actinota (after HAECKEL) ; a radiolarian with symmetrical

shell.

Eadiolaria sinking to the sea bottom and accumulating
there in great masses form a radiolaria-ooze similar to the
globigerinae-ooze ; and just as with the Globigerinae, the
remains of the ancient Radiolaria formed thick layers on
the floor of the ancient oceans, which have since been up-
lifted and now form certain rock strata. That kind of
rock called Tripoli, found in Sicily, and the Barbados
earth from the island of Barbados, both of which are used

20 ANIMAL LIFE

as polishing powder, are composed almost exclusively of
the siliceous shells of ancient and long-extinct Radiolaria.

10. Antiquity of the Protozoa, — All the animals of the
ocean depend upon the marine Protozoa (and the marine
Protophyta, or one-celled plants) for food. Either they
prey upon these one-celled organisms directly, or they prey
upon animals which do prey on these simplest animals.
The great zoologist already quoted says : " The food sup-
ply of marine animals consists of a few species of micro-
scopic organisms which are inexhaustible and the only
source of food for all the inhabitants of the ocean. The
supply is primeval as well as inexhaustible, and all the life
of the ocean has gradually taken shape in direct depend-
ence upon it." That is, the marine simplest animals are
the only marine animals which live independently; they
alone can live or could have lived in earlier ages without
depending on other animals. They must therefore be the
oldest of marine animals. By oldest we mean that their
kind appeared earliest in the history of the world. As it
is certain that marine life is older than terrestrial life — that
is, that the first animals lived in the ocean — it is obvious
that the marine Protozoa are the most ancient of animals.
This is an important and interesting fact. Zoologists try
to find out the relationships and the degrees of antiquity
or modernness of the various kinds of animals. We have
seen that the Protozoa, those animals which have the sim-
plest body structure and perform the necessary life pro-
cesses in the simplest way, are the oldest, the first animals.
Tlrs is just what we would expect.

11. The primitive form. — We find among the simplest
animals a considerable variety of shape and some manifest
variation in habit. But the points of resemblance are far
more pronounced than the points of difference, and are of
fundamental importance. The composition of the body of
one cell, as opposed to the many-celled structure of the
bodies of all other animals, is the fact to be most distinctly

THE LIFE OF THE SIMPLEST ANIMALS 21

emphasized. The shape of this one-celled body varies.
With the most primitive or simplest of the " simplest ani-
mals," like Amceba, for example, there is no " distinction
of ends, sides, or surfaces, such as we are familiar with in
in the higher animals. Anterior and posterior ends, right
and left sides, dorsal and ventral surfaces are terms which
have no meaning in reference to an Amoeba, for any part
of the animal may go first in locomotion, and when crawl-
ing the animal moves along on whatever part of its
surface happens to be in contact with foreign bodies."
The one shape most often seen among the Protozoa, or
most nearly fairly to be called the typical shape, is the
spherical or subspherical shape. Why this is so is readily
seen. Most of the Protozoa are aquatic and free swim-
ming. They live in a medium, the water, which supports
or presses on the body equally on all sides, and the body is
not forced to assume any particular form by the environ-
ment. The body rests suspended in the water with any
part of its surface uppermost or any part undermost. As
any part of the surface serves equally well in many of the
Protozoa for breathing or eating or excreting, it is obvious
that the spherical form is the simplest and most conven-
ient shape for such a body. It is interesting to note that
the spherical form is the common shape of the egg cell of
the higher animals. Each one of the higher, multicellular
animals begins life (as we shall find it explained in another
chapter of this book) as a single cell, the egg cell, and
these egg cells are usually spherical in shape. The full
significance of this we need not now attempt to under-
stand, but it is interesting to note that normally the whole
body of the simplest animals is a single spherical cell, and
that every one of the higher animals, however complex
it may become by growth and development, begins life as a
single spherical cell.

12. The primitive but successful life. — Living consists of
the performing of certain so-called life processes, such as

22 ANIMAL LIFE

eating, breathing, feeling, and multiplying. These pro-
cesses are performed among the higher animals by various
organs, special parts of the body, each of which is fitted to
do some one kind of work, to perform some one of these
processes. There is a division or assignment of labor here
among different parts of the body. Such a division of
labor, and special fitting of different parts of the body for
special kinds of work does not exist, or exists only in
slightest degree among the simplest animals. The Amceba
eats or feels or moves with any part of its body ; all of the
body exposed to the air (air held in the water) breathes ;
the whole body mass takes part in the process of repro-
duction.

Only very small organisms can live in this simplest way.
So all of the Protozoa are minute. When the only part of
the body which can absorb oxygen is the simple external
surface of a spherical body, the mass of that body must be
very small. With any increase in size of the animal the
mass of the body increases as the cube of the diameter,
while the surface increases only as the square of the diam-
eter. Therefore the part of the body (inside) which re-
quires to be provided with oxygen increases more rapidly
than the part (the outside) which absorbs oxygen. Thus
this need of oxygen alone is sufficient to determine the
limit of size which can be attained by the spherical or sub-
spherical Protozoa.

That the simplest animals, despite the lack of organs
and the primitive way of performing the life processes, live
successfully is evident from their existence in such ex-
traordinary numbers. They outnumber all other animals.
Although serving as food for hosts of ocean animals, the
marine Protozoa are the most abundant in individuals of
all living animals. The conditions of life in the surface
waters of the ocean are easy, and a simple structure and
simple method of performance of the life processes are
wholly adequate for successful life under these conditions.

THE LIFE OF THE SIMPLEST ANIMALS 23

That the character of the body structure of the Protozoa
has changed but little since early geologic times is ex-
plained by the even, unchanging character of their sur-
roundings. The oceans of former ages have undoubtedly
been essentially like the oceans of to-day — not in extent
and position, but in their character of place of habitation
for animals. The environment is so simple and uniform
that there is little demand for diversity of habits and conse-
quent diversity of body structure. Where life is easy there
is no necessity for complex structure or complicated habits
of living. So the simplest animals, unseen by us, and so
inferior to us in elaborateness of body structure and habit,
swarm in countless hordes in all the oceans and rivers and
lakes, and live successfully their simple lives.

CHAPTER II

THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS

13. Colonial Protozoa,— When one of the simplest animals
multiplies by fission, the halves of the one-celled body sepa-
rate wholly from each other, move apart, and pursue their
lives independently. The original parent cell divides to
form two cells, which exist thereafter wholly apart from
each other. There are, however, certain simple animals
which are classed with the Protozoa, which show an inter-
esting and important difference from the great majority of
the simplest animals. These are the so-called colony-form-
ing or colonial Protozoa.

These colonial Protozoa belong to a group of organisms
called the * Volvocinae. The simplest of the Volvocinae are
single cells, which live wholly independently and are in
structure and habit essentially like the other Protozoa we
have studied. They have, however, imbedded in the one-
celled body a bit of chlorophyll, the green substance which
gives the color to green plants and is so important in their
physiology. In this respect they differ from the other
Protozoa. Among the other Volvocinae, however, a few or
many cells live together, forming a small colony — that is,

* These colonial organisms, the Volvocinae, are the objects of some
contention between botanists and zoologists. The botanists call them
plants because they possess a cellulose membrane and green chroma-
tophores, and exhibit the metabolism characteristic of most plants ; but
most zoologists consider them to be animals belonging to the order
Flagellata of the Protozoa. In the latest authoritative text-book of
zoology, that of Parker and Haswell (1897), they are so classed.
34

THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 25

there is formed a group of a few or many cells, each cell
having the structure of the simpler unicellular forms.
These cells are held together in a gelatinous envelope, and
the mass is usually spherical in shape. In most of the
colonies each of the cells possesses two or three long, pro-
toplasmic, whiplash-like hairs, called flagella, and by the
lashing of these flagella in the water the whole group swims
about.

14. Gonium. — If, when one of the simplest animals di-
vided to form two daughter cells, these two cells did not
move apart, but remained
side by side and each di-
vided to form two more,
and each of these divided
to form two more, and
these eight divided each
into two, each cell com-
plete and independent but
all remaining together
in a group — if this pro-
cess should take place we
should have produced a
group or colony of sixteen
cells, each cell a complete
animal capable of living
independently like the
other simplest animals,

but all holding together

B

to form a tiny, flat, plate- FIG. 12.— Gonium pectorale (after STEIN). A,
like Colony. NOW, this is colony seen from above; B, colony seen

• 7 from the side.

precisely what takes place

in the case of those colonial Protozoa belonging to the genus
Gonium (Fig. 12). When the mother cell of Gonium di-
vides, the daughter cells do not swim apart, but remain
side by side, and by repeated fission, until there are sixteen
cells side by side, the colony*is formed. Each cell of the

26

ANIMAL LIFE

colony eats and breathes and feels for itself ; each can and
does perform all the processes necessary to keep it alive.
When ready to multiply, the sixteen cells of the Gonium
colony separate, and each cell becomes the ancestor of a
new colony.

15. Pandorina. — Another colony usually composed of six-
teen cells is Pandorina, but the cells are arranged to form
a spherical instead of a plate-like colony (Fig. 13). In Pan-
dorina morum the colony consists of sixteen ovoid cells in
a spherical jelly-like mass. Each cell has two flagella, and
by the lashing of all the flagella the whole colony moves
through the water. Food is taken by any of the cells, is
assimilated, and the cells increase in size. When Pan-
dorina is ready to multiply, each cell divides repeatedly
until it has formed sixteen daughter cells. The inclosing
gelatinous mass which holds the colony together dissolves,

and the daughter colonies be-
come free and swim apart.
Each colony soon grows to the
size of the original colony.
This kind of multiplication or
reproduction may be continued
for several generations. But
it does not go on indefinitely.
After a number of these gener-
ations has been produced by
simple division, the cells of a
colony divide each into eight
instead of sixteen daughter
cells. The daughter cells are
not all of the same size, but
the difference is hardly notice-

The eight cells resulting from the repeated division
of one of the original cells separate and swim about inde-
pendently by means of their flagella. If one of these cells
comes near a similar free-swimming cell from another

FIG. 13.— Pandorina sp. (from Na-
ture). The cells composing the
colony are beginning to divide to
form daughter colonies.

able.

THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 27

colony, the two cells conjugate (Fig. 14) — that is, fuse to
form a single cell. This new cell formed by the fusion of
two, develops a tough enveloping membrane of cellulose
and passes into what is called
the " resting stage." That is,
the cell remains dormant for a
shorter or longer time. It may
thus tide over a drought or a
winter. It may become dry or
be frozen, yet when suitable
conditions of moisture or tem-
perature are again present the
outer wall breaks and the pro-
toplasm issues as a large free-
swimming cell, which soon di-
vides into sixteen daughter
cells which constitute a new
colony.

16. Eudorina.— Another colo-
nial protozoan which much re-
sembles Pandorina, but differs
from it in one interesting and
suggestive thing, is Eudorina.
In Eudorina elegans (Fig. 15)
the colony is spherical and is
composed of sixteen or thirty-
two cells. Each of these cells
can become the parent of a new
colony by simple repeated divi-
sion. But this simple mode of
reproduction, just as with Pan-
dorina, can not persist indefi-
nitely. There must be conjuga-
tion. But the process of mul-
tiplication, which includes conjugation, is different from
that process in Pandorina, in that in Eudorina the conju-

B

FIG. 14. — Pandorina morum (after
GOEBEL). Three stages in the
conjugation and formation of the
resting spore. A, two cells just
fused; B, the two cells completely
fused, but with flagella still per-
sisting ; C, the resting spore.

28

ANIMAL LIFE

gating cells are of two distinctly different kinds. When
this kind of multiplication is to take place in the case of
Eudorina elegans, to choose a common species, some of
the cells of a colony divide into sixteen or thirty -two

minute elongated cells, each
provided with two flagella.
These small cells escape

FIG. 15.— Eudorina elegant. A, a mature colony (from Nature); B, formation of
the two kinds of reproductive cells.

from the envelope of the parent cell, remaining for some
time united in small bundles. Other cells of the colony
do not divide, but increase slightly in size and become
spherical in shape. When a bundle of the small cells
comes into contact with some of these large spherical
cells the bundle breaks up, and conjugation takes place
between the small flagellated free-swimming cells and the
large non-flagellate spherical cells. Each new cell formed
by the fusion of one of the small and one of the large cells
develops a cellulose wall and assumes a resting stage.
After a time from each of these resting spores a new colony
of sixteen or thirty-two cells is formed by direct, repeated
division.

17. Volvox. — Another interesting colonial protozoan is
Volvox. The large spherical colonies of Volvox globator

THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 29

(Fig. 16) are composed of several thousand cells, arranged

in a single peripheral layer about the hollow center of

the ball. The cells are ovoid, and each is provided with

two long flagella which pro-

ject out into the water. The

lashing of the thousands of

the flagella give the ball-

like colony a rotary motion.

The cells are held together

by a jelly-like intercellular

substance and are connect-

ed with each other by fine

protoplasmic threads which

extend from the body pro-

toplasm of one cell to the

cells surrounding it. When

the colony is full grown and

ready to reproduce itself

certain cells of the colony

undergo great changes.

Some of them increase in

size enormously, having re-

serve food material stored

in them, and they may be

called the egg cells of the

colony. Eeproduction may

now occur by simple divi-

sion of one of these great

egg cells into many small

cells, all held together in a

Common envelope. These Fl«- 16.— A, Volvox minor, entire colony

form a daughter colony

which escapes from the

mother colony and by growth and further division comes to

be a new full-sized colony. Or reproduction may occur in

another, more complex, way. Certain cells of the colony

B

£ST or "%L

30 ANIMAL LIFE

divide into bundles of very small, slender cells, each of
which is provided with flagella. The remaining cells of
the colony (that is, those which have not swollen into egg
cells or divided into many — sixty-four to one hundred and
twenty-eight — minute, flagellate cells) remain unchanged for
a while and finally die. They take absolutely no part in
reproducing the colony. One of the minute free-swim-
ming cells fuses with one of the enormous egg cells, the
new cell thus formed being a resting spore. From this
resting spore a new colony develops by repeated division.

18. Steps toward complexity.— Within the group of Vol-
vocince there are plainly several steps on the way from
simplicity of structure to complexity of structure. Gonium,
Pandorina, Eudorina^ and Volvox form a series proceeding
from the simplest animals toward the complex animals.
In Gonium the cells composing the colony are all alike in
structure, and each one is capable of performing all the
processes or functions of life. In Pandorina and Eudorina
the cells are at first alike, but there is, as the time for
reproduction approaches, a differentiation of structure ;
the cells of the colony, all of which take part in the process
of reproduction, come to be in certain generations of two
kinds — an inactive large kind which may be called the egg
cells, and a small, active, free-swimming kind which seeks
out and conjugates with, or, we may say, fertilizes the egg
cells. In Volvox there is a new differentiation. Only cer-
tain particular and relatively few cells take part in repro-
ducing the colony; most of the cells have given up the
power or function of reproduction. These cells, when the
time of multiplication comes, simply support the special
reproductive cells. They continue to waft the great colony
through the water by lashing their flagella ; they continue
to take in food from the outside. The reproductive cells
devote themselves wholly to the business of producing new
colonies, of perpetuating the species. And this matter of
reproduction is less simple than in the other Volvocince.

THE LIFE OP THE SLIGHTLY COMPLEX ANIMALS 31

At least there is much more difference between the two
kinds of reproductive cells. The egg cells are compara-
tively enormous, and they are stored with a mass of food
material. The fertilizing cells are very small, but very
active and very different from the egg cells. We have in
Volvox the beginnings of a distinct division of labor and
an accompanying differentiation of structure. Certain
cells of the colony do certain things, and are modified in
structure to fit them specially for their particular duties.
The steps from the simplest structure toward a complex
structure are plainly visible.

'lO^Individual or colony. — Is the Gonium colony, the
Pandorina colony, or the Volvox colony a group of several or
many distinct organisms, or is it to be considered as a sin-
gle organism ? With Gonium, which we may call the sim-
plest of these colonial organisms, the colony is composed
of a few wholly similar cells or one-celled animals, each
fully capable of performing all the life processes, each
wholly competent to lead an independent life. In fact,
each does, for part of its life, live independently, as we
have already described. In the case of Pandorina and Eu-
dorina, while all the cells are for most of the lifetime of the
colony alike and each is capable of living independently,
at the time of reproduction the cells become of two kinds.
A difference of structure is apparent, and for the perpetua-
tion of the species the co-operation of these different kinds
of cells is necessary. That is, it is impossible for a single
one of the members of the colony to reproduce the colony,
except for a limited number of generations. With Volvox
this giving up of independence on the part of the individual
members of the colony is more marked. There is a real in-
terdependence among the thousands of cells of the colony.
The function of reproduction rests with a few particular
cells, and for the perpetuation of the species there is demand-
ed a co-operation of two distinct kinds of reproductive cells.
The great majority of the cells take no part in reproduo

32 ANIMAL LIFE

tion. They can perform all the other life processes ; they
move the colony by lashing the water with their flagella ;
they take in food and assimilate it ; they can feel. All the
cells of the great colony, too, are intimately connected by
means of protoplasmic threads. The protoplasm of one
cell can mingle with that of another cell; food can go
from cell to cell. The question whether the Volvox colony
is a group of distinct organisms or is a single organism
made up of cells among which there is a simple but obvi-
ous difference in structure and function ; in other words,
whether Volvox is a colony of one-celled animals, of Pro-
tozoa, or is a multicellular animal, one of the Metazoa (for
so all the many-celled animals are called), is a difficult one
to decide. Most zoologists class the Volvocinae with the
Protozoa — that is, they incline to consider Gonium, Pan-
dorina, Volvox, and the other Volvocinae as groups or col-
onies of one-celled animals.

20. Sponges. — If the VolvocincB be considered to belong
to the Protozoa, the sponges are the simplest of all the
many-celled animals. Sponges are not free-swimming ani-
mals, except for a short time in their young stage, but are
fixed, like plants. They live attached to some solid sub-
stance on the sea bottom. They resemble plants, too, in
the way in which the body is modified during growth by
the environment. If the rock to which the young sponge
is attached is rough and uneven, the body of the sponge
will grow so as to fit the unevenness ; if the rock surface is
smooth, the body of the sponge will be more regular. Thus
a sponge may be said to have no fixed shape of body ; indi-
viduals of the same species of sponge differ much in form.
The typical form of the sponges is that of a short cylinder
or vase attached by one end and with the upper free end
open (Fig. 17). Many individuals of one kind usually live
together in a close group or colony, and they may be so
attached to each other as to appear like a branching plant.
This branching may be very diffuse, and the branches

THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 33

may become so interwoven with each other as to form a
very complex group. A sponge is composed of many cells
arranged in three layers — that is, the body of a sponge is a
cylinder closed at one end whose wall is composed of three
layers of cells. The outer layer of
cells is called the ectoderm, and the
cells composing it are flat and are
all closely attached to each other.
The inner layer is called the endo-
derm, and its cells are thicker than
those of the ectoderm ; they are
also closely attached to each other.
Sometimes they are provided with
flagella like the flagellate Protozoa.
The flagella are, however, not for the
purpose of locomotion, but for creat-
ing currents in the water, which
bathes the interior of the open cylin-
drical body. The middle layer,
called the mesoderm, is composed of
numerous separate cells lying in a
jelly-like matrix. From these meso-
derm cells fine needles or spicules
of lime or silica often project out
through the ectoderm. These mi-
nute sponge spicules are of a great
variety of shapes, and they form a
sort of skeleton for the support of F™-
the soft body mass. All over the
outer surface of the body are scat-
tered fine openings or pores, which
lead through the walls of the body

into the inner cavity. This cavity is of course also con-
nected with the outside by the large opening at the free or
apical end of the body.

There is hardly any differentiation of parts among the
4

of the simplest
sponges, Calcolynthus pri-
migenlus (after HAECKEL).
A part of the outer wall is
cut away to show the in-
side.

ANIMAL LIFE

sponges. As in the Protozoa, there are no special organs
for the performance of special functions. The sponge
feeds by creating, with its flagella, water currents which

flow in through the many fine
pores of the body and out from
the inner body cavity through
the large opening at the free
end of the body. These cur-
rents of water bear fine parti-
cles of organic matter which
are taken up by the cells lining
the pores and body cavity, and
assimilated. There are no
special organs of digestion.
Each cell takes up food and
digests it. The water cur-
rents also bring air to these
same cells, and thus the sponge
breathes. Although the
sponge as a whole can not
move, does not possess the
power of locomotion, yet the
protoplasm of the cells has
the power of contracting, just
as with the Protozoa, and the
pores can be opened or closed
by this cellular movement.
Practically, thus, the only
movements the sponge can
. Thebody is represented make are the movements made
as cut in two longitudinally. The by the individual cells.

large cells of the inner layer are the

egg cells. Eeproduction is accom-

plished by a process of divi-
sion, or by a process of conjugation and subsequent division.
In its simplest way multiplication takes place by a group
of cells separating from the body of the parent sponge,

PIG. 18. — One of the simple sponges,
Prophysema primordiale (after

THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 35

becoming inclosed in a common capsular envelope, and by
repeated division and consequent increase in number of
cells becoming a new sponge. This is reproduction by
" budding." The " buds," or small groups of cells which
separate from the parent sponge, are called gemmules.
Reproduction in the more complex way occurs as follows :
Some of the free amoeboid cells of the mesoderm (the mid-
dle one of the three layers of the body wall) become en-
larged and spherical in form. These are the egg cells.
Other mesodermic cells divide into many small cells, which
are oval with a long, tapering, tail-like projection. These
cells are active, being able to swim by the lashing of the
tapering tail. These are the fertilizing cells. The two
kinds of reproductive cells may be formed in one sponge ;
if so, they are formed at different times. Or one sponge
may produce only egg cells, another only fertilizing or,
as they are called, sperm cells. Conjugation takes place
between a sperm cell and an egg cell. That is, one of the
small active sperm cells finds one of the large, spherical,
inactive cells and penetrates into the protoplasm of its
body. The two cells fuse and form a single cell, which
may be called the fertilized or impregnated egg. This fer-
tilized egg, remaining in the body mass of the parent
sponge, divides repeatedly, the new cells formed by this
division remaining together. The young or embryo sponge
finally escapes from the body of the parent sponge, and
lives for a short time as an active free-swimming animal.
Its body consists of an oval mass of cells, of which those on
one side are provided with cilia or swimming hairs. The
cells of the body continue to divide and to grow, and the
body shape gradually changes. The young sponge finally
becomes attached to some rock, the body assumes the typi-
cal cylindrical shape, an aperture appears at the free end,
and small perforations appear on the surface. The sponge
becomes full grown.

Those of us who do not live in the vicinity of the sea-

36 ANIMAL LIFE

shore where sponges are found can not observe the struc-
ture and life history of living specimens. There are, how-
ever, among the thousand and more kinds of sponges a few
kinds that live in fresh water, and these are so widely
spread over the earth that examples of them can be found
in almost any region. They belong to the genus Spongilla,
and thirty or more species or kinds of Sponyilla are known.
In standing or slowly flowing water, Spongilla grows erect
and branching, like a shrub or miniature tree ; in swift
water it grows low and spreading, forming a sort of mat
over the surface to which it is attached. Eeproduction
takes place very actively by the process of budding. The
budded-off gemmules are spherical in shape, and the cells
of each gemmule are inclosed in an envelope composed of
siliceous spicules of peculiar shape. These gemmules are
formed in the body substance of the parent sponge toward
the end of the year, and are set free by the decaying of
that part of the body of the parent sponge in which they
lie. They sink to the bottom of the pond or brook, and
lie there dormant until the following spring. Then they
develop rapidly by repeated division of the cells and
growth.

It is not the purpose here to describe the many and
interesting kinds of sponges which inhabit the ocean. The
sponge of the bathroom is simply the skeleton of a large
sponge or group of sponges. The skeleton here is not
composed of lime or silica, but of a tough, horny substance,
which is secreted by cells of the mesodermal layer of the
body wall of the sponge. This substance is called spongin,
and is a substance allied to silk in its chemical composi-
tion. All the commercial sponges, the spongin skeletons,
belong to one genus — Spongia. These sponges grow espe-
cially abundantly in the Mediterranean and Eed Seas, and
in the Atlantic Ocean off the Florida reefs, and on the
shores of the Bahama Islands. The sponges are pulled
up by divers, or by means of hooks or dredges. The

THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 37

living matter soon dies and decays, leaving the horny
skeleton, which when cleaned and trimmed is ready for
use.

The most beautiful sponges are those with siliceous
skeletons. The fine needles or threads of glass, arranged
often in delicate and intricate pattern, make these sponges
objects of real beauty.

21. Polyps, corals, and jelly-fishes. — The general or typ-
ical plan of body structure of those animals which come
next in degree of complexity to the sponges can be best
understood by imagining the typical cylindrical body of a
sponge modified in the following way: The middle one
of the three layers of the body wall not to be composed
of cells in a gelatinous mass, but to be simply a thin non-
cellular membrane; the body wall to be pierced by no
fine openings or pores, so that the interior cavity of the
body is connected with the outside only by the single
large opening at the free end, and this opening to be sur-
rounded by a circlet of arm-like processes or tentacles,
continuations of the body wall and similarly composed.
Such a body structure is the general or fundamental one
for all polyps, corals, sea-anemones, and jelly-fishes. The
variety in shape and the superficial modifications of this
type-plan are many and striking ; but, after all, the type-
plan is recognizable throughout the whole of this great
group of animals. Perhaps the simplest representative of
the group is a tiny polyp which grows abundantly in the
fresh-water streams and pools, and can be readily obtained
for observation. It is called Hydra.

22. Hydra,— The body of Hydra (Fig. 19), which is
very small and appears to the unaided eye as a tiny white
or greenish gelatinous particle attached to some submerged
stone, bit of wood, or aquatic plant, is a simple cylinder
attached by one end to the stone or weed. The other free
end is contracted so as to be conical, and it is narrowly
open. Around the opening are six or eight small waving

38 ANIMAL LIFE

tentacles. The wall of the cylinder is composed of an
outer and an inner layer of cells and a thin non-cellular
membranous layer between them. The tentacles are hol-
low and are simple extensions of the body wall. The cells
of the outer layer, or ectoderm, are not all alike. Some
are smaller than the others and appear to be crowded in

FIG. 19.— The fresh- water polyp, Hydra vulgaris. A, in expanded condition, and
in contracted condition; B, cross section of body, showing the two layers of
cells which make up the body wall.

between the bases or inner ends of the larger ones. The
inner ends of the large cells are extended as narrow-pointed
prolongations directed at right angles with the rest of the
cell. These processes are very contractile and are called
muscle processes. Each one is simply a continuation of
the protoplasm of the cell body, which is especially con-
tractile. Some of the smaller ectoderm cells are very

THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 39

irregular in shape and possess specially large nuclei. These
cells are more irritable or sensitive than the others and
are called nerve cells. The ectoderm cells of the base or
foot of the Hydra are peculiarly granular, and secrete a
sticky substance by which the Hydra holds fast to the
stone or weed on which it is found. These cells are called
gland cells. Imbedded in many of the larger ectoderm
cells, especially those of the tentacles, are small oval sacs,
in each of which lies folded or coiled a fine long thread.
When the tentacles touch one of the small animals which
serve Hydra as food, these fine threads shoot out from
their sacs and so poison or sting the prey that it is
paralyzed. The tentacles then contract and bend inward,
forcing the captured animal into the mouth opening
in the center of the circle of tentacles. Through the
mouth opening the prey enters the body cavity of Hydra
and is digested by the cells lining this cavity. These cells
belonging to the inner layer of the body wall or endoderm
are mostly large, and each contains one or more contractile
vacuoles. From the free ends — the ends which are next to
the body cavity — of these cells project pseudopods or fine
flagella. These projections are constantly changing : now
two or three short, blunt pseudopods are projecting into
the body cavity ; now they are withdrawn, and a few fine,
long flagella are projected. In addition to these cells there
are in the endoderm, especially abundant near the mouth
opening and wholly lacking in the tentacles and at the
base of the body, many long, narrow, granular cells. They
are gland cells which secrete a digestive fluid. The food
captured by the tentacles and taken in through the
mouth opening disintegrates in the body cavity, or diges-
tive cavity, as it may be called. The digestive fluid se-
creted by the gland cells of the endoderm acts upon it,
so that it becomes broken into small parts. These par-
ticles are probably seized by the pseudopods of the other
endoderm cells and are taken into the body protoplasm

40 ANIMAL LIFE

of these cells. The ectoderm cells do not take food
directly, but receive nourishment only through the endo-
derm cells.

Hydra is not permanently attached. It holds firmly
to the submerged stone or weed by means of the sticky
secretion from the ectodermal gland cells of its base, but it
can loosen itself, and by a slow creeping or gliding move
along the surface of the stone to another spot. Even when
attached, the form of the body changes ; it extends itself
longitudinally, or it contracts into a compact globular mass.
The tentacles move about in the water, and are continually
contracting or extending.

Like Volvox and the sponges, those other slightly com-
plex animals we have already considered, Hydra has two
methods of multiplication. In the simpler way, there
appears on the outer surface of the body a little bud which
is composed, at first, of ectoderm cells alone ; but soon it is
evident that it is a budding, or outpushing, of the whole
body wall, ectoderm, endoderm, and middle membrane. In
a few hours the bud has six or eight tiny, blunt tentacles,
a mouth opening appears at the free end, and the little
Hydra breaks off from the parent body and leads an inde-
pendent existence. In the more complex way, two kinds of
special reproductive cells are produced by each individual,
viz., large, inactive, spherical egg cells, and small, active
sperm cells, each with an oval part or head (consisting of
the nucleus) and a slender, tapering tail-like part (consist-
ing of the cytoplasm). The egg cell lies inclosed in a layer
of thin, surrounding cells, which compose a capsule for it.
When the egg cell is ready for fertilization this capsule
breaks, and one of the active sperm cells finds its way to
and fuses with the egg cell. The fertilized egg cell now
divides into several cells, which remain together. The
outer ones form a hard capsule, and thus protected the
embryo falls to the bottom, and after lying dormant for
awhile develops into a Hydra.

THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 41

23. Differentiation of the body cells. — In Hydra we have
the beginnings of complexity of structure carried a step
further than in the sponges. The division of labor among
the cells composing the body is more pronounced, and the
structural modification of the different cells to enable them
better to perform their special duties is obvious. Some of
the cells of the body specially devote themselves to food-
taking ; some specially to the digestion of the food ; some
are specially contractile, and on them the movements of
the body depend, while others are specially irritable or
sensitive, and on them the body depends for knowledge of
the contact of prey or enemies. In the lasso cells — those
with the stinging threads — there is a very wide departure
from the simple primitive type of cells. There is in Hydra
a manifest differentiation of the cells into various kinds of
cells. The beginnings of distinct tissues and organs are
foreshadowed.

The individuals of Hydra live, usually, distinct from
each other. There is no tree-like colony, as with the sponges.
But most of the other polyps do live in this colonial manner.
The new polyps which develop as buds from the body of
the parent do not separate from the parent, but remain
attached by their bases. They, in turn, produce new
polyps which remain attached, so that in time a branching,
tree-like colony is formed.

24. Medusae or jelly-fishes. — Most of the other polyps
differ from Hydra also in producing, in addition to ordi-
nary polyp buds, buds which develop into bell-shaped struc-
tures called medusae (Fig. 20). These medusae consist of a
soft gelatinous bell- or umbrella-shaped body, with a short
clapper or stem which has an opening at its free end.
From the edge of the bell or umbrella four pairs of tenta-
cles project. The medusae usually separate from the parent
polyp and live an independent, free-swimming life. These
are the beautiful animals commonly known as jelly-fishes.
The medusae or jelly-fishes produce special reproductive

ANIMAL LIFE

cells, a single medusa producing only one kind of such cells
—that is, producing either egg cells alone or sperm cells
alone. The active sperm cells produced by one medusa
find their way to an egg cell producing medusa, and fuse
with or fertilize these egg cells. The
fertilized egg develops into a small,
oval, free-swimming embryo called a
planula, which finally attaches itself
to a stone or bit of wood or seaweed,
and grows to be a simple cylindrical
polyp attached at its base and with
mouth and tentacles at its free end.
This polyp gives rise by budding to
new polyps, which remain attached
to it, and gradually a new tree-like
colony is formed. From this polyp
or this colony new medusae bud off,
swim away, and finally produce new
polyps. Thus there is in the life of
the polyps what is called an alterna-
There are two kinds of individuals
which evidently belong to the same species of animal, or,
put in another way, one kind of animal has two distinct
forms. This appearance of one kind of animal in two
forms is called dimorphism. We shall see later that one
kind of animal may appear in more than two forms ; such
a condition is called polymorphism. In alternation of gen-
erations we have the polyp animal appearing in one genera-
tion as a fixed cylindrical polyp, while in the next generation
it is a free-swimming, umbrella-shaped medusa or jelly-fish.
The polyps which are dimorphic — that is, have a polyp
form of individual and a medusa form of individual — show
more differentiation in structure than the simple Hydra.
This further differentiation is especially apparent in the
medusae or jelly-fishes. Here the nerve cells are aggregated
in little groups arranged along the edge of the umbrella

FIG. 20.— A medusa, Eucope.
—After HAKCKEL.

tion of generations.

THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 43

to form distinct sense organs. The muscle processes are
better developed, and the digestive cavity is differentiated
into central and peripheral portions. In these dimorphic
polyps the fixed polyp individuals reproduce by the simple
way of budding, while the medusa individuals reproduce
by producing special reproductive cells of two kinds, which
must fuse to form a cell capable of developing into a new
polyp.

25. Corals. — There are many kinds of polyps and jelly-
fishes, and they present a great variety of shape and size
and general appearance. Many polyps exist only in the
true polyp form, never producing medusae. Others have

PIG. 21. — A polyp, or sea-anemone (Metridium dianthus).

only the medusa form. Some live in colonies, and others
are always solitary. The animals we know as corals are
polyps which live in enormous colonies, and which exist
only in the true polyp form, not producing medusas. They

PIG. 22.— Coral island (Nanuku Levu, of the Fiji group). (After a photograph
by MAX AGASSIZ.)

FIG. 23.— Shore of a coral island, with cocoanut palms. (After a photograph.)

THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 45

form a firm skeleton of lime (calcium carbonate), and after
their death these skeletons persist, and because of their
abundance and close massing form great reefs or banks and
islands. Coral islands occur only in the warmer oceans.
In the Atlantic they are found along the coasts of southern
Florida, Brazil, and the West Indies ; in the Pacific and
Indian Oceans there are great coral reefs on the coast of
Australia, Madagascar, and elsewhere, and certain large

FIG. 24.— Organ-pipe coral.

groups of inhabited islands like the Fiji, Society, and
Friendly Islands are composed exclusively of coral islands.
More than two thousand kinds of living corals are known,
and their skeletons offer much variety in structure and
appearance. Brain coral, organ-pipe coral (Fig. 24), the
well-known red coral from Italy and Sicily, used as jewelry,
and the sea pens and sea fans are among the better known
and more beautiful kinds of coral skeletons.

26. Colonial jelly-fishes. — While many of the medusae or
jelly-fishes are another form of individual of a true fixed
polyp, many of the larger and more beautiful jelly-fishes do
not exist in any other form. Some of these larger jelly-
fishes are several feet in diameter, and when cast up on the
beach form a great shapeless mass of soft, jelly-like sub-

46 ANIMAL LIFE

stance. The bodies of all jelly-fishes are soft and gelatinous,
the body substance containing hardly one per cent of solid
matter. It is mostly water. Many jelly-fishes are beauti-
fully and strikingly colored, and as they swim slowly about
near the surface of the ocean, lazily opening and shutting
their iridescent, umbrella-like bodies, they are among the
most beautiful of marine organisms. When one of the
jelly-fishes is taken from the water, however, it quickly loses
its brilliant colors, and dries away to a shapeless, shrivel-
ing, sticky mass.

Some of the most beautiful of the jelly-fishes belong
to a group called the Siphonophora. These jelly-fishes are
elongate and tube-like rather than umbrella- or bell-shaped,
and they are polymorphic — that is, there are several dif-
ferent forms of individuals belonging to a single kind
or species. The Siphonophora are all free-swimming, but
nevertheless form small colonies. In the Mediterranean
Sea and in other southern ocean waters the surface may be
covered for great areas by these brilliantly colored jelly-fish
colonies, each of which looks, as a celebrated German natu-
ralist has said, like a swimming flower cluster whose parts,
flowers, stems, and leaves seem to be made of transparent
crystal, but which possess the life and soul of an animal.
An abundant species of these Siphonophora (Fig. 25) is com-
posed of a slender, flexible, floating, central stem several feet
long, to which are attached thousands of medusa and polyp
individuals representing several different kinds of forms,
each kind of individual being specially modified or adapted
to perform some one duty. The central stem is a greatly
elongated polyp individual, whose upper end is dilated and
filled with air to form a float. This individual holds up
the whole colony. Grouped around this central stem just
below the float are many bell-shaped bodies which alter-
nately open and close, and by thus drawing in and expelling
water from their cavities impel the whole colony through
the water. These bell-shaped structures are attached me-

THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 47

dusa individuals, whose
business it is to be the
locomotive organs for the
colony. These medusae
are without tentacles, and
take no food and produce
no young. They have
given up the power of
performing these other
life processes, and devote
themselves wholly to the
business of locomotion.
From the lower end of the
central stem rises a host of
structures, among which
several distinct kinds are
readily perceived. One
kind is composed of a pear-
shaped hollow body open
at its free end, and bear-
ing a long tentacle which
is furnished with numer-
ous groups of stinging
cells. These are the polyp
individuals whose especial
business it is to capture
and sting prey and to eat
it. These individuals are
the food -getters for the
colony. Scattered among
these stinging, feeding
polyps, are numerous
smaller individuals with
oval, closed body, each
bearing a long, slender
thread. These threads

FIG. 25.— A colonial jelly-fish, Phys&phora
(after HAECKEL). At the top is the float
polyp, around its stem the swimming
medusa1, and below are the feeding, feel-
ing, protecting, and reproducing polyps
and medusifi.

48 ANIMAL LIFE

are very sensitive, and the polyps bearing them have for
special function that of feeling or being sensible of stimuli
from without. They are the sense organs or sense indi-
viduals of the colony. Finally, there are two other kinds
of structures or individuals which produce the special
reproductive cells for the perpetuation of the species.
These are the modified medusa individuals, and one kind,
larger than the other, produces the active sperm cells,
while the other produces the inactive egg cells.

27. Increase in the degree of complexity. — In the corals,
sea-anemones, and jelly-fishes there is plainly much more
of a division of labor among the various parts of an indi-
vidual and much more modification of these parts— that is,
much more structural complexity than among the sponges
and Hydra. And these, in their turn, are more complex than
are the colonial Protozoa, the Volvocinae. There is a great
difference in degree of complexity among the slightly com-
plex animals. But the various groups of these animals
which we have studied can all be arranged roughly in a
series beginning with the least complex among them and
ascending to the most complex. And in this series, and
in the always accompanying division of labor among the
different parts, the gradual increase in complexity is beau-
tifully shown.

From an animal composed of many structurally simi-
lar cells, each cell capable of performing all the life pro-
cesses, we pass to an animal composed of cells of a few
different kinds, of slight structural diversity. Each kind
of cell devotes itself especially to a certain few life pro-
cesses or functions. Next we find an animal in which the
cells of one kind are specially aggregated to form a single
part of the body which is specially devoted to the perform-
ance of a single function. This diversity among the cells
increases, this aggregation of similar cells to form special
parts or organs increases, and the division of labor or
assignment of special functions to special organs becomes

THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 49

more and more pronounced. Among the more complex
polyps and jelly-fishes the contractile cells form distinct
muscle fibers and muscles ; the sensitive cells form dis-
tinct nerve cells and nerve fibers which are arranged in a
primitive nervous system ; the digestive cavity becomes
complex and composed of different portions ; the reproduc-
tive cells are formed by special organs, and the distinction
between the egg cells and the sperm cells — that is, be-
tween the female reproductive elements and the male
reproductive elements — becomes more pronounced.

We have followed this increase or development of struc-
tural and physiological complexity from simplest animals
to fairly complex ones. The principle of this development
of complexity is evident. It will not be profitable to at-
tempt to follow in detail this development among the
higher animals. The complex animals are complex be-
cause their life processes are performed by special parts of
their body, which parts are specially modified so as to perf orrfi
these processes well. The animals which are more complex
than those we have studied differ from these simply in the
degree of complexity attained. In order to understand
this better we shall not further consider special groups of
animals, but special processes or functions, and attempt to
see how the modification and increase in complexity of
structure goes hand in hand with the increase of elaborate-
ness or complexity in the performance of function.

CHAPTER III

THE MULTIPLICATION OF ANIMALS AND SEX

28. All life from life. — On the performance of the f unc-
tion of reproduction or multiplication depends the exist-
ence or perpetuation of the species. Although an animal
may take food and perform all the functions necessary to
its own life, it does not fulfill the demands of successful
existence unless it reproduces itself. Some individuals of
every species must produce offspring or the species becomes
extinct. We have seen in our study of the simple animals
that the function of reproduction is the first function to
become differentiated in the ascent from simplest animals
to complex animals. The first division of labor among the
cells composing the bodies of the slightly complex animals
and the first structural differences among the cells are
connected with the performance of the function of repro-
duction or multiplication.

We are all so familiar with the fact that a kitten
comes into the world only through being born, as the off-
spring of parents of its kind, that we shall likely not appre-
ciate at first the full significance of the statement that all
life comes from life ; that all organisms are produced by
other organisms. Nor shall we at first appreciate the im-
portance of the statement. This is a generalization of
modern times. It has always been easy to see that cats
and horses and chickens and the other animals we famil-
iarly know give birth to young or new animals of their
own kind ; or, put conversely, that young or new cats and
horses and chickens come into existence only as the off-
60

THE MULTIPLICATION OF ANIMALS AND SEX 51

spring of parents of their kind. And in these latter days of
microscopes and mechanical aids to observation it is even
easy to see that the smaller animals, the microscopic organ-
isms, come into existence only as they are produced by the
division of oijier similar animals, which we may call their
parents. But in the days of the earlier naturalists the
life of the microscopic organisms, and even that of many
of the larger but unfamiliar animals, was shrouded in
mystery. And what seem to us ridiculous beliefs were
held regarding the origin of new individuals.

29. Spontaneous generation. — The ancients believed that
many animals were spontaneously generated. The early
naturalists thought that flies arose by spontaneous genera-
tion from the decaying matter of dead animals ; from a
dead horse come myriads of maggots which change into
flesh flies. Frogs and many insects were thought to be
generated spontaneously from mud. Eels were thought to
arise from the slime rubbed from the skin of fishes. Aris-
totle, the Greek philosopher, who was the greatest of the
ancient naturalists, expresses these beliefs in his books. It
was not until the middle of the seventeenth century-
Aristotle lived three hundred and fifty years before the
birth of Christ— that these beliefs were attacked and be-
gan to be given up. In the beginning of the seventeenth
century William Harvey, an English naturalist, declared
that every animal comes from an egg, but he said that the
egg might " proceed from parents or arise spontaneously or
out of putrefaction." In the middle of the same century
Eedi proved that the maggots in decaying meat which pro-
duce the flesh flies develop from eggs laid on the meat by
flies of the same kind. Other zoologists of this time were
active in investigating the origin of new individuals. And
all their discoveries tended to weaken the belief in the
theory of spontaneous generation.

Finally, the adherents of this theory were forced to
restrict their belief in spontaneous generation to the case

52 ANIMAL LIFE

of a few kinds of animals, like parasites and the animalcules
of stagnant water. It was maintained that parasites arose
spontaneously from the matter of the living animal in
which they lay. Many parasites have so complicated and
extraordinary a life history that it was only after long and
careful study that the truth regarding their origin was dis-
covered. But in the case of every parasite whose life his-
tory is known the young are offspring of parents, of other
individuals of their kind. No case of spontaneous genera-
tion among parasites is known. The same is true of the
animalcules of stagnant water. If some water in which
there are apparently no living organisms, however minute,
be allowed to stand for a few days, it will come to be
swarming with microscopic plants and animals. Any or-
ganic liquid, as a broth or a vegetable infusion exposed for
a short time, becomes foul through the presence of innumer-
able bacteria, infusoria, and other one-celled animals and
plants, or rather through the changes produced by their
life processes. But it has been certainly proved that these
organisms are not spontaneously produced by the water or
organic liquid. A few of them enter the water from the
air, in which there are always greater or less numbers of
spores of microscopic organisms. These spores (embryo or-
ganisms in the resting stage) germinate quickly when they
fall into water or some organic liquid, and the rapid suc-
cession of generations soon gives rise to the hosts of bacteria
and Protozoa which infest all standing water. If all the
active organisms and inactive spores in a glass of water are
killed by boiling the water, " sterilizing " it, as it is called,
and this sterilized water or organic liquid be put into a
sterilized glass, and this glass be so well closed that germs
or spores can not pass from the air without into the steril-
ized liquid, no living animals will ever appear in it. It is
now known that flesh will not decay or liquids ferment
except through the presence of living animals or plants.
To sum up, we may say that we know of no instance of the

THE MULTIPLICATION OF ANIMALS AND SEX 53

spontaneous generation of organisms, and that all the ani-
mals whose life history we know are produced from other
animals of the same kind. " Omne vivum ex vivo," All life
from life.

FIG. 26.— The multiplication of Amoeba by simple fission.

30. The simplest method of multiplication. — In our study
of the simplest and the slightly complex animals we became
acquainted with the simplest methods of multiplication
and with methods which are more complex. The method

54 ANIMAL LIFE

of simple fission or splitting — binary fission it is often called,
because the division is always in two — by which the body
of the parent becomes divided into two equal parts— into
halves — is the simplest method of multiplication. This is
the usual method of Amoeba (Fig. 26) and of many other of
the simplest animals. In this kind of reproduction it is
hardly exact to speak of parent *and children. The chil-
dren, the new Amwbce, are simply the parent cut into
halves. The parent persists ; it does not produce off-
spring and die. Its whole body continues to live. The
new Amoeba take in and assimilate food and add new mat-
ter to the original matter of the parent body ; then each
of them divides in two. The grandparent's body is now
divided into four parts, one fourth of it forming one half
of each of the bodies of the four grandchildren. The pro-
cess of assimilation, growth, and subsequent division takes
place again, and again, and again. Each time there is given
to the new Amoeba an ever-lessening part of the actual
body substance of the original ancestor. Thus an Amceba
never dies a natural death, or, as has been said, "no Amoeba
ever lost an ancestor by death." It may be killed outright,
but in that case it leaves no descendants. If it is not killed
before it produces new Amosbce it never dies, although it
ceases to exist as a single individual. The Amceba and
other simple animals which multiply by direct binary
fission may be said to be immortal, £nd the " immortality
of the Protozoa " is a phrase which you will be sure to meet
if you begin to read the writings of the modern philosoph-
ical zoologists.

31. Slightly complex methods of multiplication.— Most of
the Protozoa multiply or reproduce themselves in two
ways — by simple fission and by conjugation. Paramce-
cium, for example, reproduces itself for many generations
by fission, but a generation finally appears in which a dif-
ferent method of reproduction is followed. Two individu-
als come together and each exchanges with the other a part

THE MULTIPLICATION OF ANIMALS AND SEX 55

of its nucleus. Then the two individuals separate and
each divides into two. The result of this conjugation is
to give to the new Paramwcia produced by the conjugat-
ing individuals a body which contains part of the body
substance of two distinct individuals. The new Paramoe-
cia are not simply halves of a single parent ; they are parts
of two parents. If the two conjugating individuals differ
at all — and they always do differ, because no two individual
animals, although belonging to the same species, are exactly
alike — the new individual, made up of parts of each of them,
will differ from both. "We shall, as we study further, see
that Nature seems intent on making every new individual
differ slightly from the individual which produces it ; and
the method of multiplication or the production of new indi-
viduals which Nature has adopted to produce the result is
the method which we have seen exhibited in its simplest
form among the simplest animals — the method of having
two individuals take part in the production of a new one.
The further study of multiplication among animals is the
study of the development and elaboration of this method.
32. Differentiation of the reproductive cells. — Among the
colonial Protozoa the first differentiation of the cells or
members composing the colony is the differentiation into
two kinds of reproductive cells. Reproduction by simple
division, without preceding conjugation, can and does take
place, to a certain extent, among all the colonial Protozoa.
Indeed, this simple method of multiplication, or some modi-
fication of it, like budding, persists among many of the com-
plex animals, as the sponges, the polyps, and even higher
and more complex forms. But such a method of single-
parent reproduction can not be used alone by a species for
many generations, and those animals which possess the
power of multiplication in this way always exhibit also the
other more complex kind of multiplication, the method of
double-parent reproduction. Conjugation takes place be-
tween different members of a single colony of one of the

56 ANIMAL LIFE

colonial Protozoa, or between members of different colonies
of the same species. These conjugating individuals in the
simpler kinds of colonies, like Gonium, are similar; in
Pandorina they appear to be slightly different, and in Eudo-
rina and Volvox the conjugating cells are very different from
each other (Figs. 15 and 16). One kind of cell, which is
called the egg cell, is large, spherical, and inactive, while
the other kind, the sperm cell, is small, with ovoid head
and tapering tail, and free-swimming. In the simpler colo-
nial Protozoa all the cells of the body take part in repro-
duction, but in Volvox only certain cells perform this func-
tion, and the other cells of the body die. Or we may say
that the body of Volvox dies after it has produced special
reproductive cells which shall fulfill the function of multi-
plication.

Beginning with the more complex Volvocinas, which we
may call either the most complex of the one-celled animals
or the simplest of the many-celled animals, all the complex
animals show this distinct differentiation between the re-
productive cells and the cells of the rest of the body. Of
course, we find, as soon as we go up at all far in the scale of
the animal world, that there is a great deal of differentia-
tion among the cells of the body : the cells which have to
do with the assimilation of food are of one kind ; those on
which depend the motions of the body are of another kind ;
those which take oxygen and those which excrete waste
matter are of other kinds. But the first of this cell differ-
entiation, as we have already often repeated, is that shown
by the reproductive cells ; and with the very first of this
differentiation between reproductive cells and the other
body cells appears a differentiation of the reproductive
cells into two kinds. These two kinds, among all animals,
are always essentially similar to the two kinds shown by
Volvox and the simplest of the many-celled animals — namely,
large, inactive, spherical egg cells, and small, active, elon-
gate or " tailed " sperm cells.

THE MULTIPLICATION OF ANIMALS AND SEX 57

33. Sex, or male and female. — In the slightly complex
animals one individual produces both egg cells and sperm
cells. But in the Siphonophora, or colonial jelly-fishes, stud-
ied in the last chapter, certain members of the colony pro-
duce only sperm cells, and certain other members of the
colony produce only egg cells. If the Siphonophora be
considered an individual organism and not a colony com-
posed of many individuals, then, of course, it is like the
others of the slightly complex animals in this respect. But
as soon as we rise higher in the scale of animal life, as soon
as we study the more complex animals, we find that the
egg cells and sperm cells are almost always produced by
different individuals. Those individuals which produce
egg cells are called female, and those which produce sperm
cells are called male. There are two sexes. Male and
female are terms usually applied only to individuals, but
it is evidently fair to call the egg cells the female reproduc-
tive cells, and the sperm cells the male reproductive cells.
A single individual of the simpler kinds of animals pro-
duces both male and female cells. But such an individual
can not be said to be either male or female ; it is sexless —
that is, sex is something which appears only after a certain
degree of structural and physiological differentiation is
reached. It is true that even among many of the higher
or complex animals certain species are not represented by
male and female individuals, any individual of the species
being able to produce both male and female cells. But this
is the exception.

34. The object of sex. — Among almost all the complex
animals it is necessary that there be a conjugation of male
and female reproductive cells in order that a new individual
may be produced. This necessity first appears, we remem-
ber, among very simple animals. This intermixing of body
substance from two distinct individuals, and the develop-
ment therefrom of the new individual, is a phenomenon
which takes place through the whole scale of animal life.

58 ANIMAL LIFE

The object of this intermixing is the production of va-
riation. Nature demands that the offspring shall differ
slightly from its parents. By having the beginnings of its
body, the single cell from which the whole body develops,
composed of parts of two different individuals, this differ-
ence, although slight and nearly imperceptible, is insured.
Sex is a provision of Nature which insures variation.

35. Sex dimorphism. — As we have seen, almost every
species of animal is represented by two kinds of individuals,
males and females. In the case of many animals, espe-

FIG. 27. — Bird of paradise, male.

cially the simpler ones, these two kinds of individuals do
not differ in appearance or in structure apart from the
organs concerned with multiplication. But with many
animals the sexes can be readily distinguished. The male
and female individuals often show marked differences,
especially in external structural characters. We can read-

THE MULTIPLICATION OF ANIMALS AND SEX 59

ily tell the peacock, with its splendidly ornamental tail
feathers, from the unadorned peafowl, or the horned ram
from the bleating ewe. There is here, plainly, a dimor-
phism— the existence of two kinds of individuals belonging
to a single species. This dimorphism is due to sex, and
the condition may be called sex dimorphism. Among some
animals this sex dimorphism, or difference between the
sexes, is carried to extraordinary extremes. This is espe-
cially true among polygamous animals, or those in which
the males mate with many females, and are forced to fight
for their possession. The male bird of paradise, with its
gorgeous display of brilliantly colored and fantastically
shaped feathers (Fig. 27), seems a wholly different kind of
bird from the modest brown female. The male golden and
silver pheasants, and allied species with their elaborate
plumage, are very unlike the dull-colored females. The
great, rough, warlike male fur seal, roaring like a lion, is
three times as large as the dainty, soft-furred female, which
bleats like a sheep.

Among some of the lower animals the differences be-
tween male and female are even greater. The males of
the common cankerworm moth (Fig. 28) have four wings ;

FIG. 28.— Cankerworm moth ; the winged male and wingless female.

the females are wingless, and several other insect species
show this same difference. Among certain species of white
ants the females grow to be five or six inches long, while
the males do not exceed half an inch in length. In the

60

ANIMAL LIFE

case of some of the parasitic worms which live in the bod-
ies of other animals, the male has an extraordinarily de-
graded, simple body, much smaller than that of the female
and differing greatly from that of the female in structure.
In some cases even — as, for example,
the worm which causes " gapes " in
chickens — the male lives parasiti-
cally on the female, being attached to
the body of the female for its whole
lifetime, and drawing its nourish-
ment from her blood (Fig. 29).

A condition known as partheno-
genesis is found among certain of
the complex animals. Although the
species is represented by individu-
als of both sexes, the female can
produce young from eggs which
have not been fertilized. For ex-
ample, the queen bee lays both fer-
tilized and unfertilized eggs. From
the fertilized eggs hatch the work-
ers, which are rudimentary females,
and other queens, which are fully-
FIG. 29.-The parasitic worm Developed females ; from the unfer-

(Syngamus trachealis), which A _

causes the " gapes " in fowls, tilized eggs hatch only males — the
The male is attached to the drones. Many generations of plant

female, and lives as a para- , .

site on her. lice are produced each year parthe-

nogenetically — that is, by unferti-
lized females. But there is at least one generation each
year produced in the normal way from fertilized eggs.

Some of the complex animals are hermaphroditic — that
is, a single individual produces both egg cells and sperm
cells. The tapeworm and many allied worms show this
condition. This is the normal condition for the simplest
animals, as we have already learned, but it is an excep-
tional condition among the complex animals.

THE MULTIPLICATION OF ANIMALS AND SEX 61

36. The number of young. — There is great variation in the
number of young produced by different species of animals.
Among the animals we know familiarly, as the mammals,
which give birth to young alive, and the birds, which lay
eggs, it is the general rule that but few young are pro-
duced at a time, and the young are born or eggs are laid
only once or perhaps a few times in a year. The robin lays
five or six eggs once or twice a year ; a cow may produce
a calf each year. Rabbits and pigeons are more prolific,
each having several broods a year. But when we observe
the multiplication of some of the animals whose habits are
not so familiar to us, we find that the production of so few
young is the exceptional and not the usual habit. A lob-
ster lays ten thousand eggs at a time ; a queen bee lays
about five million eggs in her life of four or five years. A
female white ant, which after it is full grown does nothing
but lie in a cell and lay eggs, produces eighty thousand
eggs a day steadily for several months. A large codfish
was found on dissection to contain about eight million

If we search for some reason for this great difference in
fertility among different animals, we may find a promis-
ing clew by attending to the duration of life of animals,
and to the amount of care for the young exercised by the
parents. We find it to be the general rule that animals
which live many years, and which take care of their young,
produce but few young ; while animals which live but a
short time, and which do not care for their young, are very
prolific. The codfish produces its millions of eggs ; thou-
sands are eaten by sculpins and other predatory fishes be-
fore they are hatched, and other thousands of the defense-
less young fish are eaten long before attaining maturity.
Of the great number produced by the parent, a few only
reach maturity and produce new young. But the eggs of the
robin are hatched and protected, and the helpless fledglings
are fed and cared for until able to cope with their natural

62 ANIMAL LIFE

•»

enemies. In the next year another brood is carefully reared,
and so on for the few years of the robin's life.

Under normal conditions in any given locality the num-
ber of individuals of a certain species of animal remains
about the same. The fish which produces tens of thousands
of eggs and the bird which reproduces half a dozen eggs a
year maintain equally well their numbers. In one case a
few survive of many born ; in the other many (relatively)
survive of the few born ; in both cases the species is effect-
ively maintained. In general, no agency for the perpetua-
tion of the species is so effective as that of care for the
young.

CHAPTEE IV

FUNCTION AND STRUCTURE

37. Organs and functions.— An animal does certain things
which are necessary to life. It eats and digests food, it
breathes in air and takes oxygen from it and breathes out
carbonic-acid gas ; it feels and has other sensations ; it pro-
duces offspring, thus reproducing itself. These things are
done by the simplest animals as well as by the complex
animals. But while with the simplest animals the whole
body (which is but a single cell) takes part in doing each
of these things, among the complex animals only a part
of the body is concerned with any one of these things.
Only a part of the body has to do with the taking in of
oxygen. Another part has to do with the digestion of
food, and another with the business of locomotion. These
parts of the body, as we know, differ from each other, and
they differ because they have different things to do. These
different parts are called organs of the body, and the things
they do are called their functions. The nostrils, tracheae,
and lungs are the organs which have for function the pro-
cess of respiration. The legs of a cat are the organs which
perform for it the function of locomotion. The structure
of one of the higher animals is complex because the body
is made up of many distinct organs having distinct func-
tions. The things done by one of the complex animals are
many ; around each of the principal functions or necessary
processes, as a center, are grouped many minor accessory
functions, all helping to make more successful the accom-

63

64 ANIMAL LIFE

plishment of the principal functions. "While many of the
lower animals have no eyes and no ears, and trust to more
primitive means to discover food or avoid enemies, the
higher animals have extraordinarily complex organs for
seeing and hearing, two functions which are accessory only
to such a principal function as food-taking.

38. Differentiation of structure, — We have seen, in our
study of the slightly complex animals, how the body be-
comes more and more complex in proportion to the degree
in which the different life processes are divided or assigned
to different parts of it for performance. With the gradu-
ally increasing division of labor the body becomes less
homogeneous in structure; a differentiation of structure
becomes apparent and gradually increases. The extent of
the division of labor and the extent of the differentiation
of structure, or division of the body into distinct and dif-
ferent parts and organs, go hand in hand. An animal in
which the division of labor is carried to an extreme is an
animal in which complexity of structure is extreme.

39. Anatomy and physiology. — Zoology, or the study of
animals, is divided for convenience into several branches
or phases. The study of the classification of animals is
called systematic zoology; the study of the development
of animals from their beginning as a single cell to the time
of their birth is called animal embryology ; the study of
the structure of animals is called animal anatomy, and the
study of the performance of their life processes or functions
is called physiology. Because the whole field of zoology is
so great, some zoologists limit themselves exclusively to one
of these phases of zoological study, and those who do not
so definitely limit their study, at least give their special at-
tention to a single phase, although all try to keep in touch
with the state of knowledge in other phases. In earlier
days the study of the anatomy of animals and of their
physiology were held to be two very distinct lines of in-
vestigation, and the anatomists paid little attention to

FUNCTION AND STRUCTURE 65

physiology and the physiologists little to anatomy. But
we have seen how inseparably linked are structure and
function. The structure of an animal is as it is because
of the work it has to do, and the functions of an animal
are performed as they are performed because of the special
structural condition of the organs which perform them.
The study of the anatomy and the study of the physiology
of animals can not be separated. To understand aright
the structure of an animal it is necessary to know to
what use the structure is put ; to understand aright the
processes of an animal it is necessary to know the struc-
ture on which the performance of the processes depends.

40. The animal body a machine. — The body of an animal
may be well compared with some machine like a locomotive
engine. Indeed, the animal body is a machine. It is a
machine composed of many parts, each part doing some
particular kind of work for which a particular kind of
structure fits it ; and all the parts are dependent on each
other and work together for the accomplishment of the
total business of the machine. The locomotive must be
provided with fuel, such as coal or wood or other readily
combustible substance, the consumption of which furnishes
the force or energy of the machine. The animal body
must be provided with fuel, which is called food, which
furnishes similarly the energy of the animal. Oxygen must
be provided for the combustion of the fuel in the locomo-
tive and the food in the body. The locomotive is com-
posed of special parts : the firebox for the reception and
combustion of fuel; the steam pipes for the carriage of
steam ; the wheels for locomotion ; the smoke stack for
throwing off of waste. The animal body is similarly com-
posed of parts : the alimentary canal for the reception and
assimilation of food ; the excretory organs for the throwing
off of waste matter ; the arteries and veins for the carriage
of the oxygen and food-holding blood ; the legs or wings
for locomotion.
6

66 ANIMAL LIFE

The locomotive is an inorganic machine ; the animal is
an organic machine. There is a great and real difference
between an organism, a living animal, and a locomotive, an
inorganic structure. But for a good understanding of the
relation between function and structure, and of the com-
position of the body of the complex animals, the compari-
son of the animal and locomotive is very instructive.

41. The specialization of organs.— The organ for the per-
formance of some definite function in one of the higher
animals may be very complex. The corresponding organ
in one of the lower animals for the performance of the
same function may be comparatively simple. For example,
the organ for the digestion of food is, in the case of the
polyp, a simple cylindrical cavity in the body into which
food enters through a large opening at the apical or free
end of the body. The digestive organ of a cow is a long
coiled tube, comprising many regions of distinct structural
and physiological character and altogether extremely com-
plicated. An organ in simple or primitive condition is
said to be generalized ; in complex or highly modified con-
dition it is said to be specialized. That is, an organ may
be modified and complexly developed to perform its func-
tion in a special way, in a way differing in many particu-
lars from the way the corresponding organ in some other
animal performs the same general function. The speciali-
zation of organs, or their modification to perform their
functions in special ways, is what makes animal bodies
complex, for specialization is almost always in the line of
complexity. Later we shall see more clearly how specializa-
tion is brought about. For the present we may study
one of the more important organs of the animal body for
the sake of having concrete examples of some of the gen-
eral statements made in this discussion of function and
structure.

42. The alimentary canal — The organ which has to do
with the taking and digesting of food is called the ali-

FUNCTION AND STRUCTURE

mentary canal. In some of the higher animals this is a
very complex organ. In the cow, one of the cud-chewing
mammals or ruminants, it consists of several distinct por-
tions, which differ among themselves very much (Fig. 30).
First, there is the mouth, or opening for the entrance of
the food. The mouth is sup-
plied with teeth for tearing
off and chewing the food,
with a tongue for manipu-
lating it, and with taste pa-
pillae situated on the tongue
and palate for determining
the desirability of the food.
Into the mouth a peculiar
fluid (the saliva) is poured
by certain glands, organs ac-
cessory to the alimentary
canal. The herbage bitten
off, mixed with saliva, and
rolled by the tongue into a
ball, passes back through a
narrow tube, the oesophagus,
and into a sac called the ru-
men, or paunch. Here it
lies until the cow ceases for
the while to take in food,
when it passes back again
through the oesophagus and
into the mouth for mastica-
tion. After being masticated it again passes downward
through the oesophagus, and enters this time another sac
called the reticulum, lying next to the rumen. From here
it passes into another sac-like portion of the alimentary
canal called the omasum,, where it is strained through
numerous leaf-like folds which line the walls of this part
of the canal. From here the food passes into a fourth

t....

FIG. 30.— Alimentary canal of the ox
(after COLIN and MULLER). a, rumen
(left hemsiphere) ; b, rumen (right hem-
isphere) ; c, insertion of oesophagus ; d,
reticulum ; e, omasum ; /, abomasum ;
g, duodenum ; h and i, jejunum and
ileum ; j, caecum ; 1c, colon, with its
various convolutions ; I, rectum.

68

ANIMAL LIFE

sac-like part of the canal, called the abomasum. Here
the process of digestion goes on. The four sacs— rumen,
reticulum, omasum, and abomasum — are called stomachs,
or they may be considered to be four chambers forming
one large stomach. In the abomasum, or digesting stom-
ach, digestive fluids are poured from glands lining its
walls, and the food becomes converted into a liquid called
chyle. The chyle passes from the stomach into a long,
narrow, tubular portion of the canal called the intestine.
The intestine is very long, and lies coiled in a large mass
in the body of the cow. The intestine is divided into
distinct regions, which vary in size and in the character
of the inner wall. These parts of the intestine have
names, as duodenum, jejunum, ileum, caecum, colon, etc.
Part of the intestine is lined inside with fine papillae,
which take up the chyle (the digested food) and pass it
through the walls of the intestine to other special organs,
which pass it on to the blood, with which it becomes mixed
and carried by an elaborate system of tubes to all parts of
the body. Part of the grass taken into the alimentary
canal by the cow can not be digested, and must be got rid
of. This passes on into a final posterior part of the intes-
tine called the rectum, and leaves the body through the
anus or posterior opening of the alimentary canal. The
whole canal is more than twenty times as long as the body
of the cow ; it is composed of parts of different shape ; its
walls are supplied with muscles andblooi-vessels ; the inner
lining is covered with folds, papillae, and gland cells. It is
altogether a highly specialized organ, a structurally com-
plex and elaborately functioning organ.

Let us now examine the alimentary canal, or organ of
digestion, in some of the simpler animals.

The Protozoa, or simplest animals, have no special organ
at all. When the surface of the body of an Amoeba, comes
into contact with an organic particle which will serve as
food, the surface becomes bent in at the point of its con-

FUNCTION AND STRUCTURE

69

tact with the food particle, and the body substance simply
incloses the food (Fig. 3). Food is taken in by the sur-
face. The whole outer surface of the body is the food-
taking organ. In the simplest many-celled animals, the
sponges, there is no special food-taking and digestive organ.
Each of the cells of the body takes in and assimilates food
for itself. The sponge is like a great group of Amcebce
holding fast to each other, but each looking out for its own
necessities. Among the m

polyps, however, there
is a definite organ of
digestion — that is, food
is only taken and di-
gested by certain parts
of the body. The sim-
ple polyp's body (Fig. ^ ^ i
31) is a cylinder or vase
closed at one end and
open at the other end,
and attached by the
closed end to a rock.
The opening is usually
of less diameter than
the diameter of the
body, and it is sur-
rounded by a number
of tentacles, whose
function it is to seize the food and convey it to the mouth
opening. There are, of course, no teeth, no tongue, none
of the various parts which are in or are part of the mouth
of the higher animals. The polyp's mouth is simply a
hole or opening into the inside of the body. This body
cavity, or simplest of all stomachs, is simply the cylindrical
or vase-shaped hollow space inclosed by the body wall.
This space extends also into the tentacles. There is no
other opening, no posterior or anal opening. We can not

FIG. 31. — Obeliasp.,a simple polyp; vertical sec-
tion, highly magnified, m, mouth opening ;
al. s., alimentary sac. — After PARKER and
HASWELL.

70

ANIMAL LIFE

speak of an oesophagus or intestine in connection with this
most primitive of alimentary sacs. The cells which line
the sacs show some differentiation ; some are gland cells
and secrete digestive fluids ; some are amoeboid and are
provided with pseudopods or flagella for seizing bits of
food. The food caught by the tentacles comes into the ali-
mentary sac through the opening or primitive mouth, and

PIG. 32.— Diagrammatic sketch of a flat-
worm (Planaria), showing the
branched alimentary canal, al. c.~
After JIJIMA and HATSCHEK.

PIG. 33.— Sea-cucumber (Holothurian)
dissected to show alimentary canal,
al. c.— After LEUCKART.

what of it is digestible is, by the aid of the gland cells and
the amoeboid cells, taken up and assimilated, while the rest
of it is carried out by water currents again through the
single opening.

In the flat worms (Fig. 32) like Planaria (small, thin,
flattened worms to be found in the mud at the bottom of
fresh-water ponds) the mouth opens into a short, narrow
tube which may be called an oesophagus. The oesophagus

FUNCTION AND STRUCTURE

71

connects the mouth with the rest of the alimentary canal,
which gives out many side branches or diverticula, which
are themselves branched, so that the
alimentary sac or stomach is a system
of ramifying tubes extending from a
central main tube to all parts of the
body of the worm. There is no
anal opening. In the round or thread
worms, of which the deadly Trichina
is an example, the alimentary canal

is a simple straight tube with both \&L G,

anterior or mouth opening and pos-
terior or anal opening. In the sea-
urchins and sea-cucumbers (Fig. 33)
the alimentary canal is a simple tube
with two openings, but it is longer
than the body between mouth and
anus, and so is more or less bent or
coiled. In the earthworm the ali-
mentary canal (Fig. 34), although a
simple straight tube running through
the body, plainly shows a differentia-
tion into particular regions. Behind
the mouth opening the alimentary
tube is large and thick - walled and
is called the pharynx; behind the
pharynx it is narrower and is called
the oesophagus. Behind the oesopha-
gus it expands to form a rounded,
thin-walled chamber called the crop,
and just behind this there is another
rounded but very thick-walled cham- FIG. 34.-Earthworm diBsected

. -i T-I J.T to show alimentary canal,

ber called the gizzard. From the ^ c

gizzard back the alimentary canal is

about uniform in size, being rather wide and having thick,

soft walls. This portion of it is called the intestine. The

72 ANIMAL LIFE

posterior part of the intestine, called the rectum, leads to
the anal opening. There is some differentiation of the
inner surface of the canal. In the great group of mol-
lusks, of which the common fresh-water clam or mussel is
an example, the alimentary canal (Fig. 35) shows much
variation. The microscopic plants, which are the food of
the mussel, are taken in through the mouth and pass into
a short oesophagus, thence into a wide stomach and there
digested. Behind the stomach is a long, much-folded, nar-
row intestine which winds about through the fleshy " foot "
and finally reaches the surface of the body, and has an
anal opening at a point opposite the position of the mouth.
Among the insects there is a great range in degree of
complexity of the alimentary canal. The digestive organs
are, however, in most insects in a condition of high speciali-
zation. The mouth opening is provided with well-developed

Cll C

FIG. 35.— Pond mussel dissected to show alimentary canal, al. c.— After HATSCHEK

and CORI.

biting and masticating or piercing and sucking mouth parts ;
pharynx, oesophagus, stomach, and intestine are always dif-
ferentiated and sometimes greatly modified. In the com-
mon cockroach, for example (Fig. 36), the mouth has a
complicated food-getting apparatus, and the canal, which

FUNCTION AND STRUCTURE

73

is much longer than the body of the insect, and hence

much bent and coiled, consists of a pharynx, oesophagus,

fore-stomach or proventriculus,

true digesting stomach or ven-

triculus, intestine, and rectum

which opens at the posterior

tip of the body. The inner

lining of the canal shows much

differentiation in the different

parts of the canal, and there

are numerous accessory glands

connected with various parts of

the canal.

Finally, among the highest
animals, the vertebrates, we
find still more elaborate special-
ization of the alimentary canal.
As an example the alimentary
canal of a cow has already been
described in detail.

43. Stable and variable char-
acteristics of an organ. — In
spite of all this variation in

the structure and general character of the alimentary
canal, there are certain characteristics which are features
of all alimentary canals. In the examination of an organ
we must ever distinguish between its so-called constant or
stable characteristics and its inconstant or variable charac-
teristics. The constant characteristics are the fundamen-
tally essential ones of the organ ; the variable ones are the
special characteristics which adapt the organ for the pecul-
iar habits of the animal possessing it — habits which may
differ very much from those of some other animal of similar
size, similar distribution, similar abundance.

44. Stable and variable characteristics of the alimentary
canal. — A tiger or a lion has an alimentary canal not more

PIG. 36.— Cockroach dissected to show
alimentary canal, al. c.— After HAT-
SCHEK and Com.

74 ANIMAL LIFE

than three or four times the length of its body, while a
sheep has an alimentary canal twenty-eight times as long
as its body. The tiger is carnivorous ; the sheep her-
bivorous. Associated with the different food habits of the
two animals is a striking difference in the alimentary
canals. Animals like the horse or cat, which chew their
food before swallowing it, have a slender oesophagus ; ani-
mals like snakes which swallow their food whole have a
wide oesophagus. Birds, that have no teeth and hence
can not masticate or grind their food in their mouths, usu-
ally have a special grinding stomach, the gizzard, for this
purpose. And so we might cite innumerable examples
of these inconstant or variable characteristics of the ali-
mentary canal. On the other hand, the alimentary canals
of all the many-celled animals except the lowest agree in
certain important characteristics. Each alimentary canal
has two openings, one for the ingress of food and one for
the exit of the indigestible portions of the matter taken in,
and the canal itself stretches through the body from mouth
to anus as a tube, now narrow, now wide, now suddenly
expanding into a sac or giving off lateral diverticula, but
always simply a lumen or hollow inclosed by a flexible mus-
cular wall. The inner lining of the wall is provided with
secreting and absorbing structures. Indeed, we can reduce
the essential characters of the alimentary canal to even
more simple features. The organ of digestion or assimila-
tion of all the many-celled animals is merely a surface with
which food is brought into contact, and which has the
power of digesting this food by means of digestive secre-
{ tions, and of absorbing the food when digested. This sur-
face is small or great in extent, depending upon the amount
of food necessary to the life of the animal and the difficulty
or readiness with which the food can be digested. This
surface might just as well be on the outside of the animal's
body as on the inside, if it were convenient. In fact, it is
on the outside of some animals. Among the Protozoa the

FUNCTION AND STRUCTURE 75

digesting surface is simply the external surface of the body.
And not alone among the one-celled animals. Many of the
parasitic worms which live in the bodies of other animals,
and the larvae or " grubs " of many insects which lie in the
tissues of plants bathed by the sap, have no inner alimen-
tary canal, but take food through the outer surface of the
body. But in these cases the food is ready for immediate
absorption, so that no special treatment of it is necessary,
hence no complex structures are required.

Even were no such special treatment of the food neces-
sary in the case of the larger animals, it would still be im-

©

FIG. 37. — Diagram illustrating increase of volume and surface with increase of
diameter of sphere.

possible for the simple external surface of the body to serve
for food absorption, because of the well-known relation
between the surface and the mass of a solid body. When
a solid body in the form of a sphere increases in size, its
mass or volume increases as the cube of the diameter, while
the surface increases only as the square of the diameter
(Fig. 37). The external surface of minute animals a few
millimeters in diameter can take up enough food to supply
the whole body mass. But among large animals this food-
getting surface is increased as the square of the diameter of

76 ANIMAL LIFE

the body, while the volume or food-using surface of the
body is increased as the cube of its diameter. The food sup-
plying can not keep pace with the food using. Hence it is
absolutely essential that among large animals the food-tak-
ing surface be increased so that it will remain in the same
favorable proportion to the mass of the animal as is the
case among the minute animals, where the simple external
body surface is sufficient to obtain all the food necessary.
This increase of surface, without an accompanying increase
of size of the animal, is accomplished by having the digest-
ing and assimilating surface inside the body and by having
it greatly folded. The surface of the alimentary canal is,
after all, simply a bent-in continuation of the outer surface
of the body. It is open to the outside of the body by two
openings, and wholly closed (except by its porosity) to the
true inside of the body. By the bending and coiling of
the alimentary canal, and by the repeated folding of its
inner wall, the alimentary surface is greatly increased.
The necessity for this increase accounts largely for the
complexity of the alimentary canal.

But it is not alone this necessity for increased surface
that accounts for the great specialization of the alimentary
canal in such animals as the insects and the vertebrates.
The structural differences in different portions of the canal,
resulting in the differentiation of the canal into distinct
parts, or the differentiation of the whole organ into distinct
subordinate organs, each with a special work or function to
perform, are the result of the necessity for the special
manipulation of the special kinds of foods taken. Animals
which feed on other animals must have mouth structures
fit for seizing and rending their prey, and the alimentary
canal must be specially modified for the digestion of flesh.
Animals which feed on vegetable substances must have
special modifications of the alimentary canal quite different
from those of the carnivores. Some insects, like the mos-
quito, take only liquid food, the sap of plants, or the blood

FUNCTION AND STRUCTURE YY

of animals ; others, like the weevils, feed on the hard, dry
substance of seeds and grains ; others, like the grasshop-
pers and caterpillars, eat green leaves ; and still others eat
other insects. The alimentary canal of each of these kinds
of insects differs more or less from that of the other kinds.
The specialization of the alimentary canal depends then
upon the necessity for a large food-digesting and absorbing
surface, and on the' complex treatment of the food. The
character of this specialization in each case depends upon
the special kind or quality of food taken by the animal in
question.

45. The mutual relation of function and structure. — The
structure of an animal depends upon the manner in which
the life processes or functions of the animal are performed.
If the functions are performed in a complex manner, the
structure of the body is complex ; if the functions are per-
formed in simple manner, the body will be simple in struc-
ture. With the increase in degree of the division of labor
among various parts of the body, there is an increase in
definiteness and extent of differentiation of structure.
Each part or organ of the body becomes more modified and
better fitted to perform its own special function. A pecul-
iar structural condition of any part of the body, or of the
whole body of any animal, is not to be looked on as a freak
of Xature, or as a wonder or marvel. Such a structure has
a significance which may be sought for. The unusual
structural condition is associated with some special habit
or manner of performance of a function. Function and
structure are always associated in Xature, and should always
be associated in our study of Nature.

CHAPTER V

THE LIFE CYCLE

46. Birth, growth and development, and death. — Certain
phenomena are familiar to us as occurring inevitably in the
life of every animal. Each individual is born in an imma-
ture or young condition ; it grows (that is, it increases in
size), and develops (that is, changes more or less in struc-
ture), and dies. These phenomena occur in the succession
of birth, growth and development, and death. But before
any animal appears to us as an independent individual —
that is, outside the body of the mother and outside of an
egg (i. e., before birth or hatching, as we are accustomed to
call such appearance) — it has already undergone a longer
or shorter period of life. It has been a new living organ-
ism hours or days or months, perhaps, before its appear-
ance to us. This period of life has been passed inside an
egg, or as an egg or in the egg stage, as it is variously
termed. The life of an animal as a distinct organism be-
gins in an egg. And the true life cycle of an organism is
its life from egg through birth, growth and development,
and maturity to the time it produces new organisms in
the condition of eggs. The life cycle is from egg to egg.
Birth and growth, two of the phenomena readily apparent
to us in the life of every animal, are two phenomena in the
true life cycle. Death is a third inevitable phenomenon in
the life of each individual, but it is not a part of the cycle.
It is something outside.

47. Life cycle of simplest animals. — The simplest animals
have no true egg stage, nor perhaps have they any true

78

THE LIFE CYCLE

79

death. The new Amoebae are from their beginning like the
full-grown Amoeba, except as regards size. And the old
Amoeba does not die, because its whole body continues to
live, although in two parts — the two new Amoebae. The life
cycle of the simplest animals includes birth (usually by
simple fission of the body of the parent), growth, and some,
but usually very little, development, and finally the repro-
duction of new individuals, not by the formation of eggs,
but by direct division of the body.

48. The egg. — In our study of the multiplication of ani-
mals (Chapter III) we learned that it is the almost univer-

FIG. 38. — Eggs of different animals showing variety in external appearance, a, egg
of bird ; b, eggs of toad ; c, egg of fish ; d, egg of butterfly ; e, eggs of katydid
on leaf ; /,egg-case of skate.

sal rule among many-celled animals that each individual
begins life as a single cell, which has been produced by the

80 ANIMAL LIFE

fusion of two germ cells, a sperm cell from a male indi-
vidual of the species and an egg cell from a female indi-
vidual of the species. The single cell thus formed is called
the fertilized egg cell, and its subsequent development
results in the formation of a new individual of the same
species with its parents. Now, in the development of this
cell into a new animal, food is necessary, and sometimes a
certain amount of warmth. So with the fertilized egg cell
there is, in the case of all animals that lay eggs, a greater
or less amount of food matter — food yolk, it is called — gath-
ered about the germ cell, and both germ cell and food yolk
are inclosed in a soft or hard wall. Thus is composed the
egg as we know it. The hen's egg is as large as it is be-
cause of the great amount of food yolk it contains. The
egg of a fish as large as a hen is much smaller than the
hen's eg£ , it contains less food yolk. Eggs (Fig. 38) may
vary also in their external appearance, because of the dif-
ferent kinds of membrane or shells which may inclose and
protect them. Thus the frog's eggs are inclosed in a thin
membrane and imbedded in a soft, jelly-like substance ;
the skate's egg has a tough, dark-brown leathery inclosing
wall ; the spiral egg of the bull-head sharks is leathery and
colored like the dark-olive seaweeds among which it lies ;
and a bird's egg has a hard shell of carbonate of lime. But
in each case there is the essential fertilized germ cell ; in
this the eggs of hen and fish and butterfly and cray-fish and
worm are alike, however much they may differ in size and
external appearance.

49. Embryonic and post-embryonic development. — Some
animals do not lay eggs, that is they do not deposit the fer-
tilized egg cell outside of the body, but allow the develop-
ment of the new individual to go on inside the body of the
mother for a longer or shorter period. The mammals and
some other animals have this habit. When such an ani-
ma7 issues from the body of the mother, it is said to be
born. When the developing animal issues from an egg

THE LIFE CYCLE

81

which has been deposited outside the body of the mother,
it is said to hatch. The animal at birth or at time of hatch-
ing is not yet fully developed. Only part of its development
or period of immaturity is passed within the egg or within
the body of the mother. That part of its life thus passed
within the egg or mother's body is called the embryonic life
or embryonic stages of development ; while that period of
development or immaturity from the time of birth or hatch-
ing until maturity is reached is called the post-embryonic
life or post-embryonic stages of development.

50. First stages in development. — The embryonic develop-
ment is from the beginning up to a certain point practically
identical for all many-celled animals — that is, there are cer-

FIG. 39. — First stages in embryonic development of the pond snail (Lymnceus). a.
egg cell ; b, first cleavage ; c, second cleavage ; d, third cleavage ; «, after numer-
ous cleavages ; /, blastula (in section) ; g, gastrula, just forming (in section) ;
k, gastrula, completed (in section).— After RABL.

tain principal or constant characteristics of the beginning
development which are present in the development of all
many-celled animals. The first stage or phenomenon of
development is the simple fission of the germ cell into
halves (Fig. 39, b). These two daughter cells next divide so
that there are four cells (Fig. 39, c) ; each of these divides,

and this division is repeated until a greater or lesser num-

7

82 ANIMAL LIFE

ber (varying with the various species or groups of animals)
of cells is produced (Fig. 39, d). The phenomenon of re-
peated division of the germ cell, and usually the surround-
ing yolk, is called cleavage, and this cleavage is the first
stage of development in the case of all many-celled animals.
The first division of the germ cell produces usually two equal
cells, but in some of the later divisions the new cells formed
may not be equal. In some animals all the cleavage cells are
of equal size ; in some there are two sizes of cells. The germ
or embryo animal consists now of a mass of few or many
undifferentiated primitive cells lying together and usually
forming a sphere (Fig. 39, e), or perhaps separated and scat-
tered through the food yolk of the egg. The next stage of de-
velopment is this : the cleavage cells arrange themselves so
as to form a hollow sphere or ball, the cells lying side by side
to form the outer circumferential wall of this hollow sphere
(Fig. 39,/). This is called the Uastula or blastoderm stage
of development, and the embryo itself is called the blastula
or blastoderm. This stage also is common to all the many-
celled animals. The next stage in embryonic development
is formed by the bending inward of a part of the blasto-
derm cell layer, as shown in Fig. 39, g. This bending in
may produce a small depression or groove ; but whatever the
shape or extent of the sunken-in part of the blastoderm, it
results in distinguishing the blastoderm layer into two
parts, a sunken-in portion called the endoUast and the
other unmodified portion called the ectoblast. Endo- means
" within," and the cells of the endoblast often push so far
into the original blastoderm cavity as to come into contact
with the cells of the ectoblast and thus obliterate this cavity
(Fig. 39, Ti). This third well-marked stage in the embry-
onic development is called the gastrula * stage, and it also

* This gastrula stage is not always formed by a bending in or in-
vagination of the blastoderm, but in some animals is formed by the
splitting off or delamination of cells from a definite limited region of

THE LIFE CYCLE 83

occurs in the development of all or nearly all many-celled
animals.

51. Continuity of development. — In the case of a few of
the simple many-celled animals the embryo hatches — that
is, issues from the egg at the time of or very soon after
reaching the gastrula stage. In the higher animals, how-
ever, development goes on within the egg or within the
body of the mother until the embryo becomes a complex
body, composed of many various tissues and organs. Al-
most all the development may take place within the egg,

a

FIG. 40.— Honey-bee, a, adult worker ; b, young or larval worker.

so that when the young animal hatches there is necessary
little more than a rapid growth and increase of size to
make it a fully developed, mature animal. This is the case
with the birds : a chicken just hatched has most of the
tissues and organs of a full-grown fowl, and is simply a
little hen. But in the case of other animals the young
hatches from the egg before it has reached such an ad-
vanced stage of development ; a young star-fish or young
crab or young honey-bee (Fig. 40) just hatched looks very
different from its parent. It has yet a great deal of devel-
opment to undergo before it reaches the structural condi-
tion of a fully developed and fully grown star-fish or crab
or bee. Thus the development of some animals is almost

the blastoderm. Our knowledge of gastrulation and the gastrula stage
is yet far from complete. %

84 ANIMAL LIFE

wholly embryonic development — that is, development with-
in the egg or in the body of the mother — while the devel-
opment of other animals is largely post-embryonic or larval
development, as it is often called. There is no important
difference between embryonic and post-embryonic develop-
ment. The development is continuous from egg cell to
mature animal, and whether inside or outside of an egg it
goes on regularly and uninterruptedly.

52. Development after the gastrula stage. — The cells which
compose the embryo in the cleavage stage and blastoderm
stage, and even in the gastrula stage, are all similar ; there
is little or no differentiation shown among them. But from
the gastrula stage on development includes three important
things : the gradual differentiation of cells into various
kinds to form the various kinds of animal tissues ; the
arrangement and grouping of these cells into organs and
body parts ; and finally the developing of these organs
and body parts into the special condition characteristic of
the species of animal to which the developing individual
belongs. From the primitive undifferentiated cells of the
blastoderm, development leads to the special cell types of
muscle tissue, of bone tissue, of nerve tissue ; and from the
generalized condition of the embryo in its early stages de-
velopment leads to the specialized condition of the body of
the adult animal. Development is from the general to the
special, as was said years ago by the first great student of
development.

53. Divergence of development, — A star-fish, a beetle, a
dove, and a horse are all alike in their beginning — that is,
the body of each is composed of a single cell, a single struc-
tural unit. And they are all alike, or very much alike,
through several stages of development ; the body of each
is first a single cell, then a number of similar undifferen-
tiated cells, and then a hollow sphere consisting of a single
layer of similar undifferentiated cells. But soon in the
course of development the embryos begin to differ, and as

OF THE

UNIVERSITY )

OF /

THE LIFE CYCLE 85

the young animals get further and further along in the
course of their development, they become more and more
diiferent until each finally reaches its fully developed ma-
ture form, showing all the great structural differences be-
tween the star-fish and the dove, the beetle and the horse.
That is, all animals begin development alike, but gradually
diverge from each other during the course of development.
There are some extremely interesting and significant
things about this divergence to which attention should be
given. While all animals are alike structurally* at the
beginning of development, so far as we can see, they do not
all differ at the time of the first divergence in development.
This first divergence is only to be noted between two kinds
of animals which belong to different great groups or classes.
But two animals of different kinds, both belonging to some
one great group, do not show differences until later in their
development. This can best be understood by an example.
All the butterflies and beetles and grasshoppers and flies
belong to the great group of animals called Insecta, or in-
sects. There are many different kinds of insects, an4 these
kinds can be arranged in subordinate groups, such as the
Diptera, or flies, the Lepidoptera, or butterflies and moths,
and so on. But all have certain structural characteristics
in common, so that they are comprised in one great group
or class — the Insecta. Another great group of animals is
known as the Vertebrata, or back-boned animals. The class
Vertebrata includes the fishes, the batrachians, the reptiles,
the birds, and the mammals, each composing a subordinate
group, but all characterized by the possession of a back-

* They are alike structurally, when we consider the cell as the unit
of animal structure. That the egg cells of diiferent animals may dif-
fer in their fine or ultimate structure, seems certain. For each one of
these egg cells is destined to become some one kind of animal, and no
other ; each is, indeed, an individual in simplest, least developed con-
dition of some one kind of animal, and we must believe that difference
in kind of animals depends upon difference in structure in the egg itself.

86 ANIMAL LIFE

bone, or, more accurately speaking, of a notochord, a back-
bone-like structure. Now, an insect and a vertebrate di-
verge very soon in their development from each other ; but
two insects, such as a beetle and a honey-bee, or any two
vertebrates, such as a frog and a pigeon, do not diverge
from each other so soon. That is, all vertebrate animals
diverge in one direction from the other great groups, but
all the members of the great group keep together for some
time longer. Then the subordinate groups of the Verte-
brata, such as the fishes, the birds, and the others diverge,
and still later the different kinds of animals in each of
these groups diverge from each other. In the illustration
(Fig. 41) on the opposite page will be seen pictures of the
embryos of various vertebrate animals shown as they appear
at different stages or times in the course of development.
The embryos of a fish, a salamander, a tortoise, a bird, and
a mammal, representing the five principal groups of the
Vertebrata, are shown. In the upper row the embryos are
in the earliest of all the stages figured, and they are very
much alike. They show no obvious characteristics of
fish or bird. Yet there are distinctive characteristics of
the great class Vertebrata. Any of these embryos could
readily be distinguished from an embryonic insect or worm
or sea-urchin. In the second row there is beginning to be
manifest a divergence among the different embryos, al-
though it would still be a difficult matter to distinguish
certainly which was the young fish and which the young
salamander, or which the young tortoise and which the
young bird. In the bottom row, showing the animals in a
later stage of development, the divergence has proceeded
so far that it is now plain which is a fish, which batrachian,
which reptile, which bird, and which mammal.

54. The laws or general facts of development.— That the
course of development of any animal from its beginning to
fully developed adult form is fixed and certain is readily
seen. Every rabbit develops in the same way ; every grass-

ilcirn cinder

Jortofse Chick

. 4i._ Different vertebrate animal in successive embryonic stages. I, first
or earliest of the stages figured ; II, second of the stages ; HI, third or
latest of the stages.— After HAECKBL.

38 ANIMAL LIFE

hopper goes through the same developmental changes from
single egg cell to the full-grown active hopper as every
other grasshopper of the same kind — that is, development
takes place according to certain natural laws, the laws of
animal development. These laws may be roughly stated as
follows : All many-celled animals begin life as a single cell,
the fertilized egg cell ; each animal goes through a certain
orderly series of developmental changes which, accom-
panied by growth, leads the animal to change from single
cell to the many-celled, complex form characteristic of the
species to which the animal belongs ; this development is
from simple to complex structural condition ; the develop-
ment is the same for all individuals of one species. While
all animals begin development similarly, the course of devel-
opment in the different groups soon diverges, the diver-
gence being of the nature of a branching, like that shown
in the growth of a tree. In the free tips of the smallest
branches we have represented the various species of ani-
mals in their fully developed condition, all standing clearly
apart from each other. But in tracing back the develop-
ment of any kind of animal, we soon come to a point where
it very much resembles or becomes apparently identical
with some other kind of animal, and going further back we
find it resembling other animals in their young condition,
and so on until we come to that first stage of development,
that trunk stage, where all animals are structurally alike.
To be sure, any animal at any stage in its existence differs
absolutely from any other kind of animal, in that it can
develop into only its own kind of animal. There is some-
thing inherent in each developing animal that gives it an
identity of its own. Although in its young stages it may be
hardly distinguishable from some other kind of animal in
similar stages, it is sure to come out, when fully developed,
an individual of the same kind as its parents were or are.
The young fish and the young salamander in the upper row
in Fig. 41 seem very much alike, but one embryo is sure to

THE LIFE CYCLE 89

develop into a fish and the other into a salamander. This
certainty of an embryo to become an individual of a certain
kind is called the law of heredity. •

55. The significance of the facts of development. — The
significance of the developmental phenomena is a matter
about which naturalists have yet very much to learn. It is
believed, however, by practically all naturalists that many
of the various stages in the development of an animal cor-
respond to or repeat the structural condition of the ani-
mal's ancestors. Naturalists believe that all backboned or
vertebrate animals are related to each other through being
descended from a common ancestor, the first or oldest
backboned animal. In fact, it is because all these back-
boned animals — the fishes, the batrachians, the reptiles, the
birds, and the mammals — have descended from a common
ancestor that they all have a backbone. It is believed that
the descendants of the first backboned animal have in the
course of many generations branched off little by little from
the original type until there have come to exist very real and
obvious differences among the backboned animals — differ-
ences which among the living backboned animals are familiar
to all of us. The course of development of an individual ani-
mal is believed by many naturalists to be a very rapid, and
evidently much condensed and changed, recapitulation of
the history which the species or kind of animal to which the
developing individual belongs has passed through in the
course of its descent through a long series of gradually chang-
ing ancestors. If this is true, then we can readily under-
stand why the fish and the salamander and tortoise and
bird and rabbit are all so much alike in their earlier stages
of development, and gradually come to differ more and
more as they pass through later and later developmental
stages.

Some naturalists believe that the ontogenetic stages are
not as significant in throwing light upon the evolutionary
history of the species as just indicated. Some think that

90 ANIMAL LIFE

when the earlier stages of one species correspond pretty
closely with the early stages of another, we have a good
basis for making up our minds about relationship between
the two species. But it is certainly not obvious why we
should have a similarity among the younger stages of dif-
ferent animals and no correspondence among the older
stages of more recent animals with the younger stages of
more ancient ones. But on the other hand it is certainly
true that a too specific application of the broad generaliza-
tion that ontogeny repeats phylogeny has led to numerous
errors of interpreting genealogic relationship.

56. Metamorphosis. — While a young robin when it hatches
from the egg or a young kitten at birth resembles its par-
ents, a young star-fish or a young crab or a young butterfly
when hatched does not at all resemble its parents. And
while the young robin after hatching becomes a fully grown
robin simply by growing larger and undergoing compara-
tively slight developmental changes, the young star-fish or
young butterfly not only grows larger, but undergoes some
very striking developmental changes; the body changes
very much in appearance. Marked changes in the body of
an animal during post-embryonic or larval development
constitute what is called metamorpliic development, or the
animal is said to undergo or to show metamorphosis in its
development. Metamorphosis is one of the most interest-
ing features in the life history or development of animals,
and it can be, at least as far as its external aspects are con-
cerned, very readily observed and studied.

57. Metamorphosis among insects. — All the butterflies and
moths show metamorphosis in their development. So do
many other insects, as the ants, bees, and wasps, and all the
flies and beetles. On the other hand, many insects do not
show metamorphosis, but, like the birds, are hatched from
the egg in a condition plainly resembling the parents. A
grasshopper (Fig. 42) is a convenient example of an insect
without metamorphosis, or rather, as there are, after all,

THE LIFE CYCLE

91

a few easily perceived changes in its post-embryonic devel-
opment, of an insect with an "incomplete metamorpho-
sis." The eggs of grasshoppers are laid in little packets
of several score half an inch below the surface of the
ground. When the young grasshopper hatches from the
egg it is of course very small, but it is plainly recognizable
as a grasshopper. But in one important character it dif-
fers from the adult, and that is in its lack of wings. The
adult grasshopper has two pairs of wings ; the just hatched
young or larval grasshopper has no wings at all. The
young grasshopper feeds voraciously and grows rapidly.

FIG. 42. —Post-embryonic development (incomplete metamorphosis) of the Rockj"
Mountain locust (Melanoplus spretus). a, b, c, d, e, and /, successive develop-
mental stages from just hatched to adult individual.— After EMERTON.

In a few days it molts, or casts its outer skin (not the
true skin, but a thin, firm covering or outer body wall com-
posed of a substance called chitin, which is secreted by the
cells of the true skin). In this second larval stage there
can be seen the rudiments of four wings, in the condition
of tiny wing pads on the back of the middle part of the
body (the thorax). Soon the chitinous body covering is
shed again, and after this molt the wing pads are mark-
edly larger than before. Still another molt occurs, with
another increase in size of the developing wings, and after
a fifth and last molt the wings are fully developed, and

92

ANIMAL LIFE

the grasshopper is no longer in a larval or immature condi-
tion, but is full grown and adult.

For example of complete metamorphosis among insects
we may choose a butterfly, the large red-brown butterfly

FIG. 43.— Metamorphosis of monarch butterfly (Anosia plexippus).
c, pupa ; d, imago or adult.

o, egg ; b, larva ;

common in the United States and called the monarch or
milkweed butterfly (Anosia plexippus). The eggs (Fig.
43, a) of this butterfly are laid on the leaves of various kinds
of milkweed (Asclepias). The larval butterfly or butterfly
larva or caterpillar (as the first young stage of the butter-

THE LIFE CYCLE

93

flies and moths is usually called), which hatches from the
egg in three or four days, is a creature hearing little or
no resemblance to the beautiful winged adult. The larva
is worm-like, and instead of having three pairs of legs
like the butterfly it has eight pairs; it has biting jaws
in its mouth with which it nips off bits of the green milk-
weed leaves, instead of having a long, slender, sucking
proboscis for drinking flower nectar as the butterfly has.
The body of the crawl-
ing worm-like larva
(Fig. 43, V) is greenish
yellow in color, with
broad rings or bands of
shining black. It has
no wings, of course. It
eats voraciously, grows
rapidly and molts. But
after the molting there
is no appearance of
rudimentary wings; it
is simply a larger worm-
like larva. It continues
to feed and grow, molt-
ing several times, until
after the fourth molt it
appears no longer as an
active, crawling, feed-
ing, worm-like larva, but as a quiescent, non-feeding pupa
or chrysalis (Fig. 43, c). The immature butterfly is now
greatly contracted, and the outer chitinous wall is very
thick and firm. It is bright green in color with golden dots.
It is fastened by one end to a leaf of the milkweed, where
it hangs immovable for from a few days to two weeks.
Finally, the chitin wall of the chrysalis splits, and there
issues the full-fledged, great, four-winged, red-brown butter-
fly (Fig. 43, d). Truly this is a metamorphosis, and a start-

FIQ. 44.— Metamorphosis of mosquito (Culex).
a, larva ; b, pupa.

ANIMAL LIFE

ling one. But we know that development in other animals
is a gradual and continuous process, and so it is in the

case of the butterfly.
The gradual chang-
ing is masked by the
outer covering of the
body in both larva
and pupa. It is only
at each molting or
throwing off of this
unchanging, unyield-
ing chitin armor that
we perceive how far
this change has gone.
The longest time of
concealment is that
during the pupal or
chrysalis stage, and
the results of the
changing or develop-
ment when finally re-
vealed by the split-
ting of the pupal
case are hence the
most striking.

58. Metamorphosis of the toad. — Metamorphosis is found
in the development of numerous other animals, as well as
among the insects. Certain cases are familiar to all — the
metamorphosis of the frogs and toads (Fig. 46). The eggs
of the toad are arranged in long strings or ribbons in a
transparent jelly-like substance. These jelly ribbons with
the small, black, bead-like eggs in them are wound around
the stems of submerged plants or sticks near the shores of
the pond. From each egg hatches a tiny, wriggling tad-
pole, differing nearly as much from a full-grown toad as
a caterpillar differs from a butterfly. The tadpoles feed on

FIG. 45.— Larva of a butterfly just changing into
pupa (making last larval molt). Photograph
from Nature.

THE LIFE CYCLE

95

the microscopic plants to be found in the water, and swim
easily about by means of the long tail. The very young
tadpoles remain underneath the surface of the water all the
time, breathing the air which is mixed with water by means
of gills. But as they become older and larger they come
often to the surface of the water. Lungs are developing
inside the body, and the tadpole is beginning to breathe as
a land animal, although it still breathes partly by means of
gills, that is, as an aquatic animal. Soon it is apparent that
although the tadpole is steadily and rapidly growing larger,
its tail is growing shorter and smaller instead of larger. At
the same time, fore and hind legs bud out and rapidly take

FIG. 46.— Metamorphosis of the toad (partly after GAGE). At left the strings of eggs,
in water the various tadpole or larval stages, and on bank the adult toads.

form and become functional. By the time that the tail
gets very short, indeed, the young toad is ready to leave the
water and live as a land animal. On land the toad lives, as

96

ANIMAL LIFE

we know, on insects and snails and worms. The metamor-
phosis of the toad is not so striking as that of the butter-
fly, but if the tadpole were inclosed in an unchanging
opaque body wall while it was losing its tail and getting its
legs, and this wall were to be shed after these changes were
made, would not the metamorphosis be nearly as extraordi=

Fia. 47.— Metamorphosis of sea-
urchin. Upper figure the adult,
lower figure the pluteus larva.

nary as in the case of
the butterfly? But in
the metamorphosis of
the toad we can see the
gradual and continuous
character of the change.

59. Metamorphosis among other animals. — Many other
animals, besides insects and frogs and toads, undergo meta-
morphosis. The just-hatched sea-urchin does not resemble
a fully developed sea-urchin at all. It is a minute worm-
like creature, provided with cilia or vibratile hairs, by means
of which it swims freely about. It changes next into a curi-
ous bootjack-shaped body called the pluteus stage (Fig, 47).
In the pluteus a skeleton of lime is formed, and the final
true sea-urchin body begins to appear inside the pluteus,

THE LIFE CYCLE

97

developing and growing by using up the body substance of
the pluteus. Star-fishes, which are closely related to sea-
urchins, show a simi-
lar metamorphosis,
except that there is
no pluteus stage, the
true star-fish-shaped
body forming, with-
in and at the expense
of the first larval
stage, the ciliated
free-swimming stage.

A young crab just
issued from the egg
(Fig. 48) is a very
different appearing
creature from the
adult or fully devel-
oped crab. The body
of the crab in its
first larval stage is
composed of a short,
globular portion, fur-
nished with conspicuous long spines and a relatively long,
jointed tail. This is called the zoe'a stage. The zoe'a
changes into a stage called the megalops, which has many
characteristics of the adult crab condition, but differs espe-
cially from it in the possession of a long, segmented tail,
and in having the front half of the body longer than wide.
The crab in the megalops stage looks very much like a
tiny lobster or shrimp, The tail soon disappears and the
body widens, and the final stage is reached.

In many families of fishes the changes which take place

in the course of the life cycle are almost as great as in the

case of the insect or the toad. In the lady-fish (Albula

vulpes) the very young (Fig. 49) are ribbon-like in form,

8

FIG.

48. — Metamorphosis of the crab, a, the zoe'a
stage ; &, the megalops ; c, the adult.

98

ANIMAL LIFE

with small heads and very loose texture of the tissues, the
body substance being jelly-like and transparent. As the fish
grows older the body becomes more compact, and therefore

is

Fio. 49.— Stages in the post-embryonic development of the lady-fish (Albula vulpes),
showing metamorphosis.— After C. H. GILBERT.

shorter and slimmer. After shrinking to the texture of an
ordinary fish, its growth in size begins normally, although

THE LIFE CYCLE

99

it has steadily increased in actual weight. Many herring,
eels, and other soft-bodied fishes pass through stages simi-
lar to those seen in the lady-fish. Another type of devel-
opment is illustrated in the sword-fish. The young has a
bony head, bristling with spines. As it grows older the
spines disappear, the skin grows smoother, and, finally, the
bones of the upper jaw grow together, forming a prolonged
sword, the teeth are lost and the fins become greatly modi-
fied. Fig. 50 shows three of these stages of growth. The

a

FIG. 50.— Three stages in the development of the sword-fish (Xiphias gladius).
a, very young ; b, older ; c, adult.— Partly after LUTKBN.

flounder or flat-fish (Fig. 51) when full grown lies flat on
one side when swimming or when resting in the sand on
the bottom of the sea. The eyes are both on the upper
side of the body, and the lower side is blind and colorless.
When the flounder is hatched it is a transparent fish, broad
and flat, swimming vertically in the water, with an eye on
each side. As its development (Fig. 52) goes on it rests
itself obliquely on the bottom, the eye of the lower side
turns upward, and as growth proceeds it passes gradually

100

ANIMAL LIFE

around the forehead, its socket moving with it, until both
eyes and sockets are transferred by twisting of the skull to

FIG. 51.— The wide-eyed flounder (Platophrys lunatus). Adult, showing both eyes on
upper side of head.

the upper side. In some related forms or soles the small
eye passes through the head and not around it, appearing
finally in the same socket with the other eye.

Thus in almost all the great groups of animals we find
certain kinds which show metamorphosis in their post-
embryonic development. But metamorphosis is simply
development; its striking and extraordinary features are
usually due to the fact that the orderly, gradual course of
the development is revealed to us only occasionally, with
the result of giving the impression that the development is
proceeding by leaps and bounds from one strange stage to

FIG. 52. — Development of a flounder (after EMERY). The eyes in the young flounder
are arranged normally, one on each side of head.

another. If metamorphosis is carefully studied it loses its
aspect of marvel, although never its great interest.

THE LIFE CYCLE

101

60. Duration of life. — After an animal has completed its
development it has but one thing to do to complete its life
cycle, and that is the production of offspring. When it
has laid eggs or given birth to young, it has insured the
beginning of a new life cycle. Does it now die ? Is the
business of its life accomplished ? There are many animals
which die immediately or very soon after laying eggs. The
May-flies — ephemeral insects which issue as winged adults
from ponds or lakes in which
they have spent from one to
three years as aquatic crawl-
ing or swimming larvae, nutter
about for an evening, mate,
drop their packets of fertil-
ized eggs into the water, and
die before the sunrise — are
extreme examples of the nu-
merous kinds of animals
whose adult life lasts only long
enough for mating and egg-
laying. But elephants live for
two hundred years. Whales
probably live longer. A horse
lives about thirty years, and so
may a cat or toad. A sea-
anemone, which was kept in an aquarium, lived sixty-six
years. Cray-fishes may live twenty years. A queen bee
was kept in captivity for fifteen years. Most birds have
long lives — the small song birds from eight to eighteen
years, and the great eagles and vultures up to a hundred
years or more. On the other hand, among all the thou-
sands of species of insects, the individuals of very few in-
deed live more than a year ; the adult life of most insects
being but a few days or weeks, or at best months. Even
among the higher animals, some are very short-lived.
In Japan is a small fish (Solaux) which probably lives

"*

102

ANIMAL LIFE

but a year, ascending the rivers in numbers when young in
the spring, the whole mass of individuals dying in the fall
after spawning.

Naturalists have sought to discover the reason for these
extraordinary differences in the duration of life*c* different
animals, and while it can not be said that the reason or
reasons are wholly known, yet the probability is strong that
the duration of life is closely connected with, or dependent
upon, the conditions attending the production of offspring.
It is not sufficient, as we have learned from our study of
the multiplication of animals (Chapter III), that an adult
animal shall produce simply a single new individual of its
kind, or even only a few. It must produce many, or if it
produces comparatively few it must devote great care to
the rearing of these few, if the perpetuation of the species
is to be insured. Now, almost all long-lived animals are
species which produce but few offspring at a tinie, and
reproduce only at long intervals, while most short-lived ani-
mals produce a great many eggs, and these all at one time.
Birds are long-lived animals; as we know, most of them
lay eggs but once a year, and lay only a few eggs each-time.
Many of the sea birds which swarm in countless numbers
on the rocky ocean islets and great sea cliffs lay only a
single egg once each year. And these birds, the guillemots
and murres and auks, are especially long-lived. Insects, on
the contrary, usually produce many eggs, and all of them
in a short time. The May-fly, with its one evening's lifetime,
lets fall from its body two packets of eggs and then dies.
Thus the shortening of the period of reproduction with the
production of a great many offspring seem to be always
associated with a short adult lifetime ; while a long period
of reproduction with the production of few offspring at a
time and care of the offspring are associated with a long
adult lifetime.

There seems also to be some relation between the size
of animals and the length of life. As a general rule,

THE LIFE CYCLE 103

large animals are long-lived and small animals have short
lives.

61. Death. — At the end comes death. After the animal
has completed its life cycle, after it has done its share toward
insuring the perpetuation of its species, it dies. It may
meet a violent death, may be killed by accident or by. ene-
mies, before the life cycle is completed. And this is the
fate of the vast majority of animals which are born or
hatched. Or death may come before the time for birth or
hatching. Of the millions of eggs laid by a fish, each egg
a new fish in simplest stage of development, how many or
rather how few come to maturity, how few complete the
cycle of life !

Of death we know the essential meaning. Life ceases
and can never be renewed in the body of the dead animal.
It is important that we include the words " can never be
renewed," for to say simply that " life ceases," that is, that
the performance of the life processes or functions ceases,
is not really death. It is easy to distinguish in most cases
between life and death, between a live animal and a dead
one, yet there are cases of apparent death or a semblance of
death which are very puzzling. The test of life is usually
taken to be the performance of life functions, the assimila-
tion of food and excretion of waste, the breathing in of oxy-
gen, and breathing out of carbonic-acid gas, movement,
feeling, etc. But some animals can actually suspend all
of these functions, or at least reduce them to such a mini-
mum that they can not be perceived by the strictest exami-
nation, and yet not be dead. That is, they can renew
again the performance of the life processes. Bears and
some other animals, among them many insects, spend the
winter in a state of death-like sleep. Perhaps it is but sleep ;
and yet hibernating insects can be frozen solid and remain
frozen for weeks and months, and still retain the power of
actively living again in the following spring. Even more
remarkable is the case of certain minute animals called Ro-

104 ANIMAL LIFE

tatoria and of others called Tardigrada, or bear-animalcules.
These bear-animalcules live in water. If the water dries
up, the animalcules dry up too ; they shrivel up into form-
less little masses and become desiccated. They are thus
simply dried-up bits of organic matter; they are organic
dust. Now, if after a long time — years even — one of these
organic dust particles, one of these dried-up bear-animal-
cules is put into water, a strange thing happens. The body
swells and stretches out, the skin becomes smooth instead
of all wrinkled and folded, and the legs appear in normal
shape. The body is again as it was years before, and after
a quarter of an hour to several hours (depending on the
length of time the animal has lain dormant and dried) slow
movements of the body parts begin, and soon the animal-
cule crawls about, begins again its life where it had been
interrupted. Various other small animals, such as vinegar
eels and certain Protozoa, show similar powers. Certainly
here is an interesting problem in life and death.

When death comes to one of the animals with which
we are familiar, we are accustomed to think of its coming
to the whole body at some exact moment of time. As we
stand beside a pet which has been fatally injured, we wait
until suddenly we say, " It is dead." As a matter of fact,
it is difficult to say when death occurs. Long after the
heart ceases to beat, other organs of the body are alive —
that is, are able to perform their special functions. The
muscles can contract for minutes or hours (for a short time
in warm-blooded, for a long time in cold-blooded animals)
after the animal ceases to breathe and its heart to beat.
Even longer live certain cells of the body, especially the
amoeboid white blood-corpuscles. These cells, very like
the Ammba in character, live for 'days after the animal is,
as we say, dead. The cells which line the tracheal tube
leading to the lungs bear cilia or fine hairs which they
wave back and forth. They continue this movement for
days after the heart has ceased beating. Among cold-

THE LIFE CYCLE

105

blooded animals, like snakes and turtles, complete cessa-
tion of life functions comes very slowly, even after the
body has been literally cut to pieces.

Thus it is essential in defining death to speak of a
complete and permanent cessation of the performance of
the life processes.

A grasshopper (Melanoplus differentials) killed by disease caused by
parasitic fungus. On golden-rod.

CHAPTER VI

THE PRIMARY CONDITIONS OF ANIMAL LIFE

62. Primary conditions and special conditions. — Certain
primary conditions are necessary for the existence of all
animals. We know that fishes can not live very long out
of water, and that birds can not live in water. These,
however, are special conditions which depend on the spe-
cial structure and habits of these two particular kinds of
backboned animals. But the necessity of a constant and
sufficient supply of air is a necessity common to both ; it is
one of the primary conditions of their life. All animals
must have air. Similarly both fishes and birds, and all
other animals as well, must have food. This is another one
of the primary conditions of animal life. That backboned
animals must find somehow a supply of salts or compounds
of lime to form into bones is a special condition peculiar
to these animals. Other animals having shells or teeth
composed of carbonate or phosphate of lime are subject to
-the same special demand, but many animals have no hard
parts, and therefore need no lime.

63. Food. — All the higher plants, those that are green
(chlorophyll-bearing), can make their living substance out
of inorganic matter alone — that is, use inorganic substances
as food. But animals can not do this. They must have
already formed organic matter for food. This organic mat-
ter may be the living or dead tissues of plants, or the living
or dead tissues of animals. For the life of animals it is
necessary that other organisms live, or have lived. It is
this need which primarily distinguishes an animal from a

106

THE PRIMARY CONDITIONS OF ANIMAL LIFE 107

plant. Animals can not exist without plants. The plants
furnish all animals with food, either directly or indirectly.
The amount of food and the kinds of food required by
various kinds of animals are special conditions depending
on the size, the degree of activity, the structural character
of the body, etc., of the animal in question. Those which
do the most need most. Those with warmest blood, great-
est activity, and most rapid change of tissues are most
dependent on abundance, regularity, and fitness of their
food. As we well know, an animal can live for a longer or
shorter time without food. Men have fasted for a month,
or even two months. Among cold-blooded animals, like the
reptiles, the general habit of food taking is that of an occa-
sional gorging, succeeded by a long period of abstinence.
Many of the lower animals can go without food for surpris-
ingly long periods without loss of life. But the continued
lack of food results inevitably in death. Any animal may
be starved in time.

If water be held not to be included in the general con-
ception of food, then special mention must be made of the
necessity of water as one of the primary conditions of ani-
mal life. Protoplasm, the basis of life, is a fluid, although
thick and viscous. To be fluid its components must be
dissolved or suspended in water. In fact, all the truly
living substance in an animal's body contains water. The
water necessary for the animal may be derived from the
other food, all of which contains water in greater or
less quantity, or may be taken apart from the other
food, by drinking or by absorption through the skin.
Sheep are seldom seen to drink, for they find almost
enough water in their green food. Fur seals never drink,
for they absorb the water needed through pores in the
skin.

64. Oxygen. — Animals must have air in order to live,
but the essential element of the air which they need is its
oxygen. For the metabolism of the body, for the chemical

108 ANIMAL LIFE

changes which take place in the body of every living ani-
mal, a supply of oxygen is required. This oxygen is de-
rived directly or indirectly from the air. The atmosphere
of the earth is composed of 79.02 parts of nitrogen (includ-
ing argon), .03 parts of carbonic acid, and 20.95 parts of
oxygen. Thus all the animals which live on land are en-
veloped by a substance containing nearly 21 per cent of
oxygen. But animals can live in an atmosphere containing
much less oxygen. Certain mammals, experimented on,
lived without difficulty in an atmosphere containing only
14 per cent of oxygen ; when the oxygen was reduced to 7
per cent serious disturbances were caused in the animal's
condition, and death by suffocation ensued when 3 per
cent of oxygen was left in the atmosphere. Animals which
live in water get their oxygen, not from the water itself
(water being composed of hydrogen and oxygen), but from
air which is mechanically mixed with the water. Fishes
breathe the air which is mixed with or dissolved in the
water. This scanty supply therefore constitutes their at-
mosphere, for in water from which all air is excluded no
animal can breathe. Whatever the habits of life of the
animal, whether it lives on the land, in the ground, or in
the water, it must have oxygen or die.

65. Temperature, pressure, and other conditions. — Some
physiologists include among the primary or essential gen-
eral conditions of animal life such conditions as favorable
temperature and favorable pressure. It is known from ob-
servation and experiment that animals die when a too low
or a too high temperature prevails. The minimum or
maximum of temperature between which limits an animal
can live varies much among different kinds of animals. It
is familiar knowledge that many kinds of animals can be
frozen and yet not be killed. Insects and other small ani-
mals may lie frozen through a winter and resume active
life again in the spring. An experimenter kept certain
fish frozen in blocks of ice at a temperature of —15° C.

THE PRIMARY CONDITIONS OF ANIMAL LIFE 109

for some time and then gradually thawed them out un-
hurt. Only very hardy kinds adapted to the cold would,
however, survive such treatment. There is no doubt that
every part of the body, all of the living substance, of these
fish was frozen, for specimens at this temperature could be
broken and pounded up into fine ice powder. But a tem-
perature of —20° C. killed the fish. Frogs lived after being
kept at a temperature of —28° C., centipeds at —50° C.,and
certain snails endured a temperature of —120° C. without
dying. At the other extreme, instances are known of ani-
mals living in water (hot springs or water gradually heated
with the organisms in it) of a temperature as high as 50° C.
Experiments with Ammbce show that these simplest animals
contract and cease active motion at 35° C., but are not killed
until a temperature of 40° to 45° C. is reached. The little
fish called blob or miller's thumb ( Coitus ictalops) has been
seen lying boiled in the bottom of the hot springs in the
Yellowstone Park ; but it must have entered these springs
through streams of a temperature little below the boiling
point.

The pressure or weight of the atmosphere on the sur-
face of the earth is nearly fifteen pounds on each square
inch. This pressure is exerted equally in all directions, so
that an object on the earth's surface sustains a pressure on
each square inch of its surface exposed to the air of fifteen
pounds. Thus all animals living on the earth's surface or
near it live under this pressure, and know no other condi-
tion. For this reason they do not notice it. The animals
that live in water, however, sustain a much greater pres-
sure, this pressure increasing with the depth. Certain
ocean fishes live habitually at great depths, as two to five
miles, where the pressure is equivalent to that of many
hundred atmospheres. If these fishes are brought to the
surface their eyes bulge out fearfully, being pushed out
through reduced expansion ; their scales fall off because of
the great expansion of the skin, and the stomach is pushed

HO ANIMAL LIFE

out from the mouth till it is wrong side out. Indeed, the
bodies sometimes burst. Their bodies are accustomed to
this great pressure, and when this outside pressure is sud-
denly removed the body may be bursted. Sometimes
such a fish is raised from its proper level by a struggle
with its prey, when both captor and victim may be de-
stroyed by the expansion of the body. Some fishes die on
being taken out of water through the swelling of the air
bladder and the bursting of its blood-vessels. If an animal
which lives normally on the surface of the earth is taken
up a very high mountain or is carried up in a balloon to a
great altitude where the pressure of the atmosphere is
much less than it is at the earth's surface, serious conse-
quences may ensue, and if too high an altitude is reached
death occurs. This death may be in part due to the diffi-
culty in breathing in sufficient oxygen to maintain life, but
it is probably chiefly due to disturbances caused by the
removal of the pressure to which the body is accustomed
and is structurally adapted to withstand. A famous bal-
loon ascension was made in Paris in 1875 by three men.
After the balloon had reached a height of nearly 24,000
feet (almost five miles) the men began to lose conscious-
ness. On the sinking of the balloon to about 20,000 feet
the men regained consciousness again and threw out bal-
last so that the balloon rose to a height of over 25,000 feet.
This time all three became wholly unconscious, and on the
balloon sinking again only one regained consciousness.
The other two died in the foolhardy experiment. All liv-
ing animals are accustomed to live under a certain pres-
sure, and there are evidently limits of maximum or mini-
mum pressure beyond which no animal at present existing
can go and remain alive.

But in the case both of temperature and pressure con-
ditions it is easy to conceive that animals might exist which
could live under temperature and pressure conditions not
included between the minimum and maximum limits of each

THE PRIMARY CONDITIONS OF ANIMAL LIFE HI

as determined by animals so existing. But it is impossible
to conceive of animals which could live without oxygen or
without organic food. The necessities of oxygen and organic
food (and water) are the primary or essential conditions
for the existence of any animals.

Of course, we might include such conditions, among
the primary conditions, as the light and heat of the sun,
the action of gravitation, and olher physical conditions
without which existence or. life of any kind would be im-
possible on this earth. But we here consider by " primary
conditions of animal life " rather those necessities of living
animals as opposed to the necessities of living plants.
Neither animals nor plants could exist without the sun,
whence they derive directly or indirectly all their energy.

66. Difference between animals and plants. — It is easy to
distinguish between the animal and plant when a butterfly
is fluttering about a blossoming cherry tree or a cow feed-
ing in a field of clover. It is not so easy, if it is, indeed,
possible, to say which is plant and which is animal when
the simplest plants are compared with the simplest ani-
mals. It is almost impossible to so define animals as to
distinguish all of them from all plants, or so to define
plants as to distinguish all of them from all animals.
While most animals have the power of locomotion, some,
like the sponges and polyps and barnacles and numerous
parasites, are fixed. While most plants are fixed, some of
the low aquatic forms have the po sver of spontaneous loco-
motion, and all plants have some power of motion, as espe-
cially exemplified in the revolution of the apex of the
growing stem and root, and the spiral twisting of tendrils,
and in the sudden closing of the leaves of the sensitive
plant when touched. Among the green or chlorophyll-
bearing plants the food consists chiefly of inorganic sub-
stances, especially of carbon which is taken from the car-
bonic-acid gas in the atmosphere, and of water. But some
green-leaved plants feed also in part on organic food.

112 ANIMAL LIFE

Such are the pitcher-plants and sun-dews, and Venus-fly-
traps, which catch insects and use them for food nutrition.
But there are many plants, the fungi, which are not green
—that is, which do not possess chlorophyll, the substance
on which seems to depend the power to make organic
matter out of inorganic substances. These plants feed on
organic matter as animals do. The cells of plants (in their
young stages, at least) have a wall composed of a peculiar
carbohydrate substance called, cellulose, and this cellulose
was for a long time believed not to occur in the body of
animals. But now it is known that certain sea-squirts
(Tunicata) possess cellulose. It is impossible to find any
set of characteristics, or even any one characteristic, which
is possessed only by plants or only by animals. But nearly
all of the many-celled plants and animals may be easily
distinguished by their general characteristics. The power
of breaking up carbonic-acid gas into carbon and oxygen
and assimilating the carbon thus obtained, the presence of
chlorophyll, and the cell walls formed of cellulose, are char-
acteristics constant in all typical plants. In addition, the
fixed life of plants, and their general use of inorganic sub-
stances for food instead of organic, are characteristics
readily observed and practically characteristic of many-
celled plants. When the thousands of kinds of one-celled
organisms are compared, however, it is often a matter of
great difficulty or of real impossibility to say whether a
given organism should be assigned to the plant kingdom
or to the animal kingdom. In general the distinctive
characters of plants are grouped around the loss of the
power of locomotion and related to or dependent upon it.

67. Living organic matter and inorganic matter, — It would
seem to be an easy matter to distinguish an organism — that
is, a living animal or plant— from an inorganic substance. It
is easy to distinguish a dove or a sunflower from stone, and
practically there never is any difficulty in making such dis-
tinctions. But when we try to define living organic matter,

THE PRIMARY CONDITIONS OF ANIMAL LIFE H3

and to describe those characteristics which are peculiar to
it, which absolutely distinguish it from inorganic matter,
we meet with some difficulties. At least many of the char-
acteristics commonly ascribed to organisms, as peculiar to
them, are not so. The possession of organs, or the composi-
tion of the body of distinct parts, each with a distinct func-
tion, but all working together, and depending on each other,
is as true of a steam-engine as of a horse. That the work
done by the steam-engine depends upon fuel is true ; but
so it is that the work done by the horse depends upon fuel,
or food as we call it in the case of the animal. The oxida-
tion or burning of this fuel in the engine is wholly compar-
able with the oxidation of the food, or the muscle and fat
it is turned into, in the horse's body. The composition of
the bodies of animals and plants of tiny structural units,
the cells, is in many ways comparable with the composition
of some rocks of tiny structural units, the crystals. But
not to carry such rather quibbling comparisons too far, it
may be said that organisms are distinguished from organic
substances by the following characteristics : Organization ;
the power to make over inorganic substances into organic
matter, or the changing of organic matter of one kind, as
plant matter, into another kind, as animal matter ; motion,
the power of spontaneous movement in response to stimuli ;
sensation, the power of being sensible of external stimuli ;
reproduction, the power of producing new beings like them-
selves ; and adaptation, the power of responding to external
conditions in a way useful to the organism. Through adap-
tation organisms continue to exist despite the changing of
conditions. If the conditions surrounding an inorganic
body change, even gradually, the inorganic body does not
change to adapt itself to these conditions, but resists them
until no longer able to do so, when it loses its identity or
integrity.

CHAPTER VII

THE CROWD OF ANIMALS AND THE STRUGGLE FOR
EXISTENCE

68. The crowd of animals. — All animals feed upon living
organisms, or on their dead bodies. Hence each animal
throughout its life is busy with the destruction of other
organisms, or with their removal after death. If those
creatures upon which others feed are to hold their own, there
must be enough born or developed to make good the drain
upon their numbers. If the plants did not fill up their
ranks and make good their losses, the animals that feed
on them would perish. If the plant-eating animals were
destroyed, the flesh-eating animals would in turn disappear.
But, fortunately, there is a vast excess in the process of
reproduction. More plants sprout than can find room to
grow. More animals are born than can possibly survive.
The process of increase among animals is correctly spoken
of as multiplication. Each species tends to increase in
geometric ratio, but as it multiplies its members it finds
the world already crowded with other species doing the
same thing. A single pair of any species whatsoever, if not
restrained by adverse conditions, would soon increase to
such an extent as to fill the whole world with its progeny.
An annual plant producing two seeds only would have
1,048,576 descendants at the end of twenty-one years, if
each seed sprouted and matured. The ratio of increase is
therefore a matter of minor importance. It is the ratio of
net increase above loss which determines the fate of a spe-
cies. Those species increase in numbers whose gain exceeds
114

THE STRUGGLE FOR EXISTENCE H5

the death rate, and those which " live beyond their means "
must sooner or later disappear. One of the most abundant
of birds is the fulmar petrel, which lays but one egg yearly.
It has but few enemies, and this low rate of increase suf-
fices to cover the seas within its range with petrels.

It is difficult to realize the inordinate numbers in which
each species would exist were it not for the checks produced
by the presence of other animals. Certain Protozoa at their
normal rate of increase, if none were devoured or destroyed,
might fill the entire ocean in about a week. The conger-
eel lays, it is said, 15,000,000 eggs. If each egg grew
up to maturity and reproduced itself in the same way in
less than ten years the sea would be solidly full of conger-
eels. If the eggs of a common house-fly should develop, and
each of its progeny should find the food and temperature it
needed, with no loss and no destruction, the people of a city in
which this might happen could not get away soon enough to
escape suffocation from a plague of flies. Whenever any in-
sect is able to develop a large percentage of the eggs laid, it
becomes at once a plague. Thus originate plagues of grass-
hoppers, locusts, and caterpillars. But the crowd of life is
such that no great danger exists. The scavenger destroys
the decaying flesh where the fly would lay its eggs. Minute
creatures, insects, bacteria, Protozoa are parasitic within
the larva and kill it. Millions of flies perish for want of
food. Millions more are destroyed by insectivorous birds,
and millions are slain by parasites. The final result is that
from year to year the number of flies does not increase.
Linnaeus once said that " three flies would devour a dead
horse as quickly as a lion." Equally soon would it be de-
voured by three bacteria, for the decay of the horse is due
to the decomposition of its flesh by these microscopic plants
which feed upon it. " Even slow-breeding man," says Dar-
win, " has doubled in twenty-five years. At this rate in less
than a thousand years there would literally not be standing
room for his progeny. The elephant is reckoned the slow-

ANIMAL LIFE

est breeder of all known animals. It begins breeding when
thirty years old and goes on breeding until ninety years
old, bringing forth six young in the interval, and surviving
till a hundred years old. If this be so, after about eight
hundred years there would be 19,000,000 elephants alive,
descended from the first pair." A few years more of the
unchecked multiplication of the elephant and every foot of
land on the earth would be covered by them.

Yet the number of elephants does not increase. In gen-
eral, the numbers of every species of animal in the state of
Nature remain about stationary. Under the influence of
man most of them slowly diminish. There are about as
many squirrels in the forest one year as another, about as
many butterflies in the field, about as many frogs in the
pond. Wolves, bears, deer, wild ducks, singing birds, fishes,
tend to grow fewer and fewer in inhabited regions, because
the losses from the hand of man are added to the losses in
the state of Nature.

It has been shown that at the normal rate in increase of
English sparrows, if none were to die save of old age, it
would take but twenty years to give one sparrow to every
square inch in the State of Indiana. Such an increase is
actually impossible, for more than a hundred other species
of similar birds are disputing the same territory with the
power of increase at a similar rate. There can not be food
and space for all. With such conditions a struggle is set
up between sparrow and sparrow, between sparrow and
other birds, and between sparrow and the conditions of life.
Such a conflict is known as the struggle for existence.

69. The struggle for existence.— The struggle for exist-
ence is threefold: (a) among individuals of one species,
as sparrow and sparrow; (#) between individuals of differ-
ent species, as sparrow with bluebird or robin ; and (c) with
the conditions of life, as the effort of the sparrow to keep
warm in winter and to find water in summer. All three
forms of this struggle are constantly operative and with

THE STRUGGLE FOR EXISTENCE H7

every species. In some regions the one phase may be more
destructive, in others another. Where the conditions of
life are most easy, as in the tropics, the struggle of species
with species, of individual with individual, is the most
severe.

No living being can escape from any of these three
phases of the struggle for existence. For reasons which we
shall see later, it is not well that any should escape, for " the
sheltered life," the life withdrawn from the stress of effort,
brings the tendency to degeneration.

Because of the destruction resulting from the struggle
for existence, more of every species are born than can
possibly find space or food to mature. The majority fail
to reach their full growth because, for one reason or an-
other, they can not do so. All live who can. Each strives
to feed itself, to save its own life, to protect its young.
But with all their efforts only a portion of each species
succeed.

70. Selection by Nature. — But the destruction in Nature
is not indiscriminate. In the long run those least fitted to
resist attack are the first to perish. It is the slowest ani-
mal which is soonest overtaken by those which feed upon
it. It is the weakest which is crowded away from the feed-
ing-place by its associates. It is the least adapted which is
first destroyed by extremes of heat and cold. Just as a
farmer improves his herd of cattle by destroying his weak-
est or roughest calves, reserving the strong and fit for par-
entage, so, on an inconceivably large scale, the forces of
Nature are at work purifying, strengthening, and fitting to
their surroundings the various species of animals. This
process has been called natural selection, or the survival of
the fittest. But by fittest in this sense we mean only best
adapted to the surroundings, for this process, like others in
Nature, has itself no necessarily moral element. The song-
bird becomes through this process more fit for the song-bird
life, the hawk becomes more capable of killing and tear-

ANIMAL LIFE

ing, and the woodpecker better fitted to extract grubs from
the tree.

In the struggle of species with species one may gain a
little one year and another the next, the numbers of each
species fluctuating a little with varying circumstances, but
after a time, unless disturbed by the hand of man, a point
will be reached when the loss will almost exactly balance
the increase. This produces a condition of apparent equi-
librium. The equilibrium is broken when any individual or
group of individuals becomes capable of doing something
more than hold its own in the struggle for existence.

When the conditions of life become adverse to the exist-
ence of a species it has three alternatives, or, better, one of
three things happens, namely, migration, adaptation, extinc-
tion. The migration of birds and some other animals is a
systematic changing of environment when conditions are
unfavorable to life. When the snow and ice come, the fur-
seal forsakes the islands on which it breeds, and which are
its real home, and spends the rest of the year in the open
sea, returning at the close of winter. Some other animals
migrate irregularly, removing from place to place as condi-
tions become severe or undesirable. The Rocky Mountain
locusts, which breed on the great plateau along the eastern
base of the Eocky Mountains, sometimes increase so rapidly
in numbers that they can not find enough food in the scanty
vegetation of this region. Then great hosts of them fly
high into the air until they meet an air current moving
toward the southeast. The locusts are borne by this cur-
rent or wind hundreds of miles, until, when they come to
the great grain-growing Mississippi Valley, they descend
and feed to their hearts' content, and to the dismay of the
Nebraska and Kansas farmer. These great forced migra-
tions used to occur only too often, but none has taken place
since 1878, and it is probable that none will ever occur
again. With the settlement of the Eocky Mountain plateau
by farmers, food is plenty at home. And the constant fight-

THE STRUGGLE FOR EXISTENCE H9

ing of the locusts by the farmers, by plowing up their eggs,
and crushing and burning the young hoppers, keeps down
their numbers.

Another animal of interesting migratory habits is the
lemming, a mouse-like animal nearly as large as a rat, which
lives in the arctic regions. At intervals varying from five
to twenty years the cultivated lands of Norway and Sweden,
where the lemming is ordinarily unknown, are overrun by
vast numbers of these little animals. They come as an
army, steadily and slowly advancing, always in the same
direction, and " regardless of all obstacles, swimming across
streams and even lakes of several miles in breadth, and
committing considerable devastation on their line of march
by the quantity of food they consume. In their turn they
are pursued and harassed by crowds of beasts and birds of
prey, as bears, wolves, foxes, dogs, wild cats, stoats, weasels,
eagles, hawks, and owls, and never spared by man ; even
the domestic animals not usually predaceous, as cattle,
foals, and reindeer, are said to join in the destruction,
stamping them to the ground with their feet and even eat-
ing their bodies. Numbers also die from disease apparently
produced from overcrowding. None ever return by the
course by which they came, and the onward march of the
survivors never ceases until they reach the sea, into which
they plunge, and swimming onward in the same direction
as before perish in the waves." One of these great migra-
tions lasts for from one to three years. But it always ends
in the total destruction of the migrating army. But the
migration may be of advantage to the lemmings which re-
main in the original breeding grounds, leaving them with
enough food, so that, on the whole, the migration results in
gain to the species.

But most animals can not migrate to their betterment.
In that case the only alternatives are adaptation or destruc-
tion. Some individuals by the possession of slight advan-
tageous variations of structure are able to meet the new

120 ANIMAL LIFE

demands and survive, the rest die. The survivors produce
young similarly advantageously different from the general
type, and the adaptation increases with successive genera-
tions.

71. Adjustment to surroundings a result of natural selec-
tion.— To such causes as these we must ascribe the nice
adjustment of each species to its surroundings. If a species
or a group of individuals can not adapt itself to its environ-
ment, it will be crowded out by others that can do so. The
former will disappear entirely from the earth, or else will be
limited to surroundings with which it comes into perfect
adjustment. A partial adjustment must with time become
a complete one, for the individuals not adapted will be
exterminated in the struggle for life. In this regard very
small variations may lead to great results. A side issue
apparently of little consequence may determine the fate of
a species. Any advantage, no matter how small, will turn
the scale of life in favor of its possessor and his progeny.
"Battle within battle," says a famous naturalist, "must be
continually recurring, with varying success. Yet in the
long run the forces are so nicely balanced that the face of
Nature remains for a long time uniform, though assuredly
the merest trifle would give the victory to one organic being
over another."

72. Artificial selection. — It has been long known that the
nature of a herd or race of animals can be materially altered
by a conscious selection on the part of man of these indi-
viduals which are to become parents. To " weed out " a
herd artificially is to improve its blood. To select for re-
production the swiftest horses, the best milk cows, the most
intelligent dogs, is to raise the standard of the herd or
race in each of these respects by the simple action of hered-
ity. Artificial selection has been called the "magician's
wand," by which the breeder can summon up whatever
animal form he will. If the parentage is chosen to a defi-
nite end, the process of heredity will develop the form

THE STRUGGLE FOR EXISTENCE 121

desired by a force as unchanging as that by which a stream
turns a mill.

From the wild animals about him man has developed
the domestic animals which he finds useful. The dog
which man trains to care for his sheep is developed by
selection from the most tractable progeny of the wolf which
once devoured his flocks. By the process of artificial selec-
tion those individuals that are not useful to man or pleas-
ing to his fancy have been destroyed, and those which con-
tribute to his pleasure or welfare have been preserved and
allowed to reproduce their kind. The various fancy breeds
of pigeons — the carriers, pouters, tumblers, ruff-necks, and
fan-tails — are all the descendants of the wild dove of Eu-
rope (Columba livia). These breeds or races or varieties
have been produced by artificial selection. So it is with
the various breeds of cattle and of hogs and of horses
and dogs.

In this artificial selection new variations are more rap-
idly produced than in Nature by means of intercrossing
different races, and by a more rapid weeding out of un-
favorable— that is, of undesirable — variations. The rapid
production of variations and the careful preservation of
the desirable ones and rigid destruction of undesirable
ones are the means by which many races of domestic ani-
mals are produced. This is artificial selection.

73. Dependence of species on species.— There was intro-
duced into California from Australia, on young orange trees,
a few years ago, an insect pest called the cottony cushion
scale (Iccrya purchasi). This pest increased in numbers
with extraordinary rapidity, and in four or five years threat-
ened to destroy completely the great orange orchards of
California. Artificial remedies were of little avail. Finally,
an entomologist was sent to Australia' to find out if this
scale insect had not some special natural enemy in its
native country. It was found that in Australia a certain
species of lady-bird beetle attacked and fed on the cottony

122 ANIMAL LIFE

cushion scales and kept them in check. Some of these
lady-birds ( Vedalia cardinalis) were brought to California
and released in .a scale-infested orchard. The lady-birds,
having plenty of food, thrived and produced many young.
Soon the lady-birds were in such numbers that numbers of
them could be distributed to other orchards. In two or
three years the Vedalias had become so numerous and
widely distributed that the cottony cushion scales began to
diminish perceptibly, and soon the pest was nearly wiped
out. But with the disappearance of the scales came also a
disappearance of the lady-birds, and it was then discovered
that the Vedalias fed only on cottony cushion scales and
could not live where the scales were not. So now, in order
to have a stock of Vedalias on hand in California it is neces-
sary to keep protected some colonies of the cottony cushion
scale to serve as food. Of course, with the disappearance
of the predaceous lady-birds the scale began to increase
again in various parts of the State, but with the sending of
Vedalias to these localities the scale was again crushed.
How close is the interdependence of these two species !

Similar relations can be traced in every group of ani-
mals. When the salmon cease to run in the Sacramento
River in California the otter which feeds on them takes, it
is said, to robbing the poultry-yards ; and the bear, which
also feeds on fish, strikes out for other game, taking fruit
or chickens or bee-hives, whatever he may find.

CHAPTER VIII

ADAPTATIONS

74. Origin of adaptations. — The strife for place in the
crowd of animals makes it necessary for each one to adjust
itself 'to the place it holds. As the individual becomes
fitted to its condition, so must the species as a whole. The
species is therefore made up of individuals that are fitted
or may become fitted for the conditions of life. As the
stress of existence becomes more severe, the individuals fit
to continue the species are chosen more closely. This
choice is the automatic work of the conditions of life, but
it is none the less effective in its operations, and in the
course of centuries it becomes unerring. When conditions
change, the perfection of adaptation in a species may be
the cause of its extinction. If the need of a special fitness
can not be met immediately, the species will disappear.
For example, the native sheep of England have developed
a long wool fitted to protect them in a cool, damp climate.
Such sheep transferred to Cuba died in a short time, leav-
ing no descendants. The warm fleece, so useful in Eng-
land, rendered them wholly unfit for survival in the tropics.
It is one advantage of man, as compared with other forms
of life, that so many of his adaptations are external to his
structure, and can be cast aside when necessity arises.

75. Classification of adaptations.— The various forms of
adaptations may be roughly divided into five classes, as fol-
lows : (a) food securing, (#) self-protection, (c) rivalry, (d)
defense of young, (e) surroundings.

The few examples which are given under each class,

123

124 ANIMAL LIFE

some of them striking, some not especially so, are mostly
chosen from the vertebrates and from the insects, because
these two groups of animals are the groups with which be-
ginning students of zoology are likely to be familiar, and
the adaptations referred to are therefore most likely to be
best appreciated. Quite as good and obvious examples could
be selected from any other groups of animals. The student

FIG. 54.— The deep-sea angler (C'oiynolophus reinhardti), which has a dorsal spine
modified to be a luminous "fishing rod and lure," attracting lantern-fishes
(Echiostoma and ^Ethophora). An extraordinary adaptation for securing food.
(The angler is drawn after a figure of LUTKEN'S.)

will find good practice in trying to discover examples shown
by the animals with which he may be familiar. That all
or any part of the body structure of any animal can be
called with truth an example of adaptation is plain from
what we know of how the various organs of the animal
body have come to exist. But by giving special attention
to such adaptations as are plainly obvious, beginning stu-

ADAPTATIONS

125

dents may be put in the
way of independent ob-
servation along an ex-
tremely interesting and
attractive line of zoolog-
ical study.

76. Adaptations for
securing food. — For the
purpose of capture of
their prey, some carniv-
orous animals are pro-
vided with strong claws,
sharp teeth, hooked
beaks, and other struc-
tures familiar to us in
the lion, tiger, dog, cat,
owl, and eagle. Insect-
eating mammals have
contrivances especially

FIG. 55.— The brown pelican, showing gular
sac, which it uses in catching and holding
fishes that form its food.

FIG. 56. — Foot of the bald eagle, show-
ing claws for seizing its prey.
(CH4PMAN.)

adapted for the catching of insects. The
ant-eater, for example, has a
curious, long sticky tongue
which it thrusts forth from
its cylindrical snout deep
into the recesses of the ant-
hill, bringing it out with its
sticky surface covered with
ants. Animals which feed on
nuts are fitted with strong
teeth or beaks for crack-
ing them. Similar teeth are
found in those fishes which
feed on crabs, snails, or sea-ur-
chins. Those mammals like
the horse and cow, that
feed on plants, have usually

FIG. 57.— Giraffes feeding.

ADAPTATIONS

127

broad chisel-like incisor teeth for cutting off the foliage,
and teeth of very similar form are developed in the dif-
ferent groups of plant-
eating fishes. Molar
teeth are found when it

FIG. 58.— Scorpion, showing the special devel-
opment of certain mouth parts (the maxil-
lary palpi) as pincer-like organs for grasp-
ing prey. At the posterior tip of the body
is the poisonous sting.

FIG. 59.— Head of mosquito (fe-
male), showing the piercing
needle-like mouth parts which
compose the "bill."

is necessary that the food should be crushed or chewed,

and the sharp canine teeth go with a flesh diet. The

long neck of the giraffe

(Fig. 57) enables it to

browse on the foliage of

trees.

Insects like the leaf-
beetles and the grasshop-
pers, that feed on the
foliage of plants, have a

. . . FIG. 60.— The praying-horse (Mantis) with

pair Of jaWS, broad but fore legs developed as grasping organs.

128

ANIMAL LIFE

sharply edged, for cutting off bits of leaves and stems.
Those which take only liquid food, as the butterflies and
sucking-bugs, have their mouth parts modified to form a
slender, hollow sucking beak or proboscis, which can be

thrust into a flower nectary,
or into the green tissue of
plants or the flesh of animals,
to suck up nectar or plant sap
or blood, depending on the
special food habits of the in-
sect. The honey-bee has a
very complicated equipment
of mouth parts fitted for tak-
ing either solid food like pol-
len, or liquid food like the
nectar of flowers. The mos-
quito has a "bill" (Fig. 59)
composed of six sharp, slender
needles for piercing and lac-
erating the flesh, and a long
tubular under lip through
which the blood can flow into
the mouth. Some predaceous
insects, as the praying-horse
(Fig. 60), have their fore legs
developed into formidable
grasping organs for seizing and
holding their prey.

77. Adaptation for self-de-
fense.— For self -protection, car-

FIG. 61.— Acorns put into bark of tree . r

by the caiifomian woodpecker nivorous animals use the same

(Melanerpes formicivorus bairdii). weapOnS to defend themselves
— From photograph, Stanford Uni- . .

versity, California. which Serve to SCCUre their

prey; but these as well as

other animals may protect themselves in other fashions.
Most of the hoofed animals are provided with horns, struc-

ADAPTATIONS

129

FIG. 62.— Section of bark of live oak tree with acorns placed in it by the Californian
woodpecker (Melanerpes formicivorus bairdii).— From photograph, Stanford
University, California.

tures useless in procuring food but often of great effective-
ness as weapons of defense. To the category of structures
useful for self-defense belong the many peculiarities of col-
oration known as "recognition marks." These are marks,
10

130

ANIMAL LIFE

not otherwise useful, which, are supposed to enable mem-
bers of any one species to recognize their own kind among
the mass of animal life. To this category belongs the
black tip of the weasel's tail, which re-
mains the same whatever the changes
in the outer fur. Another example is
seen in the white outer feathers of the
tail of the meadow-lark as well as in
certain sparrows and warblers. The
white on the skunk's back and tail
serves the same purpose and also as a
warning. It is to the skunk's advan-
tage not to be hidden, for to be seen in
the crowd of animals is to be avoided
by them. The songs of birds and the
calls of -various creatures serve also as
recognition marks. Each species knows
and heeds its own characteristic song
or cry, and it is a source of mutual
protection. The fur-seal pup knows
its mother's call, even though ten thou-
sand other mothers are calling on the
rookery.

The ways in which animals make
themselves disagreeable or dangerous
to their captors are almost as varied as the animals them-
selves. Besides the teeth, claws, and horns of ordinary
attack and defense, we find among the mammals many
special structures or contrivances which serve for de-
fense through making their possession unpleasant. The
scent glands of the skunk and its relatives are noticed
above. The porcupine has the bristles in its fur specialized
as quills, barbed and detachable. These quills fill the
mouth of an attacking fox or wolf, and serve well the pur-
pose of defense. The hedgehog of Europe, an animal of
different nature, being related rather to the mole than to

FIG. 63.— Centiped. The
foremost pair of legs is
modified to be a pair of
seizing and stinging or-
gans. An adaptation
for self-defense and for
securing food.

ADAPTATIONS

131

the squirrel, has a similar armature of quills. The armadillo
of the tropics has movable shields, and when it withdraws its

FIG. 64. — Flying fishes. (The upper one a species of Cyp&durus, the lower of Exocce,-
tus.) These fishes escape from their enemies by leaping into the air and sailing
or "flying" long distances.

head (which is also defended by a bony shield) it is as well
protected as a turtle.

FIG. 65.— The horned toad (Phi~ynosoma blainvillei). The spiny covering repels many

enemies.

Special organs for defense of this nature are rare among
birds, but numerous among reptiles. The turtles are all

132

ANIMAL LIFE

protected by bony shields, and some of them, the box-tur-
tles, may close their shields almost hermetically. The
snakes broaden their heads, swell their necks, or show their
forked tongues to frighten their enemies. Some of them

FIG. 66.— Nokee or poisonous scorpion-fish (Emmydrichthys vulcanus) with poison-
ous spines, from Tahiti.

are further armed with fangs connected with a venom gland,
so that to most animals their bite is deadly. Besides its
fangs the rattlesnake has a rattle on the tail made up of a

FIG. 67. — Mad torn (Schilbeodes furiosus) with poisoned pectoral spine.

succession of bony clappers, modified vertebrae, and scales,
by which intruders are warned of their presence. This
sharp and insistent buzz is a warning to animals of other
species and a recognition signal to those of its own kind.

ADAPTATIONS 133

Even the fishes have many modes of self-defense through
giving pain or injury to those who would swallow them.
The cat-fishes or horned pouts when attacked set immov-
ably the sharp spine of the
pectoral fin, inflicting a
jagged wound. Pelicans
who have swallowed a cat-
fish have heen known to
die of the wounds inflicted
hy the fish's spine. In
the group of scorpion-
fishes and toad-fishes are
certain genera in which
these spines are provided
with poison glands. These
may inflict very severe
wounds to other fishes, or
even to birds or man. One of this group
of poison-fishes is the nokee (Emmydrich-
thys, Fig. 66). A group of small fresh-
water cat-fishes, known as the mad toms
(Fig. 67), have also a poison gland attached
to the pectoral spine, and its sting is most
exasperating, like the sting of a wasp.
The sting-rays (Fig. 68) of many species FIG. ea— A sting-ray
have a strong, jagged spine on the tail, JEXLT**
covered with slime, and armed with broad
saw -like teeth. This inflicts a dangerous wound, not
through the presence of specific venom, but from the dan-
ger of blood poisoning arising from the slime, and the
ragged or unclean cut.

Many fishes are defended by a coat of mail or a coat of
sharp thorns. The globe-fishes and porcupine-fishes (Fig.
69) are for the most part defended by spines, but their
instinct to swallow air gives them an additional safeguard.
"When ojie of these fishes is disturbed it rises to the surface,

134

ANIMAL LIFE

gulps air until its capacious stomach is filled, and then
floats belly upward on the surface. It is thus protected
from other fishes, though easily taken by man. The torpe-
do, electric eel, electric cat-fish, and star-gazer, surprise and

stagger their captors by
means of electric shocks.
In the torpedo or electric
ray (Fig. 70), found on
the sandy shores of all
warm seas, on either side
of the head is a large
honeycomb-like structure
which yields a strong
electric shock whenever
the live fish is touched.
This shock is felt severe-
ly if the fish be stabbed
with a knife or metallic
spear. The electric eel
of the rivers of Para-
guay and southern Bra-
zil is said to give severe
shocks to herds of wild
horses driven through
the streams, and similar
accounts are given of the
electric cat-fish of the
Nile.

Among the insects,

one floating belly upward, with inflated the possession of stingS

SS^aWD fr°m 8PeCimen8 ^ ^ is not uncommon. The

wasps and bees are fa-
miliar examples of stinging insects, but many other kinds,
less familiar, are similarly protected. All insects have
their bodies covered with a coat of armor, composed of a
horny substance called chitin. In some cases this chitin-

FIG. 69. — Porcupine-fish (Diodon hystrix), the
lower ones swimming normally, the upper

ADAPTATIONS

135

ous coat is very thick and serves to protect them effectu-
ally. This is especially true of the beetles. Some insects
are inedible (as mentioned in Chapter XII), and are con-
spicuously colored so as to be readily recognized by in-
sectivorous birds. The birds, knowing by experience that
these insects are ill-tasting, avoid them. Others are ef-
fectively concealed from their enemies by their close
resemblance in color and marking to their surroundings.
These protective resem-
blances are discussed in
Chapter XII.

78. Adaptation for rivalry.
— In questions of attack and
defense, the need of meeting
animals of their own kind as
well as animals of other
races must be considered. In
struggles of species with
those of their own kind, the
term rivalry may be applied.
Actual warfare is confined
mainly to males in the breed-
ing season and to polyga-
mous animals. Among those
in which the male mates
with many females, he must
struggle with other males for
their possession. In all the
groups of vertebrates the
sexes are about equal in num-
bers. Where mating exists,
either for the season or for

life, this condition does not involve serious struggle or
destructive rivalry.

Among monogamous birds, or those which pair, the
male courts the female of his choice by song and by display

FIG. 70.— Torpedo or electric ray (Nar-
cine brasiliensis), showing electric
cells.

ADAPTATIONS

137

of his bright feathers. The female consents to be chosen
by the one which pleases her. It is believed that the hand-
somest, most vivacious, and most musical males are the
ones most successful in such courtship. With polygamous
animals there is intense rivalry among the males in the
mating season, which in almost all species is in the spring.
The strongest males survive and reproduce their strength.
The most notable adaptation is seen in the superior size
of teeth, horns, mane, or spurs. Among the polygamous
fur seals (Fig. 71) and sea lions the male is about four times

FIG. 72.— A wild duck (Ay thy a) family. Male, female, and prsecocial young.

the size of the female. In the polygamous family of deer,
buffalo, and the domestic cattle and sheep, the male is larger
and more powerfully armed than the female. In the polyg-
amous group to which the hen, turkey, and peacock belong
the males possess the display of plumage, and the structures
adapted for fighting, with the will to use them.

79. Adaptations for the defense of the young. — The pro-
tection of the young is the source of many adaptive struc-
tures as well as of the instincts by which such structures are

138

ANIMAL LIFE

utilized. In general, those animals are highest in develop-
ment, with best means of holding their own in the struggle

FIG. 73.— The altricial nestlings of the Blue jay (Cyanocitta

for life, that take best care of their young. The homes
of animals are elsewhere specially discussed (see Chapter

ADAPTATIONS

139

XV), but those instincts which lead to home-building
may all be regarded as useful adaptations in preserving the
young. Among the lower or more coarsely organized

IG. 74.— Kangaroo (Macropus rufus) with youug iu pouch.

140

ANIMAL LIFE

birds, such as the chicken, the duck, and the auk, as with
the reptiles, the young animal is hatched with well-devel-
oped muscular system and sense
organs, and is capable of running
about, and, to some extent, of feed-
ing itself. Birds of this type are
known as prcecocial (Fig. 72), while
the name altricial (Fig. 73) is ap-
plied to the more highly organized
forms, such as the thrushes, doves,
and song-birds generally. With
these the young are hatched in a
wholly helpless condition, with in-
effective muscles, deficient senses,
and dependent wholly upon the
parent. The altricial condition de-
mands the building of a nest, the
establishment of a home, and the
continued care of one or both of

lata) cut open to show young the parents.

^atoSn" TheverylowestmammaUknown,
the duck-bills (Monotremes) of

Australia, lay large eggs in a strong shell like those of a
turtle, and guard them with great jealousy. But with
almost all mammals the egg is very small and without
much food-yolk. The egg begins its development within
the body. It is nourished by the
blood of the mother, and after birth
the young is cherished by her, and
fed by milk secreted by specialized
glands of the skin. All these features
are adaptations tending toward the
preservation of the young. In the

division of mammals next lowest to the Monotremes — the
kangaroo, opossum, etc. — the young are born in a very im-
mature state and are at once seized by the mother and

ill!

FIG. 76.— Egg-case of the cock-
roach.

ADAPTATIONS

141

thrust into a pouch or fold of skin along the abdomen,
where they are kept until they are able to take care of
themselves (Fig. 74). This is an interesting and ingenious
adaptation, but less specialized and
less perfect an adaptation than the
conditions found in ordinary mam-
mals.

Among the insects, the special
provisions for the protection and
care of the eggs and the young are
wide-spread and various. Some of
those adaptations which take the
special form of nests or "homes"
will be described in a later chapter

(see Chapter XV). The eggS of FIG- 77.— Giant water-bug (Ser-

the common cockroach are laid in ^£ J£le carrying egg8
small packets inclosed in a firm wall

(Fig. 76). The eggs of the great water-bugs are carried on
the back of the male (Fig. 77) ; and the spiders lay their
eggs in a silken sac or cocoon, and some of the ground or

FIG. 78.— Cocoon inclosing the pupa of the great Ceanothus moth. Spun of silk by the
larva before pupation.

running spiders (Lycosidcp) drag this egg-sac, attached to
the tip of the abdomen, about with them. The young
spiders when hatched live for some days inside this sac,
feeding on each other! Many insects have long, sharp,

142

ANIMAL LIFE

piercing ovipositors, by means of which the eggs are de-
posited in the ground or in the leaves or stems of green
plants, or even in the hard wood of tree-trunks. Some of

the scale insects se-
crete wax from their
bodies and form a
large, often beautiful •
egg-case, attached to
and nearly covering the body in
which eggs are deposited (Fig.
79). The various gall insects lay
their eggs in the soft tissue of
plants, and on the hatching of
the larvae an abnormal growth
of the plant occurs about the
young insect, forming an in-
closing gall that serves not only
to protect the insect within,
but to furnish it with an abun-
dance of plant-sap, its food. The
young insect remains in the gall
until it completes its develop-
ment and growth, when it
gnaws its way out. Such insect galls are especially abun-
dant on oak trees (Fig. 80). The care of the eggs and the
young of the social insects, as the bees and ants, are de-
scribed in Chapter IX.

FIG. 79. — The cottony cushion scale
insect (Icerya purchasi), from
California. The male is winged,
the female wingless and with a
large waxen egg-sac (e.s.) attached
to her body. (The lines at the left
of each figure indicate the size of
the insects.)

ADAPTATIONS 143

80. Adaptations concerned with surroundings in life. — A

large part of the life of the animal is a struggle with the
environment itself; in this struggle only those that are
adapted live and leave descendants fitted like themselves.
The fur of mammals fits them to their surroundings. As
the fur differs, so may the habits change. Some animals
are active in winter ; others, as the bear, hibernate, sleep-
ing in caves or hollow trees or in burrows until conditions
are favorable for their activity. Most snakes and lizards
hibernate in cold weather. In the swamps of Louisiana,

FIG. 80.— The giant gall of the white oak (California), made by the gall insect Andri-
cus californicus. The gall at the right cut open to show tunnels made by the
insects in escaping from the gall. — From photograph.

in winter, the bottom may often be seen covered with water
snakes lying as inert as dead twigs. Usually, however,
hibernation is accompanied by concealment. Some animals
in hibernation may be frozen alive without apparent injury.
The blackfish of the Alaska swamps, fed to dogs when
frozen solid, has been known to revive in the heat of the
dog's stomach and to wriggle out and escape. As animals
resist heat and cold by adaptations of structure or habits,
so may they resist dryness. Certain fishes hold reservoirs

144 ANIMAL LIFE

of water above their gills, by means of which they can
breathe during short excursions from the water. Still
others (mud-fishes) retain the primitive lung-like structure
of the swim-bladder, and are able to breathe air when, in the
dry season, the water of the pools is reduced to mud.

Another series of adaptations is concerned with the
places chosen by animals for their homes. The fishes that
live in water have special organs for
breathing under water (Fig. 82).
Many of the South American mon-
keys have the tip of the tail adapted
for clinging to limbs of trees or to
the bodies of other monkeys of its
own kind. The hooked claws of the
bat hold on to rocks, the bricks of
chimneys, or to the surface of hollow
trees where the bat sleeps through
the day. The tree-frogs (Fig. 83) or
tree-toads have the tips of the toes
swollen, forming little pads by which
they cling to the bark of trees.

Among other adaptations relat-
ing to special surroundings or con-
ditions of life are the great cheek
pouches of the pocket gophers,
which carry off the soil dug up by
the large shovel-like feet when the
gopher excavates its burrow.

Those insects which live under-

FIQ. 81.— Insect galls on leaf.

ground, making burrows or tunnels

in the soil, have their legs or other parts adapted for dig-
ging and burrowing. The mole cricket (Fig. 84) has its
legs stout and short, with broad, shovel-like feet. Some
water-beetles (Fig. 85) and water-bugs have one or more of
the pairs of legs flattened and broad to serve as oars or pad.
dies for swimming. The grasshoppers or locusts, who leap,

ADAPTATIONS

145

have their hind legs greatly enlarged and elon-
gated, and provided with strong muscles, so as
to make of them "leaping legs." The grubs

FIG. 82. — Head of rainbow tront (Salmo irideus) with gill cover bent back to show
gills, the breathing organs.

or larvae of beetles which live as " borers " in tree-trunks
have mere rudiments of legs, or none at all (Fig. 86).
They have great, strong, biting jaws for cutting away
the hard wood. They move simply by wriggling along
in their burrows or tunnels.

Insects that live
in water either come
up to the surface to
breathe or take down
air underneath their
wings, or in some
other way, or have
gills for breathing the
air which is mixed
with the water. These
gills are special adap-
tive structures which present a great variety of form and
appearance. In the young of the May-flies they are deli-
cate plate-like flaps projecting from the sides of the body.
They are kept in constant motion, gently waving back and
11

FIG. 83.— Tree-toad (Hyla regilla).

146

ANIMAL LIFE

forth in the water so as to maintain currents to bring fresh
water in contact with them. Young mosquitoes (Fig. 87)

do not have gills, but come
up to the surface to breathe.
The larvae, or wrigglers,
breathe through a special

FIG. 84.— The mole cricket (Gryllotalpa),
with fore feet modified for digging.

FIG. 85.— A water-beetle (Hydroph-
Uus).

tube at the posterior tip of the body, while the pupae have
a pair of horn-like tubes on the back of the head end of
the body.

81. Degree of structural change in adaptations. — While
among the higher or vertebrate animals, especially the
fishes and reptiles, most remarkable cases of adaptations
occur, yet the structural changes are for the most part ex-
ternal, never seriously affecting the development of the
internal organs other
than the skeleton. The
organization of these
higher animals is much
less plastic than among
the invertebrates. In
general, the higher the type the more persistent and un-
changeable are those structures not immediately exposed

FIG. 86.— Wood-boring beetle larva (Prionus).

ADAPTATIONS

147

to the influence of the struggle for existence. It is thus
the outside of an animal that tells where its ancestors
have lived. The inside, suffering little change, whatever
the surroundings, tells the real nature of the animal.

82. Vestigial organs. — In general, all the peculiarities of
animal structure find their explanation in some need of
adaptation. When this need ceases, the structure itself
tends to disappear or else to serve some other need. In
the bodies of most animals there are certain incomplete
or rudimentary organs
or structures which
serve no distinct use-
ful purpose. They are
structures which, in the
ancestors of the ani-
mals now possessing
them, were fully devel-
oped functional organs,
but which, because of a
change in habits or con-
ditions of living, are of
no further need, and
are gradually dying out.
Such organs are called
vestigial organs. Ex-
amples are the disused
ear muscles of man, the
vermiform appendix in
man, which is the reduced and now useless anterior end
of the large intestine. In the lower animals, the thumb or
degenerate first finger of the bird with its two or three little
quills serves as an example. So also the reduced and elevated
hind toe of certain birds, the splint bones or rudimentary
side toes of the horse, the rudimentary eyes of blind fishes,
the minute barbel or beard of the horned dace or chub, and
the rudimentary teeth of the right whales and sword-fish.

FIG. 87. — Young stages of the mosquito.
a, larva (wriggler) ; b, pupa.

148

ANIMAL LIFE

Each of these vestigial organs tells a story of some past
adaptation to conditions, one that is no longer needed in
the life of the species. They have the same place in the
study of animals that silent letters have in the study of
words. For example, in our word knight the Ic and gh are
no longer sounded ; but our ancestors used them both, as
the Germans do to-day in their cognate word Kneclit. So
with the French word temps, which means time, in which
both p and s are silent. The Eomans, from whom the
French took this word, needed all its letters, for they spelled
and pronounced it tempus. In general, every silent letter
in every word was once sounded. In like manner, every
vestigial structure was once in use and helpful or necessary
to the life of the animal which possessed it.

:1

Horns of two male deer interlocked while fighting. Permission of G. O. SHIELDSS
publisher of Recreation.

CHAPTER IX

ANIMAL COMMUNITIES AND SOCIAL LIFE

83. Man not the only social animal — Man is commonly
called the social animal, but he is not the only one to
which this term may be applied. There are many others
which possess a social or communal life. A moment's
thought brings to mind the familiar facts of the communal
life of the honey-bee and of the ants. And there are many
other kinds of animals, not so well known to us, that live
in communities or colonies, and live a life which in greater
or less degree is communal or social. In this connection
we may use the term communal for the life of those ani-
mals in which the division of labor is such that the indi-
vidual is dependent for its continual existence on the com-
munity as a whole. The term social life would refer to a
lower degree of mutual aid and mutual dependence.

84. The honey-bee. — Honey-bees live together, as we
know, in large communities. We are accustomed to think
of honey-bees as the inhabitants of bee-hives, but there
were bees before there were hives. The "bee-tree" is
familiar to many of us. The bees, in Nature, make their
home in the hollow of some dead or decaying tree-trunk,
and carry on there all the industries which characterize
the busy communities in the hives. A honey-bee com-
munity comprises three kinds of individuals (Fig. 88) —
namely, a fertile female or queen, numerous males or
drones, and many infertile females or workers. These
three kinds of individuals differ in external appearance
sufficiently to be readily recognizable. The workers are

149

ANIMAL LIFE

smaller than the queens and drones, and the last two differ
in the shape of the abdomen, or hind body, the abdomen of
the queen being longer and more slender than that of the

FIG. 88.— Honey-bee, a, drone or male ; b, worker or infertile female ; c, queen or
fertile female.

male or drone. In a single community there is one queen,
a few hundred drones, and ten to thirty thousand workers.
The number of drones and workers varies at different
times of the year, being smallest in winter. Each kind of
individual has certain work or business to do for the whole
community. The queen lays all the eggs from which new
bees are born; that is, she is the mother of the entire
community. The drones or males have simply to act as
royal consorts ; upon them depends the fertilization of the
eggs. The workers undertake all the food-getting, the
care of the young bees, the comb-building, the honey-mak-
ing— all the industries with which we are more or less
familiar that are carried on in the hive. And all the
work done by the workers is strictly work for the whole
community ; in no case does the worker bee work for itself
alone ; it works for itself only in so far as it is a member
of the community.

How varied and elaborately perfected these industries
are may be perceived from a brief account of the life his-
tory of a bee community. The interior of the hollow in
the bee-tree or of the hive is filled with " comb " — that is,
with wax molded into hexagonal cells and supports for
these cells. The molding of these thousands of symmet-'

ANIMAL COMMUNITIES AND SOCIAL LIFE 151

rical cells is accomplished by the workers by means of their
specially modified trowel-like mandibles or jaws. The wax
itself, of which the cells are made, comes from the bodies
of the workers in the form of small
liquid drops which exude from the skin
on the under side of the abdomen or
hinder body rings. These droplets
run together, harden and become flat-
tened, and are removed from the wax
plates, as the peculiarly modified parts
of the skin which produce the wax
are called, by means of the hind legs,
which are furnished with scissor-like
contrivances for cutting off the wax
(Fig. 89). In certain of the cells are
stored the pollen and honey, which
serve as food for the community. The
pollen is gathered by the workers from
certain favorite flowers and is carried
by them from the flowers to the hive
in the "pollen baskets," the slightly
concave outer surfaces of one of the
segments of the broadened and flattened
hind legs. This concave surface is lined
on each margin with a row of incurved

,./»-,. -U • i_ i_ i j j_i -n FIG. 89. — Posterior leg of

Stin nairS WniCn hold, the pollen maSS worker honey-bee. The

securely in place (Fig. 89). The " honey "

is the nectar of flowers which has been

sucked up by the workers by means of

their elaborate lapping and sucking

mouth parts and swallowed into a sort

of honey-sac or stomach, then brought

to the hive and regurgitated into the

cells. This nectar is at first too watery to be good

honey, so the bees have to evaporate some of this water.

Many of the workers gather above the cells containing

concave surface of the
upper large joint with
the marginal hairs is
the pollen basket ; the
wax shears are the cut-
ting surfaces of the
angle between the two
large segments of the
leg.

152 ANIMAL LIFE

nectar, and buzz — that is, vibrate their wings violently.
This creates currents of air which pass over the exposed
nectar and increase the evaporation of the water. The
violent buzzing raises the temperature of the bees' bodies,
and this warmth given off to the air also helps make evap-
oration more rapid. In addition to bringing in food the
workers also bring in, when necessary, " propolis," or the
resinous gum of certain trees, which they use in repairing
the hive, as closing up cracks and crevices in it.

In many of the cells there will be found, not pollen or
honey, but the eggs or the young bees in larval or pupal

condition (Fig. 90).
The queen moves
about through the
hive, laying eggs.
She deposits only one
egg in a cell. In
three days the egg
hatches, and the
young bee appears
as a helpless, soft,
white, -footless grub

FIG. 90.— Cells containing eggs, larvse, and pupa; of or larva. It is Cared
the honey-bee. The lower large, irregular cells . -, pprtain nf ihp
are queen cells.-After BENTON. W C

workers, that may be

called nurses. These nurses do not differ structurally from
the other workers, but they have the special duty of caring
for the helpless young bees. They do not go out for pollen
or honey, but stay in the hive. They are usually the new
bees — i. e., the youngest or most recently added workers.
After they act as nurses for a week or so they take their
places with the food-gathering workers, and other new
bees act as nurses. The nurses feed the young or larval
bees at first with a highly nutritious food called bee-jelly,
which the n arses make in their stomach, and regurgitate
for the Iarv96. After the larvas are two or three days old

ANIMAL COMMUNITIES AND SOCIAL LIFE 153

they are fed with pollen and honey. Finally, a small mass
of food is put into the cell, and the cell is " capped " or
covered with wax. The larva, after eating all the food, in
two or three days more changes into a pupa, which lies
quiescent without eating for thirteen days, when it changes
into a full-grown bee. The new bee breaks open the cap
of the cell with its jaws, and comes out into the hive, ready
to take up its share of the work for the community. In a
few cases, however, the life history is different. The nurses
will tear down several cells around some single one, and
enlarge this inner one into a great irregular vase-shaped
cell. When the egg hatches, the grub or larva is fed bee-
jelly as long as it remains a larva, never being given ordi-
nary pollen and honey at all. This larva finally pupates,
and there issues from the pupa not a worker or drone bee,
but a new queen. The egg from which the queen is pro-
duced is the same as the other eggs, but the worker nurses
by feeding the larva only the highly nutritious bee-jelly
make it certain that the new bee shall become a queen
instead of a worker. It is also to be noted that the male
bees or drones are hatched from eggs that are not ferti-
lized, the queen having it in her power to lay either ferti-
lized or unfertilized eggs. From the fertilized eggs hatch
larvae which develop into queens or workers, depending on
the manner of their nourishment; from the unfertilized
eggs hatch the males.

When several queens appear there is much excitement
in the community. Each community has normally a single
one, so that when additional queens appear some rearrange-
ment is necessary. This rearrangement comes about first
by fighting among the queens until only one of the new
queens is left alive. Then the old or mother queen issues
from the hive or tree followed by many of the workers.
She and her followers fly away together, finally alighting
on some tree branch and massing there in a dense swarm.
This is the familiar phenomenon of "swarming." The

154: ANIMAL LIFE

swarm finally finds a new hollow tree, or in the case of the
hive-bee (Fig. 91) the swarm is put into a new hive, where
the bees build cells, gather food, produce young, and thus

FIG. 91. — Hiving a swarm of honey-bees. Photograph by S. J. HUNTER.

found a new community. This swarming is simply an emi-
gration, which results in the wider distribution and in the
increase of the number of the species. It is a peculiar but
effective mode of distributing and perpetuating the species.
There are many other interesting and suggestive things
which might be told of the life in a bee community : how
the community protects itself from the dangers of starva-
tion when food is scarce or winter comes on by killing the
useless drones and the immature bees in egg and larval
stage ; how the instinct of home-finding has been so highly
developed that the worker bees go miles away for honey
and nectar, flying with unerring accuracy back to the hive ;
of the extraordinarily nice structural modifications which
adapt the bee so perfectly for its complex and varied busi-
nesses ; and of the tireless persistence of the workers until

ANIMAL COMMUNITIES AND SOCIAL LIFE 155

they fall exhausted and dying in the performance of their
duties. The community, it is important to note, is a per-
sistent or continuous one. The workers do not live long,
the spring broods usually not over two or three months,
and the fall broods not more than six or eight months;
but new ones are hatching while the old ones are dying,
and the community as a whole always persists. The queen
may live several years, perhaps as many as five.* She lays
about one million eggs a year.

85. The ants. — There are many species of ants, two
thousand or more, and all of them live in communities and
show a truly communal life. There is much variety of
habit in the lives of different kinds of ants, and the degree
in which the communal or social life is specialized or elab-
orated varies much. But certain general conditions pre-
vail in the life of all the different kinds of individuals —

a

FIG. 92.— Female (a), male (ft), and worker (c) of an ant (Camponotus sp.).

sexually developed males and females that possess wings,
and sexually undeveloped workers that are wingless (Fig.
92). In some kinds the workers show structural differ-

* A queen bee has been kept alive for fifteen years.

156 ANIMAL LIFE

ences among themselves, being divided into small workers,
large workers, and soldiers. The workers are not, as with
the bees, all infertile females, but they are both male and
female, both being infertile. Although the life of the ant
communities is much less familiar and fully known than
that of the bees, it is even more remarkable in its speciali-
zations and elaborateness. The ant home, or nest, or formi-
cary, is, with most species, a very elaborate underground,
many-storied labyrinth of galleries and chambers. Certain
rooms are used for the storage of food ; certain others as
" nurseries " for the reception and care of the young ; and
others as stables for the ants' cattle, certain plant-lice or
scale-insects which are sometimes collected and cared for by
the ants. The food of ants comprises many kinds of vege-
table and animal substances, but the favorite food, or " na-
tional dish," as it has been called, is a sweet fluid which is
produced by certain small insects, the plant-lice (Aphidse)
and scale-insects (Coccidae). These insects live on the sap
of plants ; rose-bushes are especially favored with their pres-
ence. The worker ants (and we rarely see any ants but
the wingless workers, the winged males and females appear-
ing out of the nest only at mating time) find these honey-
secreting insects, and gently touch or stroke them with their
feelers (antennae), when the plant-lice allow tiny drops of
the honey to issue from the body, which are eagerly drunk
by the ants. It is manifestly to the advantage of the ants
that the plant-lice should thrive ; but they are soft-bodied,
defenseless insects, and readily fall a prey to the wander-
ing predaceous insects like the lady-birds and aphis lions.
So the ants often guard small groups of plant-lice, attack-
ing, and driving away the would-be ravagers. When the
branch on which the plant-lice are gets withered and dry,
the ants have been observed to carry the plant-lice care-
fully to a fresh, green branch. In the Mississippi Valley a
certain kind of plant-louse lives on the roots of corn. Its
eggs are deposited in the ground in the autumn and hatch

ANIMAL COMMUNITIES AND SOCIAL LIFE 157

the following spring before the corn is planted. Now, the
common little brown ant lives abundantly in the corn-
fields, and is specially fond of the honey secreted by the
corn-root plant-louse. So, when the plant-lice hatch in the
spring before there are corn roots for them to feed on, the
little brown ants with great solicitude carefully place the
plant-lice on the roots of a certain kind of knotweed which
grows in the field, and protect them until the corn ger-
minates. Then the ants remove the plant-lice to the roots
of the corn, their favorite food plant. In the arid lands of
New Mexico and Arizona the ants rear their scale-insects
on the roots of cactus. Other kinds of ants carry plant-
lice into their nests and provide them with food there.
Because the ants obtain food from the plant-lice and take
care of them, the plant-lice are not inaptly called the ants'
cattle.

Like the honey-bees, the young ants are helpless little
grubs or larvae, and are cared for and fed by nurses. The
so-called ants' eggs, little white, oval masses, which we
often see being carried in the mouths of ants in and out of
an ants' nest, are not eggs, but are the pupae which are
being brought out to enjoy the warmth and light of the
sun or being taken back into the nest afterward.

In addition to the workers that build the nest and col-
lect food and care for the plant-lice, there is in many
species of ants a kind of individuals called soldiers. These
are wingless, like the workers, and are also, like the work-
ers, not capable of laying or of fertilizing eggs. It is the
business of the soldiers, as their name suggests, to fight.
They protect the community by attacking and driving
away predaceous insects, especially other ants. The ants
are among the most warlike of insects. The soldiers of a
community of one species of ant often sally forth and
attack a community of some other species. If successful
in battle the workers of the victorious community take
possession of the food stores of the conquered and carry

158 ANIMAL LIFE

them to their own nest. Indeed, they go even further ; they
may make slaves of the conquered ants. There are numer-
ous species of the so-called slave-making ants. The slave-
makers carry into their own nest the eggs and larvae and
pupae of the conquered community, and when these come
to maturity they act as slaves of the victors — that is, they
collect food, build additions to the nests, and care for the
young of the slave-makers. This specialization goes so far
in the case of some kinds of ants, like the robber-ant of
South America (Eciton), that all of the Eciton workers have
become soldiers, which no longer do any work for them-
selves. The whole community lives, therefore, wholly by
pillage or by making slaves of other kinds of ants. There
are four kinds of individuals in a robber-ant community —
winged males, winged females, and small and large wing-
less soldiers. There are many more of the small soldiers
than of the large, and some naturalists believe that the few
latter, which are distinguished by heads and jaws of great
size, act as officers. On the march the small soldiers are
arranged in a long, narrow column, while the large soldiers
are scattered along on either side of the column and appear
to act as sentinels and directors of the army. The obser-
vations made by the famous Swiss students of ants, Huber
and Forel, and by other naturalists, read like fairy tales,
and yet are the well-attested and often reobserved actual
phenomena of the extremely specialized communal and
social life of these animals.

86. Other communal insects. — The termites or white ants
(not true ants) are communal insects. Some species of
termites in Africa live in great mounds of earth, often
fifteen feet high. The community comprises hundreds of
thousands of individuals, which are of eight kinds (Fig 93),
viz., sexually active winged males, sexually active winged
females, other fertile males and females which are wingless,
wingless workers of both sexes not capable of reproduc-
tion, and wingless soldiers of both sexes also incapable of

ANIMAL COMMUNITIES AND SOCIAL LIFE 159

reproduction. The production of new individuals is the
sole business of the fertile males and females ; the workers
build the nest and collect food, and the soldiers protect the
community from the attacks of marauding insects. The
egg-laying queen grows to monstrous size, being sometimes

FIG. 93. — Termites, a, queen ; J, male ; c, worker ; d, soldier.

five or six inches long, while the other individuals of the
community are not more than half or three quarters of
an inch long. The great size of the queen is due to the
enormous number of eggs in her body.

The bumble-bees live in communities, but their social
arrangements are very simple ones compared with those of
the honey-bee. There is, in fact, among the bees a series
of gradations from solitary to communal life. The inter-
esting little green carpenter-bees live a truly solitary life.
Each female bores out the pith from five or six inches of
an elder branch or raspberry cane, and divides this space
into a few cells by means of transverse partitions (Fig. 94).
In each cell she lays an egg, and puts with it enough food
— flower pollen — to last the grub or larva through its life.

160

ANIMAL LIFE

She then waits in an upper cell of the nest until the young
bees issue from their cells, when she leads them off, and
each begins active life on its own account. The mining-

FIG. 94.— Nest of carpenter-bee.

FIG. 95.— Nest of Andrena, the mining-bee.

bees (Andrena), which make little burrows (Fig. 95) in a
clay bank, live in large colonies — that is, they make their
nest burrows close together in the same clay bank, but each
female makes her own burrow, lays her own eggs in it, fur-
nishes it with food — a kind of paste of nectar and pollen —
and takes no further care of her young. Nor has she at
any time any special interest in her neighbors. But with
the smaller mining-bees, belonging to the genus Halictus^
several females unite in making a common burrow, after
which each female makes side passages of her own, extend-

ANIMAL COMMUNITIES AND SOCIAL LIFE 161

Q

ing from the main or public entrance burrow. As a well-
known entomologist has said, Andrena builds villages com-
posed of individual homes, while Halictus makes cities
composed of apartment houses. The bumble-bee (Fig. 96),
however, establishes a real community with a truly com-
munal life, although a very simple one. The few bumble-
bees which we see in winter time are queens; all other
bumble-bees die in the autumn. In the spring a queen
selects some deserted nest of a field-mouse, or a hole in
the ground, gathers pollen which she molds into a rather
large irregular mass and puts into
the hole, and lays a few eggs on the
pollen mass. The young grubs or
larvee which soon hatch feed on the
pollen, grow, pupate, and issue as
workers — winged bees a little small-
er than the queen. These workers
bring more pollen, enlarge the nest,
and make irregular cells in the pol-
len mass, in each of which the queen
lays an egg. She gathers no more
pollen, does no more work except
that of egg-laying. From these new
eggs are produced more workers, and
so on until the community may come
to be pretty large. Later in the sum-
mer males and females are produced
and mate. With the approach of
winter all the workers and males die,
leaving only the fertilized females,
the queens, to live through the win-
ter and found new communities in
the spring.

The social wasps show a communal life like that of the
bumble-bees. The only yellow-jackets and hornets that
live through the winter are fertilized females or queens.
12

FIG. 96.— Bumble-bees, a,
worker ; 6, queen or fer-
tile female.

162

ANIMAL LIFE

When spring comes each queen builds a small nest sus-
pended from a tree branch, and consisting of a small comb
inclosed in a covering or envelope open at the lower end.
The nest is composed of " wasp paper," made by chewing
bits of weather-beaten wood taken from old fences or out-
buildings. In each of the cells the queen lays an egg.
She deposits in the cell a small mass of food, consisting of
some chewed insects or spiders. From these eggs hatch
grubs which eat the food prepared for them, grow, pupate,
and issue as worker bees, winged and slightly smaller
than the queen (Fig. 97). The workers enlarge the nest,
adding more combs and making many cells, in each of
which the queen lays an egg. The workers provision the
cell with chewed insects, and other broods of workers are

rapidly hatched. The
community grows in
numbers and the nest
grows in size until it
comes to be the great
ball-like oval mass
which we know so well
as a hornets' nest (Figs.
98 and 99), a thing to be
left untouched. Some-
times the nest is built
underground. When
disturbed, they swarm
out of the hole and
fiercely attack any in-
vading foe in sight.
After a number of
broods of workers has
been produced, broods of males and females appear and
mating takes place. In the late fall the males and all of
the many workers die, leaving only the new queens to live
through the winter.

FIG. 97.— The yellow-jacket (Vespa), & social
wasp, a, worker ; b, queen.

ANIMAL COMMUNITIES AND SOCIAL LIFE 163

The bumble-bees and social wasps show an intermediate
condition between the simply gregarious or neighborly

FIG. 98. — Nest of Vespa, a social
wasp. From photograph.

FIG. 99.— Nest of Vespa opened to show
combs within.

mining-bees and the highly developed, permanent honey-
bee community. Naturalists believe that the highly or-
ganized communal life of the honey-bees and the ants is
a development from some simple condition like that of the
bumble-bees and social wasps, which in its turn has grown
out of a still simpler, mere gregarious assembly of the
individuals of one species. It is not difficult to see how
such a development could in the course of a long time take
place.

87. Gregariousness and mutual aid. — The simplest form
of social life is shown among those kinds of animals in
which many individuals of one species keep together, form-
ing a great band or herd. In this case there is not much
division of labor, and the safety of the individual is not
wholly bound up in the fate of the herd. Such animals are

164 ANIMAL LIFE

said to be gregarious in habit. The habit undoubtedly is
advantageous in the mutual protection and aid afforded
the individuals of the band. This mutual help in the case
of many gregarious animals is of a very positive and obvious
character. In other cases this gregariousness is reduced to
a matter of slight or temporary convenience, possessing but
little of the element of mutual aid. The great herds of
reindeer in the north, and of the bison or buffalo which
once ranged over the Western American plains, are examples
of a gregariousness in which mutual protection from ene-
mies, like wolves, seems to be the principal advantage gained.
The bands of wolves which hunted the buffalo show the
advantage of mutual help in aggression as well as in pro-
tection. In this banding together of wolves there is active
co-operation among individuals to obtain a common food
supply. What one wolf can not do — that is, tear down a
buffalo from the edge of the herd— a dozen can do, and all
are gainers by the operation. On the other hand, the vast
assembling of sea-birds (Fig. 100) on certain ocean islands
and rocks is a condition probably brought about rather by
the special suitableness of a few places for safe breeding
than from any special mutual aid afforded ; still, these sea-
birds undoubtedly combine to drive off attacking eagles
and hawks. Eagles are usually considered to be strictly
solitary in habit (the unit of solitariness being a pair, not
an individual) ; but the description, by a Russian naturalist,
of the hunting habits of the great white-tailed eagle (Hali-
cetos albicilla) on the Eussian steppes shows that this kind
of eagle at least has adopted a gregarious habit, in which
mutual help is plainly obvious. This naturalist once saw an
eagle high in the air, circling slowly and widely in perfect
silence. Suddenly the eagle screamed loudly. "Its cry
was soon answered by another eagle, which approached it,
and was followed by a third, a fourth, and so on, till nine
or ten eagles came together and soon disappeared." The
naturalist, following them, soon discovered them gathered

ANIMAL LIFE

about the dead body of a horse. The food found by the
first was being shared by all. The association of pelicans in
fishing is a good example of the advantage of a gregarious
and mutually helpful habit. The pelicans sometimes go
fishing in great bands, and, after having chosen an appro-
priate place near the shore, they form a wide half-circle
facing the shore, and narrow it by paddling toward the
land, catching the fish which they inclose in the ever-nar-
rowing circle.

The wary Eocky Mountain sheep (Fig. 101) live to-
gether in small bands, posting sentinels whenever they
are feeding or resting, who watch for and give warning
of the approach of enemies. The beavers furnish a well-
known and very interesting example of mutual help, and
they exhibit a truly communal life, although a simple
one. They live in " villages " or communities, all helping
to build the dam across the stream, which is necessary to
form the broad marsh or pool in which the nests or houses
are built. Prairie-dogs live in great villages or communi-
ties which spread over many acres. They tell each other by
shrill cries of the approach of enemies, and they seem to
visit each other and to enjoy each other's society a great
deal, although that they afford each other much actual
active help is not apparent. Birds in migration are grega-
rious, although at other times they may live comparatively
alone. In their long flights they keep together, often with
definite leaders who seem to discover and decide on the
course of flight for the whole great flock. The wedge-
shaped flocks of wild geese flying high and uttering their
sharp, metallic call in their southward migrations are well
known in many parts of the United States. Indeed, the
more one studies the habits of animals the more examples
of social life and mutual help will be found. Probably most
animals are in some degree gregarious in habit, and in all
cases of gregariousness there is probably some degree of
mutual aid.

FIG. 101.— Rocky Mountain or bighorn sheep. By permission of the
publishers of Outing.

168 ANIMAL LIFE

88. Division of labor and basis of communal life. — We have
learned in Chapters II and IV that the complexity of the
bodies of the higher animals depends on a specialization or
differentiation of parts, due to the assumption of different
functions or duties by different parts of the body ; that the
degree of structural differentiation depends on the degree
or extent of division of labor shown in the economy of the
animal. It is obvious that the same principle of division of
labor with accompanying modification of structure is the
basis of colonial and communal life. It is simply a mani-
festation of the principle among individuals instead of
among organs. The division of the necessary labors of life
among the different zooids of the colonial jelly-fish is plain-
ly the reason for the profound and striking, but always
reasonable and explicable modifications of the typical polyp
or medusa body, which is shown by the swimming zooids,
the feeding zooids, the sense zooids, and the others of the
colony. And similarly in the case of the termite commu-
nity, the soldier individuals are different structurally from
the worker individuals because of the different work they
have to do. And the queen differs from all the others, be-
cause of the extraordinary prolificacy demanded of her to
maintain the great community.

It is important to note, however, that among those ani-
mals that show the most highly organized or specialized
communal or social life, the structural differences among
the individuals are the least marked, or at least are not the
most profound. The three kinds of honey-bee individuals
differ but little; indeed, as two of the kinds, male and
female, are to be found in the case of almost all kinds of
animals, whether communal in habit or not, the only unu-
sual structural specialization in the case of the honey bee, is
the presence of the worker individual, which differs from
the usual individuals in but little more than the rudimen-
tary condition of the reproductive glands. Finally, in the
case of man, with whom the communal or social habit is so

170 ANIMAL LIFE

all-important as to gain for him the name of " the social
animal," there is no differentiation of individuals adapted
only for certain kinds of work. Among these highest
examples of social animals, the presence of an advanced
mental endowment, the specialization of the mental power,
the power of reason, have taken the place of and made
unnecessary the structural differentiation of individuals.
The honey-bee workers do different kinds of work : some
gather food, some care for the young, and some make wax
and build cells, but the individuals are interchangeable ;
each one knows enough to do these various things. There
is a structural differentiation in the matter of only one
special work or function, that of reproduction.

With the ants there is, in some cases, a considerable
structural divergence among individuals, as in the genus
Atta of South America with six kinds of individuals —
namely, winged males, winged females, wingless soldiers,
and wingless workers of three distinct sizes. In the case
of other kinds with quite as highly organized a communal
life there are but three kinds of individuals, the winged
males and females and the wingless workers. The workers
gather food, build the nest, guard the " cattle " (aphids),
make war, and care for the young. Each one knows enough
to do all these various distinct things. Its body is not so
modified that it can do but one kind of thing, which thing
it must always do.

The increase of intelligence, the development of the
power of reasoning, is the most potent factor in the devel-
opment of a highly specialized social life. Man is the
example of the highest development of this sort in the ani-
mal kingdom, but the highest form of social development
is not by any means the most perfectly communal.

89. Advantages of communal life. — The advantages of
communal or social life, of co-operation and mutual aid, are
real. The animals that have adopted such a life are among
the most successful of all animals in the struggle for exist-

ANIMAL COMMUNITIES AND SOCIAL LIFE

ence. The termite individual is one of the most defense-
less, and, for those animals that prey on insects, one of
the most toothsome luxuries to be found in the insect
world. But the termite is one of the most abundant and
widespread and successfully living insect kinds in all the
tropics. Where ants are not, few insects are. The honey-
bee is a popular type of a successful life. The artificial
protection afforded the honey-bee by man may aid in its
struggle for existence, but it gains this protection because
of certain features of its communal life, and in Nature the
honey-bee takes care of itself well. The Little Bee People
of Kipling's Jungle Book, who live in great communities in
the rocks of Indian hills, can put to rout the largest and
fiercest of the jungle animals. Co-operation and mutual
aid are among the most important factors which help in
the struggle for existence. Its great advantages are, how-
ever, in some degree balanced by the fact that mutual help
brings mutual dependence. The community or society can
accomplish greater things than the solitary individuals, but
co-operation limits freedom, and often sacrifices the indi-
vidual to the whole.

CHAPTER X

COMMENSALISM AND SYMBIOSIS

90. Association between animals of different species. — The
living together and mutual help discussed in the last chap-
ter concerned in each instance a single species of animal.
All the various members of a pack of wolves or of a com-
munity of ants are individuals of the same species. But
there are many instances of an association of individuals
of different kinds of animals. The number of individuals
concerned, however, is usually but two — that is, one of
each of the two kinds of animals. In many cases of an
association of individuals of different species one kind
derives great benefit and the other suffers more or less
injury from the association. One kind lives at the expense
of the other. This association is called parasitism, and is
discussed in the next chapter. In some cases, however,
neither kind of animal suffers from the presence of the
other. The two live together in harmony and presumably
to their mutual advantage. In some cases this mutual
advantage is obvious. This kind of association is called
commensalism or symbiosis. The term commensalism may
be used to denote a condition where the two animals are
not so intimately associated nor derive such obvious mu-
tual advantage from the association, as in that condition
of very intimate and permanent association with obvious
co-operative and marked advantage that may be called
symbiosis. A few examples of each of these interesting
conditions of association between which it is impossible to
make any sharp distinction, will be given.
172

COMMENSAL1SM AND SYMBIOSIS 173

91. Commensalism. — A curious example of commensalism
is afforded by the different species of Remoras (Echenididce)
which attach themselves to sharks, barracudas, and other
large fishes by means of a sucking disk on the top of the
head (Fig. 103). This disk is made by a modification of

PIG. 103.— Remora, with dorsal fin modified to be a sucking plate by which the
fish attaches itself to a shark.

the dorsal fin. The Eemora thus attached to a shark may
be carried about for weeks, leaving its host only to secure
food. This is done by a sudden dash through the water.
The Remora injures the shark in no way save, perhaps, by
the slight check its presence gives to ths shark's speed in
swimming.

Whales, similarly, often carry barnacles about with
them. These barnacles are permanently attached to the
skin of the whale just as they would be to a stone or
wooden pile. Many small crustaceans, annelids, mollusks,
and other invertebrates burrow into the substance of living
sponges, not for the purpose of feeding on them, but for
shelter. On the other hand, the little boring sponge
( Cliona) burrows in the shells of oysters and other bivalves
for protection. These are hardly true cases of even that
lesser degree of mutually advantageous association which
we are calling commensalism. But some species of sponge
" are never found growing except on the backs or legs of
certain crabs." In these cases the sponge, with its many
plant-like branches, protects the crab by concealing it from
its enemies, while the sponge is benefited by being carried
about by the crab to new food supplies. Certain sponges

ANIMAL LIFE

and polyps are always found growing in close association,
though what the mutual advantage of this association is
has not yet been found out.

Among the coral reefs near Thursday Island (between
New Guinea and Australia) there lives an enormous kind
of sea-anemone or polyp. Individuals of this great polyp
measure two feet across the disk when fully expanded.
In the interior, the stomach cavity, which communicates
freely with the outside, by means of the large mouth open-
ing at the free end of the polyp, there may often be found
a small fish (Amphiprion percula). That this fish is pur-
posely in the gastral cavity of the polyp is proved by the
fact that when it is dislodged it invariably returns to its
singular lodging-place. The fish is brightly colored, being
of a brilliant vermilion hue with three broad white cross
bands. The discoverer of this peculiar habit suggests that
there are mutual benefits to fish and polyp from this habit.
" The fish being conspicuous, is liable to attacks, which it
escapes by a rapid retreat into the sea-anemone ; its enemies
in hot pursuit blunder against the outspread tentacles of
the anemone and are at once narcotized by the 'thread
cells ' shot out in innumerable showers from the tentacles,
and afterward drawn into the stomach of the anemone and
digested."

Small fish of the genus Nomeus may often be found
accompanying the beautiful Portuguese man-of-war (Phy-
salia) as it sails slowly about on the ocean's surface (Fig.
104). These little fish lurk underneath the float and
among the various hanging thread-like parts of the Phy-
salia, which are provided with stinging cells. The fish are
protected from their enemies by their proximity to these
stinging threads, but of what advantage to the man-of-
war their presence is is not understood. Similarly, several
kinds of medusae are known to harbor or to be accompanied
by young or small adult fishes.

In the nests of the various species of ants and termites

COMMENSALISM AND SYMBIOSIS

175

many different kinds of other insects have been found.
Some of these are harmful to their hosts, in that they feed
on the food stores gathered by the industrious and provi-
dent ant, but others appear
to feed only on refuse or use-
less substances in the nest.
Some may even be of help to
their hosts. Over one thou-
sand species of these rryn-rie-
cophilous (ant -loving) and
termitophilous (termite -lov-
ing) insects have been re-
corded by collectors as living
habitually in the nests of ants
and termites. The owls and
rattlesnakes which live with
the prairie-dogs in their vil-
lages afford a familiar exam-
ple of commensalism.

92. Symbiosis. — Of a more
intimate character, and of
more obvious and certain mu-
tual advantage, is the well-
known case of the symbiotic
association of some of the
numerous species of hermit-
crabs and certain species of
sea-anemones. The hermit-
crab always takes for his
habitation the shell of an-
other animal, often that of
the common whelk. All of

the hind part of the crab lies inside the shell, while its
head with its great claws project from the opening of the
shell. On the surface of the shell near the opening there
is often to be found a sea-anemone, or sea-rose (Fig. 105).

FIG. 104.— A Portuguese man-of-war
(Physalia), with man-of-war fishes
(Nomeus gronovii) living in the
shelter of the stinging feelers.
Specimens from off Tampa, Fla.

ANIMAL LIFE

This sea-anemone is fastened securely to the shell, and has
its mouth opening and. tentacles near the head of the crab.
The sea-anemone is carried from place to place by the her-
mit-crab, and in this way is much aided in obtaining food.
On the other hand, the crab is protected from its enemies
by the well-armed and dangerous tentacles of the sea-anem-

Pie. 105.— Hermit-crab (Pagurus) in shell, with a sea-anemone (Adamsia palliata,
attached to the shell.— After HERTWIG.

one. In the tentacles there are many thousand long,
slender stinging threads, and the fish that would obtain
the hermit-crab for food must first deal with the stinging
anemone. There is no doubt here of the mutual advan-
tage gained by these two widely different but intimately
associated companions. If the sea-anemone be torn away
from the shell inhabited by one of these crabs, the crab
will wander about, carefully seeking for another anemone.
When he finds it he struggles to loosen it from its rock
or from whatever it may be growing on, and does not rest
until he has torn it loose and placed it on his shell.

There are numerous small crabs called pea-crabs (Pin-
notheres) which live habitually inside the shells of living

COMMENSALISM AND SYMBIOSIS 177

mussels. The mussels and the crabs live together in per-
fect harmony and to their mutual benefit.

There are a few extremely interesting cases of symbiosis
in which not different kinds of animals are concerned, but
animals and plants. It has long been known that some
sea-anemones pos-
sess certain body
cells which con-
tain chlorophyll,
that green sub-
stance character-
istic of the green
plants, and only
in few cases pos-
sessed by animals.
When these chlo-
rophyll -bearing
sea-anemones were
first found, it was
believed that the
chlorophyll cells

,, * , , , FIG. 106.— The crab Epizoanthus paguriphilus, with

really belonged to the sea-anemone Parapagurus pilosiramus on its

the animal's body, shell,
and that this con-
dition broke down one of the chiefest and most readily
apparent distinctions between animals and plants. But
it is now known that these chlorophyll-bearing cells are
microscopic, one-celled plants, green algae, which live ha-
bitually in the bodies of the sea-anemone. It is a case
of true symbiosis. The algae, or plants, use as food the
carbonic-acid gas which is given off in the respiratory
processes of the sea-anemone, and the sea-anemone breathes
in the oxygen given off by the algae in the process of ex-
tracting the carbon for food from the carbonic-acid gas.
These algae, or one-celled plants, lie regularly only in the
innermost of the three cell layers which compose the wall
13

178

ANIMAL LIFE

or body of the sea-anemone (Fig. 107). They penetrate
into and lie in the interior of the cells of this layer whose
special function is that of digestion. They give this inner-
most layer of cells
a distinct green
color.

There are other
examples known of
the symbiotic asso-
ciation of plants
and animals ; and
if we were to fol-
low the study of
symbiosis into the
plant kingdom we

FIG. 107.— Diagrammatic section of sea-anemone, a,
the inner cell layer containing alga cells, the two
isolated cells at right being cells of this layer with
contained algae; b, middle body wall layer; c, outer
body wall layer.— After HERTWIG.

should find that in
one of the large
groups of plants,
the familiar lichens
which grow on

rocks and tree trunks and old fences, every member lives
symbiotically. A lichen is not a single plant, but is always
composed of two plants, an alga (chlorophyll-bearing) and
a fungus (without chlorophyll) living together in a most
intimate, mutually advantageous association.

CHAPTER XI

PARASITISM AND DEGENERATION

93. Eelation of parasite and host.— In addition to the vari-
ous ways of living together of animals already described,
namely, the social life of individuals of a single species and
the commensal and symbiotic life of individuals of differ-
ent species, there is another kind of association among ani-
mals that is very common. In cases of symbiosis the two
animals living together are of mutual advantage to each
other ; both profit by the association. But there are many
instances in the animal kingdom of an association between
two animals by which one gains advantages great or small,
sometimes even obtaining all the necessities of life, while
the other gains nothing, but suffers corresponding disad-
vantage, often even the loss of life itself. This is the asso-
ciation of parasite and host ; the relation between two ani-
mals whereby one, the parasite, lives on or in the other, the
host, and at the expense of the host. Parasitism is a com-
mon phenomenon in all groups of animals, although the
parasites themselves are for the most part confined to the
classes of invertebrates. Among the simplest animals or
Protozoa there are parasites, as Gregarina, which lives in
the bodies of insects and crustaceans ; there are parasitic
worms, and parasitic crustaceans and mollusks and insects,
and a few vertebrates. When an animal can get along
more safely or more easily by living at the expense of some
other animal and takes up such a life, it becomes a parasite.
Parasitism is naturally, therefore, not confined to any one
group or class of animals.

179

180 ANIMAL LIFE

94. Kinds of parasitism,— The bird-lice (Mallophaga),
which infest the bodies of all kinds of birds and are found
especially abundant on domestic fowls, live upon the out-
side of the bodies of their hosts, feeding upon the feathers
and dermal scales. They are examples of external parasites.
Other examples are fleas and ticks, and the crustaceans called
fish-lice and whale-lice, which are attached to marine ani-
mals. On the other hand, almost all animals are infested by
certain parasitic worms which live in the alimentary canal,
like the tape-vrorm, or imbedded in the muscles, like the
trichina. These are examples of internal parasites. Such
parasites belong mostly to the class of worms, and some of
them are very injurious, sucking the blood from the tissues
of the host, while others feed solely on the partly digested
food. There are also parasites that live partly within and
partly on the outside of the body, like the Sacculina, which
lives on various kinds of crabs. The body of the Sacculina
consists of a soft sac which lies on the outside of the crab's
body, and of a number of long, slender root-like processes
which penetrate deeply into the crab's body, and take up
nourishment from within. The Sacculina is itself a crus-
tacean or crab-like creature. The classification of para-
sites as external and internal is purely arbitrary, but it is
often a matter of convenience.

Some parasites live for their whole lifetime on or in the
body of the host, as is the case with the bird-lice. Their
eggs are laid on the feathers of the bird host ; the young
when hatched remain on the bird during growth and devel-
opment, and the adults only rarely leave the body, usually
never. These may be called permanent parasites. On the
other hand, fleas leap off or on a dog as caprice dictates ;
or, as in other cases, the parasite may pass some definite
part of its life as a free, non-parasitic organism, attaching
itself, after development, to some animal, and remaining
there for the rest of its life. These parasites may be called
temporary parasites. But this grouping or classification,

PARASITISM AND DEGENERATION

183

protozoan is as simple as an animal's body can be, being
composed of but a single cell, degeneration can not occur
in the cases of these parasites. There are, besides Grega-
rina, numerous other parasitic one-celled animals, several
kinds living inside the cells of their host's body. One
kind lives in the blood-corpuscles of the frog, and another
in the cells of the liver of the rabbit.

97. The tape-worm and other flat-worms. — In the great
group of flat-worms (Platyhelminthes), that group of ani-
mals which of all the principal animal groups is widest
in its distribution, perhaps a major-
ity of the species are parasites. In-
stead of being the exception, the
parasitic life is the rule among these
worms. Of the three classes into
which the flat -worms are divided
almost all of the members of two of
the classes are parasites. The com-
mon tape-worm (Tcenia) (Fig. 108),
which lives parasitically in the intes-
tine of man, is a good example of
one of these classes. " It has the
form of a narrow ribbon, which may
attain the length of several yards,
attached at one end to the wall of
the intestine, the remainder hanging
freely in the interior." Its body is
composed of segments or serially
arranged parts, of which there are

about eight hundred and fifty altogether. It has no mouth
nor alimentary canal. It feeds simply by absorbing into
its body, through the surface, the nutritious, already di-
gested liquid food in the intestine. There are no eyes
nor other special sense organs, nor any organs of locomo-
tion. The body is very degenerate. The life history of
the tape-worm is interesting, because of the necessity of

FIG. 108.— Tape-worm (Tcenia
solium). In upper left-
hand corner of figure the
head much magnified. —
After LETJCKABT.

184

ANIMAL LIFE

two hosts for its completion. The eggs of the tape-worm
pass from the intestine with the excreta, and must be
taken into the body of some other animal in order to de-
velop. In the case of one of the several species of tape-
worms that infest man this other host must be the pig.
In the alimentary canal of the pig the young tape-worm
develops, and later bores its way through the walls of the
canal and becomes imbedded in the muscles. There it lies,
until it finds its way into the alimentary canal of man by
his eating the flesh of the pig. In the intestine of man
the tape-worm continues to develop
until it becomes full grown.

In a lake in Yellowstone Park
the suckers are infested by one of
the flat-worms (Ligula) that at-
tains a size of nearly one fourth
the size of the fish in whose in-
testines it lives. If the tape-worm
of man attained such a compara-
tive size, a man of two hundred
pounds' weight would be infested by
a parasite of fifty pounds' weight.

98. Trichina and other round-
worms. — Another group of animals,
many of whose numbers are para-
sites, are the round-worms or thread-
worms (Kemathelminthes). The
free-living round-worms are active,
well - organized animals, but the
parasitic kinds all show a greater
or less degree of degeneration. One

of the most terrible parasites of man is a round-worm called
Trichina spiralis (Fig. 109). It is a minute worm, from
one to three millimetres long, which in its adult condition
lives in the intestine of man or of the pig or other mam-
mals. The young are born alive and bore through the walls.

FIG. 109. — Trichina spiralis
(after CLAUS). a, male ; b,
encysted form in muscle ; c,
female.

PARASITISM AND DEGENERATION 185

of the intestine. They migrate to the voluntary muscles
of the hosts, especially those of the limbs and back, and
here each worm coils itself up in a muscle fiber and be-
comes inclosed in a spindle-shaped cyst or cell (Fig. 109, #).
A single muscle may be infested by hundreds of thousands of
these minute worms. It has been estimated that fully one
hundred million encysted worms have existed in the mus-
cles of a " trichinized " human body. The muscles undergo
more or less degeneration, and the death of the host may
occur. It is necessary, for the further development of the
worms, that the flesh of the host be eaten by another mam-
mal, as the flesh of the pig by man, or the flesh of man by
a pig or rat. The Trichince in the alimentary canal of
the new host develop into active adult worms and produce
new young.

In the Yellowstone Lake the trout are infested by the
larvae or young of a round-worm (Bothriocephalus cordiceps)
which reaches a length of twenty inches, and which is
often found stitched, as it were, through the viscera and
the muscles of the fish. The infested trout become feeble
and die, or are eaten by the pelicans which fish in this
lake. In the alimentary canal of the pelican the worms
become adult, and parts of the worms containing eggs
escape from the alimentary canal with the excreta. These
portions of worms are eaten by the trout, and the eggs give
birth to new worms which develop in the bodies of the
fish with disastrous effects. It is estimated that for each
pelican in Yellowstone Lake over five million eggs of the
parasitic worms are discharged into the lake.

The young of various carnivorous animals are often
infested by one of the species of round-worms called " pup-
worms " ( Untinaria). Eecent investigations show that
thousands of the young or pup fur-seals are destroyed each
year by these parasites. The eggs of the worm lie through
the winter in the sands of the breeding grounds of the fur-
seal. The young receive them from the fur of the mother

I

PARASITISM AND DEGENERATION 187

and the worm develops in the upper intestine. It feeds on
the blood of the young seal, which finally dies from anaemia.
On the beaches of the seal islands in Bering Sea there are
sometimes hundreds of dead seal pups which have been
killed by this parasite (Fig. 110).

99. Sacculina. — Among the more highly organized ani-
mals the results of a parasitic life, in degree of structural
degeneration, can be more readily seen. A well-known para-
site, belonging to the Crustacea — the class of shrimps, crabs,
lobsters, and cray-fishes — is Sacculina. The young Sac-
culina is an active, free-swimming larva much like a young
prawn or young crab. But the adult bears absolutely no
resemblance to such a typical crustacean as a cray-fish or
crab. The Sacculina after a short period of independent
existence at-
taches itself to
the abdomen of
a crab, and
there completes
its develop-
ment while liv-
ing as a para-
site. In its
adult condition
(Fig. Ill) it is
simply a great

i -]•-! FIG. 111. — Sacculina, & crustacean parasite of crabs, a, at-

ac» tached to a crab, with root-like processes penetrating the

bearing many crab's body ; b, removed from the crab.

delicate r o o t-

like suckers which penetrate the body of the crab host and
absorb nutriment. The Sacculina has no eyes, no mouth
parts, no legs, or other appendages, and hardly any of the
usual organs except reproductive organs. Degeneration
here is carried very far.

Other parasitic Crustacea, as the numerous kinds of
fish-lice (Fig. 112) which live attached to the gills or to

188

ANIMAL LIFE

other parts of fish, and derive all their nutriment from the
body of the fish, show various degrees of degeneration. With
some of these fish-lice the female,
which looks like a puffed-out worm,
is attached to the fish or other aquatic
animal, while the male, which is per-
haps only a tenth of the size of the
female, is permanently attached to
the female, living parasitically on her.
100. Parasitic insects, — Among
the insects there are many kinds
that live parasitically for part of
their life, and not a few that live as
FIG. 112. -Fish-louse (Ler- parasites for their whole life. The

nceocera). a, adult ; b, larva. , . _. , , -,0\ T ,-,

true sucking lice (Fig. 113) and the

bird-lice (Fig. 114) live for their whole lives as external
parasites on the bodies of their host, but they are not
fixed — that is, they retain
their legs and power of loco-
motion, although they have
lost their wings through de-
generation. The eggs of the
lice are deposited on the hair
of the mammal or bird that

FIG. 113.— Sucking louse (Pediculm) of
human body.

FIG. 114.— Bird louse (Lipeurus densus).

serves as host ; the young hatch and immediately begin to
live as parasites, either sucking the blood or feeding on the

PARASITISM AND DEGENERATION 189

hair or feathers of the host. In the order Hymenoptera
there are several families, all of whose members live during
their larval stage as parasites. We may call all these hy-
menopterous parasites ichneumon flies. The ichneumon
flies are parasites of other insects, especially of the larvae of
beetles and moths and butterflies. In fact, the ichneumon
flies do more to keep in check the increase of injurious and
destructive caterpillars than do all our artificial remedies
for these insect pests. The adult ichneumon fly is four-
winged and lives an active, independent life. It lays its
eggs either in or on or near some caterpillar or beetle grub,
and the young ichneumon, when hatched, burrows about in
the body of its host, feeding on its tissues, but not attacking
such organs as the heart or nervous ganglia, whose injury
would mean immediate death to the host. The caterpillar
lives with the ichneumon grub within it, usually until nearly

FIG. 115.— Parasitized caterpillar from which the ichneumon fly parasites have
issued, showing the circular holes of exit in the skin.

time for its pupation. In many instances, indeed, it pu-
pates, with the parasite still feeding within its body, but it
never comes to maturity. The larval ichneumon fly pupates
either within the body of its host (Fig. 115) or in a tiny
silken cocoon outside of its body (Fig. 116). From the
cocoons the adult winged ichneumon flies emerge, and
after mating find another host on whose body to lay their
eggs.

One of the most interesting ichneumon flies is Thalessa
(Fig. 119), which has a remarkably long, slender, flexible
ovipositor, or egg-laying organ. An insect known as the

190

ANIMAL LIFE

pigeon horn-tail (Tremex columla) (Fig. 117) deposits its
eggs, by means of a strong, piercing ovipositor, half an inch
deep in the trunk wood of growing trees. The young or

FIG. 116.— Caterpillar with cocoons of the pupae of ichneumon fly parasites, and
(above) one of the adult ichneumon flies. The lines indicate natural dimensions.

larval Tremex is a soft-bodied white grub, which bores
deeply into the trunk of the tree, filling up the burrow be-
hind it with small chips. The Thalessa is a parasite of the
Tremex, and " when a female Thalessa finds a tree infested
by Tremex, she selects a place which she judges is opposite

PARASITISM AND DEGENERATION

191

a Tremex burrow, and, elevating her long ovipositor in a
loop over her back, with its tip on the bark of the tree (Fig.

FIG. 117.— The pigeon horn-tail (Tremex
columbd), with strong boring ovipositor.

FIG. lia— Thalessa lunator boring— After

COMSTOCK.

Fio. 119.— The large ichneumon fly
Thalessa, with long flexible oviposi-
tor. The various parts of this ovi-
positor are spread apart in the fig-
ure ; naturally they lie together to
form a single piercing organ.

118), she makes a derrick out

of her body and proceeds with

great skill and precision to drill a hole into the tree. When

the Tremex burrow is reached she deposits an egg in it.

192

ANIMAL LIFE

FIG. 120.— Wasp (Polistes), with female Stylops para-
site (a?) in body.

The larva that hatches from this egg creeps along this
burrow until it reaches its victim, and then fastens itself to
the horn-tail larva, which it destroys by sucking its blood.

The larva of Thales-
sa, when full grown,
changes to a pupa
within the burrow
of its host, and the
adult gnaws a hole
out through the bark
if it does not find the
hole already made by
the Tremex."

The beetles of
the family Stylopidae
present an interest-
ing case of parasit-
ism. The adult males are winged, but the adult females
are wingless and grub-like. The larval stylopid attaches
itself to a wasp or bee, and bores into its abdomen. It
pupates within the abdomen of the
wasp or bee, and lies there with its
head projecting slightly from a su-
ture between two of the body rings
of its host (Fig. 120). The adult
finally issues and leaves the host's
body.

Almost all of the mites and ticks,
which are more nearly allied to the
spiders than to the true insects, live
parasitically. Most of them live as
external parasites, sucking the blood
of their host, but some live under-
neath the skin like the itch-mites
(Fig. 121), which cause, in man, the disease known as
the itch.

FIG. 121.— The itch-mite

PARASITISM AND DEGENERATION 193

101. Parasitic vertebrates. — Among the vertebrate ani-
mals there are not many examples of true parasitism. The
hag-fishes or borers (Myxine, Heptatrema, Polistotrema) are
long and cylindrical, eel-like creatures, very slimy and very
low in structure. The mouth is without jaws, but forms a
sucking disk, by which the hag-fish attaches itself to the
body of some other fish. By means of the rasping teeth on
its tongue, it makes a round hole through the skin, usually
at the throat. It then devours all the muscular substance
of the fish, leaving the viscera untouched. When the fish
finally dies it is a mere hulk of skin, scales, bones, and
viscera, nearly all the muscle being gone. Then the hag-
fish slips out and attacks another individual.

The lamprey, another low fish, in similar fashion feeds
leech-like on the blood of other fishes, which it obtains by
lacerating the flesh with its rasp-like teeth, remaining at-
tached by the round sucking disk of its mouth.

Certain birds, as the cow-bird and the European cuckoo,
have a parasitic habit, laying their eggs in the nests of
other birds, leaving their young to be hatched and reared
by their unwilling hosts. This is, however, not bodily para-
sitism, such as is seen among lower forms.

102. Degeneration through quiescence. — While parasitism
is the principal cause of degeneration among animals, yet
it is not the sole cause. It is evident that if for any other
reason animals should become fixed, and live inactive or
sedentary lives, they would degenerate. And there are not
a few instances of degeneration due simply to a quiescent
life, unaccompanied by parasitism. The Tunicata, or sea^
squirts (Fig. 122), are animals which have become simple
through degeneration, due to the adoption of a sedentary
life, the withdrawal from the crowd of animals and from
the struggle which it necessitates. The young tunicate is
a free-swimming, active, tadpole-like or fish-like creature,
which possesses organs very like those of the adult of the
simplest fishes or fish-like forms. That is, the sea-squirt

14

194

ANIMAL LIFE

begins life as a primitively simple vertebrate. It possesses
in its larval stage a notochord, the delicate structure which
precedes the formation of a backbone, extending along the

upper part of the body,
below the spinal cord. It
is found in all young ver-
tebrates, and is charac-
teristic of the class. The
other organs of the young
tunicate are all of verte-
bral type. But the young
sea-squirt passes a period
of active and free life as
a little fish, after which
it settles down and at-
taches itself to a stone or
shell or wooden pier by
means of suckers, and re-
mains for the rest of its
life fixed. Instead of go-
ing on and developing
into a fish-like creature, it
loses its notochord, its
special sense organs, and

other organs ; it loses its complexity and high organiza-
tion, and becomes a " mere rooted bag with a double neck."
a thoroughly degenerate animal.

A barnacle is another example of degeneration through
quiescence. The barnacles are crustaceans related most
nearly to the crabs and shrimps. The young barnacle just
from the egg (Fig. 123, /) is a six-legged, free-swimming
nauplius, very like a young prawn or crab, with single eye.
In its next larval stage it has six pairs of swimming feet,
two compound eyes, and two large antennae or feelers, and
still lives an independent, free-swimming life. When it
makes its final change to the adult condition, it attaches

PIG. 132.— A sea-squirt, or tunicate.

PARASITISM AND DEGENERATION

195

itself to some stone or shell, or pile or ship's bottom, loses
its compound eyes and feelers, develops a protecting shell,
and gives up all power of locomotion. Its swimming feet
become changed into grasping organs, and it loses most of
its outward resemblances to the other members of its class
(Fig. 123, e).

FIG. 123.— Three adult crustaceans and their larvae, a, prawn (Peneus), active and
free-living ; b, larva of prawn ; c, Sacculina, parasite ; d, larva of Sacculina ;
e, barnacle (Lepas), with fixed quiescent life ; /, larva of barnacle.— After
HAECKBL.

Certain insects live sedentary or fixed lives. All the
members of the family of scale insects (Coccidae), in one
sex at least, show degeneration, that has been caused by
quiescence. One of these coccids, called the red orange
scale (Fig. 124), is very abundant in Florida and California
and in other orange-growing regions. The male is a beau-
tiful, tiny, two-winged midge, but the female is a wingless,

196

ANIMAL LIFE

footless little sac without eyes or other organs of special
sense, which lies motionless under a flat, thin, circular, red-
dish scale composed of wax and two or three cast skins of
the insect itself. The insect has a long, slender, flexible,
sucking beak, which is thrust into the leaf or stem or fruit
of the orange on which the " scale bug " lives and through
which the insect sucks the orange sap, which is its only

PIG. 124.-The red orange scale of California, a, bit of leaf with scales ; b, adult
female ; c, wax scale under which adult female lives ; d, larva ; e, adult male.

food. It lays eggs under its body, and thus also under the
protecting wax scale, and dies. From the eggs hatch active
little larval scale-bugs with eyes and feelers and six legs.
They crawl from under the wax scale and roam about over
the orange tree. Finally, they settle down, thrusting their
sucking beak into the plant tissues, and cast their skin.
The females lose at this molt their legs and eyes and

PARASITISM AND DEGENERATION 197

feelers. Each becomes a mere motionless sac capable only
of sucking up sap and of laying eggs. The young males,
however, lose their sucking beak and can no longer take
food, but they gain a pair of wings and an additional pair
of eyes. They fly about and fertilize the sac-like females,
which then molt again and secrete the thin wax scale over
them.

Throughout the animal kingdom loss of the need of
movement is followed by the loss of the power to move, and
of all structures related to it.

103. Degeneration through other causes.— Loss of certain
organs may occur through other causes than parasitism and
a fixed life. Many insects live but a short time in their
adult stage. May-flies live for but a few hours or, at most,
a few days. They do not need to take food to sustain life
for so short a time, and so their mouth parts have become
rudimentary and functionless or are entirely lost. This is
true of some moths and numerous other specially short-
lived insects. Among the social insects the workers of the
termites and of the true ants are wingless, although they
are born of winged parents, and are descendants of winged
ancestors. The modification of structure dependent upon
the division of labor among the individuals of the com-
munity has taken the form, in the case of the workers, of a
degeneration in the loss of the wings. Insects that live
in caves are mostly blind ; they have lost the eyes, whose
function could not be exercised in the darkness of the cave.
Certain island-inhabiting insects have lost their wings,
flight being attended with too much danger. The strong
sea-breezes may at any time carry a flying insect oft3 the
small island to sea. Only those which do not fly much sur-
vive, and by natural selection wingless breeds or species are
produced. Finally, we may mention the great modifications
of structure, often resulting in the loss of certain organs,
which take place to produce protective resemblances (see
Chapter XII). In such cases the body may be modified in

198 ANIMAL LIFE

color and shape so as to resemble some part of the envi-
ronment, and thus the animal may be unperceived by its
enemies. Many insects have lost their wings through this
cause.

104. Immediate causes of degeneration. — When we say
that a parasitic or quiescent mode of life leads to or causes
degeneration, we have explained the stimulus or the ulti-
mate cause of degenerative changes, but we have not
shown just how parasitism or quiescence actually produces
these changes. Degeneration or the atrophy and disap-
pearance of organs or parts of a body is often said to be
due to disuse. That is, the disuse of a part is believed by
many naturalists to be the sufficient cause for its gradual
dwindling and final loss. That disuse can so affect parts
of a body during the lifetime of an individual is true. A
muscle unused becomes soft and flabby and small. Whether
the effects of such disuse can be inherited, however, is open
to serious doubt. Such inheritance must be assumed if
disuse is to account for the gradual growing less and final
disappearance of an organ in the course of many genera-
tions. Some naturalists believe that the results of such
disuse can be inherited, but as yet such belief rests on no
certain knowledge. If characters assumed during the life-
time of the individual are subject to inheritance, disuse
alone may explain degeneration. If not, some other imme-
diate cause, or some other cause along with disuse, must
be found. Such a cause must be sought for in the action of
natural selection, preserving the advantages of simplicity of
structure where action is not required.

105. Advantages and disadvantages of parasitism and de-
generation.— We are accustomed, perhaps, to think of degen-
eration as necessarily implying a disadvantage in life. A
degenerate animal is considered to be not the equal of a non-
degenerate animal, and this would be true if both kinds of
animals had to face the same conditions of life. The blind,
footless, simple, degenerate animal could not cope with the

PARASITISM AND DEGENERATION 199

active, keen-sighted, highly organized non-degenerate in
free competition. But free competition is exactly what
the degenerate animal has nothing to do with. Certainly
the Sacculina lives successfully ; it is well adapted for its
own peculiar kind of life. For the life of a scale insect,
no better type of structure could be devised. A parasite
enjoys certain obvious advantages in life, and even extreme
degeneration is no drawback, but rather favors it in the
advantageousness of its sheltered and easy life. As long
as the host is successful in eluding its enemies and avoid-
ing accident and injury, the parasite is safe. It needs to
exercise no activity or vigilance of its own ; its life is easy
as long as its host lives. But the disadvantages of para-
sitism and degeneration are apparent also. The fate of the
parasite is usually bound up with the fate of the host.
When the enemy of the host crab prevails, the Sacculina
goes down without a chance to struggle in its own defense.
But far more important than the disadvantage in such par-
ticular or individual cases is the disadvantage of the fact
that the parasite can not adapt itself in any considerable
degree to new conditions. It has become so specialized,
so greatly modified and changed to adapt itself to the one
set of conditions under which it now lives ; it has gone so
far in its giving up of organs and body parts, that if pres-
ent conditions should change and new ones come to exist,
the parasite could not adapt itself to them. The independ-
ent, active animal with all its organs and all its functions
intact, holds itself, one may say, ready and able to adapt
itself to any new conditions of life which may gradually
come into existence. The parasite has risked everything
for the sake of a sure and easy life under the presently
existing conditions. Change of conditions means its ex-
tinction.

106. Human degeneration. — It is not proposed in these
pages to discuss the application of the laws of animal life
to man. But each and every one extends upward, and can

200 ANIMAL LIFE

be traced in the relation of men and society. Thus, among
men as among animals, self-dependence favors complexity
of power. Dependence, parasitism, quiescence favor de-
generation. Degeneration means loss of complexity, the
narrowing of the range of powers and capabilities. It is
not necessarily a phase of disease or the precursor of death.
But as intellectual and moral excellence are matters associ-
ated with high development in man, dependence is unfa-
vorable to them.

Degeneration has been called animal pauperism. Pau-
perism in all its forms, whether due to idleness, pampering,
or misery, is human degeneration. It has been shown that
a large part of the criminality and pauperism among men
is hereditary, due to the survival of the tendency toward
living at the expense of others. The tendency to live with-
out self-activity passes from generation to generation.
Beggary is more profitable than unskilled and inefficient
labor, and our ways of careless charity tend to propagate
the beggar. That form of charity which does not render
its recipient self-helpful is an incentive toward degenera-
tion. Withdrawal from the competition of life, withdrawal
from self-helpful activity, aided by the voluntary or invol-
untary assistance of others — these factors bring about de-
generation. The same results follow in all ages and with
all races, with the lower animals as with men.

CHAPTEK XII

PROTECTIVE RESEMBLANCES, AND MIMICRY

107. Protective resemblance defined. — If a grasshopper
be startled from the ground, you may watch it and deter-
mine exactly where it alights after its leap or flight, and
yet, on going to the spot, be wholly unable to find it. The
colors and marking of the insect so harmonize with its sur-
roundings of soil and vegetation that it is nearly indistin-
guishable as long as it remains at rest. And if you were
intent on capturing grasshoppers for fish-bait, this resem-
blance in appearance to their surroundings would be very
annoying to you, while it would be a great advantage to
the grasshoppers, protecting some of them from capture and
death. This is protective resemblance. Mere casual obser-
vation reveals to us that such instances of protective resem-
blance are very common among animals. A rabbit or grouse
crouching close to the ground and remaining motionless
is almost indistinguishable. Green caterpillars lying out-
stretched along green grass-blades or on green leaves may
be touched before being recognized by sight. In arctic
regions of perpetual snow the polar bears, the snowy arctic
foxes, and the hares are all pure white instead of brown
and red and gray like their cousins of temperate and warm
regions. Animals of the desert are almost without excep-
tion obscurely mottled with gray and sand color, so as to
harmonize with their surroundings.

In the struggle for existence anything that may give
an animal an advantage, however slight, may be sufficient
to turn the scale in favor of the organism possessing the

201

202 ANIMAL LIFE

advantage. Such an advantage may be swiftness of move-
ment, or unusual strength or capacity to withstand unfa-
vorable meteorological conditions, or the possession of such
color and markings or peculiar shape as tend to conceal the
animal from its enemies or from its prey. Resemblances
may serve the purpose of aggression as well as protection.
In the case of the polar bears and other predaceous ani-
mals that show color likenesses to their surroundings, the
resemblance can better be called aggressive than protective.
The concealment afforded by the resemblance allows them
to steal unperceived on their prey. This, of course, is an
advantage to them as truly as escape from enemies would be.

We have already seen that by the action of natural
selection and heredity those variations or conditions that
give animals advantages in the struggle for life are pre-
served and emphasized. And so it has come about that
advantageous protective resemblances are very widespread
among animals, and assume in many cases extraordinarily
striking and interesting forms. In fact, the explanation
of much of the coloring and patterning of animals depends
on this principle of protective resemblance.

Before considering further the general conditions of
protective resemblances, it will be advisable to refer to
specific examples classified roughly into groups or special
kinds of advantageous colorings and markings.

108. General protective or aggressive resemblance. — As
examples of general protective resemblance — that is, a gen-
eral color effect harmonizing with the usual surroundings
and tending to hide or render indistinguishable the animal
— may be mentioned the hue of the green parrots of the
evergreen tropical forests ; of the green tree-frogs and tree-
snakes which live habitually in the green foliage ; of the
mottled gray and tawny lizards, birds, and small mam-
mals of the deserts ; and of the white hares and foxes
and snowy owls and ptarmigans of the snow-covered arc-
tic regions. Of the same nature is the slaty blue of the

204 ANIMAL LIFE

gulls and terns, colored like the sea. In the brooks most
fishes are dark olive or greenish above and white below.
To the birds and other enemies which look down on them
from above they are colored like the bottom. To their fish
enemies which look up from below, their color is like the
white light above them, and their forms are not clearly
seen. The fishes of the deep sea in perpetual darkness are

FIG. 126.— Alligator lizard (Gerrhonottts sdndcauda) on granite rock. Photograph
by J. O. SNYDEB, Stanford University, California.

inky violet in color below as well as above. Those that
live among sea-weeds are red, grass-green, or olive, like
the plants they frequent. General protective resemblance
is very widespread among animals, and is not easily appre-
ciated when the animal is seen in museums or zoological
gardens — that is, away from its natural or normal environ-
ment. A modification of general color resemblance found
in many animals may be called variable protective resem-
blance. Certain hares and other animals that live in
northern latitudes are wholly white during the winter when
the snow covers everything, but in summer, when much of
the snow melts, revealing the brown and gray rocks and

PROTECTIVE RESEMBLANCES, AND MIMICRY 205

withered leaves, these creatures change color, putting on
a grayish and brownish coat of hair. The ptarmigan of
the Eocky Mountains (one of the grouse), which lives on
the snow and rocks of the high peaks, is almost wholly
white in winter, but in summer when most of the snow is
melted its plumage is chiefly brown. On the campus at
Stanford University there is a little pond whose shores are
covered in some places with bits of bluish rock, in other
places with bits of reddish rock, and in still other places
with sand. A small insect called the toad-bug ( Galgulus
oculatus) lives abundantly on the banks of this pond.
Specimens collected from the blue rocks are bluish in
color, those from the red rocks are reddish, and those from
the sand are sand-colored. Such changes of color to suit
the changing surroundings can be quickly made in the case
of some animals. The chameleons of the tropics, whose
skin changes color momentarily from green to brown,
blackish or golden, is an excellent example of this highly
specialized condition. The same change is shown by a
small lizard of our Southern States (AnoUus), which from its
habit is called the Florida
chameleon. There is a lit-
tle fish (OUgocottus snyderi)
which is common in the tide
pools of the bay of Monterey,
in California, whose color
changes quickly to harmo-
nize with the different colors
of the rocks it happens to
rest above. Some of the tree-
frogs show this variable col-
oring. A very striking in-
stance of variable protective
resemblance is shown by the

chrysalids of certain butterflies. An eminent English nat-
uralist collected many caterpillars of a certain species of

FIG. l27.-Chryealid of swallow-tail but-
terfly (Papilio), harmonizing with the
bark on which it rests.

206

ANIMAL LIFE

butterfly, and put them, just as they were about to change
into pupae or chrysalids, into various boxes, lined with paper
of different colors. The color of the chrysalid was found

FIG. 128. — Chrysalld of butterfly (lower left-hand projection from stem), showing pro-
tective resemblance. Photograph from Nature.

to harmonize very plainly with the color of the lining of
the box in which the chrysalid hung. It is a familiar fact
to entomologists that most butterfly chrysalids resemble in

PROTECTIVE RESEMBLANCES, AND MIMICRY 207

color and general external appearance the surface of the
object on which they rest (Figs. 127 and 128).

109. Special protective resemblance.— Far more striking
are those cases of protective resemblance in which the ani-
mal resembles in color and shape, sometimes in extraor-
dinary detail, some particular object or part of its usual
environment. Certain parts of the Atlantic Ocean are
covered with great patches of sea-weed called the gulf-weed
(Sargassum), and many kinds of animals — fishes and other
creatures — live upon and among the algae. Xo one can
fail to note the extraordinary color resemblances which exist
between those animals and the weed itself. The gulf-weed
is of an olive-yellow color, and the crabs and shrimps, a cer-
tain flat-worm, a certain mollusk, and a little fish, all of
which live among the Sargatmm, are exactly of the same
shade of yellow as the weed, and have small white markings
on their bodies which are characteristic also of the tiargas-
fsurn. The mouse-fish or Sargassum fish and the little sea-
horses, often attached to the gulf weed, show the same traits
of coloration (Fig. 129). In the black rocks about Tahiti
is found the black nokee or lava-fish (Emmydrichthys vul-
canus) (Fig. 66), which corresponds perfectly in color and
form to a piece of lava. This fish is also noteworthy for
having envenomed spines in the fin on its back. The
slender grass-green caterpillars of many moths and butter-
flies resemble very closely the thin grass-blades among
which they live. The larvae of the geometrid moths, called
inch-worms or span-worms, are twig-like in appearance,
and have the habit, when disturbed, of standing out stiffly
from the twig or branch upon which they rest, so as to re-
semble in position as well as in color and markings a short
or a broken twig. One of the most striking resemblances
of this sort is shown by the large geometrid larva illus-
trated in Fig. 130, which was found near Ithaca, New York.
The body of this caterpillar has a few small, irregular spots
or humps, resembling very exactly the scars left by fallen

Fia. 129.— The mouse-fish (Pterophryne histrio) in the Sargassum or gulf-weed. The
fishes are marked and colored so as to be nearly indistinguishable from the masses
of the gulf- weed. In the lower right-hand corner of figure are two sea-horses, also
shaped and marked so as to be concealed.

PROTECTIVE RESEMBLANCES, AND MIMICRY 209

buds or twigs. These caterpillars have a special muscular
development to enable them to hold themselves rigidly for

PIG. 130. — A geometrid larva on a branch. (The FIG. 131. — A walking-stick insect
larva is the upper right-hand projection from (Diapheromera femorata) on

the stem.) twig.

long times in this trying attitude. They also lack the
middle prop-legs of the body, common to other lepidopter-
15

210

ANIMAL LIFE

ous larvae, the presence of which would tend to destroy the
illusion so successfully carried out by them. The common
walking-stick (Diapheromera) (Fig. 131), with its wingless,
greatly elongate, dull-colored body, is an excellent example
of special protective resemblance. It is quite indistinguish-
able, when at rest, from the twigs to which it is clinging.
Another member of the family of insects to which the walk-
ing-stick belongs is the famous green-leaf insect (Phy Ilium)

(Fig. 132). It is found in
South America and is of a
bright green color, with broad
leaf -like wings and body, with
markings which imitate the
leaf veins, and small irregu-
lar yellowish spots which
mimic decaying or stained
or fungus-covered spots in
the leaf.

There are many butter-
flies that resemble dead
leaves. All our common
meadow browns ( Grapta),
brown and reddish butter-
flies with ragged-edged wings,
that appear in the autumn

\

FIG. 132.— The green-leaf insect
(Phyttium).

and flutter aimlessly about ex-
actly like the falling leaves,
show this resemblance. But
most remarkable of all is a

large butterfly (Kallima) (Fig. 133) of the East Indian
region. The upper sides of the wings are dark, with
purplish and orange markings, not at all resembling a
dead leaf. But the butterflies when at rest hold their
wings together over the back, so that only the under sides
of the wings are exposed. The under sides of Kallima's
wings are exactly the color of a dead and dried leaf, and

PROTECTIVE RESEMBLANCES, AND MIMICRY 211

the wings are so held that all combine to mimic with ex-
traordinary fidelity a dead leaf still attached to the twig by
a short pedicle or leafstalk imitated by a short tail on the

FIG. 133.—Xallima, the " dead-leaf butterfly."

hind wings, and showing midrib, oblique veins, and, most
remarkable of all, two apparent holes, like those made in
leaves by insects, but in the butterfly imitated by two small
circular spots free from scales and hence clear and trans-

212 ANIMAL LIFE

parent. With the head and feelers concealed beneath the
wings, it makes the resemblance wonderfully exact.

There are numerous instances of special protective
resemblance among spiders. Many spiders (Fig. 134) that

FIG. 134.— Spiders showing unusual shapes and patterns, for purposes of
aggressive resemblance.

live habitually on tree trunks resemble bits of bark or small,
irregular masses of lichen. A whole family of spiders,
which live in flower-cups lying in wait for insects, are white
and pink and party-colored, resembling the markings of the
special flowers frequented by them. This is, of course, a

FIG. 135.— A pipe-fish (Phyllopteryx) resembling sea-weed, in which it lives

special resemblance not so much for protection as for ag-
gression ; the insects coming to visit the flowers are unable
to distinguish the spiders and fall an easy prey to them.

110. Warning colors and terrifying appearances. — In the
cases of advantageous coloring and patterning so far dis-

PROTECTIVE RESEMBLANCES, AND MIMICRY 213

cussed the advantage to the animal lies in the resemblance
between the animals and their surroundings, in the incon-
spicuousness and concealment afforded by the coloration.
But there is another interesting phase of advantageous
coloration in which the advantage derived is in render-
ing the animals as conspicuous and as readily recogniz-
able as possible. While many animals are very inconspicu-
ously colored, or are manifestly colored so as to resemble
their surroundings, generally or specifically, many other
animals are very brightly and conspicuously colored and
patterned. If we are struck by the numerous cases of imi-
tative coloring among insects, we must be no less impressed
by the many cases of bizarre and conspicuous coloration
among them.

Many animals, as we well know, possess special and
effective weapons of defense, as the poison-fangs of the
venomous snakes and the stings of bees and wasps. Other
animals, and with these cases most of us are not so well
acquainted, possess a means of defense, or rather safety, in
being inedible — that is, in possessing some acrid or ill-
tasting substance in the body which renders them unpala-
table to predaceous animals. Many caterpillars have been
found, by observation in Nature and by experiment, to be
distasteful to insectivorous birds. Now, it is obvious that
it would be a great advantage to these caterpillars if they
could be readily recognized by birds, for a severe stroke by
a bird's bill is about as fatal to a caterpillar as being wholly
eaten. Its soft, distended body suffers mortal hurt if cut
or bitten by the bird's beak. This advantage of being
readily recognizable is possessed by many if not all ill-
tasting caterpillars by being brilliantly and conspicuously
colored and marked. Such colors and markings are called
warning colors. They are intended to inform birds of the
fact that the caterpillar displaying them is an ill-tasting
insect, a caterpillar to be let alone. The conspicuously
black-and-yellow banded larva (Fig. 43, b) of the common

214 ANIMAL LIFE

Monarch butterfly is a good example of the possession of
warning colors by distasteful caterpillars.

These warning colors are possessed not only by the ill-
tasting caterpillars, but by many animals which have spe-
cial means of defense. The wasps and bees, provided with
stings — dangerous animals to trouble — are almost all con-
spicuously marked with yellow and black. The lady-bird
beetles (Fig. 136), composing a whole family of small beetles

Fro. 136.— Two lady-bird beetles, conspicuously colored and marked.

which are all ill-tasting, are brightly and conspicuously col-
ored and spotted. The Gila monster (ffeloderma),ihe only
poisonous lizard, differs from most other lizards in being
strikingly patterned with black and brown. Some of the
venomous snakes are conspicuously colored, as the coral
snakes (Elaps) or coralillos of the tropics. The naturalist
Belt, whose observations in Nicaragua have added much to
our knowledge of tropical animals, describes as follows an
interesting example of warning colors in a species of frog :
'•'In the woods around Santo Domingo (Nicaragua) there
are many frogs. Some are green or brown and imitate
green or dead leaves, and live among foliage. Others are
dull earth-colored, and hide in holes or under logs. All
these come out only at night to feed, and they are all
preyed upon by snakes and birds. In contrast with these
obscurely colored species, another little frog hops about in

PROTECTIVE RESEMBLANCES, AND MIMICRY 215

the daytime, dressed in a bright livery of red and blue.
He can not be mistaken for any other, and his flaming
breast and blue stockings show that he does not court con-
cealment. He is very abundant in the damp woods, and I
was convinced he was uneatable so soon as I made his
acquaintance and saw the happy sense of security with
which he hopped about. I took a few specimens home
with me, and tried my fowls and ducks with them, but
none would touch them. At last, by throwing down pieces
of meat, for which there was a great competition among
them, I managed to entice a young duck into snatching up
one of the little frogs. Instead of swallowing it, however,
it instantly threw it out of its mouth, and went about jerk-
ing its head, as if trying to throw off come unpleasant
taste."

Certain animals which are without special means of
defense and are not at all formidable or dangerous are yet
so marked or shaped and so behave as to present a threat-
ening or terrifying appearance. The large green caterpil-
lars (Fig. 137) of the Sphinx moths — the tomato-worm is a
familiar one of these larvae — have a formidable-looking,

PIG. 137.— A "tomato-worm" larva of the Sphinx moth, Pldegethontius Carolina,
showing terrifying appearance.

sharp horn on the back of the next to last body ring.
When disturbed they lift the hinder part of the body, bear-
ing the horn, and move it about threateningly. As a mat-
ter of fact, the horn is not at all a weapon of defense, but is
quite harmless. Numerous insects when disturbed lift
the hind part of the body, and by making threatening mo-

216

ANIMAL LIFE

tions lead enemies to believe that they possess a sting.
The striking eye-spots of many insects are believed by some
entomologists to be of the nature of terrifying appearances.
The larva (Fig. 138) of the Puss moth (Cerura) has been
often referred to as a striking example of terrifying appear-
ances. When one of these larvae is disturbed, " it retracts

its head into the
first body ring in-
flating the mar-
gin, which is of a
bright red color.
There are two in-
tensely black spots
on this margin in the
appropriate position for
eyes, and the whole ap-
pearance is that of a large
flat face extending to the
outer edge of the red mar-
gin. The effect is an in-
tensely exaggerated cari-
cature of a vertebrate
face, which is probably
alarming to the verte-
brate enemies of the cat-
erpillar. . . . The effect is also greatly strengthened by two
pink whips which are swiftly protruded from the prongs
of the fork in which the body terminates. . . . The end
of the body is at the same time curved forward over the
back, so that the pink filaments are brandished above the
head."

111. Alluring coloration. — A few animals show what are
called alluring colors — that is, they display a color pattern
so arranged as to resemble or mimic a flower or other lure,
and thus to entice to them other animals, their natural prey.
This is a special kind of aggressive resemblance. A species

PIG. 138.— Larva of the Puss moth (Cerura).
Upper figure shows the larva as it appears
when undisturbed ; lower figure, when dis-
turbed.— After POULTON.

PROTECTIVE RESEMBLANCES, AND MIMICRY 217

of predatory insect called a " praying-horse " (allied to the
genus Mantis), found in India, has the shape and color of
an orchid. Small insects are attracted and fall a prey to it.
Certain Brazilian fly-catching birds have a brilliantly colored
crest which can be displayed in the shape of a flower-cup.
The insects attracted by the apparent flower furnish the fly-
catcher with food. An Asiatic lizard is wholly colored like
the sand upon which it lives except for a peculiar red fold
of skin at each angle of the mouth. This fold is arranged
in flower-like shape, " exactly resembling a little red flower
which grows in the sand." Insects attracted by these
flowers find out their mistake too late. In the tribe of
fishes called the " anglers " or fishing frogs the front rays
of the dorsal fin are prolonged in shape of long, slender fila-
ments, the foremost and longest of which has a flattened
and divided extremity like the bait on a hook. The fish
conceals itself in the mud or in the cavities of a coral reef
and waves the filaments back and forth. Small fish are at-
tracted by the lure, mistaking it for worms writhing about
in the water or among the weeds. As they approach they
are ingulfed in the mouth of the angler, which in some of
the species is of enormous size. One of these species is
known to fishermen as the "all-mouth." These fishes
(Lopliius piscatorius), which live in the mud, are colored
like mud or clay. Other forms of anglers, living among
coral reefs, are brown and red (Antennarius), their colora-
tion imitating in minutest detail the markings and out-
growths on the reef itself, the lure itself imitating a worm
of the reef. In a certain group of deep-sea anglers, the sea-
devils ( Ceratiidce), certain species show a still further spe-
cialization of the curious fishing-rod. In one species ((70-
rynolophus reinhardti) (Fig. 54), living off the coast of
Greenland at a depth of upward of a mile, the fishing-rod
or first dorsal spine has a luminous bulb at its tip around
which are fleshy, worm-like streamers. At the abyssal
depths of a mile, more or less, frequented by these sea-

218 ANIMAL LIFE

devils thex'e is no light, the inky darkness being absolute.
This shining lure is therefore a most effective means of
securing food.

112. Mimicry. — Although the word mimicry could often
have been used aptly in the foregoing account of protective
resemblances, it has been reserved for use in connection
with a certain specific group of cases. It has been reserved
to be applied exclusively to those rather numerous instances
where an otherwise defenseless animal, one without poison-
fangs or sting, and without an ill-tasting substance in its
body, mimics some other specially defended or inedible ani-
mal sufficiently to be mistaken for it and so to escape
attack. Such cases of protective resemblance are called
true mimicry, and they are especially to be observed among
insects.

In Fig. 139 are pictured three familiar American butter-
flies. One of these, the Monarch butterfly (Anosia plexip-
pus), is perhaps the most abundant and widespread butter-
fly of our country. It is a fact well known to entomologists
that the Monarch is distasteful to birds and is let alone by
them. It is a conspicuous butterfly, being large and chiefly
of a red-brown color. The Viceroy butterfly (Basilarchia
archippus), also red-brown and much like the Monarch, is
not, as its appearance would seem to indicate, a very near
relative of the Monarch, belonging to the same genus, but
on the contrary it belongs to the same genus with the third
butterfly figured, the black and white BasilarcMa. All the
butterflies of the genus Basilarchia are black and white
except this species, the Viceroy, and one other. The Vice-
roy is not distasteful to birds ; it is edible, but it mimics the
inedible Monarch so closely that the deception is not de-
tected by the birds, and so it is not molested.

In the tropics there have been discovered numerous
similar instances of mimicry by edible butterflies of inedi-
ble kinds. The members of two great families of butterflies
(Danaidae and Heliconidae) are distasteful to birds, and are

FIG. 139.— The mimicking of the inedible Monarch butterfly by the edible Viceroy.
Upper figure is the Monarch (Anosia plexippus} ; middle figure is the Viceroy
(Basilarchia archippus) ; lowest figure is another member of the same genus
(Basilarchia), to show the usual color pattern of the species of the genus.

220

ANIMAL LIFE

mimicked by members of the other butterfly families (espe-
cially the Pieridae), to which family our common white
cabbage-butterfly belongs, and by the swallow-tails (Papi-
lionidae).

The bees and wasps are protected by their stings. They
are usually conspicuous, being banded with yellow and black.
They are mimicked by numerous other insects, especially
moths and flies, two defenseless kinds of insects. This
mimicking of bees and wasps by flies is very common, and
can be observed readily at any flowering shrub. The flower-
flies (Syrphidae), which, with the bees, visit flowers, can be
distinguished from the bees only by sharp observing. When
these bees and flies can be caught and examined in hand, it
will be found that the flies have but two wings while the
bees have four.

A remarkable and interesting case of mimicry among
insects of different orders is that of certain South Ameri-
can tree-hoppers (of the family Membracidae, of the order
Hemiptera), which mimic the famous leaf -cutting ant
(Saula) of the Amazons (Fig. 140). These ants have the
curious habit of cutting off, with their sharp jaws, bits of

green leaves and carry-
ing them to their nests.
In carrying the bits of
leaves the ants hold them
vertically above their
heads. The leaf-hoppers
mimic the ants and their
burdens with remarka-
ble exactitude by having
the back of the body ele-
vated in the form of a
thin, jagged-edged ridge no thicker than a leaf. This part
of the body is green like the leaves, while the under part
of the body and the legs are brown like the ants.

Some examples of mimicry among other animals than

FIG. 140.— Tree-hopper (Membracidae), which
mimics the leaf-cutting ant (Sauba) of Bra-
zil. (Upper right-hand insect is the tree-
hopper.)

PROTECTIVE RESEMBLANCES, AND MIMICRY 221

insects are known, but not many. The conspicuously
marked venomous coral-snake or coralillos (Elaps) is mim-
icked by certain non-venomous snakes called king-snakes
(Lampropeltis, Osceola). The pattern of red and black
bands surrounding the cylindrical body is perfectly imi-
tated. But whether this is true mimicry brought about
for purposes of protection may be doubted. Instances
among birds have been described, and a single case has
been recorded in the class of mammals. But it is among
the insects that the best attested instances occur. The
simple fact of the close resemblance of two widely related
animals can not be taken to prove the existence of mimicry.
Two animals may both come to resemble some particular
part in their common environment and thus to resemble
closely each other. Here we have simply two instances
of special protective resemblance, and not an instance of
mimicry. The student of zoology will do well to watch
sharply for examples of protective resemblance or mimicry,
for but few of the instances that undoubtedly exist are as
yet known.

113. Protective resemblances and mimicry most common
among insects, — The large majority of the preceding exam-
ples have been taken from among the insects. This is
explained by the fact that the phenomena of protective
resemblances and mimicry have been studied especially
among insects; the theory of mimicry was worked out
chiefly from the observation and study of the colors and
markings of insects and of the economy of insect life.
Why protective resemblances and mimicry among insects
have been chiefly studied is because these conditions are
specially common among insects. The great class Insecta
includes more than two thirds of all the known living
species of animals. The struggle for existence among the
insects is especially severe and bitter. All kinds of " shifts
for a living " are pushed to extremes ; and as insect colors
and patterns are especially varied and conspicuous, it is

222 ANIMAL LIFE

only to be expected that this useful modification of colors
and patterns, that results in the striking phenomena of
special protective resemblances and mimicry, should be
specially widespread and pronounced among insects. More-
over, they are mostly deficient in other means of defense,
and seem to be the favorite food for many different kinds
of animals. Protective resemblance is their best and most
widely adopted means of preserving life.

114. No volition in mimicry. — The use of the word mim-
icry has been criticised because it suggests the exercise of
volition or intent on the part of the mimicking animal.
The student should not entertain this conception of mim-
icry. In the use of "mimicry" in connection with the
phenomena just described, the biologist ascribes to it a
technical meaning, which excludes any suggestion of voli-
tion or intent on the part of the mimic. Just how such
extraordinary and perfect cases of mimicry as shown by
Phyllium and Kallima have come to exist is a problem
whose solution is not agreed on by naturalists, but none of
them makes volition — the will or intent of the animal — any
part of his proposed solution. Each case of mimicry is the
result of a slow and gradual change, through a long series
of ancestors. The mimicry may indeed include the adop-
tion of certain habits of action which strengthen and make
more pronounced the deception of shape and color. But
these habits, too, are the result of a long development, and
are instinctive or reflex — that is, performed without the
exercise of volition or reason.

115. Color; its utility and beauty. — The causes of color,
and the uses of color in animals and in plants are subjects
to which naturalists have paid and are paying much atten-
tion. The subject of " protective resemblances and mim-
icry" is only one, though one of the most interesting,
branches or subordinate subjects of the general theory of
the uses of color. Other uses are obvious. Bright colors
and markings may serve for the attraction of mates ; thus

PROTECTIVE RESEMBLANCES, AND MIMICRY 223

are explained by some naturalists the brilliant plumage of
the male birds, as in the case of the bird-of-paradise and
the pheasants. Or they may serve for recognition charac-
ters, enabling the individuals of a band of animals readily
to recognize their companions ; the conspicuous whiteness
of the short tail of the antelopes and cotton-tail rabbits,
the black tail of the black-tail deer, and the white tail-
feathers of the meadow-lark, are explained by many natu-
ralists on this ground. Eecognition marks of this type
are especially numerous among the birds, hardly a species
being without one or more of them, if their meaning is cor-
rectly interpreted. The white color of arctic animals may
be useful not alone in rendering them inconspicuous, but
may serve also a direct physiological function in preventing
the loss of heat from the body by radiation. And the dark
colors of animals may be of value to them in absorbing heat
rays and thus helping them to keep warm. But " by far
the most widespread use of color is to assist an animal in
escaping from its enemies or in capturing its prey."

The colors of an animal may indeed not be useful to
it at all. Many color patterns exist on present-day birds
simply because, preserved by heredity, they are handed
down by their ancestors, to whom, under different condi-
tions of life, they may have been of direct use. For the
most part, however, we can look on the varied colors and
the striking patterns exhibited by animals as being in some
way or another of real use and value. We can enjoy the
exquisite coloration of the wings of a butterfly none the
less, however, because we know that these beautiful colors
and their arrangement tend to preserve the life of the
dainty creature, and have been produced by the operation
of fixed laws of Nature working through the ages.

CHAPTEE XIII

THE SPECIAL SENSES

116. Importance of the special senses. — The means by
which animals become acquainted with the outer world
are the special senses, such as feeling, tasting, smelling,
hearing, and seeing. The behavior of animals with regard
to their surroundings, with regard to all the world outside
of their own body, depends upon what they learn of this
outer world through the exercise of these special senses.
Habits are formed on the basis of experience or knowledge
of the outer world gained by the special senses, and the
development of the power to reason or to have sense de-
pends on their pre-existence.

117. Difficulty of the study of the special senses. — We are
accustomed to think of the organs of the special senses as
extremely complex parts of the body, and this is certainly
true in the case of the higher animals. In our own body,
the ears and eyes are organs of most specialized and highly
developed condition. But we must not overlook the fact
that the animal kingdom is composed of creatures of widely
varying degrees of organization, and that in any considera-
tion of matters common to all animals those animals of
simplest and most lowly organization must be studied as
well as those of high development. The study of the spe-
cial senses presents two phases, namely, the study of the
structure of the organs of special sense, and the study of
the physiology of special sense — that is, the functions of
these organs. It will be recognized that in the study of
how other animals feel and taste and smell and hear and

224

THE SPECIAL SENSES 225

see, we shall have to base all our study on our own experi-
ence. We know of hearing and seeing only by what we
know of our own hearing and seeing ; but by examination
of the structure of the hearing and seeing organs of cer-
tain other animals, and by observation and experiments,
zoologists are convinced that some animals hear sounds
that we can not hear, and some see colors that we can
not see.

While that phase .of the study of the special senses
which concerns their structure may be quite successfully
undertaken, the physiological phase of the study of the
actual tasting and seeing and hearing of the lower animals
is a matter of much difficulty. The condition and char-
acter of the special senses vary notably among different
animals. There may even exist other special senses than
the ones we possess. Some zoologists believe that certain
marine animals possess a " density or pressure sense " —
that is, a sense which enables them to tell approximately
how deep in the water they may be at any time. To
certain animals is ascribed a " temperature sense," and
some zoologists believe that what we call the homing in-
stinct of animals as shown by the homing pigeons and
honey-bees and other animals, depends on their possession
of a special sense which man does not possess. Eecent
experiments, however, seem to show that the homing of
pigeons depends on their keen sight. In numerous animals
there exist, besides the organs of the five special senses
which we possess, organs whose structure compels us to be-
lieve them to be organs of special sense, but whose func-
tion is wholly unknown to us. Thus in the study of the
special senses we are made to see plainly that we can not
rely simply on our knowledge of our own body structure
for an understanding of the structure and functions of
other animals.

118. Special senses of the simplest animals. — In the Amcela
(see Chapter I), that type of the simplest animals, with
16

226 ANIMAL LIFE

one-celled body, without organs, and yet with its capacity
for performing the necessary life processes, there are no
special senses except one (perhaps two). The Amoeba can
feel. It possesses the tactile sense. And there are no
special sense organs except one, which is the whole of the
outer surface of the body. If the Amceba be touched with
a fine point it feels the touch, for the soft viscous proto-
plasm of its body flows slowly away from the foreign ob-
ject. The sense of feeling or touch, the tactile sense, is
the simplest or most primitive of the special senses, and
the simplest, most primitive organ of special sense is the
outer surface or skin of the body. Among those simple
animals that possess the simplest organs of hearing and
perceiving light, we shall find these organs to be simply
specialized parts of the skin or outer cell layer of the
body, and it is a fact that all the special sense organs of
all animals are derived or developed from the outer cell
layer, ectoblast, of the embryo. This is true also of the
whole nervous system, the brain and spinal cord of the
vertebrates, and the ganglia and nerve commissures of
the invertebrates. And while in the higher animals the
nervous system lies underneath the surface of the body,
in many of the lower, many-celled animals all the ganglia
and nerves, all of the nervous system, lie on the outer
surface of the body, being simply a specialized part of
the skin.

119. The sense of touch. — In some of the lower, many-
celled animals, as among the polyps, there are on the skin
certain sense cells, either isolated or in small groups, which
seem to be stimulated not alone by the touching of foreign
substances, but also by warmth and light. They are not
limited to a single special sense. They are the primitive
or generalized organs of special sense, and can develop into
specialized organs for any one of the special senses.

The simplest and most widespread of these special
senses with, as a whole, the simplest organs, is the tactile

THE SPECIAL SENSES

227

sense, or the sense of touch. The special organs of this
sense are usually simple hairs or papillae connecting with a
nerve. These tactile hairs or papillae may be distributed
pretty evenly over most of the body, or may be mainly con-
centrated upon certain parts in crowded groups. Many of
the lower animals have projecting parts, like the feeling
tentacles of many marine invertebrates, or the antennae
(feelers) of crabs and insects, which are the special seat
of the tactile organs. Among the vertebrates the tactile
organs are either like those of the invertebrates, or are
little sac-like bodies of connective tissue in which the
end of a nerve is curiously folded and convoluted (Fig.
141). These little touch corpuscles simply lie in the cell
layer of the skin, covered over thinly by the cuticle. Some-
times they are simply free, branched
nerve-endings in the skin. These
tactile corpuscles or free nerve-end-
ings are especially abundant in those
parts of the body which can be best
used for feeling. In man the fin-
ger-tips are thus especially supplied ;
in certain tailed monkeys the tip of
the tail, and in hogs the end of the
snout. The difference in abundance
of these tactile corpuscles of the skin
can be readily shown by experiment.
With a pair of compasses, whose FlG; .i«-Tactiie papilla of

•f; ' skin of man. n, nerve.—

points have been slightly blunted, After KOELLIKEB.
touch the skin of the forearm of a

person who has his eyes shut, with the points about three
inches apart and in the direction of the length of the arm.
The person touched will feel the points as two. Eepeat
the touching several times, gradually lessening the dis-
tance between the points. When the points are not more
than an inch to an inch and a half apart, the person
touched will feel but a single touch — that is, the touching

228 ANIMAL LIFE

of both points will give the sensation of but a single con-
tact. Eepeat the experiment on the tip of the forefinger,
and both points will be felt until the points are only about
one tenth of an inch apart.

120. The sense of taste. — The sense of taste enables us to
test in some degree the chemical constitution of substances
which are taken into the mouth as food. We discriminate
by the taste organs between good food and bad, well-tasting
and ill-tasting. These organs are, with us and the other air-
breathing animals, located in the mouth or on the mouth
parts. They must be located so as to come into contact
with the food, and it is also necessary that the food sub-
stance to be tasted be made liquid. This is accomplished
by the fluids poured into the mouth from the salivary
glands. With the lower aquatic animals it is not improb-
able that taste organs are situated on other parts of the
body besides the mouth, and that taste is used not only to
test food substances, but also to test the chemical char-
acter of the fluid medium in which they live.

The taste organs are much like the tactile organs, ex-
cept that the special taste cell is exposed, so that small par-
ticles of the substance to be tasted can come into actual
contact with it. The nerve-ending is usually in a small
raised papilla or depressed pit. In the simplest animals
there is no special organ of taste, and yet Amoeba and
other Protozoa show that they appreciate the chemical con-
stitution of the liquid in which they lie. They taste — that
is, test the chemical constitution of the substances — by
means of their undiiferentiated body surface. The taste
organs are not always to be told from the organs of smell.
Where an animal has a certain special seat of smell, like
the nose of the higher animals, then the special sense
organs of the mouth can be fairly assumed to be taste
organs ; but where the seat of both smell and taste is in
the mouth or mouth parts, it is often impossible to distin-
guish between the two kinds of organs.

THE SPECIAL SENSES

In mammals taste organs are situated on certain parts of
the tongue, and have the form of rather large, low, broad
papillae, each bearing many small taste-buds (Fig. 142).
In fishes similar papillae and buds have been found in vari-
ous places on the sur-
face of the body, from
which it is believed that
the sense of taste in
fishes is not limited to
the mouth. In insects
the taste - papillae and
taste -pits are grouped

in Certain places On the FIG. 14S.— Vertical section of large papilla on
mouth parts, being es- £^£of a Calf; '•*•• taste-buds. -After

pecially abundant on

the tips of small, segmented, feeler-like processes called
palpi, which project from the under lip and from the so-
called maxillae.

121. The sense of smell. — Smelling and tasting are closely
allied, the one testing substances dissolved, the other test-
ing substances vaporized. The organs of the sense of
smell are, like those of taste, simple nerve-endings in papil-
lae or pits. The substance to be smelled must, however,
be in a very finely divided form ; it must come to the or-
gans of smell as a gas or vapor, and not, as to the organs of
taste, in liquid condition. The organs of smell are situated
usually on the head, but as the sense of smell is used not
alone for the testing of food, but for many other purposes,
the organs of smell are not, like those of taste, situated
principally in or near the mouth. Smell is a special sense
of much wider range of use than taste. By smell animals
can discover food, avoid enemies, and find their mates.
They can test the air they breathe as well as the food they
eat. In the matter of the testing of food the senses of
both taste and smell are constantly used, and are indeed
intimately associated.

230

ANIMAL LIFE

The sense of smell varies a great deal in its degree of
development in various animals. With the strictly aquatic
animals — and these include most of the lower invertebrates,
as the polyps, the star-fishes, sea-urchins, and most of the
worms and mollusks — the sense of smell is probably but
little developed. There is little opportunity for a gas or
vapor to come to these animals, and only as a gas or vapor
can a substance be smelled. With these animals the sense
of taste must take the place of the olfactory sense. But
among the insects, mostly terrestrial animals, there is an
extraordinary development of the sense of smell. It is in-
deed probably their principal special sense. Insects must
depend on smell far more than on sight or hearing for
the discovery of food, for becoming
aware of the presence of their enemies
and of the proximity of their mates
and companions. The organs of
smell of insects are situated princi-
pally on the antennae or feelers, a
single pair of which is borne on the
head of every insect (Fig. 143). That
many insects have an extraordinarily
keen sense of smell has been shown
by numerous experiments, and is con-
stantly proved by well-known habits.
If a small bit of decaying flesh be in-
closed in a box so that it is wholly
concealed, it will nevertheless soon
eating beetle, showing be found by the flies and carrion
^etles that either feed on carrion
or must always lay their eggs in de-
caying matter so that their carrion-eating larvae may be
provided with food. It is believed that ants find their
way back to their nests by the sense of smell, and that
they can recognize by scent among hundreds of individ-
uals taken from various communities the members of their

THE SPECIAL SENSES

231

own community. In the insectary at Cornell University,
a few years ago, a few females of the beautiful promethea
moth (Callosamia promethea) were inclosed in a box,
which was kept inside the insectary building. No males
had been seen about the insectary nor in its immediate
vicinity, although they had been sought for by collectors.
A few hours after the beginning of the captivity of the
female moths there were forty male prometheas fluttering
about over the glass roof of the insectary. They could not

FIG. 144.— Promethea moth, male, showing specialized antennae.

see the females, and yet had discovered their presence in
the building. The discovery was undoubtedly made by the
sense of smell. These moths have very elaborately devel-
oped antennae (Fig. 144), finely branched or feathered,
affording opportunity for the existence of very many smell-
ing-pits.

The keenness of scent of hounds and bird dogs is famil-
iar to all, although ever a fresh source of astonishment as
we watch these animals when hunting. We recently
watched a retriever dog select unerringly, by the sense of
smell, any particular duck out of a pile of a hundred. In

232 ANIMAL LIFE

the case of man the sense of smell is not nearly so well
developed as among many of the other vertebrates. This
inferiority is largely due to degeneration through lessened
need; for in Indians and primitive races the sense of
smell is keener and better developed than in civilized
races. Where man has to make his living by hunting, and
has to avoid his enemies of jungle and plain, his special
senses are better developed than where the necessity of
protection and advantage by means of such keenness of
scent and hearing is done away with by the arts of civi-
lization.

122. The sense of hearing. — Hearing is the perception
of certain vibrations of bodies. These vibrations give rise
to waves — sound waves as they are called — which proceed
from the vibrating body in all directions, and which, com-
ing to an animal, stimulate the special auditory or hearing
organs, that transmit this stimulation along the auditory
nerve to the brain, where it is translated as sound. These
sound waves come to animals usually through the air, or,
in the case of aquatic animals, through water, or through
both air and water.

The organs of hearing are of very complex structure
in the case of man and the higher vertebrates. Our ears,
which are adapted for perceiving or being stimulated by
vibrations ranging from 16 to 40,000 a second — that is, for
hearing all those sounds produced by vibrations of a rapid-
ity not less than 16 to a second nor greater than 40,000 to
a second — are of such complexity of structure that many
pages would be required for their description. But among
the lower or less highly organized animals the ears, or au-
ditory organs, are much simpler.

In most animals the auditory organs show the common
characteristic of being wholly composed of, or having as
an essential part, a small sac filled with liquid in which
one or more tiny spherical hard bodies called otoliths are
held. This auditory sac is formed of or lined internally by

THE SPECIAL SENSES

233

auditory cells, specialized nerve cells, which often bear
delicate vibratile hairs (Fig. 145). Auditory organs of this
general character are known among the polyps, the worms,
the crustaceans, and the mollusks. In the common cray-
fish the " ears " are situated in the basal segment of the
inner antennae or feelers (Fig. 14G). They consist each of
a small sac filled with liquid in which
are suspended several grains of sand
or other hard bodies. The inner

FIG. 145. — Auditory organ of a mollusk. a, audi-
tory nerve ; b, outer wall of connective tissue ;
c, cells with auditory hairs ; d, otolith.— After
LEYDIG.

PIG. 146. — Antenna of
cray - fish, with audi-
tory sac at base. —
After HUXLEY.

surface of the sac is lined with fine auditory hairs. The
sound waves coming through the air or water outside strike
against this sac, which lies in a hollow on the upper or
outer side of the antennae. The sound waves are taken up
by the contents of the sac and stimulate the fine hairs,
which in turn give this stimulus to the nerves which run
from them to the principal auditory nerve and thus to the
brain of the cray-fish. Among the insects 'other kinds of
auditory organs exist. The common locust or grasshopper

234

ANIMAL LIFE

has on the upper surface of the first abdominal segment
a pair of tympana or ear-drums (Fig. 147), composed sim-
ply of the thinned, tightly stretched chitinous
cuticle of the body. On the inner surface of this

FIG. 147.— Grasshopper, showing auditory organ (a. 0.) in first segment of abdomen.
(Wings of one side removed.)

ear-drum there are a tiny auditory sac, a fine nerve lead-
ing from it to a small auditory ganglion lying near the
tympanum, and a large nerve leading from this ganglion
to one of the larger ganglia situated on the floor of the

a.o

FIG. 148.— A cricket, showing auditory organ (a. o.) in fore-leg.

thorax. In the crickets and katydids, insects related to
the locusts, the auditory organs or ears are situated in the
fore-legs (Fig. 148).

Certain other insects, as the mosquitoes and other midges

OF THE ^

UNIVERSITY )

OF

THE SPECIAL SENSES

235

or gnats, undoubtedly hear by means of numerous delicate

hairs borne on the antennae. The male mosquitoes (Fig.

149) have many hundreds of these long, fine antennal hairs,

and on the sounding of a tuning-fork these hairs have been

observed to vibrate strongly. In the base of each antenna

there is a most elaborate organ,

composed of fine chitinous

rods, and accompanying nerves

and nerve cells whose function

it is to take up and transmit

through the auditory nerve to

the brain the stimuli received

from the external auditory

hairs.

123. Sound -making. — The
sense of hearing enables ani-
mals not only to hear the
warning natural sounds of
storms and falling trees and
plunging avalanches, but the
sounds made by each other.
Sound-making among animals
serves to aid in frightening
away enemies or in warning
companions of their approach,
for recognition among mates

and members of a band or species, for the attracting and
wooing of mates, and for the interchange of information.
With the cries and roars of mammals, the songs of birds,
and the shrilling and calling of insects all of us are familiar.
These are all sounds that can be heard by the human ear.
But that there are many sounds made by animals that
we can not hear — that is, that are of too high a pitch for
our hearing organs to be stimulated by — is believed by nat-
uralists. Especially is this almost certainly true in the case
of the insects. The peculiar sound-producing organs of

FIG. 149. — A male mosquito, showing
auditory hairs (a. h.) on the an-
tennae.

236 ANIMAL LIFE

many sound-making insects are known ; but certain other
insects, which make no sound that we can hear, neverthe-
less possess similar sound-making organs.

Sound is produced by mammals and birds by the strik-
ing of the air which goes to and comes from the lungs
against certain vibratory cords or flaps in the air-tubes.
Sounds made by this vibration are re-enforced and made
louder by arrangements of the air-tubes and mouth for
resonance, and the character or quality of the sound is
modified at will to a greater or less degree by the lips and
teeth and other mouth structures. Sounds so made are
said to be produced by a voice, or animals making sounds
in this way are said to possess a voice. Animals possessing
a voice have far more range and variety in their sound-
making than most of the animals which produce sounds in
other ways. The marvelous variety and the great strength
of the singing of birds and of the cries and roars of mam-
mals are unequaled by the sounds of any other animals.

But many animals without a voice — that is, which do not
make sounds from the air-tubes — make sounds, and some
of them, as certain insects, show much variety and range
in their singing. The sounds of insects are made by the
rapid vibrations of the wings, as the humming or buzzing
of bees and flies, by the passage of air out or into the body
through the many breathing pores or spiracles (a kind
of voice), by the vibration of a stretched membrane or
tympanum, as the loud shrilling of the cicada, and most
commonly by stridulation — that is, by rubbing together
two roughened parts of the body. The male crickets and
the male katydids rub together the bases of their wing
covers to produce their shrill singing. The locusts or
grasshoppers make sounds when at rest by rubbing the
roughened inside of their great leaping legs against the
upper surface of their wing covers, and when in flight by
striking the two wings of each side together. Numerous
other insects make sounds by stridulation, but many of

THE SPECIAL SENSES 237

these sounds are so feeble or so high in pitch that they are
rarely heard by us. Certain butterflies make an odd click-
ing sound, as do some of the water-beetles. In Japan,
where small things which are beautiful are prized not less
than large ones, singing insects are kept in cages and
highly valued, so that their capture becomes a lucrative
industry, just as it is with song birds in Europe and Amer-
ica. Among the many species of Japanese singing insects
is a night cricket, known as the bridle-bit insect, because
its note resembles the jingling of a bridle-bit.

124. The sense of sight. — Rot all animals have eyes.
The moles which live underground, insects, and other ani-
mals that live in caves, and the deep-sea fishes which live
in waters so deep that the light of the sun never comes
to them, have no eyes at all, or have eyes of so rudimentary
a character that they can no longer be used for seeing.
But all these eyeless animals have no eyes because they
live under conditions where eyes are useless. They have
lost their eyes by degeneration. There are, however, many
animals that have no eyes, nor have they or their ancestors
ever had eyes. These are the simplest, most lowly organ-
ized animals. Many, perhaps all eyeless animals are, how-
ever, capable of distinguishing light from darkness. They
are sensitive to light. An investigator placed several indi-
viduals of the common, tiny fresh- water polyp (Hydra) in a
glass cylinder the walls of which were painted black. He
left a small part of the cylinder unpainted, and in this part
of the cylinder where the light penetrated the Hydras all
gathered. The eyeless maggots or larvae of flies, when
placed in the light will wriggle and squirm away into dark
crevices. They are conscious of light when exposed to it,
and endeavor to shun it. Most plants turn their leaves
toward the light ; the sunflowers turn on their stems to
face the sun. Light seems to stimulate organisms whether
they have eyes or not, and the organisms either try to get
into the light or to avoid it. But this is not seeing.

238

ANIMAL LIFE

The simplest eyes, if we may call them eyes, are not
capable of forming an image or picture of external objects.
They only make the animal better capable of distinguish-
ing between light and darkness or shadow. Many lowly
organized animals, as some polyps, and worms, have certain
cells of the skin specially provided with pigment. These
cells grouped together form what is called a pigment fleck,
which can, because of the presence of the pigment, absorb
more light than the skin cells, and are more sensitive to
the light. By such pigment-flecks, or eye-spots, the animal
can detect, by their shadows, the passing near them of mov-
ing bodies, and thus be in some measure informed of the
approach of enemies or of prey. Some of these eye-flecks
are provided, not simply with pigment, but with a simple
sort of lens that serves to concentrate rays of light and

make this simplest
sort of eye even
more sensitive to
changes in the in-
tensity of light
(Fig. 150).

Most of the
many - celled ani-
mals possess eyes
by means of which
a picture of exter-
nal objects more or less nearly complete and perfect can
be formed. There is great variety in the finer structure
of these picture-forming eyes, but each consists essentially
of an inner delicate or sensitive nervous surface called the
retina, which is stimulated by light, and is connected with
the brain by a large optic nerve, and of a transparent light-
refracting lens lying outside of the retina and exposed to
the light. These are the constant essential parts of an
image - forming and image -perceiving eye. In most eyes
there are other accessory parts which may make the whole

FIG. 150.— The simple eye of a jelly-fish (Lizzia
koettikeri)—MteT O. and R. HBRTWIO.

THE SPECIAL SENSES

239

eye an organ of excessively complicated structure and of
remarkably perfect seeing capacity. Our own eyes are
organs of extreme structural complexity and of high de-
velopment, although some of the other vertebrates have
undoubtedly a keener and more nearly perfected sight.

The crustaceans and insects have eyes of a peculiar
character called compound eyes. In addition most insects
have smaller simple eyes. Each of the compound eyes is
composed of many (from a few, as in certain ants, to as
many as twenty-five thousand, as in certain beetles) eye ele-
ments, each eye element seeing independently of the other
eye elements and seeing only a very small part of any ob-
ject in front of the whole eye. All these small parts of
the external object seen by the many distinct eye elements
are combined so as to form an image in mosaic — that is,
made up of separate small parts — of the external object.
^^ If the head of a dragon-fly be exam-

Ov ined, it will be seen that

x two thirds or more of the

FIG. 151. — A dragon-fly, showing the large com-
pound eyes on the head.

PIG. 152.— Some of the facets
of the compound eye of a
dragon-fly.

whole head is made up of the two large compound eyes
(Fig. 151), and with a lens it may be seen that the outer
surface of each of these eyes is composed of many small
spaces or facets (Fig. 152) which are the outer lenses of
the many eye elements composing the whole eye.

CHAPTEE XIV

INSTINCT AND REASON

125. Irritability. — All animals of whatever degree of
organization show in life the quality of irritability or re-
sponse to external stimulus. Contact with external things
produces some effect on each of them, and this effect is
something more than the mere mechanical effect on the
matter of which the animal is composed. In the one-
celled animals the functions of response to external stimu-
lus are not localized. They are the property of any part of
the protoplasm of the body. Just as breathing or digestion
is a function of the whole cell, so are sensation and response
in action. In the higher or many-celled animals each of
these functions is specialized and localized. A certain set
of cells is set apart for each function, and each organ or
series of cells is released from all functions save its own.

126. Nerve cells and fibers. — In the development of the
individual animal certain cells from the primitive external
layer or ectoblast of the embryo are set apart to preside
over the relations of the creature to its environment.
These cells are highly specialized, and while some of them
are highly sensitive, others are adapted for carrying or
transmitting the stimuli received by the sensitive cells, and
still others have the function of receiving sense-impressions
and of translating them into impulses of motion. The
nerve cells are receivers of impressions. These are gathered
together in nerve masses or ganglia, the largest of these
being known as the brain, the ganglia in general being
known as nerve centers. The nerves are of two classes.

240

INSTINCT AND REASON 241

The one class, called sensory nerves, extends from the skin
or other organ of sensation to the nerve center. The nerves
of the other class, motor nerves, carry impulses to motion.

127. The brain or sensorium.— The brain or other nerve
center sits in darkness surrounded by a bony protecting
box. To this main nerve center, or sensorium, come the
nerves from all parts of the body that have sensation,
the external skin as well as the special organs of sight,
hearing, taste, smell. With these come nerves bearing sen-
sations of pain, temperature, muscular effort — all kinds of
sensation which the brain can receive. These nerves are
the sole sources of knowledge to any animal organism.
Whatever idea its brain may contain must be built up
through these nerve impressions. The aggregate of these
impressions constitute the world as the organism knows it.
All sensation is related to action. If an organism is not
to act, it can not feel, and the intensity of its feeling is
related to its power to act.

128. Reflex action. — These impressions brought to the
brain by the sensory nerves represent in some degree the
facts in the animal's environment. They teach something
as to its food or its safety. The power of locomotion is
characteristic of animals. If they move, their actions must
depend on the indications carried to the nerve center from
the outside; if they feed on living organisms, they must
seek their food ; if, as in many cases, other living organ-
isms prey on them, they must bestir themselves to escape.
The impulse of hunger on the one hand and of fear on the
other are elemental. The sensorium receives an impression
that food exists in a certain direction. At once an impulse
to motion is sent out from it to the muscles necessary to
move the body in that direction. In the higher animals
these movements are more rapid and more exact. This is
because organs of sense, muscles, nerve fibers, and nerve
cells are all alike highly specialized. In the star-fish the
sensation is slow, the muscular response sluggish, but the

17

242 ANIMAL LIFE

method remains the same. This is simple reflex action, an
impulse from the environment carried to the brain and
then unconsciously reflected back as motion. The impulse
of fear is of the same nature. Strike at a dog with a whip,
and he will instinctively shrink away, perhaps with a cry.
Perhaps he will leap at you, and you unconsciously will try
to escape from him. Reflex action is in general uncon-
scious, but with animals as with man it shades by degrees
into conscious action, and into volition or action " done on
purpose."

129. Instinct. — Different one-celled animals show differ-
ences in method or degree of response to external influences.
The feelers of the Amceba will avoid contact with the feel-
ers or pseudopodia of another Amoeba, while it does not
shrink from contact with itself or with an organism of un-
like kind on which it may feed. Most Protozoa will discard
grains of sand, crystals of acid, or other indigestible object.
Such peculiarities of different forms of life constitute the
basis of instinct.

Instinct is automatic obedience to the demands of ex-
ternal conditions. As these conditions vary with each kind
of animal, so must the demands vary, and from this arises
the great variety actually seen in the instincts of different
animals. As the demands of life become complex, so may
the instincts become so. The greater the stress of envi-
ronment, the more perfect the automatism, for impulses to
safe action are necessarily adequate to the duty they have
to perform. If the instinct were inadequate, the species
would have become extinct. The fact that its individuals
persist shows that they are provided with the instincts
necessary to that end. Instinct differs from other allied
forms of response to external condition in being hereditary,
continuous from generation to generation. This suffi-
ciently distinguishes it from reason, but the line between
instinct and reason and other forms of reflex action can
not be sharply drawn.

INSTINCT AND REASON 243

It is not necessary to consider here the question of the
origin of instincts. Some writers regard them as " inherited
habits," while others, with apparent justice, doubt if mere
habits or voluntary actions repeated till they become a
" second nature " ever leave a trace upon heredity. Such
investigators regard instinct as the natural survival of those
methods of automatic response which were most useful to
the life of the animal, the individuals having less effective
methods of reflex action having perished, leaving no pos-
terity.

An example in point would be the homing instinct of
the fur-seal. When the arctic winter descends on its home
in the Pribilof Islands in Bering Sea, these animals take
to the open ocean, many of them swimming southward as
far as the Santa Barbara Islands in California, more than
three thousand miles from home. While on the long swim
they never go on shore, but in the spring they return to
the northward, finding the little islands hidden in the arc-
tic fogs, often landing on the very spot from which they
were driven by the ice six months before, and their arrival
timed from year to year almost to the same day. The per-
fection of this homing instinct is vital to their life. If
defective in any individual, he would be lost to the herd
and would leave no descendants. Those who return be-
come the parents of the herd. As to the others the rough
sea tells no tales. We know that, of those that set forth, a
large percentage never comes back. To those that return
the homing instinct has proved adequate. This must be so
so long as the race exists. The failure of instinct would
mean the extinction of the species.

130. Classification of instincts. — The instincts of animals
may be roughly classified as to their relation to the indi-
vidual into egoistic and altruistic instincts.

Egoistic instincts are those which concern chiefly the
individual animal itself. To this class belong the instincts
of feeding, those of self-defense and of strife, the instincts

244 ANIMAL LIFE

of play, the climatic instincts, and environmental instincts,
those which direct the animal's mode of life.

Altruistic instincts are those which relate to parent-
hood and those which are concerned with the mass of indi-
viduals of the same species. The latter may be called the
social instincts. In the former class, the instincts of par-
enthood, may be included the instincts of courtship, re-
production, home-making, nest-building, and care for the
young.

131. Feeding. — The instincts of feeding are primitively
simple, growing complex through complex conditions.
The protozoan absorbs smaller creatures which contain
nutriment. The sea-anemone closes its tentacles over its
prey. The barnacle waves its feelers to bring edible crea-
tures within its mouth. The fish seizes its prey by direct
motion. The higher vertebrates in general do the same,
but the conditions of life modify this simple action to a
very great degree.

In general, animals decide by reflex actions what is
suitable food, and by the same processes they reject poisons
or unsuitable substances. The dog rejects an apple, while
the horse rejects a piece of meat. Either will turn away
from an offered stone. Almost all animals reject poisons
instantly. Those who fail in this regard in a state of
nature die and leave no descendants. The wild vetches or
" loco-weeds " of the arid regions affect the nerve centers of
animals and cause dizziness or death. The native ponies
reject these instinctively. This may be because all ponies
which have not this reflex dislike have been destroyed.
The imported horse has no such instinct and is poisoned.
Very few animals will eat any poisonous object with which
their instincts are familiar, unless it be concealed from smell
and taste.

In some cases, very elaborate instincts arise in connec-
tion with feeding habits. With the California woodpeckers
(Melanerpes formicivorus lairdii) a large number of them

INSTINCT AND REASON 245

together select a live-oak tree for their operations. They
first bore its bark full of holes, each large enough to hold
an acorn. Then into each hole an acorn is thrust (Figs.
61 and 62). Only one tree in several square miles may be
selected, and when their work is finished all those inter-
ested go about their business elsewhere. At irregular in-
tervals a dozen or so come back with much clamorous dis-
cussion to look at the tree. When the right time comes,
they all return, open the acorns one by one, devouring
apparently the substance of the nut, and probably also the
grubs of beetles which have developed within. When the
nuts are ripe, again they return to the same tree and the
same process is repeated. In the tree figured this has been
noticed each year since 1891.

132. Self-defense. — The instinct of self-defense is even
more varied in its manifestations. It may show itself
either in the impulse to make war on an intruder or in the
desire to flee from its enemies. Among the flesh-eating
mammals and birds fierceness of demeanor serves both for
the securing of food and for protection against enemies.
The stealthy movements of the lion, the skulking habits of
the wolf, the sly selfishness of the fox, the blundering good-
natured power of the bear, the greediness of the hyena, are
all proverbial, and similar traits in the eagle, owl, hawk,
and vulture are scarcely less matters of common observa-
tion.

Herbivorous animals, as a rule, make little direct resist-
ance to their enemies, depending rather on swiftness of
foot, or in some cases on simple insignificance. To the lat-
ter cause the abundance of mice and mouse-like rodents
may be attributed, for all are the prey of carnivorous beasts
and birds, and even snakes.

Even young animals of any species show great fear of
their hereditary enemies. The nestlings in a nest of the
American bittern when one week old showed no fear of
man, but when two weeks old this fear was very manifest

246

ANIMAL LIFE

(Figs. 153 and 154). Young mocking-birds will go into
spasms at the sight of an owl or a cat, while they pay little
attention to a dog or a hen. Monkeys that have never
seen a snake show almost hysterical fear at first sight of
one, and the same kind of feeling is common to most
men. A monkey was allowed to open a paper bag which

FlQ. 153.— Nestlings of the American bittern. Two of a brood of four birds one week
old, at which age they showed no fear of man. Photograph by E. H. TABOR,
Meridian, N. Y., May 31, 1898. (Permission of Macmillan Company, publishers of
Bird-Lore.)

contained a live snake. He was staggered by the sight,
but after a while went back and looked in again, to repeat
the experience. Each wild animal has its special instinct
of resistance or method of keeping off its enemies. The
stamping of a sheep, the kicking of a horse, the running
in a circle of a hare, and the skulking in a circle of some
foxes, are examples of this sort of instinct.

INSTINCT AND REASON

247

133. Play. — The play instinct is developed in numerous
animals. To this class belong the wrestlings and mimic
fights of young dogs, bear cubs, seal pups, and young
beasts generally. Cats and kittens play with mice. Squir-

FIG. 154.— Nestlings of the American bittern. The four members of the brood of
which two are shown in Fig. 153, two weeks old, when they showed marked fear
of man. Photograph by F. M. CHAPMAN, Meridian, N. Y., June 8, 1898. (Per-
mission of Macmillan Company, publishers of Bird-Lore.)

rels play in the trees. Perhaps it is the play impulse which
leads the shrike or butcher-bird to impale small birds and
beetles on the thorns about its nest, a ghastly kind of orna-
ment that seems to confer satisfaction on the bird itself.
The talking of parrots and their imitations of the sounds
they hear seem to be of the nature of play. The greater

248 ANIMAL LIFE

their superfluous energy the more they will talk. Much of
the singing of birds, and the crying, calling, and howling of
other animals, are mere play, although singing primarily be-
longs to the period of reproduction, and other calls and
cries result from social instincts or from the instinct to
care for the young.

134. Climate. — Climatic instincts are those which arise
from the change of seasons. When the winter comes the
fur-seal takes its long swim to the southward; the wild
geese range themselves in wedge-shaped nocks and fly high
and far, calling loudly as they go ; the bobolinks straggle
away one at a time, flying mostly in the night, and most of
the smaller birds in cold countries move away toward the
tropics. All these movements spring from the migratory
instinct. Another climatic instinct leads the bear to hide
in a cave or hollow tree, where he sleeps or hibernates till
spring. In some cases the climatic instinct merges in the
homing instinct and the instinct of reproduction. When
the birds move north in the spring they sing, mate, and
build their nests. The fur-seal goes home to rear its young.
The bear exchanges its bed for its lair, and its first business
after waking is to make ready to rear its young.

135. Environment. — Environmental instincts concern
the creature's mode of life. Such are the burrowing instincts
of certain rodents, the woodchucks, gophers, and the like.
To enumerate the chief phases of such instincts would be
difficult, for as all animals are related to their environ-
ment, this relation must show itself in characteristic in-
stincts.

136. Courtship. — The instincts of courtship relate chiefly
to the male, the female being more or less passive. Among
many fishes the male struts before the female, spreading
his fins, intensifying his pigmented colors through muscu-
lar tension, and in such fashion as he can makes himself the
preferred of the female. In the little brooks in spring
male minnows can be found with warts on the nose or head,

INSTINCT AND REASON 249

with crimson pigment on the fins, or blue pigment on the
back, or jet-black pigment all over the head, or with varied
combinations of all these. Their instinct is to display all
these to the best advantage, even though the conspicuous
hues lead to their own destruction. Against this contin-
gency Nature provides a superfluity of males.

Among the birds the male in spring is in very many
species provided with an ornamental plumage which he
sheds when the breeding season is over. The scarlet, crim-
son, orange, blue, black, and lustrous colors of birds are
commonly seen only on the males in the breeding season,
the young males and all males in the fall having the plain
brown gray or streaky colors of the female. Among the
singing birds it is chiefly the male that sings, and his voice
and the instinct to use it are commonly lost when the young
are hatched in the nest.

Among polygamous mammals the male is usually much
larger than the female, and his courtship is often a
struggle with other males for the possession of the female.
Among the deer the male, armed with great horns, fight
to the death for the possession of the female or for the
mastery of the herd. The fur-seal has on an average a
family of about thirty-two females (Fig. 71), and for the
control of his harem others are ready at all times to dispute
the possession. But with monogamous animals like the
true or hair seal or the fox, where a male mates with a
single female, there is no such discrepancy in size and
strength, and the warlike force of the male is spent on out-
side enemies, not on his own species.

137. Reproduction. — The movements of many migra-
tory animals are mainly controlled by the impulse to repro-
duce. Some pelagic fishes, especially flying-fishes and fishes
allied to the mackerel, swim long distances to a region
favorable for a deposition of spawn. Some species are
known only in the waters they make their breeding homes,
the individuals being scattered through the wide seas at

250 ANIMAL LIFE

other times. Many fresh-water fishes, as trout, suckers,
etc., forsake the large streams in the spring, ascending the
small brooks where they can rear their young in greater
safety. Still others, known as anadromous fishes, feed
and mature in the sea, but ascend the rivers as the im-
pulse of reproduction grows strong. Among such species
are the salmon, shad, alewife, sturgeon, and striped bass in
American waters. The most noteworthy case of the ana-
dromous instinct is found in the king salmon or quinnat
of the Pacific coast. This great fish spawns in November.
In the Columbia Eiver it begins running in March and
April, spending the whole summer in the ascent of the
river without feeding. By autumn the individuals are
greatly changed in appearance, discolored, worn, and distort-
ed. On reaching the spawning beds, some of them a thou-
sand miles from the sea, the female deposits her eggs in
the gravel of some shallow brook. After they arc fertilized
both male and female drift tail foremost and helpless down
the stream, none of them ever surviving to reach the sea.
The same habits are found in other species of salmon of
the Pacific, but in most cases the individuals of other spe-
cies do not start so early or run so far. A few species of
fishes, as the eel, reverse this order, feeding in the rivers
and brackish creeks, dropping down to the sea to spawn.

The migration of birds has relation to reproduction as
well as to changes of weather. As soon as they reach their
summer homes, courtship, mating, nest-building, and the
care of the young occupy the attention of every species.

138. Care of the young. — In the animal kingdom one of
the great factors in development has been the care of the
young. This feature is a prominent one in the specializa-
tion of birds and mammals. When the young are cared for
the percentage of loss in the struggle for life is greatly re-
duced, the number of births necessary to the maintenance
of the species is much less, and the opportunities for spe-
cialization in other relations of life are much greater.

INSTINCT AND REASON 251

In these regards, the nest-building and home-making
animals have the advantage over those that have not these
instincts. The animals that mate for life have the advan-
tage over polygamous animals, and those whose social or
mating habits give rise to a division of labor over those
with instincts less highly specialized.

The interesting instincts and habits connected with nest
or home building and the care of the young are discussed
in the next chapter.

139. Variability of instincts. — When we study instincts
of animals with care and in detail, we find that their regu-
larity is much less than has been supposed. There is as
much variation in regard to instinct among individuals as
there is with regard to other characters of the species.'
Some power of choice is found in almost every operation of
instinct. Even the most machine-like instinct shows some
degree of adaptability to new conditions. On the other
hand, in no animal does reason show entire freedom from
automatism or reflex action. " The fundamental identity
of instinct with intelligence," says an able investigator, " is
shown in their dependence upon the same structural mech-
anism (the brain and nerves) and in their responsive adap-
tability."

140. Reason. — Reason or intellect, as distinguished from
instinct, is the choice, more or less conscious, among re-
sponses to external impressions. Its basis, like that of in-
stinct, is in reflex action. Its operations, often repeated,
become similarly reflex by repetition, and are known as
habit. A habit is a voluntary action repeated until it be-
comes reflex. It is essentially like instinct in all its mani-
festations. The only evident difference is in its origin.
Instinct is inherited. Habit is the reaction produced with-
in the individual by its own repeated actions. In the
varied relations of life the pure reflex action becomes inade-
quate. The sensorium is offered a choice of responses. To
choose one and to reject the others is the function of intel-

252 ANIMAL LIFE

lect or reason. While its excessive development in man
obscures its close relation to instinct, both shade off by
degrees into reflex action. Indeed, no sharp line can be
drawn between unconscious and subconscious choice of
reaction and ordinary intellectual processes.

Most animals have little self-consciousness, and their
reasoning powers at best are of a low order ; but in kind,
at least, the powers are not different from reason in man.
A horse reaches over the fence to be company to another.
This is instinct. When it lets down the bars with its teeth,
that is reason. When a dog finds its way home at night by
the sense of smell, this may be instinct ; when he drags a
stranger to his wounded master, that is reason. When a
jack-rabbit leaps over the brush to escape a dog, or runs in
a circle before a coyote, or when it lies flat in the grass as a
round ball of gray indistinguishable from grass, this is in-
stinct. But the same animal is capable of reason — that is,
of a distinct choice among lines of action. Not long ago a
rabbit came bounding across the university campus at Palo
Alto. As it passed a corner it suddenly faced two hunting
dogs running side by side toward it. It had the choice of
turning back, its first instinct, but a dangerous one ; of
leaping over the dogs, or of lying flat on the ground. It
chose none of these, and its choice was instantaneous. It
ceased leaping, ran low, and went between the dogs just as
they were in the act of seizing it, and the surprise of the
dogs, as they stopped and tried to hurry around, was the
same feeling that a man would have in like circumstances.

On the open plains of Merced County, California, the
jack-rabbit is the prey of the bald eagle. Not long since a
rabbit pursued by an eagle was seen to run among the
cattle. Leaping from cow to cow, he used these animals
as a shelter from the savage bird. When the pursuit was
closer, the rabbit broke cover for a barbed wire fence.
When the eagle swooped down on it, the rabbit moved a
few inches to the right, and the eagle could not reach him

INSTINCT AND REASON 253

through the fence. When the eagle came down on the
other side, he moved across to the first. And this was con-
tinued until the eagle gave up the chase. It is instinct
that leads the eagle to swoop on the rabbit. It is instinct
again for the rabbit to run away. But to run along the line
of a barbed wire fence demands some degree of reason. If
the need to repeat it arose often in the lifetime of a single
rabbit it would become a habit.

The difference between intellect and instinct in lower
animals may be illustrated by the conduct of certain mon-
keys brought into relation with new experiences. At one
time we had two adult monkeys, " Bob " and " Jocko," be-
longing to the genus Macacus. Neither of these possessed
the egg-eating instinct. At the same time we had a baby
monkey, " Mono," of the genus Cercopithecus. Mono had
never seen an egg, but his inherited impulses bore a direct
relation to feeding on eggs, just as the heredity of Macacus
taught the others how to crack nuts or to peel fruit.

To each of these monkeys we gave an egg, the first that
any of them had ever seen. The baby monkey, Mono,
being of an egg-eating race, devoured his egg by the opera-
tion of instinct or inherited habit. On being given the
egg for the first time, he cracked it against his upper teeth,
making a hole in it, and sucked out all the substance.
Then holding the egg-shell up to the light and seeing that
there was no longer anything in it, he threw it away. All
this he did mechanically, automatically, and it was just as
well done with the first egg he ever saw as with any other
he ate. All eggs since offered him he has treated in the
same way.

The monkey Bob took the egg for some kind of nut.
He broke it against his upper teeth and tried to pull off
the shell, when the inside ran out and fell on the ground.
He looked at it for a moment in bewilderment, took both
hands and scooped up the yolk and the sand with which it
was mixed and swallowed the whole. Then he stuffed the

254 ANIMAL LIFE

shell itself into his mouth. This act was not instinctive.
It was the work of pure reason. Evidently his race was
not familiar with the use of eggs and had acquired no in-
stincts regarding them. He would do it better next time.
Eeason is an inefficient agent at first, a weak tool; but
when it is trained it becomes an agent more valuable and
more powerful than any instinct.

The monkey Jocko tried to eat the egg offered him in
much the same way that Bob did, but, not liking the taste,
he threw it away.

The confusion of highly perfected instinct with intellect
is very common in popular discussions. Instinct grows
weak and less accurate in its automatic obedience as the
intellect becomes available in its place. Both intellect and
instinct are outgrowths from the simple reflex response to
external conditions. But instinct insures a single definite
response to the corresponding stimulus. The intellect has
a choice of responses. In its lower stages it is vacillating
and ineffective ; but as its development goes on it becomes
alert and adequate to the varied conditions of life. It
grows with the need for improvement. It will therefore
become impossible for the complexity of life to outgrow
the adequacy of man to adapt himself to its conditions.

Many animals currently believed to be of high intelli-
gence are not so. The fur-seal, for example, finds it way
back from the long swim of two or three thousand miles
through a foggy and stormy sea, and is never too late or too
early in arrival. The female fur-seal goes two hundred
miles to her feeding grounds in summer, leaving the pup
on the shore. After a week or two she returns to find him
within a few rods of the rocks where she had left him.
Both mother and young know each other by call and by
odor, and neither is ever mistaken, though ten thousand
other pups and other mothers occupy the same rookery.
But this is not intelligence. It is simply instinct, because
it has no element of choice in it. Whatever its ancestors

INSTINCT AND EEASON 255

were forced to do the fur-seal does to perfection. Its in-
stincts are perfect as clockwork, and the necessities of
migration must keep them so. But if brought into new
conditions it is dazed and stupid. It can not choose when
different lines of action are presented.

The Bering Sea Commission once made an experiment
on the possibility of separating the young male fur-seals,
or " killables," from the old ones in the same band. The
method was to drive them through a wooden chute or run-
way with two valve-like doors at the end. These animals can
be driven like sheep, but to sort them in the way proposed
proved impossible. The most experienced males would
beat their noses against a closed door, if they had seen a
seal before them pass through it. That this door had been
shut and another opened beside it passed their comprehen-
sion. They could not choose the new direction. In like
manner a male fur-seal will watch the killing and skinning
of his mates with perfect composure. He will sniff at their
blood with languid curiosity ; so long as it is not his own
it does not matter. That his own blood may flow out on
the ground in a minute or two he can not foresee.

Eeason arises from the necessity for a choice among ac-
tions. It may arise as a clash among instincts which forces
on the animal the necessity of choosing. A doe, for ex-
ample, in a rich pasture has the instinct to feed. It hears
the hounds and has the instinct to flee. Its fawn may be
with her and it is her instinct to remain and protect it.
This may be done in one of several ways. In proportion as
the mother chooses wisely will be the fawn's chance of sur-
vival. Thus under difficult conditions, reason or choice
among actions rises to the aid of the lower animals as well
as man.

141. Mind. — The word mind is popularly used in two
different senses. In the biological sense the mind is the
collective name for the functions of the sensorium in men
and animals. It is the sum total of all psychic changes,

256 ANIMAL LIFE

actions and reactions. Under the head of psychic functions
are included all operations of the nervous system as well as
all functions of like nature which may exist in organisms
without specialized nerve fibers or nerve cells. As thus de-
fined mind would include all phenomena of irritability, and
even plants have the rudiments of it. The operations of
the mind in this sense need not be conscious. With the
lower animals almost all of them are automatic and uncon-
scious. With man most of them must be so. All func-
tions of the sensorium, irritability, reflex action, instinct,
reason, volition, are alike in essential nature though differ-
ing greatly in their degree of specialization.

In another sense the term mind is applied only to con-
scious reasoning or conscious volition. In this sense it is
mainly an attribute of man, the lower animals showing it
in but slight degree. The discussion as to whether lower
animals have minds turns on the definition of mind, and
our answer to it depends on the definition we adopt.

Pro. 155. — A "pointer" dog in the act of "pointing," a specialized instinct.
(Permission of G. O. Shields, publisher of Recreation.)

CHAPTER XV

HOMES AND DOMESTIC HABITS

142. Importance of care of the young. — The nest-building
and domestic habits of animals are adaptations, but adapta-
tions of behavior or habit rather than of structure, and are
based on instinct, intelligence, and reason. These instincts
and habits are among the most important shown by animals,
because on them depends largely the continuance of the
species. Of primary importance in the perpetuation of the
species is the possession by animals of adaptations of struc-
ture and behavior, which help the individual live long enough
to attain full development and to leave offspring. But in
the case of many animals a successful start in life on the
part of the offspring depends on the existence in the par-
ents of certain domestic habits concerned with the care and
protection of their young. The young of many animals de-
pend absolutely, for a part of their lifetime, on this parental
care. In these cases it is quite as necessary for the continued
existence of the species that the habits that afford this care
be successful as that the parent should come successfully to
mature development and to the production of offspring.

143. Care of the young, and communal life. — The nest-
building or home-making habits and the continued per-
sonal care of the young for a part of their lifetime are most
highly developed and widespread among the birds, mam-
mals, and insects ; and it is both among the insects and
the higher vertebrates that we find most developed those
social and communal habits discussed in Chapter IX. The
principal activities of an animal community have to do

18 257

258 ANIMAL LIFE

with the protection and sustenance of the young, and the
care of the young is undoubtedly a chief factor in the de-
velopment of communal life.

144. The invertebrates (except spiders and insects).—
Among the lower invertebrates parental aid to the young is
confined almost exclusively to exhibitions of pure instinct.
And this is true of many of the higher animals also. Eggs
are deposited in sheltered places, and in such places and
under such circumstances that the young on hatching will
find themselves close to a supply of their natural food. The
depositing of eggs in water by parents with terrestrial hab-
its whose young are aquatic, is an example. The toad,
which lives on land, feeding on insects, has young which
live in water and feed on minute aquatic plants (algae).
The dragon fly, that hawks over the pond or brook with
glistening wings, has young that crawl in the slime and
mud at the bottom of the pool. With most animals, after
laying eggs, the parents show no further solicitude toward
their offspring. The eggs are left to the vicissitudes of
fortune, and the parents know nothing of their fate. In
many cases the parent dies before the young are hatched.
The mammals and birds are the only two great groups ex-
cepted, although there are numerous specific exceptions
scattered among the lower invertebrates, fishes, batrachians,
and higher invertebrates, notably the insects.

There are no instances of care of the young after hatch-
ing among the sponges, polyps, worms, or star-fishes and
sea-urchins, and but few among the crustaceans and mol-
lusks. But there are in some of these groups a few cases
of nest or home building in a crude and simple way. Cer-
tain sea-urchins (Fig. 156) and worms and mollusks bore
into stones, and remain in the shelter afforded by the cav-
ity. A shell-fish (Lima hiams) cements together bits of
coralline, shells, and sand to form a crude nest or hiding-
place. The cray-fish digs a cylindrical burrow in the ground
in which it can hide.

HOMES AND DOMESTIC HABITS

259

145. Spiders.— Most spiders spin silken cocoons or sacs
in which to deposit their eggs. Some spiders carry this
egg-filled cocoon about with them for the sake of protect-
ing the eggs. After hatching, the spiderlings remain in the
cocoon a short time, feeding on each other ! Thus only the

FIG. 156.— Sea-nrchins living in holes bored into rocks of the seashore below high-
tide line.

strongest survive and issue from the cocoon to earn their
living in the outer world. "With certain species of spiders
the young after hatching leave the cocoon and gather on
the back of the mother and are carried about by her for
some time. In connection with their webs or snares many
spiders have silken tunnels or tubes in which to lie hidden,
a sort of sheltering nest. Those spiders that live on the
ground make for themselves cylindrical burrows or holes
in the ground, usually lined with silk, in which they hide
when not hunting for food. Especially interesting among
the many kinds of these spider nests are the burrows of
the various trap-door spiders. These spiders are common
in California and some other far Western States. The bur-

260 ANIMAL LIFE

row (Fig. 157) or cylindrical hole is closed above by a silken,
thick, hinged lid or door, a little larger in diameter than
the hole and neatly beveled on the edge, so as to fit tightly
into and perfectly cover the hole when closed. The upper
surface of the door is covered with soil, bits of leaves, and
wood, so as to resemble very exactly the ground surface
about it. "We have found these trap-door nests in Cali-
fornia in moss-covered ground, and here the lids of the nests
were always covered with green, growing moss.

An English naturalist who studied the habits of these
trap-door spiders found that if he removed the soil and bits
of bark and twigs, or the moss, from the upper surface of
the lid the spider always re-covered it. It is, of course,
plain that by means of this covering the nest is perfectly
concealed, the surface of the closed door not being dif-
ferent from the surrounding ground surface. This natu-
ralist finally removed the" moss not only from the surface
of a trap-door, but also from all the ground in a circle of a
few feet about the nest. The next day he found that the
spider had brought moss from outside the cleared space
and covered the trap-door with it, thus making it very con-
spicuous in the cleared ground space. The spider's instinct
was not capable of that quick modification to allow it to do
what a reasoning animal would have done — namely, cov-
ered the trap-door only with soil to make it resemble the
cleared ground about it.

Another interesting nest-making spider is the turret-
spider, that builds up a little tower (Fig. 158) of sticks and
soil and moss above its burrow. The sticks of which this
burrow are built are an inch or two in length, and are
arranged in such manner as make the turret five-sided.
The sticks are fastened together with silk, and the turret
is made two or three inches high. This turret-building
spider is one of those that carry about their egg-cocoon
with them. A female of this spider in captivity was ob-
served to pay much attention to caring for this cocoon.

262

ANIMAL LIFE

" If the weather was cold or damp, she retired to her tunnel ;
but if the jar in which she lived was set where the sun

could shine upon it, she soon re-
appeared and allowed the cocoon
to bask in the sunlight. If the
jar was placed near a stove that
had a fire in it, the cocoon was
put on the side next the source
of warmth ; if the jar
was turned around, she
lost no time in moving
the cocoon to the warmer
side. Two months after
the eggs were laid the
young spiders made their
appearance and immediately
perched upon their mother, many
on her back, some on her head,
and even on her legs. She car-
ried them about with her and fed
them, and until they were older
they never left their mother for
a moment."

146. Insects. — So much space
has already been devoted to an
account of the elaborate nest-making and domestic habits
of the bees, ants, and termites (see Chapter IX), that we
need in this place merely refer to that account. It is
among these social insects that the most interesting and
highly specialized habits connected with the care of the
young and the building of homes are found.

Many insects make for themselves simple burrows or
nests in the ground or in wood. The young or larvae of
certain moths burrow about in the soft inside tissue of
leaves, and the whole life of the moth except its short adult
stage is passed inside the leaf. These larvae are called leaf-

FIG. 158.— Nest of the turret-
spider.

HOMES AND DOMESTIC HABITS 263

miners. The larvae of some moths and of many hymenop-
terous insects live in galls on live plants. These galls are
simply abnormal growths of plant tissue, and are caused hy
the irritating effect on the tissue of the larvae which hatch
from eggs that have been thrust into the soft plant sub-
stance by the female insects. In the familiar galls on the
golden-rod live the larvae of a small moth, and in the vari-
ous kinds of oak galls live the young of the numerous spe-
cies of Cynipiclce, the hymenopterous gall insects. The tiny
larvae of some of the midges live in small galls on various
plants. To this last group of gall-making insects belongs
the dreaded Hessian fly, the most destructive insect pest of
wheat.

Among the bees and wasps only a few species, compara-
tively, are communal or live in communities. But nearly
all the wasps and bees, whether social or solitary in habit,
build nests for their young and provide the young with
food, either by storing it in the nest or by hunting for it
and bringing it to the nest as long as the young are in the
larval condition. The "mud-daubers" or thread-waisted
wasps make nests of mud attached to the lower surface of
flat stones, to the ceiling of buildings, or in other out-of-
the-way and safe places. These nests usually have the form
of several tubes an inch or so long placed side by side. In
each of the tubes or cells an egg is laid, and with it a
spider which has been stung so as to bo paralyzed but
not killed. When the young wasp hatches from the egg
as a grub or larva, it feeds on the unfortunate spider.
Others of the solitary wasps make nests in the ground
or in wood, and all of them provision their nests with
some particular kind of insect or spider. Some use only
caterpillars, some plant-lice, and some grasshoppers. Simi-
larly the solitary bees make nests in the ground as do the
mining-bees, or in wood as do the carpenter-bees, or by
cutting and fastening together leaves, as do the leaf-cutting
bees. The bees provision their nests, not with paralyzed

264 ANIMAL LIFE"

insects, but with masses of pollen or pollen mixed with
nectar.

147. The vertebrates. — It is among the vertebrates, espe-
cially in the higher groups, the birds and mammals, that
we find the care of the young most perfectly undertaken
and most widespread.

Among the fishes, the lowest of the vertebrates, most
species content themselves with the laying of many eggs in
a situation best suited for their safe hatching. But some
species show interesting domestic habits. The female cat-
fish swims about with her brood, much as a hen moves
about with her chickens. Some of the larger ocean cat-
fish of the tropics receive the eggs or the young within the
mouth for safety jyn time of danger. Certain sunfishes care
for their young, keeping them together in still places in the
brook. They also make some traces of a nest, which the
male defends. The male salmon scoops out gravel to make
a shallow nest, in which the female deposits her eggs. The
male then covers the eggs. The males of the species of
pipe-fish and sea-horses receive the eggs of the female into
a groove or sac between the folds of skin on the lower part
of the tail. Here they are kept until the little fishes are
large enough to swim about for themselves. The brave
little sticklebacks build tiny nests about an inch and a half
or two inches in diameter, with a small opening at the top.
The eggs are laid in this nest, and the young fish remain in
it some time after hatching. The male parent jealously
guards the nest, and fights bravely with would-be intruders.

The batrachians and reptiles rarely show any care for
their young. The eggs of most batrachians are laid in the
water and left by the female. The males of the Surinam
toad receive the eggs in pits of the spongy skin of the back,
where they remain until the young hatch. The eggs of
snakes are laid under logs or buried in the sand, and no
further attention is given them by the parents.

Among the birds, on the other hand, nest-building and

HOMES AND DOMESTIC HABITS

265

care of the young are the rule, and a high degree of devel-
opment in these habits is reached. All of us are familiar
with many different kinds of nests, from the few twigs
loosely put together hy the mourning-dove to the firm,
closely knit, wool or feather lined nest of the humming-
bird (Fig. 159), and the basket-like hanging nest of the

FIG. 159.— Nest and eggs of the Rufus humming-bird (Trochilus rufus). Photograph
by J. O. SNYDBB, Stanford University, California.

2C6

ANIMAL LIFE

oriole (Fig. 161). Not all birds make nests. On the rocky
islets of the northern oceans, where thousands of puffins
and auks and other maritime birds gather to breed, the
eggs are laid on the bare rock. At the other extreme is
the tailor bird of India, which sews together leaves by
means of fibrous strips plucked from a growing plant to

FIG. 160.— Neet and young of the Rufus humming-bird ( Trochilus rufus). Photograph
bjr J. O. SNTDEB, Stanford University, California.

HOMES AND DOMESTIC HABITS

267

FIG. 161. Baltimore orioles and nest ; the male in upper left-hand corner of figure.

form a long, bag-like nest (Fig. 162). In the degree of
care given the nestlings there is also much difference. The
robin brings food to the helpless young for many days, and

268

ANIMAL LIFE

finally teaches it to fly and to hunt for food for itself.
Young chickens are not so helpless as the nestling robins,
but are able to run about, and under the guiding

care of the hen mother to /^ pick up food for

themselves.

Among the mam-
mals the young are
always given some
degree of care. Ex-
cepting in the case
of the duck-bills, the
lowest of the mam-
mals, the young are
born alive — that is,
are not hatched from
eggs laid outside the
body— and are nour-
ished after birth for
a shorter or longer
time with milk
drawn from the
body of the mother.
Before birth the
young undergoes a
longer or shorter

period of development and growth in the body of the
mother, being nourished by the blood of the mother. The
nests or homes of mammals present varying degrees of
elaborateness, from a simple cave-like hole in the rocks
or ground to the elaborately constructed villages of the
beavers with their dams and conical several-storied houses
(Fig. 163). The wood-rat piles together sticks and twigs
in what seems, from the outside, a most haphazard fashion,
but which results in the construction of a convenient and
ingenious nest. The moles and pocket-gophers (Fig. 165)
build underground nests composed of chambers and gal-

FIG. 162.— Tailor-bird (OrnUhotomus sutorius)
and nest.

2TO

ANIMAL LIFE

FIG. 164.— Nest of the Californian bnsh-tit (Psaltriparus minimus). Photograph by
G. O. SNYDEK, Stanford University, California.

leries. The prairie-dogs make burrows in groups, forming
large villages.

The devotion to their young displayed by birds and
mammals is familiar to us. The parents will often risk or

HOMES AND DOMESTIC HABITS

271

suffer the loss of their own lives in protecting their off
spring from enemies. Many mother birds have the instinct
to flutter about a discovered nest crying and apparently
broken-winged, thus leading the predatory fox or weasel to

FIG. 165.— Nest and run-way of the pocket-gopher.

fix his attention on the mother and to leave the nest un-
harmed. This development of parental care and protec-
tion of the young reaches its highest degree in the case of
the human species. The existence of the family, which is
the unit of human society, rests on this high development
of care for the young.

CHAPTEE XVI

GEOGRAPHICAL DISTRIBUTION OF ANIMALS

148. Geographical distribution.— Under the head of dis-
tribution we consider the facts of the diffusion of organ-
isms over the surface of the earth, and the laws by which
this diffusion is governed,

The geographical distribution of animals is often known
as zoogeography. In physical geography we may prepare
maps of the earth which shall bring into prominence the
physical features of its surface. Such maps would show
here a sea, here a plateau, here a range of mountains,
there a desert, a prairie, a peninsula, or an island. In po-
litical geography the maps show the physical features of
the earth, as related to the states or powers which claim
the allegiance of the people. In zoogeography the realms
of the earth are considered in relation to the types or
species of animals which inhabit them. Thus a series of
maps of the United States could be drawn which would
show the gradual disappearance of the buffalo before the
attacks of man. Another might be drawn which would
show the present or past distribution of the polar bear,
black bear, and grizzly. Still another might show the
original range of the wild hares or rabbits of the United
States, the white rabbit of the Northeast, the cotton-tail of
the East and South, the jack-rabbit of the plains, the snow-
shoe rabbit of the Columbia Eiver, the tall jack-rabbit of
California, the black rabbits of the islands of Lower Cali-
fonia, and the marsh-hare of the South and the water-hare
of the canebrakes, and that of all their relatives. Such a
272

FIG. 166.— Map showing the distribution of the clouded Skipper butterfly (Lerema
accius) In the United States. The butterfly is found in that part of the country
shaded in the map, a warm and moist region. — After SCUDDEK.

•no

FIG. 167.— Map showing the distribution of the Canadian Skipper butterfly (Erynnis
manitoba} in the United States. The butterfly is found in that part of the
country shaded in the map. This butterfly is subarctic and subalpine in dis-
tribution, being found only far north or on high mountains, the two southern
projecting parts of its range being in the Rocky Mountains and in the Sierra
Nevada Mountains.— After SCUDDEB.
19

274 ANIMAL LIFE

map is very instructive, and it at once raises a series of
questions as to the reasons for each of the facts in geo-
graphical distribution, for it is the duty of science to sup-
pose that none of these facts is arbitrary or meaningless.
Each fact has some good cause behind it.

149. Laws of distribution. — The laws governing the dis-
tribution of animals are reducible to three very simple
propositions. Every species of animal is found in every part
of the earth having conditions suitable for its maintenance,
unless —

(a) Its individuals have been unable to reach this re-
gion, through barriers of some sort ; or —

(b) Having reached it, the species is unable to maintain
itself, through lack of capacity for adaptation, through
severity of competition with other forms, or through de-
structive conditions of environment ; or —

(c) Having entered and maintained itself, it has become
so altered in the process of adaptation as to become a spe-
cies distinct from the original type.

150. Species debarred by barriers. — As examples of the
first class we may take the absence of kingbirds or meadow-
larks or coyotes in Europe, the absence of the lion and
tiger in South America, the absence of the civet-cat in New
York, and that of the bobolink or the Chinese flying-fox in
California. In each of these cases there is no evident rea-
son why the species in question should not maintain itself
if once introduced. The fact that it does not exist is, in
general, an evidence that it has never passed the barriers
which separate the region in question from its original
home.

Local illustrations of the same kind may be found in
most mountainous regions. In the Yosemite Valley in
California, for example, the trout ascend the Merced River
to the base of a vertical fall. They can not rise above this,
and so the streams and lakes above this fall are destitute
of fish.

GEOGRAPHICAL DISTRIBUTION OF ANIMALS 275

151. Species debarred by inability to maintain their ground.

— Examples of the second class are seen in animals which
man has introduced from one country to another. The
nightingale, the starling, and the skylark of Europe have
been repeatedly set free in the United States. But none of
these colonies has long endured, perhaps from lack of adap-
tation to the climate, more likely from severity of competi-
tion with other birds. In other cases the introduced species
has been better fitted for the conditions of life than the
native forms themselves, and so has graduallv crowded out
the latter. Both these cases are illustrated among the rats.
The black rat, first introduced into America from Europe
about 1544, helped crowd out the native rats, while the
brown rat, brought in still later, about 1775, in turn practi-
cally exterminated the black rat, its fitness for the condi-
tions of life here being still greater than that of the other
European species.

Certain animals have followed man from land to land,
having been introduced by him against his will and to the
detriment of his domestic animals or crops. To many of
these the term vermin has been applied. Among the ver-
min or " animal weeds " are certain of the rodents (rats,
mice, rabbits, etc.), the mongoose of India, the English
sparrow, and many kinds of noxious insects. Of all the
vermin of this class few have caused such widespread de-
struction of property as the common European rabbit intro-
duced into Australia. The annual loss through its presence
is estimated at $3,500,000.

It often happens that man himself so changes the en-
vironment of a species that it can no longer maintain it-
self. Checking the increase of a species, either by actually
killing off its members or by adverse change in its sur-
roundings, is to begin the process of its destruction. Cir-
cumstances become unfavorable to the growth or reproduc-
tion of an animal. Its numbers are reduced, fewer are
born each year, and fewer reach maturity, it grows rare,

276 ANIMAL LIFE

is gone, and the final step of extinction may often pass
unnoticed.

But a few years ago the air in the Ohio Valley was dark
in the season of migration with the hordes of passenger
pigeons. The advance of a tree-destroying, pigeon-shooting
civilization has gone steadily on, and now the bird which
once crowded our Western forests is in the same region an
ornithological curiosity. The extinction of the American
hison or " buffalo," and the growing rarity of the grizzly
bear, the wolf, and of large carnivora generally, furnishes
cases in point. When Bering and Steller landed on the
Commander Islands in 1741, the sea-cow, a large herbivo-
rous creature of the shores, was abundant there. In about
fifty years the species, being used for food by fishermen,
entirely disappeared. In most cases, however, a species
that crosses its limiting barriers, but is unable to main-
tain itself, leaves no record of the occurrence. We know, as
a matter of fact, that stray individuals are very often found
outside the usual limit of a species. A tropical bird may
be found in New Jersey, a tropical fish on Cape Cod, or a
bird from Europe on the shores of Maine. Of course,
hundreds of other cases of this sort must escape notice ;
but, for one reason or another, the great majority of these
waifs are unable to gain a new foothold. For this reason,
outside of the disturbances created by man, the geographical
distribution of species changes but little from century to
century ; and yet, when we study the facts more closely,
evidences of change appear everywhere.

152. Species altered by adaptation to new conditions.

Of the third class or species altered in a new environment
examples are numerous, but in most cases the causes in-
volved can only be inferred from their effects. One class
of illustrations may be taken from island faunse. An island
is set off from the mainland by barriers which species of
land animals can very rarely cross. On an island a few waifs
of wave and storm may maintain themselves, increasing in

JTie. 168— The manatee, or sea-cow ( Trichechus latirostris). A living species of sea-
cow related to the now extinct Steller's sea-cow.

278

ANIMAL LIFE

Fio. 169.— On the shore of Narborongh Island, one of the Galapagos Islands, Pacific
Ocean, showing peculiar species of sea-lions, lizards, and cormorants. Drawn
from a photograph made by Messrs. SNODGBASS and HELLER.

numbers so as to occupy the territory; but in so doing
only those will survive that can fit themselves to the new
conditions. Through this process a new species will be
formed, like the parent species in general structure, but
having gained new traits adjusted to the new environment.

GEOGRAPHICAL DISTRIBUTION OF ANIMALS 279

The Galapagos Islands are a cluster of volcanic rocks
lying in the open sea about six hundred miles to the west
of Ecuador. On these islands is a peculiar land fauna, de-
rived from South American stock, but mostly different in
species. Darwin noted there "twenty-six land birds; of
these, twenty-one, or perhaps twenty-three, are ranked as
distinct species. Yet the close affinity of most of these
birds to American species is manifest in every character, in
their habits, gestures, and tones of voice.7'

Among land animals similar migrations may occur, giv-
ing rise, through the adaptation to new conditions, to new
species. The separation of species of animals isolated in
river basins or lakes often permits the acquisition of new
characters, which is the formation of distinct species in
similar fashion. On the west side of Mount Whitney, the
highest mountain in the Sierra Nevada of California, there
is a little stream called Volcano Creek. In this brook is a
distinct species or form of trout, locally called golden
trout. It is unusually small, very brilliantly colored, its
fins being bright golden, and its tiny scales scarcely over-
lap each other along its sides. This stream flows over a
high waterfall (Agua Bonita) into the Kern Eiver. The
Kern River is full of trout, of a kind (Salmo gilberti) to
which the golden trout is most closely allied. There can
not be much doubt that the latter is descended from the
former. With this assumption, it is easy to suppose that
once the waterfall did not exist, or that through some
agency we can not now identify certain fishes had been
carried over it. Once above it, they can not now return,
nor can they mix with the common stock of the river.
Those best adapted to the little stream have survived.
The process of adaptation has gone on till at last a distinct
species (or sub-species *) is formed. In recent times the

* In descriptive works the name species is applied to a form when
the process of adaptation seems complete. When it is incomplete, or

FIG. 171. — Three species of jack-rabbits, differing in size, color, and markings, but
believed to be derived from a common stock. The differences have arisen
through isolation and adaptation. The upper figure shows the head and fore legs
of the black jack-rabbit (Lepus insularis), of Espiritu Santo Island, Gulf of
California ; the lower right-hand figure, the Arizona jack-rabbit (Lepus alleni),
specimen from Fort Lowell, Arizona ; and the lower left-hand figure is the San
Pedro Martir jack-rabbit (Lepus martirensis), from San Pedro Martir, Baja
California.

282

ANIMAL LIFE

hand of man has carried the golden trout to other little
mountain torrents, where it thrives as well as in the one
where its peculiarities were first acquired.

Other cases of this nature are found among the blind
fishes of the caves in different parts of the world (Fig. 172).

In general, caves are
formed by the ero-
sion or wearing of
underground rivers.
These streams are
usually clear and cold,
and when they issue
to the surface those
fishes that like cold
and shaded waters
are likely to enter
them. But to have
eyes in absolute dark-
ness, in which no use
can be made of them,
is a disadvantage in
the struggle for life.
Hence the eyed species die or withdraw, while those in which
the eye grows less from generation to generation, until its
function is finally lost, are the ones which survive. By such
processes the blind fishes in the limestone caves of Ken-
tucky, Indiana, Tennessee, and Missouri have been formed.

FIG. 172. — Fishes showing stages in the loss of eyes
and color. A, Dismal Swamp fish (Chologaster
avitus), ancestor of the blind fish ; B, Agassiz's
cave fish ( Chologaster agassizi) ; C, cave blind
fish (Typhlichthys subterr emeus).

rather when specimens showing intergradation of characters are known,
the word sub-species is used. The word variety has much the same
meaning when used for a subdivision of a species, but it is a term
defined with less exactness. Thus the common fox ( Vulpes pennsyl-
vanicus) is a distinct species, being separate from the arctic fox or the
gray fox or the fox of Europe. The cross fox ( Vulpes pennsylvanicus
decussatus) is called a sub-species, as is the silver fox ( Vulpes pennsyl-
vanicus argentatus), because these intergrade perfectly with the common
red fox.

GEOGRAPHICAL DISTRIBUTION OP ANIMALS 283

To processes of this kind, on a larger or smaller scale,
the variety in the animal life of the globe is very largely
due. Isolation and adaptation give the clew to the forma-
tion of a very large proportion of the " new species " in
any group.

153. Effect of barriers. — It will be thus seen that geo-
graphical distribution is primarily dependent on barriers or
checks to the movement of animals. The obstacles met
in the spread of animals determine the limits of the spe-
cies. Each species broadens its range as far as it can. It
attempts unwittingly, through natural processes of increase,
to overcome the obstacles of ocean or river, of mountain or
plain, of woodland or prairie or desert, of cold or heat, of
lack of food or abundance of enemies — whatever the bar-
riers may be. Were it not for these barriers, each type or
species would become cosmopolitan or universal. Man is
pre-eminently a barrier-crossing animal. Hence he is found
in all regions where human life is possible. The different
races of men, however, find checks and barriers entirely
similar in nature to those experienced by the lower animals,
and the race peculiarities are wholly similar to characters
acquired by new species under adaptation to changed con-
ditions. The degree of hindrance offered by any barrier
differs with the nature of the species trying to surmount it.
That which constitutes an impassable obstacle to one form
may be a great aid to another. The river which blocks the
monkey or the cat is the highway of the fish or the turtle.
The waterfall which limits the ascent of the fish is the
chosen home of the ouzel. The mountain barrier which
the bobolink or the prairie-dog does not cross may be the
center of distribution of the chief hare or the arctic blue-
bird.

154. Relation of species to habitat.— The habitat of a
species of animal is the jegion in which it is found in a
state of Nature. It is currently believed that the habitat
of any creature is the region for which it is best adapted.

284 ANIMAL LIFE

But the reverse of this is often true. There are many cases
in which a species introduced in a new territory, through
the voluntary or involuntary influence of man, has shown a
marvelous adaptation and power of persistence. The rapid
spread of rabbits and pigs as wild animals in Australia, of
horses and cattle in South America, and of the English
sparrow in North America, of bumble-bees and house-
flies in New Zealand, are illustrations of this. Not one
of these animals has maintained itself in the wild state
in its native land as successfully as in these new countries
to which it has been introduced. The work of introduc-
tion of useful animals illustrates the same fact. The shad,
striped bass, and cat-fish from the Potomac River, intro-
duced into the Sacramento River and its tributaries by the
United States Fish Commission, are examples in point.
These valued food-fishes are nowhere more at home than in
the new waters where no species of their types had ever
existed before. The carp, originally brought to Europe
from China, and thence to the United States as a food-
fish, becomes in California a nuisance, which- can not be
eradicated, destroying the eggs and the foodstuff of far
better fish.

In all mountain regions waterfalls are likely to occur,
and these serve as barriers, preventing the ascent of trout
and other fishes. On this account in the mountains of Cali-
fornia, Colorado, Wyoming, and other States, hundreds of
lakes and streams suitable for trout are found in which no
fishes ever exist. In the Yellowstone Park this fact is es-
pecially noticeable. This region is a high volcanic plateau,
formed by the filling of an ancient granite basin with a vast
deposit of lava. The streams of the park are very cold and
clear, in every way favorable for the growth of trout ; yet,
with the exception of a single stream, the Yellowstone
River, none of the streams was found to contain any fish
in that part of it lying on the plateau. Below the plateau
all of them are w^ll stocked. The reason for this is ap-

Is

1!
§§.

ll

286 - ANIMAL LIFE

parent in the fact that the plateau is fringed with cataracts
which fishes can not ascend. Each stream has a canon or
deep gorge with a waterfall at its head, near the point
where it leaves the hard bed of black lava for the rock
below (Fig. 173). So for an area of fifteen hundred square
miles within the Yellowstone National Park the streams
were without trout because their natural inhabitants had
never been able to reach them. When this state of things
was discovered it was easy to apply the remedy. Trout of
different species were carried above the cascades, and these
have multiplied with great rapidity.

The exception noted above, that of the Yellowstone
River itself, evidently needs explanation. An abundance
of trout is found in this river both above and below the
great falls, and no other fish occurs with it. This anomaly
of distribution is readily explained by a study of the tribu-
taries at the head waters of the river. When we ascend
above Yellowstone Lake to the continental divide, we find
on its very summit that only about an eighth of a mile of
wet meadow and marsh, known as Two Ocean Pass (Fig.
174), separates the drainage of the Yellowstone from that
of the Columbia. A stream known as Atlantic Creek flows
into the Yellowstone, while the waters of Pacific Creek on
the other side find their way into the Snake River. These
two creeks are connected by waterways in the wet meadow,
and trout may pass from one to the other without check.
Thus from the Snake River the Yellowstone received its
trout, and from the Yellowstone they have spread to the
streams tributary to the upper Missouri.

This case is a type of the anomalies in distribution of
which the student of zoogeography will find many. But
each effect depends upon some cause, and a thorough study
of the surroundings or history of a species will show what
this cause may be. In numerous cases in which fishes have
been found above an insurmountable cascade, the cause is
seen in a marsh flooded at high water, connecting one

II

2 *

11

ge «

288 ANIMAL LIFE

drainage basin with another. An example of this is found
in Lava Creek in Yellowstone Park. Above Undine and
Wraith Falls, both insurmountable, are found an abun-
dance of trout. A marsh dry in summer connects Lava
Creek with Black Tail Deer Creek, a tributary of the
Yellowstone and without waterfall. From the Yellow-
stone through this creek and marsh the trout find their
way into Lava Creek. In California numerous anomalies
have been noted, as the occurrence of Tahoe trout in
Feather Eiver and in the Blue Lakes of Amador, which are
on the other side of the main crest of the Sierra Nevada
from Lake Tahoe, and the occurrence of the Whitney
golden trout in Lone Pine Creek, another similar instance.
In each case naturalists have found the man who actually
carried the species across the divide. If this matter had
been investigated a generation later, these cases would have
been unexplainable anomalies in geographical distribution.
Eeal causes are almost always simple when they are once
known.

The ways in which species may cross barriers in a state
of Nature are as varied as the creatures themselves, and far
more varied than the actual barriers. By the long-con-
tinued process of adjustment to conditions with the inces-
sant destruction of the unadapted, the various organisms
have become so well fitted to their surroundings that the
casual observer may well suppose that each inhabits the
region best fitted for it. Men have even thought that the
conditions of life have been fitted to the creatures them-
selves, so perfect is this relation.

155. Character of barriers to distribution. — Taking the
animal kingdom as a whole, the two great barriers modify-
ing distribution are the presence of the sea and changes in
temperature. It is only in rare cases that any land ani-
mals can cross either of the great oceans, and these rare
cases relate chiefly to the arctic regions. For this reason
the land faunae of Africa, South America, and Australia

GEOGRAPHICAL DISTRIBUTION OF ANIMALS 289

have developed almost independently of one another. To
the fresh-water fishes the sea forms equally a barrier, and

even the shore-fishes very rarely pass across great depths.
Relatively few of the shore-fishes of Cuba, for example, ever
cross the deep Florida Straits, and none of those of Cali-
20

290 ANIMAL LIFE

fornia ever reach Honolulu, nor are Hawaiian shore-fishes
ever seen on the coast of California. For these reasons
natural boundaries of the great realms of distribution are
found in the sea.

The other great check to distribution is found in heat
and cold. Most of the tropical animals can not endure
frost. The arctic animals, however fierce or active, are
enfeebled by heat. The timber line, north of which and
above which frost occurs the year round, therefore serves

PIG. 176. — Alligators ; animals found only in the warm waters of tropical and sub-
tropical regions.

as a boundary of limitation. Another equally marked is
the frost line. Even the fishes of the tropics are extreme-
ly sensitive to slight cold. Off Florida Keys the cutlass-
fish is sometimes seen stiff and benumbed on the water,
where the temperature is scarcely below 60° Fahr. A
"norther" on the Gulf of Mexico will sometimes bring
fishes which live in considerable depths to the surface,
through chilling the water. These barriers are rarely
crossed by localized species, but many forms, especially
birds, keep within a relatively uniform temperature through
migration. The summers are spent in the north or in the
mountains, the winters in districts that are warmer.

GEOGRAPHICAL DISTRIBUTION OF ANIMALS 291

The climate, as distinct from the temperature, also
greatly influences many species. In the Eastern United
States and in the extreme Northwest, as in Europe and
much of Asia, the atmosphere is humid all the year long.
Eains occur at intervals in the summer, and rain or snow in
the winter. The green season is from spring to fall, and the
resting of plants is in the winter. To this condition the
native animals adapt themselves, and this would seem to
be the natural order of things.

But as we pass the Western plains of Nebraska, Kan-
sas, and Texas this condition is materially changed. For
part of the year rainfall is practically unknown. The air
becomes dry, and under the cloudless sky the greater part
of the vegetation ripens its seed and perishes. This is the
arid climate. When the rainfall is very scant the region
is never covered with verdure, and is known as desert.
Such great desert tracts are found in parts of Wyoming,
Utah, Nevada, Idaho, Colorado, Arizona, New Mexico, Cali-
fornia, as well as in the northern parts of Mexico. In some
cases the deserts are exposed to great heat, forming an
ultra-torrid region, as in Death Valley in California and in
certain parts of Arizona.

But the arid region is not as a whole desolate. In many
parts rain falls more or less heavily for part of the year,
bringing a rank growth of annual grasses and of verdure
in general. In California this rainfall is in the winter, the
coldest part of the year, and the country is green from
November or October to June or May. In Mexico and
northward to Colorado the chief rainfall is in midsummer,
the warmest part of the year, and the summer is the time
of verdure.

To all these conditions the plant life must adapt itself
and with this the animal life. But the species that have
become fitted to the arid habitat have undergone some
change in the process and may have become different spe-
cies. It is, then, not easy for them to recross the barrier

292 ANIMAL LIFE

of climate to compete with those forms already adapted.
For this reason a marked change of climate like a marked
change of temperature forms a natural barrier to distribu-
tion and serves to circumscribe a natural fauna.

Closely associated with climate is the nature of forest
growth, the growth of grass, and in general the development
of conditions which serve for food or shelter to animals.
These conditions depend in part on soil, partly on climate
and topography, and partly on the acts of man. The for-
est and forest soils, acting like a great sponge, retain the
waters of the rainy season, and thus regulate the size of
the streams. The stream that changes least in volume is
most favorable to the life of fishes, frogs, and water ani-
mals generally. The destruction of forests on the moun-
tain sides acts adversely to the life of these creatures as
well as to the interests of the farmer below whose lands
the streams should water. When the forests are destroyed,
the great host of wood creatures, the bears, squirrels, war-
blers, various birds, beasts, and insects of the woods can no
longer maintain themselves, and grow rare and disappear.
For reasons that are obvious the conditions that produce
forest, prairie, canebrake, sage -desert, cactus - desert, and
the like are potent in regulating the distribution of the
species.

Still another set of conditions depends on the food sup-
ply. The planting of orchards tends to multiply greatly
the number of individuals of those species which prey upon
fruit. When food is abundant the severity of the struggle
for life is relaxed and individuals increase in number. A
species may be put to great stress by the disappearance of
the animal or plant on which it has depended. Each
change made by man among the wild animals or plants
may have far-reaching effects upon others. The coyote or
prairie-wolf destroys sheep in the ranges of the West. It
is thinned out by means of the bounty upon its scalp.
Then the jack-rabbit, on which it also feeds, greatly in-

GEOGRAPHICAL DISTRIBUTION OF ANIMALS 293

creases in abundance, injuring fruit trees and grain fields.
It is then necessary to pay for its destruction also.

To destroy hawks or owls because they catch chickens
may increase the numbers and destructiveness of field-mice
on which they also prey. To shoot robins, linnets, and
other birds that destroy small fruits is likely to increase
greatly the insect pests on which these birds also feed.
The inter-relations of species and species are so close that
none should be exterminated by man unless its habits and
relations have been subjected to careful scientific study.
Still less should any new ones be introduced without the
fullest consideration of the possible results. For example,
the mongoose, a weasel-like creature, was introduced from
India into Jamaica to kill rats and mice. It killed also the
lizards, and thus produced a plague of fleas, an insect which
the lizards kept in check. The English sparrow, intro-
duced that it might feed on insects inhabiting shade-trees,
has become a nuisance, crowding out better birds and not
accomplishing the purpose for which it was brought to the
United States.

To most kinds of animals a mountain range must act as
a barrier to distribution. In a region having high moun-
tains a species will become in time split up into several,
because the individuals in one valley will be isolated from
those of another. The fauna of California furnishes many
illustrations of this, as among its mountain chains are
many deep valleys shut off from each other and having
different peculiarities of temperature. For this reason two
counties of California differ much more widely in their
fauna than do two counties in Illinois. But Illinois as a
whole has more different kinds of animals than California,
because no barrier anywhere prevents their entrance. The
State has, we may say, its doors wide open to immigrants
from all quarters. The same is true of Iowa or of Kansas
or Kentucky. Illinois has a richer fauna than Iowa, be-
cause its extension is north and south, and it therefore

294 ANIMAL LIFE

covers a wider range of climate. Kentucky lias a richer
fauna than Iowa because it includes a greater variety of
conditions. New England was called by Professor Agassiz
a " zoological island," because of the relatively small num-
ber of its native animals, especially of species inhabiting its
rivers. The cause of this is found in its isolation, being
shut off from the Middle States by mountain ranges, while
it is bounded on two sides by the sea.

156. Barriers affecting fresh-water animals. — The animals
inhabiting fresh-water streams are affected by differences in
temperature and elevation much as land animals are. They
tend to spread from stream to stream whenever they can
find their way. An isolated stream is likely to have its
peculiar fauna just as island life is likely to differ from
that of the mainland. The same species wanders widely
within the limits of a single river basin. If a kind of fish
establishes itself anywhere in the Mississippi Valley, it may
find its way to every stream in the whole basin. If it likes
cold spring water, as the rainbow-darter does, we may look
for it in any cold spring. If, like the long-eared sun-fish,
it frequents deep pools in the brooks, we may look for it
under roots of stumps and in every " swimming hole." If,
like the channel-cat, it chooses the ripples of a river, we
may fish for it wherever ripples are. The larger the whole
river basin the more species find their way into it, and
therefore the greater the number of species in any one of
its streams.

Each species finds its habitat fitted to its life, and then
in turn is forced to adapt itself to this habitat. Any other
kind of habitat then appears as a barrier to its distribu-
tion. Thus to a fish of the ripples a stretch of still water
becomes a barrier. A species adapted to sandy bottoms
will seldom force its way through swift waters or among
weeds or rocks. The effect of waterfalls as barriers is else-
where noticed. In some streams the dam made by a colony
of beavers has the same effect. Mill-dams and artificial

GEOGRAPHICAL DISTRIBUTION OF ANIMALS 295

waterfalls have checked the movements of many species,
while others have been helped by artificial channels or
canals. Streams that run muddy at times are not favor-
able for animal life. Still less favorable is the condition
frequent in the arid region in which streams are full to
the banks in the rainy season and shrunk to detached
pools in the dry months.

The stream that has the greatest variety of animals in it
would be one (1) connected with a large river, (2) in a warm
climate, (3) with clear water and (4) little fluctuation from
winter to summer, (5) with little change in the clearness of
the water, (6) a gravelly bottom, (7) preferably of lime-
stone, and (8) covered in its quiet reaches and its ripples
with water-weeds. These conditions are best realized in
tributaries of the Ohio, Cumberland, Tennessee, and Ozark
Rivers among American streams, and it is in them that the
greatest number of species of fresh-water animals (fishes,
cray-fishes, mussels, etc.) has been recorded. These streams
approach most nearly to the ideal homes for animals of the
fresh waters. The streams of Wisconsin, Michigan, and the
Columbia region have many advantages, but are too cold.
Those of Illinois, Iowa, northern Missouri, and Kansas are
too sluggish, and sometimes run muddy. Those of Texas
and California shrink too much in summer, and are too
isolated The streams of the Atlantic coast are less iso-
lated, but none connect with a great basin, and those of
New England run too cold for the great mass of the spe-
cies. For similar reasons the fresh-water animal life of
Europe is relatively scanty, that of the Danube and Volga
being richest. The animal life of the fresh water of South
America centers in the Amazon, and that of Africa in the
Nile, the Niger, and the Congo. The great rivers of Si-
beria, like the Yukon in Alaska and the Mackenzie River
in British America, have but few forms of fresh- water ani-
mals, though those kinds fitted for life in cold, clear water
exist in great abundance.

296 ANIMAL LIFE

157. Modes of distribution. — The means and modes of mi-
gration and distribution are obvious in the case of animals
that can fly or swim or make long journeys on foot. An
island can be visited and become peopled by birds from the
nearest mainland. Fishes and marine mammals can travel
from ocean to ocean. But many animals have no means
of crossing watery barriers. " Oceanic islands, that have
been formed de novo in mid-ocean and are not detached
portions of pre-existing continents, are almost invariably
free from such animals as are incapable of traversing the
sea. If sufficiently distant from any continent, oceanic
islands are generally without mammals, reptiles, and am-
phibia, but have both birds and insects and certain other
invertebrates which are transported to them by involuntary
migration."

As suggested in the last sentence, migration may be
passive or involuntary. For example, those minute ani-
mals that can become dried up and yet retain the power
of renewing their active life under favorable conditions are
sometimes carried in the dried mud adhering to the feet of
birds, and may thus become widely distributed. Parasites
are carried by their hosts in all their wanderings. Some
animals, as rats and mice, are carried by ships and railway
trains and thus widely distributed.

158. Fauna and fauna! areas. — The term fauna is applied
to the animals of any region considered collectively. Thus
the fauna of Illinois comprises the entire list of animals
found naturally in that State. It includes the aboriginal
men, the black bear, the fox, and all its animal life down
to the Ammba,. The relation of the fauna of one region
to that of another depends on the ease with which bar-
riers may be crossed. Thus the fauna of Illinois differs
little from that of Indiana or Iowa, because the State con-
tains no barriers that animals may not readily pass. On
the other hand, the fauna of California or Colorado differs
materially from that of adjoining regions, because a moun-

GEOGRAPHICAL DISTRIBUTION OF ANIMALS 297

tainous country is full of barriers which obstruct the diffu-
sion of life. Distinctness is in direct proportion to isola-
tion. What is true in this regard of the fauna of any region
is likewise true of its individual species. The degree of
resemblance among individuals is in strict proportion to the
freedom of their movements. Variation within the limits
of a species is again proportionate to the barriers which
prevent equal and free diffusion.

159. Realms of animal life. — The various divisions or
realms into which the land surface of the earth may be
divided on the basis of the character of animal life have
their boundary in the obstacles offered to the spread of the
average animal. In spite of great inequalities in this regard,
we may yet roughly divide the land of the globe into seven
principal realms or areas of distribution, each limited by
barriers, of which the chief are the presence of the sea and
the occurrence of frost. There are the Arctic, North Tem-
perate, South American, Indo-African, Lemurian, Patago-
nian, and Australian realms. Of these the Australian
realm alone is sharply defined. Most of the others are sur-
rounded by a broad fringe of debatable ground that forms
a transition to some other zone.

The Arctic realm includes all the land area north of the
isotherm of 32°. Its southern boundary corresponds closely
with the northern limit of trees. The fauna of this region
is very homogeneous. It is not rich in species, most of the
common types of life of warmer regions being excluded.
Among the large animals are the polar bear, the walrus, and
certain species of " ice-riding " seals. There are a few spe-
cies of fishes, mostly trout and sculpins, and a few insects.
Some of these, as the mosquito, are excessively numerous
in individuals. Reptiles are absent from this region and
many of its birds migrate southward in the winter, finding
in the arctic only their breeding homes. When we consider
the distribution of insects and other small animals of wide
diffusion we must add to the arctic realm all high moun-

GEOGRAPHICAL DISTRIBUTION OP ANIMALS 299

tains of other realms whose summits rise above the timber
line. The characteristic large animals of the arctic, as the
polar bear or the musk-ox or the reindeer, are not found
there, because barriers shut them off. But the flora of the
mountain top, even under the equator, may be character-
istically arctic, and with the flowers of the north may be
found the northern insects on whose presence the flower
depends for its fertilization. So far as climate is concerned
high altitude is equivalent to high latitude. On certain
mountains the different zones of altitude and the corre-
sponding zones of plant and insect life are very sharply
defined (Fig. 178).

The North Temperate realm comprises all the land be-
tween the northern limit of trees and the southern limit of
frost. It includes, therefore, nearly the whole of Europe,
most of Asia, and most of North America. While there
are large differences between the fauna of North America
and that of Europe and Asia, these differences are of minor
importance and are scarcely greater in any case than the
difference between the fauna of California and that of our
Atlantic coast. The close union of Alaska with Siberia
gives the arctic region an almost continuous land area from
Greenland to the westward around to Norway. To the
south everywhere in the temperate zone realm the species
increase in number and variety, and the differences between
the fauna of North America and that of Europe are due in
part to the northward extension into the one and the other
of types originating in the tropics. Especially is this true
of certain of the dominant types of singing birds. The
group of wood-warblers, tanagers, American orioles, vireos,
mocking-birds, with the fly-catchers and humming-birds so
characteristic of our forests, are unrepresented in Europe.
All of them are apparently immigrants from the neotropical
realm where nearly all of them spend the winter. In the
same way central Asia has many immigrants from the Indian
realm to the southward. With all these variations there

GEOGRAPHICAL DISTRIBUTION OF ANIMALS 3Q1

is an essential unity of life over this vast area, and the rec-
ognition of North America as a separate (nearctic) realm,
which some writers have attempted, seems hardly practi-
cable.

The Neotropical or South American realm includes
South America, the West Indies, the hot coast lands of
Mexico, and those parts of Florida and Texas where frost
does not occur. Its boundaries through Mexico are not
sharply denned, and there is much overlapping of the north
temperate realm along its northern limit. Its birds espe-
cially range widely through the United States in the sum-
mer migrations, and a large part of them find in the North
their breeding home. Southward, the broad barrier of the
two oceans keeps the South American fauna very distinct
from that of Africa or Australia. The neotropical fauna is
richest of all in species. The great forests of the Amazon
are the dreams of the naturalists. Characteristic types
among the larger animals are the snout or broad-nosed
(platyrrhine) monkeys, which in many ways are very distinct
from the monkeys and apes of the Old World. In many of
them the tip of the tail is highly specialized and is used as
a hand. The Edentates (armadillos, ant-eaters, etc.) are
characteristically South American, and there are many
peculiar types of birds, reptiles, fishes, and insects.

The Indo- African realm corresponds to the neotropical
realm in position. It includes the greater part of Africa,
merging gradually northward into the north temperate
realm through the transition districts which border the
Mediterranean. It includes also Arabia, India, and the
neighboring islands, all that part of Asia south of the limit
of frost. In monkeys, carnivora, ungulates, and reptiles
this region is wonderfully rich. In variety of birds, fishes,
and insects the neotropical realm exceeds it. The monkeys
of this district are all of the narrow-nosed (catarrhine)
type, various forms being much more nearly related to
man than is the case with the peculiar monkeys of South

302

ANIMAL LIFE

America. Some of these (anthropoid apes) have much
in common with man, and a primitive man derived from
these has been imagined by Haeckel and others. No
creature of this character is yet known, but that it may
have once existed is not impossible. To this region be-
long the elephant, the rhinoceros, and the hippopotamus,
as well as the lion, tiger, leopard, giraffe, the wild asses,
and horses of various species, besides a large number of
ruminant animals not found in other parts of the world.
It is, in fact, in its lower mammals and reptiles that its

most striking dis-
tinctive characters
are found. In its
fish fauna it has
very much in com-
mon with South
America.

The Lemurian
realm comprises
Madagascar alone.
It is an isolated di-
vision of the Indo-
African realm, but
the presence of
many species of
lemur and an un-
specialized or
primitive type of
lemur is held to
justify its recogni-
tion as a distinct

realm. In most other groups of animals the fauna of Mada-
gascar is essentially that of neighboring parts of Africa.

The Patagonian realm includes the south temperate
zone of South America. It has much in common with the
neotropical realm from which its fauna is mainly derived,

FIG. 179. — A lemur (Lemur varius).

GEOGRAPHICAL DISTRIBUTION OF ANIMALS 303

but the presence of frost is a barrier which vast numbers of
species can not cross. Beyond the Patagonian realm lies
the Antarctic continent. The scanty fauna of this region
is little known, and it probably differs from the Patagonian
fauna chiefly in the absence of all but the ice-riding species.

The Australian realm comprises Australia and the
neighboring islands. It is more isolated than any of the
others, having been protected by the sea from the invasions
of the characteristic animals of the Indo- African and tem-
perate realms. It shows a singular persistence of low or
primitive types of vertebrate life, as though in the process
of evolution the region had been left a whole geological
age behind the others. It is certain that if the closely
competing fauna of Africa and India could have been able
to invade Australia, the dominant mammals and birds of
that region would not have been left as they are now — mar-
supials and parrots.

It is only when barriers have shut out competition that
simple or unspecialized types abound. The larger the land
area and the more varied its surface, the greater is the
stress of competition and the more specialized are its char-
acteristic forms. As part of this specialization is in the
direction of hardiness and power to persist, the species from
the large areas, as a whole, are least easy of extermination.
The rapid multiplication of rabbits and foxes in Australia,
when introduced by the hand of man, shows what might
have taken place in this country had not impassable barriers
of ocean shut them out.

160. Subordinate realms or provinces. — Each of these great
realms may be indefinitely subdivided into provinces and
sections, for there is no end to the possibility of analy-
sis. No school district has exactly the same animals or
plants as any other, as finally in ultimate analysis we find
that no two animals or plants are exactly alike. Shut off
one pair of animals from the others of its species, and its
descendants will differ from the parent stock. This differ-

304 ANIMAL LIFE

ence increases with time and with distance so long as the
separation is maintained. Hence new species and new
fauna or aggregations of species are produced wherever
free diffusion is checked by any kind of barrier.

161. Faunal areas of the sea. — In like manner, we may
divide the oceans into faunal areas or zones, according to
the distribution of its animals. For this purpose the fishes
probably furnish the best indications, although results very
similar are obtained when we consider the mollusks or the
Crustacea. The fresh-water fishes are not considered here,
as in regard to their faunal areas they agree with the land
animals of the same regions. Perhaps the most important
basis for primary divisions is found in the separation from
the localized shore-fishes of the cosmopolitan pelagic species,
and the scarcely less widely distributed bassalian species or
fishes of the deep sea.

The pelagic fishes are those which inhabit the open sea,
swimming near the surface, and often in great schools.
Such forms are mainly confined to the warmer waters.
They are for the most part predatory fishes, strong swim-
mers, and many of the species are found in all warm seas.
Most species have special homing waters, to which they
repair in the spawning season. Often there will be special
regions to which they never resort, either for breeding or
for food. At other times a certain species will appear in
numbers in regions where it has hitherto been unknown.
For example, the frigate-mackerel (Auxis thazard), homing
in the East Indies and the Mediterranean, appeared in
great numbers in 1880 off the coast of New England. Typ-
ical pelagic fishes are the mackerel, tunny, dolphin, flying-
fish, opah, and some species of shark. This group shades
off by degrees into the ordinary shore-fish, some being partly
pelagic, venturing out for short distances, and some are
pelagic for part of the year only. To the free-swimming
forms of classes of animals lower than fishes, found in the
open ocean, the name Plankton is applied.

GEOGRAPHICAL DISTRIBUTION OF ANIMALS 305

The lassalian fauna, or deep-sea fauna, is composed of
species inhabiting great depths (2,500 feet to 25,000 feet)
in the sea. At a short distance below the surface the
change in temperature from day to night is no longer felt.
At a still lower depth there is no difference between winter
and summer, and still lower none between day and night.
The bassalian fishes in-
habit a region of great
cold and inky darkness.
Their bodies are subjected
to great pressure, and the
conditions of life are prac-
tically unvarying. There
is therefore among them
no migration, no seasonal
change, no spawning sea-
son fixed by outside con-
ditions, and no need of
adaptation to varying en-
vironment. As a result, all
are uniform indigo-black
in color, and all show more
or less degeneration in
those characters associated
with ordinary environ-
ment. Their bodies are
elongate, from the lack of
specialization in the ver-
tebrae. The flesh, being
held in place by the great

pressure of the water, is soft and fragile. The organs of
touch are often highly developed. The eye is either exces-
sively large, as if to catch the slightest ray of light, or else
it is undeveloped, as if the fish had abandoned the effort
to see. In many cases luminous spots or lanterns are de-
veloped by which the fish may see to guide his way in the
21

FIG. 180.— A crinoid (Rhizocrinm loxoten-
sis). A deep-sea animal which lives,
fixed plant like, at the bottom of the
ocean.

306 ANIMAL LIFE

sea, and in some forms these shining appendages are highly
developed. In one form (^Ethoprora) a luminous body cov-
ers the end of the nose, like the head-light of an engine.
In another (Ipnops) the two eyes themselves are flattened
out, covering the whole top of the head, and are luminous
in life. Many of these species have excessively large teeth,
and some have been known to swallow animals actually
larger than themselves. Those which have lantern-like
spots have always large eyes.

The deep-sea fishes, however fantastic, have all near rel-
atives among the shore forms. Most of them are degener-
ate representatives of well-known species — for example, of
eels, cod, smelt, grenadiers, sculpin, and flounders. The
deep-sea crustaceans and mollusks are similarly related to
shore forms.

The third great subdivision of marine animals is the
littoral or shore group, those living in water of moderate
depth, never venturing far into the open sea either at the
surface or in the depths. This group shades into both
the preceding. The individuals of some of the species are
excessively local, remaining their life long in tide pools or
coral reefs or piles of rock. Others venture far from home,
and might well be classed as pelagic. Still others ascend
rivers either to spawn (anadromous, as the salmon, shad,
and striped bass), or for purposes of feeding, as the robalo,
oorvina, and other shore-fishes of the tropics. Some live
among rocks alone, some in sea-weed, some on sandy shores,
some in the surf, and some only in sheltered lagoons. In
all seas there are fishes and other marine animals, and
each creature haunts the places for which it is fitted.

CLASSIFICATION OF ANIMALS*

In this diagram of classification every animal referred to in this book, either by its
vernacular or its scientific name, is assigned to its proper class and branch. Of the
species mentioned by their scientific names, only the genus name is given in this list.

KINGDOM ANIMALIA

BRANCH I. PROTOZOA

CLASS I. Rhizop oda.

Amce'ba, Globigeri'nae, Radiola'ria.
CLASS II. Mycetozo'a.
CLASS III. MastigSph'ora.

Volvocm'eae, Go'nium, Pandori'na, Eudorl'na, Vol'vox.
CLASS IV. Sporozo a.

Gregarl 'na.
CLASS V. Infuso'ria.

Param&'ciiim, Vorticel'la.

BRANCH II. PORlF'ERA

CLASS I. Porifera.

Sponges, Calcolyri thus, Prophyse'ma, SpongU'la, Spon'gia,
Cll'ona.

BRANCH III. CCELfiN'TERA'TA (se-lgn-te-ra'-ta)

CLASS I. Hydroz5'a.

Hy'dra, Euco'pe, Siphonoph'ora, Physoph'ora, Obe'lia, sea-anem'-
one, pol'yp, Physa'lia, Parapdg 'urus.
CLASS II. Scyphozo'a (sl-fo-zo'-a).

Jelly-fish, Llz'zia.
CLASS III. Actinozo'a.

CSr'als, MeM'dium.
CLASS IV. Ct^nbph'ora (ten-oph'-o-ra).

* The arrangement of branches (or phyla) and classes here used is that adopted in
Parker and HasweH's Text-Book of Zoology (1897).

307

308 ANIMAL LIFE

BRANCH IV. PLATYHELMlN'THES

CLASS I. Turbella'ria.

Piano! ria,.

CLASS II. Tremato'da.
CLASS III. CestS'da.

Tape-worm, Tce'nia, Llg'ula, flat-worm.
APPENDIX TO PLATYHELMINTHES — CLASS Nemertin ea.

BRANCH V. NEMATHELMlN'THES

CLASS I. Nemato da.

Syn'gamus, round-worm, Trfchl'na, Bothrioceph' alus, pup-
worm, Unctna'ria.
CLASS II. Acanthoceph ala.
CLASS III. Chaetbg'natha (ke-tog'-na-tha).

BRANCH VI. TROCHELMlN'THES

CLASS I. Rotif'era.

Rotato'ria.

CLASS II. Dlnophi lea.
CLASS III. Gastrbt'richa.

BRANCH VII. M6LLDSCOI'DA

CLASS I. P61yzo a.
CLASS II. FhSrS'nida.
CLASS III. Brachibp'oda.

BRANCH VIII. ECHfNODER'MATA

CLASS I. Asteroi dea.

Starfish.

CLASS II. Ophiuroi'dea.
CLASS III. Echinoi'dea.

Sea-urchin.
CLASS IV. Hblothuroi'dea.

Sea-cucumber.
CLASS V. Crinoi'dea.

Crinoid, Rhizocri 'nus.
CLASS VI. Oystoi'dea.
CLASS VII. Blastoi'dea.

CLASSIFICATION OF ANIMALS 309

BRANCH IX. ANNULA'TA

CLASS I. ChaetSp'oda (ke-t5p'-o-da).

Earth-worm.

APPENDIX TO THE CH^TOPODA — CLASS Myzostom ida.
CLASS II. Gephyre a (jef-e-re'-a).
CLASS III. Archi-annelida.
CLASS IV. Hirudin'ea.

BRANCH X. ARTHRftP'ODA

CLASS I. Crusta'cea.

Lobster, cray-fish, crab, barnacle, Le'pas, hermit-crab, Pag'urus,
pea-crab, PmnotMres, Epizoan'thus, fish-lice, whale-lice, Saccu-
ll'na, Lernceo'cera, prawn, Pene us.

APPENDIX TO CRUSTACEA— CLASS Trilobi ta.

CLASS II. Onychbph'ora.

CLASS III. Myriap'oda. .
Cen'tiped.

CLASS IV. InsSc'ta.

Water-beetle, water-bug, canker-worm moth, bee, white ant,
cockroach, mosquito, weevil, grasshopper, caterpillar, butterfly,
katydid, beetle, Dip'tera, Lepidop'tera, monarch butterfly, Ano'sia,
Cu'lex, Melari oplus, May-fly, locust, cottony-cushion scale, Ice'rya,
lady-bird, Vedalia, praying-horse, Man'tis, Ser'phus, Cecro'pia,
gall insect, An'dricus, mole-cricket, Gryllotal' pa, Hydroph'ilus,
Prlo'nus, Campono 'tus, plant-lice, Aph'idae, Coc'cidae, Aphis-lion,
ant, Ec'iton. termite, bumble-bee, carpenter-bee, Andre'na, Halic'-
tus, yellow- jacket, hornet, Ves'pa, wasp, At' ta, bird-lice, Malloph'-
aga, flea, louse, Pedic'ulus, Lipeu'rus, Hymenop 'tera, ichneumon
fly, Thales'sa, horn-tail, Tre'mex, PoUs'tes, Stylop'idae, Sty'lops,
red orange-scale, toad-bug, Gal'gulus, inch- worm, span-worm,
geometrid, walking-stick, Diapherom'era, Phyl'lium, meadow
brown, Orap'ta, Kal'lima, sphinx-moth, tomato-worm, Phlege-
tJion'tius, puss-moth, Ceru'ra, viceroy butterfly, Basilar' chia, Da-
na'idae, Helicon'idas, Pier'idae, Papnion'idae, Syr'phidae, flower-flies,
tree-hopper, MembrSc'idae, Hemip'tera, Sau'ba (saw'-ba), carrion-
beetle, Callosd'mia, prome'thea, cricket, cica'da, dragon-fly, Cynip'-
idae, Hessian-fly, mud-dauber, Lerema, Eryrinis, skipper butterfly,
Schistocer'ca.

CLASS V. Arach'nida.

Tardig'rada, bear-animalcule, scorpion, Lycos'idae, tick, itch-
mite, Sarcop'tes, spider, trap-door spider, turret-spider, Ctent'za.

310 ANIMAL LIFE

BRANCH XI. MtiLLtJS'CA

CLASS I. Felecyp'oda.

Clam, pond-mussel, Ll'ma.
CLASS II. Amphineu'ra.
CLASS III. GastrSp oda.

Pond-snail, Lymnae'us, whelk.

APPENDIX TO THE GASTROPODA — CLASS Scaphbp'oda AND Rho dope.
CLASS IV. CephalSp'oda.

BRANCH XII. CHORDA'TA

SUB-BRANCH I. AdelochSr'da. CLASS Adeloohorda.

SUB-BRANCH II. Urochor'da. CLASS Urochorda.
Sea-squirts, Tunica'ta.

SUB-BRANCH III. Vertebra ta. DIVISION A. Acra'nia. CLASS Acra-
nia. DIVISION B. Crania ta.

CLASS I. Cyclostbniata.

CLASS II. Pis'ces (pis'-sez).

Codfish, sculpin, skate, lady-fish, Al'lula, sword-fish, Xtph'ias,
flounder, Platoph'rys, Salanx, Cot'tus, blob, miller's- thumb,
conger-eel, Rem'ora, Exocm'tus, flying-fish, Cypselu'rus (sip-se-lu -
rus), deep-sea angler, lantern-fish, Corynol ophus, Eclitos'toma,
^thoph'ora, nokee, scorpion-fish, Emmy drfoh' thy s, mad-tomr
Schilbeodes, cat-fish, horned pout, toad-fish, sting-ray, globe-fish,
porcupine-fish, torpedo, electric eel, electric cat-fish, star-gazer, elec-
tric ray, Urol'ophus, Dl'odon, Narci'ne, Ra'ja, black-fish, mud-fish,
trout, Sal'mo, chub, horned dace, Echeneididae Amphiprion, No'-
meus, hag-fish, Myxl'ne, Heptatre'ma, Polistotre'ma, lamprey,
Oligocot'tus, mouse-fish, lava-fish, Pterophryne, pipe-fish, Phyllop'-
teryx, anglers, Lo'phius, Antenna' rius, Ceratiide, minnow, mack-
erel, sucker, salmon, shad, alewife, sturgeon, striped bass, qum'nat,
eel, sun-fish, stickle-back, carp, cutlass-fish, rainbow darter, chan-
nel-cat, Au'xis, tunny, dolphin, opah, shark, ^EtJiopro'ra, Ip'nops,
cod, smelt, grenadier, rob'alo, corvl'na, CJiologas' ter, TyphUch' thys,
blind-fish.

CLASS III. Amphibia

Toad, frog, salamander, tree-frog, Hy'la.

CLASS IV. Reptil'ia.

Tortoise, snake, horned toad, Phrynoso'ma, rattlesnake, lizards,
An'olis, chameleon, Gila monster, Heloder'ma, JZlaps, coralil'los,
Lamproptt'tis, Oscedla, alligator.

CLASSIFICATION OF ANIMALS 3H

CLASS V. A'ves.

Bird-of-paradise, peacock, pheasant, robin, pigeon, chicken, eagle,
vulture, guil'lemot (gil'-e-mot), murre (miir), auk, ful'mar, peVrel,
sparrow, bluebird, woodpecker, owl, Colum'ba, pelican, MelanSr'pes,
cormorant, meadow-lark, warbler, turkey, blue jay, Aythya, Cya-
nocitta, Uria, cow-bird, cuckoo, parrot, ptarmigan, whippoor-
will, Antros' tomus, gull, tern, fly-catcher, bittern, mocking-bird,
shrike, bobolink, goose, humming-bird, oriole, puffin, tailor-bird,
king-bird, nightingale, starling, skylark, passenger-pigeon, ouzel
(ooz'-el), linnet, tanager, vireo, wood-warbler, Phalacro' corax,
Troch'ilus, Ormthot'omus, PsalMp' arus.

CLASS VI. Mamma'lia.

Horse, ram, fur-seal, rabbit, cat, ox, tiger, lion, sheep, elephant,
whale, bear, wolf, squirrel, lem'ming, fox, dog, weasel, stoat, rein-
deer, otter, ant-eater, giraffe', skunk, porcupine, hedgehog, arma-
dillo, Callorhl'nus, sea-lion, deer, buffalo, kangaroo, Mac'ropus,
duck-bill, Mon'otreme, monkey, gopher, elk, bison, prairie-dog,
big-horn, hare, antelope, black-tail deer, hound, mole, hyena, mice,
rodent, woodchuck, jack-rabbit, Maca'cus, Cercoptth' ecus, beaver,
wood-rat, pocket-gopher, coyo'te, civet-cat, flying-fox, mon'goose,
sea-cow, Vul'pes, walrus, musk-ox, ape, rhmoc'eros, hippopot'amus,
leopard, ass, le'inur, Trich! gchit*, manatee, Le'pus, OdobcB'nus,
Le'mur.

GLOSSARY

[Only those terms are defined in this glossary that are not explained in the text.
In the case of the terms defined or explained in the text, reference is made to the
number of the paragraph in which the explanation occurs. The pronunciation of the
vernacular and scientific names of the animals mentioned in the text is given in the
Classification.]

Abomasum: 42.

Adaptation : 67, 74.

Albuminous : said of substances containing albumen,

Alimen tary canal : 42.

Alluring coloration : 111.

Altricial: 79.

Altruistic instinct: 129.

Amoe'boid : having the changing form of an Amreba.

Anad romous : said of fishes that go from the sea up rivers to lay their
eggs.

Anatomy: 39.

Animalcule : an animal of microscopic smallness.

Anten nae : the " feelers," the most anterior pair of appendages of in-
sects and insect-like animals ; situated on the head, and the seat of
organs of special sense.

Anthropoid: man-like.

A'nus : 42.

Appen'dix vermifor mis : 82.

Artificial selection : 72.

Assim ilate : to receive food and transform it into a homogenous part
of the body substance.

At51T : a ring-shaped coral island nearly or quite inclosing a lagoon.

Atrophy : a stoppage of the growth or development of a part or organ.

Auditory : referring to the sense of hearing.

Aut6m atism : the state of being automatic ; involuntary action.

313

314 ANIMAL LIFE

Bassalian: 160.

Biologist : student of animals and plants.

Blastoderm: 50.

Blas'tula: 50.

Budding : the process of reproduction among animals in which a small
part of the body substance of an animal grows out from the sur-
face, separates from the parent, and develops into a new individual.

Cae'cum (se-kum) : 42.

Carniv'orous : flesh-eating.

Cat arrhine : nostril downward ; said of the narrow-nosed Old- World

monkeys.
Cell : 2.
Cellulose : a peculiar compound insoluble in all ordinary solvents,

forming the fundamental material of the structure of plants, and

also contained in the mantle of tunicates.
Chi tin: 57.
ChlS'rophyll : 13.

Chro'matophore : a color-bearing granule or sac.
Chro mosome : 2.
Chrys alis : 57.
Chyle: 42.
Cilia: 5.
Cleavage: 50.
C51on: 42.
Commen salism : 90.
Com'munal: 83.
Conjugation: 5.
Contractile vacuole : a vacuole that dilates and contracts regularly,

and is supposed to have an excretory function.
Cyst : 98.
Cytoplasm: 2.

Degeneration: 95.
Development: 46.
Differentiation : the setting apart of special organs for special work ;

progressive change from general to special ; specialization.
Digestion : the process of dissolving and chemically changing food so

that it can be assimilated by the blood and furnish nutriment to

the body.

Dimor phism : 24.

Dlvertic'ulum : a blind pouch arising from another larger pouch. 44.
Duode'num: 42.

GLOSSARY 315

Ec'toblast: 50.
Ec'toderm: 20.
Egg-cell: 20.
Egois'tic instinct : 129.
EmbrybTogy: 39.
Embrybn ic : 49.
En'doblast: 50.
En do derm : 20.

Environment : an organism's surroundings taken collectively.
Ex'cretory: referring to excretion, as excretory organs, the organs
which get rid of waste matter in the animal body.

Fau'na (fawna) : 157.
Fertilized egg : 20.
Fission: 4.
FlageTla: 13.
Function: 37.

Ganglion (pi. ganglia) : a nerve-center composed of an aggregation of

nerve-cells.
GSs'trula: 50.
Gem' mule: 20.
Generalization: 41.

Geologist : student of the structure and history of the earth.
Gregarious: 87.
Growth: 46.

Habit: 139.

Herbiv'orous : plant-eating.

Heredity: 54.

Hermaphroditic: 35.

Hlberna tion : passing the winter in a death-like sleep.

Homoge neous : of the same composition or structure throughout.

ileum: 42.

Inorganic : not being nor having been a living organism ; not organic.

Insectiv'orous : insect-eating.

Instinct: 128.

Intellect: 139.

Intercellular : outside of and between the cells.

Jejunum: 42.

316 ANIMAL LIFE

Lagoon' : a pool or lake ; the still water inclosed within an atoll.

Larva: 57.

LSpidBp terous : referring to the Lepidoptera, or moths and butterflies,

Littoral: 160.

Lu men : the cavity of a tubular organ. 44.

Medu'sa: 24.

Megalops: 59.

Mes'oderm: 20.

Metab olism : the act or process by which dead food is built up into
living matter, and living matter is broken down into simpler prod-
ucts within a cell or organism.

Metamorphosis: 56.

Migration: 70.

Millimeter : about one twenty-fifth of an inch ; a term used in the
metric system of measure.

Mimicry: 112.

Mind: 140.

Molt: 57.

M5nSg amous : said of animals in which a male mates with only a
single female.

Multiplication: used in the text usually synonymously with repro-
duction.

Myrmec8ph ilous : said of insects which are found inhabiting the nests
of ants.

Natural selection: 70.

N5'tochord : an elastic rod, or row of cells, formed in the early embryo
of chordate animals (including all the vertebrates and some others),
which lies below the dorsal nervous tube and above the ventral ali-
mentary tube.

Nu'cleus (pi. nuclei) : 2.

CEsSph'agus: 42.

Olfactory : referring to the sense of smell.
Oma'sum: 42.
Organ: 37.

Organic : referring to the matter of which animals and plants are com-
posed.

Organism : a living being, plant or animal.
Orientation: 95.
Otolith: 121.

GLOSSARY 317

Papilla (pi. papillae) : a small nipple-like process, as the papillae of the
skin or tongue.

Parasite: 93.

Parthenogenesis: 35.

Pelagic : inhabiting the surface of mid-ocean. 160.

Pharynx: 42.

FhysibTogy: 39.

Plat yrrhine : broad-nosed ; said of the New- World monkeys.

Plu'teus: 59.

Folyg'amous : said of animals in which a single male mates with sev-
eral females : 35.

Pblymor'phism : 24.

Polyp: 21.

Post-embryonic : 49

Praeco cial (pre-co'-shal) : 79.

Predatory: feeding on other animals.

Propolis: 84.

Protective resemblance: 107.

Protoplasm: 2.

PrSventric'ulus : 42.

Pseu'dopod : 4.

Psy'chic: 140.

Pupa: 57.

Reason: 139.

Recognition mark : 77, 115.
Rec'tum: 42.
Reflex action : 127.
Reproduction: 67.
Respiration: 4.
Retic'ulum: 42.
Retina: 123.
Ru'men : 42.
Ruminant: 42.

Saliva: 42.

Sensation: 67.

Sensorium : 126.

Silica : the mineral of which quartz, sand, flint, etc., are composed.

Spawn, v. : to lay eggs.

Specialization: 41.

Species: 151.

318 ANIMAL LIFE

Sperm cell : 20.

Spiracle : one of the breathing openings of an insect situated on the

side of the abdomen or thorax.
Spon'gin : 20.
Stimulus (pi. stimuli) : that which excites action in plant or animal

tissue.

Strata : layers, usually said of rocks.
Stridula tion : 122.
Sub-species : 151.
Symbiosis: 90.

Tactile : referring to the sense of touch. 118.

Tadpole: 58.

Ten'tacle : a protruding flexible process or appendage, usually of the

head of invertebrate animals, being used as an organ of touch,

prehension, or motion.
Termitoph ilous : said of insects inhabiting the nests of termites.

Vac'uole : a minute cavity containing air, water, or a chemical secre-.

tion of the protoplasm, found in an organ, tissue, or cell.
Vestigial : 82.
Vis'cera : the organs in the great cavities of the body, commonly used

for the organs in the abdominal cavity.

Yolk (yok) : 48.

Zoea: 59.

ZSogebg'raphy : 147.

Zo'oid : one of the more or less independent members of a colonial or

compound organism.
Zoologist : a student of animals.

INDEX

Abomasum, 68.

Actinocephalus otigacanthus (ill.),
14.

Adaptations, 113, 123; classifica-
tion of, 123 ; concerned with sur-
roundings, 143 ; degree of struc-
tural change in, 146 ; for defense
of young, 137 ; for rivalry, 135 ;
for self-defense, 128 ; for secur-
ing food, 125 ; origin of, 123.

Agassiz's cave-fish (illus.), 282.

Albula vulpes, metamorphosis of
(illus.), 98.

Alimentary canal, 66 ; of cock-
roach (illus.), 73 ; of earthworm
(illus.), 71 ; of flatworm (illus.),
70; of Holothurian (illus.), 70;
of mussel (illus.), 72 ; of Obelia
(illus.), 69 ; of ox (illus.), 67 ; of
Planaria (illus.), 70; of sea-
cucumber (illus.), 70.

Alligator (illus.), 290.

Alluring coloration, 216.

Alternation of generations, 42.

Altricial, 140.

Amoeba, 5; multiplication of (ill.) 53.

Ammba polypodia (illus.), 8.

Anatomy, 64.

Andrena, nest of (illus.), 160.

Andricus californicus, galls of
(illus.), 143.

Angler, deep-sea (illus.), 124.

Animals, life of simplest, 1 ; many-
celled, 2 ; one-celled, 2 ; slightly
complex, 24.

Anosia plexippus, metamorphosis
of (illus.), 92 ; mimicked by Ba-
silarchia archippus (illus.), 219.

Antenna of cray-fish (illus.), 233 ;
of leaf-eating beetle (illus.), 230.

Antennae, specialized, of prome-
thea moth (illus.), 231.

Antrostomus vociferus (illus.), 203.

Ants (illus.), 155.

Anus, 68.

Appearance, terrifying, 212.

Arctic realm, 297.

Area, faunal, 296.

Artificial selection, 120.

Auditory organ of cray-fish (illus.),
233; of cricket (illus.), 234; of
grasshopper (illus.), 234 ; of mol-
lusk (illus.), 233; of mosquito
(illus.), 235.

Auditory organs, 232.

Australian realm, 303.

Aythya (illus.), 137.

Barbadoes earth, 19.

Barnacle, adult and larva (illus.),
195; metamorphosis of (illus.),
101. 319

320

ANIMAL LIFE

Barrier, mountains a, to distribu-
tion, 293 ; sea a, to distribution,
288 ; temperature a, to distribu-
tion, 290.

Barriers affecting fresh-water ani-
mals, 294 ; effect of, 283 ; species
debarred by, 274; to distribu-
tion, character of, 288.

Basilarchia archippus mimicking
Anosia plexippus (illus.), 219.

Bassalian fauna, 305.

Beavers, nest of (illus.), 269.

Beetle, larva of (illus.), 146.

Beetles, lady-bird (illus.), 214.

Bird, egg of (illus.), 79.

Bird-louse (illus.), 188.

Bird of paradise (illus.), 58.

Birds, nest-making habits of, 264.

Birth, 78.

Bittern, nestlings of (illus.), 246,
247.

Blastoderm, 82.

Blastula, 82.

Blue jay (illus.), 138.

Brain, 241.

Budding, 13.

Bumble-bee, 159, (illus.), 161.

Bush-tit, nest of California (illus.),
270.

Butterfly, egg of (illus.), 79 ; mon-
arch, metamorphosis of (illus.),
92.

Caecum, 68.

Calcolynthus porimigenius (illus.),
33.

Calf, taste buds of (illus.), 229.

CallorTiinus alascanus (illus.),
136.

Camponotus (illus.), 155.

Canadian skipper butterfly, distri-
bution of (illus.), 273.

Canal, alimentary, 66.
Cankerworm-moth (illus.), 59.
Care of young of mammals, 268.
Carpenter-bee, nest of (illus.), 160.
Caterpillar parasitized (illus.), 189,

(illus.), 190.

Cave blind-fish (illus.), 282.
Ceanothus (illus.), 1.41.
Cell, animal, 2 ; egg, 21, 56 ; plant,

2 ; products, 3 ; wall, 3.
Cells, brood, of honey-bee (illus.),

152; nerve, 240; reproductive,

29, 55.

Cellulose, 24, 27.
Centiped (illus.), 130.
Cerura, larva of (illus.), 216.
Chalk, 18.
Chick, embryonic stages of (illus.),

87.

Chitin, 91.
Chlorophyll, 24.

Chologaster agassizi (illus.), 282.
Chologaster avetus (illus.), 282.
Chromatophore, 24.
Chromosome, 3.
Chrysalid of butterfly, showing

protective resemblance (illus.),

206.

Chrysalis, 93.
Chyle, 68.
Cilia, 9.
Cleavage, 82.
Climate, influencing distribution,

291 ; instincts of, 248.
Clouded skipper butterfly, distri-
bution of (illus.), 273.
Coccidium oviforme (illus.), 14.
Cockroach, alimentary canal of

(illus.), 73; egg case of (illus.),

140.
Cocoon of Cecropia moth (illus.),

141.

INDEX

321

Colon, 68.

Colonial jelly-fishes, 45 ; Protozoa,

24.

Colony, 31.
Color, 222.

Coloration, alluring, 216.
Colors, warning, 212.
Commensalism, 172, 173.
Communal life, 168; advantages

of, 170.

Communities, animal, 149.
Conditions, primary, of animal

life, 106.

Conjugating cells, 28.
Conjugation, 11, 27, 55.
Contractile vacuole, 10.
Coral, brain, 45 ; island (illus.),
44 ; organ-pipe (illus.), 45 ; red,
45.

Corals, 37-43.
Corynolophus reinhardti (illus.),

124.

Cottony cushion scale (illus.), 142.
Courtship, instincts of, 248.
Crab, metamorphosis of (illus.), 97 ;

with sea-anemone (illus.), 177.
Cray-fish, auditory organ of (illus.),

233.
Cricket, auditory organ of (illus.),

234.

Cricket, mole (illus.), 146.
Crinoid (illus.), 305.
Crop, 71.

Crowd of animals, 114.
Crustaceans, adults and larvae

(illus.), 195.
Cteniza californica, nest of (illus.),

261.

Cyanocitta cristata (illus.), 138.
Cycle, life, 78.
Cypselurus (illus.), 131.
Cytoplasm, 3.
22

Dead-leaf butterfly (illus.), 211.

Death, 103.

Deep-sea angler (illus.), 124.

Deer, horns of (illus.), 148.

Defense of the young, 137.

Degeneration, causes of, 197, 198;
human, 200 ; through quiescence,
193.

Desiccation, 104.

Development, 78 : continuity of,
83 ; divergence of, 84 ; embry-
onic, 80; first stages in (illus.),
81 ; laws of, 86 ; metamorphic,
90; of flounder (illus.), 100; of
locust (illus.), 91 ; of vertebrates,
(illus.), 87; post-embryonic, 80;
significance of facts of, 89.

Diapheromera femorata (illus.),
209.

Differentiation, 41 ; of structure,
64.

Dimorphism, 42 ; sex, 58.

Diodon hystrix (illus.), 134.

Dismal Swamp fish (illus.), 282.

Distribution, character of barriers,
to, 288; geographical, 272; in-
fluenced by climate, 291; laws
of, 274; modes of, 296; moun-
tains a barrier to, 293 ; of Cana-
dian Skipper butterfly (illus.),
273 ; of clouded Skipper butter-
fly (illus.), 273 ; of Erynnis mani-
toba (illus.), 273; sea a barrier
to, 288; temperature a barrier
to, 290.

Diverticula, 74.

Division of labor, 22, 168.

Dog, pointer (illus.), 256.

Dragon-fly, eye of (illus.), 239.

Duck, family (illus.), 137.

Duodenum, 68.
Duration of life, 101.

322

ANIMAL LIFE

Earthworm, alimentary canal of
(illus.), 71.

Ectoblast, 82.

Ectoderm, 33.

Egg case of Californian barn-door
skate (illus.), 140 ; cockroach (il-
lus.), 140.

Egg cell, 21, 56.

Egg, fertilized, 35 ; of bird (illus.),
79;, of butterfly (illus.), 79; of
fish (illus.), 79; of katydid (il-
lus.), 79 ; of skate (illus.), 79 ; of
toad (illus.), 79.

Electric ray. (illus.), 135.

Embryology, 64.

Embryonic development, 80; of
the pond snail, 81.

EmmydricJithys vulcanus (illus.),
132.

Endoblast, 82.

Endoderm, 33.

Environment, instincts of, 248.

Epizoanthus paguriphilus, with
sea-anemone (illus.), 177.

Erynnis manitoba, distribution of
(illus.), 273.

Eucope (illus.), 42.

Eudorina, 27.

Eudorina elegans (illus.), 28.

Exocwtus (illus.), 131.

Eye of dragon-fly (illus.), 239 ; of
jelly-fish (illus.), 238.

Fauna, 296; bassalian, 305 ; littoral,
306 ; pelagic, 304.

Faunal areas of the sea, 304.

Feeding habit of Californian wood-
pecker (illus.), 128, 129 ; ofMela-
nerpes formicivorus bairdii
(illus.), 128, 129; instincts of,
244.

Female, 57.

Fish, egg of (illus.), 79; embry-
onic stages of (illus.), 87 ; -louse
(illus.), 188.

Fishes, man-of-war (illus.), 175;
nest-making habits of, 264.

Fission, 9 ; binary, 54.

Flagella, 25.

Flagellata, 24.

Flatworm, alimentary canal of
(illus.), 70.

Flounder, development of (illus.),
100 ; wide-eyed (illus.), 100.

Flying fishes (illus.), 131.

Food, adaptations for securing,
125; necessary to animal life,
106.

Form, primitive, 20.

Fossil, 18.

Fresh-water animals, barriers af-
fecting, 294.

Function, 63.

Fur seal (illus.), 136.

Galapagos Islands, animals of

(illus.), 278; locusts of (illus.),

280.
Gall, giant, of white oak (illus.),

143.

Galls, insect, on leaf (illus.), 144.
Gapes, worm which causes, 60.
Gastrula, 82.
Gemmule, 35.
Generalization, 66.
Generation, spontaneous, 51.
Generations, alternation of, 51.
Geographical distribution, 272.
Geometrical larva on branch (illus.),

209.
Gerrhonotus scincicauda (illus.),

204.

Giraffe (illus.), 126.
Gizzard, 71.

INDEX

323

Globigerina-ooze, 18.

Globigerinae, 16.

Gonium, 25-30.

Gonium pectorale (illus.), 25.

Grasshopper, auditory organ of

(illus.), 234.

Green-leaf insect (illus.), 210.
Gregarina, 13, 182.
Gregarina polymorpha (illus.), 14.
Gregarinidas, 14.
Gregariousness, 163.
Growth, 78.
Gryllotalpa (illus.), 146.

Habit, 251.

Habitat, relation of species of, 283.

Habits, domestic, 257.

Habits, nest-making, of birds, 264 ;
of fishes, 264 ; of insects, 262 ; of
invertebrates, 258; of spiders,
259 ; of vertebrates, 264.

Hearing, sense of, 232.

Heliosphc&ra actinota (illus.), 19.

Heredity, 89.

Hermaphroditism, 60.

Hermit-crab, with the sea-anemone
(illus.), 176.

Hibernation, 103.

Hiving honey-bees (illus.), 154.

Holothurian, alimentary canal of
(illus.), 70.

Homes, 257.

Honey-bee (illus.), 150; adult and
larva (illus.), 83; leg of (illus.),
151 : life history of, 149.

Honey-bees, hiving a swarm of
(illus.), 154.

Host, relation of parasite to, 179.

Human degeneration, 200.

Humming-bird, nest of rufus
(illus.), 265, 266.

Hydra, 37.

Hydra vulgans (illus.), 38.
Hydrophilus (illus.), 146.
Hyla regilla (illus.), 145.

Icerya and Vedalia, 121.

Icerya purchasi (illus.), 142.

Ileum, 68.

Individual, 31.

Indo- African realm, 301.

Inorganic matter, 112.

Insect galls on leaf (illus.), 144.

Insects, metamorphosis of, 90; nest-
making habits of, 262 ; parasitic,
188.

Instinct, 242.

Instincts, altruistic, 243 ; classifi-
cation of, 243 ; concerned with
care of the young, 250 ; egoistic,
243; of climate, 248; of court-
ship, 248 ; of environment, 248 ;
of feeding, 244 ; of play, 247 ; of
reproduction, 249; of self-de-
fense, 245 ; variability of, 251.

Intellect, 254.

Intestine, 68.

Invertebrates, nest-making habits
of, 258.

Irritability, 8, 240.

Island, coral (illus.), 44.

Itch-mite (illus.), 192.

Jack-rabbits, showing variation

(illus.), 281.

Jay, Canada (illus.), 138.
Jejunum, 68.

Jelly-fish, eye of (illus.), 238.
Jelly-fishes, 37 ; colonial, 45.

Kallima (illus.), 211.
Kangaroo (illus.), 139.
Katydid, egg of (illus.), 79.
Lady-bird beetles (illus.), 214.

324

ANIMAL LIFE

Lady-fish, metamorphosis of (illus.),
98.

Larva, 92; of the mosquito, 93;
of butterfly pupating (illus.), 94 ;
of the honey-bee, 152.

Leaf -cutting ant mimicked by tree-
hoppers (illus.), 220.

Lemur (illus.), 302.

Lemur varius (illus.), 302.

Lemurian realm, 302.

Lepas, adult and larva (illus.), 195 ;
metamorphosis of (illus.), 101.

Lerema accius, distribution of
(illus.), 273.

Lernceocera (illus.), 188.

Life cycle, 78.

Life, communal, 168 ; duration of,
101 ; primitive, 21 ; processes,
21 ; social, 149.

Light, influence of, on animals,
237.

Lipeureus densus (illus.), 188.

Littoral fauna, 306.

Lizard, alligator (illus.), 204.

Lizzia koellikeri, eye of (illus.),
238.

Locust, post-embryonic develop-
ment of (illus.), 91.

Locusts of Galapagos Islands
(illus.), 280.

Louse, sucking (illus.), 188.

Macropus rufus, 139.

Mad Tom (illus.), 132.

Male, 57.

Mammals, care of young of, 268.

Manatee (illus.), 277.

Man-of-war, Portuguese (illus.),

175.

Mantis (illus.), 127.
Many-celled animal, 2.
Marine Protozoa, 15.

Marks, recognition, 22, 129, 223.

Medusae, 41.

Megalops, 97.

Melanerpes formicivorus bairdii,
feeding habit of (illus.), 128, 129.

Membracidae mimicking Sauba
ant (illus.), 220.

Mesoderm, 33.

Metamorphosis, 90 ; of Albula vul-
pes (illus.), 98 ; of Anosia plex-
ippus (illus.), 92; of barnacle
(illus.), 101 ; of butterfly (illus.),
92; of crab (illus.), 97; of in-
sects, 90; of lady-fish (illus.),
98; of Lepas (illus.), 101; of
mosquito (illus.), 93 ; of sea-ur-
chin (illus.), 96; of sword-fish
(illus.), 99; of toad, 94, (illus.),
95 ; of Xiphias gladius (illus.),
99.

Metazoa, 32.

Metridium dianthus (illus.), 43.

Micro-organism, 16.

Migration of lemming, 118 ; of lo-
cust, 118.

Mimickry, 218.

Mind, 255.

Mining-bee, nest of (illus.), 160.

Molt, 91.

Monarch butterfly (illus.), 219 ;
mimicked by Viceroy butterfly
(illus.), 219.

Monogamy, 135.

Mosquito, auditor organ of (illus.),
235 ; head of (illus.), 127 ; meta-
morphosis of (illus.), 93; young
stages of (illus.), 147.

Moth, cankerworm (illus.), 59.

Mountains as barriers to distribu-
tion, 293.

Mouth parts of mosquito (illus.),
127.

INDEX

325

Mouse-fish in gulf-weed (illus), 208.
Mt. Orizaba (illus.), 300.
Multiplication, 50 ; of animals,

114 ; simplest method of, 53 ;

slightly complex meihods of, 54.
Murres, Pallas's (illus.). 165.
Mussel, alimentary canal of (illus.),

72.
Mutual aid, 163.

Narcine brasiliensis (illus.), 135.

Natural selection, 117.

Neotropical realm, 301.

Nerve cells, 240.

Nerve fibers, 240.

Nest-making habits of birds, 264 ;
of fishes, 264; of insects, 262;
of invertebrates, 258 ; of spiders,
259 ; of vertebrates, 264.

Nest of Baltimore oriole (illus.),
267; of beavers (illus.), 269; of
Californian bush-tit (illus.), 270 ;
of Cleniza californica (illus.),
261 ; of pocket - gopher (illus.),
271; of rufus humming-bird
(illus.), 265, 266; of tailor-bird
(illus.), 268 ; of trap-door spider
(illus.), 261; of turret - spider
(illus.), 262.

Nokee (illus.), 132.

Nomeus gronovii (illus.), 175.

North Temperate realm, 299.

Nuclear membrane, 3.

Nucleus, 3.

Obelia, alimentary sac of (illus.),

69.

CEsophagus, 67.
Omasum, 67.
One-celled animals, 2.
Organ, 63; auditory, of cray-fish

(illus.), 233; of cricket (illus.),

234 ; of grasshopper (illus.), 234 ;
of mosquito (illus.), 235 ; of mol-
lusk (illus.), 233.

Organic matter, 112.

Organs, auditory, 232; of smell,
229; of sound-making, 235; of
taste, 228; of touch, 226; spe-
cialization of, 66 ; vestigial, 147.

Oriole, Baltimore, nest of (illus.),
267.

Ornithotomous sutorius, nest of,
268.

Osprey Falls (illus.), 285.

Otolith, 232.

Ox, alimentary canal of (illus.) 67.

Oxygen, necessary to animal life,
107.

Pagurus (illus.), 176.
Pandorina, 26.
Pandorina sp. (illus.), 26.
Pandorina morum (illus.), 27.
Papilio, chrysalid of (illue.), 205.
Papilla, tactile, of skin of man

(illus.), 227.
Papilla?, 67.
Paramcecium, 9.
Paramcecium aurelia (illus.), 10.
Paramcecium caudatum (illus.), 1 1.
Paramcecium plutorinum (illus.),

11.
Parasite of caterpillar (illus.), 190 ;

relation to host, 179.
Parasites, simple structure of, 181.
Parasitism, 179 ; kinds of, 180.
Parthenogenesis, 60.
Patagonian realm, 302.
Pediculus (illus.), 188.
Pelagic fauna, 304.
Pelican, brown (illus.), 125.
Peneus, adult and larva (illus.),

195.

326

ANIMAL LIFE

Pharynx, 71.

Phlegethontius Carolina, larva of

(illus.), 215.
Phrynosoma Uainvillei (illus.),

131.

PJiyllium (illus.), 210.
Phyllopteryx (illus.), 212.
Physalia (illus.), 175.
Physiology, 64.
Physoptora (illus.), 47.
Pocket-gopher, nest of (illus.), 271.
Pointer dog (illus.), 256.
Polistes, parasitized by Stylops

(illus.), 192.
Polygamy, 59.
Polymorphism, 42.
Polyps, 37.

Polystomella strigillata (illus.), 17.
Porcupine-fish (illus.), 134.
Post-embryonic development, 80.
Pigeon horn-tail (illus.), 191.
Pipe-fish (illus.), 212.
Planaria, alimentary canal of

(illus.), 70.
Plankton, 304.
Plants, difference between animals

and, 111.

Platophrys lunatus (illus.), 100.
Play, instincts of, 247.
Pluteus, 96.
Praecocial, 140.

Prawn, adult and larva (illus.), 195.
Praying-horse (illus.), 127.
Pressure, a condition of animal

life, 109.

Primitive form, 20.
Primitive life, 21.
Prionus, larva of (illus.), 146.
Processes, life, 21.
Promethea moth (illus.), 231.
Prophysema primordiale (illus.),

34.

Protective resemblance, 201.

Protoplasm, 3; chemical constitu-
tion of, 4 ; physical constitution
of, 4.

Protozoa, 1 ; colonial, 24 ; marine,
15.

Psaltriparus minimus, nest of
(illus.), 270.

Pseudopod, 5.

Pterophryne histrio in Sargassum
(illus.), 208.

Pupa, 93.

Puss moth, larva of (illus.), 216.

Quiescence, degeneration through,
193.

Rabbit, embryonic stages of (illus.),
87.

Radiolaria, 16.

Radiolaria-ooze, 19.

Raja binoculata (illus.), 140.

Realm, Arctic, 297; Australian,
303; Indo-African, 301; Lemu-
rian, 302; Neotropical, 301;
North Temperate, 299 ; of ani-
mal life, 297 ; Patagonian, 302 ;
South American, 301.

Realms, subordinate, 303.

Reason, 251.

Recognition marks, 22, 129, 223.

Rectum, 68.

Reflex action, 241.

Remora (illus.), 173.

Remora remora (illus.), 125.

Reproduction, 9, 50; instincts of,
249.

Reproductive cells, 28, 55.

Resemblance, aggressive, 202 ; gen-
eral protective, 202; protective,
201 ; special protective, 207 ; va-
riable protective, 204.

INDEX

327

Respiration, 7.

Resting spore, 28.

Reticulura, 67.

Rhizocrinus loxotensis (illus.), 305.

Rivalry, adaptations for, 185.

Rookeries, fur-seal (illus.), 169.

Rumen, 67.

Sacculina (illus.), 187.

Sacculina, adult and larva (illus.),
195.

Salamander, embryonic stages of
(illus.), 87.

Saliva, 67.

Salmon leaping (illus.), 289.

Salmo viridens (illus.), 145.

Sarcoptes (illus.), 192.

Sauba, ant mimicked by Membra-
cidce (illus.), 220.

Scale, red orange (illus.), 196.

Schilbeodes furiosus (illus.), 132.

Schistocerca (illus.), 280.

Scorpion (illus.), 127.

Scorpion-fish (illus.), 132.

Sea, a barrier to distribution, 288.

Sea, faunal areas of, 304.

Sea-anemone, 37; with algas in
body (illus.), 178.

Sea-cow (illus.), 277.

Sea-cucumber, alimentary canal of
(illus.), 70.

Sea-squirt (illus.), 194.

Sea-urchin, metamorphosis of (il-
lus.), 96.

Sea-urchins (illus.), 259.

Seal, fur (illus.), 136; pups killed
by parasite (illus.), 186; rook-
eries (illus.), 169.

Selection,artificial,120;natural,117.

Self-defense, adaptations for, 128 ;
instincts of, 245.

Sensation, 8.

Senses, special, 224 ; of the simplest
animals, 225.

Sensorium, 241.

Serphus (illus.), 141.

Sex, 57 ; object of, 57 ; dimorphism,
58.

Shark-clinging fish (illus.), 125.

Sheep, bighorn (illus.), 167 ; Rocky
Mountain (illus.), 167.

Sight, sense of, 237.

Simplest animals, life of, 1.

Siphonophora, 46.

Skate, egg case of California barn-
door (illus.), 140 ; egg of (illus.),
79.

Skin of man, tactile papilla of
(illus.), 227.

Smell, sense of, 229.

Smelling organs, 229.

Smelling pits of leaf-eating beetle
(illus.), 230.

Social life, 149.

Sound-making, 235 ; organs, 235.

South American realm, 301.

Specialization, 66 ; of organs, 66.

Special senses, 224.

Species, altered by adaptation to
new conditions, 276; debarred
by barriers, 274 ; debarred by in-
ability to maintain their ground,
275 ; definition of, 279 ; relation
of, to habitat, 283.

Sperm cell, 35, 56.

Sphinx moth, larva of (illus.), 215.

Spicules, sponge, 33.

Spiders (illus.), 212; nest-making
habits of, 259.

Sponges, 32.

Spongin, 36.

Spontaneous generation, 51.

Spore, 15, 52 ; resting, 28.

Sting-ray (illus.), 133.

ANIMAL LIFE

Structure, 63; differentiation of, 64.

Struggle for existence, 116.

Sty lops parasitizing Polistes (illus.),
192.

Sub-species, definition of, 282.

Surroundings, adaptations con-
cerned with, 143.

Swallow-tail butterfly, chrysalid
of (illus.), 205.

Sword-fish, metamorphosis of (il-
lus.), 99.

Symbiosis, 172, 175.

Syngamus trachealis (illus.), 60.

Systematic zoology, 64.

Tactile organs, 226.

Tactile papilla of skin of man (il-
lus.), 227.

Tadpole, 94.

Tcenia, solium (illus.), 183.

Tailor-bird, nest of (illus.), 268.

Tape-worm (illus.), 183.

Taste buds of calf (illus.), 229.

Taste organs, 228.

Taste, sense of, 228.

Temperature a barrier to distribu-
tion, 290 ; a condition of animal
life, 108.

Tentacle, 37.

Termite, 158, (illus.), 159.

Terrifying appearances, 212.

Thalessa lunator (illus.), 191.

Toad, egg of (illus.), 79; horned
(illus.), 131 ; metamorphosis of,
94, (illus.), 95.

Torpedo (illus.), 135.

Tortoise, embryonic stages of (il-
lus.), 87.

Touch, sense of, 226.

Trap-door spider nest (illus.), 261.

Tree-hoppers mimicking leaf-cut-
ting ant (illus.), 220.

Tree-toad (illus.), 145.

Tremex columba (illus.), 191.

Trichechus latirostris (illus.), 277.

Trichina spiralis (illus.), 184.

Tripoli, 19.

Trochilus rufus, nest of (illus.),

265, 266.
Trout, rainbow, head of (illus.),

145.

Two Ocean Pass (illus.), 287.
Tunicate (illus.), 194.
Turret-spider, nest of (illus.), 262.
Typhlichthys subterraneus (illus.),

282.

Uncinaria, killing fur-seal pups

(illus.), 186.

Uria lomvia arra (illus.), 165.
Urolophus goodei (illus.), 133.

Vacuole, 10; contractile, 10.

Variety, definition of, 282.

Vedalia and Icerya, 121.

Vertebrates, early stages in devel-
opment of (illus.), 87 ; nest-mak-
ing habits of, 264 ; parasitic, 193.

Vespa (illus.), 162 ; nest of (illus.),
163.

Vestigial organs, 147.

Viceroy butterfly mimicking Mon-
arch butterfly (illus.), 219.

Voice, 236.

Volvocinse, 24.

Volvox, 28.

Volvox globator (illus.), 29.

Volvox minor (illus.), 29.

Vorticella, 12.

Vorticella microtoma (illus.), 12.

Walking-stick insect (illus.), 209.
Warning colors, 212.
Walrus, Atlantic (illus.), 298.

INDEX

Wasps, social, 161.

Water -beetle (illus.), 146; bug,

giant (illus.), 141.
Whippoorwill (illus.), 203.
Woodpecker, Californian, feeding

habit of (illus.), 128, 129.

Xiphias gladius, metamorphosis
of (illus.), 99.

Yellow-jacket (illus.), 162.

Yolk, 80.

Young, adaptations for defense of,
137 ; care of the, 250, 257 ; num-
ber of, 61.

Zoea, 97.

Zoogeography, 272.
Zoology, systematic, 64.

(5)

THE END

TWENTIETH CENTURY TEXT-BOOKS

ANIMAL FORMS

A TEXT-BOOK OF ZOOLOGY

BY

DAVID STARR JORDAN, PH. D.

PRESIDENT OF LELAND STANFORD JUNIOR UNIVERSITY
AND

HAROLD HEATH, PH. D.

ASSISTANT PROFESSOR OF ZOOLOGY, LELAND STANFORD JUNIOR
UNIVERSITY

NEW YORK

D. APPLETON AND COMPANY
1907

COPYRIGHT, 1902
BY D. APPLETON AND COMPANY

Published May, 1902

PEEFACE

THE present volume is designed to meet the needs of
the beginning student of zoology. Accordingly, technical
and scientific names have been avoided as far as possible,
and those used are fully explained in the text or elsewhere.
The opening chapters deal with the characteristics of living
things, and, in contrasting animals and plants, attempt to
bring into relief the distinguishing marks of all animals.
Then follows a discussion of the cell and protoplasm, pre-
paring the way for the examination of a series of animals
representative of each of the great groups, from the sim-
plest to the most complex. These are considered from the
view-point of structure ; but considerable attention is also
paid to the functions of their parts, to their habits and life-
history, so that while the representatives examined are, for
the sake of simplicity, relatively few in number, they are, it
is believed, thoroughly typical. Hence, with a knowledge
of the facts presented, the student should have a broad
view of the animal kingdom, and a foundation on which to
base future study and observation. It is perhaps unneces-
sary to add that from the study of books alone no one can
really make such knowledge his own. A personal acquaint-
ance with even a few animals in their native haunts, and
an understanding of the structure and the function of their

vi ANIMAL FORMS

parts gained from dissection and experiment, is essential to
a full comprehension of what the student learns from text-
book and teacher.

The greater number of illustrations are new, and have
been drawn or photographed from living or preserved ma-
te"rial. When not otherwise accredited, the drawings have
been made by Miss Mary H. Wellman and J. Carter Beard,
to whom the authors extend their sincere thanks. Our
obligations are also due to Mr. Walter K. Fisher, who has
made the drawings of the vertebrate dissections ; to Messrs.
A. L. Melander and C. T. Brues, of Chicago, 111. ; Mr. Wm.
H. Fisher, of Baltimore, Md. ; Eev. H. K. Job, of Kent,
Conn. ; Mr. Wm. Graham, of Pasadena, Cal. ; and Dr. E. W.
Shufeldt, of New York city, for numerous photographs.

DAYID STARE JORDAN,
HAROLD HEATH.

CONTENTS

CHAPTER PAGE

I. — INTRODUCTION 1

II. — THE CELL AND PROTOPLASM .7

III.— THE PROTOZOA 11

IV. — THE SPONGES 19

V. — THE CCELENTERATES 29

VI.— THE WORMS 44

VII. — ANIMALS OF UNCERTAIN RELATIONSHIPS .... 66

VIII.— MOLLUSKS 72

IX. — ARTHROPODS. CLASS CRUSTACEA 93

X. — ARTHROPODS. CLASS INSECTS 114

XI. — ARTHROPODS. CLASS ARACHNIDA 133

XII.— ECHINODERMS 140

XIII.— THE CHORDATES 151

XIV.— THE FISHES 154

XV.— THE AMPHIBIANS 174

XVI.— THE REPTILES 184

XVII.— THE BIRDS 201

XVIII.— THE MAMMALS 225'

vii

ANIMAL FORMS

CHAPTER I

INTRODUCTION

1. Divisions of the subject. — Biology is the science which
treats of living things in all their relations. It is sub-
divided into Zoology, the science which deals with animals,
and Botany, which is concerned with plants. The field
covered by each of these branches is very extensive.
Within the scope of zoology are included all subjects bear-
ing on the form and structure of animals, on their devel-
opment, and on their activities, including the consideration
of their habits and the wider problems of their distribution
and their relations to one another.

These various subjects are often conveniently grouped
under three heads : Morphology, which treats of the form
and structure or the anatomy of organisms ; Physiology,
which considers their activities; and Ecology, which in-
cludes their relations one to another and to their surround-
ings. All the phases of plant or animal existence may be
considered under one or another of these three divisions.

2. The difference between animals and plants.— Generally
speaking, we have little difficulty in seeing that the objects
about us are either living or lifeless ; but the boundary line
between the two great divisions of living things, the animals
and plants, can not always be so clearly drawn. This is es-
pecially true of the simpler forms of life which frequently
combine both animal and plant characteristics ; but in the

1

2 ANIMAL FORMS

greater number of more highly developed species the line
of separation is clearly marked. It is very easy, for example,
to distinguish the oak-tree or the rose from a horse or a
"butterfly, and, as we shall see, the differences are not based
merely on outward appearance.

In the oak-tree, for example, the roots reaching down
into the earth, with the branches and leaves spreading out
into the air and sunlight, are admirably fitted for taking up
the food, which consists of very simple materials, less com-
plex than those forming the diet of an animal. This
permits a continuous existence in one place, and accord-
ingly we note the entire absence of locomotion and the or-
gans controlling it, which form so conspicuous a part of the
body of an animal. Also in the production of flowers and
seeds, and in the growth of the seed into the tree, we detect
many characteristics peculiar to plants.

3. Characteristics of an animal.— On the other hand, the
squirrel, for example, or any other animal, is unable to sub-
sist on water, air, and elements from the soil. These crea-
tures demand the highly diversified materials found in the
bodies of plants and of animals. Such being the case, they
do not remain anchored to one spot (except in a relatively
few cases), but are compelled to lead an active existence.
The power of voluntary movement, or movement in response
to internal impulse, is thus the first and one of the most
striking peculiarities of animals.

In the second place, the food of plants enters the body
in a soluble condition and is readily transferred to the or-
gans requiring it. While in the animals, the nutritive ma-
terials pass into the body in an insoluble state and de-
mand a varied preliminary treatment, usually within a
special digestive tube, before they are fit to be absorbed.
In the squirrel, by way of illustration, the food is first
ground to a pulp by the action of the teeth, and, moistened
with saliva, is swallowed and passed into the stomach,
where it is subjected to the solvent action of the gastric

ANIMAL FORMS

juice. From the stomach it is made to enter the intestine,
and is further acted upon by fluids from the liver, the

pancreas, and the glands of the
intestines themselves. Thus
treated it becomes changed from
an insoluble state into a fluid
which readily penetrates the
coats of the digestive tract.

Many of the organs of the
body are placed at a considera-
ble distance from the food as
it comes through the coats of
the stomach and intestine. In
order to supply them with the
necessary nourishment a distrib-
uting apparatus is required.
This is the office performed by
the circulatory system, for as
rapidly as the food penetrates
the walls of the digestive tract
it enters the blood, and by the
beating of the heart is driven
to all parts of the body, which
are thus continually kept in a
state of repair. The blood serves
also to remove waste substances
from the various structures or
organs of the animal body and
to transfer them to the kidneys,
skin, or lungs, which effect their
removal from the body.

4. Muscular and nervous sys-
tems.—Owing to the fact that
animals, as a rule, are compelled
to move about in search of food, we find two highly devel-
oped systems, the muscular and nervous, which are absent

FIG. 2.— Diagram of heart and blood-
vessels of the squirrel or other
mammal, a.o., aorta ; h, vessels
of head ; La., left auricle ; Lex.,
vessels of lower extremities ; Iff.,
lung ; l.v., left ventricle ; p.a.,
pulmonary artery ; p.v., pulmo-
nary vein ; r.a., right auricle ;
r.v., right ventricle ; #., vessels of
viscera. Arteries are represented
by heavy walls.

INTRODUCTION 5

in the plants. The first of these, constituting what is usu-
ally known as the lean meat, is a relatively complex system
of organs, differing widely according to the work performed.
In the higher animals — the squirrel, for example — there are
not less than five hundred muscles, which are under the
control of the nervous system.

The nervous system consists of the brain and spinal
cord, which in the squirrel are concealed and protected

FIG. 3.— Skeleton of squirrel, showing its relation to the body.

within the skull and back-bone. From them many nerves
pass outward to the muscles, and as many pass inward from
the eye, ear, nose, tongue, or skin. By the action of these
sense-organs the animal determines the nature of its sur-
roundings, detects its food, recognizes the presence of its
enemies, and is thus able to direct its movements to the
greatest advantage.

5. Multiplication of animals. — The organs thus far con-
sidered serve to perpetuate the animal as an individual ;
but some provision must also be made for the continuance
of the race. In the economy of nature each animal before

6 ANIMAL FORMS

its death should leave offspring to take the place of the
parent when it falls from the ranks. This is effected in
various ways. In some of the simpler animals the body
may divide into two equal parts, each of which becomes a
complete individual. In other cases the animal detaches
a relatively small portion of its body, much as a gardener
cuts a slip from a plant, and this likewise develops into a
new organism. In the greater number of animals, very
clearly illustrated by the birds, eggs are produced which
under favorable conditions develop into an organism resem-
bling the parents.

6. Summary. — Animals are thus seen to lead active, busy
lives, collecting food, avoiding enemies, and producing and
and caring for their young. While the activities of all
animals are directed to their own preservation and to the
multiplication of their kind, these processes are carried on
in the most diverse ways. The manner in which an organ
or an organism is made, and the method by which it does
its work, are mutually dependent one on the other. As
there is an enormous number of species of animals, each
differently constructed, there is, accordingly, a very great
variety of habits. As we shall see, the lower forms are
remarkably simple in their construction, and their mode of
existence is correspondingly simple. In the higher types
a much greater complexity exists, and their activities are
more varied and are characterized by a high degree of elabo-
ration. In every case, the animal, whether high or low, is
fitted for some particular haunt, where it may perform its
work in its own special way and may lead a successful life
of its own characteristic type.

CHAPTER II

THE CELL AND PROTOPLASM

7. Cells. — If we examine very carefully the different parts
of a squirrel under the high powers of the microscope
we find that they are composed of a multitude of small
structures which bear the same relations to the various
organs that bricks or stones do to a wall ; and if the inves-
tigation were continued it would be found that every or-
ganism is composed of one or more of these lesser elements
which bear the name of cells. In size they vary exceedingly,
and their shapes are most diverse, but, despite these differ-
ences, it will be seen that all exhibit a certain general re-
semblance one to the other.

8. Shape of cells. — In many of the simpler organisms
the component cells are jelly-like masses of a more or less
spherical form, but as we ascend the scale of life the condi-
tion of affairs becomes much more complex. In the squir-
rel, for example, we have already noted the presence of
various organs for carrying on different functions, such as
those of digestion, circulation, and respiration ; and, further,
the cells composing these various parts have been modified
in accordance with the duties they have to perform. In
the muscles the cells are long and slender (Fig. 4, D) ;
those forming the nerves and conveying sensations to and
from all parts of the body, like an extensive telegraph sys-
tem, are excessively delicate and thread-like ; in the skin,
and lining many cavities of the body, where the cells are
united into extensive sheets, they range in shape from high
and columnar to flat and scale-like forms (Fig. 4, E, F, G).

7

8 ANIMAL FORMS

The cells of the blood present another type (Fig. 4, B) ; and
so we might pass in review other parts of the body, and con-
tinue our studies with other groups of animals, always find-
ing new forms dependent upon the part they play in the
organism.

9. Size of cells. — Also in the matter of size the greatest
variations exist. Some of the smallest cells measure less
than one micromillimeter (^TO-^ of an inch) in diameter.
Over five hundred million such bodies could be readily
stowed away into a hollow sphere the size of the letter be-
ginning this sentence. In a drop of human blood of the
same size, between four and five million blood-cells or cor-
puscles float. And from this extreme all sizes exist up to
those with a diameter of 2.5 or 5 c.m. (one or two inches),
as in the case of the hen's or ostrich's egg. On the average
a cell will measure between .025 to .031 m.m. (TTiVfr an(^
-g-J-o of an inch) in diameter, a speck probably invisible to
the unaided eye. While the size and external appearance
of a cell are seen to be most variable, the internal structures
are found to show a striking resemblance throughout. All
are constructed upon essentially the same plan. Differ-
ences in form and size are superficial, and in passing to a
more careful study of one cell we gain a knowledge of the
important features of all.

10. A typical cell. — Some cell, for example that of the
liver (Fig. 4, A), may be chosen as a good representative of
a typical cell. To the naked eye it is barely visible as a
minute speck ; but under the microscope the appearance is
that of so much white of egg, an almost transparent jelly-
like mass bearing upon its outer surface a thin structure-
less membrane that serves to preserve its general shape and
also to protect the delicate cell material within. The com-
parison of the latter substance to egg albumen can be car-
ried no further than the simple physical appearance, for
albumen belongs to that great class of substances which
are said to be non-living or dead, while the cell material

THE CELL AND PROTOPLASM

9

or protoplasm, as it is termed, is a living substance. We
know of no case where life exists apart from protoplasm,
and for this reason the latter is frequently termed the
physical basis of life.

In addition to the features already described, the proto-
plasm of every perfect cell is modified upon the interior to

FIG. 4.— Different types of cells composing the body of the squirrel or other highly
developed animal. A, liver-cell; /, food materials; n, nucleus. B, blood-cell.
C, nerve-cell with small part of its fiber. D, muscle fiber. E, cells lining the
body cavity. F, lining of the windpipe. G, section through the skin. Highly
magnified.

form a well-defined spherical mass known as the nucleus.
Other structures are known to occur in the typical cell.
Experiment shows that the nucleus and cell protoplasm are
absolutely indispensable, whatever their size and shape, and

10 ANIMAL FORMS

therefore we are at present justified in defining the cell as
a small mass of protoplasm enclosing a nucleus.

11. Structure of protoplasm. — When seen under a lens
of moderate power protoplasm gives no indication of any
definite structure, and even with the highest magnifica-
tion it presents appearances which are not clearly under-
stood. According to the commonly accepted view, it con-
sists of two portions, one, the firmer, forming an excessively
delicate mesh work (Fig. 4, A) enclosing in its cavities
the second more fluid part. Therefore, when highly mag-
nified, the appearance would be essentially like a sponge
fully saturated with water ; but it should be remembered
that in the protoplasm the sponge work, and possibly the
fluid part, is living, and that both are transparent.

There are reasons for thinking that the structure and
the composition of protoplasm may change somewhat under
certain circumstances. It certainly is not everywhere alike,
for that of one animal must differ from that of another, and
different parts, such as the liver and brain, of the same form
must be unlike. These differences, however, are minor
when compared to the resemblances, for, as we shall see,
this living substance, wherever it exists, carries on the pro-
cesses of waste, repair, growth, sensation, contraction, and
the reproduction of its kind.

CHAPTEE III

THE PROTOZOA

12. Single-celled and many-celled animals.— In almost
every portion of the globe there are multitudes of animals
whose body consists of but a single cell ; while those forms
more familiar to us, and usually of comparatively large
size and higher development, such as sponges, insects,
fishes, birds, and man himself, are composed of a multitude
of cells. For this reason the animal kingdom has been
divided into two great subdivisions, the Protozoa including
all unicellular forms and the Metazoa embracing those of
many cells.

13. Single-celled animals. — The division of the Protozoa
comprises a host of animals, usually of microscopic size,
inhabiting fresh or salt water or damp localities on land in
nearly every portion of the globe. The greater number
wage their little, though fierce, wars on one another with-
out attracting much attention ; others, in the sharp struggle,
have been compelled to live upon or within the bodies of
other animals, and many have become notorious because of
the diseases they produce under such circumstances. A
few are in large measure responsible for the phosphores-
cence of the sea ; and still others have long been favorite
objects of study because of their marvelous beauty. Adapted
for living under diverse conditions, the bodily form differs
greatly, and yet all conform to three or four principal types,
of which we may gain a good idea from the study of a few
representative forms.

11

12

ANIMAL FORMS

14. The Amoeba. — Among the simplest one-celled ani-
mals living in the ooze at the bottom of nearly every fresh-
water stream or pond is the Amceba (Fig. 5, A), whose body
is barely visible to the unaided eye. Under the microscope

Fio. 5. — A, the Amoeba, highly magnified, showing c. v., pulsating vacuole ; f, food
particle ; n, nucleus. The arrows show the direction of movement. B, shape of
same individual 30 seconds later. C, an amoeba-like animal (Difflugia) partially
enclosed in a shell. D, an Amoeba in the process of division. E, Gromia, another
shelled protozoan (after SCHULZE).

it is seen to consist of an irregular, jelly-like mass of proto-
plasm totally destitute of a cell wall. Unlike those animals
with which we are familiar, the body constantly changes its
shape. A rounded bud-like projection will be seen to appear
on one side of the body and the protoplasm of adjacent
regions flows into it, thereby increasing its extent. Similar
projections at the opposite end of the cell are withdrawn,
and their substance may flow into the newly formed lobe,
which gradually swells in size and pushes forward. Thus,
by constantly advancing the front part of the body and

THE PROTOZOA 13

retracting the hinder portion, the cell glides or flows along
from place to place.

Upon meeting with any of the smaller organisms upon
which it lives, projections from the body are put out which
gradually flow around the prey and it becomes pressed into
the interior of the cell. The process is not unlike pushing
a grain of sand into a bit of jelly. There is no mouth.
Any point on the surface serves for the reception of food.
Oxygen gas also is taken into the body all over the surface,
and wastes and indigestible material are cast out at any
point. Nothing exists in these simple forms comparable to
the complex systems of organs that carry on these processes
in the squirrel.

The bodily size of animals is limited, and to this general
rule the Amoeba is no exception, for upon gaining a certain
size, the nucleus divides into two exactly similar portions,
and very soon afterward the rest of the body separates into
two independent masses of equal size (Fig. 5, D), each of
which, when entirely free, contains a nucleus. In this way
two daughter amoebae are formed possessing exactly the
characters of the parent save that they are of smaller size ;
but it is usually not long before they reach their limit of
growth, when division occurs again, and so on, generation
after generation.

It not infrequently happens, however, that the pond or
stream, in which the Amoeba and other Protozoa live, dries
up for a portion of the year. In such an event the body
assumes a spherical shape, develops a firm, horn-like mem-
brane about itself, and thus encysted it withstands the sum-
mer's heat and dryness and may be transported by the wind,
or otherwise, over great distances. When the conditions
again become favorable the wall ruptures and the Amoeba
emerges to repeat its life processes.

15. Some relatives of the Amoeba. — All amceba-like forms,
to the number of perhaps a thousand species, possess this
same method of locomotion, but many present some inter-

14 ANIMAL FORMS

esting additional characters. For example, the form repre-
sented in Fig. 5, C, constructs a sac-like skeleton of tiny
pebbles cemented together, into which it may withdraw for
protection. Others construct similar envelopes of lime or
flint, and still others, as they continue to grow, build on
additional chambers, giving rise to a great variety of forms
often of wonderful beauty. In the tropics, particularly,
some of the shelled Protozoa are so abundant that they may
impart a whitish tinge to the water, and in some places
their empty shells on falling to the bottom form immense
deposits. The chalk cliffs of England are in large measure
made up of such shells.

16. The Infusoria. — A little over two hundred years ago
it was discovered that wherever water remained stagnant it
became favorable for the rapid multiplication of a large
number of species of Protozoa which live in such situations.
These are known as Infusoria, and, like the preceding spe-
cies, are usually of microscopic size and of the most varied
shapes. The first striking feature of their organization is
the presence of a delicate though relatively firm external
cell membrane known as the cuticle, which preserves a defi-
nite shape to the body. Such a method of locomotion as
exists in the preceding group is consequently an impossi-
bility, but other and more highly developed structures per-
form the office. These latter organs are of two types, and
their general characteristics may be readily understood
from an examination of a few species living in the same
localities as the Amceba.

17. The Euglena. — The first type exists in the common
fresh-water organism known as Euglena, represented in
Fig. 6, A. Here the spindle-shaped body is surrounded by
a delicate cuticle perforated at one point, where a funnel-
shaped depression, the gullet, leads into the soft proto-
plasmic interior. From the base of this depression the
protoplasm is drawn out in the form of a delicate whip-like
process known as the flagellum. This structure, always

THE PROTOZOA

15

permanent in form, constantly beats backward and forward
with great rapidity in a general direction represented in
the diagram (Fig. 6, c). The movement from a to b is
much more rapid than the reverse, from b to «, which
results, like the action of the human arm in swimming, in
driving the organism forward. Not only does the flagel-
lum serve the purpose of locomotion, but it also produces
currents in the water which
may serve to bear minute
organisms down into the
"gullet, whence they read-
ily pass into the soft pro-

FIG. 6. — Flagellate Infusoria. A,
Euglena viridis ; c, pulsating
vacuole ; e, eye-spot ; g, gullet ;
w, nucleus ; t, flagellum. B, Co-
dosiga, with collar surrounding
the flagellum. C, diagram illus-
trating the action of the flagel-
lum. All figures greatly enlarged.

FIG. 7. — Paramcecium aurelia, a
ciliate infusorian. c, cilia; c.v.,
pulsating vacuoles ; /, food
particles ; g, gullet ; m, buccal
groove ; n, nucleus.

toplasm of the body, there to undergo the processes of di-
gestion and assimilation. In some forms the protoplasm in
the region of the flagellum is drawn out in the form of a
collar (Fig. 6, B), whose vibratory motion also aids in con-
veying and guiding food into the body.

18. The Slipper Animalcule. — The second type of loco-
motor organ may be understood from a study of the
24

16

ANIMAL FORMS

Slipper Animalcule (Paramwcium, Fig. 7), abundant in
stagnant water. In this form the cuticle surrounding the
somewhat cylindrical body is perforated by a great number
of minute openings through which the
internal protoplasm projects in the form
of delicate threads. Each process,
termed a cilium, works on the same
principle as the flagellum, but it beats
with an almost perfect rhythm and in
unison with its fellows, drives the an-
imal hither and thither with considera-
ble rapidity.

On one side of the body is a furrow
which deepens as it runs backward and
finally passes into the gullet (#), which
leads into the interior of the body.
Throughout the entire extent it is lined
with cilia which create strong currents
in the surrounding water and in this
way conduct food down the gullet into
the body. Embedded in the outer sur-
face of the body, in among the cilia,
are also a number of very minute sacks,
each containing a coiled thread which
may be discharged against the body of
any intruder, so that this form is sup-
plied with actual organs of defense.
Two pulsating vacuoles (c.v.) or simple
kidneys are also present, consisting of a
central reservoir into which a number
of radiating canals extend.

19. The Bell Animalcule and other
species,— The Bell Animalcule ( Vorti-
cella, Fig. 8) is often found in the same situations as the
Slipper Animalcule, which in certain respects it resembles.
It is generally attached by a slender stalk, and where many

FIG. 8.—Vorticella, an at-
tached ciliate infusori-
an, highly magnified, a,
fully extended individ-
ual ; c.v., pulsating va-
cuole ; g, gullet ; n, nu-
cleus, b, contracted
specimen, c, small free-
swimming individual,
which unites with a sta-
tionary individual (one
partly united is shown
in specimen b).

THE PROTOZOA 1Y

are growing together they appear like a delicate growth
of mold upon the water weed. The stalk is peculiar in
being traversed by a muscle fiber arranged in a loose spiral,
which upon any unusual disturbance contracts together
with the body into the form shown in Fig. 8, b.

These few examples serve to show the general plan of
organization and the method of locomotion of the Infuso-
ria ; but, as upward of a thousand species exist, with widely
differing habits, many interesting modifications are present.
Some have been driven in past time to adopt a parasitic
mode of life within the bodies of other animals. At pres-
ent they are devoid of locomotor organs, and as they absorb
nutritive fluids through the surface of the body all traces
of a mouth are also absent. The reproductive processes
also are peculiar, but they do not concern us now.

20. Characteristics common to the Protozoa. — We have
now studied the principal structures which serve in loco-
motion among these simple one-celled forms, also the means
by which they catch their food, and we shall now glance at
the internal processes, which are much the same in all.

After the food has been taken into the cell, it is prob-
ably acted upon by some digestive fluid, for it soon assumes
a granular appearance and finally undergoes complete solu-
tion. In every case the oxygen is absorbed through the
general surface of the body, and uniting with the living
substance, as in the squirrel, liberates the energy necessary
for the performance of the animal's life work. The wastes
thus produced in a large number of forms simply filter out
from the body without the agency of anything comparable
to a kidney, but in several species they are borne to a
definite spot, the pulsating vacuole (Figs. 5, 7, 8, c.v.), where
they gradually accumulate into a drop about the size of the
nucleus. The wall between it and the exterior now gives
way and the excretions are passed out. In active indi-
viduals this process may be repeated two or three times a
minute, but it is usually of less frequent occurrence.

18 ANIMAL FORMS

The loss in bodily waste is continually made good by
the manufacture of the food into protoplasm, and if the in-
come be greater than the outgo growth ensues. But, as in
all other forms, growth is limited, and ultimately the cell is
destined to divide, resulting in two new individuals. This
process may be repeated many times, but not indefinitely,
for sooner or later various members of the same species
unite in pairs temporarily or permanently, exchange nu-
clear material, and separate again with apparently renewed
energy and the ability to divide for many generations.

21. Simple and complex animals. — It is important to note
that these same processes of waste, repair, growth, feeling,
motion, and multiplication are the same as those of the
squirrel, and, furthermore, are common to all living crea-
tures, so that the difference between animals is not in their
activities, but in their bodily mechanisms ; and according to
the perfection of this, the animal is high or low in the
scale. Comparing, for example, the Amceba and Slipper
Animalcule, which are relatively low and high Protozoa, we
find in the former that any part of the body serves in loco-
motion and in the capture of food, while in the latter these
same functions are performed by definite structures, the
and gullet. Now it is well known that a workman is
able to make better watch-springs, when this is his sole
duty, than another who must make all parts of the watch ;
and likewise where a definite task is performed by a defi-
nite structure, it is more efficiently done than where any
and every part of the body must carry it on. So the
Amceba, in which definite tasks are performed by any part
of the body indifferently, is less perfect and thus lower than
the Paramcecium, where these functions are performed by
special organs. As we ascend the scale of life we find this
division of labor among special parts of the body more
complete, the organs and therefore the animal more com-
plex, and better fitted to carry on the work of its life.

CHAPTER IV

THE SPONGES

22. Their relation to the Protozoa. — While the greater
number of one-celled forms are not united with their fel-
lows, there are several species where the reverse is true. In
Fig. 9, for example, a fresh-water form known as Pandorina
is represented, consisting of sixteen cells embedded in a
spherical, jelly-like substance,
each one of which is precisely
like its companions in form
and activity. The aggregation
may be looked upon as a colo-
ny of sixteen Protozoa united
together to derive the benefit
of increased locomotion and
a larger amount of food in
consequence. As a result of
such a union they have not
lost their independence, for if
one be separated from the main
company it continues to exist.

From such a simple colonial

type we may pass through a series of several more complex
forms which reach their highest development in the beau-
tiful organism, Volvox (Fig. 10). In this form the indi-
vidual members, to the number of many thousand, are ar-
ranged in the shape of a hollow sphere. The united efforts
of the greater number, which bear on their outer surfaces
two flagella, drive the colony with the rolling movement

19

FIG. 9.— Pandorina (from Nature).
Highly magnified.

ANIMAL FORMS

from place to place. As just indicated, some individuals
lack the flagella, and their subsequent careers show them
to be of a peculiar type. Sooner or later each undergoes

a series of divisions form-
ing a little globe of cells,
which migrates into the in-
terior of the parent sphere
and develops into a new
colony. Within a short time
the walls of the parent
break, liberating the im-
prisoned young, which con-
tinue the existence of the
species while the parent or-
ganism soon decays.

Under certain circum-
stances, instead of develop-
ing colonies by such a meth-
od, some of the cells may
store up food matters and
become eggs, while others,
known as sperm-cells, de-
velop a flagellum, and sep-
arating from the colony
swim actively in the sur-
rounding water, where each
finally unites with an egg.
This union, like that of the
two individuals in Vorticel-
la (Fig. 8, #, c), results in

PIG. 10.-A, Volvox minor, entire colony tne P<>Wer of division, and
(from Nature). B, C, and D, reproduc- the egg enters upon its de-
tive cells of Volvox globator. All highly , -, . . j .

magnified. velopment, dividing again

and again. The cells so pro-
duced remain together, form a sphere, and finally develop
a Volvox colony.

THE SPONGES 21

In such associations as Volvox an important step has
been taken beyond that of Pandorina, for there is a division
of the labors of the colony among its various members,
some acting as locomotor cells while others are germ-cells.
These are now so dependent one upon the other that they
are unable to exist after separation from the main com-
pany, just as a part of the squirrel is incapable of leading
an independent existence. A higher type of organism has
thus arisen intermediate between the simple one-celled
animals and those of many cells, especially the sponges — a
relation which is more readily recognized after an examina-
tion of the latter.

23. Development of the sponge.— Like all many-celled
animals, the sponge begins its life, however, as a single cell
— the egg — which is in this case barely visible to the sharp
unaided eye. Fertilized by its union with a sperm cell,
development commences, and the first apparent indication
of the process will be the division of the cell into halves
(Fig. 11, A, B). Each half redivides into four, these again
into eight cells, and this process is repeated, giving the
young sponge the general form of Pandorina. The divi-
sions of the cells still continue and result in the formation
of a hollow globe of cells (called the Uastula, Fig. 11, E, F)
similar to Volvox, and at this point the young larva leaves
the parent.

The next transformation consists in a pushing in of one
side of the sphere, just as one might press in the side of a
hollow rubber ball. The depression gradually deepens, and
finally results in the formation of a two-layered sac known
as the gastrula (Fig. 11, G). At this stage of its existence
the sponge settles down for life in some suitable spot, by
applying the opening of its sac-like body to some foreign
object. In assuming the final form a new mouth breaks
through what was once the bottom of the sac, canals per-
forate the body wall, a skeleton is developed, and the char-
acteristic features of the adult are thus attained.

22

ANIMAL FORMS

24. Distribution. — The sponges are aquatic animals, and,
with the exception of one family consisting of relatively
few species, all are inhabitants of the sea in every part of

FIG. 11. — Diagrams illustrating the development of a sponge. A, egg-cell ; n, nucleus.
B, C, D, 2-, 4-, and 16-cell stages. E, Uastula, F, section through somewhat
older larva. G, gastrula. H, young sponge. I, section through somewhat
younger larva than H.

the glohe. The larger number occupy positions along the
shore, becoming especially abundant in the tropics ; but
other species occur at greater depths, several species living

THE SPONGES 23

between three and four miles from the surface. Unlike
the majority of animals, all members of this group are
securely fastened to some foreign object, such as rocks, the
supports of wharves, or with one extremity embedded in
the sand. As we have seen, the young enjoy a free-swim-
ming existence and are swept far and wide by means of
tidal currents, but sooner or later these migrations are
terminated in some suitable locality, where the sponge
passes the remainder of its existence. During this time
some species may never exceed the size of a mustard-seed,
while others attain a diameter of three feet, or even more.
Sponges also vary exceedingly in shape, some having the
form of thin encrusting sheets, others being globular, tubu-
lar, cuplike, or highly branched (Fig. 12).

25. The influence of their surroundings. — In by far the
larger number of cases an animal possesses the bodily form
of the parent. External agencies may modify this to some
extent, but usually only to a limited degree. A squirrel,
for example, resembling its parent, may grow to a relatively
large or stunted size according to the food supply, and it
may become strong or weak according to the amount of
exercise, and various other changes may result owing to
outside causes ; but as a result of these influences the
animal is rarely so modified that one is unable to distinguish
the species. Many of the sponges, however, are exceptions
to this general rule. If, for example, some of the young
of a certain parent develop in quiet water or in an un-
favorable locality, they will usually be low, flat, and un-
branched ; while the others, growing in swiftly running
waterways, develop into tall, comparatively delicate and
highly branched individuals. Under such circumstances
not only does the external form become modified, but
the internal organization may undergo profound change.
The entire organism is plastic and readily molded by
the influence of its surroundings, and the consequent
lack of definite characters often renders it impossible

ANIMAL FORMS

to assign such forms to a definite position among the

sponges.

26. Structure of a simple sponge.— In the simpler sponges

the body is usually vase-shaped (Fig. 13), with the base

fastened to some foreign
object, while at an oppo-
site end an opening leads
into a comparatively large
internal cavity. This lat-
ter space is also put in
communication with the
exterior by a multitude of
minute pores which pene:
trate the body wall. In

FIG. 12.— Various forms of sponges, natural size. (From Nature.)

the living condition currents of water continually pass
through these smaller canals, and out of the large termi-
nal opening, thus bringing within reach of the body minute

THE SPONGES

floating organisms or organic remains which serve as food.
The mechanism by which this process is effected, and the
various other structures of the body, are in large part invis-
ible from the exterior, requiring the
study of thin sections of the sponge
to make them clearly understood.

Under the microscope such a sec-
tion shows the body of a sponge to
consist of an immense number of va-
riously formed cells constituting three
distinct layers (Fig. 14). Not only
do these layers consist of different
kinds of cells, but the duties per-
formed by each are different. For ex-
ample, a glance at Fig. 14 will show
that in the inner layer certain colum-
nar cells exist, provided with a fla-
gellum and encircling collar, the ap-
pearance being strikingly like certain
of the Protozoa (Fig. 6, B). During
life their whip-like processes, lashing
backward and forward in perfect uni-
son, produce currents of water which
continually pass through the body.
The food thus entering the animal is
taken up by the cells of the inner
layer as it passes by. The supply,
however, is usually more than suffi-
cient to meet the demands of this
layer, and the excess is passed on to
the middle and outer layers. The
exact method by which this occurs is still a matter of
doubt, but there seems to be little question but that
each cell of the body receives its food in a practically un-
modified condition, requiring that it digest as well as
assimilate. The oxygen necessary to this latter process

FIG. 13.— One of the sim-
plest sponges (Calcolyn-
thus primigenius (after
HAECKEL). A portion
of the wall has been re-
moved to show the in-
side.

ANIMAL FORMS

is absorbed by all parts of the body in contact with the

water.

27. Skeleton of sponges. — When it is remembered that

the protoplasm composing the cells of the sponge has about

the same consistence
as the white of egg,
it will be readily un-
derstood why the
greater number of
sponges possess a skel-
eton. Without such
a support the larger
globular or branched
forms could not ex-
ist, and even in the

smaller members there would be danger of a collapse of the

body walls and consequent stoppage of the food supply,

owing to the closure of the pores. So in all but a very few

thin or fiat forms a skeleton appears in the young sponge

almost before growth

has fairly begun, and

this increases with the

body in size and com-
plexity. It is formed

by the activity of the

cells of the middle layer,

and may be composed

either of a lime com-

FIG. 14.— Portion of wall of sponge, showing three
layers, e, outer layer ; i, inner layer, consisting
of collared cells ; m, middle layer, consisting of
irregular cells, among which are the radiate spic-
ules and egg-cells.

FIG. 15.— Different types of sponge spicules.

pound resembling mar-
ble, or of flint, or of a
horn-like substance resembling silk, or these may exist in
combination in certain species. When consisting of either
of the first-named substances it is never formed in one
continuous piece, but of a vast multitude of variously shaped
crystal-like bodies termed spicules (Fig. 15). These occur
everywhere throughout the body, firmly bound together

THE SPONGES 27

by means of cells, or so interlocked that they form a rigid
support to which the fleshy substance is bound and through
which the numerous canals penetrate.

In a relatively few species only does the skeleton con-
sist of horn, though there are many in which horn and flint
exist together. In the former event, if the skeleton be
elastic and of sufficient size, it becomes valuable to others
than the naturalist, for the familiar sponges of commerce
are the horny skeletons of forms usually taken in the West
Indies or in the Mediterranean Sea. In these localities the
animals are pulled off by divers, or with hooks, and are then
spread out in shallow water where the protoplasmic sub-
stance rapidly decays. The remaining skeleton, thoroughly
washed and dried, is ready for the markets of the civilized
world.

Examining a bit of such a " sponge " under a magnify-
ing glass, it will be seen that the skeleton is not composed
of various pieces, but of one continuous mass of branching
fibers, which interlace and unite in apparently the greatest
confusion ; yet in the living animal these were perfectly
adapted to the position of the canals and the general needs
of the animal.

Besides being a scaffold-work to which the fleshy portions
of the body are fastened, the skeleton serves also for pro-
tection. In some species, needle-like spicules as fast as
they are formed are partly pushed out over the entire sur-
face of the body, giving the appearance of a spiny cactus ;
or in other cases they are arranged in tufts about the canals,
effectually preventing the entrance of any marauder.
Thus perfectly protected, the sponges have but few natural
enemies, and hence it is that in favorable localities they
grow in great profusion.

28. Race histories and life histories.— We have now traced
living things from their simplest beginnings, where they
exist as single cells, and have seen that in bygone times
similar forms have united into simple colonies, and these

28 ANIMAL FORMS

through a division of labor among the constituent cells
have resulted in Volvox-like colonies. There are the strong-
est reasons for the belief that as these simple forms scat-
tered into various surroundings and underwent changes to
meet the shifting conditions, they assumed different de-
grees of complexity that have resulted in the animal forms
of the present day.

It may have been noticed also that the sponge in its
development passes through these stages : a single-celled
egg ; later, a young form similar to Pandorina, then growing
to look like Volvox, and finally assuming its permanent form.
The history of the race of sponges and their development
through a long line of ancestry of increasing complexity is
thus told by the sponge as it develops from the egg into
the adult ; and, so far as we know, all the many-celled ani-
mals in their growth from the egg repeat more or less
clearly the stages passed through by their forefathers.

CHAPTER V

THE CCELENTBRATES

29. General remarks,— This division of the many-celled
animals includes the jelly-fishes, sea-anemones, and corals.
A few species live in fresh water, but the majority are con-
fined to the sea, being found everywhere from the shore-
line and ocean surface to the most profound depths.
Adapted to different surroundings and modes of life, they
constitute a vast assemblage of the most bewildering di-
versity. In some cases their resemblance to plants is re-
markable, and the term zoophyte or " plant animal," occa-
sionally applied to them, is the relic of former times when
naturalists confounded them with plants. Even to-day
certain species are sometimes collected and preserved as
seaweeds by the uninformed.

The general plan on which all coelenterates are con-
structed is a simple sac, in some respects resembling that
of the lower sponges, yet, since the modes of life of the
members of the two groups are usually quite unlike, we
shall find many profound differences between them.

30. The fresh-water Hydra. — The bodily plan comes out
most clearly in the Hydra (Fig. 16, A, D), which occurs
upon the stems and leaves of submerged fresh-water plants
in this and other countries. Its body, of a green or grayish
color, according to the species, scarcely ever attains a diam-
eter greater than that of an ordinary pin nor a length ex-
ceeding half an inch. One end of the cylindrical organism
is attached to some foreign object by means of a sticky
secretion, but as occasion requires it may free itself, and by

29

30

ANIMAL FORMS

means of a " measuring-worm " movement travel to another
place.

Examined under a hand lens, the free end of the body
will be found to support six to eight prolongations known

as tentacles, which
serve to convey
food to the mouth,
centrally located
in their midst.
This opening, un-
like that of the
sponges, is the
only one leading
directly into the
large central gas-
tric cavity which
occupies nearly
the entire animal
(Fig. 16, D).( As
in the sponge, the
cells of the body
are arranged in
the form of defi-
nite layers, but the
middle one is rep-
resented only by
a thin gelatinous
sheet.

31. Organs of
defense. -- These
are the so-called
lasso or nettle-cells
(Fig. 16, C). Some
of the cells of the outer layer possess, in addition to the
elements of the typical cell, a relatively large ovoid sac
filled with a fluid, and also a spirally wound hollow thread

FIG. 16.— The fresh-water Hydra. A, entire animal, de-
veloping a new individual (enlarged 25 times). B, C,
nettle-cells (after SCHNEIDER) ; D, section through
the body.

THE CCELENTERATBS 31

provided with barbs near its base. On the outer extremity
of the nettle-cell projects a delicate bristle-like process, the
trigger hair. These cells are especially abundant on the
tentacles (Fig. 16, A, D), forming close, knob-like eleva-
tions or " batteries," thus rendering it practically impossi-
ble for any free-swimming organism to avoid touching them
in brushing against the tentacles. In such an event the dis-
turbances conveyed through the trigger hair set up in some
unknown way very rapid changes in the cell. This causes
the sac to discharge the coiled thread and barbs into the
body of the intruder, which is rendered helpless by the par-
alyzing action of the fluid conveyed through the thread.
Thus benumbed it is rapidly borne to the mouth and swal-
lowed. In time new nettle-cells develop to take the place
of those discharged and consequently worthless.

32. Digestion of food. — Upon the interior of the body of
Hydra and all of the coelenterates the food, by reason of its
large size, is incapable of being taken into the various cells.
It is necessary, therefore, to break it up into smaller masses,
and this is accomplished through the solvent action of the
digestive fluid poured over it from some of the cells of the
adjacent inner layer. When subdivided, the granules swept
about the gastric cavity by the beating of the flagella (Fig.
16, D) are seized by the processes on the free surfaces of the
remaining inner layer cells, where they undergo the final
stages of -digestion ; then in a dissolved state they become
absorbed and assimilated by all the cells of the body.

33. Methods of multiplication. — Very frequently, espe-
cially if the Hydra has been well fed, two or three pro-
cesses arising as outpushings of the body wall may be
noted upon the sides of the animal (Fig. 16, A, D). If
these be watched from time to time they are found to in-
crease in size, and finally, upon their free extremities, to
develop a mouth and surrounding tentacles. Up to this
point growth has taken place as a result of the assimilation
of nutritive substances supplied from the parent ; but a con-

25

32

ANIMAL FORMS

striction soon occurs which
separates the young from the
parent, and from that time
on the two lead independent
existences. At other times
this asexual method of mul-
tiplication is replaced by sex-
ual reproduction, where new
individuals arise from fertil-
ized eggs. Both eggs and
sperm arise in Hydra and in
some other animals in the
same individual, but in all
such cases the eggs are fertil-
ized by sperm which escape
from some other individual.
The fertilized egg, surround-
ed by a firm coat, separates
from the parent, drops to the
bottom, and after a period of
rest develops into a little Hy-
dra which hatches and enters
upon a free existence.

FIG. 17.— Different types of Hydrozoan
colonies. From Nature, the lower
species magnified about 50 diameters.

THE CCELENTERATES 33

34. Hydrozoa, or Hydra-like animals.— Attention has al-
ready been directed to the fact that the structure of Hydra
is the simplest of the coelenterates ; nevertheless, the thou-
sand or more species belonging to this class which present
a much more complicated appearance (Fig. 17) possess
many fundamental Hydra-like characters. It is owing to
this fact that this assemblage of forms has been placed in
the class of the Hydrozoa, or Hydra-like animals.

With but very few exceptions the members of this class
are marine, usually living near the shore-line, where at
times their plant-like bodies occur in the greatest profusion
attached to rocks, seaweeds, or the bodies of other animals,
particularly snails and crabs. Fig. 17 (upper colony) gives
a good idea of one of the more complex forms, whose tree-
like body attains in some cases the relatively giant height of
from 15 to 25 c.m. (six to ten inches). In early life it bears
a close resemblance to a Hydra. Buds form in much the
same way, but they retain permanently their connection with
the parent, and in turn bear other buds, until finally the form
shown in the figure is attained. In the meantime root-like
processes have been forming which afford firm attachment
to the object upon which the body rests. Also during this
process the cells of the outer layer form a horny external
skeleton ensheathing the entire organism except the ter-
minal portions (the hydranths, Fig. 18, B) bearing the ten-
tacles. The gastric cavities of all communicate, and the
food captured by one ministers in part to its own needs
and, swept through the tubular stalks and roots, is also
shared by all other members.

35. Jelly-fishes and the part they play.— During the pro-
cess of growth a number of stubby branches arise which
differ from the ordinary type in shape, and also in many
cases as regards color. . These club-like, fleshy portions de-
velop close-set buds (Fig. 18, c) which early assume a bell-
like shape, the point of attachment corresponding to the
handle, while the clapper is represented by a short, slender

34 ANIMAL FORMS

process bearing on its end an opening which becomes the
mouth (Fig. 18, A). Around the margin of the bell nu-
merous tentacles develop, and at the same time the gelati-
nous substance situated between the outer ^and inner layers
of the bell expands to a relatively enormous degree, giving
it an increasing globular form and glassy appearance.

B

FIG. 18.— A jelly-fish (Gonionemus), slightly enlarged. The stalked mouth is shown
in dotted outline. B, C, enlarged portions of a hydroid colony bearing the
mouth and tentacles ; j, a capsule within which the jelly-fish develop ; D, dia-
gram of jelly-fish, illustrating its method of locomotion.

Finally, vigorous movements rupture the connection with
the parent, and this newly developed outgrowth, usually '
small, becomes an independent organism popularly termed
a jelly-fish. While the external form of the jelly-fish appears
to be widely different from the hydranths, a more careful
study shows the difference to be superficial. Some zoolo-
gists believe that jelly-fishes are simply buds which have
become fitted to separate and swim away from the colony '
in order to distribute the young, as described hereafter.
When the stalked colonies are very abundant the jelly-

THE (XELENTERATES 35

fishes may be liberated in such multitudes that the upper
surface of the ocean for many miles may be closely packed
with them in numbers reaching far into the millions. In
these positions they are carried both by oceanic currents
and through the alternate expansion and contraction of the
bell, a movement resembling the partial closing and open-
ing of an umbrella. In the jelly-fish the contraction is
more vigorous and rapid than the opening in the velum or
veil (Fig. 18, b) which is so narrowed that the water in
the subumbrella space (a) is driven through it with con-
siderable force, which results in driving the body in the
opposite direction.

The life of a jelly-fish is perhaps of short duration, last-
ing not more than a few hours in some species, up to two
or three weeks in others, but during that period they pro-
duce multitudes of eggs which develop into minute free-
swimming young. These settle down on some rock or sea-
weed, and soon develop a Hydra-like body which, after the
fashion described above, grows into another tree-like colony.

36. Alternation of generations.— It will be noticed that
the offspring of the jelly-fishes are not jelly-fishes, but stalked
colonies, and these latter forms give rise to jelly-fishes.
This is known as the alternation of generations, the jelly-
fish generation alternating with the colonial form. This
characteristic is of the greatest service in preventing the
extermination of the race. Were the stalked forms to
give rise directly to other stationary colonies, it is obvious
that before long all the available space in the immediate
locality would be filled. The food supply, always lim-
ited, would not suffice, and starvation of some or imper-
fect development of all would result ; but by means of the
free-swimming jelly-fish new colonies are established over
very extensive areas, and favorable situations are held
by all.

37. More complex types. — As mentioned above, there are
perhaps upward of a thousand species of Hydrozoa, all with

36

ANIMAL FORMS

essentially the same structure but with various modes of
branching (for some of the commoner modes, see Fig. 17)-
In some of the higher forms a division of labor has arisen
among various members of the association which has led to
most interesting results. For example, Fig. 19 represents
a species of hydroid found investing the shells of sea-snails
occupied by hermit crabs (Fig. 60). To the unaided eye
its appearance is that of a delicate vegetable growth, but
when placed under the microscope it is found to consist of

a multitude of Hydra-like
animals united by a hollow
branching root system con-
necting the gastric cavities
of all of them (Fig. 19).
Certain individuals (a)
with tentacles and a mouth
resemble a Hydra ; others,
without a mouth and ten-
tacles, are reduced to a
club-like form (b) liberally
supplied with nettle-cells
upon their free extremi-
ties; while the third type
(c), likewise devoid of a mouth, possesses rudiments of ten-
tacles below which are borne numerous clumps of repro-
ductive cells. The first type, the only one possessing a
mouth, captures the food, and after digesting it distributes
the greater portion to the remaining members by means of
the connecting root system ; those of the second form, de-
fending the others by means of their nettle-cells against
the inroads of a foreign enemy, are the soldiers of the colo-
ny; while the third type produces the eggs from which
new individuals develop.

In some of the higher Hydrozoa, the Portuguese man-
of-war (Fig. 20), this division of labor has reached a more"
advanced stage of development, and in addition the entire

FIG. 19.— An enlarged portion of a hydroid
colony (Hydractinia), showing (a) the
nutritive polyp, (5) the defensive polyp,
and (c) the reproductive polyp.

THE CCELENTERATES

37

colony is fitted for a free-swimming existence. What cor-
responds ordinarily to the attached stalk in other forms
terminates in a bladder-like expansion, distended with
gas, that serves as a float. From it are suspended individ-
uals resembling great stream-
ers sometimes many feet in
length, without mouths, but
loaded with nettle-cells that
enable them to capture the
food, which is conveyed to the
second type, the nutritive
polyps. Each of these is a
simple tube bearing a mouth,
and within them the food is
digested and distributed by
means of a branching gastric
cavity extending throughout
the entire colony. Then there
are individuals like mouthless
jelly-fishes which bear the
eggs and care for the perpet-
uation of the colony ; and be-
sides these there may be some
whose duty it is to defend the
rest, and others whose active
swimming movements, to-
gether with the wind, drive
the colony about. Thus uni-
ted, sharing the food supply
and working for the general welfare of all, the members of
this colony live in greater security and with less effort than
if, as separate individuals, each was fighting the battles of
life alone.

38. Scyphozoa, — The greater number of the larger and
more conspicuous jelly-fishes are included under this term.
In general shape and locomotion they resemble those of the

PIG. 20.— A colonial jelly-fish (Physalia).
From Nature.

38

ANIMAL FORMS

preceding group (Fig. 21), but, while the latter are generally
very small, these forms are commonly from four to twelve
inches in diameter, and some measure one to two meters
(three to six feet) across the bell. They are also distin-
guished by means of tentacles which extend from the cor-
ners of the mouth sometimes to a distance of several feet,

and together with the
marginal tentacles are
formidable weapons for
capturing small crabs,
fishes, and other ani-
mals which serve as
food. In turn these
forms serve as the food
of many whales, por-
poises, and numerous
fishes which hunt them
down, though the
amount of nourishment
they contain is prob-j
ably relatively small
owing to the fact that
in their composition
there is a large percent-
age of water (99 per
cent in some species). The lobed margin of the bell, the
absence of a definite swimming organ or velum, and the
character of several of the internal organs, distinguish the
larger from the smaller jelly-fish ; but the greatest differ-
ence, however, is in the method of development.

39. Development. — The eggs arise from the inner layer
of the jelly-fish and drop into the gastric cavity, where each
develops into a ciliated two-layered sac in some respects
like that of a young sponge. Swimming away from the
parent, they finally settle down, and attaching themselves
(Fig. 22, a) assume the external form and habits of the sea-

FIG. 21.— A jelly-fish (Rhizostoma), about one-
fourth natural size.

THE CCELENTERATES

39

anemones, described in the next section. In the course of
time remarkable changes ensue, which first manifest them-

FIG. 22.— Stages in the development of a scyphozoan jelly-fish, a, the attached
young, which in b has separated into a number of disks, each of which becomes a
jelly-fish, c. — After KORSCHELT and HEIDEB.

selves in a series of grooves encircling the body. These
grow deeper, and the body of the animal finally comes to
resemble a pile of sau-
cers with the edge of
each developed into a
number of lobes (Fig.
22, I). One after an-
other each saucer, to
preserve the simile,
raises itself from the
top of the pile and
swims away, and is
clearly seen to be a
jelly-fish, though con-
siderably unlike the
adult. As growth pro-
ceeds, however, it un-
dergoes a series of transformations which result in the
adult form.

FIG. 23.— An attached scyphozoan jelly-fish
(Halidystus). Natural size, from Nature.

40

ANIMAL FORMS

40. Sea-anemones. — In its external appearance the sea-
anemone (Fig. 24) bears some resemblance to the Hydra, but
is of a much larger size (1 to 45 c.m., or \ inch to 1-J feet
in diameter), and is frequently brilliantly colored. The
number of tentacles is also more numerous, and the mouth
leads into the body by means of a slender esophagus (Fig.
25). Numerous partitions from the body wall extend in-
ward, and many unite to the esophagus, keeping the latter

FIG. 24.— Sea anemones (the two upper figures) and solitary coral polyps.

in position. Below the esophagus each partition projects
into the great cavity of the body and bears upon its inner
free edge several important structures. The first of these,
known as the mesenteric filaments (Fig. 25), appearing like
delicate frills, plays an active part in the digestion of the
food. Associated with these are long, slender threads,

THE CCELENTERATES

41

closely packed with innumerable lasso-cells, which may be
thrown out through openings in the body wall when the
animal is attacked. Lasso-cells are also very numerous on
the tentacles, which are thus to some extent defensive, but
are chiefly active in capturing the crabs and small fish
which serve as food.

The partitions also carry eggs which may undergo the
first stages of their growth within the body, and when
finally able to swim
are sent out through
the mouth opening
by hundreds to seek
out favorable situa-
tions, there to set-
tle down and re-
main. In some spe-
cies the young may
sometimes arise as
buds, as in Hydra
(Fig. 24), and in
others the animals
have been described
as splitting longi-
tudinally into two
equal -sized young.

41. Corals.— The
coral polyps also

belong to this group, showing a very close resemblance to
the sea-anemones. In most cases they develop a firm skel-
eton of lime, commonly known as " coral," which serves to
protect and support the body. In a few species the polyps
throughout life are solitary, and with skeleton comparative-
ly simple (Fig. 24) ; but the larger number of species be-
come more complex by developing buds, which retain their
connection with the parent, and in turn produce other out-
growths with the ultimate result that highly branched

FIG. 25.— Longitudinal section through the body of a
sea-anemone, oe, esophagus ; m. /., mesenterial
filaments ; r., reproductive organs.

42 ANIMAL FORMS

colonies are produced (Fig. 26). At the same time the
outer layer of the body is continually forming a skeleton
which encloses the colony as a sheath, except at the ter-
mination of each branch, where the mouth and tentacles
are located. In certain species — for example, the sea pens
(Pennatuld) and sea fans (Gorgonia)—& skeleton may be

FIG. 26.— Small portions of coral colonies, with some of the polyps expanded.

formed of myriads of lime spicules, somewhat like those
of the sponge, which are bound together by the fleshy
substance of the body; but the skeleton of most of the
common forms in the ocean, and the coral found in
general collections, is stony. According to their method
of branching, such specimens have received various popu-
lar names, such as brain, stag-horn, organ-pipe, and fun-
gous corals.

THE CCELENTERATES 43

Nearly all species, like the sea-anemones, are brilliantly
colored during life, and several are highly phosphorescent.
All are marine, and while they are found everywhere, from
the shore-line to great depths, the more abundant and
larger species inhabit the clear, warm waters of the tropics
down to a depth of one hundred and sixty feet. In such
regions the stag-horn corals especially grow in the wildest
profusion, and become tall and greatly branched. Except
in quiet water they are continually being broken by the
waves, beaten into fragments, and the resulting sand is
deposited about their bases. As a result of this continu-
ous growth and erosion, there have been formed from coral
sand mixed with the shells of mollusks and the skeletons
of various Protozoa several of the islands along the Florida
coast and many of those of the Pacific, some of them
hundreds of miles in extent.

CHAPTER VI

« '

THE WORMS

4> General Characteristics.— The bodies of the animals
comprising the two preceding groups are exposed on all
sides equally to the water in which they live and are radi-
ally symmetrical ; but in the worms, one side of the body
is fitted for creeping, and for the first time we note a well-
marked dorsal (back) and ventral (under) surface. In the
former, the body, like a cylinder, may be divided into simi-
lar halves by any number of planes passing lengthwise
through the middle ; but in the worms, the right and left
halves only are exposed equally to their surroundings, and
there is, accordingly, only one plane which divides the body
into corresponding halves, so that these animals, like all
higher forms, are bilaterally symmetrical. In creeping, also,
one end of the body is directed forward and it thus be-
comes correspondingly modified. It usually bears the
mouth, and may be provided with eyes, feelers, or organs
of touch, and various other structures which enable the
worm to recognize the nature of its surroundings. The
nervous and muscular systems are better developed than in
the foregoing groups, and we note a greater vigor and defi-
niteness in the animal's movements, and in various ways the
worms appear better able to avoid or ward off their enemies,
recognize and select their food, and in general adapt them-
selves to the conditions of life.

The division of the worms is a very large one, and in
some respects difficult to define, owing to the close resem-
44

THE WORMS

45

blance which many of them show to animals in other
groups. All the invertebrates, therefore, except the crabs
and insects, were placed in one group until subsequent
study made it possible to classify them more exactly. Ac-
cording to the general shape of the body, and the arrange-
ment of internal organs, worms are divided into a number
of groups, chief among which are the flatworms, the thread
or roundworms, and the ringed worms or annelids.

THE FLATWORMS

43. Form and habitat. — The flatworms, as their -me
indicates, are much flattened, leaf-like forms, some species
living in damp places on land,
in fresh - water streams or
ponds, or along the seacoast,
while a variety of other spe-
cies are parasitic. The free
forms (Fig. 27) are usually
small, barely reaching a length
greater than five or seven cen-
timeters (2 to 3 inches), but
some of the parasitic species
(Fig. 31) attain the great
length of six to thirteen me-
ters (20 to 40 feet).

The free-living forms usu-
ally occur on the under side
of stones, and frequently are
so delicate that a touch is
sufficient to destroy them. A
few species are almost trans- FIG. ST.-A, fre8h-water natworm (Pto-

. naria) ; B, marine flatworm (Lepto-

parent, While many are COl- piana}. Enlarged, from Nature.

ored to harmonize completely

with their surroundings, so that, even though fragile and
defenseless, they escape the attacks of enemies by being
overlooked. The night-time or dark days are their hunting

46

ANIMAL FORMS

season, and at such periods they may be found moving about
with a steady gliding motion (due to cilia covering the en-
tire body), varied occasionally by a looping, caterpillar move-
ment, or by swimming with a flapping of the sides of the
body. When watched at such times they may sometimes
be seen to snatch up small worms, snails, small crabs and
insects, which serve as food.

More closely examining one of these forms, for example,
the species usually found on the under side of sticks and
stones in our shallow fresh-water streams (Fig. 27, A), we note
that the forward end is not developed into a well-defined

head as in the higher worms,
but is readily determined by
the presence of very simple
eyes and tentacles, while the
lower creeping surface is dis-
tinguished by a lighter color
and the presence of the
mouth. Through this small
opening a slender .proboscis

m

(in reality the pharynx) may
be extended some distance,
and may be seen to hold the
small organisms upon which
it lives until they are suffi-
ciently digested to be taken
into the body.

44 Digestive system.— In
the smaller flatworms, some
FIG. 28.-Anatomy of fresh-water flat- of wnich are scarcely larger

worm (Planaria). exs, excretory sys- than many of the Protozoa

rnCLTr^. Toet the ^entary canal is a sim-
ous system. pie unbranched tube ; but in

the larger forms such an ap-
paratus is replaced by a greatly branched digestive tract
which furnishes an extensive surface for the rapid absorp-

THE WORMS

47

tion of food, and extending deep into the tissues of the
body, carries nutriment to otherwise isolated regions. In
the fresh-water forms and their allies there are three main
branches of the intestine (Fig. 28), while in many of those
from the sea there are several, and their arrangement
affords a basis for their general classification.

45. Excretory system. — In the sponges and coelenterates
the wastes are cast out by the various cells into the gastric
cavity or at once to the exterior with-
out the aid of any pronounced system

of vessels; but in the flatworms sev-
eral of the organs are deeply buried
within the tissues of the body and a
drainage system becomes a necessity.
This consists of a paired system of ves-
sels extending the length of the ani-
mal (Fig. 28) and provided with numer-
ous branches, some of which open at
various points on the surface of the
body, while the others terminate in
spaces (Fig. 29, s) among the organs in
what are known as flame-cells. The
substances which accumulate in these
spaces are gathered up by the flame-
cell, poured into the space it contains, and by means of the
vibratory motion of its flagellum (/), a movement bearing
a fancied resemblance to the flickering of a flame in the
wind, are borne through the tubes to the exterior.

46. Nervous system and sense-organs. — In the sponges no
definite nervous system is known to exist, the slight move-
ments which the cells are able to undergo being regulated
somewhat as they are in the Protozoa. Among the coelen-
terates certain of the cells scattered over the surface of the
body are set aside as nerve-cells, and, more or less united by
means of fibers extending from them, convey impulses over
the body. In the flatworms the larger number of nerve-cells

26

FIG. 29.— Flame-cell of flat-
worm (after LANG). /,
flagellum ; n, nucleus;
s, spaces among the or-
gans of the body ; v,
waste materials.

48 ANIMAL FORMS

are collected into two definite masses (Fig. 28, B), which
constitute a simple brain on which the eyes are situated
and from which bundles of nerve fibers pass to all parts of
the body, the two extending backward being especially
noticeable. As in the squirrel, these are distributed to the
muscles and other organs to regulate their activity, while
those distributed to the skin, especially in the forward
part of the body, convey stimuli produced by touch. The
branches connecting with the eyes enable the animal to
distinguish light from darkness, but are probably too sim-
ple to allow it to clearly distinguish objects of the outside
world. The sense of smell and possibly that of taste are
also present, but are relatively feeble.

Some other characters of this class will be noted in the
consideration of the two following classes.

47. Parasitic flatworms (trematodes)— parasitism.— Men-
tion has already been made of the associations of two ani-
mals as " messmates " for mutual benefit, such as the Hy-
dractinia growing on the surface of the shell inhabited by
the hermit crab, to which it gives protection by means of
its nettle-cells, while in turn being borne continually into
regions abounding with food. More frequently, however,
one animal derives benefit from another without making
any compensation. For example, many species of flatworms
live within the shells of certain snails and upon the bodies
of sea-urchins and starfishes, where they gather in their
food supply safe from the attacks of enemies. Such asso-
ciations are probably without much if any inconvenience to
the animal thus inhabited, and it also appears probable
that the tenants are transients, using the mollusk or star-
fish only as a temporary home. But from this condition of
affairs it is only a short step to the parasitic habit, where
the association becomes permanent and the occupant is
provided with various structures which prevent its sepa-
ration from its host. This latter kind of union occurs
throughout the group of trematodes ; all are parasitic, and

THE WORMS

49

their internal organization, so closely resembling that of
the free-living forms as to need no further description, in-
dicates that they are
descendants of the lat-
ter. In the greater
number the body is
flat, and a few species
still retain their outer
coat of cilia ; but since
these are no longer of
service as locomotor
organs they have gen-
erally disappeared, and
in their place numer-
ous adhesive organs,
such as spines, hooks,
and suckers (Fig. 30),
have arisen, which en-
able the animals to
hold on with great te-
nacity. Thus attached
to its host, and using
it as a convenient and
comparatively safe
means of locomotion,
the parasite may still

continue to capture small animals for food or may derive
its nourishment from the tissues of the host. In addition
there are numbers of internal parasites, living almost ex-
clusively in the bodies of vertebrate animals, scarcely a sin-
gle one escaping their ravages.

48. Life history. — In the external parasites the young
hatch out and with comparative ease make their way to
another host ; but the young of an internal parasite, inhab-
iting the alimentary canal, have a very slight chance in-
deed of ever reaching a similar location in another host.

FIG. 30. — A parasitic flatwonn (Epidella). m
month ; o, opening of reproductive system ;
s, sucker and spines for attachment. The di-
gestive system is stippled ; nervous system
black. Enlarged 8 times, from Nature.

50 ANIMAL FORMS

For this reason an almost incredible number of eggs is laid,
and some extraordinary measures are employed in effecting
the desired result. Probably the best-known example is that
of the liver fluke inhabiting the bile-ducts in the sheep.
Each worm lays several hundred thousand eggs, which make
their way from the host, and if they chance to fall "in pools
of water or damp situations may proceed to develop, other-
wise not. If the surroundings be favorable, the young, like
little ciliated Infusoria, escape from their shells and rest-
lessly swim or move about for a short time, and if during
this time they come in contact with certain species of
snails living in these situations they at once bore into their
bodies. Here they produce other young somewhat resem-
bling a tadpole, that now make their escape from the snail.
In a short time each one crawls upon a blade of grass, and
surrounds itself with a tough shell, where it may remain for
several weeks. If the grass on which they rest be eaten by
a sheep, they finally make their way to the bile-ducts and
there become adult. The life cycle is now complete ; the
young form has found a new host ; and the process shows
how wonderfully animals are adapted to the conditions which
surround them, and how closely they must conform to these
conditions in order to exist.

49. The tapeworms (cestodes), — The cestodes, or tape-
worms, are also parasitic flatworms in which the effects of
such a mode of life are strongly marked. They occur
almost exclusively in the bodies of vortebrate hosts and
exhibit a great variety of bodily forms, in some cases resem-
bling rather closely the trematodes, but in others strikingly
different. In the latter type the body is usually of great
length (from a few centimeters to upwards of sixteen meters
(50 feet)), and terminates in a "head" (Fig. 31) provided,
in the different species, with a great variety of hooks and
spines and numbers of suckers for its attachment to the
body of the host. From the head the body extends back-
ward in the gradually enlarging ribbon-like body, slender at

THE WORMS

51

first and scarcely showing the segments which finally be-
come so prominent a feature.

When carefully examined, a two-lohed brain is found
in the " head," and from it nerves extend the entire length
of the body, followed throughout their
course by the tubes of the excretory
system ; also each segment contains a
perfect reproductive system, so that
even if it be separated from the others
it may continue to exist for a consid-
erable length of time. Furthermore,
the tapeworms are surrounded by the
predigested fluids of their host; a
special alimentary canal is therefore
superfluous, and all traces of it have
disappeared.

50. Development— As the animal
clings in this passive way to the body
of its host the segments, loaded with
eggs ready for development, separate
one after another from the free end
of the body, pass to the exterior, and

slowly crawling about like independent organisms, lay great
numbers of eggs, which may find an intermediate host as in
the life cycle of the liver fluke, and so in time find their
permanent resting-place. Fortunately in all these parasitic
forms, though an inconceivably great number of eggs are
laid, only a comparatively few reach maturity. Even these,
however, may cause at times great destruction among the
higher, and especially our domestic, animals, often doing
damage amounting to many millions of dollars per year.

51. The tapeworm in relation to regeneration,— It has
been known for more than one hundred and fifty years that
some of the lower animals possess to a surprising degree
the ability to regenerate parts of the body lost through
injury. The Hydra, hydroids, and some of the jelly-fishes

FIG. 31.— Tapeworm (Tcenia
solium). In upper left-
hand corner of figure is
the much enlarged head.
—After LEUCKART.

52 ANIMAL FORMS

may be cut into a number of pieces, each of which will
develop into a complete individual ; and this power of recov-
ery from the injuries produced by enemies is of the great-
est service in the perpetuation of the species. This ability
is also present in certain flatworms, and some species are
known which voluntarily separate the body into two por-
tions, each of which becomes an adult. In other species a
similar process results in the formation of a chain of six
individuals, placed end to end, the chain finally breaking
up into as many complete worms. It is possible that the
tapeworm may also be looked upon as a great chain of
united individuals produced by the division of a single
original parent, which becomes adapted for attaching the
others until they separate. These latter are capable only of
a very sluggish movement, and, devoid of mouth and ali-
mentary canal, are not able to digest their food, but their
life work is to so lay their eggs that they may develop into
other individuals, and for this they are well adapted.

NEMATODES (THREADWORMS)

52. General characters.— This class of worms is com-
posed of an enormous number of different species, some para-
sitic, others free all or a portion of their lives, and in view of
the fact that they inhabit the most diverse situations it is
remarkable that they are so uniform in their structure. In
all the body is slender, and the general features of its organ-
ization may be readily understood from an examination of
the " vinegar eel " (Fig. 32, A). This small worm (not an
eel), a millimeter or two in length, lives on the various forms
of mold that grow in fermenting fruit juices, especially
after a little sugar or paste has been added. A tough cuti-
cle surrounds the body, preserving its shape and at the
same time protecting the delicate organs against the action
of the acids in which it lives. Through this may be seen
great bands of muscles extending the entire length of the
body and producing the wriggling movements of swimming

THE WORMS

53

or crawling. They also give support to a brain, which is in
the form of a collar encircling the pharynx near the head,
and to the great nerves which extend from it. Still fur-
ther within the transparent body the alimentary canal may
be distinguished as a straight tube
passing directly through the ani-
mal. The alimentary canal lies
freely in a great space, the body
cavity, traces of which may exist
in the flatworms in the form of
hollow spaces into which the
kidneys open. It is possible that
in this form also the kidneys open
into this space, and it is roomy
enough besides to afford lodg-
ment for the reproductive organs
in addition to a large amount of
fluid which is probably somewhat
of the nature of blood. A space in
some respects similar to this occurs
in all the animals above this group,
and as we shall see, it is often cu-
riously modified and serves for a
number of different and highly im-
portant purposes. In the round-
worms the fluid it contains proba-
bly acts in the nature of a blood
system, distributing the food and
oxygen to various parts of the body and carrying the wastes
to the kidneys for removal.

53. Multiplication. — In the matter of the production of
new individuals the greatest differences exist. In some
threadworms, for example the " vinegar eel," eggs develop
within the body and the young are born with the form of the
parent. In other cases the eggs are laid in the water, where
they, too, may directly grow to the adult condition ; but in

FIG. 32. — Thread- or round-
worms. A, vinegar eel ( An-
guittuld) ; m, month ; ph.,
pharynx ; i, intestine ; ov..
developing young. B, Tri-
china. From Nature, greatly
enlarged.

54 ANIMAL FORMS

the greater number of species the development is round-
about, and one or more hosts are inhabited before the young
assume the adult condition. Such is the case with the
dreaded Trichina (Fig. 32, B), which infests the bodies of
several animals, particularly the rat. When these forms
are introduced into the alimentary canal of the rat, for
example, they soon lay a vast quantity of eggs, sometimes
many millions, which develop into young that bore their
way into the muscles of the body, where they may remain
coiled up for years. If the body of the rat be eaten by some
carnivorous animal, these excessively small young are lib-
erated during the process of digestion and rapidly assume
the adult condition in the alimentary canal, likewise giving
rise to young which pursue again this same course of
development.

Another example of a complicated life history is in
the Gordius or " horsehair snake " (a true worm and not a
snake) frequently seen in the spring in pools where it lays
its eggs. These eggs develop into young which bore their
way into different insect larvae, which are in turn eaten by
some spider or beetle, and the worm thus transferred to a
new host. In this they grow to a considerable size, and
then make their exit from the body of the host and finally
become adult.

54. Spontaneous generation. — It is only within compara-
tively recent years that such life histories have been under-
stood. Formerly the sudden appearance of these and other
forms in various situations were accounted for on the ground
that they arose spontaneously without the intervention of
any living creature. Even yet we hear of the transforma-
tion of horsehairs into hairworms, and of frogs, earthworms,
and several other animals from inorganic matter, but such
assertions are based on superficial observations, and at the
present time no exception is known to the law that living
creatures arise from preexisting living parents. " All life
from life " (omnium vivum ex vivo) is a universal law.

THE WORMS 55

ANNELIDS OR SEGMENTED WORMS

55. The earthworms and their relatives.— Leaving the
groups of the parasitic animals, which have been driven from
the field of active existence and in many ways are degraded
by such a mode of life, we pass on to the higher free-living
worms, where brilliant colors, peculiar habits, or remarkable
adaptations render them peculiarly interesting. In consid-
ering first their general organization, we may use the earth-
C

m

FIG. 33.— Earthworm (Lumbricus terrestris). m, mouth ; c, girdle or clitellum.

worm (Fig. 33) (sometimes called angle-worm or fish-worm)
as a type because of its almost universal distribution.

The body is cylindrical, shows well-marked dorsal and
ventral surfaces, and, as in all of the annelids, is jointed,
each joint being known as a segment. Anteriorly it tapers
to a point, and the head region bearing the mouth is ill-
defined, unlike many sea forms, yet serves admirably for
tunneling the soil in which all earthworms live. In this
process the animal is also aided by bristles or setce which
project from the body wall of almost every segment and
may be stuck into the earth to afford a foothold.

56. Food and digestive system. — The earthworms are
nocturnal animals, seldom coming to the surface during the
day except when forced to do so by the filling of their tun-
nels with water or when pursued by enemies. At night
they usually emerge partially, keeping the posterior end of
the body within the burrow, and thus they scour the sur-
rounding areas for food, which they appear, in some cases
at least, to locate by a feeble sense of smell. They also
frequently extend their habitations, and in so doing swallow
enormous quantities of earth from which they digest out
any nutritive substances, leaving the indigestible matter in

56 ANIMAL FORMS

coiled " castings " at the entrance of the burrows. In thus
mixing the soil and rendering it porous they are of great
service to the agriculturist.

Although earthworms are omnivorous they also manifest
a preference for certain kinds of food, notably cabbage,
celery, and meat, which leads us to think that they have a
sense of taste. All these substances are carried into their
retreats and devoured, or are used to block the entrance
during the day. The food thus carried into the body is
digested by a system (Fig. 34) composed of several portions,

FIG. 34. — Earthworm (Lumbricus) dissected from left side, b, brain ; c, crop ;
outer opening of male reproductive system ; dv, dorsal blood-vessel ; g, gizzard ;
h, pulsating vessels or " hearts " ; i, intestine ; k, kidney ; m, mouth ; n. c., nerve-
cord ; oe, esophagus ; 0, ovary ; od, oviduct ; ph, pharynx ; r, testes ; s.r., sem-
inal receptacles ; v.v., ventral vessel.

each of which is modified for a particular part in the pro-
cess. The mouth (m) leads into a muscular pharynx (ph)
whose action enables the worm to retain its hold on various
objects until swallowed, and this in turn* is continuous with
the esophagus. From here the food is passed into the thin-
walled crop (c),and from this storehouse is gradually borne
into the gizzard (^), whose muscular walls reduce it to a fine
pulp now readily acted upon by the digestive fluids. These,
resembling in their action the pancreatic juice of higher
animals, are poured out from the walls of the intestine into
which the food now makes its way ; and as it courses down
this relatively simple tube the nutritive substances are ab-
sorbed while the indigestible matters are cast away.

57. Circulatory system. — In all the groups of animals up
to this point the digested food is carried through the body
by a simple process of absorption, or in the threadworms by

THE WORMS

57

means of the fluid in the body cavity ; but in the earthworm
the division of labor between different parts of the body is
more perfect, and a definite blood system now acts as a
distributing apparatus. This consists primarily of a dorsal
vessel lying along the dorsal surface of tfee alimentary canal
(Fig. 34), from which numerous branches are given off to
thq body wall, and to the digestive system through which
they ramify in every direction before again being collected
into a ventral vessel lying below the digestive tract. In
some of the anterior segments a few of the connecting
vessels are muscular and unbranched, and during life pul-
sate like so many hearts to force the blood over the body,
forward in the dorsal vessel, through the " hearts " into the
ventral vessel, thence into the dorsal by
means of the small connecting branches.
Some of the duties of this vascular
system are also shared by the fluid of
the body cavity, which is made to cir-
culate through openings in the parti-
tions by the contractions of the body
wall of the animal in the act of crawl-
ing. In this rough fashion a consider-
able amount of nutritive material and
oxygen are distributed to various or-
gans, and wastes are carried to the kid- FlG 35._Diagram of earth-
neys to be removed. worm kidney. &,biood-

T i 11 £ J.T- vessel ; /, funnel open-

58. Excretion.— In nearly all of the ing into body cavity;
segmented worms there is a pair of o, outer opening; *,

j. /TV 04 OK\ septum; w, body wall.

kidneys to every segment (Figs. d4, 65).
Each consists of a coiled tube wrapped in a mass of small
blood-vessels, and at its inner end communicating with the
body cavity by means of a funnel-shaped opening. In
some unknown way jhe walls of the kidney extract the
waste materials from the blood-vessels coursing over it and
pass them into its tubular cavity. At the same time the
cilia about the mouth of the funnel-shaped extremity are

58 ANIMAL FORMS

driving a current from the body-cavity fluids, which wash
the wastes to the exterior.

59. Nervous system. — The nervous system of the earth-
worm consists first of a brain composed of two pear-shaped
masses united together above the pharynx (one shown in
Fig. 34), from which nerves pass out to the upper lip and
the head, which are thus rendered highly sensitive. Two
other nerves also pass out from the brain, and, coursing
down on each side of the pharynx like a collar, unite below
it and extend side by side along the under surface of the
digestive system throughout its entire extent. In each
segment the two halves of this ventral nerve-cord are united
by a nerve, and -others are distributed to Various organs,
which are thus made to act and in proper amount for the
good of the body as a whole.

In its relation to the outside world the chief source of
information comes to the earthworm through the sense of
touch, for definite organs of sight, taste, and smell are but
feebly developed, while ears appear to be entirely absent.
Nevertheless these are sufficient to enable it to lead a suc-
cessful life, as is evidenced by the great number of such
worms found on every hand.

60. Egg-laying.— In digging up the soil where earth-
worms abound one frequently finds small yellowish or
brownish bodies looking something like a gi*aiij of wheat.
These are the cocoons in which the earthworms lay their
eggs, and the method by which this is performed is unique.
We have already noted the presence of a swollen girdle
(the clitellum) about the body of the worm. At the breed-
ing season this throws out a fluid which soon hardens into
an encircling band. By vigorous contractions of the body
this horn-like collar is now slipped forward, and as it passes
the openings of the reproductive organs the eggs and
sperms are pushed within it. They thus occupy the space
between the worm and the collar, and when the latter is
shoved off over the head its ends close as though drawn to-

THE WORMS

59

gether by elastic bands. A sac, the cocoon, is thus pro-
duced, containing the eggs and a milky, nutritive substance.
In a few weeks the worm
develops and, bursting the
wall of its prison, makes its
escape.

61. Distribution. — The
earthworms and their allies
are found widely distributed
throughout the world, and
all exhibit many of the
characters just described.
The greatest differences
arise in their mode of life :
some are truly earthworms,
but others are fitted for a
purely aquatic existence in
fresh water or along the
seacoast ; a few have taken
up abodes in various ani-
mals and plants, and in
some of these situations they
extend far up the sides of
the higher mountains. In
all, the head is relatively
indistinct, the number of

bristles 011 each Segment FlG- 36.— A marine worm (Nereis). A, ap-

few, and for this and other ES^*""1* 8eason' and B'

reasons all are included in

the subclass Oligochaeta, or " few-bristle " worms. ,

62. Nereis and its allies. — In many of the above-men-
tioned situations members of a more extensive group of
worms are found, with highly developed heads and many
bristles arranged along the sides of the body. These are
the Polychaetes or " many-bristle " worms, and .as a repre-
sentative we may take Nereis (Fig. 36), a very common

60 ANIMAL FORMS

form along almost any seashore. The body presents the
same segmented appearance as the earthworm, but the
head (Fig. 37, A) is provided with numerous sense organs,
chief among which are four eyes and
several tentacles or "feelers."

The segments behind the head

FIG. 37.— A, head and one of the lateral appendages (B) of a marine worm (Nereis
brandtii) ; al, intestine ; /, " gill " ; k, kidney ; ra, nerve cord ; s, bristles for loco-
motion.

differ very little from one another, and, unlike those of
the earthworm, each bears a pair of lateral plates (Figs.
36, 37, B) or paddles with many lobes, some of which bear
numerous bristles. By a to-and-f ro movement these organs
aid in pushing the animal about, or may enable certain spe-
cies to swim with considerable rapidity.

As in all otn^r worms, respiration takes place through
the surface of the body, the area of which is increased by
the development, on certain portions of the paddles (para-
podia), of plates penetrated with numerous blood-vessels,
which thus become special respiratory organs or gills
(Fig. 37, B).

In their internal organization the Polychaetes are con-
structed practically on the same plan as the earthworms,
the principal difference being in the reproductive system.
In the earthworm this is restricted to some of the forward
segments, while in the present group the eggs and sperms

THE WORMS

61

are developed in almost every segment, whence they are
finally swept to the exterior through the tubes of the kid-
neys (Fig. 37, B).

The Nereis and its immediate relatives are all active
forms, and by means of powerful jaws, which may be quickly
extended from the lower part of the mouth cavity, they
capture large numbers of small crustaceans, mollusks, and
worms which happen in their path. Others more distantly
related make their diet of seaweed, and
many living on the sea bottom swallow
great quantities of sand, from which they
absorb the nutritious substances.

63. Sedentary forms. — Preyed upon by
many enemies, a large number of species
have been forced to abandon an active ex-
istence save in their early youth, and to
construct many interesting devices for their
protection. Numerous species, shortly after
they commence to shift for themselves)
build about their bodies tubes of lime (Fig.
39), from which they may emerge to gather
food and into which they may dash in times
of danger. As the worm grows the tube is
correspondingly enlarged, and these tubes,
in all stages of construction and variously
coiled, may be found on almost every avail-
able spot at the seashore, and may often
be seen on the shells of oysters in the
markets.

In other species the tube is like thin
horn, and may be further strengthened or
concealed by numerous pebbles, bits of carefully selected
seaweeds, or highly tinted shells, which give them a very
attractive appearance. Such species usually develop out
of immediate contact with other forms, but a few live
so closely associated together that their twisted tubes

FIG. 38.— A common
marine worm (Po-
ly nee, brevisetosa),
with extended pro-
boscis and over-
lapping plates cov-
ering the back.

62

ANIMAL FORMS

form great stony masses, sometimes several feet in dia-
meter.

64. Effects of an inactive life. — In many species such a
sedentary life has resulted in the almost complete disap-
pearance of the lateral appendages, which therefore no
longer serve as organs of respiration, and this function has
been shifted accordingly on to other structures. These
new organs are situated principally on the exposed head,

FIG. 39.— Sedentary tube-dwelling marine worms, upper left hand Sabella (one-half
natural size), the remainder Serpula (enlarged twice). From life.

and Fig. 39 shows the general appearance of some com-
mon species. The corners of the mouth have expanded
into great plumes, sometimes wondrously colored like a
full-blown flower, and these, bounteously supplied with
blood-vessels, act as gills. When disturbed, the plumes are
hastily withdrawn into the tube, and some of the so-called
serpulids (Fig. 39, bottom of figure) close the entrance with
a funnel-shaped stopper. While the plumes are primarily
respiratory organs, they also act as delicate feelers, and may
even bear a score or more of eyes ; and in addition, being

THE WORMS

63

covered with cilia, create the currents of water which
bring minute organisms serving as food within reach of
the mouth.

65. Development.— Unlike the earthworms, the Poly-
chaetes lay their eggs in the sea water, where they are left
alone to develop as best they may. Both the male and
female Nereis, as the egg-laying time approaches, undergo
remarkable changes in their external appearance, resulting
in the form shown in Fig. 36, A.

They are now active swimmers, and
thus are able to scatter the fertilized
eggs over wide and more or less favor-
able areas. The young also for a
time are free-swimming, but finally
end their migrations by settling to
the sea bottom, where they gradually
attain the adult condition.

As in some of the flatworms, re-
production may also occur asexually
by the division of the animal into two
or more parts, each of which subse-
quently becomes a complete indi-
vidual. In other species growth of
various parts may result in two com-
plete worms at the time of separation ;
and from such forms we may trace a
fairly complete series up to those in
which the original parent breaks up Fie.4o.-A
into twenty to thirty young.

66. The leeches. — At first sight
the leeches (Fig. 40), or at least the
smaller, more leaf-like forms, might

be mistaken for flatworms, especially for some of the para-
sitic species. As in the latter, the mouth is surrounded by
a sucker, and another is located at the hinder end of the
body, but beyond this point the resemblance ceases. The
27

I

P.

si.

ld). Right-hand figure il-
lustrates alimentary canal.

ph, pharynx ; e, crop ; p,
;^.pouche8: '*" "*

64 ANIMAL FORMS

outer surface is delicately marked off into eighty or a hun-
dred rings, of which from three to five are included in one
of the deeper true segments corresponding to those of
other annelids. From two to ten pairs of simple eyes are
borne on the head, and owing to the fact that they are
active swimmers, or move by caterpillar-like looping, loco-
motor spines are unnecessary and absent. In their internal
organization, however, there are many features which in-
dicate a close relationship with the Oligochaetes or few-
bristle worms. The nervous, circulatory, and certain char-
acteristics of the excretory systems are decidedly similar,
but, on the other hand, there are some facts difficult to
explain, which have led some zoologists to believe that
the relationship of these animals can not at present be
determined.

67. Haunts and habits.— The leeches usually dwell in
among the plants in slowly running streams, but some
occur in moist haunts on land, and a considerable number
live in the sea. All are " bloodsuckers " — fierce carnivo-
rous worms, whose bite is so insidiously made that the vic-
tim frequently is ignorant of their presence. Fishes, frogs,
and turtles are the most frequently attacked, but cattle and
other animals which come down to drink also become their
prey. In some of the tropical countries the land-leeches
are present in large numbers secreted among the leaves, and
so severe are their attacks that various animals, even man,
succumb to their united efforts. Adhering by their suck-
ers, they puncture the skin, some using triple jaws, and
fill themselves until they become greatly distended, when
they usually drop off and digest the meal at leisure. In
certain species the intestine is provided with lateral
pouches (Fig. 40), which serve to store up the food until
the time for digestion arrives. A full meal is sufficient
with some species to last for two or three months, and the
medicinal or horse-leech when gorged with food may con-
sume a year in digesting it.

THE WORMS 65

68. Egg-laying. — The eggs of some leeches are stored
up in a cocoon like that of the earthworm, which is attached
to submerged plants or placed under stones. When the
young are able to lead independent lives they emerge with
the form of the parent. A leaf -like form, Clepsine, some-
times found adhering to turtles, fastens the eggs to the
under side of its body, and the young when hatched
remain there for several days, adhering by their posterior
suckers.

CHAPTER VII

ANIMALS OF UNCERTAIN RELATIONSHIPS

IN" this chapter we shall consider in a brief way a number
of different groups of animals whose relationships are un-
certain. Up to the present time the study of their habits,
structure, and development has been of too fragmentary
or unrelated a character to enable the majority of zoologists
to agree upon their classification. Nevertheless, many of
them are highly interesting and attractive,
often very common, and in some respects
they hold important positions in the animal
kingdom.

69. The rotifers or wheel-animalcules. —
The rotifers or wheel-animalcules are rela-
tively small and beautiful organisms, rarely
ever longer than a third of an inch, but at
times so abundant that they may impart a
reddish tinge to the water of the streams
and ponds in which they live. At first
sight they might be mistaken for one-celled
animals, but the presence of a digestive
tract and of reproductive elements soon dis-
pels such a belief. Examined under the
microscope, the more common forms are
seen to possess an elongated body terminat-
ing at the forward end in two disk-like expansions beset
along the edges with powerful cilia. These serve to drive
the animal about, or, when it remains temporarily attached

FIG. 41.— A wheel-
animalcule (Rotifer).

ANIMALS OF UNCERTAIN RELATIONSHIPS 67

by the sticky secretion of the foot, to sweep the food-par-
ticles down into the mouth. Through the walls of the
transparent body such substances are seen to pass into the
stomach, where they are rapidly hammered or rasped into
a pulp by the action of several teeth located there. In
the absence of a circulatory system the absorbed food is
conveyed by the fluid of the body-cavity, which also con-
veys the wastes to the delicate kidneys. Several other
features of their organization are of much interest, espe-
cially to the zoologist, who believes that he gains from
their simple structure some ideas of the ancestors of the
modern worms, mollusks, and their allies. During the
summer the rotifers lay two sizes of " summer eggs,"
which are remarkable for developing without fertilization.
The large size give rise to females, the smaller to males, the
latter appearing when the conditions commence to be un-
favorable. The " winter eggs," fertilized by the males and
covered with a firm shell, are able for prolonged periods to
withstand freezing, drought, or transportation by the wind.
The adults also are able under the same adverse conditions
to surround themselves with a firm protective membrane
and to exist for at least a year. Once again in the presence
of moisture the shell dissolves, and in a surprisingly short
space of time they emerge, apparently none the worse for
the prolonged period of quiescence.

70. Gephyrea.— There is a comparatively large group of
worm-like organisms, over one hundred species in all, which
at present hold a rather unsettled position in the animal
kingdom. Some of the more common forms (Fig. 42)
living in the cracks of rocks or buried in the sand, usually
in shallow tide pools along the seashore, have a spindle-
shaped body terminated at one end by a circlet of tentacles
which surround the mouth. On account of their external
resemblance to many of the sea-cucumbers (Fig. 92), they
were earlier associated in the same group ; but an examina-
tion of their internal organization inclines many zoologists

ANIMAL FORMS

to the belief that the ancestors of some of these animals
were segmented worms whose present condition has arisen
possibly in accordance with their sluggish habits. This
view is strengthened by the fact that in a very few species

the larvae are dis-
tinctly segmented,
but lose this char-
acter in becoming
adult. As before
mentioned, the
greater number of
species live in bur-
rows in the sand
or crevices in the
rocks, from which
they reach out and
gather in large
quantities of sand.
As these substances
pass down the in-
testine the nutri-
tive matters are di-
gested and absorbed, while the indigestible matters are
voided to the exterior. When large numbers are associated
together they are doubtless important agents in modifying
the character of the sea bottom, thus acting like the earth-
worms and their relatives.

71. The sea-mats (Polyzoa), — The sea-mats or Polyzoa
constitute a very extensive group of animals common on
the rocks and plants along the seashore, and frequently
seen in similar situations in fresh-water streams. A few
lead lives as solitary individuals, but in the greater number
of species the original single animal branches many times,
giving rise to extensive colonies. In some species these
extend as low encrusting sheets over the objects on which
they rest; while in others the branches extend into the

m~

FIG. 42.— A gephyrean worm (Dendrostoma). Specimen
on left opened to show £, kidney, m, muscle bands,
and n.c., nerve-cord.

ANIMALS OF UNCERTAIN RELATIONSHIPS

69

surrounding medium and assume feathery shapes (Fig. 43),
which often bear so close a resemblance to certain plants

FIG. 43.-Lamp-ehells or Brachiopods (on left of figure), fossil and living, and (on
right) plant-like colonies of sea-mats.

that they are frequently preserved as such. What their
exact position is in the animal scale it is somewhat difficult
to say ; but judging especially from their development, it
appears probable that they are distant relatives of the seg-
mented worms.

70 ANIMAL FORMS

72. Lamp-shells or Brachiopods. — Occasionally one may
find cast on the beach or entangled in the fishermen's
lines or nets a curious bivalve animal similar to the form
shown in Fig. 43. These are the Brachiopods, or lamp-
shells. The remains of closely related forms are often
abundant as fossils in the rocks (Fig. 43). Over a thousand
species have been preserved in this way, and we know that
in ages past they flourished in almost incredible numbers
and were scattered widely over the earth. Unable to adapt
themselves to changing conditions or unable to cope with
their enemies, they have gradually become extinct, until
to-day scarcely more than one hundred species are known.
These are often of local distribution, and many are com-
paratively rare.

For a long period the Brachiopods, owing to their pecul-
iar shells, were classed together with the clams and other
bivalve mollusks. The presence of a mantle also strength-
ened the belief ; but closer examination during more recent
years has shown that the shells are dorsal and ventral, and
not arranged against the sides of the animal as in the
clams. Another peculiar structure consists of two great
spirally coiled " arms," which are comparable in a general
way to greatly expanded lips. The cilia on these create, in
the water currents which sweep into the mouth, the small
animals and plants that serve as food. The internal organ-
ization resembles in a broad way that of the animals con-
sidered in the previous section, and it now appears that
both trace their ancestry back to the early segmented
worms.

73. Band or nemertean worms. — In a few cases band or
nemertean worms have been discovered in damp soil or in
fresh-water streams. These are commonly small and incon-
spicuous, and are pigmies when compared with their marine
relatives, which sometimes reach a length of from fifty to
eighty feet. Many of the marine species (Fig. 44) are often
found on the seashore under rocks that have been exposed

ANIMALS OF UNCERTAIN RELATIONSHIPS

71

B

by the retreating tide. They are usually highly colored
with yellow, green, violet, or various shades of red, and are
so twisted into tangled
masses that the differ-
ent parts of the body
are indistinguishable.
As the animal crawls
about, a long thread-
like appendage, the pro-
boscis, is frequently shot
out from its sheath at
the forward end of the
body and appears to be
used as a blind man
uses his stick. At other

times, when Small WOrmS spine-tipped proboscis.

and other animals are

encountered, the proboscis is shot out farther and with
greater force, impaling the victim on a sharp terminal spine
(Fig. 44). The food is now borne to the mouth, located
near the base of the proboscis, is passed into the digestive
tract, traversing the entire length of the body, and is far-
ther operated on by systems of organs too complex to be
considered here.

FIG. 44.— A band or nemertean worm. A, entire
worm ; B, head, bearing numerous eyes and

CHAPTEE VIII

MOLLUSKS

74. General characters. — For very many years the mol-
lusks — that is, the clams, snails, cuttlefishes, and their allies
— have been favorite objects of study largely because of the
durability, grace, and coloration of the shell. The latter
may be univalve, consisting of one piece, as in the snails, or
bivalve, as in the clams and mussels, and may possess almost
every conceivable shape, and vary in size from a grain of
rice to those of the giant clam (Tridacna) of the East Indian
seas, which sometimes weighs five hundred pounds. These
external differences are but the expression of many internal
modifications, which, while adapting these animals for dif-
ferent modes of life, are yet not sufficient to disguise a
more fundamental resemblance which exists throughout
the group. In some respects the mollusks show a close
resemblance to the annelid worms, but, on the other hand,
the body is usually more thick-set and totally devoid of any
signs of segmentation. In every case the skin is soft and
slimy, demanding moist haunts and usually the protection
of a shell, and the body is modified along one surface to
form a foot or creeping disk which serves in locomotion.
The internal organization is somewhat uniform,, and will
admit of a general description later on. Mollusks are
divided into three classes, viz. : The Lamellibranchs, em-
bracing the clams ; the Gasteropods, or snails ; and the
Cephalopods, or cuttlefishes, squids, and related forms.

75. Lamellibranchs (clams and mussels). — Numerous rep-
resentatives of this class, such as the clams and mussels,

72

MOLLUSKS 73

occur along our seacoasts or are plentifully distributed in
the fresh-water streams and lakes. > They are distinguished
from other mollusks by a greatly compressed body, which
is enclosed in a shell consisting of two pieces or valves
locked together by a hinge along the dorsal surface. Eais-
ing one of these valves, the main part of the body may be
seen to occupy almost completely the upper (dorsal) part
of the shell (Fig. 45), and to be continued below into the
muscular hatchet-shaped foot (/£.), which aids the clam in
plowing its way through the sand or mud in which it lives.
Arising on each side of the back of the animal and extend-
ing its entire length is a great fold of skin, which com-
pletely lines the inner surface of the corresponding valve
of the shell. These are the two mantle lobes (m) instru-
mental in the formation of the shell, and enclosing between
them a space containing the foot and a number of other
important structures, the most conspicuous of which are
the gills (</), consisting of two broad, thin plates attached
along the sides of the animal and hanging freely into the
space (mantle cavity) between the mantle and the foot.
Owing to this lamella-like character of the branchia or gills
the class derives its name, lamellibranch. To illustrate the
relations of these various organs to one another the clam has
been compared to a book, in which the shells are repre-
sented by the cover, the fly-leaves by the mantle lobes, the
first two and last two pages by the gills, and the remaining
leaves by the foot. In the clams, however, the halves of
the mantle, like the halves of the shell, are curved, and
thus enclose a space, the mantle cavity, which is partly
filled by the gills and foot.

Unlike the other mollusks which usually lead active
and more aggressive lives, the clams show scarcely a sign of
a head and tentacles, and other sense organs are likewise
absent from this region. The mouth also lacks definite
organs of mastication, and as devices for catching and
holding food are not developed, the food is brought to the

74:

ANIMAL FORMS

mouth by means of the cilia on the great triangular lips or
palps which bound it on each side (Fig. 45, A,^?).

h

FIG. 45. — Anatomy of fresh-water clam. A, right valve of shell removed ; B, dissec
tion to show internal organs, a, external opening of kidney ; a.a., the anterior
muscle for closing the shell ; 5, opening of reproductive kidney ; c, brain ; ft.,
foot ; g, gill ; h, heart ; i, intestine ; k, kidney ; I, liver ; m, mantle (upper fig.),
mouth (lower fig.) ; p, palp (upper fig.), foot nerves (lower fig.) ; p.a., hinder
muscle for closing the shell ; s, space through which the water passes on
leaving the body , " t, stomach ; v, nerves supplying^viscera.

Between the halves of the shell in the hinge region is a
horny pad that acts like a spring, and without any muscu-
lar effort on the part of the clam keeps the shells open.

MOLL USES 75

These are also united by two great adductor muscles, located
at opposite ends of the animal (Fig. 45, A, «.«., p.a.), which
in times of disturbance contract and firmly close the shell.
Upon their relaxation the shell opens, the clam extends its
foot, and plows its way leisurely through the mud, or re-
mains buried, leaving only the hinder portion of its gaping
shell exposed. Through this opening a current of water
is continually passing in and out, owing to the action of
the cilia covering the gills, and by placing a little car-
mine or coloring matter in the ingoing stream we may
trace its course through the body. Passing in between the
mantle and the foot it travels on toward the head, giving
off small side streams which are continually made to enter
minute openings in the gills, whence they are conducted
through tubes in each gill up to a large canal at its base,
where it is carried backward to the exterior. In this pro-
cess oxygen gas is supplied to the number of blood-ves-
sels traversing the gills, and at the same time considerable
quantities of minute organisms and organic debris are
hurried forward toward the head, where they encounter the
whirlpools made by the cilia on the lips and are rapidly
whisked down into the mouth and swallowed.

75. Rock- and wood-boring clams. — Other similar forms
are rendered even more secure through their ability to
bore in solid rock. In the common Piddock, for example
(Fig. 46), the shell is beset with teeth like a rasp, which
gradually enlarge the cavity as the animal grows, until it
becomes a prisoner with no means of communication with
the exterior save the small opening through which the
siphons project. This is also the case with the Teredo,
frequently called the shipworm, which swims about for
some time during early life and then, about the size of a
small pinhead, settles down upon the timbers of wharves
or unsheathed ships, into which it rapidly tunnels.
Throughout life its excavation is extended sometimes to a
distance of two to three feet, and imprisoned yet safe at

76 ANIMAL FORMS

the bottom of its burrow, it extends its slender siphons up
the tube and out of the entrance for its food supply.
Often hundreds of individuals enter the same piece of
wood, which becomes thoroughly riddled within a short

FIG. 46.— The piddock (Zirphcea crispata), a rock-boring mollusk. Natural size,

from life.

time, and though giving no outward sign of weakness may
collapse with its own weight. Incalculable damage is thus
rendered to the shipping interests, and in consequence
much has been done to check their ravages, but they are
fai from being completely overcome.

76. Other stationary species. — A large number of other
species, while small and inconspicuous, are also free to

MOLLUSKS

77

move about, but as they become larger they lose this ability
either wholly or periodically. In the edible mussels (Myti-
lus, Fig. 47), for example, which are associated in great
numbers on the rocks along our coasts, the foot early be-
comes long and slender and capable of reaching out a con-
siderable distance from the shell to attach threads (byssus),
which it spins, to foreign objects. These are remarkably
strong, and when several have been spun it becomes a mat-
ter of much difficulty to dislodge them. After remaining
anchored in one situation for a while the mussel may vol-

Fio. 47.— The edible mussel (MytUus edulis), showing the threads by which it is
attached. Natural size, from life.

untarily free itself, and in a labored fashion move to some
other more favorable spot where it again becomes attached,
but there are numerous species, such as " fan shells "
(Pinna), scallops, Anomia, and a few fresh-water forms,
where the union is permanent.

Finally, in the oysters, some of the scallops, and a num-
ber of less familiar forms, the young in very early life drop
down upon some foreign object to which the shell soon
becomes firmly attached, and in this same spot they pass
the remainder of their lives. The oyster usually falls upon
the left half of its shell, which becomes deep and capacious
enough to contain the body, while the smaller right valve

78 ANIMAL FORMS.

acts as a lid. As locomotion is out of the question, the foot
never develops, and the shell is held by only one adductor
muscle, whose point of attachment in the oyster is indicated
by a brown scar in the interior of the shell.

77. Internal organization. — It is thus seen that the ex-
ternal features of the clam are variously modified, according
to the life of the animal, but the internal organization is
much more uniform. In nearly every species the food con-
sists of floating organisms, which are driven by the palps
into the mouth and on to the simple stomach, where it is
subjected to the solvent action of the fluids from the liver
(Fig. 45, B, I) before entering the intestine. This latter
structure is usually of considerable length, and in the active
species extends down into the foot, and it is also peculiar in
passing through the ventricle of the heart. Traversing
the intestine the nutritive portion of the food is absorbed,
and is conveyed over the body by a circulatory system more
highly developed than in the higher worms. On the dorsal
side of the clam, in a spacious pericardial chamber, the
large heart is situated (Fig. 45, /*), consisting of a median
highly muscular ventricle surrounding the intestine and of
two thin auricles, one on either side. From the former,
two arteries with their numerous branches convey the blood
to all parts of the body, where it accumulates, not in capil-
laries and veins, but in spaces or sinuses among the mus-
cles and various organs, constituting a somewhat indefinite
system of channels which lead to the gills and kidneys.
In these organs the blood delivers up the waste which it
has accumulated on its journey, and absorbing a supply of
oxygen, it flows into the great auricles, which in turn pass
it into the ventricle to circulate once more throughout
the body.

The excretory apparatus, consisting usually of two kid-
neys, of which one may degenerate in many snails, bears a
close resemblance to that of the annelids. In the clam, for
instance, each consists of a bent tube symmetrically ar-

MOLLUSKS 79

ranged on each side of the body (Fig. 45, B, &), and the inner
ends (a), corresponding to the ciliated funnel of the anne-
lid kidney, open into the pericardial cavity. The walls
are continually active in extracting wastes from the blood
supplied to them, and these, together with the substances
swept out from the pericardial cavity, traverse the tube and
are carried to the exterior. In other mollusks the kidney
may be more compact, or greatly elongated, or otherwise
peculiar, but in reality they bear a close resemblance to
those of the clam.

78. Nervous system.— The nervous system, like the ex-
cretory, differs considerably in different mollusks, yet the
resemblances are fairly close throughout. In the clam the
cerebral ganglia corresponding to the " brain " in annelids
is located at either side, or above the mouth, and from it
several nerves arise, the larger passing downward to two
pedal ganglia (p) embedded in the foot and to the visceral
ganglia (v) far back in the body (Fig. 45, B). These nerve
centers continually send out impulses which regulate the
various activities of the body and also receive impressions
from without. These come chiefly through the sense of
touch, for in the clams the other senses are usually either
feebly developed or altogether absent.

79. Development. — In the mollusca new individuals al-
ways arise from eggs, which are commonly deposited in the
water and there undergo development. In the fresh-water
clams the reproductive organ is usually situated in the foot
(Fig. 45), while in the oyster and similar inactive species it is
attached to the large adductor muscle. In these latter, and
in many other marine forms, the eggs are shed directly into
the sea, where they are left to undergo their development
buffeted by winds and waves and subject to the attack of
numerous enemies. Under such circumstances the chances
of survival are slight, and for this reason eggs are laid in
vast numbers, which have been variously estimated for the
oyster, for example, from two to forty million. Develop-

28

80 ANIMAL FORMS

ment proceeds at first much as in the sponge, but soon the
shell, foot, gills, and various other molluscan structures
put in an appearance, and the few surviving young which
have been free-swimming now settle down in some favor-
able spot, and attach themselves or burrow according to
their habit.

80. Life history of fresh-water clams. — The life history of
our common fresh-water clams is perhaps one of the most
remarkable known among mollusks. The parent stores the
eggs, as soon as they are laid, in the outer gill plate, and
there, well protected, they undergo the first stages of their
development, which results in the formation of minute
young enclosed in a bivalve shell beset with teeth. These
are often readily obtained, sometimes as they are escaping
from the parent, and when examined under the microscope
are seen to rapidly open and close their shells in a snapping
fashion when in the least disturbed. In a state of nature
this latter movement may result in attaching the young to
the fins or gills of some passing fish, which is necessary to
its further development. Within a short time it becomes
completely embedded in the flesh of its host, from which,
as a parasite, it draws its nourishment, and during the
next few weeks undergoes a wonderful series of transforma-
tions resulting in a small mussel, which breaks its way
through the thin skin of the fish and drops to the bottom.

81. The gasteropods. — The gasteropods, including snails,
slugs, limpets, and a host of related forms, fully twenty
thousand different species in all, are found in most of our
fresh-water streams and lakes and in moist situations on
land, while great numbers live along the seashore and at
various depths in the ocean, even down as far as three
miles. Examining any of them carefully we find many of
the same organs as in the clams, but curiously changed and
adapted for a very different and usually active life. In our
common land snails (Fig. 48), which we may well examine
before passing on to a general survey of the group, the first

MOLLUSKS

81

striking peculiarity is in the univalve shell, with numerous
whorls, into which the animal may at any time withdraw
completely. Ordinarily this is carried on the back of the
spindle-shaped body, which is fashioned beneath into a great

FIG. 48.— The slug (Ariolimax) and common snail (Helix). From life.

flat sole or creeping surface that bears on its forward bor-
der a wide opening through which mucus is continually
issuing to enable the snail to slip along more readily. Slime
also exudes on other points on the surface of the body and
affords a valuable protection against excessive heat and
drought.

Unlike the clams, the forward end of the body is devel-
oped into a well-marked head bearing the mouth and a
complicated mechanism for gathering and masticating food,
together with two pairs of tentacles, one of which carries the
eyes. On the right side of the animal, some distance behind
the head, is the opening of the little sac-like mantle cavity
(Fig. 48) which contains the respiratory organs, and into
which the alimentary canal and the kidneys pour their
wastes. The relation of these organs to the mantle cavity
is the same as in the clams, though the cavities differ much
in size and position.

82. Other snails. The shell. — Extending our acquaint-
ance to other species of snails, we find the same general
plan of body, although somewhat obscured at times by

ANIMAL FORMS

FIG. 49.— The chiton, armadillo-snail or sea-era -
dle. The left-hand figure shows mouth in

many modifications. A foot is generally present, also a
more or less well-developed head, and the body is usually
surrounded by a shell which varies widely in shape and
size in different species. In the common limpets the early
coiled shell is transformed into an uncoiled cap-like one,
and in the keyhole limpets is perforated at its summit. The

chitons or armadillo-
snails (Fig. 49), often
found associated with
the limpets, carry a
most peculiar shell con-
sisting of eight plates,
which enables the ani-
mal to roll up like an
armadillo when dis-
turbed. A shell is by

center of proboscis, the broad foot on each n° means a necessity,

side of which are numerous small gills. The however, f Or in many
right-hand figure shows the mantle and shell, • h fh

composed of eight plates. From life, one- bPe( es>

half natural size. beautiful naked snails

or Nudibranchs (Fig.

50) common along our coasts, it may be entirely absent,
or, as in the ordinary slugs, reduced to a small scale em-
bedded in the skin.

83. Respiration, — A considerable quantity of oxygen is
absorbed through the skin, as in all mollusks, but the chief
part of the process is usually taken by the plume-like gills,
one or two in number, which are located in the mantle
cavity. In the chitons (Fig. 49) the number of gills is
greater, amounting in some species to over a hundred,
while in the Nudibranchs (Fig. 50) gills are absent, their
places being taken by more or less feathery expansions of
the skin on the dorsal surface.

Many of the gasteropods left exposed on the rocks by a
retreating tide retain water in the mantle cavity, from
which they extract the oxygen until submerged again.

MOLLUSKS 83

Others breathe by means of gills while under water, and by
the surface of the' body and the moist walls of the mantle

FIG. 50.— Three different species of naked marine snails or Nudibranchs. Natural
size, from life.

cavity when exposed. In some of the small Littorinas
attached so far from the sea as to be only occasionally
washed by the surf this latter method may prevail for days
together — in fact they live better out of water than in it.
It is not difficult to imagine that such forms, keeping in
moist places, might wander far from the sea, and, losing
their gills, become adapted to a terrestrial life. It is
believed that in past times this has actually occurred, and
that our land forms trace their descent from aquatic ances-
tors. To-day they breathe by a lung — that is, they take
oxygen through the walls of the mantle cavity, as the slug
may be seen to do, though in some species traces of the old
gill yet remain.

84. Food and digestive system. — Many mollusks live upon
seaweeds, and the greater number of terrestrial forms are
fond of garden vegetables or certain kinds of lichens, but,
on the other hand, the latter, together with a large number
of marine snails, are carnivorous. In all cases the food
requires to be masticated, and, unlike the clams, the mouth
is usually provided with horny jaws, and an additional

ANIMAL FORMS

FIG. 51.— A small portion of the radula or
tongue-rasp of a snail (Sycotypus).

masticatory apparatus which consists of a kind of tongue
with eight to forty thousand minute teeth in our land
forms (Fig. 51), while in certain marine snails they are
beyond computation. With the licking motion of the
tongue this rasp tears the food into shreds before it is
swallowed, and in the whelks or borers it serves to wear a
circular hole through the shells of other mollusks, which

are thus killed and devoured.
This latter process is facili-
tated by the secretion of the
salivary glands, which has a
softening effect upon the
shell. Ordinarily the saliva

of snails exercises some di-
gestive action.

In the stomach of some
snails are teeth or horny
ridges which also are instrumental in crushing the food,
and in numerous minor respects peculiarities exist in differ-
ent species according to the nature of the food ; but in its
general features the digestive tract is similar to that of
the clams.

The processes of circulation and excretion are also car-
ried on by means of systems which show a certain resem-
blance to those of the clams. As might be expected, certain
differences exist, sometimes very great, but they are of too
technical a nature to concern us further.

85. Sense-organs of lamellibranchs and gasteropoda.—
The eyes of mollusks differ widely in their structure and
the position they occupy in the body. In our common
land snails two pairs of tentacles are borne on the head,
the lower acting as feelers, while each of the upper ones
bears on its extremity the eye, appearing as a minute black
dot (Fig. 48). In this same position the eyes of many
marine snails occur, but there are numerous species in
which there are other accessory eyes. In many of the

MOLLUSKS 85

limpets, for instance, there are numbers of additional eyes
carried on the mantle edge just under the eaves of the
shell, and forming a row completely encircling the body.
(In the scallops there are two rows of brilliantly colored
eyes, set like jewels on the edges of the mantle just within
the halves of the shell.) In the chitons the eyes of the
head disappear by the time the animal attains maturity,
and in some species at least their place appears to be taken
by great numbers of eyes, sometimes thousands, which are
embedded in the shells. On the other hand, eyes are com-
pletely absent in certain species of burrowing snails and in
several living in the gloomy depths of the sea far from the
surface ; they appear to be absent also from fresh-water
clams ; but the fact that certain species close their shell
when a shadow falls upon them, leads to the belief that
while actual eyes are not present the skin is extremely
sensitive to light. This is also the case with many snails.

86. Smell. — Since the sense of sight is generally unde-
veloped in the mollusks, they rely chiefly upon touch and
smell for recognizing the presence of enemies and food.
Tentacles upon the head and other parts of the body, and
a skin abundantly supplied with nerves, show them to pos-
sess a high degree of sensibility ; but in the greater num-
ber of species the sense of smell is of chief importance.
Many experiments show that tainted meat and strongly
scented vegetables concealed from sight and several feet
distant from many of our land and sea mollusks will attract
them at once. In these forms the sense of smell appears to
be located on the tentacles, but additional organs, possibly
of smell, are located on various portions of the body, usu-
ally in the neighborhood of the gills.

87. Taste and hearing. — Several mollusks appear to be
almost omnivorous, but others are decidedly particular in
their choice of food, which leads us to suspect that they
possess to some extent the sense of taste. Nerves supply-
ing the base of the mouth have also been detected, which

86 ANIMAL FORMS

may be those of taste ; but experiments along the line are
difficult to perform, and our knowledge of this subject is
far from complete. The same is true of hearing. Certain
organs, interpreted as ears and located in the foot, have
the form of two hollow sacs, containing one or more solid
particles of sand or lime, whose jarrings, when effected by
sonorous bodies, may result in hearing. On the other hand,
it is held by some that they, like the semicircular canals of
higher animals, may regulate the muscular movements
which enable the animal to keep its balance.

88. Egg-laying habits and development.— The egg-laying
habits of the gasteropods differ almost as widely as their
haunts. The terrestrial forms lay comparatively few eggs,
ranging in size from small shot to a pigeon's egg in some
of the tropical species. These are buried in hollows in the
ground or under sticks and stones, and after a few weeks
hatch out young snails having the form of the adult. The
same is also true of most of the fresh-water snails, which
lay relatively smaller eggs embedded in a gelatinous mass
frequently found attached to sticks and leaves, or on the
walls of aquaria in which they are confined. Many marine
species construct capsules of the most varied patterns
which they attach to different objects, and in these the
young are protected until they hatch. In the limpets and
many of the chitons the eggs are laid by thousands directly
in the water, and after a short time develop into free-swim-
ming young, differing considerably from the parent in ap-
pearance. Those escaping the ravages of numerous enemies
finally settle down in a favorable situation and gradually
assume the form of the adult.

89. Age, enemies, and means of defense of lamellibranchs
and gasteropods. — How much time is consumed by the young
in growing up, and the length of time they live, are ques-
tions generally unsettled. It is said that the oyster requires
five years to attain maturity, and lives ten years ; the fresh-
water clam develops in five years, and some species live from

MOLLUSKS 87

twelve to thirty years ; and the average length of life of
the snail appears to be from two to five years. Certain it
is that mollusks have numerous enemies besides man which
prevent multitudes from living lives of normal length.
Birds, fishes, frogs, .and starfishes beset them continually,
and many fall a prey to the ravages of internal parasites or
to other mollusks. Under ordinary circumstances the shell
is sufficient protection, and the spines disposed on the sur-
face in many species render the occupant still less liable to
attack. Many snails carry on the foot a horny or calcare-
ous plate known as the operculum, which closes the en-
trance of the shell like a door against intruders. Certain
noxious secretions poured out from the skin also serve as a
means of defense, and many Nudibranchs (Fig. 50) bear
nettle- cells on the processes of the body, which probably
render them distasteful to many animals. Finally, there
are numerous clams, mussels, snails, and slugs whose colors
harmonize so closely with their surroundings that they al-
most completely baffle detection, and enable them to lead
as successful a life as those provided with special organs of
defense.

90. Cephalopods. — The animals belonging to this class,
such as the squids and cuttlefishes (Fig. 52), are by far
the most highly developed mollusks. They are of great
strength, capable of very rapid movements, and several spe-
cies are many times the largest invertebrates. In almost
every case there is a well-defined head bearing remarkably
perfect eyes, and also a circle of powerful arms provided
with numerous suckers which aid in the capture of food
(Fig. 52). Posteriorly the body is developed into a pointed
or rounded visceral mass which to a certain extent is free
from the head, giving rise to a well-marked neck. Some
forms, such as the squids (Fig. 52, upper figure), are pro-
vided with fins which drive the animal forward, but in com-
mon with other cephalopods they are capable of a very rapid
backward motion. By muscular movements water is taken

88

ANIMAL FORMS

into the large mantle cavity within the body, a set of valves
prevents its exit through the same channels, and upon a
Vigorous contraction of the body walls the water is forced
Out rapidly through the small opening of the funnel, which

FIG. 52.— Cephalopoda. Lower figure, the devil-fish or octopus ( Octopus punctatus).
The upper figure represents the squid (Loligo pealii) swimming backward by
driving a stream of water through the small tube slightly beneath the eyes. Prom
life, one-third natural size.

drives the animal backward after the fashion of an explod-
ing sky-rocket. In this way they usually escape the fishes
and whales that prey upon them, but an additional device
has been provided in the form of a sac within the body,
whose inky contents may be liberated in such quantity as
to cloud the water for a considerable distance, and thus
enable them to slip away unseen into some place of safety.
Most of the cephalopods are further protected by their
ability to assume, like the chameleon, the color of the object

MOLLUSKS 89

upon which they rest. In the skin are embedded multi-
tudes of small spherical sacs filled with pigments of various
colors, chiefly shades of red, brown, and blue, each sac be-
ing connected with a nerve and a series of delicate muscles.
If the animal settles upon a red surface, for example, a
nerve impulse is sent to each of the hundreds of color sacs
of corresponding shade, causing the muscles to contract
and flatten the bag like a coin, and thus exposing a far
greater surface than before, they give the animal a reddish
hue. In the twinkling of an eye they may completely
change to another tint, or present a mottled look, and some
may even throw the surface of the skin into numerous
small projections that make the animal appear part of the
rock upon which it rests. These devices not only serve for
protection, but they also aid in enabling these mollusks to
steal upon their prey, chiefly fishes, which they destroy in
great numbers with lionlike ferocity.

The devil-fishes and a number of other species are usu-
ally found creeping along the sea bottom, generally near
shore, and are solitary in their habits, while the squids re-
main near the surface and frequently travel in great com-
panies, sometimes numbering hundreds of thousands. In
size they usually range from a few inches to a foot or two
in length, but a few devil-fishes and squids attain a greater
size, some of the latter reaching the enormous length of
from forty to sixty feet. There are many stories of their
great strength and of their voluntarily attacking people
and even overturning boats, but the latter are in almost
every case sailors' yarns.

In their external organization the cephalopods have
little to remind one of any of the preceding mollusks, and
their internal structure shows only a distant resemblance.
In the Octopi (Fig. 52) the shell is lacking ; in the squid it
is called the pen, and consists of a horn-like substance with-
out any lime deposit ; in the cuttlefishes it is spongy and
plate-like, and is a familiar object in the shops ; and, finally,

90 ANIMAL FORMS

in the nautilus it is coiled and of considerable size, and, un-
like that of any other cephalopod, it is carried on the out-
side of the animal. Interiorly it is divided by a number of
partitions into chambers, the last one of which is occupied
by the animal.

The alimentary canal shows some resemblance to that
of other mollusks, but, as in the case of the other systems
of the body, it possesses a far higher state of development.
The mouth is situated in the center of a circle of arms,
which in reality are modified portions of the foot, and is
furnished with two parrot-like jaws. From this point the
esophagus leads back into the body mass to the stomach,
which with the liver and intestine are sufficiently like
those of the clam and snail to require no further comment.

Eespiration is effected by the skin to a certain extent,
but chiefly by two gills (four in the nautilus), and the cir-
culatory system, which conveys the blood to and from these
organs and over the body with its complex heart, arteries,
capillaries, and veins, is more highly developed than in
any other invertebrate.

As might be expected in animals with so great sagacity
and cunning, the nervous system and the sense-organs reach
a degree of development but little short of what we find in
some of the vertebrates. The chief part of the nervous
system is located in the head, protected by a cartilaginous
skull, a very rare structure among invertebrates ; and while
the different ganglia may be recognized in a general way
and be found to correspond to a certain extent to those
of foregoing mollusks, they are so largely developed and
massed together that it is impossible at present to under-
stand them fully. From this point nerves pass to all
regions of the body, to the powerful muscles, the viscera,
and the organs of special sense, controlling the complex
mechanism in all its workings.

There is no doubt that the cephalopods see distinctly
for considerable distances, and a careful examination of

MOLLUSKS 91

the eye of the squids and cuttlefishes has shown them
to be remarkably complex and in many respects to be
constructed upon much the same plan as those of the
vertebrates. As to the other senses not so much is known,
but undoubtedly many species of cephalopods are possessed
of a shrewdness and cunning not shared by any other
invertebrates, save some of the insects and spiders, and are
vastly more highly organized than their molluscan rela-
tives.

91. How species originate.— We have now examined a
considerable portion of the animal kingdom, tracing its
members from their simplest beginnings as single cells,
through the formation of colonial types, and up through
the sponges, coelenterates, worms, and mollusks. It is im-
portant once more to note that they all perform the func-
tions concerned in nutrition and reproduction, and only
these. The differences which exist are those of structure.
The Hydra and the clam, for example, perform the same
duties, but their bodily apparatus differs widely, and the
completeness and perfection of the work varies accord-
ingly. The more the work to be performed by an organ-
ism is divided up among especially adapted organs, so that
each of the latter has, as far as possible, only one thing to
do, the higher is the organism.

As stated earlier in the account, it is believed that ihe
more complex animals arose from the simpler ; that if we
could trace the history of any of the great groups back
toward their first beginnings, we would find them all to
have originated from one ancestral form, that in turn owes
its descent from yet simpler forms.

Let us see something of how this has come about. We
all know that vast numbers of young are born into this
world which never come to maturity. It is said that if all
the young of the codfish were to live their allotted time,
it would be less than fifteen years before the sea would
become literally packed with them. Numerous enemies,

92 ANIMAL FORMS

the lack of food, and other agencies annihilate the larger
part. We also know that no two offspring are exactly
alike. They exhibit individual differences. One bird may
have a larger bill than another of the same brood which
excels in length of wing. As noted above, all the offspring
will not attain maturity. Those best adapted to their sur-
roundings will have the best chances of survival. The
increased length of bill or wing may be slight, but it may
be just this amount which enables the bird to probe deeper
or fly farther and thus secure the requisite amount of food.
A premium is placed on length of wing or bill generation
after generation, with the result that a long-billed species
arises distinct from the long-winged which trace their
ancestry back to the same parents. It is the same prin-
ciple which enables the breeder to increase the swiftness
of the race-horse and the strength of the draft-horse, or
the gardener to develop from the wild rose the great num-
ber of widely different varieties. In the same way other
slight peculiarities over very many generations may en-
able other forms to gradually adapt themselves to still dif-
ferent modes of life. Thus vast numbers of organisms
gradually become modified in form and complexity, and
are adapted to lives which insure them a comparative
degree of safety and less competition with other species.

The above account serves solely for purposes of illustra-
tion. No kind of bird has originated in just that way, but
as the essential force in all change of form we have the
necessity of adaptation of the individual to its surround-
ings, the death of those who can not be adapted, and the
inheritance of the advantage of the parent by its progeny,
enabling these in turn to survive and to multiply their
own kind.

CHAPTER IX

ARTHROPODS. CLASS CRUSTACEA

92. General characters. — In the Arthropods, that is, the
crabs, lobsters, shrimps, insects, spiders, and a vast host
of related forms, the body is bilaterally symmetrical, and
is composed of a number of segments arranged in a series,
as in the earthworm and other annelids. A hornlike cu-
ticle, sometimes called the shell, bounds the external sur-
face— in early life thin and delicate, but later relatively
thick, and often further strengthened by lime salts. Along
the line between the segments this coat of mail remains
thin and forms a flexible joint. Appendages also are borne
on each segment, not comparatively short and fleshy out-
growths like the lateral appendages of many of the worms,
but usually long and jointed (hence the name Arthropod,
meaning jointed foot), and variously modified for many
different uses.

93. Classification. — The species belonging to this group
outnumber the remainder of the animal kingdom. Their
haunts also are most diverse. Some are adapted for lives
in the sea and fresh water, others for widely different sit-
uations on land, and a great number are constructed for a
life on the wing. A certain resemblance exists among them
all, but the modifications which fit them for their different
habitats are also profound, and have resulted in the division
of the Arthropods into five classes. The first class ( Crus-
tacea) contains the crayfish, crabs, etc. ; the second ( Ony-
chophora) includes the curious worm-like peripatus (Fig.

I

94 ANIMAL FORMS

66) ; the third (Myriapoda, meaning myriad-footed) em-
braces the centipeds and " thousand-legs " ; the fourth (In-
secta) contains the insects ; and the fifth (Arachnida) in-
cludes the" scorpions, spiders, and mites.

94. The Crustacea. — The number of species of crusta-
ceans is estimated to be about ten thousand, and while the
greater number of these are marine, many are found in
fresh water and a few occur on land. In size they range
from almost microscopic forms to the giant crabs and
lobsters. They differ also in shape to a remarkable degree,
but at the same time there is a decided resemblance through-
out the group, except in those species which have become
modified by a parasitic habit. The characteristic external
skeleton is invariably present, and gives evidence of the
deep internal segmentation of the body. In the simple
Crustacea this is very apparent, but in the higher forms it
is usually more or less obscured, owing to the fusion of some
of the different segments, especially those of the head, as in
the crayfish (Fig. 59).

The class of the Crustacea is subdivided into two sub-
classes (Entomostraca and Malacostraca), the first containing
the fairy-shrimps (Brancliipus, Fig. 53) and their allies, the
copepods (such as Fig. 54), the barnacles (Fig. 55), and a
number of other species. In their organization all are com-
paratively simple, usually small, and the appendages show
relatively little specialization. The other subclass contains
the more highly developed and usually large-sized Crustacea,
among which are the shrimps, crayfishes, lobsters, crabs,
and a number of other forms.

95. Some simple Crustacea. — While the members of the
first subclass are minute and inconspicuous, several species
are often remarkably abundant in our small fresh-water
pools. Among these is the beautifully colored fairy-shrimp
(BrancMpus, Fig. 53), with greatly elongated body and
leaf-like appendages, whose relatively simple character leads
the zoologist to think that they are among the simplest

ARTHROPODS. CLASS CRUSTACEA 95

Crustacea, and in several points resemble the ancestral form
from which all the modern species have descended. Some
nearly related forms are provided with a great fold of the
body-wall, which may almost completely conceal the animal
from above, or it may be formed like a bivalve clam-shell,
within which the entire body may be withdrawn. This

ft

FIG. 53.— Fairy-shrimp (Branchipus). b, brood-pouch ; e, e*,
compound and simple eyes ; f, paddle-shaped feet ; h, tu-
bular heart ; i, intestine.

latter character is also found in the water-fleas (Daplinia),
very much smaller forms, and sometimes occurring in mil-
lions on the bottoms of our ponds and marshes. They are
readily distinguished from the fairy-shrimp by the short-
ness of the body, the small number of appendages, and by
their habit of using their antennae as swimming organs,
which gives to their locomotion a jerky, awkward character.
96. Cyclops and relatives. — Cyclops (Fig. 54), the repre-
sentative of a number of lowly forms belonging to the order
of Copepods, is one of the commonest fresh-water Crustacea.
The forward segments of the spindle-shaped body are cov-
ered by a large shield or carapace, the feet are few in num-
ber, and, like its fabled namesake, it bears an eye in the
center of the forehead. Nearly related species are also re-
markably abundant at the surface of the sea, at times occur-
29

96

ANIMAL FORMS

ring in such vast numbers that they impart a reddish tinge
to the water over wide areas, and at night are largely re-
sponsible for its phos-
phorescence. Many oth-
ers are parasitic in their
habits, and scarcely a
salt-water fish exists but
that at one time or an-
other suffers from their
attacks. On the other
hand, many fresh- and
salt-water fishes depend
upon the free-swimming
forms for food, and
hence, from an economic
point of view, they are
highly important organ-
isms.

97. Barnacles. — The
parasitic habit and the
lack of locomotion has
also produced marvelous
changes among the bar-
nacles, so great that
originally they were
placed among the mol-
lusks ; and as with the parasitic copepods, their true posi-
tion was only known after their life-history had been de-
termined. In the goose-barnacles * the body, attached by
a fleshy stalk to foreign objects, is enclosed by a tough
membrane, corresponding to the carapace of other Crus-
tacea, in which are embedded five calcareous plates. This

FIG. 54.— Cyclops, e. s., eggs ; i, intestine
reproductive organ.

ov,

* So called because of the belief, which existed for three hundred
years prior to the present century, that when mature these animals
give birth to geese.

ARTHROPODS. CLASS CRUSTACEA

97

is open along one side, and allows the feather-like feet to
project and produce currents in the surrounding water
which brings food within reach. In the acorn-barnacles
(Fig. 55) the stalk is absent, and the body, though possess-

FIG. 55.— Barnacles. Acorn-barnacles chiefly in lower part of figure ; goose-barnacles
above. Natural size.

ing the same general character as the goose-barnacles, is
shorter, and enclosed in a strong palisade consisting of six
calcareous plates.

The larger number of barnacles attach themselves to
the supports of wharves, the hulls of ships, floating tim-
bers, the rocks from the shore-line down to considerable
depth, and a few species occur on the skin of sharks and
whales. On the other hand, there are several species which
are parasitic, and in accordance with this mode of life ex-
hibit various degrees of degeneration. In the most extreme

98 ANIMAL FORMS

cases (Sacculind) the sac-like body, attached to the abdo-
men of crabs, is entirely devoid of appendages and any
signs of segmentation. A root-like system of delicate fila-
ments extends from the exposed part of the animal into
the host and absorbs the necessary nutriment. The mouth
and alimentary canal are accordingly absent — in fact, the
body contains little but the reproductive organs and a very
simple nervous system.

98. Structure. — In the internal organization of these
smaller crustaceans many differences may be noted, though
they are usually less profound than the external. Ordi-
narily the alimentary canal is a straight tube passing
through the body, and is provided with a pouch-like
stomach, and a more or less clearly defined liver. In
all, except the parasitic species, the external mouth-ap-
pendages masticate the food, and in a very few of the
above-described groups it may be further ground between
the horny ridges on the stomach-walls. After this pre-
liminary treatment it is subjected to the action of the
digestive juices, and when liquefied is absorbed into the
body. Here it is circulated by a blood-system of widely
different character. In many cases definite arteries and
veins are absent. The blood courses through the body in
the spaces between the different organs propelled by the
beating of the heart, which it is made to traverse. In
Cyclops (Fig. 54) even the heart is absent, and the blood
is made to circulate by contractions of the intestine. In
most of these smaller Crustacea considerable oxygen is ab-
sorbed through the body-wall ; but in several species, for
example, the fairy-shrimp (Fig. 53), special gills are devel-
oped on the appendages of the body.

99. Multiplication. — Among the Crustacea thus far con-
sidered the males are usually readily recognized owing to
their small size. The females also are usually provided
with brood-pouches in which the developing eggs are pro-
tected. In almost every case the young are born in the

ARTHROPODS. CLASS CRUSTACEA

99

form of minute larvae, provided with three pairs of append-
ages, a median eye (Fig. 56), and a firm external skeleton
or cuticle. This latter prevents the continuous growth of
the larvae or nauplius, and every few days it is thrown off,
and while the new one is forming the body enlarges. Dur-
ing this time new appendages are developed, so that after
each moult the young crusta-
cean emerges less like its
former self and more and more
like its parents. In the bar-
nacles, after several moults
have taken place, the young
become permanently attached
by means of their first anten-
nae, their thoracic feet change
into feathery appendages, and
several other changes occur.
In some of the parasitic bar-
nacles (Sacculina) the larva
attaches itself to a crab, throws
off its various appendages, and,
after other great degenerative
changes, enters its host. For
a time, therefore, their development is toward greater com-
plexity, but the later stages constitute a retrograde meta-
morphosis.

100. More complex types. — The larger, more useful, and
usually more familiar Crustacea belong to the second divi-
sion (subclass Malacostraca). It comprises such animals as
the shrimps, crayfish, lobsters, crabs, and a number of other
forms which are at once distinguished from the preceding
by the constant number of segments composing the body.
Of these, five constitute the head, eight the thorax, and
seven the abdomen. The head segments are always fused
together, and with them one or more thoracic segments
unite to form a more or less complete cephalothorax. Also,

FIG. 56.— Development of a barnacle
(Lepas). a, larva ; b, adult.

100 ANIMAL FORMS

some of the head segments give rise to a great fold of the
body-wall, the carapace, which extends backward and covers
all or a part of the thorax, with which it may firmly unite,
as in the crayfish. The appendages are usually highly spe-
cialized, and are made to perform a variety of functions.

101. The shrimps, — Among the simplest of these are the
opossum-shrimps (Fig. 57) and their relatives, small trans-

FIG. 57.— The opossum-shrimp (Mysis americana).

parent creatures often seen swimming in great numbers at
the surface of the sea or hiding among the seaweeds along
the shore. In general appearance they resemble crayfishes
or prawns, but are readily distinguished by the two-branched
thoracic feet. This " split-foot " character also occurs
among many of the preceding Crustacea, and is generally
a badge of low organization, tending to disappear in the
more highly organized forms. In this and other respects
the shrimps are especially interesting in their relation to
the preceding Crustacea, and in the fact that they may
closely resemble the ancestors of the modern prawns (Fig.
58), lobsters, crayfishes, and crabs.

102. Crayfishes and lobsters. — The last-mentioned spe-
cies and their allies, usually large and familiar forms, con-
stitute a group known as the decapods (meaning ten feet),
referring to the number of thoracic feet. Among the mem-
bers of this division probably none are more familiar than
the crayfishes, which occur in most of the larger rivers and
their tributaries throughout the United States and Europe.
It is their habit to remain concealed in crevices of rocks

ARTHROPODS. CLASS CRUSTACEA 101

or in the mouths of the burrows which they excavate, and
from which they rush upon the small fish, the larvae of

FIG. 58.— Prawn (Heplacarpus brevirostris).

many animals, and other equally defenseless creatures
which constitute their bill of fare. In turn they are
eagerly sought by certain birds and four-footed animals, and.
especially in France,
are extensively used for
food by man.

Closely related to
the crayfishes and dif-
fering but little from
them structurally are
the lobsters. In this
country they are con-
fined to the rocky coasts
from Xew Jersey to
Labrador, living upon
fish, fresh or otherwise,
various invertebrates,
and occasionally sea-
weeds. Far more than
the crayfish, the lobster
is in demand as an arti-
cle of food. By the aid

Of nets Or Various traps FIG. 59,-Tlie crayfish Ustacwl

102 ANIMAL FORMS

millions are caught each year, and to such an extent has
their destruction proceeded that in many places they are
well-nigh exterminated. At the present time, however, leg-
islation, numerous hatcheries, and a careful study of their
life habits is doing much to better matters and inciden-
tally to put us in possession of many interesting zoological
facts along this line, some of which will be mentioned later.
Frequently the prawns, especially the larger ones, and a
spiny lobster (Palinurus), are mistaken for crayfishes or
lobsters, but they differ from them in the absence of the
large grasping claws.

Along almost any coast some of these animals are to be
found, often beautifully colored and harmonizing with the
seaweeds among which they live, or so transparent that
their internal organization may be distinctly seen. Farther
out at sea other species swim in incredible numbers, feed-
ing upon minute organisms, and in turn fed upon by numer-
ous fishes and whales ; and, especially on the Pacific coast,
shrimp-fishing is an important industry.

103. The hermit-crabs.— The last of these long-tailed
decapods is the interesting group of the hermit-crabs,
which occur in various situations in the sea. In early life
they take possession of the empty shell of some snail, and
the protected abdomen becomes soft and flabby, while the
appendages in this region almost completely disappear.
The front part of the body, on the other hand, continually
grows in firmness and strength, and is admirably adapted
for the continual warfare which these forms wage among
themselves. As growth proceeds the necessity arises for a
larger shell, and the crab goes "house-hunting" among the
empty shells along the shore, or it may forcibly extract the
snail or other hermit from the home which strikes its fancy.

Many of the hermit-crabs enjoy immunity from the
attacks of their belligerent relatives by allowing various
hydroids to grow upon their homes. Others attach sea-
anemones to their shells or to one of their large claws,

ARTHROPODS. CLASS CRUSTACEA 103

which they poke into the face of any intruder. While
the anemones or hydroids are made to do valiant service

FIG. 60. — Hermit-crab (Pagurus bernhardus) in snail shell covered with Hydractinia.

with their nettle-cells, they also enjoy the advantages of
a large food-supply which is attendant upon the free ride.

104. The crabs,— The most highly developed Crustacea
are the crabs or short-tailed decapods which abound between
tide-marks alongshore, and in diminishing numbers extend
to great depths. The cephalothorax is usually relatively
wide, often wider than long, and the greatly reduced abdo-
men is folded against the under side of the thorax. Corre-
lated with the small size of the abdomen, the appendages
of that region disappear more or less, but the remaining
appendages are similar to those of the crayfish or lobsters.
All these different parts, however, are variously modified in
each species to fit it for its own peculiar mode of life. In
some forms, such as the common cancer-crab (Fig. 61), the
legs are comparatively thick-set and possessed of great
strength, enabling them to defend themselves against most
enemies. On the other hand, there are the spider-crabs
with small bodies and relatively long legs, withal weak, and

104

ANIMAL FORMS

yet so harmonizing with their surroundings that they are
as likely to survive as their stronger relatives. In this

FIG. 61.— Kelp-crab (Epialtus productus) in upper part of fignre ; to the right the
edible crab ( Cancer productus), and the shore-crab (Pugettia richii).

connection it is interesting to note that the giant crab of
Japan, the largest crustacean, being upward of twenty feet
from tip to tip of the legs, is a spider-crab, constructed on

FIG. 62.— The fiddler-crab (Gelasimus). Photograph by Miss MARY RATHBUN.

the same general pattern as our common coast forms.
Between these two extremes numberless variations exist,

ARTHROPODS. CLASS CRUSTACEA

105

some for known reasons, but more often not readily under-
stood. And not only does the form vary, but the external
surface may be sculptured or beset with spines or tubercles
which frequently render the animal inconspicuous amid its
natural surroundings. Such an effect is heightened by the
presence of sponges, hydroids, and various seaweeds which
the crab often permits to gather upon its body.

105. Pill-bugs and sandhoppers. — Finally there remain the
groups of the pill- or sow-bugs (Isopods) and the sand-fleas
or sandhoppers (Amphipods). In the first of these the
body is usually small and compressed, the thorax more or
less plainly segmented, and the seven walking (thoracic)
legs are similar. In the female each leg bears at its base a
thin membranous plate which extends inward and hori-

FIG. 63.— Isopod or pill-bug (Porcellio laevis).

zontally, thus forming on the under side of the body a
brood-pouch (Fig. 63) in which the young develop. As
one may readily discover in any of the common species,
the abdominal segments are more or less fused, and bear
appendages adapted for respiration and, in the aquatic
forms, for swimming.

106

ANIMAL FORMS

The marine isopods occur in the sand, under rocks, and
in the seaweeds ; many are parasitic upon fishes ; and the ter-
restrial forms (Fig. 63) are very common objects under old

FIG. 64.— Amphipods or sand-fleas (Gammarus, upper species, and Capretta).

logs and in cellars, where they live chiefly on vegetable mat-
ter. In the sand-fleas the body is compressed from side to
side, and while the thorax shows distinct segments, the legs
are frequently dissimilar, and some may bear pincers. One
of their most distinctive marks concerns the last three ab-
dominal appendages, which are usually modified for leaping.
The sand-fleas (Fig. 64) are familiar objects to any one
who has collected along the beach and has turned over the
cast-up seaweeds, while numbers of small species often oc-
cur among the plants in our fresh-water ponds. Some most
curious and highly modified forms, whose general appear-
ance is shown in the lower part of Fig. 64, occur among

ARTHROPODS. CLASS CRUSTACEA 107

hydroid colonies, with which their bodies harmonize in
form and color. And, lastly, most bizarre creatures, known
as " whale-lice," attach themselves to the skin of whales, of
which each species acts as host for one or more kinds.

106. Internal organization.— Most Crustacea are carnivo-
rous, preying upon almost any of the smaller animals within
convenient reach ; a much smaller number live on vege-
table food ; and there are many, such as the crayfishes, lob-
sters, and numerous crabs, which are also notorious scaven-
gers. In these latter forms the food is held in one of the
large pincers, torn into shreds by the other, and transferred
to the mouth-parts, where, as in all Crustacea, it is soon
reduced to a pulp by their rapid movements. In many
species the food is now ready for the digestive process,
but not so in the higher forms. If the stomach of any of
these, for example, the crabs or crayfishes, be opened, three
(Fig. 65, s) large teeth operated by powerful muscles will
be noted, and beyond these a strainer consisting of many
closely set hairs. In operation this " gastric mill " takes
the food passed on from the mouth-parts, and crushes and
tears it until fine enough to pass through the strainer,
whereupon it is dissolved by the juices from the liver and
is absorbed as it passes down the intestine.

The circulatory system is usually highly developed, and
consists of a heart, in some species almost as long as the
body, though usually shorter (Fig. 65), from which two or
more arteries branch to all parts of the body. Here the
blood, instead of emptying into definite veins, pours into a
series of spaces or sinuses in among the muscles and other
organs of the body, through which it makes its way back to
the heart. During this return journey it is usually made
to traverse definite respiratory organs, either situated upon
the legs or, as feathery outgrowths, upon the sides of the
body, and generally concealed under the carapace. A por-
tion of the blood is also continually sent to the kidneys,
which are located either at the base of the second antennae

108 ANIMAL FORMS

(and known as green glands), as in the crayfishes or crabs,
or on the second maxillae (shell-glands) in many of the

PIG. 65.— Dissection of crayfish, b, brain ; k, heart ; i, intestine ; k, kidney ; /, liver ;
n, nerve-cord ; r, reproductive organ ; s, stomach, showing two teeth in position.

simpler crustaceans. Their method of operation is much
like that of the kidneys in the earthworm.

107. Nervous system and special senses. — The nervous sys-
tem also shows a decided resemblance to that of the anne-
lids. The cerebral ganglia or brain is situated above the
alimentary canal in the head, and connects with the ven-
trally lying cord by a collar. As in the earthworm, this
ventral cord is double, and bears a pair of swellings or gan-
glia in each segment. In the crayfish, crabs, and other
highly modified forms, where the segments tend to fuse,
several of these ganglia may also unite, and except in early
life their number cannot be determined.

Among the less specialized Crustacea the order of intel-
ligence is low, though perhaps it may prove to be higher
than is usually supposed when such forms have been more
thoroughly studied. The following quotation relating to
the lobster applies even more to the higher forms, the
crabs : " Sluggish as it often appears when out of water and
when partially exhausted, it is quite a different animal when
free to move at will in its natural environment on the sea-

ARTHROPODS. CLASS CRUSTACEA 109

bottom. It is very cautious and cunning, capturing its
prey by stealth, and with weapons which it knows how to
conceal. Lying hidden in a bunch of seaweed, in a crevice
among the rocks, or in its burrow in the mud, it waits until
its victim is within reach of its claws, before striking the
fatal blow. The senses of sight and hearing are probably
far from acute, but it possesses a keen sense of touch and
of smell, and probably also a sense of taste."

Although enclosed in a horny and often very thick and
strong armor, the sense of touch is very keen in the
Crustacea and in arthropods generally. On many of the
more exposed portions delicate hairs or pits connected
with the nervous system occur in great abundance. Some
of these, usually on the antennae, undoubtedly serve in
detecting odors, but the remainder are considered to be
tactile. In the higher Crustacea, such as the crayfish,
lobsters, and crabs, ears are usually found, consisting of
sacs lined with similar delicate hairs, and containing sev-
eral minute grains of sand, which in many cases make their
way through the small external opening. Vibrations com-
ing through the water gently shake the grains of sand,
causing them to strike against the hairs which communi-
cate with the nervous system — a very simple ear, yet suffi-
cient for the needs of the animals.

The eyes of the Crustacea and arthropods in general are
either simple or compound. The simple and frequently
single eyes usually consist of a relatively few cells embedded
in a quantity of pigment and connected with the nervous
system. It is doubtful whether they perceive objects as
anything more than highly blurred images, and perhaps
they merely recognize the difference between light and
darkness. The compound eyes, on the other hand, are
remarkably complex structures, often borne on the tops of
movable stalks, as in the common crabs and crayfishes.
Each consists of an external transparent cornea, divided
into numerous minute hexagonal areas corresponding to as

110 ANIMAL FORMS

many internal rods of cells, provided with an abundant
nerve-supply. These latter elements may perhaps repre-
sent simple eyes grouped together to form the compound
one ; and it appears possible that each element may form
a complete image of an object, as each of our eyes is known
to do. On the other hand, many hold that the complete
eye forms only one image, a mosaic, each element con-
tributing its share.

108. Growth and development. — As we have seen, the
simpler Crustacea hatch as minute larvae (Fig. 56), and dur-
ing their growth to the adult condition are especially sub-
ject to the attacks of multitudes of hungry enemies. In
the higher forms, such as the crabs, some of these early
transformations take place while the young are still within
the egg and attached to the parent. Accordingly, the little
ones are fairly similar to their parents, and their later his-
tory is very well exemplified by the lobster.

The eggs of the lobster are most frequently hatched in
the summer months, usually July, after they have been
carried by the parent for upward of a year. The young,
about a third of an inch in length, at once disperse, undergo
four or five moults during the next month, then, ceasing
their swimming habits, settle to the bottom among the
rocks. At this time, twice their original size, they closely
resemble their parents, and their further development is
largely an increase in size. " The growth of the lobster,
and of every arthropod, apparently takes place, from in-
fancy to old age, by a series of stages characterized by the
growth of a new shell under the old, by the shedding of
the outgrown old shell, a sudden increase in size, and the
gradual hardening of the shell newly formed. Not only is
the external skeleton cast off in the moult and the linings
of the masticatory stomach, the esophagus, and intestine,
but also the internal skeleton, which consists for the most
part of a complicated linkwork of hard tendons to which
muscles are attached."

ARTHROPODS. CLASS CRUSTACEA

111

109. Peripatus (class Onychophora).— It is generally be-
lieved that the Crustacea, insects, and spiders, together
with their numerous relatives, trace their ancestry back to
animals that bore a certain resemblance to the segmented
worms. Most of these ancient types have

long been extinct, but here and there
throughout the earth we occasionally meet
with them.

Among the most interesting of these
are a few widely distributed species belong-
ing to the genus Peripatus (Fig. 66), but as
they are comparatively rare we shall dis-
miss them with a very brief description.
They usually dwell in warm countries, un-
der rocks and decaying wood, emerging at
night to feed on insects, which they ensnare
in the slime thrown out from the under
surface of the head. Their external form,
their excretory system, and various other
organs are worm-like. On the other hand,
the appendages are jointed, and one pair
has been modified into jaws. The peculiar
breathing organs characteristic of the in-
sects are also present. Peripatus therefore
gives us an interesting link between the
worms and insects, and also affords an idea
of the primitive insects from which the
modern forms have descended.

110. The centipeds and millipeds (class
Myriapoda). — Many of the myriapods — that
is, the centipeds and thousand-legged worms
— are familiar objects under logs and stones
throughout the United States. The first of these (Fig. 67)
are active, savage creatures, devouring numbers of small
animals, which they sting by means of poison-spines on the
tips of the first pair of legs. The bite of the larger tropical

30

PIG. 66.— Peripatus
(Peripatus eiseni).
Twice the natural
size.

112

ANIMAL FORMS

species especially causes painful but not fatal wounds in
man.

On the other hand, the millipeds (Fig. 68) or thousand-
legs are cylindrical, slow-going animals, feeding on vegetable

PIG. 67.— Centiped.
One-half natural size.

FIG. 68.— Thousand-legs or milliped (Julus).
Natural size.

substances without causing any particular damage, except
in the case of certain species, which work injury to crops.
When disturbed they make little effort to escape, but roll
into a coil and emit an offensive-smelling fluid, which ren-
ders them unpalatable to their enemies.

All present a great resemblance to the segmented worms,
as their popular names often testify; but, on the other
hand, many points in their organization indicate a closer
relationship to the insects. As in the latter, the head is
distinct, and bears a pair of antennas, the eyes, and two or
three pairs of mouth-parts. The trunk is more worm-like,
and consists of a number of similar segments, each bearing

ARTHROPODS. CLASS CRUSTACEA 113

one or two pairs of jointed legs. In their internal organ-
ization the character of the various systems closely resem-
bles that of the insects, and will be more conveniently
described in that connection.

Among the myriapods the females are usually larger
than the males. Some of the centipeds deposit a little
mass of eggs in cavities in the earth and then abandon
them, while others wrap their bodies about them and pro-
tect them until the young are hatched. The millipeds lay
in the same situations, but usually plaster each egg over
with a protective layer of mud. After several weeks the
young appear, often like their parents in miniature, but in
other species quite unlike, and requiring several molts to
complete the resemblance.

OF THE

UNIVERSITY

OF

|^. CHAPTER X

ARTHROPODS (Continued). CLASS INSECTS

111. Their numbers. — It has been estimated that upward
of three hundred thousand named species of insects are
known to the zoologist, and that these represent a fifth, or
possibly a tenth, of those living throughout the world. Many
of these species, as the may-flies and locusts, are represented
by millions of individuals, which sometimes travel in such
great swarms that they darken the sky. With nearly all
of these the struggle for existence is fierce and unrelenting,
and it is little wonder that such plastic animals have
changed in past times and are now becoming modified in
order to adapt themselves to new situations where food is
more abundant and the conditions less severe. Owing to
such modifications we find some species fitted for flying,
others for running and leaping, or for a life underground,
and many for a part or all of their lives are aquatic in their
habits.

112. External features.— The body of an insect— the
grasshopper, for example — consists of a number of rings
arranged end to end, as we have seen them in the Crustacea
and the segmented worms. In the abdomen these are
clearly distinct, but in the thorax, and especially the head,
they have become so intimately united that their number
is a matter of uncertainty. These three regions— head,
thorax, and abdomen — are usually clearly defined in most
insects, but they are modified in innumerable ways in ac-
cordance with the animal's mode of life.

114

ARTHROPODS. CLASS INSECTS 115

The head usually carries the eyes, a pair of feelers (an-
tennae), and three pairs of mouth-parts which may be fash-
ioned into a long, slender tube to be used in sucking, and
frequently as a piercing organ ; or they may be constructed
for cutting and biting. The thorax bears three pairs of
legs and often one or two pairs of wings. The appendages
of the abdomen are usually small and few in number, or
even absent.

113. Internal anatomy.— The restless activity of insects
is proverbial. Some appear to be incessantly moving about,
either on the wing or afoot, and are endowed with com-
paratively great strength. Ants and beetles lift many times
their own weight. Numerous insects are able to leap many
times their own length, and others perform different kinds
of work with a vigor and rapidity unsurpassed by any other
class of animals. As is to be expected, the muscular sys-
tem is well developed, and exhibits a surprising degree of
complexity. Over five hundred muscles are required for
the various movements of our own bodies, but in some of
the insects more than seven times this number exist. The
amount of food necessary to supply this relatively immense
system with the required nourishment is correspondingly
large. Many insects, especially in an immature or larval
condition, devour several times their own weight each day.
Their food may consist of the juices of animals or plants,
which they suck out, or of the firmer tissues, which are
bitten or gnawed off.

Not only do the mouth-parts stand in direct relation to
the habits of the animal and to its food, but, as we have
often noticed before, the internal organization is also
adapted for the digestion and distribution of the nutritive
substances in the most economical way. For this reason
we find the alimentary canal differing widely in the various
forms of insects. In each case it extends from the mouth
to the opposite end of the animal, and ordinarily consists
of a number of different parts. In the insect shown in

116

ANIMAL FORMS

Fig. 69 the mouth soon leads into the esophagus, which
in turn leads into the crop that serves to store up the food
until ready for its entry into the stomach ; or in some of
the ants, bees, and wasps it may contain material which

may be disgorged and fed
to the young. In many
cases the stomach is small
and ill-defined as in Fig. 69,
and again it may reach
enormous dimensions, near-
ly filling the body. It may
also bear numerous lobes or
delicate hair-like processes,
which afford a greater sur-
face for the absorption of
food. Behind the stomach
are a number of slender
outgrowths that are believed
to act as kidneys. Beyond
their insertion lies the in-
testine, which, like the
stomach, is the subject of
many modifications in the

•,.« , -, • -,

different kinds of insects.

The digested food is rap-

idly absorbed through the coats of the stomach and intes-
tine and enters a circulatory system which reminds us of
what exists in many of the Crustacea. The heart is situ-
ated above the digestive tract, and from it arteries pass out
to different parts of the body. Here the blood leaves the
vessels and is poured directly into the spaces among the
viscera, whence it is finally conducted through irregular
channels to the heart by its pulsations.

In the Crustacea the blood is made to pass through a
respiratory system usually in the form of definite gills, and
the oxygen with which it is charged is distributed to all

FIG. 69.-Cockroach, dissected to show aii-

mentary canal, al. c.— After HATSCHEK

ARTHROPODS. CLASS INSECTS 117

parts of the body. In the insects the blood serves almost
entirely to carry the food, and the oxygen is conveyed
through the animal by a remarkable contrivance found
only in the insects, the spiders, and a few related forms.

114. Respiratory system. — If we examine an insect, the
grasshopper for example, we find a number of small brown
spots on each side of the abdomen, each of which under a
magnifying-glass is seen to be perforated by a narrow slit.
Carefully opening the body, we find that each slit is in
communication with a white, glistening tube that rapidly
branches and penetrates to all parts of the animal. When
the body is expanded the air rushes into the outer openings,
on through the open tubes, and is distributed with great
rapidity to all the tissues of the body. In many insects
some of these tubes connect with air-sacs which probably
serve to buoy up the insect during its flights through
the air.

115. Wingless insects (Thysanura). — The simplest of all
insects are the fishmoths and springtails, relatively small
organisms covered with shining scales or hairs. The first
of these is occasionally seen running about in houses feed-
ing upon cloth and other substances, while the latter live
in damp places under stones and logs. They are without
wings, but are able to run rapidly and to leap considerable
distances. In addition to the ordinary appendages, the
abdomen bears what are perhaps rudimentary legs, a fact
which, together with their relatively simple structure,
strengthens the belief that the insects have descended
from centiped-like ancestors.

116. Grasshoppers, crickets, katydids, etc. (Orthoptera). —
Eising higher in the scale of insect life, we arrive at the group
of the cockroaches, crickets, grasshoppers, locusts, and
other related insects. Four wings are present, the first pair
thickened and overlapping the second thinner pair. The
latter are folded lengthwise like a fan, which is said to have
given the name Orthoptera (meaning straight-winged) to

118 ANIMAL FORMS

this group of insects. These extend all over the world,
being particularly abundant in the warmer countries, and
their strong biting mouth-parts and voracious appetites
render many of them dreaded pests to the farmer. The
cockroaches are nocturnal in their habits, racing about at
night, devouring victuals in the pantry and gnawing the
bindings of books. During the day their flat bodies enable
them to secrete themselves in crevices wherever there is
sufficient moisture.

In the grasshoppers, locusts, katydids, and crickets the
body is more cylindrical, and the hind pair of legs are often
greatly lengthened for leaping. The crickets and katydids

are nocturnal, the former re-
maining by day in burrows
which they construct in the
earth, the latter resting qui-
etly in the trees. At night

PIG. 70.— The Rocky Mountain locust.— , „ **

After RILEY, from The Insect World. tnej ±east uP<>n Vegetable

matter principally, though

some species are known to prey on small animals. Those
insects we usually term grasshoppers (properly called lo-
custs) are specially destructive to vegetation. Some spe-
cies are strong fliers, and this, connected with their abil-
ity to multiply rapidly, renders them greatly dreaded pests.
They have been described as flying in great swarms, form-
ing black clouds, even hiding the sun as far as the eye
could reach. The noise made by their wings resembled
the roar of a torrent, and when they settled upon the earth
every vestige of leaf and delicate twig soon disappeared.

The eggs of the majority of Orthoptera are laid in the
ground, where they frequently remain through the winter.
When hatched the young quite closely resemble the parents,
and, after a relatively slight metamorphosis, assume the
adult form.

117. Dragon-flies, may-flies, white ants, etc. (Pseudoneurop-
tera). — The dragon-flies, caddis-flies, may-flies, and white ants

ARTHROPODS. CLASS INSECTS

119

possess four thin and membranous wings incapable of being
folded. These possess a network of delicate nervures, giv-
ing the name Neuroptera (meaning nerve-winged) to the
class. Of the forms mentioned above, all but the white
ants lay their eggs in the water, and the developing larvse

FIG. 71.-Dragon-fly (Li bdlula pulchelld).

spend their lives in this medium until the time comes for their
complete metamorphosis into the adult. The larvae of the
caddis-flies protect themselves within a tube of stones or sticks
bound together with silken threads, which they usually
attach to the under side of stones in running water. On
the other hand, the young of the dragon- and may-flies, pro-
vided with strong jaws, are active in the search of food and
very voracious. In time they emerge from their larval skin
and the water in which they live, and after a life spent on
the wing they deposit their eggs and perish. The adult
ant-lion, a type of the related order (Neuroptera), which
has somewhat the appearance of a small dragon-fly, lays its
eggs in light sandy soil. In this the resulting larvae exca-
vate funnel-shaped pits, at the bottom of which they lie con-

120

ANIMAL FORMS

cealed. Insects stumbling into their pitfalls are pelted with
sand, which the ant-lion throws at them with a jerky motion
of the head, and are speedily tumbled down the shifting
sides of the funnel to be seized and devoured.

While the white ants are not in any way related to the
true ants, they possess many similar habits. Associated in
great companies, they excavate winding galleries in old logs
and stumps, and, further, are most interesting because of
the division of labor among the various members. The
wingless forms are divided inta the workers, which exca-
vate, care for the young, and otherwise labor for the good
of the others; and into the soldiers, huge-headed forms,

FIG. 72.— Ant-lion larva plowing its way through the sand (upper figure) while an-
other is commencing the excavation of a funnel-shaped pit similar to one on right.
Photograph by A. L. MELANDER and C. T. BRUES.

whose strong jaws serve to protect the colony. The re-
maining winged forms are the kings and queens. In the
spring many of the royalty fly away from home, shed their
wings, unite in pairs, and set about to organize a colony.
The queen rapidly commences to develop eggs, and in some

ARTHROPODS. CLASS INSECTS

121

species her body becomes so enormously distended with
these that she loses the power of locomotion and requires
to be fed. A single queen has been known to lay eggs at
the rate of sixty per minute (eighty thousand a day), and

FIG. 73.— Termites or white ants, a, queen ; b, winged male ; c, worker ; d, soldier.

those destined to royal rank are so nursed that they advance
farther in their development than the remaining sterile
and wingless forms.

118. The bugs (Hemiptera).— The large and varied group
of the bugs (Hemiptera) includes a number of semi-aquatic
species, such as the water-boatmen, often seen rowing
themselves along in the ponds by means of a pair of oar-
shaped legs, in search of other insects. Somewhat similar
at first sight are the back-swimmers, with like rowing
habits, but unique in swimming back downward. Both of
these bugs frequently float at the surface, and when about
to undertake a subaquatic journey they may be seen to
imprison a bubble of air to take along. Closely related are
the giant water-bugs (Fig. 74), which often fly from pond
to pond at night. In such flights they are frequently

122

ANIMAL FORMS

attracted by lights, and have come to be called " electric-
light bugs."

Among our most dreaded insect pests are the chinch-
bugs—small black-and-white insects, but traveling in com-
panies aggregating many millions.
As they go they feed upon the
stems and leaves of grain, which
they devour with extraordinary ra-
pidity. The squash-bug family is
also extensive, and destructive to
the young squash and pumpkin
plants in the early spring.

The lice are small, curiously
shaped bugs, which suck the blood
of other animals. The plant-lice,
also small, suck the juices of
plants, and are often exceedingly
destructive. This is especially true
of the phylloxera, a plant-louse
which causes annually the loss of
millions of dollars among the vine-
yards of this and other countries.

Even more destructive are the scale-insects, curiously mod-
ified forms, of which the wingless females may be found on
almost any fruit-tree and on the plants in conservatories,
their bodies covered with a downy, waxy, or other kind of
covering, beneath which they remain and lay their eggs,

119. The flies (Diptera). — The group of the Diptera
(meaning two-winged) includes the gnats, mosquitoes, fleas,
house-flies, horse-flies (Fig. 75), and a vast company of
related forms. Only a single pair of wings is present, the
second pair being rudimentary or fashioned into short,
thread-like appendages known as balancers, though they
probably act as sensory organs and are not directly con-
cerned with flight. The mouth-parts are adapted for pier-
cing and sucking. The eyes, constructed on the same plan

FIG. 74.— Giant water-bug (Ser-
phus dilatatus), with eggs at-
tached.

ARTHROPODS. CLASS INSECTS

123

FIG. 75.— Horse-fly (T7,

as those of the Crustacea, are comparatively large, and are
frequently composed of a great number of simple eyes
united together, upward of four
thousand forming the eye of the
common house-fly.

These insects are widely distrib-
uted throughout the world, where
they inhabit woods, fields, or houses
as best suits their needs. Their
food is varied. Some suck the
juices of plants, others attack ani-
mals, and, while many are trouble-
some pests, others, especially in the
early stages of their existence, are
of great benefit.

120. Familiar examples. — Owing
to the widely different habits and
structure of the members of this group, we shall briefly
consider two examples, the mosquito and the house-fly,
which will give us a fairly good idea of the characteristics
of all. The eggs of the mosquito are laid in sooty-look-
ing masses on the surface of stagnant pools. Within a
very short time the young hatch, and, owing to their pecul-
iar swimming movements, are known as " wrigglers." They
are then active scavengers, devouring vast quantities of
noxious substances and performing a valued service. They
frequently rise to the surface, take air into the tracheal
system, which opens at the posterior end of the body, and
descend again. After an increase in growth and many in-
ternal changes resulting in a chrysalis-like stage, they rise
to the surface, split the shell, and, using the latter as a float,
carefully balance themselves and soon fly away.

The house-fly usually lays its eggs in decaying vegetable
matter, and the young, maggot-like in form, are active
scavengers. They too undergo deep-seated changes during
the next few days, finally transforming into the adult.

124

ANIMAL FORMS

Many of this great group of the flies spend their early life
in the water or other medium acting as scavengers ; but, on
the other hand, numbers attack domestic and other animals,
and throughout their entire lives are an intolerable plague.
121. The beetles (Coleoptera).— Owing to the ease of pres-
ervation and their bright colors, the beetles have probably
been more widely collected than other insects. Fully ten

FIG. 76.— Long-horned borer (Ergateti). Larva (left-hand figure), pupa, and adult

insect.

thousand distinct species are known in North America
alone. They are all readily recognized by the two firm,
horny sheaths enclosing the two membranous wings, which
alone are organs of flight. The mouth is provided with
jaws, which are used in gnawing. Some prey on noxious
insects or upon decaying vegetable or animal matter, and
are often highly beneficial ; but others attack our trees and
domestic animals, and work incalculable damage.

ARTHROPODS. CLASS INSECTS 125

In some of the stag- or wood-beetles (Fig. 76), which
we may select as types, the adults are often found crawling
about on or beneath the bark of trees, living on sap or
small animals. The eggs laid in these situations develop
into grub-like larvae, which bore their way through living
or dead wood, and in this condition sometimes live four or
five years. They then transform into quiescent pupae (Fig.
76), which finally burst their shells and emerge in the
adult form. Others, like water-beetles and the whirligig-
beetles, whose mazy motions are often seen on the surface
of quiet streams, pass the larval period in the water.
Under somewhat different conditions we find the potato-
bugs, lady-bugs, fire-flies, and their innumerable relatives,
but the changes they undergo in becoming adult are essen-
tially the same as those described for the other members of
the order.

122. The moths and butterflies (Lepidoptera). — The moths
and butterflies occur all over the world. In their mature

FIG. 7?.— Monarch-butterfly (Anosia plexippus}. From photograph by A. L. MELAN-
DER and C. T. BRUES.

state they are possessed of a grace of form and movement
and a brilliancy of coloration that elicit our highest admi-
ration. The mouth-parts are developed into a long pro-
boscis, which may be unrolled and used to suck the nectar
out of flowers, though in many of the adult moths, which
never feed, it may remain unused. The wings, four in
number, are covered with beautiful overlapping scales that

126

ANIMAL FORMS

adhere to our fingers when handled. This feature, and
the general plan of the body, which is much the same

•:». «i *w *

FIG. ?8.--The silver-spot (Argynnis cybele). Photograph by A. L. MELANDER and

C. T. BRUES.

throughout the group, enables us to recognize most of
them at once.

123. Development and metamorphosis. — In some of the
simplest insects, as in the bugs, the young at birth resemble
their parents. In other insects the resemblance is not so
close. The young grasshopper, for example, hatches, from
an egg laid in the ground, with a ridiculously large head
and staring eyes ; still there is no difficulty in recognizing
its relationships. During the next week internal changes
take place. The shell is burst, and the grasshopper emerges,
looking more like its parents than before. This process is
repeated four or five times during the next few weeks, and
the gradual changes thus produced finally bring the young
insect to the adult form. This latter state has been attained
by an incomplete metamorphosis.

ARTHROPODS. CLASS INSECTS

127

In the flies, beetles, butterflies, and numerous insects
the differences between the newly hatched young and the
adult are vastly greater. No one looking on a caterpillar
or a grub for the first time would suspect its origin, and
the changes they undergo have attracted attention for cen-
turies. Placing any of the ordinary caterpillars with their
favorite food in a glass-covered box, we may readily watch
their transformations. Provided with biting mouth-parts
and a voracious appetite, they devour vast quantities of
vegetation for several days. Finally they cease eating, and

FIG. 79.— Life-history of silk-moth (Bombyx mori). A, adult ; B, C, D, caterpillars of
different ages ; E, F, G, silken cocoon and pupa ; H, eggs.

suspend themselves head downward by means of a kind of
cobweb. After remaining quiet a few hours, they burst
their skin, and within appears a chrysalis or pupa. In the
moths, for example, the silk-moth (Fig. 79), the caterpillar
or silk-worm, after eating the favorite mulberry leaves,
spins a silken cocoon, in which the pupa is produced. The
larvae of beetles and many other insects excavate tunnels in
wood or in the earth, and there undergo their transforma-
tions. Invariably the pupa remains quiet for days, months,
31

128 ANIMAL FORMS

or even years, but when the proper time arrives the fully
formed insect emerges, and takes to the wing.

Wonderful internal changes have been taking place
during this time. The organs fitted for the proper treat-
ment of the vegetable food of the caterpillar or grub are
destroyed, at least in part, and new systems are produced
ready for the nectar and vegetable juices which are to be
the food of the adult insect. All insects that pass through
a pupal quiescent stage are said to undergo a complete
metamorphosis.

124. The ants, bees, wasps, etc. (Hymenoptera). — The ants,
bees, and wasps are the best-known insects belonging to
this order. They are characterized by four membranous
wings, by biting and sucking mouth-parts, and the female
is often provided with a sting. All undergo a complete
metamorphosis. The eggs may be laid in the bodies of
other insects, many of which are pests, and are thus de-
stroyed ; or they may be deposited in the nests of other
insects, the foster-parents being compelled to feed them ;
or they may be placed in marvelously constructed homes,
and be the objects of the greatest attention, the parents or
attendants often risking or losing their lives in their
defense. The members of this order have long attracted
attention, largely on account of their remarkable instinc-
tive powers. They live in highly organized communities
and certain of their characteristics may be illustrated by
a study of some of the more familiar forms.

126. The ants. — The ants live in communities consisting
of anywhere from a dozen to many thousands of individuals,
according to the species. Each of these colonies contains
the queen, several young winged males and females, des-
tined as kings and queens to found new colonies, and of a
far greater number of wingless sterile females, the workers.
The workers construct the greater part of the nest, which
often consists of extensive galleries, nurseries, and grana-
ries, excavated in wood or in the earth. They also attend

ARTHROPODS. CLASS INSECTS 129

to the acquisition of food, which consists of the sweet
juices of plants, of other insects, or of leaves and seeds.
These may be fed at once, or placed in storehouses until
times of need.

Certain species of ants make carefully planned attacks
upon other weaker forms. The young are carried off, at
times only after a prolonged and fierce struggle, and all
are soon eaten, or a few may be allowed to develop and act
as slaves. Some species are unable to exist without serv-
ants, which feed them, wash them, and otherwise minister
to their comfort.

In some of their raids numerous plant-lice (delicate,
usually green, insects, such as occur on our household
plants) are often captured and carried into the nest. These
so-called " ant-cows " are carefully tended, and in return
yield up a tiny drop of a sugary fluid to the hungry ant
that solicits it.

The eggs laid by the queen develop into white worm-
like creatures, which ordinarily spin cocoons when about to
become pupae. These are incorrectly called "ant-eggs."
Many, probably on account of insufficient nourishment,
never develop reproductive organs. They become the neu-
ters or workers. The winged royalty fly away from the
colony, pair and found homes of their own, and become
surrounded by a numerous progeny.

126. The bees. — Among the bees we find a considerable
number which lead solitary lives, excavating tunnels in
earth or wood, as in the case of many of the wasps, but,
unlike them, supplying the young with honey or pollen.
Others may constitute a band of worthless insects which
steal their food from their more industrious relations, in
whose nests they also secretly deposit their eggs, leaving the
young to be nourished with food rightly belonging to others.

But it is with the social bees we are most familiar — the
bumble- and honey-bees. The former usually build in the
ground, and form colonies consisting of the queen and from

130 ANIMAL FORMS

twenty to two hundred workers. Kegular combs are not
constructed, the young at first feeding on pollen masses or
"bee-bread," and finally spinning cocoons. In the late
summer males and females appear, but as winter comes on
all perish except the queens, which seek a sheltered place,
and in the spring revive to establish new colonies.

In a wild state the honey-bees dwell in cavities of trees
and other protected places, where they form colonies,

consisting of the queen, of per-
haps two hundred males or
drones if the nest be examined
in the spring and summer, and
of a hundred times as many
sterile females, the workers.
These form among the most
highly organized insect soci-

80.-Bumblebee (Bombus). 6tleS kn°WIL A11 WOI>k f Or the

good of the colony. To each

worker is assigned a definite task, which is never shirked.
It must collect the honey, supply the wax for making the
comb, take care of the brood, or in other ways minister to
the welfare of the community. On the queen devolves
the entire task of egg-laying. She may lay three thou-
sand eggs a day, during the breeding season, for the
three or four years she lives. The drones or males,
after one nuptial flight, are killed or driven from the
hive after a life of a month or two. The unfertilized
eggs are placed in large cells, and the young fed on
pollen develop into males. The fertilized eggs may pro-
duce queens or workers at the discretion of the queen. If
the latter be desired, the eggs are placed in small cells with
a scant amount of food, which apparently causes the repro-
ductive system to remain undeveloped. The same eggs, if
placed in the large queen cells and supplied with highly
nutritious food, would have developed into queens. When
these latter appear they are vigorously attacked and killed

ARTHROPODS. CLASS INSECTS

131

by the parent if not protected by the workers. If the
young queen survive, the old queen departs with many of
her subjects, and collects them into a dense swarm attached
to a limb of a tree, where they remain until scouts return to
conduct them to their new home.

127. The wasps.— The digger-wasps are frequently to be
seen gnawing tunnels in the wood or earth, at the inner end

FIG. 81.— Nest of Vespa, a social wasp. Photograph by A. L. MBLANDEB and
C. T. BRUES.

of which an egg is laid. In some species the developing
young is nourished by food carried in to it day by day. In
other cases the parent may never see her child, dying or
abandoning it before its birth ; but before departing she is
careful to place within reach a sufficient supply of spiders,
caterpillars, beetles, or locusts that shall nourish the little
one until it becomes a motionless pupa. This stage is soon
over, and the adult wasp now digs its way to the surface.

Passing by the familiar mud-wasps or mud-daubers,
whose nests are common objects under stones or against

132 ANIMAL FORMS

the rafters of barns and houses, we arrive at the social
wasps. As the name indicates, these insects, such as the
yellow-jackets and hornets, live together in companies
which consist, as in the ants and bees, of males, females,
and workers. They also are fond of the juices of fruits,
and many of them destroy insects which may be fed to the
young. Their nests are variously situated and constructed,
but all of them agree in being composed, at least in part, of
a grayish substance which is in reality a kind of paper.
With their jaws they scrape off from old logs and fences
small particles of wood, which they probably mix with saliva,
and rolling the mass into a ball set out for home. These
pellets are then flattened out into thin sheets, and worked
up into hexagonal cells, in which the eggs are laid.

Along with the nests of the mud-daubers one frequently
notices the nests of some of the familiar wasps (Polistes),
which build cake-like nests composed of thirty or forty
hexagonal cells attached by a stalk. Somewhat similar
nests, though usually more extensive, are constructed by
the yellow- jackets in cavities in the ground. The numer-
ous combs of the hornet are surrounded by several sheets of
wood-pulp, and the whole structure is attached generally
to the limb of a tree.

In the spring the nests of all these species of wasps are
commenced by a single female, who has lived in a dormant
condition through the winter. She builds a small nest and
in time is surrounded by numerous workers, which live in
perfect harmony, enlarging the nest and rearing the young.
As autumn approaches the young males and females leave
the nest ; but the males, together with the workers, all suc-
cumb to the cold, and none but the females persist to found
a new colony the following spring.

CHAPTER XI

ARTHROPODS (Continued). CLASS ARACHNIDA

128. General characters. — In this group, comprising the
spiders, mites, and a large assemblage of related species, we
again meet with great differences in form and structure
which fit them for lives under widely different conditions.
The three regions of the body, head, thorax, and abdomen,
so clearly marked in the insects, are here less plainly de-
fined. The head and thorax are usually closely united, and
in the mites the boundaries of the abdomen are also indis-
tinct. The appendages of the head are two in number, and
probably correspond to the antennae and mandibles of other
Arthropods. In the scorpions and some species of mites
these are furnished with pincers for holding the prey, and
in other forms they act as piercing organs. Usually the
thorax bears four pairs of legs, a characteristic which readily
separates such animals from the insects.

The internal organization differs almost as much as does
the external. In many species it shows a considerable re-
semblance to that of some insects, but in others, especially
those of parasitic habits, it departs widely from such a type.
Respiration is affected by means of tracheae, or lung-books,
which consist of sacs containing many blood-filled, leaf -like
plates placed together like the leaves of a book.

Usually, as in the insects, the young hatch from eggs
which are laid, but in the scorpions and some of the mites
the young develop within the body and at birth resemble
the parent. Almost all of these organisms live either as

133

134

ANIMAL FORMS

parasites or as active predaceous animals upon other animals.
For this purpose many are provided with keen senses for
detecting their prey and poisonous spines for despatching it.
129. The scorpions. — Owing to the stout investing armor,
the strong pincers, and the general form of the body, the
scorpions might at first sight be mistaken for near relatives

of the crayfish or lobster.
A more careful examina-
tion will show that the
two pairs of pincers prob-
ably correspond to the
antennae and mandibles of
the Crustacea that have
become modified for seiz-
ing the food. The swol-
len part of the animal
lying behind the four
pairs of legs is a part of
the abdomen, of which
the slender " tail " consti-
tutes the remainder. On
the tip of the tail is a
curved spine supplied
with poison glands. Sev^
eral pairs of eyes are borne

FIG. 82.— Scorpion, showing pincer-like mouth- ,1 -, -, £ £

parts ank spine-tipped tail. °n the d°rsal SUrface °f

the head and thorax, while

on the under side of the animal several slit-like openings
lead into as many small cavities containing the lung-books.
The scorpions are the inhabitants of warm countries,
where they may be found under sticks and stones through-
out the day. At night they leave their homes in search of
food, which consists chiefly of insects and spiders. These
are seized by means of the pincers, and the sting is driven
into them with speedily fatal results. It is doubtful if the
poison causes death in man, but the sting of some of the

ARTHROPODS. CLASS ARACHN1DA 135

larger species, which measure five or six inches in length,
may produce certain disorders chiefly affecting the circula-
tion. In this country there are upward of thirty species,
most of which are comparatively small.

130. The harvestmen. — The harvestmen or daddy-long-
legs are small-bodied, long-legged creatures which resemble
in general appearance several of the spiders. They differ
from them, however, in the possession of claws correspond-
ing to the smaller ones of the scorpion, and in their method
of respiration, which is similar to that of insects. During
the day they conceal themselves in dark crevices or stride
slowly about in shaded places ; but at night they emerge
into more open districts and capture small insects, from
which they suck the juices.

131. The spiders. — The spiders are world-wide in their
distribution, and are a highly interesting group, owing
chiefly to their peculiar habits. Examining any of our
familiar species, it will be seen that the united head and
thorax are separated by a narrow stalk from the usually
distended abdomen. To the under side of the former are
attached four pairs of long legs, a pair of feelers, and the
powerful jaws supplied with poison-sacs, while eight shin-
ing eyes are borne on the top of the head. On the abdo-
men, behind the last pair of legs, are small openings into
the lung cavities which contain a number of vascular, leaf-
like projections known as lung-books. In some species
a well-marked system of tracheae are also present. At the
hinder end of the body are four or six little projections,
the spinnerets, each of which is perforated with many
holes. Through these the secretion from the glands be-
neath is squeezed out in the form of excessively delicate
threads, often several hundred in number, which harden on
exposure to the air. According to the use for which these
are intended, they may remain a tangled mass or become
united into one firm thread ; and according to the habits
of the animal, they may be used for enclosing their eggs,

136 ANIMAL FORMS

for lining their burrows, or for the construction of webs of
the most diverse patterns.

132. The habits of spiders. — Many species of spiders, some
of which are familiar objects in fields and houses, construct
sheets of cobweb with a tube at one side in which they may

FIG. 83.— A tarantula-spider (Eurypelma lentzii). Natural size. Photograph by
A. L. MELANDER.and C. T. BRUES.

lie in wait for their prey or through which they may escape
in times of danger. In the webs of the common orb- or
wheel-weavers several radial lines are first constructed, and
upon these the female spider spins a spiral web. Kesting
in the center of this or at the margin, with her foot on
some of the radial threads, she is able to detect the slight-
est tremor and at once to rush upon the entangled captive.
Some of the bird-spiders and their allies, living in trop-
ical America, and attaining a length of two inches, con-
struct web-lined burrows in the ground. From these they
stalk their prey, which consists of various insects and even

ARTHROPODS. CLASS ARACHNIDA

137

small birds. These are almost instantly killed by the poison-
fangs, and are then carried to the burrow, where the juices
of the body are extracted.

The trap-door spiders of the southwestern section of the
United States also dig tunnels, which they cover with a
closely fitting lid com-
posed of earth. Eaising
this they come out IB
search of insects, but if
sought in turn, they dash
into the burrow, closing
the door after them, and
holding it with such firm-
ness that it is rarely forced
open. If this should hap-
pen, there are sometimes
blind passage-ways, also
closed with trap-doors,
which usually baffle the
pursuer.

Finally, there are
among the thousand spe-
cies of spiders in the United States a considerable propor-
tion which construct no definite web. Many of these may
be seen darting about in the sunshine on old logs and
fences, often trailing after them a thread which may sup-
port them if they fall in their active leaping after in-
sects.

133. Breeding habits. — The male spiders are usually much
smaller than the females, and some species are only one-
fifteenth as long as the female and one one-hundredth of
its weight. They are usually more brilliantly colored, more
active in their movements, yet rarely spinning their own
webs and capturing their own food, preferring to live at
the expense of the female. At the breeding season the
males of several species^ make a most interesting display

Fio. 84. — Trap-door spider and burrow
( Ctenizd).

138 ANIMAL FORMS

of their colors, activity, and gracefulness before the females ;
and the latter, after watching these exhibitions, are said to
select the one who has " shown off " in the most pleasing
fashion. The life after this may be stormy, resulting in
the death of the male ; but ordinarily the results are not
so disastrous, and in a little while the female deposits her
eggs in cases which she spins. In these the young develop,
sometimes wintering here, and emerging in the spring to
scamper about in search of food, or to drift through the
air to more favorable spots on fluffy masses of cobweb.

Few groups of animals are more interesting objects of
study and more accessible. Their bites are rarely more
serious than those of the mosquito — never fatal ; and a
careful study of any species, however
common, will undoubtedly bring to
light many interesting and unknown
facts.

134. The mites and ticks.— The
mites and ticks are the simplest and
among the smallest of the animals
belonging to this group. To the at-
tentive observer they are rather com-
mon objects, with homes in very dif-
ferent situations. Some occur on liv-
FIG. 85.-The itch-mite (Sar- ing and decaying vegetation, in old

copies scabet). s n i i_"i

flour and unrefined sugar, while oth-
ers live in fresh water and a few in the sea. Almost all
tend toward _ parasitism. Some of the insects which they
pierce and destroy are a pest to man, but on the other hand
some are intolerable owing to the diseases they produce.

As to other parasitic organisms, degradation of structure
is manifest. The respiratory system, so important to the
active life of the insects, may be absent, the animal breath-
ing through its skin. The circulatory system may be want-
ing, the blood occupying spaces among the various organs
being swept about by the animal's movements. And many

ARTHROPODS. CLASS ARACHNIDA

139

other peculiarities have arisen which fit them for their
different modes of life.

135. The king crab (Limulus). — The king crab may be
found crawling over the bottom or plowing its way through
the sand and mud in many of the quiet bays from Maine
to Florida. The large head and thorax of these animals
are united into a horse-
shoe-shaped piece, be-
hind which lies the
triangular abdomen.
On the curved front
surface of the former
are a pair of small me-
dian eyes, and farther
outward are two larger
compound ones. On
the ventral side are
six pairs of append-
ages, instrumental in
capturing and tearing
the small animals that
serve as food, and
functioning in con-
nection with the ter-
minal spine as locomo-
tor organs. On the
ventral surface .of the abdomen are numerous plate-like flaps
which serve in respiration, and in the imperfect swimming
movements in which these animals occasionally indulge.

These relatively large and clumsy creatures are the rem-
nant of a great number of strange, uncouth animals that in-
habited the earth in past ages. Many of them show a close
resemblance to the scorpions. The anatomy and develop-
ment also show certain points of resemblance, and by some
are thought to give us an idea of the ancient type of spider-
like animal from which the modern forms have descended.

FIG. 86.— The king or horseshoe crab (Limulus
polyphemus).

CHAPTER XII

ECHINODERMS

136. General characters.— The division of the echino-
derms includes the starfishes, sea-urchins, serpent- or brittle-
stars, sea-cucumbers, and crinoids or sea-lilies. All are ma-
rine forms, and constitute a conspicuous portion of the
animals along almost any coast the world over. From
these shallow-water situations they extend to the greatest
depths of the ocean, and the bodily form possesses a great
number of variations, adapting them to lives under such
diverse conditions; and yet there is perhaps no group of
organisms so clearly denned or exhibiting so close a resem-
blance throughout. At one time it was thought that their
radial symmetry was an indication of a close relationship
to the coelenterates, but more careful study has shown them
to be much more highly developed than this latter group,
and widely separated from it. A skeleton is almost always
present, consisting of a number of calcareous plates embed-
ded in the body-wall, and often supporting numbers of pro-
tective spines, which fact has given to the group the name
Echinoderm, meaning hedgehog skin.

137. External features.— The body of a starfish (Fig. 87)
consists of a more or less clearly defined disk, from which
the arms, usually five in number, radiate like the spokes
of a wheel. At the center of the under side the mouth is
located, and from it a deep groove, filled with a mass of
tubular feet, extends to the tip of each arm. Innumerable
calcareous plates firmly embedded in the body-wall serve

140

ECHINODERMS

141

for the protection of the internal organs, and at the same
time admit of considerable movement.

In the brittle-stars (Fig. 88) the central disk is much
more sharply defined than in the preceding forms, and the
long snake-like arms are capable of a very great freedom of
movement, enabling the animal to glide over the sea-bottom,
or through the crevices of the rocks, at a surprising rate.

In several species, otherwise closely resembling those

FIG. 87.— Starfish (Asterias ocracea), ventral view. One-half natural size.

in Fig. 88, the arms divide repeatedly. These are the so-
called basket-stars, living in the deeper waters of the sea,
where they, like other brittle-stars, act as scavengers and
devour large quantities of decomposing plant or animal
remains.

At first sight the globular spiny sea-urchins (Fig. 90)
would scarcely be recognized as close relatives of the star-
fishes. A closer examination, however, shows the mouth to
be located on the under side of the body ; from it five rows
of feet radiate and terminate close to the center of the
dorsal side, and the arrangement of the plates forming the

142

ANIMAL FORMS

skeleton indicate that the sea-urchin is comparable to a
starfish, with its dorsal surface reduced to insignificant
proportions.

In the sea-urchins the calcareous plates possess a great
regularity, and are so closely interlocked that they prevent

FIG. 88.— Brittle- or serpent-stars (species undetermined). Natural size.

any motion of the body-wall. Also, each plate is usually
provided with highly developed spines, movable upon a ball-
and-socket joint. These spines serve for locomotion, and,
in some instances, for conveying food to the mouth. A
considerable number of sea-urchins show an irregularity in
form which destroys to a corresponding degree the radial
symmetry. This is due to various causes, but especially to
a compression of the body, which, in the "sand-dollars,"

ECHINODERMS 143

has resulted in the production of a thin, cake-like form
(Fig. 91).

If the spherical body of a sea-urchin were to be stretched
in the direction of a line joining the mouth and the center

FIG. 89.— Basket-star (Astrophyton). One-half natural size.

of the dorsal surface, a form resembling a sea-cucumber
(Fig. 92) would be the result. These latter organisms live
among crevices of the rocks, embedded in the mud or bur-
rowing in the sand at the bottom of the sea. In such situa-
tions they are well protected, and a highly developed skele-
ton, such as that of the sea-urchin, would not only be of
little value, but a positive hindrance to locomotion. The
skeleton, therefore, is much reduced, consisting of a few
scattered calcareous plates embedded in the fleshy body-
wall. Another peculiar feature is almost universally pres-
ent, in the form of a circlet of tentacles surrounding the
mouth, which serve either for the purpose of respiration,
for locomotion, or to convey food to the mouth.

A very good imitation of the general plan of a sea-lily
or crinoid (Fig. 93) could be made by attaching a serpent-

144

ANIMAL FORMS

star, especially one of the basket-stars, by its dorsal side
to a stalk. In the crinoids the numerous branches of the

arms are compara-
tively short, and in
the arrangement of
the internal organs
there are numer-
ous differences, but
for all that the re-
semblance of these
organisms to the
other echinoderms
is undoubted.

138. Haunts, —
The greater num-
ber of starfishes
occur alongshore,
slowly crawling
about in search of
food, or concealed
in dark crevices of
the rocks, where they may often be found as the tide goes
out, and we know that in gradually lessening numbers other
species lead similar lives at different levels far down in the
dark and gloomy depths. In these same locations the sea-
urchins occur, sometimes singly, but more usually associa-
ted in great numbers, several species excavating hollows in
the rocks, within which they obtain protection. The brit-
tle-stars and sea-cucumbers may also be found occasionally
in open view, but more often they make their way about in
search of food buried in the sand. The crinoids are usual-
ly inhabitants of deeper water, where they are found asso-
ciated often in great numbers. A few species upon attain-
ing the adult condition separate from the stalk, and are
able to move about (Fig. 94), but the remaining species
never shift their position.

FIG. 90.— Sea-urchin (Strongylocentrotus purpuratus).
Natural size.

ECHINODERMS 145

139. The organs of defense and repair of injury.— As we

have seen, the body-wall of the echinoderms is provided
with a series of plates, often bearing spines which serve as
organs of defense, and to protect the internal organs. The
starfishes and sea-urchins also possess numerous modified
spines (pedicellaria) scattered over the surface of the body,
which have the form of miniature birds' beaks, fastened to
slender muscular threads. During life these jaws continu-
ally open and close, and it is said they clean the body of
debris that settles on it ; but on the other hand there are
several reasons for the belief that they also act as organs
of defense. Thus protected, the natural enemies of echino-
derms appear to be relatively few, and are confined chiefly
to some of the fishes whose teeth are especially modified
for crushing them. In this
way, and owing to the action
of the breakers, they suffer
frequent injury, but many
species exhibit to a remark-
able degree the ability to re-
generate lost parts. Experi-
ments show that if all the
arms of a starfish be separa-
ted from the disk the latter
will within two or three *^^^^^

months renew the arms ; and FlG- w-sand-doiiar, B flat sea-urchin.

Natural size.

a single arm with a part of

the disk is able to renew the missing portions in about the

same length of time.

The brittle-stars, as their name indicates, are usually ex-
cessively delicate, often dropping all of their arms upon the
slightest provocation ; but here again the ability is present
to develop the lost portions.

Sea-cucumbers resent rough treatment by vigorously
contracting their muscular walls and removing from the
body almost the entire digestive tract, the respiratory tree,

146

ANIMAL FORMS

and a portion of the locomotor system ; but some species, at
least, renew them again. In some of the starfishes and

brittle-stars portions of the body
appear to be voluntarily de-
tached and to develop into new
individuals, and it is thought
that such self-mutilation is a
normal method of reproduction.
140. Locomotor system. — One
of the most characteristic and
remarkable features of the echi-
noderms is the water-vascular
system, a series of vessels con-
taining water which serve in the
process of locomotion. Their
arrangement and mode of opera-
tion are, with slight modifica-
tions, the same throughout the
group, and may be readily un-
derstood from their study in
the starfish.

On the dorsal surface of a
starfish, in the angle between

two of the arms, is a round, slightly elevated, calcareous
plate, the madreporic body (Fig. 95, m.p.), which under
the microscope appears full of holes, like the " rose " of a
watering-pot. This connects with a tube that passes to
the opposite side of the body, where it enters a canal
completely encircling the mouth. On this ring-canal a
number of sac-like reservoirs with muscular walls are at-
tached, and from it a vessel extends along the under sur-
face of each arm from base to tip. Each of these radial
water-mains gives off numerous lateral branches that open
out into small reservoirs similar to those located on the
ring-canal, and a short distance beyond communicate
through the wall of the body with one of the numerous

FIG. 92.— Sea-cucumber (Cucu-
maria sp.). Natural size.

ECHINODERMS

147

tube-feet, which, as we have seen, are slender tubular or-
gans, many in number, filling the grooves on the ventral
surface of each arm. This entire system of tubes and
reservoirs is full of .water, taken in, it is said, through the
perforated plate, and, when the starfish wishes to advance,
many of the little reservoirs con-
tract, forcing water into the cav-
ity of the feet, with which they
are in communication, thus ex-
tending the extremity of the tubes
a considerable distance. The
terminal sucker of each foot, act-
ing upon the same principle as
those on the cuttlefish, attaches
firmly to some foreign object,
whereupon the muscles of the
foot contract, drawing the body
toward the point of attachment.
This latter movement is similar
to that of a boatman pulling him-
self to land by means of a rope
fastened to the shore. When the
shortening of the tube-feet has
ceased, the sucking disks release
their attachment, project them-
selves again, and this process is
repeated over and over. At all
times some of the feet are con-
tracting, and a steady advance of
the body is the result.

This method of locomotion

also obtains in the sea-urchins and cucumbers, but in the
serpent-stars the tube-feet have become modified into feel-
ers, and the animal moves, often rapidly, by means of twist-
ing movements of the arms. The feet have this character
also in the crinoids, where the animal is generally without

FIG. 93. — Sea-lily or crmoicl.

148 ANIMAL FORMS

the power of locomotion. In some of the sea-cucumbers
five equidistant rows of tube-feet extend from one end of
the body to the other, and the animal crawls worm-like
upon any side that happens to be down ; but certain spe-
cies living in the sand,
where tube - feet will
not work satisfactorily,
have lost all traces of
them, and creep like an
earthworm from place
to place. In all the
sea-cucumbers the feet,
situated near the mouth,
have been curiously
modified to form a cir-

FIG. 94.— An unattached crinoid (Antedon). One- .

half natural size. clet OI tentacles, which

range in form from

highly branched to short and thick structures, and in func-
tion from respiratory organs and those of touch to con-
trivances for scooping up sand and conveying it to the
mouth.

141. Food and digestive system. — In the echinoderms the
body-wall is comparatively thin (Fig. 95), and encloses a
great space, the body-cavity, in which the digestive and re-
productive organs are contained. As the former in various
species is adapted for acting upon very different kinds of
food, it shows many modifications ; but there are a few prin-
cipal types which may be briefly considered.

In the starfishes the mouth enters almost directly into
the cardiac division of the stomach, a capacious, thin-walled
sac, much folded and packed away in the disk and bases of
the arms (Fig. 95, £). This in turn leads into the second
pyloric portion (#), with thicker walls and dorsal, to the
first, from which a short intestine leads to the exterior,
near the center of the disk. Another conspicuous and im-
portant feature is the so-called liver, consisting of a pair

ECHINODERMS

149

of closely branched, fluffy glands (Z), extending the entire
length of each arm and opening into the pyloric stomach.

The starfishes are carnivorous and highly voracious, de-
vouring large numbers of barnacles and mollusks which hap-
pen in their path. If these are small and free they are
taken directly into the stomach, but when one of relatively
large size is encountered the starfish settles down upon it,
and, slowly pushing the cardiac stomach through the mouth,
envelops it in the folds. Digestive fluids are now poured
over it, and the victim is speedily despatched and in a partly
digested condition is gradually absorbed into the body, leav-

FIG. 95.— Dissection of starfish to show : a, pyloric stomach ; b, bile-ducte (above),
cardiac stomach (be,low) ; b.c., body-cavity; /, feet; g, spines; i, intestine;
/, liver; m, mouth; m.p., madre'poric plate ; *r, reservoir; r.c., ring canal;
r.m., stomach retractor muscle ; r.v., radial vessel ; «, stone canal ; t, respira-
tory tree.

ing the shell and other indigestible matters upon the exte-
rior. Oysters and clams close their shells when thus attacked,
but a steady, continuous pull on the part of the starfish
finally opens them," and the stomach is spread over the fleshy
portions with speedily fatal results. In the interior of the
body the food is transferred to the pyloric stomach, sub-
jected to the action of the liver, and when completely dis-
solved is borne to all parts of the body.

150 ANIMAL FORMS

The digestive system of the starfishes, with its various
subdivisions and appendages, is in some respects more com-
plicated than in the other classes. This is most strikingly
the case with the serpent-stars, where the entire system for
disposing of the minute animals and plants on which it
feeds consists of a simple sac communicating with the
exterior by a single opening — the mouth.

In the sea-cucumbers large quantities of sand are taken
into the body, and the minute organisms and organic mat-
ter are digested from it. In the sea-urchins the mouth is
provided with five teeth, and the food consists of minute
bits of seaweeds, which these snip off. Such diets evidently
require a comparatively simple digestive apparatus, for in
both it consists throughout its whole extent of a tube of
equal caliber, in which the various divisions of esophagus,
stomach, and intestine are little, if at all, defined. This
is usually somewhat longer than the body, and therefore
thrown into several loops ; and in the sea-cucumbers its last
division is expanded and furnished with more highly mus-
cular walls, which aid in respiration.

142. Development. — With but a few exceptions, the eggs
of the echinoderms are laid directly in the surrounding
water, and for many days the exceedingly minute young
are borne great distances in the tidal currents. During
this period they show no resemblance to their parents, and
only after undergoing remarkable transformations do they
assume their permanent features. In every case they have
a five-rayed form in early youth, but in several species of
starfishes additional arms develop until there may be as
many as twenty or thirty.

CHAPTEE XIII

THE CHORDATES

143. General characters.— Up to the present time we have
been studying the representatives of a vast assemblage of
animals whose skeletons, if they have any at all, are located
on the outside of the body. In the corals, the mighty com-
pany of arthropods, and the echinoderms, it is external. On
the other hand, we shall find that the animals we are now
about to consider, the fishes, frogs, lizards, birds, and mam-
mals, are in possession of an internal skeleton. In some of
the simpler fishes and in a number of more lowly forms (Fig.
96) it is exceedingly simple, and consists merely of a gristle-
like rod, the notochord (Fig. 98, nc), extending the length
of the body and serving to support the nervous system, which
is always dorsal. This is also the type of skeleton found in
the young of the remaining higher animals, but as they grow
older the notochord gives way to a more highly developed
cartilaginous or bony, jointed skeleton, the vertebral column.

In the young of all these back-boned or chordate ani-
mals, the sides of the throat are invariably perforated to
form a number of gill-slits. In the lower forms these per-
sist and serve as respiratory organs, but in the higher ani-
mals they disappear in the adult. The chordates are thus
seen to be distinguished by the possession of a dorsal nerv-
ous cord supported by an internal skeleton and by the
presence of gill-slits, characters which separate them widely
from all invertebrates.

The chordates may be divided into ten classes, seven of

151

152

ANIMAL FORMS

which (the lancelets, lampreys, fishes, amphibians, reptiles,
birds, and mammals) are true vertebrates, while the others
embrace several peculiar animals of much simpler organiza-
tion.

144. The ascidians. — Among the latter are a number of
remarkable species belonging to the class of ascidians or

sea-squirts (Fig. 96).
These are abundantly
represented along our
coasts, and are readily
distinguished by their
sac -like bodies, which
are often attached at
one end to shells or
rocks. On the opposite
extremity two openings
exist, through which a
constant stream of water
passes, bearing minute
organisms serving as
food. When disturbed
they frequently expel
the water from these
pores with considerable
force, whence the name
" sea-squirt." While
many lead solitary lives,

numerous individuals of other species are often closely
packed together in a jelly-like pad attached to the rocks,
and others not distantly related are fitted to float on the
surface of the sea.

The young when hatched resemble small tadpoles both in
their shape and in the arrangement of some of the more
important systems of organs. For a few hours each swims
about, then selecting a suitable spot settles down and ad-
heres for life. From this point on degeneration ensues,

FIG. ,96.— Ascidian or sea-squirt.

THE CHORDATES 153

The tail disappears, and with it the notochord and the
greater part of the nervous system. The sense-organs van-
ish, the pharynx becomes remodeled, and numerous other
changes occur, leaving the animal in its adult condition,
with little in its motionless, sac-like body to remind one of
a vertebrate.

145. The vertebrates. — Since the remainder of this vol-
ume is concerned with the vertebrates it will be well at the
outset to gain some knowledge of their more important
characteristics. One of the most apparent is the presence
of a jointed vertebral column, composed of cartilage or
bone, which supports the nervous system. To it are also
usually attached several pairs of ribs, two pairs of limbs,
either fins, legs, or wings, and in front it terminates in a
more or less highly developed skull. In the space par-
tially enclosed by the ribs, the body-cavity, a digestive sys-
tem is located, which consists of the stomach and intestine^
together with the attached liver and pancreas. The cir-
culatory system is also highly organized, and consists of a
muscular heart, arteries, and veins which ramify through-
out the body. Breathing, in the aquatic animals, is car-
ried on by means of gills, and in the air-breathing forms
by means of lungs, which, like the gills, effect the removal
of carbonic-acid gas and the absorption of oxygen. The
nervous system, consisting of the brain situated in the
head and the spinal cord extending through the body
above the back-bone, even in the lower vertebrates, is far
more complex than in the invertebrates. The sense-organs
also attain to a high degree of acuteness, and in connec-
tion with the highly organized nervous system enable these
forms to lead far more varied and complex lives than in
any of the animals heretofore considered.

CHAPTER XIV

THE FISHES

146. General characters, — In a general way the name
fish is applied to all vertebrates which spend the whole
of their life in the water, which undergo no retrograde
metamorphosis, and which do not develop fingers or toes.
Of other aquatic chordates or vertebrates the ascidians un-
dergo a retrograde metamorphosis, losing the notochord, and
with it all semblance of fish-like form. The amphibians,
on the other hand, develop jointed limbs with fingers and
toes, instead of paired fins with fin rays. A further com-
parison of the animals called fishes reveals very great dif-
ferences among them — differences of such extent that they
cannot be placed in a single class. At least three great
groups or classes must be recognized : the Lancelots, the
Lampreys, and the True Fishes. The general characters of
all these groups will be better understood after the study
of some typical fish, that is one possessing as many fish-like
features as possible, unmodified by peculiar habits. Such an
example is found in the bass, trout, or perch. In either fish
the pointed head is united, without any external sign of a
neck, to the smooth, spindle-shaped body, which is thus fitted
for easy and rapid cleaving of the water when propelled by
the waving of the powerful tail (Fig. 97). A keel also has
been provided, enabling the fish to steer true to its course.
This consists of folds of skin arising along the middle line of
the body, supported by numerous bony spines or cartilaginous
154

THE FISHES 155

rays. These are the unpaired fins, as distinguished from
the paired ones, which correspond to the limbs of the higher
vertebrates. In the bass or perch the latter are of much
service in swimming, and are also most important organs in
directing the course of the fish upward or downward, or for

Pf

FIG. 97.— Yellow perch (Perca flavescens). df, dorsal fins ; pc, pectoral fin ; pf, pelvic
fin ; v, ventral fin.

aiding the tail in changing the course from side to side ;
or they may be used to support the animal as it rests upon
the bottom in wait for food ; and, finally, they may serve to
keep the body suspended at a definite point.

In addition to an internal skeleton the bass or perch,
like the greater number of fishes, is more or less enclosed
and protected by an external one, consisting of a beautifully
arranged series of overlapping scales, which afford protec-
tion to the underlying organs, and at the same time admit
of great freedom of movement. These usually consist of a
horny substance, to which lime is sometimes added, and
are peculiar modifications of the skin, something like the
feathers, nails, and hoofs of higher forms.

147. The air-bladder.— Xaturally a fish's body is heavier
than the water in which it lives, and there are reasons for
thinking that the air-bladder (Fig. 106, a.U.) acts in the

156 ANIMAL FORMS

bass and perch and many other fishes as a float to enable
them, without much effort, to remain suspended at a defi-
nite level. By compressing this sac, partly by its own mus-
cles and partly by those of the body-wall, the bulk of the
fish is made less, and it sinks ; upon the relaxation of these
same muscles the body expands and rises again. Deep-sea
fishes, when brought to the surface, where the pressure is
relatively slight, are found with their air-bladders so dis-
tended that they can not sink again, and the float of surface
fishes would be as useless if they were to be carried into the
depths below, so that such fishes are compelled to keep
within tolerably definite limits of depth. Morphologically
considered, the air-bladder is a modified or degenerate lung,
and in many fishes it is lost altogether.

148. Respiration. — Looking down the throat of the perch,
or any other fish, a series of slits (the gill-openings), usually
four or five in number, may be seen on each side communi-
cating with the exterior. In the sharks these outer open-
ings are readily seen, but in the bony fishes they open into
a chamber on each side of the head, covered by a bony plate
or gill-cover that is open behind. On raising these flaps
the gills may be seen composed of great numbers of bright-
red filaments attached to the bars between each slit. Dur-
ing life the fish may be seen to open its mouth at regular
intervals, and, after gulping in a quantity of water, to close
it again, contracting the sides of the throat to force it out
of the gill-openings and over the gill-filaments to the exte-
rior. During this process the blood traversing the excess-
ively thin filaments extracts the oxygen from the water and
carries it to other parts of the body.

With this information, let us return to the study of the
three classes of fishes.

149. The lancelet (Branchiostoma).— The lancelet, some-
times called amphioxus (Fig. 98), the type of the class Lepto-
cardii, is a little creature, half an inch to four inches long, in
the different species, transparent and colorless, living in the

THE FISHES 157

sand in warm seas, the nine species known being found in
as many different regions. A lancelet may be regarded as
a vertebrate reduced to its lowest terms. Instead of a
jointed back-bone, it has a cartilaginous notochord, running
from the head to the tail. A nervous cord lies above it,
enclosed in a membranous sheath. No skull is present, and
the nerve-cord does not swell into a brain. There are no
eyes and no scales. The mouth is a vertical slit, without
jaws. There is no trace of the shoulder-girdle (shoulder-
blade and collar-bone) or pelvis (hip-bone) from which

FIG. 98.— The California lancelet (Bramhiostoma californiense). Twice the natural
size, g, gills ; I, liver ; m, mouth ; n, nerve-cord ; nc, notochord.

spring the paired fins, which, in true fishes, correspond to
arms and legs. The circulatory system is fish -like, but there
is no heart, the blood being driven about by the contraction
of the walls of the vessels. Along the edge of the back and
tail is a rudimentary fin, made of fin-rays connected by mem-
brane. In the character and arrangement of its organs the
lancelet is certainly like a fish, but in degree of develop-
ment it differs more from the lowest fish than the fish does
from a mammal.

150. Lampreys (or Cyclostomes). — The class of lampreys
stands next in development (Fig. 99). The notochord gives
way anteriorly to a cartilaginous skull, in which is con-
tained the brain, of the ordinary fish type. There are eyes,
and the heart 'is developed, and consists of an auricle and
a ventricle. As distinguished from the true fish, the lam-
preys show no trace whatever of limbs or of the bones
which would support them. The lower jaw is wholly want-
ing, the mouth being a roundish sucking disk. The fins

158

ANIMAL FORMS

are better developed, but of the same structure as in the
lancelet. There is no bony matter in the skeleton, and
there are no scales. The nasal opening is single on the top
of the front of the head.

There are about twenty-five species in this class. Some
of them, called lampreys, ascend the streams from the sea

FIG. 99. — Lampreys.

in the spring for the purpose of spawning. The young
undergo a metamorphosis, at first being blind and tooth-
less. The adults feed mostly on the blood of fishes, which
they suck after scraping a hole in the flesh with their rasp-
like teeth. The others, called hag-fishes, live in the sea
and bore into the bodies of other fishes, whose muscles they
devour. All are slender, smooth, and eel-shaped.

From their structure and a few fossil remains we sup-
pose that these eel-like forms existed long ago, probably be-
fore the more highly developed sharks and bony fishes made
their appearance, but it is difficult to determine whether
their simple organization is of such long standing or is not
in part the result of semiparasitic habits, or a life spent

THE FISHES 159

largely in burrowing. Like the lancelet and other simple
chordates, they are of the greatest interest to the zoologist
who gains from them some idea of the lowly vertebrate
forms that peopled the earth long ago.

151. True fishes, — The third class, Pisces or true fishes,
to which the shark as well as the bass and perch belong has
a well-developed skeleton, skull, and brain. The lower jaw
is developed, forming a distinct mouth, and there is at least
a shoulder-girdle and pelvis ; although the fins these should
bear are not always developed, the general traits are those
we associate with the fish. Of the true fishes, there are
again several strongly marked groups, usually called sub-
classes. Of these, three chiefly interest us.

152. The sharks and skates. — Very early in the life of
the sharks (Fig. 100) and skates (Selachii or Elasmobranchii)

'

FIG. 100.— Young shark (Galeus zyopterus). One-seventh natural size.

a notochord appears, similar to that in the lancelet and the
lampreys. As growth proceeds its sheath becomes broken
up into a series of cartilaginous rings, which thus appear
like spools strung on a cord. As the fish grows older these
" spools " or vertebrae grow solid, cutting the notochord into
little disks, and great flexibility is thus secured. Cartilagi-
nous appendages also grow up and cover the spinal nerve-
cord lying above, and give strength to the unpaired fins ;
the paired fins also have their supports. The shoulder-
33

160 ANIMAL FORMS

girdle is placed behind the skull, leaving room for a distinct
neck ; strong bars of cartilage bear the gills ; others form jaws
to carry the teeth ; and a complex skull protects the brain
and sense-organs, which are of a relatively high state of devel-
opment. Throughout life the skeleton is of cartilage, with
perhaps here and there a little bone where greater strength
is required. Besides these, there are numerous minor
characters which the student will readily find for himself.

The sharks and skates or rays live chiefly in the sea,
and some reach an enormous size, the largest of all fishes.
Some are very ferocious and voracious ; others are very mild
and weak, and the development of teeth is in direct pro-
portion to their voracity of habit. In earlier geologic times
there were many more species of them than now exist.

153. The lung-fishes. — The lung-fishes (Dipnoi) are pe-
culiar forms living in some of the rivers of Australia and
the tropical regions of Africa and South America. In these
the air-bladder is developed as a perfect lung. During the
wet season they breathe like other fishes by means of gills,
but as the rivers dry up they burrow into the wet mud and
breathe by means of lungs which are spongy sacs of which
the air-bladder of other fishes is a degenerate representative.
As we shall see, they resemble in this respect the tadpoles
and some adult Amphibia (frogs and salamanders). The
paired fins are also peculiar in structure, having an elongate
jointed axis, with a fringe of rays along its length, a struc-
ture almost as much like that of the limbs of a frog as that
of a fish's fin. In fact the Dipnoi must be regarded as an
ancestral type, an ally of the generalized form from which
Amphibia and bony fishes have descended. Only four liv-
ing species of dipnoans are known, but great numbers of
fossil species are found in the rocks.

154. The bony fishes (Teleostei).— The bony fishes, or
Teleosts, are distinguished by the bony skeleton, the sym-
metrical tail, and by the development of the air-bladder as
a more or less completely closed sac, useless in respiration.

THE FISHES 161

Often this organ is altogether wanting, as in the common
mackerel. About ten thousand kinds of bony fishes are
known. The species swarm in every sea, lake, or river
throughout the earth, and some form or another among
them is familiar to every boy in the land. These fishes are
divided into about two hundred families, and these may be
arranged in fifteen to twenty orders. As these are mostly
distinguished by features of the skeleton, we need not name
them here. In Jordan and Evermann's Fishes of North and
Middle America, as well as in various other books, the stu-
dent of fishes can find the characters by which orders may
be distinguished.

155. Sturgeons and garpikes (Ganoidea). — While the great
majority of the typical fishes possess a bony skeleton, there
are a few quaint types — the ganoid fishes, such as the stur-
geons (Fig. 101) and garpikes — in which it is cartilaginous or
partly bony. In past ages these were probably the highest
type of fishes, and from their fossil remains we may con-
clude that they flourished in vast numbers ; but at present
they are almost extinct. In this country the ganoids are
represented by several species, the best known being the
sturgeons which inhabit the Great Lakes, the Mississippi,
and its tributaries; while on the East coast the common
sturgeon (Acipenser sturio) often leaves the sea and ascends
rivers. They are the largest fishes found in fresh water,
attaining a length of ten or twelve feet, and a weight of
five hundred pounds. Their food consists of small plants
and animals, which they suck in through their tube-like
mouth. The garpikes live in the larger lakes and rivers
throughout the East and Mississippi Valley. Their bodies,
from three to ten feet in length, according to the species,
are covered with comparatively large regularly arranged
square scales, and the upper jaw is elongated to form a
kind of beak, abundantly supplied with teeth. They are
carnivorous, voracious fishes, working great havoc among
the more defenseless food-fishes.

THE FISHES 163

156. The catfishes.— Among the lowest bony fishes we
may place the great group to which almost all fresh-water
fishes belong. In this group the four vertebrae situated next
the head are firmly united, and by means of certain small
lever-like bones a connection is formed between the air-blad-
der and the ear of the fish, which is sunk deep in the skull.
The air-bladder thus becomes a sounding organ in the
function of hearing. The family of catfishes possesses this
structure, and the student should look for it in the first one
he catches. The catfishes are remarkable for the long feel-
ers about the mouth, with which they pick their way on the
bottom of a pond. There are many kinds the world over.
The small ones are known as horned pout or bullhead. In
these the dorsal and pectoral fins are armed each with a
strong, sharp spine, which is set. stiff when the fish is dis-
turbed, and makes them very troublesome to handle. The
catfishes have no scales.

157. The carp-like fishes.— The still greater carp family
includes all the carp, dace, minnows, and chubs. They
have the air-bladder joined to the ear, just like the catfish,
but they lack the long feelers and the fin spines, while the
soft body is covered with scales, and there are no teeth in
the mouth. In the throat are a few very large teeth, which
the ingenious boy should find. In the sucker family these
throat teeth are like the teeth of a comb, and the mouth is
fitted for sucking small objects on the river bottom.

158. The eels. — In the great order of eels the body is
long and slim, scaleless, or nearly so, with no ventral fins.
The shoulder-girdle has slipped back from the head, so as
to leave a distinct neck, while ordinary fishes have none-
Of eels there are very many kinds — some large and fierce,
some small as an earthworm ; and one kind comes into fresh
water.

159. Herring and salmon. — In the great order which in-
cludes the herring and salmon the vertebras are all alike,
the ventral fins far from the head, and the scales smooth to

164 ANIMAL FORMS

the touch. The herring and shad are examples, as also the
salmon and trout. Some live in the great depths of the
sea, even five miles below the surface. These are very soft
in body, being under tremendous pressure. They are inky
black — for the sea at that depth seems black as ink — and
most of them have luminous spots which give them light
in the darkness. Some species have the forehead luminous,
like the headlight of an engine. Most of these deep sea
fishes are very voracious, for there is nothing for them to
feed on save their neighbors.

160. The pike, sticklebacks, etc.— Several small orders
stand between these soft-rayed, smooth-scaled fishes and

c

FIG. 108.— The blindfish and its parentage. A, Dismal Swamp fish (Chologaster
amtus\ the ancestor of (B) Agassiz's cave fish (Chologaster agassizi) and (C)
cave blindflsh (Typhlichthys subterraneus).

the form, like the perch and bass, which has many spines in
the dorsal fin. Among these transitional forms is the pike
(Fig. 103) — long, slender, circumspect, and voracious, lying
in wait under a lily-pad ; the blindfish, which lost its eyes
through long living in the streams of the great caves ; the
stickleback, small, wiry, malicious, and destructive, steal-
ing the eggs and nibbling the fins of any larger fish;
the sea-horse, often clinging with its tail to floating

166 ANIMAL FORMS

seaweed, the male carrying the eggs about in his pocket
until they hatch ; the mullet, stupid, blundering, feeding
on minute plants, crushing them in a gizzard like that of
a hen, but withal having soft flesh, good for the table ; the
flying-fishes, which sail through the air with great swiftness
to escape their enemies.

161. The spiny-rayed fishes. — In the group of spiny-
rayed fishes the ventral fins are brought forward and joined
to the shoulder-girdle. The scales are generally rough to
the touch, and the head is usually roughened also. There
are many in every sea, ranging in size from the Everglade
perch of Florida, an inch long, to the swordfish, which is
thirty. These are the most specialized, the most fish-like
of all the fishes. Leading families are the perch, in the
fresh waters, the common yellow perch, familiar to all boys
in the Northeastern States ; the darters, which are dwarf
perches, beautifully colored and gracefully formed, living
on the bottoms of swift rivers ; the sunfishes, with broad
bodies and shining scales, thriving and nest-building in
the quiet eddies ; the sea-bass of many kinds, all valued for
the table ; the mackerel tribe, mostly swimming in great
schools from shore to shore. After these come the multi-
tude of snappers, grunts, weakfishes, bluefishes, rose-fishes,
valued as food. Then follow the gurnards, with bony
heads; the sculpins, with heads armed with thorns, the
small ones in the rivers most destructive to the eggs of
trout ; and at the end of the long series a few families in
which the spines once developed are lost again, and the
fins have only soft and jointed rays. It is a curious law of
development that when a structure is once highly special-
ized it may lose its usefulness, at which point degeneration
at once sets in. Among fishes of this type are the cod-
fishes, with spindle-shaped bodies, and the flounders, with
flat bodies. The flounders lie on the sand with one side
down, and the head is so twisted that the eyes come out to-
gether on the side that lies uppermost. This side is col-

168 ANIMAL FORMS

ored like the bottom — sand colored or brown or black — and
the under side is white. When the flounder is first hatched,
the eyes are on each side of the head, and the animal
swims upright in the water like other fishes. But it soon
rests on the bottom ; it turns to one side, and as the body
is turned over the lower eye begins to move over to the
other side. Finally, we may close the series with the an-
glers (Fig. 105), in which the first dorsal spine is trans-
formed into a sort of fishing-pole with a bait at the end,
which may sometimes serve to lure the little fishes, which are
soon swallowed when once in reach of the capacious mouth.

162. Internal anatomy. — A few fishes are vegetarians, but
the greater number are carnivorous. Some swallow large
quantities of sand of the sea-bottom and absorb from it the
small organisms living there. Others are provided with
beaks for nipping off corals and tube-dwelling worms. Huge
plate-like teeth enable others to crush mollusks, sea-urchins,
and crabs, and many are adapted for preying upon other
fishes. The latter are often able to escape, owing to the
presence of numerous spines, sometimes supplied with
poison-glands; or their colors are protective, and a vast
number of devices are present which enable them with
some degree of surety to escape their enemies and capture
food.

Usually, without mastication, the food passes into the
digestive tract (Fig. 106), which in the main resembles that
of the squirrel, but varies considerably according to the
nature of the food it is required to absorb. As in other
animals, it is usually longer in the vegetable feeders. In
most fishes the walls of the canal are pushed out at the
junction of the stomach and intestine, to form numerous
processes like so many glove-fingers (the pyloric coeca, Fig
106, pyx.), which probably serve to increase the absorptive
surface. The same result is obtained in other ways, chiefly
by numerous folds of the lining of the canal.

The blood-system is much more complex in the fishes

THE FISHES

169

than in any of the invertebrates. It also differs in its gen-
eral plan from that of most adult vertebrates, owing to the
peculiar method of respiration. In almost every case the

FIG. 105.— Angler or frogfish (Lophius piscatorius}. One-tenth natural size.— After

BASKETT.

vessels returning from all parts of the body unite into one
vein leading into the heart, which consists of only one
auricle and ventricle (Fig. 106). From the heart the blood

170 ANIMAL FORMS

is forced through the gills, with all their delicate filaments,
and now, laden with oxygen and nutritious substances al-
ready absorbed from the coats of the digestive tract, it

FIG. 106.— Dissection of a bony fish, the trout (Scdmo). a.bl., air-bladder ; an., anal
opening; at/., auricle; gl.st., gills; gul., esophagus; int., intestine; kd., kidney ;
lr., liver; l.ov., ovary; opt.l., brain ; py.c., pyloric 'coeca ; sp.c., spinal cord ; spl.,
spleen ; st., stomach ; v., ventricle.

travels on to all parts of the body, continually unloading
its cargo in needy districts and waste matters in the kid-
neys before returning once more to the heart.

163. The senses of fishes.— The habits of fishes indicate
that they know considerable of what is going on in the
outside world, and their well-developed sense-organs show
the degree of their sensitiveness. A share of this informa-
tion comes through the sense of touch, which is distributed
all over the surface of the body, chiefly in the more ex-
posed regions sometimes especially provided with fleshy
feelers, like those on the chin of the catfish.

The sense of smell appears to be fairly developed, as is
that of hearing ; but there is no evidence of a sense of taste.
A few fishes chew their food, and may possibly taste it, but
there are others that swallow it whole, and in all there are
relatively a few nerves going to the tongue or floor of the
mouth.

THE FISHES

The eyes of most fishes are highly developed, and are of
the greatest use at all times. Exceptions to the rule are
found in certain species which live in caves or in the dark
abysses of the ocean. In some of these the eyes have dis-
appeared almost completely, and the sense of touch be-
comes correspondingly more acute ; in other deep-sea forms
they have grown to a large size, enabling them to distin-
guish objects in the gloom, like the owls and other noc-
turnal animals. Embedded in the skin of some of these
deep-sea fishes, and certain nocturnal ones, are peculiar
spots, composed of a glandular substance, which produces
a bright glow like that of the fireflies. These may be located
on the head or arranged in patterns over various parts of
the body, and may serve to light the fish on its way and
enable it to see its food to better advantage, or it may act
as a lure to many fishes that become victims to their own
curiosity. In those fishes which are active most of the
time the eyes are located on the sides of the head, and in
those which remain at or near the bottom they are turned
toward the top ; in every case where they can be used to
the best advantage.

164. Breeding habits. — Fishes usually lay their eggs in
spring, the salmon-trout and others in the winter. In breed-
ing-time the males often grow resplendent, and may engage
in struggles for their respective mates. In others this
ceremony is performed without show of hostility. Some
make nests, while others lay their eggs loosely in the water.

In all the salmon family the young fishes are born in
the colder fresh-water rivers, and later make their way into
the sea, where they spend the greater part of their lives.
When the time comes for them to lay their eggs they
migrate in great companies, and make their way hundreds,
perhaps thousands, of miles to the rivers in which they
spent their youth. Up these streams they rush in crowds,
leaping waterfalls and rapids, and, dashed and battered on
the rocks, many, and in some species all, die from injuries

172 ANIMAL FORMS

or exhaustion after the breeding season is passed. The
eggs, like those of the chubs, suckers, sunfishes, and cat-
fishes, are usually buried in shallow holes in the sand, and
the males of most fishes keep a faithful watch over the
young until they are able to live in safety. In some of
the sticklebacks and several marine species elaborate nests
are composed of grass or seaweeds ; some of the catfishes
carry the eggs until they hatch in their mouths or else in
folds of spongy skin on the under side of the body ; in the
pipefishes and sea-horses a slender sac along the lower sur-
face of the male acts as a brood-pouch, in which the female
places the eggs to remain until developed ; and some fishes,
such as the surf-fishes and a number of the sharks, bring
forth their young alive. On the other hand, the young of
many of the herrings, salmon, cod, perch, and numerous
other fishes are abandoned at their birth, and fall a prey to
many animals, even their parents often included.

In the former cases, where the young are protected, only
a relatively few eggs are produced : where they are aban-
doned the female often lays many millions. In every case
the number of eggs is in direct relation to the chances the
young have of reaching maturity, a few out of each brood
surviving to perpetuate the race.

165. Development and past history.— The eggs of the
higher bony fishes are usually small (one-tenth to one- third
of an inch in diameter), and the young when they hatch
are accordingly little ; in the sharks the eggs are larger,
the size of a hen's egg or even larger, and the young when
born are relatively large and powerful. These differences,
however, do not greatly affect the early development, for
in every case the head and then the trunk soon become
formed, gills arise, the nervous system appears, which is
invariably supported by a skeleton in the form of a gristly
rod— the notochord. In the lower forms of fishes this per-
sists throughout life ; but in the sharks and skates it" be-
comes replaced in the adult by another and higher type of

THE FISHES 173

skeleton, which is much more specialized with the hony
fishes.

Those who study the fossils on the rocks tell us that
the first fishes were very simple, and many believe that
their skeleton, like that of the little growing fish, consisted
only of a notochord. Many of these old forms died out
long ago, while others gradually changed in one way and
another to adapt themselves to their surroundings, the con-
stant need of adaptation having resulted in the multitude
of present-day types. Some, such as the lamprey, have
probably changed relatively only to a slight extent ; others,
like the sharks and skates, are much more altered; and
the bony fishes are far from their original low estate,
though their development has been rather toward a greater
specialization for aquatic life than an advance upward.
The little fish in its growth from the egg thus repeats the
history of its ancestral development; but as though in
haste to reach the adult condition, it omits many impor-
tant details. Moreover, the record in the Tocks is not
complete, and we have many things yet to learn of the
ancient fishes and their development from age to age to
the present day.

CHAPTEK XV

THE AMPHIBIANS

IN many respects the amphibians — toads, frogs, and sala-
manders—resemble the fishes, especially the lung-fishes
(Dipnoi). The modern amphibians are essentially fishes
in their early life, but in developing legs and otherwise
changing their bodily form they become adapted for a life
on land under conditions differing from those of the fishes.
Judging from this class of facts, we may assume that fish-
like ancestors, by the development of the lungs, became
fitted for a life on land, and that from these the amphib-
ians of our times have been derived.

166. Development. — The eggs of the Amphibia are laid
during the spring months in fresh-water streams and ponds.
They are globular, about as large as shot, and are embedded
in a gelatinous envelope (Fig. 107). They are either de-
posited singly or in clumps, or festooned in long strings over
the water-weeds. During the next few days development
proceeds rapidly under favorable conditions, resulting in an
elongated body with simple head and tail. In this condition
they are hatched as tadpoles. As yet they are blind and
mouthless, but lips and horny jaws soon appear, along with
highly developed eyes, ears, and nose. External fluffy gills
arise on the sides of the head, and slits form in the walls of
the throat, between which gills are attached, and over which
folds of skin develop, as in the fishes. A fin-fold like that
of the lancelet or lamprey appears on the tail. The brain
and spinal cord, extending along the line of the back, are
supported by a gristly notochord, and complete and com-
174

THE AMPHIBIANS

175

plex internal organs adapt the animal to a free-swimming
existence for days to come.

The tadpole is now, to all intents and purposes, a fish —
a fact most clearly recognized in its form, method of loco-

Fio. 107.— Metamorphosis of the toad.— Partly after GAGE, from Animal Life.

motion, the arrangement of the gills, and the general plan
of the circulatory system.

167. Further growth. — In the course of the next few
weeks hind limbs develop beneath the skin, through which
they finally protrude. In the same manner, fore limbs arise
at a later date. In position these organs are like the paired
fins of fishes, but they are intended for crawling or leaping
on land, and are modified in accordance with this need. As
in the higher vertebrates, the limbs develop as arms and
legs, with long fingers and toes, between which are stretched
webs of skin, which serve in swimming.
34

176 ANIMAL FORMS

In the meantime large internal changes are also taking
place. The wall of the esophagus has gradually pouched
out to form the lungs. They are richly supplied with blood-
vessels, closely resembling in their general features the
lungs of the lung-fishes. The animal now rises to the sur-
face occasionally to gulp in air, and it also continues to
breathe by means of gills. At this stage of its existence,
therefore, the larva is amphibious (two-living), and we have
the interesting example of an animal extracting oxygen
from both the water and the air. The diet of the tadpole
at this time changes from vegetable to animal substances,
and horny teeth give way to the small teeth of the frog,
and the digestive system undergoes an entire remodeling
to adapt it to its new duties. The young amphibian —
whether frog, toad, or salamander— is now a four-legged
creature, with well-developed head and tail, with lungs and
gills, though the latter are usually fast disappearing, and is
rapidly assuming those characters which will fit it for a
terrestrial or semiaquatic existence.

168. The salamanders. — The changes which now ensue in
such a larva in reaching the adult condition are relatively
slight in the lower salamanders. The external gills often
persist (Fig. 110), the lungs are also functional, and the
changes are largely those of increase of size. In the larger
number of species the gills disappear more or less com-
pletely (Fig. 108), such species often abandoning the water
for homes in damp soil or under stones and logs, returning
to it only when the time comes for their eggs to be laid.
The limbs are always relatively weak, never supporting the
body from the ground, but serving in a clumsy way to push
it from place to place. In the aquatic forms the tail con-
tinues to serve as a swimming organ. In some species the
hind legs become rudimentary, or even entirely lacking.
A still further modification occurs in a few burrowing spe-
cies, which move by wrigglings of the body, and are with-
out either pairs of legs.

THE AMPHIBIANS 177

In geological times many of the salamanders were of
great size, several feet in length, and some were enclosed
in an armor consisting of bony plates. All now living have
the skin naked, and with the exception of the giant species
of Japan, three feet in length, and a few similar forms in
America, the modern representatives are comparatively

FIG. 108.— Blunt-nosed salamander (Amblystoma opacum). Photograph by W. H.

FISHER.

feeble and measure their length by inches. Only a few, on
account of their bright colors, are particularly attractive,
while the others are usually shunned and considered re-
pulsive, chiefly because of their supposed poisonous char-
acter, though in reality few animals are more harmless.

169. Tailless forms. — In the frogs and toads the meta-
morphosis which the young undergo is almost as profound
as that which takes place with the insects. The gills, to-
gether with their blood-vessels, disappear completely. The
tail, with its muscles, nerve-supply, and skeleton, is ab-
sorbed. The cartilaginous notochord gives way to a jointed
back-bone. A skull is developed ; numerous bones form in
the limbs, affording an attachment for the powerful muscles
which make the toad, and especially the frog, expert swim-

178 ANIMAL FORMS

mers and leapers, and thus equipped they hereafter lead a
wholly terrestrial or seniiaquatic life.

170. Distribution and common forms. — All the Amphibia
are dependent upon moisture. Almost all are hatched and
developed in fresh water, and those which leave the water
return to it during the breeding season. So we find repre-
sentatives of the group all over the world having much the
same range as the fresh-water fishes. The great majority
of the salamanders are confined to the northern hemisphere,
but the toads and frogs are almost universally distributed.

Among the salamanders in this country only a relatively
few species completely retain their external gills. This is
the case with sirens and mud-puppies or water-dogs (Fig.
110), which may occasionally be seen in the clear waters
of our lakes and rivers crawling slowly about in search of
food, and every now and then rising to the surface to gulp
in air. The remainder lose their gills more or less com-
pletely, and usually leave the water for damp haunts on
land. One of the blunt-nosed salamanders, known as the
tiger salamander (Amblystoma tigrinunt), is found in moist
localities in most parts of the United States. Besides these
are numerous small species, among them the newts (Die-
myctylus)) ranging widely over the United States, living
under logs and stones and feeding upon the small insects
and worms inhabiting such situations. In several species
of salamanders the lungs disappear with age, and respira-
tion is performed solely through the surface of the skin.

The tailless amphibians are much more abundant and
familiar objects than the salamanders, and from the open-
ing of spring until late in the fall they are met with on
every hand. With few exceptions the frogs live in or about
ponds and marshes, in which they obtain protection in
troublous times and from which they derive the store of
worms and insects that serve as food. On the other hand,
the tree-frogs, as their name indicates, usually abandon the
water and repair to moist situations in trees and other vege-

THE AMPHIBIANS 179

tation. Their shrill, cricket-like calls are often heard in
the summer. The fingers and toes are more or less dilated
into disks at their tips, enabling them to climb with con-
siderable facility; and they are further adapted to their
surroundings on account of their protective colors. The
toads undergo their metamorphosis while very small, and
approach the water only at the breeding season. During
the day they remain concealed in holes and crevices, but at
the approach of evening come out in search of food.

171. Means of defense,— The food of the members of this
group consists chiefly of small fishes, insect larvae, snails,
and little crustaceans, which are swallowed whole. On the
other hand, many Amphibia prey on each other, while most
of them are eagerly sought by birds and fishes. Some, as
the toads, stalk their food only during the night-time or
depend upon their agility to escape their enemies. Others
are colored protectively, the markings of the skin resem-
bling the foliage of the earth upon which they rest, and in
some species, as the tree-toads, this color-pattern changes
as the animal shifts its position. A few species are most
brilliantly colored with red, green, yellow, or combinations
of these, in striking contrast to their surroundings. They
have apparently few enemies, possibly because of an un-
pleasant odor or taste, and it has been suggested that their
gorgeous tints are danger-signals, warning their would-be
captors from attempting a second time to devour them. At
the same time it is well known that the somber-hued toads
emit a milky secretion from the warty protuberance of
their skin which is intensely bitter, irritating to delicate
skin, and poisonous to several animals.

172. Skeleton.— As in all vertebrates, the skeleton of the
amphibian first arises as a cartilaginous rod, the notochord,
which is afterward replaced by a jointed back-bone, to
which the limbs are attached. The back-bone is anteriorly
modified into a flat, usually complex, skull. In the sala-
manders the number of vertebras is sometimes very large,

180

ANIMAL FORMS

and the body correspondingly long and snake-like ; but in
other cases parts of the vertebrae are reduced in number,
and the body is rather short and thick. In the frogs and
toads this reduction reaches its culmination, for only nine
distinct vertebrae are present, the tail vertebrae, correspond-
ing to those of the salamanders, being represented by a
rod-like bone, the urostyle, made of segments grown to-
gether.

173. Digestive and other systems. — In its main characters
the digestive tract of the amphibian (Fig. 109) resembles

p.na.

tng

FIG. 109.— Dissection of toad (Bufo). an., anal opening; au., auricle ; bL, bladder;
duo., duodenum ; Ing., lung; lr., liver; pn., pancreas ; ret., rectum ; spl, spleen;
St., stomach ; v., ventricle.

that of the fishes and the squirrel. The mouth is usually
large, and the teeth are very small, as in the frog or sala-
mander, or are lacking completely, as in the common toad.
In many salamanders the tongue, like that of a fish, is fixed
and incapable of movement. In most of the frogs and
toads it is attached to the front of the mouth, leaving its
hinder portion free, and capable of being thrown over and
outward for a considerable distance. In the throat region
gill-clefts may persist, but they usually close as the lungs
reach their development. The succeeding portions of the
canal are comparatively straight in the elongated forms, or

THE AMPHIBIANS 181

more or less coiled in the shorter species. In some cases
no well-marked stomach exists, but ordinarily the different
portions, as they are shown in Fig. 109, are well defined.

As noted above, the circulation in the tadpole is the
same as in fishes, then lungs arise, and for a time respi-
ration is effected both by gills and lungs, and the cir-
culation resembles in its essential points that of the
lung-fishes. This may continue throughout life, but more
frequently the gills and their vessels disappear, and the
circulation approaches that of the reptiles. In such forms
the heart consists of two auricles and one ventricle. Into
the left auricle pours the pure blood from the lungs ; into
the right the impure blood from the body. To some
extent these mix as they are forced into the general cir-
culation by the single ventricle. The amount of oxygen
carried is therefore smaller than in the higher air-breathers,
the amount of energy is proportionately less, and hence it
is that all are cold-blooded and of comparatively sluggish
habits.

In some species of salamanders the lungs may also dis-
appear, and breathing is carried on by the skin, as it is to
a certain extent in all amphibians. In the frogs and toads
lungs are invariably present, and vocal organs are situated
at the opening of the windpipe in the throat. These pro-
duce the characteristic croaking and shrilling, which in
many species are intensified through the agency of one or
two large sacs communicating with the mouth-cavity.

Although the brain is small in the amphibians, it is
more complex in several respects than it is in fishes.
The eyes are also usually well developed, but in some of
the cave and burrowing salamanders they are concealed
beneath the skin, and are rudimentary. The ear varies
considerably in complexity in the different species, but in
the possession of semicircular canals and labyrinth resem-
bles that of the fishes. In the frogs and toads, as one may
readily discover, the drum or tympanum is external, ap-

182 ANIMAL FORMS

pearing as a smooth circular area behind the eye. Organs
of touch, smell, and taste are likewise developed in varying
degree of perfection.

174. Breeding-habits. — While the great majority of am-
phibians mate in the spring and deposit their eggs in the
water, often to the accompaniments of croakings and pip-
ings almost deafening in intensity, several species, for
various reasons, have adopted different methods. Some of
the salamanders bring forth young alive, and several species
of toads and frogs are known in which the young are cared
for by the parent until their metamorphosis is complete.
In one of the European toads (Alytes) the male winds
the strings of eggs about his body until the tadpoles are

FIG. 110.— Salamanders. The
axolotl (the larva of Am-
blystoma tigrinitrri) and
the newt (Diemyctylus to-
rosus). P^K ,

ready to hatch ; and in a few species of tree-toads the eggs
are stored in a great pouch on the back of the parent until
the early stages of growth are over. In the Surinam toad
of South America the eggs are placed by the male on the
back of the female, and each sinks into a cavity in the
spongy skin. Here they pass through the tadpole stage
without the usual attendant dangers, and emerge with the
form of the adult.

THE AMPHIBIANS 183

Sunlight and warmth are apparent necessities for speedy
development. Tadpoles kept in captivity where the con-
ditions are generally unfavorable may require years to as-
sume the adult form. As mentioned above, the tiger sala-
mander (Amblystoma tigrinum) occurs in most parts of the
United States and Mexico. In the East this species drops its
gills in early life as other salamanders do, and assumes the
adult form, but in the cold water of high mountain lakes,
in Colorado and neighboring States, it may never become
adult, always remaining as in Fig. 110. This peculiar form
is locally known as axolotl. In this condition it breeds. It
is thus one of the very few examples of animals whose un-
developed larvae are able to produce their kind. Owing to
this* trait it was at first considered a distinct species, and
many years elapsed before its relationship to the true adult
form was discovered.

CHAPTER XVI

THE REPTILES

175. General characteristics.— In all the reptiles the gen-
eral shape of the body, and to some extent the internal
plan, is not materially different from that seen among the
amphibians. In spite of external resemblance the actual
relationship is not very close. It appears to be true that
ages ago the ancestors of the modern reptiles were aquatic
animals, possibly somewhat similar to some of the sala-
manders; but they have become greatly changed, and
are now, strictly speaking, land animals. At no time in
their development after leaving the egg do we find them
living in the water and breathing by gills. Some species,
such as the turtles, lead aquatic or semiaquatic lives, but
the modifications which fit them for such an existence
render them only slightly different from their land-inhabit-
ing relatives. The skin bears overlapping scales or horny
plates, united edge to edge, as in the turtles, enabling them
to withstand the attacks of enemies and the effects of heat
and dryness. Indeed, it is when heat is greatest that rep-
tiles are most active. In no other class of vertebrates, and
very few invertebrates, do normal activities of the body
appear to be so directly dependent upon external warmth.
In the presence of cold they rapidly grow sluggish, and
sink into a dormant state.

As in the case of all animals, habits depend upon
structure, and accordingly among the reptiles we find
many remarkable modifications, enabling them to lead
184

THE REPTILES

185

widely different lives. Nevertheless all are constructed
upon much the same plan.

176. The lizards (Sauria).— As in the amphibians, es-
pecially the salamanders, the body (Fig. Ill) consists of
a relatively small head united by a neck to the trunk,

FIG. 111.— Common lizard or swift (Scelop&rus undulatus). Photograph by W. II.

FISHER.

which, in turn, passes insensibly into a tail, usually of con-
siderable length. Two pairs of limbs are almost always
present, and these exhibit the same skeletal structure as
in the amphibians; but in their construction, as in the
other divisions of the body, we note a grace of propor-
tion and muscular development which enable the lizards
to execute their movements with an almost lightning-like
rapidity. The mouth is large and slit-like, well armed with
teeth, and the eyes and ears are keen. Scales of various

186 ANIMAL FORMS

forms and sizes, always of definite arrangement, cover the
body. The scales are always colored, in some species as
brilliantly as the feathers of birds, and usually harmonize
with the surroundings of the animal, enabling it to escape
the attacks of its many enemies. Altogether the lizards
are a very attractive group of animals. As in the salaman-
ders, the vertebral column is usually of considerable length,
but it too presents a lighter appearance and a greater flexi-
bility. Slender ribs are present, and a breast-bone and the
girdles which support the limbs. Although more ossified
than in the amphibians, the skull still continues to be com-
posed here and there of cartilage. The roof also is yet
incomplete, but with the firm plates on the surface of the
head ample protection is afforded the small brain under-
neath. As above mentioned, the limbs are slender and
insufficient to support the body, which accordingly rests
upon the ground, and by its wrigglings and the pushing of
the limbs is borne from place to place. It will be recalled
that some of the salamanders living in subterranean haunts
and burrowing in the soil have no need of limbs, and the
latter have accordingly disappeared. This condition is
paralleled by certain species of lizards. The blindworms
(which are neither blind nor worms, but true lizards, though
snake-like in appearance) are devoid of limbs, as are also
the " glass-snakes." In some species the hinder pair arise
in early life, but they remain small, and ultimately disap-
pear. In almost all lizards the tail is very brittle, breaking
at a slight touch. In such case the lost member will grow
again after a time.

177. The snakes (Serpentes).— The snakes are character-
ized by a cylindrical, generally greatly elongated body, in
which the divisions into head, neck, trunk, and tail are not
sharply defined. As we have seen, this is also true of cer-
tain lizards, but the naturalist finds no difficulty in detecting
the differences between them. Another peculiarity of the
snakes is in the great freedom of movement of the bones

THE REPTILES

187

not concerned with the protection of the brain. In the
reptiles the lower jaw does not unite directly with the
skull, as in the higher animals, but to an intermediate
bone, the quadrate, which is attached to the skull. In the
snakes these unions are made by means of elastic liga-
ments. The two halves of the lower jaw are also held

FIG. 112. — Blacksnake (Bascanion constrictor). Photograph by W. H. FISHER.

together by a similar band, so that the entire palate and
lower jaw are loosely hung together. This enables the
snake to distend its mouth and throat to an extraordinary
degree, and to swallow frogs and toads but slightly smaller
than itself. Where the prey is of relatively small size, the
halves of the lower jaw alternate with each other in pulling
backward, thus drawing the food down the throat. The
food is never masticated. The teeth are usually small and
recurved, and serve only to hold the food until it may be
swallowed. The latter process is facilitated by the copious
secretion of the salivary glands, which become very active
at this time.

A further character of the snakes is the absence exter-

188 ANIMAL FORMS

nally of any trace of limbs. However, in some of the
pythons and boas hind limbs are present in the form of
small groups of bones embedded beneath the skin and ter-
minating in a claw. There thus appears to be no doubt
that the ancestors of the modern snakes were four-footed,
lizard-like creatures, which have assumed the present form
in response to the necessity of adaptation to new conditions.

More than any other order of vertebrates do the snakes
deserve the name of creeping things, and yet their method
of locomotion enables them to crawl and swim with a ra-
pidity equal to that of many of the more highly developed
animals. This depends chiefly upon certain peculiarities
of the skeleton, which consists merely of a skull, vertebral
column, and ribs. The vertebrae, usually two or three hun-
dred in number, are united together by ball-and-socket
joints, and each attaches by similar joints a pair of slender
ribs. These in turn are attached to the broad outer plates
upon which the body rests, and the whole system is operated
by a powerful set of muscles. Upon the contraction of the
muscles the ventral plates are made to strike backward
upon the ground or other rough surface, which drives the
body forward. Also, the ribs may be made to move back-
ward and forward, and the snake thus progresses like a
centiped or " thousand-legs."

178. The turtles (Chelonia). — In many respects the tur-
tles are the most highly modified of all the reptiles. The
body (Fig. 113) is short and wide and enclosed in a shell or
heavy armor, consisting of an upper portion, the carapace,
and a flat ventral plate, the plastron. The shape of the
carapace varies greatly from a low, flat shield to a highly
vaulted dome, remaining cartilaginous throughout life, as
in the soft-shelled turtles, or becoming bony and of great
strength. The two portions of the shell form a box-like
armor through whose openings may be extended the head,
tail, and limbs. As a means of protection the turtle may
retract these organs within the shell. The head is generally

THE REPTILES

189

thick-set and muscular, and provided with horny jaws
entirely destitute of teeth, like those of the birds. The
limbs also are usually short and thick and variously shaped,
and adapted for aquatic or terrestrial locomotion. The
number of vertebrae in the body and tail are relatively few,
and the thick and heavy body is devoid of the elements of
grace and agility of movement characteristic of the other
reptiles. On the other hand, the former enjoy a freedom
from the attacks of enemies not accorded to animals in
general.

At first sight the appearance of a turtle does not indi-
cate a close relationship to the other reptiles, but a more

FIG. 113. — Box -turtle (Terraptne carollr

careful examination, and especially of their development,
discloses a remarkable resemblance. The head, tail, and
limbs are essentially similar to those of the lizards, but in
the trunk region peculiar modifications have taken place.
The ribs at first separate, as in other animals, flatten
greatly, and unite with a number of bones embedded in
the skin, thus forming one great plate overlying the back
of the animal. About the circumference of the shield
other dermal or skin-bones are added, which increase the
area of the carapace, and at the same time still others have

190 ANIMAL FORMS

arisen and united on the ventral surface to form the plas-
tron. In this process the shoulder- and hip-girdles which
attach the limbs come to be withdrawn into the body, and
we have the curious example of an animal enclosed within
its back-bone and ribs. This is even more the case with
the box-turtles (Fig. 113), common in the eastern United
States, whose ventral plate is hinged so that after the
limbs, head, and tail have been withdrawn it may be made
to act like a lid to completely enclose the fleshy parts of
the body.

Scales and horny plates are present, as in other reptiles,
the former covering all parts of the body except the cara-
pace and plastron, which support the plates. In nearly all
species the latter are of considerable size, and in the tor-
toise-shell turtles are valuable articles of commerce. They
also are sculptured in a fashion characteristic of each spe-
cies, and may, like the colors of other animals, render them
more like their surroundings, and consequently incon-
spicuous.

179. Crocodiles and alligators (Crocodilia). — The alligators
(Fig. 114) and crocodiles are much more complex in struc-
ture than the lizards, though their general form is much the
same. The body is covered with an armor of thick bony
shields and horny scales. These, along the median line, are
keeled, and extending along the length of the laterally com-
pressed tail form an efficient swimming organ and rudder.
The mouth is of large size, and is bounteously supplied with
large conical teeth, which are set in sockets in the jaw, and
not fused with it, as in many of the lizards. The nose and
ears may be closed by valves to prevent the entrance of
water, and a similar structure blocks its passage beyond
the throat while the mouth is open. When large animals,
such as hogs or calves, are captured as they come to drink,
these devices enable the alligator or crocodile to sink with
them to the bottom and hold them until drowned. The
limbs, short and powerful, are efficient organs of locomo-

THE REPTILES 191

tion on land, and together with the general shape of the
body, are also well adapted for swimming.

FIG. 114.— Alligator (Alligator mississippiensis).

180. Distribution of the lizards.— In a general way the
number of reptiles is greatest where the temperature is
highest. The tropics therefore abound in species, often
of large size, and usually of bright coloration. As one
travels northward the numbers rapidly diminish, their size
is smaller, and the tints less pronounced. In all probability
not less than four thousand known reptiles exist, whose
haunts are of the most varied description.

In North America the lizards are almost exclusively
confined to the southern portions, only a very few species
extending up to the fortieth parallel. Among these the
skinks (Eumeces) are most widely distributed. The blue-
tailed skink is probably the most familiar, a small lizard
eight or ten inches in length, dark green with yellowish
streaks and a bright-blue tail. On sunny days it may
sometimes be seen darting about on the bark of trees in
search of insects, upon which it feeds.

On « of the most familiar lizards in this country is the
" glass-snake,v found burrowing in the drier soil of the
southern half of the United States east of the Mississippi.
35

192 ANIMAL FORMS

Both pairs of limbs are absent, but by wriggling movements
of the body this lizard is able to force its way through light
soil with considerable rapidity. It is a matter of some
difficulty to secure entire specimens, for with other than
the gentlest handling the tail severs its connection with
the body, as the vertebrae in this portion are extremely
brittle. This peculiarity, together with its shape, has given
it the popular name of glass-snake. Many species of liz-
ards will thus detach the tail, a habit which is a means of
protection, enabling the animal to scamper away into a
place of safety while its enemy is concerning itself with
the detached member. Later on a new tail develops,
though usually of a less symmetrical form.

181. Horned toads. — The horned toads (Phrynosoma) are
lizards peculiar to the hot, sandy deserts and plains of

FIG. 115.— Gila monster (Heloderma suspectum). One-third natural size.

Mexico and the western United States. The body is com-
paratively broad and flat, almost toad-like, and is covered
with scales and spines of brownish and dusky tint, so Iik3
dried sticks and cactus spines in form and color as to ren-
der them difficult of detection. In captivity they readily

UNIVERSITY

OF

THE REPTILES 193

adapt themselves to their new surroundings, become tame,
and feast on flies, ants, and other insects, which they cap-
ture by the aid of their long tongue. The horned toads
are perfectly harmless creatures, but when irritated some-
times perform the remarkable feat of spurting a stream of
blood from the eye toward the intruding object for a dis-
tance of several inches. This has been regarded by some
as a zoological fable ; but there are many who have watched
the horned toad in its natural state and in captivity, and
they assure us that it is a fact.

In the hot deserts of Arizona and Sonora is another
peculiar species of lizard known as the Gila monster (Hclo-
derma) (Fig. 115), having the distinction of being the only
poisonous lizard known. Further protection is afforded
by bony tubercles on the head and by scales over the
remainder of the body, all of which are colored brown or
various shades of yellow, giving the animal a peculiar
streaked and blotched appearance.

182. Distribution of the snakes. — The snakes are much
more common than the lizards. All over the United States
one meets with them, especially the garter- or water-snakes.
Of less wide distribution are the black-, grass-, and milk-
snakes, and a number of less known species, all of which
are perfectly harmless and often make interesting pets.
Some of them when cornered show considerable temper,
flatten the head and hiss violently, and imitate poisonous
forms, but venomous snakes are comparatively few in num-
ber in northern and eastern United States. In the south-
ern portions of the country they become more abundant.
Along the streams and in the swamps the copperheads, and
especially the water-moccasins, often lie in wait for frogs
and fish. Both these species are especially dreaded, as they
strike without giving any warning sound, but the name
and bad reputation of the moccasin is often, especially in
the South, transferred to perfectly harmless water-snakes.
On higher ground are the rattlesnakes (Crotalus), once

194

ANIMAL FORMS

abundant but now in many regions well-nigh exterminated.
In these species the tail terminates in a series of horny

FIG. 116.— Diamond-rattlesnake (Crotalus adamanteus\ Photograph by W. H.

FISHER.

rings that produce a buzzing sound like that of the locust
when the tail is rapidly vibrated.

183. Distribution of the turtles, — The turtles are perhaps
somewhat less dependent upon warmth than other reptiles,
yet they too delight to bask in the sunshine, and soon grow
sluggish in its absence. In all our fresh-water streams and
ponds they are familiar objects, and several species extend
up into Canada. Among the turtles the soft shell, the
painted and the snapping turtles have the widest distri-
bution, scarcely a good-sized stream or pond from the Gulf
of Mexico to Canada, and even farther north, being without
one or more representatives. All are carnivorous and vora-
cious, and the snapping turtles are especially ferocious, and
" for their size are the strongest of reptiles." In the woods
and meadows the wood-tortoise and box-turtles are occa-

THE REPTILES

195

sionally met with, and at sea several turtles exist, some of
them of great size. Among these is the leather-turtle,
found in the warmer waters of the Atlantic, lazily floating
at the surface or actively engaged in capturing food. They
attain a length of from six to eight feet, and a weight of
over a thousand pounds, and are sometimes captured for
food when they come ashore to bury their eggs in the sand.
By this same method the loggerheads, the hawkbills, and
the common green turtles are also captured in consider-
able numbers. These are of smaller size, and the second
named is of considerable value, as the horny plates cover-

FIG. 117.— Hawkbill turtle (Eretmochdys imbricata).

ing the shell furnish the tortoise-shell of commerce. These
plates are removed after the animal is killed, by soaking
in warm water or by the application of heat.

184. Food and digestive system. — Some reptiles, among
which are a number of species of lizards and the box- and
green turtles, are vegetarians, but the great majority are

196 ANIMAL FORMS

carnivorous, and usually very voracious. The lizards espe-
cially devour large quantities of insects and snails, together
with small fishes and frogs. The latter figure largely in
the turtle's bill of fare, and in that of the snakes, which
also capture birds and mammals. On the other hand, many
of the reptiles prey upon one another ; and they are the
favorite food of hawks and owls and numerous water-birds,
of skunks and weasels and many other animals, which look
for them continually. Many of the turtleSj owing to their
protective armor, and the snakes because of their poison-
ous bite or great size and strength, are more or less ex-
empt, but this is not true of their eggs and young. The
smaller species depend upon keenness of sense, agility, and
inconspicuous tints. These latter may undergo changes
according to the character of the surroundings, but usually
only to a slight extent. The chameleons of the tropics
and a similarly colored green lizard on the pine-trees in
the Southern States are able to change with great rapidity
from green, through various shades, to brown.

185. Respiration and circulation. — While still in the egg
the young lizard develops rudimentary gills, and thus bears

extna.
int

FIG. 118.— Dissection of lizard (Sceloporus). an., anal opening ; au., auricle ; crb.h.,
brain ; coec., intestine ; kd., kidney ; Ling., left lung ; lr., liver ; pn., pancreas ;
sp.c., spinal cord ; spl., spleen ; st., stomach ; v., ventricle of heart.

evidence to the fact that its distant ancestors were aquatic ;
but before hatching they disappear, and lungs arise, which

THE REPTILES 197

remain functional throughout life. Corresponding to the
shape of the body, these are usually much elongated and
ordinarily paired (Fig. 118, l.lng.). The snakes are peculiar
in having the left lung rudimentary or even lacking com-
pletely, while the right one becomes greatly elongated and
extends far back into the body. In nearly all the reptiles
the amount of oxygen brought into the lungs is relatively
large and the activity of the animal is proportionately
great. The circulation of reptiles shows an advance be-
yond that of the Amphibia. As in the latter, there are
two distinct auricles ; but the chief difference arises from
the fact that the ventricle is more or less divided by a par-
tition which to a considerable degree prevents the blood
returning from the lungs from mixing with the impure
blood as it returns from its journey over the body. In the
crocodiles and alligators the partition is complete, and the
circulation thus approaches close to that of the higher
animals.

186. Hibernation. — Attention has already been called to
the fact that reptiles are very susceptible to cold, rapidly
growing less active as the temperature lowers. When win-
ter comes on they seek protected spots, and either alone
or grouped together hibernate. The various activities of
the body during this period are at very low ebb. The blood
barely circulates, breathing is imperceptible, and stiff
and insensible to the world about them they remain until
the warmth again stirs them to their former activity.
Some of our common turtles must also pass a somewhat
similar sleep while embedded far down in the mud during
the disappearance of the ponds in summer. At such times
no food is taken, but owing to their loss in weight it is
probable that a slow consumption of the body supplies the
small amount of necessary energy.

187. Nervous system and sense-organs.— At first sight one
is struck with the small size of the brain of fishes, Am-
phibia, and reptiles. Their intelligence likewise is at low

198 ANIMAL FORMS

ebb. Almost all the movements and operations of the body
appear to be carried on by the animal with little apparent
thought. Their acts, like most of the animals below them,
are said to be instinctive ; 'yet they are sufficiently well done
to enable the animal to procure its food, avoid its enemies,
and lead a successful life. As is true of other animals, the
ability of the reptile to cope with its surroundings depends
to a great extent upon the keenness of one or all of its or-
gans of special sense. In the reptiles the sense of sight is
perhaps sharpest, but there is considerable variation in this
respect. Movable eyelids are present in most lizards, to-
gether with a third, known as the nictitating membrane, a
thin, transparent fold located at the inner angle of the eye,
over which it is drawn with great rapidity. Snakes have
no movable eyelids, hence the eye has a peculiar stare.
Furthermore, their sense of sight, except in a few tree-dwell-
ing species, appears to be defective, the majority depending
largely upon the sense of touch.

In all the vertebrates a very peculiar organ known
as the pineal gland or eye is situated on the roof of the
brain. In several lizards its position is indicated by a trans-
parent area in one of the plates of the head, and by an
opening in the bones of the roof of the skull. In young
reptiles, and especially in one of the New Zealand lizards
(Hatt&ria, Fig. 119), its resemblance to an eye is decidedly
striking. Lens, retina, pigment, cornea, are all present
much as they are in some of the snails, but they finally
degenerate more or less as the animal reaches maturity.
It is a general belief that it represents the remnant of an
organ of sight, a third eye, which looked out through the
roof of the skull in some of the ancient vertebrates.

With the possible exception of the few species of reptiles
which produce sounds, probably to attract their mate, the
sense of hearing is not particularly well developed. The
senses of smell and taste are also comparatively feeble. The
latter sense is located in the tongue, which is also popularly

THE REPTILES 199

supposed to serve for the purpose of defense, and that it is
in some way related to the poison-glands. This, however,
is an error. The tongue is used primarily as an organ of

FIG. 119.— Tuatera (Sphenodan punctatus).

touch, and in snakes especially it is almost continually
darted in and out to determine the character of the animal's
surroundings.

188. Egg-laying.— The eggs of the reptiles are relatively
large and enclosed in a shell like a bird's egg, the shell,
however, being leathery rather than made of lime. These
are deposited in some warm situation, and generally left to
themselves to hatch. Under stones, logs, and leaves, or
buried lightly in the soil, are the positions most frequently
chosen by the lizards and snakes. The turtles almost
invariably select the warm sand at the edge of the water,
and after scooping a hole lay numerous spherical eggs,
usually at night. The alligators lay upward of a hundred
eggs about the size of those of a goose, and guard them
jealously until and even after they hatch. On the other
hand, the young of many lizards and snakes are born alive,
the eggs being hatched within the body.

Many reptiles are surprisingly slow in attaining maturity,
and live to an age attained by few other animals. It is a
well-known fact that turtles live fully a hundred years, and

200 ANIMAL FORMS

probably the same is true of the crocodiles and alligators
and some of the larger snakes. Their enemies are few, and
death usually results when the natural course is run.

Throughout life all reptiles periodically shed their skin,
as birds do their feathers and mammals their fur. In the
snakes and some of the.lizards the skin at the lips loosens,
and the animal gradually slips out of its old slough, bright
and glossy in the new one which previously developed. In
the others the old skin hangs on in tatters, gradually com-
ing away as they scamper through the grass.

CHAPTEE XVII

THE BIRDS

189. Characteristics. — Birds form one of the most sharp*
ly defined classes in the animal kingdom, and the variations
among the different species are relatively small. "The
ostrich or emu and the raven, for example, which may he
said to stand at opposite ends of the series, present no such
anatomical differences as may be found between a common
lizard and a chameleon, or between a turtle and a tortoise,"
and these we know to be relatively slight.

In many respects the birds resemble the reptiles, and
long ago in the world's history the relationship was much
closer than now, as we know from certain fossil remains in
this country and in Europe. One of the earliest of these
fossil birds, the Archaeopteryx, is a most remarkable com-
bination of bird and lizard. Unlike any modern bird, the
jaws were provided with many conical reptile-like teeth.
The wings were rather small, and the fingers, tipped with
claws, were distinct, not grown together, as in modern birds.
The tail was as long as the body, and many-jointed, like a
lizard's, each vertebra carrying two long feathers. The
bird was about the size of a crow, and it probably could
not fly far. Other ancient types have been discovered —
principally sea-birds — many of which existed when the
Pacific extended over the region now occupied by the
Rocky Mountains. These were all of the same generalized
type, intermediate between reptile and bird. This fact
leads us to the belief that birds descended from reptilian

201

202 ANIMAL FORMS

ancestors, and in becoming more perfectly adapted for an
aerial life have developed into our modern forms.

In the modern birds the most important peculiarities,
those which separate them from all other animals, are
correlated with the power of flight. The body is spindle-
shaped, for readily cleaving the air. The fore limbs serve
as wings. The hind limbs, supporting the weight of the
body from the ground, are usually well developed. A series
of air-chambers usually exists in powerful fliers. This
serves a purpose analogous to that of the air-bladder of a
fish, giving buoyancy. But the most characteristic mark
of a bird, as above stated, is its feathers, universally present
and never found outside the class. Like the scales of
lizards, and probably derived from similar structures, the)'
are of different forms, and serve a variety of purposes.
The larger ones, with powerful shafts, and forming the tail,
act as a rudder. Those of the wings give great expanse
with but little increase in weight, and are so constructed
that upon the down-stroke they offer great resistance to
the air, and push the bird forward, while in the reverse
direction the air slips through them readily. In flight
these movements of the wing may be too rapid for us to
follow, as in the humming-birds, though they are usually
much slower, two to five hundred a minute in many power-
ful fliers, such as the ducks, and frequently long-continued
enough to carry them many hundreds of miles at a single
flight. The remaining feathers are soft and downy, giving
roundness to the body and enabling it to cleave the air with
greater ease, and, being poor conductors of heat, they aid in
keeping the body at the high temperature characteristic of
birds. In most birds the body is not uniformly clothed in
feathers. Naked spaces, usually hidden, intervene between
the feather tracts, and on the feet and toes scales exist.

190. Molting. — As we all know, the growth of feathers,
unlike that of hair and nails, is limited, and after they have
become faded and worn out they are shed, and new ones

THE BIRDS 203

arise to take their place. This process of molting is
usually accomplished gradually, without diminishing the
powers of flight ; but in the ducks and some other birds all
the wing- and tail-f oathers drop out simultaneously, leaving
the bird to escape its enemies by swimming and diving.
The molting-process usually takes place in the fall, after
the nesting and care for the young is over, and often when
the need for a heavy winter coat commences to be felt.
Many birds also don what are called courting colors, ruffs,
crests, and highly colored patches, in the spring, previous
to the mating season, doubtless for the purpose of attract-
ing or impressing their mates. In other cases the change
appears to be related to the bird's surroundings. A most
beautiful example of this is the ptarmigans — grouse-like
birds living far to the north. During winter they are per-
fectly white and are almost invisible against the snow ; but
in the spring, as the snow disappears, the white feathers
gradually fall out and new ones arise. The latter so har-
monize "with the lichen-colored stones among which it
delights to sit, that a person may walk through a flock of
them without seeing a single bird."

There are also numerous birds, chiefly those that go in
flocks, which possess what are known as color-calls or recog-
nition-marks. These may consist of various conspicuous
spots or blotches on different parts of the head or trunk,
such as we see in the yellowhammer or meadow-lark ; or
one or more feathers of the wings or tail may be strikingly
colored, as in many sparrows and warblers. During the
time the bird remains at rest these usually are concealed
under neighboring feathers, but during flight they are
strikingly displayed. It may possibly be true, as many
have urged, that these color-signals are for the purpose
of enabling various members of the flock to readily follow
their leader ; but this and many other interesting questions
regarding the color of birds and other animals have not yet
received final answers.

204 ANIMAL FORMS

In very many animals, fishes as well as birds, the tints
on the under side of the body are usually relatively light
colored, shading gradually into a darker tint above. This
is in all probability a protective device, as was recently
shown by Mr. A. H. Thayer, an American artist. His ex-
periments show that the light from above renders the back
less dark, and that the shadow beneath is neutralized by
the light color. The bird thus appears uniformly lighted,
and this effect, together with streaks and blotches, renders
them invisible at surprisingly short distances.

191. Skeleton. — Turning now to the internal organization
of birds, we find many points in common with other verte-
brates, especially the reptiles, but many interesting modifi-
cations are also present that adapt them for flying and for
collecting their food. According to the nature of the food,
the beak may have a great variety of forms. The skull may
be thick and heavy, or thin and fragile, but these are mat-
ters of proportion of the various parts possessed by all
birds. The neck also is of differing length ; but it is in the
trunk region that the greatest changes have arisen, as we
may see in any of our ordinary birds. For example, the
vertebrae of this part of the body are more or less fused
together into rigid framework, to which are attached the
ribs that in turn unite with the breast-bone. In the fliers
the latter bears a vertical plate or keel, to which the great
muscles that move the wings are attached. The tail con-
sists, like that of the old-fashioned birds, of several verte-
brae, but these are of small size and fused together into a
little knob that supports the tail-feathers. The fore limbs
are used for flight, but there are the same bones that exist
in the fore limbs of other vertebrates — one for the upper
arm, two for the lower, a thumb carrying a few feathers,
and known as the bastard wing, and indications of several
bones that form the hand. In the hind limb the resem-
blance is equally apparent, though its different parts are
of relatively large size to support the body. It is interest-

THE BIRDS 205

ing to note that the knee has been drawn far up into the
body, and that the joint above the foot is in reality the
ankle.

We thus see that the bird's skeleton presents the same
general plan as that of the lizard, for example ; but in order
to combine the elements of strength, lightness, and com-
pactness essential to successful flight, it has been necessary
to remodel it to a considerable degree.

192. Other internal structures,— The lungs of birds con-
sist of two dark-red organs buried in the spaces between the
ribs along the back. Each communicates with extensive
thin-walled air-sacs extending into the space between the

FIG. 120.— Anatomy of a bird. a«., auricle ;,cbl. and crb.h., cerebellum and cerebral
hemispheres (divisions of the brain) ; duo., intestine (with portion removed) ;
giz., gizzard ; kd., kidney ; r.lng , lung; tr., trachea or windpipe ; vent., ven-
tricle.

various organs, and in many birds of flight they even extend
into the bones of the body, and thus decrease their weight.
" The enormous importance of this feature to creatures
destined to inhabit the air will be readily understood when
we learn that a bird with a specific gravity of 1.30 may
have this reduced to only 1.05 by pumping itself full of air."
As we know, air is taken into the body in order that the
oxygen it contains may combine with the tissues of the
body to liberate the energy necessary for the work of its

206 ANIMAL FORMS

life. The life of birds is at high pressure, hence their need
of much oxygen. They habitually breathe deeper breaths
than other animals. The air passing into the body trav-
erses the entire extent of the lung on its way back to the
air-sacs, with the result that large quantities of oxygen are
taken into the body. This is distributed by a circulatory
system of a more highly developed type than in any of the
preceding groups of animals. The ventricles of the heart
no longer communicate with each other, and the pure and
impure blood never mingle. Furthermore, the beating of
the heart is comparatively rapid, rushing the oxygen as
fast as it enters the blood to all portions of the body. The
result is that everywhere heat is being generated, so neces-
sary to life and activity.

In the lower animals no special means are employed to
husband the energy thus produced, but in the birds the
body is jacketed in a non-conducting coat of feathers which
prevents its dissipation. For this and other reasons the
birds, summer and winter, maintain an even and relatively
high temperature (102°-110°). Like the mammals, birds
are warm-blooded animals, full of energy, restlessly active
to an extent realized in few of the cold-blooded animals.

193. Digestive system. — This life, at high pressure, de-
mands a relatively large amount of food to make good the
losses due to oxidation. The appetites of some growing
birds is only satiated after a daily meal equal to from one
to three times their own weight, and after reaching adult
size the amount of daily food required is probably not less
than one-sixth their weight. The nature of the food is
exceedingly varied, and the digestive tract and certain ac-
cessory structures are obviously modified in accordance
with it. The beak, always devoid of teeth in the living
form, varies extremely according to the work it must per-
form. The same is true of the tongue, and many correlated
modifications exist in the digestive apparatus. In the
birds of prey and the larger seed-eating species, such as the

THE BIRDS 207

pigeons and the domestic fowls, the esophagus dilates into
a crop, in which the food is stored and softened before being
acted upon by the gizzard. The latter is the stomach, pro-
vided with muscular walls, especially powerful in the seed-
eaters, and with an internal corrugated and horny lining
which, in the absence of teeth, serves to crush the food. In
some species, such as the domestic fowls and the pigeons,
this process is aided by the grinding action of pebbles
swallowed along with the food. The remaining portions,
with pancreas and liver, vary chiefly in length, and are
sufficiently shown in Fig. 120 to require no further descrip-
tion.

194. Nesting-habits.— A few birds, such as the ostriches
and terns, merely scoop a hollow in the earth, and make no
further pretense of constructing a nest. On the other
hand, some birds, such as the humming-birds and pewees,
build wonderful creations of moss, lichens, and spider-webs,
lining it with down, and concealing it so skilfully that
they are not often found. Every bird has its own particular
ideas as to the fitness of its own nest, and the results are
remarkably different, and form an interesting feature in
studying the habits of birds. Usually the female takes
upon herself the choice of the nest and its construction ;
but these duties are in some species shared by the male.
After the eggs are laid, the male may also aid in their
incubation, or may carry food to the female. In other
species — for example, the pigeons and many sea-birds — the
parents take turns in sitting upon the eggs and in the sub-
sequent care of the young. Finally, there are certain birds,
such as the cuckoo and cowbirds, which take advantage of
the industry of other species and deposit an egg or two in
the nests of the latter. All the work of incubation and
care of the young is assumed by the foster-parents, which
sometimes neglect their own offspring in their desperate
attempts to satisfy the appetites of the rapidly growing and
unwelcome guests.
36

208 ANIMAL FORMS

The eggs of birds are relatively large, and are often
delicately colored. In some species the blotches and streaks
of different shades are probably protective, as in the plovers
and sandpipers, whose eggs blend perfectly with their sur-
roundings, but many other cases exist not subject to such
an explanation.

The young require a high degree of heat for their devel-
opment, and this is usually supplied by the parent. In a
very general way the length of sitting, or incubation, is
proportional to the size of the egg, being from eleven to
fourteen days in the smaller species, to seven or eight weeks
in the ostriches. Before hatching, a sharp spine develops
on the beak, and with this the young bird breaks its way
through the shell. Among the quails, pheasants, plovers,
and many other species, the young are born with a covering
of feathers, wide-open eyes, and the ability to follow their
parents or to make their own way in the world. Such
nestlings are said to be precocial, in distinction to the alincal
young of the more highly specialized species, such as the
sparrows, woodpeckers, doves, birds of prey, and their allies,
which are born helpless and depend for a considerable time
on the parents for support.

Some of the owls, crows, woodpeckers, sparrows, quails,
etc., remain in the same localities where they are bred.
They are resident birds. Most kinds of birds, at the ap-
proach of winter, migrate toward the southern warmer
climes, some species traveling in great flocks, by day or
night, and often at immense heights. In some cases this
movement appears to be directly related to the food-supply ;
but there are many apparent exceptions to such a theory,
and it is possible that many birds migrate for other reasons.
Certain species migrate thousands of miles, along fairly
definite routes, the young, sometimes at least, guided by
the parents, which in turn appear to remember certain
landmarks observed the year before. Sea-birds, in their
journeys northward or southward, keep alongshore, occa-

THE BIRDS 209

sionally veering in to get their bearings or to rest, espe-
cially in the presence of fogs.

195. Classification. — Most zoologists make two primary
divisions of the living types of birds — those like the ostrich
with flat breast-bones, and the other the ordinary birds, in
which the breast-bone has a strong keel for the attachment
of the powerful muscles used in flight. This distinction is
not of high importance, but we may use it as a convenience
in the description of a few typical forms belonging to sev-
eral orders into which these two divisions are subdivided.

196. The ostriches, etc. (Ratitse).— From specimens in-
troduced or from pictures we are doubtless familiar with
the ostriches and with some of their relatives. The African
ostrich (Struthio camelus, Fig. 121) is the largest of living
birds, attaining a height of over seven feet, and is further
characterized by a naked head and neck, two toes, and
fluffy, plume-like feathers over parts of the body. They
are natives of the plains and deserts of, Africa, where they
travel in companies, several hens accompanying the male.
When alarmed, they usually escape by running with a swift-
ness greater than that of the horse, but if cornered they
defend themselves with great vigor by means of their
powerful legs and beaks. Their food consists of insects,
leaves, and grass, to which is added sand and stones for
grinding the food, as in the domestic fowl. The American
ostriches or rheas, are smaller ostrich-like birds, living on
the plains of South America. Their habits are essentially
the same as those of the African species.

197. The loons, grebes, and auks (Pygopodes).— The birds
in this and some of the following orders are aquatic in
their habits. All have broad, boat-like bodies, which, with
the thick covering of oily feathers, enables them to float
without effort. The legs are usually placed far back on
the body — a most favorable place for swimming, but it ren-
ders such birds extremely awkward on land. The grebes
are preeminently water-birds. The pied-billed grebe or dab-

FIG. 121.— African or two-toed ostrich (Struthio camelus). Photograph by WIL-
LIAM GKAHAM.

THE BIRDS 211

chick (Podilymbuspodiceps), for example, found abundantly
on the larger lakes and streams throughout the United
States, captures its food, sleeps, and breeds without leaving
the water. The loons living in the same situations as the
dabchick are also remarkable swimmers and divers. Of
the three species found in this country, the common loon
or diver ( Gavia imber) attains a length of three feet, and is
otherwise distinguished by its black plumage, mottled and
barred with white, which is also the color of the under
parts. The auks, murres (see frontispiece), and puffins are
marine, and, like their inland relatives, are expert swim-
mers and divers, strong fliers, and spend much of their
time on the open sea. During the breeding-season they
assemble in vast numbers on rugged cliffs along the shore,
and lay their eggs on the bare rock or in rudely constructed
nests.

198. The gulls, terns, petrels, and albatrosses (Longi-
pennes). — The birds belonging to this group are among the
most abundant along the seacoast, and several species make
their way inland, where they often breed. All are char-
acterized by long, pointed wings and pigeon or swallow-like
bodies, which are carried horizontally as the bird waddles
along when ashore. Many are excellent swimmers and
powerful fliers, especially the petrels and albatrosses, which
sometimes travel hundreds of miles at a single flight.

The gulls are abundantly represented along our coasts,
where they frequently associate in companies, usually rest-
ing lightly on the surface of the water, or wheeling lazily
through the air on the lookout for food. The terns are
of lighter build than the gulls and are more coastwise in
their habits, and are further distinguished by plunging like
a kingfisher for the fishes on which they live. Both the
gulls and terns breed in colonies, every available spot over
acres of territory being occupied by their nests, which are
usually built of grass and weeds placed on the ground.

The petrels and albatrosses are at home on the high

212

ANIMAL FORMS

seas, rarely coming ashore except at the breeding-season.
Some species of the former are abundant off our shores?
especially the stormy petrel (Procellaria pelagica) or Mother
Carey's chickens ( Oceanites oceanicus), which are often seen
winging their tireless flight in the wake of ocean vessels.
Among the dozen or so albatrosses few reach our shores.
The wandering albatross (Diomedea exulans), celebrated in
story and as the largest sea-bird (fourteen feet between the
tips of its outstretched wings), is an inhabitant of the
southern hemisphere, and only rarely extends its journeys
to more northern regions.

199. Cormorants and pelicans (Steganopodes). — The cor-
morants and pelicans are comparatively large water-birds

FIG. 122.— White pelicans (P. erythrorhynchus) and whooping-crane (Grus ameri-
cana). Photograph by W. K. FISHER.

usually abundant along the seashore and in many sections
of the United States. The cormorants or shags are glossy

THE BIRDS 213

black in color, with hooked bills, long necks, and short
wings, which give them a duck-like flight. The much
larger pelicans (Fig. 122) are at once distinguished by long
bills, from which is suspended a capacious membranous sac.
All these birds are sociable in their habits, breeding, roost-
ing, and fishing in great flocks. Their food consists of
fishes, which the shags pursue under water and capture in
their hooked beaks ; while the pelicans, diving from a con-
siderable height or swimming rapidly on the surface, use
their pouches as dip-nets. The nests, usually built of sea-
weed or of sticks, are placed on rocky cliffs or on the
ground in less elevated places.

200. Ducks, geese, and swans (Lamellirostres). — The birds
of this order, with their broad, flat, serrated beaks, short
legs, and webbed feet, are well known, for in a wild or
domesticated state they extend all over the earth. All are
excellent swimmers, many dive remarkably well, and are
strong on the wing. While a considerable number breed
within the United States, their nesting-grounds are gener-
ally farther north, and in the early spring it is not unusual
to see them migrating in flocks from their warmer winter
homes. Among the ducks, the mergansers, mallards (from
which our domestic species have been derived), the teals,
and the beautiful wood-duck remain with us the year
round, dwelling on quiet streams and shallow ponds, living
on fish, Crustacea, and seeds. In the more open waters of the
larger lakes and along the seacoast we find the canvasback,
the scaup-ducks, and the eiders (Fig. 123) which supply the
famous down of commerce. Of the few species of geese
which inhabit the United States, the Canada goose (Branta
canadensis) is perhaps the most familiar. During their
migrations to the nesting sites they fly in V-shaped flocks,
their " honks " announcing the opening of spring. The
brant (B. lernida) is also common in the eastern part of
the country, where it, like its relations, lives on vegetable
substances entirely. The swans are familiar in their semi-

FIG. 123.— American eider-duck (Somateria dresseri).

THE BIRDS 215

domesticated state, but the two beautiful wild swans found
in this country are rarely seen.

201. The herons and bitterns (Herodines). — The herons
and bitterns are also aquatic in their habits, but, unlike the
swimming-birds, they seek their food by wading. Adapting
them for such an existence, the legs and neck are usually
very long, and the bill, longer than the head, is sharp and
slender. Among the relatively few species in the United
States, the great blue heron (Ardea herodias) is widely dis-
tributed, and may often be seen standing motionless in
some shallow stream on the lookout for fish, or it may
wander away into the meadows and uplands to vary its diet
with frogs and small mammals. Even more familiar is the
little green heron or poke (Ardea virescens), which also is
seen widely over the country. The night-herons, as their
name indicates, stalk their prey by night, and during the
day roost in companies — a characteristic common to most
herons. The bitterns or stake-drivers are at home in reedy
swamps, where they live singly or in pairs, and throughout
the night, during times of migration, utter a booming noise
resembling the driving of a stake into boggy ground. As
a rule, the herons breed as they roost — in companies — build-
ing bulky platforms, usually in trees. The bitterns, on the
other hand, secrete their nests on the ground in the rushes
of their marshy home.

202. Cranes, rails, and coots (Paludicolae).— In their ex-
ternal form the cranes and rails resemble the herons, but
in their internal organization they differ considerably.
They likewise inhabit marshy lands, but usually avoid
wading, picking up the frogs, fish, and insects or plants
along the shore or from the surface of the water. The cranes
are comparatively rare in this country, yet one may occasion-
ally meet with the whooping-cranes ( Grus americana) and
sand-hill cranes (Grus mexicand), especially in the South
and West. They are said to mate for life, and annually
repair to the same breeding- grounds, where they build their

216 ANIMAL FORMS

nests of grass and weeds on the ground in marshy places.
The rails are more abundant, though rarely seen on ac-
count of their habit of skulking through the swamp
grasses. Only rarely do they take to the wing, and then
fly but a short distance, with their legs dangling awk-
wardly. Closely related to them are the coots or mud-hens
(Fulica americana), which may be distinguished, however,
by their slaty color, white bills, and lobed webs on the toes,
and consequent ability to swim. All over the United
States they may be seen resting cm the shores of lakes or
quiet streams, or swimming on the surface gathering food.
The nest consists of a mass of floating reeds, which the
young abandon almost as soon as hatched.

203. The snipes, sandpipers, and plovers (Limicolae), — The
snipes, sandpipers, and plovers are usually small birds,
widely scattered throughout the country wherever there
are sandy shores and marshes. In most species the legs
are long, and in connection with the slender, sensitive bill
fit the bird for picking up small animals in shallow water
or probing for them deep in the mud. During the greater
part of the year they travel in flocks, but at the nesting-
season disperse in pairs and build their nests in shal-
low depressions in the earth. The eggs are usually
streaked and spotted, in harmony with their surroundings,
as are the young, which leave the nest almost as soon as
hatched.

Fully fifty species of these shore-birds live within the
confines of the United States. Among these the woodcock
(Philohela minor) and snipe ( Gallinago delicata) are abun-
dant in many places inland, where they probe the moist soil
for food, and in turn are eagerly sought by the sportsman.
Even more familiar are the sandpipers and plovers, which
are especially common along the seacoast, and are also
abundantly represented by several species far inshore.
Among the latter are the well-known spotted sandpiper or
" tip-up " (Actitis macularia) and the killdeer plover

THE BIRDS

217

alitis vocifera), which inhabit the shores of lakes and
streams throughout the country.

204. Quail, pheasants, grouse, and turkey s(Gallinse).— The
quail, grouse, and our domestic fowls are all essentially

(Lophortyx californicus). Two- thirds natural size.

ground-birds, and their structure well adapts them to such
a life. The body is thick-set, the head small, and the beak
heavy for picking open and crushing the seeds and berries

218 ANIMAL FORMS

upon which they live. The legs and feet are stout, and
fitted for scratching or for running through grass and
underbrush. Protective colors also prevent detection, hut
if close pressed they rise into the air with a rapid whirring
of their stubby wings, and after a short flight settle to the
ground again. During the breeding-season the male usu-
ally mates with a number of hens, which build rough nests
in hollows in the ground, where they lay numerous eggs.
The young are precocial.

The quail or bob-white (Colinus virginianus) and the
ruffed grouse (Bonasa umbellus) occur throughout the
Eastern States. Over the same area the wild turkey
(Meleagris gallopavo) once extended, but is now almost
extinct. The prairies of the middle West support the
prairie-hen (Tympanuchus americanus), and the valleys
and mountains of the far West are the home of several
species of quails, some of which are beautifully crested.

205. Pigeons and doves (Columbse). — The pigeons and
doves belong to a small yet well-defined order, with upward
of a dozen representatives in the United States. They are
of medium size, with small head, short neck and legs, and
among other distinguishing characters frequently possess a
swollen, fleshy pad in which the nostrils are placed. In
former years the passenger-pigeon (Ectopistes migrator ius),
inhabiting eastern North America, was probably the most
common species in this country. Their flocks contained
thousands, at times millions, of individuals, which often
traveled hundreds of miles a day in search of food, to return
at night to definite roosts — a trait which enabled the hunter
to practically exterminate them. At present the mourning-
or turtle-dove (Zenaidura macroura) is the most familiar
and wide-spread of the wild forms. The domestic pigeons
are all descendants of the common rock-dove (Columba
livid) of Europe, the numerous varieties such as the tum-
blers, fantails, pouters, etc., being the product of man's
careful selection. In the construction of the nest, usually

THE BIRDS 219

a rude platform of twigs, and in the care of the young
both parents have a share. The young at hatching are
blind, naked, and perfectly helpless, and are fed masticated
food from the crops of the parents until able to subsist on
fruits and seeds.

206. Eagles, hawks, owls, etc. (Raptores). — The birds of
prey, all of which belong to this order, are carnivorous,
often of large size and great strength, and are widely dis-
tributed throughout this country. The vultures live on
carrion, some of the small hawks and owls on insects, while
the majority capture small birds and mammals by the aid of
powerful talons. In every case the beak is hooked, and the
perfection of the organs of sight and hearing is unequaled
by any other animal, man included. They live in pairs,
and in many species mate for life. As a rule, the female
incubates the eggs, and the male assists in collecting
food.

Among the vultures, the turkey-buzzard (Cathartes aura)
is most abundant throughout the United States, especially
in the warmer portions, where it plays an important part
as a scavenger. Of the several species of hawks, the white-
rumped marsh-hawk (Circus huclsonius), the red-tailed
hawk (Buteo lorealis), the red-shouldered hawk (Buteo
lineatus), and above all the bold though diminutive spar-
row-hawk (Falco sparverius) are the most abundant and
familiar. In the more unsettled regions live the golden
eagle (Aquila chrysaetus) and bald eagle (Haliaetus leuco-
cepJialus). The owls are nocturnal, and not so often seen
as the other birds of prey, yet the handsome and fierce
barn or monkey-faced :>wl (Strix pratincold), and the larger
species, such as the great gray owl (Scotiaptex cinereua),
and the beautiful snowy owl (Nyctea nyctea}, are more or
less common, and occasionally seen. Much more abundant
is the little screech-owl (Megascops asio), and in the West-
ern States the burrowing-owl (Speotyto cunicularia], which
lives in the burrows of the ground-squirrels and prairie-

220

ANIMAL FORMS

dogs. Fiercest and strongest -of the tribe is the great
horned owl (Bubo virginianus).

FIG. 125. — Golden eagle (Aquila chrysaetus).

207. Cuckoos and kingfishers (Coccyges).— Omitting the
order of parrots (Psittaci), whose sole representative in this
country is the almost exterminated Carolina parrakeet

THE BIRDS 221

(Conurus caroUnensis), we next arrive at the cuckoos and
kingfishers, which differ widely in their habits. The black-
or yellow-billed cuckoos or rain-crows are shy, retiring
birds, with drab plumage, and though seldom seen are often
fairly abundant, and are of much service in destroying
insects. Unlike their shiftless European relatives, which
lay their eggs in the nests of others birds, they build their
own airy homes in some bush or hedgerow, and raise their
brood with tender care. The belted kingfisher (Ceryle
alcyori) is also of a retiring disposition, and spends much
of its time on some branch overlooking the water, occa-
sionally varying the monotony by dashing after a fish, or
flying with rattling cry to another locality. Their nests
are built in holes in banks, and six or eight young are
annually reared.

208. The woodpeckers (Pici). — The woodpeckers are
widely distributed throughout the world, and are preemi-
nently fitted for an arboreal life. The beak is stout for
chiseling open the burrows of wood-boring insects, which are
extracted by the long and greatly protrusible tongue. The
feet, with two toes directed forward and two backward, are
adapted for clinging, and the stiff feathers of the tail serve
to support the bird when resting. Almost all are bright-
colored, with red spots on the head, at least in the males,
which may further attract their mates by beating a lively
tattoo with their beaks on some dry limb. The glossy
white eggs are laid in holes in trees, and both parents are
said to share the duties of incubation and feeding the
young. Among the more abundant and well-known species
is the yellowhammer or flicker (Colaptes auratus), which
extends throughout the United States. Somewhat less
widely distributed is the red-headed woodpecker (Melaner-
pes erythrocephalus), and the small black-and-white downy
woodpecker (Dryolates pubescens). This is often called
sapsucker, but incorrectly so, as, like all but one of our other
woodpeckers it feeds on insects. The yellow-bellied wood-

222

ANIMAL FORMS

pecker (Spliyrapicus varius) is a real sapsucker, living on
the juices of trees. A close relative of the red-headed
woodpecker, the California woodpecker (Melanerpes formi-
civorus), is renowned for its habit of boring holes in bark
and inserting the acorns of the live oak. Subsequently the
bird returns, and breaking open the acorns, devours the
grubs which have infested them, and apparently eats the
acorns also.

209. Swifts, humming-birds, etc. (Macrochires).— The birds
of this order are rapid, skilful fliers, and their wings are
very long and pointed. The feet, on the other hand, are

•!

Fro. 126.— Night-hawk (CJiordeiles mrglnlanus) on nest. Photograph by H. K. JOB.

small, relatively feeble, and adapted for perching or cling-
ing. Accordingly, the insects upon which they feed are
taken during flight by means of their open beaks. The
night-hawk (CJiordeiles virginianus), roosting lengthwise on
a branch by day, at nightfall takes to the wing, and high
in the air pursues its food after the fashion of a swallow.
In the same haunts throughout the United States the whip-

THE BIRDS 223

poorwill (Antrostomus vociferus) occurs, sleeping by day,
but active at night. Neither of these birds constructs nests,
but lays its streaked and mottled eggs directly on the
ground. The chimney- swifts (Chcetura pelagica), swallow-
like in general form and habits, but very unlike the swallows

FIG. 127.— Anna hummers (one day old), showing short bill and small size of body.
Compare with last joint of little finger.

in structure, frequent hollow trees or unused chimneys, to
which they attach their shallow nests. The nearly related
humming-birds are chiefly natives of tropical America, only
a few species extending into the United States. Of these
the little, brilliantly colored, and pugnacious ruby throat
(Trochilus colubris) is the most widely distributed. Its
nest, like that of other hummers, is composed of moss and
lichens bound together with cobweb and lined with down.

210. Perching birds (Passeres). — The remaining birds,
over six thousand in all, belong to one order, the Passeres
or perch ers. They are characterized by great activity,
interesting habits, frequently by exquisite powers of song,
and in addition to several other structural arrangements
have the feet adapted for perching. Their nesting habits
37

224: ANIMAL FORMS

differ widely, but in every case the young are helpless at
the time of hatching, and require the care of the parents.

The perchers constitute the greater number of the birds
living in the meadows and woods, and are more or less

FIG. 128.— Anna hummer (Calypte anna) on nest.

common, and consequently familiar everywhere. Among
the families into which the order is divided that of the fly-
catchers (Tyrannidce), the crows and jays (Corvidce), the
orioles and blackbirds (Icteridai), the finches and sparrows
(Fringillidce)) the swallows (Hirundinidce), the warblers
{MniotiltidcB), the thrushes, robins, and bluebirds (Turdidce),
are the more familiar, though the others are equally inter-
esting.

CHAPTEK XVIII

THE MAMMALS

211. General characteristics. — The mammals, constituting
the last and highest class of the vertebrates, comprise such
forms as the opossum and kangaroo, the whales and por-
poises, hoofed and clawed animals, the monkeys and man.
All are warm-blooded, air-breathing animals, having the
skin more or less hairy. The young are born alive, except
in the very lowest forms, which lay eggs like reptiles, and
for some time after birth are nourished by milk supplied
from the mammary glands (hence the word mammals) of
the mother. The skeleton is firm, the skull and brain
within are relatively large, and, with few exceptions, four
limbs are present.

Most of the mammals inhabit dry land. A number,
however, such as the whales and seals, are aquatic ; while
others, such as the beavers, muskrats, etc., though not
especially adapted for an aquatic life, are, nevertheless,
active swimmers, and spend much of their time in the
water.

Mammals tend to associate in companies, as we may
witness among the ground-squirrels, prairie-dogs, rats,
mice, and the seals and whales. In many cases they band
for mutual protection, and often fight desperately for one
another. Claws, hoofs, and nails are efficient weapons, and
spiny hairs, as on the porcupines, bony plates, such as
encircle the bodies of the armadillos, and thick skin and
hair, serve as a protection. The hair is also frequently
colored to harmonize the animal with its surroundings.

225

226 ANIMAL FORMS

Some rabbits and hares in the far north don a white coat
in the winter season.

212. Skeleton. — As in other vertebrates, the external
form of mammals is dependent in large measure upon the
internal skeleton. This consists of relatively compact
bones, the cavities of which are filled with marrow. Those
forming the skull are firmly united, and, as in other verte-
brates, afford lodgment for several organs of special sense
and for the brain, which, like that of the birds, completely
fills the cavity in which it rests. The vertebral column to
which the skull is attached differs considerably in length,
but it invariably gives attachment to the ribs, and to the
basal girdles supporting one or two pairs of limbs. Gener-
ally speaking, the number of bones in the head and trunk
of all mammals is the -same, so the variations we note in
the species about us, for example, are simply due to differ-
ences of shape and proportion. As we are aware, there is a
great dissimilarity between the length of the neck of man
and that of the giraffe, yet the number of bones in each
is precisely the same. On the other hand, the variations
occurring in the limbs are often due to the actual disap-
pearance of parts of the skeleton. Five digits in hand
and foot is the rule, and yet, as we well know, the horse
walks on the tip of its middle finger and toe, the others
being represented by small, very rudimentary, splint bones
attached far up the leg. The even-hoofed animals walk on
two digits, two smaller hoofed toes being often plainly
visible a short distance up the leg, as in the pig. In the
whales the hind limbs have completely disappeared, and in
the seals, where the fore limbs are modified, as in the
whales, into flippers, the hind limbs show many signs of
degeneration.

213. Digestive system. — Some mammals, such as man,
monkeys, and pigs, are omnivorous ; others, like the cud-
chewers and gnawers, are vegetarians ; and still others,
like the foxes, weasels, and bears, are carnivorous. In

THE MAMMALS 227

every case the food substances are acted on by a digestive
system constructed on the same general plan as that in man,
yet modified according to the specific work it is required to
perform. The teeth especially afford a valuable indication
of the animal's feeding habits, and, as we may notice later,
are also of much value in classification. They consist of
incisors used in biting, canines for tearing, and premolars
and molars for crushing and grinding.

The remaining portions of the digestive tract, esopha-
gus, stomach, and intestine, with their appended glands, are
usually not unlike those possessed by the squirrel (Fig. 1).
The chief differences are in the size of the various regions.
The stomach, for example, may be long and slender or of
great dimensions, and its surface may further be increased
by several lobes, which are especially well developed in the
ruminants or cud-chewers. The intestine, relatively longer
in the mammals than in any other class of vertebrates, also
exhibits great differences in length and size. In the flesh-
eating species its length is about three or four times the
length of the body, while in the ruminants it is ten or
twelve times the length of the animal.

214. Nervous system and sense-organs. — As before noted,
the nervous system of mammals is characterized by the
large size and great complexity of the brain. Even in the
simpler species the cerebral hemispheres (large front lobes
of the brain, Fig. 1) are well developed, and in the higher
forms of the ascending series they form by far the larger
part of the brain. The sense-organs also are highly de
veloped, and are constructed and located much as they are
in man. The greatest variations occur in the eyes. In
some of the burrowing animals they are usually small, and
in some of the moles and mice may even be buried beneath
the skin and very rudimentary. On the other hand, they
are large and highly organized in nocturnal animals ; more
so, usually, than in those which hunt their prey by day.
The ears also have different grades of perfection, which

228 ANIMAL FORMS

appear to be correlated with the habits of the animal.
Among the species of subterranean habits the sense of hear-
ing is largely deficient ; but, on the other hand, it is ex-
ceedingly keen in the ruminants, and enables them to detect
their enemies at surprisingly great distances. In these
creatures the outer ears are of large size and great mobility,
and, placed as they are on the top of the head, serve to con-
centrate the sound-waves on the delicate apparatus within.
In the mammals the sense of smell reaches its highest de-
velopment, especially among the carnivores which scent their
prey. On the other hand, it is said to be absent in the
whales and very deficient in the seals. The sense of taste,
closely related to that of smell, is located in taste-buds on
the tongue, and is also more acute than in any other class of
animals. The sense of touch, located over the surface of
the body, is especially delicate on the tips of the fingers,
the tongue, and lips, which often bear long tactile hairs,
called whiskers or vibrissce.

215. Mental qualities,— Correlated with the high degree
of perfection of the brain and sense-organs the mammals
show a higher degree of development of the intellectual
faculties than any other class of animals. In many cases
their acts are instinctive, and not the result of previous
training and experience. Just as the duck hatched in an
incubator instinctively takes to the water and pecks at its
food, or as the bee builds its symmetrical comb, many of
the mammals perform their duties day by day. On the
other hand many other mammals are also undoubtedly in-
telligent. They possess the faculty of memory ; they form
ideas and draw conclusions ; they exhibit anger, hatred, and
self-sacrificing devotion for their companions and offspring
that is different from that in man only in degree and not
in kind. In fact, intelligence differs from instinct primar-
ily in its power of choice among lines of action.

216. Classification.— Of the eleven orders into which the
mammals have been divided eight are represented in this

FIG. 129.— Three-toed sloth (Bradypus). About one-tenth natural size.

230 ANIMAL FORMS

country. Of the other three the first (Monotremes) and
simplest of the eleven is represented by the duck-mole

PIG. 130.— Australian duck-mole (Ornithorhynchus paradoxus). One-fifth natural

size.

(Ornithorhynchus) living in the Australian rivers. Its
general appearance and mode of life are illustrated in

THE MAMMALS

231

Fig. 130. The monotremes are the only mammals which
lay eggs- One or two eggs are laid in a carefully con-
structed nest where the young are hatched. Another order
(Edentata) includes a number of South and Central Ameri-
can forms, among which are the ant-eaters, armadillos,
and tree-inhabiting sloths (Fig. 129). Still another order
(Sirenia) includes the fish-shaped marine dugong and sea-
cows or manatees, of which one species is found occasion-
ally on the Florida coast. The remaining orders are de-
scribed in the succeeding sections.

217. The opossums and kangaroos (Marsupialia). — The
lowest order of mammals represented in the United States

FIG. 131.— Opossum (Didelphys virginiand). One- tenth natural size. Photograph
by W. H. FISHER.

is that of the marsupials. It includes the opossums and
kangaroos, together with a number of comparatively small
and unfamiliar animals living chiefly in and about Australia.

232 ANIMAL FORMS

The opossums, fairly abundant throughout the warmer
portions of this country, are rat-like creatures, with scaly
tails, yellowish-white fur, large head, and pointed snout.
Except at the breeding season they lead solitary lives,
sleeping in the holes of trees by day and at night feeding
on roots, birds, and fruits.

The kangaroos, familiar from specimens in menageries
or museums, chiefly inhabit the plains of Australia. The
giant gray kangaroos (Macropus giganteus), attaining a
height of over six feet, go in herds, and owing to the great
development of their hind limbs and tails are able, when
alarmed, to travel with the swiftness of a horse. Several
smaller species, some no larger than rabbits, live among
the brush, and like their larger relatives crop the grass and
tender herbage with sharp incisor teeth.

While the marsupials do not lay eggs as does the duck-
mole, they allow them to develop within the body for a
very short time only. Hence the young, when born, are
scarcely more than an inch in length, and are blind, naked,
and perfectly helpless. At once they are placed by the
mother in the pouch of skin, or marsupium, on the under
side of her body. In this the young are suckled and pro-
tected until able to gather their own food and fight their
own way.

218. Rodents or gnawers (Glires). — The rodents are a
large group of mammals, including such forms as the rats,
mice, squirrels, gophers, and rabbits. They are readily dis-
tinguished by their clawed feet adapted for climbing or
burrowing, and by large curved incisor teeth. Unlike
ordinary teeth, they grow continually, and, owing to the
restriction of the hard enamel to their front surfaces, wear
away behind faster than in front, thus producing a chisel-
like cutting edge.

The largest of our native rodents is the porcupine
(Erethizon dorsatus), which ranges from Maine to Mexico,
and attains a length of nearly three feet. Many of the hairs

THE MAMMALS 233

of the body have the form of stiff, barbed spines (Fig. 132),
readily dislodged so that the animal requires no other wea_
pon of defense. The rabbits and hares are of smaller size,
and the cottontails especially are widely distributed. West
of the Mississippi the jack-rabbits are familiar, and are

Fro. 132.— Porcupine (Hystrix cristatd). One-tenth natural size.— After BREITM.

famous for their great speed. Like the porcupines, they
feed on leaves and grass, and are often very destructive.
The mice, especially the field and white-footed mice, are
abundant in woodland and meadow throughout the United
States. The house-mouse (Mus musculus) is a native of
Europe, as is the common rat (M. decumanus), which was
imported over a century ago. The wood-rat (Neotoma)^
however, is native, and may be found in many localities
from east to west. The muskrat (Fiber zibetliicus), beaver
(Castor canadensis), and woodchuck (Arctomys monax) were
also more or less plentiful formerly, but in many localities
are well-nigh exterminated. The squirrels, on the other
hand, continue to exist in large numbers. The prairie-

234 ANIMAL FORMS

dogs, ground-squirrels, and chipmunks of the terrestrial
species are of frequent occurrence, and of the tree-dwellers
the fox, gray and red squirrels are well known in many
sections of the United States.

219. Insect-eating mammals (Insectivora). — The shrews
and moles belonging to this order are representatives of a
large group of small animals, which, unlike the major
number of rodents, live on insects. The shrews, of which
there are several species in this country, are small, mouse-
like creatures, nocturnal in their habits, and hence rarely
seen. The moles are of much larger size, and owing to
their burrowing proclivities scarcely ever appear above
ground, but excavate elaborate burrows with their shovel-
like feet, devouring the insects which fall in their way. The
common mole (Scalops aquations) extends from the eastern
seaboard to the Mississippi River, where it is replaced by
the prairie-mole (S. argenteus), which extends far to the
west, into a country inhabited by other species.

220. The bats (Cheiroptera).— The bats are also insectiv-
orous, but their habits are widely different from those of
the shrews and moles. The forearm and the fingers of the
fore limbs are greatly elongated, and are connected by a
thin papery membrane, which also includes the hind limbs
and tail, arid serves as an efficient organ of flight. During
the day they remain suspended head downward in some
dark cranny, awakening at nightfall to capture flying
insects. Several species are found in this country, the
most common being the little brown bat ( Vespertilio fus-
cus), with small, fox-like face, large erect ears, and short
olive-brown hair. The red bat (Lasiurus lorealis) is also
plentiful everywhere throughout the United States, and is
distinguished from the preceding by its somewhat larger
size and long reddish-brown fur.

221. The whales and porpoises (Cete).— The animals
belonging to this order, the whales (Fig. 133), porpoises, and
dolphins, are aquatic animals bearing a resemblance to fishes

THE MAMMALS

235

only in external form. The cylindrical body has no distinct
neck, the comparatively large head uniting directly with

the cylindrical body, which terminates in the tail with hori-
zontally placed fins. No external signs of hind limbs exist,

236 ANIMAL FORMS

while the fore limbs are short and capable of being moved
only as a whole. External ears are also absent. The eyes
are exceedingly small, those of individuals attaining a length
of from fifty to eighty feet, being in some species, at least,
but little larger than those of an ox. These are often placed
at the corners of the mouth. The nasal openings, often
known as blow-holes, are situated on the forehead, and as
the whale comes to the surface for air afford an outlet for
the stream of breath and vapor often blown high in the
air — a process known as spouting. In some of the whales,
such as the dolphin, porpoise, and sperm-whales, the teeth
persist throughout life, but in most of the larger species
they never " cut " the gum, but early disappear, and their
place is taken by large numbers of whalebone plates with
frayed edges which act as strainers. The smaller-toothed
forms (porpoises, dolphins, and several species of grampus)
are frequently seen close to the shore, where they are usu-
ally actively engaged in capturing fish. On the other hand,
the larger species, such as the humpback, right whale, and
sulfurbottom, not uncommon along our coasts, especially
to the northward, live on much smaller organisms. With
open mouth these whales swim through the water until
they collect a sufficient quantity of jelly-fishes, snails, and
Crustacea, then closing the mouth strain out the water
through the whalebone fringes and swallow the residue.

As noted above, the animals of this order are almost
wholly devoid of hair, but the heat of the body is retained
by a thick layer of fat beneath the skin. This " blubber "
also gives lightness to the body (as do the voluminous lungs),
and, furthermore, yields large quantities of oil, which in
former times made " whale-fishing " a profitable industry.
The whales bear one, rarely two offspring, which are solicit-
ously attended by the mother for a long time. The smaller
species grow to a length of from five or eight feet (por-
poises, dolphins) to twice this size (grampuses) ; while the
larger whales, by far the largest of animals, range from

THE MAMMALS 237

thirty to over a hundred feet in length with a weight of
many tons.

222. Hoofed mammals (Ungulata).— The order of hoofed
animals or ungulates includes a large number of forms like
the zebra, elephant, hippopotamus, giraffe, deer, and several
other wild species, some of which are domesticated, such
as horses, sheep, goats, and cattle. All of these animals
walk on the tips of their toes, and the claws have become
developed into hoofs. The order is divided into the odd-
toed forms (perissodactyls), such as the rhinoceros with
three toes and the horse with one, and the even-toed (artio-
dactyls), as the pigs with four, and the ox, deer, etc., with
two toes. . The even-toed forms are again divided into
those which chew the cud (ruminants) and those which do
not (non-ruminants). Xo living native odd-toed mammal
exists in this country, and of the wild even-toed species all
are ruminants. In the members of this latter group the
swallowed food passes into a capacious sac (the paunch), is
thoroughly moistened, and passed into the second division
(the honeycomb), later to be regurgitated and ground by
the powerful molars. It is then reswallowed, and under-
goes successive treatment in the other two divisions of
the stomach (the manyplies and reed) before entering the
intestine.

Among the North American ruminants, the deer fam-
ily ( Cervidce) is the best represented. In the more unsettled
regions of the East the red deer is still common, and the
same may be said of the white-tailed, black-tailed, and
mule-deer of the West. Among the woods and lakes to
the northward live the reindeer and caribou, and the largest
of the deer family, the moose, which attains the size of the
horse. Of nearly the same size is the wapiti or elk, whose
general characters are shown in Fig. 134. In all of the
above-mentioned species the horns, if present, are confined
to the male (except in the reindeer), and are annually shed
after the breeding season.

THE MAMMALS 239

The native hollow-horned ruminants (Bovidce) are at
present confined to the Western plains, and comprise the
pronghorn antelope (Antilocapra americana), the wary big-
horn or Eocky Mountain sheep (Ovis canadensis), living in
mountain fastnesses, and the buffalo or bison (Bison ameri-
canus). All of these species were formerly abundant,
especially the pronghorn and buffalo, which roamed the
plains by thousands, but their extermination has been
nearly complete, small herds only persisting in a few wild,
inaccessible regions, or protected in parks.

Our domestic sheep and cattle are probably the descend-
ants of several wild species living in Europe and other
portions of the world. Of the domesticated ungulates the
horse is the direct descendant of Asiatic wild breeds ; while
the pig traces its ancestry back to the wild boar (Sus scrofa)
of Europe, and probably a native species (S. indicus) of
eastern Asia.

223. Flesh-eating mammals (Ferae). — The order of Ferce
or Carnivora is typically exemplified by such animals as the
lions, tigers, bears, dogs, cats, and seals, forms which differ
from all other mammals by the large size of the canine teeth
(often called dog-teeth) and the molars, which are adapted
for cutting, not crushing. The limbs, terminated by four
or five flexible digits, bear well-developed claws, which, to-
gether with the teeth, serve for tearing the prey. While
the bears shuffle along on the soles of their feet, the greater
number of species, as illustrated by the dog and cat, tread
noiselessly on tiptoe. Almost all are fierce and bold, with
remarkably keen senses and quick intelligence, and are the
dreaded enemies of all other orders of mammals.

The largest land-inhabiting carnivora are the bears, of
which the brown or cinnamon bear ( Ursus americanus),
inhabiting North America generally where not extermi.
nated, and the huge grizzly ( Ursus horribilis) of the West-
ern mountains, are the best-known species. The former
lives on berries and juicy herbs, while the grizzly prefers
38

240

ANIMAL FORMS

the flesh of animals which it kills. The raccoon (Fig. 135)
(Procyon lotor) is found in wooded districts all over the
United States, and its general appearance and thieving
propensities are well known. Almost everything is accept-

able as an article of food, and its fondness for poultry and
vegetables makes it an unmitigated nuisance. The otters,
skunks, badgers, wolverenes, sables, minks, and weasels, while
differing considerably in general appearance and habits, nev-

THE MAMMALS 241

ertheless belong to one family (the weasel family, Mustelidce),
and are more or less valued for their fur. Almost all are
characterized by a fetid odor, especially the skunk, which
is notoriously offensive, and in consequence is avoided by
all other animals.

The dog family is represented by several widely distrib-
uted varieties of the red fox ( Vulpes pennsylvanicus) and
gray fox ( Urocyon cinereo-argentatus), and by the coyotes

FIG. 136.— Silver fox (Vulpes pennsylvanicus, var. argentatus). Photograph
by W. K. FISHEK.

(Canis latrans) and wolves (Canis nubilus). The domestic
dog (Canis familiaris) is probably the descendant of the
wolf, and owing to man's careful breeding during thou-
sands of years has formed several widely differing varieties.
The cat family, comprising the most powerful, savage,
and keenest-scented carnivora, is represented by the lion,
tiger, jaguar, and hyena. In this country the group is
represented by the lynx (Lynx canadensis), the wildcat
(Lynx rufus),' and the panther or puma (Felis concolor),
which attains the length of nearly five feet. The domestic
cat has, like the dog, been domesticated for centuries, and
has possibly descended from an African species (Felis

FIG. 137.— Panthers (Felis concolor) and peccaries (Dicotyles torquatus).

THE MAMMALS

243

caff r a), which was held sacred by the Egyptians, who em-
balmed them by thousands.

224:. Man-like mammals (Primates).— The last and high-
est order of mammals, the Primates, includes the lemurs,
monkeys, and man. The first of these are strange squir-
rel-like forms living chiefly in trees in Madagascar and
neighboring regions where they feed on insects. The apes
and monkeys are divided into Old and New World forms,
which differ widely from each other. The American species
are marked by flat noses, with the nostrils far apart. All are
arboreal, many have long prehensile tails, and the digits bear
nails, not claws. Among them are several species of marmo-
sets, the howling monkeys (Myocetes), the spider-monkeys
(Ateles), and the capuchins (Cebus), all of which are more or
less common in captivity. In the Old World apes, on the
other hand, the nostrils are close together and are directed
downward, the tail is never
prehensile, and in some cases
is rudimentary,and may even
disappear. The lowest spe-
cies (the dog-like apes) in-
clude the large, clumsy ba-
boons, among them the fa-
miliar blue-nosed mandrill
(Cynocsphalus maimon) and
several other species of light-
er frame, such as the long-
tailed monkey (Cercopithe-
cus) (Fig. 139), the tailless
Macacus, common in menag-
eries, and the Barbary ape, in-
habiting northern Africa and

extending Over into Spain. FIG. 138.— Baby orang-utan. From life.

The remaining anthro-
poid or man-like apes include the gibbons (Hylolates), orang-
utan (Simia), gorilla (Gorilla), and chimpanzee (Anthropo-

FIG. 139. — A monkey ( Cercopithecus) in a characteristic attitude of watchfulness.

THE MAMMALS

245

pithecus). The gibbons, inhabiting southeastern Asia, pos-
sess arms of such length that they are able to touch their
hands to the ground as they stand erect. They are thus
adapted for a life in the trees, where they spend most of their
time feeding on fruit, leaves, and insects. In the same dis-
trict the orang occurs, walking when on the ground on its
knuckles and the sides of its feet. It prefers the life in
the trees, however, in
which it builds nests
serving for rest and
concealment. The go-
rilla (Fig. 140), the
largest of apes, attain-
ing a height of over
five feet and a weight
of two hundred
pounds, is a native of
Africa, where it lives
in families and sub-
sists on fruits. The
same region is the
home of the chimpan-
zee, which in its vari-
ous characteristics ap-
proaches most nearly
to man.

Man (Homo sapi-
ens) is distinguished

by the inability to oppose the big toe as he does his thumb —
a feature associated with his erect position — and by the rela-
tively enormous size of the brain. Even in an average four-
year-old child or an Australian bushman the brain is twice as
large as in the gorilla. With this relatively great develop-
ment of the nervous system is correlated superior mental
faculties, which together with social habits and powers of
speech exalt man to a position far above the highest ape.

FIG. 140.— Gorilla (Gorilla).

246 ANIMAL FORMS

As usually understood, the family of man (Hominidce)
contains but a single species, cosmopolitan and highly vari-
able. This species is "now split up into many subspe-
cies or races, the native man of this continent, or ' Ameri-
can Indian,' being var. americanus. Other races now
naturalized in America are : the Caucasian race, var. euro-
pceus\ the Mongolian race, var. asiaticus\ and the negro
race, afer. The first of these is an immigrant from Europe,
the second from Asia, and the third was brought hither
from Africa by representatives of var. europceus to be used
as slaves."

CLASSIFICATION OF ANIMALS

The following table of classification is designed to show the systematic position
of the more important animals mentioned in the text.

ANIMAL KINGDOM

ONE-CELLED ANIMALS:

BRANCH I. PROTOZO'A (protos, first ; zoon, animal)
Class I. Rhizop'oda (rhiza, root ; pous, foot).

Amce'ba, Difflu'gia.

Class II. Infuso ria (organisms found in infusions).
Order 1. Flagella'ta (flagellum, a whip).

Euglen'a, Codosig'a, Pandori'na, Vol'vox.
Order 2. Cilia'ta (cilium, an eyelash).
Paramce'cium, Vorticel'la.

MANY-CELLED ANIMALS (Metazoa):

BRANCH II. PORIF ERA (porus, pore ; fero, to carry)
Class I. Forifera (or sponges).

BRANCH III. COELENTERA TA (animals with combined body and

stomach cavity)
Class I. Hydrozo'a (hydra, water-serpent ; zoon, animal).

Hy'drOf Gfonionemus, Hydractin'ia, Portuguese man-of-war.
Class II. Scyphozo'a (scyphos, cup ; zoon, animal).

Rhlzos 'toma, Halidys'tus.
Class III. Actinozo'a (actis, a ray ; zoon, animal).

BRANCH IV. PLATYHELMIN'THES * (platus, flat ; helminthos,

a worm)

Class I. Plato'da (platus, flat; eidos, likeness).
Plana'ria, Leptdpla'na.

* For purposes of convenience, the flatworms (Platyhelminthes),
roundworms (Nematelminthes), and segmented worms (Annelids) are
combined in one branch (The Worms) in the text.

247

248 ANIMAL FORMS

Class II. Tremato da (trematodes, pierced with holes, from the
erroneous belief that the suckers are holes into the body).

Liver fluke, Epidel'la.
Class III. Cesto'da (cestos, a girdle ; eidos, likeness).

Tapeworm.

BRANCH V. NEMATELMINTHES (nema, thread; helminthos, a

worm)
Class I. Nemato'da.

Vinegar eel (Anguillula), Trichl'na, horsehair snake (Gordius).

BRANCH VI. NEMERTIN E A (nemertes, a sea-nymph)
BRANCH VII. ROTIF ERA (rota, a wheel; fero, to carry)

BRANCH VIII. ANNELIDA (annelus, a ring)
Class I. Cheet5poda (chaite, bristle; pous, foot).
Order 1. Polychae'te (polus, many ; chaite, bristle).

Ner'eis, Poly no' e, Ser'pula, Sabel'la.
Order 2. Oligochae te (oligos, few ; chaite, bristle).

Earthworm (Lum'lricus).
Class II. Hirudin'ea (hirudo, a leech).

Leeches.

Class III. Gephyrea (gephura, a bridge, because these animals were
once supposed to bridge the gap between the worms and sea-
cucumbers).

BRANCH IX. MOLLUSCOI DA

Class I. Pblyzo'a (polus, many ; zoon, animal — colonial animals).

Polyzoa, sea-mats.
Class II. Brachiop'oda (brachion, arm ; pous, foot).

Lamp-shells (brachiopods).

BRANCH X. MOLLUS'CA (mollis, soft)
Class I. Lamellibranchia'ta (lamella, a plate ; branchia, gill).

Clams, mussels, oysters, ship-worm (Tere'dd).
Class II. Gastrbp'oda (gaster, stomach ; pous, foot).

Snails, slugs, armadillo snails, naked snails, nudibranchs.
Class III. Cephalbp'oda (cephale, head ; pous, foot).

Squids, cuttlefishes, devil-fishes (Oc'topus), nautilus.

BRANCH XI. ECHINODER'MATA (echinos, a hedgehog; derma,

skin)

Class I. Asteroi'dea (aster, star; eidos, likeness).
Starfishes.

CLASSIFICATION OF ANIMALS 249

Class II. Ophiuroi'dea (ophis, serpent; oura, tail; eidos, likeness).

Serpent- or brittle-stars, basket-stars.

Class III. Holothuroi dea (holothurion, a kind of water polyp;
eidos, likeness).

Sea-cucumbers.
Class IV. Crinoi dea (crinon, lily ; eidos, likeness).

Sea-lilies or crinoids.
Class V. Echinoi dea (echinos, hedgehog; eidos, likeness).

Sea-urchins.

BRANCH XII. ARTHROP ODA (arthron, joint ; pous, foot)
Class I. Orusta'cea (crusta, a crust or shell).

Fairy-shrimp (Brdnchip'us), water-fleas (DapJi 'nia), cop'epod,
Cy' clops, goose barnacle, acorn barnacle, Saccull'na, opossum-
shrimp, prawn, lobster, crayfish, cancer-crab, rock-crab, pill-
bug or i'sopod, sand-hopper or amphi'pod.
Class II. Onyc5ph'ora (onyx, claw ; pJiero, to carry).

Pertp'atus).
Class III. Myriop oda (myrios, numberless; pous, foot).

Cent'iped, thousand-legs.

Class IV. Insec'ta (insectum, cut in, owing to the grooves surround-
ing the body).

Fishmoth, springtail, cockroach, grasshopper, cricket, katydid,
locust, dragon-fly, caddis-fly, may-fly, white ants or termites,
ant-lion, water-boatman, water-bug, back-swimmer, chinch-
bug, squash-bug, lice, plant-lice, Phyllox' era, scale-insect,
gnat, mosquito, flea, house-fly, stag-beetle, wood-beetle, water-
beetle, potato-beetle, ladybug, firefly, moth, butterfly, ants,
honey-bees and bumblebees, wasps, hornets, yellow-jackets.
Class V. Arach nida (arachne, spider).

Garden-spider, taran'tula, bird-spider, trap-door spider, mite,
tick, king-crab or horseshoe crab.

BRANCH XIII. CHORDATA (chorda, a cord, referring to the

notochord)

SUBBRANCH I. Adelbchcr' da. Class Adelochorda.
SUBBRANCH II. Urochor'da. Class Urochord-

Sea-squirts, Tunica'ta, Ascid'ians.
SUBBRANCH III. Vertebra ta (vertebratus, jointed).

Division A. Acra'nia (a, without; cranion, skull). Class Lepto-

cardii.

Lancelet (BranchiSs'toma = Amphiox'us).
Division B. Crania'ta.

250 ANIMAL FORMS

Class I. Cyclostbm'ata (cyclos, circle ; stoma, mouth).
Hagfishes, lamprey.

Class II. Pis'ces (piscis, fish).

Shark, skate or ray, lung-fish, sturgeon, garpike, catfish, horned
pout, bullhead, carp, dace, chub, minnow, eel, herring, shad,
salmon, trout, pike, stickleback, blindfish, sea-horse, mullet,
flying-fish, perch, darter, sunfish, sea-bass, mackerel, snapper,
grunt, weakfish, bluefish, rose-fish, gurnard, sculpin, codfish,
flounder, angler.

Class III. Amphibia (amphi, double; bios, life).

Siren, mud-puppy, water-dog, tiger salamander, axolotl, toad,
frog, tree-frog.

Class IV. Reptil'ia (reptans, creeping).

Skink, " glass-snake," swift, chameleon, horned toad, Gila mon-
ster, blacksnake, grass-snake, milk-snake, rattlesnake, copper-
head, water-moccasin, soft-shell turtle, snapper, painted turtle,
box-turtle, leather-turtle, loggerhead, hawkbill, crocodile, alli-
gator.

Class V. A'ves (avis, bird).

Ostrich, loon, grebe, auk, murre, puffin, gull, tern, petrel, alba-
tross, cormorant, pelican, duck, goose, swan, heron, bittern,
crane, rail, mud-hen or coot, snipe, woodcock, sandpiper, kill-
dee plover, quail, grouse, wild turkey, prairie-chicken, pigeon,
dove, eagle, hawk, owl, turkey-buzzard, cuckoo, kingfisher,
woodpecker, sapsucker, swift, humming-bird, night-hawk,
whippoorwill, crow, jay, swallow, warbler, thrush, robin,
bluebird.

Class VI. Mammalia (mamma, breast).

Duck-mole, ant-eater, sloth, armadillo, sea-cow, opossum, kanga-
roo, porcupine, mouse, rat, muskrat, woodchuck, beaver, rabbit,
squirrel, chipmunk, prairie-dog, shrew, mole, bat, whale, gram-
pus, dolphin, porpoise, zebra, elephant, giraffe, deer, antelope,,
goat, sheep, horse, cow, pig, buffalo, bear, raccoon, otter, skunk,
badger, wolverene, sable, mink, weasel, dog, fox, wolf, cat,
lynx, panther, monkey, ape, baboon, gibbon, orang-utan,
gorilla, chimpanzee, man.

INDEX

Acipenser sturio (illus.), 162.

Acorn-barnacle (illus.), 97.

Air-bladder, 155.

Albatross, 212.

Alligator mississippiensis (illus.),

191.

Alternation of generations, 35.
Altricial, 208.
Amoeba (illus.), 12 ; structure and

ha'bits, 12.

Amoeba-like protozoa, 12.
Amphibian, development of, 174 ;

distribution, 178; anatomy and

habits, 179.
Amphibious (amphi, double ; bios,

life), 176.

Amphioxus (illus.), 157.
Amphipod (illus.), 106.
Anatomy (anatemno, to cut up),

defined, 1.
Angler (illus.), 169.
Angle-worm (illus.), 55.
Anguillula aceti (illus.), 53.
Animals, characteristics of, 2 ; sim-
ple and complex, 18.
Animals and plants compared, 2.
Annelids, 55.

Anosia plexippus (illus.), 125.
Ant, 128 ; white (illus.), 120.
Anteater, 231.
Antedon (illus.), 148.
Antelope, 239.

Ant-lion (illus.), 120.

Apes, 243.

Arachnida, characteristics of, 133.

ArchoBopteryx, 201.

Argynnis cybele (illus.), 126.

Ariolimax columbianus (illus.), 81.

Armadillo, 231.

Arthropods, general features of,

93.

Ascidian (illus.), 152.
Asexual reproduction, 31.
Asterias ocracea (illus.), 141.
Astrophyton (illus.), 143.
Auk, 209.
Axolotl, 183.

Back-swimmer, 121.
Balancers, 122.
Band-worm (illus.), 70.
Barnacles (illus.), 96.
Bascanion constrictor (illus.), 187.
Basket-star (illus.), 141. .
Bass, 166.

Bat, brown, 234; red, 234.
Bear, 239.
Beaver, 232.
Bees, 238.
Beetles, 124.

Bell animalcule (illus.), 16.
Bilateral symmetry, 44.
Biology (bios, life; logos, a dis-
course) defined, 1.

251

252

ANIMAL FORMS

Birds, characteristics of, 201 ; anat-
omy of, 204 ; habits of, 207.

Bird spider, 136.

Bittern, 215.

Black-snake (illus.), 193.

Blastula (illus.), 21.

Bombus (illus.), 130.

Bony fish (illus.), 160.

Botany, defined, 1.

Brachiopod (illus.), 70.

Bradypus tridactylus (illus.), 229.

Branchiostoma californiense (il-
lustration), 157.

Branchipus (illus.), 94.

Brittle-star (illus.), 141; regenera-
tion of, 145.

Bugs, 121.

Bumblebee (illus.), 130.

Butterflies, 125.

Buzzard, 219.

Byssus, 77.

Caddis-fly, 119.

Calcolyntlius primigenius (illus.),

25.

Calypte anna (illus.), 224.
Cancer productus (illus.), 104.
Caprella (illus.), 106.
Carapace, of Crustacea, 95, 99 ; of

turtle, 188.
Carp, 163.

Cat, domestic, 239 ; wild, 239.
Catfish, 163.
Cell (cella, a little room), 7 ; shape

and size, 7; typical (illus.), 8.
Centipeds (illus.), 111.
Cephalopod (illus.), 87.
Cephalothorax, 99.
Cercopithecus (illus.), 244.
Cervus canadensis (illus.), 238.
Cestode (illus.), 50.
Cete, 234.

Cheiroptera, 234.

Chelonia, 188.

Chinch-bug, 122.

Chiton (illus.), 82.

Chologaster avetus (illus.), 164 ; C.
agassizi (illus.), 164.

Chordate, characteristics of, 151.

Chordeiles virginianus (illus.),
222.

Chub, 163.

Cilium (cilium, an eyelash), 16.

Circulatory system, use of, 4.

Clam (illus.), 72 ; anatomy, 72, 78 ;
rock- and wood-boring, 75.

Clitellum, 58.

Coccyges, 220.

Cockroach, 118.

•£oelenterate,s, general character-
istics of, 1&*

Coleoptera (illus.), 124.

Columbse, 210.

Complex animals, characteristics
of, 18.

Compound eyes, 109.

Coot, 215.

Copperhead, 193.

Corals (illus.), 41.

Cormorant, 212.

Correlation of function and struc-
ture, 6.

Cottontail, 233.

Courting colors, 203.

Crab, hermit (illus.), 102 ; cancer
(illus.), 103; rock (illus.), 104;
fiddler, 104.

Crane (illus.). 215.

Crayfish (illus.), 101.

Cricket, 118.

Crinoid (illus.),- 143.

Crocodile, 190.

Crocodilia (illus.), 190.

Crotalus adamanteus (illus.), 222.

INDEX

253

Crustacea, 93 ; anatomy of, 98, 107 ;

multiplication of, 98, 110.
Cteniza (illus.), 137.
Cuckoo, 220.
Cucumaria (illus.), 146.
Cuticle, 14.
Cuttlefish, 87.
Cutworm, 112.
Cyclops (illus.), 95; anatomy of,

98.
Cyclostomes, 157.

Daddy-long-legs, 130.

Decapods, 102.

Deer, 222.

Dendrostoma (illus.), 68.

Devil-fish (illus.), 87.

Didelphys virginiana (illus.), 231.

Diemyctylus torosus (illus.), 182.

Digestive tract, use of, 3.

Diptera (illus.), 119.

Division of labor, 21.

Dog, 223.

Dolphin, 221.

Dove, 205.

Dragon-fly (illus.), 118.

Duck-mole (illus.), 216.

Ducks (illus.), 200.

Eagle, golden, 205; bald (illus.),
205.

Earthworm (illus.), 55; anatomy,
55 ; distribution, 59.

Echinoderms, 141 ; locomotor sys-
tem, 146 ; development of, 150.

Ecology, 1.

Eel, 163.

Egg, fertilization of, 20, 21.

Elk (illus.), 222.

Encystment of protozoa, 13.

Epialtus productus (illus.), 104.

Epidella squamula (illus.), 49.

Eretmochelys imbricata (illus.),

195.

Esox (illus.), 165.
Euglena (illus.), 17.
Eurypelma lentzii (illus.), 136.

Fairy shrimp (illus.), 94.

Felis concolor (illus.), 242.

Ferse, 239.

Firefly, 124.

Fish, general characters of, 154;

respiration, 155 ; anatomy, 168 ;

breeding habits, 171.
Fishmoth, 117.
Fish- worm (illus.), 55.
Flagellum (flagellum, a whip), 14.
Flea, 122.
Flicker, 221.

Flies, 122 ; development of, 123.
Flounders, 168.
Fox (illus.), 241.
Frog, 178.

Cfammarus (illus.), 106.

Gallime, 217.

Ganglion (ganglion, a swelling), a
swelling of the nerve-cord due
to the accumulation of nerve-
cells, 79.

Ganoidea, 161.

Garpike, 161.

Garter-snake, 193.

Gasteropod (illus.), 80; anatomy and
physiology, 81.

Gastric mill (illus.), 107.

Gastrula (diminutive of gaster,
stomach), 21.

Geese, 213.

Gelasimus (illus.), 104.

Gephyrean worms (illus.), 67.

Gila monster (illus.), 193.

Glass-snake, 191.

254

ANIMAL FORMS

Glires, 232.

Gnat, 122.

Gonionemus vertens (illus.), 34.

Goose barnacle (illus.), 96.

Gordius, 54.

Gorilla (illus.), 245.

Grasshopper (illus.), 117.

Grebe, 209.

Green gland, 108.

Grouse, 218.

Grua americana (illus.), 212.

Gull, 211.

Habitat (habitare, to dwell), 45.

Hagfish, 157.

Halicetus leucocephalus (illus.),
220.

Haliclystus (illus.), 39.

Harvestman, 135.

Hawks, 219.

Helix (illus.), 81.

Heloderma suspeclum (illus.), 192.

Hemiptera, 121.

Heptacarpus brevirostris (illus.),
101.

Hermit-crab (illus.), 102.

Herodines, 215.

Herons, 215.

Herring, 163.

Homo sapiens, 245.

Honey-bee, 130.

Hoofed animals, 237.

Horned toads, 192.

Hornet, 132.

Horse-fly (illus.), 119.

Horsehair-snake, 54.

Horseshoe-crab (illus.), 139.

Humming-bird (illus.), 222.

Hydra, structure of, 29 ; multipli-
cation of, 31 ; regeneration of,
51.

Hydractinia (illns.), 36, 103.

Hydranth, 33.

Hydrozoa, 34 ; regeneration of, 51.
Hymenoptera (illus.), 128.
Hystrix cristata (illus.), 233.

Incubation (incumbo, to rest upon),

207.

Infusoria, 17.
Insectivora, 234.
Insects, numbers, 114; anatomy,

115 ; respiration, 117.
Isopod (illus.), 101.

Jelly-fish, of Hydrozoa, 33 ; of Scy.

phozoa, 37.
Julus (illus.), 112.

Kangaroo, 232.
Katydid, 117.
Keyhole-limpet, 82.
King-crab (illus.), 139.
Kingfisher, 221.

Lacertilia, 185,
Lamellibranch (illus.), 72.
Lamellirostres, 213.
Lamprey (illus.), 157.
Lamp-shell (illus.), 70.
Lancelet (illus.), 157.
Lasso-cell, 30.
Leeches (illus.), 63; haunts and

habits, 64.
Lemur. 243.
Lepas (illus.), 99.
Lepidoptera (illus.), 125.
Lepomis megalotis (illus.), 167.
Leptoplana (illus.), 45.
Lice, 122.
Life histories and race histories,

27.

Limicolae, 216.
Limulus polypJiemus (illus.), 139.

INDEX

255

Littorina, habits of, 83.

Liver-fluke, 49.

Lizard (illus.), 185.

Lobster, 102.

Locust (illus.), 117.

Long-horned borer beetle (illus.),

124.

Longipennes, 211.
Loon, 209.

LopTiius piscatorius (illus.), 169.
LopTiortyx californicus (illus.), 217.
Lumbricus terrestris (illus.), 55.
Lung-fish, 160.
Lynx (illus.), 241.

Macrobdetta (illus.), 63.

Macrocheira, 222.

Mammals, characteristics of, 225;
anatomy, 226 ; classification, 228.

Man, 245.

Many-celled animals, 11.

Marsupialia, 231.

Marsupium (marsupium, a purse
or bag), 232.

May-fly, 118.

Megaptera versabilis (illus.), 235.

Mesenteric filaments, 40.

Messmates, defined, 48.

Metamorphosis, retrograde, 99 ; in-
complete, 126 ; complete, 128.

Metazoa, defined, 11.

Mice, 233.

Millipeds (illus.), 111.

Minnow, 163.

Mite (illus.), 138.

Mole, 234. ^

Mollusks, general characters of, 72.

Molt, of Crustacea, 99, 110; of
birds, 202.

Monarch butterfly (illus.), 125.

Monkeys, 243.

Morphology, defined, 1.
39

Mosquito, 122; development of,

123.

Moths (illus.), 125.
Mud-hen, 215.
Mud-puppy (illus.), .178.
Multiplication of animals, 5.
Muscle-cell (illus.), 7.
Muscular system, use of, 4.
Mussel, 77.

My sis americana (illus.), 100.
Mytilus edulis (illus.), 77.

Nauplius (illus.), 99.
Nematoda (illus.), 52.
Nemertean worm (illus.), 70.
Nereis (illus.), 59.
Nerve-cell (illus.), 7.
Nettle-cell (illus.), 30.
Neuroptera, 118.
Newt (illus.), 178.
Night-hawk (illus.), 222.
Notochord, 151.
Nucleus (illus.), 9.

Octopus punctatus. (illus.), 87.
Oligochaetes, 59.

Operculum (operculum, a lid), 87.
Opossum (illus.), 231.
Opossum-shrimp (illus.), 100.
Orang-utan (illus.), 243.
Orb- weaving spider, 136.
OrnithorJiynchus paradoxus (illus-
tration), 230.
Orthoptera, 117.
Ostrich (illus.), 209.
Oyster, 77.
Owls, 219.

Pagurus bernhardus (illus.), 103.
Paludicolae, 215.
Pandorina (illus.), 19.
Panther (illus.), 241.

ANIMAL FORMS

Paramcecium (illus.), 15.

Parapodium (para, alongside of;
pous, foot), 60.

Parasitism, 48.

Passeres, 223.

Pelican (illus.), 212.

Pelicanus erythrorhynchus (illus.),
212.

Perca flavescens (illus.), 155.

Perch, 166.

Perching birds, 223.

Peripatus eiseni (illus.), 111.

Pheasant, 217.

Physalia (illus.), 37.

Physiology (physis, the nature of
a thing ; logos, a discussion), 1.

Pici, 221.

Piddock (illus.), 75.

Pigeons, 218.

Pike (illus.), 164.

Pill-bug (illus.), 105.

Pineal gland, 198.

Planaria (illus.), 45.

Plants and animals, differences be-
tween, 1.

Plants, characteristics of higher, 2.

Plant-lice, 122 ; in ant-nests, 129.

Plastron, 188.

Plover,.killdee, 216.

Polychjetes, 59 ; sedentary (illus.),
61 ; development of, 63.

PolynoR brevisetosa (illus.), 61.

Polyzoa (illus.), 68.

Porcellio scdber (illus.), 105.

Porcupine (illus.), 232.

Porpoise, 234.

Portuguese man-of-war (illus.), 36.

Prairie-dog, 233.

Prawn (illus.), 100.

Precocial birds, 208.

Primates, 243.

Proboscis, of flatworms, 46.

Procyon lotor (illus.), 240.
Protoplasm (protos, first; plasma,

anything molded), 9 ; structure

of, 10.
Protozoa, 11; characteristics of,

17; colonial, 19.
Pugettia richii (illus.), 104.
Pulsating vacuoles, 16, 17.
Pygopodes, 209.

Quail (illus.), 217.

Rabbit, 233.

Raccoon (illus.), 240.

Race histories and life histories, 27.

Radial symmetry, 44.

Rail, 215.

Rain-crow, 220.

Raptores, 219.

Rat, house-, 233 ; wood-, 233 ;
musk-, 233.

RatitjB, 209.

Rattlesnake (illus.), 191.

Recognition-marks, 203.

Regeneration, 51.

Reproduction, sexual and asexual,
5,32.

Reptiles, general characteristics,
185 ; distribution, 191 ; anat-
omy, 195.

Retrograde metamorphosis, 99.

Rotifer (illus.), 66.

Ruminant, 237.

Sabella, 62.

Salamander (illus.), 176; distribu-
tion, 178; structure of, 179.
Salmon, 163.
Sand-dollar (illus.), 142.
Sandhopper (illus.), 105.
Sandpiper, 216.
Sapsucker, 221.

INDEX

257

Sarcoptes scabei (illus.), 138.

Scelophorus undulatus (illus.), 185.

Scorpion (illus.), 134.

Scyphozoa, 37 ; development of, 38.

Sea-anemone, 40.

Sea-cucumber (illus.), 143 ; regen-
eration of, 145.

Sea-lily (illus.), 143.

Sea-mat (illus.), 68.

Sea-urchin (illus.), 141.

Sea-squirt (illus.), 152.

Sedentary life, effect of, 62.

Segments, of worms, 55 ; of arthro-
pods, 94.

Segmented worms, 55.

Serpentes, 186.

Serpent-star (illus.), 141.

Serphus dilatatus (illus.), 122.

Serpula (illus.), 62.

Serpulids, 62.

Seta, 55.

Sexual reproduction, 32.

Shark (illus.), 159.

Shell-gland, 108.

Shipworm, 75.

Shrew, 234.

Shrimp, fairy, 94 ; opossum (illus.),
100.

Silk-moth (illus.), 127.

Silver-spot butterfly (illus.), 126.

Simple animals, characteristics of,
18.

Single-celled animals, 11.

Sinus, blood, 78.

Siren (illus.), 178.

Slug (illus.), 80. . •/

Snail, common (illus.), 80; arma-
dillo (illus.), 82 ; naked (illus.), 82.

Snakes, 186 ; distribution of, 193.

Snipes, 216.

Somateria dresseri (illus.), 214.

Species, origin of, 91.

Sperm-cell, 20.

Sphenodon punctatus (illus.), 199.

Spicule, of sponge (illus.), 26; of
coral, 42.

Spiders, organization of, 135 ; hab-
its, 136.

Spinnerets, 135.

Spiny-rayed fishes, 166.

Sponge, development of (illus.),
21; distribution, 22; shape and
structure, 23.

Spontaneous generation, 54.

Springtail, 117.

Squalus acanthias (illus.), 159.

Squash-bug, 122.

Squid (illus.), 87.

Squirrels, 233.

Starfish (illus.), 140 ; regeneration,
145; structure, 146.

Steganopodes, 212.

Stickleback, 164.

Structure and function, correla-
tion of, 6.

Strongylocentrotus purpuratus (il-
lustration), 144.

Struthio camelus (illus.), 210.

Sturgeon (illus.), 161.

Sunfish (illus.), 166.

Symmetry, radial and bilateral, 44.

Swan (illus.), 213.

Swift, 222.

Tcenia solium (illus.), 51.

Tapeworm (illus.), 50; develop-
ment, 51 ; in relation to regen-
eration, 51.

Tarantula (illus.), 137.

Teeth, use of, 2.

Teleostei, 160.

Termites (illus.), 120.

Tern, 211.

Terrapene Carolina (illus.), 189.

258

ANIMAL FORMS

Thousand-legged worms (illus.),
111.

Threadworms (illus.), 52.

Thysanura, 117.

Tick, 138.

Tiger salamander, 178.

Toad (illus.), 178.

Trap-door spider (illus.), 137.

Trematode (illus.), 48; develop-
ment, 51.

Trichina spiralis (illus.), 53.

Trigger hair, 31.

Turkey, 218.

Turtles, 188; structure, 189; dis-
tribution, 194.

TypJilichthys sulterraneus (illus.),
163.

Ungulata, 237.

Vacuole, pulsating, 16 ; use of, 17.
Velum (illus.), 35.
Vespa, nest of (illus.), 131.
Vinegar eel (illus.), 53.
Volvox (illus.), 19; multiplication

of, 20.
Vertebrates, characteristics of, 145 .

classification, 143.

Vorticella (illus.), 16.

Vulpes pennsylvanicus (illus.), 241.

Wasps, 128 ; habits of, 131.
Water-boatman, 121.
Water-bug (illus.), 121.
Water-dog (illus.), 178.
Water-flea, 95.
Whale, humpback (illus.), 235;

sperm, 236.
Whale lice, 107.
Wheel-animalcule (illus.), 66.
Wheel- weaving spiders, 136.
Whippoorwill, 222.
White ant (illus.), 120.
Wood-beetle (illus.), 124.
Woodchuck, 233.
Woodcock, 216.
Woodpeckers, 221.
Worms, general characters of, 44°

classification, 45.

Yellowhammer, 221.
Yellow-jacket, 132.

Zirphcea crispata (illus.), 76.
Zoology, 1.
Zoophyte, 29.

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