A Synopsis of the

GENERAL MORPHOLOGY

OF ANIMALS

BY EDWIN GRANT CONKLIN

GENERAL MORPHOLOGY OF ANIMALS

A Synopsis of the

GENERAL MORPHOLOGY
OF ANIMALS

By EDWIN GRANT CONKLIN

Professor of Biology in Princeton University

PRINCETON
PRINCETON UNIVERSITY PRE

1927

COPYRIGHT, 1927, BY PRINCETON UNIVERSITY PRESS

PRINTED AT THE PRINCETON UNIVERSITY PRESS
PRINCETON, NEW JERSEY, U.S.A.

CONTENTS

PAGE

A. TAXONOMY i

I. DISTINCTIVE CHARACTERS OF ANIMALS AND PLANTS
II. SUBDIVISIONS OF ANIMALS AND PLANTS 2

1. Protophyta and Protozoa

2. Metaphyta and Metazoa

3. Transitions from Unicellular to Multicellular Forms

4. Chief Subdivisions of the Plant Kingdom 3

5. Chief Subdivisions of the Animal Kingdom

6. Binomial Nomenclature

B. PRINCIPLES OF MORPHOLOGY 4

I. DIFFERENTIATION AND INTEGRATION

1. Grades of Organization

2. Methods of Differentiation 5

3. Structure and Function

II. INDIVIDUALITY 6

1. Definition

2. Grades of Individuality

(a) Ultra-microscopic units

(b) Visible cell structures

(c) Cells

(d) Tissues 7

(e) Organs

(f) Systems

(g) Antimeres

Metameres 8

(h) Persons
(i) Corms
(j) Societies

V

CONTENTS

PAGE

III. HOMOLOGY AND ANALOGY

1. General or Meristic Homology 9

(a) Bilateral Homology (Homotypy)

(b) Serial Homology (Homodynamy)

2. Special Homology (Homogeny, Homophyly)

3. Homoplasy (False Homology or "Convergence")

4. Causes of Homology

(a) The Cause of General Homology . 10

(b) The Cause of Special Homology

IV. ORGANIC SYMMETRY 11

C. MORPHOLOGY OF METAZOA 12

I. STAGES OF METAZOAN DEVELOPMENT

1. The Fertilized Egg 14

2. The Cleavage Stages

3. The Blastula

4. The Gastrula

II. MODIFICATIONS AND COMPLICATIONS OF THE GASTRULA .... I5

1. Axes and Symmetry

(a) Protaxonia

(b) Heteraxonia l6

2. Mesoderm and Body Cavities 19

3. Metamerism 20

III. ORGAN SYSTEMS 23

1. Integumentary System

(a) Epidermis

(b) Dermis 25

2. Skeletal System

3. Motor System 28

(a) Amoeboid movement

(b) Ciliary Movement 30

(c) Muscular Movement 32

vi

CONTENTS

PAGE

4. Alimentary System 34

Digestion

Intracellular Digestion 35

Extracellular Digestion
Openings into Digestive Cavity

Fore, Mid, and Hind-Guts 37

Digestive Cells and Glands 40

Movements of Food in Alimentary Canal

5. Respiratory System

Branchiae, Gills 41

Tracheae, Lungs 44

6. Circulatory System 47

Blood- Vascular System

Circulation and Respiration 49

7. Excretory System 54

Nephridia

Proto-nephridia ^^

Meta-nephridia

Kidneys of Chordata ; Pronephros ^7

Mesonephros j'S

Metanephros ^g

Theories of Excretion 61

Excretory Ducts as Sex Ducts

8. Reproductive System 62

Sexual Reproduction; Sexes

Ovaries and Testes 63

Gonads of Vertebrates 64

Semination 65

Secondary Sexual Characters

Asexual Reproduction, or Monogony 66

9. Nervous System

( 1 ) Diffuse nervous system 69

(2) Linear nervous system

(3) The ganglionic type 70

vii

CONTENTS

PAGE

(4) The tubular type of nervous system 71

10. Sense Organs 7^

(1) Organs of smell and taste 74

(2) Organs of hearing and equilibration 76

Statocysts

Arthropod Statocysts and Ears 78

Lateral Line Organs of Vertebrates 79

The vertebrate ear

(3) Visual Organs 81

Vesicular Eyes

The paired eyes of Vertebrates 83

The compound eyes of Arthropods 85

Table of Classification and Morphology of Animals

Vlll

INTRODUCTION

THE following pages give a brief synopsis of that
part of the course in General Biology in Princeton
University which deals with the General Morphology of
Animals; other parts of that course are General Physiology,
Ecology and Biogeny (Genetics and Evolution). The Gen-
eral Morphology is printed in this form because of the
difficulty of finding elsewhere any such brief summary and
because of the repeated requests of students for such a
syllabus. In general the whole subject of animal morphology
is here dealt with from the genetical (embryological and
evolutionary) point of view because it is easier to under-
stand complicated structures when they are seen in the
process of becoming and also because by this method fun-
damental resemblances or homologies are more readily ap-
preciated. In so brief a statement covering so wide a field
only bare outlines can be given ; very much has been omitted
and many statements which should have been qualified have
been given baldly, but it has seemed best in an elementary
course to present only the leading principles, processes and
structures of animal morphology without confusing the be-
ginner with a multitude of details or with many qualifica-
tions or exceptions. It is expected, of course, that students
will use this synopsis only in connection with lectures, labo-
ratory work, and assigned readings.

GENERAL MORPHOLOGY

GENERAL Morphology deals with the forms and
structures of living things and is studied by methods
of observation, comparison, development, and experiment.
By observing and comparing the resemblances and differences
among organisms they are classified into groups that are
more or less alike; such classification is known as Taxonomy.
By methods of comparison, development and experiment,
the factors or causes which determine resemblances and dif-
ferences in structure are studied, such studies being known
as Comparative or Experimental Anatomy or Embryology,

A. TAXONOMY (= CLASSIFICATION)

All organisms are classified according to their structure
into two kingdoms, plants and animals, a few main branches
of each kingdom, and many minor subdivisions. Animals and
plants are alike in the more fundamental features of ( i ) Pro-
toplasmic and Cellular organization, (2) Metabolism, (3)
Reproduction, (4) Irritability.

I. DISTINCTIVE CHARACTERS OF ANIMALS
AND PLANTS

Animals generally have : Plants generally have :

1. No cellulose cell-walls 1. Cellulose cell-walls

2. Holozoic nutrition 2. Holophytic nutrition

3. Excretory organ (in green plants only)

4. Movements relatively 3. No excretory organ
free 4. Relatively little move-
ment

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MORPHOLOGY OF ANIMALS

However, there may be exceptions to each of these criteria
and then the decision as to whether the organism is a plant or
animal may be difficult if not impossible to make.

II. SUBDIVISIONS OF ANIMALS AND PLANTS

Both animals and plants are subdivided into one-celled
and many-celled forms and each of these is further subdi-
vided as is shown in the tables, pages 1 1, 12, 13 of the Labo-
ratory Directions, and at the end of this Synopsis.

1. Protophyta are one-celled plants, and Protozoa are one-
celled animals in which the entire body consists of a single
cell, which may be independent, or may be joined with
others like itself to form a colony of similar cells.

2. Metaphyta are many-celled plants and Metazoa are
many-celled animals in which the body consists of many
cells which differ more or less from one another.

3. Transitions from Unicellular to Multicellular forms. In
plants there is a complete series 'of intermediate forms
from the single cell to the solid aggregate of cells as fol-
lows:

(^) Single cell (e.g. Spherella),

(^) Linear aggregate of cells (e.g. Spirogyra),

(r) Superficial aggregate of cells (e.g. Ulva),

(<i) Solid aggregate of cells (e.g. mushrooms and all

higher plants).

Among animals a great gap exists between Protozoa
and Metazoa, there being no animals living today that are
intermediate between the two. However, all Metazoa, in
their development from the egg, pass from a unicellular to
a multicellular condition through the cleavage stages^ the
bias tula and tht gas trul a (Fig. 1).

: 2 :

MORPHOLOGY OF ANIMALS

4. Chief subdivisions of the Plant Kingdom are into two Sub-
kingdoms, 4 Divisions, at least 6 Subdivisions, about 50
classes, 130 orders, several thousand genera, and nearly
half a million species (see Laboratory Directions p. 11).
The "Subdivisions" of plants correspond more or less to
the "Phyla" of animals, while the Classes, Genera, and
Species in the two kingdoms are of approximately co-
ordinate value.

5. Chief Subdivisions of the Animal Kingdom. The main
subdivisions of the animal kingdom are called Phyla;
these differ in adult structure so widely that it is difficult
or impossible to find connecting links between them.
These phyla are subdivided into Subphyla, Classes, Or-
ders, Families, Genera, Species. At present we recognize
14 phyla of animals, 50 classes, about 200 orders, about
50,000 genera, more than half a million species. The
phyla, orders and classes of the animal kingdom are shown
in the table, pages 12 and 13, of the Laboratory Directions
and in the tabular classification at the end of this publi-
cation.

6. Binomial Nomenclature. The entire classification of any
plant or animal involves its assignment to its proper
Phylum, Class, Order, Family, Genus, and Species, but
its scientific name consists merely of the Latin name of
the genus and species to which it belongs (e.g. Euglena
viridis, Amoeba proteus). This method of naming ani-
mals and plants was instituted by Linnaeus (1707-1778)
and is known as binomial nomenclature.

In the following pages continual reference is made to
the different phyla, classes, and genera of animals and it is

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MORPHOLOGY OF ANIMALS

absolutely necessary that the student should knoiv the
location in the table of classification at the end of this
Synopsis of every group mentioned in the text^ that he
should see actual specimens of each of them in the museum
or laboratory^ and that he should study carefully the
figures in the text and their descriptions if he is to get any
proper understanding of the morphology of animals.

B. PRINCIPLES OF MORPHOLOGY

In comparing the structures of different organisms (Com-
parative Morphology) there are a few principles applicable
to all plants and animals.

I. DIFFERENTIATION AND INTEGRATION

Organizations means differentiation and integration;
every living thing, even the simplest, is composed of dif-
ferent parts (differentiation) which parts are united into a
single whole (integration). Many lifeless things are or-
ganized, that is, they are composed of different parts that
are united into a single whole, — as, for example, the solar
system, a chemical molecule, a watch or any other machine.
But living things are so highly and so peculiarly organized
that they are called "organisms." It is protoplasmic and
cellular organization that is distinctive of living things.

First among the principles of comparative morphology is
the fact that many organisms differ in the degree and num-
ber of their differentiations; those in which there are rela-
tively few differentiations are commonly called ''lower,"
those with many differentiations "higher" organisms.
1. Grades of organization from the relatively simple to the

relatively complex, or from the low to the high, are found

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MORPHOLOGY OF ANIMALS

(a) in plants and animals now living on the earth, (b) in
the history of organisms through past geological eras, (c)
in the development of animals and plants from egg cells
to their adult condition.

2. Methods of Differentiation. The method by which in-
creasing differentiation is brought about is best seen in the
process of individual development. Here the single egg
cell divides into many cells which come to differ from
one another in both structure and function, {a) In this
process there is a distribution of different structures to
different cells and a concomitant distribution of functions
among these cells; that is, there is "morphological di-
vision of substance" and "physiological division of labor."
{b) But increasing differentiation does not consist mere-
ly, nor chiefly, in the division of structures and functions
already present and their segregation into separate cells,
but also in the appearance of new structures and functions
in the later stages that were not present in the earlier ones.
These new structures and functions were not created ex
nihilo^ but have come from new combinations of factors
already present, that is from transformation of old struc-
tures and functions rather than from new formation.
These new differentiations of development are, therefore,
the results of "creative synthesis," just as water with its
many peculiar properties is the result of the chemical
synthesis of two atoms of hydrogen and one of oxygen.

3. Structure and function are not separate or independent
entities, they are merely two aspects of one thing, namely,
organization. From the morphological point of view a
living thing is a system of structures, from the physio-

c 5 ]

MORPHOLOGY OF ANIMALS

logical, a system of functions, but these never exist sepa-
rately. Changes in function inevitably involve changes in
structure and vice versa^ but neither is the cause of the
other, any more than one side of a coin is the cause of the
other side. Morphology and physiology are merely two
aspects of one thing, namely life.

II. INDIVIDUALITY '

All living things from the simplest to the most complex are
distinct but not wholly independent individuals.

1. Definition. An organic individual may be defined as any
unit capable of maintaining and of reproducing itself, or
in other words, capable of assimilation and reproduction.

2. Grades of Individuality. The degree of differentiation
of any organism is directly proportional to the number of
unlike units that constitute it and to the degree of their
unlikeness. The following grades of organic individuals,
from the smallest and simplest to the largest and most
complex, are recognized:

(<^) Ultra-microscopic units^ such as inheritance factors,
genes, etc. These invisible, but real units, have the power
of assimilation, growth, and division.
{b') Visible cell structures^ such as chromomeres, chromo-
somes, plastosomes, chromatophores, etc. These units also
assimilate, grow and divide; in both the ultra-micro-
scopic units (^) and these microscopically visible ones (Z?)
division is always into like portions, that is, it is non-
differential.

(r) Cells are more complex units composed of the preced-
ing simpler ones (^a and Z?), each consisting of cytoplasm

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MORPHOLOGY OF ANIMALS

and nucleus. They may exist independently as "free cells,"
or they may be united with other cells, when they are
called "fixed cells." They are the smallest units capable of
independent existence, the ultimate independent units of
organic structure and function. Every cell reproduces by
division. The division of the nucleus is typically into
equal halves, but that of the cytoplasm may be equal or
unequal, — that is the nuclear division is non-differential,
but the division of the cytoplasm may be either differen-
tial or non-differential.

(^) Tissues are groups of similar cells together with their
products (secretions, etc.). In animals there are two fun-
damental types of tissues which give rise to all others;
these are epithelia which are composed of cells closely
united side by side to form layers, and mesenchyme which
consists of cells loosely connected by processes into a more
or less spongy mass. In the animal embryo epithelial tissue
usually gives rise to the covering and lining layers of the
body and the glands, also much of the muscular, nervous
and germinal tissues; while mesenchyme produces chiefly
connective, skeletal, vascular and fatty tissues.
{e) Organs are groups of different tissues, each perform-
ing specific functions. Thus the heart is an organ composed
chiefly of muscular and connective tissues having the
specific function of pumping blood.

(/) Systems are groups of organs, each performing some
general function, such as the heart and vascular system
for the circulation of blood, the nervous system for the
reception, transmission and coordination of stimuli, etc.
{g) Antimeres (homo-typical parts) are bilaterally or

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MORPHOLOGY OF ANIMALS

radially symmetrical portions of the body, such as the
right and left arms and legs, each composed of several
systems and organs.

Metameres (homo-dynamic parts) are serially symmet-
rical portions of the body, such as the successive somites
(segments) of the body of an earthworm or lobster, each
composed of many systems and organs. When the somites
are all alike they are said to be homonomous^ when they
differ one from another they are heteronomous,
(h) Persons are separate, distinct individuals, composed
of many or all of the preceding units, such as an earth-
worm, a frog, or a man.

(d) Corms are groups of persons morphologically united,
such as a colony of sponges, hyroids, polyzoa, or ascidians.
(;) Societies are groups of individuals physiologically
united, such as a colony of ants, bees, or men.

III. HOMOLOGY AND ANALOGY

The general functions of living things, such as metabolism,
reproduction, sensitivity, are found in all plants and ani-
mals, but the various organs and systems by which these
functions are performed differ widely in different phyla.

Analogy is similarity in function but not in structure.
Thus the wings of insects and birds are analogous, though
differing widely in structure.

Homology is resemblance in structure and position, though
the function may or may not be similar. Thus the wing of a
bird, the fore-leg of a horse, and the arm of a man are
homologous, because they show fundamental resemblances
in position and structure, although they differ in function.

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MORPHOLOGY OF ANIMALS

Homology may exist between similar parts of the same
organism, or between similar parts of different organisms;
the former is called General Homology, the latter Special
Homology.

1. General or Meristic Homology is similarity between parts
which are duplicated or repeated in the same body, such
as arms, legs, fingers, teeth, etc. These repetitions may be
bilateral or serial in position, and accordingly we recognize
(^) Bilateral Homology {Homotypy) such as the re-
semblance between the right arm and the left, etc.

{b) Serial Homology {H o7nodynamy) such as the re-
semblances between arms and legs, the series of fingers,
teeth, etc.

2. Special Homology {Homogeny^Homophyly) is similarity
between corresponding parts of different organisms, such
as the fore-limbs of all vertebrates, the eyes of all insects,
etc.

3. Homoplasy {False Homology or ''Convergence'') is ap-
parent resemblance but fundamental difference between
parts of different organisms, as for example the eyes of
vertebrates and cephalopods. Such superficial resemblance
may be attributed to the influence of similar environments
and needs upon fundamentally different hereditary types ;
that is, starting from different types these structures have
"converged" toward superficially similar ones.

4. Causes of Homology. There is only one scientific explana-
tion of the cause of all real homolog}^, whether general or
special, and that is similarity of source or origin, or com-
mon inheritance from the same ancestors, just as the
fundamental resemblances of all Africans, or of other

MORPHOLOGY OF ANIMALS

human races, are due to inheritance from common
ancestors.

{a) The Cause of General Homology. General homology
is the result of common origin from the same substance
or primordium in the egg or embryo. The different fingers
on one hand come from a single hand-pad in the embryo
which partially divides into the several fingers; the same is
true of the teeth in each jaw, etc. The arm and leg come
from a common limb-ridge in the embryo which divides
into an anterior and a posterior portion, and the resem-
blance between one somite and another in the earthworm
is due to the incomplete division of the body into suc-
cessive somites. The resemblances between right and left
halves of the body (bilateral homology) must be traced
back to a still earlier period in the egg or embryo when a
bilateral division of embryonic substances occurs. Thus all
forms of general homology are attributable to origin from
common substances or primordia in the egg or embryo,
which substances undergo partial division, the same being
a form of incomplete asexual reproduction.
{b) The Cause of Special Homology must be traced back
to still earlier sources, namely to descent from similar
germ cells or germplasms. The only scientific explanation
of the homologies between the arm of a man and the fore-
leg of a horse is that both have had common ancestors.
Special homology is thus attributable to descent from
common ancestors, or to the division of a common inher-
itance material and its distribution to different individuals
in the process of sexual reproduction. It is therefore a
powerful argument in favor of evolution.

MORPHOLOGY OF ANIMALS

IV. ORGANIC SYMMETRY

Many inorganic things, such as molecules and crystals,
have varying forms of symmetry depending upon the spatial
relation of their atoms and molecules. Likewise the con-
stituent parts of an organism, such as cells, organs, antimeres,
and metameres, bear certain spatial relations to one another,
and this is what is meant by organic symmetry or asymmetry.

In any solid body there are typically three axes in the
three dimensions of space and each of these axes has two
poles. If all the axes and poles are alike we have spherical
symmetry, a condition which is found in few if any living
things. When the two poles of one axis differ from each
other (polar differentiation) while all the other poles and
axes are alike we have radial symmetry, a condition which is
found in many egg cells as well as in some adult organisms.
When all three axes differ from one another, while the two
poles of only two of these axes differ from each other, we
have bilateral symmetry, a condition which is found in a
great number of plants and animals. These and other forms
of symmetry are summarized in the following table :

SYMMETRY

ANIMAL
EXAMPLES

AXES POLES

Homaxial Homopolar

Single heteraxial Single heteropolar
Triple heteraxial " "

Double
" " Triple "

Probably the higher forms of symmetry and asymmetry
were originally derived from the lower forms by progressive

n 11 :

1. Spherical

(ideal)

2. Radial

3. Biradial

4. Bilateral

5. Asymmetry

Few if any

Hydra, jellyfish

Ctenophores

Almost all animals

above Ctenophores

Paramecium, Snails

MORPHOLOGY OF ANIMALS

differentiation of axes and poles. This can actually be seen
in the embryology of certain animals where radial symmetry
develops into bilateral, and the latter into asymmetry. On
the other hand, in the case of some bilateral animals the
eggs even are bilateral.

The chief axis of the adult connects the anterior and
posterior poles and is called the antero-posterior axis. The
side of the body which is usually directed downward is
ventral, the opposite side dorsal, and the axis connecting
these is the dorso-ventral axis. The axis connecting the right
and left sides of the body is the transverse axis. A plane
passing through the body in the antero-posterior and the
dorso-ventral axes and dividing the body into equivalent
right and left halves is called the median (bilateral) plane
or section. A plane passing through the dorso-ventral and
the transverse axes is a transverse plane, or section, while a
plane passing through the antero-posterior and the trans-
verse axes is called a coronal (frontal) plane or section.

C. MORPHOLOGY OF METAZOA

Metazoa are animals composed of many cells which are
arranged in at least two layers, an outer layer covering the
body, the ectoderm, and an inner layer lining the digestive
cavity, the endoderm ; in addition most metazoa have a mid-
dle layer or aggregation of cells, the mesoderm, lying be-
tween the ectoderm and endoderm. All have male and
female sex-cells (spermatozoa and ova).

I. STAGES OF METAZOAN DEVELOPMENT

In the course of development all metazoans pass through
the following stages (Fig. i ) :

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MORPHOLOGY OF ANIMALS

Fig. 1. Cleavage, Blastula and Gasirula of an Echinoderm (Eckinarachniiij). A, 2 cells; B,
4 cells ; C, 8 cells ; D, i6 cells ; E, 32 cells ; F, Optical Section of a Blastula of about 128
cells showing a hollow sphere. The cavity in the sphere is the blastocoel. G, H, I, optical
sections of young, medium and old gastrulae ; a, anus; be, blastocoel; bp, blastopore; c,
coelom ; g, gastrocoel ; m, mouth ; me, mesothelium ; ms, mesenchyme.

MORPHOLOGY OF ANIMALS

( 1 ) The Fertilized Egg which is a single cell, comparable
to a one-celled organism; (2) the Cleavage Stages, during
which the egg divides into a large number of cells, which are
roughly comparable to a protozoan colony (Fig. 1, A-D) ;
(3) the Blastula, which is typically a hollow sphere, radially
symmetrical, composed of many cells, surrounding a central
cleavage cavity, and resembling somewhat a Volvox colony
(Fig. 1, E, F) ; (4) the Gastrula (Fig. 1, G-I), which is
typically composed of two primary layers of cells, the outer
layer, the ectoder?n {ec), covering the body, the inner, the
endoderm (en)^ lining the digestive cavity or enteron {gas-
trocoel, g), which opens to the exterior through a primary
mouth or blastopore {bp), A third layer or group of cells, the
mesoderm, arises from one or both of the primary layers or
from certain of the cleavage cells before layers are formed,
and comes to lie between ectoderm and endoderm. It may
consist of loose, scattered cells {mesenchyme, ms), or of an
epithelial layer {mesothelium, me), or of both (Fig. 1,
H,I).

Such a gastrula is a real metazoan and some of the lowest
metazoa are, in their adult condition, little more than gastru-
lae (e.g. Hydra). The gastrula is the ground-form of all
metazoa and from this stage onward in the development of
higher metazoa different phyla diverge, the gastrula under-
going certain modifications and complications which trans-
form it into the adult form characteristic of each phylum.
The earliest stages of ontogeny are in many respects like the
lowest living organisms, as shown in the following table :

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MORPHOLOGY OF ANIMALS

ONTOGENETIC SERIES

1. Egg

2. Cleavage -

3. Blastula -

4. Radial Gastrula

TAXONOMIC SERIES

Protozoan
Protozoan Colony
Volvox-like Forms
Simple Sponge, Hydroid

5. Bilateral Gastrula Polyclad-like Forms

/mmmmfmminm mmmrm
A B

Fig. 2. Diagrams of Protaxonia in which the gastrula axis becomes the chief axis of the
adult. A, a simple sponge in which the gastrula becomes attached by its oral pole, the
blastopore then closes and a new opening into the gastric cavity, the osculum {os), appears
at the apical pole ; B, a Hydroid which has attached by the apical pole of the gastrula
while the blastopore becomes the mouth {m) which is then surrounded by a row of tentacles ;
C, a ctenophore or jelly-sphere which swims by means of plates of fused cilia (8 plates are
shown) ; the apical pole of the gastrula gives rise to the sense organ (jo) of the adult while
the blastopore becomes the mouth {m). (After Hatschek).

II. MODIFICATIONS AND COMPLICATIONS
OF THE GASTRULA

1. Axes and Symmetry

(a) Protaxonia are the lowest Metazoa (Spongiaria,
Cnidaria, Ctenophora) in which the chief axis of the gas-
trula becomes the chief axis of the adult (Fig. 2). In the

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MORPHOLOGY OF ANIMALS

simplest forms the symmetry is radial, but in more com-
plex forms it may be bilateral or biradial (Ctenophora,
Fig. 2, C).

{b) Heteroxonia (= Bilateratd) are all other Metazoa
except the protaxonia. In these the chief axis of the gastru-
la does not directly become the chief axis of the adult. By
the greater growth of the gastrula on one side (ultimately
posterior, Fig. 3, B-D) the oral and aboral poles of the
gastrula become widely separated on that side and nearer
together on the opposite (anterior) side; thus the axis run-
ning from the apical pole of the gastrula to the blastopore

Fig. 3. Diagrams of the origin of Heteraxonia from a typical gastrula. Chief axis of
gastrula indicated by arrow, apical (sensory) pole shaded, oral pole at blastopore. A,
typical gastrula, radially symmetrical ; B, bending of gastrular axis by greater growth of
gastrula on dorsal-posterior side and consequent establishment of bilateral symmetry; C,
stage corresponding to adult flat-worm (Turbellaria) with mouth in middle of ventral side,
without anus and with two nerve cords (shaded) running from brain aroimd oesophagus
and along ventral side of body ; D, stage corresponding to an annelid, the trunk segmented
into several somites, with mouth near anterior end of body, anus at posterior end, nervous
system consisting of brain, circumoesophageal ring and ventral chain of ganglia and with
a pair of nephridia in each somite.

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MORPHOLOGY OF ANIMALS

is bent, as shown in Fig. 3, and the gastrula becomes strik-
ingly bilateral in symmetry.

The apical pole of the gastrula usually becomes the
anterior pole of the adult. There is frequently a sense
organ at this pole of the gastrula (Fig. 3, so) and it is
often called the ''sense pole" ; the brain and higher sense
organs of the adult usually develop at or near this pole.
When the gastrular axis bends through approximately
90° the oral pole of the gastrula (blastopore) comes to lie
on the ventral side of the adult in many invertebrates
(some flatworms, Fig. 3, C, and arthropods) ; if it bends
through more than 90° the blastopore approaches the an-
terior end (apical pole) of the adult (annelids, Fig. 3, D,
mollusks). These relations are reversed in the lower verte-
brates (Amphioxus, ascidians, amphibians) where the
blastopore is at iirst dorsal and later posterior in position
(Fig. 4, C-E). In general among invertebrates the blasto-
pore becomes the mouth of the adult, and the anus, when
present, is a new formation ; in the echinoderms and lower
vertebrates it becomes the anus (in part), the mouth being
a new formation; in arthropods it usually closes com-
pletely, the mouth and anus being new formations.

These relations of gastrula and adult may be sum-
marized as follows :

GASTRULA ADULT

Apical Pole becomes Anterior Pole

i ventral side (invertebrates)

( dorsal " (vertebrates)

i mouth (most invertebrates)

/ ( anus (echinoderms, some chordates)

1: 17 3

MORPHOLOGY OF ANIMALS

Fig. 4. Early Development of Frog's Egg. A, Late Cleavage; R, Blastula; C, Gastrula ; D, E,
F, young embryos. The animal pole of the egg is marked by two small polar bodies. The
left side of each figure is anterior (a), right posterior {p), upper side ventral (jy), lower side
dorsal id); bp, blastopore; c, notochord or area of embryo from which it comes; E, enteron ;
en, endoderm ; m, mesoderm; n, neural tube, or plate, or area from which it comes; jc,
segmentation cavity or blastocoel.

MORPHOLOGY OF ANIMALS

While the brain lies at or near the anterior pole in all
Metazoa, the rest of the central nervous system lies on the
ventral side of the body in most invertebrates, but on the
dorsal side in vertebrates. The heart lies on the dorsal
side in many invertebrates (annelids, arthropods) but on
the ventral side in vertebrates. The side of the body on
which most of the nervous system lies is called the neural
side; that on which the heart lies is the haemal side,

A few Heteraxonia are apparently radially symmetrical
(starfish, sea urchins, etc.), but they are really bilateral.
Such apparently radial animals develop from bilateral
larvae, probably in adaptation to a sedentary life. The
internal organs of Heteraxonia are frequently asymmet-
rical, thus the heart of man is on the left side and the
larger part of the liver on the right, but this asymmetrical
condition is derived from a bilateral stage. Similarly some
adult animals, such as snails, Amphioxus, flounders, etc.,
are strikingly asymmetrical but they are derived from bi-
lateral embryos.

Mesoderm and Body Cavities. These develop in the space
between the ectoderm and endoderm, that is in the blasto-
coel or pri?nary body-cavity (Fig. i, bc^ Fig. 5, ^, in A and
B). In the lowest Metazoa the blastocoel is filled with
scattered branched cells (mesenchyme), the spaces be-
tween cells being parts of the blastocoel (sponges, Cni-
daria, Ctenophora, flat- worms). Among higher Metazoa
the mesoderm is usually divided into an inner portion
lying next to the endoderm and an outer one next to the
ectoderm. Between these is a space, the coelom or secon-
dary body-cavity (r, Fig. 5, C and D). This is lined by

C >9 3

MORPHOLOGY OF ANIMALS

flattened mesoderm cells, the peritoneum^ and is usually
divided into right and left halves by two longitudinal par-
titions, the dorsal and ventral mesenteries^ one of which
lies dorsal to the alimentary canal, the other ventral to it
(^, %\ Fig. 5, C) ; in some animals, one or both of these mes-
enteries may disappear (Fig. 5, D). In segmented animals
the coelom may be further divided into a series of cham-
bers by transverse partitions, the dissepiments or septa
Figs. 3, D; 12, A), which may disappear more or less com-
pletely in some cases. The excretory and sexual organs
are developed in large part from the walls of the coelom
and project into its cavity. The portion of the coelom
surrounding the heart is usually separated from the re-
mainder and is called the pericardial cavity; while
in the highest vertebrates (mammals) the anterior portion
of the coelom which contains the lungs is separated by the
diaphragm from the posterior part containing the abdom-
inal viscera.
3. Metamerism. A further complication of the gastrula is
introduced in segmented animals by the repetition of the
principal organs of the body in a series, one behind the
other Fig. 3, D) ; such repetition is known as metameric
segmentation, and each segment of the body is called a
somite (annelids, arthropods, vertebrates). In the simplest
cases each somite has its own section of the coelom and
its own sensory, nervous, muscular, alimentary, respira-
tory, excretory, and sexual organs, and each may bear a
pair of limbs or locomotor organs. Each somite in short,
contains all the important organs and may properly be
called a little body (i.e. somite).

[ 20 ]

MORPHOLOGY OF ANIMALS

Fig. S- Diagrammatic Transverse Sections of the bodies of A, Coelenterate ; B, Flatworm ;
C, Annelid ; D, Vertebrate ; a, blastocoel filled with mesenchyme cells {m) ; G, Gastrocoel
lined with endodermal epithelium ; g, gonocoel or cavity of sex ducts and glands, probably
homologous with c, coelom of animals with true body cavity ; n, nerve tube ; nc, notochord.
The walls of the coelom are composed of a thin layer of mesothelium (peritoneum), a thick
muscular layer (solid black), a thin layer of mesenchyme cells (ot) in what was earlier the
blastocoel {a), and covering the outside of the body the ectodermal epithelium, while endo-
dermal epithelium lines the gastrocoel (G) ; d, dorsal mesentery ; v, ventral mesentery, which ^
has almost disappeared in the vertebrate (D).

In the higher segmented animals the various somites
are no longer alike (homonomous), but show physiological
division of labor, some being differentiated for one func-
tion and some for another (heteronomous). In this way
some of the organs named above disappear in certain seg-
ments, while others become greatly enlarged or modified.

C 21 3

MORPHOLOGY OF ANIMALS

Finally this specialization of the somites is carried a step
farther in higher arthropods and vertebrates, in which we
have an intimate fusion of metameres and a coalescence of
organs in certain regions, such as to more or less com-
pletely mask the fundamental segmentation. This is espe-
cially true of the vertebrates, the lower forms of which
show segmentation of the axial skeleton (vertebrae and
ribs) and attached muscles, of the nerves, of the gills and
their blood vessels, and of the excretory and sexual organs ;
while in the higher vertebrates (reptiles, birds, and mam-
mals) segmentation is limited in the adult to the axial
skeleton, muscles, and nerves. The fusion of somites is
most pronounced in the anterior part of the body; such
fusion leads to the formation of a head {cephalization).
The head of insects contains three or four somites (Fig.
15, A), while the vertebrate head is composed of not fewer
than nine.

Primitively the limbs are all alike and a pair is borne on
each somite (many annelids) ; however in higher annelids
and arthropods they disappear entirely from certain
somites and in others undergo great modifications of struc-
ture to fit them for particular functions. In the case of
vertebrates they are limited to but two pairs, and it is
probable that these are derived from a continuous lateral
limb-ridge by the suppression of an intermediate portion.

The great modifications and complications which have
been briefly sketched lead far from the simple form of
the gastrula, which is the ground-form of all Metazoa.

1:22 n

MORPHOLOGY OF ANIMALS

III. ORGAN SYSTEMS

Organ systems are two or more organs associated in a
common function. We recognize the following systems:
(i) Integumentary, (2) Skeletal, (3) Motor, (4) Alimen-
tary, (5) Respiratory, (6) Circulatory, (7) Excretory, (8)
Reproductive, (9) Nervous, (10) Sensory.
1. Integumentary System

(^) Epidermis. In all animals the outer covering of the
body consists of a layer of epithelial cells, derived from
the ectoderm and known as the epidermis. Beneath this
layer a basement 7nembrane is present, which in some
animals is thick and serves for protection and support
(Cnidaria, Platoda). This epidermis is frequently ciliated
and it always contains gland and sensory cells and in
addition may contain nerve and muscle cells as well as
stinging cells (Cnidaria). In some animals the epidermis,
which in these cases is called hypodermis^ secretes on its
outer surface a cuticular covering which may be a thin
and flexible membrane or cuticle (hydroids, trematodes,
cestodes, annelids, rotifers), or it may be thick and flexible
(nemathelminths) or dense and inflexible except at the
joints (arthropods). In other cases the epidermis secretes
skeletal structures in certain regions only, thus giving rise
to calcareous shells (corals, moUusks, brachiopods). In
arthropods this epidermal secretion is particularly dense
and tough and is known as chitin; it may become calcified
in certain portions. In mollusks the superficial epidermis
remains naked except in a certain region, the embryonic
shell-gland, where it first secretes a cuticular covering and
then forms beneath this a dense calcareous layer, the shell ;

c 23 1

MORPHOLOGY OF ANIMALS

A

de \

Fig. 6. Sections of the Integument of A, young salamander ; ep, epidermis only two cells
thick, de, dermis consisting of branched cells and a felt-work of fibers ; B, embryo shark
showing formation of denticle (scale) ; its core of dentine d, resting on a plate of bone b,
both from the dermis de, and its outer surface covered by enamel e from the columnar
epidermal cells c ; C, developing tooth, ep, epidermis of mouth infolded to form enamel cells
ec which secrete enamel (black line) ; dp, dental papilla from dermis, covered on outer side
by odontoblasts o which secrete dentine dt. (B and C after Wiedersheim).

at the margins of the shell gland (mantle edges) the secre-
tion of these layers continues throughout life. In reptiles,
birds, and mammals the superficial epithelium (epider-
mis) becomes many layers thick and the outer layers of
cells die and are transformed into horny or cuticular sub-
stance, an adaptation to life out of water (Fig. 7). In
these three classes of vertebrates there are also a number
of characteristic epidermal outgrowths: in reptiles these
take the form of horny scales or plates; in birds
they appear as feathers which are only modified scales;
and in mammals as hair (Fig. 7), while nails or claws
are formed from the epidermis in all of these classes. In
the mammals there are also epidermal ingrowths which

1: 24]

MORPHOLOGY OF ANIMALS

may give rise to various types of glands, such as sweat,
oil, wax, and milk glands, all of which are epidermal in
origin (Fig. 7).

{b) Dermis. Beneath the surface epithelium, which is
always ectodermal in origin, there is in many animals a
fibrous or connective tissue layer known as the dermis or
corium. This layer is derived from mesenchyme and is
sometimes called "leather-skin," since leather comes from
it. It is especially well developed among echinoderms and
vertebrates, in both of which it may give rise to skeletal
spicules or plates, thus forming a dermal exoskeleton
(Fig. 10, ^/, sp). Among the vertebrates this is especially
well developed in fishes, the scales which cover the body
being of dermal origin ; in some cases these dermal scales
are covered by enamel which is derived from the epider-
mis. The same is also true of the teeth of vertebrates ; the
inner portion or dentine is of dermal origin, while the
enamel comes from the epidermis; teeth are in fact only
modified scales (Fig. 6, B and C).

Skeletal System

An internal skeleton, not the product of the integument,
is present in relatively few invertebrates, but is found in
all vertebrates and is always derived from mesenchyme.
Such a skeleton is found in sponges in the form of cal-
careous, silicious and horny spicules; in cnidarians and
ctenophores, as a supporting jelly; in many invertebrates,
as a system of connective-tissue cells and fibres; in cepha-
lopods and certain arthropods, as cartilages surrounding
the central nervous system.

On the other hand the possession of a primitive axial

C 25 •}

MORPHOLOGY OF ANIMALS

Fig. 7. Section of human skin; ep epidermis, sc stratum corneum, sg stratum granulosum,
sm stratum mucosum, mp Malpighian layer, all of epidermis ; h hair, hf hair follicle, kb
hair bulb, og oil gland, m muscle of hair, sp sweat pore, sd sweat duct, sg sweat gland, all
epidermal ingrowths ; n nerve, np nerve papilla, hp hair papilla, / sub-cutaneous fat, all
in dermis.

skeleton, the notochord (Fig. 12, B, nc)^ is one of the chief
characteristics of the Chordata; in addition to this there
are generally present in this phylum many other skeletal
elements which are usually cartilaginous or bony. In all

MORPHOLOGY OF ANIMALS

true vertebrates the notochord becomes surrounded by
cartilage, and the whole is then constricted into a series of
segments, the centra of the vertebrae; from these centra
cartilaginous arches grow dorsally around the spinal cord,
while other skeletal arches, the ribs^ surround the trunk
and later articulate with the vertebral column; finally
the ribs may be united ventrally, thus forming the ster-
num', these parts constitute the axial skeleton (Fig. 8).
The degree of development of notochord and vertebral
column in embryos and adults of different classes of verte-
brates is shown in the following table :

AXIAL
SKELETON

UNSEGMENTED
CELLULAR ROD

SEGMENTED

VERTEBRAL

COLUMN

EACH VERTEBRA

COMPOSED OF

SEVERAL BONES

EACH VERTEBRA

A SINGLE

BONE

Amphioxus
and Lowest
Chordates

In adult

Sharks

In embryo

In adult

Bony fishes

Amphibia

Reptiles

In early
embryo

In later
embryo

In young and
some adults

Birds and
Mammals

In earliest
embryo

In early
embryo

In young

In adult

In addition there is the skeleton of the head (the skull)
and that of the limbs (the appendicular skeleton). In the
lower vertebrates and in the embryos of all higher forms
the skull consists of a cartilaginous cranium partially sur-
rounding the brain, and of paired cartilaginous rods form-
ing the skeleton of the jaws and gill arches. In higher ver-
tebrates these cartilaginous elements undergo ossification,

1:27 n

MORPHOLOGY OF ANIMALS

and in addition dermal bones are formed which partially
overlie this cartilaginous basis. The appendicular skeleton
consists of two limb girdles partially surrounding the trunk
and axial skeleton, namely the pectoral and the pelvic
girdles^ and of the skeleton of the limbs themselves (Fig.
8). In the fishes these girdles and limbs are peculiar and
it is difficult to homologize their skeletal parts with those
of higher forms; in all vertebrates above the fishes, how-
ever, the relations of these parts are similar and their
homologies are not difficult to determine.
The corresponding parts and bones of the

Fore Limbs and

Hind Limbs

PARTS

BONES

PARTS BONES

Upper arm

Humerus

Thigh Femur

Fore arm

Ulna and Radius

Shank Tibia and Fibula

Wrist

Carpals

Ankle Tarsals

Palm

Metacarpals

Sole Metatarsals

Fingers

Phalanges

Toes Phalanges

3. Motor System

All animals at some time in their lives have the power of
locomotion, though in some cases this is lost before adult
life is reached and the animal becomes fixed like a plant
(hydroids, sponges, crinoids, molluscoids, and many para-
sites). However, in all these cases certain parts of the
body preserve the power of movement, though the animal
as a whole is incapable of locomotion. Animal movement
is of three fundamental types: amoeboid, ciliary, and
muscular.

(^) Amoeboid movement is limited to individual cells
and is manifested especially by free cells. It consists of a

1: 283

MORPHOLOGY OF ANIMALS

Fig. 8. Human Skeleton. Axial skeleton consists of vertebral column (fc), ribs {rb) and
sternum {st). Skull consists of cranium (cr), bones of the face, jaws (,md) and hyoid arch.
Appendicular skeleton consists of the shoulder girdle {cl, clavicle and sc, scapula), the
pelvic girdle (zV, ilium, is, ischium, pb, pubis) and the skeleton of the arm and the leg. The
bones in the arm and hand are humerus Ch), ulna {u) and radius (r), 8 carpals {c), 5
metacarpals {mc), 14 phalanges (,ph) ; the bones of the leg and foot are femur (/), patella
ipt), tibia {tb), and fibula {fb), 7 tarsals it), 5 metatarsals {mt) and 14 phalanges ipk).

MORPHOLOGY OF ANIMALS

streaming of semi-fluid protoplasm and is typically illus-
trated by the proteus animalcule, Amoeba. In this proto-
zoan small lobes, or pseudopodia^ may appear anywhere
on the body, and into one or more of these the endoplasm,
with all that it contains, may be seen to stream, at the
same time being withdrawn from the other lobes. This
flowing may continue for some time in a given direction,
the outflow of protoplasm at one end of the body being
compensated for by the inflow at the other end, thus pro-
ducing an actively progressive movement; this is, there-
fore, a vortex, the current moving forward through the
middle and backward at the periphery (Fig. 9, A). The
causes of this movement are obscure, but in some cases
it seems to be associated with temporary inequalities in
the tension of the surface layer; at points where the ten-
sion of this layer is reduced, an outflow of protoplasm
occurs, forming a lobe or pseudopod, into which proto-
plasm from the main body continues to flow so long as the
tension is least at this place. Several points of reduced
tension may exist at the same time on the surface of an
amoeboid cell, so that several lobes or pseudopodia are
found radiating from a common center. In other cases
such movement is, perhaps, due to the general contrac-
tility of protoplasm, local contraction in one part of a
cell causing an outflow in another part.
(^) Ciliary movement consists in the rhythmical beat-
ing of innumerable small protoplasmic threads {cilia)
which project from the free surfaces of certain cells and
which act somewhat like oars. Among one-celled organ-
isms the entire cell may be covered by these cilia; in all

Cso;]

MORPHOLOGY OF ANIMALS

B

c yi

Fig. 9. Types of Motor Cells. A, amoeboid cell, arrows show direction of flow ; B, ciliated
cells, showing successive stages in the stroke (1-7) and in the recovery (7-12) ; C, smooth
muscle cell, the peripheral portion of cell converted into contractile fibrillae ; D, striated
muscle cell, with four nuclei, one side of cell filled with fibrillae, each consisting of a series
of nodes and internodes, the latter with a granule at its middle.

multicellular animals they are limited to the free borders
of certain epithelial cells. The beating of a cilium includes
two movements, — the stroke, which is rapid and by which
the cilium is sharply bent in one direction, and the re-
covery of the original position which is relatively slow
and weak. It is probable that the cause of this beating is
the unequal contraction of the protoplasm on different
sides of the cilium, by which it is bent first in one direction
and then in another. All the cilia covering a free surface
beat in unison, the stroke being in one direction, and the
movement is so timed that beginning at one end of a
ciliated tract it seems to pass in a wave-like movement to
the other end (Fig. 9, B).

1:31 :

MORPHOLOGY OF ANIMALS

{c) Muscular movement^ the principal type of move-
ment in higher animals, is caused by the contraction of
muscle fibres consisting of a kind of protoplasm especially
differentiated for this purpose (Fig. 9, C, D). During the
contraction or expansion of a muscle there is no change in
its volume, the shortening of a fibre in one axis being com-
pensated for by its expansion at right angles to that axis.

All of these types of movement are found in certain
Protozoa and in many Metazoa. Amoeboid movements
are, however, usually restricted to free cells without mem-
branes or dense cortical layers of protoplasm, such as
certain egg cells, embryonic cells, endoderm cells, excre-
tory, pigment, and lymph cells of various Metazoa ; in no
case is this type effective in the movement of large bodies.
In the larvae of all phyla except the nemathelminths
and arthropods, locomotion is brought about, at least in
part, by cilia, and even among the adult forms of many
lower metazoans this is the principal type of locomotion
(ctenophores, turbellarians, nemertines, rotifers). Among
the nemathelminths and arthropods cilia are usually lack-
ing throughout the whole life-cycle. In large animals
locomotion is effected entirely by muscular contractility,
while cilia are limited to certain regions where by their
beating they produce currents. Muscle fibres are found
in all Metazoa; they are of two kinds, striped and non-
striped or smooth (Fig. 9, C, D) ; the latter are of very
wide distribution throughout the Metazoa, the former
are limited to a few phyla (mollusks, arthropods, chor-
dates). Smooth muscle is contractile to a much greater
extent than striped muscle, but is much slower in action.

1:32 3

MORPHOLOGY OF ANIMALS

The muscular system may consist of isolated fibres such
as are found in many cnidarians, platodes, and rotifers, or
these fibres may be united into bundles or sheets as is the
case in most higher animals; these groups of muscles show
many differences and can be compared only in a general
way.

In general the arrangement of the body muscles depends
upon the presence or absence of a skeleton. Animals such
as annelids, which have no skeleton, usually have the
body musculature arranged in the form of two coats, an
outer layer of circular fibres and an inner of longitudinal
ones; while the intestinal musculature is also arranged in
two coats, the outer (next the coelom) longitudinal and
the inner circular (Fig. 5, C, D). If an exoskeleton is
present, as in arthropods, these muscular layers of the body
wall are broken up into bundles which become attached
to the skeleton ; if an endoskeleton is present, as in verte-
brates, the muscles become attached to the bones, many of
which serve as levers.

The locomotor apparatus of echinoderms is unique,
consisting of a great number of tube-feet, which are hol-
low muscular tubes, closed at the end by a sucking disk.
The cavity of each tube is connected with the water-vas-
cular {ambulacral) system within the body, from which
water can be forced into the tube-feet. In this way they
are protruded until the sucking disk touches and becomes
attached to some object; then by contraction of the mus-
cles of the tube-foot the water is forced back into the
water system, and by simultaneous action of many of these
feet the body is slowly warped along (Fig. 10).

[ 33 3

MORPHOLOGY OF ANIMALS

FIG. 10. Cross section of arm of Starfish ; ep, epidermis ; d, dermis containing calcareous
plates ipl) and spines {sp) ; c, coelom ; gp, gastric pockets, lined with endoderm (en); rn,
radial nerve, lying in epidermis ; tf, tube foot, containing a water tube which communicates
with an ampulla (am) ; s, sucking disk at end of foot.

4. Alimentary System

With the exception of a few internal parasites which
absorb their food in a digested condition from the bodies
of their hosts, some form of digestive system is present
in all animals.

Digestion is the process of rendering insoluble foods
soluble and dialyzable. One of the distinguishing charac-
teristics of animals is that they, unlike plants, take in,
through a mouth opening, solid food, much of which is in
an insoluble condition. This process is called ingestion.
By the process of digestion some of this insoluble food
is rendered soluble, and hence capable of diffusing to all
parts of the organism, where by a process known as assimi-
lation some of it is built up into the substance of the proto-

C 34 !]

MORPHOLOGY OF ANIMALS

plasm itself. After the substances rendered soluble by-
digestion have been removed from the food, the indigesti-
ble remnants are cast out of the body in solid form
{egestion).

Intracellular Digestion. Among the Protozoa digestion
occurs within the body of a single cell, that is, it is intra-
cellular. The same is true of sponges and some Cnidaria,
in which the food, consisting of microscopic particles, is
ingested and digested by certain epithelial cells, lining the
endodermal cavities. In all animals above the sponges
intracellular digestion is limited to the endoderm cells
and to certain free cells, such as white blood-corpuscles
(leucocytes^ and it is of decreasing importance as one
ascends the scale.

Extracellular Digestion. In all animals except the
lowest, digestion occurs principally in a digestive cavity
surrounded by cells which pour their secretions into the
cavity. By the action of these secretions certain insoluble
food substances are transformed into soluble ones. This
digestive cavity is in all cases derived from the enteron or
primitive digestive cavity of the gastrula, and in the sim-
plest cases it is little more than a sac whose walls may be
folded into ridges or septa, thus enlarging the digestive
surface (Anthozoa), or they may be extended to form
tubular canals, the gastro-vascular system, by means of
which the digested food is also distributed to all parts of
the animal (Scyphozoa, Ctenophora, Turbellaria, Fig.
11, A).

Openings into Digestive Cavity. In all Cnidaria except
the lowest class, and in all animals above the Cnidaria,

C 35 3

MORPHOLOGY OF ANIMALS

^.

^"o:

^^l

FIG. 11. Diagrams of a Flat-worm {Planarian). A, showing gastro-vascular system in
black, water-vascular (excretory) system as unshaded tubules ending in flame-cells, muscular
pharynx, containing oesophagus, cross-hatched ; B, showing female reproductive organs on
left, single ovary at anterior end of oviduct, which receives along its sides vitellaria, or yolk
ducts ; testes, numerous shaded spheres on right ; uterus and copulatory organs, posterior
to pharynx ; C, showing nervous system consisting of brain at anterior end, two nerve cords
running posteriorly through body and giving off numerous lateral branches. (After Hatschek).

MORPHOLOGY OF ANIMALS

the ectoderm surrounding the mouth is folded in at the
mouth opening, thus forming an ectodermal tube, or
stomodaeum, which opens at the inner end into the gastric
cavity. Among chordates this ectodermal invagination
forms only the mouth cavity, the oesophagus being derived
from the endoderm. In all Cnidaria, Ctenophora, and
Platoda there is but one opening into the gastric cavity,
the mouth, and through this single opening food is taken
in and undigested remnants cast out. In the Nemertinea,
and with a few exceptions in all higher animals, there
is a second opening into the gastric cavity, namely the
anus, through which the ejecta pass out. The anus is
formed by an infolding of the ectoderm which meets and
fuses with a portion of the gastric wall; this terminal
ectodermal portion of the digestive tract is the hind-gut, or
proctodaeum.

Fore^ Mid, and Hind-Guts. With the formation of an
anus the digestive tract becomes tubular, with mouth at
one end and anus at the other, and the entire canal is
divisible into three portions, an ectodermal stomodaeum
or fore-gut, an endodermal mid-gut, and an ectodermal
hind-gut. The relative development of these three por-
tions differs much in difFerent phyla; for example among
chordates the fore-gut is limited to the mouth-cavity, and
the hind-gut to an insignificant terminal portion of the
intestine, while the mid-gut gives rise to all the inter-
vening portions of the digestive tract (Fig. 12, B). Among
arthropods, on the other hand, the mid-gut is limited to an
extremely small portion of the digestive tube between the
stomach and the intestine, while all the remaining portions

1 31 :

MORPHOLOGY OF ANIMALS

are derived from the fore and hind-guts. In all the higher
animals the fore and mid-guts may be subdivided into
mouth-cavity, pharynx, oesophagus, stomach, and intes-
tine (Fig. 12, A and B) and in some cases these portions
may be further subdivided, as for example in birds where
the oesophagus gives rise to an enlargement, the crop, the
stomach is divisible into a glandular stomach and a grind-
ing stomach, or gizzard, and the intestine consists of two
portions, the small and the large intestine. Finally into a
portion of the hind-gut the excretory and sexual ducts as
well as the intestine may open, in which case this common
chamber is called the cloaca (Fig. 12, B, cV).

Various portions of the fore-gut may be armed with
teeth, usually of a horny character among invertebrates,
and of calcareous material in vertebrates; salivary glands
open into it, and both mouth and pharynx may be pro-
trusible. The digestive and absorptive surfaces of the mid-
gut may be increased in three ways, — either ( 1 ) by an in-
crease in length, in which case it becomes folded or coiled,
or (2) by folds which project into the canal, or (3) by
diverticula, that is blind sacs or tubes, which open out
from the canal ; in many higher forms all of these methods
coexist in the same individual. The extent of the digestive
surface depends primarily upon the character of the food ;
if the latter is highly nutritious the digestive surfaces are
much smaller than where it is poor in nutrition. In car-
nivorous mammals, for example, the alimentary tract is
from four to five times the length of the body, whereas
in certain herbivora it may be from twenty to thirty times
the length of the body.

MORPHOLOGY OF ANIMALS

^o

MORPHOLOGY OF ANIMALS

Digestive Cells and Glands. In the simplest Metazoa
it is probable that all the cells lining the digestive cavity
are alike and that they all secrete the same digestive fluids;
in more complex animals the cells differ in structure in
different portions of the tract. Some of these cells form
diverticula or blind tubes opening out from the canal ; thus
digestive glands are formed which pour particular diges-
tive secretions into the alimentary canal. Those most gen-
erally present are the salivary glands, opening into the
fore-gut, and the liver and pancreas (or where both are
united, as often happens among the invertebrates, the
hepato-pancreas), which open into the mid-gut.

Movements of Food in Alimentary Canal. In all of the
lower invertebrates except the round worms the food is
moved about in the alimentary tract by means of cilia
or by general contractions of the body. In all higher
forms the contraction of muscle fibres surrounding the
canal plays an important part in this movement, though
cilia may also be present. In the chordates both longi-
tudinal and circular muscles surround the canal and by
their rhythmical contractions produce a wave-like con-
striction of the canal (^peristalsis^ ^ which passes along the
canal from mouth to anus.

5. Respiratory System.

Respiration consists in the exchange of gases between
the body and the medium which surrounds it. The gas
given off from the body is principally carbon-dioxide, one
of the products of combustion within the body, while that
which must be supplied to it is oxygen. Since oxidation
is the one essential feature of destructive metabolism

MORPHOLOGY OF ANIMALS

which occurs in all living matter, it follows that respira-
tion is a universal function in organisms. In small and
simple animals this exchange of gases takes place directly
between the living cells and the surrounding medium and
occurs all over the surface of the body. In more complex
forms with body fluids the exchange takes place between
the cells and the body fluid or blood {internal respiration)
and then between this fluid and the external medium (^.x:-
ternal respiration). There are no special organs for in-
ternal respiration. External respiration may take place
through the general integument of the body without the
aid of any specific organs, as is the case in all small animals
and in many larger ones, — for example flat worms, round
worms, rotifers, small annelids, and even some vertebrates,
such as the lungless salamanders. However, in most ani-
mals of any considerable size, special organs exist to
facilitate this exchange.

Branchiae^ Gills, In aquatic animals vascular processes
are present which serve to bring the blood into close rela-
tion with the water. These processes, which are called
branchiae or gills^ are covered by a thin epithelium
through which an interchange of gases contained in the
blood and in the water can readily take place. To facili-
tate this interchange the gills are usually much folded
or branched so as to afford a large surface, and they are
frequently covered by cilia which serve to keep the water
in motion, while at the same time the blood is circulated
through them. The simplest type of gill is a ciliated ten-
tacle, which may also serve other functions, as in Mollus-
coida and some MoUusca (Fig. 13, A); such gills may

1:41 3

MORPHOLOGY OF ANIMALS

m-

ac .h .p

aa

Fig. 13. Respiratory and Circulatory Organs of Snail and Crayfish. A, Snail (Paludina) ; a,
auricle; v, ventricle of heart; aa, abdominal aorta; ac, cephalic aorta; k, kidney; /, liver;
g, gills ; V, vein carrying blood from body to gills ; bv, branchial vein carrying blood from
gills to heart (blood goes from heart to all parts of body, comes back to kidney and gills
and then goes to heart); t, tentacles; m, mouth; eg, cerebral ganglion with eye on its
anterior side ; pg, pedal ganglion with statocyst on its posterior side ; /, foot. (After Leydig).

B, Crayfish ; h, heart with 3 ostia (openings) on each side into pericardium (/>) ; aa, ab-
dominal aorta ; ac, cephalic aorta ; as, sternal artery. Arteries are shown as empty tubes,
veins and sinuses as black. Blood goes out from heart to all parts of body, comes back
through veins and sinuses to gills, and then goes to the pericardium and heart. (After Claus),

MORPHOLOGY OF ANIMALS

become branched or plume-like or may fuse together into
plates (Lamellibranchia). Gills are situated on those parts
of the body where they will be most exposed to currents of
water, and they occur in the most extraordinarily different
positions in different phyla; thus they may be found on
the limbs (some annelids, crustaceans, Fig. 13, B), on or
around the head (sedentary annelids, molluscoids), along
the sides of the body (primitive mollusks), on the lateral
walls of the pharynx (chordates), or as outgrowths of
the hind-gut (holothurians). Homology being "corre-
spondence in the relative position and connection of
parts," there can of course be no homology between struc-
tures occurring in such diverse positions, and yet within a
given phylum they may be homologous and of high mor-
phological value (for example chordates).

In the chordates a series of gill-clefts opens right and
left through the walls of the pharynx (Fig. 12, B), and
in the lower classes of this phylum the gills are found as
highly vascular plates or tufts on the outer side of the
arches lying between these clefts; water is taken in
through the mouth and then forced out through the gill-
clefts and thus over the gills. In the higher classes of the
phylum (reptiles, birds, and mammals), imperforate gill-
clefts and gill-arches are present during embryonic life,
though at no time in their entire life history do these
animals have gill filaments and respire water. The con-
stancy of gill-clefts and arches among vertebrates gives
this character a high value in determining the affinities of
such doubtful forms as Balanoglossus, Cephalodiscus, and
Tunicata.

1:433

MORPHOLOGY OF ANIMALS

The fate of the embryonic gill-pouches in different
classes of vertebrates is shown in the accompanying table:

Embryonic
Gill Pouches

Become in adult

1^/ Pair

2nd Pair

2trd Pair

Ath Pair

5th Pair

Lower Fishes

Spiracle

and
Thymus

Open Slit

and
Thymus

Open Slit

and
Thymus

Open Slit

and
Thymus

Open Slit

and
Thymus

Lower Amphibia

Never
Open

((

«

"

Never
Open

Higher Amphibia

Never

Open

Eustachian

Tube and

Middle

Ear

Open in Larva, closed in Adult

Reptiles, Birds
Mammals

"

Never open at any stage.

Remnants of various pouches become

Thymus, Tonsils, Parathyroids.

Tracheae^ Lungs. In animals which do not dwell in
water, and in some few which do (insect larvae, lung-
fishes, etc.), certain infolded portions of the body wall or
of the pharynx occur into which air is drawn and from
which it is again expelled. Among invertebrates these in-
folded portions are generally derived from the skin;
among vertebrates from a portion of the alimentary canal,
the pharynx. In the case of insects and allied forms
(Tracheata) these infolded portions have the form of
much branched tubes, the tracheae, which reach to all
parts of the body, the terminal twigs of the tracheal sys-
tem of tubes being found in connection with almost every
bit of tissue in the body (Fig. 15, B). These tracheae open
to the exterior though closeable pores, the spiracles, situ-
ated on the sides of the body (Fig. 15, A) ; air is taken in

MORPHOLOGY OF ANIMALS

er--

FiG. 14. Gill Clefts of Vertebrate Embryos. A, Shark ; B, Chick, C, Man ; g, the gill clefts
(black lines) between which are the gill arches (unshaded) ; M, mid-brain, H, hind-brain ; e,
eye, er, ear; ft., fore-limb, hi, hind-limb. (After Scott).

through these pores and by means of the tracheal tubes
penetrates to all parts of the body, the exchange of gases
taking place directly between the tissues and the tracheae
(Fig. 15, B). Among the vertebrates the lungs are an
evaginated portion of the pharynx. The swim-bladder,
which in most fishes is a hydrostatic apparatus, (Fig. 12,
B, /), in the lung fishes (Dipnoi) becomes highly vascular
and may serve as a lung. In all higher vertebrates the lung
is paired, and its walls which in the lower classes are rela-
tively simple, become much folded and very richly sup-

C45 :

MORPHOLOGY OF ANIMALS

3 2 1

Fig. 15. Respiratory and Sensory Organs of Grasshopper. A, Surface view, showing head
<A), thorax (i) and abdomen (a) ; the head bears antennae {an), eyes (e) and mouth palps
(^p) ; the thorax bears 3 pairs of legs and 2 pairs of wings (here removed) ; the abdomen
consists of 7 segments and a terminal portion, each thoracic and abdominal segment bears
a spiracle (1-10), and the last thoracic segment has a tympanum or ear drum (0- (After
Hatschek).

A, Internal Air Sacs {as) and Tracheae ; c, cephalic trachea, d, dorsal trachea ; st, stigmatal
trachea into which spiracles (4-10) open ; v, ventral trachea. (After Packard).

plied with blood vessels. The exchange of gases here takes
place between the blood and the air within the lung, and
in most vertebrates the oxygen-carrying capacity of the
blood is increased by the presence of haemoglobin (the
coloring matter of red blood-corpuscles) which enters
into a loose chemical combination with oxygen.

1:463

MORPHOLOGY OF ANIMALS
6. Circulatory System

The physiological significance of the circulation of fluids
within the body is the distribution of nutriment and, in
some cases, of oxygen to all parts. In the simplest Metazoa
(Cnidaria, Ctenophora) there is no need of a special cir-
culatory apparatus other than that which is furnished by
the gastric cavity itself; this may branch and extend to
various parts of the body of a jellyfish or hydroid colony,
thus forming a g astro-vascular system^ through which the
distribution of nutriment takes place; the branched gastric
cavity of certain turbellarians also serves a similar func-
tion (Fig. 11, A). Circulation of body fluids also occurs
in many lower animals without the aid of any special cir-
culatory apparatus; in such cases lymph, containing the
products of digestion, is distributed through all the inter-
cellular spaces in the primary body-cavity, and by the
contractions of the general musculature of the body it is
kept in irregular movement.

Blood-Vascular System. With the single exception of
the nemerteans a blood-vascular system is found only
among animals with a secondary body-cavity, or true
coelom, and is lacking even in some of these, particularly
such as are quite small or are evidently degenerate forms.
With a few exceptions it is present in mollusks, echino-
derms, annelids, arthropods, and all chordates. In its sim-
plest form it consists of branching and anastomosing tubes
which contain blood. The walls of the tubes are composed
of flattened epithelial cells (endothelium) which may be
surrounded on the outside by muscle and connective tissue
fibres. The blood which fills these vessels consists of a

1:47 3

MORPHOLOGY OF ANIMALS

fluid or plasma within which floating cells or corpuscles
are almost invariably present. With the increasing com-
plexity of this system the walls become thicker by increase
of the muscular or connective tissue coats, and in certain
parts of the system the vessels become larger. The mus-
cular walls may be pulsatile throughout the entire length
of a vessel, or this function may be limited to a small por-
tion of a large vessel, which is then known as a heart;
even in the highest animals the heart is only a differentia-
tion of a simple pulsatile blood vessel. The vessels leading
away from the heart are arteries, those through which the
blood flows back to the heart are veins, while the small
thin-walled vessels connecting these two, and through the
walls of which the plasma escapes into the tissues, are
capillaries. In annelids there is a large longitudinal vessel
on the dorsal side and another on the ventral side of the
body, which are connected in each somite by commissural
vessels. The dorsal vessel is pulsatile along its whole
length, and peristaltic contraction waves can be seen in
a living worm to pass from its posterior to its anterior
end ; correspondingly the blood flows forward in the dor-
sal vessel, down through the commissural vessels into the
ventral one, and then backward through the latter to the
posterior portion of the body, where the blood ascends
through the commissural vessels to the dorsal vessel, after
which the same circuit is repeated. Throughout this whole
course the blood flows through vessels with definite walls,
and therefore the circulation is said to be closed.

In mollusks and arthropods a heart is present which is
more complicated and more complete than in annelids. In

1:483

MORPHOLOGY OF ANIMALS

arthropods this generally consists of a thick-walled, pul-
satile tube lying on the dorsal side of the body and extend-
ing through several somites; in each somite are a pair of
openings, the ostia, which open into the heart from the
pericardium, and through which returning blood enters
the heart (Fig. 13, B). In the moUusks also the heart is
of a compact type and is divided into auricular and ven-
tricular portions. Primitively two auricles are present,
though in some gasteropods this number is reduced to one ;
in all mollusks there is but one ventricle (Fig. 13, A). In
primitive arthropods and mollusks the blood flows out of
the ventricle at both its anterior and posterior ends; in
more highly differentiated members of these phyla, out of
the anterior end only. The vascular system of arthropods
is not a continuous system of vessels; the arteries soon end
in lacunar spaces in the tissues, and from these spaces the
blood is gathered into large sinuses and thence flows back
to the heart. These lacunar spaces and sinuses are not true
vessels, since they do not have definite walls, but they are
derived from the primary and secondary body-cavities;
the circulation is therefore an open one. In mollusks the
vascular system is more extensive than among arthropods,
but here also the circulation is open, the arteries being
connected with the veins by a system of lacunar spaces
instead of by capillaries. Finally among the echinoderms
and chordates the circulation is closed as among the anne-
lids; that is, the blood throughout its entire circuit is con-
tained within definite vessels.

Circulation and Respiration. The manner in which
blood is supplied to the respiratory organs is of great im-

MORPHOLOGY OF ANIMALS

portance in explaining the structure of the circulatory
organs in air-breathing vertebrates. In annelids, crusta-
ceans, and mollusks the blood flows directly from the heart
to all parts of the body, whence it is gathered into trunks
which carry it to the gills; from these organs it is then
returned oxygenated to the heart (Fig. 13, A, B). In
fishes the blood passes from the heart directly to the gills,
whence it is gathered into the dorsal aorta and distributed
to all parts of the body; it is then returned laden with
waste products from the tissues to the heart (Fig. 16, A).

hr

Fio. 16. Diagrams of the Course of Circulation in different classes of Vertebrates. A, Fish,
blood goes from the heart {h) to the gills {br), thence to the swim bladder (/) and body {k)
and then back to the heart. B, Frog, blood goes from the heart (^h) through 3rd and 4th aortic
arches to the body {k) and then back to the heart (A) ; through the 6th aortic arch to the
lungs and then back to the heart. C, Birds and Mammals, blood goes from left side of heart
{hk) through the 3rd and 4th aortic arches to the body {k) and then back to the right side of
the heart, whence it goes to the lungs (/) and then back to the left side of the heart. (After
Hatschek).

The heart of fishes consists of one auricle and one ven-
tricle ; it is essentially a simple tube more or less bent upon

1:503

MORPHOLOGY OF ANIMALS

itself (Fig. 17, A and A^j. In air-breathing amphibia a
part of the blood passes directly from the heart to the
lungs, whence it returns to the heart oxygenated, while
a part of it goes at once to the body (Fig. 16, Bj ; the
former is known as the pulmonary, the latter as the sys-
temic circulation. In these animals the heart is incom-
pletely divided by a partition which separates the
auricular chamber into two auricles, but which leaves the
ventricle undivided (Fig. 17, B). The blood returning
from the body is carried into the right auricle, while that
from the lungs goes into the left; in the ventricle both
kinds of blood mingle to a certain extent, though by a
peculiar arrangement of folds and valves the larger part
of the oxygenated blood which enters the left auricle is
pumped to the anterior part of the body, while the blood
from the right auricle goes to the lungs and to the posterior
part of the body. Finally in all birds and mammals and in

A

B

0

Fig. 17. Diagrams of Hearts of different classes of Vertebrates. A, Heart of Fish viewed
from left side ; i auricle and l ventricle ; arrow shows course of blood through the heart,
entering the thin-walled auricle (a), passing through the thick-walled ventricle {v) and
issuing from the heart through the conus arteriosus {c) into the ventral aorta; the valves arc
flaps protruding into the ventricle and conus. Ai, same viewed from ventral side, B, Heart of
Amphibians, 2 auricles and l ventricle. C, Heart of Birds and Mammals, 2 auricles and 2
ventricles; course of blood shown by arrows. (After Hatschek).

n5i :

MORPHOLOGY OF ANIMALS

rda

Fig. i8. Diagram of Embryonic Aortic Arches and their adult Derivatives in different
classes of Vertebrates, all viewed from ventral side. A, Embryonic Aortic Arches of all
vertebrates, six pairs (1-6), running from the ventral aorta (fa) between the gill-slits to the
right {,rda) and left dorsal aortae which join posteriorly to form the single dorsal aorta
(da) ; c, carotid artery going to head ; ta, truncus arteriosus where it issues from the heart.

B, Bony Fishes (Teleostei) with arches 3-6 preserved, each breaking up in the gill filaments.

C, Tailed Amphibia (Urodela), arches 3-6 preserved, 3d becoming the common carotids (cc)
and 4th, the great arches of the aorta (aa), while 5th and 6th are reduced and the latter
gives off a pulmonary artery (p) to the lungs. D, Reptiles, 3rd and 4th arches preserved as
in C, 5th arch disappears, 6th becomes pulmonary {p). E, Birds, 3rd arch as in D, 4th
preserved only on right side, 5th and 6th as in D. F, Mammals, 3rd arch as in D and E,
4th becomes the great arch of aorta {aa) on left, and sub-clavian (j) on right, 5th and 6th
as in D and E. (After Boas).

MORPHOLOGY OF ANIMALS

the highest reptiles (Crocodilia) the heart is completely
divided by a partition into two auricles and two ventricles,
and a double circulation, systemic and pulmonary, is
established (Fig. 17, C). The blood from the left ventricle
goes at once to all parts of the body, whence it returns to
the right auricle ; it then falls into the right ventricle and
is pumped from that to the lungs; there it is oxygenated
and returned to the left auricle, and then from the left
ventricle it is again sent out to all parts of the body (Fig.
16, C).

The transformations of the embryonic aortic arches,
lying in the pharyngeal walls between the gill-slits, in
different classes of vertebrates is shown in Fig. 18 and in
the following table :

A^tl^Arches } ^"°"^^ ^^ ^^"^^ '^^^''

\ St, 2nd Pairs

Zrd

4fk

Stk

6tk

Fishes

Disappear

Persists

Persists

Persists

Persists

Lower
Amphibia

«

Common
Carotid

Right
and Left
Arches
of Aorta

Right and Left
Pulmonary

Higher
Amphibia
and
Reptiles

«

Disappear

Birds

«

Right
Arch
of Aorta

(<

Mammals

((

"

Left Arch
of Aorta

<<

"

The condition of the heart in the embryos and the
adults of different classes of vertebrates is summarized in
Fig. 17 and in the accompanying table:

: 53 2

MORPHOLOGY OF ANIMALS

Heart with

1 Auricle
1 Ventricle

2 Auricles
1 Ventricle

2 Auricles
2 Ventricles

Fishes

In adult

Amphibia and

In embryo

In adult

Lower Reptiles

Crocodilia

Birds

Mammals

In early
embryo

In later
embryo

In adult

Excretory Syste?n

Excretion is the process of removing nitrogenous waste
products, particularly urea and allied compounds, from
the body. These waste substances are formed as the result
of protein combustion within the body, and as this form
of metabolism is universal among animals nitrogenous
waste substances are everywhere formed. With few excep-
tions all animals possess some form of excretory organ ; in
fact this is one of the distinguishing characteristics of
animals as contrasted with plants. In Protozoa the excre-
tory organ is a pulsatile vacuole which gradually fills with
fluid containing these waste products and then suddenly
contracts, forcing this fluid out of the body. In coelenter-
ates excretion is probably performed by isolated gland
cells, so that no specific organ exists for this function. Even
in higher animals excretion is performed to a limited ex-
tent by individual cells or small glands, for example the
chlorogogue cells of annelids, the dermal glands of Crus-
tacea, and the sweat glands of mammals.

Nephridia. In all higher animals a special excretory
organ exists; this usually consists of minute tubules which
take up the waste substances and pass them through the
tubules to the exterior ; such an excretory tubule is known

i: 54 3

MORPHOLOGY OF ANIMALS

by the general name of nephridium. The forms of
nephridia differ considerably in different phyla, but two
principal types may be recognized; these are the proto-
nephridia, or water-vascular system, and the meta-
nephridia.

Proto-riephridia are found in flat-worms (Fig. ii. A)
and rotifers; that is, among worm-like animals without a
true coelom; they are also found as the larval excretory
organ ("head kidneys") in annelids. They consist of more
or less branched tubules opening at one or more places
to the exterior, while the inner end of each tubule is closed
by a single large cell which bears a tuft of long cilia pro-
jecting into the lumen of the tubule (Fig. 19, A). This
tuft beats with undulatory movement and looks somewhat
like the flickering flame of a candle, whence it is called a
''flame' and the large cell which bears it is a ''flame cell.'"
The tubule itself is usually composed of a single series of
long glandular cells ("drain pipe" cells), with the lumen
running through the middle of each cell. In larger branches
of the protonephridium the walls of the tubule may be
formed of many cells which are ciliated on the side next
the lumen. These cilia as well as the "flame" drive fluids
within the lumen to the exterior. It is probable that these
fluids are transuded body fluids containing nitrogenous
waste substances which first appear as vacuoles in the
flame cells and then discharge into the lumen of the tubule
(Fig. 19, A)

Meta-nephridia are found among annelids, moUusks,
molluscoids, prototracheates, and chordates, while a modi-
fled form exists in crustaceans. Typically each meta-

: S5 1

MORPHOLOGY OF ANIMALS

tnt

tnt

Fig. 19. Types of Excretory Organs. A, Protonephridium of flat-worm or rotifer (Acoelo-
mata) ; fc, flame-cells with excretory vacuoles and "flame" of cilia in lumen ; dp, "drain
pipe" cells of tubule; np, nephropore opening to exterior; int, integument. B, Metanephrid-
ium of annelid (Coelomata) ; /, ciliated funnel opening into coelom ; ini, integument. C,
Metanephridium (mesonephric tubule) of frog; /, ciliated funnel opening into coelom; g,
glomerulus, containing a plexus of capillaries, between the entering artery and the emerg-
ing vein ; the latter spreads out over the walls of the tubule ; the tubule opens into the
segmental duct (sd), which empties into the cloaca.

nephridium consists of a tubule opening to the exterior at
one end and into the body-cavity (or some portion of it
such as the pericardium) at the other (Fig. 20, A). Where
it opens into the body-cavity the tubule is widened and
covered with long cilia and is known as the ciliated funnel
or nephrostome (Fig. 19, B and C, /). Following this is
the glandular portion of the tubule, consisting of a single
series of perforated ("drain pipe") cells, and following
this is an epithelium, composed of many cells, which form
the wall of the lumen. The latter is ciliated throughout,
and by the action of these cilia, together with those of the
ciliated funnel, coelomic fluid is drawn into the tubule
through the funnel and then driven to the exterior. Finally

[ 96 1

MORPHOLOGY OF ANIMALS

the terminal portion of the tubule, which is derived as an
invagination from the ectoderm, serves as a collecting tube
or reservoir (Fig. 19, B). Generally a single pair of these
tubules is found in unsegmented animals, such as Mol-
lusca and MoUuscoida; this number may be reduced,
however, as in the Polyzoa where they are entirely lack-
ing, or in certain Gasteropoda where one of them is sup-
pressed, or it may be increased as in the case of certain
Cephalopoda (Tetrabranchia), where two pairs are pres-
ent. In segmented animals, such as annelids, prototrache-
ates, and chordates, it is probable that originally one
pair existed in every somite, and this is still approximately
the case in some of the simpler members of these phyla
(Fig. 20), while in higher forms they are limited to certain
segments and have disappeared from others (Fig. 2 1 ) . The
segmental character of these organs is so characteristic in
the phyla last named that they are called segmental organs.

Kidneys of Chordata; Pronephros. In the Chordata
these organs undergo modifications which deserve
especial mention. They lie in the dorsal portion of the
body-cavity on each side of the notochord. Only in Am-
phioxus do they open individually to the exterior ; in other
chordates the peripheral ends of the tubules unite on each
side of the body into a duct which extends backwards and
opens into the cloaca near the anus; this is the segmental
duct (Fig. 20, B). This earliest system of segmental tu-
bules in chordates is known as the pronephros^ and it ex-
tends throughout the entire trunk of the lowest vertebrates
(cyclostomes), though in all higher forms it is limited to a
few anterior somites just back of the head (hence called

1: 57 3

MORPHOLOGY OF ANIMALS

np-

i

:^^

I

Fig. 20. Diagrams of Excretory System in Annelid (A) and in primitive Vertebrate (B) ;
/, ciliated funnel ; g, glomerulus ; np, nephropore ; sd, segmental duct ; cl, cloaca, cut off
from the intestine at z ; a, anus. (After Semper).

"head kidney") and is usually a purely embryonic organ
(Fig. 21, A).

Mesonephros. In all vertebrates above the cyclostomes
longer and more complicated tubules are formed in the
somites behind the pronephros, which also open into the
segmental duct at one end and into the body-cavity at the
other. Near the ciliated funnel a knot of blood vessels forms
on the side of the tubule and infolds its wall; this
is the glomerulus or Malphigian corpuscle (Fig. 19 C, <7).
Many of the tubules in this region then lose their ciliated

i: 58 n

MORPHOLOGY OF ANIMALS
funnels and no longer open into the body-cavity, the fluid
which passes through the tubule being transuded plasma
from the glomerulus; at the same time the single pair of
tubules originally present in each somite may give rise
to others by budding, so that several may be found in each
somite. This second edition of the nephridial system of
vertebrates is known as the mesonephros^ and it is the per-
manent excretory organ of iishes and amj^hibians (Fig.
21, C), while it is only an embryonic organ in reptiles,
birds and mammals (Fig. 21, B, mes).

Metanephros. Finally, in the last-named classes, the
definitive kidney or metanephros appears in several of the
somites posterior to the mesonephros (Fig. 2 1, D, E, met^.
Its tubules, while similar to those of the mesonephros, are
still more complex, having no trace of a ciliated funnel,
and by budding, very many of them are formed in each
somite. The duct into which they open is called the
ureter (Fig. 21, D, E, 2/), and it is an outgrowth from
the segmental duct. It is thus to be seen that the very com-
plex excretory system of vertebrates can be derived, step
by step, from the simple nephridial system of such in-
vertebrates as the annelids.

The stage of development of the kidney system in

KIDNEY SYSTEM

PRONEPHROS

MESONEPHROS

METANEPHROS

Fishes

In embryo

In adult

Amphibia

Reptiles

Birds

Mammals

In early
embryo

In later embryo.
Transformed
into sex ducts
in adult male

In adult

C 59 3

MORPHOLOGY OF ANIMALS

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MORPHOLOGY OF ANIMALS

embryos and adults of different classes of vertebrates is
shown in the accompanying table :

Theories of Excretion. There are two conflicting theo-
ries as to the method of excretion, {a) The theory of
Heidenhain holds that the cells of the nephridial tubules
take urea out of the surrounding blood capillaries and pass
it into the tubules, while the fluid which comes through
the ciliated funnels or glomeruli merely flushes out the
tubules. Excretion on this theory consists in selection of
urea by the cells of the tubules, {b) The theory of Ludwig
maintains that fluids containing urea pass into the tubules
through the funnels or glomeruli and the cells of the
tubules then take up pure fluid from the tubules and pass
it back into the surrounding capillaries. The cells of the
tubule therefore merely concentrate the solution of urea
in the tubules. The latter theory is the more probable.

Excretory Ducts as Sex Ducts. Finally, the nephridia
may carry off from the body-cavity not only coelomic
fluid, but also cells which are set free into this fluid; some
of these cells in the annelids may be loaded with urates
which are thus carried to the exterior (chlorogogue cells),
but the most important of the cells which thus escape from
the coelom are the sex cells, ova and spermatozoa. The
excretory ducts may be especially modified for carrying
off these sex cells, in which case they are known as gono-
ducts. Even among the vertebrates the oviducts and sper-
miducts are derived from the nephric system. In some
vertebrates the oviduct is split off in the embryo from the
segmental duct and opens into the body-cavity at its an-
terior end through a pronephric funnel (Fig. 21, C, /) ; its

C 61 ] .

MORPHOLOGY OF ANIMALS

posterior portion may enlarge to form a uterus. The sper-
miduct is the segmental duct itself (Fig. 21, C, sd) ; in
Amphibia it carries off both urine and spermatozoa (Fig.
21, C) ; in animals above the amphibians, which have a
metanephros and ureter, it acts exclusively as a spermi-
duct (Fig. 21, D). Some of the mesonephric tubules
(nephridia) grow into the testes and become the vasa
eferentia and epididymis (Fig. 21, D, ep). There is thus
an intimate relation between excretory ducts and genital
ducts, and therefore these two systems in vertebrates are
usually classed together as the urino-genital system.

8. Reproductive System

Reproduction among animals is both sexual and asexual ;
the former occurs among all animals, the latter is limited
to the lower forms and to the constituent cells and organs
of higher ones.

Sexual Reproduction; Sexes. Sexual reproduction or
amphigony consists in the union of the nuclei of two cells,
the sex cells or gametes, to form a single nucleus or cell,
of double origin, the oosperm or zygote, from which a new
individual similar to the parental form develops. If the
gametes are approximately equal in form and size their
union is spoken of as conjugation, if they are very unlike
in these respects they are called ova and spermatozoa,
and their union is known as fertilization. Both conjuga-
tion and fertilization occur among the Protozoa, whereas
Metazoa reproduce by means of differentiated sex cells,
namely ova and spermatozoa. In a few animals ova have
the power of developing without previous fertilization,
the process being known as parthenogenesis. If such de-

:62 D

MORPHOLOGY OF ANIMALS

velopment without fertilization occurs in larval forms
which have not completed their development it is known
as paedogenesis. In many animals the sexes are separate, —
that is, ova and spermatozoa are produced by dif-
ferent individuals, males and females, and the species is
dioecious; in some cases, however, both kinds of sex cells
are produced by the same individual, which is then said
to be hermaphrodite^ and the species is monoecious. Sepa-
rate sexes were probably derived from hermaphrodites by
the suppression of female organs in the male and of male
organs in the females; in fact each sex has rudiments of
the organs of the opposite sex.

Ovaries and Testes. The essential reproductive organs
are the gonads, or the glands which produce ova and sper-
matozoa, namely the ovaries and the testes. In sponges
the reproductive cells are widely scattered through the
mesoderm so that in these animals ovaries and testes can-
not be said to exist. In the lowest cnidarians (Hydrozoa)
the sex cells are at first widely scattered in the ecto-
dermal epithelium, but they actively migrate to certain
portions of the hydroid stem where reproductive buds are
being formed, and, aggregating there, form gonads. A
similar migration of sex cells into the gonads has been
described in several vertebrates. In all higher animals
definite gonads are present.

No genital ducts are present in the coelenterates and
none are needed, since the sex cells can escape directly into
the water in which they live. In animals above the coelen-
terates the sex cells are mesodermal in origin, and in most
cases form a part of the epithelium lining the coelom. In

C63 3

MORPHOLOGY OF ANIMALS

animals without a true coelom the sex cells arise in tubes
or glands the cavities of which may perhaps represent
the coelom (Fig. 5, B). In flatworms the gonads, espe-
cially the testes, occur in considerable numbers in a single
individual (Fig. 11, B). In round worms they are limited
to one or two elongated tubes ; in rotifers, mollusks, mol-
luscoids, and echinoderms they are confined to one or at
most a few sex glands, while in segmented animals they
are found in primitive forms in every body somite, though
with advancing organization they become limited to a
few somites or even to one (Fig. 12, A). In most animals
above the coelenterates some form of duct (gonoduct)
exists for carrying the sex cells to the exterior; among the
flatworms, roundworms, and rotifers these gonoducts are
never the protonephridia, though they may possibly repre-
sent the coelom of higher animals. In higher forms the
gonoducts are usually metanephridia, or modified excre-
tory ducts.

Gonads of Vertebrates. In vertebrates the sexes are sepa-
rate. A single pair of ovaries or testes is located in the dorsal
part of the body-cavity near the kidney ( Fig. 2 1 , ^ ) , but in
almost all mammals the testes descend from this position
and come to lie outside the abdominal cavity in the scro-
tum^ where a lower temperature favors the development
of spermatozoa. The ovaries remain in their original po-
sition and when the ova in them are ripe they break out
into the body-cavity and are then carried to the exterior
through the oviducts (Fig. 21, od). Spermatozoa of ver-
tebrates never escape into the body-cavity but pass out

c 643

MORPHOLOGY OF ANIMALS

through the mesonephric tubules and segmental ducts
(epididymis and vas deferens, Fig. 21, D, ef^ vd).

Semination. In many aquatic animals the ova and sper-
matozoa escape directly into the water, and there the eggs
are fertilized and undergo development; it is probable
that in these animals the escape of ova stimulates the
males to eject spermatozoa so that both kinds of sex cells
are shed at about the same time. In such cases enormous
numbers of sex cells are produced and very many are
wasted. A slight advance over this condition is found in
those animals (frogs, bony fishes, etc.) in which the open-
ings of the male and female ducts are placed close to-
gether at the time of shedding of the sex cells; this is
known as external copulation. In other cases the spermato-
zoa only escape from the body, and by means of currents
of water they are carried into the body of the female,
where they fertilize the ova in situ^ as in sponges, or they
are carried into certain receptacles, into which the eggs
also are collected, as in fresh-water mussels. In other
animals copulatory organs exist which serve to introduce
spermatozoa into the sex ducts of the female, thus in-
creasing the chances for the fertilization of the ova; this
is known as internal copulation. In many cases copulation
occurs rarely, sometimes but once, and the spermatozoa
are stored in a seminal receptacle which opens into or
near the oviduct. Internal copulation is a necessity in all
land animals and in most parasites, and it also occurs in
many aquatic forms (flatworms, round worms, rotifers,
gasteropods, cephalopods, annelids, arthropods).

Secondary Sexual Characters. In certain animals the

[ 65 1

MORPHOLOGY OF ANIMALS

sexes differ not only with respect to the sexual apparatus
but also in many other regards, which are known as secon-
dary sexual characters; when such differences are very
marked they constitute what is known as sexual dimor-
phism. In such cases the male is sometimes very degen-
erate in form, being occasionally only a small fraction of
the size of the female and entirely lacking alimentary
canal, sense organs, and nervous system (rudimentary
males of rotifers, barnacles, etc.).

Asexual reproduction^ or monogony^ consists in the for-
mation of new individuals by division of an old one. In
one-celled organisms and in the constituent cells of higher
animals this takes the form of cell division or the partial
division of cell aggregates. In the lower Metazoa asexual
reproduction is not limited to cell division, but the entire
body or portions of it may undergo constriction and sub-
sequent partial or complete division, thus giving rise to
new individuals. This division may be into equal parts, in
which case it is called fission ; or into unequal parts, when
it is known as budding or gemmation. In animals which
reproduce both sexually and asexually there is more or
less regular alternation of one method with the other; this
is known as alternation of generations or metagenesis. The
alternation of amphigony with parthenogenesis is called
heterogony.

9. Nervous System

Sensation and coordination are manifestations of pro-
toplasmic irritability, or that capacity of receiving and
responding to stimuli characteristic of every cell. Animals,
even the simplest, are sensitive to a variety of stimuli,

[ 66]

MORPHOLOGY OF ANIMALS

among which may be mentioned mechanical, chemical,
thermal, and electrical, as well as light, gravity, etc. These
stimuli acting on the organism, start changes in the proto-
plasm (impulses) which are transmitted to portions of the
body distant from the point first stimulated and call forth
the coordinated activities of many different parts. In
higher animals there are special sense organs for receiving
certain of these stimuli and specialized protoplasmic
fibres (nerve fibres) for transmitting impulses, while nerve
centers (nerve cells) for coordinating activities appear
very far down in the animal scale. In the lowest animals,
however, there are neither nervous system nor sense
organs, and yet through the irritability of the general
protoplasm these functions are performed.

In somic of the higher Protozoa there are specialized
parts of the protoplasm which serve for receiving and
transmitting stimuli, but in the lower forms of this
phylum these differentiations are lacking; and of course
there are no specialized sensory or nerve cells in any pro-
tozoan. The same is true of sponges, where none of the
cells are differentiated for receiving and transmitting
stimuli, i.e., there are no specialized sensory or nerve cells.
In all other phyla, however, certain cells of the body are
set apart for these particular functions, and the greater
the differentiation in these respects the more definite and
varied are the sensations, the more swiftly impulses are
transmitted to the motor system, and the more compli-
cated are the responses.

The elements of the nervous system are nerve cells and
fibres, the latter being merely outgrowths of the former

C 67 ]

MORPHOLOGY OF ANIMALS

Fig. 22. Nerve Cells and their Origins. A, Epithelial nervous system, the five large nerve
cells and the many nerve fibers (small circles) being part of the surface epithelium B ana
C, Epitheliogenous nervous systems. B, Delaminate Nervous System, the nerve cells and
fibers having split off from the surface epithelium ; C, Tubular Nervous System, the nerve
cells and fibers having folded in and then separated from the surface epithelium. D, Unipolar
Nerve Cell of an Annelid with one nerve fiber ( + ). E, Multipolar Nerve Cell of Electric Ray,
the fiber marked + is the neurite or axon, the others are dendrites. (After Hatschek).

MORPHOLOGY OF ANIMALS

(Fig. 22, D, E). A nerve cell with all of its outgrowths
is called a neuron. In practically all Metazoa these cells
are derived from ectoderm, and in a good many animals
the sense organs and entire nervous system remain
throughout life a part of the superficial epithelium which
covers the body (Coelenterata, Chaetognatha, certain An-
nelida, Molluscoidea, many Echinodermata, Balanoglos-
sus) ; such a nervous system is said to be epithelial (Fig.
22, A). In all other Metazoa the nervous system, though
formed from epithelium, separates from it in the process
of developmicnt, so that brain, ganglia, and nerve trunks
come to lie some distance from the surface of the body; this
is known as an epitheliogenous nervous system (Fig. 22,
B, C) . In addition to the two classes just mentioned, which
are based on the relations of the nerve cells to the body
layers, four types of nervous system are found among
Metazoa which are based upon the relations of the nerve
cells to one another; these are (i) the diffuse type, (2)
the linear type, (3) the ganglionic type, and (4) the
tubular type.

(1) A diffuse nervous system^ consisting of nerve cells
and fibres scattered throughout the superficial epithelium,
is the simplest type known and is found among such
animals as hydroids and sea anemones; the nerve cells are
here connected together by means of fibres into a nerve
plexus.

(2) The next step in increasing complexity is repre-
sented by a linear nervous system found in jellyfishes,
echinoderms, fiat- worms (Fig. 11, C) ; here many nerve
cells and fibres are aggregated into definite lines or strands,

C69 3

MORPHOLOGY OF ANIMALS

thus forming a centralized nervous system; other nerve
cells remaining scattered throughout the epidermis serve
to connect the nerve strand with the muscles.

(3) The ganglionic type. In ctenophores, a sense organ
from Which nerves radiate, is found at the apical pole
(Fig. 2, C, so)^ and in a great many of the higher animals
the earliest formed and most generally present portion of
the nervous system is a group of nerve cells, or ganglion,
which appears at the apical pole of the gastrula, and be-
comes in the adult the cerebral ganglion^ or brain, lying on
the dorsal side of the oesophagus (Figs. 3, 1 1, 12, A) . Nerve
trunks are always given off from this ganglion, and very
generally two of them run down on each side of the
oesophagus to its ventral side, thus forming a circum-oeso-
phageal nerve ring (Fig. 3, D). In different phyla longi-
tudinal nerve trunks may be given off from different parts
of this ring; in the case of annelids and arthropods this
ring connects on the ventral side of the oesophagus with
the ''ventral nerve chain ^'' which consists of a bilateral
pair of ganglia in each somite connected with those in
front and behind by nerve cords. The first one in the chain
is the suh'oe so phageal ganglion^ connected with the cere-
bral ganglion by the circumoesophageal connectives (Fig.
23, A). In the moUusks the nervous system consists of a
pair of supra- and sub-oesophageal ganglia {^cerebral and
pedal) which with their connectives form an oesophageal
ring. To these is usually added a pair of plural and parietal
ganglia forming a loop which extends back into the body,
while ventral trunks {pedal cords) may be present in the
foot (Fig. 13, A).

1:703

MORPHOLOGY OF ANIMALS
A

B

Bstr

Fig. 23. A, Nervous System of Crayfish, consisting of cerebral ganglion (eg), circumoesoph-
ageal ring (or), sub-oesophagial ganglion (so) and ventral chain (vc) of 12 ganglia. (After
Huxley). B, Sense Cells in Epidermis of Earthworm; E, epidermis; sz, sense cells with fiber
running into the ventral chain of ganglia (Bstr). (After Heidenhain).

(4) The tubular type of nervous system is found only
among the chordates; here the nervous system develops
from an epithelial plate {^neural plate) on the dorsal sur-
face of the embryo, which becomes invaginated in such
a way as to form a longitudinal groove, the neural groove.
This then separates from the epithelium as a tube (Fig.
22, C), which in all vertebrates is enlarged at its anterior

1: 71 3

MORPHOLOGY OF ANIMALS

end to form the brain (Fig. 4, E, n). This neural tube,
while apparently a continuous structure, is really com-
posed of segments, the neuromeres^ one neuromere being
formed in each body somite ; the neuromeres are thus com-
parable to the ganglia of the ventral chain of arthropods
and annelids. This segmentation of the central nervous
system of vertebrates, is indicated even in the adult by the
segmental arrangement of the spinal and cranial nerves.
In the embryonic development of all vertebrates the brain
consists of three enlargements or vesicles, the fore-brain^
mid-brain^ and hind-brain (Fig. 24, A); the first gives
rise to the cerebrum and 'tween brain of the adult, the
second remains as the mid-brain^ while the third gives rise
to the cerebellum and medulla (Fig. 24, C). The portion
of the neural tube posterior to the brain becomes the
spinal cord of the adult. With the differentiation of nerve
cells and fibres in the walls of the neural tube these walls
increase greatly in thickness, while the originally large
cavity of the tube becomes restricted in size, forming in
the adult the ventricles of the brain and the central canal
of the cord (Fig. 24, K).

10. Sense Organs

The simplest sense organs are the scattered sensory cells
found in the superficial epithelium of many animals ; these
may be solitary or aggregated into buds. They are elon-
gated epithelial cells with a hair-like process at the free
border and a fibre at the deeper end connecting with the
branches of a nerve cell (Fig. 23, B). They are organs of
general sensation, — that is, they are capable of receiving

MORPHOLOGY OF ANIMALS

MORPHOLOGY OF ANIMALS

various kinds of stimuli, such as mechanical, thermal, elec-
trical, and chemical, and are therefore largely undifferen-
tiated, though probably chiefly subserving the sense of touch.
These integumentary sense organs are found in almost every
group of animals. Among the vertebrates they are pres-
ent in primitive form over the general body surface; in the
fishes and amphibia they are aggregated into buds, forming
the lateral line organs^ while among the vertebrates which do
not dwell in water, deeper-lying organs, of modified type, are
found (^tactile cells^ corpuscles^ and hulbs^ Fig. 7, np). In ad-
dition to these organs of general sensation, higher Metazoa
generally possess specific sense organs namely, those differen-
tiated for the reception of particular kinds of stimuli.
These are organs of ( 1 ) smell and taste, (2) equilibrium and
hearing, (3) vision.

(1) Organs of smell and taste are organs for receiving
chemical stimuli; they are present in all vertebrates and in
many invertebrates. Their structure is extremely simple,
being but slightly modified from the type of the primitive
organs described above. In fact the olfactory sense cells of
vertebrates are merely scattered sensory cells, while the
organs of taste (taste buds. Fig. 25, A) are simply aggre-
gations of such cells. Throughout the Metazoa the organs
of taste and smell are generally located in ciliated pits or
depressions of the integument either on the head or at least
near the mouth or respiratory organs. In these positions they
serve in the one case to test food and in the other the quality
of the medium used in respiration. Among fishes the olfac-
tory organs are located in pits on the front of the head; in all
air-breathing vertebrates these pits open posteriorly into the
mouth-cavity or pharynx, and thus form the anterior part of

: 74 3

MORPHOLOGY OF ANIMALS
A

Fig. 25. Organs of Taste and Smell. A, Taste bud in epithelial layer. B, .Antennule of
Crayfish, au, auditory sac on basal joint; ex, exopodite ; en, endopodite. C, Portion of exopo-
dite bearing many hollow, olfactory hairs (<?). D, Olfactory hairs much enlarged, a, face
view; b, side view. (After Huxley).

MORPHOLOGY OF ANIMALS

the respiratory tract. The organs of taste are, of course, in
or near the mouth. Among the moUusks a sense organ which
is probably olfactory in function, the osphradium^ is located
near the gills. Among the arthropods we find notable modi-
fications of these organs owing to the fact that the entire
body-surface is there covered with an impermeable chitinous
coat. These sense organs are here peculiar hollow tubes or
cones, which are borne on the anterior portion of the body,
usually on the antennae and mouth parts; these hairs are
filled with fibrillar protoplasm which connects with sense
cells at the base of the hair (Fig. 25, B, C, D).

(2) Organs of hearing and equilibration are differen-
tiated for the reception of vibratory or mechanical stimuli ;
they are very widely represented throughout the animal
kingdom. It is advisable to consider these two organ systems
together, since the two functions which they subserve are
united in the same general organ in the vertebrates, while
in lower forms it is by no means easy to distinguish between
the two. It has long been customary to speak of all vesicular
sense organs containing free solid bodies as auditory in func-
tion, but it is much more likely that in the lower Metazoa
they serve to acquaint the animal with its bodily positions, —
that is, that they are organs of equilibration ; they are stato-
cysts rather than otocysts.

Statocysts. In many respects the simplest type of organs
of this class is found among certain jellyfishes. It here con-
sists of a short tentacle situated in a depression of the ecto-
derm and bearing a solid body or statolith near its free end;
by the movement of the tentacle the hairs or protoplasmic
processes of surrounding sensory cells are stimulated (Fig.

MORPHOLOGY OF ANIMALS

sh'-^

B

Fig. 26. Statocysts. A, of Jellyfish, si, statolith, sk, sensory hairs. (After Hatschck). B, of
Gasteropod; ac, auditory cells; n, nerve. (After Claus).

MORPHOLOGY OF ANIMALS

26, A). In other jellyiishes the sensory cells may entirely
enclose the tentacle, thus forming a vesicle or statocyst. The
organs of equilibration and hearing of most vertebrates,
as well as of most invertebrates, can be traced back to this
simple type. The sensory cells forming the walls of the stato-
cyst are similar to tactile cells, — that is, they bear processes
projecting into the cavity of the statocyst (Fig. 26, B).
By the movements of the statolith, usually a calcareous con-
cretion, these cells are stimulated and the impulses thus
generated are conveyed away by the nerve fibre. Statocysts of
this type are possessed by mollusks, certain annelids, tur-
bellarians, and brachiopods.

Arthropod Statotcysts and Ears. In the case of arthro-
pods organs of a different type are usually found, owing to
the fact that the body is here covered with chitin and that
the fine protoplasmic processes or cilia are absent. Among the
higher crustaceans the auditory organ usually consists of a
cavity in the basal joint of the first antenna (Fig. 25, B),
which is open to the exterior and which contains water and
grains of sand; the wall of the cavity bears chitinous pro-
cesses or auditory hairs which have a nervous connection at
their base; these hairs are stimulated by the movement of
water and sand within the auditory sac. Many insects have
a true tone-perceiving organ, the chordotonal organ; in
principle this consists of a few elongated cells, the chord,
one end of which is attached directly to the integument,
while a ligament runs from the other end to an opposite point
of the integument; when this apparatus is thrown into vibra-
tion, impulses are conveyed to the nerve cells attached to
some portion of the chord. In other insects (Orthoptera) a

c 78 :

MORPHOLOGY OF ANIMALS

tympanal organ may be present, consisting of a vibrating
membrane overlying a trachael chamber (Fig. 15, A, /) ;
sense cells are present between the membrane and chamber,
and when the membrane is set into vibration by sound waves
the sense cells are stimulated.

Lateral Line Organs. Among aquatic vertebrates (fishes
and amphibians) a system of integumentary sense buds is
found along the right and left sides of the body and over the
head, which is known as the lateral-line system. The func-
tion of these organs is to receive vibrations of low frequency
such as waves in water; probably they are organs also of
equilibration. In all vertebrates it is probable that the
auditory organs, as well as the organs of smell and taste,
have been derived from integumentary sense organs homolo-
gous with those of the lateral line.

The Vertebrate Ear. In the process of development the
ear of vertebrates appears as a pit-like invagination of the
skin which is then infolded to form a vesicle; this vesicle
then becomes partly divided into two chambers, the utricle
and the saccule. In most vertebrates the former bears three
pairs of semi-circular canals which are organs of equilibra-
tion, while the latter gives rise to a recess, the lagena, or
cochlear duct, which is the specific auditory organ. Cal-
careous concretions or otoliths are frequently present in the
utricle (Fig. 27, A). These sensory portions of the auditory
organ are known as the inner ear; to this is added in all
animals above the frogs and toads a middle ear or tympanum
which develops out of the first gill cleft and which transmits
the sound waves from the surface to the inner ear. Finally,
in mammals there are folds of the integument around the

C 79 3

MORPHOLOGY OF ANIMALS

Fig. 27. Human Ear. A, Internal Ear, sensory portion, infolded from ectoderm of embryo ;
u, utricle ; s, saccule, cd, cochlear duct ; asc, anterior-vertical semi-circular canal ; psc,
posterior-vertical semi-circular canal. B, Section through Ear; em, external auditory meatus; d,
ear drum ; os, 3 ossicles or chain of bones across tympanic chamber, t ; e, eustachian tube ;
ph, pharynx; fr, foramen rotundum ; fo, foramen ovale; sc, vertical semi-circular canal; c,
cochlea; sm, scala media or cochlear duct; an, auditory nerve. (After Tschermak).

MORPHOLOGY OF ANIMALS

tympanic membrane which serve to collect sound waves and
which constitute the external ear (Fig. 27, B).

(3) Visual Organs are differentiated for the reception of
light stimuli. Animals without any trace of eyes are sensitive
to light (certain protozoans, annelids, many larvae), and it
is therefore certain that protoplasm may be directly stimu-
lated by light without the intervention of any special organ.
In its simplest form an eye consists of one or a few trans-
parent cells partially surrounded by pigment in the form
of a cup, so that the light can enter only from one side; the
pigment not only absorbs light rays, but it optically isolates
the cells within from those without this cup (some jellyfish,
turbellarians, annelids). The function of such an eye is prob-
ably to determine the direction of light, since it could
give no image of luminous objects. A slight advance over
this simplest type of eye is found in the cup-shaped eyes of
certain mollusks; here certain superficial epithelial cells are
infolded to form a cup; some of these cells are deeply pig-
mented, while other intermediate cells remain clear and
unpigmented. The latter are the sensory cells and are con-
nected at their bases with nerve fibres. If this cup-shaped eye
becomes infolded still further and its opening grows smaller,
it forms a "pin-hole camera" type of eye (Nautilus,
Haliotis, Fig. 28, A).

Vesicular Eyes. If the optic cup finally closes altogether,
it forms a vesicular eye such as is present in certain mollusks
and annelids. The wall of this vesicle, which is turned toward
the epidermis, is transparent and may become thickened
to form a lens; the opposite wall of the vesicle is pigmented
and is known as the retina. In such an eye the free ends of

C81 3

MORPHOLOGY OF ANIMALS

"^m^^fnKM^.

Fig. 28. Types of Eyes. A, Eye of Haliotis (Mollusk) of "pin-hole camera" type ; e,
epidermis; cv, cavity of eye; R, retina of direct type; A^, optic nerve. (After Hatschek). B.
Pineal (unpaired) eye of a lizard (Hatteria) ; i, integument ; c, connective tissue capsule ; /,
lens; r, retina; n, nerve. (After Parker and Haswell). C and D, Diagrams of two stages in
formation of paired eyes of vertebrates. C, evagination of optic vesicles from fore-brain.
D, invagination of epidermis into optic vesicles to form lens, /, and infolding of retina, r,
which thus becomes inverse. (After Boveri).

the retinal cells are turned toward the cavity of the vesicle,
while the opposite ends, which are directed away from the
vesicle, are prolonged into fibres; such an eye has a direct
retina. This type of eye reaches its highest development
among the cephalopods, where it bears a striking

e, though

MORPHOLOGY OF ANIMALS
superficial, resemblance to the vertebrate eye. A rudimentary
eye of this type is present in some vertebrates as the pineal
eye. This is an unpaired structure on the dorsal side of the
'tween brain, and in certain reptiles it is plainly a vesicular
eye with direct retina (Fig. 28, B).

The paired eyes of vertebrates are also vesicular, but in
them the retina is inverse^ that is, the free ends of the retinal
cells are directed away from the cavity of the vesicle, while
the ends which bear the fibres are directed towards it. The
explanation of this remarkable condition is found in the
study of the development of these eyes. They arise as lateral
evaginations of the walls of the embryonic fore-brain, are
then constricted from the brain, and become vesicles con-
nected with the fore-brain by only a stalk. At this stage the
vertebrate eye is like the invertebrate one save only that it
has arisen from the neural instead of the superficial epi-
thelium. All the cells which form the vesicle have their free
ends directed toward its cavity, while their basal ends are di-
rected away from it (Fig. 28, C). The outer wall of this optic
vesicle is then infolded until it comes into contact with the
inner wall, thus forming a cup open toward the skin. The
ectoderm over the opening of the optic cup is then infolded
to form the lens^ which completely separates from the skin
and lies in the mouth of the cup. The infolded wall of the
cup alone forms the retina, and therefore the free ends of
the retinal cells are directed away from the lens and from
the cavity of the cup (Fig. 28, D). The lens and optic cup
are then surrounded by fibrous and vascular coats, the scle-
rotic and choroid', a chamber is formed in front of the lens
which is filled with aqueous humor, while one behind the

C83 3

MORPHOLOGY OF ANIMALS

..e
— c
ac

a

Fic. 29. A, Human Eye, horizontal section ; e, corneal epithelium ; r, cornea ; ac, anterior
chamber ; i, iris ; cb, ciliary body ; /, lens ; pc, posterior chamber ; p, blind spot at point of
entrance of optic nerve ; m, macula, point of clearest vision ; r, retina ; ch, choroid ; sc,
sclerotic coats. B, Compound Eye of Arthropod, composed of many individual eyes or om-
matidia ; /, layer of corneae ; 2, crystalline cones; 3, retinulae, or sensory layer; n, nerve.
Dotted lines indicate that each ommatidium receives light from only a small part of the
arrow, thus constituting mosaic vision. (After Hatschek).

MORPHOLOGY OF ANIMALS

lens and in front of the retina is filled with vitreous humor
(Fig. 29, A).

The compound eye is another type found chiefly among
arthropods. It consists of a large number of closely packed
single eyes or ommatidia^ each of which is surrounded by pig-
ment and is optically isolated from the others. Each omma-
tidium consists of (1) a polygonal cornea at the surface,
(2) a crystalline cone below this, and (3) the retinula or
group of retinal cells which are connected with nerve fibres.
The cornea and crystalline cone are refractive and serve in
the capacity of a lens, while the retinula alone is the sensory
element (Fig. 29, B).

A summary of the Morphology of Animals is given in the
appended Table.

1:85 3

Morphology of Animals

CHIEF SUBDIVISIONS OF THE ANIMAL KINGDOM

Flagellau (Euglena)

PRINCIPAL ORGAN SYSTEMS

Pore*. Canals, Gas

ts. Eyes. Stalocysti p
ties Eyes. Slatocjit

;i.Ci.r

f. CTENOPHORA. Jell:

V. PLATYHELMINTHES.
tra°h""os'l?>leol|^tr.

. ROTIFHM. Mitre

VII. NEMATHELMINTHES

:, ANNELJD.V Ringed

covered br dense e.
chilin; wilhout cilia.

XI MOLLUSCOIDEA.

, ECH.mODERMATA

XIV. CHORDATA

canal on ventral side s ...
gill sliU opening throng),
laleral walla of pSarynt

Oi.hi..roidea (Brittle.

, Suckers, Mnsdes

!:SI:

fbS;:'c5r

W, Gonads. Dncu. Copnl

S.J. Gonad.. Dncl

Dor..?'"^^:;',,!!-^.

lany layered Epidermis, Set

Connective Tissue c

Hypodermis Setae and Cutid

Epitheliogcnou
^''ventrS^Sin

'" fj'o Cilia

*"' <"'&.

.lesand^oi

Fore-Mid-Hind (

'"^alplghian Tubes"
lpilI*'fXs. Coxal'ciar

rf.«. Gonads. Ducis, Copuli
rf.?. Gonads. Ducts. Copula
1^,9, Gonads. Ducts. Copula

SimoU Eyei
Auditory "cR^n.

J.^.9. Gonads, Duel:

Cerebral. Pedal a

'cssels. No Capillaries

(<.?. Gonads. Ducts. Copula
'<f,9. Gonads, Duds. Copula
J9, Gonads. Ducts. Copula

Tentacles. Siaiocysis, Eyes, a

Fore-Mid-Hind Gut

Brancliiac, Amb. Sjsl

»filh Dermal Skeleton

:omp*^%«,¥ma,

I a>i'. cmiilll'. 1

d Altmentanr Syi
.-ind Glands (mouti PKary

Brain. (Cerebrum aoc

Eye Spots.— Olf. Organ

Imtll and Taste Organs
Eyes, Ears. Smell and Taste

i^::±g:s&tf