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A LABORATORY COURSE
IN GENERAL ZOOLOGY

A GUIDE TO THE DISSECTION AND
COMPARATIVE STUDY OF ANIMALS

BY

HENRY SHERRING PRATT, PH.D.

PROFESSOR OF BIOLOGY IN HAVERFORD COLLEGE

GINN AND COMPANY

BOSTON • NEW YORK • CHICAGO • LONDON
ATLANTA • DALLAS • COLUMBUS • SAN FRANCISCO

COPYRIGHT, 1927, BV HENRY SHERRING PRATT
ALL RIGHTS RESERVED

PRINTED IN THE UNITED STATES OF AMERICA

827.8

GINN AND COMPANY • PRO-
PRIETORS • BOSTON • U.S.A.

PREFACE

This book is a revised edition of the author's "Course in In-
vertebrate Zoology," to which have been added the two dis-
sections, abbreviated and simpUfied, of the perch and the frog
from his ''Course in Vertebrate Zoology." The main purpose of
this combination is to enable classes using the book to study rep-
resentatives of vertebrates together with invertebrates, and thus
to become acquainted with the structure of all the phyla of the
animal kingdom.

In making this revision the author has not changed the char-
acter of the work or the order of the dissections ; all those ap-
pearing in the ''Invertebrate Zoology" are also given here. The
plan of the course is to study each of the larger groups of the
animal kingdom as a whole as far as possible, instead of detached
types of the different groups taken more or less at random. The
attention is directed constantly to the main structural features
which characterize the entire group under consideration, and the
study is thus made comparative.

In order that the systematic position of the animals examined
and their larger affinities may be easily kept in mind, the names
of the most important systematic groups to which the animal
being studied belongs are placed at the head of each dissection,
and a synopsis of the whole animal kingdom has been added in
an appendix.

The course begins with arthropods, because the natural suc-
cession of forms in the animal kingdom, from the lowest to the
highest, is more apparent in them than in any other group, except
possibly in vertebrates, and it is easy for a beginner, by studying
them, to learn the real significance of the blood relationship of
animals. Whether, however, the student begins the course with
insects or with crustaceans, and whether the first insect taken
up is the wasp or the grasshopper, will be matters for the deci-

iv PREFACE

sion of the teacher. The course has been arranged so that any
of these methods may be adopted.

Many teachers prefer to begin a laboratory course in general
zoology with a frog or a fish, as convenient and relatively simple
forms to use in introducing a class to laboratory methods of study,
and this is one of the reasons for the expansion of this work by
the introduction of these dissections into it.

While the comparative feature runs through the dissections in
this course, each is complete in itself and does not depend upon
any others. The teacher is thus enabled to give his class such
dissections as he wishes and is not compelled to adopt the entire
series to have his course complete. He can also reverse the order
of the studies, if it suits his purpose better, beginning with the
Protozoa and working upward to the higher groups.

An important feature of the plan of the course has been to give
the student such practical directions that he can go on with his
work intelligently and profitably without having an instructor
constantly at his elbow. Far too much of the time of the average
youthful student is often wasted in the laboratory because the
instructor does not happen to be at hand at critical times to direct
his work. The student will often do the work wrong in conse-
quence, or perhaps he will not do anything at all ; in either case
his time is wasted and perhaps his material spoiled.

In most of the dissections the directions are so arranged that
the student can complete the study with a single specimen, and
in the longer dissections with two specimens, or at most three,
and the order in which the different systems of organs are taken
up in each dissection is made dependent upon this feature. The
necessity of practicing economy of material is thus inculcated,
and the habit is acquired of studying and handling each speci-
men with care and judgment.

H. S. PRATT

Haverford, Pennsylvania

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CONTENTS

PHYLUM ARTHROPODA ^^^^

A Wasp i

A Large Beetle 4

A Fly 5

A Large Grasshopper 7

An Insect Larva : A Caterpillar i5

A Centiped (Lithobius) i7

A Spider i8

A Crayfish or a Lobster 21

A Crab 33

A Sow Bug {Porcellio, Oniscus, or Armadillidium) 36

A Fresh-water Shrimp (Gammarus) or a Sand Flea (Talorchestia) 38

Caprella 39

Larval Decapods 40

A Free-swimming Copepod (Cyclops) 42

A Daphnid 45

A Nauplius Larva 47

PHYLUM ANNELIDA

The Sandworm (Nereis) 49

An Earthworm 54

PHYLUM VERMES
SUBPHYLUM PLATYHELMINTHES

A Planarian Worm 62

A Tapeworm ^5

SUBPHYLUM BRYOZOA

BUGULA ^9

V

:? 7 /, 3 3

vi CONTENTS

PHYLUM MOLLUSCA

PAGE

A Fresh-water Clam {Anodonta or Unio) 72

The Oyster 81

The Hard-shell Clam (KiE^£/5 3f£/?CiiiV^/i/^) 85

A Land Snail 93

A Squid {Loligo peali) 103

PHYLUM ECHINODERMATA

A Starfish 113

A Sea Urchin 119

A Sea Cucumber 124

PHYLUM CCELENTERATA

Hydra 127

A Tubularian Hydromedusan (Pennaria) 131

A Campanularlan Hydromedusan {Obeli a or Campanularia) . . 136

A Medusa (Gonionemus) 141

A Sea Anemone (Metridium) 143

PHYLUM PORIFERA

A Sponge (Grantia) 145

PHYLUM PROTOZOA

Paramecium 148

Vorticella 152

A Flagellate (Euglena viridis) 156

Amoeba 158

PHYLUM CHORDATA

SUBPHYLUM TUNICATA

A Simple Ascidian (Molgula) 160

CONTENTS vii

SUB PHYLUM VERTEBRATA

PAGE

A Frog 165

The Perch 198

APPENDIX

A Synopsis of the Classification of Animals 223

GLOSSARY 231

INDEX 243

APPARATUS AND MATERIAL

The apparatus necessary for this course of study need not be exten-
sive. Each student should be provided with the following instruments :
either one medium-sized scalpel or two, a small one and one of medium
or large size ; two pairs of scissors, a large straight pair and a small
pair, preferably with curved tips; either one pair of medium-sized
forceps or two pairs, a small one and one of medium size, both straight
and with corrugated tips; two dissecting needles, a probe, a blow-
pipe, a hand lens.

Each student should have a shallow dissecting pan, in the bottom
of which is a layer of black wax ; the depth of the pan should be about
an inch and a half. If the lobster is dissected, however, a deeper pan
without wax will also be needed. The student should also be provided
with a number of pins which may be conveniently kept, while not in
use, stuck in a large cork.

It is intended that most of the drawings of dissections should be
outhnes, usually more or less diagrammatic, made with a hard draw-
ing pencil in a large blank book, the paper of which is good and firm,
or upon sheets of drawing paper. The general use of colors by a class
is not recommended, not because the use of them is not often helpful,
but because in a class of young students it is difficult to prevent their
abuse by many. The careless student will often be tempted to sub-
stitute the use of colors for careful drawing. Outhne drawings of a
dissection on a sufficiently large scale, and carefully made and labeled,
will invariably be perfectly clear.

For the study of many of the animals or parts of them in this
course, a compound microscope will be needed ; a dissecting micro-
scope will also be useful throughout the course, although not indis-
pensable. The student should be provided with a number of glass
slides and thick cover glasses. Water may be used as a medium for
making temporary mounts of most of the objects examined under the
microscope. A solution made of equal parts of water and glycerine,
however, is usually preferable to water, as it will not dry up and,

X APPARATUS AND MATERIAL

besides, renders the object more transparent. None of the animals
studied here need to be stained and mounted in balsam or other per-
manent medium. In the case, however, of the tapeworm, the hydroids,
and perhaps one or two of the other forms, the animal can be studied
with greater profit if thus stained and mounted, and it is recommended
that the student be provided with such specimens.

As a rule the material needed can be easily obtained. Most of the
animals studied as well as microscopic preparations may be purchased
from the supply department of the Marine Biological Laboratory at
Woods Hole, Massachusetts ; General Biological Supply House, 761-
763 East 69th Place, Chicago; New York Biological Supply Co.,
34 Union Square, New York ; Michigan Biological Co., 324 South
State Street, Ann Arbor, Michigan; H. Edward Hubert, 3615 Mel-
pomene Street, New Orleans ; Southern Biological Supply Co., Natu-
ral History Building, New Orleans; Pacific Biological Laboratory,
Pacific Grove, Washington ; or other dealers in such suppHes. Powers
& Powers, Station A, Lincoln, Nebraska, will furnish live protozoans
and hydras, also microscopic slides of these animals.

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BRARY

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GENERAL ZOOLOGY

PHYLUM ARTHROPODA

Class : Tracheata. Division : Insecta. Order : Hymenopkra

A WASP

Observe the size, shape, color, and external anatomy of the
animal. The body is small in size ; it is bilaterally symmetrical,
that is, it has a right and a left side which are alike ; it has a dor-
sal and a ventral side which are unlike, and also a forward and
a hinder end which are unlike, the forward or anterior end being
distinguished by the possession of important organs of special
sense and the mouth. All of these features are characteristic of
rapidly moving animals. Can you explain why ? On the ventral
side are the legs, which are also called appendages or extremities ;
on the dorsal side of the insect are the wings.

The external surface of the animal is very smooth. This feature
is also correlated with rapid motion. Do you know how? The
animal is encased in a hard shell, called the cuticula, which is
composed largely of a very hard and resistant substance called
chitin, and serves the double purpose of a protection for the in-
ternal soft parts and a surface for the attachment of muscles. It
is, in fact, the skeleton of the animal and is called an exoskeleton,
in contradistinction to an internal supporting structure, which
would be called an endoskeleton. All invertebrate animals, ex-
cept some of the lowest, are provided with a cuticular exoskeleton,
but it is only the arthropods in which it is composed largely of
chitin. In fact, the possession of such a hard and resistant external
covering is one of the reasons why insects have so successfully
maintained themselves in the universal struggle for existence.

2 PHYLUM ARTHROPODA

Observe that the body of the animal is composed of a number
of serially arranged segments. These are called somites, or meta-
m_ereSj and the segmented type of structure presented by the in-
sect body is called a m-etameric type of structure. Observe that
the body is sharply divided into three divisions — the head,
thorax, and abdomen.

The head is unsegmented and bears on its anterior and dorsal
surface a pair of long, jointed feelers, or antennae, which are im-
portant sense organs, a pair of large compound eyes, and three
small, dotlike eyes, called ocelli, which it may be necessary to
look for with a hand lens ; on its ventral side are the mouth parts,
the organs which taste, grasp, and masticate the food. Examine
these mouth parts carefully with a hand lens ; notice that there
is a short, overhanging upper lip, beneath which is a pair of power-
ful jaws having a lateral, or side, position instead of a dorso-
ventral one like the jaws of vertebrates. Beneath the jaws are
two other pairs of mouth parts, the maxillae and the under lip,
which, however, will not be studied at present ; notice the two
pairs of elongated and segmented palps, which are probably or-
gans of taste.

The thorax is composed of three somites, or metameres, which
are called, respectively, the prothorax, the mesothorax, and the
metathorax. Each somite bears a pair of legs on its ventral sur-
face, and the mesothorax and metathorax bear each a pair of
wings on the dorsal surface ; thus the organs of locomotion of
the animal are concentrated in the thorax. Find the sutures
between the thoracic segments. The dorsal cuticula of each
thoracic segment is called the tergum; the ventral cuticula,
the sternum ; and that of each lateral side, the pleurum. Thus
we speak of the presternum, the mesosternum, and the meta-
sternum, etc.

In the abdomen the dorsal and the ventral portions of the
cuticula are composed each of a distinct plate in each somite,
which are called the tergite and the sternite, respectively. The
abdomen bears no appendages ; it contains most of the vegeta-
tive organs of the animal. At its hinder end are the vent, or anus,
and, in the female, the sting. Look with the aid of a hand lens

A WASP 3

for the spiracles, the external openings of the tracheal or respira-
tory system ; they appear as a straight row of minute dots on
each side of the abdomen and the thorax, one dot being on each
segment on each side. In dark-colored wasps it may be impos-
sible to see them with a hand lens. They are usually distinct in a
caterpillar or a grasshopper, or may be seen in the wasp by re-
moving a portion of the cuticula from the side of the body and
examining the inner surface under the microscope.

Exercise 1. Draw an outline of the side view of the wasp on a scale of
4 or 5, indicating the segmentation and all the parts observed.
The three thoracic segments may be difficult to distinguish at first,
but if it is kept in mind that each one of them bears a pair of legs,
the task will be easy. Number on your drawing the thoracic and
abdominal segments, and carefully label all the different parts and
organs.

Exercise 2, Draw an outline of the face on a scale of lo, showing
exactly the relative length and the segmentation of the antennae,
the position of the compound eyes and ocelli and the upper lip, and
label them all.

Exercise 3, Remove a metathoracic leg and draw an outline of it on a
scale of 5. Its different segments, beginning with the proximal one,
that is, the one nearest the body, are the following : the coxa, by
which the leg articulates with the body ; the trochanter, a very
small segment ; the femur, or thigh, a long segment ; the tibia, or
shank, also long ; the tarsus, or foot, which is composed of five
small segments, the last one of which bears the two claws. Label
all of these.

Exercise 4. Remove a mesothoracic wing, extend it, and draw a
picture of it on a scale of 5, indicating its venation.

4 PHYLUM ARTHROPODA

Class : Tracheata. Division : Insecta. Order : Coleoptera
A LARGE BEETLE

Compare the animal with the wasp. We notice in the first
place the heavier and clumsier body and the smaller head. The
animal is evidently much less active and also less intelligent than
the wasp. We notice also that the wings lie close to the body
instead of being raised above it. The forward, or meso thoracic,
wings are hard and thick ; they are not used for flight, but cover
the metathoracic pair and the hinder part of the body and thus
form an additional protection to the back. They are called the
wing-covers, or elytra. The entire body of most beetles, in fact,
has a thicker cuticula and, consequently, a more effective ex-
ternal covering than that of the wasp. This feature may be cor-
related with the smaller intelligence of the animal. Opening the
elytra, we notice beneath them the membranous metathoracic
wings with which the animal flies ; we notice also that they are
folded transversely as well as longitudinally. These wings are
wanting in some of the running beetles, where the wing-covers
are sometimes fused. Note the scutellum, the small triangular
plate between the base of the wing-covers. Find the eyes and
note their small size. Are ocelli present ? Find the antennae ; in
some beetles they are often concealed beneath the sides of the
head.

Exercise 1. Draw an outline of the dorsal aspect of the beetle on a
scale of 4 or 5. First, however, spread and pin the right wing-cover
and wing. Number the thoracic and abdominal segments and label
all the parts observed.

Exercise 2. Draw an outline on the same scale of the ventral aspect

of the beetle, tracing carefully the sutures between the segments.

Number the thoracic and abdominal segments.
Exercise 3, Remove a mesothoracic leg and draw an outline of it on

the scale of 5. Label the segments.
Exercise 4. Remove a wing and draw an outline of it on a scale of 5,

tracing in the veins.

A FLY 5

Class : Tracheaia. Division : Insecta. Order : Diptera
A FLY

Kill several bluebottle flies or large house flies, without muti-
lating them, and impale one specimen on a slender insect pin or
a needle. Stick the pin or needle into a cork or a small piece of
wood, in order to be able to handle it easily, and study the exter-
nal anatomy of the fly with the aid of a hand lens.

Observe the compact body of the animal, and note that it is
distinctly divided, like that of the wasp, into three divisions:
the head, the thorax, and the abdomen. Observe the color and
the hairy surface of the body, including the legs and the wings.
These numerous hairs are projections of the cuticula, and perform
a useful function as tactile organs ; that is, they are sensitive to
vibrations of the atmosphere, and thus function as sense organs
in that they aid in giving the animal a knowledge of its surround-
ings. Note the three pairs of long, strong legs and the single pair
of wings. The fly has unusual locomotory powers. Correlated
with these powers are the long cuticular hairs just mentioned, and
also the very large composite eyes. An active, rapidly moving
animal like the fly needs well-developed organs of orientation.
The eyes are larger in the male than in the female, and are closer
together on the top of the head. The two sexes may thus be
distinguished.

Between the large eyes are the three minute accessory eyes, or
ocelli. Note the peculiar form of the small antennae, with their
pinnate terminal portion. Extend the proboscis and observe its
complex structure and the oral lobes at the lower end. The fly
eats only fluid food, which it sucks up through its proboscis.

The thorax is of relatively large size, being almost entirely filled
with the very extensive musculature of the legs and wings. The
three thoracic somites are of unequal size. The middle one is
the largest and bears the wings. Note that the hinder margin of the
basal portion of the wing is divided into three prominent lobes.
The posterior thoracic somite is the smallest and bears the bal-
ancers, which are the morphological equivalents of the second

6 PHYLUM ARTHROPODA

pair of wings, possessed by most insects. These are a pair of
minute, white, knobbed organs, which project backward from the
posterior wall of the somite, each one being covered by the basal
lobe of the wing on that side. They have a sensory fimction.

The abdomen is composed of eight somites in the male fly and
nine in the female. Of these, however, four somites are much
larger than the others and make up the greater part of the abdo-
men. The sixth, seventh, and eighth in the male are very small
and rudimentary. In the female the posterior four form a long,
tubular ovipositor, which is usually telescoped into the abdomen
but can often be squeezed out by a little pressure. Each of the
five anterior abdominal somites has a pair of spiracles. Find
them.

Exercise 1. Draw an outline of the dorsal aspect of the fly on a scale
of about lo, indicating the segmentation and the parts observed,
including the venation of the wings. Label all the parts observed.

Exercise 2. Turn the fly over on its back and draw one of its legs on
a large scale. The names of the different segments of the leg may
be obtained from Exercise 3 on page 3. Note, between the two
claws on each foot, the two pulvilli — the hairy adhesive pads by
means of whose sticky secretions the fly can walk on an inverted
surface.

Exercise 3, Draw on a large scale a side view of the head with the
proboscis extended. Note carefuUy the form of the antennae and
of the proboscis. The latter is homologous to the under lip, or
labium, of other insects.

A LARGE GRASSHOPPER 7

Class: Tracheata. Division: Insecta. Order: Orthoptera
A LARGE GRASSHOPPER

Observe the shape, color, and external anatomy of the animaL
Note the long, vermiform body and the large head. The body, as
in all insects, is made up of a number of serially arranged seg-
ments, called somites, or metameres, which fall into two divi-
sions, the thorax and the abdomen. The head is unsegmented,
being composed of a number of completely fused somites, and
bears upon its dorsal and anterior surface a pair of long, jointed
feelers, or antennae, which are important sense organs, a pair of
large compound eyes, and three small, dotlike eyes, called ocelli,
which it may be necessary to look for with a hand lens ; on its
ventral side are the mouth parts, the organs with which it tastes,
grasps, and masticates its food. Examine these mouth parts with
a hand lens. Observe the long, broad upper lip and pass a needle
under its ventral edge. Back of the upper lip will be seen the
strong mandibles, and by pressing these to the right and left the
two remaining pairs of mouth parts, the maxillae and the under
lip, will be seen. Note the two pairs of jointed palps belonging to
them, which are probably organs of taste. These parts will all be
studied later in detail.

The thorax is made up of three somites, which are called the
prothorax, the mesothorax, and the metathorax. Notice that the
thorax is not separated from the abdomen by a constriction, as
it is in the wasp, but that it may be easily distinguished from the
abdomen by its greater diameter. The prothorax is movable, as in
the beetle, and its dorsal and lateral surfaces are covered by a
large shield. On the ventral side of the prothorax, between the
prothoracic legs, is, in many grasshoppers, a short projection.
The mesothorax and metathorax are united immovably with the
abdomen and are covered by the two pairs of wings. The anterior,
or mesothoracic, wings are parchment-like and are not functional
in flying, but, like the wing-covers of beetles, are held out at right
angles to the body during flight. The metathoracic wings are
membranous and when at rest are folded longitudinally like a fan

8 PHYLUM ARTHROPODA

beneath the forward wings. Each somite bears a pair of legs on its
ventral surface. The cuticula of each thoracic somite is composed
of a number of distinct plates.

The abdomen is made up of eleven somites, which, however,
are not all perfect segments, the ventral portion of several of the
terminal somites being wanting. The posterior end of the abdo-
men is different in the two sexes, the female possessing an ovi-
positor, by means of which she buries her eggs in the ground.
The ventral portions of the ninth, tenth, and eleventh somites are
wanting in the female, the last complete somite being the eighth.
The dorsal portions of the three terminal somites, however, are
present. Projecting from the hinder end of the abdomen is the
ovipositor, which consists of two pairs of short, movable, curved,
and pointed structures. One of these pairs is dorsal in position,
and the anus is at its base ; the other is ventral, and at its base is
the external opening of the oviduct. Extending from the posterior
border of the tenth somite is another pair of pointed projections,
called cerci, which may have a sensory function. Just beneath
each cercus is a plate called a podical plate, which forms the hinder
end of the body. Between the two podical plates on the dorsal
side of the animal is the small, triangular eleventh somite.

In the male an additional ventral plate, called the genital plate,
forms the hinder end of the body ; the podical plates and the cerci
are large. The eleventh somite is a small, triangular plate, dor-
sally situated, beneath which is the anus.

On the lateral side of the first abdominal segment note the
auditory organ, a large, circular opening covered by a membrane.
With the aid of a hand lens find the spiracles of the thorax and
the abdomen. Ten pairs are present, one pair on the anterior
margin of both the mesothoracic and the metathoracic segments,
and one pair on each of the eight anterior abdominal segments,
that on the first abdominal segment being just within the margin
of the auditory organ.

Exercise 1. Spread out and pin down all four wings and draw an
outline of the dorsal aspect of the grasshopper on a scale of 2 to 4.
Number the thoracic and the abdominal segments and label all
the parts observed.

A LARGE GRASSHOPPER 9

Exercise 2. Cul off the wings from the left side of the body and draw
an outline of the side view of the thorax and the two anterior
abdominal segments on a scale of 5 or 6. Note that both the meso-
thorax and the metathorax are divided by a diagonal suture into
two portions. Number the segments and label all the parts.

Exercise 3. Draw a side view of the posterior end of your specimen
(whether male or female) on a scale of 5 or 6, showing accurately
the arrangement of all the parts, and label them all.

Exercise 4, Draw an outline of the ventral surface of the thorax on a
scale of 5 or 6. Note the dovetailing of the anterior margin of the
metathorax with the posterior margin of the mesothorax and that
of the first abdominal somite with the metathorax ; note also the
attachment of the legs.

Exercise 5. Remove a metathoracic leg and draw an outline of it on
a scale of 3. The segment by which it articulates with the body is
the coxa ; the next segment is the trochanter, which in the grass-
hopper, however, is not a free segment, but is fused with the follow-
ing one, the femur ; the latter is the largest segment of the leg and
has V-shaped muscle impressions on its surface ; the next segment
is the tibia, or shank ; the end segment is the tarsus, or foot, which is
made up of five smaller segments ; the terminal one of these bears
two claws, between which is a structure called the pulvillus. This
organ is an adhesive pad which enables the animal to walk and
spring on smooth surfaces. Label all of these parts.

Exercise 6, Draw an outline of the face on a scale of 5 or 6. The
large plate which forms the top, front, and sides of the head, in
which the eyes, ocelli, and antennae are situated, is called the epi-
cranium. The sides of the epicranium, back of the eyes, are the
genae, the top is the vertex, and that part which forms the anterior
surface is the front. Ventral to the epicranium is a broad, short,
median plate called the clypeus, beneath which is the upper lip.
The antennae are the first pair of appendages. Label all parts.

The Mouth Parts. These consist of the median upper lip, or
labrum, the paired mandibles, the paired maxillae, the median
hypopharynx, and the paired under lip, or labium. The paired
mouth parts are the second, third, and fourth pairs of appendages,
the antennas being the first pair.

Exercise 7. Remove the labrum with scissors and draw it on a scale
of 5-

lo PHYLUM ARTHROPODA

Exercise 8. With strong forceps remove the dark-colored mandibles
and draw the inner surface of one of them on a scale of 5.

Exercise 9. Remove the maxillae, which lie just back of the mandi-
bles, being careful to take out the entire structure. Mount them
on a glass slide in water with the posterior side uppermost, and
examine them under the microscope. Note the following parts :
the basal segment, or cardo, by which the maxilla articulates with
the head ; the stipes, the broadest segment of the structure ; the
inner and the outer lobes, which project from the distal edge of
the stipes ; and the mgixillary palp, which projects from the lateral
edge of the stipes. Draw a maxilla on a scale of 5 and label all of
these parts.

Exercise 10, Note between the maxillae and just in front of the labium
a small median projection, the hypopharynx. Remove the labium,
taking care to leave none of it in the animal, and mount it on a slide.
Note the basal segment, by means of which the labium articulates
with the head, and the two jointed labial palps, which project from
the lateral edges. The labium is a second pair of maxillae fused in
the median line. Trace the homologies between the parts of the
labium and those of the maxillae. Draw the labium on a scale of
5 and label its parts.

Internal Anatomy. Take the grasshopper in the hand and with
a pair of fine, sharp scissors cut a slit through the body wall a
little to one side of the mid-dorsal line from one end of the body
to the other, using great care not to injure the organs within.
Place the animal, dorsal side up, in a shallow pan with a wax-
covered bottom containing water. First, with two strong pins,
pin the head to the wax and then the extreme hinder end of the
body ; then carefully spread the cut edges of the body wall as
widely as possible to the right and left and pin them down, using
many pins on each side. Observe the organs as they lie in the
body cavity. In the thorax will be seen the strong locomotory
muscles. Lying immediately beneath the dorsal abdominal wall
in the median line is the heart ; this may have been destroyed by
the incision, but if not, it may be recognized as a narrow, trans-
parent tube of the diameter of a needle, flanked by paired tri-
angular muscles which hold it to the body wall. Immediately
beneath the heart is a loose network of yellowish, fatty tissue,

A LARGE GRASSHOPPER ii

called the fat body, which covers the viscera. Remove this care-
fully. The alimentary canal will be disclosed, a large tube running
through the median axis of the body ; above its abdominal por-
tion are the paired reproductive glands, from which a duct passes
on each side around the alimentary canal to the ventral side of the
animal. Notice the silvery air tubes, or tracheae, on each side of
the alimentary canal ; also observe the tangled mass of delicate
brown threads, the urinary, or Malpighian, tubules, between the
reproductive glands and the alimentary canal.

Exercise 11. Make a sketch of the animal on a scale of 5, showing
the internal organs, and label them all.

The Digestive System. With fine scissors sever the alimentary
canal at its extreme posterior end, where it joins the anus. With
great care draw it forward between the ducts of the reproductive
organs and from beneath those organs, loosening it from the sur-
rounding tissues with a needle. Identify the following divisions
of the alimentary canal : (i) the pharynx, the space just back of
the mouth ; (2) the oesophagus, the narrow tube which runs up-
ward from the pharynx and, bending back, enters the thorax,
where it enlarges to form a pouch called the crop ; (3) the salivary
glands, a pair of delicate, branched organs, one on each side of
the crop, the ducts of which run forward to the pharynx ; (4) the
gastric caeca, eight elongated sacs which encircle the base of the
crop; (5) the stomach, a large tube which extends back to the point
where the delicate urinary, or Malpighian, tubules join the alimen-
tary canal ; (6) the intestine, the hinder portion of the alimentary
canal, which ends at the anus and is made up of three divisions.

The Excretory System. The kidney of the insect consists of
the Malpighian tubules. These are delicate, tubular glands,
about fifty in number in the grasshopper, which unite with the
alimentary canal and discharge their products into it at the point
of juncture of the stomach and the intestine. They extend freely
into the body cavity and excrete urinary wastes from the blood,
in which they lie immersed.

Exercise 12. Make a drawing of the alimentary canal and the Mal-
pighian tubules on a scale of 7 and label all of the parts.

12 PHYLUM ARTHROPOD A

The Reproductive System : the Female Organs. The two ovaries

are closely bound together by a web of connective tissue and
tracheae so as to form a single mass, which Hes above the intestine.
If your specimen is a female, part this mass along the median line
and with a needle gently remove some of the connective tissue
surrounding it. Examine it with a hand lens ; each side is a sepa-
rate ovary and will be seen to be a collection of parallel, tapering
tubules, their smaller ends being in the median line, their longer
ends projecting back to the tubelike oviduct. These tubules are
called ovarioles ; it is in them that the eggs develop. How many
tubules do you count on each side ? Notice the elongated eggs in
each ovariole. How many do you see in each one ? The two ovi-
ducts proceed from the ovaries to the ventral side of the animal,
where they unite to form a median tube, the vagina, which opens
to the outside between the ovipositors. Just above the vagina is
a small sac, the receptaculum seminis, which is connected by a
long, sinuous duct with the exterior. The sac becomes filled
with spermatozoa during pairing, which fertilize the eggs as they
pass out of the vagina.

The Male Organs. The paired testes which secrete the sperma-
tozoa lie above the intestine, bound together by connective tissue
and fat. Each testis consists of a bundle of elongated tubes, with
which a duct called the sperm duct connects posteriorly. The
two sperm ducts run, one on each side of the intestine, to the
ventral side of the animal, where they meet to form a median
tube, which leads to the genital pore just beneath the anus.

Exercise 13. Make a semidiagrammatic drawing representing all the
parts of the reproductive tract of your specimen.

The Respiratory System. The spiracles have already been
noted. They are the external openings of the tracheae, a system of
fine air tubes which extend throughout the entire body of the
insect and through which fresh air is introduced into every part
of it. The blood is thus constantly aerated, and there is never
any venous blood present. This arrangement results in a very
active metabolism and is one of the causes of the extraordinary
amount of energy which characterizes most insects. With the

A LARGE GRASSHOPPER 13

aid of a hand lens examine the tracheae in different parts of
the body. They may be easily detected by their silvery gleam.
Notice the arrangement of the main tracheal trunks, including
those which connect with the spiracles, also the arrangement of
the air sacs, which are expansions of tracheae. Mount a small
portion of the fatty tissue containing tracheae in water or glycerine
and examine them with a compound microscope. Notice the
spiral threads which line the tracheae. Find the connections of
the tracheae with the spiracles.

Exercise 14, Make a drawing of a trachea seen under a high power of
the microscope.

The Circulatory System. This system is very simple in insects,
being correlated with the great complexity of the respiratory sys-
tem. Instead of the blood being carried to the respiratory organs
to be aerated, as is the case in vertebrates, rendering necessary
a complicated system of blood tubes connecting the remotest
parts of the body with the respiratory organs, the respiratory
organs are themselves a system of tubes which introduce air to
every part of the body. The insect has a blood fluid which lies
in the body cavity. The only circulatory vessel present is the
tubular heart. This organ, whose position has already been noted,
has a closed hinder end and segmental valvular openings along
its sides. By its contractions the blood is sent into the forward
portions of the body, whence it flows back into the hinder por-
tions and enters the heart again through the valvular openings.
To observe the heart of an insect is not always easy, because of
its position so near the dorsal body wall and its great delicacy
of structure. An easy method is to mount a live, transparent,
aquatic insect larva, such as that of the mosquito, on a sKde in
water and observe it under a compound microscope. The heart
and its action may be easily studied.

The Nervous System. Cut off the alimentary tract at its for-
ward end, taking care not to injure the two nerve connectives
which pass to the brain, and remove all the viscera from the body.
The nerve cord will be seen lying on the ventral body wall of the
abdomen, in the median line, often concealed by fat. It will be

14 PHYLUM ARTHROPODA

seen to be double and to contain, in the abdomen, five enlarge-
ments, the ganglia, from each of which fine nerves radiate. Trace
the nerve cord from the abdomen into the thorax. It is here pro-
tected by hard projections of the body wall, which must be care-
fully removed. Four large ganglia will be found here, the three
posterior ones of which are the thoracic ganglia. The one in the
forward portion of the prothorax really belongs to the head and
is called the suboesophageal ganglion. From it a pair of nerve
connectives passes to the dorsally situated supraoesophageal
ganglion, or brain. The brain is the largest ganglionic mass in the
body and is situated in the top of the head between the eyes. Lay
bare the brain. Notice the optic lobes going to the eyes, and be-
tween them the much smaller ocellar lobes sending nerves to the
lateral ocelli. Beneath the optic lobes are the antenna! lobes,
which send nerves to the antennae ; near them in the median line
is the median ocellar lobe, which sends a nerve to the median
ocellus.

Exercise 15. Make a large sketch of the nervous system as far as
observed, representing it in an outline of the animal's body, and
show in which segments the different ganglia occur.

Exercise 16. Draw a diagram representing a side view of a grass-
hopper on a scale of 3 or 4, in which the segmentation, the relative
position of the heart, the alimentary tract, and the nervous system
are accurately indicated.

A CATERPILLAR 15

Class : Tracheata. Division : Inseda. Order : Lepidoptera

AN INSECT LARVA: A CATERPILLAR

Notice that the head, thorax, and abdomen are not set off from
one another. The body is thus wormHke in form, there being
almost no speciaHzation of the body parts. Determine how much
of the body is thorax and how much abdomen. The thorax bears
three pairs of jointed legs, each one terminating in a single hook.
The abdomen also bears several pairs of legs, which are not like
those of the thorax. How many are there and in what do they
differ from the thoracic legs ? Find and count the spiracles, which
are usually easily seen.

Exercise 1, Draw an outline representing a side view of the animal
on a scale of from 2 to 6 ; number the thoracic and abdominal
segments, show the spiracles, and label all the parts.

Study the head with the aid of a hand lens. Notice the pair
of large, convex plates which, with the small, median, triangular
plate, form the wall of the head. Near the lower edge of each of
the convex plates are several minute ocelli ; count them. On the
ventral side of the head find the antennae ; how many joints are
there in each ? The mouth parts are between the antennae. The
labium is bilobed, and beneath it are the dark-colored mandibles.
Just back of these are the maxillae and the labium, the latter being
a median, elongated, conical organ between the maxillae. The
external opening of the silk glands is in the labium.

Exercise 2. Draw a front view of the head on a scale of 7.

Internal Anatomy. With fine scissors make a longitudinal in-
cision the length of the animal in the dorsal integument, a short
distance to one side of the median line. Turn the integument to
the right and left and pin it down. If it has not been destroyed,
observe the heart. It is a straight, transparent tube lying in the
mid-dorsal line just beneath the integument. Note the large,
tubular alimentary tract surrounded by dehcate, glistening tra-
cheae and by the white and often filamentous fat. Its forward

i6 PHYLUM ARTHROPODA

portion is the oesophagus ; the middle and largest portion is the
stomach ; the narrow portion back of the stomach is the intestine,
which communicates with the anus. In the forward portion of
the body cavity, along the wall of the oesophagus, is a pair of del-
icate, tubular salivary glands which extend forward and conamu-
nicate with the mouth. Note and trace the course of the much
larger, tubular silk glands on the ventral body wall ; they are also
a single pair and communicate with an opening in the labium.

Find and carefully trace the course of the six Malpighian tubes,
which lie along the stomach and join it at its posterior end.

Exercise 3. Draw an outline of the opened animal on a scale of 6,
showing the organs above described as far as they have been ob-
served. Represent the segmentation and show accurately the posi-
tion of the organs in their proper segments.

Sever the oesophagus and remove the stomach and the intestine
from the body. Study the nervous system. Note the arrangement
of the tracheae with reference to the spiracles. Note the longitu-
dinal muscle bands which form a part of the body wall ; note also
their segmental arrangement.

Exercise 4, Draw an outline of the opened body on a scale of 6,
showing and numbering the segments. Draw in it the nervous
system, representing accurately the number of ganglia and plac-
ing them in the proper segments, together with the tracheae and
muscles.

The reproductive system consists of two small sexual glands and
a duct leading from each. There is no external pore.

A CENTIPED 17

Class: Tracheata. Division: Myriapoda
A CENTIPED (LITHOBIUS)

Myriapods are wormlike animals which live under logs and
stones, beneath the bark of decaying stumps and trees, and in
other dark, damp places. The two main groups of myriapods
may be easily recognized by the differences in shape and habits, —
the centipeds (Chilopoda) being flattened and very active ani-
mals with one pair of legs to a segment, the millipeds (Diplopoda)
being usually cylindrical animals v/ith short legs, two pairs of
which are present on most of the segments.

Observe the vermiform body, the well-marked segmentation, and
the segmented legs ; note also the lack of specialization among the
segments, there being no division into thorax and abdomen. The
animal is plainly an arthropod, but it is not an insect ; it is a
lower animal than an insect, because its body shows less special-
ization. Note the single pair of antennae and the insect-like mouth
parts, which consist of a pair of mandibles and two pairs of
maxillae ; also the large hooklike appendages just back of the
head. These latter are homologous to the first pair of legs ; they
are the principal organs of prehension and are provided with poi-
son glands which open on the inner surface near the end. Note
the anal feelers ; these are homologous to the hindermost legs
and enable the animal to perceive what is back of it.

Exercise 1. Draw an outline of the dorsal aspect of the animal on a

scale of 5 and label all the organs observed.

Exercise 2. Draw a ventral view of the head on a scale of 10, showing
the cephaHc appendages in position.

Exercise 3. Remove, under a dissecting microscope, the prehensile
hooks and the mouth parts, beginning with the posterior ones and
working forward, and the antennae. Mount them on a slide and
draw an outline of each. Compare the different structures of the
mouth parts with those of the insect and label them all.

The Internal Organs. The digestive, circulatory, respiratory,
excretory, and nervous systems are similar to the same systems in
insects. They will not be studied in this dissection..

1 8 PHYLUM ARTHROPOD A

Class: Arachnoidea. Division: Arachnida
A SPIDER

As large a spider as possible should be obtained for this study.
If a small one is used, it is usually well to stick a slender insect pin
through it, in order to be able to handle it easily, and it should be
studied with the aid of a hand lens. Observe the form and color
of the animal. The body is unsegmented (although the body of
the embryo spider is distinctly segmented) and is made up of two
parts, the cephalothorax and the abdomen. What does the embry-
onic segmentation indicate as to the ultimate relationships of
spiders ? Observe the hairs which cover the body and legs. They
are projections of the cuticula and are important sense organs,
being sensitive to vibrations of the atmosphere. They thus aid in
giving the animal information as to what is going on about it.

The Cephalothorax. This division of the body is equivalent to
the head and thorax of insects. Observe carefully the eight eyes
at or near its forward end, both the size and the arrangement of
which vary much in the various species of spiders. The ventral
surface bears the six pairs of appendages, the mandibles, the
pedipalps, and the four pairs of legs.

The mandibles, the anterior pair, occupy a vertical position at
the front end of the body and consist each of a basal portion and
a terminal claw, near the tip of which is the pore from which
poison is injected into the bite. In consequence of the vertical
position of its mandibles the spider can strike only an insect
which is beneath it.

The second pair of appendages are the pedipalps. These are
leglike and contain one less segment than the legs. The basal
segment of the pedipalp is called the maxilla. The two maxillae
are flattened structures situated on the underside of the cephalo-
thorax just back of the mandibles, their forward, medial margins,
which cover the mouth, being used to lacerate and squeeze the
food so that the animal juices can be sucked up. Spiders prey
exclusively upon living animals, but they can take in only liquid
food. The pedipalps of the female spider differ in shape from

A SPIDER 19

those of the male, and the two sexes may be distinguished in this
way. In the female the pedipalp looks exactly like a small leg ;
in the male the terminal portion is expanded and very complex
in structure, being used by the animal in the act of pairing.

The third, fourth, fifth, and sixth pairs of appendages are the
legs ; they are used by the spider for a variety of purposes be-
sides walking. They are important as tactile organs, their great
length increasing their usefulness in this respect, and they un-
doubtedly compensate the animal in a certain degree for the lack
of antennas. The legs are also of use in spinning and manipulat-
ing the web, the complex structure of the claws being associated
with this function.

The median plate between the maxillae on the ventral side of
the body is the labium ; the one between the bases of the legs is
the sternum.

The Abdomen. The dorsal surface is usually marked by several
pairs of depressions, which mark the points of attachment of mus-
cles. At the hinder end, on the ventral surface, are three pairs of
spinnerets. Study these carefully with the aid of a hand lens. At
the end of each spinneret are numerous microscopic holes, from
which is exuded the semifluid silk. This is made up of many soft
strands, which harden as they unite to form the thread.

A study of the embryology of the spider shows that the spin-
nerets are homologous to abdominal legs.

Note the spiracles, the external openings of the respiratory
organs, the tracheae and the lungs. A short distance in front of
the spinnerets in the ventral surface of the abdomen is the single,
median, minute tracheal spiracle, which it is often difflcult to see.
The lung spiracles are a pair of large slits near the anterior end of
the abdomen, each one at the lateral end of a transverse fold of
the integument. Between them in the median line is the genital
pore. In the female spider it is covered by a large and complex
plate called the epigynum.

Exercise 1. Cut off the legs on the right side of the body and draw
an outline of a side view of the spider on a scale of from 5 to 10,
putting in only the basal portion of the legs but all the pedipalps
and the mandibles. Carefully label all the parts observed.

20 PHYLUM ARTHROPODA

Exercise 2. Draw an outline of the ventral aspect of the body on the
same scale, putting in and labeling all the parts observed.

Exercise J. Draw the front end of the body on a scale of lo, showing
the mandibles and the eight eyes.

Exercise 4, Draw the pedipalp on a scale of 6.

Exercise 5. Draw one of the legs on a scale of 6.

Exercise 6. Cut off the terminal joint of one of the fourth pair of
legs and study it under a compound microscope, noting the shape
of the claws and the hairs which often surround them. Draw them.

The internal anatomy of the spider will not be studied in this
dissection.

A CRAYFISH OR A LOBSTER 21

Class : Crustacea. Division : Malacostraca. Order : Decapoda
A CRAYFISH OR A LOBSTER

These two animals are very common, the one in fresh and the
other in salt water. In external form and internal anatomy they
are exceedingly similar to each other, and the same directions for
dissection may be made to apply to either. In habits and general
method of life the animals also resemble each other ; they move
about at or near the bottom of the water, preferring regions which
are rocky or stony, and feed upon small animals of all kinds and
upon carrion.

Observe the shape, color, and external anatomy of the animal.
It is bilaterally symmetrical ; the body is composed of a number
of serially arranged segments, which are called somites, or meta-
meres ; the dorsal and the ventral sides of the body are unlike,
the latter being characterized by the possession of a series of
paired and jointed appendages metamerically arranged, that is,
each somite, or metamere, bears a pair of appendages ; the ante-
rior and the posterior ends are also unlike, the former being
characterized by the possession of organs of special sense and the
mouth. The external covering of the body is a chitinous cuticula
which constitutes an exoskeleton. All these features are equally
characteristic of insects and myriapods.

As in all crustaceans, and also in insects, the body of the animal
falls into three distinct divisions : the head, thorax, and abdo-
men. The first two of these body divisions do not, however,
articulate freely with each other as they do in insects, but in com-
mon with all the higher crustaceans, they are fused together and
form a single structure, which is called the cephalothorax. The
dorsal and the lateral surfaces of this division show no segmenta-
tion, because of the fusion of the somites and the presence of a
hard, shieldlike structure covering it, which is called the cara-
pace, but on the ventral side the segmentation is distinctly seen.
Extending along the entire ventral surface of the animal are the
paired appendages. Their metameric significance may not be
seen in the cephalothorax, but it will be distinctly seen in the

22 PHYLUM ARTHROPOD A

abdomen, where each somite except the last, or telson, bears
a pair of appendages.

The animal is capable of two sorts of locomotion. By powerful
strokes of the broad, finlike end of the abdomen it swims rapidly
backward, and it can also walk on its thoracic legs. It is well pro-
vided with special sense organs. Most important to it are the
two pairs of feelers, or antennae, which are characteristic of all
crustaceans, and the compound eyes on movable stalks. It also
possesses, in a pair of small cavities on the upper surface of the
basal joints of the first or shorter pair of antennae, peculiar sense
organs, which were formerly supposed to be ears, but are now
known to be balancing organs. With the aid of them the animal
maintains its equilibrium.

The body of the crayfish or the lobster, as of all the higher
crustaceans, is made up of twenty somites, or body segments, of
which the thirteen anterior somites form the cephalo thorax, and
the seven posterior ones the abdomen.

The Cephalothorax. The anterior five somites forming this
body division are cephalic, the remaining eight are thoracic, and
all are covered dorsally and laterally by the carapace. The pro-
jection running forward from the anterior end of the carapace is
called the rostrum. A transverse groove is seen near its middle ;
this is the cervical suture and marks the boundary between the
head and the thorax. In the crayfish two semicircular, longi-
tudinal grooves extend backward from the outer ends of the cer-
vical suture, which separate the sides of the carapace from the
median, dorsal portion. The sides of the carapace are called the
branchiostegites ; they cover lateral folds of the dorsal integu-
ment of the animal, which extend over the sides of the body and
inclose between themselves and it the spaces within which lie the
gills. These spaces, the gill chambers, thus communicate freely
with the surrounding water. Pass the handle of a scalpel or
other flat object beneath the lower edge of the branchiostegite
and it will go into the gill chamber. During life a current of
water passes constantly into the gill chamber along this lower
edge, where it bathes the gills and then passes out at the for-
ward end.

A CRAYFISH OR A LOBSTER 23

Study the ventral side of the cephalo thorax. The most impor-
tant organs here are the appendages. At the anterior end of
the body are the two pairs of antennae, the longer pair being the
second. On the lower surface of the basal joint of each of the
latter is an opening ; these are the external openings of the kid-
neys, or green glands. Back of the antennae is the mouth. It is
bounded in front by a liplike structure called the labrum, and at
the sides by the strong mandibles. Press the mandibles aside and
pass a probe into the mouth. Between the mouth and the large
claws are five pairs of appendages which assist in the act of eating ;
they are two pairs of delicate leaf like maxillae, just back of the
mouth, and three pairs of larger maxillipeds, back of them. They
are best identified by beginning with the hinder pair of maxilli-
peds, which is just in front of the large claws, and working for-
ward, placing a needle or knife between the appendages as they
are identified. Back of the maxillipeds come the large grasping
claws, or chelipeds, which form the principal weapons of offense
and defense of the animal, and in the largest lobsters are powerful
enough to crush a man's arm. Note the difference between the
right and the left claw, if any. Back of the chelipeds are four
pairs of walking legs. In the male animal the paired external
openings of the genital organs are at the base of the last pair of
walking legs, in the female at the base of the third pair from the
last. Find them.

The Abdomen. The seven somites forming this body division
are all free and jointed with one another. Note the difference in
the thickness of the cuticula on the dorsal and the ventral sur-
faces, also its thinness at the joints. The appendages on the abdo-
men have various uses. They probably have a general respiratory
function. In the male the first two pairs are functional in pairing ;
in the female the first five pairs hold the eggs from the time they
are laid until the young are hatched. The last pair in both sexes
is large and broad and with the end segment forms the swimming
fin. The end segment is called the telson ; it bears no appendages ;
the anal opening is in its ventral side.

The natural color of the animal is usually a greenish black, but
hot water or a preservative turns it red.

24 PHYLUM ARTHROPODA

Exercise 1, Draw an outline of the dorsal side of the animal and
label all the parts described above.

Cut off the right branchiostegite with the scissors, taking care
not to injure the gills beneath. Push aside the gills and notice the
thin integument which forms the lateral wall of the cephalo-
thorax. Observe the method of attachment of the gills. They are
feathery, thin-walled expansions of the body wall and are at-
tached either to it or to the basal portions of the legs. They
present a very large surface to the surrounding water, and the
blood circulating through them is thus oxygenated. Notice the
epipodites, the skinny flaps which project from the basal joints of
many of the legs and separate the gills of a segment from those of
the next. They are not prominent in the crayfish.

Exercise 2. Without displacing the gills or epipodites make a sketch
of them as they lie in the gill chamber.

Exercise 3. Draw a diagram representing an ideal transverse section
of the body wall in the region of the walking legs ; show the rela-
tions of the branchiostegites, the legs, and the gills to the body.

The Appendages. Of these there are nineteen pairs, each somite
of the body, with the exception of the last one, bearing a pair.
There are thus thirteen cephalothoracic appendages, of which
five are cephalic and eight thoracic, and six abdominal append-
ages. All these appendages, except the first pair, however much
they may differ from one another, are modifications of a single
primitive type of structure. This type has been least modified in
certain of the abdominal appendages. We shall, consequently,
study these first.

Exercise 4. The abdominal appendages are called swimmerets or
pleopods. Cut off the right swimmeret of the fourth abdominal
somite close to the body, draw it on a large scale, and label all its
parts. It consists of a basal piece, the protopodite, and two terminal
branches, the medial, or endopodite, and the lateral, or exopodite.
This type of structure is characteristic of all crustacean appendages
except the pair belonging to the first somite; those appendages
which apparently differ from this type are modifications of it.

Exercise 5. Remove and draw on a large scale the right-hand
sixth swimmeret. It is quite different from the last one drawn.

A CRAYFISH OR A LOBSTER 25

and is sometimes called a uropod, but yet has the typical parts.
Label its parts.

Exercise 6, a. If the animal is a male, remove and draw the right-
hand first and second swimmerets. These are modified from the
typical structure to serve as copulatory organs.

Exercise 6, &. If the animal is a female, remove and draw the right-
hand first swimmeret.

Exercise 7. The five pairs of walking legs (including the chelipeds)
are called periopods and belong to the thorax. Remove and draw
the right-hand fourth periopod, disregarding the gill attached to it,
and label the parts. It consists of seven segments, of which the two
proximal segments, those nearest the body, constitute the protopo-
dite, and the five distal ones, those farthest from the body, the
endopodite. The exopodite is not present.

Exercise 8. The cheliped is composed of the same segments as the
other periopods. With a strong knife split the claw lengthwise into
two equal halves. Examine ^the muscles controlling the movable
limb of the claw. There is a strong adductor muscle which closes it,
and a weaker extensor muscle which opens it. Make a diagram-
matic drawing illustrating them.

Exercise 9. The three pairs of appendages directly in front of the
chelipeds are the maxillipeds ; they are thoracic appendages which
assist in the process of eating. Remove with forceps and scissors
the right-hand third (that is, the posterior) maxilliped ; draw it on
a large scale, disregarding the gill which may be attached to it, and
carefully label the protopodite, exopodite, and endopodite.

Exercise 10. Remove with the forceps the right-hand second maxil-
liped and draw it on a large scale.

Exercise 11. Remove and draw the right-hand first maxilliped. The
two large basal segments are the two segments of the leaflike
protopodite, the endopodite is a very small structure next to the
protopodite, and the exopodite is a much longer structure next to
the endopodite. A large epipodite is present. Label all of these.

Exercise 12. The two pairs of delicate appendages in front of the
maxillipeds are the maxillae ; they are cephalic appendages. Re-
move with forceps the right-hand second maxilla and draw a large
outline of it. The protopodite is wide and leaf like ; the endopodite
is small ; the exopodite is wide and with the epipodite forms a broad
plate, called the scaphognathite, which is used by the animal to
maintain a current of water from the gill chamber.

26

PHYLUM ARTHROPODA

Exercise 13. Remove carefully and draw the right-hand first maxilla.
The protopodite is wide and leaflike and similar to that of the
second maxilla ; the endopodite is extremely small ; the exopodite
is wanting.

Exercise 14. Extract with strong forceps and draw the right-hand
mandible. The biting portion of it, together with the first joint of
the small palp, forms the protopodite ; the two terminal joints of
the palp are the endopodite ; the exopodite is wanting.

Exercise 15. Remove at its base and draw the right-hand second
(the longer) antenna. The exopodite is a short, stiff, platelike
expansion ; the endopodite is the long, slender, terminal portion.

Exercise 16. Remove and draw the right-hand first antenna, or
antennule as it is also called. No exopodite and endopodite are
present in this appendage throughout the Crustacea, there being
typically but one terminal branch. In the crayfish and lobster this
terminal branch is double.

Exercise 17. Construct in your notebook a table showing the relation
of the appendages and somites, as follows :

No. OF Somite

Name of Appendage

Prot.

Ex.

End.

I

2

Head . . . <

3

4

- 5

r 6

First maxilliped

+

+

4-

7

8

Thorax . . -

9

lO

II

12

'14
15
16

Abdomen . ^

17
18

19

< 20

A CRAYFISH OR A LOBSTER

27

Write opposite the number of each somite, in your notebook,
first, the name of the appendage belonging to it, then indicate
what parts that appendage possesses by a '' + " and what parts
it lacks by a ^^ — " under the appropriate head, as is shown in
the case of the sixth somite in the table on page 26.

The Gills. Remove the left-hand branchiostegite. Place the
animal in water and study the gills on the left side. These organs
may occur on the eight thoracic somites, and on each segment
they may be attached either to the basal joint of the leg, when
they are called podobranchiae, to the flexible joint by which the
leg articulates with the body, when they are called arthro-
branchise, or to the body wall just above the leg, when they are
called pleurobranchiae. A single thoracic somite may bear on
each side four gills, — a pleurobranch, two arthrobranchs, an an-
terior and a posterior, and a podobranch, — but on most of the
somites a less number is present.

Exercise 18. Construct in your notebook a table showing the ar-
rangement and number of the gills and also of the epipodites and
their relations to the somites bearing them, as follows :

No. OF

Somites

PODO.

Ant. Arth.

Post. Arth.

Pleu.

Epip.

Total

6

7
8

9
10
II
12
13

Begin with somite 13 and indicate by a '' + " under the proper
head opposite the number of each somite the presence of the gill
or epipodite, and by a '' — " its absence.

The Internal Organs. With strong scissors and forceps care-
fully remove the shell from the entire dorsal surface of the animal,
taking great care not to disturb the organs lying beneath. Notice
just beneath the shell a pigmented membrane. This is the under-
skin, or hypodermis, the matrix of the shell. Entirely remove the

28 PHYLUM ARTHROPODA

under-skin. Study the organs as they He, without disturbing them.
Notice in the cephalothorax, first, the large sadike stomach just
back of the rostrum and connected by muscles with the anterior
body wall. On each side of the stomach will be seen the cut ends
of a mass of muscle fibers. These are the mandibular muscles.
Demonstrate their connection with the mandibles. Just back of
the stomach is the white, shield-shaped heart, from the ante-
rior end of which five delicate arteries proceed, a median artery
and two pairs of lateral ones. Find these arteries and trace them
forward as far as possible without breaking them. On both sides
of the stomach and the heart and partly beneath them are the
liver and the reproductive organs. The former is a pair of large,
soft, and usually light-green organs which may fill a large portion
of the cephalothorax and may extend back into the abdomen.
The latter, if the animal is a female, are a pair of brownish or
yellowish organs, the ovaries, in which the ova can often be seen ;
they are situated beneath the heart and in front of and behind it,
and vary in size and also in color with the development of the
ova. When these are approaching maturity the ovaries are the
most prominent organs in the body cavity, and often extend far
back into the abdomen. In the male animal the reproductive
glands, the testes, are white in color and very slender, and occupy
the same position as the ovaries in the female. Note the coiled
sperm duct on each side.

Study the musculature and the other organs of the abdomen.
There are two systems of muscles here. On the dorsal side are
longitudinal muscles, the extensors, which extend or straighten
the abdomen. Separate these muscles carefully along the median
line and observe beneath them the dehcate, colorless abdominal
artery which carries the blood from the heart throughout the
abdomen. Trace it forward to the heart. Notice the lateral
branch arteries. How many pairs are there? Just beneath this
artery lies the intestine, which often contains dark-colored fecal
matter. Beneath it and filling most of the space within the abdo-
men are the flexor muscles, which are very complex, and whose
function it is to bend or flex the abdomen. It is by the use of
these two sets of muscles that the animal swims.

A CRAYFISH OR A LOBSTER 29

Exercise 19. Draw an outline of the animal's body, showing the
segmentation and the organs mentioned above, and label them all.

The Circulatory System. The heart lies within an inclosed space
called the pericardial sac, the walls of which, the pericardium, will
have been partially destroyed by the removal of the under-skin.
The heart, the abdominal artery with its lateral branches, and
the five anterior arteries have been studied and drawn. Care-
fully press aside the heart and note the median dorsoventral
artery which leaves the abdominal artery near the heart and
passes ventrally. This artery supplies with blood a ventral
longitudinal artery, which lies in a midventral position in the
thorax and abdomen.

Remove the dorsal abdominal artery and the heart from the
body and float them in clean water. Note the six valvular open-
ings of the heart, two being on the dorsal side, two on the ventral,
and one on each of the lateral sides. These can be seen by blowing
on the heart through a blowpipe.

Exercise 20. Draw a dorsal view of the heart showing the valves
there present.

The course of the circulation of the blood is the following : by
the contraction of the heart the blood is sent through the arteries
to all parts of the body ; after bathing the different tissues it col-
lects in a space in the ventral portion of the body cavity in which
lie the ventral nerve chain and the ventral abdominal artery, and
passes toward the gills ; from this space it then passes to the gills
through afferent veins, one of which runs to each gill and along
the outer edge of it; it then runs through the delicate gill fila-
ments, where it is aerated, and passes by efferent veins on the
inner edges of the gills back to their base ; here six larger bran-
chial veins collect the blood and carry it to the pericardial sac,
whence it is taken through the valvular openings into the heart.

Exercise 21. Draw a diagram representing the entire circulatory
system.

The Reproductive System : the Female Genital Organs. The
position of the ovaries has already been observed. In the cray-

30 PHYLUM ARTHROPODA

fish their forward portions are paired, and their hinder portions
are fused and He in the median Hne. In the lobster, however, no
such fusion takes place, but the two ovaries are united by a
bridge midway in their length. Find the paired oviducts which
lead from the ovaries to the genital openings. Remove both
ovaries and oviducts from the body and float them in water.

Exercise 22y a. Make a diagrammatic sketch of them.

The Male Genital Organs. The position of the testes has been
already noted. In the crayfish they are similar in shape and posi-
tion to the ovaries in the female animal, but are more slender ;
in the lobster they are a pair of long white tubes which extend
forward as far as the stomach and back into the abdomen. Find
the paired sperm ducts, which are long, convoluted tubes con-
necting the testes with the external genital openings. Remove
the sperm ducts with the testes from the body and float them in
a pan of water.

Exercise 22 ^ b. Make a diagrammatic sketch of the male reproduc-
tive tract.

Cut open a sperm duct and examine its contents under a high
power of the microscope. Star-shaped spermatozoa will be seen ;
if, however, the specimen has not been well preserved they will no
longer be present.

Exercise 22y c. Draw a spermatozoon, if present.

The digestive tract consists of the mouth, oesophagus, stomach,
intestine, into which open the paired livers, and the rectum. Pass
a probe through the mouth into the stomach and notice the dorso-
ventral course of the oesophagus, which joins the mouth with the
stomach. The paired ducts which unite the two lobes of the liver
with the intestine join that organ just back of the stomach. Find
them. With scissors sever the oesophagus just ventral to the
stomach, taking care not to injure the brain, which Hes in front
of the stomach, or the two slender nerve connectives, which he on
either side of the oesophagus. Sever the rectum near the anus.
Remove the entire digestive tract from the body and place it in a

A CRAYFISH OR A LOBSTER 31

pan of clean water. The liver is so soft that it may not be possible
to remove it entire. Notice the boundary between the intestine
and the somewhat larger rectum. In the crayfish the rectum is
much longer than the intestine; in the lobster the opposite is
true. In the lobster notice the blind gut, or appendix, which joins
the rectum near its anterior end.

Exercise 23. Make a diagrammatic sketch of the digestive tract.

Cut open the stomach by a ventral incision and wash it out.
Observe its chitinous lining and the dark-brown, chitinous teeth.
This chitinous lining is a continuation of the cuticula which
covers the external surface of the body and is molted with the
cuticula. During certain parts of the year a pair of large, cal-
careous bodies called gastroliths are embedded in the lining of the
stomach. They remain in the stomach after the molting of the
cuticula and furnish lime for the new cuticula, which at once
grows rapidly.

Exercise 24. Make a sketch of the inner surface of the stomach
showing the teeth.

The Excretory System. Notice in the extreme forward end of
the body cavity, just in front of and below the stomach, a pair of
pale, greenish bodies. These are the kidneys, or green glands.
Each one is made up of two portions, the smaller glandular por-
tion, next to the body wall, and the larger saccular portion, or
urinary bladder, next to the stomach. From the latter the ureter
leads to the external openings which have already been noted.

Exercise 25. Draw a view of the forward end of the body cavity
showing the kidneys as they lie in position.

The nervous system consists of a ventral double nerve cord
lying in the midventral line in the body cavity and extending
the length of the animal, with paired ganglia at intervals, also of
a brain situated just back of the eyes, which is united with the
ventral nerve by two nerve connectives, passing one on each side
of the oesophagus. The ventral ganglia have typically a meta-
meric significance, but many of the somites have lost their ganglia,

2,2 PHYLUM ARTHROPOD A

so that there are fewer ganglia than somites. The double nature
of the ventral nerve is best seen in the thorax.

Remove all the muscles and the viscera from the body. The
ventral nerve cord will be seen in the abdomen lying in the mid-
ventral line. Notice the gangha. How many do you count?
Notice the lateral nerve branches. In the cephalothorax the nerve
cord is concealed beneath transverse ridges of the ventral wall of
the shell. Cut these with scissors and expose the nerve, beginning
at the hinder end of the cephalothorax and working forward.
How many thoracic ganglia do you find ? Just back of the oesoph-
agus is the large suboesophageal ganglion which is connected with
the brain by the two connectives already mentioned. The brain,
or supraoesophageal ganglion, is just back of the eyes.

Exercise 26. Draw an outline of the body and in it the nervous sys-
tem, showing accurately the number of ganglia and the segments
in which they lie, together with the lateral nerves.

Exercise 27. Draw a diagram representing an ideal sagittal section
of the animal in which the relative position of the principal systems
of organs is accurately shown.

A CRAB 33

Class : Crustacea. Division : Malacostraca. Order : Decapoda

A CRAB

The crab is a representative of the more highly speciaUzed
of the two divisions of the Decapoda, the Brachyura, which in-
clude those decapods with short, weak abdomens. The lobster
and the crayfish represent the other and less highly specialized
of the two divisions, the Macrura, which comprise those decapods
with long abdomens.

Compare the crab with the lobster or the crayfish. Note the
broad, shield-shaped cephalo thorax and the abdomen bent under
it. The abdomen of the male crab is narrow and that of the
female is broad. Which sex is your animal ? In what ways is the
higher specialization of the cephalothorax and the abdomen of
the crab shown ?

The body of the crab is composed of twenty somites, like that
of the crayfish and the lobster, thirteen of which belong to the
cephalothorax and seven to the abdomen. The cephalothorax is
covered by a carapace. Notice the short, transverse suture which
separates the cephalic from the thoracic portion. At the ends of
this suture notice the longitudinal depressions which mark off the
lateral branchial areas and separate the branchiostegites from the
median portion of the carapace. The branchiostegites are not ap-
plied closely to the body as they are in the lobster and the cray-
fish, but stand out from it, very much increasing the transverse
axis of the cephalothorax and making it longer than the longi-
tudinal axis. This feature of its structure makes it easy for the
crab to run sideways. Notice that the ventral edge of the bran-
chiostegite is closely applied to the body, so that the respiratory
water could hardly enter the gill chamber along this edge as it
does in the crayfish and the lobster. An opening is present, how-
ever, at the base of the chehped through which the water enters.
Pass a probe into the branchial chamber through this opening.
Notice the prominent stalked eyes ; also the two pairs of delicate
antennae. Examine and identify the mouth parts and the thoracic
legs ; they will be found to correspond to those of the crayfish or

34 PHYLUM ARTHROPODA

the lobster. Find the openings of the genital organs ; in the male
on the ventral surface of the last cephalothoracic segment and in
the female in the third segment from the last.

The abdomen is relatively small and weak and usually remains
folded beneath the cephalothorax. It lacks the swimming fin ;
crabs of most species cannot swim. The common blue crab, how-
ever, swims very well by means of the fifth pair of periopods.
The number of abdominal segments is variable, fusion having
taken place between certain of the somites. This number is also
not the same in the male and the female of the same species.
Raise the abdomen from the cephalothorax and observe the
swimmerets on its ventral surface. In the female note the long
chitinous hairs which fringe the swimmerets. It is to them that
the eggs and newborn young are attached. The only swimmerets
present in the male are the first two pairs, which are functional
in pairing.

Exercise 1. Draw a dorsal view of the animal with the abdomen
extended, being careful not to omit the antennae and the eyes, and
label all the parts observed.

Exercise 2. Construct in your notebook a table showing the relation
of the appendages and somites similar to that made use of with the
lobster or the crayfish. (See page 26.)

The Gills. With stout scissors cut off the right branchiostegite
and expose the gills. These will be found to be quite different
from those in the lobster or the crayfish, pleurobranchiae only
being present. Note the enormously elongated epipodite of the
first maxilliped which extends across the gills to the hinder part
of the branchial chamber.

Exercise 3. Construct a table showing the relation of the gills to the
somites similar to that made use of in the dissection of the lobster
or the crayfish. (See page 27.)

Exercise 4. Draw a diagrammatic cross section representing an out-
fine of the body wall in the region of the walking legs ; show in this
the relation which branchiostegites, legs, and gills bear to the body.

Internal Anatomy. With strong scissors and forceps remove
the shell from the entire dorsal surface of the body, taking care

A CRAB 35

not to injure the organs within. The arrangement of the organs
will be seen to be similar to that in the crayfish or the lobster.
The livers are a pair of extensive yellowish organs. The anterior
portion of each of these passes laterally into the cavity of the
branchiostegite ; the posterior portion passes backward beneath
the heart. In the male animal the testes are whitish organs which
follow the course of the livers; the sperm ducts are slender,
coiled tubes which lie on each side of the heart. In the female
animal the ovaries also accompany the livers ; the oviducts are a
pair of tubes which pass to the genital openings, themiddle portion
of each being expanded to form a large sac, the receptaculum
seminis.

Exercise 5. Draw an outline of the body and the organs as they lie
in it. Label all carefully.

Remove all the viscera, taking care not to injure the brain and
the circumoesophageal nerves, and examine the nervous system.
The brain is just back of the eyes, as in the lobster or the cray-
fish, and is united with the ventral nerves by means of the lateral
circumoesophageal connectives which pass on each side of the
oesophagus. There is, however, no long ventral nerve cord with
segmental ganglia, but a single large ganglionic mass, in the
shape of a ring, which occupies a central position in the cephalo-
thorax, and from which nerves radiate to the different append-
ages. The dorsoventral artery passes through this ring. Expose
the entire nervous system.

Exercise 6. Draw a semidiagrammatic view of the nervous system,
being careful to represent accurately the nerves radiating from the
ganglionic ring and those going from the brain to the eyes and to
the antennae.

36 PHYLUM ARTHROPODA

Class : Crustacea. Division : Malacostraca. Order : Isopoda
A SOW BUG {PORCELLIO, ONISCUS, OR ARMADILLIDIUM)

This animal is one of the few terrestrial crustaceans. It may
be found at any time of the year under stones, logs, etc., and
in other moist, dark places, where it lives on decaying organic
matter.

The animal must be studied with the aid of a hand lens or a
dissecting microscope. Compare the animal with the crustaceans
already studied. Notice the flattened body. It is composed of
twenty somites, of which five are cephaHc, eight are thoracic,
and seven are abdominal, and much less fusion has taken place
among them than is the case in the decapods. The head and the
thorax are not covered by a carapace and thus are not joined to-
gether to form a cephalo thorax. The apparent head is composed
of six fused somites, five of which are cephalic and one thoracic.
The remaining seven thoracic somites are free and movable.
Count them. Count the abdominal segments. Six will be found,
the last two abdominal somites being fused together.

Find the eyes : they are not on stalks, but are sessile. Only
one pair of antennae appears, the first pair being rudimentary.
Notice the pair of anal feelers which extend back from the hinder
end of the body. These are homologous to the last pair of ap-
pendages, like the cerci of orthopterous insects, and have a similar
function, that of orienting the animal as to what is behind it.

Study the ventral side of the animal. Notice if it be a male or a
female. The male has a long, dark-colored, tube-shaped copulatory
organ which extends from the forward border of the abdomen
backward. The female, besides lacking this organ, may have a
brood sac on the ventral surface of the thorax, which is composed
of plates attached to the medial side of the first five pairs of
walking legs and contains eggs or young. Has your specimen a
brood sac?

The Appendages. First observe the seven pairs of walking legs ;
they are the thoracic legs numbering from two to eight ; gills are
wanting in them. The gills, instead of being thoracic structures,

A SOW BUG

37

as in the decapods, are attached to the abdominal legs. With a
fine needle separate the flattened appendages of the first five
abdominal segments. The endopodite serves as the gill, while the
exopodite is large and platelike and covers the endopodite.

Observe the appendages of the head. But one pair of antennse
will be seen, the second, the first pair being rudimentary in sow
bugs; this feature is apparently an adaptation to a terrestrial life.
The other mouth parts consist of one pair of mandibles, two pairs
of very dehcate maxillae, and a pair of maxillipeds which are
platelike and cover the other mouth parts. The maxillipeds
belong to the first thoracic somite, which is fused with the five
cephalic to form the head.

Exercise, Draw a dorsal view of the animal on a scale of lo.
Number the thoracic and the abdominal segments.

^S PHYLUM ARTHROPODA

Class : Crustacea. Division : Malacostraca. Order : Amphipoda

A FRESH-WATER SHRIMP (GAMMARUS) OR A
SAND FLEA {TALORCHESTIA)

The fresh- water shrimp is common in many places in pools and
streams, and may be easily caught with a fine net ; the sand flea is
a marine animal and is extremely common along all of our shores.

Notice the compressed and translucent body ; this latter fea-
ture is extremely widespread among the smaller aquatic ani-
mals. Can you explain what is the advantage to a small aquatic
animal of being translucent or transparent ? Note the two pairs
of long antennae. In common with all the higher Crustacea, the
body is composed of twenty somites, of which five are cephalic,
eight thoracic, and seven abdominal. Like the isopod, the animal
has no carapace, the eyes are sessile, and the apparent head is
composed of six fused somites, five being cephalic and one tho-
racic. There are thus seven free thoracic segments. Note the
broad movable plates, the epimeral plates, which extend down-
ward from the ventral side of certain of the thoracic segments ;
note the differences in form between the thoracic appendages.
The abdomen is composed of six free segments, the sixth and
seventh somites being fused. Count them. The first three pairs of
abdominal legs are swimming legs, the last three are jumping legs.

Study the appendages, beginning with the free thoracic ones.
With fine needles separate the legs and observe the gills attached
to the posterior borders. How many bear gills? In the female
observe the brood sac when it is present ; it is formed by plate-
like projections of the inner side of certain thoracic feet. In the
abdomen observe the biramous appendages ; they bear no gills.
The cephalic appendages are those typical of Crustacea. In front
of the mouth is a median lip called the labium. Then come the
mandibles and two pairs of maxillae. The pair of appendages, the
maxillipeds, belonging to the first thoracic somite (which is fused
with the cephalic somites) form a kind of lower lip.

Exercise. Draw an outline of the side view of the animal on a large
scale. Number the thoracic and the abdominal segments.

CAPRELLA 39

Class: Crustacea. Division: Malacostraca. Order; Amphipoda

CAPRELLA

This is a very common marine amphipod which is found along
our shores cHnging to hydroid colonies or to seaweed. It is an
interesting animal because it illustrates an extreme degree of
modification from the typical amphipod type, — a modification
which is the result of its mimicry of the form of the hydroid or
seaweed on which it lives.

Notice the irregular, cylindrical form and the small number of
appendages. The apparent head is composed of seven fused
somites, of which five are cephalic and two are thoracic, the first
of these latter bearing a pair of maxillipeds, and the second a pair
of legs. There are thus six free thoracic segments, of which the
first, fourth, fifth, and sixth bear legs which are without gills and
the second and third bear gills but no legs. The abdomen has lost
its segmentation and its appendages and has been reduced to a
mere protuberance at the end of the thorax.

Exercise. Draw a large outline of the side view of the animaL
Number the segments and label the parts observed.

40 PHYLUM ARTHROPODA

Class : Crustacea. Division : Malacostraca. Order : Decapoda

LARVAL DECAPODS: THE ZOEA OF THE CRAB; THE
MEGALOPA OF THE CRAB ; THE MYSIS STAGE OF THE

LOBSTER

These names have been given to certain larval forms of the
crab and the lobster, as well as to those of other of the higher
crustaceans. It is as zoeae that the crab and the higher crusta-
ceans generally leave the egg. The zoea of the crab grows into
the megalopa, which in time grows into the adult animal. The
stage in which the lobster is born is more advanced than the
zoea and is called the mysis stage. All of these larvae are minute
animals and are more or less common in the surface waters of the
sea along the Atlantic coast.

Mount several zoeae of the crab on a slide and study them under
the microscope. The body will be seen to be divided into two
body divisions, a cephalothorax and an abdomen. The former
is covered with a delicate carapace, from which project one or
more spines. When the animal is newly born it possesses the
typical five pairs of cephalic appendages, and the anterior two or
three pairs of thoracic appendages, that is, the maxillipeds, which,
however, are used for locomotion. The remaining thoracic and
the abdominal appendages are wanting, but appear as the animal
increases in size, those anteriorly situated appearing first. The
animal has two stalked eyes.

Exercise 1. Draw a side view of a zoea on a large scale, representing
accurately the appendages, and label the parts observed.

Mount a megalopa and study it under the microscope. You will
observe that it is much larger than the zoea, that it has acquired
a relatively much larger cephalothorax and abdominal append-
ages, and is much more crablike than the zoea. But it still has
a long abdomen, and at the end of this is a swimming fin. The
megalopa is a swimming animal, like the adult lobster, but it is
gradually assuming the characters of the adult crab. Its two
anterior maxillipeds have lost their locomotory character, which

LARVAL DECAPODS 41

they possessed in the zoea, and have assumed their final form and
function. Identify all the mouth parts.

Exercise 2, Draw a dorsal view of the animal, with the legs extended,
on a large scale.

Mount several lobster larvae in the mysis stage and study them
under the microscope. The lobster is born in a more advanced
condition than is the crab. The zoea stage of the lobster is passed
through in the egg, and when the young animal emerges from the
egg it resembles Mysis, a schizopodous crustacean, and hence is
said to be in the mysis stage. The general form of the animal does
not differ much from that of the adult. The abdomen bears no ap-
pendages. The cephalothorax is very nearly like that of the adult
and bears the same appendages. The third maxilliped, however,
is a locomotory appendage, as it is in Mysis, and with the five
periopods is used for swimming. Notice the biramous character
of each periopod.

Exercise 3. Draw a side view of the animal on a large scale.

42 PHYLUM ARTHROPODA

Class : Crustacea. Division : Entomostraca. Order : Copepoda
A FREE-SWIMMING COPEPOD (CYCLOPS)

These minute animals are representatives of the division of
Crustacea called the Entomostraca. All the crustaceans here-
tofore studied belong to the higher group called Malacostraca.
Copepods are extremely common in both fresh and salt water.
They may be obtained in almost any permanent pool of water in
the woods or fields or from the surface water of the sea, often in
large quantities, and are easily kept in aquaria. The animals
should be studied alive if possible, and their swimming move-
ments observed. They propel themselves, not by means of the
feet, but by the long antennae. It is the rowing motion of these
organs which give the animals their jerky movements in the
water. Place them on a slide under a cover glass with a few
strands of cotton, and examine them under a microscope; in
order to prevent the cover glass from crushing them it is well to
support it with a Httle wax at two of the corners. If the pressure
of the cover glass does not suffice to keep them quiet, the with-
drawal of some of the water from under the cover glass with
blotting paper will probably accomplish this result, but care
must be taken not to let the water dry up.

Observe the cylindrical body and the two pairs of long an-
tennae with their sense hairs ; also the long spines at the end of
the abdomen. Note the division of the body into abdomen and
cephalothorax, and also that the latter is not covered by a cara-
pace. If the animal is a female it may be carrying a pair of egg
sacs filled with eggs extending from the anterior end of the abdo-
men. Note the median eye, also the intestine and muscle fibers,
through the transparent body wall.

The body is made up of fifteen somites, the head, thorax, and
abdomen each containing five. The head is relatively large, and
its somites are fused together ; they bear the cephalic appendages
common to all crustaceans. The first pair of antennae is longer
than the second ; in the male it is secondarily modified to form
clasping organs, by which the female is held during pairing. In

A FREE-SWIMMING COPEPOD 43

Cyclops, which is the commonest fresh-water genus of the Co-
pepoda, the first thoracic somite is fused with the head, leaving
only four free thoracic somites. The abdomen bears no append-
ages. In the female the first two abdominal somites may be
fused together.

Exercise 1. Draw a large outline of the dorsal aspect of a copepod,
not putting in any appendages except the antennae. Represent
accurately the sense hairs on the antennae and the caudal bristles.
Number the thoracic and abdominal segments and carefully label
all the parts.

Study the appendages. The thoracic appendages are bira-
mous. They do not bear gills, and the fifth pair is rudimentary.
The cephalic appendages consist of two pairs of antennae, one
pair of mandibles, and two pairs of maxillae.

Exercise 2, Draw a side view of an animal showing the appendages

in position.
Exercise 3. Draw an outline of a thoracic leg on a large scale, showing

accurately all the joints and hairs and its biramous form.

Compare the copepod with the young larva of the crab or the
lobster. Enumerate the points of structural similarity between
them.

Internal Anatomy. This can be best studied in the live animal.
The alimentary tract is straight and of large diameter, and often
contains dark-colored fecal matter. The mouth has a ventral
position, as in other crustaceans, and the anus is dorsal. There
is no liver or other accessory glandular organ. The circulatory
system in Cyclops consists of the colorless blood fluid alone,
there being no heart. The blood is, however, kept in circulation
by the rhythmic contractions of the intestine. Other copepods
possess a dorsal heart. There are no special respiratory organs,
respiration being effected through the body wall. The excretory
system consists of a pair of coiled kidney tubes, called the shell
glands, which lie in the forward part of the cephalothorax.

The reproductive system consists of median or paired organs in
the dorsal portion of the cephalothorax above the intestine. In
the female the ovaries are often conspicuous as a pair of large

44 PHYLUM ARTHROPODA

branched organs. The oviducts are paired and lead to the exter-
nal sexual openings in the first abdominal segment. Appended to
the first abdominal segment may be a pair of egg sacs containing
fertilized eggs which are cemented together by means of a secre-
tion of the oviduct. In the male the reproductive gland is the
median testis, which communicates by means of paired sperm
ducts with the external sexual openings, which are also in the
first abdominal segment. The spermatozoa collect in the terminal
portion of each sperm duct and form there a small mass known
as a spermatophore. The two spermatophores, during the act of
pairing, pass to the female and fertilize the ova. The male animals
are much less numerous than the females.

The reproductive glands of the copepod can be observed as
above described only during times of sexual activity. At other
times they can be seen only in part or not at all.

The muscular system can be easily seen to consist of striated
muscle fibers. Longitudinal as well as converging fibers will be
seen at each appendage.

The nervous system may be seen in favorable specimens as a
ventral strand in the cephalothorax connecting with the large
dorsal brain.

Exercise 4. Draw a side view of the animal, showing as many of the
internal organs as you have observed.

A DAPHNID 45

Class : Crustacea. Division : Entomostraca, Order : Phyllopoda
A DAPHNID

The Daphnidae are a family of small fresh-water crustaceans
common in lakes and pools. They should be studied under the
microscope and alive if possible. Place one or more on a slide
with a few strands of cotton under a cover glass two corners of
which are supported by wax, and draw off enough water to keep
them quiet ; also observe several in a watch glass. The body of
the animal will be seen to differ in shape from those crustaceans
already studied. It is but indistinctly segmented, and, except the
head, is entirely covered by a bivalve shell. This shell is the
cuticular covering of paired folds of the dorsal integument, one
fold covering each side of the body. Beneath the opening of the
valves of the shell appear the appendages and the abdomen ; on
the surface of the shell a meshwork of fine lines can usually be
seen. Notice the large, median eye ; it may often be seen to
tremble sHghtly. The shell has a deep, ventral indentation near
the base of the antennae.

The first pair of antennae is very small, but may be easily seen
projecting downward just back of the eye. The second pair of
antennae is very long and biramous, the two branches being the
exopodite and endopodite; they are the principal organs of
locomotion. Just back of the antennae is a large flap, called the
upper lip, and back of this are the large mandibles. There is but
a single pair of maxillae, and they are so small that they will
probably not be seen. Four to six pairs of thoracic appendages
follow, the function of which is probably exclusively respiratory.
How many are present in your specimen? Notice the leaflike
surface of these appendages (whence the name Phyllopoda) and
their complex form. The short abdomen articulates with the
thorax and is bent beneath it, where it may be seen often moving
rapidly back and forth.

Exercise 1. Draw an outline of a side view of the animal on a large
scale and label the appendages and other parts observed.

46 PHYLUM ARTHROPODA

Internal Organs. The digestive tract passes from the mouth,
which is ventrally placed and lies back of the ventral cleft in the
shell, first forward, then turns dorsally and finally posteriorly and
extends back to the anus near the end of the abdomen. Near
the anterior bend of the digestive tract a pair of colored, curved
pouches communicate with it ; they are the livers. The sachke
heart may be seen beating rapidly above the intestine. It pos-
sesses a pair of lateral openings into which the blood streams
from the body cavity with each dilation, and an anterior opening
through which it is sent into the forward part of the body with
each contraction. There are no other blood vessels.

The Reproductive Organs. The daphnids which are usually seen
are all parthenogenetic females, the males making their appear-
ance at certain times of the year only. The female animal is
larger than the male, and may be distinguished by its brood sac.
This is a large space just beneath the dorsal wall of the thorax
in which the eggs and the young brood are carried. The ovaries
are a pair of tubular organs alongside the intestine, which com-
municate, by means of short oviducts, with the brood sac. The
ovaries are easily detected by the presence of large ova in them.
These are in groups of four, of which but one, the third, is
destined to become an egg, the other three being nutritive cells
by which it is nourished.

During the greater part of the summer the eggs pass into the
brood pouch unfertilized and develop there parthenogenetically,
producing only females. The young animals pass out of the
brood chamber through a posterior opening ; they soon become
adult and in their turn give birth to parthenogenetic females.
The eggs which thus develop are called summer eggs. At certain
times of the year, however, as in the autumn, males are also born.
They fertilize the females, and the fertilized eggs then produced
differ from those which were unfertilized in possessing thicker
shells. They are called winter eggs and are able to resist the cold
of winter or the effect of drought. In the spring the winter eggs
develop into parthenogenetic females again.

Exercise 2. Draw an outline of the animal and place in it all the
internal organs you have observed.

A NAUPLIUS LARVA 47

Class : Crustacea. Division : Entomostraca
A NAUPLIUS LARVA

In an aquarium containing copepods or ostracods there are
sure to be numbers of the young larvae of these animals. They
are minute, free-swimming forms and are called nauplii, and may
be recognized by the triangular or oval, unsegmented body, which
bears three pairs of appendages and a median eye. Nauplii of
marine entomostracans may also be met with in large numbers
among the small animals obtained by skimming the surface
waters of the sea with a fine net.

Examine in a watch glass under a microscope water contain-
ing sediment taken from a jar in which are copepods or ostra-
cods. Find a nauplius ; the ostracod nauplius differs from that
of the copepod by being inclosed in the characteristic bivalve
ostracod shell. If marine plankton is at hand, look for several
kinds of nauplii in it.

Study the structure of a nauplius. Observe the unsegmented
body ; if the animal is not newly born, signs of segmentation may
have begun to appear. Observe the three pairs of segmented ap-
pendages ; the segmentation, however, is often indistinct. These
appendages are homologous to the first and second pair of an-
tennse and the pair of mandibles of the adult animal. As in the
adult, the first pair is uniramous ; the second and third pairs are
biramous. Both of the latter two pairs are used for locomotion,
although it is probable that they also act as jaws. The median
eye will be seen, and the straight digestive canaL

Exercise 1. Draw a nauplius on a large scale and label all the parts
above mentioned.

The nauplius larva is of great theoretical significance. It ap-
pears as the youngest, free-swimming, larval stage of almost all
the entomostracans and of several of the malacostracans, and
those malacostracans which are born in a later period of their
development pass through a nauplius stage (that is, a stage in
which the body is unsegmented and bears three pairs of append-

48 PHYLUM ARTHROPODA

ages) while they are still in the egg. This universal occurrence
of the naupHus larva seems to indicate that it repeats substan-
tially the structure of the primitive ancestor of all crustaceans.
In its further development and growth the nauplius larva in-
creases in size, gradually becomes segmented, and acquires new
appendages, its growth and the specialization of its organs
advancing from the anterior toward the posterior end. The
appendages, which were originally typical, unmodified crusta-
cean appendages, become differentiated to form the first and
second pairs of antennae and the mandibles, and finally the size
and structure of the adult are attained.

Exercise 2. Look for several nauplii which are somewhat advanced
in development and draw outlines of them.

PHYLUM ANNELIDA

Class : Chcetopoda. Order : Polychceta
THE SANDWORM (NEREIS)

Nereis is a common marine worm which Hves in the sand along
the shores of our northern and middle states. Its food consists of
various kinds of small marine animals, which it catches with its
formidable, protrusile proboscis. A specimen should be selected
for study in which the proboscis is not thrust out.

Observe, in the first place, the long, segmented, and some-
what flattened body, the pair of appendages on each segment,
and the distinct head with special sense organs at the forward
end ; observe also that the body tapers toward the hinder end,
where is a pair of special sense organs, the long caudal feelers.
All these characters indicate an animal possessed of the power
of rapid locomotion. Count the somites, or body segments ; note
that they are almost exactly alike. This lack of specialization is
in sharp contrast to the condition of the somites in most arthro-
pods. Observe carefully the appendages ; they differ from those
of the arthropod in that each one is an unjointed expansion of
the body wall, whereas the arthropod appendage is segmented.
Each one is made up of several lobes and is provided with long
bristles, or setae. Note the absence of a hard shell, the external
integumentary covering being the glistening cuticula, which has
not been stiffened by the presence of calcareous salts.

Observe the head and the forward portion of the body. An
annelid's body is composed genetically of two portions: the
prosoma, or primitive head, and the metasoma, or the primitive,
segmented trunk. The prosoma may be further divided into the
prostomium, which lies in front of the mouth and contains the
brain and the principal organs of special sense, and the metasto-
mium, which contains the mouth. In Nereis the prostomium

49

so

PHYLUM ANNELIDA

bears the following special sense organs : a pair of palps, large
cylindrical projections extending forward at its anterior end ; a
pair of tentacles, two delicate organs between the palps; and
two pairs of eyes, small, beadlike organs near the base of the
palps. Carefully identify all these organs and notice whether
the palps and tentacles are jointed. The metastomium is, in
Nereis, fused with the first two somites of the metasoma, or trunk,
and the segment thus formed is called the peristomium. It bears
the mouth and four pairs of long, flexible sense organs called the
peristomial cirri. Carefully observe, with the aid of a hand lens,
their exact position. These cirri are morphologically not cephalic
organs, as are the palps and the tentacles, but are remnants of
appendages of the first two somites.

Exercise 1, Make an outline of the dorsal aspect of the head and the
first five or six somites on a scale of 5. Number the somites. Care-
fully label all the parts.

Exercise 2, Draw a side view of the head on a scale of 5. Take special
care to represent accurately the position of the peristomial cirri.

Exercise 3, Find a specimen, if possible, with the proboscis thrust
out and draw a dorsal view of its head.

Note the tapering of the body at the hinder end. The worm
grows in length at this end. The posterior somites are the
youngest and hence the smallest.

Exercise 4. Make a sketch of the hinder end of the animal. The long
sense organs at the extreme end are called caudal cirri. In which
direction do they project?

The appendages in annelids are called parapodia. Carefully
examine the parapodia at different parts of the body and see if
they are all alike.

Remove a parapodium from the middle of the body ; mount it
on a slide in water and study it with the aid of a hand lens or a
microscope. Compare it with the parapodia still on the animal
and determine which is its dorsal and which its ventral side. It
can be divided into two distinct portions, the dorsal and the
ventral portions, called the notopodium and the neuropodium,
respectively, each of which is stiffened by an internal, chitinous.

THE SAND WORM 51

supporting rod, called the aciculum. Find the two acicula. The
large dorsal lobe of the notopodium is a respiratory organ, a gill.
It contains branching blood vessels which can be easily seen.
Attached to its dorsal edge is a slender, vibratile sense organ, the
dorsal cirrus. Beneath the gill are two lobes, one bearing bristles,
or setae. The neuropodium is made up of two lobes, one of them
setae-bearing, beneath which is a ventral cirrus.

Exercise 5, Draw a parapodium on a scale of 6 and label the parts.

Internal Anatomy. Make an incision with fine, sharp scissors
in the mid-dorsal line of the integument of the anterior third of
the animal, taking great care not to injure the viscera which lie
beneath. The body cavity, or coelom, will be seen to be divided
into compartments corresponding to the somites, by transverse
partitions which are called septa. Holding the cut edge of the
integument with forceps, cut the septa where they join it, and
then spread out and pin down the body wall, using many pins on
each side.

The Digestive Organs. The mouth leads into the large pharynx,
which is composed of an anterior and a posterior portion. With
sharp scissors cut open the pharynx along the mid-dorsal line
and note the number and arrangement of the chitinous teeth em-
bedded in its inner surface. Notice the delicate muscles passing
from it to the body wall, by means of which the pharynx can be
thrust out of the mouth and drawn back again. They are the
protractors and the retractors. A pharynx which is thus pro-
trusile is called a proboscis. Just back of it is the narrow oesoph-
agus with which a pair of small tubular glands communicate.
Back of the oesophagus is the stomach intestine, which extends
to the anus. Observe the mesenteries. These are longitudinal
partitions, in structure like the septa, one of which attaches the
stomach intestine to the dorsal and the other to the ventral body
wall. Press the intestine aside and see the ventral mesentery.

The Circulatory System. Nereis has two distinct circulatory
fluids, the colorless, or coelomic, and the red blood fluid. The first
consists of a plasma in which float amoeboid blood cells ; it cir-
culates freely in the body cavity, or coelom, being forced by the

52 PHYLUM ANNELIDA

movements of the body from one segment to another through
small openings in the septa. The red blood consists of a red
plasma, in which float colorless blood cells, and circulates in
closed tubes. The most important of these blood vessels are two
longitudinal tubes, the dorsal and the ventral arteries, which lie
in the median line, one above and the other below the alimentary
canal. The former, the dorsal artery, pulsates and drives the
blood toward the forward end of the body and distributes it to
lateral segmental arteries. Observe these and determine how
many there are in each segment ; also note the capillary network
into which the dorsal artery breaks up at its anterior end. The
dorsal portions of the lateral arteries carry the blood to the gills
and other organs, whence it collects again in the ventral portions
of these arteries and is conducted to the ventral artery. In this
vessel the blood flows toward the hinder end of the body.

Exercise 6. Draw a view of the opened animal on a scale of 5, showing
the organs above described. Label all the organs carefully.

Sever the ahmentary tract at the oesophagus and remove the
stomach intestine from the body. Observe the muscle bands in
the body waU ; note the difference in direction and size of the
different bands. Observe the muscles at the base of the acicula.

The Excretory System. The kidneys of the animal consist of a
pair of very slender, tortuous tubes called nephridia, which lie
in the body cavity against the ventral body wall in each somite
except the last two or three; they are difficult to see because
each is covered with a mass of jellylike connective tissue. Each
nephridium opens through the body wall to the exterior in a
minute pore on the ventral surface of each somite near the base
of the parapodium. The anterior end of the nephridium passes
through the septum which forms the anterior wall of the somite
in which that organ lies, and opens into the body cavity. The
opening, which is ciliated, is called the nephrostome; it lies
against the anterior surface of a septum.

The Nervous System. Observe in the midventral line of the
body cavity the nerve cord. Trace it forward to the brain. Note
the connectives which encircle the pharynx and connect it with

THE SANDWORM 53

the brain. Remove the forward end of the nervous system from
the body, mount it in glycerine, and study it under a microscope.
Note the ganglionic enlargements and the double nature of the
nerve cord. Study the branches which pass off from the nerve
cord and from the brain.

Exercise 7, Draw a diagram representing the opened body cavity in
a number of somites on a scale of from 5 to 10, showing all the above-
mentioned features which have been observed.

The Reproductive System. There is no complicated repro-
ductive system in Nereis. The sexes are separate. The repro-
ductive glands make their appearance only during the periods of
sexual activity and then as swellings of the peritoneal lining of
the body cavity. The eggs or spermatozoa, as the case may be,
fall into the body cavity and find their way to the outside through
the nephridia or through ruptures in the body wall.

Exercise 8. Draw a diagram representing an ideal cross section in the
region of the stomach intestine ; show the stomach intestine with
its mesenteries, the blood vessels, the nerves, and the muscles.

54 PHYLUM ANNELIDA

Class : Chcetopoda, Order : Oligochceta
AN EARTHWORM

The earthworm is, to most people, the most familiar annelid.
It is distributed over the entire earth, the United States pos-
sessing several species. The animal is nocturnal in its habits. It
lives in long burrows in the ground, in which it lies during the day
and the inclement seasons of the year. Its food consists of leaves
and other vegetable substances and also of the organic matter
contained in the soil which passes through its alimentary canal.

First study the animal alive, if possible, but also have one at
hand which has been killed in very weak alcohol. Notice its color,
its cylindrical, elongated body, its very small, pointed head, and
the absence of appendages, — all of which characteristics are
correlated with its burrowing life. Note also the absence of a
hard shell, the external integumentary covering being the glis-
tening cuticula. As the animal lacks appendages, locomotion is
accomplished by means of body movements. Place the animal
on moist filter paper or a piece of moist newspaper and study
its method of locomotion. It will be observed successively to
elongate and to shorten its body, which of course would be
impossible if it were covered by a hard shell. Notice that along
the ventral and the lateral surfaces are several rows of minute
bristles, the setae ; they aid in locomotion and are under the con-
trol of muscles. Determine, by passing the animal through the
fingers and with the aid of a hand lens, how many rows there
are and their relation to the segments. Determine also whether
the setae at the forward end of the body project in the same
direction as those at the hinder end. Observe carefully the im-
portance of the setae in locomotion.

The animal is segmented externally ; that is, it is made up of
a number of somites, or metameres, like the crustacean body.
Count the somites, beginning with the segment just back of the
mouth, which is the first somite. Notice the close similarity of
the somites. This lack of specialization is always a primitive

AN EARTHWORM 55

character in a segmented animal and is in sharp contrast to the
condition of the somites in most arthropods.

Notice the moist, sHmy surface. Moisture is necessary to the
animal's existence ; this accounts largely for its nocturnal and
subterraneous habits. Notice also the red blood vessels through
the semitransparent body wall. What movement of the blood can
you detect ? What are the differences between the dorsal and the
ventral surfaces ? Notice the difference between the anterior and
the posterior ends. The forward end is the older ; the animal
grows in length by adding new somites to the hinder end. Notice
the ventral position of the mouth and the terminal position of the
anus ; note also the thickened ring around the body not far from
the forward end. This is the clitellum ; its function will be ex-
plained in speaking of the reproductive organs. The animal is
without organs of special sense ; numerous minute, tactile sense
organs which are sensitive to light and other stimuli are, however,
present. These are distributed along the body but are especially
abundant toward its anterior end. Note the greater sensitiveness
of this part of the body.

The head of the animal may be divided into two portions : the
prostomium, the median dorsal projection overhanging the mouth,
and the indistinct peristomium, which contains the mouth and is
marked off from the prostomium by fine, transverse lines. What
somites are included in the clitellum? On the ventral surface of
the fifteenth somite a pair of prominent, transverse slits will be
seen. They are the external openings of the sperm ducts. On
the fourteenth somite look with the hand lens for the two minute
openings of the oviducts ; they are difficult or impossible to see
except during the reproductive period of the animal. Other
external openings in the ventral surface which are usually too
small to see are the following : the two pairs of openings of the
sperm receptacles between the ninth and tenth and the tenth and
eleventh somites ; and those of the kidney tubules, or nephridia,
which open through the body wall to the exterior by minute
pores on either the ventral or the lateral side near the anterior
border of the somite. Look for them with a hand lens.

56 PHYLUM ANNELIDA

Exercise 1. Make a sketch on a scale of 3 of the ventral aspect of the
forward end of the animal back to the posterior border of the
cHtellum. Indicate the somites and number them.

Exercise 2. Make a similar sketch of the ventral view of the last four
somites on a scale of 3.

Internal Anatomy. Pin a large worm, which has been killed,
firmly to the wax of the dissecting pan by a strong pin at each
end ; then make an incision with fine, sharp scissors through the
integument in the mid-dorsal line from the forward end of the
animal to a point back of the clitellum, taking great care not to
cut the viscera lying beneath. It will be noticed that the body
cavity is divided into compartments, corresponding to the so-
mites, by transverse partitions, which are called septa. Holding
the cut edge of the body wall with the forceps, cut the septa where
they join it ; then spread out and pin down the body wall, using
many pins on each side.

Observe first the large alimentary canal, which passes straight
through the animal; also several pairs of conspicuous white
bodies a short distance from the anterior end, which are the
sperm sacs. If the specimen has been freshly killed, the red blood
vessels will also be seen. Study and identify in detail the follow-
ing systems of organs.

The Circulatory System. The earthworm has two circulatory
fluids, a red one and a colorless one. The latter consists of a
plasma in which float amoeboid blood cells. It is present only
in the body cavity and circulates throughout the body, being
driven by the movements of the animal from one somite to an-
other through small openings in the septa ; it will, of course, not
be visible in a dissection. The red blood consists of a red plasma
in which float colorless blood cells, and it circulates in a system of
closed blood tubes. The most important of these blood vessels are
two longitudinal vessels and numerous circular ones. Observe the
dorsal longitudinal vessel in the median line, above the alimentary
canal. It is contractile and propels the blood toward the head.
Push aside the intestine and observe just beneath it the ventral
vessel, which runs parallel to the dorsal one. Notice that these
vessels break into small branches at their anterior ends.

AN EARTHWORM 57

The circular or commissural blood vessels connect the dorsal
and the ventral vessels and have a paired and segmental arrange-
ment. They are not all of equal size. Observe the four or five
large pairs near the forward end of the animal, which pass directly
between the dorsal and the ventral vessels. They are, like the
dorsal vessel, contractile and are sometimes called the hearts.
In which somites are they ? Find the commissural vessels poste-
rior to them. These are much smaller and do not pass directly
between the longitudinal vessels, but break into capillaries be-*
tween them.

The Digestive System. The pharynx is an oval, muscular pouch
occupying four or five somites directly back of the mouth ; radi-
ating muscle fibers join it with the body wall. The oesophagus is
a slender tube following the pharynx and running between the
conspicuous sperm sacs to the crop in about the fifteenth somite.
Press aside these sacs and notice beneath them three pairs of
white glands ; these are lateral diverticula of the oesophagus and
contain calcareous crystals which have a digestive function. The
crop is a thin-walled dilation of the oesophagus which lies about
in somites 15 and 16, close to the gizzard, a muscular, thick-walled
chamber of the same size as the crop and lying in somites 17 to 19.
This is followed by the stomach intestine, a large tube with lateral
segmental pouches, which passes to the hinder end of the body ;
covering the surface of the stomach intestine is a loose mass of
yellowish-brown cells, the chloragogue cells, whose function is
unknown.

Exercise 3. Make a drawing of the opened animal on a scale of 3,
showing the segmentation and representing the organs above de-
scribed in their proper somites ; label all carefully.

Sever the alimentary tract just back of the pharynx and re-
move it from the body.

The Reproductive System. The earthworm is hermaphroditic
and possesses the following genital organs :

The Male Organs, (i) The sperm sacs. These have already
been noticed. They are large, white, irregularly lobed sacs occu-
pying somites 9 to 13 ; they vary in size with the sexual condi-

58 PHYLUM ANNELIDA

tion of the animal, being largest during periods of sexual activity.
(2) The testes. Two pairs of these organs are present, which He
beneath the sperm sacs in the tenth and eleventh somites ; they
are very minute objects and will not be seen. (3) The sperm ducts.
These are a pair of slender white tubes which extend from the
sperm sacs to the fifteenth somite, where they open, through the
conspicuous transverse slits already noticed, to the exterior. Look
first for the posterior portion of these tubes and trace them for-
ward. The spermatozoa pass from the testes, where they but
partially develop, into the sperm sacs, in which their development
is completed and where they are grouped together in balls. From
here they pass, during pairing, into the sperm ducts, and out of
the animal through the slitlike openings in the fifteenth somite.

The Female Organs, (i) The sperm receptacles. These are two
pairs of spherical white sacs beneath the sperm sacs in the ninth
and tenth somites ; they are easily seen. (2) The ovaries. These
are a pair of extremely small organs lying near the median line
and attached to the anterior septum of the thirteenth somite
near the ventral body wall ; they will hardly be seen. (3) The
oviducts. These are two minute, funnel-shaped tubes which
extend from immediately behind the ovaries through the septum
to the external opening in the next somite ; they will also hardly
be seen.

During the months of May and June earthworms meet and
pair in the nighttime. Two animals place themselves along-
side of each other, headed in opposite directions and in such a way
that the sperm receptacles of each come opposite the openings of
the sperm ducts of the other, and are held firmly together by the
secretion of glands in the integument. The sperm receptacles
of each are then filled with spermatozoa from the other animal.
The worms then separate. Sometime later the clitellum secretes
a viscid fluid which hardens and forms a tough, cylindrical mem-
brane around the body. The worm then squirms backward, caus-
ing this membrane to pass forward toward its head. As the
membrane passes the fourteenth somite, eggs are poured from
the oviducts into the viscous mass which is held between it and
the body, and at the tenth and eleventh somites spermatozoa pass

AN EARTHWORM 59

in from the sperm receptacles and at once fertilize the eggs. The
cylindrical membrane then passes completely off the worm and
its two ends close together. It forms thus a yellowish, spindle-
shaped capsule about as large as a small pea, which is called the
cocoon. In it the young animals are born.

Excretory Organs. These are the kidneys of the animal. They
consist of a pair of coiled tubules, called nephridia, which lie near
the lateral and ventral wall of the body cavity in each somite,
except the first three or four and the last one. Each nephridium
has two openings, a funnel-shaped, ciliated opening into the body
cavity, called the nephrostome, and one through the body wall
to the outside. The former in each case is attached to the anterior
side of a septum. The tube passes backward through the septum
to the next somite, in which the greater portion of it lies, and
through the wall of which it communicates with the outside. The
distal, middle, and proximal portions of the tube differ from one
another. The distal portion (that next to the nephrostome) is
very slender ; the middle portion is much thicker and has glandular
walls ; and the proximal portion is a dilated tube which probably
acts as a urinary bladder.

Exercise 4. Make a sketch of somites 8 to 20, representing diagram-
matically the reproductive organs and two or three pairs of ne-
phridia lying in their proper somites, and label all.

Crush the sperm sacs of a fresh worm, which has not been
in alcohol, mount some of the milky fluid in it, and examine it
under a compound microscope. Notice the sperm spheres and
spermatozoa.

Exercise 5. Draw a sperm sphere and a spermatozoon.

With a sharp knife or curved scissors carefully remove as much
of a nephridium as possible from the animal's body. Mount it on
a slide and examine it under a microscope.

Exercise 6. Draw it and label its divisions so far as observed.

The nervous system is essentially similar to that in arthropods.
It consists of a double cord joined by segmental, paired ganglia ;
the two cords are so closely bound together that they appear as

6o PHYLUM ANNELIDA

a single strand. Remove the sperm sacs and observe the nerve
cord as it Hes in the midventral Hne. Note the sHght sweUings ;
these are the segmental ganglia. Trace the nerve cord forward
to the region of the mouth, where it encircles the forward end of
the pharynx and joins the small brain. Observe the two ganglia
of which the brain is composed. Remove the forward portion
of the nervous system from the body. Mount it on a slide and
examine it under a microscope. Note the double nature of the
nerve cord and of the gangHa. Note accurately the lateral
branches that leave the cord ; note also the shape and branches
of the brain.

Exercise 7. Draw the nervous system on a large scale, accurately
representing all the details.

Study of a Cross Section. This is instructive because it shows
the relations of the organs to one another in their natural posi-
tions and also illustrates their finer structure. A properly stained
and mounted cross section of any portion of the body will serve
for this study.

Observe first the integument ; it is made up of the cuticula on
the outside and the cellular hypodermis beneath it. The latter is
composed, in most parts of the body, of a single layer of cells
which secretes the cuticula. Note the numerous single-celled
glands in the hypodermis. If the section passes through a seta,
notice its method of attachment and its muscles. Beneath the
integument are the body muscles. Of these there are two sys-
tems, the circular and the longitudinal muscles. The former are
a narrow band just beneath the hypodermis. The latter are much
more extensive and project into the body cavity; they are ar-
ranged in groups and will be seen of course in cross section. Near
the center of the body cavity note the large alimentary canal. If
the section is in the region of the stomach intestine, note the
longitudinal fold in the dorsal intestinal wall which very largely
increases its surface. Observe the structure of the alimentary
canal ; its cavity, or lumen, is bounded by a thick mucous mem-
brane consisting of a single layer of very long, slender cells,
around which are two muscle layers, an inner circular and an

AN EARTHWORM 6i

outer, very thin, longitudinal layer. Surrounding the muscle
layers and also forming a thick fold over the dorsal and lateral
intestinal surfaces are the pear-shaped chloragogue cells. Ob-
serve the dorsal and the ventral blood vessels, and also the com-
missural blood vessels, if any are in the section. Study carefully
the nervous system. Note the muscular sheath which surrounds
the nerve cord, and embedded in it the sections of three minute,
longitudinal blood vessels. Note the double nature of the nerve.
Note the large, pear-shaped nerve cells and the nerve fibers,
also the three large bodies in the dorsal portion of the ganglion.
These latter are called the giant fibers. Do lateral nerves join the
ganglion? If so, trace their fibers into it. Also trace their fibers
away from the ganglion and see where they go. Examine care-
fully the peritoneum. This is a layer of cells which lines the body
cavity and bounds all the organs in it.

Exercise 8, Draw the cross section and carefully label all the organs.

PHYLUM VERMES

SUBPHYLUM PLATYHELMINTHES

Class : Turbellaria
A PLANARIAN WORM

Planarian worms are very common animals in fresh-water
streams and ponds as well as in the sea ; they may be found on the
under-side of stones or on aquatic vegetation. They are fiat, elon-
gated, very soft, and contractile animals, brownish or yellowish
in color, and usually half an inch or less in length ; at the forward,
broader end is a pair of lateral sensory lobes called the auricles,
and on the dorsal surface are two black eyes; the hinder end
is pointed. A variety of forms is found, some of which are very
minute and are without an intestine or have a straight, tubular
intestine, whereas others are much larger and have a branched
intestine. The latter include most of the commoner turbella-
rians and those for which these directions have been prepared.

Study the live animal under a hand lens. Note the gliding
motion with which it moves. This is accomplished partly by the
action of the cilia which cover its surface and partly by muscular
contraction. Near the middle of the ventral surface are the
mouth and the protrusile proboscis. Mount the animal on a slide
in water beneath a cover glass and observe the action of the cilia
under a compound microscope. Note also the rhabdites, minute
rods which are thickly set in the integument.

Exercise 1. Draw an outline of the animal on a large scale, with the
eyes and proboscis, and indicate its anterior and posterior ends.

Study an animal which is under the pressure of a large cover
glass, and make out as many of the following organs as possible,
using often reflected instead of direct light.

The Digestive System. The digestive canal is usually easily

62

A PLANARIAN WORM 63

seen. The mouth is a circular opening near the center of the ven-
tral surface ; it leads into the pharynx, a cylindrical organ with
thick, muscular walls, which can be thrust out of the mouth as a
proboscis. At the base of the pharynx the intestine divides into
three trunks, one of which passes forward and the other two back-
ward to the extremities of the animal's body. Each of these
trunks gives off lateral branches which are themselves often
branched. There is no anus.

Exercise 2. Draw an outline of the animal and place in it the digestive
system in detail.

The Reproductive System. Planarians are hermaphroditic ;
the sexual organs are complicated in structure and arrangement
and difficult to observe in a Kve specimen. Near the lateral edges
of the body will be seen, among the ends of the lateral intestinal
branches, two sets of lobed organs. Of these the larger are the
yolk glands, which connect with the oviducts ; the smaller and less
apparent ones are the rounded testes. Just back of the mouth is
the uterus, which is often to be recognized by the spherical eggs
it may contain ; it passes back to a sac called the genital cloaca.
The ovaries are a pair of spherical bodies in the anterior part of
the body, and from them a pair of oviducts extends to the hinder
part of the body, receiving the lateral yolk glands on their course.
Leading from the testes are the vasa efferentia, very delicate
tubes, which pass to the conspicuous vasa deferentia. There is a
pair of the latter organs, one on each side of the mouth and phar-
ynx ; they extend to the hinder part of the animal, where they
unite to form the muscular cirrus, which opens into the genital
cloaca. The two oviducts also fuse at their hinder ends, and the
median duct thus formed opens into the genital cloaca. This
structure, which thus receives all the ducts from the genital glands,
communicates with the outside through the genital pore, a median,
ventral opening in the hinder part of the body.

Exercise 3. Draw a large outline of the animal and place in it as
much of the reproductive system as you have observed.

The Nervous System. Beneath the eye spots will be seen the
opaque brain, a large nervous mass consisting of a pair of minor

64 PHYLUM VERMES

masses united by a broad commissure. From its anterior and
lateral sides numerous sensory nerves pass to the anterior body
surface, which render this extremity a highly sensitive tactile
organ. From its posterior side a pair of large longitudinal nerve
cords passes to the hinder end of the body, being united at
intervals by transverse nerves.

The Excretory System. This is difficult to see ; it consists of
minute tubes which extend throughout the body and collect the
excreted matters from the tissues. There are two main longitudi-
nal tubes extending the length of the body, which open to the
outside through minute pores on the dorsal surface of the animal.
These tubes are not straight but coiled and give off numerous
branches, at the termination of each of which is a peculiar cell
with a vibratory process at its base called a flame cell ; they are
joined by a transverse tube at the anterior end of the animal.
Portions of the excretory system can often be seen in the com-
pressed animal, where they appear as fine lines.

Exercise 4. Draw an outHne of the animal and place in it as much of
the nervous and the excretory system as you have observed.

No special respiratory system is present in the Turbellaria, the
ciliated outer surface of the body performing this function. A
circulatory system and a blood fluid are also wanting. The
branching of the digestive and excretory systems is correlated
with this feature. Can you explain how ? As in other flatworms,
the turbellarians possess no body cavity, the primitive body
cavity being filled secondarily by a peculiar vesicular connective
tissue called parenchyma. The muscular system consists of a
layer of strong circular and longitudinal muscles just beneath the
surface of the body and of oblique muscles passing through the
parenchyma.

A TAPEWORM 65

Class : Cestodes
A TAPEWORM

Tapeworms are common parasites in the intestines of ver-
tebrate animals. Tcenia saginata, the common tapeworm infest-
ing man, may often be obtained from physicians. If it is alive
when obtained, it should be placed in a normal salt solution (a 0.75
per cent solution), in which it will keep alive for several hours,
and may then be studied. If it is dead it should be preserved in
alcohol or formalin. Tcenia serrata, a tapeworm of the dog, and
TcBuia crassicollis, which lives in the cat, are both common ani-
mals and are convenient forms for study. The intestines of adult
cats or dogs should be slit open and the worms taken out and
placed alive in a normal salt solution. They are white, bandlike
objects, six inches or more in length, which are attached by one
end to the wall of the intestine. In separating them from the
intestinal wall care should be taken not to tear them.

Study the movements and general form of the animals as they
lie in the salt solution. The worm will be seen to be made up of a
large number of segments, and to bear at the smaller end a small
rounded knob. The segments are called proglottids, and the
rounded knob, the scolex. The body of the animal is not made
up of body divisions which we can call head and trunk. The
scolex, however, may be held to represent its anterior end, the
proglottids having arisen from it by a process of terminal growth.
The scolex is thus the oldest part of the animal's body ; in fact,
it constitutes the entire parasite when it first arrives in the in-
testine of the host (as the animal is called in which a parasite
lives), the proglottids only then beginning to grow. The youngest
proglottids are those nearest the scolex; those at the opposite
end of the body are the oldest and hence the largest. Count the
proglottids. The animal attaches itself to the wall of the intestine
by means of its scolex, which is provided for this purpose with
four suckers and usually two rows of chitinous hooks ; the scolex
of TcBnia saginata lacks the hooks. Thus attached, it lies im-
mersed in the digestive fluids of its host and absorbs through the

66 PHYLUM VERMES

outer surface of its body the nutriment it needs. It is without
a digestive system.

Exercise 1. Draw an outline of the animal on a large scale, taking
care to represent the number of proglottids accurately.

The Scolex. Cut off the scolex, mount it on a slide in glycerine or
water, and examine it under the microscope. Notice the fine excre-
tory canals which occur in every part of it. Can you determine
their arrangement ? Note the numerous minute calcareous bodies.

Exercise 2, Draw the scolex on a scale of lo. Represent accurately
the suckers and the number and position of the hooks, if these are
present.

Exercise 3, Draw a single hook highly magnified.

The Proglottids. Each proglottid is composed mainly of repro-
ductive organs and circular, longitudinal, and obHque muscle
fibers embedded in a spongy tissue called parenchyma. The pa-
renchyma fills the entire primitive body cavity, which is thus
absent in this animal. Each proglottid contains a complete set
of both male and female genital organs. These are immature in
the youngest and smallest proglottids ; in those at about a third
of the distance from the anterior end of the body they are mature ;
in the largest proglottids, those at the posterior end of the body,
the uterus is so distended with eggs that most of the other genital
organs are obliterated and do not appear. Two pairs of longitudi-
nal excretory canals pass from one end of the worm to the other,
running near to and parallel with each lateral edge; in each
proglottid, also, are one or two transverse canals. One or more
pairs of longitudinal nerves run parallel with and very near the
excretory canals, which are also joined in each proglottid by a
ring commissure.

Cut off two or three proglottids from the forward end of the
body, two or three from about a third of the distance from the
anterior end, and two or three from the posterior end, and soak
them all first for a short time in a dilute solution of caustic potash
and then in one of equal parts of glycerine and water*

Place the proglottids from the hinder end of the body in dilute
glycerine between two glass slides, press them gently so as to

A TAPEWORM 67

squeeze them as thin as possible without crushing them, and
study them under the microscope. In the middle of one of the
lateral edges of the proglottid notice a slight projection containing
a depression. This is the genital papilla, and the depression is the
genital cloaca. Two canals will be seen running from the genital
cloaca toward the center of the proglottid. These are the vasa
deferens and the vagina, the former being the larger of the two.
The greater part of the proglottid will be seen to be occupied
by a much-branched organ filled with a granular substance. This
is the uterus, and the granules are eggs. Near each lateral edge of
the proglottid will be seen a straight band. These two bands are
the dorsal excretory canals ; find the transverse canal near the
hinder end of the proglottid. Between each canal and the edge
of the proglottid will be seen a delicate line running parallel with
the canal. These are the main nerves ; they are joined by trans-
verse nerves.

Exercise 4. Draw the proglottid, showing accurately all the features
that you have observed.

Study in the same way the mature proglottids. Find the
uterus. It is here a straight, narrow tube in the middle of the
proglottid, and is not yet distended with eggs. Near the center
and toward the posterior end of the proglottid will be seen an ir-
regular mass of organs. These are the paired ovaries, two large,
round bodies, one on each side of the uterus ; the median yolk
gland, which is below the end of the uterus, near the posterior
margin of the proglottid ; the shell gland, between the yolk gland
and the uterus. From the shell gland the vagina and vas deferens
proceed to the genital cloaca, the former being the smaller and
more posterior of the two. Scattered throughout the proglottid
are numerous small, round bodies, the testes, which are joined
with the vas deferens by numerous minute vasa efiferentia. Find
the excretory canals and the longitudinal nerves.

Exercise 5. Draw the proglottid, showing all the features you have
observed ; carefully label all.

Study in the same way the immature proglottids.

Exercise 6. Draw the immature proglottid,

68 PHYLUM VERMES

The tapeworm may fertilize itself or be fertilized by another
individual, and where self-fertilization takes place one proglottid
of the anunal may fertilize another, or a single proglottid may
fertilize itself. The ova from the ovaries, on being fertilized, pass
at once into the uterus. The ripe proglottids, which are filled
with eggs in which the embryo has already begun to develop,
break off from the hinder end of the worm and pass out of the
body of the host. They then break open or are crushed, and
their eggs are scattered on all sides.

The Encysted Tapeworm. The adult worm alone is found in
the intestine. The eggs, in order to develop, must pass out of the
host and fall upon something which will afterward be eaten by
another animal, called the intermediate host, which is itself
preyed upon by the host. After being thus transferred to the
stomach of the intermediate host, there hatches from each egg a
minute, spherical embryo, called the six-hooked embryo, which is
provided with three pairs of hooklike organs of locomotion. This
embryo works its way through the wall of the intestine of the
animal and migrates finally to some one of the internal organs,
where it lodges and grows into a cystlike larval form, called the
cysticercus. Within the cysticercus is a fully developed scolex,
but turned wrong side out. If now the intermediate host is
eaten by the host, the scolex turns right side out, passes into the
intestine of the latter, attaches itself to the intestmal wall, and
grows into an adult tapeworm.

The intermediate host of TcBuia saginata is the beef, in the
muscles of which the cysticercus will be found, if present. That
of Tcenia serrata is the rabbit and that of Tcenia crassicollis is
the mouse; in the former animal the cysticerci are embedded
in the peritoneum or the liver, and in the latter, in the liver.
Open the body cavity of either of these animals by a median
ventral incision and look for cysticerci. They are large, whitish
bodies and are easily detected if present. When a cysticercus is
found, it should be carefully dissected out, its outer wall slit and
the scolex exposed to view. Mount it on a slide in dilute glyceruie
and study it.
Exercise 7, Draw a view of the scolex in its cyst>

BUGULA 69

SUBPHYLUM BRYOZOA
BUGULA

Bugula turrita is a marine colonial bryozoan which is very com-
mon in the shallow waters along our coast. The colonies are a
dull yellow in color and sessile, being attached to rocks, seaweed,
and other objects. The animals are very small and must be
studied with the aid of a microscope.

Study a large piece of a colony (alive if possible) and notice the
spiral arrangement of the branches. A branch is made up of a
double row of elongated partitions or chambers, each of which is
called a zooecium. Each zooecium represents a separate individual
of the colony ; within its walls are the soft parts of the animal,
which are called collectively the polypide. The individual bryo-
zoan is thus made up of two distinct parts, the zooecium and the
polypide, the former constituting the chitinous outer wall of the
animal, the latter comprising its viscera and the tentacles. At its
upper, or distal, end the zooecium has a large opening through which
the forward end of the polypide can be protruded and into which
it withdraws itself when alarmed. The cuticula which forms the
zooecium is rendered hard by the presence of carbonate of lime ;
it is thus much more enduring than the remainder of the animal,
and after death the empty zooecium may persist long after all the
softer parts have disappeared. Look for the empty zooecia in
your specimen.

The different individuals of a colony have arisen by a process
of budding from the individuals below them in the colony. Thus
the oldest individuals are those nearest the base of the colony,
the basal one being the progenitor of the entire colony. This is
also the only individual of the colony which has not come into
existence by a process of budding ; it began its life as a free-
swimming larva which was hatched from an egg.
■ The Zooecium. Mount a small portion of the colony containing
two or three branches on a slide under a cover glass.
Exercise 1. Draw a large and accurate outline of the zooecia, leaving
' ' out the polypides. Observe very carefully the boundaries of the
zooecia and their relations to one another.

70 PHYLUM VERMES

The Polypide. Study a number of polypides, both retracted
and extended. The forward end of the polypide consists of a cir-
cular ridge, called the lophophore, which bears a row of long cili-
ated tentacles. In the midst of the circle is the mouth. The
tentacles are vibratile and serve as respiratory as well as prehen-
sile organs. It will be seen that the lophophore can be entirely
withdrawn within the zocecium.

The Digestive System. The mouth opens into the pharynx,
which leads into the oesophagus. This opens into a large saclike
stomach, the lower portion of which is lengthened into a long
pouch. From the upper end of the stomach, near the base of the
oesophagus, the short intestine leaves it and passes to the thick-
walled rectum. This leads to the anus, which is situated just out-
side the lophophore near the mouth. The digestive tract has thus
the shape of the letter V, the point of which is formed by the
stomach pouch. Passing from the stomach pouch to the lower end
of the body is a broad mesenteric strand called the funiculus. In
order to study the digestive tract satisfactorily, a polypide should
be found in which both arms of the V come into view.

The Muscular System. The retractor muscle, whose function
it is to draw in the lophophore, may, in favorable specimens, be
seen as a delicate strand which passes from the wall of the
zocecium to the pharynx.

The nervous system has not yet been observed in Bugula, but
in nearly allied Bryozoa it consists of a single ganglion between
the mouth and the anus. From it nerves radiate to the tentacles
and other organs. There are no organs of special sense.

The Reproductive Organs. The animals are hermaphroditic.
Ova develop from the peritoneal lining of the spacious body cav-
ity and will be seen, when present, lying near the stomach pouch.
Spermatozoa develop from the funiculus and, when present, form
a mass around it ; they fertilize the ova in the body cavity.

There are two methods of reproduction, the sexual, in which
the new individual develops from the fertilized egg, and the
asexual, in which the new individual arises by budding. As al-
ready stated, the entire colony, with the exception of its oldest
member, has developed in the latter way.

BUGULA 71

Exercise 2. Draw an extended individual in which the entire digestive
tract can be seen and label all the organs observed.

There are no special respiratory or excretory organs ; the entire
outer surface of the body performs these functions. The circu-
latory system is represented by the colorless blood fluid alone.
There are no circulatory vessels, the blood being contained in the
body cavity.

Avicularia and Ocecia. These are peculiar structures, found in
connection with the zocecia, which are morphologically equiva-
lent to distinct individuals. An avicularium is a small structure,
like a bird's head in shape (hence the name), which may be found
attached to the wall of some of the zocecia near the opening. It
has a movable lower jaw which can be opened and shut by two
sets of muscles. Its function is to seize and hold small animals.
These soon die in its grasp, and their disintegrated remains are
swept into the mouth by the ciHated tentacles. An ooecium is a
disklike structure which, in some parts of the colony, lies in front
of a zooecium. It serves as an egg capsule in which the embryo
develops. A single embryo will be found in each.

Exercise 3. Find a zooecium with an avicularium attached and
draw them.

Exercise 4. Find a zooecium with an ooecium attached and draw them.

PHYLUM MOLLUSCA

Class : Pelecypoda
A FRESH-WATER CLAM {ANODONTA OR UNTO)

These animals are common in most parts of the country ; they
inhabit the sandy or gravelly bottoms of fresh-water streams and
lakes. There are many species of fresh-water clams, two com-
mon genera being Anodonta and Unio, the former of which is
characterized by the thinness of the shell and the latter by its
thickness.

Study first the live animal, if possible. Its body is unsegmented
and entirely inclosed in a bilateral, bivalve shell, which is the
cuticula of the animal richly charged with calcareous salts. The
two valves of the shell cover the right and left sides of the animal
and are joined together on its dorsal side by the dark-colored
hinge ligament, while their ventral edges are open ; the animal is
thus very much compressed laterally. The anterior end of the
animal is more rounded and less elongated than the posterior end.
Which is the right-hand valve ? The elevation on each valve near
the hinge toward the forward end is called the umbo. It is the
oldest portion of the shell ; from it as a beginning point the shell
has grown in size by additions to its ventral edge. Note the par-
allel lines of growth. The ventral edges of the shell are thus the
youngest portions of them.

Exercise 1. Make a drawing of the right-hand valve, indicating the
anterior, posterior, dorsal, and ventral aspects, and showing the
lines of growth.

Exercise 2. Make a drawing of the dorsal aspect of the animal.

Kill the animal by immersing it for a few minutes in hot water
(70° C). As the shell is kept closed by the contraction of the two
muscles which pass between its valves, it will gape open as soon as

A FRESH-WATER CLAM 73

the animal is dead and the muscles are relaxed. It is the elasticity
of the hinge ligament which causes it to open.^

Examine the animal as it lies in the shell. It will be seen that
the inner surface of each valve is covered with a soft, slimy mem-
brane, the lower edge of which is parallel to the edge of the shell.
This is the mantle ; it is a double fold of the dorsal integument of
the body, one side of the body being covered by either fold. The
mantle is the matrix of the shell ; that is, it secretes the shell.
Its lower edge is provided with muscle fibers and can be extended
beyond the edge of the shell ; it also possesses sensory functions.

Observe the large, soft visceral mass hanging between the two
lobes of the mantle ; it contains the viscera of the animal. On the
lower side of the visceral mass, that is, toward the gape of the
shell, is the muscular foot, which can be extended below the edge
of the shell and is the organ of locomotion. Observe the two leaf-
like gills on each side of the visceral mass and foot ; notice also
the two large adductor muscles, one in front of and the other
behind the visceral mass, which pass from one valve of the shell
to the other and serve to close them.

Pass a knife between the mantle and the left shell and separate
them from each other. Cut the two muscles close to the shell ;
cut the elastic hinge ligament and remove the left shell.

Study the inner surface of the shell. Note the two large scars
marking the surfaces of attachment of the adductor muscles;
just above each is the scar of a much smaller muscle, the re-
tractor of the foot, and just posterior to the anterior adductor
muscle is the scar of the protractor muscle of the foot. Note the
broad line which joins the scars, running parallel with the edge
of the shell. This is the pallial line ; it is formed by the insertion
in the shell of the delicate muscle fibers at the edge of the mantle.
Do you find hinge teeth in the shell just beneath the hinge liga-
ment ? Unio has such teeth ; Anodonta is without them and is
also characterized by the thinness of its shell.

1 The shell may also be opened by inserting some sharp, wedge-shaped instru-
ment between its valves. The valves are thus pressed apart far enough to admit
the blade of a scalpel, by means of which the adductor muscles should be cut close
to the left valve of the shell. The hinge ligament should then be cut and the left
valve be removed.

74 PHYLUM MOLLUSCA

Exercise 3. Draw a view of the inner surface of the shell.

Break the shell and examine the broken edge with a hand lens.
Study the structure of the shell. It is composed of three layers :
the inner mother-of-pearl layer (which is secreted by the entire
surface of the mantle) , the prismatic layer, and the organic layer
or periostracum on the outside. The two latter layers are secreted
by the edge of the mantle ; the periostracum is very thin and easily
peeled off. Place a piece of the shell in a solution of hydro-
chloric acid ; note the effervescence which results ; note also that
an organic remnant, even of the two inner layers, is left.

Exercise 4. Draw a view of the broken edge of the shell on a scale
of 5. Show the prisms of the prismatic layer.

Place the animal in water and study it as it lies in the right
shell.^ The two halves of the mantle will be seen to envelop en-
tirely the visceral mass and the foot. Over the entire dorsal por-
tion of the visceral mass the mantle is fused with it and cannot
be separated, but the lateral and ventral portions of the mantle
lobes hang free, inclosing an extensive space, which is called the
mantle cavity. In it, on each side of the visceral mass, lie the two
leaflike gills. In front of the gills are two pairs of triangular
flaps, the oral palps, between which, in the median line and just
back of the anterior adductor muscle, lies the mouth. Find it.

Trace the irregular Hne of attachment of the mantle with the
visceral mass ; it follows the base of the gills and of the oral palps
and passes beneath both adductor muscles. Observe the edges
of the mantle and note that at the hinder end of the animal they
are darkly pigmented, and the middle point of the pigmented
line is joined with the base of the gills by a short septum. This
septum divides the posterior portion of the mantle cavity into
a dorsal and a ventral chamber. The latter is the very large
branchial chamber which contains the gills; the former is the
very small cloacal chamber. Identify these chambers. The pig-
mented edges of the mantle are at this place modified so as to
form, when the edges of the two sides of the mantle are applied

1 For the study of the soft parts of the clam it is well to have at hand also a
specimen which has been deprived of both valves of the shell.

A FRESH-WATER CLAM 75

to each other, two short tubular openings, which place these two
chambers in communication with the outside water and are
called the siphons. The ventral siphon is called the branchial or
incurrent siphon; through it water streams into the branchial
chamber bearing food and air for respiration. The dorsal siphon
is called the excurrent or cloacal siphon, and through it water
passes outward charged with fecal matter from the alimentary
tract and carbon dioxide of respiration. Note the sensory ten-
tacles on the branchial siphon.

Probe the dorsal siphon. Carefully remove the left lobe of the
mantle after cutting it with fine scissors along its line of attach-
ment with the visceral mass.

Through the transparent body wall observe the organs in the
dorsal portion of the visceral mass. Just back of the anterior ad-
ductor muscle is the liver, which can often be recognized by its
greenish color. Between the hinge ligament and the base of the
gills Hes the heart in its transparent pericardium, and beneath it
is the dark-colored kidney. The rectum may be seen passing
through the pericardium and the heart, then extending above the
posterior adductor muscle to the cloaca, where it ends with the
anus. Cut open the cloacal chamber by a slit in the side of its
siphon. Find the hinder end of the rectum and the anus. Note
just beneath the muscle a canal which accompanies the base of
the gill forward. This is the suprabranchial passage of the outer
gill ; it runs posteriorly to the cloacal chamber. Blow into it with
a blowpipe, also probe it from behind.

Exercise 5. Draw a semidiagrammatic view of the animal lying in
the right-hand valve of the shell, representing the organs above
mentioned. Carefully label all.

The Respiratory System. The gills have already been noticed.
The two gills on each side of the visceral mass are in origin
but a single organ, which is called the ctenidium. The clam is
thus provided with a single pair of ctenidia, which are homolo-
gous to those of the squid and of snails. Each gill consists of a
pair of plates, or lamellae, united at their lower edges and open
above, and further joined by vertical or dorsoventral cross

76 PHYLUM MOLLUSCA

partitions, the interlamellar partitions. The space between the
lamella is thus divided into parallel, vertical chambers, the water
tubes, which run from the bottom to the top of the gill and open
above into the suprabranchial passage. One of these passages
runs along the base of each gill, as a wide canal, to the cloacal
chamber. We have already observed the suprabranchial passage
of the outer gill. In order to observe that of the inner gill, lift up
both gills ; the inner lamella of the inner gill will, in most species
of clam, be seen not to be united with the wall of the visceral
mass along the hinder portion of the foot, but to have a free edge.
The long slitlike opening thus presented leads into the inner
suprabranchial passage. Probe it backward to the cloacal cham-
ber. Probe it also from the hinder end forward and notice that
back of the visceral mass the two inner suprabranchial passages
— that is, those belonging to the inner gills of the right and left
sides — coalesce, forming a single passage.

Study the finer structure of the gills. Place a gill on a glass slide
in a little water and with forceps and a knife carefully separate
the two lamellae. Mount a piece of a lamella in water and study
it under a compound microscope. Note the vertical interlamellar
partitions. Observe that the lamella is a delicate latticework
made up of ridges, the gill filaments, running vertically and thus
parallel with the interlamellar partitions, and of cross ridges, the
interfilamentary connections, which run between and connect
the vertical filaments. The apertures in the latticework place the
water tubes in communication with the water in the branchial
chamber. The gill filaments are provided with cilia, the action of
which causes streams of water to pass into the water tubes. The
cilia may easily be seen by mounting a piece of a gill of a freshly
killed animal on a slide and examining it under a compound mi-
croscope. The course of the respiratory water is from the bran-
chial chamber into the water tubes, through which it passes into
the suprabranchial passages and through these into the cloacal
chamber.

The outer gill and sometimes the inner gill also act as a brood
chamber in the female clam, and during the breeding season will
be found to be distended with eggs, or embryos.

A FRESH-WATER CLAM 77

Exercise 6. Draw a diagram of the respiratory system showing the
gills and their relation to the suprabranchial passages. Show the
direction of the flow of the respiratory water by means of arrows.

Exercise 7. Draw a diagram showing the minute structure of a
lamella.

The Circulatory System. With fine scissors and great care cut
open the pericardium by a slit along its dorsal border. Note the
heart, with the rectum passing through it. The heart consists of
three chambers : a median, thick-walled ventricle and two lat-
eral auricles. These latter are delicate, thin-walled organs, tri-
angular in shape, the base of the triangle lying along the dorsal
border of the gills and the apex communicating with the ven-
tricle. If the left auricle has been injured in the dissection, the
right one is easily seen by looking across the pericardial space.
From the ventricle an anterior and a posterior artery pass to
either end of the body. These arteries lie alongside the rectum,
to which the anterior one is dorsal and the posterior one is ven-
tral ; they are difficult to distinguish from it, except in speci-
mens in which the heart has been injected.

The course of the blood is the following : by the contraction of
the heart the blood is sent to all parts of the body ; on its return
course it is first conveyed through a system of lacunae to the
kidneys, and thence to the gills ; here it circulates in vessels which
run through the interlamellar partitions, the gill filaments, and
the interfilamentary connections, and is oxygenated; it then
passes into the auricles.

The Excretory System. This system consists of a pair of kid-
neys, which are dark-colored organs lying just beneath the peri-
cardium and in front of the posterior adductor muscle. Each
kidney consists of two parts, the kidney proper and the ureter.
The former is a dark, thick-walled gland which lies beneath the
ureter and communicates with it at its hinder end. The ureter is
a thin-walled vessel lying above the kidney proper, with a small
external opening on the side of the visceral mass beneath the an-
terior end of the kidney and near the base of the inner gill. It
is usually difficult to find but may be recognized by its white
lips. The kidney also possesses at its anterior end a duct lead-

78 PHYLUM MOLLUSCA

ing into the pericardial cavity. Slit open the ureter and kidney-
proper in clean water and observe their inner structure.

Exercise 8. Draw a diagram representing the pericardial cavity and
the kidney, showing the relation of the two structures to each
other. Draw the heart in the pericardial cavity, showing the rela-
tion of the auricle to the gills.

The Digestive System. The stomach and intestine are embedded
in the visceral mass and are difficult to dissect out of it. With
care and patience, however, it can be done. Find the mouth be-
tween the two pairs of palps ; note the upper and the lower lips,
which connect the upper and the lower pairs of palps respec-
tively. The mouth is seen to the greatest advantage in a specimen
which has been deprived of both valves of the shell. Trace the
rectum from the anus to the place of its entrance into the visceral
mass. Carefully remove with forceps and knife the tough white
integument which covers the left side of the visceral mass, taking
care not to disturb the organs beneath. The soft cream-colored
mass just above the foot is the reproductive gland ; the light-
greenish mass lying just above this is the liver. Embedded in these
masses lies the alimentary tract, a narrow, delicate tube, which
will be injured in the dissection unless the greatest care is taken.
Gently scrape away the soft mass which surrounds the alimen-
tary tract, laying it entirely bare. The water in the dissecting
pan must be frequently renewed to keep it clear, and great care
must be taken not to break the tract. The course of the whole
digestive tract is the following : The mouth opens into the short
oesophagus, after which the canal dilates to form the stomach.
The dark-colored liver surrounds the stomach. Back of the
stomach is the intestine, a narrow tube which runs backward and
downward to the hinder end of the visceral mass ; it then turns
upward and runs forward to a point above the stomach, where it
turns downward to the lower side of the visceral mass ; it then
bends dorsally again and runs to the point where it leaves the
visceral mass. Here the rectum begins and passes through the
heart and above the posterior adductor muscle to the anus in
the cloacal chamber.

A FRESH-WATER CLAM 79

Clams feed upon minute organisms and organic particles
contained in the water. Some of the water in the mantle cavity
is drawn by the ciliated oral palps into the mouth and passes
through the alimentary tract, where organic substances con-
tained in it are digested and absorbed. The clam usually lies
with its forward end buried in the sand and its hinder end with
the siphons projecting into the water.

Exercise 9. Draw a diagrammatic view of the digestive system.

The Reproductive System. The sexes are separate. The repro-
ductive glands (testes or ovaries) are very similar to each other
and consist of a pair of cream-colored masses which fill a large
part of the visceral mass. They communicate with the outside
through a pair of minute openings, one on each side of the visceral
mass near the base of the gills. The opening can often be located
by pressing out from them eggs or sperm. The eggs, as soon as
laid, pass into the water tubes of the outer gills of the mother,
where they hatch. The young larvae are very immature and are
called glochidia ; they leave the mother and attach themselves
to the sides of fishes by means of a pair of sharp projections on
the ventral edges of the valves of the shell, where they lead a
parasitic life. While here they undergo a metamorphosis and
finally attain the adult structure, when they detach themselves
and drop to the bottom. Look for glochidia in your specimen.

The Nervous System. This consists of three pairs of ganglia :
the cerebral ganglia, or brain, the pedal ganglia, and the visceral
ganglia, and the nerves proceeding from them. Each of the last
two pairs is joined with the brain by a pair of nerve connectives.

First find the visceral ganglia. They are a small grayish or
pinkish mass on the ventral surface of the posterior adductor
muscle with nerves radiating in all directions. Two of these
nerves, the cerebrovisceral connectives, will be seen passing for-
ward, one on each side of the visceral mass.

Next find the brain. It consists of a pair of ganglia situated
above the mouth, just behind the anterior adductor muscle. The
two ganglia are not so close together as those of the visceral pair ;
they lie on either side of the muscle and are united by a com-

8o PHYLUM MOLLUSCA

missure. Each ganglion sends out three large nerves : the
cerebrovisceral connective, which goes to the visceral ganglia, the
cerebropedal connective, which goes to the pedal ganglia, and
the pallial nerve, which passes to the mantle. Find them.

The pedal ganglia form a nervous mass buried in the foot near
its base. Make a shallow longitudinal incision in the bottom of
the foot and gently pull the flaps apart ; the pinkish mass and the
nerves radiating from it will be seen.

Exercise 10. Draw a diagram representing the nervous system.

Make several transverse sections with a razor through the
region of the heart of a clam which has been previously hard-
ened. Identify all the organs which appear.

Exercise 11. Draw a diagram representing a cross section ; carefully
label all the organs.

THE OYSTER 8i

Class : Pelecypoda
THE OYSTER

Select a large, live oyster in the shell, and if it is dirty wash it
thoroughly. The shell is sometimes covered with mud, and with
hydroids, sponges, tube-forming annelids, and other marine ani-
mals. The small, round holes made by the yellow boring sponge
are often conspicuous.

The two valves of the shell will be seen to be different in shape,
one being more or less flattened and the other much deeper and
more convex. These two valves cover the right and left sides of
the animal's body, the convex valve being on the left and the
flattened one on the right side. The oyster is a sessile animal,
after it has passed through its youthful migratory period, and is
fastened to a rock or shell or other stationary object by its left
shell. It thus lies on its left side, and the flat right shell acts as
a cover which can be raised to allow the animal to draw in water
containing food and respiratory air, and closed when danger
threatens. The very young oyster is a symmetrical animal which
swims about actively in the water. While it is still very small —
so small, in fact, that it is barely visible to the naked eye — it
settles down and fastens itself to some stationary object, and in
its subsequent growth accommodates itself more or less to the
irregularities of this substratum. This is the reason why the
shell is so often rough and irregular in shape.

The smaller end of the shell is the anterior end. The hinge
ligament is situated here, the elasticity of which keeps the shell
open except when it is closed by the contraction of the large
adductor muscle. At this end is also the umbo, the oldest part
of the shell. Note the parallel lines of growth which extend from
the umbo to the ventral and posterior sides of the shell. When the
anterior, the right, and the left sides of the shell are known, the
ventral and posterior sides can be easily determined.

Exercise 1. Make an outline drawing of the right valve, indicating
the anterior, posterior, dorsal, and ventral aspects and showing the
lines of growth.

82 PHYLUM MOLLUSCA

Remove the right valve in the following way : Break off the
edge of the shell with a hammer, insert the blade of a scalpel and
cut the large adductor muscle, which is not far from the edge but
nearer the dorsal than the ventral margin. It is important to keep
the blade close to the right valve so as not to mutilate the internal
organs. Force off the right valve and examine its inner surface.

Exercise 2. Draw the inner surface of the shell, showing the muscle
scar with its lines of growth and the hinge ligament, and label the
dorsal, ventral, anterior, and posterior sides of it.

Study the animal as it lies in the left valve. Note the soft,
shiny mantle, which covers the inner surface of the shell and has
secreted it. The mantle is a double fold of the integument which
extends ventrally from the dorsal side and covers the two lateral
sides of the body. Its lower edge is bordered by a fringe of short,
pigmented tentacles, which are the principal sense organs of the
animal ; it is also provided with muscle fibers which enable it to
be slightly extended beyond the edge of the shell.

The most conspicuous organ in the body will be seen to be the
large adductor muscle. Lying between it and the hinge ligament
is the visceral mass, containing most of the viscera. Along the
ventral side are the four gills.

Put the oyster into a pan of water and with fine scissors and
forceps remove the right mantle. Just in front of the adductor
muscle observe the pericardium. Carefully cut it away and see
the heart, which lies in the pericardial cavity ; it will be beating
if the animal is still alive. The ventricle is dorsal in position ;
the auricle is ventral, lying next to the gills, from which it re-
ceives the purified blood. The four gills lie close together, no
foot being present to separate the two right-hand from the two
left-hand gills. Just in front of the gills, at the front end of the
body, are the two pairs of large oral palps. The mouth is between
these palps, two being on each side of it. Find the mouth and
note that it lies between an under and a lower lip, each of which
is formed by the union of a pair of palps ; that is. a palp on the
right side joins one on the left and forms the upper lip, and the
other two palps join to form the lower lip.

THE OYSTER 83

Oysters feed on minute organisms contained in the water.
These are caught in the sUme which exudes from the surface of
the gills and moved forward by the action of the cilia of the gills
and the palps to the mouth.

The anus and the rectum will be seen on the dorsal side of the
adductor muscle.

Exercise 3. Make a drawing of the oyster as it lies in the left shell,
representing all the organs above mentioned. Carefully label all.

The Digestive Tract. This consists of the short oesophagus, the
stomach and the dark-colored liver which surrounds it, and the
long intestine. The mouth opens directly into the oesophagus,
which leads to the stomach. The position of this organ can easily
be determined, because it is embedded in the dark-brown liver.
Carefully scrape or cut away the side of the visceral mass and
expose the liver ; continue the process until the stomach is seen.
The intestine extends straight back from the stomach to a posi-
tion ventral to the adductor muscle and between it and the gills.
It then turns on itself and passes straight forward to the dorsal
side of the stomach, around the forward and ventral sides of it,
and thus back again to the dorsal side of the muscle, where it
ends with the anus. Most of it is surrounded by the yellow re-
productive gland. Lay bare the intestine. This can be done best
after the oyster has been hardened for a few days in a 5 per cent
solution of formalin.

Exercise 4. Make a drawing of the digestive tract in an outline of
the animal's body.

The remaining systems of organs of the visceral mass will not
be studied in this dissection.

The American oyster is a unisexual animal; the common
European oyster is hermaphroditic. The reproductive glands,
the ovaries or testes, are a pair of yellowish or whitish organs of
irregular form which occupy the larger part of the visceral mass
and surround the digestive tract and other organs. The kidneys
are also a pair of organs of irregular form which, together with a
portion of the intestine, occupy the lower and hinder part of the

84 PHYLUM MOLLUSCA

visceral mass, between the muscle and the gills. The nervous
system has been much modified by the sessile habit of life of the
oyster. The cerebral ganglia are represented by a nerve ring
which surrounds the mouth ; it is called the circumpallial nerve.
Fibers from this ring go to the pigmented sense papillae at the
margin of the mantle. The visceral ganglia lie along the antero-
ventral side of the muscle and are joined with the cerebral ring
by longitudinal connectives. The pedal ganglia are wanting.

THE HARD-SHELL CLAM 85

Class : Pelecypoda
THE HARD-SHELL CLAM {VENUS MERCENARIA)

This is a very common marine mollusk which inhabits the
sandy bottoms of the ocean along the Atlantic coast. The soft-
shell clam {Mya arenaria), which lives in mud fiats visible be-
tween tides, resembles it very much in structure and may be
used for this dissection.

Study the live animal first, if possible. Its body is unsegmented
and is entirely inclosed in a bilateral, bivalve shell, which is the
cuticula of the animal richly charged with calcareous salts. The
two valves of the shell cover the right and left sides of the animal
and are joined together on its dorsal side by the dark-colored
hinge ligament, while their ventral edges are open ; the animal is
thus very much compressed laterally. The anterior end of the
animal is truncated ; the posterior end is elongated. Which is
the right-hand valve ? The elevation on each valve near the hinge
ligament is called the umbo. It is the oldest portion of the shell ;
from it as a beginning point the shell has grown in size to its pres-
ent proportions by additions to its ventral edge. Note the parallel
lines of growth. The ventral edges of the shell are thus the young-
est portions of them.

Exercise 1. Make a drawing of the right-hand valve, indicating the
anterior, posterior, dorsal, and ventral aspects, and showing the
lines of growth.

Exercise 2. Make a drawing of the dorsal aspect of the animal.

Kill the animal by immersing it for a few minutes in hot water
(70° C). Since the shell is kept closed by the contraction of the
two muscles which pass between the valves, it will gape open as
soon as the animal is dead and the muscles are relaxed. It is the
elasticity of the hinge ligament which causes it to open.^

1 The shell may also be opened by inserting a sharp, wedge-shaped instrument
between the valves. The valves are thus pressed apart far enough to admit the
blade of a scalpel, by means of which the adductor muscles should be cut close to
the left valve of the shell. The hinge ligament should then be cut and the left
valve removed.

86 PHYLUM MOLLUSCA

Examine the animal as it lies in the shell. It will be seen that
the inner surface of each valve is covered with a soft, slimy mem-
brane, whose lower edge is parallel with the edge of the shell. This
is the mantle ; it is a double fold of the dorsal integument of the
body, one side of which is covered by either fold. The mantle is
the matrix of the shell ; that is, it secretes it. The lower edge of
the mantle is provided with muscle fibers and can be extended
beyond the edge of the shell ; it also possesses sensory functions ;
in some pelecypods eyes are situated in the mantle's edge.

Observe the large, soft visceral mass hanging between the two
lobes of the mantle ; it contains most of the viscera of the animal.
On the lower side of the visceral mass, that is, toward the gape of
the shell, is the muscular, wedge-shaped foot, which can be ex-
tended beneath the edge of the shell and is the organ of locomo-
tion. Notice the two leaflike gills on each side of the visceral
mass and foot. Observe the two large adductor muscles, one in
front of and the other behind the visceral mass, which pass from
one valve to the other and serve to close them.

Pass a knife between the mantle and the left shell and separate
them from each other. Cut the two muscles close to the shell ;
cut the hinge ligament and remove the left shell.

Study the inner surface of the shell. Note the two large scars
marking the surfaces of attachment of the adductor muscles;
just above the anterior scar is that of a much smaller muscle, the
anterior retractor of the foot. Note the broad line which joins
the scars and runs parallel with the edge of the shell except near
the posterior muscle scar, where it bends forward, forming a tri-
angular indentation. This is the pallial line ; it is formed by the
insertion in the shell of the delicate muscle fibers near the edge of
the mantle. The indentation is the pallial sinus. Note the hinge
teeth just beneath the umbo.

Exercise 3, Draw a view of the inner surface of the shell.

Break the shell and examine the broken edge with a hand lens.
Study the structure of the shell. It is composed of three layers :
the inner mother-of-pearl layer, which is secreted by the entire
surface of the mantle, the prismatic layer, and the organic layer, or

THE HARD-SHELL CLAM 87

periostracum, on the outside. The two latter layers are secreted
by the edge of the mantle ; the periostracum is very thin and is
frequently wanting on some portions of the shell. Place a piece
of the shell in a solution of hydrochloric acid ; note the efferves-
cence which results.

Exercise 4. Draw a view of the broken edge of the shell on a scale
of 5. Show the prisms of the prismatic layer.

Place the animal in water and study it as it lies in the right
shell. ^ The two halves of the mantle will be seen to envelop en-
tirely the visceral mass of the foot. Over the dorsal portion of the
visceral mass the mantle is fused with it and cannot be separated,
but the lateral and the ventral portions of the mantle lobes hang
free, inclosing an extensive space, which is called the mantle cav-
ity. In this cavity, on each side of the visceral mass, lie the two
leaflike gills. Observe the edges of the mantle. They are fused
forward of the anterior adductor muscle ; the entire ventral edges
are free and permit the foot to protrude between them ; their pos-
terior edges are richly pigmented, and are also fused and modified
to form the two siphons. These are tubes which can be extended
beyond the shells and through which water is taken into and
expelled from the mantle cavity. Probe them. Note on each side
below the posterior adductor muscle the triangular muscle which
connects the siphons with the shell. It is the siphonal retractor
muscle. Between the two siphons in the mantle cavity note the
short, transverse septum which divides the posterior portion of
the mantle cavity into two chambers, a dorsal and a ventral one.
The latter is the very large branchial chamber, which contains the
visceral mass and the gills ; the former is the very small cloacal
chamber. The ventral siphon is called the branchial or incurrent
siphon ; through it the water streams into the branchial chamber,
bearing food and air for respiration. The dorsal siphon is called
the excurrent or cloacal siphon, and through it water passes
outward from the cloacal chamber, charged with carbon dioxide
of respiration and with fecal matter from the alimentary tract.
Probe the cloacal chamber.

1 For the study of the soft parts of the clam it is well to have also at hand a
specimen which has been deprived of both valves of the shell.

88 PHYLUM MOLLUSCA

Carefully remove the left mantle lobe after cutting it with fine
scissors at its line of attachment, beginning at the forward end.
Cut ofT the siphonal muscle, leaving the siphons in position.
Place the anim^al in water and study the arrangem^ent of the
organs. Observe the position of the gills ; note in front of them
two triangular flaps, the oral palps ; in the median line between
the two pairs of oral palps is the mouth ; find it. Along the base
of the gills note an elongated passage leading posteriorly to the
cloacal chamber ; this is the suprabranchial passage of the outer
gill. Blow into this passage at its hinder end in the cloacal
chamber with a blowpipe, or probe it.

Observe again the siphonal region. Note again the short septum
which separates the branchial from the cloacal chamber, and the
opening between it and the visceral mass ; probe this opening.
Just beneath the umbo will be seen through the semitransparent
body wall a dark-colored mass, the liver, back of which are the
yellowish reproductive gland and the dark-colored organ of Keber.
Back of the latter is the pericardium, within which is the heart.
Beneath the heart and in front of the posterior adductor muscle is
the dark-colored kidney. Passing through the pericardium and
the heart and above the posterior adductor muscle to the cloaca
will be seen the rectum. It ends with the anus near the hinder sur-
face of the muscle. Open the cloacal chamber by a slit in the side
of its siphon and find the anus.

Exercise 5. Draw a semidiagrammatic view of the animal lying in the
right-hand valve of the shell, representing the organs above men-
tioned. Carefully label all.

The Respiratory System. The gills have already been noticed.
The two gills on each side are in origin but a single organ,
which is called the ctenidium. The clam is thus provided with a
single pair of ctenidia, which are homologous to those of the squid
and of snails. Each gill consists of a pair of plates, or lamellae,
united at their lower edges and open above, and further joined by
vertical or dorsoventral cross partitions, the interlamellar par-
titions. The space between the lamellae is thus divided into
parallel, vertical chambers, the water tubes, which run from the

THE HARD-SHELL CLAM 89

bottom to the top of the gill and open above into the supra-
branchial passage. This is a wide canal running along the base
of each gill to the cloacal chamber. The course of the supra-
branchial passage of the outer gill has already been noted. In
order to observe that of the inner gill, lift up both gills ; the inner
suprabranchial passage will be seen at the base of the inner gill.
Probe from the cloacal chamber into it. Notice that back of the
visceral mass the two inner suprabranchial passages coalesce and
form a single passage.

Study the finer structure of the gills. Place a gill on a glass slide
in a little water and with forceps and knife carefully separate the
lamellae. Mount a piece of a lamella in water and study it under
a compound microscope. Note the vertical interlamellar parti-
tions. Observe that the lamella is a delicate latticework made
up of ridges, the gill filaments, which run vertically and thus
parallel with the interlamellar partitions, and of cross ridges, the
interfilamentary connections, which run between and connect
the vertical filaments. The apertures in the latticework place the
water tubes in communication with the water of the branchial
chamber. The gill filaments are provided with cilia, as may easily
be seen if the gill is alive, the action of which causes streams
of water to pass into the water tubes. The course of the re-
spiratory water is from the branchial chamber into the water
tubes, through which it passes to the suprabranchial passages,
and through these into the cloacal chamber, whence it is ejected
through the cloacal siphon.

Exercise 6. Draw a diagram of the respiratory system, showing the
gills and their relation to the suprabranchial passages. Show the
direction of the flow of the respiratory water by means of arrows.

Exercise 7. Draw a diagram showing the structure of a lamella.

The Circulatory System. With fine scissors carefully cut open
the pericardium by a slit along its dorsal border and expose
the heart. Note the heart, with the rectum passing through it.
The heart consists of three chambers ; a median, thick-walled ven-
tricle and two lateral auricles. These latter are delicate, thin-
walled organs, triangular in shape, the base of the triangle lying

Qo PHYLUM MOLLUSCA

along the dorsal border of the gills and the apex communicating
with the ventricle. If the left auricle has been injured in the dis-
section, the right one is easily seen by looking across the pericar-
dial space. From the ventricle an interior and a posterior artery
pass to either end of the body. The posterior artery expands,
near the posterior end of the pericardium, to form a large, thick-
walled sac, the arterial bulb. These two arteries lie alongside
the rectum, to which the anterior one is dorsal and the posterior
one is ventral ; they are difficult to distinguish from it, except in
specimens in which the heart has been injected.

The course of the blood is the following : by the contraction of
the heart the blood is sent to all parts of the body, whence it is
conveyed through a system of lacunae to the kidneys and thence
to the gills ; here it circulates in vessels which run through the
interlamellar partitions, the gill filaments, and the interfilamen-
tary connections, and is purified ; it then passes into the auricles.

The Excretory System. This consists of a pair of kidneys which
lie just beneath the pericardium and in front of the posterior
adductor muscle. Each kidney consists of two parts, the kidney
proper and the ureter. The former is a dark, thick-walled gland
which Hes beneath the ureter and communicates with it at its
hinder end. The ureter is a thin-walled vessel lying above the
kidney proper, with a small external opening in the side of the
visceral mass near the base of the inner gill which is usually diffi-
cult to find. The kidney also possesses at its anterior end a duct
leading into the pericardial cavity. Slit open the ureter and kid-
ney proper and observe their inner structure.

Exercise 8. Draw a diagram representing the pericardial cavity and
the kidney, showing the relation of the two structures to each
other. Draw the heart in the pericardial cavity, showing the rela-
tion of the auricle to the gills.

The Digestive System. The stomach and intestine are em-
bedded in the visceral mass and are difficult to dissect out of
it. With care and patience, however, it can be done. Find the
mouth between its two pairs of palps; note the upper and
the lower lips, which connect the upper and the lower pair of
palps respectively. The mouth is seen to the greatest advantage

THE HARD-SHELL CLAM 91

in a specimen which has been taken out of both shells. Trace the
rectum from the anus through the heart to the point where it
meets the visceral mass. With forceps and knife carefully remove
the tough white integument which covers the left side of the vis-
ceral mass. The soft, cream-colored mass filling the greater part
of it is the reproductive gland ; the greenish mass above is the
liver. Embedded in these masses lies the alimentary tract, a nar-
row, delicate tube, which will be injured in the dissection unless
the greatest care is taken.- Gently scrape away the soft mass which
surrounds the alimentary tract, laying it entirely bare. The water
in the dissecting pan must be frequently renewed to keep it clear,
and great care taken not to break the canal. The course of the
whole digestive tract is the following : The mouth opens into the
short cBsophagus, after which the canal dilates to form the stom-
ach. The liver surrounds the stomach and is connected with it by
several ducts. Back of the stomach is the intestine, which first
runs backward and downward to the posterior part of the visceral
mass, after several turnings in the lower part of which it bends
upward and runs forward parallel with the posterior margin of
the visceral mass to its dorsal border, where it leaves it. Here the
rectum begins and passes through the heart and above the pos-
terior adductor muscle to the anus. A small, transparent rod is
often present in the intestine ; its function is unknown.

Clams feed upon minute organisms and organic particles con-
tained in the water. Some of the water in the mantle cavity is
drawn into the mouth by the ciliated oral palps and passes
through the alimentary tract, where the organic substances are
digested and absorbed.

Exercise 9. Draw a diagrammatic view of the digestive system.

The Reproductive System. The sexes are separate. The repro-
ductive glands (testes or ovaries) are very similar to each other
and consist of a pair of cream-colored masses which fill a large
part of the visceral mass. Their external openings are a pair of
minute pores, one on each side of the visceral mass just below
and in front of the opening of the ureter. They can sometimes
be located by pressing out from them eggs or sperm.

92 PHYLUM MOLLUSCA

The nervous system consists of three pairs of ganglia : the
cerebral ganglia, or brain, the pedal ganglia, and the visceral
ganglia, and the nerves proceeding from them. Each of the last
two pairs is joined with the brain by a pair of nerve connectives.

First find the visceral ganglia. They are a small, pinkish mass
on the ventral surface of the posterior adductor muscle, with
nerves radiating in all directions. Two of these nerves, the
cerebro visceral connectives, will be seen passing forward, one on
each side of the visceral mass.

Next find the brain. It consists of a pair of pinkish ganglia
situated above the mouth, just behind the anterior adductor
muscle. The two ganglia are not so close together as are those
of the visceral pair ; they lie on each side of the muscle and are
united by a commissure. Each ganglion sends out three large
nerves : the cerebrovisceral connective, which goes to the visceral
ganglia, the cerebropedal connective, which goes to the pedal
ganglia, and the pallial nerve, which passes to the mantle. Find
these nerves.

The pedal ganglia form a nervous mass buried in the foot near
its base. They must be sought by cutting into the foot near its
base and may be recognized by their pink color.

Exercise 10. Draw a diagram representing the nervous system.

Make several transverse sections with a razor through the
region of the heart of a clam which has been previously hardened.
Identify all the organs which appear.

Exercise 11. Draw a diagram representing a cross section ; carefully
label all the organs.

A LAND SNAIL 93

Class : Gastropoda. Order : Pulmonata
A LAND SNAIL

The edible French snail (Helix pomatia) is very common in
Europe, in many parts of which it is used for food. It is imported
into this country for the same purpose and may be obtained at
small cost in New York and other cities. It is especially adapted
for dissection, but any large Helix may be used instead. The large
slug (Limax maxima) is very similar to Helix in structure and
may also be used, but as it has no coiled shell, that feature of the
dissection would be omitted.

The snail is a terrestrial animal and feeds principally upon
leaves. It hibernates in the winter under stones and logs, after
having first closed the mouth of its shell with a thin disk of
hardened, calcified slime called the epiphragma. If it is still in
winter quarters when obtained, the epiphragma should be re-
moved and the animal placed among fresh leaves in a warm room,
when it will soon come out of its shell and begin to feed. Snails
are best killed for dissection by drowning. They should be placed
in a large, covered jar of water, when they will die extended in
from one to two days. If the air is first boiled out of the water,
the process will be accelerated, but the animal should not be
placed in water which is still hot.

Study the external characters of the animal. Its body is un-
segmented and is covered with a shell, but unlike the shell of the
pelecypod that of the snail is a univalve. As in other mollusks,
the shell is the cuticula of the animal charged with calcareous
salts, and forms an exoskeleton. In shape the shell is an elongated
cone which has been twisted to the right, forming a closely coiled
spiral. The tip of the spiral is called the apex, the opening is
called the mouth, and its axis, the columella. How many turns
does the spiral make? The apex corresponds to the umbo of
the pelecypod; it is the oldest portion of the shell, the point
from which its growth has proceeded. Note the parallel lines
of growth. The ventral edge, or mouth, of the shell is thus its
youngest part. The animal can withdraw its entire body within

94 PHYLUM MOLLUSCA

the shell, but when it is walking or feeding it protrudes its head
and foot. The visceral mass, containing all its viscera, is always
covered by the shell and has thus its exact shape ; that is, it is
an elongated cone which has suffered a dextral twisting so as to
form a closely coiled spiral. As a matter of fact, however, it is
the visceral mass which has been primarily twisted ; the shell is
twisted because it covers the visceral mass. If the spiral were to
be imagined uncoiled and extending straight up above the foot,
the apex would be the uppermost and the foot the lowermost por-
tion of the body ; the apex is thus, morphologically, the dorsal
and the foot is the ventral aspect of the animal.

As in the pelecypod, the visceral mass is inclosed in a mantle,
which is a fold of the dorsal integument, but unlike the pelecypod
it is a single fold and not a double one. This fold falls about and
covers the visceral mass on all sides, as does a thimble the fingei
it is on, and secretes the shell on its outer surface. The ventral
edge of the mantle is provided with muscles, so that it can be
protruded beyond the mouth of the shell or retracted within ii
This edge is called the collar. Find it in your specimen. On the
right side of the animal note the deep notch and the round hole
in the collar. This is the respiratory pore, which opens into the
respiratory chamber. This chamber is the mantle cavity. Probe
it gently and determine its extent. The animal being terrestrial
has no gills, but respires by means of a lung, which is a highly
vascularized portion of the wall of the mantle cavity. In a live
animal note its power to open and close the respiratory opening.

The foot of the animal forms a broad creeping disk, adapted
for locomotion on flat surfaces. Its wavelike undulations may
be observed by causing the animal to walk over a glass plate.
The head, which is wanting in the pelecypods, forms the anterior
end of the animal and bears two pairs of hollow, retractile ten-
tacles, the posterior pair carrying each an eye at its extremity.
The mouth of the animal is between and a little below the base of
the anterior pair of tentacles. Probe it and note the paired, lobed
lips. Just beneath the mouth is the broad opening of the pedal
slime gland. Probe it and note the extent of the gland. On the
right side of the head is a straight groove which extends to a

A LAND SNAIL 95

depression just behind the base of the anterior tentacle. This de-
pression is the common genital pore, the animal being hermaph-
roditic. The anus is a small opening just beneath the respiratory
pore at the end of a deep groove. It is not easily observed from
the outside.

Note the asymmetry of the animal. Its spiral twist has been
the cause of the loss of the primitive bilateral symmetry of the vis-
ceral mass and shell. They are not borne squarely above the foot,
but obliquely and to the left. The respiratory pore (that is, the
opening of the mantle cavity) and the anus have not a median
posterior position, as must have been the case in the primitive
ancestor of the animal, but have suffered displacement to the
right side. Other instances of asymmetry will be noticed as the
dissection proceeds.

Exercise 1, Draw a side view of the animal seen from the right side
as it appears when it is moving and when the head and foot are out
of the shell, and label the parts above mentioned.

Exercise 2. Draw a similar sketch of a front view of the animal.

Remove the dead animal from its shell in the following way :
place it for five minutes in strong alcohol, or for half a minute in
very hot (not boiHng) water, in order to loosen the shell ; then twist
it out of the shell ; this must be done very gently, or the animal
will be torn.

Exercise 3. Draw the shell showing its opening on the right.

Break off a portion of the edge of the shell and examine the
broken edge with the aid of a hand lens. Note the three layers
which compose the shell : the inner pearly layer, which has been
secreted by the entire surface of the mantle ; the thick middle
layer and the thin outer layer, or periostracum, which have been
secreted by the collar. The periostracum is a horny, uncalcified
layer, which is usually more or less worn off.

The Internal Organs. Take the snail, deprived of its shell, in
the hand, and remembering that the outer side of each whorl of
the spiral is on the left side of the animal and that the inner side
of the whorl is on the right, observe the extent of the mantle

96 PHYLUM MOLLUSCA

cavity. Put the blowpipe through the respiratory pore and blow
into the mantle cavity. It will be seen to extend from the collar
to the posterior side of the first whorl. Examine the mantle wall
with a hand lens and against the light. The network of blood
vessels which constitutes the lung will be seen. On the hinder
border of the mantle cavity note the kidney, an elongated, light-
colored, triangular organ ; just in front of it and beneath it — that
is, between it and the mantle cavity — is the heart within the peri-
cardium ; note the two chambers of the heart, the dorsal auricle
and the more ventrally placed and larger ventricle. Back of the
kidney is the dark-colored liver, which, with the intestine and
the light-colored reproductive tract, occupies the remainder of the
coils of the spiral. Note the rectum, a broad tube on the inner
(right) border of the mantle cavity going to the anus. Cut a small
hole in it, and through this pass a probe to the anus.

The Mantle Cavity. Lay this open in the following way : with
fine scissors cut through the collar at the respiratory pore ; then
make an incision in the mantle wall from this opening, following
the collar round the outer side of the whorl to the heart ; con-
tinue the incision across the artery leading out of the heart, and
through the delicate membrane between the liver and the kidney
to the rectum, at the inner border of the whorl. The mantle can
now be laid back and its cavity with the organs exposed. The
broad rectum will be seen running along the entire inner border of
the mantle cavity. Now make an additional incision from the
respiratory pore along the inner (lower) border of the rectum as
far as the kidney. Lay back the mantle and pin it down as flat as
possible under water. Identify the heart within the pericardium,
the kidney, and the rectum.

The Respiratory and Circulatory Systems. Observe the lung
(the network of blood vessels in the inner surface of the mantle)
and the large pulmonary vein, which runs along the kidney to the
heart. Slit open the pericardium. The two chambers of the heart
will be more distinctly seen, the thin -walled auricle into which the
vein runs and the larger ventricle. Back of the latter the aorta
passes into the viscera ; its cut end will be seen.

The process of respiration and circulation is the following:

A LAND SNAIL 97

the air is drawn into the mantle cavity through the respiratory
pore; this is accompHshed by the alternate enlarging and con-
tracting of the cavity by means of the muscular body wall which
constitutes its floor. Notice the longitudinal and the transverse
muscles in this floor. The blood circulating in the lung is oxy-
genated and passes into the heart through the pulmonary vein
as arterial blood. It is forced by the heart through the aorta, and
thence through arteries to all parts of the body, whence it returns
through blood lacunae to the lung.

The Excretory System. The large kidney has already been seen.
It is a sac the glandular projections of the walls of which almost
fill its lumen. As is the case with pelecypods, the kidney commu-
nicates with the pericardial space through a fine canal and also
with the mantle cavity by means of a ureter. The pericardial
canal is opposite the ventricle and cannot be seen easily. The
ureter may be easily traced. It is a wide canal which leaves the
kidney at its forward end near the place where the pulmonary
vein approaches the kidney ; it first runs along the inner side of
the kidney to its hinder end ; here it doubles on itself and passes
forward to the inner edge of the mantle, where it runs beside the
rectum to a point near the respiratory pore and opens into the
mantle cavity.

It will be noticed that the heart and the kidneys are both asym-
metrical organs. The heart has but one auricle ; it will be remem-
bered that in the pelecypod the auricles are paired organs ; one
of the pair must thus be wanting in the snail. There is also only
one kidney and one ureter, instead of a pair of each, as in the
pelecypod. It is the left member of the pair in each case which is
wanting.

Exercise 4. Draw a view of the inner surface of the mantle on a scale
of 3, showing the organs mentioned above ; label all.

The Digestive System. Pass a bristle through the anus into the
rectum in order to mark it. With two strong pins firmly fasten
the extreme forward end of the animal's foot and also its hinder
end to the wax of the dissecting pan. With sharp, fine scissors cut
through the floor of the mantle cavity and the collar in the median

98 PHYLUM MOLLUSCA

line ; carry the incision forward in the median line along the head
between the base of the tentacles to the mouth. Care should be
taken in making this incision not to cut the organs beneath.
Spread the flaps as widely as possible to the right and left and
pin them down, thus exposing the organs in the forward part of
the body.

The white organs on the right side of the body belong to the
reproductive system. The large, dark organ in the center, or
on the animal's left, is the stomach. Find the slender, curved
oesophagus, which leads forward from it to the dorsal side of the
large, muscular pharynx. The oesophagus is encircled by the white
nerve collar, the dorsal portion of which is the brain. If, however,
the animal died in a retracted condition, the pharynx may have
slipped back through the nerve collar, which would then encircle
the forward end of that organ. Note the two white, leaflike salivary
glands which lie close against the wall of the stomach, and trace
their ducts forward to the pharynx. Lying above and across the
oesophagus is the white, cylindrical penis, which will be seen to
extend from the genital pore at the right of the mouth and to bend
sharply on itself. The bend of the penis is connected by a long
retractor muscle to the dorsal body wall. Find it ; cut it and pin
the penis on the animal's right. Note the broad, glistening re-
tractor muscle connecting the pharynx with the ventral, posterior
body wall. Notice its shape ; with strong forceps pull it loose from
the pharynx and entirely remove it. Beneath it, note the still
larger retractor muscle running from the forward end of the head
back to the same locality. What is the function of these different
retractors ? The dark-colored sheaths and the retractor muscles of
the tentacles will also be seen ; trace these muscles to their origin.
Find the nerve which passes from the brain into each tentacle.

Separating the delicate filaments connecting the stomach with
the surrounding organs, and pushing it and the oesophagus to the
animal's left, find the large nervous mass which forms the ventral
portion of the nerve collar, and the nerves radiating from it. It is
an agglomeration of ganglia, being made up principally of the
pedal and the visceral ganglia. Press the reproductive organs
to the animal's right and pin them down. The receptaculum

A LAND SNAIL 99

seminis, a small, spherical body the size of a small shot, at the
end of a long tube, will be found in a bend of the intestine, from
which it must be separated.

We turn now to the other end of the intestine. Trace the rec-
tum from the anus to the point where it is surrounded by the
liver and carefully dissect away the integument which covers the
inner surface of the whorl. The light-colored hermaphroditic
gland will be exposed. Then remove the delicate integument
which covers the outer surface of the whorl, and the dark-brown
liver will be exposed. Press the liver away from the intestine and
completely free it, being careful not to break either liver or in-
testine. Great care should also be taken not to injure the her-
maphroditic gland, which is the yellowish mass on the inner side
of the last whorl, or the hermaphroditic duct leading away from
it. Note that the liver is composed of two masses, the smaller
of which is of spiral form and occupies the apex of the shell ; the
larger is subdivided into three lobes. Note also the two main
bile ducts which join the liver with the intestine. The visceral
artery will be seen lying upon the liver, sending branches off on
both sides, and must not be confused with the bile ducts, which it
resembles in appearance. It carries blood from the aorta to the
top of the spiral, supplying all the organs of the visceral mass. At
the point where the bile ducts communicate with the intestine,
that organ makes a sharp turn.

Spread out the digestive tract to the animal's left and pin it
down, without removing or breaking the hermaphroditic gland
or duct. The stomach will be seen to extend nearly to the liver.
Next comes the intestine, which soon makes the sharp turn above
mentioned, receives the bile ducts, and passes into the rectum
at the right side of the mantle cavity.

Exercise 5. Draw an outline of the alimentary tract from the mouth
to the anus on a scale of 2 and label all its parts.

Study the structure of the pharynx. Pass a probe into the
mouth and notice the extent of the pharyngeal cavity . Notice the
transverse, horny jaw in the roof of the mouth. With a sharp
knife split the dorsal pharyngeal wall, taking care not to injure

lOO PHYLUM MOLLUSCA

the nerve collar or reproductive organs. Notice that the connec-
tion of the oesophagus and the salivary ducts with the pharynx
is near the dorsal wall of the latter organ. Observe the thick, mus-
cular tongue, the organ by means of which the animal grinds its
food. Its surface is covered with a ribbon set with small teeth,
called the radula. This can be easily pulled off with the forceps.
Mount it on a slide in water or glycerine and study its surface
under a high power of the microscope.

Exercise 6. Make a drawing of several of the teeth.

The Reproductive System. The snail is hermaphroditic, but is
not self-fertilizing. The hermaphroditic gland, which at different
times produces both spermatozoa and ova, is situated on the inner
side of the smaller lobe of the liver. The hermaphroditic duct is a
delicate, white, convoluted tube, which goes from the hermaphro-
ditic gland to the albuminous gland, a large, white body lying near
the liver. From this organ the oviduct and vas deferens pass for-
ward to the genital opening near the mouth. These canals are
side by side and connected with each other for the first part of their
course, but may be distinguished by the character of their walls, the
oviduct having folded, glandular walls, whereas the vas deferens
is a narrow tube with thin walls. It is through the latter canal
that spermatozoa pass out from the hermaphroditic duct ; the
ova pass out through the oviduct, the glandular walls of which,
together with the albuminous gland, secrete the albumen which
surrounds them when they are extruded. Near their forward end
these two canals separate. The oviduct loses its glandular walls,
becomes cylindrical in shape, and expands to form the vagina.
This is a thick-walled vessel with which are connected the follow-
ing accessory genital organs : the receptaculum seminis, a small,
spherical organ, already mentioned, lying in the bend of the intes-
tine and joined with the vagina by means of a long tube which lies
along the oviduct ; the mucous glands, two bunches of tubular
glands; and the dart sac, a thick-walled sac which contains a
calcareous spicule. Identify these organs.

The vas deferens, after separating from the oviduct, passes
under the retractor muscles of the tentacle to the distal end of

A LAND SNAIL loi

the penis. This organ has already been noted ; it is tubular in
shape and lies in a bent position across the oesophagus. A re-
fractor muscle inserted at the bend connects it with the dorsal
body wall. At the point where the vas deferens meets it observe
the flagellum, a long, tubular sac into which spermatozoa pass
from the vas deferens and where they are massed together to form
spermatophores. Both penis and vagina communicate, side by
side, with the genital cloaca, which opens to the exterior through
the common genital pore.

When two animals pair each receives a spermatophore from
the other. This passes into the receptaculum seminis, which is
thus filled with the spermatozoa of the other animal, and these
finally fertilize the eggs as they pass into the vagina from the
oviduct.

Exercise 7. Make a semidiagrammatic drawing of the reproductive
organs on a scale of 2.

Split the dart sac and take out the dart ; mount it on a slide
in water or in glycerine and examine it under a compound
microscope.

Exercise 8, Draw the dart.

The Nervous System. Sever the oesophagus and remove the
reproductive and digestive systems, leaving the pharynx in the
body and taking care not to injure any of the nerves. The prin-
cipal ganglia are contained in the nerve collar. The two supra-
oesophageal ganglia, which constitute the brain, will be seen
joined by a broad transverse commissure. From their anterior
surface nerves run to the tentacles, and from their inner surface
a pair of nerves run to the posterior end of the pharynx, where
they meet a pair of small phar5mgeal ganglia. The supraoesophag-
eal ganglia are connected by broad connectives with the sub-
oesophageal ganglia. Remove the pharynx from the body. By
slightly scraping the suboesophageal ganglia with a small scalpel,
it will be seen to consist of two principal ganglionic masses. The
forward mass is a pair of ganglia, the pedal ganglia ; the hinder
mass consists of the large visceral ganglia, at the side of which is

I02 PHYLUM MOLLUSCA

the pair of small pleural ganglia. Observe the nerves radiating
from the suboesophageal ganglia, and determine as far as possible
to what organs they go.

Exercise 9. Make a semidiagrammatic drawing of the nervous system.

Organs of Special Sense. The eyes of the snail at the end of the
posterior tentacles have already been noted. They are easily seen
in a large animal which has its tentacles extended. The snail is
also provided with a pair of auditory organs. They consist of
two small sacs embedded in the pedal ganglia. In order to see
them cut off the suboesophageal ganglion, mount it in glycerine,
and examine it under a compound microscope. The auditory
nerves are very delicate and come from the supraoesophageal
ganglia.

A SQUID 103

Class : Cephalopoda
A SQUID {LOLIGO PEALI)

The squid is a very common marine animal. It is gregarious
in its habits and swims about in large schools in search of its food,
which consists of crustaceans, , small fishes, etc. When alarmed
by the presence of its natural enemies, which are many kinds of
fishes, it clouds and darkens the water by ejecting into it an ink-
like fluid. The fresh animals are studied with greater profit than
those which have been preserved in alcohol, as this changes the
nature and appearance of many of the organs. The best pre-
servative is formalin.

External Anatomy. Observe the cylindrical, bilaterally sym-
metrical body ; at one end is a pair of broad fins, and at the other
the movable head bearing ten arms, two of which are much
longer than the others. The mouth is at the base of and sur-
rounded by the arms, and the brown, horny beak may usually
be seen protruding partly from it. The large eyes are on the sides
of the head at the base of the arms. Each is covered by a cornea,
which is pierced by a small hole between the eye and the base of
the arms, so that sea water is admitted freely into the space be-
tween the cornea and the pupil, and may take the place of the
aqueous humor of the vertebrate eye. A transverse fold on the
side of the head between the eye and the body is the olfactory
organ. Observe the pigment spots, or chromatophores, which are
distributed over the body ; they are constantly changing in shape
and size during life, causing corresponding changes in the color
and appearance of the animal.

The head and neck project from the large mantle cavity, into
which they can be partially withdrawn by means of powerful re-
tractor muscles, in very much the same way that a snail's head
and foot can be withdrawn into its shell. The siphon, or funnel, a
large, funnel-shaped organ at the base of the head, also projects
from it and can be similarly withdrawn. Gently probe the mantle
cavity and determine its extent. The mantle constitutes the
outer surface of the body. It will be seen to be a cylindrical

I04 PHYLUM MOLLUSCA

structure with thick, muscular walls within which he all the viscera
of the animal ; its free edge is called the collar, as in the snail. It
is also necessary to observe that the mantle is not a paired organ,
as it is in the clam, but an unpaired one, as in the snail. The squid
has no foot, as has the clam or the snail, but morphological equiva-
lents of the foot are present in the arms and the siphon.

Since in all mollusks the foot or its equivalent occupies a
ventral position, and the visceral mass a dorsal position, the
arms of the squid, together with the head, must be on its ventral
side, and the opposite end with the broad fins must be dorsal ;
the animal is thus enormously extended dorsoventrally. It will
be readily seen also that the mantle falls as a cylindrical fold from
the dorsal end about the entire body, exactly as it does in the case
of the snail. In fact, if the coils of the snail's visceral mass could
be straightened out, the mantle would fall as a cylindrical fold
from its dorsal end and terminate in the collar below, in the same
way as in the squid. The morphologically posterior side of the
animal is]that on which the siphon is situated ; the anterior side is
the opposite one. In common parlance, however, the head end
of the squid is called the forward end, and the fin-bearing end,
the hinder. The side bearing the fins is likewise called the upper
side or back, and the opposite side, on which is the siphon, the
under or lower side. These terms, although incorrect in a strictly
morphological sense, are much more convenient for general use
and will be employed hereafter in these directions.

The mantle of the squid does not secrete an external shell, as
does that of the snail and the clam ; in a long pocket on the upper
side, however, is an elongate, horny structure, called the pen,
which is secreted by the mantle and is the equivalent of the shell
of other mollusks.

Make a short, shallow incision in the upper surface of the man-
tle, beginning with the collar. Turn the flaps aside and note the
brown, horny pen lying beneath. Do not remove it at present, as
the dissection of the parts beneath might be disturbed by its
removal.

Exercise 1. Make a drawing of the underside of the animal.

A SQUID 105

Note that the arms may be divided into a right and a left group,
each containing five arms. Observe a single arm ; how many-
rows of suckers has it ? Observe the structure of a sucker. Note
the difference between the two long arms and the others in the
place of origin and the arrangement of the suckers.

The Mantle Cavity. Open the mantle cavity by a longitudinal
incision through the thick mantle wall of the under side of the
body to one side of the median line, running from the collar to
the apex of the animal, taking care not to injure the delicate
organs within. Notice, in the first place, that the collar is not
attached to the head at any point of its circumference ; and also
that on the inner surface of the mantle, on the upper side of the
body in the median Hne and also on each lateral surface, there is
an elongate, cartilaginous structure which fits into a correspond-
ing cartilage on the body — an arrangement which enables the
collar to be applied very closely to the head.

Place the animal in water with the head away from you and
pin down the flaps of the mantle. Observe the soft visceral mass
within it, and notice that it is fused with the mantle only in the
median Hne of the back ; also that the pen, which is embedded in
the mantle, protects the viscera on that side. Observe the siphon
and probe it. It will be seen to be a funnel-shaped tube communi-
cating between the mantle cavity and the outside. Slit it open and
observe the flapHke valve at the forward end. Notice the lateral
pockets on each side of the siphon which open toward the mantle
cavity and occupy the space between the siphon and the median
line of the back. They are separated from the siphon by the
lateral, cartilaginous rods above mentioned. It will be seen that
while water can easily pass into the mantle cavity from the out-
side all around the neck, a contraction of the muscular wall of
the mantle would force the water out through the siphon only, as
that which is forced into the lateral pockets would at once swell
them out and close the spaces at the sides of the siphon. It is, in
fact, by thus shooting the water in the mantle cavity forcibly
through the siphon that the animal swims.

Note the two large retractor muscles of the siphon and beneath
them the two larger retractor muscles of the head.

io6 PHYLUM MOLLUSC A

Observe again the visceral mass ; it is covered by a thin, trans-
parent membrane, the body wall, the extreme thinness of which is
correlated with the thickness of the mantle which covers it. If
the animal is a female that fact may be known by the presence of
two very large, transversely striated bodies, called the nidamental
glands, which He near the center of the body, and are a part of the
reproductive system. Carefully remove these in order to expose
the organs beneath. If the animal is a male (and the student
should obtain a male if possible) it can be recognized by the
absence of nidamental glands and also by the presence of the
testis, a large, white, tubular organ which lies near the median line
toward the hinder end of the animal. In the female the ovary,
which occupies a similar position, is often very full of the gran-
ular ova.

Notice in the mantle cavity the pair of plumose gills to the
right and the left of the visceral mass, each attached to the inner
surface of the mantle by a mesentery. Between the retractor
muscles of the siphon and extending from the base of the gills
forward to the siphon is the rectum, which terminates in the anus,
with its two projecting valves. Find the valves. Beneath the
rectum is the ink bag, and both are attached to the organs be-
neath them by a mesentery. The ink bag communicates with the
rectum by means of a duct which joins it near the anus ; this duct
may be found by sHtting the rectum for a short distance back of
the anus, when the small opening may be made to appear by
squeezing the ink bag and forcing the ink into the rectum. The
ink, together with the fecal matter from the intestine and other
waste products, is voided into the sea water through the siphon ;
its function is to cloud the water and thus hide the animal from
its enemies. In the male animal notice the long, tubular penis to
the right of the rectum (the animal's left) ; if the animal is a female,
the thick-walled oviduct will be seen in a corresponding position.

At the base of each gill note a round, disk-like body ; this is a
branchial heart, from which blood is sent into the gills ; near each
branchial heart toward the median line and running forward
alongside the rectum is an elongate, transparent structure, the
kidney. The position of the kidneys may be determined by the

A SQUID • 107

two conspicuous white veins — the precaval veins — which pass
through them longitudinally from one end to the other. These
veins are wide, spongy- walled structures which run to the bran-
chial hearts and will be seen toward the median line from those
organs. Just beneath the base of the two kidneys and between
the branchial hearts is the median, or systemic, heart, into which
blood pours from the gills. Note a median artery, the posterior
aorta, which leads back from the systemic heart ; it branches into
three large mantle arteries, two of which pass to the right and
left, respectively, and enter the mantle at the side, while the other
passes into the mantle in the median Hne ; it is through these
arteries that the mantle is supplied with blood.

On each side between the base of the gill and the rectum and
extending parallel with the latter organ, notice again the delicate
kidney ; each of the pair of kidneys extends backward to a point
a short distance back of the branchial heart, and forward to a
point back of the base of the ink bag, where it communicates with
the mantle cavity through a small opening. Find the two open-
ings by lifting up the body wall with forceps and blowing on it
with a blowpipe, when they will appear.

Running back from the branchial heart on each side is a wide
vessel, the postcaval vein ; the forward end of this vein has thick,
spongy walls like those of the precavals and is easily seen ; the
greater part of it, however, has extremely thin walls and can be
seen with difficulty. Near the base of each gill note also a vessel
which runs forward and laterally into the mantle; this is the
mantle vein. Just back of this vein is a muscle which connects
the gill with the mantle ; it is the branchial retractor muscle.

Note the two large stellate ganglia in the forward part of the
inner surface of the mantle, and the radiating nerves which each
ganglion sends into the mantle.

In the hinder portion of the visceral mass in the male animal
observe on the animal's left (the observer's right), just behind the
branchial heart, a coiled tube, the vas deferens, and in the female
the thick-walled oviduct. Extending farther back and near the
median Hne is the large, white testis in the male and the large
ovary in the female.

io8 PHYLUM MOLLUSCA

Exercise 2. Make a large sketch of the mantle cavity of the animal
showing these organs, and label all.

With fine scissors and forceps carefully dissect away the deli-
cate transparent body wall and expose the organs beneath, taking
care not to injure them.

The Excretory System. The kidneys and their external open-
ings have already been observed. As in other mollusks, the kid-
neys also communicate with the pericardial space.

The Circulatory and Respiratory Systems. Pushing aside the
organs which partly conceal it, observe again the systemic heart ;
note its shape and slightly asymmetrical position. Extending
from its forward end is the anterior aorta, which takes blood to
the forward part of the body ; its course cannot be followed at
present. The hinder part of the body is supplied with blood by
the posterior aorta. This vessel, as we have already seen, leaves
the hinder end of the systemic heart ; it sends off two pairs of
small arteries to the stomach and to other viscera, and then
branches into the three mantle arteries already mentioned. Find
them all and trace them as far as possible. Observe again the two
branchial hearts. Note the branchial artery, by which blood
passes from the branchial heart to the gill; also the branchial
vein, through which it passes from the gill to the systemic heart.

Observe again the veins which bring the blood to the branchial
hearts. The precavals bring blood from the forward part of the
body. Trace them forward. They enter the kidneys near the for-
ward end of those organs and traverse their glandular walls back
to the branchial heart. Press aside the rectum and the forward
end of the kidneys and observe where the two precavals come
from beneath and enter the kidneys. With fine scissors cut the
connective tissue which binds the veins, and also the mesentery
which holds down the rectum and the ink bag, and turn these or-
gans back. Trace the two precavals forward ; they will be seen to
come from a delicate median vein which may be followed into the
head. Observe again the postcaval veins, which bring blood from
the hinder part of the body and join the branchial hearts near the
same place as the precavals. Their forward ends also traverse the
glandular walls of the kidneys and are here conspicuous ; back of

A SQUID 109

these they are much wider, but are very thin-walled and not easily
seen. Trace them as far as possible. Observe again the mantle
veins, which bring blood from the mantle to the branchial hearts.
The course of the blood is the following : it enters the branchial
hearts through the postcaval, precaval, and mantle veins ; the
contraction of these hearts sends it into the branchial arteries
which pass along the upper side of the gills ; it then traverses the
delicate, transverse filaments of the gills and becomes oxygenated,
when it collects again in the branchial veins on the opposite side
of the gills ; through these it passes to the systemic heart, whence
it is sent through the anterior and posterior aortas to the different
parts of the body.

Exercise 3. Make a diagrammatic drawing of the circulatory and the
respiratory systems.

The Digestive System. Remove the kidneys and precaval veins.
Beneath them will be seen a large, glandular, bilobed organ of some-
what doubtful function, called the pancreas. At its forward end a
pair of cylindrical organs, the liver ducts, will be seen entering it
from the liver. The pancreas is made up of anastomosing, glan-
dular projections of the walls of these ducts. Remove the gills,
branchial hearts, systemic heart, and hinder arteries. The deH-
cate body wall should be completely removed from the entire vis-
ceral mass, and great care be taken not to injure the stomach
pouch beneath. This latter organ is a large bag with thin, trans-
parent walls which extends to the extreme hinder end of the body ;
beneath it will be seen the large testis or ovary, according to the
sex of the animal. This pouch is not really a part of the stomach,
notwithstanding its name, but is a reservoir for the secretions of
the liver, which communicates with it through the liver ducts.
Carefully loosen the stomach pouch without separating it from
the body and let it float in the water of the dissecting pan. It
communicates with the thick-walled stomach, which hes just in
front of it, but food substances are prevented from passing into it
from the stomach by valves. Loosen the stomach, noticing that
it is bound to the ovary or testis by an artery, the genital artery.
At the forward end of the stomach are the intestine and the oesoph-

no PHYLUM MOLLUSCA

agus, side by side ; the former passes between the two halves
of the pancreas and ends with the rectum ; the oesophagus goes
forward side by side with the anterior aorta to the middle of the
large liver and passes through it in company with the aorta. The
oesophagus is easily found by turning the stomach over. A small
ganglion with radiating nerves will be seen by the side of the
oesophagus near its junction with the stomach.

The liver is an elongated body lying between the retractor
muscles of the head and of the siphon ; two ducts emerge from
it and pass through the pancreas to the stomach pouch. Loosen
and remove the connective tissue around the liver and raise it up ;
the oesophagus and the aorta will be seen to pass through it
toward the back of the animal and then forward to the head.

Remove the siphon and split the wall of the head ; trace the
oesophagus to its forwara end. It will be seen to pass through
the ganglionic mass which constitutes the central nervous system,
and which is surrounded by a hard, cartilaginous capsule. In
front of this it meets and ends in the bulbous pharynx. Near the
forward end of the liver, and resting upon the oesophagus, will be
seen the median salivary gland, the duct of which may be traced
to the pharynx ; near the hinder end of the pharynx is a pair of
smaller salivary glands, which also communicate with it. Trace
their ducts to the end. The alimentary canal will thus be seen
to consist of the following organs : the muscular pharynx, with
which a pair of small salivary glands and a single, large salivary
gland communicate ; the narrow oesophagus ; the thick-walled
stomach ; the stomach pouch, which communicates with the stom-
ach by a valved opening ; the elongated liver, which communi-
cates with the stomach pouch by two long ducts ; the bilobed
pancreas ; the intestine, which leaves the stomach near the point
where the oesophagus enters it ; the rectum, which is joined by
the ink bag and passes to the anus.

Exercise 4. Take the alimentary tract out of the body, pin it down,
and make a drawing of it ; label all its divisions.

Slit open the stomach and examine its ridges. Slit open the
pharynx on the upper side ; note the large, chitinous jaws and the

A SQUID III

radula. The latter organ, like the radula of snails, is used in chew-
ing the food ; examine its surface under a microscope and note the
calcareous teeth.

Exercise 5. Make a drawing of the jaws.
Exercise 6. Make a drawing of several of the teeth of the radula.

The Reproductive System: the Male. The principal genital
organs have already been observed. The single, median testis is a
large, flat organ, dorsal to the stomach pouch, in the hinder por-
tion of the visceral mass ; the genital artery joins it with the sur-
face of the stomach. The testis has no direct connection with the
vas deferens, but is surrounded by a thin, transparent membrane
within which it Ues as in a capsule, and into which the sperma-
tozoa escape. The vas deferens, which is also unpaired, commu-
nicates with this capsule. It is a long and much-twisted tube
with several wide, glandular regions, and lies, bound by connective
tissue into a compact mass, on the left side of the viscera. Take
the entire system out of the body, put it into water, loosen and
straighten out, as far as possible, the convolutions of the vas
deferens. Beginning with its hinder end we find first a narrow,
convoluted tube, and then follows a thicker, tubular portion, the
vesicula seminalis ; near the forward end of this portion is a
glandular body, the prostate gland, and a membranous sac ; a
long, straight, narrow portion comes next, which widens to form
the spermatophoric sac, within which the spermatophores are
formed ; then follows the tubular penis, which forms the for-
ward end of the tract and has already been observed lying in
the mantle cavity to the left of the rectum.
Exercise 7, a. Make a drawing of the male genital tract.
Exercise 7, b. Open the spermatophoric sac and look for spermato-
phores ; they are slender, white objects about half an inch long.
Mount several on a slide and make a drawing of one.

The Female. The single ovary, like the testis, is a very large,
elongated gland occupying the hinder end of the visceral mass and
surrounded by a capsule. The oviduct communicates with this
capsule ; it passes forward along the left side of the visceral mass,
its walls becoming thickened in its course to form the oviducal

112 PHYLUM MOLLUSC A

gland, and opens into the mantle cavity by means of a large, thick-
lipped aperture to the left of the rectum.

Two pairs of prominent accessory glands are present in the
female, the large, white, finely striated nidamental glands, which
cover up most of the other organs of the visceral mass, and
beneath them the much smaller accessory nidamental glands,
which are pink-colored in life and lie to the right and left of the
rectum ; both pairs of glands open at their forward ends into the
mantle cavity. These glands secrete the egg capsules, which pro-
tect the eggs after they are laid and while development is going
on within them.

Exercise 7, c. Make a drawing of the female organs.

The Nervous System. In the position of the principal ganglia
the squid resembles the snail, but these ganglia are difficult to
observe in a dissection because they are compactly massed to-
gether and are protected by a cartilaginous capsule, which forms
a sort of skull. The cerebral or supraoesophageal ganglia form a
large mass above the oesophagus ; broad commissures join it with
the suboesophageal mass, which is composed of the visceral, pedal,
and, in front of the latter, the brachial ganglia. Connected with
the sides of the cerebral mass are the two optic nerves, which
widen out to form the large optic ganglia, and running forward
from it are two small nerves which connect it with the suprapha-
ryngeal ganglia, a small mass above the hinder end of the pharynx.
From these ganglia small nerves pass around the oesophagus to
the pair of subpharyngeal ganglia. From the forward surface of
the suboesophageal mass, that is, from the brachial ganglia, ten
nerves pass off to the arms. These may be seen on the inner
surface of the head after the removal of the pharynx. From the
hinder surface of the visceral ganglia pleural nerves run to
the stellate ganglia in the mantle. Trace these nerves from the
stellate ganglia to their source.

The Pen. Make a longitudinal sHt in the mantle on the back
of the animal and remove the pen ; it will be seen to lie quite
loosely in its sac.

Exercise 8. Draw the pen.

PHYLUM ECHINODERMATA

Class : A steroidea
A STARFISH

Several species of starfishes are common along our coasts, the
most familiar being Asterias vulgaris, the common New England
form, which is found along the entire Atlantic coast, and Asterias
forbsii, which is common south of Cape Cod. They are remark-
ably sluggish creatures which live on the sea bottom, moving
slowly, often in large numbers, from place to place and feeding
on the various mollusks which come in their way.

Two specimens will be needed for this dissection, a dried one
for the study of the hard parts, and one that is fresh or has been
preserved in formalin or alcohol for the study of the internal and
other soft parts. To prepare a dried starfish the live animal
should be placed in fresh water for half an hour. It should then
be placed in alcohol for an hour, and then dried thoroughly. If
only preserved material is at hand, the animals may be simply
dried. The fresh water and alcohol expand the body wall of the
animals and prevent it from collapsing after death.

Study the external characters of a fresh or a preserved speci-
men. Observe the color and the flattened, radiate body form.
The body is composed of a central dis^ from which radiate five
arms, or rays. All these rays are normally of equal length.
Specimens are often found, however, in which the length of the
rays is unequal. This is due to the fact that starfishes often lose
one or more of their rays by accident ; the missing member is soon
replaced by a new ray, but while it is growing out it will be shorter
than the others. The spaces between the rays are called inter-
rays. In the center of the under surface of the disk is the mouth ;
hence this surface of the animal is called the oral surface. Its
upper surface is called the aboral surface.

113

114 PHYLUM ECHINODERMATA

In the aboral surface of the disk notice the red madreporite (in
preserved specimens it may have lost its color and be white).
Examine it on the dried specimen with the aid of a hand lens or
the low power of a compound microscope and notice its porous
structure. In the aboral surface is also the anus ; it is a very
small opening and will be difficult or impossible to see in the
specimens at hand. Note the short fixed spines covering the
entire aboral surface. Each one is a part of small calcareous
plate buried beneath the integument. The entire body wall of
the animal is made up largely of these plates, which give it its
stiffness. The plates are not, however, connected with one an-
other except by muscles and connective tissue, and the animal's
arms are flexible and freely movable. Demonstrate this fact
with your specimen. In the dried animal this flexibility no
longer appears, as the entire body wall has been rendered rigid
by the drying. In the soft places between the plates note
the delicate, tubular projections of the integument ; they are
the contractile papulae, and are organs of respiration and excre-
tion and possibly also of sensation. With the aid of a hand lens
find, around the base of each spine, the pedicellariae ; these are
minute, pincer-like organs of somewhat uncertain function, but
they probably aid in keeping the surface of the animal free
from particles of dirt and from minute organisms which might
be harmful.

The two arms which inclose the madreporite between their
bases are called the bivium; the remaining three, the trivium.
How can a plane be passed through the body so as to divide
it into two symmetrical halves?

Exercise 1. Make a life-size drawing of the aboral aspect of the animal
and label all the features observed.

On the oral surface observe the deep groove which extends
from the mouth along each arm to its tip. This is the ambulacra!
groove. Observe the two rows of movable spines which fringe
each side of the groove ; note also the five pairs of movable spines
which surround the mouth. Separate these spines and observe
the mouth surrounded by a circular membrane, the peristome.

A STARFISH 115

From the sides of each ambulacral groove two zigzag rows of
soft tentacles project. These are the ambulacral feet ; they are
muscular tubes with sucker disks at their ends and are the organs
of locomotion. Scrape the feet from a portion of the groove and
examine its sides ; note the slender, transverse calcareous plates
which form it, and the round openings between them, called the
ambulacral pores, through which branches from the feet project
into the body cavity. Note the zigzag nature of each of the two
rows of these pores. Notice also the delicate cord which extends
along the median line of the groove ; it is the main nerve of the
arm ; it proceeds from a nerve ring in the central disk to the tip
of the arm. Follow it to the tip and note the red pigment spot
with which it ends. This is the eye. In preserved specimens the
pigment may have lost its color.

Exercise 2. Make a life-size drawing of the oral aspect of the animal
and label all these features.

Scrape off several pedicellariae, mount them on a slide, and ex-
amine them under a compound microscope. By pressing on the
cover glass with a needle, the jaws can be made to open and shut ;
try it.

Exercise 3. Draw a pedicellaria on a large scale.

Cut off the aboral wall of the severed arm of the dried specimen
and scrape away the remains of the internal organs and the am-
bulacral feet. Study the inner surface of the ambulacral groove.
Note the two rows of slender transverse plates which form the
sides of the groove, and on each side between every two plates,
the minute ambulacral pore.

Exercise 4. Make a drawing on a scale of 3 of the inner surface of the
ambulacral groove, showing the plates and the pores.

Cut off the aboral wall of the central disk of the dried specimen,
scrape away the remains of the internal organs, and study the ar-
rangement of the plates in the inner surface of the oral body wall.
Note the circular mouth protected by converging spines, and also
the membranous peristome. Observe the convergence of the five

ii6 PHYLUM ECHINODERMATA

arms about the peristome ; note also the interradial partitions
which separate the base of the arms.

Exercise 5. Make a hfe-size drawing showing these features.

Internal Anatomy. Remove the entire aboral body wall from
the trivium and the central disk of the fresh or preserved speci-
men, with the exception of the madreporite which must not be
removed, being very careful not to injure the organs beneath.
Study the internal organs and observe the following systems :

The Digestive System. Observe the large sacHke stomach,
which almost fills the central disk. Its walls are much folded,
and five short, baglike pouches extend from it into the five arms.
When the animal feeds the stomach is everted and thrust out
through the mouth and about its prey. It is drawn in again by
means of five pairs of retractor muscles, which connect the stom-
ach pouches with the inner surface of the ambulacral grooves.
Find the pair of retractors belonging to each stomach pouch.
Communicating with the aboral portion of the stomach are five
large, radial digestive glands, which are usually called livers. Each
gland almost fills an arm ; it is made up of two main trunks, from
which project numerous side branches ; the two ducts leading
from the two trunks in each arm unite to form a single duct which
passes to the stomach. Each trunk is suspended from the aboral
wall of the arm by two mesenteries. Find the mesenteries in one
of the bivial arms. Study the structure of the livers. The stomach
is connected with the mouth by a short oesophagus, and from its
upper surface a short, slender intestine passes to the anus. Con-
nected with the intestine is a small branched diverticulum, the
intestinal caecum. The intestine, together with its caecum, may
have been removed when the aboral body wall was taken off. If
this is the case look for them on the portion of the aboral wall
which was taken off and notice also the position of the anus.

The Reproductive System. The sexes of the starfishes are sep-
arate. The sexual organs are branched glandular organs, ten in
number, which lie in the proximal portion of the rays and open
to the outside through minute pores in the aboral walls of the
interrays. Two glands will be found in each ray extending from

A STARFISH 117

the base of the ray toward its tip. The actual size of these organs
depends entirely upon the sexual condition of the animal. In
young or immature animals they may be no more than half an
inch long or less, while in reproducing animals they may extend
almost to the tip of the ray. The testis of the male and the ovary
of the female animal do not differ from each other in general ap-
pearance. In the mature female, however, the ovaries have a
light-yellow color, and in the mature male the testes are white
and are less voluminous than the ovaries.

Exercise 6. Make a semidiagrammatic drawing of the animal showing
the details of the digestive and reproductive systems ; label all.

Remove the stomach and the reproductive organs from the
body, taking care not to injure the sinuous stone canal which is
at one side of the former.

The Ambulacral System. This is the most characteristic system
of organs in the Echinodermata. In the starfish it consists of the
following organs : a circular canal, called the ring canal, surround-
ing the mouth ; connected with this canal are nine minute, lobated
sacs called the racemose or Tiedemann's vesicles, two being lo-
cated in each interray except the one in which is the stone canal,
where but one is present ; five radial canals, which pass from the
ring canal along the median line of the ambulacral grooves to the
tips of the arms ; the ambulacral feet, which are connected with
the radial canals by short branch canals, and also project through
the ambulacral pores into the body cavity, where they expand to
form small sacs called ampullae ; a sinuous canal, called the stone
canal, which connects the ring canal with the madreporite ; the
madreporite, a porous plate by means of which the entire system
is placed in communication with the outside sea water.

In studying this system find first the madreporite and the stone
canal, and trace the latter from the madreporite to the ring canal.
Remove the spines which project over the peristome and find the
ring canal. It is a delicate tube, of about the diameter of a needle,
which surrounds the mouth, running around the base of the arms
at the point where the peristome joins them ; it is thus, like the
radial canals, outside the body cavity. Remove some of the am-

ii8 PHYLUM ECHINODERMATA

bulacral feet from a ray and find again the delicate radial canal
which Hes along the middle of the ambulacral groove. Trace it to
the ring canal. Cut the aboral body wall from one of the bivial
rays, remove the liver, and observe the ampullae. Press them and
notice that the feet are thereby extended.

The ambulacral system will be seen to be a system of tubes
extending throughout the body and in communication with the
sea water. They are filled with a fluid which is not pure sea
water, but is rather a watery serum in which float amoeboid cells.
This fluid is driven into the tubelike ambulacral feet, which
thereby acquire rigidity and are extended. The system is the
locomotory system of the animal. It moves by extending the
feet, attaching the sucker disks at their ends to some stationary
object, and then drawing them in. The animal is thus able to
pull itself slowly along. The ambulacral system possibly also
exercises excretory and respiratory functions.

Exercise 7. Draw a diagram of the ambulacral system.

There are no special respiratory and excretory organs. These
functions are exercised by the papulae and possibly the ambula-
cral feet.

The nervous system consists of a circumoral nerve ring, which
lies just beneath the ambulacral ring canal, and five radial nerves,
which proceed from it along the median line of the ambulacral
grooves to the tips of the arms, tlach radial nerve ends with a
pigment eye. There are no other organs of special sense. The
main nerves of the starfish do not lie within the body cavity but
in the integument and can thus be seen from the outside. There
are, however, in addition to these nerves, other less important
ones which are internal. We have already observed the radial
nerves in the median line of the ambulacral grooves; the ring
nerve can also be seen as a slight ridge just beneath the ring canal.

Exercise 8. Draw a diagram representing the nervous system.

Exercise 9. Draw a diagram representing a vertical section of the
animal passing through the madreporite and the anterior ray
(that is, the middle trivial ray).

A SEA URCHIN 119

Class : Echinoidea
A SEA URCHIN

Several species of sea urchins occur along the Atlantic coast,
the most famiUar being Arbacia, the dark-colored urchin, and
Strongylocentrotus, the green urchin, the former having a more
southerly distribution than the latter. The animals live on the
sea bottom or on rocks, usually in companies, and move slowly
about from place to place, using not only the ambulacral feet
but often the spines as well, as organs of locomotion. They feed
partly upon small animals and partly upon organic substances
present in the sand and mud, which they pass through the
intestine.

Two specimens will be needed for this dissection, a dried one
for the study of the hard parts and a fresh or preserved one for
the study of the internal organs.

Observe the radiate, spheroidal body entirely covered with
movable spines. Look among the spines and find the ambulacral
feet. These can be extended in Hfe beyond the spines and are
employed by the animal as organs of locomotion. Note the five
ambulacral areas (those containing the feet), and between them
the five interambulacral areas. The flattened surface is the under,
or oral, surface, on which the animal moves ; the rounded surface
is the aboral. It will be seen that the aboral side of the sea urchin
bears ambulacral feet, whereas in the starfish the oral side alone
bears them.

In the center of the oral surface observe the mouth and the
five calcareous teeth which project from it. Surrounding the
mouth is a membrane which fills the space between the edges of
the shell and is called the peristome. Notice the ten short am-
bulacra! suckers which surround the mouth, and near them the
five groups of pincer-like pedicellariae. Observe the long, slender
stalks of these organs. Near the margin of the peristome are five
groups of ambulacral feet, which are in the ambulacral areas, and
also ten groups of short branched gills, which mark the inter-
ambulacral areas.

I20 PHYLUM ECHINODERMATA

Exercise 1. Make a drawing of the oral surface on a scale of 2.

With forceps remove some of the pedicellariae, mount them,
and study them under the microscope. Note the three minute
jaws and the long stalk. Press on the cover glass and cause the
jaws to open and shut.

Exercise 2. Make a drawing of a pedicellaria.

Study the structure and method of articulation of the spines.
Pull off several and notice their ball-and-socket joints, also the
delicate muscles by which they are moved. Notice the fluting
of the shaft.

Exercise 3. Make a semidiagrammatic drawing of a spine on a large
scale, showing the articulation of the muscles.

Remove the spines from the dried specimen and thoroughly
clean the shell. This is accomplished the most effectually by
placing it in a solution of warm caustic potash for a short time.
Great care should be taken, however, not to leave it in the solu-
tion too long or it will fall to pieces. Study the aboral side of the
shell. Observe the rows of tubercles on which the spines have
articulated ; note also the bands of minute holes, the ambulacral
pores, through which the ambulacral feet have projected. There
are ten of these bands arranged in pairs, and each pair represents
an ambulacral area, or a ray. Between the five rays are the five
interambulacral areas, or the interrays, which are somewhat
broader than the rays ; count the rays and the interrays.

The center of the aboral surface is free from spines and is made
up of several small plates. It is called the periproct and contains
in its center the minute anus. Surrounding the periproct are ten
plates, which also bear no spines. Five of these, which are larger
than the others, are situated at the ends of the interrays and are
pierced each by a small hole. These plates are called the genital
plates, and the holes are the external openings of the genital
organs. One of the genital plates is larger than the others and is
porous ; it is the madreporite. The five smaller of the ten plates
which surround the periproct are situated at the ends of the rays.
They are called the ocular plates. Each is pierced by one or two

A SEA URCHIN 121

holes, through which project minute, pigmented tentacles. Notice
that each ray and each interray is made up of two rows of plates ;
thus there are twenty rows of plates altogether. As in the star-
fish, the two rays between which the madreporite lies are called
the bivium ; the other three, the trivium.

Exercise 4. Make a drawing of the aboral side of the shell, with the
spines removed, on a scale of 2, showing accurately the boundaries
of all the plates. Label the rays, interrays, and all the other parts
observed.

The Internal Organs. Place a fresh or preserved sea urchin in a
pan of water. Carefully cut away the peristome with scissors and
remove the shell of the oral body wall on one side of the peristome
without disturbing the organs within. Observe the following
systems of organs :

The Digestive System. This is quite different from the same
system in the starfish. The mouth opens into the oesophagus,
which passes through the center of the large, cone-shaped dentary
apparatus, which is also, because of its shape, called Aristotle's
lantern. This is a complicated structure consisting of a number of
calcareous plates and muscles which project from the mouth into
the body cavity. Study its muscular attachment with the shell.
Note the protractor muscles, which pass from its upper end to the
oral body wall, by means of which the apparatus can be thrust
down and partly out of the mouth; note also the retractor
muscles, which pass from the lower part of it to the tall inner
projections of the shell.

Exercise 5. Draw a diagram showing the dentary apparatus in the
body and its muscular attachment to the shell.

The oesophagus, after leaving the dentary apparatus, passes to
the elongated intestine ; this lies close to the body wall, to which
it is attached by means of a mesentery. Carefully follow the
intestine, breaking away the wall if necessary, as it winds around
the inner surface of the shell. Note the intestinal siphon, a short
tube which lies alongside the intestine, joining it at both ends.
From the intestine a short rectum passes to the anus. In mak-
ing this dissection keep the animal immersed in clear water ;

122 PHYLUM ECHINODERMATA

remove as little of the shell as possible, and do not remove any
of the organs from the body.

The genital system is similar to that in the starfish. The sexes
are separate, but the sexual glands of the male and female are alike
in appearance. They consist of five radial, granular masses, which
lie in the upper part of the body cavity, each mass communicating
with the outside through one of the genital pores. The actual
extent of the sexual glands depends upon the sexual condition of
the animal. During the breeding season, in the summer, they
may almost fill the body cavity.

Exercise 6. Make a diagrammatic drawing of the digestive and
reproductive systems and label all their parts.

Remove the dentary apparatus from the body and examine it
carefully. It is made up principally of five triangular plates called
alveoli, the lower ends of which bear the teeth. The alveoli are
bound together by short muscles. The base of the dentary ap-
paratus is made up of a complicated system of smaller plates.

Exercise 7. Make a drawing of the dentary apparatus.

The ambulacral system is similar to that of the starfish. A ring
canal surrounds the oesophagus just inside the inner end of the
dentary apparatus and is connected with the madreporite by
means of the stone canal. This organ is a small tube which lies in
contact with the oesophagus and also, in the neighborhood of the
aboral body wall, with the rectum. From the ring canal five ra-
dial canals pass along the median lines of the rays to their aboral
ends, sending off branches to the ambulacral feet. The entire sys-
tem of tubes, except the ambulacral feet, is within the body cavity,
instead of outside it, as in the starfish. Look on the inner sur-
face of a ray for the radial canal. On each side of it observe the
row of small vesicles, the ampullae, which are the reservoirs of the
ambulacral feet. Determine the exact relation of the ampullae to
the feet, and of both to the ambulacral pores in the shell. It will
be seen that there are two rows of these latter on each side of the
radial canal. Through one of these rows the branch canals pass
from the radial canal to the ambulacral feet on the outside of

A SEA URCHIN 123

the shell ; through the other row projections of the feet pass back
into the body cavity, where they expand to form the ampullae.
There is thus a single row of feet on each side of the radial canal
in each ray.

The ambulacral system will be seen to consist of a system of
tubes extending throughout the body and communicating with
the sea water through the madreporite. It is filled with a fluid
which, as in the starfish, is not pure sea water, but is rather a
watery serum in which float amoeboid cells. This fluid can be
driven into the ambulacral feet, which acquire rigidity and are
thereby extended. The animal moves by extending the feet, at-
taching the sucker disks at their ends to some stationary object,
and then drawing them in ; it is able thus to pull itself slowly
along. Some sea urchins with long spines also move on the tips
of their oral spines as on stilts.

Exercise 8. Draw a diagram representing the ambulacral system.

Respiration and excretion are performed by the peristomial
gills and the ambulacral feet.

The nervous system cannot be studied in a dissection. It is
essentially like that of the starfish. From the ring nerve around
the oesophagus five radial nerves pass along the median line of
the rays to the ocular plates, where each terminates in a tentacle.
The entire system is within the body cavity instead of a portion
of it being outside as in the starfish.

124 PHYLUM ECHINODERMATA

Class : Holothurioidea
A SEA CUCUMBER

Two common species of holothurians which Uve in the shallow
water of the New England coast are suitable for this dissection :
Thyone briareus and Cucumaria frondosa, the former being com-
mon in Vineyard and Long Island Sounds, and the latter on the
Maine coast. The latter is the larger of the two species, being from
ten to twenty-five centimeters or more long and about half as
thick, and differs from Thyone, among other things, in having
ambulacral feet in the radial areas alone ; thus it possesses five
broad bands of these appendages, the interradial areas being
smooth. Thyone is from eight to about twenty centimeters long
and five centimeters thick, and is covered with ambulacral feet,
the five broad radial bands meeting in the interradial areas. It
will be used as the basis of this dissection.

Observe the form and color of the body. Note that the upper
and lower sides are distinctly differentiated, the former having
fewer feet than the latter. In the center of the forward end is
the mouth, surrounded by ten branched tentacles, by means of
which the animal collects the minute organisms which constitute
its food. Above the mouth and between the bases of two ten-
tacles is the genital pore. In the center of the hinder end is the
anus, surrounded by five anal teeth. The main longitudinal axis
of the body (that which joins the mouth and the anus) will thus
be seen to be very long, in sharp contrast to that of the starfish
and sea urchin, in which it is much shorter. In consequence of
this feature the body is elongate and more or less wormlike, and
the animal does not rest on the oral^ surface but on its side.

Exercise 1. Draw a side view of the animal and label carefully all
the features observed.

With strong scissors cut the body open by a longitudinal slit
along the underside. Place it in a pan of water and with large
pins fasten down the two sides of the body wall on the right and
left. Without cutting any of them, study the internal organs.

A SEA CUCUMBER 125

Note the spacious body cavity and the long, coiled intestine
which partly fills it. At the front end of the body and just behind
the tentacles is the conspicuous calcareous ring, a more or less
rigid cylinder containing five radial and five interradial plates,
which surrounds the oesophagus. Projecting from the hinder end
of the calcareous ring will be seen two large, cylindrical sacs, the
Polian vesicles, and inserted in the ring are five prominent re-
tractor muscles, by means of which the tentacles and the forward
end of the body can be invaginated. Note the position of the
genital gland and its duct. The gland is a thick bunch of slender
filaments in the forward part of the body cavity, joined with the
body wall by a mesentery, which converge to the hinder end of
the duct. This is a slender tube which passes forward on the
upper side of the body cavity and opens to the outside between
two tentacles on the upper side of the body. The sexes are sepa-
rate ; they are alike, however, in appearance. Note the respira-
tory trees, the profusely branched organs which extend from the
rectum throughout the entire length of the body cavity.

Study the course of the digestive tract. The oesophagus passes
through the calcareous ring to the thick- walled, muscular stomach,
which lies directly behind it. From the hinder end of the stomach
the long, thin-walled intestine passes, with many loops and turns,
to the short, wide rectum at the hinder end of the body. Trace
the intestine carefully, but without cutting it, throughout its en-
tire course and note the three mesenteries by which it is attached
to the body wall.

Note the thick, muscular walls of the rectum and the muscle
strands which join it with the body wall. The two branching
respiratory trees spring from the forward end of the rectum and
extend through the body cavity to its forward end. Observe care-
fully their extent. It is by the periodic dilation and contraction
of the muscular walls of the rectum that water is alternately taken
into the rectum and the respiratory trees and expelled from them,
and respiration thus carried on.

Exercise 2. Draw a diagrammatic view of the opened animal on a
large scale, showing the organs observed. Carefully label all.

126 PHYLUM ECHINODERMATA

Cut the oesophagus just back of the calcareous ring and remove
the digestive tract with the respiratory trees and the genital
gland from the body. The five retractor muscles of the calca-
reous ring may be followed to the body wall, where they will be
seen to join five longitudinal muscles which extend along the
inner surface of the body wall the length of the body. These
muscles mark the five radial areas of the body. Note the circular
muscles which also lie on the inner surface of the body wall.

Study the ambulacral system. The ring canal, which is often
difficult to see, surrounds the oesophagus at the hinder end of
the calcareous ring. The Polian vesicles, two elongated sacs which
have already been noted, extend from it into the body cavity.
They secrete and store the lymphatic fluid which fills the ambu-
lacral canals. The stone canal, which also extends from the ring
canal, is a slender tube, and the madreporite with which it ends
lies in the body cavity and not at the surface of the body. It is
probably a rudimentary structure. The five radial canals extend
from the ring canal along the radial areas of the body and beneath
the five radial muscle bands to the hinder end of the body. The
ambulacral feet extend from the radial canals through the body
wall and are seen thickly studding the outer surface of the body.
The numerous ampullae extend into the body cavity and will be
seen on the inner surface of the body wall. The tentacles are
also a part of the ambulacral system, the canals which supply
them with the ambulacral fluid springing from the radial canals.

Exercise 3, Draw a diagram of the ambulacral system.

Exercise 4. Draw a diagram representing an ideal cross section
through the middle of the body.

The calcareous spicules and plates which form a part of the
body wall and are so characteristic of the echinoderms as a group
are less conspicuous in holothurians than in any of the other
classes of the phylum, and will not be studied in this dissection.
They are of minute size, although definite in shape in the various
species, and do not form a connected skeleton as in the starfish
and the sea urchin. Pedicellariae are absent.

PHYLUM CCELENTERATA

Class : Hydrozoa
HYDRA

This small animal is of general although sporadic distribu-
tion throughout the country. It frequents ponds and streams
abounding in plant life and is best obtained by collecting water
together with water plants, sticks, and other objects from several
such places, and allowing it to stand in glass jars. The polyps
will, if present, be seen, after an interval of some time, attached
to the stems or leaves of the plants or to the sides of the jar.
They may be kept indefinitely in aquaria of this sort and will
usually multiply rapidly. Three species of Hydra are common in
this country : the brown or reddish hydra {Hydra oli gactis) , y^hich.
has tentacles much longer than the body, the green hydra (Hydra
viridissima) , and the gray hydra {Hydra vulgaris), both of the
latter species having relatively short tentacles.

Hydra is a slender, tubular animal from one eighth to one
half an inch in length ; it attaches itself by one end to some sta^
tionary object and projects pendant in the water ; at the other
end is the mouth, surrounded by from four to eleven long, thread-
like tentacles. It does not attach itself permanently to one place,
but can crawl about or swim slowly through the water. Its food
consists largely of small crustaceans which it kills or paralyzes
with the peculiar stinging organs called nettle cells, located
principally in the tentacles.

Study the animals first, if possible, without disturbing them
and with the aid of a hand lens. Note the extreme contractility
of the body. Look for individuals with distended bodies. These
have just swallowed prey. Look for budding individuals ; bud-
ding is one method of reproduction. Observe the color of the

animal.

127

128 PHYLUM CCELENTERATA

Detach a polyp by means of a pipette, place it in a watch glass
of water, and study it under a microscope.

Exercise 1. Draw several outlines of the animal on a large scale,
showing different shapes and positions it can assume. Notice the
mouth in the midst of the tentacles.

Place a polyp on a slide in water and cover it with a cover
glass, but support the corners of the latter with wax to avoid
crushing the animal. Study its structure under the microscope.
We see that the type of structure is radial and not bilaterally
symmetrical ; that the animal is tubular in shape, and that its
internal cavity opens to the outside through the mouth; that
the mouth is a small opening at the end of the conical, terminal
portion of the body, called the hypostome, at the base of which
are the tentacles. The internal cavity is called the gastrovascu-
lar space and is the common digestive and circulatory cavity of
the animal. The end by which the animal is attached is called
the foot. It is an adhesive disk containing gland cells which pro-
duce a sticky secretion.

Examine the finer structure under a high power of the micro-
scope. The body wall is made up of two layers of cells, the outer
ectoderm and the inner entoderm. The cells of the latter are
much longer and more irregular than those of the ectoderm ; their
inner surfaces are amoeboid and also often flagellate. Embedded
in the entoderm cells are the chromatophores, the bodies which
give color to the animals. In the green hydra these are chloro-
phyll bodies ; in the brown hydra they consist of a substance
similar to chlorophyll. In both cases- they are probably single-
celled algae living symbiotically with the polyp. Between these
two cell layers is a thin noncellular one called the supporting
layer. The tentacle is a hollow projection of the body wall and
has the same structure.

Among the ectoderm cells of the distal portion of the body, and
especially of the tentacles, notice the oval, highly refractive sting-
ing organs, or nematocysts. Each one consists of a spiral, thread-
like tube, with several barbs at its base, which lies coiled within
the cavity of a cell called a cnidoblast. The cavity is filled with a

HYDRA 129

poisonous fluid ; its walls form an ovoid sac, of which the tube is
the very much elongated and invaginated outer end. A minute
spine projects beyond the free surface of the cnidoblast into the
water. When the surface of the ectoderm is irritated the tube is
evaginated and shot violently out, and the poisonous fluid con-
tained in the cavity of the nematocyst is injected into any animal
that may be struck.

Exercise 2. Make a large semidiagrammatic drawing of the animal
showing the details of its structure ; label all carefully.

Methods of Reproduction. Hydra reproduces both sexually and
asexually. Well-fed polyps will soon begin to bud off new indi-
viduals. The bud makes its appearance first as a projection of the
body wall, and soon becomes a distinct branch. Tentacles and a
mouth make their appearance at the extremity of the branch, and
the young polyp is complete. It remains attached to the parent
for a while ; then detaches itself and begins an independent life.

Besides this asexual method of reproduction, which may go on
as long as the animal is well fed and vigorous, reproduction by
sexual methods also occurs at more or less irregular intervals.
Sexual organs appear in the form of projections of the ectoderm
of the body wall. Two classes of these projections appear : smaller
ones, which are testes, and larger ones, which are ovaries. The
former of these organs, which lie near the distal end of the animal,
produce spermatozoa ; the latter are near the proximal end, and
each produces a single large ovum. The green hydra is a her-
maphroditic animal, having both testes and ovary when sexually
active, whereas the brown hydra is dioecious, being either male or
female.

Exercise 3. Find a polyp with reproductive organs and make a
drawing of it.

Special respiratory, excretory, digestive, and circulatory organs
are absent in the hydra. Respiration and excretion are carried on
through the surface of the body wall. Digestion, circulation, and
absorption go on within the gastro vascular space. The animal's
prey is caught in the water with the tentacles, which sting it into

130 PHYLUM CCELENTERATA

insensibility, and then swallowed into the gastrovascular space.
The entoderm cells, which line this cavity, extend their very
plastic inner ends toward or about it, and some of them secrete a
digestive fluid. The object is then digested and mingles with the
water present in the gastrovascular space, while by the vibrations
of the fiagella currents are produced which cause this fluid to cir-
culate throughout all the parts of it.

Distinct muscular and nervous systems are absent in the hydra.
Delicate muscle fibers are present, however, in the form of long,
parallel projections of the inner ends of ectoderm cells. Nervous
elements are also present in the form of isolated ganglionic cells
situated in the ectoderm, which send delicate projections to the
muscle fibers and to the nematocysts. Both muscle fibers and
ganglion cells are present throughout the animal's body but are
most numerous in the tentacles. There are no organs of special
sense.

A TUBULARIAN HYDROMEDUSAN 131

Class : Hydrozoa. Order : Anthomedusm
A TUBULARIAN HYDROMEDUSAN (PENNARIA)

Pennaria is a marine animal and one of the commonest hydro-
medusans along our coast. As is characteristic of the group to
which it belongs, it exhibits the phenomenon of alternation of
generations. Two generations of individuals, a sexual and an
asexual, alternate with each other. The latter is called the hydroid
generation ; the animal in this stage is sessile and colonial, and
produces by budding, that is, by asexual methods, the sexual gen-
eration. This is called the medusoid generation ; in it the animal
either remains attached to the hydroid colony and is then called
a sporosac, or separates itself and becomes a free-swimming
jellyfish, which is called a medusa ; in either case the medusoid
produces by sexual methods embryos which, after a brief free
hfe, attach themselves to fixed objects and develop into the
hydroid generation.

The Hydroid Stage in Pennaria.^ In this stage these animals
form branching colonies of polyps, which are attached to the rocks
and seaweed in shallow water. The colonies are several inches
in length, and are found in thick clusters which often cover the
rocks over small spaces ; their color is a delicate pink.

Place a small portion of a colony in a watch glass of water
or alcohol, and study it under the microscope. Observe the
main stem of the colony and its branches, also the position of
the polyps on the branches. Note carefully the differences in
size between the different polyps. Which is the largest polyp?
Study the method of branching. Has the colony a main axial
stem? If it has, the oldest polyp will be the one at the tip of
the stem and the second oldest will be the one at the end of the
lowest branch.

The stem of the colony together with the branches is called the
hydrocaulus ; the rootlike projections by which it is attached at
its base are the hydrorhiza ; the polyps are called hydranths.

1 Bougainvillea or any other tubularian can also be used, with slight changes in
the directions.-

132 PHYLUM CCELENTERATA

Exercise 1. Draw a diagram representing the method of branching
and showing the arrangement of the polyps on the stem.

Mount a small branch of the colony on a slide in water and
study a number of hydranths of all sizes. Note the radial type
of structure and the tubular body, the internal cavity of which
opens to the outside through the small terminal mouth. The stem
also and its branches have an internal cavity which is a contin-
uation of that of the hydranth. The cavity of the hydranth and
the stem is called the gastrovascular space and is the common
digestive and circulatory cavity of the animal. Notice the ringed
constrictions in various parts of the stem, especially near the
polyps ; they give the stem strength and flexibility.

Study the two kinds of tentacles, the whorl of larger ones-around
the base of the hydranth and the shorter ones on the body of it.
In Bougainvillea the basal tentacles alone are present. Count the
basal tentacles. Count the short tentacles on a large and then on
a small hydranth. The larger hydranth will be found to have more
than the smaller one. Notice the small hydranth buds on the side
of the stem ; find one whose tentacles have not yet developed.
Some of the hydranths will be seen to have large ovoid projections
of variable size on their sides. These are the medusoid buds,
which become either free-swimming medusae or sessile sporosacs.
In Bougainvillea the medusoid buds do not appear on the hy-
dranths but on the stem ; they become free-swimming medusae.

Exercise 2. Make a semidiagrammatic sketch of a large hydranth
and a portion of the stem on a large scale ; label the different parts.

Exercise 3. Make a sketch on a large scale of the smallest hydranth
or hydranth bud you can find.

Mount a hydranth and a part of the stem on a slide in dilute
glycerine, and study their finer structure. First study the struc-
ture of a tentacle. It is not hollow as is the tentacle of Hydra,
but is made up of an axis consisting of a single row of large
entoderm cells around which is a layer of small ectoderm cells.
Between these two cell layers is the delicate, noncellular supporting
layer. Find the highly refractive stinging organs, or nematocysts,
at the end of the tentacle. These are the organs with which the

A TUBULARIAN HYDROMEDUSAN 133

animal kills its prey. Each one consists of a spiral, threadlike
tube, with several barbs at its base, which lies coiled within the
cavity of a cell called the cnidoblast. The cavity is filled with a
poisonous fluid ; its walls form an ovoid sac, of which the tube is
the very much elongated and invaginated outer end. A minute
spine projects beyond the free surface of the cnidoblast into the
water. When the surface of the ectoderm is irritated the tube is
evaginated and shot violently out, and the poisonous fluid con-
tained in the cavity of the nematocyst is injected into any animal
that may be struck. Look for nematocysts which have discharged
their spiral threads.

Exercise 4. Draw the distal portion of a tentacle showing its cellular
structure; show the nematocysts at the end, including several
which have been discharged.

Study the structure of the wall of the hydranth. It is made up
of an outer ectoderm and a much thicker inner entoderm, each
composed of a single layer of cells ; the inner ends of the ento-
derm cells are amoeboid and often flagellate, the function of the
flagella being to maintain in circulation the fluids in the gastro-
vascular space ; between these two layers is the thin, noncellular
supporting layer. Study the structure of the stem. Observe
its central cavity, which is a part of the gastrovascular space,
and the three layers just mentioned. In live specimens notice
the action of the flagella. Notice the cuticula which covers the
outer surface ; it is not found in the hydranth. This cuticula is
called the perisarc. It is a supporting structure and gives the
colony rigidity.

Exercise 5. Make a drawing showing the cellular structure of the wall
of the hydranth and of the stalk so far as observed; carefully
label all.

Special respiratory, excretory, digestive, and circulatory organs
are not present in the hydroid. Respiration and excretion are
carried on through the surface of the body wall. Digestion,
circulation, and absorption go on within the gastrovascular
space. The polyps feed upon small swimming animals, which
they kill or stun with their nematocysts and then swallow into

134

PHYLUM CCELENTERATA

the gastrovascular space. Digestion goes on within this space,
and waste matters are ejected from the mouth. The products
of digestion mingle with the water present in the gastrovas-
cular space and circulate throughout the colony, the internal
cavities of all the individual polyps of a colony being in com-
munication with one another. When a colony is well fed it grows
rapidly, new hydranths bud out from the stalk, and medusoid
buds appear and grow into medusae. The hydranths are fre-
quently destroyed by frost or by the beating of waves or by
fishes, but new ones quickly grow in their places.

The Medusoid Stage. The medusoids of tubularian hydro-
medusans are either sessile sporosacs or free-swimming medusae.
Pennaria produces both kinds. During a greater part and in
some cases the whole of the year they remain attached to the
colony and are thus sporosacs, but usually during the middle and
last of the summer they detach themselves from the hydroid and
become free-swimming medusae. Both conditions may be found
in the same colony and at the same time. The medusoids of
Bougainvillea are always free-swimming and with other medusae
will be found in the surface waters of the ocean. They may
usually be easily obtained by placing the live hydroid colony in
a small glass of sea water ; the medusae will be found swimming
about in the water.

Place several medusae of Bougainvillea or of any other tubu-
larian in a watch glass of sea water or, if they are preserved
specimens, in alcohol. If they are alive, observe the swimming
motions. Note the radiate type of structure. The body is bell-
shaped, having an outer convex and an inner concave side, and
on its margin are tentacles (in the medusa of Pennaria they are
rudimentary) . The convex side is called the exumbrella or aboral
side, and the concave, the subumbrella or oral side. In the center
of the latter is the proboscis-like projection called the manubrium,
at the distal end of which is the mouth, surrounded by short
oral tentacles. The mouth opens into the gastrovascular space,
which comprises the space within the manubrium and also a
system of canals in the bell-shaped body. These canals consist
of four radial tubes, which extend from the base of the manu-

A TUBULARIAN HYDROMEDUSAN 135

brium to the periphery of the body, and are there united by a
circular tube, which runs parallel with the margin and close to it.
Observe the arrangement of the marginal tentacles, and also of
the oral tentacles, if these are present. At the base of the groups
of marginal tentacles are minute sense organs, the ocelli ; they
are characterized by the presence of pigment and are sensitive
to light. Note the four swellings on the side of the manubrium ;
these are the sexual organs and are specialized portions of the
ectoderm. The sexes are separate. Around the inner margin of
the subumbrella, at the base of the tentacles, is a broad, muscular
membrane extending inward and called the velum, by means of
which the animal swims.

Exercise 6, Make a diagrammatic sketch of the medusa as far as
observed and label all its parts.

The medusa is a more highly specialized form than the polyp,
although they are homologous forms and are essentially alike in
structure. The different vegetative functions are carried on in
the medusa as they are in the hydranth. The medusa being a
free-swimming animal, however, its muscular and nervous systems
are much more highly developed than the same systems are in the
hydranth. In the latter the only muscles present are delicate
fibers, elongated projections of the inner ends of ectodermal cells,
which cause movement in the tentacles and the body of the hy-
dranth, and the nervous system is represented only by scattered
ganglion cells, which lie among the ectoderm cells. In the medusa
the velum is the principal organ of locomotion. It contains bands
of ectodermal muscle fibers, by the contraction of which the mo-
tion of the umbrella is produced which propels the animal through
the water. The nervous system consists of a double nerve ring
of ganglion cells and fibers running around the margin of the
disk, from which delicate fibers run to the velum and to the
sense organs.

136 PHYLUM CCELENTERATA

Class : Hydrozoa. Order : Leptomedusce

A CAMPANULARIAN HYDROMEDUSAN (OBELIA OR
CAMFANULARIA)

These are very common marine animals which Hve in the
shallow water along our coast. In common with other members
of the group they exhibit the phenomenon of alternation of gen-
erations. Two generations of individuals, a sexual and an asex-
ual, alternate with each other. The latter is called the hydroid
generation ; the animal in this stage is sessile and colonial and
produces by budding, that is, by asexual methods, the sexual gen-
eration. This is called the medusoid generation ; in it the animal
either remains attached to the hydroid colony (Campanularia)
and is then called a sporosac, or separates itself (Obelia) and
becomes a free-swimming jellyfish, which is called a medusa;
in either case the medusoid produces by sexual methods embryos
which, after a brief free life, attach themselves to fixed objects
and develop into the hydroid generation.

The Hydroid Stage. While in this stage these animals form
branching colonies, which are attached to seaweed, rocks, and
other objects. Place a small portion of a colony in a watch glass
in water or alcohol, and study it under the microscope. Observe
the differences in size between the different polyps, as well as their
position on the stem. Determine the method of branching. Has
the colony a main axial stem? If not, which is the oldest polyp?
Notice the ringed constrictions in various parts of the stem, espe-
cially near the polyp ; they give the stem strength and flexibility.

The stem of the colony together with the branches is called the
hydrocaulus ; its rootlike projections by which it is attached at
its base are the hydrorhiza. Observe that there are two distinctly
different kinds of polyps instead of but one, as in Pennaria or
Bougainvillea : (i) the feeding polyp, or hydranth, which is the
more numerous and bears tentacles, and (2) the reproductive
polyp, or blastostyle, which is a modified hydranth and is much
the larger and the less numerous and bears no tentacles. Notice
that the perisarc, the transparent cuticular covering of the stem,

A CAMPANULARIAN HYDROMEDUSAN 137

does not end at the base of the polyp, as is the case in the tubu-
larian hydroid, but is continued over the polyp, inclosing it as in
a cup. It is thus a protective device and is called in the case of
the hydranth the hydrotheca, and in the case of the blastostyle
the gonotheca. The feeding polyp withdraws within its hydro-
theca for protection when alarmed. The reproductive polyp never
emerges from its gonotheca, which is a closed structure, but the
medusoids or their sexual products escape into the surrounding
water through an opening in the gonotheca's free end.

Exercise 1. Draw a diagram representing the method of branching
of the colony and the arrangement of the polyps.

Mount a portion of a branch with several hydranths in water or
dilute glycerine. Study an expanded hydranth. We note the
radial type of structure and the tubular body, the internal cavity
of which opens to the outside through the terminal mouth ; also
that the stem has a cavity which is continuous with that of the
hydranth. The internal cavity of the hydranth and of the stem
is called the gastrovascular space and is the common digestive
and circulatory cavity of the animal. The portion of the hy-
dranth between the base of the tentacles and the mouth is called
the hypostome. Count the tentacles.

Exercise 2. Make a semidiagrammatic sketch of the expanded

hydranth on a large scale and label all its parts.
Exercise 3. Make a sketch of a contracted hydranth.

Study the finer structure of an expanded hydranth. First
study the structure of a tentacle. It is made up of an axis con-
sisting of a single row of large entoderm cells, around which is a
layer of small ectoderm cells. Between these two cell layers is
the delicate, noncellular supporting layer. Find the highly re-
fractive nematocysts in the tentacle. These are the stinging or-
gans with which the animal kills its prey. Each one consists of a
spiral, threadlike tube, with several barbs at its base, which lies
coiled within the cavity of a cell called the cnidoblast. The cavity
is filled with a poisonous fluid ; its walls form an ovoid sac, of
which the tube is the very much elongated ^nd invaginated outer

138 PHYLUM CCELENTERATA

end. A minute spine projects beyond the free surface of the cni-
doblast into the water. When the surface of the ectoderm is
irritated the tube is evaginated and violently shot out, and the
poisonous fluid contained in the cavity is injected into any animal
that may be struck. Look for nematocysts which have discharged
their spiral threads.

Exercise 4. Draw the distal portion of a tentacle showing its cellular
structure; show the nematocysts at the end, including several
which have been discharged.

Study the finer structure of the wall of the hydranth. It is
made up of an outer ectoderm and a much thicker inner entoderm,
each composed of a single layer of cells ; the inner ends of the
entoderm cells are amoeboid and often flagellate, the function of
the flagella being to maintain in circulation the fluids in the gastro-
vascular space ; between these two layers is the thin noncellular
supporting layer. The hydrotheca incloses aU, but it is not in con-
tact with the ectoderm. Study the structure of the stem ; it has
essentially the same structure as the hydranth ; note the outer
cuticular covering, the perisarc. Note the action of the flagella in
a live specimen.

Exercise 5. Make a drawing showing the cellular structure of the wall
of the hydranth and of the stem.

Study a blastostyle. We note that it is a cylindrical object
inclosed within its transparent gonotheca. Budding out on the
sides are the young disk-like medusae, those toward the free end
being the largest and the oldest. The blastostyle has no tentacles
and no mouth. It has an internal cavity which is a part of the
gastrovascular space of the colony, and within which the nutri-
tive fluids circulate.

Exercise 6. Make a drawing of a blastostyle.

Special respiratory, excretory, digestive, and circulatory organs
are not present in the hydroid. Respiration and excretion are car-
ried on through the surface of the body wall. Digestion, circula-
tion, and absorption go qu within the gastrovascular space. The

A CAMPANULARIAN HYDROMEDUSAN 139

colony lives upon small swimming animals, which the feeding
polyps kill or stun with their nematocysts, and then swallow into
the gastrovascular space. Digestion goes on within the feeding
polyps ; the products of digestion mingle with the water present
in the gastrovascular space and circulate throughout the colony.
The entire colony is thus nourished, and if conditions are favor-
able it will grow rapidly and produce a large number of medusae.
The polyps are frequently destroyed by frost or by the beating
of waves or by fishes, but new ones quickly grow in their places.

The Medusoid Stage. The medusoids of campanularian hydro-
medusans are either sessile sporosacs or free-swimming medusae.
The medusae are minute disk-shaped jellyfishes, about one eighth
of an inch in diameter, which may be found swimming in the sur-
face waters of the ocean. Place several in a watch glass of sea
water or, if they are preserved specimens, in alcohol. If they are
alive, observe the swimming motions. Note the radiate type of
structure. The body resembles an umbrella in shape, having a
convex and a concave side, and is bordered by a fringe of tentacles.
The former is called the exumbrella or aboral side, the latter, the
subumbrella or oral side. In the center of the latter side is the
proboscis-like projection called the manubrium, at the distal end
of which is the mouth. This opens into the gastrovascular space,
which comprises the space within the manubrium and also a sys-
tem of canals in the disk-like body. These canals consist of four
or more radial tubes, which extend from the base of the manu-
brium to the periphery of the disk, and are there united by a cir-
cular tube which runs parallel with the margin of the disk and
close to it.

Count the marginal tentacles. At the base of certain of the ten-
tacles are minute sense organs, called lithocysts, which are prob-
ably organs of equilibrium. Find them.

Near the middle of each radial tube notice a prominent swelling
on the subumbrella. These are the sexual glands and are special-
ized portions of the ectoderm. The sexes are separate in medusae,
but the sexual glands have the same appearance in the two sexes.

Around the inner margin of the subumbrella, at the base of the
tentacles, is a muscular membrane extending toward the manu-

140 PHYLUM CCELENTERATA

brium and called the velum, by means of which the animal
swims. In campanularian medusae it is often very narrow and
not easily seen; in tubularian medusae it is broad and very
noticeable.

Exercise 7. Make a diagrammatic sketch of a medusa and label all
its parts.

The medusa is a more highly specialized form than the polyp,
although they are homologous forms and are essentially alike in
structure. The manubrium of the medusa and the hypostome of
the polyp do not differ essentially from each other ; the tentacles
are also homologous structures. Consequently the exumbrella of
the medusa corresponds to the base of the polyp, and just as
the latter is attached to the stem at its base, so the medusa is
attached to the blastostyle by its exumbrella. The digestive,
excretory, respiratory, and circulatory functions are carried on
in the medusa as they are in the hydranth. But because the
medusa is a free-swimming animal, its muscular and nervous
systems are much more highly developed than are the same
systems in the hydranth.

In the latter form the only muscles present are delicate fibers,
elongated projections of the inner ends of ectodermal cells, which
cause movement in the tentacles and the body of the hydranth,
and the nervous system is represented only by scattered gan-
glionic cells, which are also of ectodermal origin. In the medusa
the velum is the principal organ of locomotion. It contains bands
of ectodermal muscle fibers, by the contraction of which the mo-
tion of the umbrella is produced which propels the animal through
the water. The nervous system consists of a double nerve ring
which runs around the margin of the disk and from which delicate
fibers pass to the velum and the sense organs.

A MEDUSA 141

Class : Hydrozoa. Order : Trachomedusm
A MEDUSA (GONIONEMUS)

This animal is a better form to study, on account of its larger
size, than the minute tubularian or campanularian jellyfish. It
is a very common medusa at Woods Hole, but its range of distri-
bution is very limited, although it has also been found in Long
Island Sound.

Place the medusa in a small dish of water, which should be set
upon a dark background. The water should be deep enough to
permit the jellyfish to be readily turned over. If it is alive, study
the pulsations of the bell, by means of which it swims. With a
simple lens or a compound microscope study its form and color.
Note the radiate type of structure. Unlike the bilaterally sym-
metrical animals, the medusa has no dorsal, ventral, anterior, or
posterior side.

The outer, convex surface of the bell-shaped body is called the
exumbrella or the aboral side, and the concave underside is called
the subumbrella or the oral side. From the center of the latter
extends a large, dark-brown projection called the manubrium, at
the distal end of which is the mouth, surrounded by four recurved
lips. At the base of the manubrium is the stomach, a four-sided
sac from the four corners of which the four straight radial canals
extend to the periphery of the body, where they are united by the
ring canal, which runs around the margin of the bell. The radial
and ring canals, together with the stomach and the cavity of the
manubrium, form the gastrovascular space, the entodermal lining
of which is colored brown.

Directly beneath the four radial canals and projecting slightly
into the subumbrella space are the four reproductive organs,
which are also brown in color and present a corrugated appear-
ance. The sexes are separate, but the animals are not dimorphic.

Observe the number and arrangement of the tentacles, of which
an adult medusa possesses from sixty to eighty. Note the spiral
arrangement of the nettle cells on each tentacle, and also the ad-
hesive pad near its outer end. It is by means of the nettle cells in

142 PHYLUM CCELENTERATA

these pads that the animal anchors itself to seaweeds and other
objects when at rest. Note the exact point above the margin of
the bell where the tentacles are inserted. In the basal portion of
each tentacle is a conspicuous, pigmented body ; this is a hollow
bulb which is connected with the ring canal. Between the ten-
tacles are the lithocysts, mmute projections from the margin of
the bell which are probably equilibrial in function.

Observe the velum, the membrane which extends around the
inner margin of the bell toward the manubrium. It is the prin-
cipal organ of locomotion and contains bands of ectodermal
muscle fibers by the contractions of which the motion of the bell
is produced which propels the animal through the water. Similar
bands of muscle fibers are also present in both the subumbrella
and the exumbrella.

Exercise 1. Draw a semidiagrammatic view of the exumbrella on a
scale of from 5 to 10, showing the tentacles extended and all the
organs which have been observed.

Exercise 2. Draw a side view of the animal on the same scale, show-
ing the velum, the manubrium, and all the other organs observed.

Exercise 3. Draw a semidiagrammatic view of the subumbrella on
a scale of 5 to 10, showing the velum, the manubrium, and the
other organs observed.

The hydroid generation of Gonionemus is a minute, solitary
polyp which lives attached to the bottom in shallow water ; it
will not be studied here. The polyp is only about one millimeter
in height and has four tentacles which can be extended two milli-
meters. It is thus very much smaller than the medusa, which has
a height of about nine millimeters and a diameter of about twenty.
The polyp forms new polyps by budding, but has never been
observed forming the medusae ; so it is not known how these
originate.

A SEA ANEMONE 143

Class : Anthozoa. Order : Zoantharia
A SEA ANEMONE (METRIDIUM)

This animal, which is the largest sea anemone along the North
Atlantic coast, is often plentiful on rocks, shells, and docks in
shallow water. Place an expanded individual in a deep dish of
water and observe its shape, color, and method of attachment.
The upper end of the columnar body is called the disk ; in its
center is the elongate, sUtlike mouth, surrounded by the numer-
ous tentacles. The lower end of the animal is called the foot. It
is not permanently attached to the substratum ; the animal has
some locomotory powers and can slowly move from place to place.

Study the form of the mouth. Note the thickened lips at each
angle of the mouth ; these form a ciliated groove, called the si-
phonoglyph, through which the genital products reach the out-
side. In some individuals only one siphonoglyph is present.

Study the surface of the disk and the tentacles. The former is
frequently expanded and thrown into folds and lobes. The ten-
tacles are elongated diverticula of the disk and are hollow. They
are charged with nettle cells and are the principal organs of de-
fense and offense. They are also useful in feeding; after the
nettle cells have stung the small animals which constitute the
food of the sea anemone, the tentacles place them in its mouth.
The tentacles are not all the same size, those nearer the mouth
being the larger and the older.

Note the character of the columnar body. It is pierced by
small pores through which long, white, glandular threads, armed
with nettle cells and called acontia, may be thrust when the
animal is irritated.

Exercise 1. Draw the expanded animal, showing the column and the
disk, with the mouth and the tentacles.

Internal Anatomy. Cut the animal into halves by a longitu-
dinal incision passing at right angles to the mouth and from the
disk to the foot. The mouth will be seen to open into a flattened
tube with more or less corrugated walls, called the gullet. Note

144 PHYLUM CCELENTERATA

the formation of the siphonoglyphs. The gullet leads into the
gastrovascular space, which is the general internal cavity.

The most prominent structures in this cavity are the mesen-
teries, which are longitudinal partitions extending from the outer
wall of the body inward toward its center. These mesenteries will
be seen to occur in pairs ; six of these pairs, called the primary
mesenteries, join the body wall with the wall of the gullet. The
pair at each angle of the gullet which inclose the siphonoglyphs
between them are called the directives. Between the six pairs of
primary mesenteries are secondary, tertiary, and quartemary
pairs. The gastrovascular space is thus divided into a large
number of partially separated longitudinal chambers.

Note carefully the structure of the free edges of the mesenteries
below the gullet. The thickened, corrugated structure which forms
the edge is the mesenterial filament ; it contains digestive glands.
From the base of the mesentery extend the acontia. The repro-
ductive organs, the testes and ovaries, are also located in the
mesenteries, lying alongside the mesenterial filaments. The sexes
are separate and similar in appearance.

Note carefully the position of the longitudinal muscle bands,
one of which is present on the surface of each mesentery. It is
by means of these muscles that the body is contracted. A circu-
lar muscle in the disk closes the mouth by its contraction and aids
in drawing in the tentacles.

Exercise 2. Draw a semidiagrammatic view of the cut surface of the
animal, showing these features.

Make a cross section through the gullet and study the arrange-
ment of the mesenteries, the relation of the primary mesenteries
to the gullet, and the longitudinal muscles.

Exercise 3. Draw a diagram of the cross section, showing these
features.

Make a cross section through the body beneath the gullet.

Exercise 4. Draw a diagram of the cross section, showing the ar-
rangement of the mesenteries.

PHYLUM PORIFERA.>j.

Class: Calcarea v^ ^ \^

A SPONGE (GRANTIA)

Grantia is a noncolonial sponge which is common along the
New England coast. It is a small, cylindrical animal, about half
an inch in length, and occurs in small groups attached to rocks
or other objects below low-water mark.

Place several specimens in a watch glass of water, and study
their shape and external characters with the aid of a hand lens.
Observe the cylindrical body and at one end of it a small opening
surrounded by straight, needlelike spicules ; the opposite end is
the one by which the animal was attached. The opening is called
the osculum or excurrent opening. Notice the smaller spicules
and the openings of numerous minute pores which cover the sides
of the body. Growing out from the base of the larger individuals
may often be seen small ones, which in the course of time will
become independent animals. Note the evident radial symmetry
of the animal.

Exercise 1. Make a drawing of an animal on a scale of 5.

Split a dried sponge with a sharp knife into two equal halves
and study it under a dissecting microscope. Observe the large
central cavity. Large numbers of openings will be seen in its wall ;
they are the mouths of the radial canals, which are projections of
the central cavity into the body wall. Examine carefully the cut
edges of the body wall ; observe the radial canals, which are cut
longitudinally here. Notice also the shorter and less regular in-
current canals, which lie between the radial canals and open to
the outside through external incurrent pores. There are thus two
systems of canals in the body wall : (i) the radial canals, which
are a part of the central cavity, and (2) the incurrent canals,

14s

146 PHYLUM PORIFERA

which open to the outside. These two systems of canals com-
municate with each other by means of minute openings, so that
water which enters the incurrent canals from the outside through
the external incurrent pores passes freely into the radial canals,
and thence into the central cavity. From here it passes out
through the osculum.

Exercise 2. Make a semidiagrammatic drawing of the inner surface
of the body wall and the cut edge of the animal, showing the fea-
tures described above.

Isolate the spicules of a sponge by boiling a portion of it in a
caustic-potash solution. Mount some of them in water and ex-
amine them under a high power of the microscope. Find the
three different kinds of spicules : the long straight ones which
guard the osculum, the short straight ones which guard the ex-
ternal incurrent pores, and the triradiate ones which are within
the body wall and give it rigidity and firmness ; some of the latter
project into the central cavity.

Exercise 3, Draw an outline of each kind of spicule on a large scale.

Make thin sections of a sponge by placing it between two pieces
of elder pith or of cork, and shaving off the sections with a sharp
razor or scalpel. Obtain in this way cross, longitudinal, and tan-
gential sections. Mount them in water and study them under the
microscope. Slides of stained sections can also be used.

Study a cross section in which the canals have been cut longi-
tudinally. Observe the radial and the incurrent canals and their
relations to one another. Note the arrangement of the spicules
which guard the incurrent pores, also of those triradiate spicules
which project into the central cavity.

Exercise 4, Make a drawing of several canals showing these features.

Study a tangential section in which the canals appear in cross
section and study the arrangement of the triradiate spicules
around them.

Exercise 5. Make a drawing illustrating it.

A SPONGE 147

Specialized reproductive organs are not present in Grantia.
The sexual elements will be found in the form of large, spherical
bodies buried in the wall of the sponge. Fertilization takes place
here, and when development begins the young embryos escape
into the sea water through the canals. For a while the embryo is
a free-swimming animal, but it finally fastens itself to a rock and
develops into the adult sponge. Besides this sexual reproduction,
the sponge also reproduces asexually by budding. Each distinct
cluster of individuals probably represents the gemmated progeny
of a single individual.

Special respiratory, excretory, digestive, circulatory, nervous,
and locomotory organs are wanting in Grantia. Respiration and
excretion are carried on through the entire surface of the body.
The animal feeds on minute organisms and particles of organic
matter suspended in the water which streams into the canal sys-
tem through the incurrent pores. The radial canals are lined with
peculiar entoderm cells called collar cells, each one of which
possesses a flagellum. The action of the flagella produces the
current of water through the canals, from which the collar cells
obtain and ingest food particles. Circulation is from cell to cell.

PHYLUM PROTOZOA

Class : Infusoria
PARAMECIUM

Paramecium, often called the slipper animalcule, is one of the
commonest of the larger infusorians. It is a minute, single-celled
animal, being just on the limit of vision, and is often present in
standing water which contains decaying vegetable matter. It
may usually be obtained by permitting vegetable matter from a
pond to stand in water for a week or two. In shape it is an elon-
gated ellipsoid with a wide, slightly twisted, longitudinal groove,
called the oral groove, on one side ; the surface which contains
the groove may be called the ventral surface, and the opposite
surface, the dorsal. The animal is colorless and transparent, ex-
cept when it contains within its body colored food particles.

Mount a drop of water containing Paramecia and some decay-
ing matter together with a few strands of cotton on a slide, using
a large, thick cover glass, and study the animals under a low
power of the microscope. They will be seen swimming rapidly
about, but will gradually collect around the decaying matter or
be entangled in the cotton. If they do not become quiet in a
few minutes, it is because there is too much water under the
cover glass, and some of it should be withdrawn with a piece of
blotting paper. Care should be taken that the water does not
entirely evaporate.

Observe the unsymmetrical shape of the animal, and the dif-
ference between the anterior and the posterior ends. Notice the
rolling over of the animal as it swims through the water; the
peculiar spiral twist of the body is correlated with this motion,
but does not necessarily cause it, since the animal may at times
revolve in the direction opposite to that of the twist. It is in con-
sequence of this peculiar revolving motion that the animal is able

148

PARAMECIUM 149

to maintain a course through the water which is practically
straight. The great majority of swiftly moving animals are bi-
laterally symmetrical, and move in straight lines because of that
feature of their structure, but Paramecium, together with most
free infusorians, has an unsymmetrical form and consequently
would tend to move in circles without making progress, if it
were not for the revolution of its body on its long axis.

Exercise 1. Draw several simple outlines of the body showing its
shape as seen in different positions.

Exercise 2. Draw an outline of an ideal cross section through the
middle of the body.

Study the structure of the body, using a high power of the
microscope when necessary. Study the action of the hairlike,
vibratile cilia which cover the outer surface of the animal and
by means of which it moves. They are usually difficult to see in
the live animal because of their very rapid motion, but by vary-
ing the light and the focus of the microscope they will be brought
into view, and in the dead animal are plainly visible. Determine
the direction in which the cilia move. Are they all of the same
length? Note the delicate, transparent cuticula which covers the
body ; it appears as a highly refractive line.

The body has no internal cavity, and the protoplasm of which
it is composed is in two distinct layers, the ectosarc and entosarc.
The former is the thick, firm, transparent outer layer which, with
the cuticula, gives permanent shape to the body ; it often appears
obliquely striated. The entosarc is the semifluid, granular mass
which forms the remainder of the body. From near the anterior
end the oral groove runs obliquely along the ventral side of the
body to a point back of the middle, getting deeper as it goes. At
its inner end the groove becomes a closed tube, which extends into
the entosarc and ends with the mouth. Notice the trichocysts,
slender, radially arranged bodies which fill the ectosarc. They
are organs of defense, which remind one of the nematocysts of the
Coelenterata ; when the animal is attacked by an enemy or irri-
tated by a sharp reagent it discharges long, delicate bristles from
the trichocysts, which project beyond the cilia and protect them.

I50 PHYLUM PROTOZOA

They may also leave the body when the irritation is very severe
and form a protective halo of bristles surrounding the animal.

Observe the granular nature of the entosarc, and the spherical
food vacuoles within it. These are particles of food, usually com-
posed of bacteria or minute particles of other vegetable sub-
stances surrounded by water, which circulate within the semifluid
entosarc. Watch the entosarc closely, and observe the currents
in it. Determine the direction of the currents and whether the
direction is ever changed. The food vacuoles form at the inner
end of the oral groove, where the particles of which they are com-
posed have been swept by the cilia of the groove. Watch the
formation of them ; this may be done by putting grains of car-
mine or indigo into the water.

Observe the pulsating vacuoles. These are the excretory organs
of the animal. They are globular drops of clear liquid, two in
number, which appear near the aboral surface of the body, not
far from either end, and break through the ectosarc into the sur-
rounding water. They do not appear simultaneously, but alter-
nate with each other. When a vacuole has disappeared, radiating
canals of clear fluid gradually form about the spot where it was
located, bringing the fluid which is to form the next vacuole at
that end. Time the formation of the pulsating vacuoles ; how
many form in a minute ?

Observe the macronucleus, a large, ovoid structure near the
center of the body. At its side are either one or two minute
micronuclei, according to the species, Paramecium caudatum hav-
ing one and Paramecium aurelia two ; they may be seen if the ani-
mal is killed by adding a i per cent solution of acetic acid to the
water or a few drops of aceto-carmine solution.

Exercise 3. Make a large semidiagrammatic drawing of a Para-
mecium showing all these details, and label all.

Paramecium has no special vegetative organs except the pul-
sating vacuoles. Food vacuoles are taken into the entosarc
through the mouth. Here they circulate for some time, while
the water forming the vacuole is absorbed and the food parti-
cles that it contains are digested. The indigestible matters are

PARAMECIUM 151

collected at a spot just back of the mouth and are there ejected
from the body through a temporary opening in the ectosarc,
which forms for that purpose ; the water of the food vacuole is
collected in the pulsating vacuoles and ejected. Respiration is
carried on through the external surface of the body. The organs
of locomotion are the cilia, which are distributed evenly over the
surface of the body ; they are hairlike projections of the ectosarc
through pores in the cuticula. Sensation is exercised by the entire
surface of the body. Place a minute crystal of salt on the slide
and observe the reaction of the animals to it.

Reproduction is asexual, by division. A transverse constric-
tion appears in the surface of the middle of the animal's body and
deepens until it is divided in two. Each half becomes an inde-
pendent animal and grows to full size. Look among a large num-
ber of animals for one which is dividing.

A process which is universal among infusorians is conjugation.
Two individuals place the ventral surfaces of their anterior ends
together. In this position their bodies fuse together and an inter-
change of micronuclear matter takes place between them. The
two individuals then separate.

Conjugation was formerly supposed to be a process by which
weak and infertile animals renewed their strength and vitality.
It is now supposed to be rather a preparation for unfavorable
hfe conditions. The change in the structure of the micronucleus
leads to a change in the essential characters of the animals, and
thus gives them additional powers of environmental adaptation
and a better chance to survive unfavorable conditions.

Exercise 4. Look for dividing and also for conjugating individuals.
Observe them carefully and draw them.

152 PHYLUM PROTOZOA

Class: Infusoria
VORTICELLA

This infusorian differs from Paramecium in being a sessile ani-
mal, and in that the cilia are not equally distributed over all parts
of the body but are confined to certain parts of it. Vorticella and
its allies are often called bell animalcules. The animal consists
of a bell-shaped body at the end of a long stalk which is perma-
nently attached to some object in the water. Around the upper
and wider margin of the body is a row of large cilia. A deep
oral groove, which is also bordered by cilia, extends from the
margin toward the center of the animal and bears the mouth at
its inner end.

A number of genera of bell animalcules are found in both fresh
and salt water. Vorticella is noncolonial and possesses a con-
tractile spiral stalk. Carchesium and Zoothamnium are colonial
and differ from each other in that in the former each individual
animal contracts independently, whereas in the latter the entire
colony always contracts as a unit ; in both, the colonies are large
and easily visible, appearing often like white mold on the object
of attachment. Epistylis is colonial with a noncontractile stalk.
These animals are all common.

Mount a drop of water on a slide, together with some vegetable
or other substance to which Vorticella is attached, and study it
under the microscope. (Any other bell animalcule will do equally
well.) Observe the shape of the animal ; tap on the slide with a
pencil and cause it to contract ; note the marginal cilia and the
current they set up in the water ; find the oral groove and note
that the current in the water tends to sweep small objects into it.

Notice the partial radial symmetry of the animal ; this body
form is due to its sessile habit of life. Paramecium, which is a
rapidly moving animal, is not radially symmetrical. Can you
explain why a sessile organism tends to be radial ?

Exercise 1. Draw a careful outline of the expanded animal on a large
scale, and another of the contracted animal, and label the parts
mentioned above.

VORTICELLA 153

Study the structure of the body. It consists of a single cell, as
does Paramecium, and is composed of two protoplasmic layers :
the ectosarc, which is the firm external layer, and the entosarc,
the more fluid protoplasm of which the inner portion of the
animal is composed. Covering its outer surface is the cuticula,
which, with the ectosarc, gives the animal its permanent shape.
The stalk is a continuation of the ectosarc and of the cuticula.
Its inner portion alone, that is, the axis, is contractile; its
cuticula simply accommodates itself by assuming a spiral shape.
Note the longitudinal striations in the ectosarc at the base of
the bell.

Observe the granular nature of the entosarc and the spherical
food vacuoles within it ; note the circulation of the latter in the
granular protoplasm. Each food vacuole is composed of particles
of organic matter in a minute globule of water, which collect in
the oral groove and are then driven into the mouth. Watch the
formation of them ; this is done easily by placing grains of indigo
or carmine in the water.

Vorticella has a single pulsating vacuole, which is in the upper
part of the body. It is the organ of excretion of the animal and
consists of a globule of clear liquid which collects near the surface
of the body and is then discharged through the ectosarc into the
water. As in Paramecium, the water which is ingested as a part
of the food vacuoles is discharged through the pulsating vacuole
together with renal products. Time the formation of the pulsat-
ing vacuoles ; how many form a minute ?

Observe the macronucleus ; it is a narrow, elongated structure
and is easily seen ; near it is the small, spherical micronucleus.

Exercise 2, Make a large semidiagrammatic drawing of a Vorticella
showing these details, and label all.

Vorticella has no special vegetative organs except the pulsating
vacuole. The food particles which are ingested into the entosarc
are there digested, and waste matters are egested through a tem-
porary anus in the upper portion of the body. Respiration is
carried on through the external surface of the body. Organs of
locomotion are present in the cilia, by which the animal can swim

154 PHYLUM PROTOZOA

about if it is broken from its stalk. The axial fiber in the stalk is
a delicate, striated muscle fiber. Sensation is exercised through
the external surface.

Vorticella reproduces asexually, by a longitudinal division.
The process begins at the upper end of the body and proceeds to
the base, so that finally there are two individuals upon a single
stalk. One of these now separates itself from the stalk, assumes
a cylindrical form, and, having developed a band of tempo-
rary cilia near one end, swims away to find a place for itself.
It soon attaches itself, loses the temporary cilia, and develops
a stalk.

In the case of the colonial Vorticellidae both of the individuals
produced by the process of division remain on the stalk. In
Zoothamnium the colony is dimorphic ; it contains nutritive in-
dividuals which are similar to Vorticella, and reproductive indi-
viduals which are large and globose and do not feed. The latter
separate themselves from the parents and swim off and found new
colonies. This dimorphism and division of labor remind one of the
Hydromedusae. In Vorticella, as in Paramecium, reproduction is
largely a matter of sufiicient nutrition, well-nourished animals
reproducing faster than poorly nourished ones. Conjugation also
occurs ; it is brought on by the same conditions as in Paramecium
and is highly important to the well-being of the race. The process
is, however, somewhat different from conjugation in Paramecium.
An individual divides into from two to eight parts. These free
themselves from the stalk, acquire each a basal band of cilia, and
swim about in the water until they come in contact with indi-
viduals of the ordinary kind, with which they fuse. A permanent
conjugation is then effected instead of a temporary one as in
Paramecium.

Conjugation, it will be noticed, while it is not a sexual process,
is closely allied to such a process, and it is probably through it that
sexuality arose in the organic world. In Paramecium and Vor-
ticella we have two important steps in the development of sexu-
ality. In the former animal the conjugating individuals are of the
same size, or isogamous, and the fusion of the two individuals is

VORTICELLA 155

temporary, while in the latter they are of different sizes, or het-
erogamous, and the fusion is permanent. As a result of this dif-
ferentiation in Vorticella one of the conjugating individuals is a
large, passive form, and the other is a small, active, motile form,
which finds and fuses with the passive form. A distinct fore-
shadowing of the two sexes which characterize the Metazoa is
thus present.

Exercise 3. Look among a large number of Vorticellas for conju-
gating and for dividing individuals. Observe them carefully and
draw outlines of those observed.

156 PHYLUM PROTOZOA

Class : Mastigophora
A FLAGELLATE {EUGLENA VIRIDJS)

This single-celled organism, which combines the characters of
animals and plants, is often so plentiful in pools and ditches that
it makes the water green. It is a minute, elongated protozoan, one
end of which is pointed and the other blunt ; in the latter end is
the mouth, which leads into a depression called the gullet, from
which springs a long, threadlike, vibratile flagellum. The body
is covered by a very delicate cuticula. The animal is colored green
by the presence of chlorophyll bodies in it.

Mount a drop of water containing Euglena on a slide and study
it under the microscope. Observe its shape and color ; observe
also its swimming motions and the motions of the flagellum.
The latter organ will be seen to be at the anterior end of the
body ; it is always in advance as the animal swims. The animal
revolves on its long axis as it swims and describes a spiral path
in its progress from one spot to another. In some flagellates the
flagellum is at the posterior end. Whether the flagellum in any
species of flagellate is at the anterior or the posterior end of the
body depends upon the direction the vibratile motion of the fla-
gellum takes. If the motion begins at the base of the flagellum
and proceeds toward its tip, the animal's body will be driven
ahead with the flagellum at the rear, whereas if the motion
begins at the tip of the flagellum, the body will be drawn after
it. Note the extreme plasticity of the body. It can assume a
variety of shapes, and will often be seen swimming by the alter-
nate contraction and expansion of the body, like a worm.

Exercise 1. Draw a number of simple outlines of the body showing
its shape at different times.

Study the structure of the body. The protoplasm composing
it is clear, its surface often showing delicate striations. Note the
cuticula. In the middle of the body is a large, spherical nucleus.
At the anterior end near the gullet is a clear space called the reser-
voir; find it. It receives the discharges of the pulsating vacuoles.

A FLAGELLATE 157

These are minute globules of clear liquid, which represent the ex-
cretory wastes of the animal; they collect and discharge into
the reservoir periodically, which thus acts as a urinary bladder
and in turn opens into the gullet. Near the reservoir is a red
pigment spot, which is sensitive to light ; it is the most primitive
form of an eye.

Exercise 2, Draw Euglena on a large scale with the organs mentioned.

In its life processes Euglena partakes of the nature of both
a plant and an animal. Through the agency of the chlorophyll,
bodies of greater or less size called pyrenoids are manufactured ;
these produce carbohydrates, which constitute a large part of
the food of the organism. The process goes on only during the
daytime and is a characteristic plant process. But Euglena also
ingests solid food after the manner of animals. Bacteria and
other food particles are taken into the mouth at the anterior end
and thence sink into the soft, protoplasmic body. Excretion is
effected through the pulsating vacuoles ; respiration, through the
body surface.

From time to time Euglena encysts itself. It loses its flagel-
lum, draws itself together into a spherical form, and secretes a
cyst of cellulose. After a while it either throws off the cyst and
assumes its former shape or reproduces by dividing into from two
to eight small Euglenas. Reproduction thus takes place during
the period of encystment ; also at times free individuals reproduce
by longitudinal division.

Exercise 3, Among a large number of individuals look for dividing
and also for encysted ones. Make large drawings of several.

158 PHYLUM PROTOZOA

Class : Sarcodina. Order : Rhizopoda
AMCEBA

The amoeba is a jellylike, single-celled animal which may be
found in stagnant water attached to submerged objects, or in
bottom sediment ; it is also often found in moist, damp places
which are not under water. There are many species, the largest
being within the range of the unaided vision, the smallest species
requiring high powers of the microscope to detect. Among the
common species of amoeba are Amoeba proteus, a large, often
active form with long pseudopodia, Amoeba verrucosa, a large,
sluggish form with very short pseudopodia, Amoeba Umax, a
small form which flows along without definite pseudopodia, and
Amoeba radiosa, a small, star-shaped form with slender, radiating
pseudopodia.

Mount on a slide a drop of water with sediment or scrapings
from a submerged leaf or stick containing amoebas, and find one.
Observe its shape and granular appearance. From time to time
the shape of the body changes by the thrusting out of projections
called pseudopodia. Observe the formation of pseudopodia.

Exercise 1. Draw several outlines of the animal showing its shape at
different times.

Observe the structure of the body. The protoplasm forming it
will be seen to be divisible into two layers, the ectosarc and the
entosarc ; the former is the clear, transparent layer which forms
the periphery of the body ; the latter is the granular, translucent
mass which forms the remainder of it. The ectosarc is of firmer
consistency than the entosarc, and its outer surface forms a deli-
cate cuticula. When a pseudopodium begins to form, it consists
at first of ectosarc alone, but entosarc finally enters it as it grows
larger. The entire body will often flow into a single pseudopo-
dium, in which case the animal flows in that direction. When
this happens the ectosarc of the hinder portion of the body will
be seen to wrinkle as the entosarc flows away from it.

Observe the granular nature of the entosarc and the flowing of

AMCEBA 159

the granules as they move about with the motion of the proto-
plasm. Observe the food vacuoles in the entosarc ; they are par-
ticles of food surrounded by water. Observe the pulsating vacuole,
the organ of excretion, which is usually near the hinder end of the
moving body. It will be seen to be a large globule of clear liquid
which forms near the periphery and then discharges into the sur-
rounding water. Time its pulsations ; how many form a minute ?
Add a I per cent solution of acetic acid to the water, or a few
drops of aceto-carmine solution, and find the nucleus.

Exercise 2. Make a large semidiagrammatic drawing of an amoeba
showing the features mentioned above, and label all.

Amoeba has no special vegetative organs except the pulsating
vacuole. Solid food consisting of plants and animals and particles
of organic matter is ingested in the form of food vacuoles. These
move about in the entosarc with the movements of the animal's
body and the nutritive matters are digested and absorbed. Waste
matters are then egested by being thrust out of a temporary open-
ing in the ectosarc into the water. Respiration is carried on
through the surface of the body. One reason for the active throw-
ing out of pseudopodia is the necessity of increasing the relative
area of the surface of the body for respiratory purposes.

Reproduction in Amoeba is carried on by division. The nucleus
first divides; the animal then elongates, and a transverse con-
striction appears in its middle, which is finally carried through
the body. Two animals are thus formed, each of which contains
half of the nucleus. As in other protozoans, reproduction in
Amoeba is largely dependent upon nutrition. If the nutritive
conditions surrounding them are unfavorable, the animals grad-
ually lose their vitality and reproductive powers and in the course
of time will die. Conjugation also probably occurs, as in other
Protozoa, although it has never been observed.

PHYLUM CHORDATA
SUBPHYLUM TUNICATA

Class: Ascidiacea
A SIMPLE ASCIDIAN (MOLGULA)

Ascidians are sessile, marine animals which live attached to
rocks, seaweed, and other objects in the waters along our shores.
Many ascidians are colonial animals; the young individuals,
which arise by a process of budding, remaining attached to the
parents. In a colony which is thus formed certain organs are
often possessed in common, and a very intimate relation is estab-
lished between its individual members. Molgula is noncolonial ;
it is usually found in clusters attached to rocks below low tide.

Molgula is a small, saccular animal, an inch or less in length.
Its outer covering is a thick, tough tunic, or test, which is charac-
terized by being partly composed of cellulose, a substance rarely
met with in animals. The surface of the tunic is covered with nu-
merous minute projections, among which sand and dirt lodge and
cause the dirty appearance which characterizes it, except where it
is in contact with that of other individuals.

The animal has two external body openings, the incurrent
opening, or the mouth, and the excurrent opening, each of which
is at the end of a projection of the body wall called a siphon and
is fringed by short tentacles. The tentacles may, however, have
been drawn into the openings and thus not be apparent. The in-
current siphon is at the anterior end of the body, the excurrent
siphon represents the morphologically posterior end ; the portion
of the body lying immediately between the two is the dorsal side ;
the opposite side, which is very much longer and includes the sur-
face of attachment, is the ventral side. A stream of water is drawn
into the incurrent opening, bearing the minute organisms which
constitute the animal's food and the air needed for respiration ;

1 60

A SIMPLE ASCIDIAN i6i

through the excurrent opening the water is ejected, charged with
fecal matter and reproductive products.

Exercise 1. Make a sketch of the animal on a scale of 2 or 3 ; label
the dorsal and ventral aspects and the siphons.

Beneath the tunic and in contact with it is the mantle, which is
the remainder of the body wall, the tunic being a highly modified
cuticula protecting its outer surface. Remove the entire tunic.
This may be easily done by snipping it with scissors and then pull-
ing it off with forceps ; it is not tightly joined with the mantle.
The mantle will be seen to be a transparent structure through
which the internal organs appear. Observe the white muscle
bands in the mantle, especially the transverse and longitudinal
muscles in the siphons by means of which they are extended and
contracted. Note also the short tentacles at the incurrent and
excurrent openings. Count those at each opening.

The Digestive System. The most conspicuous internal organs
are the cream-colored genital glands near the center of the body
and the alimentary canal. The latter lies on the left side of the
body, where it appears as an S-shaped structure which incloses
the former. Place the body in water with the left side uppermost
and the siphons away from you, and study the arrangement of the
organs. The incurrent opening (at your left) will be seen to have
more prominent tentacles than the excurrent opening. From the
base of the incurrent siphon the large pharynx, the most volumi-
nous organ of the body and the principal organ of respiration, will
be seen extending to the lower side of the body. Note the six
longitudinal ridges which appear as light-colored bands in the
pharyngeal wall. Find and trace a white or cream-colored line
extending in the midventral line from the base of the incurrent
siphon to the opposite side of the body. This is the endostyle ; it
is a ciliated and glandular groove which lies between two folds in
the midventral wall of the pharynx ; it extends the length of that
structure and ends posteriorly near the opening of the pharynx
into the oesophagus. Find this point. The oesophagus is short
and communicates with the stomach, and these two divisions
form the lower and thicker limb of the S-shaped digestive tract.

1 62 PHYLUM CHORD AT A

The upper limb is formed by the intestine, which passes to the
base of the excurrent siphon, where it ends with the anus. Find
these organs.

The Reproductive System. Molgula is hermaphroditic. The
sexual organs consist of a pair of large hermaphroditic glands, one
of which is seen on each of the lateral sides of the body. A short
duct runs from each gland to the base of the excurrent siphon.
On the left side the duct will be seen alongside the posterior end
of the intestine ; find it.

The Circulatory System. On the right side of the body beneath
the hermaphroditic gland will be seen the heart in its pericardium.
It is a muscular sac from each end of which proceeds a large blood
vessel. The vessel leaving the ventral end (at the observer's right)
is called the cardiobranchial vessel; it passes along the mid-
ventral side of the pharynx, beneath (external to) the endostyle,
and gives off branches which run transversely along the pharyn-
geal wall. The vessel leaving the dorsal end of the heart is called
the cardiovisceral ; it breaks up into numerous branches, which
ramify among the viscera and other parts of the body. From the
viscera the blood is collected again in a vessel called the viscero-
branchial, which passes along the mid-dorsal pharyngeal wall and
gives off transverse branches.

The heart of tunicates is peculiar in that its pulsations change
the direction of the flow of the blood alternately from the cardio-
branchial to the cardiovisceral vessels, and back again. The con-
traction of the heart is of a peristaltic nature ; it passes from one
end to the other of it for a short time ; then after a short pause
the contraction is renewed, the peristaltic motion beginning at
the opposite end and driving the blood in the opposite direction.

The Nervous System. About halfway between the two siphons,
embedded in the mantle beneath the dorsal surface of the animal,
lies a small ganglion from which nerves radiate. No organs of
special sense are present, except the tentacles and minute eyespots
at the incurrent and excurrent openings.

The Excretory System. Beneath the heart is an elongated, vesic-
ular organ which is the single, unpaired kidney ; it is ductless.
Beneath the ganglion above mentioned is a small, glandular organ

A SIMPLE ASCIDIAN 163

called the subneural gland; it has a duct which communicates with
the pharynx. The function of this gland is probably excretory ;
it is supposed to be homologous to the hypophysis of vertebrates.

Exercise 2, Make a drawing of the left side of the animal on a scale
of from 4 to 6, showing all the internal organs which appear in that
aspect. Label the dorsal and the ventral sides of the body and all
the organs.

Exercise 3, Make a drawing of the right side of the animal showing
all the organs which appear in that aspect.

Exercise 4. Make a drawing of the dorsal side showing the organs
observed there.

The Peribranchial Chamber. Cut off the excurrent siphon at its
base and with a needle or bristle probe the opening. The probe
will pass into the large space between the mantle and the pharynx.
This is the peribranchial chamber ; it surrounds the pharynx on
all sides, except in the midventral line, and communicates with
the outside water through the excurrent siphon. It is not a part
of the body cavity, but has been formed by an infolding of the
outer surface of the body. Into it, near the base of the excurrent
siphon, the digestive and genital tracts discharge their products
for removal with the current of respiratory water which streams
out of that siphon.

The Respiratory System. The principal respiratory organ is the
pharynx, which communicates with the incurrent siphon by an
opening fringed with a circular row of branched tentacles. Its
walls are pierced by numerous slitlike, ciliated openings, called
stigmata, through which the respiratory water streams from it
into the peribranchial chamber. A current of water is thus main-
tained, which passes through the incurrent siphon into the phar-
ynx, and thence through the stigmata into the peribranchial
chamber, and out again at the excurrent siphon. The stigmata are
vertical in position and are arranged in transverse rows, which
extend across the pharyngeal wall, and are separated from one
another by delicate vertical bars ; the transverse rows have be-
tween them large transverse bars, and running longitudinally
along the pharyngeal wall on each side are six large longitudinal

1 64 PHYLUM CHORD ATA

bars or ridges, which are easily seen and have already been
mentioned. Through all these bars the blood circulates, being
brought to them either by the cardiobranchial or the viscero-
branchial blood vessels, and respiration is thus carried on.

Lay the animal with the left side uppermost. Slit open the
incurrent siphon and the pharynx by inserting the point of fine
scissors in the siphon and, after cutting its wall to its base, carry-
ing the cut through the wall of the pharynx along the side of and
parallel with the midventral line to the posterior end of that
organ. Lay the pharynx open. The twelve large longitudinal
bars will be seen projecting into the pharyngeal lumen. Trace
them throughout their entire extent. Find with the aid of a dis-
secting microscope or a hand lens the rows of branched tentacles
at the base of the incurrent siphon and count them.

In the midventral line note the endostyle ; notice also that it
is a groove. Trace the endostyle forward to the base of the siphon.
At its anterior end the endostyle is continuous with a ciliated
ridge which encircles the anterior end of the pharynx and is called
the peripharyngeal ridge. This ridge is itself continuous on the
dorsal side of the animal, that is, on the side opposite to the endo-
style, with a ciliated longitudinal ridge called the dorsal lamina,
which passes along the mid-dorsal line to the opening of the
oesophagus at the posterior end of the pharynx. Trace the peri-
pharyngeal ridge and the dorsal lamina.

These organs aid in the ingestion of the animal's food. The
endostyle is a glandular and ciliated groove; the gland cells
secrete a viscid substance which catches the food particles ; the
cilia create a current which drives them toward the anterior
end. Here they meet a current created by the cilia of the peri-
pharyngeal ridges which takes them around the pharyngeal wall
to the dorsal lamina, along which they are driven posteriorly to
the opening of the oesophagus.

Between the siphons note the ganglion and subneural gland.

Exercise 5. Make a semidiagrammatic drawing showing the struc-
tures which appear in connection with the pharyngeal wall.

Exercise 6. Make a large diagram of Molgula and show the relative
positions of the different organs ; label all.

A FROG 165

SUBPHYLUM VERTEBRATA

Class: Amphibia. Order: Salientia
A FROG

The following descriptions will enable the student to identify
the commonest species of frogs.^

Rana pipiens, the leopard frog. Green or brown, with large black blotches
edged with white or yellow which lie in two irregular rows on the back ; legs
barred above ; belly pearly or yellowish ; length about 2| inches ; lives in
marshes and wet places or in the grass.

Rana palustris, the pickerel frog. Brown or greenish, with several rows
of oblong square blotches on back and sides ; length about 3 inches ; com-
mon in cold springs or streams or in the grass.

Rana clamitans, the green, or spring, frog. Green or brown, with rounded
spots all over the back ; legs with several cross bands ; beneath, pure white ;
length 3 inches ; lives in ponds and streams.

Rana catesbeiana, the bullfrog. Green or brown, with faint dark spots
above ; head often bright green ; beneath, with pale blotches ; length 5 to
8 inches ; lives in ponds and streams.

Rana sylvatica, the wood frog. Pale reddish brown; head small and
pointed, with a dark band on each side between eye and arm; length
i^ inches ; lives in the woods and in the grass.

Three specimens will be needed for a complete dissection of the
frog : one for the outer form and the greater part of the internal
organs, including the heart and the great blood vessels entering
and leaving it ; one for the other blood vessels ; and one for the
skeleton. A partial dissection can be made with a single specimen.
The animals should be preserved during the dissection in a 5
per cent solution of formalin or in cold storage.

Place the animal (alive if possible) under a glass or in a dis-
secting pan and observe its form and color. The body is short
and compact, with a large head and mouth ; the hinder end is
characterized by the lack of a tail and by the great length of
the hind legs. The color is such as to adapt it to the environ-

1 These descriptions have been modified from H. S. Pratt's ''Manual of the
Vertebrates of the United States."

1 66 PHYLUM CHORD ATA

ment in which it lives, and may change from time to time, like
that of a chameleon, although not to the same extent as in that
animal.

The skin of the frog is without scales, claws, or other hardened
integumentary structures, such as are possessed by other verte-
brates. It is, however, provided with numerous integumentary
glands which secrete a protective slime. The characteristic tri-
radiate openings of these glands may be seen with the aid
of a hand lens in skin which the frog has shed ; pieces of skin
will often be found in the water in which the animals have
been kept.

The body of the frog may be divided into two regions, the
head and the trunk. The neck region, which is wanting in
fishes and is so characteristic of land vertebrates, is just be-
ginning to make its appearance in amphibians. A distinct neck
is not present ; there is present, however, one cervical vertebra
with which the skull articulates. The caudal region is also want-
ing in the adult. In the larval frog and toad a long tail is present,
by means of which the animal swims ; it is, however, gradually
absorbed as the tadpole passes through its metamorphosis.

The Head. This body division is triangular in shape. The
mouth is large and bordered by skinny lips, which close tightly to-
gether like the cover on a box and thus prevent air from escaping
during the act of respiration. The eyes are large and protruding.
Each is protected by two eyelids, the upper one of which is large
and thick and with little power of movement ; the lower one is
semitransparent and movable.

In front of the eyes are the nostrils ; each of these is provided
with a valve which can be tightly closed. The nostrils communi-
cate directly with the mouth. Probe them with a bristle. Back
of each eye is a large, circular area, the tympanic membrane, or
ear drum, which is thus on the outer surface of the body. Be-
tween the eyes is a small, dark spot which marks the frontal organ ;
it is a rudiment of a median eye. In the male frog, of certain
species, a pair of large vocal sacs project from the hinder part of
the head in the breeding season. Probe them from the mouth, if
present, and determine their extent.

A FROG 167

The Trunk. This body division is short and shows externally
no marks of segmentation ; it bears the appendages. In the
middle of the back will be noticed a prominent hump, which in-
dicates the position of the sacrum, where the hinder appendages
articulate with the spinal column. At the posterior end of the
trunk and slightly dorsal in position is the small opening of the
cloaca, the anus.

The Appendages. Two pairs of legs are present ; each leg is
made up of three divisions, a proximal, a middle, and a distal
division. In the foreleg these correspond to the upper arm, the
forearm, and the wrist and hand respectively ; in the hind leg,
to the thigh, the shank, and the ankle and foot. The toes have
no claws.

The forelegs are relatively short and weak and do not aid in
locomotion. Four fingers are present, the thumb being rudi-
mentary. In the male frog the first finger is thickened. The
fingers are not joined by a web.

The hind legs are long and muscular and are the principal
organs of locomotion both on land and in the water. While the
animal is at rest the hind legs are folded together back of it in a
position ready for springing. In this position the three divisions
of the leg become apparent. In the distal division certain ankle
bones are much elongated and make this the longest of the three
divisions. The five toes are webbed, the medial (innermost) one
being the big toe.

Exercise 1. Draw the animal as it sits or lies before you.

If the animal is still alive, it may be killed by placing it in
a jar of water in which a small quantity of chloroform or ether
has been put.

Exercise 2, Draw a dorsal view of the extended animal showing the
features above mentioned ; carefully label all.

Exercise 3, Draw a side view of the head.

The Mouth and Pharynx. Open the mouth as wide as possible ;
cut each angle of the jaw a little, if necessary, so that the mouth
will remain open. The mouth and pharynx will be seen to be a

i68 PHYLUM CHORDATA

single space which extends back to the beginning of the oesopha-
gus. With forceps pull the tongue forward ; it is a slimy, band-
like structure which is attached only at its forward end. The
hinder end, which extends back into the pharynx, is bilobed.
The lower jaw is without teeth. Just back of the tongue in the
floor of the mouth may be felt the hyoid cartilage, which supports
the tongue.

Back of the tongue is the glottis, a median, longitudinal slit
which opens into the lungs. The glottis is in the middle of an
elliptical elevation formed by the two arytenoid cartilages ; it is
usually closed, but may be opened with a needle. Place the end
of a blowpipe in it and blow up the lungs.

The frog has two methods of respiration : (i) with the skin and
the mucous membrane of the mouth and pharynx, and (2) with
the lungs. Air is taken by regular inspirations through the nos-
trils into the mouth and pharynx, where it is acted upon by the
highly vascular mucous membrane. It is also at irregular inter-
vals taken by an act of swallowing through the glottis into the
lungs. It is expelled from the lungs by the elasticity of their
walls, which contain muscle fibers, and that of the muscular
sides of the body. Immediately after the expiration air is again
swallowed, so that the lungs are kept filled. The floor of the
mouth will be observed in the live frog to oscillate rapidly and
regularly. This act is not connected directly with the pulmonary
but rather with the pharyngeal respiration.

The opening behind the glottis into the digestive tract is the
gullet, or oesophagus. Probe it.

In the roof of the mouth note the upper jaw, in which is a row
of teeth called the maxillary teeth. Just behind them in the for-
ward part of the mouth, near the median line, are two small
groups of teeth called the vomerine teeth. On each side of these
is one of the inner openings of the nostrils. Probe them. Near the
angle of the mouth on each side is the large opening into the tym-
panic cavity, the Eustachian tube. Probe one.

Exercise 4. Draw a sketch of the opened mouth and pharynx on
a scale of about 2, and carefully label all the organs mentioned
above.

A FROG 169

The Internal Organs. Place the animal on its back in a dissect-
ing pan containing water, with its head away from you, and pin
it fast with a large pin through the tip of the jaw and one through
each of the four legs. Raise the skin of the belly with forceps,
and with scissors make an incision in it along the midventral line
the entire length of the body.

Notice the looseness of the skin and the large space between
it and the underlying muscles. This space is a lymph cavity.
Note carefully the points where the skin is attached to the mus-
cles. Note the large blood vessels on the inner surface of the
skin ; these are the cutaneous veins and arteries. The blood is
brought to the skin to be aerated, an important part of the res-
piration of the animal being carried on through the skin.

Through the semitransparent muscles in the region of the fore-
legs may be seen and felt a number of platelike bones and
cartilages. These form the pectoral (shoulder) girdle and the
breastbone, which support the fore limbs. In the midventral
line will be seen through the body wall a broad, dark line ; it is
the abdominal vein.

Observe the ventral body muscles; see page 187.

Lift up the ventral body wall with forceps, and with scissors
make a longitudinal incision through it in the median line the
length of the body, taking care not to cut the organs lying be-
neath. Pull the two flaps of the body wall gently apart and pin
them. Examine the organs which lie in the abdominal cavity,
but without disturbing any of them.

If the animal is a male, or a female which is not breeding, the
most conspicuous organs will be the large, reddish liver and the
intestine. If it be a mature female, the dark-colored, granular
ovaries may occupy a large part of the space within the body
cavity ; in this case the ovaries should be removed so that the
other organs can be studied.

Lying on the left side of the liver and wholly or partly con-
cealed by it is the elongated stomach. In front of the liver in the
median line is the conical heart within its membranous pericar-
dium. Lying between the lobes of the liver may be seen the small,
greenish, spherical gall bladder.

I70 PHYLUM CHORD ATA

Make a transverse incision in each flap of the body wall. Turn
the flaps to the side and pin them down, exposing fully the
internal organs.

In addition to the organs already mentioned one or both lungs
may be seen. They are usually shriveled, saclike organs which
lie at the forward end of the abdominal cavity, concealed by the
liver. If either is fuH of air, it should be punctured and made to
collapse. At the hinder end of the abdominal cavity, between the
base of the hind legs, the large urinary bladder will be seen ; if it
is not found readily, insert the blowpipe in the anus and inflate the
bladder. Several elongated, yellowish bodies may be seen project-
ing from between the other organs ; they are called the fat bodies.

The abdominal cavity is lined by a membrane called the perito-
neum. Note that the organs in it are attached to the walls or to
each other by thin membranes ; these are the mesenteries ; they
are folds of the peritoneum.

Exercise 5. Draw an enlarged outline of the animal and in it the
internal organs as they lie in the body cavity before they have been
disturbed ; label all carefully.

The Digestive System. This system consists of the mouth,
pharynx, oesophagus, stomach, intestine, cloaca, liver, and
pancreas.

The mouth and pharynx have already been studied. Without
cutting anything, press the liver to the animal's right and fully
expose the stomach. It will be seen to be a large, curved organ,
the anterior, or cardiac, end being near the left lung at the side of
the heart, and the posterior, or pyloric, end being near the median
line of the body. The oesophagus is a short tube, not quite as
wide as the stomach, which joins the cardiac end of that organ
with the pharynx.

From the pyloric end of the stomach, which is marked by a con-
striction, the intestine proceeds, with many turns, to the hinder
part of the body. It is composed of two divisions : the small
intestine, and the large intestine, or rectum. The small intestine
forms the greater part of it ; its anterior portion, the duodenum,
is bent forward so as to lie parallel with the stomach, and be-

A FROG 171

tween them lies the whitish, irregularly shaped pancreas. The
rectum is about half an inch long and forms the hinder part of the
intestine ; it is much wider than the small intestine and may often
be recognized by its dark color. The rectum is continuous pos-
teriorly with the cloaca, a short, wide vessel which lies between
the base of the hind legs and finds an outlet through the anus.

Observe again the extensive mesenteries which bind the divi-
sions of the digestive tract with the wall of the abdominal cavity.

At one side of the forward portion of the rectum will be seen
a dark-red, spherical body, the spleen. Press the intestine and
mesentery aside, — but without cutting them, — and observe
the flattened, dark-colored kidneys, which lie close to the dorsal
body wall. At their forward ends are the two yellow, spherical
testes, if the animal is a male, or the irregular, saclike ovaries,
if a female ; in front of these organs are the yellow, finger-shaped
fat bodies.

Observe closely the liver and pancreas. The former is com-
posed of two main lobes, one of which is subdivided into two
smaller lobes. Note carefully the connection between these two
parts. Turn the whole liver forward, — but without cutting any-
thing,— pin it there, and study its dorsal surface and the pancreas.

The pancreas is an irregular, whitish gland which lies in the
bend made by the stomach and the duodenum. Near the hinder
border of the liver note the spherical gall bladder. Find the bile
duct, which joins the liver with the duodenum. It is a slender
tube which issues from the gall bladder and, after receiving a
number of branch ducts from the liver, joins the duodenum a
short distance from the pylorus. It passes through the pan-
creas, from which it receives one or more small pancreatic ducts.
Gently squeeze the gall bladder with forceps and force the dark-
green bile into the duct ; it will thus be easy to follow. If the
bile will not flow, cut the gall bladder open and inject a carmine
solution in it with a pipette.

Exercise 6. Make a semidiagrammatic drawing of the dorsal surface
of the liver and pancreas, with an outline of the stomach and duo-
denum, showing the features just described ; carefully label all
the organs.

172 PHYLUM CHORD ATA

The study of the digestive system will be completed after
the heart has been examined.

The Heart and its Vessels. The heart of the frog is com-
posed of five divisions : a single ventricle, two auricles, the sinus
venosus, and the truncus arteriosus. Observe the pericardium,
which closely invests the heart.

The ventricle is the large, conical, posterior portion of the heart ;
by its contractions the blood is sent forward through the trun-
cus arteriosus, which is the large, cylindrical vessel springing
from its anterior end. The truncus divides into two large vessels
which pass forward and leave the pericardial space. Each of
these vessels then divides into three arteries, called the aortic
arches, through which the blood is carried to all parts of the
body. The anterior arch is called the carotid arch; it carries
blood to the head. The middle arch is called the systemic arch.
The right and left sides of this arch meet back of the heart and
form the dorsal aorta, which lies just beneath the spinal column
and distributes arterial blood to the trunk and extremities. The
posterior arch is the pulmocutaneous arch ; through it blood is
carried to the lungs and the skin for aeration.

In front of the ventricle are the right and left auricles ; they
appear dark-colored in consequence of the thinness of their walls.
On the dorsal side of the heart is a large, thin-walled, dark-
colored sac, the sinus venosus. Blood is brought to the heart
from the organs and tissues of the body by three large veins
which enter the sinus venosus : these are the right and left pre-
caval veins, which enter the forward end of the sinus, bringing
blood from the forward part of the body ; and the postcaval vein,
which enters the hinder end of the sinus, bringing blood from the
hinder part of the body. From the sinus the blood enters the
right auricle. Blood is brought to the heart from the lungs by
the pulmonary vein, which lies alongside the left precaval vein
and enters the left auricle ; this vein is formed by the union of a
right and a left pulmonary vein, which bring blood from the two
lungs.

Exercise 7. Make a drawing of the ventral aspect of the heart and the

blood vessels, so far as these have been observed, on a scale of 2 or 3. \

A FROG 173

The Digestive System {Continued). Dissect this system in the
following way. Lift up the liver with forceps, and with scissors
free its anterior border from the tissues beneath it, being careful
not to injure the lungs. Find the oesophagus, which joins the
stomach with the pharynx. Note that the lungs also join the
ventral wall of the pharynx. Take hold of the oesophagus with
forceps, lift it up, and with scissors cut across the floor of the
mouth in front of the lungs.

The forward end of the digestive tract, with the lungs, being
thus cut loose from the body, can be bent backward. With scis-
sors cut the stomach and liver loose from the tissues beneath
them ; cut the mesentery by which the intestine is joined with
the dorsal body wall, being careful not to injure the flattened
kidneys and testes or ovaries, and straighten the intestine out.
The entire digestive tract, together with the lungs, will thus be
removed from the body, except at its hinder end. Extend it in
the water and pin it there, with the lungs attached to the phar-
ynx, and the liver and pancreas attached to the duodenum by
the bile duct.

Exercise 8. Make a drawing of the digestive system, with the lungs ;
label all the parts and organs belonging to it.

Slit open the stomach and the forward end of the intestine and
note the ridges on their inner surface. Cut open a lung and note
that it is a hollow sac with a network of ridges on the inner surface.

The Urinogenital System. The urinary and the genital organs
are in close union with each other, notwithstanding their differ-
ence in function, and are conveniently studied together. The
urinary organs consist of the paired kidneys, the paired ureters,
the urinary bladder, and the cloaca.

The kidneys are two large, flattened bodies which lie close
to the dorsal body wall in the posterior portion of the body
cavity. The ureter is a straight white tube which runs from
the outer posterior border of the kidney to the dorsal wall of
the cloaca. The urinary bladder is a large, bilobed sac at the
hinder end of the body cavity which springs from the ventral
wall of the cloaca. Its opening into the cloaca can be applied

174 PHYLUM CHORD ATA

closely to the openings of the ureters, and it can thus receive
the urine from them ; in it urine is stored.

On the ventral surface of each kidney is an irregular, yellow-
ish line which is called the adrenal body, an endocrine body.

The genital organs consist of the testes in the male and the
ovaries in the female, and the ducts which conduct the genital
products to the outside.

The Male. The testes are two yellow, ovoid bodies which lie
against the ventral surface of the kidneys and are attached to
the dorsal wall of the abdominal cavity by mesenteries. Join-
ing each testis with the ventral surface of the kidney are about
a dozen fine tubules, which are suspended in the mesentery.
Through these the spermatozoa, which are formed in the testis,
make their way into the kidney and thence into the ureter. This
duct thus serves the double function of a ureter (an outlet for
urine) and a sperm duct (an outlet for sperm) and is thus a
urinogenital duct.

Many male frogs have rudimentary oviducts, which appear as
a pair of slender white cords, each lying alongside the kidney.

The Female. The ovaries differ very much in size and appear-
ance at different times of the year. In the springtime they are
often so distended with the small, spherical ova that they may
almost fill the abdominal cavity. Tf this is not their condition,
they appear as a pair of folded, dark-colored bodies which lie on
the ventral surface of the kidneys attached to the dorsal body wall
by median mesenteries. The paired ducts through which the ova
find their way to the cloaca are the oviducts. In adult females
each of these ducts is a thick-walled, twisted tube which lies in the
abdominal cavity against the dorsal body wall. Its anterior end
opens into this cavity and is situated at the side of the heart and
the posterior end opens into the cloaca. The posterior portion of
the oviduct is expanded and forms a uterus, a reservoir for ova.

The ova escape from the ovaries by the rupture of their walls
into the abdominal cavity; they then make their way to the
mouths of the oviducts and through them into the cloaca. Dur-
ing this descent of the ova the albumen which surrounds them
is secreted by the walls of the oviducts.

A FROG 175

At the anterior end of the kidneys is a pair of prominent, yel-
low, branching fat bodies. They vary much in size at different
times of the year, being largest before the breeding season and
smallest after it.

While studying the urinogenital system, the organs of which it
is composed need not be disturbed. With a strong scalpel cut
through the bony pelvis exactly in the median line between the
legs in order to expose the cloaca. The urinary bladder is a del-
icate structure which is attached to the body wall by mesen-
teries. It must be freed from these and great care taken not
to cut either it or the cloaca.

Exercise 9. Make a semidiagrammatic drawing of the urinogenital
system with the cloaca ; label carefully all its parts.

The Nervous System. This system is made up of the following
divisions : (i) the central nervous system, which is composed of
the brain and the spinal cord ; (2) the peripheral nervous system,
which is composed of {a) the paired cranial and spinal nerves and
(6) the sympathetic nervous system ; and (3) the special sense
organs.

The cranial nerves and the spinal nerves each number ten
pairs ; the former spring from the brain and the latter from the
spinal cord and place these structures in communication with
the various organs and tissues of the body. The sympathetic
nervous system lies in the body cavity in connection with the
cranial and spinal nerves and innervates certain important viscera.

Remove the urinogenital system from the body. Raise it care-
fully with forceps, and with fine scissors cut it loose from the
dorsal body wall. Note the spinal column projecting into the
body cavity, and lying ventral to it note a large blood vessel,
the dorsal aorta ; this must not be disturbed. The spinal column
is made up of nine vertebrae and a long terminal bone called the
urostyle. Identify them.
We shall study first the spinal nerves and the sympathetic sys-
tem. Each spinal nerve is joined with the spinal cord by two
roots, a dorsal and a ventral root, and passes out from the
neural canal of the spinal column through a space between two

176 PHYLUM CHORD ATA

vertebrae. At the point where these two roots meet, the dorsal
root bears a large ganglion called the spinal ganglion. This gan-
glion is embedded in a prominent white body present between the
vertebrae, called the calcareous body.

The ten pairs of spinal nerves will be seen in the body cavity,
where they appear as white strands which lie against the dorsal
body wall on each side of the vertebral column. The most con-
spicuous ones are the seventh, eighth, and ninth nerves, which
lie close together in the hinder part of the abdominal cavity. They
emerge on each side from between the seventh and eighth ver-
tebra, the eighth and ninth, and the ninth and the urostyle
respectively, and proceed straight back almost parallel with the
spinal column. These nerves are joined with one another by
short connecting branches and form a network, or plexus, called
the sciatic plexus. From this plexus issue a number of nerves
which proceed to the hinder quarters of the body and the hind
legs. Of these the largest is the sciatic nerve, which goes to the
hind leg.

Find the sciatic plexus. Follow the sciatic nerve into the leg as
far as possible.

In the forward part of the abdominal cavity, on each side, is
another much smaller nerve plexus called the brachial plexus,
which is composed of the first three spinal nerves. Of these the
second, which is the largest and most conspicuous, is a large white
cord lying at right angles to the spinal column and emerging from
between the second and third vertebrae ; it is joined by a small
branch from the first and one from the third spinal nerves, and
passes to the foreleg.

The fourth, fifth, and sixth pairs of spinal nerves are delicate
cords which emerge from between the fourth and fifth, fifth and
sixth, and sixth and seventh vertebrae, and pass obliquely back-
ward to the muscles of the back.

Find these nerves.

The sympathetic system consists of a pair of delicate longitudi-
nal nerves which lie in the abdominal cavity on either side of the
spinal column, close to the dorsal body wall. In each longitu-
dinal nerve are ten enlargements, the sympathetic ganglia, from

A FROG 177

each of which one or more short branches run to a spinal
nerve. Find these nerves and their ganglia.

The first, second, and third of these ganglia lie close to the
first, second, ^nd third spinal nerves and are joined with them by
short branches. The fourth to the ninth sympathetic ganglia,
inclusive, are situated nearer the median plane than the first
three ; they, together with the longitudinal nerve, lie alongside the
dorsal aorta, which will be noticed as a median, dark-colored tube.
The fourth sympathetic ganglion is the smallest ; the ninth is
the largest and is joined with the ninth spinal nerve by several
branches ; the tenth is small and is often wanting.

Branches from the sympathetic nerves and their ganglia pass
to the various viscera. The largest of these branches proceed
from the fourth, fifth, and sixth ganglia, and after joining to-
gether form a large nerve, called the splanchnic nerve, which
supplies the intestine and other viscera.

Study the sympathetic system; first find the longitudinal
nerves and ganglia, and then observe their relations to the spinal
nerves and ganglia.

Exercise 10. Make a semidiagrammatic drawing of the spinal nerves
and the sympathetic system. Draw first an outline of the spinal
column as it appears in the ventral aspect of the opened body
cavity ; number the vertebrae ; draw the spinal nerves and then
the sympathetic system.

The Brain and the Spinal Cord. In order to expose these organs
remove the skin and muscles from the back of the head and
trunk. Find the juncture of the skull with the backbone. By
bending the head slightly down, a space about an eighth of an
inch long, which is covered by a dark-colored membrane, may be
made to appear between the skull and the backbone. Introduce
one blade of the scissors into the skull through the opening, and
make a cut along the side of the skull between the eyes. Make a
similar cut along the other side, and with the forceps lift off the
roof of the skull, thus exposing the brain. Similarly cut through
the two sides of the neural canal, which contains the spinal cord,
and expose it.

lyg PHYLUM CHORDATA

Carefully remove the dark membrane, the pia mater, which
covers the brain, and observe its five regions : the cerebrum,
the diencephalon, the optic lobes, the cerebellum, and the medulla
oblongata.

The brain and spinal cord are hollow structures. A delicate
canal, called the central canal, runs through the center of the
cord ; in the brain this canal widens out into a number of spaces
which are called the ventricles.

The anterior and largest region of the brain is the cerebrum.
It is made up of the two lateral hemispheres. The anterior ends
of the hemispheres are fused and form the olfactory lobe, from the
anterior end of which the two olfactory nerves pass to the nose.

Back of the cerebrum is the inconspicuous diencephalon, and
behind that are the paired optic lobes, or midbrain. In the roof
of the diencephalon will be seen, with the aid of the lens, several
deHcate structures. Near the center of it arises a threadlike pro-
jection called the pineal body, which extends forward over the
diencephalon. In an early period of the larval hfe of the frog the
pineal body extends through the skull to the skin on the top of
the head between the eyes, where it joins the brown spot known
as the frontal organ, which is the rudiment of the pineal eye ; this
connection is lost before the animal becomes adult.

Back of the optic lobes and separated from them by a deep
groove is a narrow, transverse ridge, the cerebellum, and back of
that is the medulla oblongata, which is continuous with the spinal
cord. The dorsal wall of the medulla is a dark-colored, vascular
membrane called the posterior choroid plexus, beneath which is
the fourth ventricle of the brain. The triangular, depressed area
which these structures form is called the fossa rhomboidalis.

The spinal cord is the portion of the central nervous system
which lies in the neural canal of the spinal column. It is a thick
white band, oval in cross section, from which the paired spinal
nerves spring. At two points it is swollen : (i) where the spinal
nerves which form the brachial plexus, and (2) where those form-
ing the sciatic plexus, respectively, leave it. The hinder end of
the cord tapers rapidly until it becomes a fine thread which ex-
tends into the urostvle.

A FROG 179

Exercise 11. Draw the dorsal aspect of the brain and the spinal cord.

Study the lateral surface of the brain and the proximal portions
of the cranial nerves. Ten pairs of these nerves are present in the
frog; several pairs are so small, however, that they may not
be seen.

The first cranial nerve is the olfactory, which extends forward
from the olfactory lobe. Cut away the roof of the anterior portion
of the skull and follow the two olfactory nerves forward. Each
will be seen to branch a short distance in front of the olfactory
lobe and be distributed to the walls of the nasal capsule.

Cut the olfactory nerves. Dissect away the left side of the
skull and expose the left surface of the brain, preserving as far as
possible the nerves which will be seen coming from it.

Lying close to the inner wall of the skull, at the hinder end of
the orbit, is a yellowish body, often surrounded by a calcareous
sac. This is the Gasserian ganglion, and must not be injured.
Just behind the hemispheres the optic nerve, the second cranial
nerve, issues from the ventral surface of the diencephalon and
extends forward to the eye.

The third and fourth cranial nerves, the oculomotor and the
trochlear, are very small and can hardly be found ; they go to
muscles of the eyeball. The oculomotor springs from the ven-
tral surface of the midbrain, the trochlear from the dorsal sur-
face between the optic lobes and the cerebellum.

The fifth, sixth, seventh, and eighth cranial nerves, which
are the trigeminal, abducens, facial, and auditory, respectively,
arise close together from the forward end of the medulla ob-
longata. The first three of these nerves, together with the ante-
rior end of the sympathetic nerve, are united in the Gasserian
ganglion. The trigeminal nerve is the largest of these three ; it
arises from the side of the brain just beneath the cerebellum and
passes forward to the ganglion. The abducens is a very slender
nerve which arises from the ventral surface of the medulla near
the median line. The facial and auditory nerves arise behind
the trigeminus. The auditory is the larger and passes directly
to the auditory capsule ; the facial is much smaller and passes
alongside the trigeminus to the Gasserian ganglion.

i8o PHYLUM CHORDATA

The ninth and tenth cranial nerves, the glossopharyngeal
and the vagus, respectively, arise from the side of the me-
dulla, back of the auditory nerve, by four roots. These unite
to form a single nerve, which emerges from the cranial cavity
by a foramen at the side of the foramen magnum. Immediately
back of this foramen it expands into the large jugal ganglion,
from which the glossopharyngeal and the vagus proceed. The
former passes forward to the tongue ; the latter passes back-
ward along the pharyngeal wall, giving off branches which
supply the muscles of the shoulder, the larynx, heart, lungs, and
stomach.

Exercise 12. Draw the lateral aspect of the brain on a scale of 3, and
the cranial nerves so far as observed.

Study the ventral surface of the brain. Cut the cranial nerves
and remove the brain from the skull. Put it into a dish of
water and study its ventral surface. Identify the olfactory lobe,
the hemispheres, and the structures belonging to the diencepha-
lon. The optic nerves will be seen issuing from the optic chiasma,
a structure formed by the crossing of the optic nerves on
the ventral side of the brain. Behind the optic chiasma is the
infundibulum, a large median projection which is divided into
a right and a left lobe, and extending from the hinder end of
which is a flattened body called the pituitary body. This body
is lodged in a depression in the floor of the cranial cavity, and
usually remains there after the brain is removed from the skull.

The ventral portion of the midbrain is formed by the crura
cerebri, which lie beneath the optic lobe and are partly con-
cealed by the infundibulum. Arising from the crura near the
middle line may be seen the very delicate oculomotor nerves.

The medulla oblongata is but slightly wider than the spinal
cord. A longitudinal groove is present in the midventral line of
both.

Exercise 13, Draw the ventral aspect of the brain on a scale of 3.

The Vascular System. This system is made up of the follow-
ing parts : (i) the heart, a muscular pump which is continually

A FROG i8i

driving the blood to all parts of the body ; (2) the arteries, the
vessels through which the blood is carried away from the heart ;
(3) the veins, the vessels through which the blood is returned to
the heart ; (4) the capillaries, the minute vessels which connect
the veins and arteries.

Kill a frog and pin it down as directed on page 167. Make a
midventral incision through the skin from the tip of the snout
to the anus. Note the prominent cutaneous veins on the inner
surface of the skin. Identify first the abdominal vein through
the ventral wall of the abdomen. This vein lies in the body
cavity against the ventral abdominal wall and will appear as a
dark median line. Open the body cavity by a longitudinal in-
cision in the midventral line from the anus to the tip of the
lower jaw. Take great care not to cut the organs within.

Free the attachments of the Kver to the body wall, spread the
two flaps to the right and left, making a short transverse cut in
each, and pin them fast, exposing fully the heart, liver, and other
internal organs. In the female animal the ovaries should be
removed if they obscure the other organs.

The heart and the blood vessels leaving and entering it have
already been studied (page 172).

The Veins.^ The veins may be divided into two groups, which
are (i) the systemic veins, those which enter the sinus venosus,
with their branches, bringing for the most part venous blood
from the various organs and tissues; and (2) the pulmonary
veins, which enter the left auricle, bringing arterial blood from
the lungs.

We shall first study the systemic veins. These may be sub-
divided into two groups : (i) the cava! veins, which bring blood
directly to the heart ; (2) the portal veins, which bring blood
directly to the liver and kidneys, whence it goes to the heart.

1 The veins and larger arteries are usually easily studied without being injected,
the veins being colored by the blood in them. The animals should not be dissected
fresh, but the blood should be permitted to harden in the veins first. If it is wished
to inject them, this should be done through the abdominal vein in both directions
for the portal systems, and through the postcaval for the remaining systemic
veins. The arteries can be better studied if they are injected ; this should be done
through the ventricle and truncus arteriosus.

i82 PHYLUM CHORDATA

Three large caval veins are present, which enter the sinus
venosus; two of these, the right and left precavals, bring
blood from the anterior half of the body, including the forelegs ;
the other, the postcaval, brings blood from the posterior half
of the body.

Turn the apex of the heart forward and observe the two large
precaval veins which enter the sinus venosus at its forward end.
Each precaval is formed by the union of three veins, which meet
immediately in front of the heart. These are the external jugular,
the most anterior of the three, which brings blood from the head ;
the innominate vein, the middle one, which brings blood from the
brain, the shoulder, and the forearm ; and the subclavian vein,
the largest and hindermost, which brings blood from the arm and
the skin.

Several small, glandular bodies are present near the external
and internal jugular veins, of which the parathjrroid, the thyroid,
and the thymus glands are the most important. The first of these
is a small, ovoid body which lies next to the inner ventral surface
of the external jugular. The thyroid gland is a somewhat larger
body near the parathyroid but dorsal to it. The thymus is a small,
ovoid body just back of the hinder margin of the tympanic mem-
brane near the innominate vein.

The subclavian vein is formed by the union of two veins, the
brachial vein, which is one of the two veins returning blood from
the foreleg, and the great cutaneous vein, which returns it from
the skin. Follow the former vein and its branches. The latter
vein occupies a peculiar position in that it is partly respiratory in
function. It lies on the inner surface of the skin, receiving nu-
merous branches, and may be traced forward into the head, where
it receives branches from the mucous membrane of the mouth
and the pharynx. It is, however, not wholly respiratory, as it
also receives branches from muscles. ^

Exercise 14. Draw a diagrammatic sketch showing the precaval veins
and their branches so far as observed.

The postcaval is a large median vein which enters the posterior
end of the sinus venosus. It arises between the kidneys and^uns

A FROG 183

along the mid-dorsal wall of the abdominal cavity, just beneath
the doFsal aorta, to the liver, through which it goes to the heart.
Press the intestine to one side, but without cutting anything, and
observe it and its branches.

The postcaval vein receives the following branches : the renal
veins, five or six pairs in number, from the kidneys ; the sper-
matic veins (in the male) or ovarian veins (in the female), from
two to four pairs in number, which join the postcaval between the
renal veins, bringing blood from the genital organs ; the adipose
veins, a pair of veins from the fat bodies ; the hepatic veins, three
in number, from the liver.

Exercise 15, Draw a diagrammatic sketch showing the postcaval vein
and its branches so far as observed.

The Portal System of Veins. A portal vein is one which does
not go directly to the heart but either to the liver or the kidneys,
where it divides up into capillaries and distributes the venous
blood throughout these organs. In the frog two portal systems
are present : the hepatic portal, by which blood is taken to the
liver, and the renal portal, by which it is taken to the kidneys.

The Hepatic Portal System. This system is made up of two
large veins and their branches : the abdominal vein, which
brings blood from the hind legs, and the hepatic portal vein, which
brings it from the digestive tract and the spleen.

The abdominal vein is a median ventral vein which has already
been seen ; it will have been cut in opening the abdominal cavity.
It is formed by the union of the right and left pelvic veins, which
come from the hind legs. Trace the abdominal vein back and
find them.

Turn the liver forward and expose its dorsal surface ; fasten it
with pins in this position. Trace the abdominal vein forward to
the liver. Just to the left of the gall bladder it divides into three
branches, two of which go to the right and left lobes of the liver,
and the third joins the hepatic portal vein.

Study the hepatic portal and its branches. It is a short, wide
vein which lies in the mesentery and enters the left lobe of the
liver. Near the point where it enters the liver it is joined by the

1 84 PHYLUM CHORD ATA

branch of the abdominal vein just mentioned. The hepatic portal
receives numerous intestinal veins, which lie in the mesentery and
come to it from the small and large intestines. It also receives the
splenic vein from the spleen, the two large gastric veins from the
stomach, a gastroduodenal vein from the stomach and duodenum,
and a number of small pancreatic veins from the pancreas. The
anterior portion of its course is through the pancreas.

The Renal Portal System. This system is made up of veins from
the hind legs, the dorsal body wall, the kidneys, and, in the fe-
male, the oviducts ; it is joined with the abdominal vein by the
pelvic veins, which have just been observed, and with the kidneys
by the renal portal veins.

Two veins, the femoral and the sciatic, collect the blood of the
hind leg on each side of the body. The femoral is the larger of
these. It is a large vein which appears on the ventral surface of
the leg, where it receives the large pelvic vein.

The sciatic vein lies on the back of the thigh. It passes for-
ward and joins the femoral, and the vein so formed is the renal
portal, which runs forward to the outer margin of the kidney at
its hinder end. The vein then continues within the kidney close
to its lateral margin and gives off numerous branches, which
break up into capillaries in the kidney. At about the middle of
the kidney a vein comes from the side and entering the kidney
joins the renal portal. This is the dorsolumbar vein ; it collects
blood from the dorsal body wall. In the female a large number
of veins from the oviduct also enter the lateral margin of the
kidney and join the renal portal vein.

Trace the abdominal vein back and find the pelvic veins. Follow
one of the pelvic veins back to the base of the leg, where it will be
seen to join the prominent femoral. Trace this vein forward to its
point of union with the sciatic vein ; trace the renal portal vein
to the kidney. Study the distribution of the femoral and sciatic
veins in the leg. Observe the dorsolumbar vein entering the kid-
ney in the middle of its lateral margin and study its distribution.

Exercise 16. Draw a semidiagrammatic sketch showing the hepatic
and renal portal systems, together with an outline of the organs with
which the veins enter into relations.

A FROG 185

The Pulmonary Veins. The common pulmonary enters the left
auricle. It is a very short vein and is formed by the union of the
right and left pulmonaries, which come from the right and left
lungs respectively. Each pulmonary vein lies along the medial
side of the lung, the right pulmonary being somewhat longer than
the left. Turn the apex of the heart forward and find these veins.

Exercise 17, Draw a diagram of the entire venous system.

The Arteries.^ All the blood in the heart leaves it through the
truncus arteriosus, the structure of which has already been ob-
served. At its anterior end the truncus divides into a right and
a left branch, each of which after passing through the pericar-
dium again divides into three branches, the carotid, systemic,
and pulmocutaneous aortic arches. The last named of these
arches, which is the hindermost in position, branches ofi from
the others a short distance in front of the pericardium ; the other
two usually remain together a short distance before separating.

The anterior aortic arch, the carotid, passes forward and
dorsally a short distance and then divides into two vessels,
the internal and the external carotid arteries. At this point the
walls of the arteries are thickened and spongy, and the ovoid
structure thus formed is called the carotid gland; it is not a
gland, however, but probably acts as an accessory heart. The
internal carotid, the larger of the two, goes dorsally and then
forward and supplies the brain, the orbit, and the mucous mem-
brane of the roof of the mouth. The external carotid passes
directly forward and supplies the tongue and the muscles of the
lower jaw.

The posterior aortic arch, the pulmocutaneous, takes blood
to the lungs and skin to be oxygenated. On each side just back
of the carotid gland it divides into two arteries, the pulmo-
nary and the great cutaneous. The former passes back a short
distance and then divides into three arteries which traverse the
walls of the lung. The latter passes first forward and dorsally
and then backward along the inner surface of the skin.

1 The arterial system can be best studied after it has been injected ; this should
be done through the ventricle and the truncus arteriosus.

1 86 PHYLUM CHORD ATA

Exercise 18. Draw a semidiagrammatic sketch showing the distribu-
tion of these arteries so far as observed.

The middle arch, the systemic, suppUes the greater part of the
body with blood. The two sides of the arch pass dorsally, one
on each side, around the oesophagus to the dorsal side of the
body cavity, where they meet and form the dorsal aorta. This
vessel runs just beneath the spinal column to the hinder part of
the body cavity, where it divides into two arteries, the iliacs,
which go to the hind legs.

Lift up the stomach and find the dorsal aorta ; follow it for-
ward to the meeting point of the two sides of the systemic arch,
and then follow each side of the .arch to the heart. Each side of
the arch gives off the subclavian, occipito vertebral, oesophageal,
and laryngeal arteries.

These arteries leave the arch near together. The subclavian
is the largest and supplies the shoulder and foreleg. Follow it
and its branches. The oesophageal and occipitovertebral are
close together and go to the head, a branch of the latter — the
vertebral — also passing back along the dorsal surface of the
spinal column. The laryngeal arises in front of the others and
goes to the head.

Just behind the point of union of the two sides of the systemic
arch the dorsal aorta gives off the cceliacomesenteric artery.
Observe that this large artery is a continuation of and receives
most of the blood of the left systemic arch. The dorsal aorta
thus receives most of its blood from the right arch.

The coeliacomesenteric artery supplies the stomach and intes-
tine with blood. It soon divides into two branches, the coeliac
artery and the anterior mesenteric, the former supplying the
stomach, liver, and pancreas, the latter the small intestine,
rectum, and spleen.

Posteriorly to the coeliacomesenteric artery four to six pairs
of urinogenital arteries spring from the ventral surface of the dor-
sal aorta and go to the genital organs, kidneys, and fat bodies.
Several pairs of small lumbar arteries also spring from the dorsal
surface of the aorta and supply the dorsal body wall Near the

A FROG 187

hinder end of the dorsal aorta the small median posterior mesen-
teric artery leaves its ventral surface and runs to the rectum.

The common iliac arteries, which are formed by the division
of the dorsal aorta, are large vessels which supply the hind
legs. A short distance from its origin each gives off two small
arteries : the hypogastric, which supplies the rectum, bladder,
and ventral body wall ; and the femoral, which goes to the thigh.
After giving off these arteries the common iliac is known as the
sciatic artery. Follow it and its branches into the leg.

Exercise 19. Draw a semidiagrammatic sketch showing these arteries
so far as they have been observed.

Exercise 20. Draw a diagram of the entire arterial system.

The Muscular System. Most muscles in the land vertebrates
are attached at both ends by means of tendons. One end
is usually attached to a more or less fixed and the other to a
more movable portion of the body, the former being called the
origin of the muscle and the latter its insertion. The middle part
of the muscle is called the belly ; by its contraction the origin
and insertion, and with them the skeletal pieces to which they
are attached, are brought nearer together. Muscles are usually
attached to the bones and cartilages ; thick, fibrous membranes,
called aponeuroses, which often cover muscles and other organs,
may, however, serve the same purpose.

Kill a frog and completely remove the skin from the body,
without injuring any of the muscles. Inasmuch as they are more
or less transparent when fresh, it is well to let the animal lie in
alcohol or formalin before the muscles are studied.

Fasten the frog on its back, with its head away from you, by
means of a pin through the tip of the nose and one through each
foot, and, without cutting anything, study the muscles of the
ventral side of the body. On the head the broad, thin subman-
dibular muscle, the fibers of which are transverse in direction,
stretches across from one side of the lower jaw to the other.
A median tendon separates the right half of it from the left half.
A narrow strand of muscle, the subhyoid, lies at the hinder end
of the submandibular, with fibers parallel to its fibers and at-

1 88 PHYLUM CHORD ATA

tached to it in the medial area. The lateral ends of the subhyoid
iind their origin in the hyoid cartilage.

At the forward end of the trunk is a group of four muscles
which radiate from the base of the foreleg on each side toward
the medial area of the body and may be called the pectoral group.
Beneath the medial ends of these muscles between the forelegs
will be seen the delicate sternum, or breastbone. The foremost
of these muscles is the coracoradial. It is a wide muscle, the
hinder half of which is concealed beneath the muscle next be-
hind it. From its distal end a long tendon passes along the
humerus to the forearm. The function of this muscle is to bend
the forearm.

The other three muscles of this group are divisions of the pec-
toral muscle. The anterior two divisions, like the coracoradial,
have their origin in the breastbone near the median line. The
hindermost division, which is the largest, has its origin in the
outer edge of the broad aponeurosis, which occupies the median
area of the abdomen. All the divisions of the pectoral muscle
have their insertion near the proximal end of the humerus in the
upper arm and serve to bend the arm back.

Lying in front of the pectorals is the deltoid muscle. It con-
sists of two principal portions, an anterior and a posterior
portion, both of which arise near the median line along the
forward border of the pectorals. They are inserted in the hu-
merus and cover the insertions of the pectoral muscles. Deter-
mine the action of the deltoid muscle.

Back of the breastbone are the abdominal muscles, which form
the ventral and lateral walls of the abdomen. Three pairs are
present, the rectus abdominis, the external oblique, and the
transversus.

Extending from the hinder end of the breastbone to the hinder
end of the trunk is the aponeurosis just mentioned — a broad
median band of connective tissue covering the midventral area
of the abdomen. A similar aponeurosis is also present in the mid-
dorsal area.

The rectus abdominis muscles are a pair of longitudinal muscles
which lie in the midventral area beneath the aponeurosis. A

A FROG 189

narrow tendinous band called the linea alba lies in the median
line and separates the right from the left rectus. There are also
present in these muscles four or five transverse, tendinous bands
which divide them into segments. This segmentation, which
also appears in the rectus abdominis of many higher vertebrates,
including man, is an inheritance from the metameric condition
of the body muscles in the fishes and the salamanders.

The external oblique muscle forms the lateral wall of the abdo-
men on each side. It is a broad, thin muscle which extends from
the mid-dorsal to the midventral aponeurosis, its fibers having
an oblique direction. Immediately beneath this muscle is the
transversus, the fibers of which have a transverse direction.

Exercise 21. Draw the ventral aspect of the body showing these
muscles.

Study the superficial muscles of the ventral surface of the hind
leg. The longest muscle of the thigh is the sartorius. It is a long
band which extends along the middle of the thigh from the pel-
vis to the proximal end of the shank. Just in front of it is a
broad muscle, the vastus internus, which forms the anterior bor-
der of the thigh. It forms also the anterior portion of a three-
fold muscle, the triceps extensor femoris, which is the principal
extensor muscle of the thigh. The other two portions of this
muscle are on the upper side of the leg ; they are the rectus an-
ticus femoris and the vastus externus, the latter being posterior
to the former.

Posterior to the sartorius on the ventral surface are three mus-
cles, the adductor magnus, the rectus internus major, and the
rectus internus minor, the latter of which forms the hinder
margin of the thigh. These are all, together with the sartorius,
flexors of the leg.

On the lower leg, or shank, the large muscle which forms the calf
is the gastrocnemius. At its lower end is the tendon of Achilles,
which passes over the ankle and is continued in the plantar apo-
neurosis, a broad, tendinous band covering the sole of the foot.
The front side of the shank is formed by the tibialis anticus
muscle.

I go PHYLUM CHORD ATA

Exercise 22, Draw the ventral aspect of the hind leg.

Study the superficial muscles of the dorsal surface of the leg.
The anterior half of the surface of the thigh is occupied by two
muscles already noted, the rectus anticus femoris and the vastus
externus. Posterior to the last-named muscle are the biceps
femoris, the semimembranosus, and the rectus internus minor,
the latter forming the hinder margin of the leg.

On the shank will be seen the large gastrocnemius, forming the
calf of the leg ; the tibialis anticus, forming its anterior border ;
and the peroneus between the two.

Exercise 23. Draw the dorsal aspect of the leg.

Without injuring the bones trace each of these muscles to its
origin and insertion and determine which muscles are extensors
and which are flexors.

The Skeletal System. This system is made up of two portions,
the exoskeleton and the endoskeleton. The exoskeleton in ver-
tebrates consists of the hardened bony or horny structures which
develop in the skin and furnish an external protection to the an-
imal. These structures are very poorly represented in the frog.
The skin is naked, no bony or horny scales or other hardened in-
tegumental structures being present. The toes are also without
claws. The only exoskeletal structures are the teeth and certain
bones called membrane bones which form a part of the skull.
These bones are, however, so intimately joined with the other
bones and cartilages of the skull that they will be studied in
connection with them.

The endoskeleton consists of the bony and cartilaginous frame-
work of the body. It may be divided into the axial skeleton,
which includes the skull and the spinal column, and the appen-
dicular skeleton, which includes the framework of the two pairs
of appendages, that is, the legs and the girdles which join them
with the trunk. The breastbone may also be conveniently studied
with the appendicular skeleton.

The Appendicular Skeleton. The anterior appendages consist
of the forelegs and the pectoral girdle. This girdle is formed of
a right and a left half which meet midventrally ; here they enter

A FROG 191

into a close union with the breastbone and form with it a bony
and cartilaginous ring which almost completely encircles the
forward part of the trunk.

Each half of the pectoral girdle supports one of the forelegs
and is composed of two portions, a dorsal and a ventral portion.
The former portion consists of two skeletal pieces of nearly equal
size, the suprascapula and the scapula, which lie respectively on
the dorsal and lateral sides of the body. The suprascapula —
the dorsal half — is a broad, thin plate which extends upward
over the spinal column. Its broad, free dorsal end is composed
of cartilage ; the remainder of it is bone. The scapula is an elon-
gated plate of bone which extends from the suprascapula to
the ventral side of the body.

The ventral portion of the pectoral girdle consists of two
bony and three cartilaginous skeletal pieces. The two bones are
the coracoid and the clavicle, the former being the larger and the
more posterior in position, and extending from the scapula to the
midventral line. Joining them and the scapula is an irregular
cartilaginous mass in the hinder side of which is a depression, the
glenoid cavity, in which the humerus articulates.

The breastbone, or sternum, lies in the medial area of the body,
between the ventral ends of the two pectoral girdles, and is made
up partly of bone and partly of cartilage.

Remove the pectoral girdle with the foreleg from the body. In-
asmuch as it is not joined with the vertebral column it may be
removed by freeing it from the muscles in which it is embedded.
First locate accurately the delicate cartilaginous portions of
the sternum ; carefully locate also the delicate suprascapula on
each side of the body. Then insert the blade of a small scalpel
under the suprascapula on one side and free it from the muscles
which lie over it. Pass the scalpel down to the scapula and
then to the ventral portion of the pectoral girdle. Do the same
on the opposite side of the body, and finally remove the entire
girdle with the breastbone from the body. Disarticulate and re-
move the two forelegs and very carefully clean away the muscles.

The Foreleg. The skeleton of the foreleg is composed of three
divisions, a proximal, a middle, and a distal division.

192 PHYLUM CHORDATA

The proximal division, or upper arm, is composed of a single
bone, the humerus. The head of it, which is cartilaginous, fits
in the glenoid cavity and forms the shoulder joint. At the distal
end is a large round projection, on each side of which is a ridge
forming the articular surface for the bone of the next division.

The middle division, or forearm, is composed of a single bone,
the radioulna. It is formed by the fusion of the radius and ulna,
the two bones which are present in the forearm of most verte-
brates. The larger part of this bone is the radius. Its proximal
end is concave, the projecting process on it being the olecranon,
or elbow. Its distal end has two articular surfaces.

The distal division is composed of the carpus, or wrist, and the
hand. The carpal bones are six in number, arranged in two rows,
a proximal and a distal row. The hand is made up of five digits,
of which the first digit, or thumb, is very small and rudimentary.
Each of the other four digits is composed of two parts : the
metacarpus, the long proximal bone which articulates with the
carpus ; and the phalanges, two or three small bones which form
the finger. The thumb contains a metacarpus alone.

Exercise 24. Draw an outline sketch of the pectoral girdle and breast-
bone, representing them in one plane, also the foreleg in outline,
showing accurately all the bones and cartilages; label them all
carefully.

The Posterior Appendages. These consist of the hind legs, and
the pelvic girdle which joins them with the trunk.

The pelvic girdle, like the pectoral, is composed of a right and
a left half which meet ventrally and form an arch. The dorsal
ends of the arch articulate with the last vertebra of the spinal
column, and at its ventral end, on each side, is the acetabulum,
the articular surface of the hind leg. Extending backward from
the last vertebra between the two sides of the pelvic girdle is the
long bone called the urostyle, which forms the hinder part of the
spinal column.

Each half of the pelvic girdle is composed of two portions, a
dorsal and a ventral portion. The former consists of the long,
slightly arched ilium, which forms the side of the arch and

A FROG 193

articulates dorsally with the last vertebra. The ventral portion
is disk-shaped and is composed of the ventral end of the ilium, a
small, triangular bone called the ischium, and a small, triangular
cartilage called the pubis ; the pubis is anterior to the ischium in
position.

Carefully strip the muscles from the pelvic girdle, disarticulate
it from the vertebral column, and remove it and the hind legs
from the body. Disarticulate the legs and thoroughly clean the
pelvis.

The Hind Leg. The skeleton of this leg closely corresponds to
that of the foreleg. It is made up of three divisions, a proximal,
a middle, and a distal division.

The proximal division, or thigh, is composed of a single bone,
the femur, the head of which fits into the acetabulum and forms
the hip joint.

The middle division, or shank, is composed of a single bone, the
tibiofibula. It is formed by the fusion of the tibia and the fibula,
the two bones which are present in the shank of most vertebrates.
The line of division between the two is very distinct.

The distal division is composed of the tarsus, or ankle, and the
foot. The tarsal bones are five in number, arranged in two rows,
a proximal and a distal row. The proximal row consists of two
long bones, the astragalus and the calcaneum, which are united
at both ends. The latter is on the inner side of the foot and cor-
responds to the heel bone of the higher vertebrates. The distal
row is composed of three very small bones. The foot is made up
of six digits, of which one is supernumerary and rudimentary ; the
others are the five digits which characterize the typical vertebrate
foot. The supernumerary digit is on the inner side of the foot and
consists of from one to three small bones. Each of the other five
is composed of two parts : the metatarsus, a long bone which
articulates with the tarsus ; and the phalanges, two to four smaller
bones which form the toe. The first digit on the inner side of the
foot corresponds to the great toe.

Exercise 25. Draw the ventral aspect of the leg on a scale of 2, show-
ing accurately the outlines of the bones and cartilages.

194 PHYLUM CHORD ATA

The Axial Skeleton. The Vertebral Column. Strip the muscles
from the back. Disarticulate the head from the trunk.

The vertebral column is composed of nine vertebrae and a long,
unsegmented bone called the urostyle, which forms its posterior
portion. Four regions may be distinguished in it : a cervical
region, consisting of the first vertebra ; a thoracolumbar region,
consisting of the succeeding seven vertebras ; a sacral region, con-
sisting of the last vertebra ; and the urostyle, which represents a
caudal region.

A vertebra is made up of the following parts : the centrum, or
body, which is the cylindrical ventral portion ; the neural arch,
on the dorsal side of the centrum, which with it forms the neural
canal in which lies the spinal cord ; and the transverse processes,
a pair of long lateral projections. The neural arch is made up of
a pair of neural processes, which form its sides, and the median
neural spine, or spinous process, which forms its roof. On the an-
terior surface of the neural arch is a pair of articular projections
called the prezygapophyses ; on the posterior surface is a pair of
corresponding postzygapophyses. It is by these projections that
the vertebrae are locked together. Note the difference in size in
the transverse processes of the various vertebrae.

The first, or cervical, vertebra is called the atlas. It differs from
the other vertebrae principally in that it has no transverse proc-
esses (although occasionally they have been found) , no prezyga-
pophyses, and a thinner centrum. On its anterior surface is a pair
of depressions into which fit the articular processes of the skull,
the condyles. The last vertebra, or sacrum, has large, transverse
processes with which the pelvic girdle articulates. It lacks post-
zygapophyses, and on the hinder surface of the centrum is a pair
of prominences which articulate with the urostyle.

Exercise 26. Draw a view of the ventral aspect of the vertebral
column.

Exercise 27. Remove the second vertebra, clean it thoroughly, and
draw a view of its hinder end.

The Skull. The skull is composed of two regions, the cranium
and the visceral skeleton. The former incloses and protects the

A FROG 195

brain and the organs of special sense ; the latter surrounds the
mouth and pharynx, forming the framework of the jaws and
tongue.

We shall begin with the study of the lower jaw and the hyoid
apparatus, which supports the tongue. These structures belong
to the visceral skeleton. The hyoid is a thin plate of cartilage,
with a pair of long anterior and a pair of short posterior pro-
jections, which is embedded in the muscles of the lower jaw.

The lower jaw, or mandible, is a paired structure which is com-
posed on each side of two bones called the angular and the den-
tary, the former of which is posterior to the latter, and a cartilage
which articulates with the upper jaw.

The mandible is without teeth.

Without disarticulating the lower jaw, carefully dissect the
hyoid apparatus from the floor of the mouth.

Exercise 28. Remove and clean the mandible and the hyoid appa-
ratus and draw them on a large scale.

Thoroughly clean the remainder of the skull, but do not remove
the tympanum.

Observe the great flatness and breadth of the skull. At its
hinder end is the foramen magnum, the large opening through
which the spinal cord enters the brain cavity. The cranium,
which protects the brain and the special sense organs, is the
narrow medial portion of the skull. On each side of it is the large
oval opening in which the eye lies. The arch-shaped sides of the
skull are formed by the upper jaw and other portions of the
visceral skeleton.

The cranial bones fall into two distinct groups, those form-
ing the brain case, or cranium proper, and those forming the cap-
sules of the special sense organs. The primitive cartilaginous
cranium, which is partly replaced or covered by these bones, per-
sists throughout the life of the animal in great part, and appears
on the surface in a number of places. In skulls which have been
allowed to become dry this cartilage will have disappeared.

The cranium proper contains the following bones. At the
posterior end of the skull and surrounding the foramen magnum

196 PHYLUM CHORDATA

are the two exoccipital bones. Each exoccipital bears on its hinder
surface a convex articular projection, the occipital condyle, by
which the skull articulates with the atlas. At the side of each ex-
occipital will be seen a portion of the primitive cartilage.

The pair of long, flat bones which form the roof of the cranium
and lie directly in front of the occipitals are the frontoparietals.
Their anterior ends overlap a bony ring which encircles the for-
ward end of the brain case ; this is the ethmoid.

The ventral portion of the cranium is formed by two bones,
the ethmoid, just mentioned, and the parasphenoid. The latter
is a large T-shaped bone which covers the entire ventral portion
of the cranium and overlaps the ethmoid ; its lateral portions
extend to the auditory capsules. At the sides of it, back of the
ethmoid, will be seen the primitive cartilage. The large foramen
of the optic nerve will be seen in the side of the cranium between
the frontoparietal and the parasphenoid.

The Special-Sense Capsules. These are the auditory capsules
at the hinder end of the skull and the nasal capsules at its for-
ward end. The optic capsules do not ossify.

The auditory capsules are fused with the sides of the cranium
proper and consist largely of cartilage. On the ventral side of the
skull the lateral projections of the parasphenoid bone cover them.
On the dorsal and anterior sides a bone called the prootic is pres-
ent ; it will be seen abutting the hinder part of the frontoparietal
bone. Between the prootic and parasphenoid bones is the large
foramen of the trigeminal nerve.

Ventral to the prootic on the side of the skull is a depression ;
at the bottom of this is a large hole called the fenestra ovalis,
which looks inside the auditory capsule. The depression is the
tympanic cavity, — the middle ear of higher vertebrates, — which
in the fresh skull is covered laterally by the tympanic membrane,
or ear drum. The fenestra ovalis is joined with the tympanic
membrane by a small bony and cartilaginous rod called the col-
umella. This small structure, like the ossicles of the mammalian
ear, conveys the sound waves from the tympanic membrane to
the inner ear. Skulls from which the tympanic membrane has
been removed have often lost it.

A FROG 197

The two nasal capsules lie side by side, fused with the anterior
end of the cranium proper, and are also composed largely of car-
tilage. The ringlike ethmoid bone, which, as we have seen, forms
the anterior end of the brain case, also forms the posterior end
of the nasal capsules. Two pairs of membrane bones are present
in these capsules, the dorsal nasals and the ventral vomers. The
former are a pair of large bones which lie in a transverse position
covering the cartilage just in front of the ethmoid ; the latter are
a pair of bones also in front of the ethmoid, on the ventral surface
of the skull, each bone bearing a group of small teeth.

The upper jaw and the other remaining portions of the visceral
skeleton still remain to be described. The upper jaw forms two
distinct arches, an outer, or maxillary, arch, and an inner, or pala-
topterygoid, arch.

The three bones of the maxillary arch, on each side, are the
quadratojugal, the small posterior bone ; the maxillary, the long,
thin bone which bears most of the teeth ; and the premaxillary,
the small anterior bone which forms the anterior end of the skull.

The two bones of the palatopterygoid arch, on each side, are
the pterygoid and the palatine. The former is a large bone which
lies at the hinder end of the skull, medial to the quadratojugal
and the maxillary, and is best seen on the ventral surface. The
palatine is a slender bone which lies on the ventral surface and
extends from the maxillary at the forward end of the pterygoid
transversely to the ethmoid.

The bones and cartilages by which the lower jaw is suspended
from the cranium are called the suspensorium. In the frog it is
formed on each side by a small cartilage, the quadrate, and a bone,
the squamosal. The quadrate Hes at the extreme lateroposterior
end of the skull, in close connection with the quadratojugal bone
and between the pterygoid and the squamosal ; the lower jaw ar-
ticulates with its outer surface. The squamosal is a T-shaped
bone which lies on the dorsal surface of the skull in the region
of the auditory capsule ; it supports the tympanum.

Exercise 29. Draw a view of the dorsal aspect of the skull. Show
accurately the outlines of the bones and cartilages.

Exercise 30. Draw a similar view of the ventral aspect.

198 PHYLUM CHORDATA

Class : Pisces. Order : Teleostei
THE PERCH

The perch is one of the commonest fresh-water fishes. It is
found almost everywhere in streams, ponds, and lakes, where it
lives on small aquatic animals of every kind. Any other bony
fish may be used in place of the perch ; the differences in structure
which exist will not confuse the dissection. Two or three speci-
mens will be needed for each student, but a partial dissection
can be performed with a single specimen. During the progress of
the dissection they should be kept in a 5 per cent solution of
formalin or in cold storage.

Observe the form and external markings of the animal. The
body of the perch, like that of the majority of teleosts, is elon-
gated and laterally compressed, with a wedge-shaped head and
a terminal mouth ; the hinder end is less compressed than the
forward portion and terminates with the tail fin, which is the
principal organ of locomotion. The other fins are all of good size
and may be used both for purposes of locomotion and as a means
of defense against attack, the sharp spines with which most of
them are provided being formidable weapons.

The entire body, with the exception of a part of the head and
the fins, is covered with scales, which overlap one another poste-
riorly. Examine them carefully on different parts of the body
and note their arrangement and difference in size. If the fish is
fresh, note the slimy, transparent epidermis which covers the
scales ; if it is not fresh, scrape off some of the dried epidermis.
Note the lateral line which runs along each side of the body
parallel with the back its entire length ; it is an organ of special
sense, being sensitive to vibrations in the water.

Observe the color bands and their arrangement. Are they bi-
laterally symmetrical ? Note that the color consists of an aggre-
gation of small dots, except where it forms solid masses. These
dots are pigment cells; they are just beneath the epidermis in
the outer layer of the dermis and may be scraped off with the
epidermis. Note the structure of a single dot ; it will be seen to

THE PERCH 199

consist of a black central kernel — the body of the cell — sur-
rounded by a halo of fine dots, which constitute its outlying pro-
jections. Many fishes have the power of changing their color in
a remarkable degree, although it is probably a reflex action in
them and not under the control of the will. It is accomplished
by the often very rapid variation in the extent of these pigment
cells, which in such cases are amoeboid.

The body may be divided into three regions, the head, trunk,
and tail, the boundary between the latter two regions being the
anus. There is no neck.

Vertebrates which live in the water differ much from those
living on the land in the arrangement of the body regions. Water
animals must force their way through a dense medium, and
hence the forward portion of the body is rigid and usually more
or less wedge-shaped. A neck region is thus absent, since a neck
region is essentially flexible. Even in those mammals which have
adapted themselves to a wholly aquatic life, as the cetaceans,
the neck region is so much reduced that the head and trunk are
in direct contact with each other. In every animal which moves
rapidly, however, at least one flexible body region must be pres-
ent, where the body can turn when the direction of movement
is to be changed. In the fish this is accomplished in the caudal
region ; in most mammals it is in the lumbar region.

The Head. The head of fishes differs from that of land verte-
brates in that it contains the organs of respiration and the heart.
The head of the perch is flattened ventrally and dorsally, with
the large mouth at its anterior and the gills at its posterior end.
The opening of the mouth is bounded ventrally by the paired
mandibles, and dorsally by the paired premaxillae, above which
on each side is the flattened maxilla. The large eyes are without
Hds. A transparent membrane called the conjunctiva passes over
the front of the eye and is continuous with the epidermal layer
of the skin ; a deep fold of the skin is also present around the eye,
joining it with the skin of the head, and yet permitting it con-
siderable freedom of motion in its socket.

In front of the eyes are two pairs of nostrils ; there is, however,
but a single pair of nasal capsules, each capsule having two ex-

200 PHYLUM CHORDATA

ternal openings. Note the difference in shape of these two nostrils
and the valve which overhangs the anterior one. The nasal cap-
sules do not open posteriorly into the mouth, but are wholly
sensory in function.

At the posterior end of the head is the large operculum, or gill
cover, and at its hinder margin, the gill openings. Note the sharp
protective spine which projects back from each gill cover. Along
the hinder and lower border of the gill cover is the branchiostegal
membrane, supported by seven parallel, bony rays, the branchi-
ostegal rays, which forms a valve guarding the gill opening. Un-
derneath the gill cover on each side will be seen the four gill
arches, which bear the red gills, and the clefts between the arches.
Note the rudimentary gill, which appears as a red patch on the
inner surface of the gill cover in front of the first gill arch.

Cut off the left gill cover and probe between the gill clefts into
the pharynx. Observe carefully the form and position of the gill
arches and the double row of gill filaments on each ; also the gill
rakers, the row of spiny projections on the side of each arch,
which prevent food from passing through the clefts.

The Trunk and Caudal Region. These two regions pass gradu-
ally into each other ; they bear the appendages. At the posterior
end of the trunk are the anus and the genital and urinary pores.
The anus is the largest and most anterior of these three openings ;
the other two are minute and are situated behind it on a small
papilla. Behind this is often a transverse depression.

The Appendages. Two kinds of appendages are present, the
paired fins and the median fins. The latter are alone present in
the lowest fishes; they are simply dorsal and ventral flattened
expansions of the body which are stiffened by bony rays. In the
perch two dorsal fins and one ventral, or anal, fin are present, and
one caudal fin. Note carefully which of these fins have sharp,
spiny rays, and in which the ends of the rays are divided and
flexible. Observe that the two dorsal fins are nearly continuous
with each other.

The paired fins are also expansions of the body wall, stiffened
by bony rays; they are homologous to the appendages of the
higher vertebrates. Two pairs are present : an anterior pair,

THE PERCH 20I

the pectoral fins ; and a posterior pair, the ventral, or pelvic, fins.
The former are nearly vertical in position and are situated on the
side of the trunk just behind the gill cover. They are supported
by a bony arch within the body wall just back of the gills, which
is called the pectoral girdle. The pelvic fins are a short distance
behind them and are nearly horizontal in position. Note that in
all the fins the rays with split tips are segmented.

Exercise 1. Draw an outline of the right side of the animal ; do not

draw the scales. Label the organs carefully.
Exercise 2. Draw the ventral view.

The Internal Organs. It will be well first to cut off the sharp
tips of the dorsal fins to keep them from hurting the hands. The
internal organs will be exposed by removing the left side of the
body wall. After placing a probe in the anus to mark it, make a
straight incision through the body wall from just in front of the
anus to the mouth, cutting through the midventral point of the
lower jaw. Care must be taken not to cut the organs which lie in
the body cavity. Now lay the fish in a dissecting pan with the
head to your left and its ventral side toward you, lift up the cut
edge of the body wall with forceps, and with scissors cut away
the left body wall and remove it, taking great care not to injure
any of the internal organs. The ribs of the left side and the
muscles attached to them, the left pectoral fin, and the left pelvic
fin will thus have been removed.

It is best in doing this to work from the midventral incision up-
ward, as in this way the internal organs are brought gradually
into view as the work proceeds. The liver, intestine, and repro-
ductive organs, the latter sometimes very large, will first be seen ;
then, dorsal to them, the air bladder. This organ adheres closely
to the body wall and special care must be taken not to injure it ;
its ventral wall forms a wall across the body cavity which is
tough and strong, its lateral walls becoming thinner dorsally.

Place the animal in a pan of water and examine the organs,
without, however, disturbing them. Note the glistening perito-
neum, the membrane which lines the abdominal cavity ; it passes
along the ventral side of the air bladder. The mesenteries.

202 PHYLUM CHORDATA

the membranes which support the intestine and the other organs
in the abdominal cavity, are folds of the peritoneum.

The body cavity is made up of two divisions, the larger and
posterior of which is the abdominal cavity, the anterior and very
much smaller one being the pericardial cavity. The former is
lined by the peritoneum and contains most of the viscera ; the
latter is lined by the pericardium and contains the heart. These
two cavities are separated from each other by the false diaphragm,
which is composed of the posterior wall of the pericardium and
the anterior wall of the peritoneum ; it is not homologous to the
diaphragm of mammals.

In the abdominal cavity the largest organ is the air bladder,
which extends the entire length of the cavity and occupies the
dorsal half of its space. On the inner surface of its ventral wall a
pair of red patches composed of a network of capillaries will be
noticed. The air bladder is a hydrostatical apparatus by means
of which the fish can maintain its position in the water at different
depths without conscious effort.

At the anterior end of the abdominal cavity and just back of
the false diaphragm is the large red liver ; at the posterior end,
running back to the anus, is the intestine, which is usually in-
closed in fat. The anterior portion of the intestine forms a coil
lying a httle to the right of the median plane, within which, often
embedded in fat, lies the spleen. On the animal's left, more or less
covered by the hinder border of the liver, is the stomach, a large,
cylindrical body; alongside of it are several elongated pyloric
appendages.

The gonads, or genital glands, consist of the paired testes in the
male and the single ovary in the female, and lie dorsal to the in-
testine, extending from the hinder end of the cavity forward. If
the female is studied during the breeding season, the ovary may
occupy a large portion of the abdominal cavity and conceal the
other organs.

In the pericardial cavity, dorsal to the thick muscles between
the gills, will be seen the heart. It consists of the median ven-
tricle, a large, muscular organ, at the sides of which appears the
deep-red auricle ; at the back of (dorsal to) this organ is the sinus

THE PERCH 203

venosus, a large, deep-red sac which communicates with the
auricle. In front of the ventricle is the large bulbus arteriosus,
which is the beginning of the aorta.

Exercise 3. Make a semidiagrammatic sketch of the left side of the
fish, showing the opened body cavity and its organs as they appear
before they have been disturbed ; label them all carefully.

The Digestive System. This consists of the mouth, pharynx,
oesophagus, stomach with the pyloric appendages, intestine, and
liver.

The Mouth and Pharynx. Cut away the left half of the lower
jaw and the gill arches ; the mouth and pharynx are thus fully
exposed. They will be seen to form a single large space extending
from the opening of the mouth to the oesophagus, the pharynx
being the portion of the space which contains the gills.

The teeth are very small and are present not only in the upper
and lower jaws but also on the roof of the mouth and the roof
and floor of the pharynx. Examine them carefully with the aid
of a hand lens. There are three groups of teeth on the roof of the
mouth, a small median patch of vomerine teeth, and a pair of
lateral patches of maxillary teeth. Note carefully the position of
the teeth on the gill arches.

Just within the margin of each jaw is a transverse membrane ;
probe behind them. These two membranes are the oral valves,
which prevent the water from flowing out again through the
mouth during respiration. Breathing consists of two actions,
the inspiration and the expiration. At the inspiration the oral
valves open and the branchiostegal membranes, which form
valves at the opening of the gill cover on each side, close ; at the
expiration the oral valves close and the branchiostegal valves
open, allowing the respiratory water to pass out through the
gill clefts.

Observe carefully the form and arrangement of the gill arches.
Note the gill rakers, the short, spiny projections on the gill
arches which prevent solid substances from passing out through
the gill clefts. Cut out a gill arch and examine the gills on it.
Observe that a double row of gill filaments is present. Study

204 PHYLUM CHORDATA

carefully the arrangement of these filaments with reference to
the gill arches and with reference to each other. In fishes the gills
are outgrowths of the wall of the pharynx. In the ventral wall
of the mouth is the tongue ; note its relation to the gill arches.

Exercise 4, Draw a sketch of the mouth and pharynx showing both
dorsal and ventral surfaces with the features above described.

Study the remainder of the digestive tract. Observe the short,
wide oesophagus, which joins the pharynx with the stomach.
Turn the left lobe of the liver to one side and observe the ante-
rior, or cardiac, end of the stomach. Note the shape of the liver
and the mesentery which attaches it to the anterior abdominal
wall ; cut this mesentery. Note the mesentery which joins the
stomach with the ventral wall of the air bladder, and cut it.
Note that a part of the intestine lies free and is not attached to
the body wall by a mesentery. Cut the oesophagus and remove
the entire digestive tract from the body, retaining, however, its
posterior attachment at the anus.

Study the various parts of the digestive tract. The stomach
has three distinct regions: an anterior region, which extends
straight back from its cardiac portion and ends posteriorly in a
blind sac ; a posterior region, which leaves the anterior region at
right angles near its middle and extends to the beginning of the
intestine; and the pyloric appendages, three long, cylindrical
blind sacs. Just back of these appendages is a slight constriction
which marks the pyloric, or hinder, end of the stomach.

The intestine begins at the pylorus and extends to the anus.
It is composed of three divisions : the duodenum, which includes
the anterior loop of the intestine, between the limbs of which the
spleen lies ; the small intestine ; and the rectum. The boundary
between the last two divisions is the circular ridge about an
inch in front of the anus. The liver is a large gland which com-
municates with the intestine by means of the bile duct. This
duct emerges from the gall bladder, which lies against the pos-
terior surface of the liver, receives a number of branch ducts
from the liver, and joins the intestine near the base of the pyloric
appendages. A pancreas has not been found in the perch.

THE PERCH 205

Exercise 5. Draw a semidiagrammatic sketch of the digestive tract
and label carefully all its parts.

The Urinogenital System : the Male Genital Organs. The testes

are a pair of white, elongated bodies which lie in the abdominal
cavity just ventral to the air bladder, to which they are joined
by a mesentery. They taper toward the hinder end and finally
fuse together, the median portion thus formed passing directly
to the genital pore just behind the anus. The actual size of the
testes depends upon the sexual condition of the animal ; during
the breeding season they are large and may extend into the
anterior portion of the abdominal cavity.

The Female Genital Organs. The ovary is a median body which
lies in the abdominal cavity between the intestine and the air
bladder, and is joined to the latter by a mesentery. It is an
elongated sac filled with small ova and varies in size with the
sexual condition of the animal ; no oviduct is present, the hinder
part of the ovary becoming gradually smaller and finally com-
municating with the outside through the genital pore just back
of the anus.

Exercise 6. Make a sketch of the genital organs.

The Urinary Organs. Remove the testes or the ovary. Dissect
the air bladder away from the body wall and remove it. Note
the thinness of its dorsal wall, where it lies just beneath the kid-
neys. These organs are a pair of slender, deep-red bands which
lie close against the dorsal body wall, one on each side of the
vertebral column. Their anterior ends unite just dorsal to the
oesophagus and form a large, dark-colored median mass called
the head kidney, which extends across the body cavity. Dissect
away the remains of the oesophagus and note the exact shape
and extent of the head kidney. The ureters are a pair of tubes
which run along the entire length of the medial borders of the
kidneys, joining at their hinder ends. The single vessel thus
formed then passes to the minute urinary pore just back of the
genital pore ; a small urinary bladder projects from the median
portion of the ureters.

2o6 PHYLUM CHORDATA

Exercise 7, Draw a sketch of the urinary system within an outline of
the body and the body cavity.

The nervous system consists of (i) the central nervous system,
which includes the brain and the spinal cord ; (2) the peripheral
nervous system, which includes the paired cranial and spinal
nerves and the sympathetic nervous system ; and (3) the special
sense organs.

We shall study first the special sense organs and then the
brain. The peripheral nerves will not be studied in this dissec-
tion. The special senses are located in the integumentary sense
organs, the nasal capsules, the eyes, and the ears.

The integumentary sense organs consist of (i) minute, scattered
sense buds which occur principally on the head but also on the
body, and (2) of the lateral line. This line is a straight canal in
the integument which extends along the side of the body from
the head to the caudal fin, with branches also upon the head, in
which are groups of sensory cells.

The nasal capsules are a pair of sacs situated in front of the
eyes and communicating with the outside by two nostrils on each
side ; no communication with the mouth is present. Cut off the
outer wall of one of the capsules and observe the delicate folds
of the sensory epithelium radiating from a central point.

Exercise 8, Make a drawing of the capsule on a large scale.

The Eyes. Observe the central pupil, through which light is
admitted to the interior of the eye ; the iris, which surrounds the
pupil ; and the transparent cornea, which lies in front of both and
forms the outer coating of the eye. Observe carefully the circular
fold of the skin which surrounds the front of the eyeball.

With strong scissors cut away the circular ridge of the skull
which surrounds the eye, and remove the muscles of the head
just beneath and back of it. Remove also the slimy fold of the
skin just mentioned, which surrounds the front of the eye. Note
that a transparent layer of the skin passes over the cornea and
may be peeled off ; this is the conjunctiva.

Observe the position of the eye in the orbit ; it is held in place

THE PERCH 207

by the optic nerve, which joins it at its inner end, and by six
small muscles. These muscles have their origin in the wall of the
orbit, and their insertion in the outer coating of the eyeball, the
movements of which they control.

Study these muscles. Press the eyeball downward and note on
its medial side the insertions of two muscles ; the anterior one is
the superior oblique, which goes from the eyeball to the inner,
anterior wall of the orbit ; the posterior one is the superior rectus,
which goes from the eyeball to the inner posterior wall of the
orbit. Push the eyeball backward and note the inferior oblique
muscle, which has its insertion on its anteroventral surface and
passes parallel with the superior oblique to the inner anterior
wall of the orbit. On the posterior side of the eyeball is the in-
sertion of the external rectus muscle, which runs to the inner
posterior wall of the orbit. Cut the superior oblique muscle at its
insertion in the eyeball ; beneath it will be seen the insertion of
the internal rectus, which runs back to the inner posterior wall
of the orbit. Cut all these muscles at their insertion in the eyeball
and pull it gently outward and forward ; the inferior rectus will
be seen, whose insertion is on the inner side of the eyeball and
which runs to the inner posterior wall of the orbit. Cut this
muscle and the optic nerve and remove the eyeball from the orbit.
Note the origins of the four rectus muscles in the posterior wall of
the orbit, and of the two oblique muscles in the forward wall of
the orbit.

Exercise 9. Draw a sketch of the orbit showing its muscles and the
optic nerve.

Study the eyeball. Its wall is composed of three coats, the
sclera, the choroid, and the retina. The tough outer covering is
the sclerotic coat, or the sclera, of which the cornea is the portion
in front. Cut the eyeball in two lateral halves ; remove the other
eye and cut it in an anterior and a posterior half ; study the in-
terior of both under water. Just back of the pupil is the spherical
crystalline lens, held in place by the iris ; do not remove it. This
is the shape of the lens in all vertebrates which live under water.
In a dense medium like water vision is necessarily limited in

2o8 PHYLUM CHORDATA

range, and fishes can see only those objects which are close to
them. An eye with a spherical lens is shortsighted.

The inner coating of the eye is the retina. Between it and the
sclerotic coat is the choroid coat, which is black on its inner sur-
face and lies just beneath the retina, and silvery on its outer sur-
face, just within the sclera. The portion of the choroid which
extends over the front of the eye is the iris, the central opening
of which is the pupil ; delicate muscles in the iris control the size
of the pupil.

Note the blind spot, the point where the optic nerve enters the
eye. From near this spot a slender projection of the choroid,
called the falciform process, extends through the retina to the
side of the lens, to which it is attached.

The two large chambers of the eye are the one between the
iris and the cornea, which is filled with the watery aqueous humor,
and that between the retina and the lens, which is filled with the
jeliylike vitreous humor.

Exercise 10. Draw a diagram representing the structure of the eye.

The Ear. The auditory organ consists of the membranous
labyrinth alone, which is embedded in the cranium back of the
eye ; no external opening exists. To dissect it out is a difficult
task, which will not be done in this dissection.

The Brain. If the brain has been kept in strong formalin, as
directed, it will have been hardened and will be in good condition
for dissection ; if, however, its condition is for any reason no
longer suitable, a fresh animal must be taken.

Remove all the skin and the thick muscles from the head and
the high-arched dorsal portion of the trunk just back of it. With
a strong scalpel or scissors cut away the roof of the skull and the
dorsal wall of the neural canal of the spinal column. The brain
does not nearly fill the cavity of the skull but is surrounded by a
granular, fatty tissue ; carefully remove this substance and ex-
pose the brain.

Study its dorsal surface. It is made up of five divisions. The
first and anterior division is the cerebrum ; it consists of a pair of
hemispheres, at the anterior ends of which project the olfactory

THE PERCH 209

lobes. The third division, or midbrain, consists of the paired
optic lobes, the largest part of the brain, between which and the
cerebrum appears a small, median, sunken area, the diencepha-
lon, the second division. Projecting dorsally from this division
is the long and slender pineal body, which is the rudiment of
a third optic nerve ; it may have been removed in exposing the
brain. Back of the optic lobes are the fourth division, the cere-
bellum, and the fifth division, the medulla oblongata, which is
continuous with the spinal cord. Note the longitudinal median
groove in the medulla and spinal cord, and the paired lateral
swellings, the restiform bodies, at the anterior end of the former.

Exercise 11. Draw a dorsal view of the brain on a scale of 3 or 4.

The Vascular System. This is made up of the following organs :
(i) the heart, which receives venous blood from the tissues and
forwards it to the gills ; (2) the arteries, which carry {a) venous
blood from the heart to the gills, and (h) arterial blood from the
gills to the tissues ; (3) the veins, which carry venous blood to the
heart ; and (4) the capillaries.

The Venous System. Two distinct systems of veins are pres-
ent : one is composed of the systemic veins, which carry blood
directly to the heart ; the other is composed of those which carry
blood first to the liver, and is called the portal system.

We shall first study the latter system. Take a fresh animal and
open its body cavity by a midventral incision. Cut away both
the right and the left body walls between the incision and the air
bladder. The portal system consists of a pair of intestinal veins
which lie alongside the intestine, a splenic vein from the spleen,
several gastric veins from the stomach, and a pneumatocystic
vein from the air bladder, all of which unite to form the single
large portal vein. This vein passes to the hinder surface of the
liver, where it breaks into branches which carry the blood to all
parts of that organ.

The veins of this system are often difficult to dissect because of
the fat in which they usually lie embedded. First find the two
intestinal veins and free them from the fat. Lift up the spleen
and the duodenal loop in which it lies and find the point of union

2IO PHYLUM CHORD ATA

of these two veins ; find also the splenic vein, the gastric veins
from the stomach proper and the pyloric appendages, and the
pneumatocystic vein from the air bladder. Note the exact ar-
rangement of these veins ; note also the branching of the portal
vein on the hinder surface of the liver.

Exercise 12, Draw a semidiagrammatic view of the portal system.

The Heart and the Pericardial Space. Cut away all the muscles
between the gills, being very careful not to injure the heart or the
artery which issues from its forward end ; the pericardial cavity
will be thus exposed.

The heart in fishes stands in close relation to the gills, and lies
in the posteroventral portion of the head between them. It is
made up, as we have seen, of three parts : the large muscular
ventricle, the deep-red auricle, and the sinus venosus, the vessel
which lies across the hinder end of the pericardial space. Blood
is brought from the various tissues to the sinus venosus by the
veins, from which it flows into the auricle, and from it into the
ventricle.

The ventricle sends the blood forward into the bulbus arte-
riosus, a thick-walled vessel in front of the ventricle, which is
the beginning of the aorta. The muscular walls of the bulbus
are highly elastic, and when they are distended they exert a
constant pressure upon the blood which is passing through it ;
the blood thus flows forward in a constant stream and without
pulse beats.

Exercise 13. Draw a semidiagrammatic view of the pericardial cavity
with the heart and the bulbus arteriosus.

The Arterial System.^ The forward continuation of the bul-
bus arteriosus forms the ventral aorta. This vessel sends ofT
four pairs of afferent branchial arteries which carry blood to
the gills. Four pairs of efferent branchial arteries then run

1 The ventral aorta and afferent branchial arteries may be injected through the
bulbus arteriosus. In order to inject the remainder of the arterial system, cut off
the tail a short distance in front of the caudal fin and inject forward in the caudal
artery. This is the uppermost of the two vessels which lie in the canal formed by
the bony arches on the ventral side of the spinal column.

THE PERCH 211

from the gill arches dorsally to the median plane, where they
form the dorsal aorta.

Springing from the dorsal end of the first (anterior) efferent
branchial artery, on each side, is the large carotid artery, which
suppUes the head ; it soon divides into two branches, which pass
above and below the eye. The dorsal aorta passes along the
dorsal side of the body cavity, just beneath the spinal column,
to the posterior end of the body ; in the caudal region it becomes
the caudal artery and Hes in the ventral arches of the vertebrae.
It gives off the paired spinal arteries along its entire course;
the cceliac artery, a large median vessel which leaves the aorta
a short distance back of the branchial arteries and, breaking
up into a number of branches, supplies the digestive tract, air
bladder, and genital glands; and the two subclavian arteries,
which leave the aorta just back of the cceliac artery and go
to the pectoral fins.

First study the ventral aorta and the afferent branchial ar-
teries. Entirely remove the lower jaw and the left operculum,
but do not disturb the gill arches ; with scissors cut off the gills
from the arches. Follow the ventral aorta from the bulbus
arteriosus forward between the ventral ends of the gill arches.
Find the points where the four afferent arteries on the left side
leave the aorta, and trace the course of each along the hinder
side of the gill arches ; the transparency of the skin covering the
gill arches permits this to be done easily.

The remaining arteries will be studied after the veins.

Exercise 14. Draw the arteries just observed.

The Venous System (Continued)} The following are the sys-
temic veins, which carry blood directly to the heart. The short
hepatic vein enters the sinus venosus in the median plane. Press
the liver away from the false diaphragm and find it. Joining each
end of the transverse sinus venosus is a large and conspicuous

^ The veins are easily studied without injection, as the death of the animal leaves
them filled with blood. If it is wished to inject them, however, this may be done
through the caudal vein, which is the lowermost of the two vessels in the ventral
canal of the vertebral column..

212 PHYLUM CHORD AT A

duct, or sinus, called the Cuvierian duct. The two Cuvierian ducts
lie along the anterior end of the abdominal cavity, parallel with
each other and just in front of the liver, between the sinus
venosus and the head kidney. Inasmuch as the head kidney is
dorsal in position and the sinus venosus ventral, the Cuvierian
ducts have an almost vertical position in the body. Trace these
vessels from the sinus venosus dorsally ; they will be found just
behind the posterior gill arch. The dorsal ends of these two
ducts are connected by a horizontal sinus which lies just beneath
the spinal column.

Two pairs of prominent veins, the jugulars, or anterior cardi-
nals, and the posterior cardinals, bring blood from the anterior
and the posterior portions of the body, respectively, to the hori-
zontal sinus of the Cuvierian ducts. Just before joining the sinus
the two jugulars unite and form a median vein ; the two posterior
cardinals also unite and form a median vein.

The two posterior cardinals are large and prominent veins and
lie just beneath the vertebral column, partly embedded in the
kidneys ; the dorsal aorta lies between them. The left cardinal
is much larger than the right, and much longer, and begins its
course at the posterior end of the body as the caudal vein ; this
vein lies in the ventral arches of the spinal column and receives
the paired spinal veins.

In the trunk region it receives the left spinal and renal veins.
The right cardinal begins its course in the hinder part of the body
cavity and receives the right spinal and renal veins. Find the
cardinals in the mass of the kidneys and trace them forward to
the median cardinal and the Cuvierian ducts.

Find the median jugular vein and trace it and the two jugulars
and their branches as far as possible.

Entering the horizontal sinus are three additional veins. Two
of these are the small, paired subclavian veins, which bring blood
from the pectoral fins; the third is the intestinal vein, which
brings blood from the stomach, intestine, and genital organs.
Follow the course of these veins and their branches.

Exercise IS. Draw a diagram of the venous system, so far as observed.

THE PERCH 213

The Arterial System {Continued). We shall now study the ef-
ferent branchial arteries and the dorsal aorta and its branches.
The afferent arteries have already been seen ; the efferent ar-
teries lie immediately posterior to them on the gill arches. Make
a cross section of a gill arch and note these two arteries; the
afferent is the larger of the two. Remove the ventral wall of the
pharynx and the heart. Dissect away the mucous membrane
which covers the roof of the mouth and pharynx. Trace the
efferent arteries dorsally to the dorsal aorta. Follow the aorta
and the arteries which branch from it, as already explained (see
page 211).

Exercise 16. Draw a diagram showing the efferent branchial arteries,
the dorsal aorta and its branches, so far as observed. Carefully
label all.

Exercise 17. Draw a diagram of the entire vascular system.

The Body Muscles. Skin the side of the body. Note the great
muscle which forms the entire side and extends from the head
to the caudal fin. This complex muscle is made up of a succes-
sion of muscle segments, which are separated from one another
by connective-tissue septa. Each muscle segment is a plate, be-
tween two septa, and consists of parallel fibers which run across
from one septum to the other. Note the zigzag shape of the edge
of a muscle segment, as it appears on the surface of the body ;
note also that each muscle segment may be divided into a dorsal
and a ventral half.

Exercise 18. Make an outline of the fish and draw in it a few of
the muscle segments and septa.

Cut the tail off an inch or two behind the anus and study the
cut surface. The muscle segments appear here in groups of con-
centric circles. This appearance is due partly to the fact that
the inner edge of each muscle segment, which is attached to the
spinal column and to its dorsal and ventral processes, is much
anterior in position to the outer edge which appears on the sur-
face, and also partly to the zigzag shape of the muscle segments.
Cut horizontal and dorsoventral sections of the muscles and
prove this.

214 PHYLUM CHORD ATA

Observe the other structures in the cross section : the skin
with the scales and the lateral line, and the spinal column with
the neural arch on its dorsal side and the haemal arch on its ven-
tral side, the former containing the spinal cord and the latter the
caudal artery and vein.

Exercise 19. Draw the cross section.

The Skeletal System. The skeleton of the fish is made up of
two distinct portions, the exoskeleton and the endoskeleton.
The former is of dermal origin and consists of the scales and the
teeth. The scales are embedded in the dermis, or inner layer of
the skin; covering them on the outside is a thin layer of the
dermis and all the epidermis, which, however, are often broken
through by the sharp posterior tips of the scales.

Pull out several scales. Note the parallel lines of growth.

Exercise 20. Make a sketch of a scale.

The endoskeleton forms the bony and cartilaginous frame-
work of the body. It may be divided into (i) the axial skeleton,
which includes the skull and the vertebral column, with the ribs ;
and (2) the appendicular skeleton, which includes the skeleton
of the paired and the median fins.

To prepare the endoskeleton for study remove all the viscera ;
immerse the animal in hot water in order to soften the muscles,
and then carefully remove them from the skeleton. Separate
the bones from one another as little as possible at first, but leave
enough of the ligaments to bind them together. Be very careful
not to remove any of the bones of the head, some of which are
small and easily lost. If any of the bones are removed from the
skeleton in preparation, they must be carefully preserved. It is
of great practical use to have a dried skeleton at hand for com-
parison during the dissection. It is not necessary that this skele-
ton be thoroughly cleaned and mounted, but any skeleton that
has been dried after the muscles have been removed will answer
the purpose.

The Axial Skeleton. The Vertebral Column and the Ribs. The
vertebral column consists of a succession of bony vertebrae closely

THE PERCH 215

connected by intervertebral ligaments. They are deeply bicon-
cave ; the two concavities are joined by a central canal and all
these spaces are filled with the jellylike notochord. The noto-
chord thus forms a continuous structure which runs the length of
the vertebral column.

The vertebral column may be divided into two regions : the
trunk region, in which ribs are present, and the caudal region,
in which they are absent. Each trunk vertebra is composed of
a biconcave body, or centrum, from the dorsal side of which
arises the neural arch, and from the ventrolateral side of which
projects a pair of long, curved haemal processes, which form the
ribs. The neural arch is composed of a pair of neural processes
and a long, median neural spine ; it incloses the spinal cord.

The ribs are long, slender bones which form extensions of the
haemal processes. The ribs of teleosts are not homologous to
those of the higher vertebrates, but represent the distal ends of
the haemal processes. Extending from the ribs is also a series of
long, slender bones which lie in the muscle segments and may
be homologous to the ribs of the higher vertebrates; they are
called the intermuscular bones.

The caudal vertebrae differ from the others in that the haemal
processes meet in the midventral plane and unite in a haemal
spine, forming thus the haemal arch, and inclosing a space in
which lie the caudal vein and artery ; they do not bear ribs.

The haemal processes of the caudal vertebrae are thus homol-
ogous to the ribs of the trunk region. This homology is easily
seen by following the haemal processes forward from the caudal
region : at the point where the two regions meet the caudal
haemal processes separate from each other and become ribs.

Count the trunk vertebrae and caudal vertebrae. Note how
the former grade into the latter. The spinal column ends pos-
teriorly with a fan-shaped bone called the urostyle.

Exercise 21. Draw a posterior and also a side view of one of the trunk
vertebrae ; draw the same views of a caudal vertebra.

Make a sagittal section of a vertebra. Examine the concavities
with the aid of a lens and note the lines of growth. The centrum

2i6 PHYLUM CHORD ATA

increases in size by the addition of successive layers of bone to
the outside, each of which is a Httle larger than the previous one
and hence overlaps it ; the biconcave shape is the result of this
method of growth.

Exercise 22. Draw the section on a large scale showing these
features.

The Skull.^ The skull is composed of two portions: the
cranium, which protects the brain and the special sense organs ;
and the visceral skeleton, which surrounds the anterior end of the
alimentary canal and consists of the framework of the jaws and
gill arches. These two portions of the skull have had a very dif-
ferent origin and are different in character and appearance. The
cranium forms the entire dorsal half of the skull, and its con-
stituent bones and cartilages are for the most part immovably
knit together, so that they form a single compact whole. The
visceral skeleton forms the ventral half of the skull, and its bones
and cartilages are mostly loosely joined with one another and
with the cranium.

The Visceral Skeleton. Without at first removing any of its
parts, we shall now study the skeleton of the jaws and the gill
arches. The visceral skeleton is made up of a series of seven
arches called the visceral arches, which surround the mouth and
pharynx. Each visceral arch is a paired structure consisting of a
right and a left side which meet in the midventral plane. Each
side is also made up of several segments which fall approximately
into a dorsal and a ventral half.

The visceral arches may be divided into two groups: an
anterior group, consisting of the first two arches, which form the
skeleton of the jaws, the tongue, and the gill cover ; and a pos-
terior group, consisting of the last five arches, which support the
gills and are called the gill or branchial arches. Identify these
two groups.

We shall first study the anterior group. The first visceral arch
is the mandibular arch. The dorsal and ventral halves of it, to-

* For a study of the skull a large fish like a cod is a much more convenient ob-
ject than a perch.

THE PERCH 217

gether with certain other bones, form the skeleton of the upper
jaw, the roof of the mouth, and the lower jaw. The second vis-
ceral arch is the hyoid arch, the dorsal half of which enters into
relation with the mandibular arch and becomes the suspensorium
of the jaws, — that is, the connecting link between them and the
cranium, — and the ventral half of which forms the support of
the tongue and bears the opercular bones and the branchioste-
gal rays.

The Mandibular Arch. Identify the following bones forming
the lower jaw, the ventral half of this arch. At its proximal end
is the large articular bone, by means of which it articulates with
the quadrate bone of the upper jaw. The forward and middle
surfaces of the lower jaw are formed on each side by the large
dentary bone, which bears teeth. A small additional bone, the
angular, is present at the hinder end of the jaw.

Identify the bones of the upper jaw. Forming the forward end
and the lateral margins of this jaw are two pairs of bones, the
premaxillae and the maxillae. The former bear teeth and meet
each other in the middle Hne in front; the latter are without
teeth, being a pair of flattened rods back of the premaxillae, with
which their anterior ends articulate.

The hinder end of the upper jaw is made up of a series of paired
bones. The quadrate bone is a large, triangular bone at the hinder
end of the upper jaw on each side, with which the lower jaw ar-
ticulates. The metapterygoid is a large bone lying just above the
quadrate and back of the orbit. The ectopterygoid is an elongated
bone which projects in front of the quadrate. The endopterygoid
is a thin plate above the anterior end of the ectopterygoid and
forming a part of the roof of the mouth. In front of the last two
bones is the palatine, which helps form the roof of the mouth and
bears teeth.

Surrounding the orbit and forming its margin on all except the
dorsal side is a row of bones called the suborbitals, which are very
easily lost in the cleaning. The most anterior one is a large bone
between the orbit and the maxilla, on the surface of which will
be seen radiating canals belonging to the lateral-line system of
sensory canals*

2i8 PHYLUM CHORD ATA

The Hyoid Arch. The dorsal half of this arch forms the sus-
pensorium. It is formed by the hyomandibular and the sym-
plectic bones. The former is a large bone which extends from the
cranium ventrally, back of the metapterygoid to the symplectic ;
the latter is a small bone which lies just dorsal to the hinder end
of the quadrate ; the jaws are thus joined with the cranium.

The ventral half of the arch is formed by a row of bones, on
each side, which extend ventrally from the junction of the sym-
plectic and the hyomandibular and are called collectively the
hyoid bones. These bear the branchiostegal rays. In the middle
area between the right and left sides of the hyoid arch is the
basihyal bone, which supports the tongue.

Articulating with both the dorsal and ventral halves of the
hyoid arch on each side are bony plates and rods which support
structures guarding the openings of the gill slits : these are the
opercular bones and the branchiostegal rays. There are four
opercular bones : the preopercular lies along and back of the
hyomandibular and symplectic bones ; the opercular is a broad
bone just back of the preopercular, the hinder border of which is
drawn out into a long spine ; the interopercular and subopercular
lie along the ventral borders of the preopercular and the opercular
respectively.

Articulating with the ventral half of the hyoid arch are
seven slender arched bones, the branchiostegal rays, which sup-
port the branchiostegal membrane.

Exercise 23. Draw a semidiagrammatic view of the side of the skull
on a scale of 2, and carefully label all the bones just mentioned.

Disarticulate and remove the five gill arches and the hyoid arch
from the body. In the midventral area and joining these arches
is a row of median bones called the copulae ; the copula of the
hyoid arch is the basihyal, which supports the tongue and has
already been described. Note the gill rakers, the row of short
processes which spring from the inner surface of the gill arches ;
they act as strainers which permit water to pass through the gill
slits but keep solid objects out of them. Note gill rays, which
are on the outer surface of the arches and support the gills.

THE PERCH 219

Exercise 24, Make a drawing of the ventral aspect of the gill and
hyoid arches on a scale of 2 ; carefully label their various parts.

Remove the bones of the visceral skeleton and study the
cranium.

The cranium is 11 complicated structure, made up of a large
number of bones and cartilages. The bones are so tightly bound
together that the sutures are often indistinguishable until the
skull has been soaked a long time or boiled in a solution of dilute
caustic potash.

Two distinct regions are present in the cranium : (i) the cra-
nium proper, which is the brain case and comprises the bones
and cartilages in the medial portions of the skull ; and (2) the
capsules of the special sense organs, which protect the optic,
auditory, and nasal organs.

Observe the general character of the cranium. Note the small
brain cavity and the foramen magnum, the opening at the hinder
end of the cranium by which the spinal cord enters it. Note the
orbit, which occupies a large space on the side of the cranium ;
the portion of the cranium in front of it forms the nasal capsule ;
that back of it on each side is the auditory capsule. Note the
two fiat spines which project back of the auditory capsule on
each side.

Study the posterior aspect of the cranium. The hinder end
is formed of four occipital bones. The basioccipital is ventral ;
its concave posterior surface articulates with the centrum of the
first vertebra. The two exoccipitals inclose the foramen magnum
between themselves ; each bears a broad articular surface which
meets a corresponding one on the neural arch of the first vertebra.
The foramen of the vagus nerve is in the exoccipital on each side.
The supraoccipital is dorsal and bears a broad median spine. On
each side of these bones and forming the posterolateral part of
the cranium is the auditory capsule.

Exercise 25, Draw the hinder end of the skull on a scale of 2, showing
outlines of these bones.

Study the dorsal aspect of the cranium. Extending in front
of the supraoccipital are the two large frontal bones ; these form

2 20 PHYLUM CHORD ATA

the greater part of the roof of the skull and the medial walls of
the orbits. Immediately back of the frontal bone on each side
is the small, irregular parietal.

The auditory capsule is formed of an intricate complex of bones
at the posterolateral corner of the skull. From the hinder end
of the capsule two prominent spinelike processes project ; these
processes on the two sides, together with the median supra-
occipital spine, form the five prominent projections at the hinder
end of the skull.

Of the two lateral processes the more dorsal is the epiotic
process ; it is formed by the small epiotic bone, which lies lateral
to the supraoccipital and behind the parietal. The more ventral
process is the parotic process. It is formed by two bones, the
more dorsal being the pterotic, the more ventral the opisthotic ;
the latter contains the foramen of the glossopharyngeal nerve.
Directly in front of the opisthotic is the prootic bone, through
the anterior border of which go the trigeminal and facial nerves,
and directly above which is the sphenotic. These five otic bones
form the auditory capsule.

Just in front of the prootic and sphenotic are two bones, the
orbitosphenoid and the alisphenoid, the former being the more
ventral of the two ; they belong to the cranium proper, lie in the
lateral wall of the brain case, and are not seen from above.

In front of the frontals and forming the anterior end of the
cranium are the paired nasals, which form the roof of the nasal
capsule.

Exercise 26, Draw the dorsal aspect of the cranium on a scale of 2.

Study the ventral aspect. Three bones form the medial por-
tion of the ventral wall of the cranium : the basioccipital at the
hinder end, the vomer at the forward end, and the long, slender
parasphenoid, or parabasal, between. The vomer bears teeth and
forms the ventral wall of the nasal capsule.

In front of the orbits and directly above the vomer are the me-
dian ethmoid and the two lateral ethmoids, which form the
anterior end of the cranium proper and the hinder end of the
nasal capsules.

THE PERCH 221

The cranium, as we have already seen, is composed of the
cranium proper and the special-sense capsules. The former,
which is the brain case, contains the following bones: the oc-
cipitals, parietals, frontals, sphenoids, ethmoids, and parabasal.
The sense capsules inclose the auditory, optic, and olfactory
organs. The auditory capsule is made up of the otic bones. The
optic capsule does not ossify, but remains largely membranous ;
the sclera of the eyeball, however, contains cartilage, and the
suborbitals appear around the orbit. The nasal capsule contains
the nasals and the vomer ; the ethmoids also enter into relations
with the hinder part of it.

Exercise 27. Draw the ventral aspect of the cranium on a scale of 2,
showing outlines of the bones.

Exercise 28. Draw the lateral aspect.

Boil the cranium until the bones can be separated from one
another, and study them carefully.

The Appendicular Skeleton. Four median fins are present :
an anterior and a posterior dorsal fin, a ventral fin, and a caudal
fin. The framework of the two dorsal fins and that of the ventral
fin are essentially alike; each consists of a series of elongated
bones, the basals and radials. The former are long, flattened
plates which lie embedded between the muscles of the right and
left sides of the body, alternating with the haemal and neural
spines. Articulating with the outer end of each of these bones is
a radial, or fin ray, a straight, bony rod. Note the two kinds of fin
rays, the sharp, stiff ones, and the jointed, flexible ones. The ante-
rior basal does not bear a radial. The caudal fin is without basals,
the radials joining directly with the urostyle and the neural and
haemal spines.

Exercise 29. Draw the skeleton of a dorsal fin and that of the cau-
dal fin.

The Paired Fins. The pectoral fin consists of about fifteen fin
rays and four small basals, the latter articulating with the pec-
toral girdle. This structure is formed of several bones, of which
two articulate with the basals, the former of these bones being

2 22 PHYLUM CHORDATA

dorsal to the latter. In addition to these are a number of bones
which join the fin with the cranium.

Exercise 30. Draw the pectoral fin and girdle.

The Ventral, or Pelvic, Fins. Each of these consists of about
five radials. At their proximal ends they articulate with a large
plate which represents the fused basals. These two plates lie
alongside of each other in the median area ; no pelvic girdle is
present.

Exercise 31. Draw the pelvic fins.

APPENDIX

A SYNOPSIS OF THE CLASSIFICATION OF ANIMALS
PHYLUM I. PROTOZOA
Single-celled animals, aquatic and microscopic.

Class I. Sarcodina. Protozoans with more or less protractile pseu-
dopodia.

Order i. Rhizopoda. Pseudopodia without axial filament and usually
very retractile. Ex. Amoeba,

Order 2. Heliozoa. Fresh-water Sarcodina with siliceous skeleton and
raylike pseudopodia, each with an axial filament. Ex. Actinospherium.

Order 3. Radiolaria. Marine Sarcodina with siUceous skeleton. Ex.
Polycystina.

Class 2. Mastigophora (Flagellata). Protozoans with one or more vibra-
tile flagella. Ex. Euglena.

Class 3. Sporozoa. Protozoans which are internal parasites and have
no locomotory organs as adults. Ex. Gregarina.

Class 4. Infusoria. Protozoans with cilia or sucking tentacles.

Order i. Ciliata. Ciliate infusorians. Ex. Paramecium.

Order 2. Suctoria. Infusorians with sucking tentacles. Ex. Acineta.

PHYLUM II. PORIFERA

Sessile, mostly colonial animals without specialized organs or tissues;
body wall pierced by numerous pores or canals and usually stiffened by
either calcareous or siliceous spicules and either with or without spongin
fibers.

Class 1. Calcarea. Sponges with calcareous spicules and of simple struc-
ture. Ex. Grantia.

Class 2. Hexactinellida. Glass sponges with six-rayed siliceous spicules.
Ex. Euplectella.

Class 3. Demospongiae. Massive sponges with either siliceous spicules
or spongin fibers or both. Ex. Spongilla.

223

2 24 APPENDIX

PHYLUM III. CCELENTERATA

Radiate animals with a single but often branched internal cavity and no
ccelom.

SuBPHYLUM I. Cnidaria. Coelenterates provided with nettle cells.

Class I. Hydrozoa (Hydromedusae). Hydroid polyps and jellyfish, the
former without mesenterial ridges and the latter with a velum.

Order i. Hydrariae. Fresh-water hydroids of simple structure. Ex. Hydra.

Order 2. Hydrocorallinae. Coral-hke marine hydrozoans. Ex. Millepora.

Order 3. Tubulariae. Hydroids without hydrotheca; medusae with gon-
ads on the manubrium. Ex. Pennaria.

Order 4. Campanulariae. Hydroids with hydrotheca ; medusae with gon-
ads on the subumbrella. Ex. Obelia.

Order 5. Trachomedusae. Hydroids (when present) minute and of sim-
ple structure; medusae usually large with gonads on the subumbrella.
Ex. Gonionemus.

Order 6. Narcomedusae. Hydroids wanting ; medusae with lobed rim.
Ex. Cunina.

Order 7. Siphonophora. Free-swimming colonial hydrozoans. Ex. Phy-
saHa.

Class 2. Scyphozoa (Scyphomedusae). Hydroids and jellyfish, the for-
mer with mesenterial ridges and the latter without a velum and often of
large size. Ex. Aureha.

Class 3. Anthozoa. Sea anemones and corals; soHtary or colonial poly-
poid cnidarians without medusoid generation.

Order i. Alcyonaria. Anthozoans with eight mesenterial ridges and eight
pinnate tentacles. Ex. Corallium.

Order 2. Zoantharia. Anthozoans with numerous mesenterial ridges and
numerous simple tentacles. Ex. Metridium.

SuBPHYLUM II. Ctenophora. Coelenterates with eight bands of cihated
ridges on outer surface. Ex. Mnemiopsis.

PHYLUM IV. VERMES

The lower worms. Animals of primitive structure and without paired
locomotory appendages or distinct head.

SuBPHYLUM I. Platyhelminthes. Flatworms ; no anus present in most
forms and body cavity filled with a vesicular connective tissue called
parenchyma.

Class I. Turhellaria, Mostly free-living flatworms with ciliated outer
surface. Ex. Planaria.

Class 2. Trematodes, Flukes. Small parasitic flatworms with mostly
a branched digestive tract and an anterior mouth. Ex. Fasciola.

APPENDIX ^25

Class 3. Cestodes. Tapeworms. Elongated, usually segmented para-
sitic flatworms without digestive tract. Ex. Taenia.

Class 4. Nemertea. Nemertean worms. Elongated, mostly free-
swimming flatworms with a protrusile proboscis and a cilated outer surface.
Ex. Cerebratulus.

SuBPHYLUM 11. Nemathelminthes. Roundworms or threadworms;
mostly parasitic. Ex. Ascaris.

SuBPHYLUM III. Trochelminthes (Rotifera). Minute, aquatic worms
with mouth surrounded by cilia. Ex. Rotifer.

SuBPHYLUM IV. Bryozoa. Minute, sessile, colonial animals with a ridge
bearing ciHated tentacles around the mouth. Ex. Bugula.

SuBPHYLUM V. Brachiopoda. Sessile, marine, mollusk-like animals with
a dorsal and a ventral shell. Ex. Terebratulina.

SuBPHYLUM VI. Phoronidea. Sessile, marine worms living in tubes and
with a tentacular ridge around the mouth. Ex. Phoronis.

SuBPHYLUM VII. Chaetognatha. Minute, transparent, marine worms
with a slender body, two or three pairs of horizontal fins, and paired pre-
hensile bristles around the mouth. Ex. Sagitta.

SuBPHYLUM VIII. Sipunculoidea. Elongated, marine worms, the an-
terior portion of which can be invaginated and is usually surrounded by
tentacles, Ex. Sipunculus.

PHYLUM V. ANNELIDA

The higher worms. Elongated, segmented worms which have paired,
unsegmented appendages, and a usually distinct head.

Class I. Archiannelida. No parapodia or setae. Ex. Polygordius.

Class 2. Chaetopoda. With setae, segmentally arranged.

Order i. Polychaeta. Mostly marine chaetopods with parapodia on which
are numerous setae. Ex. Nereis.

Order 2. Oligochaeta. Earthworms. Mostly fresh-water or land chaetopods
without parapodia and with few setae. Ex. Lumbricus.

Class 3. Hirudinea. Leeches. Annelids with a sucker at each end and no
appendages or setae. Ex. Hirudo.

Class 4. Myzostomida. Disk-shaped parasites of echinoderms with five
pairs of parapodia. Ex. Myzostoma.

PHYLUM VI. ARTHROPODA
Externally segmented animals with segmented appendages.

Class I. Crustacea. Aquatic, gill-bearing arthropods; two pairs of an-
tennae present.

2 26 APPENDIX

Division i. Entomostraca. Small, simply constructed crustaceans with a
variable number of body segments and without abdominal appendages.

Order i. Phyllopoda. Entomostracans with flat, leaflike appendages.

Suborder i. Branchiopoda. Elongated phyllopods with segmented body.
Ex. Branchipus.

Suborder 2, Cladocera. Laterally compressed phyllopods, the body
of which is not distinctly segmented and is inclosed in a bivalve shell ; sec-
ond pair of antennai are swimming organs and project from the shell.
Ex. Daphnia.

Order 2. Copepoda. Elongated entomostracans with distinctly seg-
mented body and without gills ; the female often carries one or two egg sacs.
Ex. Cyclops.

Order 3. Ostracoda. Minute, laterally compressed entomostracans with
entire body inclosed in a bivalve shell. Ex. Cypris.

Order 4. Cirripedia. Sessile, hermaphroditic entomostracans with body
inclosed in a calcareous shell ; barnacles. Ex. Lepas.

Division 2. Malacostraca. Crustaceans with a constant number (twenty)
of body segments and nineteen pairs of appendages; abdominal append-
ages present.

Subdivision i. Phyllocarida. Primitive malacostracans with carapace
and with leaf like thoracic feet. Ex. Nebalia.

Subdivision 2. Arthrostraca. Malacostracans with usually seven free
thoracic body segments, and with sessile eyes.

Order i. Amphipoda. Laterally compressed arthrostracans with gills on
thorax. Ex. Gammarus.

Order 2. Isopoda. Dorsoventrally depressed arthrostracans with gills
on the abdomen. Ex. Oniscus.

Subdivision 3. Thoracostraca. Malacostracans with carapace covering
the head and all or some of the thorax, and with stalked eyes.

Order i. Schizopoda. Small thoracostracans with carapace covering en-
tire thorax, and with one pair of maxillipeds. Ex. Mysis.

Order 2. Stomatopoda. Thoracostracans with three free thoracic body
segments and large abdomen. Ex. Squilla.

Order 3. Cumacea. Small thoracostracans with reduced carapace. Ex.
DiastyHs.

Order 4. Decapoda. Large thoracostracans with carapace covering
entire thorax, and with three pairs of maxillipeds.

Suborder i. Macrura. Elongated decapods with large abdomen. Ex.
Homarus.

Suborder 2. Brachyura. Broad decapods with reduced abdomen. Ex.
Cancer.

Class 2. Arachnoid ea. Arthropods lacking antennae and with body
usually consisting of cephalothorax and abdomen.

APPENDIX 227

Division i. Xiphosura. Large marine arachnoideans with a long, spike-
like telson. Ex. Limulus.

Division 2. Arachnida. Usually air-breathing arachnoideans with six
pairs of appendages.

Order i. Scorpionida. Large arachnids with a long, segmented abdomen
ending in a poisonous sting. Ex. Scorpio.

Order 2. Palpigradi. Minute arachnids with a long, segmented caudal
tilament. Ex. Kcenenia.

Order 3. Pedipalpi. Arachnids with a constriction between the cepha-
lothorax and the segmented abdomen. Ex. Thelyphonus.

Order 4. Solifugae. Arachnids with a constriction between the head and
thorax. Ex. Galeodes.

Order 5. Pseudoscorpionida. Arachnids without a constriction be-
tween cephalothorax and abdomen ; pedipalps chelate and very long. Ex.
Chelifer.

Order 6. Phalangida. Arachnids with extremely long, slender legs and a
segmented abdomen. Ex. Phalangium.

Order 7. Araneae. Spiders. Arachnids with a constriction between the
cephalothorax and the unsegmented abdomen. Ex. Agelena.

Order 8. Acarina. Mites. Arachnids with body not divided into ceph-
alothorax and abdomen, and unsegmented. Ex. Hydrachna.

Order 9. Linguatulida. Parasitic arachnids with ringed, vermiform body.
Ex. Pentastomum.

Order 10. Tardigradi. Minute, aquatic arachnids. Ex. Macrobiotus.

Order 11. Pycnogonida. Sea spiders. Marine arachnids with very long
legs. Ex. Pallene.

Class 3. Tracheata. Air-breathing arthropods with one pair of an-
tennae.

Division i. Onychophora. Wormlike tracheates with indistinctly seg-
mented body and appendages. Ex. Peripatus.

Division 2. Myriapoda. Wormlike tracheates with distinctly segmented
body and appendages.

Order i. Progoneata. Body mostly cylindrical and with two pairs of
legs to a segment. Ex. Julus.

Order 2. Chilopoda. Centipeds. Flattened myriapods with one pair of
legs to a segment. Ex. Lithobius.

Division 3. Insecta. Insects. Tracheates with body divided into head,
thorax, and abdomen; with three pairs of legs and usually two pairs of
wings.

Order i. Aptera. Minute, wingless insects without metamorphosis. Ex.
Lepisma.

Order 2. Pseudoneuroptera. Insects with two pairs of net-veined wings,
biting mouth parts, and incomplete metamorphosis, Ex, Dragon fly.

228 APPENDIX

Order 3. Orthoptera. Insects with two pairs of wings (the first pair being
usually parchment-like), biting mouth parts, and incomplete metamorphosis.
Ex. Grasshopper.

Order 4. Neuroptera. Insects with two pairs of net- veined wings, biting
mouth parts, and complete metamorphosis. Ex. Ant lion.

Order 5. Coleoptera. Beetles. Insects with two pairs of wings (of which
the first pair are elytra), biting mouth parts, and complete metamorphosis.
Ex. Potato beetle.

Order 6. Hemiptera. Bugs. Insects with two pairs of wings, or wingless,
with sucking mouth parts in form of a jomted proboscis, and incomplete
metamorphosis. Ex. Aphis.

Order 7. Lepidoptera. Butterflies and moths. Insects with two pairs of
scale-covered wings, with biting or sucking mouth parts in form of a long,
unjoin ted proboscis, and complete metamorphosis. Ex. Bombyx.

Order 8. Diptera. Insects with one pair of wings, sucking mouth parts,
and complete metamorphosis. Ex. House fly.

Order 9. Hymenoptera. Insects with two pairs of wings, biting and
Ucking mouth parts, and complete metamorphosis. Ex. Bee.

PHYLUM VII. MOLLUSCA

Animals without paired locomotory appendages and with a soft, unseg-
mented body which is usually inclosed in a calcareous shell.

Class I. Amphineura. Symmetrical mollusks without a shell or with
one composed of eight pieces in a longitudinal row. Ex. Chiton.

Class 2. Scaphopoda, Symmetrical mollusks with a cylindrical shell.
Ex. Dentalium.

Class 3. Gastropoda. Snails. Mollusks with an asymmetrical, spiral
shell and a single mantle cavity.

Order i. Opisthobranchiata. Marine snails with posterior gills. Ex.
.^olis.

Order 2. Pulmonata. Fresh-water and land snails, without gills but with
lungs. Ex. Helix.

Order 3. Prosobranchiata. Mostly marine snails with anterior gills.
Ex. Fulgur.

Class 4. Pelecypoda. Symmetrical mollusks with a bivalve shell and
paired mantle cavities. Ex. Unio.

Class 5. Cephalopoda. Mollusks with a large head, which bears a num-
ber of long arms, and with a single mantle cavity.

Order i. Tetrabranchiata. Cephalopods with four gills and a large
convoluted shell. Ex. Nautilus.

Order 2. Dibranchiata. Cephalopods with two gills and either eight or.
ten arms ; shell when pr<;*ent^ concealed in the mantle. Ex. LoUgo.

APPENDIX 229

PHYLUM VIII. ECHINODERMATA

Radially symmetrical animals with calcareous plates or spicules in the
body wall.

Class I. Crinoidea, Sea lilies. Echinoderms which are sessile through-
out Hfe or only as larvae. Ex. Comatula.

Class 2. Asteroid ea. Starfish. Flattened, star-shaped echinoderms with
an ambulacral furrow on the under side of each ray. Ex. Asterias.

Class 3. Ophiuroidea, Brittle stars. Flattened echinoderms with long,
vibratile arms and without ambulacral furrows. Ex. Amphiura.

Class 4. Echinoidea. Sea urchins. Spheroidal or flattened echinoderms
without arms. Ex. Arbacia.

Class 5. Holothurioidea. Sea cucumbers. More or less wormlike echi-
noderms with oral tentacles. Ex. Synapta.

PHYLUM IX. CHORDATA

Animals with a dorsal central nervous system, an internal skeletal system,
consisting in the simplest cases of the notochord, and paired pharyngeal
slits and arches.

SuBPHYLUM I. Enteropneusta. Wormlike chordates with a large pro-
boscis in front of the mouth. Ex. Balanoglossus.

SuBPHYLUM II. Tunicata. Chordates in which the body is inclosed in a
tunic ; a large pharyngeal chamber and a ventral heart present.

Class I. Larvacea. Minute, free-swimming timicates with a long tail.
Ex. Appendicularia.

Class 2. Thaliacea. Free-swimming, transparent tunica tes. Ex. Salpa.

Class 3. Ascidiacea. Sessile, saccular tunicates, either shnple or colonial.
Ex. Molgula.

SuBPHYLUM HI. Leptocardia. Elongated, fishHke chordates, compressed
laterally and attenuated at both ends. Ex. Amphioxus.

SUBPHYLUM IV. Vertebrata. Chordates with distinct head, bearing
organs of special sense, with red blood, and usually with two pairs of
appendages.

Class I. Pisces. Fishes. Aquatic vertebrates which breathe by means
of gills, and usually with bony scales and paired fins. Ex. Perca.

Class 2. Amphibia. Amphibians. Vertebrates with giUs during a part
or all of their life, and usually with lungs ; scales mostly absent. Ex. Rana.

Class 3. Reptilia. Reptiles. Vertebrates with body covered with horny
scales or plates and without gills. Ex. Coluber.

Class 4. Aves. Birds. Feathered vertebrates whose anterior appendages
are wings. Ex. Callus.

Class 5. Mammalia. Mammals. Hair-covered vertebrates which suckle

their young. Ex. Felis.

GLOSSARY

Abdomen: in invertebrates, the posterior body division; in vertebrates

the ventral area back of the thorax.
Aboral: the side of the body opposite the mouth in a radiate animal.
Aciculum: a chitinous supporting rod in the parapodia of annelids.
Acontia: long threads armed with nettle cells in sea anemones.
Adductor muscle : a muscle which draws an organ toward the axis of the

body.
Air sacs : tracheal enlargements in insects.
Algae : very simple, green plants.
Alimentary tract: the digestive canal, the organ which ingests, digests, and

absorbs the food.
Alternation of generations: the alternate succession of sexual and asexual

generations in plants and animals.
Alveolus: a pyramidal ossicle which supports one of the five teeth in the

dentary apparatus of the sea urchin.
Ambulacral feet: tubular projections with sucker disks at their ends in

echinoderms.
Ambulacral groove: the elongated groove on the oral side of the rays of

the starfish.
Ambulacral pores: minute openings in the body wall in the starfish and

the sea urchin.
Ampulla: a saclike projection of the ambulacral foot in echinoderms.
Anal feelers: paired posterior projections.
Analogous: having a similar function.

Antenna: a segmented sensory appendage on the head of arthropods.
Anterior: at or toward the front end of the body.
Anus : the posterior opening of the digestive canal.
Aorta: a large artery leading directly from the heart.
Appendage : a projection from some part of the body.
Appendix : a short diverticulum of the intestine.
Aristotle's lantern: the dentary apparatus of the sea urchin.
Artery: a blood vessel carrying blood away from the heart to the tissues.
Arthrobranch: a gill attached to the joint between the leg and the body

in crustaceans.
Articulate : composed of a series of homologous segments.
Asexual: reproduction by division or budding and not through the agency

of the sexes.

I 231

232 GLOSSARY

Auricle : a chamber of the heart which receives the blood from the veins.
Avicularium : a structure shaped like a bird's head attached to the zooecium
in Bryozoa.

Balancers: the homologues of the metathoracic wings in Diptera.
Bilateral symmetry: having the right and left sides alike.
Bivalve : a shell composed of two distinct and equivalent parts or valves.
Bivium : the two rays of a starfish or a sea urchin which inclose the madre-

porite between them ; the two rays forming the upper side of the holothu-

rian's body.
Blastostyle: the reproductive polyp of a campanularian hydroid.
Body cavity: an internal space in the body in which lie the viscera ; thecoelom.
Body wall : the outer portion of the body, which usually bounds the body

cavity toward the inside.
Brachial : relating to the arms.
Branchial: relating to the gills.
Branchial heart: a lateral heart in the squid.
Branchiate : bearing gills.

Branchiostegite: a paired lateral fold of the body wall in crustaceans.
Brood sac: a chamber in which the eggs develop in certain crustaceans.
Bud: an outgrowth of an animal which becomes a new individual.

Caecum: a saclike appendage of the digestive tract.

Calcareous: formed of carbonate of lime.

Carapace: the shell covering a portion or all of the cephalothorax in

crustaceans.
Cardo : the basal division of the maxilla in insects.
Cellulose: the woody cell wall of plant cells.
Cephalothorax: a body division formed by the fusion of the head and the

thorax in arthropods.
Cercus: a paired projection at the posterior end in certain insects.
Cerebral: relating to the cerebrum, or brain.
Cheliped: the large grasping claw in many crustaceans.
Chitin: a hard and very resistant substance present in the cuticula of

arthropods.
Chloragogue cells: glandular cells surrounding the digestive canal of the

earthworm.
Chlorophyll: the green coloring matter of plants.
Chromatophores: pigment bodies.

Cilia: the numerous vibratory projections on the surface of certain cells.
Cirrus: a filamentous, sensory appendage of annelids; a protrusile, copu-

latory organ of flatworms.
Clitellum: a thickened, glandular region on the earthworm which secretes

the cocoon.

GLOSSARY 233

Cloaca: a tubular or saclike space which receives the discharge of various

organs.
Clypeus: a median sclerite in the face of insects just back of the upper lip.
Cnidoblast: a stinging cell in Cnidaria which contains the nematocyst.
Cocoon: a case containing one or more developing animals.
Coelom: the body cavity.

Collar: the ventral edge of the mantle in gastropods and cephalopods.
Colon: a division of the intestine.
Columella: the axis of a spiral snail's shell; a minute rodlike bone in the

ear of frogs.
Compound eye: an eye made up of a number of separate elements, or

ommatidia, in arthropods.
Conjugation: the fusion of two protozoans and interchange of nuclear

matter.
Connective tissue: a tissue whose principal function is to support and hold

in place other tissues and organs.
Coxa: the proximal segment of an insect's or a spider's leg, by which it

articulates with the body.
Crop: a dilated portion of the oesophagus.
Ctenidium: a respiratory organ in mollusks.
Cuticula: the outer layer of the integument of most invertebrates.
Cyst: a capsule containing an animal usually in a state of suspended ani-
mation.
Cysticercus: a cyst containing a tapeworm scolex.

Dentary apparatus: the five teeth and their supporting structure in the

sea urchin.
Development: the series of changes in the early life of any animal by which

it passes from the condition of a fertilized egg to that of the adult.
Dimorphism: the condition in which a species exists in two distinct forms,

as, for instance, male and female.
Distal: a position away from the point of attachment — opposed to

proximal.
Diverticulum: a saclike projection of a tubular organ.
Dorsal: on or toward the back.
Dorsal lamina: a ciliated ridge in the mid-dorsal line of the pharynx in

the ascidian.

Ectoderm: the outermost layer of cells in the Porifera and Coelenterata.
Ectosarc: the outermost layer of nongranular protoplasm in protozoans.
Elytra: the hard wing-covers in beetles.
Embryo: a young animal which is passing through its developmental

stages, usuaUy within the egg membranes or in the maternal uterus.
Encyst: the act of an animal in forming a cyst about itself.

234 GLOSSARY

Endopodite: the innermost of the two terminal branches of the typical

crustacean leg.
Endoskeleton: an internal supporting structure.
Endostyle: a ciliated and glandular groove in the midventral Hne of the

pharynx in ascidians.
Entoderm: the innermost layer of cells in the Porifera and Coelenterata.
Entosarc : the inner granular protoplasm in protozoans.
Epicranium: the sclerite forming the dorsal, median, and lateral walls of

the head in insects.
Epigynum: a cuticular plate covering or accompanying the female genital

pore in many species of spiders.
Epiphragma: the disk of calcified slime with which a land snail can close

the opening of its shell.
Epipodite: a membranous projection of the protopodite in crustaceans.
Excretion; the ehmination of nitrogenous wastes from the body.
Excurrent: passing outward.
Exopodite: the outermost of the two terminal branches of the typical

crustacean leg.
Extensor muscle : a muscle that extends an organ.
Extremity: a paired lateral or ventral appendage of the body of an animal,

used primarily for locomotion, although in many cases having second-
arily some other function.
Exumbrella: the aboral side of a medusa.

Femur: a segment of an insect's or a spider's leg; the thigh bone in ver-
tebrates.

Fertilization: the union of the spermatozoon and the ovum.

Flagellum: a vibratory threadlike projection of certain cells.

Flame cell: the terminal cell of an excretory tubule of flatworms.

Flexor muscle: a muscle that bends an organ.

Food vacuole : a globule of water containing food particles.

Front: the anterior median portion of the epicranium.

Funiculus: a mesenteric strand connecting the stomach pouch with the
body wall in bryozoans.

Funnel : the siphon of a cephalopod.

Ganglion: an aggregation of nerve cells.

Gastrolith: a calcareous body sometimes present in the stomach of crusta-
ceans.

Gastrovascular space: the central cavity in Coelenterata.

Gastrula: a stage in the development of the embryo in which two cell
layers only are present, the ectoderm and the entoderm.

Gena: the lateral portion of the epicranium in insects.

Genital plate: a sclerite at the posterior end of the abdomen in the male
grasshopper.

GLOSSARY 235

Giant fibers: three large fibers in the dorsal portion of the nerve cord in

the earthworm.
Gill : an organ for the breathing of air contained in the water.
Gill filament: ciliated vertical ridges on the surface of the gills.
Gizzard: a portion of the alimentary tract with thickened muscular walls;

a chewing stomach.
Gland: an organ which produces a secretion.
Glochidium: the larval form of Anodonta and Unio, which lives a parasitic

life in the skin of fishes.
Glottis: the opening from the pharynx into the trachea.
Gonotheca: the cuticular outer covering of the blastostyle.
Green gland: the kidney of a malacostracan crustacean.

Haemal: pertaining to the blood system.

Head: the anterior body division of the higher animals.

Heart: a muscular tubelike or saclike organ which propels the blood.

Hermaphroditic: having the two sexes united in one animal.

Hinge ligament: the flexible portion of a bivalve shell which joins the
two valves.

Homologous: having had a similar origin.

Host: the animal which harbors a parasite.

Hydranth: a feeding polyp in a hydroid colony.

Hydrocaulus : the stem of a hydroid colony.

Hydroid: the sessile, asexual generation of the Hydromedusae.

Hydrorhiza: the rootlike projections of a hydroid colony by which it is
attached.

Hydrotheca: the cuticular outer covering of the hydranth in campanularian
hydroids.

Hypodermis: the cellular layer which forms the inner portion of the integu-
ment of most invertebrates.

Hypopharynx: a median projection from the ventral wall of the pharynx
in insects.

Hypophysis: a ventral projection of the brain in vertebrates.

Hypostome: the projection of a hydroid's body which bears the mouth.

Imago: an insect which has completed its metamorphosis ; an adult insect.

Incurrent: passing inward.

Integument: the outer covering of an animal.

Interfilamentary connections: cross ridges which join the gill filaments in

clams.
Interlamellar partitions: vertical walls which join the two lamellae of a

clam's gills.
Intermediate host: the animal which harbors the larval form of a parasite.
Interray: one of the divisions of the radiate body of echinoderms.
Intestine: a division of the digestive tract.

236 GLOSSARY

Kidney: an excretory organ which eliminates nitrogenous wastes from the
body.

Labium: the lower lip of insects.

LabruHi: the upper lip of insects and of some crustaceans.

Lamella: a leaf like or plateHke structure.

Larva: a young animal which has left the egg and is leading a free life, but

which has not yet completed its development.
Lateral: a position to the right or left of the median Hne.
Ligament: a tough connecting band.
Ligula: the anterior portion of the labium in insects.
Lithocyst: a marginal sense organ in certain medusae.
Liver: a digestive gland.
Lophophore: a circular or horseshoe-shaped ridge bearing tentacles in

Bryozoa.
Lumen: the cavity within a tubular organ.

Macronucleus: the large nucleus of an infusorian.

Madreporite: a porous plate through which fluids enter the ambulacral

system.
Malpighian tubules: the kidney of insects and certain other arthropods.
Mandible : the anterior pair of mouth parts in arthropods ; the lower jaw

of vertebrates.
Mantle : the integumental fold in mollusks which secretes the shell.
Manubrium: the projection of a medusa's body which bears the mouth.
Maxillae: the paired mouth parts immediately behind the mandibles in

arthropods ; the upper jaw of vertebrates.
Maxillipeds: the anterior thoracic appendages which assist in mastication

in crustaceans.
Medusa: a medusoid which becomes a free-swimming jellyfish.
Medusoid: the sexual generation of a hydromedusan.
Megalopa: a larval stage of the crab.
Mentum: a division of the labium in insects.
Mesentery: a lamella which supports some one of the viscera.
Mesothorax: the second thoracic somite in insects.
Metamere : one of the serial, homologous body segments, together with its

appendages, which form the body of an articulate animal.
Metamorphosis : the quiescent period in the life of a holometabolic insect

during which it changes from a larva to an imago.
Metasoma: the primitive segmented trunk of an articulate animal.
Metastomium: the posterior portion of the prosoma of an annelid.
Metathorax: the third thoracic somite in insects.
Metazoa: the division of the animal kingdom comprising the many-celled

animals.

GLOSSARY 237

Micronucleus: the smaller of the nuclear bodies in infusorians.

Mother-of-pearl: the inner layer of the shell of mollusks.

Molt: to shed the cuticula or the outer portion of it.

Mouth parts: the masticatory appendages on the head of arthropods.

Mysis stage: a larval form of the lobster.

Nauplius : a larval form of crustaceans.

Nematocyst: the stinging organ in the Coelenterata which is within the

cnidoblast.
Nephridium: a urinary tubule in anneHds.

Nephrostome: the ciliated opening of a nephridium into the body cavity.
Nerve commissure: a nerve connecting the two members of a pair of

gangUa.
Nerve connective: a nerve connecting two ganglia not of the same pair.
Nettle cell: the stinging organ in the Coelenterata.
Neuropodium: the ventral division of the parapodium of an annehd.
Nidamental glands: the large glands which secrete the egg capsules in

the squid.
Notopodium: the dorsal division of the parapodium of an annelid.
Nucleus : a spheroidal body in a cell, the center of its activities.

Ocellus: a minute, primitive eye.

Ocular plate: the plate at the aboral end of a ray of the sea urchin.

(Esophagus: the gullet, the division of the digestive canal leading from the

pharynx to the stomach.
Ommatidium: a single element of the compound eye of an arthropod.
Ooecium: a structure in Bryozoa in which the embryo develops.
Operculum: the flap covering the gill slits in fishes.
Oral: relating to the mouth.

Oral groove: a groove leading to the mouth in ciliate infusorians.
Osculum: the cloacal opening in sponges.
Ossicles: the calcareous plates in the body wall of echinoderms.
Ovarioles: the tubules forming the ovary of an insect.
Ovary: the female sexual gland.

Oviduct: the tube leading from the ovary toward the outside.
Ovipositor: the organ by means of which certain insects deposit their eggs.
Ovum: the female sexual cell, the egg.

Pallial line: the line along which the margin of the mantle is attached to

the shell in mollusks.
Pallial sinus: the indentation in the pallial line caused by the insertion of

the siphonal retractor muscle.
Palp: a sensory organ near the mouth.
Pancreas: a digestive gland.
Papulae: the delicate projections of the body wall in the starfish.

238 GLOSSARY

Paragnatha: delicate lamellae just behind the mandibles in the crayfish
or the lobster.

Parapodium: the appendage of annelids.

Parasite: an animal which attaches itself to another and lives upon its
nutritive fluids.

Parenchyma: a vesicular connective tissue which fills the body cavity of
flatworms and leeches.

Parthenogenesis: reproduction by means of unfertilized eggs.

Pectoral: relating to the thorax or breast.

Pedicellariae: minute pincer-like organs present on the external surface of
starfishes and sea urchins.

Pedipalps: the second pair of appendages in the Arachnida.

Pelvic: relating to the region of the hind Hmbs.

Pen: the shell of the squid.

Pericardium: the membrane surrounding the heart.

Periopods: the thoracic appendages posterior to the maxillipeds in crus-
taceans.

Periostracum: the outer layer of the molluscan shell.

Periphery: the outer surface of a body.

Periproct: the region immediately around the anus.

Perisarc: the cuticular outer covering of a hydroid.

Peristome: a membrane surrounding the mouth in echinoderms.

Peristomium: the posterior portion of the head in most annelids, consisting
of the metastomium and the anterior somites of the metasoma.

Peritoneum: the membrane Hning the body cavity.

Pharynx: the division of the alimentary tract immediately back of the
mouth.

Plankton: a collective term referring to all small forms of life in the surface
waters of the sea or of fresh water.

Planula: the larva of many Coelenterata.

Pleopod: an abdominal appendage in crustaceans.

Pleurobranch: a gill attached to the body wall in crustaceans.

Plexus: a network.

Podical plates: paired sclerites at the posterior end of the abdomen in
certain insects.

Podobranch: a gill attached to the leg in crustaceans.

Polian vesicle : a sac extending from the ring canal in echinoderms.

Polymorphism: the condition in which a species exists in several distinct
forms.

Polyp : a sessile individual of any of the lower animals.

Polypide: the soft parts of a bryozoan.

Posterior: at or toward the hinder end of an animal.

Proboscis: a prehensile or sucking organ in certain animals, usually a por-
tion of the pharynx.

GLOSSARY 239

Proglottid: a tapeworm segment.

Prosoma: the primitive head of annehds made up of the prostomium and
the metastomium.

Prostate gland: the gland which secretes the fluid in which the sperma-
tozoa are suspended.

Prostomium: the anterior portion of the head of annelids.

Pro thorax: the first thoracic segment in insects.

Protopodite : the basal segment of a crustacean's leg.

Protractor muscle: a muscle which extends the organ to which it is
attached or moves it forward.

Proximal : a position toward the point of attachment — opposed to distal.

Pseudopodium: a retractile process in rhizopods.

Pulsating vacuole : a globule of excretory fluid in many protozoans.

Pulvillus : an adhesive pad on the foot of insects.

Pupa: the stage in the life of a holometabolic insect when it is undergoing
its metamorphosis.

Pyrenoids: protein bodies in Euglena producing starch.

Racemose vesicles: minute diverticula of the ring canal in starfishes.
Radial symmetry: having the parts or organs arranged symmetrically

about a common center.
Radial tubes: a portion of the gastrovascular space in the medusa.
Radula: the band of calcareous teeth in the pharynx of gastropods and

cephalopods.
Ray: one of the main divisions of the radiate body of echinoderms.
Receptaculum seminis: a receptacle for sperm in the female animal.
Rectum: the posterior division of the digestive tract.
Respiratory tree : a branched diverticulum of the rectum in holothurians.
Retractor muscles: muscles which draw in an organ to which they are

attached.
Rostrum: a projection of the carapace in crustaceans.

Sagittal: pertaining to a median dorsoventral plane in an animal's body.
Salivary glands : digestive glands at the anterior end of the digestive tract.
Scaphognathite : the elongated epipodite of the second maxilla in certain

crustaceans.
Sclerite: a small plate forming a portion of the cuticula of a segment

in insects.
Scolex: the anterior end of a tapeworm.
Scutellum: a small sclerite in the tergum of the thoracic segments in

insects.
Segment: one of a number of serial divisions of an animal's body or of

an organ.
Septum: a plate forming a division wall between two spaces.

240 GLOSSARY

Sessile : fixed to one place, without locomotory powers — of an animal ;
not on a stalk or stem — of an organ.

Seta: a bristle.

Sexual: reproduction through the agency of the two sexes.

SheU gland: the kidney of entomostracans.

Siphon: the organ through which water enters or leaves the mantle cavity
in mollusks and ascidians.

Siphonoglyph: a ciliated groove in the angle of the gullet in Anthozoa.

Somite : one of the serial, homologous body segments which form the body
of an articulate animal.

Spermatophore: a capsule of spermatozoa.

Spermatozoon: the male sexual cell.

Sperm duct: a duct leading from the testis.

Sperm receptacle: a sac for the storing of sperm in the female animal.

Sperm sac: a sac for the storing of sperm in the male animal, a seminal
vesicle.

Sperm sphere: a mass of spermatozoa in the earthworm.

Spicule: a minute, calcareous or siliceous body in sponges and echinoderms.

Spinnerets: the appendages on a spider from which the silk exudes.

Spiracle: an external opening in the tracheal system.

Sporosac: a sessile medusoid, one which remains attached to the parent
hydroid.

Sternum: the breastbone.

Stigmata: the respiratory openings in the pharyngeal wall in Molgula.

Stipes: a division of the maxilla in insects.

Stomach: a division of the digestive tract.

Stomach pouch: a diverticulum of the stomach.

Stone canal: a tube joining the madreporite with the ring canal in echi-
noderms.

Submentum : the basal segment of the labium in insects.

Subneural gland: a glandular body in ascidians.

Subumbrella: the oral surface of a medusa.

Supporting layer: the noncellular layer between the ectoderm and ento-
derm in Hydrozoa.

Swimmeret: an abdominal appendage of a crustacean, a pleopod.

Symbiotic: the living together of two dissimilar organisms, each being
dependent upon the other.

Systemic heart: the median heart of the squid.

Tactile: relating to the sense of touch.

Tarsus: the foot of an insect or a spider; the ankle of vertebrates.

Telson: the terminal segment of a crustacean.

Tentacle: an elongated, unsegmented tactile organ.

Tergum: the dorsal surface.

GLOSSARY 241

Terminal: toward or at the posterior or the distal end.

Test: the tunic of the ascidian; the rigid shell of the sea urchin.

Testis: the male sexual gland.

Thorax: the body division of arthropods following the head ; the chest of
vertebrates.

Tibia: the segment of an insect's leg ; a bone in the shank of vertebrates.

Tiedemann's vesicles: minute diverticula of the ring canal of the star-
fish.

Trachea: a respiratory tube.

Trichocyst: a cyst containing a defensive bristle in the ectosarc of Infu-
soria.

Trivium: the three rays of an echinoderm opposite to the bivium.

Trochanter: the segment of an insect's or a spider's leg, between the coxa
and the femur.

Trochophore: a larval form common to polychaetous annelids.

Tunic: the outer cuticular covering of tunicates.

Umbo: the protuberance above the hinge on the shell of a pelecypod.

Ureter: a tube forming the outlet of the kidney.

Uropod: the sixth swimmeret of the macruran decapod, that which forms

the swimming tail.
Uterus: a dilated portion of the oviduct in which the egg or the developing

animal is detained.

Vagina: the terminal division of the female reproductive tract.

Vas deferens: a duct leading from the testis toward the external opening,

the sperm duct.
Vegetative organs: those organs which have to do with the processes of

nutrition, growth, and the expulsion of wastes.
Vein: a vessel which brings blood toward the heart.
Velum: the circular muscular membrane of a medusa.
Ventral: on or toward the underside of an animal.

Ventricle: a chamber of the heart from which blood is sent over the body.
Viscera: the organs within the body cavities.
Visceral mass: the compact group of organs comprising the principal

viscera in moUusks.

Wing-covers: the first pair of wings of a beetle.

Zoea: a larval form of the crab and of certain other crustaceans.
Zooecium: the outer cuticular covering of a bryozoan.

INDEX

Amoeba, 158
Amphibia, 165
Amphipoda, 38, 39
x\nnelida, 49, 54
Anodonta, 72
Anthozoa, 143
Arachnida, 18
Arbacia, 119
Armadillidium, 36
Arthropoda, i
Ascidiacea, 160
Asterias forbsii, 113
Asterias vulgaris, 113
Asteroidea, 113

Beetle, 4

Bell animacule, 152
Bougainvillea, 131
Bryozoa, 69
Bugula, 69

Calcarea, 145

Campanularia, 136

Caprella, 39

Carchesium, 152

Caterpillar, 15

Centiped, 17

Cephalopoda, 103

Cestoda, 65

Chaetopoda, 49, 54

Chordata, 160, 165, 197

Clam (fresh- water), 72

Clam (hard-shell), 85

Ccfilenterata, 127, 131, 136, Ui, US

Coleoptera, 4

Copepoda, 42

Crab, 33

Crayfish, 21

Crustacea, 21, 33, 36, 38, 39, 40, 4^,

45, 47
Cyclops, 42

Daphnid, 45
Decapoda, 21, 33, 40
Diptera, 5

Earthworm, 54
Echinodermata, 113, 119, 124
Echinoidea, 119
Epistylis, 152
Euglena, 156

Flagellate, 156

Fly, 5

Fresh- water clam, 72
Fresh-water shrimp, 38
Frog, 165

Gammarus, 38
Gastropoda, 93
Gonionemus, 141
Grantia, 145
Grasshopper, 7

Hard-shell clam, 85
Helix pomatia, 93
Holothurian, 124
Hydra, 127

Hydromedusan, 131, 136
Hydrozoa, 127, 131, 136
Hymenoptera, i

Infusoria, 148, 152
Insect larva, 1 5
Insecta, i, 4, 5, 7> ^5
Isopoda, 36

243

244

INDEX

Land snail, 93
Larval decapods, 40
Lepidoptera, 15
Limax maxima, 93
Lithobius, 17
Lobster, 21
Loligo peali, 103

Mastigophora, 156
Medusa, 141
Megalopa, 40
Metridium, 143
Molgula, 160

MoUusca, 72, 81, 85, 93, 103
Mussel (fresh- water), 72
Mya arenaria, 85
Myriapoda, 17
Mysis, 40

Nauplius larva, 47
Nereis, 49

Obelia, 136
Oligochaeta, 54
Oniscus, 36
Orthoptera, 7
Oyster, 81

Paramecium, 148

Pelecypoda, 72, 81, 85

Pennaria, 131

Perch, 198

Phyllopoda, 45

Pisces, 198

Planarian worm, 62

Platyhelminthes, 62, 65

Polychaeta, 49

Porcellio, 36

Protozoa, 148, 152, 156, 158

Pulmonata, 93

Rana, 165

Salientia, 165

Sand flea, 38

Sandworm, 49

Sea anemone, 143

Sea cucumber, 1 24

Sea urchin, 119

Shrimp (fresh- water) , 38

Simple ascidian, 160

Slipper animalcule, 148

Snail, 93

Sow bug, 36

Spider, 18

Squid, 103

Starfish, 113

Strongylocentrotus, 119

Taenia crassicollis, 65
Taenia saginata, 65
Taenia serrata, 65
Talorchestia, 38
Tapeworm, 65
Teleostei, 198
Trachomedusa, 141
Tubularian hydromedusan, 131
Tunicata, 160
Turbellaria, 62

Unio, 72

Venus mercenaria, 85
Vermes, 62, 65, 69
Vertebrata, 165, 197
Vorticella, 152

Wasp, I

Zoantheria, 143
Zoea, 40
Zoothamnium, 152