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BOOK 592.4. P44 c. 1
PETRUNKEVITCH # MORPHOLOGY OF
INVERTEBRATE TYPES

3 T1S3 Domsno 7

MORPHOLOGY OF INVERTEBRATE
TYPES

^!^^

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MORPHOLOGY oj

OF INVERTEBRATE m
TYPES

BY
ALEXANDER PETRUNKEVITCH, Ph.D.

ASSISTANT PROFESSOR OF ZOOLOGY IN THE SHEFFIELD SCIENTIFIC
SCHOOL OF YALE UNIVERSITY

THE MACMILLAN COMPANY
1929

All rights reserved

^,

Copyright, 1916

By the MACMILLAN COMPANY

Set up and electrotyped. Published July, igifi.

PRINTED IN THE UNITED STATES OF AMERICA BY
THE BERWICK & SMITH CO.

PREFACE

Owing to the rapid advance of Experimental Zoology, the
older branches of morphological study such as Comparative
Anatomy and Embryology, have been left in the background.
In order to acquire familiarity with modern methods and
problems, it became necessary to curtail the time allotted to
Morphology. As a natural consequence the laboratory guides
in general use in this country are more elementary than is de-
sirable. The result has proved in many ways unsatisfactory.
The student is left with only a superficial knowledge of the
structure, life and development of those very animals which
in his later studies he is going to use for experiments.

But the high tide in Experimental Zoology has been reached
and passed. A general tendency to return to morphological
studies is manifest in recent years in the great increase in the
number of papers dwelling upon the anatomy of invertebrates.
It seems, therefore, desirable to place in the hands of the student,
a manual which will enable him to lay the foundation for a
knowledge of invertebrate anatomy.

The usual method of teaching this subject consists in having
the student dissect a series of types, but while doing this he is
supposed to pay attention only to structures of a general char-
acter and to skip details. This method is open to grave criti-
cism. GeneraHzation should come as the result of extended
observation. If we substitute generalization for observation, we
do the student's work for him and defeat our own end. By this
I do not mean to say that the student should be offered only a
knowledge of minute facts and left to his own resources to sift
the material and draw conclusions. On the contrary! The

Vi PREFACE

teacher should guide the student until the latter learns to dis-
tinguish important from trivial, generalized from specialized, and
correlated from disconnected facts. Only in this way will the
student learn scientific observation and inductive thought as
opposed to mere statement of facts and random conclusions.
But in order to accomplish this, the student should first of all
learn to observe everything that can be observed from a thor-
ough dissection with simple methods.

Accordingly the treatment of the subject of this manual
differs somewhat from the usual. Each chapter consists of two
parts: a monograph in which a description is given of the animal
selected as representative of its class and instructions for the
students to follow in dissection. The descriptions while short,
are sufiftciently detailed to include obvious structures of specific
value. The monographs are based partly on work done by
others, partly on my own dissections and investigations. To
these latter are due some of the divergencies from generally ac-
cepted opinions as well as statements not to be found elsewhere
in print. I have examined all important papers which have
been published up to the present time and if I do not give any
list of these it is: first, because the student has no need of them;
second, because the teacher will know how to get at the sources;
and, third, because a comprehensive list would make the book
too long and in consequence, too expensive, while a partial list
would be of no great value.

The instructions have been followed successfully by my
students for two years, and have thus stood the test of the
laboratory. The student is expected to read the descriptive
part at home, the day before the exercise. The accompanying
figures will help him to understand the subject. At the same
time he will not be able to copy them in the laboratory instead
of making original drawings from the specimen he dissects, for
the simple reason that they either represent the structure of
some other, though nearly related species, or are diagrammatic.
Some of these figures are copies from other books; some are

PREFACE Vli

combination drawings or modified in one respect or another in
accordance with my own ideas and dissections.

A few words of explanation are necessary in regard to the
choice of material. I tried to use only American species but
this proved difiicult in the case of Trematodes and leeches. I
have examined many species of Trematodes but none of them
is comparable to the common, small, European liver fluke,
Dicrocoelium lanceatum. This species does not occur in the
Western Hemisphere, but it is a truly typical representative of
its class and so common in Europe that it may be bought of
foreign firms in unlimited quantities. The medicinal leech is
for sale alive in this country and there is therefore no special
advantage in giving preference to indigenous species which one
would have to collect since they are not on the market. I tried
also to avoid species which are usually studied in the Freshman
year in the course in General Biology. Experience has shown
me that the students resent repetition and surely there is no
need for it. As long as the student is required to take General
Biology or Elementary Zoology preparatory to Invertebrate
Zoology, a diversity of material is of distinct advantage. An
unfortunate exception had to be made in the case of the Earth-
worm, but I do not know of any convenient substitute. A con-
cession to tradition had to be made in the case of the locust. In
my opinion a roach is to be preferred, because it is easier to
dissect and because it may be kept alive in winter; but private
conversation with various colleagues has convinced me that it
would not do to leave out the grasshopper. Some groups had
to be omitted altogether on account of lack of space. If the
book meets with favorable reception, new chapters may be
added in a later edition.

The detailed descriptions in the manual of the anatomy of
animals which the student is given to dissect, will relieve the
teacher of the necessity of lecturing on this subject. His time
may be better spent in giving a general account of the group
to which each animal belongs and in treating broader aspects

viii PREFACE

of the problems involved in the study of Morphology. The
whole aim of the teacher should be to attain a perfect balance
between laboratory work and lectures, for the one teaches
methods, the other, results.

Alexander Petrunkevitch.
New Haven, Conn.
May, 1916.

TABLE OF CONTENTS

PAGE

Preface v

Table of Magnifications xi

List of things to be furnished by the Laboratory xii

List of things to be furnished by the Student xiii

Paramecium caudatum Ehrenberg i

Grantia ciliata Fabricius lO

Pennaria tiarella McCrady i6

Sertularia pumila Linneus 23

TiMA FORMOSA L. AgASSIZ 27

Gonionemus murbachh Mayer 33

AURELIA AURITA (L.) VAR. FLAVIDULA PeRON ET LeSUEUR... 35

MeTRIDIUM marginatum MiLNE-Ed WARDS 45

Dendroccelum lacteum (Muller) 55

Dicroccelium lanceatum Stiles and Hassall 62

Tenia saginata Goeze 72

ascaris lumbricoides linneus var. suilla 79

LuMBRICUS TERRESTRIS L. MiJLLER 88

Nereis virens Sars 97

HiRUDO medicinalis Linneus 105

Daphnia pulex Muller 113

HOMARUS AMERICANUS MiLNE-Ed WARDS 122

SCHISTOCERCA AMERICANA DrURY I45

Agelena nevia Walckener 158

ASTERIAS FORBESI (DeSOR) 174

Ophiopholis aculeata (Linneus) 183

Pentacrinus asteria (caputmeduse) (Muller) 189

Arbacia punctulata (Lamarck) 191

Thyone briareus (Lesueur) 202

Venus mercenaria Linneus 208

LiMAx maximus Linneus 220

Loligo pealii Lesueur 231

Molgula manhattensis (De Kay) 256

ix

TABLE OF MAGNIFICATIONS

Bausch and Lome Optical Co.
Objective Eyepiece

5x

10 X

i6 mm. (73)

50 X

100 X

8 mm. (V3)

100 X

200 X

4 mm. (Va)

215 X
Spencer Lens Co.

430 X

Objective

Eyepiece

4X

8x

16 mm.

50 X

75 X

8 mm.

125 X

210 X

4 mm.

210 X
Ernst Leitz

465 X

Objective

Eyepiece

I

IV

3 (16.2 mm.)

51 X

103

4 (10. 0 mm.)

91 X

182

6 (4 . 0 mm.)

240 X

460

A LIST OF THINGS TO BE FURNISHED BY THE
LABORATORY FOR EVERY STUDENT

1. Microscope. Stand with rack and pinion, micrometer
screw, mirror, substage with Abbe Condenser and Iris diaphragm,
triple revolving nosepiece. Objectives i6 mm. (Va inch),
8 mm. (Vs inch), and 4 mm. (^6 inch); Huyghenian eyepieces
of the magnifying power of 5 and 10 x.

2. Dissecting microscope; stand and one lens of 10 diameter
magnification.

3. Dissecting tray, round, about 6 or 7 inches in diameter.

4. Dissecting tray, rectangular, 10 x 16 inches.

5. Syracuse dish.

6. Stender dish, 25 mm. high, 50 mm. in diameter.

7. Finger bowl.

8. Watch glass.

9. Two pipettes.

10. One dozen pig's bristles.

11. Stiff, black horsehair (from horse's tail).

12. Filter paper.

13. Lens paper.

14. Reagents: 10% ether in water; 20% solution of caustic
potash; quince seed jelly; Methylgreen saturated in 1% acetic;
hydrochloric acid.

15. Prepared slides and specimens for dissection.

xu

A LIST OF THINGS WHICH THE STUDENT HIMSELF
SHOULD PROVIDE AND KEEP IN THE LABORA-
TORY

1. Loose leaf notebook 8 x lo V2 inches, with white draw-
ing paper.

2. Ruler with inch-centimeter scale.

3. Divider.

4. Soft pencil.

5. Hard pencil.

6. Set of colored pencils.

7. Eraser.

8. Dissecting instruments: large scissors with one blunt end,
fine scissors, two handles for needles, sewing needles, two
forceps, spatula, tube for blowing into organs, razor.

9. One package of pins.

10. Slides.

11. Cover glasses.

12. A hard tooth brush.

13. A towel.

MORPHOLOGY OF INVERTEBRATE
TYPES

MORPHOLOGY OF INVERTEBRATE
TYPES

PARAMiECIUM CAUDATUM Ehrenberg

Material. Paramaecium caudatum may be obtained easily
in immense quantities from hay infusions inoculated a few days
before the exercise. Fixation, staining and washing may be
accomplished without difficulty by the use of a centrifuge. The
stained specimens are then transferred gradually into 95%
alcohol, picked up with a fine pipette and squirted onto a shde
covered with a thin layer of glycerine-albumen. The slide may
be finished in the usual manner. Live Protozoa are found in
any aquarium prepared for the purpose some two weeks in ad-
vance. It is advisable to use mud from the bottom of various
pools and to put it into separate dishes. I have obtained excel-
lent cultures of Amoeba several times from infusions of horse
manure.

Every student should receive a small aquarium with live
Protozoa, a small flask with hay infusion containing Para-
maecium, and prepared slides of Paramaecium stained in Iron
Haematoxyhn.

Descriptive Part

Paramaecium caudatum is one of the best known Protozoa
and occurs all over the world in fresh water pools. It belongs
to the Class Ciliata characterized by the presence of cilia
which function as organs of locomotion. Compared with an
Amoeba, Paramaecium is a highly differentiated creature. Yet
it has none of the organs characteristic of higher animals. Its

1

2 MORPHOLOGY OF INVERTEBRATE TYPES

body consists of a single cell which has therefore to perform
withii^ itself the various functions of life, such as nutrition,
sensation and reproduction. Nature has assigned these func-
tions to various parts of the cell, which may be compared with
organs of multicellular animals. One must bear in mind, how-
ever, that no homology exists between cell-parts and organs,
and that even the analogy is quite superficial. It is advisable,
therefore, to use the term organellum in treating the structures
of unicellular organisms and to speak of sensory, nutritive,
protective, reproductive and other organella.

The body of Paramaecium is asymmetrical, i. e., it cannot be
divided into two like parts. The anterior end is wider and
more blunt than the posterior one. Paramaecium appears al-
most round in a cross-section, yet there is a distinct difference
between two surfaces one of which may be regarded as dorsal
and the other as ventral. The latter is characterized by the
presence of a large depression or peristome which extends from
the anterior end to somewhat beyond the middle of the body
and is as wide as half the width of the animal. At the bottom
of the peristome is the mouth or cytostome which leads into a
short, curved cytopharynx with an undulating membrane. Half-
way between the lower end of the latter and the posterior end
of the body is a small opening functioning as an anus and called
cytopyge. There are two pulsating vacuoles, one about one-
quarter of the entire length from the anterior end, the other
about the same distance from the posterior end of the animal.
In about the middle of the body is a large macronucleus and a
small micronucleus.

Protoplasmic structures. The protoplasm of Paramaecium
shows the characteristic differentiation into ectoplasm and
endoplasm. The latter is devoid of particular structures. It
has a finely granulated appearance and contains food-vacuoles
with food in various stages of digestion, and the two nuclei.
The ectoplasm, on the other hand, presents many structures
and is subdivided into three layers: pellicula, alveolar layer and

AnK

Fig. I. — Stylonychia
mytilus O. F. M. viewed
from the underside. From
Arnold Lang's Lehrbuch
der Vergleichenden Ana-
tomic, somewhat modi-
fied. I, upper lip; 2, af-
ferent canal of the pul-
sating vacuole ; 3, adoral
plates; 4, peristome; 5,
afferent canal of the pulsating
vacuole; 6, right edge of the per-
istome; 7, pulsating vacuole; 8,
posterior half of the macronucles;
9, posterior micronucles; 10, cy-
topyge (situated on the dorsal
surface); 11, anal cirri; 12, tail
bristles; 13, ventral cirri; 14, tac- Post.

til cilia; 15, cytostome; 16, prae-

oral row of cilia; 17, right wall of peristome; 18, anterior half of macronu-
cleus; 19, undulating membrane; 20, frontal cirri.

4 MORPHOLOGY OF INVERTEBRATE TYPES

cortical layer. The pellicula is a thin elastic membrane covering
the entire body. It has a distinctly striated appearance due to
the fact that the surface is divided into small hexagonal fields
separated from each other by ridges. From the center of each
field arises a long ciliiim. Under the base of the cilium is a
motor center in the shape of a small grain from which a fibre
runs toward the endoplasm. The rhythmic beating of the cilia
produces a pressure on the water and propels the animal in a
wide spiral course. At the posterior end of the body are several
immobile cilia probably with tactile function. In the angles
formed by the ridges of the pellicula as well as in the middle of
some ridges are the insertion points of the trichocyst ends.
The alveolar layer is situated immediately under the pellicula
and consists of a single layer of alveoh with walls at right angles
to the surface. The innermost layer of the ectoplasm is the
cortical layer which contains the trichocysts and the pulsating
vacuols. The trichocysts are protective organella. They are
present in immense numbers and have the shape of small spindles
with a thickened end directed toward the surface and a rod-like
process inserted in the ridges of the pellicula, as described above.
When the animal is irritated the trichocysts ''explode" and
appear as long and thin threads acting as harpoons. The Pul-
sating or contractile vacuoles are excretory organella. They con-
sist of a central collecting vacuole and from seven to ten afferent
vacuoles. The central vacuole is distended to its limit when the
afferent vacuoles are contracted and is in systole when the
afferent vacuoles are in diastole. The afferent vacuoles empty
their content into the central vacuole which in its turn empties
the fluid to the outside through a small excretory pore situated in
the pellicula over the center of the vacuole.

We have seen already that the mouth or cytostome is situated
at the bottom of the peristome and leads into a short, curved
cytopharynx. The undulating membrane which is attached to the
wall of the cytopharynx is in continuous motion, drawing food
particles toward the posterior end of the cytopharynx where the

PARAMECIUM CAUDATUM EHRENBERG 5

latter ends in the endoplasm. A food vacuole is formed around
the food thus drawn in. It detaches itself from the end of the
cytopharynx and moves through the endoplasma first down-
ward, then upward and again downward. During this cyclosis
or travelling through the endoplasma the food particle is digested
and at the end of the process the waste matter is emptied through
the cytopyge.

Reproduction. Asexual reproduction is the usual method
and consists in a transverse division. This is initiated by changes
in the micronucleus which is essentially a reproductive organ-
ellum. It becomes elongated and its chromatic substance shows
an arrangement similar to that in mitosis of cells in higher
animals, yet without the presence of a centrosome. The cyto-
stome, too, becomes elongated and compressed in the middle to
the shape of a narrow slit. The cytopharynx produces pos-
teriorly a new cytopharynx. Two new pulsating vacuols are
formed so that each daughter individual will receive an old and
a new vacuole. Next the macronucleus becomes also elongated.
A constriction appears in the middle of the animal which at
this stage of its life is almost twice as long as usual. Finally both
nuclei divide in two, their division is followed by a division of the
protoplasm and the two halves of the original Paramaecium be-
come independent individuals. Each is provided with all nec-
essary organella, assumes the shape of a normal Paramaecium
and is soon ready to divide again. Such transverse division may
go on for from 80 to 100 generations, but sooner or later a re-
organization of the nuclear apparatus has to take place in order
that the race should not die out. This is accomplished by a
process recently discovered by Woodruff. The micronucleus
divides twice. Three of the four micronuclei thus formed dis-
integrate and are resorbed by the protoplasm. The fourth
micronucleus alone persists. The macronucleus which is es-
sentially a vegetative nucleus, breaks up and is also resorbed
by the protoplasm. A new macronucleus is formed from the
micronucleus in a way similar to that in conjugation.

6 MORPHOLOGY OF INVERTEBRATE TYPES

Sexual reproduction consists in conjugation. In this process
the reorganization of the nuclear apparatus is combined with
amphimixis. Two individuals approach each other and bring
their ventral (oral) surfaces into close contact. The macronuclei
remain for a while undisturbed, but later show signs of degenera-
tion. After the separation of the conjugants they break up into
small parts which are gradually resorbed by the protoplasm.
The micronuclei, on the other hand, play the chief role in con-
jugation. They divide in both individuals twice in succession.
Of the four micronuclei thus formed, three disintegrate and are
resorbed by the protoplasm. The fourth micronucleus of each
conjugant divides into a stationary or female nucleus and a mi-
grating or male nucleus. The latter leaves the individual in
which it was formed and enters the other conjugant. Here it
fuses with the stationary nucleus forming a synkarion. When
both individuals have exchanged their migrating nuclei and
formed synkaria, conjugation is finished. The individuals
separate, but their nuclear apparatus is not normal. Now the
reconstruction of the macronucleus begins. The synkarion di-
vides three times in succession giving rise to eight nuclei. Three
of these nuclei disintegrate and are resorbed by the protoplasm,
one assumes the shape and function of the micronucleus, while
the remaining four grow and change eventually into macronuclei.
The exconjugant is now ready for a new division. First the
micronucleus divides in two; then two of the four future macro-
nuclei pass to the one end, and two to the other end of the di-
viding Paramascium. When the division is completed each
daughter cell has one micronucleus and two macronuclei. Yet
another division must take place. The micronucleus divides
again while of the two macronuclei one passes to one end and
the other to the other end of the animal. Thus the balance of
nuclei is restored in the third generation and each individual
is now in possession of a single micronucleus and a single
macronucleus.

PARAMECIUM CAUDATUM EHRENBERG

i

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Fig. 2. — Diagram representing the various stages in the process of conjugation
and of the following reconstruction of the macronucleus in Paramaecium. From
Arnold Lang's Lehrbuch der Vergleichenden Anatomie. Ma, macronucleus of the
conjugant; Mi, micronucleus of the conjugant; a\ h^, stationary micronuclei; a^, bo,
migrating micronuclei; 01^2, 02^1, synkarion; I, the two conjugants A and B; II,
the four individuals of the second generation produced by the transverse division of
the exconjugants; III, the eight individuals of the third generation, each with a
single macronucleus and micronucleus; the crown indicates the disintegration of
the corresponding nucleus.

Instructions

I. Put a small drop of hay infusion containing live Paramaecia
on a slide, add a small drop of quince seed jelly, cover with
a cover glass and examine under low power (100 diameters).
When the animals have quieted down examine a paramaecium
under higher power (200 diameters) and make a drawing show-
ing outline of body, peristome, cilia, pulsating vacuoles, food

8 MORPHOLOGY OF INVERTEBRATE TYPES

particles, macronucleus. Label anterior and posterior end,
dorsal and ventral surface.

2. Find under low power a specimen in the process of division.
Examine it at 200 diameters and make a drawing showing as
many structures as are visible.

3. If possible, try to find a couple of Paramaecia in conjugation
and make a drawing of them.

4. Take another drop of the same infusion, put a drop of the
methylgreen solution in 1% acetic acid on a cover glass, turn the
cover glass over, so that the drop will be suspended from the
underside and allow the cover glass to drop suddenly on the
drop of hay infusion from a height of about one inch. This will
fix the animals and stain the nucleus after a while. Examine
under high power (400 diameters) and make a drawing showing
all structures brought out by the stain.

5. Examine the prepared slide under high power (400 di-
ameters) and make a drawing showing trichocysts, macronucleus
and, if visible, micronucleus.

6. Examine the prepared slide with Paramaecia in division
under high power (400 diameters) and make a drawing of it.

7. Examine the prepared slide of Paramaecia in conjugation
under high power (400 diameters) and make a drawing of it.

8. Additional exercise. Place a drop of water from the side of
an aquarium on a slide, add a small drop of quince seed jelly,
cover with a cover glass and examine under low power (100 di-
ameters). Find as many species of Protozoa as possible, examine
them one after the other under high power (400 diameters),
identify them with the aid of a proper book and make drawings.
Label Class and Order to which the species belongs.

9. Additional exercise. Take a drop of water from the bottom
of the aquarium with some sediment and do as in the preceding
exercise.

10. Additional exercise. Cut off with fine scissors the head and
the last abdominal segment of a mealworm (larva of a beetle,
Tenebrio molitor). Take the end of the protruding alimentary

PARAMyECIUM CAUDATUM EHRENBERG 9

canal firmly with a forceps and pull it out. Place it on a slide
and squeeze out the contents. Remove the alimentary canal,
cover the drop with a cover glass and examine under low power.
Find two associated individuals of the mealworm gregarine and
make a drawing of them. Label protomerite, deuteromerite
with the nucleus, exoplasma and endoplasma.

11. Additional exercise. Examine a prepared sHde with a
stained transverse section through an earthworm in the region
of the sexual organs. Find as many stages of the gregarines as
possible and make drawings of them. One may count on find-
ing encysted individuals in stages of sporogony and spores of
the second generation with eight sporozoits in each.

12. Additional exercise. Examine a prepared slide with trans-
verse sections through the alimentary canal of the centipede
Lithobius and find as many stages of the coccidium as possible.
Stages of schizogony, macrogametocytes and microgametocytes
may be found in the intestinal endothehal cells, cysts in the
lumen of the intestine.

GRANTIA CILIATA Fabricius

Material. Grantia ciliata is very common on the Atlantic
Coast. It is found in shallow water just below the low tide mark.
Specimens intended for general study as well as for sections
showing the arrangement of spicules, should be preserved in
alcohol. Specimens intended for microscopic study must be
preserved in a special manner. The best method is that of
Minchin. It consists in preserving the specimens in the place
where they are collected. The collector should take with him a
1% aqueous solution of osmic acid, distilled water and Ranviers
picrocarmin. As soon as the specimens have been brought to the
surface they must be plunged into a mixture of equal parts of
osmic acid and sea water. After five minutes the specimens must
be washed in several changes of distilled water and allowed then
to remain for two hours in picrocarmin. They are then ready to
be washed again in distilled water and to be transferred into
alcohol. Before imbedding the specimens must be decalcified
by any one of the usual methods. The sections may be stained
for five minutes in a solution composed of one part of i% aqueous
solution of nigrosin and nine parts of a saturated aqueous solu-
tion of picric acid. Every student should receive for study one
alcoholic specimen and the following four prepared slides: a
transverse section through an alcoholic specimen, not stained;
spicules isolated by boihng in a solution of potassium hydrate; a
transverse section through a decalcified specimen preserved by
the method of Minchin; a tangential longitudinal section through
a similar specimen.

Descriptive Part

Grantia ciliata is a common representative of the Phylum
Porifera and belongs to the group of calcareous sponges. Unlike

Fig. 3. — Transverse
section through Sycon
gelatinosum from Parker
& Haswell, Textbook of
Zoology. IC, incurrent
canal; R, radial tube
(flagellated chamber) ;
sp, triradiate spicules;
sp', spicules of cortex;
dc, cortex; sp" , triradial
spicules of atrium; ec,
ectoderm; en, endoderm;
pm, external incurrent
pore; pp, prosopyle; ap,
apopyle; di, diaphragm;
exc, excurrent passage;
P. G., atrium; em, early
embryo; em'y late em-
bryo.

12 MORPHOLOGY OF INVERTEBRATE TYPES

the majority of sponges it is not a colonial form, so that indi-
viduals produced by budding are connected with the mother
individual only at the base. It is devoid of locomotion and
remains throughout its life attached to the surface of the rock or
pile on which it grows. A great number of calcareous spicules
support the soft tissues and play the role of a primitive skeleton.

General anatomy. Grantia ciliata has more or less the
shape of a hollow cylinder with a blind base. It is built on the
principle of radial symmetry. The free end terminates in a wide
opening or osculum surrounded by a crown of long oscular
spicules. Smaller cortical spicules cover the whole surface of the
sponge like bristles of a brush. More or less concealed by them
are numerous small external pores which lead into the so-called
incurrent canals. These canals are arranged radially to the
longitudinal axis of the sponge. They end blindly and do not
reach the central cavity. This cavity is called the atrium. The
surface of the wall, which faces the atrium is also perforated by
numerous, regularly arranged pores called apopyles. They lead
into radial tubes, which are arranged similarly to the incurrent
canals, i. e., radially to the longitudinal axis of the sponge. They
too, end bhndly, but at the end opposite to the blind end of the
incurrent canals, under the so-called cortex or a layer of tissue
underlying the external surface of the sponge. The incurrent
canals communicate with the radial tubes by means of numerous
pores or prosopyles. Water containing food in the shape of
minute animal and plant life is drawn into the incurrent canals
through the external pores by the motion of the iiagella of the
cells lining the radial tubes. From the incurrent canals it passes
into the radial tubes through the prosopyles, from the radial
tubes into the atrium through the apopyles and is finally thrown
out of the body through the osculum. Thus the acts of respira-
tion and nutrition are accomplished at the same time. Respira-
tion and digestion take place in the radial tubes.

Miscroscopic anatomy. The walls of the sponge are com-
posed of three distinct layers of cells: ectoderm, mesenchyme

GRANTIA CILIATA FABRICIUS 13

and endoderm. The mesenchyme is often called mesoderm,
but since it has not the same origin as the mesoderm of higher
animals, the term mesenchyme is preferable.

Ectoderm. The outside covering of the walls of the
sponge and the lining of the incurrent canals is formed by a
single layer of ectodermal cells. These cells are polygonal in
shape, quite flat and belong to the type of pavemental epi-
thelium. In the incurrent canals larger cells are found scattered
irregularly among the common ectodermal cells. These larger
cells are perforated and are therefore called porocytes. The
pore of a porocyte is nothing but a prosopyle which has already
been mentioned.

Endoderm. The lining of the atrium as well as of the
radial tubes is formed by a single layer of endodermal cells.
These belong to two different types. The cells lining the atrium
look very much like the ectodermal cells and belong to the same
type although they are of different origin. The cells lining the
radial tubes are called collar cells or choanocytes. They are long,
with a collar at their inner edge and a long flagellum protruding
far into the hollow space of the tube. Owing to these cells the
radial tubes have been also termed the "flagellated chambers."

Mesenchyme. The mesenchyme is a gelatinous sub-
stance or mesoglcea containing cells and spicules. It fills out
the space between the incurrent canals and the radial tubes and
forms the cortex under the ectodermal layer on the surface of
the sponge. Some of the cells found in the mesoglcea look like
common connective cells with several processes and are called
collencytesY^ Others assume the shape of long spindles and,
being contractile, function as muscles. Such cells are found
surrounding the apopyles and the osculum where they form a
real sphincter. Some cells have the ability of amoeboid motion
and are called amcehocytes. Still other cells are said to have
nervous function. All these various types of cells are supposed
to be modified coUencytes. Another type of cells found in the
mesogloia are the more or less round sclerohlasts or cells produc-

14 MORPHOLOGY OF INVERTEBRATE TYPES

ing spicules. The spicules in Grantia belong to two types. The
oscular spicules and the spicules imbedded in the cortex have
the shape of longer or shorter needles. Every needle is produced
as a calcareous secretion of a single scleroblast. The spicules
found in the mesogloea between the radial tubes and the incur-
rent canals are composed of three radii. These triradial spicules
are so situated in the wall of the sponge that one of the three
rays coincides with a radius and points toward the surface of
the sponge. Every triradial spicule is produced as a calcareous
secretion of three scleroblasts.

Reproduction. Asexual reproduction consists in the forma-
tion of a bud near the base of the sponge. The bud grows and
soon an osculum appears at its free end. In this way loosely
connected colonies of several individuals may be formed.
Sexual reproduction is preceded by the formation of eggs and
sperm cells. Both are modified amoebocytes. In the case of the
egg it is a considerably grown and rounded up amoebocyte.
In the case of the sperm an amoebocyte is first transformed
into a so-called spermatogonial cell from which a number of
spermatozoa develop. The fertilized egg remains for a long
time in the mesogloea under the endoderm of the radial tubes.
It develops gradually into a ciliated larva or amphiblastula
and escapes by way of a radial tube and the osculum. The
larva attaches itself with the blastopore end to a suitable sur-
face. Later the osculum breaks through at the pole opposite
to the blastopore.

Instructions

1. Place an alcoholic specimen in a shallow dish with water
and examine under the dissecting microscope. Make a quarter
page drawing showing general shape, oscular spicules, and
cortical spicules covering the surface.

2. Cut the specimen longitudinally into two symmetric
halves. Use for that purpose a razor. Again examine the
specimen under dissecting microscope, but this time the cut

GRANTIA CILIATA FABRICIUS 15

surface. Make a half page drawing showing the osculum with
its spicules, atrium, apopyles or pores in the wall of the atrium,
and the radial tubes and incurrent canals in the cut surface of
the wall.

3. Examine under low power (50 diameters) a prepared slide
with a cross-section through Grantia showing the arrangement
of spicules. Make a half page drawing showing four of the
radial tubes indicated by the rows of spicules.

4. Examine under microscope (200 diameters) a prepared
slide of isolated spicules. Make a drawing of a triradial spicule
and of shorter and longer needle-like spicules.

5. Examine under low power (100 diameters) a stained cross-
section. Make a half page drawing showing four radial tubes.
Label external pore, incurrent canal, radial tube, and apopyle.

6. Examine the same section under high power (400 diame-
ters). Find a place with an embryo. Make a drawing showing
the three layers of cells; label choanocytes (collar cells), ectoderm^
mesenchyme, and embryo. If possible, find and show on the
drawing a prosopyle.

7. Examine under low power (200 diameters) a stained tan-
gential section. Determine which rings represent the cross-
sections of radial tubes by the shape of the cells facing the center
of the ring. In doing so, remember that the radial tubes are
lined with endodermic choanocytes (collar cells), while the
incurrent canals have fiat ectodermal cells. Make a half page
drawing showing several of the tubes and canals and label them.

8. Additional exercise. Boil an alcoholic specimen in a test
tube containing a solution of potassium hydrate. When all
the tissues are dissolved, allow the spicules to settle, carefully
pour off the liquid and wash the spicules twice in clean water.
Pick up a number of spicules by means of a pipette and transfer
them on to a shde. Place them under the microscope and focus.
Add now a small drop of a 10% solution of hydrochloric acid
and observe how the spicules dissolve with the formation of
small gas bubbles, proving their calcareous nature.

PENNARIA TIARELLA McCrady

Material. P. tiarella is very common along the Atlantic
Coast. Medusae may be preserved in either osmic acid or
formalin. Polyp colonies may be preserved by any one of the
various methods proposed for the fixation of coelenterata. The
student should receive a colony of polyps, a specimen of medusa,
a ready made slide of a polyp, stained and somewhat compressed,
and a cross-section through a polyp and one through the stem.

Descriptive Part

Pennaria tiarella is a typical representative of the class
Hydrozoa and has a complete alternation of generations or
metagenesis. The polyp is gymnohlastic, i. e., neither the polyps
themselves, nor the medusa buds are protected by a covering
of the so-called perisarc.

Hydrosome or polyp colony. The colony is attached to
the surface on which it grows, by root-like processes — the
hydrorhiza. From the hydrorhyza runs a more or less straight
stem or hydrocaulus with many alternating side branches which
in turn give rise to simple or slightly branched ramuli. The
perisarc or the membrane covering of the colony forms ringlets
on the stem beyond every branch and at the base of every ramu-
lus and ends at the base of every polyp or hydra nth. The origi-
nal and therefore oldest hydranth of the colony is the one at
the top of the main stem. The next oldest hydranth is at the
end of the branch nearest to the hydrorhiza. The youngest
hydranth of the main stem is always the one nearest the top

Fig. 4. — Obelia sp. from Parker & Haswell's Textbook of Zoology.
A, portion of a colony with certain parts shown in longitudinal section;
By medusa; C, the same with reversed umbrella; D, the same, oral aspect;
Bd. J, 2, buds; bis, blastostyle; coe, coenosarc; ect, ectoderm; end, endoderm;

z6

eut, enteric cavity (ccelenteron) ; gth,
gonotheca; hth, hydro theca; /, litho-
cyst; m. bd, medusabud; mnb, manu-
f^ brium; nisgl, mesogloea; mth, mouth;
p, perisarc; P. i, 2, j, polyps; rod. c,
radial canal; /, tentacle; vl, velum.

l8 MORPHOLOGY OF rWKRTEBKATK TYPES

hydranlh and Ihe same is true for every branch. The reason
for it lies in the monopodial method of branching. The original
or founder-polyp becomes elongated at its base, producing the
first or main stem. As the main stem grows in length the first
bud appears near its base. This bud becomes the end polyp
of the first or oldest branch. The main stem continues to grow
and produces a second bud between the first branch and the
top polyp, then a third bud and so on. The same process ap-
plies to the branches. The main stem with its root and branches
is hollow and this cavity is naturally in direct continuation with
the cavity of every hydranth. This cavity is the coslenteron or
gastro-vascular cavity. The hydranth consists of a short stem
or peduncle and a flask-shaped head. The mouth is at the end
of a conical hypostome. At the base of the hypostome are two
or three verticels, each composed of from 5 to 7 short oral ten-
tacles. These tentacles terminate in a knob ojnematocysts. Near
the base of the head is a circle of from 12 to 16 long basal ten-
tacles with nematocysts arranged in little groups along each
tentacle. Each group of nematocysts appears as a little swell-
ing of the tentacle. All tentacles are solid (not hollow). The
medusa buds appear on the head, between the oral and basal
tentacles. They become either free-swimming medusae or re-
main rudimentary and sessile.

Microscopic structure. The walls of the hydrocaulus or
stem, of the branches, and of the hydrorhyza are known under
the name of cosnosarc and are composed of three concentric
layers. The outer layer secreting the perisarc is the ectoderm. It
is composed of "indijferent" epithelial cells. Some of these cells
in the hydrorhyza are modified into adhesive cells. The middle
layer has the appearance of a thin membrane. It is a structure-
less, non-cellular mesoglcea. The inner layer is the endoderyn and
is composed of so-called circulatory endodermal cells. The walls
of the hydranth are composed of the same three layers and here,
too, the mesogloea remains structureless. But the elements
which enter into the formation of the ecto and endoderm are

PENNARIA TIARELLA :McCRADY

19

icnc.

Fig. 5. — Schematic drawing of a section through the wall of a hydrozoon,
after Dalage and Herouard, somewhat modified, ep^ ectoderm; nisg, meso-
gloea; end, endoderm; ggl, ganglionic cell; mel, muscular fibres; nf, nerve fibre;
c, palpocil; cue, cnidocil; pd, peduncle; cpl, external layer of capsule; cpi,
internal layer of capsule; cr, hooks;/, filament; op, operculum.

different from those in the hydrocaulus. The ectoderm con-
sists of an epitheHal and a subepitheUal layer. The epithelial
layer is composed of indifferent and a few myo-epithelial cells,
i. e., cells with a contractile process or muscular fibre at their
base. All ectodermal muscular fibres belong to the longitudinal
system of fibres. In the suh-epUhelial layer are found among

20 MORPHOLOGY OF INVERTEBRATE TYPES

small so-called interstitial cells, genital cells and nerve cells.
The endoderm consists of my o- epithelial flagellated cells and
a few gland cells. The former are much larger than the
corresponding ectodermal cells and each cell has a long flagel-
lum on the surface facing the cavity of the polyp. Their
contractile processes form the system of circular muscular
fibres. The h3T)ostome is different from the rest of the hy-
dranth head in this respect that both circular and longitu-
dinal muscle fibres are more numerous, that genital cells are
absent and that among the cells of the ectoderm sensory cells
are found. The structure of the tentacles is different in several
respects. The tentacles are solid and the core of the tentacle is
formed by a single row of large endodermal cells. They have the
shape of short cylinders and their protoplasm is quite vacuolated.
Circular muscle-fibres are absent. The epithelial layer of the
ectoderm consists of myo-epithelial cells, sensory cells and nema-
tocysts. The subepithelial layer contains interstitial cells,
nerve cells and cnidohlasts or mothercells of the nematocysts.
The nematocysts or nettlecells are arranged in batteries at the
end of the oral tentacles and in the swellings of the basal ten-
tacles. A nematocyst has the shape of an ellipsoid. It has an
outer capsule with a short spine or cnidocil at the free end close
to the apical opening of the capsule. This opening is closed by a
plug. Inside the outer capsule is an inner capsule containing a
spirally coiled w^ filament. The filament is a hollow tube and its
wall is continuous with the wall of the inner capsule. When the
cnidocil comes into contact with an object or when the polyp
receives a chemical stimulus the nematocyst "explodes," i. e.,
the filament is suddenly everted through the apical opening.
The nematocysts serve both as organs of defence and for the
purpose of kilUng or stunning small animals which form the
food of the polyp.

Medusome or free swimming medusa. This is the sexual stage
in the life cycle of Pennaria tiarella and the sexes are separate.
Both sexes look however alike but for a somewhat slenderer

PENNARIA TIARELLA McCRADY 2i

manubrium of the male. The medusa has the shape of an elon-
gated bell or thimble. Its outer surface is called the exumbrella,
the inner surface the subumbrella. The opening of the bell is
partly closed by a circular velum. From the apex of the subum-
brella the manubrium hangs down into the subumbrella cavity
like the tongue of a bell. The mouth is at the free end of the
manubrium. At the free edge of the bell are four equidistant
knobs — rudimentary tentacles. Both exumbrella and subum-
brella are covered with ectodermal cells. The manubrium is
hollow and its cavity leads directly into a central cavity which
is known under various names such as the central gaslrovascular
cavity or the stomach and which represents the central part of
the coelenteron. Radiating from the central cavity are Jour
radial canals which run in the wall of the bell toward its edge
where they open into a circular canal. Four gonads develop in
the ectodermal layer of the manubrium at the time of maturity.
The microscopic structure of the medusa of Pennaria tiarella is
in its essential features the same as in Tima and will be con-
sidered in connection with that species.

Instructions

1. Place a complete colony of polyps, i. e., a hydrosome, in
a shallow dish with water and examine it under the dissecting
microscope. Make a half page drawing showing the hydrorhyza
or root, the hydrocaulus or stem with its branches and their
ramuli, and the hydranths or polyps. Label the founder polyp at
the top of the stem and the end polyps of the branches in the
sequence of their age as ai, bi, C] . . . . Label the polyps of
two branches in the sequence of their age as a2, as, a4 . . ., bo,
bs, b4 . . . .

2. Examine a small branch under low power (50 diameters).
Make a half page drawing showing two hydranths of which at
least one should have medusa buds. Label perisarc or membrane
covering the stem, coenosarc or the wall of the stem; on the

22 MORPHOLOGY OF INVERTEBRATE TYPES

hydranth label mouth, hypostome, oral tentacles, basal ten-
tacles and medusa buds.

3. Examine under high power (400 diameters) a prepared
sHde of a polyp and make a drawing showing an oral tentacle
with the knob of nematocysts at the end. Label supporting
endoderm, mesogloea and ectoderm.

4. Examine on the same sHde a basal tentacle and make a
drawing of it showing the swellings containing the batteries
of nematocysts, supporting endoderm and ectoderm.

5. Examine under high power (400 diameters) the prepared
slide with a cross-section through a hydranth. Make a drawing
showing ectoderm, mesogloea and endoderm.

6. Examine in the same manner the cross-section through the
hydrocaulus; make a drawing and label it.

7. Place a medusa in a Syracuse dish with water under the
dissecting microscope. Make a quarter page drawing showing
the side view of the medusa. Label exumbrella, subumbrella,
velum, manubrium, mouth, central gastro-vascular cavity, ra-
dial canals, circular canal and rudimentary tentacles, one at
base of each radial canal.

8. Additional exercise. Put a live fresh- water hydra on a
slide in a drop of water, cover it up with a cover glass and press
it with the pomt of a needle until the animal will break up into
small fragments. Examine under high power and find the iso-
lated exploded nematocysts. Make a drawing of one of them
showing the structure, and label parts.

SERTULARIA PUMILA Linnseus

Material. S. pumila is found growing on fucus and in
tide-pools along the Atlantic Coast. The student should receive a
colony with gonosomes (it is better to put the whole lot the night
before in glycerine), a stained colony, a cross-section through a
hydranth, and a longitudinal section through a gonosome.

Descriptive Part

Sertularia pumila is another common representative of the
class Hydrozoa. But unlike Pennaria it has no alternation of
generations, there being no medusa stage. It differs from
Pennaria also in that it belongs to the group of calyptoblastic
polyps in w^hich the perisarc forms a protective covering or theca
for the hydranth and for the blastostyle. The method of bud-
ding is monopodial with terminal btid,^ not with terminal or top
polyp as in Pennaria. The stem has as in Pennaria "unlimited"
growth, but the terminal bud never develops into a polyp. In-
stead, it produces at regular intervals two opposite buds which
remain sessile and become polyps. The result is similar to that in
sympodial budding inasmuch as the oldest polyps are nearest to
the hydrorhyza, but every pair of polyps has the same age, the
stem is not composite and the polyps take no part whatever in
its formation. The creeping hydrorhyza does not present any-
thing particular in its structure. The hydrocaulus or stem is

1 The sympodial method is more common in calyptoblastic polyps and may
be best studied in Obelia. The polyps have limited growth. The polyp
nearest the hydrorhyza is the oldest, the end polyp the youngest. The main
stem is composite, the stem of every new polyp adding to its length. There
are gradations between the sympodial method and the monopodial method
with terminal bud.

23

24 MORPHOLOGY OF INVERTEBRATE TYPES

straight, divided into regular internodes each with a pair of
opposite hydranths. Branches are always opposite, arising at
the base of the hydranths. Every internode giving rise to
branches has therefore two hydranths and two branches. The
ramuli or branches of the second order arise asymmetrically,
i. e., one ramulus to an internode of a branch. The colony is
largely composed of trophosomes, i. e., common hydranths whose
main function is nutrition, and a few gonosomes or modified
polyps which depend for their nutrition on the trophosomes and
whose function is sexual reproduction. The gonosomes are
asymmetrical, a single gonosome being produced by an internode
in front of the trophosomes.

Trophosome. We have already seen that the hydranths
of Sertularia pumila are protected by a theca. As there are two
hydranths for each internode, there are also two hydrotheccB
for each internode. They sit on a '^ shelf ^' of the internode and
the base of each hydro theca presents a regular diaphragm with
a circular opening the diameter of which is considerably smaller
than the diameter of the hydrotheca itself. The coenosarc of
the stem is therefore distinctly constricted by the diaphragm,
through which it has to pass to form the pol}qD. The free end
of the hydrotheca is provided with an oval opening or aperture.
The edge of the aperture is emarginate, produced into tw^o
opposite teeth. The aperture may be closed by a two-flapped
operculum or lid when the hydranth is entirely withdrawn into
the hydrotheca. This operculum is best likened to a double door
only that the so-called adcaiiline flap, i. e., the one whose hinge
is nearest to the stem, is much smaller than the abcauline flap.
The hydranth is provided with a special ectodermal fold which
arises considerably below the tentacles and is attached to the
edge of the hydrotheca. This fold forms an almost complete
contractile sheath and serves as a protractor.^ The hydranth
has a single circle of usually sixteen flliform tentacles situated at
the base of the hypostome. They have the shape of the basal
^ NtUting mistook the optical section of the sheath for two protractors.

SERTULARIA PUMILA LINN^US 25

tentacles of Pennaria with nematocysts grouped in batteries.
The microscopic structure of the hydranth is generally speaking
the same as in Pennaria.

Gonosome. The reproductive polyp or gonosome is much
larger than the trophosome. The gonotheca, usually called
gonangium has the shape of a ovoid sac with a truncated end and
is attached to the stem by a very thin and short pedicel or collar.
An opening is formed later when the so-called acrocyst pro-
trudes through the truncated end to the outside. The blasto-
style or modified polyp develops a gonophore which appears as an
outgrowth of the ectoderm filled with reproductive cells. The
gonophore soon outgrows the blastostyle and almost fills the
gonangium. At the distal end of the gonophore a cuticle is
produced and the end of the gonophore protrudes now through
the gonangium. The cuticle of the gonophore expands forming a
globular sac or acrocyst into which the eggs pass. The remains
of the gonophore in the gonangium appear as irregular strands
called gubernacula. A new acrocyst is formed with the produc-
tion of a new gonophore. The male gonangia are more slender.
The egg develops into a planula which forms a new colony.

Instructions

1. Put a colony into a Syracuse dish with glycerine and exam-
ine it under the dissecting microscope. Make a half page draw-
ing showing hydrorhyza, hydrocaulus and branches with tropho-
somes and gonosomes. Number the polyps i-i, 2-2, 7,-^ and
so on, beginning with the polyps nearest to the hydrorhyza, to
show the monopodial method of branching w4th terminal bud.
Nimiber the branches a, b, c . . . and the polyps of one branch
ai-i, a2-2, a3-3 . . . and so on.

2. Cut off a piece of the stem with expanded pol3q)s and put it
on a slide in a drop of glycerine. Cover it with a cover glass and
examine under low power (50 diameters). Make a half page
drawing of two internodes showing the coenosarc of the stem

26 MORPHOLOGY OF INVERTEBRATE TYPES

with the shelf to which the polyps are attached, the perisarc,
the hydrothecac with their terminal aperture and two-flapped
operculum, the diaphragm at the base of the hydrotheca, the
hydranths with the single circle of filiform tentacles, hypostome,
mouth and ectodermal fold servdng as protractor.

3. Examine in the same manner a gonosome. Make a half
page drawing of two internodes of the stem carrying the gonan-
gium, the blastostyle, gonophore and gubernacula or strands
representing the remains of the previous gonophore which had
been emptied.

4. Examine in the same way the terminal bud (200 diameters)
and make a drawing of it.

5. Examine under high power (400 diameters) the prepared
slide with a stained colony and make a drawing of a tentacle
showing its structure.

6. Examine under high powTr (400 diameters) the cross-sec-
tion through a hydranth and make a drawing showing hy-
drotheca, ectodermal fold, ectoderm, mesoglcea, endoderm and
ccelenteron.

7. Additional exercise. Find among the colonies a gonosome
in which an acrocyst has been formed above the gonangium.
Cut it off with a piece of the stem, place on a slide in a drop of
glycerine, cover with a cover glass and examine under low power
(50 diameters). Make a drawing showing details.

TIMA FORMOSA L. Agassiz

Material. Tima formosa is not uncommon in the North
Atlantic. Along the New England coast the mature medusae are
found in March, April and May and disappear before June.
North of Cape Cod they are found in autumn and winter. The
specimens should be preserved in formalin. For microscopic
study a specimen must be preserved in i% osmic acid.

Descriptive Part

Tima formosa belongs to the class Hydrozoa and has a life-
cycle with complete metagenesis. The hydrosome has been bred
from a fertilized egg by Agassiz, but is very little known, quite
insufficiently in fact, to recognize it in nature. The medusome is
one of the largest American hydromedusae.

The edge of the medusa is a perfect circle and the body, which
is called the umbrella may be best likened to a bell. The outer
surface of the bell is called the exumbrella, the inner surface the
subumbrella. The exumbrella has the shape of a paraboloid and
its axis is somewhat longer than the radius of the base. Owing
to the thickness of the bell at its apex the axis of the subumbrella
is considerably shorter. Moreover, the subumbrella has a
peculiar shape inasmuch as its median portion hangs down
through the opening of the bell, in the shape of a cone called the
peduncle. At the end of the peduncle is a short manubrium with
a mouth-opening surrounded by four long lips with transverse
folds. The mouth leads into the quadrangular cavity of the
manubrium, which is called the stomach. Four radial canals
arise from the corners of the stomach and run at even distances
from each other in the wall of the peduncle toward its base, bend

27

28 MORPHOLOGY OF INVERTEBRATE TYPES

over and continue to the edge of the bell where they open into a
circular canal. The stomach with its system of canals represents
the coelenteron of the medusa. It is usually known under the
name of gastro-vascular syskju. The broad, contorted bands
accompanying the radial canals are the gonads. Around the edge
of the medusa are thirty-two tentacles; of these four long tenta-
cles are at the points where the radial canals open into the circu-
lar canals and four halfway between them, eight medium long
ones halfway between the preceding and sixteen short tentacles
each halfway between the long and medium ones. The tentacles
are hollow and their cavity communicates with the circular canal.
Each tentacle is swollen at the base and is therefore subdivided
into a bulb and shaft. Between each pair of tentacles are three
small protuberances or rudimentary tentacles of which there are
therefore 96 all together. There are 128 simple marginal sense
organs or statocysts, alternating with the tentacles and protuber-
ances. The entrance to the subumbrella is somewhat constricted
by a flat, circular diaphragm or velum. The velum is the organ
of locomotion and when it contracts a pressure is produced on
the water in the bell-cavity, which forces the medusa to move in
the direction of its apex.

It is evident that the medusa is built on the principle of radial
symmetry. The axis of the bell, passing through the apex or
centre of the exumbrella and the mouth is the longitudinal axis.
The bell may be divided into symmetrical octants by four
planes intersecting in the longitudinal axis. Two of these planes
are called perradial and run through the radial canals; they
divide the stomach diagonally. The other two planes, called
interradial run halfway between the radial canals; they divide
each side of the stomach in two. A plane dividing two opposite
octants into two halves is called adradial. There are therefore
four adradial planes, but neither of them divides the medusa into
two symmetric halves. In accordance with the above termi-
nology the tentacles receive the names of the planes to which
they belong. There are four perradial, four interradial and

TIMA FORMOSA L. AGASSIZ

29

Fig. 6. — Schematic section through a hydromedusa in the region of the
velum after Delage and Herouard (Zoologie Concrete), cnlc, circular canal;
cntty tentacular canal; msg, mesogloea; air, radial canal; mslso, muscular
fibres of subumbrella; mclvt, muscular fibres of velum; hrty nettle cell battery;
ne, external nervous ring; ni, internal nervous ring.

eight adradial tentacles. Of the remaining sixteen tentacles,
eight are per-adradial (halfway between a per-radial and adra-
dial tentacle) and eight interadradial (halfway between an in-
terradial and an adradial tentacle).

Body layers. The body of the medusa is composed of

So MORPHOLOGY OF INVERTEBRATE TYPES

the same three layers as the body of the polyp. These layers
are ectoderm, mesoglcca and endodcrm. The mesogloea gives
the bulk to the body. It is a transparent, gelatinous, structure-
less substance containing much more water than organic matter.
It fills out the space between the ectoderm and the endoderm
and is not only found in the umbrella and peduncle, but forms
also a thin layer in the velum, manubrium and tentacles. The
endoderm forms the lining of the gastro-vascular system and
of the tentacles. The ectoderm forms the covering of the exum-
brella, subumbrella, peduncle, manubrium, tentacles and velum.
To the ectoderm belong further the gonads, the muscular
system and the nervous system with the sense organs.

Endoderm. The endoderm of the medusa is composed
of the same elements as that of the polyp except that myo-
epithelial cells are absent. The lining of the stomach is
formed by a single layer of digestive endoderm and unicellular
glands; the lining of the canals is formed by circulatory
endoderm; and the lining of the tentacles by supporting
endoderm.

Body covering. The covering of the exumbrella consists of
a single layer of ectodermal flat epithelial cells. Near the margin
of the exumbrella nematocysts are found also between the flat
cells. The ectodermal layer of the subumbrella is more compli-
cated. It consists of two distinct layers, a superficial or epithelial
layer and a deeper or subepithelial layer. It must be borne in
mind, however, that the subepithelial layer has been differen-
tiated from the epithelial layer. The epithelial layer is composed
of indifferent columnal epithelial cells, myo-epithelial cells and
sensory cells. The latter, however, are very few in number.
The subepithelial layer is composed of ganghonic cells, nerve
fibres, striated and unstriated muscle fibres. The same elements
are found in the ectoderm of the peduncle and manumbrium.
There are, however, many sensory cells on the manubrium and
nematocysts on the lips. The ectoderm of the tentacles is com-
posed of indilTerent cells, myo-epithelial cells, sensory cells and

TIM A FORMOSA L. AGASSIZ 31

nematocysts. The latter are spindle-shaped and immense in
number.

Muscular system. As already stated the muscular sys-
tem of Tima consists of subepithelial striated and non-striated
fibres. To the former belong the circular fibres of the sub-
umbrella, of the peduncle and of the subumbral side of the velum,
to the latter the radial fibres of the subumbrella and the longitu-
dinal fibres of the peduncle, manubrium and tentacles. In the
tentacles the longitudinal fibres are found only in the shape of
one strand in their velar side, while circular fibres are entirely
absent. There are no endodermal muscle fibres in Hydro-
medusae.

Nervous system. The nervous system consists of a
subepithelial network of ganglionic cells and fibres in the sub-
umbrella, peduncle and manubrium, and of two rings in the base
of the velum. The rings are usually termed the central nervous
system. They too are composed of ganglionic cells and fibres.
One ring is subumbral in position, the other exumbral; the rings
are concentric and are separated from each other by the meso-
glcea of the velum. The subumbral ring innervates the circular
muscles of the velum. The exumbral ring is the heavier of the
two and innervates the tentacles, the marginal sense organs and
sensory cells. The two rings are connected with each other by
fibres.

Statocysts. Tima formosa has one hundred twenty-eight
simple statocysts along its margin, alternating with the
protuberances and tentacles. The statocysts are more or less
round bodies of very small size. They consist of ectodermal
cells and each statocyst contains from fifteen to twenty con-
cretions arranged in a hemisphere. The function of the stato-
cysts is that of controlling the equilibrium of the body.

Reproductive system. The reproductive organs of Tima
are simple gonads. They are nothing but specialized ectodermal
cells of the subumbrella and peduncle in the region of the radial
canals. They appear as four long sinusoid bands. The repro-

32 MORPHOLOGY OF INVERTEBRATE TYPES

ductive cells are dehisced directly into the bell cavity and
through the bell-opening to the outside. The sexes are separate.
Fertilization is left to chance and takes place outside of the
body, in the water.

Instructions

1. Examine a specimen of Tima formosa in a finger bowl with
water. Make a half page drawing showing the paraboloid ex-
umbrella, velum, bell cavity, peduncle, manubrium with the
four lips, the four radial canals, gonads, circular canal, tentacles.
Determine the perradial, interradial and adradial planes. Label
all structures including the bulb and shaft of a tentacle, and the
planes.

2. Cut off a piece of the margin with two tentacles and a por-
tion of the velum. Put it on a slide in glycerine, exumbral side
uppermost and examine under low power (50 diameters) without
a cover glass. Make a drawing showing the hollow bulbs of the
two tentacles, three protuberances and four statocysts.

3. Cut off a piece of the velum and place it on a slide in a drop
of glycerine, subumbral side uppermost. Cover it up with a
cover glass and examine under high power (400 diameters).
Make a drawing showing the circular muscles.

4. Examine in the same manner the shaft of a tentacle.
Make a drawing showing muscular strand, nematocysts, en-
doderm and cavity.

5. Examine under high power the prepared slide with a radial
section through the velum and bell edge. Make a drawing show-
ing the circular canal, mesogloea, subumbral and exumbral
nervous rings and the two ectodermal layers of the velum with
the mesogloea between them, and the muscular fibres in the
subepithelial subumbral layer of the velum.

GONIONEMUS MURBACHII Mayer

Material. G. murbachii is common in a certain locality
in Woods Hole and may be obtained from the Biological Labora-
tory. Specimens for microscopic study should be preserved in
1% osmic acid. Every student should study a specimen pre-
served in formahn and the following prepared shdes: a radial
section through the velum and bell edge, a cross-section of a
tentacle, a cross-section through the manubrium and a cross-
section through the peduncle. The general plan of structure
is so similar to that in Tima that a separate description is not
needed and the specific characters will be pointed out in the
instructions.

Instructions

I. Examine a specimen of Gonionemus in a stender dish with
water. To see it best turn it so that it shows more than its pro-
file, enabhng you just to see the bell opening. Make a half
page drawing of it in this position. The drawing should show:
the almost hemispherical exumbrella, the powerful velum, the
bell cavity and the subumbrella; hanging down from the centre
of the subumbrella the short peduncle with the manubrium and
four lips surrounding the mouth; the four radial canals and
under them the sigmoid bands which are the gonads; the cir-
cular canal near the margin; the tentacles each with a swollen
base or bulb and thin shaft. (In a live specimen there is a bright
green pigment spot in the endoderm at the base of every tentacle,
but the color disappears in formalin.) In making the drawing
observe that the peduncle and manubrium have not the shape
of a square in a cross-section but that of a cross. The number
of tentacles varies in Gonionemus and depends largely upon the

33

34 MORPHOLOGY OF INVERTEBRATE TYPES

age of the individual. There are usually from sixty to eighty
tentacles.

2. Cut off a piece of the bell-margin between the radial canals,
put it on a shde in a drop of glycerine, exumbral side uppermost.
Examine through the microscope under low power (loo diam-
eters). Make a drawing showing the circular canal and the
margin of the bell with the base of several tentacles and the
statocysts with the single concretion in them. There are usually
half as many statocysts as tentacles.

3. Cut off a piece of the velum, put it on a slide in a drop
of glycerine, subumbral side uppermost, cover up with a cover
glass and examine under high power (400 diameters). Make a
drawing showing the epithelial cells and circular muscle fibres.

4. Sever a tentacle and examine in the same manner that
region of it where it is bent at right angles. Make a drawing
showing the rings of nematocysts and the cup-like adhesive pad.

5. Examine under high power (400 diameters) the prepared
slide with a cross-section of a tentacle. Make a drawing
showing the central cavity, supporting endoderm composed of
large cells with small nuclei, mesogloea and ectoderm with ovoid
nematocysts and muscular fibres.

6. Examine the radial section through the edge of the bell
and the velum. Make a half page drawing showing the circular
canal, mesogloea, ectoderm of the subumbrella, subumbral
ectodermal layer of the velum with the circular muscles, exum-
bral ectodermal layer, subumbral nervous ring and exumbral
nervous ring.

7. Examine a cross section through the manubrium (50 di-
ameters) and make a drawing showing the cross-shaped cavity
of the stomach, mesogloea and endoderm.

AURELIA AURITA (L.) varietas FLAVIDULA
Peron et Lesueur

Material. Mature individuals of Aurelia flavidula may
be collected in summer and preserved in formalin. Scyphos-
tomae and strobilae are common on seaweed in October. Ephyrae
may be found in March and April. Material for microscopic
study should be fixed in i% osmic acid. The student should
study a mature medusa, a scyphostoma and a strobila preserved
in formalin, and the following prepared microscopic slides: a
cross-section through an arm of a mature medusa with embryos,
an ephyra stained in toto, a cross-section through a scyphos-
toma, a median longitudinal section through a scyphostoma.

Descriptive Part

Aurelia aurita is a typical representative of the Class
Scyphozoa. It is a cosmopolitan jelly-fish common in European
seas and the Atlantic and Pacific Oceans. The American variety
is known under the name of Aurelia flavidula. The life cycle
of AureHa flavidula consists in an alternation of generations com-
bined with metamorphosis or development of the adult from a
larva. The whole life cycle is completed in one year and the
preponderance lies with the sexual stage or medusa, compared
with which the polyp is very small. The medusae begin spawning
toward the end of July. The eggs develop in small breeding
pouches of the mouth-arms of the female. Here they reach the
stage of a ciliated planula. The planulae leave their mother in
October and soon attach themselves to some seaweed or rocks.
The mouth appears and tentacles grow out around the peristome.
Thus a scyphopolyp or scyphostoma is formed. The scyphos-
toma begins to grow and produces by a peculiar method of

6S

36

MORPHOLOGY OF INVERTEBRATE TYPES

transverse fission a column or strobila of about a dozen ephyrcE.
The ephyrae begin to separate in March or April, appear soon
as small medusae, grow rapidly and become mature in summer.
Medusa. The medusa is built on the principle of radial
symmetry. It has the shape of a disc or better of a round con-

FiG. 7. — Aurelia labiata Ch. & Eys, from Mayer's Medusae of the World.
One mouth-arm has been cut off to show the structure of the medusa.

vex concave lens. The oral surface or suhumbrella is very slightly
concave. The aboral surface or exumbrella has almost the shape
of a hemisphere when the medusa is fully contracted. The longi-
tudinal axis running from the centre of the subumbrella to the
centre of the exumbrella is shorter than the radius of the disc.

AURELIA AURITA 37

The medusa may be divided into eight symmetric sectors
or octants by four planes intersecting each other at 45° in the
longitudinal axis. For the sake of convenience two of the planes
intersecting at right angles are called perradial, and the other
two interradial. The planes which divide the disc into sixteen
sectors are called adradial. Neither of the adradial planes di-
vides the medusa into symmetric halves. The various organs
are said to be perradial, interradial or adradial in accordance
with the name of the plane which bisects them.

The margin of the medusa is not an unbroken circle. There
are eight notches in the margin corresponding to the four planes
of symmetry. At the bottom of each notch is a sense organ or
rhopalium. Four of them are perradial and four interradial.
On each side of a rhopalium is a large marginal lappet. The
margin of the medusa is covered with numerous small tentacles.
These tentacles arise from the aboral surface at a very short
distance from the margin and alternate with small marginal
lobules. The tentacles are hollow, distended at their base; their
cavity is in direct communication wdth the circular canal of the
gastro-vascular system (coelenteron) and is lined with endoderm.
Each tentacle has on its subumbral side longitudinal muscle
fibers in the subepithelial layer of the ectoderm and a row of
broken rings of nematocysts on its exumbral side.

Body covering and muscular system. The body covering
consists of a single layer of ectoderm. On the exumbrella the
cells of the covering belong to the type of very flat epithelial
cells with numerous nematocysts arranged in batteries on little
protuberances, especially prominent tow^ard the edge of the disc.
On the subumbrella the number of nematocysts is small and
they are irregularly scattered among the common epithelial
cells. The subepithelial layer is a well developed system of
circular and radial muscular fibres. The circular fibres are
especially numerous toward the edge of the disc. The radial
fibres run from the centre toward the tentacles. The contraction
of the disc is accomplished by the joint action of the circular and

38 MORPHOLOGY OF LWERTEBRATE TYPES

radial fibres; the expansion is due to the elasticity of the meso-
gloea. There is no velum in Aurelia, as is true for the entire Class
of Scyphozoa which therefore are called Acraspedce. What is
known as velarium in some Scyphomedusae is not a fold of the
ectoderm but a fold of the subumbrella containing endodermal
canals. There is no velarium in Aurelia and the application of
this name to the border of the disc between the base of the
tentacles and the margin is not correct.

Mesogloea. The bulk of the medusa is formed by a
gelatinous, elastic mesogloea. The mesogloea is not structure-
less as in Hydromedusae, but contains stellate and bipolar
cells.

Gastro-vascular system. We have seen that the ccelen-
teron of the Hydromedusae is already considerably more com-
plicated than that of the Hydropolyps. The coelenteron of the
Scyphozoa is still more complicated and appears in the shape of a
highly differentiated gastro-vascular system. The month is
situated at the end of a short manuhriiim, and has more or less
the shape of a square. The angles of the opening are perradial in
position. The edge of the manubrium is drawTi out to a con-
siderable length and forms four mouth arms which are also
perradial. Each of the four angles of the mouth continues as a
longitudinal groove to the end of the mouth-arm. Both edges of
the groove are fringed wdth a row of minute lahial tentacles.
When both edges are in close contact the mouth appears not as a
square but as a cross or as a longitudinal fissure. It may be
added that the mouth-arms of the female are stouter than those
of the male.

The mouth leads into the central cavity or stomach. The
stomach is produced into four large interradial gastric poticheSy
between which the four perradial canals are situated. The
openings leading from the stomach into the pouches are called
the gastric ostia. On the floor of each gastric pouch is a groove
formed by two folds of the lining. These gonadial grooves extend
from the ostia to the middle of the pouches where they become

AURELIA AURITA 39

considerably wider. They serve tiie purpose of carrying the
genital cells from the gonads which are situated in the gastric
pouches, to the stomach. On the floor of the pouches, surround-
ing the gonadial groove is a horseshoe-shaped genital ridge and to
the inside of it and closely applied to it a row of gastric filaments.
The system of radial canals is quite complicated and subject to
great variations although it follows a definite plan. One can
always recognize /cwr perradial branching c^xidXs, four interradial
blanching canals and eight adradial straight canals. The per-
radial canals arise from the corners of the stomach between the
gastric pouches and soon give off two opposite branches while
the main stem runs straight to the periphery and opens into the
circular canal near the base of the perradial rhopalium. The
primary branches subdivide several times and spread over the
space between the central branch and the adjoining adradial
canals. Their terminal branches open into the circular canal.
The interradial canals arise from the gastric pouches and branch
in the same manner as the perradial canals. But the primary
branches are formed so close to the beginning of the canal that
they produce the impression of two independent canals. The
adradial canals have no branches. They arise from the gastric
pouches and run straight to the periphery, opening into the
circular canal halfway between the rhopalia. The circular canal
follows the edge of the disc between every rhopalium. When it
reaches a rhopalium it forms a horseshoe bend around its base.
The cavities of all tentacles and rhopalia open also into the cir-
cular canal. The entire gastro-vascular system, from the edge of
the mouth to the last ramifications of the canals is naturally
lined with endoderm. The majority of the cells of which the
layer of endoderm is composed are cylindrical ciliated epithelial
cells. In the stomach are many glandular cells. In the gastric
filaments numerous nematocysts occur. The grooves of the
mouth-arms are also lined with endoderm. The labial tentacles
are rich in nematocysts.

Subgenital pits. When one examines the subumbrella

40

MORPHOLOGY OF INVERTEBRATE TYPES

of Aurelia one cannot fail to notice four interradial, oval openings
which appear as if they were leading into the gastric pouches.
In reahty they lead into blind suhgenital pits situated under the

gastric pouches. The
pits are lined with ec-
toderm and have no
connection whatever
with the gastric
pouches. Neither have
they anything to do
with the reproductive
functions of Aurelia.
It is probable that the
subgenital pits help
respiration inasmuch
as they increase the
surface of the subum-
brella and allow the
water to come nearer
to the reproductive
organs.

Nervous system.
The nervous system
consists of a subepi-
thelial network of ganglionic cells and fibres and of eight
nervous centres or ganglia, one at the base of every rhopalium.
The subepithelial network is found in the subumbrella between
the epithelial layer and the layer of muscular fibres. The func-
tion of this network is chiefly that of a motor system.

Sense organs. We have seen that Aurelia has eight
marginal sense organs or rhopalia. A rhopalium is a very com-
plicated organ with various functions. It is protected by the
two marginal lappets and its cavity stands in direct communica-
tion with the circular canal. The rhopahum itself consists of a
large dorsal protective fold or hood with the aboral olfactory pit

Fig. 8. — Tentaculocyst of Aurelia aiirita,
longitudinal section. Diagram after Eimer,
from E. Ray Lanckester's Treatise on Zo-
ology. A, aboral (superior) olfactory pit; B,
adoral (inferior) olfactory pit; c, exumbrella;
Con, statocyst; Ent, radial canal continued
into tentaculocyst; End, endoderm; Oc, ocellus;
T, tentaculocyst; Or, subumbrella; H, bridge
between the two marginal lappets (hood) .

AURELIA AURITA 41

at its base and a small club or tentaculocyst under it. An oral ol-
factory pit is situated at the base of the tentaculocyst. The
tentaculocyst is hollow except at its end which is developed as a
statocyst or an organ of equilibrium and is filled with statolyths.
On the aboral surface of the tentaculocyst, a little in front of the
olfactory pit, is a simple pigment spot or aboral ocellus. On the
oral surface is a well developed, pigmented, cupped ocellus of the
inverted type in which the cones are turned away from the light.
The canal which runs from the circular canal into the tentacu-
locyst, forms two blind canals at its base.

Reproductive system. The sexes are separate and the
reproductive organs are simple, ductless gonads. When the
gonads are fully developed they appear as four colored rings
broken only by the narrow gonadial grooves. They are situated
in the gastric pouches and are endodermal in origin. The re-
productive cells are simply dehisced into the gastric pouches and
reach the stomach through the gonadial grooves. The fertilized
eggs are found later in the grooves of the mouth-arms and de-
velop in special pouches there. Here they reach the planula
stage and leave then the mother.

Scyphostoma. The structure of the scyphostoma has
been often misinterpreted owing to the extreme contractility
of its muscle-bands which change the position of the mouth.
Thus it happened that investigators have described a manu-
brium and a gullet where such organs do not exist in reality.
The stem is quite short and thin. The tentacles are attached in a
circle around a fiat peristome from the middle of which arises a
conical hypostome. At the extreme end of the hypostome is the
square mouth. The tentacles are solid and normally sixteen in
number, although as many as twenty-four have been observed.
The corners of the mouth mark the two perradial planes. In the
interradii of the peristome are four pits which are usually sup-
posed to be homologous with the subgenital pits of the adult
medusa. These pits are called septal funnels. They are formed
by the invagination of the ectoderm and lead into the wall of the

42 MORPHOLOGY OF INVERTEBRATE TYPES

tasniolae. The tceniolcB are longitudinal folds of the endodermal
lining of the polyp cavity. The four taeniolcC are interradial in
position and subdivide the cavity of the polyp into four per-
radial chambers. Each taeniola has a strong band of longitudinal
muscle fibres which are attached to the blind end of the septal
funnel. The ectodermal subepitheUal muscle fibres are circular.
Ephyra. We have seen already that the scyphostoma
produces about a dozen ephyrae by a process of strobilization.
An ephyra which has just detached itself has a flat body divided
into eight narrow lobes or rays, four of which are perradial and
four interradial. Each lobe has two end lappets (marginal
lappets of various authors. According to Schewiakov they be-
come later the sensory lobules of the rhopalia). Between the
two lappets is a not yet completely developed rhopalium. The
square mouth is situated at the end of a short manubrium. The
stomach is subdivided into chambers and gives off eight blind
canals, one for each ray. Four interradial gastric filaments are
present and their number grows with the growth of the ephyra.
The transformation into a medusa is gradual with the more
rapid growth of the disc between the lobes, till the star-shaped
body assumes the shape of a disc.

Instructions

I. Put a mature medusa on its back in a dissecting tray filled
with water and examine the specimen with naked eye. Deter-
mine the perradial planes by the position of the mouth-arms and
the interradial ones by that of the gonads. Find the openings of
the subgenital pits. Move the mouth-arms carefully apart with
two fingers till the mouth is wide open, introduce then a black
horse hair through the mouth into one of the perradial canals,
pushing the hair gently till it reaches the circular canal. Intro-
duce another hair through a stomach ostium into an interradial
canal, and a third hair into an adradial canal. Make a full page
drawing showing the following structures: Four mouth-arms

AURELIA AURITA 43

with the fringe of labial tentacles, longitudinal groove and
brood-pouches filled with developing eggs; mouth, four gastric
pouches with gastric ostia, gonadial grooves, gonads and gastric
filaments; four perradial canals, of which only two running at
right angles to each other should be drawn with all branches, of
the other two merely their beginning; one interradial canal with
all its branches in the quadrant between the two finished per-
radial canals; two adradial canals of the same quadrant; the cir-
cular canal; the eight rhopalia or marginal sense organs; the
tentacles all around the edge of the disc; the four openings of the
subgenital pits over the centres of the gastric pouches. Label all
above structures as well as the perradial, interradial and adradial
planes.

2. Cut off with scissors a piece of the margin with a rhopalium.
Place it in a Syracuse dish in as little water as possible and turn
the piece so that its exumbral surface would be uppermost.
Examine under low power (50 diameters) the rhopalium and the
surrounding structures. Make a half page drawing showing
the hollow tentacles with swollen base, the protective fold, in
it the tentaculocyst, the two marginal lappets, the pigment spot,
and the protuberances with nematocysts on the exumbrella.

3. Turn the piece over and examine it under higher power
(100 diameters). Make a half page drawing showing the horse-
shoe bend of the circular canal, the ends of the radial canals, the
canals leading from the circular canal into the tentacles, the
canal leading into the tentaculocyst with two blind canals at
its base, the marginal lappets, the pigmented cupped eye, and
the statocyst at the end of the tentaculocyst.

4. Transfer the piece onto a slide, add a drop of glycerine,
cover with a cover glass and examine the ectodermal covering
of the subumbrella under higher power (200 diameters). Make
a drawing showing the circular muscles and the nematocysts
scattered over the surface.

5. Cut off another piece of the margin of the medusa with
scissors and put it on a slide with its exumbral surface upper-

44 MORPHOLOGY OF INVERTEBRATE TYPES

most, add a drop of glycerine and cover with a cover glass. Ex-
amine the covering of the exumbrella under high power (400
diameters). Choose a protuberance with some exploded nem-
atocysts and make a drawing showing an entire protuberance
with all nematocysts.

6. Examine a tentacle on the same slide and make a drawing
showing the rings of nematocysts.

7. Cut off a piece of a mouth-arm, place it on a slide in a drop
of glycerine, cover with a cover glass and examine under high
power (200 diameters). Make a drawing showing the tentacles
with the nematocysts evenly distributed over them, and the
brood-pouches with embryos.

8. Cut off a piece of the floor of a gastric pouch with gastric
filaments, put it on a slide in the same manner as in the preceding
exercise and examine under high power (200 diameters). Make
a drawing of a gastric filament showing the cavity and the walls
with nematocysts evenly distributed over them.

I

METRIDIUM MARGINATUM Milne-Edwards

Material. M. marginatum is quite common in larger
tide-pools and below the low-water mark along the Atlantic
Coast. Specimens must be stupefied by the addition of magne-
sium sulphate in increasing quantity. When reaction to stimuli
has been inhibited the solution may be replaced by weak for-
malin. Chromic acid is also recommended but has no particular
advantage. For microscopic study pieces of stupefied specimens
may be preserved in i% osmic acid or in any of the reliable
fixing fluids. Every student should receive two specimens and a
cross-section through an acontium.

Descriptive Part

Metridium marginatum is a representative of that group
of the class Anthozoa which is known under the common name
of sea-anemones. The majority of Anthozoa are colonial forms
and possess a calcareous skeleton. Metridium on the other hand
is not a colonial animal and does not possess such a skeleton.
Yet its structure is nevertheless typical of an Antho-polyp and
like all Anthozoa it has no medusa stage, the polyp itself pro-
ducing reproductive cells.

External features and gastro-vascular system. The body
of Metridium marginatum may be divided into a hrosid foot and a
cylindrical column or scapus crowned by a festooned capitulum
carrying numerous hollow tentacles. The largest tentacles are
nearest the mouth. The oblong mouth is situated at the end
of the polyp in the middle of a flat peristome. In the wall of
the column are numerous pores or cinclides which leads into the
gastro-vascular space. These pores are always closed and

45

46

MORPHOLOGY OF INVERTEBRATE TYPES

I
I

-^%cl.

Fig. 9. — Morphologic type of a Hexactinia after Y. Delage & E.
Herouard, Zoologie Concrete, acn, acontia; h, mouth; d I, primary mesen-
tery; d II, secondary mesentery; d III, tertiary mesentery; cU, cincHdes;
dd, dorsal (directive) mesentery; dp, foot; dtt, capitulum; entd, mesenteric
filament; gtx, gonad; Ug and Ig, subdivisions of an endocoele; mdc, meso-
glcea; 0, outer septostoma; oi, inner septostoma; p, tentacular pore; ph,
gullet; sph, sphincter capituli; spgd, dorsal siphonoglyphe; spgv, ventral
sophonoglyphe; // I, l\\ lenlacles of the first to fourth \-erticel.

METRIDIUM MARGINATUM MILNE-EDWARDS 47

therefore invisible. But when a liA^e Metridium is subjected to
strong stimuli, long white threads or acontia are suddenly pro-
jected through the cinclides to the outside. The transverse
folds of the wall are produced by the contraction of longitu-
dinal muscle fibres. The mouth of Metridium leads into a gullet
or pharynx which almost reaches the foot and opens into the
gastro-vascular space or stomach. We have seen that a gullet
had been erroneously ascribed to scyphostoma owing to the
temporary invagination of the hypostome under the influence
of stimuli. In Metridium the invagination is permanent and
the gullet is therefore homologous to the hypostome of scy-
phostoma. The lining of the gullet is thrown into longitudinal
ridges, except in the siphonoglyphes which are ciliated furrows
running from the opposite ends of the mouth down the whole
length of the gullet. The number of individuals with a single
siphonoglyphe is about equal to that with two siphonoglyphes.
The gastro-vascular space is subdivided by longitudinal mesen-
teries or partitions. Of these, six pairs are attached to the
wall and to the gullet and are called complete or primary mesen-
teries. They have in their wall longitudinal muscle hands often
called musde-bajtners. In diglyphic specimens (with two siphon-
oglyphs) two pairs of the primary mesenteries are called directive
mesenteries. They are attached to the siphonoglyphs and their
muscle-banners are directed away from each other. In each of
the other four pairs the banners are directed toward each other.
In monoglyphic specimens a single pair of directive mesenteries
is present and in the remaining five pairs the banners are directed
toward each other. The diglyphic type presents therefore two
planes of symmetry, intersecting at right angles, while the
monoglyphic type is strictly bilateral. The space between the
gullet and the body wall is divided by the primary mesenteries
into twelve chambers, but the mesenteries being arranged in
pairs, the chambers between the pairs are much larger than the
chambers formed by the two mesenteries of the same pair.
The former are called cxocceles, the latter endocceles. The endo-

4S MORPHOLOGY OF INVERTEBRATE TYPES

and exocoeles communicate with each other below the gullet,
where the inner edge of the primary mesenteries is free and where
all chambers open into the central cavity. The chambers com-
municate with each other also at the anterior end. This com-
munication is established by means of large round or oval open-
ings in the mesenteries. The openings are arranged in two
circles; one surrounding the gullet close under the inner surface
of the peristome, the other somewhat lower and close to the wall.
There are therefore altogether twenty-four openings, two in
each mesentery. These openings are called inner and outer
mesenteric ostia or septostomata. The free edge of the primary
mesenteries forms a thickened and twisted mesenteric filament.
Near the base of the mesentery the filament becomes free,
changes somewhat its structure and appears in the shape of a
long and thin thread or acontium. Each exoccele is subdivided
by incomplete meseiiteries, which are attached only to the wall of
the polyp but not to the gullet and which have only outer septo-
stomata. There are usually one pair of secondary, two pairs of
tertiary, and four pairs of quaternary incomplete mesenteries in
each exoccele, or altogether forty- two pairs of incomplete mesen-
teries in the six exocoeles. Each incomplete mesentery has a
mesenteric filament along its free edge and an acontium at the
base. In the wall of the incomplete mesenteries the gonads are
situated and appear as a single row of bead-like bodies. When
fully developed they fill almost completely the exocoeles. Small
gonads develop also near' the free edge of the primary mesen-
teries, except the directives, below the gullet.

Microscopic anatomy, (a) Ectoderm. The ectoderm forms
the covering of the body, of the tentacles, and of the peristome,
and the lining of the gullet. Although composed of a single layer
of cells it may be divided into an epithelial and a subepithelial
layer. The former consists of ciliated columnar cells, mucous
glands, albuminous glands, nematocysts, and sensory cells; the
latter of nematoblasts, ganglionic cells, nerve fibres, muscle cells
and muscle fibres, (b) Endoderm. The endoderm forms the

etw. z

cn.hl

TT.. .^ —Transverse section of a tentacle of Ammonia Sulcata ^iter Schneider.
./J sup^rtin?XroftVei:;!Sfferentcells (De^^

so

MORPHOLOGY OF INVERTEBRATE TYPES

lining of the gastro-vascular system, including the mesenteries,
the covering of the gullet, and the acontia. The gonads are also
endodermal. The epithelial layer of the endoderm consists of
columnar flagellated cells and glands as well as numerous long
nematocysts in the acontia. The subepithelial layer consists of
ganglionic cells, nerve fibres, muscular fibres and nematoblasts
in the acontia. (c) Mesoglcea. The mesogloea is found be-

FiG. II. — Nervous system of an actinia after Wolff.

tween the two preceding layers in the wall, tentacles, peristome,
gullet and mesenteries. It is formed by a striated substance in
which multipolar connective cells are enclosed.

Muscular system. The muscular system in Metridium
is highly developed. Both ecto- and endodermal muscles are
formed by myo-epithelial cells, but in many cases the fibre may
become independent of its mother cell. All ectodermal fibres
are longitudinal in position. The endodermal muscle fibres of
the wall and tentacles are circular. At the anterior end of the

METRIDIUM MARGINATUM MILNE-EDWARDS 51

column they form a real sphincter columtuB. The muscles of the
primary mesenteries, all endodermal, are arranged in a definite
manner. On one surface of the mesentery is the longitudinal
muscle banner and a radial basal muscle. On the other sur-
face are transverse muscle fibres, oblique muscle fibres, and a
basal radial muscle.

Nervous system. The nervous system consists of a
subepithelial ectodermal and a subepithelial endodermal net-
work of ganglionic cells and fibres. The network is especially
rich in the gullet, peristome and tentacles. It is very probable
that there is a direct connection between the two networks in
the shape of fibres which pass through the mesogloea. Sense
organs are absent but sensory cells with tactile and chemo-
tactile functions are found both in the ecto- and endoderm.

Reproductive system. The reproductive organs have
been already described. They are in the shape of ductless gonads
situated in the mesenteries. The sexual cells are dehisced into
the exocoeles. The sexes are separate. The ciliated planula
develops into a polyp.

Asexual reproduction. Reproduction by budding from
the base of the column near the foot is not uncommon. Occasion-
ally a longitudinal fission of adult specimens also occurs. If the
fission is not completed the result is a specimen with two mouths
and gullets.

Instructions

1. Examine a specimen in a deep finger bowl filled with
water. Make a life-size drawing showing the side view. Label
foot, column or scapus, capitulum, and tentacles.

2. Make a life-size drawing of Metridium showing the capit-
ulum from above. Label tentacles, peristome, mouth and si-
phonoglyphs. Label the type to which the specimen belongs, as
monoglyphic or diglyphic, depending upon the presence of one or
two siphonoglyphs.

3. Take the specimen in your left hand, press it gently with

52 MORPHOLOGY OF INVERTEBRATE TYPES

your fingers and cut it with large scissors in two a little below
the capitulum. The cut must be made at right angles to the
axis of the polyp in the same manner as you would cut across
rubber tubing. Put the two halves in separate finger bowls. If
the cut was not quite neat, trim the edges and mesenteries with
scissors, till you obtain a perfect cross-section. This method is
vastly superior to a cross-section made by a razor. Examine the
cut surface of the upper half. Make a half page drawing showing
gullet, siphonogl>T>hs, primary mesenteries with inner septo-
stomata, incomplete mesenteries, and between them the open-
ings leading into the tentacles, circular muscle. Label also
the one or two pairs of primary mesenteries which are the
directives.

4. Take the lens from the dissecting microscope and examine
the cut surface of the gullet. Make a drawing showing the
three layers: ectoderm, mesogloea, and endoderm.

5. Examine the cut surface of the lower half. Make a half
page drawing showing primary mesenteries with their muscle-
banners, incomplete mesenteries, mesenteric filaments, gonads,
gullet with siphonoglyphs. Label also the directive mesenteries,
exocoeles, and endocceles.

6. Cut the lower half in the same manner as before, but this
time close to the foot. The cut should be below the gullet, but
if it happened to be above the end of the gullet, trim the sur-
face with scissors. Make a half page drawing showing primary
mesenteries, incomplete mesenteries, gonads, and acontia.

7. Take another specimen. Introduce the blunt end of the
large scissors into the mouth and cut the specimen into two
symmetric halves. The cut must be made through the corners
of the mouth so that each siphonoglyph will be cut in two.
Put both halves in a finger bowl with water and choose the one
which has been less damaged. Trim the edge to the primary
mesenteries until you have a clean longitudinal section with the
gullet in the middle and a complete mesentery on each side of it.
Make a half page drawing showing capitulum with tentacles,

METRIDIUM MARGINATUM MILNE-EDWARDS 5.3

peristome, gullet with longitudinal ridges and siphonogl)^hs,
both complete mesenteries with their muscles and inner and
outer septostomata, mesenteric filaments, acontia, gonads (if
present), bases of incomplete mesenteries converging to the
centre of the foot, and the sphincter columnas in the wall under
the capitulum.

8. Carefully cut out both primary mesenteries beginning
at the centre of the foot and passing one blade of the scissors
under the mesentery. When the cut has reached the wall,
continue it in the same manner all the way to the capitu-
lum. Then cut in the same manner the gullet, beginning at
its lower end, until you reach the mouth. Lift the mesentery
carefully with a forceps and cut across the peristome. Place
both mesenteries for future examination in a dish with water.
The removal of the primary mesenteries exposes the incomplete
mesenteries in the exocoeles on both sides of the gullet. Make a
half-page drawing showing all structures including the mesen-
teric filaments, gonads, and acontia.

9. Place one of the removed primary mesenteries on a slide
and examine against a window. Make a full-size drawing show-
ing the entire mesentery with the strips of the walls to which it
is attached. Label foot, column, capitulum, peristome, gullet,
mesenteric filament, acontium, gonad (if present), sphincter
columnae, inner septostoma, outer septostoma, longitudinal
muscle (banner), oblique muscle, transverse muscles. The two
basal muscles are not always discernible. Owing to the trans-
parency of the mesentery, the muscles of both sides are equally
well visible. In reality, the longitudinal muscle is on one side,
while the transverse and oblique muscles are on the other
side.

10. Put a small piece of an acontium on a slide in a drop of
glycerine, cover with a cover glass and press the latter until
the acontium is completely mashed. Examine under the micro-
scope at 400 diameters. Make a drawing of a long, spindle-
shaped nematocyst with the filament in it.

54 MORPHOLOGY OF r.VVERTl-:HRATr: TYPES

11. Examine the prepared slide with a cross-section through
an acontium. Make a drawing showing the row of nematocysts.

12. Additional exercise. Touch a Hve Metridium with the
end of a pencil until at least one acontium is projected through
a cinclide. Make a sketch showing the animal in this con-
dition.

DENDROCCELUM LACTEUM (Miiller)

Material. Live specimens of D. lacteum should be kept
in a small aquarium with clean water with a few algae and several
dead leaves at the bottom. They will live a considerable time
if fed on daphnids, copepods and small dipterous larvae. This
species is much better for study than Phagocata gracihs, but
is rarer and for that reason must be collected considerably be-
forehand. The best method of fixation for in toto mounts or
cross-sections is hot sublimate (saturated aqueous solution)
poured suddenly into the dish in which the animal has been
allowed to creep on a dead leaf. If after fixation the animal still
adheres to the leaf, it can be sectioned with it; but usually it
will let go without losing the ciliated epithelium as is invariably
the case when it is fixed in a glass beaker without a dead leaf.
Specimens with a well-filled alimentary canal should be mounted
in toto, without stain of any kind. The differentiation of in-
ternal organs in Planarians is impossible even with Alumcarmin
which gives such wonderful results in flukes and tapeworms.
The internal anatomy, with the single exception of the digestive
organs, must therefore be studied on serial sections. If possible,
every student should get a complete series of sections and find
those among them which are mentioned below.

Descriptive Part

Dendroccelum lacteum is a hermaphroditic fresh-water Tur-
bellarian (Planarian), common both in Europe and this country.
The body is strongly flattened dorso-ventrally. The mouth
opens on the ventral surface considerably behind the middle of
the body, and still farther behind lies the joint opening of the

55

56

MORPHOLOGY OF INVERTEBRATE TYPES

reproductive organs. At the anterior end which is the seat of
the central nervous system, are two lateral projections, the
tentacles, and behind them, on the dorsal surface, two eyes.
Between the tentacles is a retractile grasping organ.
Integument. The body covering is composed of a single layer

■^/pf^/r^i^Hm<it/-ffini^^r^t'4,t,(^^,

Fig. 12. — Transverse section through the skin of Dendrocodum lacteum
after Hallez, modified. GL, gland cell; BM, basal membrane; OM, oblique
muscles; LM, longitudinal muscles; RC, rhabdite mother cell; RH, rhabdites;
EP, epithelial cells; CM, circular muscles; NC, nerve cells; DVM, dorso- ventral
muscles; TM, transverse muscles; P, parenchyma.

of cilitated epithelial cells lying on a hasal membrane. Between
the epithelial cells numerous unicellular gla}id cells are found
which secrete slime, together with branches of bipolar nerve cells
and little rods or rhabdites which possibly serve as protection
and support to the tender epithelium. The rhabdites are pro-
duced by special cells in the parenchyma.

Muscular system. The muscular system, which forms a real

DENDROCCELUM LACTEUM 57

muscular skin bag and is the main organ of locomotion, lies im-
mediately beneath the basal membrane. The movement of the
epithelial cilia is of use to the animal only when it glides, ventral
surface up, on the surface of the water. A well developed layer
of circular muscles situated under the basal membrane is followed
inwardly by a much weaker layer of oblique muscles. The well
developed layer of longitudinal muscles antagonistic to the cir-
cular muscles comes next, and finally a weak layer of transverse
muscles, adjoining the mesenchyme or parenchyma. Traversing
the whole body of the animal from the dorsal to the ventral
surface are the dorso-ventral muscles.

Parenchyma. The space between the dorsal and ventral
transverse muscles is occupied by a mesenchyme or parenchyma.
In it the cells producing the rhabdites are situated, together with
slime glands which open to the outside on the ventral surface of
the animal near the edge. The different organs are also im-
bedded in the parenchyma.

Digestive system. The alimentary canal begins with a
mouth situated on the ventral side considerably behind the mid-
dle of the body. It leads into a spacious mouth cavity, often
called the pharyngeal sheath for the reason that it incloses the
protrusible pharynx. The pharynx represents a muscular tube
covered with epithelium on the inside as well as on the outside
and occupying almost the entire mouth cavity. The opening
of the free end of the pharynx is known under the name of
pharyngeal mouth, and when the pharynx is protruded to the
outside through the mouth proper, the pharyngeal mouth is
the organ which grasps the animal used for food. Round the
pharyngeal mouth the openings of pharyngeal glands are found,
which are situated in the parenchyma. The pharynx leads into
a blind stomach consisting of three main tubes with numerous
side branches. One of the main tubes is median in position and
runs forward almost to the anterior end of the animal, the two
others run backward, right and left of the mouth cavity to the
posterior end. There is no anus.

58 MORPHOLOGY OF INVERTEBRATE TYPES

Specialized organs of circulation and respiration are absent.

Excretory system. The excretory system consists of two
longitudinal canals, running the whole length of the body and
anastomosing anteriorly; they are the so-called collecting canals.
The capillary canals, present in great numbers and each closed
distally by a flame cell open into the collecting canal. The col-
lecting canals open to the outside on the dorsal surface by means
of short tubes. There are altogether four pairs of excretory pores.

Nervous system. The nervous system consists of a brain oc-
cupying the anterior end of the animal and of two longitudinal
nerves extending to the posterior end of the animal. The brain
is composed of two lobes connected with each other by a trans-
verse commissure and supplies with nerves the tentacles and the
median lobe as well as the eyes. The longitudinal nerves are
connected with each other by numerous commissures and give
off numerous branches which innervate the entire body.

Reproductive system. The great majority of Turbell-
arians including D. lacteum are hermaphrodites. The male and
female genital organs open into a genital cloaca communicating
with the outside by means of a common genital opening which is
situated on the ventral surface at some distance behind the
mouth. The male genital organs consist of numerous testes be-
ginning shortly behind the ovaries and extending to or even
beyond the region of the common genital opening, of two rather
short vasa deferejitia which open into a penis pouchy a muscular
penis and a group of penis glands (prostata) . There seem to be
no efferent ducts leading from the testes to the vasa deferentia;
the spermatozoa reach the latter through the interstices of the
parenchyma. As already stated, the penis opens into the
genital cloaca and lies usually invaginated in the penis pouch.
During copulation the penis becomes evaginated and protrudes
through the common genital opening.

The female genital organs consist of three groups of organs not
directly connected with each other, but each with a separate
opening leading into the genital cloaca. The first group consists

DENDROCCELUM LACTEUM

59

of two ovaries situated in
the anterior end of the
body shortly behind the
eyes and iwo oviducts run-
ning more or less parallel
to each other and uniting
posteriorly of the common
genital opening, where they
form a common or single
oviduct. Both oviducts
receive throughout their
length a great number of
yolk glands. The second
organ which has a separate
opening into the genital
cloaca is the uterus with its
uterine canal. Both are
single; the uterus lies in
front of the penis pouch, ex-
tending to the region of the
mouth. The uterine canal
is dorsal to the penis pouch.

The third organ with a
separate opening into the
genital cloaca is the mus-
cular bursa copulatrix which
functions as a female organ
of copulation.

From the foregoing it is

clear that the genital cloaca

has five openings: (i) the

common genital opening

Fig. 13. — Reproductive Organs of Dendroccelum lacteum after Hallez.
O, eye; OV, ovary; T, testis; OD, oviduct; VD, vas deferens; U, uterus;
PR, prostata; BC, bursa copulatrix; GO, genital opening; F, yolk glands;
PU, pharynx; M , mouth; P, penis; GC, genital cloaca; CO, common oviduct.

..JPH

6o MORPHOLOGY OF INVERTEBRATE TYPES

leading to the outside, (2) the opening of the penis, (3) the open-
ing of the common oviduct, (4) the opening of the uterine canal
and (5) the opening of the bursa copulatrix.

The cocoon in which the eggs are inclosed is fonned in the case
of Dendroccelum lacteum in the genital cloaca. The develop-
ment is direct.

Instructions

1. Make a half page drawing of a live or preserved specimen
showing the shape of the body, tentacles, median lobe, eyes,
alimentary canal, mouth, genital cloaca and the common genital
opening.

2. Place a live specimen on a slide in a drop of water under
cover glass and examine the edge of the animal under high powder
(400 diameters). If the light is adjusted correctly the beating of
the cilia of the epithelium will be observed without diffi-
culty.

3. Make a drawing from a prepared slide of a cross-section
through the region of the eyes. Label eyes and brain.

4. Make a drawing of a cross-section through the region
of the ovaries. Label ovaries and stomach with its
branches.

5. Make a drawing of a cross-section through the region
somewhere between the ovaries and the pharynx. Label epithe-
lium, muscular system, testes, oviducts, yolk glands, anterior
stomach canal wdth its branches, longitudinal nerves and
parenchyma.

6. Make a drawing of a cross-section through the region of
the pharynx. Label all organs including the vasa deferentia,
mouth cavity, pharynx, and pharyngeal glands.

7. Make a drawing of a cross-section through the region of
the uterus and label parts.

8. Make a drawing of a cross-section through the region of
the penis pouch and label parts.

DENDROCCELUM LACTEUM 6i

9. Make a drawing of a cross-section through the region
of the genital cloaca and label parts.

10. Examine section mentioned in No. 5 under high power
(400 diameters) and make a drawing of the epithelium showing
epithelial cells, gland cells and rhabdites.

DICROCCELIUM LANCEATUM Stiles and Hassall

=Distomum lanceolatum

Material. D. lanceatum is by far the best species of
Trematodes for class study. Although it does not occur in the
United States, at least not as an indigenous species, it is pref-
erable to other Trematodes on account of the clearness with
which all the parts of the complicated reproductive organs may
be seen even by the inexperienced student. D. lanceatum is so
common in Europe, especially in Germany, that there should be
no difficulty in procuring any number of specimens. The best
results are obtained, when the specimens have been preserved in
a weak formalin solution and stained in alum carmin. Prepared
slides of Distomum in toto may be bought of H. Bocker in
Wetzlar at the listed catalog price of Mark 1.5 per slide. The
lung flukes of frogs used in this country as representative types
of Trematodes are far inferior to D. lanceatum, and possess
moreover no Laurer's canal. Each student should receive a
prepared slide of D. lanceatum stained and mounted in toto and,
if possible, a complete series of cross-sections.

Descriptive Part

Dicrocoelium lanceatum is a typical representative of the
Class Trematodes of the Phylum Platyhelminthes. It is the
commonest liver fluke of sheep in Europe, found often in hun-
dreds in the gall ducts. Its body is quite flattened dorso-
ventrally. The anterior end may be recognized by the mouth
sucker and the ventral surface by a similar sucker or acetabulwn
situated in the median line about one-flfth the animal's length

62

DICROCCELIUM LAXCEATUiM STILES AND HASSALL 63

Fig. 14. — Opisthioglyphe endoloba (Duj). After Loos. B, acetabulum;
C, cirrus pouch; Dg, branch of intestine; Dsl, yolk gland; EP, excretory pore;
G, genital pore; K, ovary; L. C, Laur^r's canal; M- mouth; PH, pharyn.x;
Ri receptaculum seminis; T, testis; U, uterus.

,^^ >^^

64 MORPHOLOGY OF INVERTEBRATE TYPES

from the mouth sucker. Of the five openings to the outside
found in the majority of trematodes, three are on the ventral
surface (the mouth and the male and female genital openings),
one at the posterior end of the body (the excretory opening) , and
one on the dorsal surface (opening of the Laurer's canal). An
anus is not present. Locomotion is accompHshed by the com-
bined action of the muscular system of the body aided by the
muscular system of the suckers.

Integument. Unlike Turbellarians, D. lanceatum has no
epithelium, a condition due to its mode of development and true
to all Trematodes, The body is covered with a cuticle secreted
by special cells which are found between the muscles and in the
parenchyma.

Muscular system. The muscles underlying the cuticle
belong to three distinct layers. First come the circular muscles.
Then a thin layer of diagonal muscles. The third or innermost
layer is composed of longitudinal muscles. Traversing the body
of the animal from one surface to the other and attached to the
cuticle on both sides are the dorso-ventral muscles.

The acetabulum or ventral sucker is a shallow cup-shaped
muscular organ of attachment composed of radial, equatorial
and meridional fibres. Special muscles attached to the acetabu-
lum serve for the locomotion of the entire sucker.

Parenchyma. All the space between the muscular system
and the various organs is occupied by a special tissue composed
of a homogeneous substance traversed by a network of fibres
and containing many nuclei. This tissue is called parenchyma
and is a mesenchyme.

Digestive system. The mouth opens ventrally close to
the anterior end of the body and leads into the mouth cavity
formed by the mouth sucker. The mouth sucker is composed
mainly of radial and a network of diagonal muscular fibres. The
anterior edge of the sucker forms an upper lip, the posterior
edge a lower lip. Both can be moved separately by means of
special muscle fibres in the sucker, belonging to the diagonal

DICROCCELIUM LANCEATUM STILES AND HASSALL 65

group. The mouth cavity leads into a muscular pharynx which
is globular and considerably smaller than the mouth sucker. Its
walls are composed mainly of radial muscle fibres. The pharynx
protrudes somewhat into the mouth cavity, thus forming an
upper and a lower pharyngeal lip. A protractor and a retractor
muscle control the forward and backward movement of the
pharynx. The last section of the foregut or stomadeum is the
(Esophagus, a short and thin tube with muscular walls and an
inner lining formed by a cuticle with short spines directed back-
wards. Unicellular salivary glands situated above the oesophagus
open by means of long ducts into the pharynx and mouth cavity.

The midgut or intestine into which the oesophagus opens con-
sists of two blind arms running backwards for a considerable
distance. It is lined with epithelial cells and has a rather poorly
developed muscular layer composed of longitudinal and circular
fibres.

Excretory system. The excretory system consists of a
pair of canals which begin in the posterior end of the body, run
forward as far as the vicinity of the mouthsucker, turn more or
less sharply and run backward, finally uniting in a single excre-
tory canal. This canal occupies the median line of the posterior
quarter of the body and opens at the posterior end of the animal.
A number of smaller canals open into the two excretory canals.
Each of these small canals ends in a funnel closed by a so-called
' ' flame cell . ' ' The walls of the canals are formed by a thin cuticle
and are surrounded by the parenchyma. Inasmuch as their walls
have no cellular structure the canals may be regarded as remains
of the body cavity. The single median canal is provided with
longitudinal muscular fibres and a sphincter controlling the
excretory opening.

Nervous system. The central nervous system consists
of a pair of cerebral ganglia situated above the pharynx. The
right and the left ganglion are connected with each other by a
commissure which is dorsal to the pharynx. Each ganglion
gives rise to three anterior and three posterior longitudinal

66 MORPHOLOGY OF INVERTEBRATE TYPES

nerves. Two of these six nerves are ventral, two dorsal and
two lateral. Their ramifications supply with nerves the vari-
ous organs. The six nerves running backward anastomose freely
with each other by means of numerous transverse commissures.
Ganglionic cells are found throughout the length of the main
stems. The mouth sucker and the pharynx receive each a
pair of special nerves from the cerebral ganglia. Special sense
organs are absent.

No circulatory or respiratory system is present.

Reproductive system. D. lanceatum is a true hermaphro-
dite like the great majority of the Trematodes, i. e., it has a
complete set of male and female sexual organs in the same in-
dividual. The genital openings are separate, both on the ventral
surface in the median ventral line. The female genital opening
lies in front of the acetabulum and about on the same level with
the two branches of the intestine where they merge in the
oesophagus. The male genital opening lies immediately behind
the female opening. A common genital cloaca, so common in
Trematodes, is wanting in D. lanceatum. The male genital
apparatus consists of a pair of testes, their ducts and an organ
of copulation. The testes are situated between the two branches
of the intestine. They are not symmetrical in their position.
The first testis lies immediately behind the acetabulum, while
the second testis lies somewhat to the left immediately behind
the first one. Accordingly the ducts or vasa deferentia are not
of equal length and since both run forward, that of the posterior
or second testis is the longer. Both ducts meet in the median
line immediately in front of the acetabulum. From here on
they continue as a single duct or ductus ejaculatorius. This
duct is inclosed in a so-called cirrus pouch and presents three
sections, the seminal vesicle, the median or prostatic section and
the terminal or cirrus section. The latter may be protruded
through the male genital openmg by a process of evagination or
turning inside-out when it functions as the cirrus or male organ
of copulation. The walls of the cirrus have circular and longi-

DICROCCELIUM LANCEATUM STILES AND HASSALL 67

tudinal muscle fibres. The cirrus pouch itself has muscular
walls and harbors many unicellular glands called the prostata
which open by means of separate ducts into the median section
of the ejaculatory duct.
The female genital apparatus consists of a single ovary,

SH..

Fig. 15. — Interrelation of various ducts in the female reproductive or-
gans of Dicroccelium lanceatiim after Leuckart. OV, ovary; RS, receptacu-
lum seminis; SH, MehUs' gland; VD, vitello duct; UT. uterus; OD, ovi-
duct; L. C, Laurer's canal; OH, opening of Laurer's canal.

an oviduct, a receptacle for the sperm, two yolk glands with
their ducts, MehHs' gland, a uterus and a Laurer's canal. The
ovary lies immediately behind the second testis, and the con-
siderably smaller receptaculum seminis lies immediately behind
the ovary, close to the dorsal surface of the animal. The oviduct
is a very short tube and soon receives the Laurer's canal and
duct of the receptaculum. The latter is about as long as the ovi-

68 MORPHOLOGY OF INVERTEBRATE TYPES

duct. The Laurer's canal is several times longer than the ovi-
duct. It runs forward at an angle to the longitudinal axis of
the body and opens on the dorsal surface considerably to the
left of the plane of symmetry, on the level of the posterior edge
of the second testis. Its function is as yet not well understood.
The uterus is in direct continuation with the oviduct and from
this it is evident that the duct of the receptacle and the Laurer's
canal mark the end of the oviduct and the beginning of the
uterus. In many Trematodes the space at the juncture of the
four canals is considerably widened and is then known under the
name of "ootype." But in D. Lanceatum no such ootype is
present. The uterus is a very long duct, about twelve times
longer than the animal itself and is consequently coiled up. The
diameter of the uterus at its beginning is no wider than the
diameter of the other three ducts but it becomes very soon con-
siderably wider. The course of the uterus is at first backward,
till it reaches close to the posterior end of the body. From here
on it runs forward, passes the testes and ends in the female
genital opening which has been already mentioned. The coils
of the descending (backward) portion of the uterus run parallel
with and ventral to those of the ascending portion. This is
the reason why in an animal stained in toto the uterus appears
as if it were branched. When fully developed the uterus oc-
cupies the entire space between the two branches of the intestine
and almost all the space to the rear of them. The two yolk
glands are situated in the middle third of the body to the out-
side of the intestinal branches. Each gland is in reality a com-
posite of a number of small glands which open separately into
the longitudinal yolk duct (of which therefore there are two, one
on each side of the body). The longitudinal yolk ducts are
connected with the transverse yolk ducts which run approximately
at right angles with the axis of the animal, meet in the middle
and form a short common yolk duct which opens into the uterus
close to the duct of the receptacle. The transverse yolk ducts
are ventral to the intestinal branches. MehHs' gland consists

DICROCCELIUM LANCEATUM STILES AND HASSALL 69

of a number of unicellular glands surrounding the uterus imme-
diately behind the opening of the common yolk duct. Each
gland opens into the uterus by means of a separate duct.

Apparently both cross and self-fertilization are possible. In
both cases the organ of copulation is the cirrus, but in the case of
self-fertiUzation it is introduced into the female genital opening
of the same individual. The sperm travels all the length of the
uterus till it reaches the duct of the receptacle and finally
the receptacle itself. The eggs produced in the ovary slip
down the oviduct and become fertilized by the sperm stored
in the receptacle as they pass by the opening of its duct. Yolk
from the yolk glands is added to each fertilized egg as it passes
by the opening of the common yolk duct. Finally a shell is
formed around each egg from material furnished by the yolk ^
gland and the now completely formed egg travels down the
uterus followed by other eggs, until the uterus is completely
filled. A well developed uterus may harbor over a million eggs.

Development. The eggs develop in the uterus but the
little embryo remains inclosed in the eggshell and does not leave
it until the egg has been swallowed by a snail. The eggshell
loses its little lid under the influence of the digestive juices and
sets free the larva which is called miracidium and is provided
with a ciliated ectoderm. This larva finally manages to get into
the liver of the snail, but in doing so it strips itself of its ciliated
ectoderm. The further development of D. lanceatum has never
been observed, but we must assume that it is essentially the
same as in its better known near relative, the large liver fluke
Fasciola hepatica. Here the larva grows into the new stage
known as sporocyst. The sporocyst soon produces by a special
kind of internal budding a number of redicB or larvae of the
second generation. Each redia produces by a similar process a

^ The gland formerly known under the name of " shell-gland " and
described here as Mehlis' gland does not produce the shell, but probably
secretes a liquid in which the eggs are suspended. See Goldschmidt, Zool.
Anz. 1909.

70 MORPHOLOGY OF in\I::ktebkatj-: types

number of cercarice, or larvae of the third generation. The
cercaria is already similar to the fluke except that it has a tail
and that its reproductive organs are not yet developed. The
cercariae leave the snails, swim for a while in water with the
aid of their tail, sooner or later climb up a grass-blade, lose the
tail and become encysted. If now swallowed by a sheep, the
young fluke becomes liberated from its cyst, travels up the
gall duct and soon matures. In the case of Fasciola hepatica
a single egg may produce on the average ten rediae and each
redia forty cercariae. Thus a single egg will produce about
four hundred adult flukes and the progeny of a single fluke reach
the considerable number of 400,000,000 (four hundred million).
This immense productivity counterbalances the loss incurred by
the species in the passive transmission to the intermediary and
final host.

Instructions

1. Examine a prepared slide of D. lanceatum under the dis-
secting microscope to get a general view of the various organs.
Place now the slide under the microscope (50 diameters),
and make a full page drawing showing all organs. The
drawing ought to show the digestive system including mouth
sucker, pharynx, oesophagus and intestine; acetabulum or ven-
tral sucker; the common excretory canal with the excretory
opening at posterior end of animal; the male reproductive organs
including both testes, vasa deferentia, cirrus pouch with ejac-
ulatory duct in which one may sometimes distinguish the ves-
icula, the prostata portion, the prostata and the cirrus; the fe-
male reproductive organs including ovary, receptaculum seminis,
oviduct, yolk glands, longitudinal and transverse yolk ducts.
(In some cases Laurer's canal, the common yolk duct, the duct
of the receptacle and Mehlis' gland will also be visible.) Label
all parts including male and female genital openings.

2. Examine under low power (50 diameters) the series of
cross-sections through D. lanceatum. Pick out for special study

DICROCCELIUM LANCEATUM STILES AND HASSALL 71

the sections through (i) the cerebral gangHa, (2) the acetabulum,
(3) the anterior testis, (4) the posterior testis, (5) several succes-
sive sections following the end of the testis. Make diagram-
matic drawings of each of these sections to show the relative
position of the organs and of the various ducts.

3. Additional exercise. Put a snail in a small dish with
water, break its shell, make an incision in the liver and collect
the escaping rediae and cercarias into the water. These larval
stages belong to other species of flukes than D. lanceatum and
may be readily procured if one has a sufficient number of snails.
Examine them while alive. They may be afterwards killed in
a sublimate fixing fluid and stained with haematoxylin.

TiENIA SAGINATA Goeze

Material. Taenia saginata may be obtained at almost
any hospital. It should be presented in 2-4% formalin (avoid
alcoholic specimens as they become intransparent). Stain in
Grenacher's Alumcarmin for twenty-four hours. Decolorize for
a few seconds in water with hydrochloric acid, wash in water for
twenty-four hours. Press the proglottids gently between two
slides by tying the slides with a thread and place the slides into
50% alcohol for twenty-four hours. Transfer slides into 95%
alcohol for twenty-four hours, then into a jar with absolute
alcohol with desiccated copper sulphate for twenty-four hours;
hence into xylol for twenty-four hours. Cut the thread, separate
the slides and lifting the proglottid with a spatula transfer it for
twenty-four hours into a thin solution of xylol dammar. In-
close in thick xylol dammar. This method gives remark-
ably beautiful slides showing all organs. The proglottids with
uterus filled with eggs should be treated in the same manner
but without any stain. The scolex is too rare to be used for class
work. Dipylidium caninum ( = Taenia elliptica sive cucum-
erina) found in quantities in almost all dogs and cats may be
used for this purpose. Material preserv^ed in sublimate is pref-
erable for cross-sections which may be stained in Bohmer's
or Delafield's haemotoxylin. In place of Taenia saginata T.
crassicollis, T. serrata or T. marginata may be used.

Descriptive Part

Taenia saginata is a common human tapeworm. A full grown
specimen measures usually about eight metres in length although
specimens of only four metres and such of ten metres length are

72

i

T^NIA SAGINATA GOEZE

73

not uncommon and cases have been reported where the worm at-
tained the considerable length of thirty-six metres. As the body
of the worm is very contractile the length is subject to great
variations even if the age and number of proglottids is the same.
The scolex (unfortunately often called ''head") is at the fore-
most end of the body
and is followed by a
long strobila or chain of
upwards of one thou-
sand proglottids which
increase gradually in
length and width. The
oldest proglottids are
at the posterior end of
the worm. Daily sev-
eral of these proglot-
tids break off and leave
the intestine of the pa-
t i e n t spontaneously,
and daily new proglot-
tids are formed by the
scolex by a process of
strobilization or seg-
mentation of its
"neck." If the entire
chain is broken off at
the scolex, it takes the
latter from ten to
twelve weeks to pro-
duce a new chain. The internal organization of each proglot-
tid is the same as that of the preceding and following pro-
glottids, but sexual mature proglottids are found only, roughly
speaking, in the middle third of the chain. The posterior
proglottids show a degeneration of the sexual organs owing to
the enormous development of the uterus under pressure of the

Fig. i6. — Nervous system in the scolex of
Tcenia ccenuriis after Niemiec. DN , dorsal
nerve; LN, lateral nerve; VN, ventral nerve.

74 MORPHOLOGY OF INVERTEBRATE TYPES

immense quantity of eggs. The entire chain of proglottids is
strongly compressed dorso-ventrally, but owing to the lateral
position of the genital openings the distinction between the dorsal
and ventral surfaces is not so simple as is the case with tape-
worms in which the uterus has an opening of its own in the
median ventral line. It is customary to call dorsal the surface
which is further removed from the ovary.

The scolex is almost square in a transverse section and com-
pared with the chain is very small, for it measures only about
1.5 mm. square. It has four suckers and between them a poorly
developed and modified rostellum appearing as a small sucker.
Besides a highly developed muscular system it has a central
nervous and an excretory system, both of which continue
through the entire chain of proglottids.

The mature proglottids are most suitable for the study of the
internal organization of the tapew^orm for only in them all parts
of the reproductive system are found. An examination of a
cross-section shows that the outside covering is a cuticle pro-
duced by special cells situated in the parenchyma or the tissue
which fills out all the spaces betw^een the different organs and
muscles. An epithelial epidermis is lacking. The muscular
system is highly developed. It consists of a thin sheet of circular
muscles followed by a thin sheet of longitudinal muscles beyond
which are the cuticle-producing cells. A second set of longitu-
dinal muscles extends all the way to the transverse muscles which
run from one side of the proglottis to the other. Dorso-ventral
muscles traverse the parenchyma and serve to flatten the body.
The central portion inclosed between the dorsal and ventral
transverse muscles contains all the organs with exception of the
peripheral nerves, four of the longitudinal nerves and the col-
lecting tubules of the excretory system. The parenchyma is a
mesenchymatous tissue and its cells produce usually calcareous
bodies found in great quantities throughout the parenchyma.

Digestive organs are absent.

The nervous system consists of ten longitudinal nerves, three

T^NIA SAGINATA GOEZE 75

pairs of which are lateral, one pair dorsal and one pair ventral.
The dorsal and ventral nerves lie outside the transverse muscles.
Of the three lateral nerves of each side the middle one is by far
the largest. The longitudinal nerves give off minute branches
to the sense cells found in the parenchyma. In the scolex the
longitudinal nerves are connected with each other by com-
missures with ganglionic enlargements forming the central
nervous system.

The excretory system consists typically of two pairs of longi-
ttidinal canals opening to the outside on the last proglottid. Of
these the inner pair is considerably less developed than the outer
pair and seems to be missing in older proglottids. Close to the
posterior edge of each proglottis is a transverse canal connecting
the two longitudinal canals. The collecting tubules which are
numerous are very thin tubes opening into the longitudinal
canals. The distal end of the tubules is closed by a flame
cell.

Reproductive system. Taenia saginata is a hermaphrodite
and each proglottid has a complete set of reproductive organs.
The genital papilla or the wall of the genital atrium is situated
either on the left or on the right side of the proglottis. The male
reproductive organs consist of several hundreds of small round
testes situated to the inside of the dorsal transverse muscles ex-
tending over almost the entire proglottis. The vasa eferentia
open near the middle of the proglottis into the vas deferens, a
canal which appears under low power as a straight line running
to the genital atrium, but which in reality is convoluted through-
out its entire length. The end portion of the vas deferens, which
is inclosed in a muscular cirrus pouch, may be protruded as a
cirrus through the male genital opening into the atrium.

The female reproductive organs are much more complicated.
The female genital opening situated in the atrium under the male
genital opening, leads into a thin, straight tube running parallel
with the vas deferens. This tube is the vagina. Towards the
middle of the proglottis the vagina bends downward and forms

I'^G. 17. — Mature Proglottis of Tania Csnurus after Leuckart. T, testis; LC,
lateral canal; C, cirrus; GA, genital cloaca; RS, receptaculum seminis; OD, oviduct;
FC, fertilization canal; VD, vitello duct; U, uterus; VE, vas eflferens; DE, ductus
ejaculatorius; VA, vagina; OV, ovary; SC, sperm canal; SH, Mehlis' gland; VI,
yolk gland.

T^NIA SAGINATA GOEZE 77

the receptaculum seminis. Immediately beyond this the vagina
receives the oviduct and proceeds then as the so-called fer-
tilization canal to the oot5^e where it unites with the yolk duct
and the uterus. The two ovaries although having the appearance
of more or less round bodies, are in reality composed of a number
of tubular follicles. Their ducts unite in the median line of the
proglottis and form a short common oviduct which, as already
stated, opens into the vagina behind the receptacle for the
sperm. The yolk gland is single, situated close to the posterior
end of the proglottis. The yolk duct is short, straight and opens
into the fertilization canal in the ootype in the same place with
the uterus. The uterus appears as a blind tube of considerable
size running forward almost to the anterior edge of the proglottis.
In maturing proglottids this tube is simple, but under the pres-
sure of eggs it begins to develop branches and in the rearmost
proglottids occupies almost the entire space. There are then
from twenty to thirty-five branches on each side of the median
canal of the uterus. The Mehlis' gland or ootype has an almost
circular shape and consists of numerous glands each opening
separately into the fertilization canal at its place of union with
the uterus and yolk duct.

Self-fertilization is usual, but cross-fertilization occurs. The
sperm is stored in the receptacle. The eggs are fertilized in the
fertilization canal, and on being supplied with yolk and sur-
rounded with a shell pass into the uterus. They reach the out-
side through a disintegration of the tissues or are swallowed with
the entire proglottis. The development is combined with a
metamorphosis and a change of hosts, the intermediary host
being cattle. The first larval stage is known as an oncosphcBra
which on reaching the muscles (or sometimes internal organs)
becomes transformed into a cysticercus.

Instructions

I. Examine under dissecting microscope the prepared slide
of a mature proglottis. It shows the longitudinal and transverse

78 MORPHOLOGY OF INVERTEBRATE TYPES

excretory canals, genital papilla with the atrium and all parts of
the reproductive system except the efferent canals and the
oviduct. Make a half page drawing and label all parts.

2. Examine under dissecting microscope the prepared slide of
a proglottis with a fully developed uterus. Count its lateral
branches on each side. Make a half page drawing showing the
outline of the proglottis and the uterus.

3. Examine under low power a cross-section through a pro-
glottis. Make a half page drawing and label the cuticle, sub-
cuticular cells, longitudinal, dorso-ventral and transverse mus-
cles, testes and uterus, longitudinal excretory canals and the
lateral longitudinal nerves.

4. Examine a prepared sHde of a scolex of Dipylidium cani-
num (a common tapeworm of dogs and cats). Unlike the scolex
of T. saginata it has a well developed rostellum with hooks.
Make a drawing showing rostellum, hooks, suckers, and neck.

ASCARIS LUMBRICOIDES Linnaeus varietas
SUILLA

Material. A. suilla is so common in pigs that it may be
found in great quantities at any slaughterhouse. The specimens
should be placed directly into Perenyi's fixing fluid in which they
may remain for any length of time. Material thus fixed is good
both for dissection and microscopic anatomy. Sections should
be stained in haematoxylin and eosin. Every student should
receive one male, one female, a cross-section showing the nervous
ring and a cross-section through the middle of the worm.

Descriptive Part

Ascaris suilla is a common parasite of the pig and is closely
related to the human round worm. It is a typical representative
of the Class Nematoda.

External features. Ascaris suilla has the shape of a long
cylinder attenuated at both ends. At one end is the mouth
surrounded by three finely toothed triangular lips, one of which
is dorsal and the other two ventro-lateral in position. Extend-
ing practically through the whole length of the animal are four
equidistant lines: a median ventral line, a median dorsal line, and
two lateral lines. About two millimeters from the anterior end
is a small excretory pore in the median ventral line. The posterior
end of the female is practically straight and the anus appears as a
transverse sHt with two anal lips on the ventral side about two
millimeters in front of the tail end. The female genital opening
is also median and ventral in position and is situated about one-
third the entire length from the head. The position of the
opening in younger females is nearer the middle of the body.

79

MORPHOLOGY OF INVERTEBRATE TYPES

i^a

The posterior end of the male is strongly
curved and provided with two hard
bristles called spicula which may be
protruded through the anus and which
serve as organs of copulation. The
male has no external genital opening.
Instead, the ejaculatory duct opens into
the cloaca.

Body wall. The body wall consists
of a cuticle, subcuticle and a layer of
longitudinal muscles. The cuticle is
composed of the following eleven lay-
ers: (i) Outer membrane, (2) outer
cortex layer, (3) inner cortex layer,
(4) layer of iibrillae, (5) homogeneous
layer, (6) band layer, (7) outer fibrous
layer, (8) middle 'fibrous layer, (9) inner
fibrous layer, (10) basal layer, (11) in-
ner membrane. Of these lavers the

Fig. 18. — Anchylostoma duodenale; male
hookworm, after Schulthess, from Blanchard,
Zoologie Aledicale. a, buccal capsule; b, oesoph-
agus; c, ner\'Ous ring; d, excretory pore;
e, posterior end of oesophagus; /, cuticle; g,
muscular layer; h, anterior end of intestine; /,
right cer\-ical gland; k, left cervical gland;/,
duct of the left cervical gland; ;», intestine; n,
testis; 0, blind end of testis; p, seminal vesicle;
g, ejaculatory duct; r, anterior end of spicule;
s, spicules; /, left lateral papilla; 11, anal papilla;
V, chitinous piece behind the anal glands; w, z,
chitinous rays of the caudal bursa.

ASCARIS LUMBRICOIDES LINN^US . 8 1

homogeneous layer is the thickest, equal to or even exceeding in
thickness all other layers taken together. The cuticle is non-
porous ^ and transversely plicated. Under the cuticle is a thin
syncitial subcuticle containing nuclei. The cuticle is a product
of this layer which may be best termed hypodermis.^ Under the
latter is a single layer of longitudinal muscles. Each muscle cell
consists of a protoplasmic core or cell body and a contractile sheath.
The protoplasm is highly vacuolated. It contains the nucleus
and a system of supporting fibres. The contractile sheath is
drawn out on one side of the cell into a flat spindle which is
longer than the cell body and is differentiated into longitudinal
fibres. In a cross-section the fibres appear therefore arranged in
two rows, at first parallel and barely separated from each other,
then diverging and partly inclosing the protoplasmic core. The
fibres are inserted directly into the cuticle. The protoplasmic
core with its supporting fibrillae sends out processes to the nerves
in the longitudinal lines. The layer of longitudinal muscles is
divided by these lines into four longitudinal fields or bands.
In some Nematodes there are only two muscle cells in a cross-
section of a band. But in Ascaris there are several muscle cells
to a cross-section of a band and Ascaris belongs therefore to the
so-called polyniiaria.

Body cavity. Ascaris has no true body cavity. The
cavity which one sees in dissecting the worm is in reality a sys-
tem of intracellular spaces, or large vacuoles in a few enormous
cells which fill out the space between the longitudinal muscles
and the alimentary canal and reproductive organs. The vacuoles
are so large and so many that the walls between them are quite
thin and have been overlooked until recently. Ascaris and
with it all other Nematodes shows more relation to the

^ What has been described as a system of minute canals proved to be the
fibrillar layer.

2 The subcuticle is usually called ectoderm. The term hypodermis,
first proposed for a similar layer in Arthropods, is less confusing since there
are in Ascaris other organs of ectodermal origin.

MORrilOLOGY OF IXVIORPIOISK A Tl': rvpi^s

parenchymal oils Hat worms
than to higher invertebrates
with a calome.

The vacuoles contain a
fluid rich in albuminous sub-
stances. This fluid has an
unpleasant odor and is highly
poisonous to some individuals,
producing a strong irritation
of mucous membranes.

Muscular system. Loco-
motion in Ascaris is accom-
plished by the alternating
contraction and relaxation of
the longitudinal muscle-
bands. When all four bands
have been contracted simul-

FiG. 19. — Wilsonema, a nema-
tode found in the soil in the United
States, after N. A. Cobb, from
Yearbook of Dept. Agriculture for
1914, I, lateral view of female; II,
dorso-ventral view of the head of
the same individual; 111, enlarged
lateral view of the tail end; a,
ventral appendage which, together
with the corresponding dorsal ap-
pendage, acts as a sieve; b, lateral
tactile organ associated with the
sifting apparatus; c, internal ele-
ments (supports?) of the sieves; </,
inflated vahailar apparatus; e, am-
phid; /, cuticle; g, pharynx; //,
nerve ring; 7, excretory pore; j,
oesophagus; h, threefold valve of
the cardiac bulb; /, cardia; ?;;,
wall of intestine; n, flexure of
anterior ovarj-; 0, vulva; p, egg; q,
lumen of intestine; r, rectum; s,
lateral wings; /, anus; «, caudal
seta; v, spinneret; u\ anal muscles;
X, apical portion of spinneret; y,.
one of the three caudal glands.

ASCARIS LUMBRICOIDES LINN^US 83

taneously, expansion is due to the elasticity of the cuticle.
Besides the longitudinal muscles there are several special muscles
in Ascaris, which will be considered in connection with other
organs.

Digestive system. The digestive system of Ascaris is
very simple and consists of a straight tube running from the
mouth to the anus. It may be divided into three sections:
the oesophagus or foregut, the intestine or midgut and the
rectum or hindgut. The cesophagus is short and shows a tri-
radial lumen in cross-section. Being a stomadeum it is naturally
lined with a heavy cuticle. The wall of the oesophagus is formed
by myo-epithehal cells of special structure. Their fibres, ar-
ranged radially, form real dilatator es oesophagi, i. e., they widen
the lumen, while the cuticle lining serves as their antagonist.
A few gland cells are found in the wall of the oesophagus com-
pletely inclosed by the myo-epithelial cells. The intestine or
midgut is a straight canal which is so much flattened dorso-
ventrally that it almost has the appearance of a band; its wall
is composed of high, columnal epithelium. Two enormous muscle
cells form a network of fibres around the hind end of the intes-
tine and act as dilatatores. The same two cells furnish in the
male also the muscular fibre-net of the ejaculatory duct. The
short rectum is lined with a cuticle and is surrounded by a ring
of six large gland cells. The position of the anus has been de-
scribed above.

Excretory system. The excretory system is inclosed in
the side lines. The side line appears as a heavy ridge separating
the longitudinal muscles. Already with naked eye one can
observe a longitudinal line dividing the side line into two lines,
and this division appears still clearer in a cross-section. The
structure of a side line is quite complicated and is composed of
various elements of which the subcuticle is the least conspicuous,
being in the side lines no more developed than in other regions
of the body wall. The main tissue of the side line is divided
into upper and lower symmetric halves by the cells of the

84 MORPHOLOGY OF INVERTEBRATE TYPES

inierstiiial lamella at Ihe end of which is the excretory canal.
The entire canal is nothing but a long cylindrical cavity inside
of a single enormous cell extending almost from end to end of
the worm. The canal bends downward a little behind the
nervous ring and unites with the excretory canal of the other
side forming a short single diict. We have seen already that
this duct opens ventrally in the median line and that this ex-
cretory opening is situated about two millimeters from the an-
terior end. The wall of the common duct is also formed by
a single cell. Accompanying the excretory canals throughout
their length in the side lines are two pairs of excretory glands, one
pair in each side line. In a cross-section they appear as large
ovals situated in the main tissue of the line, one above, the other
below the interstitial lamella.

Attached to the inner edge of the side lines not far from their
anterior end are two pairs of phagocytic organs w^hich have the
appearance of small tufts. Their function is to retain and to
make substances harmless which cannot be removed by the
excretory canals.

Nervous system. The nervous system of Ascaris is com-
posed of several cephalic ganglia or nerve centres situated in the
region of the oesophagus, a nerve ring surrounding the latter,
eight longitudinal nerves, six anterior sensory nerves and an
anal ganglion or nerve centre in the posterior end of the body.
The largest and best developed of the anterior ganglia is the
so-called ventral cephalic ganglion. It has two lobes and is con-
nected with the other ganglia and the rest of the nervous system
through the nerve ring which represents a large cephalic com-
missure. Connected with the nervous ring and running for-
wards are six bundles of sensory nerves supplying the sense
organs of the lips. Posteriorly the nerve ring gives off six
longitudinal motor ner\^es. One of these is ventral and is the
heaviest of all; another is dorsal, two are subdorsal and two
sub-ventral. The dorsal nerve is inclosed in the dorsal median
line, the ventral in the ventral median line. The other four

ASCARIS LUMBKICOIDES LINN.EUS 85

nerves run in the subcuticle. The lateral lines inclose also each
a longitudinal lateral nerve which, however, takes its origin from
the lateral cephalic ganglion. The longitudinal nerves of each
side are connected with the dorsal and ventral nerves by thin
commissures which are asymmetric and form right and left
semicircles.

The sense organs are represented only by six oral papillm
situated on the lips and in the case of the male by from seventy
to seventy-five anal papillcB. Higher sense organs are absent.

Circulatory and respiratory organs are absent.

Reproductive system, (a) Female. About one-third from
the anterior end of the body is a small transverse slit in the mid-
ventral line. This is the genital opening or vulva. It leads into
a short vagina which soon splits into two very long, convoluted
tubes. Each of these tubes is ten times as long as the entire
worm and may contain over ten million eggs. The portion of the
tube, which is nearest the vagina has a considerable diameter
and represents the uterus. The middle portion is the oviduct
and the thin end portion is the ovary. The ovary ends blindly.
Inclosed in the ovary is a long central rod or rachis round which
the young eggs are arranged at first regularly, then more or
less in small clusters. The division line between the ovary and
the oviduct is not apparent externally, but the oviduct is lacking
the rachis and contains free eggs. The end of the oviduct is
somewhat distended and functions as a sperm receptacle. Here
the eggs are fertilized. The uterus contains therefore only
fertilized eggs and its walls secrete an albuminous substance in
which the eggs are inclosed.

(b) Male. The male reproductive organs are situated in the
posterior third of the body. They consist of a single tube
w^hich opens into the cloaca or the short portion of the ali-
mentary tract between the rectum and the anus. The tube is
from seven to eight times as long as the entire worm and is
clearly divided into three uneven portions. The longest portion
is the testis which has the shape of a thin convoluted tube.

86 MORPHOLOGY OF INVERTEBRATE TYPES

Inclosed in the testis is a rachis similar to that in the ovary
and the sperm cells are grouped around it. The rachis is not
as long as the entire terminal portion and that part of the
thin tube, which is free from the rachis may be termed vas def-
erens. The central portion is much shorter and thicker. It is
the vesictda seminalis. The end portion is again much thinner
and quite short, being no more than seven or eight millimeters
long. It has a musculature of its own and serves as ductus
ejaculatorius.

In the immediate vicinity of the ejaculatory duct are two
pouches. They open into the cloaca, dorsal to the intestinal
canal, and contain two chitinous bristles or spicula which serve
as organs of copulation.

Development. The fertilized eggs develop directly and
the shell protects the embryo from desiccation or climatic
changes. When swallowed with water or vegetables, the embryo
leaves the egg in the stomach of the host and matures in the
intestine.

Instructions

1. Put a female Ascaris into a dissecting tray with water and
examine the anterior end with the aid of a lens. Make a drawing
showing the ventral view of the anterior end. Label lips, ventral
line and excretory pore.

2. Open the specimen by a longitudinal slit which should be
made parallel to and halfway between the dorsal and left lateral
Hnes. Pin down the body wall. Make a full page drawing
showing oesophagus, intestine, rectum, phagocytic organs if
visible, vulva, vagina, uteri and oviducts with ovaries.

3. Open a male specimen in the same manner. Show in the
drawing the digestive organs, ductus ejaculatorius, vesicula
seminalis and testis with vas deferens; also the spicula.

4. Examine prepared slide with a cross-section through the
nervous ring under low power (50 diameters) and make a drawing
showing all parts.

ASCARIS LUMBRICOIDES LINN^US 87

5. Do the same with the cross-section through the middle of
the worm.

6. Examine the sUde under high power (400 diameters).
Make the following drawings:

(a) Cuticle. Label outer membrane, cortex layer, layer of
fibrillae, homogeneous layer.

(b) Side line. Label interstitial lamella, excretory canal.

(c) Muscle cell. Label protoplasmic core, nucleus, contractile
sheath.

(d) Intestine.

(e) Uterus.

(f) Oviduct.

(g) Ovary. Label rachis and eggs.

LUMBRICUS TERRESTRIS, L. Muller

Material. L. terrestris may be kept alive in moist soil
or preserved in Perenyi's fluid for dissection. Specimens for
transverse microscopic sections should be fed for several days on
milk or olive oil in a jar free from soil and fixed in either Perenyi's
or Petrunkevitch's sublimate mixture. Every student should
receive one large specimen killed by slow narcotizing in alcohol,
and the following transverse sections: through the region of the
seventh to tenth segments (oesophagus), through the region
between the twentieth to thirtieth segment and through the
clitellum.

L. terrestris has been selected on account of its considerable
size. The genus Lumbricus should not be confused with either
Eisenia (common species Eisenia foetida and rosea) or Helodrilus
(common species Helodrilus caliginosus, chloroticus). For dis-
tinctive characters see Das Tierreich, Oligochaeta by Michaelsen.

Descriptive Part

Lumbricus terrestris is an earthworm equally common in
Europe and the United States and a typical representative of the
Class Ohgochaeta. It is a hermaphrodite like the majority of the
species belonging to the same class.

External anatomy. The body of Lumbricus is segmented
externally. The number of segments varies from one hundred
and ten to one hundred and eighty, a condition similar to that in
Nereis, showing that in this respect the Oligochasts have not yet
attained the degree of fixed segmentation found in leeches. The
anterior end of the worm is thicker and almost round in a cross-
section. The posterior end is distinctly flattened dorso-ventrally.

88

LUMBRICUS TERRESTRIS L. MULLER

89

The mouth is situated on the first
segment and is surmounted by a pro-
stomium. The anus is terminal on
the last segment. About one-quarter
or one-third the entire length from
the anterior end is found a shiny belt
occupying six or seven segments; this
is the clitellum. Careful counting of
the segments will reveal that the
clitellum begins with the thirty-first
or thirty-second and ends with the
thirty-seventh segment, in other
words, it has always a definite posi-
tion. By passing a finger from the
rear end of the worm forward along
the ventral surface and on each side,
one will feel the stiff bristles situated
in each segment and forming two
ventral and two lateral lines. Each
line consists of two rows and each
segment has therefore eight bristles
or chet(B commonly marked as a, b,
c, and d, beginning with the ventral
bristles. The first and last segment
have no bristles. The ventral bristles
of the twenty-sixth segment situated
on glandular papilla are often, but
not always modified as long genital
chetce. There are two openings of
seminal receptacles between the
ninth and tenth segments and two
similar openings between the tenth
and eleventh segments in the lateral
lines; two female genital pores on
the fourteenth segment just above

XVIII

Fig. 20. — Sparganophilus
tamesis, general anatomy after
Bentam, from Cambridge Nat-
ural History. I-XVIII, seg-
ments; 1, 4, 6, perivisceral ves-
sels (6 is one of the hearts); 2,
3, 7, dorsal vessel; 5, sper-
matheca; 8, sperm sacs; 9,
intestino-tegumentary vessels;
10, ovary; ir, 12, integument-
ary vessels.

yo MORPHOLOGY OF INVERTEBRATE TYPES

the b bristles; two male genital pores on the fifteenth segment
between the bristles b and c; two nephridiopores or excretory
openings above the b bristles in every segment, except the an-
terior three and the last or anal segment ; finally a single row of
dorsal pores in the mid dorsal line between the segments, be-
ginning with the seven-eight intersegmental line. These open-
ings connect the body cavity with the outside.

Body wall. The body wall is composed of a cuticle,
hypodermis, circular muscles, longitudinal muscles and a thin
layer of parietal or somatic peritoneum. The cuticle which is a
product of the underlying hypodermis, is rather fine and pre-
sents when viewed from the surface a system of fine stria tion.
The lines intercross each other more or less at right angles and in
many points of intersection there are small pores. The under-
lying hypodermis consists of a single layer of columnal epithelium.
Between the common cells glandular and sensory cells are found.
In the clitellum the h>^odermis is considerably thickened and
richly supplied with minute blood-vessels which run between the
cells. This region is, moreover, very rich in large gland cells
which become especially active during copulation and the for-
mation of the cocoon. The papillae of the twenty-sixth segment
contain in them capsulogenous glands producing the chief sub-
stance of which the cocoon is formed. Immediately under the
hypodermis is a heavy layer of circular muscles the function of
which is to elongate the body. Between the fibres of these
muscles pigment granules are found. To the inside of the cir-
cular muscles lies a still heavier layer of longitudinal muscles.
They form four bands, a right and left lateral ba^id situated
between the lateral and ventral lines of bristles, a ventral hand
occupying the space between the ventral lines and a much
broader dorsal band. The longitudinal muscles have a very
peculiar structure. They consist of a central lameUa with a
series of parallel lamellae attached to the former on each side.
A transverse section of a longitudinal muscle has therefore the
appearance of a plume or feather. The function of the longitu-

LUMBRICUS TERRESTRIS L. MULLER 91

dinal muscles is to shorten the body when all bands contract
simultaneously or to curve it in the direction of that band which
contracts singly. In this case the opposite bands act as antag-
onists.

The chetae or bristles sit in special sacks formed by a depres-
sion of the h3^odermis. The base of the sack is connected with
special muscle fibres. These fibres serve to move the chetae thus
helping in locomotion.

Coelome or body cavity. The body cavity of Lumbricus,
inclosing all organs, is divided into chambers by transverse
intersegmental septa. The septa are lacking only in the anterior
six segments and the last posterior one where they are replaced
by a loose network of muscular and connective fibres. The septa
have a muscular structure and the peritoneum which lines the
body wall extends over all septa. The body cavity is filled with a
liquid containing amoebocytes and chloragogue cells which will be
mentioned in connection with the digestive system. Each
coelomic chamber communicates with the outside through a pair
of nephridia and a dorsal pore. The dorsal pores have been
already mentioned. If a worm is subjected to a strong stimulus
or irritation a drop of coelomic fluid appears from every dorsal
pore. It is probable that we have herein an arrangement to keep
the surface of the worm moist in case of excessive dryness or
heat.

Digestive organs. The alimentary canal runs in a straight
line from the mouth to the anus. It begins with the mouth
cavity. This is followed by a muscular, ovoid pharynx which
extends to the end of the sixth segment. The pharynx is pro-
vided with numerous muscles attached to the body wall and
acting as dilators. The oesophagus is a laterally compressed tube
extending as far back as the thirteenth segment. In the eleventh
and twelfth segments it receives three pairs of small calciferous
glands. These glands secrete calcium carbonate. The oesopha-
gus is surrounded by five pairs of ''hearts." The crop has the
shape of an ovoid with a diameter several times that of the

92 MORPHOLOGY OF INVERTEBRATE TYPES

oesophagus. It occupies the fourteenth and fifteenth segments.
The gizzard has about the same size as the crop but is provided
with powerful musculature. It occupies the sixteenth, seven-
teenth, and eighteenth segments. The gizzard opens into the
intestine which extends all the way to the anus. The absorbing
surface of the intestine is considerably increased by the presence
of a longitudinal dorsal fold or typholosole. The microscopic
structure of the intestine presents some interesting peculiarities.
The intestinal cavity is lined with striated cuticle produced or
secreted by a layer of high columnal epithelial cells. To the
outside of this layer comes the vascular layer containing blood
vessels. This is followed by the muscular layer. The outermost
layer of the intestine is formed by large chloragogue cells which
are supposed to possess an excretory function and which are also
found floating in the coelomic fluid.

Excretory system. The excretory system of Lumbricus
consists of paired nephridia or segmental organs. One pair of
nephridia is found in every segment except the anterior three and
the anal segment. Each nephridium is a coiled tube which
opens with a ciliated funnel or nephrostome into the body cavity
of the preceding segment and with a nephridiopore to the out-
side above the b bristle. The nephridium may be divided into
three portions. The first, transparent portion following upon the
funnel is a long thin tube with fine blood-vessels between its
loops. The median or glandular portion is considerably thicker
than the preceding one and is opaque because of the secretion of
the glandular cells of its wall. The terminal portion is the short-
est and widest and plays the role of a collecting bladder.

Circulatory system. The circulatory system of the earth-
worm is characterized by the presence of five longitudinal blood-
vessels extending from end to end of the animal. The segmental
character of the system is evidenced by the presence of connect-
ing blood-vessels or loops. The largest longitudinal vessel is
the dorsal vessel which runs in the mid-dorsal line above the
intestine. This vessel shows swellings in every segment in the

LUMBRICUS TERRESTRIS L. MULLER 03

region of the intestine, but becomes a thinner and perfectly
cyHndrical tube in the region of the oesophagus and finally bi-
furcates above the pharynx. The dorsal blood-vessel is con-
tractile and its pulsations push the blood forward, toward the
head. In the other four longitudinal blood-vessels the blood
runs from the head backwards and neither of them is contractile.
The ventral vessel is the largest among them, although consider-
ably smaller than the dorsal vessel. It Hes in the mid-ventral
line between the intestine and the nervous system and also
bifurcates in the region of the pharnyx. The dorsal and ven-
tral blood-vessels are connected with each other by means of
five pairs of aortic loops or lateral hearts which are simply pul-
sating blood-vessels surrounding the oesophagus in the seventh
to eleventh segments. The aortic loops push the blood from the
dorsal into the ventral vessel. A further connection between
the two vessels is established by means of their branches both
in the pharyngeal and anal regions of the animal. The subneural
blood-vessel runs along the mid-ventral line of the nervous sys-
tem. The subneural and dorsal blood-vessels are connected
with each other in every segment by right and left parietal
vessels, the first pair of which belongs to the twelfth segment,
i. e., follows immediately behind the last aortic loop. In the
region of the aortic loops the connection between the subneural
and dorsal vessels is estabhshed through the intervention of the
lateral oesophageal vessels. The blood runs in the parietal vessels
from the subneural into the dorsal vessel. The last two of the
main five longitudinal blood-vessels are the lateral neural vessels.
They run at the sides of the nervous system and are connected
with the subneural vessel by short transverse vessels of which
there is a pair in every segment. The intestinal canal receives
the blood supply from the ventral vessel through small intes-
tinal branches and the blood returns to the dorsal vessel through
similar dorsal branches. The body wall receives its blood supply
through lateral cutaneous branches of the ventral vessel. Organs
of respiration are absent and the blood is oxidized in the body

94 MORPHOLOGY OF IN\^ERTEBR.\TE TYPES

wall. Here the branches of the ventral vessel anastomose with
branches which return the oxidized blood to the dorsal vessel.
The dorsal vessel receives, therefore, mixed blood, the larger
part of which has been oxidized while some of it is venous. The
blood itself is red, but the color is contained in the plasm and
not in the corpuscles.

Nervous system. The nervous system consists of a pair of
supraoesophageal ganglia or brain and a ventral chain of paired
ganglia. The brain is situated in the third segment and gives
off anteriorly two pairs of nerves. Two lateral connectives unite
the brain with the suboesophageal or first pair of ventral ganglia.
The right and left elements of the nervous chain are so closely
connected with each other that their paired origin may be
recognized only on sections. The chain appears as a whitish
cord with a swelling in every segment, which marks the ganglia.
Each swelling gives off two pairs of nerves. There is also a pair
of dissepimental nerves given off by the ventral chain halfway
between the swellings of the chain. Thus every segment has
three pairs of nerves, a condition similar to that in Nereis and
Hirudo. Higher sense organs are absent.

Reproductive system. Lumbricus terrestris is a herma-
phrodite like all Oligochaetes. The male reproductive organs
consist of two pairs of testes, two pairs of ciliated funnels, two
vasa deferentia, two seminal vesicles and three pairs of sper-
mothecae. The testes are very small and are situated in the tenth
and eleventh segments close to and above the nervous system.
They are inclosed in the seminal vesicles. The first seminal
vesicle which is situated in the tenth segment has two pairs of
spermothecae or lateral pouches and contains the first pair of
testes and the first pair of ciliated funnels. The second seminal
vesicle is situated in the eleventh segment, has one pair of large
spermothecae and contains the second pair of testes and the
second pair of ciliated funnels. The funnels perforate in both
segments the dissepimenta and open into the thin vasa defer-
entia. These two ducts are rather short, terminating in the two

LUMBRICUS TERRESTRIS L. MULLER 95

genital pores on the fifteenth segment between the b and c
bristles. The sperm matures in the vesicles.

The female reproductive organs consist of one pair of ovaries,
one pair of oviducts and two pairs of sperm receptacles. The
ovaries are situated in the thirteenth segment. The oviducts
are extremely short tubes; they begin with a wide opening in
the thirteenth segment, perforate the dissepiment and end in
the two female genital openings on the fourteenth segment just
above the b bristles. The first pair of receptacula seminis are
round white bodies situated in the ninth segment and opening
to the outside in the lateral lines between the ninth and tenth
segments. The second pair of receptacles are similar to the
first, situated in the tenth segment and open to the outside in
the lateral lines between the tenth and eleventh segments.
Self-fertilization is impossible. The sperm is stored in the
receptacles during copulation. It passes later into the cocoon
which is secreted by the clitellum at the same time as the eggs
are deposited into it. Development is direct.

Instructions

1. Place the specimen in a dissecting tray with water and make
a full page drawing showing the side view. Count the segments
carefully. Label mouth, anus and clitellum. For the position
of the genital openings compare the specimen with what has
been stated in the paragraph on external anatomy.

2. Open the specimen by a longitudinal slit parallel to the mid-
dorsal line and fasten the sides with pins. Make a full page
drawing showing dissepimenta, ahmentary canal, dorsal blood-
vessel, aortic loops, nephridia and organs of reproduction.
Label pharynx with its dilators, oesophagus, calciferous glands,
crop, gizzard and intestine; dorsal blood-vessel, five pairs of aortic
loops surrounding oesophagus, parietal vessels beginning behind
the reproductive glands and connecting the dorsal with the sub-
neural vessel; nephridia and three pairs of large spermothecae.

3. Remove the alimentary canal by cutting it across in the

96 MORPHOLOGY OF INVERTEBRATE TYPES

middle of the pharynx, Hfting with a forceps and cutting with
scissors the aortic loops and parietal vessels. The removal of the
alimentary canal will expose all reproductive organs, nervous
system, neural blood-vessels and nephridia. Make a full page
drawing showing all structures inclosed in segments seven to
seventeen. Label first seminal vesicle with first and second pair
of spermothecae, second seminal vesicle with third pair of spermo-
thecae, the two vasa deferentia; the first pair of receptacula
seminis in the ninth segment, and the second pair of receptacula
in the tenth segment if not completely hidden by the spermo-
thecas; the pair of small ovaries in the thirteenth segment, the
oviducts; and finally the setigerous glands in the eighth to
thirteenth segments.

4. Remove a complete nephridium, place it on a slide in
glycerine and examine under low pow^r. IMake a drawing show-
ing nephrostome, transparent portion, glandular portion and
terminal portion.

5. Remove reproductive organs and lift carefully the nervous
system cutting the side nerves. Place the nervous system on a
slide and examine under low power. Make a drawing showing
supraoesophageal ganglion and at least five ventral ganglia.

6. Examine under low powder the prepared section through
the region of the aortic loops. Make a drawing showing cuticle,
hypodermis, circular muscles, longitudinal muscles, dorsal
vessel, ventral vessel, nervous system, oesophagus and such
other organs as may be seen.

7. Examine under low power the prepared section through the
region of the twentieth to thirtieth segments. ]\Iake a drawing
showing a, b, c, and d bristles, cuticle, hypodermis, circular
muscles, longitudinal muscles, intestine with typhlosole and
chloragogue cells, dorsal vessel, ventral vessel, nervous system,
median and lateral subneural vessels and nephridia.

8. Examine under high power (400 diameters) the prepared
section through the clitellum and make a drawing showing the
hypodermis with gland cells.

NEREIS VIRENS Sars

Material. N. virens is a common polychagte and may
be found in abundance under rocks between tides along the
Atlantic Coast. It should be anesthetized in a weak solution
of alcohol and preserved either in formahn or alcohol. Small
specimens for cross-section are best preserved either in sublimate
or in Bouin's picro-aceto-formalin. It is advisable to harden a
very large specimen in Miiller's liquid for a long time and to cut
it into separate segments with a sharp razor taking care not to
injure the parapodia. These sections can be preserved in-
definitely in alcohol and should be used for the study of the
topography of the organs under low power.

Descriptive Part

Nereis virens is an annulated or segmented marine worm
of the Class Polychaeta. The number of segments of which its
body is composed varies with the size and age of the worm
and may be less than a hundred in small specimens and up
to about two hundred in old and large ones. This variation
is due mainly to the production of new segments in the pos-
terior end of the worm with increasing age. The head repre-
sents the first segment of the worm and is sufficiently well dis-
tinguishable from the rest of the body. When the proboscis
which will be described further on, is completely withdrawn, the
mouth appears as a large round opening overhung by appendages
of the so-called prostomium. The latter forms a part of the head,
situated in front and above the mouth and carries two pairs of
eyes, two small tentacles at the end of the median lobe and two
large lateral palpi. That part of the head on which the mouth

97

gS MORPHOLOGY OF INVERTEBRATE TYPES

opens is called the peristomium and carries four pairs of peris-
tomial tentacles. Among these the two posterior dorsal tentacles
are the longest and present a convenient character for the
identification of the sex. In males they extend as far back as
the ninth segment, while in females they barely reach the middle
of the fifth segment.

The segments of the body do not present sufficient differences
for separating them into thoracic and abdominal segments as is
the case with many other polychaets. On the contrary, with
exception of the hindmost or anal segment, they are all more
or less alike, each provided with a pair of lateral appendages or
parapodia and a pair of nephridio pores or openings of the ex-
cretory organs at the base of the parapodia. The parapodia
are organs of locomotion and respiration. Each parapodium is
composed of two lobes, a dorsal notopodium and a ventral neuro-
podium. In the notopodium we distinguish a small lower ligula
and a large upper ligula with a dorsal cirrus. Between the two
ligulas is an opening through which the chetae or bristles pro-
trude. Their base is inclosed in the so-called chetigerous sack.
One bristle is much stouter than the rest and scarcely projects
beyond the opening. It is the acicidum. In the neuropodium we
find the same parts with the dift"erence that the ventral cirrus
sits at the base of the lower ligula. The parapodia of the first
and second segments lack the lower ligula, chetae and aciculum
of the notopodium, the latter being composed only of the upper
ligula with the dorsal cirrus. The chetcT of all parapodia consist
of two parts : the shaft and the blade. The base of the latter sits
in a terminal socket of the former. The last or anal segment has
no parapodia. Instead it has two long anal cirri above the anus
which is terminal.

Body wall and muscular system. The body wall is com-
posed of a cuticle and of a muscular skin layer. The former is
produced by the hypodermis and is perforated in many places by
the openings of the unicellular glands. The muscular skin layer
consists of circular vmscles underlying the hypodermis, longi-

NEREIS VIRENS SAKS

99

tudinal muscles form-
ing two dorsal and two
ventral longitudinal
bands, and oblique
muscles running from
the sides of the dorsal
surface to the middle
of the ventral surface.
The innermost layer
of the body wall is
formed by the parietal
layer of the peritoneal
epithelium.

The coelome o r
body cavity is the
space between the
body wall and the ali-
mentary canal. It is

Fig. 21. — Circulatory
system of Nereis virens,
side view, after Turnbull.
DB, dorsal blood vessel;
V, recurrent blood vessel
connecting dorsal blood
vessel with U network; U,
network on pharynx (pro-
boscis) ; SG, salivary gland;
E, branch to the oesopha-
gus; Z>, dorsal vessel to
parapodium; C, lateral
connecting vessels; K, ten-
tacular blood vessel; G,
network; S, branch from
ventral vessel to G net-
work; r, vessel connecting
ventral blood vessel with
U network; F, ventral ves-
sel to parapodium; VB,
ventral blood vessel; L,
anterior portion of ventral
blood vessel; PR, phar-
yngeal ring vessel.

lOo MORPHOLOGY OF im-ERTEBRATF, TYPES

(lividcd by scplu or transverse partitions into a scries of cham-
bers eacli corresponding to one segment and connected with the
others 1)v openings below the intestine. The Qelome is Hned
with peritoneal epithelium.

Digestive system. The mouth leads into a i)haryn.\ which
is supj>lied with protractor and retractor muscles and may be pro-
truded in form of a proboscis. On the iiuier surface of the
j^harynx, /. c, the one which becomes the outside surface of the
proboscis when the latter is protruded, are minute chitinous
denticles and two powerful pharyiigeal jaics with a serrated edge.
The pharynx leads into a short oesophagus. Into the latter open
two digestive glands often called salivary glands. The oesophagus
leads directly into the intestine into which it projects. The
intestine is a straight tube running through the entire
length of the worm and terminating on the last segment in
an anus.

The respiratory organs have been already mentioned. They
are the ligiihr of the parapodia.

The circulatory organs consist of two longitudinal vessels of
which one is dorsal and the other ventral. Both are contractile.
The dorsal vessel situated above the intestine in the middle line
of the body between the two dorsal longitudinal muscular bands
propels the blood towards the head. The ventral vessel situated
below the intestine propels the blood toward the posterior end of
the body. Both vessels are connected in each segment by a
right and a left transverse vessel, forming a ring around the in-
testine. Each ring gives off two dorsal and two ventral branchial
vessels to the parapodia. The dorsal vessels form a network in
the ligula of the upper ramus, the ventral vessels in that of the
lower ramus.

The excretory system consists of paired nephridia, one pair
for each segment except head and anal segment. Each ne-
phridium is a coiled tube opening into the body cavity by means
of a ciliated funnel and to the outside by means of a nephridiopore
situated on the ventral surface at the base o\ the parapodium.

\i;ki;is \ iki:\s sAks loi

The funnel belongs lo (he segment preceding I hat of ihe iic-
phridiopore. Conseciuenlly, the canal perforates the septum.

The nervous system consists of a chain of ganglia, one pair
of ganglia for each segment. The first pair is dorsal to the ali-
mentary canal and is situated in the head. It is the brain or
supraoesophageal ganglion. It gives off four optic nerves to the
eyes, two palpal nerves to the palpi and two tentacular nerves to
the tentacles. The following ganglia are all ventral to the
alimentary canal. The first of these, the so-called suha^sophageal
ganglion is connected with the brain by commissures surrounding
the pharynx. Nerves supplying the peristomial tentacles arise
from small ganglia connected with the commissures. Each
abdominal ganglion sends off three pairs of nerves. Of these the
first pair supplies the muscles of the septum, the second passes
through the septum to the preceding segment, and the third
goes to the parapodia. A visceral nervous system is represented
by several dorsal and ventral ganglia on the pharynx; they are
connected with each other and with the brain.

Reproductive system. The sexes are separate. The
gonads are developed from the peritoneum and the reproductive
cells, in the absence of special ducts, reach the outside through
the nephridiopores of the posterior region of the body. Fertili-
zation takes place in the water. Development is combined with
a metamorphosis. The larval stage is known as a Trochophora.

Instructions

I. Place a specimen with withdrawn proboscis into a dissecting
tray with water. Examine with the aid of the lens the anterior
end and make a drawing about three times natural size of the
dorsal surface of the first ten segments. Label head and body.
Label on the prostomium the eyes, tentacles and palpi, on the
peristomium the peristomial tentacles. Observe the length of
the two posterior dorsal tentacles and use this character to de-
termine the sex. Label the segments of the body and the para-
podia.

^

' ^ /

\ I -"^ ^

/ » /

W / 1

' J ^^

\\ ; \

\ ' T 1
' ' / 1

'^

V 1

\ / ^.^ "^

-^^''

u^T^ — ^

/

^

r

b 4

— A \

.

'^AAV-n--

.-->C

' --f

fM

» ^//

^

n NL

/ \

f/A

n...._^. .::^^

wll-/

r

y^

111 ._::rV^__^

^J^i-^

i V ::r=.

3t^

.^r"

^^^^

D

Fig. 2 2. — Nervous system of Nereis virens after Tumbull, slightly
modified. J, jaws; h, antennal nerves; c, palpal ner\'es; /, ganglia for the

NEREIS VIRENS SARS 103

2. Make a drawing showing the side view on the same scale.
Label all parts, including mouth.

3. Make a drawing of the ventral surface of the posterior ten
segments. Label anus, anal cirri, parapodia and nephridio-
pores.

4. Place a specimen with a protruded proboscis in the same
tray and make a drawing as in No. i. Label proboscis, pharyn-
geal denticles and jaws.

5. With the aid of a forceps tear out a parapodium from one
of the middle segments of the worm. Place it in a Syracuse dish
with glycerine under the dissecting microscope and make a
quarter page drawing of it. Label the notopodium and neuro-
podium. In the former label the upper ligula with the dorsal
cirrus, chetae, aciculum and lower ligula. In the neuropodium
label upper ligula, chetse, aciculum and lower ligula with the
ventral cirrus.

6. Tear out a parapodium from the first segment behind the
head, examine in the same manner and make a drawing. Label
parts.

7. With the aid of two needles tear the parapodium to free
the chetae. Place a cheta on a slide in a drop of water under
a cover glass, examine under microscope and make a drawing
showing the shaft and blade.

8. Open the worm along its back in a line parallel to the mid-
dorsal line from head to anus and fasten the body wall with pins.
Remove ventral longitudinal muscles bit by bit. Make a draw-
ing of the anterior third showing dorsal longitudinal muscles,
septa, digestive organs, circulatory organs, nephridia and su-
pracesophageal ganglion. Label septa, pharnyx, oesophagus,
intestine, digestive glands, protractors and retractors of the
proboscis, dorsal vessel (on body wall), transverse vessels,

dorsal peristomial cirri; «\ ganglion; ;/, nerves for the dissepimenta; m,
parapodial nerves; /, parapodial branch; h, ventral chain of ganglia; C,
cerebral ganglion; o, nerve passing through dissepiment to preceding seg-
ment; k, parapodial ganglion.

I04 MORPHOLOGY OF im^RTEBRATE TYPES

ventral vessel, nervous system visible beneath it, and supraoe-
sophageal ganglion.

9. Remove alimentary canal and ventral blood-vessel and
make a drawing showing nervous system and nephridia.

10. Place the prepared isolated segment of the worm in a
Syracuse dish with water under dissecting microscope and make
a half page drawing showing parapodia, dorsal longitudinal
muscles, ventral longitudinal muscles, oblique muscles, dorsal
vessel, intestine, ventral vessel, nervous system and nephridia.

11. Examine under microscope a prepared slide with a cross-
section through Nereis virens. Make a half page drawing of it
and label all parts mentioned in No. 10 and also cuticle, cir-
cular muscles, dorsal mesentery of the intestine, gonads, parietal
and visceral layers of the peritoneum.

HIRUDO MEDICINALIS Linnaeus

Material. Live leeches may be bought in any pharmacy
and can be kept alive in an aquarium, without food, during many
months, or they may be allowed to suck the blood of frogs.
Two hours before the exercise the leeches should be placed in
io% alcohol with a few drops of chloroform. The alimentary
canal may be injected, though this is not essential. A red gela-
tine mass gives, good results, but carmin suspended in a solution
of celloidin is preferable. The mass is allowed to harden by
placing the injected leech with the needle in 70% alcohol.
To prepare material for transverse sections the leech must be
stretched in a dissecting pan by means of two pins and fixed in
a mild fixing fluid such as Perenyi's. The best imbedding me-
dium is celloidin. Transverse sections of imbedded specimens
for study under the dissecting microscope should be cut with a
common razor to the thickness of one annulus. These sections
can be preserved indefinitely in alcohol. Thin sections may be
stained in haematoxylin and eosin or orange G.

Descriptive Part

The leech is a hermaphroditic segmented worm inhabiting
stagnant fresh-water pools of Europe and imported into this
country for medicinal purposes. The development of the leech
shows that it is composed of thirty-three ^ segments, a number
characteristic of all segmented worms of the Class Hirudinei.

1 According to some investigators the head of the leech is composed of six
segments and the whole number of segments is therefore 34 and not $$. In
this case the segment with the mule genital opening would be the eleventh
and the last visible segment the twenty-seventh, and not the twenty-sixth.
It is situated between the fourth and fifth pair of nephridiopores.

105

I06 MORPHOLOGY OF INVERTEBRATE TYPES

The anterior five segments form the head which, however, is not
externally distinguishable from the following twenty-one seg-
ments which form the trunk. The posterior seven segments are
modified in the adult leech into the so-called posterior sucker.
Externally the segmentation is shown by the excretory pores,
but is otherwise hard of recognition owing to the presence of
transverse folds of the skin known as rings or annuli. The typi-
cal number of annuli for each segment in the Hirudinei is three,
but in the medicinal leech we find that the first and second
segments are composed of a single annulus each, the third and
fourth of two annuli each, the fifth and sixth of three annuli
each, the seventh of four annuli, the eighth to twenty-third of
five annuli each, the twenty-fourth, twenty-fifth and twenty-
sixth of two annuU each, making altogether one hundred and two
annuli. The skin of the leech is composed of a cuticle and a
single layer of epithelial cells, which we shall call the hypodermis.
Inserted between these cells there are many unicellular slime
glands the number of which is especially great in the ninth,
tenth and eleventh segments where they produce the cocoon
covering the eggs during oviposition. These segments repre-
sent therefore the clitellum and are equivalent to the clitellum
of the earthworm. The skin is the seat of special sense organs
in the form of sensory papiUce especially well developed in the
middle annulus of each segment. On the dorsal surface of the
animal in the anterior five segments forming the head, are five
pairs of eyes, one pair in each segment; the third and fourth
pairs are located in the first annulus, the fifth pair in the second
annulus of the corresponding segment. The color of the skin is
due to special branched pigment cells situated under the hypo-
dermis between the muscles.

The muscular system is highly developed. Immediately
under the hypodermis is a layer of circular muscles. The
function of these muscles is to extend the animal. The space
between the circular muscles and the internal organs is almost
completely occupied by a powerful layer of longitudinal muscles.

HiRUDo :medicinalis linn^us 107

Their function is antagonistic to that of the circular muscles.
Traversing both layers at right angles are the radial and the
dorso-ventral muscles the function of which is to flatten the
animal. In the posterior sucker the muscular system is rep-
resented by radial and circular fibres. In reality the entire
muscular system is still more complicated. Between the muscles
we find connective and hotryoidal tissue.

Digestive system. The alimentary canal begins with the
mouth on the ventral surface of the first segment. The mouth
is surrounded by the mouth sucker. In the mouth cavity are
three jaws one of which is median and dorsal in its position,
while the other two are oblique and ventral. The edge of each
jaw is provided with a row of teeth for the perforation of the
skin. The jaws are operated by a muscular system of their
own. Behind the jaws the mouth cavity leads into a short,
muscular pharynx into which the ducts of the salivary glands
open. It is the secretion of these glands which prevents the
coagulation of the blood of the animal attacked by the leech
and causes prolonged bleeding from the wound. Behind the
pharynx begins the midgut which is sharply divided into two
sections. The anterior section or stomach, called also crop, is
by far the most powerfully developed part of the alimentary
canal. It possesses eleven pairs of ccBca the last of which is
the longest. It is a storage place for the food. The resorption
of the food takes place in the second section or intestine which
is a much shorter and thinner tube connected with the stomach
(crop) by what is termed the funnel. The hindgut or rectufn
terminates in an anus situated on the dorsal surface at the
base of the posterior sucker.

The circulatory system consists of two lateral blood-vessels,
one dorsal and one ventral sinus and their ramifications. The
lateral vessels have muscular walls. Anteriorly and posteriorly
they are directly connected with each other, so that the blood
can pass from one lateral vessel into the other. A further con-
nection between the lateral vessels is estabhshed by the presence

io8

MORPHOLOGY or i\\i:kifi?r.\ ii: rvpi:s

of seventeen pairs of transverse branches. Some branches split
up into numerous capillary tubes between the different organs,
the botryoidal tissue and the hvpodermis. These capillary
tubes communicate with the capillaries of the sinuses. The

Fig. 23. — A nephridium of Hirudo medicinalis after Leuckart. T, tes-
tis; VE, vas efferens; VD, vas deferens; LB, lateral blood vessel; AL, apical
lobe; TL, testis lobe; F, neptrostome; ML, middle lobe; VDT, vesicle duct;
F, vesicle.

dorsal and the ventral sinuses communicate besides with each
other in the posterior end of the body. The sinuses with their
ramifications represent the ccelome. They are lined with epithe-
lial cells but have no muscular walls. There is no regular blood
circulation, but the pulsation of the lateral vessels and their
branches propels the blood in the one or the other direction.

Organs of respiration are absent, the blood being oxidized in
the skin.

The excretory system consists of seventeen pairs of ne-
phridia which open to the outside on the ventral surface through
as many nephridio pores. The first pair of nephridiopores is
situated on the first annulus of the seventh segment, while

HIRUDO MEDICINALIS LINN^US 109

the following sixteen pairs are situated on the second annulus
of the successive segments. The twenty-third segment is there-
fore the last segment possessing nephridiopores. A nephridium
is a coiled tube usually with an open funnel; but in the case
of the leech the funnel which lies in a lateral branch of the
ventral sinus is closed. The funnel of the sixth to fifteenth
nephridia is in close contact with the testes and this is the
reason why the first part of the nephridium is called the testis
lobe. The largest part of the nephridium is formed by the
middle or glandular lobe which forms together with the so-
called apical lobe a closed, compressed ring and is connected
with the vesicle by a thin vesicle duct. Both the apical and
middle as well as the testis lobe are perforated by a compli-
cated, ramified system of canals. The vesicle is the last part of
the nephridium and is connected with the nephridiopore by a
very short duct.

The nervous system consists of a brain or supraoesophageal
gangHon which lies above the pharynx and just behind the jaws,
a subcesophageal ganglion underneath the pharynx and connected
with the brain by two heavy commissures, and a chain of twenty-
one pairs of ganglia inclosed in the ventral sinus, one pair of
ganglia for each segment except in the case of the last or twenty-
first pair which represents the fused neuromeres of the last
seven segments transformed into the posterior sucker. The
brain together with the subcesophageal ganglion represents the
fused neuromeres of the anterior six segments. The sympathetic
nervous system is represented by a median nerve which runs
above the nervous chain along the wall of the stomach.

Reproductive system. The leeches are true hermaphro-
dites. The male reproductive organs in the medicinal leech
consist of nine pairs of testes situated beneath the stomach in
the thirteenth to twenty-first segments. There are two lon-
gitudinal vasa deferentia connected with the testes by short
vasa efferentia. Anteriorly each vas deferens forms a convo-
luted seminal vesicle. The next portion of each duct, connecting

no

MORPHOLOGY OF INVERTEBRATE TYPES

P.

PR

OE

Fig. 24. — ISIale reproductive organs of Aidostomum after Brandes (com-
bined from two figures and somewhat modified). P, penis; PO, penis pouch;
PR. prostata; CG, Cowper's gland; SR, sperm reservoir; DE, ductus ejacu-
latorius; YS, vesicula seminaHs; VD, vas deferens; VE, vas efferens; T, testis.

HIRUDO MEDICINALIS LINN^US iii

the seminal vesicle with the penis is a muscular ductus ejacula-
torius. The penis is single and may be protruded through the
male genital opening which is situated in the median Hne on
the ventral surface between the fourth and fifth annuli of the
tenth segment. At the base of the penis is a prostata gland.
The female reproductive organs consist of two ovaries^ two short
oviducts and a single muscular vagina with its female genital
opening between the fourth and fifth annuli of the eleventh
segment.

Instructions

1. Place a freshly killed leech on its back in a dissecting tray
with water and fasten it by means of two pins stuck through
the anterior and posterior suckers, at the same time stretching
it as far as possible. Find the genital openings. Make a full
page drawing of the ventral surface. Use the male genital
opening as a guide to the segmentation. Try to find the nephri-
diopores and the sensory papillae.

2. Remove the pins, turn the leech over, dorsal surface up,
and fasten it as before. Make a sketch of the anterior five
segments showing the eyes.

3. Make a superficial longitudinal median incision with a
very sharp scalpel in the middle segments of the body. Stick
two pins through the cut edges of the skin and force it apart.
Holding the edge of the skin with a forceps continue the incision
posteriorly and anteriorly gradually forcing the skin apart and
fastening it with pins, until the entire animal has been opened.
Make a full page drawing of the alimentary canal and label
parts.

4. Remove with the aid of a forceps the entire alimentary
canal beginning with the rectum but leaving the jaws. This
will expose all the other organs except the dorsal sinus which
was destroyed by the longitudinal incision. Try to locate all
organs without the aid of the teacher. Make a full page drawing
of the male and female reproductive systems, excretory system,

112 MORPHOLOGY OF LWICRTEIiRATL: TVPIOS

lateral blood-vessels with some of the branches and the nervous
system in the ventral sinus. Label all organs and corresponding
segments using the last pair of nephridia for guide.

5. Carefully excise with scissors an entire nephridium, put it
in a drop of water on a slide and examine under dissecting mi-
croscope. Make a drawing about live times natural size and
label parts.

6. Remove a jaw and examine it under microscope in a drop of
glycerine. Make a drawing of it about two inches long.

7. Apply a live leech to the hand or arm. When the skin is
pierced remove the leech by putting a drop of alcohol on its
head. Wipe off the blood and examine the wound. The three
incision lines meeting in the centre correspond to the cutting
edges of the jaws. Make a sketch of the wound.

8. Study the prepared cross-section of a leech under micro-
scope. Make a half page drawing of it showing the cuticle,
hypodermis, circular, longitudinal, radial and dorso-ventral
muscles, the botryoidal tissue, stomach, nephridia, lateral
vessels, dorsal sinus, ventral sinus with the nervous system,
vasa deferentia (and testes if the section contains them).

DAPHNIA PULEX MuUer

Material. Specimens of Daphnia pulex may be collected
at any time of the year, but are more abundant in spring. It
is the common large Daphnia of our fresh-water ponds. The
animals may be kept well in large aquaria, provided all hydras
have been carefully removed. Every student should receive
several Hve specimens.

Descriptive Part

Daphnia pulex is a common representative of the Suborder
Cladocera, Order Entomostraca, Class Crustacea, These little
crustaceans inhabit the fresh-water ponds of North America and
Europe and may occur in millions in small pools. They swim by
the aid of their second antennae which are developed into power-
ful organs for a jerkwise method of propulsion. Their food con-
sists mostly of microscopic algae. Owing to their ability to
propagate parthenogenetically, i. e., without fertilization, one
meets during the spring and summer with scarcely any males.

The body of Daphnia pulex, like that of all arthropods, is
segmented or composed of somites. But the somites can be
recognized only by their appendages of which there is never
more than one pair to a somite, and by the internal metamer-
ization of the organs. We shall see later that the number of
somites in higher crustaceans, is definite and that the entire
body may be readily divided into a head, thorax and abdomen;
the head is composed of six somites, the thorax of eight and the
abdomen of seven. But the Entomostraca form a group of
crustaceans in which near relatives may have a different number
of somites. Only the head is always composed of six somites and,

113

114 MORPHOLOGY OF INVERTEBRATE TYPES

in the case of all Cladocera, it is clearly separated from the rest
of the body. The somites of which the head in Daphnia pulex
is composed are as follows: (i) ocular somite, (2) first antennal
somite, (3) second antennal somite, (4) mandibular somite, (5)
first maxillary somite, (6) sixth cephalic somite devoid of ap-
pendages in the adult but corresponding to the second maxillary
somite of all other Crustacea. These six somites are fused to-
gether and even in the embryo do not articulate with each other
but represent a single unit. The first three somites are preoral,
the following postoral in the adult. The remainder of the body
is not clearly divided into a definite number of somites and is not
well separable into a thorax and abdomen. The five somites
following the last cephalic somite are apparent from the five
pairs of limbs, whereas the end of the body has no appendages
and has lost all evidence of external and internal segmentation.
Thus it is impossible to say to how many somites this portion
of the body corresponds. The end of it is used as an organ of
locomotion when the animal is moving on the bottom of the pool.
For this reason it is often called the "scratch foot." It has two
tactile bristles, a series of small teeth on the dorsal surface, and
ends in two powerful claws which are know^n under the name of
furca. The trunk of the animal is protected by a right and left
fold of the integument extending beyond the thoracic append-
ages. This fold is often spoken of as the '' shell " from its analogy
to the bivalve shell of the lamellibranch molluscs. The postero-
dorsal end of the shell is drawn out into a sharp angle which cor-
responds to the long spine found in other species of Daphnids.
The ventral, free edge of the shell carries a series of long bristles
at right angles to the surface of the shell and directed inward,
i. e., toward the plane of symmetry. These bristles afford an
excellent protection to the gills of the animal while at the same
time freely admitting water into the shell.

Integument and skeleton. The integument of Daphnia
consists of a single layer of epithelial cells or hypodermis which
secretes a thin supporting membrane on its inner surface and a

DAPHNIA PULEX MULLER

"S

heavy chitinous cuticle on the outer surface. Once produced, the
chitinous cuticle is incapable of further growth. It hardens and
becomes the skeleton. Increase in the size of the animal can be
therefore accomplished only by a process of moulting during
which the old skeleton is thrown off. The newly produced

iiO MORPHOLOGY OF INVERTEBRATE TYPES

cuticle stretches rapidly, while soft, and then hardens to take
the place of the old skeleton. Another important feature of the
external skeleton is that locomotion would be impossible if
this were not subdivided into articulating segments connected
with each other by thin and pliable cuticle. This is exactly
what occurs in all arthropods. In higher forms the articulation
is so perfect that locomotion is possible only in strictly circum-
scribed planes. But in Daphnia articulation is still very primi-
tive, special articulating surfaces are still absent and the joints
have more freedom. The cuticle appears of ten sculptured. Thus
for example the shell of Daphnia presents the appearance of
a network, due to minute ridges of the cuticle. Two other
characteristics of an external skeleton may be mentioned in
this place. One is that the muscles are of necessity inside the
skeleton, not outside as in the case of an internal skeleton such
as in vertebrates. The other is that the skeleton has to be pro-
vided with pores to admit of a connection between the nervous
system and the sense organs which convey to the former the
impressions of the outside world. We shall see later how some
of the sense organs are structured.

Appendages. The first pair of appendages are the
antennules (often called first pair of antennae), which belong to
the second cephalic somite. They are single-jointed, tiny, cy-
lindrical organs situated near the forward angle of the head.
At their ends are several olfactory bristles (aisthetascs). The
antennules of the male are considerably longer than those of the
female. The second pair of appendages are the antenna (called
also second pair of antennae). They are built on a strictly
two-branched or biramous plan and consist of several joints.
The joint by which they are attached to the head is called the
coxopodite, the one following it, the hasopodite. Together these
two joints represent the so-called protopodite of a typical bira-
mous appendage. The two branches of the antenna attached
to the basopodite are known as endopodite and exopodite. The
former is three-jointed, the latter four-jointed. The endopodite

Fig. 26. — Appendages of Daphnia similis Claus.

Ii8 MORPHOLOGY OF INVERTEBRATE TYPES

carries four long plume-like bristles, one of which sits at the end
of the second joint and three at the end of the third joint.
The exopodite carries five similar bristles of which three sit at
the end of the last, in this case of the fourth joint. We have
seen already that these antennae are used for swimming. Con-
sequently they possess their own powerful muscles arising from
the back of the head and attached to the coxopodite. The third
pair of appendages are the mandibles. They are long chitinous
plates situated at the sides of the mouth and provided with a
masticating edge. The fourth pair of appendages are the
maxillcB, two weak plates ending in four plume-like bristles.
The five pairs of thoracic limbs have a peculiar leaf-like ap-
pearance, with many bristles along their edge. The third pair
of limbs is the largest, the fourth next in size. The structure
as to detail differs in all five pairs, but the plan of structure re-
mains the same and may be best understood from the third or
largest limb. It consists of a large leaf-like, single- jointed pro-
topodite, by means of which it is attached to the body and the
edge of which is thickly covered with a row of long bristles;
a smaller, single-jointed exopodite with six plume-like bristles;
and a still smaller oval epipodite which serves as gill. The en-
dopodite is not developed and is represented only by a few
bristles situated between the bristles of the protopodite and those
of the exopodite.

Digestive system. The mouth is situated on the head in the
depth of the depression which separates the head from the trunk.
At the sides of the mouth are two mandibles and below it two
maxillae. These two pairs of appendages are the masticating
organs of the anunal. The alimentary canal is, as usual, com-
posed of three divisions, the foregut, the midgut, and the hindgut.
The foregut is represented by a short oesophagus which runs
slantingly upward and opens into the much wider midgut,
slightly projecting into it. The midgut is a cyhndrical tube
running first upward, then straight backward, and finally down-
ward. Near the anterior end the midgut receives two dorsal

DAPHNIA PULEX MULLER 1 19

blind sacs or coeca. There is no differentiation into stomach
and intestine, so that the simple midgut performs both functions.
The hindgut or rectum is restricted to the last or anal somite.
The anus opens at the end of the trunk under the base
of the furca, but morphologically it is dorsal to it (almost
terminal) .

Excretory system. The excretory system consists of a
pair of maxillary {or shell) glands. These glands are modified
nephridia. They are situated in the wall of the shell in the region
below the heart and consist of a coiled tube, one end of which
terminates in a little blind sac representing a modified nephro-
stome, while the other end opens to the outside on the ventral
surface of the sixth cephalic somite. The glands are called max-
illary because in other Entomostraca the sixth cephalic somite
has the second pair of maxillae for appendages.

Circulatory system and body cavity. The circulatory
system of Daphnia is an open one. The heart has the shape of an
ovoid and is situated dorsally to the midgut and in front of the
brood-pouch. It is composed of a few striated muscular cells
and has one pair of ostia or side openings through which the
blood returns to the heart. There are no blood-vessels. The
pulsation of the heart throws the blood forward and backward
into the body cavity where it flows between the various organs
until it reaches the branchial lacunae of the gills. Here it is
oxidized and returns to the heart through the ostia. It is there-
fore probable that the heart receives some 'Wenous" blood be-
sides the "arterial" blood which comes from the gills. The
blood is colorless and contains amoebocytes.

Respiratory system. The so-called epipodites of the
thoracic appendages are developed as gills. The animal keeps
up a constant current of water inside its shell by a continuous
beating motion of the thoracic limbs.

Nervous system and sense organs. The nervous system
of Daphnia is more or less characteristic of several orders of
Entomostraca. The brain or supraoesophageal mass consists

I20 MORPHOLOGY OF INVERTEBRATE TYPES

of an anterior median region from which a nerve runs to the
ocellus or simple eye, a pair of lobi optici or protocerebriim with
optic nerves for the compound eye, and a pair of ganglia anten-
nularia or deuterocerebrum with nerves for the antennules. The
brain is dorsal to the oesophagus and sends out a right and left
connective to the sub oesophageal mass. This mass consists of a
pair of ganglia with nerves for the antennae, and corresponds
therefore to the ganglia antennaria or tritocerebrum of higher
crustaceans in which it has become a part of the brain, while
the next pair of ganglia moved into its place to form a sub-
oesophageal mass. The suboesophageal mass and the following
chain of ganglia are ventral to the alimentary canal. This ven-
tral chain of paired ganglia is characterized by a great concentra-
tion of its elements. The last pair of ganglia belongs to the
somite carrying the fifth pair of thoracic feet.

Daphnia pulex has a pair of olfactory organs in the antennules,
a simple eye, a compound eye and a great number of tactile
hairs. A tactile hair is a process of a modified hypodermal cell
connected through the intermediation of a terminal ganglionic
cell with the central nervous system. The olfactory organs of
the antennules may be regarded as modified tactile hairs. The
simple eye is of the cup type, reminding of similar structures in
lower invertebrates. The compound eye of Daphnia, while
originally of paired origin, becomes fused into a single organ
early in the embryonic life. It is more primitive than the com-
pound eyes of higher arthropods and cannot be very efficient.
To increase its usefulness it is provided with special muscles, the
alternate contraction of which keeps the eye in continuous mo-
tion to allow shadow-images to stimulate a greater number of
nerve endings.

Reproductive system. The sexes are separate. In the
male the reproductive system consists of a pair of testes situated
one on each side of the midgut. The two vasa deferentia open
separately on the ventral surface of the trunk near its posterior
end. In the female the system consists of two long ovaries

DAPHNIA PULEX MULLER 121

situated right and left of the midgut and extending from the
heart region to the posterior bend of the body. The oviducts
are short and open separately on the dorsal surface of the trunk
into the brood-pouch. The latter is a large space under the shell
above the trunk and behind the heart. The par theno genetic
eggs develop in this brood-pouch until the young daphniae are
able to swim out of the pouch. The so-called winter eggs, i. e.,
fertilized eggs, are at first enveloped by a special sac called
ephippium in the brood-pouch, and the entire ephippium is then
deposited into the water. On sinking to the bottom of the pool
the ephippium remains there until next spring, when the eggs
develop into females.

Instructions

1. Place a Daphnia in a watch glass with a few drops of water.
Add a drop of io% ether and examine the animal under low
power (50 diameters). Make a full page drawing showing the
right side view, but omit from the drawing the left second an-
tenna. Label right second antenna, compound eye, ocellus,
first antenna, carapace, heart, furca, abdominal tactile bristles,
intestine, hepatic coeca, five thoracic limbs, shell gland, muscles
of second antenna, brood-pouch with embryos, ovary, if devel-
oped.

2. Etherize the Daphnia still more and turn it on its back.
Make a full-page drawing showing ventral view. Label carapace
with protective fringe of hair, both pairs of antennae, both
hepatic coeca, compound eye, ocellus, furca, anus, abdominal
bristles.

HOMARUS AMERICANUS Milne-Edwards

Material. Specimens of the American lobster may be
obtained from the Woods Hole INIarine Laboratory or from any
of the other marine laboratories. Every student should re-
ceive a specimen preserved in weak alcohol and another with
the circulatory system injected.

Descriptive Part

Homarus americanus or the common American lobster is a
typical representative of the Suborder Decapoda, Order Mala-
costraca, Class Crustacea. It is an inhabitant of the Atlantic
Coast, where it is found in great numbers below the low- tide mark.
Unfortunately, owing to its gastronomic qualities, the older
and larger specimens become more and more rare. The largest
lobster on record is at present in the American Museum of
Natural History in New York. It was caught in 1897 at the
Atlantic Highlands, N. J., is twenty- three and three-fourths
inches in length and weighed when alive thirty-four pounds.
Its crushing claw is fifteen inches long. The food of the lobster
consists chiefly of fish, live or dead, and to a smaller part of
invertebrates. Although much more highly developed than
the little Daphnia, the lobster shows much better the segmenta-
tion of its body.

External features and segmentation. To avoid repeti-
tion, the minute description of the external features of the lobster
will be given in the ''Instructions" at the end of this chapter.
We are here concerned with the general principles of the struc-
ture of its body.

The body of the lobster is composed of twenty-one somites

HOMARUS AMERICANUS MILNE-EDWARDS 123

Fig. 27. — Astacus fliiviatilis after Huxley. A, male; B, female; a, anus;
gg, opening of green gland (antennal gland, nephridium); Ih, upper Up or
labrum; mt, metastoma or under lip; od, opening of oviduct (9 gen. op.);
vd, opening of vas deferens {& gen. op.); i, stem of eye; 2, antennules; j, an-
tennae; 4, mandibles; 8, second maxillipeds; 9, third maxillipeds; 10, first
pereiopod; u, second pereiopod; 14, fifth pereiopod; 15-20, abdominal ap-
pendages; Z, XI, XIV, sternites of fourth, fifth, and eighth thoracic somites;
XV It sternite of second abdominal somite.

124 MORPHOLOGY OF INVERTEBRATE TYPES

or segments. Externally it may be divided into an anterior
fused portion called the cephalothorax and a posterior clearly
segmented portion called the abdomen or pleon. The cephalo-
thorax is composed of fourteen segments, the anterior six of
which belong to the head or cephalon, the posterior eight to the
thorax. The somites of one region of the body have little in
common with the somites of the other two regions, since every
component of a somite has been modified to meet the require-
ments of position and function. Thus the somites of the thorax
became fused together to give more rigidity to this region, rigid-
ity needed on account of the powerful development of the
thoracic legs or pereiopods, especially those of the first pair.

Skeleton. The skeleton of the lobster is of two kinds.
One is external, produced as a chitinous secretion by the hypo-
dermis of the integument, and is called the exoskeleton. The
other is internal, produced by infoldings of the ectoderm called
apodemes. This skeleton is known under the term of endophrag-
mal skeleton or simply ejido skeleton. The exoskeleton is thick
and caHcified. Its color is derived from pigment produced by
special cells or chromatohlasts situated below the hypodermis.
The exoskeleton of every segment being the product of the
integument, naturally has more or less the shape of a ring or
cylinder, as, for example, in the case of the abdominal somites.
The dorsal surface of such an exoskeletal ring is called the
tergum or tergite, the ventral the sternum or sternite, and the
sides the pleurcB. The calcification of the pleurae in this case
makes the ring thoroughly rigid and the pleurae themselves
httle distinct. In many arthropods, however, the pleurae re-
main as soft as the intersegmental membrane, and only the
tergite and sternite are hard. Such is the case with many
arachnids and insects. Where the somites are separated from
each other by a thin articulating membrane, there the corre-
sponding sternitcs and tergites are easily recognized. But
when adjoining somites fuse together, it becomes often diflScult
to recognize them and may even require special comparative

HOMARUS AMERICANUS MILNE-EDWARDS 125

anatomical and embryological study. Such is the case with the
cephalothorax of the lobster. Here, as we have seen, the somites
are fused together and the fused tergites form a dorsal shield
or carapace. The thoracic tergites have, moreover, produced
two lateral folds or hranchiostegites for the protection of the
gills, these folds forming part of the carapace. Nothing shows
the limits of individual tergites and it is not even quite sure,
though very probable, that the transverse so-called cervical
groove corresponds to the division line between the sixth and
seventh somites, i. e., between head and thorax. The recogni-
tion of the sternites is somewhat easier, owing to the fact that
not all of them have fused so completely. The more difficult
among them, those of the second and third cephalic somite,
are described in the "Instructions." The endoskeleton is re-
stricted to the thorax and abdomen. In the thorax it forms
a compHcated latticework connected with the sternites of the
exoskeleton and forming attachment points for muscles and
providing protection for the nervous system. We have already
seen that the endoskeleton arises as infoldings or apodemes of
the ectoderm between the segments of the body. Consequently
the endoskeleton itself is segmented, each segment consisting
of a pair of endosternites and a pair of endopleurites. The sternal
canal in which the nervous system is inclosed is formed by the
endosternites and their dorsal horizontal plates or mesophragmas.
The latter thus form the roof of the sternal canal, but this roof
is discontinuous, the canal being in reality a passage through
several arches. The paraphragmas or outer processes of the
endosternites connect the latter with the endopleurites. The
endoskeleton forms the peripheral hinges for the articulation of
the thoracic appendages in the shape of balls which are situated
on the epimeral plates. The median hinges, on the other hand,
are cup-shaped and are formed by the sternites.

Appendages. Except for the first cephalic somite which
bears the compound eyes and the last abdominal or anal somite
called the telson^ all the other somites have a pair of appendages.

126 MORPHOLOGY OF INVERTEBRATE TYPES

We shall consider them in their natural sequence. The append-
ages of the second (cephalic) somite are called the antennules
(or first pair of antennae). They are typically biramous and
consist of a two- jointed shaft or protopodite and two many-
jointed flagelli. The inner flagellum is the shorter one, the
outer flagellum bears many chemical setae or aesthetascs sup-
posedly of olfactory function. The first joint of the shaft con-
tains a statocyst with an opening to the outside.

The appendages of the third somite are the antennce (called
also second pair of antennae), characterized by the exceedingly
long flagellum. They consist of a two-jointed protopodite, a
scale-like exopodite, and an etidopodite or many-jointed flagellum.
The function of the antennae is tactile. The first joint of the
protopodite bears a papilla with the opening of the antennal or
green gland. The antennae are embryologically postoral ap-
pendages, but become soon preoral (prostomial) in position.
The appendages of the fourth somite are the powerful mandi-
bles. They consist of a single joint, probably homologous to
the coxopodite of a two-jointed shaft, and adapted to the
trituration of hard food; and of a three-jointed palpus repre-
senting the endopodite. This palpus is protected by a groove
in the mandible proper, in which it usually Hes concealed. The
triturating edge of the mandible is formed by exceedingly hard
chitin. When the mandibles are removed from their sockets
in the process of dissection, the tendons of their muscles remain
adhering to them.

The appendages of the fifth and sixth somite are the first and
second maxillce, respectively. Their function is chiefly to pass
on the food. They are quite thin, leaf-like structures. The
first maxilla consists of a two-jointed protopodite and an en-
dopodite with a short flagellum. In the second maxilla there
are present also an exopodite and an epipodite, while the en-
dopodite lacks the flagellum. Comparative study shows that
the epipodite is always derived from the exopodite. In the
second maxilla the epipodite is still short and wide. It func-

HOMARUS AAIERICANUS MILNE-EDWARDS 127

tions as a scaphognathite or bailer in driving the water out of
the branchial cavity.

The following eight, i. e., the seventh to fourteenth somites
form, as we have seen, the thorax. The first three pairs of
thoracic appendages are called maxillipeds, the last five pairs
pereiopods. The first and second maxillipeds are used for pass-
ing food, the third has mainly a masticatory function. The
second and third have besides a respiratory function, inasmuch
as they have a gill or podohranchia attached to them. The
epipodite is well developed in all three maxillipeds and in the
first it is furnished with a fold or trough for the reception of the
bailer. The exopodite of all three maxillipeds has a many-
jointed flagellum. The endopodite shows a gradual increase in
complication. In the first maxilliped it is two-jointed, in the
second four-jointed, in the third five-jointed, considerably
heavier than the exopodite. In this respect the third maxilliped
is especially interesting for it shows clearly how a monoramous
appendage such as a pereiopod has developed from a biramous
one. If the five joints of the endopodite be counted as a con-
tinuation of the two-jointed protopodite, then the limb becomes
seven-jointed — and that is the typical number of joints in a
pereiopod. Beginning with the proximal end of the limb, the
joints receive the names of coxopodite, basopodite, ischiopodite,
meropodite, carpopodite, propodite, and dactylopodite. The is-
chiopodite of the third maxilliped has a comb with a row of
about twenty teeth along its inner edge and a brush along its
outer edge. Of the five pairs of thoracic legs or pereiopods the
anterior three are chellpeds, i. e., they have a forceps or chela
at their end, whereas the last two pairs end in a simple claw.
Each pereiopod consists of seven joints of which the first two
represent the protopodite and the remaining five the endopodite.
The exopodite has completely disappeared. An epipodite and
podohranchia are present in the first four and lacking in the
fifth pair. The joints are articulated by means of hinges. The
forceps of a cheliped is formed by the propodite and dactylopo-

128 MORPHOLOGY OF INVERTEBRATE TYl'ES

dite. The main body of the propodite is called the hand, the
dactylopodite the movable finger, while the immovable finger is
simply a process of the propodite. The first pair of pereiopods,
the so-called great chelipeds, are especially powerful. Their
function is to hold and crush the food. It is a common occurrence
among crabs and lobsters that the left and right great chelipeds
are not alike. In the lobster one claw is heavy, is used for
crushing the food, and its fingers for this reason have a row of
rounded tubercles. The other claw is more slender, is used for
seizing and holding the prey, and its fingers have a row of sharp
teeth. All pereiopods are easily broken ofi by the animal if
held fast by an enemy. The breaking plane is always between
the baso- and ischiopodite. Anatomically this is due to a
special mechanism and it is interesting to remember that the
part of the limb remaining intact consists of the two joints of
the protopodite. The relation of the pereiopods to the genital
openings wiU be described later.

The following sLx pairs of appendages belong to the abdomen
and are called pleopods. The first pair is uniramous. In the
female it is quite small, in the male it is developed as a pair of
stylets for copulation. The second, third, fourth and fifth pairs
of pleopods .are biramous and very much alike in structure.
They consist of a single-jointed protopodite and flat exopodite
and endopodite, both the latter fringed with tactile hairs. These
pleopods are used for forward swimming and, in the female, for
holding and aerating the eggs. The last pair of pleopods is at
the same time the last pair of all appendages. These pleopods
belong therefore to the twentieth somite and form together with
the anal somite the tail fan. They are usually called uropods.
The exopodite is two-jointed. Both the endopodite and exo-
podite are in the shape of broad and flat paddles fringed with
tactile hairs.

Muscular system. It is clear that a hard, segmented
exoskeleton must possess a segmented muscular system of a
different type from that of segmented worms in which the in-

I30 MORPHOLOGY OF INVERTEBRATE TYPES

tegument of the body is soft and elastic. On the whole the
muscular system is too complicated to be dealt with here, but
some of the most important muscles may be mentioned. The
extension of the segmented abdomen is accomplished by mus-
cles attached to the tergites, the flexion by more powerful
muscles attached to the sternites. The thoracic appendages are
moved by muscles arising from the endoskeleton and ending in
the coxopodites. A flexor and an extensor is present in every
joint of the appendages for the next joint, except of course in the
dactylopodite which is the last joint.

Digestive system. The V-shaped mouth is ventral in
position and is provided with two lips. The upper lip or labrum
has a median keel on both its surfaces, dividing each surface into
two concave areas. The lower lip or metastoma is bifurcated.
Between the two lips at the sides of the mouth are the powerful
mandibles and behind the mouth the maxillae and maxillipeds
described above and used as mouth parts in passing the food.
The mouth leads directly into a short oesophagus which opens
into the stomach. The stomach is a large and complicated organ
forming the last section of the foregut. It is divided into three
regions, the cardiac sac, the gastric mill and the pyloric sac.
On each side of the stomach is a large gastrolithic plate. The
gastric mill in which the food is ground into a fine pulp contains
a median tooth and lateral teeth. These teeth are the inner
projections of special ossicles. The wall of the stomach has a
regular framework of articulated ossicles some of which will be
mentioned in the instructions. The grinding movement of the
gastric teeth is accomplished by the action of special muscles.
The anterior gastric muscles are attached to the so-called pro-
cephalic lobes of the tergite of the first cephalic somite. The
posterior gastric muscle is attached to the carapace at the cervical
groove. The food passes from the cardiac sac into the grinding
mill separated from the former by a valve. Here it is ground
fine and delivered into the pyloric sac. Food that is still too
coarse returns to the mill to be rewound. Parts that cannot be

HOMARUS AMERICAXUS MILNE-EDWARDS

131

ground sufficiently line pass again to the cardiac sac and are
finally vomited.

The iniestine or midgut is a thin straight tube rimning as
far back as the beginning of the sixth abdominal somite. Here
it forms a median dorsal blind sac or ccecum. The ccecimi marks
the end of the midgut. The portion of the alimentary canal
behind it is the hindgut or rectum. The anus opens ventrally
on the last somite. Connected with the intestine is a large
digestive gland, often called liver. It opens into the intestine by

Fig. 29. — Circulator}- system of the European Lobster after Gegenbaur.
From Claus-Grobben, Lehrbuch der Zoologie. C, heart; Pc, pericard;
Ac, aorta cephalica; A. ah, aorta abdominalis; As, ventral artery.

means of two short ducts between the pyloric valves. Food is
passed into the digestive gland for final digestion.

Excretory system. The excretory system of the lobster
consists of two antennal or green glands which are in reality
modified nephridia. They are more or less lentil-shaped bodies
situated in front of the stomach. Each gland consists of a blind
sac which represents a modified nephrostome, a convoluted
glandular tube, a bladder and a short duct. The latter opens on a
papilla situated on the coxopodite of the second antennae.

Circulatory system. The circulatory system of the
lobster, although highly developed, is an open system, inasmuch
as the arteries and veins open into a system of lacunae and

132 MORPHOLOGY OF INVERTEBRATE TYPES

sinuses. The heart is inclosed in a pericardium. It is situated
immediately under the carapace in the posterior region of the
latter. The heart has a peculiar shape, being flattened above,
rounded below and somewhat wider in front than behind. It is
perforated by three pairs of openings called ostia^ through which
the blood from the pericard is admitted to the heart and which
are provided with valves to prevent the return of the blood to
the pericardial sinus. One pair of the ostia is dorsal, one lateral
and one ventral. The heart gives off five arteries in front and
two behind. The anterior arteries are a single anterior aorta or
median cephalic artery which supplies the brain and eyes, two
antennal arteries which supply a number of organs, and two
hepatic arteries. Posteriorly the heart gives off the posterior
aorta or median dorsal abdominal artery which runs backward
above the intestine, giving off six pairs of lateral arteries to the
pleopods, and the sternal artery which runs down, passes through
the nerve cord and divides into two branches. The anterior
branch is called the suhneural thoracic artery and runs straight
forward under the nerve cord. The posterior branch is called the
suhneural abdominal artery and runs backward under the nerv-
ous cord. The arteries split into smaller branches, become finally
capillaries and open into lacunae. From here the blood, which is
colorless and contains amoebocytes, passes to the ventral sinus
which surrounds the nervous cord, and is driven to the gills by
the afferent branchial vessels. Having been oxidized in the capil-
laries of the gills the blood passes through the efferent branchial
vessels into the branchio-cardiac veins. There are altogether
five pairs of these veins opening into the pericardial sinus.

Respiratory system. The respiratory system of the lob-
ster consists of twenty pairs of gills. Of these six pairs are podo-
branchicB attached to the appendages of the eighth to thirteenth
somites (second maxilliped to fourth pereiopod), ten pairs are
arthrobranchicB attached to the articulating membrane of the
ninth to thirteenth somites, and four pairs pleurobranchix at-
tached to the endopleurites of the eleventh to fourteenth somites.

HOMARUS AMERICANUS MILNE-EDWARDS 133

The podobranchiae are protected by the epipodites of the same
appendage, while all branchiae are inclosed in the branchial
cavity formed on each side of the cephalothorax by the branch-
iostegites of the carapace. These two cavities are open to the
outside along the lower free edge of the carapace. The water
is kept in constant motion and driven out of the cavity by the
rhythmic action of the "bailer" which has been described above
(epipodite of the second maxilla). The gills consist of a central
stem and numerous rows of branchial filaments in which the blood
is oxidized.

Nervous system. The nervous system of the lobster is
clearly segmented inasmuch as there is typically one pair of
gangha for each somite. The ganglia of each pair are connected
with each other by transverse commissures and with the ganglia
of the following somite by longitudinal connectives. Thus the
nervous system follows the well known "ladder" type of seg-
mented invertebrates. But the high development of the lobster
is also demonstrated by the deviations from the ideal type. The
anterior three pairs of ganglia form together a more or less com-
pact brain or supraoesophageal mass, situated dorsally to the
ahmentary canal. The first pair of ganglia supplies nerves to
the compound eyes and forms the optic lobes or protocerebrum.
The second pair innerves the antennules and forms the deuter-
ocerebrum. The third pair innerves the antennae and forms the
tritocerebrum. The rest of the nervous chain is ventral to the
alimentary canal, and the fourth pair of ganglia which form the
anterior-most part of the subcesophageal mass are connected
with the tritocerebrum of the brain by two long connectives,
forming a ring around the oesophagus. The subcesophageal
mass is composed of six paired ganglia belonging to the fourth to
ninth somites. Next come five thoracic and five abdominal
paired ganglia. The ganglia situated in the sixth abdominal
somite represent the fused ganglia of the sixth abdominal and
anal somites. The connectives between the twelfth and thir-
teenth somites diverge to allow the passage of the sternal artery

134 MORPHOLOGY OF INVERTEBRATE TYPES

described above. The lobster possesses also a sympathetic or
visceral nervous system consisting of four ganglia and of nerves
given off by them. Two of the ganglia, forming a pair, are
situated at the sides of the oesophagus and are called commissural
ganglia. Of the other two ganglia one is called the oesophageal
ganglion, is median in position and single; the other is similarly
median and single, and is called the gastric ganglion. The
sympathetic nervous system supplies nerves to the alimentary
canal, heart, and other viscera.

Besides the innumerable sensory hairs which are distributed
all over the body and its appendages, the lobster has a pair of
balancing organs or statocysts and a pair of compound eyes.
A statocyst is a little sac in the coxopodite of the antennules and
communicates with the outside by means of an opening. Inside
the sac is a horseshoe-shaped sensory ridge composed of some
seventy-five plume-like hairs and about three hundred short
setce. Scattered among the hairs and setae are numerous small
statolyths. These are simply grains of sand which the lobster
introduces into the sac through the opening. This is done dur-
ing the so-called fourth larval stage. The eyes are situated at
the end of movable eye-stalks, and belong to the type of com-
pound eyes, i. e., are made up of upward of fourteen thousand
ommatidia. Each ommatidium is composed of a corneal lens,
a crystal cone formed by four cells, two pigment cells surrounding
the cone and shutting out excessively inclined rays of light,
seven retinula cells and a rhabdome or rod secreted by the latter.

Reproductive system. The sexes are separate, but the
male and female are externally very much alike. The position
of the genital openings and the structure of the first (and to
some extent of the second) pair of pleopods make, however,
the recognition of the sex simple.

Male. The openings of the sperm ducts are on the
coxopodites of the fifth pair of pereiopods (14th somite) and
are directed backward and outward. The sternite of the four-
teenth somite forms a deep V-shaped groove for the reception

HOMARUS AMERICANUS MILNE-EDWARDS 135

of the stylets. These are the first pair of pleopods specially
modified for the purpose of copulation. The endopodite of the
second pleopod has a short spur and the median spines of the
abdominal sternites are fully developed. A pair of testes, some-
times separate, sometimes united by a transverse bridge, are
situated behind the heart, partly concealed by the liver. The
sperm duct or vas deferens of each testis is a coiled up tube. Its
muscular end portion functions as a ductus ejaculatorius. The
sperm cells are provided with three stiff rays and are immobile.
They are very compHcated structures designed to "explode"
on touching the egg.

Female. The openings of the oviducts are on the coxopo-
dites of the third pair of pereiopods (12th somite) and are di-
rected backward and inward. The sternite of the fourteenth
somite is modified as a sperm receptacle and serves as an organ
of copulation and for storage of the sperm. The median spines
of the abdominal sternites are poorly developed. The first
pleopods are small and filiform, the endopodite of the second
pleopods lacks the spur present in the male. A pair of ovaries
connected by a traverse bridge occupy in the mature female
all the space on each side of the body from the stomach to the
fourth or fifth abdominal somites. The oviducts are compara-
tively short tubes. The number of eggs increases with the age
and size of the animal and reaches the total of about one hundred
thousand in females of over fifteen inches in length. In spawn-
ing the female lies on her back and as the eggs leave the oviducts
and pass by the sperm receptacle, they are fertilized; each tgg
becomes inclosed in a thin capsule; the eggs adhere to each
other and the whole batch is carried by the mother attached to
the pleopods. Development is indirect, combined with a meta-
morphosis.

Instructions

I. Examine the dorsal surface of a lobster and make a full
page drawing of it, labeling all parts mentioned below.

136 MORPHOLOGY OF INVERTEBRATE TYPES

The carapace is produced anteriorly into a sharp median
process or rostrum extending far beyond the eyes. A median
longitudinal absorption line which has an important relation to
the moulting process runs from the end of the rostrum to the
posterior edge of the carapace. Halfway between the rostrum
and the posterior edge is a transverse cervical groove between two
triangular spots. These spots represent the attachment points
of muscles the other end of which is attached to the wall of the
gill chamber; the cervical groove itself serves for the attach-
ment of the posterior gastric muscles. Beyond the triangular
spots the cervical groove continues on each side of the carapace,
ending at its anterior edge below the second pair of antennae.
Halfway between the triangular spots and the rostrum are two
oval tendon marks of tendons which bind the carapace to the
endoskeleton. Two depressions called branchiocardiac lines
run from the triangular spots toward the posterior edge of the
carapace. The region between these lines is called the areola
or cardiac region. The region in front of the cervical groove
is called the gastric region since the stomach of the lobster occu-
pies almost all the space under this region of the carapace. The
sides of the carapace, limited dorsally by the cervical and bran-
chiocardiac lines, are called the branchial regions. Since the
carapace in this region forms merely a protective covering for
the gills, the sides of the carapace are called the branchiostegites.

The abdomen is composed of seven distinct segments of which
only the tergites and the lateral lobes or pleurce are visible. The
appendages of the sixth abdominal segment form together with
the seventh abdominal segment a powerful tail-fan used in
swimming. A uropod or appendage of the sixth abdominal
segment consists of a protopodite or basal joint, a single-jointed
endopodite, and a two-jointed exopodite. The seventh abdominal
segment is called the telson.

In the position in which the lobster is being examined, several
pairs of appendages belonging to the cephalothorax are visible.
These are: first and secoiui pair of antennce, third maxillipedSy

HOMARUS AMERICANUS MILNE-EDWARDS 137

and five pairs of pereiopods. The first three pairs of pereiopods
end in a double claw or chela. The chelae of the first pair are
especially powerful. One of them, the cracker claw, is developed
more than the other which is called the toothed claw,

2. Examine the ventral surface of the lobster and make a
full page drawing of it. Identify the sex by the position of
the genital openings and by the structure of the first pair of
abdominal appendages. In the male the openings of the vasa
defer entia are situated on the inner surface of the basal joint
or coxopodite of the fifth pereiopod, and the first pair of ab-
dominal appendages are modified as organs of copulation or
stylets. In the female the openings of the oviducts are situated
on the inner surface of the coxopodite of the third pair of pereio-
pods, the fifth sternal plate forms a seminal receptacle, and the
first pair of abdominal appendages is poorly developed and
more or less filiform. Besides the appendages mentioned in
the preceding paragraph, there are visible the abdominal ap-
pendages or pleopods. Of the somites only the sternites are
visible. The telson shows the anus.

3. Take the specimen into your left hand and looking, so to
say, into the mouth of the animal, make a full-size drawing
showing the appendages surrounding it in their natural position.
The following parts are visible in this position: the first and
second pair of antennce, triangular epistome or upper lip, man-
dibles with mandibular palpi, endopodite of the second pair of
maxillipeds, endo- and exopodites of the third pair of maxillipeds.
On the basal joint of the second pair of antennae the openings
of the green glands are visible.

4. Remove tlie carapace by lifting it at the posterior edge and
cutting the muscles and tendons. The carapace will come off
with eyes and antennae attached to it. These should be re-
moved not by pulling but by cutting the joint-membrane with
sharp scissors. When the inside of the carapace has been cleaned
it will be noticed that the skeleton of the three preoral segments
remains firmly attached to the carapace. Holding the carapace

1 38 MORPHOLOGY OF INVERTEBRATE TYPES

in your left hand and looking into it from the back so that the
longitudinal axis of the body coincides with the axis of vision,
make a full size drawing showing the skeleton of the three
preoral segments. Label the articulation sockets of the eyes,
below them a more or less T-shaped sternite of the second seg-
ment separating the sockets of the first pair of antennae, and
behind this and forming together with it the epistome, the
sternite of the third segment. The lateral projections of this
sternite are hinged to the carapace and help to form the sockets
for the second pair of antennae. The two lobes in front of the
eye-sockets are the procephalic processes and serve for the at-
tachment of the anterior gastric muscles. The bridge between
them is supposedly the tergite of the first or ocular somite,
while the two sides of the oval containing the eye-sockets are
the epimeral plates of this segment. The thin bars separating
the socket of the first antenna from that of the second antenna
are the epimeral plates of the second somite. The thickened
edge of the carapace is formed by the epimeral plates of the third
somite. Attached to the base of the third sternite (and over-
hanging the mouth) is the upper lip.

5. Remove all appendages beginning with the tail-fan and
proceeding gradually forward. In doing so cut the articulation
membrane with a sharp scalpel and label every appendage for
future use. The label should be tied to the appendage and
must show the number of the segment and the side of the body,
as for example, left fourteen. This is easily done because the
telson is the twenty-first segment. The gill-bearing appendages
should be removed with the gill attached to them. There are
altogether six pairs of these so-called podobranchiae, one pair
for each somite from the eighth to the thirteenth. When all
appendages have been removed, separate the abdominal seg-
ments by cutting the membrane between them, clean and label
each segment for future study. Put the cephalothorax into a
glass beaker half filled with a 5% solution of potassium hydrate
and boil it till all tissues will be dissolved except the endophrag-

HOMARUS AMERICANUS MILNE-EDWARDS 139

mal skeleton and the stomach. Put the latter aside for future
study. Wash the skeleton in water. Place it in its natural
position, sternites down, anterior end away from you, and make
a natural-size drawing showing the entire endophragmal skeleton
viewed from above. It will be noticed that it consists of a mesh-
work of thin lamellae or apode^nes of which there are two pairs
between every pair of adjoining segments. In the position
indicated there are visible three rows of meshes. The internodes
of the median row are formed each by two horizontal plates
or mesophragmata constituting a part of the endosternites.
The walls of each mesh of the median row are formed mainly
by the endopleurites and partly by the paraphragmata or lateral
plates of the endosternites. Each paraphragma articulates
with an endopleurite. In the depth below the mesophragmata
the sternites of the exoskeleton are visible. The sternites
form the floor, the mesophragmata the roof of the sternal canal.

6. Carefully isolate one of these endophragmal segments
which have openings for pleurobranchiae by cutting its con-
nections with the adjoining segments. Make a natural size
drawing of it looking into the sternal canal. The sides of the
sternal canal are formed by the endosternites, the roof by the
mesophragmata of the endosternites, the floor by the sternite
of the exoskeleton. The endopleurites will be seen to the outside
of the endosternites. Each endopleura has a hard articulation
point for the coxopodite of the corresponding appendage.

7. Make a drawing, three-fourths view, of the abdominal
segment showing tergite, pleurae, sternite, epimeron and sockets
of pleopods. On the tergite show the portion covered by the
preceding segment and clearly separated from the rest by a
groove.

8. Make a drawing ^ of the first antenna showing the three-
jointed protopodite; the membrane covering the otocyst and the
pore leading into the otocyst on the first joint of the protopodite;

^ Drawings 8 to 16 to represent the ventral aspect of the appendages, i. e.,
showing the surface normally visible in a lobster turned on its back.

140 MORPHOLOGY OF INVERTEBRATE TYPES

the inner flagellum and the outer flagellum with chemical setae
(both flagella many-jointed).

9. Make a drawing of the seco-nd antenna showing the two-
jointed protopodite, the scale-Kke exopodite and the two-jointed
endopodite with a many-jointed flagellum; on the first joint of the
protopodite show the opening of the green gland.

10. Make a drawing of the mandible showing the protopodite
with the grinding edge, the three-jointed palpus which represents
the endopodite, the heavy adductor muscle attached by means of
a long tendon to the inner edge of the protopodite and the abduc-
tor muscle attached by a small tendon to the outer edge of the
joint.

11. Make a drawing of the first maxilla showing the two-
jointed protopodite, both joints modified as maxillary plates,
and the endopodite.

12. Make a drawing of the second maxilla showing the two-
jointed protopodite, each joint modified as a maxillary plate and
subdivided; the rudimentary endopodite; the scaphognathite or
''bailer" which drives the water out of the branchial cavity.
The anterior portion of the scaphognathite represents the
exopodite, the posterior one the epipodite.

13. Make a drawing of the first maxilliped showing the two-
jointed protopodite, both joints modified as maxillary plates,
endopodite, exopodite with many-jointed flagellum and the large
epipodite with the fold or trough at its outer edge for the bailer or
fan.

14. Make a drawing of the secoftd maxilliped showing the two-
jointed protopodite, the four-jointed ettdopodite, the exopodite
with the many-jointed flagellum, the epipodite and the rudimen-
tary gill or podobranchia. The second joint of the protopodite
shows a groove subdividing it into two not yet quite distinct
joints. Label first joint of protopodite as coxopodite, second as
baso-ischiopodite, first joint of endopodite as meropodite, next as
carpopodite, propodite, and dactylopodite.

15. Make a drawing of third maxilliped showing the two-

HOMARUS AMERICANUS MILNE-EDWARDS 141

jointed protopodite^ five- join ted endopodite, exopodite^ with flagel-
lum, epipodite and gill or podobranchia. Label coxopodite, baso-
podite, ischiopodite with comb, meropodite, carpopodite, propodite,
and dactylopodite.

16. Make a drawing of the first pereiopod with the toothed
claw showing also epipodite and podobranchia. Label all joints
and be careful to represent correctly the interlocking mechanism
between the coxo- and ischiopodite and the breaking plane be-
tween the baso- and ischiopodite. Indicate by dotted lines the
axis of every articulation for comparison with third and fifth
pereiopods.

17. Make a drawing of the third pereiopod showing all struc-
tures including epipodite, podobranchia and, if the specimen is
a female, opening of the oviduct. Label all joints and indicate
by dotted lines the axis of all articulations.

18. Do the same with the fifth pereiopod, which has an epipo-
dite, but no podobranchia. If the specimen is a male, show the
opening of the vas deferens.

19. Injected specimen. Cut away the left branchiostegite by
an incision along the branchiocardiac and left cervical line, ex-
posing the left branchial cavity. Make a natural-size drawing
showing the gills in situ. Of the outer gills the anterior six are
podobranchicB, the hindmost the pleurobranchia of the fourteenth
segment. The lamellae between the gills are the epipodites.
Protruding from under the ends of the.podobranchiae the ends
of the arthrobranchiae and pleurobranchia are visible.

20. Remove the podobranchiae and epipodites by cutting them
at their base, being careful, however, not to disturb the other
gills. This exposes all arthrobranchiae. Make a drawing of
them in situ and label anterior and posterior arthrobranchiae of
every somite.

21. Remove all arthrobranchiae. This will expose the four
pleurobranchiae, while the attachment places of the removed
gills will appear as round holes. Make a drawing and label
somites.

142 MORPHOLOGY OF INVERTEBRATE TYPES

22. Remove left half of endophragmal skeleton by cutting the
sternites and endosternites in the median line. Remove left wall
of the cephalic portion of the cephalothorax and left half of the
abdominal tergites and pleurae to the pleopods. Gradually
remove all muscles of the left half of the body in the thoracic
region and abdomen by lifting them and cutting at their at-
tachment places with scissors, and by carefully separating them
from the injected blood-vessels. Do not touch any muscles or
organs in the cephahc region. When the dissection is accom-
plished, the cephalic region will show only such structures as
lie immediately under the carapace, while the thoracic region
and abdomen will show all organs of the left half of the body ex-
cept the removed muscles. Immediately behind the cervical
groove will be seen the heart which gives o£f anteriorly a median
blood-vessel the anterior aorta (or cephalic artery) and posteriorly
the posterior aorta (or dorsal abdominal artery); the blood-
vessel running parallel with the lateral cervical groove to the
second antenna is the left antennal artery. The heavy vessel
running behind the heart down to the ventral side is the single
sternal artery. It ends in a median longitudinal ventral vessel
the forward part of which is called the anterior ventral or thoracic
artery and the hind part the posterior or ventral abdominal artery.
Immediately behind the left eye will be seen the left procephalic
lobe with the anterior gastric muscle attached by the other end
to the large stomach. The stomach is partly concealed by a
heavy muscle — the adductor of the left mandible — behind which
the posterior gastric muscle attached to the stomach and cervical
groove is visible. Under the anterior end of the antennal artery
is the flexor of the second antenna. Under the antennal artery,
occupying all the space between heart and endoskeleton and
extending all the way to the abdomen, is the left lobe of the
liver. Underneath the latter are the cut edges of the endoster-
nites which form the roof of the sternal canal. Inside that sternal
canal is the nervous chain and beneath this the ventral artery
which has been already mentioned. Extending longitudinally

HOMARUS AMERICANUS MILNE-EDWARDS 143

from the liver to the abdomen and lying on the endosternites
one sees in the depth the powerful right flexor abdominis. In
the abdomen one can see under the posterior aorta the dark
intestine clearly separated from the last section of the alimentary
canal, the rectum. If the specimen is a male, a tube will be seen
running from behind the heart to the base of the fifth leg — this
is the left vas deferens; and the testis will be found lying along-
side the intestine and partly imbedded in the liver. If the speci-
men is a female, the ovary occupies, when fully developed, all
the space from the stomach to the fourth or fifth abdominal
segment.

23. Remove the right wall of the cephalic portion, cut the
alimentary canal between oesophagus and stomach, cut the ceph-
alothorax transversely close behind the adductors of the man-
dibles, remove all organs except the green glands and nervous
system and make a drawing showing the rear view of the cut
surface. Above the oesophagus you will notice the large brain or
supraoesophageal mass and running on each side of the oesophagus
from the brain to the subcesophageal mass a long connective.
Between the muscle of the second antenna and this connective
appears one of the large green glands.

24. Open the ''hand" of the big claw by removing the entire
outer wall. Make a natural-size drawing showing the outline of
the claw, the point of articulation between the movable finger
and the hand, the large and heavy flexor and the considerably
smaller extensor.

25. Boil the stomach in a 10% solution of potassium hydrate,
wash it in water and examine first from above, then from the
side. The dorsal pouch leading into the dorsal tooth marks the
position of the gastric mill. The soft sac in front of the gastric
mill is the cardiac sac, the one behind the gastric mill the pyloric
sac. The hard transverse ossicle in front of the pouch is the
cardiac ossicle. The hard ossicle forming, as it were, a roof
over the pouch is the pyloric ossicle. The floor of the pouch is
formed by the urocardiac ossicle. The latter is articulated with

144 MORPHOLOGY OF INVERTEBRATE TYPES

the cardiac ossicle and with the prepyloric ossicle or median
tooth. The cardiac ossicle is at each end {i. e., on the right and
left) articulated with the base of the pterocardiac ossicle. To-
gether they form a sharp point. The pterocardiac ossicle is
more or less triangular in shape and its apex articulates with the
zygocardiac ossicle. The thickened edge of the latter extends to
the edge of the pouch. Here it articulates with a small triangular
ossicle — the anterior dorsolateral pyloric plate— which thus serves
to connect the zygocardiac with the pyloric ossicle. Below the
upper edge of the zygocardiac ossicle is a lateral pouch with
several dark brown notches. These are the lateral teeth which
project into the stomach and form a part of the zygocardiac
ossicle. A rod-like ossicle — the posterior intermediate cardiac bar
—runs from the lateral pouch of the zygocardiac ossicle ob-
hquely downward. At its lower end it articulates with two
closely appUed ventrolateral cardiac bars. These bars seem at first
sight to form a single ossicle and have the appearance of a curved
sabre. At the anterior end the two bars articulate with a single
anterior lateral cardiac bar which appears almost as a continua-
tion of the other two. Between the articulation point of the
zygocardiac with the pterocardiac ossicle and that of the pos-
terior intermediate cardiac bar with the ventrolateral cardiac
bars are two ossicles or intermediate cardiac bars articulating with
the small injerolateral tooth. A narrow curved ossicle— the
gastrolithic &af— runs forward from the apex of the pterocardiac
ossicles. The remaining ossicles (the number of the various
ossicles in the stomach is twenty-four and of these several are
paired) are more diflacult of observation and may be omitted.
Make a drawing of a side view of the stomach and label all parts.
26. Open the stomach in the mid-ventral Une. Make a draw-
ing of it showing cardiac sac, pyloric sac and gastric mill, con-
sisting of the median tooth, lateral teeth and small inferolateral
teeth.

SCfflSTOCERCA AMERICANA Drury

Material. Schistocerca americana or the common Amer-
ican locust is the largest grasshopper of the United States and
for that reason well adapted for dissection. Unfortunately the
dissection of the insect comes always at the wrong time of year
when live specimens are not available. It should not be dis-
sected at the beginning of the course, not only because of the
complexity of the organization, but also and chiefly because of
the impossibility of understanding its bearing upon the problem
of segmentation without some knowledge of other related forms.
The specimens should be preserved in 70% alcohol and soaked
in water for twenty-four hours before dissection. Every student
should receive two preserved specimens, one of which must be a
male, the other a female, two fresh specimens if available at the
time, and, if possible, prepared slides with sections through the
chordotonal organ, antennae, compound eyes, and various in-
ternal organs.

Descriptive Part

Schistocerca- americana, or American locust, is a typical repre-
sentative of the Order Orthoptera, Class Hexapoda or Insects.
In its structure it is much more primitive and generalized than
many other insects and shows well the metamerism of the body.

Segmentation. The body of the locust is, in all probabil-
ity, composed of twenty-one somites. The first six somites are
fused permanently together. They form the head. The next
three somites form the thorax. The abdomen shows only eleven
segments, but the eleventh segment is probably composed of
two somites.

Head and appendages. Three of the six somites of which
the head is composed are preoral in position. The first is the

145

Fi«. 30. — Meloloniha vulgaris, gen-
eral anatomy after Strauss-Diirckheim
from Mojsisovics Edlen von Mojsvar,
Leitfaden (1885). pm, palpus maxillaris;
awcf, antenna of male; an' 9, antenna

of female, represented here merely for comparison; oi, o\ facetted eyes;
gs, brain; gi, suboesophageal ganglion; g;», the three thoracic ganglia; cA,

SCHISTOCERCA AMERICANA DRURY 147

ocular somite and has no appendages. It carries the two facetted
eyes and three simple ocelli. The second somite is characterized
by the presence of a pair of many-jointed, filiform antennae.
The third somite, corresponding to the so-called intercalary
segment of the embryo, has no appendages but is sufficiently
apparent from its neuromere, the tritocerebrum, which innerves
the upper lip or labrum. The fourth somite carries a pair of
powerful toothed mandibles. The fifth somite carries a pair of
maxillcB. From a comparison of the maxillae with appendages of
crustaceans we conclude that they are composed of a two-jointed
protopodite complicated by the presence of two movable plates
used in mastication, which must be regarded as a differentiation
of the second joint of the protopodite, and an endopodite devel-
oped in the form of a palpus, while the exopodite is completely
wanting. The first joint of the protopodite is called by entomol-
ogists cardo, the second, stipes; the intermaxillary lobe of the
protopodite, lacinia, the outer, galea. The maxillary palpus is
five-jointed. The sixth somite carries the under lip or labium,
which however must be regarded as a pair of second maxillae
fused along their inner edge and slightly modified. The fused
first joints of the protopodites are known to entomologists under
the name of submentum; the fused second joints under the name
of mentum. The two lobes corresponding to the right and left
galeae of the first maxillae are called ligulce, while the laciniae are
wanting. The labial palpi are four-jointed. Projecting into the
mouth cavity from the inner surface of the mentum is a small
plate called hypo pharynx.

Except for the presence of the eyes and appendages the
somites of the head could not be recognized externally for the
reason that the skeleton of the head forms a single capsule or

chitinous endo-skeleton; oe, oesophagus; v, ventriculus; vm, vm', malpighian
tubes; i, intestine; c, colon and rectum; sph, sphincter ani; a, end of rectum;
U, testis; ds, vasa efferentia; vd, vas deferens; vs, vesicula seminalis; glm,
mucous gland; de, ductus ejaculatorius; pp, penis; vtr, tracheal sacs; tr,
tracheal tubes; c, coxa; tr, trochanter; fe, femur; //, tibia; tai-tas, tarsus;
on, claws; py, pygidium.

148 MORPHOLOGY OF INVERTEBRATE TYPES

epicranium. The front of this capsule is called frons, the back,
occiput, the top, vertex, and the two sides — cheeks or getuB. A
plate attached to the lower end of the frons and itself serving
for the attachment of the upper Up or labrum is called clypeus.
Thorax and its appendages. The thorax is composed of
three distinct somites. The first thoracic somite is called the
prothorax and carries the first pair of legs. The integument of
the prothorax forms a large dorsal sclerite or tergite usually
known under the name of pronotum. The corresponding ven-
tral sclerite or prosteruiim carries a spine. The head is joined
movably to the prothorax as the prothorax itself is joined mov-
ably to the second thoracic somite or mesothorax. The meso-
thorax is joined immovably to the third thoracic somite or meta-
thorax. This is due to the presence of a deep mcision in the
posterior margin of the mesosternum into which a corresponding
process of the metathorax fits perfectly. The posterior edge of
the latter has a similar incision for the reception of a process of
the first abdominal sternite. The mesothorax has a pair of legs
and a pair of wings which in this case are modified as covers for
the real wings of the following somite. The metathorax has a
pair of legs especially adapted to jumping and a pair of wings
which are thin and folded under the covers when not in flight.
Both meso- and metathoracic wings are simple outgrowths of
the integument and the "veins" of the wing are thickened por-
tions having the structure of a tube. The cavity of the veins
incloses special nerves and tracheal tubes and communicates
directly with the body cavity. The arrangement of the "veins"
or the so-called venation of the wings follows a definite plan
which is of great importance for a comparative study of insects.
The legs consist of five segments, the coxa by which it is attached
to the body, the trochanter — a short segment corresponding to
the ischiopodite, the femur (or meropodite) , the tibia (a fused
carpo and propodite) and a three-jointed tarsus or dactylopodite.
The ventral surface of the first joint of the tarsus is subdivided
by pads giving it the appearance as if it were composed of

SCHISTOCERCA AMERICANA DRURY 149

three joints. At the end of the tarsus are two curved claws and
between them a movable lobe, the pulvillus.

Abdomen. Generally speaking, the skeleton of an ab-
dominal somite represents a ring composed of two semicircular
sclerites connected with each other and with the preceding and
following somites by a thin cuticle. The hind edge of each ring
overlaps the anterior edge of the following ring. In the case of
the first abdominal somite, however, the tergite and the sternite
are disjointed. The tergite terminates above the third leg and
bears the chordotonal, auditory organs. The sternite has an
anterior process which fits into a corresponding incision of the
metasternum and forms a rigid juncture. The ninth and tenth
tergites of the male are partially fused and are together only as
wide as the ninth sternite. The latter has a terminal plate
attached to it known under the name of suhgenital plate. At-
tached to the posterior edge of the tenth tergite are two movable
plates, the cerci. Beneath the latter are two podical plates which
belong to the eleventh somite. The sternites of the tenth and
eleventh somites are wanting. In the female the ninth sternite
is also wanting. The last or eighth sternite has a long median
posterior process. The tenth tergite has also cerci as in the male,
but they are considerably smaller, while the triangular podical
plates of the eleventh somite are much larger. At the end of the
abdomen is an ovipositor composed of three pairs of movable
parts representing modified appendages. The median pair is
called the egg-guides and is much smaller than and partly con-
cealed by the other two pairs.

Integument. The integument consists of a single layer
of epithelial cells or hypodermis which secretes a thin supporting
membrane to the inside and a more or less thick cuticle to the
outside. The cuticle which is nothing else but the exoskeleton
is elastic only where it is very thin, i. e., between the joints,
whereas the sclerites or thickened portions of the cuticle are more
or less rigid. The animal increases in size with each moulting,
while the new cuticle is not yet hardened. The increase in size

150 MORPHor.or.v of tnvertebrati-: tvprs

of the abdomen in a gravid female is due merely to a stretching
of the interarticular cuticle to its full capacity. The grasshopper
has an eiidoskeleton in the shape of the so-called tentorium in the
head and oifurculcB in the thorax. These structures are simply
apodemes or infoldings of the integument and serve for the
attachment of muscles.

Muscular system. The muscular system shows clearly
the segmented nature of the grasshopper in the abdomen. In
the rest of the body the muscles are highly differentiated and
specialized. Of interest are the heavy muscles of the two pairs
of wings in the thorax and the muscles of the ovipositor in the
abdomen. Very powerfully developed are also the muscles of
the mandibles and those of the third pair of legs which are
adapted to jumping.

Digestive system. The alimentary canal of the grass-
hopper is clearly divided into three portions, the foregut, the
midgut, and the hindgut. The foregut begins with the mouth
which is provided with mouth parts in the shape of an upper lip,
a pair of mandibles, a pair of maxillae, and a lower lip with the
hypopharynx, all except probably the upper lip being true ap-
pendages. The pharynx occupies the greater part of the head
and leads into the oesophagus which runs backward forming a
large crop or ingluvies in the meso- and metathoracic somites.
The last section of the foregut is the gizzard or proventriculus
which in other insects is very well defined, but which in the case
of the American locust is not externally recognizable except
as the end portion of the crop. The midgut or ventriculus ex-
tends back into the seventh abdominal segment. It receives
eight double gastric coeca immediately behind the foregut.
These coeca are arranged so that one arm of each coecum is
directed forward and the other backward. The end of the midgut
is marked by numerous malpighian tubes, beyond which the
hindgut begins. The first section of the hindgut is called ileum
and is comparatively large. Near the end of the abdomen it is
constricted and the much smaller colon rises in a curve. The

SCHISTOCERCA AMERICANA DRUR Y 1 5 1

rectum is again much bigger. The anus opens between the
podical plates, dorsaliy to the ovipositor. Morphologically ter-
minal in position in all insects, the anus belongs to the twelfth
somite. A pair of salivary glands are situated one on either side
of the crop. The salivary ducts open on the ligula of the lower
lip into the mouth cavity.

Excretory system. The numerous malpighian tubes which
open into the alimentary tract between the mid- and hindgut
function as excretory organs.

Circulatory system. The heart is situated under the body
wall in the mid-dorsal line. It has the shape of a long tube and
consists of a series of ventricles typically one foi every somite,
inclosed in a pericardial sinus. Each ventricle is separated from
the preceding one by a pair of valves and communicates with
the pericardial sinus by a pair of ostia. Thus there are a pair of
ostia and a pair of valves for every somite. Anteriorly the heart
forms a short aorta which opens into the body cavity in the head.
There are no blood-vessels and the blood, which contains amce-
bocytes, circulates freely in the body cavity before it returns
to the heart. Metamerically arranged triangular muscles serve
to enlarge the pericardial sinus and to draw blood into it. The
blood current in the heart is always from the rear end forward.

Respiratory system. The American locust, together with
all other air-breathing insects, possesses a compHcated system of
tracheal tubes communicating with the outside by a series of
paired openings. These openings or spiracles {stigmata) are
twenty in number. One pair belongs to the mesothorax, one
pair to the metathorax, and the rest to the first eight abdominal
somites. In the thorax they are situated in the pleura, dorsal
to the places of attachment of the legs; in the abdomen on the
terga not far from their lateral edge, one on each side. The
spiracles lead into short tubes, opening into two longitudinal
trunks. Each trunk gives off numerous branches which pene-
trate between the cells of all organs. The smallest branches
end blindly in a terminal cell. The two trunks communicate

152 MORPHOLOGY OF INVERTEBRATE TYPES

with each other by transverse anastomoses. The microscopic
structure of a tracheal tube is very characteristic. It consists
of a single layer of cells and a spiral intima or cuticle.

Fat body. All the space between the various organs is
occupied by the fat body composed of very large cells. The
function of the fat body is not yet quite understood, but it is
known that it retains salts of the uric acid and that the quan-
tity of these salts increases with age.

Nervous system. The nervous system of the locust be-
longs to the ladder or chain type. The brain or supraoesopha-
geal mass is situated in the head above the oesophagus and is
composed of three neuromeres (three pairs of ganglia). The first
pair or the protocerebron gives off nerves to the eyes and oceUi,
the second or deuterocerebrojt to the antennae, the third or trito-
cerebron to the upper lip. The brain sends out two connectives
to the suboesophageal mass, thus forming a ring around the
oesophagus. The suboesophageal mass is also composed of three
neuromeres (three pairs of ganglia) with nerves for the postoral
mouth appendages, i. e., mandibles, maxillae and under lip. The
following two pairs of ganglia belong to the prothorax and me-
sothorax. The paired gangHon situated in the metathorax
represents the fused ganglia of this and the first abdominal
somite. The ganglia of the second and third abdominal somites
are also fused into one mass and are situated in the second ab-
dominal somite. The fourth, fifth and sixth somites have a
paired ganglion each, but the ganghon situated in the seventh
somite undoubtedly represents the result of a fusion of all the
remaining ganglia (seven to eleventh neuromeres). There is a
highly developed sympathetic or visceral nervous system connected
with the brain and supplying ner\'es to various organs.

Sense organs. Like all insects the locust is a veritable
storehouse of sense organs. The olfactory sense is mostly re-
stricted to the antennae,^ the auditory sense has for its seat the

^ Mclndoo's obser\ations on what he considers to be the true olfactory
organs in insects still need a great deal of additional proof.

ncr. slomod. p:

ner. $eni.l<ibr,

ner.mot.labr.
. _ner. sens. labr. pha.
•Tier. antn. chord.

-ner. antn,. sens. I.

•Tier. antn. sens. 9.
- -ner. antn. mot. fun.

ner. antn. mot. $ca.

ner. dil. inf. pha.

ner.eru proth.

Fig. 31. — Diagram of the nervous system of an insect after Janet (1905),

154 MORPHOLOGY OK INVERTEBRATE TYPES

tympanal organs of the first abdominal somite, while the two
facetted and three simple eyes are situated on the head. The
sense of touch is highly developed in the antennae, while nerve
endings for the perception of taste are found chiefly on the hy-
popharynx. We shall consider only the eyes and the tympanal
or auditory organs. The facetted or compound eyes are composed
of a great number of ominatidia. An ommatidium is composed
of a chitinous cornea, a crystal cone, pigment cells which isolate
one ommatidium from another and prevent reflections of rays
forming too great an angle with the axis, and finally the retinula
with a central rod or rhabdome. The image formed by each
ommatidium correspond to a small portion of the horizon only,
and the actual image is a composite of the images formed by all
facets. The retina is direct.

The simple eyes or ocelli are composed of a lens formed by the
cornea, a vitreous body which is rather poorly developed, and a
direct retina. The opinion has been recently advanced that the
function of the ocelli is to produce stereoscopic vision in con-
junction with the compound eyes, but this opinion is far from
being generally accepted.

The tympanal or auditory organs belong to the type of so-called
chordotonal organs of which there are many varieties in insects.
They are situated on the first abdominal tergite and are rather
large. In typical chordotonal organs the swinging or vibrating
element is furnished by a tendon, while in a tympanal organ it
is a mtTuhidint-tympanum, stretched within an almost circu-
lar chitinous wall or rim. The nervous apparatus of the organ
is very complicated and apparently allows of a fine perception
of various sounds.

Reproductive system. The sexes are separate and easily
recognizable owing to the presence of the ovipositor in the fe-
male. The female has two ovaries composed of a number of
tubes which sit in a row on the corresponding oviduct. The
two oviducts unite in a short vagina which opens between the
ovipositor on the subgenital plate (eighth sternite). A sperm

SCHISTOCERCA AMERICANA DRURY 155

receptacle with a long duct has a separate opening dorsal to the
former. The male has two testes with two vasa deferentia uniting
in an ejaculatory duct which opens on the dorsal surface of the
subgential plate. A number of very fine tubes open into the
vasa deferentia near their junction and serve as seminal vesicles.
Development is combined with an incomplete metamorphosis.
The immature stage is sometimes called nymph and is in many
respects like the adult, but lacks the wings.

Instructions

1. Place a specimen in a white dish with water on its right
side and make a drawing twice natural size showing the left
side. In this position the parts enumerated below will be visible
and should be labeled. Head: frons, clypeus, labrum, vertex,
left gena, below it and limited in front by the clypeus and labrum,
the left mandible; behind the mandible the left maxillary palpus;
above the gena the left compound eye, in front of the eye the
left antenna and above the latter and close to the compound
eye left the ocellus or single eye. Thorax: prothorax including
pronotum, prosternum, prosternal spine and left prothoracic
leg,^ mesothorax including mesothoracic wing, left pleuron, left
mesothoracic leg and mesosternum (mesonotum and mesotho-
racic spiracle are not visible); metathorax including pleuron,
metasternum, left metathoracic leg, and metathoracic spiracle
(metathoracic wing and metanotum not visible). Abdomen:
as many segments as are not concealed by the wings, showing
tergite, sternite and spiracle of every segment.

2. Place the specimen on its back in a dissecting tray, spread
the wings at right angles to the body and pin them down.
Make a drawing showing the ventral aspect of the animal. Label
antennae, frons, eyes, cl3T)eus, labrum, mandibles, maxillary
palpi, labium, labial palpi, prosternum with spine, prosternal

1 On the prothoracic leg label coxa, trochanter, femur, tibia and tarsus
with claws and pulvillus.

156 MORPHOLOGY OF LNVERTEBRATE TYPES

legs, mesosternum, mesothoracic legs and wings, metasternum,
metathoracic legs and wings, first to eighth abdominal sternites.
In the male the ninth sternite with the terminal subgenital
plate attached to it and the cerci; in the female the ventral
division of the ovipositor.

3. Make a drawing showing the left side view of the abdomen
of a female from the division line between the seventh and eighth
segments backwards. Label eighth, ninth, tenth and eleventh
tergites, eighth sternite, cerci, podical plates and the three
left divisions of the ovipositor (dorsal, ventral and egg-guide).

4. With the aid of a forceps remove the upper lip or labrum,
both mandibles, both maxillae, under lip or labium and hypo-
pharynx. Place them in their natural sequence and make a
drawing showing all parts. Label on the maxillae cardo or first
joint of protopodite, stipes or second joint of protopodite, lacinia
or inner maxillary plate, galea or outer maxillary plate and the
five- jointed maxillary palpi. On the labium label submentum,
mentum, hgula, and four-jointed labial palpi.

5. Make a drawing showing the side view of the metathorax
and first and second abdominal somites with wings raised to
expose the auditory organ.

6. Cut out with sharp scissors the left auditory organ together
with a piece of adjoining integument, remove all adhering tracheal
tubes, wash and examine its inner surface in water through the
dissecting microscope. Make a drawing showing the thickened
edge of the organ, the tympanum or membrane, the auditory
nerve, the ganghon.

7. Female. With the aid of fine scissors make an incision
along the ventral surface of the animal parallel to and slightly to
the left of the mid- ventral line. The incision should extend from
the ovipositor to the head. Make a similar incision parallel
to and slightly to the left of the mid-dorsal line, and remove the
left body wall. Remove in the same way the left side of the head
capsule. Make a drawing showing the organs in situ. Label (i)
alimentary canal in which the following parts may be recognized:

SCHISTOCERCA AMERICANA DRURY 157

crop, salivary glands, coeca, ventriculus, ileum (the rest of the
hindgut is covered up by the reproductive organs); (2) mal-
pighian tubes opening into the alimentary canal, marking the
end of the midgut; (3) nervous system: brain above the oesoph-
agus in the head and ventral chain below the alimentary canal
with three thoracic and five abdominal gangha; (4) heart, if
not destroyed by preserving process, will be seen in the mid-
dorsal line of abdomen above all the other organs; (5) ovarial
tubes, occupying all the space between the heart and the ali-
mentary canal in abdomen; (6) muscular system: muscles of the
fore and hind wings in the thorax and the muscles of the ovi-
positor in the abdomen; (7) endoskeleton arising from the stern-
ite of the thorax.
Additional exercises.

(a) Take a fresh specimen and remove the dorsal body wall,
including wings, by an incision along each side of the body just
above the line of the spiracles. Examine the excised wall for
the heart and make a drawing of this. Fasten down the ventral
body wall with pins in the dissecting tray with water and care-
fully push the various organs apart allowing the alimentary
canal to lie on the left and the reproductive organs on the right.
This separation, if carefully performed, will expose the entire
alimentary canal, nervous system and reproductive organs.
Make a full page drawing and label parts from the text in the
general description.

(b) Examine under microscope the sections through the
various regions of the alimentary canal and make drawings of
them.

(c) Examine under microscope the section through the au-
ditory organ and make a drawing of it.

(d) Examine under microscope a fresh tracheal tube (in
water) and make a drawing of it.

(e) Examine under microscope the sHde with the stained
ovarial tubes and make a drawing of it.

AGELENA N^EVIA Walckenaer

Material. Agelena naevia is the "common grass-spider"
and may be collected in great quantities during July and August.
Specimens should be preserved in 70% alcohol. Stronger alcohol
is injurious, formalin ruins the specimens completely. Fer-
tilized females will deposit eggs in a cocoon in confinement. The
spiderlings should be fixed in my sublimate mixture. Every
student should receive a mature female, a mature male, an im-
mature specimen, a series of prepared slides with transverse sec-
tions through a spiderling and a prepared slide with median
longitudinal section.

Descriptive Part

Agelena naevia is a very common spider inhabiting the United
States. It belongs to the Order Araneae, Class Arachnida, of
which it is a t}^ical representative. Meadows are often covered
with the webs of Agelena, which glisten like silver in the morning
dew of autumn. They have the shape of a broad sheet extending
into a long funnel in the depth of which the spider hides waiting
for its prey.

External features and segmentation. The body of
Agelena shows a clear separation into an anterior portion or
cephalothorax and a posterior portion or abdomen. The latter is
joined to the cephalothorax by a very thin stalk or pedicel. With
the loss of external and to some extent even of the internal seg-
mentation, the differentiation of the integument into sclerites
has been also considerably impaired, if not completely oblit-
erated. The tergites of the cephalothorax are fused together,
forming a single shield or carapace. A cephalic groove with sulci

158

i6o MORPHOLOGY dlMrm^ERTEBRATE TYPES '

running from it obliquely forward jnarks the limit between the
head or cephalon and thorax. This groove is an infolding of the
integument, serving for the attachment of the gastric muscles
and corresponds evidently to the cervical groove of the lobster.
The thoracic stdci running toward the lateral margins of the
carapace still represent the last traces of the external segmenta-
tion. At the anterior end of the carapace are eight beadlike
eyes. The portion of the carapace between the eyes and chelicera
is called clypeus. On the ventral surface of the cephalothorax
between the coxae of the limbs there is a plate called sternum.
It represents the fused sternites of the five thoracic somites.
Attached to the anterior edge of the sternum is a movable lower
lip. The latter protects the mouth from below as the upper lip
or rostrum does from above. Between the rostrum and the
clypeus edge are two powerful chelicera.

The pedicel which binds the abdomen to the cephalothorax
has a dorsal sclerite of peculiar shape. It is called lorum and
represents the tergite of the first abdominal somite.

The abdomen has the shape of an elongated egg. It terminates
in an anal tubercle in front of which and ventral in position are
six spinnerets. Immediately in front of the anterior spinnerets
is a small spiracle or opening leading into the tracheal tubes.
Not far from the anterior edge of the ventral surface is a trans-
verse genital fold. At the ends of the fold are two lung-slits
leading into the book-lungs. The integuments of the abdomen
are soft and do not show any traces of segmentation.

It is clear that in order to understand segmentation in Agelena,
one must seek other evidence than that afforded by the structure
of the exoskeleton. Here the anatomy of internal organs, com-
parative anatomy and embryology come to our aid. Some
Arachnida, such as scorpions and Solifugae, possess a segmented
abdomen. The nervous system retains even in adult spiders its
composition of distinct neuromeres, and the embryo shows a
transitory external segmentation of the abdomen which is even
supplied with appendages. With all that the opinion as to the

AGELENA N^VIA WALCKEN^R i6i

number of somites which enter into the formation of the arachnid
body, is still divided. Some maintain that the portion of the
head situated in front of the mouth is composed of two somites
only. In this case the chelicera would be homologous with the
antennules of the lobster. A more generally accepted opinion
is based on the assumption that the chelicera are homologous
with the antennae of the lobster, and that the preoral region is
composed of three somites. We, for our part, will assume that
four somites lie in front of the mouth and that the entire body of
an arachnid is composed of twenty-two somites, i. e., as many as
in the crustacean Nebalia and one more than in the lobster.
According to this interpretation the chelicera are homologous
with the mandibles of insects and crustaceans. They are postoral
in the embryo, but become preoral early in the development.
The twenty- two somites are distributed as follows: five belong
to the head, four to the thorax, and thirteen to the abdomen.
The four preoral somites of the head are (i) the first ocular
somite, characterized by the presence of the four lateral and
two posterior median eyes; (2) the second ocular somite, char-
acterized by the presence of the two anterior median eyes; (3) an
embryonic, evanescent segment with rudimentary appendages
which disappear completely; (4) the cheliceral somite with
chelicera for appendages. The fifth cephalic somite is the first
postoral somite. It has the pedipalpi for appendages. The four
thoracic somites have a pair of legs each. The abdomen in the
embryo of Agelena is divided into ten segments, the first six
of which have a pair of embryonic appendages each. The first
to ninth segment represent as many somites, but the tenth seg-
ment evidently corresponds to the last four segments of the
abdomen in the scorpion and is therefore the result of a fusion
of four somites. The embryonic appendages of the first, second,
third and sixth abdominal somites disappear, those of the fourth
and fifth become spinnerets. The first abdominal somite which
disappears completely in the scorpion, forms the pedicel in
Agelena. The pair of lungs and the genital opening belong to

1 62 MORPHOLOGY OF INVERTEBRATE TYPES

the second abdominal somite. The spiracle belongs to the third
somite and is situated at its posterior margin. The six embryonic
segments situated behind the spinnerets and representing the
sixth to thirteenth somites become quite rudimentary, fuse to-
gether and form the small anal tubercle at the end of which the
anus is situated.

Appendages. Agelena has two pairs of cephalic, four
pairs of thoracic, and two pairs of abdominal appendages. The
first pair of cephalic appendages is called the chelicera. They
are used for grasping the prey, for holding it during the process
of pre-digestion, and, in case of need, for defence. They are
two-jointed. The basal joint has more or less the shape of a
pyramid, the terminal joint or fang is thin, pointed, and as hard
as steel. When " closed " the fangs rest between the two margins
of the basal joints. Both joints move in a transverse plane which
is characteristic of all the true or two-lunged spiders. Under the
tip of the fang is the opening of the poison duct. The second pair
of cephalic appendages is called pedipalpi. They are six-jointed.
The first joint or coxa has a maxillary lobe or lamina. It does not
seem probable that the laminae serve either for masticating or
passing the food to the mouth. The coxa of the pedipalp in
tarantulas which ''chew" their food, has no lamina, while all
true spiders do not ''chew" but "suck" their prey. It is more
likely that the laminae are tactile organs. Their connection with
the process of predigestion will be considered later. The re-
maining five joints of the pedipalp form the feeler or palpus.
The terminal joint is simple in the female, resembles the terminal
joints of the legs and has a claw. In the male, on the other hand,
the terminal joint is modified as an organ of copulation. Its
structure is very complicated and may be treated here only in a
general way. The dorsal wall has more or less the shape of a
trough which serv^es for the protection of the copulatory ap-
paratus. The latter is the product of the ventral wall of the
terminal joint. It consists of a sperm reservoir inclosed in a thin
sac called hcematodocha, an e^nbolus or intromittent organ in the

AGELENA N^VIA WALCKEN^R 163

shape of a spiral tube, a conductor and various hard, chitinous
additional structures. The swelling of the ha^matodocha causes
the protrusion of the entire apparatus during copulation when
the embolus is introduced into the sperm receptacle of the
female.

The four pairs of thoracic appendages are seven-jointed legs.
Two of them are directed forward and two backward, so that
the inner surface of the former corresponds to the outer surface
of the latter, and vice versa. The first joint which serves for
the attachment of the leg to the cephalothorax is called coxa or
coxopodite, the second — trochanter or basopodite, the third — femur
or meropodite, the fourth — patella or carpopodite, the fifth — tibia
or propodite, the sixth — metatarsus or first dactylopodite, the
seventh — tarsus or second dactylopodite. Some of the joints have
spineSy the number and position of which furnish excellent sys-
tematic characters. At the end of the tarsus are two serrated
upper claws and a serrated median or lower claw. The first pair
of spinnerets called anterior or lower spinnerets are two-jointed
appendages of the fourth abdominal somite. It is probable
that they are homologous with the exopodite of a pleopod.
Their terminal joint is hemispherical and has comparatively
few spinning tubes. The appendages of the fifth abdominal
somite are the remaining four of the six spinnerets present in the
majority of spiders. They are biramous pleopods in which the
protopodite disappeared, the exopodite became the two-jointed
posterior or upper spinneret, while the endopodite is represented
by the much smaller, single-jointed median spinneret. The
terminal joint of the posterior spinnerets is long and thin, and
this joint as well as the median spinneret are the seat of numerous
spinning tubes.

Integument. The integument consists of a chitinous
cuticle produced by the hypodermis. The latter is composed
of a single layer of epithelial cells. A supporting membrane is
produced by the hypodermis on its inner surface. The chitinous
cuticle forming the carapace, sternum and segments of the ap-

1 64 MORPHOLOGY OF INVERTEBRATE TYPES

pendages is very hard. The articulation membranes are on
the other hand quite soft and elastic. The integument of the
abdomen is considerably thicker than that of the intersegmental
membranes, but is still elastic as is clearly demonstrated by
the considerable stretching of the abdomen under the pressure
of the growing ovaries. The entire body, excepting the articu-
lation membranes, is covered with hair. This is of at least two
types. One is long, serrated, but not branched; the other is
shorter, plumose. Every hair is the outgrowth of a single hypo-
dermal cell and is supplied with a nerve fibre. Thus the whole
surface of the body is very sensitive to touch or to air-currents
and vibrations. The spines as well as the claws and the spinning
tubes are also modified hairs. The spines sit in special sockets
and may be considerably lifted from their normal position in
which they practically lie on the surface of the limb. A new
cuticle with all its hair is produced underneath the old one and
takes the place of the latter after each moulting.

Endo skeleton. An endoskeleton is present in both
cephalothorax and abdomen. In the latter it consists of three
rudimentary endosternites serving for the attachment of muscles.
The endoskeleton of the cephalothorax is of a very peculiar
shape. It consists of a central, saddle-like plate situated between
the thoracic ganglionic mass and the ring of intestinal coeca,
and of a number of processes. Some of these processes serve
for the attachment of muscles which hold the endoskeleton as
it were suspended horizontally, others for the attachment of
muscles inserted in the coxae of the limbs.

Muscular system. The muscular system is naturally
very complicated and highly specialized. It still retains traces
of metamerization in the abdomen and thorax. Of special in-
terest are the muscles of the pumping stomach. The single
dorsal one is attached to the fold of the cephalic groove, the
two ventral muscles to processes of the endoskeleton. Very
powerful are the muscles moving the chelicera. The muscular
system of the legs and pedipalpi is characterized by the absence

AGELENA N^VIA WALCKEN^R 165

of extensors, their function being performed by the elastic inter-
articular membranes.

Glands. The most important glands are the poison
glands and the spinning glands. The poison glands are situated
in the head and occupy a large portion of its cavity. There are
two of them, one for each chelicer. The glands are sausage-
shaped, with a spiral muscle in their wall. The duct runs be-
tween the flexor and extensor of the fang and terminates in a
small opening under the tip of the latter. The action seems to
be voluntary, the spider using the poison only in self-defence.
The numerous spinning glands are situated in the abdomen.
A separate duct leads from each gland to a spinning tube on the
spinnerets. The silk is a secretion which hardens from contact
with the air. Two coxal glands of uncertain function are sit-
uated in the cephalothorax. Their ducts open between the first
and second pair of legs. It is probable that all arachnids pos-
sessed once a pair of openings at the base of all four pairs of legs.
The coxal glands are modified nephridia with a blind end. A
group of pedipalpal glands is contained in the maxillary lobe of
the pedipalpi. Each gland has a separate opening. The secre-
tion has a peptonizing ferment and is used in predigestion.

Digestive system. The digestive system of Agelena, as
that of all spiders, is peculiar in many respects. The mouth opens
ventrally so that the upper lip sits really in front of it and the
lower Up behind it. The maxillary plates of the pedipalpi are
at the sides of the lower lip and not in front of it as is the case
with the maxillae in insects. The foregut is clearly divided into
a pharynx, oesophagus and pumping stomach. The pharynx
has two chitinous plates in its intima provided with special
muscles. It leads directly upward and its posterior end is con-
siderably thicker than the oesophagus. The latter is a cylindrical
curved tube Hned with chitin. It opens into the pumping
stomach, the wall of which has three chitinous plates and works
as a pump. The upper chitinous plate serves for the attachment
of the dorsal dilatator, the ventrolateral plates for that of the

1 66 MORPHOLOGY OF INVERTEBRATE TYPES

ventral dilatatores. The contraction of these muscles widens
the lumen of the stomach. Special circular constricting fibres
reduce the volume of the stomach and force its contents into
the intestine. This section of the alimentary canal represents
the midgut and extends backward through the pedicel into the
abdomen as a rather wide and slightly curved tube. The in-
testine is connected with a so-called ccecal ring in the cephalo-
thorax. This ring is formed by two blind processes growing
out of the midgut at its juncture place with the pumping stom-
ach. Later the two processes unite in front and give off, besides,
bhnd pouches, one pair of which extends forward from the an-
terior edge of the ring, a single one — backward from the upper
wall of the posterior edge of the ring, and four lateral pairs
which end bhndly in the coxae of the legs. Within the abdomen
the intestine forms four pairs of highly branched and anastomos-
ing hitestinal diverticles which are imbedded in a mass of adipose
tissue. For a long time this tissue has been described as "liver,"
owing to the fact that it binds the diverticles of the intestine
firmly together and has the appearance of a large gland. It
forms two lobes separated by the heart and extending almost
to the mid- ventral line and incloses all the organs. The adipose
tissue makes the dissection of spiders very difficult. The hind-
gut forms a large stercoral pocket into which the malpighian
tubes open. The rectum is short and terminates in an anus on
the anal tubercle. Salivary glands are absent. The process of
feeding in Agelena is very peculiar and characteristic of true
spiders. It is combined with predigestion of the food outside
of the alimentary canal. When the fangs have pierced the body
wall of the victim, a drop appears between the tips of the spider's
maxillary lobes. This drop is supposed to be the secretion of
the pedipalpal glands, but an admixture of intestinal juice is
probable. It peptonizes all organs and tissues of the victim.
The fluid is drawn into the alimentary canal by a combined suck-
ing action of the pharynx and pumping stomach. Resorption
takes place in all portions of the midgut.

AGELENA N^VIA WALCKEN/ER 167

Excretory system. Two malpighian lubes function as organs
of excretion. They are situated in the abdomen, partly imbedded
in the adipose tissue, and open into the stercoral pocket of the
hindgut. It is probable that the coxal glands, too, have an ex-
cretory function.

Circulatory system. Agelena has an open circulatory
system in which the arteries open directly into the body cavity.
The hearl is a long tube with muscular walls, situated in the mid-
dorsal line of the abdomen, directly under the body wall, be-
tween the two lobes of the adipose tissue. It is inclosed in a
pericardium and provided with three pairs of lateral ostia
through which the blood enters. Anteriorly the heart becomes
constricted and forms the anterior aorta which runs above the
intestine, enters the cephalothorax and divides into two arteries
in the region of the pumping stomach. These arteries subdivide
and supply the appendages and the organs inclosed in the ceph-
alothorax. Posteriorly the heart gives off the posterior aorta
which extends almost to the anus. The blood which is colorless
and contains amoebocytes, reaches the lungs by way of lacunae,
while a portion of it is oxidized through the tracheal tubes. From
the lungs the blood returns to the heart by way of two pulmonary
sinuses which open into the pericard at the points nearest to
the anterior ostia. The blood oxidized through the tracheal
tubes returns to the heart through the four posterior ostia. It
is probable that the heart does not contain fully oxidized blood.
The rhythmic pulsation of the heart drives part of the blood
forward and part backward into the posterior aorta.

Respiratory system. The respiratory system of Agelena
consists of a pair of lung-books and of tracheal tubes. The
lung-hooks are modified and infolded appendages of the second
abdominal somite. They are situated in front of the genital
groove in little pockets of the integument. The opening leading
into the pocket is called the stigma or spiracle and is situated at
the rear end of the pocket. The lung itself consists of numerous
lamellce attached to the anterior end of the lining of the pocket

l68 MORPHOLOGY OF INVERTEBRATE TYPES

and comparable to the leaves of a book. The air circulates be-
tween the lamellae, while the blood is oxidized inside of the
lamella3. The system of tracheal tubes is quite separate from the
lung-books. The single tracheal spiracle is situated on the ven-
tral surface of the abdomen, in front of the spinnerets. The
common stem splits very soon into two branched tubes pene-
trating among the cells of the various organs.

Nervous system and sense organs. The central nervous
system of Agelena is characterized by a great concentration of
ganglia. The brain or supracesophageal mass is usually supposed
to consist of only two neuromeres, the protocerebron and the
deuterocerebron. But it is much more plausible that at least
four neuromeres compose the brain. The first neuromere is
represented by the first optic ganglia and supplies nerves to the
four lateral and the posterior median eyes. The second neuro-
mere is formed by the second optic ganglia and supplies nerves
to the anterior median eyes. The third neuromere apparently
disappears during development, while the fourth neuromere
supplies nerves to the chelicera. The brain is united to the
suboesophageal mass by short connectives forming a ring for the
passage of the oesophagus. The suboesophageal mass is situated
immediately above the sternum and below the endoskeleton,
so that a puncture of the sternum involves usually also a punc-
ture of the suboesophageal mass and causes paralysis of the
animal. The mass is composed of the last cephalic ganglion,
four thoracic ganglia, and at least three abdominal ganglia
(four abdominal ganglia in the scorpion). It supplies nerves to
the under lip, pedipalpi and legs. Posteriorly the mass is drawn
out into a long stem ending in another ganglionic mass in the
pedicel. This latter mass is quite small compared with the
thoracic mass and represents the fused and greatly reduced
remaining ganglia of the abdomen.

The best developed sense in Agelena is that of touch. Every
hair, bristle and spine is supplied with a nerve fibre, with the
result that the slightest touch or the gentlest vibration of the

AGELENA N.EVIA WALCKEN^R 169

web is at once perceived by the animal. Some of the hair on the
appendages is exceptionally fine and erect, sitting in the centre
of a small, disc-like or hemispherical membrane. This hair is
called trichobothria, and is supposed to convey the impressions of
air vibrations or sound. Yet real sense of hearing is wanting
in all spiders. An olfactory function has been ascribed to special,
so-called lyriform organs appearing as minute slits on the legs
near the articulations and elsewhere in the integument. But
only strongest odors, such as emitted by essential oils or acetic
acid, can be perceived and even these imperfectly. The eyes
are fairly well developed. They are situated on the carapace,
eight in number, and belong to the type of simple eyes or ocelli.
The eyes form two strongly procurved rows and are designated
in systematics as anterior middle, anterior lateral, posterior
middle, and posterior lateral eyes. But in reality the lateral
eyes of the anterior row belong together with the eyes of the
posterior row to the first somite, while the anterior median eyes
alone belong to the second somite. Moreover, the anterior
middle eyes belong to a different type than the other six eyes,
inasmuch as they possess a direct retina, while the others have an
inverted retina. An ocellus is typically composed of a chitinous
corneal lens, a vitreous or glass body and a retina. A ring of pig-
ment under the lens serves as an iris. The retina of the inverted
eyes is provided with a tapetum behind the rods. The images
formed by the eyes are very sharp, but the acuity is very slight,
so that the spider is not able to see much detail.

Reproductive system. The sexes are separate and may be
easily recognized. The legs of the male are much longer in pro-
portion to its carapace, while the abdomen of the female is con-
siderably distended by the growing ovaries. At the same time
the modified and enlarged terminal joint of the pedipalpi in the
male makes the recognition quite simple.

(a) Female. Two ovaries occupy almost all the space below
the intestine in the abdomen. Each forms anteriorly a short ovi-
duct, the two oviducts uniting in a still shorter uterus. The

)

lyo

MORPHOLOGY OF INVERTEBRATE TYPES

latter terminates in a wide, slit-shaped genital opening situated
in the genital groove on the ventral surface of the abdomen. It
belongs to the second abdominal somite. Above the genital
opening is a chitinous structure called epigynum with two open-

u.

iRs.

ov.^.J

Fig. 33. — Female reproductive organs of a spider, modified from V. v.
Engelhardt. ov, ovary; 0, oviduct; u, uterus; co, copulation opening; cd,
copulation duct; gp, genital opening; F, fertilization canal; I, II, III Rs,
first to third sperm receptacles.

ings leading into as many sperm receptacles. Thus there are alto-
gether three sexual openings. K fertilization canal connects each
receptacle with the uterus. Only the two openings leading into
the receptacles are used for copulation. The sperm is stored
for a considerable time in the receptacles. As the eggs pass the
openings of the fertiUzation canals in the uterus, they are

AGELENA-N^VIA WALCKEN^R 171

fertilized one by one and appear in the median genital
opening.

(b) Male. Two testes in the shape of long and thin coiled
tubes are situated in the abdomen under the intestine. They
continue as vasa deferentia and unite shortly before their joint
genital opening. The latter occupies the same position as the
corresponding opening of the female — in the genital groove.
The copulatory apparatus of the male has no connection what-
ever with the sexual organs producing the sperm, and is formed
by the modified last joint of the pedipalpi, as described above.
Thus it happens that Agelena, like all other spiders, has two
organs of copulation corresponding to the two openings of the
sperm receptacles in the female. When the male becomes ma-
ture with the last moult, he weaves a small "sperm" web into
which he ejects the sperm through its genital opening and pumps
it then into the sperm reservoirs of his palpi. If now the male
finds a mature female, he introduces the embolus of one pedipalp
into one of the receptacles of the female and fills it with sperm.
After a few minutes the other embolus is introduced into the sec-
ond receptacle and copulation is finished.

Development. The fertilized eggs are laid into a specially
prepared sheet which, when filled, is formed into a closed cocoon.
Development is direct. The spiderlings resemble their parents.
Maturity comes only with the last moult, before which the
spiders are unable to reproduce. The life cycle occupies an
entire year.

Instructions

I. Place a mature female in its natural position in a dish
with alcohol and examine in reflected light under dissecting
microscope. Make a half page drawing of the dorsal surface.
Label cephalothorax and abdomen. On the former label cara-
pace, cephalic groove, thoracic sulci and eyes. At the end of
the abdomen label the upper spinnerets. The heart is visible

172 MORrilOLOGY OF IXVERTEBRATK TYPES

in some specimens in the mid-dorsal line under the integument
in the abdomen.

2. Turn the specimen on its back and make a half page
drawing showing the ventral surface. Label chelicera with
fangs, pedipalpi with maxillary lobes, lower lip, sternum, legs,
pedicel, lung-books, genital fold with genital opening in middle
and a stigma at each end, epigynum, anterior or lower spin-
nerets, posterior or upper spinnerets, and anal tubercle. The
tracheal spiracle is difficult of detection, but may be recognized
by the direction of the hair surrounding it. The median spin-
nerets are not visible. Label the somites with the aid of the
description given on a previous page. Label all joints of a
pedipalp and of a leg.

3. Take the spider with your left hand and examine it through
a magnifying lens from in front. Make a drawing showing out-
line of carapace and chelicera. Label ch^jeus, anterior and pos-
terior rows of eyes ; represent the shape of each row by a dotted
line passing through the centres of the eyes. This drawing
will represent the relation of characters as used by systematists.

4. Repeat the same drawing omitting the dotted lines. Make
a single dotted line between the eyes, dividing them into two
groups corresponding with the somites. The second somite has
only one pair of eyes. Label somites.

5. Remove two terminal joints of a leg, boil them in 20%
potassium hydrate, wash in water, place on a slide in a drop of
glycerine and examine under cover glass through the microscope
(200 diameters). Make a drawing showing claws.

6. Examine the joints under high power (400 diameters), find
a common hair, a plumose hair, a spine and a trichobothrium.
Make drawings of them and label.

7. Boil the rest of the leg in 20% potassium hydrate and find
the lyriform organs. Make a drawing showing them.

8. Remove the spinnerets together with adjoining tissue and
treat in the same manner. Examine under low power and make
a drawing showing all six spinnerets.

AGELENA N/EVIA WALCKEN^R 173

9. Examine the terminal joint of an upper spinneret under
high power (400 diameters). Make a drawing showing the
spinning tubes among the hair.

10. Remove the epigynum with adjoining tissue, boil it in
20% potassium hydrate, etc.; and examine it under high power.
Make a drawing showing outHne of chitinous structures and
both receptacles.

11. Remove two terminal joints of an immature spider, treat
them in the same manner and examine under high power. Make
a drawing showing the old claws and the new claws still inclosed
in the tarsus.

12. Remove one of the pedipalpi of a mature male, treat it
in the same manner, and make a drawing showing the terminal
joint with the copulatory apparatus. Label the long, spiral
embolus and the soft haematodocha.

13. Examine under microscope the series of prepared trans-
verse sections through a spiderling. Make at least four draw-
ings, one through the region of the eyes, one through the
pumping stomach, one through the lungs and one through the
abdomen in front of the spinnerets. Label all organs.

14. Examine under microscope the prepared slide with a series
of sagittal sections through a spiderling. Choose the section
showing both heart and nervous system. Make a drawing
showing all organs. Label also all neuromeres in accordance
with the somites.

ASTERIAS FORBESI Desor

Material. Specimens of Asterias forbesi may be collected
among rocks between the tides or in tide-pools. Excellent dry
specimens may be obtained by suddenly pouring boiling water
into the dish in which the starfish has been allowed to assume
the attitude of perfect radial symmetry. For the study of in-
ternal organs and especially of the ambulacral system injected
specimens are highly recommended. The injection may be
accomplished without difficulty through one of the radial canals
and should be continued till the ambulacral feet of all radii are
injected. The specimens may be then preserved in formaHn.
Injected specimens are also for sale at most of the marine
laboratories. Very small specimens should be fixed for sections
in Perenyi's fluid for which purpose each arm is severed with a
sharp razor. Decalcifying before imbedding is imperative.
Staining in haematoxylin with counterstaining in eosin or orange
G is advisable.

Descriptive Part

Asterias forbesi is a common starfish of the Atlantic Coast.
Like all echinoderms it shows radial symmetry inasmuch as
each of its five arms contains the same set of organs. But this
radial symmetry is not the original plan of structure and a closer
examination of the starfish reveals the fact that it is built on
the principle of bilateral symmetry. This plan is determined
by the position of the so-called madreporic plate which is situated
on the dorsal surface of the central disc between two of the
arms. The plane of symmetry bisects therefore the madreporic
plate, the anus which is situated in the middle of the dorsal sur-
face of the central disc, and the arm or radius opposite to the

174

ASTERIAS FORBESI 175

madreporic plate. The two arms between which the madreporic
plate is situated form the bivium, the remaining three the trivium.
The entire dorsal surface is covered with short spines or tubercles
surrounded by groups of pedicellariae. On the ventral surface,
in the middle of the disc, is the mouth surrounded by five pairs
of oral spines. Radiating from the mouth are the ambulacral
grooves in which the ambulacral feet lie in four rows in each arm.
The edges of the grooves are covered with sharp spines. At the
end of each arm, imbedded among spines, is the terminal tentacle
with a red spot at its base — the eye.

The body wall is very thick and hard owing to the presence
of calcified plates or ossicles^ in its inner layer. The ossicles of
the dorsal surface are more or less irregular in shape and are
held together by connective tissue and muscular fibres. The
ventral surface is formed by articulated ossicles arranged in four
rows in each arm. The two middle rows are formed by the
ambulacral ossicles articulated with each other in the median
line. Between these are the ambulacral pores through which the
ambulacral feet project. The two outer rows of ossicles are called
adambulacral and they carry the sharp spines already men-
tioned. The peristome is surrounded by a pentagon of oral
ossicles. The outer layer of the body wall is formed by a ciliated
epithelium. Outside of the adambulacral plates on the ventral
surface and on the dorsal surface are numerous branchice (or
papulae) protruding through the interstices between the ossicles.
They are short, tubelike projections of the coelome or body cav-
ity, are lined on the inside with ccelomic epithelium and are
capable of being completely retracted. They serve for the
purpose of respiration. The pedicellarice are little scissor-like
appendages sitting around the tubercles and spines. They are
composed of three parts, two blades or jaws and a basal plate
with which the blades are articulated. There are two kinds of
pedicellariae, larger ones with straight edges of the blades and

^The entire skeleton of ossicles in the starfish is so articulated that it
admits an extensive downward and very slight lateral motion.

176 MORPHOLOGY OF INVERTEBRATI:: TYPES

smaller ones with curved edges. The blades are provided with
special muscles enabling them to open and close. The pedicel-
lariae are thus enabled to grasp small particles and serve as pro-
tection against small enemies and as an apparatus to keep the
surface of the starfish clean and to help in forwarding food to
the mouth.

The body cavity or coelome is very large and extends to the
end of each arm. It is lined with ciliated coelomic epithelium,
the so-called parietal layer.

Digestive system. The mouth as already mentioned is sit-
uated in the middle of the ventral disc and is surrounded by the
peristomial membrane and five pairs of movable spines. It leads
into a very short oesophagus which is followed by the immense
stomach. The cardiac part of the stomach, i. e., the one nearest the
oesophagus, forms five broad cardiac pouches and can be everted
to the outside through the mouth, when the animal is feeding,
for the purpose of predigesting the food. The digestive fluid
used in this process comes from five pairs of digestive glands
situated in the arms, each gland suspended by two mesenteries
from the roof of the arm. These glands are diverticles of the
stomach. The retraction of the cardiac portion is accomplished
by five pairs of retractors or muscles arising from the sides of
the ambulacral ridges. The pyloric portion of the stomach, i. e.,
the one nearest the intestine, is considerably smaller than the
cardiac one and has the five pyloric coeca just mentioned. The
intestine is very short and insignificant and bears five in-
testinal coeca. The anus is situated near the middle of the dorsal
surface.

Ambulacral or water-vascular system. From the per-
forated madreporic plate which is situated on the dorsal disc
between the bivium, a stout, somewhat curved hydrophoric
or sto7te canal runs toward the ventral body wall. Here it
enters the ring canal in an interradius which is inclosed in the
Ossicles surrounding the mouth. Lito the ring canal open nine
Tiedemann's bodies (there being only one T. body in the inter-

ASTERIAS FORBESI

177

lac.rd

cn.hy

org.a)c___
sin^x.-—

— cn.pd.

I.

— i^esped.

Fig. 34. — Ambulacral system of a starfish after Delage and Herouard.
lac. ann, oral lacuna; en. aim, oral (ring) canal (ambulacral); sin. 0, oral
sinus; sin. rd, radial sinus; en. rd, radial canal (ambulacral); lac. rd, radial
lacuna; c«. hy, hydrophone (stone) canal; org. ax, axial organ; sin. ax, axial
sinus; en. t, transverse canal; v, valve between transverse canal and ampulla;
ves. poly polian vesicle; en. pdb, ambulacral tube; ves. ped, ampulla; en. pd,
sucker.

radius of the hydrophone canal) which are minute glands pro-
ducing amoebocytes. The ring canal gives rise to five radial
canals each of which runs imbedded in the body wall along the
median Hne on the ventral surface of the arm ending blindly in
the terminal tentacle just before the last ossicle. Each radial
canal gives off at regular intervals transverse canals. Each pair
of transverse canals consists of a short canal on one side and a

178 MORPHOLOGY OF INVERTEBRATE TYPES

longer canal on the opposite side of the radial canal. The short
and long canals of each side are alternating. At the end of each

transverse canal is a special muscular hollow organ, consisting
of an ambulacral foot with an acetabulum or sucker and an am-
pulla. The ampullae are in the body cavity, while the ambulacral
feet protrude to the outside through the pores between the am-
bulacral plates. The first five pairs of ampullae sitting over the
ossicles in which the ring canal is inclosed, are connected with
oral tentacles. These tentacles are simply somewhat modified
ambulacral tubes. Real polian vesicles opening directly into the
ring canal are absent in A. forbesi, but the five pairs of ampullae
just mentioned are sometimes called by this name. The sea
water admitted to the system through the madreporic plate
mixes with the amoebocytes produced by the Tiedemann bodies
and is forced by the compression of the ampullae into the am-
bulacral feet which become extended and serve the purpose of
locomotion. The suckers at the end of each foot allow perfect
adhesion to any surface and are made use of also when the star-
fish intends to open the shells of a mollusc to feed upon the ani-
mals. It is probable that the ambulacral system plays also the
role of an excretory organ.

The circulatory system follows the course of the ambulacral
system under which it is situated. It is composed of a system of
sinuses and a system of lacunce with the a.xial organ. Immedi-
ately under the ambulacral ring is the oral sinus ring divided by
a perforated septmn into an external and an internal oral sinus.
The external sinus gives rise to five radial sinuses. Each radial
sinus is divided longitudinally by a septum inclosing the radial
lacuna. The radial sinus runs under the radial canal of the
ambulacral system to the end of the arm, the left and right
channel uniting in the tentacle. The radial sinuses give off
transverse sinuses to the ambulacral feet. The internal oral
sinus gives rise to the axial sinus which runs along the hydro-
phone canal with which it stands in communication by a distal
orifice. Communicating with the dorsal end of the axial sinus

ASTERIAS FORBESl

179

is the ahoral sinus-ring, which gives rise to five geniicd sinuses.
The system of laciuice is inclosed in the septa of the system of
sinuses. It is composed of an oral lacuna ring, five radial
lacuncB, transverse lacunce, axial lacunce, aboral lacuna ring and
five genital lacunce.
The axial organ is simply a glandular part of the axial lacuna,

avnls.

^ird.

Fig. 35. — Nervous system of a starfish after Delage and Herouard.
n. ose, oesophageal nerves; an. oes, esophageal nervous ring; an. c, ecto-
neural oral ring; nh, hyponeural radial nerve; an, hyponeural oral ring;
np, transverse (pedal) nerves; avnls, nervous rings of suckers; nrd, ecto-
neural radial nerve; nd, deep dorsal nerves.

in which the lacuna forms a plexus. It is connected with the
axial sinus by numerous pores.

The axial sinus, axial lacuna with the axial organ and the
hydrophone canal are all together inclosed in a peritoneum and
form the axial complex.

The nervous system consists of a superficial or ectoneural,
deep ventral or hyponeural, and deep dorsal or entoneural sys-

l8o MORPHOLOGY OF INVERTEBRATE TYPES

tern. The cctoneural nervous system follows the plan of the am-
bulacral and circulator}^ systems. It is epidermal in origin, be-
longs to the \'entral surface of the animal and is situated below
the system of sinuses covered only by the epithelium of the body
wall. Its function is sensor}^ It consists of a neroous ring sur-
rounding the peristomial membrane, five radial nerves which give
off transverse nerves and a peripheral netivork innervating the feet
and the pedicellariae. The hyponeural or motor system is struc-
tured like the ectoneural from which it is separated only by a
very thin membrane of connective tissue. The ento neural system
belongs to the dorsal coelomic epithelium and consists of five
nerves radiating from the centre.

Reproductive system. The sexes are separate, but the
structure of the reproductive organs is similar in both sexes.
Five pairs of ramified adradial gonads open to the outside through
as many genital pores which are interradial and dorsal in their
position. When fully developed the gonads extend far into the
arms, on each side of the digestive coeca.

The eggs are fertilized in the water. The development is a
metamorphosis in which the larv^al stage is represented by the
Bipinnaria.

Instructions

1. (a) Dry specimen. Make a full size drawing of the aboral
or dorsal surface of the starfish showing the central disc and all
arms, madreporic plates, tubercles and pedicellariae. Indicate
by a dotted line the plane of symm^etry and label bivium and
trivium. The anus is usually not visible but should be indicated
by a dot in the centre of the disc. The details should be drawn
only in one arm.

2. Remove with a forceps the ambulacral feet of one arm.
Make a full size drawmg of the oral or ventral surface, showing
mouth, oral spines, ambulacral grooves with four rows of am-
bulacral pores between the ambulacral ossicles, spines on the

ASTERIAS FORBESI i8i

adambulacral ossicles, ectoneural nerv'ous ring and ectoneural
radial nerve. Details should be drawn only in one arm.

3. Examine under low power the madreporic plate and make a
drawing about 5 cm. in diameter showing its structure.

4. Remove with a scalpel some of the pedicellariae, place them
in a drop of water on a slide, examine under microscope and
make a drawing showing both types of pedicellariae.

5. (b) Injected specimen. Place the specimen in a dissecting
tray with water, examine the oral surface and introduce into
drawing No. 2 (in an arm different from the one drawn in detail
from the dry specimen) the ambulacral feet, eye and tentacle.

6. Examine the aboral surface and introduce into drawing
No. I the branchiae.

7. With strong scissors make a lateral incision around the
whole animal taking care not to ruin the internal organs. Make
a circular incision around the madreporic plate. Lift with a
forceps the dorsal body wall at the end of each arm and press
the digestive glands down, tearing the mesenteries with the
handle of a scalpel. Cut the intestine near the anus and remove
the entire dorsal body wall. Make a drawing of its inner sur-
face showing the network of ossicles and spaces between them.

8. Make a full page drawing of the digestive system showing
cardiac pouches and their retractors, pyloric portion of the
stomach with five pyloric coeca or digestive glands, and intestine
with its intestinal coeca.

9. Remove with a forceps the entire digestive system taking
care not to spoil the peristomial membrane. Make a drawing
of the reproductive organs.

10. Remove the reproductive organs. Make a full size draw-
ing of the ambulacral system showing madreporic plate, hydro-
phone canal, Tiedemann's bodies, the five pairs of ampullae
situated over the oral ossicles and belonging to the oral tentacles
and the four rows of ampullae in one of the arms. Show also the
peristomial membrane and the axial organ of the circulatory
system.

1 82 MORPHOLOGY OF INVERTEBRATE TYPES

11. Remove the peristomial membrane, the ampuUse of the
oral ossicles and all ampullae of one arm. Make a full size draw-
ing showing the teeth, ring of oral ossicles with ten pores for the
ampullae and the ossicles of the arm with the ambulacral pores
between them.

(c) Cross-section of arm.

12. Make a drawing of the prepared slide showing body wall,
coelome, digestive glands with mesenteries, ampullae, ambula-
cral feet, radial canal, radial sinuses with the septum containing
the radial lacuna and the ectoneural radial nerve.

OPHIOPHOLIS ACULEATA Linn«us

Material. Ophiopholis aculeata is very common in tide-
pools along the Atlantic Coast north of Cape Cod. Dissection
of the internal organs is not satisfactory and the brittle star
should be used only for comparison with other classes of Echino-
dermata. Specimens preserved in alcohol will therefore suffice.
The study may be, however, supplemented by an examination of
cross-sections through an arm and the disc. For this purpose
very small specimens should be preserved in a fixing fluid and
decalcified before sectioning.

Descriptive Part

Ophiopholis aculeata is a representative of the Class Ophiu-
roidea. While its general plan of structure is similar to that of
the starfish, there are many important differences in structure.
The radial symmetry is apparent in the whole organization of
the brittle star, but the bilateral symmetry is still in evidence
from the position of the madreporic plate situated on the oral
or ventral surface of the animal. The body, especially on the
aboral or dorsal surface of the animal, is sharply differentiated
into a central disc and five narrow, long arms. The ossicles of the
arms are so articulated as to allow an extensive lateral motion,
whereas the up-and-down ward motion is very slight. Owing to
the fact that the arms are narrow their visible bases do not
occupy the entire periphery of the central disc but are separated
from each other by those regions of the central disc, which be-
long to the interradii. The dermal skeleton of the aboral surface
of the central disc is composed of a system of distinct plates,
while the areas 'between these plates are covered with short

183

i84

MORPHOLOGY OF INVERTEBRATE TYPES

spines. One plate is situated in the centre of the disc and is
called the central plate. It is surrounded by five radial plates.
To the outside of the radial plates are ten radiating rows of

__n.rd.

mcl.

c.g.l.

- cil

Fig. 36. — Transverse section of an arm of a brittle star after Delage
and Herouard. c. epn, epineural cavity; n. h, hyponeural oral ring; sin. rd,
radial sinus; en. rd, radial canal; n. rd, epineural radial nerve; n. mcl, nerv'es
of intervertebral muscles; mcl, intervertebral muscles; n. pgt, spine nerve;
71. cut, cutaneous nerve; c. g. I, lateral portions of the general cavity; cil,
ciliated groove or band of the cavity; c. g. a, axial cavity of the arm.

interradial and secondary radial plates [usually three of the
former for each interradius and two of the latter for each radius.
At the base of each arm are hasal plates (adradial), usually one
on each side of the distal secondary radial plate.] On the oral

OPHIOPHOLIS ACULEATA 185

surface of the animal the disc is reduced to the interradii appear-
ing as pouches between the arms, which extend to the mouth and
enter into the formation of the oral ring or pentagon. On the
inner surface of the pentagon are five interradial jaw plates each
carrying a series of ieeth. The free edge of the pentagon is fringed
with scales called oral papillce. Immediately behind this fringe
in each interradius are two narrow adoral plates followed by a
large oral plate. One of the five oral plates has a minute pore and
functions as madreporic plate. On each side of the arms, extend-
ing from the oral pentagon almost to the edge of the disc is a
narrow genital slit.

The first thing noticeable in the structure of the arms is the
absence of the ambulacral groove. Instead, the oral surface
of the arm is flat and presents a single row of plates — one plate
for each segment of the arm — the so-called ventral or under arm
plates. The aboral surface of the arm presents a similar row of
plates — the dorsal or upper arm plates. These plates are sep-
arated from each other by a row of small scales found also be-
tween the dorsal and lateral plates. Each lateral plate has five or
six movable spines. The openings for the ambulacral tentacles
are situated between the lateral and ventral plates, one opening
on each side. The tentacles are protected by a tentacle scale.
The dorsal, ventral and two lateral plates are ossifications rep-
resenting the outer skeleton of the arm. The arm of all brittle
stars has also an inner skeleton consisting of a row of articulated
vertebrcB occupying the major part of the cavity of the arm. On
removal of the aboral wall of the central disc one can see four
vertebrae of each arm between the oral pentagon and the edge
of the disc. Between them are the cavities of the interradii
for the reception of the genital organs and of the stomach
pouches.

The internal anatomy is very similar to that of the starfish
and the various systems are built on the same principle. But
there are certain differences in structure, which should be
emphasized. The digestive system is restricted to the central

i86

MORPHOLOGY OF INVERTEBRATE TYPES

gpcl—

cnpd-

n.cuL__

Fig. 2>1- — Relation between the nervous and ambulacral systems in the
arm of a brittle star after Delage and Herouard. n. cut, cutaneous nerve;
en. pd, pedal canals; g. pd, pedal ganglia; n. epn, epineural nerve; g. epn,
epineural ganglia; pd, ambulacral tubes; n. rd, epineural radial nerve;
nh, h>'poneural radial nerve; en. rd, radial canal; n. mel, nerve of inter-
vertebral muscle.

disc. Hepatic glands are absent. The stomach is a blmd sac.
Intestine and anus are wanting. The ambulacral system pre-
sents the following differences: the hydrophoric canal opens on

OPHIOPHOLIS ACULEATA 187

the oral surface of the central disc in the madreporic plate already
mentioned. There are four Polian vesicles attached to the ring
canal. Tiedemann's bodies are absent. In each arm are only
two rows of ambulacral tubes and they are developed as ten-
tacles, devoid of a sucker and useless in locomotion. The dif-
ference in the number of rows is, however, not of great impor-
tance, since we must remember that in the starfish there are al-
ternating longer and shorter transverse canals, while in the brit-
tle star all transverse canals are of the same length. The ner-
vous system is more differentiated than in the starfish. The
radial nerves have regular ganglionic enlargements in each seg-
ment of the arm; the dermal network of the ectoneural system
has disappeared. Eyes are absent, but the terminal tentacle is
present in each arm.

Ophiopholis aculeata reproduces with a metamorphosis, in
many respects similar to that of the starfish. The sexes are sep-
arate and the reproductive cells are discharged into the water.
The larva which develops from the fertilized egg is provided with
two long and four short arms and is known under the name of
Plutetis.

Instructions

1. Make a half page drawing of the aboral surface showing
central disc and arms. Label all plates on the central disc.

2. Make a drawing, on the scale of 5:1, under dissecting
microscope, of a few segments of an arm, showing dorsal plates,
row of scales separating them, lateral scales and the spines on
the lateral plates.

3. Make a half page drawing of the oral surface showing oral
pentagon with teeth, five arms, genital slits.

4. Make a large drawing of the ventral surface of the disc,
examining it under dissecting microscope. Label oral pentagon,
fringe of oral papillae or scales, jaws with teeth, oral tentacles,
adoral and oral plates, madreporic plate, genital slits, ventral
plates of the arm, ambulacral pores, tentacle scales and tentacles.

1 88 MORPHOLOGY OF INVERTEBRATE TYPES

5. Remove with scissors the entire aboral wall of the disc.
Remove with a forceps all organs. Make a drawing showing
teeth, oral pentagon, vertebrae of the arms, genital pouches of
the interradii and genital slits.

6. Break off an arm and examine the exposed cross-section
under dissecting microscope. Make a quarter page drawing
showing dorsal, ventral, two lateral plates with spines, vertebra,
ambulacral tentacles.

PENTACRINUS ASTERIA (CAPUT MEDUSAE)

Miiller

Material. This beautiful crinoid is found in the West
Indies in depths over eighty fathoms. It is too expensive for
class work and the description given here is merely for compari-
son and completeness.

Descriptive Part

Pentacrinus Asteria is a representative of the Class Crinoidea.
Its body is divided into a long stalk or peduncle and a chalice
with arms. The stalk is composed of numerous pentagonal
columnal ossicles with toothed anterior and posterior edges and
so articulated that the teeth of one columnal ossicle fit into the
spaces between the teeth of the next columnal ossicle. In the
centre of each ossicle is a canal running through the entire stalk.
Situated on the stalk at more or less regular intervals are the
cirri. Those nearest the chalice are the shortest. There are
always five cirri for each vorticellum. The columnal ossicle
bearing the cirri is somewhat larger than the intermediate
ossicles and has five articular facets for the reception of the cirri.
The cirri are many-jointed and have a central longitudinal canal
communicating with the canal of the stalk, and through this
with the canals of the arms. The first columnal ossicle of the
stalk is star-shaped with the rays of the star situated in the
interradii of the animal and appearing on the surface as small
rounded plates between the bases of the arms. This star-shaped
first columnal ossicle serves as base for the chalice and can be
seen on removal of the stalk. The chalice is composed of the
arms and the disc. Each arm has three radial joints beyond
which begins the splitting up of the arm into branches. Al-

189

190 MORPHOLOGY OF INVERTEBRATE TYPES

though the number of branches is definite (our species has forty
branches, i. e., eight branches for each arm) the method of
branching is subject to great variations. Each branch is com-
posed of numerous joints, every other joint carrying on its oral
surface two many- jointed pinnulce or small branches. Not only
the branches of the arms but the pinnulse too have an ambu-
lacral groove with openings for the ambulacral tentacles devoid of
suckers. The ambulacral grooves continue from the two pri-
mary branches of each arm onto the oral disc, each pair soon
uniting and reaching the mouth in form of a single groove.
Radiating from the mouth are therefore only five ambulacral
grooves. The anus is also on the oral disc surrounded by ossicles
which form a tube-like inclined projection. (Plate III, f. 2 of
J. Miiller. Abh. Ak. Wiss., Berlin, 1841.) Pedicellariae absent.
In the internal organization should be mentioned the presence
of numerous hydrophone canals opening into the coelome, ab-
sence of Polian vesicles and the position of the gonads in the
pinnulae with genital rhachis running through the whole length
of the arm branches. The reproductive cells reach the outside
through the rupture of the tissues of the pinnulae in more or less
predetermined places. The development is not known, but is
probably similar to that of the European crinoid Antedon.

ARBACIA PUNCTULATA Lamarck

Material. Arbacia punctulata is found in shallow water
and in tide-pools along the Atlantic Coast. It is advisable to
make an incision either in the peristomial membrane or in the
equatorial line in those specimens which are to be preserved in
formalin or alcohol for dissection.

Descriptive Part

Arbacia punctulata, or purple sea-urchin, is a common repre-
sentative of the Class Echinoidea. With exception of a small
area on the back and a somewhat larger one on the ventral side,
the entire body is covered with spines. In the centre of the
dorsal or aboral surface lies the anus protected by four calcareous
movable plates of the periproct. In the centre of the ventral
or oral surface lies the mouth armed with five sharp white teeth.
The mouth can be tightly closed by a circular Up which is a
muscular differentiation of the soft peristomial membrane. The
edge of this membrane is attached to the test or rigid calcareous
skeleton. Around the lip are five pairs of modified ambulacral
tubes — the oral papillce or suckers. Scattered over the peristome
are long-stemmed pedicellaricB. On the edge of the peristome,
attached to the peristomial membrane are five pairs of branched
organs of respiration — the branchiae. Five clusters of ambulacral
feet surround the peristome.

When the spines are removed the test appears in the shape of
a hemisphere with a rounded edge. It is composed of twenty
rows of interlocking calcareous plates extending from the peri-
proct to the peristome. Five pairs of rows consist of perforated
ambulacral plates and represent the radii, while the five pairs

IQI

192

MORPHOLOGY OF INVERTEBRATE TYPES

^ ^O

ARBACIA PUNCTULATA 193

of rows composed of adambulacral plates represent the interradii.
Meridional zig-zsig lines run in each radius and interradius
marking the articulation seams of the interlocking plates. Sur-
rounding the periproct are five large almost triangular genital
plates. They represent the terminal plates of the interradii and
each shows a large genital pore. One of these plates is somewhat
larger than the others and has a rough surface. It is the madre-
poric plate and marks the plane of bilateral symmetry. Between
the genital plates are five small ocular plates. These belong to
the radii and each has two pores for the terminal tentacle. The
term — ocular — is a misnomer, for the tentacles correspond to
the terminal tentacles of other echinoderms and do not have
the structure of eyes.

Returning to the meridional rows of plates we observe first
of all that the radii are much narrower than the interradii.
Moreover, the radial plates increase gradually in size from the
ocular plate to the edge of the peristome wkere the radius is
widest. On the other hand, the interradial plates, also gradually
increasing in size, are largest at the equator beyond which they
again diminish. As a consequence the radial and interradial
plates bordering the peristome are about equal in size. Each
radius has two median rows of rounded tubercular bases for
spines and two lateral double rows of ambulacral pores. An
examination of single plates shows that each ambulacral plate
has one tubercle and three pairs of ambulacral pores, a pair for
each foot. On the oral surface of the test the number of pores
increases gradually until near the peristome there are eight
pairs of pores for each radius as against two pairs of the aboral
surface. The equatorial adambulacral or interradial plates have
four tubercles each. This number decreases gradually towards
both poles until the last two aboral plates have only one tubercle
each, and the plates nearest the peristome only two tubercles
each. The radial plates bordering the peristome are in reality
the second oral radial plates. The first radial plates are inclosed
in the peristomial membrane. There are five pairs of them with

194 MORPiioLocv OF r.\\i;KTi:i;KATi: tvpks

pores for the five pairs of oral suckers. The first and second
adambulacral oral plates are turned over and fused with the
third and fourth plates. The edge of the peristome, turned
towards the inside of the test is thickened and presents in each
radius a pair of calcareous processes or auricles for the attach-
ment of the retractors of the Aristotle's lantern.

The spines of the aboral surface are more or less sharp-pointed
and attain their greatest length in the equatorial region. To-
ward the peristome they begin to change their shape, some
become blunt at the end, others widen at the end so that the
flat end of the spine appears wider than the base. All spines
are fluted and all have a hemispherical socket at their base for
the articulation with the hemispherical knob of the tubercles.
The spine is held in position by a muscular sheath attached to
the base of the spine and the periphery of the tubercle. This
sheath is composed of a double set of muscular fibres. The set
of external fibres is used for locomotion. The set of inner fibres
is used for the fixation of the spine in a given direction. The
relaxation of the fibres of the sheath brings the spine in its
normal upright position.

The ambulacral tubes are of three kinds. The five pairs sur-
rounding the lip are short, heavy and developed as oral suckers
or papillcB. The five clusters of tubes surrounding the peristome
as well as the tubes of the remaining part of the oral surface are
real ambulacral feet with a sucker at the end. In life they are
thin and long and may be extended beyond the spines. The
tubes of the aboral surface have no suckers and appear as
tentacles.

The pedicellaricB have a long stem and a head with three jaws.
One type of pedicellariae is found on the peristomial membrane
and on the test between the spines. The stem is thick and its
calcareous axis extends to the head. The head is large and the
jaws are wide. The second type is found only on the test. The
stem is thin, the calcareous axis does not reach to the head.
The head is small and the jaws are narrow.

ARBACIA PUNCTULATA

195

Five minute sphmridia are situated on the peristomial edge

of the test. They are modified spines and function as sense

organs. To be made visible the clusters of ambulacral feet have

to be removed when each spheridium appears as a httle bean-

3. iV.

c.a. 1^!:

cn.rd

'lacann.

Fig. 39. — Longitudinal section through the lantern of a sea urchin show-
ing the relative position of various organs. After Delage and Herouard.
anny epineural nervous ring; sin. or, oral sinus; py, pyramid (lantern);
dnt, tooth; ph, pharynx; md. t, transverse muscles; sp, sponge-organ; cnhy,
hydrophoric canal; lac. ann, oral lacuna; en. ami, ring canal; y, compas;
0, opening from oral sinus to Stewart's pouch; h, rotula; ps, Stewart's
pouch; md. i. py, interpyramidal muscles; lac. rd. ph, pharyngeal radial la-
cuna; en. rd, radial canal; sin. rd, radial sinus; n. rd, epineural radial nerve;
sq, ossicles of test.

shaped body in a small depression in the median line of the
radius.

The entire test including spines, pedicellaria, sphasridiae, etc.,
is covered with a ciliated epidermis. The inside surface of the
test forming the wall of the body cavity or coelome is lined with
a ciliated peritoneal epithelium.

196 MORPHOLOGY OF INVERTEBRATE TYPES

Digestive system. The mouth, as already mentioned, is sit-
uated in the centre of the oral surface. It is surrounded by the
circular lip and armed with five teeth placed interradially and
belonging to a complicated apparatus known under the name of
AristotWs lantern. The hollow axis of this lantern is formed by
the pharynx while the body of the lantern which has the shape
of a pyramid with a pentagonal base is composed of five com-
plicated calcareous parts or jaws and as many groups of muscles.
When isolated each calcareous jaw appears in the shape of a
triangular pyramid. The middle portion of the tooth is inclosed
between the two halves of an ossicle called the alveolus. The
elastic free upper end of the tooth is curved over the base of the
pyramid and inclosed in a pouch of the oral sinus. The horn-
like processes of the alveoli serving for the attachment of the
protractors are termed the epiphysis and, though fused with the
alveoU, are in reality separate ossicles. Radiating from the
middle of the lantern at its base are five ossicles articulated to
the alveoli and called rotulce. Below the rotulae and also radial
in position are five compasses or F-shaped ossicles, called by that
name on account of their two diverging ligaments. These long
and thin ligaments arise side by side from the head or distal
enlargement of the compass and are attached to the peristomial
edge of the two adambulacral plates on each side adjoining the
radius to which the ossicle belongs. The muscular apparatus
of the lantern is very comphcated. It consists of seven sets of
muscles aggregating no fewer than sixty individual muscles.
Of these we will consider the following: (i) Five inter pyramidal
(or interalveolar) muscles. These short muscles are attached to
the adjoining radial surfaces of the alveoli. They hold the al-
veoU together and close the teeth. (2) Five pairs of protractors.
They are attached to the epiphysis and the peristomial edge of
the test and run to the inside of and parallel to the compass-
ligaments. (3) Five pairs of retractors attached to the external
surface of the alveoli near the teeth and to the auricles. (4) Five
muscles binding together the compasses and forming the di-

ARBACIA PUNCTULATA 197

aphragm surrounding the oesophagus. All lantern muscles are
composed of smooth fibres.

The entire lantern with its muscles is inclosed in the oral sinus
formed by the so-called peripharyngeal membrane which is simply
a part of the peritoneum.

The (esophagus is a rather long tube opening into the broad and
flat stomach which has numerous folds or pouches. The stomach
makes an almost complete circle and leads into the intestine
which also forms an almost complete circle, running however
above the former in the opposite direction. The short rectum
runs directly upward and terminates in an anus in the centre
of the aboral surface of the animal. Closely applied to the inner
edge of the stomach but not otherwise connected with it runs the
siphon. It is a thin tube starting from the oesophagus not far
from its proximal end and opening into the intestine close to
the stomach. The siphon has practically the same histologic
structure as the stomach. The entire alimentary canal has a
covering of ciliated peritoneum and is suspended by mesen-
teries.

The ambulacral or water vascular system. We have seen
already that one of the genital plates serves at the same time
as a madreporic plate. The hydrophoric canal runs down to the
diaphragm where it opens into the circular canal surrounding
the oesophagus. The circular canal gives off five radial canals.
Into the circular canal, between the radial canals, open five
small sponge-like organs, erroneously called either Polian or
Tiedemann's vesicles, but histologically different from both.
The radial canals run between the two auricles of the same
radius and terminate in the terminal tentacle. They give
off transverse canals connected with the ambulacral tubes and
ampullcB. The first pair of transverse canals belongs to the
oral papillae. Each ambulacral foot or tentacle, bifurcated at
its base, communicates with its ampulla by means of two open-
ings. This is the reason why there are two pores in the am-
bulacral plates for each ambulacral tentacle or foot. The am-

iqS morphology of invertkbrate types

bulacral tubes arc muscular structures covered externally as
well as internally with ciliated epithelial cells.

Respiratory system. The five pairs of branchiae already
mentioned function as organs of respiration as do possibly also
the ambulacral tentacles of the aboral surface. The branchiae
are outgrowths of the peristomial membrane and communicate
with the oral sinus.

Circulatory system. The circulatory organs are composed
of a system of sinuses and a system of lacunar. The system of
sinuses is characterized by a complete isolation of its component
parts. The five radial sinuses end blindly at both ends. The
oral sinus has been described above and communicates only with
the five pairs of branchiae. The aboral sinus communicates only
with its five prolongations, the genital sinuses. The axial sinus
is reduced to the central cavity of the axial organ and commu-
nicates with the hydrophoric canal. The system of lacunae re-
sembles that of the starfish, though somewhat complicated by
the presence of stomachal lacunae. The axial organ which is part
of the system of lacunae, communicates with the aboral as well
as the oral circular lacuncB. The latter surrounds the oesophagus.
Five radial lacunce run from the oral lacuna to the terminal ten-
tacles. The highly branched system of stomachal lacuncB ex-
tends over the entire stomach and part of the intestine. It con-
sists of two canals with small branches opening into them along
their entire extent. One of these canals is the external stomachal
lacuna, the other the internal stomachal lacuna. They are closely
appHed to the stomach and the internal lacuna communicates
with the oral lacuna. The aboral lacuna gives rise to five genital
lacunae.

Nervous system. The nervous system includes, as in the
starfish, three distinct systems. The ectoneural system consists
of an oral ring surrounding the mouth and situated on the inner
surface of the lip, five radial nerves with their branches and a
subepidermal nervous plexus. The radial nerves are not super-
ficial but run along the inner surface of the test in so-called

ARBACIA PUXCTULATA 199

epineural cavities. The hyponeural or motor system is consider-
ably reduced and consists of five very short radial nerves with
still shorter five transverse pieces representing the broken up
ring. The entoneural system is inclosed in the aboral sinus. It
consists of an aboral pentagon or ring with five genital nerves.

Reproductive system. The reproductive organs are similar
in both sexes. They consist of five simple, interradial gonads
which open on the genital plates. Organs of copulation are
absent. The reproductive cells are emptied into the water.
The development is characterized by the formation of a larva
known as pluteus.

Instructions

1. Place a sea-urchin in a dissecting pan with water. Ex-
amine the spines and remove one of each of the three types of
spines: a long, sharp-pointed equatorial spine, a short spine with
the broadened flat end and an intermediate spine with blunt end.
Study their surface under dissecting microscope. Make a draw-
ing showing the three spines twice their natural size.

2. Remove all spines taking care not to injure the ambulacral
tubes. Place the sea-urchin with its oral surface uppermost.
Make a drawing twice natural size showing the five teeth, cir-
cular lip, peristomial membrane, five pairs of oral papillae, five
pairs of branchiae, five clusters of ambulacral feet, spine tubercles
and the pedicellariae of the peristomial membrane. Label also
radii and interradii.

3. Remove with the aid of a forceps an ambulacral foot and
examine it under dissecting microscope. Make a drawing
showing the foot and its sucker.

4. Remove an entire cluster of ambulacral feet and examine
the place under dissecting microscope. Make a drawing showing
the pairs of ambulacral pores and the minute sphaeridium.

5. Remove a pedicellaria, place it in glycerine on a slide under
a cover glass, examine under microscope and make a drawing

200 MORPHOLOGY OF INVERTEBRATE TYPES

showing the stem with the calcareous axis and the head with
three jaws.

6. Find on the test a similar pedicellaria with jaws open and
make a drawing of it.

7. Find on the test the second type of pedicellaria in which
the axis does not reach to the head. Make a drawing of it
under microscope.

8. Remove an ambulacral tube (tentacle) from the aboral
surface of the test, examine it in the same manner and make a
drawing of it.

Q. Find the five terminal tentacles on the ocular plates.

10. Carefully clean the entire aboral surface of the test with
a toothbrush or examine a dried and cleaned test. Make a
large drawing showing the periproct with the four anal plates,
five genital plates with the genital openings, five ocular plates
with pores for the terminal tentacles, five radii and five inter-
radii with the zigzag suture between the two rows of plates, the
tubercles on the interradial or adambulacral plates and the
tubercles and pairs of ambulacral pores on the radii. Once more
examine carefully the genital plates and find among them the
one which serves also as madreporic plate. Show with a dotted
line the plane of bilateral symmetry.

11. Make a drawing of the oral surface of a dry specimen in
which the spines and the peristomial membrane were removed.
Show the arrangement of the spine tubercles, the peristomial
edge of the test, the ambulacral pores and the first and second
adambulacral plates folded back upon the two following plates.

12. Holding the test in your hand so as to look into the body
cavity under an angle to the dorso-ventral axis, examine with
naked eye the peristomial edge of a radius and make a drawing
twice the natural size showing the auricles.

13. Returning to the cleaned alcoholic specimen make an
incision around the equator. Strong scissors may be used for
this purpose, but a very fine saw is preferable. Carefully open
the specimen by cutting the mesenteries of the alimentary canal,

ARBACIA PUNCTULATA 201

if necessary, and place both halves side by side in the dissecting
tray with water. The oral half will contain the Aristotle's
lantern, oesophagus, stomach and siphon, part of the axial com-
plex (hydrophone canal with the axial organ) and five ambulacral
radial canals with ampullae, as well as the ring canal. The
aboral half will contain the intestine with the rectum, five
gonads and five radial canals with ampullar. Make a full page
drawing showing both halves side by side with all organs in situ.

14. Cut the oesophagus and remove the stomach from the
oral half. This will expose the Aristotle's lantern inclosed in
the oral sinus. Carefully remove with a needle and fine forceps
the wall of the sinus, exposing all parts of the lantern. Make
a drawing twice natural size showing the base of the lantern
as viewed from above. The drawing must show the transverse
section of the oesophagus surrounded by the diaphragm, the
compass ossicles with their tendons, the curved ends of the
teeth and the auricles with the retractors.

15. Make a drawing showing the side view of the lantern
on a large scale. The drawing must show the curved ends of
the teeth, the head of the compass ossicles, the compass liga-
ments, the rotulae, the epiphyses with the protractors, the alveoli
with the interpyramidal muscles, the retractors and the auricles.

16. Take out the lantern and separate the alveoli. Make a
drawing of an isolated alveolus with the tooth.

17. Separate the two parts of an alveolus, taking care not to
break the tooth. Make a drawing of the tooth showing the
cutting edge, the central portion and the elastic end.

THYONE BRIAREUS Lesueur

Material. Thyone briareus is found along the Atlantic
Coast below low water mark on muddy bottom. Like all holo-
thurians they are liable to throw out their viscera when put in
a preserving liquid. Various methods have been recommended
to overcome this difficulty. Perhaps the simplest is that of
seizing the animal below the tentacles with a strong forceps and
plunging it into boiling water. After that they may be pre-
served in alcohol or formalin. For the study of the ambulacral
system the specimens should be injected through a Polian
vesicle.

Descriptive Part

Thyone briareus is a common representative of the Class
Holothurioidea. Although following the plan of radial sym-
metry it Httle resembles the other Echinodermata. Its body is
elongated and considerably narrowed towards the anterior end
which is provided with branched tentacles surrounding the
mouth. Moreover, the animal does not crawl on its oral surface,
but uses for this purpose the three radii thickly covered with
ambulacral feet. This surface is therefore physiologically the
ventral one, while the dorsal one is represented by the remaining
two radii. The bilateral symmetry is besides greatly accen-
tuated by the position of the anus at the end of the body and
the presence of a single genital opening between the tentacles
on the interradius opposite the median radius of the ventral
surface. The ten oral tentacles correspond to the ten oral
papillae of the sea-urchin and represent the first five pairs of
modified ambulacral tubes. The median ventral pair of tentacles
is considerably smaller than the others. It is customary to

-_-M.

Fig. 40. — Anatomy of Holothuria tuhidosa after M. Edwards, somewhat
modified. M, tentacles; F, ampullae of the tentacles; RW, radial ambulacral
canal; W, rete mirabile; L. K, left water lung; R. K, right water lung; G. A,
anastomosis of the blood vessels; HD, posterior section of intestine; MD, mid-
dle section of intestine; S, hydrophone canal; Wf, ring canal; PB, polian
vesicle; G, gonad; LM, longitudinal muscles; VD, ventral blood vessel;
DD, dorsal blood vessel; BK, dilator of the cloaca; A', cloaca; A, anus.

204 MORPHOLOGY OF INVERTEBRATE TYPES

consider the radius to which they belong, that is, the median
ventral radius of the trivium, as the fifth radius; the adjoining
left ventral radius as the fourth radius, and the right ventral
radius as the first radius; the right bivial radius as the second
and the left bivial radius as the third radius. Behind the tenta-
cles is a short collar strengthened on the inside by a calcareous
peripharyngeal ring or corona consisting of five radial and five
interradial plates. The ambulacral tubes of the ventral surface
are numerous and developed as true amhtdacral feet with ter-
minal suckers; those of the bivium or dorsal surface are less
numerous and devoid of suckers, i. e., they are modified into
ambidcLcral tentacles. Terminal tentacles are absent.

The body wall is devoid of an articulated calcareous skeleton.
Instead it is soft and muscular with irregular perforated calca-
reous plates in its dermal layer, mainly in the anterior and pos-
terior regions of the body. Perforated calcareous plates are
found also in the wall of the tentacles and of the suckers of the
feet. The external body covering consists of an epidermis which
is not ciliated. Under the dermal layer is a heavy sheet of
circidar muscles. Five longitudinal muscles each consisting of two
bundles run the whole length of the radii. The pharynx has five
strong retractors, each consisting of two bundles, and the cloaca
has numerous dilating muscles — dilatatores cloacce — which run
radially in all directions from the cloaca to the body wall. The
body cavity is lined with a ciliated peritoneal epithelium.

Digestive system. The mouth situated at the anterior end
of the body is surrounded by a circular lip but devoid of teeth
or jaws. The pharynx is a cylindrical tube provided with five
retractors already mentioned. Between the pharynx and the
short oesophagus is a constriction. The stomach is small but
muscular. The intestine is thin, very long and convoluted, runs
to the posterior end, turns back to the anterior end and turning
again runs to the posterior end where it forms a short rectum
and opens into the cloaca. The anus or opening of the cloaca
to the outside is in the centre of the aboral end of the animal,

THYONE BRIAREUS 205

morphologically in the same place as the anus of the sea-urchin.
The entire alimentary canal is provided with a muscular sheath
of internal longitudinal and external circular muscles.

Respiratory system. Two water-lungs or canals opening
into the cloaca and furnished with many lateral branches func-
tion as organs of respiration. They are attached to the body
wall and to the intestine by mesenteries. The left lung is be-
sides surrounded by a network of lacunae.

Water-vascular or ambulacral system. The ring capiat sur-
rounds the pharynx at its junction with the oesophagus. The
hydrophoric canal ends in a madreporic plate. It does not open
to the outside, but into the body cavity or coelome, since the
madreporic plate lies within the latter and not on the body
wall, a condition similar to that in Crinoidea. (The Crinoidea
have, however, numerous madreporic canals.) There are
usually two large Polian vesicles opening into the ring canal.
The five radial canals connected with the ring canal nm first
forward and give off transverse canals to the five pairs of oral
tentacles; then turn backward and run between the two bundles
of longitudinal muscles to the posterior end where they ter-
minate blindly. The radial canals give off throughout their
length transverse canals to the ambulacral feet and tentacles.
The ampullce are small and scattered all over the inner surface
of the body wall.

The circulatory system is characterized by the greater devel-
opment of the lacunar system and a reduction of the system of
sinuses. Both axial sinus and axial lacuna are absent. Five ra-
dial sinuses giving off transverse branches to the ambulacral
tubes, extend all the way to the cloaca and terminate blindly.
At the anterior end they open into the circular oral sinus. The
circular oral lacuna gives rise to five rcdial lacuna and two intes-
tinal lacun(B. The ventral intestinal lacuna runs along the intes-
tinal canal. The dorsal intestinal lacuna runs in the mesentery,
giving off a series of branches which split into a network — the so-
called rete mirahile. This network of lacunae is connected not

2o6 MORPHOLOGY OP INVERTEBRATE TYPES

only with the dorsal lacuna, but also with its collateral lacuna
which runs along the intestine in the line of attachment of the
mesentery.

The nervous system is composed of an ectoneural and a hypo-
neural system resembUng each other and separated by an ex-
tremely fine membrane. The oral nerve ring is situated in the
peristome. It gives rise to five radial nerves running to the
cloaca. The ectoneural ring gives rise to ten tentacle nerves,
the ectoneural radial nerves to a subepithelial plexus. The
hyponeural or motor system supplies nerves to all the muscles.
An entoneural system has as yet not been described and is
probably wanting.

Reproductive system. The sexes are separate, but the
anatomical structure of the reproductive organs is similar in
both sexes. There is but a single gonad composed of two clus-
ters of numerous tubes. In the case of the male, these tubes
constitute the testis and open into a small seminal reservoir. A
seminal duct or vas deferens runs forward from this reserv^oir and
terminates in an interradial genital opening at the base of the
tentacles in the mid-dorsal line. In the case of the female, the
tubes constitute the ovary and open into a common oviduct
which terminates in the genital opening. The latter has the
same situation as in the male. The sexual cells, copulation being
absent, are discharged into the water and fertilization left to
chance. Development is characterized by a metamorphosis
with a larval stage known as A uricularia.

Instructions

I. Place the specimen in a dissecting tray with water. Find
the smallest pair of tentacles belonging to the middle radius
(fifth radius) of the trivium. Turn the specimen so that this
radius would be on your right. Make a drawing of the specimen
in this position (lateral view) showing tentacles, mouth, am-
bulacral tubes, and anus.

THYONE BRIAREUS 207

2. Holding the specimen in your left hand, spread with your
fingers the tentacles and make a drawing (oral view) with the
mouth in the centre, the two smallest tentacles directed down-
ward, and the genital opening in the mid-dorsal line.

3. Place the specimen again on its right side and make a lon-
gitudinal incision, beginning close to the anus and ending at the
base of the tentacles between the smallest and the adjoining
larger tentacle (along the edge of the fifth radius). Make a
similar incision along the edge of the third radius. Cut the body
wall across under the tentacles, and in front of the anus, from
one incision to the other and remove the wall, carefully sepa-
rating it from the viscera. This will expose all organs with excep-
tion of those belonging to the radius which has been removed.
Pin down the edge of the body wall. Uncoil the alimentary canal
by cutting the mesentery close to the body wall. Cut the mes-
entery holding the gonad with its duct and push the gonad out-
side of the body cavity, taking care not to tear the duct. Push
the water lungs outside of the body cavity. Make a full page
drawing showing: outline of the body with tentacles, longitudinal
muscles, circular muscles, retractors of the pharynx, cloacal
muscles; peripharyngeal calcareous corona, oesophagus, stomach,
intestine, intestinal mesentery with the rete mirabile, water
lungs, cloaca, gonad with genital duct, ambulacral ring canal,
hydrophoric canal with madreporic plate, polian vesicles and
ampullae.

4. Put a piece of the body wall of the trivium in a test tube
with a 10-20% aqueous solution of caustic potash and boil it
over a gas flame until all the tissues have been dissolved. Allow
the particles to settle to the bottom. Pour off the liquid and
wash the sediment in water. Pick up with a hne pipette some of
the sediment and put a drop of it on a slide. Find under mi-
croscope a calcareous perforated plate of the ambulacral foot.
Make a drawing of it.

5. Do the same with a tentacle.

VENUS MERCENARIA Linnaus

Material. Live specimens of Venus mercenaria may be
bought on the market under the name of hard shell clam. To
open them they must be placed for several minutes in water
heated to 70° centigrade. After that they should be preserved
in weak formalin. Freshly killed specimens should be injected
and preserved in formalin for the study of the digestive organs.
The following is the best method. The left valve is removed in
the usual manner. The animal in its right valve is placed in a
dish with warm water. A hypodermic syringe is introduced into
the rectum a little in front of the anus and the injection con-
tinued until the mass begins to come out through the mouth.
The best mass to be used is a solution of gelatine in water made
dark red by the addition of finely powdered carmin. When the
injection is finished, the specimen is placed in 4% formalin which
gradually hardens the injection mass and makes it insoluble.
Every student should receive one freshly killed and injected
specimen. For the additional exercise, transverse sections
through hardened specimens should be studied under water.

Descriptive Part

Venus mercenaria is a common representative of the Class
Lamellibranchia which comprises all bivalves. It lives in the
mud, between tides, along the Atlantic Coast. In its structure
it is strictly bisymmetrical, the valves being right and left and
their hinge dorsal in position. The anterior or oral end of the
body may be recognized by the fact that it is broader and that the
lines of growth of the shell are procurved. The starting points
of growth in the two valves are called umbones; the first lines of

208

VENUS MERCENARIA LINN^US 20.9

growth form around each umbo and appear in the adult shell as
a distinct prominence or shoulder. The valves are held together
by a ligament and a lock. The latter is one of the few structures
which are not strictly symmetrical in Venus, for the teeth of one
shell correspond to notches in the other. The teeth are of two
kinds. In front of the ligament are prominent cardinal teeth,

gon.

Fig. 41. — Anodonia cygnea. General Anatomy, from Parker and Has-
well's Text-book of Zoology, a, anus; a. ad, anterior adductor; a. ao, an-
terior aorta; a. v. ap, auriculo-ventricular aperture; hi, urinary bladder; c.
pi. gn, cerebro-pleural ganglion; d. d, duct of digestive gland; d. gl, diges-
tive gland; d. p. a, dorsal pallial aperture; ex. sph, exhalant siphon; //, foot;
g. ap, genital aperture; gon, gonad; gid, gullet; i. l.j, inter-lamellar junction;
in. sph, inhalant siphon; int, intestine; kd, kidney; m, mantle; mih, mouth;
p. ao, posterior aorta; p. ad, posterior adductor; p. c, pericardium; pd. gn,
pedal ganglion; r. ap, renal aperture; r. an, right auricle; ret, rectum;
r. p. s, reno-pericardial aperture; st, stomach; iy, typhlosole; v, ventricle;
V. gn, visceral ganglion; w. t, water tubes.

while under the ligament are the smaller and more numerous
lateral teeth. The free edge of the shell is serrated, this being
due to its origin as a secretion of the fringed edge of the mantle
and allowing a firmer closing of the two valves. The shell is
composed of three layers: the outer layer or periostracum, the
middle or prismatic layer, and the inner or mother-of-pearl layer.

2IO MORPHOLOGY OF INVERTEBRATE TYPES

The first two are produced by the edge of the mantle, while the
third layer is a secretion of the entire surface of the mantle.
In Venus mercenaria the mother-of-pearl layer is very thick
but does not possess the aspect of the corresponding layer in
other shells. The periostracum is thin and easily worn off by
friction against the mud and sand in which the animal Hves.
The edge of the mantle or pallium leaves a distinct impression
on the inside of the shell called the pallial line. At the posterior
end of the shell this line shows a distinct triangular pallial
sinus marking the position of the retractor of the siphon. Four
muscles attached to the shell leave also their impressions on it.
These are the two powerful adductors of the valves and the two
small retractors of the foot. The impression of the anterior re-
tractor of the foot is always separate from the impression of the
much larger one of the anterior adductor of the valves and is
situated above it. The posterior retractor of the foot leaves
seldom, if ever, an impression of its own, separate from that of
the posterior adductor of the valves; usually it merges with the
latter, forming a common impression.

As has been stated, the shell is produced by the mantle or
pallium, the latter being simply a fold of the body covering.
It is not extraordinary, therefore, that both surfaces of the man-
tle, i. e., the one underlying the shell and the other facing the
mantle cavity, are lined with eipthelial cells of ectodermal origin.
There are naturally numerous glands in the outer epithehal
layer of the mantle. Between the two epithelial layers are
connective tissue and muscular fibres. The latter form three
systems: (i) muscle fibres which run in the free margin of the
mantle at right angles to its edge, (2) fibres parallel to the edge,
and (3) short fibres traversing the mantle from its outer to its
inner surface. The first of these systems forms a regular band
visible with the naked eye. Near the posterior end of the body
the right and left mantle folds are grown together in two places,
one above the other, thus forming two short tubes or siphons.
These may be readily recognized by their dark pigmentation.

VENUS MERCENARIA LINN^US 211

In some lamellibranchs they attain a comparatively enormous
length, while in others they are quite absent. The dorsal
siphon is called the cloacal or excurrent siphon, the ventral — the
branchial or incurrent siphon. These are the only channels
through which an exchange of water takes place. By the action
of the ciliated epithelium of the gills and of the mantle water
is drawn in through the branchial and expelled through the
cloacal siphon.

Inclosed in the mantle cavity is the body proper, together
with all its organs and the gills. The latter, together with the
osphradium, oral papillae and external openings of various or-
gans, are often united under the name of the pallial organ com-
plex. The lower extremity of the body forms a muscular foot
which is laterally so strongly compressed that it has been likened
to the keel of a boat. The foot is the chief organ of locomotion.
The body cavity or ccelome is reduced to the pericardium.

Digestive system. The mouth is situated at the anterior end
of the body above the anterior retractor of the foot. It is pro-
vided with an upper and an under lip, each formed by the fusion
of two oral lappets or labial palps, of triangular shape. A
pharynx is lacking and with it naturally is lacking the grating
plate or radula so characteristic of other classes of molluscs.
Salivary glands are also absent. The mouth leads directly into
the oesophagus, which is followed by the stomach. A large liver
opens into the stomach near its anterior end. The stomach is a
rather thick tube running obHquely backward and downward and
has at its pyloric end two openings. The one leads into the intes-
tine, the other into the crystal rod-sheath, a blind diverticle of the
stomach secreting a crystal rod of unknown function. The
intestine is a coiled tube lying between the pyloric end of the
stomach and the liver, under the stomach. The rectum is a very
long and thin tube. At first it runs backward to the right of the
stomach and at an almost right angle to it. It rises then at
more than a right angle until it reaches the pericard. Here it
turns again backward, perforates the ventricle of the heart and

212 MORPHOLOGY OF INVERTEBRATE TYPES

opens behind the posterior adductor of the valves into the cloacal
siphon. The stomach, intestine and ascending branch of the
rectum are imbedded in the muscular tissues of the body, the
pyloric end of the stomach and the beginning of the intestine in
the base of the foot.

Excretory system, body cavity and pericardial gland.
The majority of the molluscs possess both a primary and second-
ary body cavity. The former is represented by the system of
sinuses and lacunae by means of which a connection is established
between the arterial and venous systems. It always lacks an
epithelial covering of its own. The secondary body cavity or
coelome is reduced to the space within the pericard and the
lumen of the gonad. It is always lined with its own en-
dothelium. The cavity of the pericard communicates directly
with the mantle cavity through the excretory system. This
system consists of one pair of nephridia (called also organs of
Bojanus). They are situated on the dorsal side of the animal
between the pericard and the posterior adductor of the valves.
Each nephridium communicates by means of an open funnel
with the pericardial space, and by means of a nephridiopore
or external excretory opening, situated on a minute papilla, with
the mantle cavity. The nephridium consists of a wide tube
or sac bent on itself. The first part, communicating with the
pericardial cavity, is glandular (kidney), the second part is
non-glandular and plays the role of a urinary bladder. The
right and left nephridiopores are situated underneath the re-
spective gills immediately behind the right and left reproductive
openings.

A pericardial gland (often called Keber's organ), probably with
excretory function, lies in front of the pericard. In life it has a
dark or reddish color. Its secretion is discharged into the peri-
cardial space.

Circulatory system. The circulatory system of Venus,
as that of all other lamellibranchs, is an open system in which
the veins communicate with the arteries by means of sinuses

VENUS MERCENARIA LINN^US

213

and lacunae between the various organs, i. e., by means of spaces
devoid of a lining of their own. The heart contains arterial blood
and is composed of a single ventricle and two auricles. It is
inclosed in the pericard on the dorsal surface of the animal.
The ventricle has the shape of a pyramid with the apex pointing
anteriorly, and is perforated by the rectum. The auricles are
quite thin-walled and capable of considerable distension. The
ventricle gives rise to two aortas. The anterior aorta runs forward
above the alimen-
tary canal and
soon spHts up into
several arteries;
the posterior aorta
runs backward
below the rectum
and soon divides
into two posterior
mantle arteries.
The posterior
aorta forms a pul-
sating bulbus ar-
teriosus not far Fig. 42. — Diagram of the structure of a gill of
from the ventricle ^"^'^^"'^ cygnea after Peck, from Parker and Has-
well's Textbook of Zoology, b. v, blood vessels; /,
01 the heart and branchial filaments; /. /. j, interlamellar junction; i.
still in the pericar- /• i, interfilamentar junction; os, external ostium; os',
dial cavitv The ^^^^^^^^ ostium; wt, water tubes.

blood passes from the lacunae between the various organs into a
venous longitudinal sinus situated under the pericard, thence
through a network of veins in the nephridia into the branchial
arteries. After oxidation in the gills the blood enters the hran-
chial veins and returns to the heart through the auricles. A
very small quantity of the blood runs in other channels than
those referred to above and returns to the heart either venous
or partly oxidized in the mantle. The blood itself contains
amoebocytes and is rich in albuminous substances.

214 MORPHOLOGY OF INVERTEBR ATK TYPES

Respiratory system. Venus merccnaria has two pairs of
gills or ctenidia. However, the two gills of the same side must
be regarded morphologically as a single gill. They are out-
growths or folds of the body wall, attached to the body on each
side along an almost straight line extending from the anterior
end of the liver almost to the lower end of the adductor of the
valves. The inner gill is broader than the outer one and both
have a striated or ridged appearance. Each gill consists of two
lamellae joined at regular intervals by parallel connections called
interfoliary bridges. These bridges correspond to the grooves
between the ridges or folia of the gill and divide the space inside
the gill into water tubes. These tubes are lined with ciliated
epithelium. They open dorsally into the upper branchial cham-
ber of the gill which communicates directly with the cloacal
siphon. The folia or ridges of the gill are covered with an im-
mense number of small, ciliated filaments. The gills are natu-
rally rich in blood-vessels, the largest among which run chiefly
in the interfoliary bridges. Water is drawn into the mantle
cavity through the incurrent or branchial siphon by the com-
bined action of the ciliated epithelium of mantle and gills, passes
into the water tubes through the incurrent openings of the gill,
oxidizes the blood, collects in the upper branchial chamber and
is ejected through the cloacal siphon.

Nervous system. The nervous system of Venus mer-
cenaria is typical not only of all lamellibranchs but in a way
also of other molluscs. Although devoid of a head separate
from the rest of the body, the lamellibranchs naturally possess
a central nervous system corresponding to that in other mol-
luscs. It consists of three paired ganglia. Of these the pair of
cerebro pleural ganglia is situated above the oesophagus imme-
diately under the anterior retractor of the foot. The pair of
pedal ganglia is imbedded in the tissues of the foot close under
the place where the stomach opens into the intestine. Finally
the pair of visceroparietal ganglia is situated on the anterior
surface of the posterior adductor of the valves. The cerebro-

VENUS MERCENARIA LINN.^US

215

pleural ganglia are connected with the pedal ganglia by cerehro-
pedal connectives, and with the visceroparietal ganglia by cere-
brovisceral connectives. The cerebral ganglia furnish the anterior
pallial nerves. The visceroparietal ganglia which, like the other
h

^- \

Fig. 43. — Nervous system of Anadonta anatina after Vogt and Yung,
Anatomic Comparee. c, foot; k, pedal ganglion; /, cerebro-pedal connective;
g, cerebral ganglion; h, cerebral connective; a, anterior adductor muscle; r, g,
anterior pallial nervxs; d, liver; 5, visceral nerve; /, cerebro- visceral con-
nective; e, gill;/, edge of mantle; n, branchial nerves; w, visceral ganglion;
o, posterior pallial nerves; 6, posterior adductor muscle; p, lateral pallial
nerves.

ganglia in Venus, are of a pink or yellow color, lie so close to
each other that they have the appearance of a single ganglion.
Behind the cerebrovisceral connectives this ganglionic mass
gives off two branchial nerves and two posterior pallial nerves.

210 MORPHOLOGY OF INVERTEBRATE TYPES

Reproductive system. The sexes are separate and the
reproductive organs consist of a pair of simple gonads with a
right and left reproductive opening just in front of the corre-
sponding nephridiopore under the gill. The gonads when fully
developed fill all the space between the loops of the alimentary
canal, extending backward to the posterior end of the body.
The reproductive cells are discharged into the mantle cavity
and then pass with the water current through the cloacal si-
phon to the outside. Development is indirect, combined with
a larval stage.

Instructions

1. Place a specimen of Venus mercena-ria on edge against the
side of a tray, hinge uppermost. Make a full size drawing
showing both valves, the umbones and the ligament holding
the valves together. Label right and left valve.

2. Make a full size drawing showing the side view of the left
valve. Label umbo, lines of growth, anterior and posterior end,
dorsal and ventral surface.

3. Cut the ligament longitudinally with a very strong scalpel.
Press the ventral edges of the valves somewhat apart, carefully
introduce a fine scalpel under the left valve, between it and the
mantle, cut the anterior adductor of the valves and retractor
of the foot, and the posterior adductor and retractor as close
to the valve as possible, press down the mantle with the handle
of the scalpel and remove the left valve. Place the right valve
with the animal in it into a deep crystallization dish with water
for future examination. Examine the inner surface of the left
valve and make a full size drawing showing the umbo, Hgament,
cardinal and lateral teeth of the lock, pallial line showing at-
tachment of the mantle, attachment area of anterior adductor
of the valves and above it attachment area of the much smaller
anterior retractor of the foot, attachment area of the posterior
adductor and, if visible, above it the small area for the pos-
terior retractor of the foot, pallial sinus showing the position

VENUS MERCENARIA LINN^US 217

of the retractor of the siphon, and the serration of the valve
edge.

4. Break the shell and examine the broken edge through a lens.
Make a drawing showing the outer layer or periostracum, the
median or prismatic layer, and the inner or mother-of-pearl layer.

5. Cut off a piece of the left mantle fold near the anterior
adductor, examine it under the dissecting microscope and make
a drawing showing the fringe of papillae.

6. Cut off a larger piece of the mantle in the region of the foot.
Place it on a slide and examine with the naked eye. Make a
drawing showing the free edge of the mantle and the portion
normally adhering to the shell. In the former label fringe,
thickened portion and muscular portion.

7. Remove the entire left mantle fold by cutting it with
scissors around the retractor siphons, adductors and along its
dorsal attachment. This exposes that portion of the mantle
cavity which is called the branchial chamber. Make a full size
drawing showing the outline of the right valve, edge of right
mantle fold, anterior adductor, anterior retractor of the foot,
posterior adductor, posterior retractor of the foot, the keel-
shaped foot forming a continuation of the visceral complex, the
two triangular labial palps just above the anterior adductor;
the two left ctenidia or gills partly covering the visceral complex;
the transparent upper branchial chamber limited dorsally by a
white line — the attachment place of the gills to the body; above
this line in front, dark organs visible through the body wall
(dark brown — liver, yellow — gonad, dark — Keber's organ); the
large pericard and behind it the dark kidney.

8. Take the right valve with the animal in it into your left
hand and hold it so as to obtain a front view of the siphons (left
siphonal retractor on your left). Notice the pigmented siphons.
The upper one is the cloacal or excurrent siphon. Make an
incision in its wall and expose the anus. Make a full size draw-
ing showing left retractor of the siphon, left and right mantle
edge, anus, cloacal siphon, branchial siphon.

2l8 MORPHOLOGY OF INVERTEBRATE TYPES

9. Hold the specimen, in water, on edge, ventral side up, and
press down the outer left gill. Notice between the outer and
inner gill a white line. This is the attachment line of the gills
to each other and contains the branchial artery. Make a drawing
showing the two gills. Label also the gill-filaments and ridges
or foha.

10. Remove both gills. Place a piece of a gill on a slide and
cut it with a razor at right angles to the filaments by pressing
the razor against it (not by drawing it). Make now in the same
manner an incision parallel to and close to the first, thus ob-
taining a cross-section of the gill-filaments. Examine under
microscope (50 diameters) in a drop of water without cover glass.
Make a drawing showing three ridges or folia of the two lamellae
with their filaments, interfoliary ridges, water tubes and blood-
vessels.

11. Holding the animal on edge, ventral side up, find the
yellow visceral ganglion under the posterior retractor of the foot
with nerves forming a letter X. The two nerves running back-
ward are the pallial nerves of the siphon. The two anterior
arms of the X are formed by two pairs of nerv^es. Of these the
outer pair soon bends backward and enters the gills. This pair
of nerves is called branchial gill nerves. The two inner anterior
nerves are the cerebrovisceral connectives.

12. Remove both labial palps. Split the foot with a razor
longitudinally into two symmetrical parts. Cut off the left half
by means of an incision parallel to the attachment Hne of the
gill and running backward from the middle of the anterior
adductor. Remove the left half of the foot and find on the cut
surface of the right half the pedal ganghon — a Httle yellow or
pink body at the base of the foot. Remove the left wall of the
pericard and of the urinary bladder. Make a full size drawing
showing outline of shell; edge of mantle; both adductors of the
valves; both retractors of the foot; left retractor of siphon; left
cerebropleural ganglion, which has the appearance of a little
yellow or pink body under the anterior retractor; pedal ganglion;

VENUS MERCENARIA LINN^US 219

visceroparietal ganglion; yellowish Keber's or pericardial gland
in front of the pericard, wall of pericard; in the pericard — the
ventricle of the heart with the rectum passing through it; thin,
transparent left auricle and arterial bulb in upper corner of
pericardial cavity; urinary bladder between posterior wall of
pericard and posterior retractor of the foot; under urinary
bladder the opaque, dotted kidney or glandular portion of the
organ of Bojanus; under the attachment line of the gill and
immediately behind the posterior wall of the pericard, the
minute left excretory opening of the kidney; in front of it, the
minute left genital opening.

13. Take the injected specimen. Remove the left mantle
fold and the gills in the same manner as before. Remove Keber's
gland, left wall of pericard, left wall of urinary bladder and left
wall of cloacal chamber. With the aid of forceps and scalpel
remove carefully all muscles of the visceral complex and the
gonad. In removing the muscles take care never to pull at
more than one muscle bundle at a time and never to pull at a
deeper muscle until the bundles overlying it have been removed.
When the entire alimentary canal is exposed, it will appear as
a pink or red coiled tube. Make a full size drawing showing
outline of shell, edge of right fold of mantle, left retractor of
the siphon, siphons, anterior adductor, anterior retractor of the
foot, posterior adductor, posterior retractor of the foot, left
labial palps, dark, granulated liver; stomach extending in a curve
downward from upper edge of liver; coiled intestine, situated in
front of the stomach; thin rectum, situated behind the stomach
and rising toward the edge of the pericard, where it passes
through the ventricle, runs thence dorsally to the posterior
artery over the posterior adductor and terminates in an anus
in the cloacal chamber.

LIMAX MAXIMUS Linnaeus

Material. Limax maximus is a European species which
was introduced into this country and is now fairly common. It
is found in damp places and may be kept alive for a long time if
fed on vegetables or apples. It is by no means a typical gaster-
opod, nor even a typical pulmonate, since the great majority of
gasteropods have a helicoid body. But the size and shape of
Limax make it peculiarly adapted for dissection and after all it
possesses characteristically gasteropod features. The old method
of drowning the animal in a hermetically closed jar filled with
water which had been deprived of air by prolonged boiling, is
still giving the best results. A few drops of alcohol or chloroform
may be added to the water to hasten the relaxation of the
muscles. Freshly killed specimens are best for dissection.
Specimens preserved in formalin should be soaked in warm water
for several hours previous to dissection. Every student should
receive one specimen and a prepared slide with a section through
the hermaphroditic gland.

Descriptive Part

Limax maximus or the gray slug belongs to the order of pul-
monate gasteropods. It lives on land in damp places, feeding
on various plants. Its molluscan nature is at once apparent
from its IsLrgefoot which is its only organ of locomotion, and its
dorsal shield which is nothing but the mantle. The foot is quite
flat and extends from one end of the animal to the other. Its
border is clearly set off from the rest of the body. The head is
not distinctly separate, except on the ventral surface. Here the
7nouth is situated, which, when closed has the appearance of a

LIMAX MAXIMUS LINN^US

221

triradial slit. Above the mouth are two pairs of tentacles. The
posterior tentacles are the longer ones and carry an eye at their
end. As we shall see later, the tentacles are provided with special
muscles, are retractile and seldom fully extended even in spec-

FiG. 44. — Anatomy of Helix pomalia after Cuvier. A, anus; At, atrium;
C, ventricle; Cg, cerebral ganglion; D, intestine; Dr, mucous glands; Ed,
albuminous gland; Fl, flagellum; L, liver; M, stomach; Mr, retractor penis;
N, kidney; P, penis; PI, lung; Pr, prostata; Ps, dart sac; RS, sperm recep-
tacle; SK, columellar muscle; Sp, salivary gland.

imens which have been killed with great precaution. The shield
or mantle is oval and has an oval opening, the pneumostoma, near
its right edge, considerably beyond the middle of the entire
length of the mantle. This opening leads to the lung, and near
it, in the mantle cavity, lie the anus and the excretory opening

222 MORPHOLOGY OF INVERTEBRATE TYPES

or nephrostoma. The genital opening also is situated on the right
side, in front of the mantle, about halfway between the latter
and the anterior edge of the animal and about halfway between
the right margin of the foot and the mid-dorsal line. Limax is a
hermaphrodite, but the reproductive cells do not ripen simulta-
neously, so that the creature functions either as male or as female
at one time. During its female state, the genital opening serves
as a vulva; during the male state, the penis is protruded through
it and appears as a cylindrical body with a triangular terminal
fin. The position of the pneumostoma and of the genital opening
is characteristic of the genus.

Integument. The integument of Limax is composed of
a single layer of epithelium and of a dermis. The cuticle produced
by the epithelial cells is very thin and presents numerous pores.
Large gland cells originally belonging to the epithelial layer form
a sub-epithelial layer. Between the gland cells under the epithe-
lium are pigment cells which belong already to the dermis. The
most important elements of the latter are connective cells and
muscle fibres. An especially large caudal gland is located in the
integument near the posterior end of the body. The muscular
fibres are especially well developed in the foot.

Mantle. The shield of Limax is a true mantle, i. e., a
fold of the integument, although in this case considerably less
developed than in that of the lameUibranch Venus. The epithe-
Hal layer secretes a shell which is thin and rudimentary. More-
over, it is internal in position, lying under the epithelium, and is
apparently without function, since in other genera it is com-
pletely wanting. The pneumostoma, the position of which has
been described above, leads into the mantle cavity which is
homologous with the mantle cavity of other gasteropods, al-
though Limax is an air-breathing slug.

Muscular system. The system of muscular fibres in the
foot serves all purposes of locomotion and there are few individ-
ualized muscles in Limax. The muscles belonging to other sys-
tems will be described in their proper places. It suffices here to

LIMAX MAXIMUS LINN.^US 223

mention the retractors of the tentacles. They have a common ten-
don attached to the body wall in the mid-dorsal line inamediately
behind the mantle. The muscle runs beneath the loop of the
rectum and the coiled penis. On reaching the pharynx it spUts
into a right and left bundle, the former passing above the penis:
Each bundle gives off again a smaller bundle to the anterior
tentacle. In the posterior tentacles the retractors split up also
into two bundles, one of which is attached to the eye, the other
to the wall in the olfactory region.

Digestive system. The mouth is situated on the ventral sur-
face of the head and when closed has the appearance of a tri-
radial slit; two radii are directed obliquely forward, the third
backward. This is due to the fact that the mouth is provided
with two lips of which the upper one is almost triangular, while
the lower one is bilobed. The pharynx is almost globular and
the opening from it into the oesophagus is situated in its upper
wall. At the sides of this opening, but somewhat lower, there
are two small openings of the salivary ducts. The floor of the
pharynx is occupied by the radula. This is a chitinous curved
band with numerous rows of chitinous teeth. It is produced by
the secretion of the lining of the radular sac and may be moved
forward and backward on the floor of the pharynx by the action
of a protractor and a retractor or tensor. The former is situated
in front of the radula, under the epithelial Hning of the floor of
the pharynx; the latter under the radula itself. The oesophagus
is a comparatively thin and short tube leading into a large
stomach. Behind the oesophagus is a pair of salivary glands, one
of which, the left one, is situated somewhat in front of the other,
but both are dorsal to the stomach. Their ducts open into the
pharynx. The stomach extends through two- thirds of the an-
imal's length. It leads into a long and coiled intestine. The
rectum is somewhat larger but considerably shorter than the in-
testine and opens through an anus into the mantle cavity
near the pneumostoma. The stomach, intestine and part of
the reproductive organs are almost completely surrounded by

224 MORPHOLOGY OF INVERTEBRATE TYPES

the large liver which opens into the stomach near its pyloric
end.

Excretory system and body cavity. Li max possesses a
primary and a secondary body cavity. The former forms the
system of lacunae and blood sinuses and is devoid of its own epi-
thelial lining. The secondary body cavity or coelome is reduced
to the space within the pericard and the lumen of the hermaph-
roditic gland. It has its own endothelial lining. The excre-
tory system consists of a single nephridium situated under the
shield in contact with the pericard. A ciliated funnel or nephros-
toma leads from the pericard into the glandular portion of the
nephridium, which is usually known under the name of kidney.
The ureter is a long, curved duct beginning at the posterior end
of the glandular portion and terminating in a nephridiopore in
the mantle cavity near the pneumostoma.

Circulatory and respiratory systems. Limax has an open
circulatory system in which the central organ is represented by
the heart, while the connection between arteries and veins is
established through a system of lacunae and sinuses. The heart
is inclosed in the pericardium. It consists of a single ventricle and
a single auricle; the latter condition is due to the fact that the
animal has a single lung and consequently a single pulmonary
vein. In the normal position the auricle is in front of the ven-
tricle. The latter leads into a large aorta which soon divides
into a cephalic artery (called also aorta cephalica) and visceral
artery (called also aorta visceralis). The cephalic artery on
reaching the region of the pharynx gives off arteries to the ten-
tacles and then divides into a single buccal artery ^ and a single
recurrent pedal artery. The system of veins consists of numerous
short branches which open into two longitudinal lateral veins.
These form a renal sinus around the nephridium. Pulmonary
arteries leading from the sinus to the lung ramify into numerous
branches which give rise to equally numerous branches of the puU
monary vein. The latter conveys the oxidized blood to the heart.
The blood is practically colorless and contains amoebocytes.

LIMAX MAXIMUS LINN/EUS 225

The lung is simply the modified imier surface of the mantle
cavity. It is oval, like the external surface of the shield, but
.the rear third of it is occupied by the pericard with the heart,
the nephridium and part of the rectum. The rest of the surface
presents a network of fine blood-vessels, some of which are
arteries while others are veins. The pneumostoma or opening
leading into the pallial cavity may be closed or opened at will
by the action of a double sphincter.

Nervous system and sense organs. The nervous system
of Limax, while considerably more complicated than that of the
lamellibranch Venus, still presents the characteristic paired gan-
glia. Above the oesophagus is the pair of cerebral ganglia united
to each other by a commissiira cerebralis. Nine pairs of nerves
are given off by these ganglia to the various organs of the head;
of these the so-called buccal nerves form a pair of buccal ganglia
on the dorsal surface of the pharynx. A tenth nerve is given off
by the right cerebral ganglion to supply the penis. The posterior
region of each cerebral ganglion forms two lobes, the pedal
lobe lying more to the outside, and the visceral lobe more to the
inside. The pedal lobe of each side gives rise to a cerebropedal
connective. The two pedal ganglia lie under the cephalic artery.
Of the four pairs of pedal nerves arising from the two ganglia
the last pair is the longest and may be traced without difficulty
to the rear end of the foot beyond the visceral complex. The
visceral lobes of the cerebral ganglia give rise to a pair of cere-
bropleural connectives which end in the pleural ganglia of the
suboesophageal ganglionic mass. This mass, which lies below the
oesophagus, but above the cephalic artery, consists of five dis-
tinct ganglia. The outside pair is formed by the pleural ganglia,
then follows the pair of parietal ganglia and the centre is occupied
by a single visceral ganglion. The latter, however, must be re-
garded as the result of an early fusion of a pair of ganglia. The
suboesophageal ganglionic mass gives off nerves to the mantle,
lung and viscera.

The senses are more or less highly developed. The whole in-

Fig. 45. — The central nervous system of Helix pomaiia. Modified after
Bohmig and Meisenheimer. bg, buccal ganglion; bc7n, buccal connective;
h\, pharyngeal nerve; 60, nerve to salivary gland; 63-^4, nerves to intestine;
cb, cerebrobuccal connective; eg, cerebral ganghon; tnic, metacerebrum;
vw, mesocerebrum; pCy protocerebrum; ccm, cerebral connective; c/2, base of

UMAX MAXIMUS LINN^US 227

tegument is rich in sensory cells. Many of these may be safely
interpreted as organs of touch although the most sensitive region
is on the anterior tentacles and around the mouth. The sense
of taste is localized in special cells of the lining of the mouth
cavity. The olfactory sense seems to be restricted to the pos-
terior tentacles. These tentacles are also the seat of a pair of
well formed eyes. As regards the sense of hearing, the matter is
not so simple. There is a pair of oiocysts above the pedal ganglia.
They receive their nerves from the cerebral ganglia. They
serve as organs of equilibrium; but future investigation will
have to show to what extent the function of hearing also may be
ascribed to them.

Reproductive system. Like all pulmonate gasteropods,
Limax is a hermaphrodite and its reproductive system is very
complicated. At the rear end of the visceral complex, surrounded
by the lobes of the liver, is a hermaphroditk gland with a long and
fine hermaphroditic dtict. The gland consists of numerous lobes
but sperm and eggs never ripen at the same time. The animal
is, as we say, proterandric, i. e., it functions first as a male and
later as a female. The hermaphroditic duct finally divides,
giving rise to a rather spacious oviduct and a thin vas deferens.
The two ducts run in close contact with each other. The place
where the hermaphroditic duct divides is marked by the al-
buminous gland which opens into the oviduct and which attains
a considerable size when the animal functions as a female. The
last portion of the female duct is called the vagina and leads into
the vestibule or genital cloaca. The seminal receptacle which
functions as a bursa coptdatrix and receives the sperm during

nervus olfactorius and opticus; C3, nervus peritentacularis externus; C4,
nervus peritentacularis internus; C5, nervus acusticus; ce, nervus labialis
intemus; ct, nervus labialis medianus; cs, nervus labialis externus; C9, nervus
penis; cp, cerebropedal connective; cpl, cerebropleural connective; pcci,
pcc\\, connectives of the pedal ganglia; pi-p% pedal nerves; plpd, pleuropedal
connective;^/^, pleuroparietal connective; pig, pleural ganglion; pg\, pg2,
parietal ganglion; rg, visceral ganglion; vp, viscero-parietal connective;
VI, n. pallialis dexter externus; V2, n. pallialis dexter internus; ^3, n. intes-
tinalis; v^, nervus analis; ^5, nervus cutaneus; v^, n. pallialis sinister.

228 MORPHOLOGY OF INVERTEBRATE TYPES

copulation, opens also into the genital cloaca. The vas def-
erens separates from the female duct at the point where the
vagina begins and soon leads into the coiled penis which opens
into the vestibule. The penis is provided with a retractor penis,
a powerful muscle which arises from the dorsal body wall some-
what in front and to the left of the common retractor of the
tentacles. The penis is situated above the stomach and passes
under the retractor of the right posterior tentacle before it
reaches the vestibule. We have seen already that the vestibule
opens to the outside on the right side of the animal, in front of
the shield. This is the comfnon genital opening. When two in-
dividuals come together in the act of copulation, the penis of
the male is protruded through his common genital opening and
is introduced into the bursa copulatrix of the female through her
common genital opening. The eggs undergo an abbreviated
development in which the characteristic lar\^al stage of gaster-
opod molluscs is considerably modified and difficult of recogni-
tion.

Instructions

1. Place a specimen in a dissecting tray with water and make a
half page drawing of its right side. Label foot, mantle, pneumo-
stoma, genital opening (in front of mantle, behind tentacles),
anterior right tentacle, posterior right tentacle with eye (if
everted).

2. Make a large drawing showing the front view of Limax.
Label the four tentacles, mouth, upper lip and the cleft lower

lip.

3. Make an incision with scissors along the left side just
above the foot and extending as far as the head. Make another
incision along the right side, but extending only to the posterior
edge of the shield; remove the dorsal body wall back of the shield
by a transverse incision. Pin down the foot. Carefully turn
the shield over to the right, cutting the aorta, retractor penis
and retractor of the tentacles and freeing the rectum. Make a

LIMAX MAXIMUS LINN^US 229

drawing showing all organs in situ: immediately behind the
anterior tentacles the ovoid pharynx and above it two thin
tentacular nerves; at each side of the pharynx a dark band —
the retractors of the second tentacles ; behind these a coiled tube
passing to the right under the right retractor — the penis with
its retractor the end of which is now free, since it has been cut;
on the left side and behind the penis the two white salivary
glands and the anterior end of the thin- walled stomach; the
brown-lobed liver with two loops of the intestine imbedded in it;
three long organs behind the salivary gland and to the right of
the liver — the cephalic artery, oviduct and common retractor
of the tentacles; the aorta; at the extreme end — the hermaphro-
ditic gland; at each side of the visceral mass running along the
foot the right and left pedal nerves; the flat rectum, its end dis-
appearing in the mantle cavity; in front of it, protruding through
the wall of the mantle cavity — the cut proximal end of the
aorta; to the right of the rectum the cut attachment of the
common retractor of the tentacles and in front of this that of
the penis.

4. Pull the retractor communis tentaculorum from beneath
the intestine. Free the hermaphroditic gland and canal. Cut
the liver blood-vessel. Free the intestine from the liver and
stomach without tearing any ducts or parts and leave it on your
left. Cut the retractor of the right tentacle and free the entire
reproductive system, pushing it to the right. Make a full page
drawing showing digestive and reproductive organs. In the
former, label: pharynx, ducts of salivary glands, oesophagus,
salivary glands, stomach, liver, intestine and rectum. In the
reproductive system, label: retractor penis, penis, vas deferens,
vas efferens, bursa copulatrix, vagina, oviduct, albuminous
gland, hermaphroditic duct, and hermaphroditic gland.

5. Remove the digestive and reproductive organs. Open the
mantle cavity by carefully removing with a forceps the body
wall below it. Make a drawing showing nervous, circulatory
ard excretory systems. Label anus, pneumostoma, rectum,

230 MORPHOLOGY OF INVERTEBRATE TYPES

ureter, kidney, pericardium, auricle, ventricle, pulmonary vein
with ramifications. The nephridium is the large spongy body,
the ureter runs from its rear edge in a curve to the left (animal's
right). The heart is in contact with it.

6. Extract from under the ramifications of the pulmonary
vein the shell and make a drawing of it.

7. Remove the radula from the pharynx, place it on a slide in
water, examine under microscope (100 diameters) and make a
drawing of it.

8. Examine under high power (400 diameters) the prepared
section through the hermaphroditic gland and make a drawing
showing young eggs and spermatozoa or such stages of the male
reproductive cells as may be found.

LOLIGO PEALII Lesueur

Material. Loligo pealii is found along the Atlantic Coast
in depths below low-water mark and down to about fifty
fathoms. It should be preserved in formalin. Injection of the
circulatory system is not necessary. Each student should re-
ceive if possible one specimen of each sex.

Descriptive Part

Loligo pealii or the common squid is a typical representative
of the class Cephalopoda or those molluscs in which part of the
modified foot is situated at the end of the head and forms the
tentacles and arms surrounding the mouth. At first sight it
looks as if the squid were structured on a different plan from
other molluscs, but the difference is more of a physiological than
of a morphological kind. In order to understand the structure
of the squid one should place it head down, hind end up. In
this position the ventral surface is formed by the arms, tentacles
and funnel, the latter being also part of the modified foot. The
mouth is at the anterior end, the funnel at the posterior end of
the animal. The dorsal surface is drawn out in the shape of a
long cone in which we may recognize an antero-dorsal and a
postero-dorsal region or surface. In other words, the squid is
an excessively humpbacked animal in which this condition has
deeply modified the functions of the various regions of the body.
The squid swims forward or backward more or less in the line of
its long axis and with the postero-dorsal surface down. This po-
sition presents the least resistance to locomotion in the water.
The postero-dorsal surface functions therefore as the ventral
surface, the antero-dorsal as the dorsal surface and the dorso-

231

232

MORPHOLOGY OF INVERTEBRATE TYPES

Fig. 46. — Anatomy of Octopus after Leuckart's Wandtafeln. i, artery
of an arm; 2, nerve of an arm; j, pharynx; 4, buccal ganglion; 5, cerebral
ganglion; 6, duct of upper salivary glands; 7, funnel; 8, posterior salivary
glands; g, crop; 10, anus; 11, gill artery; 72, opening of left nephridium;
I J, gill vein; 14, gastric ganglion; 15, left auricle; 16, spiral coecum of stom-
ach; 17, nephridial sac; 18, water canal; 19, ventricle; 20, ovar>'; 21, rectum;
22, hepatic ducts; 23, mantle; 24, stomach; 25, right gill; 26, opening of the
right oviduct; 27, stellar ganglion; 28, nerve to the gastric ganglion; 29,
posterior salivary glands; 30, aorta; ji, oesophagus; 32, optic ganglion;
33, anterior salivary gland.

LOLIGO PEALII LESUEUR 233

ventral axis as the longitudinal axis. To avoid confusion the
terms anterior and posterior, dorsal and ventral will be restricted
to their morphological meaning and will have the same value as
in other molluscs. On the other hand, the terms upper and lower,
fore and hind, left and right will be applied for the various re-
gions of the body in the natural position of the swimming animal.
External Features. The body of the squid is divided into
head, neck and trunk. The mouth is surrounded by ten arms
two of which differ from the rest and are called tentacles. It is
customary to count the arms beginning at the upper surface of
the head. The fourth or last pair of arms is below the mouth
and the tentacles are between the third and fourth pair. The
third pair of arms is the longest but still considerably shorter
than the tentacles. A muscular membrane attached to the base
of the arms extends from these to the mouth. Its peripheral
portion called the buccal membrane, possesses sevejt muscular
projections each bearing two rows of small suckers on the sur-
face facing the mouth. One of these projections is situated
in the plane of symmetry above the mouth, i. e., between the
first pair of arms. The inner part of the membrane, immedi-
ately surrounding the mouth, is called the peristomial metfibrane.
In the female there is a special horseshoe organ on the buccal
membrane in the median line below the mouth. This organ
serves for the attachment of spermatophores. The four pairs
of arms are sessile, gradually tapering toward the end and all
but the third pair are trapezoidal at base. They are provided
with inner and outer marginal membranes and two rows of suck-
ers. Each sucker is composed of a short pedicel and cup the edge
of which is strengthened by a serrated chitinous ring. The
central part of the base of the cup is formed by the piston which
is the end of the pedicel. The largest suckers are at the base
of each arm. The tentacles are retractile. They are composed of
a peduncle and a terminal club on which the suckers sit in four
rows; the largest suckers are in the middle of the club. The
peduncle of the tentacles sits in a sac formed by the outer

234 MORPHOLOGY OF INVERTEBRATE TYPES

membranes of the third and fourth arm. In the adult male the
left fourth arm (the lower left arm often called. in text-books
the fifth arm when the tentacle is counted as the fourth) is
hcctocotylized; beginning with about the eighteenth to twentieth
suckers from the base of the arm the hectocotylization progresses
distally. It consists in an enlargement and swelling of the pedi-
cels and a reduction of the cups. In the thirtieth to thirty-fifth
suckers the cups disappear almost completely. The small'
suckers at the end of the club gradually become normal again.

On each side of the head is a large and \\'ell de^'eloped eye in
which one may easily recognize the transparent cornea, the
opaque iridescent iris, the pupil and the lens. In front of the
eye, near the edge of the cornea, is a small opening — the aqui-
ferous pore — which may be closed by a sphincter muscle. It
leads into a short aquiferous canal which opens by means of a
ciliated funnel into the outer chamber of the eye. On each side
of the head, beginning just behind the eye, is a fold of the skin
called the olfactory crest. Its free edge is covered up by the
mantle. On the lower surface of the squid, projecting forward
from under the mantle, is the funnel {infundibulum) often called
the siphon for the reason that its function is in part analogous
to that of the cloacal siphon of lamellibranchs. The two struc-
tures are, however, of different origin. The siphon of the lamel-
libranchs is part of the mantle, while the funnel of the squid is
part of the foot. The funnel is a hollow, somewhat flattened
conical tube open at both ends and attached to the head and
visceral mass. Its tip may be directed forward or backward at
will, and in consequence the current of water ejected through the
funnel may be forward or backward. The squid swims in the
direction opposite to that of the current.

The rest of the body of the squid is completely hidden by the
mantle, which has the shape of a long cone with two jins. The
free ventral edge of the mantle, called the collar, is drawn out
into three more or less equidistant jirojections. The anterior or
upper projection marks the end of the pen, while the two postero-

LOLIGO PEALII LESUEUR 235

lateral or lower projections mark the pallial cartilages. The
mantle is attached to the body in the median line on the back.
There are other connections, such as ligaments and muscles,
which, however, will be considered later.

Integument. The integument of the squid consists of an
epidermis and a dermis. The former is a simple layer of epithe-
lial cells, which secretes a thin cuticle and covers the entire body
including both the inner and outer surfaces of the mantle. The
epidermis, which lines the funnel, forms two pairs of glands of
unknown function. These glands are usually known under the
name of the organ of Verrill. The pen, which is an internal shell,
is also a product of the epidermis inasmuch as it is secreted by
the shell gland. Originally nothing but an invagination of the
epidermis on the dorsal surface of the embryo, the shell gland
becomes later a closed sac inside of which is inclosed the pen.
In the adult the pen is a thin and narrow, transparent, chitinous
shell extending from the upper projection of the collar to the
rear end of the mantle and situated under the latter above the
visceral mass. The median ridge of the pen represents the line in
which the two free edges of the early shell gland have grown
together to form the sac.

The dermis is composed of four layers: outer fibrous layer,
chromatophore layer, deep fibrous layer, and iridiocyst layer.
The changes in the color of the live squid are due to the second
layer and are increased by the reflection of the light from the
iridiocysts. Each chromatophore is composed of a pigment cell
and of from ten to thirty muscle cells which are attached to the
membrane of the pigment cell and receive a supply of nerve
fibres from the mantle nerves. A contraction of the muscle cells
results in an expansion of the pigment cell. On the under surface
of the fins and in all parts where the integument is not exposed to
light, both the chromatophores and iridiocysts are lacking.

Skeleton. Besides the pen which serves for the attach-
ment of the mantle the squid possesses several cartilages. These
cartilages are usually called the endoskeleton, although they do

236 MORPHOLOGY OF IXVERTEBR/VTE TYPES

not constitute a connected system. The head contains a cephalic
cartilage with several foramina or openings for the various organs
such as oesophagus, arteries, nerves, etc., and several smaller
cartilages. Other cartilages are situated in the fins, the neck, the
siphon, and the mantle. Among them, of special interest are the
nuchal cartilage which lies between the liver and the pen, artic-
ulating with the latter, and the two infundibular cartilages artic-
ulating with the two pallial cartilages.

Muscular system. The muscular system of the squid is
highly developed and complex. The foot and the mantle may be
regarded as pre-eminently muscular organs, but muscular layers
and specialized muscles are also found in connection with various
organs. Excepting the three bands of muscular fibres connecting
the mantle with the fins, the thick wall of the mantle is composed
of a sheet of circular muscle fibres, covered by and lined with
integument. Longitudinal, vertical and transverse fibres form
the muscular layer of the fins. In the funnel are both circular
and longitudinal fibres. The latter are continuous with the
heavy (siphonal) retractors of the funnel which arise one on each
side of the funnel and terminate in the pen. A pair of short
protractors of the funnel arise from the cephalic cartilage and help
to attach the funnel to the head. Inside the funnel, close to its
external opening and situated on the upper wall, is a muscular
valve preventing the entrance of water into the funnel from the
outside. On each side of the funnel is a muscular sheet or valve
projecting into the mantle cavity and preventing the passage
of water from the mantle cavity to the outside by way of the free
space between the collar and the head. A pair of heavy cephalic
retractors arise from the cephalic cartilage and terminate in the
middle of the pen, just behind the so-called nuchal retractors. The
muscular system of the arms and tentacles is too complicated to
be considered here. Some of the muscles of the various organs
will be mentioned in the course of description of the respective
systems.

Digestive system. The digestive system of the squid

LOLIGO PEALII LESUEUR 237

consists of the alimentary canal, three salivary glands, liver and
pancreas. The buccal membrane surrounding the mouth has
been described above. The mouth leads into the mouth cavity.
The walls of this cavity are formed by the biiccal bulb, a nearly
spherical, muscular body inclosed in the buccal sac and provided
with a single retractor. Inside the bulb are two powerful chitinous
jaws resembling an inverted beak of a parrot, the upper jaw
shutting into the lower one. Their action is dependent upon
special muscles. Attached to the base of the lower jaw is an
organ called the odontophore, projecting into the mouth cavity.
The odontophore consists of two parts. The lower portion of the
odontophore is a muscular tongue or ligula. At the tip of the
ligula is the opening of the salivary duct of the median or abdom-
inal salivary gland which is imbedded in the proximal (ventral)
end of the liver. The upper portion of the odontophore is called
the radula. The radula bears seven rows of sharp, chitinous
teeth and a row of plates on each side. These teeth, as well as
the chitinous band connecting and supporting them, are secreted
by the cells of the radular sac, into which the radula may be
partly withdrawn by the contraction of the retractors of the
radula. The opposite movement of the radula is brought about
by the protractors of the radula. The exposed portion of the rad-
ula is curved. The teeth, which are inclined backward, become
more or less erected when the radula is protracted; the triturating
action of the radula is therefore possible only during retraction,
and this is the reason why the retractors are stronger than the
protractors. Partially imbedded in the muscles between the
radula and the oesophagus is a pair of buccal salivary glands with
short ducts opening into the mouth cavity on the so-called
palatine lobes.

The cesophagus is a thin and long tube leading from the buccal
bulb to the stomach. On leaving the head it passes through a
deep groove in the liver to the upper surface of the latter, and
continuing in a shallow median groove extends somewhat be-
yond the middle of the liver. Here the oesophagus dips down-

238 MORPHOLOGY OF INVERTEBRATE TYPES

ward, perforates the liver, emerges on the lower surface of the
latter between the two hepatic ducts and opens into the stomach
near the middle of the visceral mass. The liver is embryologic-
ally a paired gland, but the two glands grow together during
development, and in the adult squid the liver resembles a long
cone with its base toward the head and its somewhat drawn-
out apex toward the rear. The right and left hepatic ducts enter
the nephridial sac, pass through the U-shaped pancreas and after
receiving the numerous minute ducts of the latter emerge as
hepato- pancreatic ducts. These soon unite and open as a com-
mon hepato-pancreatic duct into the stomach-pouch. The
stomach has thick muscular walls and communicates directly
with the stomach-pouch (called also coecum or spiral stomach)
which is a long sac with thin muscular walls. When fully ex-
tended the stomach pouch reaches the rear end of the body.
Close to the cardiac opening of the stomach connecting it with
the oesophagus, is the pyloric opening leading from the stom-
ach into the intestine. The cardiac and p3doric openings are
controlled by sphincters. The opening between the stomach
and the stomach-pouch is controlled by the gastric valve, that
between the pouch and the intestine by the intestinal valve, and
the one between the common hepato-pancreatic duct and the
pouch by the hepatic valve. The intestine is a short tube running
forward and downward between the two lobes of the pancreas.
The intestine soon becomes constricted and leads into the
rectum which is about half as long as the liver. The rectum ends
at the base of the funnel in an anal chamber the entrance to
which may be closed by the anal sphincter. The anus is pro-
vided with an upper and lower lip and two lateral leaf -like pro-
cesses— rectal papillce.

Embryologically and histologically the alimentary canal may
be divided into three parts. The foregut or stomadeum includes
the bulb, oesophagus and stomach, and is lined with a chitinous
cuticle. The midgut includes the stomach-pouch and the in-
testine and is lined with ciliated epithehum rich in unicellular

LOLIGO PEALII LESUEUR 239

glands. The hindgut or proctodeum is represented by the rec-
tum. Connected with the hindgut and embryologically derived
from it as an invagination of the distal end of the proctodeum is
an organ situated above the rectum and called the ink-sac. It is
shaped like an elongated gourd and its duct opens into the anal
chamber. The glandular portion of the enlarged end of the ink-
sac produces a black liquid which can be squirted through the
anus and out of the siphon, thus hiding the squid from the pur-
suit of an enemy. The iridescent color of the walls of the
ink-sac is due to iridiocysts.

Circulatory system. The circulatory system of the squid
is a closed one. There are three central pulsating organs, one
of which is arterial and is called the systemic heart, while the
other two are venous and are called the branchial hearts. The
systemic heart is situated between the two gills at their base,
below the stomach. It consists of a single chamber or ventricle.
In the European squid, Saepia officinalis, the branchial veins
are dilated at the end, thus forming two auricles. But in the
American common squid auricles are absent. The systemic
heart receives only oxidized blood through two afferent vessels —
the right and left branchial veins — and distributes it through
three efferent vessels — the anterior aorta, the posterior aorta,
and the genital artery. The openings of the branchial veins
into the systemic heart are protected each by a pair of semi-
lunar valves preventing the return of the blood to the gill. The
openings of the anterior and posterior aortas are protected each
by a single semilunar valve preventing the return of the blood to
the heart. The anterior aorta arises from the anterior projection
of the heart and runs forward to the head in close contact with
the oesophagus. In the head it gives off branches and communi-
cates through capillaries with the anterior vena cava. The
posterior aorta arises from the posterior projection of the heart
and soon splits into three pallial or mantle arteries. Of these the
median mantle artery runs forward along the median line of the
lower surface of the mantle, while the two lateral mantle arteries

Fig. 47. — Circulatory system of
the Squid (Loligo pealii) after Wil-
liams, combined from two figures.
PH, pharynx; AA. anterior aorta;
BV, branchial vein; MPA, median
pallial artery; LP A, lateral pallial
artery; GV , genital vein; AVC, an-
terior vena cava; BA, branchial ar-
tery; NAVC, nephridial portion of
anterior caval vein; PV , pallial vein;
BH, branchial heart; NVCP, ne-
phridial portion of posterior caval
vein; VCP, vena cava posterior;
GA , genital artery.

MPA.

JVCP

<z;.-GA

LOLIGO PEALII LESUEUR 241

•

pass obliquely backward to the upper surface giving off branches
to the rear half of the mantle and to the fins. The blood from
the median mantle artery passes through capillaries into the
mantle veins; that from the lateral mantle arteries into the
posterior venae cavae. The genital artery arises from the middle
of the ventral surface of the heart. Its blood passes through
capillaries into the genital vein.

The system of veins is characterized by the considerable size
of the vessels and by their special relation to the excretory sys-
tem. The anterior vena cava the branches of which collect the
blood from the head is a vessel of considerable size running in
the median line below the liver toward the nephridial sac. On
entering the latter, it splits into a right and left arm which trav-,
erse the nephridial sac and open into the respective branchial
hearts. The two arms have been called by Brooks the glandular
portion of the anterior vena cava because they are surrounded
by the urinary gland. Williams calls them the nephridial sinus
because of their wide lumen. The latter term is scarcely more
fortunate than the former, since in other cephalopods the two
arms are in no wise conspicuously enlarged. They may be
best designated as the nephridial arms of the anterior vena cava.
Into the right nephridial arm open the right pallial or mantle vein,
the right vena cava posterior, and the genital vein; into the left
the left pallial vein and the left vena cava posterior. The pallial
veins collect the blood from the forward portion of the mantle,
blood that was carried there by the median mantle artery. The
pallial veins open into the nephridial arms of the anterior vena
cava close to tne branchial hearts. The right and left posterior
venae cavae are large vessels collecting the blood from the rear
half of the mantle. On entering the nephridial sac, each vein
forms a slightly dilated nephridial portion of the {right or left)
posterior vena cava (Brooks calls this portion the glandular
portion of the posterior vena cava, while Williams considers
it to be a part of the nephridial sinus). The nephridial portion
is surrounded by the end of the urinary gland. Between the

242

MORPHOLOGY OF INVERT I-Ji RATI-: TVVKb

nephridial portion of the posterior vena cava and the nephridial
arm of the anterior vena cava is a distinct constriction.

^^ The branchial

hearts, one at the
base of each gill,
are inclosed each
in its own pericard
which is simply a
pouch of the coe-
lome. Each heart
has two openings.
One opening ad-
mits the blood
from the nephrid-
ial arm of the an-
terior vena cava
and is protected
by two semilunar
Fig. 48. — Diagram of the excretory system of valves. The other
Sepia officinalis after Vigelius (from Encyclopaedia Qpgj^jj^„ jg^^jg j^to
Britannica, Cephalopoda, Fig. 29). The nephridial ^ °
sacs are supposed to have their upper walls removed, the branchial ar-
V. c, vena cava; r. d. v. c, right descending branch of f^^y ^Lud has a
the same; r. s. v. c, left descending branch of the . . . .

same; v. b. a, vein from the ink-bag; v. w, mesen- valve consistmg Ot
teric vein; v. g, genital vein; v. a. d, right abdominal four tubercles. At-
vein; v. a. s, left abdominal vein; v. p. d, right paUial t^opVig^^ j-q each
vein; v. p. s, left pallial vein; c. b, branchial heart; t,- 1 1,

X, appendage of the same; c. v, capsule of the branchial heart IS
branchial heart; np, external aperture of the right a lymphatic or per-
nephridial sac; y, reno-pericardial orifice placing the {rnrdial dand The
left renal sac or nephridium in communication with ^

the viscero-pericardial sac, the course of which below majority of the
the nephridial sac is indicated by dotted lines; /, the smaller veins car-
similar orifice of the right side; a. r, glandular renal . v. Kl ^

outgrowths; iv. k, viscero-pericardial sac (dotted tyll^S ^"^^ Dlooa
outline). from various or-

gans are tributaries of the venous system of the anterior vena
cava. (They are not shown in Fig. 47.)

LOLIGO PEALII LESUEUR 243

Respiratory system and mantle cavity. It has been al-
ready stated that the body of the squid is surrounded by a
mantle which has the shape of a hollow, elongated cone. The
mantle is attached to the body along the antero-dorsal surface.
The free edge of the mantle, called collar, is kept in position by
these equidistant cartilages. Of these the right and left in-
fundibular cartilages articulate with the pallial cartilages of the
mantle, while the nuchal cartilage articulates with the pen in
the upper median line. We have seen besides that inside the
funnel is a valve — the infundibular valve — and on the outside
of the funnel are two lateral valves. Water is drawn rhythmically
into and forced out of the mantle cavity by the expansion and
contraction of the mantle. It enters through the free space
between the head and the collar and leaves by way of the funnel,
a reversal of the current being prevented by the valves. Sit-
uated in the mantle cavity and extending from the heart for-
ward to the base of the funnel, are two gills. As the water is
drawn into the mantle cavity by the expansion of the mantle,
it passes between the so-called pinnae of the gills and leaves the
latter by way of the branchial canals.

Each gill or ctenidium has the shape of a long, trihedral
pjrramid and consists of a core and the gill proper. The core
itself has the shape of a trihedral pyramid and is attached to
the mantle by a sheet of integument along the median line of
one of its surfaces. The other two surfaces serve as support to
the gill proper. The latter may be likened to a plume with
triangular barbs or pinnae which are attached to the core by
one edge. The branchial vein runs along the free edge of the
gill and corresponds to the stem of the plume. An eferent vessel
runs along the free edge of every pinna to the vein. The bran-
chial artery runs along the edge of the core, under the vein and
parallel to it. It gives off an aferent vessel to each pinna. These
vessels run along that edge of the pinna which is attached to
the core. The branchial artery is separated from the branchial
vein by the branchial canal which runs parallel to them and

244 MORPHOLOGY OF INVERTEBRATE TYPES

opens into the mantle cavity at the base of the gill. A ductless
branchial gland is inclosed in the core of each gill. The function
of these glands is not known.

Excretory system and coelome. The excretory system of
the squid stands in close relation to the circulatory system and
the coelome or secondary body cavity. It consists of a bilobed
sac — the nephridial sac — and a urinary gland. The sac opens
into the mantle cavity by means of two nephridiopores situated
on small papillae, one on each side of the rectum. The intestine
passes through the nephridial sac and emerges from it as the
rectum. The two nephridial arms of the anterior vena cava and
the nephridial portions of the two posterior venae cavae are in-
closed in the lobes of the nephridial sac. Surrounding these
vessels and hiding them from view are the two urinary glands
which are therefore likewise inclosed in the nephridial sac. The
coelome connects directly with the nephridial sac by means of
two short canals. It is lined with ciliated peritoneum. The
systemic heart, the branchial hearts, the stomach, the stomach-
pouch, and the gonad are contained in the coelome (visceral
sac).

Nervous system and sense organs. The nervous system
is characterized by the great concentration of the paired ganglia
of the central nervous system and the high development of the
sense organs. Four pairs of gangUa enter into the formation
of the oesophageal ring. They are inclosed in the cephalic
cartilage or skull and are almost completely fused together.
Of these four pairs of ganglia the cerebral ganglia lie above the
oesophagus, the pleural ganglia form the sides of the ring, while
the pedal ganglia and the visceral ganglia are situated below the
oesophagus. The pedal ganglia lie in front of the visceral ganglia.
Two large optic ganglia are closely connected with the pleural
ganglia. Two large stellate ganglia are situated on the inner
surface of the mantle just behind the nuchal cartilage. They
are connected with the visceral ganglia and supply nervTS to
the mantle. Several other ganglia are found in the head and

Fig. 49. — Longitudinal section through the head of the Squid showing
ganglia and cartilages. After Williams. UJ, upper jaw; IBG, infrabuccal
ganglion; SBG, suprabuccal ganglion; OE, oesophagus; PPG, propedal
ganglion; CG, cerebral ganglion; PG, pedal ganglion; VG, visceral ganglion;
PLN, pleural nerve and viscero-stellate connective; VN, visceral nerve and
viscero-branchial connective.

246 MORPHOLOGY OF INVERTEBRATE TYPES

foot. Altogether the squid has fifteen paired and one single
ganglion. The latter is the splanchnic ganglion. It supplies
nerves to the alimentar}' canal and is connected with the so-
called injrahuccal ganglia which are in turn connected with the
cerebral and pedal ganglia.

Two large eyes are situated on the head, each in its own
capsule. Each is connected with its optic ganglion by a short
optic nerve. The structure of the eye is very similar to that of
the vertebrate eye. The eyeball is provided with nine eye-
muscles. The outer chamber of the eye communicates with
the outside by means of the aquiferous pore situated imme-
diately beyond the edge of the cornea. The iris is thick and
iridescent. A small contractile fold of it projects, when in the
light, so as to give the pupil the shape of a crescent. The spheri-
cal lens is attached to the ciliary body by a sheet of connective
tissue and is brought nearer to or farther away from the retina
by the ciliary muscle. The retina is direct. Wedged in between
the eyeball and the optic ganglion, on each side of the head, are
two lymphatic glands — the so-called white bodies.

The two olfactory crests harbor in their concavity a special
sense organ possibly with olfactory function. They receive
their nerves from the pedal ganglion.

Two statocysts are situated in the cephalic cartilage below
the visceral ganglia. Each statocyst consists of a cavity lined
with epithelium. Twelve papillae project into the cavity. , In
one place the epithelium is ciliated and forms the macula statica
on which the statolyth rests. A ciliated ridge — crista statica —
winds along the walls of the cavity. A ciliated canal — Kdlliker*s
duct — leads from the cavity toward the surface. The duct ends
blindly in the adult squid, but communicates with the outside
in the embryonic stages. The statocysts receive their nerve
supply from the pedal gangUa in the shape of two eristic and
two macular nerves.

Orgatis of taste are found on the tongue or ligula. They
receive their nerve supply from the infrabuccal ganglia.

LOLIGO PEALII LESUEUR 247

Organs of touch are represented by numerous nerve end-
ings in the integument, quite especially on the arms.

Reproductive system. The sexes are separate and may
be recognized by the presence of the hectocotylized arm in the
matuie male and the horseshoe organ on the buccal membrane
of the mature female. The male reproductive organs consist of
a single testis, vas deferens, vesicula seminalis, prostata, ap-
pendix, appendicular gland, spermatophoric duct, spermato-
phoric sac and penis. The testis is situated in the coelome and
is attached to the coelomic wall by a mesentery. It has no duct,
but opens directly into the ccelome by means of a narrow slit;
thus the sperm is first discharged into the coelome. Near the
opening of the testis is a ciliated funnel which leads from the
coelome into the vas deferens. The latter opens into the vesicula
seminalis which has the appearance of a convoluted canal. This
appearance is due to its origin, but in the adult squid the vesicula
is a glandular sac with a very irregular lumen and presents three
distinct portions. A ciliated canal (Chun's canal) connects the
third portion of the vesicula with the genital sac which surrounds
the vesicula, prostata and appendix with its diverticulum and
opens into the mantle cavity by means of a short duct. The
spermatophoric duct ^ connects the vesicula with the spermato-
phoric sac. The latter leads into a muscular penis which opens
into the mantle cavity near the anus. The prostata is a large
gland, the duct of which opens into the spermatophoric duct
close to the so-called appendix with its appendicular gland.

The spermatozoa are discharged in bundles or ''ropes" from
the slit in the testis directly into the coelome. They reach the
vesicula seminalis through the vas deferens and are here in-
closed in a special case or spermatophore. The spermatophores
are stored in the spermatophoric sac before they are transferred

^The term "vas efferens" used by Brooks and later by Williams for the
designation of this canal is unfortunate, since under this term is usually
understood the small duct leading from a testis to its vas deferens in lower
invertebrates.

248 MORPHOLOGY OF INVERTEBRATE TYPES

to the horseshoe organ of the female. The spermatophores
have the shape of an elongated tube with double walls and an
ejaculatory apparatus at the rear end. The space between
the thin inner wall surrounding the sperm rope and the thick,
chitinous outer wall is filled with a liquid. At the front end
of the sperm rope is a sticky plug. The ejaculatory apparatus
consists of a flask-shaped sac prolonged into a long filament
which is partly inclosed in the case and partly free. The proxi-
mal portion of the filament is somewhat thicker than the rest and
is provided with a spiral thickening of its wall, called the spring.
The female reproductive organs consist of an ovary, oviduct,
a pair of nidamental glands and a pair of accessory nidamental
glands. The ovary is a large organ situated in the coelome.
The eggs drop into the coelome and pass into the oviduct through
the ciliated funnel of the latter. The oviduct consists of three
portions. The first portion is a tube at first running forward,
then backward and then again forward. The second portion
is considerably thicker and glandular in its structure. It is
usually called the oviducal gland and produces spherical capsules
of the individual eggs. The last portion is comparatively short
and opens into the mantle cavity on the left side of the body
near the anus. The nidamental glands are situated on the lower
surface of the viscera. They are large, rounded bodies lying side
by side in the median line of the trunk and opening into the
mantle cavity. The nidamental glands produce the substance
of which the capsules of the &gg masses are formed. In front
of the nidamental glands are the two accessory nidamental glands,
smaller in size and oval. Their function is not known.

Instructions

I. Place a squid on its right side, head towards you, in a large
dissecting tray with water. Make a full page drawling of the
left side. Label: head, first, second, third and fourth arms, ten-
tacles, eye, aquiferous pore, funnel and mantle. In the eye label:
iris, pupil and lens; in the mantle: its free edge or collar, with its

LOLIGO PEALII LESUEUR 249

left lower projection of the pallial cartilage and dorsal median
projection marking the forward end of the pen; the left lateral
fin and the chromatophores. Label also the antero-dorsal and
the postero-dorsal surface. The long axis of the squid is the
dorso-ventral axis.

2. Take the squid in your left hand holding the first, second
and third pairs of arms between your second and third fingers
and pressing down with your thumb the tentacles and the
fourth pair of arms; make a drawing showing the front view of
the head. Label arms, tentacles, buccal membrane with its
seven muscular projections, peristomial membrane, mouth,
upper and lower mandible of the beak and, in case the specimen
is a female, the horseshoe organ of the buccal membrane.

3. Make a drawing of the right tentacle showing the peduncle
and the club with four rows of suckers.

4. Cut with a razor a large sucker longitudinally. Examine
the surface under dissecting microscope. Make a drawing show-
ing the cup with the serrated chitinous ring, the piston and the
pedicel.

5. Make a drawing of the left fourth arm showing the inner
marginal membranes and two rows of suckers. In case the arm
belongs to a mature male, it will be found to be hectocotylized,
i. e., the cups are well developed only in the proximal eighteen
pairs of suckers; from here on they diminish in size and disappear
almost completely in the thirtieth to thirty-fifth pair and become
gradually normal again toward the end of the arm.

6. Sever one of the muscular projections of the buccal mem-
brane and examine under dissecting microscope. Make a draw-
ing of it showing the two rows of suckers.

7. Make with a sharp scalpel an incision in the mantle along
the upper longitudinal median line. Notice that the pen is
completely inclosed in a sac. Carefully extract the pen and
make a drawing of it.

8. Turn the squid on its back and holding the mantle firmly
with your left hand, make in it with a sharp scalpel a deep

250 MORPHOLOGY OF INVERTEBRATE TYPES

incision a little to the left of and parallel with the median line
of the lower surface. This incision should extend throughout
the entire length of the mantle and care must be taken not to
injure the organs. Place now the squid with its head away
from you, press the sides of the mantle apart and fasten them
firmly dowTi with pins, thus exposing the mantle cavity. The
funnel appears now as a hollow and somewhat flattened muscular
cone, open at both ends. The two long, grooved cartilages of
the funnel — the infundibular cartilages are now naturally sep-
arated from the pallial cartilages of the mantle with which they
articulate. The sides of the funnel are provided with muscular
valves — one on each side — which have the appearance of flaps.
The two heavy muscles arising from the sides of the funnel and
nmning parallel to each other are the retractors of the funnel.
The two plimie-like organs situated to the outside of the retrac-
tors and attached to the mantle by a sheet of integument are
the gills or ctenidia. The vessel running along each gill is the
branchial vein. The single organ lying in the median line be-
tween the retractors of the funnel is the rectum. The anus opens
into the funnel and on its sides are the two rectal papillce. From
now on the description varies according to the sex of the spec-
imen. In case of a female proceed to instruction No. 16.

Male. In case the specimen is a male it can be recognized
as such by the presence of the penis at the left side of the rectum
(on your right). Under the rectum the sides of the iridescent
ink-sac are visible. The round bodies, one at the base of each
gill, are the branchial hearts. A small gill muscle arises from the
base of each gill and terminates in the mantle. The vessel
running from the mantle to the branchial heart on each side and
in its course more or less parallel to the gill, is the mantle vein.
The two large, bluish, semitranslucent vessels running obliquely
from the branchial hearts backwards, are the posterior vence cavce.
Three smaller vessels arise from a median point between these
veins. They are the mantle arteries. Of these the two lateral
manile arteries accompany the posterior venx cavae. The

LOLIGO PEALII LESUEUR 251

median mantle artery running to the median line of the mantle,
if not severed during the dissection, will be seen running ob-
liquely to your right and terminating in the mantle where it
splits up into finer branches. The two small papillae at the
base of the rectum are provided each with an opening. These
openings are the nephridiopores. The thin, bilobed nephridial
sac occupies all the space between the base of the rectum and
the branchial hearts. Through the wall of the sac the two
urinary glaitds can be clearly discerned. Behind the left bran-
chial heart (on your right) the following organs can be seen
through the body wall (visceral sac) : the coiled vesicula semin-
alis, the spermatophoric sac and, more toward the median line, —
the large testis extending to the rear end of the sac. Make a full
page drawing showing all organs in situ.

9. Cut the lateral valves on each side of the funnel along the
infundibular cartilage. Pull the funnel gently with your fingers
until you have completely removed it together with its retrac-
tors. Open its lower wall along the median line and fasten the
sides down with pins. Make a drawing showing the infundibtdar
valve, the four organs of Verrill and the retractors.

10. The removal of the funnel with its retractors has exposed
to view the cephalic retractors with the oesophagus and liver be-
tween them, and the two large stellate ganglia situated on the
mantle under the cephalic retractors on the level of the tips of
the gills. The body wall and the nephridial sac should now be
removed with the aid of a forceps. When this has been done,
pass the end of a curved forceps between the posterior vena cava
of each side and the corresponding lateral mantle vein and sep-
arate the vessels without tearing them. In a similar manner
separate the ink-sac from the rectum leaving intact its attach-
ment to the anal chamber and push the ink-sac to your right.
Separate the penis from the underlying tissues; push the stomach
pouch to the left exposing the end of the testis with the terminal
tendon. Find the single splanchnic ganglion on the rectum near
its base.

252 MORPHOLOGY OF INVERTEBRATE TYPES

Make a full page drawing of all organs as exposed to view.

11. Remove the posterior venae cavae and the urinary gland
together with the nephridial arms of the anterior vena cava
passing through them, thus exposing the systemic heart. Cut the
sheet of integument which fastens the right gill (on your left)
to the mantle and turn the gill so as to bring into view the
branchial artery which runs from the branchial heart along the
gill. Make a drawing showing the branchial hearts, the left
branchial vein, the right branchial artery, the systemic heart,
the base of the anterior aorta and the posterior aorta with the
three mantle arteries.

12. Remove the arterial system, branchial hearts and gills.
Carefully push to the left the digestive organs, thus exposing
to view the male reproductive organs. Make a drawing showing
the testis with its mesentery or tendon attached to the wall of a
capsule of the coelome, vas deferens, the convoluted vesi<:ida
seminalis, the thin spermatophoric duct with the prostata and
appendix connected with it at its beginning, the large sperma-
tophoric sac filled with spermatophores, and the penis.

13. Open the spermatophoric sac and remove some of the
spermatophores into a shallow dish with water. Examine under
low power (microscope) showing the double walls with the
ejaculatory apparatus at the rear end. In the latter show the
flask-shaped sac and the filament with the spring.

14. With a razor cut into the head in the median line between
the eyes both above and below the buccal bulb without injuring
the latter. Carefully remove the entire alimentary canal with
its glands allowing it to float in water on one side. Make a
drawing showing the buccal bulb with the upper and lower jaw^
the anterior aorta and the oesophagus with the liver through which
they pass, the stomach, characterized by its thick muscular walls,
the thin and large stomach pouch, the short intestine passing
between the two lobes of the pancreas and the rectum with the
rectal papillcB. Show also the two hepatic ducts connecting the
liver with the pancreas and, if possible, the hepato-pancreatic

LOLIGO PEALII LESUEUR 253

ducts; the median salivary gland which appears as a small white
body imbedded in the liver on its anterior upper surface.

15. Open the buccal bulb and remove the radula. Place it
in a drop of glycerine on a slide under cover. Examine under
low power and make a drawing showing the arrangement and
shape of the teeth.

16. Female. In case the specimen is a female it can be
recognized as such by the presence of two long glands situated
behind the rectum. These glands are the nidamental glands.
They extend backwards between and beyond the branchial hearts
which appear as two round bodies at the base of the gills. Two
small accessory nidamental glands are partly covered by the
forward ends of the nidamental glands. Underneath the nida-
mental glands and therefore invisible, is the bilobed nephridial
sac. The two nephridiopores are situated on small papillae at
the sides of the nidamental glands close to their forward end.
The small vessel arising from a median point between the
nidamental glands close to their rear ends is the median mantle
artery. The two large, bluish, semitranslucent vessels running
from under the rear ends of the nidamental glands obliquely
toward the mantle are the posterior vence cavcB. The tw^o small
vessels accompanying these veins are the lateral mantle arteries.
The mantle veins run from the mantle to the branchial hearts and
their course is more or less parallel to that of the gills. A small
gill muscle is attached to the base of each gill and terminates
in the mantle. Under the rectum and almost completely hidden
by it, lies the ink-sac. On the left of the rectum (your right) is
the oviduct. The oviducal gland can be seen through the body
wall immediately behind the left nidamental gland (on your
right). The large ovary, also seen through the body wall (visceral
sac) extends backward to the end of the sac. Make a full page
drawing showing all organs in situ.

17. Cut the lateral valves on each side of the funnel along the
infundibular cartilage. Pull the funnel gently with your fingers
until ycu have completely removed it together with its rc"

254 MORPHOLOGY OF INVERTEBRATE TYPES

tractors. Open its lower wall along the median line and fasten
the sides down with pins. Make a drawing showing the in-
fundibular valve, the iour, organs of V err ill and the retractors.

1 8. Remove the nidamental and accessory nidamental glands,
the nephridial sac and the body wall. When this has been done,
pass the end of a curved forceps between the posterior vena
cava of each side and the corresponding lateral mantle vein,
and separate the vessels without tearing them. In a similar
manner separate the ink-sac from the rectum leaving intact its
attachment to the anal chamber and push the ink-sac to your
right. Separate the oviduct from the underlying tissues. Push
the stomach pouch to the left exposing the ovary and the oviducal
gland. Observe the cephalic retractors exposed to view by the
previous removal of the funnel, the oesophagiis and the liver be-
tween the retractors, and the two large stellate ganglia situated on
the mantle under the retractors on the level of the tips of the
gills. Find the single splanchnic ganglion on the rectum near its
base. Make a full page drawing of all organs as exposed to view.

19. Remove the posterior venae cavae and the urinary gland
together with the nephridial arms of the anterior vena cava
passing through them, thus exposing the systemic heart. Cut
the sheet of integument which fastens the right gill (on your left)
to the mantle and turn the gill so as to bring into view the
branchial artery which runs from the branchial heart along the
gill. Make a drawing showing the branchial hearts, the left
branchial vein, the right branchial artery, the systemic heart,
the base of the anterior aorta and the posterior aorta with the
three mantle arteries.

20. Remove the arterial system (except the anterior aorta),
branchial hearts and gills. Carefully push to the left the di-
gestive organs, thus exposing to view the female reproductive
organs. Make a drawing showing the immense ovary and the
oviduct. In the latter show the first portion, the second portion
or oviducal gland and the last portion with the genital opening
not far from the anus.

LOLIGO PEALII LESUEUR 255

21. With a razor cut into the head in the median line between
the eyes both above and below the buccal bulb without injuring
the latter. Carefully remove the entire alimentary canal with
its glands, allowing it to float in water on one side. Make a
drawing showing the buccal bulb with the upper and lower
jaw, the anterior aorta and the oesophagus with the liver through
which they pass; the stomach, characterized by its thick mus-
cular walls; the thin and large stomach pouch; the short in-
testine passing between the two lobes of the pancreas and the
rectum with the rectal papillae. Show also the two hepatic ducts
connecting the liver with the pancreas and, if possible, the
hepato-pancreatic ducts; the median salivary gland which ap-
pears as a small white body imbedded in the liver on its anterior
upper surface.

22. Open the buccal bulb and remove the radula. Place it
in a drop of glycerine on a slide under cover. Examine under
low power and make a drawing showing the arrangement and
shape of the teeth.

23. (This exercise should take the place of the two preceding
ones, if the first specimen dissected was a male. In case the
first specimen was a female this exercise should take the place
of the fourteenth and fifteenth.) Cut off the head. Divide it
in two by a quick cut with a sharp razor along the plane of sym-
metry. Examine the cut surface of the left half. Compare it
with the text figure and make a drawing showing the cerebral,
pedal, visceral and propedal ganglia; the cephalic cartilage; the
buccal bulb with the upper and lower jaw and odontophore
(ligula and radula).

24. Additional exercise. Open the mantle cavity of the in-
jected specimen as in paragraph 8. Remove all organs except
the gills and genital organs without disturbing or injuring the
blood-vessels. The liver has to be removed bit by bit to prevent
the breaking of the aorta. When the circulatory system is
completely exposed make a drawing of it in color.

MOLGULA MANHATTENSIS DeKay

Material. Molgula manhattensis is quite common along
the Atlantic Coast and may be preserved by simply putting the
specimens into weak formalin. Every student should receive
two specimens.

Descriptive Part

Molgula manhattensis is a common representative of the
Order Monascida, Class Urochorda (or Tunicata), Phylum
Prochordata. How much the Tunicates have in common with
the Cephalochorda and Hemichorda (Enteropneusta), the other
two classes referred to the same phylum by modern zoologists,
is a matter for future investigators to decide. I for one do not
think that there is much in common between these three classes.
Meanwhile Molgula may be studied as a t>'pical representative
of the Tunicates.

Molgula has almost the shape of an egg. At the anterior end
it is produced into a short tube — the incurrent or buccal siphon,
not far from which is another short tube — the excurrent or cloacal
siphon. The edge of the former has six lobes, that of the latter
four lobes. The semitranslucent external wall of the animal is the
tunique. It has the consistency of hard gelatine, is nothing but
a very thick cuticle secreted by the hypodermis of the body
wall, and is always covered with grains of sand. These sand
grains are firmly attached to the tunique owing to the adhesive
quality of the radicoid filaments which cover the entire surface
of the tunique.

The tunic contains amoeboid mesodermic cells, but these
cells do not belong to it. They are immigrants from the mesen-
chyme. Numerous branched vascular diverticla* are found in

256

MOLGULA MANHATTENSIS
S.

257

Cl

QoV'atsc.

Fig. 50. — Morpholigic type of a Urochord from'Y. Delage and Herouard,
Zoologie Concrete, an, anus; h, orifice of the oesophagus; C, heart; cd, duct
of the neural gland; cl, cloaca; E, endostyle; gi, lower groove; gl, neural gland;
sJ'Pi pyloric gland; Gn, nervous ganglion; Id, dorsal notches; n, dorsal nervous
trunk; ov, ovary; oov, orifice of the ovary; otsc, orifice of the testis; pr, peri-
pharyngeal groove; s, buccal siphon; st, stomach; tsc, testis; tt, buccal
tentacles; tv, vibratile tubercle; vl, longitudinal sinus; vt, transverse sinus.

25S MORPHOLOGY OF INVERTEBRATE TYPES

the wall of the tunique. They communicate directly with the
schizocoele. The hypodermis (called also epidermis), which
secretes the tunique, is formed by a single layer of cylindrical
epithelial cells. Under the h\^odermis are longitudinal and
circular muscle fibres especially well developed in the siphons.
The two siphons are formed by an infolding or invagination of
the body wall and are lined, therefore, with a cuticle which is an
equivalent of the tunic. Inside the incurrent siphon, at its base,
is a crown of twelve branched tentacles. Four of these are of
the first order and are so situated that two are in the plane of
symmetry and two at right angles to it. The remaining eight
tentacles are of the second order and much smaller than the
former. These tentacles form a transverse sieve at the bottom
of the siphon and undoubtedly help to keep larger foreign bodies
from entering the pharynx. The excurrent siphon has no ten-
tacles.

The plan of structure of Molgula is a strange mixture of per-
fect symmetry with asymmetry. The former is the original
plan, apparent from the structure of the pharynx, the relative
position of the nervous system and endostyle to the siphons
and of the gonads to the endostyle. But the immense develop-
ment of the pharynx and its peculiar function as an organ of
respiration has brought about the displacement of the rest of
the aUmentary canal to the left side of the body and of the heart
and kidney to the right side. The plane of symmetry bisects
the endostyle and both siphons. The endostyle is ventral, while
the nervous system, situated between the siphons, is dorsal in
position.

In order to understand the structure of Molgula, one must
remember that the pharynx has developed enormously at the
expense of all other organs. Disregarding for a moment the
siphon and cloaca, one might compare the animal to a rubber
ball in which the inner space represents the pharyngeal cavity,
while all the organs are in the wall. The difference lies in the
fact that in Molgula this wall is not solid. The organs lie, in

MOLGULA MANHATTENSIS 259

reality, in cavities reduced to mere crevices communicating with
each other. These crevices represent the modified body cavity
and are called schizoccele. The latter should not be confused
with the cavity surrounding the pharynx. This cavity, com-
posed of a right and left half separated from each other by the
adhesion of its wall along the dorsal lamina and endostyle, is
called the peripharyngeal or perihranchial cavity and represents
nothing but two blind sacs of the cloacal chamber. Being blind
sacs they of course possess an inner and outer wall. The in-
testinal loop and all other organs pressed to the one or the other
side of the pharynx lie in the schizoccele, to the outside of the
outer wall of the peripharyngeal cavity.

Digestive system. We have seen already that the in-
current siphon leads into an enormous pharynx and that the
passage to the latter is protected by a transverse sieve formed by
twelve branched tentacles. The pharynx is a very complicated
organ which functions both as pharynx and gill, and it presents
many peculiar structures. There is, not far from the tentacular
sieve, a circular ciliated peripharyngeal groove formed by two
circular ridges. The endostyle, the dorsal lamina and the six
pairs of pharyngeal ridges begin at this groove. The dorsal
lamina is a straight ridge running in the mid-dorsal line from the
peripharyngeal groove to the opening of the oesophagus. The
endostyle begins at the opposite pole of the peripharyngeal groove
and runs in the mid-ventral line to within a short distance from
the opening of the oesophagus. The endostyle is a deep ciliated
groove formed, like the peripharyngeal groove by two ridges.
Each ridge has three ciliated bands separated from each other
by simple epithelial cells. The cells at the bottom of the groove
possess extraordinarily long cilia which reach beyond the edges
of the ridges. Between the end of the endostyle and the oesoph-
agus is a fine inferior groove. Starting from the peripharyn-
geal groove and evenly interspaced between the dorsal lamina
and the endostyle are sLx pairs of pharyngeal ridges which end in
the inferior groove. They are curved in the same sense as the

26o MORPHOLOGY OF INVERTEBRATE TYPES

endostyle, but the ventral pair is much more curved than the
dorsal pair. The ridges are perforated by openings called stig-
mata. The stigmata establish a direct communication between
the pharynx and the cloacal chamber by way of the peripharyn-
geal (or peribranchial) cavity into which they open. There is
also a ciliated vihratile tubercle, or better, ridge, in shape an in-
complete oval with involuted ends, situated between the peri-
pharyngeal groove and the tentacle sieve above the starting
point of the dorsal lamina. It surrounds the opening of the ad-
neural gland. As will be shown later, the pharynx is rich in
blood-sinuses. As water is drawn into the pharynx through the
incurrent siphon, the food particles pass into the oesophagus,
driven thither by the action of the endostyle, while the water
enters the peribranchial cavity through the stigmata and is again
expelled through the cloacal siphon. The opening from the
pharynx into the oesophagus is a small oval. It is situated at
the lower extremity of the dorsal lamina. The oesophagus is
quite short and is considerably constricted at the place where it
opens into the stomach. A digestive gland composed of several
tubular lobes and known under the name of liver opens into
the stomach near its cardiac end. The stomach is a curved,
U-shaped tube of considerable diameter. It runs first down-
ward and forward and then upward. Here the canal changes its
course and structure. It becomes the intestine and runs in an
opposite direction to the stomach, dorsal to it, first downward
and backward, then upward. The anus opens into the cloacal
chamber in the mid-dorsal line.

Excretory system. Molgula has no excretory organs com-
parable to those of other invertebrates. Waste products are
stored in a bean-shaped kidney which is situated on the right
side of the animal and is devoid of any duct or opening. There is
another organ of unknown function which is supposed to be an
organ of excretion. This is the adneural gla^id,^ situated dorsal to
the nervous gangHon between the two siphons. The duct of the
1 Usually called subneural.

MOLGULA MANHATTENSIS 261

gland opens into the pharynx. The opening is surrounded by the
ciHated ridge called the vibratile tubercle.

Circulatory system. The heart is an elongated organ situated
in the pericard on the right side of the animal between the right
reproductive gland and the kidney. It has no valves and the
direction of its peristaltic pulsation alone controls the direction
of the blood-current. It is a remarkable fact characteristic of
Tunicates that the direction of the pulsation and with it that of
the blood-current, changes periodically. The blood flows in both
cases out of the heart into the vascular sinuses and returns to the
heart by way of the opening opposite to the one by which it left
it. Although the sinuses are lacking a wall of their own, so that
the blood circulation is an open one, yet they are more or less
highly differentiated and almost play the role of real blood-
vessels. The most important among them are the ventral sinus,
the pericoronal sinus, and the branchial sinuses. The blood is
colorless and contains amoebocytes.

Nervous system. The nervous system consists of a simple
ganglion situated below the adneural gland between the two
siphons. Special sense organs are lacking.

Reproductive system. Molgula is a hermaphrodite. It
has a pair of ovaries and a pair of testes. The ovary and
testis of the same side of the body are so closely applied to each
other that they produce the impression of a single gland. Never-
theless each ovary has its own oviduct and each testis its own
sperm duct. The latter is a much thinner tube and runs in con-
tact with the corresponding oviduct. The glands themselves are
situated on the sides of the animal, those of the left side in the
loop of the intestine, those of the right side above the heart.
The genital openings are situated in the cloacal chamber some-
what above the anus.

Instructions

I. Place a Molgula in a dissecting tray with water, with the
anterior siphon directed away from you and the posterior siphon

262 MORPHOLOGY OF INVERTEBRATE TYPES

to your lef l. Make a drawing showing tunic, anterior or incurreni
siphon, posterior or excurrent siphon, dorsal surface between the
siphons and attachment area on opposite surface, also external ten-
tacles of siphons if not withdrawn (six incurrent, four excurrent).

2. Remove tunic by making an incision with scissors and
tearing it apart with the aid of two strong forceps. Make a
drawing of the right side. Label circular and longitudinal
muscles on both siphons; the ganglion half way between the
siphons; the hermaphroditic gland lying across the body; the
more or less bean-shaped and transparent excretory organ under
hermaphroditic gland; the heart between them; the endostyle
(a white line along right edge of animal, ending in a circular
peripharyngeal groove); the brown liver on left, extending to
the edge of the animal.

3. Make a drawing of the left side of the same specimen.
Label ganghon; endostyle; liver, oesophagus (a short tube be-
tween the lobes of the liver); stomach, intestine and left her-
maphroditic gland with duct.

4. Support the specimen between two pins stuck into the
bottom of the tray and place it so that the incurrent siphon
would be directed away from you and the excurrent siphon above
it, presenting its sides symmetrically, with the plane of sym-
metry at right angles to the bottom of the tray. Make a drawing
showing all organs w^hich are visible in this position, and label
anus, intestine, stomach, liver, oesophagus, both hermaphroditic
glands, edge of pharynx, and endostyle.

5. Remove the right body wall, exposing the wall of the
pharynx and cloacal chamber. The operation is somewhat
delicate and care must be taken not to cut through the wall of the
pharynx. The right hermaphroditic gland, the heart and the
kidney should be removed together with the body wall. Make
a drawing showing peripharyngeal ridges with ^.he stigmata;
cloacal chamber with anus and left genital opening in the depth;
liver; proximal end of cesophagus and valve separating it from
the pharynx ; endostyle.

MOLGULA MANHATTENSIS 263

6. Open the pharyngeal cavity by cutting through the wail of
the pharynx. Begin the cut below the Hver, continue along the
cndostyle and finish at the base of the cloacal siphon. Make a
drawing showing: endostyle; peripharyngeal groove; dorsal
lamina extending to the opening of the oesophagus; crown of
inner tentacles at base of the incurrent siphon ; opening of phar-
ynx into oesophagus; pharyngeal ridges terminating in the
peripharyngeal ridge and converging toward the oesophageal
opening; volute-shaped adneural gland and, above it, the gan-
gUon.

7. With sharp scissors cut a specimen from which you have
removed the tunic, transversely a little way below the si-
phons. Make a drawing of the cut surface of the part with the
siphons, looking into the pharyngeal cavity. Label body wall;
pharynx; peripharyngeal cavity; pharyngeal ridges, peripharyn-
geal groove; vibratile tubercle; ganglion; endostyle; dorsal lami-
na; incurrent siphon with four branched tentacles of the first
order and eight of the second forming a sieve.

Printed in the United States of America.

G.