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Invertebrate Zoology. Harley Jones. Vati 
Cleave. 1931. xiv - 282 pp. McGraw-Hill Book 

In writing and revising this textbook of in- 
vertebrate zoology the author has successfully 
avoided the mistake of writing for the sake of 
impressing his colleagues in the field. In the re- 
vision, stress has been taken from the taxonomic 
organization originally employed, while general 
material has been introduced such as was former- 
ly found in textbooks of general zoology. The 
index reveals one brief reference to the entoderm ; 
the ectoderm is referred to the same page while 
the mesoderm has a paragraph on the following 
page. Nematocysts are called exclusively "net- 
tling" cells ; cnidoblasts are not mentioned : neith- 
er for that matter is the coelom given a place in 
the index although it is mentioned at different 
places in the text. The echinoderms are dis- 
cussed between the Molluscoidea and the Mol- 
lusca, and one finds scarcely a hint of the pos- 
sibility of constructing a diphyletic organization 
of the animal kingdom. This text must have 
been found useful, otherwise a second edition 
would not have been called for, and a hasty sm - 
vev indicates that the revised book is an improve- 
ment. — W. C. Allee. 

reviewed in ♦'Collecting Net" 


Reviewed in the "Collectin.^, Net" 
July 25, 1931, and presented 
to the Library of the 
Marine Biological Laboratory 



A. FRANKLIN SHULL, Consulting Editor 



A. Franklin Shull, Consulting Editor 

Fernald — Applied Entomology 
Graham — Principles of Forest 

Haupt — Fundamentals of Biology 
Haupl — Laboratory Directions for 

General Biology 
Metcalf and Flint — Destructive 

and Useful Insects 
Mitchell — General Physiology 
Pearse — Animal Ecology 
Reed and Young — Laboratory 

Studies in Zoology 
Riley and Christenson — Guide to 

the Study of Animal Parasites 
Rogers — Comparative Physiology 

Rogers — Laboratory Outlines in 
Comparative Physiology 

Shull — Heredity 

Shidl, LaRue and Ruthven — Animal 

Shull, LaRue and Ruthven — Labo- 
ratory Directions in Animal 

Snndgrass — Anatomy and Physi- 
ology of the Honeybee 

Van Cleave — Invertebrate Zoology 

Wieman — General Zoology 

Wieman — An Introduction to 
Vertebrate Embryology 

Wieman and Weichert — Laboratory 
Manual for Vertebrate Embry- 


Edmund W. Sinnott, Consulting Editor 

Adams — Farm Management 
Babcock and Clausen — Genetics in 

Relation to Agriculture 
Babcock and Collins — Genetics 

Laboratory Manual 
Belling — The Use of the Micro- 

Boyle — Marketing of Argicultural 

Brown — Cotton 

Carrier — Beginnings of Agricul- 
ture in America 
Cruess — Commercial Fruit and 

Vegetable Products 
Cruess and Christie — Laboratory 

Manual of Fruit and Vegetable 

Eames and MacDaniels — Plant 

Eckles, Combs and Mactj — Milk 

and Milk Products 
Emerson — Soil Characteristics 
Fawcett and Lee — Citrus Diseases 
Fitzpatrick — The Lower Fungi — 

Gardner , Bradford and Hooker — 

Fruit Production 
Gardner , Bradford and Hooker — 

Gdumann and Dodge — Compara- 
tive Morphology of Fungi 
Hayes and Garber — Breeding Crop 

Heald — Plant Diseases 
Horlacher — Sheep Production 

Hutcheson and Wolfe — Field Crops 
Jones and Rosa — Truck Crop 

Loeb — Kegeneration 
Loliuis and Fred — Agricultural 

Lutman — Microbiology 
Maximoc — A Textbook of Plant 

Miller — Plant Physiology 
Piper and Morse — The Soybean 
F'ool — Flowers and Flowering 

Plant s 
Rice — The Breeding and Improve- 
ment of Farm Animals 
Sharp — Cytology 
Sinnott — Botany 
Sinnott — Laboratory ^Manual for 

Elementary Botany 
Sinnott and Dunn — Principles of 

Swingle — A Textbook of System- 
atic Botany 
Thatcher — Chemistry of Plant Life 
Thompson — Vegetable Crops 
Waite — Poultry Science and 

Weaver — Root Development of 

Field Crops 
Weaver and Bruner — Root Devel- 
opment of Vegetable Crops 
Weaver and Clements — Plant Ecol- 

operculum- — 
motor mass (motorium) 

opercular fibers 

dorsal disk _ _ _ 


outer dorsal lip 

dorsal furrow 

circumesophageal ring 

oral cilia 

oral disk 

adoral membranelles 

— -inner adoral lip 
—outer adoral furrow 
-outer adoral lip 
-esophagus or cytopharynx 

esophageal rectrator strands 
ventral skeletal area 

^skeletal laminae 

ventral side of body 

boundary layer (ectoplasmic) 

ectoplasm- . 

nuclear membrane 
rectal fibers 




Frontispiece.— Reconstruction of a sagittal section of a ciliate (Diplodinium 
ecaudatum) illustrating the extent to which specialization may proceed within the 
cytoplasm of a single-celled animal. {Redrawn from Sharp.) 

r ■ 




Professor of Zoology, University of Illinois 

Second Edition 



Copyright, 1924, 1931, by the 
McGraw-Hill Book Company, Inc. 


All rights reserved. This book, or 

parts thereof, may not be reproduced 

in any form without permission of 

the publishers. 


Dedicated to 


In preparing this revision, most of the chapters have been 
rewritten and a considerable number of new illustrations have 
been added. Sections on the. Protozoa and the Porifera have 
been wholly recognised and the point of view and interpretations ^ 
have been rather radically altered so as to bring the contents of 
these chapters into accord with the recent discoveries and the 
writings of the best recognized authorities. Six years of use 
of the text with undergraduate classes have revealed the fact 
that the original edition carried its material in too formidable 
a framework of taxonomic organization. Without sacrificing 
the standards of scientific accuracy, much material of general 
biological interest has been introduced into the revised edition 
to relieve the fault of overemphasis on morphology and taxonomy 
in a general invertebrate text. The severity of the taxonomic 
organization has been reduced further by the omission of specific 
sections on the orders from most of the chapters. Pertinent 
material formerly included under the ordinal headings has been 
reorganized and placed in the general discussion of the phylum 
or of the classes where it is less apt to become lost to the student 
in the intricate maze of systematic relationships of the subgroups. 

To compensate for the removal of so much cf the formal 
framework of classification from the body of the text, an outline 
of classification has been added at the close of the discussion for 
each phylum. Herein is given a terse characterization of all 
classes, subclasses, and orders generally recognized for each 
phylum. For review, for general extension of taxonomic horizon, 
and for a comprehensive view of the interrelationships of the 
various subdivisions of the phyla, these summaries give the 
material an organization which is easily grasped and assimilated. 

Throughout the preparation of the revision, a number of 

advanced students in the University of Ilhnois have read and 

criticized sections of the manuscript and have generously assisted 

in the reading of proofs. tt i \t r^ 

^ ^ Harley J. Van Cleave. 

Urbana, III. 

April, 1931. 


An understanding of structure, development, and relationships 
of animals is essential as a basis for all lines of zoological study 
and investigation. In the early history of the teaching of 
zoology, a thorough grounding in these fundamental elements of 
the science was the chief objective of the introductory courses 
offered in colleges and universities. It was through an early 
training of this sort that most of the prominent general zoologists 
of the present and the preceding generations have passed. 
Whether they remained in the older fields of general zoology or 
entered as pioneers into virgin lines of investigation, this broad 
training gave to them an understanding of animals which could 
be gained by no other method. 

There is a growing practice of placing major emphasis upon 
biological principles in the fundamental courses in college 
zoology. There are many arguments in support of such training 
based upon principles for beginners in the science. However, in 
such a course students frequently have so httle knowledge of 
animals that there is but limited opportunity for them to have 
either a full understanding of the principles or capacity for 
making applications of them. Under these conditions it becomes 
essential that the introductory course be supplemented by a 
systematic study of organisms. Even in the instances where the 
initial course follows essentially the old type method of instruc- 
tion, the number of forms covered is inadequate and should be 
followed by further studies. 

Because of their relations to human anatomy and development 
and as specific material for instruction of premedical students, 
specialized courses in vertebrate zoology are very generally 
given. For students who are seeking training either as teachers 
or as investigators, there is fully as much need for specific courses 
pertaining to the invertebrates. 

There have been some admirable large treatises dealing with 
the invertebrates, and there have been several American labora- 
tory guides intended for such a course but there have been few 
books available as student textbooks. In the present work, the 
writer has endeavored to collate materials which will serve as a 


class room text and reference work at the same time. An 
introductory course in college zoology is assumed as a prerequisite 
to a course for which this book is designed as a text. 

More material has been included than could ordinarily be 
covered in a single semester. This offers the instructor greater 
opportunity of selection in organizing his work than is possible 
when only the materials for a specific course are presented. 

In zoological textbooks, it is frequently the custom to give 
a detailed description of a representative of each of the major 
groups. The writer has had a firm conviction that specific 
information of this nature is more readily grasped by students 
when they approach it through laboratory study of type forms 
with a mind unbiased by minute textbook descriptions. 

When a highly characteristic representative of a group is 
described in full detail in the text, the instructor has the alter- 
native of choosing some less characteristic or less easily available 
species for laboratory study or of permitting his laboratory 
instruction to become largely routine verification of statements 
set forth in the text. 

By avoiding duplication in the text of materials indented for 
laboratory approach, it has been possible to place greater emphasis 
upon biological principles and generalizations in the treatment 
of each group. 

A considerable number of my colleagues, and especially those 
in the University of Illinois, have rendered valuable assistance in 
reading and criticising portions of the manuscript and in offering 
suggestions during its preparation. It is a pleasure to acknowl- 
edge the very able assistance of Bernice F. Van Cleave whose 
criticisms of the early drafts of the manuscript and aid in reading 
the proofs have been of very great value. 

Various publishers have granted the use of cuts or the per- 
mission to reproduce illustrations. Henry Holt and Co. and P. 
Blakiston's Son and Co. have supplied cuts for a considerable 
number of illustrations. The John Wiley and Sons, The Com- 
stock Publishing Co., Gustav Fischer, and the MacMillan Co. 
have granted permission to redraw illustrations and in some few 
instances have furnished cuts. Acknowledgment of these 
courtesies is further made in the legends accompanying the 

^"^^-^S"^^^- H. J. Van Cleave. 

Urbana, Illinois, 
November, 1923. 



Preface to the Second Edition ix 

Preface to the First Edition xi 


Introduction 1 

Phylum Protozoa 20 

Introduction to the Metazoa 59 

Phylum Porifera 81 

The Goelenterates and Ctenophores 88 

Phylum Plathelmint'hes 105 

Phylum Nemathelminthes 128 

Phylum Trochelminthes 138 

Phylum Coelhelminthes (Annelida) 148 

Phylum Molluscoidea 159 

Phylum Echinoderma 165 



xiv ' • CONTENTS 


Phylum Mollusca 184 

Phylum Arthropoda — Introduction and Class Crus- 
tacea 203 

Phylum Arthropoda Exclusive of Crustacea and 

Insecta 225 

Phylum Arthropoda {Concluded) 236 

Phylogeny 253 

Index 263 



This book deals with that portion of the field of zoology which 
concerns all animals other than those included in the single 
phylum Chordata. Its scope is thus expressed in negative 
terms, for all animals lacking a backbone are invertebrates. 
Early in the development of modern classification the term 
Vertebrata was introduced as the name of a phylum or branch to 
include the four highest classes of the Linnean system. Popu- 
larly, then, all animals have become recognized as either verte- 
brates or invertebrates. More recently, three small groups of 
animals, including Amphioxus, the sea squirts, and Balano- 
glossus, have been shown to possess characters which seem to 
point to definite relationships with the Vertebrata, even though 
they lack a vertebral column. One of these characters is the 
presence of a notochord, a structure which in the embryology 
of the vertebrates is the forerunner of the vertebral column. 
As a consequence, the Leptocardii, the Tunicata, and the Enter- 
opneusta as prochordates have been combined with the true 
Vertebrata to form a phylum which bears the name Chordata. 

Technically, then, the terms chordate and non-chordate furnish 
a more sound basis for distinguishing the highest phylum of 
animals from all the lower phyla combined, but the widespread 
and popular acceptance of the terms vertebrate and invertebrate 
have operated against the general introduction of these terms. 
Strictly speaking, the prochordates are invertebrates but they 
are not discussed in this book. 

In the classification here adopted, twelve non-chordate phyla 
are recognized, as follows: Protozoa, Porifera, Coelenterata, 
Ctenophora, Plathelminthes, Nemathelminthes, Trochelminthes, 
Coelhelminthes, MoUuscoidea, Echinoderma, Mollusca, and 



Primary and Subdivisions. — It should be recalled that each 
phylum is made up of classes, orders, families, genera, and 
species in descending sequence and that each primary division 
is capable of still further arrangement into sub- and supergroups. 
Thus, when a class comprises a large number of orders, it fre- 
quently becomes convenient to combine the orders within the 
single class into two or more groups by uniting those orders 
which seem to have essential features in common into a subclass 
or a superorder. Each major or primary group may thus have 
within it secondary groups of greater scope than the next-lower 
primary division. 

Basis for Classification. — The entire system of classification" 
of animals is based upon interpretation and judgment. There 
are no absolute units of measure which determine how many 
species or how many classes any phylum is to include. Every 
attempt at classification in some measure aims to express varying 
degrees of relationship between different organisms, and every 
decision regarding relationships is based upon fixed premises. 
Proofs of phylogenetic relationships are not procurable through 
direct observation, but evidences are offered in the structure and 
development of the individual and frequently through fossil 
remains of extinct forms which serve as bridges or connecting 
links between our modern groups. Man's knowledge of all 
organisms is, at best, but fragmentary. A comparison even of 
the known facts of structure, development, and habits of two 
organisms involves much interpretation in determining the 
relative importance which is to be ascribed to each set of facts. 
Some structures and organs are highly variable even among the 
offspring of the same parents, while other characters may be 
relatively fixed in members of one group and highly variable in 
those of another. Obviously, then, all facts are not of equal 
significance in determining relationship and there is no arbitrary 
means of predicting which are of value and which are worthless 
in making comparisons. 

In common parlance, one organism is said to be higher or 
lower than another without any conscious analysis of how the 
decision has been reached. Such an expression, in order to carry 
weight, is reached only after numerous comparisons have been 
made and a decision has been reached as to what differences are 
essential and what incidental. 


Primitive vs. Degenerate. — Simplicity of organization is not 
always a safe guide in the interpretation of relationships, for sim- 
plicity of form and structure may be either primitive or secon- 
darily derived as a consequence of degeneracy. A tapeworm 
(Fig. 67) is simpler in many points of organization than a plana- 
rian (Fig. 60 c), because the tapeworm lacks a digestive system 
and special sense organs, both of which are present in the planarian. 
Primitively, every organism must be able to digest and assimilate 
its own food materials. Dependence upon some other individual 
for performing a process essential to life cannot be a primitive 
condition but is secondarily acquired and has been accompanied 
by degeneration of the digestive organs in perfectly adjusted 
parasites. The tapeworm is consequently degenerately and not 
primitively more simple than the planarian. 

Mutability of Group Concepts. — Throughout the entire system 
of classification, different premises and definitions lead to wholly 
divergent conclusions. A group which has been considered as 
a class by one zoologist may be set apart as an independent 
phylum by another, just because the same observed facts receive 
different interpretation and the same terms are defined differ- 
ently. In elementary zoological courses, a rigid system of classi- 
fication is frequently taught, but the advanced student must 
sooner or later appreciate the fact that group concepts are man- 
made devices adopted for man's convenience in his discussions 
of organisms which seem to be related. Shift of emphasis or 
new facts and new interpretations play an important part in 
formulating any scheme of classification, for, after all, group con- 
cepts are constructed to include organisms and are not necessarily 
expressions of natural laws with which the organisms must of 
necessity agree. 

The Law of Priority. — Scientific names of the larger subdivi- 
sions of the animal kingdom are subject to considerable differ- 
ences in usage, for there are no fixed rules governing the acceptance 
or rejection of names pertaining to phyla, classes, and orders. 
In contrast with this, the use of names for species, genera, and 
families is definitely controlled by rules or laws. An Inter- 
national Commission on Zoological Nomenclature was established 
by the International Zoological Congress (1895) to formulate a 
code of rules or laws governing the problems of naming animals. 
One of the basic principles of this code has been the law of pri- 
ority. According to this law the valid name of a genus or of a 


species can be only that scientific name under which it was first 
described, provided that the name is binomial and has not been 
used previously for some other animal. The same specific name 
may be used for any number of different animals belonging to 
different genera. 

The tenth edition of Linnaeus' Systema Naturae was published 
in 1758. Since the appearance of this work marks the first 
general application of binary nomenclature in zoology, this date 
has been accepted by the International Commission as the 
starting point for application of the law of priority. 

It frequently happens that after certain generic and specific 
names come into general use some one discovers that an earlier 
writer had applied a different binomial name to the same animal. 
Then the generally accepted name has to be dropped in favor of 
the prior name. Confusion and inconvenience frequently result 
from the application of this law but it seems to be the only safe 
means whereby scientific names may be accepted. 

It also frequently happens that a generic name has been used 
previously for some other genus of animals and in this instance 
only the earliest use of the name stands as valid, for the second 
time the name is used it is considered as a homonym. The name 
Trichina was applied to a parasitic worm in 1835, but in 1830 
the same name had been assigned to a genus of insects. It 
therefore became necessary to rename the worm, and even after 
the name Trichina came into very general and popular use the 
worm was renamed Trichinella. 

Since the name of a family is formed by adding the suffix -idae 
to the stem of the name of the type genus, family names are also 
governed by the same laws which govern the use of generic 

Phylogenetic Relationships. — The arrangement of animal 
groups in a list or in a book of necessity follows a sequence which 
places the lowest at one end and the highest at the other. Fre- 
quently, this arrangement carries with it the idea that classifica- 
tion intends to express a linear relationship of all forms, that 
each group has arisen from or has evolved from the one preceding. 
Such a hypothesis was held by some of the early zoologists, but 
the more commonly accepted idea of today postulates that our 
present-day animals are not directly related but that two groups 
bear closer or more distant relationship to each other chiefly 
through an extinct form which is an ancestor of both. Thus, 


for example, among the flat worms the Tiirbellaria are regarded as 
the most primitive class in the phylum Plathelminthes, while the 
Trematoda are considered as a higher class. This does not mean 
that trematodes had their origin from the turbellarians but prob- 
ably signifies that both trematodes and turbellarians had a com- 
mon origin through some ancestral group of past time which was 
neither trematode nor turbellarian. 

Thus, instead of a linear arrangement illustrating the modern 
idea of phylogenetic relationship of the animal groups, this 
relationship is best expressed as a branching treelike structure 
(Fig. 1) in which the the phylum protozoa 

various groups are repre- 
sented by the branches. 
Closely related groups 
would be represented by 
a forking of a common 
ancestral branch. One 
point wherein the tree 
comparison may not be 
carried too far is that 
many of the branches 
connecting modern-day 
forms are dead or extinct. 
Through the field of 
paleontology we have an 
imperfect picture of these 
connecting branches in 
the records left as fossil 
animals — imperfect be- 
cause there are so many 
groups like the naked 
Protozoa and the worms 
of which but few fossils ^^^- ^^ 
are known. Because of 
the significance of these extinct animals in considering our modern 
fauna, attention is directed to some of the more important fossil 
forms throughout the body of this book. The necessity of 
considering our fauna as a product of the animal life of the past 
makes it seem advisable to include here a reference table show- 
ing the sequence of some of the more important geological periods 
with some of the dominant forms of life characteristic of each. 

Di'&iincfl^ Animal 

jj Plani--Lil^ 



Phylogenetic arrangement of the classes 
in the phylum Protozoa. 


Geologic Chronology for North America 
(Slightly modified from Lull) 


Major divisions 


Dominant life 

Coenozoic. . 



Late Proterozoic. . 
Early Proterozoic. 



Late Mesozoic 
Early Mesozoic 

Late Paleozoic or 

Middle Paleozoic 

Early Paleozoic 



Late Tertiary 
Early Tertiary 










Age of man 

Age of mammals 
Rise of archaic mammals 

Extreme specialization of 

Rise of birds and flying 

Rise of dinosaurs 
Extinction of ancient life 

Rise of land vertebrates, 
modern insects, and am- 

Rise of insects and prim- 
itive reptiles 

Rise of echinoderms and 

Rise of amphibia 

Rise of scorpions and lung 

Rise of corals, nautilids, 
and armored fishes 

Rise of shelled animals and 
dominance of trilobites 

Age of primitive marine 
invertebrates, but very 
few fossils known 

No fossils; probably only 
single-celled organisms 


Many of the early naturalists believed in spontaneous genera- 
tion of life, that is, that non-living matter gives rise to living 
organisms continuously. As early as the seventeenth century 
Redi and some other leaders in science advanced evidences that 
the more complex organisms cannot originate in this manner. 
It is only within the past century, however, that the spontaneous 
generation of the lower organisms was finally discredited and 
disproved. That all life comes only from living things was only 
then established as an axiom. Power of reproducing its like is 
an inherent property of protoplasm which sharply differentiates 
it from all lifeless matter. Though all forms of life possess this 


power of reproduction, the detailed steps in the genesis of new 
individuals are far from uniform throughout the animal kingdom. 
Of the varied methods of reproduction encountered among ani- 
mals, common features permit them all to be classified as either 
asexual or sexual. Any reproductive process which involves the 
genesis of new individuals through the functioning of specialized 
cells, termed the gametes or germ cells, is sexual. Conversely, 
any reproductive process which does not involve the functioning 
of germ cells is asexual. 

Asexual Reproduction. — In the simplest forms of animal life, 
the body of a single individual becomes divided into two or more 
parts each of which by the growth processes assumes approxi- 

e\S® 'S 

Fig. 2. 

Fig. 3. 


Fig. 4. 

Figs. 2-4. — Diagrams to illustrate the methods of asexual reproduction: 
2, binary fission in Paramecium; 3, multiple fission or spore formation in a proto- 
zoan; 4, budding in Acanthocystis. 

mately the size of the original organism which produced it. This 
kind of reproductive process wherein no germ cell functions is 
termed asexual and depends upon the power of part of an organ- 
ism to reproduce the whole (Figs. 2 to 4). If the products result- 
ing from such a division of the body are approximately equivalent, 
the term fission is applied to the process. Fission is further 
recognized as simple or binary (Fig. 2) and multiple (Fig. 3) 
depending upon whether two or numerous individuals result. 
In binary fission, the direction of the dividing plane is frequently 
indicated by specifying whether the fission is longitudinal or 
transverse. Fission is characteristic of many Protozoa and 
occurs in isolated instances through many metazoan groups, 
but in the latter asexual reproduction through the formation of 
buds is more frequent. In budding (Figs. 4 and 36), a relatively 
small part of the body of a parent individual becomes modified 
as a starting point of a new individual. Only after development 
has gone to certain stages is the bud recognizable as similar to 



the parent. Ultimately, the bud may separate from the parent 
and become an independent organism, or, in some instances, 
it remains attached permanently and successive generations 
retain bodily connection, thereby producing a colony. 

Buds usually occur on the external surface of the parent indi- 
vidual, but in some instances groups of cells within the body 
become surrounded by a membrane and form what are desig- 
nated as gemmules or internal buds. Typically, these are highly 
resistant bodies which arc liberated by the disintegration of the 

Fig. 5. — Eudorina elegans Ehrenberg. A, adult colony, X475; B, daughter 
colony produced by division of one of the cells of ^4, X730; C—E, development of 
spermatozoa from a mother cell; F, spermatozoa. {From Shull, LaRue, and 
Ruthven after Wed and Goehcl). 

body of the dead parent and as resting gemmules tide the species 
over times and conditions which are unsuited for a vegetative 
period. The gemmules of sponges and statoblasts of the Polyzoa 
are among the best examples of internal buds or gemmules. In 
both origin and structure, these are multicellular. 

Sexual Reproduction. — There are relatively few Metazoa which 
rely upon asexual reproduction exclusively. More frequently a 
process involving the specialization of cells for reproduction is 
encountered. Any cell specialized for this purpose is designated 
as a gamete or germ cell, and reproduction through such an 
agency is termed sexual reproduction. Even among the Protozoa 
(Fig. 5) germ cells are encountered. Despite their relative sim- 
plicity in organization, there are comparatively few protozoans 
for which a complete developmental history is known. For 
some of the most commonly known forms, ignorance of anything 



beyond an asexual multiplication has led to a widespread belief 
that simple division is the only means of reproduction. How- 
ever, in the Protozoa which have been thoroughly investigated, 
it has been found that an asexual cycle (schizogony) is frequently 
followed by a sexual cycle (sporogony). Presence of a compli- 
cated life cycle, involving an alternation of generations, has 
thus been established for some single-celled creatures (see Fig. 

Specialization of Gametes. — In its most characteristic form, 
sexual reproduction involves the complete fusion of two special- 
ized gametes to form a zygote (Fig. 6). In some instances, 

Fig. 6. — Diagram of fertilization and cleavage. It is assumed that maturation 
of the egg has been completed before the entrance of the spermatozoon. {After 

however, the fusing cells are termed isogametes because they 
seem to be alike in form and function. Though isogametes have 
considerable significance in hypothetical discussions of the origin 
and differentiation of sex cells, they are of relatively infrequent 
occurrence among animals. Even in some instances in which 
fusing gametes are indistinguishable in size the two react differ- 
ently to cytological stains and thus give evidence of a probable 
differentiation even though morphological differentiation is 

The gametes of most animals show two distinct lines of differ- 
entiation. The enlargement of the cell through accumulation of 
yolk or deutoplasm is characteristic of macrogametes or egg 
cells. In some instances, the ovum does not contain all of the 
stored food material but is accompanied by special storage cells 
as yolk cells or follicle cells. Special protective envelopes or 



shells are frequently developed about the egg cell. The male 
germ cell or microgamete is usually very minute and represents 
a specialization for effective locomotion. 

In many kinds of animals, but a single type of germ cell is 
produced by any given individual. Those which produce macro- 
gametes or ova are designated as females, while those which pro- 
duce microgametes or spermatozoa are called males. When 
both kinds of germ cells occur in the body of the same individual 
such a one is said to be hermaphroditic. Usually this condition 
involves the presence of two distinct gonads but in some molluscs 
there is a hermaphroditic gonad which produces both eggs and 

Fig. 7. — Two cell stage in development of the egg of Ascaris tncgalocephala, 
showing chromatin diminution in somatic cells. Germ cells are derived from 
blastomere marked s which retains all of its chromatin. (After Boveri). 

Early Isolation of Germ Cells. — Long before germ cells become 
functional they are readily distinguishable from the other cells 
of the body. The exact point at which this differentiation of 
germ cells from somatic cells is accomplished has been determined 
for only a few animals but in these instances the cells destined to 
form the gametes are distinguishable at a remarkably early period 
in development of the young. In Ascaris megalocephala, a 
nematode of the horse, the first division of the fertilized egg (Fig. 
7) separates two cells only one of which (s) retains all of its 
chromatin intact. Portions of the chromosomes in the other 
cell are cast off into the cytoplasm. It is only from the cell with 



unaltered nuclear content that the germ cells have their origin. 
But even in this instance the separation of germ cells from somatic 
cells is not so direct as this narration might imply. In each 
of about the first five divisions of the cell with its full complement 
of chromatin one of the two resulting cells undergoes a chromatin 
diminution but at the end of the fifth or sixth cleavage the germ 
plasm has been isolated and in all subsequent divisions every 
blastomere gives rise to either 
somatic or germ cells. 

In several other instances, it 
has been noted that the cells 
which later produce the gametes 
are distinguishable early in em- 
bryonic life. In insect embryos, 
the cells from which the gametes 
are formed are distinctly larger 
than the somatic cells. Thus 
in the fly Miastor, the primor- 
dial germ cells (Fig. 8 gc) are 
readily distinguishable from the 
somatic cells (cl) early in the 
cleavage of the egg. 

Gametogenesis and Fertiliza- 
tion. — Even though the cells 
which later go to form the 
gametes are early distinguish- 
able from the somatic cells, they 

must pass through a COmpli- Fig. 8.— Development of a centro- 
Cated series of changes before l e c i t h a l egg of the fly Miastor. 
. , , , ^ . . Blastula stage showing germ cells (gc) 

they are capable of union m 
fertilization. These changes are 
collectively termed gametogene- 
sis or, in the male, spermato- 
genesis and in the female, oogenesis (Fig. 9 B-D). Three 
periods are recognizable in gametogenesis: a multiplication 
period wherein the relatively small number of primordial germ 
cells is greatly augmented; a growth period, which involves fun- 
damental changes in the nuclear organization of the cell and 
in relative size; and finally a maturation period during which the 
chromosomes in the gametes are reduced to one-half the number 
characteristic of the somatic nuclei. The modified nuclear divi- 

at posterior extremity readily distin- 
guishable from the other blastomeres 
{cl) . {From Shull, LaRue, and Ruthven 
after Hegner). 



sion resulting in a reduction of number of chromosomes is known 
as meiosis. The chromosome number in cells which have not 
undergone maturation is said to be diploid, because each cell 
contains two sets of chromosomes, one of which is derived from 
its male parent and the other from the female parent. Following 
maturation, the gametes are said to have the haploid or reduced 
number of chormosomes. 

Fig. 9. — Maturation of the egg and fertilization in Ascaris. A, spermatozoon 
about to enter egg; B, spermatozoon inside egg; egg nucleus in anaphase of first 
maturation division; C, completion of first polar body; D, late anaphase in forma- 
tion of second polar body; E, masturation of egg completed, male and female 
pronuclei, each with two chromosomes, meeting; F, formation of first cleavage 
spindle from centrosome of spermatozoon, with two paternal and two maternal 
chromosomes in late prophase. {From. Sharpe after Hcrtwig). 

The reduction in chromosome numbers is in preparation for 
fertilization or the union of two gametes to form a fertilized egg 
or zygote (Fig. 9). By this reduction phenomenon the fertilized 
egg, and consequently all of the cells resulting from its mitotic 
division, retain a constant number of chromosomes from genera- 
tion to generation. Through fertilization a sperm cell with the 
haploid number of chromosome unites with an egg cell with the 
haploid number to form a zygote whose nuclear composition is 


In the maturation process, two kinds of chromosomes are usu- 
ally distinguishable. These are the ordinary chromosomes, 
which are designated as the autosomes, and others which differ 
from them both in appearance and in behavior and are termed 
the sex-chromosomes or heterosomes. 

In some instances, the immature germ cells of each sex have 
a pair of sex-chromosomes. Typically, the sex-chromosomes of 
the female are both alike and are called the X-chromosomes. 
On the other hand, in the immature germ cells of the male one 
of the sex-chromosomes is frequently smaller than the other 
and is designated as the Y-chromosome. Under these conditions 
the autosomes of the maturing male germ cells are distributed 
evenly and equally among the mature cells, but the sex-chromo- 
somes become segregated so that any individual cell in addition 
to its autosomes contains either an X- or a Y-chromosome but 
never both. In the male, then, half of the sperm cells contain 
the Y-chromosome but no X, while the other half contain an 
X-chromosome but no Y. Every mature egg cell contains the 
autosomes and a single X-chromosome. Fertilization of an egg 
cell by a spermatozoon containing the Y-chromosome produces a 
zygote from which only a male could develop, but fertilization 
by a spermatozoon with an X-chromosome produces a zygote 
from which only a female could develop. 

The Y-chromosome may be entirely lacking in some species. 
Under these conditions, there are two kinds of male sex cells 
formed: one with the autosomes plus an X-chromosome, the 
other with autosomes alone. 

While the dimorphism in the germ cells is usually characteristic 
of the male, there are some instances recorded, as in the birds, 
in which all of the male germ cells are alike, but the female germ 
cells show chromosome differences. 

Composition and Cleavage of Zygote. — Except for the possi- 
bility that one of the fusing germ cells may have one more 
chromosome than the other, the two gametes contribute equally 
to the chromatin content of the zygote. The cytoplasm of the 
zygote is that contained in the mature egg except for a practically 
insignificant amount brought in by the entering sperm cell. The 
stored food material, which provides the energy requisite to the 
life processes of the embryo, is furnished by the deutoplasm of 
the egg cell, except in those forms which have accessory yolk 
cells accompanying the egg and those which receive nourishment 


from the parent individual. The so-called middle piece of the 
sperm cell contains a centrosome. Barring some very unusual 
circumstances, this centrosome brought in by the male pronucleus 
forms a mitotic spindle within which the chromatin of both the 
male and female pronuclei becomes commingled. The mitotic 
division of the fertilized egg which ensues is followed by cleavage 
of the surrounding cytoplasm. Through a continued sequence 
of mitosis and cleavage, large numbers of cleavage cells or 
blastomeres are formed. 

Cleavage Patterns. — The relative size and arrangement of 
these blastomeres is greatly influenced by the amount and distri- 
bution of the stored material within the egg. If the yolk is evenly 
distributed, the egg is said to be homolecithal and it undergoes 
a complete cleavage resulting in the formation of numerous cells, 
all of which are practically uniform in size. However, deuto- 
plasm is heavier than the surrounding cytoplasm, and in many 
instances tends to accumulate at the vegetative pole of the egg. 
The term telolecithal is applied to such an egg. Yolk serves 
as a mechanical obstruction to the paths of the cleavage planes. 
Consequently, if the vegetative pole is heavily yolk laden, cleav- 
age is restricted to a disc of cytoplasm at the animal pole. How- 
ever, in some telolecithal eggs, the entire cell cleaves, but, since 
the mitotic spindle tends to take a position in the center of the 
cytoplasm of the cell, spindles will be formed nearer the animal 
than the vegetative pole and as a consequence the resulting cells 
are unequal in size. The cells at the animal pole are much 
smaller than those at the vegetative pole. Some arthropod eggs 
have the yolk collected in the center and are therefore said to 
be centrolecithal. Cleavage in such an egg is restricted to the 
layer of cytoplasm surrounding the yolk (Fig. 8) and is referred 
to as superficial cleavage. 

The blastomeres resulting from cleavage of the fertilized egg 
assume various arrangement patterns. In some groups of ani- 
mals, the blastomeres fail to follow any orderliness in their 
formation and arrangement. Instances of this sort are desig- 
nated as indeterminate cleavage. This condition stands in sharp 
contrast with that found in some other groups, the members of 
which have cleavage processes so orderly that it is possible to 
predict with exactitude the direction which successive cleavage 
planes will take. Determinate cleavage, as this is called, renders 
it possible to trace the history of each blastomere, to follow 


through a cell Hneage the succession of blastomeres, and to 
tell what organ or structure of the larva or adult is ultimately to 
be formed from any given blastomere. This recognition of a 
fixed arrangement of the cells is especially characteristic of anne- 
lids and molluscs (Fig. 10). The annelidan cross and the mollus- 
can cross are terms which are applied to the fixed cleavage 
patterns characteristic of these respective groups. The orderly 
processes which give rise to fixed relations of the cells of the 
embryo are carried throughout the entire life of some organisms, 
for scattered through the various phyla there are examples of 
animals which in the adult stage possess an absolutely fixed 

a b c 

Fig. 10. — Diagrams showing cleavage pattern in MoUusca and Annelida, 
a, blastomeres of Nereis iaftcr Wihon) ; h and c, blastomeres of Crepidula. 
The stippled cells form the Molluscan Cross. The four unshaded cells at the 
pole of the embryo are termed the apical cells, while the unshaded cells radiating 
from these comprise the Annelidan Cross. {After Conklin). 

number of cells or of nuclei in all or in part of the organs of 
the body. Such a condition is spoken of as cell constancy or 
nuclear constancy. 

Blastula and Gastrula. — All eggs pass through a stage called 
the blastula wherein the blastomeres are usually arranged in a 
single layer. Typically, the cells surround a central fluid-filled 
cavity, but in centrolecithal eggs the fluid-filled cavity is replaced 
by a solid yolk mass. The cells in certain regions of the blastula 
multiply more rapidly than the surrounding cells. Typically, in 
the homolecithal egg this unequal growth causes part of the wall 
of the blastula to be forced within the blastula cavity as an 
inpocketing or invagination. This stage in development is 
termed a gastrula and is composed typically of an outer layer of 
cells, the ectoderm, and an inner layer, the entoderm, which 
surrounds a central cavity, the gastrula cavity or archenteron. 
Primitively, this archenteron is in communication with the exte- 


rior through an opening, the blastopore. A gastrula such as that 
just described occurs only in the homolecithal type of egg. In 
telolecithal eggs, with total cleavage, the small blastomeres at the 
animal pole multiply much more rapidly than do the heavily yolk- 
laden vegetative cells. As a consequence, the cells from the 
animal pole grow down and surround those of the vegetative pole 
which thereby become the entoderm, while the small surface cells 
are recognizable as ectoderm. In such a gastrula, there is no 
archenteron within the entoderm. Gastrulation by this method 
is termed epibolic gastrulation. In telolecithal eggs with only 
partial cleavage, a modified invagination occurs through a shoving 
in of cells near the edge of the cleavage disc. As mentioned 
above, blastula formation in the centrolecithal eggs gives rise to a 
layer of cells arranged around the central yolk mass. In later 
development, each cell of this blastula undergoes a cleavage 
parallel to the surface of the egg. Thereby a two-layered condi- 
tion or a gastrula is attained and the process is spoken of as 

Mesoderm and Later Development. — The addition of the third 
body layer, the mesoderm, between the ectoderm and the ento- 
derm is one of the most conspicuous and most significant features 
in later development of all animals above the coelenterates. 
Development from this point onward is subject to so many indi- 
vidual differences that few general statements may be made. 
In many of the more generalized groups, the attainment of the 
gastrula stage marks the beginning of an independent existence. 
By means of cilia the larva has the power of movement and 
through the blastopore food is taken into the archenteron where it 
is digested and assimilated. However, in many groups and espe- 
cially in terrestrial and fresh-water forms, the individual is carried 
far past the gastrula stage while still confined within the egg 
membranes. In the extreme of such cases, the individual 
which emerges from the egg is essentially like the adult except in 
size and stage of development of the reproductive organs. 
Throughout its development, the young of such a form would be 
referred to as an embryo. On the other hand, if the individual 
produced from the egg lacks some structures characteristic of the 
adult and possesses others which are lost in later development, the 
young is ordinarily termed a larva. Many different larval forms 
are encountered in the various invertebrate groups but these are 
so numerous and have such diverse forms that descriptions of 


them will be given in the discussion of the groups in which each 
type belongs. It should be mentioned, however, that these 
larval stages are frequently considered as having great phylo- 
genetic significance. Groups having fundamentally similar 
larvae are usually considered as having developed from a common 
ancestral form, for there are many evidences supporting the law 
of biogenesis which states that ontogeny is a brief recapitulation 
of phylogeny. 

Parthenogenesis is sexual development without fertilization. 
The eggs of many kinds of animals undergo a modified type of 
maturation and are then capable of development without fertili- 
zation. It is not uncommon for parthenogenesis and true sexual 
reproduction to alternate in the life cycle of the same species. 
The parthenogenetic habit has become thoroughly established in 
some species of animals. Frequently, in such instances, males are 
extremely rare and there are some species which reproduce 
parthenogenetically in which males have never been observed. 

Paedogenesis and Polyembryony. — The gonads of some indi- 
viduals become functional before the body reaches adult form. 
In some of the dipterous insects (flies of the genus Miastor, for 
example), the larvae become precociously mature and as maggots 
produce mature eggs. These eggs undergo parthenogenetic 
development within the body of the larva and a new genera- 
tion of larvae is produced within the body of each. This 
type of precocious parthenogenetic development is termed 

It has been demonstrated that in some instances a single 
fertilized egg may give rise to more than one individual. This 
condition, which has been termed polyembryony, occurs in both 
invertebrates and vertebrates and is especially characteristic of 
some insects. This power of development of an entire individual 
from a portion of an embryo calls to mind the fact that experi- 
mentally the blastomeres of many of the marine invertebrates 
may be isolated and each blastomere thus separated forms a 
complete individual. 

Breeding Habits. — Regarding breeding habits, numerous 
different conditions exist. In the Protozoa and among many 
lower Metazoa, isolated gametes are set free into the surrounding 
medium and fertilization occurs entirely apart from the bodies 
of the parent individuals. Motility, at least of the male gametes, 
and chemical emanations from the female gamete bring the two 


germ cells together and thus insure fertilization. Under such 
conditions, the larva or young of Metazoa passes through its 
embryological stages until it is capable of independent mainte- 
nance. Among many Metazoa, this stage is reached with the 
gastrula, for at this time the larva contains organs differentiated 
sufficiently to enable it to ingest and utilize food from the outside 
world. For its metabolism prior to this time, the embryo has 
been dependent upon the food substances stored within the egg. 
Correlated with increase in quantity of stored food material 
within the egg, the young of many species undergo complete 
development upon the materials thus furnished, as mentioned 
above, and never lead an independent larval existence. 

Frequently, the eggs are fertilized while still within the body of 
the female. This may involve a copulation whereby sperm cells 
are deposited within the body of the female by the male through 
some sort of intromittent organ, or, in some instances, as in 
the rotifers and leeches, sperm cells are united in groups called 
spermatophores which penetrate the body wall of the female. 
Parts of the body of the female may be modified as a seminal 
receptacle for receiving the sperm cells from the male, though 
fertilization of the eggs may be deferred for some time, even 
several years, after copulation has taken place. In hermaphro- 
ditic forms, there are several distinct methods of fertilization. 
Reciprocal copulation (Fig. 78), whereby sperm cells from one 
hermaphroditic individual fertilize the eggs of the other, occurs in 
many instances. Self-fertilization is also encountered in some 
hermaphroditic organisms. 

Birth Habits. — When eggs are discharged before cleavage has 
begun, the species is said to be oviparous. Frequently, the egg 
is retained within the body of the female until the young is fully 
formed and the larva emerges from the egg membranes before it 
leaves the body of the parent. This condition is designated as 
viviparous. Any condition intermediate between these two 
extremes is termed ovoviviparous. In this condition, the egg has 
at least started to divide before it leaves the body of the parent. 
In popular, and sometimes in scientific, usage the term oviparous 
is applied to any condition in which the female brings forth eggs, 
regardless of the stage of development of the contained ovum or 
embryo. The term ovoviviparous is in this instance restricted 
to that condition in which the embryo is liberated from the egg 
membranes just before it leaves the maternal body. 


General References 

The references here cited contain information bearing upon most of the 

chapters throughout this book but are not repeated at the end of individual 


Bronn, H. G. (various authors). 1880 . "Klassen und Ordnungen des 

Tier-reichs." Leipzig. 

Delage, Y. et Herouard, E. 1896-1903. "Traite do zoologie concrete.'' 
Paris, Schleicher Freres. 

Harmer, S. F. and Shipley, A. E. (various authors). 1891-1909. "The 
Cambridge Natural History." London, Macmillan. 

Hertwig, R. (translated and edited by Kingsley, J. S.). 1912. "A Man- 
ual of Zoology." New York, Holt. 

Johnson, M. E. and Snook, H. J. 1927. "Seashore Animals of the Pacific 
Coast." New York, Macmillan. 

Lang, A. (translated by Bernard, H. M. and M.). 1891. "Text-book of 
Comparative Anatomy." London, Macmillan. 

Lankester, Sir R. (various authors). 1909. "A Treatise on Zoology." 
London, Black. 

Parker, T. J. and Haswell, W. A. 1928. "A Text-book of Zoology," 
Vol. 1. London, Macmillan. 

Sedgwick A. 1898. "A Student's Text-book of Zoology." London, 

VoGT, C. und Yung, E. 1885-1888. "Lehrbuch dor praktischen vergleich- 
enden anatomie." Brunswick. 

Ward, H. B. and Whipple, G. C. (various authors). 1918. "Fresh-water 
Biology." New York, Wiley. 

von Zittel, K. A. 1900. "Text-book of Palaeontology." English transla- 
tion by Eastman, C. B. London, Macmillan. 


The Cell as an Individual. — The Protozoa represent the 
simplest organization of animal protoplasm to form independent 
units or individuals. Though in the typical examples the indi- 
viduals comprise single cells (Figs. 15, 19, 20), the cytoplasm 
of these cells has undergone various lines of differentiation and 
specialization. Thus these cells are far from simple or "undif- 
ferentiated" when considered from the point of view of adapta- 
tion for numerous different functions (see Frontispiece). The 
chief point of distinction between the cell of a protozoan and one 
of a many-celled animal (metazoan) lies in the fact that in the 
former all of the functions of a living animal are executed by a 
single mass of protoplasm. Any specialization for different func- 
tions in the single cell must be restricted to specialization of a 
part within that cell. On the other hand, the Metazoa are com- 
posed of numerous cells, groups of which have become specialized 
for limited functions. Consequently, for the rest of the func- 
tions characteristic of living matter these cells of limited function 
in the Metazoa become more or less dependent upon the other 
cells united with them to form an aggregate termed an individual. 
It thus becomes obvious that differentiation in Protozoa usually 
involves specialization of the parts within a single cell, while in 
the Metazoa entire cells become specialized for limited functions. 
In this light it is readily understood that "undifferentiated" as 
applied to a protozoan cell does not imply "unorganized" for 
the organization found there is frequently much higher (see 
Frontispiece) than in many kinds of metazoan cells which have 
undergone histological differentiation (Figs. 37-46). 

Protozoa as Non-cellular Organisms. — Since the concept of the 
Protozoa as simple organisms has led to such widespread mis- 
understanding of the true nature of these complicated, though 
minute, animals, some protozoologists have chosen to consider 
Protozoa as non-cellular. The advocates of this position hold 
that the body of a protozoan with its specialized regions repre- 



sents a mass of protoplasm which has undergone modifications 
for varying functions though the body of the protoplasmic mass 
has never become divided into cellular units. 

Some other workers prefer to retain the morphological compari- 
son of a protozoan as a single cell while they draw their contrasts 
on the side of differences in physiology or function. In his 
"Biology of the Protozoa" Dr. Calkins has well expressed his 
view in the words: 

. . . while a single protozoan is to be compared structurally with a single 
isolated unit tissue cell of a metazoan as a bit of protoplasm differen- 
tiated into cell body, or cytoplasm, and nucleus, it is a very different 
unit physiologically. In its vital activities it should be compared not 
with the unit tissue cells, but with the entire organism of which the 
tissue cell is a part . . . it is still a cell and at the same time a complete 
organism performing all of the fundamental vital activities within the 
confines of that single cell. 

The complexity of structure within a single-celled protozoon is 
well illustrated in the figure of Diplodinium which faces the title 
page of this book. The multiplicity of specialized parts in this 
animal does not permit one to think of it as a simple organism. 

Metazoan Tendencies. — There is no broad gap separating the 
simple, single-celled Protozoa on the one side from the Metazoa 
on the other. Many Protozoa, especially among the Mastigo- 
phora and Ciliata, show distinct tendencies toward specialization 
of cells. Frequently, the cells resulting from the division of a 
single one remain attached, thus forming a group which functions 
as a unit and is termed a colony (Fig. 5). Within such a group 
only part of the cells may retain the powers of reproduction while 
the others carry on all of the remaining functions for the colony. 
This marks an early separation of two kinds of cells, the germ cells 
as distinct from the body or somatic cells (Fig. 5). Separate germ 
cells or gametes occur in many different kinds of Protozoa, in fact 
it is not uncommon for two different kinds of gametes to make 
their appearance in this group as microgametes (male germ cells) 
and macrogametes (female germ cells), but in all these instances 
all of the somatic cells remain similar. Thus, there is a finely 
graded series of changes which connects the single-celled condi- 
tion and the colony bearing only one kind of somatic cells and 
one or more types of germ cells. Most zoologists agree that this 
marks the limit of histological differentiation in the Protozoa. 


If histological differentiation of the somatic cells occurs, the 
organism is recognized as a metazoan. 

Definition. — -An inclusive definition of the Protozoa might be 
given as follows: The Protozoa constitute that phylum of the 
animal kingdom which includes all single-celled animals and cell 
aggregates in which there is no histological differentiation of the 
somatic cells. 

Organization. — Because of their small size, Protozoa^ were 
entirely unknown to the early scientists. Their study dates 
from the introduction of the microscope. Most of the early 
observers maintained that Protozoa are made up of complete 
systems of organs such as are found in the higher animals. 
Dujardin denied the presence of organ systems. Definite com- 
parisons with single cells of the Metazoa were first made by 
de Bary as early as 1843 but it was left for von Siebold (1848) 
to describe them as unicellular. 

In lax usage, the term organ is still used in referring to those 
parts of the protozoan cell which have become adapted to special 
functions. More correct usage restricts this term to cell groups. 
Accepting this limitation, it becomes necessary to designate 
differentiated structures in Protozoa as cell organs or organ- 
ellae. This distinction has led to a rather common usage of 
terms such as cytostome (cell mouth), cytopyge (cell anus), 
cytopharynx (cell pharynx), in referring to the organellae of 

The five classes of this phylum differ so widely in structure and 
degree of specialization of parts that little may be said that would 
apply equally to all animals included here. While most Protozoa 
are small in size, the plasmodia of the Mycetozoa may cover a 
surface several inches in diameter. At the opposite extreme 
stand the Sporozoa, many of which pass through a spore stage 
in which the individual is less than 1 micron in diameter. These 
represent about the smallest cells known in the animal kingdom. 

The cytoplasm of a protozoan cell is usually divided into a 
covering ectoplasm and a more distinctly granular internal mass, 
the endoplasm. Within the ectoplasm, there are extreme differ- 
ences in organization, for in some instances it is reduced to an 
extremely thin layer while in others it is stratified into several 
distinctly separable regions. In the Microsporidia, and some 
other forms, no differentiation of the cytoplasm into layers has 
been demonstrated. 


Habitats.^ — ^Protozoa are encountered in extremely diverse 
habitats. All classes, with the exception of the Sporozoa, have 
numerous species which occur as free-living organisms in both 
fresh and salt water. Soil-inhabiting species are not uncommon. 

Many species are widely distributed over the face of the earth 
wherever conditions favorable for their existence are found. 
This broad distribution seems to be associated with the power of 
entering into an inactive state during which the body is sur- 
rounded by a protecting wall. This encysted condition may 
come as part of a definite life cycle or may result from inhospitable 
conditions, such as the evaporation of the water in which active 
specimens are living. In this state the inactive cysts may be 
carried extremely great distances by the wind or other agencies 
of distribution. Encysted forms are excellent examples of sus- 
pended animation, for when the cysts fall into favorable con- 
ditions the cyst wall is lost and the protozoan becomes active after 
its period of dormancy. By this means many species have 
attained almost cosmopolitan distribution, so that students in 
Europe, Asia, Africa, and South America find in their pools and 
lakes many of the same species that North American students 
encounter. Forms of the open ocean and of the deep seas seem 
to be more restricted in their distribution. Even different regions 
in the same ocean may yield characteristically different lists of 
protozoan fauna. The limitations of distribution in the ocean are 
even more striking when we consider that certain species live 
only at the surface of the water while some genera have never 
been discovered anywhere except in the extreme depths of the 

Many forms occur as cysts in the soil but some, especially 
rhizopods and flagellates, remain active in ordinary garden soil 
and are thought to be important because they feed upon bacteria 
of the soil. Little is known of the actual role of Protozoa in 
connection with the problems of soil biology. While active 
Protozoa doubtless feed upon important soil bacteria, no one has 
yet proved that they exert an injurious effect upon the soil. 

The parasitic habit has become the exclusive condition among 
the Sporozoa, but in each of the other classes the same habit is 
encountered to a greater or less degree. Some species have the 
faculty of leading either an independent or a parasitic existence, 
as opportunity is presented to the individual animal. Espe- 
cially in tropical countries the diseases of man and of his domestic 


animals are very frequently the result of protozoan parasites. 
Malaria, sleeping sickness, dysentery, Rocky Mountain spotted 
fever, Texas fever are some of the diseases for which the causal 
agent is a protozoan. 

Food Habits. — In food habits, the Protozoa display great 
diversity. Many of the chlorophyll-bearing Mastigophora are 
plantlike in their metabolism. Under the influence of sunlight 
their chlorophyll synthesizes food substances in a purely plant- 
like manner. These forms in which the food is built up from 
simple compounds by the processes of photosynthesis are 
designated as autotrophic. 

Most of the Protozoa are holozoic in their metabolism and require 
complicated organic compounds as foods. These organic foods 
may be ingested as solid particles either through the action of 
pseudopodia as in Amoeba and other Rhizopoda or through a 
cytostome as in Parameciuin and many other ciliates and many 
flagellates. In the endoplasm, these solid food particles undergo 
digestion in food vacuoles before they are assimilated. In the 
instances just cited the food material may be either living or 
dead plant or animal matter. However, in some species the 
predaceous habit is rather firmly fixed. Thus Didinium lives 
largely upon paramecia the bodies of which it ingests through a 
highly specialized cytostome located at the tip of a proboscis. 
The Suctoria through their hollow tentacles are enabled to 
suck out the protoplasm from other organisms and utilize it 
as food. 

Organic matter may be absorbed through the body surface in 
many Protozoa. This is especially true of forms which lack a 
cytostome. These organisms dependent upon the absorption 
of elaborated food stuffs may be either parasitic upon living 
organisms or may utilize decomposing organic matter. In the 
latter instance, they are said to be saprophytic. 

Cultures. — Under usual conditions. Protozoa are present in 
stream, pond, or lake water in relatively small numbers. Various 
factors in the environment cooperate in keeping the numbers of 
any given species from becoming excessive. If individuals of a 
given species multiply unusually, forms which feed upon this 
species, or depend upon it in other ways, will naturally increase, 
thereby tending to reduce the excessive numbers. Thus the 
"balance of nature" works here even as among the higher forms 
of life. If food is present in excess and natural enemies are lack- 



ing, the balance is broken and immense numbers make their 
appearance. This is what happens when an infusion or culture 
medium, rich in food material, is allowed to stand in the labora- 
tory to produce a protozoan culture. 

There is a tendency for the forms appearing in laboratory cul- 
tures to follow a regular though not absolute sequence in the 
order of their appearance. The first forms to become abundant 
are frequently minute flagellates (monads). According to the 
observations of Woodruff these are followed in succession by the 
Ciliata: Colpoda, Hypotrichates, Paramecium, and Vorticella. 
Only in very old cultures following the foregoing series did he 
find Amoeba in relative abundance. 

'^Yy-f-j ^H 

Fig. 11. 

Fig. 12. 

Fig. 13. 

Figs. 11-13. — Diagrams to illustrate the methods of asexual reproduction. 
11, binary fission in Paramecium; 12, multiple fission or spore formation in 
a protozoan; 13, budding in Acanthocystis. 

Though most species of the free-living, fresh-water Protozoa 
will reproduce in laboratory cultures, there are some which refuse 
to do so and though abundant in the plankton are not found in 
laboratory cultures. Examples are some of the dinoflagellates 
and ciliates of the family Tintinnidae. 

Reproduction. — Various forms of reproduction are encountered 
in this group. The one most frequently found is that of binary 
fission. In this process, the nucleus usually undergoes an indirect 
division and this is followed by constriction and finally separation 
of the cytoplasm to form two new individuals (Fig. 11). Com- 
monly, structures become duplicated in the dividing individual 
before fission is completed. Fission not uncommonly occurs 
when individuals are in an inactive or encysted state. A type 
of multiple fission (Fig. 12) is characteristic of some groups of 
Sporozoa and Mycetozoa. In this instance, the nucleus under- 
goes a series of divisions. Each of the nuclear masses thus 
formed becomes surrounded by a layer of cytoplasm and upon 


the rupture of the original cell wall numerous small cells, called 
spores, are liberated. 

In the indirect nuclear divisions mentioned above, there are 
frequently one or more points wherein the process differs from 
mitosis as it occurs in metazoan cells. When the centrosome 
occurs within the nuclear wall, the entire process of division may 
be accomplished without the disappearance of the nuclear 
membrane. Frequently the chromatin is present as one or more 
large bodies which are termed karyosomes. These constrict and 
divide without chromosome formation, in a manner somewhat 
resembling amitosis. The term promitosis has been applied to 
this primitive type of indirect nuclear division to distinguish it 
from the more elaborate mitosis. 

In some flagellates and rhizopods the chromatin is not con- 
fined to a nucleus but is spread more or less uniformly throughout 
the cytoplasm. Such distributed chromatin granules have been 
observed by some investigators to form minute secondary nuclei 
(as in Actinosphaerium) which become the nuclei of conjugating 

Because of the important role that chromosomes have assumed 
in discussions of heredity in the Metazoa, much interest has 
centered around a discussion of the nature of chromatin masses 
in Protozoa. Some investigators maintain that in many species 
of Protozoa definite chromosomes appear in specifically character- 
istic numbers. 

Many Protozoa have a definite life cycle more intricate than 
a simple succession of fission states. Frequently, this cycle 
involves two distinct types of adult individuals: sporonts, which 
give rise to gametes, and schizonts, which give rise to asexual 
individuals. There may thus be an alternation of generations 
involving two distinct types of developmental cycle: a cycle 
of sporogony, during which gamete formation and fertilization 
occurs, and one of schizogony, during which no sexual process 
is involved. These alternating cycles in the free-living forms are 
correlated with environmental conditions, especially seasonal 
changes, while in the parasitic species thay are frequently corre- 
lated with change of host. In the malarial organisms (Fig. 29), 
for example, schizogony continues during development of the 
organism in the vertebrate host and sporogony involving the 
fertilization of gametes is restricted to the sojourn of the parasite 
in the body of the mosquito. 


Budding (Fig. 13) differs from fission chiefly in the relative 
sizes of the resulting parts. In fission, two or more approxi- 
mately equivalent parts result from the partition of one individ- 
ual. Through budding, on the other hand, the identity of a 
parent organism is retained, for the buds arise as smaller out- 
growths from the body of the producing individual. 

Conjugation in the Protozoa involves either the temporary or 
permanent fusion of two or more individuals of the same species. 
A simple type is found in the fusion of the cytoplasm of a number 
of individuals to form a Plasmodium, as in the Mycetozoa. More 
frequently the nuclei are involved in either a permanent fusion 
resulting in true fertilization or a temporary fusion involving an 
exchange of nuclear material. 

Endomixis. — An intricate process of nuclear reconstruction 
without the fusion of individuals occurs in some Protozoa. 
Details of this phenomenon, which is termed endomixis, have 
been observed especially in Paramecium by Doctors L. L. Wood- 
ruff and Rhoda Erdmann. They found that in Paramecium the 
macronucleus gives off budlike fragments at regular time inter- 
vals and that these fragments are absorbed in the cytoplasm. 
While the macronucleus is being broken up, the micronucleus 
undergoes a series of divisions, differing in detail in the different 
species but resulting in the formation of eight products. In 
Paramecium caudatum, which has a single micronucleus, a series 
of three divisions produces the eight micronuclear bodies; while 
in P. aurelia, which has two micronuclei, only two divisions of 
the micronuclei occur. Part of the eight micronuclear bodies 
disintegrate while of the remainder some form micronuclei, others 
macronuclei. Thus wholly new nuclear equipment is formed 
from a portion of a micronucleus after a manner strikingly similar 
to the nuclear reorganization following conjugation. In Para- 
mecium caudatum, endomixis occurs at regular intervals of about 
60 days, while in Paramecium aurelia the period is approximately 
30 days. 

In some other ciliates endomixis has been observed but in 
these it usually occurs during an inactive period of encyst ment. 

Heredity in the Protozoa. — Studies on heredity in the Protozoa 
have been distinctly hampered by the lack of conclusive infor- 
mation on the entire life cycle in all but a relatively few species. 
Some forms, as far as known, reproduce by simple division. 
Here, obviously, any mechanism for inheritance would be simple 


indeed. On the other hand, those species which undergo con- 
jugation possess all of the possibilities for recombinations of 
nuclear material from two parents which characterize the highly 
complicated mechanism of biparental inheritance in the Metazoa. 
Intermediate between these two extremes lie those species in 
which endomixis occurs. This process, with its striking simi- 
larity if not full equivalence to parthenogenesis in the Metazoa, 
introduces the possiblility of recombinations of the gene-bearing 
chromatin within the body of the single individual. 

Many types of bodily change have been recorded among Pro- 
tozoa, seemingly due wholly to environmental changes such as 
those in the medium, in temperature, etc. Even certain types 
of mutilation are passed on to the progeny for a limited number 
of generations. But the evidence is fairly conclusive that here, 
as in the Metazoa, changes originating during the life of the indi- 
vidual are not permanent. When abnormal individuals produced 
by a changed environment or by mutilation are returned to 
normal surroundings the body eventually assumes its character- 
istic form. 

Extensive work on selection has been carried on by H. S. 
Jennings and others who have found that a single individual of 
a "wild" population may contain a great number of hereditary 
characteristics stored up in the single individual through its past 
history of chromatin interchange during conjugation. When 
such wild individuals are isolated and their offspring are reared 
under uniform conditions, extremely diverse individuals may be 
secured, evidently the result of recombinations of characters. 
For one species of Paramecium, Jennings discovered eight dis- 
tinct races each reproducing its own kind. 

Colony formation frequently results from incomplete separa- 
tion of cells following division. The products of rapid division 
may remain in union for a short time to form a temporary aggre- 
gate which ultimately separates into its individual cells, or they 
may remain permanently associated to form a colony. The con- 
nections and relationships between the individuals of a colony 
are highly varied. The cells of the colony, regardless of their 
arrangement, usually have connections of protoplasm passing 
from one to another. By this means communication between 
the cells is carried on readily and the actions of the indi- 
vidual cells are coordinated so that the group behaves as a 



The individual cells in many colonies are not bound closely 
together but are loosely associated by protoplasmic connections 
(Fig. 14) or by individual stalks of attachment. The appearance 
of unity is greatly strengthened in many colonies by the presence 
of a regularly shaped gelatinous mass (Fig. 5) in which the 
cells are embedded. Branching or arboroid colonies (Fig. 14) 
frequently occur in the Peritricha and in some Mastigophora. 
Highly developed colonies (Fig. 5) are 
common in the Mastigophora, especially 
among the Euglenoidea. Because of 
the similarity between various masti- 
gophoran colonies and the cleavage and 
blastula stages in the development of 
Metazoa, this group has frequently 
been cited as the one most directly in 
line with metazoan phylogeny. How- 
ever, some workers are inclined to the 
view that while colony formation is less 
characteristic of the Ciliata, speciahza- 
tions found there demand consideration ch^ytomo7ad?na°^ ^"^mnlblyol 

in any discussion of the phylogeny of seHularia Ehrenberg. A, ar- 
ii ^ T\/r^+^ ^„ rangement of cells in treelike 

the Metazoa. ,„l„^y. ^ individual in its 

The general plan of classification by cuplike sheath. {From Shuii, 
Doflein has been adopted in the ^?^f"'' °"^ ^"'^'"' ''^''' 
present work. In this system, relation- 
ships are more clearly shown than in the older systems which 
involve recognition of four equivalent classes. In the remainder 
of this chapter, the following subphyla and their included classes 
are discussed: 

Subphylum I. 

Class 1. 

Class 2. 

Class 3. 
Subphj'lum II. 

Class 4. 

Class 5. 









In the subphylum Plasmodroma are assembled all Protozoa 
which never develop cilia, while the subphylum Ciliophora 
includes those which have cilia at least during part of their 



Class Mastigophora 

The presence of one or more flagella is practically the only 
character common to all Mastigophora. More than superficial 
examination is necessary to distinguish some Mastigophora 
bearing numerous flagella from ciliates, and on the other hand 
the boundary between Mastigophora and Sarcodina is obscured 
through the presence of temporary flagella on some of the 
Sarcodina and of pseudopodia on some of the Mastigophora 
(Fig. 15). In addition to these confusing relationships with other 
classes of Protozoa, there are some forms which are so distinctly 


-MastlgcUa vitrea. 
Goldschmidt) . 

{After Fig. 16. — Isolated flagellum of 
Euglena showing axial filament within 
surrounding sheath. {After Butschli). 

plantlike that they are claimed alike by zoologists and botanists. 
Body shape is far from constant in many forms, the degree of con- 
stancy depending upon the character of the body surface. In 
some, the pellicle is either wanting or so thin as to permit of 
free amoeboid movements. Limitation to change in shape is 
secured in some by the presence of supporting structures of 
organic matter such as the axostyle (Figs. 21 and 22). 

Each flagellum (Fig. 16) consists of a firm axial filament, part 
of which is encased in a more fluid contractile sheath. Distally, 
this filament extends a short distance beyond the sheath and 
constitutes an "end piece." Proximally, the axial filament con- 
tinues through the cytoplasm to the blepharoplast, though in 
some instances the basal granule from which the flagellum arises 
may be separate from the blepharoplast and connected with it 


only by a delicate thread, the rhizoplast. There is thus devel- 
oped a system of centers associated with the base of the flagellum, 
connected by fine protoplasmic fibers. The name kinetic ele- 
ments has been applied to this system in the belief that it con- 
trols or directs the activity of the flagellum. In most of the 
parasitic, but lacking in the free-living flagellates, there occur 
one or more masses of deeply staining substance known as the 
parabasal body (Fig. 21) associated with the kinetic elements. 
These are thought to be a store of material to be used by the 
kinetic elements in their functioning. Kofoid and his followers 
refer to this whole group of blepharoplast, nucleus, rhizoplasts, 
and parabasal body as a neuromotor apparatus. 

Among the more characteristic Mastigophora, the flagella 
occur at the anterior extremity. If they are directed forward 
and by their movement pull the body along, they are called 
trachella; if they are directed backward and by their movement 
propel the body ahead, they are designated as pulsella. The 
trachella are the more common type. In many Mastigophora, 
flagella arising at the anterior extremity are directed backward 
along the side of the body as trailing flagella. These may be 
either free or fused with the side of the body as an undulating 
membrane as in the Trypanosomes (Fig. 23). 

Even in some chlorophyll-bearing Mastigophora there is a 
cytopharynx, though it is not always functional for ingestion of 
solid food. Contractile vacuoles usually empty into the cyto- 
pharynx, either directly or through communicating canals and 

A single nucleus is usually present, though in some of the 
parasitic forms there may be two (Giardia, Fig. 21) or many 
nuclei, as in members of the family Calonymphidae, parasitic in 

On the basis of relationships and general methods of metabo- 
lism, the Mastiogophora may be divided into two subclasses. 
The distinctly plantlike forms are included within the Phytoma- 
stigina, while the ones displaying more pronounced animal char- 
acters are grouped within the subclass Zoomastigina. 

Subclass Phytomastigina 

Members of this subclass have one to four flagella located at 
the anterior extremity. The cytoplasm is usually very finely 
granular, not heavily vacuolated, and without distinct boundary 



between ectoplasm and endoplasm. Typical members of this 
group (Fig. 17) possess chromatophores, frequently brown or 
yellow in the lower forms and green in the higher. Especially in 
the latter, chlorophyll is found. While members of this subclass 
are characteristically capable of manufacturing their own food, 
there are but few which are entirely lacking in power of taking 
in either fluid or solid foods. Contractile vacuoles are usually 
present in the fresh-water forms either as a simple pulsating 
vacuole or as a more highly complicated system of vacuoles and 
reservoirs. Division is usually by longitudinal binary fission. 

Fig. 19. 

Figs. 17-19. — Typical examples of Phytomastigina. 17, Chromulina showing 
chromatophores and a distinct shell {redrawn from Hofcndcr); IS, Chilomonas 
Paramecium, a form without chloroplasts, common in laboratory cultures 
(redrawn from Dofiein); 19, Peranema trichophorum, a species with a long 
flagellum, only the tip of which vibrates {redrawn from Dofiein). 

In most cases the cell organs become duplicated before separa- 
tion of the body begins. In some species of Euglena, "division 
cysts" are formed, and in these instances reproduction does not 
occur during motile stages but only after the flagellum has been 
cast off and the individual has entered a quiescent phase. Many 
of the members of this subclass have but a delicate pellicle or 
lack it entirely and are thus capable of extreme changes in body 
form as exemplified in the peristaltic or "euglenoid" movements 
of the bodies of Euglena and some of its colorless allies, Astasia 


and Peranema (Fig. 19). In contrast with these stand the dino- 
flagellates some of which have distinct shells made of a substance 
closely allied to cellulose, covering the entire body. 

The members of this subclass include forms of extremely 
diverse size. While some species are only about 2 microns in 
length, others, such as the Noctiluca, one of the organisms caus- 
ing phorphorescence in the ocean, reach as much as 1.5 mm. in 

This subclass includes the much discussed Volvox family, the 
most plantlike of all Protozoa. Members of this family illus- 
trate all the important steps in colony formation and in the 
history of reproduction. In Spondylomorum, reproduction is 
wholly asexual, each of the sixteen cells of the colony by fission 
producing a new colony. In Gonium, while asexual reproduction 
occurs, a zygote may result from the permanent union of isoga- 
metes. Male and female sex cells occur in Pandorina, Eudorina 
(Fig. 5), and Volvox, the colonies of some species of the last two 
being sexually differentiated into male and female colonies. 

The subclass Phytomastigina includes five orders, which are 
listed and some of the distinctive characteristics given in the 
table of systematic arrangement of the Protozoa at the close of 
this chapter. Though motile fiagella are characteristic, many 
species of Phytomastigina have the ability to discard their 
fiagella and still live an active life. In this immotile state, 
feeding and reproduction may continue normally and there is 
no impervious wall formed as in instances of encystment. In 
many of these plantlike flagellates the surface cytoplasm of the 
cell has the ability to secrete a gelatinous substance which passes 
through the pellicle and forms a matrix surrounding the cell. 
Frequently the cells resulting from division remain associated. 
Then the whole mass becomes enclosed as a colony in a gelatinous 
matrix, only the fiagella extending through it to the outside. 

Another type of surface secretion results in the deposition of 
cellulose or other materials to form a shell or test surrounding all 
or part of the cell body. In some instances these tests are highly 
ornamented and frequently they are composed of plates joined 
together in specifically characteristic arrangement and number 
so that species, in the dinoflagellates for example, may be differ- 
entiated on the characters furnished by the test. 

The planthke fiagellates are very widely distributed in both 
fresh and salt water. Many genera of the dinoflagellates are 


exclusively marine and in this group is now included the large 
Noctiluca noted for its phosphorescence. Most of the Volvox 
family are limited to fresh water where they occur as conspicuous 
elements of the plankton, frequently so abundant as to impart 
distinctive colors, odors, and tastes to the water. One family 
includes species, especially of the genus Chrysidella, which live 
symbiotically within the bodies of other animals and are the 
"yellow cells" of the Radiolaria and Foraminifera. While some 
members of this subclass live as parasites on or in other animals, 
most of the hosts are invertebrates, and hence as parasites they 
have been relatively unimportant and have received little 


The Zoomastigina include the orders of flagellates the members 
of which are distinctly animal in their nutrition or receive their 
food by absorption. Hence in food habits they are classed as 
holozoic and saprozoic. Chlorophyll and chromatophores, so 
characteristic of the Phytomastigina, are lacking in this subclass, 
consequently the autotrophic method of nutrition is lacking. 
The kinetic and locomotor elements are very highly specialized 
especially in the parasitic forms. Colony formation is common, 
particularly the arboroid type. There are no instances in which 
fertilization is definitely known. Fission may occur in either a 
free-swimming or an encysted state, and multiple fission may 
result in the formation of a number of individuals within the 
membrane of the original cell. Encystment is practically 

Some of the most important members of the Mastigophora 
belong here because of the prevalence of the parasitic habit in the 
Protomastigina. Four orders are recognized in this classification, 
though authorities differ in the number and grouping of the forms 
within this subclass. The orders are not clearly defined units 
but some probably represent groups of convenience. 

The simplest members of this subclass show confusing mixtures 
of flagellate and rhizopod characters, for forms like Mastigamoeba 
and Mastigella (Fig. 15) possess numerous pseudopodia and a 
permanent flagellum. 

The minute monads and the colonial Anthophysa are frequently 
encountered in laboratory cultures. A delicate collar surrounds 
the base of the flagellum in the choanoflagellates in such close 



mimicry of the collared cells of sponges (Fig. 50 B) that some zoolo- 
gists have seen a possible origin of the sponges from these 

Chilomastix (Fig. 20), Giardia (Fig. 21), and Trichomonas 
(Fig. 22) are genera which have species living very commonly as 
intestinal parasites of man and represent unusual degree of 
development of a neuromotor apparatus. 


Fig. 20. 

-Chilomastix mesnili (Wenyon). A, active stage; B, encysted. 
Kofoid and Swezy). 


Flagellates living in the gut of termites represent the most 
highly specialized members of the Mastigophora and the most 
bizarre types of protozoan structure. Their bodies are so closely 
set with fiagella that they were originally considered as ciliates. 
In the bodies of their hosts, according to the discoveries of 
L. R. Cleveland (1923), they digest the wood eaten by the termites. 
The symbiotic interdependence between the flagellates and their 
host is revealed by the fact that the Protozoa die if the termites 
are deprived of wood and the termites are unable to subsist on 
wood diet if freed of the flagellates. Trichonympha, Teretonym- 
pha, and Stephanonympha are representative genera of these 



peculiar flagellates. In the last-named genus are at least 100 
nuclei and similar duplication of the groups of kinetic elements. 
Trypanosomes (Fig. 23) occur as blood parasites in all classes 
of vertebrates. In some instances, they are restricted to a single 
vertebrate host species and in so far as has been observed these 
are harmless to the host. Such is the condition which exists 





Fig. 21. 

Fig. 22. 

Figs. 21 and 22. — Typical parasitic Zoomastigina showing, especially, the 
organization of the neuromotor apparatus. 21, Giardia microti Kofoid and 
Christiansen. Parabasals are two curved, dark bodies behind the axostyle. 
{After Kofoid and Christiansen). 22, Trichomonas augusta Alexieff. {After 
Kofoid and Swesy). 

between the rat host and its normal parasite Trypanosoma lewisi 
which occurs in rats all over the earth. This species is trans- 
mitted from one rat to another by the rat flea. The trypano- 
somes are taken into the stomach of the flea when blood is sucked 
from an infected rat. The trypanosomes penetrate the tissue 
cells of the flea and undergo multiple fission. After considerable 
change in form they penetrate into the rectum of the flea and are 
passed out with the feces. Reentrance into the rat is not by the 
bite of the flea but occurs when the rat licks its fur. 


In contrast with the non-pathogenic T. lewisi stand some other 
species of the same genus which produce fatal diseases in man 
and other animals in Africa and South America. T. gambiense 
and T. rhodesiense are the organisms responsible for the true 
sleeping sickness in man. A single genus of fly known as the tsetse 
fly (Glossina) transmits the trypanosomes to man. Rhodesian 
sleeping sickness (transmitted by Glossina morsitans) is more 
virulent than the Gambian variety (carried by G. palpalis). One 
of the great difficulties experienced in control measures is the fact 
that the trypanosomes seem to live normally in the bodies of ante- 
lope and other game animals which thus serve as reservoirs for the 
spread of the disease. When the 
tsetse fly sucks the blood of an 
infected person, the trypanosomes 
undergo development in the sali- 
vary glands of the fly and when 
they reach the infective stage are 
injected into the blood of other membrane 

human hosts by the bite of the fly. 
The part of the cycle in the inverte- 
brate host is thus radically different Wv ^^i^ J/uckus 
from that of T. lewisi. In the BJepharoplast — ■■ 
blood of an infected man the try- 
panosomes are never present in Fig. 23. — Trypanosoma theileri 
1 , mi r 1 (Bruce"), from blood of a cow. 

large numbers, ihey are lound {After Luhe). 
in lymphatic glands and in the 

cerebrospinal fluid. Infection is accompanied in its early stages 
by fever, enlargement of lymphatic glands and spleen, anaemia, 
and wasting of the body. The lethargic condition which 
generally precedes death, giving rise to the name "sleeping 
sickness," accompanies the invasion of the central nervous 
system by the trypanosomes. 

In addition to human sleeping sickness, Chagas' disease in 
South America is due to a trypanosome. Many species attack 
domestic animals, while others have been recorded from fishes. 
Amphibia, reptiles, and birds. The genus Leishmania is likewise 
important because of the human diseases Kala-azar and oriental 
sore produced by some of its species. 

The spirochaetes represent a group of organisms of very great 
importance and yet of highly problematical relationships. Some 
investigators have thought that they are protozoans related to 


the typanosomes, but in structure, in habits, and in reactions 
they seem to be more closely related to the bacteria. Treponema 
pallidum, the spirochaete which causes syphilis, is one of the most 
important representatives of this group. Some of the species 
of the genus Spiroschaudinnia produce a disease in man known as 
relapsing fever. 

In his "Biology of the Protozoa," G. N. Calkins (1926) rules 
the spirochaetes out of the Protozoa, "as their main character- 
istics place them much closer to the bacteria than to Protozoa. 
Their transverse division and spore formation through coccoid 
bodies are not duphcated amongst flagellated Protozoa, but are 
distinctly Spirillum-like." 

Class Sarcodina 

The Sarcodina usually lack a true cell wall, though in many 
instances skeletons or other protective coverings have been 
developed. The cytoplasm is characteristically more fluid than 
in other classes of Protozoa. The majority of Sarcodina are 
floating forms tending toward spherical body shape, but creeping 
forms attain flattened bodies or become cylindrical. 

Pseudopodia are the most characteristic feature of the Sar- 
codina in spite of the extremely different appearances which these 
structures may present in the various orders. In most instances, 
pseudopodia serve both for locomotion and for the ingestion of 
food. No less than four types of pseudopodia are generally 
recognized by students of Protozoa: axopodia, my:xopodia, filo- 
podia, and lobopodia. In the order named these represent 
progressive modification from motile, ancestral flagella. Each 
axopodium (Fig. 24 C) is provided with an axial filament arising 
from a kinetic element in the endoplasm. The filament is 
invested by a sheath of ectoplasm endowed with powers of 
streaming back and forth along the filament. Thus structurally 
there is very close agreement between an axopodium and a 
flagellum. Myxopodia (Fig. 24 B) lack an axial filament but 
possess a core of denser protoplasm. When they come in contact 
they tend to fuse forming a mesh or network of cytoplasm 
as in the Foraminifera. Filopodia lack all axial structures 
and consist of a wholly homogeneous cytoplasm. Lobopodia 
(Fig. 24 A), the form of pseudopodia encountered in Amoeba, 
are the most transitory type. For their formation they are 
dependent wholly upon the fluidity of the protoplasm. 



The nucleus is frequently single, though binucleate or multi- 
nucleate individuals are common in some orders. The nuclei in 
these latter are essentially similar rather than manifesting the 
functional differentiation which is characteristic of the macro- 
and micronuclei of the ciliates. 




Fig. 24.— Types of pseudopodia. ^-1, eruptive types of lobopodium; B, 
myxopodia of Foraminifera; C, axopodium of Heliozoa. {Redrawn from Calkins). 

Binary and multiple fission and budding arc all represented in 
the types of reproduction among the Sarcodina, and some forms 
pass through a definite life cycle involving conjugation (Arcella). 
The relatively simple organization of the Rhizopoda has led 
many students to assume that only a simple cleavage of the cell 
is involved in the reproductive process. It is probable that 
simple fission is not the sole phenomenon in the ontogeny of many 
forms. Encystment, so common a phenomenon in fresh-water 
and parasitic Sarcodina, has not been observed in Foraminifera 
and Radiolaria, the two chief groups of marine forms. 



Many important parasites, especially among the Amoebae and 
Mycetozoa, occur in this group. The Radiolaria (Fig. 25) and 
Foraminifera (Fig. 26) have been of great importance because 
of the part played by their shells in the formation of sedimentary 


The Heliozoa and Radiolaria are united to form the subclass 
Actinopoda. The first of these are chiefly fresh-water forms, the 
latter exclusively marine. As chiefly floating forms, the members 
of this subclass are many of them spherical, with pseudopodia of 

Fig. 25. — Two characteristic forms of radiolarian shells. (After Haeckel). 

lobose type or with axopodia. A dense cortical layer of the cyto- 
plasm so characteristic of Amoeba is lacking in the Actinopoda. 
The cytoplasm is highly alveolar and in the fresh-water forms 
(Heliozoa) bears one or more contractile vacuoles in the ecto- 
plasm. A sharp separation of ectoplasm and endoplasm is 
marked in the Radiolaria by a chitinous membrane called the 
central capsule (Fig. 25 A). The cytoplasm within the central 
capsule contains one or many nuclei, while the extracapsular 
ectoplasm is devoid of nuclei and is differentiated into more or 
less specialized zones. Minute openings in the capsular mem- 
brane permit of communication between the ectoplasm and 

Silicious skeletons of marvelously intricate design (Fig. 25 A) 
or simple latticework cover the body in some members of this 
subclass, though others may be naked or provided with varying 
arrangements of spicules, spines, and plates. 



Reproduction involves binary fission, budding, and multiple 
fission. In the Radiolaria the spores formed in the endoplasm 
within the capsular membrane may be similar (isospores) or 

dissimilar in size (anisospores). 
to be gametes, definite infor- 
mation on this point is lacking. 
Food consists largely of liv- 
ing organisms captured by 
lobose pseudopodia. 

Subclass Rhizopoda 

Though these spores appear 

The amoeba-like forms (Fig. 
27), the Foraminifera (Fig. 26), 

A B T 

Fig. 26. — Shells of Foraminifera. A, 

Siphogenerina striatula; B, S. columnel- 

and the slime molds are the laris, seen in longitudinal section; C, 

most characteristic representa- Cydamina orbicularis, side view. {After 

^ Ciishman) . 

tives of this subclass. All 

forms of pseudopodia excepting the axopodia are found but 
rarely combined in the same individual. A test is very com- 
monly present. It may be of pseudochitin to which other 
material such as silica are cemented or may consist of calcium 
carbonate laid down between two chitinous membranes as in 

the Foraminifera. 

The genus Amoeba, so 
commonly met in elementary 
zoological laboratories has 
many relatives that live a 
parasitic existence in the 
bodies of numerous inverte- 
brate and vertebrate hosts. 
The majority of these depen- 
dent amoebae are either 
harmless parasites or com- 
mensals, though Endamoeba 
histolytica (Fig. 27) and one or two other species cause dysen- 
tery in man. 

Some shelled rhizopods such as Arcella, Euglypha, Centropyxis, 
and Difflugia are apparently but amoebae that have acquired 
houses and are hence included in the same order (Amoebida) . In 
contrast, the Foraminifera (Fig. 26) have anastamosing pseudo- 
podia and shells showing varying degrees of complexity. The 
foraminiferan shells are of extremely diverse natures but usually 

Fig. 27. — Endamoeba histolytica in vegeta- 
tive state. {Redrawn from Hegner) . 


closed at one pole and open at the other. In form, the shell 
ranges from a simple, single chamber (Monothalmia) to complex 
series (Fig. 26 B) of spirally arranged chambers (Polythalmia). 
Growth of the latter is through the periodic formation of new 
chambers of increasing dimensions. Foramina serve to com- 
municate between the various chambers of the shell, all of which 
are filled with protoplasm. As the protoplasm increases in bulk, 
it protrudes beyond the limits of the old shell and a larger chamber 
is added to the chambers previously formed. Pseudopodia of 
different types protrude through either a single opening or numer- 
ous small apertures. In some of the smaller forms, the shell is 
almost wholly of organic material formed by the protoplasm. 
More commonly, silicious or calcareous matter is also present, and 
frequently foreign bodies are incorporated directly into the sub- 
stance of the shell. Chalk deposits, the nummilitic limestones, 
and various other sedimentary rocks have been formed largely 
of the fossil shells of Foraminifera. 

Members of the suborder Polythalmia (Fig. 26) are exclusively 
marine. The shells, which are of extremely diverse types, may 
be either pure lime or partly of organic matter. Those of some 
fossil forms are relatively immense in size {Psammonyx vulcanis, 
5 to 6 cm.). Some idea of the rate of reproduction of these forms 
may be gained from the fact that chalk deposits many feet in 
thickness are composed almost exclusively of their shells (Bilo- 
culina). In Norfolk, England, the chalk measures have an 
average thickness of 1,450 feet. It is reported that nearly 
50,000,000 square miles of ocean bed is covered with a globigerina 

The slime molds are frequently considered as fungi, but though 
they are border-line forms between plants and animals the 
structure and development of the vegetative body gives proof of 
close relationship with other rhizopods. The young stages are 
small amoeboid forms which may, by direct transformation, 
develop a flagellum. Transition between these two stages is 
accomplished very readily. Both reproduce by ordinary fission 
and each has the power of ingestion of solid food material and 
absorption of organic fluids. The flagellate form may become a 
microcyst from which either the flagellate (myxoflagellate) or the 
amoeboid (myxamoeba) form may emerge. 

Logs, stumps, and fallen leaves in moist woodland are especially 
favorable places for discovering sporangia of slime moulds in all 


their diversity of beautiful forms and pleasing colors. Though 
most have no direct economic importance, Plasmodiophora 
brassicae is parasitic upon cabbage and related plants. 

Class Sporozoa 

The Sporozoa are Protozoa without locomotor structures in the 
adult stage. While representing great diversity of structure, 
habitats, and life cycle, they all agree in having the parasitic 
habit firmly established and in reproducing by spores which are 
usually enclosed in a firm shell. In some instances, especially 
where alternation of hosts is introduced, the spore shell may be 
wanting and in still other instances more than one spore may be 
included within a single shell. Alternation of generations is 
widely distributed in this group. Sporozoa are usually intra- 
cellular parasites, at least during the early stages in develop- 
ment. In most instances the spores give characters more readily 
available for identification than do the vegetative stages. Nutri- 
tion is exclusively by absorption through the body surface. 

Representatives of many groups ranging from Protozoa to 
mammals serve as hosts to sporozoan parasites. Pathological 
conditions in the host frequently result from infection by these 
parasites. The organisms which cause malaria (Fig. 29) are by 
far the most important members of this class. 

Subclass Telosporidia 

In members of this subclass spore formation occurs only at the 
end of the vegetative period and sporulation results in the 
destruction of the vegetative individuals. Typically, the adult 
stage has a single nucleus but in some a multinuclear stage is 
found. Infection of a new host is through a stage called a 
sporozoite which is usually an intracellular parasite. The sporo- 
zoite either develops directly into a sexual individual or first 
passes through an asexual multiplicative cycle. Following ferti- 
lization, spores are formed which give rise to the sporozoites. 

The gregarines, the Coccidia, and the malarial organisms are 
the chief representatives of this subclass. Of these the gregarines 
five in invertebrates exclusively, being found chiefly in arthropods 
and annelids where they dwell as parasites in the lumen of the 
digestive tract and of glands opening into it. Typically the 
gregarine body is divided into an anterior protomerite and a 
posterior deutomerite separated by an ectoplasmic septum. Of 



these only the latter bears a nucleus. Frequently, especially in 
the young stages, there is an outgrowth of the protomerite carry- 
ing various structures for attachment to the host. This is termed 
the epimerite. Since no mouth opening is present, all food is 
absorbed directly through the body wall. A myoneme layer 
within the deeper region of the ectoplasm renders body move- 
ments possible. 

Fig. 28. ■ — Life cycle of Eimeria schubergi. Stage V to VIII, a period of 
schizogony; XIo and Xlla beginning of development of male and female gam- 
etes in sporogony period of the cycle. {From Shall, La Rue, and Ruthven). 

The Eugregarinaria are the typical gregarines which in their 
life cycle involve only a propagative phase. This suborder is 
usually subdivided into two groups, the cephaline gregarines 
which bear an epimerite at least in the early developmental 
stages, and the acephaline gregarines which never possess an 
epimerite. The cephaline gregarines, of which there are numer- 
ous genera and species, occur chiefly in the alimentary canal of 
arthropods. Gregarina, Stenophora, and Leidyana are a few of 
the numerous genera. The acephaline gregarines are chiefly 
coelom parasites, either lying free in the coelom or sometimes 


within organs located in the coelom. Monocystis from the 
sperm sacs of oHgochaet worms is the typical genus. 

The Coccidia (Fig. 28) dwell typically within the epithelial cells 
and are wholly lacking in powers of locomotion. Both verte- 
brates and invertebrates are subject to attack and one species of 
the genus Isospora parasitizes man. 

The Haemosporidia live chiefly in the red blood corpuscles of 
vertebrates and are thus different in habits from the lumen- 
dwelling gregarines and the epithelial tissue-inhabiting Coccidia. 
As a typical example of the group stand the organisms {Plasmo- 
dium vivax, P. malariae, Laverania falciparum) which are the 
causal agents of malaria. The life cycle involves both schizogony 
and sporogony. 

The amoeboid organisms of malaria, called schizonts (Fig. 29 ; 
1 to 7), occur in the erythrocytes of persons afflicted with 
the disease. Each schizont grows until it almost fills the 
corpuscle (6). Then spore formation takes place and upon rup- 
ture of the corpuscle numerous spores or merozoites are liberated 
into the blood stream, ready, if they escape the attack of the- 
leucocytes, to enter new erythrocytes and thus continue an auto- 
infection through an indefinite period. The chills characteristic 
of the disease are coincident with the periods of sporulation. 

Following a series of these asexual generations, the spore upon 
entering a corpuscle produces a gametoblast (9a, 96). In later 
development, each gametoblast develops into either a single 
female gamete or a group of male gametes. Further develop- 
ment of these gametoblasts depends upon their being taken into 
the body of a mosquito of the genus Anopheles which secures 
them with the blood of a malarial patient. The mosquito is not 
able to transmit malaria to a new human host immediately follow- 
ing the introduction of the organisms into the body of the 
mosquito. It is only upon completion of the sexual cycle (13 
to 27) of the parasite within the tissues of the insect that the 
bite of the mosquito may transmit the disease. In the stomach of 
the mosquito, each microgametoblast gives rise to a number of 
motile microgametes (156). Each of these upon fusion with a 
macrogamete (16) forms an ookinete or zygote (17). The zygote 
penetrates the wall of the stomach and becomes encysted (18). 
While in this condition sporoblasts are formed and each of these 
gives rise to numerous spindle-shaped sporozoites (25). These 
are liberated into the body cavity of the mosquito (26) and are 



A 5 6 

Fig. 29. 


carried by the circulatory system to various parts of the body 
including the salivary glands (27). From this location they are 
able to pass along the hypopharynx and thus are introduced into 
the blood stream of any human being bitten by the mosquito. 

Several different kinds of malaria are distinguishable both from 
clinical evidence and from morphological and developmental 
differences of the causal organisms. Spring tertian or benign 
malaria, of which Plasmodium vivax is the cause, is characterized 
by the presence of a fever recurring on alternate days. The 
quartan type of malaria, which is caused by P. malariae, is dis- 
tinguished by fevers with two-day intervals between attacks. 
Infections of Lavcrania falciparum produce malignant or perni- 
cious malaria with daily recurrence of the fever. 

Subclass Neosporidia 

Members of this subclass are multinuclear in their adult stage. 
Spore production is continued over a considerable period of time 
so that in any one individual, spores in different stages of develop- 
ment are to be found. Unlike the Telosporidia, spore formation 
does not bring the life of the individual to an end. Spore forma- 
tion is indirect, for the body forms a number of sporoblast mother 
cells. These in turn give rise to the sporoblasts which become 
transformed into the spores. 

The Neosporidia include the Myxosporidia parasitic almost 
exclusively on fishes, Microsporidia in insects but occasionally 
in fishes, and the Sarcosporidia in muscle cells of mammals. Of 
the Myxosporidia many species invade the gills, integument, and 
muscles of fishes producing conspicuous cysts filled with minute 
spores. The Microsporidia have considerable economic interest, 
for the pebrine disease of silkworms is a serious menace to the 
silk industry and the Nosema disease attacks honey bees. 

Fig. 29. — Life cycle of Plasmodium vivax, the tertian malarial parasite. Stages 
1-12 and 13c-17c in human blood stream; stages 13a-14a, 136-156, and 16-27, 
in body of mosquito. 1, the infecting stage for man, the sporozoite; 3-7, stages 
in schizogony resulting in the formation of merozoites (7). 9a-12a, formation of 
macrogametocyte; 13a— 14a, maturation of microgamete. 96-126, formation 
of microgametocyte; 136-156, maturation of the microgametes. 16, fertiliza- 
tion; 17, zygote or ookinete. 18-25, stages in development -wathin cysts in 
stomach wall of mosquito; 26, sporozoites liberated from cysts making their way 
through the body fluids to the salivary glands of the mosquito (27), where they 
are ready to infect a man bitten by the mosquito. 13c-17c, stages in formation 
of merozoites from macrogametocyte which fails to leave the human body. 9a 
and 96-(25) are stages in sporogony. (From Liihe after combinations of drawings 
by Graasi and Schaudinn). 



Pasteur's name is closely associated with the Microsporidia 
because of his discovery of the cause and means of control of 

The Myxosporidia have bivalve spores which usually contain 
one, two, or four polar capsules in addition to the amoeboid body 
called the sporo plasm (Fig. 30). The polar capsules are observa- 

Anierior end 
■foramen of pole 
— Shell 

Fig. 30. 

■Polar caps 
led polar 

lef' o'fsj 

— Po lienor end 

-Diagrammatical front and side views of a Myxobolus spore. 



ble in fresh material without previous treatment. Two nuclei 
are found in the sporoplasm. The vegetative stage (Fig. 31) 
occurs either as a tissue parasite or in cavities. In the cavities 
the trophozoite forms plasmodia of various shapes. The tissue- 
inhabiting forms are usually oval, rounded, or elongated and may 
be either free or encysted. Spores are formed in the endoplasm 

of the plasmodial trophozoite. 
Gall bladder, uriniferous 
tubules, and urinary bladder 
of fishes are common seats of 
infection by the free organ- 
inhabiting forms, while gills 
and muscles of fishes are 
especially favorable tissues for 
the encysted forms. Cysts, 
which may attain a diameter of 
several millimeters, result from the hypertrophy of host tissues 
surrounding what was originally a single parasite. By endoge- 
nous budding of this parasite, spores in all stages of their develop- 
ment are being produced continuously. Leptotheca, Myxidium, 
Myxobolus, Henneguya, and Chloromyxum are characteristic 

Fig. 31. — Vegetative stage of a myxo- 
sporidian, Leptotheca ohlmacheri. {After 
Kudo) . 


In the Microsporidia, each spore contains a single polar capsule 
which is observable only after treatment with proper reagents. 

Sarcosporidia occur in the muscles of various mammals. The 
encysted form reaches a length of several millimeters and finally 
becomes a mass of sickle-shaped spores in clumps separated from 
each other by partitions. Detailed structure of the spore is 
not known. The encysted forms of the genus Sarcocystis within 
the muscles of mammals have long been known under the name 
of Miescher's corpuscles, but the complete life cycle is not 


The subphylum Ciliophora includes all Protozoa which possess 
cilia at least through part of their active life, and in typical 
instances the nuclear material has become separated into two 
specialized bodies, one controlling the generative processes (the 
micronucleus) and the other directing the general activity of the 
cell (the macronucleus) . Instead of referring to this condition 
as binucleate, it is more proper to consider the nucleus as dimor- 
phic, for the micro- and macronuclei combined perform the 
functions ordinarily exercised by the single nucleus of metazoan 
cells and of those Protozoa which have single nuclear bodies. 
In some degenerate forms (Opalina and Ichthyophthirius) there 
is no distinction of micro- and macronuclei, for two or more similar 
nuclei occur. There are two classes within this subphylum: the 
Ciliata, in which cilia are retained throughout life, and the 
Suctoria, in which cilia are lost after the young individuals 
become attached to some object. 

Class Ciliata 

As mentioned earlier in this chapter the ciliates represent a 
degree of complication of parts (see Frontispiece) not found else- 
where even in the differentiated metazoan cells. It has been 
maintained by some that the body of the ciliate with its speciali- 
zation and differentiation of the nuclei represents a differentiation 
analogous to the separation of somatic and germ cells in the 
Metazoa. Though the Mastigophora are commonly cited as the 
protozoan group through which relationships with the Metazoa 
are traced, there is considerable evidence that this phylogenetic 
significance is at least shared by the Cihata. 


Both free-living and parasitic forms are common. Though the 
cell is commonly covered by a pellicle or cell wall, there are 
definite openings for the ingestion of food and discharge of waste. 
A cytostome leads through a cytopharynx down into the endo- 
plasm and a cytopyge for the elimination of solid waste is present 
though usually not observable except at the time of elimination. 

The Cytoplasm. — The cytoplasm is divided into an ectoplasm 
and an endoplasm of which the former comprises a number of 
rather sharply differentiated regions not uniformly encountered 
in the various groups. The alveolar layer is the outermost and 
is marked by a striated appearance due to the arrangement of 
the alveoli of the cytoplasm. Beneath this lies the trichocyst 
layer in which the spindle-shaped trichocysts are embedded with 
their tips at the surface ready for discharge as long, stiff organs 
of defense. A contractile layer underlies this one. It consists 
of myonemes which ordinarily run parallel to the rows of cilia. 
Between or external to the myonemes are found the basal granules 
from which the cilia take their origin as they pass between the 
alveoli of the alveolar layer to the outer surface. A spongy zone 
of ectoplasm traversed by fluid-filled spaces and channels overlies 
the endoplasm and with the contractile vacuoles and their 
associated radial canals represents an excretory layer. 

The endoplasm is less highly organized. It comprises a fluid 
cytoplasm within which are contained the nuclei, mitochondria, 
Golgi apparatus, and various inclusions such as food and water 
vacuoles, excretory granules, and sometimes symbiotic algae. 

Trichocysts, commonly associated with the ectoplasm, have 
their origin in the endoplasm and later migrate to their peripheral 
location. Trichites are elongated rods usually surrounding the 
mouth, apparently giving support and protection to the body. 
Similar supporting rods are frequently combined to form a tube 
known as a pharyngeal basket leading from the mouth into the 

The Nuclei. — The macronucleus is extremely variable in 
shape, while the micronucleus is usually a single rounded mass. 
In the Peritricha, the macronucleus is frequently a long ribbon- 
shaped structure and in the Heterotricha it usually assumes a 
distinctly beaded or moniliform condition. An exceptional dis- 
tribution is found in the genus Trachelius the members of which 
have a single large macronucleus and thirteen micronuclei. The 
number of micronuclei is not constant even for the species 


within the same genus. Thus in Paramecium, the common 
species P. caudatimi has a single micronucleus, P. aurelia has 
two, P. polycarium has four, and P. multimicronucleata may have 
as many as eight. In spite of its importance in normally direct- 
ing reproductive processes, the micronucleus is not an absolute 
essential in cell life, for amicronucleate races of Paramecium, 
Didinium, and other ciliates have been cultured for long periods. 

Reproduction is by binary fission, usually in the free state 
but in some forms accompanying encystment. Preparatory to 
fission the micronucleus divides by mitosis and the macronucleus 
by amitosis. In several ciliates, chromosomes of a fixed number 
have been observed in the dividing micronucleus though never 
in the macronucleus. Mouth and other structures are frequently 
duplicated in the dividing individual before fission is completed. 

Conjugation of varied forms occurs in this group. It always 
results in reorganization of the protoplasm involving the absorp- 
tion of the macronucleus by the cytoplasm and reorganization 
of a new macronucleus and micronucleus from products of a 
fertilization nucleus. The old idea of conjugation as essential 
to prevent senescence has been shown to be unfounded, for 
Woodruff has carried a race of Paramecium through 11,000 
generations without opportunity for conjugation. This he 
accomplished by keeping isolated individuals under observation 
and removing one after each body division. 

In the Peritricha, of which Vorticella is an example, a modified 
form of conjugation between, a microconjugant and a macro- 
conjugant occurs. By two or three successive divisions, some 
individuals produce four or eight microconjugants which after 
acquiring an aboral ring of cilia become free-swimming. One of 
these microconjugants fuses with an ordinary individual, which 
is the macroconjugant. In brief, the micronucleus of the 
microconjugant undergoes division to form eight parts of which 
seven degenerate and only one persists. Later, this one divides 
into two. During this same time the micronucleus of the 
macroconjugant has divided into four parts of which three 
degenerate and the remaining one redivides to form two nuclei. 
In both conjugants, the macronucleus has undergone disinte- 
gration. In each conjugant, one of the two pronuclei which 
originated from the micronucleus disappears. The remaining 
nucleus of the microconjugant passes over into the cytoplasm of 
the macroconjugant and fuses with the single nuclear mass 


remaining there. As a consequence, the macroconjugant now 
has a fusion nucleus and the microconjugant which now has no 
nucleus drops off and disintegrates. The nucleus of the remain- 
ing individual divides into eight parts of which one becomes a 
micronucleus and the other seven macronuclei. Through a 
series of divisions of the micronucleus and fission of the cyto- 
plasm, seven individuals ultimately result from the conjugation. 

Endomixis, comparable to parthenogenesis of the Metazoa, has 
been discussed earlier in this chapter. 

Arrangement of Cilia. — On the basis of arrangement of the 
cilia, five orders of ciliates are recognizable. The cilia on the 
body surface are primitively arranged in meridional bands 
radiating from a terminal mouth (as in Prorodon) but in many 
forms the mouth has shifted its position, and with it the arrange- 
ment of body ciliation becomes more complicated. The cilia 

Fig. 32. — Diagram showing wave motion of cilia. {After Vcrworn). 

which are not fused in bands or tufts are usually in meridional or 
spiral arrangement and occur either in furrowlike depressions or 
each cilium within the center of a small depression. In Para- 
mecium, these depressions are hexagonal or rhombic. At the 
angles of these polygons and in the middle of some of the sides 
are located the points of the trichocysts. Each cilium is com- 
posed of a firm axial filament covered by a sheath of more fluid 
contractile substance. The filament takes its origin from a basal 
granule and in many instances minute fibrils extend inward from 
the basal granules to the wall of the macronucleus. Various 
modifications of cilia occur. Non-motile tactile bristles have the 
same general structure as the typical motile cilium. In the 
pharynx of many ciliates, all the cilia of a row become fused to 
form an undulating membrane or several adjoining rows may 
fuse to form a membranelle. In the Hypotricha, tufts of cilia 
are fused to form brushlike structures called cirri. 

Movements of cilia are coordinated by a system of fibrils which 
comprise a neuromotor system. These fibrils have been most 
highly specialized and most thoroughly studied in some of the 
Hypotricha — Euplotes, for example (Fig. 34). Cilia which have 
not been modified to form membranelles or cirri move in rhythmic 
waves (Fig. 32) along the rows of unmodified cilia. 



In many forms, such as Vorticella, which is attached by a 
stalk, the cilia create water currents carrying food to the animal 
and do not ordinarily produce locomotion. Under such circum- 
stances dissemination of the species is accomplished during the 

Fig. 33. — Balantidium minutum Fig. 34. — Neuromotor system in 
Schaudinn. {After Schaudinn). Euplotes karpa, showing system of 

fibrils connecting the cirri. (After 
Prowazek) . 

reproductive period when individuals break loose from their stalks 
and for locomotion use cilia which are formed at this time only. 

Class Suctoria 

The Suctoria are characterized by the complete lack of cilia 
in the adult stage and the presence of tentacles by means of which 

Fig, 35. — Suctoria (after various writers). A, colony of Dendrosoma; B, 
Rhyncheta; C, Ophryodendron; D, Tokophrya; E, ciliated young of Sphaero- 
phrya; F, diagram of capitate and styliform tentacles arising from ectoplasm. 
(From Hertwig's Manual of Zoology by Kingsley, Courtesy of Henry Holt and Co.). 

other organisms are captured and the protoplasm drawn into the 
body of the suctorian. Cilia occur on the body of the free-swim- 



ming larva (Fig. 35 E) but are lost when the animal assumes the 
adult form after becoming sessile. The tentacles are hollow, 
usually terminating in a suckerlike knob. The macronucleus is 
extremely variable in form. In some colonial representatives its 
branches extend throughout the branches of the colony. A 
micronucleus has never been demonstrated for some suctorians. 
Binary fission, so common in the Ciliata, is rare in the suctorians. 
Both external and internal buds are of common occurrence. The 
embryos when liberated are furnished with bands of cilia for 
locomotion. Conjugation, in nature similar to that found in 
Ciliata, may take place either between 
similar individuals or between a fixed 
individual and a free-swimming bud, 
suggesting the condition found in the 
Vorticella. Podophrya, Acineta, Ephe- 
lota (Fig. 36), Tokophrya, and Dendro- 
soma are typical genera. 

Interrelationships of the Classes 
OF Protozoa 

The Protozoa represent the simplest 
present-day organization of animal life. 
It is probable, however, that at one 
,^\^\ ■^?"^'^, ^"°*°".^J^ time there existed simpler living sub- 

{Ephelota biUschhana) with 

five daughter buds. (After stancc which would be Considered animal 
Caikms). jj^ nature. This most primitive form of 

life was probably more homogeneous in 
its make-up, for morphological differentiation of the nucleus and 
cytoplasm and regional differentiation of the cytoplasm such as 
we find in even the simplest Protozoa have probably resulted 
from long periods of progressive evolution. 

There are numerous evidences that plant life existed on the 
earth before animals came into being. The Mastigophora, 
through the Phytomastigina, display unquestioned relationships 
with the members of the plant kingdom (Fig. 1). Because of 
this fact some biologists consider the Mastigophora as the most 
primitive class of the Protozoa. 

On the other hand, the rhizopod organization is, on the whole, 
less intricate than that of the Mastigophora or of any other proto- 
zoan. Consequently, on the criterion of simplicity of organiza- 


tion, some consider the Sarcodina as the most primitive class of 
the phylum. It remains as a possible explanation that the 
Sarcodina represent the simplifying effects of a regressive 
evolution. In their vegetative stages, the Sporozoa show con- 
vincing evidences of rhizopod affiliations through the presence 
of pseudopodia and in their general organization. Sporozoa 
probably had their rise through some ancestral form of our 
present-day Sarcodina. 

In the Ciliophora is encountered the highest morphological 
and physiological differentiation found in Protozoa. Without 
doubt this subphylum stands at the apex of protozoan evolution. 
The Suctoria have been given off as a side branch from the main 
ciliate stem, as evidenced by the fact that suctorians, in their 
development, pass through a ciliated larval stage. 

Questionable Protozoa. — In discussing the Mastigophora, 
attention was called to the fact that many of the Phytomastigina 
possess characters which point to plant affinities as well as animal 
relationships. The problem of assigning such forms to either 
plant or animal kingdom depends upon establishing definite 
criteria for separating these two kingdoms. The zoologist con- 
fronts a much more difficult task when he comes to consider some 
of the extremely minute bodies which have been named as pos- 
sibly the causal agents of certain diseases. In some such 
instances there is yet no ground for proof that the bodies under 
discussion are actually hving cells. Thus the Negri bodies 
found in the nervous system of animals suffering from hydro- 
phobia are thought by some to be degeneration products of 
diseased cells, while others believe them to be protozoan cells. 
Similarly, for many diseases, minute bodies which in their 
behavior resemble Protozoa more closely than bacteria have been 
observed, though they defy actual assignment to any known 
group of the Protozoa. The bodies associated with Rocky 
Mountain spotted fever, trench fever, and typhus fever, so very 
minute that they pass through filters, are considered as Protozoa 
by some workers but there is no general agreement as to their 

As pointed out previously, the spirochaetes have been shuffled 
back and forth between the Protozoa and the bacteria because 
of the lack of decisive characters that would compel them to be 
lined up with one group or the other. This problem of the rela- 
tionships of the lower plants and animals is far from settled. 


In fact, at the present time many bacteriologists are now main- 
taining that bacteria may belong to the animal kingdom. 

Outline of Classification 

Phylum Protozoa. — Single-celled animals or cell aggregates in which there 
is no histological differentiation of somatic tissues. 

A. Subphylum Plasmodroma. — Protozoa lacking cilia throughout life. 
I. Class Mastigophora. — One or more vibratile flagella. 

a. Subclass Phytomastigina. — Plantlike flagellates. 

1. Order Chrysomonadina. — One or two flagella; small; frequently 
with pseudopodia; yellow-brown chromatophores; spore formation 
in cysts. Dmobryon (Fig. 14), Chromulina (Fig. 17), Chnjsopyxis, 
Uroglena, Synura. 

2. Order Cryptomonadina. — One or two flagella; small; constant 
body form; yellow, l)rown, blue, or green chromatophores; chiefly 
marine. Chilomonas (Fig. IS), Chrysidella, Cyathomonas, 

3. Order Dinoflagellata. — Two flagella, one usually in circular 
furrow; yellow-brown chromatophores; test usually present; 
chiefly marine. Ceratiurn, Nodiluca, Peridinium, Gymnodiniiim, 

4. Order Euglenoidina. — Relatively large; one or two flagella; 
green or red chromatophores or wanting; complex vacuole system; 
usually a cytostome; chiefly fresh-water. Euglena, Phaciis, 
Peranema (Fig. 19), Astasia, Trachelomonas. 

5. Order Phytomonadina. — Two flagella; green or yellow chroma- 
tophores; colony formation characteristic; sex differentiation in all 
degrees. Chlamydomonas, Gonium, Spondylomorum, Pleodorina, 
Pandorina Eudorina, Platydorina, Volm x. 

b. Subclass Zoomastigina. — Animal-like flagellates; no chromato- 
phores; no chlorophyll; vacuole simple; kinetic and locomotor 
elements highly differentiated. 

6. Order Protomastigina. — One to few flagella; pseudopodia 
common; one nucleus. Mastigamoeha, Mastigella (Fig 15), Actino- 
moTMS, Dimorpha. 

7. Order Protomonadina. — One or two, rarely three, flagella; one 
nucleus; minute; many parasitic. Trvvanosoma (Fig. 23), Leish- 
mania, Crithidia, Monas, Bodo, Bicosoeca, Anthophysa. 

8. Order Polymastigina. — Few to many flagella; one, two, or 
many nuclei; many parasitic in digestive tract; colony formation 
unknown. Giardia, Chilomastix, Stephanonympha, Dinenympha. 

9. Order Hypermastigiua. — Many flagella; one nucleus; 
parasitic in insects; highly specialized. Lophomonas, Joenia, 

n. Class Sarcodina. — Usually lack cell membrane; floating or sus- 
pended forms or with pseudopodia for locomotion. 

a. Subclass Actinopoda. — Unusuallj" spherical; typically floating; 

axopodia; highly alveolar. 


1. Order Heliozoa. — Stiff, radial pseudopodia; frequently open- 
work skeleton. Actinophrys, Actinosphaerium, Acanthocystis (Fig. 
4), Clathrulina. 

2. Order Radiolaria. — Shell or spicules; central capsule; exclusively 
marine. Acanthomctra, Acanthosphaera, Sphaerocapsa. 

b. Subclass Rhizopoda. — Pseudopodia without axial filaments; 
typically creeping. 

3. Order Proteomyxa. — Naked; pseudopodia reticulose or filose. 
Nuclcaria, Vampyrdla. 

4. Order Mycetozoa. — Terrestrial or semiterrestrial; plasmodium 
formation. Arcyria, Stemonitis, Dictydium.,Cornatrichia. 

5. Order Foraminifera. — Typically myxopodia; often complex 
calcareous shells. Globigerina, Allogromia, Polystomella, Rotalia, 
Cyclammina (Fig. 26), Siphogenerina (Fig. 26). 

6. Order Amoebida. — Naked or simple, one-chambered shell; 
lobopodia or filopodia. Amoeba, Endamoeba (Fig. 27), Pelomyxa, 
Arcella, Difflugia, Centropyxis, Euglypha. 

III. Class Sporozoa. — Exclusively parasitic; reproducing by spores; 
locomotor structures lacking in adult condition. 

a. Subclass Telosporidia. — Spore formation at close of trophic stage 
only; intracellular parasites during part of life cycle. 

1. Order Gregarinida. — In lumen of arthropods and annelids. 
Gregarina, Monocystis, Stenophora. 

2r~0r3er Coccidiomorpha. — Typically intracellular. Isospora, 
Eimcria (Fig. 28), Haemogregarina, Leucocytozoon. 

b. Subclass Neosporidia. — Reproduction throughout trophic stages. 

3. Order Cnidosporidia. — Sporoblast bivalve, containing one or 
more polar capsules; important pathogenic parasites of inverte- 
brates and vertebrates. Myxobolus (Fig. 30), Lcptothcca (Fig. 31), 
Myxidium, Nosema, Glugea. 

4. Order Sarcosporidia. — Muscle of vertebrates, particularly 
mammals. Sarcocystis. 

B. Subphylum Ciliophora. — Protozoa having cilia at least in some stages. 
rV. Class Ciliata. — Simple or compound cilia throughout vegetative 


1. Order Holotricha. — Generally with uniform body cilia; lacking 
a specialized zone of membranelles. Prorodon, Opalina, Coleps, 
Didinium, Dileptus, Colpoda, Colpidium, Paramecium, Lacrymaria. 

2. Order Heterotricha. — Body covered with fine cilia; left-handed 
oral spiral of membranelles. Balantidium (Fig. 33), Stentor, Spir- 
ostomum, Bursaria, Nyctolherus. 

3. Order Oligotricha. — General body ciliation lacking; adoral 
cilia usually a complete ring of membranelles, the only organ of 
locomotion. Hallcria, Strombidium, Tintinnus, Tintinnidium, 
Entodinium, DipJodinium. 

4. Order Hypotricha. — Flattened dorsoventrally; motile organs on 
ventral surface only; oral membranelles in left-handed spiral. 
Kerona, Urostyla, Uroleptus, Stylonychia, Oxytricha, Euplotes. 

5. Order Peritricha. — Typically stalked; oral cilia in right-handed 


spiral; body cilia rarely prosont. Vorticella , Cydochaeta, Tricho- 
(lina, (\)thurn.ia, Zoolhamnium, Carchesium, Epistylis, Opercularia. 
V. Class Suctoria. — Cilia confined to buds or embryos; adults bear 
tentacles for sucking and piercing; usually attached by a stalk. Den- 
drosoma (Fig. 35 A), Tokophrya (Fig. 35 D), Sphaerophrya (Fig. 35 E), 
Podophrya, Acineta, Ephelota, Rhynchaeta, Ophryodendron (Fig. 35 C). 


Calkins, G. N. 1926. "Biology of the Protozoa." New York, Lea and 

Craig, C. F. 1911. "The Parasitic Amoebae of Man." Philadelphia, 

. 1926. "A Manual of the Parasitic Protozoa of Man." Phila- 
delphia, Lippincott. 
DoBELL, C. G. 1921. "The Intestinal Protozoa of Man." London, John 

Bale, Sons and Danielson. 
DoFLEiN, F. 1916. "Lehrbuch der Protozoenkundc," 4th ed. Jena, G. 

Haeckel, E. 1862-1888. "Die Radiolarien (Rhizopoda radiosa): Einc 

Monographic." Berlin. 
Kent, W. S. 1880-1882. "A Manual of the Infusoria (etc.)." London, 

D. Bogue. 
Leidy, J. 1879. "Fresh-water Rhizopods of North America." Rept.U.S. 

Geol. S^irvey, Vol. 12. 


In the foregoing chapter, attention has been called to the tend- 
encies which some Protozoa display toward the specialization 
of their cells. Most of this specialization is expressed in increased 
complexity of intracellular organization. In some instances, 
however, groups of cells, termed colonies, act as the unit or indi- 
vidual. Within colonies some of the cells frequently become 
specialized as gametes for reproductive purposes but all of the 
somatic cells remain alike. In the Protozoa, any specialization 
of somatic cells affects all equally, so there is no differentiation. 

It seems probable that the Metazoa must, at some time, have 
originated from protozoan ancestors, but all evidences of the 
means of this transition have been lost and both Protozoa and 
Metazoa have gone far along independent lines of evolution 
since the Metazoa came into existence. The lack of fossil forms 
bridging this gap is not surprising, for both the Mastigophora 
and the Ciliata, through the ancestors of which this genesis 
might have taken place, have no hard parts which could be 
preserved as fossils. 

There are a few present-day animals which have been termed 
the Mesozoa because they have been considered as intermediate 
between Protozoa and Metazoa, but their simplicity is probably 
due to degeneracy, as is shown in the section of this chapter 
devoted to them. 

Embryology frequently offers a clue to and outlines the steps 
in phylogeny (recapitulation theory). All Metazoa, in their 
development, typically pass through a single-celled stage — the 
fertilized egg. This fact readily furnishes the basis for an analogy 
of metazoan origin from the single-celled organisms. In the 
development of the fertilized egg into the adult organism, there 
is a time when large numbers of fundamentally similar cells (the 
blastomeres) are produced through the partition of the single 
cell. It is only in later development that these cells through 
histological differentiation assume different forms and come to 



perform diverse limited functions. This subject of histological 
differentiation, which marks off the Metazoa from all Protozoa, 
is discussed in the second section of this chapter. A brief 
discussion of the organ systems of the invertebrates concludes 
the chapter. 

A. The Mesozoa 

A small group of relatively simple many-celled animals com- 
posed of a layer of ectoderm cells covering a single or several 
entoderm cells has frequently been considered as standing inter- 
mediate between the Protozoa and the Metazoa and has conse- 
quently been termed the Mesozoa. The phylogenetic significance 
which has thus been attached to this group is a subject regarding 
the tenability of which very grave doubts are justifiable, for all 
of the more important representatives of the group are internal 
parasites. It seems probable that their simplicity of organiza- 
tion is an accompaniment of degeneracy directly traceable to 
adaptation to the parasitic existence. Not even the known facts 
regarding the development of these forms aid in pointing out 
relationships with any other group of organisms. 

The Dicyemidae and the Heterocyemidae include species which 
are parasitic in cephalopods, while Rhopalura, which represents 
the Orthonectidae, lives as a parasite in various invertebrate 

B. Histological Differentiation 

Differentiation of Tissues. — As emphasized in the foregoing 
chapter, all Metazoa differ from the Protozoa in that the cells 
of the former become specialized for restricted functions, that is, 
undergo histological differentiation. Certain groups of cells 
become similarly specialized for carrying out one specific function 
more effectively than that function may be executed by unmodi- 
fied protoplasm. Such a group of similarly specialized cells is 
termed a tissue. A tissue is made up not only of units of cyto- 
plasm and their contained nuclei, but frequently the cytoplasm 
forms substances which are essential in the effective functioning 
of the tissue. These substances, which are termed plasmic 
products, may be either inconspicuous or so prominent that the 
cells which produce them are obscured. 

Classification of Tissues. — Since the function of a tissue is one 
of its most important characteristics, the classification of tissues 


is usually built around this character. Even though differentia- 
tion leads along numerous different lines in the metazoan body, 
the general results are so nearly uniform in different animal 
groups that but four chief classes of tissues are commonly 
recognized, namely; epithelium, connective tissue, muscle tissue, 
and nervous tissue. 

In addition to these chief classes of tissues, many other kinds 
are recognizable but often these are modifications of one or more 
general classes. Thus the nettling cells of a Hydra or the pig- 
ment cells of a squid may be considered as special classes of cell 
differentiation. But since these special types of cells are rather 
readily referable to an epithelium or a connective tissue or 
one of the other types, there is no distinct advantage in creating 
additional classes of tissues. 

Both form and structure of an organism depend upon the 
grouping of the component tissues. Thus both morphology and 
physiology go back for their final basis to the study of the several 
tissues and frequently to the form and structure of the individual 
component cells. 

Epithelial Tissues 

Definition and Classification. — Any tissue which covers 
a surface is called an epithelium. Phylogenetically, tissues of 
this class are the most primitive, for the coelenterates are essen- 
tially nothing but epithelial in structure. In like manner, in 
individual development among all the Metazoa, the first division 
of labor for the cells of the embryo occurs with the formation of 
the gastrula stage when nothing but surface coverings is present. 

Form vs. Function. — Varied forms of cells and sundry func- 
tions occur in tissues in this group. If an epithelium comprises 
but a single layer of cells, it is termed simple, while an epithelial 
layer two or more cells in thickness is designated as stratified 
epithelium. The form of the component cells leads to the use of 
such descriptive terms as cuboidal, columnar, and squamose, 
but, since function is a much more important criterion for 
classification, epithelia are grouped as protective, formative or 
glandular, ciliated, communicative or sensory, and germinal. 

Ciliated and Flagellated Epithelium. — Even in the gastrula 
stage of ontogeny some or all of the cells are frequently provided 
with cytoplasmic threads, the movements of which are primarily 
for locomotion. Ciliated or flagellated cells as a means of loco- 



motion are only infrequently found in adult motazoans but do 
occur in the flatworms (Fig. 62), the rotifers (Fig. 72), and 
molluscs (Fig. 91). More frequently, however, cilia and flagella 
on epithelia of Metazoa produce movements of the medium (P'ig. 
37) rather than affect locomotion. Thus in the sponges the 
flagellated cells produce the water currents which carry the food 
supply. The same is true of the ciliated mantle and gills of the 

Protective Epithelium. — Because of their location on external 
surfaces epithelia are frequently modified for a protective func- 
tion. Stiff fibrils in the cytoplasm (Fig. 38) and thickenings of 

Fig. 37. — Glandular and ciliated 
epithelium showing unicellular glands 
in entoderm of earthworm, Eisenia 
rosea. (After Schneider, courtesy of 
Giittar Fischer). 

Fig. 38. — Protective epithelium 
from a flatworm, Planocera folium. 
(After Schneider, courtesy of Gustav 
Fischer) . 

the outer margin of the cell are two means whereby mechanical 
protection is secured. Frequently, an external epithelium may 
form extracellular materials which are deposited in a layer or in 
a succession of layers over the surface of the body. The cuticula 
characteristic of so many invertebrates is thus produced by an 
underlying epithelium which is frequently termed a hypodermis. 
In many instances, inorganic salts are deposited in this cuticula 
until a shell-like armor of resistant plates is formed. 

Glandular or formative epithelium represents one of the com- 
monest modifications of this class of tissue. Even in the gastrula 
stage the entoderm of the embryo has begun to be specialized as 
a glandular tissue and the cytoplasm and its inclusions render the 
entoderm obviously different from the ectoderm. Cells scattered 
singly through an epithelium may become specialized as gland 
cells, as in the case of the mucous cells (Fig. 37) of the earthworm. 
Frequently, these unicellular glands become enlarged and through 



elongation the single cells extend down into the underlying tissues 
much deeper than the ordinary epithelial cells which surround 

Flat surfaces or large areas of an epithelium may become 
differentiated for a glandular function, as in the foot of Hydra, 
but in most instances specialized glands require an increased sur- 
face for their functioning as secretory or excretory organs, and 
consequently areas of glandular epithelium become invaginated 
either as a simple tube or as a series of branched tubes and cham- 
bers the walls of which are composed of gland cells. A duct 
usually keeps such a compound gland in communication with the 
surface from which the gland cells were originally invaginated. 

Sensory or Communicative Epithelium. — Naturally, the only 
communication which an organism may have with the out- 
side world is through its surfaces. Consequently, any sensory 
mechanism must be associated with 
the surface of the body and is there- 
fore epithelial in origin. Sensory cells 
located in the epithelium have com- 
munication with nerve endings through 
which they are able to transmit stimuli 
to the central nervous system. Hairs 
or bristles on the surface of epithelial 
cells render them especially susceptible 
to touch stimuli, so isolated cells or 
groups of cells thus modified form 
various tactile organs. An association 
of pigment with epithelial cells having 
rich nerve supply usually signifies an 
optic organ of some type and is des- 
ignated as a retina. Balancing organs 

or statocysts (Fig. 39) are usually modified epithelia specialized 
for receiving tactile stimuli through the movements of a body 
called a statolith or an otolith. 

Germinal Epithelium. — Germ cells of all Metazoa have their 
origin in an epithelium. In many of the groups, the presence of 
a germinal epithelium is readily observable. In some of the 
simple Metazoa, as, for example, the coelenterates, the germ 
cells may arise as modifications of epithelial cells which are not 
even necessarily grouped to form a gonad. The gonads of many 
groups are bounded by an epithehum, the cells of which trans- 





39. — Statocyst of Goni- 
onemus. (Orig.) 



form into the germ cells. Morphologically, the female germ cells 
become differentiated from ordinary epithelial cells chiefly 
through the addition of deutoplasm or yolk and in many instances 
through acquiring additional membranes. On the other hand, 
the male germ cells become highly modified and undergo exten- 
sive rearrangements of their parts, chiefly as an adaptation for 
more effective movement. 

Connective or Supporting Tissue 

Definition. — Tissues which fill in between and connect other 
tissues, give support to them and to the body in general, are 
grouped under the general category of connective or supporting 

tissues. It has been shown that 
the individual cells in their form 
and function give the chief char- 
acter to epithelial tissues, but in 
the connective tissues plasmic 
products are usually the most 

Fig. 40. — Cartilage of an adult 
squid. {Frorn Dahlgren and Kcpner, 
courtesy of The Macmillan Company). 


Fig. 41. — Portion of a section of 
Cerebratulus marginatus, showing much 
branched connective tissue cells. 
{After Schneider, courtesy of Gustav 

Fischer) . 

conspicuous elements. In some tissues these products are 
intracellular as in the spicules formed by certain cells of the 
sponges. Fat or adipose tissue results from an accumulation of 
oil or fat droplets within the cytoplasm until they coalesce to 
form a single large drop around which the cytoplasm of the cell 
is spread in a thin, attenuated layer. 


More frequently, the plasmic products are intercellular. 
In such instances, the cells of a connective tissue are embedded in 
a non-protoplasmic matrix the specific nature of which may be 
highly variable. In its simplest condition, this matrix is com- 
posed of a gelatin-like mass within which the cells are embedded 
to form a homogeneous or gelatinous connective tissue (Fig. 40). 
Frequently fibrillae are formed within the gelatin (Fig. 41) 
giving greater firmness. Depending upon the arrangement of 
these fibrillae, their composition, and composition of the matrix, 
several kinds of connective tissue are recognizable, as, for 
example, fibrous connective tissues, elastic tissue, cartilage, etc. 

Muscle Tissue 

Movement is an inherent property of all protoplasm. Even 
in the Protozoa, portions of the cytoplasm frequently become 
altered as fibers for movement more effective than that which 

Fig. 42. — Muscle fibers from subumbrella of jellyfish, Carmarina hastaia. 
{After Schneider, courtesy of Gu&tav Fischer). 

characterizes undifferentiated, viscous protoplasm. These par- 
tial specializations of the protozoan cell for movement are termed 
myonemes. Partial differentiation of cells for contraction arc 
not infrequent in the Metazoa. The inner margins of the ecto- 
derm cells of Hydra contain contractile threads within the cyto- 
plasm, and consequently the covering cells of the Hydra are 
frequently termed epithelio-muscular cells. In the nematodes, 
the mesoderm cells forming the innermost layer of the body wall 
undergo a partial differentiation to form a series of longitudinal 
muscle fibers. The peripheral parts of these mesoderm cells 
become specialized for contraction while an undifferentiated 
mass of cytoplasm containing the nucleus protrudes into the body 

Frequently, mesenchyme cells become elongated or spindle- 
shaped and develop myofibrils within their cytoplasm. Such 
cells when grouped together form the commonest type of smooth 
muscle tissue. 



Striated muscles occur even in forms as low as the medusae 
(Fig. 42), where they remain on the surface of the body. They 
usually originate as epithelial cells. The nuclei of cells which are 
to form striated muscle undergo a series of divisions without sub- 
sequent division of the cytoplasm. In the cytoplasm of the poly- 
nuclear cells thus formed, numerous fibrils make their appearance. 
The fibers are surrounded by a sarcoplasm containing the nuclei, 
and the whole muscle element is covered with a sheath which is 
termed the sarcolemma. Each fibril is composed of two different 
kinds of substance, one called the isotropic substance which 
does not stain readily, and another, the anisotropic substance 
which is doubly refractive and stains deeply. The two sub- 
stances alternate regularly along the fibril, and in adjoining 
fibrils they are in alignment so that in stained 
preparations under the microscope a bundle of 
^IfJm^. fibrils has a cross-striped appearance. Peculiar 
groupings of fibrils occur in some muscle cells 
(Fig. 43). 

Nervous Tissue 

Irritability, or the power of reacting to stimuli, 
is inherent in all protoplasm but even in the single- 
FiG 43 — celled Protozoa it has been shown that parts of the 
Muscle fiber cytoplasm become specialized as a neuromotor 

from adductor , o u • t a- u u i 

apparatus, feuch specialization has been demon- 
strated for some of the ciliates in which it seems 
probable that stimulation is propagated along 
certain tracts or fibrils more efficiently than 
through undifferentiated cytoplasm. In most 
Metazoa, a nervous system is developed for transmitting 
sensory and motor impulses through the linking together of 
highly specialized nerve cells or neurons. Nervous impulses 
are propagated through a plexus of scattered cells in the hydroid 
stage of the coelenterates. 

Characteristically, a nerve cell consists of a nucleated cell 
body of cytoplasm from which one or more cytoplasmic processes 
are given off. There is a definite polarity in nerve cells. A 
given fibril propagates an impulse in only one direction. When 
there are several cytoplasmic processes from the cell body of a 
neuron those which transmit the impulse inward toward the cell 
body are termed dendrites. Normally, but a single process 

muscle of mus- 
sel, Anodonta 
(After Schnei- 



carries impulses outward from the cell body and this process 
is called a neurite or an axon. 

In most of the higher Metazoa, the nerve 
cells are grouped into masses which are called 
ganglia. The neurites and dendrites form 
part of the fibrous portion within the ganglia 
and continue in groups out from the ganglia 
as important constituents of the nerves of the 
peripheral nervous system. 

By very special histological technic the 
cytoplasm of a neruon and its processes is 
shown to contain various fibrillar structures 
(Fig. 44). It is thought that nerve impulses 
are transmitted along these fibers rather than fibriilae in ganglion 
through the general cytoplasm. In addition, cell of a leech. {After 
much of the cytoplasm of nerve cells contains 
deeply staining granules or masses of questionable significance, 
which are termed the tigroid bodies. Frequently, a region 

I/nplan fahon cone 

Fig. 4-4. — Neuro- 


Fig. 45. 

Fig. 46. 

Fig. 45. — Nerve cell from ganglion of crayfish. {After Dollcy). 
Fig. 46. — Neuroglia cells from nerve cord of earthworm, Eisenia rosea. 
{After Schneider) . 

within the cytoplasm of the cell body directly continuous with 
the cytoplasm of the axon is devoid of tigroid bodies. This 
implantation cone, as it is sometimes called, is so marked in 


arthropods that sections of nerve cells (Fig. 45) have the 
appearance of being highly vacuolated. 

Interspersed between the neurons and fibers of a ganglion 
are varying amounts of mesodermal connective tissue and a 
highly modified type of ectoderm cells (Fig. 46) which are called 
neuroglia. The neuroglia cells of invertebrates usually have 
some portion of the cell body on the surface of the ganglion, while 
a fiber-bearing, supporting portion extends between the nervous 
elements of the ganglion. 

In function, structure, and origin, nervous tissue and the 
sensory epithelium discussed earlier are intimately associated. 

Morphological Changes in the Nucleus 

In the foregoing discussion, attention has been focused upon 
the changes in the form and structure of the cytoplasm and of the 
plasmic products which are involved in histological differentia- 
tion. Omission of the nucleus from this discussion does not 
imply that it is not concerned in differentiation. There are, 
however, few conspicuous changes in form or appearance of the 
nucleus in comparison with the radical changes in the extra- 
nuclear portions of the cells. As the controlling center of 
most of the activities of the cell, doubtless the nucleus must have 
varying functions depending upon the line along which the cell 
is specialized, but there are few reflections of this in the morphol- 
ogy of the nucleus. The nuclei in some highly active tissues 
become irregular or even much branched but these morphological 
changes seem to be correlated with rate of activity rather than 
with actual differentiation of the nucleus in a particular kind of 

C. The Organ Systems 

Just as the individual cells in the many-celled animals become 
dependent one upon another through histological differentiation 
so also do the individual tissues become interdependent. Few 
tissues function as utterly isolated units, for they become com- 
bined in groups which are known as organs. These organs are 
the readily recognizable morphological units of which the meta- 
zoan body is composed. All of the tissues which form an organ 
cooperate in the performance of some function. Obviously, not 
all tissues in an organ function to the same extent, but usually 
one kind of tissue becomes the dominant or essential tissue and is 


aided in carrying out its functions by the other tissues of the 
same organ. 

Thus, in a digestive organ the essential process of digestion is 
made possible through the presence of glandular epithelia which 
form substances necessary for the digestion of food. The 
epithelia are frequently aided in this function by cooperative 
muscles under whose action the content of the digestive tract is 
kept agitated so that all of the food material comes into contact 
with the digestive fluids produced by the epithelia. 

When identical or similar organs are grouped for the perform- 
ance of the same or related functions, such a complex is termed 
an organ system. All of the activities which characterize the 
living animal are so fundamentally interrelated that the numer- 
ous organs and tissues of the metazoan body are capable of being 
grouped under a relatively small number of systems. Yet these 
major organ groups, the systems, have their interrelations and 
function not as independent units but as a correlated whole — 
the organism or the individual. 

Throughout the following sections of this book constant 
reference is made to the digestive, circulatory, respiratory, 
excretory, locomotor, reproductive, and nervous systems. The 
organs of which each system is composed in the various phyla and 
classes are not necessarily identical in structure or origin, for 
frequently the same function is carried on by conspicuously 
different organs in different groups. A brief discussion of the 
modifications of the several organ systems is given here, but for a 
full understanding and appreciation of this discussion, the 
student should again read this section after having completed the 
laboratory study of a number of metazoan forms. 

The Digestive System. — The power of transforming non-living 
food substances into living matter or protoplasm is a distinctive 
property of all living organisms. This is not accomplished 
by a direct transmutation or by a simple act of endowing the 
food with life. Since living protoplasm is in a liquid or semi- 
liquid state the process of digestion or liquefying of food is 
the essential first step in the transformation of food into living 
matter. In some degenerate forms of animal life which have 
become adapted to a purely parasitic existence, as, for example, 
the tapeworms and the Acanthocephala, food taking is restricted 
to the direct absorption of liquids by the cells of the body. 
Organs of digestion are entirely wanting in such instances, 


for the entire digestive process is performed by the organs of 
the host. 

In some of the Protozoa and in a few of the Metazoa, sohd 
food substances are taken directly into the protoplasm of the 
organism. These ingested particles, though within the proto- 
plasm, remain foreign to it until they have undergone physical 
and chemical changes which render them capable of assimilation 
by the protoplasm. This type of digestion is termed intra- 
cellular digestion. Among the sponges and some of the flat- 
worms, in addition to the Protozoa, the entire digestive process 
is intracellular. 

Ontogenetically, the archenteron of the gastrula stage repre- 
sents the first organ specialized for digestion. In many animals, 
the larva upon reaching the gastrula stage is an independent 
organism capable of self-maintenance. Food taken through the 
blastopore of such a gastrula is rendered liquid through the action 
of secretions formed by the entoderm cells. Thus the gastrula 
cavity becomes a space within which intercellular digestion takes 
place and is termed an archenteron or primitive digestive system. 

Among the Metazoa, there are some animals in which the 
digestive system of the adult never reaches a state of complexity 
appreciably higher than that of the archenteron. Among the 
Coelenterata, the polyp of the Hydrozoa is essentially similar to 
the archenteron, for it is a simple entodermal sac with but a single 
opening, the mouth. In many other coelenterates, there is a 
beginning of a separation of the distributive function from the 
central digestive cavity. Thus in the jellyfishes, the gastro- 
vascular system consists of a central cavity from which a periph- 
eral system of pouches or canals passes to the outlying parts of 
the body and thereby aids in the distribution of the digested food. 

In most of the Metazoa above the coelenterates, the blastopore 
of the gastrula is not retained, as the mouth of the adult for 
the blastopore closes during development of the embryo and for a 
time the entodermal digestive tube is but a sac without communi- 
cation with the outside. Communication is later established by 
an infolding of the body-covering at the anterior extremity to 
form an ectodermal antechamber to the entodermal digestive sac. 
Such an entodermal digestive sac with a single opening lined with 
ectoderm (the stomodaeum) is characteristic of the Turbellaria. 

In the nemertines and in all organisms higher than the flat- 
worms, there is characteristically a second ectodermal invagina- 


tion to meet the entoderm. This second ectodermal invagination 
is normally located at the posterior extremity as a proctodaeum 
terminating in an anal aperture. Forms in which both stomo- 
daeum and proctodaeum occur are said to have a complete 
digestive system. In its simplest condition, this consists of a 
continuous tube the extremities of which are ectodermal and 
the true digestive middle portion of which is formed of, or at 
least is lined with, entoderm. Most of the highly developed 
digestive systems represent only modifications of this simple 
tubular condition. Different regions become modified for 
limited functions. With these specializations, changes both 
in form and in structure arise and regions such as esophagus, 
stomach, and intestine are marked off. Jaw^s or organs for the 
comminution of food are frequently developed in the stomodaeum. 
Glands grow out from the walls of the tube and remain attached 
to the digestive tract only through ducts. Muscles, connective 
tissue, and vessels of the circulatory system become associated 
with the entodermal tube to form a more highly complicated 
digestive system. In some instances, the proctodaeum receives 
the ducts of the reproductive system and is then termed a cloaca. 

Circulatory System. — Whether digestion is intercellular or 
intracellular, the products of digestion are directly available to 
only part of the cells of the body while the remainder of the cells 
must rely upon some agency external to themselves for furnishing 
them with the materials essential for anabolism. In some of the 
simpler Metazoa, this is accomplished by direct transfer of the 
materials from cell to cell, but in the highly complicated organ- 
isms such transfer is not possible because some cells are so far 
removed from the organs where digestion takes place. In such 
instances, a special system for the distribution of digested food 
has made its appearance as a specialized circulatory sj^stem. 

Many steps in the development of complexity of the cir- 
culatory system are observable in the different animal groups. 
It has already been pointed out that the gastro vascular system of 
coelenterates serves for both digestion and distribution but even 
in this phylum many steps in the differentiation of the two sys- 
tems are found. Trematodes and Turbellaria among the flat- 
worms also represent the condition of a combined gastro vascular 

In its simplest form, as exemplified by the hydroid polyp, 
this system is a simple sac or pouch, the walls of which are formed 


of two layers of cells, the ectoderm and the entoderm. The most 
distant cells of such an organism are so slightly removed from 
the store of digested food material that distribution is by direct 
transfer from cell to cell. In the medusae of the same phylum, 
the digestive sac is relatively small and located near the center of 
a much enlarged body within which transfer from cell to cell is not 
so readily possible. Under these conditions a distributive system 
arises, intimately associated with but somewhat distinct from 
the digestive system. The tubes or pouches of such a gastro- 
vascular system mark the beginnings of a separate system for 

Among the coelomate animals, digested food materials fre- 
quently pass through the wall of the digestive tract into the 
coelomic cavities where it becomes recognizable as the body 
fluid. Only in the simple coelomate animals is this fluid entirely 
free within the body cavities, for it becomes more or less confined 
within a system of definite channels comprising the vessels of the 
circulatory system. When the fluid is continuously within 
vessels, the system is termed a closed system, but when in any 
part of its course the fluid is emptied into the body cavity or into 
sinuses the system is designated as an open circulatory system. 
Muscles become associated with at least part of the vessels which 
then function as a pumping organ or heart. In many inverte- 
brates, the most conspicuous pumping organ is the vessel, or 
part of the vessel, which lies dorsal to the digestive tract, but 
hearts are frequently located in other regions or the body, as, for 
example, the gill hearts of molluscs and the modifications of the 
circular vessels in the annelids. 

In some instances, notably in the molluscs, the heart becomes 
differentiated into various chambers. Those which receive the 
blood are termed auricles and those which force the blood out 
into the vessels are called ventricles. The heart presents an 
interesting instance of independent or parallel development in 
invertebrates and vertebrates. A three-chambered heart is 
fairly common in the molluscs, while the fishes have but a two- 
chambered heart. 

Respiratory System. — The energy manifested by a living 
organism results largely from the union of oxygen with proto- 
plasm and its contained food substances. Highly complicated 
organic compounds are oxidized or broken down into simpler 
waste substances, thereby transforming the potential energy of 


the complex chemical compounds into the kinetic energy of the 
living organism. The term respiration is applied to the process 
of admission of oxygen into living protoplasm and the subsequent 
giving off of carbon dioxide. More strictly this process should 
be termed aerobic respiration, for there are some organisms which 
obtain their energy release in the total absence of oxygen and 
these are said to undergo anaerobic respiration. Since aerobic 
respiration is by far the more common, the term respiration when 
not modified is usually taken to mean this type. 

Respiration is a process essential for the existence of every 
living cell, yet in the many-celled metazoans respiration as a 
cellular process becomes masked or lost sight of through the 
introduction of organs which facilitate the process for the entire 
organism. Since the respiratory process involves an exchange 
of oxygen and carbon dioxide, any surface which serves for this 
exchange must be moist and delicate in order to permit a diffusion 
of the gases. In animals which are diploblastic, as the coe- 
lenterates, conditions for respiration are not essentially different 
from those found in the single-celled Protozoa, for practically 
every cell has a surface exposed to the water through which the 
gases may diffuse. Even in some of the coelomate animals, 
such as the earthworms, the body surface provides an area 
sufficient for the respiratory exchange but in these as well as in 
all the higher animals the body fluid plays an important part 
in that it absorbs or loosely combines the gases within the body. 

In many animals, the body surface is unable to supply all of 
the cells with oxygen because of insufficiency or because of dry- 
ness which inhibits diffusion. Special organs for respiration 
have been developed in all such forms. Roughly, in inverte- 
brates, these modifications may be classified as gills, tracheae, 
book-lungs, and lung sacs. 

Of gills, there are two distinctly different kinds, blood gills 
and tracheal gills. The former usually consist of portions of 
the body wall drawn out into thin filaments or thin lamellae the 
cavities of which are continuous with the body cavity and are 
filled with body fluids or contain blood vessels through the 
walls of which the respiratory exchange takes place. Tracheal 
gills are evaginations or invaginations of the body wall within 
which or around which air tubes or tracheae are distributed. 
Gills occur on almost any part of the body, wherever their func- 
tion may be carried out. 


Tracheae are invaginations of the body wall to form a system 
of air tubes which carry atmospheric oxygen directly to all parts 
of the body in insects. The intimate structure of the tracheal 
system is discussed in greater detail in Chapter XV. 

In the arachnids, an invagination of the body wall leads into 
an organ called a book-lung, the walls of which are composed of 
much-folded delicate membranes through which oxygen is taken 
from the air-filled cavity of the book-lung and carbon dioxide 
is given off. 

In some molluscs, the cavity which ordinarily bears gills 
lacks these structures and has become secondarily modified as a 
lung sac into which atmospheric air is periodically admitted and 
from which the air bearing carbon dioxide is ejected. 

Excretory System. — As a result of katabolic processes, wastes 
are formed within every living cell. The carbon dioxide men- 
tioned above is only one of the important waste substances. 
Water, ammonia, urea, and other nitrogenous compounds accu- 
mulate within the living protoplasm. Many of these wastes are 
soluble in water and tend to diffuse through the walls of the cells. 

In many of the Protozoa, contractile vacuoles facilitate the 
collection and elimination of these wastes. Among the coe- 
lenterates and the Porifera, direct diffusion through the cell 
surfaces into the surrounding water suffices for the elimination 
of these katabolic products. In the Metazoa, the first specializa- 
tion for their elimination is the protonephridial system of flat- 
worms, rotifers, and trochophore larvae. This system consists 
of a series of tubules associated with flame cells. Such a system 
may be provided with an enlarged excretory vesicle within which 
the excretory matter accumulates before it is discharged through 
the excretory pore. 

A great many of the coelomate animals have a metanephridial 
system which picks up excretory wastes from the coelomic cavities 
and from the body fluid. The unit of structure in such a system 
is essentially a funnel-like nephrostome which communicates 
with the outside by means of a tubule. The nephridia are paired 
organs segmentally arranged in annelids, while in the Crustacea 
they are considerably modified in form and are limited to one or 
two pairs. 

A group of thin-walled Malpighian tubes communicates with 
the intestine of insects and discharges excretory wastes from the 
body fluid into the digestive canal. 



Reproductive System. — In some Metazoa, the gonads or 
essential organs of reproduction are the only organs involved in 
the reproductive process. Thus, in the coelenterates, the germ 
cells are dehisced directly from the body surface or into the 
gastro vascular cavity. Ducts which lead from the gonads to the 
exterior are present in most other Metazoa, These, and other 
accessory organs, comprise the reproductive system. The gonads 
may be single, paired, or multiple. Frequently, a gonad becomes 
subdivided- into follicles, each of which has more or less the 
appearance of an independent organ. 

Fig. 47. — Different relations of nophridia and sexual ducts in chaetopods 
(after Goodrich). I, hypothetical primitive condition, gonoducts (g) and 
protonephridium (p) independent; II, ciliated grooves discharge into duct of 
protonephridium as in Phyllodoceids and Goniads; III, ciliated grooves and met- 
anephridium (n) open independently as in Dasybranchus; IV, ciliated grooves 
open into canal of metanephridium as in Syllids, Spionids, etc. (From Hertwig's 
Manual of Zoology by Kingdey, courtesy of Henry Holt and Co.). 

When each individual bears the organs of but one sex, the 
species is said to be dioecious. Very frequently among the 
invertebrates a single individual may produce both eggs and 
sperms and is then said to be hermaphroditic or monoecious. 
Hermaphroditism commonly results from the occurrence of a 
full set of gonads and accessory organs for each sex in the body 
of the same individual but in some snails both eggs and sperms 
are formed in the same gonad, which is therefore designated as a 
hermaphroditic gonad. 


In most instances, the ducts which carry the germ cells from 
the ovaries and spermaries are at least in part (Fig. 47) derived 
as modifications of excretory ducts. In the marine annelids 
the varied relations of ducts for elimination of waste and libera- 
tion of germ cells are well illustrated. In some forms the gono- 
ducts and the excretory ducts are wholly independent (Fig. 47, 
III), while in other forms the gonoducts or ciliated groove may 
become united with either a protonephridial tube (II) or a 
metanephridium (IV). 

Vasa deferentia of the male are usually directly continuous with 
the gonads, while the oviducts of the female have communication 
with the ovaries only through the body cavity. A portion of the 
oviduct of the female is frequently enlarged as a uterus within 
which the eggs are held for either a brief time or through the full 
period of the development of the embryos. 

Following copulation, sperm cells are frequently stored within 
the body of the female in a receptaculum seminis from which they 
escape to fertilize the eggs which are produced over a considerable 
period of time. The vitellaria are also accessory glands of the 
female. They furnish nutritive materials and in some instances 
supply the substances of which the eggshell is formed. In many 
animals, there are no separate vitellaria and frequently in these 
only a portion of the ovarian cells become functional ova, while 
others act as nurse cells or follicle cells which supply the ova 
with additional reserve food material. 

Copulatory organs are of common occurrence. An intromit- 
tent organ known as the cirrus is frequently characteristic of the 
male. This may be a modification of the terminal portion of the 
vas deferens or, as is often the case, may be purely accessory 
structures such as spicules or appendages modified to aid in the 
transfer of sperm. Clasping organs are especially characteristic 
of the males of some arthropods, and in the same group oviposi- 
tors are frequent accessories of the female system. 

Locomotor System. — Numerous specialized structures for 
locomotion are encountered in the single-celled organisms. 
Cilia, flagella, pseudopodia, and myonemes are characteristic 
partial differentiations of cells for movement. The first two of 
these especially are carried over into the Metazoa as modifications 
of epithelial cells for locomotion or the production of movement. 
Many larval Metazoa perform locomotion by means of cilia exclu- 
sively, but the flatworms and the rotifers represent the highest 


adults in which locomotion results from ciliary action. In the 
Metazoa, movements and locomotion more frequently result 
from the activity of contractile fibers which represent different 
degrees of specialization or differentiation of muscle cells. In 
some instances, there are scattered contractile elements, but more 
commonly the contractile elements are associated to form 
continuous sheets or bundles of muscles. 

The dermomuscular sac of flatworms represents a definite 
stage in the development of a locomotor system. Alternate 
contraction and relaxation of the longitudinally and circularly 
directed fibers in such a sac produce successive shortenings and 
elongations of the body. As a consequence locomotion results. 
When hard skeletal parts make their appearance, groups of mus- 
cle cells become attached to these hard parts and with fixed points 
of attachment perform more effective movements. Even the 
setae of an earthworm serve for muscle attachment and aid in 
locomotion, thereby suggesting an extremely early stage in the 
differentiation of a locomotor skeleton. The exoskeleton of the 
arthropods is the most highly organized locomotor skeleton 
found in the invertebrates. 

Nervous and Sensory System, — Histological elements differ- 
entiated for transmission of nervous impulses assume sundry 
forms of organization in the metazoan body. The hydroid or 
polyp stage of the coelenterates is usually provided with a 
scattered or diffuse arrangement of nerve cells, but in all of the 
Metazoa above the coelenterates the nerve cells are grouped to 
form definite tracts of nerve tissue. In these tracts occur not 
only the cell bodies of the neurons and cytoplasmic fibers with 
their specializations as already described but also supporting 
elements of which the neuroglia cells are the most important. 
Usually, there is a tendency for the cell bodies of the neurons to 
become massed in certain parts of the nerve tracts. These 
regions are designated as ganglia. 

The nervous systems of medusae are interesting in that they 
exhibit a series of steps toward the centralization of the nervous 
system. In the hydromedusae, a nerve ring without any differ- 
entiation of ganglia parallels the margin of the bell. In the 
scyphomedusae, the nerve ring has become more specialized and 
shows localization of the cell bodies to form ganglia. 

Among metameric animals, there is a tendency for ganglion 
formation in each somite, but this primitive scheme is in many 


instances altered by the fusion of two or more successive ganglia 
to form a single mass. This tendency is especially pronounced in 
the anterior region of the body where a single prominent ganglion, 
the brain, is located. 

In many of the worms, two or more lateral nerve cords pass 
through the body, while in some of the annelids and in the arthro- 
pods these have been fused to form a single chain of ganglia 
ventral to the digestive tract. An examination of a cross-section 
of such a nerve cord, however, usually furnishes evidences of its 
double nature. 

That portion of the nervous system which contains the ganglia 
and consequently is the controlling center for directing the activ- 
ity of the organism is designated as the central nervous system, 
while from it fibers and bundles of fibers pass to the various parts 
of the body as a peripheral nervous system. Largely through the 
latter are the sense organs brought, into relationship with the cen- 
tral nervous system. A sympathetic system in some of the higher 
arthropods controls the activity of some of the internal organs. 

As has been pointed out, there are numerous modifications of 
epithelial tissues for receiving stimuli and transmitting nervous 
impulses to the central nervous system of the invertebrates. 
Concerning many of these sensory organs our knowledge is but 
fragmentary, for they are organs which have no counterpart in 
the human body and consequently direct knowledge of their 
functions is beyond the scope of human experience. In some 
instances, their functions may be inferred from the location and 
the mechanics of their structure and through the reactions of the 
organism when the organ in question is stimulated. 

Tactile organs are very commonly developed as hairs or bristles 
from the surfaces of epithelial cells, or, as in the case of bristles of 
arthropods, they may be formed as outgrowths of specialized cells. 
Tactile organs may be scattered fairly uniformly over the 
surface of the animal, but sometimes specific organs are especially 
adapted for receiving tactile stimuli, as, for example, the tentacles 
of coelenterates, worms, and snails and the antennae and palpi 
of insects and crustaceans. In many such instances, other sen- 
sory organs are associated with those of touch. Thus the 
olfactory organs of insects are commonly found on the antennae 
along with other sensory organs of uncertain function. In some 
instances, the antennae are provided with hairs which seem to be 
associated with an auditory function. 



Both taste and olfactory organs of the invertebrates are rather 
simple in their organization. They are especially developed in 
the insects, where they frequently occur as a sac or depression 
within which there is a bristle or conelike projection. 

Organs of sight, hearing, and balance are frequently much 
more highly organized than those organs previously mentioned. 
The association of pigment with sensory epithelial cells frequently 
denotes the presence of a retina or some sort of structure for 
reception of light stimuli. Yet there are some organisms, such as 
the earthworms, which respond to hght stimulation without hav- 
ing any specialized sense organs for the reception of light stimuli. 
Pigment spots near the bases of the tentacles in many jellyfishes 
are usually thought to have a light-percipient function and in 
some instances even have lenslike bodies associated with them. 

In the turbellarians, there are eyes in which the sensory 
epithelium forming the retina has undergone considerable speciali- 
zation, and in many of the marine annelids highly complicated 
optic organs are found. The 
highest development of an 
invertebrate eye is found in 
cephalopods, the eyes of which 
very closely resemble those of 
vertebrates, though the two 
seem to have no phylogenetic 
relationship but apparently 
have arisen independently of 
each other. 

Compound eyes are highly 
characteristic of arthropods. A 
compound eye is composed of an 
aggregation of similar elements 
called ommatidia the number of 
which is indicated by the facets 
or rounded or hexagonal markings on the surface of the eye. 
Each ommatidium is bounded externally by a biconvex cornea 
beneath which is located a fluid or a sohd conehke lens. Beneath 
this are one or more chitinous rods termed the rhabdomes. 
Pigment cells surround each ommatidium and nerve fibers pass 
off from the base of the rhabdomes. 

The term ocellus is frequently taken as synonymous with 
a simple eye, but from point of view of structure there are two 

Fig. 48. 

Median ocellus of Acilius. 
{After Patten) . 


very distinctly different types of ocelli. Those of one type, 
exemplified by the median ocelli of insects (Fig. 48), are but 
little less complicated than the compound eyes, while the lateral 
ocelli are much simpler. 

The simple, unpaired median eye of the lower crustaceans 
shows many features in common with the eyes of the flatworms. 

There are numerous modifications for auditory and balancing 
functions in invertebrates but most of these are recognizable as 
some sort of otocyst or statocyst (Fig. 39), which depends upon 
the movement of a free or suspended body, the otolith or statolith, 
against the walls of the vesicle for stimulating the central nervous 
system and causing the organism to right itself. In some of the 
insects is found a highly specialized auditory organ in which a 
tympanum receives sound waves and transmits the stimulus 
through an underlying space to the nerve endings in the sensory 


Dahlgren, U. and Kepner, W. A. 1908. "A Text-book of the Principles 
of Animal Histology." New York, Macmillan. 

Kellicott, W. E. 1913. "A Textbook of General Embryology." New- 
York, Holt. 

KoRSCHELT, E. and Heider, K. 1895-1900. "Textbook of the Embryology 
of Invertebrates." (English translation.) London, Sonnenschein. 

McBride, E. W. 1914. "Text-book of Embryology." Vol. 1, Inverte- 
brata. London, Macmillan. 

Schneider, K. C. 1902. "Lehrbuch der Vergleichenden Histologic der 
Tiere." Jena, G. Fischer. 


The word sponge usually calls to mind the sponges of commerce 
which are only one type of the skeletal remains from animals 
belonging to the phylum Porifera. A mass of parenchymatous 
tissue penetrated by numerous pores covers this framework in 
the living animal. The living commercial sponges, though 
slimy to the touch, have a fairly solid fleshy body which is 
described by one author in the following manner: " In appearance 
and consistency and the manner in which it cuts with a knife, 
a living sheep's-wool sponge is not unlike a piece of beef liver, 
perforated with holes and canals." Sponges vary so much in 
shape that little can be said regarding their form. Without 
exception, they are attached to some object and have lost powers 
of free locomotion. Along with the lack of locomotor powers 
there is the correlated lack of nervous tissue. In fact, there are 
no specialized organs of any kind in the sponges. 

Distribution. — Sponges live under the most varied conditions 
of aquatic existence from polar regions to the tropics and in 
both salt and fresh water, but they seem to be more influenced by 
conditions of the immediate environment than by mere geograph- 
ical location. Some species are characteristic of the shore hnes, 
even flourishing between tide marks, while others live only in 
the greatest depths of the ocean. Only a single family (Spon- 
gillidae of the class Demospongia) has become adapted to life in 
fresh- water, but in the species of this family distribution is so 
much facilitated by the reproductive buds or gemmules that 
most of the genera are practically cosmopolitan. 

Fossil sponges appear in all strata of the Earth's crust back to 
the Cambrian but even the oldest fossils are fairly closely similar 
to forms of today, so they throw little light upon the problem 
of the origin of sponges. 

Cell Layers. — The cell layers of the sponges offer but httle 
opportunity for comparison with the remainder of the Metazoa. 
The main bulk of the body consists of a solid mass of mesoderm 




cells. The cells covering the surface of the body and those 
lining the internal cavities have an entirely different history 
from the ectoderm and entoderm cells of other many-celled 
animals. For this reason many zoologists maintain that the 
sponges represent a line of development wholly independent 
of all the higher animals and consequently set off the sponges 
from the Metazoa, applying to this phylum the rank of a separate 
subkingdom, the Parazoa. Certain chambers inside the sponge 
are lined with flagellated cells which, because of their location 
and function in capturing and digesting food, have frequently 
been considered as entodermal cells. In Fig. 49, the location 



Fig. 49. — Morphological types of sponges. A, Ascon type; B, Sycon type; C, 
Leucon type. {From Korschelt and Heidcr). 

of the flagellated cells is indicated by the hairlike parallel lines. 
In the course of embryological development these flagellated 
cells have been shown to arise from those cells of the embryo 
which in normal development give rise to the ectoderm. In 
fact, in the protozoan genus Proterospongia (Fig. 50 B), which 
is thought by many to be like the ancestor bridging the gap 
between the flagellate Protozoa and the sponges, the collared 
flagellate cells are found on the outside of the body. 

Morphological Types. — Three types of sponge structure are 
usually recognized (Fig. 49). In the simplest sponges, of the 
Ascon type (A), the body is a thin-walled sac having a single 
large opening, the osculum, at one extremity and numerous 
minute incurrent pores through the body wall communicating 
with a central cavity. This central cavity, which is frequently 
called a stomach, is lined with flagellate cells, but since digestion 


is intracellular the cavity is not a true digestive cavity and there- 
fore not a stomach. Opening and closing of the pores leading 
into this flagellated chamber are under control of the animal. 
When the pores are open, the action of the flagella causes water 
currents bearing food material to pass through the flagellated 
chamber and out through the osculum. During this passage, 
food is removed from the water and ingested by the cells. Leu- 
cosolenia and Olynthus are examples of this simplest type of 
sponge organization. 

The Sycon (Fig. 49 B) differs from the foregoing chiefly in the 
fact that the central cavity is lined with pavement epithelium 
and the flagellated cells have withdrawn to small radial chambers 
or ampullae embedded in the thickened wall. The central 
cavity is now a distinct cloaca with which each ampulla communi- 
cates by means of a small opening called the apopyle. As the 
name implies, the radial chambers are arranged radially about 
the cloaca. Alternating with the radial chambers are the incur- 
rent canals which pass from the exterior inward toward, but not 
into, the cloaca. Minute openings, the prosopyles, communi- 
cate between the incurrent canals and the adjacent ampullae, 
thus allowing water currents bearing food material to pass 
through the radial chambers, into the cloaca and out through the 
osculum. Grantia and Sycon are two typical genera whose 
representatives are built upon the Sycon plan. 

As they acquire greater bulk, some sponges reach a size wherein 
direct communication between the ampullae and the exterior 
and cloacal surfaces is no longer possible. Intricately branched 
incurrent and excurrent canals establish these surface relations 
for the deeply embedded ampullae (Fig. 49 C). Sponges of this 
character are of the Leucon type and are typified by such genera 
as Leucilla and Oscarella. 

Physiology of Sponges. — Regardless of the type, all sponges 
are fundamentally alike in their methods of securing food. 
Movements of the flagella in the flagellated chambers produce 
water currents through the canal system of the sponge. Food 
particles entering the flagellated chambers are ingested by the 
cells, and the process of digestion is wholly intracellular. From 
the incurrent streams of water the sponge derives its nourishment 
and its oxygen for respiration while the same water stream as 
it leaves the body by way of the osculum carries the metabolic 



Reproduction. — Asexual reproduction through budding is 
common in sponges. By this means masses of individuals are 
produced thereby forming colonies. These colonies may be 
either groups of distinctly separable individuals with only a 
common region near the base, or they may be so intimately 
fused as to render the recognition of individuals impossible. 
Sponges possess great power of regeneration. In the commercial 
sponge fisheries, advantage is taken of this fact and small frag- 
ments are planted and allowed to grow before harvesting. 

Fig. 50 A. — Embryo of Grantia. Blastula stage within embryonic chamber 
of parent individual. {After Dendy). 

Fig. 50 B.- — Proterospongia, showing collared flagellate cells. {Redrawn from 

Kent) . 

Gemmule formation is another type of reproduction found in 
fresh-water sponges. Groups of cells called gemmules or internal 
buds become separated from the surrounding tissues by confining 
membranes. These gemmules are capable of withstanding 
desiccation and other adverse circumstances which are fatal 
to the living sponge and are therefore of great importance in 
the life cycle of sponges. 

Sponges are usually hermaphroditic, but since in any individual 
the germ cells of the male usually mature before those of the 
female they are said to be protandrous. Eggs are fertilized 
within the parent and there undergo segmentation (Fig. 50 A). 
The formation of the germ layers and the transformation of the 
larva present conditions wholly unlike these found typically 
in the Metazoa. The blastomeres resulting from cleavage of 


the fertilized egg become arranged as an ovoidal blastula (Fig. 
50 A) one pole of which is rounded and the other flattened, but 
from this point onward conditions are not in harmony with the 
usual plan of embryonic development. The cells at the more 
pointed pole become extremely granular, while those at the op- 
posite pole become much elongated and each develops a flagellum. 
The large columnar cells grow down and partly enclose the mass 
of granular cells. At about this stage, the larva breaks through 
the embryonic chamber in which it has developed and by way 
of the osculum uses the flagella to swim freely in the water. 
Upon leaving the embryonic chamber of the parent, the granular 
cells which have up to this time been partially invaginated into 
the blastocoel come to he on the surface of the embryo. The 
larva is now an ovoid body one pole of which consists of flagel- 
lated cells and the other of granular cells. In other invertebrate 
larvae, the ciliated or flagellated cells are the ones which remain 
on the surface of the body and form the skin. Hence they are 
called ectoderm cells. But in the sponges the flagellated cells 
have a different future and later come to lie inside the body. 
This free-swimming larval stage is called an amphiblastula. 
Cells which are the forerunners of the mesoderm occur in the 
cavity of the amphiblastula. After a day or more of free- 
swimming existence the amphiblastula settles down with the 
pole bearing the flagellated cells in contact with some object. 
The flagellated cells become invaginated to form a cavity within 
the granular cells and from these the collared flagellate cells 
develop. The system of pores and canals and the osculum later 
make their appearance, thus laying down the general form and 
organization of the sponge, though the details of arrangement 
of the parts take considerable time for growth and development. 

During the period of transformation from the larva some of the 
mesenchyme cells from the interior of the larva break through the 
layer of granular cells and become distributed over the surface. 
These apparently mesodermal cells of the larva thus form the 
body covering, adding another unparalleled chapter to the history 
of sponge development, for in no other instance is there a record 
of the skin of an animal being formed from mesoderm. 

Skeleton. — The skeletal structures (Fig. 51) which give the 
chief character to the Porifera are spicules formed by mesoderm 
cells called scleroblasts. Characteristically each of the large 
spicules of the mature sponge starts as a microscopic crystal 



within the cytoplasm of a scleroblast but as the spicule grows it 
ultimately extends the limits of the mother cell and breaks out 
from its walls. In some other instances spicules are formed by 
the combined action of a group of scleroblasts. 

In the classification of sponges, the nature and form of the 
skeletal structures are of great importance. Spicules may be 
formed of either calcareous or silicious material in a great variety 
of patterns. A network of a substance called spongin (Fig. 
51, 1) ,is another type of supporting structure formed by cells 
called spongioblasts. This spongin, which is closely allied to 
silk in its chemical composition, may appear either alone or 
along with silicious spicules. 

Fig. 51. — Skeletal structures of sponges {after Schulzc and Maas). 1, spongin 
fiber surrounded by spongioblasts;' 2-7, different types of spicules. {From 
Hertwig's Manual of Zoology by Kingslcy, courtesy of Henry Holt and Co.). 

Economic Importance. — Aside from the market value of 
sponges gathered for domestic use and utilization in the arts, 
sponges have relatively little direct value. They have no im- 
portant enemies, but many kinds of crustaceans, worms, and 
other animals find shelter within their canals and some species 
of sponges have been found only on the bodies of other animals, 
especially crabs, where they apparently hold a symbiotic relation- 
ship. The spicules seem to afford a great measure of protection, 
for practically no animals feed upon sponges. Some of the boring 
sponges do damage to the shells of molluscs into which they 
burrow for protection. 

Outline of Classification 

Phylum Porifera.^^Many celled; body penetrated by pores communicating 
with one or more internal chambers or canals, at least one of which is lined 


with collared-flagellate cells. Skeleton a framework of inorganic spicul(>s 
or of organic fibers or both. Aquatic. 

I. Class Calcarea. — Ocean dwellers, in shallow water; chiefly radial 
in form; spicules of carlwnate of lime, simple or triradiate. 

1. Order Homocoela.- — Gastral epithelium a single sac. Clathrina, 

2. Order Heterocoela. — Gastral epithelium lining canals or 
chaml^ers around a central cloaca. Gxmjitia, Leucortis. 

II. Class HexactineUida. — Deep-sea forms; chiefly radial in form; 
skeleton a network of silicious spicules. Hyalonema, Ewplectella. 

in. Class Demospongia. — Marine and fresh water; skeleton of spongin 
or of silicious spicules or both, sometimes wholly lacking; canal system 

1. Order Tetraxonida. — Typically tetraxon spicules. Geodia, 

2. Order Monaxonida. — Typically simple spicules. Cliona, Sub- 
erites, SpongiUa, Carterius, Ephydatia. 

3. Order Keratosa. — Spongin; no true spicules. Euspongia, 

4. Order Myxospongia. — Without skeleton. Oscarella. 


(See references cited at close of Chapter I) 

Dendy, a. 1893. "Observations on the Structure and Classification of the 
Calcarea heterocoela." Quart. Jour. Micr. Set., 35: 159-257. 

Haeckel, E. 1872. "Die Kalkschwamme, eine Monographie." Berlin. 

Moore, H. F. 1908. "The Commercial Sponges and the Sponge Fisheries." 
Bull. U. S. Bur. Fish., 28: 399-511. 

Parker, G. H. 1910. "The Reactions of Sponges, with a Consideration 
of the Origin of the Nervous System." Jour. Exp. Zool., 8: 1-41. 

Wilson, H. V. 1907. "On Some Phenomena of Coalescence and Regener- 
ation in Sponges." Jour. Exp. Zool, 5: 245-258. 

1910. "Development of Sponges from Dissociated Tissue Cells." 
Bull. U. S. Bur. Fish., 30: 1-30. 



Coelenterates are diploblastic, radially symmetrical animals, 
usually bearing tentacles and provided with nettling cells. In 
fundamental structure, the bodies are but slightly modified from 
the plan of arrangement of the gastrula stage which occurs in 
the embryology of all higher Metazoa. Because of their form, 
Cuvier included them along with the echinoderms in his type 
Radiata. Extreme differences in structure preclude the possi- 
bility of uniting these two groups on the basis of superficial 
agreement in disposition of their parts. The echinoderms 
have a true body cavity or coelom in addition to the digestive 
cavity. In the coelenterates, however, only one' cavity is 
found. Phylogenetically, this single cavity represents the gas- 
trula cavity from which in higher forms of life both digestive 
cavity and body cavity arise during later development. Since, 
in the coelenterates, these two cavities are not differentiated, 
Leuckart suggested the name Coelenterata which they now bear, 
carrying with it the idea of lack of specialization of coelom from 
the enteron. 

Morphological Types. — Body form among the coelenterates is 
usually referable to one of tw.o types, the hydroid or polyp form 
and the medusoid or jellyfish form. While these two forms are 
clearly differentiated one from the other, in fundamental struc- 
ture they are essentially alike. In each, tentacles are usually 
present and function in grasping food and bringing it into the 
mouth. These tentacles, and in some instances the body also, are 
supplied with nettling cells which are tubular threads coiled 
within a small bladderlike structure. As a means of defence or of 
offence, these threads are discharged from the nematocysts with a 
force sufficient to carry them even through the chitinous body 
covering of small crustaceans introducing into the wound an 
irritating fluid. By their use, some coelenterates, the Portuguese 
man-of-war for instance, are capable of inflicting painful injury 
even to man. 



The term polyp, which is appHed to the hydroid individual, is 
derived from the Latin name of the cuttlefish (Polypus) because 
of a superficial resemblance between the two animals. 

Cell Layers. — The body wall is composed of an external layer 
of cells called the ectoderm and an internal layer lining the 
gastrovascular cavity called the entoderm. Between these lies 
a non-cellular, gelatinous substance termed the mesoglea. The 
extent and importance of this mesoglea in the various coelen- 
terates varies extremely. In hydroids it is commonly a very 
delicate, inconspicuous layer, while in jellyfishes it has become the 
most conspicuous as well as the most bulky part of the whole 
organism. Structures having their origin in either ectoderm or 
entoderm tend to pass into the mesoglea, thus rendering it more 
highly complicated and causing it to partake of the nature of a 
definite mesoderm. The extreme of this tendency is found in the 
Ctenophora, a group which is frequently included as a class of the 
Coelenterata but here recognized as an independent phylum. 

Tissues and Organs. — In degree of differentiation, the medu- 
soid represents much the higher type. Structures frequently 
either wanting or of low organization in the hydroid individuals 
are well represented in the medusae even of the same species. 
Muscle occurs as a partial differentiation of epithelial cells in 
hydroids of the Hydrozoa and Scyphozoa but sheets and bundles 
of muscle tissue are recognizable in the medusae. Specialized 
sensory organs are almost exclusively associated with the more 
distinctly centralized nervous system of the medusoid forms. 
In these, organs of equilibrium appear in extremely diverse 
stages of development, representing conditions varying from 
simple, exposed, modified tentacles or sensory clubs to the highly 
complicated statocysts with accessory protective vesicles (Fig. 
39). The reproductive organs typify the lowest degree of 
specialization, for no ducts or other accessory sexual organs of any 
sort accompany the gonads. 

Modifications of the Digestive System. — In its most primitive 
condition, the coelenteric or gastrovascular cavity of coelenterates 
is a simple bag with a single opening, the mouth. This is the 
condition in the polyp of the Hydrozoa. In the medusae, there 
are frequently diverticula from the central chamber of the 
digestive system which provide greater space for the digestion 
of food and serve for the delivery of digested food material to the 
more distant parts of the body. In many instances, these diver- 


ticula have the form of definite vessels, called the radial canals, 
which may be united at their distal ends by a common vessel 
termed the circumferential canal. Further complication of the 
digestive system is found in the Anthozoa wherein a definite 
infolding of ectoderm projects for some distance from the mouth 
opening into the digestive chamber as an esophagus. In this 
same group, the cavity becomes divided by a series of partitions 
called mesenteries or septa which provide additional surface 
for the processes of digestion and assimilation of the food 

Metagenesis. — Both types of individuals described above 
occur in the course of the life cycle of the Hydrozoa and the 
Scyphozoa. An asexual hydroid generation gives rise to a sexual 
medusoid generation. This condition of direct alternation 
between two generations of different type has been termed meta- 
genesis. The occurrence of hydroid and medusoid forms in 
the developmental cycle of a single species has not been under- 
stood long. It is, then, not surprising that all of the earlier 
treatises considered the hydroid and medusoid generations, 
even of the same species, as belonging to distinct and independent 
groups of the animal kingdom. 

Habitat. — In habits, the coelenterates are chiefly marine, 
though Hydra, Protohydra, Cordylophora, and a few rare 
medusae occur in fresh-water habitats. 

At the seashore, the anemones and the colonial hydroids are 
among the most interesting animals on pilings, on seaweeds, and 
on rocks, while the fragile bodies of the free-floating jellyfishes 
with their play of colors and graceful pulsations are the most 
conspicuous of the free-floating plankton organisms. Because of 
their size and abundance, coelenterates are among the most prom- 
inent of the organisms which render the ocean phosphorescent. 

Food Relations. — Most coelenterates feed upon microscopic 
organisms which are captured by the tentacles and killed by the 
nettling cells. Fishes and crustaceans of considerable size are 
included in the food of some of the jellyfishes, however, and even 
the minute fresh-water hydras at times feed upon newly hatched 
fish. In turn, the soft bodies of coelenterates frequently fall 
prey to other animals in spite of the protective nature of the 
nettling cells. The discovery of nettling cells in some flat worms 
and molluscs threw some doubt upon the nematocysts as dis- 
tinctive organs of the coelenterates but it is now known that the 


nettling cells acquired their unusual location when coelenterates 
were used as food. 

Class Hydrozoa 

The Hydrozoa are coelenterates usually having an alternation 
of generations of which the hydroid is frequently the more 
conspicuous. The hydropolyp lacks longitudinal folds of the 
entoderm and the mouth opens directly into the coelenteric 
cavity. The hydromedusa, when present, has a smooth bell 
margin; the concave surface is partially enclosed by a membrane, 
the velum, which extends inward from the edge of the bell. 
In the members of this class the gonads may be borne either on 
the radial canals or on the manubrium. The germ cells are 
expelled from the gonads directly through the surface of the body. 

Life History. — The life cycle of Obelia serves well to illustrate 
metagenesis as it occurs in the Hydrozoa. The hydroid colony 
starts as a single polyp which, by continued budding, produces a 
series of individuals alternating along a common stalk. Such a 
colony is attached to some object by means of a rootlike structure 
termed the hydrorhiza. In a young colony, all of the individuals 
are alike and all function as vegetative zooids. Starting near 
the base of the colony, reproductive individuals begin to make 
their appearance. These are sac- or vase-shaped zooids, termed 
gonangia, budded off in the axils between the vegetative indi- 
viduals and the common stalk. Each gonangium is composed 
of a central core upon which buds are formed. As development 
proceeds, these buds display medusan characters and upon 
reaching full development become detached and emerge from 
the open end of the gonotheca as free-swimming jellyfish. The 
medusae produce germ cells which, upon fertilization, undergo 
cleavage to form a ciliated larva, termed a planula, but little 
higher in organization than the gastrula. After a short period 
of free existence the planula becomes attached to some object 
and transforms into a hydroid through the development of a 
circle of tentacles and of a hydrorhiza. From this single zooid, 
an entire colony is produced through repeated budding. 

While budding is the usual method of reproduction during the 
asexual generation of Hydrozoa, it occurs but rarely in the medu- 
soid generation. Sarsia and a few other genera are peculiar in 
that young jellyfishes are budded from the manubrium or from 
the margin of the bell at the ends of radial canals. 



The Gonophore and Its Reduction. — An individual among the 
coelenterates which bears the gonads is termed a gonophore. 
Ordinarily, this is a free-living medusoid individual (Fig. 52 A) 
with gonads either on the manubrium or on the radial canals of 
the gastrovascular system. In some representatives of the class 
Hydrozoa, the medusoid generation is lacking. This condition 
is usually the result of a process which is designated as gonophore 
reduction. Usually, the gonophores have their origin as products 
of asexual reproduction from the hydroid individuals. Under 
ordinary circumstances, they undergo development to a certain 
stage while still attached to the hydroid and are then liberated. 

Fig. 52. — Gonophore reduction. A, free gonophore, an independent medusa; 
B, a naedusa which remains permanently attached to hydroid colony and fails 
to develop either tentacles or mouth; C, still further reduction of gonophore to a 
simple manubrium (the spadix) surrounded by gonads; D, gonads produced 
around a slight entodermal rudiment of the manubrium as in Hydra. {After 
Kingsley) . 

In some instances, however, freedom is never gained, then the 
gonophores remain permanently attached to the parent hydroid 
(Fig. 52 B-D) and organs essential to the independent existence 
of the gonophore fail to develop to a functional stage or degen- 
erate entirely. By the selection of different examples, a finely 
graded series of steps in the degeneration of the medusoid may 
be secured. A greatly reduced gonophore comprises little more 
than an ectodermal bag within which the germ cells are arranged 
about a core of entoderm called the spadix. The spadix repre- 
sents the remains of a degenerated manubrium. The term 
sporosac is applied to such a reduced gonophore (Fig. 52 C) 
attached to the body of a hydroid individual. In some species 
of Corymorpha, medusae-bearing gonads are formed. Though 
the medusae pulsate in futile fashion for several days, they never 
break away from the parent polyp. The ultimate in gonophore 



suppression is found in members of the genus Hydra, wherein 
the gonads occur in the ectoderm with practically no remnant 
of spadix or other medusoid remains (Fig. 52 D). 

Suppression of the Hydroid. — In contrast with the gonophore 
reduction discussed above stands the suppression of the hydroid 
generation characteristic of representatives of the order Trachy- 
linae. In these, development of the medusa proceeds directly 
from the larva derived from the fertilized egg without the inter- 
vention of the polyp generation. 

Polymorphism in Hydrozoa. — The original plan of two body 
forms alternating in the life cycle of the Hydrozoa has become 
much modified in members of one order called the siphonophores 
(Siphonophora). These are free-float- 
ing or swimming colonial forms in 
which the parts have become so 
interdependent and so highly special- 
ized for limited functions that it 
becomes difficult to distinguish whether 
the individuals of the colony are 
hydroid or medusoid." A continuous 
tube of the digestive system connects 
all the individuals of a colony and each 
performs but a limited service for the 
entire colony. One individual consists 
of only an air-filled bladder or pneu- 
matophore (Fig. 53 A), which regulates 
the position of the colony at or 
beneath the surface of the ocean. 
Nectocalyces, or swimming bells (B, C), 


Diagram of a 

frequently occur just below the float siphonophore colony (Phy- 
1 ■ 1 ,^ 1 .,1 sophorida). A, pneumato- 

and provide the colony with a means p^ore; b, c, swimming bells; 
of locomotion. These are lacking in -D. protective zooid; E, sporo- 

P ,, . -Ill • 1 sac; F, G, dactylozooids; H, 

one of the most widely known sipho- fg^jing polyp; I, nettling 
nophores, the Portuguese man-of-war cells. {After Claus). 
(Physalia), whose greatly enlarged 

pneumatophore acts as a sail. The remainder of the colony 
consists of feeding zooids H, dactylozooids as sensory organs F 
and G, sporosacs E, bracts or organs of defence D, and batteries 
of nettling cells /. The individuals are so completely interde- 
pendent and are so highly specialized for carrying on limited 
functions for the colony as a whole that the colony is frequently 


likened unto a single individual with organ systems resembling 
those found in higher Metazoa. 

Dimorphism. — A condition of two unlike forms in the same 
generation is not unusual in the Hydrozoa. This condition is 
most commonly encountered as the specialization of vegetative 
and reproductive zooids in the same colony as described for 
Obelia. Dimorphism of a less common type occurs in the stag- 
horn coral Millcpora, wherein gasterozooids provided with 
mouth and tentacles and dactylozooids lacking the mouth are 
associated in the same colony. Neither of these represents the 
gonophore generation, for rudimentary jellyfishes of both sexes 
are produced, usually in chambers within the coral-like skeleton 
of the colony. 

Histological Differentiation. — As was stated in the introduc- 
tory discussion of the characters of the phylum, the medusa 
represents a higher type of differentiation than is found in the 
hydroid polyp. Scattered nervous elements and sensory cells 
characteristic of the polyp are replaced by a centralized nerve 
ring with which statocysts and light-percipient organs are asso- 
ciated in the hydromedusa except in the Anthomedusae, which 
are without statocysts. Hertwig has called attention to the 
fact that no less than eight distinct lines of differentiation occur 
in the ectoderm cells of the hydromedusa. There are myoblasts, 
nerve cells, indifferent cells, sensory cells, cnidoblasts, gland 
cells, pigment cells, and germ cells. Since a number of these 
are represented by several distinct types of histological differen- 
tiation, as, for example, the several distinct types of sensory 
and gland cells, these simple diploblastic animals become more 
highly organized than is ordinarily understood. 

Body Covering and Skeleton. — An investing membrane 
surrounds some hydroid forms (Obelia) but is wanting in others 
(Hydra, Tubularia). This investing membrane may be either a 
delicate perisarc covering all or only part of the zooid or, in 
some instances (Millepora) part of a massive calcareous skeleton. 
The expansion of perisarc around a vegetative zooid is termed a 
hydrotheca, while that around a reproductive zooid is termed a 

Class Sc3rphozoa 

The Scyphozoa are coelenterates having an alternation of 
generations of which the medusoid is the more conspicuous. 


Medusae of the Scyphozoa are often less that an inch in diameter, 
though one species of Cyanea on the Atlantic coast is said to 
reach a diameter of 7 feet across the bell and to have tentacles 
120 feet long. The scyphopolyp or scyphistoma when present is 
distinguishable from the hydropolyp through the presence of 
four longitudinal folds of the entoderm called the taeniolae. In 
the development of the medusae, these form the gastral tentacles 
or phacellae. When the germ cells leave the gonade they are 
liberated into the gastrovascular cavity, not directly to the 
exterior as in the Hydrozoa. The scyphomedusa lacks a velum, 
has a more or less notched margin, and bears entodermal gonads. 
While Scyphozoa are usually dioecious, some genera are herma- 
phoroditic. Form and structure of the medusa are more readily 
understood when the development has been outlined. 

Fig. 54. — Larval stages in the development of Aurelia. A, scyphistoma; B, 
strobila; C, ephyra. (Orig.). 

Life History. — The medusae of Aurelia produce germ cells, 
which, upon fertilization, undergo cleavage and develop into a 
ciliated larva called a planula. After a brief free existence, the 
planula becomes attached to some object and transforms into an 
individual superficially like a Hydra, to which the names scyphi- 
stoma (Fig. 54 A) or scyphopolyp have been apphed. By a 
series of transverse constrictions, the scyphistoma becomes 
separated into a pile of saucerlike structures (Fig. 54 B) and in 
this stage is called a strobila. As the constrictions become 
deeper, a series of ephyrae become detached from the strobila. 
At the time of their liberation, each ephyra (Fig. 54 C) has eight 
prominent armlike projections arranged radially about the mar- 
gin of the body. Growth of the medusa from this ephyra 


involves a rather conspicuous metamorphosis. The eight arms 
mentioned above mark off eight radii of the mature jellyfish. 
The four radii passing through the angles of the mouth are known 
as the perradii, the other four as the interradii. Axes falling 
between the bases of the eight arms of the ephyra are designated 
as the adradii. By more rapid growth of the adradial zones the 
notches of these regions become filled up, thus giving the young 
jellyfish a slightly crenated circular outline. The marginal 
tentacles make their appearance in the adradial regions. 

Though alternation of generations described for Aurelia is the 
customary type of development among the Scyphozoa, in some 
genera a free-swimming medusa develops directly from the ferti- 
lized egg. True asexual reproduction does not occur in the 
medusa of Scyphozoa. 

Body Form. — There is great diversity in form of body among 
the Scyphozoa. Aurelia has a flattened, saucer-shaped bell 
while some of the genera from the tropics have almost cubical 
bodies (Cubomedusae). The cubical appearance is heightened 
by the presence of only four tentacles on the margin of the bell. 
Some of the relatives of the Aurelia lack marginal tentacles 
altogether and in the members of the suborder Rhizostomae a 
series of folds around the manubrium bear eight small mouths in 
addition to the one at the end of the manubrium. Members of 
the genera Haliclystus and Lucernaria have the aboral surface 
drawn out as a peduncle by which the animal becomes attached. 
In these same genera, marginal tentacles are lacking but the bell 
is drawn out into eight arms each of which is provided with a 
cluster of diminutive tentacles. 

Class Anthozoa 

All organisms belonging to this class conform to a polyp form 
of organization. The anthozoan polyp differs fundamentally 
from the characteristic hydrozoan polyp in the presence of an 
ectodermal esophagus and of longitudinal partitions called 
septa or mesenteries (Fig. 55) partially dividing the gastrovascular 
cavity. Well-developed bands of muscles are found in connec- 
tion with the septa. The mesoglea contains numerous cells, 
thus having the appearance of a simple connective tissue and 
supplying firmness and a fleshy consistency to the body in many 
forms. Both solitary and colonial forms occur in this exclusively 
marine group of almost universally sessile organisms. Examples 



of the solitary forms are the sea anemones and of the colonial 
forms are the corals, gorgonians, sea pens, and sea pansies. 

Fig. 55. — Section of a young sea anemone, ss, sagittal plane; tt, transverse 
axis; I, II, III, septa of first, second and third orders; ek, ectoderm; en, ento- 
derm; /, mesenterial filament; m, muscle; r, directive septa. {After Boveri, from 
Hertwig's Manual by Kingsley, courtesy of Henry Holt and Co.). 

Fig. 56. — Transverse section of Zoantharian. AB, plane of symmetry; x, 
siphonoglyphe. {From Hertwig's Manital by Kingsley, courtesy of Henry 
Holt and Co.). 

Anthozoa occur in greatest abundance in tropical seas where their 
skeletal remains build massive coral reefs. 


Digestive System. — The mouth is usually oval or slit-liko and 
leads through an inturned tube of ectoderm, called the esophagus, 
into the gastrovascular cavity. Like the mouth, this esophagus 
is usually compressed so that a sagittal axis is recognizable 
marking off a biradial symmetry. In some instances, one or two 
grooves run the length of the esophagus at the poles of the sagittal 
axis. These grooves (x, Figs. 56, 57) are termed the siphono- 
glyphes. The esophagus extends only part way from the oral 
toward the pedal surface. It is held in position by the primary 
septa (Fig. 55, /), folds of entoderm and mesoglea which extend 
from the body wall inward to meet the esophagus and thus at the 
oral extremity completely divide the gastrovascular cavity into 
a number of pockets or chambers. 

Number and Arrangement of Septa. — Since each chamber of 
the gastrovascular cavity continues beyond the body proper into 
a tentacle, there is usually a direct relation between the number 
of chambers and the number of tentacles. In addition to the 
primary septa, many others reach only part way toward the 
esophagus. These are of varying lengths and on the basis of 
their length and order of development are termed secondary, 
tertiary, and so forth (Fig. 55, II, III). The number of primary 
mesenteries is of considerable importance in distinguishing the 
various orders of the Anthozoa. In the Alcyonaria (Octocoralla), 
there are eight septa. In practically all representatives of this 
order, the muscle ridges of all septa are directed toward the same 
pole of the sagittal axis. Edwardsia is an exception to this rule 
and marks a step in transition from the Alcyonarian to the 
Zoantharian (Hexacorallan) type, for in Edwardsia the position 
of the muscles on one pair of septa at one pole of the sagittal 
axis is reversed. The Zoantharia usually have one pair of 
primary septa at each pole of the sagittal axis and two lateral 
pairs on each side of the body. The two pairs lying in the 
main axis (Fig. 55 ?•) are called the directives. The muscle 
ridges on these face outward, while in all the remaining pairs 
the ridges of one pair face each other. In development, the 
primary septa are the first to appear. Between these, come the 
secondary and later the tertiary septa. With the increase in 
number of septa, there is usually a corresponding increase in 
number of tentacles, but this correlation is not absolute. 

Other Organs. — In addition to muscles, the gonads, mesen- 
terial filaments, and acontia are important structures associated 


with the septa. The germ cells, which have their origin in the 
ectoderm, come to lie in the mesoglea near the free margin of the 
septa. The ruffled free margin of each septum is edged with a 
thickened mass of epithelial cells called the mesenterial filament, 
thought to be of use in holding and compressing food particles, 
thereby aiding in digestion. Near the pedal disc, these mesen- 
terial filaments are frequently modified to form long threadlike 
organs, the acontia, which are provided with numerous nettling 
cells. Through the mouth or through minute pores in the body 
wall the acontia are thrust to the outside of the body where they 
serve as efficient defensive organs. 

Reproduction. — Most Anthozoa, with the exception of the sea 
anemones, possess some sort of skeletal structures, either as 
solid deposits formed by the zooids or as spicules in the colony 
wall. These structures are of such diverse natures in the different 
groups that they cannot be described here. 

Sexual development involves the formation of a planula 
through cleavage of the fertilized egg. The planula, after a 
brief free-swimming life, settles down and undergoes a transfor- 
mation to the polyp form. Budding is of common occur- 
rence and gives rise either to separate individuals or to colony 


Sea anemones, true stony corals, and black corals are examples 
of the Zoantharia which may be recognized from other Anthozoa 
by the presence of numerous simple or unbranched hollow 
tentacles. The corals are colonial, while the anemones occur as 
solitary polyps. The esophagus is provided with two siphono- 
glyphes and the gastrovascular cavity bears numerous paired 
mesenteries, typically occurring in multiples of six. Except for 
the directives (mentioned above) the septa are arranged in pairs 
with the muscle bands of each pair facing one another. Edward- 
sia is an exception with its eight mesenteries and sixteen or more 
tentacles. Various other modifications of the rule occur in 
other representatives of this subclass. 

There is great diversity in extent of development of skeletal 
material in this subclass. Skeletal structures are lacking in the 
sea anemones, the stony corals develop massive calcareous 
skeletons, while the black corals have a branched, hollow axial 
skeleton of horny material lacking lime spicules. 



Subclass Alcyonauia 

Eight feathered tentacles and eight single mesenteries (Fig. 57) 
characterize members of this subclass which are also frequently 
referred to under the name Octocoralla. Colonial forms, 
frequently with polymorphism of the indi- 
viduals, are common. Budding to form a 
colony is usually stoloniferous. 

As in the preceding subclass, here also we 
find diversity in skeletal structure. Horny 
or solid calcereous rods give form and 
support to the colonies of sea fans (Gorgo- 
nians) and precious coral, while only clus- 
tered spicules occur within the tissues of the 
sea pens and sea pansies. 

Fig. 57. — Trans- 
verse section of an 
Alcyonarian. {From 
Hertwig's Alaniial by 
Kingsley, courtesy of 
Henry Holt and Co.). 


The Ctenophora have very commonly 
been considered as a class of the Coelenterata. The absence of 
marginal tentacles, lack of netting cells, centralization of the 
nervous mechanism, and transformation of the mesoglea into a 
mesoderm mark the chief arguments in favor of considering the 

A B 

Fig. 58. — A ctenophore, Pleurohrachia rhododaclyla Agassiz. .-1, lateral view 
B, aboral view. {After L. Agassiz). 

ctenophores as an independent phylum. The most striking 
external feature is the presence on the surface of the body of 
eight meridional bands of swimming combs or plates (Fig. 58) 
each of which is composed of a linear series of short rows of cilia. 


Two tentacles arising in lateral ectodermal depressions or 
pockets are characteristic of most representatives of this phylum. 
At the aboral pole occurs a slight depression within which is 
located a single otocyst. In the transverse plane this depression 
is continued as two narrow ciliated areas called the polar plates. 

Body form is not constant throughout the group. Many indi- 
viduals are ovoid, others have a somewhat shortened dorsoventral 
axis, and still others are drawn out into a narrow band. 

The nettling cells characteristic of coelenterates are wanting in 
the ctenophores. In their place structures called adhesive cells 
occur upon the tentacles. 

Digestive System. — The mouth leads into a tube called the 
stomach, but since it is ectodermal in origin it is more properly 
a stomodaeum. This stomach opens into an entodermal sac, 
the funnel. The stomach and funnel are both flattened with the 
broad axis of one at right angles with that of the other. From 
the funnel are given off laterally two perradial vessels each of 
which divides dichotomously to form the four interradial vessels, 
(Fig. 58 B). By another dichotomous division these form 
the eight adradial vessels each of which communicates with 
one of the meridional vessels directly underlying the rows of 
combs. Each perradial vessel gives off a branch called the 
paragastric canal, which runs parallel to the stomach and ends 
blindly. At its aboral extremity the funnel gives off a funnel 
vessel which, after forming two or four branches, proceeds to 
the aboral pole and there empties through two or more openings 
called the excretory pores. 

Development. — Ctenophores undergo direct development from 
the fertilized egg. There is no alternation of generations. One 
of the most interesting facts regarding the development of the 
egg is that the materials for the formation of the adult structures 
seem to be definitely arranged within the egg even before the 
first cleavage. Evidence of this has been gained from the fact 
that injury to or removal of a bit of the cytoplasm leads to the 
formation of a ctenophore lacking in some organ or structure 
such as the reduction in number of combs or loss of a part of the 
digestive system. 

Classes and Orders. — On the presence of or lack of tentacles, 
two classes are recognizable: the Tentaculata and the Nuda. Of 
these, the former includes three orders and the latter a single 
order, the Beroida. Members of the order Cydippida (Pleuro- 


brachia, Hormiphora, etc.) have ovoid or pear-shaped bodies 
bearing two tentacles arising from tentacular sacs or sheaths and 
into which they may be retracted. The Lobata includes those 
which have two large oral lobes and numerous minute lateral 
tentacles. In the young, two large tentacles are present but in 
later stages only their bases are present and these are without 
a sheath. Venus' Girdle (Cestus) is an example of the order 
Cestida members of which have the body much compressed in 
the vertical plane. Representatives of the order Beroida lack 
tentacles and the wide mouth and gullet occupy most of the 

Outline of Classification 

A. Phylum Coelenterata. — Diploblastic; radially symmetrical; bearing 
tentacles and nettling cells; a single gastrovascular cavity with one opening; 
no anus; aquatic. 

I. Class Hydrozoa. — Alternation of generations typical; medusa with 
velum; pol>p without entodermal folds. 

1. Order Leptolinae. — Hydroid fixed. 

a. Suborder Anthomedusae. — Polyp without theca; gonads on 
manubrium; Hydra, Clava, Cordylophora, Coryne, Tu bular ia, 
Bougairwillia, Hydradinia. 

b. Suborder Leptomedusae. — Polyp with theca; gonads on 
radial canals. Obelia, Go iiionemu s. Campanidaria, Sertularia, 
Aequorea, Plumularia. 

2. Order Trachylinae. — Medusa develops directly from egg. 

a. Suborder Trachymedusae. — Tentacles on bell margin; 
gonads on radial canals. Petasus, Trachijnema, Liriope, Cam- 
panella, Aglantha. 

b. Suborder Narcomedusae. — Tentacles arise from aboral 
surface of bell some distance from margin; gonads on manubrium. 
Cunina, Aeginopsis, Cunocantha. 

3. Order Hydrocorallina. — Massive calcareous exoskeleton. Mil- 
lepora, StTjIasUr. 

4. Order Siphonophora. — Pelagic; colonial with marked poly- 
morphism. Phmadia, Vclella. 

II. Class Scyphozoa. — Usually alternation of generations; medusa 
without velum; gastric tentacles; polyp with four entodermal folds. 

1. Order Stauromedusae. — Conical or vase-shaped medusa; no 
marginal sense bodies. Tessera, Haliclystus, Lucernaria. 

2. Order Peromedusae. — Cup-shaped medusa; four interradial 
sense bodies. Pericolpa, Pcriphylla. 

3. Order Cubomedusae. — Cuboidal; four perradial sense bodies. 

4. Order Discomedusae. — Saucer-shaped medusa; eight or more 


a. Suborder Semostomae. — Four large perradial arms surround 
mouth. Aur^a, Cyanra. 

b. Suborder Rhizostomae. — No marginal tentacles; many- 
mouths. Stomolophus. 

III. Class Anthozoa. — Polyp only; esophagus; mesenteries. 

A. Subclass Zoantharia. — Numerous mesenteries and tentacles. 

1. Order Actinaria. — Solitary" ; no skeleton. Metridium, Sagartia, 
Crihjjua, Ccrianthus, Edwardsia. 

2. Order Madreporaria. — Colonial; calcareous skeleton. Astrgji,' 
gia, Orbicella, Meandrina, Coeloria, Favia, Madrepom, Pontes. 

3. Order Antipatharia. — Colonial; treelike horny skeleton. Anti- 
pathcs, Cirripathes. 

B. Subclass Alcyonaria. — Eight mesenteries and branched tentacles. 

1. Order Alcyonacea. — Skeleton of spicules or small, irregular 
bodies; axial rod lacking. Tubipora, Alcyonium. 

2. Order Gorgonacea. — Treelike; calcareous or horny skeleton. 
Gorgofda, CoraUium, Euplexaura. 

3. Order Pennatulacea. — Colony with one end usually buried in 
sea-l3ottom. RcjiiUa, Pennatida, Plilosarcus. 

B. Phylum Ctenophora. — Eight meridional bands of ciliated plates; adhe- 
sive cells; centralized nervous system; pelagic. 

I. Class Tentaculata. — Typically with two aboral tentacles. 

1. Order Cydippida. — Body ovoid or pear-shaped; two retractile 
tentacles arising from aboral tentacular sacs. Pleurobrachia, 

2. Order Lobata. — Two large oral lobes; no tentacular sacs; 
numerous lateral tentacles. Bolinopsis, Deiopea. 

3. Order Cestida. — Pdbbonlike; two tentacular sacs and numerous 
lateral tentacles. Cestus. 

n. Class Nuda. — No tentacles; no oral lobes. 

1. Order Beroida. — Laterally compressed; gullet occupies most of 
interior of body. Beroe. 


(See general references at close of Chapter I) 
Agassiz, a. 1865. "North American Acalephae." Mus. Co?np. Zool., 

Harvard, Illustr. Cat. 2. 
Agassiz, L. 1850. "Contributions to the Natural History of the Acalephae 

of North America." Mem. Amer. Acad. Arts Sci., Boston, 4: 221-374. 
. 1860-1862. "Contributions to the Natural History of the United 

States." Boston. 
Chun, K. 1880. "Die Ctenophoren des Golfes von Neapel und der 

angrenzenden Meeres-Abschnitte." Vol. I, Fauna und Flora des 

Golfes von Neapel. 
Haeckel, E. 1879-1880. "System der Medu.sen. I, Cra.spedoten; II, 

Acraspeda." Jena. 
Mayer, A .G. 1910. "Medusae of the World." Carnegie Inst., Wash- 
ington, Publ. 109. 


Mayer, A. G. 1912. "Ctonophores of the Atlantic Coast of North 

America." Carnegie Inst., Washington, Publ. 162. 
McMuRRicH, J. P. 1891. "Contributions on the Morphology of the 

Actinozoa. Ill, The Phylogeny of the Actinozoa." Jour. Morph., 5: 

Nutting, C. C. 1901. "The Hydroids of the Woods Hole Region." Bull. 

U. S. Fish Comm. for 1899: 325-386. 


The Plat hel mint hes comprise a phylum of wormlike animals 
with bodies usually flattened, devoid of body cavity, and lacking 
true segmentation. The bulk of the body is composed of a mass 
of connective tissue termed parenchyma within which the various 
organs are embedded. Externally, this is covered by either a 
ciliated epithelium or a non-cellular cuticula. A body muscula- 
ture is present, the fibers of which are usually circular, longitu- 
dinal, and diagonal in arrangement. Frequently, in addition, 
fibers are also found penetrating the body from dorsal to ventral 
surfaces. The nervous system usually consists of two or more 
longitudinal strands which, near one end of the body, bear a pair 
of ganglia called the brain. In many instances, the longitudinal 
nerve trunks are connected by cross-commissures which give 
the system a ladderlike appearance. 

Typically, the digestive system is composed of an ectodermal 
pharynx and a mesenteron, for the proctodaeum is lacking. 
Exceptions to this are found in the cestodes, which lack all evi- 
dence of a digestive system, and in the nemertines, which have a 
complete system terminating posteriorly in an anus. A few 
species of trematodes possessing an anus have also been described. 
Excretion is by means of a system of tubules, the protonephridia, 
which ramify throughout the body and end in minute structures 
called the flame cells. 

Most of the flatworms are hermaphroditic. The reproductive 
organs consist not only of the primary organs or gonads but to 
them are added many accessory organs and glands not found in 
lower organisms. There is great diversity in methods of repro- 
duction in this phylum. The usual bisexual method is fre- 
quently supplemented by fission, budding, and parthenogenetic 

Form, habits, and details of structure differ so profoundly in 
the members of the various classes that few features beyond those 
already mentioned are held in common by all members of this 



Interrelationships of the Classes. — The Turbellaria (Fig. 60) 
represent the most generahzed group of the Plathelminthes. 
Modifications in the classes Trematoda and Cestoda represent, 
chiefly, adaptation to the parasitic habit. Extreme development 
of organs of generation and fixation (Fig. 63 A) and reduction of 
locomotor organs, sensory apparatus, and other structures charac- 
teristic of free existence characterize these two parasitic groups. 

The nemertines are very commonly considered as the highest 
class of the flatworms. It is coming to be very generally recog- 
nized, however, that they are more closely allied to the higher 
worm groups. 

Development. — Some of the flatworms, especially the fresh- 
water Turbellaria and the monogenetic termatodes, undergo 
direct development from the fertilized egg. More commonly^ 
metamorphosis is involved. There are numerous different types 
of larvae characteristic of the various flatworm groups. Miiller's 
larva with eight lobes, the margins of which are outlined by a 
continuous band of cilia, occurs in many polyclads. This seems 
to represent a primitive type of larva from which many others 
have been differentiated. 

The pilidium of nemertines characteristically undergoes a 
complicated metamorphosis during which but a portion of the 
larva is utilized in the transformation to the adult form while the 
remainder is cast off. 

Development in the trematodes frequently involves an alter- 
nation of generations. The generation which lives in snails, 
and is usually considered as larval (Fig. 65 A-C), undergoes a 
parthenogenetic cycle of reproduction, while the adults of the 
same species reproduce the first larval stage (Fig. 65 A) by means 
of fertilized eggs. 

In addition to the sexual forms of reproduction leading to 
formation of larvae mentioned above, many of the flatworms 
reproduce asexually. Some species of Turbellaria undergo 
transverse fission (Fig. 62). In some of the fresh-water planar- 
ians (Fig. 61) this seems to be the chief if not the only means of 
reproduction. This method of naturally reproducing an entire 
individual from a portion of another rests upon the development 
of power of regeneration. 

Relationships to Other Phyla. — Through the Ctenophora and 
the Turbellaria, the two phyla Coelenterata and Plathelminthes 
seem to have a fairly close phylogenetic relationship. Muscula- 



ture, nervous system, and reproductive organs which have their 
origin as purely epitheUal structures in the lower coelenterates 
become associated with the mesoglea in the higher coelenterates 
and transform this layer into a true mesoderm in the Ctenophora. 
This trend of development and specialization of the mesoderm 
is carried still further in the highly organized parenchymatous 
body of the Plathclminthes. 

Fig. 59. — ^4. Coeloplana adult in dorsal view. B. Larva of Coeloplana directly- 
after hatching. {Redrawn from Komai) . 

There are some organisms which show striking combinations 
of flatworm and ctenophore characteristics. Coeloplana and 
Ctenoplana are two such genera to which unparalleled phylo- 
genetic significance has been attached. Coeloplana in its general 
form (Fig. 59 A) resembles a flatworm, but on its dorsal surface 
it bears a sense organ like the one found in similar position on 
the ctenophores. The dorsal tentacles and the tentacular sacs 


likewise resemble those of a ctenophore. Furthermore, in its 
life history Coeloplana passes through larval stages which bear 
ciliated combs (Fig. 59 B) of a form which have been thought 
to be distinctive of the ctenophores. Ctenoplana closely resem- 
bles Coeloplana except that it carries the rows of combs through 

A B c 

Fig. 60. — General organization in the three classes of Turbellaria. A, a 
Rhabdocoel; B, a Polyclad; C, a Triclad. The nervous system is shown in 
black; digestive system in cell outlines and nuclei; .4, testes closely stippled, 
yolk gland coarsely stippled, ovary at posterior extremity on right; B and C, 
male reproductive system stippled on left side of body, female organs on right. 
{From von Graff). 

to its adult condition. Thus Coeloplana resembles a flatworm 
and Ctenoplana a ctenophore, while each combines confusing 
characteristics of both groups. 

Class Turbellaria 

The Turbellaria range in size from minute, microscopic forms 
to some which attain several inches in length. The polyclads 
and triclads are usually distinctly flattened and in outline 
range from disc-shaped to lanceolate and long ribbonlike 



forms. Among the rhabdocoels is found even greater diversity 
of form. Some are spindle-shaped, while others are distinctly 
flattened. The ectoderm is covered with ciHa (Fig. 62) which by 
their movement produce a smooth gliding locomotion. Currents 
produced by these cilia are responsible for the name of the class. 
Turbellaria are mostly free-living, aquatic organisms, though 
some have acquired the parasitic habit and others have become 
adapted to living in or on moist soil. 

Fig. 61. — Axial gradient in Planaria. -4, Planaria showing location of incip- 
ient heads; B, curve showing rise in metabolic gradient at levels marked in .4. 
{Redrawn from. Child). 

Axial Gradient. — When two new individuals are formed from 
parts resulting from either natural or accidental separation of the 
body of a planarian in a transverse plane, there is always a 
tendency for the front piece to produce a tail and the hind one a 
head. In planarians which normally divide by fission, the 
posterior end of the body represents additional potential individu- 
als even before there is any evidence of separation. Professor 
C. M. Child has given experimental proof of this. In his physio- 


logical studies ho found that the head end of the worm has the 
highest rate of metabolism. This rate tends to decrease in the 
region behind the head. The term axial gradient has been 
applied to this physiological differentiation of regions along the 
axis of the body. When a certain level is reached (Fig. 61) the 
rate of metabolism shows a sudden increase. This point on 
the axis of the body coincides with the region where fission would 
first take place, and the sudden increase in rate of metabolism 
marks the location of a new incipient head. Behind this point 
Dr. Child found in long individuals a series of regions showing 
alternation between regions of increased and diminishing rates 
of metabolism. Each rise in metabolic rate marks the location 
of a potential new head. 

The Orders. — A pharynx and blind intestine comprise the 
digestive tract of the turbellarians. The muscular pharynx is 
frequently enclosed in a pocket within the body and in feeding 
is thrust out like a proboscis. 

Upon the nature of the intestine the three orders are distin- 
guishable. Among the Polycladidea (Fig. 60 B), the mesenteron 
consists of a central space from which numerous branches pass 
into the parenchyma. These branches become greatly sub- 
divided and frequently anastomose. In the Tricladidea, three 
main branches (Fig. 60 C) lead off from the pharynx, one directed 
anteriorly and two posteriorly. Each of these has numerous 
lateral diverticula which nearly reach the margins of the body. 
In the Rhabdocoelida, the mesenteron (Fig. 60 A) forms a simple 
sac-shaped or rodlike intestine. The so-called Acoela lack a 
cavity in the mesenteron which has been described as a digestive 

Nervous System. — A simple, linear type of central nervous 
system is characteristic. In the Polycladidea, there is a consid- 
erable plexus of nerve branches with a brain near the anterior 
extremity and a few main trunks. The triclad central nervous 
system is essentially a pair of lateral nerve trunks which pass 
posteriorly from the brain with irregular transverse commissures. 
In some forms (Gunda), these connecting branches are so regular 
that they constitute a distinctly ladder type of system. The 
rhabdocoel nervous system is distinctly similar to that of the 
triclad type. Highly specialized sensory organs are not common 
in the Turbellaria. Eyes of a simple type are usually found 
above the brain. In the polyclads, these occur in large numbers 



and may also be developed along the margins of the body. In 
the triclads, a single pair of eyes is usually present, as is also 
the case with the rhabdocoels, though these last may have two 
pairs or even a single eye. The sense of touch is very highly 
developed in the Turbcllaria and tentacles are 
frequently found, but the entire body is also 
highly sensitive, rendered so especially by the 
presence of sensory hairs. 

Reproduction. — Except for some rhabdocoels, 
the Turbellaria are hermaphroditic. The repro- 
ductive organs differ considerably in the different 
orders. In most of the fresh-water forms, the 
eggs undergo a simple, direct development but 
some of the polyclads have a more complicated 
developmental cycle. The larval polyclad is 
called a Miiller's larva. Asexual reproduction 
by transverse fission occurs in some rhabdocoels, 
frequently in the event of rapid fission, giving 
rise to a complex chain of individuals. Such a 
condition is shown in Fig. 62 in which the order 
of formation of the successive individuals is 
indicated. Similar chains are potentially present 
in the planarians, which undergo fission as shown 
in Fig. 61 and described in the section on axial 

Class Trematoda 

The trematodes or flukes are exclusively para- 

. . ,. . . , , ., Fig. 62.— a 

sitic, living either as ectoparasites upon or as r h a b d o c o e i 
endoparasites within the bodies of various ani- (Microstomum) 

, „. .... . . 1 J , in the process of 

mais. bmce parasitism is an acquired and not a fission into six- 
primitive mode of life, the bodies of all trematodes teen zooids. 

1 1 • f 1 • X r Roman numer- 

are more or less modmcd as an accompaniment oi ^is indicate the 
the parasitic habit. Simplicity of structure in this order of the 

, 1 , , 1 . ■ -,■ ■ fission planes. 

class denotes degeneracy and not primitive sim- (After von Graff). 
plicity. While there are no free-living forms, 
most trematodes in their fundamental structure present clear 
evidences of close relationship with the Turbellaria. They vary 
from less than one millimeter to several centimeters in length. 
A cuticula, frequently supplied with spines, covers the body 
surface of all adult trematodes, while some larval forms (Fig. 



65 A) possess a ciliated covering. The epithelium characteristic 
of the body covering of the Turbellaria has become profoundly 
modified in this group and the cells are scattered through the 
underlying parenchyma. 

Most flukes are hermaphroditic and the conspicuous repro- 
ductive organs are of prime importance in the classification of 
the group (Fig. 66 A). A few, the Schistosomes (human blood 
flukes), for example, are unisexual. Suckers, hooks, and spines 

h/foufh ' 


Genifal pore 






Fig. 63 A. — GyrodactyJus clcgans, a 
monogenetic trematode. {After Luhe) . 

Fig. 63 B. — Opisthorchis fcl incus, a 
digenetic trematode. {After Stiles and 
Hassall ) . 

are developed in varying combinations for securing attachment to 
the host. The number and arrangement of these structures are 
of particular significance in the grouping of the trematodes. Two 
subclasses, Monogenea and Digenea, are recognized. 

Subclass Monogenea 

The monogenetic trematodes are usually external body para- 
sites but in some instances they have migrated inward to loca- 
tions such as the mouth cavity, the respiratory organs, the cloaca, 
or the urinary bladder which are in direct communication with 
the body surface. The chief organ of attachment (Fig. 63 A) is 


usually located at the posterior extremity and consists of varied 
forms of sucking discs and combinations of hooks and spines.' 
These hermaphroditic individuals produce eggs which undergo 
direct development without complicated larval changes and 
without alternation of hosts. The young parasites may immedi- 
ately attach themselves to the body of the host which sheltered 
the parent fluke. 

Two orders are recognized on the basis of structure of the 
posterior organ of fixation. In members of the order Mono- 
pisthocotylea (Gyrodactylus (Fig. 63 A), Dactylogyrus, Ancyro- 
cephalus, Nitzschia), this posterior organ is a single structure 
provided with extremely varied combinations of hook and spines 
for attachment to the skin or gills of fishes. In the Polyopistho- 
cotylea (Alicrocotyle, Polystoma, Diplobothrium, etc.), each 
individual bears two or more posterior suckers supplemented by 
hooks or spines. Skin and gills of fishes and body surface, 
urinary bladder, and pharynx of amphibians and reptiles are the 
chief seats of infestation by representatives of this second order. 

Subclass DiGENEA 

Almost all of the Digenea are internal parasites which require 
at least two hosts for completion of the life cycle. Instead of the 
direct development characteristic of the monogenetic trematodes, 
the digenetic forms always pass one generation or more in a 
mollusc before the stage capable of living inside the final host is 
attained. There is thus an alternation of generations accom- 
panying an alternation of hosts. ]\Ian and his domestic animals 
act as hosts for numerous species of the digenetic trematodes, 
hence they hold much of interest for the student of medical 

Digenetic trematodes are provided with one or two suckers 
the anterior of which usually surrounds the mouth opening. 
Conspicuous chitinous hooks, so common in the Monogenea, are 
lacking in the Digenea, though relatively smaller spines are of 
common occurrence. The ventral sucker, or acetabulum, is 
usually near the middle of the body, though in some instances 
it is entirely wanting (monostomes). From the mouth the 
digestive system continues backward as a single tube which 
divides to form two lateral branches, the ceca, for digestion and 
distribution of the food. Some forms seem to depend entirely 
upon absorbing their nutriment from the host directly through 



the body surface. The excretory system is highly characteristic. 
This consists of a series of finely branching tubes following a 
definite pattern of arrangement distinctive for each family of 

flukes. At the posterior 

end of the body the tubes 
empty into a median blad- 
der. Each collecting tubule 
terminates distally in a 
flame cell the most charac- 
teristic feature of which is a 
tiny tuft of cilia extending 
into the lumen of the tube. 
In the adult worm (Fig. 
66 A), the male reproduc- 
tive organs consist normally 
of two testes which com- 
municate with the genital 
pore through the vas 
deferens and a cirrus. The 
female organs comprise an 
ovary of variable form from 
which an oviduct leads 
through a modified region 
called the ootype into the 
uterus. Between the ovary 
and the ootype the oviduct 
may receive two canals, one 
from the vitelline receptacle 
and the other from the 
Laurer's canal and the 
receptaculum seminis. As 
ova pass from the ovary 
down the oviduct, sperma- 
tozoa from the receptacu- 
lum seminis and yolk and 
shell material from the vitel- 
line receptacle pass with 
them into the ootype. Here 
each fertilized ovum and a number of yolk cells become sur- 
rounded by a shell to form an egg. The ootype is a modified 
portion of the oviduct in the region of the so-called shell gland 

Fig. 64. — Photograph of a sheep-liver 
fluke stained and cleared for microscopic 
study. (Orig.). 



or Mehlis' gland. The vitelline material which supphes both 
the yolk cells and the substance of the shell is produced in 
glandular bodies, the vitellaria, usually distributed along the 
margins of the body (Fig. 64), and communicating with a 
vitelline receptacle by means of smaller tubules which combine 
to form two transverse ducts. 

Life Cycle. — As an illustration of the developmental cycle of 
the Digenea, that of the sheep-liver fluke, Fasciola hepatica (Fig. 
64), has been chosen. The adult flukes live in the bile ducts of 
the sheep's liver where, by their 
presence and by their munching 
off portions of the lining of the 
ducts, they produce a diseased con- 
dition known as liver rot. As eggs 
are discharged from the parent fluke, 
they pass down the bile duct into 
the intestine and out of the body of 
the host along with the feces. The 
embryo within the eggshell develops 
into a small ciliated larva known as 
a miracidium (Fig. 65 A) which is 
released from the shell only when 
surrounding conditions are proper. 
In swampy ground or following a 
heavy dew or rain, this miracidium 
is enabled to swim in search of a 
suitable snail before the trematode 
can go further in its development. 
Upon coming in contact with a snail 
of suitable species, the miracidium 

makes use of a small boring spine and cercariae; £>, free cercaria; £;, 

on its anterior extremity to pene- y^/^^^,^ '^''^'^^- ^-^^"'^^^ "^'''' 
trate the soft tissues of the snail and 

comes to lie in the liver of the snail. Here it undergoes a trans- 
formation, becoming a bag-shaped structure called a sporocyst 
(B). Parthenogenetic reproduction occurs within the sporocyst 
to form numerous individuals called rediae. Each redia (C) is 
a simply organized individual possessing a pharynx and tubular 
intestine in addition to a single birth pore through which young 
are discharged. Within each redia develops either a new genera- 
tion of rediae or individuals of another type termed cercariae. 

Fig. 65. — Stages in the develop- 
ment of the sheep-liver fluke 
(Faficiola hepatica). A, miraci- 
dium; B, sporocyst; with rediae 
developing internally; C, redia, 
with second generation of rediae 


A cercaria (D) is a minute fluke which in addition to the rudi- 
mentary organs of the adult contains also a strongly developed 
posterior tail for locomotion. The cercariae leave the body 
of the snail and for a time are free-swimming creatures. Finally 
the cercaria crawls onto vegetation where after losing its tail it 
becomes surrounded by a calcareous cyst wall (E). In this 
condition, it remains inactive until the plant bearing the cyst is 
taken into the stomach of a sheep or other suitable animal. 
Under the digestive action, the young fluke is liberated from the 
cyst and occurs free in the digestive tract. As it passes into 
the intestine, its chance of ever reaching maturity rests 
upon the discovery of the opening of the bile duct and migration 
through it into the liver. The successful individual becomes 
established in the bile passages where after a few weeks of growth 
it has reached adult size and begins to produce eggs, thus closing 
the cycle. 

In the foregoing instance the cercaria enters its vertebrate 
host in a purely passive manner. Similarly some of the most 
dangerous human flukes, especially in the Orient, are taken into 
the digestive tract in an encysted state. In contrast, the blood 
flukes belonging to the genus Schistosoma have active cercariae 
which gain entrance to the human body by penetrating the skin. 

In so far as is known, a mollusc is essential for the development 
of all digenetic trematodes. Frequently, other hosts are added, 
either as essential links in the life cycle or as faculative adapta- 
tions, until the complete developmental cycle involves a number 
of different species which act as host to a single parasite. 

Class Cestoda 

Like the trematodes, all cestodes are parasitic but through 
fundamental structures show marked relationships with the less 
highly specialized representatives of this phylum. The only 
sure criterion for the separation of Cestoda from other parasitic 
flatworms is their entire lack of a digestive system. While the 
typical cestode is made up of a chain of segments or proglottids 
and a scolex for attachment to the host, there are some, the 
monozoic cestodes or Cestodaria, in which the body is composed 
of but a single unit. Though these latter are usually not more 
than a few millimeters in length, the segmented forms frequently 
attain a length of several meters and may be divided into several 
thousand proglottids. 


Unsolved Problems. — The question of the orientation of the 
cestode body has never been decided with certainty. Many 
zoologists maintain that the scolex is the anterior extremity 
of the chain because here the chief nervous centers are found. 
Other investigators contend just as stohdly that since in develop- 
ment from the larval stage the scolex is at the posterior extremity 
of the larva this scolex must represent the morphological posterior 
extremity of the adult chain. Another question upon which 
there is just as radical division of opinion is that upon the deter- 
mination of what constitutes an individual. Is the entire chain 
an individual or a colony of individuals? Continuity of nervous 
and vascular systems throughout the chain with some modifica- 
tions at the extremities not found in the individual proglottids 
presents evidences of unity of the entire chain. On the other 
hand, since the reproductive organs are about the only structures 
remaining in the proglottids, complete duplication of these in 
each proglottid gives support to the argument that each proglot- 
tid is an individual, groups of which have remained united to 
form a colony as a result of incomplete separation following 
asexual reproduction to form the chain. 

Organization. — That part of the strobila which is located 
between the scolex and the proglottids is frequently not divided 
into segments but as a more or less sharply defined region is 
termed the neck. This is the budding zone where new pro- 
glottids are being formed. Thus in age the scolex is the oldest, 
then come the proglottids in order from the free extremity toward 
the neck. Few structures are evident in the neck and in the 
small proglottids most recently formed, for there is a gradual and 
progressive development of the organs (organogenesis) repre- 
senting all stages between the fully formed organs of the terminal 
proglottids and the merest traces of fundaments in the proglot- 
tids just behind the neck. The sexual organs of the terminal 
proglottid are the oldest and consequently mature first. Upon 
reaching full maturity, the gravid proglottids are frequently 
severed from the remainder of the strobila. 

In many genera the tapeworm continues to live and to produce 
more proglottids indefinitely just as long as the scolex and neck 
region remain attached to the wall of the digestive tube of the 

Peculiarities of Chain Formation. — In some forms (Ligula), 
there are no partitions between the proglottids but the organs 


are duplicated successively in an undivided body. A secondary 
strobilization is known to take place in some cestodes wherein 
the original proglottids undergo a secondary subdivision, each 
giving rise to a number of proglottids which may eventually 
separate as independent strobila. One proglottid in each new 
chain becomes modified as a pseudoscolex. 

Subclass Cestodaria 

The Cestodaria or monozoic cestodes rather closely resemble 
trematodes, but the lack of digestive organs necessitates their 
inclusion as a subclass along with the true cestodes. There is no 
sharp differentiation between scolex and proglottids in the Ces- 
todaria, and but one set of reproductive organs usually occurs 
in each individual. They occur in both marine and fresh-water 
hosts. Members of the genus Archigetes become mature in 
oligochaetes, Gyrocotyle in the spiral valve of elasmobranchs, 
and Caryophyllaeus, Glaridacris, and Amphilina in the body 
cavity or digestive tract of fishes. The organ of attachment is 
variously modified. 

Subclass Cestoda (s. sir.)^ 

The poly zoic cestodes include all the typical tapeworms which 
have a scolex followed by a succession of organs usually divided 
in chain fashion into a series of proglottids. These cestodes 
reach the adult state in the digestive tract of vertebrates, every 
class from fishes to mammals serving in the capacity of host. 
There is no restriction to a single type of larval host as in the 
digenetic flukes, for a wide variety of both invertebrates and 
vertebrates act as host for larval tapeworms. 

Reproductive Organs. — Each proglottid is furnished with a 
complete set of reproductive organs (Fig. 66 B), and in some 
instances two sets occur in each. The male organs consist of 
from one to more than a hundred testes (Fig. 67) embedded 
within the parenchyma and connected by vasa efferentia and vas 
deferens with the cirrus which communicates with the outside. 
The genital ducts of both sexes usually open in a common genital 
pore which is either on the lateral margin of the proglottid or 

^ Sensu stricto indicates that a word is used in a limited or restricted 
sense. In this instance the word Cestoda is used for the entire class, but 
in a restricted sense the same word is used to designate the true cestodes 
from the Cestodaria. 



on the ventral surface. When it is marginal there is no necessary 
regularity in its disposition, for in adjacent proglottids it may be 
either on the same or on opposite margins. In cestodes which 

. Cirrus. 



Ovar^ --'.' 

_ . .. Shell gland 

'~ ■Seminal rTecepfade - 

.'•^Vilellan'a ■ 

Laurers canal 

Fig. 66. — Diagrams showing the fundamental plan of organization of sexual 
organs in ^-1, trematodes; B, cestodes. (Grig.). 

have two sets of genital organs, the pores occur on both lateral 
margins of each proglottid. 

The arrangement of the female organs differs greatly in the 
different genera. The vagina opens through the common genital 

/ transverse 
excretory duct 

vitelline duct 

cirrus sac 
^genital atrium 


longitudinal ' uterus 

testicular follicles vas deferens 

circular muscles 


Fig. 67. — Stereogram showing arrangement of organs in a proglottid of a fish 
tapeworm (Thysanocephalum). (After Causey). 

pore and in addition in some instances the uterus has an inde- 
pendent orifice to the exterior. A typical arrangement (Fig. 
67) is as follows: The vagina, as it passes through the proglottid, 


bears a dilated portion for the storage of sperm cells called the 
receptaculum seminis or seminal receptacle. As the duct con- 
tinues beyond this receptaculum it receives two side branches, 
the oviduct which connects it with the ovary and the vitelline duct 
which communicates with the vitellaria or vitelline glands. The 
oviduct frequently bears a modification called the oocapt for 
forcing the eggs along their course. When eggs and spermatozoa 
meet, fertilization occurs. The fertilized eggs and the substance 
from the vitelline glands pass into a structure similar to the 
ootype of the trematodes in the region of the shell gland. Here 
shells are formed around the eggs which are then passed on into 
the uterus. In older proglottids, the uterus frequently develops 
egg-filled pouches which occupy practically the entire volume of 
the proglottid. Eggs are rarely discharged from the uterus, for 
more frequently the entire egg-filled proglottid is liberated and 
carried out of the body of the host. 

Other Organs. — The nervous and excretory systems extend the 
length of the strobila and parts in individual proglottids do not 
represent complete units. Ganglia located in the scolex send 
nerve trunks backward through the proglottids, usually as two 
lateral branches fairly close to the lateral margins of the body. 
Specialized sensory organs are wholly wanting. Parallel to the 
longitudinal nerve trunks run the main canals of the excretory 
system. These in turn receive smaller canals the branches of 
which ramify through the parenchyma and terminate in flame 

Reproduction. — Several different methods of development 
occur in the Cestoda. Typically, the egg undergoes a cleavage 
within the shell to form an embryo, bearing six minute hooks, 
called an onchosphere. The onchosphere may in some instances 
be provided with cilia for locomotion (as in Diphyllobothrium, 
the fish tapeworm of man), but more frequently it is borne within 
embryonic membranes as an immotile body. Upon introduction 
into a suitable host, the onchosphere develops into a larval form 
differing with the various groups of the cestodes. Among the 
lower cestodes, which live chiefly in fishes, this larva is a small 
solid body called a plerocercoid (Fig. 68 A). A larval stage 
called the cysticercus or bladderworm stage (C) is found among 
the higher cestodes. The name cysticercoid is applied to larval 
forms {B) which seem to be intermediate between the plerocercoid 
and the cysticercus. 


The taeniae which dwell in man have a cysticercus, while one 
of the commonest tapeworms of dogs (Dipylidium) and of rats 
(Hymenolepis) have a cysticercoid larva. Some of the worms 
having a cysticercus undergo asexual reproduction in the larval 
stage, developing several scolices within a single bladder (genus 
Coenurus) or at times budding off a series of daughter cysts each 
with one or more scolices (genus Echinococcus). 

Life History. — As an illustration of cestode development the 
life cycle of Taenia saginata, the beef tapeworm of man, will be 
outlined. The mature individual of Taenia saginata occurs in the 
intestine of man. As the older proglottids become gravid, they 
become detached from the end of the chain and pass out of the 

Fig. 68. — Schematic drawings of cestode larvae. A, plerocercoid; B, cysticer- 
coid; C, cysticercus. (Orig.). 

body along with the feces. These isolated proglottids have inde- 
pendent powers of movement and may crawl away from the feces 
onto grass or other vegetation where they might be taken into 
the stomach of a grazing cow, along with the grass. In the 
stomach of the cow, the walls of the proglottid are digested 
away and the shells of the embryos open, thus liberating the 
onchospheres in the digestive cavity of the cow. Each oncho- 
sphere by the action of its hooks bores into the wall of the diges- 
tive tract and may enter the blood stream by which it is carried 
to remote parts of the body. Especially in the muscular and 
connective tissues, the onchospheres come to rest and undergo a 
transformation to large saclike structures, the cysticerci, each 
bearing an inverted scolex. Each cysticercus is surrounded by a 
cyst wall formed by the action of the surrounding host tissues. 
Here it lies without going further in its development unless 
introduced into the body of another animal which is suitable as a 
host. Cysticerci in thoroughly cooked infected beef are harmless. 
If beef containing the living cysticerci is eaten by man, the 
digestive action liberates the cysticercus from its confining cyst, 


the inturned scolex becomes everted and secures attachment to 
the hning of the ahmentary canal of the new host. The scolex 
is not affected by the digestive processes of the host but the cyst 
appended to its free extremity disintegrates, leaving only the 
scolex and neck which, through growth, produce the entire 

The Scolex. — The scolex shows great diversity in form. In 
members of the order Pseudophyllidea, the scolex is usually 
provided with two longitudinal sucking grooves (Marsipometra, 
Bothriocephalus, Abothrium, Ligula), though some instances 
these are highly modified (Bothrimonus) and occasionally are 
united to form an unpaired terminal adhesive organ (Cyatho- 
cephalus). The scolex in members of this order is usually 
unadorned except in Triaenophorus which bears conspicuous 
chitinous hooks and in the primary scolex of Haplobothrium 
which bears four proboscides, therein resembling the Trypa- 
norhyncha. Members of the order Trypanorhyncha, which 
occur chiefly in the digestive tract of marine fishes, have a 
proboscis of varied form provided with four terminal, long, 
cylindrical proboscides covered with minute spines (Tetrarhyn- 
chus). In the order Tetraphyllidea are included cestodes with 
a scolex bearing four cuplike bothria or suckers and with each 
proglottid bearing numerous vitelline follicles (Proteocephalus, 
Corallobothrium). The order Cyclophyllidea includes forms 
having four cuplike or saucer-shaped suckers and usually a 
terminal organ between the suckers termed a rostellum. In 
this order, the vitellaria are usually posterior to the ovary and 
occur in a single compact mass. This order of numerous families 
includes most of the cestode parasites of the higher vertebrates 
(Taenia, Anaplocephala, Hymenolepis, Dipylidium, Davainea, 
and many other genera). 

Class Nemertinea 

The nemertines are usually included as a class under the 
Plathelminthes because of their agreement with other fiatworms 
in: (1) structure of the nervous system, (2) presence of a pro- 
toncphridial excretory system, and (3) the strong development of 
the mesoderm which renders the body highly parenchymatous. 
On the other hand, they differ from other fiatworms in: (1) the 
specialization of a vascular system so that distribution of nourish- 
ment is not cared for by a combined gastrovascular system, (2) 



the development of an anal opening at the posterior extremity of 
the digestive tract, and (3) the presence of a closed sac, the 
proboscis sheath, which surrounds the proboscis and by some is 
considered as representing the beginning of a coelom. In 
attempting to show relationships with the coelomate animals, 
this last point is of considerable importance. Other workers 
have thought that the cavities of the gonads of nemertines are 
really coelomic sacs. 

Nemertines are chiefly marine, living usually in burrows in 
mud or sand, and in some species attaining a length of 90 feet. 
Some of the smaller forms inhabit fresh water or live in moist 
soil. The body surface is ciliated and frequently brilliantly 

ps pm 

pn po 

Fig. 69. — Diagram of a nemertine. b, brain; c, ciliated pit; d, dorsal nerve 
trunk; di, dorsal blood vessel; g, gastric ceca; i, intestine; /, lateral nerve trunk; 
Iv, lateral blood-vessel; p, proboscis, retracted; pm, proboscis muscles; pn, proto- 
nephridial tube; po, its opening; ps, cavity of proboscis sheath. {After Kingsley, 
courtesy of Henry Holt and Co.), 

colored. Numerous mucous glands produce secretions which 
may form a tube within which the animal dwells. The body 
wall contains an outer circular and an inner longitudinal layer 
of muscles which are so effective that a worm which is 15 feet or 
more when fully extended may shorten to less than 2 feet in 

The proboscis (Fig. 69) is one of the most characteristic struc- 
tures of the nemertine. This is a hollow muscular tube turned 
into the body at the anterior extremity and when thus inverted 
extends far back through the body within a saclike cavity called 
the proboscis sheath. By contraction of the fluid-filled sheath, 
the proboscis is everted and thrust out from the anterior part 
of the body. At the tip of the extruded proboscis, there is fre- 
quently a sharp-pointed stylet which, of course, is at the extreme 
posterior end of the proboscis when it is retracted and inverted 
within the sheath. Retraction of the everted proboscis is accom- 
plished by means of a retractor muscle which runs from the tip 


of the proboscis to the base of the sheath. Abundant nerve 
supply indicates that the proboscis is highly tactile, but it is 
also an organ of defence. When extruded, it coils around the 
prey and at the same time a viscid secretion from the proboscis 
sheath prevents the prey from escaping. 

The mouth is located at the anterior extremity of the body or 
just ventral to the opening of the proboscis sheath. The tubular 
ectodermal esophagus opens into the tubular intestine (mesen- 
teron) which usually has paired lateral diverticula along its 
course to the anus. The anus may be either at the posterior 
extremity or in some instances the digestive tract does not enter 
the tail region. 

Reproduction. — The sexes are usually separate. The gonads, 
which are lateral in position, occur between the intestinal diver- 
ticula. Each ovary or spermary is a saclike organ which usually 
opens on the dorsal surface by a pore. Both eggs and sperma- 
tozoa are discharged from the body through the pores, and 
fertilization takes place outside the body of the worm. Cleavage 
of the fertilized egg usually results in the formation of a helmet- 
shaped larva known as the pilidium. Cilia occur on the lapets at 
the lower margins of this larva and also in a patch at the opposite 
pole known as the apical plate. The apical plate is the chief 
nervous center of the larva. Development of the adult from 
this larva involves a complicated metamorphosis. By the 
growth of two infoldings of the ectoderm, a part of the body 
containing the digestive system of the larva is surrounded and 
cut off from the remainder of the body. In later development, 
these ectodermal infoldings form the body wall of the adult worm 
and only the parts of the larva enclosed by them are utilized 
in the production of the young worm, for the remainder of the 
larva is cast off during the metamorphosis. In some instances, 
development is direct, without involving the pilidium, while in 
still others a reduced creeping pilidium, frequently termed 
Desor's larva, takes its place. 

Vascular System. — Typically, the vascular system has three 
main longitudinal trunks, two lateral and a median dorsal vessel 
which lies between the intestine and the proboscis sheath. 
Transverse loops connect the two lateral vessels. The fluid 
contained in this system is usually colorless. This is the first 
instance in the animal kindgom where the function of distribution 
is taken over by an independent system, for in lower forms the 


functions of digestion and distribution are performed by a 
gastrovascular system. 

Excretory System. — The excretory system consists of two 
longitudinal tubules which run parallel to the lateral vessels of 
the circulatory system and through their course give off small 
branches which terminate in flame cells. Either a single pore or 
several pores communicate with the exterior. 

Nervous System. — The central nervous system consists of a 
pair of ganglia from which two lateral and one median dorsal 
nerve pass backward through the body. Details of structure and 
arrangement of the nerve trunks differ considerably in the 
different orders. In some instances (Protonemertini), the 
nervous system remains in the superficial layers of the body 
external to the musculature. A pair of ciliated grooves on the 
sides of the head, frequently called cerebral organs, are closely 
connected with the brain and have a sensory function. Eyes 
and tactile organs are usually developed. 

Outline of Classification 

Phylum Plathelminthes.- — Triploblastic, wormlike animals; without body 
cavity; lacking true segmentation. 

I. Class Turbellaria. — Ciliated; chiefly free-living. 

1. Order Polycladidea. — Many branches to digestive tract; 
marine. Planocera, Leptoplana, Stylochus. 

2. Order Tricladidea. — Three main branches to digestive tract, 
one anterior and two posterior. Planaria, Bdelloura, Dendrocoelum. 

3. Order Rhabdocoelida. — Digestive tract a simple sac. Micro- 
stomuni, Stenostomum, Prorhynchiis. 

II. Class Trematoda. — Parasitic; non-cellular cuticula covers body; 
suckers for attachment; alimentary canal present. 

a. Subclass Monogenea, — Ectoparasitic; development direct; suck- 
ers and hooks for attachment. 

1. Order Monopisthocotylea. — Posterior attachment organ single. 
Gyrodactylus, Dactylogyrus, Nitzschia. 

2. Order Polyopisthocotylea. — Posterior attachment organ double 
or multiple. Polysiovia, Microcotyle, Sphyranura. 

h. Subclass Digenea. — Endoparasitic; alternation of generations and 
alternation of hosts; one or two suckers. 

1. Order Gasterostomata. — Anterior sucker imperforate; mouth 
on midventral surface. Bucephalus. 

2. Order Prostomata. — Anterior sucker surrounds mouth which is 
at or near anterior tip. 

a. Suborder Aspidocotylea. — Oral sucker wanting or poorly 
developed; ventral sucker powerful disc. Aspidogaster, 


b. Suborder Monostomata. — No ventral sucker. Notocotylus, 

c. Suborder Strigeata. — Usually two suckers; cercaria fork- 
tailed. Slrigca, SchistDHoma 

d. Suborder Amphistomata. — Acetabulum terminal or sub- 
terminal, highly developed, posterior to reproductive organs. 
Gastrodiscus, Diplodiscus, Allassostoma, Watsonius. 

e. Suborder Distomata. — Oral and ventral sucker present; 
acetabulum usually anterior to reproductive organs. FasciaLa, 
Fasciolopsis, Opisthorchis, Paragonimus, Clonorchis, Telorchis, 

m. Class Cestoda. — Parasitic; body covering a cuticula; no digestive 
system; scolex for attachment. 

a. Subclass Cestodaria. — Body not divided into proglottids; a single 

set of reproductive organs. Gyrocotyle, Archigetcs, Amphilina, 

Caryophyllaeus, Glaridacris. 

h. Subclass Cestoda. — Body usually divided into proglottids; 

reproductive organs oft repeated. 

1. Order Pseudophyllidea. — One terminal or two opposite sucking 
grooves. Abothrium, Bothriocephalus, Diphyllobothrium, Sparga- 
num, Marsipometra; Ligida, Cyathocephalus, Triaenophorus. 

2. Order Trypanorhyncha. — Two or four suckers and four hook- 
covered protrusible proboscides on scolex. Tetrarhynchus. 

3. Order Tetraphyllidea. — Four sucking cups; in fish and reptiles. 
Dinobothriu m, Proteocephalus, Corallobothrium. 

4. Order Cyclophyllidea. — Four cuplike suckers and an apical 
organ of varied form. Taenia, Hymenolepis, Echinococcus, Ana- 
plocephala, Dipylidium, Davainea. 

IV. Class Nemertinea. — Ciliated; proboscis and proboscis sheath, 
dorsal to mouth; blood system; digestive canal complete; aquatic. 

1. Order Protonemertini. — Nervous system outside the muscles; 
no stylet. Carindla. 

2. Order Mesonemertini. — Nervous system in muscles; no stylet. 

3. Order Metanemertini. — Nervous system inside muscles; usually 
with stylets. Stichostcynma, Geonemertes. 

4. Order Heteronemertini. — Several layers of muscles; nervous 
system in muscles; no stylet. Lineus, Cerebratalus. 


(See general references cited at close of Chapter I) 
Chandler, A. C. 1930. "Introduction to Human Parasitology." New 

York, Wiley. 
Child, C. M. 1915. "Senescence and Rejuvenescence." Universitj^ of 

Chicago Press. 
Curtis, W. C. 1902. The Life History, the Normal Fission, and the 

Reproductive Organs of Planaria maculata. Proc. Boston Hoc. Nat. 

Hist., 30: 515-559. 


Faust, E. C. 1929. "Human Helminthology." Philadelphia, Lea and 

VON Graff, L. 1882-1899. "Monographie der Turbellarien." Leipzig. 
. 1904-1908. "Turbellaria. I Abt., Acoela und Rhabdocoelida." 

Bronn's Klassen und Ordnungen der Tierreichs, Vol. 4. 
Hegner, R., Root, F. M. and Augustine, D. L. 1929. "Animal Parasi- 
tology." New York, Century. 
HuBRECHT, A. A. W. 1883. On the Ancestral Forms of Chordata. 

Quart. Jour. Micr. Sci., 23: 349-368. 
. 1887. "Report on the Nemertea." Report Challenger, Zool., 

Vol. 19. 
LANGy A. 1884. Die Polycladen des Golfes von Neapcl. Fauna u. Flora 

desGolfes von Neapel, Monograph 11. 
WiLHELMi, J. 1909. Die Tricladen des Golfes von Neapel. Fauna u. 

Flora des Golfes von Neapel, Monograph 32. 
Wilson, C. B. 1900. Habits and Early Development of Cerebratulus 

lacteus. Quart. Jour. Micr. Sci., 43: 97-198. 



The Nemathelminthes or roundworms are threadlike or eyhn- 
drical worms which have a body cavity and lack segmentation. 
The body is covered with a heavy cuticula. Highly developed 
sensory and locomotor organs are wholly lacking. Reproduction 
is invariably sexual, and in the course of direct development 
distinctive free larval stages are lacking. Since the splanchnic 
layer of the mesoderm is wanting, the body cavity is not a true 
coelom but is termed a pseudocoel. Three classes are usually 
recognized as belonging to this phylum, the Nematoda (or thread- 
worms), the Gordiacea (or hair snakes), and the Acanthocephala 
(or spiny headed worms). There is considerable doubt as to the 
correct location of the Acanthocephala in the system, for evi- 
dences indicating flatworm relationships are not wanting. The 
three classes differ so fundamentally in structure that it seems 
best to offer individual treatment of the groups rather than 
discuss characteristics of the phylum further. 

Class Nematoda 

The nematodes or nemas, as they are frequently called, have 
characteristically elongated, cylindrical bodies (Fig. 70 B) 
covered by a resistant cuticula. They occupy almost every con- 
ceivable habitat capable of supporting life. Fresh-water, marine, 
and soil-inhabiting species are extremely numerous and as para- 
sites both of plants and of animals they are of high importance. 
In length, they range from a fraction of a millimeter to more than 
a meter. There are no appendages and no segmentation, though 
in some free-living forms striations, cuticular scales, or bristles 
may give a superficial appearance of segmentation. 

Histology. — In section, the body wall is seen to be composed of 
an external non-cellular layer, the cuticula, directly beneath 
which lies the subcuticula or hypoderm. A layer of partially 
differentiated muscular cells lines the outer wall of the body cav- 
ity and comprises the chief bulk of the body wall. Each of these 




epithelio-muscular cells has only a small portion of its bulk differ- 
entiated into contractile substance lying next to the subcuticula, 
while the remainder of the cytoplasm, containing the nucleus, 
protrudes as a rounded mass 
into the body cavity. 

This muscular layer is not a 
continuous lining of the body 
cavity, for it is interrupted by 
slight breaks in four regions 
which are designated as the 
dorsal, ventral, and lateral lines. 
The fairly conspicuous thicken- 
ing of the subcuticula which 
stands in the middle of each 
lateral surface is known as a 
lateral line. Less pronounced 
intrusions of the subcuticula 

'Nerve nnj 

■ExcreforLj pore 


- Ovary 




Fig. 70 A. — The nervous system of a 
nematode. {After Brandes)-. 

Fig. 70 B. — Semidiagrammatic 
drawing of a female free-living 
nematode. {After J agerskiold) . 

occur also in the middorsal and midventral lines. In the lateral 
hues are borne the ducts of the excretory system. Near the 
anterior extremity of the body, these are united and communi- 


cate with the exterior through a single excretory pore on the 
ventral surface of the body. In the dorsal and ventral lines are 
found the two main longitudinal nerve trunks (Fig. 70 A) which 
connect with a nerve ring near the anterior extremity of the body. 
From this same ring are given off a number of smaller longi- 
tudinal branches, some extending forward and others a short 
distance backward. 

Digestive System. — The digestive system is practically a 
straight tube (Fig. 70 B) with a mouth at the anterior extremity 
and the anus on the ventral surface slightly in front of the 
posterior tip. The mouth is usually surrounded by liplike organs 
and, in the case of parasitic forms, frequently bears chitinized 
structures for grappling the host tissues. Behind the mouth, 
there frequently occurs a muscular esophagus with an especially 
conspicuous pharyngeal bulb. The stomach-intestine continues 
posteriorly from the esophagus as a usually flattened tube. 
In the male, the genital ducts open into the posterior region 
of the digestive tube which is thus transformed into a cloaca. 

Reproductive Organs. — The male is usually recognizable by its 
smaller size and frequently by the more pointed and curved 
posterior extremity. From the cloaca of the male, copulatory 
spicules are frequently protruded. Reproductive organs in the 
sexes are very simple in structure, for normally they consist of a 
gonoduct which is directly continuous with the gonad. This tube 
in the male is usually single, while in the female (Fig. 70 B) it is 
frequently bifurcated with the opening of a single vagina occurring 
on the ventral surface of the body. In some forms, the ducts and 
gonads are extremely long and coiled throughout the greater 
part of the length of the body, while in others they are a single 
straight rod. 

The eggs of the horse Ascaris (A. megalocephala) have provided 
one of the most widely used materials for the intimate study of 
chromosomes and of the mechanics of mitosis and cleavage. The 
works of Boveri, Biitschli, and zur Strassen on the eggs of Ascaris 
stand as classics in the field of cytology. 

Method of Reproduction. — Nematodes reproduce only sexu- 
ally. Eggs are fertilized within the oviduct of the female 
and are discharged either before development has proceeded far 
or are retained until the young are fully developed. In some 
instances, the eggs are not laid but are retained in the body 
of the female until the young have left the egg membranes and 


are thus brought forth viviparously. Parthenogenesis occurs 
in some instances, as mentioned below. 

Life Cycles. — A number of extremely interesting and important 
facts are connected with the developmental cycle of the parasitic 
nematodes. Some species are parasitic throughout their entire 
life (Trichinella and blood Filariae), while many are free-living 
for at least part of their existence (hookworms as larvae, Mermis 
as adults). Still others are facultative parasites living either 
free or parasitic as opportunities are offered. Of this last condi- 
tion, Rhabditis hufonis serves as an excellent example. The 
young of this species live in mud where they undergo sexual 
reproduction, both males and females being found. Larvae pro- 
duced by these sexual individuals may continue to produce in the 
same manner as the parents, or in case they find their way into 
the lungs of a frog they become established as parasites and 
develop only parthenogenetically. The parthenogenetic eggs 
pass out with the feces and in the mud again pass through the 
dioecious phase of the life cycle as free individuals. 

Ascaris lumbricoides is a large worm, about 8 inches long, 
which lives in the intestines of man and the pig. Though the 
Ascaris in these two hosts look exactly alike, experiments so far 
indicate that larvae hatched from a worm living in a pig fail to 
develop normally in man, and vice versa. Until very recently 
it has been assumed that infestation by this species is by direct 
introduction of the young into the digestive tract of a new 
host individual where it becomes established immediately. 
Recent investigations have demonstrated that the larvae of these 
ascarids, when introduced into the digestive tract of a new host 
individual, undergo extensive migration through the organs and 
tissues of the body, reaching the final position only after having 
passed by way of the circulatory system into the lungs. Heavy 
infestations by these larvae cause serious pulmonary disorders, 
as the larvae penetrate the lung tissues and travel by way of the 
respiratory passages into the mouth and down the digestive 
tract to the intestine, where they reach maturity. 

After entering the host the female requires only about 2 months 
to reach maturity. Some idea of the danger of this species as a 
human parasite may be gained from the fact that a single female 
has been found to give off 200,000 eggs daily with a total produc- 
tion of 27,000,000 eggs by a single worm. The eggs are very 
resistant, for the heavy shells prevent even strong chemicals 


from penetrating and injuring them. There are evidences that 
eggs of Ascaris may remain ahve in the soil for a period of 5 to 6 
years. This adds very greatly to the difficulty of eradicating 

Extensive migrations are also carried on in the body of the host 
by the hookworms (Ancylostoma of the Old World and Necator 
of the New World). The adult worms, which cause very serious 
loss of blood and affect the entire body of the host, occur in the 
intestine of man where the thin-shelled eggs are produced and 
eliminated along with fecal matter. In the soil, the young 
worms hatch and feed for a while on the fecal matter. A new 
skin forms beneath the old one, which is finally shed, and the 
larva enters upon the second period of its life. After another 
molt, the larva is ready to infest a new host individual. This it 
does either by entering the body along with contaminated water 
or food or by active penetration of the skin. Hands and feet 
are the chief inroad for the larvae, which attack the skin exposed 
to the soil and follow along hair follicles or between epidermal 
scales to the lymph spaces. Once in the lymph stream they are 
carried passively to the subclavian, thence to the heart, and on 
with the blood stream to the lungs. Here the larvae leave the 
capillaries, enter the air sacs, and wander through the bronchi, up 
the trachea into the esophagus, and down through the stomach 
to the intestine where they become attached to the wall by 
means of their specially adapted sucking mouth. About 7 
to 10 weeks from the time the larva enters the skin, eggs begin 
to appear in the feces, indicating that the worms have reached 
maturity and the life cycle is thus completed. 

Still a different condition is found in the life cycle of Trichinella 
spiralis. There are two distinct stages in the parasitism by this 
worm: the sexually mature worm which occurs in the digestive 
tract and the encysted larvae in the muscles. Both sexes occur 
in the intestine of man and of other mammals of which the pig 
and rats are the most important hosts. The gravid female 
pierces the wall of the intestine with her posterior extremity. 
With the genital pore inserted in a lacteal vessel, the young are 
liberated and carried by the lymph and blood streams into the 
tissues of the host's body. It is not known to what extent this 
migration is active or passive. Upon reaching muscular tissue, 
the larvae become encysted and all further development is 
contingent upon the muscles containing the encysted larvae 


being introduced into the digestive system of some other mammal, 
for the larvae have no independent means of ever leaving the 
body of the host individual which sheltered the mature female 
in its intestine. The chief source of the adult worms in the 
human intestine is from the larvae encysted in pork, while 
the hog in turn receives its intestinal form through eating the 
viscera of other hogs in slaughter yards or from eating rats 
which have become infested. 

Blood-inhabiting nematodes such as the Filariae are often 
carried from one host to another through the bite of blood-sucking 
insects. Some species which cause elephantiasis through the 
occlusion of blood and lymph vessels are carried by mosquitoes. 

Free-living Nematodes. — There are numerous species of 
free-living nematodes representing many genera. In the genus 
Iota the body is covered with scales which give it a superficial 
appearance of segmentation. Tylenchus, Dorylaimus, Mermis, 
and Rhabditis are names of characteristic genera of free-living 
nematodes, though species in some of these genera may be 

Class Gordiacea 

Superficially, the Gordiacea resemble the nematodes, but in 
finer details of structure they have little in common with them. 
Some systematists are inclined to disregard these differences 
and therefore include the Gordiacea as a subclass of nematodes. 
The adults live free in the water where the females lay strings of 
eggs which develop into small larvae. These larvae enter the 
body of some insect where they undergo development to the 
adult body form. 

The hair snakes, so commonly found in watering troughs 
and in ponds and streams, are the adults of Gordiacea which 
have escaped from the bodies of crickets or other insects. They 
bear no lateral lines, and the nearly cylindrical body with blunt 
anterior extremity and irregularly roughened cuticula serve to 
differentiate the Gordiacea from the nematodes. 

Gordius, Paragordius, and Chordodes are genera in this class. 

Class Acanthocephala 

The Acanthocephala are absolutely parasitic in habits. There 
is no trace of digestive organs in the mature worms and even in 
the development of the larva there are no specializations of the 



entoderm to form even a rudiment of an alimentary canal. As 
adults they occur normally in the digestive tract of vertebrates. 
The first larval host is practically always an arthropod, though 
young individuals have been found frequently in other hosts 


Recspfade — U- 

HHrachns of- 



. Bursa 

A B 

Fig. 71. — Morphology of the Acanthocephala. A, general organization of 
male of Acanthocephalus ranac (Schr.) ; B, interior of caudal extremity of a young 
female of Neoechinorhynchus emydis, showing genital tract. {After Van Cleave). 

which probably act as intermediate hosts. In the digestive 
tract of the vertebrate, the acanthocephalan has an elongated 
flattened body form but upon removal to water or killing fluids 
the liquids distend the body to cylindrical form. One of the 
most characteristic structures is the proboscis (Fig. 71 A) at the 


anterior extremity of the body bearing hooks for grapphng into 
the host tissues. This proboscis is frequently capable of inver- 
sion into the anterior extremity of the body inside an organ 
termed the proboscis receptacle or sheath. In some instances 
the proboscis may be retracted within the anterior region of the 
body without being inverted into the receptacle. 

Nuclear Constancy. — In all members of the family Neoechino- 
rhynchidae, the individuals of each species are constructed on 
exactly the same plan as far as the cellular elements are con- 
cerned. Each organ and each tissue contains a definitely fixed 
number of nuclei in the identical positions for every member of a 
given species. In these forms, the size of the individual depends 
wholly upon the size of the component cells, for all worms of 
a given species are made up of the same number of cells. This is 
a condition wholly different from that found in some other 
animals, for in some species the size of the individual is depen- 
dent upon the number of cells in its body. Typical arrange- 
ment of nuclei in the organs of a single system is shown in 
Fig. 71 B. 

The body wall is composed of an external layer of cuticula 
which overlies a syncitial mass called the subcuticula. This 
subcuticula forms by far the greatest bulk of the animal and is 
provided with a few rounded giant nuclei, finely dendritic nuclei, 
or numerous small nuclei. Two muscle layers, one with the 
fibers directed longitudinally and the other with the fibers 
directed circularly, mark the internal limit of the body wall and 
bound the body cavity. 

At the anterior extremity of the body proper, two organs of 
variable shape and of undetermined function, the lemnisci, 
extend into the body cavity alongside of the proboscis recep- 
tacle, apparently as continuations of the subcuticula. 

The central nervous system consists of a single ganglionic 
mass within the proboscis receptacle. In the different genera, 
this occupies a position varying from the posterior extremity of 
the receptacle to a point near the anterior end of the receptacle. 
Small branches are given off to the surrounding organs, and 
usually a pair of structures called the retinacula pass through the 
wall of the receptacle out to the body wall. 

Internal Organization. — Inversion of the proboscis is accom- 
plished by a pair of invertor muscles which run from the tip of the 
proboscis to the base of the receptacle through which they con- 


tinue and pass on through the body cavity to an insertion on the 
body wall as the retractors of the receptacle. 

A sheath passes backward from the posterior extremity of the 
proboscis receptacle as the suspensory ligament which holds the 
reproductive organs in place. The male organs consist of a pair 
of testes and a group of cement glands which communicate with 
the cirrus. The cirrus is contained within an evertible structure 
at the posterior end of the body known as the copulatory bursa. 
When protruded, this is a bell-shaped structure in the center of 
which the cirrus is located. 

In the female, there is no persistent gonad. Egg masses are 
formed very early and after fertilization these are broken up into 
individual embryos each of which becomes surrounded by a 
series of three embryonic membranes. The hard-shelled embryos 
thus formed are usually ovoid or spindle-shaped in form. The 
embryos are held for some time within the female's body cavity 
which becomes filled with them. Finally, they are discharged 
through an apparatus known as the selective apparatus (Fig. 
71 B) which passes them down the uterus and out of the genital 

All classes of vertebrates harbor these parasites. Macracan- 
thorhynchus hirudinaceus found in hogs is one of the most commonly 
known species. The genera Echinorhynchus and Neoechinor- 
hynchus are represented by several species in American fishes. 
Several genera, including numerous species, infest the intestines 
of birds and mammals. The genus Moniliformis occurs normally 
in rodents but is also at least a facultative parasite of man. 

Outline of Classification 

Phylum Nemathelminthes, — Body covering a cuticula; wormlike; 

I. Class Nematoda. — Complete digestive tract; lateral lines along 

sides of ])ody; body cavity a pseudococl. 

1. Order Trichosyringata. — Esophagus a small tul)e with cliitinous 
lining. Trichindla, Triciwstrongylus. 

2. Order Myrosyringata. — Esophagus . prominent; muscular. 
Ascaris, Heterodera, Rhabditis, Strongyloidcs, Syngamus, Ancy- 
lostoma, Necator, Haemonchus, Filaria, Loa, Dioctophyvie. 

n. Class Gordiacea.^ — Body cavity lined with epithelium; no lateral 
lines; larva in insects, adult in water. Gordius, Paragordius. 
III. Class Acanthocephala. — Digestive organs lacking; always parasitic; 
proboscis a hook-covered introvert. Echinorhynchus, Acanthocephalus, 
Neocchinorhynchus, Gigantorhynchus, Macracanthorhynchus, Monili- 



(See general references cited at close of Chapter I) 
Cobb, N. A. 1914. The North American Free-Uving Fresh-water Nema- 
todes. Trans. Amer. Micr. Soc, 33: 69-134. 
. 1915. Nematodes and Their Relationships. U. S. Dept. Agr. 

7eor6ooA-, 1914: 457-490. 
Hall, M. C. 1916. Nematode Parasites of Mammals. Proc. U. S. Nat. 

Mus., 50: 1-258. 
Loess, A. 1905 and 1911. The Anatomy and Life History of Agchylostoma 

duodenale, Dub., A monograph. Cairo. 
LtJHE, M. 1911. "Acanthocephalen." Die Siisswasserfauna Deutsch- 

lands. Heft 16. Jena. 
Martini, E. 1916. Die Anatomie der Oxyiiris curvula. Zeltschr. 

wissensch. Zool., 114: 137-543. 
May, H. G. 1920. Contributions to the Life Histories of Gordius robustus 

Leidy and Paragordiu.s varius (Leidy). III. Biol. Monograph, Vol. 5, 

No. 2. 
Ransom, B. H. and Schwartz, B. 1919. Effects of Heat on Trichinae. 

Jour. Agr. Res., 17: 201-221. 
Ransom, B. H. and Foster, W. D. 1920. Observations on the Life 

History of Ascaris lunibricoides. U. S. Dept. Agr. Bull. 817. 
Stiles, C. W, 1903. Report upon the Prevalence and Geographic 

Distribution of Hookworm Disease (Uncinariasis or Anchylostomiasis) 

in the United States. U. S. Hygienic Lab. Bull. 10. 
Van Cleave, H. J. 1919. Acanthocephala from the Illinois River, with 

descriptions of species and a synopsis of the family Neoechinorhynchidae. 

III. Nat. Hist. Survey Bull, 13: 225-257. 
Yorke, W. and Maplestone, P. A. 1926. "The Nematode Parasites of 

Vertebrates." Philadelphia, Blakiston. 


Many of the higher invertebrates in their development pass 
through a larval stage known as the trochophore. In most 
instances where a trochophore is involved, it later by meta- 
morphosis gives rise to an adult animal which in organization 
is fundamentally different from the simple larva. There are, 
however, a few organisms which in their adult state are not 
essentially different from the trochophore type of organization. 
The most characteristic of these is the group of the Rotifera. 
In addition to the rotifers, there is a small group of minute 
fresh-water organisms known as the Gastrotricha which are in 
some respects similar to the Rotifera. These two groups are 
united to form a phylum to which the name Trochelminthes is 
frequently applied. 

By some, representatives of this phylum are thought to repre- 
sent precociously mature larvae, or an instance of what might 
be termed phylopaedogenesis. 

Class Rotifera 

The Rotifera, or wheel animalcules, are microscopic animals 
which in fundamental structure closely resemble the trochophore. 
In size and superficial appearance, they might be mistaken for 
Protozoa and were so considered by many of the early workers. 
Close observation reveals in them miniature organ systems and 
demonstrates their true metazoan natures. Most rotifers live in 
fresh water though a few dwell in seas. No body of water is too 
large or too small for them, for tiny temporary pools frequently 
support a varied fauna of these minute organisms. Many are 
capable of withstanding desiccation and one type of eggs is highly 
resistant. These two facts go far toward explaining the prac- 
tically cosmopolitan distribution of many species, for the dried 
individuals or eggs could be transported great distances or 
might even be carried by the winds. Some genera are commonly 
represented in the fresh-water plankton, others are character- 




istically associated with vegetation, while still others live on 
muddy bottoms of ponds and streams. 

Gross Morphology. — The body, which is extremely variable in 
shape, usually consists of a trunk and a tail. In many genera a 
flexible cuticula covers the trunk, but in others there is a firm. 






I l\ 

• » 1 gland 




Fig. 72.- — A rotifer, Hydatina senta, viewed from ventral surface. (Orig.). 

shell-like covering called the lorica. The anterior extremity 
of the trunk is usually modified to form a structure known as 
the trochal disc (Fig. 72) which is one of the most char- 
acteristic structures of the rotifers. This disc bears cilia in 
highly variable arrangement and is capable of retraction within 
the anterior region of the trunk. The mobile posterior extremity 
is usually recognizable as a tail with some sort of adaptation for 


attachment, though in many forms (Asplanchna, Pedahon, etc.) 
this is lacking. Individuals of some species are permanently 
attached. In this instance (Melicerta, Floscularia), they may 
secrete a tube or may form a case, partly of foreign matter, 
within which the rotifer can withdraw when disturbed. A few 
species form colonies by secreting a gelatinous material within 
which the foot of each individual becomes embedded. In free- 
swimming forms, the trochal disc is the chief organ of locomotion, 
though at times the rotifer may loop along like a leech and in some 
instances outgrowths from the body wall form hollow limblike 
appendages (as in Pedalion) by means of which the animal skips 
through the water. 

Trochal Disc. — The plan of ciliation of the trochal disc is 
usually reducible to various modifications of one or two bands 
of cilia. In the simplest condition, a single circle of cilia edges 
the margin of the circular disc. Distortion of this circle at 
certain points results in the formation of either blunt ciliated 
lobes (Floscularia) or long ciliated arms (Stephanoceros) but in 
each of these instances the cilia are arranged in a single continu- 
ous row. In many instances, a second band of cilia is introduced 
parallel to the first and in some of these also the ciliated bands 
may become lobed. Almost always when two rows of cilia are 
present the mouth occurs between the two on the ventral surface 
of the disc. In Trochosphaera, which lacks a trochal disc, there 
is an equatorial preoral circle of cilia with a few cilia postoral 
in distribution. This condition, as well as the general internal 
organization of Trochosphaera, corresponds very closely to 
conditions found in the trochophore larvae of higher inverte- 
brates. It is because of this close agreement that some contend 
that rotifers phylogenetically represent trochophore larvae 
which have attained full sexual development precociously. 

Digestive System. — The digestive system comprises a mouth 
located on the ventral surface of the trochal disc, an esophagus 
with its elaborate mastax for triturating food, a stomach, and an 
intestine which opens near the posterior extremity through an 
anus. The mastax is highly characteristic of the rotifers but is 
subject to considerable modifiabilityof form. In many instances, 
there are three heavily chitinized parts discernable : an incus and 
two mallei, but either of these elements may be wanting. By 
action of the muscular esophageal wall, the parts of the mastax 
are worked together as an effective crushing organ which reduces 


the food ready for digestion, when it passes on into the stomach. 
A pair of digestive glands is usually associated with the stomach. 
Both the stomach and intestine are lined with cilia. In the 
genus Asplanchna, the stomach ends blindly, for there is no 

Excretory System. — Coiled nephridial tubes lying in the 
body cavity bear flame cells at the ends of their lateral branches. 
These excretory tubules discharge their waste into a urinary 
bladder and thence into the cloaca. 

Nervous System. — A single ganglion, usually located in the 
anterior dorsal region of the body, gives off nerves to the sur- 
rounding organs and from it a pair of lateral nerve trunks pass 
posteriorly to the tail. One or more simple pigment spots, and 
sometimes more complicated eyes, are often associated with the 
brain and, aside from tactile hairs, represent about the only 
development of sensory apparatus. 

Reproduction. — Rotifers are bisexual, though usually the male 
is much reduced and in some instances has never been observed. 
At times, the male lives as a parasite on the female. Since they 
have no digestive organs, many males are very short lived. In 
describing rotifer structure, the body of the female is considered 
as typical. Most rotifers are oviparous or ovoviviparous, 
though some (Asplancha, Philodina) are viviparous. Partheno- 
genetic development, as commonly found here, frequently 
involves two different sizes of eggs, of which the larger produce 
only females and the smaller only males. Insemination of the 
female to produce fertilized eggs seems to be accomplished by per- 
foration of the body wall at any point to introduce spermatozoa 
into the body cavity. Fertilized eggs thus produced differ from 
the parthenogenetic eggs in the possession of heavy, resistant 
shells and are designated as winter eggs. 

Sexual Organs. — The female organs usually consist of a single 
ovary (two in Philodina) and a vitellarium of highly variable 
form, though in a few cases there are two gonads with no dis- 
tinction between ovary and vitellarium. In parthenogenetic 
development, the young are produced within the body of the 
female and are usually liberated by rupture of the body wall of 
the parent. Winter eggs are usually carried inside the body some 
time before they are discharged through the oviduct and then lie 
dormant for a period before the young are hatched. Organiza- 
tion of the male is frequently simple, due to the degeneration of 



the digestive organs which occur as degenerate strings of tissue, 
near the posterior region, to which the male gonad is attached. 
A special copulatory organ in the form of a protrusible cirrus 
is often present. 

Rotifers as Experimental Animals. — Because of the readiness 
with which they may be reared in cultures, rotifers have been 
widely used in the study of problems concerned with the deter- 
mination of sex. For instance, in some 
species the ratio of males to females may be 
controlled directly by regulation of the 
temperature or by the type of food given to 
the parthenogenetic females. 

Nuclear Constancy. — For a number of 
rotifers, it has been shown that each organ 
is built of a fixed number of cells or at least 
contains a constant number of nuclei. An 
especially thorough study along this line has 
been published by Eric Martini for Hyda- 
Una senta, each individual of which contains 
a total of 959 nuclei distributed in fixed 
numbers through the various organs and 
tissues of the body. 

Class Gastrotricha 

Though the Gastrotricha are here included 
as a class along with the Rotifera under the 
phylum Trochelminthes, their relationships 
with the rotifers are far from firmly estab- 
lished. Some zoologists maintain that the 
Gastrotricha are more directly related to the 
Nematoda. They are rarely more than 0.5 
mm. in size and though they occur relatively 
ovary. X400 {After frequently in protozoan and rotifer cul- 

Zelinka, from \\ ard and 

Whipple's Fre^h-water turcs, their Small sizc and rapid movements 
Biology, reprinted by fgnder closc examination difficult. Though 

permission of John 

Wiley and Sons, Inc.). widely distributed, they are restricted to 
fresh water. 
In most instances, there is a head set off from the body proper 
by a slight constriction. The body is flattened on the ventral 
surface and convex on the dorsal. The ventral surface is 
furnished with two longitudinal bands of cilia near the median 

Fig. 73. — Chaeton- 
otus maximus, one of 
the Gastrotricha, in 
ventral view. Ex, kid- 
ney; M, muscles; B, 
brain; E, egg; O, esoph- 
agus; /, intestine; Ov, 


line, by the action of which locomotion is accomplished. The 
body proper may be either smooth or covered with plates, spines, 
or bristles. The mouth, which is borne at the anterior extremity of 
the head, is usually surrounded by a circle of delicate oral bristles. 
In addition, there are frequently lobes on the sides of the head 
from which groups of sensory hairs protrude. 

The internal organization (Fig. 73) is relatively simple. The 
digestive tract runs as a straight tube through the axis of the 
body. In the body musculature, only a few longitudinal strands 
of muscle but no circular muscles have been demonstrated. The 
brain occupies much of the head region. The excretory organs 
are protonephridia. 

Only females are known, yet it is uncertain whether these are 
truly parthenogenetic or are hermaphroditic, and the male 
gonads have never been observed. The ovary occupies the 
posterior region of the body cavity and as fully formed eggs 
push anteriorly in the body cavity, they frequently distort the 
shape of the gravid female. 

Chaetonotus and Lepidoderma are the most representative 
genera in the North American fauna. 

Outline of Classification 

Phylum Trochelminthes. — Microscopic, triploblastic, unsegmented Meta- 
zoa; as adults usually resembling trochophore; mouth usually surrounded 
by cilia; aquatic. 

I. Class Rotifera. — A crown of cilia (corona) typically at anterior 
extremity; posterior extremity usually terminating in a foot or jointed 
tail; pharynx bears mastax. 

a. Subclass Monogononta. — Ovary single. 

1. Order Notommatida. — Mouth not near center of corona. 
Proales, Notommata, Synchaeta, Polyarthra, Distyla, Monostyla, 
Rattulus, Diurella, Hydatina, Anuraea, Notholca, Brachionus, 
Schizocerca, Asplanchna. 

2. Order Floscularida. — Mouth near center of corona. Flos- 
cularia, Microdon, Apsilus, Stephanoceros. 

3. Order Melicertida. — Two parallel wreaths of cilia with a furrow 
between; outer wreath always shorter. Pterodina, Pompholyx, 
Pedalion, Triarthra, Trochosphaera, Melicerta, Conochilus. 

b. Subclass Digononta. — Two ovaries. 

1. Order Bdelloida. — Fresl) water. Rotifer, Philodina. 

2. Order Seisonida. — Marine; parasitic. Seison, Paraseison. 

II. Class Gastrotricha.^Body spindle-shaped; flattened ventral surface 
bearing two rows of cilia; cuticular spines on back; ring of cilia (?) 
around mouth. Chaetonotus, Lepidoderma. 



(See general references cited at close of Chapter I) 

Harking, H. K. 1913. Synopsis of the Rotatoria. U. S. Nat. Mus., 
Bull. 81. 

Hudson, C. F. and Gosse, P. H. 1889. "The Rotifera or Wheel Animal- 
cules." London. 

Jennings, H. S. 1896. The Early Development of Asplanchna Herrickii 
DeGuerne. Bull. Mus. Comp. Zool., Harvard, Vol. 30, No. 1. 

. 1900. Rotatoria of the United States with Especial Reference 

to those of the Great Lakes. Bull. U. S. Fish Comm., 19: 67-104. 

Martini, E. 1912. Studien iiber die Konstanz histologischer Elemente, 
III. Hydatina senta. Zeitschr. Wissensch. Zool., 102: 425-645. 

Shull, a. F. 1918. Effect of Environment upon Inherited Characters in 
Hydatina senta. Biol. Bull., 34: 335-350. 

Stokes, A. C. 1887. Observations on Chaetonotus. The Microscope, 7: 
1-9, 33-43. 

. 1896. "Aquatic Microscopy for Beginners." 3d ed., pp. 178- 193. 

Wesenburg-Lund, C. 1923. "Contributions to the Biology of the Roti- 
fera. I, The Males." Kopenhagen. 



The Coelhelminthes or Annelida comprise a number of worm- 
like forms which possess a coelom and are usually segmented. 
This group contains many of the forms included under the 
"Vermes" of Linnaeus and other early workers. It has been 
said frequently that the Vermes constituted a wastebasket 
into which forms which could not be placed elsewhere were 
dumped. ]\Iuch of the advance in modern classification of the 
animal kingdom has centered around the recognition of groups 
of wormlike forms bearing common characteristics and establish- 
ing for them rank as independent phyla and classes. Thus the 
Plathelminthes,* Nemathelminthes, Molluscoidea, and Trochel- 
minthes have been removed from the old group Vermes and 
each has been elevated to the rank of an independent phylum. 
There yet remains an assemblage of segmented worms, arrow- 
worms, and gephyrean worms which seem to have enough features 
in common to warrant their retention for the present in a single 
major group of the animal kingdom. These forms, which 
collectively are known as the Coelhelminthes, agree in possession 
of a coelom and in having the nervous system of a uniform type. 
An excretory system is lacking in members of the class Chaetog- 
nathi and in some representatives of the other groups, but 
characteristically a metanephridial system is found. The 
Gephyrea, which lack segmentation, agree with the typical 
annelids in passing through a larval stage known as trochophore. 

Class Chaetognathi 

The chaetognaths, or arrow-worms, are small forms with body 
shape admirably adapted for life at the surface of the ocean 
where they move about with great rapidity in search of food. 
The body, which is almost as transparent as crystal, is usually 
about 15 mm. long though one species reaches a length of 70 mm. 
It is divided into three segments: head, trunk, and tail. Each 
of these divisions is separated from adjoining segments by a 




transverse septum, and a longitudinal mesentery divides each 
coelomic cavity into a right and a left part. From the sides of 
the head (Fig. 74) there extend a series of bristles which act as 
jaws in seizing prey, hence the name Chaetognathi, or bristle 
jaws. In preserved specimens, these jaws are frequently folded 
close against the sides of the head. 

Swift movements are produced by muscular contractions of 
the body and are directed by fins which occur as outgrowths 
of the body wall. These are a fan-shaped tail fin and one or 
two pairs of lateral fins on the trunk. Some species undergo 

marked migrations between 
the surface and deeper water 
twice daily. In Southern 
California one species be- 
comes most abundant at the 
surface of the ocean just 
after sunrise and again just 
after sunset. The central 
nervous system consists of a 
cerebral ganglion on the 
dorsal surface of the esopha- 
gus which is connected with 
a ventral ganglion near the 
middle of the trunk by two 
long esophageal connectives. 
A pair of eyes lie close behind 
the brain with which they are connected by a pair of nerves. 
Papillae scattered over the surface of the body probably 
have a tactile function. A peculiarly modified region on 
the dorsal surface just behind the head has been interpreted 
as olfactory. 

The individuals are hermaphroditic. Ovaries occur in the 
posterior part of the trunk cavity and communicate with the 
exterior through an oviduct on each side which opens laterally 
near the posterior extremity of the trunk segment. The testes 
are contained in the coelomic cavities of the tail segment. Sper- 
matozoa, liberated directly into the cavity, are carried out 
through a delicate sperm duct which is frequently dilated near 
its extremity to form a seminal vesicle. 

In development, the fertilized egg undergoes cleavage to form 
a typical gastrula. Two lateral folds of the entoderm extend 

Fig. 74. — Head of Sagitta showing bristle 
like jaws extended. [Orig.). 


down into the archenteron dividing this cavity into three parts. 
Of these, the two lateral cavities, lying between each fold and 
the body wall, later become the coelomic cavities, while the 
central space between the two folds constitutes the mesenteron. 
During the gastrula stage, two entoderm cells opposite the blasto- 
pore become recognizable as the rudiments of the gonads. By 
a single division, these two cells form two pairs of which the 
anterior pair later forms the female gonads and the posterior 
pair the male gonads. The young resemble the parents except 
in size. 

Sagitta, the arrow-worm, is the most characteristic genus of 
the Chaetognathi of which Spadella and Krohnia are other 
recognized genera. 

Class Chaetopoda 

The Chaetopoda are among the most characteristic of Coel- 
helminthes. Metamerism is sharply marked. The coelom is 
divided into successive chambers by transverse septa which 
correspond with the external constrictions of the body wall, and 
even internal organs such as nervous, excretory, and circulatory 
systems bear the marks of metamerism. Most of the segments 
bear bristles or setae which, by their number and arrangement, 
give a basis for classification into subclasses. In the Polychaeta, 
the setae occur in outgrowths of the body wall called parapodia 
which function as oarlike organs in swimming. 

The digestive system, though a straight tube, usually shows 
specialization into regions. The mouth typically lies on the 
ventral surface very near the anterior extremity beneath the 
terminal somite called the prostomium. In the two subclasses, 
the prostomium is highly variable in degree of specialization. 
Among the Oligochaeta it is frequently a small, inconspicuous 
lobe with generalized sense organs similar to those found on 
other segments of the body, while among the Polychaeta it more 
frequently bears highly specialized tactile organs and eyes. 

The circulatory system consists of at least two main longi- 
tudinal trunks, one dorsal and the other ventral, connected by 
lateral vessels in each segment. Frequently, additional longi- 
tudinal vessels occur. Some of the lateral vessels in the anterior 
region of the body are specialized as pumping organs or hearts. 
These, with the dorsal vessel, propel the blood through the system 
by their pulsations. 


The central nervous system consists of a ventral chain with a 
ganglion in each segment. This chain, which is really composed 
of two fused cords, frequently shows its double nature in its 
ladder-like appearance, and in cross-section the arrangement of 
cells and fibers give still further evidence. In the anterior region 
of the body, the two cords separate to pass around the pharynx 
on the dorsal surface of which occurs the largest ganglion, called 
the suprapharyngeal ganglion or brain. 

Excretion is by means of metanephridia in the adults and 
protonephridia in larvae of forms which have the trochophore. 
Characteristically, each somite is provided with a pair of meta- 
nephridia the funnel or nephrostome of each of which is attached 
to the posterior septum while the tubule penetrates the septum 
and passes into the cavity of the adjacent somite before opening 
to the outside through the nephridiopore. Modified nephridial 
ducts (Fig. 47) are frequently utilized for the discharge of the 
germ cells. Primitively, separate ducts called ciliated grooves 
or coelomoducts serve for the discharge of the germ cells, but 
these frequently become associated with the nephridia, the tubules 
of which then becomes the gonoducts. 

Embryology. — Annelid eggs undergo cleavage of determinate 
type which results in the formation of a definite pattern in the 
arrangement of the blastomeres. From polar view the cells are 
arranged in the form of a cross. So characteristic is this pattern 
(Fig. 10) that the term annelidan cross has been applied to 
it. Later development results in the formation of a trochophore 
except in fresh-water and terrestrial forms. The structure and 
transformation of this larva will be described later. Asexual 
reproduction is not uncommon. In some forms, there is but 
slight specialization of the individual somites. This condition, 
known as homonomous metamerism, facilitates asexual repro- 
duction and frequently leads to the formation of a chain of 
individuals through the differentiation of the metameres of a 
single worm. 


The sexes are separate in the polychaetes. Usually, the 
anterior somites are rather highly specialized to form a distinct 
head bearing eyes and tactile organs. The somites have out- 
growths from their lateral margins which are known as parapodia 
and serve as rudimentary appendages. Each parapodium is sup- 



ported by numerous bristles or setae which are arranged in two 
bundles. The lobe of the parapodium surrounding the ventral 
bundle of setae is called the neuropodium, while that surrounding 
the dorsal bundle is the notopodium. In Nereis, the notopodia 
are supplied with numerous blood vessels and function as gills. 
Among other polychaetes, gills are frequently developed as long 
filamentous outgrowths, either along the sides of the body or 
restricted to certain areas. Each lobe of the parapodium 
frequently bears a fleshy sensory pro- 
jection termed the cirrus. In some 
instances, dorsal scales covering the 
back of the worm represent modified 
dorsal cirri. 

Dimorphism. — Among the poly- 
chaetes, two different types of indivi- 
duals are frequently encountered in the 
same species, the atoke or sexless and 
the epitoke or sexual individual. Before 
their relationships were understood, 
these phases were frequently considered 
as distinct species and even as different 
genera. In some instances, the atoke 
(Fig. 75) has the power of budding to 
produce sexual epitokes which become 
separated as free individuals. This is 
the condition found in the palola worm 
{Eunice viridis) the epitoke of which 
appears in extreme numbers in the tropical South Seas and is 
relished by the Samoans as a food. The atoke remains in the 
corals at the bottom of the sea where it proceeds to regenerate a 
new posterior end. 

In some of the common species of marine annelids there are 
striking differences in appearance of the immature worms and 
those filled with mature germ cells. These differences are so 
conspicuous that before they were understood, mature and 
immature worms of the same species were ascribed to different 
genera. In some species of the common clam worm (Nereis) the 
peculiarly modified mature specimens were given the generic 
name Heteronereis. When the young worms approach maturity, 
the segments in the posterior half of the body become storehouses 
for the germ cells, taking on an entirely different appearance 

Fig. 75. — A Palola worm, 

Eunice viridis (Gray), show- 
ing differentiation of body 
into an enlarged atoke and 
a posterior epitoke. (After 
W oodworth) . 



from those of the front half of the same worm. Thus an anterior 
atoke and a posterior epitoke are recognizable in the body of the 
mature Nereis, though the two regions never become separated 
as in the palola worm. The most conspicuous of the changes in 
the epitoke region concern the parapodia. These increase in 
size while the original setae are shoved out and are replaced by 
new ones of entirely different shape and arrangement. The 

lobes of the parapodia develop 
new outgrowths which render 
them more effective swim- 
ming organs. Even though 
the parapodia of the immature 
Nereis seem to be admirably 
fitted for swimming, the 
worm rarely becomes pelagic 
except at the onset of sexual 

Development. — Practically 
all polychaetes are marine 
and in development pass 
through the trochophore 
stage. In organization, the 
polychaetes represent a more 
primitive condition than the 

Fig. 76. — The development of Poly- oligochaetes. Though the 

gordius. A, young trochophore; fi.elonga- ^y ip-^pViaptPK flrp frpoupntlv 

tion of posterior cone; c, D, stages in oDiigocnactes are irequentiy 

transformation of trochophore; £, anterior simpler in strUCturC, their 
extremity of young Polygordius following „;^„14„;^^ :„ ^.y^p r-pmilt of 
metamorphosis. {After Fraipont). Simplicity IS tne reSUlI OI 

degeneracy, for it seems prob- 
able that in becoming adapted to life in fresh water or to the 
terrestrial existence they have departed from the generalized 
structure and mode of development which characterize the more 
primitive members of the class. 

The trochophore (Fig. 76, A) is typically pear-shaped or in 
the form of two cones joined by their bases, one dorsal and the 
other ventral in position. Surface ciliation of this larva may 
follow any one of a number of patterns. A tuft of cilia at the 
apical plate and a preoral circle near the equator of the larva 
are the most constant, but other bands may also appear. The 
digestive system consists of a mesenteron which communicates 
with the mouth opening through an esophagus and with a 


terminal anus through an intestine. The region between the 
digestive tube and the outer body wall is filled with a gelatinous 
substance through which run strands of muscle and nerve and 
the tubules of the protonephridial system. Near the posterior 
extremity of the early trochophore, there usually occur a pair 
of cells called the teloblasts which are the forerunners of the 
mesoderm. As the larva elongates, these teloblasts continue 
to divide, forming bands of mesoderm cells on either side of the 
digestive tract. With the elongation of the posterior cone of the 
trochophore in the transformation of the larva into the adult 
worm, these mesoderm bands become divided into primitive 
segments (Fig. 76 C) within which the coelomic cavities later 
make their appearance. The segments of the young worm thus 
formed are usually provided with provisional setae which are 
later thrown off and replaced by 
the permanent setae. 

Both free-living and sessile 
polychaetes are found. Modifi- 
cations of body form associated 
with these differences in habits 
have furnished a basis for the 
separation of two orders. 

Free Living. — In the order 
Errantia are included all of the 
free-swimming polychaetes with a 
well-developed head but with all 
the remaining segments practi- 
cally homonomous, bearing para- 
podia of approximately uniform 
character. The pharynx is ever- 

,•11 -1 e ii u ■ c Fig. 77. — Lateral view of a ser- 

tible and frequently bears a pair of ^^^.^ ^^^^ Spirorbis. removed from 

formidable jaws which are used in its shell. The ovoid bodies within 
, • AT • iU 1 the club-shaped operculum are 

capturmg prey. Nereis, the clam- d e v e i o p i n g e m b r y o s. (After 
worm, is one of the commonest ciaparede). 
examples of this order. Aphro- 

dita has the dorsal surface so covered with bristles as to war- 
rant the common name sea mouse. Lepidonotus bears twelve 
pairs of broad overlapping scales on its back. Syllis reproduces 
by means of lateral clusters of buds, and Autolytus undergoes 
asexual reproduction through the formation of buds at the 
posterior extremity. 


Tube Dwellers. — Members of the order Sedentaria are tube 
dwellers. Some live in burrows in the sand (Arenicola), others 
form membranous tubes (Myxicola, Manyunckia), and still 
others live in calcified tubes within which they withdraw (Hy- 
droides, Spirorbis) and close the opening with a modified tentacle, 
the operculum. The Sedentaria lack the jaws characteristic 
of the free-moving Errantia and display much greater diversity 
in structure of the anterior and posterior regions of the body. 
Numerous filamentous gills and tentacles frequently adorn the 
head and anterior body somites which usually protrude from the 
tube or burrow (Fig. 77), while the parts constantly encased 
have weakly developed parapodia. Bright colors frequently 
occur on the gills and anterior region of some of the tube dwellers. 
When such forms stick their heads from their tubes, they have 
all the brilliance of full-blown flowers. 

Subclass Oligochaeta 

The oligochaetes are as characteristically terrestrial and fresh- 
water inhabitants, as are the polychaetes marine. In many 
ways, they bear evidences of degeneracy as an accompaniment 
of the change from marine to fresh-water or land habitat. Pe- 
lagic larvae and parapodia are entirely lacking in all members of 
the subclass, while gills occur in only a few forms and the sensory 
apparatus represents a very low stage of specialization. Setae 
occur in pairs, rows, or bundles but never have parapodia associ- 
ated with them. The sexes are never separate. The male and 
female gonads occur in different segments. In the Naididae 
and Aeolosomatidae, asexual as well as sexual reproduction 

Near the anterior end of the worm, usually not far removed 
from the openings of gonoducts, the body wall of a number of 
somites is supplied with numerous glands which in the height of 
sexual development form a thickened collar-like band over the 
dorsal and lateral surfaces of the body known as the "clitellum." 
This clitellum produces secretions which harden to form a capsule 
or cocoon for containing the eggs after they are laid. In the 
earthworms, fertilization is reciprocal. During copulation (Fig. 
78) a spermiducal pore of each individual is opposite the opening 
of a receptaculum seminis of the other so that sperm cells of each 
individual pass into the receptaculum of the other, thereby 
accomplishing cross-fertilization. 



Various systems of subdividing the Oligochaeta have been 
proposed, most of which have as a basis either the habits or 
structural characters correlated with aquatic or terrestrial habits. 

Members of the order Microdrili are small oligochaetes of 
relatively few segments and usually aquatic in habits. Eyespots 
are frequently present. 

In the order Megadrili are found the larger oligochaetes, which 
are commonly known as earthworms. The bodies of these con- 

FiG. 78.- 

-Flashlight photograph of earthworms in copulation. 
by Alvin R. Cahn), 

{Original photo 

tain numerous segments (Moniligaster, Perichaeta, Microscolex, 
Diplocardia, Helodrilus, Lumbricus). 

Class Hirudinea 

The Hirudinea, or leeches, are annehds with a fixed number of 
somites (generally 34), but superficially each somite is subdivided 
by constrictions (Fig. 79) into a number of annulations, so the 
number of external rings does not correspond to the number of 
internal divisions of the coelom. The coelom is very greatly 
reduced but the small pouches furnish the basis for the determina- 
tion of the number of somites. With the exception of one genus 
(Acanthobdella) leeches have no setae. The posterior extremity 
bears a sucker on its ventral surface and usually a second sucker 
is developed around the mouth. Both of these are organs of 
attachment, and in addition the oral sucker aids in the ingestion 
of food. In addition to a graceful undulating swimming, free 
in the water, leeches may also progress by fixing the anterior 



sucker to some object, then drawing the body into a loop and 
fixing the posterior sucker near the same spot. 

Shght development of the coelom and the dorsoventral flat- 
tening of the body give leeches a distinct flatworm appearance. 
Segmentation, presence of a coelom, development of a chtellum, 

Fig. 79. — Organization of a leech, Placobdella parasitica. I-XXVII, somites; 
phg, pharyngeal glands; oe, esophagus; s, atrium or spermatophore sac; v.s, 
seminal vesicle; t, testes; ov, ovary. C Redrawn from Whitman in Ward and 
Whipple's Fresh-water Biology and reprinted by permission of John Wiley and Sons, 
Inc.) . 

presence of a distinctly annelid type of nervous system, and 
general organization, however, demonstrate close relationships 
with the Oligochaeta. Michaelsen proposes a group Clitellata 
to include the Oligochaeta and Hirudinea as two coordinate 


groups. This same author suggests that leeches may be oUgo- 
chaetes modified for predatory habits. 

The digestive system does not He loosely in a spacious coelom as 
does that of the Chaetopoda, for the coelom is largely encroached 
upon by a parenchyma which leaves a system of blood-filled 
sinuses in addition to the definite circulatory system. The 
terminal buccal chamber opens into a muscular pharynx which, 
in turn, leads by way of an esophagus into a stomach or crop. 
This latter may be either a straight tube or may give rise to 
from one to twenty pairs of lateral diverticula before emptying 
into the intestine. A small rectum leads to the anus on the dorsal 
surface near the posterior extremity. 

A met anephri dial system similar to that found in the Oligo- 
chaeta serves for excretion. Usually, not more than seventeen 
pairs of nephridia occur, for they are lacking from the somites of 
both extremities and from some of the clitellar somites. The 
central nervous system consists of a brain and a ventral chain of 
ganglia of which there are frequently twenty-three. 

Leeches are hermaphroditic. Fertilization occurs either by 
reciprocal copulation or through the implantation of spermato- 
phores on the skin. Spermatozoa escape from these and pene- 
trate the tissues to the ovarian sacs where fertilization occurs. 
Development from the egg is direct. In many leeches a cocoon 
is formed by the clitellum for sheltering the eggs and young 
as in the oligochaetes. In some species, however, the eggs are 
carried on the ventral surface of the parent's body and even the 
young remain attached there for some time after hatching. 

Hirudinea get their vernacular name of bloodsuckers from 
the fact that many species are permanent or temporary ecto- 
parasites on the bodies of other animals. Many species, and 
especially the young, are predaceous in habit, feeding upon other 
small organisms, and resort to the parasitic habit only when 
opportunity is offered. Most leeches are inhabitants of fresh 
water, though some are marine and a few are terrestrial. Those 
which feed upon blood have either three jaws supplied with sharp 
teeth (Gnathobdellida) which lance the skin of the host or a 
conical proboscis evertible from the pharynx (Rhynchobdellida) 
for piercing. Glandular secretions from the leech hinder coagu- 
lation of the blood and render its wounds difficult to staunch. 

Two orders are commonly recognized, the Rhynchobdellida 
and the Gnathobdellida. Of these, the former are jawless 


while the latter have jaws. Species of the genus Piscicola which 
live on fishes, Glossiphonia, and Placobdella are characteristic 
of the first order, while the medicinal leech {Hirudo medicinalis) 
and Macrobdella, which so commonly attacks bathers, are 
examples of the Gnathobdellida, 

Class Archiannelida 

As the name signifies, the Archiannelida seem to represent 
a primitive type of annelid organization which may have con- 
siderable significance in solving the problem of the origin of the 
higher annelids. The body, which shows only slight indica- 
tion of segmentation externally, has a coelom completely divided 
into somites. Both parapodia and setae (Fig. 76 E) are lacking. 
The nervous system is distinctly more simple than that character- 
istic of other annelids, for it remains in direct contact with the 
epidermis and shows no centralization to form ganglia. In the 
genus Protodrilus, there are two ventral nerve cords connected 
by transverse commissures, but in Polygordius there is a single 
cord. Representatives of both of these genera are exclusively 
marine. The prostomium bears a pair of tentacles and in addi- 
tion to these a pair of ciliated grooves are the only structures 
which seem to have a sensory function. 

In the development of Polygordius, a typical trochophore 
(Fig. 76) occurs the formation and metamorphosis of which have 
been worked out in great detail. 

Appendix to the Archiannelida 

Dinophilus and some other simple wormlike forms are of 
questionable systematic position. Some zoologists maintain 
that they show possible relationships with the Archiannelida, 
while others consider them as more closely related to the Trochel- 
minthes. Members of the genus Dinophilus are minute marine 
worms which live among seaweeds. The body consists of a head, 
five or six trunk segments, and a tail segment. The adult worm 
rather closely resembles the larva of marine polychaetes. 

Class Gephyrea 

The Gephyrea are marine worms which differ from the remain- 
ing annelids in the lack of segmentation, parapodia, and setae. 
Development involves a modified trochophore larva. The 


undivided coelom contains a complete digestive system the intes- 
tine of which extends posteriorly some distance, then coils back on 
itself to a dorsal anal opening toward the anterior extremity of 
the body. The extended anterior extremity of the body is pro- 
vided with tentacles or a lobed tentacular fold within which the 
mouth is located. The entire anterior region is capable of inver- 
sion within the body. 

A single pair of nephridia comprise the excretory apparatus 
and serve as gonoducts. The nervous system originates in the 
anterior extremity as a dorsal cerebral ganglion which is joined 
with the ventral longitudinal nerve cord by a pair of lateral 
branches. The ventral nerve cord bears no ganglia but gives off 
lateral branches to the body wall and to the internal organs. 

Though the sexes are usually distinct, persistent gonads 
are not present. The germ cells have their origin in masses or 
ridges of cells in the lining of the body cavity and either undergo 
full development in this location or are discharged early into the 
coelom where development is completed. 

Sipunculus and Phascolosoma are characteristic genera of 
the Gephyrea. 

Outline of Classification 

Phylum Coelhelminthes. — Coelomate worms; usually segmented. 

I. Class Chaetognathi. — Marine, pelagic; three segments; bristle 
jaws. Sagitta, Spadella, Krohnia. 

II. Class Chaetopoda. — Numerous segments with setae; ventral chain 
of ganglia; blood vascular system. 

a. Subclass Polychaeta. — Parapodia; numerous setae; marine; 
separate sexes; no clitellum; trochophore larva. 

1. Order Errantia. — Free-swimming. Nereis, Aphrodita, Lcpido- 
notus, Syllis, Autolytus, Halosydna. 

2. Order Sedentaria. — Tube dwellers. Clymenella, Arenicola . 
Myxicola, Sabellaria, Hydroides, Spirorbis, Chaetopterus, Aj'i phj ijui^r^ 

b. Subclass Oligochaeta. — Clitellum; no parapodia; few setae; not 

1. Order Microdrili. — Small; few segments; aquatic; a.sexual 
reproduction common. Tubifex, Dero, Nais, Chaetogaster, Aeolo- 
soma, Pristina, Mesenchytraexis. 

2. Order Megadrili. — Large; many segments; usually terrestrial; 
sexual. Moniligaster, Perichaeta, Helodrilus, Diplocardia, Lum- 

III. Class Hirudinea. — Typically thirty-four somites; more annulations 
than somites; one or two suckers. 

1. Order Rhynchobdellida. — No jaws; introvert at anterior end. 
Glossiphonia, Placobdella, Piscicola. 


2. Order Gnathobdellida. — Mouth large; usually jaws present; no 
proboscis. Macrobdella, Hirudo, Haemopis, Dina. 

IV. Class Archiannelida. — Marine; no setae or parapodia; segmented 
internally; no ganglia in nervous system. Polygordius, Protodrilus. 

V. Class Gephyrea. — No segmentation or parapodia; trochophore; 

1. Order Inermia. — No setae; anus dorsal. Phascolosoma. 

2 Order Armata. — Few setae; anus posterior. Sipunculus, 



(See general references cited at close of Chapter I) 
CLAPERfeoE, E. 1868. "Les Annelides Chetopods du Golfe de Naples." 

Fraipont, J. 1887. Le genre Polygordius. Fauna u. Flora d. Golfes 

von Naepel, Monograph 14. 
Gerould, J. H. 1906. The Development of Phascolosoma. Zool. 

Jahrb., 23: 79-162. 
Hatschek, B. 1878. Studien iiber Entwicklungsgeschichte der Anneliden. 

Arbeiten Zool. Inst. Wien., 1 : 277-404. 
Hertwig, O. 1880. "Die Chaetognathen, Ihre Anatomie, Systematik, 

und Entwicklungsgeschichte." Jena. 
Moore, J. P. 1905. Hirudinea and Oligochaeta collected in the Great 

Lakes Region. U. S. Bur. Fish. Bull, 25: 157-171. 
Nachtrieb, H. F. Hemingway, E. E. and Moore, J. P. 1912. Leeches of 

Minnesota. Geol. and Nat. Hist. Survey of Minn., Zool. Series, V. 
Sedgwick, W. T. and Wilson, E. B. 1904. "An Introduction to General 

Biology." New York, Holt. 
Stephenson, J. 1930. 'The Oligochaeta." London, Oxford Press. 


The phylum Molluscoidea contains three classes which agree 
in the possession of a ridge called the lophophore at the anterior 
extremity of the body bearing a crown of ciliated tentacles. The 
three classes: Polyzoa, Brachiopoda, and Phoronida, present so 
many individual peculiarities that few statements may be made 
that would apply equally to the organization of the members of 
all three. Consequently, the structure and characteristics of 
the individual classes will be discussed separately. 

Fig. 80. — Diagrams to show contrast in general organization in; A, an endo- 
proct bryozoan; B, an ectoproct bryozoan. The digestive system is shown in 
solid black. Compare the position of the anal opening in the two figures. 

Class Polyzoa 

Individuals of the class Polyzoa (or Bryozoa) bear very close 
superficial resemblance to hydroid polyps from which they are 
readily distinguishable because the polyzoan zooids possess a 



complete alimentary tract terminating in an anus near the ante- 
rior extremity of the body. The position of the anal opening 
with reference to the circle of tentacles serves as a basis for the 
discrimination between the two orders (Fig. 80), which show 
extreme differences in internal structure. In the order Ecto- 
procta, the anus occurs outside the tentacular ring, while in the 
Endoprocta it is within the circle formed by the tentacles. The 
lack of a coelom in the Endoprocta renders their relationship to 
the other Polyzoa open to question. Some investigators have 
considered that the Endoprocta possibly bear relationship to 
the Rotifera. The majority of the Polyzoa are colonial. The 
individuals of a colony are bound together by an organic con- 
necting material. The individuals are in many instances cov- 
ered with a gelatinous, horny, or calcified layer forming an 
exoskeleton. Polyzoa are most abundant in salt water, though a 
number of genera appear in fresh water. 

I. Order Ectoprocta 

The Ectoprocta are colonial forms which frequently attain 
considerable size. Individuals of a colony are variously arranged 
in branching pattern (Bugula, Plumatella), in fiat mats (Crista- 
tella), as encrusting layers (Microporella, Cribrillina), or as 
soUd gelatinous masses (Pectinatella), which may attain a size 
of a foot or more in diameter. The individual zooids uniting 
to form these different types of colony assume a number of dis- 
tinctly different shapes. The lophophore is capable of retraction 
within the anterior part of the zooid through the agency of special 
retractor muscles. In addition to the action of this introvert, 
the entire zooid is frequently able to withdraw into the interior 
of the colony. In some instances, either an operculum or a series 
of lobes is drawn into the aperture to close it when the zooid is 

Modified individuals occur in many Ectoprocta. Avicularia 
are modified zooids shaped much like a bird's head the beak of 
which is capable of grasping objects and holding them until 
they disintegrate. The food fragments are then secured by the 
tentacles. Vibracularia, another type of modified individual, are 
long whiplike structures which, in their development, seem to be 
modified avicularia. 

The funiculus is a double strand of tissues which passes from 
the bend in the alimentary canal through the coelom to the 


aboral extremity of the zooid. In addition to the budding which 
gives rise to the colony formation, sexual reproduction may also 
take place. Most ectoprocts are hermaphroditic. Ovaries 
and spermaries make their appearance either in the lining of 
the coelom or in the tissues of the funiculus. The gonads dehisce 
into the coelom where fertilization takes place. In some species 
special chambers, called ovicells, are provided for containing 
the developing embryos. Following cleavage, a free-swimming 
larva of variable form in different species makes its appearance. 
This larva undergoes a transformation to form a zooid from which 
a colony later develops by budding. Statoblasts, or internal 
buds, are characteristic of many fresh-water ectoprocts. These 
are surrounded by chambers which upon drying become filled 
with air and serve to float the statoblasts. In the fresh-water 
genera, Pectinatella and Cristatella, the free larval stage has 
been retained. 

II. Order Endoprocta 

In members of this order, the anal opening occurs within the 
circle of tentacles. With the exception of one genus, Urnatella, 
the entire group is marine. The body is usually cup-shaped, 
enclosing at its open end a cavity called the vestibule (Fig. 80-4), 
which contains the mouth and the anus. The rim of this 
cavity bears the tentacles, which are capable of being withdrawn 
into the vestibule. The space between the alimentary canal and 
the body wall is filled with a gelatinous matrix. This lack of an 
undisputed coelom, together with the fact that the excretory 
system is protonephridial, furnishes ground for doubting any close 
relationship between the Endoprocta and the Ectoprocta. 
Pedicellina, Urnatella, Loxosoma are characteristic genera. 

The larva formed by the endoprocts undergoes a less con- 
spicuous transformation than that of the ectoprocts. After 
becoming fixed, it transforms into a zooid from which other indi- 
viduals arise by budding. 

Class Brachiopoda 

Because of the presence of a bivalve shell, brachiopods are 
frequently confused with molluscs. The shell of the brachiopod 
is, however, composed of a dorsal and a ventral valve (Fig. 81), 
while the valves of the bivalve molluscs are lateral. In the 
brachiopods, the valves are articulated in their posterior regions 



and from the posterior end of the animal a stalk or peduncle for 
attachment is frequently developed. The valves are lined by a 
mantle by whose action the shell is secreted. The bristles borne 
in the edge of the mantle are of the same type as those found in 
annelids and seem to point to a relationship between brachiopods 
and annelids. The ventral shell in many cases bears a short 
beaklike projection posterior to the hinge and it is through this 
that the peduncle passes. The shells so closely resemble the 
most primitive type of oil lamp that the common name "lamp 
shell" is very generally applied to shells of this group. 

Manfle lobe 

Doiiol valve 


Di'gesh've gland 





Fig. 81. — Semidiagrammatic sagittal section of a brachiopod {Magellania 
lenticularis) . {Redrawn from Parker and Haswell, with the permission of Mac- 
m.illan Co.). 

In members of the order Inarticulata (Lingula, Crania, and 
Discina), the foregoing description does not apply, for the two 
valves are similar and the hinge is wanting. In the Articulata, 
where a hinge is present, the valves are not held open by an 
elastic hinge ligament as in the Acephala of the molluscs, but 
both opening and closing of the valves are accomplished by 
muscular action. Closure of the shell is by means of a pair of 
adductor muscles which are attached to the dorsal shell but unite 
to form a single muscle before reaching their insertion on the 
ventral valve. Two pairs of divaricator muscles pass between 
the ventral valve and that part of the dorsal valve posterior to the 
hinge. By their contraction the valves are opened. From each 
valve a pair of muscles known as the adjustors pass to an insertion 
on the peduncle. It is through contraction of these last muscles 
that the animal is able to shift the position of the entire body. 


Much of the space within the shell is occupied by a pair of 
conspicuous, spirally coiled arms or lophophores. In the order 
Articulata, the dorsal valve frequently bears a calcareous loop 
which supports the lophophore. Each arm on its outer margin 
bears a longitudinal groove bordered by a row of small tentacles. 
Water currents produced by the ciha on the tentacles and in the 
groove carry food particles toward the mouth. 

The U-shaped digestive tract consists of a mouth opening in 
the middle of the lophophore, a dorsally directed gullet which 
empties into an expanded stomach, and from this a ventrally 
directed intestine which ends blindly except in members of the 
order Inarticulata. Cilia line the entire digestive tract. 

Two transverse septa divide the coelom into three somites, but 
the shortening of the chief axis of the body has been accompanied 
by a coiling of the digestive tube and consequently the arrange- 
ment of the septa is somewhat confused and difficult to observe. 
The coelomic cavities extend into the arms and the mantle 
lobes. One or two pairs of nephridia communicate with the • 
coelomic pouches and serve as both excretory and reproductive 
ducts. The gonads are borne chiefly in the coelomic cavities of 
the mantle. The sexes are usually separate. 

The brachiopods pass through a trochophore larval stage. 
During cleavage and embryonic development the eggs are held in 
brood pouches. At the time of its liberation the larva is divided 
into three segments. The fully grown larva becomes attached 
by its posterior end and from this region the peduncle develops. 
The large midregion or mantle segment of the larva increases in 
size and secretes the shell characteristic of the adult. 

Brachiopods are exclusively marine. Though represented by 
relatively few living species, they reached an extreme state of 
species formation in the Silurian and Devonian periods. Tere- 
bratulina, Terebratula, and Waldeheimia are characteristic 
modern genera of the Articulata. 

Class Phoronida 

The relations of members of the single genus Phoronis have 
been much under discussion among zoologists. Wormlike in 
form, these marine organisms dwell in membranous or leathery 
tubes. The body is long, cylindrical, and unsegmented, at one 
extremity bearing numerous ciliated tentacles arranged in the 
form of a lophophore as characteristic of members of the phylum 


Molluscoidea. Both mouth and anus occur on the extremity 
bearing the lophophore. The larva, which is known as actino- 
trocha, is a modified trochophore. 

Outline of Classification 

Phylum MoUuscoidea.^ — Triploblastic; aquatic; lophophore bearing ciUated 
tentacles; anus near anterior end. 

I. Class Polyzoa. — Zooids small; usually colonial; alimentary canal 

1. Order Ectoprocta.- — Anus outside lophophore; coelomate. 

a. Suborder Gymnolaemata. — Lophophore circular; chiefly 
marine. Crisia, Stenopora, Bugula, Membranipora, Paludicella, 

b. Suborder Phylactolaemata.^ — Lophophore horseshoe- 
shaped; fresh water. PlumateUa, Fredericella, Pedinatella, 

2. Order Endoprocta. — Anus within lophophore; no coelom. 
Loxosoma, PediccUina, UrnateUa. 

II. Class Brachiopoda. — Bivalve shell; marine; solitary; usually a 
peduncle for attachment. 

1. Order Inarticulata. — Valves of shell without hinge; shell 
chiefly organic; anus present. Lingula, Crania, Discina, Glotiidia. 

2. Order Articulata. — Valves of shell hinged; shell limy; no anus. 
Terebr alula, Waldheimia, Magellania, Laqueus. 

III. Class Phoronida. — Marine; wormlike; in cylindrical sand tube. 


(See general references cited at the close of Chapter I) 

Allmann, G. J. 1856. "Monograph of the Fresh-water Polyzoa." 
London. Ray Society. 

Davenport, C. B. 1890. CristateUa: The Origin and Development of the 
Individual in the Colony. Bull. Mus. Comp. Zool, Harvard, 20: 

■ . 1893. On Urnatella gracilis. Bull. Mus. Comp. Zool, Har- 
vard, 24: 1-44. 

Morse, E. S. 1902. Observations on Living Brachiopods. Mem. 
Boston Soc. Nat. His., 5. 

Osburn, R. C. 1910. The Bryozoa of the Woods Hole Region. U. S. 
Bur. Fish. Bull, 30: 203-266. 



The Echinoderma are radially symmetrical, coelomate animals 
with usually a subdermal skeleton of calcareous plates and a 
vascular system known as the ambulacral system, used chiefly in 
locomotion. All representatives of this phylum are marine in 
habits. Here are included the starfishes, serpert stars, sea 
urchins, sand dollars, sea cucumbers, sea lilies, and some groups 
which are known only from fossil remains. Of the echinoderms 
living at the present time, five distinct types of organization 
are represented, hence there are five recent classes in the phylum. 

A B 

Fig. 82. — Diagrams to represent the formation of the rudiments of the coelom 
and of the water-vascular system in an echinoderm as viewed from the dorsal 
surface. Left hydrocoel rudiment in solid black; right stippled. A, constriction 
of hydrocoel rudiments from coelom; B, left hydrocoel acquires communication 
with exterior through stone canal; C, left hydrocoel encircles stomodaeum and buds 
off rudiments of five radial canals, coelomic pouches increase in size and surround 
the stomach. (Orig.). 

The radial symmetry is not perfect, for some structures are 
distinctly bilateral in their arrangement. In the classification 
of Cuvier, the echinoderms were included along with the coelen- 
terates within the group Radiata. The superficial resemblance 
in arrangement of parts seems to be an adaptation to the sessile 
habit, developed independently in these two groups, and does 
not indicate any phylogenetic relationship. Radial symmetry of 
Coelenterata is primitive, while that of the Echinoderma is 
only secondarily derived from a bilateral condition, as is evi- 
denced by the marked bilaterality of the larvae. Details of the 



transformation from the one type of symmetry to the other will 
be discussed in detail. 

Development of the Water -vascular System. — During early 
larval development, an echinoderm is distinctly bilateral in 
form (Fig. 84). Radial arrangement of the parts characteristic 
of the adult makes its appearance only following the formation 
of the mesoderm. Mesothelial sacs (Fig. 82) are formed as lateral 
outpocketings of the entoderm just as in embryos of many other 
groups. Both the right and the left mesoderm pouches undergo 
a constriction (A) which separates each into an anterior and a 
posterior sac. The posterior sacs continue to increase in size 
and ultimately form (C) the right and left coelomic cavities. 
The anterior sacs are the rudiments of the water-vascular system 
and are termed the hydrocoel sacs. The left hydrocoel acquires 
communication with the body wall through a tubular outgrowth 
(B), which becomes the stone canal of the water-vascular system. 
The right hydrocoel fails to develop but remains vestigial and 
finally disappears. The entire water- vascular system is thus 
formed from the left hydrocoel. As it increases in size, it encircles 
the esophagus of the larva and becomes the ring canal of the 
vascular system. Five radial pouches extend outward from this 
ring canal and as they increase in length they become recogniz- 
able as the radial vessels one of which passes along each arm of 
the adult. The water-vascular system thus has its origin from 
one of the mesodermal pouches of a bilaterally symmetrical larva 
which in later development assumes a radial arrangement of its 

The skeletal system is one of the most characteristic features 
of the echinoderms. Though details of arrangement and extent 
of development are highly variable, certain of the plates are 
fairly constant in their fundamental relations throughout a num- 
ber of classes. The skeletal plates have their origin in the 
mesoderm and lie near the surface of the body directly beneath 
the outer body covering. Spines, frequently associated with 
these plates, suggest the meaning of the name Echinoderma 
which is Latin for spiny skin. In some instances (Echinoidea), 
the plates are rigidly articulated to form a continuous shell or 
test, unchangeable in form, within which most of the organs lie. 
In other classes (Asteroidea, Ophiuroidea, and Crinoidea), at least 
some of the plates are movable and permit of some flexibility in 
the parts which they cover. Among the Holothuroidea, the 


skeletal plates are so poorly developed that their presence in the 
soft body wall is restricted to minute discs and anchor-shaped 
bodies scattered through the tissues. 

Other Organ Systems. — The locomotor and skeletal systems 
described above are the most distinctive of the systems found in 
echinoderms. There are no highly specialized organs for excre- 
tion, no definite circulatory system, and the nervous system 
represents a very low order of specialization. Excretion is 
accomplished through the action of amoebocytes in the coelomic 
fluid, and excretory products are liberated through gills variously 
located on or in the bodies of the several classes. 

Like all higher Metazoa, the nervous system has its origin 
in the skin of the embryo, but in echinoderms it remains perma- 
nently associated with the skin and never migrates to an internal 
position affording greater protection. The nerve ring and its 
branches are of lowly organization, for there is no concentration 
of nerve cells to form a brain, and even ganglia are lacking. 
Echinoderms are well supplied with sense organs but most of them 
are of generalized type. Tactile organs are extremely diversified, 
including both ordinary and modified tube-feet and certain 
kinds of spines. Pedicellariae are highly sensory in addition to 
their function of defense. Eyes are found commonly in only the 
starfishes and the sea urchins. Other sensory organs will be 
mentioned under the various classes. 

The tubular digestive system, with both mouth and anus, 
displays great diversity in the various classes. 

The sexes are separate. The gonads are single in the sea 
cucumbers, but in other classes there are five gonads, each with 
its duct but with no other accessory organs, for the germ cells 
are liberated into the water for fertilization and subsequent 

Researches on Echinoderm Eggs. — An enumeration of the 
researches that have been conducted upon the eggs of echino- 
derms would call for practically a complete history of the develop- 
ment of the fields of embryology and cytology. Starting with the 
pioneer descriptive embryology by Alexander Agassiz, we find 
some of the most detailed and most superbly illustrated studies on 
cleavage and on the larval forms that have been made for any 
group of animals. Hans Driesch used sea urchin eggs to test 
the powers of isolated cells of the embryo in the formation of a 
whole larva. Working on the eggs and sperms of starfish and 



sea urchins, Frank Lillie developed an explanation of why the 
sperm cell moves toward the egg cell. A specific substance 
called fertilizin is given off by the eggs into the water and serves 
as a chemical stimulus to direct the sperm cells to the eggs. The 
nature of the ultimate structure of protoplasm and the role of 
membranes in fertilization and in other life processes have been 
largely conducted upon echinoderm eggs. Jacques Loeb used 
sea urchin eggs in his carefully devised experiments on partheno- 
genesis to determine the factors that induce eggs to undergo 
cleavage without fertilization. These are only samples of the 
hundreds of researches in which the germ cells of echinoderms 
have played an important part in developing our ideas of living 

Orientation. — The main axis of the body extends between the 
oral and aboral poles. This arbitrary morphological orientation 
is frequently not in accord with the natural or physiological orien- 
tation as determined by the natural position of the body of the 
living animal. In Asteroidea, Ophiuroidea, and Echinoidea 

the oral surface is ventral 
in position, while in the 
Crinoidea the mouth is 
directed upward, and in 
the Holothuroidea the 
chief axis is parallel to 
the surface on which the 
animal rests. Typically, 
the parts of the body are 
arranged in radial manner 
about the main axis. In 
most echinoderms, the 
ambulacral system occu- 
pies certain radial regions 
of the body where the 
skeletal plates are perforated for the tube feet. These regions 
are designated the ambulacral areas, while the plates between 
the ambulacral areas constitute the interambulacral areas. 
Many of the echinoderms have on the oral and aboral surfaces 
series of plates which seem to be constant enough in their 
appearance in the various groups to be considered homologous. 
The crinoids and the extinct Mastoids and cystoids are seden- 
tary. Coi related with this habit, they possess a stalk of attach- 

FiG. 83.— Starfish devouring an oyster. 
(From Linville, Kelly, and Van Cleave, 
General Zoology). 


ment which is a prominent morphological character in these 
groups. Recognition of this character is expressed in the estab- 
lishing of two subphyla: the Eleutherozoa, which lack a stalk, and 
the Pelmatozoa, which characteristically carry a stalk. 

Economic Importance. — Starfishes are among the worst 
enemies of the oysters (Fig. 83) and cause great losses in that 
industry. IXIost of the other echinoderms have little direct 
economic importance. They feed chiefly on seaweeds, and in 
turn some species serve as food for fishes. 


The echinoderms which are devoid of a stalk and are con- 
sequently capable of free locomotion have been assembled 
within the subphylum Eleutherozoa. The common starfishes 
(Asteroidea), the brittle stars and serpent stars (Ophiuroidea), 
the sea urchins and sand dollars (Echinoidea), and the sea 
cucumbers (Holothuroidea) are the classes included within this 

Class Asteroidea 

The Asteroidea are the common starfishes. In these, the body 
is composed of a central disc from which usually five arms radiate. 
Arms may occur in multiples of five, though a few species fail to 
adhere to the pentagonal form. In the species which have more 
than five rays the extra rays are commonly added after the larva 
has metamorphosed to a five-rayed condition. New rays continue 
to bud off between the older ones until more than twenty rays are 
found in full-grown individuals of a number of species. A sharp 
differentiation of arms and disc is wanting in the cushion stars. 
The coclomic cavity of the disc is continued into the rays and 
many of the viscera thus extend into the arms. The mouth occurs 
on the ventral surface, in the center of a membrane called the 
peristome. Along the oral surface of each ray extend the tube 
feet, which are confined to a depression called the ambulacra! 

Starfishes are found in all the oceans and at all depths. They 
dwell on the bottom, though they frequently crawl onto sub- 
merged piles and wharves. Many species live wholly in deep 
water, while others are readily observed in the tide pools. 

Arrangement of Plates. — Each skeletal plate of the ambulacra! 
grooves is provided with notches in the margins which articulate 


with other ambulacral plates. The notches in adjacent plates 
coincide, so the opening for each ambulacrum is between two 
plates. The ambulacral pores thus formed are in parallel rows. 
Distally, each double row of ambulacral plates terminates in a 
single ocular plate. This is so named because of the sense organ 
which it bears. The rigid body of a dried or preserved specimen 
gives little idea of the powers of movement possessed by the 
arms of the living starfish. The plates are articulated and their 
movements are controlled by body muscles in a manner which 
permits of considerable flexibility and freedom of movement 
in the arms. Laterally, yet on the oral surface, the ambulacral 
plates are bordered by a row of interambulacral plates which 
usually bear movable spines. A series of less regularly arranged 
adambulacral plates edges each row of interambulacrals. 

The aboral surface of each arm is made up of a series of plates 
of considerably variable arrangement in different members of the 
group. The disc of the Asteroidea lacks the regularity in 
arrangement of its plates so characteristic of some other classes of 
echinoderms. The madreporite is the only conspicuous plate 
which is constant in position, and even this is variable in some 
species, for in these more than one madreporite occurs. Both the 
disc and the rays are typically covered with scattered spines. 
The areas between these spines are ciliated and bear numerous, 
small, hollow filaments, the branchiae or gills, which are direct 
continuations of the coelom. Through the thin walls of these 
gills the body fluids are able to carry on the respiratory process. 

Pedicellariae. — Surrounding the body spines and scattered 
over the general surface of the body, there are frequently minute 
pincher-like organs called pedicellariae. Each of these is a small 
calcareous organ at the end of a strongly muscular stalk. In 
various species, these differ in form though two types are 
commonly found. In one type, the two jaws of the pinchers are 
articulated at their bases with a separate small calcareous body, 
while in another type the two jaws cross each other at their 
bases and continue beyond the crossing in handle-like processes 
where the muscles for operating the jaws are attached. Pedicel- 
lariae serve to remove small foreign bodies from the skin of the 
starfish and probably also serve as protection against small 
organisms which might attack the body of the starfish. 

Plates of the Disc. — The five genital plates, which are located 
interradially on the dorsal surface of the disc, are perforated by 


the external openings of the gonoducts. The five pairs of gonads 
He within the eoelom at the bases of the arms. One genital plate 
is in most instances enlarged and perforated by numerous pores 
which serve for the entrance of water into the water-vascular 
system. In some instances, this sieve or madreporite is not 
single but occurs as two or more plates. 

S3niimetry. — In the typical condition of a single madreporite, 
the eccentric position of this organ is the most conspicuous 
external evidence of deviation from the radial type of symmetry. 
Since a plane passing through the madreporite and through the 
arm on the opposite side of the disc bisects the body, the animal 
is in reality bilaterally symmetrical. For convenience of refer- 
ence, the arm opposite the madreporite is called the anterior arm 
or ray. This, and the two adjacent rays, constitute the tri- 
vium, while the two remaining rays, between which the madre- 
porite is located, comprise the bivium. 

The digestive system opens to the exterior through the ventral 
mouth. Small objects are ingested through the mouth; but 
because of the small size of the peristome, large objects cannot 
be taken into the body. Mussels and oysters (Fig. 83), which 
serve as food for the starfish, are digested outside the body 
through the peculiar provision which admits of the starfish 
everting the stomach through the mouth opening. The everted 
stomach surrounds large food masses and, after digesting them, 
is again drawn through the mouth opening into the body. A 
rather conspicuous constriction divides the stomach into dorsal 
and ventral chambers. The mouth opens directly into the 
ventral or cardiac chamber of the stomach while dorsal to this 
lies the pyloric chamber. From the cardiac chamber, a gastric 
pouch extends into each ray. A pair of hepatic ceca, occupying 
much of the space within each ray, communicate by a common 
duct with the pyloric chamber near the base of each arm. An 
intestine of minute size leads from the pyloric chamber to the 
aboral surface of the disc where it either opens through an eccen- 
tric anus or ends blindly. Small, branched ceca are given off 
from the intestine in some starfishes. The fact that these 
ceca undergo rhythmic pulsations suggests the possibility that 
they may have a function as respiratory organs. 

The water -vascular system is a series of tubes or canals of 
which the main parts comprise a ring canal surrounding the 
esophagus and a series of radial vessels given off from this, one 


to each arm. The ring canal communicates with the exterior 
by way of a stone canal which opens through the madreporite. 
In many Asteroidea, there is but a single madreporite and stone 
canal, but in some two or more of these structures are present. 
This latter condition is frequently associated with the powers of 
asexual reproduction. Small tufts of tubules called Tiedemann's 
bodies, interradial in position, are connected with the ring canal. 
Within these organs the amoeboid lymph cells which occur in 
the water-vascular system are formed. In addition, long- 
stalked vesicles called Polian vesicles jointhe ring canal interradi- 
ally in some forms of Asteroidea. To these, also, have been 
ascribed the function of lymph glands. 

The longitudinal canal in each arm passes along the median 
line of the ventral surface just outside (that is, ventral to) the 
ambulacral plates. At the tip of each ray, the ambulacral 
canal ends in a single tactile organ. Most Asteroidea have four 
longitudinal rows of tube-feet in the ambulacral area of each 
arm, but in some (e.g., Henricia) there are but two longitudinal 
rows. By opposite lateral branching, canals are given off along 
the course of the longitudinal canal and communicate with the 
tube-feet. In the typical condition of four longitudinal rows of 
feet, the two lateral branches arising from the same level on the 
longitudinal canal are of unequal length. This condition usually 
alternates in adjacent pairs, so on each side of the longitudinal 
canal a long and a short transverse canal alternate. Thus the 
tube-feet fall into two parallel rows on each side of the longitudi- 
nal canal. Each ambulacrum is a muscular tube which, at its 
inner end, bears a muscular sac called an ampulla. By contrac- 
tion of the walls of the ampulla, the fluid in the tube-foot is put 
under compression. Relaxation of the muscles in the foot per- 
mits the tube to elongate greatly. At the same time, the hydro- 
static pressure causes a cuplike disc at the end of the foot to 
flatten, and in case the disc comes in contact with some object it 
adheres to it. Shortening of the tube is accomplished by allowing 
the fluid to turn back into the ampulla. When several feet 
become attached to an object, by their concerted contraction 
they drag the whole body of the starfish, and in this manner 
locomotion is accomplished. 

The Sexes and Reproduction. — Though starfishes are of 
separate sexes, there are no external features which distinguish 
the sexes. Some species have been found to reach maturity in 



less than a year. The gonads increase in size with the approach 
of the breeding season. Mature eggs are discharged directly into 
the water. Sperm cells, discharged by sexually mature males, 
are likewise set free in the water, where fertilization takes place. 
Total, equal cleavage gives rise to a free-swimming ciliated 
blastula. Following gastrulation, the rounded body changes 
form (Fig. 84 A-D) and becomes known as the bipinnaria with a 
number of ciliated lobes. In later development, the bipinnaria 

Fig. 84. — Development of starfish larvae. .4, early larva, oral view; B, the 
same, lateral view; C, more advanced larva, oral view showing extent of ciliated 
bands; D, bipinnaria larva; E, larva in brachiolarian stage. {Redrawn from A, 
Agassiz) . 

becomes further modified in form and is known as a brachiolarian 
larva (Fig. 84 E). Each of these larval forms is distinctly 
bilaterally symmetrical, yet by an intricate metamorphosis the 
radial symmetry of the adult starfish is superimposed. In some 
asteroids, development has become modified, resulting in the 
elimination or reduction of the free larval stages. In some 
instances (Leptasterias hexactis), the female carries the eggs and 
embryos in a brood pouch until the fully formed young are 
able to shift for themselves. Powers of regeneration are devel- 
oped in an extreme degree among the Asteroidea. 


The nervous system consists of a circumoral nerve ring with 
a radial nerve extending along each ambulacral area. The 
system is peculiar in that it retains a superficial location in the 
ectoderm. Minute branches are given off to the various organs. 
There are no centralized ganglia in this system but experiments 
indicate that the nerve ring serves as a coordination center. 

Musculature.— Though there are no conspicuous muscle 
bundles encountered in the dissection of a starfish, muscle tissues 
play exceedingly important roles in the structure of this animal. 
The walls of the tube-feet are largely composed of muscle. 
Minute muscles manipulate the pedicellariae. Spines are moved, 
and the skeletal plates are articulated, by muscles. The stomach 
is everted by muscular action, and special bundles of muscles 
retract it. Movements of the arms are produced by delicate 
sheets of muscles just beneath the dorsal wall of each ray. 

The common starfishes belong to the genus Asterias. Cteno- 
discus has a pentagonal body without conspicuous arms. Solas- 
ter and Pycnopodia have numerous arms. Henricia is a northern 
genus with only two rows of feet on each ray. 

Class Ophiuroidea 

The brittle stars or serpent stars have highly flexible arms 
radiating from a central circular or pentagonal disc. Though 
superficially resembhng the asteroids, they differ radically from 
them in details of organization. The ambulacral system is 
much reduced and fails to function in locomotion but serves 
rather as a series of tactile organs. Writhing movements of the 
arms produce locomotion. The ambulacral plates are withdrawn 
into the interior of each ray where they are fused together to 
form a jointed rodlike structure the units of which are called the 
vertebrae. Both internally and externally, each arm is composed 
of a large number of similar segments. In the basket stars, the 
arms become finely branched. The digestive system is confined 
to the disc and lacks an anus. Bursae are thin-walled sacs, 
leading inward from the ventral surface of the disc, which serve 
for respiration and into which the gonads open. 

On the oral surface of the disc, five interradial groups of plates 
project in toward the mouth to form the jaws which are operated 
by muscles for masticating food or for selecting food particles. 
At the base of each jaw there are usually three plates, a large 


oral shield and two smaller adoral shields. One of the oral shields 
becomes modified to form the madreporite. 

Development involves a bilaterally symmetrical larva (Fig. 
85) known as the pluteus or, better, the ophiopluteus. 

Ophiura, Ophiopholus, and Amphioplus are typical genera of 
brittle stars. Astrophyton is the basket star with its finely 
divided arms. 

Class Echinoidea 

The sea urchins and sand dollars are usually globular, hemi- 
spherical, or disc-shaped. The shape, which is unalterable in any 
given species, is determined by the arrangement of the skeletal 
plates. These are immovably united to form a firm shell or test. 

Fig. 85. — Ventral view of young ophiopluteus of Ophiothrix fragilis. {Redrawn 
from MacBride, courtesy of MacmiUan Co.). 

Spines usually cover most of the test except at the oral and aboral 
poles. Surrounding the mouth, there is a circular opening where 
the plates are replaced by a membrane termed the peristome. 
Normally, the anus occurs at the pole opposite the mouth in a 
region called the periproct, while in some instances it occurs on 
the margin of the disc. The skeletal plates are arranged in 
meridional bands part of which bear openings through which the 
ambulacral feet protrude and are therefore termed ambulacral 
areas. The non-perforated plates between two adjacent ambu- 
lacral areas are designated as an interambulacral area. 

Each ambulacral area terminates at the periproct in a single 
ocular plate homologous to the ocular plate at the end of each 
arm in the Asteroidea. A series of genital plates alternate with 
the ocular plates around the periproct, and each marks the 
termination of an interambulacral area. One of the genital 
plates is modified to serve as a madreporite. 

In its fundamental arrangement, the water-vascular system is 
essentially like that described for the asteroids. From the 
madreporite, the stone canal leads into the circumesophageal 


vessel. Interradially, five Polian vesicles communicate with 
the circular canal and a radial vessel passes along each ambulacral 
area on the inner surface of the test. In the ambulacral plates, 
there are two perforations for each tube-foot. Through one of 
these, the lateral branch of the radial canal passes to the tube-foot 
and through the other the foot is in communication with its 
ampulla. In many forms, the ambulacra are aided in the loco- 
motor process by the highly developed mobile spines which are 
articulated with the surface of the test and are operated by special 

One of the most characteristic structures of the echinoid is 
the Aristotle's lantern. From the oral surface of the animal, the 
five teeth with which this organ is supplied are visible in the 
center of the peristome. The main part of the lantern lies within 
the body cavity. The teeth are at the tips of a set of jaws which 
are operated by muscle bundles attached to a calcareous frame- 
work of intricate pattern surrounding the mouth cavity. From 
the aboral surface of the lantern is given off a short esophagus. 
This in turn leads into the stomach, a part of which is greatly 
dilated and flattened and extends almost around the body. The 
intestine bends backward in the opposite direction from that of 
the course of the stomach and in the case of the urchins passes 
to a median dorsal anus, while in the sand dollars it passes along 
the posterior interambulacrum to an anal opening on or near the 
margin of the disc. The siphon occurs as a branch from the 
esophagus which parallels the course of the stomach for some 
distance, then reunites with it. It seems probable that this 
heavily ciliated tubule may have a respiratory function and may 
also be of service in washing refuse from the intestine. 

When a sea urchin is opened for dissection by removal of the 
aboral wall of the test, the gonads are usually the most conspicu- 
ous structures first encountered. These are five large masses, 
interambulacral in position, connected at the aboral pole by a 
band of tissue termed the genital rachis. From each gonad a 
gonoduct passes to the opening in the adjacent genital plate. 
The larva (Fig. 86 ^) which results from the cleavage and later 
development of the fertilized egg is termed a pluteus. Since the 
term pluteus is also applied to the larva of Ophiuroidea, the 
name echinopluteus is frequently utilized. 

Respiration is performed to a considerable extent by the water- 
vascular system. In some echinoids, only part of the tube-feet 



are ambulatory while the remaining ones lack the sucking disc 
and seem to have chiefly respiratory and tactile functions. A 
pair of branched, filamentous gills occurs on the margin of the 
peristome opposite each interambulacral area of the sea urchin. 
The siphon, as already mentioned, is also thought to aid in 

The nervous system is fundamentally the same type as that 
described for the Asteroidea, comprising a nerve ring from which 
five primary branches are given off. In addition to pedicellariae, 
there are small organs known as sphaeridia scattered over the 

s/phon ^y- ^^^ ^\^ 

Shmachf >] 



Anus nedvm 

86 B. — Anatomy of keyhole urchin, 
Mellita pentapora. {After Coe). 

Fig. 86 A. — Dorsal view of 
young echinopluteus of Echi- 
nus esculentus. {Redrawn 
from MacBride, courtesy of 
Macmillan Co.). 

body surface of many species. These are thought to function 
as organs of equilibrium. Specially modified tube-feet on the 
peristome are associated with the sense of taste. 

Arbacia, Strongylocentrotus, and Toxopneustes are genera 
of the common sea urchins. The tropical genus Clypeaster 
includes some of the largest urchins. This, with the sand dollar 
(Echinarachnius) and the keyhole urchin (Mellita, Fig. 86 B) 
represents the order Clypeastroidea. Members of the order 
Spatangoidea, of which Spatangus is an example, are usually more 
or less heart-shaped. There are several groups of echinoids 
which are known only from fossil remains. 

Class Holothuroidea 

The sea cucumbers are elongated echinoderns lacking a 
definite skeleton, with a mouth at one extremity surrounded by 



STone canct^-K 




Calcareous ring 
Retracf or muscle 

a circle of branched tentacles {Y\g. 87) and an anus at the opposite 
extremity. Since the mouth end goes forward in locomotion, 
it is frequently called the anterior extremity in the holothurians. 
Typically, the body is five sided and on each side bears a double 
row of tube-feet. In some species, the three sides constituting 
the ventral surface have the tube-feet more highly developed than 
they are on the two dorsal ambulacral areas. Some few forms 

have feet irregularly scat- 

tered over the body sur- 
face, while some burrowing 
species lack feet. The 
body wall is highly muscu- 
lar. The alternate use of 
longitudinal and circular 
muscles enables the cucum- 
ber to creep like a worm. 
Though there is no con- 
tinuous skeleton, the body 
wall is rather firm. This 
is due in large measure to 
the presence of microscopic 
calcareous plates embedded 
in the tissues. The form of 
the plates is highly variable 
in different species and they 
serve as important features 
in classification. In some 
species, a calcareous ring 
of ten plates surrounds the esophagus and serves as a support 
for the tentacles. In a few forms, Psolus for example, the 
body is encased in hard scales or plates. 

The ring canal of the ambulacral system is located around 
the esophagus just behind the tentacles. From the ring canal the 
radial canals pass to the posterior extremity of the body. In the 
first part of their course, they run anteriorly and give off branches 
to the oral tentacles which are in reality highly modified tube-feet. 
One or more Polian vesicles are frequently present. The stone 
canal, instead of opening to the exterior, bears one or more 
madreporites which open into the coelom. As in other echino- 
derms, the water-vascular system is rather closely paralleled by a 
blood-vascular system the vessels of which form extensive 



- Muscle 
Cloacaf muscles 



Fig. 87. 

-Anatomy of a holothurian, Thyone. 
{After Coe). 


anastomoses on the alimentary canal. The nervous system is 
of the type previously described for other echinoderms, but the 
quick responses given by the holothurians seem to indicate that 
the nervous and sensory organs are more highly developed than 
in other members of the phylum. 

The digestive canal is held in definite position by mesenteries. 
The esophagus passes into a stomach which is followed by a tubu- 
lar intestine. The main course of this tube is, in most species, 
posterior in the median dorsal interradius, then anterior in the 
left ventral interradius, and finally posterior in the right dorsal 
interradius to the cloaca. From the walls of the cloaca, there 
are usually a pair of minutely branched respiratory trees which, 
by the muscular action of the cloaca, are filled with water and 
serve as respiratory organs. 

A genital pore occurs in the anterior region of the body. 
This is the opening of the single, much-branched gonad. In 
development of the embryo, a larval form known as an auricularia 
is produced. Sea cucumbers have marked powers of regenera- 
tion. Individuals may automatically eject much of the internal 
organs and yet be able to regenerate them. 


The Pelmatozoa are echinoderms which, during the whole or 
at least the early stages of their existence, are fixed by a jointed, 
flexible stalk or are attached by the dorsal or aboral surface of 
the body. The principle organs are enclosed in a cup-shaped or 
spherical test, called the calyx, the walls of which contain a 
system of calcareous plates and ambulacral or food grooves 
leading to the mouth. The crinoids are the only living examples 
of this group, which also includes the extinct Cystoidea and 

Class Crinoidea 

The crinoids, or sea lilies, are usually provided with a long stalk 
or column at one end of which is attached the calyx with its 
movable arms. In numerous forms, lateral projections called 
cirri are borne along the stalk. In those instances where the stalk 
is lacking, the cirri are frequently attached directly to the base of 
the calyx. Occasionally, there are free-swimming crinoids, but in 
their development these pass through a fixed stage (Fig. 88), thus 
giving evidence that the free condition is not primitive in mem- 



bers of this class. Of present-day forms, most are restricted to 
the greater depths of the ocean. Though distinctly local in their 
distribution, they occur in great numbers, as must have also 
been the case in past geological times when they were abundant 
enough to form beds of rock of considerable thickness. The 
joints of the stems are very conspicuous in many limestone 


The calyx is usually a globular or 
cup-shaped capsule which holds the 
more important internal organs. 
This cup is formed of two or more 
circles of plates. The ring of plates 
next to the point of attachment to the 
stalk or column and extending upward 
to the projections of the arms com- 
prises the base of the calyx, within 
which there may be either a single 
circle of plates called the basals or two 
circles. In this last instance, the 
plates next to the stalk are termed 
the infrabasals, while the others are 
called the basals. A series of plates 
designated as the radials follows the 
cycle of basal plates. An arm has its 
origin with each radial plate. The 
arms are formed of a series of plates 
Fig. 88.— Fixed larva of a continuous with the radials and may 

cv'inoid, Antedon rosacea. {After i ..i ■ ■, ^ ^ ^ -r 

Carpenter). "^ either Simple or branched. In 

some of the more highly organized 
fossil forms, and in all of the recent crinoids, the arms are 
furnished with pinnules alternating on opposite sides. In these, 
the gonads are borne. Arms and pinnules are traversed on the 
ventral surface by an ambulacral groove at the bottom of which 
there is a tubular extension of the coelom. 

That part of the calyx which lies between the bases of the arms 
may be either in the form of a membrane with thin calcareous 
ossicles embedded in it or in the form of a series of plates making a 
continuous disc. In or near the center of this area, the mouth 
opening (Fig. 89) is plainly discernible, while in an eccentric 
interradial position the anus usually occurs. The grooves 
mentioned in the description of the arms continue across the 



oral disc to the mouth opening. In the case of distinctly dicho- 
tomously branched arms, there may be but a single groove 
across the disc to represent each pair of arm branches. The 
grooves are ciliated and serve as channels along which food 
particles are borne to the mouth opening. Ambulacra line the 
margin of the groove along each arm but they function as 
tactile organs for they lack suckers and ampullae. The nerv- 
ous and circulatory systems follow the course of the ambulacral 
system in muchthe same manner as already described for other 

In its normal position, the crinoid directs the oral surface and 
tentacles upward. Its position is thus the reverse of that 
normally assumed by the echinoids, asteroids, and ophiuroids. 

The full course of development is known for but a single species, 
Antedon rosacea. The germ cells are dehisced from the pinnules 
of the arms to the outside of which the eggs become attached. 
Fertilization of the heavily yolk-laden 
eggs is followed by cleavage resulting 
in the formation of a free-swimming 
ciliated larva, in which there is no 
communication between the mouth 
and the stomach. In this respect, 
the larva resembles that of some of 
the highly modified larvae of echino- 
derm groups rather than the typical 
free-living echinoderm pluteus, auri- 
cularian, or bipinnarian. Calcareous 
plates begin to make their appearance 
early in the development of the larva. 
After a few days of free-swimming arms to mouth. (After King- 

n J sley in Hertwig's Manual, court- 
existence, the larva becomes faxed esy of Henry Holt and Co.). 

(Fig. 88) and undergoes a compli- 
cated series of changes which lead to the differentiation of 
calyx and stalk. The larva in this condition is said to be in the 
Pentacrinus stage. In later development, there is consider- 
able resorption of skeletal plates characteristic of the larva. 
Ultimately, the animal becomes detached from the stalk and 
capable of independent movement. 

Fig. 89. — Oral area of crinoid 
(Antedon), showing by dotted 
lines the course of the intestine 
from mouth (m) to anus (a) ; g, 
ciliated grooves leading from 


Outline of Classification 

Phylum Echinoderma. — Triploblastie; coelomate; radially symmetrical; 
calcareous plates in skin; water-vascular system; marine. 

A. Subphylum Eleutherozoa. — Echinoderms without stalk. 

I. Class Asteroidea. — A disc with five radiating arms; arms not sharply 
set off from disc; ambulacral feet in grooves on oral surface. 

1. Order Forcipulata. — Marginal plates inconspicuous; stalked 
pedicellariae; tube-feet with sucking discs. Asterias, Pisaster, 
Evasterias, Leptastcrias, Orthasterias, Astrometis, Pycnopodia. 

2. Order Spinulosa. — Marginal plates inconspicuous; pedicellariae 
wanting or at least not stalked; tube-feet with sucking discs. 
Henricia, Solaster, Crossaster, Asterina. 

3. Order Phanerozonia. — Marginal plates large; pedicellariae usu- 
ally sessile or in pits; tube-feet pointed or with discs. Ceramaster, 
Mediaster, Dermasterias, Linckia, Astropecten, Luidia. Ctenodiscus. 

II. Class Ophiuroidea. — Arms slender, sharply set off from disc; no 
ambulacral grooves. 

1. Order Ophiurae. — Arms unbranched. Ophioderma, Ophiura, 
Ophionereis, Ophiothrix, Ophiopholis, Amphioplus. 

2. Order Euryalae. — Arms branched. Astrophyton, Gorgono- 

III. Class Echinoidea. — No arms; rigid test; globe or disc-shaped. 

1. Order Regularia. — Test nearly globular; mouth and anus in 
dorsoventral axis; large spines; Aristotle's lantern present. 
Arbacia . Strongylocentrotus, Toxopneustes. 

2. Order Clypeastroidea. — Test more or less flattened; anus at 
margin of test; spines very small; Aristotle's lantern present. 
Mellita; Clypeaster, Dendraster, Echina r achni us . 

3. Order Spatangoidea. — Test heart-shaped; mouth and anus 
eccentric; no Aristotle's lantern. Spatangus, Lovenia. 

rV. Class Holothuroidea. — No continuous skeleton; plates small, 
scattered in skin; elongate; cylindrical; mouth surrounded by a circle 
of tentacles. 

1. Order Pedata. — With tube-feet. TJuione, Cucumaria, Psolus, 

2. Order Apoda. — Without tube-feet. Synapta, Synaptula, Lepto- 

B. Subphylum Pelmatozoa. — Stalked at least during part of life. 

I. Class Crinoidea. — Temporarily or permanently attached by a stalk; 
five feathery arms radiating from a cuplike disc. Antedon, Penta- 
crinus, Rhizocrinus, Metacrinus. 

II. Class Cystoidea. — Fixed, stalked, or sessile; varying number of 
unbranched arms; wholly fossil. Lower Silurian to Carboniferous. 

III. Class Blastoidea. — Stalked; arms lacking; wholly fossil. Upper 
Silurian to Carboniferous. 

rV. Class Edriasteroidea. — Theca of irregular plates; arms unbranched 
and lying on theca; wholly fossil. Cambrian to Carboniferous. 



(See general references at close of Chapter I) 
Aqassiz:, a. 1872-1874. Revision of the Echini. Mus. Comp. Zool., 

Harvard, iLlustr. Cat. 7. 
Agassiz:, A. 1877. North American Starfishes. Mem. Mus. Comp. ZuoL, 

Harvard, Vol. 5, No. 1. 
Clark, H. L. 1901. Synopsis of North American Invertelirates, XV. 

The Holothuroidea. Amer. Nat., 35. 
. 1904. The Echinoderms of the Woods Hole Region. U. S. 

Fish Comm. Bull, 1902. 
CoE, W. R. 1912. Echinoderms of Connecticut. Conn. Gcol. and Nat. 

Hist. Survey, Bull. 19. 
Fisher, W. K. 1906. The Starfshes of the Hawaiian Islands. U. S. 

Bur. Fish. Bull, 1903: 987-1130. 
Mead, A. D. 1900. The Natural History of the Starfish. ['. ,S. Fish 

Comm. Bull, Vol. 19. 


The phylum Mollusca includes essentially bilaterally symmet- 
rical, unsegmented Metazoa in which the coelom has been 
reduced by invasion of connective tissue and of musculature 
until only the pericardial cavity and the lumen of the gonads 
represent its remains. The typical bilateral symmetry is 
frequently lost or obscured through secondary coiling or torsion 
of parts of the body. Head, visceral mass, foot, mantle, and 
shell are characteristic structures, but in many instances one or 
more of these are lacking and all are subject to great variability 
in the various groups. A rasplike organ known as the radula is 
associated with the mouth of most molluscs except the Acephala. 
This is a bandlike membrane bearing numerous cross rows of 
chitinous teeth for rasping off food material. 

A modified trochophore, termed the veliger, is a larval form 
common to all of the major groups of molluscs, though in the 
modified development of some forms it has been suppressed. 
A larval organ, known as the shell gland, is typical though not 
always retained. Reproduction is exclusively sexual. 

The usual means of locomotion is by crawling on an unpaired 
foot. Powers of locomotion are lost in some sessile forms and 
in still others swimming is made possible through modifications 
of various structures. The mantle, when present, secretes the 
shell and bounds a cavity which may either contain the gills 
or function directly as a lung. The central nervous system is 
typically composed of three pairs of ganglia of which each pair 
is associated with a special sensory organ. The cerebral ganglia 
communicate with the eyes, the pedal ganglia with the statocysts, 
and the visceral ganglia with the osphradia or olfactory organs. 

In size, Mollusca range from minute individuals barely visible 
to the eye to some of the largest known invertebrates. One of 
the giant squids reaches a length of 18 feet. 

The Mollusca have continued as important and conspicuous 
components through all the geological periods in which animal 
remains have been found. 



In detail of structure, the members of the various classes 
differ so greatly that morphology will be discussed separately 
under the headings for the five classes. 

Class Amphineura 

The Amphineura seem to represent the most primitive group 
of the Mollusca. The chitons are the typical representatives. 
They are strictly marine, living at all depths but occurring 
in special abundance in shallow water where they move freely 
over the rocks by the action of the powerful foot. The head 
region is not sharply set off. There are no tentacles and usually 
no eyes. The eight transverse plates with which the dorsal 
surface is covered overlap like the tiles on a roof. The arrange- 
ment of the plates rather strongly suggests origin from segmented 
ancestors, though present-day molluscs lack metamerism. The 
plates are so articulated that the animal may roll into a ball. 
Sometimes, they are completely covered by the mantle, but more 
commonly the mantle only partially covers the plates and extends 
beyond them along the sides of the body where it is covered 
with spines. Mantle folds on the ventral surface produce a 
series of small gills or ctenidia. Nerves penetrate the skeletal 
plates and in the outer, less dense layer of each plate there are 
frequently small sense organs called aesthetes and in some 
instances eyes. The mouth and anus are located near the 
anterior and posterior extremities of the body on the ventral 
surface. The internal organs are disposed bilaterally along the 
median plane marked off by the oral-anal axis. 

All representatives of this class are marine. The oval or 
flattened body, which is bilaterally symmetrical, may or may 
not have a differentiated head. The nervous system consists 
of a pair of cerebral ganglia connected by a circumesophageal 
ring and four longitudinal nerve cords which pass, two laterally 
and two ventrally, through the body. There is no centralization 
of the nerve cells in these cords to form ganglia. In many 
instances, the longitudinal cords are connected by numerous 

Some amphineurans are simpler in structure than the chitons. 
These wormlike forms without shells occur at fairly great 
depths of the ocean where they burrow in the mud or sand or 
are associated with various colonial coelenterates. The mantle 
covers the body completely and bears calcareous spicules instead 


of producing a definite shell. The foot is lacking and in its 
place there occurs a longitudinal, ventral, ciliated groove. The 
evidence seems to indicate that this is a degenerate rather than a 
primitive type of amphineuran. There are only a few genera of 
which Chaetoderma, Neomenia, and Dondersia are characteristic. 

Class Acephala (Pelecypoda) 

As is indicated by the name, the members of this class have 
no specialized head. Many writers on the Mollusca use the name 
Pelecypoda for the members of this class. This alternative 
name refers to the hatchet-shaped foot. The Acephala have a 
shell composed of two lateral valves. Typically the hinge 
of these valves is dorsal, though its position is modified in many 
instances. On the valves are usually found conspicuous con- 
centric lines, known as the lines of growth. The umbo, a slight 
prominence on the dorsal surface around which the lines of 
growth are distributed, is the oldest part of the shell. An 
elastic hinge ligament tends to hold the margins opposite the 
hinge gaping open. The valves of the shell are closed only by the 
contraction of adductor muscles which extend from one valve to 
the other through the body. Immediately within the protective 
shell lie the right and left lobes ( the mantle, secretions from 
which form the shell. 

While the entire animal is usually capable of withdrawing 
completely into the shell, the development of the siphon in some 
of the burrowing clams is so great that the siphons cannot be 
pulled into the shell. In the peculiar wood-boring shipworm 
(Teredo) the shells are diminutive and are modified for boring 
wood, while protection of the body which is ordinarily afforded 
by the shell is given by the wooden tunnel within which the 
worm-shaped animal dwells permanently. Submerged timbers 
and wooden ships are frequently completely destroyed by the 
tunnels of shipworms. Another marine group of peculiar habits 
is that which comprises the rock borers. In these (pholads), the 
front edge of the shell is used as a rasp to bore into solid rocks. 

The members of this class contain many forms of direct 
economic importance. The oysters, scallops, and clams are 
used as food. Shells of the fresh-water mussels and some marine 
forms are used in the manufacture of buttons. The members 
of this class play important roles in the food chains of organisms 
of both fresh water and seas. 


Members of the class Acephala show extreme range of size. 
Many species of the finger-nail shells of the genus Pisidium are 
under 2 mm. in length, while the giant Tridacna from the Indian 
and Pacific oceans reaches a length of more than a meter. 

Structure of the Shell. — The shell is covered externally by a 
thin, organic cuticula. The main bulk of the shell is composed 
of calcium carbonate. In some species, the innermost layer of 
the shell is composed of thin layers arranged parallel to the sur- 
face. These lamellae are minute enough to diffract the light and 
thereby produce an iridescence. The nacre, or mother-of-pearl, 
as this layer is termed, occurs in many fresh-water mussels and 
is especially conspicuous in the pearl oyster (Meleagrina). One 
type of pearls is formed by the deposition of nacre about foreign 
objects which have been introduced between the shell and the 
mantle. Many of the Acephala have a non-iridescent, porcela- 
neous lining of the shells. A short distance from the margin of 
the shell the mantle is joined to it by a line of muscle fibers called 
the mantle muscle. The line on the shell formed by the attach- 
ment of this muscle is termed the pallial line. 

Adductor Muscles. — In many of the Acephala there are large 
adductor muscles, one attached near the anterior and the other 
near the posterior end of the shell. In the common sea mussels 
(Mytilus) the anterior adductor is greatly reduced so that closure 
of the shell depends chiefly upon an enlarged posterior adductor. 
In the oysters and pectens there are two adductors in the larva 
of which the anterior later entirely atrophies, leaving a single 
large posterior adductor near the middle of the shell. 

The Siphons. — When the shell is nearly closed, the margins 
of the two mantle lobes are pressed together tightly except in the 
posterior region. Here the mantle lobes remain lightly separated 
to form two openings termed the siphons. Of these, the ventral 
or inhalant siphon is for the inflow of fresh water into the mantle 
chamber and over the gills, while the dorsal or exhalant siphon 
discharges water from the mantle cavity and carries the feces 
along with the water. The margins of the mantle adjacent to 
the siphons are, in some instances, fused together, thus leaving the 
siphons as permanent openings. The mantle in the region of the 
siphons frequently elongates and produces a siphon tube which 
projects beyond the shell. Both siphons may be united in a 
single tube or there may be two tubes entirely or only partially 


The Gills and Palpi. — The mantle lobes overlie the gills which 
are typically paired structures on each side of the body. In the 
most primitive Acephala the gills are featherlike structures 
similar to those found in the Gastropoda. Gills of all of the 
higher Acephala are derivable from this simple condition. As the 
flattened filaments elongate, the distal ends of the filaments 
become recurved and thus each filament becomes V-shaped. In 
the sea mussels the gill filaments may be either independent or 
united by interlocking cilia, while in the fresh-water bivalves and 
the marine clams the walls of adjoining filaments become grown 
together to form a continuous sheet. 

Cilia covering these gills bring water currents through the 
siphon into the mantle chamber thereby making respiration 
possible and at the same time bringing into the mantle cavity 
microorganisms and other organic matter which finally enter the 
mouth and serve as food. A pair of small flaplike structures, 
the labial palpi, surround the mouth opening and aid in the 
selection of food. 

The Foot. — The foot is highly characteristic in the Acephala, 
though in some instances, as in the oyster, it has become degener- 
ate. In the adult stage the oyster loses all power of locomotion, 
for the left valve of the shell becomes permanently attached to a 
rock or some other object. Among the scallops (Pecten) the 
foot is degenerate, but locomotion is accomplished by a rapid 
snapping movement of the shells produced by the single, heavy 
adductor muscle. The foot is, typically, a hatchet-shaped 
muscular organ which, by protrusion through the gaping valves 
of the shell, becomes fixed in the sand or mud, then by contrac- 
tion of the muscles the animal is drawn a slight distance for- 
ward. Posterior to the foot, there frequently occurs a byssus 
gland secretions from which form heavy silken threads by 
means of which permanent or temporary attachment is 

The visceral mass is a softer body, lying dorsal to the foot, 
within which various organs are located. As the name indicates, 
there is no head in the Acephala. The digestive system has its 
beginning in a mouth at the anterior extremity of the body, 
located between the small leaflike labial palpi. In the visceral 
mass, the digestive tube makes a number of coils and in most 
Acephala terminates in a rectum which passes through the peri- 
cardium and also through the ventricle. Gonads and liver 


occupy most of the visceral mass surrounding the ahmentary 
canal. A gelatinous rod, called the crystalline style, frequently 
occurs in the stomach. Recent investigations seem to indicate 
that this organ aids in separating food from foreign particles and 
probably contains a store of enzymes for use in the digestive 

Circulatory System. — The heart, which lies in the dorsal part 
of the visceral mass, is surrounded by a pericardium and consists of 
a single ventricle and two auricles. A system of arteries carries 
the blood from the heart to the tissues, where they frequently 
terminate in sinuses. The veins which return the blood to the 
heart pass first through the excretory organ and then through the 
gills. Separate vessels supply the mantle with blood and return 
the blood directly to the heart without passing through the 
excretory organ or gills. 

Excretory Organs. — The pair of nephridia characteristic of 
Acephala are frequently called the organs of Bo j anus after their 
discoverer. They lie immediately below the pericardium. Each 
nephridium is a wide tube, bent upon itself, one end of which 
opens into the pericardial cavity and the other, non-glandular in 
structure, serves as a urinary bladder with its opening usually on 
a minute papilla into the inner cavity of the gill chamber. 
Another gland of excretory function, called Keber's organ, lies 
anterior to the pericardium into which it discharges. 

Nervous System. — A pair of cerebropleural ganglia is located 
one on each side of the mouth near the base of the labial palpus. 
A transverse supraesophageal commissure connects this pair of 
ganglia. Two nerve cords originate in each cerebropleural 
ganglion and connect this ganglion with the two other nerve 
centers on each side of the body. One of these passes ventrally 
to communicate with the pedal ganglion, embedded in the muscles 
of the foot, and the other passes posteriorly to a region ventral 
to the posterior adductor where the parietal and viceral gangha 
of the more primitive molluscs have become fused to form a 
single posterior ganglion. 

Sense Organs. — There are but few highly specialized sensory 
organs. Statocysts are frequently found near the pedal ganglia, 
though they seem to have nerve connections with the cerebro- 
pleural ganglia. Patches of sensory epithelium called osphradia, 
and thought to have olfactory functions, are located near the 
base of the gills. The labial palpi and the edges of the mantle 



are highly sensory. The small projections from that part of the 
mantle which forms the siphons are especially sensitive to touch. 
Eyes (Fig. 90) are distributed over the margins of the mantle in 
some forms such as the scallops. 

Reproduction. — Most of the Acephala are dioecious. In 
many of the marine forms the gonads discharge their germ cells 
directly into the water where they undergo fertihzation and pass 
through cleavage and early larval development to form free- 
swimming larvae. In the fresh-water mussels, however, the eggs 

Fig. 90. 

-A section through an eye from the mantle margin of Pecten. {Slightly 
modified from Patten). 

after discharge from the ovary are passed into brood pouches 
or marsupia located within the gills. Either in the marsupia, 
or en route to them, sperm cells, brought in with the water 
currents by way of the siphon, fertilize the eggs. Each egg 
undergoes cleavage to form a larva known as a glochidium (Fig. 
91 A-D). This larval form is provided with a thin bivalve shell 
operated by a single adductor muscle. Other soft parts within 
the shell are not clearly organized except for the presence in some 
species of a "larval thread" of uncertain significance and of 
minute sensory hairs. The glochidia, when discharged from the 



gills of the mother, are unable to continue development inde- 
pendently. Almost without exception the larval mussel must 
exist for some time as a parasite (Fig. 91 £") on the gills or fins of a 
fish. During this parasitic existence the glochidia undergo 
a metamorphosis and finally leave the host (Fig. 91 /^) as juvenile 
mussels. Glochidia are frequently referred to as "hooked" or 
"hookless." In the hooked forms, the ventral margin of each 
shell bears a hinged hook. Glochidia of this type usually become 
attached to fins or scales of fishes (as in Lasmigona and Strophi- 
tus). Valves of the bookless glochidia are sometimes provided 

Fig. 91. — Development of fresh-water mussels. A, axe-head type of bookless 
glochidium of Proptera alata, anterior end view; B, lateral view of same; C, 
bookless glochidium (Ligumia suhrostrata) posterior end view; D, lateral view of 
same; E, glochidium of Actinonaias carinata encysted in gill of rock-bass; F, 
young mussel, same species as C, one week after close of parasitic life, showing 
lines of growth beyond glochidial shell and protruded foot with its cilia. {After 
Lef&vre and Curtis). 

with small non-jointed spines (Fig. 91 A) as in the axe-head type 
or may be entirely devoid of spines (Fig. 91 C and D). Most of 
the bookless glochidia become attached to the gills of fish hosts. 
The fingernail shells are hermaphroditic and differ from the 
other fresh- water Eulamellibranchia in that the young are 
born alive and do not pass through a larval stage outside the 
body of the parent. 

In the classification here adopted, the number and arrange- 
ment of the gills and of the adductor muscles serve as the chief 
characters for the recognition of orders. Some students of the 
MoUusca and paleontologists more frequently utilize a system of 
classification based largely upon hinge structure. 


Class Scaphopoda 

An external tubular shell, open at both ends and more or less 
curved, covers the body of the scaphopods. The shape of the 
shell is responsible for the common name tooth shell so fre- 
quently apphed to members of this class. Jaws and radula are 
present but in the paired liver and general arrangement of the 
nervous system the Scaphopoda resemble the Acephala. Gills 
are wanting and the rudimentary heart possesses only a ventricle. 
The foot, which is rather long and conical, extends from the larger 
opening of the shell and bears two lateral lobes. Dentalium is 
a modern genus of this class all members of which are marine. 

Class Gastropoda 

The snails, limpets, slugs, and sea hares are examples of the 
Gastropoda. Representatives of this class usually have head, 
visceral mass, foot, and mantle, though in some instances one or 
more of these may be wanting. While most gastropods live in 
the oceans, many have become adapted to life in fresh water and 
some to terrestrial existence. A shell, when present, is composed 
of a single piece. In some snails, either a limy or a horny disc 
called the operculum serves for closing the opening when the 
animal is withdrawn into the shell but is not considered as 
comparable to a second valve. The foot is usually flattened so 
that it presents a large ventral surface upon which the animal 
crawls, but in many of the pteropods the foot is modified as a fin 
used for swimming. The head, which is located anterior to the 
foot, bears hollow tentacles, eyes either at the base or tips of the 
tentacles, and a mouth. The mantle covers the dorsal surface 
of the body and bounds a spacious mantle cavity which contains 
the gills in the water-breathing forms. A siphon, through which 
water enters and leaves the branchial chamber, is frequently 
formed by an outgrowth from the edge of the mantle. The shape 
of the shell is often greatly influenced by the presence of the 
siphon. In the pulmonate snails, the mantle cavity is largely 
replaced by a highly vascular sac or lung into which air is admit- 
ted through a single opening, the spiracle. 

Gills occur in all marine snails and in some fresh-water forms. 
Lungs seem to have been developed in connection with the change 
to terrestrial habits, for land snails are the most characteristic 
pulmonates. Aquatic pulmonates such as the common genera 


Physa, Planorbis, and Lymnaea probably were derived from 
terrestrial ancestors whose descendants have again returned to 
the aquatic habitat. 

The visceral mass hes dorsal to the foot. With increase in its 
size, it frequently assumes a spiral form, and the mantle, which 
is carried with it, continues to secrete a shell at the free margin. 
As a result, the shell assumes a spiral form. The shell may be 
either simple cone-shaped or anything between this and a highly 
complex spiral. In the spiral type of shell, the coiled chamber 
usually surrounds a calcareous pillar which marks the axis of the 
shell and is termed the columella. In any given species, the 
coiling of the shell about the columella is normally in a fixed 

Fig. 92. — Diagrams to show shifting of organs in the development of the 
streptoneurous condition of gastropods. All diagrams as though viewed from 
dorsal surface. A, an orthoneurous gastropod with posterior anus, and gills, 
ganglia, and auricles arranged in perfect bilateral symmetry; B, hypothetical 
intermediate condition in torsion; C, streptoneurous condition with gill, auricle, 
and parietal ganglion of original left side now on right side of body. {After Lang) . 

direction. Most coiled shells when held with the apex point- 
ing upward and the aperture facing the observer have the 
aperture on the right side and are therefore said to be dextral. 
Some snails are characteristically sinistral {e.g., Physa). jMuscles 
from the body attached to the columella prevent the snail from 
being able to come clear out of the shell. 

Since growth takes place through the addition of new shell 
material by the mantle around the aperture, each whorl is 
successively larger than the preceding one. When a siphon is 
present, the aperture is drawn into an elongated process for 
containing it. 

The nervous system consists of two cerebral ganglia, the pairs 
of pedal and visceral ganglia, and two or three additional pairs 
all of which are united by commissures. The cerebrovisceral 


commissures may be either parallel (Fig. 92 A, orthoneurous) 
or crossed (Fig. 92 C, streptoneurous) as a result of torsion 
within the body of the animal. Alimentary canal, nephridia, 
gills, circulatory and nervous systems are all affected in their 
arrangement by this torsion, which is usually toward the right 
side. The anus may open into the mantle cavity either on the 
right side or in extreme instances may occupy a location near 
the head. In this latter instance, gills, nephridia, and other 
organs which belong primitively on the left side of the body 
become shifted in position to the right, and vice versa. 

Digestive System. — The mouth may be a mere opening on 
the ventral surface of the head or the opening at the end of a 
tubular introvert or proboscis. The buccal cavity is provided 
with an effective mechanism for breaking up food. This consists 
of a rasplike radula whose teeth tear off particles of food. In 
many instances, the radula is supplemented by a cutting or 
crushing jaw of chitinous nature. The form, number, and size 
of these radular teeth provide some of the most reliable criteria 
for separating genera and species. The esophagus leads from 
the mouth cavity to the stomach and this in turn into the 
intestine, though the divisions of the digestive tract are usually 
not sharply marked off in the convolutions within the visceral 
mass. The anal opening is rarely at the posterior end of the 
body. It is more commonly lateral or at the front end of the 
body as the consequence of shifting of position of organs due to 
torsion of the body. Various glands are associated with the diges- 
tive system. Of these the liver which extends into the smaller 
whorls of the shell and the salivary glands are the most important. 
In some of the carnivorous snails, the salivary glands secrete 
sulphuric acid. This acid aids the radula in perforating the 
shells of other molluscs. 

Respiratory Organs. — Typically there are two gills contained 
in the mantle cavity of the gastropods but this condition is 
subject to various modifications. Very commonly as an accom- 
paniment of torsion the primitively right gill degenerates, 
leaving but one gill in most of the Streptoneura. In many 
instances, true gills become replaced by other modifications for 
respiration. Among the nudibranchs portions of the body 
surface may become modified as gills. The true gills are vestigial 
in the limpets and secondary branchiae are formed as a series of 
folds between the mantle and foot. The most pronounced 


modification of the respiratory system is found in the pulmonale 
snails. Here the walls of the mantle cavity become richly 
supplied with blood vessels and the cavity becomes recognizable 
as a lung. A small opening permits air to be taken directly into 
the lung. This is the usual method of respiration in most of the 
land snails. The aquatic pulmonates seem to be closely related 
to the land forms, and members of the genera Physa, Planorbis, 
and Lymnaea have a lung similar to that found in land snails. 

Sense Organs. — The tentacles are the most conspicuous 
sensory organs. The eyes may be either at the bases of or at 
the tips of the tentacles but in some forms they are borne at the 
tips of a second pair of tentacles. 

Torsion of Body. — Modifications of the body correlated with 
torsion (Fig. 92) have in some instances resulted in loss of gill, 
nephridium, and osphradium of the primitively left side. As in 
the other Mollusca, the number of auricles is directly correlated 
with the number of respiratory organs, consequently with the 
loss of one gill the heart comprises but a single auricle and a single 
ventricle. When the lungs or gills are located in the front of 
the heart (Prosobranchia and Pulmonata), the auricles are 
anterior to the ventricle, but when placed behind the heart 
(Opisthobranchia and Pteropoda), the auricle is posterior to 
the ventricle. 

Reproduction. — The gonad is always single. In some instances, 
the sexes are separate but many genera are hermaphroditic. 
In the latter, though there is a single hermaphroditic gonad, each 
individual bears the accessory sexual organs of both sexes. There 
is great diversity in reproductive habits. In many species eggs 
are laid singly or in small groups, in others they are deposited in 
masses surrounded by a gelatinous substance, while in many of 
the marine snails complicated capsules are formed for containing 
the eggs and an albuminous fluid which serves to nourish the 
embryos. The course of development is fairly uniform through- 
out the group, involving the presence of a veliger larva. In 
terrestrial and fresh-water forms, the veliger stage is passed 
within the eggshell and not as a free larval stage. Some gastro- 
pods are viviparous. 

Subclass Prosobranchia 

In the Prosobranchia or Streptoneura are included those 
gastropods which have the cercbrovisceral commissures of 


the nervous system crossed or twisted into the form of a figure 
eight (Fig. 92 C). The sexes are separate. A shell is almost 
always present and is usually provided with an operculum. 

I. Order Aspidobranchia 

The order Aspidobranchia includes prosobranchs with but 
little concentration of the nervous system. The limpets (sub- 
order Docoglossa) include marine forms with a non-spiral shell 
and bearing either a single true ctenidium or a secondarily 
developed pallial gill beneath the mantle margin or with both 
true and secondary gills. Patella and Acmaea are representa- 
tive genera. The abalones (Haliotis) and the genera Fissurella 
and Trochus exemplify the suborder Rhipidoglossa in which 
both limpetlike and spiral shells occur. 

II. Order Ctenobranchia 

This order includes large numbers of marine, fresh-water, 
and terrestrial Streptoneura with the shell usually coiled in a 
more or less elevated spiral. The heart has but one auricle. 
Due to torsion, the primitively right gill is shifted to the left side 
of the body. Both shelled and naked forms are included within 
this order. The members of the suborder Heteropoda are free- 
swimming and pelagic marine molluscs in which the foot is 
modified to form a vertical fin. Carinaria, with its delicate, 
glassy shell, and Atlanta are examples. In the suborder Platy- 
poda the foot is flattened ventrally. Littorina, Crepidula, 
Natica, Strombus, Pleurocera, Murex, and Terebra are examples 
of this highly diversified suborder. 

Subclass Euthyneura 

The cerebrovisceral commissures are not crossed in the 
members of this subclass but form a single loop. The indi- 
viduals are hermaphroditic. The shell, which is typically spiral 
or flattened, is frequently vestigial or wanting. In the order 
Opisthobranchia are included marine forms of which the tecti- 
branchs (Aplysiidae or sea hares and pteropods) and nudi- 
branchs are examples. 

The order Pulmonata comprises chiefly terrestrial and fresh- 
water forms. The walls of the mantle cavity are modified to 
form a lung into which air is taken in respiration. Most of the 



aquatic forms depend upon periodic visits to the surface of 
the water for renewing the oxygen supply in the lung, though 
in some instances the lung has become adapted secondarily for 
water respiration. Physa, Lymnaea, Planorbis, and the limpet- 
shaped Ancylus are fresh-water pulmonates. The slugs (Limax, 
Arion, and Agriolimax) and many land snails (Polygra, Helix, 
Pupa, Bulimulus) are also pulmonate Euthyneura. 

Class Cephalopoda 

The cephalopods are the most highly organized molluscs. 
In habits, they are exclusively marine. Among present-day 
forms, they include the squids (Fig. 93 A), cuttlefishes, devilfishes 

Fig. 93. — Cephalopods. .4, ventral view of a squid, Loligo opalescens; B, dorsal 
view of Polypus bimaculatus. (After Berry). 

(Fig. 93 B), and Nautilus. When a shell is present it is almost 
always internal, for only Nautilus of our recent cephalopods 
fives within a shell. In most instances (except in Nautilus), a 
head is well defined. A pair of strong jaws, each in shape 
resembfing the beak of a parrot, are located within the mouth 
cavity. The eight or ten characteristic arms which surround 
the mouth and the funnel-shaped siphon are modifications of 


the foot. The arms are provided with heavily rimmed sucking 
discs arranged in rows. In some forms the effectiveness of these 
grasping organs is increased by the presence of a strong clawhke 
hook within the cavity of each disc. The siphon is a highly 
muscular tube through which water is drawn into the mantle 
cavity for respiration and the body wastes are forcibly expelled. 
The jet of water issuing from the siphon drives the squid or 
devilfish backward with remarkable speed and constitutes one 
of the chief means of locomotion. The devilfish can move 
forward only by using the arms in crawling, but movement of the 
fins on the sides of the body of a squid enable it to swim either 
forward or backward. A fleshy mantle encloses a cavity which 
contains the gills and at the same time forms a protective covering 
for most of the other viscera. At will many cephalopods may 
eject a cloud of ink from an ink sac. The pigment sepia is 
derived from the cuttlefish, for the ink sac which is characteristic 
of the Decapoda stores quantities of this pigment. When 
the animal is disturbed, clouds of this ink are shot from the 
mantle cavity through the siphon and serve as an effective cover 
under which the cuttlefish moves off to safety. 

Some cephalopods have unusual powers of changing color. 
Variously colored pigment cells in the skin may expand or 
contract, producing wonderful combinations. At one moment 
the entire body may be of a uniform color which may be changed 
instantaneously by a play of flashes of varied hues over the body. 
Some species of squid can produce almost every tint of the 

A pair of eyes, on the sides of the head beneath the bases of 
the tentacles, very closely resemble the eyes of vertebrates. 
In finer structure and origin, the cephalopod and vertebrate 
eyes are widely different, for despite their superficial resemblances 
the two seem to have had entirely independent origin. The 
cornea of the cephalopod eye is perforated and thereby allows 
water to enter the anterior chamber, while it is not perforated in 
the vertebrate eye. The arrangement of the sensory cells, 
the retina, is just the reverse in the two types of eye. The 
vertebrate retina is said to be inverted, for light passing through 
the eye strikes the sensory cells of the retina on the same end 
that bears attachment to the nerve endings. In contrast with 
this, the direct retina of the cephalopod eye is so organized 
that the light passing through the eye falls upon the free ends of 


the sensory cells. In Nautilus, much simpler eyes are found, 
for lens, vitreous body, cornea, and iris are wanting. 

Two kinds of hearts are present. The systemic heart is of 
typical molluscan type with its single ventricle and two auricles 
corresponding to the number of gills and receiving blood from 
them, but in addition to this heart there is a branchial heart at the 
base of each ctenidium which forces the blood through the gills. 

Cephalopods are of importance especially because of the part 
which they play in food chains. Squids and devilfish feed upon 
fishes and in turn serve as food for still larger fishes and for the 
sperm whale. They are used extensively for bait and in many 
regions as food for man. 

The sexes are separate. The spermatophores of the male are 
frequently stored in arms which become more or less modified. 
In most instances, this modification involves only sufficient 
change to adapt the arms as accessory copulatory organs but in a 
few genera the entire arm bearing the spermatophores becomes 
severed from the body of the male and acquires independent 
powers of locomotion. When first observed, these wormlike 
castaway arms were thought to be entire organisms and were 
described under the name Hectocotylus before their relationship 
to the cephalopod body was understood. The term hectocotyli- 
zation is used to indicate this type of spermatophore transfer 
through the agency of dissevered tentacles. The heavily yolk- 
laden telolecithal eggs undergo partial discoidal cleavage. The 
organs of the young cephalopod are formed from the blastoderm; 
first as flattened projections, but as these grow the young animal 
becomes recognizable, at first appended to the bulky yolk sac by 
the head end. 

Largely on the basis of the number of gills, two subclasses of 
cephalopods are recognized, the Tetrabranchia with four gills and 
the Dibranchia with two. 

Subclass Tetrabranchia 

The genus Nautilus contains the only living representatives of 
the Tetrabranchia. A well-developed, chambered shell within 
which the animal lives, the presence of four gills, four auricles, 
and four nephridia, a divided siphon, and many tentacles without 
hooks or suckers characterize the living examples of this subclass. 
In past geological ages, numerous tetrabranchs flourished, the 
fossil shells of which show many interesting evolutionary ten- 





dencies in development. But little is known of the habits and 
development of Nautilus, for though the empty shells are cast 
ashore in quantities in the Pacific and Indian oceans the animal 
is rarely found alive. 

Subclass DiBRANCHIA 

In this subclass are included those cephalopods having two 
branched gills in the mantle cavity, two nephridia, two auricles, 
highly organized eyes, and eight or ten arms bearing suckers or 
hooks. On the basis of the number of arms, two orders are 

recognized, namely, Decapoda — 
with ten arms (devilfishes and 
Argonauta) — and 0<}topoda — 
with eight arms (squid, cuttlefish 
and Spirula). 

Some sort of shell is usually 
present, in the form of an internal, 
coiled, chambered shell as in 
Spirula; a long, horny pen of 
purely organic matter as in the 
squids (Loligo); or a highly cal- 
careous plate known as the 
"cuttlebone" of the cuttlefish 
(Sepia, Fig. 94). In the argo- 
nauts, the female is provided 
with a thin, single-chambered 
shell, but the male argonauts 
and all other representatives of 
the order typically lack a shell. 
An ink sac (Fig. 94) is usually 
present. The powerful, muscu- 
lar arms are used both in swim- 
ming and in the capture of prey. A constriction separates the 
head from the body proper and marks the anterior boundary of 
the mantle. At this level, on the ventral surface occurs the 
respiratory opening. 

Man He 

Fig. 94. — Diagram of median section 
of Sepia. {From Lang). 

Outline of Classification 

Phylum Mollusca. — Triplol^lastic; iinsegmented; coelom greatly reduced; 

shell or shell gland at least during larval development. 

1. Class Amphineura. — Nervous system not concentrated; shell when 
present composed of eight transverse calcareous plates; marine. 


1. Order Placophora. — Shell dorsal, of eight transverse plates; 
foot broad; gills lateral, simple, mantle folds. Chito n, Crypto- 
chiton, Trachydermon, Amicula, Chaetopleura, Lepidochitona, 

2. Order Aplacophora. — ^Elongate; covered bj^ a mantle; spicules 
instead of shell; gills posterior. Neotnenia, Dondersia, Chaeto- 

n. Class Acephala. — No head; shell of two valves; aquatic. 

1. Order Protobranchia. — One pair plumelike gills; Yoldia, 
Nucidn, Lcda, Solenomya. 

2. Order Filibranchia. — Gills platelike, filaments V-shaped; 
anterior adductor reduced. Mytilus . Modiola, Area, Anomia, 

3. Order Pseudolamellibranchia. — Gills platelike, in vertical 
folds; but one adductor. Pccicn, Ostrea, Meleagrina. 

4. Order Eulamellibranchia. — Gill filaments grown together to 
form a continuous sheet; adductors approximately equal. Mya, 
Venus, Cardiitni, Ensatella, Teredo, Unto, Anodonta, Lampsilis, 
Qiiadrula, Sphaerium, Pisidium, Pholas, Barnea. 

5. Order Septibranchia. — Gills a horizontal partition in mantle 
cavity. Silenia, Cuspidaria. 

III. Class Scaphopoda. — Shell tubular; open at each end. Dentalium, 


rV. Class Gastropoda. — Shell single, usually spiral; radula. 

a. Subclass Prosobranchia. — Nervous system twisted in figiire 
eight; sexes separate. 

1. Order Aspidobranchia. — Nervous system little concentrated; 
one or two gills; two auricles. Haliotus, Trochus, Fissurelln, 
Patella, Acmaea. 

2. Order Ctenobranchia. — Usually one gill; one auricle. Cari- 
naria, Atlanta, Littorina, Crepidula, Natica, Strornbus, Pleurocera, 
Campeloma, Murex, Terebra, Buccinum, ,Busycon. 

b. Subclass Euthyneura. — Nervous system not twisted; hermaphro- 

1. Order Opisthobranchia. — Gills. Bulla, Aeolis, Navanax, Tethys, 

2. Order Pulmonata. — Lungs. Physa, Planorbis, Lymnaea, Fer- 
rissia, Limax, Arion, Agriolimax, Polygyra, Helix, Pupa, Gastro- 

V. Class Cephalopoda. — Distinct head with arms bearing suckers; shell 
frequently reduced, internal. 

a. Subclass Tetrabranchia. — Four gills; chambered shell. Nautilus;. 

b. Subclass Dibranchia. — Two gills; arms in circle around mouth; 
tubular funnel; shell internal. 

1. Order Decapoda. — Ten arms, two of which are tentaciilar; 
suckers stalked. Loligo, Sepia, Spirula, Rossia. 

2. Order Octopoda. — Eight arms; suckers sessile. Argonaula, 



(See general references cited at the close of Chapter I) 
Baker, F, C. 1928. The Fresh Water Mollusca of Wisconsin. Wis. 

Geol. and Nat. Hist. Survey, Madison. 
Berry, S. S. 1912. A Revision of the Cephalopods of Western North 

America. U. S. Bur. Fish. Bull, 30: 267-336. 
CoKER, R. E., Shira, a. F., Clark, H. W., and Howard, A. D. 1921. 

Natural History and Propagation of Fresh-water Mussels. U. S. 

Bur. Fish. Bull, 37: 77-181. 
Drew, G. A. 1911 and 1919. Sexual Activities of the Squid, Loligo Poalii 

(Les). Jour. Morphol, 22: 327-359; 32: 379-435. 
Kellogg, J. L. 1900. Observations on the Life History of the Common 

Clam, Mya arenaria. U. S. Fish Comm. Bull, 1899: 193-202. 
. 1915. Ciliary Mechanisms of Lamellibranchs. Jour. Morphol, 

26: 625-702. 
Lefever, G. and Curtis, W. C. 1912. Studies on the Reprodviction and 

Artificial Propagation of Fresh-water Mussels. U. S. Bur. Fish. Bull, 

30: 105-301. 
Ortmann, a. E. 1911. A Monograph of the Najades of Pennsylvania. 

Mem. Carnegie Museum, Vol. 4, No. 6. 



The arthropods are segmented animals distinguishable from all 
others in that they bear paired, jointed appendages and have a 
chitinous exoskeleton. This is by far the largest phylum in the 
animal kingdom, for it contains more than three times as many 
species as all the remaining phyla put together. In spite of the 
immense numbers, the group is clearly recognizable as a natural 

In many respects, the arthropods represent the highest 
development found in the non-chordate animals. In earlier 
chapters, it has been shown that the bodies of the higher seg- 
mented worms are composed of a linear repetition of similar 
rings or somites. The somites of these worms often bear appen- 
dages, called parapodia, but these in their highest development 
are mere flaplike folds of the body wall and are never jointed. 
The tendency toward speciahzation of appendages has been 
carried much further in the arthropods, for here they have become 
more highly organized and they are definitely articulated. 
Metamerism of the body also finds higher expression in the 
arthropods, for in most arthropods the segments of the body 
have undergone greater specialization and more distinct regional 
differentiation than is encountered in any annelids. Since the 
segments of the worms show so little differentiation, the metamer- 
ism of the worms is said to be homonomous, while that of the 
arthropods is very distinctly heteronomous. In most instances 
at least head and trunk regions are recognizable and in many 
cases three body divisions are distinctly set off as head, thorax, 
and abdomen. 

The body covering or integument of arthropods is composed 
of a chitinous material overlying the hypodermis. To permit 
movement of the body, joints occur in this otherwise unwieldy 
exoskeleton. These joints are termed sutures, and skeletal 
areas bounded by sutures are designated as sclerites. Inorganic 



substances, especially lime, are frequently added to the chitin, 
thus giving the skeleton additional strength and thereby afford- 
ing greater protection to the underlying parts. The skeletal 
plates are more or less telescoped at the more prominent joints, 
so the body surface presents an uninterrupted armor, 

Ecdysis. — With age, the chitinous covering increases in thick- 
ness and becomes an effective barrier to further growth of the 
parts which it encases. Growth is then rendered possible only by 
periodic shedding of the cuticula in a process known as molting 
or ecdysis. Immediately following the ecdysis, the body cover- 
ing is extremely soft and pliable and during this period there is 
rapid increase in size. Frequency of ecdysis varies greatly in 
different groups of the arthropods but it is of much more frequent 
occurrence in early stages of development. The Crustacea 
continue to shed the skin periodically throughout life. In the 
insects, attainment of the adult form marks the last ecdysis 
through which the individual passes, except in the mayflies which 
molt once after the wings become functional. 

Number of Segments. — In the fundamental structural plan 
of an arthropod, each segment bears a pair of appendages. 
Fusion of segments and degeneration of appendages on some 
segments frequently obscure this plan, but even in such instances 
evidences of the primitive condition are often still observable 
in the embryo, for the vestiges of appendages occur here even 
though they may be entirely wanting in the adult. 

Characteristic Organ Systems. — ^The chief organ of the 
circulatory system, a dorsal vessel or a heart, lies dorsal to the 
digestive canal. The circulatory system is of the open type, for 
the body fluid is not restricted to vessels throughout its course 
but is frequently spilled into sinuses or lacunae. These sinuses 
are so prominent that they are frequently mistaken for a coelom. 
These cavities are a development of a portion of the circulatory 
system, hence they are designated as a haemocoel, for they are 
not a true coelom. 

The central nervous system consists of a ladderlike chain of 
ganglia (Fig. 123) and a brain or supraesophageal ganglion of 
which all but the brain is ventral to the digestive tract. 

Various modifications for respiration and excretion are found, 
but these will be discussed under the several classes. 

Development is always sexual but modified types of sexual 
development are found in the establishment of parthenogenetic 


and paedogenetic habits in some groups. Polyembryony has 
been demonstrated in some insects. Hermaphroditism is rare. 
The centrolecithal egg undergoes partial, superficial cleavage in 
most instances. So many different larval stages are involved in 
the various groups of arthropods that the developmental cycle 
will be considered separately for the individual classes of the 

Classes. — There is much disagreement regarding the number 
of classes into which the Arthropoda should be divided. Until 
fairly recently, four or five classes were considered sufficient to 
express the extent to which differentiation has proceeded in this 
phylum. With the increase in our knowledge of the arthropods 
no less than eleven or twelve groups merit recognition as classes. 
Even a conservative judgment would demand that the Crustacea, 
the Acerata, the Onycophora, the Diplopoda, the Chilopoda, the 
Symphyla, the Myrientomata, and the Insecta be recognized 
as classes. Some writers maintain that in addition to these, 
several other groups such as the Pycnogonida, the Tardigrada, 
the Linguatulida, and the Pauropoda should be admitted to the 
rank of classes. 

Class Crustacea 

The cuticula of most crustaceans has become hardened 
through the addition of carbonate and phosphate of lime to the 
organic chitinous skeleton. Members of this class are typically 
aquatic, though some (the sow bugs and land crabs) have become 
modified for terrestrial existence. All modern representatives of 
this class have two pairs of antennae. A carapace is very fre- 
quently present. This has its origin as a fold at the head end and 
grows backward as a continuous shield protecting the underl3nng 
parts and obscuring the external evidences of segmentation of the 
parts covered. In some instances, the entire body is enclosed in 
this shell-like carapace (Fig. 99), which gives to these crustaceans 
a confusing external resemblance to molluscs. More frequently, 
the head and all or part of the thorax are covered by the carapace. 

Appendages. — All of the present-day crustaceans have two 
pairs of antennae, though the trilobites (Fig. 96), all of which are 
extinct, have but a single pair. The appendages are typically 
Y-shaped and are said to be of the biramous or schizopodal type. 
The stem of the Y, which provides attachment with the body 
wall, is called the protopodite and is composed of two segments, 


namely, a proximal coxopodite and a distal basipodite. Distally, 
the basipodite characteristically bears two branches of which 
the one nearer the median line of the body is termed the endopo- 
dite and the other the exopodite. Many of the crustacean 
appendages have become modified so that the biramous con- 
dition is not observable, as, for example, the walking legs of the 
crayfish and lobster in which the protopodite bears but a single 
series of segments to form the leg. A study of the larvae of the 
lobsters (Fig. 95) gives proof that the distal part of the leg is 
the endopodite, for in larval lobsters the thoracic legs are dis- 
tinctly biramous and it is only in later development that the 
exopodite of these appendages disappears. There are no free 
larval stages in the crayfishes, for the entire development is com- 
pleted within the egg, but since other structures of the crayfish 
homologize so directly with those of the lobster, the leg of the 
crayfish is likewise considered as being composed of a protopodite 
and an endopodite. 

While the schizopodal or biramous appendages are character- 
istic of most crustaceans, there are some in which a more primitive 
type of appendage is found. In the Phyllopoda, there are leaf- 
like feet on the thorax (Fig. 97) though the antennae are bira- 
mous. The phyllopod appendages bear a number of lateral 
processes called endites projecting from a central axis of podo- 
meres or foot segments. This type of appendage seems to be 
more generalized than the biramous type and the latter may 
have originated as a modification of the foliaceous type. 

The number of body segments is highly variable. The 
anterior five somites are fused with the prostomium to form the 
head. This may be united with some or all of the thorax to 
form a cephalothorax. The abdominal somites are highly 
variable in number. 

Respiratory and Excretory Organs. — Respiration is usually 
by means of gills, though in some instances there are no modified 
structures for respiration, because the entire body surface func- 
tions in this capacity directly. The gills are very frequently 
borne within a gill chamber formed by the walls of the carapace 
and the body wall. Special organs of excretion are the green 
glands and the so-called shell glands which open on the bases of 
the second antennae and the second maxillae respectively. 
These two organs occur together in the larvae, but in adult 
crustaceans one or the other fails to develop. They agree 


in fundamental structure. Each consists of a terminal vesicle 
which communicates with an external pore by a slender, greatly 
coiled tubule. 

The digestive canal is largely ectodermal and is consequently 
lined with chitin. The stomodaeum includes not only the 
pharynx but also a dilated portion, modified for grinding, which 
is called the stomach. Much of the intestine is proctodaeum. 
The mesenteron is a relatively short region into which a paired 
digestive gland, the hepatopancreas, empties. 

Sensory Organs. — Both antennae and antennules are tactile 
organs, innervated from the brain, but in some instances the 
antennae are the chief locomotor organs. Otocysts are found 
only in the Malacostraca, where they occupy a position in the 
protopodite of the antennules (Decapoda) or on the last abdom- 
inal appendages (Schizopoda). Paired compound eyes are char- 
acteristic of many crustaceans but there are some blind forms. 
An unpaired X-shaped "nauplius eye" occurs in the larvae of 
most crustaceans and is retained as the optic organ in the adults 
of the lower Crustacea. 

Development. — The Crustacea typically pass through one or 
more larval stages before reaching adult organization. When 
direct development occurs, it has either resulted from a sup- 
pression of larval stages, or these stages have been passed before 
the embryo leaves the egg. The nauplius is one of the most 
important as well as most characteristic of the larval stages of 
Crustacea, for it is almost universal among the lower orders. 
This larva is composed of three segments bearing three pairs of 
appendages. The foremost of these appendages are simple and 
later develop into the antennules, while the second and third are 
biramous and in later development form the antennae and 
mandibles respectively. There is a single unpaired eye. 

In a characteristic instance, as in the development of Cyclops, 
the nauplius after its first molt becomes a metanauplius bearing 
the rudiments of three pairs of appendages in addition to those 
found in the nauplius. These additional rudiments represent 
the two pairs of maxillae and one pair of maxillipeds. A pair 
of very rudimentary thoracic appendages also make their 
appearance on the metanauplius. In each of three successive 
molts, the Cyclops larva acquires an additional pair of these 
stumphke rudiments of the thoracic legs, so with the close of the 
larval period there are four pairs of the thoracic rudiments, and 



following a final larval molt the larva has practically attained 
the adult form. 

Among the parasitic copepods, the nauphus and metanauplius 
stages are passed within the egg membranes, and the larva at 
hatching is termed a copepodid because of its fairly close resem- 
blance to the general organization of the free-living copepod. 

The zoea (Fig. 103 B) is characteristic of the Malacostraca 
where it is usually the first larval stage but in a few instances it is 
preceded by a nauplius. It consists of a cephalothorax bearing 

Fig. 95. — Typical stages in the development of the lobster. A, first swimming 
stage or mysis; B, second larval stage, with abdominal appendages; C, fourth 
larval stage showing loss of exopodites from walking legs. (After Herrick). 

biramous appendages and an abdomen without appendages. 
The head bears a pair of lateral compound eyes. Typically, 
the zoea by several molts transforms into a mysis stage (Fig. 
95 A), in which even the posterior thoracic appendages are 
biramous and are of use in swimming. In later development, 
the exopodite of these biramous thoracic appendages disappears 
and leaves the unbranched walking legs of the adult. 

In the Brachyura (crabs), the thoracic appendages of the 
zoea, which are later destined to become the walking legs, develop 
in the free-swimming larva as only budlike rudiments and never 
acquire the biramous condition characteristic of the mysis larva. 
Thus in the crabs the mysis stage has been eliminated and the 



zoea in passing to the adult condition transforms through a more 
advanced type of larva (Fig. 103) which is termed the megalops. 
Six subclasses of the Crustacea are here considered: the 
Trilobita, which are represented by fossil remains only, the 
Phyllopoda, the Copepoda, the Ostracoda, the Cirripedia which 
are exclusively marine, and the Malacostraca. 

Subclass Trilobita 

The trilobites are of importance because they represent a 
primitive type of crustacean which has not persisted to recent 
times but occurred in abundance during Cambrian times when 

Fig. 96. — A trilobite, Triarthrus Becki. A, dorsal; B, ventral aspect. {After 
Beecher, from Zittell). 

the oldest fossil-bearing rocks were formed, reached an extreme 
development in the Silurian, and disappeared in the Permian. 
In habits, they were exclusively marine. Their remains are 
among the most popularly known fossils. The body contains 
a variable number of somites (Fig. 96) which are grouped into 
head, thorax, and abdomen. Each segment bears a pair of 
jointed appendages. The head bears five pairs all of which are 
biramous except the antennules and they are simple. A pair of 
compound eyes usually occurs on the head. 

A pair of longitudinal grooves separates the body into a central 
axis and two lateral pleural lobes. This "trilobed" condition of 
the body suggests the origin of the group name. 

About 2,000 species have been described. Proetus, Asaphus, 
Dalmanites, Triarthrus (Fig. 96), Harpes, Ctenocephalus, Para- 


doxides, and Homalonotus are examples of the 200 or more 
described genera. 

Subclass Phyllopoda 

Of the present-day Crustacea, the Phyllopoda are the most 
primitive. The name intimates one of the outstanding charac- 
ters of the group, the presence of leaflike appendages, of which 
there are ten or more pairs located on the trunk. Here belong 
the fairy shrimps (Branchiopoda) and that host of minute, 
shelled crustaceans known as the water fleas (Cladocera). Fresh- 
water, marine, and brackish-water forms are included within the 
confines of this group the members of which have little more than 
the leaf like appendages as a common characteristic. In the two 
orders, Branchiopoda and Cladocera, members of the former have 
numerous segments, very distinctly marked, while members of 
the latter order have few somites and these are frequently not 
clearly defined. Even within each order, body form and struc- 
ture are subject to great variation. There is usually a pair of 
conspicuous eyes and frequently in addition there is a small 
median eye. In some instances, the paired eyes, distinct in the 
young, fuse to form a single eye, but even then two optic nerves 
are retained, so the double nature of the eye is still observable. 
The sexes are distinct, though males are much less numerous than 

Members of the two orders Branchiopoda and Cladocera are 
separable on the basis of the number of trunk appendages. In 
the former, there are ten or more pairs of trunk appendages, while 
in the Cladocera the appendages of this region do not exceed six 

I. Order Branchiopoda 

With the exception of one genus, Artemia, the Branchiopoda 
are exclusively fresh-water phyllopods which occur in all parts of 
the world. Eubranchipus (the fairy shrimp. Fig. 97), Apus, 
and Estheria represent three distinctly different types of struc- 
ture within this order. In habits, all of these are peculiar in that 
they are restricted to small pools and especially the temporary 
pools which are formed by spring rains and disappear during the 
summer. In these temporary pools, they appear in very great 
numbers in early spring, then after a few days or weeks they dis- 
appear entirely. An examination of the mud after the pool has 
dried up reveals large numbers of the eggs which are capable of 


withstanding desiccation. In fact, the eggs of some forms refuse 
to develop if they are returned to water immediately and 
develop normally only after being dried out. From the egg is 
derived a larva in the nauplius or metanauplius stage. Many 
phyllopods swim upon the back with the ventral surface upper- 
most. This is especially characteristic (Fig. 97) of Eubranchipus. 
Eubranchipus and the brine shrimp Artemia are examples of 
the suborder Anostraca, the members of which have no carapace 
and bear stalked eyes. The members of suborder Notostraca, 
of which Apus and Lepidurus are examples, include branchiopods 
with a shield-shaped carapace covering part of the trunk and 
with sessile eyes. In the suborder Conchostraca are grouped 

Fig. 97. — Eubranchipus vernalis in normal position for swimming. {After 
Packard, from Kingsley's Hertwig, with permission of Henry Holt and Co.). 

forms which have a bivalve carapace enclosing the entire animal. 
Estheria, Limnadia, and Limnetus, with their strikingly mollus- 
can appearances, are examples of this suborder. 

II. Order Cladocera 

The Cladocera are small phyllopods, rarely more than 3 mm. 
in length, with a distinct head and usually with a carapace cover- 
ing the trunk and legs. They are commonly called water fleas. 
Daphnia is a characteristic genus. In a few instances (Leptodora 
and Polyphemus), the carapace is greatly reduced and serves only 
as a brood sac, leaving the trunk and legs entirely free. 

The head holds a median compound eye which has usually 
resulted from the fusion of two lateral eyes and is capable of 
rotation within a capsule. The antennules are sensory but the 
second antennae are modified as swimming organs and constitute 
the chief organs for locomotion. In addition, the head bears a 
pair of mandibles and a pair of greatly reduced maxillae. 

A dorsal heart, just back of the head, is the only circulatory 
organ, for vessels are wanting. An ostium on each side of the 


heart allows the blood to enter, and contraction of the heart 
forces the blood through a single anterior opening. There are 
no specialized organs for respiration. That portion of the body 
within the shell is divisible into two regions, one, the body proper 
which is not sharply segmented but bears six pairs of foliaceous 
appendages, and the other, an unsegmented postabdomen. 
The trunk appendages function chiefly in creating water currents 
through the space within the shell. 

Most Cladocera react positively to weak light and negatively 
to strong light, but these reactions may be reversed by other 
stimuli acting as controlling factors in their behaviour. Thus, 
in cold water, Cladocera may respond positively to a light 
stimulus which would repel the same individuals living at a higher 

Reproduction is largely parthenogenetic. The eggs are stored, 
frequently in considerable numbers, in a brood case formed 
within the dorsal portion of the carapace. Here they undergo 
full development without ever having a free larval stage. Under 
ordinary circumstances, all of the parthenogenetic eggs produce 
females. When unfavorable conditions arise, however, not all 
of the parthenogenetically developed individuals are females, but 
some males are hatched from parthenogenetic eggs. In the 
sexual cycle which follows, each female produces only one or 
two large thick-shelled eggs. These are true sexual eggs which 
require fertilization before development is initiated. The 
fertilized eggs pass into a brood sac the walls or portions of the 
walls of which become modified as an enclosing envelope (ephip- 
pium) within which the resting eggs lie until the return of 
conditions favorable for their development. 

The Cladocera, along with some other crustaceans, have 
great value in that they are important as food for many aquatic 
animals, especially fish, while they in turn utilize the smaller 
algae which are abundant under conditions in which they are 
found. Daphnia and Bosmina are characteristic shelled forms, 
while Leptodora and Polyphemus have greatly reduced shells. 
Most cladocerans live in fresh water. 

Subclass CoPEPODA 

Copepods inhabit both fresh and salt water. Some forms 
have become so highly modified through adaptation to the 
parasitic habit that they are recognizable only with difficulty as 



arthropods. In the free-Hving forms, the body is usually 
elongated and distinctly segmented. The appendages are char- 
acteristically biramous, though some have become so modified 
that they have lost their biramous nature. Six pairs of append- 
ages are borne on the head and four or five on the anterior region 
of the trunk, while the posterior region of the trunk lacks append- 
ages. The last abdominal segment is forked. 

The impaired nauplius eye is characteristic of copepods and 
prompted the application of the name Cyclops to one genus. 

In many instances, the eggs when discharged from the female 
are surrounded by a gelatinous substance and remain attached 
to the body as prominent egg sacs. Conspicuous egg sacs are 
highly characteristic of the female Cyclops. Larvae hatching 
from the eggs are in the nauplius stage. 

Copepods occur in such abundance and are important food 
items for such numbers of different animals that they have 
considerable economic impor- 
tance. Many fishes feed almost 
exclusively upon small crusta- 
ceans of which the copepods are 
the most numerous. Even some 
whales subsist largely upon 
copepod diet. 

There are a number of fami- 
lies displaying an extensive array 
of bizarre body forms. In many 
instances, all of the appendages 
excepting those modified for 
attachment to the host have 

Parasitic Copepods. — In the foregoing, only the free-living 
copepods have been considered. There are immense numbers 
of copepods which dwell as parasites upon other animals. Fishes 
are especially prominent hosts of those parasites, which are 
popularly known as fish lice. Both the body and gills are sub- 
ject to attack. Some of these parasitic copepods e.g., the genus 
Argulus (Fig. 98) of the order Branchiura have a pair of com- 
pound eyes, fully developed swimming feet, and a modification of 
the first maxillipeds to form a pair of sucking discs for securing 
attachment to the host. These argulids are not permanent 
parasites, for especially at the breeding season they leave the 

Fig. 98. — Dorsal view of a female 
parasitic copepod, Argulus versicolor. 
{After Wilson). 



body of the host and swim free in the water. In contrast with 
these stand some other parasitic copepods the bodies of which 
have become so greatly reduced as an adaptation to the parasitic 
habit that they are more Hke a simple worm in appearance than 
like an arthropod. It is only through the unaltered larval 
stages, the nauplius and the metanauplius, that the affinities 
of these degenerate adults are recognizable. 


Ostracoda are abundant in various fresh- and salt-water 
habitats. The entire animal is enclosed within a bivalve shell 

First antennae 
\ Sernvd antennae 
\ \ Eye 

Branchial Plate of Mandible 
1 Stomach 

Fuod hnlls 

Branchial setae 


Dorsal seta 

— Sub-terminal clatv 

Terminal claw 

\ '•Terminal aeiu 
. Mandibular palp ^^ Firstfoot 

Labrum Branchial plate of maxilla 

Fig. 99. — General anatomy of an ostracod, Cypris virens Jurine. (After 
Vavra) . (Reprinted by permission from Ward and Whipple's Fresh-water Biology, 
published by John Wiley and Sons, Inc.). 

Natatory setae 

(Fig. 99), but when the valves open the appendages protrude. 
Segmentation of the body is very indistinct or wanting. The 
head region bears two pairs of antennae, both used in swimming; 
the mandibles; and two pairs of maxillae. Ordinarily, the trunk 
region bears but two pairs of legs. 

Most ostracods are omnivorous. Both parthenogenetic and 
true sexual reproduction occur. The eggs produce nauplii which 
undergo a number of molts before reaching the mature form. 


Cypris is free-swimming, while Candona is of a burrowing habit. 
In the marine forms (Cypridinidae) a heart is present but in 
fresh- water ostracods there is no heart. 

Subclass ClRRIPEDIA 

Most Crustacea are free-moving animals except the parasitic 
forms and the members of the entire group known as the Cirri- 
pedia or barnacles. Not only are the barnacles sessile but as a 
consequence of this habit they show very marked degeneration 
of many structures. Adaptation to the sessile life also helps 
to explain the fact that hermaphroditism, which is so uncommon 
in Arthropods, is the usual condition among the Cirripedia. 
Though occasionally males are found, they are usually small, 
degenerate forms known as complemental males which very 
frequently live within the shell of the female, at times as parasites. 

The body is enclosed in a membranous mantle which, in most 
instances, is encased in calcareous plates. It is not surprising 
that until less than a century ago barnacles were thought to be 

In development, the Cirripedia hatch from the egg as a naup- 
lius. Because of its resemblance to the ostracod, Cypris, a 
second larval stage with a bivalve shell, is termed the cypris stage. 
The free-swimming larva comes into contact with some object 
to which it becomes attached. The first antennae, with their 
associated cement glands, aid in the fixation. Calcified plates 
usually make their appearance in the mantle folds and form a 
protective shell surrounding the animal. Within this shell, the 
body of the barnacle is peculiarly oriented, for, as is sometimes 
said, the barnacle stands on its head and kicks food into its mouth. 
Water currents bearing food to the mouth are set up by the 
beating of the usually six pairs of plumelike biramous appendages 
of the trunk. The mouth is surrounded by a pair of mandibles 
and two pairs of maxillae. 

The goose barnacles {Lepas anatifera) have the region of 
attachment (Fig. 100) drawn out into a characteristically 
elongated stalk. A mediaeval myth maintains that the goose 
develops from these goose barnacles. The neckless barnacles 
(Balanus, for example), which encrust rocks and other submerged 
objects, have a flattened base of attachment from which the 
skeletal plates arise directly. Barnacles are exclusively marine 
and choose stones, wood, animals, plants — ^in short, any object — 



as a place for attachment. When they become attached to the 
bodies of other animals, it is a very common thing for them to 
become dependent upon the animal which offers shelter, and as a 
consequence there is here shown in great detail the development 
of the parasitic habit. Various species become more or less 
dependent upon whales and sharks, while Sacculina becomes 
attached to the abdomen of decapod crabs and undergoes a para- 
sitic degeneration rarely equaled. All traces of appendages 
and of digestive organs are lost, and, as the name implies. 




Tesf-f's ■ 




- Vas deferens 

Fig. 100.— Barnacle. 

Sacculina becomes a mere saclike structure with a rootlike 
system penetrating the body of the host for absorbing food. The 
relationships of this degenerate form are shown only through the 
larval stages which are fully characteristic of the barnacles. 

Subclass Malacostraca 

The Malacostraca are frequently spoken of as the higher 
Crustacea. Lobsters, crayfishes, crabs, shrimps, prawns, sow 
bugs, scuds, and sand fleas are common names of some represen- 
tatives of this subclass. The body segments, so highly variable 
in number in members of the foregoing groups, are in the Mala- 
costraca almost rigidly limited to twenty or twenty-one. Usuallj^ 
the head consists of a prostomium and five additional somites, 
the thorax of eight, and the abdomen of seven of which the 
last (the telson) bears no appendages. 



The antennal glands are the typical excretory organs of adults, 
though larvae and some Isopoda have a shell gland communi- 
cating with the second maxillae for excretion. The genital 
opening of the male is characteristically upon the coxopodite of 
the eighth thoracic appendage and of the female on the sixth. 

Within the Malacostraca, a considerable number of orders are 
recognized, but for the purposes of this text attention is limited 
to but six of them. 

I. Order Phyllocarida 

A few marine species belonging to the genus Nebalia (Fig. 101) 
show unusual combinations of phyllopodan and malacostracan 

Fig. 101. — Male of the genus Nebalia. {AfUr Claus). 

characters. In number of somites and in location of the genital 
openings, they agree with the higher Crustacea, yet the thoracic 
appendages are leaflike. 

II. Order Schizopoda 

These small, mostly marine, Malacostraca with compound eyes 
on movable stalks bear a delicate carapace covering the cephalo- 
thorax. The eight appendages of the thorax are biramous 
swimming organs with both exopodite and endopodite and in one 
family (Mysididae) bear gills projecting freely into the water. A 
postabdominal somite bears appendages which with the telson 
form a caudal fin. The use of this fin causes the animal to swim 
backward as do crayfishes and lobsters. This order includes a 
number of pelagic marine forms and the family Mysididae some 
species of which inhabit fresh water. Mysis relicta occurs in the 



Great Lakes at considerable depths and is identical with speci- 
mens found in similar lakes of northern Europe. 

III. Order Stomatopoda 

In the Stomatopoda, the posterior three thoracic segments bear 
complete biramous appendages which are used in swimming, 
while of the five anterior to these all but the first bear appendages 
modified as prehensile maxillipeds. The terminal segment of 
these prehensile appendages folds into a groove of the preceding 
segment as a knife blade folds into its handle. 

The carapace is short, for at least the posterior four segments 
of the thorax are not included within it. Antennules and stalked 
eyes are borne on two movable, inde- 
pendent somites at the anterior extremity 
of the head. Gills are borne on the 
abdominal appendages. These are chiefly 
burrowing forms living in mud. Squilla 
(Fig. 102), Gonodactylus, Chloridella are 
characteristic genera. 

IV. Order Decapoda 

The lobsters, crayfishes, shrimps, and 
crabs agree with the Schizopoda in hav- 
ing a carapace covering the entire cepha- 
lothorax. The head bears stalked eyes, 
antennules, antennae, mandibles, and two 
pairs of maxillae. Of the thoracic append- 
ages, the anterior three pairs are modified 
as maxillipeds while the remaining five 
pairs are locomotor and lack an exopodite. 
All of the walking legs bear either pinch- 
ers or claws. The first pair of walking 
legs, which are known as the chelipeds, 
bear extremely strong pinchers termed 
the chelae. One jaw of the pinchers is 
the movable distal segment of the endopodite called the finger, 
while the other jaw is an immovable outgrowth from the next 
to the last segment forming the thumb. 

■ The Decapoda have marked powers of regeneration. If an 
appendage is removed, a new one begins to develop beneath the 
old shell but does not become evident externally until the shell 


102. —Squilla 
{From Bigclow). 



is shed at the next molt. Injury is frequent in nature, and 
consequently the claws are rarely of equal size. An injured claw- 
is autotomously broken off by the animal at a special joint where 
there is but little exposed to heal over. 

Abdominal Appendages. — Most of the abdominal segments 
bear small biramous appendages termed swimmerets, except 
in the crabs (Fig. 103) which have a rudimentary abdomen. 
The appendages of the sixth abdominal somite are enlarged 
and with the telson from a powerful tail fin. In the males, 
the first and second abdominal appendages are highly modified 
as a copulatory organ for the transfer of sperm. In the females, 
the swimmerets serve as a place for the attachment of the eggs 
during development. 


Fig. 103. — The rock crab, Cancer irroratus. A, adult male, about one-fourth 
size; B, larva in zoea stage; C, larva in megalops stage. {After Rathhun) . 

Respiration. — Gills are borne on the basal joints of certain 
of the maxillipeds and legs and on the body wall. These are 
usually contained within a branchial chamber formed by the 
overhanging lateral folds of the carapace. A process of the 
second maxilla becomes modified as a gill bailer the action of 
which pumps water through the gill chamber. The gill chamber 
retains water, thus keeping the gills moist and permitting 
decapods to remain out of water for some time without injury to 
the gills. In some crabs, the branchial chamber has become 
modified as a lung in correlation with terrestrial habits. 

The circulatory system is of the open type. The heart 
lies within a pericardial sinus in the dorsal region of the thorax. 
Five arteries arise from the heart. Three of these pass ante- 
riorly to the organs of the head and thorax, one passes along the 
median dorsal line of the abdomen, and the last is directed ven- 
trally either from the posterior boundary of the heart or as a 


branch from the posterior vessel soon after the latter leaves the 
heart. Blood is received into the heart from the pericardial sinus 
through openings in the heart wall called ostia. The capillaries 
of the arteries allow the blood to pass into a large sternal sinus. 
In the thorax, offshoots from the sternal sinus pass into each gill 
as an afferent branchial vessel or vein. Within the branches of 
the gills, each afferent vein communicates with an efferent 
branchial vein from which the blood is returned to the peri- 
cardial sinus. 

Digestive System. — From the mouth opening between the 
mandibles on the ventral surface of the head, the digestive 
system passes as a short tube, the esophagus, into a spacious 
chamber termed the stomach. The latter is divided into two 
regions: a large anterior sac, the cardiac chamber, which bears 
a chitinous organ for grinding food; and a smaller posterior 
pyloric chamber. The intestine, which proceeds from the 
pyloric chamber, receives the ducts from the two lateral hepato- 
pancreases. On the wall of the cardiac chamber, there are fre- 
quently hard, rounded masses of lime, the gastroliths. 

Excretory Organs. — Within the head, near the base of each 
antenna, lie the excretory organs, the green glands. Each 
discharges to the exterior by a pore located on the base of the 

The central nervous system consists of a ventral chain of gan- 
glia the number and disposition of which are correlated with the 
extent of the development of the abdomen. A brain, near the 
anterior extremity of the body, communicates with the ventral 
chain of ganglia by means of a pair of circumesophageal connec- 
tives. Six thoracic and six abdominal ganglia comprise the chain, 
except in the crabs, which have a rudimentary abdomen and here 
the entire ventral chain becomes fused to form a single ganglionic 

Reproduction. — The gonads frequently consist of two lateral 
and a single median lobe from which a pair of ducts lead to the 
genital openings on the ventral surface of certain walking legs. 
The eggs undergo superficial cleavage and give rise to either a 
larva or a young adult the larval stages of which are all passed 
through while in the egg. Considering the relative uniformity 
in structure of the adult decapods, there is a surprisingly great 
number of larval forms. Typically, the larva which leaves 
the egg is a zoea. From this develops a mysis stage (Fig. 95 A) 


with even the thoracic appendages biramous. In later develop- 
ment, the exopodites of these biramous thoracic appendages 
disappear (Fig. 95 C) leaving the unbranched walking legs of 
the adult. In the crabs, the thoracic appendages of the zoea, 
which are later destined to become the walking legs, develop in 
the free-swimming larva as rudimentary budlike appendages only 
(Fig. 103) and never acquire the biramous condition characteristic 
of the mysis. Thus in the crabs the mysis stage has been elimi- 
nated and the zoea in passing to the adult form involves a more 
advanced type of larva which has been called the megalops (C). 
Some of the prawns have the zoea preceded by a nauplius and 
metanauplius and thus show unique combinations of larvae of 
both the lower and the higher crustaceans. 

V. Order Amphipoda 

Almost all of the Amphipoda are aquatic. Characteristically, 
the body is compressed laterally. The common name of beach 
fleas or sand fleas is due to the leaping movements with which 

Fig. 104. — An amphipod, Orchcstia palustris. (After Kunkel). 

they spring into the air when out of the water. The head bears 
six pairs of appendages. There are seven segments in the thorax. 
The first of these bears a pair of small appendages which function 
as maxillipeds. 

In the anterior region of the thorax, the sides of the body are 
frequently prolonged ventrally by epimeral plates which are 
borne upon the legs and serve to enclose the gills or gill sacs. 
In the same region, scales from the two sides of the body fre- 
quently enclose a brood pouch ventral to the ventral body wall. 
Within this pouch, eggs and the young are carried. 

The abdominal appendages are of two types. On the anterior 
three somites of the abdomen are borne biramous feet with 
many joints and numerous hairs. Posterior to these the append- 
ages are biramous but the branches are not segmented and form 


springing organs. Gammarus and Hyallela are common fresh- 
water genera, while Orchestia (Fig. 104) and Caprella are marine. 

VI. Order Isopoda 

A dorsoventral flattening of the body is characteristic of 
most isopods. The first thoracic segment is coalesced with the 
head. Though the body superficially shows sharp marks of 

segmentation, there is a tendency for 
the abdominal somites to fuse. All of 
the abdominal appendages are similar 
and usually biramous. Gills are borne 
on the ventral side of the abdomen and 
in the terrestrial species are capable of 
utilizing moist air for respiration. A 
brood sac is borne ventral to the thorax 
as in the amphipods. 

Isopods, or sow bugs or pill bugs as 
they are called popularly, are found 
Fig. 105.— a fresh-water in both fresh-water (Asellus, Fig. 105) 
\7fSr°Kunket^''' ''°'^'^'''''' ' and salt-water (Idotea) habitats, and 

a number of genera are terrestrial 
(Porcelho, Oniscus). In these last, the head bears but a single 
pair of antennae. 

Very great degeneracy has accompanied the acquisition of 
the parasitic habit in a number of forms. 

Outline of Classification 

Phylum Arthropoda. — Segmented; with jointed, paired appendages; 
chitinous exoskeleton. 

I. Class Crustacea. — Skeleton limy; two pair antennae; chiefly aquatic. 

a. Subclass Trilobita. — Exclusively marine; fossil only; two longi- 
tudinal grooves separate body into three lobes. Triarthrus. 

b. Subclass Phyllopoda. — Most appendages leaflike. 

1. Order Branchiopoda. — Ten or more pairs trunk appendages; 
usually with carapace. Apus, Lepidurus, Estheria, Limnadia, 
Limnetus, Artemia, Branchinecta, Eubranchipus. 

2. Order Cladocera. — Not more than six pairs trunk appendages; 
usually a carapace including all but head. Dapkjxia, Lcptodora 
Polyphemus, Sida, Pleuroxus, Chydorus, Bosmina, Moina. 

c. Subclass Copepoda. — No carapace; trunk appendages biramous, 
four or five pairs; no appendages on abdomen. 

1. Order Eucopepoda. — No compound eyes. C yclop s, Epischura, 
Diapiomus, Ergasilus, Lernaea. 

2. Order Branchiura. — Two' compound eyes. Axgulus. 


d. Subclass Ostracoda. — Carapace over whole body; two pairs 
trunk appendages. Cypris, Candona. 

e. Subclass Cirripedia. — Marine; attached; usually enclosed by cal- 
careous plates. Lepas, Balanus. 

f. Subclass Malacostraca. — Twenty or twenty-one somites; all 
except last bear paired appendages. 

1. Order Phyllocarida. — Marine; thoracic appendages leaf like. 

2. Order Schizopoda. — Eyes stalked; gills thoracic; trunk appen- 
dages biramous. Mysis. ' 

3. Order Stomatopoda. — Eyes stalked; gills abdominal. Squilla, 
Gonodactylus, Chloridella. 

4. Order Decapoda. — Carapace covers cephalothorax; five pairs 
uniramous legs on thorax. Cambarus, Astactis, Homarus, Crago, 
Peneus, Palaemon, Palaemonetes, Can cer, Callinedes, Carcinus, 
Gelasimus, Libinia, Macrocheira, Upogebia, Pagurus, Emerita, 

5. Order Amphipoda. — Laterally compressed; gills thoracic; two 
types of abdominal appendages. Gammarus, Hyallela, Orchestia, 
Caprclla, Amphithoe. 

6. Order Isopoda. — Dorsoventrally depressed; gills abdominal; 
abdominal appendages (except uropods) similar. Asellus, Man- 
casellus, Idothea, Porcellio, Oniscus. 


(See general references cited at close of Chapter I) 

BiRGE, E A. 1895. Plankton Studies on Lake Mendota, L Vertical 
Distribution of the Pelagic Crustacea during July 1894. Trans. 
Wis. Acad. Sci., Arts, and Letters, 10: 421-484. 

BiRGE, E. A. and Juday, C. 1912. A Limnological Study of the Finger 
Lakes of New York. U. S. Bur. Fish. Bull, 32: 527-609. 

Brooks, W. K. and Herrick, F. H. 1892. The Embryology and Meta- 
morphosis of the Macroura. Proc. U. S. Nat. Acad., 5: 325-576. 

Churchill, E. P., Jr. 1919. Life History of the Blue Crab. U. S. Bur. 
Fish. Bull, 36: 93-128. 

Darwin, C. 1851-1853. "A Monograph of the Sub-class Cirripedia." 
London, Ray Societj'. 

Herrick, F. H. 1911. Natural History of the American Lobster. U. S. 
Bur. Fish. Bull, 29: 149-408. 

Herrick, C. L. and Turner, C. H. 1895. Synopsis of the Entomostraca 
of Minnesota. Geol and Nat. Hist. Surv. Minnesota, Zool Series, 2. 

Huxley, T. H. 1901. An Introduction to the Study of Zoology, Illus- 
trated bj' the Crayfish. New York. 

Kunkel, B. W. 1918. The Arthrostraca of Connecticut. ConJi. Geol. 
and Nat. Hist. Survey Bull 26. 

Marsh, C. D. 1907. Revision of the North American species of Diapto- 
mus. Trans. Wis. Acad. Sci., Arts, and Letters, 15: 381-488. 

1910. A Revision of the North American Species of Cyclops. 
Trans. Wis. Acad. Sci., Arts, and Letters, 16: 1067-1134. 


Ortmann, a. E. 1906. The Crawfishes of the State of Pennsylvania. 

Mem. Carnegie Mus. Pittsburgh, 2: 343-523. 
Packard, A. S. 1883. A Monograph of the Phyllopod Cnistacea of North 

America, with Remarks on the Phyllocarida. Ann. Rep. U. S. Geol. 

Patten, W. 1887. Eyes of Molluscs and Arthropods. Jotir. Morph., 

1 : 67-92. 
. 1887a. Studies on the Eyes of Arthropods. Jour. Morph., 1: 

193-226, 2: 97-190. 
Rathbun, Mary J. 1917. The Grap.soid Crabs of America. U. S. Nat. 

Mus., Bull. 97. 
Rathburn, R. 1893. Natural History of Economic Crustaceans of the 

United States. Bull. U. S. Fish Comm. 1889: 763-830. 
Richardson, Harriet. 1905. A Monograph of the Isopods of North 

America. U. S. Nat. Mus., Bull. 54. 
Sharpe, R. W. 1897. Contributions to a knowledge of the North Amer- 
ican fresh water Ostracoda. Bull. III. Lab. Nat. Hist., 4: 414-484. 
Wilson, C. B. 1902 to date. Numerous articles on parasitic copepods 

in Proceedings U. S. Nat. Mus. 



Class 2, Acerata 

The horseshoe crabs, spiders, scorpions, mites, a number of 
degenerate forms, and some important extinct animals which 
are known only from fossil remains are grouped as a class to which 
the name Acerata is applied. This name signifies the lack of 
antennae common to all of the members of the class. The body- 
usually consists of a cephalothorax and abdomen, though in the 
mites these two divisions are not separated. Six pairs of appen- 
dages upon the cephalothorax are arranged about the mouth. 
The bases of one or more pairs of these appendages are modified 
to serve as mandibles. The abdomen is composed of a variable 
number of somites which, in the embryo, bear appendages, but 
these are lost or highly modified in the adult, except in the 

Many modifications of the respiratory organs are found in 
this group. Gills occur on the abdomen of some but correlated 
with the air-dwelling habit the gills become drawn into the body 
where they are found as lung books which open to the exterior 
through narrow slits. In some instances, the lung books are 
replaced by tracheae which penetrate to all parts of the body as 
in the insects. 

The basal segment of the abdomen bears the genital opening. 
In development, there is no metamorphosis. 


The extinct eurypterids and the horseshoe crabs appear 
strikingly hke Crustacea. In details of organization and in 
development, however, they show an intimate relationship with 
scorpions and other Acerata and are consequently recognized as 
constituting a subclass to which the name Gigantostraca has 
been applied. 



Of the six pairs of cophalothoracic appendages, the first are 
preoral, and the remaining five pairs, which are for walking, have 
their bases modified as masticating organs. In addition to a pair 
of lateral compound eyes, the cephalothorax bears a pair of 
median ocelli. 

Members of the order Eurypterida have a small cephalothorax 
and a large twelve-jointed abdomen. These forms flourished 

during past geological ages and in 
structure seem to be intermediate 
between the scorpions and the mem- 
bers of the order Xiphosura, of which 
Limulus, the horseshoe crab, is the 
only living example. In Limulus, the 
telson is long and spikelike. The 
abdomen bears six pairs of appendages 
of which the first forms a broad, flat 
operculum which overlaps the follow- 
FiG. 106.— Ventral view of ing five pairs of platelike appendages, 

trilobite larva of Limulus i (• i • i i -n tt 

Polyphemus. {After Kingdey) . ^ach of whlch bears a gill. UpOn 

hatching from the egg, the young 
Limulus is said to be in the trilobite stage (Fig. 106) because 
of its resemblance to the organisms of that group. 

Subclass Arachnida 

Scorpions, spiders, mites, harvest men, and some less commonly 
known Acerata are grouped under a common subclass Arachnida. 
Most of these are air-breathing forms in which the 
cephalothorax bears one pair of pedipalps lateral or immediately 
posterior to the mouth and a pair of preoral appendages termed 
the chelicerae. In addition to these appendages, four pairs of 
walking legs are just as characteristic of the arachnids as three 
pairs are for the insects. As an exception to the foregoing, it 
should be noted that as a rule young mites have but three pairs 
of legs (Fig. 108 C), and in some gall mites only two pairs of 
legs are found. 

True jaws are entirely lacking. Few arachnids swallow solid 
objects. The chelicerae and pedipalps are frequently modified 
for crushing the prey, and in some instances the bases of some of 
the walking legs serve the same function. Usually, only the body 
juices of the victim are taken into the stomach and this is accom- 
plished by action of a muscular sucking stomach. 



Some species of arachnids are eyeless, but more commonly there 
are from two to twelve simple eyes. Body form and structure 
are so highly variable in the members of this subclass that they 
will be discussed under the more important orders individually. 

I. Order Scorpionida 

Scorpions are tropical and subtropical in their distribution, 
occurring in the United States only in the southern part. Some 
look much like crustaceans, for in addition to the four pairs of 
walking legs there are a pair of large chelate pedipalps which are 
easily mistaken for legs. The 
chelicerae also bear chelae. Pos- 
terior to the cephalothorax, which 
bears the appendages, is the 
abdomen. This is conspicuously 
divided into two regions. Seven 
broad somites continue backward 
from the cephalothorax as the 
preabdomen and these are followed 
by a series of six somites of smaller 
size which constitute the postabdo- 
men. The terminal somite of the 
postabdomeu bears a sharp spine 
known as the sting with which a 
pair of poison glands are associated. 
This serves as an effective organ 
for killing insects upon which 
the scorpions feed and produces 
wounds painful even to man. 
Lung books occur in the last four 
somites of the preabdomen on the 
ventral wall of which they open as 

paired spiracles. From three to six pairs of eyes are commonly 
borne on the cephalothorax. A pair of comblike organs of 
undetermined function (Fig. 107), referred to as the pectines, 
occur on the ventral wall of the second preabdominal somite. 

II. Order Araneina 

Spiders are the most popularly known representatives of the 
arachnids and comprise the order Araneina. The body is divided 
by a deep constriction between the cephalothorax and abdomen. 

Fig. 107. — A scorpion (Pandi- 
nus) from ventral surface. {From 
Versluys and Demoll). 



The four prominent, pairs of legs not only serve for walking and 
jumping but the posterior pair also aid in the formation of the 
characteristic silken webs. Except in members of one family, 
the abdomen is saclike, unsegmented, and joined to the cephalo- 
thorax by a narrow stalk. At the caudal end, the abdomen bears 
a small conical portion which represents a greatly reduced 

Spinning organs are located on the ventral surface near the 
caudal extremity of the abdomen and consist, usually, of three 
pairs of spinnerets. These are fingerlike in form and are thought 
to represent rudiments of two pairs of abdominal appendages. 
Spinning tubes are distributed over the terminal portion of each 
spinneret and through these the fluid is expelled, which, upon 
contact with the air, hardens to form silk. An additional spin- 
ning organ known as the cribellum occurs in some spiders. This 
consists of a median, ventral, sievelike plate anterior to the 
spinnerets bearing very numerous spinning tubes. 

The tarantulas (Eurypelma) and the orb weavers (many 
genera in the subfamily Araneinae) are among the extremely 
numerous representatives of this order. 

Ill, Order Acarina 

The mites and ticks have a broad, unsegmented abdomen which 
is not constricted at its union with the cephalothorax. As a 

Fig. 108. — Acarina. Boophilus annulatus, the tick which carries Texas fever; 
A, female; B, male; C, young with only three pairs of legs. (After Banks). 

consequence the entire body appears saclike, though in some 
instances cephalothorax and abdomen are distinguishable. The 
part usually termed the abdomen includes two somites which in 
reality belong to the thorax. Frequently, the segments bearing 
the chelicerae and pedipalps are more or less distinct from the rest 


of the body and are then designated as a beak or rostrum. 
Though the typical number of legs is four pairs, the newly 
hatched young usually have but three pairs (Fig. 108 C), and in 
one family (Eriophyidae) only two pairs are present in the adults. 
The Acarina are of great biological importance. As parasites 
of man and of other animals they have great economic signifi- 
cance, and particularly the ticks, as carriers of disease-producing 
organisms, have received considerable attention. The southern 
cattle tick (Boophilus annulatus, Fig. 108) is the carrier of the 
organism (a protozoan, Babesia) which causes Texas fever or 
tick fever in cattle. The cattle industry in the South is greatly 
hampered by outbreaks of this disease. Through misunder- 
standing of the status of the generic name Boophilus, this tick has 
been referred to frequently in literature under the name Magaro- 
pus. The itch mites {Sarcoptes scabei, Fig. 109) burrow within 
the skin of man and produce a disease known 
as the itch. Mites of poultry and of other 
birds and of mammals also belong to the 
Acarina. The water mites (Hydrachnida) 
usually live as free adults in water, though the 
larvae are frequently encountered as parasites 
on molluscs, insects, and other animals. 

IV. Other Orders of Arachnida 

There are a considerable number of small, Fig. i0 9. — An 
though very sharply marked, groups of ^^^^^ mite, Sar- 

'^ "^ '^ "^ 7 o 1 copies humams . 

arachnids members of which fail to come {After Banks). 
within the foregoing descriptions of the larger 
groups. These are frequently recognized as comprising several 
independent orders. Chief of these are the Pseudoscorpionida. 
the Solpugida, the Phalangida, and the Linguatulida. 

The pseudoscorpions resemble the scorpions except that 
there is no differentiation of pre- and postabdomen and no sting. 
The chelicerae function as spinning organs. Chelifer is a com- 
mon genus. 

The Solpugida are comparatively rare. The head bears 
a pair of extremely large chelicerae which serve for crushing the 
prey and long, leglike pedipalps which seem to function chiefly 
as feelers. The first thoracic segment is fused with the head, 
but the remaining three are free. The genus Eremobates is 
represented by several species in the southern United States. 


The Phalangida are commonly called the harvest men or 
daddy-long-legs. As the latter name implies, the legs are inordi- 
nately long. Respiration is by means of tracheae. They feed 
largely on mites. 

The Linguatulida or Pentastomida are arachnids the bodies 
of which have been so much modified in adaptation to the para- 
sitic habit that the adults are distinctly wormlike. It is only 
through the larval stages that arachnidan relationships are 
evident, for the larva bears two pairs of legs. All but the hooks 
of these legs become lost in metamorphosis, and these are retained 
near the anterior extremity as organs of fixation. Numerous 
genera have been differentiated among these highly modified 
arachnids. Lungs, respiratory passages, and digestive system 
are usual seats of infestation. Pentastoma, Porocephalus. 
Armillifer, and Linguatula are characteristic genera. 

Fig. 110. — A male pycnogonid, Nymphon stroemii, c, chelicerae; o, ovigerous 
legs; p, pedipalpi; r, rostrum. {After Kingsley, from Hertwig's Manual of 
Zoology, by Kingsley, courtesy Henry Holt and Co.). 

Class 3. Pycnogonida 

The pycnogonids are small, exclusively marine arthropods 
which cling to seaweeds and hydroids and at times are dredged 
in great numbers from deep waters. The body consists of 
a cephalothorax and vestigial abdomen. The cephalothorax 
usually bears a terminal suctorial proboscis and seven pairs of 
jointed appendages. The appendages next to the proboscis 
bear chelae. Four pairs of appendages are usually used in 
walking. The third pair of appendages are in some species modi- 
fied in the male for holding the eggs and are termed the ovigerous 
appendages. Reproductive organs open on the second segment 
of certain of the legs. 

Though the abdomen is reduced to a mere vestige, without 
appendages and unsegmented, it contains two pairs of ganglia. 


The pycnogonids rather closely resemble the spiders, but the 
presence of seven pairs of appendages is a character not encoun- 
tered in the Arachnida. There is considerable question as to 
the proper place to include the members of this aberrant group. 
Nymphon (Fig. 110) is a characteristic genus. 

Class 4. Tardigrada 

The water-bears or tardigrades are microscopic organisms 
living in both fresh and salt water. The body is provided with 
four pairs of unsegmented appendages each of which bears 
terminal claws. The number and form of these claws differ 
in the various genera. Neither antennae nor mouth parts are 
found on the head. Thus far neither respiratory nor circulatory 
organs have been demonstrated. 

A single gonad opens into the cloaca. The nervous system 
consists of a brain, a subesophageal ganglion, and a ventral 
chain of four ganglia. 

There is a possibility that the tardigrades may belong to 
the annelid group, though the internal organization seems to 
indicate degenerate arthropod relationships. Macrobiotus is 
the name of one genus. 

Class 5. Onycophora 

The genus Peripatus and several other genera closely allied 
to it present such a mixture of annelidan and arthropodan 
characters that much significance is usually attached to these 
forms as a possible connecting link between the worms and the 
arthropods. The various species, which have been encountered 
in Africa, Australia, New Zealand, Central and South America, 
and the West Indies, comprise several genera which together 
seem to warrant their being grouped as an independent class. 

The body is wormlike or caterpillar-like in form, without exter- 
nal marks of segmentation but with numerous paired legs the 
number of which varies in different species. Though these legs 
have a ringed appearance, they are not distinctly jointed, and in 
this respect they seem to be intermediate between the parapodia 
of worms and the jointed legs of arthropods. The metameric 
arrangement of the legs is paralleled by some features of the 
internal organization. Near each pair of legs, the ventral nerve 
cords bear a slight enlargement, though ganglia are not differen- 
tiated. The base of each leg carries a nephridial opening. Thus 



the nephridia are also metameric in their arrangement. The 
head bears three pairs of appendages. At the anterior extremity, 
it bears a pair of ringed antennae and behind these a pair of 
oral papillae. Two pairs of hooked plates within the mouth 
cavity have been regarded as mandibles. Spiracles in longitu- 
dinal rows, or scattered, communicate internally with respiratory 
tubes which are in the form of tracheae. The genital ducts, 
which are modified nephridia, opens just anterior to the anus. 
The Onycophora are viviparous. In habits, they are nocturnal, 
living during the day under bark and in decaying wood. 

Class 6. Myrientomata 

The minute arthropods included within the single order 
Protura are recognized by some as comprising an independent 
class to which the name Myrientomata has been applied. The 
representatives of this class are somewhat 
similar to the Thysanura in body form, but 
antennae and cerci are both lacking. The 
thorax bears three pairs of legs and on the 
abdomen there are the vestiges of three pairs 
of appendages. 

Most of the described species live in 
humus. The group has been so recently 
established and the species have been so 
little studied that relationships to other 
arthropods have not been well established. 


In the older literature, a number of 
tracheate arthropods with numerous legs 
Pauropus were considered as a single class under the 
name Myriapoda. These forms present so 
many differences in structure that at least 
four distinct classes are now recognized under the names 
Diplopoda (Class 7), Chilopoda (Class 8), Symphyla (Class 9), 
and Pauropoda (Class 10). 

The Chilopoda show many evidences of close relationship 
with the insects, while members of the genus Scolopendrella, 
which represent the Symphyla, show remarkable combinations 
of diplopodan and insectan characters. Except for the eleven 
or twelve pairs of legs, Scolopendrella very closely resembles the 

Ficj. Ill 
huxlcyi. {After 
yon) . 



insects belonging to the order Thysanura. The pauropods are 
minute forms usually under one millimeter in length (Fig. Ill) 
bearing nine pairs of functional legs. 

Class 7. Diplopoda 

The thousand-legs, or millipeds, are terrestrial - arthropods 
with the body consisting of two regions, head and trunk. The 
cylindrical body is composed of numerous segments each of 
which seems to bear two pairs of short legs inserted near the 
median line of the body. The apparent doubling in number of 
appendages on each somite, which is so uncommon in arthropods, 
seems to be explained on the grounds that each seeming segment 
is, in reality, a double somite, for on the ventral surface the 
double nature of the skeletal plates is evident. Some few of 
the segments, especially near the anterior extremity, remain 
simple and bear but a single pair of legs each. The distinct head 
bears a pair of short antennae, usually lateral groups of ocelli, and 
the mouth parts. The latter comprise an unpaired upper lip, a 
pair of mandibles, and one or two additional pairs of jaws. 

Most millipeds are harmless and of little importance, for they 
feed largely upon decaying vegetable matter. Julus, Glomeris, 
Polyxenus, and Spirobolus are commonly encountered genera. 

Class 8. Chilopoda 

The centipeds are terrestrial tracheate arthropods with only 
two body regions in which each of the flattened trunk somites 
bears but a single pair of legs. On the first trunk somite these 
are modified as poison jaws bearing ducts of poison glands. The 
legs are lateral in position and are not borne near the median 
ventral line as in the diplopods. 

The antennae are long and many jointed. The head also bears 
a pair of mandibles and two pairs of maxillae. The common 
house centiped, Scutigera forceps, is a representative of this class. 
Lithobius, Geophilus, and Scolopendra are other genera. 

Outline of Classification 

Phylum Arthropoda, continued. 

II. Class Acerata. — No antennae; body usually cephalothorax and 
abdomen; usually six pairs cephalothoracic appendages; abdomen 
almost always lacking appendages in adult; respiration by abdominal 
gills, lung books or trachcao. 

a. Subclass Gigantostraca. — Marino; gills on abdominal appendages 
two to six; a pair compovuid eyes and one pair median ocelli. 


1. Order Xiphosura. — Cepthalothorax large; abdominal somites 
fused, terniiuating in a long spine. Limulus. 

2. Order Eurypterida. — Extinct; cephalothorax small; abdomen 
twelve jointed; Eurypterus, Fterygotus. 

b. Subclass Arachnida. — Chiefly air breathers; usually four pairs 
walking legs, one pair pedipalps, and one pair chelicerae on 

1. Order Scorpionida. — Abdomen segmented, posterior region 
slender, flexible, frequently ending in sting; pedipalps and cheli- 
cerae chelate. Buthus, Uropleclus, Pandinus, Centrums. 

2. Order Araneina. — Deep constriction between cephalothorax 
and abdomen; abdomen unsegmented; spinning organs near caudal 
extremity. Epeira, Agalena, Eruypelma, Lycosa. 

3. Order Acarina. — Broad, unsegmented abdomen, not constricted 
at union with cephalothorax; frequently with piercing beak. 
Boophihis, Sarcoptes, Dermacentor, Hydrachna. 

4. Order Pseudoscorpionida. — Resembling ^scorpions but abdomen 
not divided into two regions; no sting; chelicerae spinning organs. 

5. Order Solpugida. — Extremely large chelicerae on head. Eniero- 
bates, Galeodes, Dalames. 

6. Order Phalangida. — Four pairs exceedingly long legs. Liobo- 
num, Phalangium. 

7. Order Linguatulida. — Wormlike; two pairs degenerate legs of 
which only claws remain. Pentastoma, Linguatula, Arviillifer, 

III. Class Pycnogonida. — Marine; abdomen vestigial; cephalothorax 
bearing terminal suctorial proboscis and seven pairs very long legs. 
Nymphon, Pycnogonum. 

IV. Class Tardigrada. — Microscopic; aquatic; four pairs unsegmented 
appendages, each with terminal claws. Marcobiotus. 

^V. Class Onycophora. — Wormlike; numerous, short, paired legs, ringed 
but not jointed; tracheae. Peripatus. 
VI. Class Myrientomata. — Minute; antennae and cerci lacking. 

1. Order Protura. — Three pairs legs on thorax; three pairs vestigial 
legs on abdomen. Acerentomon. 
^ VII. Class Diplopoda. — Terrestrial; body cylindrical; numerous seg- 
ments, mostly bearing two pairs of short legs inserted pear midline. 
Julus, Glomcris, Spirobolus, Polyxenus, Parajidus. 

VIII. Class Chilopoda. — Terrestrial; tracheate; numerous segments; 
body flattened; single pair legs to a somite, lateroventral. Lithobius, 
Geophilus, Scutigcra. 

IX. Class Symphyla. — Eleven or twelve pair legs. Scolopendrella. 

X. Class Pauropoda. — Minute; nine pairs legs. Pauropus. 


(See general references at close of Chapter I) 
Banks, N. 1904. A Treatise on the Acarina, or Mites. Proc. U. S. 
Nat. Mus., 28: 1-114. 


Banks, N. 1908. A Revision of the Ixodidae, or Tricks, of the United 

States. U. S. Dept. Agr., Bureau Eutomol., Tech. Ser., 15. 
CoMSTOCK, J. H. 1912. "The Spider Book." Garden City, N. Y., 

Doubleday Page. 
Hunter, W. D. and Hooker, W. A. 1907. Information concerning the 

North American Fever Tick. U. S. Dept. Agr., Bureau Entomol. 

Bull. 72. 
KiNGSLEY, J. S. 1892-3. The Embryology of Limuhis. Jour. Morphol., 

7: 35-68; 8: 195-268. 
Montgomery, T. H. Jr. 1909. On the Spinnerets, Cribelhim, Coluhis, 

Tracheae, and Lung-books of Araneads. Proc. Acad. Nat. Set. Phila. 

May, 1909: 299-320. 
NuTTALL, G. H. F. Numerous papers on the morphology, classification, 

and biology of ticks. Parasitology, Cambridge, England. 
Packard, A. S. 1880. The Anatomy, Histology, and Embryology' of 

Limulus polyphemus. Mem. Boston Sac. Nat. Hist., 1880. 
Patten, W. 1893. On the Morphology and Physiology of the Brain and 

Sense Organs of Limulus. Quart. Jour. Micr. Sci., 35: 1-96. 
. 1912. "The Evolution of the Vertebrates and Their Kin." 

Philadelphia, Blakiston. 
Stiles, C. W. 1910. The Taxonomic Value of the Microscopic Structures 

of the Stigmal Plates in the Tick Genus Dermacentor. U. S. Hygienic 

Lab. Bull. 62. 


Class 11. Insecta 

The members of this class are air-breathing arthropods with 
distinct head, thorax, and abdomen, except in some larvae and 
in some modified adults. They have a single pair of antennae, 
three pairs of thoracic legs, and usually one or two pairs of wings 
in the adult stage. The opening of the reproductive organs is 
near the caudal extremity of the body. As chiefly terrestrial 
animals, insects have become adapted to the greatest variety of 
conditions and have been so successful that they outnumber all 
other animals both in species and in individuals. Practically 
every possible relationship of organic beings is found in a list of 
the habits of insects. Both as predators and as parasites upon 
plants and upon other animals and in their relations to the spread 
of disease, they occupy a position of economic importance not 
excelled by any other animal group. 

Tjrpes of Metamorphosis. — Most insects in their development 
pass through conspicuous changes in form between the time 
that the individual leaves the egg and the time it reaches full 
maturity. In these changes, which are collectively known as 
metamorphosis, the young lacks some structures or organs 
characteristic of the adult. These are attained only in later 
development, and frequently some structures or organs of the 
young are distinctive and become lost during later development. 
Numerous gradations occur between the condition in which the 
young is but slightly different from the adult and that in which 
it bears practically no resemblance to the adult to which it later 
transforms. As a consequence, several different types of meta- 
morphosis are recognizable. Insects which undergo a complete 
metamorphosis, that is, those which involve profound changes 
in form (Fig. 112) and have a pupal stage which is usually inac- 
tive, are said to be holometabolous or are referred to as the Holo- 
metabola. Egg, larva, pupa, and imago are all distinguishable 
in the Holometabola. The organs of one stage in development 




are not necessarily carried over directly into the following stage. 
Many of the larval organs disappear as a result of phagocytic 
or chemical action, and through histogenesis new organs of the 

A * c 

Fig. 112. — Development of a holometabolous insect. A, larva; B, puparium; 
C, imago of fly {Phormia regina) . {From Folsom's Entomology, courtesy of P. 
Blakiston's Son and Co,). 

adult are formed. In the Holometabola, wings and legs of the 
adult do not develop externally on the larva but develop inter- 

FiG. 113. — Development of a heterometabolous insect. Six successive 
instars of the squash bug, Anasa tristis. A to E, nymphs; F, adult or imago. 
(From Folsom,'s Entomology, courtesy of P. Blakiston's Son and Co.). 

nally as imaginal buds (Fig. 126) which emerge and become free 
only later in development. 

The Heterometabola include those insects which transform 
without a true pupal period. The stages between the egg and 


the imago fairly closely resemble the general bodily structure 
of the imago (Fig. 113) and are termed nymphs. The lack of 
functional wings usually differentiates the nymph from the adult, 
but external wing pads from which the wings later develop are 
characteristic of many nymphs. 

The Thysanura and CoUembola are wingless and throughout 
life retain essentially the forms they have at hatching. There 
are some changes, but these are so inconspicuous that many are 
inclined to refer to the insects of these two orders as the 

Appendages of the Head. — The head of an insect bears a single 
pair of antennae, the eyes, and the mouth parts (Fig. 114). 
These last comprise an unpaired labrum or upper lip, a pair of 
mandibles, the hypopharynx, the maxillae, and the labium. 
The labium, or lower lip, is in reality a second pair of maxillae 
of which at least the basal portions are fused along the median 
line. Both the maxillae and the labium are composed of several 
distinct sclerites and bear palpi, designated respectively as 
maxillary and labial palpi. All of the appendages of the head 
are articulated with immovable parts, forming the head capsule. 

Modifications of Mouth Parts.^ — The mouth parts are subject 
to numerous modifications of form and function. As described 
above, they are suited for holding and chewing food, but in many 
groups only liquid food is talcen and in these groups some of 
the mouth parts are modified to form a sucking tube. The 
most significant of these suctorial modifications are found in the 
Hemiptera, the Lepidoptera, the Diptera, and the Hymenoptera. 

The jointed beak of the Hemiptera consists of a troughlike 
labium partially covered above at its base by the labrum. 
Within this trough the elongated maxillae and mandibles are 

The long, coiled proboscis of the Lepidoptera is formed of 
parts of the maxillae, while the labrum, mandibles, and labium 
are greatly reduced or wanting. 

The female mosquito has a piercing type of mouth parts. 
The labrum and epipharynx are fused and with the hypopharynx 
form the food channel. The linear mandibles and maxillae are 
used in puncturing the skin of the victim, while the labium 
forms a sheath for the other mouth parts. 

Suctorial and mandibulate functions are both performed by 
the mouth parts of the honeybee. The mandibles are used for 



cutting, crushing, and other purposes as in strictly mandibulate 
types. The maxillae and the labial palpi are folded to form a 
sheath within which an elongated portion of the labium serves 
as a lapping tongue. 

Sclerites and Sutures of the Head. — A considerable number of 
sclerites are fused to form the head capsule. In many instances, 
the sutures separating the sclerites are visible, and both sutures 
and sclerites bear definite names. In the generalized insects, 
as, for example, in the Orthoptera, the epicranial suture (Fig. 
114) is one of the best and most constant landmarks. This 
suture originates at the margin of the occipital opening (through 
which the vicera of the head and thorax are continuous) and 

Epkroinial suiure 





Fig. 114. Fig. 115. Fig. 116. 

Figs. 114-116. — Morphology of an insect head. 114, frontal aspect of cock- 
roach head; 115, lateral aspect; 116, dissected frontal aspect. {Redrawn from 
Comstock with permission). 

extends as a median suture over the dorsal surface of the head. 
At its ventral extremity, the epicranial suture bifurcates, and 
thus its form is that of an inverted Y. Between the arms of the 
Y, there is an unpaired sclerite, called the front, which in most 
insects bears the median ocellus. An additional unpaired sclerite 
ventral to the front is the clypeus. On its ventral border the 
clypeus is articulated with the labrum or upper lip. 

The several paired sclerites of the head, including the lateral 
surfaces (Fig. 115) and the parts dorsal to the arms of the epi- 
cranial suture, constitute what is termed the epicranium. The 
vertex is just dorsal to the front. It is between the compound 
eyes and usually bears the paired ocelli. The occiput extends 
between the vertex and the occipital foramen mentioned above. 
The genae and the postgenae form the lateral portions or cheeks 
of the epicranium. A chitinous supporting structure called the 
tentorium (Fig. 116) is found within the head. This usually 



consists of a central plate from which two or three pairs of arms 
pass to the exoskeleton of the head. 

Sensory Organs. — Both simple (Fig. 117) and compound eyes 
are found in the insects. Compound eyes occur on the sides of 
the head in most adult insects except some generalized and some 
parasitic forms. Practically all insects which have a complete 
metamorphosis (the Holometabola) have simple eyes in the 
larval stage. 

Aside from the eyes, other sensory organs in insects show 
remarkable lack of uniformity in location upon the body and in 
localization and organization. Even the antennae, which are 
popularly thought of as tactile, have in some species of insects 
no less than seven different types of microscopic sensory organs. 
Most of these are probably tactile, auditory, and olfactory, 

though frequently it has been 
necessary to assume functions 
upon the basis of structure and 
position and by interpretation of 
the reactions of the insect rather 
than to determine them by actual 
demonstration. Hairs of gradu- 
ated lengths upon the feathery 
antennae of mosquitoes and moths 
vibrate in response to sound 
waves through a range which in 
the mosquito coincides with the 
pitch of the mosquito hum. 
Another type of auditory organ, 
of widely different structure and 
location, is the tympanal organ, 
which consists of a drumlike membrane for the reception of 
sound waves. In the common grasshoppers, this tympanal 
organ is located on each side of the first abdominal somite, while 
in katydids a similarly constructed organ is found on the second 
joint of each front leg. 

End organs of taste and smell are usually located on the maxil- 
lary palpi, the epipharynx, the hypopharynx, and the labial 
palpi, but they are not restricted to any given organ or appendage. 
Tactile organs and sensory structures of undetermined func- 
tions occur as modifications of the body covering over most of 
the surface of an insect. 

Fig. 117. — Median ocellus of 
beetle, Acilius. (After Patten) 


Somites of the Head. — As in the other arthropods, paired 
appendages are considered as a criterion for the determination of 
the number of somites in the head. There are evidences that the 
eyes, antennae, mandibles, maxillae, and labium are borne on 
distinct segments. Beyond this, study of early embryological 
stages furnishes evidence that an additional pair of appendages 
starts to form in the embryo but never becomes functional or is 
at most rudimentary in the adult. Traces of this embryonic 
appendage are between the fundaments of the antennae and 
mandibles. Thus in position these correspond to the second 
antennae of the crayfish and may be homologized with them. 
It thus seems probable that the head of an insect has resulted 
from the fusion of six original somites only five of which have 
retained their appendages or their rudiments in the adult insect. 

The Thorax. — The thorax is the region which bears the legs and 
wings when they are present in the nymphs and adult insects. It 
is composed of three more or less firmly united segments. In 
order, backward from the head, these are: prothorax, mesothorax, 
and metathorax. In many insects, the last two bear wings. 
Each somite is composed of several sclerite groups which, accord- 
ing to their location, are recognized as comprising the parts of the 
tergum (dorsal wall), sternum (ventral wall), and pleura (lateral 
walls) . 

The Legs. — Each leg is articulated to the wall of the thorax 
partly by means of small articular sclerites in the region of articu- 
lation of the sternum and pleuron. Five divisions or regions are 
recognizable in each leg. From articulation outward, these are: 
coxa, trochanter, femur, tibia, and tarsus. In most insects, each 
region is but a single segment except the tarsus which commonly 
has five segments. Correlated with highly variable modifications 
in function the legs display numerous modifications in form. 

The Wings. — In many of the winged insects the mesothorax 
and the metathorax each bears a pair of wings, but in instances of 
only one pair of wings these are usually borne upon the meso- 
thorax. Rudiments of the second pair of wings are frequently 
present as halteres or balancing organs upon the metathorax. 
Some striking modifications of the primitively membranous 
wings occur. In beetles, the mesothoracic wings are thickened, 
horny structures, the elytra, modified for the protection of the 
metathoracic wings and the dorsal surface of the body. In some 
of the bugs, the bases of the front wings (hemelytra) are horny 



while the tips are membranous. In Orthoptera, the entire front 
wings are somewhat thickened and are designated as tegmina. 
Structure and Origin of the "Wings. — Typically, the wings of 
insects are two pairs of membranous appendages which develop 
as saclike folds of the body wall. In fully developed wings, this 
saclike nature is obscured because the two walls of the sac 
become so closely applied that they appear as a single membrane, 
and a very delicate one at that. A framework upon which this 

Cosia Subcosh I 

Subcosh 2 

Medici 4' 

ki Anal 


'■Media 3 
. Cubi-tus I 

Fig. 119. 

Fig. 120. 

Figs. 118-120. — Insect wing venation. 118, hypothetical primitive type of 
wing venation with cross- veins added; 119, forewing of Hyptia showing great 
reduction in number of veins; 120, forewing of a May fly showing great increase in 
number of veins. {Redrawn from Corns took with permission). 

membrane is supported is composed of hollow tubes or veins 
which originate as modified tracheae of the respiratory system. 
In the early stages of development, the tracheae are accompanied 
by nerves and blood-filled outgrowths of the body cavity. The 
pattern which this framework assumes is spoken of as the "wing 
venation." In most groups, this pattern shows remarkable con- 
stancy among the members of the same species. Between genera 
and the larger groups there is usually considerable variation in the 
details of arrangement of the veins. It seems, however, that the 
wings of all insects are directly homologous and that their princi- 
pal individual veins are likewise homologous. Comparisons of 
numerous wings and the study of the arrangement of the tracheae 


before the wings reach their functional form have led to the 
formulation of a hypothetical plan of primitive venation (Fig. 
118) from which all wing patterns are derivable, though all of the 
homologies are not readily observable by the beginner in this 

Plan of Wing Venation. — In the plan of primitive venation 
(Fig. 118), eight principal veins are present and most of these may 
be branched. Beginning with the anterior margin when the 
wings are at right angles to the body, these veins are: costa, 
subcosta, radius, media, cubitus, first anal, second anal, and third 
anal. Changes from the hypothetical type occur either by the 
addition of new veins or branches (Fig. 120) or through the 
reduction in the number of veins (Fig. 119) through atrophy or 
through coalescence of two or more veins. Cross-veins fre- 
quently connect two of the longitudinal veins. 

The Abdomen and Its Appendages. — The abdomen bears a 
highly variable number of segments in the various groups of 
insects. A study of insect embryology shows that the abdomen 
consists normally of eleven segments, but in later development 
adjacent segments may coalesce and in some adults they are 
telescoped one with another. The wall of each segment is 
composed of a tergum and a sternum united by a pair of pleural 
membranes. The segments are typically without appendages 
and are approximately similar except near the caudal extremity 
where certain segments are more or less modified. In the Thy- 
sanura, rudimentary abdominal limbs occur and in the embryos of 
some other insects each segment may bear a pair of rudimentary 
appendages. Those of the first seven abdominal segments are 
usually lost during early embryonic life, while the last two 
or three pairs frequently persist to form the genitalia — the 
genital claspers of the males and the ovipositors of the females 
(Fig. 121). 

A true ovipositor consists of three pairs of valves, called gono- 
pophyses, arranged as a dorsal and a ventral pair surrounding an 
inner pair. The inner valves form a channel through which 
the eggs are conveyed. There is strong evidence that these 
three pairs of gonopophyses represent the paired appendages 
of the eighth, ninth, and tenth abdominal somites and are seri- 
ally homologous with the thoracic legs. In the Hymenoptera, 
the ovipositor is modified as a stinging organ and has poison 
glands associated with it. 



The inner pair of gonopophyses of the males are modified as an 
intromittent organ or cirrus, while the other two pairs of gono- 
pophyses are frequently modified as clasping organs which func- 
tion in copulation. Through many groups the external geni- 
talia show remarkable constancy of form within each species. In 
many instances, the forms grouped together as single species 
by early writers are at present being separated into several 
clearly distinct species chiefly on the basis of characters furn- 
ished by a study of the male genitalia. The last (eleventh) 
abdominal somite in many insects bears a pair of caudal append- 
ages known as cerci. Both in form and in function these are 
highly variable. 

9 10 t1 

89 10 i\ 

Fig. 121. — Extremity of abdomen of a grasshopper, Mdanoplus dlffere?itialis; 
A, male; B, female. The terga and sterna are numbered, c, cercus; d, dorsal 
valves of ovipositor; c, egg guide; p, podical plate; s, spiracle, s.p., suranal plate; 
V, ventral valves of ovipositor. (From Folsom's Entomology) . 

The Respiratory System. — The internal organization of insects 
is essentially like that of the other arthropods already described. 
The respiratory system offers one of the most conspicuous points 
of difference from most other arthropodan groups. This is a 
system of air tubes, called tracheae and tracheoles, which carries 
air to all parts of the body (Fig. 122). This system is in com- 
munication with the outside air through openings in the body 
wall, termed spiracles, of which there are normally ten pairs, 
arranged, as a rule, two pairs on the thorax and eight on the 
abdomen. Opening and closing of the spiracles for the admission 
or expulsion of air is under control of the insect. From each 
spiracle a short trachea commonly leads to a main tracheal trunk 
of which there is one on each side of the body. Branches from 
these two main trunks penetrate into even the minutest parts 
of the body. 

Tracheae arise as invaginations of the body wall, and con- 
sequently the infolded chitinous covering of the body continues 



within the tracheae as a chitinous internal hning of the tubes. 
Within the tracheae the chitin is not disposed in a uniform layer 
but assumes the form of a coiled spiral thread lining the inner wall 
of the tracheal vessel. Small tubes lacking the spiral chitinous 
threads form the most minute subdivisions of the tracheal system 
and are designated as tracheoles. 
Tracheae in some insects may 
become modified as enlarged sacs 
which serve as air reservoirs. 

Modifications of the Respira- 
tory System. — Of the typical 
respiratory system with spiracles 
communicating directly between 
the tracheae and the outside air, 
there are many modifications. 
In the CoUembola, and in many 
aquatic larvae, there are no 
specialized organs for respira- 
tion, for that function is per- 
formed directly through the skin. 
Gills occur in many aquatic 
nymphs and larvae. These are 
of two distinct types, tracheal 
and blood gills. In the former, 
lateral or caudal evaginations of 
the body wall are furnished with 
numerous tracheae which are 
continuous with the vessels of 
the tracheal system within the 
body and conditions suitable for 
a respiratory exchange are thus 
provided. Even a portion of 
the digestive tract may be 
appropriated as a respiratory 
organ, as in the rectal tracheal 
gills of the dragonfly nymphs. 
Blood gills are usually threadlike evaginations from the body 
wall of aquatic insects. The spaces within these gills are in 
direct communication with the fluid-filled body cavity, and 
through the delicate walls of the gills a respiratory exchange is 
made possible without requiring the presence of tracheal tubes. 

Fig. 122.— Tracheal system of an 
insect, a, antenna; 6, brain; I, leg; n, 
nerve cord; p, palpus; s, spiracle; .si, 
spiracular branch; t, main tracheal 
trunk; v, ventral branch; t'.s., \dsceral 
branch. (From Folsom's Entomology, 
after Kolbe). 



A few insects, especially larvae, live under conditions which 
exclude the presence of oxygen. This is especially true of some 
larvae living in very deep water. Under these conditions, 
anaerobic respiration is carried on. 

The central nervous system consists of a brain or supraeso- 
phageal ganglion and a longitudinal nerve chain ventral to the 
digestive tract as in all of the Arthropoda. Typically each 
thoracic and abdominal somite is supplied with a ganglion 

Fig. 123. — Different degrees of concentration of the ventral cord of arthropods 
(from Gegenbauer) . A, termite (after Lespes); B, water beetle (after Blanchard); 
C, fly (after Blanchard); D, scorpion spider (after Blanchard). a, abdomen; 
(7^, 0^, ganglia of ventral cord; gi, infraesophageal ganglion; gs, supraesophageal 
ganglion; o, eye; p'p'\ walking feet; tr, lung books; 1, chelicerae; 2, pedipalpi. 
(From Hcrtwig's Manual of Zoology by Kingsley, courtesy Henry Holt and Co.). 

(Fig. 123 A). It is, however, a very common thing for the nerve 
chain to undergo concentration as a result of which the ganglia 
in adjoining somites fuse (B and C). The extent of such longi- 
tudinal condensation of the nervous system varies greatly in 
different arthropods and even shows remarkable changes during 
the life of a single individual. Thus, in the honeybee, the larva 
has a distinctly metameric nerve chain through the thorax and 
abdomen but when the adult stage is reached the thoracic ganglia 
has been reduced to two and the abdominal to five. 

A sympathetic nervous system may include a ventral trunk 
associated with the central nervous system and some very 
delicate branches near the dorsal part of the body. Branches 



from this sympathetic system pass chiefly to the dorsal vessel 
and the tracheal system. 

Embryology. — The fertilized egg nucleus of insects undergoes 
several divisions and many of the resulting nuclei migrate from 
the yolk mass to the superficial layer of the cytoplasm. The 
layer of cytoplasm containing these nuclei undergoes cleavage 
and there is thus formed a layer of cells surrounging a central 
yolk mass (Fig. 8). This is essentially a blastula stage in devel- 
opment, and the cell layer is fre- 
quently termed the blastoderm. 
Those nuclei which remain within the 
yolk become surrounded by cyto- 
plasm and are designated as the 
yolk cells. The blastoderm becomes 
thickened in one region and forms 
the germ band from which the ven- 
tral surface of the embryo later 
develops. The course of this develop- 
ment follows two different paths in 
different insect groups. These are 
known as the overgrown and the 
invaginated types of development. 

The former of these involves much g'""^''^ ^^p^ of insect develop- 

ment. ,4, amniotic folds start- 

the less complicated narration. The 
germ band (Fig. 124) sinks below the 


Fig. 124. — Two diagrammatic 
sagittal sections to show over- 

ing to cover the germ band 
(cross-lined) ; B, amniotic folds 
completed. {From Korschelt and 

surface of the surrounding blastoderm Hcider). 
to form a groove. As this groove 

deepens, the walls of the blastoderm fold up over the germ 
band. When the folds of the blastoderm meet, they fuse and the 
double wall thus formed encloses a cavity known as the amniotic 
cavity, which lies between the germ band and the before-men- 
tioned double wall. The outer layer of cells, which comprises 
the outer margin of the double wall, is termed the serosa and is 
directly continuous with the blastoderm surrounding the yolk. 
The inner cell layer, which lines the amniotic cavity, is termed 
the amnion. The cells of the germ band comprise a layer of 
ectoderm adjacent to the amniotic cavity, and underlying this 
ectoderm is a mass of cells which represent both the entoderm 
and the mesoderm of the embryo. The surface of this germ 
band, which represents the ventral surface of the developing 
insect, becomes traversed by a series of transverse grooves to 



form the primitive segments of the embryo. Upon these 
primitive segments, paired outgrowths occur. These are the 
fundaments of the legs and other appendages. At first these 
appendages are all similar in appearance, but as development 
proceeds some of them become suppressed while the remaining 
ones begin to take on different forms depending upon the kind 
of appendage each is to form in the adult animal. Lateral and 
dorsal parts of the insect body result from the growth and 
extension of the germ band after the ventral structures have been 
laid down„ 

A B C D E 

Fig. 125. — Diagrammatic median sections to illustrate development of a 
Libellulid egg. ^4, development of germ band; -B, invagination of germ band; 
C, development of amniotic fold; D, closure of opening into amniotic cavity and 
dtevelopment of rudiments of appendages; E, migration of embryo from amniotic 
cavity back to surface of egg. {From. Korschelt and Heider after Brandt), 

A much more complicated condition exists when the germ band 
undergoes invagination. In such an instance, the germ band 
makes its appearance (Fig. 125 A) on the posterior ventral 
surface of the egg. One end of the germ band sinks into the 
underlying portion of the egg and, submerged within the egg, 
begins to grow forward (B) toward the anterior pole of the egg, 
carrying with it the blastoderm which was attached to the 
posterior margin of the band. Practically all of the germ band 
thus invaginates. The cavity formed within the egg by this 
invagination is the amniotic cavity, one wall of which is composed 


of the cells of the germ band and the other of the blastoderm cells. 
The amniotic cavity is then cut off from the exterior. The 
invagination which has brought the germ band to lie within the 
amniotic cavity has seriously altered the orientation of the germ 
band with reference to other parts of the egg. The ventral 
surface of the embryo, which originated on the ventral external 
surface of the egg, has by invagination come to be directed 
toward the dorsal side of the egg. While in this position (C and 
D), the embryonic appendages make their appearance. Soon 
afterward the embryo is everted from the amniotic cavity and 
again comes to lie (E) on the surface of the egg with its parts 
coinciding with the original orientation of the egg. 

In the embryological development of most of the Orthoptera, 
Trichoptera, Diptera, Lepidoptera, and Hymenoptera the over- 
grown or superficial type of germ band occurs. The invaginated 
or immersed type of germ band is found in some of the Odonata, 
Coleoptera, Thysanoptera, and Hemiptera. Transitional con- 
ditions, which seem to be intermediate between these two types, 
are found in some of the Coleoptera and the Orthoptera. 

Internal Metamorphosis and the Imaginal Discs. — In both 
types of metamorphosis, an insect undergoes radical changes in 
its internal organization before reaching the adult stage. Even 
organs which are found ahke in the larval and adult stages do not 
pass over directly from one stage to the next, but through the 
processes of histolysis the tissues of one stage disappear and 
entirely new tissues are formed through the processes known as 
histogenesis. In the Holometabola, where entirely new struc- 
tures such as wings and legs become functional for the first time 
with attainment of the adult form, still more profound internal 
changes accompany metamorphosis. During the larval and 
pupal stages, rudiments of the legs, wings, and head appendages 
make their appearance as internal buds. These imaginal discs, 
as they are called, are, throughout their early development, 
enclosed within internal sacs (Fig. 126 A). The hypoderm of the 
body wall invaginates to form these sacs, and the rudiment of the 
appendage which each of these sacs contains lies thus within a 
cavity entirely surrounded by hypoderm. Only in later develop- 
ment (Fig. 126 C and D) do the sacs open and allow the develop- 
ing appendages to extend freely beyond the body surface. 
The development and transformation of the imaginal discs of 
the head and thorax of a fly are shown in Fig. 126. 



In the classification of Linnaeus, there were but seven order 
of insects recognized. These have, in recent times, been sub- 
divided and new orders have been added until today there are 
more than twenty well-defined orders of the Insecta. In the 
accompanying table, these orders are listed and some of the 
characters which are usually considered of importance are 
furnished for each. 

The Importance of Insects. — Insects have such significance 
in their relations to human welfare that it is difficult to give even 
a faint idea of their importance without going into extensive 
details, for volumes have been written upon this phase of the 
subject. The honeybees, the silk worms, and the various insects 

of legs v,_ 

A B C D 

Fig. 126. — Development and transformation of imaginal discs in head and 
thorax of A, larva; and B-D, pupa of a fly (Musca). Wing rudiments are omit- 
ted. {From Korschclt and Heider after VanRees, in part after Kowalevsky) . 

which pollinate flowers are so widely and so popularly known that 
their importance need not be discussed here. As pests, the 
Hemiptera occupy a place of extreme significance, with the 
chinch bug {Blissus leucopterus) as a menace to grain farming 
and the hundreds of species of scale insects which attack trees 
and shrubs as outstanding examples. Among the Dipt era, 
numerous disease-carrying flies and mosquitoes, the Hessian 
fly which destroys growing grain, and warbles and bots which 
attack domestic animals demand attention. The gipsy moth 
in its ravages upon shade trees, the coddling moth in its molesta- 
tion of the apple industry, and the numerous cutworms are 
examples of important Lepidoptera. Extensive damage to cotton 
crops through the boll weevil and much of the injury by wood- 
boring and leaf-eating insects are attributable to species of the 

Predaceous insects such as the coccinelid beetles, the dragon 
flies, and the Neuroptera aid materially in checking the damage 


























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wrought by other insects. Likewise, the Hymenoptera and the 
Diptera include many useful species in that they are frequently 
parasitic upon and cause the death of destructive insects. The 
illustrations cited here are only a start in the enumeration of the 
economically significant representatives of the class Insecta. 

Outline of Classification 

Phylum Arthropoda, concluded. 

XI. Class Insecta. — Three pairs legs; typically two pairs wings; head, 

thorax, and abdomen distinct; air breathers. 
See table giving The Orders of Insects (page 251). 


(See general references at close of Chapter I) 

COMSTOCK, J. H. 1924. "An Introduction to Entomology." 3d. ed. 
Ithaca, N. Y., Comstock. 

and Comstock, A. B. 1915. "A Manual of the Study of Insects." 

Ithaca, N. Y., Comstock. 

EssiG, E. O. 1929. "Insects of Western North America." New York, 

FoLSOM, J. W. 1922. "Entomology with Special Reference to Its Ecologi- 
cal Aspects. 3d ed. Philadelphia, Blakiston. 

Herms, W. B. 1915. "Medical and Veterinary Entomology." New 
York, Macmillan. 

Herrick, G. W. 1914. "Insects Injurious to the Household and Annoy- 
ing to Man." New York, Macmillan. 

Kellogg, V. L. 1908. "American Insects." New York, Holt. 

LuTZ, F. E. 1921. "Field Book of Insects." New York, Putnam. 

MacGillivray, a. D. 1923. "External Insect-anatomy." Urbana, 111., 
Scarab Co. 

Metcalf, C. L. and Flint, W. P. 1928. "Destructive and Useful 
Insects, Their Habits and Control." New York, McGraw-Hill. 

Needham, J. G. and Lloyd, J. F. 1930. "The Life of Inland Waters." 
Springfield, 111., Thomas. 

Sanderson, E. D. and Peairs, L. M. 1920. "Insect Pests of Farm, 
Garden, and Orchard." New York, Wiley. 

Snodgrass, R. E. 1910. Anatomy of the Honeybee. U. S. Dept. Agr., 
Bur. Entomol., Tech. Scr:, 18. 

Wheeler, W. M. 1893. A Contribution to Insect Embryology. Jour. 
MorphoL, 8: 1-160. 


The phylogenetic relationships of organisms and the origin 
of species have long been topics of more than ordinary interest to 
the scientists. In one of the most generally maintained of the 
early conceptions, all relationship was denied because it was 
thought that each species represents an independent, super- 
natural creation. Such was the belief of even so recent a scien- 
tist as Linnaeus whose systematic arrangement and classification 
of plant and animal forms furnish one of the best clues to an 
interpretation of the relationships of many groups. The natural 
origin of species from preexisting forms of life was first given 
strong support through the careful observations and generaliza- 
tions of Charles Darwin, whose book "The Origin of Species" 
(1859) has probably been more discussed than any other scientific 
work. With the establishing of a belief in blood relationship 
between the various animals a new interest in the tracing out 
of that relationship became awakened. 

At the present time there is much ground for lack of agreement 
concerning how species have come into existence, but among 
scientists there is no longer any doubt that species, genera, 
and even phyla have moved in a continuous procession since the 
inception of life upon this planet. Even a cursory survey of the 
evidences from paleontology reminds one of the facts that there 
are no indications of vertebrates existing upon the earth prior 
to the Lower Cambrian period of the earth's history and that 
in general the faunas of successive periods and eras show advance- 
ment beyond, yet undeniable relationships with, the preexisting 
life. It is not the purpose of this chapter to prove the ideas of 
continuity, development, and differentiation of living organisms, 
for this is now accepted by those who take the trouble to examine 
the evidences as little less than axiomatic. Nor yet is it the 
intention to speculate upon the methods or processes whereby 
these changes in the fauna and the creation of different types 
have come about. Even among the best informed scientists 
there is yet lack of unanimity of opinion upon these subjects. 



Any attempt at classification in some measure endeavors to 
convey a concept of relationships. Groupings or separations are 
based upon an evaluation of the presence or the lack of common 
characteristics. In most instances, the possession of fundamen- 
tal characters of like nature is considered as indicative of common 
origin. As pointed out in Chapter I, the student is apt to con- 
sider phylogenetic relationships from an erroneous point of view, 
assuming that such kinship is traced between the respresentatives 
of present-day groups when as a matter of fact all of our existing 
organisms have undergone some degree of differentiation since 
their origin. It is only through common ancestry that these 
lines of kinship are traced. 

Sources of Evidences of Relationships.^ — Obviously, then, 
relationships are not observable directly but frequently may be 
deduced from evidences gathered from one or more of the fields 
of paleontology, embryology, and comparative anatomy. Even 
these available sources in many instances give incomplete and 
inconclusive evidence of an organism's past history. Present-day 
organisms are but points in the lines of evolutionary progress. 
The source and the termination of any such line are but rarely 
discovered. Through the study of paleontology and embryology 
we gain a glimpse here and there of portions of the path which 
various organisms have followed in reaching their present posi- 
tion in the lines of evolutionary progression. 

Paleontology. — In determining a broad outline of the course 
of phylogeny among the invertebrates, the science of paleontology 
offers but fragmentary and woefully incomplete data concerning 
the ancestral forms. The Cambrian period has disclosed repre- 
sentatives of most of the important invertebrate phyla, yet, 
as Schuchert has said, more fundamental evolution had taken 
place up to this time than subsequently. Studies of the rocks 
of the preceding periods have revealed rare and imperfect and 
almost indecipherable evidences of a marine fauna including 
Protozoa and marine worms. Careful studies by Walcott and 
others have furnished evidences that practically all of the impor- 
tant invertebrate phyla were represented in the fauna of pre- 
Cambrian times. However, the pre-Cambrian rocks, have under- 
gone metamorphosis under the action of heat and pressure 
until their contained fossil remains have become concealed or 
even destroyed. 


Because of the incompleteness of our knowledge of pre-Cam- 
brian life, the evidences of phylogeny of the lower forms must 
be sought elsewhere than in the fossil remains. In individual 
development, there is furnished a clue to racial history. 

Law of Biogenesis or Recapitulation Theory. — Since the days 
of von Baer, it has been commonly observed that individuals 
which are markedly different in appearance as adults have stages 
in their development when the embryos have much the same 
appearance. Structures common in the embryos may have 
entirely different fates in the adults. The works of Haeckel 
did much to popularize this observation. So generally did the 
principle seem possible of application that it became expressed as 
a law — the law of biogenesis- — ^which states that ontogeny is a 
brief recapitulation of phylogeny. 

Thus, since the course of individual development more or less 
faithfully repeats racial history, those characters which make 
their appearance early in the course of embryology represent a 
heritage from distant ancestors of the race. The more distant 
the ancester the more numerous the offspring and consequently 
the more kinds of animals which would display these ancestral 
characters in the course of their development. Such ancestral 
characters, appearing early in development, have been desig- 
nated as palingenetic. In contrast with them, the coenogenetic 
characters which appear relatively late in development are more 
nearly specific and are found in much smaller groups of 

There are many instances in which individual development has 
become so highly modified that the operation of the biogenetic 
law seems to be invalidated. In some instances, the processes of 
ontogeny become so much shortened that whole chapters in the 
racial history are deleted in the course of individual genesis. 
These and other facts have led some investigators to discredit 
the law entirely. It seems probable, however, that in many 
instances valuable light is thrown upon racial history through 
the study of embryology. 

Gastraea Theory. — Since a gastrula stage occurs in the onto- 
geny of practically all of the Metazoa, Haeckel propounded a 
theory which endeavored to establish a blood relationship among 
all Metazoa through an ancestral form to which he applied the 
name of the Gastraea. The Gastraea he considered as a hypo- 
thetical form which in its adult organization displayed the 


characters of the embryonic gastruhi. Though our present-day 
Coelenterata are said to stand on a level of the gastrula in funda- 
mental structure, the Gastraea must not be confused with the 
members of this phylum which have undergone great differentia- 
tion and progressive evolution. 

Without discrediting the Gastraea theory in the least, it 
represents but a step in the right direction, for do not the various 
gastrulae in turn originate from a single-celled condition — ^the 
fertilized egg? Thus all animal forms may be traced along their 
phylogenetic paths to a common ancestry in the one-celled 
organisms. Even Haeckel recognized this fact at the time when 
he propounded his Gastraea theory. The highways between 
the single-celled condition and our highly diversified Metazoa 
have not all been surveyed. Only here and there are there 
indications of the routes which have been taken. Some forms 
seems to have gone on an independent track early in the course 
of evolution. Others seem to have traveled long distances 
together before the ancestral stock became diversified to form 
various ones of our present-day animals. Some groups, such 
as the Vertebrata, the Echinoderma, and the Nematoda, seem 
to have left no conclusive evidence regarding their relation- 
ships to any other animal groups. Speculation plays a large 
part in attempting to decipher the faint lines which are directed 
toward but never lead to any of the usual roads of descent. 

In the following section attention will be directed to some of 
the probable ancestral lines of the various Metazoa. 

Metazoan Tendencies in the Protozoa. — In Chapter II, it has 
been pointed out that within the Protozoa are found many 
indications of tendencies toward metazoan conditions. Chief 
among these are colony formation and isolation of germ plasm 
from the soma. Much has been made of the parallel between 
the blastula stage in the embryology of the Metazoa and the 
spherical arrangement of the cells in colonies such as Volvox. 
By one or several paths of descent, the ancestors of our present- 
day Protozoa have probably been the direct or indirect source of 
all the remaining phyla. 

Porifera. — While the Porifera are usuallj^ accorded a position 
as the lowest phylum of the Metazoa, there are numerous 
reasons for assuming that the simplicity of sponges is attributable, 
in part, to degeneracy or to regressive evolution. The collared 
cells of the Porifera are the exact counterparts of the bodies of 


choanoflagellates. Proterospongia (Fig. 50 B) is a unique form 
which might resemble the ancestors of the Porifera and in an 
unusual manner stands intermediate between the choanoflagel- 
lates and the sponges. This is a colony of choanoflagellates the 
individual cells of which are embedded in a matrix containing 
some wandering amoeboid cells comparable to the mesoderm cells 
of the Porifera. 

Coelenterata and Ctenophora. — It seems probable that a 
primitive gastrula of the nature of Haeckel's hypothetical 
Gastraea must have had its origin from a spherical protozoan 
colony by more rapid growth at one pole which resulted in an 
invagination to form the gastrula cavity. The planula, some 
modification of which occurs in the larval development of all 
coelenterates and of the ctenophores, is but a slightly modified 
gastrula. As MacBride has so well pointed out, a planula-like 
ancestor probably gave origin to the members of these two phyla. 

Plathelminthes.^ — ^In an earlier chapter it has been stated 
that Coeloplana (Fig. 59) and Ctenoplana are modern genera 
which stand intermediate between the flatworms and the cteno- 
phores and display an odd combination of characters some of 
which had come to be considered as diagnostic for one or the 
other of the two phyla. It is probable that through some form 
similar to these the flatworms have arisen from a ctenophore-like 

Nemathelminthes. — Regarding the origin and relationships of 
the Nemathelminthes practically nothing is known. They 
stand as peculiarly isolated forms whose history has never been 
deciphered. The method of mesoderm formation seems to 
preclude any close relationships with the annehd worms. There 
has been no well-founded theory advanced as to their origin or 
relationships. The numerous suggestions which have been 
offered are based upon very minor details. 

The Trochophore. — In the Plathelminthes, Coelhelminthes, 
Molluscoidea, Trochelminthes, and Mollusca there occur larvae 
which represent only minor modifications of the trochophore 
(Fig. 76). Thus a direct relationship among these forms is 
traceable through the larvae. In fundamental structure the 
trochophore has been likened to the ctenophores. It has already 
been shown that the Plathelminthes are readily derivable from 
the ctenophore organization. The metamerism, so characteristic 
of the Annelida, has been explained on various grounds. Begin- 


nings of metamerism are readily observable in the flatworms. 
Regular arrangement of the lateral diverticula from the digestive 
tract and orderly disposition of the gonads between these divertic- 
ula indicate preparations for a segmentation of the body before 
the septa make their appearance as in the annelids. 

Echinoderma. — The echinoderms offer but few clews to an 
explanation of their origin. Their radial symmetry is not 
primitive, and, since the larvae are bilaterally symmetrical 
throughout, it seems probable that the members of this group 
have evolved from a bilaterally symmetrical coelomate ancestor. 
MacBride has emphasized the fact that echinoderm larvae 
represent a primitive type not directly related to the trochophore. 
Echinoderm larvae show more resemblance to the tornaria larva 
of Balanoglossus than to any of the lower Metazoa. In some 
instances this fact has been utilized in tracing a possibile ancestry 
of the vertebrates through a line which passes back through the 
Enteropneusta (Balanoglossus) to a primitive type from which 
the echinoderms and the Enteropneusta have arisen. 

Arthropoda. — The arthropods have many points in their 
morphology which are likewise shared by the annelids. Even 
the parapodia of the annelids are not so markedly different from 
the foliaceous appendages of the lower Crustacea. The Onyco- 
phora (Peripatus) were for a long time considered as worms, but 
closer investigation revealed characters which have been looked 
upon as diagnostic of the Arthropoda. Standing as they do, mid- 
way between the arthropods and the annelids, the Onycophora 
show a transition from one group to the other which may well be 
taken as indicative of a wormlike ancestry for the arthropods. 

In this phylum there arises an interesting conflict between the 
evidences presented by embryology and morphology. On the 
basis of morphological study the arthropods seem to have their 
closest affinities with the elongate annelidlike forms, but in 
development all of the lower Crustacea pass through a character- 
istic larval form known as the nauplius. This larval form is 
very short and bears but three pairs of appendages. There is no 
conclusive method of weighing the relative merits of the two 
lines of evidence. It seems, however, that the nauplius might 
be an adaptation to a free-swimming existence and thus may 
have undergone changes a full record of which has not been 
retained in the much shortened history of the race which is pre- 
sented by ontogeny. Some who see in the nauplius a significant 


ancestral form have considered it as a modified trochophore 
not fundamentally different in structure from some of our present- 
day rotifers. The swimming appendages of rotifers like Pedalion 
are thought by some to represent incipient arthropod appendages. 

Ancestry of the Vertebrates. — Some of the most widespread 
interest in the blood relationships of animals centers around the 
question of the origin of the Chordata. Even though the verte- 
brates seem to have made their appearance at a time when a 
fairly complete picture of the fauna is observable through the 
fossil remains, there is little light thrown upon the origin of 
the vertebrates through the study of paleontology. Fossils 
of some of the heavily armored fishes, the Ostracoderms, closely 
resemble the fossil arachnids known as the Merostomata and our 
modern Limulus which some one has called a "living fossil." 
The study of comparative anatomy, however, seems to indicate 
that the vertebrates must have had a simpler beginning than this 
would indicate. 

The three subphyla of the Prochordata, represented by 
Amphioxus (or Branchiostoma), Balanoglossus, and the tuni- 
cates, have characters which permit them to be classified with 
the Chordata yet in most instances display a low type of general 
organization which seems to relate them to the non-chordate 
forms. Each of these groups has been considered as a possible 
ancestral stock from which the true vertebrates have had their 
origin. Since these forms lie outside the scope of this textbook, 
their relationships to the problem of vertebrate phylogeny will 
not be considered in detail, but attention will be directed to some 
of the invertebrate groups through which the ancestry of the 
vertebrates has been derived by various investigators. 

As has been pointed out in an earlier discussion, the Metazoa 
in general, including the Vertebrata, develop from a single 
cell, the zygote. Thus a possible ancestral history of the verte- 
brates leads back to the single-celled organisms. This rela- 
tionship is so distant, however, that many attempts have been 
made to find satisfactory evidences of vertebrate geneology 
through the higher groups of the invertebrates. Early metamer- 
ism of the vertebrate embryos indicates that some segmented 
organism gave rise to the vertebrate line, but further than this 
the evidences are capable of broadly divergent interpretations. 
In consequence, numerous theories have been advanced. Space 
does not permit a detailed account of all of these but some 


of the more important ones have been chosen for discussion. 
The coelenterates, the nemertines, the annehds, and various 
arthropods have been championed as the possible origin of the 
vertebrate series. 

Annelid Theory. — The metameric condition of annelids, the 
relation of their nephridia to the coelom, and the fundamental 
relationships of vascular and nervous systems to the digestive 
tract closely resemble the conditions found in the lower verte- 
brates and in vertebrate embryos. Semper and other investiga- 
tors have shown that by inverting the position of the body of 
an annelid the fundamental systems are brought into almost 
complete harmony with their arrangement in the vertebrates. 
Such a shift in the orientation of the body is not at all uncommon 
in various animal groups. Many of the crustaceans and molluscs 
move with their ventral surface in a dorsal position. Even the 
notochord, which is distinctive for the Chordata, has a counter- 
part in the bundles of supporting fibers which accompany the 
annelidan nerve chain. Recent publications of Delsman have 
awakened a new interest in the annelid theory of vertebrate 

Nemertine Theory. — Hubrecht has maintained that the nemer- 
tines stand in the direct line of ancestry of the vertebrates. 
One of the chief arguments in favor of this theory is the possible 
homology between the proboscis sheath of the nemertine and 
the notochord of the chordate. The lateral nerve cords of 
the nemertine could assume a dorsal position as in the chordates 
without a complete change in the orientation of the body such 
as is necessitated in the annelid theory. 

Arachnid Theory. — Patten has seen in the arachnids, especially 
in forms like the scorpions and Limulus, many points of structure 
in direct harmony with vertebrate organization. Through 
comparisons of these arachnids with fishlike Ostracoderms he 
has built up an elaborate theory showing a possible origin of 
the vertebrates from arachnidan ancestry. 

Conclusion. — Each of the numerous theories, of which the 
foregoing are characteristic examples, is based upon a group 
of facts derived from studies in comparative anatomy and embry- 
ology. Yet no one theory depicts a satisfying genealogy of 
the vertebrate group. At most, the various hypotheses furnish 
ground for a belief in kinship between the vertebrate and the 
invertebrate. Kinship or common origin would explain most 


of the facts which have been arrayed as a proof or demonstration 
of vertebrate origin from the invertebrates. 

Similarity in structure and development and even homologies 
between the members of two animal groups do not prove that 
one has originated from the other. At most, they point to a 
common heritage. Our various theories demonstrate undis- 
puted interrelationships between the chordate phylum and the 
non-chordates, but the key to the ancestry of the vertebrates 
lies hidden, possibly lost, in some form of past ages which has 
been an ancestor alike to the vertebrates and the higher inverte- 
brates and through which both of these groups have inherited 
the characters which they hold in common. A search for this 
ancestral form among the highly differentiated animal forms of 
today is little short of hopeless. 


Delsman, H. C. 1922. "The Ancestry of Vertebrates as a Means of 
Understanding the Principle Features of Their Structure and Develop- 
ment." Weltevreden (Java). 

Gaskell, W. H. 1908: "The Origin of the Vertebrates." London, Long- 
mans Green. 

HuBRECHT, A. A. W. 1883: On the Ancestral Form of the Chordata. 
Quart. Jour. Micr. Sci., N. S., 23: 349-368. 

Lull, R. S. 1917: "Organic Evolution." New York, Macmillan. 

MacBride, E. W. 1914. "Text-book of Embryology." Vol. 1, "Inverte- 
brata." London, Macmillan. 

OsBORN, H. F. 1917. "The Origin and Evolution of Life." New York, 

Patten, W. 1912. "The Evolution of the Vertebrates and Their Kin." 
Philadelphia, Blakiston. 

Semper, C. 1875. Die Stammesverwandtschaft der Wirbelthiere und 
Wirbellosen. Arbeit. Zool. Zoot. Inst. Wurzburg, 2: 25-76. 

Walcott, C. D. 1899: Pre-Cambrian Fossiliferous Formations. Bull. 
Geol. Soc. Amer., Apr. 6, 1899: 199-244. 

Wilder, H. H. 1909. "History of the Human Body." New York, Holt. 


Abalone, 196 

Abdominal appendages of Insecta, 

Abothrium, 122, 126 
Acanthobdella, 153 
Acanthocephala, 128, 133, 136 
Acanthocephalus, 136 
Acanthocystis, 57 
Acanthonietra, 57 
Acanthosphaera, 57 
Acarina, 228, 234 
Acephala, 184, 186, 201 
Acephaline gregarines, 44 
Acerata, 205, 225, 233 
Acerentomon, 234 
Acetabulum, 113 
Acineta, 54, 58 
Acmaea, 196, 201 
Acoela, 110 
Acontia, 99 
Actinaria, 103 
Actinomonas, 56 
Ac'tinophrys, 57 
Actinopoda, 40, 56 
Actinosphaerium, 57 
Actinotrocha, 164 

Adductor muscles of Acephala, 186, 
of Brachiopoda, 162 
Adhesive cells, 101 
Adjustor muscles, 162 
Adoral shields, 175 
Adradii, 96 
Aeginopsis, 102 
Aeolis, 201 
Aeolosoma, 157 
Aeolosomatidae, 152 
Aequorea, 102 
Aerobic respiration, 73 
Aesthetes, 185 

Agalena, 234 
Agassiz, A., 167 
Aglantha, 102 
Agriolimax, 197, 201 
Alcyonacea, 103 
Alcyonaria, 98, 100, 103 
Alcyonium, 103 
Allassostoma, 126 
Allogromia, 57 
Alternation of generations, 9 

Coelenterata, 90 

Protozoa, 43 
Alveolar layer of cytoplasm, 50 
Ambulacral area, 168, 175 
Ambulacral groove, 169 
Ambulacral system, 165 
Ametabola, 238 
Amicula, 201 
Amnion, 247 
Amoeba, 40, 41, 57 
Amoebida, 57 
Amoebocytes, 167 
Amphiblastula, 85 
Amphilina, 118, 126 
Amphineura, 185, 200 
Amphioplus, 175, 182 
Ampliioxus, 259 
Amphipoda, 221, 223 
Amphistomata, 126 
Amphithoe, 223 
Amphitrite, 157 
Ampullae, 83, 155 
Anaerobic respiration, 73, 246 
Anal veins, 243 
Anaplocephala, 122, 126 
Ancestry of vertebrates, 259 
Ancylostoma, 132, 136 
Ancjdus, 197 
Ancyroccphalus, 113 
Anisotropic, 66 
Annelida, 145 




Annelid theory, 260 

Annelidan cross, 15, 148 

Annulations of leeches, 153 

Anodonta, 201 

Anomia, 201 

Anopheles, 45 

Anoplura, 251 

Anostraca, 211 

Antedon, 181, 182 

Antennae, 205, 207, 209, 214 215 

218, 236 
Antennal gland, 217 
Antennules, 207, 211 
Anthomediisae, 102 
Anthophysa, 34, 56 
Anthozoa, 96, 103 
Antipatharia, 103 
Antipathes, 103 
Anuraea, 143 
Aphrodita, 151, 157 
Apical plate, 150 
Aplacophora, 201 
Aplysiidae, 196 

Aplysina, 87 

Apoda, 182 

Apopyle, 83 

Appendages, 140, 203 
biramous, 205, 206, 208 
crustacean, 205 
foliaceous, 206, 212 
insectan, 238 
phyllopodan, 206 
polychaet, 148 
rotifer, 140 
schizopodal, 205, 206 

Apsilus, 143 

Apus, 210, 222 

Arachnid theory, 260 

Arachnida, 229, 234 

Araneina, 227, 234 

Arbacia, 177, 182 

Arboroid colony, 29 

Area, 201 

Arcella, 39, 41, 57 

Archenteron, 15, 70 

Archiannelida, 156, 158 

Archigetes, 118, 126 

Arcyria, 57 

Aronicola, 152, 157 
Argonauta, 200, 201 
Argvdus, 213, 222 
Arion, 197, 201 
Aristotle's lantern, 176 
Armata, 158 
Armillifer, 230, 234 
Arrow-worms, 145 
Artemia, 210, 222 
Arthropoda, 203, 222, 233, 258 
Articular sclerites, 241 
Articulata, 162, 163, 164 
Asaphus, 209 

Ascaris, 130, 131, 136 * 

germ cells, 10, 12 
lumbricoides, 231 
maturation, 12 
megalocephala, 130 

Ascon, 82 

Asellus, 222, 223 

Asexual reproduction, 7 

Aspidobranchia, 196, 201 

Aspidocotylea, 125 

Aspidogaster, 125 

Asplanchna, 140, 143 

Astacus, 223 

Astasia, 56 

Asterias, 182 

Asterina, 182 

Asteroidea, 166, 169, 182 

Astrangia, 103 

Astrometis, 182 

Astropecten, 182 

Astrophyton, 175, 182 

Atlanta, 196, 201 

Atoke, 149 

Aurelia, 95, 103 

Auricles, 72 

Auricularia, 179 

Autolytus, 157 

Autosomes, 13 

Autotomy, 219 

Autotrophic, 24 

Avicularia, 160 

Axial gradient, 109 

Axon, 67 

Axopodia, 38 

Axostyle, 30 




Babesia, 229 
Balanoglossus, 258 
Balantidium, 57 
Balamis, 215, 223 
Barnacles, 215 
Barnea, 201 
de Bary, 22 
Basal granules, 30, 52 
Basals, 180 
Basipodite, 206 
Basket stars, 174 
Bdelloida, 143 
Bdelloiira, 125 
Beach fleas, 221 
Beak, 229 
Beroe, 103 
Beroida, 101, 103 
Bicosoeca, 56 
Binary fission, 7 
Biogenesis, law of, 17, 255 
Bipinnaria, 173 
Biramous appendages, 205 
Birth habits, 18 
Bivium, 171 
Bladderwprm, 120 
Blastoderm, 247 
Blastoidea, 179, 182 
Blastomeres, 14 
Blastopore, 16, 70 
Blastula, 13 
Blepharoplast, 30, 31 
Blood gills, 73, 245 
Blood-suckers, 155 
Bodo, 56 

Bojanus, organs of, 189 
Bolinopsis, 103 
Book lungs, 73, 74 
Boophilus, 229, 236 
Bosmina, 212, 222 
Bothrimonus, 122 
Bothriocephalus, 122, 126 
Bougainvillia, 102 
Boveri, 130 
Brachiolaria, 173 
Brachionus, 143 
Brachiopoda, 159, 161, 164 

Bracts, 93 

Brain, 78, 110, 141, 148, 204 

Branchiae, 170 

Branchinecta, 222 

Branchiopoda, 210, 222 

Branchiura, 208, 222 

Breeding habits, 17 

Bristle jaws, 146 

Brittle stars, 174 

Brood sac, 190, 211, 212, 221 

Bryozoa, 159 

Buccinum, 201 

Bucephalus, 125 

Budding, 7, 27 

internal, 8, 48 
Bugula, 160, 164 
Bulimulus, 197 
Bulla, 201 

Bursa, copulatory, 136 
Bursae, 174 
Bursaria, 57 
Busycon, 201 
ButschH, 130 
Buthus, 234 
Buttons, 186 
Byssus gland, 188 


Cadulus, 201 
Calcarea, 87 
Calkins, G. N., 21, 38 
Callinectes, 223 
Calonympha, 31 
Calyx, 180 
Cambarus, 223 
Campanella, 102 
Campanularia, 102 
Campelonia, 201 
Cancer, 223 
Candona, 215, 223 
Caprella, 222, 223 
Capsular membrane, 40 
Carapace, 205, 206 
Carchesium, 58 
Carcinus, 223 
Cardium, 201 
Carinaria, 196, 201 
Carinella. 126 



Carterius, 87 
Cartilage, 64, 65 
Caryophyllaeiis, 126 
Cattle tick, 229 
Cell as an individual, 20 

Cell constancy, 15 

Cell lineage, 15 

Cell organs, 22 

Cellulose, 33 

Cement glands, 136 

Centipeds, 233 

Central nervous system, 77, 78, 94, 
110, 125, 130, 135, 141, 146, 
148, 167, 174, 177, 184, 189, 
193, 204, 220, 246 

Centrolecithal, 14 

Centropyxis, 41, 57 

Centrosome, 14 

Centrums, 234 

Cephaline gregarines, 44 

Cephalopoda, 197, 201 

Cephalothorax, 206, 225 

Cephalothrix, 126 

Ceramaster, 182 

Ceratium, 56 

Cercariae, 115 

Cerebral ganglia, 193 

Cerebratulus, 126 

Cerebro-pleural ganglion, 189 

Cerianthus, 103 

Cestida, 103 

Cestoda, 116, 126 

Cestodaria, 118, 126 

Cestus, 102, 103 

Chaetoderma, 201 

Chaetogaster, 157 

Chaetognathi, 145, 157 

Chaetonotus, 143 

Chaetopleura, 201 

Chaetopoda, 147, 157 

Chaetopterus, 157 

Chagas' disease, 37 

Chalk, 42 

Charybda, 102 

Chelae, 218, 227 

Chelicerae, 226, 227 

Chelifer, 229, 234 

Cheliped, 218 

Child, CM., 109 

Chilomastix, 35, 56 
Chilomonas, 56 
Chilopoda, 205, 232, 234 
Chitons, 185, 201 
Chlamydomonas, 56 
Chloridella, 218, 223 
Chloromyxum, 48 
Chlorophyll, 24, 32 
Choanoflagellates, 34, 35, 257 
Chordata, 1 
Chordodes, 133 
Chromatophores, 32 
Chromosomes, 11, 26 

autosomes, 13 

in Protozoa, 26 

X-, 13 

Y-, 13 
Chromulina, 56 
Chronology, geologic, 6 
Chrysidella, 34, 56 
Chrysomonadina, 56 
Chrysopyxis, 56 
Chydorus, 222 
Cilia, 29, 49, 52 
Ciliata, 29, 49, 57 
Ciliated grooves, 148, 186 
Ciliophora, 29, 49, 57 
Circulatory system, 71 
Circumferential canal, 90 
Cirri of ciliates, 52 

of crinoids, 179 
Cirripathes, 103 
Cirripedia, 209, 215, 223 
Cirrus, 76, 136, 149 
Cladocera, 210, 222 
Clam worm, 149, 151 
Clams, 186 
Clasping organs, 76 
Classification, basis of, 2 
Clathrina, 87 
Clathruhna, 57 
Clava, 102 
Cleavage, determinate, 14 

indeterminate, 14 

patterns, 14 
Cleveland, L. R., 35 
Cliona, 87 
Clitellata, 154 
Clitellum, 152 



Cloaca, 83, 130 

Clonorchis, 126 

Clymenella, 157 

Clypeaster, 177, 182 

Clypeastroidea, 177, 182 

Clypeus, 239 

Cnidosporidia, 57 

Coccidia, 43, 45 

Coccidiomorpha, 57 

Cocoon, 155 

Coelenterata, 88, 102, 257 

Coelhelminthes, 145, 157, 257 

Coelomoducts, 148 

Coeloplana, 107, 257 

Coeloria, 103 

Coenogenetic, 255 

Coenurus, 121 

Coleoptera, 251 

Coleps, 57 

Collared flagellate cells, 82, 256 

Collembola, 238, 251 

Colony, 11, 28, 34, 59, 84 

Colpidium, 57 

Colpoda, 57 

Columella, 193 

Comatrichia, 57 

Combs, 100 

Commission on nomenclature, 3 

Complemental males, 215 

Compound eyes, 79, 207, 209, 217, 

226, 240 
Conchostraca, 211 
Congress, International zoological, 3 
Conjugation, 27, 51 
Connective tissue, 64 
Conochilus, 143 
Constancy, cell, 15 

nuclear, 15, 135, 142 
Contractile vacuole, 40, 50 
Copepoda, 209, 212, 222 

parasitic, 208, 213 
Copepodid, 208 
Copulation, 18, 142, 152 

reciprocal, 18, 155 
Copulatory bursa, 130 
Copulatory organs, 142, 219 
Corallium, 103 
Corallobothrium, 122, 126 
("oral reefs, 97 

Corals, 97, 100 
Cordylophora, 90, 106 
Corrodentia, 251 
Corymorpha, 92 
Coryne, 102 
Costa, 243 
Cothurnia, 58 
Cotylaspis, 125 
Coxa, 241 
Coxopodite, 206 
Crabs, 205, 208, 216, 218 
Crago, 223 
Crania, 162, 164 
Crayfish, 216, 218 
Crepidula, 196, 201 
Cribellum, 228 
Cribillina, 160 
Cribrina, 103 
Crinoidea, 166, 179, 182 
Crinoids, 179 
Crisia, 164 

Cristatella, 160, 161, 164 
Crithidia, 56 
Crop, 155 
Crossaster, 182 
Crustacea, 203, 205, 222 
Cryptochiton, 201 
Cryptomonadina, 56 
Cryptomonas, 56 
Crystalline style, 189 
Ctenidia, 185, 196 
Ctenobranchia, 196, 201 
Ctenocephalus, 209 
Ctenodiscus, 174, 182 
Ctenophora, 88, 100, 103 

phylogeny of, 257 
Ctenoplana, 107, 257 
Cubitus, 243 
Cubomedusae, 96, 102 
Cucumaria, 182 
Cultures, 24 
Cunina, 102 
Cunocantha, 102 
Cuspidaria, 201 
Cuticula, 105, 128, 205 
Cuttlebone, 200 
Cuttlefish, 197, 200 
Cuvier, 88, 165 
Cyanea, 95, 103 



Cyathocephalus, 122, 126 
Cyathomonas, 56 
Cyclammina, 57 
Cyclochaeta, 58 
Cyclocoelum, 126 
Cyclophyllidae, 122, 126 
Cyclops, 207, 213, 222 
Cydippida, 101, 103 
Cypridinidae, 215 
Cypris, 215, 223 
Cypris stage of Cirripedia, 215 
Cysticercoid, 120 
Cysticerciis, 120 
Cystoidea, 179, 182 
Cytopharynx, 22, 31 
Cytoplasm, 20, 22 

regional differentiation of, 50 
Cytopyge, 22, 50 
Cytostome, 22, 50 


Dactylogyrus, 113, 125 
Dactylozooids, 93, 94 
Daddy-long-legs, 230 
Dalmanites, 208 
Daphnia, 211, 222 
Darwin, 253 
Datames, 234 
Davainea, 122, 126 
Decapoda, 200, 201, 207, 223 
Deiopea, 103 
Delamination, 16 
Delsman, 260 
Demospongia, 87 
Dendraster, 182 
Dendrocoelum, 125 
Dendrites, 66 
Dendrosoma, 54 
Dentalium, 201 
Dermacentor, 234 
Dermaptera, 251 
Dermasterias, 182 
Dermomuscular sac, 77 
Dero, 157 
Desor's larva, 124 
Determinate cleavage, 15 
Deutomerite, 43 
Deutoplasm, 9, 14 

Devilfishes, 197, 200 

Diaptomus, 222 

Dil)ranchia, 199, 200, 201 

Dictydium, 57 

Dicyemidae, 60 

Didinium, 24, 57 

Differentiation, histological, 20, 60 

Difflugia, 41, 57 

Digenea, 112, 113, 125 

Digenetic trematodes, 113 

Digestion, 69 

Digestive parenchyma, 110 

Digestive system, 69 

Digononta, 143 

Dileptus, 57 

Dimorpha, 56 

Dimorphism, 94, 149 

Dina, 158 

Dinenympha, 56 

Dinobothrium, 126 

Dinobryon, 56 

Dinoflagellata, 33, 56 

Dinophilus, 156 

Dioctophyme, 136 

Dioecious, 75 

Diphyllobothrium, 126 

Diplobothrium, 113 

Diplocardia, 153, 157 

Diplodinium, 21, 57 

Diplodiscus, 126 

Diploid, 12 

Diplopoda, 205, 232, 233, 234 

Diptera, 251 

Dipylidium, 121, 122, 126 

Directives, 98 

Disc, 169 

Discina, 162, 164 

Discoidal cleavage, 199 

Discomedusae, 102 

Distomata, 126 

Distyla, 143 

Diurella, 143 

Divaricator muscles, 162 

Docoglossa, 196 

Doflein, 29 

Dondersia, 201 

Dorsal vessel, 204 

Dorylaimus, 133 

Driesch, Hans, 167 



Dujardin, 22 
Dysentery, 24, 41 


Earthworm, 153 
Ecdysis, 204 

Echinarachnius, 177, 182 
Echinococcus, 126 
Echinoderma, 165, 258 
Echinoidea, 166, 175, 182 
Echinopluteus, 176 
Echinorhynchus, 136 
Echiurus, 158 
Ectoderm, 15 
Ectoplasm, 22, 27 
Ectoprocta, 160, 164 
Edriasteroidea, 182 
Edwardsia, 99, 103 
Egg, 10, 14, 75 
Egg sacs, 213 
Eimeria, 57 
Eleutherozoa, 169, 182 
Elytra, 241 
Embioptera, 251 
Embryo, 16 
Embryology, 254, 255 
Emerita, 223 
Emerobates, 234 
Encystment, 34 
Endamoeba, 41, 57 
Endites, 206 
Endomixis, 27, 52 
Endoplasm, 22, 50 
Endopodite, 206 
Endoprocta, 160, 161, 164 
Energy release, 72 
Ensatella, 201 
Enteropneusta, 1, 258 
Entoderm, 15 
Entodinium, 57 
Enzj'mes, 189 
Epeira, 234 
Ephelota, 54, 58 
Ephemerida, 251 
Ephippium, 212 
Ephydatia, 87 
Ephyra, 95 
Epibolic, 16 

Epicranial suture, 239 

Epicranium, 239 

Epimeral plates, 221 

Epimerite, 44 

Epischura, 222 

Epistylis, 58 

Epithelio-muscular cells, 65, 129 

Epithelium, 61, 62, 63 

Epitoke, 149 

Erdmann, Rhoda, 27 

Eremobates, 229 

Ergasilus, 222 

Eriophyidae, 229 

Errantia, 151, 157 

Estheria, 210, 211, 222 

Eubranchipus, 210, 222 

Eucopepoda, 222 

Eudorina, 8, 33, 56 

Euglena, 56 

Euglenoid movement, 32 

Euglenoidina, 56 

Euglypha, 41, 57 

Eugregarinaria, 44 

Eulamellibranchia, 201 

Eunice, 149 

Euplectella, 87 

Euplexaura, 103 

Euplotes, 52, 57 

Euryalae, 182 

Eurypelma, 228, 234 

Eurypterida, 226, 234 

Eurypterids, 225 

Euspongia, 87 

Euthyneura, 196, 197, 201 

Evasterias, 182 

Exopodite, 206 

Exoskeleton, 77, 203 


Fairy shrimp, 210 
Fasciola, 115, 126 
Fasciolopsis, 126 
Fat tissue, 64 
Favia, 103 
Femur, 241 
Ferrissia, 201 
Fertilization, 11, 18 

reciprocal, 18, 153 

self, 18 



Filaria, 131, 133, 136 

blood inhabiting, 131 
Filibranchia, 201 
Filopodia, 3S 
Finger-nail shells, 191 
Fish-lice, 213 
Fission, 7, 25, 27 
Fissurella, 196, 201 
Flagella, 30, 83 
Flagellated chambers, 83 
Flame cells, 74, 105, 125 
Floscularia, 140, 143 
Floscularida, 143 
Flukes, 111 
Follicle cells, 9 
Foot of Mollusca, 184 
Foraminifera, 39, 40, 41, 57 
Forcipulata, 182 
Fredericella, 164 
Front, 239 
Funiculus, 160, 161 
Funnel of Ctenophora, 101 


Galeodes, 234 
Gametes, 8, 21 

specialization of, 9 
Gametoblast, 45 
Gametogenesis, 11 
Gammarus, 222, 223 
Ganglia, 67, 77 
Gastrocopta, 201 
Gastrodiscus, 126 
Gastropoda, 192, 201 
Gasterostomata, 125 
Gastraea theory, 255 
Gastral tentacles, 95 
Gastroliths, 220 
Gastrotricha, 138, 142, 143 
Gastrovascular system, 70, 89 
Gastrula, 15 
Gelasimus, 223 
Gemmules, 8, 81, 84 
Genae, 239 
Genital claspers, 243 
Genital plates, 170 
Genital rachis, 176 
Genitalia, 243 

Geodia, 87 

Geologic chronology, 6 
Geonemertes, 126 
Geophilus, 233, 234 
Gephyrea, 156, 158 
Germ band, 247 
Germ cells, 8, 9, 10, 11 
Germ plasm, 11 
Giardia, 31, 35, 36, 56 
Gigantorhynchus, 136 
Gigantostraca, 225, 233 
Gill bailer, 219 
Gills, 73, 149, 167, 188, 206 
Glandular epithelium, 62, 69 
Glaridacris, 118, 126 
Globigerina, 57 
Glochidium, 190 
Glomeris, 233, 234 
Glossina, 37 
Glossiphonia, 156, 157 
Glottidia, 164 
Glugea, 57 

Gnathobdellida, 155, 158 
Golgi apparatus, 50 
Gonads, 10, 63 
Gonangia, 91 
Gonionemus, 102 
Gonium, 33, 56 
Gonodactylus, 218, 223 
Gonoducts, 148 
Gonophore reduction, 92 
Gonopophyses, 243 
Gonotheca, 94 
Gordiacea, 128, 133, 136 
Gordius, 133, 136 
Gorgonacea, 103 
Gorgonia, 97, 103 
Gorgonocephalus, 182 
Gradient, axial, 109 
Grantia, 83, 87 
Green gland, 206, 220 
Gregarina, 43, 44, 57 
Group concepts, 3 
Gunda, 110 
Gymnodinium, 56 
Gymnolaemata, 164 
Gyrocotyle, 118, 126 
Gyrodactylus, 113, 125 




Haeckel, 255 

Haemocoel, 204 

Haemogregarina, 57 

Haemonchus, 136 

Haemopis, 158 

Haemosporidia, 45 

Hair snakes, 128 

Haliotis, 196, 201 

Halosj'dna, 157 

Halteres, 241 

Halteria, 57 

Haliclystus, 96, 102 

Haplobothriuni, 122 

Haploid, 12 

Haplozoon, 56 

Harpes, 201 

Harvest men, 226, 230 

Head, 109, 145, 184, 192, 197, 203, 

218, 236, 239, 241 
Head capsule, 238 
Heart, 204, 211 
Hectocotj'lization, 199 
Heliozoa, 40, 57 
Helix, 197 
Helodrilus, 153, 157 
Hemelytra, 241 
Hemigrapsus, 223 
Hemiptera, 251 
Henneguya, 48 
Henricia, 172, 174, 182 
Hepatopancreas, 207, 220 
Heredit}' in Protozoa, 27 
Hermaphroditic gonad, 10, 75, 195 
Hermaphroditism, 10, 75, 205 
Heterocoela, 87 
Heterocyemidae, 60 
Heterodera, 136 
Heterometabola, 237 
Heteronemertini, 126 
Heteronereis, 149 
Heteronomous metamerism, 203 
Heteropoda, 196 
Heterosomes, 13 
Heterotricha, 50, 57 
Hexactinellida, 87 
Hexacoralla, 98 
Hinge ligament, 186 

Hirudinea, 153, 157 

Hirudo, 156, 158 

Histogenesis, 237, 249 

Histological differentiation, 20, 60 

Histolysis, 249 

Holometabola, 236, 237 

Holothuroidea, 166, 177, 182 

Holotricha, 57 

Holozoic, 24, 34 

Homalonotus, 210 

Homarus, 223 

Homocoela, 87 

Homolecithal, 14 

Homonomous metamerism, 148, 203 

Hormiphora, 102 

Horse-shoe crabs, 225 

Hubrecht, 260 

Hyallela, 222, 223 

Hyalonema, 87 

Hydatina, 143 

Hydra, 90, 93, 102 

Hydrachna, 234 

Hydrachnida, 229 

Hydractinia, 102 

Hydrocoel sacs, 166 

Hydrocorallina, 102 

Hydroid, 88 

Hydroides, 152, 157 

Hydromedusa, 91 

Hydrophobia, 55 

Hydropolyp, 91 

Hydrorhiza, 91 

Hydrotheca, 94 

Hydrozoa, 89, 91, 102 

Hymenolepis, 121, 122, 126 

Hymenoptera, 251 

Hypermastigina, 56 

Hypodermis, 128, 203 

Hypopharynx, 238 

Hypotricha, 52, 57 

Ichthyophthirius, 49 
Idotea, 222, 223 
Imaginal buds, 237, 249 
Imago, 236 

Implantation cone, 67 
Inarticulata, 162, 164 



Incurrent canals, 83 
Incus, 140 

Indeterminate cleavage, 14 
Inermia, 158 
Infrabasals, 180 
Infusoria, 49 
Ink sac, 198 
Insecta, 205, 236, 252 
Interambulacral, 168, 175 
Internal buds, 8 

International commission on nomen- 
clature, 3 
Interradii, 96 
Intromittent organ, 18, 76 
Introvert, 160, 190 
Invaginated development, 247 
Invertebrates, 1 
Iota, 133 
Irritability, 66 
Ischnochiton, 201 
Isogametes, 9 
Isopoda, 222, 223 
Isoptera, 251 
Isospora, 45, 57 
Isotropic, 66 
Itch, 229 

Jellyfish, 88 
Jennings, H. S., 28 
Joenia, 56 
Julus, 233, 234 


Kala-azar, 37 
Karyosomes, 26 
Keber's organ, 189 
Kerona, 57 
Keyhole urchin, 177 
Kinetic elements, 31, 34 
Kofoid, C. A., 31 
Krohnia, 147, 157 

Labial palpi, 239 
Labium, 239 

Labrum, 239 
Lacrymaria, 57 
Lacunae, 204 
Lamp shells, 162 
Lampsilis, 201 
Land crabs, 205 
Land snails, 192, 195 
Laqueus, 164 
Larva, 16, 236 
Lasmigona, 191 
Lateral line, 129 
Laurer's canal, 114 
Law of priority, 3 
Leda, 201 
Leeches, 153 
Leidyana, 44 
Leishmania, 37, 56 
Lemnisci, 135 
Lepas, 215, 223 
Lepidochiton, 201 
Lepidoderma, 143 
Lepidonotus, 151, 157 
Lepidoptera, 251 
Lepidurus, 211, 222 
Leptasterias, 173, 182 
Leptocardii, 1 
Leptodora, 211, 222 
Leptolinae, 102 
Leptomedusae, 102 
Leptoplana, 125 
Leptosynapta, 182 
Leptotheca, 48, 57 
Lernaea, 222 
Leucilla, 83 
Leuckart, 88 
Leucocytozoon, 57 
Leucon, 83 
Leucortis, 87 
Leucosolenia, 83, 87 
Libinia, 223 
Ligula, 117, 122, 126 
Lillie, Frank, 168 
Limax, 197, 201 
Limnadia, 211, 222 
Limnetus, 211, 222 
Limpets, 192, 194 
Limulus, 226, 234 
Linckia, 182 
Lines of growth, 186 



Lineus, 126 

Linguatula, 229, 230, 234 
Linguatulida, 205, 230, 234 
Lingula, 162, 164 
Linnaeus, 4, 145, 253 
Liobonum, 234 
Liriope, 102 
Lithobius, 233, 234 
Littorina, 196, 201 
Liver, 194 
Loa, 136 
Lobata, 102, 103 
Lobopodia, 38 
Lobster, 216, 218 
Locomotor system, 76 
Loeb, Jacques, 168 
Loligo, 200, 201 
Lophomonas, 56 
Lophophore, 159, 160, 163 
Lorica, 139 
Lovenia, 182 
Loxosoma, 161, 164 
Lucernaria, 96, 102 
Luidia, 182 
Lumbricus, 153, 157 
Lung, 192, 195 
Lung book, 225, 227 
Lung sacs, 73 
Lvcosa, 234 
Lymnaea, 201 


Macracanthorhynchus, 136 
Macrobdella, 156, 158 
Macrobiotus, 231, 234 
Macrocheira, 223 
Macroconjugant, 51 
Macrogametes, 10, 21 
Macronucleus, 49, 50 
Madrepora, 103 
Madreporaria, 103 
Madreporite, 170 
Magellania, 164 

Malacostraca, 207, 209, 216, 223 
Malaria, 42, 45 
Malarial organisms, 26, 42, 43 
Mallei, 140 
Mallophaga, 251 
Malpighian tubes, 74 

Mancasellus, 223 
Mandibles, 207, 225 
Mantle of Brachiopoda, 162, 163 
Cirripedia, 215 
MoUusca, 184, 187 
Manubrium, 91, 92, 96 
Manyunckia, 152 
Margaropus, 229 
Marsipometra, 122, 126 
Marsupia, 190 
Martini, E., 142 
Mastax, 140 
Mastigamoeba, 34, 56 
Mastigella, 34, 56 
Mastigophora, 29, 30, 56 
Matrix, 33 
Maxillae, 207 
Maxillipeds, 207 
Meandrina, 103 
Mecoptera, 251 
Media, 243 
Mediaster, 182 
Medusoid, 88 
Megadrili, 153, 157 
Megalops, 209, 220 
Mehlis' gland, 115 
Meiosis, 12 
Meleagrina, 187, 201 
Melicerta, 140, 143 
Melicertida, 143 
Mellita, 177, 182 
Membranelle, 52 
Membranopora, 164 
Mermis, 131, 133 
Merozoites, 45 
Mesenchyme, 65 
Mesenchytraeus, 157 
Mesenterial filaments, 99 
Mesenteries, 96, 98, 146, 179 
Mesoderm, 16 
Mesoglea, 89 
Mesonemertini, 126 
Mesothorax, 241 
Mesozoa, 59, 60 
Metacrinus, 182 
Metagenesis, 90 
Metamerism, 147, 203 

heteronomous, 203 

homonomous, 148, 203 



Metamorphosis, 236 
Metanauplius, 207, 211 
Metanemertini, 12G 
Metanephridial sj'stem, 74, 155 
Metathorax, 241 
Metazoa, 20, 59 
Metazoan tendencies, 21 
Metridium, 103 
Miastor, primordial cells in, 11 

paedogenesis in, 17 
Michaelsen, 154 
Microconjugant, 51 
Microcotyle, 113, 125 
Microdon, 143 
Microdrili, 153, 157 
Microgametes, 10, 21 
Microgametoblast, 45 
Micronucleus, 49, 50 
Microporella, 160 
Microscolex, 153 
Microsporidia, 47 
Microstomum, 125 
Middle piece, 14 
Miescher's corpuscles, 49 
Millepora, 94, 102 
Milliped, 233 
Miracidium, 115 
Mites, 225, 226, 228 
Mitochondria, 50 
Mnemiopsis, 103 
Modiola, 201 
Moina, 222 
Mollusca, 184, 200 
Molluscan cross, 15 
Molluscoidea, 159, 164 
Molting, 204 
Monads, 34 
Monas, 56 
Monaxonida, 87 
MoniHformis, 136 
MoniUgaster, 153, 157 
Monocystis, 57 
Monoecious, 75 
Monogenea, 112, 125 
Monogenetic trematodes, 112 
Monogononta, 143 
Monopisthocotylea, 113, 125 
Monostomata, 126 
Monostomes, 113 

Monostyla, 143 
Monothalmia, 42 
Mouth parts of insects, 238 
Miiller's larva, 106 
Murex, 196, 201 
Muscle tissue, 65, 66 
Mussels, 186, 190 
My a, 201 
Mycetozoa, 40, 57 
Myofibrils, 65 
Myonemes, 44, 50 
Myriapods, 232 
Myrientomata, 205, 232, 234 
Myrosyringata, 136 
Mysididae, 217 
Mysis, 208, 217, 220, 223 
Mytilus, 187, 201 
Myxamoeba, 42 
Myxicola, 152, 157 
Myxidium, 48, 57 
Myxobolus, 48, 57 
Myxoflagellate, 42 
Myxopodia, 38 
Myxospongia, 87 
Myxosporidia, 47 


Nacre, 187 

Naididae, 152 

Nais, 157 

Narcomedusae, 102 

Natica, 196, 201 

Nauplius, 207, 211 

Nautilus, 197, 199 

Navanax, 201 

Nebaha, 217, 223 

Necator, 132, 136 

Nectocalyces, 93 

Negri bodies, 55 

Nemas, 128 

Nemathelminthes, 128, 136, 257 

Nematocysts, 88 

Nematoda, 128, 136 

Nemertinea, 122, 126 

Nemertine theory, 265 

Nemertines, 122 

Neoechinorhynchus, 136 

Neomenia, 201 



Neosporidia, 47, 57 

Nereis, 149, 157 

Nerve ring, 94, 130, 167 

Nerve tissue, 66 

Nervous system, 77, 89, 94, 105, 
110, 125, 130, 135, 141, 146, 
148, 167, 174, 177, 184, 189, 
193, 204, 220, 246 
condensation of, 246 

Nettling cells, 88, 93 

Neurite, 67 

Neuroglia, 68, 77 

Neuromotor apparatus, 35 

Neuromotor system, 52 

Neuron, 66, 77 

Neuroptera, 251 

Neuropodium, 149 

Nitzschia, 113, 125 

Noctiluca, 33, 56 

Nomenclature, international com- 
mission on, 3 

Non-cellular organisms, 20 

Non-chordate, 1 
phyla, 1 

Nosema, 47, 57 

Notholca, 143 

Notocotylus, 126 

Notommata, 143 

Notommatida, 143 

Notostraca, 211 

Notochord, 1 

Notopodium, 149 

Nuclear-constancy, 15, 135, 142 

Nuclearia, 57 

Nucula, 201 

Nuda, 101, 103 

Nudibranchs, 194, 196 

Nyctotherus, 57 

Nymph, 238 

Nymphon, 231, 234 


Obelia, 102 
Occiput, 239 
Ocellus, 79, 226, 239 
Octocoralla, 98, 100 
Octopoda, 200, 201 
Ocular plates, 175 

Odonata, 251 

Olfactory organs, 79, 146, 184, 240 

Oligochaeta. 147, 150, 152, 157 

Oligotricha, 57 

Olynthus, 83 

Ommatidia, 79 

Onchosphere, 120 

Oniscus, 222, 223 

Onycophora, 205, 231, 234 

Oocapt, 120 

Oogenesis, 11 

Ookinete, 45 

Ootype, 114 

Opalina, 49, 57 

Opercularia, 58 

Operculum, 152, 160, 192, 196 

Ophioderma, 182 

Ophionereis, 182 

Ophiopholus, 175 

Ophiothrix, 182 

Opisthorchis, 126 

Ophiura, 175, 182 

Ophiuroidea, 166, 174, 182 

Ophrj'odendron, 58 

Ophisthobranchia, 195, 196, 201 

Oral shields, 175 

Orb weavers, 228 

Orbicella, 103 

Orchestia, 222, 223 

Organ systems, 68 

Organellae, 22 

Organogenesis, 117 

Organs, 68 

Oriental sore, 37 

Orthasterias, 182 

Orthonectidae, 60 

Orthoneurous, 194 

Orthoptera, 251 

Oscarella, 83, 87 

Osculum, 82 

Osphradia, 184, 189 

Ostium, 211, 220 

Ostracoda, 209, 214, 223 

Ostrea, 201 

Otocyst, (see statocyst), 80 

Otolith, 63, 80 

Overgrown development, 247 

Ovicells, 161 

Oviducts, 76 



Ovigerous legs, 230 
Oviparous, 18 
Ovipositors, 76, 243 
Ovoviviparous, 18 
Oxytricha, 57 
Oysters, 186, 187 

Paedogenesis, 17, 205 
Pagurus, 223 
Palaemon, 223 
Palaemonetes, 223 
Palingenetic, 255 
Pallia! line, 187 
Palola worm, 149 
Palpi, 188 
Paludicella, 164 
Pandinus, 234 
Pandorina, 33, 56 
Parabasal bodies, 31 
Paradoxides, 209 
Paragonimus, 126 
Paragordius, 133, 136 
Parajulus, 234 
Paramecium, 51, 57 
Parapodia, 147, 148 
Paraseison, 143 
Parasitic habits, 23, 111 
Parazoa, 82 
Parenchyma, 81, 105 

digestive, 110 
Parietal ganglion, 189 
Parthenogenesis, 17, 212 
Pasteur, 48 
Patella, 196, 201 
Patten, 260 

Pauropoda, 205, 232, 233, 234 
Pauropus, 234 
Pebrine disease, 47 
Pecten, 187, 188, 201 
Pectinatella, 160, 161, 164 
Pectines, 227 
Pedal ganglion, 184, 189 
Pedalion, 140, 143 
Pedata, 182 
Pedicellariae, 167, 170 
Pedicellina, 161, 164 
Pedipalpi, 226 

Peduncle, 162, 163 

Pelecypoda, 186 

Pelmatozoa, 169, 182 

Pelomyxa, 57 

Peneus, 223 

Pennatula, 103 

Pennatulacea, 103 

Pentacrinus, 181, 182 

Pentastoma, 230, 234 

Pentastomida, 230 

Peranema, 33, 56 

Pericardial sinus, 220 

Pericardium, 188 

Perichaeta, 153, 157 

Pericolpa, 102 

Peridinium, 56 

Peripatus, 231, 234, 258 

Peripheral nervous system, 67, 78 

Periphylla, 102 

Periproct, 175 

Perisarc, 89 

Peristome, 175 

Peritricha, 50, 57 

Peromedusae, 102 

Perradii, 96 

Petasus, 102 

Phacellae, 95 

Phacus, 56 

Phalangida, 229, 230, 234 

Phalangium, 234 

Phanerozonia, 182 

Pharyngeal basket, 50 

Phascolosoma, 157, 158 

Philodina, 143 

Pholads, 180, 201 

Phoronida, 159, 103, 164 

Phoronis, 163, 164 

Phosphorescence, 33 

Phylactolaemata, 164 

Phyllocarida, 217, 223 

Phyllopoda, 206, 209, 210, 222 

Phylogenetic relationships, 2, 4, 253 

Phylogeny, 253 

Physa, 193, 201 

Physalia, 93, 102 

Phytomastigina, 31, 56 

Phytomonadina, 56 

Pilidium, 106, 124 

Pill bugs, 222 



Pinnules, 180 
Pisaster, 182 
Piscicola, 156, 157 
Pisidium, 187, 201 
Placobdella, 156, 157 
Placophora, 201 
Plakina, 87 
Planaria, 109, 125 
Planocera, 125 
Planorbis, 193, 201 
Planula, 91, 95, 99, 257 
Plasmic products, 60, 64 
Plasmodiophora brassicae, 43 
Plasmodium, 45 
Plasmodroma, 29, 56 
Plathelminthes, 105, 125, 257 
Platvdorina, 66 
Platypoda, 196 
Plecoptera, 251 
Pleodorina, 56 
Plerocercoid, 120 
Pleura, 241 

Pleurobrachia, 101, 103 
Pleurocera, 196, 201 
Pleuroxus, 222 
Plumatella, 160, 164 
Plumularia, 102 
Pluteus, 176 
Pneumatophore, 93 
Pneumonoeces, 126 
Podomeres, 206 
Podophrya, 54, 58 
Poison glands, 227, 233 
Polar capsules, 48 
Polian vesicle, 172, 176, 178 
Polyarthra, 143 
Polychaeta, 147, 148, 157 
Polycladidea, 110, 122 
Polyembryony, 17, 205 
Polygordius, 156, 158 

development, 156 
Polygyra, 197, 201 
Polymastigina, 56 
Polymorphism, 93 
Polyopisthocotylca, 113, 125 
Polyp, 88, 89 
Polyphemus, 211, 222 
Polypus, 201 
Polvstoma, 113, 125 

Polystomella, 37 
Polythalmia, 42 
Polyxenus, 233, 234 
Polyzoa, 159, 164 
Pompholyx, 143 
Porcellio, 222, 223 
Porifera, 81, 86 

morphological types, 82 

phylogeny, 256 
Porites, 103 
Porocephalus, 230, 234 
Portuguese man-of-war, 88, 93 
Postabdomen, 212, 227 
Posterior ganglion, 170 
Postgenae. 239 
Prawns, 216 
Preabdomen, 227 
Predaceous habit, 24 
Primitive segments, 151 
Primitive vs. degenerate, 3 
Primordial germ cells, 11 
Priority, law of, 3 
Pristina, 157 
Proales, 143 
Proboscis, Acanthocephala, 134 

Gasteropoda, 194 

leech, 155 

Nemertinea, 123 

snail, 194 
Proboscis receptacle, 135 
Proboscis sheath, 123 
Prochordates, 1, 259 
Proctodaeum, 71 
Proetus, 209 
Proglottids, 116 
Promitosis, 26 
Prorhynchus, 125 
Prorodon, 52, 57 
Prosobranchia, 195, 201 
Prosopyles, 83 
Prostomata, 125 
Prostomium, 147, 206, 216 
Protandrous, 84 
Proteocephalus, 122, 126 
Proteomyxa, 57 
Proterosjjongia, 82, 257 
Prothorax, 241 
Protobranchia, 201 
Protodrilus, 156, 15S 



Protohydra, 90 
Protomastigina, 56 
Protomerite, 43 
Protomonadina, 56 
Protoneinertini, 126 
Protonephridia, 74, 105, 148 
Protopodite, 205 
Protozoa, 20 
Protura, 232, 234 
Provisional setae, 151 
Psammon^TC vulcanis, 42 
Pseudocoel, 128 
Pseudolamellibranchia, 201 
Pseudophyllidea, 122, 126 
Pseiidopodia, 34, 38 
Pseudoscolex, 118 
Pseudoscorpionida, 229, 234 
Psolus, 178, 182 
Pterodina, 143 
Pteropoda, 192, 195 
Pterygotus, 234 
Ptilosarcus, 103 
Pulmonata, 195, 196, 201 
Pulsella, 31 
Pupa, Insecta, 236 

Molhisca, 197, 201 
Pycnogonida, 205, 230, 234 
Pycnogonum, 234 
Pycnopodia, 174, 182 

Quadrula, 201 



Rachis, 176 

Radial canals, 90 

Radial chambers, 83 

Radial symmetry, 88, 165 

Radial vessels, 166 

Radials, 180 

Radiata, 88 

Radiolaria, 39, 40, 57 

Radius, 243 

Radula, 184, 194 

Rattulus, 143 

Recapitulation theory, 59, 255 

Receptaculum seniinis, 76, 114 

Reciprocal fertilization, 18, 153 

Redia, 115 

Redi, 6 

Regeneration, 84, 106, 173, 218 

Regularia, 182 

Relapsing fever, 38 

Relationships, phylogenetic, 2, 4, 253 

Renilla, 103 

Reproduction, 6 

asexual, 7 

sexual, 7, 8 
Reproductive system, 75 
Respiration, 72 
Respiratory system, 72 
Respiratory trees, 179 
Retina, 63, 79, 198 
Rhabditis, 131, 133, 136 
Rhabdocoelida, 110, 125 
Rhabdomes, 79 
Rhipidoglossa, 196 
Rhizocrinus, 182 
Rhizoplast, 31 
Rhizopoda, 41, 57 
Rhizostomae, 96, 103 
Rhopalura, 60 
Rhyncheta, 58 
Rhynchobdellida, 155, 157 
Ring canal, 166 
Rocky Mountain fever, 55 
Rossia, 201 
Rostellum, 122 
Rostnmi, 229 
Rotalia, 57 
Rotifer, 143 
Rotifera, 138, 143 
Roundworms, 128 


Sabellaria, 157 
Sacculina, 216 
Sagartia, 103 
Sagitta, 147, 157 
Sand dollars, 165, 175 
Sandfleas, 216 
Saprophytic habit, 24 
Saprozoic, 34 
Sarcocystis, 49, 57 
Sarcodina, 29, 38, 56 



Sarcolemma, 66 

Sarcoplasm, 66 

Sarcoptes, 234 

Sarcosporidia, 49, 57 

Sarsia, 91 

Scallops, 186, 188 

Scaphopoda, 192, 201 

Schistosoma, 116, 126 

Schistosomes, 112 

Schizocerca, 143 

Schizogony, 9, 26 

Schizonts, 26 

Schizopoda, 207, 223 

Sclerites, 203, 230, 239 

Scleroblasts, 85 

Scolex, 116, 122 

Scolopendra, 233 

Scolopendrella, 232, 234 

Scorpionida, 227, 234 

Scorpions, 225, 226, 227 

Scuds, 216 

Scutigera, 233, 234 
Scyphopolyp, 95 
Scyphistoma, 95 
Scyphozoa, 94, 102 
Sea anemones, 97, 99 
Sea cucumbers, 165, 177 
Sea fans, 100 
Sea hares, 196 
Sea Hlies, 165, 179 
Sea mouse, 151 
Sea pansies, 97, 100 
Sea pens, 97, 100 
Sea urchins, 165, 175 
Sedentaria, 152, 157 
Seison, 143 
Seisonida, 143 
Selective apparatus, 136 
Self fertihzation, 18 
Seminial receptacle, 18 
Semostomae, 103 
Semper, 260 
Sepia, 198, 200, 201 
Septa, 96 
Septibranchia, 201 
Serosa, 247 

Serpent stars, 165, 174 
Sertularia, 102 

Setae, 147, 152 
provisional, 150 

Sex-chromosomes, 13 

Sexual reproduction, 8 

Sheep-liver fluke, 115 

Shell of Brachiopoda, 161 
of Mollusca, 184, 187 

Shell gland, 114, 184, 206, 217 

Shipworms, 186 

Shrimps, 216, 218 

Sida, 222 

von Siebold, 22 

Silenia, 201 

Sinuses, 155, 203 

Siphogenerina, 57 

Siphon, 176, 177, 186, 187, 192, 197 

Siphon of Echinoidea, 176 

Siphonaptera, 251 

Siphonoglyphes, 98, 99 

Siphonophora, 93 

Sipunculus, 157, 158 

Sleeping sickness, 37 

SUme moulds, 41, 42 

Slugs, 192, 197 

Snails, 192 

Solaster, 174, 182 

Solenomya, 201 
Solpugida, 229, 234 
Somatic cells, 11, 21 
Sow bugs, 205, 216, 222 
Spadella, 147, 157 
Spadix, 92 
Sparganum, 126 
Spatangoidea, 177, 182 
Spatangus, 177, 182 
Spermatogenesis, 11 
Spermatophores, 18, 155, 199 
Spermatozoa, 10 
Sphaeridia, 177 
Sphaerium, 201 
Sphaerocapsa, 57 
Sphaerophyra, 58 
Sphyranura, 125 
Spiders, 225, 226, 227 
Spinnerets, 228 
Spinning organs, 228 
Spinning tubes, 228 
Spinulosa, 182 
Spiracle, 232, 244 



Spirol)olus, 233, 234 

Spirochaets, 37, 55 

Spirorbis, 152, 157 

Spiroschaiidinnia, 38 

Spirostomum, 57 

Spirilla, 200, 201 

Spondylomorum, 33, 56 

SpongiUa, 87 

Spongillidae, 81 

Spongin, 86 

Spongioblasts, 86 

Spontaneous generation, 6 

Sporangia, 42 

Sporoblasts, 45, 47 

Sporocyst, 115 

Sporogony, 9, 26 

Sporonts, 26 

Sporoplasm, 48 

Sporosac, 92, 93 

Sporozoa, 29, 43, 57 

Sporozite, 43, 45 

Squid, 184, 200 

Squilla, 218, 223 

Starfishes, 165 

Statoblasts, 8, 161 

Statocyst (see otocyst), 63, 80, 89, 

94, 184, 189 
Statolith, 63, 80 
Stauromedusae, 102 
Stemonitis, 57 
Stenophora, 44, 57 
Stenopora, 164 
Stenostomum, 125 
Stentor, 57 

Stephanoceros, 140, 143 
Stephanonympha, 35, 56 
Sternal sinus, 220 
Sternum, 241 
Stichostemma, 126 
Sting, 227 

Stomatopoda, 218, 223 
Stomodaeum, 70, 207 
Stomolophus, 103 
Stone canal, 172 
zur Strassen, 130 
Strepsiptera, 251 
Streptoneurous, 194 
Strigea, 126 
Strigeata, 126 

Strobila, 95, 117 

Strombidiiun, 57 

Stroml)us, 196, 201 

Strongyloeentrotus, 177, 182 

Strongyloides, 136. 

Strophitus, 191 

Stychopus, 182 

Stylaster, 102 

Stylet, 123 

Stylochus, 125 

Stylonichia, 57 

Subcosta, 243 

Subcuticula, 128 

Suberites, 87 

Suckers, 112, 153 

Sucking discs, 198 

Suctoria, 29, 49, 53, 58 

Supporting tissue, 64 

Sutures, 203, 239 

Swimmerets, 219 

Sycon, 83 

Syllis, 151, 157 

Sympathetic nervous system, 78, 246 

Symphyla, 205, 232, 234 

Synapta, 182 

Synchaeta, 143 

Syngamus, 136 

Synura, 56 

Syphihs, 38 

Systema Naturae, 4 


Tactile organs, 63, 78 
Taenia, 121, 122, 126 
Taeniolae, 95 
Tarantulas, 228 
Tardigrada, 205, 234 
Tarsus, 241 
Taste organs, 79 
Tectibranchs, 196 
Tegmina, 242 
Telolilasts, 150 
Telolecithal, 14 
Telorchis, 126 
Telosporidia, 43, 57 
Telson, 216, 217 
Tentacles, 88, 161, 163, 195 
Tentorium, 239 



Terebra, 196, 201 
Terebratula, 163, 164 
Terebratulina, 163 
Teredo, 186, 201 
Teretonympha, 35 
Tergiim, 241 

Termites, Protozoa of, 35 
Tessera, 102 
Tethys, 201 

Tetrabranchia, 199, 201 
Tetraphyllidea, 126 
Tetrarhynchus, 122, 126 
Tetrazonida, 87 
Texas fever, 229 
Thorax, 241 
Thousand-leg, 233 
Thyone, 182 
Thysanoptera, 251 
Thysanura, 238, 251 
Tibia, 241 
Tick fever, 229 
Ticks, 228 

Tiedemann's bodies, 172 
Trigoid bodies, 67 
Tintinnidium, 57 
Tintinnus, 57 
Tissues, 60 
Tokophrya, 54, 58 
Tooth shells, 192 
Torsion in gastropods, 194 
Toxopneustes, 182 
Tracheae, 73, 225, 244 
Tracheal gills, 73, 223 
Trachella, 31 
Trachelomonas, 56 
Tracheoles, 244 
Trachydermon, 201 
Trachylinae, 93, 102 
Trachymedusae, 102 
Trachynema, 102 
Trematoda, 112, 125 
Trench fever, 55 
Treponema pallidum, 38 
Triaenophorus, 126 
Triarthra, 143 
Triarthrus, 209, 222 
Trichinella, 4, 131, 132, 136 
Trichites, 50 
Trichocyst, 50 

Trichodina, 58 

Trichomonas, 35, 36 

Trichonympha, 35, 56 

Trichoptera, 251 

Trichostrongylus, 136 

Trichosyringata, 136 

Tricladidea, 110, 125 

Tridacna, 187 

Trigonia, 201 

Trilol)ita, 222 

Trilobite larva of Limulus, 226 

Trilobites, 205, 209 

Triopha, 201 

Triviimi, 171 

Trochal disc, 139, 140 

Trochanter, 241 

Trochelminthes, 138, 143 

Trochophore, 138, 145, 148, 150, 156, 

163, 184, 257 
Trochosphaera, 140, 143 
Trochus, 196, 201 
Trypanorhyncha, 122, 126 
Trypanosoma, 31, 36, 56 
Trypanosomes, 31, 36 
Tse-tse fly, 37 
Tube-feet, 167 
Tubifex, 157 
Tubipora, 103 
Tubularia, 102 
Tubulipora, 164 
Tunicata, 1 
Turbellaria, 108, 125 
Tylenchus, 133 
Tympanum, 80, 240 
Typhus fever, 55 


Umbo, 186 

Undulating membrane, 31, 52 

Unio, 201 

Upogebia, 223 

Urnatella, 161, 164 

Uroglena, 56 

Uroleptus, 57 

Uroplectus, 234 

Urostyla, 57 

Uterus, 76, 114 



Vacuole, contractile, 40, 50 

Vagina, 130 

Valves, brachiopod, 161 

mollusc, 186 
Vampyrella, 57 
Vasa deferentia, 76 
Velella, 102 
Veliger, 184, 195 
Velum, 95 
Venation, 242 
Ventral sucker, 113 
Ventricles, 72 
Venus, 201 
Venus, flowerhasket, 87 

girdle, 102 
Vermes, 145 
Vertebrae, 174 
Vertebrata, 1, 259 
Vertex, 239 
Vestibule, 161 
Vibracularia, 160 
Visceral ganglion, 184, 189, 193 
Visceral mass, 184, 188 
Vitellaria, 76, 104, 109, 128 
Viviparous, 18 
Volvox, 33, 56, 256 
Vorticella, 51, 53, 58 


Waldeheimia, 163, 164 
Water-bears, 231 

Water fleas, 210, 211 
Water mites, 229 
Water-vascular system, 166 
Watsonius, 126 
Wheel animalcules, 138 
Wing pads, 238 
Wings, 241, 242, 243 
Winter eggs, 141 
Woodruff, L. L., 25, 27, 51 
Worms, arrow, 147 

flat, 105 

round, 128 

spinyheaded, 128 


X-chromosomes, 13 
Xiphosura, 225, 234 

Y-chromosomes, 13 
Yoldia, 201 
Yolk cells, 9, 114 


Zoantharia, 98, 99, 103 

Zoea, 208, 220 

Zooids, 91, 159 

Zoological Congress, International, 3 

Zoomastigina, 31, 34, 56 

Zoothamnium, 58 

Zygote, 9, 13 



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