Handbook of Animal Diversity

Richard E. Blackwelder

• Publisher: CRC Press
• Year: 2019
• Print ISBN: 9781315893907
• eBook ISBN: 9781351089906
• Topic: Medicine
• Subtopic: Veterinary Medicine

This book is a summary of the diversity between and within the classes of animals. It is intended for reference on all aspects of animals that can be studied comparatively, but such comparisons requires that the occurrence of the feature in question beknown for more than just one or two groups. It is in large part a book on invertebrate animals because the vertebrates from only a small part of the diversity of animals.

Part I
Introduction

“They are concerned with the diversities of the world instead of with its unities.”

G. B. Shaw

Chapter 1
INTRODUCTION TO DIVERSITY

TABLE OF CONTENTS

I. Diversity

II. Problems of Viewpoint

A. Organismic Viewpoint

B. Human Orientation

C. Treatment of Adults Exclusively

D. Generalization

III. The Nature of Organisms

A. Protozoa as Animals

B. Cellularity

1. The Cell as a Unit

2. Cellularity of Protozoa

C. Individuality

D. Individuality vs. Coloniality

E. Polymorphism

General References

I. Diversity

In modern biology books the word diversity is little used. It is well known that there are more than a million kinds of animals on earth and that they belong to at least several dozen groups that are often rather distinct from each other. But it is said that they all consist of cells made of similar protoplasm, originating in the same reproductive processes, developing by similar processes of differentiation and growth, and functioning by means of basically similar organs. This book may restore the balance to the word diversity, by its constant repetition, and it will show that the similarities as described above are either unwarranted generalizations or are meaningful only in some restricted sense while being surrounded by the almost endless diversity of animals.

The diversity is actually much greater than can be shown here. First, much diversity is lost to human knowledge through the extinction of types of animals evolved in earlier ages. These would be included if possible, but the only diversity that can be known for any of these groups now is that shown by the hard parts that were fossilized, and these give only indirect clues to bodily functions. There are more of these extinct groups than is usually recognized and together they hold almost all the keys to the course of evolution among animals as a whole. They are included whenever the available information warrants.

Second, biochemical diversity is limited by the nature of molecules and chemical bonds. The bonds are much more diverse than they were once thought to be. The molecules of living matter are limited by the apparently universal use of carbon in their basic structure and of water as the universal fundamental constituent of tissues and source of hydrogen and oxygen. The boiling and freezing points of water come close to setting limits to the existence of life, and actually do so for animals.

Third, there is a subtlety in the interaction of chemicals, genes, and internal and external environments that cannot be treated by comparative methods. The details of diversity seem endless, but the real extent of diversity in the mechanisms is either unknown or inexpressible.

Fourth, the interactions of animal organs, especially the nervous and endocrine system, are so complex in producing what is called animal behavior that the diversity is difficult to tabulate. Only a few aspects, such as parasitism, can be extensively dealt with here.

It is thus seen that diversity is ubiquitous and greatly overshadows such unity (better called similarity) as does exist. This diversity has largely been known for many decades, but it is so abundant and all pervasive, and often so deeply hidden in the literature of science, that general books give a seriously inadequate account of it. One could say that much of the diversity is forgotten by recent generations. It is hoped that these pages will restore the balance by tabulating the diversity that has been assembled and by providing a framework into which others can fit the no-doubt-large number of instances of which the authors are ignorant.

II. Problems of Viewpoint

The reasons for the inadequate treatment of diversity in all textbooks are partly historical and partly curricular. These center around the fact that zoology courses often are forced to serve other goals than to make known the animal kingdom. They particularly serve the interests of men and women in medicine, nursing, public health, agriculture, and other “practical” fields. For these purposes, the interest is in Homo sapiens, but stand-ins are usually used in such activities.

These and other problems of viewpoint are dealt with under the following four headings.

A. Organismic Viewpoint

As biochemical knowledge and laboratory capabilities have increased in recent years, there has been a tendency to assume that only this “modern” experimental approach is fruitful in biology. It certainly has been fruitful, but it is a mistake to think that it is even now the only useful or the only interesting pursuit in biology. In this modern approach, it seems to be believed that time is better spent on the finer details revealed by new techniques and instruments. A student of invertebrates countered the argument thus: “The organismal viewpoint . . . maintains that biology should study entire living organisms, not merely the physics and chemistry of their isolated parts.”

A person who examines the literature on the structure and physiology of animals in general will see at once that, although much is known, it is so widely scattered that it is unavailable to nonspecialists and so has never been treated comprehensively or comparatively. In this absence of ready knowledge of the comparative features of animals in general there has been a strong tendency to assume that in the “basic aspects” animals are all much alike. It is the authors’ purpose to show the inadequacies of this assumption by displaying part of the existing diversity.

B. Human Orientation

Most of physiology and embryology, and much of the other branches of zoology, have been dominated by the study of the one species Homo sapiens. This was inevitable because of the immediate benefits to mankind, especially in regard to human health, and because man was the most interesting organism to many early workers. When other animals were studied, it was to compare with humans, again primarily for the increase of knowledge about the human organism.

By the time other animals came to be studied seriously, a large and detailed science of the biology of mankind had accumulated and, to record this, an extensive terminology. These terms were then applied to the lower animals, whether or not they were really appropriate. For instance, one may say that a jellyfish has eyes, even though there is little similarity between those simple light-sensitive organs and mammal eyes. One can speak of tracheae in both humans and insects, although there is no structural or developmental similarity and only a very general functional similarity.

The alternative would have been to have duplicate terms in vertebrates and invertebrates, and probably also between higher and lower invertebrates, which no doubt would have seemed a frightening prospect to zoology students of those days. Because this was usually not done, confusion has resulted, and gradually many different new terms had to be adopted for the fundamentally different situations that occur in the endless diversity in the animal kingdom.

Must one then conclude that this human-oriented terminology is unfortunate? In part yes, because, for example, it is unfortunate for any student to think that because “respiration”, the sequence of biochemical reactions in the protoplasm, is basically the same in all animals, it follows that all animals “breathe”. In the sense that this term is used in humans, insects might be said to breathe, with their very different tracheal breathing system, but a truly aquatic or parasitic animal certainly does not breathe. Thus, one must interpret such an expression each time from knowledge of the animal involved, because of recognition of the great differences that actually occur. However, generalizing is difficult; it is true that all animals require “food” and energy from the outside, but it is not true that all animals “feed” or “eat” in the vertebrate sense.

In studying any animal group outside of the vertebrates, and sometimes even among them, it is necessary to be alert to prevent being misled by a term that seems to say that this feature is similar to the one by that name in humans. Instead of trying to avoid such terms in this book, we try by tabulating the diversity to show that the general terms must be used with care and understanding. Where the structural differences are very great, it is probably better to use a different term, if one is available. For example, the kidneys in mollusks and humans are both excreting nitrogenous waste products, but they are not at all similar in structure, location, or embryonic origin. They are both “kidneys” only if that word is defined as equivalent to “excretory organ”.

Thus, a major feature of this book is the attempt to bring terminologies into agreement with the true facts of structure and function in the diverse groups of animals. New terms are very seldom suggested, but distinctions between existing “synonyms” and careful definitions of terms are frequently used to make it possible to display the existing diversity.

C. Treatment of Adults Exclusively

There is a basic illogicality in nearly all treatments of animals, whether the subject is structure, function, or behavior, in that there is a tacit (and usually unrecognized) assumption that adult animals are alone involved. There is no way to justify this position, as the life of an individual extends from the moment it is isolated as an individual (zygote or fragment) until its death (or division).

Everything that occurs during the entire life is part of that animal. To cite only what occurs in the adult is utterly inadequate in a study of any organism, as we will try to make clear at many points in this book. We will cite structures, functions, and behaviors irrespective of the stage of the life history, which means citing these features in all stages in which they are known. Of course the subject of development itself covers all stages, and it could be treated as a time frame for discussing all the other features.

Inasmuch as animals are always in a state of change, even when they appear to be stable for a period, it is possible to define “an animal” as a life history. In speaking about one stage, one is in reality stopping the sequence, like stopping a movie projector on a single frame. The animal never stops in this continuous sequence of stages, although it appears to proceed now at a slower rate and now at a faster one. However, some so-called life cycles cover the lives of two or more individuals; in studying individuals it is necessary to watch out for interruption of the cycle by some hidden reproduction that brings an end to the individual long before the life cycle of the species is completed.

D. Generalization

Throughout this book there are references to or quotations of statements made in other books that seem to be inadequate for some reason. The most common reason is that the statement is a generalization that inadequately reflects the extent of the diversity of animals. Generalizing is not automatically bad, but in scientific writings it is dangerous; the pitfalls are many. For example, a physiologist recently wrote:

“I have deliberately restricted myself to such parts of the subject as are susceptible to broad generalization; but the broader the generalization the more exceptions it has to admit of. In disregarding such exceptions I realize that I have laid myself open to the charge of over-simplification . . . but the omissions have been deliberate. I cannot see much point in stating a generalization only to smother it with qualifications.”

This is a very interesting comment, because the writer provides all the arguments necessary to refute his own contention. A generalization that couldbe smothered with exceptions is not worth stating. Where real exceptions occur, the generalization should acknowledge them as part of the facts being generalized. We agree that there is not much point in making such an unqualified generalization — in fact it is usually much better not to mislead the reader by making any statement that is just not true.

In this book also there are doubtless some generalizations. If these have known or even suspected exceptions, we have attempted to make that fact part of the generalization. Throughout this book an effort is made to emphasize exceptions, give examples rather than final, complete lists, and leave them all open to other interpretations, further expansion, and more detailed analysis. This, therefore, allows for addition of further...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Preface
• The Authors
• Table of Chapters
• Part I Introduction
• Part II The Origin of Individuals
• Part III Development of Individuals
• Part IV Adult Individuals
• Part V Behavior
• Part VI Coloniality
• Part VII End of Existence
• Part VIII Classification
• Index