Animal biology : An elementary text-book

Preface To Second Edition

The call for a second edition of this volume has enabled me to revise the text, to substitute in several cases improved woodcuts, and to add a brief Classification of the Types, and a Glossary. C. Ll. M.

University College, Bristol,

April 1889.

Preface

In preparing this volume I have endeavoured to meet the requirements of those who intend to present themselves for the London Intermediate and Preliminary Scientific, for the Oxford and Cambridge Local, and for other examinations of like range based upon the type system.

Special attention has been paid to Embryology.

The larger portion of the work has been devoted to the Vertebrate types, because I have had in view, in the first place, the requirements of those who intend to follow a medical career, and who will, I believe, after working through such a course as is developed in this volume, be able to make a better start, and thereafter more rapid progress, in their study of human anatomy and physiology than would be otherwise possible ; and, in the second place, because there are many who are led to take a deep interest

VI

PREFACE.

in human anatomy and physiology, hut who are of necessity able to make their study practiced only through the dissection of the lower Vertebrates.

The illustrations have been engraved by Mr. George E. Lodge, after outline sketches of my own, chiefly from dissections or preparations made in the Biological Laboratory of the University College, Bristol. The dissections and preparations have in many cases been made in accordance with instructions given, in text or figures, by previous authors — especially by Professor Howes, in his excellent Atlas of Biology, by Professors Foster and Balfour in their Elementary Embryology, and by Professor T. J. Parker in his Zootomy. In the few cases in which my indebtedness is more direct I have placed the name of the author in brackets. With hardly any exceptions they are to be regarded, not as pictures, but as outline sketches, to serve as guides to practical work, and to be considered only in relation to the actual objects displayed by dissection. In a few cases they are to be looked upon as purely diagrammatic, to aid in the comprehension of the text. In my opinion, the more widely different such pure diagrams are from pictures the better.

My sincere thanks are due, and are here most gratefully tendered, to Professors W. K. Parker, F.B.S. ; G. B. Howes, F.Z.S. ; William Barnsay, Ph. D. ; and Mr. G. Munro Smith,

for valuable aid and advice.

April 1SS7.

C. Ll. M.

Contents

CHAP. PAGE

I. Introductory, ...... xi

Part I. Vertebrate Anatomy And Physiology

As exemplified by the Frog , the Pigeon or Fowl , and the Rabbit , with occasional Reference to other Types.

II. Natural History ancl External Characters, . . 1

III. General Anatomy, ...... 21

IV. General Physiology, . .... 55

V. General Histology, ...... 63

VI. General Embryology, ... 86

VII. The Genesis of Tissues and Organs, . . .117

VIII. The Skeleton, . . . . . .139

IX. Nutrition and Metabolism, . . . . .177

X. The Heart and Circulation, . , . .194

XI. The Brain and Nerves, ..... 215

Vlll

CONTENTS.

Part II. Some Invertebrate Types

CHAP. PAGE

XII. The Crayfish 231

XIII. The Cockroach, 257

XIV. The Earthworm, 278

XV. The Snail, 291

XVI. The Fresh-water Mussel, •. . • • • 305

XVII. The Liver-Fluke and Tapeworm, . . . .322

XVIII. The Hydra, 335

XIX. Vorticella and Paramcecium, .... 347

XX. The Amoeba, 354

Appendix

Classification of Types, . . . • • .361

INTRODUCTORY.

b

Chapter I. 1ntr0duct0r Y

We live in a world that is teeming with life. The air, the surface of the land, the waters of ocean, river, and pond, swarm with living organisms, each more or less perfectly adapted to the conditions of its existence. Many problems arise with regard to this world of living things. What is their form and structure 1 How do they move and breathe and reproduce their kind1? How, and on what do they feed ; and how does the food minister to their growth or their activity 1 How are they distributed over the earth’s surface 1 What is the method and manner of their origin 1 These, and other questions of like nature, arise in connection with the world of life. And the science that deals with these problems is the Science of Biology.

This science branches into two main departments, in accordance with the division of living things into Animals and Plants. The two branches start, indeed, from a common stem, for there are certain characteristics common to plant life and animal life, and the lowest plants are scarcely to be distinguished from the lowest animals. But the structure and functions of the vast majority of animals differ so markedly from the structure and functions of the vast majority of plants, and the problems of animal life differ so materially from the problems of plant life, that the Science of Animal Biology , which deals with the former, is justly entitled to a distinct position as a separate branch of study. It is with this branch that this volume deals.

The essence of science is organisation and exactness. Most

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ANIMAL BIOLOGY.

[Chap. I.

people have some knowledge of the animal world in which they live and of which they form a part. But this general information, useful and interesting as it is, lacks that organisation and exactness which is a distinguishing mark of science. * To rise from general information to science, we must use the methods of science, which are : (1) observation and experiment; (2) inference and hypothesis ; (3) verification. The beginner will, however, wisely rest content with repeating the observations and experiments (not merely reading about them but repeating them), and conscientiously verifying the inferences, of his masters in science. This he must do if he is to learn science as science, and not as history. To learn about science is valuable. But to be taught science itself through the direct teachings of Nature is far more valuable. The student of science must learn his facts at first hand, and must regard hooks as guides to that object. Observation, experiment, and verification are to be regarded as primary duties by every student of science as such. They are therefore incumbent on the student of Animal Biology.

What is the nature of those living things on the scientific study of which we are now to enter 1 We are more or less accpiainted with a considerable number of very different kinds of living animals, such as dogs, butterflies, worms, sea-anemones, star-fish, jelly-fish, and so forth. There are also simpler and more minute forms of life, with which, however, the student is presumably at present unfamiliar. Taking such animals as these, therefore, what is there about them to distinguish them on the one hand from not-living things, and on the other hand from plants 1 Let us endeavour to organise and make exact the general knowledge on these questions which we already possess.

A marked characteristic of life is growth. But we speak also of the growth of not-living things, of clouds, of the river in flood, of crystals from solution, of tapioca grains on boiling- What then distinguishes living growth 1 We cannot, perhaps, describe it better than by saying, (1) that it is an organic growth, that is a growth of the various organs of the living animal in due proportion ; (2) that it is a growth, not merely by the

INTRODUCTORY.

xiii addition of new material, but by the incorporation of that new material into the very substance of the old ; and (3) that the material incorporated during growth differs from the material absorbed from without, which has thus undergone a chemical transformation within the animal. The growth of the organism is dependent upon the continued absorption of new material from without, and its transformation into the substance of the body.

But, after a while, the growth of the living animal ceases. It is, as we say, fully grown. Why, then, continue the process of intussusception , as it is called, that is, the incorporation of new material from without 1 Because, if it be not continued, the animal wastes away and dies. And thus a new fact comes to light, that of constant waste, which must be made good by constant repair. So that we may say that a living animal is a centre of continual waste and repair, of nicely-balanced constructive and destructive processes.

Only so long as the constructive processes outbalance the destructive processes does growth continue. During the greater part of a healthy man’s life, for example, the two processes, waste and repair, are in equilibrium. In old age waste slowly but surely gains the mastery ; and, at death, it sets in unchecked by l’epair.

So far, then, a living animal is a centre of continual waste and repair, which may or may not be accompanied by growth. Let us now look at this growth a little more closely. There is something more than growth in the passage of the infant into the man. There is development as well. But take a more marked case. In spring and early summer there is plenty of frog-spawn in the ponds. A number of blackish specks of the size of mustard seeds are imbedded in a jelly-like mass. They are frogs’ eggs. They seem unorganised. But watch them, and the organisation will gradually appear. The egg will be hatched, and give rise to a little fish-like creature (see Fig. 2, p. 3). This will gradually grow into a tadpole, with a powerful swimming tail. Legs will appear. The tail will shrink in size and be drawn into the body. The tadpole will have developed into a

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ANIMAL BIOLOGY.

[Chap. I.

frog. S©, too, in the case of the chick. We know that the egg we eat for breakfast, if it had been placed for three weeks or so under a hen, would have developed into a little chicken. But not only the frog and the chick, but the dog, the worm, the butterfly, the star-fish, are one and all developed from an egg which is at first just a little speck of living matter. So that we may say that living animals, during their growth, pass from a comparatively simple condition to a comparatively complex condition by a process of change which we call development.

Now it is clear that, since we have no knowledge of dead matter springing into existence as living matter, life on the earth would soon cease if there were not something more than growth, development, decay, and death. Since death is the heritage of living things, we have the necessity for repvduction. This process is essentially the detachment of a part of the parent organism, which part itself, in turn, develops, reproduces, decays, and dies. In the higher animals reproduction becomes possible when growth and development are ceasing. The excess of repair over waste is seen, not in growth, but in the periodic detachment of a portion of the organism to continue its kind.

Another fact must now be introduced since it is one that is eminently characteristic of living things. In any animal the line along which the series of changes (growth, development, etc.) takes place is not indeterminate, but is determined by inheritance. Every mammal, e.g., begins life as a minute speck of an egg. There is often nothing about the egg to tell us to what particular animal it will give rise. And yet this is already quite determined by inheritance. So that we may say, summing up so far, that a living animal is a centre of continual waste and repair ; it undergoes a series of successive developmental changes constituting its life-history, the special nature of which is determined by inheritance ; it reproduces its kind by the detachment of a portion of its own substance.

And what is that substance 1 The essential constituent of a living animal is protein, composed of carbon, hydrogen, oxygen, and nitrogen, with a little phosphorus and sulphur. This, with much water, forms the chief constituent of protoplasm. At pre-

INTRODUCTORY.

xv sent it is only known as the product of life. The protoplasm, moreover, is not distributed throughout the body in a continuous uninterrupted mass, but is disposed in separate individual particles called cells. If you scrape gently the inside of your lower lip with a pen-knife, you will remove some of the cells of that part of the body. Mounted in a little saliva, and examined under the microscope,1 they will be seen (Fig. 24, ii.) to be flattened plates of irregular shape, sometimes curled up at the edges, and containing a rounded spot at the centre, called the nucleus. Of such nucleated cells (or their products), differing in form and appearance in different parts, the whole body is compounded.

Is there anything further to be added 1 It may be suggested that animals move about and are guided by feelings. Now, with regard to the feelings, it will be well to leave them on one side. Each of us knows a great deal about his own feelings, and very little of the feelings of his neighbours. Our knowledge of the feelings of animals is only arrived at by a complex process of inference ; and with regard to the lowest animals we are completely ignorant whether they have feelings or not. Leaving the feelings on one side, therefore, we may notice that moving about is only a particular and conspicuous manifestation of that general activity or vital energy which is characteristic of the animal organism. A large proportion of this activity arises from the fact that the organism is eminently sensitive — using this term not as necessarily implying feeling, *but in the same sense as a photographer would use the word when speaking of a sensitive plate. The animal is sensitive in its prompt and ready response and reaction to the stimulus of surrounding conditions, just as a sensitive plate promptly reacts under the

1 The student must provide himself with a good working compound microscope. The instrument I am in the habit of recommending to my students in the University College, Bristol, is a stand on the Hartnaeh model, with a No. 2 or 3 Zeiss ocular, and Zeiss’ objectives A and E, ora “ 1-inch” and “ |th-inch” of English make. The former is spoken of in this book as loio power, the latter as high power. Such an instrument may be obtained from any first-class instrument maker. The student is warned against ..purchasing low-priced second-hand instruments by inferior makers.

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ANIMAL BIOLOGY.

[Chap. L.

stimulus of light. But while a large proportion of the activity of the organism is clue to its sensitiveness, a certain proportion, which increases as we rise in the scale of organisation, is spontaneous, for the animal contains the springs of action within itself. It will be well, therefore, for us to replace the particular statement, that animals move about, by the more general, comprehensive, and exact statement, that they exhibit certain activities, prompted from within or called forth by surrounding conditions. So that we may now finally state the characteristics of living animals as follows : — A living animal consists of an aggregate of protoplasmic cells, together with certain cell-products ; it is a centre of waste and repair ; it undergoes a series of developmental changes constituting its life-history, the special nature of which is determined by inheritance ; it exhibits certain activities by which it maintains its relation to surrounding conditions ; it reproduces its kind by the detachment of a portion of its own substance.

This definition or description applies to all the higher animals ; but we shall learn hereafter that there are many of the lower animals that are not cell-aggregates, but are each of them constituted by a single cell. These unicellular animals are called protozoa or cytozoa, while the multicellular organisms, in which, as we shall see, different cells have different modes of activity, are called metazoa or liistozoa.

Let us now consider the essential points in which one of these higher animals — the animals with which we are ordinarily acquainted — differs from one of the higher plants — the plants we see around us. But first we may note the points of resemblance. They run almost through the whole description given above. Both animal and plant are cellular and protoplasmic ; both grow by intus-susception ; both are centres of waste and repair ; of both there is a developmental life-history ; in both is reproduction similar in principle. The activities of the animal, however, differ widely from those of the plant. But the main difference is in the nature of the food, and the manner of its intus-susception. Plants can build up protoplasmic matter out of such inorganic materials as contain the requisite elements.

INTROD UCTOR Y.

XVII

Animals cannot do this. They require ready-made protoplasm in the form of vegetable or animal matter. Plants raise the inorganic into the organic; animals then take up the process and often carry it forward to more complex products. But no animal can raise the inorganic into the sphere of the organic. That is the function of the plant. Plants alone can manufacture protoplasm. Animals require the manufactured article.

But the manufactured article is not directly incorporated as such into the substance of the consumer. The protoplasm of sheep cannot become the protoplasm of man without being largely taken to pieces, chemically, and then put together again. Hence the necessity for digestive and assimilative organs to enable the animal to do this. Even the protoplasm of the mother’s milk must be digested before it can be assimilated by the young. In plants anything of the kind is exceptional and subsidiary. They take the elements of protoplasm from the air that bathes their leaves, and from the water that bathes their roots. Hence the branching and spreading form of that part which is exposed to the air, and the far-reaching ramifications of that part which is implanted in the earth. We may sum up this distinction by saying that animals differ from plants in that they require protoplasmic food-stuff which must undergo a more or less complex process of digestion, within or without their bodies, before it can be assimilated.

Out of this main distinction there flows a secondary distinction of some importance. Plants manufacture organic tissues out of such inorganic raw materials as contain the requisite elements. One of these raw materials is the carbonic acid gas of the air, from which the green plant abstracts the carbon, returning the oxygen to the air as a by-product for which it has no use. But carbonic acid gas is one of the chief products of that continued waste which is entailed by the ceaseless activity of the animal. And to enable that waste — which, as we have seen, is essential to animal life — to continue, the animal requires a more or less liberal supply of oxygen. So that animals and the higher green plants perform opposite and complementary functions in the economy of nature. By the plant carbonic acid gas xviii ANIMAL BIOLOGY. [Chap. I.

is decomposed, and oxygen set free into the air. In the animal the carbonic acid gas is recomposed and breathed forth into the air.

The higher animals thus differ from the higher plants (1) in that they recompose the carbonic acid gas which the plants decompose ; (2) in the nature and amount of their activities ; and especially (3) in requiring protoplasmic food-stuff, which must undergo a process of digestion before it can be assimilated.

Having now gained some idea of the nature of those living things which it will be our task to study, let us proceed to consider what is the nature of the problems that are likely to be suggested by the study of animal organisation and animal life. And that we may not fall into generalities beyond the reach of the student, let us consider the special case of such an animal as the common frog. In the first place, the fully-grown frog has a tolerably constant external form and appearance, by which it may always be recognised and distinguished from other organisms, such as the toad, or the edible frog of the Continent. Dissection after death further shows that it has a definite and tolerably constant internal structure. A number of organs such as heart, liver, stomach, kidneys, are found within the body, and these have a constant form and constant relations to the nerves and blood-vessels which ramify throughout the body. And minute examination with the microscope further shows that there is a definite and constant minute structure. The organs are seen to be built up of cells or cell-products aggregated in special ways into what are called tissues. So that we have to consider: (1) the general form and structure of the organism ; (2) the special form and structure of the organs ; and (3) the minute form and structure of the tissues.

But the frog does not stand alone among organisms. Hence it is necessary to compare its general structure, the structure of its organs, and the structure of its tissues, with the general, organic, and minute structure of other organisms, with the object of ascertaining what are the points of resemblance and the points of difference. By this means the range and importance of the structural problems is enormously increased.

xix

Now all those problems which deal with form and structure are called morphological problems ; and Morphology , which deals with these problems, forms a well-marked department of Animal Biology. From what has just been said, moreover, it follows that this department comprises three divisions :

1. The morphology of organisms (zoology).

2. The morphology of organs (anatomy).

3. The morphology of tissues (histology).

But the form and structure of the frog does not remain the same in all periods of its existence. Beginning life as an egg, it is hatched as a tadpole, and only reaches the full stature of froghood after several metamorphoses. There is, therefore, a Development of the organism as a whole, of the organs within the organism, and of the tissues of which the organs are composed. Hence there arises a second and most important set of morphological problems of development, which may be divided into

1. The development of organisms

( a. of the individual (ontogeny). ( b. of the race (phytogeny).

2. The development of organs (organology)

3. The development of tissues (histogenesis)

embryology.

So far we have only considered the problems that arise out of a study of form or structure. We have regarded the frog merely as a piece of mechanism. But every structure has its special office. Every organ in the frog’s body has its particular work to do, its function as it is called. The muscles contract or shorten, and so produce motion, the heart beats and ministers to the circulation of the blood, the lungs are for respiration, the eye for seeing, the glands for secreting, and so forth. In these various organs, moreover, special tissues perform the special part of the work, and within these tissues special cells are developed. Finally, all these functions are to be made subservient to the general good of an organism which is set in the midst of more or less complex surrounding conditions. And then the frog himself, as one organism among many, has to perform his special function in the economy of Nature. Hence arises a new

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ANIMAL BIOLOGY.

[Chap. I.

set of problems connected with functions and the chemical or metabolic changes which accompany the exercise of function. These fall within the department of Physiology. And just as there is a morphology of organisms, of organs, and of tissues, so too is there —

1. The physiology of organisms.

2. The physiology of organs.

3. The physiology of tissues and tissue cells.

Other biological problems fall under the head of Distribution. Frogs do not occur all the world over. They have a definite distribution in space. Nor do they occur in all geological strata. They have a definite distribution in time. Hence the problems connected with the distribution of animals fall naturally under two heads :

1. The distribution in space (chorology).

2. The distribution in time (chronology).

Finally, there is a fourth department of Animal Biology, which deals with the causes of those fads which are studied in the other three departments. This is of recent growth. To the questions, What are the structures and functions of tissues, organs, and organisms ? What is the series of developmental changes undergone 1 What is the distribution in space and time? is added a further question, How has all this been brought about ? And thus to the three departments of Biology which deal Avith the facts of structure, the facts of function, and the facts of distribution, is added a fourth, called JEtiology, which deals with the causes to which these facts are due. And this fourth department is co-extensive with the other three, which may be grouped together under the head of Descriptive Biology. Descriptive Biology thus deals with facts, and .^Etiology with causes. And since no fact can be said to be understood until we have some knowledge of its manner of causation, it is clear that ^Etiology is necessary to and supplementary to the otherwise incomplete science of Descriptive Biology. Descriptive Biology without HCtiology lacks life : iEtiology not founded on

INTRODUCTORY.

xxi

Descriptive Biology is baseless speculation. Both must grow together and minister to each other’s wants.

It only remains, in this introductory chapter, to indicate the aim and scope of this volume.

The number of animals— each a centre of so many biological problems— is well-nigh countless. So is the number of houses in England. And yet, on entering the house of an acquaintance whom we have never before visited, we know at once what we shall find. The hall and passages, the dining-room, drawingroom, library ; kitchen and offices below ; bedrooms above — we already more than half know them all. And why ? Because we already know the type of house that belongs to a certain grade of society. If we visit a lord or a labourer, our expectations are different. We know that some men have to live in hovels, in which one room has to suffice for all the needs of the famity. We know that at the other end of the scale of social life there is a separate room for every function of that more complex life. So is it also with animals. There is a lowly type of animal, where a single cell constitutes the whole house, and all the functions of life have to be performed in and through that single cell. There are others composed of many cells, in which differentiation of structure and specialisation of function have been carried far. But a special type of structure is characteristic of each grade of animal life, just as a special type of house characterises each grade of social life. And if we know the type which marks that special grade, we more than half know what will be the state of matters with regard to differentiation of structure and specialisation of function in any individual case. If we know, for example, the typical structure of a mammal, a bird, and an amphibian, we know pretty well what to expect in any other mammal, bird, and amphibian, and are some way on our road towards a knowledge of the structure of a fish or a reptile. And so with regard to other grades of animal life.

It is the object of this volume to assist the student in acquiring such a knowledge of some of the more important facts of the morphology and physiology of certain typical animals as may form

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[Chap. I.

a base-line sufficiently accurate and extensive to enable him, by further study and research, to carry on his survey of the animal kingdom. Space will not permit of the study of the facts of distribution. And with regard to ^Etiology, the aim will be rather to pave the way for a study of causes by an accurate presentation of facts than to deal at any length and more than incidentally with the theory of Evolution or the Doctrine of Descent.

PART I.

VER TERRA TE A NA TOM Y A ND PH YSIOL OGY

AS EXEMPLIFIED BY

THE FROG, THE PIGEON, AND THE RABBIT, with occasional Reference to other Types.

Chapter II. Natural History And External Characters

1. The Frog. — The common frog, liana temporaria, is tolerably abundant in summer in clamp places and by the side of ponds. In winter it is not readily to be found, for at that season of the year frogs hibernate, often in groups, buried in the mud and under water. The warmth of spring rouses them from their torpor, and they then congregate and pair with much sound of croaking. The female lays a number of eggs, about one-tenth of an inch in diameter, each of which is surrounded by a thin layer of albumen. And as they are laid the male pours upon them a fertilising fluid. If a little of this fluid, or the water into which it is shed, be examined under the microscope (high power), a great number of minute active bodies of delicate tapering form will be seen. They are the spermatozoa, the essential elements in fertilisation. After the eggs have been fertilised by the entrance into each egg of a spermatozoon, the albuminous coating of the yolk swells to many times its original thickness by the absorption of water; so that the frog-spawn then has the appearance of a white gelatinous mass, made up of largish jelly-like spheres, in the midst of each of which is the dark ovum, which is seen, on closer inspection, to have a darker and a lighter hemisphere.