Comparative anatomy and physiology

Preface

40^

The reader who is sufficiently acquainted with the progress in vertebrate physiology during the last phase of physiological methods, and who knows how scattered and incomplete are the investigations which have been made by the same kind of physical and chemical inquiries on invertebrate animals, will not expect to find in the present volume any complete statement of the physiology of animals, in the sense in which that term is now used. Such observations as have been made without especial reference to the vital processes of man are, for the most part, very valuable and suggestive ; but the time to write a text-book of Comparative Physiology, as we now understand it, has not yet arrived.

All that I have attempted to do in this little book has been to illustrate the details of structure by a notice of such experimental inquiries as I have convinced myself, or have adequate reason to believe, are, in their broad outlines, correctly stated. I have much more attempted to make use of what were long since called the experiments that Nature makes for us, by mi Comparative Anatomy and Physiology.

referring to, sometimes perhaps insisting on, the different methods by which similar results are attained by different animals. That which I have most constantly kept before myself, and which I hope the student will faithfully bear in mind, is, that there has been an evolution of organs as well as of animals, and that he who desires to understand the most complicated organs must first know the structure of such as are more simply constituted.

In pursuit of this object, I have written about organs rather than about groups of animals ; but I have added an index in which the various parts of an animal are collected under the head of its name ; so that the student who desires to use this manual as a zoological text-book will have no difficulty in selecting the portions of the chapters which bear on a particular form or set of forms.

I have departed a little from the ordinary method of writing a handbook, in somewhat plentifully interspersing the names of my authorities for various statements. I have done this, not only because it recommends itself to my sense of justice, but because zoological science is just now advancing so rapidly that many observations and suggestions have to be incorporated, even in a text-book, before they become the general property of zoological workers. My indebtedness to the personal teaching and the published writings of Professor Ray Lankester must be

Preface .

IX by no means thought to be limited to the statements with which his name will be found to be connected ; indeed, I owe him more than I can well express.

I have been careful to acknowledge the source whence the illustrations are taken, and I have to return my thanks to the Publication Committee of the Zoological Society ; to Professor Flower, who only added one more to a number of acts of personal kindness when he generously put at my disposal all the wood-blocks which were in his own possession ; and to those other friends who have allowed me to copy figures from their works.

As this manual is written on lines that are rarely followed, I shall be greatly obliged for any suggestions as to its improvement, or for corrections of any errors which may have found their way into it.

*

F. JEFFREY BELL.

King’s College , May , 1885.

Contents

CHAPTER PAGE

I.— Introductory 1

II.— Amoeba 18

Chapter I. Introductory

Comparative anatomy is the science of the structure of animals, considered in their relation to one another; comparative physiology deals with the functions of the parts of which these animals are made up, and, by examining different forms that present various kinds of activities, it throws light on the essential properties of living matter.

The study of animals is but a paid of the wider science of the study of organised matter generally, the science of biology, which takes plants as well as animals for the objects of its investigations. Under the head of biological studies we have, therefore, to group (a) those which regard organisms as working machines, capable of performing various functions ; these studies are physiological, whether animals or plants be separately or simultaneously examined ; (b) in the second place, the parts of which the organism is made up may be investigated, and our studies are then said to be anatomical, if we concern ourselves with isolated types, as does the student of human anatomy ; or they are morphological, when we compare organisms and their parts one with another, and try to draw out the significance of isolated facts, and to learn their bearing on the general scheme of the organisation of living matter. b— 16

The present work is concerned only with Animals ; but, as there is a fundamental resemblance between Plants and Animals, it is in the first place necessary to enquire into the characters and modes of activity of living matter, pure and simple, without any question as to whether it be animal or vegetable.

LIVING MATTER.

Animals and plants have at least this in common, that they are both fashioned out of a material which, in all its essential characters, is common to them both ; and, whether one would be a zoologist, or student of animals, or a botanist, or student of plants, it is, in the very first place, necessary that he should have some clear and exact comprehension of what are the characters and what are the modes of action of that primary fashioning substance which forms the material basis of living creatures, and which is known as protoplasm. The fact that the sciences of zoology and botany have to do with this 1 ‘ physical basis ” of living matter separates and distinguishes them at once from such studies as chemistry or jfiiysics, with which the phenomena of life have no necessary connection.

Living: is distinguished from not- living- matter by several important and easily recognisable characters. It would seem to have a fundamental and characteristic composition ; it has the power of continuing to exist by taking into (nutrition), and making part of itself (assimilation) other living or even not-living matter. Nutrition and assimilation lead to growth, and this growth is succeeded by a stage in which the additional material obtained is used for the purposes of reproduction. After a time a living organism may be seen to be unable to withstand the action of the surrounding forces in the midst of which it has lived, grown, and reproduced itself ; in other words, its activity diminishes and

Chap. i. ] Characters of Living Matter. 3

diminishes, until at last it dies. From this dead matter, living material can never, by any process now known to us, be produced ; for, so far as we know, living matter can only proceed from other living matter.

As the chemist is only able to acquire definite information with regard to the chemical composition of living matter by the use of certain treatments which deprive it of life, we cannot speak with certainty of more than the broad outlines of its composition ; but this, at least, may be said : in living matter (protoplasm), the four chemical elements, oxygen, hydrogen, nitrogen, and carbon, are always found, and with them there would seem also to be associated small quantities of sulphur and phosphorus. It is possible, if not certain, that protoplasm is a compound of a number of the so-called proteid bodies, and it is quite certain that what chemists call its “ atomic composition ” is very high. One of the most complex bodies known to us is that constituent of the brain which is called protagon ; and its “ atomic composition ” has been determined to be C^HgogNgPOgg, or no less than 509 atoms. When such a body is active, fresh chemical changes are always taking place within it ; it is in a condition of unstable equilibrium ; the result of such change, so far as it affects the living matter, is loss or waste; in addition to this, living matter is always taking up fresh oxygen, and forming carbonic acid, of which it has to free itself. These activities combined require, as may be supposed, the addition of fresh material from without; that is to say, living matter demands food. The food so taken in may or may not be similar in composition to the organism itself ; but, as the living creature has wasted through all its parts, the fresh material has not merely to bo taken in, it has also to be assimilated. When a crystal, placed in a solution of its own material, grows, it does so by merely laying on the fresh molecules outside those already formed ; protoplasm, on the other hand, makes the fresh food, which may or may not, indeed need not, have the same composition as itself, an essential part and parcel of itself.

In the next place we observe, that while a crystal under the conditions just now mentioned will grow so long as it is supplied with matter of similar chemical constitution, living matter only grows when assimilation goes on at a quicker rate than destruction or waste. Save for the difficulties of experimenting, there is no reason why all the sulphate of copper in the world should not (a) be brought into one huge crystal, and (£) so remain. It is not so with living matter ; for every organism there appears to be a limit of growth, and when that is reached, all the succeeding matter assimilated goes for a different purpose. The organism, ceasing to grow, begins to reproduce its kind, and, in the very simplest cases, produces an individual exactly similar to itself. This act of reproduction appears to be, next to sustentation, the primary work of every organism, and when that is completed, we often observe that the parent organism begins to lose its activity ; it becomes the prey of other living organisms ; or, undergoing gradual decay, the complex mass of albuminous matter, which we call protoplasm, and associate with life, falls away into constituent molecules of a less high degree of chemical complexity.

Assimilation, growth, reproduction, death, are, as here explained, four phases in the history of living matter which at once and sharply distinguish it from crystalline or other dead material.

Nor is this all ; if we set one crystal against another of similar composition, or if we try to rouse or stimulate a crystal, we get no response. With living matter the case is very different ; roused either by chap, i.] Characters of Living Matter. 5

some apparent friend or enemy in the water, or by a touch from our needle, as we observe it under the microscope, a mass of living matter will be found to be irritable. In consequence of this irritability it undergoes some change converting latent into actual energy, and this is most frequently and most easily seen to be some change in space, or in the relations of its parts ; these are due to what is known as the contractility of living matter. In other cases, the production of heat, light, or electricity, is the expression of irritability.

We have next to observe, that within the area of any given mass of protoplasm, there may be movements of its parts ; some of the granules seem to stream in a more or less regular course between those on either side of them, in a way which can best be understood by supposing the observer to be raised above and to be able to note the movements of a great crowd of passengers in a busy street ; some move faster than and overtake others, some collect into more or less small crowds ; others, having moved onward for a certain distance, turn aside or turn back. This streaming movement of protoplasm is highly characteristic, and affords a proof that the problem of the motile activity of protoplasm can only be explained by the study of the parts of which it is made up.

Lastly, thin layers of non-granular protoplasm are sometimes to be observed gliding over firm bodies ; by these means the whole mass is enabled to progress in a forward direction.

The study of streaming movements shows us that the constituent particles do not move around any fixed point, but freely as the particles of a fluid substance. So far as we can see, these movements are not the result of any external cause ; did we choose to allow that a simple mass of protoplasm had a “ will,” we might well call them “ spontaneous” or “ voluntary;”

without going so far, we must allow that they appear to be due to the protoplasm itself ; they are selfmoved or automatic.

Living matter, then, is irritable and automatic ; irritability finds expression in contractility, or in the production of such forces as heat, light, or electricity.

With regard to its general physical and chemical characters, we have to note that it is possessed of great cohesive powers, and yet is very extensile ; it does not mix with water, but it swells by imbibition ; it may expel the contained fluid in the form of rounded vacuoles, and bubbles of gas are sometimes apparent in it. It is ordinarily colourless, and refracts light more strongly than water ; it is in most, and probably in all cases, slightly alkaline in reaction.

Before we leave the general consideration of protoplasm, we must point out two foreign elements which have to be considered. The first of these is the presence in protoplasm, as we ordinarily observe it, of various more simple chemical compounds, which have the form of granules ; these, which may be fatty or starchy bodies, are conveniently grouped together under the head of metaplasm ; they may be regarded as owing their origin to the changes that are constantly taking place in the molecular constitution of the protoplasm, or, in other words, as waste products not yet eliminated.

The second is a general motion of a protoplasmic mass, especially when of particularly small size ( e.g . bacteria) ; this movement of the body as a whole is not a vital, but a purely physical phenomenon, as may be demonstrated by the simple experiment of rubbing up a little gamboge in a drop of water, when exactly the same movement is to be observed. This approximation and separation of small particles is a phenomenon which has attracted the attention of the physicist, by

The Cell.

whom it must be explained ; it was, however, first observed by an eminent botanist, and is consequently known as the Brownian movement.

The term cell is not unfrequently applied to every separate mass of living matter, but, in consequence of the associations connected with this term, it is better to make use of the more elaborate though perhaps more intelligible nomenclature which enables us to distinguish between the different characters of “elementary organisms.” When attention was first directed to these objects, the botanist observed that in each mass of protoplasm there was a jiortion which, by various characters, could be easily distinguished from the rest, and which might be very appropriately spoken of as the nucleus ; in addition to this, he saw that the outer portion of the protoplasm was enclosed as in a wall ; he spoke, therefore, of the whole as a cell, with a cell wall, and a contained nucleus.

Later on it was found that the protoplasm (or “ sarcode,” as it was originally called) of animals was not to be distinguished from that of plants, and it was then also seen that it was only in very rare cases that this animal protoplasm was enclosed in a cell wall. Thereby the very first conception of a cell was destroyed, but the name was still retained as a convenient term.

Still later researches revealed the at first astonishing fact that organisms could and did exist in which that specially modified portion of the protoplasm which had been called the “ nucleus ” was, to all appearance, altogether absent ; some naturalists, and especially some physiologists, now regard the nucleus as no essential part of the cell. On the other hand, it seems better to recognise in our nomenclature the present conditions of our knowledge, and to use for the “ elementary organism ” some other definite term than that around which so many battles have

S Comparative Anatomy and Physiology.

been fought, and with which, perhaps, no few superstitions are or have been connected.

We will, therefore, follow^ those who have agreed to the suggestion of Prof. Haeckel, and will use for the elementary organism, whether or no provided with a nucleus, the useful and suggestive term of plastid. This plastid, or unit of organic structure, is composed of protoplasm ; it may be without a nucleus, when it is a cytod (or cell-like body), or it may have within it a denser mass, which is very feebly, if at all, contractile, the nucleus; in which case it is a cell. This nucleus is ordinarily provided with one or more smaller uucleoli, and, possibly, always has a distinct investing membrane. It would appear to have a special chemical composition, inasmuch as while a cell when treated with a ten per cent, salt solution leaves a precipitate, no such precipitate is stated to be found when a cytod is subjected to the same reagent. The body so precipitated has been called nuclein.

Protoplasm, then, is presented to us in the form of plastids, and these plastids may either be without (cytods) or have (cells) distinct nuclei. All organisms are composed of one or more cells, or, in other words, are either unicellular or multicellular. The former, as much as the latter, are capable of exhibiting all the essential phenomena of life.

TISSUES AND ORGANS.

When we examine the different stages in the history of a developing animal, or compare a series which commences with low and passes through more highly developed forms, we find a gradual increase in the complexity of the parts ; of this we have already had an example in comparing the cytod with the cell, and we shall observe it in every chapter of this work. This increase in complexity is termed the process of €lifferentiatiou.

Tissues and Organs.

In making a general survey of animals we find that the lowest consist only of simple cells ; later on, the cells are found not to live an independent existence, but to be associated one with another, and different groups of cells are seen to be differentiated in various ways. The result of this is that sets of cells come to have different characters (some are contractile, others irritable, and so on), and these different sets are what are known as tissues ; secondly, we observe that these tissues become connected with one another in different proportions and relations, so as to give rise to those parts of the adult which take on particular duties, and are known as organs.

Looked at in a general way, and without taking any notice of exceptional cases, we observe that there are tissues in an animal which are not found in a plant ; these, which are distinguished as the animal tissues, are such as have a relation to movement or sensation ; in other words, the muscles and nerves are animal tissues. On the other hand, plants preserve, protect, and sustain themselves, and the corresponding tissues in animals are always spoken of as the vegetative ; of these we may find convenient examples in that outer layer of the body which is spoken of as epithelium, or that supporting tissue which is known as bone.

The classification of organs is a little more complex, but it will be convenient to give it now, so that time and space may be saved in the future.

In the first place it is clear that the vegetative functions fall under three great heads ; an animal has to care for itself, to adapt itself to or move through its surroundings, and to reproduce its kind. And, in the second place, it is just as obvious that it has to perceive what is going on around it, and to act accordingly. We have, then :

io Comparative Anatomy and Physiology.

(1) Organs of internal relations.

i. Protective. — Examples : Skin, shell.

ii. Nutritive. — Examples : Digestive tract (nu-

trient) ; heart and blood-vessels (circulatory).

iii. Purifying'. — Gills, lungs (carbonic acid) ; kidneys (nitrogenous products).

(2) Organs of external relations.

iv. Locomotor. — Limbs, etc. (compounded of skeletal and muscular tissues).

v. Prehensile. — Limbs, etc. (compounded of

skeletal and muscular tissues).

vi. Offensive. — Teeth, claws, electrical, odorous

organs.

(3) Reproductive.

(a) Germ-producing glands : testes, ovaries, which are

essential.

(j8) Copulatory : penes, etc., which are accessory.

(4) Sensory.

(a) Organs which receive impressions ; eye, ear, etc., brain.

(j8) Organs which Stimulate other organs ; brain.

METHODS OF COMPARISON.

When an anatomist has acquired a positive knowledge of a certain number of selected forms of life, he proceeds to convert his empirical acquaintance with facts into science by reasoning upon the information which he has acquired. In this operation he makes great use of the fertile method of comparison, remembering the words of Buffon, “ Ce n’est qu'en comparant que nous pouvons juger .” Like things, however, must be compared with like, or confusion will inevitably result. We must, therefore, lay down certain rules to guide us in these kinds of enquiries, for, though no one would attempt to compare a heart with a long, many would, at first, more willingly compare the leg of a man with that of a cockroach, than the fin of a perch with the wing of the sparrow ; yet the latter is the more justifiable proceeding.

Homology.

ii

The reason for this is plain, the moment we clearly understand what object the comparative anatomist has before him ; it is that of coming to some general conclusions as to identity or community of structure ; for this purpose, then, he is not to compare parts that have the same function, but those that are formed in the same kind of way. The physiologist, on the other hand, looking at organs as parts of a machine, examines together those that do the same thing. When we compare parts morphologically, we must not be content merely with an analogy between them, we must be careful that there is a homology or real resemblance.

The first criterion of homological parts is their development from similar embryonic structures ; such, are the wing of a bird and the leg of a horse. But a further question now arises ; why have these wings and legs, which in their completed condition are so different from one another, a similar structure in the embryo 1 The answer to this is given by the doctrine of descent, which supposes that the bird and the horse had in the past a common ancestor, provided with limbs simpler in structure than those of either bird or horse, but having essentially that which they have now, or which they have had, and from which they are both derived ; a true and complete homology of parts is, then, only to be found between animals which have had a common ancestor provided with the part to be compared. This complete homology may be conveniently spoken of as tiomogeny (Ray Lankester).

It is very necessary to have before the mind this idea of community of descent, because we shall constantly meet with cases in which, with a very close resemblance in structure and mode of development, there is not a complete identity in descent. For example, all mammals and all birds are provided with four cavities in their hearts : two auricles and two ventricles ; but it is certain that, whatever was the animal that was the nearest common ancestor to the two, it had only one ventricle. The right and left ventricles of the hearts of birds and mammals are, then, not homologous in the sense of being homogenetic ; they have been acquired independently by the two groups, in consequence of certain physiological needs ; they are the result of similar modifying forces, and are homoplastic, but not homogenetic parts.

DEVELOPMENT.

The last point to which the student must be introduced is one of the very greatest importance. If we study the animal kingdom throughout, we find that, starting from the simplest mass of protoplasm, we are gradually led to the complex and elaborate structural and functional arrangements which are found in so highly organised an animal as man himself. If, on the other hand, we study the developmental history of a highly organised form, we find that it starts from a simple mass of protoplasm, the egg, or ovum, as this plastid is called ; this cell gradually becomes more and more elaborated, and takes on the more complex arrangement which may be seen in its parent ; we observe, that is, that not only are there a number of stages in the different representatives of the animal world, but that there are also a number of stages in the structural history of every individual ; and we may go yet a step farther, and say that in a broad and general way there is a complete parallelism between the two.

The results of the investigations and considerations which flow from a study of the facts here indicated are best expressed in an aphorism, which may at once be laid to heart, and which will be abundantly proved by a study of development and comparative anatomy :

E VOL UTION.

“ The history of the individual is a compressed epitome of the history of the race.”

Those, therefore, who desire to obtain a complete knowledge of animals or, indeed, of any one animal, must not be contented with an account of the anatomy of the adult ; they must direct their attention also to its development, and become the students of Embryology, while they must no less take care to study the history of the animal, or of its allies, in the past ages of the world, or to know something of its Palaeontology.

These two branches, Embryology and Palaeontology, are of the greatest assistance in an endeavour to obtain some clear idea of the morphology of animals ; but a weapon no less sure and no less important is that of comparison, by means of which similar parts in different organisms are studied and explained ; no better aid to safe judgment can be afforded, and it must be used unceasingly and unsparingly.

EVOLUTION.

The great maze and mass of facts which are found in works on zoology or comparative anatomy are hardly to be held together without the bond of philosophy ; the grouping of facts, and, still more, the grouping of animals, must be always more or less unintelligent, mechanical, and artificial, unless we make some use of some kind of explanation. That which we shall use here will be founded on the belief that there is a blood relationship, or relationship by descent and inheritance, between every member of the animal kingdom; and that, were it possible to know all the facts, we could make a genealogical tree for animals, which should be as exact and definite as the family tree which is drawn up by the genealogist or the herald. That division of biology which busies itself with genealogical problems is known as Phylogeny.

All the steps in the differentiation and elaboration of organised beings are examples of that process of evolution, which, when based on a belief in the existence of a blood relationship between animals, is known as the doctrine of descent. In an attempt to understand how this has worked, we make reference to two different series of facts. We have, in the first place, to make certain generalisations as to the way in which the differences have been brought about, and we have, in the second, to consider what are the essential properties of living matter which may be regarded as the determining factors in the evolution of organised material.

The generalisations made from a number of ob^ served facts may, if this definition be borne in mind, be called the laws of evolution; they have been thus enunciated by Professor Huxley :

(1) There has been an excess of development of some parts in relation to others.

(2) Certain parts have undergone complete or partial suppression.

(3) Certain parts, which were originally distinct, have coalesced.

Let us apply these laws to a concrete example, and select for study the fore-foot of a camel. In the more primitive mammalia there were five fingers or digits, each connected by a metacarpal or palm bone with the wrist, and these five sets of digits and metacarpals were of subequal size. In the hoofed group of animals the first of these, or thumb, disappeared, as in the case of the modern pig ; the two that were now outermost, the second and fifth, became smaller and smaller, as in the sheep or deer, and finally, as in the camel, disappeared altogether. Here we have various stages of law 2. This loss of the outer was accompanied by an increase in the size of the median digits and metacarpals (law 1), and in the more or less complete fusion of

Chap, i.] Heredity and Variability. 15

the third and fourth metacarpals one with the other (law 3) ; the result of this last process being the formation of a bone which at its lower end only gives any obvious indication of its primitively double nature.

The characteristics of protoplasm which appear to be the determining factors of evolution, are (1) its power of producing an organism like to itself; and (2) the fact that no child or parent, or any two children, are exactly similar one to another. The first of these principles is known as that of heredity, the second as that of variability. It is obvious that the second principle only comes into action because of the differences in the surroundings of every individual plastid ; the greater the homogeneity of the surroundings, the greater the likenesses between the plastids. The law of heredity may consequently be compared to the first law of motion (Gasquet).