Zoology : a description of types of animal structure

Preface

The present Manual is intended to serve as an introduction to the study of Comparative Anatomy and Physiology, and may be used by students preparing for the elementary stage of the zoological section of the Science and Art Examinations. It is based on the method adopted by the leading teachers of Zoological Science — namely, that of selecting a few of the leading types of animal life, as exemplified by common animals, and of initiating the student into the study of zoology, through the description of the structure and physiology of these typical forms. In this way alone, is it possible to give the beginner broad and comprehensive views of animal organisation, on which a sound knowledge of the modifications of animal structure may be afterwards built up.

The descriptions have in most cases been verified by the Author’s observations ; and he has not hesitated to introduce here and there considerations of a somewhat theoretical, nature, but which bear more or less directly on the maxims and precepts of zoology. The examination of a zoophyte has thus afforded a text for a discussion on the * individuality ’ of animals ; whilst the nature of ‘ homologies ’ and the physiological differences between animals have been illustrated by the examination of the lobster’s exoskeleton, and by the comparison of Amoeba with higher forms, respectively.

It may lastly be remarked, that in no case can the study of living organisms be satisfactorily carried on without the continual and direct examination of the objects described. These lessons must therefore be regarded as constituting a guide-book to practical work in zoology, in which either teacher or student, provided with a microscope, and having access to a few common animals, may find simplified descriptions of the leading types of the zoologist’s study. The Author would also hope that, incidentally, the use of the Manual may promote the extension of biological knowledge, which constitutes one of the most powerful sources of culture at the command of the modern educationist.

Edinburgh, April 1877.

Chapter I. Animal Life In Its Lower Forms— Amceba

1. The objects that surround us in the world may be conveniently divided into two great groups or series, according as they are living objects or lifeless ones. To the living objects we apply the term organic , whilst the lifeless things are known as inorganic objects. The science which investigates the details of living or organic beings, and makes us familiar with every part of their history, is named Biology (Gr. bios, life ; and logos, science). The inorganic objects of our world fall to be considered by such sciences as Geology, Chemistry, Natural Philosophy, and the like.

2. Living beings are themselves susceptible of division into two great groups. These groups we term familiarly animals and plants. Hence the science of Biology necessarily comprehends two divisions — Zoology , which investigates the life-history of animals ; and Botany , dealing with

ZOOLOGY.

the details of plant-structure and life. It is well at the outset to remark, that although practically, and for the sake of convenience, we may make a distinction between animals and plants, and between the respective sciences which deal with them, no sharp line of separation can in reality be drawn. In some cases, it may be absolutely impossible to determine the exact nature of an organism, or to say whether it is an animal or plant ; and thus the biologist must become acquainted with the structure and history of both groups of living beings, if he is to perfectly represent his science. Zoology and Botany, in fact, require to be studied together, in view of the intimate relations which exist between the animal and plant worlds.

THE AMOEBA AND ITS LIFE-HISTORY.

3. The subject of the present series of lessons is the animal creation. As an example of the manner in which all animals, high and low alike, must be studied, we shall select one of the lowest animal organisms, and endeavour to embody in its life-history the chief details which await the consideration of the zoologist. The Amoeba , or ‘ Proteus-animalcule ’ (fig. 1) as it was formerly named, is one of

Fig. 1. — Amoeba : a, young Amoeba radiosa, with pseudopodia protruded; b, older specimen of A. radiosa , shewing nucleus, tt, and food-vacuoles (both highly magnified).

the most familiar organisms with which the microscopist is acquainted. Various species of these animalcules are

LIFE-HISTORY OF AMCEBA.

found in fresh water which has lain stagnant for some time, in damp earth and mud, in infusions of organic matter, and also in the sea. They are all of minute or microscopic size, and may vary in diameter from the -uiirth to the uTvth part of an inch. When one of these beings is watched by the aid of a microscope, the body is seen to consist of a speck of colourless jelly-like matter, to which the names protoplasm, bioplasm , and sarcode have been given. This protoplasm is a highly complex substance, and, as exemplified by the amoeba, may of itself form an entire and complete animal ; whilst many of the lowest plants are similarly composed of this matter in its simplest and most primitive condition.

4. PROTOPLASM is an albuminous substance, and one of a great class of compounds to which the name of Protein compounds has been applied. From its constant association with living beings, it has been named the 1 physical basis of life.' The bodies of all living organisms are composed of this substance in their earliest state. In the higher animals, the protoplasm becomes developed or differentiated into many different tissues ; whereas in lower organisms it retains much of, or all its primitive simplicity. The life or vitality of protoplasm is not certainly known to exist under any other circumstances than when derived from pre-existing and living protoplasm. Its chemical composition shews protoplasm to consist of carbon, hydrogen, nitrogen, and oxygen ; and on physical examination, it is found to undergo contractio7i when stimulated by electricity ; whilst, when subjected to a temperature of from 30° to 450 or 50° Centigrade (86°, 1 1 30, 1220 Fahr.), it coagulates or undergoes a process known as ‘ heat-stiffening.’

5. Structure of Amoeba. — The body of the amoeba (fig. 2, A) is seen to be of a firmer consistence towards its outer surface than internally. And whilst the name of ectosarc , b, has been given to the firmer and outer layer of the body, the term endosarc , a, is applied to the more fluid internal layer. The latter portion of its body may exhibit a granular appearance, produced by the presence of numerous minute solid particles in the general protoplasm. Within the ectosarc, and sometimes within the endosarc, one or more clear

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ZOOLOGY.

spaces may be seen to exist, and these spaces exhibit motions of expansion ( diastole ) and contraction {systole). They are termed contractile vesicles (fig. 2, A, B, c), and are apparently permanent organs. These vesicles are not to

Fig. 2. — Morphology of Amoeba : A, diagrammatic figure of Amoeba ; a, endosarc ; b, ectosarc; c, c, contractile vesicles; n, nucleus. B, Amoeba, princeps; c, contractile vesicle; n, nucleus; v, so-called ‘villous’ region. C, Amoeba encysted. D, further stage of C, shewing interior of cyst occupied by pseitdonavicellce. E, two pseudonavicellse, or young Amoebae liberated (all highly magnified).

be confused with certain other clear spaces of temporary nature, named food-vacuoles. The only remaining structure exhibited by the amoeba is a solid particle situated in the endosarc, and named the nucleus or endoplast (fig. 2, A, B, n ). This body may contain a smaller particle, named the nucleolus ; and both structures are rather characteristic of many lower animals allied to the amoeba, and included with it in the sub-kingdom Protozoa.

A living amoeba is soon seen to merit its name of Proteus-animalcule, or, indeed, its ordinary designation (Gr. amoibos , changing), since it may be observed to be continually altering the shape and configuration of its body ; passing or literally flowing from one shape into another, and moving about by pushing out its soft protoplasmic body into certain processes named pseudopodia. These processes, usually of blunt, finger-like nature, appear generally to be emitted from any part of the body ; but in some cases they

PHYSIOLOGY OF AMCEBA.

II have been described as being more limited in their origin. When the animalcule wishes to move or to seize particles of food, part of the body is pushed out into one or more pseudopodia. The ectosarc appears to be chiefly concerned in the formation of these processes ; but the endosarc also seems to pass into the pseudopodia, which thus become prolongations of both layers of the body-protoplasm. It is the power of emitting pseudopodia which enables these animalcules to alter their shape, as already described ; and this power, moreover, is a notable characteristic of the group ( Rhizopodd ) of organisms of which the amoeba is a typical representative.

6. Physiology of Amoeba. — When a particle of food approaches the amoeba, it seizes the minute morsel by means of the pseudopodia. The particle enters the soft protoplasm of the body apparently by passing through the ectosarc, which latter, it may be presumed, readily admits solid matters, and as readily closes the temporary fissures formed by their entrance. These animalcules, and others more or less nearly related to them, may be fed artificially when under microscopical examination, by strewing in the water in which they are contained some coloured substance, such as indigo or carmine, reduced to a very fine powder. The particles of coloured matter are well seen against the usually colourless bodies of the animalcules, and the process of seizing and of digesting this artificial food can therefore be readily studied. Ehrenberg, the great German microscopist and naturalist, was the first to employ this convenient mode of observation. The food of the amoeba consists of the minute vegetable organisms and smaller animalcules which inhabit its native waters ; and the nutriment simply passes into the internal layer or endosarc, and is there digested.

No distinct mouth exists, and food can therefore be received by any part of the body-margin. Neither do we find any digestive apparatus or stomach-sac, such as exists in most higher animals, to be present. Around each foodparticle received into the body a clear space soon forms ; and when the food is dissolved or digested, this clear space

ZOOLOGY.

remains apparent for a time as a ‘food- vacuole/ which, however, soon closes up and disappears. The food-vacuoles are in all probability caused by the presence of water or of some other fluid. If any part of the food is found to be indigestible, it is simply cast out of the body through a temporary opening formed in the ectosarc, just as when the food was received or ingested ; whilst in som£ amoebae a special portion of the body, named the villous region (fig. 2, B, v), has been alleged by some observers to be that by or at which indigestible portions of food are rejected.

7. Circulation in Amosba. — The result of the digestion of food in all living beings is to produce a fluid— named blood in animals, and sap in plants — from which the body can gain a supply of fresh nutrient matter, and thus repair the wear and tear which is inevitably associated with the act of living and being. This fluid is usually circulated throughout the body so as to bring it in contact with each tissue or part of the body ; and in higher animals, the heart and blood-vessels or analogous organs perform this duty. Although the simplicity of the amoeba’s body forbids us to expect to find therein the perfection of detail exhibited by higher beings, it nevertheless appears probable that the contractile vesicles represent a circulatory apparatus in its most primitive state. At the moment of contraction or systole of these vesicles, tubes or canals are described by some authorities as having been observed to pass from the vesicles to the surrounding protoplasm ; and we know that in certain near neighbours of the amoeba — the Infusorian animalcules— these radiating spaces or canals actually occur. But apart from these observations, the mere fact of these vesicles contracting so regularly would strongly suggest the probability and correctness of the idea that their function is that of circulating some fluid or other throughout the organism.

Innervation of Amoeba. — No traces whatever of a nervous system or organs of sense, whereby the amoeba might be brought into relation with the outer world and thus gain a knowledge of its surroundings, can be discerned. That the organism can and does exercise some means of gaining

REPRODUCTION OF AMOEBA.

that knowledge, is evident from its behaviour to the foodparticles, which it stretches out its pseudopodia to seize when they touch its body, and also from its apparently avoiding objects which lie in its miniature path, so to speak. We know that animals of much higher rank and structure than the amoeba (for example, sea-anemones) appear to feel and to exhibit irritation when touched, and in many such organisms no nervous system can be detected. Even some plants may exhibit symptoms of irritability, as is well seen in the sensitive plant {Mimosa pudica), and the Venus’ fly-trap {Dioncea muscipula ), and nothing analogous to a nervous system has yet been discovered in plant tissues. We are therefore forced to conclude that the power of appreciating sensations may probably exist as a diffused property or attribute of the protoplasm of which the bodies of lower organisms are composed, and apart from the development or presence of any distinct nervous system.

8. Reproduction. — The structure and processes we have detailed as taking place in a living amoeba are evidently sufficient to perpetuate the life of the single and individual animal alone. Death is continually busy in the ranks of living beings, and to repair the constant loss of individuals which thereby results, the process of reproduction comes into play. The process of nutrition repairs the loss of the animal viewed as an individual ; the process of reproduction repairs that of the species or race. The amoeba may reproduce its kind in two chief fashions. The first process, named fission , is that of simple division of the body. It is named an asexual process of reproduction because the elements of sex do not take part therein. When this process of fission occurs, we see either a single pseudopodium become detached from the amoeba’s body, or the body itself may divide into two parts. In either case, each separate portion moves away, and with or without a process of further growth, develops into a perfect amoeba.

Then, secondly, the amoeba may be seen to undergo a peculiar process known as encystation. In the latter case, the movements of the animalcule cease, its body assumes a rounded shape, and the ectosarc is replaced by a greatly

ZOOLOGY.

thickened layer or cyst (fig. 2, C). Hence the organism is said to be encysted. The nucleus and contractile vesicles disappear, and then the endosarc is observed to break up or divide into numerous little bodies, named pseudonavicellce (fig- 2, D), which, when liberated by the rupture of the parentcyst, soon appear in the likeness of young amoebae (fig. 2, E). Whether or not this process may be termed sexual , and whether or not the nucleus and nucleolus may be held to represent the elements of the male and female sexes, is still a disputed point. But analogy seems to indicate that the belief in the sexual nature of this form of reproduction, is neither unwarranted nor untenable.

9. Classification. — The naturalist has learned to recognise

different species or kinds of amoebae, and to distinguish one species from another by differences in general form, and in the manner in which the pseudopodia are protruded, and by other characteristics. Hence he is enabled by comparison, to make a classificatio7i of these organisms, and to relate them to organisms with which they most nearly agree in their structure and development.

A common form of amoeba, for example, is the Amceba radiosa (fig. 1), in which the pseudopodia appear as radiating processes. In Amoeba pi'inceps (fig. 2, B), another species, the pseudopodia are said to be given off from a limited part of the body only ; and the same character distinguishes another species — Amoeba Umax. The further extension of this principle of the recognition of likenesses, and consequent arrangement of these and allied organisms, would lead us to note a great variety of organisms essentially resembling the amoeba, in the simple protoplasmic nature

Fig. 3. — Foraminifer : Gromia, one of the Foraminifera, highly magnified, shewing calcareous shell, and delicate interlacing pseudopodia.

HOW LIVING BEINGS ARE STUDIED.

of their bodies, in their want of definite organisation, and in the general presence of a nucleus, &c. The wide relationship thus constituted, is sufficient to unite such animals to form one of the primary divisions of the animal world — known as the Protozoa, which group includes the Foraminifera (fig. 3), Infusorian animalcules , the Rhizopoda (of which Amoeba is the type), Spojiges , and some less familiar organisms.

HOW LIVING BEINGS ARE STUDIED.

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10. Having regarded the amoeba from the several points of view in which the zoologist must consider every living being, we may briefly try to form some estimate of the amount of knowledge we have thus obtained, and also of the manner in which that knowledge has been acquired. The thorough or biological examination of any living being comprehends a knowledge not only of its morphology or structure , but also of its physiology or functions , and of its distribution or place and sphere of habitation. The due understanding of any piece of machinery, such as a steamengine, involves an almost similar collection of details. The mechanic who understands his business must know the structure of his machine, and the relations of one part to another. He may thus be said to possess a knowledge of its structure or morphology. But if the engine is viewed only as a motionless piece of mechanism, the mechanic, however perfectly he may know its structure, can hardly be said to be perfectly or thoroughly acquainted with it. The moving mass of machinery has to be very differently regarded from the motionless engine, and a new study opens before the observer, in the investigation of the active functions of the structures with which he has already made himself familiar. So is it with living organisms. To examine the morphology of an amoeba is only to half discharge the task of the biological observer. We must also watch the organism in life, and note how the functions of its body are carried on, and how the organism thereby maintains its existence.

The Morphology or structure of animals and plants will i6

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therefore include the consideration of their (i) Anatomy , or the structure of the fully-grown body ; (2) Development , or the history of the changes and stages through which the organism passes in its growth from its youngest state to the adult period of existence ; and (3) Taxonomy or Classification , through which we may compare and relate organisms to one another from a knowledge of their structure.

Similarly, a knowledge of the Physiology of any animal or plant implies an understanding of how organisms live, and of the three great functions which, as seen in the amoeba, contribute to the maintenance of their own existence and that of their species. We therefore note (1) the function of nutrition , by which the animal or plant nourishes itself, or how food is received, digested, and applied to the uses of the organism ; (2) the function of reproduction , whereby it produces new individuals competent to carry on the life of the species in time ; and (3) the function of relation or innervation , through which the living being becomes cognisant of its surroundings, and relates itself to them through the operation of its nervous system or analogous means.

The third department of biological science is that of distribution. This branch deals with the habitat of animals and plants, and shews us, (1) through geographical distribution , or distribution in space, in what areas or regions of the existing world the various groups of living beings are distributed ; or (2) through geological distribution, or that in time, how animals and plants were represented in past epochs of this world’s history— epochs indicated by the arrangement of the rocks comprising the crust of the globe, and by the fossils or petrified remains of living things therein contained.

II. LIFE AND ORGANISATION. — From the examination of the amoeba we may thus gain an idea of the manner in which the researches of the zoologist are to be pursued. But the investigation of the animalcule may afford an illustration of other and equally important points in connection with the study of living beings. We thus learn that, of itself, a minute speck of structureless protoplasm may con-

PROTOPLASM AND LIFE.

stitute a living organism, capable of living, as perfectly, in so far as the result to the organism is concerned, as the highest being. Then also we note that the presence of definite structure , or the possession of tissues and orgcuis — shortly named organisation — is not necessary for the presence or maintenance of the living state. We find forms, of even lower grade than amoeba — exemplified by the

Fig. 4. — Monera : Protomyxa aurantiaca, one of the Monera, or lowest Protozoa, destitute of nucleus and contractile vesicles. The animal is represented as seizing food.

Monera (fig. 4), and Foraminifera (fig. 3) — in which a nucleus and contractile vesicles are wanting. Thus life is not in any sense dependent upon the presence of organised structures, since these lower organisms live, literally without possessing any machinery to live with. We may observe also, in connection with this latter point, that the vital properties of protoplasm confer upon this substance the power not only of digesting food, but also of exercising all other functions in connection with the living state. Upon what these powers or properties depend, we do not know. At the most, we can only form some idea of the conditions necessary for the existence of life. Of the exact nature of

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life itself we are as ignorant now as were the philosophers of the classic ages.

RELATIONS OF ANIMALS AND PLANTS.— Then lastly, we may observe in the amoeba the chief characteristics which enable us to settle the identity of animals, and to distinguish them from plants. As many lower plants exhibit the power of contractility and of motion that is seen in the amoeba — the former of which powers we have already noted to be an inherent property of protoplasm itself— we cannot maintain that the amoeba is an animal, or, indeed, that any other organism is an animal, on these two grounds. We note also that the chemical analysis of animal and plant protoplasm has not as yet enabled us to detect any decided or reliable difference between the two groups of living beings. The fact that the amoeba does not inclose its body within a sac or wall-membrane of the starchy substance named cellulose— this latter substance being found in plant-structures — may be regarded as a proof of its animal nature. But we find surer points of distinction in the fact, that, like all animals, and unlike the vast majority of plants, the amoeba inhales and absorbs oxygen, and emits or exhales carbonic acid j and further, that like other animals, the animalcule can only subsist on protein or organised matter , which has already and previously been elaborated and converted into that state. The amoeba, or any other animal, has no power of manufacturing new protoplasm from such inorganic materials as carbonic acid, water, and ammonia. But as plants possess this power of converting inorganic matters into protein or organic compounds, the nature of the food may be regarded as a reliable ground for separating animals (and with them the amoeba) from their vegetable neighbours. Another and subsidiary point of distinction which might satisfy us of the amoeba’s animal nature, consists in the fact that food is received directly into the interior of the body in animals ; whilst in plants the nutritive organs consist mainly of external surfaces exemplified by the root and leaves. The entire animal world, it may be lastly noted, comes to be ultimately dependent for support on plants ; since animals obtain from plants the

THE COMMON HYDRA.

ready-made protein compounds required for the maintenance of their frames. And strange as it may appear, the lion, flesh-feeder though the ‘ king of beasts ’ may be, is in reality dependent upon the grasses on which he treads. Since the flesh of the antelopes and other game on which he subsists, merely represents so much vegetable matter modified and converted into animal tissue. The annihilation of plants, would thus in truth mean and include the utter extinction of animal life.

Chapter II. The Hydra, Or Common Fresh-water Polype

12. DIFFERENCES BETWEEN DIFFERENT ANIMALS.— When we compare the amoeba as an animal belonging to a low grade of structure with more highly organised beings, the differences between the compared organisms are seen to consist of two sets or kinds. Animals differ from one another either morphologically — that is, in the structure of their bodies ; or physiologically — that is, in respect of the degree of perfection in which the functions of their bodies are performed. One animal, in other words, is said to be higher than another, because, on examination of the body of the former, we find certain principles therein represented, to which the names morphological type and specialisation of function or physiological division of labour have been applied. The lower animal, on the contrary, does not exhibit these principles, for the due illustration of which we may select one or two very simple but typical animals belonging to a group of the animal world known as the Ccelenterata.

13. EXAMINATION OF HYDRA. — Attached to the duckweed and other plants which grow in our ponds and ditches, certain small animals, scientifically known as Hydrce , and popularly as the ‘ Common Fresh- water polypes’ (fig. 5), may be found in plenty, especially in the

ZOOLOGY.

warmer months of the year. If water, along with confervse or the lower vegetable organisms which float on its

surface, be taken from a stagnant pool, and allowed to remain in a clear glass vessel, the hydrae which are therein contained, will be found to have clustered on the side of the vessel next to the light. These little organisms may be distinguished by the unassisted sight, and may attain a maximum length of half an inch.

More commonly, however, they are of smaller size. Each hydra presents for examination a little tubular body, fixed to the duckweed or other matter by one extremity — which may be named the proximal end or hydrorhiza — and possessing at the unattached or distal extremity a mouth , surrounded by a circle of larger or shorter tentacles or feelers , numbering from four to ten, or even more. The body may be coloured green or brown ; the former colour being due to the development of the same substance — chlorophyll — which gives to plants their green hues. The organism thus described, including a mouth, tentacles, and body-cavity, is known to zoologists as a polypitej this latter term being applied to the nutritive element in the body of all animals allied to the hydra ; whilst the entire organism and body of the hydra, or any of its allies, is named the hydrosoma.

When a hydra is touched or irritated, we observe the tentacles to be temporarily withdrawn, and the body also for a time to contract upon itself. The organism, therefore, possesses means for receiving and acting upon sensations, and thus exercises in some way or other the function of relation or hmervation. The hydras are not permanently rooted to fixed objects, but possess the power of moving about in the

Fig. 5. — Hydrozoa : Hydra fusca, the ‘ Long-armed Hydra,’ shewing a young bud at b, and a more advanced bud at c (magnified).

STRUCTURE OF HYDRA.

water. They may "be observed to crawl somewhat after the fashion of leeches, by alternately fixing the mouth and root ; whilst occasionally they may be met with floating on the surface of the water they inhabit.

14. Structure and Histology of Hydra. — The morphology of the hydra may be very easily understood. The tubular body, when microscopically examined, is found to be composed of two layers or membranes, closely apposed. The outer layer is named the ectoderm (fig. 6, A, B, ec).

Fig. 6. — Morphology of Hydra: A, longitudinal diagrammatic section of Hydra ; ec, ectoderm ; en, endoderm ; m, mouth ; s, somatic or body-cavity ; t, t, tentacles. B, transverse section of hydra (references as in A). C, one of the cells of the ectoderm largely magnified, shewing the processes of the cell. D, a ‘thread-cell’ in its normal state. E, the thread-cell of hydra, with thread everted. F, microscopic section of body-wall of hydra ; a, cells of ectoderm ; b, middle layer ( mesoderm ), formed by internal processes of ectodermal cells ; c, cells of endoderm, some bearing cilia. The nuclei and thread-cells are shewn in the cells of the ectoderm, and nuclei in those of the endoderm.

and the inner is known as the endoderm , en. The latter forms the lining membrane of the interior of the body-cavity or somatic cavity (fig. 6, A, s), as it is termed ; and the tentacles, t, which are hollow organs, consist of simple tubular extensions of the cavity of the body, and are each composed of ectoderm and endoderm layers. The more minute or microscopic examination of the layers of the hydra’s body, shews that each membrane is made up of certain minute elements named celts. These