Elementary zoology

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ELEMENTARY PRACTICAL ZOOLOGY

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FRANK E. BEDDARD, M.A. (Oxon.), F.R.S.

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Preface

The present volume is a guide to the elementary zoology required by the Science and Art Department. It embraces all the types comprised in the syllabus issued by that department, with a little additional matter, such as would naturally be given in a course of lectures preparatory for that examination.

As regards the systematic side of zoology, the candidate for the examination is required to know “ the characters of the classes of Vertebrates,” and to refer “ common Vertebrates to their classes.” He is also expected to have a similar knowledge of the Artbropoda and of the Mollusca.

Since the types of animals selected for special study comprise several others among the chief groups of the animal kingdom, I have thought in advisable to include a general classification. This is exceedingly brief, but may serve to present the student with a rough notion of the characters of the different groups of animals. I have endeavoured to use in these definitions anatomical characters, which should be familiar to the student after the structure of the types has been mastered.

Inasmuch as the knowledge required from the student who is a candidate for the Science and Art examinations embraces some of the more important generalizations of biology, such as the cellular constitution of animals, the phenomena of reproduction, etc., it is hoped that this book may serve the needs of others who desire an acquaintance with the elements of the subject without a view to any particular examination.

Many distinguished authorities hold that in treating of animal structure it is desirable to commence with the higher

VI

Preface.

forms and gradually work down to the lower forms. It is argued that to do this is advantageous, since the student commences with what must be the more familiar part of the subject. Some rough notion of human anatomy is possessed by most persons ; whereas the very methods by which the lower animals are studied are new to the beginner. There is, however, no transition between a dissection of a frog with scalpel and scissors and the examination of an amoeba with the microscope. The plunge into an unfamiliar region of the subject must be made some time ; and why not at the very commencement ? Besides, to begin with the low forms and to gradually work to the higher has the undoubted advantage of presenting the facts in a logical sequence.

I therefore begin with the amoeba, and deal with the other types in, so far as is possible, an ascending order.

The types which are described here, as well as the general facts of animal structure, have been dealt with by so many zoologists in so many text-books, that it is hard to illustrate them by fresh drawings. It is quite useless, for example, to attempt to improve upon the excellent figures of the crayfish appendages to be found in Mr. Huxley’s “ The Crayfish.” Such illustrations I have copiously borrowed from various sources which are duly acknowledged in the case of each cut.

Among these will be noticed a few new illustrations, either copied from the original memoirs which they illustrate or modified from existing wood-cuts in text-books ; these are Figs. 1-7, 9-13, 22-25, 2 7—32, 34, 37-41, 43, 45-49, 52> 63> 73, 74, 76. For these I am indebted to the skill of Mr. R. E. Holding.

F. E. BEDDARD.

Contents

CHAPTER PAGE

I. The Unicellular Animals : The Amceba (Amceba, Various Species) ; The Bell Animalcule (Vorti-

cella) 1

Chapter I. The Unicellular Organisms

The Amceba (Amoeba, various species).

Water from any stagnant pool, particularly if a drop be taken from the surface of the mud at the bottom, will be frequently found to contain examples of an organism known as Amoeba. This term is used rather loosely for a number of creatures — fresh-water, marine, terrestrial (in damp earth) — which agree in being composed of a speck of seemingly jelly-like substance, and in moving by a flowing motion, accompanied by the thrusting out of processes of the irregularly shaped body. Some little experience of these organisms will soon show that there are many kinds of amoebae, which have been classed by naturalists in different species and even genera ; 1 some, for instance, are larger, much larger, than others. The form of the thrust-out processes of the body — the pseudopodia , as they are termed — differs from species to species; the more or less granular appearance of the body is another character which varies in the different kinds ; and there are other points of difference. In all these organisms, however, the body has no fixed form ; it is simply an irregular mass of living matter ; hence the name of “ Proteus animalcule ” was applied to it by some of the earlier observers. When an amoeba, preferably one of the larger species, is examined, it is seen to consist of

The terms “genera” and “species” are explained later (see p. 173).

B

- Elementary Zoology.

a mass of semi-fluid matter, which in the middle of the body is filled with granules, and is clear, or clearer, peripherally. In

F ig. i. — Various species of Amoeba:, highly magnified, characterized by the differing forms of their pseudopodia. (After Mcebius.)

the living form sometimes — but it is better shown by staining reagents — a denser central body may be observed of a spherical

The Unicellular Organisms.

shape, which has no fixed position, but moves about with the movements of the animal ; this is termed the nucleus. In many amoebae a clear vesicle also exists (the contractile vacuole ), which, if kept under the eye for a few minutes, will be noticed to suddenly contract — apparently expelling its fluid contents — and to gradually fill out again, repeating the process continually.

This is positively the entire anatomy of the amoeba, stated, of course, very briefly. No animal simpler in structure than this organism is certainly known. Some amoebae lack the contractile vacuole; but all consist of a mass of the jelly-like living matter ( protoplasm ) and a nucleus or nuclei. It has been stated that there are amoeboid creatures without a nucleus ; but this does not appear to be by any means certain. And we shall see later that it is improbable.

The amoeba not only has the certain definite structure that has just been briefly described ; it also acts ; the living matter of which it is built up performs certain functions. We have to consider the physiology (the functions) of the amoeba, as well as its structure (its morphology). The movements of the animal have been already referred to. It is continually in motion, and the movements may be retarded by cold, increased by warmth, and again retarded and stopped by too great warmth. Various chemical substances produce similar effects. The living matter, therefore, of which the amoeba is composed is capable of movement, and is irritable — that is, responds to stimuli. If an amoeba be watched for some time it will be seen to feed. It takes in nourishment by simply flowing over and engulphing a minute plant or other organism ; it literally gets outside its food. After a time the ingested food particle will be seen to gradually disappear, and the indigestible residue may be seen to be thrust out of the body. Furthermore, the contractions of the contractile vacuole expel from the body other waste substances. If an amoeba be kept in filtered water, in which there is no vegetable or animal matter, it will die after a longer or shorter time. Nor will it avail it that the water be impregnated with the various chemical elements, or compounds of them, that make up its body. The amoeba needs organized matter to feed upon.

Elementary Zoology.

The body of the amoeba, therefore, is constantly wasting away and being as constantly renewed by the taking in of food, which must be in the form of living or dead animal or vegetable matter.

But the animal performs other functions.

When the creature has attained to a certain size, which differs in different species, and even in individuals, it divides into two; the nucleus divides and then the protoplasm, so that where there was one amoeba there are two. This process under favourable circumstances is continually repeated. But it has its limits. After a certain number of generations have been thus produced by simple fission — the number varying with the species, and not being accurately fixed — this method of reproduction ceases. Another kind of generation comes into play. Two amoebte approach and fuse together, the nuclei joining and the protoplasm being commingled. After a longer or shorter period of conjugation the two may become surrounded in a delicate cyst, and break up into a number of minute spores, which gradually attain to the size of the parents after the rupture of the case ; or the two may separate, and, refreshed by the union of the nuclei and protoplasm, go on dividing by the process of fission. This conjugation must not be confused with the ingestion of one amoeba by another, though in some cases it is doubtless difficult to distinguish between the two — between hunger and love.

The question may be asked, Why should an amoeba divide? why should it not go on growing indefinitely? The question is easier to ask than to answer. In considering the matter it must be borne in mind that possibly the viscid and semifluid protoplasm cannot hold together in droplets above a certain size, and (more important) that the surface does not increase in extent pari passu with the contained mass ; hence the power of ingesting food and excreting waste products may be not sufficiently rapid to keep pace with the growing mass. Accordingly the animal divides into more conveniently sized pieces.

The amoeba then moves, responds to stimuli, feeds, excretes, grows, and reproduces itself. These are among the most important characteristics of living matter; and in all living beings the same phenomena are observable, in addition to respiration, which is the taking in of oxygen.1

By these characters living may be distinguished from nonliving matter. This living matter, “ the physical basis of life,” or protoplasm , as it is usually called, is a viscous semifluid substance with granules interspersed. A certain vagueness of meaning has at times attached itself to the expression protoplasm. It must not be regarded as a substance of definite chemical composition. It is a mixture of various substances, whose exact relations cannot, from the nature of the case, be accurately ascertained. For in order to manipulate it the protoplasm must be killed; and dead protoplasm is an altogether different thing from living protoplasm. In dead protoplasm the actual elements which compose it can, of course, be accurately enumerated; these have been found to be carbon, hydrogen, oxygen, nitrogen, sulphur, and a few others, such as calcium, phosphorus, potassium, sodium, magnesium, and iron. The first five are combined to form various proteids, i.e. albumens, globulins, etc. But any account of the chemical nature of the substances would take too long a space, and requires a detailed treatment at the hands of a chemist. Attention, therefore, will be directed only to the bare outline given above, and to the fact that protoplasm is not a chemical but a morphological expression for a complex substance exhibiting the properties already referred to.

Recent microscopical research into the nature of protoplasm has revealed the fact that it has a definite structure, that its particles are disposed in a regular fashion ; but the interpretation of the observed facts has differed greatly. The two principal views of the constitution of protoplasm are known respectively as the “network theory” and the “foam theory.” According to the first view, the protoplasm is disposed in a network of denser protoplasm, the meshes of which are filled by the more

1 This function is separated from the others, since the taking in of free

the "1 "CSKiraftl0-n^ \ "0t quite absolutely universal. Thus there are SL?r°»UC'n,CtCrla’ which not only do not take in free oxygen but ate y t . They must obtain their oxygen from compounds.

fluid parts ; in this case the granules are largely an optical delusion, and represent the nodal points of the meshwork : the granules are also partly “ formed ” substances, i.e. bodies not protoplasm, but produced by the activity of the protoplasm, just as the droplets of fat in a fat cell are formed substances produced by the activity of the protoplasm. A mass of protoplasm, then, on this view, may be roughly compared to a sponge.

Fig. 2. — A portion of the body of a Multinucleate infusorian Ofialina, to show network structure of protoplasm. Very highly magnified. (After Butschli.)

According to the foam or honeycomb theory the living substance of a cell is comparable to a mass of froth, of which the air in the bubbles is formed by less viscid, the walls of the bubbles by more viscid, protoplasm. It will be apparent that so far this view of the constitution of protoplasm does not differ widely from the network view. The protoplasm of various cells and certain simple organisms appears to demonstrate that this foam theory is the true explanation of the arrangement of protoplasm. It is especially well seen in an amoeboid creature,

The Unicellular Organisms.

Pelemyxa , not far removed from the more common forms of amoebae, and chiefly differing by the presence of a large number of nuclei. Professor Butschli, moreover, has succeeded in artificially compounding a substance which has many of the physical properties of protoplasm to all appearance. He made a fine emulsion of oil and a solution of such a substance as common salt. This mixture not only showed the foam-like

N, nuclei ; vac, vacuoles ; Ch. V., vesicles (tinged green by chlorophyll).

structure, but it exhibited movements ; and masses of it even divided in a very life-like fashion. In the case of the “ artificial protoplasm ” the walls of the bubbles were formed by the oil, and the alkaline solution occupied their interior. This mixture, however, bears a relation to true protoplasm like that of a waxwork figure to a man; it is not a kind of monster of Frankenstein. And, ingenious though the experiments

Elementary Zoology.

undoubtedly are, they really prove nothing, since protoplasm is admittedly not an oily substance, though it may often contain oil drops formed by its own activity.

Having considered the protoplasm which builds up the bulk of the amoeba’s body, we may turn to the nucleus. The nucleus is a spherical to elliptical structure, encircled by a definite wall, and showing a granular appearance. One or more rounded bodies in its interior are termed nucleoli. At one time the nucleus was regarded simply as a denser bit of protoplasm in the interior of the protoplasm, denser perhaps by reason of its central position and the consequent pressure of the surrounding substance. It is now known to be a perfectly distinct structure. This opinion is based upon its minute structure, which has been of late years more elaborately investigated, and by the physiological importance which it has been proved to possess. This is evidently incompatible with a mere central thickening of the protoplasm. The nucleus very commonly exhibits a reticular arrangement of its contents, the denser network consisting largely of a substance which has been termed chromatin. But the constitution of the nucleus is more fully entered into below (p. 127) in connection with its multiplication. As to the importance of the nucleus, it seems probable, in the first place, as already stated, that a nucleus is always present in living bodies. There are apparently a few exceptions, such as those minute organisms so often connected with disease, and generally known as bacteria. Figures showing a structure like nuclei in those organisms have been published, but not to the satisfaction of everybody. The view has been advanced that the whole body of the minute plant is a free nucleus with but a slender rim of protoplasm. This may be the case, but the matter cannot be fully gone into here ; it is sufficient for the present purpose to insist upon the certainly almost (and probably quite) universal presence of a nucleus. The importance of the nucleus in the protoplasm is shown by the striking part that it takes in the division of the amoeba; it initiates this division. Experiments have been made with large amoebae, and with allied organisms, which tend to show that when the creature is torn up by fine needles into small pieces, it is only those fragments which have a nucleus (in the case of the multinucleate forms such as Priomyxa, already referred to), or a fragment of the nucleus where there is but one, that can reproduce the amoeba of which they are a fragment. Bits of detached protoplasm, minus all trace of the nucleus, lead for a time what has been described as a “pseudo-existence,” but ultimately decay; during this brief existence, they may perform movements, but these have an abnormal character without the dominating nucleus to direct them. Again, the nucleus of many tissues in the higher animals has been observed to preside over such functions as the formation of yolk in ova, the budding of plant cells ; and, in short, it is clear that the functions of the protoplasm are largely directed by the nucleus.

Before leaving the amoeba, there is one important event in its life that must be referred to. At times, when circumstances are unfavourable — if, for example, the medium in which the animal is living becomes too dry — the amoeba will surround itself with a delicate skin excreted by the protoplasm. In this encysted condition it can survive a degree of dryness which would be fatal to it in its naked and unprotected condition.

The Bell Animalcule ( Vorticclla ).

The organism known as the Bell Animalcule belongs to the same great division as that which contains the amoeba, but to a different group ; to this group the name “ Infusoria ” has been given, originally from the fact that various members of it were to be found in organic infusions. The Vorticclla is a social, but not a colonial, form common in fresh water ; numbers are found living closely together. The creature, as is shown in the accompanying figure (Fig. 4), has somewhat the appearance of a wine-glass supported on a long stalk. When the animal is undisturbed, this stalk is elongated to its full ; if the slide upon which a number of the organisms have been placed for examination be jarred, they will contract, and the long stalk is then seen to be thrown into a spiral. At the same time a circle ofrapidly moving filaments, which deck the free end of the infusorian, are retracted also.

Elementary Zoology.

The vorticella is, like the amoeba, an unicellular animal.1 So far it does not present us with an advance of structure upon the last described organism, which is one of the simplest of organized creatures. But the vorticella is a useful example of a single cell, which is highly specialized in many and different directions ; it is still a unit of structure, but illustrates the very great amount of differentiation which can occur in a cell.

DISC

Fig. 4. — Vorticella, highly magnified. (After Biitschli). mth, mouth ; vest', funnel-shaped vestibule ; mac. nuc, macronucleus ; mic. nuc, micronucleus.'

The body of the vorticella is somewhat wineglass shaped ; it has a thick rim above, which is, when the animal is fully extended, slightly everted. This rim is fringed with vibratile cilia. Just inside this rim, on one side of the body, is a funnel-shaped depression leading some way into the interior of the animal ; this is the aperture through which food particles are taken in. The body itself has a delicate, slightly hardened, outer layer, the cuticula ; beneath this is a firm layer, the 1 This anticipates what is dealt with on p. 14.

ectosarc, and the rest of the body within this, again, is made up of a more granular and softer protoplasm, the endosarc. The stalk is seen to contain a central core in the shape of a delicately striated filament ; at its attachment to the body, the filament spreads out fan-wise, and forms a layer of excessively fine fibres lying in the ectosarc. This is termed the myophan layer , and is of a muscular nature. It is the muscle in the stalk which enables the contraction already spoken of to be effected. The figure (Fig. 4) shows the layer plainly. The vorticella possesses a contractile vacuole ; this undergoes

regular expansions and contractions. A nucleus is present, which has a horseshoe form, and near it a smaller body, the micronucleus. The large nucleus is termed the niacronuclcus.

Like the amoeba, then, the vorticella is a mass of protoplasm with a nucleus ; like the amoeba also, the protoplasm is distinguishable into an outer and inner layer, and there is a contractile vacuole ; but in other respects the vorticella is more advanced in structure : the ectoplasm is again subdivided into a outer cuticular and an inner layer, below which again is the myophan layer, concentrated in the stalk to form the muscle fibre ; while instead of pseudopodia, appearing when and where they are wanted, are definite and persistent outgrowths of the ectosarc, which have the power of independent movement. Yet there 'is not so wide a difference as might be thought between the heavy, slowly flowing, pseudopodia and the actively vibratile cilia. Some amoebae have very slender pseudopodia, while organisms belong to the protozoa have long thin fixed processes, from which to cilia is not a long step. Approaching the matter from the other side, cilia have been observed in the living animal to “melt down” into pseudopodia.

The vorticella multiplies by division, and also after what may be termed a sexual union. A bell divides down the middle, and two vorticellae are the result. This division results in the production of two kinds of individuals ; one is like the parent form, the other is a locomotive body with a ring of cilia at each end of the body. The locomotive body, after a longer or shorter interval, settles down and becomes a stalked vorticella ,l Or one of these locomotive individuals will attach itself to a stalked bell, and become fused with it, the nuclei becoming broken up, fusing with those of the other individual, ultimately reacquiring their original form. Ordinary division occurs after this which may be compared to a sexual process. It has been shown that, after a certain number of generations produced by simple division there is a need for this sexual union to restore the exhausted protoplasm.

1 Sometimes, particularly if the water be too foul, a vorticella will detach itself from its stalk and lead a free existence, forming later a new stalk, and settling down to a sedentary life.

Chapter II. The Hydra ( Hydra Virtdis , Hydra Gr1sea, Etcx

The Hydra is a small organism, not more than half an inch in length when fully extended— and that would be a large specimen — which is common in ponds and other pieces of still fresh water. The animal exists in, at any rate, two forms in this country ; one -set of individuals are green, the others brown or nearly colourless. Hydra has the general shape shown in the accompanying figure (Fig. 6) ; it consists of a tubular body surmounted by a wreath of tentacles which grow out from the base of a conical expansion ; this bears the mouth at its summit. The whole body is retractile, the retracted hydra having an oval to round form. The hydra belongs to a large group of animals containing the jelly-fish of our seas, the “ Portuguese man-o’- war,” and a variety of soft-bodied creatures which, as will be stated later, form one of the primary divisions of the animal kingdom. This creature leads a stationary life, adhering to a leaf of duck-weed or a fragment of stone or stick, and by waving its arms freely in the surrounding water, catches and narcotizes by means of the thread cells — to be described presently— minute worms, crustaceans, etc., which form its food. The green hydra is coloured green by chlorophyll , a pigment which is nearly universal in the vegetable kingdom, being only absent in the fungi and in a few parasitic plants belonging to higher groups.

This chlorophyll is also present in a few other animals, even higher in the scale than hydra, such as the two Planarian worms, Vortex viridis and Convuluta schulzei. It also exists in a few Infusorians. It is not, however, safe to jump to the conclusion that when an animal is coloured green it is

Elementary Zoology.

by this particular pigment. Green birds and green lizards, for example, do not in any case owe their;.colour to chlorophyll.

In order to prove conclusively that a given green pigment is or is not chlorophyll, it should be submitted to three tests— chemical, physiological, and morphological.

Chlorophyll is, as a rule, associated with certain structures, the chlorophyll corpuscles, which are nucleus, like masses of protoplasm tinged with the chlorophyll. This is not the invariable case ; but it is safe to regard a green pigment contained in special corpuscles as being chlorophyll, though the converse cannot be asserted. This may be termed a morphological test.

Chlorophyll has certain definite chemical reactions and characters. In the first place it shows a definite absorption spectrum with characteristic dark bands. It is soluble in alcohol, and the solution is fluorescent. By transmitted light it is green, by reflected light reddish. There are, of course, a variety of other chemical methods of deciding whether the pigment is chlorophyll.

Finally, there is the physiological method. Protoplasm can, in the presence of chlorophyll, split up the carbonic acid of the air into oxygen and carbon, combining the carbon with the water in the protoplasm to form — usually starch, but sometimes some other substance, e.g. oil, composed of the elements carbon, oxygen, and hydrogen.

Tried by the first two of these tests, the green colour of hydra is chlorophyll.

Experiments, however, appear to show that it is of no great physiological use to its possessor.

Opinions differ as to whether the chlorophyll is really a product of the cells of the hydra (in which case there is a most interesting point of likeness between animal and vegetable protoplasm), or whether the so-called chlorophyll corpuscles of hydra are not to be looked upon as small unicellular plants. If so, this illustrates what is termed symbiosis ; as the small plants derive advantage (shelter, etc.) from their association with the host, while (though, as already said, this is not so clear) they confer advantages in the shape of starchy or other matters formed by them upon their host.

When a transverse or longitudinal section is made through the hydra, the body is seen to consist of two layers of cells surrounding a central cavity, which latter communicates with the exterior through the mouth aperture before referred to. When a hydra is teased up in water with fine needles, these cells are dissociated from each other, and float freely about. Each may then be seen to consist of a piece of protoplasm, with -a more or less centrally placed nucleus. The boundaries of the cells may also be recognized in the sections. The whole body of the hydra is thus built up of a number of structural units or cells, each one of which is the equivalent of a single amoeba or vorticella. The hydra, and all the animals lying above it in the series, is “multicellular;” the amoeba and the vorticella are