Chapter XX. Protozoa and Sponges

We are so accustomed to see certain of the vital functions of animals performed by special organs or tissues, that we wonder when we find creatures which move without limbs, contract without muscles, respire without lungs or gills, and digest without a stomach or intestines. But thus we are taught that the function is independent of the organ, and, as it were, prior to it ; though in nine hundred and ninety-nine cases out of a thousand it be associated with it. In truth, the simplest forms of animal life display very little of that division of labour, the minuteness of which increases as we ascend the organic scale ; the common tissue is not yet differentiated* (to use the awkward term which is becoming fashionable among physiologists) into organs, but it is endowed with the power of fulfilling various oifices, and performing many functions. In all probability, the function is but imperfectly performed; the specialisation of certain tissues, and their union into organs, and the complexity of such combinations, no doubt, perform the given function in a far more complete degree; and it is the number and elaborateness of these that constitute one animal higher in the scale than another. The human lung is no doubt a more complete breathing apparatus than the entire ciliated surface of an Infusory,f and the human eye sees more perfectly than

The term is used to express the cellular development of an organisation from its original germ.

The name given to a class of minute animals from their being generally developed in infusions of animal or vegetable matter.

the loose aggregation of pigment granules on the edge of a Medusa. But this diversity is essential to creation, as the great and wondrous plan which we see it to be ; and, meanwhile, we may rest satisfied that the humble requirements of the lowest organism are met adequately by its humble endowments.

This evening I propose to show you some of these humble conditions of animal life ; the lowest of the lowly. I have here two or three phials of very rich water from the fresh-water ponds in the neighbourhood. All collections of water are not equally productive; and very far indeed is the popular notion from correctness, that every drop of water which we drink contains millions of animalcules. You may find many collections of clear water, springs, streams, and pools, from which you may examine drop after drop in succession, with the highest powers of the microscope, and scarcely discover a solitary animalcule. Again, it is not stagnant and fetid pools that are the richest in vitality ; though no doubt you will always obtain some forms abundantly enough in such conditions. According to my own experience—an experience of many years —the paucity or profusion of animal life in any given collection of water can never be determined beforehand ; the season, the situation, the aspect, the character of the country, and many other ud suspected conditions, influence the result, at which yet one may often give a shrewd guess. Generally speaking, small ponds, in which there is a good deal of vegetation below the surface, and particularly if this be of a minutely divided character, such as Myriophyllum, Chara, &c, and whose surface is well covered with duckweed (Lemna), yield well ; and, in collecting, it is desirable so to dip that some of the fine loose sediment of the bottom may flow into your phial, and then to pluck up one or more of the filamentous water-plants, and introduce these into it.

Now to examine such a collection, proceed as I am about to show you. Hastily glancing with the pocket-lens over the foliage, and selecting such filaments as seem the most loaded with dirty floccose matter, I pluck off with pliers one or two, together with one or two of the cleaner ones that are higher up on the plant, nearer the growing point. Having laid these on the lower glass of the live-box, I take up with the tip of a fine tube, or a pipette, a minute quantity of the water at the bottom, which flows in as you see, carrying a few granules of the sediment. This drop I discharge upon the glass of the live-box, put on the cover, and place the whole on the stage of the microscope.

First, let us use a low power, one hundred diameters or so, in order to take a general glance at what we have got. Here is an array of life, indeed ! Motion arrests the eye everywhere. " The glittering swift and the flabby slow" are alike here; clear crystal globules revolve giddily on their axes ; tiny points leap hither and thither like nimble fleas ; long forms are twisting to and fro ; busy little creatures are regularly quartering the huntingground, grubbing with an earnest devotedness amoncr the sediment as they march up the stems; here are vases with translucent bodies protruding from the mouths; here are beauteous bells, set at the end of tall threads, ever lengthening and shortening ; here are maelstroms in miniature, and tempests in far less than a teapot ; rival and conflicting currents are whirling round and round, and making a series of concentric circles among the granules. Surely here is material for our study.

I see an object slowly creeping along the glass, which will be just the thing for our purpose. It is the Proteus {Amoeba diffluens)* Let me put on a higher power, and submit it to your observation.

The names given to this little creature are expressive of its character. Amoeba is from the Greek a/ictj3t) (amoibe), signifying change ; and Proteus in mythology was an old prophet of the sea, who when seized assumed all manner of forms to avoid uttering his predictions.

You see a flat area of clear jelly, of very irregular form, with sinuosities and jutting points, like the outline of some island in a map. A great number of minute blackish granules and vesicles occupy the central part, but the edges are clear and colourless. A large bladder is seen near one side, which appears filled with a subtle fluid.

But while you gaze on it, you perceive that its form is changing : that it is not at two successive moments of exactly the same shape. This individual, which when you first looked at it was not unlike England in outline, is now, though only a few minutes have passed, something totally different; the projecting angle that represented Cornwall is become rounded and more perpendicular; the broken corner that we might have called Kent has formed two little points up in the position of Lincolnshire; the large bladder, which was in the place of the Eastern counties, is moved up to the Durham coast, and is, moreover, greatly diminished ; and other like changes have taken place in other parts.

Lo ! even while speaking of these alterations, they have been proceeding, so that another and a totally diverse out-

FOKMS OF AMCEBA.

Successively drawn from one individual . line is now presented. A great excavation takes the place of Dorset ; Kent is immensely prolonged ; the bladder has quite disappeared, &0.; but it is impossible to follow these changes, which are ever going on without a moment's

intermission, and without the slightest recognisable rule or order. The projections are obliterated or exaggerated ; the sinuosities are smoothed, or deepened into gulfs, or protruded into promontories ; firths form here, capes there; but not by starts, but evenly, and with sufficient rapidity to be appreciable to the eye while under actual observation; though the alterations are more striking if you take your eye off the object for a few seconds, and then look again ; and still more so if you try to sketch the outline. Individuals vary greatly in dimensions; this specimen is about one hundred and twentieth of an inch in long diameter, but others I have seen not more than one-tenth as large as this, and some twice as large.

Disregarding now this peculiarity of change of form, which has procured for it the name of the old sea-god that was so difficult to bind, we will concentrate our attention on some other points not less interesting. That great bladder undergoes changes besides those gradual alterations of place which are dependent on the general form. It slowly but manifestly increases in size up to a certain extent, when it rather suddenly diminishes to a point, and immediately begins to fill again, as slowly as before. These alternations go on with some regularity, and we cannot observe them without becoming convinced that it is a process of filling and emptying; that the bladder gradually fills with a fluid which is either secreted by its walls or percolates into it from the surrounding tissue ; which fluid, when full, the bladder discharges by a sudden contraction of its outline. But whither the fluid goes it is difficult to determine ; I have never been able, in this or in any other instance of its occurrence (though this contractile bladder is characteristic of the extensive classes Infusoria and Rotifera) to see any issue of fluid from the bodyatthe moment of contraction; and therefore conclude that it is discharged into the body, perhaps back again into the tissues from whence it was taken up, and from whence it was about to be collected again. Hence it is probably the first obscure rudiment of a circulation ; the lluids impregnated with the products of digestion being thus collected, and then diffused throughout the soft and yielding tissues.

The smaller bladder-like spaces that you see in considerable numbers in the substance of the animal, are collections of fluid contained in excavations of that substance, which are called vacuoles, differing from vesicles, inasmuch as they seem to have no proper wall or inclosing membrane, but to be merely casual separations of the common substance, such as would be made by drops of water in oil. These vacuoles appear to be connected with the digestive function ; for very many of them are not clear, but are occupied with granules more or less opaque, and of exceedingly various dimensions. That these collections of granules are food you will see by the following experiment.

I mingle a little carmine with the water, just enough to impart a visible tinge to it, and close the live-box again. Already you perceive that some of the tiny globules are become turbid and red, and that their opacity and colour are deepening perceptibly. We see by this that the particles of carmine have been taken into the jelly-like sarcode, and are accumulating in little pellets surrounded by fluids, in these casual hollows of its substance. The process is rendered still more obvious when, as is often the case, some Diatomacean,* with a hard siliceous shell, becomes the food of the Amoeba. The apparently helpless jelly spreads itself over the organism, so as soon to envelop it; the flesh, which, having no skin, can unite with itself whenever the parts come into contact, closes over the

The Diatomacece, a term formed from the Greek cia (dia), through, and rkfivtiv (temneio), to cut, are so called from the ease with which their masses may be broken or cut through ; whence their popular name of brittle-worts. They are usually considered to belong to the vegetable kingdom.

Diatom, which is thus brought into the midst of the sarcode, a vacuole being new-made for its reception. This, then, performs the part of a temporary stomach : the digestible portions of the prey are extracted, and then the insoluble shell of flint is, as it were, gradually squeezed to some part of the exterior, and gradually forced out, the vacuole disappearing with it, or perhaps retaining a minute portion of the fluid, and thus perpetuating itself for a while. This is the earliest condition in which the process of digestion can be recognised.

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