Pond-Life—Apparatus and Instructions for Collecting Objects—Methods of Examination—Sponge—Infusoria.

Of all departments of microscopic research the most fascinating and the most popular is that which deals with what is known by the generic name of “pond-life.” The minute forms of the animal creation included in this term are of such exquisite beauty, and allow the processes of their life-history to be followed with such facility, from the cradle (when they have one) to the grave (which is very often the body of another, larger, organism), that there is none which has attracted more observers. Indeed, the first application of the microscope, by Leeuwenhoek, early in the seventeenth century, was to the observation of these forms of life.

X.

A few words may be said, in the first place, as to the outfit. A very useful part of it is a walking-stick, to which can be attached either a net for capturing the larger forms of life, or a hook for collecting the weeds, to which many forms of great interest and beauty are attached (Fig.15). The stick is telescopic, and can also have attached to it a bottle, which, put into the water at any desired spot,—say, amongst a clump of weeds, or near the bottom, upside down, and then suddenly reversed,—will bring away samples of the inhabitants of the neighbourhood. When these are sparsely distributed through the water, the latter may be concentrated by the use of a bottle round the neck of which is firmly tied a coarse calico bag, funnel-shaped, and supported by a wire ring, somewhat as shown in the illustration. Muslin is, however, too coarse for many organisms. This net is immersed in the water so that the ring is just above the surface, and one bottleful after another poured through. The water strains off, the organisms are left behind. The immersion is necessary to reduce the pressure to which delicate organisms would otherwise be subjected. When the bottle is full, or sufficiently concentrated as to its contents, the latter are poured into one of the ordinary collecting-bottles, of which half a dozen at least should always be taken.

On reaching home, and as often as possible on the way, the corks should be removed, as these organisms soon use up the air in the water.

For examination a glass trough of considerable size, say three inches in length, half an inch in depth, and two inches in height, should be half filled with the water, and examined with the pocket magnifier. With a little practice it will be found easy to take up not only the larger organisms, but even very minute ones, with one of the dipping-tubes with a long tapering point already referred to. The organism, when “spotted,” is followed by eye and tube, the finger being held over the mouth of the latter, and at the critical moment the finger is removed, and the organism swept into the tube by the in-rushing water. Now wipe off the excess with a clean handkerchief, “spot” the organism in the tube again, and carefully absorb the superfluous water with a piece of blotting paper; and finally, gently but sharply blow the remainder on to the plate of the live-box, put on the cover, and examine with a one-inch power. If, as often happens, the organism sticks to the side of the tube, a little more water must be drawn in, and the process repeated. The use of the cotton-wool trap spoken of previously will often be very helpful in the examination of actively moving organisms.

In the case of weeds, a small portion should be placed in the trough and carefully examined from end to end, first with the pocket lens and then with the one-inch power. Let us now consider the objects most likely to be met with.

A piece of stick may be coated with a white layer, feeling rough to the touch, and full of small holes. The chances are that this will be a piece of fresh-water sponge, Spongilla fluviatÍlis, and by dark-field illumination particles may be seen to enter at some orifices and be ejected at others. With a very high power and a very thin section, properly prepared, these holes will be seen to be the mouths of channels which are lined by the most delicate organisms possible, each having a minute body crowned with a tiny crystal cup, in the middle of which is a long cilium, or flagellum, as it is here called (PlateXIII. Fig.1). The currents are produced by the combined action of these flagella. In point of fact, the sponge is a colony of minute animals working harmoniously for the common good. If the specimen be found in winter the sponge will be full of tiny balls, the “gemmules” of the next season’s growth. The roughness is due to the flinty spicules, which are at once the scaffolding and the protection of the sponge, and by boiling the sponge in a mixture of nitric acid and water (half and half) these spicules will be set free, and may be washed, allowed to settle, washed again, dried, and mounted in balsam. The gemmules are coated by very beautiful spicules, consisting of two wheels connected by a rod. These may be treated in the same way. The life-history of the common sponge is as yet but imperfectly known.

Perhaps the lowest form of life is the Amoeba, shown in PlateIX. Fig.1, a mere lump of jelly, which flows along, and when it comes into contact with any likely subject for digestion flows round it, encloses it, absorbs what it can from it, and leaves it behind. A near relative of the Amoeba is the Arcella (Fig.2), which is simply an Amoeba with a shell. Being unable to swim, these organisms are naturally to be most often found at the bottom of the collecting bottle, and it is always advisable to take up a portion of the dÉbris with a dipping tube, which is then held upright on a slide with the finger upon it until the dirt settles on to the slide, when it is removed, a cover-glass put upon the dirt, and a quarter-inch power used for examination. Many forms will be discovered in this way which would otherwise escape observation.

Another curious organism, of great size (comparatively) and extreme beauty, is the sun animalcule (ActÍnophrys), which has a round body and long tentacles (Fig.3), to which free-swimming organisms adhere, and by the combined action of the neighbouring ones are drawn to the body and received into it; one cannot say swallowed.

Fig.6, PlateIX., shows the curious arithmetical process whereby the Infusoria multiply by division, a groove appearing at one point, rapidly deepening, and finally separating the animal completely into two. The species is the ChÍlodon, a flattened creature, ciliated all over, having a set of teeth arranged in the form of a tube, and at its fore-part a kind of membranous lip. A similar phenomenon, in an earlier stage, is shown in Fig.26, PlateXIII., the organism in this case being EuplÓtes.

It has been said that sponges are colonies of extremely minute organisms, each furnished with a membranous collar or funnel, the whole looking like an exquisite wine-glass without a foot. These organisms are not always grouped in colonies, however. Many are free-growing, and may be found attached to the stems of water-plants, but they are extremely minute, and will hardly be noticed until the microscopist has acquired considerable experience, nor even then—with such an instrument as we have postulated—will he see more than a tiny pear, with a straight line, the margin of the cup, on each side of its summit. The flagellum will be quite invisible.

Some similar organisms may, nevertheless, be found which, though still minute, are within the range of a properly managed quarter-inch objective. Such an one, of extreme beauty, is the DinobrÝon shown in PlateXIII. Fig.3. Each “zoÖid,” as the separate animals are called, among the Infusoria, or each generation of zoÖids, stands upon its parent and has two flagella. When alarmed, the zoÖid sinks to the bottom of its cell, and withdraws its flagella. In Fig.2 (EuglÉna) we have a similar zoÖid, but of far greater size, and free-swimming. It is a very common object, and possesses a red eye-speck close to the “contractile vesicle.” All Infusoria have the latter, some a great number, as in Fig.9. The vesicle contracts at regular intervals, and is then simply blotted out, but reforms in the same place, so that it is probably the heart or the urinary bladder of these minute animals.

The lovely rosette shown in Fig.4 is the Synura, a spherical colony of zoÖids, each of which has two flagella, and is in addition clothed with rows of cilia. A beautiful sight it is to watch these colonies rolling through the field of view. Not uncommon, especially in brackish water, is the Peridinium (Fig.5), with its plate armour, long flagellum, and girdle of cilia. A gigantic species of the same family is common in sea-water, and will be easily recognised by its body, not much larger than that of Peridinium, being furnished with three long arms, curiously bent. It is called Ceratium, and is sometimes present in such abundance as to thicken the water, near the surface of which it swims.

We now come to a class of Infusoria which is characterised by the possession of a complete covering of cilia, arranged in rows all over the body. The number of these is enormous; we can only glance at a few types, by mastering which the observer will, at all events, know whereabouts he is. The first we will take is the Coleps (Fig.6), a very common kind, whose body is marked by a series of geometrical lines, so that the organism looks very much like an elongated geographical globe. These markings are on the tunic, which is of a brownish colour. Very different is the Trachelocerca (Fig.7), with its long flexible neck, which is in constant movement from side to side as the creature swims along. As seen in the figure, the neck is clear and the head has a fringe of longer cilia.

The Trachelius (Fig.8) is perhaps the largest of all the Infusoria, being readily visible to even an inexperienced eye. Its body is richly furnished with contractile vesicles, and the protoplasm is curiously reticulated. We may here remark that the Trachelius is especially prompt in doing what most of these organisms do when put under pressure in a live-box, namely, in performing a kind of harakiri. The outline first becomes irregular, then the body rapidly swells and finally comes to pieces, the fragments dancing mockingly away under the influence of their still-moving cilia. The remedy is to use the cotton-wool trap and the lightest possible pressure.

A very elegant organism is shown in the bottom right-hand corner of the Plate (Fig.25). It is the Loxophyllum, and has a strongly marked contractile vesicle.

Another large form is Amphileptus (Fig.9), already referred to as having a large number of contractile vesicles arranged in a regular row; and more massive still is Bursaria (Fig.10), a very curious organism, very much like a purse indeed, and possessing a wonderful arrangement of cilia inside the funnel. These are arranged like a ladder, a series of rows of short stiff cilia, which move at short intervals in unison, and tend to sweep down into the cavity any small particles of food. This arrangement is here described for the first time, and appears to be quite unlike anything else among the Infusoria. Not unlike Bursaria, but having no ladder, and being furnished with a delicate membranous pouch in front of the slit of the purse, is Condylostoma, which we shrewdly suspect to be the young form of Bursaria. This is a point which requires elucidation.

One of the most beautiful of all these forms is shown in Fig.11, Folliculina, a type of a large group characterised by the possession of a transparent case, of extremely elegant form, within which the animal retreats on the slightest alarm.

Fearless and independent, as becomes its size, is the trumpet-shaped Stentor (Fig.12), which may easily be seen when present, as it is in almost every good gathering of water-weed. The particular form drawn (S. MÜlleri) does not make a case, but many members of the genus do, and it is very common to see a stem almost covered with them. Such a sight, once seen under dark-field illumination, will never be forgotten. The method of multiplication of the Stentors (by division) is extremely easy to watch, and very instructive.

A curious organism is Trichodina (Fig.13), which, though a free-swimmer, is always parasitic upon the body of some higher animal. We have found it sometimes upon Hydra, and always in hundreds upon the stickleback. The next group of Infusoria is distinguished by the body’s being only ciliated at particular points, usually round the mouth, or what acts as such. The first form is Vorticella (Fig.14), a beautiful vase-like creature upon a stem. Down the stem runs a muscular fibre, and on the least shock the fibre contracts and draws the stem into a beautiful spiral, whilst the cilia are drawn in, and the zoÖid assumes the appearance of a ball at the end of a watch-spring. An exquisite sight is a colony of VorticellÆ, for these actions are always going on, as, for example, when one member of the family touches another, which is quite sufficient to provoke the contraction.

XI.

POLARIZED LIGHT.

Many compound tree-like forms of Vorticella are known, one of which, Carchesium (Fig.15), may serve as a type of all. In the case of this organism, the colony contracts in sections on a moderate shock; in the second, Zoothamnium, as a whole; whilst in Epistylis the stalks are rigid, and the individuals contract singly. When the shock is violent, the appearance presented by the two former is that shown in Fig.16. In all three cases the colonies are usually so large that they are visible as trees to the naked eye, and some members of the group are extremely common. Moreover, they are often parasitic, as, for example, upon Cyclops, which is frequently loaded with them.

Another compound form is Ophrydium, a colony of which (not unusually large) is shown of the natural size in Fig.18, with a single zoÖid, magnified, by the side of it, in Fig.19.

Lastly, we have an exquisite group of organisms related to Vorticella, but possessing a transparent envelope, the forms of which are most varied, but always graceful. Vaginicola (Fig.17) is a good example of this, and Cothurnia (Fig.20) still more so. Many of these organisms, too, are furnished with a plate, attached either to the head or to the body, which plate, when they withdraw into their cases, closes the latter perfectly, as in the case of the exquisite Pyxicola (Fig.21).

A very interesting but singularly obtrusive organism is the Stylonychia (Figs.22, 23). How often has it happened to us to have an interesting object nicely in the field of view, and then to have it knocked out of sight by the blundering incursion of this burly fellow, who runs so rapidly by means of his “styles” that he gives nothing time to get out of the way. He is of interest to us, however, as the representative of a class in which the body is not ciliated, or very partially and slightly so, usually round the mouth. We have frequently found Stylonychia, in company with Vorticella and Paramoecium (PlateIX. Fig.6), in the water in which flowers have been standing for a few days; sometimes the numbers are so great as to make the water quite milky.

One more form must conclude this short sketch of the great Infusorial family. It is the Acineta (Fig.24), which, attached by its foot-stalk, and devoid of cilia, patiently waits, with outspread arms, to receive and embrace smaller members of the family as they dance merrily about. Alas! its embrace is as fatal as that of the image of the Virgin which bore beneath its robe spikes and daggers, for the victim struggles vainly to escape, and the nourishment from its body is rapidly absorbed.

And here we take our leave of a group which, simple as is the construction of the animals which it includes (for every one, great and small alike, is composed of a single cell), is yet full of beauty and interest. He who wishes to pursue the matter further will find in Saville Kent’s Manual of the Infusoria a perfect mine of information, to which we gladly acknowledge our indebtedness, both now and in time past.