Common Objects of the Microscope

CHAPTER VIII

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.

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.

Fig. 15.

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 (Plate XIII. 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 Amœba, shown in Plate IX. 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 Amœba is the Arcella (Fig. 2), which is simply an Amœba 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, Plate IX., 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, Plate XIII., 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 Plate XIII. 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.

XI.

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 Paramœcium (Plate IX. 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.

CHAPTER IX

Fresh-water Worms—Planarians—Hydra—Polyzoa—Rotifers—Chætonotus—Water-Bears.

The fresh-water worms form a large and well-defined group, and a few words regarding them may be useful.

They are very common, and very difficult to find information about, most of the work relating to them having been done in Germany. At the same time, they are so highly organised and so transparent that the process of their life-history may be easily followed.

One large group has the peculiarity of multiplying by division, the last joints or segments being devoted to the formation of the new individual. At one time of the year the ordinary sexual process of reproduction takes the place of this method, and each worm is then surrounded by a belt such as may be seen in the common earthworm under similar conditions. Further information on this subject is greatly needed.

The type is the common Naïs, which has a body of thirty segments or more, two eye-specks on the head, and a double row of bristles along the back; whilst below, each segment carries strong hooked bristles, nearly buried in the body, by means of which the worm crawls. Inside the mouth is a large proboscis, which can be protruded, and this leads into a stomach which is merely an enlargement of the intestine which succeeds it. The circulation of the blood (which is colourless) can be easily watched. It begins at the tail with a contraction of the dorsal vessel, passes up to the head, and then down below the intestine to the tail again. The intestine is ciliated inside, and it is by a current of water carried into the intestine by these cilia that the blood is aërated.

In the next genus, Dero, this is clearly seen, for the tail (Plate XIV. Fig. 1) is opened out into a wide shield, from which rise four, six, or even eight finger-like processes. These parts are all ciliated, and contain a network of blood-vessels. The worm lives in a case which it builds in the mud, and the way to find it is to put some of the mud into a glass beaker with water, and allow it to stand. If there be members of this family in it, their tails will be seen protruding above the water. Pour out the mud sharply, fill up with water, and allow the dirt to subside, and the worms may then be made to leave their cases by pressure by a camel hair pencil on the lower end of the tube, and may be caught with the dipping tube and placed in the live-box. They have no eyes, otherwise the general outline of the body closely resembles that of Naïs.

Slavína (Fig. 2) has a row of touch-organs, like pimples, round each segment, and is a dirty looking creature, with an inordinately long first pair of bristles, but this reaches its acme in Pristina (Fig. 3) (sometimes, though wrongly, called Stylaria) parasita, which has three long sets of bristles upon the back, and keeps these in constant wing-like motion. The true Stylaria has a long trunk, set right in the head, and tubular (Fig. 6); it grows to a considerable length, and when in the stage of fission it is very funny to see the two proboscides waving about, one on the middle, as well as the original one at the head. There is also a form with a shorter proboscis of the same kind.

Bohemilla has a tremendous array of saw-like bristles upon the back, whilst Chætogaster has none at all in this position, and few below. Æolosoma has merely tufts of hair instead of bristles, and swims freely. It is easily recognised by the red, yellow, or green pigment spots in its skin, and by the ciliated head. Rarest of all the family is the one which connects it with the ordinary Tubifex, the red worm which lives in masses in the mud of brooks and ponds, the waving tails protruding above the water, and being instantly withdrawn when a foot is stamped upon the bank. Their Naid cousin is Naidium, and has red blood, but multiplies by fission, which Tubifex does not.

Another group of worms is the Planarians, small leech-like worms, black, white, or brown, which are rarely absent from a gathering. The would-be investigator will find in them an abundant field for work, as they are but very imperfectly known or studied.

The great enemy of all these worms is the Hydra, a good idea of which may be formed from Plate IX. Fig. 13. There are three species, all of which are fairly common. They capture their prey in exactly the same way as sea-anemones and the marine hydroid forms, so numerous and varied.

Nor must we omit to notice the exquisitely beautiful Polyzoa, such as Lophopus (Plate XIV. Fig. 4), with its ciliated tentacles and transparent social home; Fredericella (Fig. 5), with its graceful stems, and their still more graceful inhabitants; and the wonderful Cristatella, whose colonies form bodies which crawl over the stems of water plants. But for grace, beauty, and variety, the Rotifers assuredly outshine all their fellow inhabitants of our ponds and streams.

We can only take a few types, and of all these the most common is the common Rotifer (Plate IX. Fig. 10). It is there shown in the act of swimming, but it can withdraw its “wheels” and creep like a leech, protruding its foot as it does so. It is distinguished by the two eye-spots on the proboscis from Philodina, in which they are on the breast, and Callidina, which has none. When at ease in its mind, the animal protrudes its wheels, and by their action draws in particles of food, these passing down to the incessantly moving jaws, which act like a mill and crush the food before it passes on to be digested. The movement of the jaws may even be seen in the young Rotifer whilst still in the egg within the body of the parent, and as the egg reaches its full development other eggs again are visible within it, so that we may have three generations in one individual. The males of most of the Rotifera are unknown. Those that are known are very lowly organised, having only the ciliary wreath and the reproductive organs, and are only found at certain seasons of the year. For the remainder of the time parthenogenesis is the rule, just as among the Aphides. We select a few individuals for illustration as types. Those who wish to pursue this study further must be referred to the monumental work of Hudson and Gosse.

The common Rotifer, already referred to, may be taken as the type of the Bdelloida, or leech-like class, so called from their mode of “looping” themselves along. The group is a comparatively small one in comparison with the next, the Ploïma, or free-swimmers. We can only select from the vast variety a few species, first of those classed as illoricated, from their being without a loríca, or case, and then of the loricated, which possess it. A very large and common form is Hydátina (Plate XIV. Fig. 7), which lives by choice in the reddish pools of water found often by the roadside. It shows the whole organisation of the class magnificently; the ciliary wreath on the head, with the striped muscles which draw the latter back, the powerful jaws, the digestive canal with its crop and intestine, the ovary with the developing eggs, the water-vascular system with the curious vibratile tags, and finally, the cloaca, which receives the waste of the body and expels it at intervals.

Another form, also common, especially in clear water, is Synchæta (Fig. 8), very much like a kite or peg-top in shape, which has the power of attaching itself by a glutinous thread, and spinning round at a tremendous rate. Then there is the gigantic Asplanchna (Fig. 9), which has no opening below, so that the waste must be discharged by the mouth; and curious Sacculus, which gorges itself with chlorophyll until it looks like a green bag with a string round it, but clear and sparkling. Of the Notommatæ there is a whole host, but we can only mention the beautiful N. Aurita (Fig. 10), with an eye of a beautiful violet colour, composed of several spherules massed together, and two curious ear-like processes on the head, from which it takes its name. Some of the Ploïma have powers of leaping which must be noticed. The Triarthra (Fig. 11) has three arms, or what we may call such, which it can stretch out suddenly and leap to a considerable distance, whilst in Polyarthra the arms become a whole cluster of broad saw-like bristles.

We pass on to note a few species of the mail-clad or loricated Rotifers, chief among which the great Euchlanis (Fig. 12), a noble-looking fellow, calls for our attention, his great size rendering him easily visible to the naked eye. It is difficult to avoid using the masculine gender, but, of course, all those figured and described are of the gentler sex. Salpina, too (Fig. 14), with its box-like lorica, armed with spines at each of the upper angles, and having three below, is quite easily recognised, and very common. Brachionus (Fig. 13) has a shield-shaped case, well furnished with spines, symmetrically arranged at the top, and an opening below for the flexible wrinkled tail, like the trunk of an elephant. Pterodina (Fig. 15) has a similar tail, but a round case, and the head is much more like that of the common Rotifer when extended. Anuræa (Fig. 16), on the other hand, has no tail, and its case is shaped like a butcher’s tray, with a handle at each corner. Dinocharis (Fig. 17) has a roof-like case, with long spines on the root of the tail, and a forked stiff foot. Noteus (Fig. 18) is much like Pterodina, except in its foot, which more nearly resembles that of Dinocharis.

The list might be indefinitely extended, but sufficient has probably been said to enable the tyro to find his bearings in this large, beautiful, and interesting class.

We pass on to notice in conclusion two or three of the fixed forms, of which a beautiful example is the Melicerta ringens (Plate IX. Fig. 7), whose building operations have a never-ending charm. Particles of débris are accumulated in a curious little cavity in the chin, in which they are whirled round, and mixed with a secretion which binds them together, and when a brick is made the head is bent down and the brick applied to the desired spot with mathematical regularity. By supplying fine particles of innocuous colouring matters, the Melicerta may be made to build a variegated case. The most remarkable specimen known is the one figured in Hudson and Gosse’s work, which was found by the present writer in a specimen of water from which he had already obtained five-and-twenty species of various kinds of Rotifer; the water was collected by an inexperienced person, and there was only a pint of it. It had, moreover, been kept for three weeks, and the moral of that is, to preserve one’s gatherings, and keep an aquarium into which they may be poured when done with for the moment. New forms will often develop with startling rapidity, their eggs having been present in the original gathering. The young form of Melicerta, shown in Plate XIV. Fig. 20, is strangely unlike its mother, and much more nearly resembles its father.

Another group of extreme beauty is the Flosculariæ (Fig. 19), several species of which are very common. They will be easily known by their appearance, which resembles a shaving brush when closed; whilst, when opening, the shaving brush resembles a cloud of delicate shimmering threads, which at last stand out straight, radiating all round the head of the creature, and forming the trap by means of which it catches its prey. Finally, there is the lovely Stephanoceros (not, unfortunately, very common), with its five symmetrically placed and gracefully curved arms, perhaps the most lovely of all Rotifers, with its exquisitely transparent body, sparkling with masses of green and golden brown. He who finds this has a treasure indeed, and will be encouraged to prosecute his studies in this “Fairyland of Microscopy.”

Two irregular forms call for a word of remark. The first is Chætonotus (Plate XIII. Fig. 27), which stands on the borderland of the Infusoria and the Rotifers, neglected as a rule by the students of both; and the second the Tardigrada (Plate XIV. Fig. 21), or water-bears, which have feet like those of the red wriggling larva of Chironomus, whose silky tubes are common enough on submerged walls and on the stems of plants, these feet consisting of a mass of radially arranged hooklets, which can be protruded or withdrawn at will; whilst the head of the water-bear is far more like that of a louse, pointed and hard, and suited for burrowing about, as the animal does, among the rubbish at the bottom of the bottle. Both the genera just referred to will repay careful study, as little is known of their life-history or development.

A few words must be devoted, in conclusion, to the Entomostraca, those shrimp-like animals which, like their marine relatives, act as scavengers to the community. Fig. 22 is a portrait of Cypris, a not very handsome form, but one very commonly found. Its shell is opaque, so that the internal organs are difficult to observe. Far different in this respect is the beautiful Daphnia, the water-flea par excellence, whose carapace is of crystalline clearness, so that every movement of every one of the internal organs may be followed with the greatest facility. There are many species of the genus, and some of them are very common, so that the opportunity of examining these lovely objects is easily obtained. Plate XIV. Fig. 23, shows the most common of all the class under notice, the Cyclops, so named from the fact that, like the fabled giants of classical literature, it has a single eye in the middle of its forehead. It is often loaded with Infusoria, especially Vorticella and Epistylis, already described, to such an extent that its movements are greatly hampered.

XII.

XII.

We have not space to figure more of these creatures, but other forms will be found not inferior in interest to those mentioned. The most curious of all are those which earn a dishonest and lazy living by attaching themselves to the bodies of other and larger animals, chiefly fish. One of the largest is the Argulus, the bane of aquarium keepers, which is of considerable size, and attacks gold-fish, and in fact almost any fish to which it can obtain access.