Common Objects of the Microscope

XI.

POLARIZED LIGHT.

XI.

Many compound tree-like forms of Vorticella are known, one of which, Carchesium (Fig. 15 (#x15_x_15_i10)), 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 (#x15_x_15_i10). 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 (#x15_x_15_i10), with a single zoöid, magnified, by the side of it, in Fig. 19 (#x15_x_15_i10).

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 (#x15_x_15_i10)) is a good example of this, and Cothurnia (Fig. 20 (#x15_x_15_i10)) 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 (#x15_x_15_i10)).

A very interesting but singularly obtrusive organism is the Stylonychia (Figs. 22 (#x15_x_15_i10), 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 (#x12_x_12_i37)), 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 (#x15_x_15_i10)), 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 (#x15_x_15_i43)) 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 (#x15_x_15_i43)) 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 (#x15_x_15_i43)) (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 (#x15_x_15_i43)); 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 (#x12_x_12_i37). 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 (#x15_x_15_i43)), with its ciliated tentacles and transparent social home; Fredericella (Fig. 5 (#x15_x_15_i43)), 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 (#x12_x_12_i37)). 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 (#x15_x_15_i43)), 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 (#x15_x_15_i43)), 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 (#x15_x_15_i43)), 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 (#x15_x_15_i43)), 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 (#x15_x_15_i43)) 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 (#x15_x_15_i43)), 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 (#x15_x_15_i43)), 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 (#x15_x_15_i43)) 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 (#x15_x_15_i43)) 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 (#x15_x_15_i43)), 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 (#x15_x_15_i43)) has a roof-like case, with long spines on the root of the tail, and a forked stiff foot. Noteus (Fig. 18 (#x15_x_15_i43)) 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 (#x12_x_12_i37)), 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 (#x15_x_15_i43), is strangely unlike its mother, and much more nearly resembles its father.

Another group of extreme beauty is the Flosculariæ (Fig. 19 (#x15_x_15_i43)), 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 (#x15_x_15_i10)), 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 (#x15_x_15_i43)), 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 (#x15_x_15_i43) 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 (#x15_x_15_i43), 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.

The gills of the stickleback will furnish examples of the curious Ergasilus, which consists chiefly of an enormous pair of hooks and two long egg-bags, the latter, in varying form, being carried by many of the Entomostraca.

Upon the fins of the same fish will be found the remarkable Gyrodactylus, a worm-like animal which attaches itself by a large umbrella-like foot, in the centre of which are two huge claws. The head is split down the middle for some distance. We may mention, in concluding our notice of the external and involuntary guests of the unlucky stickleback, that its skin is usually frequented by hosts of the Trichodina described in the last chapter. Of the internal parasites, want of space forbids us to speak.

CHAPTER X

Marine Life—Sponges—Infusoria—Foraminifera—Radiolaria—Hydroid Zoophytes—Polyzoa—Worms—Lingual Ribbons and Gills of Mollusca—Star-Fishes and Sea-Urchins—Cuttle-Fish— Corallines—Miscellaneous Objects.

Great as is the range of objects presented to the student of fresh-water life, the latter field is limited indeed as compared with that afforded by the sea. The Infusoria and Rotifers furnished by the latter are, indeed, much fewer in number and variety, but the vast host of sponges, polyzoa, hydroids, crustacea, molluscs, ascidians, and worms, to say nothing of the wealth of vegetable life, renders the sea the happy hunting-ground of the microscopist.

Whether it be along the edge of the water, as the tide retreats, especially after a gale; or in the rock-pools; or, perhaps best of all, upon those portions of the shore left uncovered only by the lowest spring-tides, the harvest is simply inexhaustible. Stones turned up will exhibit a world in miniature. Encrusted with green or pink sponges, or with gelatinous masses of ascidians, fringed at its edges with hydroids, coated above with polyzoa, a single one will often supply more work than could be got through in a week of steady application.

A description of the fresh-water sponge already given may serve very well to indicate the general outlines of the organisation of the marine ones too. The spicules of the latter are, however, not always flinty; very often, as in the case of Grantia (Plate IX. Figs. 8 (#x12_x_12_i37) and 14), they are calcareous, a point which can be settled by the application of a little nitric acid and water. If lime be present there will be strong effervescence, and the separation of the spicules can only be effected by gently warming a portion of the sponge in caustic potash solution, pouring the resulting mass into water, and allowing the spicules to settle. The washing and settling must be repeated several times, and a portion of the deposit may then be taken up with a dipping-tube, spread upon a slide and dried, and then covered in balsam solution. The forms are endless, and the same sponge will often supply three or four, or even more. Among them may be seen accurate likenesses of pins, needles, marlin-spikes, cucumbers, grappling-hooks, fish-hooks, porters’-hooks, calthrops, knife-rests, fish-spears, barbed arrows, spiked globes, war-clubs, boomerangs, life-preservers, and many other indescribable forms. The flinty forms must be prepared by boiling, as described in speaking of the mounting of diatoms in Chapter XI., except that, of course, only one settlement is required after thorough washing.

Every one who has been by or on the sea on a fine summer night must have noticed the bright flashes of light that appear whenever its surface is disturbed; the wake of a boat, for example, leaving a luminous track as far as the eye can reach. This phosphorescence is caused by many animals resident in the sea, but chiefly by the little creature represented at Fig. 9 (#x12_x_12_i37), the Noctilúca, myriads of which may be found in a pail of water dipped at random from the glowing waves. A tooth of this creature more magnified is shown immediately above.

A large group of microscopic organisms is known to zoologists under the name of Foraminifera, on account of the numerous holes in their beautiful shells, most of which are composed of carbonate of lime, though some are horny and others are composed of aggregations of minute grains of sand, the forms in one class often closely imitating those in another. It is of the shells of these minute animals that the “white cliffs of old England” are very largely composed, and those who desire to understand the part which these tiny creatures have played, and are playing, in geology, will do well to study Huxley’s fascinating essay on “A Piece of Chalk.”

XIII.

XIII.

The inhabitants of these shells are Amœbæ, mere masses of jelly, and some forms may be found sliding over the weeds in almost every rock-pool. The anchor-mud, already spoken of, always contains some, and many forms may be found in the sand from sponges, which should be passed through a series of wire sieves, of increasing fineness, and the residuum in each case be examined dry under a one-inch power. The shells may be picked up with a needle which has been slightly greased by being passed over the hair, and they may be mounted by sticking them to the slide with thin starch paste, putting on a cover-glass properly supported, and then running turpentine under the cover-glass, heating to expel the air, and finally filling up with balsam. Or, as opaque objects, they may be mounted in a cell dry. The forms are endless, but all are beautiful, and a few examples are given in Plate IX. Fig. 4 (#x12_x_12_i37) (Miliolína), and Plate XII. Fig. 7 (#x14_x_14_i24), which is a portion of the shell to show the holes, Fig. 13 (#x14_x_14_i24) (Polystomella), Fig. 14 (#x14_x_14_i24) (Truncatulína), Fig. 15 (#x14_x_14_i24) (Polymorphína), Fig. 16 (#x14_x_14_i24) (Miliolína, partly fossilised), Fig. 18 (#x14_x_14_i24) (Lagéna). and Fig. 20 (#x14_x_14_i24) (Biloculína).

Allied to these are the lovely Radiolaria, whose shells, constructed on a similar plan, are composed of flint. They are found in remarkable profusion in the deposit from Cambridge, Barbados, but also in a living state at even enormous depths in the ocean. The present writer has obtained many forms from Challenger soundings, and the great authority on this subject is Haeckel’s report in the official accounts of the expedition of the above-named vessel.

The Hydroid Zoophytes are represented by several examples. These creatures are soft and almost gelatinous, and are furnished with tentacles or lobes by which they can catch and retain their prey. In order to support their tender structure they are endowed with a horny skeleton, sometimes outside and sometimes inside them, which is called the polypidom. They are very common on our coasts, where they may be found thrown on the shore, or may be dredged up from the deeper portions of the sea.

Fig. 13 (#x12_x_12_i37) is a portion of one of the commonest genera, Sertularia, showing one of the inhabitants projecting its tentacles from its domicile. Fig. 15 (#x12_x_12_i37) is the same species, given to show the egg-cells. This, as well as other zoophytes, is generally classed among the sea-weeds in the shops that throng all watering-places.

The form just referred to is a near relative of the Hydra, already described, and belongs to the same great family as the sea-anemones. One form, shown in Fig. 26 (#x12_x_12_i37), is the Hydra Tuba, long thought to be a distinct animal, but now known to be the young form of a jelly-fish, or Medusa. The Hydra Tuba throws off joints at intervals, each of which becomes a perfect jelly-fish. One of them is shown in Fig. 27 (#x12_x_12_i37). Fig. 28 (#x12_x_12_i37) represents a very small and pretty Medusa, the Thaumantias. When this animal is touched or startled, each of the purple globules round the edge flashes into light, producing a most beautiful and singular appearance. Fig. 29 (#x12_x_12_i37) exhibits the so-called compound eye of another species of Medusa, though it would appear that these are really connected with the nervous system of the animal, and have to do with the pulsating contractions of the bell by which it is propelled through the water.

In my Common Objects of the Sea-Shore the Actíniæ, or Sea-Anemones, are treated of at some length. At Fig. 16 (#x12_x_12_i37) is shown part of a tentacle flinging out the poison-darts by which it secures its prey; and Fig. 17 (#x12_x_12_i37) is a more magnified view of one of these darts and its case.