If we examine one of these sponges in a watch glass of water to which we have added a very little carmine powder, we can easily see the water currents entering the pores and coming from the oscula. Towards autumn, if we open a river sponge we shall see little yellow bodies about the size of a pin’s head; they are the buds from which the young sponges arise. One of these must be removed very carefully and placed in a watch glass of water; if our specimen be placed in the sun, we shall not have many days to wait before we find that it has given rise to an active transparent little creature, to a young sponge in fact. By repeating our experiment with carmine, by the aid of the microscope, we can see the water currents passing through its body.
Wherever we collect our pond water we are certain to find some of the wheel animalculæ or rotifers. There are such a number of different kinds that we might describe several, and yet not mention the one that any of our readers had happened upon. They may be recognised, because they are always so transparent that all their internal organs may be plainly seen and they always have two or more discs or lobes, at their forward end, fringed with fine whip-like threads. These little threads are constantly in motion, so that they appear like little revolving wheels hence the name, wheel animalcule. Nearly all these creatures have a sucker or false foot at their tail end by means of which they attach themselves to some support or they may swim freely in the water. The study of rotifers has been the life-work of some scientists and they will afford any microscopist, who is interested in them with abundant occupation.
Amongst the water weeds we shall probably find another small but striking creature, known as the sun animalcule, Actinophrys Sol. Why exactly it is called the sun animalcule we cannot say, probably it has earned its title from the fact that it resembles the conventional idea of the sun, with light rays radiating all round its circumference. The creature is whitish-grey in colour, spherical in shape and just visible to the naked eye. All over the surface of its body there are a number of apparently empty spaces, known as vacuoles—they probably account for its peculiar colour. Everywhere, it is studded with long, slender-pointed rod-like outgrowths. But rarely, the sun animalcule exhibits any movement and for long periods the only signs that it is living occur at feeding time. Its food consists of water animals and plants, of varying size. When a small animal comes into contact with one of the pointed rods, which radiate from the animalcule, it appears to be held there in some unaccountable manner and, after a pause, it begins a fateful journey by sliding down the rod to the spherical body of its captor. Then it is passed into one of the vacuoles and digestion very soon takes place. That it does not do so immediately is shown by the fact that the wheels of a wheel animalcule, which has been passed to the vacuole of our subject, continue their movements for a considerable period. When larger animal food is partaken of, a different method is pursued by the sun animalcule. A water-flea, for instance, coming into contact with one of the rods will struggle violently in its efforts to escape. Then the sun animalcule shows real signs of life, for some of the other rods bend over and hold the captive so that it, eventually, is passed to a vacuole.
In our [chapter on agriculture] we mention a peculiar flat worm known as the liver fluke. This unpleasant creature has one or two relatives who make their home in ponds and, though they are not more beautiful than the liver fluke to look upon, they are quite harmless. One of these flat worms is about 3/4-inch long and not unlike an indian club in shape. It moves about, with some speed either by the aid of a sucker on its head or by a curious gliding movement. The other pond-frequenting flatworm is more like the liver fluke, its leaflike oval body, about 1/2-inch long, is pointed fore and aft. It is a common sight to see it gliding here and there in search of still smaller animals off which it may make a meal.
The sea mat and bird’s head, two common animal colonies of the sea-shore, possess pond-dwelling relatives of the greatest interest to the microscopist. Like the seaside forms they dwell in colonies. One of the commonest is known as Lophopus Crystallinus and it may be found attached to duckweed, the curious little plant whose tiny leaves float upon the surface of the water. Lophopus, when at rest, resembles a little piece of jelly. If we are patient and watch it under our microscope we shall see it expand, sending forth a number of stalks, each one tipped with a horse-shoe shaped feathery tentacle. Each of these tentacles belongs to a separate animal which with its fellows forms a colony. Not so interesting are the branched, threadlike colonies of Plumatella Repens which may be sought upon the leaves of water plants. Our ponds can furnish no more extraordinary object for our microscope than a colony of Cristatella Mucedo; it is curious in appearance and still more curious from the fact that though a colony of animals it acts like a single individual in crawling over the weeds and stones in shallow, sun-kissed water. The Cristatella colony is jelly-like and greenish in colour, in length it may grow to a couple of inches. Its under surface is flat, whilst from its upper, convex surface the little animals forming the colony wave their brush-like tentacles in the water.
In searching the various pond weeds for specimens we are sure to meet with various jelly-like masses; these must always be examined carefully. They may be the egg-masses of interesting water creatures; various water-snails, for instance, protect their eggs with a jelly-like covering.
If we meet with any large fresh-water mussels, sometimes called swan mussels, we shall probably find the fleshy parts of the molluscs swarming with minute creatures, which we may conclude are parasites. Mussels, like nearly all living creatures, have their parasites it is true, but what we have discovered will almost certainly prove to be young mussels. We must examine them under the microscope to make sure. Then we shall see, if young mussels they be, very minute and very thin-shelled little creatures; the edges of their shells are armed with teeth and are quite unlike the highly polished and smooth shells of the parent mussel. We shall also see a long thread issuing from the animal within the shell. If we are examining the young mussel in water we shall notice that it is constantly snapping its two shells together.
Had we left these young mussels undisturbed, a very curious life they would have led. They would have remained attached to their parent for some little time probably, or some of them might have fallen to the muddy bed of the pond. Their behaviour in either case would be the same. The long threads that we have already examined would have floated in the water and, directly they were touched by a passing fish, the little shells would begin to snap violently. The lucky ones would not snap in vain for they would close upon the fin or tail of the fish and then their snapping would cease. Like the bulldog, these little mussels may take a long time in getting hold but when once they have managed to fasten their teeth into anything it is well-nigh impossible to make them let go. Usually they never let go, but are carried away by the fish. In most cases, the irritation they set up in the flesh of their new-found foster parent causes a swelling to occur with the result that the little mussels are engulfed. Within the flesh of the fish they go through a series of changes; they lose their teeth and their tell-tale thread, they become in fact miniatures of the adult mussels, then they manage to escape from the fish, settle down in the mud and fend for themselves.
The crabs and lobsters which we know so well have fresh water relatives in nearly every pond. Many of the creatures we have examined have needed careful search to discover their whereabouts; not so the fresh water crustacea, as they are called. Their activity, their curious movements in the water compel attention.
The fresh water shrimp is a curious little creature, sometimes he paddles his three pairs of hind legs and sometimes he jerks his body in a ludicrous manner, in either case he manages to propel himself rapidly through the water. He is about half an inch long, brown in colour and with a curved body not unlike a shrimp. If we examine him under the microscope we notice that his front legs are bent forwards, whilst his hind legs are bent backwards. The male water flea is much larger than the female, a fact which probably accounts for the fact that these little animals often carry their wives about with them by seizing them with their fore legs.