CHAPTER V
ANIMAL LIFE IN THE PONDS AND STREAMS

The enthusiastic microscopist will probably never lack material for his instrument, whatever branch of microscopical work he may decide to make his own. To the student of Pond Life, either animal or vegetable, there is granted a never-ending store of beautiful and interesting objects. Because one pond has been thoroughly searched and all that it can offer has been carefully examined, we must not conclude that no other pond will be worth our attention. Though indeed many little animals occur over and over again in practically every pond, there are other equally interesting animals which only occur in certain localities and for these we must keep a sharp look-out.

The apparatus needed for the collection of the denizens of ponds and streams, need consist of no more than a net with a very fine mesh and a jar in which to bring our captures home; for, of course, animals which dwell in water need not be dried on the journey home. Various useful accessories for the student of pond life are sold at very reasonable rates by most scientific instrument makers.

We shall find many representatives of the animal world in our pond and exceedingly interesting most of them will prove. From the mud we may obtain the “protean animalcule,” known to scientists as Amœba Proteus, the most lowly of all animals. Though this creature is plentiful and just visible to the naked eye, he is not easy to separate from his surroundings. He is almost colourless and therefore paler than the mud. Having secured him on the end of a glass rod, let us examine him in a drop of water on a slide. At first he will remain motionless, as a protest against being disturbed; we shall not have to wait long, however, for soon one part of his body will be seen to protrude and then grow larger and larger till it forms a false foot; other parts may follow suit, till he is more elongate than oval, and he moves in the direction of his false foot with a curious gliding motion. His pace is not great and has been calculated at a twenty-fifth of an inch in an hour. Really the “protean animalcule” is little more than an animated drop of jelly, a fact we can substantiate by watching him feed. His food consists of minute water plants such as diatoms, and when one of these plants comes within his line of march he simply surrounds it with his false feet and, as it were, flows around it. When he has digested all he can he flows away from the undigested portions; he has no mouth or any of the organs usually associated with animal anatomy.

While hunting for our Amœba, it is highly probable that a very active little slipper-shaped organism may have forced himself upon our attention. From his shape he has earned the popular name of the slipper animalcule. He is rather more highly organised than the Amœba for he possesses a mouth, as we shall see when we are able to examine him. So rapidly does he swim, however, that something must be done to curb his activity; he may either be killed with a drop of weak acid or we may put a little tuft of cotton wool on our drop of water and a coverslip lightly over that. The threads of cotton wool will form a network, in the meshes of which the active little animalcule will be confined. Careful observation will show that he is, like a slipper, more pointed at one end than at the other; that there is a funnel-shaped orifice, his mouth, at one side of his body; and that he is covered with little threads which lash the water with rhythmic movement and propel him with considerable rapidity. These little threads also send currents of water to his mouth, and in the water is his food.

Having examined these two free swimming denizens of the pond, we may advantageously turn our attention to some of the weeds growing therein. Careful examination with our pocket lens will almost certainly reveal a number of minute living creatures attached to the submerged stems and leaves. It is impossible to describe all the interesting creatures we might find: we must content ourselves with two, because they are common inhabitants of many ponds. One interesting little creature we are almost certain to find, the Hydra. He may be green or he may be brown but in structure he will remind us of a small sea anemone. When we put him under the microscope he has the appearance of a mass of jelly attached to the water plant on which we found him. He will soon open himself out, however, and we shall see that his free end is provided with a number of tentacles; these he waves about in the water to draw small swimming creatures to his mouth which is situated in the centre of the group of tentacles. Any luckless creature, coming within reach of the Hydra, is at once stung with one of the barb-shaped stings which stud his tentacles and then passed to his mouth. The Hydra is wonderfully tenacious of life; it is said that he has been turned inside out, like a glove finger, without suffering any inconvenience. Probably the specimen we are examining will have a swelling on the side of its body, which might be mistaken for the result of some injury; it is nothing of the kind; it is merely a bud which will grow into a young Hydra and, when old enough, become detached from its parent and float away to another plant. Under a fairly high magnification, we shall almost certainly see something gliding rapidly over the body of the Hydra; its movements are too quick to allow of careful examination; the creature which is almost as elusive as the slipper animalcule is a parasite of the Hydra.

Not quite so common as our last object but still common enough to be mentioned here is the beautiful “bell animalcule.” Like the Hydra, this creature, except in its very young stages, remains affixed to a water plant. In shape the “bell animalcule” resembles a wineglass on a long delicate stem; round the part corresponding to the rim of the glass, there is a fringe of the hair-like, water-lashing structures with which so many of these lowly creatures are provided, and these structures also surround the entrance to the funnel-shaped mouth. When undisturbed, the bell animalcule has its slender stalk fully extended and its little threads lash the water vigorously, causing currents, containing food material, to travel towards its mouth. A sharp tap on the microscope slide will cause the creature to contract, the threads cease their lashing and the stalk contracts spirally, so that the body of the animalcule is drawn close to the object to which it is attached. By degrees the spiral uncoils and the little threads resume their lashing.

Sometimes, as we examine our bell animalcule, we may be fortunate enough to see it splitting into two parts to form two separate individuals. This curious process should be watched carefully. The upper part of the bell splits first and, by degrees the whole bell divides into two equal parts so that we have a pair of bells on a single stalk. The next stage consists of the formation of a ring of whip-like structures round the base of one of the bells; both bells, by the way, have the circlet of whips round their upper edges. Soon after these additional little whips are formed, the owner of them breaks away from the stalk and swims about in the water for a time, finally coming to rest on a suitable water weed. Then the lower ring of whips has served its purpose and in its place a long stalk grows; from this time forward the new bell animalcule will never move from the position it has chosen. This form of increase, this simple splitting takes place over and over again but by degrees the little animal appears to become exhausted and the process slows down or stops.

The partially exhausted Vorticella may gain increased vitality by fusion with another individual and this process also we may have the luck to see though it is less frequent than the simple splitting. Sometimes a bell may be observed to divide, not into halves, but into two unequal parts. The smaller of these parts may divide again into from two to eight parts, each one of which, having developed a fringe of little whips, swims off on its own account. These little barrel-shaped swimming forms, instead of settling down and forming stalked bells, seek an exhausted creature, fuse with it near the base of its bell and finally become absorbed by it. The result of this fusion is that the bell animalcule takes on a new lease of life and once more begins to divide actively.

In our search for specimens for our microscope we may come across a very common pond dweller, closely related to the bell animalcule, known by the name of Carchesium Spectabile. We cannot fail to recognise its family likeness to the form we have already studied, for it consists of a large number of stalked bells growing on a single parent stem. It is really a little colony of bells. When one of the young individuals of Carchesium settles down in the spot it has selected for its dwelling-place, it grows a stalk just as Vorticella did, and it divides later into two individuals. Now in Vorticella only the bell divides, in Carchesium part of the stalk divides also and, instead of swimming away to find a new home it remains attached to the parent stalk. When this has happened several times a goodly colony is formed.

There are few fresh-water animals more commonplace and apparently uninteresting when observed casually than the pond sponge and the river sponge. Yet, if we take either of them home and examine them with the aid of our microscope, we shall be delighted with our specimens. In reality they are of absorbing interest and, at certain times of the year we may easily obtain young sponges, and capital objects they make for the microscope.

Before we describe our two specimens let us try to explain what manner of creature a sponge really is. If we examine any bath sponge, we notice that it is perforated with many small holes and some larger ones. Some sponges show this better than others. The small holes are pores, the large ones mouths, but, as we shall see in a moment we must not run away with the idea that they in any way resemble our familiar idea of a mouth. These large holes are called oscula by scientists, but we wish to avoid scientific words as much as possible. The simplest sponge of all, consists of a little bag, which remains affixed by its base to a seaweed. All over its sides there are many pores and at its tip there is a single osculum; it is known as the Purse Sponge and is common round our coasts. The inside of the purse sponge is lined with cells, each one of which is tipped with a little whip which waves about unceasingly. The waving of the whip causes water to flow through all the pores into the hollow bag and out again by way of the osculum. Although most sponges, including our fresh-water forms, are much more complicated than the purse sponge, the same thing happens in them all, water is drawn in by way of the pores and forced out by way of the oscula.

The best place to seek for the fresh water sponges is on the under sides of floating wood, broken tree branches and the like. Their appearance depends upon whether they have been growing in a well-lighted spot or in darkness; they contain chlorophyl as do the higher plants, and sponges grown in the light are green, those which the light has not reached are buff-coloured or corn-yellow. The pond sponge is brighter green than its river frequenting relative and is a coarser creature altogether. It often forms little finger-like outgrowths, whereas the river sponge is more leaflike.

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.

The water louse we may also encounter, he is not nearly so interesting as the fresh water shrimp. He is closely related to the wood louse which we all know, and has a similar flattened body.

Very much smaller though even more interesting is the common water flea. By day these animals retire to the mud at the bottom of the pond but, morning and evening, they swim actively with a curious jerky motion. We must examine our specimen carefully for he is of more than ordinary interest. We cannot fail to observe how transparent he is, so much so that all his internal organs can be plainly seen, but let us deal with his exterior first of all. His large eyes are plainly visible, but his most conspicuous feature is the pair of large branched feelers, by means of which he swims. If we examine several specimens, one or more is certain to be a female, then we may probably observe an egg in process of formation in the brood pouch, a large, elongated cavity, just below the back of the animal. Immediately above the brood pouch, the heart is situated and, if we can induce its owner to keep still for a moment or two, the heart-beats may be plainly seen.

Not unlike the water flea as it swims about in the water of our collecting jar is the curious, transparent little creature known as Cypris. Although so transparent its body is contained in a pair of shells, very similar to those of the mussel; a fact which formerly led to its being classed with the shell-fish. We may well examine this little fellow under the microscope for much of his structure may be made out through his shell. His very conspicuous eye is sure to attract our attention; he possesses but a single eye and seems to make up for the lack of a second by having a very large one. Two pairs of feelers project beyond the shell in front. Of his two pairs of legs the foremost, or at least their tips, hang down below the shell, but the last pair, as we can see through the shell, are turned upwards. At the hinder end of the body, there are two long bristles, they may best be seen when Cypris is swimming. Of what use exactly these bristles may be to their owner is not definitely known, but it is thought that they are of service in keeping his shell clean.

Another active little animal, quite as common as the water flea, is sure to attract our attention. It is no larger than the water flea but much more elongated; some specimens are bigger than others, and the bigger ones are the females. Its name is Cyclops and, though so common, it has no popular name.

Cyclops is so named on account of the fact that it possesses but a single eye; it is, however, rather an interesting creature in other respects, so we will study it more closely. Looking down upon the creature, we see that the front part of its body is composed of an undivided shield, behind which there are four plates and behind these again there are in the male five and in the female four segments or rings, at the extreme tip there is a forked tail, each fork being furnished with a number of bristles. On the head are two pairs of organs, one pair long the other pair short and, if we observe Cyclops in the act of swimming, we shall see that the long pair of organs play the chief part. In the centre of the front of the head there is a black or red patch—the eye. Very frequently we may meet with a specimen carrying a relatively large bladder-like body on each side of its abdomen. These bladders which are each about one-third as long as the creature which carries them, are egg-sacs.

If we are able to secure one or two specimens with egg-sacs attached, we can study the young Cyclops without much difficulty. Take a small glass tube—a test-tube as used by chemists will serve admirably—partly fill it with pond water and add a little water weed, then introduce the egg-bearing females and place in the light. We must watch the tube from day to day, and before long it will be evident that the young ones have arrived in the world, for we shall have no difficulty in seeing dozens of little white specks swimming about in the water and settling on the sides of the tube. We must remove one carefully on the end of a glass rod or on a paint brush and examine it in a drop of water under the microscope. This young creature is totally unlike its parent, it is oval and possesses three pairs of stiff bristles, of which the first pair are simple and the other two pairs are branched. Although the bristles are used solely for swimming at this stage, it may be of interest to mention that in the adult Cyclops they become transformed into jaws and the two pairs of organs we have already examined. At the front end of the oval body we can closely distinguish the single eye, which persists throughout life.

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