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.
CHAPTER VI
PLANT LIFE IN PONDS AND STREAMS
In this chapter we shall confine ourselves to the true water-dwelling plants, as distinct from those, such as the water lilies, which though never found growing on dry land, appear undecided whether they will be water plants or land plants. Looking at the matter from a more scientific point of view, all our pond plants will be much lower in the scale of development than the water lilies and other flowering plants.