The common leech is well known in medicine. It is curiously enough an inhabitant of ponds and lakes, and in such conditions has no opportunities for tasting the warm blood for which it develops such a liking when opportunity offers. This is really a remarkable fact, that the animal should be placed in a position naturally in which its most natural tendencies should remain unsatisfied.
The progression of the leech is performed by undulating movements and the prehensile action of the suckers—head and tail. The eyes are ten in number, near the mouth and at the top of the head. The mouth is furnished with numerous tri-radiate teeth, but in some leeches they are not sharp, as in the “medicinal” variety. It is known that both sexes are represented in every leech, but they are not self-reproductive.
The earth-worm, so familiar to all, has been lately raised into importance. It lives in clay, and bores its way through the ground. It feeds upon organic matter contained in the earth, and when it has assimilated the nourishing particles, it ejects the remainder in small heaps of soft dirt, which are visible after rain particularly. The worm, by its burrowing, turns up the land, and vastly increases its fertility.
Fig. 841.—Wheel animalcule (rotifer vulgaris).
The earth-worm in its outer structure resembles the leech, but, as any one will at once perceive, the worm is not furnished with suckers by which it can assist itself to move. Instead of these rounded terminals the worm is finely pointed, and thus capable of boring its way through the earth. Progression is accomplished by moving first the head portion and then the next, so that a regular series of movements is necessary. The minute spines or bristles of the worm prevent its body being retracted by muscular effort.
The vital organs are rather forward of the centre of the body, and so if a worm be cut behind them it will survive and reproduce a tail. But the portion cut off will not be found alive, nor is it capable of forming a new perfect worm as generally supposed.
There are many other orders of worms which we can only indicate—viz., the Tubicolæ which surround themselves with a hard case; the Errantia, or sea-worms, sand-worms, etc., like the lobworm used for bait, and the naiads of our fresh-water ponds, all of which are suited to the aquatic life they lead. Indeed, of all the annelides, the earth-worm is the only specimen that is suited for living upon land. As regards the last mentioned, we may add that worms do not prey upon dead bodies as is so generally imagined. They are vegetable feeders, and do not burrow very deeply.
The transparent condition of the Rotifers renders them easy of observation under the microscope, and we find a nervous system, intestines, and a developed stomach. They are fresh-water inhabitants.