the LEECH; a genus of insects belonging to the order of vermes intestina. The body moves either forward or backward. There are several species, principally distinguished by their colour. The most remarkable are the following. 1. The medicinalis, or medicinal leech, the form of which is well known, grows to the length of two or three inches. The body is of a blackish brown colour, marked on the back with six yellow spots, and edged with a yellow line on each side; but both the spots and the lines grow faint, and almost disappear, at some seasons. The head is smaller than the tail, which fixes itself very firmly to anything the creature pleases. It is viviparous, and produces but one young one at a time, which is in the month of July. It is an inhabitant of clear running waters, and is well known for its use in bleeding.
2. The sanguifuga, or horse-leech, is larger than the former: Its skin is smooth and glossy; the body is depressed; the back is dusky; and the belly is of a yellowish green, having a yellow lateral margin. It inhabits stagnant waters.
3. The geometra, or geometrical leech, grows to an inch and a half in length; and has a smooth and glossy skin of a dusky brown colour, but in some seasons greenish spotted with white. When in motion, its back is elevated into a kind of ridge; and it then appears as if measuring the space it passed over like a compass, whence its name. Its tail is remarkably broad; and it holds as firmly by it as by the head. It is common on stones in shallow running waters; and is often found on trout and other fish after the spawning season.
4. The muricata, or murred leech, has a taper body, rounded at the greater extremity, and furnished with two small tentacula or horns strongly annulated and rugged upon the rings, the tail dilated. It inhabits the Atlantic Ocean, and is by the fishermen called the sea leech. It adheres to fish, and generally leaves a black mark on the spot.
The organs of generation in leeches are formed like those of the sea and land snails. See Helix.—The leech's head is armed with a sharp instrument that makes three wounds at once. They are three sharp tubercles, strong enough to cut through the skin of a man, or even of an ox or horse. Their mouth is as fleety nippel the sucker; by the working of this piece of mechanism, the blood is made to rise up to the conduit which conveys it to the animal's stomach, which is a membranaceous skin divided into 24 small cells. The blood which is sucked out is there preserved for several months almost without coagulating, and proves a store of provision to the animal. The nutritious parts, pure and already digested by animals, have no call to be disengaged from heterogeneous substances; nor indeed is there an anus discoverable in the leech, mere transpiration seems to be all that it performs, the matter fixing on the surface of its body, and afterwards coming off in small threads. Of this an experiment may be tried by putting a leech into oil, where it keeps alive for several days; upon being taken out and put into water, there appears to loosen from its body a kind of slough shaped like the creature's body. The organ of respiration, though unascertained, seems to be situated in the mouth; for if, like an insect, it drew its breath through vent holes, it would not subsist in oil, as by it they would be stopped up.
It is only the first species that is used in medicine; being applied to tender parts upon the vessels in order to draw off the inspissated blood with which they are overcharged, or to phlebotomize young children. If the leech does not fasten, a drop of milk is put on the spot it is wished to fix on, or a little blood is drawn by means of a slight puncture, after which it immediately settles. Prudence requires it should be held fast with a piece of rush, lest it should find its way into the anus when used for the hemorrhoids, or penetrate into the oesophagus if employed to draw the gums; otherwise it would make the greatest havoc either in the stomach or intestines. In such a case, the best remedy is to drink salt water; which is the method practised to make it loose its hold when it sticks longer than was intended. Oil of tartar, volatile alkali, pepper, and acids, make it also leave the part on which it was applied. If, on the contrary, it is intended it should draw a larger quantity of blood, the end of its tail is cut off. It then sticks continually to make up the loss it sustains. The discharge occasioned by the puncture of a leech is easily stopped with brandy or other astringents.
At Ceylon, travellers who walk bare legged are molested by the great numbers of leeches concealed under the grass.—All leeches vary in their colours at some seasons, but they are generally of a dusky greenish brown or yellow, and often variegated. They are said to be very restless before a change of weather, if confined in glasses.