HIRUDO, the leech, in natural history, a genus of the Vermes Intestina class and order. Body truncated at both ends, ob long, unarmed, moves onward by elongat ing the body, and then contracting itself into an argil, adhering alternately by the head and tail. The body moves either forward or backward. There are seven teen species, pri nci pally distinguished by their colAir. The most remarkable are the following : H. 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 any thing the creature pleases. It is vivipar ous, and produces but one young at a time, which is in the month ofJuly. It is an inhabitant of clear running waters in Europe, and is well known for its use in bleeding. The species used in the Unit ted States is probably distinct from this.
H. muricata, or muricated leech, has a taper body, rounded at the greater ex tremity, and furnished with two small tentacula, or horns, strongly annulated and rugged upon the rings, the tail dilat ed. It inhabits the Atlantic ocean, andis by the fishermen called the sea-leech. It adheres to fish, and generally leaves a black mark on the spot.
The mouth of the leech is armed with a sharp instrument, that makes three wounds at once, and may be compared to the body of the pump, and the tongue or fleshy nipple to the sucker : by the work ing of this piece ofmechanism, 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 twenty four-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 di gested by animals, have no call to be dis engaged from the heterogenous sub stances : nor indeed is there an anus dis coverable in the leech ; mere transpira tion seems to be all that it performs, the matter fixing on the surface of its body, and afterwards coming off in smallthreads. Of this on experiment may be tried, by puttin a leech into oil, where it keeps alive r 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 or
gan 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 the skin in order to draw off blood. With this view they are employed to phlebotomise young children. If the leech does not fasten, a drop of sugared milk is put on the spot it is wished to fix on, or a little blood is drawn by means of a slight punc ture, after which it immediately settles. The leech, when fixed, should be watch ed, lest it should find its way into the anus when used for the hemorrhoids, or pene trate into the oesophagus, if employed to draw the gums, as it would make,great havoc in the stomach or intestines. In such a case, the best and quickest reme dy is to swallow some salt ; which is the method practised to make it loose its hold when it sucks longer than was in tended.
H. sanguisuga, horse leech : elongated, olive brown, with an ochre-yellow mar ginal band : found in stagnant waters, ditches, and ponds : from four to six iuches long : body above dull with an ochre margin on each side ; be neath paler, with sometimes a few black spots ; tail thicker than the head. This species sucks blood with great avidity, and in large quantities. H. viridis,: body depressed, oblong, green, with a trans parent margin and pointed tail. This species has been described by Dr. Shaw in the " Transactions of the Linnxan So. ciety :" it inhabits clear cold waters, is about the eighth of an inch long, and, like most of the genus, has a power of repro duction almost equal to that of the po lype ; for if the animal be divided in eve ry direction, the parts will become per fect animals, and may be again divided and again reproduced. It is of a grass green colour, with a transparent border all round. It has since been referred to the genus Planaria. H. geometra, or geometrical leech : body filiform, green tsh, spotted with white; both ends dilata ble, and equally tenacious. It inhabits fresh waters : moves as if measuring like a compass, whence the name : it is found on trout and other fish, after the spawn ing season.