HIRUDO, the LEECH; a genus of insects belonging to the order of vermes intestina. The body moves either forward or backward. There are nine species, principally distinguished by their colour. The most remarkable are the following.
1. The medicinalis, or common leech, hath an oblong brown body, marked with six yellow spots, and is an inhabitant of ponds, ditches, and other stagnant waters. This animal is well known for the purpose of bleeding, children especially. This practice is as old as the days of Pliny, who gives the creature the name of hirudo sanguifuga. In his time, leeches were used instead of cupping-glasses for persons of plethoric habits, and those who were troubled with the gout in the feet. He asserts, that if they left their head in the wound, as was sometimes the case, the wound was incurable; and he informs us, that Messalinus, a person of consular dignity, lost his life by such an accident. Some imagine, that leeches have a poisonous quality, because the wound they make is not always easily healed; but this depends on the habit of the body, and will also happen when the lancet is used. To make leeches fasten soon, keep them hungry, and rub the part to which they are to be applied with warm milk or blood. If they stick longer than is thought convenient, they must not be pulled off; but if their heads are touched with common salt, they soon fall off of themselves. If they are thought not to have drawn a sufficient quantity of blood, apply cloths wrung out of warm water upon the orifice; or, if convenient, put the part into warm water; and thus the bleeding may be prolonged. They are to be kept in bottles not quite filled with water, which ought to be renewed every three or four days at farthest. A little sugar may be added to the water in which they are kept. For the cases in which the application of leeches is advisable, see (the Index subjoined to) MEDICINE.
2. The sanguifuga, or horse-leech, hath a depressed body; in the bottom of the mouth are certain great sharp tubercles or whitish caruncles. The slenderest part is about the mouth, and the thickest about the tail. The tail itself is very slender; the belly of a yellowish green; the back dusky. This species is also a blood-sucker, though not used in medicine. The instruments with which both species perforate the skin, are found, on a nice dissection, to be a number of very fine teeth disposed in a regular order on three ribs, or jaws, placed between the aperture of the lips and the bottom of the mouth; each of them along a strong muscle of its own length. Hence the wound made by leeches consists of three cuts proceeding like radii from a centre, and making equal angles with each other. This structure of the wound is most distinctly seen when the swelling has gone down, and the skin is clean, which is usually on the fourth day.—Leeches are able to live in oil; and when taken out of this liquid and put into water again, they throw off a tender skin or film, of the regular shape of the whole body. Their being able to live in this fluid shews, that they respire by the mouth: which is also further proved, by gently warming the water in which they are kept; for then the animals being uneasy, breathe hard, and very visibly. These animals may in some shape answer the purposes of barometers; for when preserved in glasses, they predict bad weather by their great restlessness and change of place.
3. The geometra, or geometrical leech, is a native of the same places with the two former. It hath a filiform body, greenish, spotted with white; both ends dilatable, and equally tenacious. It moves as if measuring the spaces it passes over like a compass, whence its name. It is found on trout and other fish after the spawning season.
4. The muricata, or tuberculated leech, hath a taper body, rounded at the greater extremity, and furnished with two small horns; strongly annulated and tuberculated upon the rings; the tail dilated. It inhabits the sea, adheres strongly to fish, and leaves a black mark on the spot.
5. The myxine, or hag, is about eight inches long, with a slender body, carinated beneath, and an adipose or rayless fin round the tail, and under the belly. It inhabits the ocean; enters the mouths of fish, when on the hooks of lines that remain a whole tide under water; and devours the whole, except the skin and bones. The Scarborough fishermen often take it in the robbed fish, on drawing up their lines. Linnaeus attributes to it the property of turning water into glue.