2. One species has been used in medical treatments to bleed patients or to eat away putrid flesh from a wound.
3. Any of numerous carnivorous or bloodsucking annelid worms that constitute the class Hirudinea, that typically have a flattened segmented lance-shaped body with well-marked external annulations (ringlike structures, segments, or parts), a sucker at each end, a mouth within the anterior sucker, and a large stomach with pouches of large capacity at the sides.
Such pouches are hermaphroditic (having both male female reproductive organs); usually, with direct development, and which occur chiefly in fresh water, although a few are marine and some tropical forms are terrestrial.
Leeches were used as a method of bloodletting, a practice common up to the middle of the 19th century.
In modern times, leeches have been used to evacuate periorbital hemorrhage (black eye) and to remove congested venous blood from the suture lines of re-implanted fingers.
Sometimes, a patient's veins are too weak to carry blood and it builds up, causing venous congestion and since leeches are a source of hirudin, an anticoagulating principle secreted by their buccal glands and leech saliva contains several active substances including inhibitors of platelet aggregation, they are used to decrease such venous congestion.
The leech's saliva contains substances that anesthetize the wound area, dilate the blood vessels to increase blood flow, and prevent the blood from clotting and so attaching leeches to the body draws the blood away gradually and painlessly.
Leeches are particularly useful in plastic surgery; such as, breast reconstruction and where a part of the body has become severed and had to be sewn back on.