Medicinal leech (Hirudo medicinalis). Leeches are parasites that feed on blood. They attach themselves to the skin using suckers and create a wound with three sharp jaw plates, which contain over 600 teeth. They remain attached to their host until they are full. Their saliva contains hirudin, a chemical which prevents blood from clotting. A normal wound may bleed for up to 20 minutes before clotting, but a leech wound can bleed for up to 10 hours. They are used in medicine to reduce blood coagulation, relieve pressure from pooling blood and stimulate circulation.