Leech. Surgeon supervising a medicinal leech (Hirudo medicinalis) feeding on a human hand. Leeches are parasites which feed on blood. They attach themselves to the skin using suckers and create a wound with their three sharp jaw plates. Their saliva contains hirudin, a chemical which prevents the blood from clotting. They are used in modern medicine to drain blood from haematomas (accumulations of blood) and to draw blood into transplanted tissues until circulation is established. A leech may drink between 10 and 15 millilitres of blood in one meal, which will feed it for months.