Once used extensively to treat illnesses believed to be caused by excess blood, the medicinal leech, Hirudo medicinalis, is a
bloodsucking worm that can be found in ponds and lakes of the eastern United States. It inflicts a painless bite from a sucking
disk at each end of its body. The leech’s saliva, injected into the wound, contains a substance that prevents the host’s blood from
clotting. Leeches are sometimes still used to drain pooled blood from under the skin.
