There are several kinds of leeches. All are moderately short, flattened worms. A leech has a small, toothed sucker on its head and a second, larger sucker at its tail end. The front sucker enables it to attack the skin of animals and slowly remove blood, which is its food.
   Most species of leech live in fresh water. A few live in the sea. Land species occur in moist, tropical grasslands or forests.
   Leeches are segmented worms related to the earthworm. The outside of the body has more ring-like folds than correspond to the true body segments inside. The internal organs resemble those of an earthworm, but are adapted for parasitism.
   One European species, the medicinal leech, was used for centuries by early doctors to draw blood as a supposed cure, and to remove the purplish-brown clots from bruises. Recently, scientists have isolated an anti-clot chemical (hirudin) from leech salivary glands. It is used in studies of blood-clotting diseases.