Leeches have relatively few segments and no bristles. The body is often cylindrical with one sucker at each end. The front sucker surrounds the mouth and is usually smaller than the rear one. Anus is placed just in front of the rear sucker. Marine leeches feed on body fluids from fishes. They leave their hosts only to attach cocoons to rocks, shells and other hard surfaces.
Only two orders of marine leaches are known in European water. Arhynchobdellida are leeches with a straw-like instrument used to puncture the skin of a fish or another host. Members of the order Rhynchobdellida lack this "straw".