Many parasitic leeches are able to parasitize a wide variety of hosts. Most of the marine and some of the freshwater leeches are fish parasites. The medicinal leech, Hirudo medicinalis, is one of a group of aquatic bloodsucking leeches with jaws. The digestive tract of bloodsuckers produces an anticoagulant, hirudin, which keeps the engorged blood from clotting. A few leeches attach permanently to the host, leaving only to reproduce.
Predatory leeches are active at night and hide by day.
Leech Reproduction Like the oligochaetes, leeches are hermaphroditic and cross-fertilizing, although fertilization is internal. In other species the sperm are not enclosed and are transferred directly into the body of the partner by copulation. A courtship display is seen among some leeches at the time of mating. The young emerge as small copies of the adults. Listing from Electric Library , a pay service.