Leeches are annelids comprising the subclass Hirudinea. There are freshwater, terrestrial, and marine leeches. Like the Oligochaeta, they share the presence of a clitellum. Like earthworms, leeches are hermaphrodites. Some, but not all, leeches are hematophagous.The European medical leech, Hirudo medicinalis, and some congeners, as well as some other species, have been used for clinical bloodletting for thousands of years, although most leeches do not feed on human blood, but instead prey on small invertebrates, which they eat whole.