Leech, the popular name of worms belonging to the sub-class Hirudinea. The best-known form is the common leech (Hirudo medicinalis), which is used to suck blood from congested parts. Though not so often employed as formerly, it is reared" in large numbers in ponds in Hungary. It possesses three horny jaws with finely-serrated edges, which in cutting through the skin are used like saws, producing a wound which bleeds freely, but heals quickly. Another species is the Horse-leech, common in ponds in Britain.