Leeches are parasitic worms that are closely related to the earthworm. They have soft, muscular, segmented bodies that can lengthen and contract. Leeches typically have suckers at both ends and their body is muscular and relatively solid.
The majority of leeches live in freshwater but some can be found in terrestrial and marine environments. The best-known species attach themselves to a host with a sucker and feed on blood. A minority of leech species are predatory, mostly preying on small invertebrates.
Leeches have been used in medicine from ancient times until the 19th century to draw blood from patients. In modern times, leeches are used to treat joint diseases and extremity vein diseases. Leeches secrete hirudin when they bite their prey, an anticoagulant that is sometimes used in microsurgery to treat blood-clotting disorders.