Leeches are unlike other annelids in several ways. They have a fixed number of segments (usually 34), a dorso-ventrally flattened body, both an anterior and posterior sucker (usually), no parapodia, and usually no setae. The coelom is not subdivided by septa in most species, and it has been filled with muscle and connective tissue.
Leeches are hermaphroditic. Development is direct as in oligochaetes.
Most leeches are found in freshwater habitats, but a few are marine and some are terrestrial (but they require warm, moist conditions). Most are either carnivorous or parasitic. Medicinal leeches were used for centuries by physicians to control diseases that were believed to be caused by an excess of blood. Interest in using leeches has revived recently among surgeons trying to reattach severed limbs or digits, for it turns out that leeches are able to do a better job of controlling swelling in the reattached limb (while minute veins grow and reconnect) than can surgeons. Scientists have also shown much interest in the anticoagulant that leeches secrete as they feed.
Approximately 500 species of leeches have been described.
Phil Myers (author), Museum of Zoology, University of Michigan-Ann Arbor.