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leech, predacious or parasitic annelid worm of the class Hirudinea, characterized by a cylindrical or slightly flattened body with suckers at either end for attaching to prey. The leech, like other annelids, is segmented, but its numerous surface folds obscure the internal segments. In many forms the mouth has three small jaws equipped with sharp teeth. The digestive tract has lateral pouches that hold enough of the leech's staple food, blood, to last for months. The reproductive system is complex; leeches are hermaphroditic and cross-fertilizing. Nearly all leeches are aquatic, abounding in freshwater ponds in temperate regions. Some are permanent parasites of man, horses, cattle, fish, and mollusks, but most are merely predatory. The salivary secretions of the leech contain hirudin, an anticoagulant. Medicinal leeches, once used by physicians to bleed patients suffering from almost any ailment, are now used in some parts of the world chiefly in the treatment of bruises such as black eyes. Certain small leeches of the E Mediterranean region may enter the bodies of humans and animals through drinking water and lodge as parasites in the mouth or the respiratory passages. Leeches are classified in the phylum Annelida, class Hirudinea.
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