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, but they also are found in the tropics, in polar oceans, and in deserts. Some are permanent parasites of humans, horses, cattle, fish, and mollusks, but most are merely predatory. The salivary secretions of the leech contain hirudin, an anticoagulant, and other substances that promote blood flow. The medicinal leech ( Hirudo medicinalis ), a European freshwater leech once used by physicians to bleed patients suffering from almost any ailment, are now used to remove pooled blood from under skin grafts and other reconstructive surgeries, to treat bruises (such as black eyes), and to treat some osteoarthritis. 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. The giant Amazon leech can grow as large as the forearm of an adult human being. Leeches are classified in the phylum Annelida, class Hirudinea.