leaches

Leeches are a group of segmented worms in the subclass Hirudinea, within the phylum Annelida. They comprise roughly 600 to 700 described species found in freshwater, terrestrial, and some marine environments. Most leeches are dorsoventrally flattened and lack the bristles (setae) common to many other annelids. They typically have two suckers, a smaller anterior one around the mouth and a larger posterior one for attachment and locomotion.

Feeding strategies vary: many leeches are hematophagous, attaching to vertebrate hosts and feeding on blood for days, while others are predators or scavengers of invertebrates. Hematophagous species often secrete saliva containing anticoagulants such as hirudin and, in some species, anesthetics to facilitate feeding.

Reproduction in leeches is usually sexual; most species are hermaphroditic and cross-fertilize. After mating, a clitellum forms a cocoon in which eggs are deposited; juveniles hatch and grow to maturity.

Ecology and human relevance: Leeches inhabit a wide range of habitats and play roles as predators, parasites, and prey within food webs. The medicinal leech, Hirudo medicinalis, has a long history in medicine; today leeches are still used in microsurgery to relieve venous congestion and promote tissue survival under controlled conditions. Some leech species are kept as aquarium inhabitants or used as bait.

Conservation: Habitat degradation and water pollution affect many species; some are threatened by habitat loss and collection.