leeches

Leeches are segmented worms in the phylum Annelida, belonging to the subclass Hirudinea. Most species are freshwater, but some are terrestrial or marine. They are flattened dorsoventrally and typically have two suckers, an anterior and a posterior, used for feeding and locomotion. Their bodies usually comprise around 32–34 segments, though the exact number varies by species. Leeches lack many of the bristles (chaetae) found in other annelids, and many have smooth, mucus-covered skin.

Leeches feed in two main ways: predatory species that ingest small invertebrates, and hematophagous species that feed on the blood of vertebrates, including mammals. Blood-feeding leeches inject saliva containing anticoagulants such as hirudin and local anesthetics to facilitate feeding. After feeding, they may swell substantially and can go long periods between meals.

Most leeches are hermaphroditic, and cross-fertilization occurs between individuals. After mating, many species produce cocoons secreted by the clitellum that contain the eggs. Juveniles hatch from these cocoons and grow into adults through molts.

Leeches are most diverse in freshwater habitats, where they act as predators of invertebrates and serve as prey for larger animals. Some species are kept as fishing bait, and the European medicinal leech, Hirudo medicinalis, has a long history in medicine. In modern medicine, leeches are used to relieve venous congestion after reconstructive surgery, and compounds in their saliva have been investigated for anticoagulant properties.

Conservation: Habitat loss and water pollution threaten many species. Some are protected in certain regions and are bred for medical use.