Among the freshwater leeches are the Glossiphoniidae, dorso-ventrally flattened animals mostly parasitic on vertebrates such as turtles, and unique among annelids in both brooding their eggs and carrying their young on the underside of their bodies.The terrestrial Haemadipsidae are mostly native to the tropics and subtropics, while the aquatic Hirudinidae have a wider global range; both of these feed largely on mammals, including humans. A distinctive family is the Piscicolidae, marine or freshwater ectoparasites chiefly of fish, with cylindrical bodies and usually well-marked, bell-shaped, anterior suckers. Not all leeches feed on blood; the Erpobdelliformes, freshwater or amphibious, are carnivorous and equipped with a relatively large, toothless mouth to ingest insect larvae, molluscs, and other annelid worms, which are swallowed whole. In turn, leeches are prey to fish, birds, and invertebrates.The name for the subclass, Hirudinea, comes from the Latin hirudo (genitive hirudinis), a leech; the element -bdella found in many leech group names is from the Greek βδέλλα bdella, also meaning leech. The name Les hirudinées was given by Jean-Baptiste Lamarck in 1818. Leeches were traditionally divided into two infraclasses, the Acanthobdellidea (primitive leeches) and the Euhirudinea (true leeches). The Euhirudinea are divided into the proboscis-bearing Rhynchobdellida and the rest, including some jawed species, the "Arhynchobdellida", without a proboscis.The phylogenetic tree of the leeches and their annelid relatives is based on molecular analysis (2019) of DNA sequences.
It leads successively into the pharynx, a short oesophagus, a crop (in some species), a stomach and a hindgut, which ends at an anus located just above the posterior sucker. The stomach may be a simple tube, but the crop, when present, is an enlarged part of the midgut with a number of pairs of ceca that store ingested blood. The leech secretes an anticoagulant, hirudin, in its saliva which prevents the blood from clotting before ingestion. A mature medicinal leech may feed only twice a year, taking months to digest a blood meal. The bodies of predatory leeches are similar, though instead of a jaw many have a protrusible proboscis, which for most of the time they keep retracted into the mouth. Such leeches are often ambush predators that lie in wait until they can strike prey with the proboscises in a spear-like fashion. Predatory leeches feed on small invertebrates such as snails, earthworms and insect larvae. The prey is usually sucked in and swallowed whole. Some Rhynchobdellida however suck the soft tissues from their prey, making them intermediate between predators and blood-suckers. Blood-sucking leeches use their anterior suckers to connect to hosts for feeding. Once attached, they use a combination of mucus and suction to stay in place while they inject hirudin into the hosts blood. In general, blood-feeding leeches are non host-specific, and do little harm to their host, dropping off after consuming a blood meal. Some marine species however remain attached until it is time to reproduce.