Both groups are hermaphrodites and have a clitellum, but leeches typically differ from the oligochaetes in having suckers at both ends and ring markings that do not correspond with their internal segmentation.

The majority of freshwater leeches are found in the shallow, vegetated areas on the edges of ponds, lakes and slow-moving streams; very few species tolerate fast-flowing water.

[6] 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.

[7] The terrestrial Haemadipsidae are mostly native to the tropics and subtropics,[8] while the aquatic Hirudinidae have a wider global range; both of these feed largely on mammals, including humans.

[6] A distinctive family is the Piscicolidae, marine or freshwater ectoparasites chiefly of fish, with cylindrical bodies and usually well-marked, bell-shaped, anterior suckers.

[9] 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.

Acanthobdella Erpobdelliformes Hirudiniformes Glossiphoniidae Piscicolidae Ozobranchidae The most ancient annelid group consists of the free-living polychaetes that evolved in the Cambrian period, being plentiful in the Burgess Shale about 500 million years ago.

[22][18] Leeches show a remarkable similarity to each other in morphology, very different from typical annelids which are cylindrical with a fluid-filled space, the coelom (body cavity).

[30][31] Of these segments, the first five are designated as the head and include the anterior brain, several ocelli (eyespots) dorsally and the sucker ventrally.

[35] Each cocoon is fixed to a submerged object, or in the case of terrestrial leeches, deposited under a stone or buried in damp soil.

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.

In Hirudo medicinalis, these supplementary factors are produced by an obligatory mutualistic relationship with the bacterial species, Aeromonas veronii.

[40] In addition, leeches produce intestinal exopeptidases which remove amino acids from the long protein molecules one by one, possibly aided by proteases from endosymbiotic bacteria in the hindgut.

Their large size makes leeches convenient as model organisms for the study of invertebrate nervous systems.

[26] Leeches move using their longitudinal and circular muscles in a modification of the locomotion by peristalsis, self-propulsion by alternately contracting and lengthening parts of the body, seen in other annelids such as earthworms.

They use their posterior and anterior suckers (one on each end of the body) to enable them to progress by looping or inching along, in the manner of geometer moth caterpillars.

[44] The Hirudinidae and Erpobdellidae can swim rapidly with up-and-down or sideways undulations of the body; the Glossiphoniidae in contrast are poor swimmers and curl up and fall to the sediment below when disturbed.

[46] Leech bites are generally alarming rather than dangerous, though a small percentage of people have severe allergic or anaphylactic reactions and require urgent medical care.

Symptoms of these reactions include red blotches or an itchy rash over the body, swelling around the lips or eyes, a feeling of faintness or dizziness, and difficulty in breathing.

[49] An externally attached leech will detach and fall off on its own accord when it is satiated on blood, which may take from twenty minutes to a few hours; bleeding from the wound may continue for some time.

[51][52] Leech saliva is commonly believed to contain anaesthetic compounds to numb the bite area, but some authorities disagree.

[59][60] Pliny the Elder reported in his Natural History that the horse leech could drive elephants mad by climbing up inside their trunks to drink blood.

[62] In Old English, lǣce was the name for a physician as well as for the animal, though the words had different origins, and lǣcecraft, leechcraft, was the art of healing.

[63] William Wordsworth's 1802 poem "Resolution and Independence" describes one of the last of the leech-gatherers, people who travelled Britain catching leeches from the wild, and causing a sharp decline in their abundance, though they remain numerous in Romney Marsh.

Leeches can reduce swelling in the tissues and promote healing, helping in particular to restore circulation after microsurgery to reattach body parts.

[66][67] Other clinical applications include varicose veins, muscle cramps, thrombophlebitis, and joint diseases such as epicondylitis and osteoarthritis.

They showed that mammal mitochondrial DNA, amplified by the polymerase chain reaction, can be identified from a leech's blood meal for at least four months after feeding.

[75][76] Exposure to synthetic estrogen as used in contraceptive medicines, which may enter freshwater ecosystems from municipal wastewater, can affect leeches' reproductive systems.

Although not as sensitive to these compounds as fish, leeches showed physiological changes after exposure, including longer sperm sacs and vaginal bulbs, and decreased epididymis weight.