Leech is a small sucker with the mouth at the anterior end. A large sucker is at the posterior end. All leeches have 34 body segments. The length of the body generally ranges from minute to about 20 cm or even longer when the animal stretches.

Leeches breathe through the skin. The digestive system contains a crop, or pouch, in which food can be stored for several months. One to four pairs of eyes are located at the anterior end. Individuals are hermaphroditic¡Xi.e., functional reproductive organs of both sexes occur in the same individual. Leeches are not self-fertilizing, however, for the sperm of one individual fertilizes only the eggs of other individuals. The eggs are laid in a cocoon, which may be deposited on land or in water. Development and growth are direct, without a larval stage.

Leech occurs primarily in freshwater and on land. Some species of leeches are predators of other animals; some eat organic debris; others are parasitic. Aquatic leeches may feed on the blood of fishes, amphibians, birds, and mammals, or they may eat snails, insect larvae, and worms. True land leeches feed only on the blood of mammals. Three jaws set with sharp teeth make a Y-shaped incision in the flesh.

The leech's saliva contains substances that anesthetize the wound area, dilate the blood vessels to increase blood flow, and prevent the blood from clotting. The anticoagulant hirudin, which is extracted from the body tissues of the European medicinal leech (Hirudo medicinalis), is used medically.

Land leeches await their victim in damp vegetation, poising one end in the air. The victim is often unaware that he has been bitten until blood is discovered running from the wound. Blood flow may continue because of the anticoagulant still present.

Leeches that attack humans belong to the family Gnathobdellidae. Some species have been used medically for centuries. In Europe the use of leeches to drain off blood reached its height of popularity in the 19th century.

Diseases commonly treated with leeches included mental illness, tumours, skin disease, gout, and whooping cough. A common treatment for headache was to apply several leeches to each temple and allow them to draw blood. In addition to H. medicinalis of Europe, the Algerian dragon (H. troctina) was used. Gnathobdella ferox was commonly used in Asia. After H. medicinalis was introduced into North America, it established itself there as a wild species. It grows to 10 cm in length and is green, with four to six brown stripes.

Other land leeches that attack humans are primarily of the genus Haemadipsa in Asia, the Philippines, the East Indies, and Madagascar. Leeches of the genus Philaemon are parasitic on humans in Australia.

Aquatic leeches, particularly Limnatis nilotica, may enter the body in drinking water. Some may enter the excretory openings of persons who bathe in infested waters. L. nilotica, which inhabits lakes and streams of southern Europe, North Africa, and the Middle East, attains lengths of up to 12 cm. The younger, smaller specimens are most likely to enter the body. When ingested with drinking water they may first attach themselves to the linings of the nose or throat and then be inhaled into the lungs. A person infected with many such leeches may suffer from anemia resulting from loss of blood.

In external wounds secondary infection is more likely to occur than anemia. Leeches can cause suffocation and death of the host by blockage of the breathing passages; in Asia, in particular, domestic animals commonly die in this way.

Leech is distributed in most areas. Caught in summer and autumn, the leech is boiled to death in boiling water, cut into lengths and dried in the sun or low heat for use when raw or after being scorched with talcum.