Posted by Jelio Mir on

Alternative title: Hirudinea
The leech, (subclass Hirudinea), any of about 650 species of segmented worms (phylum Annelida) characterized by a small suction cup, which contains the mouth, at the anterior end of the body and a large supter located at the posterior end. All leeches have 34 body segments. The length of the body varies from minutes to about 20 cm (8 inches) or even more when the animal stretches. Leeches appear mainly in freshwater and on land. Members of the species Rhynchobdellida appear both in the sea and in fresh water. A member of this species, the giant Amazon leech (Haementaria ghilianii), can grow up to half a meter (about 18 inches) in length. This leech uses its 15 cm (6 inch) proboscis as a hypodermic needle to suck blood from its host. Some species of leeches are predators of other animals, some eat organic debris and others are parasites.

Leeches breathe through the skin. The digestive system contains a culture or bag, in which food can be stored for several months. One to four pairs of eyes are located at the anterior end. Individuals are hermaphroditic; that is, the functional reproductive organs of both sexes occur in the same individual. Leeches do not self-fertilize, but for an individual's sperm they fertilize only the eggs of other individuals. The eggs are laid in a cocoon, which can be laid on land or in water. Development and growth are direct, without a larval stage of free life.

Aquatic leeches can feed on the blood of fish, amphibians, birds and mammals or eat snails, insect larvae and worms. True earthworms feed only on mammalian blood. Three jaws set with sharp teeth make a Y-shaped incision in the flesh. Leech saliva contains substances that anesthetize the wound area, dilate blood vessels to increase blood flow and prevent blood clotting. The anticoagulant hududine, which is extracted from the body tissues of the European medicinal leech (Hirudo medicinalis), is used to prevent the formation of blood clots after surgery; another chemical isolated from Amazonian leeches is used to dissolve existing blood clots.

European medicinal leech (Hirudo medicinalis)After attaching its head sucker to the skin, the leech uses its three jaws with razor-sharp teeth to make a neat Y-shaped cut. Salivary ductules between the teeth secrete several pharmacologically active substances, including a local anesthetic and the potent anticoagulant hirudin.

European medicinal leech (Hirudo medicinalis). After attaching the suction cup to the skin, the leech uses its three jaws with sharp teeth to make a Y-shaped cut. The salivary ducts between the teeth secrete several pharmacologically active substances, including a local anesthetic and a strong hirudin anticoagulant. Encyclopedia Britannica, Inc.

Ground leeches are waiting for their victim in the damp vegetation, raising one end in the air. The victim is often ignored for being bitten until blood is discovered flowing from the wound; blood flow may continue due to the anticoagulant still present.
Many leeches that attack humans belong to the family Gnathobdellidae. Some species have been used for medical purposes for centuries; In Europe, the use of leeches to drain blood reached the height of its popularity in the nineteenth century. Diseases commonly treated with leeches included mental illness, tumors, skin diseases, gout and whooping cough. A common treatment for headaches was to apply several leeches to each temple and allow them to draw blood. Leeches have also been used to prevent excessive accumulation of blood following certain surgical procedures (for example, reassembling severed fingers).
In addition to H. medicinalis from Europe, the Algerian dragon (H. troctina) was used. Gnathobdella ferox has been commonly used in Asia. After H. medicinalis was introduced to North America, it settled there as a wild species. It grows up to 10 cm (4 inches) long and is green with four to six brown stripes.

Other terrestrial leeches that attack humans are mainly of the genus Haemadipsa from Asia, the Philippines, the East Indies and Madagascar. Philaemon leeches are parasitic on people in Australia.
Aquatic leeches, especially Limnatis nilotica, can enter the body in drinking water. Some can enter the excretory openings of people bathing in infested waters. L. nilotica, which lives in lakes and streams in southern Europe, North Africa and the Middle East, reaches lengths of up to 12 cm (4.75 inches), but younger, smaller specimens are more likely to enter body. When ingested with drinking water, they can first attach to the linings of the nose or throat and then be inhaled into the lungs.

Tyrannobdella rex, a member of the Praobdellidae family and native to the remote Amazon River in Peru, seems to prefer the mucous membranes found in mammalian nasal cavities. This leeches look for their victims while bathing, making an attachment with the help of a single jaw made of eight large teeth.

A person infected with many such leeches may suffer from anemia resulting from blood loss. Secondary infection is more likely to occur in external wounds than anemia. Leeches can cause suffocation and death of the host by blocking the airways; in Asia, in particular, domestic animals usually die.

This article was most recently reviewed and updated by John P. Rafferty, editor.