Leech

Leech, common name for carnivorous or bloodsucking worm once widely used by physicians and barbers for bloodletting and still used for this purpose in some regions of the world. In modern medicine, bloodletting is no longer practiced, but leeches continue to be used to relieve blood congestion in certain delicate operations, where such use is less likely to cause infection than other techniques. Leeches are widespread in marine waters and are found in fresh water and on land in temperate and tropical regions.

The animals are flattened ringed worms, measuring from 5 mm to 46 cm (0.2 to 18 in) in length and are equipped with a sucking disk at both the anterior and posterior ends. Typically, the body is made up of 33 segments; the external rings, which do not correspond to the internal segments, vary in number among different species. Each ring contains about five or six warty prominences, or papillae, which serve as sense organs. On the anterior end of the worm are several eyes. In some leeches, the anterior mouth contains three toothed plates with which the animal pierces the skin of its prey. Blood ingested by leeches is mixed with salivary juices containing an anticoagulant substance known as hirudin (which can be extracted and has been used in medicine to prevent blood clotting). The blood passes into a dilated, branched stomach, or crop, where it is stored for several months before being completely digested. A leech consumes about three times its weight in one feeding and then subsists for months on the stored food. Leeches are hermaphroditic, each specimen containing several pairs of testes and one pair of ovaries. Typical species lay their eggs in mucous cases known as cocoons; upon hatching, the young of some forms attach themselves to the underside of an adult and are carried about with it until they can live independently. Aquatic leeches can swim; both aquatic and terrestrial species move over solid surfaces by muscular expansion and contraction; some also move by attaching themselves with one sucker and then somersaulting onto the other sucker.

Scientific classification: Leeches make up the class Hirudinea of the phylum Annelida. Leeches are divided into four orders. Acanthobdellida is a primitive order parasitic on salmon. Gnathobdellida contains the typical leeches, with toothed plates in their anterior suckers and red blood in their circulatory systems. The order Rhynchobdellida is characterized by the replacement of the toothed plates with an anterior proboscis and by the absence of red coloring matter in the blood. It contains all the marine species and a few freshwater species. Members of the order Pharyngobdellida lack teeth, although one of two stylets may be present. They are both terrestrial and aquatic. The class is sometimes alternately divided into three subclasses.