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A class of the annelid worms commonly known as leeches. These organisms are parasitic or predatory and have terminal suckers for attachment and locomotion. Most inhabit inland waters, but some are marine and a few live on land in damp places. The majority feed by sucking the blood of other animals, including humans.

Leeches differ from other annelids in having the number of segments in the body fixed at 34, chaetae or bristles lacking, and the coelomic space between the gut and the body wall filled with packing tissue (see illustration). In a typical leech the first six segments of the body are modified to form a head, bearing eyes, and a sucker, and the last seven segments are incorporated into a posterior sucker.

General structure of a leechenlarge picture
General structure of a leech

The mouth of a leech opens within the anterior sucker, and there are two main methods of piercing the skin of the host to obtain blood: an eversible proboscis or three jaws, each shaped like half a circular saw, placed just inside the mouth. The process of digestion is very slow, and a meal may last a leech for 9 months. The carnivorous forms have lost most or all of their gut diverticula and resemble earthworms in having a straight, tubular gut. Leeches are hermaphroditic, having a single pair of ovaries and several pairs of testes.

The importance of leeches as a means of making incisions for the letting of blood or the relief of inflammation is declining, and in developed countries the bloodsucking parasites of mammals are declining, because of lack of opportunity for contact with the hosts. In other countries they are still serious pests.

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



(leeches), a class of annelid worms. Leeches are mostly black, yellowish brown, or greenish. Their flattened or, less frequently, cylindrical body consists of a prostomium and 33 body segments. The cutaneous coverings of the segments are divided into three to five (or more) annuli. There are two suckers: one anterior and one posterior. Leeches range in length from 0.2 to 15 cm. Most species have from one to five pairs of eyes. The anterior and posterior ganglia of the ventral nerve cord merge in the subpharyngeal and posterior nerve masses owing to the formation of suckers. The musculature is well developed. The secondary body cavity, or coelom, is reduced, and the spaces between organs are filled with mesenchyme. Most of the blood vessels are formed from vestiges of the coelom that have acquired muscular walls. Respiration is cutaneous. Some species have respiratory bladders or branchial gills.

The majority of leeches suck the blood and body juices of various vertebrates and invertebrates. Those that are not bloodsuckers are predators that swallow their prey whole. The salivary glands of bloodsucking species excrete hirudin, a substance that hinders clotting of the blood. Some leeches have a proboscis in the anterior part of the digestive tract. Others have toothed jaws to enable them to destroy the cutaneous coverings of their prey. The jaws of predatory leeches are for the most part reduced. Reproduction is sexual, and all leeches are hermaphroditic. The fertilized eggs are deposited in cocoons.

Most leeches live in fresh waters, where they are often numerous and play an important role in the breakdown of matter. The remaining species live in the sea or in damp soil. Some fishes eat leeches, as do desmans. Several leeches, such as the medicinal leech (Hirudo medicinalis), are used in the treatment of several diseases. Fish and bird leeches may cause mass destruction of their hosts. The horseleech (Haemopis), which is found in the USSR in Transcaucasia and Middle Asia, enters the throat of horses with drinking water. The leeches may lodge in the animal’s throat and cause suffocation.

There are two subclasses of leeches: old leeches and present-day leeches. The first subclass includes two species of the genus Acanthobdella, which have preserved a number of features of oligochaete worms—the ancestors of leeches. The present-day leeches comprise the order Rhyncobdellae, which includes the families Glossiphoniidae and Ichtyobdellidae, and the order Gnathobdellida, which includes the families of jawed leeches and pharyngeal leeches. Of the approximately 400 species of leeches (according to other data, 300), about 60 are found in the USSR.


Rukovodstvo po zoologii, vol. 2. Moscow-Leningrad, 1940.
Zhizn’ presnykh vod SSSR,[vol.] 2. Moscow-Leningrad, 1949.
Zhizn’ zhivotnykh, vol. 1. Moscow, 1968.
Lukin, E. I. Piiavky. Kiev, 1962. (Fauna Ukrainy, vol. 30.)
Mann, K. H. Leeches (Hirudinea). Oxford, 1962.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


(invertebrate zoology)
A class of parasitic or predatory annelid worms commonly known as leeches; all have 34 body segments and terminal suckers for attachment and locomotion.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
According to Franzen's observations (1974 and 1977) annelides spermatozoa are of the primitive type en Polychaetes and Archiannelides and of the modified type in Oligochaetes and Hirudinea, which is correlated with fertilization modalities.
AF315053 Lamellibrachia barhami AF168742 AF315043 AF315044 AF315045 AF315047 Oasisia alvinae AF168743 AF315052 Ridgeia piscesae AF168744 AF315048 AF315051 AF315054 Ridgeia piscesae GB X79877 b Riftia pachyptila AF168745 AF315049 AF315050 Tevnia jerichonana AF168746 AF315042 Monilifera Sclerolinum brattstromi AF315061 AF315046 Annelida Alvinellidae Paralvinella palmiformis AF168747 Chaetopterida Chaetopterus variopedatus U67324 c Hirudinea Haemopis sanguisuga X91401 d Hirudo medicinalis AF315058 Oligochaete Enchytraeus sp.
One group consisted of organisms living on macrophytes, branches or on ot her substrates (Hirudinea, Ephemeroptera, Trichoptera, Odonata, Coleoptera, Megaloptera), and which are relatively sensitive to fish predation.
All organisms were identified to genus, except the leeches (Hirudinea), earthworms (Oligochaeta), and water mites (Acari).
Occurrence and relative abundance of parasitic leeches (Hirudinea) on aquatic turtles in a wetland environment in Tennessee.
TABLE 1.--Total number of prey delivered by male and female Eastern Screech-Owls identified to the lowest taxonomic category possible and arranged in order of abundance Prey N Insects Coleoptera (N = 166 larval individuals) 490 Orthoptera 220 Lepidoptera 64 Odonata 10 Cicadidae 3 Tettigoniidae 1 Unidentified insects 44 Crayfish Family Astacidae 81 Amphibians Family Ranidae 23 Leeches Subclass Hirudinea 20 Mammals Peromyscus spp.
La abundancia de ciertos taxa, tales como Andesiops peruvianus, Massartellopsis (Ephemeroptera), Austrelmis (larva y adulto, Elmidae, Coleoptera), Hydroptilidae (Coleoptera), Simuliidae (Diptera), Hydrobiosidae y Odontoceridae (Trichoptera) y Glossiphonidae (Hirudinea), se incrementaron hacia el lado negativo del eje 1, mientras que Limnophora y Orthocladiinae (Diptera) incrementaron su abundancia hacia el lado positivo del eje 1 (Cuadro 4).
In order to simplify analysis, the following dietary categories were used: molluscs (Mollusca), leeches (Hirudinea), zooplankton, crustaceans (Crustacea, represented solely by waterlouse Asellus aquaticus), mayflies (Ephemeroptera), caddisflies (Trichoptera), chironomid larvae and pupae (Chironomidae), dipterans (Diptera, except Chironomidae), terrestrial insects, other (Odonata, Heteroptera, Megaloptera, Coleoptera), fish eggs, fish fry and detritus (detritus, sand and macrovegetation residue; not included in analysis).
To that end, Eldridge (1992) identified four functional groups of aquatic invertebrates: (1) passive dispersers such as leeches (Hirudinea), amphipods (Amphipoda), isopods (Isopoda), and gastropods (Gastropoda); (2) those that can withstand drought and freezing such as some beetles (Coleoptera) and true flies (Diptera); (3) those that lay eggs in moist-soils of drying wetlands during summer such as some dragonflies and damselflies (Odonata) and true flies; and (4) those that leave shallow, ephemeral wetlands to winter in larger, more-stable aquatic systems such as some true bugs (Hemiptera) and some beetles.