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Leeches are segmented worms that belong to the phylum Annelida and comprise the subclass Hirudinea. Like the oligochaetes, such as earthworms, leeches share a clitellum and are hermaphrodites. Nevertheless, they differ from the oligochaetes in significant ways. For example, leeches do not have bristles and the external segmentation of their bodies does not correspond with the internal segmentation of their organs. Their bodies are much more solid as the spaces in their coelom are dense with connective tissues. They also have two suckers, one at each end. The majority of leeches live in freshwater environments, while some species can be found in terrestrial and marine environments, as well. The best-known leeches, such as the medicinal leech, ''Hirudo medicinalis'', are hematophagous, feeding on vertebrate blood and invertebrate hemolymph. Most leech species, however, are predatory, feeding primarily by swallowing other invertebrates. Almost 700 species of leeches are currently recognized, of which some 100 are marine, 90 terrestrial and the remainder freshwater taxa. Leeches, such as the ''Hirudo medicinalis'', have been historically used in medicine to remove blood from patients. The practice of leeching can be traced to ancient India and Greece, and continued well into the 18th and 19th centuries in both Europe and North America. In modern times, leeches are used medically in procedures such as the reattachment of body parts and reconstructive and plastic surgeries and, in Germany, treating osteoarthritis. ==Taxonomy and systematics== Leeches are presumed to have evolved from certain Oligochaeta, most of which feed on detritus. However, some species in the Lumbriculidae are predatory and have similar adaptations as found in leeches. As a consequence, the systematics and taxonomy of leeches is in need of review. While leeches form a clade, the remaining oligochaetes are not their sister taxon, but in a diverse paraphyletic group containing some lineages that are closely related to leeches, and others that are far more distant. There is some dispute as to whether Hirudinea should be a class itself, or a subclass of the Clitellata. The resolution mainly depends on the eventual fate of the oligochaetes, which as noted above, do not form a natural group as traditionally circumscribed. Another possibility would be to include the leeches in the taxon Oligochaeta, which would then be ranked as a class and contain most of the clitellates. The Branchiobdellida are leechlike clitellates that were formerly included in the Hirudinea, but are just really close relatives. The more primitive Acanthobdellidea are often included with the leeches, but some authors treat them as a separate clitellate group. True leeches of the infraclass Euhirudinea have both anterior and posterior suckers. They are divided into two groups: Arhynchobdellida and Rhynchobdellida * Rhynchobdellida are "jawless" leeches, armed with a muscular, straw-like proboscis puncturing organ in a retractable sheath. The Rhynchobdellae consist of two families: * * Glossiphoniidae are flattened leeches with poorly defined anterior suckers. * * Piscicolida have cylindrical bodies and usually well-marked, bell-shaped, anterior suckers. The Glossiphoniidae live in freshwater habitats; the Pisciolidae are found in seawater habitats. * Arhynchobdellida lack a proboscis and may or may not have jaws armed with teeth. Arhynchobellids are divided into two orders: * * Gnathobdela: This order of "jawed" leeches, armed with teeth, includes the quintessential leech: the European medical (bloodsucking) leech, ''Hirudo medicinalis''. It has a tripartite jaw filled with hundreds of tiny, sharp teeth. The incision mark left on the skin by the European medical leech is an inverted Y inside a circle. Its North American counterpart is ''Macrobdela decora'', a much less efficient medical leech.〔(【引用サイトリンク】title=freshwater leech )〕 Within this order, the family Hirudidae is characterized by aquatic leeches and the family Haemadipsidae by terrestrial leeches. In the latter are ''Haemadipsa sylvestris'', the Indian leech and ''Haemadipsa zeylanica'' (''yamabiru''), the Japanese mountain or land leech. * * Pharyngobdella: These so-called worm-leeches consist of freshwater or amphibious leeches that have lost the ability to penetrate a host's tissue and suck blood. They are carnivorous and equipped with a relatively large, toothless mouth to ingest worms or insect larvae, which are swallowed whole. The Pharyngobdella have six to eight pairs of eyes, as compared with five pairs in Gnathobdelliform leeches, and include three related families. The Erpobdellidae are some species from freshwater habitats. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「leech」の詳細全文を読む スポンサード リンク
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