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Oligochaeta is a class of animals in the phylum Annelida, which is made up of many types of aquatic and terrestrial worms, and including all of the various earthworms. Specifically, the oligochaetes contain the terrestrial megadrile earthworms (some of which are semiaquatic or fully aquatic), and freshwater or semiterrestrial microdrile forms, including the tubificids, pot worms and ice worms (Enchytraeidae), blackworms (Lumbriculidae) and several interstitial marine worms. With around 10,000 known species, the Oligochaeta make up about half of the phylum Annelida. These worms usually have few setae (chaetae) or "bristles" on their outer body surfaces, and lack parapodia, unlike polychaeta. ==Common characteristics== Oligochaetes are well-segmented worms and most have a spacious body cavity (coelom) used as a hydroskeleton. They range in length from less than up to in the 'giant' species such as the giant Gippsland earthworm and the Mekong worm ''Amynthas mekongianus'' (Cognetti, 1922).〔Blakemore, Robert J., Csaba Csuzdi, Masamichi T. Ito, Nobuhiro Kaneko, Maurizio G. Paoletti, Sergei E. Spiridonov, Tomoko Uchida & Beverley D. Van Praagh (2007). Megascolex (Promegascolex) mekongianus Cognetti, 1922: its extent, ecology and allocation to Amynthas (Oligochaeta: Megascolecidae). Opuscula Zoologica. 36: 19-30 (Aug. 2007) ().〕 The first segment, or prostomium, of oligochaetes is usually a smooth lobe or cone without sensory organs, although it is sometimes extended to form a tentacle. The remaining segments have no appendages, but they do have a small number of bristles, or chaetae. These tend to be longer in aquatic forms than in the burrowing earthworms, and can have a variety of shapes. Each segment has four bundles of chaetae, with two on the underside, and the others on the sides. The bundles can contain one to 25 chaetae, and include muscles to pull them in and out of the body. This enables the worm to gain a grip on the soil or mud as it burrows into the substrate. When burrowing, the body moves peristaltically, alternately contracting and stretching to push itself forward. A number of segments in the forward part of the body are modified by the presence of numerous secretory glands. Together, they form the clitellum, which is important in reproduction. 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「Oligochaeta」の詳細全文を読む スポンサード リンク
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