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Gastrotrich
The gastrotrichs (phylum Gastrotricha), commonly referred to as hairybacks, are a group of microscopic (0.06-3.0 mm), worm-like, pseudocoelomate animals, and are widely distributed and abundant in freshwater and marine environments. They are mostly benthic and live within the periphyton, the layer of tiny organisms and detritus that is found on the seabed and the beds of other water bodies. The majority live on and between particles of sediment or on other submerged surfaces, but a few species are terrestrial and live on land in the film of water surrounding grains of soil. Gastrotrichs are divided into two orders, the Macrodasyida which are marine (except for two species), and the Chaetonotida, some of which are marine and some freshwater. Nearly eight hundred species of gastrotrich have been described.
Gastrotrichs have a simple body plan with a head region, with a brain and sensory organs, and a trunk with a simple gut and the reproductive organs. They have adhesive glands with which they can anchor themselves to the substrate and cilia with which they move around. They feed on detritus, sucking up organic particles with their muscular pharynx. They are hermaphrodites, the marine species producing eggs which develop directly into miniature adults. The freshwater species are parthenogenetic, producing unfertilised eggs, and at least one species is viviparous. Gastrotrichs mature with great rapidity and have lifespans of only a few days.
==Etymology and taxonomy==
The name "gastrotrich" comes from the Greek γαστήρ ''gaster'', meaning "stomach", and θρίξ ''thrix'', meaning "hair".〔(【引用サイトリンク】title=Gastrotrich )〕 The name was coined by the Russian zoologist Élie Metchnikoff in 1865.〔 The common name "hairyback" apparently arises from a mistranslation of "gastrotrich".
The relationship of gastrotrichs to other phyla is unclear. Morphology suggests that they are close to the Gnathostomulida, the Rotifera, or the Nematoda. On the other hand, genetic studies place them as close relatives of the Platyhelminthes, the Ecdysozoa or the Lophotrochozoa.〔 As of 2011, around 790 species have been described. The phylum contains a single class, divided into two orders: the Macrodasyida and the Chaetonotida. Edward Ruppert ''et al.'' report that the Macrodasyida are wholly marine,〔 but two rare and poorly known species, ''Marinellina flagellata'' and ''Redudasys fornerise'', are known from fresh water. The Chaetonotida comprises both marine and freshwater species.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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