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Nematomorpha (sometimes called Gordiacea, Nematomorpha commonly known as horsehair worms or Gordian worms) are a phylum of parasitoid animals superficially similar to nematode worms in morphology, hence the name. They range in size in most species from long and can reach in extreme cases up to 2 metres, and in diameter. Horsehair worms can be discovered in damp areas such as watering troughs, swimming pools, streams, puddles, and cisterns. The adult worms are free living, but the larvae are parasitic on arthropods, such as beetles, cockroaches, mantids, orthopterans, and crustaceans. About 351 freshwater species are known and a conservative estimate suggests that there may be about 2000 freshwater species worldwide. The name "Gordian" stems from the legendary Gordian knot. This relates to the fact that nematomorpha often tie themselves in knots.〔Piper, Ross (2007), ''Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals'', Greenwood Press.〕 ==Description and biology== Nematomorphs possess an external cuticle without cilia. Internally, they have only longitudinal muscle and a non-functional gut, with no excretory, respiratory or circulatory systems. The nervous system consists of a nerve ring near the anterior end of the animal, and a ventral nerve cord running along the body. Reproductively, they are dioecious, with the internal fertilization of eggs that are then laid in gelatinous strings. Adults have cylindrical gonads, opening into the cloaca. The larvae have rings of cuticular hooks and terminal stylets that are believed to be used to enter the hosts. Once inside the host, the larvae live inside the haemocoel and absorb nutrients directly through their skin. Development into the adult form takes weeks or months, and the larva moults several times as it grows in size.〔 The adults are mostly free living in freshwater or marine environments, and males and females aggregate into tight balls (''Gordian knots'') during mating.〔 In ''Spinochordodes tellinii'' and ''Paragordius tricuspidatus'' which have orthopterans as their vector, the infection acts on the insect's brain and causes it to seek water and drown itself, thus returning the nematomorph to water.〔 — according to Thomas ''et al.'', the "infected insects may first display an erratic behaviour which brings them sooner or later close to a stream and then a behavioural change that makes them enter the water", rather than seeking out water over long distances.〕 ''P. tricuspidatus'' is also remarkably able to survive the predation of their host, being able to wiggle out of the predator that has eaten the host. 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「Nematomorpha」の詳細全文を読む スポンサード リンク
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