The two phyla make up the group Nematoida in the clade Cycloneuralia. Their closest relatives are the nematodes. The phylum is placed along with the Ecdysozoa clade of moulting organisms that include the Arthropoda. Male mermithids have one or two spicules just before the end apart from having a thinner, smoother cuticle, without areoles and a paler brown colour. Unlike nematomorphs, mermithids do not have a terminal cloaca. Nematomorphs can be confused with nematodes, particularly mermithid worms. See also: List of bilaterial animal orders The adults are mostly free-living in freshwater or marine environments, and males and females aggregate into tight balls ( Gordian knots) during mating. Development into the adult form takes weeks or months, and the larva moults several times as it grows in size.
Once inside the host, the larvae live inside the haemocoel and absorb nutrients directly through their skin. The larvae have rings of cuticular hooks and terminal stylets that are believed to be used to enter the hosts. Adults have cylindrical gonads, opening into the cloaca.
Reproductively, they have two distinct sexes, with the internal fertilization of eggs that are then laid in gelatinous strings. The nervous system consists of a nerve ring near the anterior end of the animal, and a ventral nerve cord running along the body. Internally, they have only longitudinal muscle and a non-functional gut, with no excretory, respiratory or circulatory systems. Nematomorphs possess an external cuticle without cilia. This relates to the fact that nematomorphs often coil themselves in tight balls that resemble knots.
The name "Gordian" stems from the legendary Gordian knot. About 351 freshwater species are known and a conservative estimate suggests that there may be about 2000 freshwater species worldwide. The adult worms are free-living, but the larvae are parasitic on arthropods, such as beetles, cockroaches, mantises, orthopterans, and crustaceans. Horsehair worms can be discovered in damp areas, such as watering troughs, swimming pools, streams, puddles, and cisterns. Most species range in size from 50 to 100 millimetres (2.0 to 3.9 in) long, reaching 2 metres in extreme cases, and 1 to 3 millimetres (0.039 to 0.118 in) in diameter. Nematomorpha (sometimes called Gordiacea, and commonly known as horsehair worms, hairsnakes, or Gordian worms) are a phylum of parasitoid animals superficially similar to nematode worms in morphology, hence the name. Phylum of parasitoid animals, horsehair worms
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