Nematomorpha

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Nematomorpha

Kingdom:	Animalia
Phylum:	Nematomorpha Vejdovsky, 1886

Classes

Nectonematoida
Gordioidea

Nematomorpha (sometimes called Gordiacea, and commonly known as Horsehair worms or Gordian worms) are a phylum of parasitic animals which are morphologically and ecologically similar to nematode worms, hence the name. They are, on average, 1 meter long, and 1 to 3 millimetres in diameter. Horsehair worms can be discovered in damp areas such as watering troughs, streams, puddles, and cisterns. The adult worms are free living, but the larvae are parasitic on beetles, cockroaches, grasshoppers and crustaceans. About 320 species have been described.

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. Reproductively, they are dioecious, with the internal fertilization of eggs that are then laid in gelatinous strings.

Nematomorphs can be confused with nematodes, particularly Mermithid worms. Unlike Nematomorphs, Mermithids do not have a terminal cloaca. 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.^[1]

Image:Spinochordodes in Meconema.jpg

Spinochordodes tellinii and its grasshopper host

In Spinochordodes tellinii, which has grasshoppers as its vector, the infection acts on the grasshopper's brain and causes it to seek water and drown itself, thus returning the nematomorph to water.^[2] They are also remarkably able to survive the predation of their host, being able to wriggle out of the predator which has eaten the host cricket.^[3]

Relationships within the phylum are still somewhat unclear, but two classes are recognised:

Class Nectonematoida: Marine, planktonic, with a double row of natotory setae along each side of the body; with dorsal and ventral longitudinal epidermal cords, blastocoelom spacious and fluid filled; gonads single; larvae parasitise decapod crustaceans
Class Gordioidea: Freshwater and semiterrestrial; lack lateral rows of setae; with a single, ventral epidermal cord; blastocoelom filled with mesenchyme in young animals but becomes spacious in older individuals; larvae parasitise grasshoppers and crickets

References

^ Malcolm S. Bryant, Robert D. Adlard & Lester R.G. Cannon 2006. Gordian Worms: Factsheet. Queensland Museum. [1]
^ Thomas, F.; Schmidt-Rhaesa, A., Martin, G., Manu, C., Durand, P., and Renaud, F. (May 2002). Do hairworms (Nematomorpha) manipulate the water seeking behaviour of their terrestrial hosts?. J. Evol. Biol. 15 (3): 356–361. ISSN 1010-061X. — 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.
^ Ponton, Fleur; Camille Lebarbenchon, Thierry Lefèvre, David G. Biron, David Duneau, David P. Hughes, and Frédéric Thomas (April 2006). Parasitology: Parasite survives predation on its host. Nature 440 (7085). doi:10.1038/440756a.