Nematomorpha (sometimes called Gordiacea, and commonly known as horsehair worms or Gordian worms) are a phylum of parasitoid animals superficially morphologically similar to nematode worms, hence the name. They range in size in most species from 50 to 100 centimetres (20 to 39 in) long and can reach in extreme cases up to 2 metres, and 1 to 3 millimetres (0.039 to 0.12 in) 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 beetles, cockroaches, 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.
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.
In Spinochordodes tellinii, which has orthopterans as its vector, the infection acts on the insect's brain and causes it to seek water and drown itself, thus returning the nematomorph to water. They are also remarkably able to survive the predation of their host, being able to wriggle out of the predator that has eaten the host.
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.
The phylum is placed along with the Ecdysozoa clade of moulting organisms that include the Arthropoda. Fossilized worms have been reported from Early Cretaceous Burmese amber dated to 100–110 million years.
Relationships within the phylum are still somewhat unclear, but two classes are recognised. The five marine species of nematomorph are contained in Nectonematoida. Adults are planktonic and the larvae parasitise decapod crustaceans, especially crabs. They are characterized by a double row of natotory setae along each side of the body, dorsal and ventral longitudinal epidermal cords, a spacious and fluid-filled blastocoelom and singular gonads.
The approximately 320 remaining species are distributed into two families and seven genera within order Gordioida. Gordioidean adults are free-living in freshwater or semiterrestrial habitats and larvae parasitise insects, primarily orthopterans. Unlike nectonematiodeans, gordioideans lack lateral rows of setae, have a single, ventral epidermal cord and their blastocoels are filled with mesenchyme in young animals but become spacious in older individuals.
- Hanelt, B, F. Thomas, and A. Schmidt-Rhaesa (2005). "Biology of the phylum Nematomorpha". Advances in Parasitology 59: 244–305. doi:10.1016/S0065-308X(05)59004-3.
- Zhang, Z.-Q. (2011). "Animal biodiversity: An introduction to higher-level classification and taxonomic richness". Zootaxa 3148: 7–12.
- Poinar Jr., G (2008). "Global diversity of hairworms (Nematomorpha: Gordiaceae) in freshwater". Hydrobiologia 595 (1): 79–83. doi:10.1007/s10750-007-9112-3.
- Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.
- Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 307–308. ISBN 0-03-056747-5.
- 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. doi:10.1046/j.1420-9101.2002.00410.x. — 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.
- Schmidt-Rhaesa, Andreas (2002). "Two Dimensions of Biodiversity Research Exemplified by Nematomorpha and Gastrotricha". Integrative and Comparative Biology 42 (3): 633–640. doi:10.1093/icb/42.3.633. PMID 21708759.
- 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): 756. doi:10.1038/440756a. PMID 16598248.
- Malcolm S. Bryant, Robert D. Adlard & Lester R.G. Cannon 2006. Gordian Worms: Factsheet. Queensland Museum. 
- Poinar George, Ron Buckley (2006). "Nematode (Nematoda: Mermithidae) and hairworm (Nematomorpha: Chordodidae) parasites in Early Cretaceous amber". Journal of Invertebrate Pathology 93 (1): 36–41. doi:10.1016/j.jip.2006.04.006. PMID 16737709.
- Pechenik, 'Biology of the Invertebrates, 2010, pg 457.
- "Gordioidea". Integrated Taxonomic Information System.
- "Chordodidae". Integrated Taxonomic Information System.
- Pechenik, Jan A. (2010). "Four Phyla of Likely Nematode Relatives". Biology of the Invertebrates (6th International ed.). Singapore: Mc-Graw Hill Education (Asia). pp. 452–457. ISBN 978-0-07-127041-0.
Further reading 
- Baker GL, Capinera JL (1997). "Nematodes and nematomorphs as control agents of grasshoppers and locusts". Memoirs of the Entomological Society of Canada 171: 157–211.
- Hanelt B, Thomas F, Schmidt-Rhaesa A (2005). "Biology of the phylum Nematomorpha". Advances in Parasitology 59: 244–305. doi:10.1016/S0065-308X(05)59004-3.
- Poinar GO Jr (1991). "Nematoda and Nematomorpha". In Thorp JH, Covich AP. Ecology and Classification of North American Freshwater Invertebrates. San Diego, CA: Academic Press. pp. 249–283.
- Thorne G (1940). "The hairworm, Gordius robustus Leidy, as a parasite of the Mormon cricket, Anabrus simplex Haldeman". Journal of the Washington Academy of Science 30: 219–231.
- Nematomorpha Biodiversity Project website. Provides pictures, life cycle information and searchable databases.
- horsehair worms on the UF / IFAS Featured Creatures Web site
- Video of worm and beetle host in pet's water bowl (see bottom of page), submitted by R. Preston and collected by E. M. Preston from central California, USA.
- Videos of a cricket infected with a Gordian worm with the worm emerging to mate, as well as emerging from predators that have eaten the infected cricket. From Nature, April 2006.
- Nematomorph worm on YouTube — explains the proteome research done on this worm
- The cricket suicide (video)
- http://www.youtube.com/watch?v=4E5vUUtSWT4&feature=youtu.be -inside a spider