Nematomorpha

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Horsehair Worm, member of a group of long, slender, unsegmented worms that resemble horsehair. The name of their phylum, Nematomorpha, means “threadlike.” Long ago, people thought that these worms were actually hairs from a horse's tail that had come alive. The immature worms, or larvae, are all parasites, feeding within the bodies of leeches and arthropods such as insects and crustaceans. The adults are free-living—that is, they do not depend on a host.

Horsehair worms are 1 to 3 mm (0.04 to 0.12 in) in diameter and up to 1 m (3.3 ft) long. There are two classes of horsehair worms. The first, known as nectonemes, live within the bodies of crabs and their relatives. The adults are planktonic, drifting in the currents of the open ocean. The body has two rows of bristles down each side, which aid in buoyancy. Nectonemes have only one gonad, or sex organ. The second class of horsehair worms is made up of the gordian worms, so named because they appear to tie themselves in knots, much like the complex Gordian knot of Greek mythology. The larvae parasitize leeches and arthropods such as grasshoppers, crickets, beetles, millipedes, and centipedes. The adults occur worldwide in freshwater or moist terrestrial habitats. Gordian worms possess a pair of gonads.

The body of the adult horsehair worm is covered by a relatively thick outer cuticle, or skinlike covering. Underlying the cuticle is a layer of longitudinal muscle running the length of the body. The horsehair worm has no circular muscle, and thus its movement is limited to lashing and curling. The digestive system is simple, even in the free-living adults. There are no excretory, circulatory, or respiratory organs. The larvae probably take up nutrients directly through the body wall during the parasitic phase. Feeding has not been observed in the adults, and most have no mouth, but they grow considerably after they leave the host at the end of the larval stage. It is therefore likely that they do feed by some means. The nervous system consists of a nerve bundle at the head end and a nerve cord running down length of the body. Around the head of some species is a ring of pigmented tissue that may be light-sensitive, but horsehair worms have no eyes. The outer layer of the cuticle is often covered with bumps, some of which may detect touch and some of which produce a lubricant.

Scientific classification: Horsehair worms make up the phylum Nematomorpha. The nectonemes make up the class Nectonematoidea, which has only one genus, Nectonema. The gordians make up the class Gordioidea.

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