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(Nematomorpha, Gordiacea), a class of invertebrates of the subphylum Nemathelminthes. The body is hairlike (the folk name is “live hair”). Coloring ranges from white to light brown and almost black. Adults are from several cm to 1.5 m in length, with a thickness of 0.5-2 mm. Adults live in fresh water or in seas (Nectonema). They lay millions of tiny eggs shaped like long milky-white cords.
The tiny nematomorph larvae parasitize in two hosts. In the body of the first (most often in the muscles of the larvae of moths, dragonflies, or mayflies) the larvae become encysted and remain there until they are swallowed by the second and final host (most frequently large arthropods). They develop to the sexually mature state in the body cavity of the second host. The full development cycle takes about 18 months. The class comprises 12 genera; six are found in the USSR, and comprise 17 species, including Chordodes longipilus and Gordius aquaticus. Horsehair worms are found as pseudoparasites in humans and domestic animals. There was formerly a folk superstition that these worms became embedded in the skin during bathing.