Nematomorpha
. 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 knot