Nematomorpha [key], small (about 230 species) phylum of pseudocoelomates; the horsehair worms. Most are very slender, elongated creatures found in ponds and streams, whose larvae live as parasites in arthropods. They emerge as adults for a brief time, then mate and die. A small number are planktonic in marine habitats and live as larvae in crabs or shrimps. Adults are simplified externally and internally; they have no excretory or circulatory systems and only a vestigial digestive tract. The female produces long strings of eggs. After hatching, the larva penetrates any convenient aquatic animal, but its development stops until it has found its way into an appropriate host, typically an insect. The adult nematomorph emerges when the host is in or near water; it molts once after emerging and takes up its brief adult existence.
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