Any of more than 15,000 named and many more unnamed species of worms in the class Nematoda (phylum Aschelminthes). Nematodes include plant and animal parasites and free-living forms found in soil, freshwater, saltwater, and even vinegar and beer malts. They are bilaterally symmetrical and usually tapered at both ends. Some species have separate sexes; others are hermaphroditic. They range from microscopic to about 23 ft (7 m) long. Nematode parasites can occur in almost any body organ but are most common in the digestive, circulatory, or respiratory system. Hookworms, pinworms, and eelworms are nematodes. See alsofilarial worm, guinea worm, trichina.