Any of several slender, cylindrical worms of the group Nematoda, which some scientists consider to be a class of the aschelminths and others to be a separate phylum. Most nematodes are tiny and live in enormous numbers in water, soil, plants, and animals. They have a simple structure, with a long hollow gut separated from the body wall by a fluid-filled space. Several nematodes, such as pinworm, roundworm, filaria, and hookworm, are parasites on animals and humans and cause disease. One species, Caenorhabditis elegans (usually called C. elegans ), was one of the first animals to have its entire genome sequenced and is important in biological research as a model organism.