Roundworms, or nematodes, are members of the phylum Nematoda (from the Greek term nematos, meaning “thread”) and are numerous in two respects: 1) number of known and potential species; and 2) the total number of these organisms in a habitat. Approximately 12,000 species of nematodes have been named, but it has been estimated that if all species were known, the number would be closer to 500,000. Nematodes live in a variety of habitats ranging from the sea to soil. Six cubic inches (100 cubic centimeters) of soil may contain several thousand nematodes, a square yard (.85 square meters) of woodland or agricultural soil may contain several million of them, and good topsoil may contain billions per acre.