The many thousands of species of nematodes in the phylum Nemata are a group of animals commonly placed into four categories: parasites of man and animals; parasites of plants; species living in marine and brackish water; and free-living soil and fresh-water species. No single character or criterion can be used to distinguish nematodes from other similar animals, although one feature-not structural–sets nematodes apart, and that is the tremendous variation in size. Nematodes vary in length from the 1/125" ectoparasites of plants to the 25' long parasite of whales.