The nematodes or roundworms are in the phylum Nematoda. They are a diverse animal with a very broad range of environments. Nematode species can be difficult to distinguish, and although over 25,000 have been described of which more than half are parasitic or microscopic big , the total number of nematode species has been estimated to be about 1 million. Unlike cnidarians and flatworms, nematodes have tubular digestive systems with openings at both ends.