The nematodes /ˈnɛmətoʊdz/ or roundworms comprise the phylum Nematoda. They are a diverse animal phylum inhabiting a very broad range of environments. Nematode species can be difficult to distinguish; and although over 28,000 have been described,[2] of which over 16,000 are parasitic, the total number of nematode species has been estimated to be about 1 million.[3] Unlike cnidarians andflatworms, nematodes have tubular digestive systems with openings at both ends.