The "roundworms" or "nematodes" (phylum Nematoda) are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 80,000 have been described, of which over 15,000 are parasitic. It has been estimated that the total number of described and undescribed roundworms might be more than 500,000. Unlike cnidarians or flatworms, roundworms have a digestive system that is like a tube at both ends.
