Nematode
The nematodes ( /ˈnɛmətoʊdz/) or roundworms are traditionally regarded as the phylum Nematoda or Nemathelminthes. As such, they would be the most diverse phylum of pseudocoelomates, and one of the most diverse of all animal phyla, but discussion is in progress to determine whether the phylum is to be split or not.
Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. 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.
Read more about Nematode: Habitats, Taxonomy and Systematics, Anatomy, Reproduction, Free-living Species, Parasitic Species, Epidemiology