Common house cat infected with parasite
Nematodes on fig roots.
In the stomach of hedgehog.
The nematodes /ˈnɛmətoʊdz/ or roundworms constitute 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, 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.