The
nematodes 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 25,000 have been described, of which more than half are
parasitic, the total number of nematode species has been estimated to be about 1 million. Unlike the phyla
Cnidarians and Platyhelminthes (
flatworms), nematodes have tubular
digestive systems with openings at both ends.