|Unidentified roundworm from wet soil|
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.
The body strucure is fairly constant. A thick cuticle gives protection, and acts as a kind of hydrostatic skeleton. Cell numbers in the organs are often constant within a species. Caenorhabditis elegans has had its genetics studied intensively, and had its genome analysed in 1998. DNA sequence analysis of nematodes is quite advanced, including trees of species relationship.