|Eophasma jurasicum, a fossilized nematode|
Nematode species look very similar to one another. 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 structure is fairly constant. A thick cuticle gives protection, and acts as a kind of hydrostatic skeleton. 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.
Cell numbers in the organs are constant within a species. Nematodes have a fixed, genetically determined number of cells, a phenomenon known as eutely. The male C. elegans has 1031 cells, a number which does not change after cell division stops at the end of the larval period. Growth is solely due to an increase in the size of individual cells.