; phylum Aschelminthes)
A class of worms (a phylum in some classifications) which vary greatly in size up to about 5 cm, although most are microscopically small. The whole body is covered by a tough cuticle that has flanges, and may also have ridges or spines. Nematodes are spindle-shaped (the nearly perfect cylindrical shape is a diagnostic feature), have bilateral symmetry, with strikingly radial or biradial arrangements of structures around the mouth (also a diagnostic feature), and are unsegmented. They lack true cilia, have a pseudocoelom, and the head is not distinct from the rest of the body. There is a very large number of species, morphologically all similar, occurring as parasites in plants and animals, and as free-living forms. They are first known from rocks of Carboniferous age.
Subjects: Zoology and Animal Sciences.