An almost perfectly cylindrical, unsegmented worm, its body covered in a tough cuticle (‘skin’) with flanges and in some species spines or ridges, and a head not clearly distinct from the body. Most eelworms are microscopically small, but some are up to 5 cm long. They comprise the phylum Nematoda and occur worldwide in nearly every type of environment. More than 25,000 species have been described, but the total number may exceed one million. About half are parasites.