Introduction to the Nematodes
These helminths are also refered to as roundworms. They have very elongated cylindrical and unsegmented bodies, covered in a tough inert cuticle, which may, depending on the species, have striations, lateral spines, terminal expansions or other modifications. Internally the cuticle is formed from an underlying hypodermis, (which may either be a syncytium or made up of cells),which has four longitudinal thickenings, the two lateral thickenings containing excretory canals. Over this is a muscular layer, lining the fluid filled pseudocoelomate body cavity (i.e.. not lined with an epithelium as in a true coelom), within which the internal organs are held. In almost all cases there are separate sexes, sexual reproduction occurring in the definitive host in parasitic species.
Note: The classification and taxonomy of nematodes, as has been pointed out, is under continual discussion, and is thus rather confusing. For example, many older systems do not have a phylum Nematoda, the nematodes being grouped in the class Nematoda, part of a much larger grouping, the phylum Nemathelminthes, and including the Acanthocephala, (now also grouped in their own phyla).