Marine species vary from 83 um to about 50 mm in length. The animal parasites range from about 1 mm to 6 or 7 meters in length, the largest being a nematode parasitic in whales. Plant parasites range from 0.25 mm to approximately 12 mm in length.
Nematodes are elongate, cylindrical, unsegmented worms.
They are bilaterally symmetrical. In males the digestive and the reproductive systems join posteriorly to form a cloaca which opens on the ventral side near the terminal end. The cuticle is predominately protein with traces of lipids and carbohydrates. About 17 to 19 amino acids have been identified. The primary function of the cuticle is to provide a certain rigidity and to protect the nematode from harmful materials in the environment.