Jain Biology: 5. Phylum Nematoda (Nemathelminthes):

These are unsegmented round worms. There is a complete alimentary canal with a mouth and an anus; circulatory and respiratory systems are absent; nervous system is in thc form of a nerve ring; sexes are usually separate. Many are parasitic in animals and cause serious diseases. There are many types as: Ascaris, common intestinal parasite, 12 to 27 cms. in length; Guinea worm causes disease in men called 'naru'; is thread-like and is as long as 90 cms.