Nematoda- The nematoda are the most diverse of the phylum. They are also some of the most diverse of all animals. They have about 1,000,000 different species. The nematode species are very tough and difficult to distinguish. They have digestive systems with openings at both ends unlike the cnidaria. Many nematodes excrete salt through their underside.
Secernentea- These are the main clases of nematodes. This type of species is in the Animalia kingdom. It is characterized by many caudal papillae and an excretory system that posseses many lateral canals. They have no circulatory or respiratory system. This is true in all nematodes. the secerntea contai several parasites. Some of those are the Rhabditia and Spiruria.
Enoplea- These are a class of round worms. Most are free-living worms. At the beginning they were created as a subclass. There was then a dismantling of a artificial group which gave them their class of their own. They contain two distinct lineages. They are sometimes reinstated as subclasses dorylaimia and enoplia. They are also apart of the Animalia kingdom. But. some believe that they are as distant from each other as from the chromodorea.
Chromadorea- These are a class of round worms. They contain a single subclass and several orders. Their subclass is chromadoria. the chromadoria are able to split up if the orders are found to form several different clades. Many suspect that the chromadorea may not be monophyletic. The members of this classes body usually has annules. The amphids on their bodies usually spiral and elaborate. Also each of them have three esophageal glands. Finally, they usually are found living in marine sediments.