Nematoda
Nematodes, who are most commonly referred to as roundworms, have the most diverse in a phylum of invertebrate animal with a three layered body. It's whole body structure separates it from others.. There have been over 28,000 described, 16,000 or more of which are parasites. Roundworms have tubular digestive systems with little openings at both sides. Unlike cnidarians or flatworms who have one opening. Their bodies are symmetrical, but their heads are not. They also only have one recognized class which is the secernentea.
Citation:http://en.wikipedia.org/wiki/Nematode
Citation:http://en.wikipedia.org/wiki/Nematode