• PHYLUM 4. ROUNDWORMS, OR NEMATHELMINTHES.
These worms have long, cylindrical bodies with two-side symmetry. They are built on the tube-within-a-tube plan. This is, they have a mouth and anus at opposite ends of the digestive tube, and the tube is inside a body cavity, or coelom. Some species look rather like an earthworm. But roundworms are never segmented, while earthworms always are. A great many the roundworms are parasites of animals or plants. Some of these cause serious diseases of people, animals and crops. . There are about 80,000 species and roundworms