The flatness of these worms enables them to move wastes and gasses quickly through the cells. Roundworms Roundworms have tissues that are organized into organs, also a body cavity that makes the worm rigid. They have a digestive tract with 2 openings. Nematodes are the most abundant animals on the Earth.
Some common nematodes are brainworms, pinworms, hookworms, and filarial worms.