• PHYLUM 3. FLATWORMS, OR PLATYHELMINTHES.
These have three well-developed layers. They are flattened, back to belly. They have two-sided symmetry. In spite of having mesoderm(the third germ layer), they still have a saclike body cavity with a single opening. They are not segmented. The tapeworm with its jointed body is sometimes regarded as a single segmented animal, but probably this is a colony of animals. Nearly all flatworms are parasites that live in the bodies of other animals. One free-living kind is planaria, a gentle slip of a worm that biology teachers get out of ponds and show to their classes There about 5,000 species of known flatworms.