The phylum Platyhelminthes comprises about 20,000 species. They are commonly known as flatworms and include free-living aquatic flatworms (class Turbellaria), as well as the mostly parasitic flukes and tapeworms (classes Monogenea, Trematoda, and Cestoda). Collectively, they are primitive organisms that are flat, soft-bodied, and symmetrical.
Recent molecular studies suggest that the Platyhelminthes as a whole may have arisen as two independent groups from different ancestral groups. If this is correct, most of the flatworms may belong to the Lophotrochozoa, a large group within the animal kingdom that includes molluscs and earthworms, while the rest are much more primitive.