Platyhelminthes
Platyhelminthes are better known as Flatworms. Flatworms are symmetrical and are made up of three layers of cells.
Other than a gut, these organisms have no cavities. Because of this features, these organisms must depend on diffusion, a process of the gradual spreading out of matter, which makes it so they must retain their flat shape.
Flatworms are internal parasites, or organisms that survive by feeding on matter inside another organism's organs.
It is believed that most flatworms descend from the same ancestors as many mollusks and earthworms, while the rest seemed to evolve from the more basic roots of animals.
Flukes, planarians, and marine flatworms are all Platyhelminthes.