Platyhelminthes
Platyhelminthes are also known as flatworms. Flatworms have no body cavity other than the gut and they lack an anus. They have one pharyngeal organ which both takes in food and expels waste. Because of the lack of any other body cavity, in larger flatworms the gut is often very widely branched in order to transport food to all parts of the body. They must get fluids and oxygen through diffusion, and no cell can be too far from the outside, making a flattened shape necessary.