What is Platyhelminthes and what are the defining characteristics of this phylum?

The simplest animals that are symmetrical and triploblastic (composed of three fundamental cell layers) are the Platyhelminthes, the flatworms. Flatworms have no body cavity other than the gut, and they also lack an anus; the same pharyngeal opening both takes in food and expels waste. Because of the lack of any other body cavity, in larger flatworms the gut is often very highly branched in order to transport food to all parts of the body. The lack of a cavity also constrains flatworms to be flat; they must breathe by diffusion, and no cell can be too far from the outside, making a flattened shape necessary.