Flatworms (Phylum Platyhelminthes) are a primitive, albeit successful, soft-bodied invertebrate. They do not have an internal body cavity or internal organs, and only a very simple digestive cavity, meaning they must take in oxygen and nutrients by diffusion. Most flatworms are parasitic, but those that aren't are usually predators or scavengers. Marine flatworms are found in a variety of forms and colours, often spectacular.