Flatworms (Platyhelminthes) are phylum of unsegmented, soft, flat invertebrates.
Flatworms have no body cavity which is why they are flat. The advantage of a flat body is facilitated respiration and circulation. Flatworms do not have respiration and circulation system. But, oxygen diffuses across the skin and reach all parts of the body thanks to a flattened shape. Also, it is easy to hide from predators because they can get almost everywhere thanks to the body shape.