The flatworms, or Platyhelminthes, are a phylum of relatively simple ,unsegmented soft-bodied invertebrates. Unlike other species like them, they are acoelomates (having no body cavity), and have no specialized circulatory and respiratory systems, which restricts them to having flattened shapes that allow oxygen and nutrients to pass through their bodies by diffusion. The digestive cavity has only one opening for both the ingestion (intake of nutrients) and egestion (removal of undigested wastes); as a result, the food cannot be processed continuously.