Phylum: Platyhelminthes
Platyhelminthes is another name for flatworms. This phylum consists of are a phylum of relatively simple unsegmented, soft-bodied invertebrate animals. Animals in this phylum have no body cavity, no specialized circulatory and no respiratory organs. There are only 20,000 known species in this phylum.
Flatworms
Flatworms, like any other animal, take in oxygen and release carbon dioxide. But flatworm do not undergo cellular respiration. Instead they undergo diffusion to obtain the oxygen they need, and release the carbon dioxide they don't. This is mainly the reason for flatworms being flat. Every cell must respire, so no cell can be too far from their surface.
Tapeworms
Tapeworms are parasitic animals. They can not live on their own so they remain in the digestive system of their host. This does not allow them to breath oxygen then. Instead tapeworms use anaerobic respiration. This allows them to break down sugar into ethanol or lactic acid, but without oxygen. This is a less efficient process the aerobic respiration but tapeworms do not require much energy.
Flukes
Flukes is another name for trematoda. Flukes are another parasitic worm in the Platyhelminthes phylum, and have a flat oval body. Trematodas do not possess any repiratory organs. They obtain the oxygen they need through diffusion. Their small flat bodies allows all cells to obtain the oygen they need to repire because they're so close to the surface of their bodies. The small pores on their body's surface also allows any respiratory waste, like carbon dioxide, to excrete.