Platyhelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates. They have no body cavity or specialized circulatory or respiratory system. As a result, they are flat (hence the name flatworms) to maximize the skin surface area avaible for oxygen diffusion. Over half of known flatworm species are parasitic.

​

Common flatworm species include flukes and tapeworms.

​

They take in oxygen but have no formal respiratory system. This is because they are very simple animals. They went through such small evolution that they never developed a complex respiratory system. They breathe through their skin which they have to keep moist which is why most live in water or watery environments (Platyhelminthes).

​

The phylum Platyhelminthes is more evolved in terms of their respiratory system when compared to the phylum Porifera, as they have evolved to maximize their surface area. Flatworms usually have longer, curved bodies, which allows them to respire more efficiently in general when compared to the phylum Porifera. Once again we see a great example of how structure determines function with the respiratory systems of flatworms.