The flatworms, or Platyhelminthes, Plathelminthes, or platyhelminths are a phylum of relatively simple unsegmented, soft-bodied invertebrates
Flatworms 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.