Platyhelminthes are a simple phylum made up of invertebrate, unsegmented, soft-bodied organisms
Flat worms are soft-bodied invertebrate animals from the platyhelminthes phylum. 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.
Tapeworms are parasitic flatworms that live in human and animal digestive tracks. They are also very primitive evolutionary wise like flatworms and do not have a respiratory system. They also take in oxygen from the skin.
Trematoda are flattned oval or worm-like animals. They are part of the platyhelminthes phylum. They have no respiratory organs and also obtain oxygen through gas exchange. They are also called flukes