DIFFUSION IN MULTICELLULAR ORGANISMS
Many multicellular animals are small enough that they don't require a specialized gas exchange system. Even relatively large animals, such as flatworms (phylum Platyhelminthes), can have adaptations that eliminate the need for a complex respiratory (and circulatory) system. If flatworms were spherical, the center of the animal would have the same problem as a spherical cell—its volume couldn't be supplied with oxygen fast enough via the surface. But platyhelminths are very dorsoventrally flattened—"flatworms." This flattening makes them very thin and gives them a large ratio of surface area to volume. The flatworm's respiratory surface area is large enough to service its relatively low volume.