How does diffusion supply oxygen to a flatworm’s body tissues?

Flatworms do not possess a specialized respiratory system; instead, they rely on diffusion to meet their oxygen requirements. Because these organisms have a flattened body shape, the distance between their external environment and their internal tissues is kept small, which allows oxygen to move directly across the body wall and into the cells.12

This process is effective because the flatworm's body structure ensures that its metabolizing cells are located close enough to the surface for gases to be exchanged efficiently without the need for complex respiratory organs or an internal circulatory system.213