A learner makes a note: a flatworm relies largely on diffusion because its thin body brings cells close to the environment or gut. The missing scientific idea is:

Flatworms lack a specialized circulatory system. Their flattened body shape and branched gut help reduce diffusion distance for gases and nutrients. This MCQ belongs to the Platyhelminthes topic in Animal Diversity-I and is written to test concept understanding rather than memorized wording.