During a Zoology practical, flatworms lack lungs and blood vessels yet survive because most cells are close to surfaces or a branched gut. Select the most accurate identification.

Correct Answer: D

diffusion suits small or flattened animals

Diffusion can meet gas exchange needs in small, thin, or flattened animals. As body size and thickness increase, specialized circulatory and respiratory systems become more important. This MCQ belongs to the Comparative anatomy topic in Animal Diversity-I and is written to test concept understanding rather than memorized wording.