A learner makes a note: a comparison between flatworms and roundworms emphasizes one-way movement of food. The missing scientific idea is:

A complete digestive tract has separate mouth and anus, allowing one-way flow and specialization of regions. This improves feeding efficiency compared with a single-opening gastrovascular cavity or incomplete gut. This MCQ belongs to the Comparative anatomy topic in Animal Diversity-I and is written to test concept understanding rather than memorized wording.