A preserved flatworm appears thin from top to bottom and lacks a body cavity. The best answer is:

Platyhelminthes are dorsoventrally flattened acoelomate worms. Their flattened body helps diffusion of gases and nutrients because they lack specialized circulatory and respiratory systems. This MCQ belongs to the Platyhelminthes topic in Animal Diversity-I and is written to test concept understanding rather than memorized wording.