Why is it logical that platyhelminthes have flattened bodies? why is it logical that platyhelminthes have flattened bodies? they lack a coelom, so their body cannot form a rounded tube-within-a-tube design. they are sit-and-wait predators that hide from passing prey by flattening themselves against the substrate. a flat body provides a large surface area for gas exchange, which compensates for their lack of gas-exchange organs. they have simple bodies and evolved early in the diversification of protostomes?