All are correct about acoelomates except:

Explanation

The correct answer is Option B: Well-developed respiratory system. Acoelomates, such as those in Phylum Platyhelminthes, lack a well-developed respiratory system and rely on diffusion through their body surface for gas exchange. Option A is incorrect because acoelomates have a sac-like digestive system. Option C is incorrect as the body cavity of acoelomates is filled with parenchyma. Option D is incorrect because their excretory system is simple, consisting of structures like protonephridia, rather than being well-developed.