In platyhelminths, the differentiation of the body is of higher order than in case of coelenterates. In coelenterates, the body comprises of a hollow cavity in the centre which is not differentiated further. However, in case of platyhelminths, the body is differentiated into specialised parts. They have an outer covering called as ectoderm, an inner lining of the central cavity calles as endoderm, and another specialied layer of cells in between the ectoderm and the endoderm called as mesoderm. 

The mesoderm layer alows tissues to be organised into organs, which forms various organ systems in platyhelminths, like the reproductive system, the digestive system, nervous system, etc.

This is why platyhelminths are considered to be better evolved than coelentrates.