Question

why animals belonging to the phylum platyhelminthes have poorly developed digestive system

Answer 1

There are some flatworms (Platyhelminthes) which are parasitic means feed on their host for their food, one of them is Tapeworm. It is a intestinal parasite and its body is designed to absorb digested food from its host so it does not need well developed digestive system. But there are some flatworms like Planaria, which are free living and have well developed digestive system.