As in it the digestive system is either absent or not complete, hence it allows it's host to digest the food n then this digested food is been assimilated by the worm via the porous outer skin layers
parasitic platyhelminthes have succkers and hook for attachment.Digestive system in them is generally absent.Thus they obtain nourishment through their skin.But in some if digestive system is present it is incomplete i.e. with only one opening i.e.mouth