The digestive system is incomplete because it is not a tube with a mouth at one end and anus at the other. There is only one opening, so it is described as a sac-like system. Food enters the mouth and is digested in a cavity. Any non-digestable materials must be regurgitated back through the mouth.
Flatworms follow the pattern of digestion just like all other members in the platyhelmithes phylum. The food is entered through the mouth and the waste is also discharged through the mouth.
Tapeworms do not feed off humans, they feed off the food we digested. Tapeworms lack a digestive tract and absorb nutrients directly across the skin or cuticle.
Flukes diet is based of other animals blood. Its digestive system consists of a sac like stomach, which opening is a mouth that acts as a sucker that extract the blood for food.