Many flatworms are parasites which means they live inside or on another organism and they take food from their host. Planarians are scavengers so they feed on dead or decaying material and they are also predators. They feed like a vacuum cleaner. They glide onto its food and inserts a feeding tube into it. Digestive juices flow out of the planarian and into the food. The juices begin to break down the food while it is still outside the worm's body. Then the planarian sucks up the partly digested bits. Tapeworms absorb food from their host's digestive system. Roundworms have a digestive system that is like a tube, open at both ends. Food travels in one direction.