What organisms has the most complex digestive system. jellyfish pigeon sponge flatworm?

The digestive system of a flatworm is the most complex of the organisms listed. Flatworms have a gastrovascular cavity, which is a single opening that serves as both a mouth and an anus. The gastrovascular cavity is lined with gastrodermis, which is a specialized tissue that secretes digestive enzymes. The food is broken down in the gastrovascular cavity and then absorbed into the body.