Abstract
Polyclad flatworms possess an intricately branched digestive system combining features of a gastrovascular cavity as well as a gastrointestinal tract. Nonetheless, the functions of this system remain unconfirmed, due to a lack of effective observation methods. This paper presents a novel staining method to facilitate the analysis of this highly branched digestive system. Video recordings obtained during ingestion revealed sequentially bidirectional gastrovascular flows and a corresponding occurrence of regular contractions. Tissue sections revealed that the contractions can be attributed to a radial arrangement of muscles around the gastrointestinal tract. The highly branched digestive system of the flatworm revealed evidence of bidirectional flow and sequential peristalsis, which may allow for a diet of greater diversity than is possible in animals with only a gastrovascular cavity. The proposed staining technique opens up new avenues for research on the digestive behavior of lower organisms.