How do flatworms move using circular, diagonal, and longitudinal muscles?

Flatworms (phylum Platyhelminthes) possess a complex muscular system located beneath the epidermis that allows them to alter their body shape and move effectively. This system consists of three primary layers of muscle fibers: an outer layer of circular muscles, a middle layer of diagonal fibers, and a deeper layer of longitudinal muscles.1

These muscle sets work in various combinations to make the flatworm's body long and thin, short and fat, or to bend it to either side. In addition to these layers, flatworms have dorsoventral muscle fibers that run between the upper and lower surfaces of their flattened bodies.1

For larger flatworms, these muscles are used to pass waves of muscular contraction along the body, which enables them to crawl in a snail-like fashion. While smaller flatworms often rely on ciliary activity for movement, the muscular contractions become necessary for the propulsion of larger species.12