The Polycladida represents a highly diverse clade of free-living marine turbellarian flatworms. They are known from the littoral to the sublittoral zone (extending to the deep hot vents), and many species are common from coral reefs. Only a few species are found in freshwater habitats. Often polyclads are associated with other invertebrates, such as bivalve mollusks, sponges, corals, or ascidians.