An order of marine Turbellaria which are several millimeters to several centimeters in length and whose leaflike bodies have a central intestine with radiating branches. Most of these platyhelminth species live in the littoral zone on the bottom, on seaweed or on other objects, or as commensals in the shells of mollusks and hermit crabs. None are parasitic. Except in warm waters, they are seldom brightly colored.