Marine triclads are unable to swim freely and need some sort of substrate in order to be able to move about. The animals are able to glide upside down, hanging to the surface film.
Marine planarians are capable of two types of locomotion, viz. the gliding movement, which is common in planarians, and the looping or caterpillar type of locomotion. During gliding the cyanophilous, subepidermal glands secrete the mucous over which the animals move. The propulsion results from (1) muscular waves that pass backwards from the front end of the body and which involve the longitudinal muscles of the ventral body surface, and (2) from ciliary motion.
The looping type of locomotion, in contrast to gliding, is characterized by conspicuous changes in the shape of the animal. It starts when the planarian attaches its hind end firmly to the substrate. Next, the front end is extended forwards through the action of circular muscles on the hydrostatic skeleton. Then the front end adheres to the substrate and the posterior body end is drawn up by means of the shortening of the longitudinal subepidermal muscles.
Marine triclads show a negative phototactic reaction to light, excepting the ectosymbiotic bdellourids which are positively phototactic.