Although some species occur in fresh and/or brackish water, most species of marine planarian have been collected from a marine habitat. The animals usually live on that part of the shore which is subjected to tidal action. Here the specimens may be found in coarse sand, under stones or other objects, or on algae. They may occur also in rock pools and salt marshes. However, marine triclads are not confined to the shore region because some of them have been collected from considerable depths.
Marine triclads have been reported from all continents, including Antarctica. Furthermore, species have been found on various islands in the Pacific and Atlantic Oceans. It is without doubt that many more species remain to be discovered, particularly from areas at tropical latitudes, where much less sampling has been done than on European and North American coasts.