The lined top shell, of the prosobranch species Phorcus lineatus, lives inside cracks in rocks and coral reefs in the seas of Europe. It often climbs out of the water to rest on man-made structures or rock surfaces. When in danger, Phorcus lineatus withdraws completely inside its shell and closes the entrance with its protective lid, or operculum. Unlike limpets, the one-toothed top shell is unable to attach itself to the substrate with enough force to prevent it from being washed away by the waves.