1. Litorina rudis, palliata and litorea are found in definite zones. 2. Individuals of Litorina litorea, located on vertical surfaces between the tidal marks, exhibit oscillatory movements which correspond to those of the tides but they do not exhibit such movements when they are located either on flat horizontal surfaces between the tidal marks or on any kind of a surface below the low-tide mark. 3. The primary directive force for rhythmical movements is the surface film of water. The secondary directive forces are the quiescent position of desiccated individuals, character of the surfaces, moisture and food. 4. Litorina litorea has no established rhythm in the absence of tidal changes.