1. Locomotor activity has been continuously recorded by aktograph in Carcinus maenas (L.), using (a) ‘dock crabs’ from non-tidal docks, and (b) ‘shore crabs’ from the intertidal zone.

2. Dock crabs, kept in continuous light, either dim or bright, show a temperature-independent rhythm. During the first 24 hr. large peaks occur after dusk and just before dawn, with smaller peaks during the day. Each daytime peak occurs about 12-13 hr. later than a night peak. Subsequently all the peaks drift, occurring about 50 mm. later each day.

3. Dock crabs kept in a day/night light regime do not show this drift.

4. Shore crabs, after 4-8 weeks in non-tidal aquaria, behave in the same way as dock crabs.

5. It is suggested that the drifting of the peaks represents an expression of endogenous tidal rhythmicity and a ‘multiple clock’ hypothesis is postulated to account for the control of rhythrnicity.

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