ii] HABITS OF CONVOLUTA 63 



Some hours elapse, and, as the tide is running off 

 the roscoffensis zone, curiosity, or its after-effect, 

 provokes us to inspect the cup on the laboratory 

 table. Even as we look into it, a faint green colour 

 steals over the surface of the sand, and, in a minute 

 or two, it is almost black with a dark crowd of 

 C. roscoffensis. Now curiosity joins with astonish- 

 ment to beget a new idea. More cups are found 

 that the observation may be repeated and coincidence 

 put out of court. Each time we repeat the observation 

 on fresh batches of animals we obtain the same result. 

 As on the shore in the roscoffensis zone, so in the 

 laboratory the upward and downward movements of 

 Convolute march with the movements of the tide. As 

 the tide recedes from their home upon the shore, the 

 sojourners in the laboratory rise up : as the tide rises 

 over it, they sink down. In the absence of all apparent 

 external stimulus, C. roscoffensis, obedient to its 

 custom, yet keeps time with the tide. The rhythm of 

 the tides is reflected by the movements of the animal. 

 For eight successive tides (Fig. 13) the animals in the 

 laboratory maintain their rhythm, synchronous with 

 the ebb and flow of the waters over the roscoffensis 

 zone : then, though the rhythmic movement up and 

 down may yet continue, its temporal periodicity 

 loses precision, and, finally, the rhythm is worn down. 

 This stage reached, the animals exchange a working 

 day of double, six-hour shifts, two up, two down, for 



