66 PLANT-ANIMALS [CH. 



memory of past vibrations caused by the making 

 tides invading periodically the C. roscoffensis zone, 

 how much more certain should be the effects of 

 present vibrations. Yet, if the vessel containing the 

 animals is so exposed that a steady drip of water falls 

 upon the surface of the sand contained in the vessel, 

 C. roscoffensis clings to its periodic habit. As soon as it 

 perceives the vibrations it descends and remains below 

 the sand. When, however, the time for its uprising 

 arrives, it rises to the surface, and, in spite of injuries, 

 remains upon the surface. It seems difficult of belief 

 that the memory of a particular kind of blow can be 

 a more powerful spur to action than the actual 

 receipt of an unceasing series of blows of a like kind. 

 The original suggestion which, though it is not ac- 

 cepted by the author of the memory hypothesis, 

 seems to fit the facts, seeks to explain the periodicity 

 of upward and downward movement exhibited by C. 

 roscoffensis by connecting it with tonic light effect. 



In support of this it may be mentioned that 

 C. roscoffensis fails to exhibit its tidal rhythm except 

 when it is subjected to a fairly high light intensity 

 during its period of " upuess." Thus, even in a room 

 at some little distance from the window, the movement 

 does not keep tidal time. 



Again, other observations indicate that the spell 

 of illumination counts for something in determining 

 the precision of the movements. Thus, if three 



