i] THE HABITAT OF CONVOLUTA 23 



C. roscoffensis is gone. On stormy days, when the 

 making tide announces its landward progress angrily 

 thundering like ramping clouds of warrior horse 

 the reverberations of the sand send signals to the 

 colonies which make their dispositions underground 

 long before the breaking waves can reach or damage 

 them. All these ordered goings and comings may 

 the observer see on any day on any beach in Brittany. 

 But, to discover more precisely the physiological 

 methods of these purposeful movements, the labora- 

 tory must take the place of the beach, and simple 

 scientific methods must supplement bare observation. 

 In this way, it is possible to refer movements, so pur- 

 poseful as to suggest volition, to simple, non-conscious, 

 nervous responses to one or more of several stimuli, 

 the chief of which are gravity and light. 



Before, however, we investigate the living animals 

 in the laboratory we may note yet another example 

 of rhythmic behaviour in our plant-animals. 



However carefully the observer seeks at low water 

 among the exposed weeds of the paradoxa zone, he will 

 find no animals bearing ripe eggs. As the tides be- 

 come large enough to permit of approach on foot to 

 that zone, the animals which he obtains are, for the most 

 part, minute, immature specimens. On succeeding days, 

 the catch consists of larger animals, till, during the 

 latest spring tides, it is composed chiefly of adults, many 

 of which may contain unripe eggs. Then comes a period 



