n] HABITS OF CONVOLUTA 67 



batches of C. roscoffensis, collected directly after 

 the colonies emerge, are put in darkness for periods 

 of one, two, and three hours respectively, and are 

 then exposed to the light, that which had only one 

 hour's run in darkness descends first, and that which 

 had two hours' darkness descends next. 



Taking the results of these experiments into con- 

 sideration and bearing in mind the condition of 

 lethargy which C. roscoifensis may manifest, in its 

 natural station, after long light-exposures, we are led 

 to frame some such hypothesis as the following, in 

 order to account for the periodic tidal movements 

 exhibited by this animal. 



Phototropism and background reaction lead C. 

 roscoifensis to the most illuminated parts of that 

 region of the beach which provides it with a con- 

 tinuous, filmy stream of water. 



Independently of its tropistic effect, light exerts 

 a tonic effect on the physiological state of the 

 animals. Under the combined influences of tropistic 

 and tonic light-stimuli, C. roscoffensis is held at 

 attention in the "up" position : in other words, whilst 

 subject to this constant rain of phototonic stimuli, it 

 remains negatively geotropic. True, if the sand is 

 agitated, the vibrations set up suffice to change the 

 sign of its response to gravity and send it geotroping. 

 Nevertheless it is easy to show that the response of 

 C. roscoffensis to the vibration- stimulus is less marked 



52 



