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. roscoffensis 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. 

 roscoffensis 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 be tapped, 

 the vibrations set up suffice to change the sign of the 

 animal's response to gravity and send it geotr oping. 

 Nevertheless it is easy to show that the response of 

 C. roscoffensis to the vibration-stimulus is less marked 



52 



