n] HABITS OF OONTOLUTA 61 



lethargy or light-rigor here attributed to exposure 

 for long periods to high light intensities is due the 

 fact that, whereas, on some days, the C. roscoffensis 

 patches on the shore disappear before the water of 

 the making tide reaches them, yet, on other days, 

 the multitude of animals composing a patch lies 

 motionless and indifferent to the approach of the in- 

 coming tide. Not till the first wave sweeps over them, 

 do the animals throw oif their sloth and disappear. 



We have now to attempt to apply the knowledge 

 we have obtained of the tropistic responses of C. 

 roscoifensis to light and gravity, and of the tonic 

 effects of light, to the elucidation of the most strikingly 

 picturesque feature of the behaviour of this animal, 

 that of its tidal uprising and downlying. Almost as 

 soon as the water of the falling tide has run off the 

 roscoffensis zone, the green colonies appear, and, 

 before the making tide invades it, they vanish. The 

 purposes of ascent and descent are obvious. By its 

 ascent, the animal reaches the light without which 

 for reasons we shall discover subsequently it cannot 

 live; by its descent, C. roscoffensis maintains its 

 situation on the shore and escapes the waves. 



As our study of its tropisms makes clear, these 

 movements of ascent and descent may be induced in 

 the laboratory by subjecting the animals to appro- 

 priate stimulation. Vibrations produced by tapping 



