THE BIONOMICS OF CONVOLUTA ROSCOFFENSIS. 401 



the black half is — in the case where wliite ground is nearest 

 the light, 50 : ; in the case where black ground is nearest 

 the light, 30 : 20. 



Thus the background modifies phototropic response; the 

 animals tend to remain on a white ground and to shun a black 

 ground, and to such an extent that when for the execution 

 of a phototropic movement the passage from a white over to 

 a black ground is involved, a large proportion — two thirds — 

 of the animals fail to perform that movement. 



Phototropism is, then, not an inevitable reaction. Under 

 certain conditions its sign is reversed ; under others it fails 

 to take effect. We give a further illustration of what we 

 feel is an important and general fact, that a " reflex " has 

 something of the uncertainty which attaches to a movement 

 in which volition is concerned, and something of the complex 

 of conditions which in the latter class of actions constitute 

 " choice." 



Periodic Tidal Effect. 



If, as the Convolutas expose themselves on the sand after 

 the tidal water has withdrawn from above them, a small 

 patch of some thousands is scooped up into a vessel, the 

 animals immediately disappear beneath the sand scooped up 

 with them. As soon as the vessel ceases to be shaken — when 

 laid on the ground in the open, for example — the green 

 patch of Oonvoluta reappears. It persists for some hours, 

 but then melts away again, only to appear once more after a 

 few hours spent below the surface. 



This coming and going, apparently so erratic, is obedient 

 to a simple rule. If, after watching the patch disappear 

 beneath the sand of the vessel, one looks away to the 

 strip of sand whence the animals were taken, one finds 

 that the returning tide is already lapping its edges ; and 

 inspection shows that the colonies of Cunvoluta known 

 to inhabit that stretch of sand arc all now beneath the 

 surface. If, again, one remarks the time at which the tide 



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