ii] HABITS OF CONVOLUTA 55 



would occur were the dominating stimulus alone 

 applied. This happens, as we have seen, when 0. 

 roscoffensis is subjected to both light- and heat- 

 stimulation. 



So also, in the case of light and gravitational stimuli 

 acting simultaneously, the mode of response of C. 

 roscoffensis shows that the latter stimulus may be 

 ignored. 



Thus, if animals are placed in water in a tall 

 glass cylinder on a steady table, they rise to the 

 surface of the water and congregate on the side 

 toward the light. If the light-conditions are modi- 

 fied by the interposition of a black card or plate 

 of ground glass between the source of light and the 

 top of the water, C. roscoffensis relaxes its hold and 

 swims downward to take up a new position just 

 below the edge of the screen. Geotropism is sub- 

 ordinated to phototropism. 



Subjected to simultaneous stimulation by light 

 and gravity, C. roscoffensis behaves exactly like a 

 green plant placed under similar conditions. Though 

 the stem of a green plant is negatively geotropic, 

 yet, if it is illuminated from below, the plant, 

 ignoring gravitational stimulation, directs the tip of 

 its stem downwards toward the source of light. 



The behaviour of both plant and animal would 

 seem to indicate that, in the reflex groundwork 

 of nervous activity, something akin to the pheno- 



