spatially Determined Reactions 209 



ments on the behavior of certain insects to light. Bohn 

 says of the moUusk Littorina that when its tissues are 

 neither very wet nor very dry,' it ceases to respond with a 

 fatal necessity to light ; " the animal seems, as it were, to 

 disengage itself from the influence of external forces, seems 

 no longer to behave like a pure machine: it goes to the 

 stones and seaweed where it may find shelter and nourish- 

 ment as if it saw and was conscious of them" (80). 



§ 61. Mutual Influence of Light and Gravity Orientations 



Orientation to light and orientation to gravity are not 

 without mutual influence in determining the behavior of 

 an animal. Supposed instances of this have been noted 

 in the case of the periodically changing geotropism of 

 Convoluta roscoffensis (253) and in the copepods observed 

 by Esterly (210). The relations of gravity and light re- 

 sponses in the larvae of the squid, a cephalopod moUusk, 

 seem to be as follows. The larvae have a tendency to rise 

 to the surface of the water both in darkness and in light, 

 suggesting negative geotropism. Two test tubes were 

 arranged by Loeb, one lying horizontally and at right angles 

 to a window, the other inclined at an angle of 45 degrees 

 from the upright position, and with the upper end directed 

 away from the window. Larvae were placed in both tubes ; 

 those in the former showed positive phototropism by collect- 

 ing at the end nearest the window, but those in the latter 

 gave evidence that their negative geotropism was stronger 

 than their positive phototropism by rising to the upper end, 

 although it was farthest from the source of light (428). 

 It is not usual for geotropism thus to come off victorious in 

 a contest with other tendencies. Jennings says, "As a 

 general rule the reaction to gravity is easily masked by 



