Spatially Determined Reactions 183 



animal. Supposed instances of this have been noted in the 

 case of the periodically changing geotropism of Convoluta 

 roscoffensis (140) and in the copepods observed by Esterly 

 (113). The relations of gravity and light responses in the 

 larvae of the squid, a cephalopod mollusk, 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 phototaxis by collecting at the end nearest the win- 

 dow, but those in the latter gave evidence that their negative 

 geotropism was stronger than their positive phototaxis by 

 rising to the upper end, although it was farthest from the 

 source of light (239). It is not usual for geotropism thus to 

 come off victorious in a contest with other tendencies. Jen- 

 nings says, " As a general rule the reaction to gravity is easily 

 masked by reactions to other stimuli " (211, p. 150). In the 

 mollusks observed by Bohn, the tendency in ascending or 

 descending the rocks is to orient the body in the line of the 

 greatest slope. When light and gravity are acting together 

 upon the animal, its movement seems to be a resultant of the 

 two, but if the mollusk is made to move on a vertical plane, 

 gravity thus exerting its maximal force, the influence of the 

 light disappears altogether; and if the animal is put in an 

 upside-down position by further tipping of the surface, the 

 sense of its phototropism is reversed; that is, it may be 

 repelled instead of attracted by a dark screen (55). 



A curious tendency has been noted by many observers in 

 insects with both eyes blinded ; namely, to fly straight up into 

 the air. Forel thought they did so because in no other 



