HELIOTROPISM OP ANIMALS 59 



returned into the shade ; but if this did not happen, as was 

 more usually the case, the animal continued its movement 

 into the sunlight. The animals did not always check their 

 movements in passing from a shaded area into the sunlight. 

 Often they moved without delay from the shade into the 

 sunlight. The following observation shows that the rays of 

 light which strike the head mainly determine the orientation : 

 When I placed a fully grown animal on a board, and pushed 

 the board from the shade into the sun, so that only the head 

 of the animal was struck by sunlight, the larva immediately 

 placed its median plane in the direction of the sun's rays. 

 When, however, I put only the aboral pole into the sunlight, 

 this orientation did not occur. Animals from which the 

 first few segments of the oral pole had been amputated no 

 longer oriented themselves toward the light. Yet little weight 

 is to be given to vivisection experiments, which are followed 

 only by an inhibition of the effects of a stimulus. 



When I allowed^ the sun's rays to fall on the plane of the 

 board perpendicularly, the animals moved over it in all 

 directions. As in this case the animals could not follow the 

 direction of the rays of light, it had no other influence upon 

 them than to increase their restlessness, and no uniform 

 orientation resulted. 



It could be shown very beautifully in these full-grown 

 larvae that essentially only the more refrangible rays are con- 

 cerned in exercising a directing influence upon these animals. 

 I placed a large number of fully grown larvse on the middle 

 of a horizontal board in a darkened room, and exposed them 

 to the sun's rays which made but a small angle with the 

 horizon. Within ten to twenty seconds every animal had 

 placed its median plane in the direction of the rays of light, 

 and moved exactly parallel to the shadow of a vertical object 

 which had been thrown upon the board for comparison. I 

 treated a new lot of animals in exactly the same way, but 



