HELIOTROPISM OF 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 
larvee 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 larve 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 
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