288 STUDIES IN GENERAL PHYSIOLOGY 
from the behavior of heliotropic animals, are best shown in 
the following experiment: AB (Fig. 66) is the plane of the 
window; abc, the vessel containing the animals. At one 
point be the outer surface of the vessel is made impermeable 
to rays of light by means of black paper. It can easily be 
seen that at be a small section of the 
circle is not struck by the rays of light 
ie falling upon it from without. If the 
of Planaria at the beginning of the exper- 
iment are brought to the window side 
a of the vessel, but so that they are not 
ae ee struck by the light, all, or almost all, the 
animals are found in a few hours, or on the following day, 
under the opaque paper be where the intensity of the illumi- 
nation is least. If the same experiment is made with nega- 
tively heliotropic Limulus larve, the larvee move to the room 
side a of the vessel, and remain there permanently. It is 
clear, under these circumstances, that when these Planarie 
are left quietly for some days in a cylindrical vessel abcd 
(Fig. 67), all the animals finally collect at the two sides c 
and d, as was observed by Dr. Wheeler. Heliotropic animals 
in the same vessel either go immedi- 
ately to the window side a or the room 4 +— 
side b of the vessel, and remain there. 
This mode of reaction to changes in °K 
the intensity of light occurs probably 
also in angleworms; perhaps, too, in 
A ————__ 9 
other animals. It is, moreover, pos- FIG. 67 
sible that heliotropism and photokinesis are associated in 
certain animals’—a subject which I still wish to investigate.’ 
1. g., Spirographis spallanzanii. 
2T had previously noticed that in some animals, which I at that time considered 
negatively heliotropic, the typical heliotropic experiment did not succeed very well. 
lattributed this to secondary circumstances. I now consider it possible, however, 
that the experiments which I described, for example those on the larvee of beetles, 
indicate as much the existence of photokinesis as negative heliotropism. I shall 
make further experiments in this direction. 
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