268 STUDIES IN GENERAL PHYSIOLOGY 
Nor do the animals collect at D on the room side of the dish, 
but rather at H. The movements, therefore, occur in the 
direction of the rays of light. If the experiment is to be 
demonstrated to others, a shadow may be thrown into the 
vessel by a rod, in which case one can see directly that the 
- animals move parallel to 
the shadow. 
Attention need scarcely 
be called to the fact that 
if rays of light strike the 
animal simultaneously 
from various directions, 
and the animal is able to 
move freely in all direc- 
ay tions, the more intense rays 
FIG. 64 will determine the direc- 
tion of the progressive movements. 
That it makes no difference to the negatively heliotropic 
Limulus larve whether they go from regions of less intense 
light to regions of greater intensity—that is to say, from 
the “dark” into the “light”—but that only the direction 
of rays of light determines the direction of the progressive 
movements, is shown by the following experiment. Let AB 
in Fig. 64 again be the plane of the window; SS, the hori- 
zontal projection of the sun’s rays falling into the room 
obliquely from without and above. The horizontal part of 
the window frame casts the shadow CD upon the table. The 
strip CD will, of course, be illuminated by reflected daylight. 
I placed the vessel ef containing the Limulus larvee upon 
the table so that the window side e of the dish lay in the 
shadow, while the room side f of the dish was in the sunlight. 
At the beginning of the experiment the larvae were collected 
in the shadow on the side of the dish nearest the window. 
They at once began to move to the room side in the path of 
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