96 STUDIES IN GENERAL PHYSIOLOGY 
that the animals could not erect themselves. They were so 
placed in the aquarium that their longitudinal axes lay per- 
pendicular to the side ab (Fig. 10) of the aquarium, and their 
pedal extremities Mf touched the glass wall ab. The side a 
faced and was parallel to the plane of the window. The 
animals fastened 
a ! M 3 themselves to the wall 
ab, and then began 
to react, in their char- 
acteristic way, to the 
ae light, by which the 
head was turned and 
the tube became con- 
7 cave toward the 
FIG. 10 source of light. The 
tube ALN assumed the position MN,. As soon, however, 
as the tentacles touched the glass wall ab, the tip N turned 
away from the glass wall. The heliotropic bending gradually 
affected all the elements of the tube MN, so that the 
Spirographis finally reached the position MN,, in which it 
remained throughout the period of observation—four 
months. 
I repeated this experiment a number of times, always 
with the same result. 
7. The heliotropic phenomena of Spirographis took place 
both in direct sunlight and in diffuse daylight. The minimum 
light intensity just sufficient to bring about these phenomena 
is very small. I have not yet studied the effect of rays of 
different refrangibility in producing these phenomena. Since 
thus far the more refrangible rays have proved to be the 
most effective heliotropically both in plants and animals, it 
is to be suspected that Spirographis also will prove no excep- 
tion. 
8. As is well known, Sachs has formulated the law that 
s3 
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