266 STUDIES IN GENERAL PHYSIOLOGY 
This leaves still two possibilities: the oral, or the aboral, 
pole may be turned to the source of light. When the former 
is the case, the animals are called positively heliotropic; 
when the latter is the case, negatively heliotropic. In the 
case of sessile animals orientation was brought about by the 
light without any complicating secondary phenomena, and 
when light fell upon them from one side only, heliotropic 
curvatures resulted just as in plants. Spirographis spallan- 
zanii gave rise to positively heliotropic curvatures; while the 
stolons of Sertularia gave rise to negatively heliotropic cur- 
vatures under certain conditions. If, however, the animals 
are able to move freely, a complicating feature appears, in- 
asmuch as the animals execute progressive movements, and 
these take place in the direction of the rays of light, as the 
median plane of the animals is brought into this direction. 
If the animals are positively heliotropic, progressive move- 
ments must occur toward the source of light. If the 
animals are negatively heliotropic, they must move away 
Jrom the light. The difference between this idea and that 
of former authors is recognized immediately. According to 
my idea, the fact whether the animals go toward the light or 
away from it, is a consequence of their orientation by the 
light—a fact which former authors overlooked. Moreover, the 
direction of the progressive heliotropic movements lies in 
the direction of the rays of light —another fact whichhad been 
universally overlooked. The former conception, that certain 
animals seek the “light,” while others seek the “darkness,” 
is completely refuted by the fact, which I discovered, that 
positively heliotropic animals can be forced to go in the 
direction of the rays of light from sunlight into the shade, 
and to remain there; while negatively heliotropic animals 
can be compelled to move in the direction of the rays of 
light, from the shade into direct sunlight, and remain there. 
A few experiments will better illustrate the nature of helio- 
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