272 STUDIES IN GENERAL PHYSIOLOGY 
hand to show that the phenomena of the liberation of energy 
produced by the light show different characteristics in posi- 
tively and negatively heliotropic animals to correspond with 
this theory. Negatively as well as positively heliotropic 
animals execute progressive movements under the influence 
of light, independently of their orientation, and a difference 
could be expected only in the efforts which the animal must 
make to execute the given progressive movements. It might 
be thought that in positively heliotropic animals light brings 
about a condition of the muscles or the nervous system in 
which the liberation of energy is made easier, while in nega- 
tively heliotropic animals a condition of the muscles is 
brought about by the light in which the liberation of energy 
is made more difficult. I have given some observations in 
sec. £of this paper which seem to sustain such an assumption. 
Before doing this, however, I wish to acquaint the reader with 
aseries of new facts which deal with the transformation of 
positive heliotropism into negative, and vice versa. 
III. ON THE TRANSFORMATION OF POSITIVE HELIOTROPISM 
INTO NEGATIVE HELIOTROPISM, AND THE REVERSE 
1. In my earlier papers I was able to describe such ani- 
mals only as were constantly positively or negatively helio- 
tropic. Later, Groom and I described some observations at 
Naples on the behavior of the nauplii of Balanus perforatus, 
and certain other marine animals, which were at times nega- 
tively heliotropic, and at other times positively heliotropic.’ 
We found that the intensity of the light determines the sense 
of heliotropism in these animals. Above a certain intensity 
light makes these animals negatively heliotropic, and this 
the more quickly the greater the intensity of the light. 
By lamplight the animals were always positively heliotropic. 
I have made further experiments on pelagic animals at 
1“ Der Heliotropismus der Nauplien von Balanus perforatus,” Biologisches 
Centralblutt, Vol. X. 
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