TRANSFORMATION OF HELIOTROPIO ANIMALS 287 
which move under the influence of light. If heliotropism 
exists in this case, it is possible that its sense varies. The 
difficulties which confront one in investigating this problem 
are greater than in the case of the Polygordius larve. Even 
though one assumes that certain elements of the retina are 
heliotropic, nevertheless one of the important conditions in 
our sight—namely, the perception of differences in inten- 
sity —cannot possibly be attributed to heliotropic reactions; 
for we saw that heliotropic animals moved not only out of 
the dark into the light, but also in the reverse direction, if 
only the direction of the orienting rays remained the same. 
It is possible that the perception of differences in the inten- 
sity of light by our eyes depends upon specific elements of 
the retina which react especially to changes in the intensity 
of the rays of light. Be this as it may, there are certain 
animals which are not, or at least not very markedly, oriented 
by the rays of light; which are, therefore, not outspokenly 
heliotropic, but which react very promptly to differences or, 
more correctly, to changes in the intensity of the light. 
These I will term photokinetic' animals. A species of the 
fresh-water Planarian, for which I am indebted to Dr. 
Wheeler, is photokinetic. If the animals are put into a 
large dish of water, they creep about in every direction. 
They are not oriented by the light. Yet one observes a dif- 
ference in the behavior of the animals, depending upon 
whether they move from regions of more intense light to 
regions of less intensity, or the reverse. A decrease in the 
intensity of the light tends to make the animals come to 
rest, while an increase in the intensity increases their ten- 
dency to move. It thus happens that these animals gradually 
collect in such places in the dish where the intensity of the 
light is a relative minimum. This phenomenon, and at the 
same time the difference in the behavior of such animals 
1In German, unterschiedsempfindlich. 
Digitized by Microsoft® 
