TRANSFORMATION OF HELIOTROPIC 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 larvae. 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 1 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 



l In German, unterschiedsempfindlich. 



