VIII 



THE ARTIFICIAL TRANSFORMATION OF POSITIVELY 

 HELIOTROPIC ANIMALS INTO NEGATIVELY HELIO- 

 TROPIC AND VICE VERSA 1 



THE new facts contained in the following pages deal 

 chiefly with the task of rendering positively heliotropic 

 animals negatively heliotropic, and rice versa. I think also 

 that I have discovered a difference in positively and in nega- 

 tively heliotropic animals with regard to the liberation of 

 energy. As both series of observations may give us some 

 clue in regard to the nature of heliotropic phenomena in 

 general, I have briefly repeated here the description of the 

 simple facts of heliotropism, and have prefaced it with a 

 short theoretical explanation. A later part in this paper 

 treats of the behavior of animals, which, though not helio- 

 tropic, still react to the light by movements. These I shall 

 term photokinetic (unt&rschiedsempfindlicK). In the con- 

 cluding part of this paper are given the results of some further 

 experiments bearing 011 the causes of depth-migration and 

 depth-distribution in marine animals. 



I. THE SIMPLE FACTS OF HELIOTROPISM 



1. All former authors who have studied the behavior of 

 animals toward light have, without exception, been of the 

 opinion that animals " preferred" either light or darkness, 

 and correspondingly either sought the light places in spare 

 or shunned them. Five years ago I showed that there is a 

 large number of animals which are oriented by the light. 

 and in such a way that they are forced to place their axes 

 or planes of symmetry in the direction of the rays of light. 



i PflQgers Archiv, V.>1. LIV (ISD'ii, p. si. 



265 



