288 STUDIES IN G-ENEKAL PHYSIOLOGY 



from the behavior of heliotropic animals, are best shown in 

 the following experiment: AB (Fig. 66) is the plane of the 

 window; abc, the vessel containing the animals. At one 

 point be the outer surface of the vessel is made impermeable 

 to rays of light by means of black paper. It can easily be 

 seen that at be a small section of the 

 circle is not struck by the rays of light 

 *,--' falling upon it from without. If the 



c ^ >v Planaria at the beginning of the exper- 



\ ) iment are brought to the window side 



a of the vessel, but so that they are not 



FIG. 66 struck by the light, all, or almost all, the 



animals are found in a few hours, or on the following day, 

 under the opaque paper be where the intensity of the illumi- 

 nation is least. If the same experiment is made with nega- 

 tively heliotropic Limulus larvse, the larvae move to the room 

 side a of the vessel, and remain there permanently. It is 

 clear, under these circumstances, that when these Planarise 

 are left quietly for some days in a cylindrical vessel abed 

 (Fig. 67), all the animals finally collect at the two sides c 

 and d, as was observed by Dr. Wheeler. Heliotropic animals 

 in the same vessel either go immedi- 

 ately to the window side a or the room * + 

 side b of the vessel, and remain there. 

 This mode of reaction to changes in 

 the intensity of light occurs probably 

 also in angleworms; perhaps, too, in 

 other animals. It is, moreover, pos- 

 sible that heliotropism and photokinesis are associated in 

 certain animals 1 a subject which I still wish to investigate. 2 



IE, gr., Spirographis spallanzanii. 



2 1 had previously noticed that in some animals, which I at that time considered 

 negatively heliotropic, the typical heliotropic experiment did not succeed very well. 

 1 attributed this to secondary circumstances. I now consider it possible, however, 

 that the experiments which I described, for example those on the larvse of beetles, 

 indicate as much the existence of photokinesis as negative heliotropism. I shall 

 make further experiments in this direction. 



