266 STUDIES IN GENERAL PHYSIOLOGY 



This leaves still two possibilities: the oral, or the aboral, 

 pole maybe turned to the source of light. When the former 

 is the case, the animals are called positively heliotropic; 

 when the latter is the case, negatively heliotropic. In the 

 case of sessile animals orientation was brought about by the 

 light without any complicating secondary phenomena, and 

 when light fell upon them from one side only, heliotropic 

 curvatures resulted just as in plants. Spirographis spallan- 

 zaiiii gave rise to positively heliotropic curvatures; while the 

 stolons of Sertularia gave rise to negatively heliotropic cur- 

 vatures under certain conditions. If, however, the animals 

 are able to move freely, a complicating feature appears, in- 

 asmuch as the animals execute progressive movements, and 

 these take place in the direction of the rays of light, as the 

 median plane of the animals is brought into this direction. 

 If the animals are positively heliotropic, progressive move- 

 ments must occur taininl the source of light. If the 

 animals are negatively heliotropic, they must move away 

 from the light. The difference between this idea and that 

 of former authors is recognized immediately. According to 

 my idea, the fact whether the animals go toward the light or 

 away from it, is a consequence of their orientation by the 

 light a fact which former authors overlooked. Moreover, the 

 direction of the progressive heliotropic movements lies in 

 the direction of the rays of light another fact which had been 

 universally overlooked. The former conception, that certain 

 animals seek the "light," while others seek the "darkness," 

 is completely refuted by the fact, which I discovered, that 

 positively heliotropic animals can be forced to go in the 

 direction of the rays of light from sunlight into the shade, 

 and to remain there; while negatively heliotropic animals 

 can be compelled to move in the direction of the rays of 

 light, from the shade into direct sunlight, and remain there. 

 A few experiments will better illustrate the nature of helio- 



