134 ^Journal of Comparative Neurology and Psychology. 



motion of the animal. Other possible complications are covered 

 by the unequivocal statement, made and emphasized in this 

 and the following papers on the same subject, that the main 

 feature in all phenomena of heliotropism is the fact that symmetri- 

 cal points of the photosensitive surface of the animal must be 

 struck by the rays of light at the same angle. It is in full harmony 

 with this fact that if two sources of light of equal intensity and 



Fig. 7. Representation of the direction of movement of Volvox when subjected to light from two 

 sources, a, plate glass aquarium 8 cm. long and 8 cm. wide; h, 222 volt Nernst glower, 66 cm. from 

 aquarium (distance from aquarium constant); c, no volt glower, (distance from aquarium variable); 

 d, screen; e, point of introduction of Volvox; /, direction of light rays; i, 2, 3, and 4, courses of Volvox 

 exposed to light from both glowers: i,with no volt glower 199 cm. from aquarium; 2, with no volt 

 glower 99 cm. from aquarium; 3, with no volt glower 49 cm. from aquarium; 4, with no volt glower 

 24 cm. from aquarium; xy, course of Volvox when exposed to light from glower h only; y — s, course 

 when exposed to light from glower c only. 



distance act simultaneously upon a heliotropic animal, the animal 

 puts its median plane at right angles to the line connecting the 

 two sources of light. This fact was not only known to me but 



