Spatially Determined Reactions 175 



this is the case in Daphnia was shown by Radl, who placed 

 the animal under a microscope in such a way that only the 

 eyes could be moved. When the light coming from below 

 was diminished, the eyes rolled upward; when the light 

 coming from above was diminished, they rolled downward. 

 The precise positive phototaxis of Daphnia, Ra"dl thinks, is 

 primarily an eye movement, the body being turned to follow 

 the eyes (354). Indeed, Radl is of the opinion that in all 

 animals having eyes, the essential feature of phototropism is 

 eye-orientation, wholly analogous to fixation in the case of 

 human vision (356). In amphipods, blackening of one eye 

 of a positively phototropic animal caused a turning toward the 

 blackened side, as if the animal were trying to restore the 

 missing illumination; similar experiments upon negative 

 animals produced turning toward the other side (181). Like 

 phenomena have been observed in other Crustacea, in 

 mollusks, annelids, and insects. Bonn, like Radl, is in- 

 clined to explain the light tropisms of animals with eyes as 

 entirely due to an effect, either tonic or inhibitory, according 

 as the animal is positive or negative, of light acting through 

 the eyes upon the muscles of the same side of the body. If 

 one eye received more light than the other, a positive animal 

 would turn toward the darker side because the muscles on 

 the side toward the light would be more strongly stimulated. 

 A negative animal would turn toward the light because of 

 inhibition of muscular activity on that side. Orientation 

 may then be effected in a normal animal when the eyes 

 equally illuminated (55). 



: 01 



ion \ 



are ] 



70. Influences Affecting the Sense of Light Orientations 

 In no class of animal responses to stimulation is the effect 

 more dependent upon the cooperation of a number of condi- 

 tions than in those involving orientation to light. Many in- 



