PEARSE. — THE REACTIONS OF AMPHIBIANS TO LIGHT. 205 



stated, they are apparently brought about by unequal stimulation on 

 the two sides of the body. This tropic character applies to the reac- 

 tions whether they are induced by stimulation through the skin or 

 eyes or through the simultaneous stimulation of both. In general, it 

 may be said that the photic responses are of a typically reflex character 

 and show little evidence of powers of association. 



IV. SUMMARY. 



(1) The following amphibians were found to be positively photo- 

 tropic: Diemyctylus viridescens, Rana clamata, R. palustris, Bufo 

 fowleri, B. americanus ; and the negatively phototropic species studied 

 were : Necturus maculosus, Cryptobranchus allegheniensis, Ambly- 

 stoma punctatum, Plethodon cinereus erythronotus. 



(2) Most of the species mentioned under (1), after the removal of 

 their eyes, gave photic responses which were like those of normal 

 individuals. 



(3) The photic reactions of eyeless amphibians are not due to the 

 direct stimulation of the central nervous system or the exposed ends 

 of the optic nerves by light, but to the action of the skin as a 

 photoreceptor. 



(4) Mechanical stimulation (handling) does not change the charac- 

 ter of the photic reactions, though it makes them more evident by 

 inducing locomotion. 



(5) Toads which are stimulated by light through the eyes alone 

 react in the same manner as individuals stimulated through the skin 

 or through both the skin and the eyes. 



(6) The movements of eyeless toads stimulated unilaterally by light 

 from above are toward the illuminated side ; and toads stimulated 

 through one eye only by light from in front do not go toward the light 

 but turn toward the illuminated side. The photic reactions are there- 

 fore due to differences in light intensity on the two sides of the body 

 and the direction of the rays is ineffective. 



(7) After the eyes have been removed, Cryptobranchus and Nec- 

 turus are most responsive when the tail is illuminated, but the skin of 

 the toad is apparently of equal sensitiveness on all parts of the body. 



(8) A prolonged period of time passed in light or dark had no 

 effect on the nature of the phototropic responses of the toad. 



(9) Cryptobranchus is strongly photokinetic, but in the other am- 

 phibians tested this quality was not strongly developed. 



(10) When normal amphibians were used, blue light was the most 

 effective in the production of tropic responses, but when eyeless indi- 



