PEARSE. — THE REACTIONS OF AMPHIBIANS TO LIGHT. 177 



the light, and this reaction was repeated on five successive trials. 

 These reactions could not have been due to the direct stimulation of 

 the optic nerves by light, as they were not exposed to such stimulation. 

 The results are in agreement with those of Graber ('83), who filled the 

 orbits of Triturus with black wax, and of Dubois ('90), who covered 

 the eyes of Proteus with a mixture of gelatine and lampblack. Both 

 these observers obtained phototropic reactions by stimulating the skin. 



(i) Conclusions. 



From the experiments described it may be said that photic sensi- 

 tiveness is general in the skin of amphibians. While there is consid- 

 erable variation in the phototropism of different species, and even 

 of individuals of the same species, the reactions brought about by 

 stimulation through the skin alone are like those produced when both 

 the skin and eyes act as photoreceptors. 



B. The Influence of Mechanical Stimulation on the Photic 

 Reactions of the Toad. 



In the experiments with terrestrial amphibians and light the obser- 

 vations were always made after the animals had been handled by the 

 experimenter, and, though the response was decided in most cases and 

 of such a nature as to attribute it to light, it is not impossible that 

 mechanical stimulation through handling may have been responsible 

 for more or less of it. In order to test this matter five toads which 

 were known to be positively phototropic were placed successively in a 

 box, the floor of which measured thirty-eight by ninety centimeters. 

 The sides and floor of this box were of slate, and the ends were closed 

 by glass heat-screens containing a layer of water 3.75 centimeters 

 thick. The roof consisted of a coarsely woven black cloth stretched 

 on a wooden frame, and the observations were made through the meshes 

 of this cloth. A lamp giving a light intensity of 220 candle-meters 

 was changed from one end to the other at five-minute intervals for a 

 period of fifteen minutes. Four of the individuals when first placed in 

 the apparatus went toward the light, and then wandered back and 

 forth without evident reference to it, and apparently tried to escape 

 from the enclosure. The fifth animal sat in the centre of the box, 

 turning from one side to the other for three minutes, and then went 

 away from the light. When the lamp was changed from one end of 

 the apparatus to the other, only one of the individuals turned imme- 

 diately and went toward it ; the other four were apparently indifferent. 

 In a later experiment, however, two toads were observed to be persist- 



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