PEARSE. — THE REACTIONS OF AMPHIBIANS TO LIGHT. 193 



to the effect of this heat from the apparatus was made in the following 

 way : On two occasions when a toad had gone successively ten times 

 toward the light, an opaque screen was interposed in such a way 

 that the light was cut off but the radiating heat from the apparatus 

 was allowed to reach the toad. In both instances the individuals 

 gave ten reactions without apparent reference to the heated appa- 

 ratus, thus showing that the reactions had not been brought about 

 by heat. 



In order to test the sensitiveness of the toad to increased tempera- 

 ture, two eyeless individuals were suspended in such a way that the 

 hind legs could be dipped into water. Neither of these animals made 

 any movement under this method of treatment when the water was at 

 room temperature (20° C). The temperature of the water was then 

 raised five degrees at a time, and there was no response until a temper- 

 ature of 40° C. to 45° C. had been reached, when the animals quickly 

 withdrew their legs from the hot water. It was evident, from these 

 results, that the toad did not respond readily to increase in tempera- 

 ture. Reese ( :06) found that Cryptobranchus also was comparatively 

 insensitive to changes in the temperature of the surrounding medium, 

 but, if the temperature was raised above 40° C, violent motor reactions 

 occurred. 



While these observations showed that amphibians might not be 

 very sensitive to thermic stimulation, the possibility was not excluded 

 that the assumed photic reactions might in reality be due to stimula- 

 tion of the skin receptors by heat. If the positively phototropic 

 reactions of blinded toads were due to the stimulation of such recep- 

 tors, it ought to be possible to obtain similar reactions through the 

 use of radiant heat instead of light. To ascertain if this were possible, 

 an apparatus was arranged in which steam was passed through a verti- 

 cal brass pipe which measured seven millimeters in diameter. The 

 eyeless toads were placed near this pipe, and their reactions tested in 

 the same manner as had previously been done with light. All these 

 experiments were performed in the dark, but before and after the heat 

 experiments each individual was tested with light (1.24 candle-meters) 

 to ascertain whether it was positively phototropic or not. The 

 method of experimenting in the dark was to orient the toad by using 

 a mark at a known distance from the source of heat ; then to listen 

 until a movement was heard ; after which the position of the animal 

 was ascertained by feeling for it with the hand. In Table XVII. the 

 signs -f , — , and are used to indicate movements in relation to the 

 steam pipe as a source of heat, as they have previously been used for 

 sources of light. As this table shows, toads placed near (10 to 20 cm.) 

 vol. xlv. — 13 



