286 bulletin: museum of comparative zoology. 



colors in the spectrum. There were movements toward both Hghts 

 in any pair, the larger percentage being always to the blue, or to the 

 light nearer the blue end of the spectrum. The percentage of move- 

 ments to red light in pairs with the other lights, was only slightly 

 higher than the percentage of movements toward the dark, when these 

 lights were used singly. 



c. Reactions with the Skin as Receptor. 



Having demonstrated that the eyes were concerned in the reactions 

 in which the whole body was exposed, it remained to test the reactions 

 of toads in which only the skin was exposed to balanced pairs of 

 monochromatic lights. In order to protect the eyes from the light, 

 the hoods described on p. 269 were again used. 



The results of the tests are given in Table 9. Seven sets of twelve 

 toads each were tested, and therefore a total of 1344 trials was made 

 in each single pair of lights, or 8064 trials in all. For the first three 

 sets, four of the lot of twelve toads which had been employed for the 

 first three sets of tests where both the skin and the eyes were exposed, 

 and for the first set of tests where only the eyes were exposed, were 

 used. Eight of this lot of twelve had died during the course of 

 former experiments, and their places were therefore supplied by eight 

 other toads. Three of the present lot of twelve also died, during the 

 course of these experiments, and their places were supplied by three 

 other animals. For the next two sets, a new lot of twelve, and for 

 the last set, another new lot of twelve toads, were again selected. 

 One of each of these last two lots also died during the course of these 

 experiments, and their places were supplied by two other animals. 

 There were thus a total of 41 separate toads used for these tests. 



It will be seen, by referring to Table 9, that in all of the pairs there 

 were some movements toward both lights, with the larger percentage 

 always toward the blue, or to that light of a given pair which, in the 

 spectrum, is nearer the blue. In the pairs in which red occurred, 

 there were also movements to this light, but only when paired with 

 blue were these movements to red light more numerous than the 

 negative responses to the lights when used singly. It was here, in 

 these pairs of red with the other lights, that were most clearly brought 

 out the differences in sensitiveness, when the light was received through 

 the eyes, or through both the skin and the eyes, and that when it was 

 received through the skin only. If the pairs in which red occurred 

 be considered, it wuU be seen that the percentage of movements 



