PEARSE. — THE REACTIONS OF AMPHIBIANS TO LIGHT. 167 



B. Methods. 



The experiments described in the present paper have been devoted 

 (1) to extending the range of our knowledge of photic reactions among 

 the amphibians, (2) to ascertaining more fully the nature of the photo- 

 receptors involved, and (3) to determining how great a part the central 

 nervous system takes in these reactions. It gives me great satisfaction 

 to express my indebtedness to Professor G. H. Parker, under whose 

 direction the work was accomplished. 



All the experiments which are described in the succeeding pages were 

 carried on in a dark room, the temperature of which usually varied 

 between 17° C. and 21° C. The source of the light was a six-glower 

 Nernst lamp, and as the amount of light it gave out varied under dif- 

 ferent conditions, the intensity used is given under the descriptions of 

 the various experiments. All the amphibians used were collected in the 

 vicinity of Cambridge, Massachusetts, with the exception of Necturus, 

 which came from Venice, Ohio ; Cryptobranchus from Oil City, Penn- 

 sylvania, and, through the kindness of Professor A. M. Banta, from 

 Marietta, Ohio ; and Diemyctylus from Jaffrey, New Hampshire. The 

 aquatic species were kept in a large aquarium tank, four meters long 

 by one and a half wide, in a cool basement room. The terrestrial 

 forms were kept in cages, the floors of which were covered with earth 

 and dead leaves, and individuals upon which operations had been per- 

 formed were placed on a bed of moist excelsior in glass jars. Little 

 trouble was experienced in keeping the animals in good condition. 

 The frogs and toads were fed with meal worms, which they ate readily 

 throughout the winter. The other species were not fed, though Cryp- 

 tobranchus may have eaten frogs, which were kept for other purposes 

 in the aquarium with it ; and as one of those animals lived for two 

 years, it is not improbable that it obtained such food from time to 

 time. The experiments were carried out in the autumn and winter 

 months (October 1 to April 1) of two different years. 



Of the aquatic species used, Cryptobranchus was the most reactive. 

 For experimental purposes Bufo was the most satisfactory of the land 

 forms, both on account of its extreme activity and its greater ability to 

 withstand dryness. Both Bufo fowleri and B. americanus were used, 

 but the experiments on the two species were not kept separate. Dr. 

 L. J. Cole informs me that Acris is much better than Bufo for work 

 of this nature, but I have not had an opportunity to try it. The term 

 " amphibians " in this paper does not include caecilians, whose reac- 

 tions to light are, so far as I know, unstudied. 



