290 ALFRED C. REDFIELD 



been found that an induced current of electricity will stimulate 

 the terminations of sensory nerves which are found in the month 

 and cloaca and thus bring about a contraction of the melano- 

 phorqs. Stimulation of receptors in the ear are probably re- 

 sponsible for setting up the state of nervous excitation which 

 produces a contraction of the melanophores when the horned 

 toad is held upon its back. One cannot say exactly what nerves 

 are stimulated when an attempt is made to pry open the mouth 

 of a horned toad, but this procedure also sets up a nervous dis- 

 turbance which produces a contraction of the melanophore pig- 

 ment. It seems clear that there are no specific receptors involved 

 in the reaction; any receptor, the stimulation of which produces 

 the necessary state of nervous excitement, is capable of initiating 

 the reaction by which the melanophore pigment is contracted. 



3. Receptors involved in adaptive reactions 



The reactions of the melanophores to the color of the substra- 

 tum upon which the horned toads are kept depend upon stimuli 

 received through the eyes. If horned toads which have been 

 kept upon a bed of dark cinders are transferred to a bed of White 

 sand, they become noticeably paler after one day and reach the 

 maximum state of contraction of the melanophore pigment within 

 five days. If such animals are blindfolded by fastening bits of 

 woolen cloth over their eyes with celloidin and adhesive tape, 

 they will retain the dark color, although they are kept upon a light 

 colored background for several weeks. 



The failure of the melanophore pigment to contract is not due 

 to any influence of the bandage, for animals which have had only 

 one eye bandaged become pale as rapidly as specimens which have 

 not been bandaged at all. Plate 2 shows a nmnber of animals 

 which illustrate such an experiment. In figure 3, a is a horned 

 toad which has been kept upon cinders, h an animal which has 

 been kept upon sand without blindfolding. The contrast in the 

 pigmentation occurring under the two conditions is obvious. 

 Figures 4 and 5 show four horned toads all of which have been 

 upon sand for ten days : d and /, having had the use of their eyes* 



