RETINAL PIGMENT OF PLANORBIS 361 



dark the pigment is limited to the base of the. rod. Chun ('03) 

 confirmed these general conclusions on deep-sea cephalopods, 

 as did Hess ('05) by making a comparison of the pigment re- 

 sponses in pelagic and httoral forms. 



Temperature is likewise a controlling factor in determining 

 the ultimate distribution of pigment in many bodj^ melanophores. 

 In general (Parker, '06), low temperature has an influence similar 

 to that of light, inasmuch as it favors pigment expansion; high 

 temperature, on the contrary, like darkness, induces pigment 

 contraction. 



A similar condition liolds for the retinal pigment of certain 

 animals. Congdon ('07, p. 547) found that: "In both Palae- 

 monetes and Cambarus the proximal retinal pigment migrates 

 distally when the temperature is lowered and proximally when 

 it is raised." The writer (Arey, 16") found an identical ten- 

 dency in the retinal ])igment of fishes. Working with the 

 frog's retina, Gradenigro ('85) first showed that at a tempera- 

 ture of 30°C. the pigment expands, thereby closely simulating 

 the distribution characteristic of light. Later Herzog ('05) 

 confirmed this discovery, and further stated that maximal 

 expansion likewise ensues when the temperature api^roaches 

 the freezing point, the contracted condition typical of darkness 

 being obtainable only between the temperatures of 14° antl 18°C. 

 A reinvestigation of this matter recently made by the writer 

 (Arey, 16") has convinced him that Herzog's results are sub- 

 stantially correct and that the behavior of the retinal pigment 

 of the frog to temperature must indeed be considered exceptional; 

 the explanation for this is presumably to be found in the exist- 

 ence of a superimposed nervous control, resulting in the oblitera- 

 tion of the more primitive response. 



Positional changes of the retinal pigment of gasteropods in 

 response to definite physiological stimuli have never been re- 

 corded. Smith ('06), in his work on the structure of the eyes 

 of pulmonate gasteropods, noticed that the retinal pigment 

 of Planorbis trivolvis Say showed processes of variable length, 

 and accordingly he performed a few experiments to determine 

 whether the pigment distribution could be correlated with 



