RETINAL PIGMENT OF PLANORBIS 379 



A few experiments seemed to indicate that both ether and 

 carbon dioxide did completely check the pigment movements 

 when Planorbis was introduced from dar! ness into light, j^et 

 the snails were very susceptible to the anaesthetics and in no 

 case survived. By careful experimentation it might be possible 

 to find concentrations at which movements of the pigment would 

 be prevented, yet the pigment cells and the animals (as deter- 

 mined by controls) survive. After several trials, in all of which 

 the snails did not outlive the 3 to 4 hours subjection to the 

 anaesthetics necessary for light adaption, the work was dis- 

 continued. 



It is evident that the cliicf vnhw of the few determinations 

 made lies in their suggest i\'eness and not in the actual results 

 gained, which, because of the absence of a])propriate controls, 

 are properly open to criticism. Hence one may be allowed 

 merely to suggest that the ])igment in the excised eyes of Planor- 

 bis, similarly to that in Ameiurus, is able to overcome the prob- 

 able anaesthetic effect of accunmlated wastes in the light only, 

 whereas the weaker response in the dark fails to appear at all 

 because of the presence of the same catabolic products. 



DISCUSSIOxN 



In 190(3 Parker analyzed the results of many workers con- 

 cerning the influence of light and temperatiu'e ujDon melanophores 

 and stated his conclusions in the following generalization (p. 

 413) : *'It is probable that in all melanophores in which there is 

 a migration of ]ngment, light or low tem])erature will induce 

 a migration toward the source of illumination and the absence 

 of light or a high temperature a migration in the reverse direc- 

 tion." Certain cases, how(»\ei-. may be cited which do not 

 conform to Parker's dictiun. tSuch reversed behavior to light 

 is exhibited by the melanophores of the frog (Harless, '54), the 

 eel (Steinach, '91) and Triton (Hertel, '07). A lack of conform- 

 ity is likewise shown in the temperature responses of the frog's 

 retinal pigment (Herzog, '05; Arey, '16='). It is not improbable 

 that, in these animals, a more or less active nervous control 



