RETINAL PIGMENT OF PLANORBIS 381 



in which the pigment granules under illumination would migrate 

 away from the source of light instead of toward it would be 

 produced." 



It has already been pointed out that the exceptional behav- 

 ior of the bod}' chromatophores of the frog, eel, and Triton 

 probably is the result of a nervous control. As far as direct 

 responses are concerned, Parker's further assertion ('OG, pp. 

 411-412), for all we know to the contrary, is true: "Hence it 

 seems probable that the melanophores, retmal pigment cells, 

 and other like structures in which dark pigment granules ex- 

 hibit migratory movements, are restricted as to these possibili- 

 ties, and that in light they always transport their ])igment 

 toward the source and never in the reverse direction." 



Since the pigmented cells of the gasteropod eye have no 

 demonstrable nervous connections, this condition, if true, ren- 

 ders these cells wholly indifferent in the process of light i)er- 

 ception. It follows, therefore, that such cells are not comparable 

 to the pigment-bearing retinular cells of certain arthropods 

 in which the ])igment granules, contained within the sensory 

 cells themselves, change ])()siti(ni in light and in darkness. The 

 situation in the gasteropod, howe\'er, is quite similar to that 

 in the vertebrate eye in tliis respect. 



In several crustaceans (e.g., Parker, '99, upon the am])hii)od, 

 Gammarus) it has been shown that in darkness the pigment 

 of the retinulai- cells mo^es proximally. thereby leaving part 

 of the rhabdome devoid of pigment, whereas in light the rhab- 

 dome again receives a pigment sheath. These responses were 

 interpreted as having the following significance. In the light, 

 the rhabdome, surrounded by pigment, is protected from over- 

 stimulation by light reflected internally from the white pigment; 

 in dim light, on the contrary, the efficiency of the visual apparatus 

 is increased by the withdrawal of pigment, whereby the reflect- 

 ing mechanism enables the eye to make the best use of the 

 available diffuse light. Theories involving the principles of 

 oA-er-stimulation as well as of optical isolation have also found 

 many supporters among those who attempt to explain the 

 phenomenon of pigment migration in the vertebrate eye (Arey, 

 '15). 



