smith: eyes of pulmonate gasteropods. 269 



amphipods. 1 In these arthropods the position of the rhabdome is such 

 that it can be protected by pigment migration, but in the pulmonates 

 the rod is wholly distal to the pigment. Hence, whatever the purpose 

 of the pigment migration, it cannot here be for the same purpose as in 

 the arthropods cited. Some other method of explanation is necessary* 

 However, unless the pigment moves in response to efferent impulses 

 (which is improbable), the reaction is probably a direct response to light 

 stimulation. To that extent the "indifferent" cells may be sensitive. 

 It is true that I have not yet been able to demonstrate a migration of 

 pigment in either Helix or Limax, but in the eye of Planorbis some 

 migration certainly occurs. Not having been able as yet to determine 

 the exact conditions under which it occurs, I can only suppose from 

 analogy that the migration is a response to light. There seems to be 

 less need of pigment migration in snails which have their eyes on retrac- 

 tile tentacles than in those species which, like Planorbis, have their eyes 

 immovably fixed in the surface of the head. The shape of the cells, the 

 position of the pigment in some cells as compared with that in others, 

 and the apparent need of pigment migration in the eye of Flanorbis, all 

 point to a probable responsiveness of its pigment cells to light; but the 

 pigment cells are clearly " indifferent " in the sense that they do not 

 transform radiant energy into nerve impulses and transmit them to the 

 central apparatus, for they have no connection with the central nervous 

 apparatus. 



Hesse (:02 b , p. 610) ascribes to the pigment of the retina a secondary 

 function, that of insuring "optical isolation." To illustrate with Gam- 

 marus, the pigment which surrounds the rhabdome absorbs the oblique 

 rays and prevents their reflection back into the rhabdome. Confusion 

 of images is thus prevented. In dim light the migration of pigment 

 proximad makes it possible for the animal to utilize the reflected light 

 for directive movements. Somewhat similar explanations would hold in 

 the eyes of cephalopods and vertebrates. But in gasteropods the rods 

 are not isolated from each other by pigment, for they are all distal to 

 the pigment cells. No matter how intense the light, the rod is- exposed 

 to it, unless the eye be withdrawn into the body, as in Limax, or the 

 head within the shell, as in Planorbis. The pigment zone as a whole 

 absorbs the light which passes through the rods, and thus prevents re- 

 flections within the cup-shaped rod zone, and consequently any confusion 

 as to the direction of the source of light. Any other kind of " optical 



1 Somewhat similar pigment migration is known in the eyes of cephalopods 

 and vertebrates, but the mechanism of the movement is not known. 



