382 LESLIE B. AREY 



Tlio iiitcrpictatioJi of iuij>;nit()ry movcinents in the retinal 

 pigjnent of giusteropods, or to go further, the interpretation of 

 the presence of such pigment at all, from the standpoint of 

 preventing overstimulation or of securing optical isolation, 

 involves greater difficulties than is the case in arthropods. In 

 the snail, the brush-like rods, which have been described as the 

 photo-receptive portions of the sensory cells, are entirely distal 

 to the pigmeirt zone, hence these elements are at all times ex- 

 posed to the full strength of light. It has been pointed out, 

 however, that the pigment may serve as a background to pre- 

 vent reflection and thus it indirectly produces a limited 'opti- 

 cal isolation.' 



The accumulation of pigment far from the cell bases, and the 

 slight extent to which it can be induced to move proximally, 

 suggests that its chief utility may exist in a relationship with 

 that portion of the sensory cell which it immediately surrounds. 

 In this connection, a statement by Smith ('06, p. 270) is 

 suggestive : 



Aside from preventing internal reflections within the rod zone, 

 it is possible that the pigment is directly protective to that part of 

 the visual cell which is surrounded by it. We do not know that the 

 middle part of the sensory cell is not sensitive to light. Neither do 

 we know how or where light vibrations are transformed into nervous 

 impulse. If the transformation takes place in the middle zone, the 

 pigment may serve some purpose there. 



Having thus seen that the presence of retinal pigment of 

 gasteropods is only to be interpreted with difficulty, how much 

 greater is the task of assigning explanations to the feeble move- 

 ments exhibited by this pigment. It may be said that explana- 

 tions of the presence of retinal pigment and of its movements 

 are self-inclusive, an explanation of one necessarily involving the 

 other. That presence and mobility represent two more or less 

 distinct factors, follows, however, from the fact that in some, 

 and perhaps in most gasteropods (e.g., in Helix and Limax, 

 Smith, '06) the retinal pigment is non-motile. 



Theorists who have viewed the migratory movements of 

 retinal pigment in those eyes where striking changes occur as 

 indicative of the prevention of overstimulation or the procural 



