SMITH : EYES OF PULMONATE GASTEROPODS. 275 



furnishes us with a number of very large sensory cells whose details are 

 not obscured by pigment. It is doubtful whether, in the whole animal 

 kingdom, there is a more suitable example for illustrating the fibrillar 

 endings than in the rods of Limax. The accessory retina quickly per- 

 mits a view of the fibrils in the cell body by both the vom Rath and 

 the Bethe methods. The method of Prentiss is less certain for these 

 reasons : (1) it employs methylen blue, (2) the rods lie within a capsule 

 which is difficult of penetration, and finally, (3) the mantles of the rods 

 seem to offer special difficulties to the differentiation of their fibrils. 



The opinions of investigators regarding the nature of the rods have 

 varied greatly. The rod-zone was at first mistaken for the entire retina 

 by Krohn ("37, '39). No one concerned himself with the nature of the 

 rods until Hensen pointed out the axial fibrils. The relatively clear 

 anatomical results of Babuchin did not suggest to him any special func- 

 tion for the fibrils. He assigned light-reception to the pigmented cells, 

 which carried no rods. The rods entirely escaped the notice of Car- 

 riere and Simroth. Hensen was the first to commit himself on the 

 structure of the mantle, which he thought was a secretion of the pig- 

 mented- cells. Along with the acceptance of the opinion that the recip- 

 ient organs in other groups of animals were cuticular structures, the 

 same idea was applied by him to the mantle of the rod of gasteropods. 

 Hilger urged this conception for Limax, but missed the striae which 

 Babuchin had described twenty years earlier, in spite of the improve- 

 ments in technique which had meanwhile been made. Hilger believed 

 that each pigment cell of the retinal group produced its quota of the 

 mantle of the projectng rod cells. Patten spoke against the conception 

 that the mantle, being the recipient surface, could be inert, and he 

 introduced the idea of neurofibrils. He believed, apparently, that the 

 fibrils were imbedded in a common cuticular mass, of which the lens and 

 vitreous humor formed a part, and he thought the fibrils were best 

 differentiated by dissolving away the cuticula. Such a course is unneces- 

 sary for the rod of Limax, because it shows the mantle fibrils in the 

 fresh state. Patten says (p. 618) "the vitreous body, the lens, and 

 the retinidial layer, at the edge of the optic cup merge into each other 

 and by means of a gradual series of changes pass into the cuticula of 

 the hypodermal cells surrounding the optic cup," thus making the 

 vitreous humor, as I have said, a cuticular structure. It does not 

 appear, however, that this is a correct view. 



There are several facts which point to the mantle of the rods as con- 

 sisting of something besides fibrils. Hesse intimated as much, for he 



