smith: eyes of pulmonate gasteropods. 271 



slightly larger. The cytoplasmic contents of the two are not conspic- 

 uously different either in form or in structure. The indifferent cells are 

 not tli in and highly refractive at their proximal ends as are the pigment 

 cells of the chief retina, but their basal -ends resemble, instead, those of 

 the cornea. It is therefore too much to say with Henchman ('97, 

 p. 429), respecting the accessory eye : " In one respect only does it differ 

 from the chief eye : the cells corresponding to the pigment cells of the 

 retina contain no pigment. In other respects it presents the same his- 

 tological conditions and a similar arrangement of histological elements," 

 as in the chief retina. The one constant feature is the presence of the 

 two kinds of cells, but they have such an irregular arrangement and fre- 

 quently are so bent on themselves (Plate 4, Fig. 55) that the knife 

 seldom cuts any of them lengthwise in the eyes which I have examined. 

 The indifferent cells of the accessory retina bear little resemblance to 

 the pigment cells of the chief retina, but they do resemble corneal cells, 

 by which they are almost entirely circumscribed laterally. I am, there- 

 fore, disposed to believe that the degree of differentiation in the non- 

 sensory cells of the accessory retina puts them into much closer 

 structural relationship to the corneal cells than to the pigment cells of 

 the chief retina. 



Hesse gives to the accessory retina a copious vitreous humor, which is 

 continuous with that of the chief retina. The vitreous humor between 

 cornea and lens is made very much more voluminous in his figure than 

 it is in the eyes which I have sectioned. My photomicrographs (Plate 1, 

 Fig. 2 ; Plate 2, Fig. 18) show that there is only a very small amount of 

 vitreous humor in front of the lens. In looking over serial sections of 

 ten eyes, chosen at random, eight showed no connection between the 

 vitreous humor of the two retinas. The two others were doubtful owing 

 to a tearing of the section by the shattered lens. As has already been 

 stated, the accessory retina may have a small lens, though usually there 

 is no accessory lens, and sometimes there is no accessory vitreous 

 humor. In one case the accessory lens was in the most ventral part of 

 the accessory retina, lying against the capsule. 



The use of the polariscope in the study of the fresh rods of Limax 

 establishes two facts : (1) The fibrillae are present as a normal mechan- 

 ism in the living rod ; their presence in sections, therefore, cannot be 

 explained on the ground that they may be artifacts produced by the fix- 

 ing or staining fluids ; (2) The fibrils are doubly refractive to light. 

 The rod of Limax adds one more example to those presented by Howard 

 (:03), who showed that the light-recipient organs in several groups of 



