smith: eyes of pulmonate gasteropods. 243 



becomes the cornea, whereas the remainder develops into the retina. 

 The cornea (Fig. 2) is an unpigmeuted layer of cells which are long, 

 slender, prismatic, and translucent. Their arrangement is in general 

 radial, although not strictly so, for their long axes are directed towards 

 the front of the lens, more than half of which lies in the distal hemi- 

 sphere of the eye. The more proximal marginal cells of the cornea 

 thus either stand at right angles to the chief axis of the eye or their 

 central ends may even point a little toward the distal pole of the 

 eye. 



The retina makes up the remaining, or proximal, two-thirds of the 

 optic sac. The peripheral ends of the retinal cells are everywhere in 

 contact with the capsule. The marginal cells of the retina pass in along- 

 side the corneal cells, so that the margins of the two structures are in 

 contact. In general the retinal cells, like the corneal, have a radial 

 arrangement. The position of the pigment makes it convenient to dis- 

 tinguish three concentric zones in the retina, which, however, are some- 

 what arbitrary divisions, for, as I have stated, the retina is an epithelial 

 structure which is only one cell thick. The peripheral zone (Fig. 1, 

 rtn. ex.), in contact with the capsule, is unpigmeuted. It contains the 

 nucleated portions of the retinal cells. The middle zone (rtn. m.) con- 

 tains the pigment, except for which light from in front would enter the 

 peripheral zone. The internal or central zone (rtn. i.) lies between the 

 pigment zone and the lens, and contains the rods, — the light-recipient 

 structures of the retina. 



The retina consists of two, and only two, kinds of cells, — pigmented 

 and unpigmeuted. Both are attached to the capsule at their basal ends, 

 and stand side by side in a radial fashion. They will be described in 

 detail subsequently. The pigment cells are only about two-thirds as 

 long as the unpigmeuted cells, and they are indifferent ; that is, non- 

 sensory. They support the unpigmeuted cells, which are the sensory 

 elements of the optic apparatus. The pigment cells are the more 

 numerous, particularly toward the margin of the retina, and they are 

 arranged around the sensory cells in such a way as to suggest the ap- 

 pearance of ommatidial groups of cells. That part of the sensory cell 

 which lies distal to the pigment zone is the rod. 



Fach rod consists of two parts (Plate 1, Fig. 2): (1) a club-shaped 

 axis (ax. bac), which Henchman ('97) rightly interpreted as a longitudi- 

 nally fibrous, distal prolongation of the sensory cell ; in the figure it has 

 the appearance of a dark core ; (2) a thick mantle (" Stiftchensaum " of 

 Hesse), which Babuchin ('65) was the first to describe as radially stri- 



