smith: eyes of pulmonate gasteropods. 245 



The Histology of the Eyes of Limax maximus and 



Helix pomatia. 



Having described the general relations to each other of the two kinds 

 of retinal cells, I shall now consider each in more detail. Except where 

 it is expressly stated otherwise, the descriptions refer to Limax. Inas- 

 much as the pigment cells are simpler and less important than the sen- 

 sory cells, they will be considered first. 



If the eye be macerated sufficiently, very instructive views may be 

 obtained by preparing the retina between two cover-glasses in the way 

 already described. By this process the retinal cells are more or less 

 completely separated from one another and from the capsule. The pig- 

 ment cells, isolated and entire, are obtained more easily than the sensory 

 cells by this means. They are elongated and club-shaped with dark- 

 brown granular pigment filling the enlarged distal ends (Fig. 10, com- 

 pare also Fig. 9, Plate 1). As cross sections show (Figs. 5, 6, 12), the 

 enlarged ends are prismatic, owing to mutual pressure. Near the middle 

 of its length the cell tapers rather abruptly into a slender, cylindrical 

 stalk, whose diameter continues to diminish slowly toward the basal 

 attachment. This stalk-like portion of the cell is clear, unpigmented, 

 and narrower than the nucleus, which consequently produces an enlarge- 

 ment in it. Proximally the slender stalk branches into a number of 

 fine, root-like processes (rdl'S), which continue on into the connective- 

 tissue capsule, where they are lost to view in its meshes. It is not 

 unusual in macerated preparations to find two or three separate pigment 

 cells clinging by these roots to the inner face of a piece of the capsule. 

 None of these root-like processes have, as some investigators suppose, 

 the nature of dendrites or neurites. They do not find their way out of 

 the capsule, but end among its fibres. They are, therefore, organs of 

 attachment, whereby the pigment cells maintain their proper place in the 

 retina. The nucleus (nl 1 .) occupies a place near the upper end of the 

 stalk ; it is ellipsoidal and nearly twice as long as thick. In macerated 

 preparations it is homogeneous, non-granular, highly refractive and shows 

 no nucleolus. 



Tangential sections through the retina at the level of the pigment 

 zone must, of course, show cross sections of a certain number of the 

 retinal cells. Such cross sections have been figured and described re- 

 peatedly, but the mutual relations of the cells have never been por- 

 trayed as well as they are shown in depigmented sections (Plate 1, Figs. 

 3, 5-8, 11, 12). If the pigment cells are not too far decolored, they 



