246 HISTOLOGY 



bundles near the outer basement membrane to a point one half the 

 thickness of the retina, where they stop and are continued distally as the 

 visual rods. Between the visual cells are found the sustentacular cells, 

 which extend as thin fibrillar bodies from the basal membrane up to the 

 tops of the visual cells. Here they expand into a pigmented cyto- 

 plasm, which surrounds and lies between the ends of the visual cells. 

 The nuclei of the visual cells are round and full, while those of the susten- 

 tacular cells are very much smaller and elongated. 



The visual rods are nearly as wide and as long as the cells from which 

 they come. Passing down their center almost to the tip is an axial 

 filament, which is of considerable thickness and is fibrillar. The ends of 

 the rods are frequently shrunken by the process of preparation, but, 

 when seen to advantage, they are full and rounded and almost touch the 

 vitreous body. The material of this body extends, as very regularly 

 arranged threads, up between the rods as far as the pigment layer, where 

 it is lost. The writers believe that these threads represent a flow of 

 secretion from some cells in the retina to supply the growing vitreous 

 body. Just which cells produce this secretion was not determined. 

 Judging from what is known of other worms, it must be the pigment 

 cells. 



The nerve fibers form series of bundles lying near the basal mem- 

 brane. They are seen cut in transection in Figure 218. Also one of the 

 visual cells may be seen sending a process into the bundles. The basal 

 membrane is very well marked both in form and staining power. It is 

 lined externally by thin flat cells, one of whose nuclei is to be seen in the 

 figure. 



In all the lower mollusk forms the sensory cells possess much the 

 same kind of visual rod or rhabdome. The rhabdome is varied to a 

 rather more elaborate and fan-shaped rod in Helix. 



There remain yet two very complex mollusk eyes to be described, 

 that which is found on the mantle edge or back of many plecypod mol- 

 lusks as Pecten, Spondylus, and Oncidium, and that of the dibranchiate 

 Cephalopods. These are most complex in structure and probably the 

 most efficient of the eyes of invertebrate animals. 



The eyes of Pecten are found scattered on the edge of its mantle folds 

 on short stalks, or lying directly on the surface. Among the former 

 is the eye of Pecten irradians, a common American form found on the 

 eastern coast of the United States. 



The entire eye is covered with a simple epithelium bearing a very 

 insignificant cuticle and resting on a well-developed basement mem- 

 brane. Most of these cells are tall and are heavily pigmented in their 

 proximal ends, the nucleus lying midway in the cell at the point where 

 the pigment ceases. The area forms a very broad band about the equa- 



