VII 



MOLLUSOA—THE SENSORY ORGANS 



173 



retinal cells lie on the inner side of the limiting membrane, and are 



thus turned to the source of light _ajid at the same 



time to the cavity of the primary vesicle. The '■r-:':r:\^-^k:w.fl. t 

 retina is covered on its inner side by a somewhat y:f,^^:fvgX-f:'-^!ry-- 

 thick membrana limitans. 



2. The eye is surrounded, except on the side 

 turned to the surface of the body, by a eartila- 

 ginous capsule, which resembles the sclerotica in 

 the vertebrate eye ; this cartilage, where it covers 

 the retina, is perforated like a sieve, so that the 

 optic nerves can pass through it. 



3. Immediately underneath the cartilaginous 

 floor of the retina lies a very large ganglion opticum, 

 in the form of a massive cerebral lobe. From this 

 rise the nerves which run to the retina through the 

 perforations of the cartilaginous capsule. 



4. The two halves of the lens, which are unequal 

 in size (the outer being the smaller), consist of 

 homogeneous concentric laminae. 



5. The cavity of the primary vesicle (between 

 the retina and the lens) is filled with perfectly 

 transparent fluid. 



It has been proved that, as in the Arthropoda 

 and Vertebrata, the pigment granules of the rod 

 cells, which in the dark lie at the base of the cell, 

 under the influence of light travel towards its free 

 end. 



i. 



in 



and the Eyes 

 Pecten (Fig. 



Fig. 149.— Two retinal 

 cells of a Ceplialopod, 

 much magnified (after 

 Grenadier). 1, Meui- 

 lirana limitans ; 2, piii- 

 ment ; 3, secreted 

 tlire.ads ; 4, nerve fibre ; 

 5, rod ; 6, pigment ; 7, 

 limiting cell ; 8, limiting 

 menjbrane ; 9, retinal 

 cell ; 10, nerve fibre. 



The Dorsal Eyes of Oneidium 

 at the edge of the Mantle 

 150) and Spondylus. 



These eyes have been said to resemble vertebrate 

 eyes in structure, because in them the visual rods 

 are turned away from the light, being directed 

 inwards towards the body. 



They are vesicular eyes, but in them it is the 

 outer wall of the vesicle, that turned to the light, which becomes the 

 retina, while the inner wall (which in other Molluscs forms the retina) 

 is a pigmented epithelium. At the same time the outer or retinal wall 

 is invaginated towards the inner pigmented wall, as is the endoderm 

 towards the ectoderm in the formation of the gastrula. The conse- 

 quence of this is, that the cavity which in other Mollusca is filled by the 

 gelatinous mass (lens) disappears, and the vesicle becomes a flattened 

 thick-walled plate (Peden) or cup {Oneidium), consisting of a pigment 

 layer and a retina. The body epithelium which passes over the eye is 

 unpigmented and transparent, and here becomes the cornea. Beneath 



