VISUAL TISSUES 



245 



sac and the closing of the epidermal layers. This condition can be seen 

 in Figure 217, representing the eye of the common Florida conch, 

 Strombus gigas. 



This beautiful eye is well developed in all particulars, although not so 

 complex as that of the cephalopods. The eye-sac invagination has been 

 cut off from a well-developed epithelial cornea to which a thick layer of 

 connective tissue has been added. The sac ...-,.,..,...., 



lies, almost free, in a large blood space in 

 which blood coagulum and blood cells can be 

 seen. The sac is connected with the sur- 

 rounding tissues by a few plate-like strands 

 of connective tissues. The blood space is 

 supplied with the blood by a vessel which can 

 be traced, together with the nerve, through 

 the entire length of the long eye-stalk. 



The lining epithelium of the eye-sac is 

 developed into the visual cells, the supporting 

 elements (and several other cells). This is 

 done in a far greater degree on the posterior 

 side of the sac, where these cells form the 

 retina. This retina diminishes in thickness 

 as it is examined more toward the front, 

 until first the rods suddenly terminate and 

 later the pigment is lost, likewise abruptly, 

 and then there is left only a very thin layer of 

 simple epithelium lying on its outer basal 

 membrane and forming a transparent layer 

 to let the light-rays pass in. 



This anterior region of the eye-sac is in 

 direct contact with the lens, a spherical body 

 with a denser outer shell. The lens is a non- 

 cellular structure probably formed by the an- 

 terior cells of the sac which touch it. It is 

 not as large as the inside of the cavity of the 

 sac, but there remains between it and the retina 

 a crescentic (in section) space filled with the 

 vitreous fluid. In two other worms, it will be 

 remembered, this fluid was secreted by spe- 

 cial cells. This does not hold in the conch, 

 for the fluid is secreted from the entire sur- 

 face of the retina, as we shall presently show. 



The visual cells form the largest part of this retina, which is wonder- 

 fully clear and easy to study (Fig. 218). They extend from the nerve 



FIG. 21.8. Enlarged portion of 

 retina of Strombus gigas. vis.c., 

 visual cell whose yjsual rod 

 (vis.rd.) projects above from 

 the distal surface of the pig- 

 ment layer; ac.c., nucleus of 

 accessory cell which makes and 

 operates the pigment and also 

 forms the vitreous fluid ; vit.fl., 

 vitreous fluid which emerges 

 in thin streams from between 

 the visual rods; nv.f., nerve 

 fiber bundles. X 920. 



