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by black pigment which absorbs incident light. In dim illumination the 

 processes are retracted and the guanin cells, now exposed, reflect light 

 reaching them back upon the retina. This mechanism provides more light 

 for the rods in dark-adaptation, and provides a sharper image in the light- 

 adapted eye. A tapetum is sometimes wanting (abyssal shark Somniosus, 

 pelagic ray My/iobatis, etc.) (146). 



In the eyes of some teleosts there occur photomechanical changes 

 consisting of migrations of retinal pigment and movements of the visual 

 cells. These changes are most conspicuous in duplex retinae (provided 



Fig. 8.12. Occlusible Chorioidal Tapetum of Elasmobranchs 



(Left) Section through chorioid of light-adapted eye, showing pigmented processes 

 expanded on external surfaces of guanin plates, shielding them from light which has 

 passed through the retina. (Right) dark-adapted condition, showing pigmented processes 

 retracted, whereby the guanin can reflect light back through the visual cells, gp, guanin 

 plates ; cc, chorioidal capillaries ; n, nucleus of guanin cell ; pc, layer of migratory chori- 

 oidal pigment cells ;pe, pigment epithelium of retina (devoid of pigment); uc, unmodified 

 chorioid; pp, process of chorioid pigment cells. (Diagram based on Mustelus, from 

 Walls (146), after Franz.) 



with rods and cones) and are concerned with light- and dark-adaptation 

 of the retina. 



In teleost eyes provided with retinal photomechanical mechanisms, the 

 retina is backed by a layer of pigmented epithelium having long processes 

 which extend between the visual cells. When the fish is exposed to bright 

 light, pigment granules migrate down these processes, forming sleeves 

 about the visual cells and shielding them from oblique rays. In dim light 

 the pigment migration is reversed. The cones and rods of many teleosts 

 are also contractile. In the light-adapted condition the cones lie outside 

 the pigmented region or migrate away from it, while the rods move in the 

 direction of the pigment and are partially shielded. In darkness the rods 



