VISUAL SYSTEM: STATE OF THE ART 



53 



adaptation (1) the cone inner segment expands and (2) the cone fiber, that 

 portion connecting the cell nucleus to the inner segment, elongates (Figure 9). 

 This was apparently the first description of physiological changes in the 

 cone fiber of any vertebrate during light adaptation. However, it raises the 

 question of where the tissue displaced by the cone pedicles goes. Does the 

 outer plexiform layer become wrinkled? Unfortunately, Wang presents only 

 line drawings of this phenomenon. Photographs would help in clearing up 

 this question. 



Tamura and Niwa (1967) reported on the presence of cones in the 

 stingray Dasyatis akajei, but the neko shark Heterodontus japonicus was said 

 to be cone -free. In a followup investigation, Niwa and Tamura (1975) 

 examined the retinas of nine elasmobranch species, confirming the presence 

 of cones in Dasyatis but denying it in Mustelus, Heterodontus, Triakis, 

 Orectolobus, Glyphus (= Prionace), Etmopterus, and Discobatus. Species dif- 

 ferences may account for these observations, but cones definitely occur in 

 Prionace (Gruber et al. 1975) and probably in Mustelus (Stell and Witkovsky 

 1973a). 



Anctil and Ali (1974) found "a few cones" in the hammerhead Sphyrna 

 lewini; they were difficult to identify among the numerous slender rods. 



Ginglymostoma Cone Photoreceptor 



light adapted 



adapting 



dark adapted 



ELM 



cone fiber 



Figure 9 Photomechanical movements in Ginglymostoma cirratum. 

 While the rods are immobile, the cone fibers apparently elongate during 

 dark adaptation. This arrangement does not change the relative position 

 of rods and cones as occurs during photomechanical movement of 

 teleosts. (Redrawn from Wang (1968); Ph.D. dissertation.) 



