24 



VISION 



reported (Nicol 1964). A summary of results led to provisional grouping 

 of elasmobranchs on the basis of tapetal occlusability . Nicol thus recognized 



1) The occlusable tapetum, which darkens when pigments migrate over 

 the reflecting plates, found in pelagic species from well-illuminated waters; 



2) The nonocclusable, permanently bright tapetum occupying the entire 

 fundus, found primarily in deep-sea squaloids; 



3) The nonocclusable tapetum occupying part of the fundus, occurring in 

 certain littoral and benthic elasmobranchs (the condition of Scyliorhinus is 

 typical. Its tapetum is wanting ventrally, but where it exists it is permanently 

 bright); 



4) The partly occlusable tapetum, also found in benthic elasmobranchs, 

 characterized by partial expansion of the tapetal melanophores in the light- 

 adapted eye. 



Thus, even concerning the apparently simple question of tapetal occlus- 

 ability, the existing situation turned out to be rather complex. 



Detailed anatomical, physiological, and physical properties of the tapetum 

 were reported by Denton and Nicol (1964). Their histological study pro- 

 duced two noteworthy observations: (1) tapetal plates are aligned differently 

 in different parts of the eye so as to reflect light directly back through the 

 same optical path of entrance (Figure 5) and (2) nonpigmented processes of 

 the melanophore cells appear to be permanently extended in fixed pathways 

 over the tapetal plates. Denton and Nicol thus suggested that pigment gran- 

 ules, not the entire pigment cell, migrate during occlusion. This agrees with 

 the earlier ideas of Bernstein (1961) and Gilbert (1963) and later with 

 Kuchnow and Martin (1970a). 



Denton and Nicol also studied kinetics of occlusion in Squalus, using fixed 

 and fresh tapeta. The occluded (light-adapted) tapetum becomes completely 

 shiny after 1 h in darkness; the tapetum of a dark-adapted dogfish darkens 

 after about 2 h in bright light. Pharmaceutical agents and anesthetics had 

 little or no effect on expansion of pigments in isolated tapeta. Stripping the 



DORSAL 



Figure 5 Section through the center of 

 the eye of Prionace glauca showing the 

 orientation of tapetal plates behind the 

 retina. The guanine-coated plates are so 

 arranged as to reflect light back through 

 the path of entry. (Modified from Denton 

 and Nicol (1964). By kind permission 

 of the authors and the Journal of the 

 Marine Biological Association of the 

 United Kingdom © 1964 Cambridge 

 University Press Limited.) 



