48 



VISION 



Receptor Cells— Photoreceptors of elasmobranchs have been continu- 

 ously investigated since as early as 1866 (by Schultze). Recently, however, 

 a basic change in our understanding of the elasmobranch retinal receptor 

 types and correlated function has occurred (Figure 8). 



Hannover (1840) and later Muller (1851) and Schultze (1866) distin- 

 guished two types of vertebrate photoreceptors— the rods and cones. A for- 

 mulation known as the Duplexity Theory of vision which assigns separate 

 visual functions to each receptor type has come down from these original 

 findings. The basic assumption is that functioning of the retina in dim light 

 (scotopic conditions) is mediated by the rod system, while cones take over 



CONE 



Figure 8 Light micrograph of the photoreceptors of Carcharodon 

 carcharias. The maximum rod-cone ratio in this species was 4:1. ELM = 

 external limiting membrane; HC = horizontal cell; IS - inner segment; 

 ONL = outer nuclear layer; RE = retinal epithelium; ROS = rod outer 

 segment. (From Gruber et al. (1975). Reprinted from the Bulletin of 

 Marine Science by permission.) 



