616 REPTILES 



The types of visual cells (Fig. 177a) are the same three as in the 

 turtles (c/. Fig. 176b, p. 612), and are respectively homologous with 

 them (see Plate I). The oil-droplets have long since been discarded 

 from the cones, however,* and the rods are rich in rhodopsin and greatly 

 outnumber the cones, instead of constituting a minority of the visual 

 cells as in even the most photophobic of chelonians. 



In the region backed up by the tapetum, it might be expected that the 

 cones would have become diminished in numbers or even eliminated, 

 to make that much more room for sensitive rods. Instead, the cones have 

 been retained; but their outer segments — even in the 'area centralis' — 

 have been made as rod-like as possible {i.e., heavy and cylindrical — Fig. 

 177b). Within this single retina we may thus observe a local, partial 

 transmutation of cones into rods. These 'intermediate' visual cells are 

 interestingly like the droplet-bearing elements of Sphenodon (Fig. 179, 

 p. 621) in their morphology — and no doubt, to a degree, in their 

 physiology. 



(C) Sphenodon 



See also pages: 216 visual cells 



78 rhodopsin 224 pupil 



135, 138 relationships 251 Fig. 100 



189-90 Fig. 82, visual cells, fovea 274 scleral ossicles 



200-2 oil-droplets 339-40 parietal eye 



206 vision 497, 519-20 color vision 



This single living member of the Rhynchocephalia was originally 

 thought to be a lizard, and was placed in the lacertilian family Agamidae. 

 Its true nature transpired at a time when the rhynchocephalians were 

 supposed to be very primitive. Anatomically, Sphenodon is indeed 

 'generalized' as compared with the highly specialized — though far older 

 — chelonians and crocodilians. But its position in modern taxonomy is 

 near the lizards. 



All sorts of efforts have been made to see the Sphenodon eye as the 

 'most primitive' sauropsidan optic; but it is nothing of the kind. So far 

 as the eye is concerned, Sphenodon can best be described as a pre-lizard 

 which has gone off the beaten track into nocturnality. 



*In Alligator and perhaps in all; but mentions of colorless oil-droplets (in unnamed 

 species!) occur in even recent literature. 



