THE RETINA OF SPHENODON 



621 



The visual cells (Fig. 179; cf. Figs. 176b, 177a, 181; pp. 612, 615, 

 626) clearly explain the persistence of the fovea, which has been lost in 

 other nocturnal reptiles whose diurnal relatives are foveate (e.g., geckoes, 

 xantusiids, pygopodids). The matching single and double elements are 

 about equal in numbers and greatly predominant. They are manifestly 

 homologous with the single and double cone? of turtles and crocodilians; 

 but Sphenodon has converted them into physiological rods by enlarging 

 their outer segments and largely bleaching their oil-droplets — which, 

 however, have been retained (contrast Fig. 177b, p. 615). 



The third, tiny type of element is very 

 scanty. Never more than twenty can be 

 counted in a 10 [I sagittal section of the large 

 (17mm.) eye. It is an unmodified droplet- 

 free cone, obviously useless to the animal 

 and on its way to total disappearance. By 

 reference to Plate I, it will be seen that this 

 element must be the same cotylosaurian- 

 eosuchian droplet-free cone which has be- 

 come a rod in the turtles and, independently, 

 in crocodilians (and still once more in the 

 birds or in their dinosaurian ancestors — see 

 p. 661). It seems thus to be a little-cone- 

 which-makes-a-better-rod. Why, in converting 

 over to nocturnality, Sphenodon elected in- 

 stead the two droplet-bearing elements for 

 transmutation into rods, cannot be explained. 

 But the droplet-free element has obviously 

 proven unsatisfactory in modern reptiles as 

 a cone — not only to Sphenodon, where it is 

 even excluded from the fovea — ordinarily a pure-cone region in other 

 vertebrates (not one shows in the field of the photograph in Fig. 82, 

 p. 189) — but also in the lizards, which eliminated it entirely (see Fig. 

 180a, p. 626). 



Sphenodon very probably owes its long survival as a 'living fossil' to 

 its adoption of nocturnality, which was facilitated by the transmutation 

 of diurnal-ancestral cones into low-threshold elements, and expresses 

 itself elsewhere in the eye in the simplified optic nerve, the slit pupil, the 

 shallowed fovea, the enlarged lens (and cornea), the loss of the conus 

 papillaris (see p. 653), and the reduced accommodatory apparatus. 



Fig. 179 — Representative visual 



cells of Sphenodon punctatum: 



single rod, double rod, and cone. 



X 1000. 



