REPTILES 



original snakes. And these losses and defects were so numerous that the 

 snakes had almost to invent the vertebrate eye all over again. Nothing 

 like this tremendous feat has occurred in any other vertebrate group, 

 so far as we can tell. No other vertebrates except the placental mam- 

 mals* have had to do any 'rebuilding' at all. Wherever else the eyes have 

 degenerated, they have remained degenerate as long as their owners 

 survived. We can perhaps understand now why a legless lizard is not a 

 snake simply because it is legless. The snake-shaped lizards such as 

 Ophisaurus and Pygopus originated above-ground, and escaped the pain- 

 ful period of near-extinction which the true snakes experienced and 

 which they have so gloriously survived. 



The Ophidian Retina — Apart from the visual-cell layer, the strata 

 of the modern snake retina are quite orthodox 

 in structure, and it is unlikely that they have 

 undergone any drastic reconstruction as a con- 

 sequence of the underground babyhood of the 

 Ophidia; for, wherever the eye has become 

 vestigial but has retained a functional retina 

 (e.g., in cascilians), one notes that though the 

 visual cells are reduced to nuclei each bearing 

 a mere knob of cytoplasm, the nuclear and 

 plexiform layers are still present and distinct. 

 The phylogenetic steps between the Typh- 

 lopi-[\ke condition and the mammalian-like 

 retina of the Boidas are lost, and we can only 

 guess at them (Plate I) . The boas and pythons 

 all have the same retina, exemplified in Figure 

 184 by Tropidophis, which has only single 

 cones and rhodopsin-bearing rods. The cones 

 here (as in all snakes) lack not only oil-drop- 

 lets but also paraboloids and myoid extensi- 

 bility. In all these respects, they indicate 

 plainly that they were never derived directly 

 from above-ground lizard cones. 



Between the Boidse and the great central 

 family Colubridse there is again a great gulf, 

 which may be partly filled if ever the retina of 



And perhaps the cod family (see pp. 398-9, and footnotes on pp. 586 and 588). 



O.N 



fe €' 



(?) 



•G. 



Fig. 184 — Retina of one of 



the Boidse, Tropidophis mel- 



anurus. x 500. 



P.E.- pigment epithelium; R.- 

 rods; C.- cones; L.- Limitans; 

 O.N.- outer nuclear layer; 

 I.N.- inner nuclear layer; G.- 

 ganglion-cell layer. 



