xv. io SNAKES 411 



from the stump. The new sensory innervation of the skin is derived 

 entirely from the surviving dorsal roots, whose cells become greatly 

 enlarged. 



10. Suborder Ophidia 



The snakes are obviously descended from lizards of some kind, but 

 their precise mode of origin is obscure. Some workers believe that 

 their nearest living relatives are the platynotid lizards (monitors, &c). 

 There is evidence that the snakes passed through a burrowing stage in 

 their early history, although no known platynotids show marked fos- 

 sorial adaptation. A burrowing ancestry is particularly suggested by 

 the structure of the eye, which, as Walls has pointed out, differs 

 widely from that in typical lizards (Fig. 238). Thus there are no 

 scleral ossicles or cartilages in snakes, and accommodation is brought 

 about in a manner unusual for reptiles, involving displacement of the 

 lens. The visual cells include cones of a peculiar type, which have 

 apparently been derived from rods. The yellow retinal droplets that 

 serve to protect lacertilian retinae from excessive light are absent, 

 and instead some diurnal snakes protect their retinas by a yellow- 

 tinted lens. These features can all be interpreted on the supposition 

 that the ophidian eye was once drastically reduced, but has sub- 

 sequently been refurbished in response to the needs of life above 

 ground. Other characters that seem to point in the same direction 

 include the structure of the ears which, as in many burrowing lizards, 

 have apparently degenerated. The ear drums, tympanic cavities, and 

 Eustachian tubes are absent, and the columella auris articulates with 

 the quadrate. It seems unlikely that snakes can hear airborne sounds 

 at all well, though doubtless they are sensitive to ground vibrations 

 transmitted through the bones of the jaw. 



The snakes show many other interesting peculiarities, the most 

 obvious being the complete absence of limbs. Only in a few of the 

 more primitive forms such as the boas and pythons can rudiments of 

 the hind limbs and their girdles be found; in these snakes claws may 

 be present externally on either side of the cloaca and are said to play 

 a part in coitus. 



Locomotion is produced by the lateral undulation of the body, 

 which exerts pressure on surrounding objects and pushes the snake 

 forwards; the enlarged transverse ventral scales of most species help 

 to prevent slipping. A few snakes (e.g. some boas and vipers) can also 

 progress by muscular movements of the ventral scales, with their 

 bodies stretched out almost in a straight line. The spine is strengthened 



