228 CHARACTERS OF VERTEBRATE ANIMALS 



The elongated cylindrical form is well suited for gliding- 

 through thick undergrowth or herbage, and it may be for climb- 

 ing, while some snakes are modified for swimming and others for 

 burrowing. The resemblance to the limbless lizards is striking, 

 but here as elsewhere too much importance must not be attached 

 to similarity of shape, as this may result from the same or similar 

 habits in animals which are not closely related. No snake ever 

 possesses the least trace of fore-limbs, even their girdles being 

 entirely absent, as also is the sternum. Snake-like lizards, on 

 the other hand, may have very small fore-limbs or external 

 traces of the same; and if not, as in the blind-worm, dissection 

 shows that the girdles are represented, nor is the sternum ever 

 absent. As regards hind-limbs, these are generally entirely 

 absent in snakes, but vestiges are in some cases to be found one 

 on each side of the cloacal aperture. 



The head is small and flattened, passing without any 

 perceptible neck into the trunk, and that again into a gradually- 

 tapering tail. In nearly all cases the mouth is a wide slit, and 

 a small rounded nostril can be seen on each side of the snout 

 at or near its tip. The stony stare which is so characteristic 

 of a snake is due to a very peculiar arrangement. Instead of 

 upper and lower eyelids, there is a circular area of transparent 

 skin by which the eye is protected, and occupying the space 

 between this and the front of the eyeball is a tear-chamber always 

 full of the secretion of the tear-gland. 



Numerous overlapping scales cover the body, these being 

 replaced on the head and under surface of the body by 

 horny plates. Ventral shields, as those in the latter position 

 are termed, are never to be seen in the limbless lizards. Snakes 

 exhibit a great variety of colouring and marking, by which 

 protection or other purposes are served, as will be seen else- 

 where. One very characteristic habit, not peculiar to this group, 

 however, is the periodical shedding of the outer part of the skin. 

 The snake wriggles out of this slough, turning it inside out in 

 the process. 



The internal organs of snakes (fig. 144) exhibit a number 

 of peculiarities, dependent partly on the elongated shape of 

 the body, partly on the way in which prey is secured and 

 swallowed, and partly on the manner of locomotion. The first 

 factor, for example, influences the number of the vertebrae, and 



