SNAKES. 109 



but also the breathing of the animal. This casting 

 of the skin, furthermore, is accomplished in a very 

 remarkable manner, inasmuch as it is forced off 

 by the formation on the inner skin of a layer of 

 very fine hairs, which serve the purpose of releas- 

 ing the old skin by their rigidity of position. 

 Strangely enough, a precisely similar mechanism 

 is adopted by the Crustacea for ridding them- 

 pelves of the old shell when this has become too 

 small. These hairs are known as casting hairs. 

 Having performed their primary purpose, they 

 are retained for purely decorative ends, becoming 

 transformed into the curious, and often micro- 

 scopic, stripes, ridges, or spikes which ornament 

 the edges of the scales in the snakes and other 

 reptiles which change the skin after this fashion, 

 and the carapace of the Crustacea. 



The eye is peculiar in that, like the Gecko 

 and certain burrowing lizards it has no eyelids. 

 Instead, it is covered by a horny shield, which 

 may be compared to a watch glass. This shield, 

 as in the case of the Gecko, is probably formed 

 by the transformation of the nictitating mem- 

 brane. The snake, however, differs from the 

 Gecko in that the eye, behind its shield, is 

 immovable. 



The nostrils are situated at the end of the 

 snout. The sense of smell appears to be well 

 developed, some snakes indeed, as in the common 

 English Grass-snake (Tropidonotus natrix), hunting 

 its prey by smell rather than by sight and 

 sound. 



The tongue is long, slender, forked at the top, 

 and very sensitive, serving entirely as a delicate 



