532 GENERAL CHARACTERS OP REPTILES. 



Birds ; but we only rarely find a prolongation having an analogy 

 with the marsupium ( . 340). The eyelids are generally to the 

 number of three, but are sometimes entirely wanting; in 

 Serpents, for example, the skin is continuous without inter- 

 ruption in front of the eyes ; and only presents in this point 

 sufficient transparency, not to oppose any obstacle to the passage 

 of light, an arrangement which gives to these animals a re- 

 markable fixity of look. The auditory apparatus is much less 

 complete than in Mammalia, or even than in Birds. The external 

 ear is almost always entirely wanting ; and the tympanum is at 

 the surface of the head, either uncovered, or concealed beneath 

 the skin ; sometimes there does not exist even a trace of it. The 

 cavity of the tympanum is usually but very imperfectly closed 

 in by the bones of the cranium, and communicates by a large slit 

 with the pharynx, of which it sometimes seems to be only an 

 appendage ; the small bones of the ear are generally wanting ; 

 and the cochlea is often rudimentary. The nasal fossae are but 

 little developed ; and the sense of taste seems to be very obtuse 

 in all these animals. The tongue is sometimes thickened and 

 fleshy ; but in general it is thin, dry, very protractile, and cleft 

 towards the end ; Serpents and Lizards (Fig. 309) afford us 

 examples of this. The tongue of Frogs 

 presents a singular peculiarity j it is fleshy, 

 and fixed to the jaw by its anterior ex- 

 tremity, whilst its tip, usually directed 



backwards, is free and can be turned forwards. Lastly, in the 

 Chameleon, this organ becomes a very remarkable instrument 

 of prehension ; for the animal can dart it out to a distance which 

 exceeds the length of its body and it is terminated by a kind 

 of adhesive cushion, to which the flies and other insects, on 

 which this slow and awkwardly-moving reptile feeds, are easily 

 attached. 



473. There are few Reptiles which live on vegetable matter 

 alone. Almost all are carnivorous, and, with some exceptions, 

 only seek a living prey, which they swallow generally without 

 dividing it ; hence the choice of the animals on which they are 

 nourished is regulated, so to speak, by the size of their mouth. 



