CLASS OF EEPTILES. 313 



well organized for flight as the preceding, and the first still 

 more completely webbed; finally, the swan, the goose, and 

 the duck (Fig. 234), whose bill is covered with a soft skin 

 instead of a horny envelope. 



CLASS OF REPTILES. 



454. The class Eeptiles comprises all vertebrate animals 

 whose respiration is from birth aerienne and incomplete. 

 They have lungs, like mammals and birds ; but their circula- 

 tory apparatus is always so arranged that a portion of the dark 

 blood mingles with the arterial, without having traversed the 

 respiratory organs ; and generally this admixture takes place in 

 the heart, which has in that case but a single ventricle commu- 

 nicating with two auricles ( 108). Finally, the skin of reptiles 



Fig. 269. Lezard Vert Piquet^.* 



is generally or almost always covered with scales. In their 

 general form they resemble mammals more than birds ; but 

 in this respect they vary. The tortoise (Fig 270), lizard 

 (Fig. 269), and serpent (Fig. 271) have very different forms. 

 Their limbs, when present, are so short, that they seem rather 

 to creep than walk, and their steps are not directed in the 

 axis of the body, but laterally : hence the word reptile. 



455. Their skeleton presents in its structure variations 

 much greater than what occurs in hot-blooded vertebrate 

 animals. All its component parts may in their turn be want- 

 ing, excepting the head and vertebral column ; but the bones 

 composing it preserve great analogies and homologies with 

 the hot-blooded vertebrata, and exhibit analogies with the 

 other classes. 



* The green lizard : lacerta viridia. 



