348 



CLASS OF EEPTILES. 



454. The class Reptiles 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 tra- 

 versed the respiratory organs ; and generally this admixture 

 takes place in the heart, which has in that case but a single 

 ventricle communicating with two auricles ( 108). Finally, 

 the skin of reptiles 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. 

 345), lizard (Fig. 844), and serpent (Fig. 346), have very 

 different forms. 



Fig. 344. The Green Lizard ; Lacerta viridis. 



Their head is almost always small, and their body very 

 elongated ; some, as serpents, are entirely without limbs, or 

 have only vestiges of them (Fig. 272) ; but most of these 

 animals, the lizard for example, have four limbs, constructed 

 so as to serve for walking or swimming. Nevertheless, their 

 limbs, when present, are so short as to allow the body to 

 touch the soil, and in place of being directed parallel with the 

 axis of the body, and consequently to move in this direction, 

 move perpendicularly to their axis, and from side to side. 

 Hence the name 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 



