CHAPTER V. 



CLASS OF REPTILES. 



462. THE class of Reptiles comprises all the cold-blooded 

 Vertebrated animals, whose respiration (in the perfect state, at 

 least) is aerial and incomplete. They have lungs, as Mammalia 

 and Birds ; but their circulating apparatus is always arranged 

 in such a manner, that a part of the venous blood is mixed with 

 arterial blood without having traversed the respiratory organ. 

 This mixture usually takes place in the heart ; which only 

 possesses a single ventricle, into which open the two auricles. 

 In their general form, Reptiles approach Mammalia nearer than 

 Birds ; but they offer in this respect many variations, as may 

 be seen by comparing together, a Tortoise (Fig. 326), a Croco- 

 dile (Fig. 329), a Serpent (Fig. 339), and a Frog (Fig. 342). 

 Their head is almost always small, and their body very much 

 lengthened out; some, as Serpents, are entirely destitute of 

 members, or have only traces of them ; but the greater number 

 of these animals, the Lizard and Frog for instance, have four 

 limbs, formed so as to serve for walking or swimming. Further, 

 these members are generally too short to prevent the trunk from 

 dragging on the earth ; hence, most Reptiles are accustomed to 

 crawl rather than walk ; from which circumstance it is, that 

 their name is derived.* 



463. The skeleton in this class presents much greater varia- 

 tions in its structure, than in warm-blooded Vertebrata. All 

 the parts of which it is composed are wanting in one or another 

 group, excepting the head, and the vertebral column ; but the 

 bones of which these are composed, always preserve a great 

 resemblance to those of Mammalia and Birds, and are easily 



* The Latin repto means, to crawl or creep. 



M M 2 



