INTRODUCTION. 



The Reptilia constitute a class of vertebrated animals 

 of which the structural characters are as follow : — Thev 

 have cold blood, — that is to say, their power of pro- 

 ducing animal heat is so limited as scarcely to be appre- 

 ciable, and not sufficient, therefore, to keep up any 

 standard temperature of the body, nor to prevent it from 

 following all the thermal variations of the atmosphere or 

 water by which they are surrounded. 



The integument is covered with hard and dry cuticle 

 in various modifications of form, in some constituting 

 broad plates, in others imbricated scales. The heart is 

 in all cases trilocular, — that is to say, it is composed of 

 two auricles and a single ventricle ; the respiration is 

 exclusively pulmonary throughout life, and their repro- 

 duction is oviparous. The Amphibia, or Batrachia, which 

 are included in the Reptilia by Cuvier and many other 

 naturalists, differ from them, however, in various essential 

 and important characters. The heart particularly is bilo- 

 cular : the integument is naked, and the respiration is 

 carried on by means of branchiae during the earlier period 

 of life, changing — in some totally, and in others parti- 

 ally — to the pulmonary character in the adult condi- 

 tion. 



