360 REPTILES. 



nor a true fish, but a sort of ambiguous production 

 sharing the attributes of both. Let us, however, 

 examine their structure a little more closely. 



In Eeptiles the circulation is arranged in such a 

 manner, that the heart, at each contraction, sends 

 into the lungs only a small portion of the blood 

 received from the various parts of the body ; so 

 that the bulk of the circulating fluid returns to the 

 system without having passed through the lungs, and 

 undergone the process of respiration. 



It is respu'ation that communicates to the blood its 

 heat, and to the muscles their irritability. We find, 

 therefore, that Eeptiles have cold blood, and that 

 their muscular power is, upon the whole, less than 

 that of the quadrupeds and birds. Accordingly, their 

 movements are generally confined to those of creep- 

 ing and swimming ; and although many of them can 

 leap and run quickly upon some occasions, tlieir 

 general habits are lazy, their digestion excessively 

 slow, their sensations obtuse, and in cold and tem- 

 perate climates they pass almost the entire winter in 

 a state of lethargy. Not possessing warm blood, 

 they have no occasion for clothing capable of re- 

 taining heat, and they are consequently covered with 

 scales, or simply with a naked skin. As another 

 consequence of their w^ant of vital warmth, no Keptile 

 sits upon its eggs, which frequently have only a 

 membranous envelope, and are left to be hatched 

 entirely by the heat of the sun, or of the soil in 

 which they are deposited. 



The class of Eeptiles is of great extent, and em- 

 braces many forms of animals that differ widely from 

 each other, both in their structure and habits ; they 

 may, however, be grouped under four principal 

 sections, characterized as in the following Table : — 



