LIZARDS 813 



CHAPTER XIII. 



DIVISION I.— VERTEBRATES. 



CLASS III.— REPTILES. 



This class, whicli includes lizards, serpents, tortoises, toads, 

 and frogs, have cold blood, and a circulation less perfect than 

 those of the preceding classes. Only a portion of the blood re- 

 ceived from the body by the heart is sent to the lungs to be sub- 

 jected to the influence of air, whilst the remainder, mixed with a 

 part of that which has undergone the change produced in respi- 

 ration, is returned again into the circulation. The majority of 

 the animals of this class have two auricles to the heart, but only 

 one ventricle ; the red blood from the lungs is poured into the left 

 auricle, while the black blood from the body passes into the right. 

 The two kinds of blood are immediately transferred from the 

 auricles to the ventricle, where they mingle together, and, by the 

 contraction of the ventricle, are forced to the lungs and the body 

 through two distinct vessels. 



Unlike the Mammalia and Birds, the vessels of Reptiles are 

 filled with an imperfect fluid, which is not adapted to impart a 

 high degree of life and vigor ; and, therefore, as the animal heat 

 is always proportioned to the amount of respiration, they are cold- 

 blooded. Their lungs being small, and the circulation slow, they 

 consume but little air, and are able to live for a considerable time 

 without it. In general they are sluggish and indolent, have but 

 little sensitiveness, and digest their food very slowly. The brain 

 is small, and the nervous system imperfect ; and, although they in- 

 crease their species by means of eggs, they take no pains to hatch 

 them. 



I. — LIZARDS. 



This order includes a very considerable variety, and is com- 

 posed of the true lizards, the chameleon, the dragons, the croco 

 27 



