352 



GENERAL ZOOLOGY 



are so quick that it is difficult for the eye to follow them. 

 When alarmed the scales can be raised, the throat patches 

 swollen, and the head elevated. The harmless little creature 

 then looks quite formidable. In order to provide for growth 

 the scaly skin is cast periodically. 



Internal Structure. The internal structure is, in general, 

 similar to that of the amphibians, but in several respects it 







2*7 



Fig. 179. The pine lizard. (Reduced) 



indicates a higher type of animal. Thus the lizard 

 breathes by lungs at all periods of its life. The 

 heart has a longitudinal partition extending par- 

 tially through the posterior chamber (Fig. 180), 

 so that there is an incomplete separation into 

 right and left ventricles. The heart is thus three- 

 chambered, being composed of one ventricle and two auri- 

 cles. The arteries leading out from the ventricles are so 

 arranged that impure blood returning from the tissues of 

 the body is not completely mixed in the ventricle with the 

 purified blood from the lungs. When both auricles contract, 

 the blood is forced into the ventricle. But the purified blood 

 from the left auricle does not mix completely with the im- 

 pure blood from the right auricle. Arteries which open from 

 the left side of the ventricle receive most of the blood from 



