CLASS REPTILES. 



309 



are more complex than those of the lung-fishes or the 

 amphibians. In respect to circulation which we have 

 seen to be so closely related to the respiration, the reptiles 

 are intermediate between the lower and higher vertebrates. 

 The temperature of their blood is controlled by the out- 

 side conditions and not by the rate of oxidation on the 

 inside, and hence they are called cold-blooded. The heart 

 in most of them is three-chambered, as in the amphibians ; 

 but in the crocodiles there are two complete auricles 

 and two ventricles as in the birds and mammals. In 

 these the blood from the system and the blood from the 

 lungs do not mix, as happens in the amphibians and the 

 lower reptiles. The general arrangement of the arteries 

 and veins is much as in the higher vertebrates. 



318. The Chief Orders of Reptilia. For a view of the 

 numerous extinct types of reptiles the student should 

 refer to the geologies. The four modern orders are to be 

 considered as remnants, highly modified to meet special 

 conditions of life. 



The Lacertilia, or Lizards (Figs. 124 and 125). These 

 little animals are shaped much like the salamanders, 

 with which they are often confused. They are easily 

 distinguished from them by the rather stouter legs, the 

 presence of scales in the skin, and the presence of claws 

 on the toes. In this group are included, beside the forms 

 commonly recognized, the chameleon, which is gifted 

 with great power of changing color; the horned toad, of 

 the western part of the United States, which is not a toad 

 at all ; the Gila monster, of the Southwest, which is one of 

 the largest of the lizards and the only one known to be 

 poisonous; and the legless "glass-snakes" of the South, 

 so called because of the ease with which the body breaks 



