108 A COURSE ON ZOOLOGY. 



reproduces its species by eggs ; but the mother does not 

 sit on the eggs ; they are laid in places which are most 

 likely to be favorable for the hatching. 



From what has already been said of the mixture of 

 venous and arterial blood, the conclusion can easily be 

 drawn that the blood that circulates in the lizard is not 

 pure. Hence there is a less activity in the respiratory 

 combustion, since the blood is less charged with oxygen ; 

 consequently the temperature of the animal is lower, and 

 reptiles are called cold-blooded animals, in comparison 

 with birds and mammals, which are spoken of as warm- 

 blooded animals. These terms must not be interpreted 

 literally. It would be more exact to say that reptiles 

 have a variable temperature, that is, the temperature is 

 about that of surrounding objects, while birds and mam- 

 mals are animals of constant temperature. When it is 

 cold, the latter are able to increase the respiratory com- 

 bustion and to maintain the bodily heat; while, when 

 the weather is warm, a more restricted diet and less 

 active movement are followed by a diminished combus- 

 tion, and consequently the high external temperature 

 meets an internal balance. Lizards, and in general all 

 reptiles, have no power to maintain such equilibrium of 

 temperature. 



The Frog. The frog belongs to the class of batra- 

 chians. It has a naked, clammy skin, covered with a 

 thin epithelial layer, which is continually being renewed. 

 It has two pair of feet and has no tail. 



The thoracic and abdominal cavities are not separated 

 by a diaphragm. The digestive apparatus is not unlike 

 that of the lizard ; the jaws are provided with small 

 teeth. 



In the respiratory apparatus, the lungs are only two 



