XIX] CLASSIFICATION 451 



embryo and has the function of a respiratory organ. The skin 

 develops horny scales. 



Clas-s IV. AVBS. 



Gnathostomata agreeing with Reptilia in most points, but 

 having a constant temperature independent of that of the surround- 

 ing medium. The skin is provided with feathers instead of scales 

 and the fore limb is used as a wing. 



Class V. MAMMALIA. 



Gnathostomata agreeing in many points with Reptilia, but 

 clothed with hair instead of scales. The body, like that of Aves, 

 has a constant temperature independent of that of the surrounding 

 medium. The young are nourished after birth by the secretion 

 of certain glands of the mother termed milk glands or mammary 

 glands. 



292 



