MAMMALIA. 387 



representation (Fig. 287) these air-tubes are shown at one side, 

 and the lung in its natural con- 

 dition on the other, The reader 

 is thus furnished with the means 

 of comparing these important 

 organs in man, with those which 

 have been already exhibited (Fig. 

 242) as existing in birds. 



Covering. While scales form 

 the characteristic covering of 

 fishes, and feathers of birds, hair 

 may be said to be that of the 

 mammalia. It is not invariably 

 present, and it undergoes many 

 modifications in its appearance. 

 We term it wool upon the sheep; 

 the same material becomes spines 

 upon the Hedgehog, and "quills 

 upon the fretful Porcupine" 

 (Fig. 288). It even assumes an 

 aspect still more extraordinary, 

 and is converted into bony 



Fig. 287. AIR-TUBES, AND 

 LUNGS OF MAN. 



Fig. 288. POUCUPINE. 



plates in the defensive covering of the Armadillo (Fig. 311). 



