40 The Bird 



is provided with a thick, callous pad, which, by constant 

 use, is thus ke])t Ijare of feathers. In addition, the under 

 sides of the degenerate wings are also free of plumage, 

 owing no doubt to the continual close application of 

 these organs to the sides of the body. The other bare 

 areas are almost obliterated. ))ut the legs are bare, thus 

 allowing perfect freedom in action. 



Some birds, such as vultures and cassowaries, have lost 

 all feathers on the head and neck, or other portions of 

 the l)ody, from various causes, as for cleanliness, or, in 

 some cases, })rol)abl}' for ornament. This will be spoken 

 of more in detail in a later chapter. 



Moult 



The waste of internal tissues and organs in animals 

 is repaired by means of the l^lood wdiich brings them 

 fresh material and carries away worn-out cells, as it 

 traverses arteries and veins. Entire parts, as the tails of 

 tadpoles, may even be absor}:)ed; but, in general,^ skin 

 structures when old and worn out are cast off and renewed 

 from the low^er, or derm, la}'er. This takes place in various 

 waj's. The skin, even to the covering of the e^'eballs, 

 may come off entire, as is the case among snakes, or por- 

 tions peel off and tear away, as in lizards. Warm-blooded 

 animals also shed, or cast, their outside covering; mam- 

 mals shedding their coats of hair, and birds their feathers. 

 In the latter class this process is called moidting. 



The nestling down and the feather which replaces it 

 can hardly be considered as separate structures, as the 



