BIRDS ORGANISATION DIAGRAM 4 57 



CLASS BIRDS 



DIAGRAM 4. 



BIRDS are vertebrate animals which are always easily recog- 

 nised by having only two legs and the body covered with 

 feathers. They have also very warm blood. Nearly all birds 

 fly, and their body seems formed to cleave the air. The beak 

 forms a point in front, like the prow of a ship, and the body 

 ends behind in a tail, by which the bird directs its flight, as a 

 boat is guided by the rudder. The whole structure of the bird 

 is arranged for flight ; the feathers overlap one another like the 

 tiles of a roof, for gliding through the air ; the wings, which 

 correspond with the arms, spread out, and fold back against the 

 body ; they are moved by very large muscles, which form the 

 greater part of the flesh of the bird. These are those found on 

 each side of the breast attached to a bone called the sternum, 

 which is furnished with a sort of keel in the middle. Therefore 

 the more the keel of the sternum is developed, the easier the 

 bird flies. It is easy to see the difference in this respect between 

 the sternum of a fowl which never rises from the ground, and 

 that of a duck, which sometimes makes long journeys. The 

 wings are provided with long feathers which spread out like a 

 fan when the bird opens its wing, and fold one over the other 

 when it closes it. These feathers are sometimes very long, and 



