CHAPTER IX 



columba: a chordate with wings 



Externals. — The birds are principally distinguished by the 

 possession of feathers, and the modification of the fore limbs 

 into wings. The hind limbs continue to serve for terrestrial 

 locomotion, but it is important to note that although the birds 

 have evolved the habit of standing on two legs only, the body 

 is still carried in a horizontal position. The mouth is toothless 

 but bordered by a horny beak, the external nostrils are on each 

 side of the upper beak, a little way behind the tip. The eyes 

 have upper and lower lids, and also a " third eyelid," or 

 nictitating membrane. The ear-drum is no longer flush with 

 the surface of the skin, but sunk at the bottom of a tube, which 

 is the external auditory meatus. The alimentary and urino- 

 genital systems open at the cloaca. The tail is very much 

 shortened, and on its dorsal side is the uropygial gland. 

 This gland, which is the only one to be found in the skin of 

 birds, produces a secretion with which the bird preens its 

 feathers, and makes them waterproof. There are scales on 

 the legs, and claws at the ends of the toes (in a very few cases 

 also on the fingers), but no dermal ossifications of any kind are 

 present. 



Feathers. — The feathers are arranged on the surface of the 

 body in definite tracts, called pterylae. Feathers are formed 

 by the epidermis (see p. 224), and are of different kinds in the 

 various regions of the body. Those visible on the outer 

 surface of the bird are called penna?, which include the quill 

 or flight-feathers and the contour feathers of the adult bird. 

 Their typical structure may now be described. 



A penna consists of a stalk (quill or rachis) carrying a vane. 



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