THE PIGEON AS A TYPE OF BIRDS. 629 



in the lungs ; the ends of some of these branches are continued 

 into surrounding air-sacs ; these may be continued into the 

 bones, and end in minute air-spaces. The trachea has bonv 

 rings, a larynx (without vocal chords} at its upper end, and a 

 syrinx or song-box (with vocal chords] at the origin of the 

 bronchi. Expiration is the more active part of the respiratory 

 process. 



The (metanephric] kidneys are three-lobed, and lie embedded 



in the pelvis ; the ureters open into the cloaca ; there is no 



bladder ; the urine is semi-solid, and consists chiefly of urates. 



Water must be mainly got rid of by evaporation from the 



ivalls of the air-sacs and air-passages. 



The testes lie beside the kidneys ; the vasa deferentia run 

 outside the ureters, and open into the middle region of the 

 cloaca. The right ovary atrophies, the right oviduct is rudi- 

 mentary. There is rarely any copulatory organ, but it is 

 large in ostriches, ducks, geese, and some other birds. 



The eggs have much yolk and hard calcareous shells. The 

 segmentation is meroblastic and discoidal. The allantois is 

 chiefly respiratory, though it helps in absorbing the nutritive 

 substance of the egg, and acts as a receptacle for the embrvo^s 

 waste products. 



THE PIGEON (Columba) AS A TYPE OF BIRDS. 



The numerous varieties of domesticated pigeon (pouter, 

 fantail, tumbler, etc.) are all descended from the rock-dove, 

 Columba livia, and afford vivid illustrations of variation, 

 and of the results of artificial selection. Certain variations, 

 e.g. in beak or tail, crop up, we know not how ; and similar 

 forms are bred together until a new breed is established. 

 The diet of seeds, the wooing of mates, the feeding of 

 the young by both parents, are well known. 



External characters. --The form of the body, well suited 

 for rapid flight, ceases to be graceful when stripped of its 

 feathers. The cere above the nostrils, the third eyelid in 

 the anterior upper corner of the orbit, the external opening 

 of the ear concealed by the feathers, the preen gland on 

 the dorsal surface at the root of the tail, and the cloacal 

 aperture, are external features easily recognised. 



The feathers most important in flight are the twenty-three 



