THE PIGEON AS A TYPE OF BIRDS. 505 



shortness of the large intestine, the presence of a cloaca, 

 may be noted. 



The heart is four-chambered ; the single aortic arch curves 

 to the right side ; only the pulmonary artery rises from the 

 right ventricle ; the valves between the right auricle and the 

 right ventricle are in part themselves muscular; the red 

 blood-corpuscles are oval and nucleated ; the temperature 

 of the body is from 2°-i4° F. higher than that of Mammals, 

 . and much higher than that of the cold-blooded Reptiles. 



The lungs are fixed to the dorsal wall of the thorax ; the 

 bronchial tubes expand in irregular branches in the lungs ; 

 the ends of some of these branches are continued into 

 air-sacs surrounding the lungs ; these air-sacs are in com- 

 munication with air-spaces in the bones. The trachea is 

 supported by bony rings, and has a larynx at its upper end, 

 and a syrinx or song-box at the origin of the bronchi. 



The 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. 



The testes lie beside the kidneys ; the vasa deferentia run 

 beside the ureters and open into the middle region of the 

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

 mentary. 



The eggs have much yolk and a hard calcareous shell. 

 The segmentation is meroblastic and discoidal. The allan- 

 tois is chiefly respiratory, though it may also help in absorbing 

 the nutritive substance of the egg. 



The Pigeon ( Columba) considered as a type of Birds. 



The varieties of domesticated pigeon with which we are 

 familiar, are all descended from the rock-dove, Columba 

 livia, and afford very vivid illustrations of variation, and of 

 the results of artificial selection. The power of rapid flight, 

 the diet of seeds, the affection between mates and between 

 the parents and their young are well known. 



Form and External Characters. — The form, well suited 

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

 feathers. The cere overarching the nostrils, the third eyelid 

 hidden in front of the eyeball, the external opening of the 

 ear almost concealed by the feathers, the preen-gland on the 



