352 SYSTEMATIC ZOOLOGY. 



to increase the respiratory surface, and also to lessen the 

 specific gravity of the bird. They are also possibly of use 

 in changing the position of the centre of gravity during 

 flight. 



The heart has four chambers, the single ventricle of 

 lower forms being divided into right and left ventricles. 

 The large blood-vessels which lead from the heart are, in 

 the embryo, much like those of the fish; but with develop- 

 ment some parts are altered and others suppressed (fig. 

 117, D), so that the result is more modified than in the forms 

 already discussed. Thus the left half of the third arch, 

 except for an artery (subclavian) going to the wing of that 

 side, has entirely disappeared, while the right half, here 

 called the arch of the aorta, connects the left ventricle with 

 the dorsal aorta. From this the first and third arches, 

 modified into carotids, seem to arise. The second arch 

 is completely suppressed, while the sixth arch, arising 

 from the right ventricle, carries the blood to the lungs and 

 forms the pulmonary artery. In returning from the 

 body the venous blood is emptied into the right auricle 

 and passes thence, through the right ventricle, to the lungs 

 for aeration; while that from the lungs goes to the other 

 side of the heart, and thence to all parts of the body. 

 Hence there is here no mixing of arterial and venous 

 blood in the heart. 



In the reproductive organs a constant feature is the 

 suppression of the right ovary, a rudiment of it existing 

 in a few forms. In the breeding-season the oviduct is 

 very large, and from its walls are secreted the white and 

 the shell of the egg. The eggs are large, and are always 

 enclosed in a limy shell. There is quite a difference in the 

 condition in which the young hatch from the egg. Some 

 are nearly naked and very helpless (altrices), while others 



