Rasic Structure <>f Vertebrates 



Asymmetry of paired blood-vessels, even the major trunks, is very 

 common. In reptiles the chief artery of the body, the dorsal aorta, is 

 symmetrically connected with the ventral heart by a pair of large 

 arteries forming a so-called "aortic arch." In adult birds only the right 

 vessel of this aortic arch, complete in the embryo, persists; in mam- 

 mals only the left vessel persists (Fig. 71). The very small arteries and 

 \eins are quite devoid of symmetry in their arrangement. 



XASAL PHARYNX 



Fig. 6. Diagram of the alimentary 

 canal. (Courtesy, Jackson: "Morris' Hu- 

 man Anatomy," Philadelphia, The Blak- 

 iston Company.) 



The right and left lobes of the liver and of the lungs are commonly 

 asymmetric in number, size, form, and position. The right and left 

 kidneys in mammals may not be exactly opposite each other. There is 

 more or less functional modification of symmetry. In a decidedly 

 right-handed person, the right arm and hand are more strongly de- 

 veloped than the left. 



One of the most striking cases of modified symmetry is that of the 

 "flatfishes" such as the flounder, sole, and halibut. These fishes ha- 

 bitually rest at the bottom of the water, lying flat on one side; in some 

 species on the right side, in others on the left. The external form of the 



