ORGANIC EVOLUTION 



203 



accompaniment of the shift from aquatic to terrestrial life. 

 Before it happens the gills begin to atrophy and new chan- 

 nels begin to be developed. A carotid artery springs from 

 the anterior side of the foremost branchial vessel on each 

 side, to carry blood from the heart into the head. A short 

 cut is developed between the dorsal and ventral portions of 

 the branchial vessels of the second pair, and these become 

 the aortic arches of the adult salamander, forming a con- 



FIG. 130. Diagram of types of circulatory apparatus in vertebrates x, a 

 lung fish (Ceratodus) ; y, a frog and z, a mammal. (For the sake of clear- 

 ness the auricle is turned backward, straightening out the sigmoid flexure.) 

 /, 2, 3, 4, the four branchial arches, becoming, in y and z\ i, the carotid; 2, 

 the aortic, and 4, the pulmonary arches; a, auricle; v, ventricle; /, lung; c, 

 cava; r, the single root of the dorsal aorta in mammals. 



tinuous uninterrupted channel from the heart to the dorsal 

 aorta, which ultimately becomes the main channel of the 

 circulation. Thus we see that in metamorphosis the fourth 

 arch that springs from the arterial trunk becomes the pulmo- 

 cutaneous artery; the first arch becomes the carotid, the 

 second becomes the aortic, the third atrophies, or becomes 

 variously connected with adjacent arches, and the bran- 

 chial circulation first laid down becomes so altered as to be 

 recognizable only by most careful comparison. 



