THE VASCULAR SYSTEM 



359 



kinds of blood in the dorsal aorta. In mammals the sinus venosus 

 becomes merged with the walls of the right auricle. 



Evolution of the Aortic Arches. The device of oxygenating blood in 

 pharyngeal gills is peculiar to chordates. Nevertheless aortic arches 

 connecting ventral and dorsal aortae in the pharyngeal region occur in 



Fig. 299.— Modifications of the aortic arches in different vertebrates. A, primi- 

 tive scheme; B, dipnoan; C, urodele; D, frog; E, snake; F, Uzard; G, bird; H, mam- 

 mal, c, celiac artery; da, dorsal aorta; dh, ductus Botalli; ec, ic, external and internal 

 carotids; p, pulmonary artery; 5, subclavian; va, ventral aorta. Vessels carrying venous 

 blood black; those with mixed blood shaded; those which disappear, dotted outlines. 

 (From Kingsley's "Comparative Anatomy of Vertebrates," after Boas.) 



annelids. It is a matter of opinion whether this point of resemblance 

 between annelids and chordates has a phylogenetic significance or is 

 simply a case of convergence. In chordates the number of aortic arches 

 is correlated with the number of visceral arches. Amphioxus has the 

 largest number, nineteen pairs, of primary visceral arches among chor- 

 dates. The aortic arches are correspondingly numerous. The largest 

 number of aortic arches in vertebrates occur in some species of cyclostomes, 

 fifteen pairs in bdellostoma stouti. Although some elasmobranchs have 



