2 7 4 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



is greatest in the myxinoids, where the number of clefts varies (p. 239) ; 

 seven or eight in the notidanid sharks; and, as recent investigations 

 tend to show, probably six in the embryos of all other vertebrates. The 

 history of these arches differs greatly in the different classes (fig. 280), 

 there usually being a reduction in number by the more or less complete 



FIG. 280. Modifications of the aortic arches in different vertebrates, after Boas. 

 A, primitive scheme; B, dipnoan; C, urodele; D, frog; E, snake; F, lizard; G, bird; H, 

 mammal, c, coeliac artery; da, dorsal aorta; db, ductus Botallii; ec, ic, external and internal 

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

 blood black, those which disappear, dotted. 



abortion of one or more pairs as well as a modification of those that per- 

 sist, accompanying changes in the respiratory system. 



With the development of gills (ichthyopsida) each aortic arch be- 

 comes divided into two portions, an afferent branchial artery convey 

 ing blood from the ventral aorta to the gills and an efferent branchial 

 artery (sometimes called a branchial vein) carrying it from the gills 



