274 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 
Fic. 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. ¢, cceliac 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 
