dorsal aorta 



„ , gill raker 



erterent artery 



adductor branchialis muscle 

 cross connection 



nerve efferent ortei 



external constrictor muscle 



gill ray 



bronchial oi 



afferent artery 

 original channel (afferent artery) 



— ventral aorta 



B 



Figure 11-21. Interrelationships of vessels in the arch of a shark, A and B, semidiagrammotic 

 anterior view of two stages (A and D of Figure 1 1-20) of arch development; C, cross section of arch. 

 (After Goodrich, 1930) 



extend outward in the gill filaments: one afferent and two 

 efferent vessels in each filament. 



In Myxine the aortic arches are similar to those of the 

 lamprey with the exception that each aortic arch serves the 

 hemibranchs of a single pouch, a unique situation. The af- 

 ferent artery is associated with the outer opening of the 

 pouch, the efferent with the inner. There are interconnec- 

 tions between the arteries of a series, and it is quite possible 

 that the pouch relationship is secondary, resembling that 

 observed in the efferent system of the shark. The pouches 

 are difficult to homologize with those of the lamprey or the 

 other vertebrates because the arches cannot be numbered. 



General observations on aortic arches 



There appears to be a consistent plan of arch formation 

 among both agnaths and gnathostomes. This constancy is a 

 reflection of the similarity of having branchial bars which 

 primitively bore gills. There is no fi.\ed number of aortic 

 arches, no fixed type of ventral aorta or of dorsal vessel or 



vessels. The lack of a premaxillary arch in any vertebrate 

 casts doubt on the existence of such a segment. The loss of 

 the first (mandibular) arch as a simple entity in all 

 vertebrates is of interest, and it might be significant in terms 

 of the original presence of jaws even in the agnaths. 



The usual discussion of the evolution of the aortic arches 

 in tetrapods implies that the observed adult conditions are 

 retentions of phylogenetic stages suggesting an amphibian to 

 reptile to mammal sequence. Actually parallel modifications 

 are indicated in the loss of the second and, usually, the fifth 

 arches and in various asymmetries. The living amphibians 

 have maintained symmetry in the reduction of arches, while 

 shortening the ventral aorta. The living reptiles have lost 

 the truncus and have developed separate outlets (4) for the 

 carotid, pulmonary, and right and left systemic arches. The 

 reptile pattern may have been derived from that observed 

 in the salamander. The bird has carried the reptile pattern 

 one step further with the loss of the left systemic arch. The 

 mammal has separate openings (2) for the joined carotid 

 and systemic arches, and the pulmonary stems, and has re- 



basilar 



orbital a 



mandibular artery 



1 ^-—mesenteric 

 I' muscle 



subclavian artery 



bronchial bar valve 



efferent artery 

 efferent artery 



hypobranchial 



B 



septum 



Figure 11-22. Semidiagrammotic lateral view of aortic arches of a chimoerid, Hydro/ogus, A, and 

 a cross section through an arch, B. 



THE AORTIC ARCHES 



353 



