vi AETERIAL SYSTEM 399 



they are mainly in details. In the Elasmobranohs perhaps the most 

 conspicuous of these is to be seen in the relations of the efferent 

 vessels, each of which emerges, not from an ordinary aortic arch 

 traversing a gill septum, but from a vascular loop surrounding a 

 gill-cleft, the general arrangement being that shown in Fig. 188, C. 

 According to Dohrn (1886) this arrangement comes about in the 

 following way. As the walls of the clefts form lamellae and 

 develop respiratory activity, two branches grow downwards, one 

 anterior and one posterior, from the dorsal end of each aortic arch 

 (Fig. 188, A, VI). These branches become connected together 

 by cross bridges as shown in the figure (an). The aortic arch now 

 undergoes reduction and eventually becomes obliterated, just ventral 

 to the point where the two branches are given off (Fig. 188, A, 

 arch III), so that the arch is now divided into two distinct parts — 

 a ventral afferent and a dorsal efferent — the latter prolonged 

 ventralwards into the two branches. Of these the posterior branch 

 of each pair becomes somewhat reduced in size, it develops a 

 secondary connexion at its upper end. with the efferent vessel next 

 behind and loses its connexion with its original efferent vessel. 

 The result is that, after this has happened, each efferent vessel 

 again possesses two branches at its ventral end but these, instead of 

 passing into the same gill septum, pass into two adjoining septa 

 one in front of and one behind the intervening cleft (Fig. 188, B). 

 Eventually the ventral ends of each pair of branches become joined 

 so that the cleft is now surrounded by a complete efferent loop — 

 the loops of successive clefts being connected together by a single 

 persisting anastomotic vessel (Fig. 188, C). 



In Chlamydoselachus the modification of the aortic arches just 

 described does not take place. 



The number of aortic arches corresponds with that of the visceral 

 arches and is normally six. 



In correlation with the presence of the pseudobranch on the 

 posterior face of the mandibular arch in Elasmobranohs the first 

 aortic arch in these fishes is well developed but its primitive 

 relations with the arterial scheme become much obscured owing 

 primarily to the development of large new afferent and efferent 

 channels connected with the pseudobranch, which carries in its 

 train the reduction of both the ventral and the dorsal portions of 

 the original aortic arch. 



In the case of the second aortic arch, correlated with the fact 

 that the anterior face of this visceral arch has lost its respiratory 

 function, there is developed only a single, posterior, efferent 

 downgrowth instead of two as is the case with the arches farther 

 back. A wide anastomosis between this and the first aortic arch 

 just below the spiracle provides the secondary afferent vessel to 

 the pseudobranch which as already mentioned supplants the 

 primitive afferent vessel formed by the ventral portion of the 

 first arch. 



