176 



THE ELASMOBRANCH FISHES 



collector behind it. Soon the gill cleft is snrroiinded ventrally by these vessels, 

 but dorsally the loop is not yet completed around the cleft. A second important 

 change then takes place. A branch from the posterior efferent-collector passes 

 backward above the cleft and connects with the anterior efferent-collector 

 following. There is thus completed an efferent-collector loop around the entire 

 cleft characteristic of the adult. 



In certain forms, even in the adult, the posterior efferent-collector may re- 



ps.... 



Fig. 165 



Fig. 164 



Fig. 164. Arteries derivative of the first efferent-collector, Squahis suoldii. (L. H. Bennett, 

 orig.) 



hr.ef}, first branchial efferent ; e.c, external carotid ; ef.c}--, first and second efferent- 

 collector arteries; hy.ef., hyoidean efferent; ps., pseudobranchial artery. 



Fig. 165. Cross-trunks from eighth to ninth efferent-collectors, Squalus sucklii. (L. H. Ben- 

 nett, orig.) 



ct., cross-trunk ; ef.c}'^, eighth and ninth efferent-collectors. 



tain a dorsal commissural connection with the anterior efferent-collector of the 

 same gill. This interesting condition obtains in the adult Chlamydoselachus 

 (fig. 160). In this form the posterior efferent-collector sends a branch to join 

 the efferent-collector back of it, but it also retains an embryonic attachment 

 with the anterior efferent of its own holobranch. Usually, however, the branch 

 connecting it to the succeeding efferent-collector becomes so important that 

 the original connection of the posterior efferent-collector to its own efferent 

 anteriorly is entirely lost in the adult. In any event there results in the adult 

 a complete efferent circuit around the gill cleft, each circle being composed of 

 the posterior efferent-collector of one gill and the anterior efferent-collector 

 of the gill following. Around all the clefts (except the last) and the spiracle, 

 complete loops are formed as thus described. 



Since there are no demibranchs posterior to the last cleft no efferent-col- 

 lector lies back of this cleft and hence a circuit is incomplete around it. The 

 last posterior efferent-collector, that one in front of the last cleft, is usually 



