THE ELASMOBRANCH FISHES 173 



second branchial afferent joins the common trnnk of the last two so that, in 

 them, only two stems leave the ventral aorta. 



Each afferent continues around the base of the cartilaginous gill arch in 

 front of the branchial rays, giving off smaller afferent arterioles both to the 

 anterior and to the posterior filaments of a gill {Squalus sucklii, fig. 161, 

 hr.af.). In their course upward the afferents grow smaller and smaller and 

 terminate in the dorsal part of the branchial region. An interesting arrange- 

 ment is reported by Allis (1911) for Chlamydoselachus (fig. 160) in which the 

 afferent arteries, except the hyoidean and last branchial, instead of ending 

 dorsally, bifurcate, one branch («/.") passing over the cleft anteriorly to join 



Fig. 160. Branchial arteries of Chlaviydoselachus. (From Allis.) 



af.", anterior division of afferent; af.^, posterior division of afferent; hr.af, fifth bran- 

 chial afferent ; efc, efferent-collector ; hy.af., hyoidean afferent artery ; or., orbital artery, 



the afferent in front, the other (af.^) passing dorsally and back over the suc- 

 ceeding cleft to join the following afferent. In this arrangement, which is not 

 greatly unlike that of Squalus, the afferents are connected into a series of 

 closed loops around all the clefts. 



CAPILLAEIES 



The capillaries in the gill filaments or folds connect the arterioles of the 

 afferents (see fig. 145, a.h.) with a similar series of efferent arterioles (e.h.) 

 leaving the gills. They form a net so complex that it is impossible to trace an 

 individual capillary. The wall of each capillary is made up of a single layer of 

 cells, forming the effective membrane through which the exchange of gases is 

 made. If a longitudinal section could be made through the entire length of a 

 single capillary it would begin where the thicker walled afferent arteriole ends 

 and end with another thicker walled arteriole, the beginning of the efferent- 

 collector type of vessel. 



EFFEEENT-COLLECTOES 



Blood brought to the gills by the afferents passes into the capillaries of the 

 gill filaments. Here it is oxygenated and is then sent down the gill filaments 

 (e.b.-, fig. 145) into efferent-collectors (efc.) lying at their bases. The efferent- 



