THE HEART AND ARTERIES OF POLYODON 



421 



in each holobranch of sharks, as Alhs seems to suppose. As 

 shown in the figure, numerous nutrient arteries (a.nu.) are given 

 off all along the gill. Largest and most important of these are 

 the hypobranchials discussed below, which arise from the ante- 

 rior ventral divisions of the efferent arteries. 



a fil. e. a. til. tr. lym. 



a. fi 



a. fil 



e. br 



rsp. 



o. fil. 



'ym. 



a. br. a. 



br. e. 



Fig. 8 Stereogram of a region near the middle of a gill. The basal parts of 

 two and one-half gill-filaments are shown on each side. 



The arrangement of vessels near the middle of a gill is shown 

 semidiagramatically in figure 8. This arrangement is described 

 by Allen ('07, p. 106). The efferent artery (a.br.e) lies deep- 

 est and next above it is the afferent artery (a.br.a). The third 

 vessel (lym.) still higher is the branchial vein or lymphatic of 

 Allen's account. The afferent filamentar artery at its origin 

 immediately gives off a short spur (a.fil.s.) to supply the region 

 lateral to the main vessel and below its own point of origin. 



