INTERNAL ANATOMY. 49 



The two last-named vessels only are represented in the 

 tongue-bars, and differ in their arrangement in the latter 



o 



in so far as the external vessel is enclosed within the 

 skeletal rod. 



The blood which circulates in the tongue-bars flows into 

 them, not from the branchial artery, but from the primary 

 bars through the cross-bars of the pharynx. The vessels 

 of each gill-bar unite above into a single efferent vessel, 

 which conducts the blood into the dorsal aorta of either 

 side. So that while efferent vessels issue alike from both 

 primary and tongue-bars, the afferent vessels, which lead 

 the blood directly from the branchial artery into the gill- 

 bars, are confined to the primary bars (Fig. 20). The 

 blood, having been oxygenated during its passage through 

 the gill-bars, past which a constantly renewed stream of 

 water is kept flowing, enters the dorsal aorta, and is then 

 carried backwards to the region of the intestine. The 

 two halves of the dorsal aorta, which we have already 

 noted on either side of the hyperpharyngeal groove, be- 

 come united into a common trunk behind the pharynx, so 

 that in the region of the intestine there is a single dorsal 

 aorta (cf. Fig. 28), from which lateral branches are given 

 off to the wall of the intestine. These then break up into 

 capillaries, which anastomose freely together, and so form 

 a perfect vascular network round the intestine. Finally, 

 the blood emerges from this capillary system into a large 

 vein lying below the digestive canal, the sub-intestinal vein. 

 Here it flows in a forward direction until it reaches the 

 base of the hepatic ccecum. At this point the vein appears 

 to stop short, but in reality breaks up into another system 

 of capillaries surrounding the liver. 1 From these again 

 the blood is collected into the large multiple hepatic vein 

 above the ccecum. Here it flows backwards as far as 



