1 84 COMPARATIVE ANATOMY 



The blood-vascular system of Amphioxus differs from that 

 of other vertebrates in the absence of a definite heart. It 

 presents, however, a very characteristic vertebrate feature 

 namely, a hepatic-portal system. The blood from the in- 

 testine and posterior end of the body is collected into 

 a large sub-intestinal vein which runs forward below the 

 intestine till it reaches the hepatic caecum. Here it breaks 

 up into a number of capillaries surrounding the liver. From 

 these the blood is collected into a hepatic vein lying on 

 the dorsal side of the caecum ; the hepatic vein runs back- 

 wards to the point where the caecum joins the intestine, 

 then turns sharply forwards and runs below the floor of the 

 pharynx in the endostylar ccelom as the branchial aorta. 

 The branchial aorta contracts rhythmically from behind for- 

 wards, and it is obvious that in position ajid function it 

 corresponds to the heart and truncus arteriosus of higher 

 vertebrates. The branchial artery gives off lateral vessels 

 which run, right and left, into each primary gill-bar. Each 

 lateral vessel is dilated at its base into a contractile bulbil, 

 and immediately beyond it divides into three branches, which 

 run up the bar, one to the outside of the ccelomic canal, one 

 internally to the skeletal rod, and one near the internal end 

 of the bar (fig. 45, C, z> 3 , z> 2 , v l ). In the tongue-bars there 

 is no coelomic vessel, but an external and an internal vessel 

 are present, the former embedded in the skeletal rod. The 

 vessels of the tongue-bars have no connection below ' with 

 the branchial artery, but receive their blood supply through small 

 lateral vessels running in the synapticula. The two or three 

 vessels of each gill-bar unite above to form a single efferent 

 branchial vessel which leads direct into the dorsal aorta. 

 Before opening into the efferent branchial vessels the ccelomic 

 vessel of each primary bar and the external vessel of the tongue- 

 bar next behind it give off branches which unite to form a 

 plexus surrounding the excretory tubule overlying each primary 

 gill-cleft. The blood from the plexus is carried to the dorsal 

 aorta by efferent vessels. In the pharyngeal region there are 

 two dorsal aortae, one lying on each side of the hyperbranchial 

 groove (fig. 44, ao). Behind the pharynx the two aortae unite 

 into a single vessel which runs back above the intestine, giving 

 off numerous branches which form a rich capillary network on 

 the walls of the gut. From these capillaries the blood passes 



