VASCULAR SYSTEM. 



29 



The vascular system. There is no heart, but the larger 

 vessels are peristaltically contractile. Lying on the ventral 

 side of the endostyle in the pharyngeal wall there is a 

 longitudinal sub -pharyngeal vessel, which corresponds to 

 the heart and ventral aorta of the Vertebrata. Anteriorly it 

 terminates by branching to the lips. Laterally all along 

 its course it gives off branches, 

 which have on their bases small 

 contractile swellings called bul- 

 bils, to the primary bars. These 

 ascend dorsalwards and open 

 into the aortic root of their own 

 side. The secondary bars receive 

 their blood supply from the 

 primary through the transverse 

 bars. The roots of the aorta lie 

 on each side of the hyperpharyn- 

 geal groove (Fig. 10). In the 

 intestinal region they unite to 

 form the single dorsal aorta, 

 which gives off branches to the 

 intestine and lateral body walls. 

 Both the aortic roots are con- 

 tinued forward as the carotid 

 arteries. These are joined to- 

 gether by a transverse anasto- 

 mosis, and the right vessel gives 

 off a large much convoluted 

 branch which passes ventral- 

 wards at the level of the velum 

 and ends blindly (Fig. 9). The 

 anterior part o*f the right carotid 

 has the form of a plexus, and 

 gives off branches to the oral cirri of both sides. 



The intestinal blood is collected into a sub-intestinal vein lying 

 in the ventral wall of the intestine. This vein is not a simple 

 vessel, but consists of a plexus of vessels, frequently communi- 

 cating and lying side by side. Anteriorly the sub -intestinal vein 

 appears to break up into a capillary system on the wall of the 

 hepatic caecum. The blood of the caecum is collected into the 



FIG. 13. Portion of the excretory canal 

 of a young Amphioxus with its soleno- 

 cytes, from the living animal (after 

 Goodrich). 1 solenocyte ; 2 tube of 

 solenocyte ; 3 excretory canal ; 4 

 flagellum of solenocyte. 



