402 



CEPHALOCHOEDA 



[CH. 



The blood system is exceedingly simple. The blood from the 

 alimentary canal is brought back by a sub-intestinal vein, which 

 like a broad river is often subdivided into two or three parallel 

 channels which then reunite with one another ; in a word it is more 

 a plexus than a tube. It runs to the tip of the liver on its outer 

 side, returns on its inner side, and pursues its course then as a 

 single channel under the pharynx, where it is called the ventral 

 aorta. In this region it is contractile, deriving its muscles from 

 the walls of the ventral coelornic tube. Vertical branchial vessels 



FIG. 198. Diagrammatic transverse section of Amphioxus to show the relation 

 of the excretory and genital organs. 



1. Nerve-corcL 2. Notochord. 3. Myotome. 4. Upper sclerotome 

 which gives rise to the fin-rays. 5. Lower sclerotome. 6. Developing 

 genital organ. 7. Dorsal coelomic canal crossed by the solenocytes 



of the nephridium communicating with the branchial coelomic canal in 

 the gill-bar: the lower ref.-liiie points to the branchial coelomic canal. 

 8. Nephridium. 9. Dorsal aorta. 10. Pharynx. 11. Atrial 

 cavity. 



called arterial arches are given off; these ascend in the gill 

 septa, that is, the portions of the wall of the pharynx intervening 

 between the gill-slits. Arriving at the dorsal line of the pharynx 

 these vessels empty into two longitudinal vessels, the dorsal 

 aortae, which further back unite into one (Figs. 189 and 190). 

 The tongue-bars also contain vessels emptying into the dorsal 

 aortae ; these communicate with the branchial vessels through 

 what are called synapticulae, that is, cross pieces tying the 

 tongue-bar to both sides of the gill-slit which it divides (Fig. 191). 



