CIRCULATOEY SYSTEM. 189 



bar. The aortic arches open above into the dorsal 

 aoi'tiu. Those of the primary pharyngeal bars alone 

 arise directly from the cardiac aorta. The ccelomic 

 aortic arches of adjacent bars communicate with 

 each other by vessels in the horizontal bars of the 

 pharynx. 



8. The dorsal aortse are a pair of longitudinal vessels into 

 which the aortic arches of both primary and second- 

 ary gill-arches open at their dorsal ends. They lie 

 just beneath the notochord, one on each side of the 

 epibranchial groove, and project into the dorsal 

 ccelomic canals. 



The dorsal aortaa carry the aerated blood back- 

 wards : they unite at the hinder end of the pharynx 

 to form a single vessel, which runs back along the 

 dorsal surface of the intestine. 



4. The portal veins are vessels running in the ccelom along 



the ventral surface of the intestine : they collect the 

 blood from the intestine and carry it forwards to the 

 liver, where they break up into capillaries. 



5. The hepatic veins are vessels which collect the blood 



from the liver and carry it backwards along the 

 dorsal surface of the liver. On reaching the junc- 

 tion of the liver and intestine, they unite to form a 

 single vessel, which runs forwards along the ventral 

 surface of the .pharynx and becomes the cardiac 

 aorta. 



6. A longitudinal vessel runs along the inner side of each 



atrial fold, and supplies the myotomes and reproduc- 

 tive organs. 



H. The Lymphatic System. 



Distinct from the ccelomic spaces is a system of lymphatic 

 spaces associated with the myotomes, nerve-roots, and other 

 parts ; of these the most conspicuous are a pair of large meta- 

 pleural canals running longitudinally within the metapleural 

 folds. 



