THE VASCULAR SYSTEM 



267 



truncus arteriosus, and distributed to the numerous gills by a series of 

 paired afferent vessels, the aortic arches. Contractile enlargements or 

 bulbils of these vessels aid in the propulsion of blood to the gills. Neph- 



ridia similar to those of annelids are associated with the gills and pre- 

 sumably assist in the eUmination of nitrogenous wastes. From the 

 efferent vessels the blood passes to the paired dorsal aortae, the anterior 

 extensions of which correspond to the internal carotid arteries of verte- 

 brates. Posterior to the gills the paired aortae unite to form the median 



