XX] CIRCULATORY SYSTEM 461 



two hemispheres and has nervous tissue on its roof as well as its 

 floor. The cerebellum is developed into a great flap which projects 

 back and covers the thin roof of the medulla oblongata (Fig. 207). 

 It has also lateral outgrowths called cerebellar lobes. 



The alimentary canal is considerably longer than the body and 

 is consequently folded. It has, as a matter of fact, a U -shape: 

 the first limb and a part of the next constitute the 

 cfnl entary stomach, which is marked off from the intestine by 

 a constriction and a powerful development of the 

 circular muscles forming a sphincter or circular muscle. To the 

 posterior aspect of the loop is attached the prominent spleen. The 

 intestine, although outwardly straight, is probably derived from a 

 corkscrew coil by the adhesion of successive turns : for the " spiral 

 valve " which, as we said, is merely a ventral infolding, has a very 

 strongly marked spiral course. The liver opens by the bile-duct 

 into the beginning of the intestine, and close to its opening is 

 situated that of the duct of the pancreas. A small gland of 

 unknown function, the rectal gland, opens into the hinder end 

 of the intestine. 



The pericardium is almost completely separated from the rest of 

 the coelom, communicating only by two narrow holes with it. The 

 heart has the typical structure described in the last chapter (see 

 p. 430). In the conus there are at least two transverse rows of 

 pocket- valves, occasionally more. The arterial arches arising from 

 the ventral aorta run up between successive gill-sacs and break 

 up into capillaries on the surface of the gills : from these the blood 

 is collected by vessels in the form of loops completely surrounding 

 the gill-sacs. From these loops four pairs of epibranchial vessels 

 arise and run backwards in the dorsal wall of the pharynx con- 

 verging to form the single dorsal aorta, which supplies blood to all 

 the hinder part of the body. The last gill-sac has a gill only on 

 its anterior border ; the blood from this does not reach the dorsal 

 aorta directly but is connected by a transverse vessel with the loop 

 surrounding the preceding gill-sac. The dorsal aorta gives off on 

 each side a subclavian artery to the pectoral fin and then four 

 median arteries which run down through the mesentery and supply 

 the alimentary canal. These are named the coeliac, anterior 

 mesenteric, lienogastric and posterior mesenteric arteries 

 respectively (Fig. 227). The most anterior, the coeliac, has two 

 important branches, (1) one supplying the liver and the proximal 

 part of the stomach with arterial blood, and (2) the other supplying 

 the anterior part of the intestine and the pancreas. The anterior 



