150 BULLETIN OK THE BUREAU OF FISHERIES. 



mantle break up into a fine lacunar network that envelops the pallial muscles (fig. 

 133, Pal A). 



The gastro-intestinal arteries are a pair of large vessels leading off to the right and 

 left, respectively, from a very short thick trunk, the cceliac artery, that arises from the 

 ventral surface of the arterial bulb (fig. 132, GIA, p. 149). They immediately divide 

 into two branches, one of which passes forward sending out many small vessels to the 

 stomach, recurrent intestine, terminal intestine, and liver; the other of which runs 

 posteriorly supplying blood to the direct intestine, recurrent intestine, and surrounding 

 tissues. The anterior gastro-intestinal arteries are more or less symmetrical in their 

 course, while the posterior ones are not. The left posterior gastro-intestinal artery lies 

 to the left of the direct intestine to which it sends a rich supply of blood. The right 

 posterior gastro-intestinal artery lies on the right side of the direct intestine deeply 

 imbedded in the tissues. To expose this artery to view it is necessary to remove the 

 rectum and overlying tissues and move the recurrent intestine slightly to the right. 

 The right gastro-intestinal artery is slightly larger than its corresponding vessel on the 

 left side and further differs from it by giving off a large trunk, the intestinal artery 

 (fig. 132, 1 A) that runs posteriorly along the recurrent intestine, giving off small branches 

 that spread over it. In its course backward from the point of origin of the intestinal 

 artery the right posterior intestinal artery gives off two or three short trunks from its 

 right side that pass directly to the recurrent intestine, where they divide into small 

 vessels that spread out over the surface of the intestine. This artery also furnishes 

 several vessels that carry blood to the right side of the direct intestine and to the 

 rectum. 



The recurrent arteries (fig. 132, RRA and LRA) are a pair of long trunss that 

 arise from the lateral sides of the anterior gastro-intestinal arteries immediately after 

 their point of origin. They pass outward and then turn abruptly backward, passing 

 over the median side of the posterior retractor muscles to the anterior wall of the 

 posterior adductor muscle, where they turn downward and forward and give out 

 numerous branches to the walls of the lateral cavities and the mesosoma. 



The pericardial artery (fig. 132, PA) is a single median vessel that arises from the 

 ventral side of the cceliac trunk between the points of origin of the gastro-intestinal 

 arteries. It runs posteriorly on the middle part of the floor of the pericardial chamber 

 terminating in the region of the anus. In its course it gives off numerous small vessels 

 which go to the base of the pericardial chamber, the direct intestine, the recurrent 

 intestine, and adjacent tissues. 



The hepatic arteries consist of several pairs of short vesseis, usually three in num- 

 ber, which branch off from the aorta at right angles and penetrate directly into the 

 liver, where they divide into many branches that form a rich network throughout the 

 gland. 



The terminal arteries arise from the anterior end of the aorta which forks at a posi- 

 tion about midway between the point of origin of the intermediate pallial arteries and the 

 anterior extremity of the body. The two resulting branches continue forward and 

 downward to the anterior extremity of the body, where they turn back sharply on their 

 respective sides to form the anterior pallial arteries which traverse the lower edge of the 

 mantle for about one-half its length. In their course the terminal arteries also give 

 rise to small vessels that go to the anterior part of the liver, the genital glands of the 



