I48 BULLETIN OF THE BUREAU OF FISHERIES. 



A VC, p. 149) in which are valves that permit the blood to flow toward the ventricle 

 only. On the other hand, the auricles communicate with large afferent oblique veins 

 on their respective sides. The afferent oblique veins enter the auricles in the ventral 

 anterior region where they greatly enlarge and become continuous with the walls of the 

 auricles (fig. 133, AOV, p. 149). The walls of the auricles are covered with a brown 

 colored, spongy tissue which presents a rough, irregular surface. These are the peri- 

 cardial glands (fig. 131, PG), which are described under the excretory system. The 

 auricles lie almost free in the pericardia} cavity. They are attached to the floor and 

 lateral walls of the chamber at their posterior extremities by numerous small blood 

 vessels which empty directly into the auricles. Anteriorly the auricles are held in 

 place by the afferent oblique veins with which they are continuous. 



ARTERIAL SYSTEM. 



The blood leaves the heart by a single anterior aorta, from which it is distributed 

 to the body through five channels which are as follows : (1) Three pairs of pallial arteries 

 which supply the mantle and pallial muscles; (2) a pair of gastro-intestinal arteries 

 which go to the stomach, intestines, posterior retractor muscles, posterior adductor 

 muscle, lateral cavities, and mesosoma; (3) a single pericardial artery which carries 

 blood to the walls of the pericardium, the direct and recurrent intestines, and the border, 

 ing genital glands ; (4) three pairs of hepatic arteries which go to the liver; and (5) a pair 

 of terminal arteries which furnish branches to the anterior parts of the body. 



The aortic bulb marks the beginning of the aorta and gives rise to several of the most 

 extensive arteries. It arises as a bulbous swelling from the anterior end of the ventricle 

 immediately below and just anterior to the point where the rectum penetrates into 

 the ventricle. 



The anterior aorta, which is the largest of the arteries, arises from the anterior end 

 of the bulb and runs forward on the dorsal surface of the body immediately below the 

 hinge ligament (fig. 133, A, p. 149). When it reaches the point just below the anterior 

 end of the hinge ligament it divides into two large right and left trunks that pass outward 

 to the outer surface of the mantle and then bend sharply backward to supply numerous 

 small vessels to the ventral anterior portion of the mantle. These are the anterior 

 pallial arteries (fig. 133, A PA). A very small artery continues forward and down- 

 ward from the bifurcating point of the aorta to send out fine branches over the dorsal 

 surface of the oesophagus. 



A second pair of arteries of minor extent arise from the mid region of the anterior 

 aorta and send branches to the anterior portion of the mantle folds. Because of their 

 position the author has named them the intermediate pallial arteries (fig. 133, IPA). 

 Sometimes two pairs of these intermediate pallial arteries are present. 



The posterior pallial arteries are a pair of large vessels which arise from the ventro- 

 lateral surfaces of the aortic bulb. The trunks pass out to the surface where they 

 fork into anterior and posterior vessels that in turn subdivide into numerous branches 

 that supply the entire middle and posterior portions of the mantle and the posterior 

 adductor muscle besides sending numerous small vessels into the liver (fig. 133, PPA). 

 The main branches of the three pallial arteries give off many minor branches that 

 continue to divide and subdivide into still smaller vessels which make a fine network 

 throughout the whole mantle. The trunks that terminate at the periphery of the 



