THE COMMON MUSSELS. 17 



figured the anterior pallial artery of the other side to the one just 

 described sprung from a point between the aortic bulb and the anterior 

 retractors, but considerably in front of the aortic bulb. There seems 

 to be considerable variation in M. magellanicus with respect to the 

 anterior pallial arteries ; but in no specimens were they found to start 

 from the aortic bulb. 



In M. lotus and edulis then, to summarise the above, the aorta 

 and the two anterior pallial arteries start together from the aortic 

 bulb; in M. magellanicus the anterior pallial arteries start from some 

 point of the aorta in front of the aortic bulb, either from the anterior 

 end as a direct continuation of the aorta or from some point midway 

 between the anterior retractors and the aortic bulb (Fig. 23) . The 

 anterior aorta in M. latus is continued into two circumpallial arteries 

 that pass round the margins of the mantle on the outer sides. The 

 anterior aorta in M. edulis ends in capillaries on the ventral edge of 

 the mantle near the anterior end, after dividing into two. The ante- 

 rior aorta in M. magellanicus is continued on one side, at any rate, 

 into the anterior pallial artery, which is not marginal. 



In M. edulis and magellanicus the upper part of the liver and the 

 dorsal surface about the course of the anterior aorta is supplied by 

 somewhat strong, although short, arteries from the aorta, so that the 

 anterior aorta in these two species is well furnished with short 

 arteries, giving off spurs on each side (Figs. 30 and 31). 



In M. latus the upper part of the liver and the pallial surface 

 above it are supplied chiefly by the anterior pallial arteries, and the 

 deeper parts of the liver by the gastro-intestinal artery yet to be 

 described. The anterior aorta has then very few large lateral 

 branches until it reaches the anterior end of the mantle, where 

 branches are usually sent off to the anterior part of the mantle. 

 These small pallial arteries sometimes spring from the anterior part 

 of the circumpallial arteries (Figs. 21 and 29). 



Returning now to the aortic bulbs : From the posterior underside 

 of the aortic bulb there arise two arteries, one passing forwards and 

 the other backwards (g a and p ao, Fig. 24) . The artery that passes 

 back springs from the right side of the aortic bulb, and is the largest 

 of all the arteries. It passes along the median line of the floor of 

 the pericardial cavity, standing out prominently when injected. It 

 is called by Sabatier the pericardial artery, but is in reality the 

 posterior aorta. The artery that passes forward from the underside 

 of the aortic bulb is a smaller artery, but is yet of considerable 

 calibre. It rises from the left side of the bulb below the rectum 

 (g a, Fig. 24), and passes forward along the pyloric csecurn on the 

 2 



