THE COMMON MUSSELS. 19 



which bifurcates on the lower lip to supply the ventral side of the 

 inner labial palps. Posterior to the origin of this tentacular artery, 

 and almost opposite the posterior point of attachment of the inner 

 labial palps, the anterior ventral artery gives off on the lower side 

 another branch. This artery runs back in the median line, conceal- 

 ing the anterior ventral artery, and is easily seen on the ventral 

 surface in injected specimens. It passes up the middle of the 

 anterior side of the foot : hence it may be called the pedal artery 

 (p a, Figs. 24 and 28). The anterior ventral artery also gives off 

 branches to the liver above it, and to the anterior retractors. 



These are the main branches from the anterior aorta in M. latus, 

 in which it thus supplies in part the dorsal surface of the body in 

 front of the heart, and also in part the underlying mass of liver; it 

 supplies also the supra-aesophageal cavity, the anterior edges of the 

 mantle, the labial palps, the muscles of the foot and byssus (except 

 the posterior parts of the posterior retractors of the byssus), and the 

 foot itself. All these parts are supplied by the anterior aorta and its 

 branches . 



The posterior aorta, or pericardial artery. In M. latus it lies in 

 the median line of the floor of the pericardial cavity between the 

 pyloric caecum and the recurrent intestine. It gives off many 

 branches on each side in the floor of the pericardium, but these are 

 very variable in size and position. One of the strongest branches on 

 each side goes to the edge of the floor of the pericardium, and enters 

 the mantle, there dividing into two strong branches — the posterior 

 pallial arteries (pp a, Figs. 21 and 29), which supply the mantle-lobes 

 below the middle retractors and the pericardial region. Leaving, for 

 the present, the smaller branches : the posterior aorta passes back in 

 the median line over the posterior adductor. Anterior to the anus 

 it bifurcates, and a branch passes down each side of the posterior 

 adductor by the sides of the rectum (p m a, Fig. 21). These two forks 

 curve down over the posterior adductor, and one enters each mantle- 

 lobe (pma, Figs. 21, 24, and 29), passing backwards obliquely to the 

 margins of the mantle, which they reach a little above the posterior 

 junction of the mantle-lobes. These two forks are visible from the 

 outside of the mantle for some distance before they reach the margin 

 (Fig. 29 : in this figure the dotted line shows the part concealed). On 

 arriving at the margin, the main part of each branch of the posterior 

 aorta turns downward and follows the ventral edge of the mantle-lobe 

 of its side, forming the posterior circumpallial or marginal arteries. 

 These posterior marginal arteries run forward and fuse with the 

 anterior marginal arteries formed by the bifurcation of the anterior 



