ANATOMY OF THE PEARL OYSTER. 63 



the two, passes backwards into the tissue surrounding the exposed part of the rectum 

 and runs therein parallel with the latter to a point slightly above the anus (about 

 Jj- inch in 2^-year-old specimens). Here it changes its course, turning forwards into 

 the hinder tendinous portion of the adductor muscle and immediately divides into 

 two branches (Plate VII., figs. 4 and 5). One of these turns to the right, passing 

 through the muscle parallel to and a little beneath its surface. Just before reaching 

 the insertion of the muscle it turns abruptly at right angles and passes into the 

 mantle as the right posterior pallial artery (Art.p.p., Plate VII., figs. 4 and 5). Its 

 neighbour on the left the left posterior pallial artery (Art.p.p.') passes under the 

 rectum and into the tendinous part of the adductor, emerging and entering the left 

 mantle lobe in a similar manner to the right branch. 



Each of these arteries after entering the mantle runs forward along its junction 

 with the adductor till opposite the paired pallial sense organ (S.o., Plate VII., fig. 5). 

 Curving forwards each then runs out to the inwardly projecting tongue of the mantle 

 edge opposite the posterior gill tips. Here an anterior and a posterior branch are 

 given off which run forwards and backwards respectively within the thickened pallial 

 margin, parallel to and just beneath the pallial gutter. 



The anterior branch meets a similar branch from the anterior pallial artery, the 

 two so fused being the common pallial artery (Plate VI., fig. 2, Art.c.p.). 



The anterior aorta (Ao.A., Plate VI., fig. 2) passes forwards from the heart, above 

 and to the left of the rectum, and then bends to the right and runs above and to the 

 right of the stomach and oesophagus. 



Of the many branches given off by this arterial trunk, by far the most important 

 and largest is the first, the unpaired visceral artery (Art.vis., Plate VI., fig. 2). 

 This branches off immediately after the dorsal aorta crosses the rectum. It is rather 

 wider than the continuation of the aorta. Turning ventrally, it penetrates the central 

 portion of the viscero-pedal mass, crossing in its course to the left of the descending 

 intestine and then giving off branches into the gonad and to the intestines. 



Returning to the dorsal aorta, we find it supplying numerous arteries to right and 

 left and downwards the chief of these being the hepato-pedal artery (Art.h.p., 

 Plate VI , fig. 2), given off just above the junction of the oesophagus with the 

 stomach. Then branches are given off to the right and left labial palps, and beyond 

 the mouth the aorta ends in two diverging branches the right and left anterior 

 pallial arteries (Art.a.p.), which pass ventrally within the mantle edge to fuse with 

 the posterior pallial arteries as the common pallial artery {Art.c.p.). 



The hepato-pedal artery, like the visceral, passes downwards to supply the viscero- 

 pedal mass. Level with the floor of the stomach it bifurcates, the anterior branch 

 going forwards to the foot as the pedal artery, the posterior branch the hepatic 

 artery turning back to traverse the digestive gland. From the pedal a branch goes 

 forwards and bifurcates to form twigs going right and left to the palps. 



The blood stream, carried by the ultimate ramifications of the arteries, passes into 



