16 STUDIES IN BIOLOGY. 



The anterior pallial arteries diverge from the aorta and pass down 

 over the middle retractors of their respective sides into the mantle- 

 lobes, soon dividing into two or more large branches, whose ramifica- 

 tions extend over the outer side of the mantle (Fig. 29) . The anterior 

 pallial arteries originate as I have above described in both M. latus 

 and edulis, but in M. magellanicus there is a somewhat different ar- 

 rangement {a pa, Fig. 23, and ap a, Fig. 30), as the anterior pallial 

 arteries do not spring from the aortic bulb, but from the aorta at 

 points more or less distant from the aortic bulb. It will be well to 

 describe first the course of the anterior aorta in the three species 

 before describing the anterior pallial arteries of M. magellanicus. 



The aorta, which is easily made out on the dorsal surface for some 

 distance, gradually leaves the median line in M. latus and sinks 

 deeper {a ao, Figs. 21 and 24), being lost sight of until near the an- 

 terior retractors, where it becomes visible in the thin roof of the 

 supra-sesophageal cavity. After it is first lost sight of on the dorsal 

 surface, halfway between the anterior and middle retractors, it passes 

 along the inner wall of the supra-sesophageal cavity to the left side, 

 giving off an artery, where this cavity begins, to the right side (A, 

 Fig. 24). It continues clown the left wall of this cavity, being 

 visible on the upper surface only while passing the left anterior 

 retractor, when it begins to incline towards the median line. If the 

 supra-resopliageal cavity be laid open the aorta is readily seen on its 

 wall to the left side. At the anterior end of the mantle the aorta is 

 again in the median line (Fig. 21), and passes down over the anterior 

 end of the mantle to the ventral side, where it divides into two strong 

 arteries, the circumpallial or marginal arteries (a ma, Figs. 21 and 

 24), which follow round the edges of the mantle. These circumpallial 

 arteries send off numerous branches, which ramify in the pallial 

 muscles round the margin of the mantle-lobes (Fig. 29) . 



The above description of the aorta up to the anterior end of the 

 mantle applies in the main points to M. edulis and magellanicus as 

 well as to M. latus, but from this point there is considerable differ- 

 ence. In M. edulis the anterior aorta forks into two at the anterior 

 end of the mantle as in M. latus, but the two branches pass only a 

 little way along the ventral side when they break up into capillaries 

 (Figs. 31 and 22) . The specimens of M. magellanicus that I had for 

 injection were spirit specimens, and therefore not in very good con- 

 dition. The anterior aorta however in M. magellanicus was found to 

 curve round on one side just above the anterior adductor, and then, 

 passing back horizontally a little below the middle of the mantle, to 

 become the anterior pallial artery (Figs. 23 and 30) . In the specimens 



