THE COMMON MUSSELS. 29 



The ascending veins are represented by fine channels in the sub- 

 stance of the mantle, which are easily made out on the inner side 

 of the mantle (Fig. 2) ; but these channels have no definite walls, and 

 injection introduced into them spreads at once through the neigh- 

 bouring substance. There is no marginal sinus. The horizontal 

 vein, as far as I can make out, is represented by a long lacunar 

 space, from which the blood probably enters the organ of Bojanus. 



I have never succeeded in making any injection, fluid or otherwise, 

 enter the gill-filaments ; but there is an efferent vessel along the 

 dorsal edge of the ascending lamella of each gill. These efferent 

 vessels are frequently injected and pass forward to between the bases 

 of the labial palps, as described in M. cdulis. Their course is marked 

 in Fig. 2 by the dotted lines marking the lines of attachment of the 

 ascending gill-lamellag. 



Large intermuscular sinuses exist as in M. edulis, and there is a 

 fine plexus of veins over the surface of the mesosoma, but these seem 

 to have porous or incomplete walls, as the injection immediately 

 diffuses into the surrounding tissue. The whole tissue in the 

 neighbourhood of the organ of Bojanus is lacunar, and, in injecting 

 from the foot, is readily filled with injection. 



The horizontal vein is similar in position in M. latus and edulis, 

 but it communicates with the auricle in somewhat different points 

 (Figs. 33 and 34). The oblique vein, as Sabatier terms it, in M. 

 edulis, passes up over the anterior side of the posterior retractors to 

 enter the anterior end of the auricle. If the oblique vein in M. latus 

 had a corresponding position it would pass up over the anterior side 

 of the middle retractors to the auricle ; but this is not its position. 

 It lies between the middle and the posterior retractors, and enters the 

 auricle about the middle of its length (o v, Fig. 33). This seems 

 another result of the differences between the retractors of M. latus 

 and edulis. M. magellanicus resembles M. edulis in this point. The 

 oblique vein of M. edulis and magellanicus lies in a large " couloir " 

 or passage opening into the pericardium, but there is no such passage 

 or opening into the pericardium in M. latus (coul, Figs. 31 and 34). 



To summarise, then, the condition of the venous system in M. 

 latus : The blood conveyed from the heart by the pallial arteries and 

 aorta on the outer side of the mantle returns for the most part in the 

 case of the mantle by short channels, branching among the repro- 

 ductive products and growing wider as they ascend on the inner side 

 of the mantle. These represent the ascending veins of M. edulis, but 

 are not furnished with definite walls. The blood from these ascending 

 channels seems to collect in a large sinus occupying the position of 



