MOLLUSC A. 27 



To this double circle of the mantle and the gills, 

 there must also be. added another special system for 

 the great muscle. 



A portion of the venous blood appears to return 

 from the mantle in the vicinity of the branchial cavity in 

 two large vessels situated on the interior of the mantle 

 leaves (Fig. 6,jm'). These lie on either side, near 

 the bases of the gills, and on approaching the muscle 

 spht into two branches, which encircle its edges on the 

 outer convex side, but not on the anterior or concave 

 side.* 



From these branches numerous smaller tubes arise 

 which penetrate the muscle, passing into the interior at 

 intervals all the way around. 



. The aerated blood from the mantle which these con- 

 vey into the muscle, and the arterial blood from the 

 artery, before described, which penetrates the muscle 

 on the anterior side, is all collected into one large vein 

 on the lower side (Fig. 6, y), and emptied into the 

 median vein {x). 



The extraordinary complication of the gill structure 

 in the oyster makes it an unfavorable type for studying 

 the course of the blood in these organs, but the ease 

 with which the larger arteries and veins can be filled 

 by injections, from the heart or auricular antechamber, 

 makes it available in this respect. 



* Here they are replaced by two other of the mantle system 

 of veins (2), which go directly to the venous trunks, and empty 

 into the auricles on either side. One thing quite noticeable in 

 these arteries and veins is the effect of pressure upon their size. 

 Thus the vertical tubes carry blood into the gills, are dilated at 

 the bottom where the blood tends to accumulate ; the alternate 

 return verticals and the reservoirs in the partitions are dilated at 

 the upper ends from the same cause. 



