24 



MOLLUSC A. 



ately above the heart, and just back of the great muscle 

 and body, is collected in a set of branches which dis- 

 tribute themselves over that region, and convey it to 

 the auricle by a vein running close to the anterior edge 

 of the muscle on the inner side of the mantle. (Figs. 



These veins can usually be distinguished by the 

 clear, transparent look of their walls, and by the way 

 in which they lead to the great sinus in front of the 

 auricle at the ventral corner of the cavity containing 

 the heart. To see the remaining parts of the venous 

 system, dissection is necessary. 



Cut off the mantle leaves completely, and then slit 

 open carefully the largest venous vessel (Figs. 5, 6, x) 

 at the fleshy junction of the inner lamince of the middle 

 pair of gills. This will be found to receive, near the 

 anterior end, two large branches running right and left, 

 and other branches from the bases of the palpi. Along 

 the bottom of its channel several large veins from the 

 body and many smaller ones open, which come in some 

 cases from the spongy capillary tissue about the intes- 

 tine, and in others from the capillary tissue of the more 

 adjacent parts of the body (Figs. 5-7, 12). This is, 

 in fact, the main stem or reservoir of the venous system 

 of the body and viscera, and from this the blood re- 

 ceived from all of the regions of the body is thrown 

 into the gills, and then, after being aerated in its pas- 

 sage through the tubes and capillaries, is again col- 

 lected in the special vessels already described, and 

 passed onward to the heart, mingling, when it arrives 

 at the two main stems of the venous system leading to 

 the heart, with the blood returning from the mantle, 

 and partly also from the viscera. 



