MOLLUSC A. 



23 



difficult to follow out, even without injections, and 

 may be partly seen by the eye without resorting to dis- 

 sections. 



The best way to study these is, as with the arteries, 

 to start from the part of the heart to which they lead ; 

 in this case the auricles. If the oyster has been care- 

 fully opened, and the mantle turned back on either 

 side, the auricles may be seen to connect with a nar- 

 row, triangular space with thin walls (Figs. 5, 12, 15). 

 Tliis space has two main branches, one leading an- 

 teriorly up on to the side of the body (^z') , and the other 

 posteriorly on the inner side of the mantle to the base 

 of the outer gill lamina (7) . Fig. 6 shows this last and 

 its branches, as well as the anterior one and its 

 branches (/''). The anterior one branches near the 

 bases of the gills, and is really the stem into which the 

 venous vessels of the anterior parts of the gills and 

 mantle empty ; and the blood, also, from part of the 

 viscera (Fig. 5, vv^, passes down this main channel to 

 the auricle without going through the gills at all. The 

 posterior vessel and its branches (Fig. 6) connect in 

 the same way with the vessel at the base of the outer 

 lamina of the outer gill, and also pass on beyond this 

 to the outer part of the mantle. The vessels at the 

 bases of the inner gills are connected with that at the 

 base of the outer lamina and mantle by the cross-bars 

 of the bottom of the branchial cavity which contain 

 large tubes (Fig. 6). In tliese vessels and tubes the 

 blood from the posterior parts of the gills and mantle 

 is collected and transmitted to the posterior venous 

 branch, by which it is taken to the auricle (Fig. 5). 



The blood from that region of the mantle immedi- 



