228 OILMAN A. DREW. 



Both auricles and ventricle are composed of interlacing muscle 

 fibers, and are capable of great extension. In preserved speci- 

 mens, the heart is usually contracted and is not very conspic- 

 uous. In such contracted hearts the cavities of auricles and 

 ventricle are practically obliterated. 



The heart lies in a somewhat triangular, spacious pericardial 

 cavity that is dorsal to the posterior half of the adductor muscle, 

 and ventral to the posterior portion of the liver. Posteriorly it 

 is covered only by a somewhat thick, muscular membrane which 

 separates it from the mantle chamber. 



As already mentioned, two blood vessels leave the ventricle 

 (Figs, i and 3), one from each end. Although they are not so 

 placed in reference to the ways the terms are generally used in 

 describing Pccten, the two ends correspond to the anterior and 

 posterior ends of the ventricle in most forms of lamellibranchs. 

 The posterior aorta is much the smaller of the two, leaves the 

 heart ventral to the intestine (actually anterior to it) and divides 

 immediately after leaving the heart, into two vessels, one of 

 which, the smaller, follows along the intestine, supplying it and 

 surrounding portions with blood. The other vessel turns almost 

 at right angles upon leaving the aorta and enters the adductor 

 muscle, where it divides into a system of vessels that supply the 

 muscle with blood. 



The anterior aorta is much larger than the posterior aorta, 

 and supplies all of the remainder of the body. It leaves the 

 ventricle dorsal to (actually posterior to) the intestine and very 

 soon gives rise to a vessel which passes into and supplies the 

 wall that separates the pericardial cavity from the mantle cham- 

 ber. From the pericardium the anterior aorta follows along the 

 postero-dorsal border of the liver to the base of the ear. Here 

 it gives rise to the branch (Fig 3, ppa) which passes posteriorly 

 to the extreme upper margin of the mantle that lines the ear, 

 giving off along its course a number of branches, which supply 

 this portion of the mantle. Here it divides into two vessels, a 

 right and a left, each of which bends abruptly ventrally (Fig. 

 5, ppa) and follows along the margin of the respective mantle 

 lobe about opposite the line of attachment of the infolded ridge 

 of the mantle, alongside but external to the pallial nerve. 



