fibers along the borders of the pits. The arrangement gives the 

 outside of the auricle a pebbly appearance that is very striking. 

 Both auricles and venticle 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 both 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. II and 13), one from each end. Although they are not 

 so placed in reference to the ways the terms have been used in 

 describing this form, the two ends correspond to the anterior 

 and posterior ends of the ventricle in most forms of lamelli- 

 branchs. 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 rernainder 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 a branch (fig. 13, 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. 

 9, ppa.) and follows along the margin of the respective mantle 

 lobe about opposite the line of attachment of the infolded ridge 



36 



