49 



gone oxidation in the gills, is distributed through the 

 body by two series of vessels ; posteriorly it leaves the 

 heart by the right and left posterior pallial arteries (Art.pp., 

 Art.pp'., fig. 30, PI. V.), and reaches the siphons and the 

 posterior mantle margin ; anteriorly it traverses the aorta 

 (Ao.), which soon bifurcates; one branch, the anterior 

 pallial artery (Art. pa.), passes over the anterior adductor 

 and reaches the anterior mantle margin; the other branch 

 descends the anterior part of the viscero-pedal mass as the 

 viscero-pedal artery (Art.vp.). In its course this supplies 

 blood to the labial palps through the right and left labial 

 arteries (A.lab.), and again bifurcates, one branch, the 

 visceral artery (Art. v.), penetrates the visceral mass and, 

 passing first to the straight portion of the intestine, sup- 

 plies the whole length of the latter, the other continues 

 on to the bend of the foot. 



Further than this, it is impossible to trace the afferent 

 blood channels. Both in the mantle margin and in the 

 viscero-pedal mass the arteries become lost in an irregular 

 system of lacunae, lying principally in the interspaces 

 between the muscle bundles. This lacunar system re- 

 presents the capillary and venous portions of the vascular 

 system of a more highly organised animal, and it is here 

 that the interchange between blood stream and tissues, in 

 the metabolism of the latter, is effected. Two foci exist 

 towards which the blood circulating in this lacunar system 

 converges. From the anterior and posterior margins of the 

 mantle lobes it flows in the ventral mantle edge towards 

 the centre, then dorsally through the spaces in the interior 

 of the thin mantle lobes towards the umbonal parts of the 

 latter. Here there is a direct communication between the 

 intrapallial lacunas and the renal sinus, but the greater 

 portion of the blood, after bathing the tubules of the peri- 

 cardial gland, reaches the anterior corners of the auricles. 



