9226 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. . 
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, Pl. 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 (A7t.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 (A7t.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 1s 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 lacunz, 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 lacunae 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, 
