SEA-FISHERIES LABORATORY. Ds te 
the anterior margin of the viscero-pedal mass the aorta 
bifurcates; one vessel runs straight forward over the 
anterior adductor to the mantle lobes, forming the anterior 
pallial artery (A7t.p.a.), the other passes straight down- 
wards as the viscero-pedal artery. In its course this gives 
off a vessel from its anterior side which soon bifurcates, 
forming the right and left labial arteries (A.lab.). Still 
further down a large vessel is given off from the posterior 
side, the visceral artery (Art.v.), this runs back horizon- 
tally until it meets the straight portion of the intestine ; 
branches are given off which supply blood to the rest of 
the gut. The main vessel is continued beyond this branch 
to the ventral surface of the foot as the pedal artery 
(Act p:): 
Only the above described vessels, constituting the 
arterial portion of the vascular system, have definite walls. 
The further course of the circulation lies in irregular 
lacune between the various organs, between muscle 
bundles, and in cavities in the connective tissues. ‘The 
first focus of this system of venous channels is the renal 
sinus (Sin ven., figs. 4 and 30), an irregular blood space 
surrounding the tubules of the renal organ. Anteriorly 
this begins as a pair of blood spaces lying underneath the 
pericardium on each side of the middle line of the body 
(‘“‘venee cavee’’). These unite into a large median cavity 
in the middle of the renal organ, from which the blood 
filters outwards round the system of tubules constituting 
that organ. Blood enters the renal sinus dorsally from 
the mantle lobes, and anteriorly from a vertical sinus in 
the posterior part of the viscero-pedal mass (Sv.p., figs. 
11 and 30). This communicates with the renal sinus 
through an opening in the muscular wall of the former, 
where the posterior retractor muscles of the foot take 
origin from the muscular body-wall. Here the arrange- 
