40 



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 (Art.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 

 (Art. 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 

 lacunae 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.ren., 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 

 ("venae cavae"). 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 (Sin.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- 



