464 J. F. ILLINGWORTH, 



laterally in the visceral wall just within the abortive epipodium. These 

 tiny arteries like the pedal extend to the posterior end of the animal, 

 w^here each finally connects with the pedal artery of that side. I 

 have not seen these lateral branches of the pedal system described ; 

 so to distinguish them I shall call them the marginal pedal arteries, 

 and the two large pedal trunks simply pedal arteries. The former 

 furnish an abundant supply of blood to the thickened columellar muscles 

 near the anterior end of the visceral cavity. They also give off capil- 

 lary branches all along the wall. Arteries similar to these lateral 

 pedal are found in Cryptochiton. They branch from the anterior end 

 of the pedal trunk in exactly the same way that they do here, though 

 with the material I had at hand I was not able to trace the con- 

 nection with the pedal at the posterior end. 



The gastric aorta {G.Ao) makes a turn forward upon leaving the 

 aortic enlargement and runs down and back along the ventral wall 

 of the stomach. The first part of this aorta gives several arteries to 

 the upper part of the stomach, and from the point where it turns 

 downward an artery passes to the anterior loop of the intestine. 

 Numerous capillary branches all opening from this aorta cover the 

 under side of the stomach. The terminal branches supply the inner 

 part of the liver and the part of the intestine imbedded in this organ. 



The genital aorta {Gen.Ao) contains almost one third of the ar- 

 terial blood. This vessel leaving the posterior angle of the aortic en- 

 largement, proceeds back and to the right. Two small branches are 

 given off to the dorsal wall of the greater end of the stomach; also a 

 branch of considerable size to the right side of the liver. Upon 

 reaching the reproductive organs the aorta bifurcates, the larger branch 

 going into the right lobe of the genital mass. Each branch turns 

 forward, one to the right, the other to the left. They lie exposed on 

 the inner wall of the genital cavity, where they break up into capil- 

 laries and are distributed to all parts of the organ. 



The mantle circulation in this group is well developed. The blood 

 that is carried to the mantle by the palliai artery does not pass 

 through the ctenidia before returning to the heart which leads us to 

 beheve that the mantle acts as an aerating organ. The arterial blood, 

 after passing through the capillaries of the palliai artery, is collected 

 into a large oval canal, the palliai sinus (Fig. M P. s) which is situated 

 directly above the palliai artery, and lies very close to it at the angle 

 where the mantle cavity opens on each side. The posterior part of 

 the palliai sinus is joined by its smaller anterior connective. From 



