MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 185 



From the anterior side of the marginal pallial artery a regular scries of small arteries pass 

 into the portion of the mantle in front of the marginal artery, which we have noticed to he espe- 

 cially muscular and slightly thicker than the middle portions of the mantle. 



From the posterior side of the marginal artery and from the anterior portion of the pallial 

 artery numerous vessels pass into the middle portions of the mantle. 



The common septal artery bends around the anterior side of the heart immediately after its 

 origin and then passes backward along the ventral surface of the heart. It is here covered by 

 the portion of the pallio-visceral ligament which incloses the heart and hangs suspended in a 

 mesentery-like fold of the ligament. Running backward under the left side of the heart, the 

 artery arrives at the posterior viscero-pericardial opening, through which it passes by follow- 

 ing the right edge. This edge being attached to the gonad, the artery passes directly upon the 

 surface of the ovary or testis, as the case may he. Passing over the lower edge onto the anterior 

 face of the gonad, the artery readies the gastric ligament, along the edge of which it passes to the 

 posterior wall of the body near the base of the siphuncle. The common septal artery does not 

 appear to give off any branches to the gonad. 



Arrived at the posterior wall of the body, the artery divides into a right and a left septal 

 artery. These ramify over the portion of the body wall which faces the septum. It will be 

 remembered that this portion of the body wall is hounded by the dorsal and the posterior ventral 

 aponeurotic bands. The branches of the septal arteries are rigidly confined to the septal area of 

 the body wall (Willey). 



The siphuncular artery arises as a branch of one of the septal arteries, sometimes of one, 

 sometimes of the other. Entering the base of the siphuncle. the artery extends through it to the 

 end. Other smaller branches of the septal arteries may also enter the base of the siphuncle. 



VENOUS CIRCULATION. 



Only a portion of the venous system appears to be closed. The blood passes from the 

 arterial capillaries into sinuses, which in one way or another are placed in communication with 

 the vena cava. In the mantle there seems to be a quite extensive closed circulation. 



The vena cava lies in the ventral wall of the body, extending from the cephalic cartilage to 

 the posterior limit of the mantle cavity. It possesses thin muscular walls of its own, which 

 appear to he innervated by two nerves springing from the pleuro-visceral ganglia. In its anterior 

 portion the vena cava is hounded laterally by the inner faces of the shell muscles, which, touching 

 each other ventrally, are separated dorsally. A triangular space is thus formed, which is occupied 

 by the vena cava. 



The dorsal wall of the vena cava, which is flush with the inner surface of the body wall, is 

 perforated by numerous holes of varying size. 



The vena cava is in communication anteriorly with numerous extensive blood spaces in the 

 tissues surrounding the buccal mass and at the bases of the tentacles. Some of these spaces 

 appear much like definite branches of the vena cava. The vena cava also receives several large 

 veins from the shell muscles and the body wall, but these vessels are not constant in either number 

 or position. Two veins on each side of the cephalic region pass through the body of the cartilage 

 into the anterior end of the vena cava. 



Through the openings in its dorsal wall by which the vena cava is in communication with 

 the hsemoccel blood enters the vena cava through the latter cavity from the sinuses of the body 

 wall, and probably from the viscera contained in the hamiocoel and its extensions. 



At the posterior limit of the mantle cavity the vena cava divides into a right and a left branch, 

 which branches almost immediately subdivide into two branches passing to the anterior and 

 posterior gills of either side, the branchial arteries. Each of the branchial arteries, on its way 

 to the gill, passes through the posterior wall of a renal sac, where it sends branches into the 

 renal and pericardial appendages. (Text-fig. !», p. i6±.) 



Welley says that the veins of the mantle '"are collected into two main trunks, which lie on 



