322 MR. A. HAJSrCOCK ON THE AXATOMT 



manner of a peritoneum, and is so attached to the wall of the 

 chamber. 



A large trunk vessel passes from the dorsal extremity of the 

 heart, and immediately divides into three branches, one of which 

 advances between the mantle and the lining membrane along the 

 dorsal region at the back of the endostyle ; another passes in the 

 opposite direction down the dorsal margin to the bottom of the 

 branchial sac. These two form the great dorsal branchial channel, 

 and are equivalent to the ventral or thoracic sinus of Milne-Ed- 

 wards ; and they both communicate with the dorsal extremities 

 of the transverse channels of the branchial sac. The third branch 

 turns off at right angles to this great dorsal channel, close to the 

 point where it is united to the heart, and, in company with another 

 vessel, to be shortly described, penetrates the mantle and goes to 

 ramify in the test. 



From the other or ventral extremity of the heart there are two 

 large trunk vessels given off, one to each side of the stomach. 

 These ramify over the digestive organs, and supply a minute net- 

 work spread over both sides of the visceral mass ; this network 

 may be termed the visceral plexus. It is in direct communica- 

 tion with a similar plexus of blood-channels or sinuses that lies 

 between the mantle and the lining membrane of the right side ; 

 and this latter is continuous with another plexus similarly 

 situated in the left side of the mantle ; these together form what 

 we shall call the pallial plexus. The trunk branch that supplies 

 the left side of the stomach and the portion of the visceral plexus 

 there situated divides into two large stems, one of which inclines 

 towards the intestine, the other towards the oesophagus ; the 

 former passes for some little distance along the intestinal tube, 

 and then, leaving it, penetrates the mantle in the dorsal region, 

 and goes associated with the third branch from the dorsal extre- 

 mity of the heart, already described, to ramify in the test. Thus 

 originates the double vessel that carries the nourishing fluid to 

 and from that envelope or tunic. The stem that goes towards the 

 oesophagus passes along by the side of the lower extremity of the 

 intestine, and, just before reaching the anus, turns aside to join a 

 large vessel that extends along the ventral margin from one end 

 to the other of the branchial sac. This, which is the great ven- 

 tral branchial channel, is the dorsal sinus of Milne-Edwards. It 

 communicates with the ventral extremities of the transverse 

 branchial channels ; and its lower extremity bifurcates, a branch 

 passing on each side of the mantle. 



