20 STUDIES IN BIOLOGY. 



aorta, and thus complete a circumpallial circuit between the anterior 

 and posterior aortas in each mantle-lobe {a m a and pma, Figs. 21, 

 24, and 29) . A small part of each fork of the posterior aorta turns 

 upward, to supply the margin behind the posterior adductor (Fig. 29) . 



Returning now to the pericardial cavity : the pyloric caecum and 

 the posterior retractors of the byssus are supplied by a branch from 

 the left side of the posterior aorta. Just behind the aortic bulb the 

 posterior aorta gives off a small artery to the middle retractors of the 

 byssus, and at the hinder end of the cavity it gives off on its upper 

 side a small artery that passes forward on the underside of the rectum 

 (D, Fig. 24). These are small arteries; but on the ventral side a 

 little in front of the posterior adductor the posterior aorta gives off a 

 large trunk that passes downward in the median line (Fig. 24). 

 Almost immediately this trunk gives off on its posterior side a large 

 artery that enters the posterior adductor (Fig. 24, B) and ramifies 

 throughout it. The rest of the downward trunk passes down in front 

 of the muscle to near the parieto-splanchnic ganglia, still in the 

 median line, when its main branch turns forward at a little more 

 than a right angle, and passes forward nearly horizontally to the 

 anterior end of the mesosoma, giving off in its course lateral branches 

 to the reproductive organs and other surrounding organs. The hori- 

 zontal part I call the posterior ventral artery, and its course is towards 

 the upper side of the mesosoma in the median line (p v a, Fig. 24). 

 From the angle where the posterior ventral artery turns forward, 

 several small arteries are sent backwards to ramify in the membranes 

 about the parieto-splanchnic ganglia. The descending trunk also 

 gives off on its posterior side a small artery (Fig. 23, C), which, keep- 

 ing close to the adductor, comes to ramify near the ganglia also, 

 this region being well supplied with small arteries. 



The posterior aorta and its forks above the adductor give off many 

 small branches. One of these runs forward on the outer side of the 

 pyloric caecum (Fig. 24), while others supply the mantle-lobes above 

 the posterior adductor, forming small pallial arteries. Behind the 

 posterior adductor each fork of the aorta often sends forward an artery 

 into the mantle below the posterior adductor (Fig. 29). All the 

 pallial arteries previously mentioned are on the outer side of the 

 mantle-lobes, but the branches of the posterior aorta send off behind 

 the posterior adductor each a small artery that runs along the line of 

 attachment of the gills near the posterior adductor on the inner side 

 of the mantle-lobes. 



Gastro-intestinal artery. This artery, as already mentioned, 

 springs from the underside of the aortic bulb to the left, and passes 



