THE COMMON MUSSELS. 25 



is also unpaired, (g a, Fig. 24), and nearly median under the aorta. 

 The arteries to the mesosoma and to the muscles, that is, the two 

 ventral arteries, with the arteries to the foot and even to the ventral 

 side of the inner palps, are all unpaired and median. 



In M. edulis the anterior and posterior aorta are also unpaired 

 and median, but the latter is a very small, insignificant artery, supply- 

 ing only the floor of the pericardium, and not the underlying organs 

 ( p ao, Figs. 25 and 26 : in Fig. 25 the posterior aorta or pericardial 

 artery is left white to show upon the dark gastro-intestinal artery). 

 All the important arteries to the stomach, liver, intestine, and to the 

 sides of the body are paired arteries. From the underside of the 

 aortic bulb in M. edulis there springs a short trunk (the coeliac trunk 

 of Sabatier), which gives rise to three branches or arteries, the median 

 being the small pericardial artery or posterior aorta above mentioned ; 

 the lateral branches are large, and are the gastro-intestinal trunks (Fig. 

 25). These trunks divide into the anterior and posterior gastro- 

 intestinal arteries (g a, Figs. 25 and 26) . The anterior gastro-intes- 

 tinal arteries pass forward on each side of the pyloric caecum, and 

 supply the stomach, intestines, &c. They also give rise near their 

 base to two long arteries which run back one on each side of the body, 

 and are called by Sabatier the recurrent arteries (r a, Figs. 25 and 

 26). The recurrent arteries supply the parts of the genital organ on 

 the sides of the body, and their lower branches supply the sides of the 

 mesosoma. The posterior gastro-intestinal arteries run back one on 

 each side of the pyloric caecum, and supply between them the parts 

 of the alimentary canal below the pericardium. Thus, in M. edidis 

 the anterior and posterior gastro-intestinal arteries and the recurrent 

 arteries forming a great part of the arterial system are paired and 

 lateral ; in M. latus these are replaced by the gastro-intestinal artery, 

 the posterior aorta or pericardial artery (which is very small in M. edidis), 

 the anterior and the posterior ventral arteries, which are all unpaired 

 and median. 



Another remarkable point of difference between these arterial 

 systems is the want of the marginal arteries in M. edulis, which are 

 so strongly developed in M. latus. In M. edulis the anterior aorta 

 bifurcates at the anterior end, but it does not pass far round the 

 mantle before it breaks up into capillaries (Fig. 31), and there is no 

 account given by Sabatier (7) of any continuation of the pericardial 

 artery or of any of the posterior gastro-intestinal arteries into the 

 posterior part of the mantle. The muscular borders of the mantle in 

 M. edulis are supplied by fine capillaries from the ends of the branches 

 of the pallial arteries, which approach much nearer the margin of the 



