THE COMMON MUSSELS. 15 



passage of food. As I mentioned above, the pyloric caecum is closely- 

 applied to and almost united with the direct intestine through 

 the whole course of the latter in M. latus (Figs. 1 and 14). The 

 separation of the pyloric caecum and intestine has not proceeded so 

 far in M. edulis as in M. latus and several other Lamellibranchs, or 

 else, which is less likely, there has been a fusion of two originally 

 distinct tubes. I think this explanation preferable to the suggestion 

 that in two allied species, while the whole disposition of the alimen- 

 tary canal is otherwise exactly similar, and also the other organs 

 easily comparable, a short part of the alimentary canal should be 

 occupied by totally different structures — in one by part of the 

 stomach, and in the other by part of the intestine and a caecum of 

 the stomach. 



The continuation of the pyloric caecum into the mantle is perhaps 

 peculiar to M. latus, but I have not had any other species than the 

 three above described to dissect for comparison. I have examined 

 the shells of various other species of Mytilus, and find that the muscle 

 impressions agree with those of M. edulis and magellanicus, the 

 anterior adductor being present and the impression of the posterior 

 retractors continuous, except in the case of M . smaragdinus, from 

 China, which exactly resembles the young of M. latus, except that the 

 shell is somewhat thinner than that of M. latus. This resemblance 

 has been noted by Professor Hutton (8). 



Circulatory System. 



The heart is placed on the dorsal side just behind the middle, in 

 a large cavity, the pericardial cavity or pericardium (Fig. 1), between 

 the middle retractors of the byssus and the posterior adductor, and 

 lying above the posterior retractors of the byssus. The heart con- 

 sists of a single central cavity, the ventricle, and of two wings, the 

 auricles (aur, Fig. 21). The ventricle is pierced longitudinally by 

 the rectum, which passes through the pericardial cavity (Fig. 1). 

 The auricles open into the sides of the ventricle, and, widening out 

 into a fan-shape, are attached by the wider ends to the sides of the 

 floor of the pericardial cavity. The ventricle is blind at the hinder 

 end, and the aorta sets out from the anterior end (a ao, Fig. 21) . 



Arteries. The main artery or aorta passes forward on the right 

 upper side of the rectum from the anterior end of the ventricle, and 

 soon widens out into a large aortic bulb (a b, Figs. 21 and 24), which 

 gives rise to several arteries. From the upper anterior side arise 

 three trunks, the median one being the anterior aorta, and the two 

 lateral ones the anterior pallial arteries (a p a, Fig. 21) . 



