XI.] THE FRESH- WATER MUSSEL. 107 



manner, and, finally, a rectum, which lies in the middle line 

 of the dorsal aspect of the body, traverses the pericardium 

 and the heart which lies therein, and finally ends in the anus. 



As the mouth is below and behind the anterior adductor and 

 the rectum passes in front of and above the posterior adductor, 

 it is clear that the alimentary canal, as a whole, lies between 

 the two adductor muscles. 



Digestion, that is solution of the proteinaceous and other 

 nutritive matters contained in food, is effected in the stomach 

 and intestine; and the nutritious fluid, thus formed, transudes 

 through the walls of the alimentary cavity and passes into 

 the blood contained in the blood-vessels which surround it. 

 This blood is thence carried into a large sinus, which occupies 

 the middle line of the body under the pericardium and 

 between the organs of Bojanus (see Laboratory work 5), 

 and receives the greater part of the blood returning from 

 all parts of the body. From this median vena cava, branches 

 are given off to the gills and open into the extensive vascular 

 network which those organs contain. From this, again, 

 trunks lead towards the pericardium and open into one or 

 other of the two auricles of the heart, which communicate 

 by valvular apertures with the ventricle. The ventricle gives 

 off two aortic trunks, one of which, the anterior, runs forwards 

 in the middle line, above the rectum, while the other runs 

 backwards, below the rectum. From these two aortae branches 

 are given off which divide into smaller ramifications for the 

 different regions of the body, and for the viscera, and finally 

 terminate in channels which answer to the capillaries of the 

 higher animals. 



The pericardia] cavity, in which the heart is lodged, is 

 situated in the posterior half of the dorsal region of the body. 

 Through its thin dorsal wall, and, still better, when it is care- 

 fully laid open, the heart can be seen beating. The auricles 

 contract and, after them, the ventricle ; the wave-like contrac- 



