88 THE FRESHWATER MUSSEL 



2. The heart consists of three chambers, a median ventricle, 



and right and left auricles ; the latter returning to 

 the ventricle the blood from the right and left gills 

 respectively. 



a. The right auricle is a triangular sac with very thin 



transparent walls. The base of the triangle lies 

 along the dorsal border of the gills of the right 

 side, from which it receives blood ; the apex is 

 attached to the side of the ventricle about the 

 middle of its length, and opens into it by a 

 valved aperture. 



To see the auricle, press the gills down gently. 



b. The left auricle is similar to the right, and returns 



the blood from the left gills. 



c. The ventricle is an elongated thick- walled muscular 



sac of a reddish colour, lying in the middle of 

 the pericardial cavity, and receiving the auricles 

 laterally. Its posterior end is enlarged and 

 bilobed. 



The ventricle surrounds the rectum, which 

 runs straight through the pericardial cavity from 

 end to end, apparently traversing the ventricle, 

 though not doing so in reality. In the early 

 stages the ventricle Hes completely below the 

 rectum, but during development its anterior end 

 extends up at each side, and the two processes 

 meet each other and unite above the rectum, 

 completely surrounding it. 



3. The arteries. 



There are two main arteries, one arising from 

 each end of the ventricle, which convey the aerated 

 blood from the heart to all parts of the body. Their 

 main branches may be followed by injecting them 

 from the ventricle. 



a. The anterior aorta is a continuation of the anterior 

 end of the ventricle : it runs forwards along the 



