102 THE FRESHWATER MUSSEL. 



of the gills. It contains the heart and is traversed 

 by the rectum. Its walls are thin and semitrans- 

 parent behind; thicker and spongy in front. 

 Through them the pulsations of the heart can be 

 seen. 

 Open the pericardial cavity by a longitudinal incision 



along the right side, and cut away as much of its walls as is 



necessary to expose the heart fully. 



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 mantle-lobes and 

 gills of the right and left sides 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 and mantle-lobe. 



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 lies 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. 



