XI.] THE FRESH- WATER MUSSEL. 113 



3. Now remove the animal completely from its shell, by 

 detaching the other mantle-lobe from the valve to 

 which it is fixed, and cutting through the attachments 

 of the adductor muscles to that valve. The thick 

 dorsal ^border of the animal and the continuity of 

 the mantle-lobes will now be more readily made out 

 than they could be previously (2. b. @.). 



4. The heart. 



a. On the dorsal border of the animal is a clear 

 space, where the mantle is very thin and covers 

 in a cavity filled with fluid. This cavity is the 

 pericardium, and through its walls the heart can 

 be seen beating. 



b. Pin the Anodon out in water between two pieces 

 of loaded cork, or paraffin, so that its dorsal border 

 is upwards, a mantle-lobe spread over each bit of 

 cork, and its foot and gills hanging down between 

 the two pieces : then carefully cut away the- dor- 

 sal side of the pericardium without injuring the 

 heart. 



c. The heart will now be exposed ; it is a yellow- 

 ish transparent sac, exhibiting regular contrac- 

 tions and composed of a median and two lateral 

 chambers. 



a. The ventricle, or median chamber ; an oval sac, 

 from each end of which a large vessel (anterior 

 and posterior aorta) is continued ; running 

 through the middle of the ventricle is seen 

 part of the alimentary canal. All parts of 

 the wall of the ventricle do not contract to- 

 gether; but a sort of wave of contraction 

 M. 8 



