XL] THE FRESH-WATER MUSSEL. 117 



tions and composed of a median and two lateral 



chambers. 



^\ a. The Denticle, 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 

 passes, from one end of it to the other, like 

 the peristaltic contraction of the intestine in 

 one of the higher animals. 



^A /5. The auricles; one of these will be seen on each 

 side if the ventricle be gently pushed out of 

 the way: each is a somewhat pyramidal sac, 

 continuous with the ventricle at the apex of 

 the pyramid. 



5. The organs of Bojanus. 



a. Divide the alimentary canal at the posterior part 

 of the pericardiac chamber and turn it and the 

 heart forwards, so as to lay bare the floor of the 

 pericardium. Running along the middle line 

 of this floor will be seen a large blood-sinus, 

 the great vena cava; on each side of this, the 

 floor is formed by the roof of a transparent sac 

 (the non-glandular part of the organ of Bojanus), 

 through which is seen a dark brown mass (the 

 glandular part of the organ of Bojanus). 



b. At the extreme front end of the pericardiac floor, 

 immediately under the point at which the in- 

 testine enters the cavity, will be found a pair of 

 oval openings; pass into each a bristle, tipped 



