XI.] THE FRESH-WATER MUSSEL. in 



The pericardial 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 carefully laid open, the heart can be seen beating. The 

 auricles contract, and, after them, the ventricle ; the wave- 

 like contraction of the latter being much the more easily 

 visible. The lips of the auriculo-ventricular apertures are so 

 disposed that the blood is impeded from flowing back into 

 the auricles, when the ventricles contract, and is forced out, 

 either forwards or backwards, through the two aortse. From 

 these it finds its way to the capillaries, and returns from them 

 to the vena cava; whence it is carried, through the organs of 

 Bojanus, to the branchiae. Here it becomes purified of car- 

 bonic anhydride, and receives oxygen from the water in 

 which the branchiae are plunged ; and it is finally brought 

 back in an arterialized condition to the heart. 



The heart is therefore systemic and propels aerated 

 blood. 



The majority of the vessels which convey the blood 'from 

 the vena cava to the branchiae, traverse the walls of the 

 dark-coloured organs the organs of Bojanus which has 

 already been mentioned ; and it is probable that they here 

 part with their nitrogenous waste matters the organ of 

 Bojanus, in all probability, playing the part of a kidney. 

 The cavity of the organ of Bojanus communicates, on the 

 one hand, with the pericardium, and, on the other, with the 

 exterior, by an aperture which is situated close to the attach- 

 ment of the inner gill to the walls of the body. Thus the 

 cavity of the pericardium communicates directly with the 

 exterior, though by a roundabout way. But it also com- 

 municates directly with the venous system, by sundry small 

 apertures placed in the anterior part of its floor. Hence 

 it must contain a mixture of blood and water. 



