v.] THE FRESH-WATER MUSSEL. 3II 



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

 dark-coloured organs — the organs of Bojanus — which have 

 already been mentioned; and they here part with their 

 nitrogenous waste matters — the organ of Bojanus playing 

 the part of a kidney. The cavity of the organ of Bojanus 

 coinmunicates, on the one hand, with the pericardium; and, 

 on the other, with the exterior, by an aperture to which 

 reference has already been made. Thus the cavity of the 

 pericardium communicates directly with the exterior, though 

 by a roundabout way. 



The organ of Bojanus consists of a pair of modified tubes, 

 each of which can, like the nephridium of the worm, be 

 resolved into three segments — an internal non-glandular, a 

 middle glandular, and a terminal muscular one. The rela- 

 tions of the whole to the pericardium in this animal, are as 

 those of any one pair of segmental organs to the body-cavity 

 in the worm; and if so be that the pericardium oi Anodonta 

 is, like that of the Frog, a direct derivative of the body-cavity, 

 the excretory organ is in no way anomalous in its rela- 

 tionships. 



The digestive gland of this animal is mainly concerned 

 in the elaboration of a digestive fluid. The production 

 of combustible carbo-hydrate material — a function by no 

 means confined to the liver in other animals — goes on to a 

 considerable extent in other far removed parts of the body, 

 glycogen being formed. The connective-tissues and espe- 

 cially the mantle lobes are remarkable for its presence, it 

 being elaborated in the form of large intra-cellular vesicles, 

 as the product of activity of certain of the connective-tissue 

 corpuscles \ 



The nervous system of the Anodonta consists mainly of 



1 Similar glycogenous cells are met with in the walls of the lacunar 

 spaces and on the * mesenteries ' of the Snail. 



