V.] THE FRESH-WATER MUSSEL. 311 



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 

 communicates, 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, like the nephridium of the worm, can be 

 resolved into three segments an internal non-glandular, 

 a middle glandular, and a terminal muscular one. An 

 outselling current is induced by the ciliary action of the 

 lining epithelium of each of these organs, and their relations 

 to the pericardium of this animal, are as those of any one 

 pair of segmental organs to the body- cavity in the worm. 

 If so be that the pericardium of Anodonta is, like that of 

 the Frog, a direct derivative of the body-cavity, the excretory 

 organ is in no way anomalous in its relationships. 



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 1 . 



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. 



