FRESH-WATER MUSSEL. 285 



PLATE VI. 



FRESH-WATER MUSSEL (Anodonta cygnea), 



Dissected so as to show its muscular and nervous systems, as well as certain other organs 



in relation with them. 



THE animal has been taken out of the shell ; the gills and the 

 mantle have been removed on the left side, together with the labial 

 tentacles and parts of the pericardium, as well as of the organ of Bojanus 

 or nephridium of the same side. 



a. Right mantle lobe, free along its ventral edge. 



a'. Fimbriated portion of mantle corresponding to the inferior siphonal 



notch by which water is drawn into the branchial cavity. 

 a". Dorsal raphe along which the two halves of the mantle meet. 



b. Foot. The muscular portion is strongly contracted. 



c. Gills of right side. 



/ Union of external gill to the mantle between the inferior and superior 

 siphonal notches. 



d. Anterior adductor. 



e. Posterior adductor. 



f. Posterior retractor of the foot, inserted into either valve, anteriorly 



and superiorly to the posterior adductor, the scar or muscular 

 impression of the two being more or less confluent. Its muscular 

 expansion in the foot is especially well developed along the free 

 or ventral edge of the foot, and it inter-digitates very freely with the 

 protractor pedis, though it lies for the most part at a lower level 

 than that muscle. 



g. Protractor of the foot. This fan-shaped muscle spreads over the 



external surface of the foot, from an insertion into the shell, a little 

 superiorly to the point where the pallial line joins the impression 

 of the anterior adductor. It acts, consequently, as an antagonist 

 to the preceding and succeeding muscles. Its impression is distinct 

 in this animal from that of the adductor. 



h. Anterior retractor of the foot. The fibres of this muscle take origin 

 from a point in the shell, towards the dorsal aspect of the anterior 

 adductor, though some way from its dorsal border. They spread 

 thence into the foot especially along its anterior edge, and down 

 as far as its anterior angle, for the most part at a deeper level than 

 the preceding muscle. Some of the fibres, however, spread over the 

 visceral mass dorsally. 



h! The line points to the position of the smaller retractor muscles with 



