XI.] THE FRESH- WATER MUSSEL. 105 



gape, it will be seen to be sucked in; while, after a short time, 

 a current of the same substance will flow out from an opening 

 between the two edges of the mantle on the dorsal side of 

 the posterior end of the body; and these 'inhalent' and 

 * exhalent ' currents go on, so long as the animal is alive and 

 the valves are open. Any disturbance, however, causes the 

 foot, if it was previously protruded, to be retracted, while the 

 edges of the mantle are drawn in and the two valves shut with 

 great force. On the other hand, in a dead Anodonta the valves 

 always gape, and if they are forcibly shut spring open again. 

 The reason of this is the presence of an elastic band, which 

 unites the dorsal margins of the two valves, for some dis- 

 tance, and is put on the stretch when the valves are forcibly 

 brought together. During life they are thus adducted by 

 the contraction of two thick bundles of muscular fibres, which 

 pass from the inner face of one valve to that of the other, 

 one at the anterior and the other at the posterior end of the 

 body, and are called the anterior and posterior adductors. 



The animal can be extracted from the shell without 

 damage, only by cutting through these muscles close to their 

 attachments. It is bilaterally symmetrical, the foot proceed- 

 ing from the middle of its ventral surface ; the mouth is 

 median and situated between a projection, which answers to 

 the under surface of the anterior adductor muscle, and the 

 superior attachment of the foot. On each side of the mouth 

 are two triangular flaps with free pointed ends the labial 

 polpi and behind these, on each side, two broad, plate- 

 like organs, with vertically striated outer surfaces, are 

 visible. These are the gills or branchice. In the dorsal 

 region, the integument is soft and smooth ; on each side, 

 it is produced into two large folds, the lobes of the mantle 

 or pallium, which closely adhere to the inner surface of the 

 valves of the shell, and end, ventrally, in the thickened 

 margins already mentioned. They pass into one another in 



