ro8 ELEMENTARY BIOLOGY. [CHAP. 



so as to fall towards the 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 disturb- 

 ance, 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 distance, and is put on 

 the stretch when the valves are forcibly brought together. 

 During life they are thus addncted 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 pro- 

 ceeding 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 

 palpi and behind these, on each side, two broad, plate-like 

 organs, with vertically striated outer surfaces, are visible. 

 These are the gills or branchia. In the dorsal region, the 

 integument is soft and smooth ; on each side, it is produced 

 into large folds, the lobes of the mantle or pallium, which 

 closely adhere to the inner surface of the valves of the shell, 



