306 ELEMENTARY BIOLOGY. [CHAP. 



waters. If some finely dividing colouring matter, such as 

 indigo, Is dropped into the water, 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 disturbance, however, 

 causes the foot, if it was previously protruded, to be re- 

 tracted, while the edges of the mantle are drawn in and the 

 two valves shut with great force. This adduction results 

 from 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 valves of the dead Anodonta 

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

 again. The reason of this is the presence of an elastic 

 band or ligament, which unites the dorsal margins of the 

 two valves, for some distance, and is put upon the stretch 

 when the valves are forcibly brought together. The natural 

 tendency to divarication of the valves resulting from this, is 

 held in check during life by the contractions of the afore- 

 named adductor muscles. These respond to a nervous 

 stimulus, and, on the death of the animal, that being with- 

 drawn, the full force of the elasticity comes into play, the 

 ,valves becoming divaricated to their utmost extent. 



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. There is a 

 total absence of any distinct head segment such as is seen 

 in the Snail. The mouth is median and situate between 



