THE FRESH-WATER MUSSEL. 



95 



your toes to listen to what I am saying ! The mussel is 

 not a " long-eared animal ; " indeed, its ears are so very 

 small, that you can hardly see them with the naked 

 eye. If you get a young mussel and look closely, 

 you will find two little bags, called ear-sacs, one in each 

 side of the foot (Fig. 103). They are full of fluid, and 



Fig. 93. 



Mantle. 



.Mantle. 



Gills. Foot. Gills. 



Mussel cut crosswise, showing the Position of Foot, Gills, and Mantle. 



in the fluid are some very little stones, called otoliths, 

 which keep up a trembling motion. These little stones 

 make the sounds stronger, just as the pebbles on the 

 beach increase the sound of the waves. Though these 

 ears are so small, they are very sharp ; for, if you make 

 .the least noise when the foot is out of the shell, the 



