54 PEARLS AND PEARLING 



ing its way through the mud or sand. The foot holds 

 the mussel up while it is feeding and is used to move 

 the animal from one place to another. By contracting 

 and expanding the foot while it is in the ground the 

 mussel is able to push forward and plow through the 

 bottom of the river. The foot also holds the mussel from 

 being swept away by the swift water in many cases. Al- 

 though the shell may be almost closed, the foot which is 

 expanded in the ground can hold so tightly that a great 

 effort is required to lift the mussels. 



The mouth is located under the anterior adductor 

 muscle and opens into the abdominal sac. It is carried 

 through the intestinal canal, and finally empties near the 

 posterior addustor. The animal has a sort of a heart, 

 surrounding the intestinal canal, which beats regularly. 



Mussels have gills, or branchae, which separate the 

 oxygen from the water and answer to the gills of the 

 fishes or the lungs of land animals. They are fastened 

 to the mantle and the top of the abdominal sac in some 

 cases, and hang down between the two on both sides of 

 the sac and reach from the posterior end of the shell 

 almost to the anterior end. They are very delicate and 

 beautiful and are of vital importance to the mussel. 



The palpi are just in front of them on both sides of 

 the abdominal sac and under the mantle. 



This pair of smaller, triangular flaps, extending be- 

 hind are constantly in motion when the animal is feeding. 

 In the posterior part of the mussel the mantle shows two 

 small openings, one above the other. One or both of 

 these is fringed and may be seen expanded when the 



