96 MOLLUSKS 



When the two adductor muscles are cut free from the 

 valves, a round or oval surface is seen which is marked off 

 from the rest of the interior of the shell. These areas 

 are called muscle scars (Figure 94). 



When the empty clam shell is examined, it is found 

 that the hinge, sometimes called the hinge ligament, is 

 elastic. This is shown by compressing the two valves and 

 seeing how promptly they open when the pressure is taken 

 off. Where the two valves come in contact just beneath 

 the hinge ligament, a blunt projection of one shell fits into 

 a depression in the other. These are called the hinge 

 teeth. 



LABORATORY STUDY 



Live clams can be secured in the market during the school year. The 

 dissection of the clam is too difficult, but the arrangement of the organs 

 in the mantle cavity can be studied. The position of the adductor 

 muscles, foot, gills, palps, heart, etc., should be observed. Examine a 

 small portion of a gill under the microscope for cilia. A variety of 

 shells of clams should be studied in which hinge, muscle scars, and hinge 

 teeth are examined. Compare clam and snail shells. 



92. Locomotion. — The movements of the fresh water 

 mollusks are extremely slow. In the clam the foot is 

 forced out of the shell by the blood, which flows into it 

 and causes the foot to be greatly enlarged. Muscles 

 attached to the shell and front of the foot contract and 

 pull the shell forward over the extended foot. 



93. Food. — The food of the clam consists of microscopic 

 plants and animals that . are caught in a sticky fluid 

 (mucus) on the gills, as the water passes through them. 

 The food, together with the mucus, is moved into the 

 mouth by means of cilia. The mouth is simply an open- 

 ing into the body and the cilia are on triangular flaps or 

 lips (palps) on either side of the mouth. From the mouth 

 food passes into the digestive canal, where the nutritious 

 parts are absorbed (Figure 95). 



