THE CLAM AND OTHER BIVALVES 201 



In the act of swimming the valves open and close quickly, 

 by the alternate action of the hinge ligament and the large 

 adductor muscle. On closing, the valves catch a quantity of 

 water between the mantle folds. The water escapes under 

 pressure from within, through a round opening at either 

 end of the straight flange of the hinge. The resulting 

 action of these jets of water backward, against the body 

 of water outside, is to force the larger and broader end 

 of the animal forward. When the scallop ceases swimming, 

 it immediately falls to the bottom. 



Shipworms. The shipworms are marine bi- 

 valve mollusks that have taken 

 to burrowing in wood. The 

 shell covers but part of the 

 long, wormlike body. Piles 

 and wharves and other wooden 

 structures on the water fronts 

 become honeycombed and are 

 completely undermined by the 

 burrows of these animals. They 

 cause great damage in many 

 |y regions along the coast. 





a* 





Fig. 106. Group of living scallops, (x J) 



