30 PEARLS [CH. 



bivalves there are two adductor muscles. Where 

 one only is present, it corresponds to the posterior 

 of the two. This adductor muscle can contract with 

 considerable force. The force required to open an 

 edible oyster, an edible mussel or a cockle is probably 

 known to most readers. This force is being ex- 

 pended in overcoming the strength of the adductor 

 muscles. Whilst, however, the bivalves can resist 

 very considerable pulls made for a short period, a 

 very weak pull is sufficient, if sustained for some 

 time, to break down the oyster's protection. This is 

 the cause of much trouble, for one of the enemies of 

 the bivalve, the starfish, opens the shell merely by 

 pulling on both valves with comparatively little force, 

 but with an insistence that is to be admired. The 

 process is interesting, because for a long time it was 

 supposed that the starfish poisoned the mollusc, when 

 of course after death the shell would open auto- 

 matically. This is not the case, as the starfish 

 simply applies its arms to the two valves of the shell, 

 attaches its numerous " tube feet " and steadily pulls. 

 When its perseverance has been rewarded by the 

 divarication of the two valves, the stomach is ex- 

 truded and the animal proper of the mollusc digested. 



In addition to the adductor muscle, there are 

 several other large and definite muscles which are 

 connected with the foot. 



There are six of these altogether. All are 



