 The main muscular system in bivalves is the posterior and anterior adductor muscles , although the anterior muscles may be reduced or even lost in some species . These strong muscles connect the two valves and contract to close the shell . They work in opposition to the ligament which tends to pull the valves apart . In sedentary or recumbent bivalves that lie on one valve , such as the oysters and scallops , the anterior adductor muscle has been lost and the posterior muscle is positioned centrally . In file shells that can swim by flapping their valves , a single , central adductor muscle occurs . These muscles are composed of two types of muscle fibres , striated muscle bundles for fast actions and smooth muscle bundles for maintaining a steady pull . 
