 The mantle suspender muscles attach the mantle to the shell and leave an arc @-@ shaped scar on the inside of the valve , the pallial line . The paired pedal protractor and retractor muscles operate the animal 's foot . Some bivalves , such as oysters and most scallops , are unable to extend their foot and in them , these muscles are absent . Other paired muscles control the siphons and the byssus . 
