 Visible on the inside of most empty bivalve valves is a shiny curved line that runs more or less parallel to the outer margin of the shell and often connects the two adductor muscle scars ( if the animal had two adductor muscles ) . This line ( known as the pallial line ) exists because , parallel to the opening edge of the bivalve 's shell , the mantle is attached to the shell by a continuous narrow row of minute mantle retractor muscles . The function of these small muscles is to pull the loose edge of the mantle up out of harm 's way when this is necessary because of minor predation attempts . In many bivalves , the mantle edges fuse at the posterior end of the shell to form two siphons , through one of which water is inhaled , and the other expelled , for respiration and suspension feeding . Often , a pocket @-@ like space occurs into which the siphons fit when they are retracted . This is visible on the inside of the valve as an indentation on the pallial line which is known as the pallial sinus . 
