 Razor shells can dig themselves into the sand with great speed to escape predation . When a Pacific razor clam ( Siliqua patula ) is laid on the surface of the beach it can bury itself completely in seven seconds and the Atlantic jackknife clam , <unk> <unk> , can do the same within fifteen seconds . Scallops and file clams can swim by opening and closing their valves rapidly ; water is ejected on either side of the hinge area and they move with the flapping valves in front . Scallops have simple eyes around the margin of the mantle and can clap their valves shut to move sharply , hinge first , to escape from danger . Cockles can use their foot to move across the seabed or leap away from threats . The foot is first extended before being contracted suddenly when it acts like a spring , projecting the animal forwards . 
