Scallops are number 1 on The Most Extreme Movers. They spend more of their time sitting on the seabed, dining on plankton that drift by. They are enclosed by two hard shells, and have no legs, no wings, and not even a tail. But when a hungry sea star comes along, they need to take evasive action. They use jet propulsion. Flexing the muscles that keep the two shells together squirts water out their sides and this propels them forward.