Bones are hardly prerequisite for movement. Consider arthropods and worms, for example. Lots of movement - no bones about it! Larval mollusks float in the water with no control over their movement. As adults, they have a variety of locomotory behaviors depending on the class. Gastropods, monoplacophorans, and chitons have a muscular foot on which they crawl around, usually on a film of mucus. Cephalopods generally use jet propulsion by muscular contraction of a funnel. Bivalves, if they move at all, and scaphopods pull themselves into sediment by muscular foot contractions; scallops may use jet propulsion through the water. Aplacophorans use cilia and mucus to crawl across the sea floor and muscles to wrap around food. All these means of locomotion except the cilia require muscular action but as you can see this takes many forms. The subject of molluscan movement is a complex one because mollusks themselves are so diverse!