Locomotion type mostly correlates to the feeding habit. e.g. Herbivorous mollusc are slower than carnivorous molluscs.
Terrestrial Snails: Foot developed to a flat sole on the ventral side. Wave like motion of the foot carries the snail forward. Gland at the foot produces mucus that the snail crawls on.
Aquatic Snails: Pallial cavity filled with air provides the snail buoyancy to crawl on the water’s undersurface.
Gastropods can use sabre shaped Operculum to propel themselves forward.
Some marine gastropods have lost their protective shells and have developed foot like parapodia that enable them to glide in water.
Locomotion in cephalopods is facilitated by ejecting a stream of water for propulsion: “jet” propulsion.
Bivalves are largely sessile and do not move.