How are the adaptations of molluscs related to their lifestyle?

Some shellfish have a leg. Depending on the lifestyle, it can perform different functions. If the mollusk lives on the bottom, the leg can be an organ of movement, fixation to the bottom or stones, serve for digging food or burying in the sand. In immobile molluscs, the leg is reduced or absent. The shell serves as a protection, but when additional organs appear for this, the shell may be absent. In mollusks leading a sedentary lifestyle, there is no head and almost no developed sense organs (class bivalve molluscs).