The phylum Mollusca is an extremely vast group of animals with a resultantly vast array of physical attributes, although some features are shared by all creatures who are a part of it. For example, all mollusks have a mantle. A mantle is a fold of skin who's job in some organisms it is to produce the shell by secreting calcium carbonate, while in other mollusks its just an epidermus layer covering the visceral mass ( soft tissue organ mass). All mollusks also have a shell at some point in there life cycle, though some shed them after their larva stage and others have receded and became vestigial structures. Another shared trait is the presence of a muscular foot. It has evolved to serve many different purposes for different animals. From being a balance sensor to being a means of movement or secreting mucus.