Their shells have also been used as money in some preindustrial societies .
Molluscs have , for centuries , also been the source of important luxury goods , notably pearls , mother of pearl , Tyrian purple dye , and sea silk .
Molluscs ' shells have also been used as money in some preindustrial societies .
Molluscs have , for centuries , also been the source of important luxury goods , notably pearls , mother of pearl , Tyrian purple dye , and sea silk .