Mollusks have been exploited by people around the world for thousands of years. Both shelled (snails, clams, scallops, conchs) and unshelled mollusks (squid and octopus) have been popular food items since prehistoric times. Several early societies used shells such as cowries for money. Today, mollusk shells are often collected and sold. Even mollusk waste products have value. A pearl is just shell layers that the mollusk uses to cover debris that gets under its shell. All shelled mollusks make them, but it is the pearl oysters that sometimes make the symmetrical, shiny ones, essentially decorative pieces of dirt. Over the years, mollusks have been used for many other purposes: dyes, decorative inlays, medicines, blades, fishing lures, tweezers, and horns. Overharvesting has endangered many mollusks species, and cultivation of mollusks had emerged as one solution.