Oyster

The name oyster is used for a number of different groups of mollusks which grow for the most part in marine or brackish water. The shell, usually highly calcified, surrounds a soft body. Gills filter plankton from the water, and strong adductor muscles are used to hold the shell closed. Some of these groups are highly prized as food, both raw and cooked.

The oyster is the root of an idiomatic saying "The world is your oyster", which means that to achieve something in this world, you have to grab the opportunity.