When a tiny parasite enters an oyster's shell, mantle cells cover it with nacre until this is formed
... nacre until the foreign body is enclosed in the shell-like substance, 
forming... [DOC] [TLE] Natural Pearl VS Cultural Pearls | Shecy Pearls - Pearl Jewelry [PAR] Pearls are formed inside the shell of certain mollusks as a defense mechanism ... 
intruder or parasite enters a bivalve mollusk, and settles inside the shell.