Nacre is secreted by the ectodermic cells of the mantle tissue of certain species of mollusk. In these mollusks nacre is continually deposited onto the inner surface of the animal's shell (the iridescent nacreous layer, also known as mother of pearl), both as a means to smoothen the shell itself and as a defense against parasitic organisms and damaging detritus.
When a mollusk is invaded by a parasite or is irritated by a foreign object which the animal cannot eject, a process known as encystation entombs the offending entity in successive, concentric layers of nacre. This process eventually forms what we call pearls and continues for as long as the mollusk lives.