What produces the layers of calcium carbonate that form a pearl?

The formation of pearls occurs within certain mollusks, particularly bivalves like oysters, where the mantle tissue plays a crucial role. The mantle is a significant organ that secretes layers of calcium carbonate, which is the primary material of a pearl. When an irritant, such as a grain of sand or a parasite, becomes trapped inside the mollusk's shell, the mantle begins to secrete nacre (also known as mother-of-pearl) around the irritant. This nacre is composed of aragonite or calcite crystal layers, along with organic materials. Over time, the continuous layering of these materials around the irritant results in the formation of a pearl.

Other options do not contribute to pearl formation in a significant way. The mesoderm refers to one of the three primary germ layers in an embryo and does not directly produce pearls. Cnidocytes are stinging cells found in cnidarians like jellyfish and corals and are unrelated to mollusks or pearls. The visceral mass is a part of the mollusk's anatomy containing internal organs, but it is the mantle that directly facilitates the secretion of calcium carbonate making it essential for pearl formation. Therefore, the mantle is the correct choice regarding the production of pearl