ttps:////»https://www.w3.org/TR/REC-html40/loose.dtd»>
They are basically protective shells of marine molluscs, soft-bodied animals. When they are born, a temporary shell forms around them, called protoconch. When the mollusk grows and reaches young age, the definitive shell begins to form, replacing the first one. It is the so-called mantle – tissue similar to the skin, involving the vital parts of the animal – that secretes the substances that form the four layers of the shell. The main component is calcium carbonate, extracted from ocean water. There are also organic elements, such as proteins, produced by the animal itself. “As the mollusk grows, the mantle eliminates more carbonates and proteins, and the shell also increases in size”, says oceanographer Ceci Pereira Moreira de Souza, from the University of São Paulo. In addition to the shells formed by two parts (known as the bivalve system), there are also the cone-shaped structures produced by molluscs of the class known as gastropods.