The normal instinctive action of the mollusk is self-protective and adaptive. By the secretive action of its mantle it gathers from the water in which it lives, material to build a shell with a rough and rugged exterior for its enemies, and adapted to resist the chemical activities by which it is surrounded, and a perfectly smooth lining suitable as an interposition for its own delicate organism. Barring accidents, the building functions of the animal are employed only in the extension of the shell to meet the needs of its own growth and protection. But should a particle of secretion intended for the shell, harden within the folds of the oyster's mantle, or some parasite or other intruder present itself within the nacre-forming sphere, the instinctive action which lines the rougher part of the shell is also directed toward the foreigner, and it is at once covered with a like deposit. This is the birth of a pearl, and it grows layer by layer as long as it remains within the scope of the nacre building instinct. These layers, or skins as they are called, are seldom iridescent. Occasionally a pearl of that character is found, but it is generally from a fresh-water mussel, and the nacreous plates are of unusual tenuity. Although the pearl like the lining of the mollusk's shell is composed of carbonate of lime in series of thin waves lapping each other, each series constituting a plate or separable layer, there is a distinct difference in construction. Whereas the lining is a series of horizontal layers, the pearl is made up of concentric layers, each addition enveloping those preceding it. These skins however are not always absolutely distinct and separate. Instead of being like a succession of globular skins, each completely covered by its successor, the growth is often spiral and the construction is as if the nucleus had been rolled one, two, or three complete revolutions in a continuous plate of nacre, and the spiral envelope then finally merged into another plate and the process repeated. That which to a casual glance, therefore, appears to be six rings of nacre in a sectional cut, is in reality, several spirals of two or three turns each. It is also noticeable that whereas the wave edges, with all their eccentricities, trend generally in one direction in the shell nacre, in the pearl, the lines twist and curl with a concentric tendency, as though the waves had been laid on by turning or rolling the pearl in the material of which it is composed.