Many suggestions have been made as to the origin of pearls. According to one theory, pearls arise from secretions caused by the presence of some foreign object, such as a grain of sand, within the shell of the oyster. But it has been shown that fine pearls are not thus produced, but only pearly concretions. Another theory is that pearls are the result of disease in the oyster. M. Seurat of the Oceanogaphic Museum of Monarco, believes that the origin of pearls is to be ascribed to the presence of a parasite. The species of parasite differs with the species of oyster, but this mode of origin is general with all fine pearls. To restore the luster of a "dead" pearl the outer tarnished envelopes may be removed by acids. Thus the effect produced upon a tarnished pearl by causing a fowl to swallow it is accounted for by the dissolvent action of the gastic juice.