The "pearl-oyster" is not a true oyster, that is, not a member of the family to which the edible oysters belong, but it is a member of the same class, that is, it is a bivalve mollusc. Pearls are obtained from a number of different "pearl-oysters," but the finest pearls and mother-of-pearl come from the tropical species _Meleagrina margaritifera_. This pearl-oyster "has an extensive distribution, being found in Madagascar, the Persian Gulf, Ceylon, Australia, Philippine Islands, South Sea Islands, Panama, West Indies, etc." Mother-of-pearl is simply the inner lining of the shell, which is composed of numerous thin layers of carbonate of lime so arranged that the edges of the successive layers produce many fine striæ very close together. The beautiful iridescence of this inner shell-lining is caused by the complicated diffraction and reflection (interference effects) of the light by the fine striæ and the translucent superposed thin plates of shell material. Pearls are simply isolated deposits of shell material usually around some particle of foreign substance which has found lodging in the mantle-cavity. Sometimes small objects are purposely introduced into the shell in order to stimulate the formation of pearls. The pearl-fishers go out in boats and dive to the bottom, filling baskets with pearl-oysters. These are piled up in a bin and left to die and decompose. "When the flesh is pretty thoroughly disintegrated, it is washed away with water, great care being taken that none of the pearls loose in the flesh are lost. When the washing is concluded the shells themselves are examined for pearls which may be attached to the interior of the valves." The principal pearl-fishery is that on the coast of Ceylon; pearl-fishing has been carried on here for over 2000 years. [Illustration: FIG. 107.--_Martesia xylophaga_, a Pholad, in Panama mahogany. (Photograph by C. H. Snow; permission of Amer. Soc. Civil Engineers.)] The ship-worm (_Teredo_) is an interesting member of this class of bivalve molluscs, because of its unusual habits, and strangely modified body form. The teredo is long and worm-like in general appearance, with a small bivalve shell at one end and two elongated siphons at the other. The young teredo is a free-swimming ciliated embryo like the young of the other bivalve molluscs, but it soon settles on a piece of submerged wood, usually the pile of a wharf, or the bottom of a ship, and burrows into this wood. As it grows it enlarges and deepens its tube-like burrow, and lines it with a calcareous deposit. The burrow may be a foot long or longer, and when thousands of teredos attack a pile or the bottom of a ship, the wood soon becomes riddled with holes. These boring molluscs do great damage to wharves and ships. In Holland where they were first discovered they caused such injuries to the piles and other submerged wood which supported the dikes and sea-walls that they seriously threatened the safety of the country.