. The animans and man; an elementary textbook of zoology and human physiology. has to betaken in cultivating oysters to furnish proper conditionsfor growth and development. The young oysters whenfirst attached are called spat; when a little older thisspat, now called seed, may be transplanted to new beds,which are stocked in this way. In fact some beds haveconstantly to be thus restocked, the young oysters producedon them not finding good places to attach themselves, andso swimming away. Sometimes pieces of slate, pottery, etc.,are strewed about the oyster-beds to serve as collectors,that is,
. The animans and man; an elementary textbook of zoology and human physiology. has to betaken in cultivating oysters to furnish proper conditionsfor growth and development. The young oysters whenfirst attached are called spat; when a little older thisspat, now called seed, may be transplanted to new beds,which are stocked in this way. In fact some beds haveconstantly to be thus restocked, the young oysters producedon them not finding good places to attach themselves, andso swimming away. Sometimes pieces of slate, pottery, etc.,are strewed about the oyster-beds to serve as collectors,that is, as places for the attachment of the young oysters. 174 THE ANIMALS AND MAN The extent of the acreage of the American oyster-beds islarger than that of any other country. The Baltimoreoyster-beds on the Chesapeake River and its tributariescover 3,000 acres, and produce an annual crop of 25,000,000bushels. The pearl-oyster is not a true oyster, that is, not amember of the family to which the edible oysters belong,but it is a member of the same class, that is, it is a bivalve. FIG. 85. Pholas sp., a mollusc burrowing in sandstone. (Photographby C. H. Snow; permission of the American Society of Civil Engineers.) mollusc. Pearls are obtained from a number of differentpearl-oysters, but the finest pearls and mother-of-pearlcome from the tropical species Meleagrina pearl-oyster has an extensive distribution, beingfound in Madagascar, the Persian Gulf, Ceylon, Australia,Philippine Islands, South Sea Islands, Panama, WestIndies, etc. Mother-of-pearl is simply the inner liningof the shell, which is composed of numerous thin layers ofcarbonate of lime so arranged that the edges of the suc-cessive layers produce many fine striae very close beautiful iridescence of this inner shell-lining is causedby the complicated diffraction and reflection (interferenceeffects) of the light by the fine striae and the translucent ARTHROPODS AND MOLLUSCS 175 superposed thin
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