Elements of comparative zoology (1904) Elements of comparative zoology elementsofcompar1904king Year: 1904 206 SYSTEMATIC ZOOLOGY. luscs, the precious pearls. These pearls are really the shell-forming secretions of the mollusc around some foreign body and they receive their beauty from the way in which the shell is deposited around the centre. Fresh- water mussels, to be referred to a few lines below, also form pearls of value. The shell of the pearl-oyster also has its value, for it furnishes the mother-of-pearl used for buttons, knife-handles, for inlaying, etc. The pearl- oysters occur in
Elements of comparative zoology (1904) Elements of comparative zoology elementsofcompar1904king Year: 1904 206 SYSTEMATIC ZOOLOGY. luscs, the precious pearls. These pearls are really the shell-forming secretions of the mollusc around some foreign body and they receive their beauty from the way in which the shell is deposited around the centre. Fresh- water mussels, to be referred to a few lines below, also form pearls of value. The shell of the pearl-oyster also has its value, for it furnishes the mother-of-pearl used for buttons, knife-handles, for inlaying, etc. The pearl- oysters occur in the Indian Ocean, and also in the Bay of Panama. The salt-water mussels (fig. 44) so abundant on the FIG. 44.—Salt-water mussel (Mytilus edulis). mud flats all along northern shores have a peculiar gland in the foot which secretes strong silky threads (byssus) by which these animals anchor themselves. The common species, which occurs both in Europe and New England, is called the edible mussel; but not infrequently severe sickness follows its use as food. The fresh-water mussels or Unios are especially abundant in America, the Mis- sissippi basin being their centre. They are useless as food, owing to their strong taste. There are possibly a hundred species of these forms in America; over six hundred so-called species have been described. In their siphonal structure they form a transition to the next group.
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