OYSTERS, CLAMS, MUSSELS 



229 



the same way as that already described for the fresh-water 

 pearls, may have, when large and of perfect shape and luster, 

 a very great value. The recently attained perfection in the 

 making of imitation pearls may somewhat lessen the market 

 value of true pearls, but the pearl-fisheries are still of great 

 importance. 



For more than two thousand years Ceylon has been the 

 center of the pearl-fisheries industry, but many valuable pearls 

 and much better mother-of-pearl shells are found in the waters 

 of other tropical islands. 



FIG. 105. Inner side of a pearl shell. (Reduced). 



Teredos. Very unlike any other members of this class in 

 general appearance and habits are the teredos or ship-worms 

 that so often do great damage to any timber that is in salt 

 water. The young teredo is a free-swimming embryo like the 

 young of other molluscs, but it soon settles on some piece of 

 submerged wood and begins to burrow into it. As it grows 

 and develops its small bivalve shell, it bores deeper into the 

 wood, lining its burrow with a shell-like calcareous deposit. 

 As the ends of the siphons are kept close to the entrance of 

 the burrow, the animals soon become very much elongated and 



