ARTHROPODS AND MOLLUSCS 175 



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 pur- 

 posely introduced into the shell in order to stimulate the 

 formation of pearls. The pearl-fishers go out in boats and 



FIG. 86. Martesia xylophaga, a Pholad, burrowing in Panama mahogany. 

 (Photograph by C. H. Snow; permission of the American Society 

 of Civil Engineers.) 



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. 



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 



