MOLLUSC A : THE MOLLUSCS 251 



decompose. " When the flesh is pretty thoroughly dis- 

 integrated, 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 



FIG. 107. Martesia xylophaga, a Pholad, in Panama mahogany. (Photo- 

 graph by C. H. Snow; permission of Amer. Soc. Civil Engineers.) 



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 



