MUSSELS, CLAMS, OYSTERS, SNAILS, SQUIDS 111 



leads a truly parasitic life for two months, after which it 

 undergoes a metamorphosis and falls to the bottom again, 

 there to begin an independent existence. 



Distribution and economic importance. — This moUusk 

 is widely distributed in the fresh-water streams and lakes 

 of the United States. The pearly layers of its shells furnish 

 material for large quantities of buttons, and pearls of con- 

 siderable value are often found within its shells. Accord- 

 ing to recent investigations, it has been shown that pearls, 

 at least in the mussel, result from the irritation caused by 

 the presence of a minute roundworm parasite in the man- 

 tle of this mollusk. The larva of this parasitic worm, es- 

 saying to bore through the mantle, rests for a time among 

 the loosely connected cells of the latter. As a result, the 

 cells of the outer laj'er of the mantle are stimulated to de- 

 posit a hard substance, called nacre, around the resting 

 larva. This nacre, owing to the fine ridges running across 

 it, produces an iridescent object which we call a pearl. 

 It is now thought that all pearls are formed in this way, 

 although it was formerly held that an undeveloped egg or 

 grain of sand lodging between the shell and mantle would 

 stimulate the latter to deposit nacre and thus form a pearl. 



The mussel is protected from its chief enemies, the 

 raccoon, mink, and otter, by remaining on the bottom 

 partly hidden in the mud and by withdrawing into its 

 hard calcareous shell. 



