122 CLAMS, OYSTERS, AND MUSSELS 



also as a means of varying the specific gravity. If a snail 

 floating at the surface of the water be touched, the lung 

 will force out a bubble of air, thus causing its body to sink 

 quickly out of the way of danger. 



The slug has the power to spin a path of silk to protect 

 its body from irritating substances. The silk also serves 

 as a means of traveling; for example, when the slug is 

 descending a tree. 



Economic importance of the Mollusca. — This is probably 

 the most important group, economically, that has been 

 discussed. The class containing the clams and oysters 

 stands first in economic importance. The oyster industry 

 is carried on in nearly every seacoast town and village from 

 Massachusetts to Texas. The industry reaches its highest 

 development in Chesapeake Bay and vicinity. The busi- 

 ness gives employment to thousands of persons, and the 

 value of the oysters sold amounts to millions of dollars 

 annually. 



Mention must be made of the products obtained from 

 the pearl oyster. It annually furnishes large quantities of 

 valuable pearls, and the mother-of-pearl obtained from 

 the shells of this moUusk forms, in the aggregate, a product 

 of great value. Buttons, knife handles, penholders, um- 

 brella handles, etc., are made from mother-of-pearl. Men- 

 tion has already been made of the economic importance 

 of the fresh-water mussel. The clams, periwinkles, and 

 some snails form a considerable source of food supply. On 

 the other hand, the shipworm does great damage by boring 

 into piles, wharves, and ships. The slugs often become 

 injurious in gardens, but the squid is of value for cod 

 bait while the cuttlefish furnishes cuttle bone and the 

 material for sepia ink. 



