122 CLAMS, OYSTERS, AND iMUSSELS 



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

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

 will force out a )3ubl:ile of air, thus causing its body to sink 

 cjuickly out of the way of danger. 



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

 its botly from irritating substances. The silk also serves 

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

 descending a tree. 



Economic importance of the MoUusca. — This is probably 

 the most important group, economicalh', 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 fm-nishes large fjuantities of 

 valuable pearls, and the mothei-of-pcarl obtained from 

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

 of great value. Buttons, knife handles, penhoklers, 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. Tlie clam.s, periwinkles, and 

 some snails form a consideral^le source of food supply. On 

 tlie otlier hand, the shipworm does great ilamage by boring 

 into piles, wliarves, and sliips. The slugs often become 

 injurious in gardens, but tlie stiuid is of value for cod 

 bait while the cuttlefisii funushes cuttle bone and the 

 material for se])ia ink. 



