PHYLUM MOLLUSCA 255 



group up around and eventually cover the parasites. Thus a cyst 

 is produced about the glochidium and within this structure the 

 larval clam undergoes metamorphosis. It shortly breaks loose from 

 its host, drops to the stream or pond bed, and leads an independent 

 life. The rapid dissemination of mussels in a river system can be 

 accounted for by the movements of their fish-hosts. 



Economic Relations of the Phylum 



Mollusks have been used as food by man from the beginning of 

 civilization. Oysters, clams, scallops, snails, and the arms of cuttle- 

 fish are found in the menus of peoples all over the world. It has 

 been estimated that the oyster industry along the Atlantic Sea- 

 board approximates 40,000,000 dollars annually. Along the Texas 

 coast alone. Federal statistics show that 51,719 barrels of oysters 

 were sold in 1932. Buttons are made from the shells of the large 

 heavy river clams and along the Ohio, Missouri, and Mississippi 

 rivers the button industry amounted to 5,000,000 dollars in 1931. 



Within some of the clams are found pearls which are formed by 

 some irritating particle, such as a parasite or sand grain that be- 

 comes lodged between the mantle and the shell. Iridescent protec- 

 tive layers of mother-of-pearl are deposited around the foreign par- 

 ticle, thus producing the pearl. The Japanese have been success- 

 ful in artificially stimulating pearl production by planting small 

 objects, such as pieces of mother-of-pearl, between the mantle and 

 shell of pearl-oysters. 



Pulverized clam shells are also being used as a calcium supple- 

 ment to chicken feed. Shells have also been used as a medium of 

 exchange. The wampum of the eastern coast of North America 

 consisted of strings of cylindrical beads made from brightly colored 

 clam shells. Shells have always been and still are used for orna- 

 mentation. Crushed shells are used in road construction. 



Some mollusks are injurious to human interests. Among these 

 might be mentioned the marine snail, Urosalpinx cinerea, which 

 drills into and feeds on oysters and other pelecypods; the common 

 shipworm, Teredo navalis, attacks the wood of ships and pilings, 

 making extensive excavations. Certain species of snails serve as the 

 intermediate host of parasitic flatworms or flukes. The liver fluke 



