THE ECONOMIC BIOLOGY OF INVERTEBRATES 471 



we hear called the " ears " are really the muscles that held the 

 shell together and it is this muscle only which we eat in the case 

 of the scallop. 



Clams are found along our whole Atlantic coast; oysters are 

 abundant south of Cape Cod with Chesapeake Bay as the center 

 of the industry, having the largest production of any region in the 

 world. The Pacific coast and foreign shores also furnish these 

 succulent bivalves, but even so, Chesapeake oysters are in demand 

 in the best markets of Europe, and the oyster yields the most 

 valuable water crop in existence. It is the leading fishery product 

 in fifteen different states. Aside from their value as food, mol- 

 lusc shells furnish us with " mother-of-pearl " for buttons, handles, 

 and ornaments, with crushed shell for chicken feeding, and with 

 the precious pearl of the jewelry store. 



These latter are found in " pearl oysters " (not the edible species) 

 and are caused by the entrance within the shell of a grain of sand or 

 the irritation of a parasitic worm, which makes the oyster secrete 

 layer after layer of shell substance, to cover the offending particle, 

 much as the hand protects itself from irritation by growing a 

 callous layer. The most valuable pearls are found in the Persian 

 Gulf and on the coasts of Ceylon. Fresh-water clams furnish the 

 irregular " baroque " pearls and are found largely in the Mis- 

 sissippi and its branches. 



Shells have always been used for ornaments and formerly passed 

 for money as well, the " cowrie " of Africa and the " wampum " 

 of our Indians being two examples. Wampum consisted of beads 

 cut from the colored parts of clam shells. 



Snails and slugs are another group of molluscs, which, especially 

 in France, are valued as food. They do considerable harm in 

 gardens where they eat young seedlings and leaves. The shiny 

 trails so often seen on sidewalks are left by the slugs in their travels. 



A near relative is the abalone of the California coast, whose 

 beautiful rainbow colored shell is used for ornaments and for a 

 great deal of inlaying work. 



The third group of molluscs is called the cephalopods and in- 

 cludes the squid, cuttle fish, and octopus. Man uses squid for 



