MOLLUSCA. 225 



ally in this country. The only way in which this can be 

 increased is by scientific "farming." This implies the 

 proper preparation of suitable shallows, and the planting 

 of the young oysters there, and the protection of them 

 from their enemies, including man, until they reach 

 maturity, which is for about five years. This is made 

 possible by the great power of multiplication which the 

 oyster has. It is claimed that a single oyster may pro- 

 duce 60,000,000 eggs. 



Other forms that have been used more or less as food 

 are the fresh- water mussel, the long-necked clam, and 

 several species of marine mussels. Large species of 

 snails are prized as food in some regions. 



Most mollusks secrete a pearly substance at some part 

 of the mantle. This is normally spread out thinly over 

 the inside of the shell and is called "mother-of-pearl." 

 Pearl buttons, knife handles, and the like, are made from 

 the shells of our fresh-water clams. This is quite an 

 industry on some of the tributaries of the Mississippi. 



In a few species, under certain abnormal conditions of 

 irritation of the mantle, possibly due either to disease 

 or to the presence of some irritating substance next the 

 mantle, there is the formation of the pearl in a massive 

 form. These are concentric layers of mother-of-pearl. 

 Pearls have long been much prized as ornaments. They 

 have been found in great quantities in some of the graves 

 of the mound-builders. The finest pearls come from the 

 marine pearl oyster (Meleagrina margaretifera) , though 

 good ones are found frequently enough in our fresh-water 

 forms to add interest to the search. Small ones are 

 frequently found in the common oyster. Squid are used 

 as bait by fishermen. Some of the boring forms do much 

 damage to piles and other subterranean timbers placed 



