THE CLAM AND OTHER BIVALVES 



205 



cally exterminated. The United States Bureau of Fisheries 

 has studied the problems of mussel propagation very care- 

 fully. In many of the streams commercial catching of mus- 

 sels is prohibited in the hope of natural restoration. Fishing 

 crews carry on rescue work to save fishes stranded in back- 

 water ponds and bayous after high water. Before the 

 rescued fishes are returned to the streams they are placed 

 in tanks containing liberal quantities of glochidia. By this 

 means each fish may carry 

 several hundred young mus- 

 sels back into the streams. 



Pearls. True pearls of fine 

 quality have been found in 

 many species of mussels in all 

 parts of the Mississippi Val- 

 ley. In nature, pearls are 

 formed around some irritat- 

 ing object. Parasitic worms 

 and eggs of some of the small 

 mites infecting the mussels 

 are the most frequent bodies 

 around which pearls are 



formed (Fig. 110). In reality, pearls are tombs incasing the 

 bodies of intruders. Even sand grains at times become the 

 nucleus around which certain kinds of pearls are formed. 



Japanese scientists recently discovered methods of pro- 

 ducing "culture" pearls by inserting objects inside the man- 

 tle. When perfect spheres cut from shells are introduced 

 into the mantle, the culture pearls are almost perfect. 



Relation to Environment. There are more than five hun- 

 dred varieties of fresh-water mussels in the United States. 

 Of these, some live only in muddy streams, and others are 

 limited to sandy or to rocky bottoms. In streams, shallow 

 waters, where riffles are formed, harbor the greatest number 



Fig. 110. Section of mantle fold of 

 mussel showing how a pearl is formed 



1, cells of mantle; 2, external epithe- 

 lium of mantle ; 3, internal (ciliated) ep- 

 ithelium ; If, position of shell ; 5, pearl ; 

 6, remains of parasite. (After Jameson) 



