204 GENERAL ZOOLOGY 



this happens in a favorable place, it burrows into the mud 

 and begins the life of its adult kin. 



Economic Importance. The distribution of fresh-water 

 mussels has become a matter of considerable economic im- 

 portance, especially in the states of Iowa and Illinois. 

 Many factories have been established for the purpose of 



Fig. 109. Mussel fisherman with boat equipped for capturing fresh- 

 water mussels 



Photograph by F. M. Woodworth, courtesy of F. C. Baker 



making pearl buttons from the valves. According to the 

 annual report of the United States Bureau of Fisheries, ap- 

 proximately twenty-six thousand tons of mussel shells with 

 a value of over one million dollars were taken (Fig. 109) 

 by fishermen in 1922. These shells furnished material for 

 over eight million dollars' worth of manufactured products, 

 such as buttons and pearl novelties. 



So many mussels have been taken from the streams 

 (Fig. 109) in many regions that they have become practi- 



