280 



CIVIC BIOLOGY 



muskrats until discovery of their pearls and still more valuable shells 

 turned prosaic farming districts into " pearl fisheries " and developed a 

 nourishing industry. As with everything else, from forests to clams, 

 when it is discovered to possess commercial value, the American public 

 has hastened to kill the goose that laid the golden eggs ; so the waters, 

 many of them, are already depleted and the shell industry is in danger. 



FIG. 127. Mussel in sand, moving in direction of large arrow, foot pro- 

 truding from anterior, and inhalent and exhalent, siphons from posterior. 



end of shell 



As a consequence the United States Bureau of Fisheries has made exten- 

 sive surveys to discover the distribution of useful species, and has estab- 

 lished a biological station at Fairport, Iowa, to study practical methods 

 of propagating mussels in the great Mississippi basin. All this is likely 

 to prove a futile expenditure of time and money, unless communities can 

 learn to control their piratical impulses and members, and unite upon 

 rational plans for conservation of these industrial resources. 



It is estimated that a " niggerhead " requires from fifteen to eighteen 

 years to grow to a diameter of four and one-half inches, and pearls of 

 value are never found in mussels less than five years old. Other species 

 of nearly equal value may be grown in a shorter time possibly in from 

 four to six years. Growth lines on the shell are commonly taken to indi- 

 cate age, or at least the years required for a mussel to reach adult size. 



