PEARLS AND PEARL-DIVING 



A grain of sand will sometimes work its way between 

 the body and one of the valves, thereby causing an 

 irritation which the mollusc at once proceeds to check 

 by gradually covering the intruder with carbonate of 

 lime films, one upon another like the coats of an onion, 

 till it is as smooth and polished as the inside of the shell 

 itself. Again, when a dog-whelk or other boring animal 

 that preys on oysters has succeeded in drilling a hole 

 through the shell from the outside, the little creature 

 within will sometimes plug up the aperture with its 

 secretion and laboriously spread layers over the nucleus 

 thus formed. The result in either of these cases is that 

 a pearl will be found adhering to one of the shells. 



The more valuable pearls, however, are found loose 

 inside the mantle of the mollusc, or at least slightly 

 connected with it ; and these have a somewhat different 

 origin. It often happens that one of the ova is lifeless, 

 and, not being thrown out with the rest at spawning 

 time, gradually increases in size, because, though infertile, 

 it is still supplied with blood-vessels from the parent 

 body; then hardens and becomes an even greater source 

 of irritation than any foreign object would be till it has 

 been " insulated " with nacre and made a pearl of. These 

 will be the globular and pear-shaped pearls pear-shaped, 

 because at times the connecting link or pedicle between 

 the egg and the body is also covered with the films. One 

 of the latter sort has been taken weighing 3f oz. troy, 

 2 in. long and 4 in circumference. I believe it is still to 

 be seen at the South Kensington Museum. 



Any shell-bearing mollusc may contain pearls, though 



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