PEARL PRODUCTION. 5 



Dr. GEORGE HARLEY in 1889. GIARD has recently pointed out that a considerable 

 resemblance between the pearl and an animal calculus is compatible with the parasitic 

 theory. Calculi commonly form around a nucleus, and many parasites are known to 

 have calcified cysts deposited over them. Some pearls, as we shall show below, not 

 of the finest quality, are probably formed as calculus-like growths independently of 

 vermean parasites. Even when the parasite is present as a nucleus and causes the 

 initial stimulation, it must be remembered that the pearl is produced by the molluscan 

 host, not by the parasite, and so has been justly compared by more than one writer 

 to an animal gall. 



There are two papers by HARLEY in the ' Proceedings of the Royal Society.' The 

 first (vol. 43, p. 461) dealt with the chemical composition of pearls, and the second 

 (vol. 45, p. 612) with the structural arrangement of the mineral matters, and there 

 HARLEY states two views, the one that they are " diseased concretions " comparable 

 with " other morbid calculi," and the second that they are " misplaced pieces of 

 organised shell." He recognises various kinds of nuclei, organic and inorganic, but 

 also admits that pearls may sometimes begin " by the mere aggregation and coalescence 

 of mineral molecules." (See our ' Calcospherules,' p. 27, below). 



COMPOSITION OF PEARL AND NACRE. 



In the paper on the " Composition of the Pearl and of Nacre," G. HARLEY and 

 H. S. HARLEY ('Roy. Soc. Proc.,' 1888, p. 461) give the following as their analysis 

 of " pure white pearls " (British, Australian and Ceylonese) : 



Carbonate of lime 9172 



Organic matter (animal) . 5 '9 4 



Water 2'23 



Loss 0-11. 



They also, for comparison, quotq from WATTS' ' Dictionary of Chemistry ' * the 

 following analysis of mother-of-pearl : 



Carbonate of lime 66'00 



Water 31'50 



Organic matter 2'50, 



and express their surprise at the large amount of water found. 



This difference between these two substances, produced in the same animal in a 

 similar manner, and supposed to be so closely related to one another, is so very great 

 that we felt that it was desirable to have another analysis made especially since it 

 is not stated in WATTS' Dictionary who made the analysis quoted, nor what shell was 



* Vol. iii, p. 1057, 1882. 



