V] PEARLS 53 



resembling in composition and microscopic structure 

 certain layers of the shell of the mollusc in which 

 the pearl has been formed. It may be of almost any 

 colour and may lack altogether the lustre and general 

 appearance that is commonly associated with pearls. 

 As a matter of fact pearls are nothing more than 

 abnormal products, calcareous pathological bodies 

 formed, OAving to some irregularity, in the tissues of 

 certain shellfish. 



Yon Hessling gave the following as a definition of 

 pearls. "Pearls are shells converted into a spherical 

 form ; they possess the same histological, physical, 

 and chemical peculiarities, in so far as these do not 

 belong specially to the rounded shape, and both 

 undergo, in all the difierent stages of formation, the 

 same fate." 



As w^e have already, for this special purpose, 

 described the shell of a bivalve in the account of the 

 pearl oyster, it w ill be easy to apply this knowledge 

 to the structure of pearls. 



It is necessary, however, before doing this to draw 

 attention to other calcareous bodies w^hich are found 

 in molluscs, which may possess a pearly lustre, and 

 yet are not true pearls. These bodies are known 

 as blisters. They are very often formed owing to 

 foreign objects such as sand grains getting between 

 the shell and the mantle ; they often arise in the 

 process of repair of shell perforations. 



