54 PEARLS [CH. 



" Blisters " have a different mode of origin, there- 

 fore, and the}' are usually attached to the shell. They 

 may be very large, and are often cut away from 

 the shell and used in cheap jewellery. It is blister 

 formation that has been encouraged, as we shall see 

 in a later part of this chapter, by artificial processes 

 for trade purposes. 



True pearls themselves are of two kinds, 



(a) Cyst pearls (Herdman) or Parenchyma pearls 

 (Jameson), 



(b) Muscle pearls (Herdman). 



Of these, the former are the most important com- 

 mercially, and have formed the subject of most of the 

 scientific work on the origin of pearls. They are 

 usually spherical pearls which occur singly in the 

 mantle or in the soft tissues of the pearl oyster or 

 mussel. They may occur even some distance away 

 from the surface, embedded in the digestive gland or 

 so-called liver. 



Muscle pearls were distinguished first by Herdman 

 (British Association Report, Southport Meeting, 1903). 

 They occur usually in large numbers in the muscular 

 tissue, where the muscles are attached to the shell. 

 Fig. 5 indicates the positions where these pearls are 

 found in the pearl oyster of Ceylon. 



Now, putting the definition of von Hessling in 

 another form, it may be said that pearls are bodies 

 consisting of layers of calcareous material with an 



