413 THE MICROSCOPE. 



Pearls are usually met with in the Meleagrina Margari- 

 tifera, " Pearl Oyster," which, however, does not belong to 

 the family Ostracea. They are likewise found in the 

 Mussel known as Mya Margaritifera, and an inferior kind 

 in many Mussels of the rivers of Great Britain ; and at one 

 time, the pearl-fishery of Ireland was justly celebrated. 

 Naturalists somewhat differ in their opinions as to the 

 mode in which pearls are formed. Some think that they 

 are produced by particles of sand getting into the stomach ; 

 the animal, to prevent the roughness of these particles from 

 injuring its delicate structure, covers them over with a 

 secretion from a gland, and by continual additions, they 

 gradually increase in size. Mussels, in which artificial 

 pearls were said to have been formed by the Chinese, have 

 frequently found their way to this country. It is now, 

 however, pretty generally admitted to be a disease ; and 

 that pearls are matured on a nucleus, consisting of the 

 same matter as that from which the new layers of shell 

 proceed at the edge of the Mussel or Oyster. The finest 

 kind are formed in the body of the animal, or originate in 

 the pearly-looking part of the shell. It is from the size, 

 roundness, and brilliancy of pearls, that their value is 

 estimated. 



The microscope discloses a difference in the structure of 

 pearls : those having a prismatic cellular structure have a 

 brown horny nucleus, surrounded by small imperfectly- 

 formed prismatic cells ; there is also a ring of horny matter, 

 followed by other prisms, and so on, as represented in fig. 

 208 ; these transverse sections of pearls from Oysters show 

 successive rings of growth or deposit. 



In a segment of a transverse section of a small purple 

 pearl from a species of Mytilus, (fig. 209,) all trace of 

 prismatic structure has disappeared, and only a series of 

 fine curved or radiating lines are seen. The pearl consists 

 of a beautiful purple- coloured series of concentric laminae ; 

 and many of them have a series of concentric zones, and 

 axe of a yellow tint. The most beautiful sections for micro- 

 scopic examination are obtained from Scotch Pearls. 



True pearls, being composed entirely of nacre, are 

 ibeautiful in their colouring. Some, again, are made up 

 f nacre and prismatic cellular structure; the centre 



