266 THE NATURAL HISTORY OF 



As the pearl oyster is an inhabitant of the Polar as well 

 as of other seas, it may not be deemed uninteresting to the 

 scientific as well as the general reader to be made ac- 

 quainted with the manner in which the pearl is secreted 

 by this animal ; for the following lucid description, which I 

 here insert, I am indebted to a valuable contemporary.* 



OBSERVATIONS ON THE FORMATION OF PEARL. 



" Pearl is a calcareous secretion of molluscous animals 

 deserving notice. It is secreted only by the fish of bivalves, 

 and principally by such as inhabit shells of foliated structure, 

 as sea and fresh water muscles, oysters, the Pinna?, &c. A 

 pearl consists of carbonate of lime, in the form of nacre, and 

 animal matter arranged in concentric layers around a nu- 

 cleus. Each layer is presumed, but I know not on what 

 grounds, to be animal ; so that a pearl must be of slow 

 growth, and those of large size can only be found in full- 

 grown oysters. ' It is the nacral lining of the central cell 

 that produces the lustre peculiar to the pearl, which cannot 

 be given to artificial ones.' 



"Pearls, as Mr. Gray justly observes, are merely the in- 

 ternal nacred coat of the shell, which has been forced, by 

 some extraneous cause, to assume a spherical form. They 

 are, therefore, not properly ' a distemper in the creature 

 that produces them,' and cannot, under any view, be com- 

 pared with Calculi in the kidney of man;f for, though ac- 

 cidental formations, and, of course, not always to be found 

 in the shellfish which are known usually to contain them, 

 still they are the products of a regular secretion, applied, 

 however, in an unusual way, either to avert harm or allay 

 irritation. That in many instances they are formed by the 

 oyster, to protect itself against aggression, is evident ; for, 

 with a plug of this nacred and solid material it shuts out 

 worms and other intruders which have perforated the softer 



* Magazine of Natural History, vol. v. for 1832. 

 t Lister, Hist. An. Ang: p. lbO. 



