354 



THE OCEAN WORLD. 



favourable circumstances, and the nacre so useful in many industrial 

 arts. Fine pearls and nacre have, in short, the same origin. The 

 nacre invests the whole interior of the shell of Meleagrina margariti- 

 fera ; being the same secretion which in the pearl has assumed the 



_ 164. Meleagrina margaritifera (Linnseus). 

 Outside of the shell. Inside of the shell. 



globular form : in one state it is deposited as nacre on the walls of the 

 bivalve, in the other as a pearl in the fleshy interior of the animal. 

 This nacre is therefore at once a calcareous and horny matter, which 

 the animal secretes, and which it attaches to the interior walls of the 



Fig. 165. Meleagrina margaritifera (Linnaeus). 



shell during the several periods of its development. Pearls are formed 

 of the same substance, only in place of being deposited upon the 

 valves in beds, the material is condensed and agglomerated in small 

 spheroids, which develop themselves either on the surface of the 



