126 STUDIES IN ANIMAL LIFE. 



But, although' pearls depend upon the healthy, 

 not the diseased activity of the mantle, it is clear 

 that there must be some unusual condition present 

 for their formation, since the secretion of nacre 

 does not spontaneously assume the form of pearls. 

 What is the unusual condition ? Naturalists are at 

 present divided into two camps, fighting vigorously 

 for victory. The one side maintains that the origin 

 of a pearl is this : an egg of the oyster has escaped 

 and strayed under the mantle, or the egg of a para- 

 site has been deposited there; this egg forms the 

 nucleus round which the nacre forms, and thus we 

 have the pearl. The other side maintains with 

 great positiveness that any thing will form a nucleus, 

 a grain of sand no less than the egg of a parasite. 

 'Tis a pretty quarrel, which we may leave them to 

 settle. Some aver that grains of sand are more nu- 

 merous than any thing else ; but Mdbius says that 

 of forty -four sea pearls and fifteen fresh-water pearls 

 examined by him, not one contained a grain of sand ; 

 and Filippi, who has extensively investigated this 

 subject, denies that a grain of sand ever forms the 

 nucleus of a true pearl. Both Mlippi and Kiichen- 

 meister* declare that a parasite gets into the mussel 

 or oyster, and its presence there stimulates an active 

 secretion of nacre. 



There are pearls, according to Mobius, which 

 consist of three different systems of layers, like the 

 shells in which they are formed ; with this differ- 

 * See their interesting essays in Mullee's Archiv., 1856. 



