184 UTILIZATION OF MINUTE LIFE. 



form, is of a greenish tint on the outside, and of a 

 beautiful pearly lustre in the interior. It consti- 

 tutes mother-of-pearl, which is an important article 

 of commerce at the present day. The pearls for 

 which this mollusc is also sought are small, acci- 

 dental excrescences found in the shell, often buried 

 in the animal's body, but most commonly seen 

 adhering to one of the valves of the shell itself. 

 Like other animals of the mussel kind, Avicula 

 margaritifera secretes a byssus, by which long silken 

 thread it adheres to submarine objects. 



Other Mollusca which inhabit the ocean have 

 been observed to produce pearls. Such are the 

 common oyster (Ostrea), many mussels (Mytilus), 

 and some bivalves belonging to the genus Perna. 

 They are also produced by certain fresh-water 

 mussels (Unio). 



The exact nature of a pearl has been the object 

 of much discussion. Some inquirers imagine it to 

 be the result of a particular disease, which causes 

 the animal to produce these pearly concretions, by 

 occasioning in some parts of the shell an unwonted 

 production of calcareous matter. This being pro- 

 duced abundantly and suddenly, does not spread 

 itself uniformly over the interior surface of the valve 

 of the shell, but constitutes those little concretions 

 we call pearls. 



In the opinion of others, pearls are regarded as 



