Ill] THE PEARL OYSTER 23 



described in being secreted normally by the entire 

 external surface of the mantle. Consequently, it can 

 increase in thickness throughout the life of the 

 animal. It is built up of very delicate lamellae or 

 tiny platelets which are arranged overlapping one 

 another (fig. 3), parallel to the surface of the shell. 

 The edges of the lamellae are zigzag, and as the sur- 

 face of the mother-of-pearl layer exhibits parts of the 

 faces and also the edges of the lamellae, a surface is 



Fig. 3. Nacre or Motlier-of-Pearl laminae in surface view 

 (highly magnilied). 



formed which possesses by virtue of interference 

 phenomena the so much-desired iridescent lustre. 



It is this nacreous layer which gives the pearls 

 their beauty, and it is this layer which is often 

 deposited over sand grains, or objects placed by man 

 between the shell and mantle. Further reference to 

 the nacreous layer will be made later. 



The last layer to which reference will be made 

 here is found between the muscle and shell. It is 



