34 



SHELL GALLEEY. 



it. Recent investigations, however, prove that pearls are chiefly due 

 to the presence of the larval stages of Nematode Worms. These 

 become enclosed in minute sacs in the skin of the mantle of the 

 mollusc, and in course of time are surrounded with superimposed 

 layers of nacre. Pearls thus formed in the Pearl Oyster are the 

 round pearls, which are so highly valued. 



"Blister-pearls," or "attached-pearls," as they are sometimes 

 called, are those which are attached to the inner surface of the 

 shells, and are produced either by the intrusion of extraneous bodies 



Fio-. 28. 



Pearl-Oyster (Margarilifera margaritifera). Case 147. 



iDetween the animal and the shell, or by the secretion of nacre over 

 holes in the shells made from the outside by boring molluscs, worms, 

 or sponges. These pearls, as a rule, are of comparatively little 

 money value. 



The nacre is generally of the well-known pearly-white colour, 

 very rarely dark, and occasionally almost black. The action of the 

 animal in secretmg successive layers of nacre over any foreign body 

 which intrudes between the mantle-folds, and thus converting it 

 into a pearl, is strikingly illustrated by two specimens in which, in 



