ACEPHALA. 247 



of shells takes place : that description is applicable 

 to the Bivalves ; the fringed edges of the mantle 

 secreting the stony matter in the form of a viscid 

 cement, and depositing it around the shell by being 

 projected and turned back over its margin, while the 

 colouring glands supply the various tints ; and on the 

 other hand the whole general surface of the mantle 

 depositing a layer on the inside of the shell desti- 

 tute of colour, and forming the beautiful substance 

 called nacre, or mother of pearl. Wounds or ir- 

 ritation in any part of the mantle cause an in- 

 creased secretion of pearly matter in that part, and 

 hence the origin of those highly esteemed, and 

 beautiful productions, called pearls. Minute boring 

 worms often perforate the shell of the Pearl-bearing 

 Bivalves among others, and even penetrate to their 

 bodies. The mantle thus irritated, throws out pearly 

 matter in great abundance on the wounded spot, 

 which increasing, layer by layer, soon forms a globu- 

 lar excrescence, a pearl of greater or less size and 

 purity. Sometimes, moreover, a minute atom of 

 sand, or fragment of stone, may become introduced 

 between the mantle and the shell. In this case 

 the secretion is copiously poured out, coating over the 

 little annoyance until a detached pearl is formed, 

 the more valuable, because perfect in its whole cir- 

 cumference. A slight account of the mode in which 

 these, costly articles are obtained, may not be un- 

 acceptable or out of place, though the animal pro- 

 ducing them in most abundance seems to be not 

 a true Oyster, as commonly supposed, but a species 



