CHAPTER XII.
	Pearls.
	P EARLS are lustrous concretions, consisting essen­tially of carbonate of lime interstratified with animal membrane, found in the shells of certain mollusks. They are believed to be the result of an abnormal secre­tory process caused by an irritation of the mantle of the mol-lusk consequent on the intrusion into the shell of some foreign body, as a grain of sand, an egg of the mollusk itself, or per­haps some cercarian parasite. It has also been suggested that an excess of carbonate of lime in the water may cause the development of the pearl. Accepting the former theory as the more probable one, it is easy to understand how this foreign body, which the mollusk is unable to expel, becomes encysted or covered as by a capsule, and gradually thickens, assuming various forms—round, elongated, mallet-shaped—and is some­times as regular as though it had been turned in a lathe. Charles L. Tiffany, who has given considerable attention to this subject, suggests that the mollusk continually revolves the enclosed particle in its efforts to rid itself of the irritation, or possibly that its formation is due to a natural motion which is accelerated by the intruding body. In regard to the formation of pearls, the following general statements may be made : Whatever may be the cause or the