STUDIES IN ANIMAL LIFE. 125 



women wear as necklaces. Disease would be the 

 very worst cradle for pearls. The idea of disease 

 originated in a fanciful supposition of pearls being 

 to the oyster and mussel what gall-stones and uri- 

 nary calculi are to higher and more suffering ani- 

 mals. Keaumur, to whom we owe so many good 

 observations and suggestive ideas, came near the 

 truth when, in 1717, he showed that the structure of 

 pearls was identical with the structure of the shells 

 in which they grow. He attributed their formation 

 to the morbid effusion of coagulating shell-material. 

 I presume you know that shells are formed by a 

 secretion from the mantle? The mantle is that 

 delicate semi-transparent membrane which you ob- 

 serve, on opening a mussel, lining the whole inte- 

 rior of the shells, and having at its free margins a 

 sort of fringe of delicate tentacles which are sensi- 

 tive and retractile. A microscopic examination of 

 these fringes shows them to be glandular in struc- 

 ture that is, they are secreting organs. The whole 

 mantle, indeed, is a secreting organ, and its secre- 

 tion is the shell-material: the fringes secrete the 

 coloring matters of the shell, and enlarge its circum- 

 ference ; the rest of the mantle secretes the nacre, 

 or mother of pearl, and increases the thickness of 

 the shell. Now it is obvious that the formation of 

 pearl nacre and of pearls depends on the healthy 

 condition of the mantle, not on its diseases. If the 

 mantle be injured, the nacre is not secreted at all, 

 or in less quantities. 



