PEARL. 227 



ficult, after reading some experiments of this kind, not to 

 give our assent to the supposition that some portion of the 

 lime had, indeed, been developed by the vital powers of 

 the animal ; but since a physiologist, distinguished for his 

 philosophic caution, has laid it down as an axiom "that 

 no organic agent has the power either of creating material 

 elements, or of changing one such element into another,"* 

 we must hesitate, at least if we believe calcium to be an 

 elementary body ; for then the mollusks must apparently 

 obtain it from without, extracting it by molecules from the 

 food and ambient water, and making it, through the agency 

 of their secreting organs, enter into new compositions and 

 decompositions. They are thus greatly employed in pre- 

 serving the due quality of our lakes, rivers, and seas, ex- 

 tracting from the water unceasingly those impure earthy 

 additions which are as unceasingly made from the death and 

 decomposition of successive generations of their own races 

 and of the Crustacea, from the frittering down of corals and 

 of rocks by storms, atmospherical influences, and the still 

 more powerful workings of the sea, and by the attrition of 

 running water on calcareous soils, f From such sources the 

 water is impregnated with that calcareous earth which it is 

 the ordered duty of these animals to remove ; and this they 

 do by reconsolidating it, and making it pass into new forms 

 of every variety and beauty, as I shall afterwards explain. 



2. PEARL is another calcareous secretion of molluscous 

 animals deserving notice. It is secreted by the fish of 

 bivalves ; J and principally by such as 

 inhabit shells of foliated structure, as lg ' 



Pearl-mussels (Meleagrina), sea and 

 fresh-water mussels, oysters, the Pinnse, 

 &c. A pearl consists of carbonate of 

 lime in the form of nacre, and animal 

 matter, arranged in concentric layers 

 around a nucleus, as it is well shown 

 in our figure (Fig. 41), copied from a 

 beautiful engraving appended to a pa- 

 per of Sir E. Home's in the " Philosophical Transactions." 

 Each layer is presumed, but I know not on what grounds, to 

 be annual ; so that a pearl must be of slow growth, and those 



* Prout's Bridgew. Treatise, p. 431. 



t On this subject see Zool. Journal, i. 96, 97. Bostock's Physiology, ii. 

 386. Good's Study of Medicine, ii. 31. Turner'sSac. Hist, of the World,!. 97. 



I Pearls " are not confined, as has been asserted, to bivalve shells, though 

 they are more frequent in them than in others." BAIRD, in Chambers' s Mis- 

 cellany, No. 167, p. 10. 



Q 2 



