THE BIRTH OF A PEARL.

Golden Bay Argus, Volume VI, Issue 72, 14 October 1897, Page 2

THE BIRTH OF A PEARL.

When an oyster feels something tickling his skin he docs not—-indeed, he obviously cannot — proceed to scratch the spot and apply counterirritation, as a quadruped or a bipid would. The only policy left to the mollusc in such a plight is to ftDce off his enemy with a layer or two of mother-o’-pearl, mere scientifically known under the term of “ nacre.” In consequence of this many otter things happen inside the shell, and among them pretty frequently the birth of a pearl. It should be known, however, to start with, that all the sliell-bi-aiing molluscs have the power of secreting calcite, which is a form of carbenate ot lime, from the outer layer of cells which forms the cuticle or skin of their soft, not to sav sloppy bodies. In this way they build up their shells. At the Royal Institution on Tuesday Professor Stewart had a good deal to say aboutthe “ Lamellibranchs,” as the oyster group are called, and incidentally discussed their pearlforming habits. The limy secretion which they give off is laid iu extremely thin, semi-tran-sparent films,and this is just the reason why the “ nacre ” is so beautifully iridisceut, for very thin films have the property of producing what are scientifically known as “ interference ” effects and breaking ordinary light up into the colouis of the spectrum. Now 7 , if some tiny foreign body, such as a graiu of sand, a “diatom,” or even a minute shrimp or other crustacean, happens to find its way iusiile the mantle border of the oyster, it sets up a good deal of irritation, and iu self-defence the poor oyster is obliged to cover up tho iutmder with a layer of " nacre ” So around the grain of sand, we will say, the mollusc goes on d< positing thin layois of nacre one after the other, like the skins of an onion, until the point of iriilatiou is completely encysted and a pearl Las been formed. This piocess is carried out, or attempted to be carried out, whatever the intruder may be, so that Professor Stewart has eveu seen little fish encased in pearl within the bivalve's shell. The Chinese, who never seem to neglect an opportunity to improve upon Nature,have long taken advantage of this habit to artificially induce the growth of peails by filling up the oyster with all kinds of irritating things. Ordinary pear-shaped pearls are excited by means of a tiny 11-shaped piece of wire thrust into the mantle-bordi r, but it is not uncommon to insert little metallic images of the “ great god Biuih,’’and subsequently obtain a cast of the same in pearl. These are much valued as charms. The finest Oriental pearls are those found within the mantle of the oysti r close to the lips of the shell, or iu the soft pails near to the hinge.