114 Pearls. 



extracts the oxygen from water, either salt or fresh, 

 we are, however, still untaught. The water is 

 admitted between the "mantle lobes" into the 

 " pallial chambers," where it is oxygenated : the 

 oyster evidently retains a considerable quantity of 

 gas within itself, many shells being discovered by 

 the divers, simply by the betraying bubbles of gas 

 emitted by the oyster, in the act of closing its 

 shell. The effete water is renewed by diffusion, as 

 there is no regular pulsating movement to eject it. 



In most fishes there is a special arrangement to 

 guard against the admission of foreign substances 

 to the respiratory organs, the branchial arches being 

 developed into a kind of fringe. In the invertebrates 

 however, there is no special apparatus for that pur- 

 pose, and when, after storms or other disturbing 

 causes, the water becomes thickly charged with 

 sand, mud, and other substances in suspension, it 

 is evident that the water admitted within the pallial 

 chamber of the oyster must be equally thick, and 

 it can hardly be doubted but that some particles 

 of this suspended matter are accidentally retained 

 entangled in the tissues of the oyster, especially if 

 the latter happen to be weakened by disease. 



The healthier the appearance of the oyster, and 

 the greater the amount of water emitted when 



