THE PEARL OYSTER. 921 



betraying bubbles of gas emitted by the oyster, in the act 

 of closing its shell, The effete water is renewed by diffu- 

 sion, 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 respira- 

 tory organs, the branchial arches being developed into a 

 kind of fringe. In the invertebrates, however, there is no 

 special apparatus for that purpose, and when, after storms 

 or other disturbing causes, the water becomes thickly 

 charged with sand, mud, and other substances in suspen- 

 sion, it is evident that the water admitted within the pal- 

 lial chamber of the oyster must be equally thick, and it 

 can hardly be doubted but that some particles of this sus- 

 pended 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 opened by the 

 knife, the less probability will there be of finding any 



Pearl It is within the mantle that many of 



the Pearls are found, and the inference is that the interior 

 surfaces of this integument secrete the fine pearly layers 

 around the nucleus of what is to become a Pearl ; whilst 

 the layers of the shell are secreted mainly by the exterior 

 surfaces of the mantle. With the first layer deposited 

 around it, the intruder becomes a Pearl, and if this nucleus 

 is of animal or vegetable matter, decomposition, or pro- 

 bably absorption, will in time leave an apparently empty 

 space, the cavity being lined and discoloured by the residue, 

 a hollow Pearl is thus formed 



The oyster is not entirely dormant, and its movements, 

 together with the varying position of the Pearl within its 

 tissues, probably regulate the shape assumed by the con- 



