the oxygen from water, either salt or fresh, we are, however, still
untaught. The water is admitted between the " mantle lobes " into the "
palliai 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.
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 purpose,
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 palliai 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