BIVALVE MOLLUSCA. 325 
is as exactly and neatly cut as if it had been perforated by the 
sharpest tool, and that a corroding dissolvent could not act with this 
regularity, but would attack the harder and more tender parts un- 
equally. This objection, which M. Quatrefages opposes to the idea 
of a chemical solvent, appears to us to admit of no reply. But, 
Fig. 129.—Pholas dactylus having hollowed out a shelter in a block of gneiss. 
while opposing unassailable reasons against the two theories, M. 
Quatrefages does not leave us without a reasonable explanation of a 
very puzzling phenomenon. “Let us not forget,” he says, “ that 
the interior of the gallery is constantly saturated with water ; conse- 
quently all the points of the walls which are not protected by the 
tube are subjected to constant maceration. In this state a mechanical 
action, even though inconsiderable, would suffice to clear away the bed 
of fibre thus softened, and, if this action is in any degree continuous, 
