THE ACEPHALOUS MOLLUSKS. 189 
shares this extraordinary opinion ; but this cannot explain their 
presence in logs of wood. Others have supposed that the current 
of water produced in the process of their respiration wore out 
the cavity by its continual erosion; but this theory again is not 
supported by fact, for the pholas finishes its excavation in a few 
months, which could not possibly be the case if water were the 
only agent. Another supposition is, that the foot and the edge 
of the mantle are filled with siliceous particles, like sand-paper, 
which, by continual friction, rub and wear down the rock, and 
MODIOLAS IN ROCK. 
file away its substance. Other naturalists suggest that the pholas 
may have the power of secreting a corroding liquid, by which 
the rock is eaten away. But then how is the shell itself to 
escape the action of the liquid ? 
To De Blainville belongs the honour of suggesting that a 
simple movement of the shell, constantly repeated, would in 
time pierce the stone. Observation has since decided that his 
opinion is correct; the shell itself cannot be worn away by this 
process, because it is composed of aragonite, which is harder 
than the rock in which the animal burrows. The mollusk bores 
down a considerable depth, and then hollows out its home to 
accommodate its increasing bulk. Hence, as the engraving 
shows, their lithodomes are bottle-shaped—wide at the bottom, 
with narrow necks. 
