These creatures have much more powerful means of boring than the
solens, for not only do they imbed themselves in hard clay, but even in
stone and lava. In what manner this is effected has been the subject of
much dispute; it is almost certain that the process is not mechanical,
for their soft body, and the fragile nature of their shells, seem an
insuperable bar to such a proceeding. Some authors have asserted that
the hole is formed by means of an acid secreted by the animal, which
acts chemically on the stone; but there are two reasons against this
solution of the difficulty; first, no acid liquid has been discovered
in the living animal, and, secondly, although acid would act upon
stone, it would have no effect whatever on lava.
Another curious part of the history of these shell-fish is their
phosphorescence, which is so bright, that it has been asserted, if
eaten in the dark without their being cooked, it appears as if the
person devouring them was swallowing phosphorus.
[Illustration: _Pholas dactyloides._]
Although not used as food in this country, they are not uncommonly
eaten on the shores of the Mediterranean, where some large species are
found.
The species represented in the engraving is found on the British
coasts, imbedded in clay; its shell is of a delicate white, beautifully
carved. The projecting piece at the lower part of the shell in the
engraving is the long foot of the animal; on the right hand, the hole
from which a shell has been removed is shown.