Strange as it seems, these animals have bored these holes in the stone;
and they are capable of boring in far harder rock than this; even in
compact limestone. The actual mode in which this operation is performed
long puzzled philosophers. Some maintained that the animal secreted
an acid which had the power of dissolving not only various kinds of
stone, but also wood, amber, wax, and other substances, in which the
excavations are occasionally made. But it was hard to imagine a solvent
of substances so various, and to know how the animals’ own shells were
preserved from its action; while, confessedly, no such acid had ever
been detected by the most careful tests.
Others maintained that the rough points which stud the shell enable it
to serve as a rasp, which the animal, by rotating on its axis, uses
to wear away the stone or other material; but it was difficult to
understand how it was that the shell itself was not worn away in the
abrasion.
Another zoologist, rejecting this hypothesis, maintained that the edges
of the mantle and the short thick foot are the instruments employed;
and that, though these fleshy organs seem little fitted for such
work, they are really endowed with the requisite power in the shape
of crystals of flint which are deposited thickly in their substance.
Strange to say, however, other accurate observers fail to detect these
siliceous crystals, and therefore reject the hypothesis.
Another suggested that the stone was removed in invisible particles by
the constant action of currents in the water, produced by vibratile
cilia seated on the soft parts of the animal; but this supposition was
found untenable on examination.
Actual observation in the aquarium has proved that the second
hypothesis is the true one. M. Cailliaud in France and Mr. Robertson
in England have demonstrated that the Pholas uses its shell as a
rasp, wearing away the stone with the asperities with which the
anterior parts of the valves are furnished. Between these gentlemen a
somewhat hot contention was maintained for the honour of priority in
this valuable discovery. M. Cailliaud himself used the valves of the
dead shell, and imitating the natural conditions as well as he could,
actually bored an imitative hole, by making them rotate. Mr. Robertson,
at Brighton, exhibited to the public living Pholades in the act of
boring in masses of chalk. He described it as “a living combination
of three instruments, viz., a hydraulic apparatus, a rasp, and a
syringe.” But the first and last of these powers can be considered
only as accessory to the removing of the detritus out of the way,
when once the hole was bored, the _rasp_ being the real power. If you
examine these living shells, you will see that the fore part, where
the foot protrudes, is set with stony points arranged in transverse
and longitudinal rows, the former being the result of elevated ridges
radiating from the hinge, the latter that of the edges of successive
growths of the shell. These points have the most accurate resemblance
to those set on a steel rasp in a blacksmith’s shop. It is interesting
to know that the shell is preserved from being itself prematurely worn
away by the fact, that it is composed of arragonite, a substance much
harder than those in which the Pholas burrows. Yet we see by the
specimens before us that such a destructive action does in time take
place, for some of these have the rasping points much more worn than
others, many of the older ones being nearly smooth.
The animal turns in its burrow from side to side when at work, adhering
to the interior by the foot, and therefore only partially rotating to
and fro. The substance is abraded in the form of fine powder, which
is periodically ejected from the mouth of the hole by the contraction
of the branchial siphon; a good deal of the more impalpable portions
being deposited by the current as it proceeds, and lodging as a soft
mud between the valves and the stone. Mr. Hudson,[28] who watched some
Pholades at work in a tide-pool in the chalk, observed the periodic
ejection of the cloud of chalk-powder, and noticed the heaps of the
same material deposited around the mouth of each burrow. The discharges
were made with no regularity as to time. Mrs. Merrifield[29] records
a curious fact. “A lady, watching the operations of some Pholades
which were at work in a basin of sea-water, perceived that two of
them were boring at such an angle that their tunnels would meet.
Curious to ascertain what they would do in this case, she continued
her observations, and found that _the larger and stronger Pholas bored
straight through the weaker one_, as if it had been merely a piece of
chalk rock.”
Mr. Ross, of Rhyl, having a Pholas in his aquarium, prepared a piece of
wood, by excavating a shallow cavity, about a quarter of an inch deep,
in which he set the animal, whose shell was two inches long. “After
a short time the animal attached its foot to the bottom of the hole
and commenced swaying itself from side to side, until the hole was of
sufficient depth to allow it to proceed in the following manner:--It
inflated itself with water, apparently to its fullest extent, raising
its shell upwards from the hole; then holding by its muscular foot, it
drew its shell gradually down. This would have produced a perpendicular
and very inefficient action, but for a wise provision of nature. The
edges of the valves are not joined close together, but are connected
by a membrane (extension of the mantle), and instead of being joined
at the hinge (umbo) like ordinary bivalves, they possess an extra
plate, attached to each valve of the shell, which is necessary for the
following operation:--In boring, this mollusk, having dilated itself
with water, draws down its shell within the hole, gradually closing the
lower anterior edges until they almost touch. It then raises its shell
upwards, gradually opening the lower anterior edges, and closing the
upper, thus boring both upwards and downwards. The spines are placed
in rows, like the teeth of a saw; those towards the lower part of the
shell being sharp and pointed, while those above, being now useless,
are not renewed.”[30]