The Pholas, &c.
(ROCK-BORERS.)
'He that of greatest works is finisher
Oft does them by the meanest minister.'
XVI.
At certain parts of the Scottish coast, the 'dykes,' or walls built
near the road-side, are constructed entirely of rough-hewn pieces of
hard sandstone rock, brought from the neighbouring shore. Sometimes a
dyke will extend for two or three miles, without presenting an isolated
fragment, in which the honeycomb-like perforations of certain species
of the boring Mollusca are not more or less apparent.
A fragment of soft sandstone lies before me, measuring three and a half
inches in length, and two inches in breadth, which, small though it be,
contains no fewer than seventeen cylindrical tunnels. Each of these
exhibits so wonderful a skill in construction, that human hands could
not surpass it, though aided by 'all the means and appliances to boot,'
of mechanical agency.
It is generally stated that the Pholas never intrudes itself into the
apartment occupied by a neighbouring 'worker.' The Pholas, however,
often intrudes on its neighbour; and such intrusion is manifested in
the small piece of stone alluded to in no less than four instances.
Here let me observe, that it is not always the larger mollusc that
bores through the smaller one; it as frequently happens that the latter
deserves the brand of wanton aggressor. Both cases are common enough,
and, indeed, must of necessity occur, wherever at any time a colony of
various sized Pholades are clustered together in a small portion of
rock.
A fragment of rock riddled by the Pholas is a much more pleasing
sight than can be witnessed at the sea-shore in connection with that
animal under usual circumstances. For this reason: When visiting the
habitat of the boring bivalves, a host of small circular holes are
sometimes seen; at other times the surface of the same portion of the
beach appears comparatively sound, and it is only by striking a smart
blow with a hammer upon the ground, that we render scores of orifices
instantly observable in all directions, from each of which is thrown a
small jet of water. This phenomenon is caused by the Pholades in alarm
retracting their siphons, which had hitherto filled the entire extent
of the tunnels. At such a locality, if a piece of rock be excavated,
various specimens of these boring molluscs, shrunk to their smallest
possible size, will be discovered at the base of the cavities, which
are invariably of a conical form, tapered at the top, and gradually
enlarging as they descend.
It must be evident, then, that neither the likeness of the animal, nor
the formation of its singular dwelling-place, can be seen by the casual
wanderer along the sea-shore.
It will also be apparent to the intelligent reader, that when once the
Pholas is located in a certain spot, he becomes a tenant for life;
for never by any chance whatever, can the poor miner leave his rocky
habitation by his own unaided exertions, even were he so inclined. As
he grows older and increases in size, nature teaches the animal to
enlarge his habitation in a proportionate and suitable manner.
During the period of the boring process, the orifice becomes clogged
above the shell with the _debris_ of the rock, and this, if allowed to
accumulate would speedily asphyxiate the animal. To get rid of such
an unpleasant obstruction, the Pholas retracts, and closes the end of
its siphons, then suddenly extends the 'double barrelled' tube to its
full length, until it reaches the entrance of the tunnel. This movement
often repeated, causes portions of the pulverized stone to be forced
outwards at each operation.
It is interesting to watch the curious manner in which the end of the
principal siphon is alternately closed and spread out when it reaches
the water, like a man inspiring heavily after any unusual exertion; it
is then made again to descend, and renew its task, as above described.
In extracting that portion of the _debris_ which is deposited at
the _base_ of the cavity, below the body of the industrious miner, a
different plan is adopted. Wherever a Pholas is at its labours, there
are always deposited within a circumference of several inches round the
tunnel, myriads of short thin threads, which are squirted out from the
smaller siphon.
The nodules on examination are found to be composed of pulverized
rock, which is drawn in at the pedal opening, and ejected in the
above manner, thereby effectually clearing the lower portion of the
orifice. It was suggested to me that these threadlike objects were the
fœcal matters of the Pholas, but this idea was soon dispelled by the
assistance of the microscope; and, moreover, from the fact that the
threads are never visible when the animal is in a quiescent state, but
only when it is busily engaged in its mysterious task of boring.
I was for some time puzzled to find any aperture in connection with
the club-like foot of the Pholas (_P. crispata_), although several of
the bivalves were sacrificed to the cause of science. But what the
microscope and scalpel in this instance failed to unfold, attentive
watching of the animal in the aquarium made palpably apparent, in the
following manner.
I had on one occasion captured about a dozen Pholades, some of which
were embedded in the solid rock, others detached.
The first mentioned, I knew would be quite safe among the blennies
and crabs, from the untempting and unedible nature of their siphonal
tubes. Very different was the case with the defenceless, disentombed
specimens. These were intended as food for their finny companions, who
happened to be particularly fond of a change of diet. My 'pack' had
subsisted for some time on Mussels, and on such excellent food, had
become impudent, corpulent, and dainty. But overgorged epicures though
they were, I knew that although everything else failed, a 'real live'
Pholas placed before them would serve to speedily whet their appetites.
A splendid specimen of the siphoniferous bivalve was dropped into the
tank, the base of which it had no sooner reached, than the fishes, with
eager eyes and watering mouths, came hovering like a flock of vultures
round the welcome meal thus unexpectedly placed before them.
One rascal, who seemed to be cock of the walk, came forward and made
the first grip at the delicate fleshy foot, that in appearance was as
white as a newly fallen snowflake. The pedal organ was, of course,
instantly and forcibly withdrawn, so much so, indeed, as to be almost
hidden from view, except at its extreme base. In this position it
remained for several seconds. When the finny gourmand again boldly
advanced to take a second mouthful, to my intense surprise he was,
apparently, blown to a distance of several inches. I could scarcely
credit the evidence of my senses. Another and another of the fishes
were in their turn served in like manner as their leader. In a short
time, however, the poor mollusc failed to repulse his enemies, and
finally fell a passive victim to their gluttonous propensities.
Now comes the important question, 'How is the boring operation
performed?' How can this simple animal, with its brittle shell, and
soft fleshy body, manage to perforate the sandstone, or other hard
substances, in which it lives?
For hundreds of years this query has been asked, and various are the
replies which from time to time have been given. Singular to state,
although specimens of the Pholas, and its allies the Saxicavæ, are to
be procured in abundance in many parts of the kingdom, the subject is
not even yet positively settled.
There have been many theories advanced, some the result of fancy
or guess-work; others, of practical study. All these have their
supporters, but none have, by common consent, been adopted by
physiologists as the true one.
Having for several years made this subject a study of personal
observation, I believe I may venture to state, that I have succeeded
in casting a feeble ray of light upon it; and, although the result of
my labours may not be deemed conclusive, I may at least claim some
credit for my endeavours to clear up a most difficult, though deeply
interesting point in natural history.
The various theories promulgated on this knotty point are generally
classed under five heads: 1st, That the animal secretes a chemical
solvent--an acid--which dissolves the substance in which it bores.
2d, That the combined action of the secreted solvent, and rasping by
the valves, effects the perforations. 3d, That the holes are made by
rasping effected by silicious particles studding the substance of
certain parts of the animal. 4th, That currents of water, set in action
by the motions of vibratile cilia, are the agents. 5th, and lastly,
That the boring mollusca perforate by means of the rotation of their
shells, which serve as augurs.
Of all the above, the first which is quite a fancy theory, seems to
meet with greatest favour among certain naturalists. But as it is
rather puzzling to find a chemical solvent, which will act equally upon
sandstone, clay, chalk, wax, and wood, this hypothesis can only be
looked upon by practical men as ingenious, but incorrect. Even were it
proved that the animal really possessed the power of secreting an acid
sufficiently powerful, the question naturally arises, How can the shell
escape being affected in like manner with the much harder substance in
which it is situated?
The second theory, or the combined action of rasping and the secreted
solvent, is, for obvious reasons, equally objectionable.
The third theory, which endeavours to account for the wearing away of
the rock by means of silicious particles situated in the foot and other
parts of the animal, has been for some time proved to be erroneous,
from the fact, that the combined skill of some of our best anatomists
and microscopists has failed to discover the slightest presence of any
particles of silex in the Pholadidæ, although these are believed to
exist in other families of the boring acephala.
The fourth theory, that of ciliary currents as an accessory agent in
boring, is worthy of greater consideration, chiefly from the evidence
we possess of the immense power which the incessant action of currents
of water possess in wearing away hard substances.
We come now to what may be considered the most important of the
theories above enumerated, viz., the mechanical action of the valves of
the Pholas in rasping away the rock, &c. This hypothesis is one which
most naturally suggests itself to the mind of any impartial person, on
examining, for instance, the rasp-like exterior of the shell of _Pholas
crispata_.[12] But as I shall endeavour to show, although the shell
forms the principal, it does not by any means constitute the _sole_
agent in completing the perforating process.
[12] Specimens of this species, I may mention, have always formed the
subject of my experiments, and therefore are alone alluded to in the
following remarks.
Mr. Clark, a clever naturalist, considers with Mr. Hancock that the
powerfully armed ventral portion of the _mantle_ of the closed boring
acephala is fully adequate to rub down their habitations, and that the
theories of mechanical boring, solvents, and ciliary currents, are
so utterly worthless and incapable of producing the effects assigned
to them, as not to be worth dwelling upon for one moment. Mr. Clark,
therefore, comes to the conclusion that 'the foot is the true and
sole terebrating agent in the Pholas.' This 'fact' he considers to
be 'incontestably proved,' for the following reason, viz., because
he had discovered specimens of this bivalve with the foot entirely
obliterated,--which phenomenon, Mr. Clark states, is caused by
the animal having arrived at its full growth, at which period the
terebrating functions cease; and as 'nature never permanently retains
what is superfluous,' the foot is supposed gradually to wither away,
and finally disappear.
This, I suspect, is another 'fancy' theory. Although I have excavated
hundreds of Pholades, some of giant-like proportions, it has never
been my lot to witness the foot otherwise than in a healthy and fully
developed condition.
Another writer, having no opportunity of viewing the living animal,
does not consider it difficult to imagine the Pholas 'licking a hole'
with its foot, from the fact that he (Mr. Sowerby) managed to make 'a
sensible impression' upon a piece of kitchen hearthstone. 'I had,' he
says, 'not patience to carry the experiment any further, but as far
as it went, it left no doubt on my mind that, with the foot alone,
and without any silicious particles, without a chemical solvent, and
without using the rasping power of its shell, our little animal could
easily execute his self-pronounced sentence of solitary confinement for
life.'
Such an inconclusive statement as this would, I feel certain, never
have been penned, had its author been so fortunate as to have had
opportunity of watching a Pholas at work.
But, as Professor Owen truly observes, 'Direct observation of the
boring bivalves in the act of perforation has been rarely enjoyed, and
the instruments have consequently been guessed at, or judged of from
the structure of the animal.' Such, evidently, is the case with Mr.
Sowerby, and several other writers who treat on this subject.
Here we may call attention to the folly of naturalists endeavouring
to tag a pet theory upon all the boring acephala, to the exclusion of
every other. Such a system is defended upon the principle that, 'it
is much more philosophical to allow that animals, so nearly allied as
these in question, are more likely to effect a similar purpose by the
same means, than that several should be adopted. Surely this is more
consistent with the unity of the laws of nature, and that beautiful
simplicity which is everywhere prevalent in her works.'
How much more shrewd and philosophical are the opinions of such a man
as Professor Owen, who, when speaking of the mechanical action of the
valves of _P. crispata_, says, 'To deny this use of the Pholas shell,
because the shell of some other rock-boring bivalves is smooth, is
another sign of a narrow mind.' Again, this learned author forcibly
remarks, in direct opposition to the writer previously quoted, '_The
diversity of the organization of the boring molluscs plainly speaks
against any one single and uniform, boring agent at all_!'
The more I study this subject, the more does the truth of the
last-mentioned statement become apparent to my mind.
An examination of engravings of the shells, or even of the Pholas
itself, when lying loose in the tank, or quietly seated in the rock,
extending and retracting its siphons, fails to give one the slightest
idea of its extraordinary appearance when enlarging its dwelling. At
such times it seems to be a totally different animal, and to have
suddenly acquired a most marvellous degree of power, energy, and
perseverance, forming a striking contrast to its usual quiet, passive
habits.
In the first place, as I have elsewhere written, it retracts its tube
to, and even under, the level of its shell, just as a man, about to
urge onwards some heavy mass with his shoulders, would depress his
head to increase and concentrate his muscular power. Then follows an
expansion of the neck or upper part of the ventral border, from whence
the siphons protrude. This movement closes the posterior portions of
the valves below the hinge, and brings their serrated points together.
The next act on the part of the animal is to place its foot firmly
at the base of the hole; when leaning forward, it makes a sweeping
movement fully half round the cavity, pressing firmly-upon the umboes,
which nature has strengthened for the purpose by two curved teeth
fixed on the inside of the valves. At this stage it again reclines on
its breast, and tilting up the shell as much as possible, it makes
another motion round to its former position, leaning upon its back. By
these intricate movements, which the Pholas appears to accomplish by
a contraction almost painfully strong, it opens the rasping points of
the valves. These execute a very peculiar scooping movement at the base
of the cavity, and the animal having got so far, prepares itself for
further exertion by a short rest.
The specimen whose movements I have attempted to describe, lived in
my possession for a considerable time. It bored so completely through
the piece of rock in which it was embedded, that the whole of its foot
dropped through the aperture, and remained in this position for months,
the animal, in consequence, being unable to change its position even in
the slightest degree. Each movement of this specimen, both before and
while the hole at the base of the cavity was gradually being enlarged,
was watched, and every striking and interesting feature that occurred
noted down at the moment. Various queries were put and answered, as
far as possible, by direct ocular demonstration of the labours of the
animal in the vase before me.
I consider myself to have been singularly fortunate in being able
to view the actions of the creature from beneath, in consequence of
the hole being bored through the rock. This circumstance allowed me
distinctly to see what was going on at the base of the orifice.
My early observations have fortunately been confirmed in other captive
Pholades, which at various periods have been domesticated in my tanks.
I am convinced, then, that the shell forms the _principal_ agent in
boring the animal's dwelling, without either acid or flinty particles.
The late lamented Professor Forbes held that if this were the case,
the rasping points on the surface of the valves would soon be worn
down,--an appearance which, he says, is never seen. With all respect
for such an eminent name, I must state that he was in error. Not only
are the edges at certain times worn, but the rough surface is worn
nearly smooth, appearing in certain parts of a white colour, instead of
a light drab, as usual.
But the reader may ask, if certain parts of the valves are occasionally
worn smooth, and the animal works so vigorously, how is it that they
are never rasped through? This is a very natural question, and one that
I put to myself repeatedly.
I have made frequent and careful observations while the animal was
actually at work, in order to satisfy myself upon this point, and have
always perceived that the particles of softened rock fell from, and on
each side of, the large and well-developed _ligament_ that binds the
hinge, and extends to the lowest points of the valves. Moreover, this
leathery substance always seemed scraped on the surface. I cannot,
therefore, but believe that the ligament aids very materially in
rubbing off the rock, or at all events, in graduating the pressure of
the valves during the process, and that this curious organ, instead
of being worn away, may, like the callosity upon a workman's hand,
increase in toughness the more labour it is called upon to perform.[13]
[13] Mr. Clark says, 'M. Deshayes, in his comment on Pholas, in the
last edition of Lanarck, mentions the hinge as scarcely existing, and
not being _a veritable ligament_.' How different from the fact; and
I will observe, that '_if there is a genus better provided than any
other of the bivalves with ligamental appendages, it is Pholas.... The
Pholas is iron-bound as to ligament_, which in it is far more powerful
in securing the valves, than is the shell of any other group of the
acephala, of similar fragility and tenuity!'
The reason why so few specimens of the Pholades exhibit a worn shell
may be thus explained: As the animal only bores the rock in sufficient
degree to admit of its increased bulk of body, it only requires to bore
occasionally, and there may be often an interval of many months, during
which time nature may have renewed the serrated edge and rough surface
of the valves, and thus enabled the creature to renew its wondrous
operations.
We now come to a consideration of the foot, which, as many writers
aver, forms the 'sole terebrating agent.'
Although this sweeping statement is incorrect, I will freely admit that
the foot constitutes an agent second only in importance to the shell of
the animal. A casual examination of any Pholas perforation will show
that the foot could not have been the only instrument by which the
cavity was formed, from the peculiar rings that line the lower portion
of its interior. These rough appearances, I feel convinced, could be
formed by no other means than the rotatory motion of the shelly valves.
The valves, however, could not rotate and press against the surface of
the rock, were it not for the aid which the foot affords to the animal,
by its being placed firmly at the base of the hole, and thus made to
act as a powerful fulcrum.
This supposition fully accounts for the lowest extremity of the rocky
chamber being always smooth, and hollowed out into a cup-like form by
the action of the fleshy foot above alluded to.
The foot for a long time was a complete puzzle to me: I was unable to
satisfy my mind as to how it acquired its seeming extraordinary power.
The phenomenon was fully explained when I became aware of the presence
of that mysterious organ the hyaline stylet, situated _in the centre of
the foot_. The use of this springy muscle, which is, as we have shown
in the previous chapter, by naturalists erroneously considered to be
the gizzard of the animal, is, I believe, _solely to assist the Pholas
in its boring operations_.
Perhaps some of my readers would like to know how to procure a sight
of the stylet; if so, their wishes may be easily gratified. Take up a
disentombed Pholas in your hand, and with a sharp lancet or point of a
pen-knife, briskly cut a slit in the extreme end of the foot, and, if
the operation be done skilfully, the object of your search will spring
out of the incision to the extent, it may be, of a quarter of an inch.
If not, a very slight examination will discover the opal gelatinous
cylinder, which may be drawn out by means of a pair of forceps.
When extracted and held between the finger and thumb by its smaller
end, the stylet will, if struck with a certain degree of force, vibrate
rapidly to and fro for some seconds, in the same manner as a piece of
steel or whalebone would be affected, under like circumstances.[14]
[14] In the _Athenæum_ (Nos. 1632 and 1636), were kindly published two
letters from the author on the above subject, under the respective
dates January 26th, and February 28th, 1859.
So long as a Pholas exhibits only the ends of its siphons to the eyes
of a greedy crab, it is perfectly safe from attack. It is only when
the fleshy foot is unprotected that it falls a prey to some hungry
crustacean.
The toughness of the siphonal orifices is, I believe, a most important
point, for, as I shall endeavour to explain, the siphonal tubes
constitute important accessory excavating agents, to those already
enumerated.
We all know that the hole which each young Pholas makes, when first
he takes possession of his rocky home, is extremely minute,--not
larger than a small pin's head; now, it stands to reason, that if the
shell was the only terebrating agent, the opening of the cavity in
question would always remain of the same size, or, perhaps, on account
of the action of the water, a slight degree larger than its original
dimensions. Such, however, is not the case.
Here is a fragment of rock exhibiting several Pholas holes. The
aperture of one of these, which I measure, is nearly half-an-inch in
diameter, while in juxtaposition with it is situated another cavity,
measuring across the entrance less than the eighth part of an inch.
The reader will at once perceive, if the foot and shell were the sole
augurs, that as the animal descended deeper into the rock, the siphonal
tube, as it enlarged in proportion to other parts of the animal, would
have to be drawn out to an extremely fine point to fit the opening of
the tunnel. But as this is not the state of matters, the conclusion
forces itself upon us, that that portion of the orifice situated above
the shell of the animal must be enlarged by the constant extension and
retraction of the siphons, aided by currents of water acting on the
interior surface of the cavity.
This same theory will also serve to explain how it is that all Pholades
situated at the same depth in the rock, are not all of a uniform size.
I have frequently seen a piece of rock exhibit the peculiarity of two
burrows of vastly different proportions as regards breadth, being
precisely the same depth from the surface of the stone. This appears
to me equally wonderful and puzzling at first sight, as the 'boring'
question.
What age is attained by any species of the rock-borers before they
arrive at full growth, there are no means of knowing. This point,
like several others in the history of these animals, still remains a
mystery, nor is it likely soon to be cleared up. The largest specimen
of _P. crispata_ that I have seen is at present in my possession. Each
valve measures three and a half inches in length, by two inches in
breadth. Some foreign specimens of this species, and especially of _P.
dactylus_, are, however, frequently found of much larger dimensions.
On no occasion have I ever examined any Pholas excavation that had lost
its conical shape, a fact that seems to prove that the successive
stages of the boring operation must have taken place solely in
consequence of the animal not having reached its adult form.[15] For
had the shell attained its full development, and its owner continued
to labour, and rasp away the rock, the sides of the cavity at its base
would necessarily present a parallel appearance--a phenomenon which is
never witnessed.
[15] The above remark holds good, even although (_as is frequently the
case_) the animal wilfully deviates from the straight path, and bores
its tunnel in a curved form.
From this we may conclude that the depth of the perforation, which is
seldom many inches, depends entirely upon the growth of the mollusc.
When keeping specimens of the Pholas for observation, the usual plan is
to chip away the rock to the level of the valves, so that the whole of
the animal's siphonal tubes may be distinctly seen, however slightly
these organs may be extended. This plan, I found, did very well for a
time, but I was annoyed to witness, that in the course of a few months,
the siphons ceased to be either advanced or retracted,--they having
become, as it were, rudimentary.
To obviate such contingency, the writer adopted the following scheme.
To place in the tank a Pholas completely embedded in a fragment of
rock, so that nothing but the tips of its siphons, when extended to the
utmost, were visible, would not afford much pleasure to the student. I
therefore managed to saw away the rock in such a manner, as to leave a
narrow slit along the entire length of the tunnel, so as to expose the
slightest movement of the animal within. Having natural support for its
siphons, I expected that these organs would be constantly retracted and
extended; but such was not the case; at least for so long a period as I
had anticipated.
After repeated experiments, I have now discovered that whether the
siphons be protected as above described or not, they will always be
vigorously exercised if the animal be placed in shallow water, so that
its tubes when fully extended will reach the surface of the fluid.
The conclusion, from what has been stated, is, that the Pholas can
no longer be considered a weak and helpless animal. Possessed of a
rasp-like shell, a horny ligament, retractile tubes, a strong muscular
foot, and a powerful spring or stylet, it is not by any means difficult
to conceive that these agents when they are all brought into play, are
fully equal to the task of excavating the rocky chamber in which the
animal lives.