Glimpses of Ocean Life 26

Glimpses of Ocean Life 26

'But,' you ask, 'what then was the opal stick?' Ay, there's the rub,

for even the greatest naturalists cannot positively agree as to the

use and purpose of this mysterious organ. Yet it is to be found (as we

have seen) in the foot of the Pholas, in the Mussel, the Cockle; and,

in fact, it occurs in almost all bivalves both great and small. It is

termed the 'hyaline cylindrical stylet,' and is very _lucidly_ and

scientifically described as 'an elastic spring to work the corneous

plate or attritor, and by the muscular action of the foot and body, to

divide and comminute the food, and especially the minute crustaceous

and testaceous alimentary matters received into the stomachial cavity.

It appears then that this appendage acts as _a gizzard_, and the

bivalve mollusca are thus supplied with a masticatory apparatus very

analogous to the gizzards of some of the gasteropoda.'

 

Now, the simple fact that I have stated above, of the hyaline stylet

being found in the _foot_, and not in the stomach, at once proves

that it cannot possibly act as a _gizzard_ to the Pholas, or any other

bivalve in which it is known to exist.

 

In the succeeding chapter I shall endeavour, from personal observation,

to shed a slender ray of light upon the function of the stylet.

 

 

 

 

CHAPTER XVI.

 

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.