Glimpses of Ocean Life 27

Glimpses of Ocean Life 27

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_.