Glimpses of Ocean Life 19

Glimpses of Ocean Life 19

Being tired of wandering about, No. 1 then extended his foot along the

base of the vase to a certain point, and there let it rest for a few

seconds. When again withdrawn, to my great delight, I saw the first

thread of a new byssus had been constructed.

 

As my principal object was to become acquainted with the mode of

formation of the beard, I did not feel satisfied with merely watching

the movements of the animal from above. After a brief interval another

thread was spun. I bore in mind the words of Reaumur, who says, 'The

Mussel never spins more than four or five threads in the twenty-four

hours.' Aware that no time must be lost, though still afraid to disturb

the mollusc lest it might suspend its labours, I instantly detached

my specimen, and again turned its shell round so as to bring the

opening of the valves against the face of the glass. The creature did

not seem at all offended at his handiwork having been destroyed, but

still obstinately refused to let me see the working of its foot. Again

was the shell rolled over, and again did I replace it in its former

position. This time, in order to keep it from being shifted, a stone

was deposited upon the valve. Nothing daunted, the animal gradually

separated the valves of its shell, and at the same time advanced and

elevated its foot to the exact position that I had so long desired.

 

The spinner, when at its full length, was pressed firmly upon the

flat surface of the glass, and there allowed to remain for a while.

Suddenly, at nearly _half an inch distance from its extreme end_ (or

point), a little mouth was seen to form, about the size of a large

pin's head, from which there issued a milk-white fluid, that gradually

hardened and became fixed to the glass. This object being light in

colour, had a pretty effect when contrasted with the rich brown tint

of the spinner. Shortly afterwards the foot rolled over and withdrew

into the shell, leaving behind it the silken thread which had just been

spun. The 'little mouth,' above described, was, if I may so term it,

the mould in which the end of the thread was cast.

 

In the course of two hours a bundle of byssus threads, sixteen in

number, were produced by this industrious little labourer.

 

Having thus seen that the foot is useful to the Mussel as an instrument

of progression _before_ the beard is formed, let me now endeavour

to show that it is, at times, of equal service for the same object,

_after_, and when the mollusc is anchored thereby to any particular

spot.

 

When we remember that this anchorage is formed of a harp-like set of

strings, amounting to ten or even _a hundred_ in number, it does seem

an almost incredible fact that the Mytilus is enabled to change its

station, even when living in single blessedness.

 

To see a Mussel 'flit,' is a sight one may often watch and wait

for without success. On the other hand, when least expected, the

self-willed mollusc may commence operations. When about to take up

a new home, the animal shaves off its beard entirely, or in more

scientific language, 'rejects its byssus' altogether. In order to

excite the locomotive instincts of my specimens, I used to cut all

the threads of their cable except one. The animal being suspended, of

course its whole weight was then thrown upon a single fibre. Such a

state of insecurity was by no means agreeable, and I generally found in

the course of a few hours that fresh threads were rapidly thrown out,

and an entirely new byssus formed; the old one, which was broken off at

the root, being left behind as useless.

 

Another singular peculiarity of the Mussel which came under my

observation has not been, so far as I am aware, noticed by previous

naturalists. I allude to the power which the animal possesses of

lengthening out the root or stem of the beard, apparently to an

unlimited extent. This power appears to be seldom exercised, for

although I have had hundreds of specimens of the Mytili, in only one

instance have I witnessed the phenomenon in question.

 

A large specimen of this bivalve, procured accidentally from a fishwife

in the street, was dropped into the aquarium, and placed close against

the surface of the glass. The animal seemed highly delighted with its

change of situation, for in a few moments the valves were opened, and

a long draught of water taken in to bathe its branchiæ, and furnish

a hearty meal. Having satisfied its appetite, the next process, of

course, was to find out what kind of a home he had been introduced

into. The foot, a noble specimen, was soon protruded, and one after

the other, in rapid succession, various threads were formed. By next

morning the animal, advancing by a series of easy stages, had reached

the surface of the water, which was exactly five inches deep. I knew

it would not remain long in this position, and was anxious to discover

what plan would next be adopted. Several courses were open to him.

For instance, like a marine Captain Cook, he might circumnavigate his

little _Globe_,--or he might let go his cable and drop plump to the

bottom,--or he could follow the route I had often seen taken by his

relations, viz., to journey back to the place from whence he started.

It pleased him, however, to strike out into a new path,--to devise a

method of his own. While located near the top of the tank, he threw out

exactly ninety-eight threads, not certainly for security, but merely,

it would appear, for pleasure.

 

Then slowly but surely, day by day, he lengthened out the stem of his

byssus tree, until it reached the extreme length of nearly five inches.

To what further degree it would have been extended, had the mollusc not

reached the base of the tank, it is impossible to conjecture.

 

No sooner did the shell touch terra firma, than the cable which had

taken so long to spin was immediately broken off. I have succeeded in

keeping the same animal by me for the last twelve months, but have seen

no attempt at a renewal of the operation, in the progress of which I

had taken so lively an interest. I may add that this Mussel taught me

another lesson; it was this: in my early studies regarding the habits

of the Mytilus, I had adopted a certain theory of the manner in which

the beard was formed; and having watched so long, and witnessed the

process so repeatedly, I thought myself justified in forming certain

conclusions. My belief was that the creature could not form more than

one thread at a time, _without withdrawing its foot into the shell_, as

I believed, in order to procure a fresh supply of material. That this

notion was erroneous, this animal proved to my entire satisfaction. Not

only may one, but two, three, four, and even six threads be attached

to any selected object, the point of the foot being passed from one

position to another, without the organ being withdrawn into the valves

until the whole of the threads are formed. How many more the Mussel is

capable of producing at one 'stretch,' I have no means of knowing, but

six is the largest number that any of my specimens in such case have

ever fabricated.

 

The general idea seems to be that the Mussel works in the same manner

as the spider, who emits a drop of liquid against some foreign

substance, which, being allowed to harden somewhat, is then drawn out

as the spider recedes. This notion, I may state, is quite erroneous.

When the sucker of which we have spoken is formed, _the thread is

completed_. It is true that the foot as it retires into the shell

generally glides down the newly-constructed filament, but this is not

of necessity, nor does such circumstance invariably occur. Indeed,

while busily engaged in attaching a disc to the glass, the muscles of

the foot will contract, and thus throw open the folds of the groove,

situated in the middle of that organ; when thus exposed, the byssus

thread may be seen in the furrow, stretched like the string of a harp

or dulcimer.

 

While the end of the thread is being attached to a certain spot, a

conspicuous muscular action is perceived going on in the foot, which

alternately swells and contracts, as if something were being pumped

up through the byssal channel, until it reached a certain point.

There being dilated and spread out in successive layers, it assumes a

trumpet-like disc, which is firmly fixed to the foreign object. Indeed,

I am by no means certain that the thread is not, when first produced,

exactly like a trumpet in shape. It also conveys the idea of being

blown out in a similar manner to a piece of bottle glass. After being

exposed to the air for some little time, the hollowness of the thread

is not so apparent as when it is newly fabricated.

 

The mucous fluid, from which the fibres are formed, is secreted in

a gland situated at the base of the foot, whence it is apparently

expelled at the will of the animal into the furrow already referred

to, and is there spun into threads. The toughness of these filaments,

considering that each is finer than the thinnest strand of silk, is

remarkable. Their strength, however, may be easily accounted for,

when we know that each is composed in reality of innumerable delicate

threads, bound together by a subtle gelatinous fluid. This phenomenon

may be made out quite distinctly with a common hand lens, if the

following simple experiment be adopted: Make a Mussel construct its

thread in such a way that the disc of each is planted on the face of

the glass. Then place the fine point of a common needle upon the outer

edge of a chosen disc or sucker, and gently draw the former away to

a little distance, and you will find that by so doing the stretched

string becomes _peeled_. Continue this process carefully, and before

the thread gives way you will have divided it into a dozen parts at

least, all of which are visible to the naked eye, but clearer when the

hand lens is used, and still more distinctly and beautifully defined,

of course, if the microscope be brought into play.

 

The foot of the Mussel appears to be firmly strapped on, as it were,

to certain transverse muscles, by a contraction of which the animal

closes its shell with surprising force. This strap, composed of a

powerful tendon which passes under the adductor muscles, is attached at

either end to the base of the foot. Thus we account for the remarkable

strength which is evidently seated in the foot, and makes it of so much

importance to the animal. At first sight nothing appears more easy

than to pluck out this organ by the roots, but an attempt will prove

the experiment to be more difficult than many persons suppose.

 

The colour of the foot varies considerably in different specimens,

even of the same species. Some, for instance, are of a chestnut brown;

others of a kind of mauve or purple, covered with a peach-like bloom

during life; others, again, are of a deep-toned umber, while not a few

are pearly white, and streaked sometimes with pink like a tulip.

 

The peculiarity of the Mussel to attach itself to foreign substances

has been taken advantage of for the benefit of man, and a curious

instance is exhibited at Bideford in Devonshire, at which town there is

a bridge of twenty-four arches, stretching across the Torridge river

near its junction with the Taw. 'At this bridge the tide flows so

rapidly that it cannot be kept in repair by mortar. The corporation,

therefore, keep boats in employ to bring mussels to it, and the

interstices of the bridge are filled by hand with these mussels. It is

supported from being driven away entirely by the strong threads these

mussels fix to the stonework.'

 

Like most other writers who quote this strange account, I have not

had ocular proof of its accuracy.[9] That it is quite probable I can

readily believe, as a pretty experiment will partly prove it to any

spirited aquarian. Following out the above idea of the bridge at

Bideford, I managed to build an exceedingly pretty centre piece for my

tank.

 

[9] Since writing the above, I have received the following interesting

epistle from Mr. Edward Capern, the celebrated 'poet and rural postman'

of Bideford, who kindly sought out the information I desired, relative

to the present state of the bridge above alluded to:--

 

Bideford, January 27, 1859.

 

DEAR SIR,--I have inquired of the bridge warder, and he informs me that

the feoffees of the bridge command mussels to be brought up by the

cart-load, to protect the foundation, which is laid on _rubble_.

 

I am pleased that it has been in my power to procure this information

for you--I am, dear sir, faithfully yours,

 

EDWARD CAPERN.

 

Having no ready means of making a rock arch, I collected such pieces

of rock, stones, &c., with weeds attached, as I thought would answer

my purpose, and then proceeded to fabricate the object of my wishes in

the following simple way: First were laid two stones parallel to each

other at three or four inches apart. Upon these I placed a large piece

of rock in a transverse direction. Between the interstices a number

of small mussels were then inserted. When fully satisfied that the

bivalves had moored themselves, I gradually piled one piece of rock

upon another until the structure reached the desired height, each piece

being bound to its neighbour by means of the byssus threads of the Mytili.