Glimpses of Ocean Life 28

Glimpses of Ocean Life 28

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.

 

 

 

 

CHAPTER XVII.

 

The Sea-Mouse.

 

(APHRODITE ACULEATA.)

 

 

'For seas have ...

As well as earth, vines, roses, nettles, melons,

Mushrooms, pinks, gilliflowers, and many millions

Of other plants, more rare, more strange than these,

As very fishes living in the seas.'

 

 

 

 

[Illustration:

 

1 THE SEA MOUSE (_Aphrodite aculeata_)

2 THE DORIS

3 YOUNG OF THE DORIS

4 EOLIS PAPILLOSA]

 

 

 

 

XVII.

 

 

Beauteous stars also the sea contains, as numberless, though not so

brilliant in appearance as those which stud the firmament of heaven;

flowers, too, grow beneath the wave, and rival in loveliness the gems

which adorn our fields and hedge-rows. Nay, more, like the land, the

ocean owns its various grasses, its lemons, and cucumbers, its worms,

slugs, and shelly snails, its hedgehogs, its birds, its ducks and geese

(_anatidæ_), its dogs, its hares, and lastly its _mice_ (_aphroditæ_.)

The latter objects, despite their unprepossessing name, being in no

wise less interesting than those above mentioned.

 

The _Aphrodite aculeata_ is, perhaps, one of the most gorgeous

creatures that inhabits the seas of our British coast. Its body is

covered with a coating of short brown hairs, but as these approach the

sides of the animal, they become intermixed with long dark bristles,

the whole of which are of an iridescent character. In one respect

this creature bears no resemblance to its namesake of the land, being

extremely slow and sluggish in its movements (at least according to

our experience) when kept in confinement. Some writers, however, affirm

that the Aphrodite possesses the power, although seldom exercised, of

both running and swimming through the water with considerable speed.

 

In general the animal loves to tenant the slimy mud, and wherever the

writer has happened to come upon a specimen at the sea-shore, its back

has always been thickly coated with sand or dirt. The Sea-Mouse, then,

unlike the peacock, can never be deemed an emblem of haughty pride,

yet has nature in her lavish beauty endowed this humble inhabitant of

the deep with a richness of plumage, so to speak, fully equal in its

metallic brilliancy to that which decorates the tail of the strutting

bird we have mentioned. As the bristles of the Aphrodite are moved

about, tints--green, yellow, and orange, blue, purple, and scarlet--all

the hues of Iris play upon them with the changing light, and shine with

a metallic effulgence. Even if the animal, when dead, is placed in

clear water, the same varied effect is seen as often as the observer

changes his position.

 

Not only are the _Setæ_ worthy of notice on account of their lustrous

beauty, but also for their shape, and the important part they play in

the economy of the animal. These lance-like spines seem to be used by

the Aphrodite as weapons of defence, like the spines of the hedgehog

or porcupine. In some species they are like harpoons, each being supplied with a double series of strong barbs.