BARNACLES AND ACORN-SHELLS.
Occasionally in strolling along a beach after a storm we shall
encounter some wreckage that came ashore with the last wave of the
incoming tide, and so failed to be washed off again. It may be a spar,
a rudder, a stern-board with a name upon it that tells a tale of a
vessel that has gone down. It may come in clean, with the splintered
wood looking as though just smashed, and we may judge from such
appearances how long it is since the catastrophe happened. On the other
hand, it may bear evidence of having floated in the sea for a long
period before getting into a current running coastwards. Such evidence
will consist in the wood being heavily soaked with water, or in its
surface being covered with hundreds of writhing snake-like creatures
with pale blue heads. We have met under such circumstances, with balks
of timber with scarcely an inch of their surface not covered with this
foreign growth; with casks on which they grew all round the edges of
the heads and the hoops.
A few months ago there drifted into our “porth” a small keg-buoy with
a long thick hawser attached, and the submerged half of the buoy had
a fine crop of the writhing things hanging from it, whilst they hung
from the rope in clusters a few inches apart. The finder very kindly
hauled it upon the rocks, and coiled the hawser round it that I might
photograph the entire lot. As it lay there in the autumn sunshine it
looked a very pretty group, and I regret that the camera would not
reproduce the snaky movements, nor the fine colouring.
Now the creature is no other than the Ship-Barnacle (_Lepas
anatifera_), one of the chief obstacles to speed in the old days of
“the wooden walls of England.” When a ship had made an ocean voyage
it was necessary to dock her and scrape off the enormous quantities
of Barnacles that not merely added to her weight, but offered strong
opposition to her passage through the waters. To-day, what with steel
vessels and patent anti-fouling compositions with which to paint the
ship’s bottom, the poor Barnacles find their world much narrower than
formerly, and with fewer openings for the enterprise of their race.
Should you come across such a barnacle-ridden waif of the sea, consider
it carefully. You shall find in it matter of interest, and, in addition
to its provision of something for your imagination to play round, in
your efforts to get a clue to the vessel of which the wreckage once
formed part, the life-story of the Barnacle itself is a romance.
[Illustration: SHIP-BARNACLE.]
Before we attempt to tell this story briefly, let us look at one of
the specimens before us. The long and evidently muscular neck ends in
a composite shell, which is seen to be composed of four portions, or
valves hinged together, opening in front, and strengthened at the back
by a fifth valve, a long, narrow, and curved piece. At short intervals
the two halves into which this “shell” is obviously divided part in
front, and out comes a mass of coiled up, slender, and hairy processes
which separate and uncoil as though attempting to catch some invisible
body, then coil up again and withdraw as though they had really caught
it and meant to keep it. Now this is the principal, one might almost
say the sole occupation of their adult lives, but writhing is another
to which they pay some attention. Probably it may strike you as a
monotonous, perhaps senseless way of spending one’s days; but it is
quite evident, from the great numbers of Barnacles crowded within a few
square feet, and all looking prosperous, that it is a paying game.
It must be remembered that however clear and crystalline the sea-water
appears, there is really great truth in the remark of the scientific
luminary, who said that the sea was a kind of thin soup or broth,
holding enormous quantities of animal and vegetable matter in solution,
most of it invisible to the unassisted vision. Whoever possesses a
retentive hand like that of the Barnacle, has only to spread the palms
and fingers wide, then close them tightly, to have _something_ enclosed
therein. Such is the Barnacle’s experience; and it is by the mere
opening and shutting of his hand that he gets a good living. Strictly
speaking, this hand is _not_ his hand, but a number of feet and hands
which correspond with the limbs of the crabs, lobsters, and shrimps.
Strange as the assertion may sound, unlike as the creatures appear, the
Barnacles belong to the same great class (Crustacea) as the animals
described in the last two chapters, though they are partly separated
from them and put into an order (Cirripedia) by themselves. No wonder
if you hesitate to accept this statement as a fact; you are in good
company, for no less a naturalist than the great Cuvier failed to see
the relationship.
That this order is an important one will appear when it is stated
that the great Charles Darwin wrote an important work in two volumes,
devoted to the “recent” Cirripedes, and two other volumes on the
“fossil” species of the order.
These Cirripedes are divided into two main groups--the pedunculated or
stalked Cirripedes, represented by the lively Barnacles before us, and
the sessile or stalkless Cirripedes, of which the familiar Acorn-shell
of the littoral rocks are the examples.
Now these two groups may strike you as having little in common, and
yet their early history is practically identical, one group with the
other. Longfellow was quite right when he stated that “things are
not what they seem,” at least, they are not _always_ what they seem;
conversely, they do not always seem what they are. We must not be
content with taking a couple of creatures at one particular stage in
their existence, and say these organisms differ so widely from each
other that we must put them into equally widely separated classes or
groups; we must try to find out and compare all the stages in their
life-histories, before we can talk of separating or bringing together,
except in the most temporary fashion, there to be kept, as it were,
in quarantine until we have found out what we wish to know concerning
their antecedents.
No one, until he had evidence of the successive stages in the life
of a butterfly, would dream of putting such dissimilar things as a
caterpillar and a butterfly into the same order; yet their wonderful
course of development was long ago traced out, and it is within the
power of any person to check off the whole progress from the batch of
elegant eggs laid on a cabbage leaf, through the ravenous worm-like
caterpillar stage, and the apparently inanimate chrysalis to the
beautiful white butterfly that can take no solid food, and which by
depositing another batch of exactly similar eggs, completes the cycle,
and so assures us we have made no mistakes in our observations.
In a like manner we can watch the series of stages, utterly unlike
each other, through which a crab, a lobster, a shrimp or a Barnacle
passes before it attains the adult condition; and when we find the
early forms of the Barnacle agreeing in a very curious way with stages
in the life-history of typical Crustaceans, we are perfectly justified
in grouping them in the same class of animal life. We have, in fact,
pierced through the disguise with which some of the adult forms have
sought to hide their identity, and have found out their true characters.
It must be confessed that the course of development in some of these
creatures partakes of the character of what has been termed “an
Irishman’s rise.” In the case of the caterpillar and the butterfly,
everybody recognises that development is progress, that the butterfly
is a higher being than the caterpillar. But in others development
spells retrogression. Such is undoubtedly the case with the Cirripedes,
and with certain crustaceans which lead the life of parasites. The
course of development in the Barnacles and Acorn-shells has been very
succinctly stated by Darwin.
“The larvæ in the first stage have three pairs of locomotive organs,
a simple single eye, and a probosciformed mouth, with which they
feed largely, for they increase much in size. In the second stage,
answering to the chrysalis stage of butterflies, they have six pairs
of beautifully constructed natatory legs, a pair of magnificent
compound eyes, and extremely complex antennæ; but they have a closed
and imperfect mouth, and cannot feed: their function at this stage is
to search out by their well-developed organs of sense, and to reach
by their active powers of swimming, a proper place on which to become
attached and to undergo their final metamorphosis. When this is
completed they are fixed for life: their legs are now converted into
prehensile organs; they again obtain a well-constructed mouth; but they
have no antennæ, and their two eyes are now re-converted into a minute,
single, simple eye-spot. In this last and complete state, Cirripedes
may be considered as either more highly or more lowly organized than
they were in the larval condition. But in some genera the larvæ become
developed into hermaphrodites, having the ordinary structure, and
into what I have called complemental males; and in the latter the
development has assuredly been retrograde, for the male is a mere sack,
which lives for a short time, and is destitute of mouth, stomach, and
every other organ of importance, excepting those for reproduction.”
In this early condition these Cirripedes much resembled the minute
so-called water-fleas that swarm in our fresh-water ponds and streams,
and when upon the point of their last change they laid their heads down
upon the spot selected for their future station in life. Then a natural
marine glue, that sets under water, exuded from their antennæ, and they
became fixtures, head downwards. The two valves of their old shells
were thrown off, and the new ones, largely composed of carbonate of
lime, grew up from the base.
Some of the Barnacles on our buoy are apparently dead, and one of these
we can take to pieces. Taking off one half of the compound shell, we
find the creature attached to the floor of the chamber, evidently on
its back. From the upper end there arise the twelve limbs, six on each
side, and each one dividing into two branches, each branch a beautiful
feather with a wonderfully jointed, supple, purple-black stem, closely
fringed with purple hairs. It is from this plume-like cluster of
curling limbs that the order obtains its name (Latin, _cirrus_, a
curled lock of hair, and _pes_, a foot = curl-footed).
When the shell opens and the trunk which supports all these limbs is
thrust forward, each branch separates from its fellows and becomes
almost straight, spreading out its hairs as widely as possible. Thus
extended, the entire plume of feathers sweeps through a limited space
of water, and many minute creatures are entangled in its hairs, and so
brought into the currents that flow towards the Barnacle’s mouth.
Huxley has described the Barnacle as standing on its head and kicking
food into its mouth; but we question whether this partakes of his usual
accuracy of description. So far as we have been able to make out the
process, the food particles are strained off from the sea-water by this
exquisite net, and brought, not kicked to the mouth.
It is to this plume of feathers that the Barnacle owes its specific
name, _anatifera_ = goose-bearing. It was formerly thought to be a
vegetable production, whose fruit, when ripe, gaped open, and dropped
out an embryo bird, which fell into the water and developed into a
Bernicle Goose. Gerarde, three centuries ago, wrote a wonderful and
circumstantial account of the whole business, which he declared he
had seen with his own eyes; and every writer of popular works on the
sea since then has seen fit to reproduce his account as one of the
curiosities of natural history. I have no intention of doing so, for it
is time it had a little rest after being so hard worked. For a similar
reason I have in this book utterly ignored Montgomery’s “Pelican
Island;” and the equally hackneyed quotations from Southey, Crabbe, and
Coleridge, that have been a boon to some of my predecessors in filling
their pages, I have also put upon a retired list.
Cirripedes, not being so completely boxed up as the majority of
crustaceans, can enlarge their dwellings by additions to the edges of
the shells, and therefore do not need to throw off the entire envelope
from time to time. But it is difficult to entirely get rid of racial
characteristics, even when there is no special need to retain them; and
so we find the Cirripedes casting the skins of their bodies from time
to time, though the limy shell is made to serve for all their life.
There is a smaller species of Necked Barnacle (_Scalpellum vulgare_),
the shelly portion of which, seen edgeways, looks like a penknife,
whence the Latin name. It is usually found growing among corallines; it
is figured in accompanying group.
[Illustration:
PYRGOMA. SCALPELLUM. PORCATE BARNACLE.
ACORN-SHELL.
]
There is a peculiar little Barnacle called _Pyrgoma anglicum_, which
is parasitical upon the pretty Devonshire Cup-coral (_Caryophyllia
smithii_). It is shown on the coral in the upper left-hand side of our
illustration above, and may be looked for in any of the localities
where this coral occurs. It attaches itself to the outer edge of the
plates of the corallum.
Let us turn now to the more familiar Acorn-shells (_Balanus
balanoides_) that crust the rocks between tide-marks. We might have
used the expression “too-familiar,” for whoever has had to put a
bare foot upon them in bathing or swimming from the rocks, will have
had cause for remembering their sharp edges. It is not easy to keep
the Ship-barnacle in an aquarium; but a flake of rock, or a disused
limpet shell, crusted with _Balanus_, is conveniently kept in a glass
of sea-water, and will long continue at once a thing of beauty and
a wonder to friends who are ignorant of natural history. These are
sessile Cirripedes, that is, they have no stalks upon which to writhe,
but _sit_ directly upon the rock.
If we scrape one of these Acorn-shells off the rock with our useful
putty-knife, we shall find that it has a thin base of shelly matter
upon which it reposes much as the Ship-barnacle does upon the floor
of its shelly chamber. But it will be seen that the sloping outer
walls of the Acorn-shell are firmly cemented together, and allow of no
movement; the top, however, is open, but the animal within is protected
by an interior door of four pieces, that opens in the middle like the
cellar flaps seen in connection with business basements. These doors
“butt” together accurately, and open easily by pressure from inside.
Then out comes a more beautiful and delicate “hand” even than that
of the Barnacle, for this is so fine and transparent that it looks a
thing of spun glass. There is the same movement as in the Barnacle,
the everlasting grasping at something, the opening and shutting of
the cellar flaps. Its earlier history is also similar to that of its
stalked relation. There is a larger species of Acorn-shell known as
the Porcate Barnacle (_Balanus porcatus_), the name having relation to
the form of the conical shell; porcate signifying that it has ridges
between the furrows that mark its outside. Other species, smaller, some
almost flat, will be found on some parts of our coast, but we would
refer our readers to Mr. Darwin’s work[5] for the further study of the
Cirripedes.
[5] A Monograph of the Cirripedia, 2 vols. Ray Society.