THE MOLLUSCS, ETC.
The curious floral forms of the Zoöphytes have, as yet, attracted the
greatest degree of attention among the constructors of marine Aquaria;
yet other forms of ocean life offer equal, if not superior,
opportunities for curious observation, and are equally well calculated
to bear the confinement of the tank. Among them all, none more than the
Molluscs, especially the shell-bearing division, which merits the
careful attention of the student of Nature, as forming some of her most
singular manifestations.
The knowledge of the nature and structure of many of the most curious
shell-bearing Molluscs is of very recent date, with the exception of
those which possess obvious qualities which have fitted them for
articles of diet or commerce; such as the Oyster, Mussel, Cockle, &c.,
as eatable species—and the Purples, Sepias, and Cuttle-fish, as
containing valuable dyes. With the exception of such as these, the
pearl-yielding Bivalves, and a few others, nothing was known, with a few
remarkable exceptions, of the animals that create and inhabit the
beautiful shells that have so long been ranked among the most elegant
objects of the cabinets of the curious. Many of these were, in fact,
scientifically classified by learned naturalists before the nature of
the animal, of which they formed the mere senseless husk, was even
guessed at. The ordinary collector did not even desire to know anything
of the creature which produced the shell he most prized; it was
sufficient for him that it was estimated as “rare” by his brother
collectors—rarity being a quality more highly prized than even beauty.
With this feeling, prices as great were given for single shells as ever
enthusiastic Hollander paid for a coveted bulb during the height of the
Tulipomania. No amount of guineas was too much, at a sale of shells, for
such a contested prize as a _Many-ribbed harp_, a _Gloria maris_, a
_Cedo nulli_, or a _Voluta Junonia_. But that race of idle
shell-fanciers has given place to a race of true conchologists, who are
investigators as well as collectors, and whose labours are daily
developing unexpected and valuable knowledge from those long obscure
pages of the great book of Nature.
The marine Aquarium may be made the means of many curious discoveries
regarding the habits and organization of the shell-bearing Molluscs;
and, with this feeling, I may direct the attention to several of their
singular characteristics, in order that they may serve as clues to the
detection of others.
The term Mollusc, from _mollis_, soft, is intended to express that the
whole class are invertebrate; that is, entirely without spine, or any
bony support to their curious fleshy forms. The term was invented by the
illustrious Cuvier, but is objectionable as a distinctive one, the
characteristic on which it is founded being shared by other distinct
classes of animals. When, however, the application of a term is well
understood, its inner signification becomes unimportant; it is,
therefore, now too late to criticise the one invented and applied by the
prince of modern naturalists.
Among the interesting facts detected by recent science, it has been
shown that many of the seemingly shapeless masses of soft substance,
scarcely to be termed flesh, possess all the senses of the higher
animals. In the _Cephalopoda_, the organs of sight and hearing are both
well developed; and Professor Owen considers that the Nautilus possesses
even an organ of passive smell. The _Gasteropoda_ too, are, according to
Siebold, nearly all furnished with ears and eyes, the former organs
being described as forming round capsules, conspicuously visible near
the roots of the tentacles.
Some of the _Conchifera_, also, are furnished with numerous eyes, which,
like those of the Scallops and Clams of our own shores, are also placed
among their tentacula.
It appears probable, says Dr. Johnston, that many have also the sense of
taste, as they are observed to select particular articles of food in
preference to others, and there is no other sense that appears fitted to
regulate the choice. The mouth, as it is termed, of many of the
molluscous tribe is furnished, as among the _Gasteropoda_, with a fringe
formed of filaments, which may be organs of touch, and they have also a
complicated breathing apparatus.
The strength of these boneless creatures is something very
extraordinary, and almost incomprehensible. The _Strombus gigas_, a
soft, snail-like creature, carries a shell which often weighs more than
five pounds; the _Cassis tuberosa_ supports one nearly as heavy, and the
naked Molluscs, that have no shell to carry, have other modes of
exhibiting strength of a very extraordinary character.
The shells of the clothed Molluscs are senseless, being permeated by no
vessels, and are formed by the animal itself from a secretion with which
its outer integuments are invested, and which may be described as lime
in a state of solution. The thickened edge of the mantle, by means of
which the form is given to the shell, and the general manipulation
effected, is furnished, as may be seen with the aid of a moderate lens,
with a minute and highly sensitive fringe, the cilia of which are of
various colours, corresponding in tone and position to the tints which
decorate the exterior of the shell. The coloured cilia or fringes have
doubtless a dyeing power, which colours the calcareous solution at the
time it is added to the shell by their plastic instinct. The solution
becomes a hard testaceous substance so soon as it leaves the body of the
animal, and is deposited in architectural layers upon the beautiful
structure of the shell, by the “trowel” and “brushes” of the edge of the
mantle.
This process is beautifully described in Jones’s “Animal Kingdom,” with
all the details relating to the successive ridges on the shell, which
mark the age of the animal; it having been ascertained what time is
required for the completion of each story of the edifice.
The power of locomotion is one of the most curious subjects for
observation in the structure of shell-coated Molluscs, and for this
purpose the marine Aquarium offers many advantages. Other classes of
animals have been distinguished by the number of their feet; we have,
for instance, a tribe of worms termed centipedes, or hundred-footed
creatures; and, to pass over many gradations, to the superior
grass-feeding and carnivorous animals, we find them termed quadrupeds,
or four-footed creatures; while the human race, along with birds, has
been termed biped. Why, therefore, may we not coin a word for our
present purpose, and call these curious Molluscs monopeds, or
single-footed creatures?—for they walk with a single foot, being
compelled to do so by the very simple fact that they have no other. This
limb, or foot, being gradually protruded, its bearing against some
substance forces them forward, and when the foot has attained its full
distension it is drawn in, and a new bearing obtained, and by the
repetition of this process, a certain amount of locomotion is effected.
Some species float on the surface by means of this foot. Having crawled
up a rock to the height of the surface of the water, the foot is
protruded and exposed to the air, when it becomes suddenly dried, and in
that state serves as a cork, which enables the animal to float away
close under the surface of the water. But if any agitation of the water
wet this floating apparatus, or the animal withdraw it voluntarily
beneath the water for that purpose, the creature immediately sinks to
the bottom.
The swimming power of this race of creatures is equally curious. The
_Cephalopoda_, by the ejection of a jet of water, propel themselves
rapidly in the opposite direction, and by the repetition of the jet at
regular intervals, a beautiful power of motion is obtained, as regular,
and with less labour than that of ordinary swimming by means of the
action of fins or other oar-like limbs adapted to the purpose.[3] The
_Pteropoda_, however, in their little shells, translucent as glass, swim
by the action of small fin-like paddles placed near the head.
[Footnote 3: Some species effect leaps by an analogous
contrivance—collecting water within the closed mouth, and then emitting
it at a gush from a small portion of the aperture, suddenly opened,
which propels the creature to a considerable distance, as it were, at a
single bound.]
The _Bivalves_ do not make so clever a use of their single foot as the
_Univalves_. The foot in this tribe appears to be furnished with a
terminal hook, which, when the foot is protruded, clings to some
substance, and the animal is drawn up to that point, when the operation
has to be repeated; this appears likely to produce but a slow rate of
progress, yet some of the sand-boring _Bivalves_ manage, when alarmed,
to conceal themselves with great rapidity by that means.
The _Mollusca_, as feeders, are divided into three classes—those which
take only liquid food, the vegetable feeders, and the carnivorous
species.
Those which are only able to take food in a liquid form, are such as
have no means of seizing prey, their food consisting of the countless
myriads of infusorial animalcules which float in the sea-water, and
which are carried into the orifice of the stomach or mouth by the
current. Of these, the _Dunicata_, _Brachiopoda_, and _Conchifera_, are
examples.
The liquid feeders exhibit a very low form of molluscous life, but other
classes are furnished with means of defence and aggression, equal to
those of terrestrial quadrupeds, and much more extraordinary in their
form. Some of the carnivorous _Univalves_, for instance, feed upon the
_Bivalves_ by drilling a hole through the solid shell, and withdrawing
the animal piecemeal, as required.
The _Eolis papillosa_ has been observed tearing away the tentacles of
different species of Sea-Anemone with extraordinary voracity, and the
tribe must therefore very evidently be excluded from the Aquarium. The
Cuttles, also, are to be avoided from the same cause; they are fierce
tyrants of the deep, that would make sad havoc among the delicate
creatures with which we delight to furnish our tanks of glass. The
curious substance termed Sea-Grapes, which are the eggs of this
creature, might, however, be placed in the tank, and the progress of
development watched, without fear of injury to the other inmates.
The full-grown Cuttle is, nevertheless, so curious a creature that, in a
tank prepared with that special view, his habits might furnish food for
much curious observation—indeed, carefully fed up, he might form very
excellent food himself; his German name, _Kuttel_, signifying tripe, the
flavour of which his flesh is said to resemble. The common Squid, which
is eaten by the poor of our coasts, is a kindred species, and is also
said to have a similar flavour. Molluscs of this class, as well as the
disgusting-looking Poulp, or many-feet, are seen in profusion in the
markets of the south of Europe, and are as highly prized as the Oyster
with us. The ancients carried their taste for them so far as to feed
them up artificially; and at the nuptial feast of Iphicrates, one
hundred _Polypi_ and _Sepiæ_, as we are informed, were disguised with
different sauces, each imparting a different flavour. The land Molluscs
were also much sought as a table delicacy, a species of the large
Garden-Snail being bred for that purpose, and fed upon a prepared paste,
which so accelerated their growth that we learn, from the industrious
Pliny, of their attaining to enormous dimensions; the shells of some of
the finest being capable of holding eighty measures of water, called
quadrants. But in speaking of Molluscs, I must principally confine
myself to such as are suitable for an Aquarium.
Among the Sea-Snails of our own coast, which are still eaten by the
lower orders, is the Periwinkle, considered by some superior in flavour
to the Oyster or Shrimp. This creature, the _Littorina littorea_, is one
of the most useful creatures in an Aquarium, cleansing it from all
decaying vegetation, which is its natural food. The Periwinkle varies
much both in size and colour, the ground tone of the shell being
sometimes red, orange, or even scarlet, sometimes with and sometimes
without handsome black bands. Such as are coloured in this attractive
manner should obviously be selected as inmates of the Aquarium, in
preference to the dull-coloured varieties; and a few of the small yellow
kind, _Littorina littoralis_, may be added by way of variety, though
they do not succeed so well in confinement as the other species.
The Whelk, _Buccinum undatum_, another of the snail-like Molluscs of our
coasts, which is considered good eating by the lower orders, and often
seen on fish-stalls at particular seasons, is well worthy a place in our
miniature sea; especially under certain circumstances, when the shell of
this creature assumes a most singular aspect, well calculated to excite
the wonder of the young naturalist. It is sometimes found surmounted by
a mass of living substance, which might be taken for the body of the
creature, residing in preference on the roof of its dwelling during the
summer months, as it may be observed spreading a set of tentacles, from
a mouth-like orifice, for the collection of food. Within, however, a
pair of protruding eyes are seen glaringly on the watch for prey, and
another set of food-clutching machines may be noticed beneath them,
ready for their work, and only awaiting the opportunity. They look much
like the claws of a lobster, and if any suitable object comes within
their reach, it is seized by one or both of these two-fingered hands,
and carried to the yawning mouth beneath; but before it reaches that
evidently impatient receptacle, a brightly-shining crimson finger,
ornamented with two white stripes, darts from beneath those claws and
mouth, and, snatching away the rich morsel, disappears as suddenly as it
came, leaving the expectant mouth and astonished claws both empty. The
mystery of this seemingly compound creature having, as it were, a
first-self living outside the house and getting a separate living, a
second-self located in the front parlour, and prevented from eating its
own dinner by a third-self residing in the back parlour, may be easily
explained, now that the persevering observations of our naturalists have
solved it. It is as follows:—
The internal dweller in the front parlour is the Hermit Crab (_Paguras
Prideauxii_), a creature seldom contented with its own pretty solid
habitation, but ever seeking some further protection, which it generally
finds in an empty Whelk-shell. It is, moreover, very particular as to
_fit_, and other details; for it has been observed, when looking out for
a house, to try and reject many before finally adopting an abode. The
inhabitant of the back parlour is the Sea-Worm, _Nereis bilineata_, a
creature which, instinctively knowing the voracious propensities of the
Crab, and determining to share his abundant feasts, seizes his
opportunity, when mine host of the Whelk-shell is pretty well surfeited
and in a semi-dormant state, to sneak past the dangerous claws into the
“back parlour,” which is the interior of the narrow spiral of the
shell—a form of apartment which affords him a most comfortable and
convenient home, in which, by the superfluous voracity of the Crab, he
is furnished with board as well as lodging. The external tenant of the
Whelk-shell is a parasitic Sea-Anemone, known as the Cloak-Anemone, from
its power of nearly enveloping the object to which it attaches itself,
by means of the extension of its stem or body. It is known in scientific
classification as _Adamsia palliata_, having been made a separate genus,
and its specific name ingeniously taken from that of the Roman cloak,
the well-known pallium of the classical writers.
Almost invariably, when the Hermit Crab is discovered inside the
Whelk-shell, the _Adamsia_ is found outside; and the Hermit is seldom
without his dinner assistant, the prettily-striped _Nereis_. This fact
is so well known to fishermen, that when in search of this worm, which
is an excellent bait, they never fail to break the shells tenanted by
the Hermit Crab, and are seldom disappointed in finding the object of
their search in his company.
Another parasitic Anemone, still more fond of travelling, the _Actinia
parasitica_, often selects the back of the Crab himself (generally
_Paguras Bernhardus_), and in that position is hurried along, in the
sidling gallop of his steed, in a way that must often prove
inconvenient; for in passing under ledges of rock, the Crab, doubtless,
only takes his own measure. Yet, in such cases, the Anemone probably
knows how to take care of himself; and when _Bernhardus_ becomes
skittish and adventurous, “draws in his horns,” as many other bold
spirits are obliged to do at certain crises of their career; and in this
state, presenting only a semi-spherical mass of tough leathery
substance, he can fearlessly allow himself to be driven beneath stony
archways, or under impending branches of the marine forests, by his
ferocious Jehu, with less chance of injury than the outside passenger of
a terrestrial stage-coach passing beneath the low gateway of some
inn-yard.
Our largest native shell of the Whelk tribe is the _Fusus antiquus_,
often used by the Shetland islanders as a lamp; for which purpose it is
suspended horizontally, the cavity holding the oil, and the wick
projecting from the canal.
The Whelks belong to the interesting family _Muricidæ_, some of which,
natives of our own coasts, are very pretty objects for the Aquarium. It
was the _Murex trunculus_ which yielded the Tyrian purple, different
species affording distinct tones of colour. In form, these shells are
somewhat like our common Whelk, but finely marked with broad, dark,
spiral stripes. The ancient mode of extracting the dye, as described by
Pliny, was verified by Mr. Wild, in 1838, in a very interesting manner.
In the neighbourhood of the site of the ancient Tyre, he found, in the
rocks on the sea-shore, a vast number of round cavities, evidently the
work of the hammers and chisels of long ages past. These cavities varied
in size, from that of a small flower-pot to that of a cauldron, and
round about them still lay scattered immense masses of the remains of
the shells and bodies of the _Murex_, in many instances aggluminated
together. They had evidently been pounded in those cavities, exactly as
described by Pliny, and the dye extracted according to the formulæ so
graphically detailed by the ancient naturalist.
The _Purpura lapillus_ of our own shores yields a similar dye, and may
be kept in our Aquaria as a reserve bottle of “marking-ink;” for the
ingenious Mr. Gosse has shown how its dye may be thus used for household
purposes. The shell is a small white univalve, with one or more bands of
pale brown. It perishes on being immersed in fresh-water; and a thick
vein of yellowish white, near the head, contains the dye, which is a
liquid of a creamy thickness and of pale, indistinct colour. But if it
be painted in the forms required, as a cipher, or any other ornament, on
linen, or any other textile fabric, with a camel’s-hair pencil, and
exposed to the air, it rapidly assumes a yellow tone; which first
changes to green, then blue, till at last it becomes a full strong
indigo, exhibiting plainly all the forms that have been traced. A
crimson-red change next ensues, and the final colour, which is indelibly
permanent, is a reddish purple.
There is also a large naked Mollusc, one of the _Aplysia_, that pours
forth, under excitement, a secretion of rich purple hue; but the colour
is considered valueless as a dye, from its extreme volatility, though it
is stated that it may be rendered permanent by means of nitric acid.
The common _Planorbis corneus_, a shell coiled in the form of a ram’s
horn, has a similar property; but the colour of the fluid is still more
volatile. The purple liquid, however, contained in another of our native
shells (_Scalaria_) is very permanent.
It is well known that the ink of the Chinese, which we term Indian ink,
is prepared from the Cuttle, and the Cuttles of the Mediterranean Sea
furnished the principal black inks and dyes of the Greeks and Romans. It
is a kind of _Sepia_, in fact, that still furnishes the rich brown
colour which bears the name of the animal from which it is derived.
The common _Sepia vulgaris_ might form an appropriate specimen for a
marine Aquarium, many of its habits being singular; and its power of
enveloping itself in a cloud of its own rich dye may often be observed
when it is irritated by the presence of a real or fancied danger. It has
the faculty of propelling itself hither or thither by the emission of a
jet of water, as described in speaking of the locomotive power of other
Molluscs, with the additional faculty of guiding its motions by the
rapid movement of two fin-like paddles, which, when in agitation,
produce an effect not unlike the fluttering action of the wings of a
moth. This little _Cephalopod_ has large projecting eyes, and a group of
arms that hang listlessly down when the fins are in motion. It changes
colour fitfully and beautifully, exhibiting in the course of such
changes pretty metallic spots and rings, which appear and disappear, now
like gold, now like silver, as seen through a semi-opaque substance. The
whole creature is at one moment of a dusky gray tone, but fitfully
changes to white or deep brown when alarmed. These little creatures are
exceedingly voracious, and when one was observed by Mr. Gosse to seize
another of its own species, the victim shot out its defence of dark
black fluid.
Some of the _Trochus_ tribe of shells look pretty in an Aquarium, but at
present their treatment is so imperfectly understood, that they do not
seem to do well. The specimens can, however, be renewed as required.
_Trochus ziziphinus_, the pearly _Trochus_, the animal of which is of a
rich orange colour striped with black, moves freely about, and forms a
very attractive object. The animal of _T. granulatus_ is larger and
handsomer, but shy, and displays little activity in confinement. The
small _Trochus_, _T. cinerarius_, if placed in an Aquarium, may be
observed rasping down the minute Confervæ that grow on the inside of the
glass; and the curious method of the operation, and the singular
instruments with which it is performed, may be observed by the aid of a
small pocket lens.
_Limpets_—those curious bonnet-shells, as they are termed in some
places, which are found in the form of a flattened and inverted funnel,
adhering closely to the flat rocks of the sea-shore in all the European
seas—are more curious than they appear at a first glance, and have
characteristics that well repay the labour of persevering observation. I
should always place a few in an Aquarium.
The common Limpet (_Patella vulgata_) has a power, which appears
extraordinary when the soft substance of its body is considered, of
excavating, more or less deeply, a portion of the rock which it makes
its home. It is supposed to leave its hollow in the night, returning
infallibly to its home in the morning. This habit might be watched in an
Aquarium, and, if verified, a very interesting fact would be
established, which at present remains somewhat doubtful, although Mr.
Lukis, of Guernsey, marked a Limpet, and found it return to its haunt.
These creatures belong to the order named _Cyclobranchiata_, from the
breathing apparatus being arranged in a circle round the body. The
pretty British shell, vulgarly called the Ark of Noah, but which is the
_Arca tetagrona_, should be tried in Aquaria, as well as the elegant
Heart-shell of the beautiful genus _Isocardia_; the movements and habits
of the latter, as described by the Rev. J. Bulwer, being very curious
and interesting.
The pretty little Cowry is an object that must not be passed over in
silence, when treating of objects fitted for the marine Aquarium,
although, in a little book of this extent, many others must of necessity
be omitted. This beautiful little creature, _Cypræa Europæa_, carrying
its porcelain-like dwelling on its back, is enabled to move steadily
along by the action of its single foot. When in action, the mantle, as
it is termed, which is the general covering of the body, is greatly
distended, and protrudes from the shell, which it perfectly encloses,
folding up at the sides, and meeting at the top, the joining being
scarcely perceptible, and the whole surface fitting so tightly to the
shell, that the little ribbings are seen distinctly through it. It is
curious to observe the act of respiration, and all the associated
phenomena of this wonderful little animal. The foot is pale orange, the
mantle delicate olive, spotted with black and studded with protruding
glands of yellow. It is, in short, when in a state of activity, a most
curious and beautiful creature, of whose appearance and habits
thousands, who only know and admire the deserted shell, can have no
idea.
The bivalves, of which all are acquainted with at least one kind—the
delicious edible Oyster—offer many animals suited to Aquaria. The
curious Razor-shell, but for its habit of burrowing, would form a very
curious object; and the Cockles, from the rich colour of their beautiful
fringes, when the shells are partially opened for feeding and breathing,
are very beautiful objects.
The means of movement of the common Scallop, or Cockle, and other
bivalves, by means of a single fleshy “foot,” have been described in
speaking of Molluscs in general; but the spinous Cockles, _Cordium
aculeatum_ and _C. tubercutum_, have been termed the aristocracy of the
Scallop tribe. The valves of the largest open three-quarters of an inch,
and the visible portion of the spongy-looking fleshy mantle is of a
pellucid orange colour; at the end is protruded a double tube, thick and
short, enveloped in a fringe of cirrhi or tentacles. The foot, which has
been compared to a tongue, is smooth, glossy, and semi-transparent, like
scarlet cornelian, and enables the creature to move about with great
activity in an Aquarium; some that were sent, by the Rev. C. Kingsley,
to Mr. Gosse, having startled that gentleman by the noise they were
creating among the pebbles and other objects of his tank, by their rapid
movements.
Many other kinds of shells might be mentioned if space permitted, but I
must content myself with mentioning, _en passant_, the polished _Donax_,
which, when the animal displays itself, exhibiting its bright yellow
colouring, with its curious stripes and gay pink fringes, would form a
real ornament to the drawing-room sea; and just hinting at the
introduction of a specimen of the _Tritonia Hombergi_, remarkable for a
power of producing an audible sound like the click of a steel wire.
The pretty little bivalve, the _Lima hians_, also forms a very
attractive addition to the Aquarium, especially in motion, when its long
orange fringes form a train or tail like that of a fiery comet, as Dr.
Landsborough has observed, as it glides along, propelled by the
discharge of a jet of water, the mechanism for the propulsion of which
forms its swimming apparatus.
[Illustration]
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