CHAPTER VI.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, ofthe 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 aMany-ribbed harp, aGloria maris, aCedo nulli, or aVoluta 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 habitsand 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, frommollis, 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 theCephalopoda, the organs of sight and hearing are both well developed; and Professor Owen considers that the Nautilus possesses even an organ of passive smell. TheGasteropodatoo, are, according to Siebold, nearly all furnished with ears and eyes, the former organs being describedas forming round capsules, conspicuously visible near the roots of the tentacles.
Some of theConchifera, 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 theGasteropoda, 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. TheStrombus gigas, a soft, snail-like creature, carries a shell which often weighs more than five pounds; theCassis tuberosasupports 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 bythe 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 ofshell-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 ofthe 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. TheCephalopoda, 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]ThePteropoda, however, in their little shells, translucent as glass, swim by the action of small fin-like paddles placed near the head.
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.
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.
TheBivalvesdo not make so clever a use of their single foot as theUnivalves. 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; thisappears likely to produce but a slow rate of progress, yet some of the sand-boringBivalvesmanage, when alarmed, to conceal themselves with great rapidity by that means.
TheMollusca, 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, theDunicata,Brachiopoda, andConchifera, 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 carnivorousUnivalves, for instance, feed upon theBivalvesby drilling a hole through the solid shell, and withdrawing the animal piecemeal, as required.
TheEolis papillosahas been observed tearing away the tentacles of different species of Sea-Anemone with extraordinary voracity, and the tribe must therefore very evidently be excludedfrom 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 hundredPolypiandSepiæ, 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, theLittorina 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 ofvariety, 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, ornamentedwith 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 tofit, 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 asAdamsia 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, theAdamsiais found outside; and the Hermit is seldom without his dinner assistant, the prettily-stripedNereis. 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, theActinia parasitica, often selects the back ofthe Crab himself (generallyPaguras 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 whenBernhardusbecomes 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 theFusus 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 familyMuricidæ, some of which, natives of our own coasts, are very pretty objects for the Aquarium. It was theMurex trunculuswhich yielded the Tyrianpurple, 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 theMurex, 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.
ThePurpura lapillusof 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, containsthe 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 theAplysia, 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 commonPlanorbis 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 theGreeks and Romans. It is a kind ofSepia, in fact, that still furnishes the rich brown colour which bears the name of the animal from which it is derived.
The commonSepia vulgarismight 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 littleCephalopodhas 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 whenalarmed. 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 theTrochustribe 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 pearlyTrochus, the animal of which is of a rich orange colour striped with black, moves freely about, and forms a very attractive object. The animal ofT. granulatusis larger and handsomer, but shy, and displays little activity in confinement. The smallTrochus,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 appearat 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 namedCyclobranchiata, 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 theArca tetagrona, should be tried in Aquaria, as well as the elegant Heart-shell of the beautiful genusIsocardia; 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, fromthe 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 aculeatumandC. 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 polishedDonax, which, when the animal displays itself, exhibiting its brightyellow 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 theTritonia Hombergi, remarkable for a power of producing an audible sound like the click of a steel wire.
The pretty little bivalve, theLima 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.