Natural History, Mollusca/Mollusca
The four Classes of animals which have been considered in the preceding volumes of this series we have seen to have one character in common; viz. the possession of a bony framework within the body, of which a jointed spine is the most essential element. This character, which unites those four Classes into one great group, and gives to that group the name Vertebrata, by which it is distinguished among naturalists, we have seen, however, by slow degrees, deteriorated, if I may use such an expression, from bone to cartilage, and gradually diminished in its development, until, in the lowest of the Fishes, it can scarcely be recognised at all.
I come now to treat of animals in which the bony skeleton no longer exists. The conditions of their existence do not require such a scaffolding on which to build the constituent muscles: many are habitually immersed in water, a fluid the density of which supports their soft bodies; their motions generally lack the precision, energy and variety of those which belong to Vertebrate animals; and where this is not the case, as in the Articulate Classes, the skeleton which affords attachment to the muscles, is not internal, but invests the body, while its substance differs essentially from bone in its chemical composition and its structure.
An immense assemblage of living creatures are included in this category; creatures differing widely from each other in the most important characteristics, so that they cannot be grouped together. The term Invertebrata, by which they are sometimes designated, indicates indeed only a negative character, and we shall be greatly mistaken if we suppose (misled by such a term) that the animals which have a skeleton, and those which are destitute of one, constitute two primary divisions of living beings, of equal or co-ordinate importance.
Several divisions of Invertebrate animals do, in fact, exist, each one of which is equal in rank to the Vertebrata. One of these will form the subject of the present volume, commonly known by the name of Mollusca; a term invented by the illustrious Cuvier, from the word mollis (soft), and evidently suggested by the softness of their boneless bodies. The appellation can scarcely be considered happy, for the character so indicated is very trivial, and is shared by other animals of totally different structure:—objectionable, however, as it is, it has been generally adopted, and I shall not hesitate to make use of it.
As the great Vertebrate Division includes the four distinct Classes of Beasts, Birds, Reptiles and Fishes, so does the great Division of Mollusca contain six Classes, distinguished by characters which I shall presently enumerate. I must, however ever, first indicate those which they possess in common, and by which they are naturally grouped together.
The nervous system demands our first attention. Instead of a great mass of nervous substance accumulated in one place, and a lengthened spinal cord proceeding from it, giving out threads to all parts of the body, as in the Vertebrata, we find the nervous centres numerous, unsymmetrical and disposed in various parts of the system, no one having so decided a predominance over the others in bulk, as to merit the appellation of a brain. There is, however, one mass larger than the rest, which is always placed either above the gullet (œsophagus), or encircling it, in the form of a thickened ring; and from this the nerves that supply the organs of sense invariably originate. This mass, or ganglion, must undoubtedly be regarded as the representative of the brain; for in the most highly organized animals of the Division, the Squids and Cuttles (Cephalopoda), this encirling mass is enclosed and defended by a case of cartilage, the lingering rudiment of a bony skull.
The accompanying engraving, which is copied from Professor Grant's "Outlines of Comparative Anatomy," will give the reader an idea of the system of nerves and ganglia, with some of the other organs, as they appear in Bulla lignaria, a large and handsome shelled Mollusk found on the British coasts.
In the above figure the chief ganglion forms a ring, (marked ee,); anterior to this there is a small ganglion, not seen, because situated below the bulb of the gullet (a), just before the insertions of its diverging muscular bands (c), and behind the salivary glands (b). The brain-ring (e) has on each side a large three-lobed ganglion (f), whence numerous nerves pass to the surrounding parts, and two long branches (h) extend backwards along the sides of the abdomen, to two ganglia (i,i), placed above the muscular foot.
NERVES OF BULLA Behind these are two sympathetic ganglia, (k, k), which send threads to the digestive system, the ovary (o), the oviduct (p), the uterine sac (q) , the vulva (m), and the urinary organs (n). This may be considered as a fair average sample of the nervous system in the Mollusca, being selected from a Class presenting neither the highest, nor the lowest forms of organization.
The nervous centres are, for the most part, grouped without regard to symmetry, those of one side not corresponding to those of the other; and this irregularity is characteristic of the whole Division, not only in the nerves, but in the other organs of the body. Some zoologists have derived from this peculiarity, a name for the Division, sufficiently expressive, though too uncouth for general adoption, that of Heterogangliata.
All the senses common to the higher animals are found in the Mollusca, though some are, doubtless, wanting in the humbler Classes of the Division. In the Cephalopoda, the organs of sight and hearing are distinct and well developed, and Professor Owen is of opinion that the Nautilus, an animal of this Class, possesses an organ of "passive smell." The Gasteropoda are almost invariably furnished with eyes; and, according to M. Siebold and other zoologists, with ears also, a pair of round capsules, placed near the bases of the tentacles, and enclosing one or more crystalline globules, called otolites. Some of the Conchifera are furnished with numerous eyes, placed among the tentacles, examples of which are found in the Clams and Scallops (Pecten) of our own shores. I scarcely know a more beautiful sight of the kind, than is presented by the edges of the mantle in one of our Scallops. If you ever have an opportunity of procuring a living specimen, which is not difficult to find at low water, on most of our rocky shores, place it in a glass of sea-water, and watch its movements. Soon the beautiful painted shells will begin to open, and the fleshy mantle will be seen to occupy the interval, like a narrow veil extending perpendicularly from each shell. The edge of each of these veils will now be seen, if you examine it with a pocket lens, to be fringed with long white threads, which are the tentacles, or organs of touch; and amongst them lie scattered a number of minute points, having the most brilliant lustre, and bearing a close resemblance to tiny gems. Indeed, the mantle has been aptly compared to one of those pincushions which are frequently made between pairs of these very shells, the eyes representing a double row of diamond-headed pins, set round the middle. It is observable that the Bivalves, which are thus profusely furnished with eyes, are also
THE GREAT SCALLOP.
endowed with the faculty of precise and vigorous motion. It does not appear clear that any of this Class possesses a distinct sense of hearing.
The faculty of taste is plausibly conjectured, rather than proved, to belong to the Mollusca. "It seems necessary," says Dr. Johnston, "to suppose the existence of this sense in all Mollusca, for they select particular articles of food in preference to others; and we know no other sense which is fitted to regulate the choice." The organs appropriated to this faculty are probably the margins and internal surface of the mouth, and the tentacles which in some species are placed close to this orifice.
Every one who has touched a crawling Slug or Snail, must have had a practical proof of the delicacy of its sense of touch. The whole surface of the body, invested with a soft, flexible, and mucous skin, contracts on the slightest contact with any unexpected substance, and is, doubtless, an extended organ of feeling, probably much more sensitive than the naked skin of our bodies. But, besides this, most, if not all of these animals are furnished with organs of special touch called tentacles, which serve to collect and convey impressions of the proximity, the form, the hardness, and perhaps other qualities, of those bodies which the animals may desire to investigate. The mantle, also, in many of the Gasteropoda, is fringed with a number of filaments, often curiously branched, which are probably accessary organs of touch.
The sensitiveness manifested by some of the large Gasteropoda, the great Conchs of the West Indies, for example, to the presence of other bodies, even without contact, and which the Rev. Lansdown Guilding attributed to the sense of hearing, may, perhaps, rather be considered as a modification of feeling, capable of appreciating the pulsations of the atmosphere. The experiments of this naturalist, not to be vindicated from the charge of cruelty, are thus described. "I lately suspended," he says, "a number of large Strombi by the spire, that the animal, when dead, might fall from the shell. They had remained in this situation several days, till the body, weak and emaciated, hung down nearly a foot from the aperture, and the eyes had become dim. I found that even before my shadow could pass over them, they were aware of my presence, and endeavoured to withdraw into the shell. I then cut off the eyes, with the thick cartilaginous tentacula in which they were lodged, but the animals still continued to be sensible of my near approach, while hanging in this mutilated and painful condition." Dr. Johnston records a manifestation of feeling, somewhat analogous to this, in one of the common shellfish of our sandy shores. "On a summer evening," he remarks, "I have observed the common Spout-fish (Solen siliqua), extended along the surface of the fine sand in which they burrow, enjoying, apparently, the calmness and mildness of the season, take alarm and instantaneously descend when I was yet distant several yards: and I can explain this and similar facts only on the supposition of the existence of a sense of touch feelingly alive to impressions impalpable to our grosser sense."
The respiration is aquatic in most of the Mollusca. The breathing organs, in most cases, resemble in essential points the gills of fishes, consisting of a great number of leaves, often minutely subdivided. They are chiefly formed of blood-vessels, covered with rows of vibrating cilia, by the constant motion of which, currents of water are perpetually hurled along the entire surface of the breathing organ, communicating oxygen, the vital principle, to the blood as they go, through the thin walls of the vessels. In many species, as the Bivalves, the gills form two large comb-like plates; in others they are arranged in the form of a feather; a beautiful tribe, known as naked-gilled, have these organs placed on the outside of the body, sometimes forming prominent warts or papillœ, disposed in rows, or in tufts, sometimes resembling little branching trees, and at others, arranged as a number of elegant plumes, set, like the petals of an exquisite flower, around a circle.
In the Tunicata, examples of which may be found on our rocky beaches, closely adhering to the under surface of stones at low water, and looking like shapeless masses of a substance something between gristle and jelly,—the breathing organ is developed to a very great extent. It occupies a capacious chamber in the interior of the animal, the two sides of which are studded on their inner surfaces with little oval cells, arranged in a regular pattern of rows. Each of these cells is formed by an oval ring of cilia, which, when in full play, present a most beautiful and interesting spectacle. The accompanying figure, taken from the life, is a magnified representation of a tiny creature, not larger than a pin's head, but as transparent as the purest crystal. The oblong rings conspicuously seen are the ciliary cells of the breathing organ; but no figure can convey an adequate impression of the beauty of the sight, when the observer gazes upon forty or more of these ovals, all set round their interior with what look like the cogs on a watch-wheel, dark and distinct, running round and round with an even, moderately rapid, ceaseless motion.
One large tribe of the Gasteropoda comprises animals, however, which breathe air, and are terrestrial in their habits. Of these the Slugs and Snails of our fields and gardens afford familiar examples. The delicately-formed, and often brilliantly-painted shells, which throng the damp woods in tropical countries, likewise belong to this group, and furnish the most highly prized treasures of our conchological cabinets. In these the air is
inhaled into an ample chamber, the interior of which is lined with cilia. After parting with its oxygen, it is again expelled; the common orifice for both processes being situated on the side of the body. Any one may have ocular evidence of the existence of this organ, by watching our common Garden Snail. If you look at its right side, just behind the tentacle, or horn, that carries a black eye at its point, you will see a large hole suddenly open, where before there was no trace of it. After remaining open for a few moments, the margin will leisurely contract again, until it is perfectly closed, and as invisible as before. This is the breathing orifice; and during the interval that you saw it open, the aerial contents of the chamber were expelled, and a copious draught of fresh air was inspired. The process is repeated with tolerable regularity about once every fifteen seconds.
The blood in the MOLLUSCA is thin, transparent, and colourless; or at most presents only a pale bluish-white hue. It is, however, contained in a system of distinct vessels, through which it circulates, having for the source of its motion a well-developed, complex, pulsating heart.
Besides the system of vessels which carry the blood, there is another system, most conspicuous in the aquatic tribes, which has been called the system of aqueducts. They communicate with the element in which the animal lives and moves, and are filled with it at will, as the galleries and canals of a sponge are filled with the liquid in which it is immersed. The chief use of these water-canals appears to be the distension and expansion of the foot, to render it better fitted for locomotion, yet so as not to interfere with the privilege, essential to most of these animals, of withdrawing the whole of the body within a shell. Some of the marine Mollusca, when in a state of activity, protrude a soft foot, far exceeding in dimensions the whole bulk of the shell; yet let the creature be disturbed, and the whole is suddenly withdrawn into the cavity, so completely that not a trace of it is visible. "When shrunk within its shell," observes Dr. Johnston, "you might well deem any animal that could hide itself there, all too small and weak to carry about a burden larger and heavier than itself,
THE GIANT STROMBUS.
and that safety might be here advantageously exchanged for relief from so much heaviness of armour, and from such an impediment to every journey. There is in my small cabinet a fine specimen of Cassis tuberosa, which measures fully ten inches in length, and upwards of eight in breadth, another of Strombus gigas is nearly one foot in length. The weight of the former is four pounds two ounces; that of the latter, four pounds nine ounces; yet the snail creeps under this load at apparent ease. Nor are you much surprised when you see it actually in motion, for the seeming disproportion between the contained animal and containing shell has disappeared. On issuing from its shell, like an Eastern Genii freed from his exorcism, the animal has grown visibly, — has assumed a portlier size and more pedestrious figure. The body has suddenly become tumid and elastic, the skin and exterior organs stretched and displayed, the foot has grown in length and in breadth, and, with additional firmness, it has acquired at the same time the capability of being directed, bent, and modified in shape, to a considerable degree, as the surface of the road traversed may require. Thus it is with nearly all the cephalous mollusca; and by a similar disposition of aqueducts, the foot of the Bivalves is equally adapted to every act subservient to their locomotion and more especially to the act of burrowing; for had the foot not been so framed as to permit of an enlargement superior to the size of the shell, it seems obvious that the furrow could not have been made large enough to contain the latter. The same, too, with many Gasteropods which burrow in the sand when in search of prey. The Buccina and most carnivorous mollusca have this ability, dependent on the system of aqueducts we have been describing; and you must observe, that from the manner in which the shell is attached to the body by the large retractor muscle, it so happens that this is drawn into the furrow always with the notch in the aperture uppermost, so that, when completely buried, the animal is still enabled to communicate with the water by its respiratory siphon."
Beyond the rudimentary strip of cartilage that in some of the Cephalopoda represents the vanishing spine of the Vertebrata, the Mollusca have no internal skeleton. But in the great majority of cases, the soft parts are protected and supported by what we may call an external skeleton, of the substance familiar to us as shell. Lime is the essential element of this substance, as it is also of bone: but shell is a carbonate of lime, while the earthy part of bone is a phosphate.
When we consider the beauty and variety that are presented by shells, the important part they play in the economy and habits of the animals, and the use that is made of them in systematic arrangement, it becomes a question of high interest to inquire in what manner they are formed.
"The shells themselves are absolutely deprived of vitality, permeated by no vessels, and as incapable of expansion by any internal power as the rocks to which they are not uncommonly attached; so that the young naturalist is necessarily at a loss to conceive either the mode of their formation, or the origin of all the gaudy tints and external decorations that render them the ornaments of our cabinets.
"The simple apparatus by means of which shells are constructed, is the external membranous layer that invests the body of the mollusk,—the mantle, as it has been termed; and, whatever the form of the shell, it owes its origin entirely to this delicate organ. . . . .
"It is the circumference, or thickened margin of the mantle alone which provides for the increase of the shell in superficial extent. On examining this part, it is found to be of a glandular character, and moreover not unfrequently provided with a delicate and highly sensitive fringe of minute tentacula. Considered more attentively, it is seen to contain in its substance patches of different colours, corresponding both in tint and relative position with those that decorate the exterior of the shell.
"When the animal is engaged in increasing the dimensions of its abode, the margin of the mantle is protruded, and firmly adherent all round to the circumference of the valve with which it corresponds. Thus circumstanced, it secretes calcareous matter, and deposits it in a soft state upon the extreme edge of the shell, where the secretion hardens and becomes converted into a layer of solid testaceous substance. At intervals this process is repeated, and every newly-formed layer enlarges the diameter of the valve. The concentric strata thus deposited remain distinguishable externally, and thus the lines of growth marking the progressive increase of size may easily be traced.
"It appears that at certain times the deposition of calcareous substance from the fringed circumference of the mantle is much more abundant than at others: in this case ridges are formed at distinct intervals; or, if the border of the mantle at such periods shoots out beyond its usual position, broad plates of shell, or spines of different lengths, are secreted, which, remaining permanent, indicate, by the interspaces separating successively deposited growths of this description, the periodical stimulus to increased action that caused their formation.
"Whatever thickness the shell may subsequently attain, the external surface is thus exclusively composed of layers deposited in succession by the margin of the mantle, and, seeing that this is the case, nothing is more easy than to understand how the colours seen upon the exterior of the shell are deposited, and assume that definite arrangement characteristic of the species. We have already said that the border of the mantle contains, in its substance, coloured spots: these, when minutely examined, are found to be of a glandular character, and to owe their peculiar colours to a pigment secreted by themselves; the pigment so furnished being therefore mixed up with the calcareous matter at the time of its deposition, coloured lines are found upon the exterior of the shell wherever these glandular organs exist. If the deposition of colour from the glands be kept up without remission during the enlargement of the shell, the lines upon its surface are continuous and unbroken; but if the pigment be furnished only at intervals, spots or coloured patches of regular form, and gradually increasing in size with the growth of the mantle, recur in a longitudinal series wherever the paint secreting glands are met with. . . . .
"While the margin of the mantle is thus the sole agent in enlarging the circumference of the shell, its growth in thickness is accomplished by a secretion of a kind of calcareous varnish, derived from the external surface of the mantle generally; which, being deposited layer by layer over the whole interior of the previously existing shell, progressively adds to its weight and solidity. There is, moreover, a remarkable difference between the character of the material secreted by the marginal fringe, and that furnished by the general surface of the pallial membrane; the former we have found to be more or less covered by glands appointed for the purpose, situated in the circumference of the mantle, but as these glands do not exist elsewhere, no colouring matter is ever mixed with the layers that increase the thickness of the shell, so that the latter always remain of a delicate white hue, and form the well-known iridescent material usually distinguished by the name of nacre, or mother of pearl."
This lucid description of the process specifically applies to the Conchifera, or Bivalves; but the formation of the shell in the Gasteropoda is not marked by any important point of difference.
No species of this great Division of animals is furnished with limbs, properly so called: unless we may consider as such the long flexible tentacular arms of the Poulpes and Cuttles (Cephalopoda) which are used as instruments of an ungainly sort of crawling, as well as for seizing prey and dragging it to the mouth: yet various modes of locomotion are by turns practised among the Mollusca. In one extensive Class, the Gasteropoda, of which the Limpet and the Snail are examples, an even gliding movement is that which is most characteristic; a broad expanded muscular disk, called the foot, being applied to the surface over which the animal crawls. Many of the aquatic members of the Class are able to float at the surface by the aid of the same organ. They crawl to the top of the water up the stem of a plant, or the side of a rock, and stretching out the bottom of the foot along the surface, the back being downward, it presently dries by contact with the air. While it remains dry, it will float the animal, which then glides along as if on a solid body, crawling in fact, on the inferior surface of the air: but if by any agitation of the water, or by the will of the animal, the foot become overflowed, the state of suspension is ended, and the creature falls to the
POULPE AND CALAMARY.
bottom. The Pond-snails (Limneus), with olive-coloured fragile shells, that inhabit every little pool and lakelet, may be seen in a summer's day, by scores, enjoying the air in this manner.
The wide ocean between the tropics is inhabited by a shell, resembling in appearance that of a Snail, but tinged with blue (Ianthina), which is furnished with an elaborate apparatus for swimming,—
"Like little wanton boys that swim on bladders."
To the hinder part of the foot is attached a kind of float, consisting of many small bladders of thin membrane, united in a group, and looking somewhat like coarse froth. By means of these, the shell floats securely on the broad sea.
Another bladder-swimmer, and like the former, an ocean-species, is the Litiopa. "This is a small snail, born amid the gulf-weed, where it is destined to pass the whole of its life. The foot, though rather narrow and short, is of the usual character, and, having no extra hold, the snail is apt to be swept off its weed; but the accident is provided against, for the creature, like a spider, spins a thread of the viscous fluid that exudes from the foot, to check its downward fall, and enable it to regain the pristine site. But suppose the shock has severed their connexion, or that the Litiopa finds it necessary to remove, from a deficiency of food, to a richer pasture, the thread is still made available to recovery or removal. In its fall, accidental or purposed, an air-bubble is emitted, probably from the branchial cavity, which rises slowly through the water, and as the snail has enveloped it with his slime, this is drawn out into threads as the bubble ascends; and now, having a buoy and bladder whereon to climb to the surface, it waits suspended until that bubble comes into contact with the weeds that everywhere float around."
A species of Cerithium, found at the mouths of rivers in tropical countries, has the silkworm-like habit of spinning threads, by which it suspends itself from the mangrove-roots; and our own, freshwater Snails have the power of suspending themselves in the same manner. Mr. Warington, in an interesting paper on the habits of some aquatic animals kept in confinement, thus records the curious fact:—
LIMNEUS AURICULARIS."In watching the movements of the Limnei, I was for some time under the impression that they had a power of swimming or sustaining themselves in the water, as they would rise from the bottom of the pond, a portion of the rockwork, or a leaf of the plants, and float for a considerable period, nearly out of their shells, without any apparent attachment, and by the contortions and gyrations of their body and shell, move some little distance, in a horizontal direction, from the point which they had left. On more carefully watching this phenomenon, however, I found they were attached by a thread or web, which was so transparent as to be altogether invisible, and which they could elongate in a similar way to the spider: they also possessed the power of returning upon this thread by gathering it up, as it were, and thus drawing themselves back to the point which they had quitted. These facts were clearly proved in the following manner: A Limneus stagnalis had glided its way along a young and short leaf of the Vallisneria, which terminated below the surface of the water, and having readied the extremity, launched itself off from it; after moving about with a sort of swimming or rolling motion in a horizontal direction for some time it lowered itself gradually, and in effecting this, the long flexible leaf of the Vallisneria was bent with an undulating motion, corresponding exactly with every movement of the snail, clearly showing that it had a firm attachment to the extremity of the leaf. On another occasion a L. glutinosus gradually rose from the surface of a piece of submerged rock, and when at the distance of about three or four inches from it, stayed its progress, floating about in a circumscribed horizontal direction for some time; at last it rose suddenly and rapidly to the surface, evidently from the rupture of its thread of attachment. The most convincing proof, however, of this fact, that I can, perhaps, adduce, and one that I have often repeated with all the before-mentioned Limnei, is that when the snail has been some inches distant from the supposed point of attachment, a rod or stick has been carefully introduced, and slowly drawn on one side between them in a horizontal direction, and by this means the snail can be made to undulate to and fro, obeying exactly the movement of the rod: this requires to be done very gently, as, if too much force is used, the web is broken, and the snail rises rapidly to the surface."
The wide expanse of ocean from the equator to the poles is tenanted by a class of swimmers, small, indeed, in the number of its species, but countless in the hosts of individuals of which they are composed; the Pteropoda. Some of these inhabit shells, which for delicacy and transparency, the thinnest glass. They possess a pair of large membranous fins, which closely resemble the wings of a butterfly, and by using these organs in a flapping manner, the little animals swim briskly about.
Perhaps some of my readers will expect me to include the beautiful Paper Nautilus (Argonauta), among swimming Mollusks; seeing that the poets have claimed for it the honour of teaching navigation to man:—
"Learn of the little Nautilus to sail,
Montgomery, the poet of the ocean, thus beautifully expresses the popular notions concerning it:—
"Light as a flake of foam upon the wind,
The accuracy of modern research, however, has proved this to be but a pleasant fable. The Argonaut is a Cuttle-fish, and crawls along the bottom, like its fellows, by means of its slender, flexible, tentacular arms, as represented in the preceding engraving, (fig. 2); while the pair that are furnished with a broad fleshy disk, have an office very different from that of sails, namely, that of forming, repairing, and protecting the thin and papery shell. (See fig. 3.) Its only swimming power appears to be that which it possesses in common with all Cephalopoda, of shooting along in a backward direction, by the force of a jet of water from the funnel, as shown at fig. 1, where it is represented as swimming towards the point a.
Among the Tunicata there are some singular tribes which swim freely in the sea. "The , translucent as their native waters, and often united in chains, after a pattern peculiar to each species, are driven along the surface with considerable quickness by alternate contractions and expansions, and by the propulsion they receive from a current of water, which is made continually to traverse the long diameter of the body, sucked in by the posterior aperture, and issuing in a stream through that on the side of the mouth. Hence the body is always pushed backwards—a circumstance that has misled some naturalists to describe the posterior aperture for the true mouth. The Pyrosomata are a still more singular family of the same order. Each seeming individual of this genus is, in fact, a numerous colony of little mollusca, every one in its own cell, distinct, yet inseparably connected with its fellows. Collected into the figure of a gelatinous cylinder, open at one extremity and closed at the other, and roughened externally by a multitude of tubercles disposed sometimes in rings and sometimes irregularly, they float in the Australian seas like stars of this lower world, shedding around them a halo of light, brilliant indeed, but surpassed in beauty by those other colours of the creatures which it serves to disclose; colours which come and go at pleasure, glorying as it were, in their subtle changes, passing rapidly from a lively red to aurora, to orange, to green, and to azure blue; a magic scene, compelling more than the admiration of every beholder."
Bivalve Mollusca in general have much less power of shifting their locality than Univalves. Many appear to be absolutely stationary, at least during their adult existence. But others, as the Cockle, have a most versatile organ, known as the foot, capable of being protruded from between the valves, which, among its various uses, serves the purpose of locomotion. It is in general applied in this manner. Being stretched out to its utmost extent, its point is made to hook downward into the sand or mud, and the body with the shell is then dragged down by the muscular contraction of the foot. In most cases, this mode of progression is sufficiently slow and awkward, but some of the sand-borers are able to conceal themselves thus with surprising rapidity.
Others of this Class are vigorous leapers; and of some the bounds are so vivacious, forcible and sudden, that they might almost be compared with the flight of a bird, or the shooting of a fish. The Clams or Scallops (Pecten) and their elegant relatives the Limæ, are eminent among our native species for this faculty. The mode in which the leaps are effected is always described as being the opening and sudden closing of the valves or shells; but, in the case of the former genus, I have recently found that the real organ of motion is the mantle. The edges of this being firmly closed, when the interior is filled with water, the fluid is forcibly ejected from the lips, which are relaxed for the purpose at any point according to the will of the Scallop; and by the jet of water striking on the surrounding element, the whole animal is shot to a considerable distance in the opposite direction.
Most of the MOLLUSCA are, as I have already intimated, inhabitants of the waters, and these are divided between the seas and the freshwaters, in a ratio somewhat like that which these divisions of the waters of our globe bear to each other. Of the marine kinds some dwell permanently and exclusively at the bottom of the deep sea, whence they are to be procured only by dredging. A species of Crania has been brought up from a depth of 255 fathoms. Others inhabit the open ocean, habitually or occasionally swimming on the top of the waves, or resting on the floating sea-weeds of warm latitudes. Many species confine themselves to the vicinity of the shore, where each peculiar situation and locality has its proper kinds. The sands, the ooze and mud of harbours, the boulders and loose stones of the wave-washed beach, the sides, ledges, and pools of rocks, uncovered at the recess of every tide, are all inhabited by species peculiar to the respective locality. Some species strictly marine are able to endure protracted exposure to the air, as the Periwinkle and the Limpet, the most familiar of shell-fish, which every visitor to the sea-side habitually sees clustered on the rocks close to the limits of high-water mark.
The fresh-water Mollusca manifest a similar choice of situation, though a less latitude is permitted for its exercise. Some are peculiar to large rivers, some to estuaries, others to lakes, and yet others to small ponds and ditches.
In general the habits of one species of a genus when ascertained, are found to indicate those of all its fellows of the same genus; as for instance not only is our common Cockle (Cardium edule) a burrower in the ooze at the margin of the sea, but all other species of the genus Cardium have similar habits of life. Yet Mr. Gray has enumerated a considerable catalogue of species, which break this rule, classing them under four divisions. 1st, where species of the same genus are found in more than one kind of situation, as on land, in fresh and in salt-water; 2nd, where one or more species of a genus, most of whose species inhabit fresh-water, are found in salt or brackish water; 3rd, where, on the contrary, one or more species of a genus, whose species generally inhabit the sea, are found in fresh-water; and 4th, where the same species is found both in salt and fresh-water." 
Of those species which dwell upon the land and breathe the air, most affect moist situations. The common Garden Snail, as is well known, retires to crevices and corners in continued dry weather, where it closes its shell with a temporary door to prevent the evaporation of its vital juices, and patiently waits the return of congenial humidity. The first shower prompts the sensitive creatures to venture abroad, and we see them crawling by dozens over our borders and garden walks, imbibing from the steaming surface the grateful moisture. The damp woods of warm countries are the situations which most reward the researches of the laborious collector of land-shells; but there are some which are found in the driest places, as stony plains, and the summits of arid hills.
Many terrestrial Mollusca which ordinarily inhabit moist places, are enabled, by a precaution similar to that adopted by our own Snail in drought, to sustain life, in such a state of retirement and suspension of their usual habits, not for a few days or weeks only, but even for many years. Numerous examples have occurred in which the land-shells of distant countries have been brought to England, alive but torpid, and have been kept shut up in drawers for twelve, eighteen, and even twenty months; manifesting no signs of life until moistened, when they presently crawled about, and began to eat. But the most singular example of this protracted sleep on record, is that of Mr. Simon's Snails, which must surely have been the veriest Rip Van Winkles among Mollusca. The following account is from the Philosophical Transactions; and the facts seem to have been carefully investigated, and well authenticated:—
"Mr. Stuckey Simon, a merchant of Dublin, whose father, a fellow of the Royal Society, and a lover of natural history, left to him a small collection of fossils and other curiosities, had among them the shells of some snails. About fifteen years after his father's death (in whose possession they continued many years), he by chance gave to his son, a child about ten years old, some of these snail-shells to play with. The boy put them into a flower-pot, which he filled with water, and the next day into a basin. Having occasion to use this, Mr. Simon observed that the animals had come out of their shells. He examined the child, who assured him that they were the same he had given him, and said he had also a few more, which he brought. Mr. Simon put one of these into water, and in an hour and a half after, observed, that it had put out its horns and body, which it moved but slowly, probably from weakness. Major Vallancy and Dr. Span were afterwards present, and saw one of the snails crawl out, the others being dead, most probably from their having remained some days in the water. Dr. Quin and Dr. Rutty also examined the living snail several different times, and were greatly pleased to see him come out of his solitary habitation after so many years' confinement. Dr. Macbride and a party of gentlemen at his house, were also witnesses of this surprising phenomenon. Dr. Macbride has thus mentioned the circumstance: — 'After the shell had lain ten minutes in a glass of water that had the cold barely taken off, the snail began to appear, and in five minutes more we perceived half the body pushed out from the cavity of the shell. We then removed it into a basin, that the snail might have more scope than it had in the glass; and here, in a very short time, we saw it get above the surface of the water, and crawl up towards the edge of the basin. While it was thus moving about, with its horns erect, a fly chanced to be hovering near, and, perceiving the snail, darted down upon it. The little animal instantly withdrew itself into the shell, but as quickly came forth again, when it found the enemy had gone off. We allowed it to wander about the basin for upwards of an hour, when we returned it into a wide-mouthed phial, where Mr. Simon had lately been used to keep it. He presented me with this remarkable shell, and I observed, at twelve o'clock, as I was going to bed, that the snail was still in motion; but next morning I found it in a torpid state, sticking to the side of the glass."
In treating of the food of the Mollusca, Dr. Johnston divides them into three classes; first, those which take their food in a liquid form, or suspended in water; secondly, those which are more properly carnivorous; and, thirdly, those which feed on vegetable matter.
Under the first division are comprised all those which have no distinct head, including the three classes, Tunicata, Brachiopoda and Conchifera. None of these have any power of pursuing prey, nor any organs for mastication. Yet any one who has ever examined with a microscope, either the sea-water, which appears to the naked eye pure and simple, or the impalpable sediment which lies upon the bottom, will be at no loss to discover abundant organic matter fitted to supply nutriment to these headless, generally stationary, and apparently helpless creatures. Countless millions of Infusorial animalcules sport in the clear water, altogether unappreciable by our senses, while vegetables clothed with flinty shells, the Diatomaceæœ of botanists, equally numerous and equally minute, crowd the mud on the floor of the sea.
In order that these minute bodies should afford nutriment to the headless Mollusca, a simple but effective contrivance is provided. The currents which ceaselessly play over the breathing organs, produced by the cilia which cover them, not only bring water to be respired, but come charged with the various organic particles, both animal and vegetable, that occur in the vicinity. It is, therefore, merely necessary that the orifice of the stomach, which for convenience sake may be called the mouth, be situated in the course of the currents, and be endowed with the power of selecting and retaining such substances as are suitable for digestion.
The remaining classes divide themselves into flesh-eaters, and those which live upon vegetable diet, the preponderance, however, being, as well as can be estimated, rather with the former. Not a few of the Univalves feed upon their Bivalve relatives, not seizing the opportunity, as has been pretended, of killing their victim as it lies incautiously with gaping shells; but by drilling a small hole through one of the valves, and extracting the fleshy parts, particle by particle. Some species devour dead fishes and other putrifying animal matters with avidity. Many of the elegant naked-gilled tribes prey on each other, though their proper food consists of zoophytes. I have found the large Eolis papillosa tear away the tentacles of different species of sea-anemones, which seemed to be its natural food.
The Cephalopoda, including the Cuttles, the Poulpes, and the Squids, are fierce and predatory, the tyrants of the deep. Furnished with many long arms, stretching in all directions, and studded with rows of adhesive suckers, they seize with ruthless grasp any passing fish or other animal, whose strength is inferior to their own, and drag it to a hard and sharp horny beak, the mandibles of which resemble those of a parrot's bill, and being moved by powerful muscles are enabled either to crush the shells in which their victim may be enclosed, or to tear it to pieces if it be a fish, or other animal of muscular or sinewy tissues.
In speaking of the vegetable-feeding Mollusca, the ravages committed by those pests of our gardens, the Slug and Snail, will occur to every one. Other species of the same genera are never or rarely seen in gardens, but devour the herbage of the roadside, the bank, or the hedge. Many, particularly those which inhabit the woods of foreign countries, devour the leaves of trees. The plant-eaters among the marine tribes live upon the various kinds of sea-weeds, of which there is a sufficient variety to gratify a taste much more epicurean than it probably is in reality. The common Periwinkle and the Limpet are both vegetable feeders, and there is a pretty little species of the latter genus which invariably, I believe, confines itself to one plant: this is the Patella pellucida, distinguished by having on its summit three or four lines of blue, most brilliantly gemmeous. It feeds on the tangle, (Lammaria digitata) eating away a cavity for itself, just large enough to contain its body, in the substance of the cartilaginous stem, commonly beneath the shelter of the arching roots. I have pulled up the tangles by dozens at low spring-tide, and have scarcely ever found one that had attained certain dimensions without finding a little parasitical Limpet embedded in its substance.
If we measure the interest which we take in any section of created beings by their powers of conferring benefit or inflicting injury on our own race, we shall find the Mollusca not unworthy of our regard in both these respects. Many of them are used as human food, and that not by savage nations only, but by ourselves and by all classes of society. The Limpet, the Periwinkle, the Whelk, the Mussel, and the Cockle, are commonly sold in the streets of our sea-port towns and large cities, though these are certainly more prized by the lower classes of society than by those of more cultivated tastes. There is no doubt that many if not all of our larger Bivalves might be added to the list, and probably some of these might prove not unworthy of a place among more delicate viands. I have myself tried the large Pholas dactylus, that bores the sandstone, and have found its substance tender, and its taste agreeable.
The Cuttle-fishes, though but little used among ourselves, are prized by most other nations. Mr Couch, speaking of the common Squid, declares that it is excellent, and compares it to tripe, a resemblance to which the kindred genus owes its name, for Kuttel in German signifies tripe. Among the people of Southern Europe the Cuttles are in high repute for the table, and this taste has been handed down from the ancient Greeks and Romans. The classics frequently allude to them as among the greatest delicacies. At the nuptial feast of Iphicrates a hundred Polypi and Sepiæ were served up. Among the Greeks generally they were disguised with various condiments and sauces; and the Poulpe, or Many-feet, (Polypus, the Octopus of modern zoology) was the most highly esteemed. Dr. Johnston quotes the "good old story" of Philoxenus in illustration of the gourmand taste for this ill-looking Cephalopod.
"Of all fish-eaters,
Snails appear to have found equal favour with the ancients. The Romans were accustomed to keep these animals in snail-sties, or Cochlearia, where they fattened them with nutritive pastes artificially made. The species was probably the Helix pomatia, which is considerably larger than our garden snail, but the dimensions which they are said to have attained under these favourable circumstances are so enormous as to be utterly incredible. The Illyrian snails were most esteemed for their size, and Pliny informs us that it was a matter of emulation among the amateur snail-feeders of that day to excel each other in the bulk to which their pets should attain; vaunting their most prodigious specimens, as prize pigs and oxen are boasted of among ourselves: "And in time men grew to take such a pride and glory in this artificial feat, namely, in striving who should have the biggest, that in the end one of their shells ordinarily would contain eighty measures called quadrants."
Our continental neighbours still enjoy a dish of snails, and several attempts have been, from time to time, made to introduce them at English tables, but with very little success.
But among all the edible Mollusca, there is none that can compete with the Oyster. To speak of the universality of the esteem would be superfluous; but some statistical particulars may not be uninteresting, as showing the importance of this shell-fish in a commercial view. "The number of vessels immediately employed in the dredging for oysters on the Essex coast are about 200, from twelve to forty or fifty tons burden each, employing from 400 to 500 men and boys. The quantity of oysters bred, and taken, and consumed annually, mostly in London, is supposed to amount to 14,000 or 15,000 bushels. All the fisheries connected with this part of the coast, are stated to employ a capital supposed to amount to from £60,000 to £80,000."
It is, however, not only as supplying food that the benefits of the Mollusca are to be estimated: they are useful also in the arts. The pearl, that splendid auxiliary to costume in all ages, glittering on the tiara of the Assyrian monarchs and on the diadem of our own sovereign, is a production of this class of animals, This, it is true, has no merit beyond its beauty, but the substance called mother-of-pearl is, as is well known, much used in the arts.
CAMEO CUT IN A CASSIS.
It consists of the interior layers of many species of shells, principally Bivalves; but some among the Univalves have an interior brilliantly nacreous. The Top-shells (Trochus), several species of which occur on our own shores, are very rich in this respect, and the widely-gaping Ear-shells (Haliotis) are most gorgeous.
The elegant articles called cameos, so much used as clasps and brooches, are formed out of the substance of large shells: the ponderous Helmets (Cassis) of the West Indies are chiefly used for this purpose. A great excellence in the art consists in the careful cutting away of the material so that the ground shall display one colour, commonly a dark hue, while the design is carved in another, commonly the pure white, which overlays the brown.
Shells, being composed of carbonate of lime, are capable of being burned to a quick-lime, having all the essential properties of that made from stone. On some parts of our own shores where limestone is scarce, shell-lime is extensively burned; but in other countries, as Holland and the United States of America, scarcely any other is used, either for building purposes or for manuring land.
MONEY COWRY. Among the subordinate uses of shells may be mentioned that in China and Japan the valves of a sort of Oyster (Placuna), which are as large as a plate, flat, and transparent, are used in windows and for other purposes where we employ glass; and among the semi-barbarous of Africa, a little species of Cowry (Cypræa moneta) is universally recognised as a money currency. The natural secretions of the Mollusca are not much used among us. The Indian ink used by artists is generally understood to be in part, at least, composed of the black liquor of some species of Cuttle; and an ink is prepared in Italy from this liquor, which, according to Cuvier, differs from the genuine China ink only in being a little less black. Among the ancients this secretion was certainly the basis of the ordinary writing ink, and the soft blackish-brown colour, known as sepia, is at this day manufactured from it.
The Tyrian purple, the most celebrated manufacture of that famous crowning city whose merchants were princes, was the juice of a shell-fish. Several species were employed to communicate various tints, but the principal was the Murex trunculus, one of the commonest shells of the Mediterranean, which may be compared for size and general appearance to our familiar Whelk.
PURPURA. But there is a shell occurring by myriads on our own rocky shores, which has a like property; it is the Purpura lapillus, a small white univalve, surrounded by one or more bands of brown more or less distinct. I have myself been entertained with making experiments on the purple dye of this shell-fish, which, perhaps, some of my readers may like to imitate. In order to this, having collected a few of the animals, which adhere to the rocks between tide-marks, break the shells with the blow of a hammer, taking care not to crush the animals: throw them then into a basin of fresh water, in which they will die in a few minutes. Examining them now, you will find, just behind the head, under the overlapping edge of the mantle, a thick vein of a yellowish white hue, filled with a substance resembling cream: this is the dye in question. It is thick and glutinous, so that you cannot well apply it with a pen; but with a camel's-hair pencil you may paint, as it were, upon linen or cotton cloth any lines, the initials of your name, for example. When you have done this you will perhaps be disappointed, for the marks as they dry will be but just discernible, displaying only a pale yellow tint with not the slightest approach to purple, but exhaling an insufferable odour of garlic.
Place your linen in the light of the sun, and look at it again in half-an-hour, or, if you please, watch its changes. The marks have by this time passed from yellow into pea-green, and are now of a full grass-green; under the influence of the light the change proceeds rapidly, the yellow element gradually disappearing, and the blue element becoming more and more prominent, until through the stages of deep-green, sea-green, and greenish-blue, the colour at length appears a full indigo. The red element now begins to be apparent, and rapidly increases in intensity until the hue is a dull, reddish purple. In my own experiments this was the ultimate tint obtained; a tint perfectly indelible as long as the texture of the material remained, neither light, nor time, nor washing, nor the application of chemical agents having now the least influence either in changing its hue or causing it to fade. I have seen it stated that if the cloth be washed in scalding water and soap, it comes out from the lather changed from the reddish purple hue to a fair bright crimson; with me, however, the soap and the hot water had no appreciable influence in brightening the colour. My experiments were performed in winter, and I will not affirm that the intensity of a summer's sun would not in some degree have modified the result. There appears to me one objection to this material ever having been used to dye large surfaces of uniform colour; for from the admixture of mucus with the colouring matter, when any quantity of the latter is collected, the hue is found to imbue the cloth in a mottled or blotched manner, some parts being much darker than others. What method the ancients had of avoiding this appearance I do not know.
I have seen it repeatedly stated that the slimy liquor remaining in the shell of the common snail, when the animal is crushed, is an admirable cement for glass or china, resisting both heat and moisture. I have tried it both simple, and mixed, as sometimes directed, with quick-lime in powder, but am compelled to confess, that I found it utterly worthless, the adhesion being in every case no greater than if I had used spittle for the purpose.
Let us now see what rank the Mollusca can assume among those creatures which inflict direct injury upon man. The ravages committed by various species of snails and slugs are often annoying, and sometimes serious. There are probably few of my gentle readers who have a garden at their disposal, who have not been disappointed of their crops of spring flowers by the nightly depredations of these pests. The border has been well dug and smoothed, the seed has been carefully sown, and the spot has been eagerly watched from day to day; but no sooner have the tender seed-leaves appeared above ground in a slender green line or circle, than night after night they are gnawed away, until nothing remains but the brown earth, and the label which tells where the seed had been. But to the farmer the consequences are often much more important. In wet seasons the slugs increase with such rapidity in the fields, that a wheat-crop after one of clover, tares, or beans, is very uncertain, and may be said generally to fail. The damage annually done to corn, clover, and turnips, by these apparently insignificant creatures, is very great. In France and the South of Europe, the vineyards are subject to similar attacks from the vine snail (Helix pomatia). The buds and opening leaves of the vine are gnawed off by them as they appear, and the hopes of the autumnal vintage are often blasted.
But much more lamentable than any of these are the injuries inflicted upon shipping, and the piers and defences of maritime towns, by the ship-worm (Teredo navalis). Ranging over extensive seas from the tropics to the shores of Northern Europe, this boring worm, or rather Mollusk with a worm-like form, is incessantly engaged in devouring and destroying all kinds of woodwork that is immersed in the sea. Linnæus long ago styled it the calamity of ships, and there is no maritime nation which has not confessed the formidable power of this subtle enemy. In the years 1731 and 1732, the United Provinces were under a dreadful alarm; for it was discovered that these mollusks had made such depredations on the piles which support the banks of Zealand and Freisland, as to threaten them with total destruction, reclaiming from man what he had with unexampled labour wrested from the ocean. A few
TEREDO.years after they fortunately abandoned the dikes; but fearful of the return of an enemy more powerful than even the Grand Turk, who boasted that he would exterminate them with a host armed with spades and shovels, the Dutch offered a reward of value, to any one who should discover a remedy to ward off their attacks, and ointments, varnishes, and poisonous liquors, were recommended by the hundred. The exact amount of the damage done at this visitation—which Sellius, unable to discover any natural cause for it, says was sent by the Deity to punish the growing pride of the Hollanders—I have not been able to ascertain. Writers in general speak of it as "very great;" and Dr. Job Baster mentions the Teredo as an animal "which has done so many millions damage to these countries." In our own country it has done, and continues to do, extensive mischief. The soundest and hardest oak cannot resist these noxious creatures; but in the course of four or five years, they will so drill it as to render its removal necessary, as has repeatedly happened in the dockyard of Plymouth. To preserve the timbers used there, and exposed to them, the plan now adopted is to cover the parts under water with short broad-headed nails, which, in salt water, soon invests the whole with a strong coating of rust impenetrable by their augers. The plan appears to have proved effectual, for, in the harbours of Plymouth and Falmouth, where the Teredo was once abundant, it is now rare or not to be found; but in other parts it has still a residence, and within these few years it has materially injured or destroyed many of the piles used in the construction of the pier at Port Patrick, on the coast of Ayrshire; and the Limnoria terebrans, a crustaceous insect, co-operating with it, the result of their united efforts can hardly fail to be the utter and speedy destruction of all the timber in the pier."
Another kind of injury is dependent on the fact that certain species, which are generally eatable and even wholesome, become at certain times highly poisonous. Some foreign species are liable to this fatality, particularly oysters, both in the East and West Indies. But we need not go to distant countries for cases in point. The Mussels of our own rocks, though generally sold and eaten by many persons without fear, are well known to be fickle in their qualities, and many cases are on record in which their use has proved fatal. One of these, well authenticated, and investigated by scientific medical men, occurred at Leith in June 1827. Many of the poor of this town were poisoned by eating mussels which had been collected in the docks.
MUSSELS. "The town," says Dr. Combe, "was in a ferment, and the magistrates with great propriety issued a warning against the use of the mussels. Many deaths were reported, and hundreds of individuals were stated to be suffering under the effects of the poison. Luckily, matters were not so deplorable; but we ascertained that in addition to the man mentioned before, the companion of our patient, an elderly woman, had died. In all about thirty cases occurred, with great uniformity of symptoms, but varying very much in severity; but none, so far as I know, have left any permanent bad effects."
The cause of this occasional liability to become poisonous seems involved in almost total obscurity. Dr. Johnston, who discusses at some length the many loose and vague conjectures that have been hazarded on the subject, has shown, that not one of them is tenable, unless it be that in some cases the poisonous principle proceeds from some particular food which, not fatal to the Mollusks, yet generates a diseased condition of the body, deadly to other creatures. The Leith mussels, he adds, were living in a dock, where we may presume they were nurtured and fattened amid putrescent matters; and Dr. Coldstream, than whom no one is better qualified to decide the point, gave it as his opinion that the liver was larger, darker, and more brittle than in the wholesome fish, and satisfied Dr, Christison that there was a difference of the kind. It must be confessed, however, that these observations leave the question pretty nearly where it was before.
Some peculiar secretions of the Mollusca remain to be noticed. And first the black liquor or ink of the Cuttles and Squids, which has already been mentioned as useful to man, but which is doubtless much more useful to the animals themselves. These animals, when in danger, are known to pour forth from a funnel-like orifice a liquor of a blackish-brown colour in considerable abundance; this fluid, readily diffusing itself and mingling with the surrounding water, produces such a cloud of obscurity as frequently enables the crafty animal to escape, enveloped in the mist of its own making,—as the deities in Homer are represented as concealing their favourite heroes.
Somewhat analogous to this is a secretion of a rich purple hue produced and poured forth under excitement, by those large and naked mollusks, the Aplysiæ. I have found one of these animals, on being put into a vessel of clean sea-water, change the whole to a brilliant purple in a very few minutes; and on the water being renewed even again and again, produce the same result. This was a West Indian species, but there is one found occasionally upon our own coasts which has the same property.
This liquor must not be confounded with that which constitutes the purple dye of Murex, Purpura, &c. already mentioned, for it is so volatile as to be unsuitable for the purposes of dyeing. According to Cuvier, the secretion in drying assumes the beautiful deep hue of the sweet Scabious, and remains unaltered by long exposure to the air. Nitric acid, in small quantity, heightened the tint, but a larger dose changed it to a dirty orange colour, while potash turned it to a dingy vinous grey.
PLANORBIS CORNEUS. Both the acid and the alkali precipitated many white flakes from the fluid. The purple tint is readily transferred to spirit when the animal is immersed in it; the tincture retains this colour for awhile, but at length becomes of a deep clear red, like that of port wine.
A very common shell in ponds and ditches, (Planorbis corneus,) coiled up like a ram's horn, is said to have the same property; a purple fluid is poured out from beneath the mantle, but it is so fugitive that no application can prevent its speedily turning to a dull rusty colour.
Colonel Montagu mentions one of our marine shell-fish (Scalaria clathrus) as secreting a purple juice. "It may be collected either from the recent or dried animal, by opening the part behind the head; and as much can be procured from five individuals as is sufficient, when mixed with a few drops of spring-water, to cover half a sheet of paper. Neither volatile nor fixed alkali materially affects it; mineral acids turn it to a bluish green, or sea-green; sulphuric acid renders it a shade more inclining to blue; vegetable acids probably do not affect it, since cream of tartar did not in the least alter it. These colours, laid on paper, were very bright, and appeared for some months unchanged by the action of the air or the sun; but, being exposed for a whole summer to the solar rays, in a south window, they almost vanished. The application of alkali to the acidulated colour always restored it to its primitive shade, and it was as readily changed again by mineral acid."
I have already mentioned some thread-spinners among the Mollusca; there are others which have the power of forming threads of silky substance much stronger and more durable than those of our pond snails. The Common Mussel (Mytilus edulis) is one of these marine silk-worms; and we have a good many others. The bundle of threads, familiar to many of my readers as the beard of the shell-fish, is the substance in question, termed by naturalists byssus, a Greek word originally signifying silk; and the use to which it is applied by the animal itself is that of a cable to moor itself to the solid and immovable rock, that it may not be washed away by the violence of the waves. The mode in which the threads are formed, and the organ by which they are secreted, are thus described by Professor Rymer Jones:—
"The foot in the Mussel is of small dimensions, being useless as an instrument of progression. By its inferior aspect it gives attachment to the horny threads of the byssus, which are individually about half an inch in length, or as long as the foot itself, by which, in fact, they are formed, in a manner quite peculiar to certain families of Conchifera; no other animals presenting a secreting apparatus at all analogous, either in structure or office, to that with which these creatures are provided. The manner in which the manufacture of the byssus is accomplished is as follows: A deep groove runs along the under surface of the foot, at the bottom of which thin horny filaments are formed by an exudation of a peculiar substance, that soon hardens and assumes the requisite tenacity and firmness. While still soft, the Mussel, by means of its foot, applies the extremity of the filament, which is dilated into a kind of little sucker, to the foreign substance whereunto it wishes to adhere, and fastens it securely. Having accomplished this the foot is retracted; and the thread, of course, being drawn out of the furrow where it was secreted, is added to the bundle of byssus previously existing, all of which owed its origin to a similar process."
Whoever has attempted to wrench up a Mussel from one of those shallow rock-pools, in which they lie as closely packed as paving stones, will have had proof of the great strength of these threads, no small violence being required to detach one. But there is an example on record, where the strength of the threads has been turned to such account as to give this Mollusk a second claim to be included in the list of such species as are beneficial to man.—"At the town of Bideford, in Devonshire, there is a long bridge of twenty-four arches 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 by the tide entirely by the strong threads these mussels fix to the stonework; and by an act, or grant, it is a crime liable to transportation for any person to remove these mussels, unless in the presence and by the consent of the corporative trustees."
There are bivalve shells allied to the mussel, called Pinna, usually of very large size, but of thin and delicate structure. The threads spun by these are long, fine, glossy, and produced in great abundance; they are capable of being twisted like silk, and the inhabitants of Sicily weave them into a sort of cloth remarkable for its softness and warmth, but which refuses to take any dye. In the British Museum, together with some very fine specimens of the shells of this Mollusk, there is a pair of gloves made of its byssus; but articles made of this material are very costly, and cannot be considered in any other light than that of curiosities. Pope Benedict XV, in 1754, had a pair of stockings presented to him which were woven from the silk of the Pinna. These were the subject of general admiration, from the extreme delicacy of their texture—well shown by the minuteness of the box in which they were enclosed.
The mention of the ship-worm naturally presents to the mind another tribe of boring Mollusca,—those which perforate hardened clay, and even stone. These, belonging to various genera, are sufficiently common on our own coasts. Different species of Pholas excavate their burrows, which resemble the holes bored by augers or large gimlets in wood, clay, and sandstone; the Venerupis in shale and similar friable rocks, the Lithodomi and Saxicavæ in the limestone, and the Gastrochæna in limestone, fluor, and granite. A curious example of the boring powers of one of these species, the Modiola lithophaga occurs at Pozzuolo, in the Bay of Naples, where a colony of these Mollusks had settled themselves in the pillars of the temple of Jupiter Serapis during the period of its submersion. At the height of ten feet above the base of the three standing pillars which remain, and in a position exactly corresponding in all, is a zone of six feet in height, where the marble has been scooped into cells by these Mollusca. The holes are to the depth of four inches; and it is observed that the nodules of quartz and feldspar, which sometimes occur in the hard limestone of the pillars, are untouched.
Many theories have been invented to account for the singular power exercised by these animals, such as the following; that the animals entered the rock while it was in a soft and plastic state, and that it afterwards hardened around them—that the animal poured out some peculiar fluid which had the chemical property of dissolving the rock—that the latter was ground away by the roughnesses on the shells as they revolved, as if by the action of a rasp or file—that the minute particles of the stone were one by one separated and driven off by the force of currents of water, produced by vibrating cilia: but all these theories appear to be set aside by the discovery of Mr. Albany Hancock, one of the highest living authorities on the subject. This gentleman finds that the excavating instrument is the anterior portion of the animal, either the foot and the edges of the mantle, or the edges of the mantle solely. These organs are fitted for the office they are to perform, not only by their position and figure, and their pliability and muscular structure,—made more than commonly muscular for the duty,—but also by being armed with a rough layer of numerous crystalline particles of various sizes and shapes, chiefly five- and six-sided, and all having one or more elevated points near the centre. These crystals are imbedded in the surface of the boring foot and thickened edges of the mantle; and, consisting, probably, of silex or flint, either pure or in combination with some animal matter, they form a sort of file,—superior, however, to any of our workmen's files in this, that the surface keeps itself always in a proper state of roughness for trituration. This is done by an organic law, which causes the crystals to be constantly shed, and as constantly renewed.
All the borers above alluded to are Bivalves, and I know of no other Mollusk which can properly be classed with them. A common Gasteropod, however, the familiar Limpet (Patella vulgata), excavates the rock on which it lives to the extent of making a depression, more or less deep, exactly corresponding to the shape and size of the margin of its shell. When one removes a Limpet from its firm adhesion and finds a hollow beneath it, evidently made to contain its body, one is ready to conclude that the animal is a permanent tenant of the spot, never moving from it; and when we learn that the food of the Limpet consists of sea-weeds, we wonder how it is possible that a stationary animal can find vegetable food. But the truth is, I believe, that the Limpet wanders away from its hollow during the night, returning to it as a home by an infallible instinct on the approach of morning. The mode in which the excavation is performed is the same as that just mentioned in the case of the borers, the whole under surface of the foot being furnished with sharp crystals of flint imbedded in its substance.
In general the stony shells of the Mollusca afford them, a sufficient protection, but a few species construct for themselves nests. A native example of this instinct is described in interesting terms by the Rev. D. Landsborough, who obtained it in Lamlash Bay: —
"The most interesting, though not the rarest, thing we got was Lima hians. I had before this some specimens of this pretty bivalve, and I had admired the beauty and elegance of the shell; but hitherto I had been unacquainted with the life and manners of its inhabitant. Mr. and Miss Alder had got it in the same kind of coral at Rothesay, so that when Miss Alder got a cluster of the coral cohering in a mass, she said, 'O, here is the Lima's nest!' and breaking it up, the Lima was found snug in the middle of it. The coral nest is curiously constructed, and remarkably well fitted to be a safe residence for this beautiful animaL The fragile shell does not nearly cover the Mollusk, the most delicate part of it, a beautiful orange fringe-work, being altogether outside of the shell. Had it no extra protection, the half-exposed animal would be a tempting mouthful—quite a bonne-bouche to some prowling haddock or whiting; but He who tempers the wind to the shorn lamb, teaches this little creature, which He has so elegantly formed, curious arts of self-preservation. It is not contented with hiding itself among the loose coral, for the first rude wave might lay it naked and bare. It becomes a marine-mason, and builds a house or nest. It chooses to dwell in a coral grotto; but in constructing this grotto it shows that it is not only a mason, but a rope-spinner, and a tapestry-weaver, and a plasterer. Were it merely a mason, it would be no easy matter to cause the polymorphous coral to cohere. Cordage, then, is necessary to bind together the angular fragments of the coral, and this cordage it spins; but it spins it as one of the secrets of the deep. Somehow or other, though it has no hand, it contrives to intertwine this yarn which it has formed, among the numerous bits of coral, so as firmly to bind a handful of it together. Externally, this habitation is rough, and therefore better fitted to elude or to ward off enemies. But though rough externally, within all is smooth and lubricous, for the fine yarn is woven into a lining of tapestry, and the interstices are filled up with a fine slime, so that it is smooth as plaster-work …
"When the Lima is taken out of its nest, and put into a jar of sea-water, it is one of the most beautiful marine animals you can look upon. The shell is beautiful; the body of the animal within the shell is beautiful; and the orange fringe-work, outside of the shell, is highly ornamental. Instead of being sluggish, it swims about with great vigour. Its mode of swimming is the same as that of the scallop. It opens its valves, and, suddenly shutting them, expels the water, so that it is impelled onwards or upwards; and when the impulse thus given is spent, it repeats the operation, and thus moves on by a succession of jumps. When moving through the water in this way, the reddish fringe-work is like the tail of a fiery comet.
The filaments of the fringe are probably useful in catching its prey. They are very easily broken off, and it is able that they seem to live for many hours after they are detached from the body, twisting themselves like so many worms."
Thus we have slightly touched a few of the details of the history of this great division of animated beings; and we discover that they are not less rich in interesting endowments and faculties, in various contrivances and compensations, in singular habits and instincts, than other animals higher in the scale of organization. But it is only when we study the Mollusca as living beings, that we discover these points of varied interest. The mere collection of shells, however curious their forms and brilliant their colours, would impart but a small amount of knowledge when separated from the animals to which they belong. "The shell-collector of former days looked upon his drawers, if they were rich in rare species or varieties, as containing an assemblage of gems; and, indeed, the enormous prices given for fine and scarce shells, joined with the surpassing beauty of the objects themselves, almost justified the view which the possessor took of his cabinet of treasures. They were to him really 'les delices des yeux et de l'esprit;' and the energetic zeal with which he collected, and the sacrifices that he made to procure a fine and perfect Many-ribbed harp, a Gloria maris, or Cedo nulli, among the cones; an Aurora or Orange-cowry, a Voluta aulica, or Voluta Junonia, &c., were only comparable to the extravagances of those visited by the tulip mania when it was at its height. But though they were the delight of his eyes, they were, in nine cases out of ten, little more to the owner of them: they were mere trinkets on which he looked dotingly, without knowing, and scarcely wishing to know, the organization of the animal whose skeleton only was before him."
In these days, however, the examination of the shell is considered by all who possess any claim to science, as subordinate to the history of the entire animal.
Naturalists arrange the Mollusca in six classes, named Cephalopoda, Pteropoda, Gasteropoda, Conchifera, Brachiopoda, and Tunicata. Of these the first three are sometimes distinguished as Encephala, or furnished with a head; the last three as Acephala, being destitute of that organ.
- That is, "having dissimilar nerve-knots."
- Zool. Journ. iv. 172.
- Introduction to Conchology, p. 199.
- Introduction to Conchology, p. 173.
- Jones's Animal Kingdom, p, 385.
- Johnston's Conchology, p. 134.
- Annals of Natural History. October, 1852.
- Johnston's Intr. to Conch. p. 125.
- Philos. Trans. 1835; Part ii.
- Phil. Trans, (abridged) xiii. 566.
- Holland's Pliny, vol. i, p. 267.
- Encyc. Brit. Supp. vol. iv. p. 269.
- Introduction to Conchology, p. 11.
- Edin. Med. and Surg. Journal, xxix. p. 88.
- Test. Brit. Supp. p. 122.
- Animal Kingdom, p. 383.
- Ann. and Mag. K. H. Oct. 1848.
- Excursions to Arran, p. 319.
- Penny Cyclop., art. Malacology.