Mollusc

“ The body of the head-bearing mollusc is attached to the shell internally by one or more muscles, and if we examine the interior surface we are generally able to distinguish the impressions or scars denoting the points of attachment. ”

“ Where the typical molluscan body is well developed it is composed of four principal parts: a head, with the mouth, feelers, eyes, and other organs of special sense; a trunk containing the internal organs; a foot which is a thick muscular mass not at all foot- or leg-like in shape, but which is the organ of locomotion by means of which the mollusc crawls; and a mantle which is a fold of the skin enclosing most of the body and which produces the shell. Such a typical molluscan body is possessed by most of the snails. ”

“ Every one who has searched a few rock pools will have seen the familiar Hermits, and will probably have been interested in their varied antics. First you observe the shell of a mollusc—a Trochus, Periwinkle, or a Whelk—travelling at an abnormal rate for a member of its class. You approach closely to make an inquiry into the matter, when the motion suddenly ceases, and the shell instantly drops into position with its mouth close to the surface below. ”

“ Mollusc—Mollusca (L. mollis, soft) . The primary division of the animal kingdom. It contains most shell-fish, slugs, &c.
Monograph (Gr. monos, one; grapho, I write) . A written description of a single thing, or class of things.
Multivalve (L. multus, many; valvæ, folding doors) . Shells composed of many pieces or valves, as the Chiton.
Nudibranchiate (L. nudus, naked; branchiæ, gills) . An order of gasteropods, in which the gills are exposed, as the Eolis, Doris, &c.
Oxygen. A gas which is one of the constituent parts of water and of atmospheric air. It is essential to animal life. ”

“ From the mollusc, form undecided, matter still fit to be converted to any form, the superabundant strength of the young world, richly plethoric, abounding in alimentation, there must at an early period have proceeded two forms, contrary in appearance, but tending and qualified to the same end. ”

“ This confusion is the more unpardonable, because even the most ancient of scribes always spoke of this mollusc as the “ten-armed one,” while a reference to any standard work on Natural History will show even the humbler cuttle-fish with their full complement of arms—that is, ten. ”

“ Which belonging to that division of Natural History called the Mollusca, from the Latin Mollis—soft; these Molluscous animals, then, are animals having a soft body, and no internal skeleton. You may be quite sure that a Mollusk will never break its bones, because it has none to break; it has a shell, however, which may be broken, at least in some cases, for all Mollusks have not snug habitations of the kind; but wander about the watery or earthy world in which they live, quite naked ”

“ And although this consideration belongs more especially to the study of the animal itself, yet the habits of the animal materially influence the structure of the shell.
The Terrestrial or Land Molluscs live on land, breathe air, and feed on plants and trees.—Those who find pleasure in horticultural pursuits will at once call to mind a too familiar example of these Molluscs in the common garden snail. ”

“ The Mollusca, as feeders, are divided into three classes—those which take only liquid food, the vegetable feeders, and the carnivorous species.
Those which are only able to take food in a liquid form, are such as have no means of seizing prey, their food consisting of the countless myriads of infusorial animalcules which float in the sea-water, and which are carried into the orifice of the stomach or mouth by the current. ”

“ SNAILS, CUTTLEFISHES, SQUIDS, OCTOPUS, TUSK SHELLS, BIVALVE MOLLUSCS, OYSTERS The Molluscs have no limbs. The body is surrounded by a membraneous sac, from the secretions of which in many species a chalky shell is formed. The organs of circulation, digestion, and respiration are well developed. The under side of the body is thickened into a fleshy “foot,” by which locomotion is effected, and there is a well-marked head. ”

“ We have another class of animal life, the articulata,
in the same zone; and we have some three hundred species of molluscs through the silurian. Nay, so large is the last named class at this early period, that it is denominated by American geologists "the age of molluscs." I must remind our readers that during the whole of this immeasurable age, we have not a single authenticated land plant; nay, further, we have fish, the creation of the fifth day, before aught of the third day's creation appears. ”

“ And when we come to examine the land and fresh-water Mollusca, it is found not merely that some Mollusca are terrestrial and others fluviatile, but that certain species haunt the hills and others the valleys, some the recesses of woods and others the open meadow sides, some prefer the limestone rocks, others the sandy or clayey districts, some live only in still or gently moving waters, while others are never found except where the current is rapid and powerful. ”

“ Molluscs.—Molluscs are soft-bodied animals, in most cases protected by a hard, external shell, but they differ essentially from crustaceans and all other arthropods in not being segmented, and in not possessing jointed limbs. Familiar and instructive examples of two classes of the group are found in the fresh-water mussel and the garden snail.
The fresh-water mussel (Fig. 233) is to be found in streams, along the bed of which it ploughs its way by means of a muscular [Pg 381] foot (ft.) ”

“ In conclusion, it may be interesting to know that these molluscs progress, while undisturbed, literally on their heads, with all the eight arms which surround the head acting as feet as well as hands to convey food to the ever-gaping mouth; but when moving quickly, as in flight, or to attack, they eject a stream of water from an aperture in the neck, which drives them backwards at great speed, all the arms being close together. ”

“ WE now come to the Molluscs, or Soft-bodied Animals, of which there are a very great many. Some of them live in shells, like the oyster and the whelk, and are often spoken of as “shell-fishes.” But they are not really fishes at all, for they have no bones as fishes have, and are made in quite a different way. And there are just a few of them which have no shells at all.
One of these is that very curious creature which we call the Cuttle. You may sometimes find it in the rock-pools, lurking in the crevices among the rocks, or hiding under the masses of sea-weeds which grow round the edges. ”

“ There is another kind, covered with a shell, such as the oyster; and another, protected by a softer shell, such as the crab. Another kind is that of the mollusca, such as the cuttlefish; and lastly, the family of insects. All these are destitute of (red) blood.
Here, then, we have a general classification of animals, which it is important to notice, as we may have occasion afterwards to compare it with arrangements proposed by other naturalists. It may be reduced to the following form:—
Red-blooded Animals. ”

“ Among the shell-bearing molluscous animals, the little shell which contains the creature in its youngest stages remains ever after as an integral part of the outer calcareous case. As the inhabitant grows, it continues to add band after band to the outer edge of the shell, each of which, whilst preserving the general symmetry and proportions of the whole structure, increases its dimensions in every way. ”

“ From the oyster to the snail we have passed, step by step, along successive stages of one-sided over-development. The facts have shown that a bivalve mollusk could not have descended from a univalve. As all mollusks in early life have the axis of the body straight, and the parts symmetrically arranged on either side, we may infer that bilateral symmetry characterized the remote ancestors of the molluscan type. Now, while a mollusk is bisymmetrical or nearly so, if the mantle secretes a shell it must be in in two parts, or, as in Chiton, in many parts. ”

“ Linnæus divides it into five orders:
I. Intestina, Intestinal Animals: simple, naked, and destitute of limbs: for example, the earth-worm, the guinea-worm, the leech, and the ascaris: 7 genera, 24 species.
II. Mollusca. Simple, naked animals, furnished with limbs: the slug, the sea-mouse, the sea-anemone, the cuttlefish, the sea-nettle, the star-fish, and the sea-urchin: 18 genera, 110 species.
III. Testacea, Shell-fish. Soft, simple animals, covered with a shell which is usually calcareous. This order includes 36 genera, and 814 species. ”

“ If the observations of recent embryologists are to be credited, many mollusks and molluscoidea exhibit a similar evolution. At any rate, even in undoubted mollusca, the elements of their segmentarian origin are abundantly visible.
If we take the genus arca, for instance, as a typical mollusk, it is easy to follow the segments of which it is composed. Each half of this bivalve has its own organization, as complete as in any segment of any annulosa. Each has its own heart, its own nerve-ganglia and branches, its own aërating apparatus and systemic circulation, such as it is. ”

“ The shell is shaped something like that of a whelk, but is very smooth and porcelain-like, and is generally white, ornamented with several coloured bands. It is, however, one of the most variable of shells, differing not only in colour but in form. It always inhabits the belt of the shore between tide-marks, and preys upon other Molluscs, such as the mussel and periwinkle, literally licking them to pieces with its long riband tongue. ”

“ For following up this branch of inquiry the study of the distribution of the mollusca offers special advantages. There are numerous marine, fresh-water, and terrestrial species and genera. They are slow moving; they have not the means of transporting themselves great distances, like insects, for example, that may easily and often pass over arms of the sea, or fly from one country to another. ”

“ Let it be noted, also, that those tertiary species which have continued with little change down to our days are the marine animals of the lower grades, especially mollusca. Their low organization, moderate sensibility, and the simple conditions of an existence in a medium like the ocean, not subject to great variation and incapable of sudden change, may well account for their continuance ”

“ Describe the various types of eyes found in mollusks, and their location. Describe the tongue or lingual ribbon of the snail, and its use. What is the operculum of snails? its use? Physiology. Describe the circulation of water through the siphons and mantle cavity of a clam. How is it caused? What three uses has it? What are the principal facts about the development of fresh-water clams? Describe the circulation of blood in a clam. What various methods of locomotion are found among mollusca? Economics. ”

“ The whole number of species of molluscous animals known is estimated at about twelve thousand recent and fifteen thousand fossil. Many of the living species furnish wholesome food, and some are esteemed as delicacies. The marine shells, by the immense numbers in which they are produced, perform an important office in abstracting from the sea-water its excess of calcareous matter and thus aid in maintaining its purity. ”

“ Besides, molluscs have many enemies, which destroy them both on land and in water. In the water,—fish, frogs, newts, ducks and other water-fowl, and on land many birds, the hedgehog, toads, etc., largely depend upon them for food. ”

“ Mollusca, and the soft parts of a few of the testaceous kinds, are represented by coloured wax models in the exhibited series of shells arranged in the Bird Gallery.
‘The whole of the exhibited collection is in an excellent state of preservation. The system or scale on which the genera, species, and local varieties of shells are exhibited, with their names and localities, gives to the ordinary visitor a power of comparing his own specimens, and, in most instances, of determining them, without the necessity of special application to the keeper or assistant in the department. ”

“ Of the molluscous life of the age in this region we, know almost nothing, since the marine deposits which contain its remains are not now above the ocean-level, and the fresh-water and estuary beds exposed to our observation have yet yielded none. Of the land-animals scarcely any traces have been found except their footprints. ”

“ Their mode of diffusion.—Notwithstanding the proverbially slow motion of snails and mollusks in general, and although many aquatic species adhere constantly to the same rock for their whole lives, they are by no means destitute of provision for disseminating themselves rapidly over a wide area. "Some Mollusca," says Professor E. Forbes, "migrate in their larva state, for all of them undergo a metamorphosis either in the egg or out of the egg. ”

“ Fish scales should also be procured, and specimens should be taken from the lateral line.
Molluscs of all kinds afford many beautiful objects, and the observer should be very careful to examine the wonderful tongue-ribbon of the snail, the slug, the periwinkle, the whelk, and other similar molluscs. If meant to be examined by polarized light, the tongue-ribbon should be mounted in Canada balsam.
Crystals should always form part of a collection. Take those of common salt, nitre, sugar, chlorate of potash, salicine, &c. ”

“ Molluscs live on land, in fresh water and in the ocean. No depths of the ocean abysses are too great for the octopi, no coast but has its many shells, hardly a pond or stream is without its mussels and pond-snails, and in all regions the land-snails and slugs abound.
[Pg 245]
Body form and structure.—The molluscs are not to be mistaken for any other of the lower animals; they have a structure peculiarly their own. In them the body is not articulated or segmented as with the worms and arthropods, nor radiate as in the echinoderms, nor plant-like as with the sponges and polyps. ”