Mollusca

“ The fresh-water Mollusca secrete a strong periostracum, in order to protect the shell against the corrosive influence of the carbonic acid gas with which the water is surcharged. The shells of deep-sea Mollusca, living, as they do, in water probably undisturbed by currents of any kind, have to protect themselves against the same eroding influence, and do so in the same way. ”

“ Land Mollusca are, as has been remarked, fettered to the soil. Quadrupeds, birds, fishes, and reptiles are provided with organs of motion which enable them to overpass barriers of various kinds. Even plants, although themselves incapable of motion, may be conveyed in every direction by means of seeds, which are either wafted by the wind or adhere to the skin of animals. But the Mollusca have no such regular means of transport, and are, in a large number of instances, limited to districts of a certain character of soil, or producing certain kinds of vegetation. ”

“ 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 ”

“ There is only one order, which includes the cuttle-fish, nautilus, and belemnites.
2. Pteropoda, or Wing-footed Mollusca. These animals have two membranous feet or arms, like wings, proceeding from the neck. There is only one order, which contains six genera, the best known of which is the Hyalæa, the shell of which is commonly called Venus’s chariot.
3. Gasteropoda, or Body-footed Mollusca. All these animals crawl with the flat part of the body, which acts as a kind of sucker. There are nine orders in Cuvier’s system. ”

“ 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. ”

“ The Mollusca are characterised by having no internal skeleton, nor external horny case, as in the Articulata; they are rather soft, and either void of solid covering, or possessed of "shells" composed chiefly of earthy matter (chalk) , and in one or two pieces, called valves, hence the names, uni-valve and bi-valve; all shells of this description belong to the Mollusca, some of which have no means of locomotion, as Oysters; others have a "foot" covered with a muscular expansion, called the "mantle," by which they glide onwards, as the Snail. ”

“ The class Mollusca—pulpy animals—forms a grand division which man has been pleased to make in the animal kingdom, and immediately below the Vertebrata and above the Annulosa, which again stand above the Cœlenterata, which includes the polyps, sea-anemones, hydras, and corals, which last are more highly organized than the Protozoa. ”

“ 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. ”

“ 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. ”

“ As a rule, among the Mollusca, the shell forms a passive mode of resistance to the attacks of enemies. Bivalves are [63] enabled, by closing their valves, to baffle the assault of their smaller foes, and the operculum of univalves, both marine and land, serves a similar purpose. Many land Mollusca, especially Helix and Pupa, as well as a number of Auriculidae, have the inside of the aperture beset with teeth, which are sometimes so numerous and so large that it is puzzling to understand how the animal can ever come out of its shell, or, having come out, can ever draw itself back again. ”

“ The chief types of Mollusca were already differentiated at the beginning of the geological record, and the metamerism which occurs in the Cephalopoda has been evolved within the limits of that class.
External Characters.—The characteristic organs of Mollusca are the mantle and shell, the foot, the ctenidia and the radula, of which all but the last are external. The original form was bilaterally symmetrical, and this symmetry is retained in all the classes except the Gastropoda. At the anterior end the head is differentiated ”

“ The great province of the Mollusca, in which, for our present purpose, we may include some aberrant and rudimentary Molluscoids, is now best known to us by its medium types, the univalve and bivalve Shell-fishes; the higher group of the Cuttle-fishes and Nautili, though not uncommon, being much less numerous, and one at least of the lower groups, the Lamp-shells or Brachiopods, being represented in the modern world by but few forms. ”

“ Their organs of vegetative life attain a disproportionate development. On the other hand, what are termed the "organs of relation" present a wide range of variation, as may be imagined from the fact that while some of the simplest representatives of the Mollusca consist of mere motionless sacs or bags, containing organs of digestion, respiration, circulation, and generation, its more complex forms are active predatory creatures, endowed with remarkable and varied powers of locomotion, and with sense-organs as keen and as highly developed as those of insects. ”

“ 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. ”

“ There is therefore a marked contrast between the Mollusca of the tropics and of the temperate zones, while different regions in the same latitude, whether within or without the tropics, often show great diversity in their fauna. Every region is thus characterised by its Mollusca. The Mollusca, for instance, of Australia or of South Africa characterise those countries quite as much as do the kangaroo and the emu, the hartebeest and the ostrich; there is nothing like them anywhere else in the world. ”

“ The most striking difference in the Mollusca, as compared with the Articulata, is seen in the entire want of that segmentation which is so apparent in the Insects or Centipedes. The body of an Oyster, a molluscous animal, is a soft mass, and, though possessed of organs, never exhibits the slightest trace of joints, as seen in the higher worms, insects, etc. ”

“ Had ‘Atlantis’ ever existed, we should have expected to find a considerable proportion of the Mollusca common to all the groups, and perhaps to Europe as well, and there would apparently be no reason why a genus which occurred in one group should not occur in all. As a fact, we find the species extremely localised throughout, and genera occur and fail to occur in a particular group without any obvious reason. ”

“ 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. ”

“ Many fix themselves to the shells of Mollusca, and are thus borne along by them to short distances. Others, like some species of sea-pens, float about in the ocean, and are usually believed to possess powers of spontaneous motion. But the most frequent mode of transportation consists in the buoyancy of their eggs, or certain small vesicles, which are detached, and are capable of becoming the foundation of a new colony. ”

“ Although with the Mollusca sexual selection does not seem to have come into play; yet many univalve and bivalve shells, such as volutes, cones, scallops, &c., are beautifully coloured and shaped. The colours do not appear in most cases to be of any use as a protection; they are probably the direct result, as in the lowest classes, of the nature of the tissues; the patterns and the sculpture of the shell depending on its manner of growth. The amount of light seems to a certain extent to be influential ”

“ 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 ”

“ The Pulmonata [15] are asymmetrical air-breathing non-marine Mollusca, generally, but not always, furnished with a shell. The sexes are always united in the same individual, and the operculum is always wanting, except in Amphibola. ”

“ Characteristics of Abyssal Mollusca.—Large shells appear to be rare in the great ocean depths, and are usually very fragile; even moderately-sized specimens are far from common. The only group in which species occur larger than the usual size is the Nudibranchs, which are represented by at least one form larger than an orange. ”

“ It will be observed that in the foregoing allusions to organic remains, the testacea or the shell-bearing mollusca are selected as the most useful and convenient class for the purposes of general classification. In the first place, they are more universally distributed through strata of every age than any other organic bodies. Those families of fossils which are of rare and casual occurrence are absolutely of no avail in establishing a chronological arrangement. ”

“ Certain Hymenoptera make use of dead shells of Helix hortensis in which they build their cells. [214] Magnus believes that in times when heavy rains prevail, and the usual insects do not venture out, certain flowers are fertilised by snails and slugs crawling over them, e.g. Leucanthemum vulgare by Limax laevis. [215]
Mollusca as Food for Man.—Probably there are few countries in the world in which less use is made of the Mollusca as a form of food than in our own. There are scarcely ten native species which can be said to be at all commonly employed for this purpose. ”

“ Fresh-water Mollusca, too, or their ova, are exceedingly likely, from their extreme abundance, to be transported by water-birds, which fly without alighting from one piece of water to another. Again, the isolation of one river system from another is, in many instances, by no means well marked or permanent, and a very slight alteration of level will frequently have the effect of diverting the supplies of one watershed into another. ”

“ That large group of animals has not a single terrestrial representative upon any point of the surface of the globe; and during all periods of the history of our earth, we find that they have always been limited to the liquid element. And they are not only aquatic, they are chiefly marine, as but exceedingly few of them are found in fresh waters. Among Mollusca we find almost the same adaptation. Their element also is the sea. The number of fresh-water species is small, compared with that of marine types; and we find terrestrial species in only one of their classes. ”

“ Mollusca, crustacea, annelides, and zoophytes are almost peculiar to this element, having but few scattered representatives on earth; but, amidst all its varied inhabitants, there are none more exclusively confined to its realms, none that rule them with such absolute sway, none more remarkable for number, variety of form, beauty of colour, and, above all, for the infinite advantages which they yield to man, than the great class of fishes. ”

“ In number, variety, and beauty, the species which inhabit the large rivers of North America present a striking contrast with the few and homely British fresh-water mussels; nor have we, in a fossil state, any shells of this family at all comparable with those living types. [372] The earliest fossil Mollusca referred to the genus Unio appear in certain layers of clay and argillaceous ironstone belonging to the Carboniferous system of Derbyshire, Coalbrook Dale, &c. ”