The term Mollusc, from mollis, soft, is intended to express that the whole class are invertebrate; that is, entirely without spine, or any bony support to their curious fleshy forms. The term was invented by the illustrious Cuvier, but is objectionable as a distinctive one, the characteristic on which it is founded being shared by other distinct classes of animals. When, however, the application of a term is well understood, its inner signification becomes unimportant; it is, therefore, now too late to criticise the one invented and applied by the prince of modern naturalists.
Among the interesting facts detected by recent science, it has been shown that many of the seemingly shapeless masses of soft substance, scarcely to be termed flesh, possess all the senses of the higher animals. In the Cephalopoda, the organs of sight and hearing are both well developed; and Professor Owen considers that the Nautilus possesses even an organ of passive smell. The Gasteropoda too, are, according to Siebold, nearly all furnished with ears and eyes, the former organs being described as forming round capsules, conspicuously visible near the roots of the tentacles.
Some of the Conchifera, also, are furnished with numerous eyes, which, like those of the Scallops and Clams of our own shores, are also placed among their tentacula.
It appears probable, says Dr. Johnston, that many have also the sense of taste, as they are observed to select particular articles of food in preference to others, and there is no other sense that appears fitted to regulate the choice. The mouth, as it is termed, of many of the molluscous tribe is furnished, as among the Gasteropoda, with a fringe formed of filaments, which may be organs of touch, and they have also a complicated breathing apparatus.
The strength of these boneless creatures is something very extraordinary, and almost incomprehensible. The Strombus gigas, a soft, snail-like creature, carries a shell which often weighs more than five pounds; the Cassis tuberosa supports one nearly as heavy, and the naked Molluscs, that have no shell to carry, have other modes of exhibiting strength of a very extraordinary character.
The shells of the clothed Molluscs are senseless, being permeated by no vessels, and are formed by the animal itself from a secretion with which its outer integuments are invested, and which may be described as lime in a state of solution. The thickened edge of the mantle, by means of which the form is given to the shell, and the general manipulation effected, is furnished, as may be seen with the aid of a moderate lens, with a minute and highly sensitive fringe, the cilia of which are of various colours, corresponding in tone and position to the tints which decorate the exterior of the shell. The coloured cilia or fringes have doubtless a dyeing power, which colours the calcareous solution at the time it is added to the shell by their plastic instinct. The solution becomes a hard testaceous substance so soon as it leaves the body of the animal, and is deposited in architectural layers upon the beautiful structure of the shell, by the “trowel” and “brushes” of the edge of the mantle.
This process is beautifully described in Jones’s “Animal Kingdom,” with all the details relating to the successive ridges on the shell, which mark the age of the animal; it having been ascertained what time is required for the completion of each story of the edifice.
The power of locomotion is one of the most curious subjects for observation in the structure of shell-coated Molluscs, and for this purpose the marine Aquarium offers many advantages. Other classes of animals have been distinguished by the number of their feet; we have, for instance, a tribe of worms termed centipedes, or hundred-footed creatures; and, to pass over many gradations, to the superior grass-feeding and carnivorous animals, we find them termed quadrupeds, or four-footed creatures; while the human race, along with birds, has been termed biped. Why, therefore, may we not coin a word for our present purpose, and call these curious Molluscs monopeds, or single-footed creatures?—for they walk with a single foot, being compelled to do so by the very simple fact that they have no other. This limb, or foot, being gradually protruded, its bearing against some substance forces them forward, and when the foot has attained its full distension it is drawn in, and a new bearing obtained, and by the repetition of this process, a certain amount of locomotion is effected. Some species float on the surface by means of this foot. Having crawled up a rock to the height of the surface of the water, the foot is protruded and exposed to the air, when it becomes suddenly dried, and in that state serves as a cork, which enables the animal to float away close under the surface of the water. But if any agitation of the water wet this floating apparatus, or the animal withdraw it voluntarily beneath the water for that purpose, the creature immediately sinks to the bottom.
The swimming power of this race of creatures is equally curious. The Cephalopoda, by the ejection of a jet of water, propel themselves rapidly in the opposite direction, and by the repetition of the jet at regular intervals, a beautiful power of motion is obtained, as regular, and with less labour than that of ordinary swimming by means of the action of fins or other oar-like limbs adapted to the purpose.[[3]] The Pteropoda, however, in their little shells, translucent as glass, swim by the action of small fin-like paddles placed near the head.