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.