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."[4]
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."[5]
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
