This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(1309). Simple as the structure of the hinge is in the Ostracea, in other Bivalves it frequently exhibits far greater complexity, and the opposed valves present prominent elevations and deep fossa) which lock into each other, and thus form a very secure articulation of great strength and solidity. In such cases the arrangement of the elastic ligament for opening the valves is slightly modified, being placed externally instead of within the shell, but its action in antagonizing the adductor muscles is still equally efficacious.
(1310). We must, in the next place, solicit the attention of the reader to a very important subject connected with the economy of this class of Mollusks, viz. the growth and formation of their shells. Infinitely diversified are the forms presented by their testaceous valves, and equally various the colours which not unfrequently adorn their external surfaces. Some exhibit a beauty and delicacy of sculpture of a most exquisite character; others, covered with large spines, or festoons of calcareous plates, puzzle the beholder to comprehend how the growth of such parts, in the situations which they occupy, can be effected with so much regularity of arrangement. The shells themselves are 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.
(1311). 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.
(1312). In order to simplify as much as possible our description of the process whereby the shell is formed, it will be necessary to consider it under two points of view: first, as relates to the enlargement of the valves in length and breadth; and secondly, as regards their increase in thickness, - very different parts of the mantle being employed in the attainment of these two ends.
(1313). 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 (fig. 247, h; fig. 248, e), 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.
(1314). 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 (fig. 250).
(1315). 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.
(1316). 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 formed 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.
(1317). The carbonate of lime (for such is the earth whereof the shells of bivalves are principally composed) is, at the moment of its deposition, imbedded in a viscid secretion that forms a kind of cement; and on dissolving the shell in a dilute acid, the animal material thus produced remains in the shape of a delicate cellulosity, in the interstices of which the chalky particles had been entangled. If the proportion of the above-mentioned secretion be abundant, it not unfrequently, by hardening on the exterior of the shell, constitutes what has been very inaptly termed its epidermis, representing a comparatively soft external skin of semicorneous texture. If exceedingly thick, the epidermic layer thus formed becomes loose and shaggy, giving the shell a hirsute appearance; but, both in its structure and origin, such pilose investment has no claim to be considered analogous to the hair of animals possessing an epidermis properly so called.