(1318). 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 coloured 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.

(1319). Local irritation of various kinds is found to stimulate the mantle to increased action, so as to cause the pearly matter to be secreted more abundantly at the part irritated. Thus there are various minute boring Annelidans that, in the exercise of their usual habits, perforate the shells of oysters, and penetrate even to the soft parts of their bodies. Stimulated by the presence of these intruders, the mantle beneath the place attacked secretes nacre in inordinate quantities to repair the injured portion of the shell, and prominent nuclei are soon formed, which, enlarging by the addition of continually-added layers of nacreous matter, become so many pearls adherent to the interior of the shelly valves.

(1320). Or pearls may owe their origin to another cause. It not unfrequently happens that sharp angular substances, such as grains of sand or fragments of stone, are conveyed between the valves, and become imbedded in the delicate tissue of the mantle. Thus irritated, the mantle throws out copiously the peculiar iridescent material which it secretes, and with it coats over the cause of annoyance, wrapping it in numerous concentric laminae of nacre, and thus forming the detached and globular pearls so valuable in commerce.

(1321). One other circumstance connected with the growth of bivalve shells requires explanation. From the earliest appearance of the shelly valves until the period when the included mollusks arrive at their mature size, the adductor muscle or muscles have been of necessity perpetually changing their position, advancing gradually forward as the enlargement of the shells was accomplished, so as to maintain in the adult precisely the same relative situations as they originally did in the young and as yet minute animal. Taking the Oyster for an example, it is quite obvious that the adductor muscle, which at first was connected with the thin and minute lamellae forming the earliest shell, has, during the entire growth of the animal, become further removed from the hinge, and transferred from layer to layer as the shell increased in thickness, till it has arrived at the position occupied by it in connexion with the last-formed stratum that lines the interior of the ponderous valves of the full-grown Oyster. The manner in which this progressive advance of the adductor muscle is effected is not at first easily accounted for, seeing that it is always fixed and firmly adherent at all points of its attachment.

In order to understand the circumstances connected with its apparent removal, it is necessary to premise that a thin layer of the mantle itself is interposed between the extremities of the muscle and the inner surface of the shell, forming the bond of connexion between the two, and, like the rest of the pallial membrane, assisting in increasing the thickness of the shell by adding layers of nacre to its inner surface. Particle after particle is laid on by a kind of interstitial deposit between the mantle and the extremity of the adductor muscle, but so gradually, that the firm attachment between the muscle and the shell is not at all interfered with; and as the animal grows, the transference of the muscle from layer to layer is thus slowly and imperceptibly effected.

(1322). We have as yet limited ourselves almost exclusively to a description of the simplest forms of Conchifera, namely those belonging to the Ostraeean family, which, being generally incapable of locomotion, are deprived of a foot, and are recognizable by having the two lobes of the mantle unconnected with each other around their entire circumference. On turning our attention to the organization of the mantle in other families, we find that in them it no longer offers the same simple arrangement, but, the two lobes becoming gradually more and more completely united along their edges, the bodies of the mollusks are by degrees enclosed by the pallial membranes, and seem, as it were, sacculated. Moreover, sometimes the mantle is prolonged into membranous tubes of considerable length called siphons, through which the water is conveyed to the gills, and excrementitious matters expelled from the body. In the Mussels (Mytilacea) the edges of the mantle are partially joined, so as to present two apertures, through one of which the foot is protruded, while the other, the smaller of the two, gives issue to the excrement.

A third family (Chamacea) has the circumference of the two divisions of the mantle still more intimately united, leaving three distinct fissures - one for the passage of the foot, another for the entrance of water to the branchiae, and a third for the ejection of matter from the rectum. Of these, some are of gigantic dimensions, and fix themselves by a strong byssus. One species, indeed (Tridacne gigas), is so enormous in its size, that its shells alone not unfrequently weigh upwards of two hundred pounds, and hatchets are employed to chop its thick and tendinous cables from the rock to which it holds.