CONCHIFERA "Molluscous animals enveloped in a shell composed of car bonate of lime, forming two valves, connected by a joint, and applied the one to the right, the other to the left side ; mantle more or less open ventrally, generally with two orifices behind ; gills four-leaved, no head, mouth placed at the angle of the gills, furnished with lips and palps, sexes distinct ; young undergoing a metamorphosis." " The great majority of Molluscs which inhabit bivalve shells constitute a very numerous and extensive class, distinguished by certain characters possessed by them in common. Encased in dense and massive coverings of such construction as to pre clude the possibility of their maintaining more than a very im perfect intercourse with the external world, and deprived even of the means of communication with each other, we might natu rally expect their organization to correspond in its general feeble ness with the circumscribed means of enjoyment and limited capabilities of locomotion allotted to them. Numerous species, indeed, are from the period of their birth firmly fixed to the rock ,which gives them support, by a calcareous exudation that cements their shells to its surface, as is familiarly exemplified in the case of the common Oyster ; or else, as the Muscles anchor themselves securely and immovably by unyielding cables of their own con struction. The Scallop, unattached but scarcely better adapted for changing its position, rudely flaps together the valves of its expanded shell, and thus by repeated jerks, succeeds in effecting a retrogressive movement. The simple apparatus by means of which shells are constructed, is the external membranous layer that invests the body of the Mollusc, the mantle, as it has been termed, and, whatever the form of the shell, it owes its origin en tirely to this delicate organ. When the animal is engaged 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 concentrix 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 per manent, indicate, by the interspaces separating successively de posited growths of this description, the periodical stimulus to increased action that caused their formation.
" Whatever thickness this 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 colors seen upon the exterior of the shell are deposited, and as sume that definite arrangement characteristic of the species. The border of the mantle contains in its substance colored spots ; these when minutely examined, are found to be of a glandular character, and to owe their peculiar colors to a pigment secreted by themselves ; the pigment so furnished, being therefore mixed up with the calcareous matter at the time of its deposition, colored lines are formed upon the exterior of the shell wherever theie glandular organs exist. If the deposition of color 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 colored 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.
" The organs of hearing consist of a pair of transparent cap sules filled with a clear fluid. Each contains a glassy globule, which constantly maintains a very singular and rotary motion, that instantly ceases when the capsule is ruptured. These cap sules are situated in the foot.
" Organs of vision are much more obvious, at least in many species, being often highly colored, very numerous, and promi nently situated."