(1288). The great majority of Mollusks 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 preclude the possibility of their maintaining more than a very imperfect intercourse with the external world, and deprived even of the means of communication with each other, we might naturally expect their organization to correspond in its general feebleness 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 Mussels, anchor themselves securely and im-moveably by unyielding cables of their own construction. 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; while the Cockles, destined to burrow in the sand, are furnished with a tongue-like foot, by which they dig the holes wherein they lie concealed, and crawl, or even leap about, upon the shore.

Many, as the Pholades, penetrate the solid rocks and stones, and excavate therein the caverns that they inhabit; or, in the case of the Teredo, with dangerous industry bore into the bottoms of ships or submerged wood of any description, and silently destroy by their insidious ravages the piers or dykes which human labour has erected.

(1289). Following our usual custom, we shall select for examination one of the most simply organized bivalves for the purpose of illustrating the general structure which characterizes the class; and in the common Scallop (Pecten Jacobaea) we have a species well adapted to exhibit the principal features of their economy. On separating the two valves of the shell in the animal before us, we at once perceive that each is lined internally with a thin and semitransparent membrane (fig. 247, a, h), which, like the shells, encloses the body of the mollusk in the same way that the leaves of a book are contained between its covers. The circumference of these outer membranes, which form the mantle, is, in this case, quite free and unconnected, except in the immediate vicinity of the hinge that unites the two valves. The borders of the mantle are thickened, and surrounded with a delicate fringe of retractile filaments; they moreover present a decidedly glandular appearance, and secrete colouring matter of various tints, similar to those seen upon the exterior of the shell: the glandular margins of the mantle form, in fact, the apparatus by which the extension of the shell is effected; and by them its outer layer is secreted, and in many cases painted with gorgeous hues, as will be explained more at length hereafter.

Pecten Jacobaea.

Fig. 247. Pecten Jacobaea.

(1290). Between the lobes of the mantle are seen the branchiae (b, g), always consisting of four delicate leaves, composed of radiating fibres of exquisite structure, and generally attached to the circumference of the body by their fixed extremities, but elsewhere perfectly free, so as to float loosely in the water, which finds free admission to them. The mouth (l) is situated between the two inner laminae of the branchiae, in a kind of hood formed by the union of the gills at their origin; it is a simple orifice, without any kind of dental apparatus, but bordered by four thin and membranous lips (k) placed on each side of the aperture.

(1291). The valves, which are opened by the elasticity of a compressible ligament interposed between them at the hinge, are closed by the contraction of a powerful muscle (c), which passes directly from one to the other, and around this adductor muscle the viscera of the body are disposed: the stomach, liver, and generative system are imbedded in the mass (d, e, f); the convolutions of the intestine may be traced occasionally (n, o); and the termination of the rectum (m) is visible externally, situated upon that side of the adductor muscle which is opposite to the mouth. In the neighbourhood of the oral aperture is placed a retractile fleshy organ (i), which, although in Pecten it exhibits very-rudimentary dimensions, expands in other species to such a size as richly to merit the name of foot usually applied to it.

(1292). Whoever for a moment reflects upon the arrangement of the branchial apparatus, and the position of the oral orifice, consisting, as it does, of a simple aperture unprovided with any prehensile organs, must perceive that there are two circumstances connected with the economy of a Conchiferous Mollusk, and those not of secondary importance, by no means easily accounted for. It is, in the first place, absolutely essential to the existence of these animals that the element in immediate contact with the respiratory surfaces should be renewed as rapidly as it becomes deteriorated; or suffocation would inevitably be the speedy result of an inadequate supply of fresh and aerated water - to secure which, especially when the valves of the shell are closed, no adequate provision seems to exist. Secondly, it is natural to inquire, how is food conveyed into the mouth? for in an animal, itself fixed and motionless, and at the same time, as in the case of the creature we are now considering, quite deprived of any means of seizing prey, or even of protruding any part of its body beyond the margins of its abode in search of provision, it is not easy to imagine by what procedure a due supply of nutriment is secured.

Wonderful, indeed, is the elaborate mechanism employed to effect the double purpose of renewing the respired fluid, and feeding the helpless inhabitant of these shells. Every filament of the branchial fringe, examined under a powerful microscope, is found to be covered with countless cilia in constant vibration, causing by their united efforts powerful and rapid currents, which, sweeping over the entire surface of the gills, hurry towards the mouth whatever floating animalcules or nutritious particles may be brought within the limits of their action, and thus bring streams of nutritive molecules to the very aperture through which they are conveyed into the stomach, - the lips and labial fringes acting as sentinels to admit or refuse entrance, as the matter supplied be of a wholesome or pernicious character. So energetic, indeed, is the ciliary movement over the entire extent of the branchial organs, that, if any portion of the gills be cut off with a pair of scissors, it immediately swims away, and continues to row itself in a given direction as long as the cilia upon its surface continue their mysterious movements.