In some of the acephalous mollusks the siphons are too large to be received within cover of the shell, in which case the retractor muscle is generally small, inasmuch as it is then of little use (Mya, Glycimeris); but in those species in which the siphons are of mid dling size, or not so large as to be incapable of entering the shell, the retractile muscle is of considerable size and. power (Tellina, Psammobia).
3. The shell.—The lobes of the mantle appear to be the efficient parts in determining the form of the shell, and it is by their thick edges that this coverina is in great part secreted. The whole of the Conclliferous acephala,without exception, are included within a bivalve shell, the two parts of which are joined by a point in their upper edge, to which the title of hinge has been given by naturalists, and very pro perly, because it is in truth upon it that the motions of the valves take place.
General structure.—When examined with due attention, the shell is found to be composed of two kinds of laminw very distinct from one another (a, b, fig. 363) ; the one, secreted from within outwards by the edges of the mantle, present themselves under the form of greatly elongated cones, the thick parts of which are turned towards the outer surface (a, c, c, fig. 363); the other, in parallel layers, secreted by the central and posterior parts of the mantle, line the interior of the shell, and in many species at length fill up the cavity of the hooks. These two layers of the shell are frequently found in cer tain fossil species almost com pletely separated from one an other. At other times the inner layer is seen to have been dissolv ed away, whilst the external one continues without appearing to have undergone any great change.
It is in the genera Chama, My Pinna, Spondylus more es pecially that the two laminw of which a bivalve shell is formed can be studied to greatest advantage, and this study is of importance as leading to a more accurate knowledge of certain fossil genera, in regard to the charac ter of which some uncertainty has always pre vailed, by reason of one of the constituent portions of their shell always being found dis solved, as in Patillus. In some genera the ex ternal layer is very readily distinguished, from having a fibrous structure (a, a, fig. 369), a structure observed more especially in the shells of the Pinna family and those of the Malleacea. The two layers of the shell are in the inverse ratio of one another in point of thickness : the external layer, extremely thin towards the hook, increases continually towards the edges, whilst the inner layer, thick at the hook, becomes thinner and thinner as it ap proaches the edges, around which it is usually exceeded a little by the outer layer. A fact well deserving of attention is this :—that the muscular impressions and the whole articular aspect of the hinge are formed in the substance of the inner layer of the shell, and these parts, of so much consequence, do not leave a trace upon the external layer when this alone is pre served. It is only from having neglected to study
the structure of the shell with sufficient attention that naturalists have found themselves at a loss to discover the true characters of certain fossil genera, as Podopsis, Spherulites, which, in consequence of their position in porous chalky beds, never occur with more than the outer layer of their shell in a good state of preser vation.
The hinge.—The part of the edge of a shell by which the two valves are conjoined, is, as we have already had occasion to state, deno minated the hinge. This part is entirely formed by the inner layer of the shell. The part of the shell, of various length and thickness, upon which the hinge occurs, is called its cardinal edge. ln the hinge two structures are appa rent : ist, an elastic ligament, the position of which is variable; 2d, projections and corres ponding cavities on either valve, destined un doubtedly to give additional strength to their union.
1. The ligament.—The ligaments of bivalve shells are distinguished into two kinds, acco'rd ing to their structure and their position : they are internal when they are completely hidden by the cardinal edge of the shell ; they are external when they appear on the outside be yond this edge. The internal ligament is com posed of a great number of highly elastic fibres, parallel to one another, and perpendi cular to the valves they connect. They are secreted by a lamina of the mantle, projecting upon the back of the animal, and penetmting between the edges of the two shells. The fibres of the ligament secreted when the shell is partially open, are of too great length when it is. shut, so that when the valves are ap proximated to one another these fibres are forcibly compressed, and their elasticity is brought into play, by which it is only necessary for the animal to relax its adductor muscles in order to have the fibres of the ligament, in their effort to regain their natural length, force the valves apart from one another to a deter minate extent. When the ligament is external, it rests upon the prominent parts of the cardinal edge, parts to which the title of nymphw has been given (a, a, fig. 365). When the ligament is of this kind, it consists of two distinct layers, one external, thin, and very strong, composed of transverse fibres, which extend from one nymplia to the other, and are stron9,1y inserted within a groove hollowed out of the base of each of them. The other portion of the external ligament is of precisely the same structure as that of internal ligiments, and is comprised between the nymph and the outer layer, of which we have just made mention. The action of this ligament is also precisely the same : it forces the valves apart when the animal ceases to maintain its adductor muscles in a state of contraction.