Part 6

Aperture.

The aperture or opening of the spiral tube, was formerly described as the mouth; a term calculated to convey an erroneous impression, when applied to a part of the shell which has no correspondence with the mouth of the animal. The word aperture is used by modern writers in a general sense, including the cavity, its edge, and the canals. The cavity itself is distinguished in various shells as to its shape, which depends much upon the degree of modification produced by the last whorl. In some cases, as in Cyclostoma, where the aperture stands apart from the last whorl, the shape is round, or nearly so. The Scalaria presents a good example of this. In others, where the inner edge or lip, wrapping over the body whorl is nearly straight, the aperture is semi-lunar, or half-moon shaped: this is remarkable in the "Neritacea" of Lamarck, named, on that account, "hemi-cyclostomata" by De Blainville. In a great number of instances, the lower part of the body whorl enters obliquely into the upper part of the aperture, the result being a pyriform, or pear-shaped opening. The aperture is described as long when it is largest in the direction of the axis, and wide, in the contrary case. The anterior is the part at the greatest distance from the apex, and the body whorl; the posterior, the part nearest to the apex. Thus some apertures are described as posteriorly contracted and anteriorly widened, or the reverse. A linear aperture is one contracted in its whole length, as in Cypræa. When the whorls are angulated, a trigonal aperture is the result, as in many species of Trochus. Some are transversely oval, that is in an opposite direction to the axis, and others longitudinally oval. When the whorls are formed with two outer angles, a somewhat quadrated aperture is formed. There are other variations too numerous to mention.

Apertures.

The entire edge of the aperture described generally, is named the Peritrême, but this term can only be conveniently applied in cases where, in some at least of its characters, it is the same all round, so that one descriptive term is applicable to the whole. As, however, this is of rare occurrence, it is found convenient in descriptions to separate the rim from the outer lip. In a great number of instances, this is done naturally, by a canal, or notch at the anterior or lower extremity, and by the posterior union of that part which overlays the body whorl with the other portion. At these two points the outer and inner lips separate from each other: we therefore describe the

Canals of the Aperture.

When there is neither notch nor canal, anteriorly or posteriorly, interrupting the edge of the aperture, it is described as entire. When there is a notch or sinus at the anterior extremity, it is said to be emarginated. When the edge of this notch is expanded, and drawn out in the form of a beak, it is said to be canaliferous, or to have a canal. When, in addition to this, the lips are thickened and contracted posteriorly near their junction, and drawn out so as to form a groove, it is said to be bi-canaliculated, or to have two canals. The anterior canal is said to be long or short, according to the proportion which it may bear to the rest of the shell. Thus the canal of Ranella ranina (fig. 393 in the plates), may be described as short; while that of Murex haustellum, (fig. 396, pl.) is long. When it is wide near the aperture, and becomes gradually contracted towards its termination, it is said to be tapering, as in Pyrula (fig. 388, pl.); when the termination is sudden, it is described as truncated. If, on placing the shell upon a plane, with the aperture downwards, the canal is seen to rise upwards, it is recurved. In Buccinum and Nassa it is turned suddenly over the back, and forms a short, curved elevation; it is then described as recurved and varicose. If the edges meet, so as to form a tube, it is said to be closed, as in some species of Murex and Typhis. The posterior canal is, in some cases, free, or standing out from the spire, as in some species of Ranellæ; while in others it is decumbent, running up the sides of the spire, as in Rostellaria (fig. 402, pl.).

Canals.

Lips, or edges of the Aperture.

The part of the edge of the aperture next to the body whorl is named the inner, or columellar lip. Posteriorly it commences at the point of union with the outer lip, where that touches the body whorl, the junction being generally marked by an angle, and sometimes by a canal. Anteriorly it terminates where there is generally seen a notch or canal, or sudden angle, from which the outer lip proceeds. The part which setting out from the body whorl, and proceeds outwards at a distance from the axis, till it reaches the anterior canal or notch (or its place in case of absence) is named the outer lip. In many cases the edges are united in such a manner, that it is difficult to distinguish where the inner lip terminates, and the outer lip commences: when this is the case, it is usual to describe the margin or peritrême, as a whole, without distinguishing the parts. The outer lip, sometimes called the right lip, or labrum of continental writers, is sometimes acute, not being of thicker substance than the remainder of the shell. In other cases it is obtuse, or thickened and rounded at the edge. When thickened and turned backwards it is described as reflected; when, on the other hand, it is turned inwards towards the axis, as in the Cyprædæ, it is inflected, or involute. When it is toothed, a distinction must be observed as to whether the dentations are external or internal. If the teeth are small and numerous, it is denticulated; if larger, it is dentated; when expanded into a kind of wing, as in some species of Strombus and Rostellaria, it is described as alated; and a family in Lamarck's system is named "Alatæ," from this very circumstance. In some of those which are expanded, the expansion is divided into separate, attenuated portions, they are then said to be digitated.

Outer Lips.

The inner lip, sometimes named the columellar lip, or "labium," is subject to similar variations as to thickness, dentition, &c. That portion of it which lies upon the body-whorl is frequently distinguished from that which intervenes between it and the notch or canal. De Blainville, restricting the term lip "bord gauche" to the former portion, applies the term "columella" to the latter; and in some instances this may be the more convenient method of describing the part in question. The columellar lip is sometimes detached entirely from the body of the shell, as in Murex haustellum; in others it is decumbent, or lying over the last whorl, although quite distinct, and in some cases, thickened, callous, or tumid.

At the lower or anterior part, sometimes called the columella, there are in many instances flattened, laminated folds; these are particularly conspicuous in the genera Cymba and Melo, where, being obliquely spiral and laminar, they are extremely elegant, presenting to the eye graduated repetitions of the line of beauty. In other cases, as in the Turbinellæ, they are more horizontal and thickened.

In some cases the columella is swelled into a varicose mass; as in Oliva, Ancillaria, &c.; it is then described as tumid or varicose. It is sometimes tortuous, and sometimes straight, and is susceptible of many variations, too minute and particular to be described in this part of the work.

COLUMELLAR LIPS.

OPERCULUM.

The aperture of many species of shells remains constantly open; but in a great number of species it is occasionally closed, whenever the animal is retracted within the shell, by a calcareous or horny piece called the operculum. This must be distinguished in the first instance from another kind of calcareous covering, which in some univalve shells serves to close the aperture during a certain portion of the year. This piece, named the epiphragm, although hardened and shelly in appearance, is no real part of the animal or of the shell; being only a secretion temporarily hardened, for the purpose of defending the animal from external influences during the hibernating or torpid season, to be dissolved when that season is at an end. On examining this piece, it will be observed that it is not formed in regular layers like the rest of the shell; while the true operculum is of a regularly laminated structure, having a nucleus and receiving obliquely deposited additions, either in a lateral spiral or concentric direction. It is attached to the posterior part of the foot on the upper surface; and when the animal retires within its shell, that part of the foot enters last, drawing the operculum after it, and thus closing the aperture.

The opercula of various shells differ in the first place as to their chemical composition. They are calcareous when formed principally of calcareous matter, like the rest of the shell, as in Neritina, Nerita, and some others. They are corneo-calcareous, when upon an internal lamina of horny consistency there is a thickened layer of shelly matter. This is the case with shells of the genus Turbo and Phasianella, which are on this account distinguished from those of the genus Trochus; the opercula of the latter being horny or corneus.

The size of the operculum is distinguished by comparison with the rest of the shell; thus, those of Strombus, Cassis, &c. are small; while those of Cyclostoma and others are large, filling up the cavity at its outer edge.

The direction in which the successive layers are deposited, forms another ground of distinction. The disc is formed in some instances of a series of whorls, the apex or nucleus being more or less central; if these whorls are numerous, the operculum is described as multispiral, as in shells of the genus Trochus; if few, as in Cyclostoma, it is paucispiral. In some instances the flattened spire consists of but one whorl, it is then unispiral; and when scarcely one turn is completed, it is described as subspiral. When the layers are applied upon each other in such a manner that the nucleus is central, and the edges of the subsequent layers are extended beyond each other all around, so as to form rims, the operculum is described as concentric; if the nucleus is lateral, or at one side without being spiral, it is lammellated; and when it forms a terminal point, enlarging in the form of a finger-nail or claw, it is unguiculated. In the operculum of a Neritina, there is a lateral process, by means of which it is locked into the columella, the term articulated is then applied. In that of Navicella, there is also a process which appears to radiate from the nucleus, it has therefore been described as a radiated operculum.

Opercula of Spiral Univalves.

BIVALVE SHELLS.

Bivalve shells, named Conchacea by Lamarck, are those which consist of two principal portions united to and folded upon each other by means of a hinge. The pieces united compose the shell, while each piece separately is called a valve. Considering the bivalve shell as a whole, it will be necessary, in the first instance, to describe the position in which it is to be observed, in order to give the student a clearly defined notion of what is intended, when terms expressive of height, depth, length, breadth, &c. are used, as well as when the anterior and posterior extremities are spoken of. For this purpose, we must suppose the animal to be living and creeping along the bed of the sea by means of its foot; where this foot protrudes, will be the ventral margin, and the opposite part the dorsal margin of the shell. There will then be a valve on each side; and if we further suppose the animal to be walking forward with its back to the observer, the right and left valves will correspond with his right and left sides.

Measurements.

The length will be measured from anterior (a) to posterior (p), and the lines of growth running in the same direction will consequently be longitudinal or concentric; transverse of some authors. The height will be from the umbones (u), to the ventral margin, and lines or bands in that direction are termed radiating; longitudinal, according to some authors.

The points from which the growth of the shell commences, are called the umbones; these usually turn towards the anterior part of the shell: if this circumstance fails to point out the anterior, it may in many cases be distinguished by the muscular impressions of the mantle. If this has a sinus or winding, it is always near the posterior muscular impression; and in all cases where there is an external ligament, it is on the posterior side.

There is sometimes an impression near the front of the umbones, which forms a semicircle on each valve; the space within this semicircle is called the lunule (wood-cut, fig. 71 and 72, l. l. l.); a corresponding depression, when it exists on the posterior margin near the umbones, is named the escutcheon.

Hinge.

The hinge of the shell is on the dorsal margin, and is composed of the various apparatus by which the two valves act upon each other in opening and shutting. It consists of a ligament, which is placed on the dorsal margin, just at the back of the umbones, and unites the two valves together; the cartilage or thick gristly elastic substance, sometimes found close to the ligament, to which it then forms an inner coating, and sometimes received into a pit within the shell. It serves the purpose of keeping the shell open when not forcibly closed by the adductor muscles. An inner layer of shelly matter upon which are placed teeth, and pits to receive them on the two valves reciprocally. Each of these it will be necessary to treat of more at large; observing, at the same time, that in some species of Bivalves these parts may be wholly or partially wanting. Thus we meet with some shells, such as the Muscle, without teeth; and there is the group containing Pholas, &c. the hinge of which is destitute of teeth and ligament, the two valves being kept together by loose cartilages, and by the contracted space in which they are confined.