Part 3
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.
_Ligament and Ligamentary Cartilage of the Hinge._
These two distinct substances have been described by many writers as though, composing the same mass, they were of one substance; but the difference may very easily be explained. The _true ligament_ is external, being fixed on the edge of one valve behind the umbones, and passing over in an arch to the corresponding edge of the other, very correctly retaining the name of _ligament_, because it serves the purpose of binding the two together. The thick, elastic substance, which Mr. Gray names the _cartilage_, is sometimes found in connexion with the ligament, so as to form one mass with it, although it is always separable and placed within it: it is sometimes placed quite within the shell, and separated from the ligament, in a pit or hollow formed for its reception in the hinge lamina, near the centre. It is found in both valves, and being elastic, the portion in one valve presses against that in the other, so as to keep the valves apart, unless voluntarily closed by the adductor muscles of the animal. The ligament is sometimes spread over an external area, as in Arca, while the cartilage is placed in several grooves of the same area, beneath the outer covering.
_Hinge lamina, Teeth and Fulcrum of the Ligament._
In a great variety of cases, there is a thickening of the substance of the shell within, under the dorsal margin; this is named the hinge lamina. It is sometimes merely callous; but in many cases it has raised _teeth_ in both valves, those in one valve entering into corresponding cavities in the other. Those which are placed immediately below the umbones, and seem to take their rise from beneath them, are called _cardinal teeth_; those at a distance from the umbones, which are seen to lie along the upper margin of the shell are named _lateral teeth_.
When the cardinal teeth terminate in a double point, which is not unfrequently the case, they are said to be _bifid_. The lateral teeth, in various species, are distinguished as terminating _near_ to, or at a _distance_ from the umbones. In the Nuculæ and Arcæ there is a row of teeth placed across the hinge lamina. In which case, the lateral cannot be distinguished from the cardinal teeth.
_Muscular Impression._
Lamarck divides the Bivalve shells into two general orders; the first is named "Dimyaria," having two adductor muscles; and the second, "Monomyaria," having but one. These adductor muscles are used for the purpose of drawing the valves together, being composed of contractile fibrous gristle, fastened firmly to the inner surface of each valve. The place where they are thus fixed may be seen when the animal is removed, by depressed areas, which are generally pretty well defined, and are named _muscular impressions_. Where there is but one adductor muscle, there will be but one of these impressions near the centre of each valve, but in the Dimyaria, where there are two, the impressions are seen, one on the anterior, and one on the posterior of each valve, just below the _hinge_ lamina. They are sometimes _complex_, that is composed of several portions in a group; but in general, they are simple and well defined.
They are also described as large or small, in proportion to the size of the shell; regular or irregular in form. The animal is attached to the inner surface by the fibrous portions of the mantle, which creates a linear impression or _cicatrix_, commonly described as the _palleal impression_, or muscular impression of the mantle. It runs near the ventral margin from one muscular impression to the other, sometimes in a smooth _continuous_ line or band, and sometimes in an interrupted series of small impressions. Near the point of union with the posterior muscular impression, there is sometimes a more or less considerable winding inwards towards the centre of the shell, and back again towards the point of union. This is named the _sinus_, and is distinguished as being _angular_ or _rounded_, large or small, according to the species. When it enters towards the centre of the shell in a tongue-shaped outline it is said to be _ligulate_. Where it exists it affords a certain index to the posterior side of the shell; as it is the region through which the excretory tubes pass.
_Umbones._
These are the prominent points of the dorsal edge, where the growth of the shell commenced, and are called beaks, by some English writers. In some instances they are close to each other; in others they are rendered distant from each other by the intervention of areas in the hinge, as in Spondyli, &c. In Pectunculus they are _straight_; in Venus _curved_ towards the anterior margin; in Isocardia, _spiral_; in Chama, _decumbent_; in Diceras, _free_. In shells subject to external corrosion, the process commences at the umbones.
GENERAL CHARACTER OF BIVALVES.
When the _breadth_ is spoken of, the distance between the most convex parts of both valves, when closed, is intended; but when an expression implying _thickness_ is used, it refers to the substance of each valve: it is important to bear this in mind, as many persons have been misled by descriptions in which the distinction has not been attended to. Glycimeris (fig. 67 in the plates) is a _thick_ shell, but Anatina (fig. 69 in the plates) is a _broad_ one.
_Regularity._
A great number of Bivalves are extremely regular in their form. These are generally locomotive, and consequently free from those obstructions in growth occurring to stationary shells, which being confined in a particular position, or to a particular spot, modify their shape according to the substance with which they come in contact, and thus become irregular. This is generally the case with shells which are attached to submarine substances, such as Spondyli, Oysters, &c.; and the degree of irregularity will depend upon the extent of surface involved in the attachment. In the case of fixed shells, the attached valve is usually termed the under valve, and the other which moves freely upon the hinge, is termed the upper valve.
_Form and Proportions._
Bivalves are said to be _equivalve_ when the two valves correspond in extent, breadth, and thickness; and of course _inequivalve_ in the contrary case. They are _equilateral_ when a line drawn from the umbones to the ventral margin would divide the shell into two nearly equal parts; and of course _inequilateral_ in the opposite case, which occurs in the great majority of instances.
A Bivalve is said to be _compressed_, when the distance is small from the most prominent part of one valve to that of the other. It is _cylindrical_ when lengthened, and more or less rounded in its breadth, as in Lithodomus (fig. 161 in the plates). It is _cordiform_ when the shape presents a resemblance to an imaginary heart, as in Cardium cardissa (fig. 122 in the plates), and in the Isocardia (fig. 126 in the plates). It is _linguiform_ when it resembles a tongue in shape, as in Vulsella (fig. 185 in the plates); _rostrated_ when it protrudes at either extremity, and terminates in a kind of point, as in Sanguinolaria Diphos (fig. 99 in the plates); _truncated_ when it ends in a square or angle, as if cut off; an example of which may be seen in Solen (fig. 60 in the plates).
Other Bivalves are distinguished as being _auriculated_, having processes flattened and expanded on either side of the umbones, as in Pecten (cut, fig. 82). When there is one of these on each side of the umbones, it is _bi-auriculated_; when only on one side, it is _uni-auriculated_. When the expansion is very broad, as in Unio alatus (fig. 142 in the plates), and in the Hammer Oyster (cut, fig. 83), the term _alated_ is used.
With regard to these alated species of _Uniones_, it is necessary to observe that they are also "_adnate_," as it is termed; the two valves being joined to each other by the dorsal edge of the expanded parts, and united so completely in substance with each other, that they cannot be separated without being broken. Many other terms are used to express difference in Bivalves, but being generally applicable to Univalves and Multivalves, as well as to them, they will be found explained at large in the alphabetical part of the work.
MULTIVALVE SHELLS.
These are of three different kinds; first, the "_dorsal_," as they are termed by Linnæus, because they form a ridge in the back of the animal. They are composed of eight pieces, or separate valves, placed in a longitudinal series, being joined to each other by inserted lamina, and named _Articulata_ by De Blainville, on that account. The genus Chiton is the only example of this kind of Multivalves.
The second kind, M. De Blainville terms the _lateral_ bivalves, the pieces being placed in pairs on each side of the animal; these compose the "Pedunculated Cirripedes."
They differ considerably in the number and arrangement of the valves; the small ones, which are found near the peduncle in some species, are sometimes termed accessary valves; those which form the edge through which the bunch of Cilia protrude, are termed _ventral_, and those on the opposite side _dorsal_. The extremity joining the peduncle is the basal, or anterior; and the upper extremity is the apsiral, or posterior. The peduncle is the medium of attachment to submarine substances, to which this well known tribe of shells adhere.
The third kind are termed _coronular_ by De Blainville, and compose the order Sessile Cirripedes of Lamarck; they consist of a number of valves placed against each other side by side in a circle, supported on a plate, or tube, or cup, and closed by an operculum composed of two or more valves.
The _basal support_ is sometimes thick and flat, sometimes forming an elongated tube, and sometimes hollowed out into a cup. In other species it is altogether wanting. The operculum always consists of more than one piece, generally of two pairs: they are either articulated to each other by serrated edges, and placed against each other conically, as in Balanus, or they lie flat in two pairs against each other. Through the ventral pair the _cirrhi_ protrude.
The _parietal_ valves, composing the principal part of the shell, vary in number, form and position. The _anterior_ valves are placed on the same side with the cirrhi; the _posterior_, those on the opposite side; and those which remain between on each side are the lateral valves. In many cases, particularly in Balanus, each valve is separated into the _prominent_ and _depressed_ areas, and the inserted lamina. In some instances, the parietal portion is formed by a single rounded piece.
In the accompanying cut (87), the prominent areas are distinguished by the letters _pr_, and the depressed areas by _r_; the posterior valves of the operculum are marked _p. o._, and the anterior _a. o._ The basal valve (fig. 88) belongs to a Balanus. Fig. 89 is an Acasta, the cup-shaped base of which is represented at fig. 90.