Cojvchology (Gr.Cojvchology 050079 a shell, andCojvchology 050080 a discourse), the science which treats of the testaceous parts of the mollusca. Formerly these were made the basis for classifying the animals to which they belong; but the enlarged study, introduced by Cuvier, of the anatomical structure of the bodies contained within the shells, has developed an apparent want of correspondence between the latter and the organization of the more important parts, upon which the habits and true character of the animal must mainly depend. (See Mollusca.) In the old classification, molluscous animals possessed of the same peculiarities of internal structure were widely separated by the differences of their external covering; some animals, as the cirripedes, comprising nearly'all the old division of multivalves, were included with the mollusca. because of their shelly covering, and thus they continued until their structure at last was found to refer them to the articulata; and some mollusca possessed of no shell, as the nudibranchs, would properly be excluded from the arrangement that comprised others hardly differing from them, except in being provided with this appendage, or perhaps but a poor apology for it.

And then, as some species have the faculty of leaving and returning to their shells, it might occur, in ignorance of the relations of each part to the other, that the shell should be referred to one department and the body to another of the animal kingdom. Indeed, by reason of the changes of form and color that take place in the growth of some shells, the same species have been described at different ages of the individual under different names.

But though the important divisions of the mol-lusea are now based on their internal organization, the study of the shells is nevertheless not to be neglected in the pursuit of the general science of malacology, or the study of the mollusca; for the minor divisions into species are still generally controlled by differences in the forms of the shell; and in the important department of palaeontology, it is only the outer coverings of the ancient mollusca, or casts of these in more durable materials, that are presented to us, by which we may determine their character. - Shells consist of carbonate of lime secreted by the animal, and intermixed with some animal matter; they are usually heavier where calcareous formations prevail, the mollusca which bear them also being more abundant than in granitic and other districts deficient in lime. The secretion is first of an albuminous matter through the pores of the outer mantle of the embryo mollusk; this is generally succeeded by an admixture of crystalline particles of carbonate of lime; new layers form beneath these, and the shell gradually gains thickness and extension. In the species in which it is least developed it appears as a hollow cone or plate, which serves as a protection to the breathing organ and heart.

The mantle it covers in other species is in part the same important organ. The protuberances and ridges seen on many univalve and bivalve shells appear in the course of their growth by the margin of the mantle turning out at a considerable angle, and thus building up a plate in this position for a certain distance. This growth then ceases, the mantle retracts, or may be regarded as changing itself into the shelly layers, and then it extends in the original direction, carrying out the shell with it, till it turns again to form a second plate or ridge; and so the process goes on. The spines upon shells are produced by the mantle sending out from its margin tentacula, which secrete from their surface the calcareous matter, and thus form slender tubes. These become solid, as the fleshy matter, while it retracts and is absorbed, continues still to deposit the shelly layers. As each successive layer in the growth of shells extends beyond the line of the next older layer, the edges of these must appear upon the outer surface as marked lines of growth. Many mol-lusks possess the power of altering and enlarging their shells to adapt them to their growth, which they appear to do as if by an intelligent will.

Thus the murices remove the spines that interfere with the extension of the shell; and others enlarge the aperture, when it has become too contracted for the body, by wearing away or dissolving its walls. The growing cyprcea removes the inner wall of its shell, and extending its mantle around upon the outside, adds there, in successive layers, more than was taken from within. The shell not only thus changes in size,but the form of the adult becomes altogether different from that of the young. The distinguishing marks of shells are the number of parts of which they are composed, and their peculiar forms and prominences. They possess also different textures and various colors, often of great beauty. Some consist of a single piece, as those univalves which are not provided with the horny or calcareous operculum, such as is attached to the posterior part of the foot of other species, and which, when the foot is drawn in, serves to close the aperture as with a tight and perfectly fitting cover. In these the shell is in two pieces; so also is it in most of the bivalves, and the resemblance of the arrangement has led some conchologists to suppose that the operculum may be regarded as the equivalent of the dextral valve of the con-chifera; but the anatomical structure forbids this conclusion.

In the terebratula the shell is in three parts, and in some of the pholades and chitons it is in four or more. The peculiar forms of the aperture in some of the univalves are found to bear constant relations to the internal organization of the animal, and so far are therefore to be relied on as indicative of its habits and character. Such are the grooved and notched siphons in some of the spiral univalves peculiar to marine mollusca, which breathe by two gills furnished with a fleshy tube for supplying to them the water. The textures of shells are described as porcellanous, pearly or nacreous, fibrous, horny, and some are glassy and translucent. These textures are produced by the different manner in which the particles of mineral and animal matter are arranged, which is best exposed when the shells are partially decomposed by the disappearance of the animal substance, a condition often met with in fossil shells. In some of them the complicated arrangement of the three strata of plates, which make up the porcellanous structure, is exhibited in the broken edges of the cyprseas, cones, etc. The outer and inner layers present the edges of the plates to the surface, while those of the middle one lie in groups at different angles.

Each plate is composed of minute prismatic cells, arranged at angles of 45° with the plane of the plate, and these meet the cells of the adjoining plates at right angles. The animal matter of the porcellanous shells is soluble gelatine. The pearly shells are in alternating layers of very thin albuminous membrane and carbonate of lime, which by their minute undulations give the pearly lustre. This structure is the least permanent, and in some geological formations the shells that were provided with it have disappeared, leaving only their casts, while those of fibrous texture are preserved unchanged. This texture is produced by an arrangement of prismatic cells of carbonate of lime, which extend continuously through the successive layers that make up the shell. It is well exhibited in the heavy fossil inoceramus, which sometimes is found falling into fibrous blocks, the grain transverse to the surface of the shell. The oyster shell, in which the prismatic cells are irregularly arranged in the successive layers, separates into plates parallel with its surface - an example of laminated structure.

In the brachiopoda the structure is so peculiar that it may be detected in the smallest fragment of shell; it consists of elongated and curved cells matted together, and often perforated by circular holes arranged in quincunx order. Colors, however beautifully exhibited upon the surfaces of shells, are to them no more distinctive features than to the minerals and flowers upon which they are also brilliantly displayed. They are most richly developed upon those surfaces most exposed to the light, and in the class of shells that inhabit shallow waters. In some instances they have proved as permanent as the shell itself, being preserved in a few of the fossils of various geological formations, even as old as those of the Devonian period. The color is usually limited to the surface beneath the epidermis, which is the membrane of animal matter coating the entire shell, and protecting it from the action of destructive chemical agents. But some of the shells of porcellanous texture present different colors in adjoining layers, a peculiarity which adapts them for the carved work in colors described in the article Cameo. - The shells of' a single piece appear in various forms; most of them are in spiral convolutions, which form logarithmic curves.

In some the spire is wound around an axis called a columella, which is entirely concealed within the whorls; in others these wind about without coming in contact, the shell then resembling a corkscrew. Some are smooth conical bodies, terminated at the base of the cone by an insignificant spire, and with a long aperture on one side. In some the spiral form is very prominent in the young individuals, and is afterward entirely concealed under the successive layers of growth. In a few instances the convolutions are so flattened that no conical form is produced. The argonaut is an example of the last, a shell almost as delicate as paper, and so light as to be no impediment to the animal's rising to the surface by means of its distended air cells. The haliotis (sea-ear) is also an example of a very flat and in this instance obscure spire. The aperture of this shell is open, resembling the ear in form, whence its name. To the animal the shell is a covering shield, which protects him as he draws it down, and holds by his broad foot firmly to the rock. Many other mollusca similarly provided are distinguished by shells shaped like a shield, as umbrella; or like a boat, as navicella; or of more conical form, as the patella or limpet.

The univalves take also the form of a tube open at each end. - In describing shells, they are supposed to be in the position which the animals assume when in motion and progressing from the observer. The aperture of the univalves is downward, and the spire points backward and upward; the two sides are distinguished as right and left. The whorls wind obliquely from the apex of the spire from left to right in the greater number of species; those in which the direction is reversed are known as reverse or sinistral shells. Individual exceptions occur in the species of each group. Bivalve shells of the class conchifera, in which all are included that breathe by two pairs of gills, take when in motion a position with the hinge of the valves upward. Two prominences called the umbones are observed, one on each valve bending toward each other; the beak or apex of each of these corresponds to the apex of the spire of the univalves. It often inclines toward one end of the shell; this is always the anterior extremity, and the opposite the posterior. The latter is also marked in many bivalves by a notch or sinus in the pallial impression upon the inner surface of the shells, always opening, when it occurs, toward this extremity.

The pallial impression is the mark of the margin of the mantle where it was attached to the shell. The position of some bivalves, as the large scallops, oysters, etc, is indicated only by the large muscular impression which is always on the posterior end unless it be between the umbones. The bivalve being then placed in its natural position for progressing from the observer, the valves are designated, one as the right, the other as the left. If the two be of the same size and shape, as is the case in the unattached con-chifera, the shell is said to be equivalve; but when one valve is attached to a foreign body, this is larger and deeper than the other, and the shell is said to be inequivalve; if one end be longer than the other, the shell is inequilateral. This measurement is made along the horizontal line of greatest length from each extremity to the vertical line dropped from the highest point of the umbones to the base. The anterior end is usually found to be shorter than the posterior. The upper margin is called the dorsal, and the lower the ventral. The breadth is the length of the vertical line from the dorsal margin to the base, and the thickness is the distance between the valves when closed.

The valves may join closely together, or gape open at one or both ends, or even be furnished with a vertical opening by corresponding notches in the margin of the valves. These peculiarities of the form of the shell have relation to the internal organization of the animal, the openings being one for the purpose of allowing the protrusion of the foot for propelling the shell, another for the byssus by which it is firmly anchored, and another for the long siphon, the respiratory organ, which serves to inhale the water to be filtered by the gills, and then to pass it back. Bivalve shells are strongly marked on their inner surface, near the dorsal margin, by the depressions for the muscles which draw the valves together, and some present prominent apophyses for the attachment of these adductors. The shells also bear the impressions of the attachment of the foot, byssus, siphons, and mantle. But the most important marks are the articulations of the hinge, called teeth. These, by their different numbers and positions along the dorsal margin, serve as distinguishing marks of species, and even of genera; a distinction to which they are properly entitled from the fact that their varying forms indicate corresponding changes in the organization of the soft parts.

The ligaments which hold the valves together are an external horny one lying behind the umbones, which is stretched by the closing of the valves, and an internal short fibrous one, which is compressed endwise as the valves are drawn together by the adductor muscles. As these are relaxed by death or otherwise, the shells are pushed partly open by the elasticity of the cartilage. The marks upon the inner surface of the valves left by these organs are important features in the description and classification of bivalve shells. - In the embryo of these animals the umbones are the first formed portions of their shelly covering. In most genera this is carried out with the growth of the body, so as always to serve as its complete protection; but in others it attains to the extent of only a partial covering, and in these instances other curious provisions are made for completing the necessary defence. The teredos are slender worm-like animals, that sometimes attain a length of 2 1/2 ft., and one species even reaches 3 ft. By close observation only are the two shells perceived at the larger extremity of this long siphonal tube. They cover the visceral organs, but admit between them in front the fleshy foot, by means of which the creature is able to burrow into wood.

Here he excavates long tubes adapted to his form, and finishes them with a smooth calcareous lining, thus providing an artificial covering for the unprotected portion of the body. Securing himself permanently to the inner extremity, his communication without is thereafter only through the siphonal tube. The pholades burrow into limestone and other stony substances which are less hard and tenacious than their own valves; other shells even are penetrated by them. The front portion of these is ribbed like a rasp, and is thought to answer the purpose of one in forming cavities. The whole shell is of unusual hardness, its structure more resembling that of aragonite than of calcareous spar. Being thin and without ligaments, it is strengthened by accessory valves upon the dorsal margin. In some attached bivalves, as the oyster, the embryonic umbo often takes the form of the surface to which the valve is fastened, and this form is afterward retained. With the growth of the animal the original shield loses its importance, and as in the univalves the spire, which corresponds to the umbo, is often deserted, and becoming dead and brittle falls away (in which case it is said to be decollated), so in the bivalves the umbones wear out and the layers of shell scale off, leaving the summit ragged and unsightly; in this condition they are said to be decorticated.

The fresh-water univalves and bivalves are especially subject to this erosion. The injury that might result from this removal of the shell is obviated in some cases by the deposit of new layers within, or by the construction of partitions, or, as in some tubular shells which continue to advance upward with the growth of the coral that encloses them, by filling the deserted portion with solid calcareous matter. Some peculiarities of the shells of the other class of bivalves, the bra-chiopoda (by some recent authors placed among the articulates), are given under that head. These valves are distinguished as dorsal and ventral, instead of right and left. The ventral is commonly the larger one, and is surmounted by a prominent perforated beak, through which the organ passes by which it is attached to foreign bodies. This valve resembles in shape the antique lamps, and the hole in the beak corresponds to that for the wick; they are hence known as lamp shells. - The class cephalopoda includes many genera whose shells are concealed within the soft parts. Many of these are known only as fossils, and nothing is left to indicate their former existence but the numerous stony pointed bodies and pens, which were at the same time the skeleton and partial receptacle of the soft portion of the animal.

One of these calamary or squid-like fossils, belemnoteuthis, found in the Oxford clay at Chippenham, England, was so perfectly preserved that the muscular mantle, fins, ink bag, funnel, eyes, and tentacles with their horny hooks, were all distinguishable. A similar discovery, described by Buckland, is noticed in the article Belemnite. Some of the fossil ink bags found in the lias are nearly a foot in length, and are invested with a brilliant pearly layer; the ink forms excellent sepia. In the sepiadae of this class the shell is the spongy calcareous substance known as cuttle bone. It is a broad laminated plate, as long and wide as the body in which it is enclosed. It grows to an extraordinary size, one being noticed by Banks and Solander in the Pacific, near the equator, which was about 6 ft. in length. To the same class belongs the nautilus pompilius, the only living representative of the order tetrabranchiata, of which fully 1,400 fossil species are known, among which are the ammonites. Its light shell is separated into air chambers by numerous partitions, which greatly increase its strength and add but little to its weight.

The four species of argonauts are also cephalopods; their shell is thin and translucent like paper, and serves as a boat to the animal that sits unattached within, his long tentacles stretching forth from the projected head, and the short funnel beneath it forcibly spurting forth water, the recoil of which sends the animal backward. This is the shell which Aristotle called the nautilus, and fancifully described, as many others have since, under the name of paper nautilus, as floating in fine weather upon the surface with its sails spread to the breeze. - The whole number of species of molluscous animals known is estimated at about 12,000 recent and 15,000 fossil. Many of the living species furnish wholesome food, and some are esteemed as delicacies. The shells are principally composed of carbonate of lime, the other ingredients, of the oyster for instance, being 1.2 per cent, of phosphate of lime, and 0.5 of animal matter; the shells of the brachiopods consist largely of phosphate of lime. They are therefore well adapted for being burnt to quicklime, used as calcareous fluxes in smelting ores, or applied as fertilizers to the soil.

The marine shells, by the immense numbers in which they are produced, perform an important office in abstracting from the sea water its excess of calcareous matter, and thus aiding to maintain its purity. As objects of beauty, shells have always been admired, and frequently appropriated to uses as ornaments. Some varieties were used by the Athenians as ballots, with the name upon them of the person to be banished, whence the term ostracism. Some shells, as the cyprcea moneta, have served the purposes of coin among rude nations. Others, as the marine avicula margaritifera and the fresh-water unio margaritiferus, are noted for the pearls which are secreted between their valves around some foreign substance. Mother-of-pearl is the polished shell of nacreous structure. Rare species of shells are highly prized by collectors, and single specimens have been sold for exorbitant sums. Woodward ("Manual of the Mollusca") cites the sums once paid for such, as 100 guineas for a carinaria, now worth only 1s.; 40 for a wentletrap, now worth 5s. The conus gloria maris has fetched £50 more than once, and cyprcea umbilicata sold for £30 in 1850.

Univalves.   1. Navicella. 2. Umbrella. 3. Patella.

Univalves. - 1. Navicella. 2. Umbrella. 3. Patella.

Bivalve Shell.   a, a, bosses; b, b, hinges; c, principal teeth.

Bivalve Shell. - a, a, bosses; b, b, hinges; c, principal teeth; d, d, lateral teeth; e, e, muscular impressions; f, pallial impression; g, g, sides of the shell; h, ligament; k, ventral edge; o, front edge; r, umbo.