The popular names of ‘shells,’ ‘shell-fish,’ and the like, as commonly applied to the Mollusca, the intrinsic beauty and grace of the shells themselves, resulting in the passion for their collection, their durability and ease of preservation, as compared with the non-testaceous portion,—all these considerations tend to unduly exalt the value of the shell as part of the organism as a whole, and to obscure the truth that the shell is by no means the most important of the organs.

At the same time it must not be forgotten that the old systems of classification, which were based almost entirely on indications drawn from the shell alone, have been strangely little disturbed by the new principles of arrangement, which depend mainly on structural points in the animal. This fact only tends to emphasise the truth that the shell and animal are in the closest possible connexion, and that the shell is a living part of the organism, and is equally sensitive to external influences.

A striking instance of the comparative valuelessness of the shell alone as a primary basis of classification is furnished by the large number of cases in which a limpet-shaped shell is assumed by genera widely removed from one another in cardinal points of organisation. This form of shell occurs in the common limpet (Patellidae), in Ancylus (Limnaeidae), Hemitoma (Fissurellidae), Cocculina (close to Trochidae), Umbrella and Siphonaria (Opisthobranchiata), while in many other cases the limpet form is nearly approached.

Roughly speaking, about three-quarters of the known Mollusca, recent and fossil, possess a univalve, and about one-fifth a bivalve shell. In Pholas, and in some species of Thracia, there is a small accessory hinge plate; in the Polyplacophora, or Chitons, the shell consists of eight plates (see Fig. 2, p. 8), usually overlapping. A certain proportion of the Mollusca have no shell at all. In many of these cases the shell has been present in the larva, but is lost in the adult.

The shell may be

(1) External, as in the great majority of both univalves and bivalves.

(2) Partly external, partly internal; e.g. Homalonyx, Hemphillia, some of the Naticidae, Scutum, Acera, Aplustrum (Figs. 148 and 149).

(3) Internal; e.g. Philine, Gastropteron, Pleurobranchus, Aplysia, Limax, Arion, Hyalimax, Parmacella, Lamellaria, Cryptochiton, and, among bivalves, Chlamydoconcha.

(4) Absent; e.g. all Nudibranchiata and Aplacophora, many Cephalopoda, a few land Mollusca, e.g. all Onchidiidae, Philomycus, and Vaginula.

The Univalve Shell.—In univalve Mollusca the normal form of the shell is an elongated cone twisted into a spiral form round an axis, the spiral ascending to the left. Probably the original form of the shell was a simple cone, which covered the vital parts like a tent. As these parts tended to increase in size, their position on the dorsal side of the animal caused them gradually to fall over, drawing the shell with them. The result of these two forces combined, the increasing size of the visceral hump, and its tendency to pull the shell over with it, probably resulted in the conversion of the conical into the spiral shell, which gradually came to envelop the whole animal. Where the visceral hump, instead of increasing in size, became flattened, the conical shape of the shell may have been modified into a simple elliptical plate (e.g. Limax), the nucleus representing the apex of the cone. In extreme cases even this plate dwindles to a few calcareous granules, or disappears altogether (Arion, Vaginula).

Varieties of the Spiral.—Almost every conceivable modification of the spiral occurs, from the type represented by Gena, Haliotis, Sigaretus, and Lamellaria, in which the spire is practically confined to the few apical whorls, with the body-whorl inordinately large in proportion, to a multispiral form like Terebra, with about twenty whorls, very gradually increasing in size.

As a rule, the spire is more or less obliquely coiled round the axis, each whorl being partially covered, and therefore hidden by, its immediate successor, while the size of the whorls, and therefore the diameter of the spire as a whole, increases somewhat rapidly. The effect of this is to produce the elevated spire, the shell of six to ten whorls, and the wide aperture, of the normal type of mollusc, the whelk, snail, periwinkle, etc.

Sometimes, however, the coil of the whorls, instead of being oblique, tends to become horizontal to the axis, and thus we have another series of gradations of form, from the excessively produced spire of Terebra to the flattened disc of Planorbis, Polygyratia, Euomphalus, and Ammonites. The shell of many species of Conus practically belongs to the latter type, each whorl folding so closely over its predecessor that the spiral nature of the shell is not perceived until it is looked at at right angles to the spire.

In some cases the regularly spiral form is kept, but the whorls are completely disconnected; e.g. some Scalaria, Spirula; among fossil Cephalopoda, Gyroceras, Crioceras, and Ancyloceras; and, among recent land Mollusca, Cylindrella hystrix and Cyathopoma cornu (Fig. 151). Sometimes only the last whorl becomes disconnected from the others, as in Rhiostoma (see Fig. 180, p. 266), Teinostoma, and in the fossil Ophidioceras and Macroscaphites. Sometimes, again, not more than one or two whorls at the apex are spirally coiled, and the rest of the shell is simply produced or coiled in an exceedingly irregular manner, e.g. Cyclosurus, Lituites, Orygoceras, Siliquaria (Fig. 153), Vermetus. In Coecum (Fig. 170, p. 260) the spiral part is entirely lost, and the shell becomes simply a cylinder. In a few cases the last whorl is coiled irregularly backwards, and is brought up to the apex, so that the animal in crawling must carry the shell with the spire downwards, as in Anostoma (Fig. 154), Opisthostoma (Fig. 208, p. 309), Strophostoma, and Hypselostoma (Fig. 202 A, p. 302).

Some genera of the Capulidae, in which the shell is of a broadly conical form or with scarcely any spire, develop an internal plate or process which serves the purpose of keeping the animal within the shell, and does the work of a strong attachment muscle. In Mitrularia this process takes the form of a raised horse-shoe; in Crucibulum it is cup-shaped, with the edge free all round; in Galerus, Ergaea, Crepipatella, and Trochita we get a series of changes, in which the edge of the cup adheres to the interior of the shell, and then gradually flattens into a plate. In Crepidula proper this plate becomes a regular partition, covering a considerable portion of the interior (Fig. 155 G). Hipponyx secretes a thin calcareous plate on the ventral surface of the foot, which intervenes like an operculum between the animal and the substance to which it adheres.

Sinistral, or Left-handed Shells.—The vast majority of univalve spiral shells are normally dextral, i.e. when held spire uppermost, with the aperture towards the observer, the aperture is to the right of the axis of the spire. If we imagine such a shell to be a spiral staircase, as we ascended it we should always have the axis of the spire to our left.

Sinistral or ‘reversed’ forms are not altogether uncommon, and may be grouped under four classes:—

(1) Cases in which the genus is normally sinistral; (2) cases in which the genus is normally dextral, but certain species are normally sinistral; (3) cases in which the shell is indifferently dextral or sinistral; (4) cases in which both genus and species are normally dextral, and a sinistral form is an abnormal monstrosity.

In all cases of sinistral monstrosity, and all in which a sinistral and dextral form are interchangeable (sections 3 and 4 above), the position of the apertures of the internal organs appears to be relatively affected, i.e. the body is sinistral, as well as the shell. This has been proved to be the case in all specimens hitherto examined, and may therefore be assumed for the rest. The same uniformity, however, does not hold good in all cases for genera and species normally sinistral (sections 1 and 2). As a rule, the anal and genital apertures are, in these instances also, to the left, but not always. In Spirialis, Limacina, Meladomus, and Lanistes the shell is sinistral, but the animal is dextral. This apparent anomaly has been most ingeniously explained by Simroth, Von Ihering, and Pelseneer. The shell, in all these cases, is not really sinistral, but ultra-dextral. Imagine the whorls of a dextral species capable of being flattened, as in a Planorbis, and continue the process, still pushing, as it were, the spire downwards until it occupies the place of the original umbilicus, becoming turned completely ‘inside out,’ and we have the whole explanation of these puzzling forms. The animal remains dextral, the shell has become sinistral. A convincing proof of the truth of this is furnished by the operculum. It is well known that the twist of the operculum varies with that of the shell; when the shell is dextral, the operculum is sinistral, with its nucleus near the columella, and vice versâ. In these ultra-dextral shells, however, where it is simply the method of the enrolment of the spire that comes in question, and not the formation of the whorls themselves, the operculum remains sinistral on the apparently sinistral shell.

The reverse case to this, when the shell is dextral but the orifices sinistral, is instanced by the two fresh-water genera Pompholyx (from N. America), and Choanomphalus (L. Baikal). A similar transition in the enrolment of the whorls may be confidently assumed to have taken place, and the shells are styled ultra-sinistral.

Yet another variation remains, in which the embryonic form is sinistral, but the adult shell dextral, the former remaining across the nucleus of the spire. This is the case with Odostomia, Eulimella, Turbonilla, and Mathilda, all belonging to the Prosobranchiata, with Actaeon, Tornatina, and Actaeonina among the Opisthobranchs, and Melampus alone among Pulmonates.

Monstrosities of the Shell.—Abnormal growths of the shell constantly occur, some of them being scarcely noticeable, except by a practised eye, others of a more serious nature, involving an entire change in the normal aspect of the creature. Scalariform monstrosities are occasionally met with, especially in Helix and Planorbis, when the whorls become unnaturally elevated, and sometimes quite disjoined from one another; carinated monstrosities develop a keel on a whorl usually smooth; acuminated monstrosities have the spire produced to an extreme length (Fig. 158); sinistral monstrosities (see above) have the spire reversed: dwarfs and giants, as in our own race, are occasionally noticed among a crowd of individuals.

More serious forms of monstrosity are those which occur in individual cases. Mr. S. P. Woodward once observed[332] a specimen of an adult Helix aspersa with a second, half-grown individual fixed to its spire, and partly embedded in the suture of the body whorl. The younger snail had died during its first hibernation, as was shown by the epiphragm remaining in the aperture, and its neighbour, not being able to get free of the incubus, partially enveloped it in the course of its growth. In the British Museum two Littorina littorea have become entangled in a somewhat similar way (Fig. 160 B), possibly as a result of embryonic fusion. Double apertures are not uncommon[333] in the more produced land-shells, such as Cylindrella and Clausilia (Fig. 160 A). In the Pickering collection was a Helix hortensis which had crawled into a nutshell when young, and, growing too large to escape, had to carry about this decidedly extra shell to the end of its days. A monstrosity of the cornucopia form, in which the whorls are uncoiled almost throughout, is of exceedingly rare occurrence (Fig. 161).

Some decades ago ingenious Frenchmen amused themselves by creating artificial monstrosities. H. aspersa was taken from its shell, by carefully breaking it away, and then introduced into another shell of similar size (H. nemoralis, vermiculata, or pisana). At the end of several days attachment to the columella took place, and then growth began, the new shell becoming soldered to the old, and the spiral part of the animal being protected by a thin calcareous envelope. A growth of from one to two whorls took place under these conditions. The individuals so treated were always sordid and lethargic, but they bred, and naturally produced a normal aspersa offspring.[334] In the British Museum there is a specimen of one of these artificial unions of a Helix with the shell of a Limnaea stagnalis.

Composition of the Shell.—The shell is mainly composed of pure carbonate of lime, with a very slight proportion of phosphate of lime, and an organic base allied to chitin, known as conchiolin. The proportion of carbonate of lime is known to vary from about 99 p.c. in Strombus to about 89 p.c. in Turritella. Nearly 1 p.c. of phosphate of lime has been obtained from the shell of Helix nemoralis, and nearly 2 p.c. from that of Ostrea virginica. The conchiolin forms a sort of membranous framework for the shell; it soon disappears in dead specimens, leaving the shell much more brittle than it was when alive. Carbonate of magnesia has also been detected, to the extent of ·12 p.c. in Telescopium and ·48 p.c. in Neptunea antiqua. A trace of silica has also occasionally been found.

When the shell exhibits a crystalline formation, the carbonate of lime may take the form either of calcite or aragonite. The calcite crystals are rhombohedral, optically uniaxal, and cleave easily, while the aragonite cleave badly, belong to the rhombic system, and are harder and denser, and optically biaxal. Both classes of crystal may occur in the same shell.

Two main views have been held with regard to the formation and structure of the shell—(1) that of Bowerbank and Carpenter, that the shell is an organic formation, growing by interstitial deposit, in the same manner as the teeth and bones of the higher animals; (2) that of Réaumur, Eisig, and most modern writers, that the shell is of the nature of an excretion, deposited like a cuticle on the outside of the skin, being formed simply of a number of calcareous particles held together by a kind of ‘animal glue.’ Leydig’s view is that the shell of the Monotocardia is a secretion of the epithelium, but that in the Pulmonata it originates within the skin itself, and afterwards becomes free.[335]

According to Carpenter, when a fragment of any recent shell is decalcified by being placed in dilute acid, a definite animal basis remains, often so fine as to be no more than a membranous film, but sometimes consisting of an aggregation of ‘cells’ with perfectly definite forms. He accordingly divides all shell structure into cellular and membranous, according to the characteristics of the animal basis. Cellular structure is comparatively rare; it occurs most notably in Pinna, where the shell is composed of a vast multitude of tolerably regular hexagonal prisms (Fig. 162 B). Membranous structure comprises all forms of shell which do not present a cellular tissue. Carpenter held that the membrane itself was at one time a constituent part of the mantle of the mollusc, the carbonate of lime being secreted in minute ‘cells’ on its surface, and afterwards spreading over the subjacent membrane through the bursting of the cells.

The iridescence of nacreous shells is due to a peculiar lineation of their surface, which can be readily detected by a lens. According to Brewster, the iridescence is due to the alternation of layers of granular carbonate of lime and of a very thin organic membrane, the layers very slightly undulating. Carpenter, on the other hand, holds that it depends upon the disposition of a single membranous layer in folds or plaits, which lie more or less obliquely to the general surface, so that their edges show as lines. The nacreous type of shell occurs largely among those Mollusca which, from other details in their organisation, are known to represent very ancient forms (e.g. Nucula, Avicula, Trigonia, Nautilus). It is also the least permanent, and thus in some strata we find that only casts of the nacreous shells remain, while those of different constitution are preserved entire.

Porcellanous shells (of which the great majority of Gasteropoda are instances) usually consist of three layers, each of which is composed of a number of adjacent plates, like cards on edge. The inclination of the plates in the different layers varies, but that of the plates in the inner and outer layer is frequently the same, thus if the plates are transverse in the middle stratum, they are longitudinal in the inner and outer strata, and, if longitudinal in the middle, they are transverse in the other two. Not uncommonly (Fig. 163 B) other layers occur. In bivalves the disposition and nature of the layers is much more varied.

In Unio the periostracal or uppermost layer is very thin; beneath this is a prismatic layer of no great depth, while the whole remainder of the shell is nacreous (Fig. 162 A). Many bivalves show traces of tubular structure, while in the Veneridae the formation and character of the layers approaches closely to that of the Gasteropoda. Further details may be gathered from Carpenter’s researches.[336]

Formation of Shell.[337]—The mantle margin is the principal agent in the deposition of shell. It is true that if the shell be fractured at any point, the hole will be repaired, thus showing that every part of the mantle is furnished with shell-depositing cells, but such new deposits are devoid of colour and of periostracum, and no observation seems to have been made with regard to the layers of which they are composed. As a rule the mantle, except at its margin, only serves to thicken the innermost layer of shell.

It is probable that the carbonate of lime, of which the shell is mainly composed, is separated from the blood by the epithelial cells of the mantle margin, and takes the crystalline or granular form as it hardens on exposure after deposition. The three layers of a porcellanous shell are deposited successively, and the extreme edge of the mouth, when shell is forming, will contain only one layer, the outermost; a little further in, two layers appear, and further still, three. The pigment cells which colour the surface are situated at the front edge of the mantle margin.

Shelly matter is deposited, and probably secreted, not only by the mantle, but also in some genera by the foot. This is certainly the case in Cymbium, Oliva, Ancillaria, Cassis, Distortio, and others, in several of which the foot is so large that the shell appears to be quite immersed in it.[338]

The deposition of shell is not continuous. Rest periods occur, during which the function is dormant; these periods are marked off on the edge of the shell, and are known as lines of growth. In some cases (Murex, Triton, Ranella), the rest period is marked by a decisive thickening of the lip, which persists on the surface of the shell as what is called a varix (see p. 263).

The various details of sculpture on the exterior surface of the shell, the striae, ribs, nodules, imbrications, spines, and other forms of ornamentation are all the product of similar and corresponding irregularities in the mantle margin, and have all been originally situated at the edge of the lip. Spines, e.g. those of Murex and Pteroceras, are first formed as a hollow thorn, cleft down its lower side, and are afterwards filled in with solid matter as the mantle edge withdraws. What purpose is served by the extreme elaboration of these spiny processes in some cases, can hardly be considered as satisfactorily ascertained. Possibly they are a form of sculptural development which is, in the main, protective, and secures to its owners immunity from the attacks of predatory fishes.

‘Attached’ genera (e.g. Chama, Spondylus) when living on smooth surfaces have a flat shell, but when affixed to coral and other uneven surfaces they become very irregular in shape. The sculpture of the base on which they rest is often reproduced in these ‘attached’ shells, not only on the lower, but also on the upper valve, the growing edge of which rests on the uneven surface of the base. Oysters attached to the branches of the mangrove frequently display a central convex rib, modelled on the shape of the branch, from which the plaits of sculpture radiate, while specimens fixed to the smooth trunk have no such rib. Crepidula, a genus which is in the habit of attaching itself to other shells, varies in sculpture according to that of its host. Sometimes the fact may be detected that a specimen has lived on a ribbed shell when young, and on a smooth one when old, or vice versâ. A new genus was actually founded by Brown for a Capulus which had acquired ribs through adhesion to a Pecten. A specimen of Hinnites giganteus in the British Museum must at one period of its growth have adhered to a surface on which was a Serpula, the impression of which is plainly reproduced on the upper valve of the Hinnites.[339]

Growth of the Shell.—Nothing very definite is known with regard to the rate of growth of the shell in marine Mollusca. Under favourable conditions, however, certain species are known to increase very rapidly, especially if the food supply be abundant, and if there is no inconvenient crowding of individuals. Petit de la Saussaye mentions[340] the case of a ship which sailed from Marseilles for the west coast of Africa, after being fitted with an entirely new bottom. On arriving at its destination, the vessel spent 68 days in the Gambia River, and took 86 days on its homeward voyage. On being cleaned immediately on its return to Marseilles, an Avicula 78 mm. and an Ostrea 95 mm. long (both being species peculiar to W. Africa) were taken from its keel. These specimens had therefore attained this growth in at most 154 days, for at the period of their first attachment they are known to be exceedingly minute. P. Fischer relates[341] that in 1862 a buoy, newly cleaned and painted, was placed in the basin at Arcachon. In less than a year after, it was found to be covered with thousands of very large Mytilus edulis, 100 mm. × 48 mm., the ordinary size on the adjoining banks being only about 50 to 60 × 30 mm.

Some observations have already been recorded (p. 40) on the growth of Helix aspersa. In the summer of 1858, which was very dry, especially in the south of France, the young Helices born that year were still very small in August. About the end of that month abundant rain came on, and in four or five days young H. variabilis, H. pisana, and H. aspersa, eating without cessation, as if to make up for lost time, grew more than a centimetre of shell. The lip of a young H. arbustorum has been observed to have grown, at the end of the first week in the season’s growth, 3 mm., at the end of the second week, 6·25 mm., the third, 11·5 mm., and the fourth 12·5 mm., with a finished lip.[342]

Careful observation has shown that in the growth of the shell of Helix aspersa the periostracum is first produced; it is covered with hyaline globules, 10–12 mm. in diameter, which persist even in the oldest shells. Calcareous matter is deposited on the internal face of the new periostracum, at some distance from the margin. It is secreted by a white zone or band of cells bounding the entire breadth of the mantle as applied to the peristome. Immediately behind the white zone are a series of pigment cells which not only give the shell its colour but complete the calcification of the shelly matter laid down by the white zone. When the animal has attained its full growth and the lip is finished off, the white band and the periostracum cells completely disappear, and only such cells persist as contribute to the internal thickening of the shell. Shell growth, in this species, is very rapid. If a portion of the pulmonary sac is laid bare, by removing a fragment of shell, at the end of 1½ or 2 hours there may be detected a delicate organic membrane covering the hole, and strewn with crystals of carbonate of lime. This thickens with great rapidity, and soon fills up the hole with solid matter. For two consecutive months an animal, deprived of food, has been known to reproduce this membrane daily after its removal every morning.[343] Prof. Schiedt has found that oysters, if deprived of the right valve and exposed to the light, not only develop brown pigment over the whole exposed surface of mantle and branchiae, but actually succeed in part in reproducing the valve and hinge.[344]

Deposit of Additional Layers of Shell.—Mollusca possess the power of thickening the interior of the shell, by the deposit of successive layers. This is frequently done in self-defence against the attacks of boring Mollusca, sponges, and worms. Cases may often be noticed of Ostrea, Spondylus, and other sedentary molluscs, which, unable to escape the gradual assaults of their foes, have provided against them by the deposit of fresh shelly matter. A somewhat similar plan is adopted to provide against intrusion by way of the aperture. Pearls are, in many cases, the result of shell deposition upon the eggs or even the body of some intrusive parasite (Distoma, Filaria, etc.), and are, in some countries, artificially produced by the introduction of fragments of sand, metal, etc., into living Unio and Anodonta. Little joss images are made in India and China, the nacre on which is produced by thrusting them inside living Unionidae.

A specimen of Helix rosacea, in the British Museum, into whose shell a piece of grass somehow became introduced, has partitioned it off by the formation of a sort of shelly tunnel extending throughout its entire length (Fig. 167).

Absorption of Internal Portions.—Certain genera have the remarkable property of absorbing, when they become adult, the internal portions of the whorls and the greater part of the columellar axis. The effect of this is to make the shell, when the process is complete, no longer a spiral but a more or less produced cone, and it is found that in such cases the viscera of the spire lose their spiral form, and take the shape of the cavity in which they lie. Amongst the genera in which this singular process takes place are Nerita,[345] Olivella, and Cypraea amongst marine forms, and nearly the whole of the Auriculidae[346] (Fig. 168). Conus reduces the internal subdivisions of the spire to extreme thinness. It is noticeable that these genera are all of considerable thickness of shell, and it is perhaps the result of the whole energy of the animal being directed to the formation of its external protection that the internal walls of the spire become atrophied and eventually disappear.

Decollation.—In certain genera, when the shell becomes adult, the animal ceases to occupy the upper whorls, which accordingly die and drop off, the orifice at the top having meanwhile been closed by a shelly deposit. Such shells are termed decollated. In some land genera decollation is the rule, e.g. in Cylindrella (Fig. 169), Eucalodium, and Rumina, as well as in many species of the brackish water genera, Truncatella, Cerithidea, and Quoyia. Stenogyra (Rumina) decollata, a common shell in the south of Europe, has been noticed to bang its upper whorls violently against some hard substance, as if to get rid of them.

Special Points in the Growth of Certain Genera.—In the young of Coecum the apex is at first spiral, but as growth proceeds and the long tube begins to form, a septum is produced at the base of the apex, which soon drops off. Soon afterwards, a second septum forms a little farther down, and a second piece drops off, leaving the shell in the normal cylindrical form of the adult (Fig. 170). The development of Fissurella is of extreme interest. In an early stage it possesses a spiral shell, with a slit on the margin of the outer lip of the last whorl. As growth advances, shelly matter is deposited on both margins, which results in the slit becoming a hole and the spire a mere callosity, until at last they appear to coalesce in the apex of the adult shell (Fig. 171). The singular formations of Magilus and Rhizochilus have already been described (pp. 75, 76). Cypraea, in the young stage, is a thin spiral shell with a conspicuous apex. As growth proceeds, the surface of the whorls, which are nearly enveloped by two large lobes of the mantle, becomes overlaid with new layers of shelly matter, until eventually the spire becomes embedded, and ultimately disappears from view (Fig. 172).

Patella, when young, has a nautiloid shell (see Fig. 45, p. 134), but it is a remarkable fact that we are entirely ignorant, in this commonest of molluscs, of the transition stages which convert the nautiloid into the familiar conical shell. The young shell of Pteroceras is deceptively unlike the adult, and is entirely devoid of the finger-like processes which are so characteristic of the genus (chap. xiv.).

Among the bivalve Mollusca, Anomia in a young stage is not to be distinguished from Ostrea. Soon a small sinus appears on the ventral margin, which gradually deepens and, as the shell grows round it, forms a hole for the byssus, eventually becoming fixed beneath the umbones (see Fig. 173). In Teredo the two valves of the shell proper, which is very small, become lodged in a long calcareous tube or cylinder, which is generally open at both ends (see chap. xvi.). In Aspergillum a somewhat similar cylinder is developed, but the valves are soldered to the tube, and form a part of it, the tube itself being furnished, at the anterior end, with a disc, which is perforated with holes like the rose of a watering-pot. In Clavagella the left valve alone becomes soldered to the tube, while the right valve is free within it (see chap. xvi.). Fistulana encloses the whole of its shell in a long tapering tube, which is not at any point adherent to the shell.