CHAPTER IX

THE SHELL, ITS FORM, COMPOSITION AND GROWTH--DESIGNATION OF ITS VARIOUS PARTS

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).

[Illustration: FIG. 148.--_Aplustrum aplustre_ L. Mauritius, showing the partly internal shell (=S=); =F=, foot; =LL=, cephalic lappets; =TT=, double set of tentacles. (After Quoy and Gaimard.)]

[Illustration: FIG. 149.--_Sigaretus laevigatus_ Lam., showing shell partially immersed in the foot; =F=, anterior prolongation of the foot. (After Souleyet.)]

(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.

[Illustration: FIG. 150.--Examples of shells with =A=, a flattened spire (_Polygyratia_); =B=, a globose spire (_Natica_); =C=, a greatly produced spire (_Terebra_).]

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.

[Illustration: FIG. 151.--Examples of shells with disconnected whorls; =A=, _Cyathopoma cornu_ Mf., Philippines; =B=, _Cylindrella hystrix_ Wright, Cuba. (Both × 4.)]

[Illustration: FIG. 152.--Example of a shell whose apical whorls alone are coiled, and the remainder produced in a regular curve. (_Cyclosurus Mariei_ Morel., Mayotte.)]

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).

[Illustration: FIG. 153.--_Siliquaria anguina_ Lam., showing scalariform coil of upper whorls and irregular extension of the lower.]

[Illustration: FIG. 154.--_Anostoma globulosum_ Lam., Brazil. (After P. Fischer.)]

[Illustration: FIG. 155.--Various forms of the internal plate in _Capulidae_: =A=, _Calyptraea_ (_Mitrularia_) _equestris_ Lam., E. Indies; =B=, _Crucibulum scutellatum_ Gray, Panama; =C=, _Ergaea plana_ Ad., and Reeve, Japan; =D=, _Galerus chinensis_ L., Britain; =E=, _Crepipatella dilatata_ Lam., Callao; =F=, _Trochita maculata_ Quoy, N. Zealand; =G=, _Crepidula fornicata_ Lam., N. America.]

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_.

[Illustration: FIG. 156.--_Fulgur perversum_ L., Florida. × ½.]

[Illustration: FIG. 157.--Illustration of the gradation of forms in _Ampullaria_ between a dextral (=A=) and an ultra-dextral species (=F=).]

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).

[Illustration: FIG. 158.--Monstrosities of _Neptunea antiqua_ L., and _Buccinum undatum_ L., with a greatly produced spire (from specimens in the Brit. Mus.).]

[Illustration: Fig. 159.--Monstrosities of _Littorina rudis_ Mat, The Fleet, Weymouth. (After Sykes.)]

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_.

[Illustration: FIG. 160.--Monstrosities with two apertures: =A=, _Cylindrella agnesiana_ C. B. Ad., Jamaica; =B=, _Littorina littorea_ (from specimens in the British Museum).]

[Illustration: FIG. 161.--Cornucopia-shaped monstrosity of _Helix aspersa_, from Ilfracombe. (British Museum.)]

=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.

[Illustration: FIG. 162.--=A=, Section of shell of _Unio_: _a_, periostracal layer; _b_, prismatic layer; _c_, nacreous layer. =B=, Horizontal section of shell of _Pinna_, showing the hexagonal prisms.]

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.

[Illustration: FIG. 163.--Sections of shells. =A=, _Conus_: _a_, outer layer; _b_, middle prismatic layer, with obliquely intersecting laminae above and below; _c_, inner layer. =B=, _Oliva_: _a_, outer layer; _b_, layer of crossed and curved laminae; _c_, prismatic layer, succeeded by layer of laminae at right angles to one another; _d_, inner layer. =C=, _Cypraea_: _a_, outer layer; _b_, middle layer; _c_, inner layer.]

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).

[Illustration: FIG. 164.--_Murex tenuispina_ L., Ceylon. × ⅔.]

[Illustration: FIG. 165.--_Neritina longispina_ Récl., Mauritius. (Operculum removed.)]

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]

[Illustration: FIG. 166.--A specimen of _Anomia ephippium_ L., Weymouth, taken upon _Pecten maximus_, the sculpture of which is reproduced on the upper valve of the _Anomia_, and even on a young _Anomia_ attached to the larger specimen.]

=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).

[Illustration: FIG. 167.--A specimen of _Helix rosacea_ Müll., Cape of Good Hope, into which a piece of grass has by some means become introduced. The animal has protected itself by covering the grass with a shelly layer. (From a specimen in the British Museum.)]

=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.

[Illustration: FIG. 168.--_Auricula Judae_ Lam., showing the disappearance of the partitions of the whorls, which are represented by dotted lines. (After Fischer.)]

[Illustration: FIG. 169.--=A=, Decollated (adult) form, and =B=, perfect (young) form of _Cylindrella nobilior_ Ad., Jamaica; the dotted line shows where decollation takes place.]

[Illustration: FIG. 170.--Development of _Coecum_: =A=, showing the gradual formation of septa; _a_, apex; _ap_, aperture; _ss_, first septum; _s´s´_, second septum. (After de Folin.) =B=, Adult form of _C. eburneum_ Ad., Panama. × 8.]

=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.

[Illustration: FIG. 171.--Four stages in the growth of _Fissurella_, showing how the spire gradually disappears and the marginal slit becomes an apical hole, =A=, =B=, =C=, highly magnified, =D=, natural size. (After Boutan.)]

[Illustration: FIG. 172.--Three stages in the growth of _Cypraea exanthema_ L. (From specimens taken at Panama.)]

=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.

[Illustration: FIG. 173.--Development of the byssus or the plug-hole in _Anomia_. (After Morse.)]

=Terms employed to denote various Parts of the Univalve Shell.=--The _apex_ is the extreme top of the spire, and generally consists of the embryonic shell, which may often be recognised by its entire want of sculpture. When the embryonic shell happens to be large, the apex is often mammillated, _e.g._ in _Fusus_, _Neptunea_, and some _Turbinella_; in the _Pyramidellidae_ it is sinistral.

The _suture_ is the line of junction between any two successive whorls. It may be deep, and even channelled, or very shallow, as in Fig. 150 B (p. 246).

The _spire_ is the whole series of whorls except the last or _body whorl_. A _whorl_ is a single revolution of the spiral cone round the axis. The spire may be _subulate_ (as in _Terebra_, Fig. 150 C), _turreted_ (_Scalaria_), _depressed_ (_Polygyratia_, Fig. 150 A), _conical_ (_Trochus_), _globose_ (_Ampullaria_, _Natica_, Fig. 150 B), with almost all conceivable gradations between these types. The number of whorls is best counted by placing the shell mouth downwards, and reckoning _one_ for every suture that occurs between the extreme anterior point of the shell and the apex.

[Illustration: FIG. 174.--Illustrating the technical terms applied to the various parts of a univalve shell.]

The _mouth_ or aperture may be (_a_) entire, as in _Helix_, _Natica_, _Ampullaria_, when its _peristome_ or margin is not interrupted by any notch or canal, or (_b_) prolonged at its anterior and sometimes also at its posterior end into a _canal_. The _anterior canal_ serves as a protection to the siphon,[347] the _posterior canal_ is mainly anal in function, and corresponds, in part, to the hole of _Fissurella_, the slit in _Pleurotoma_ and _Emarginula_, and the row of holes in _Haliotis_. The mouth presents every variety of shape, from the perfect circle in _Cyclostoma_ and _Trochus_, to the narrow and prolonged slit in _Conus_ and _Oliva_.

[Illustration: FIG. 175.--Anal slit in _Pleurotoma_.]

The right margin of the mouth (the left, in sinistral shells) is termed the _outer lip_ or _labrum_, the left margin the _inner lip_, _labium_, or _columella lip_.[348] In young shells the outer lip is usually thin and unfinished, while in the adult it is generally thickened into a rib, or furnished with more or less prominent teeth, or given an inward or outward curve. In some genera, especially the Strombidae, the outer lip of the adult develops long finger-like processes, which sometimes attain an extraordinary size (chap. xiv.). As growth proceeds, these marginal teeth and ribs are either dissolved and disappear, or are permanently incorporated, in the shape of _varices_, with the framework of the shell. Some shells, _e.g._ _Natica_, _Turritella_, _Actaeon_, have a permanently unfinished outer lip, even in the adult stage. The columella lip varies in shape with the mouth as a whole; thus it may be straight, as in _Conus_, or excavated, as in _Sigaretus_, _Struthiolaria_, and _Bulla_. Frequently it is continued by part of the body whorl, as in _Ficula_, _Dolium_, and _Fasciolaria_.

[Illustration: FIG. 176.--_Solarium perspectivum_ Lam., from the under side.]

[Illustration: FIG. 177.--Section of _Turbinella pyrum_ L., showing the plicae on the columella and the growth of successive whorls.]

The folds or plaits on the columella, which are often characteristic of the genus or even family (_e.g._ Fasciolariidae, Mitridae, Turbinellidae) are not merely external, but continue down the whole spire (see Fig. 177, which also shows how successive fresh growths have thickened the columella).

The whorls may be wound in a spiral, which is either hollow, as in _Solarium_, or quite compact, as in _Oliva_, _Terebra_, _Cypraea_, with every possible intermediate grade. This concavity, which varies in depth and width, is known as the _umbilicus_, and shells are accordingly spoken of as _deeply_ (_e.g._ most Trochidae and Naticidae), _narrowly_ (_e.g._ _Lacuna_, _Littorina_), or _widely_ (_e.g._ _Solarium_) _umbilicated_. When the spiral is quite flat, as in _Planorbis_ and some _Helix_, the umbilicus vanishes entirely. Shells in which the whorls are so compactly coiled on an ascending spiral that there is no umbilicus, are termed _imperforate_.

[Illustration: FIG. 178.--The slit in =A=, _Hemitoma_, =B=, _Emarginula_, =C=, _Macroschisma_, =D=, _Craniopsis_, =E=, _Puncturella_, =F=, _Fissurella_.]

_The Slit._--Many shells are furnished with a slit in the last whorl, which opens, in most cases, on the outer lip, and is sometimes of considerable depth, at others a mere notch. In the patelliform shells it is always in front of the apex. The function of the slit appears to be mainly anal, the excretory products being thus allowed to escape by a passage of their own, without soiling the clean water taken in by the branchiae. The posterior canal of some Gasteropoda probably performs a similar function. In the adult _Fissurella_ the slit becomes an apical hole (see Fig. 178 F), in the allied genera it is either immediately in front of the spire (_Puncturella_), or half-way between the spire and the anterior margin (_Rimula_), or on the margin and well marked (_Emarginula_), or a mere indentation of the margin (_Hemitoma_). In _Pleurotomaria_ it is exceptionally long, and is well marked in _Bellerophon_, _Schismope_, _Scissurella_, _Murchisonia_, and _Pleurotoma_ (where it is sutural). In _Haliotis_ and _Polytremaria_ it is replaced by a series of holes, which are closed up as the animal grows past them. Some of these holes (at least in _Haliotis_) certainly serve the purpose of admitting water to the branchiae, while others are anal. In _Trochotoma_ there are only two holes, united by a narrow fissure.

_The Tubed Land Operculates._--A group of the Cyclophoridae, which is restricted to Further India and the great Malay Islands, has developed a remarkable _sutural tube_ on the exterior of the last whorl, near the aperture, A similar tube, but more obscure, exists in _Alycaeus_. Several stages in the development of this tube may be noticed, beginning with the elevation of part of the peristome into a simple irregular shelly plate, which is continued, first into a short, and then into a long tube, which becomes soldered to the shell; finally, the tube becomes free, and the anterior part of the last whorl is disconnected from the spire (Fig. 180 A-D).

[Illustration: FIG. 179.--The slit in =A=, _Bellerophon_, =B=, _Pleurotomaria_, =C=, _Schismope_, =D=, _Polytremaria_, =E=, _Haliotis_ (not drawn to scale).]

[Illustration: FIG. 180.--Development of the tube in the tube operculates: =A=, _Pterocycus rupestris_ Bens.; =B=, _Opisthoporus birostris_ Pfr.; =C=, _Spiraculum travancoricum_ Bedd.; =D=, _Rhiostoma Housei_ Pfr.]

It is singular that the tube does not appear to be of any use to the animal, since its internal extremity, in the complete form, is closed, and does not communicate with the interior of the whorl. It may be presumed, however, that in origin the tube served as a means of conveying air to the animal when the operculum was closed. The holes in the peristome of _Pupina_, _Cataulus_, and _Anostoma_ (Fig. 154) may be compared.

[Illustration: FIG. 181.--_Eburna spirata_ Lam., E. Indies. =F=, foot; =OP=, operculum; =P=, penis; =S=, siphon; =T=, tentacles, with eyes at their base. (After Souleyet.)]

=The Operculum.=--The operculum is a cuticular development of a group of cells situated on the dorsal side of the foot, exactly over the terminal point of the fibres of the columellar muscle. It is so situated that in crawling it is generally carried free of the shell, sometimes at the extreme upper end of the foot, more usually somewhat nearer to the shell (Fig. 181). In _Pterocyclus_ it is pushed back into the umbilicus when the animal is in motion.

The operculum is present in nearly all land, fresh-water, and marine Prosobranchiata, absent in all Opisthobranchiata in the adult state, except _Actaeon_, and in all Pulmonata, except _Amphibola_. It has been lost in the following marine Prosobranchiata: many Cancellariidae and Conidae, _Oliva_ (though present in _Olivella_ and _Ancilla_), Harpidae, Marginellidae, _Voluta_ proper (though present in _V. musica_), nearly all Mitridae, Cypraeidae, Doliidae, Ianthinidae; and, of land genera, in Proserpinidae. It is evident, therefore, that its presence or absence is of limited value in classification. In some species of _Ampullaria_ and _Natica_ it is horny, in others shelly. Dall found that in a number of specimens of _Volutharpa ampullacea_, 15 p.c. had opercula, 10 p.c. traces of the operculigenous area, but no operculum, the rest no trace of either. Monstrosities of _Buccinum undatum_ sometimes occur, which have two, or in rare case three opercula.

As a rule, the operculum exactly fits the mouth of the shell. But in cases where the mouth is very large (_e.g._ _Conus_, _Strombus_, _Concholepas_, some _Bullia_), it only covers a very small portion and is quite inadequate as a protection (Fig. 62, p. 155). Again, when the shell has assumed a more or less limpet-shaped form, and habitually adheres to flat surfaces without much occasion for locomotion, the operculum becomes degraded and is probably on the way to being lost altogether. This is the case with _Navicella_ (a modified _Nerita_, see Fig. 13, p. 17), _Concholepas_ (a modified _Purpura_), _Sigaretus_ (a modified _Natica_). Probably the more completely patelliform shells of _Crepidula_, _Haliotis_, _Fissurella_, and _Patella_ have reached the stage at which the operculum has been lost entirely. In _Navicella_, besides becoming degraded, the operculum has actually become partly internal, and apparently serves the purpose of separating the viscera from the upper part of the foot, something like the shelly plate in _Crepidula_. This explains why the operculum in this genus is polished on both sides.[349]

Some authors have imagined that the operculum is homologous (_a_) to the second valve in Pelecypoda, (_b_) to the byssus. It differs, however, morphologically from the former in the essential point of not being produced by the mantle, and from the latter in not being produced by a special gland.

[Illustration:

Turbo Turbo Livona Ampullaria Natica Sarmaticus) (Callopoma)

FIG. 182.--Various forms of opercula.]

As regards shape and formation, the operculum has usually a more or less well-marked nucleus which may be central (_e.g._ _Livona_), sub-central (_Ampullaria_), lateral (_Purpura_), or terminal (_Pyrula_). As a rule, both the inner and outer surfaces are fairly flat, but in _Torinia_, _Cyathopoma_, and _Pterocyclus_ the outer surface is elevated and conically spiral, in some _Turbo_ (_e.g._ _Sarmaticus_) it is covered with raised tubercles resembling coral, while in others (_e.g._ _Callopoma_) it is scored with a deep trench. _Aulopoma_, a land genus peculiar to Ceylon, has a paucispiral operculum with hollow whorls, deceptively like a _Planorbis_; it fits over the aperture instead of into it. In _Livona_ and most Trochidae the operculum is cartilaginous and multispiral. In _Strombus_ it is narrow, curved, and often serrated like a leaf on one of the edges; in _Conus_ it is narrowly oblong and rather featureless; in _Littorina_, paucispiral and always cartilaginous. In many cases (_e.g._ _Paludina_) there is no true spiral form, but the striae are concentric to a nearly central nucleus, and thus give the appearance of a spiral. The evolution of the operculum in _Navicella_ from _Nerita_ has already been illustrated (p. 10). _Neritopsis_ has a very remarkable operculum, the striated appendage of which locks behind the columella of the shell, like the tooth in the opercula of the Neritidae.

[Illustration:

Pyrula Purpura Littorina Aulopoma Torinia × 3/2 × 3

Neritopsis Strombus Conus × 2

FIG. 183.--Various forms of opercula.]

=Terms employed to denote various parts of the Bivalve Shell.=--The _umbo_, or _beak_, is the apex of the hollow cone, of which each valve may be regarded as consisting. This apex is usually more or less twisted: it is markedly spiral in _Isocardia_, _Diceras_, some _Chama_, and especially _Requienia_, while in _Pecten_, _Lepton_, and others the spiral is altogether absent. As a rule the umbones point _forward_, _i.e._ towards the anterior end of the shell. In _Donax_, _Nucula_, and _Trigonia_, however, they point backward. The umbones are generally more or less approximated, but in _Arca_ they are widely separated.

An _equilateral_ shell is one in which the umbones are more or less central with regard to its anterior and posterior portion, while in an _inequilateral_ shell the umbones are much nearer one end than the other. On the other hand, _equivalve_ and _inequivalve_ are terms used to express the relation of the two valves to one another as a whole. Thus nearly all bivalve shells are more or less inequilateral, but a comparatively small proportion are inequivalve.

The _dorsal margin_ is adjacent to, the _ventral margin_ opposite to, the umbones. The _anterior_ and _posterior margins_ are respectively the front and hinder edges of the shell.

The muscles which serve to close the valves leave _impressions_ on the inner surface of each valve. These, when both muscles are present, are known as the _anterior_ and _posterior adductor impressions_. The impression produced by the muscular edge of the mantle, which curves downwards and backwards from the anterior adductor impression, is known as the _pallial line_. In shells with only one muscle it is represented by an irregular row of small marks, or disappears altogether (_Ostrea_). The _pallial sinus_ is produced by the muscles which retract the siphons, and is most marked in those genera in which the muscles are powerful and the siphons large (_e.g._ _Tellina_, _Mya_). It is entirely absent in genera possessing no retractile siphons.

[Illustration: FIG. 184.--Left valve of _Venus gnidia_ L.: =A=, anterior, =B=, posterior, =C=, dorsal, =D=, ventral margin, =AB=, length, =CD=, breadth of shell.

_a.m_, anterior; _p.m_, posterior adductor muscle; _p_, pallial line; _p.s_, pallial sinus; _l_, ligament; _lu_, lunule; _u_, umbo; _c_, cardinal teeth; _a.l_, anterior lateral tooth; _p.l_, posterior lateral tooth.]

[Illustration: FIG. 185.--Right valve of _Lucina tigerina_ L.: =A=, anterior, =B=, posterior, =C=, dorsal, =D=, ventral margin; =AB=, length, =CD=, breadth of shell.

_a.m_, anterior; _p.m_, posterior adductor muscles; _p_, pallial line; _l_, ligament; _u_, umbo; _c_, cardinal teeth; _a.l_, _p.l_, anterior and posterior lateral tooth.]

_Right and Left Valve._--The simplest way of distinguishing the valves as right and left is to hold the shell in such a way that the siphons point towards the observer, and the mouth away from him; in this position the valve to the right is called the _right valve_, and the valve to the left the _left valve_. If, however, the animal is not present, it may be remembered that the ligament is nearly always _behind_ the beaks, and that the beaks, as a rule, point _forward_, thus the right and left valves can generally be named by observation of the beaks and ligament. When the ligament is median to the valves (_e.g._ _Ostrea_, _Pecten_), and the beaks are not curved, the valves may be recognised by noting the fact that the impression of the adductor muscle (in these cases always single) is nearer to the posterior than to the anterior side. In a similar way the pallial impression, which only forms a sinus on the posterior side, furnishes a guide to the valves of _Donax_, in which the beaks point backward, and of _Tellina_, in which the beaks are frequently central.

In the fixed inequivalves (_e.g._ _Chama_) it is sometimes the right, sometimes the left valve which is undermost, but the fixed valve, whether right or left, is always deep, and the free valve flat. _Ostrea_ and _Anomia_ are always fixed by the _left_ valve.

The _lunule_ is a well-marked area _in front of_ and close to the umbones, usually more or less heart-shaped, and limited by a ridge. Generally it is shallow, but sometimes, as in _Dosinia_, _Opis_, and some _Cardium_, deeply impressed. A corresponding area _behind_ the umbones, enclosing the ligament, is called the _escutcheon_ (Fig. 186), but it seldom occurs.

The _ligament_ is a more or less elastic band, which unites the two valves along a line adjacent to the umbones. As a rule, the greater part of the ligament is external to the shell, but it may be entirely internal. It is placed, normally, _behind_ the umbones, but in a few cases, when the hinge line is very long (_Arca_, _Pectunculus_), it extends in front of them as well. The edges of the valves, when the ligament is mainly external, are more or less excavated for its reception. When internal it is generally contained in a groove or spoon-shaped pit, known as the _fossette_ (compare Fig. 187).

[Illustration: FIG. 186.--_Venus subrostrata_ Lam.: _es_, escutcheon; _li_, ligament; _lu_, lunule; _u_, _u_, umbones.]

The ligament consists of two distinct parts, which may occur together or separately, the external, or _ligament proper_, and the internal, or _cartilage_. Only the external portion can be seen when the valves are closed. As a rule, the two portions are intimately connected with one another, the ligament folding over the cartilage, but in some cases, _e.g._ _Mya_, _Mactra_, where the cartilage is lodged within the hinge, they are completely disconnected (Fig. 187).

In _Pecten_ the external ligament is very thin, and runs along the dorsal margin, while the internal ligament is large, solid, and situated in a shallow pit. In _Perna_, where the hinge is toothless, the ligament is folded into a number of transverse ridges, which fit into corresponding grooves in the shell.

The ligament proper is _inelastic_ and insoluble in caustic potash. The cartilage is very elastic, composed of parallel fibres, slightly iridescent, and soluble in caustic potash.

The operation of the ligament--using the word as including the whole ligamental process--is in opposition to that of the adductor muscles. When the latter close the valves, they compress the ligament, an action which its elasticity resists: thus its operation tends in part towards keeping the valves open. But when ligament and cartilage are both fully developed, they work in opposition to one another, the ligament, by its resistance to compression, preventing any straining of the adductor muscles when the valves are open, and the cartilage, for the same reason, preventing the ventral margins of the shell from closing too rapidly upon one another when the valves are being shut.

[Illustration: FIG. 187.--Hinge of =A=, right valve, and =B=, left valve of _Mulinia edulis_ King; _ca_, cardinals; _l.a_, anterior laterals; _l.p_, posterior laterals; _f_, fossette; _c_, cartilage; _l_, ligament.]

_The Hinge._--The valves of _Pelecypoda_ are generally articulated, below the umbones, by a _hinge_ which is furnished, in the majority of cases, with interlocking teeth, small pits or depressions in each valve corresponding to the teeth in the other. The teeth are distinguished as _cardinal_, or those immediately below the umbo, and _lateral_, or those to either side of the cardinals, the latter being also distinguished as _anterior_ and _posterior laterals_, according as they are before or behind the umbo (Fig. 184). In shells which are tolerably equilateral there is no difficulty in distinguishing between cardinal and lateral teeth. But when they are very inequilateral, the whole hinge may share in the inequality of growth, and an anterior lateral may be thrown backward and simulate a cardinal, or a cardinal may be thrown backward and simulate a posterior lateral (_e.g._ _Cardita_, _Unio_, Fig. 188). In many _Chama_ the cardinals are pushed up into the umbo and become a mere ridge, while the strong anterior lateral becomes nearly central and simulates a cardinal.

[Illustration: FIG. 188.--Hinges of =A=, _Cardita semiorbiculata_ Brug., and =B=, _Unio rectus_ Lam., showing how, in inequilateral shells, the lateral teeth tend to shift their position. _a.m_, anterior adductor, _p.m_, posterior adductor muscle; _c_, _c_, cardinal teeth; _p.l_, posterior lateral teeth; _l_, ligament.]

Some bivalves, _e.g._ _Anodonta_, _Ostrea_, _Pedum_, many _Mytilus_, have no hinge teeth at all, in others the laterals are wanting (_Psammobia_, _Diplodonta_). In the Arcadae the hinge consists of a number of very similar denticles, which are often serrated like the teeth of a comb (Fig. 189).

[Illustration: FIG. 189.--The hinge in _Arcadae_: =A=, _Nacula Loringi_ Ang. × 4/3; =B=, _Arca granosa_ L.; _u.a_, umbonal area.]

[Illustration: FIG. 190.--=A=, _Tridacna scapha_ Lam.; =B=, _Cardium enode_ Sowb., showing the interlocking of the ventral margins.]

Hinge-teeth are probably, in origin, derived from the crenulations or ribbings of the surface of the shell, the upper ends of which impinge upon the dorsal margin and mark it in a way which is quite recognisable when the shell is thin. Similar crenulations, resulting in interlocking of the valves, are not uncommon upon the ventral margin in certain genera (Fig. 190). The mechanical effect of these continued riblets, when fitted together on the opposing valves, would be to prevent the valves sliding upon one another while closing, or after being closed. Thus there would be a probability of their surviving, even after the ribbing had disappeared from the surface of the shell, the increased strength given by the hinge compensating for, and making it possible to do without, the extra strength supplied by the ribs. It is therefore possible that the teeth of the Nuculidae and Arcadae, which have no distinction between cardinals and laterals, represent a very ancient type, from which have been evolved the various forms of hinge in which cardinals and laterals are distinguished. Even in some forms of Arcadae (comp. _Pectunculus_) we get a hint how the transverse teeth of the typical _Arca_ may have become transformed into the longitudinal tooth of the normal lateral.[350]

The developed hinge-teeth, then, ensure the opening of the valves in one direction; they also secure their accurate closure upon one another in exactly the same plane. Exposed shells and gaping siphons matter little to animals which are protected by their burrowing propensities, but to those which live in material which can be easily penetrated by their foes, it must be of advantage to be able to buckle their armour absolutely tight. The edentulous hinge of _Anodonta_ is a degeneration from a dentate type, which retains its teeth (in _Unio_, etc.) when subject to the jar of rapid streams, but tends to lose them in the stiller waters of canals, lakes, and ponds.

_Other processes in the bivalve shell._--In _Anatina_ each umbo is fissured and strengthened on the inside by a kind of umbonal plate which carries the ligament. Some forms of _Liligna_ develop a strong internal umbonal rib, which serves as a buttress to strengthen the shell. In _Pholas_, the so-called falciform process serves as a point of attachment for the muscles of the foot and viscera. There is no ligament or hinge-teeth, the place of the latter being taken by the anterior adductor muscle, which is attached to the hinge-plate, the latter being reflected back into the shell.

In _Septifer_ the anterior adductor muscle is carried on a sort of shelf or _myophore_, and in _Cucullaea_ the posterior adductor is

## partly raised on a similar and very prominent formation.

_Length and breadth_ of bivalve shells is variously measured. Most authorities measure _length_, or ‘antero-posterior diameter,’ by a straight line drawn from the extreme anterior to the extreme posterior margin, and _breadth_ by a similar line, drawn from the umbones to a point, not very clearly marked, on the opposite ventral margin (see Figs. 184 and 185). Others, less correctly, reverse these terms. _Thickness_ is measured by the extreme distance of the opposite faces of the closed valves. As a rule, the length exceeds, and often greatly exceeds, the thickness, but in a few cases--_e.g._ the _Cardissa_ section of _Cardium_--this is reversed.

_The periostracum._--Nearly all shells are covered, at some period of their growth, by a _periostracum_,[351] or surface skin, which serves the purpose of protecting the shell against the destructive effects of the chemical action set up by water or air. It also, in some cases (see p. 258), acts as a kind of base upon which the shell is deposited. In old shells it is commonly worn away, especially at those parts which are likely to become abraded.

The form and composition of the periostracum varies greatly. Sometimes (_e.g._ _Oliva_) it is a mere transparent film, at others (_Zonites_) it is transparent, but stout and solid. It is corneous in _Solenomya_, covered with fine hairs in many _Helicidae_, in _Conus_, _Velutina_, and _Cantharus_ it is thick, fibrous, and persistent; in _Trichotropis_ and some _Triton_ it is furnished with long bristles on a thick ground (Fig. 191). In fresh-water shells it is usually rather thick, in order to protect the shell from the erosive powers of certain kinds of water. In some cases (_Mya_, _Anatina_) the periostracum is continued over the siphons, so as to form a protection throughout their whole length.

[Illustration: FIG. 191.--_Triton olearium_ L., Mediterranean, an example of a shell with a stout and hairy periostracum. × ½.]

_Erosion._--The fresh-water Mollusca generally, and marine mollusca in a few rare cases (_Purpura_, _Littorina_) are subject to _erosion_, or decay in the shell substance. In univalves erosion usually sets in near the apex (Fig. 192), where the life of the shell may be regarded as weakest, and in bivalves near the umbones. It is commonest in old shells, and rarely occurs in the very young. So long as the periostracum is present to protect the shell, erosion cannot set in, but when once it has been removed the shell is liable to the chemical changes set up in its substance by water. There is abundant evidence to show that erosion is caused by pollution of water. Out of many instances one must suffice. In a certain stream near Boston, U.S., numbers of Mollusca occurred, the shells of which were very perfect and free from disease. Some little way down stream an alkaline manufactory drained its refuse into the water. At and below this point for some distance every shell was more or less eroded, most of them seriously. Farther down, when the alkali refuse became diluted away, the shells retained their normal condition.[352]

[Illustration: FIG. 192.--Example of an eroded fresh-water shell (_Melania confusa_ Dohrn, Ceylon).]

A small percentage of lime in the water appears to produce erosion. The result of some experiments by G. W. Shrubsole, in the investigation of this point, may be tabulated as follows:[353]--

Water from Lime present Result per gall.

River Dee, near Chester 3·00 grs. acted strongly on shells Wrexham 4·00 grs. „ „ „ River Dee, near Llanderyel 0·53 grs. „ „ „ Trent Canal 8·33 grs. no action „

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