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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 nontestaceous 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).
F . 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.)
F 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.
F 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.
F . 151. Examples of shells with disconnected whorls; A, Cyathopoma cornu Mf., Philippines; B, Cylindrella hystrix Wright, Cuba. (Both × 4.)
F 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).
F 153 Siliquaria anguina Lam , showing scalariform coil of upper whorls and irregular extension of the lower.
154
F
Anostoma globulosum Lam , Brazil (After P Fischer )
F . 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.
F . 156. Fulgur perversum L., Florida. ½.
F 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).
F . 158. Monstrosities of Neptunea antiqua L., and Buccinum undatum L., with a greatly produced spire (from specimens in the Brit Mus )
F . 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.
F . 160. Monstrosities with two apertures: A, Cylindrella agnesiana C. B. Ad., Jamaica; B, Littorina littorea (from specimens in the British Museum)
F . 161. Cornucopiashaped 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.
F 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.
F . 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).
164. Murex tenuispina L., Ceylon. × ⅔.
F . 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
F .
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]
F . 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
