WHEN the shell has its apex directed upwards, and its aperture downwards and towards the reader, its spire, as is the case with the great majority of Gasteropod spiral shells, ascends obliquely towards the right. It is in consequence termed 'dextral.' In the living animal inhabiting such a shell, the heart is on the left, the generative, respiratory and anal apertures on the right of the body. The aperture of the shell in Helix as in all vegetable-feeding Gastropoda is entire, i. e. forms an unbroken curve: in carnivorous Gastropoda on the contrary it is notched or produced into a canal which lodges a process of the mantle or siphon: e. g. in the Whelk. These two varieties of aperture are known respectively as 'holostomatous' and 'siphonostomatous.' The columella or pillar formed by the contact of successive whorls of the shell lies on the left side of the aperture in the angle between the first whorl and the peristome or free margin of the aperture. It is hollow and the external opening or umbilicus is partially hidden by the peristome, not wholly as it is in the Garden Snail, Helix aspersa. The smooth rounded off edge of the peristome shows the animal to have been adult.
The shell is coarsely striated in a direction parallel to the margin of the aperture, i. e. corresponding with the lines of growth, and more delicately in a spiral direction, parallel to the five coloured bands with which it is marked. The 'apex' or 'nucleus,' the tip of the shell or the part first formed, is smooth and semi-porcellanous in appearance, thus differing, as is so often the case, from the rest of the shell. The spiral line marking the point of contact between one whorl and its successor is known as the 'suture.' The shell increases in thickness towards the peristome, and it is probable that but little addition to its substance takes place except in that region. In this particular, as well as in being univalve and possessing a much smaller amount of organic matter, the shell of a Gastropod contrasts with that of a Lamellibranch.
There is no operculum in the Pulmonata, the order of Gastropoda to which the Snails and Slugs belong. When the snail hibernates it closes the aperture of its shell by a whitish disc, the hibernaculum or epiphragma - a structure containing much organic matter and calcareous granules of variable shape irregularly scattered. It appears to be secreted by the margin of the mantle or collar (see next preparation) rather than by the foot, as has been supposed. There are sometimes two such epiphragmata, one within the other. The moveable clausilium of the pulmonate Clausilia, which covers the aperture of the shell when the animal is retracted, appears to be a structure homologous with the epiphragma.
The majority of Gasteropod spiral shells are dextral. When the spire of a shell turns in a direction opposite to what is normal, it is said to be 'reversed.' In some species this is a common occurrence, e.g. in the Common Whelk; in others it is so usual that the type of shell commonly met with is sinistral, e. g. Clausilia. The lower whorls of the shell may, in a few instances, become loose and straggling, e.g. Vermetus, Siliquaria.
The reflection of the peristome over the umbilicus in Helix is due to the presence of a 'columellar lobule' or projection of the collar. When the shell is injured at the peristome before the animal is adult, both the outer non-calcareous layer 'periostracum' or 'epidermis' and the calcareous layers internal to it are repaired: when at any other spot, or after the animal is adult, only the latter, and in this case the shell is thickened internally. The colouring matter is secreted by unicellular glands of the collar. The spiral shell, as in all Gastropoda with few exceptions, is a secondary shell, i. e. formed on the outer surface of the mantle, and not from the shell-gland of the embryo. A primary shell, i. e. one formed by this gland, rarely persists. It is found, however, as the 'nucleus' of the shell in the pulmonate Clausilia, and as the internal shells of the Slugs Arion and Limax.
The shell of the Helices is stated by Longe and Mer to consist of an external organic cuticle or periostracum; of an outer calcareous layer containing colouring matter, and composed of (1) a thin external division with confused striation, (2) a thicker internal division which consists of vertical prisms; and of an inner nacreous layer including several laminae of prisms arranged horizontally, the axes of the prisms in the different laminae being at right angles to one another. The cuticle is formed by a cutogenic apparatus situated just behind the collar. It is composed of a pallial groove into which glandular caeca open, and an epithelial organ consisting of long bottle-shaped cells which secrete granules. The epithelial organ disappears in the adult, and the glands of the pallial groove gradually undergo atrophy. The rest of the shell appears to be formed by the edge of the mantle; the nacre from its surface in general. In Zonites algirus, according to Nalepa, the cells of the pallial groove develope in spring - the period at which growth of the shell takes place - into long unicellular glands, and the cells of a ridge (? = epithelial organ of Longe and Mer) just behind the groove enlarge into flask-shaped gland cells. The cuticula is secreted by these two sets of gland cells.
It is possible that the glands may be developed in Helix, as in Zonites, in the spring of the year, and that the atrophy of the organ in the adult (supra) merely marks a period of rest. (See Nalepa, SB. Akad. Wien. lxxxvii. Abth. i. 1883.)
A layer of nacre or mother of pearl is not generally found in the calcareous shells of Gastropoda. The calcareous matter is usually arranged in three layers composed of parallel lamellae, those of the outer and inner layer usually disposed parallel to the suture, of the middle layer at right angles to it. The direction of the layers is sometimes reversed. The lamellae are vertical to the surface of the shell, and each lamella is composed of calcareous prisms, all parallel to one another, but forming an oblique angle with the surface of the shell. The prisms of adjoining lamellae are disposed in contrary directions to one another. The calcareous matter is in the form of Arragonite (Ca C03). Splinters of the shell are hard enough to scratch Calc-spar.
The Gastropod shell is composed chemically of about 1.5 per cent. of an organic substance (Conchiolin); of about 95-98 per cent. of Lime carbonate; with small quantities of Magnesium carbonate; phosphates; silica and alumina.
The inner whorls of the shell are in some Gastropoda partially absorbed. In some cases the animal withdraws from the older region of the shell. The part forsaken is closed off by a calcareous lamina secreted by the surface of the mantle. The process of withdrawal and closing off may be repeated several times.
Technical terms, structure, etc. Woodward, Manual of the Mollusca (ed. 3.), 1875, p. 28, p. 204. Keferstein, Mollusca, Bronn's Klass. und Ordn. des Thier-reichs, iii. 2. p. 899, p. 1181. Zittel, Handbuch der Palaeontologie, Abth. 1, ii. p. 153. Leydig, Haut-decke und Schale der Gastropoden, A. N. 42. 1876.
Structure and formation of Shell in Helices. Longe and Mer, C. R. xc. 1880, cf. A. N. H. (5) v. 1880., Epiphragm. Keferstein, op. cit. supra, p. 1186. Binney, Terrestrial Mollusca of United States, ii. 1851. Analysis and source of lime. Barfurth, A. M. A. xxii. 1883, p. 509.