Last update: 07/04/2006

The shell of snails is an external skeleton secreted by the dorsal face and the free edge of the coat. It is primarily made up of calcium carbonate. 

The shell constitution:

The shell consists of three superimposed principal layers:

shell layers

The periostracum (A): left varnished very resistant guard secreted by a glandular furrow of the coat edge, this external layer consisted by conchiolin fibers, nitrogenized substance with consistency cornea, and ensures the protection of the deep layers. The color pigments of the shell are usually located here.

The ostracum (BC): secreted by the edge of the coat, this average layer, more or less thick is, as its name indicates, made up of hexagonal prisms of calcite piled up in columns, perpendicularly laid out at surface of the shell and enchased in cells consisted fibres of conchiolin.

The hypostracum (D): secreted by all the dorsal surface of the coat, it results from regular stacking blades of conchiolin and limestones blades consist in crystallised aragonite spangles. All the layers are parallel between them and on the shell surface, overlapping like roof tiles. It's this laminated provision which, by luminous radiations interference, gives this iridescent shimmer, this glossed aspect and these metal colors characteristic of mother-of-pearl. Between the prismatic layer and the given lustre to layer, there is a transition course where the conchiolin fibers of pass gradually from the anticline provision (prismatic layer) at the pericline disposal (pearly layer). Moreover, the pearly layer is protected on its internal face by a hyaline layer with a lamellate aspect: the hypostracum.

In assessment, we can say that the mollusc shell consist in 89 to 99 % of calcium carbonate: the remainder, it's calcium phosphate, silicon, sulphate of calcium, magnesium carbonate, iron sesquioxide, anhydride phosphoric and sometimes cæsium and rubidium, the whole cemented by an organic screen compacts primarily made up by conchyoline, scleroprotein chitinoïdale similar of your nails or the beetles cuticule.

It's thus why, it's necessary to include calcium in snail food !

The shell increase:

The shell increases on the surface thanks to the periostracum and the ostracum, in thickness thanks to the pearly layer. In all the cases, the shell growth is discontinuous, periods of growth alternating with periods of rest related to the food availability, in the conditions of temperature and to the sexual reproduction: the parallel scratches of increase at the layer edge (and which affect the 2 external layers) testify of these discontinuities. By counting the scratches increase, one can appreciate the age and the physiological animal state. It's by its external edge that a shell increases: this one, thin and fragile at the beginning, becomes then rigid (we said while the snail is "lined").

The shell rolling up: dextral or senestral?:

At the gastropods, although the shells have an extreme diversity of aspect, of form, of ornamentation, there is only one fundamental structure: a tube rolled up on itself around a secondary axis, most of the time in the direction of the needles of a watch (i.e. towards the line: dextral rolling up), sometimes in the direction reverses (i.e towards the left: senestral rolling up ). Indeed, if the tube grew in a rectilinear way, it would very quickly become long and quite cumbersome…

If you look at a snail shell of near, you can really see the layers. Shell is rolled up because the snail doesn't manufacture the layers at the same speed, which means that the shell develops more quickly towards a sector than towards another and thus, will form a spiral upwards. 

The major part of the gastropods has a dextral shell: only a small number of species naturally presents a sinistral torsion like Busycon contrarium, which bears its name well. 

Snail kingIn some dextral species, one can meet senestral shells  exceptionally (1 case out of 20 000 for the helix pomatia).

With your opinion, which is senestral?

Source: Answer: that of right-hand side

The columellar muscle:

columelar muscleThe shell is connected to snail by the columellar muscle. If this muscle has suddenly yielded, the snail separates from his shell and die.