Periostracum

[3]: 475  The periostracum acts as a framework from which the outer layer of carbonate can be suspended, but also, in sealing the compartment, allows the accumulation of ions in concentrations sufficient for crystallization to occur.

When secreted, it consists of the soluble protein periostracin; this polymer becomes insoluble through a tanning process involving quinone.

In some species the periostracum is tufted, or forms hair-like growths which in some cases can give the fresh shell a velvety feel.

Haired shells are almost exclusively observed in species living in moist microhabitats, like layers of fallen leaves, broad-leaved vegetation, damp meadows or wet scree.

Such a correlation suggests an adaptive significance of the trait in such a habitat; it was thus speculated that the hydrophobic hairs facilitate the movement in wet environments by relieving surface tension.

These semi-rigid structures are part of the periostracum, a thin protein layer (conchiolin) secreted by the snail to cover the calcareous shell.

[5] Experiments by Pfenninger et al. (2005)[5] on genus Trochulus showed an increased adherence of haired shells to wet surfaces.

The possession of hairs facilitates the adherence of the snails to their herbaceous food plants during foraging when humidity levels are high.

The absence of hairs in some Trochulus species could thus be explained as a loss of the potential adaptive function linked to habitat shifts.