外殼膜

*, a land snail or terrestrial gastropod. The periostracum of this species has minute hairs, giving the snail a velvety feel]]
*]]
of the "ocean quahog", marine bivalve Arctica islandica, from Wales]]

外殼膜（），亦作殻皮，是軟體動物及腕足动物的外殼之外的一層包裹物；但兩個分類的外殼膜由不同材質組成，亦使這兩種外殼膜表現出不同的特徵。在軟體動物，主要為腹足綱及雙殼綱最常見，但在頭足綱如異鸚鵡螺屬之類的物種也有外殼膜。外殼膜與其包裹的外殼及外殼上的其他分層都是整個外殼的整全部分，也成為了這些物種的可分辨外觀。

Periostracum這個字由（包圍）和兩部分組成，意思就是「包圍着外殼」，包圍着外殼的鈣質層（ostracum）。

功能
The formation of a shell requires certain biological machinery. The shell is deposited within a small compartment, the extrapallial space, which is sealed from the environment by the periostracum, a leathery outer layer around the rim of the shell, where growth occurs. This caps of the extrapallial space, which is bounded on its other surfaces by the existing shell and the mantle.

The periostracum is secreted from a groove in the mantle, termed the periostracal groove. When secreted, it consists of the soluble protein periostracin; this polymer becomes insoluble through a tanning process involving quinone.

Description
Periostracum is often yellowish or brownish in color. In some species it is black. The periostracum is very often a different color than the underlying layer of the shell.

In the shells of species which have periostracum, this shell layer is quite often physically worn away or chemically eroded in the parts of the shell that are older, thus it may only still be visible in the more recently formed areas of the shell.

Periostracum can in some cases be quite thin, smooth, glossy and transparent, such that it looks almost like a thin yellow varnish, or it can be thicker and more or less opaque. When it is thick it is often relatively rough in texture and dull. In some species the periostracum is tufted, or forms hair-like growths which in some cases can give the fresh shell a velvety feel, see. In some species the periostracum adheres very tightly to the underlying shell surface, in others the periostracum is less firmly attached.

In certain marine species, such as for example certain species of cone snails, a heavy periostracum obscures the color patterns that exist on the calcareous layer of the shell.

In many aquatic species, once a shell has been removed from the water and has had time to completely dry out, then the periostracum may become brittle and start to flake or peel off of the surface of the shell.

It is not uncommon for shell collectors to deliberately remove a periostracum layer (using household bleach) if they feel that a shell is more attractive without it. However the periostracum is an important part of the shell, and is definitely of interest to malacologists. Details of the periostracum can sometimes be very helpful in identifying a species.

Hairs on gastropod shells
Haired shells occur in gastropods in several species of the Stylommatophoran families Polygyridae, Helicidae and Hygromiidae. These families are only distantly related, suggesting that this features has evolved several times independently. 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.。

參看

• 外殼 (軟體動物)
• Lorica

參考文獻