The outer covering of a shell is called the periostracum.
The periostracum (pronounced /ˌpɛriˈɒstrəkəm/) is a thin, organic layer that forms the outermost part of the shell in many shelled animals. These animals include molluscs like snails and clams, as well as brachiopods. Think of it as a "skin" on the shell.
Key Characteristics of the Periostracum:
- Composition: It is composed of conchiolin, a protein-based material.
- Function: It provides protection against abrasion, dissolution (especially in acidic environments), and boring organisms.
- Appearance: The periostracum can vary in color, texture, and thickness depending on the species and environmental conditions. It can be smooth, hairy, or even spiky. In some species, it is transparent or nearly so, while in others it is thick and opaque.
- Durability: It is often the first part of the shell to erode or wear away over time, especially in older shells or those exposed to harsh conditions.
In summary, the periostracum is a vital protective layer on the outside of many animal shells, contributing to their survival and integrity.
