Mollusca: The Shell

The shell is formed by secretions of glandular cells in the mantle. Except in the chitons, the shells of all mollusks are basically similar, differing only in certain mineralogical details. The shell is composed of an outer, prismatic layer containing densely packed cells of calcareous material secreted by the edge of the mantle; and an inner, nacreous layer of thin, laminated plates of calcareous material laid down by the entire mantle surface. When very thin, the nacreous lining of the shell is pearly and iridescent. Layers of this material may form around a grain of sand or other irritant that lodges between the mantle and the shell; this process eventually forms a pearl. Pearl oysters of the genus Pinctada are the most commercially important pearl formers.

Sections in this article:

• Introduction
• Class Cephalopoda
• Class Scaphopoda
• Class Bivalvia
• Class Gastropoda
• Class Monoplacophora
• Class Polyplacophora
• Class Aplacophora
• Reproduction
• Circulatory and Nervous Systems
• Respiration
• The Digestive Tract
• The Shell
• The Body Wall
• Anatomical Features

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