by the germ layer ectoderm) of the mantle tissue.However, nacre does not seem to represent a modification of other shell types, as it uses a distinct set of proteins.

Evolution

The fossil record shows that all molluscan classes evolved some 500 million years ago from a shelled ancestor looking something like a modern monoplacophoran, and that modifications of the shell form ultimately led to the formation of new classes and lifestyles. However, a growing body of molecular and biological data

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