The Molluscs diversified
The shell is secreted by the upper surface of the mantle, with limpets producing shell at equal rates along the edge of the mantle, in other animals the front end of the mantle secretes at a faster level than the rear end and produces a flat spiral. The maximum secretion may be to one side and develops twisted or turreted-shaped shells, or in the case of cowries the secretion is concentrated along the sides of the mantle creating a shell resembles a clenched fist. Molluscs may have either single shells (limpets), two shells or bivalves (mussels) or a number of shell plates (chitons). In some molluscs the shell has become reduced and totally internal (cuttlefish) whereas in others it is total absent (octopuses).
Limpets in their natural habitat (here northwestern Galicia). One animal was separated from the rock and turned over for the photograph to have a view of the creature inside.