It is n't a mollusk's shell, either; it 's a caddice-worm's shell.
It is a mollusk because it has a mantle, a foot, and a radula.
He desired the shell of the mollusk that burrowed in the cleft of the cliff.
He had lain there in the sand for some time, as motionless as a mollusk at low water.
The Mollusk has years in which to build her spiral, so she makes it very perfectly.