All Molluscs possess a body plan of a muscular foot, a visceral mass, and a mantle. Mollusks are also soft-bodied animals coming from the Latin root of the name molluscus meaning soft.
Molluscs' bodies characteristically are divided between a ventral foot
(as a locomotion organ) and a dorsal mantle or pallium (protecting the
body and building the shell).
if you mean a general characteristic that is supposedly shared but not all mammals share it then it will be that they give birth to their young. The platypus and echidna lay eggs instead of giving...