operculum

…the cheilostome colonies, and the operculum seems to have been significant in the evolution of the specialized zooids of this order. The avicularium type of zooid has a small body and a rudimentary polypide; the operculum, however, is proportionally larger, has strong adductor (closing) muscles, and has become, in effect,…

Prosobranchs use the operculum, the horny or calcareous disk located on the back of the foot at the posterior end, to seal the shell opening after the head and body have been retracted.

…use a calcareous or horny operculum (trapdoor) on the foot to seal off the aperture when the foot is withdrawn into the shell. In the cephalopods Nautilus and Spirula, the planospirally coiled shell consists of multiple chambers connected by a porous tube called the siphuncle. The chambers contain quantities of…

…horny or calcareous door, or operculum. The careful collector makes it a point to preserve the door with the shell in his collection. After a mollusk has died other marine animals may become occasional inhabitants of the shells.