shell sac

noun1

1. •
   The sac in which the shell of a mollusc is secreted during development; (now especially) the fold of mantle in which a cephalopod secretes its pen or gladius.

   • Baby cephalopods, even octopuses, all have a shell sac, which is comparable to the mantle folds that baby clams and snails use to make their shells. — 2017