Here's the usual ability...
Swallow Whole (Ex): A giant nautilus can try to swallow a grabbed opponent of a smaller size than itself by making a successful grapple check. Once inside, the opponent takes X+X points of crushing damage plus X points of acid damage per round from the nautilus's gizzard. A swallowed creature can cut its way out by using a light slashing or piercing weapon to deal X points of damage to the gizzard (AC X). Once the creature exits, muscular action closes the hole; another swallowed opponent must cut its own way out. A Gargantuan nautilus's interior can hold X Large, X Medium, X Small, X Tiny, or X Diminutive or smaller opponents.