OK, how is this then?

Far Swallow (Su): If the mind worm scores a critical hit with a probe worm attack, the victim must succeed on a DC 28 Will save or be immediately transported into the mind worm's interior, allowing the mind worm to immediately swallow the opponent (no grapple check required). This is a conjuration (teleportation) effect. The save DC is Charisma-based.
A swallowed creature takes 2d6+13 points of crushing damage plus 6 points of acid damage per round from the mind worm’s gizzard. A swallowed creature can cut its way out by using a light slashing or piercing weapon to deal 40 points of damage to the worm’s gizzard (AC 18). Once the creature exits, muscular action closes the hole; another swallowed opponent must cut its own way out.
A mind worm's interior can hold 1 Large, 4 Medium, 16 Small, or 64 Tiny or smaller creatures.
For this one, I think the acid and mind-affecting effects should be left on the stat block. Change the name to “immersion alertness” or something like that:
Immunities: Mind worms are immune to acid and mind-influencing effects. If it is immersed in water, the mind worm is not subject to critical hits and cannot be flanked.
Changing this:
Constitution Drain (Ex): A creature successfully hit by a probe worm suffers 1d4 points of permanent Constitution drain, or 2d4 points of permanent Constitution drain on a successful critical hit.
To this:
Constitution Drain (Ex): Living creatures hit by a probe worm’s attack take 1d4 points of Constitution drain (double damage on a successful critical hit).