One advantage to the game's structure-- each mission is completely self-contained, with its own items and hazards that don't carry over to the other missions-- is that I can approach each mission almost as its own game. I've decided to further emphasize this structure by taking a different approach to the player's health in each of the game's three missions.
The first and shortest mission is pretty combat heavy, with lots of enemies to shoot and lots of things to shoot them with. Because it's so focused on action, I'm giving the player a recharging shield. Whenever you get hit, the shield goes away for a handful of seconds. Survive during those tense moments, and your shield pops right back on. This is the approach I talked about way back in Dev. Journal # 11, and I initially thought it would be the standard approach for the entire game.
But, because the shield is a power-up-- an item that you acquire-- and because one of my "rules" is that once an item appears in a mission, it would not reappear in any other missions, I felt it would be more interesting to find a different solution.
In the case of the second mission, that solution is no solution-- that is, the player has no health or defenses. Nor, in fact, any offenses; the second mission, which is roughly twice as long as the first, is all about navigation, with each item increasing your ability to get around the level and reach previously impossible-to-reach places. It's not, however, hazard free, as the level is full of fish, torpedoes, underwater mines, machine guns, and exploding-butt-lumberjacks that will have to be avoided with tricky, precise platforming and the usage of your various items. And while this one-hit death-a-palooza is balanced out by an ability to save anywhere at anytime, it is likely the most difficult level in the game in terms of requiring the most of the player's reflexes. (However-- as I hinted in my dev journal "Difficulty as Deterrent"-- this extreme difficulty exists to encourage the player to avoid it as much as possible; breaking the sequence will transform the level into something of a cakewalk.)
The third mission is by far the longest, functioning pretty much as a complete Metroidvania-style game, with multiple bosses, paths, and items to acquire. If the other missions are pretty bare-bones and narrowly focused (the first on action, the second on navigation), this one is intended to be much broader, deeper, and robust. For this third mission, I'm going with a more traditional approach-- a health meter that is upgraded as you find Health Upgrades and refilled with pellets dropped by enemies. This mission will also feature an ammo system-- unlike the other two-- with a similar capacity upgrade and pellet-drop mechanic.