[Suicide Junkie]

Natural selection dosen't select against "easily shot" until after intelligence arrives on the planet. Or at least technology that originated from something intelligent.

If you've got a gas sac for floating, suspending the brain in the middle is not very good.
More efficient to attach it to the outside (it will naturally rotate to the bottom, like a gondola). Right near the mouth, and the feeding apparatus and any manipulators for getting said food.

Putting the sac on the end of a tail so the critter can lunge with its head to get food would be advantageous if the food is multicellular... filterfeeders won't need anything but shifting winds to blow the plankton-equivalent past them. Having multiple flotation sacs might be good if the environment occasionally pops them. It wouldn't help against predators, tho.

Floater predators could be avoided by climbing higher... the highest prey would survive, and the low/slow/weak ones would get eaten, thus driving both populations to float higher and higher.

Winged predators would be like woves on sheep. Not much that can be done except to live in herds. Face any adults outwards, maybe have some stinger tentacles to wave and discourage them unless they're actually hungry.
Although, floating higher could help somewhat here too... If it takes more work for the wingers to get up to your altitude from wherever they rest, then they'll go after the lower, easier prey.