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Note primarily that this is not a duplicate of Jet Powered Animals.

Note also that this is a suspend-disbelief question. I am not asking why you think this can't happen.

I am building a world of flying things. This world has only a slightly lower gravity than earth, but the same composition. One such thing is called the (insertname). It is capable of flying by acquiring air, puffing itself to massive size, then expelling that air from behind itself. There are humans, (primitive), on this planet, and they want to know:

What kind of biology allows for air-jet travel off-ground and out of water, and how would this creature be designed?

Edit for close-voters: This world is a parallel earth. It has all the same conditions, but also includes this fauna.

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    I really don't think a "suspended-disbelief" tag is required for Worldbuilding. I'm going to remove it. You can assume we're suspending disbelief when you leave off [tag:reality-check]. I also removed [tag:balloon-whales] since it appears your creature doesn't actually float. – Samuel Feb 22 '16 at 22:17
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    @Samuel, if you wanted such a creature to be marginally believable, then it'd have neutral buoyancy in its media and then just use the inflation jet for propulsion. This is essentially what squid do. – Jim2B Feb 23 '16 at 00:37
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    Closer-voters: Please don't vote to close without leaving commentary. The OP cannot fix the problem (and I don't see one here) if there is no commentary. – Frostfyre Feb 23 '16 at 05:09
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    @Frostfyre It's too broad, opinion based, lack of research effort, idea generation, resulting in it being unclear - that's 4 close reasons, 3 of which are represented by the votes, and 1 DV reason. – Aify Feb 23 '16 at 05:54
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    @Aify Hmm, I don't think it's that bad myself. It could do with specifying the world conditions better but we've had far more broad creature design questions. – Tim B Feb 23 '16 at 11:13
  • @CalebWoodman You can see the revision history here. It looks like you also tried to readd "suspended-disbelief" which is why I assume both were removed again. Please be more descriptive in your edit notes, for instance if you included your above comment I'll bet that only "suspended-disbelief" would have been removed, not balloon-whales. – Samuel Feb 23 '16 at 16:51
  • @TimB Just because we've had broader creature design questions doesn't mean they were all on topic - (And seeing as 3 of my top questions are creature design, I may be biased since I use those as my standard for reference to on/off-topic). Basically, my point is that a precedence of having previous broader questions during which the site scope hasn't been fully defined doesn't give reason to allow off-topic questions onto the site now. – Aify Feb 23 '16 at 17:07
  • @CalebWoodman the "balloon whales" tag is too specific a tag for WB. It would literally explicitly be used for this one question, while a "flight" + "creature design" covers any flying animal. – Aify Feb 23 '16 at 17:11
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    @Aify balloon whales is a tag that's been used before, if he wants to use it here that's fine. If you want the tag burninated then start a meta discussion :) – Tim B Feb 23 '16 at 17:13
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    @TimB Well sh.... I did not know. Uno momento please, moving for burnination! – Aify Feb 23 '16 at 17:16

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Entry nine of eighteen. Day 38.

We have heard a great sigh and humph outside our tent this morning and went out into the dawn to investigate. There, like a translucent mass of clear jelly the size of a large, child's paddling pool, was something we could not quite discern if it were dead or alive. Underneath it was a live creature - likely a mouse or some other ground dwelling mammal - which was being suffocated to death by the thing.

We withdrew to observe at a distance.

Over a period of hours we found that the jelly had dissolved the mammal right down to the bone, which remained scattered beneath it and picked-over in fragments.

In the hour that followed, the jelly appeared to grow by a magnitude of four times in the sunlight - either by production or absorption of gas with aid from the heat. It became clearer and more translucent until, like pockets of bubblewrap, it had become engorged with air. It was easy to draw a comparison here with the jellies of the deep sea discovered long before - stinging tentacles became visible like short, wiry hairs on the underside of the thing, and a repeated segmentation could be observed in the whole.

Within minutes, and hesitantly, it lifted off from the ground to return to the air from where we assumed it had come.

Within a few feet it was hard to discern, and once it had passed more than a couple of yards into the air we were no longer able to view it at all.

It was from this point forward we began to carry umbrellas as a matter of course, though we have not seen one since.

Footnote: To clarify - these are ambush predators which project their stored air behind them to descend quickly into the ground and ensnare their prey.

Vincent
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Ah, yes. The glorious Jet-Butt. Quite a fascinating creature.

Its anatomy is truly fascinating!

You see, the Jet-Butt, which is quite a small creature to begin with, puffs certain flexible membrane pockets and its sides with methane produced by its digestive system, and rises right off the ground!

It then uses the content of a third pocket, filled with regular air, as a means of propulsion by slowly (or quickly) releasing it through a muscle-controlled orifice, or Jet-Anus.

Since the process of filling its methane pockets is not as fast as all that, this flight mechanism is only used for migratory purposes.

Sadly, these majestic creatures have been known to spontaneously combust if exposed to too much direct sunlight when in flight.

AndreiROM
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Entry five of eighteen. Day 34.

These gliders, for lack of a better name, have a light, rigid bone structure which defines a half-shell of a rib cage and two, bowing, static fins which project laterally from the torso. A rudder-like tail swings horizontally behind it between two rigid projections on the rear - like noses, arching up in ridges of cartilage from the lower back. A great, wide mouth at the fore of the creature, coated in fine hairs and mucus, draws in the air required for projecting the breath through its nostrils as well as the small, microbial creatures it filters from the atmosphere.

By raising and swooping down, mouth open, it can open the channel between mouth and nostrils, using its morphology to bottleneck the air and jet it out from behind itself - while using very little energy in the process.

If alarmed or otherwise startled, the creature may inhale deeply and eject the air in a bellowing trumpet or screech, thus accelerating at great speeds.

Such creatures can grow up to sixteen feet in length, if one includes the nine feet of tail.

Neither Faraday or myself have seen any of these "gliders" upon the ground, and it would seem they would be too fragile to perch or stand. They have no external limbs, and appear to be quite comfortable to ride the thermals when not propelling across the vast prairie.

Their main predators appear to be large bracken-like lichen. Being of limited sight, we have seen these creatures become ensnared like a limp balloon - and - over a period of weeks - digested.

Faraday has collected the bones, which we intend to return in whole to the university.

Vincent
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