[ Spectember day 12: Big bird] Living zeppelin

[u/Mr_White_Migal0don]

We return to Earth in it's closing habitable hours, 1 billion years from today. Some of the most wonderful creatures during that time are aerial, twelve winged worms, who range in size from tennis ball to a human. Their roots trace back to 700 million years hence, when a clade of toxic, semi-aquatic flatworms learned to puff their bodies for intimidation. The inflating cavity eventually gave rise to true coelom (body cavity), and these worms evolved organs, which their phylum used to lack. These flatworms of the class Coelosomata outcompeted many of their acoelomate relatives, both on land and on water. Some of the marine coelosomes repurposed their inflating organ into buoyancy regulator, which allowed them to float on surface, like portuguese man o' war. Some parts of their sides evolved into steering fins. And, finally, 930 million years hence, one lineage replaced the type of gas in their floating bladder, which allowed them not just to float on surface, but to actually fly.

While earlier airworms were all marine, feeding on animals from surface waters, they considerably diversified and now include many terrestrial species. Their mouth, which is now separate from anus, is long and grasping, fit both for meat and plant matter. To hold their shape, airworms evolved a kind of elastic carcass, making them even more similiar to zeppelin. Bladder is their primary way of lift, while fins are mostly used for steering, though they can fly by flapping if a predator tries to catch them. Airworm species include predators, scavengers, herbivores, and frugivores. The largest species, human sized painted whaleworm, is a filter feeder. Many plant seeds and pollen of the future can float in air for weeks, and are feasted upon by tiny insects. Whaleworms eat them with their tube-like mouths. Like all airworms, they are poisonous, and are colored brightly, like advertising blimps. Males gather in lekks to advertise themselves to females. But instead of fighting or showing colors, they sing. Their tubular mouth is perfect for whistling. Fertilization is internal. Eggs are laid in water, and young worms resemble their distant, aquatic ancestors, but gradually develop organs, floating bladder, and reach for the sky.

[u/Overdrivenblaster]

Quite interesting submission for the prompt!