Cloud beings

Ideas of cloud-borne Venusians are not new. As far back as 1967, when there was evidence for substantial amounts of water vapor in the planet's clouds, Carl Saga and Yale biophysicist Harold Morowitz hypothesized organisms the size of Ping-Pong balls, with skins a single molecule thick. Such organisms, so the thinking went, might have originated on the surface some time in the distant past when conditions were more temperate an, as the surface heated up, migrated to the skies. Like jellyfish in terrestrial oceans, they would maintain their buoyancy with float bladders -- filled,  in the Venusian case, with hydrogen. The idea was criticized because although Morowitz and Sagan had suggested ways the cloud-borne might be cloud (sexual land asexual reproduction), they had not shown how they might have evolve, and no one could imagine an evolutionary path by which a Venusian surface dwelling organism might develop a float bladder. Sagan, though, was undeterred. He didn't abandon the cloud-borne life; he just suggested another place to look for it...

-- from Weird Life by David Toomey

See also Libertine Bubbles