Skywhale (Orion's Arm)

Skywhales are large flying invertebrates native to To'ul'h Prime in the Orion's Arm universe.

Anatomy
A Skywhale is a large, bilaterally-symmetrical, filter-feeder invertebrate, that remains afloat by the buoyancy provided by internal hydrogen gas bags. Though it has no true backbone, it has a strong dorsal network of members that prevent the gas bags from ripping apart. Its mouth is sideways-opening.

Evolution
The ancestor of the skywhale was hyperthermophile that adapted to the photic zone of the atmosphere where there are larger quantities of food available in the form of skyplankton.

It has even established symbiosis with many forms of skyplankton, by allowing it to grow on its skin, preventing parasitic forms of plankton to infect the Skywhale.