Sea jellies, often misnamed jellyfish due to their lack of a skeletal system, are boneless invertebrates belonging to the phylum Cnidaria. Unlike true fish, they lack a heart, brain, and gills, instead relying on a decentralized "nerve net" to sense environmental changes and thin, gelatinous tissue to absorb oxygen directly from the water. These resilient organisms have inhabited the world’s oceans for over 500 million years, predating dinosaurs and surviving multiple mass extinction events. Their simple yet effective biological design, which includes specialized stinging cells and a support structure known as mesoglea, has allowed them to thrive in diverse aquatic environments ranging from the frigid Arctic to tropical shores.

The diversity among sea jellies is remarkable, with species like the Lion’s Mane jelly reaching lengths of 120 feet, surpassing the adult blue whale. Perhaps most fascinating is the Turritopsis dohrnii, or "immortal jellyfish," which can reset its biological clock through a cellular process called transdifferentiation, reverting from an adult state back to its earliest development stage. Beyond their natural habitats, sea jellies have contributed to significant scientific research, including NASA experiments where thousands were sent into orbit to study how microgravity affects sensory development. This research revealed that jellies raised in space developed severe vertigo upon returning to Earth, providing critical insights into how gravity influences the internal balance organs of living creatures.

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