Natalie Angier in The New York Times:
Among nature’s grand inventory of multicellular creatures, jellyfish seem like the ultimate other, as alien from us as mobile beings can be while still remaining within the kingdom Animalia. Where is the head, the heart, the back, the front, the matched sets of parts and organs? Where is the bilateral symmetry? Yet if any taxonomic dynasty is entitled to the originalist mantle, to the designation of genuine emblematic earthling animal, and also to brand the rest of us the alien arrivistes, it is the jellyfish. A diverse group of thousands of species of gooey, saclike invertebrates found throughout the world, the jellyfish are preposterously ancient, dating back 600 million to 700 million years or longer. That’s roughly twice as old as the earliest bony fish and insects, three times the age of the first dinosaurs.
“Jellyfish are the most ancient multiorgan animal on earth,” said David J. Albert, a jellyfish expert at the Roscoe Bay Marine Biological Laboratory in Vancouver, British Columbia.