BALTIMORE — Until I met Doug Allen, the wiry, ponytailed senior aquarist who guided me through the extremely popular jellyfish exhibit at the National Aquarium, my personal experience with jellyfish consisted mainly of using them as yet another excuse not to go swimming: “Hey, I could get stung by a jellyfish!” Isn’t that what happened to 1,800 people off the coast of Florida last week? So when Mr. Allen suddenly stopped, clambered a ladder to the top of one of the tanks and called down, “You want to try holding a moon jelly?” my first impulse was to knock a few schoolchildren out of the way as I bolted for the door. My second impulse ...

Too late. A three-inch-wide moon jellyfish had been plopped in my hands, and my fear quickly dissolved into fascination. The jellyfish shimmered and glowed. With its tendrils retracted, it looked like a round bar of glycerin soap, or maybe a translucent diaphragm, and it felt equal parts firm, jiggly and slimy, like a slice of liver coated in raw egg. And for all the vigor of my fondlings, I detected no sting.

“The poison of a common moon jellyfish is very weak,” said Anders Garm, who studies jellyfish at the University of Copenhagen. “You’d have to kiss the jellyfish to feel it.” There was no risk of that, but when we parted, the jellyfish left behind a kiss of its own on the palm of my hand: a sticky film that was surprisingly hard to remove. Thanks, my little honey moon.

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?