They have no brains. No specialized digestive system. No blood. No nervous system. No vascular system. Just a one-size-fits-all cavity where food meanders its way in and is passively digested. The gastrovascular cavity is basically a hipster nonchalantly swaying to the music at a Death Cab for Cutie show, because they're not as hip and unknown as they used to be and it doesn't want to look too enthusiastic.
|"Yeah, Ben Gibbard is alright, I guess," he said, tears glistening on his cheeks as "Transatlanticism" reached its climax|
The only thing jellyfish can consciously do (if you can even call it consciously, which you probably can't) is flex a single muscle that kind of, sort of, in a way, gives them control over their motion. Like spreading your arms and legs out to slightly alter your heading while you're falling from 20,000 feet without a parachute.
The way they reproduce is weird as hell, too. A bunch of them get together and basically make a big old cocktail of sperm and eggs, which eventually leads to mass fertilization. The eggs hatch into little larvae called planulae, which stick to a firm surface and grow into polyps. Polyps are essentially what you'd expect to get if you cross-bred a jellyfish with a sea anemone, and if there are any mad scientists reading this and thinking that sounds like a good idea, please don't. You are the most boring mad scientist in history. Stop it.
|If this diagram shows up in your plot to take over the world, there are several somethings wrong with you.|
So the polyp grows up a bit, and suddenly things take a turn. In many cases, the polyp will spontaneously just...clone itself. Because jellyfish are both sexually and asexually reproductive organisms. And then sometimes, just for kicks, a few polyps will decide they're not in a place right now where they can afford to live on their own, so they'll find some roommates and form a colony. Not to save on rent money, mind you. They share a goddamn stomach. Because of course they do.
Finally, they fly the coop to become an ephyra, which is more like what you imagine when you think of a jellyfish. Then it grows tentacles, and becomes a medusa. Then it haunts your nightmares and inspires Nintendo to design a creature that can literally suck your life force out of your upper torso.
|If you played games in the '80s and '90s, you are now having PTSD-esque flashbacks.|
"Ben Gibbard 2" by Sharat Ganapati - originally posted to Flickr as Ben Gibbard. Licensed under CC BY 2.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Ben_Gibbard_2.jpg#mediaviewer/File:Ben_Gibbard_2.jpg