Rather than belabor the point that the rest of the Internet is wrong to call this artificially bioengineered construct a "jellyfish" as opposed to a "sea jelly," we're just going to get straight to the heart of it: it can swim, and it's powered by heart muscle cells harvested from rats.
The researchers at Caltech who
invented grew this thing call it "Medusoid." It's modeled after a sea jelly, which are the oldest multi-organ animals on the planet, having probably be swimming around in one form or another for the last 500,000,000 years or so. At its most basic, a sea jelly has a lot in common with an animal heart, in that it rhythmically actuates itself to control movement of a fluid, making it an interesting and slightly strange platform to use as a model for bionengineering a living pumping system.
To make their fake sea jelly, the researchers started with an eight-armed elastic membrane structure made of silicon, which (from a material properties perspective) is jellin' just like a real jelly. On top of this structure was deposited a layer of protein, which formed a framework for the growth of heart muscle cells harvested from rats. When the growth was complete, a gentle electrical stimulation (like a pacemaker) caused the rat heart cells to flex together, causing the jelly (let's just go ahead and call it a "zombie rat jelly") to begin to make swimming motions.
Despite the relative simplicity of the design, the zombie rat jelly exhibited a series of fairly complex swimming and feeding behaviors just like what'd you see in a biological (non zombie rat) jelly, and the researchers even see ways in which they can improve on the jellies that nature has come up with. But making turbo-charged jellies is just the beginning: this strategy could be used for reverse-engineering human organs, with the eventual goal of creating a pacemaker made out of biological elements instead of mechanical ones.
Oh, and the scientists also want to release their zombie rat jelly out into the wild to see if it's capable of gathering food on its own. They didn't specify what kind of food, but based on absolutely nothing, these things will probably latch onto your eyeballs and suck them out of your skull next time you go swimming.
Watch a video of the zombie rat jelly not doing that, below.