Is it a jellyfish? Is it a worm? No, it's an existential puzzle! This larva is yet more proof that nature will never stop finding ways to confound people. It develops itself inside itself, then bursts out of itself, leaving itself behind.
Nemertea, sometimes known as ribbon worms,, roam the oceans, the lakes, and even the land. They have a few unpleasant tricks, like secreting mucus from their proboscis to stop us from getting a grip on them, and making that mucus toxic to make us regret ever trying to get a grip on them. But biologists (who can never leave well enough alone) decided to take a closer look, and they found that nemertea don't just mess with our bodies, they're out to get our minds, too.
Your typical nemertean reproduces in an orderly fashion. It lays eggs, those eggs are fertilized, and the fertilized eggs hatch a miniature version of the adult nemertean. Then there are the heteronemerteans, who come out of the egg as a larva, but that larva doesn't look like a worm. It looks a bit like a jellyfish, though some describe its shape as that of a deerstalker cap with the flaps let down. This larva — which just to make things weirder has no anus — eats plankton and flaps its way around the ocean.
At some point a little thing begins to grow inside the larva. This is the juvenile worm. The larva goes on about its business, swimming and eating. The worm inside it grows. At some point, the worm stops getting nutrients from the larva and starts eating the larva itself. The juvenile, after it has eaten its fill, bursts out of its own larval self, falls down to the ocean floor, and starts its life as a juvenile ribbon worm. The process is called catastrophic metamorphosis, a name which, give credit where credit is due, biologists got exactly right.
So yes, this is a worm that has two stages of life, which both exist at the same time. What the hell, heteronemerteans? What even is that? Stop messing with my sense of self, nature! One more trick like this and I will global warm you to death.