What comes first, the medusa or the planula? You'd have to go back about 600 million years ago (give or take 50 million) to find out. As it stands now the jellyfish have a cycle like all living creatures and their drive to reproduce sustains their species. Where the circle ends and begins, no one really knows.
These creatures we call jellyfish, or jelly blubber, or just jellies are also known as scyphozoans. They are a member of the same family as corals and anemones and spend a portion of their life looking similar to both of those species. They've got no spine and no true brain. Instead they radiate from a central point and use a nerve net to detect prey, danger, temperature and light.
Interestingly, they've only got one opening (or mouth) from which they eliminate waste, reproduce and eat. Perhaps most interesting of all though, is the range of size and danger associated with the many creatures that fall under the title jellyfish. Their tentacles can be mildly sticky to completely deadly. Their sizes (top of bell to tentacle's end) can be mere millimeters to a hundred feet.
Regardless of size or the danger their tentacles present, jellyfish all produce offspring in relatively the same way with a five part cycle. Most jellyfish undergo this cycle rather quickly and exhaust their lifespan within a year. However, some cold water specimens are known to live for at least five years.
In this article the planula comes before the medusa.
THE PLANULA LARVA
This stage is marked by a little fertilized egg that swims to what will hopefully be a safe environment to grow. What they are looking for a is a place to anchor where the temperature is right, where food will drift by and where they won't be eaten. If they find all of those things, they make it to the next stage.
This stage is spent cemented to a safe place and growing rapidly. It can be likened to a cocoon in a way. The creature that the jelly fish is going to become looks nothing like the polyp attached to a rock on the sea floor. But something more is going on here than just getting bigger.
THE BUDDING POLYP
This stage is the beginning of mobility. Still anchored to the rock, the polyp is working on sending out the next forms. It's a launch pad for things to come. The polyp isn't just budding, it's dividing. Several organisms are about to be set adrift.
Ephyra are like baby jellyfish. Their tentacles aren't as long, they look chubbier and safer than they will once they're fully grown. Once they escape from the polyp they can swim or float at leisure while they wait for their bodies to morph into what will finally be an adult jellyfish.
Quite quickly an adult jellyfish is formed and ready to continue the entire cycle. Sometimes this entire process only takes a few months. The medusa stage of the jellyfish is the adult and reproductive stage. The females have developed the brooding sacs on the undersides of their feeding arms. Both males and females have developed the proper sexual organs within their stomach lining. The male can release sperm and the female can take it into her single opening, to create a whole new batch of planula larva.
Encyclopedia of the Sea, by Richard Ellis