Simple, ethereal living bubbles, jellyfish have been drifting through the oceans of this planet for more than 600 million years. Their bodies may be simple, but their lifecycles are quite complicated.
Jellyfish reproduce sexually. Each "medusa," the familiar lampshade-shaped creature with trailing tentacles, is either male or female. In most species, a male medusa releases his sperm directly into the water. The sperm then swim into the mouths of nearby females, fertilizing their eggs. Each female releases a swarm of tiny larvae called "planulae," which are covered with pulsing hair-like cilia that allow them to swim.
When a planula finds a solid surface, it settles down and develops into a polyp. The polyp is sessile (rooted in place), and looks similar to a tiny sea anemone, its cup-shaped body crowned with tentacles. The polyp is called a "scyphistome," and it undergoes the most amazing transformation in the whole lifecycle of the jelly.
The scyphistome filters food from the water with its tentacles, and continues to grow. Once it's large enough, the top end begins to bud off into "ephyra," immature medusas. The ephyra resemble a stack of plates in the cupboard. One by one, these tiny new jellyfish detach from the top of the stack, and swim away.
Over the next few weeks or months, depending on the species, the tiny medusas grow into fully grown jellyfish. Once they have reached full size, the new jellies are ready to mate, and start the whole bizarre cycle over.