The first stage in the jellyfish life cycle is the planula. A planula is a ovular shaped, microscopic organism. they move them selves around using many cilia, or tiny hairs.
Planula are the byproduct of sexual reproduction in jellyfish. At least two adult jellyfish of opposite sexes must be near each other for sexual reproduction to occur. The jellyfish will release gametes (eggs and sperm) into the water and they will meet, thus fertilizing the eggs. The eggs will begin to multiply in cells up to 64 cells. Then they become a full blastocyst and eventually form a planula. Millions of planula are formed at one time.
These planula will move around with their cilia for around 7-10 days. Then they will find a clean place and settle down on it. once they have a hard surface to attach to they will begin to metamorphose into what is known as a polyp. That is the next major stage in the jellyfish life cycle.
I have a video of cannonball jellyfish sperm cells as well.