Why can't jellyfish's life cycle not be determined?

Jellyfish have complex life cycles, where they go through no less than six different developmental stages. The life for any species of jellyfish changes frequently. Some stages are quick, and others can last for a year or longer. Like most animals, jellyfish reproduce sexually, meaning that adult jellyfish are either male or female and possess reproductive organs called gonads. When jellyfish mate, the male releases sperm in through the mouth located on the underside of its bell.

When the eggs of the female jellyfish are fertilized, they undergo embryonic development typical of all animals. After setting to the sea floor, the planula larvae attach itself to a hard surface and transforms into polyp’s larva attaches when the polyp hydroid colony is ready for the next stage, the stalk portions of their polyps begin to develop.