Life-cycle
See also: Biological life cycle and Developmental biologyMost jellyfish alternate between polyp and medusa generations during their life cycle. Additionally, there are several possible larval life-stages.
After fertilization a primitive free-swimming larval form, called the planula, develops. The planula is a small larva covered with cilia. It settles onto a firm surface and develops into a polyp. Some polyps can also asexually produce a creeping frustule larval form, which then also develops into a new polyp.
The polyp is generally a small planted stalk with a mouth that is ringed by upward-facing tentacles. The polyps are like miniatures of the closely related anthozoan (sea anemones and corals) polyps, which are also members of Cnidaria. The jellyfish polyp may be sessile, living on the bottom or another substrate such as floats or boat hulls, or it may be free-floating or attached to tiny bits of free-living plankton or rarely, fish or other invertebrates. Polyps may be solitary or colonial. Polyp colonies form by strobilation, resulting in multiple polyps which share a common stomach cavity. Most polyps are very small, measured in millimeters. They feed continuously. The polyp stage may last for years.
Eventually the polyp gives rise to the medusa stage. New medusae are usually created asexually by strobilation or budding from the polyp. The medusa is the life stage which is most typically identified as a jellyfish.
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