Members of the Cnidaria have several special features. These include the cnidocytes that are found in no other types of animals and are unique to the Phylum. A second feature is the ability of these animals to undergo budding. The bud eventually becomes detached from the parent and then lives an independent existence. Another special feature is the alternation of body forms that is present in many species of the Phylum Cnidaria.
Adult jellyfish are the medusa body form.
This body forms swims by contractions of the body. The medusa produces either sperm cells or eggs. The zygote undergoes mitosis to form swimming immature stages. The immature stage eventually settles on the ocean floor or a structure like a reef and a sedentary polyp stage is produced. The polyp undergoes budding, and the buds are small swimming medusae.