Final answer:
The precise lifespan of Helmet Jellyfish is not provided, but jellyfish from Class Scyphozoa generally have a medusa stage lasting a few months, and the lifecycle includes both medusa and polyp stages.
Step-by-step explanation:
The lifespan of the Helmet Jellyfish is not directly stated in the provided reference material. However, jellyfish, as creatures belonging to the phylum Cnidaria and more specifically the Class Scyphozoa, typically exhibit a life cycle that includes both a free-swimming medusa stage and a sessile polyp stage. Members of the Scyphozoa class are known for their bell-like shape and their ability to range in size, as exemplified by Cyanea capillata, which can reach up to two meters in diameter. Although specific lifespan details are often variable and dependent on species, environmental factors, and predation, many jellyfish have relatively short lifespans, with some medusa stages lasting only a few months.