Question: what are the structural differences between hydra and jellyfish?
Answer: Hydrozoan Life cycle. Obelia is a genus in the class Hydrozoa. Note: The Obelia life cycle is similar to that of Scyphozoan jellyfish, the true jellyfish, The major difference is that a Scyphozoan jellyfish spends most of its life as a medusa, while Obelia spends most of its life as a polyp.