True alternation of generation in the life cycle of an organism implies alternation of a diploid asexual phase with a haploid sexual phase. But in obelia and many other cnidarians, is not a true alternation of generation because both phase are diploid and polypoid and medusoid bodies are fundamentally homologous and one can be mechanically derived easily from the other. This type of alternation of generation as found in obelia called as metagenesis in which alternation of asexually polypoid and sexually medusoid bodies takes place.
Parthenogenesis : Development of embryo without fertilization of egg (ovum).
Metamorphosis : Changes from larva to adult.
Neoteny : Larva becomes sexually mature and capable to reproduce like adult.