STINGING cells.Coelenterata have three classes-Hydrozoaoften show metagenesis.An example is an obelia- Sea fur, (Poly and Medusa shows metagenesis)Scyphozoa-Medusa form is more common.An example is aurelia- Jellyfish, Moon jellyAnthozoa-The only polyp form dominant.An example is corallium- Red coralExplanation-The mature medusoid jellyfishes such as Aurelia are sexual stages.Sperms and eggs produced in the different individuals are released in water, which fuses to form a zygote.The latter develops into a fixed polyploid scyphistoma which UNDERGOES several TRANSVERSE fission to produce ephyra by the process called strobilation, a kind of sexual reproduction.Thus the sexual generation of the adult jellyfish is ALTERNATED with an asexual generation i.e., scyphistoma.Thereby ephyra larva is found in the life CYCLE of Aurelia. Larva of Obelia is planula.Larva of physalia is planula.