Which cnidarians have both polyp and medusa stages?

Many species within the classes Hydrozoa, Scyphozoa, and Cubozoa pass through both polyp and medusa stages during their life cycles.1

In these groups, the life cycle typically involves an alternation of generations:
* Medusae reproduce sexually to produce larvae, which then settle and metamorphose into polyps.1
* Polyps reproduce asexually to produce new medusae.1

While these two body forms often occur within the life cycle of a single cnidarian, there are variations in how prominent each stage is. For instance, in scyphozoans (true jellyfish), the medusa is the larger and more conspicuous form, whereas in many hydrozoans, the polyp phase is more dominant. Additionally, some species may lack a free-swimming medusa stage entirely, or the medusae may remain attached to the polyp.12

In contrast, members of the class Anthozoa—which includes sea anemones and corals—are exclusively polypoid and do not possess a medusa stage.1