Cnidarians are a phylum of aquatic invertebrates that includes:

* Jellyfish (medusa form dominant)

* Sea anemones (polyp form dominant)

* Corals (polyp form dominant)

* Hydra (polyp form dominant)

Medusa is the free-swimming, bell-shaped form with tentacles dangling from the underside.

Polyp is the sessile, cylindrical form attached to a substrate, with tentacles surrounding a mouth opening.

Some cnidarians, like jellyfish, are primarily medusa-form, while others, like sea anemones, are primarily polyp-form. However, all cnidarians go through both forms at some point in their life cycle.

Here's a breakdown:

* Jellyfish: The dominant form is the medusa, but they have a larval polyp stage.

* Sea anemones: Primarily polyp form but can reproduce by budding, producing small medusae.

* Corals: Exist as colonial polyps, and their medusae are very small and short-lived.

* Hydra: Exclusively polyp form throughout their lifecycle.

In summary, all cnidarians have the ability to exist in both medusa and polyp forms, although the dominant form varies between different species.