The distinguishing feature of this phylum are stinging cells, the cnidae. These are mislabeled in this illustration as nematocysts. Nematocysts are actually the organelles within the cnidae which contain the harpoon or snare-like structure.

Life Cycle of Obelia sp. Note the phenomenon of sequential polymorphism, aka alternation of generations

 Obelia gonangia, or feeding polyp. Note tentacles, which are missing in the reproductive polyp.

 Reproductive polyp of Obelia. These lack tentacles. Instead, there are areas which undergo budding, where the medusae are formed.

 Medusa form of Obelia.

 Hydra, a freshwater representative of the Class Hydrozoa. There are no medusa forms in this species.

 Hydra, having captured a microcrustacean, it will soon ingest the captured prey into the mouth. Note the fragments of its previous meal in the gastrovascular cavity.

 Closeup view of live corals. Corals are classified in the Class Anthozoa.

 Anemones are classified in the class Anthozoa, along with corals. Clownfish often reside within the tentacles of sea anemones. Presumably, the clownfish is acquiring a safe place to live, while the sea anemone acquires a means to draw prey towards it, because clownfish are brightly colored.