1. The phylum Cnidaria includes over 9,000 species of mostly marine animals classified into four classes: Hydrozoa, Scyphozoa, Anthozoa, and Cubozoa. 2. They are commonly called "stingers" because their tentacles contain stinging cells called cnidocytes that are used for defense, feeding, and attachment. 3. The classes differ in whether they are marine or freshwater, whether the polyp or medusa form is more prominent, and details of their gastrovascular cavity, mesogloea, and gonad placement. Examples across the classes include Hydra, jellyfish, sea anemones, corals, and box jellyfish