The four classes of cnidarian include the following except

1. Analyze the question You are asked which option is NOT one of the four classes of phylum Cnidaria (the group that includes jellyfish, sea anemones, corals and hydra).

2. Concept Phylum Cnidaria is divided into four classes (some textbooks list three, but modern classification gives four):

• Hydrozoa: Hydra, Obelia, Portuguese man-of-war
• Scyphozoa: true jellyfish (e.g. Aurelia)
• Cubozoa: box jellies
• Anthozoa: sea anemones, corals, sea fans

3. Solution Check each option:

• A. Turbellaria: this is a class of flatworms, in phylum Platyhelminthes (e.g. planarians). It is not a cnidarian class.
• B. Anthozoa: a cnidarian class.
• C. Scyphozoa: a cnidarian class.
• D. Cubozoa: a cnidarian class.

Note that Hydrozoa, the fourth class, isn't listed. Since all of B, C and D are valid cnidarian classes, the odd one out is Turbellaria.

4. Answer A. Turbellaria.

5. Tip Remember the cnidarian classes with "HSCA": Hydrozoa, Scyphozoa, Cubozoa, Anthozoa. Anything ending in "-zoa" that isn't one of these, or any name like Turbellaria, Trematoda or Cestoda, belongs to the flatworms. Turbellaria, Trematoda and Cestoda are the three classes of Platyhelminthes.