Final answer:
The class within the phylum Cnidaria that includes the most species is Anthozoa, with about 6,100 described species including sea anemones and corals.
Step-by-step explanation:
Within the phylum Cnidaria, the class that includes the most species is the Anthozoa. This class is characterized by a sessile polyp body plan only, meaning these organisms do not have a medusa stage in their life cycle. Examples of Anthozoans are sea anemones, sea pens, and corals, with approximately 6,100 described species. Sea anemones are often vibrant in color and have a cylindrical shape, attached to substrates with a mouth opening surrounded by tentacles equipped with cnidocytes.