Final answer:
The cnidarian class that includes sea pens, sea daisies, corals, and many others is the class Anthozoa. This class is characterized by creatures with a sessile polyp body plan and lacks a medusa stage.
Step-by-step explanation:
The cnidarian class that contains sea pens, sea daisies, corals, and many others is the class Anthozoa. This class includes organisms that exhibit a sessile polyp body plan exclusively, which means there are no medusa stages in their life cycle.
Examples of Anthozoans include sea anemones, which are often brightly colored, cylindrical in shape, and range from 1.8 to 10 cm in diameter. Anthozoans, which also include sea pens and corals, are known for their bright colors and are attached directly to a substrate.
Their mouth opening is surrounded by tentacles bearing cnidocytes, specialized cells that can immobilize prey with toxins.
The phylum Cnidaria, to which Anthozoans belong, consists of around 10,000 described species, exhibiting radial or biradial symmetry and being diploblastic.
They are primarily marine species, with nearly all Cnidarians living in the ocean. Moreover, corals within this class can form symbiotic relationships with green algae, gaining energy from the algae's photosynthetic processes.