The correct answer is Cnidaria. Explanation & Key Takeaways: • Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. • Cnidarians are characterized by the presence of specialized cells known as cnidocytes that they use mainly for capturing prey. • The phylum Cnidaria includes not only corals but also jellyfish, sea anemones, and hydra. • Corals primarily form symbiotic relationships with algae called zooxanthellae, which provide them with energy through photosynthesis. Related Concepts: • Phylum Cnidaria • Cnidaria is a phylum under the kingdom Animalia that includes around 10,000 species of aquatic animals. • They are predominantly marine species but also include freshwater species. • Cnidarians have a simple body plan with radial symmetry, a mouth, and tentacles. • They exhibit two main forms: the polyp (e.g., sea anemones) and the medusa (e.g., jellyfish). • Anthozoa • Anthozoa is a class within the phylum Cnidaria that includes corals, sea anemones, and sea pens. • They are exclusively marine animals. • Unlike other cnidarians, anthozoans do not have a medusa stage in their life cycle. • They are often colonial and can form extensive coral reefs which provide critical habitats for marine life. • Zooxanthellae • Zooxanthellae are a type of symbiotic algae that live within the tissues of corals. • They perform photosynthesis and provide the coral with organic compounds, which serve as a major energy source for corals. • This symbiotic relationship is essential for the growth and health of coral reefs. • Coral bleaching occurs when corals expel their zooxanthellae due to stress conditions such as increased sea temperatures. • Cnidocytes • Cnidocytes are specialized cells unique to the phylum Cnidaria. • They contain organelles called nematocysts, which can deliver a sting to capture prey or for defense. • These cells play a crucial role in the feeding mechanism of cnidarians. • The venom delivered by cnidocytes can vary from mild to extremely potent, depending on the species.