The Class Anthozoa is a group of marine invertebrates within the phylum Cnidaria, including corals, sea anemones, and sea pens. Anthozoans are exclusively polypoid (they lack a medusa stage) and are mostly sessile, attaching to substrates in shallow or deep oceans. Many, like reef-building corals, form colonies and secrete calcium carbonate skeletons, creating vital marine habitats. They capture food using tentacles equipped with stinging cells (cnidocytes) and play key roles in marine ecosystems as primary reef builders and contributors to biodiversity.