A coelenterate is a member of the phylum Cnidaria, which includes jellyfish, corals, and sea anemones. These organisms are characterized by having a simple body structure with radial symmetry, a central cavity (the coelenteron) for digestion, and specialized stinging cells called cnidocytes used for capturing prey and defense. Coelenterates are primarily aquatic and play essential roles in marine ecosystems. They exhibit both solitary and colonial forms, often showcasing diverse colors and shapes.