The Class Scyphozoa includes the "true jellyfish," marine cnidarians known for their gelatinous, umbrella-shaped bodies and drifting, often pulsating movement. They have a simple body structure with a mouth, gastrovascular cavity, and tentacles lined with stinging cells (cnidocytes) for capturing prey. Most are free-swimming medusae, with a life cycle that alternates between sessile polyps and mobile medusae. Scyphozoans are carnivorous, feeding mainly on plankton and small fish, and play important roles in marine food webs.