A nematocyst is a specialized cell found in the tentacles of certain aquatic animals, particularly in cnidarians like jellyfish, sea anemones, and corals. These cells contain a tiny, coiled structure that can rapidly eject a barbed thread when triggered, allowing the organism to capture prey or defend itself from predators. When a nematocyst fires, it can inject toxins into its target, which can immobilize or deter potential threats. This mechanism is crucial for the survival of cnidarians, as it helps them secure food and protect themselves in their marine environments.