2. Hydra
Hydras are tiny freshwater animals that belong to the same phylum as jellyfish, but they possess a unique secret. Their bodies are packed with stem cells that continuously divide and renew, allowing hydras to endlessly regenerate tissues and avoid aging entirely under optimal conditions.
Unlike many other cnidarians, hydras rarely show signs of senescence—even after years in laboratory studies. According to Scientific American, this capacity for perpetual self-renewal puts hydra among the best examples of biological immortality in the animal kingdom.