Actinistia is a group of ancient lobe-finned fish that includes the coelacanth, a species thought to have been extinct for millions of years until its rediscovery in 1938. These fish are characterized by their unique, fleshy pectoral and pelvic fins, which resemble limbs, and they are closely related to the ancestors of terrestrial vertebrates. Living primarily in deep marine environments, Actinistia are known for their distinctive, thick scales and a unique swim bladder that functions as a lung. They are considered "living fossils" due to their long evolutionary history, dating back over 400 million years, providing valuable insights into the evolution of vertebrates.