The tiny jellyfish Turritopsis dohrnii is one of the few animals that can be considered truly immortal. Jellyfish normally go through a life cycle that begins with an egg and then slowly grows
to a larva. This attaches to a rock or another solid surface where it continues to grow, becoming a polyp. Once the polyp is big enough, it starts to bud off
and the adult jellyfish, known as a medusa, drifts into the ocean. In the case of the immortal jellyfish, after it reaches its mature adult stage it can revert back to the polyp stage before growing into an adult once again. Potentially this can be repeated indefinitely, resulting in a jellyfish that won’t die
of old age.