What animals are biologically immortal

Are there immortal beings?
There really are! Freshwater polyps, approx. 2.5 cm large cnidarians have one ability above all other animals: They can replace their cells again and again with new "fresh" cells and thus regenerate themselves continuously. That makes them potentially immortal. The magic word is stem cell. Like us humans, freshwater polyps have stem cells, which are the only cell types that are able to develop into different cell types (muscle cells, fat cells, nerve cells, etc.). In contrast to the cnidarians, however, we cannot use it to regenerate our entire body. The genetic material of our cells shortens with each further cell division. At some point the telomeres (the ends of the chromosomes) have become so short that the cell stops its cell division process and inevitably dies. But this does not apply to the undifferentiated stem cells. Freshwater polyps are able to replace all cells by dividing new stem cells, which means that they are theoretically immortal - but this ability does not protect against predators.
The freshwater polyps are not the only living things that are immortal. Jellyfish of the species Turritopsis nutricula have developed a completely different technique for regenerating their cells. Through the process of transdifferentiation, the jellyfish manages to transform its cells back to their original state. So aged cells are simply transformed into young cells, which would theoretically make the jellyfish immortal.