Planarians (flatworms) have many immortal species as well. They are able to grow and degrow, changing their cell populations to the proportions found in juvenil

Planarians (flatworms) have many immortal species as well. They are able to grow and degrow, changing their cell populations to the proportions found in juveniles. They are able to divide by fission, where the head remains and the tail grows a new brain from scratch.