Hydra

Hydra are tiny invertebrates with soft bodies that resemble jellyfish. Hydras, like Turritopsis dohrnii, have the ability to live indefinitely. Hydras do not deteriorate with age, as previously reported by Live Science. These invertebrates are mostly composed of stem cells, which renew continuously by duplication or cloning. Hydras do not live forever under normal settings due to risks such as predators and illness, but they may be immortal if these external threats are removed.

Hydra has a tubular, radially symmetric body that can stretch to 10 mm (0.39 in) in length and is held in place by a simple adhesive foot known as the basal disc. The basal disc's adhesive characteristics are due to the secretion of a sticky fluid by gland cells. A mouth opening at the free end of the body is encircled by one to twelve slender, movable tentacles. Each tentacle, or cnida (plural: cnidae), is covered in stinging cells known as cnidocytes.

Age: potentially immortal