This freshwater animal shows no sign of ever aging

For four years, biologist Daniel Martinez tracked hydra individually, watching for the usual signs of aging — and found none: death rates and reproduction stayed flat the entire time. A hydra's body is made almost entirely of stem cells constantly dividing and replacing every cell in it, so the animal keeps rebuilding itself indefinitely. It can still die from injury or disease — it just never seems to wear out on its own.

— Daniel E. Martinez, Mortality Patterns Suggest Lack of Senescence in Hydra — Experimental Gerontology, Vol. 33, 1998