In recent years, scientists have increasingly discovered creatures who appear immune to the aging process or have incredible mechanisms to survive in extraordinary environments. These animals can still die from disease, injury, or consumption by other animals, but apparently cannot die of old age.
Tardigrades - So-called "Water Bears," these microscopic creatures resembling tiny, eight-legged bears can enter a state of suspended animation called cryptobiosis in which they can survive in temperatures approaching absolute zero and as high as 151° C (304° F). They also show no ill-effects at radiation levels thousands of times higher than that which would kill almost any other living creature, are immune to environmental toxins, and can survive for decades without water. In a 2007 experiment, tardigrades were launched into orbit, exposed to the vacuum of space, and returned to Earth alive.
Immortal Jellyfish - The immortal jellyfish is a miniscule species of jellyfish which, after reaching sexual maturity and reproducing, reverses the aging process and returns to a newborn polyp state, before re-aging back to sexual maturity, reproducing again, and starting the cycle all over again, in a process known as transdifferentiation. This cycle can apparently continue indefinitely.
Hydra - The hydra is a genus of microscopic animals possessing radial symmetry. The cells of the hydra are constantly dividing, and thus refresh themselves, mitigating the effects of toxins and free radicals. Many scientists believe that hydra thereby do not age, and are functionally immortal, although this has not been firmly proven.
Lobsters - Scientists have speculated that lobsters do not age. No matter how long lobsters live, their cells show negligible senescence. This is likely due the high levels of the enzyme telomerase, which constantly regenerates the telomeres of the lobster's cellular DNA. In fact, older lobsters have been shown to be more vigorous and more fertile than younger lobsters. In addition, lobsters never stop growing as they age, and there seems to be no genetic upper limit on a lobster's size.
Planarian Worms - These little guys are super cute. The also seem indestructable. Cut one in half and and you get two - the tail grows a new head and the head grows a new tail. Plus, like lobsters, they regenerate their telomeres and show negligible senescence.