Hydra is a small genus of small, fresh-water organisms of the phylum Cnidaria. Biologists are interested in Hydra because of their regenerative ability, they do not appear to die of old age, or to age at all!

Hydra have a tubular body up to 10mm long when extended. At the free end of the body there is a mouth surrounded by 1-12 tentacles called cnidae. You can learn more about the Hydra here.