Family
Hydridae
Hydra viridissima. Photo: (c) 大肚魚, some rights reserved (CC BY-NC)
Hydra /ˈhaɪdrə/ is a genus of small, fresh-water organisms of the phylum Cnidaria and class Hydrozoa. They are native to the temperate and tropical regions. Biologists are especially interested in Hydra because of their regenerative ability — they do not appear to die of old age, or indeed to age at all.
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