Hydra is a genus of tiny freshwater organisms within the phylum Cnidaria-- the phylum that includes jellyfish, sea anemones and corals, among others. Most hydra measure less than half an inch.

Hydra belong to the phylum Cnidaria. Members of this phylum are over 600 million years old and were present at the beginning of animal evolution. In their original state, ...

Hydra are simple animals that, along with jellyfish, belong to the phylum cnidaria. Cnidarians first emerged 600 million years ago. "We determined which genetic 'gateway,' or ion channel, ...