Hydra are simple animals that, along with jellyfish, belong to the phylum cnidaria. Cnidarians first emerged 600 million years ago.

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.

Hydras can be found in freshwater. “Most members of this phylum are in the ocean (corals, sea anemones and jellyfish). It is in an animal phylum (Cnidaria or Coelenterata) that lacks the ability ...

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.