Hydra are a genus of small, fresh-water organisms that are classified under the phylum Cnidaria. In addition to being related to such organisms as jellyfish, they are characterized by their tiny, tube-shaped bodies that contain several tentacles on one end.
While several species have been identified, two of the most distinctive species include h. oligactis which is brown in color and hydra viridissima which is green in color (green hydra).
* The green coloration of h. viridissima is due to the presence of zoochlorellae, an algae that lives in symbiosis with the hydra.
- are among a few cnidarians belonging to the class Hydrozoa that is found in freshwater habitats (ponds, lakes, and slow-flowing rivers and streams).
- is named after a sea snake in Greek mythology that had nine heads.