A genus of small simple fresh-water animals possessing radial symmetry. Hydra are predatory animals belonging to the phylum Cnidaria and the class Hydrozoa.They can be found in most unpolluted fresh-water ponds, lakes, and streams in the temperate and tropical regions and can be found by gently sweeping a collecting net through weedy areas. They are multicellular organisms which are usually a few millimetres long and are best studied with a microscope. Biologists are especially interested in Hydra due to their regenerative ability; and that they appear not to age or to die of old age.