Hydra is a genus of simple fresh-water animal possessing radial symmetry. Hydras 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 hydras due to their regenerative ability; and that they appear not to age or die of old age. However, presently, there is no scientific consensus on whether Hydra undergo senescence, as discussed hereinafter.