<P> Hydra / ˈhaɪdrə / is a genus of small, fresh - water organisms of the phylum Cnidaria and class Hydrozoa . They are native to the temperate and tropical regions . Biologists are especially interested in Hydra because of their regenerative ability--they do not appear to die of old age, or indeed to age at all . </P> <P> Hydra has a tubular, radially symmetric body up to 10 mm (0.39 in) long when extended, secured by a simple adhesive foot called the basal disc . Gland cells in the basal disc secrete a sticky fluid that accounts for its adhesive properties . </P> <P> At the free end of the body is a mouth opening surrounded by one to twelve thin, mobile tentacles . Each tentacle, or cnida (plural: cnidae), is clothed with highly specialised stinging cells called cnidocytes . Cnidocytes contain specialized structures called nematocysts, which look like miniature light bulbs with a coiled thread inside . At the narrow outer edge of the cnidocyte is a short trigger hair called a cnidocil . Upon contact with prey, the contents of the nematocyst are explosively discharged, firing a dart - like thread containing neurotoxins into whatever triggered the release which can paralyse the prey, especially if many hundreds of nematocysts are fired . </P> <P> Hydra has two main body layers, which makes it "diploblastic". The layers are separated by mesoglea, a gel - like substance . The outer layer is the epidermis, and the inner layer is called the gastrodermis, because it lines the stomach . The cells making up these two body layers are relatively simple . Hydramacin is a bactericide recently discovered in Hydra; it protects the outer layer against infection . </P>

Do all hydra have the same number of tentacles