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''Hydra'' is a genus of small, fresh-water animals of the phylum Cnidaria and class Hydrozoa, native to the temperate and tropical regions. 〔Gilberson, Lance (1999) ''Zoology Lab Manual'', 4th edition. Primis Custom Publishing.〕〔Solomon, E., Berg, l., Martin, D. (2002) ''Biology'' 6th edition. Brooks/Cole Publishing.〕 Biologists are especially interested in ''Hydra'' because of their regenerative ability -- they appear not to age or die of old age. ==Morphology== ''Hydra'' has a tubular, radially symmetric body up to 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. 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. ''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. The nervous system of ''Hydra'' is a nerve net, which is structurally simple compared to mammalian nervous systems. ''Hydra'' does not have a recognizable brain or true muscles. Nerve nets connect sensory photoreceptors and touch-sensitive nerve cells located in the body wall and tentacles. Respiration and excretion occur by diffusion everywhere through the epidermis. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Hydra (genus)」の詳細全文を読む スポンサード リンク
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