Light micrograph of a Hydra. Close-up of a tentacle with visible cells of nematocysts. Hydras are small fresh-water animals with a tubular body. They are invertebrates where the free end of the body is a mouth opening surrounded by one to twelve thin, mobile tentacles. Each tentacle, or cnida 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. 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. Magnification: x1250 when printed at 10 centimetres wide at its longest edge.