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- Photo title: Antique engraved illustration of the hydra. Vintage illustration of the hydra. Old engraved picture. Longitudinal cross
- Author: Romannerud
- Cover photo description:
- Antique engraved illustration of the hydra. Vintage illustration of the hydra. Old engraved picture. Longitudinal cross section of the hydra. Book illustration published 1907. Hydra has a tubular, radially symmetric body up to 10 mm (0.39 in) long when extended, secured by a simple adhesive foot known as 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. This can paralyze the prey, especially if many hundreds of nematocysts are fired.
- Image ID:262181552
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