Permission given by photographer to use on AskNature. Scanning electron micrograph showing the tip of a Hydra viridis tentacle. A meshwork of triggered nematocyst cells is visible. When hooked cnidocytes on the surface of the tentacle come in contact with prey, they release a coiled nematocyst. Extension of the nematocyst thread allows the tentacle to latch onto the prey, and this signals the hypostome, or mouth, to open.