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the immortal life of a hydra

Students often get to look at hydras - tiny, fresh-water members of the group that includes sea anemones, jellyfish, corals, and the Portuguese man'o'war. All these cnidarians have a simple body-plan: two layers of true tissue with a jelly-like layer between them, a sac-like gut with a single opening that acts as both mouth and anus, and the characteristic stinging cells - cnidocytes - that give the taxon its name. And many of them rely on endosymbiotic algae for their survival, using some of the sugars that the algae produce by photosynthesis. The image below shows part of a hydra's tentacle - you can see not only its green algal symbionts, but also a halo of discharged stings.