Green hydra. Light microscopy footage of the green hydra (Hydra viridissima). Hydra are microscopic aquatic invertebrates of the phylum Cnidaria. These freshwater predatory animals have typical cnidarian features: radial symmetry, saclike bodies with only one opening, and stinging tentacles for catching prey. They spend their entire lives as sedentary polyps and are hermaphrodites, where each hydra has testes on its stalk that can fertilise any eggs. The green hydra gets its colour from the Chlorella algae in the cells lining its gastrovascular cavity. This is an example of symbiosis. This clip was filmed with darkfield illumination.
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