Hydra. Coloured scanning electron micrograph (SEM) of Hydra sp., a minute aquatic invertebrate of the phylum Cnidaria. These 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. When conditions are good the hydra can also reproduce asexually, with young simply budding from the adult's body. Magnification: x20 when printed 10cm wide.