Phylum

Cnidaria (Cnidarians)

Hydra viridissima. Photo: (c) 大肚魚, some rights reserved (CC BY-NC)

Cnidaria (/naɪˈdɛəriə/) is a phylum containing over 10,000 species of animals found exclusively in aquatic (freshwater and marine) environments: they are predominantly marine species. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance, sandwiched between two layers of epithelium that are mostly one cell thick. They have two basic body forms: swimming medusae and sessile polyps, both of which are radially symmetrical…

— Wikipedia