Sea anemones, jellyfish, hydras, and corals are among the 9400 species of Cnidaria. These radially symmetrical invertebrates are the least morphologically complex members of Subkingdom Eumetazoa, the true metazoa. The term coelenterate is used only in reference to both the Cnidaria and the Ctenophora phyla; it is no longer used as a synonym for Cnidaria. All cnidarians are aquatic and nearly all are marine. The four classes of cnidarians are the Hydrozoa (hydras), the Scyphozoa (true jellyfish), the Anthozoa (most corals and sea anemones), and the Cubozoa (sea wasps and several other genera named for their cube-shaped bodies). Cnidarian tentacles and oral arms are replete with stinging cells called cnidoblasts, each containing an intracellular nematocyst, unique to this phylum.