Introduction to Cnidaria Jellyfish, corals, and other stingers. . . Cnidarians are incredibly diverse in form , as evidenced by colonial siphonophores, massive medusae and corals, feathery hydroids, and box jellies with complex eyes. Yet, these diverse animals are united because they are all armed with stinging cells called nematocysts, most likely as a result of common history. The name ...