The word Cnidaria is formed by combining the Greek word knide meaning thorn and the Latin word area. Nidarians are called flower of the sea. The former name of the order Nidaria is Cilenterata. The marine animal Chironex fleckeri is the world’s most poisonous jellyfish (it can kill 67 people). Animals of this phase form colonies under the sea like a tropical rain forest. They form reefs or reefs. The number of species in this category is about 10,203. Cnidaria was named by scientist Leuckart (1847).