Cnidaria \Cni*da"ri*a\, n. pl. [NL. See Cnida.]

(Zo["o]l.) A comprehensive group equivalent to the true C[oe]lenterata, i. e., exclusive of the sponges. They are so named from presence of stinging cells (cnidae) in the tissues. See Coelenterata.

Source: Webster's Revised Unabridged Dictionary (1913)