Cnidae: The intracellular secretory product diagnostic of all members of the phylum Cnidaria. It consists of a capsule containing a highly-folded, eversible tubule. Cnidae are used to capture prey, to defend against predators, to attack (or to defend against attack by) unrelated, neighboring cnidarians, and to attach to suitable substrates. Nematocysts, spirocysts and ptychocysts are the three major categories of cnida (Watson, 1988 in: Hessinger and Lenhoff, 1988; p.21).