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).