[Cnidaria] (pl. cnidae) Stinging or adhesive organelles unique to the phylum Cnidaria; sometimes called thread- or sting-capsules. A cnida is a secreted, essentially extracellular capsule within which is a coiled and folded tubule which everts and straightens on discharge.
The cell secreting it is called a cnidocyte (termed cnidoblast when immature).
Throughout the phylum, different kinds of cnidae are used in prey capture, defence and attachment. The three categories, each fulfilling a range of these functions, are nematocyst, spirocyst and ptychocyst.
In leptolids only nematocysts are formed; many of those in the epidermis have a minute cnidocil ('hair trigger') which projects from the apical end of the cell and may be large enough to be seen under the light microscopecoelom.

Alternative form for cnida : cnidae.