There is a considerable range of structure in the nematocysts of the Coelenterata. In _Alcyonium_ and in many other Alcyonaria they are very small (in _Alcyonium_ the nematocyst is 0.0075 mm. in length previous to discharge), and when discharged exhibit a simple oval capsule with a plain thread attached to it. In _Hydra_ (Fig. 124) there are at least two kinds of nematocysts, and in the larger kind (0.02 mm. in length previous to discharge) the base of the thread is beset with a series of recurved hooks, which during the act of discharge probably assist in making a wound in the organism attacked for the injection of the irritant fluid, and possibly hold the structure in position while the thread is being discharged. In the large kind of nematocyst of _Millepora_ and of _Cerianthus_ there is a band of spirally arranged but very minute thorns in the middle of the thread, but none at the base. In some of the Siphonophora the undischarged nematocysts reach their maximum size, nearly 0.05 mm. in length. [Illustration: FIG. 124.—Nematocyst (_Nem_) of _Hydra grisea_, enclosed within the cnidoblast. _CNC_, Cnidocil; _f_, thread of nematocyst; _Mf_, myophan threads in cnidoblast; _N_, nucleus of cnidoblast. (After Schneider.)] When a nematocyst has once been discharged it is usually {248}rejected from the body, and its place in the tissue is taken by a new nematocyst formed by a new cnidoblast; but in the thread of the large kind of nematocyst of _Millepora_ there is a very delicate band, which appears to be similar to the myophan thread in the stalk of a _Vorticella_. Dr. Willey[283] has made the important observation that in this coral the nematocyst threads can be withdrawn after discharge, the retraction being effected with great rapidity. The "cnidoblast" is a specially modified cell. It sometimes bears at its free extremity a delicate process, the "cnidocil," which is supposed to be adapted to the reception of the special stimuli that determine the discharge of the nematocyst. In many species delicate contractile fibres (Fig. 124, _Mf_) can be seen in the substance of the cnidoblast, and in others its basal part is drawn out into a long and probably contractile stalk ("cnidopod"), attached to the mesogloea below.