The zooids of _Millepora_ are very contractile, and can be withdrawn below the general surface of the coral into the shelter of the pore-tubes. When a specimen is examined in its natural position on the reef, the zooids are usually found to be thus contracted; but several observers have seen the zooids expanded in the living condition. It is probable that, as is the case with other corals, the expansion occurs principally during the night. The colony is provided with two kinds of nematocysts—the small kind and the large. In some colonies they are powerful enough to penetrate the human skin, and _Millepora_ has therefore received locally the name of "stinging coral." On each of the dactylozooids there are six or seven short capitate tentacles (Fig. 129, _t_), each head being packed with nematocysts of the small kind; similar batteries of these nematocysts are found in the four short capitate tentacles of the gastrozooids. The nematocysts of the larger kind are found in the superficial ectoderm, some distributed irregularly on the surface, others in clusters round the pores. The small nematocysts are about 0.013 mm. in length before they are exploded, and exhibit four spines at the base of the thread; the large kind are oval in outline, 0.02 × 0.025 mm. in size, and exhibit no spines at the base, but a spiral band of minute spines in the middle of the filament. There is some reason to believe that the filament of the large kind of nematocysts can be retracted.[299] At certain seasons the colonies of _Millepora_ produce a great number of male or female Medusae. The genus is probably dioecious, no instances of hermaphrodite colonies having yet been found. Each Medusa is formed in a cavity situated above the last-formed tabula in a pore-tube, and this cavity, the "ampulla," having a greater diameter than that of the gastrozooid tubes, can be recognised even in the dried skeleton. {260}[Illustration: FIG. 129.—Diagrammatic sketch to show the structure of _Millepora_. _Amp_, an ampulla containing a medusa; _Can.1_, canal system at the surface; _Can.2_, canal system degenerating in the lower layers of the corallum; _Cor_, corallum; _D_, an expanded dactylozooid with its capitate tentacles; _Ect_, the continuous sheet of ectoderm covering the corallum (_Cor_); _G_, a gastrozooid, seen in vertical section; _Med_, free-swimming Medusae; _t_, tentacle; _Tab_, tabula in the pore-tubes. (Partly after Moseley.)] It is not known how frequently the sexual seasons occur, but from the rarity in the {261}collections of our museums of Millepore skeletons which exhibit the ampullae, it may be inferred that the intervals between successive seasons are of considerable duration.