10. _Hydroceratinidae_.--This family contains the single Australian species _Clathrozoon wilsoni_ Spencer, in which a massive hydrorhiza bears sessile hydrothecae, containing hydranths each with a single tentacle, and numerous nematophores. See W. B. Spencer [53]. 11. _Dendrograptidae_, containing fossil (Silurian) genera, such as _Dendrograptus_ and _Thamnograptus_, of doubtful affinities. [Illustration: FIG. 60.--Portion of the calcareous corallum of _Millepora nodosa_, showing the cyclical arrangement of the pores occupied by the "persons" or hydranths. About twice the natural size. (From Moseley.)] ORDER III. Hydrocorallinae.--Metagenetic colony-forming Hydromedusae, in which the polyp-colony forms a massive, calcareous _corallum_ into which the polyps can be retracted; polyp-individuals always of two kinds, gastrozoids and dactylozoids; gonosome either free medusae or sessile gonophores. The trophosome consists of a mass of coenosarcal tubes anastomosing in all planes. The interspaces between the tubes are filled up by a solid mass of lime, consisting chiefly of calcium carbonate, which replaces the chitinous perisarc of ordinary hydroids and forms a stony corallum or _coenosteum_ (fig. 60). The surface of the coenosteum is covered by a layer of common ectoderm, containing large nematocysts, and is perforated by pores of two kinds, gastropores and dactylopores, giving exit to gastrozoids and dactylozoids respectively, which are lodged in vertical pore-canals of wider calibre than the coenosarcal canals of the general network. The coenosteum increases in size by new growth at the surface; and in the deeper, older portions of massive forms the tissues die off after a certain time, only the superficial region retaining its vitality down to a certain depth. The living tissues at the surface are cut off from the underlying dead portions by horizontal partitions termed _tabulae_, which are formed successively as the coenosteum increases in age and size. If the coenosteum of _Millepora_ be broken across, each pore-canal (perhaps better termed a polyp-canal) is seen to be interrupted by a series of transverse partitions, representing successive periods of growth with separation from the underlying dead portions. [Illustration: FIG. 61.--Enlarged view of the surface of a living _Millepora_, showing five dactylozooids surrounding a central gastrozooid. (From Moseley.)] Besides the wider vertical pore-canals and the narrower, irregular coenosarcal canals, the coenosteum may contain, in its superficial portion, chambers or _ampullae_, in which the reproductive zoids (medusae or gonophores) are budded from the coenosarc.