The method of colony formation in the Gymnoblastea is very varied. In some cases (_Clava squamata_) a number of zooids arise from a plexus of canals which corresponds with the system of root-like processes of the solitary forms. In _Hydractinia_ this plexus is very dense, and the ectoderm forms a continuous sheet of tissue both above and below. The colony is increased in size in these cases by the gemmation of zooids from the hydrorhiza. In other forms, such as _Tubularia larynx_, new zooids arise not only from the canals of the hydrorhiza, but also from the body-walls of the upstanding zooids, and thus a bushy or shrubby colony is formed. In another group the first-formed zooid produces a hydrorhiza of considerable proportions, which fixes the colony firmly to a stone or shell and increases in size with the growth of the colony. This zooid itself by considerable growth in length forms the axis of the colony, and by gemmation gives rise to lateral zooids, which in their turn grow to form the lateral branches and give rise to the secondary branches, and these to the tertiary branches, and so one; each branch terminating in a mouth, hypostome and crown of tentacles. Such a method of colony formation is seen in _Bougainvillia_ (Fig. 130). A still more complicated form of colony formation is seen in _Ceratella_, in which not a single but a considerable number of zooids form the axis of the colony and of its branches. As each axis is covered with a continuous coat of ectoderm, and each zooid of such an axis secretes a chitinous fenestrated tube, the whole colony is far more rigid and compact than is usual in the Gymnoblastea, and has a certain superficial resemblance to a Gorgoniid Alcyonarian (Fig. 133, p. 271). The branches of the colony and a considerable portion of the body-wall of each zooid in the Gymnoblastea are usually protected by a thin, unjointed "perisarc" of chitin secreted by the ectoderm; but this skeletal structure does not expand distally to {264}form a cup-like receptacle in which the oral extremity of the zooid can be retracted for protection.