X GYMNOBLASTEA ANTHOMEDUSAE 263 



however, has no aboral processes, and appears to be only 

 temporarily attached to foreign objects by the secretion of the 

 perisarc. Among the solitary Gymnoblastea several species 

 reach a gigantic size. GorymorpTia is 50-75 mm. in length, but 

 Monocaulus from deep water in the Pacific and Atlantic Oceans 

 is nearly 8 feet in length. Among the solitary forms atten- 

 tion must be called to the interesting pelagic Pelagohydra 

 (see p. 274). 



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 Geratella, 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 fenes- 

 trated 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 ecto- 

 derm • but this skeletal structure does not expand distally to 



