A COLLECTION OF HYDROIDS. 305 



depression {pit.) in Avhicli lies a tliickeniiig' of the ectodermal 

 epithelium (j&gs. 2b, 26 B.n.). This thickening has very much 

 the structure of a nematophore, it being provided with a battery 

 of large nematocysts which resemble those occurring in the 

 Plumulariida?; they measure 22-7 |U in length and 4*2^ in 

 breadth. 



In the specimens at my disposal it was not possible to 

 decide whether there were ordinary nematocysts on the 

 tentacles. 



Gonosome. —Unknown. 



This hydroid was very largely enveloped by a coralline alga 

 (fig. 26, C.W.), which formed a perfectly regular layer over the 

 greater part of the outer surface of the colony. It renders the 

 colony very firm and strong, and in this way it may possibly 

 be of some use to the hydroid. 



The hydroid is, of course, peculiar among the Sertularians 

 in the remarkable toothed condition of the margin of the 

 hydrotheca, and it is further distinguished by possessing no 

 ti-ace of operculum. 



(16) Sertularia operculata Lin. 



Sertularia operculata Liu., ' Syst.,' Hincks, T,, 'Brit. Hyd. Zooph.,' 



p. 263 ; Bale, W. M.. • Cat. of* Austr. 

 Zooph.; 1884. p. 67 ; Xutting, C. C, 

 ' American Hydroids," pt. ii " Ser- 

 tiilariidaj," lOU^. p. 64. 



Found attached to the surface of rocks ; greenish-brown in 

 colour. Height about o in. 



Teophosome. — Hydrorhiza forms a feltwork, very firmly 

 attached to the surface of the rock. Diameter 0"105 mm; 

 thickness of perisarc 15//. 



Hy drocaulus. — It branches dichotomously ; a hydro- 

 theca at each side of every axil and in contact with each 

 other. 



