<)2 THE VOYAGE OF H.M.S. CHALLENGER. 



Characters of the sub-order Hydrocorallin^e, and of the Families and 

 Genera contained in it, modified so as to represent the present knowledge 

 ON the subject. 



Sub-order Hydrocorallin^;, Moseley. 1 



Compound Hydroid stocks, growing by gemmation. Hydropliyton consisting of 

 a mesbwork of ramified ccenosarcal canals, composed of an ectoderm and pigmented 

 endoderm, lodged within channels permeating a hard calcareous support, " ccenosteum," 

 which is deposited by the ectodermal investment of the canals, and forms masses of 

 very various shape. Surface of the Hydropliyton covered with a continuous layer of 

 ectoderm. Zooids of two forms — the one provided with a mouth and gastric cavity, 

 " gastrozooid " ; the other mouthless and simply tentacular in function, " dactylozooid." 

 Tentacles, when present, mostly with knobbed extremities. A well-defined muscular 

 layer present in the zooids. Zooids lodged within chambers, " gastropores " and 

 " dactylopores," excavated in the substance of the Hydropliyton, lined by reflections of 

 the surface layer of the ectoderm, forming the " sacs " of the zooids. Zooids of the 

 two forms either scattered irregularly over the surface of the stock, or gathered into 

 groups more or less regular, in each of which a centrally-placed gastrozooid is 

 surrounded by a ring of dactylozooids. Cavities of zooids communicating with the 

 ccenosarcal mesbwork by large canal offsets. 



1. Family Milleporid^:, L. Agassiz. 



Ccenosteum irregular in growth, arborescent or encrusting, composed of a thin super- 

 ficial living layer, supported by a dead mass made up of successive preceding dead 

 layers. Pores devoid of styles, divided into a series of vertically succeeding chambers 

 by transverse calcareous partitions, " tabulae " ; usually scattered irregularly, but in 

 some species grouped with tolerable regularity into systems, in which a centrally- 

 placed gastropore is surrounded by a ring of dactylopores. Nematocysts of two kinds 

 present — the one, the three-spined form, occurring only in Hydroids ; the other ovoid 

 in shape, with a thread beset with a spiral of spines. Gastrozooids short, cylindrical, 

 with from four to six tentacles with knob-like tips, set in a single whorl. Dactylo- 

 zooids long, filiform, and tapering, with an irregular number of short knob-bearing 

 tentacles set on at irregular intervals. Gonophores unknown, but not contained 

 within special cavities in the substance of the ccenosteum " ampullae." 



1 Prelim. Report, Phil. Trans. Roy. Soc, vol. clxvii., part 1, 1877, p. 132. 



