190 CLASSIFICATION. 



All the living Hydroida are thus either destitute of a ehitiiious perisarc — Eleutheroblastea 

 nnd MoNOPSEA — or else are more or less extensively invested by this protective covering — Grjixo- 

 BLASTEA and Calyptoblastea. In no case, however, does there exist the additional support of a 

 solid chitinous rod. This is found in the extinct group of the graptolites, whose tubular offsets, 

 moreover, we have already compared, not to hydrothecoD, lint to nematophores. 



Though w(; are still entirely ignorant of many important points in the structure of grapto- 

 lites, and know nothing beyond mere surmise of the functions of their parts, or of the phenomena 

 of their lives, we find in them nevertheless a group of characters which — accepting the view 

 advocated above of their hydroid atfinities — necessitates the establishment for them of a separate 

 sub-order of IIyduoida. And even though we may regard the nature here attributed to the 

 tubular offsets or denticles as too hypothetical to form the basis of such a separation, we shall 

 have in the solid rod alone a character sufficient for this purpose.^ The name of Rhabdophora 

 may be assigned to the sub-order th\is constituted. 



Among the leading groups into which the Hydroida are thus divisible the sub-order of the 

 Eleutheroblastea is that in which we meet with the greatest morphological uniformity, the 

 entire group being represented only by the single genus Ilijdra. 



The MoNOPSEA are more diversified, for we find among them the various types of the 

 Gcryonidan and Jj]ginidan medusae. Our knowledge, however, of these medusae is still imperfect ; 

 and though much light has recently been thrown on them, especially by the researches of Haeckel, 

 who has shown close and hitherto unsuspected relations between the Geryonidan and ..^ginidan 

 forms, we are by no means sure that they are all developed on the same plan, and the group 

 MoNOPSEA can scarcely be yet regarded as possessing more than a provisional value. 



In the Gymnoblastea there is still more morphological diversity, this sub-order being repre- 

 sented by numerous fauiilies and genera. 



In the Calyptoblastea the special morphological modifications are greater than in any of the 

 others, with the exception, possibly, of the RHABnopHORA. In the Calyptoblastea, indeed, 

 these modifications necessitate a primary division of this sub-order into two subordinate gi'oups, 

 each of which includes families and genera." The characters here assumed as the basis of hydroid 

 classification may be tabulated in the following scheme : 



to the properly non-sexual form or " blastocherae." If this view be correct the Geryonidans must be 

 regarded as composed of a hydrosoma, consisting, as in other hydroids, of a trophosome and a gonosome, 

 but in which the trophosome is raedusiform instead of being, as in all the other sub-orders, hydriform. 



It is only since the earlier portion of the present Monograph was printed that the development of 

 the medusa from the egg without the intervention of a hydriform trophosome has been established on 

 sufficient evidence. Certain views, founded on the absence of evidence in favour of this phenomenon, 

 have been there expressed, and must accordingly be now received with some modification. 



^ Though the solid rod is present in all the typical graptobtes, there are some very aberrant forms 

 [Retiolites) in which it has not yet been demonstrated with certainty. It is a matter for consideration 

 whether these do not constitute a group witii affinities pointing in quite a different direction from 

 those of the true graptolites. 



" Hincks (' Hist. Brit. Zooph.') divides the HynRoiDA into three suborders, to which he assigns 

 the names of Gvunochroa, Athecata, and Thecophora. These groups are equivalent respectively to 

 the Eleutheroblastea, Gymnoblastea, and Calyptobl.\stea of the present Monograph. Had not 

 tliese last names been used by me in the earlier part of the ^Monograph, before the publication of Mr. 

 Hiiicks's work, I should have here hesitated to add to the existing heavy nomenclature of zoology. 



