CLASSIFICATION. 189 



Of the three sub-orders vvliicli tlius ])ossess a hydriform tropliosome, there is one in which 

 no perisarc is ever excreted, while the hydrosoma is never permanently attached, and the nutritive 

 buds on attaining a certain degree of maturity disengage themselves from the parent stock and 

 continue their growth independently as free organisms. The designation of ELEUTnEiiOBLASTEA 

 may accordingly be assigned to the order tlius characterised. 



In another sub-order a more or less obvious perisarc is always present, while the hydrosoma 

 is always attached, and the zooids of the trophosome never become separated as free, independently 

 developing organisms.' In this sub-order there is never developed either hydrotheca or gonan- 

 giura. So that both nutritive and generative buds are destitute of the characteristic receptacles 

 which afford protection to these parts in the following sub-order. The name of Gymnoblastea 

 suggests itself as a designation sufficiently expressive of this distinction. 



In the last of the three sub-orders of living hydroids characterised by the possession of a 

 hydriform trophosome we have the same perisarcal covering of chitine, and the same permanent 

 attachment of the hydrosoma and nutritive buds which we meet with in the Gymnoblastea ; but 

 the hydranths are always protected by a hydrotheca, and the generative buds are always included 

 in a gouangiura. These characters afford the grounds for distinguishing a third sub-order, and 

 suggest for it the name of Calyptoblastea." 



The last of the four sub-orders of the living IIydroida is distinguished by the absence of a 

 hydriform trophosome, the ovum becoming developed through direct metamorphosis into a medu- 

 siform body, just as in the other orders it is developed into a hydriform body. To the sub-order 

 characterised by this very exceptional condition I shall assign the name of Monopsea.^ 



' The detachment of the hydranths in certain Tubulariae (see above, p. 69) indicates the com- 

 mencemeut of decadence and death in these bodies, which never undergo further growth or development 

 after their separation from the parent stem. I take it for granted, also, that the free hydranths 

 described by Stimpson and by M'Crady under the names of AcavUs and Nemopsis have been detached 

 in the same way from some fixed tubularian stem. None of these cases, therefore, can be confoiuidcd 

 with the separation of the buds in the Eleutheroblastea. 



^ The chitinous covering, which often invests the gonophore in tlie gymnoblastic hydroids, and 

 which, in some cases, as in Cordylophora lacustris, attains considerable thickness, must not be con- 

 founded with a gonaugiunn. The true gonangium is always developed round a blastostyle, though the 

 latter may occasionally be forced back, and become atrophied by tlie pressure of the growing gonophore. 



So also the thin expansion of the perisarc, which may be seen extending over the base of the 

 hydranth in certain Gymnoblastea, as in Cordylophora lacustris, and more especially in various species 

 of BougainvUlia, is entirely different from a hydrotheca. The hydranth is never to any extent 

 retractable within it, as it is within the true liydrotheca, and during the contraction of the hydranth 

 this chitinous sheath merely adapts itself to the contraction, being then thrown into more or less well 

 defined transverse folds. 



' I am not disposed to regard a non-.sexual trophosome as necessarily absent from the jMonopsea 

 any more than from the other three sub-orders. As yet none but Geryonidan and yEgiuidan medusae 

 have been proved to possess the characters on which the sub-order Monopsea has been founded ; for 

 the case of Lizzia, which has been adduced as an instance of direct development of the medusa from the 

 egg, is probably based on some error of ol)servatioa (see above, p. 100). Now, in both Geryonidan 

 and .^ginidan medusae the generative elements are produced in sac-like offsets of the radiating canals, 

 and reasons have already been given for regarding these generative sacs — at least in the Geryonidan 

 forms — as true zooids ; and the medusic from wliose radiating canals they are emitted would then belong 



