THE GONOSOME. 47 



covering, and will then, while still forming part of the hydrosoma, have their surface in immediate 

 contact with the surrounding water {Si/ncorj/nc, Clava, Ilijdractinia, &c.) 



In other cases the blastostylc, with the gonophorcs which bud from it, may be surrounded 

 by a close case or capsule, formed by a layer of ectoderm with an external chitinous investment 

 {Campanularia unA Sertularia) (woodcut, fig. 2,/). I have elsewhere designated this capsule by 

 the name of " ffonanyium.'"^ The blastostylc extends through the axis of the gonangium as a 

 cylindrical column, bearing the gonophores as buds upon its sides, and generally expanded at 

 its summit into a conical plug or disc, by which the gonangium is here closed. 



In some cases the contents of the gonangium escape, when mature, by the simple rupture of 

 the summit (Phtmiilaria, &c.). In others, hoAvever, the summit is separated as a distinct lid, 

 which is then either cast oS'at once [Sertularia pumila, &c.), or it remains movably attached by 

 one spot of its edge, as by a hinge, to the margin of the aperture thus formed in the summit of 

 the gonangium {Serhdaria opercidata, Antennularla anfennina). 



In every instance where a gonangium exists the hydranths also are protected by a hydrotheca, 

 while the absence of a gonangium is always associated with the absence of a hydrotheca. The 

 diCFerence thus involved in the presence or absence of these parts corresponds to two primary 

 sections of the Hydroida, and I have distinguished all hydroids which possess a gonangium and 

 hydrotheca by the name of caltptoblastic, while the gymnoblastic hydroids are those which — 

 with the exception of the freshwater Hydras which constitute a separate section — are destitute of 

 these protective coverings. 



In by far the greater number of cases the blastostyle in the calyptoblastic genera carries 

 numerous buds, which are either sporosacs or blastocheraes, and which always increase in 

 maturity as they recede from the base and approach the summit of the gonangium (woodcuts, 

 fig. 2,y, and figs. 18 and 19). In some cases, however, the blastostyle bears but a single bud; 

 and then it often happens that this enlarges to such an extent as to fill nearly the entire cavity 

 of the gonangium, the blastostyle being pushed aside out of the axis, and becoming often 

 partially absorbed, so as to render it difBcult to demonstrate its existence^ (woodcut, fig. 3, k). 



It usually happens that a fleshy membrane abounding in thread-cells may be seen passing 

 over the whole of the generative buds while still attached to the blastostyle within the gonangium 

 (woodcuts, figs. 18,6? and 19, J). It closely confines them as in a common sac, and is probably an 

 internal layer which has separated from the original formative ectoderm on whose outer surface the 

 gonangium had been excreted. It performs an important office in the economy of the hydroid by 

 confirming the generative buds and guiding them or their contents towards the orifice of the 

 gonangium. I shall refer to it under the name ol guhernaadum. 



Sometimes the blastostyle, though in the very young state quite simple, soon breaks up, 

 from a comuion point near the base, into several distinct tubes, which again luiite in the 



^ " On the Structure and Terminology of the Reproductive System ia the Corynidse and Sertu- 

 lariadfe," 'Ann. Nat. Hist.,' July, 1860. 



" The difference presented by the gonangia, according as they coutaiu numerous gonophores or 

 only a single one, is regarded by Gegenbaur (' Generatiousweclisel,' p. 38) as of sutiicieut importance to 

 induce him to distinguish the gonangia into "polymeric" and " monomeric." I am not disposed 

 liowever, to give much weight to this difference, the number of gonophores which a gonangium may 

 contain varying with particular conditions of development. 



