THE GONOSOME. / Gl 



incomplete ; so that here the contents of the cavity at one side of the septum communicate with 

 those upon the other side, both sides communicating at the base of the leaflet with the common 

 cavity of the coenosarc. 



The leaflets, as they increase in size (B, C), direct themselves vertically from the upper surface 

 of the ramulus, and those of one side arch over so as to apjjroach those of the opposite. They are 

 at first free, but they afterwards become intimately united at their edges, the nematophores con- 

 tinuing to project as tooth-like processes, and forming an elegant serrated ridge between every 

 two leaflets. Ultimately the leaflets of one side coalesce with those of the other by their summits, 

 and thus form a completely closed chamber (D).^ 



In the receptacle thus formed the gonangia are produced. They spring from the upper side 

 of the metamorphosed ramulus, near the point where the leaflet leaves it, and represent the 

 hydrothecae which e.xist on an ordinary ramulus, and whose place they here take. They begin 

 to be produced at an early stage of the corbula, and may be easily examined in the young 

 corbula before it has become closed (B i^, Ci). The metamorphosed ramulus generally remains 

 unchanged for a short distance from its origin, and may be here seen bearing one or two ordinary 

 hydrothecae. 



About twelve gonangia are usually contained in each corbula. They are of a 

 very simple type (G), of a regular oviform figure, and with their chitinous walls thin and 

 delicate. Each gonangium seems to contain but a single sporosac, which soon comes to 

 occupy almost its whole cavity. A long, nearly cylindrical spadix extends from the base to 

 the summit of the sporosac, passing in the male through the axis of the mass of sperma- 

 togenous tissue, but in the female pushed to one side by the development of the large single 

 ovum, which here occupies almost the whole remaining portion of the cavity of the sporosac. 



There may appear some difliculty in deciding as to whether the corbula ought to be 

 regarded as properly belonging to the trophosome or to the gonosome. The truth is, that it 

 holds a place exactly intermediate between the two, and may in this respect be compared to the 

 bracts in plants; for these are in the same way intermediate between the ordinary leaves 

 and the proper floral verticils. As the bracts, however, are usually treated of in connection with 

 the inflorescence, whose limitation they frequently determine, we shall, perhaps, here also find it con- 

 venient to speak of the corbula in connection with the gonosome rather than with the trophosome.- 



^ In some other species (Jfflaophenia myriophyllum) the leaflets never coalesce, and the corbula 

 remains permanent!}' open. 



2 In a very ingenious paper, " On the Morphology of the Reproductive System in tlie Sertularian 

 Zoophytes," by Professor E. Forbes ('Ann. of Nat. Hist.,' 1844, vol. xiv, p. 385), the autlior recognises 

 in the corbulse oi Aglaojihenia plmna, and some other allied species, their true significance as meta- 

 morphosed branches. He mistakes, however, the nature of the metamorphosis, while, in accordance 

 with the prevailing view, he sees in the receptacles in question bodies in all respects corresponding to 

 the proper gonangia of the other hydroids. 



Forbes, moreover, extends his generalisation, applying it to the gonangia of the other Sertularians, 

 which he believes must be all regarded as peculiarly metamorphosed branches, with metamorphosed and 

 confluent hydrothecse, exactly in tlie same way that the floral verticils in plants may be referred to 

 verticillate, metamorphosed, and variously combined leaves. " The vesicle," he says, " is formed from 

 a branch or pinna through an arrest of individual development, by shortening of the spiral axis, and, by 

 a transformation of the stomachs (individuals) into an ovigerous placenta, the dermato-skeletous (or 



