149 PHYSIOLOGY 



The doctrine of the sexuality of tlie Hydroida now waited only for the discovery of tlic male 

 element in order to receive its complete development. This discovery was made by Ehrcnberg, 

 who, in 183S, pointed out the real nature of certain conical tubercles which at particular seasons 

 are developed on the body of the freshwater hydra, and had been by previous observers regarded 

 as a peculiar disease to which this animal was supposed to be subject, but which were now shown 

 by Ehrenberg^ to be true spermatophorous capsules, while a further and imi)ortant step in this 

 direction was made by Krohn, who a few years afterwards announced that he had, in the 

 Peniiaria Cavolinii, Ehren., found certain receptacles similar in form to the ovigerous ones long, 

 ago described by Cavolini in the same remarkable hydroid, but containing spermatozoa instead of 

 ova. Similar observations were made on Tahidaria indivisa and on Eudeiidriuni racemosum, as 

 well as on Aglaophenia pluma and the Serttdaria [Eudendriiim ?) missenensis of Cavolini, in all 

 of which Krohn succeeded in detecting spermatozoa." 



It is now certain that every species of hydroid gives origin to male and female zooids (or, in 

 case of such medusa:' as may be directly developed from the egg, to male and female se.xually 

 generated individuals), one destined for the production of ova, the other for that of spermatozoa. 

 The separation of the sexes in distinct generative zooids, or in distinct individuals of a sexually 

 generated offspring, is thus absolute and universal among the Hydroida. In by far the greater 

 number of cases the separation is carried even further than this ; for we scarcely ever meet with 

 male and female gonophores in the same colony. As an almost universal rule, then, the 

 Hydroida are dioecious ; in other words, every colony is unisexual.^ 



Some few cases of a monoecious condition, however, occur. This has been noticed by many 

 observers in the freshwater Hi/dree,"^ where, indeed, it is the most usual condition. I have found 

 it also in Plumularia pinnata, which sometimes carries on the same stem both male and female 

 gonophores, and Hincks has observed it in some other sertularian hydroids.** In Bicoryne 

 conferfa too there may generally be found, among the dense forest of stems with which this 

 hydroid invests the surface of univalve shells, some stems carrying male and others female 

 gonophores. Each stem, howevei-, carries gonophores of one sex only, though it would seem that 

 both male and female stems are united by the creeping stolon into a common colony. In 

 Hydradinia, on the other hand, whose habit is entirely similar to that of Bicoryne, we 

 never meet with the two sexes in a common colony ; perhaps even never investing the same 

 shell. 



Orlyiii of the Generative Elements. — Throughout the whole of the Hydroida the generative 

 elements originate between the endoderm and ectoderm, and, with one exceptional condition to 

 be presently described, are always formed in the walls of an organ strictly homologous with the 

 manubrium of a gynuioplithalmic medusa. 



' ' ^Nlittlieil. aus (!eii Verliandl. der Gcsellsch. iiaturf. Frcuudc in Berlii),' 1838. 



" Krohn, " Eiiiiiie Bemerkungen und Beobaclitungen ubci- die Gesclileclitverlialtuisse bei den 

 Sertulariiien," 'Miillei's Archiv,' Jahrg. 1843, S. 174. 



■' Ki'ohn had already noticed lliat, in all the species examined by liim, the tnale and female gono- 

 phores were borne on separate colonies (loc. cit., p. 181). 



* See especially Prof. Allen Thomson " On the Coexistence of Ovigerous Capsules and Sperma- 

 tozoa in the same Individuals oi Hydra viridis," in ' Proc. Roy. Soc. Edin.,' No. 30, 1845-47. 



" ' Quarterly .Juunial of Science,' July, 18G5, p. 409, note. 



