38 MORPHOLOGY. 



The condition alluded to is especially well marked in the medusa of Cladonewa and of 

 Bou(/ainvillia, and in certain forms of Oceania, in the sense in which this group has been 

 restricted hy Forbes and Gegenbaur, and generally accepted by the German zoologists ; and an 

 opportunity of studying an undescribed medusa of this type (woodcut, fig. 8), which I obtained 

 abundantly in the autumn of 1865, on the west coast of Scotland, has plainly shown that the 

 generative lobes of the manubrium cannot be regarded as true zooids. The generative elements 

 are here simply produced between the endodcrm and ectodenn of the manubrium, and the lobes 

 are nothing more than a puckered or sacculated condition of the walls in those parts where the 

 ova or spermatozoa originate. I do not, however, deny the possibility of the manubrium as well 

 as of the gastrovascular canals giving origin to true buds, to which the development of the 

 sexual elements may be confined ; when this has been shown to be the case, the medusa 

 presenting it must take its place among the blastochemes. 



Whether the medusae referable to the type of Geryonia and its allied forms ought to be 

 regarded as blastochemes is as yet uncertain, though my own opinion is in favour of so viewing 

 them. The parts which in these medusae give origin to the generative elements have been 

 described as leaf-like expansions of the radiating canals. Haeckel's observations have led him to 

 deny to them the significance of true zooids,^ while he sees in them nothing more than mere 

 lateral expansions of the canal, a portion of whose epithelium becomes here difi'erentiated into 

 ova or spermatozoa. This question as regards the Geryonida is, as we shall afterwards see, one 

 of considerable importance. Having had no opportunity of examining for myself specimens of 

 these medusae, I can bring no direct personal observations to bear upon it ; but the account 

 given by Haeckel, who has so admirably worked out the structure and history of the Geryonida, 

 does not appear, as I shall afterwards show, to necessitate the conclusion to which he arrives, 

 or to be inconsistent with the zooidal nature of the reproductive pouches. Further observations, 

 however, instituted for the express determination of this point will be needed before we can regard 

 the question as thoroughly cleared up. 



6. Homoloyical parallelism between Sjjorosac and Medusa. 



While it will be found very convenient to insist upon the differences pointed out above 

 between the adelocodonic gonophore on the one hand, and the phanerocodonic gonophore and 

 blastocheme on the other, it must not be supposed that these two forms are constructed upon 

 plans widely different from one another. We find, on the contrary, that the most exact 

 parallelism admits of being demonstrated between them ; for though they may at first sight 

 appear very different, it can nevertheless be shown that the closed generative sac of a Chiva 

 or a Hydractinin is an easily understood modification of a medusa." 



^ Haeckel, ' Die Famille der Riisselquellen,' Vorwort, viii. 



" It is now many years since I endeavoured to demon'strate that the so-called " ovarian vesicles" 

 of the Tubularince, and the fi.xed sacs contained within the gonangium of the Seriularina and Campanu- 

 larinm, are in all cases strictly homologous with the free medusa, that they possess a true medusal 

 structure in a more or less degraded or disguised condition. (" On the Anatomy and Physiology of 

 CordylopJiora," ' Phil. Trans.,' June, 1853.) 



