530 Royal Society, 



and consist of the two "foundation " membranes, in or between which 

 the generative elements, whether ova or spermatozoa, are developed. 

 This the author concludes from his observations on several genera, 

 which he gives in detail, and which add considerably to, and differ 

 in some respects materially from, what has been stated by previous 

 observers. In the ovarium, the two membranes develope between 

 them immense multitudes of ova with a dark granulous yelk and 

 clear germinal vesicle. The ova are attached to the outer surface 

 of the inner membrane. In the testis the inner membrane is pro- 

 duced into a vast number of thick pyriform sacs, which lie between 

 the two membranes, with their blind ends towards the inner surface 

 of the outer membrane; internally, they open each by a distinct 

 aperture on the fine surface of the inner membrane. The contents 

 of the sacs are spermatozoa, and cells in every stage of development 

 towards spermatozoa, which appear to be formed by the elongation 

 of the secondary cells contained in the large cells. 



The author's observations lead him to believe that the muscular 

 fibres are always developed in the outer "foundation" membrane. 

 Each fibre in Rhizostoma is made up of very small and indistinct 

 fibrils, which are transversely striated. He has not observed any 

 indubitable trace of a nervous system in the Medusae, nor of the 

 so-called blood-vascular system described by Will. 



In this section of the memoir the affinities of the Medusae are 

 Considered. In their essential characters, — viz. their construction 

 out of two membranes inclosing a variously -shaped cavity ; their 

 generative organs being external and variously developed pro- 

 cesses of the two membranes ; and the universal presence of the 

 peculiar organs called thread-cells, — they present a striking resem- 

 blance to other families of Zoophytes, as the Hydroid and Sertula- 

 rian Polypes, the Physophoridge and the Diphydae. The disc of a 

 Medusa is represented by the natatorial organ among the Diphydae 

 and Physophoridae, but has no homologue among the Hydrae and 

 Sertulariae. The cell of the Sertularian Polype rather resembles the 

 " bract " of the Diphydae than the natatorial organ, and the latter 

 family forms a connecting link between the Medusae and the Phy- 

 sophoridae. Of the two kinds of tentacles in the Medusae, the first 

 is represented in the Physophoridae and Diphydae, by the thickenings, 

 richly beset with thread-cells, that frequently occur in the lip of the 

 stomach ; in the Sertularian Polypes by the tentacles of the margin 

 of the mouth. The second kind is homologous with the prehensile 

 organs of the Diphydce and Physophorida, and with the peculiar 

 clavate processes of Plumularia. All these organs commence their 

 development as bud-like processes of the two joining membranes. 

 The peculiar clavate organs of Plumularia are developed from the 

 common tube independently of the stomach. They have not been 

 hitherto described, and were observed by the author in two species 

 of Plumularia dredged at Port Curtis. They were of two kinds, the 

 one attached to the cell of the polype, the other to the pedicle of the 

 ovary. To each species there were three processes of the former 

 kind, two above proceeding from near that edge of the aperture 



