834 



the inner and outer membranes of the circular vessel. In the Pha- 



nerocarpas the marginal corpuscles are pedunculated and protected 

 by a semilunar fold. The author describes peculiarities in this part of 

 the organization of Rhizostoma. The excretory orifices, described 

 by Ehrenberg as general in Medusa aurita, were not detected by 

 the author in Cephea ocellata. Nor does he admit the supposed 

 nerves and intertentacular ganglia of that author to be such. 



Paragraphs 29 to 36 are occupied by a minute description of the 

 tentacles of Medusae. 



The generative organs of the three groups of Medusae are always 

 portions more or less developed of the walls of the system of canals, 

 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 w^ith 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 Physophoridae 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 



