RoT/al Society. 529 



The organs of the Medusae are formed out of two distinct mem- 

 branes — foundation membranes. Both are cellular, but the inner 

 is in general softer, less transparent and more richly ciliated than 

 the outer, but contains fewer thread-cells. The outer is dense, 

 transparent, and either distinctly cellular or developed into a mus- 

 cular membrane. It may be ciliated or not, and is usually thickly 

 beset with thread-cells, either scattered through its substance or con- 

 centrated upon more or less raised papillae developed from its sur- 

 face. When the stomach is attached to the disc, the outer mem- 

 brane passes into the general substance of the disc, while the inner 

 becomes continuous with the lining membrane of the canals. There 

 is a larger or smaller space, termed by the author the " common 

 cavity," between the inner aperture of the stomach and the openings 

 of the canals, with which both communicate. This is the structure 

 of the stomach in the Cryptocarpae and Phanerocarpae ; in the 

 Rhizostomidae it is fundamentally the same, but the stomachs are 

 very minute, and collected on the edges and extremities of the ra- 

 muscules — a common stem. The Rhizostomes, ^'woac? their digestive 

 system, have the same relation to the Monostome Medusae that the 

 Sertularian Polypes have to the Hydrag, or the Coralline Polypes to 

 the Actiniae. In consequence of a very irritable contractile mem- 

 brane surrounding and overlapping the orifices of their stomachs, 

 they are seen with difficulty. This membrane consists of two pro- 

 cesses, one from each side of the perforated edge of the branch. In 

 Rhizostoma they generally remain distinct, but in Cephea they are 

 frequently united in front of and behind each aperture so as to form 

 a distinct polype-like cell. In the structure of the disc there exists 

 no difference between the Monostome and Rhizostome Medusae. 

 The author gives an account of his observations on the minute struc- 

 ture of the disc. The arrangement of the cavities and canals of the 

 disc differs in the different sections. In very many of the Crypto- 

 carpae there is a circular, valvate, muscular membrane developed 

 from the inner and under edge of the disc. In the Phanerocarpae 

 such a membrane does not seem to be present, but in Rhizostoma 

 and Cephea it is evidently replaced by the inflexed edge of the disc. 

 In the Cryptocarpae the marginal corpuscles are sessile upon the cir- 

 cular vessel. They are spheroidal vesicles, containing a clear fluid, 

 and one or more strongly-refracting bodies occasionally included 

 within a delicate cell. The marginal vesicles are placed between 

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

 nerocarpae 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, 



Phil. Mag. S, 3. No. 239. Suppl. Vol. 35. 2 M 



