ON THE ANATOMY OF THE FA^^LY OF THE MEDUS.'E I I 



tinctly cellular or developed into a muscular membrane. It may be 

 ciliated or not, but it is usually thickly beset with thread-cells, either 

 scattered through its substance or concentrated upon more or less 

 raised papillae developed from its surface. 



8. I would wish to lay particular stress upon the composition of 

 this and other organs of the Medusae out of two distinct membranes, as 

 I believe that it is one of the essential peculiarities of their structure 

 and that a knowledge of the fact is of great importance in investigating 

 their homologies. I will call these two membranes as such, and inde- 

 pendently of any modification into particular organs, " foundation 

 membranes.'' 



9. When the stomach is attached to the disc, the outer membrane 

 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 between the inner aperture of the stomach and the 

 openings of the canals, with which both communicate, and which 1 

 will therefore call the " common cavity." 



10. In the Rhizostomidae the structure of the stomachs is funda- 

 mentally the same, but they are very minute, and are collected upon 

 the edges and extremities of the ramuscules of a common stem ; so 

 that the Rhizostomidae, quoad their digestive system, have the same 

 relation to the Monostome Medusae as the Sertularian Polypes have 

 to the Hydrae, or the Coralline Polypes to the Actiniae. 



11. If one of the ultimate ramuscules be examined, it will be found 

 to consist of a thick transparent substance, similar in constitution to 

 that of the mass of the disc, through which there runs, nearer one 

 edge than the other, a canal with a distinct membranous wall ciliated 

 internally. From this '' common canal " a series of parallel diverticula 

 are given off at regular intervals, and run to the edge of the branch, 

 where xriley terminate by rounded oblique openings. It is not always 

 easy to see these apertures, but I have repeatedly satisfied myself of 

 their presence by pajssing a needle or other delicate body into them, 



figs. 28, 25. y 



12. The difficulty in seeing the openings arises in great measure 

 from the presence of a membrane which surrounds and overlaps them, 

 and being very irritable, contracts over them on being touched. The 

 membrane consists of two processes, one from each side of the per- 

 forated edge of the branch, fig. 28. In i^/zz^oi-^w^ these two processes 

 generally remain distinct, so that their bases form a common channel 

 into which all the apertures open ; but in Cephea they are frequently 

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

 polype-like cell, figs. 35, 36. 



