OF THE STOMACH — OF THE DISC. 
415 
the ramuscules of a common stem ; so that the Rhizostomidse, 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 
they terminate by rounded oblique openings. It is not always easy to see these 
apertures, but I have repeatedly satisfied myself of their presence by passing a needle 
or other delicate body into them, figs. 28, 29. 
12. The difficulty in seeing the openings arises in great measure from the pre- 
sence 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 perforated edge of the branch, fig. 28. In Rhizostoma 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. 
13. Each membranous process is composed of two membranes ; the outer of these 
is continuous with and passes into the thick transparent outer substance above men- 
tioned (11) ; the other is less transparent, more richly ciliated, and continuous with 
the lining membrane of the canals through the apertures. The two membranes are 
continuous at the free edge of the fold, and are here produced into numerous tenta- 
cula. The latter are beset with great numbers of thread-cells, and are in constant 
motion while the part retains its vitality*, fig. 29. 
14. Of the Disc. — In the Medusae monostomatoe the outer membrane of the sto- 
mach is, as I have said, continuous with the thick transparent mass of the disc, as the 
inner membrane is with the lining membrane of the canals which traverse it. The 
disc, therefore, is composed of two membranes inclosing a cavity variously shaped. 
15. I have examined the minute structure of the disc in Rhizostoma. The outer 
surface of the transparent mass is covered with a delicate epithelium composed of 
polygonal nucleated cells joined edge to edge. Among these there are many thread- 
cells. Beneath this there is a thick gelatinous mass which is made up of an appa- 
rently homogeneous substance containing a multitude of delicate fibres interlacing in 
every direction, in the meshes of which lie scattered nucleiform bodies. On the lower 
* M. Milne-Edwards, in his “ Observations sur la Structure de la Meduse Marsupiale/’ describes the 
fringe and its tentacles, but having altogether overlooked the true digestive apertures, he ascribes to the ten- 
tacles the function of villi. “ Les fransres aui garnissent les bras des rhizostomes sont done bien certainement 
des organes d’absorption, et leur structure les rend en efFet tres propres a remplir cette function, qui ici depend 
probablement tout entier d’un phenom^ne analogue a celui designe par M. Dutrochet sous le nom d’endos- 
mose.” 
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