12 ON THE ANATOMY OF THE FAMH^Y OF THE MEDUSA 



13. Each membranous process is composed of two membranes ; 

 the outer of these is continuous with and passes into the thick trans- 

 parent outer substance above mentioned (11) ; the other is less trans- 

 parent, more richly ciliated, and continuous with the lining membrane 

 of the canals through the apertures. The two membranes are con- 

 tinuous at the free edge of the fold, and are here produced into 

 numerous tentacula. The latter are beset with great numbers of 

 thread-cells, and are in constant motion while the part retains its 

 vitality ,1 fig. 29. 



14. Of the Disc. — In tY^it Medusce monostomatcB the outer membrane 

 of the stomach is, as I have said, continuous with the thick trans- 

 parent mass of the disc, as the inner membrane is with the lining 

 membrane of the canals which traverse it. The disc, therefore, is com- 

 posed of two membranes inclosing a cavity variously shaped. 



15. I have examined the minute structure of the disc in Rliiso- 

 stoina. 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 apparently 

 homogeneous substance containing a multitude of delicate fibres 

 interlacing in every direction, in the meshes of which lie scattered 

 nucleiform bodies. On the lower surface of the disc, the only differ- 

 ence appeared to be that the epithelium was replaced by a layer of 

 parallel muscular fibres. 



16. It might be said that the gelatinous substance here described 

 is a new structure, and not a mere thickening of the outer membrane ; 

 but a precisely similar change is undergone by th-e outer membrane in 

 the Diphydse, and here it can be easily traced, e.g. in the formation of 

 the bracts and in the development of muscular fibre in the outer wall 

 of the common tube. 



17. The structure of the inner membrane of the disc and its canals 

 resembles that of the corresponding tissue in the stomach, &c., but in 

 the ultimate ramifications of the canals it becomes more delicate. 



In these points there exists no difference between the Monostome 

 and Rhizostome Medusae. 



18. The three divisions, however, vary somewhat in the arrange- 

 ment of the cavities and canals of the disc. 



1 M. Milne-Edwards, in his "Observations sur la Structure de la Me'duse Marsupiale," 

 describes the fringe and its tentacles, but having altogether overlooked the true digestive 

 apertures, he ascribes to the tentacles the function of villi. " Les franges qui 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 fonction, qui ici depend probablement tout entier 

 d'un phenomene analogue a celui designe par M. Dutrochet sous le nom d'endosmose." 



