416 
MR. T. H. HUXLEY ON THE ANATOMY OF THE MEDUSiE. 
surface of the disc, the only difference 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 struc- 
ture, and not a mere thickening of the outer membrane ; but a precisely similar change 
is undergone by the outer membrane in the Diphydae, 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 arrangement of the cavi- 
ties and canals of the disc. 
In the Cryptocarpae, the common cavity .may be either small {Thaumantias) or large 
{Oceania)-, from it there proceed a number of straight unbranching canals which 
open into a circular canal running round the margin of the disc. 
In the Phanerocarpae the general arrangement is similar, but the canals frequently 
branch {Medusa aitrita, Phacelloghora) and anastomose in a reticulate manner. 
In many of the Monostome Medusae the centre of the under surface of the disc 
projects into the “common cavity” as a rounded boss (fig. 11a.), and according to 
its form and size will seem to divide the former more or less into secondary cavities. 
This appears to me to be the origin of the multiple stomachs of Medusa aurita as 
described by Ehrenberg. 
19. In the Rhizostomidae, the canals of the branched processes unite and open by 
four {Rhizostoma, CepJiea) or eight {Cassiopea}) distinct trunks into a wide curiously- 
shaped cavity, from whence anastomosing canals are given off to all parts of the disc 
(figs. 26, 26 a.). The circular vessel exists, but is not particularly obvious in conse- 
quence of anastomosing branches being given off beyond it. 
20. In very many of the Cryptocarpae {Caryhdoa, Oceania (fig. 5 a & 6.), Puly- 
xenia) 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, fig. 26 a. 
21. Of the Marginal Corpuscles. — In the Cryptocarpae the marginal corpuscles 
are sessile upon the circular vessel, figs. 8, 9, 10. They are spheroidal vesicles, con- 
taining a clear fluid, and one or more spherical strongly-refracting bodies occasion- 
ally included within a delicate cell. The marginal vesicles are placed between the 
inner and outer membranes of the circular vessel. 
In the Phanerocarpae {Phacellophora) the marginal corpuscle (figs. 25, 25 a.) is placed 
at the extremity of a short double-walled tubular pedicle projecting downwards or 
