414 
MR. T. H. HUXLEY ON THE ANATOMY OF THE MEDUSA. 
vary in form and position in different species, and may be absent ; the other organs 
are constantly present in the adult animal. 
5. Three well-marked modifications of external structure result from variations 
in the relative position of these organs. There is either — 1st, a simple stomach 
suspended from the centre of a more or less bell-shaped disc, the disc being traversed 
by canals, on some part of which the generative organs are situated, e.g. Geryonia, 
Thaumantias ; or 2ndly, a simple stomach suspended from the centre of a disc ; but 
the generative organs are placed in cavities formed by the pushing in, as it were, of 
the stomachal wall, e.g. Aui'elia, Phacelloyhora \ or 3rdly, the under surface of the 
disc is produced into four or more pillars which divide and subdivide, the ultimate 
divisions supporting an immense number of small polype-like stomachs; small aper- 
tures lead from these into a system of canals which run through the pillars, and 
finally open into a cavity placed under the disc ; the generative organs are attached 
to the under wall of the cavity, e.g. Rhizostoma, Cephea. 
6. To avoid circumlocution I will make use of the following terms (employed by 
Eschscholtz for another purpose) to designate these three classes, viz. Cryptocarp.® 
for the first, Phanerocarp^ for the second, and Rhizostomtd.® for the third. 
7. In describing the anatomy of the Medusae it will be found most convenient to 
commence with the stomach, and trace the other organs from it. 
Of the Stomach . — This organ varies extremely both in shape and in size in the 
Cryptocarpse and Phanerocarpae. But whatever its appearance, it will be always 
found to be composed of two membranes, an inner and an outer. These differ but 
little in structure ; both are cellular, but the inner is in general softer, less transparent 
and more richly ciliated, while it usually contains but few thread-cells. The outer, 
on the other hand, is dense, transparent, and either distinctly 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 papillee developed from its surface. 
8. I would wish to lay particular stress upon the composition of this and other 
organs of the Medusee 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 independently 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 aper- 
ture of the stomach and the openings of the canals, with which both communicate, 
and which I will therefore call the “common cavity.” 
10. In the Rhizostomidse the structure of the stomachs is fundamentally the 
same, but they are very minute, and are collected upon the edges and extremities of 
