426 
MR. T. H. HUXLEY ON THE AFFINITIES OF THE MEDUSA. 
the Medusse with the corresponding organs in the rest of the series ; and it is curious 
to remark, that throughout, the outer and inner membranes appear to bear the same 
physiological relation to one another as do the serous and mucous layers of the germ ; 
the outer becoming developed into the muscular system and giving rise to the organs 
of offence and defence ; the inner, on the other hand, appearing to be more closely 
subservient to the purposes of nutrition and generation. 
59. The structure of the stomach in the Medusae is in general identical with that 
of the same organ in the rest of the series. The Rhizostomidse offer an apparent 
difficulty, but it appears to me that the marginal folds in them answer to the stomachal 
membrane of the Monostome Medusse ; the apertures to the inner orifice of their 
stomach, and the common canal to their “ common cavity.” Just as in a polygastric 
Diphyes the common tube answers to the chamber into which the stomach of a 
monogastric Diphyes opens; and in Cephea JVagneri (Will) these resemblances are 
still more striking. He says that each cotyledon “ has at its apex a small round 
opening, tlie mouth, which leads to an ovate cavity, occupying the whole interior of 
the cotyledon. I consider this as the proper digestive or stomaehal cavity, and believe 
that the cotyledons have the same relation to the vessels as the so-called suckers 
{Sangrohren) of the Diphydse to the common tube {Saftrohre)*." 
60. The disc of a Medusa is represented by the natatorial organ among the 
Diphydse and Physophoridse. Take for instance the disc of Oceania or Cytceis. It 
is here a more or less bell-shaped body, traversed by radiating canals, lined by a 
distinct membrane, united by a circular canal at the margin. In the centre the radi- 
ating canals communicate freely with the chamber into which the stomach opens. 
The inner margin of the disc is provided with a delicate, circular, valvate membrane. 
The same description applies, word for word, to the natatorial organs of the Diphydse 
and Physophoridse ; the only difference being, that in the latter the stomach is 
outside the cavity (fig. 47) of the organ, instead of being, as in the Medusse, suspended 
from its centre inside, fig. 49. And even if the different texture of the two organs 
should give rise to any doubt, the genus Rosacea, in which the natatorial organ is 
perfectly soft and gelatinous, furnishes the needful intermediate form. 
61. The disc of the Medusse has no representative among the Hydrse and Sertu- 
lariadse. The cell of the Sertularian Polype rather resembles the “bract” of the 
Diphydse than the “natatorial organ” in its structure and function, and in this 
manner the Diphydse form a connecting link between the Medusse and the Physo- 
phoridse. 
62. Of the two kinds of tentacles of the Medusse, the first is represented, in the 
Physophoridse and Diphydse, by the thickenings, richly beset with thread-cells, that 
frequently occur in the lip of the stomach ; in the Sertularian Polypes {Plumularia, 
Canipanularia) by the tentacles of the margin of the mouth, which precisely resemble 
the tentacles of the fringe of Rkizostoma, or the marginal tentacles of Thaumantias, 
* Horse Tergutinse, p. 60. 
