OF THE TENTACLES. 
419 
31. The tentacles of the fringe of Rhl%ostoma and Cephea have already been de- 
scribed, fig. 13. The tentacles which beset the generative membrane closely resemble 
them, and consist of a single membrane, containing many small thread-cells, 4 i^th 
of an inch in diameter. Their cavity is filled with a homogeneous substance, some- 
times containing nuclei, similar to those of the disc (15.) ; the inner membrane takes 
no part in their formation, fig. 30. 
32. The marginal tentacles of Thaumantias are very similar (fig. 3) to the buccal 
tentacles of Mesonerna ; they consist of an outer membrane, in which numbers of 
thread-cells are imbedded, and an inner axis composed of clear cells arranged end to 
end ; they have a peculiarity, which has been already pointed out by Prof. E. Forbes, 
in being placed above the marginal vesicles instead of being alternate with them, as 
in the nearly allied genus Geryonia\ and from this fact, and from their totally differ- 
ent structure, I believe that they have a totally different origin. In Geryonia the 
tentacles belong to the second class — are processes of the circular canal ; in Thau- 
mantias they are simple processes of the outer foundation membrane, i. e. of the sub- 
stance of the disc. Perhaps this difference in structure among the tentacles may 
turn out to be a good means of generic distinction among other members of the 
class. 
33. As to the second class of tentacles. Such are the marginal tentacles of 3Ie- 
sonema, of Geryonia (Will), of Oceania and of Medusa aurita (Ehrenberg); the 
tentacles of the under surface of Phacellophora, and the interbrachial tentacles of 
Cephea. 
34. In the specimens of Mesonerna I obtained, there were not more than eight 
tentacles, placed at equal distances round the disc, which had attained their full 
development. The interval between every two was filled up by a series of bud-like 
rudimentary tentacles, and marginal corpuscles alternate with them. Each tentacle, 
in its bud-like rudimentary form, is simply a csecal process of the circular canal, and 
has therefore, like it, a double wall and an internal cavity, usually filled with granules 
in rapid motion, produced by the ciliae of the inner wall; the outer wall contains 
large thread-cells. The structure of the adult tentacle is essentially the same, but in 
the course of its growth it has become divided into a lower filamentous portion and 
an upper dilated sac, by which it communicates with the circular canal, fig. 8. 
The marginal tentacles of Oceania resemble these in all points ; they are double- 
walled, communicate freely with the circular canal, and contain an immense number 
of minute thread-cells in their outer wall, fig. 15. 
35. In Phacellophora there is no distinct marginal circular canal, but the sixteen 
radiating canals are very wide and sacciform, and communicate only by anastomosing 
marginal branches. Eight of the canals are narrower and run to the marginal cor- 
puscles. The alternate eight are very much wider, and their outer, under surface is 
beset with a curved series of long tentacles, fig. 18. Now the lower wall of the 
canals is composed of the two “ foundation membranes,” and the tentacles are simply 
