1 6 ON THE ANATOMY OF THE FAMILY OF THE MEDUSA 



the orifice of the central cavity, is prolonged at its edges into a great 

 number of short tentacles. Each of these is composed of an outer 

 wall, in which immense numbers of thread-cells are imbedded, and a 

 central axis made up of large transparent cells. This cellular axis 

 extends for some distance beyond the base of the tentacle into the 

 substance of the membrane, fig. 7. 



31. The tentacles of the fringe of Rhizostorna and Cephea have 

 already been described, fig. 13. The tentacles which beset the gener- 

 ative membrane closely resemble them, and consist of a single mem- 

 brane, containing many small thread-cells, ^ ^^ ^ 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 Tliaiiinantias are very similar (fig. 3) 

 to the buccal tentacles o{ Mesonenia ; they consist of an outer mem- 

 brane, 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. Eorbes, in 

 being placed above the marginal vesicles instead of being alternate 

 with them, as in the nearly allied genus Geryotiia ; and from this fact, 

 and from their totally different 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 Thaumantias 

 they are simple processes of the outer foundation membrane, i.e. of the 

 substance 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 Mesonenia, of Geryonia (Will), of Oceania and of Aledusa 

 <?//;7'/rt (Ehrenberg) ; the tentacles of the under suda^ceoi Phace I lop hora, 

 and the interbrachial tentacles of Cephea. 



34. In the specimens oi Mesonenia 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 mar- 

 ginal 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 cilijE 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 



