ON THE AFFINITIES OF THE FAMILY OF THE MEDUSA 25. 



by a circular canal at the margin. In the centre the radiating 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 Diphydje and Physophoridce ; 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 Medusae, 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 

 Hydrae and Sertulariadae. 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 Diphyda; form a 

 connecting link between the Medusje and the Physophoridae. 



62. Of the two kinds of tentacles of the Medusae, the first is repre- 

 sented, in the Physophoridae and Diphydje, by the thickenings, richly 

 beset with thread-cells, that frequently occur in the lip of the stomach ; 

 in the Sertularian Polypes (^Plumularia, Campanularid) by the ten- 

 tacles of the margin of the mouth, which precisely resemble the 

 tentacles of the fringe of Rhizostoina, or the marginal tentacles of 

 Thaumantias, in being composed of a single membrane covered with 

 thread-cells, and having a cellular axis. 



63. The second kind of tentacle is homologous with the prehensile 

 organs of the Diphydse and Physophoridae with the peculiar clavate 

 processes of Pluinularia, and so far as 1 can judge from descriptions 

 of their structure, with the tentacles of Hydra. 



All the organs here mentioned commence their development as 

 bud-like processes of the two primary membranes, elongating and 

 attaining the forms peculiar to their perfect state as they grow older. 

 The tentacles of the Medusa: are usually developed (as in most 

 Monostomatae) from the circular vessel of the disc, sometimes 

 {Phacellophora) from the diverging canals, sometimes, finally, from the 

 neck of the stomach (Lymnorea, Javonia). The prehensile organs of 

 the Physophoridae also have considerable variety in position. In 

 Porpita, Vitella, Angela (?), they are developed from the margin of 

 the float ; in Physophora and many others from the base or the pedicle 

 of the stomach. The prehensile organs of the Diphyda; are always 

 developed either from the base or the pedicle of the stomach. The 

 peculiar clavate organs of Plumularia are developed from the common 

 tube independently of the stomach. 



