CHAPTER FIFTH. 



THE DISCOPHORiE RHIZOSTOME^ 



SECTION I 



THE RHIZOSTOME.T; in GEJfERAL. 



Ever since the Medusa; of this type have begvm to l)e investigated, they have 

 excited great wonder, and have been represented as differing widely from the others 

 in their structure and mode of existence. While in all other Meduste a so-called 

 mouth has been observed in the centre of the lower surface of the body, through 

 which the food could readily be introduced into the main digestive cavity, with 

 the aid of the so-called arms, Rhizostoma and other Medusa? allied to it have 

 been described by Reaumur, Cuvier, Eysenhardt, and even recently by Milne-Ed- 

 wards, as destitute of mouth, and only capable of absorbing food through innu- 

 merable suckers traversing the arms and reaching the stomach through narrow 

 channels. According to these representations, the Rhizostomidae would appear widely 

 different in their structure from the other Discophorae, and they have been contrasted 

 with them, as Polystomes; but their true relation seems to have escaped the pene- 

 tration of those who sought for a solution of the difficulty. In his latest paper 

 on the classification of Acalephs, Gegenbaur once more calls attention to this problem 

 of the polystomy of the Rhizostomidas, without, however, offering a solution. It 

 seems to me to be very simple, and that a careful comparison of Aurelia, in all 

 the successive stages of its growth, may exj)lain how the Rhizostomidse may appear 

 widely different, and yet have the same structure as the common Medusae. It is 

 true, there is no central broad opening in the middle of the lower floor in Rhizo- 

 stoma, as in Aurelia ; but the margin of the arms shows innumerable minute 

 pores, communicating with narrow tubes, gradually uniting into wider channels, and 

 finally reaching the central cavity; while in Aurelia there is a quadrangular central 



