CCELENTERATA : LUCERNARIDA. 143 



roof is constituted by the under wall of the central cavity of the 

 umbrella, and its sides are open. The reproductive elements 

 are developed within radiating folded diverticula of the root 



Fig. 60. Rhizostomidae. Generative zooid of Rhizostoma 



(after Owen), a Umbrella; bb "Stomatodendra," covered Fig. 61. Generative zooid of 

 with clavate tentacles and minute polypites ; c c Anasto- Rhizostoma pulmo, reduced 



mosing network of canals. 



in size. (After Gosse.) 



of this genital cavity." According to other authorities, how- 

 ever, the apparent polypites of the " stomatoderidra " are really 

 mouth -like apertures formed by the extraordinarily complex 

 manner in which the oral lobes are folded, and there is then in 

 reality but one single central polypite, hanging from the under 

 surface of the umbrella. 



It appears, finally, that amongst the old Pulmograde Aca- 

 lephae, or amongst what would commonly be called Jelly-fishes, 

 we have the following distinct sets of beings, which resemble 

 each other more or less closely in appearance, but differ in 

 their true nature: 



1. Free medusiform gonophores of various Corynida, Cam- 

 panularida, and the Oceanic Hydrozoa. 



2. True Medusidce, entirely resembling the former in anato- 

 mical structure, but differing in the fact that their ova do not 

 give rise to a fixed zooid, but to free - swimming organisms 

 exactly like the parent hydrosoma (Trachynemida, Geryonida, 

 and ^Lginidce). 



3. Hydrozoa, which are provided with an "umbrella" (with 



