CCELENTEEATA : HYDEOZOA. 93 



pites, interspersed with clavate tentacula, is suspended from 

 the middle of the umbrella in a very singular way. The 

 main trunks of the dependent polypiferous tree, in fact, unite 

 above into a thick, flat, quadrate disc, the " syndendrium," 

 which is suspended by four stout pillars, the "dendrostyles," 

 one springing from each angle, to four corresponding points 

 on the under surface of the umbrella, equidistant from its 

 centre. Under the middle of the umbrella, therefore, is a 

 chamber, whose floor is formed by the quadrate disc, whilst 

 its 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 roof of this genital cavity/ 



It appears, finally, that amongst the old Pulmograde 

 Acalepha?, 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 Corynidce, Sertu- 

 laridce, Gampanularidce, 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 (Trachynemidce and 



3. Hydrozoa which are provided with an ' umbrella ' (with 

 all the peculiarities belonging to this structure), but which 

 reproduce themselves without the intervention of free genera- 

 tive zooids produced by fission (Pelagia). 



4. The free generative zooids of most of the Pelagidcs, with 

 an umbrella and a single polypite, the primitive hydrosoma 

 being fixed and sexless (Aurelia, Cyanea, &c.). 



5. The free generative zooids of the Rhizostomidce, with an 

 umbrella and a complex central tree bearing many polypites 

 (Rhizostowia, Cephea, &c.). 



Of these five classes of organisms, Nos. 1 and 2 constitute 

 the Gymnophthalmate Medusce of Professor E. Forbes, whilst 

 Nos. 3, 4 and 5 are the Steganophthalmate Medusce of the 

 same naturalist. 



SUB-CLASS V. GEAPTOLITID^. The organisms included at 

 present under this head are all extinct, and they are in many 

 respects so dissimilar, and their structure is so far from being 

 entirely understood, that it is doubtful if any definition can 

 be framed which will include all the supposed members of the 

 family. The following definition, however, will include all 

 the most typical Graptolites : 



