144 MANUAL OF ZOOLOGY. 



all the peculiarities belonging to this structure), but which re- 

 produce themselves without the intervention of free generative 

 zooids produced by fission (Pelagia). 



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

 an umbrella and a single polypite, the primitive hydrosoma 

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



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

 umbrella and a complex central tree bearing many polypites 

 (Rhizostoma, Cephea, &c.) 



Of these five classes of organisms, Nos. i 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. GRAPTOLITID^E ( = RHABDOPHORA, Allman). 

 The organisms mclude^^tj^resgnl under this head are all 

 extinct, and tliey~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 : 



Hydrosoma compound, occasionally branched, consisting 

 of numerous polypites united by a coenosarc ; the latter being 

 enclosed in a strong tubular polypary, whilst the former were 

 protected by hydrothecse. In the great majority of Graptolites 

 the hydrosoma was certainly unattached; but in some aber- 

 rant forms doubtfully belonging to the sub-class there is 

 reason to believe that the hydrosoma was fixed. The poly- 

 pites are never separated from the ccenosarc by any partition. 

 In many cases the hydrosoma was strengthened by a chitinous 

 rod, the " solid axis," somewhat analogous to the chitinous rod 

 recently described by Professor Allman in the singular Poly- 

 zoo n Rhabdopleura. 



From the above definition it will be seen that the nearest living allies to 

 the Graptolites are the Sertularians. In point of fact, if we do not insist 

 upon the presence of a "solid axis" as part of the definition, the Grapto- 

 lites differ from the Sertularians in no essential point, save that the hydro- 

 soma is always attached in the latter, and was certainly free in the most 

 typical examples of the former. Indeed, certain forms at present placed 

 among the Graptolites such as Ptilograptus and Dendrogi-aptus are so 

 similar to some living Sertularians, that it might be well to remove them 

 altogether from the Graptolitidcz, and to regard them as extinct representa- 

 tives of the Sertularida. 



As regards the value of the " solid axis " as an element in defining Grapto- 

 lites, it is doubtful if much stress can be laid upon its presence or absence. 

 It is true that it is present in all the most characteristic members of 'the 

 sub-class, but it has not been proved to be present in some forms, which 

 in other respects are quite typical members of the group. 



