94 MANUAL OF ZOOLOGY. 



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 hydrothecae. In the great majority of Grapto- 

 lites the hydrosoma was certainly unattached, but in some 

 aberrant forms doubtfully belonging to the sub- class there 

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



?ites are never separated from the ccenosarc by any partition, 

 n many cases the hydrosoma was strengthened by a solid 

 chitinous rod, the * solid axis,' somewhat analogous to the 

 chitinous rod recently described by Professor Allman in the 

 singular Polyzoon, 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 Graptolites differ from the Ser- 

 tularians in no essential point, save that the hydrosoma is 

 always attached in the latter, and was certainly free in the 

 most typical examples of the former. Indeed, certain forms 

 at present placed amongst the Grapiolites such as Piilograpsys 

 and Dendrograpsus are so similar to some living Sertularians, 

 that it might be well to remove them altogether from the 

 Gra2itoliticl.ee, and to regard them as extinct representatives of 

 the Sertularidce. 



As regards the value of the ' solid axis' as an element in 

 defining Graptolites, we fear that much stress cannot 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 

 seems to be certainly absent in some e.g. in Retiolites Gei- 

 nitzianus, and in all species of Bastrites and there do not seem 

 to be sufficient grounds for excluding these from the Grapto- 

 litidce on this account alone. 



Taking such a simple Graptolite as G. Sagittarius (fig. 24, 1) 

 as the type of the sub-class, the hydrosoma is found to consist 

 of the ' solid axis,' the ' common canal,' and the ' cellules.' 

 The entire polypary is corneous and flexible, and the solid 

 axis is a cylindrical fibrous rod, which gives support to the 

 entire organism, and is often prolonged beyond one or both 

 ends of the hydrosoma. The common canal is a tube which 

 encloses the coenosarc, and gives origin to a series of cellules, 

 these being little cups corresponding to ' hydrothecse,' and 

 enclosing the polypites. Not only are the essential details of 

 the structure with the exception of the solid axis strictly 

 CDmparable with that of a Sertularian, but there is good 

 evidence, as shown by Hall and the author, that the reproduc- 

 tive process was also carried on in a manner similar to what 



