CCELENTERATA : HYDROZOA. 



121 



enclosed in a strong tubular polypary, whilst the former were 

 protected by hydrothecae. In the great majority of Graptolites 

 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 polypites are 

 never separated from the coenosarc 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 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 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 Ptilo- 

 grapsus and Dendrograpsus are so 

 similar to some living Sertularians, that 

 it might be well to remove them alto- 

 gether from the GraptolititUe, and to 

 regard them as extinct representatives 

 of the Sertularida. 



As regards the value of the " solid 

 axis " as an element in defining Grap- 

 tolites, we fear that much stress cannot 

 be laid up on 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 Geinitzianus, 

 and in all species of Rastrites 

 and there do not seem to be sufficient 

 grounds for excluding these from the 

 Graptolitida on this account alone. 



Taking such a simple Graptolite as 

 G. priodon (fig. 36) 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, 



Fig. 36. A, Graptolites prio- 

 don, Bronn, preserved in 

 relief. Lateral view slightly 

 enlarged. B, Dorsal view 

 of a fragment of the same 

 species : considerably en- 

 larged. C, Front view of 

 a fragment of the same, 

 showing the mouths of the 

 cellules : much enlarged. 

 D, Transverse section of 

 the same. All from the 

 base of the Coniston Flags, 

 (Original.) 



