8o 



CCELENTERATA. 



pary was almost always strengthened by a chitinous rod or fibre, 

 which is termed the " solid axis/' and which is somewhat analo- 

 gous to the chitinous rod described by Dr Allman in the 

 singular Polyzoon, Rhabdopleura. 



From the above definition, it will be seen that the Grapto- 



lites agree with the living Sertulari- 

 ans in possessing a corneous poly- 

 pary, which not only invests the cceno- 

 sarc, but is expanded into little cups 

 or " hydrothecae, " within which 

 each polypite is protected. The 

 Graptolites, however, differ from the 

 Sertularians in the fact that the poly- 

 pary was unattached, and apparently 

 free-floating, whilst it has not, except 

 in a few cases, anything like the plant- 

 like appearance of the latter. Further, 

 the hydrothecae of the Graptolites, 

 except in the genus Rastrites, always 

 more or less overlap one another ; 

 whereas those of the Sertularians are 

 not in contact. Lastly, no Sertula- 

 rian exhibits any structure which can 

 be compared with the " solid axis " 

 of the Graptolites. 



Taking such a simple Graptolite 

 as G. priodon (fig. 28), or G. sa&tta- 



Fig. 28. Morphology of Grapto- -if A \ rlui 



utes priodon. A, Graptolites pri- rius (fig. 29, A), as the type of the sub- 



odon, Bronn, preserved in relief: place the nolvnarv is Seen to rnn si col- 

 lateral view slightly enlarged. B, Cldbb > Uie POiypary 1 



Dorsal view of a fragment of the Of three elements, which are known 



same species: considerably en- f u crk i;^ av ic " fVi^ " n->, 



larged. C, Front view of a frag- ES tne SOllO. aXIS, tn< COmmon 



ment of the same, showing the Canal" and the " Cellules." The 



mouths of the cellules: much en- /, % .... .. . . . ,. 



larged. D, Transverse section of SOlld aXIS IS a Cylindrical fibrOUS 



rod which gives support to the cor- 

 neous and flexible polypary. The 

 term " solid" is probably a misnomer; for it was almost certainly 

 hollow, and filled with living material. It appears to be absent in 

 the genus Rastrites, and in Retiolites Geinitzianus, but some un- 

 certainty rests upon this point. As a very general rule, it is 

 prolonged as a longer or shorter naked rod beyond one or both 

 ends of the polypary, and either extension may be more or less 

 dilated. Its basal prolongation, with or without an accom- 

 panying extension of the common canal, is termed the "radicle," 

 or " initial point," as marking the organic base of the frond. 

 The " common canal " is the tube in which the coenosarc 



