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 

 o as G. priodon (fig. 28), or G. sagitta- 



i&X^t^Sn^. rius (fig. 29, A), as the type of the sub- 



odo,t, Bronn, preserved in relief : c l ass> tne polypary IS Seen tO Consist 



Clements, ^ 



ment of th 

 largcd. D, 



lateral viewsHghtivn f > 



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



viewof bl a y fr a e : *s the solid axis," the common 



e, showing the canal," and the " Cellules." The 



ve^ s^on'of " solid 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 Retiolitcs Gcinitzianus, 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 crenosarc 



