FOSSIL HYDROZOA. 



81 



was enclosed ; but it commonly appears, in compressed speci- 

 mens, merely as a vacant space between the "cellules" and 

 the solid axis. The common canal gives origin, by a process 

 of budding, to the " cellules " or " hydrothecae," which are little 

 horny cups for the reception of the polypites. Each cellule 



1) 



Fig. 29. A, Young individual of Graptolites Sagittarius, His., showing the slender 

 curved base of the frond, and the extension of the axis beyond its opposite end ; B, Base of 

 another individual of the same, in which there is an extremely long " radicle ; " C, Frag- 

 ment of G. Sagittarius, much enlarged to show the cellules from a specimen in reHef ; 

 D, Specimen of Graptolites Clingani, Carr., showing the distal and proximal extensions 

 of the axis. 



rests by its base upon the common canal, is separated from its 

 neighbours by " cell-partitions," and opens at its apex by a 

 distinct aperture or " cell-mouth," through which the polypite 

 could exsert its tentaculate head. 



The reproductive process appears, in some cases, at any rate, 

 to have been carried on by the formation at certain seasons of 

 horny capsules, of much greater size than the cellules, within 

 which the generative elements were matured. In some cases 

 these " ovarian vesicles" have been found actually attached to 

 the fronds of Graptolites. In other cases, as described by the 

 writer, we find numerous bell-shaped horny capsules (fig. 30), 

 each with a little spine at its summit, scattered through the 

 rock in which the Graptolites occur, but only doubtfully 

 attached to the fronds of the latter. These we may infer to 

 have been " ovarian vesicles ; " but they differ from the bodies 

 so called in the Sertularians in becoming detached from the 

 parent colony. 



Two leading types may be distinguished amongst the Grap- 



