CCELENTERATA : GRAPTOLITIM!. 



'45 



Taking such a simple Graptolite as G. priodon (fig. 62) 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 poly- 

 pary is corneous and flexible, and the 

 solid axis is a cylindrical fibrous rod, 

 which gives support to the entire organ- 

 ism, and is often prolonged beyond one 

 or both ends of the hydrosoma. There 

 is, however, every reason for believing 

 that the so-called "solid axis" is truly 

 hollow. The common canal is a tube 

 which encloses the coenosarc. and gives 

 origin to a series of cellules, these 

 being little cups corresponding to " hy- 

 drothecae," and enclosing the polypites. 

 Not only are the essential details of the 

 structure with the exception of the 

 solid axis strictly comparable with that 

 of a Sertularian, but there is good evi- 

 dence that the reproductive process, 

 in some forms at any rate, was also 

 carried on in a manner similar to what 

 we have seen in some other Hydroida Fig- 62 ._ A) Graptolite 

 namely, by generative buds or gono- 

 phores enclosed in gonangia. 



ograptus) priodon, Bronn, 

 preserved in relief lateral 

 view, slightly enlarged ; B, 

 Dorsal view of a fragment 

 of the same species consi- 

 derably enlarged ; 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. (Orig- 

 inal.) 



No Graptolite, however, has hitherto been 

 certainly proved to have been fixed by a " hy- 

 dro rhiza," and it is only in certain aberrant 

 forms that there are any traces of a " hydro- 

 caulus." 



Besides the simple forms of Graptolites with 

 a row of cellules on one side (monoprionidian) 

 (fig. 62), there are others with a row of cellules 



on each side (diprionidian) (fig. 63). Many other modifications are known ; 

 but there is only another peculiarity which is worthy of notice here. This 

 is the occurrence in several genera of a basal corneous disc or cup, which is 

 probably the homologue of the " float "or " pneumatophore " of the Phy- 

 sophoridae. (For distribution of Graptolites see Distribution of Hydrozoa 

 in Time. 



As regards their mode of occurrence, Graptolites are usually 

 found as glistening, pyritous impressions, with a silvery lustre. 

 In some cases, however, they are found in relief. 



SUB-CLASS VI. HYDROCORALLIN^E. This name has recently 

 been proposed by Mr Moseley for two groups of marine ani- 

 mals which produce a regular skeleton of carbonate of lime, 



K 



