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MANUAL OF ZOOLOGY. 



and the solid axis is a cylindrical fibrous rod, which gives support 

 to the entire organism, 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 ccenosarc, and 

 gives origin to a series of cellules, these being little cups 

 corresponding to " hydrothecae," 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 a good evidence, as shown by Hall and 

 the author, that the reproductive process was also carried on 

 in a manner similar to what we have seen in the other Hydroida 

 namely, by generative buds or gonophores. 



No Graptolite, however, has hither- 

 to been certainly proved to have 

 been fixed by a " hydrorhiza," and 

 it is only in certain aberrant forms 

 that there are any traces of a " hy- 

 drocaulus." 



Besides the simple forms of 

 Graptolites with ,a row of cellules 

 on one side (monoprionidian) (fig. 

 37, 2), there are others with a 

 row of cellules on each side (dip- 

 rionidian) (fig. 37, 3). Many other 

 curious modifications are known ; 

 but there is only another peculi- 

 arity which is worthy of notice 

 here. This is the occurrence in 

 several genera of a basal cor- 

 neous disc or cup, which is pro- 

 bably the homologue of the " float " 

 or " pneumatophore " of the Physo- 

 phoridae. (For distribution of Grap- 



rius enlarged ; Solid axis J {Com- tolitCS SC6 ' Distribution of HydrO- 

 mon canal ; c Cellules. 2. Mono- . * 



prionidian Graptolite (G- argen- ZOa in lime.) 



tens). 3. Diprionidian Graptolite \ rAcy o r rlc rVipir mrn-lp. rf nr 



(Diplograpsus pristis, variety with AS regards tneiF mOdC 



long basal spines). currence, Graptolites are usually 



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

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



