CCELENTERATA : HTDEOZOA. 



95 



we have seen in the other Hydroida, namely, by generative 

 bnds or gonophores. 



No Ghra/ptoUte, however, has 

 hitherto 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 ' hydro caulus.' 



Besides the simple forms of Grap- 

 tolites with a row of cellules on 

 one side (monoprionidian) (fig. 24, 

 2), there are others with a row of 

 cellules on each side (diprionidian) 

 (fig. 24, 3). Many other curious 

 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 

 Physophoridse. (For distribution of 

 Graptolites see Distribution of Hy- 

 drozoa in Time.) 



As regards their mode of occur- 

 rence, Graptolites are usually found 

 as glistening, pyritous impressions, 

 with a silvery lustre. In some cases, however, they are found 

 in relief. 



Fig. 24. Morphology of Grapto- 

 lites. 1 . Portion of Graptolites 

 Sagittarius enlarged ; a. Solid 

 axis; 6. Common canal; c. 

 Cellules. 2. Monoprionidian 

 Graptolite (G. argenteus). 3. 

 Diprionidian Graptolite (Di- 

 plograpsus prist.is, variety 

 with long basal spines). 



CHAPTER XII. 



DISTRIBUTION OF THE HYDEOZOA. 



I. DISTRIBUTION OF HYDROZOA IN SPACE. The genera of Hy- 

 drozoa have a wide distribution, the mode of reproduction 

 amongst the fixed forms being such as to insure their extension 

 over considerable areas. The various species of Hydra are of 

 common occurrence in the fresh waters of Europe. Cordylo- 

 phora, the sole remaining fresh-water genus, has not been 

 found to occur out of the north temperate zone. All the 

 other Hydrozoa, without a known exception, are marine in 

 their habits. The fixed forms, viz. the Corynidce, Sertularidcc, 



