112 



MANUAL OF. ZOOLOGY. 



certain aberrant forms that 



Fig. 27. — Morphology of Graptolites. 

 I. Portion of Graptolites Sagittar- 

 ius enlarged ; « Solid axis ; o Com- 

 mon canal : c Cellules. 2. Mon- 

 oprionidian Graptolite {G. argen- 

 teus). 3. Diprionidian Graptolite 

 \Diplo^apsus pristiSf variety with 

 long basal spines). 



there are any traces of a " hydro- 

 caulus." 



Besides the simple fonns of 

 Graptolites with a row of cellules 

 on one side (monoprionidian) (fig. 

 27, 2,) there are others with a 

 row""of cellules on each side (dip- 

 rionidian) (fig. 27, 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- 

 phoridse. (For distribution of Grap- 

 tolites see Distribution of Hydro- 

 zoa in Time.) 



As regards their mode of oc- 

 currence, Graptolites are usually 

 found as glistening, pyritous im- 

 pressions, with a silvery lustre. In 

 some cases, however, they are found 

 in relief. 



CHAPTER XII. 

 DISTRIBUTION OF THE HYDROZOA. 



I. Distribution of Hydrozoa in Space. — The genera of 

 Hydrozoa 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. AH the 

 other Hydrozoa, without a known exception, are marine in 

 their habits. The fixed forms — viz., the Corynida, Sertularida, 

 and Campanularida — are represented more or less abundantly 

 in almost all seas, extending from the littoral zone to con- 

 siderable depths. The oceanic Hydrozoa, Calycop/ioridce and 



