CCELENTERATA : HYDROZOA. 



lO/ 



certain aberrant forms that there are 

 caulus." 



Besides the simple forms of Grap- 

 tolites with a row of cellules on 

 one side (monoprionidian) (fig. 26, 

 2), there are others with a row of 

 cellules on each side (diprionidian) 

 (fig. 26, 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 

 Physophoridae. (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. 



any traces of a " hydro- 



Fig. 26. Morphology of Grapto- 

 lites. i. Portion of Graptolites 

 Sagittarius enlarged ; a Solid 

 axis ; b Common Canal ; c 

 Cellules. 2. Monoprionidian 

 Graptolite (G. argenteus}. 3. 

 Diprionidian Graptolite (Diplo- 

 grapsus pristis, variety with long 

 basal spines.) 



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. All 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- 



