CCELENTERATA : HYDROZOA. 123 



CHAPTER XII. 



DISTRIBUTION OF THE HYDROZOA. 



I. DISTRIBUTION OF HYDROZOA IN SPAC. The genera of 

 Hydrozoa have a wide distribution, the mode of reproduction 

 amongst the fixed forms being such as to ensure 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 Campannlarida are represented more or less abundantly 

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

 siderable depths. The oceanic Hydrozoa, Calycophoridcz and 

 Physophorukz, are chiefly characteristic of tropical seas ; but 

 they are found also in the Mediterranean, and even in seas not 

 far from, or even within, the Arctic circle. 



II. DISTRIBUTION OF HYDROZOA IN TIME. The fine-grained 

 lithographic slates of Solenhofen and Eichstadt have yielded 

 impressions of Meduscz belonging to the existing families of 

 the SEquoridcz and Trachynemidiz ; and the Lucer?iarida are 

 represented by an ancient form of the Rhizostomidcz in the 

 same formation. With these exceptions, however, there are 

 hardly any fossil remains which would universally be 

 conceded to be of a Hydrozoal nature. The Oldhamia of 

 the Cambrian rocks of Ireland has, indeed, been regarded 

 as belonging to the Hydrozoa; but it is believed by Mr 

 Salter to be really a plant It consists of a main stem 

 with numerous secondary branches, springing from the axis 

 in an umbellate manner, but exhibiting no traces ofhydro- 

 thecae. 



The occurrence of Corynida in a fossil condition can hardly 

 be said to be free from doubt. Remains probably referable to 

 this order have been, however, recently discovered in the Pal- 

 aeozoic Rocks. The oldest of these was described by the 

 Author some years ago from the Lower Silurian rocks of Dum- 

 friesshire under the name of Corynoides. More lately a form 

 called Palaocoryne has been described from the Carboniferous 

 rocks of Scotland. Species of Hydractinia have also been 

 described from the Cretaceous, Miocene, and Pliocene 

 deposits. 



