







HYDROZOA. 



127 



far from low-water mark, while Tubularia occurs 

 at depths varying from less than one to more than 

 fifty fathoms. 

 The oceanic Hyd 



swim 



near the surface of the water, the approach of rain 

 or wind compelling them to retire for safety to the 

 more tranquil depths below. The large "jelly- 

 fishes " which, during summer and autumn, occur 

 so abundantly in our seas, are, with few exceptions, 

 the reproductive zooids of Aurelia, Cyanea, and 



Chry 



Equally numerous with these, but less 



conspicuous from their extreme transparency, 

 appear hosts of minute medusoids, while Diphyo- 

 zooids, Velella, Physalia, and one or two other 

 Physophoridce may, at rarer intervals, be de- 

 tected. 



3. Geographical Distribution. — The genera 

 of Hydrozoa are very widely distributed, renewed 

 investigations tending rapidly to diminish the 

 number of those supposed to be peculiar to certain 



regions of the globe. 



Hyd 



not yet been definitely ascertained. The other 

 fresh-water genus, Gordylophora, has been met 

 with only in Denmark, Great Britain, Ireland and 

 North America. 



Not much is known accurately of the geographi- 

 cal range of the Gorynidce ; the Sertularidce, 

 from the ready preservability of their polypary, 

 having been far more extensively studied. Sertu- 

 wria, Plumularia, Antennularia, and Gampa- 

 nulana are truly cosmopolitan, and the same may, 

 likewise, be said of some species of these genera, 

 tor example, S. operculata. Many South African 



