HTDROZOA. 127 



far from low-water mark, while Tuhularia occurs 

 at de]jths varjrins;^ from less than one to more than 

 fifty fathoms. 



The oceanic Hydrozoa in fine weather swdm 

 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 

 Ghrysaora, 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. Creos^raphical Disfrilintion. — The genera 

 of Hydrozoa are very widel}^ distributed, renewed 

 investigations tending rapidly to diminish the 

 number of those supposed to be peculiar to certain 

 regions of the globe. 



The limits of the area inhabited by Hydra have 

 not yet been definitely ascertained. The other 

 fresh-water genus, Cordylophora, has been met 

 with only in Denmark, Grreat Britain, Ireland and 

 North America. 



Not much is known accurately of the geographi- 

 cal range of the Corynidw ; the S'ertularidWy 

 from the ready preservability of their polypary, 

 having been far more extensively studied. Sertu- 

 laria, Pluonidaria, Antennularia, and Campcv- 

 nularia are truly cosmopolitan, and the same may, 

 likewise, be said of some species of these genera, 

 for example, S. opereulata. Many South African 



