126 HYDROZOA. 



Not far from Pdagia, but in a family by itself, 

 Gegenbaur has placed the genus Chm^bdea, 

 Fritz jNIiiller, however, shows, that in the closely 

 allied Tamoya, a distinct veil is certainly present, 

 while Gharyhdea itself is furnished with marginal 

 processes, which seem to represent the same ap- 

 paratus. 



Section IV. 



DISTRIBUTION OF HYDROZOA. 



I. Relations to Physical Elements. — 2. Bathymetrical Distribution. 

 — 3. Geographical Distribution. 



1. Relations to Physical Elements. — The 



Ilydrozoa, as a class, are almost exclusively marine; 

 Hydra and Cordylophora being the only fresh- 

 water genera hitherto described. 



2. Bathymetrical Distribution. — The ma- 

 rine Hydrozoa, with reference to their distribution, 

 may conveniently be divided into two groups, the 

 fixed and the oceanic. The fixed Hydrozoa, Cory- 

 nidce and Sevtidaridoe, are less abundant between 

 tide marks than at depths of a few fathoms, some 

 forms extending their range to very deep water. 

 The Coinjnidce are, perhaps, on the whole, more 

 partial to shallow waters than the Sertularidce, 

 certain species of the latter order, especially of the 

 genus Campaiudariay being found at considerable 

 depths. But the vertical distribution of several 

 forms is more limited than that of others. Thus 

 Clava and Coryne appear usually not to wander 



