126 HYDROZOA. 



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

 'Gregenbaiir has placed the genus Charyhdea. 

 Fritz Miiller, however, shows, that in the closely 

 allied Tamoya, a distinct veil is certainly present, 

 while Charyhdea itself is furnished with marginal 

 processes, which seem to represent the same ap- 

 paratus. 



Section IV. 



DISTRIBUTION OF HYDROZOA. 



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

 — 3. Geographical Distribution. 



1. Relations to Physical Elememtg;. — The 



Hydrozoa, as a class, are almost exclusively marine; 

 Hydra and Co7xlylop}iora being the only fresh- 

 water genera hitherto described. 



2. Hathymetrical Uistritotitioit. — 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 Sertulai^idw, are less abundant between 

 tide marks than at depths of a few fathoms, some 

 forms extending their range to very deep water. 

 The Corynidce are, perhaps, on the whole, more 

 partial to shallow waters than the Sertularidce, 

 certain species of the latter order, especially of the 

 genus Gampanularia, being found at considerable 

 depths. But the vertical distribution of several 

 forms is more limited than that of others. Thus 

 Glava and Coryne appear usually not to wander 



