ACTINOZOA. 233 



near the level of low water. A few appear to be 

 oceanic. Philomedusa, a mimite form, from the 

 Brazilian seas, habitually seeks shelter beneath 

 the swimming-organ of various MedusidcB and 

 LucernaridcB. 



The bathymetrical distribution of the Cteno- 

 phora, by reason of their oceanic habit, is scarcely 

 amenable to observation. Some species, during 

 the storms of winter, appear to seek considerable 

 depths, on the return of spring again approaching 

 to the surface. 



3. €reographical MistriHjiitioii. — The Cte- 

 nophora, Alcyonaria, and soft-bodied Zoantharia 

 appear to be about equally abundant in tropical 

 and temperate seas, many forms extending their 

 ranore to hiofh latitudes. Of corallio:enous Zoan- 

 tliaria two families, Turhinolidce and Madrepo- 

 ridce, are not without northern and even arctic 

 representatives, yet by far the majority of other 

 sclerodermic species are seldom found to occur 

 beyond the limits of the tropics. The reef-build- 

 ing Corals, according to Dana, will not flourish in 

 water wherein the mean winter temperature is 

 lower than 66° F. So that on either side of the 

 equator a zone of water sufficiently heated for the 

 growth of these Corals extends, the boundary lines 

 of which have of necessity a somewhat contorted, 

 irregular course, by reason of the varied com- 

 binations of circumstances influencing the local 

 distribution of heat. Even within these limits 

 other external conditions, not less essential than 

 a high temperature to the welfare of reef-build- 

 ing Corals, are often absent. But when once the 

 nature of these conditions has been carefully 



