CHAPTER VI. 



GENERAL REMARKS ON THE GEOGRAPHICAL 

 DISTRIBUTION OP ZOOPHYTES. 



99. HEAT, light, pressure, and means of subsistence, influence 

 more or less the distribution of all animals ; and to these causes 

 should be added, for water species, the nature or condition of the 

 water, whether fresh or marine, pure or impure, still or agitated. 

 Next to the character of the water, heat is the most prominent limit- 

 ing agent for marine animals, especially as regards latitudinal extent, 

 while light and hydraulic pressure have much influence in determining 

 their limits in depth. 



Although these causes fix bounds to species and families, they do 

 not necessarily confine tribes of species to as small limits. This is 

 sometimes the case, and is nearly so with a large group of zoophytes; 

 yet other tribes and orders include species whose united range com- 

 prises all the zones, from the equator to the polar ices, and every 

 depth, to the lowest affording traces of life which man has explored. 



Order Hydroidea. 



100. The Hydroidea are met with in all seas and at great depths, 

 as well as at the surface. The tropics, and the cold waters of the 

 frigid zone, have their peculiar species, and a few are found in fresh 

 waters. The rocks and common marine plants of the sea-coast, the 

 dead or living shell, or the floating Fucus of the ocean, are often 

 covered with these feathery corals ; and, about reefs, they occasionally 

 implant themselves upon the dead zoophyte, forming a mossy covering, 

 taking the place of the faded coral blossom. 



The species are most abundant, however, in the waters of the 



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