98 Mr. J. D. Dana on Zoophytes. 



XII. — On the Geographical Distribution and Classification of 

 Zoophytes*. By James D. Dana. 



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 it is nearly true of a large group 

 of zoophytes; yet other tribes and orders include species whose 

 united range comprises all the zones, from the equator to the 

 polar ices, and every depth, to the lowest which man has explored, 

 affording traces of life. 



Order Hydroidea. — 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 

 temperate zone, and are common upon some portions of our own 

 coast. 



Order Actinoidea. — The Actinoidea are marine zoophytes. 

 All oceans have their species, yet in the torrid zone they more 

 especially abound, and display most variedly their colours and 

 singular forms. 



The soft Actinidce and the Alcyonaria have the widest range, 

 occurring both among the coral reefs of the equatorial regions, 

 and, to the north and south, beyond the temperate zone. The 

 Mediterranean affords species of Gorgonia, Corallium and Alcy- 

 onium, besides numerous Actinia. The coasts of Britain have 

 also their Alcyonia and Actinia, and from far in the northern 

 seas, come the Umbellularia, and some other species of the Pen- 

 natula family. 



Among the coral-making Actinaria, the Madrepore and Astraea 

 tribes are almost exclusively confined to the coral-reef seas, — a 



* From Silliman's American Journal for March and Mav 1847. 



