§ 15. GEOGRAPHICAL DISTRIBUTION OF CORALS. 613 



of Corals by an eminent young American chemist (Mr. B. 

 Silliman), has shown, contrary to expectation, that they 

 contain a much larger proportion of fluorine than of phos- 

 phoric acid.* 



15. Geographical distribution of Corals. — Iwillfirst 

 consider the geographical distribution of the Polyparia ; in 

 the next place, describe a few of the principal varieties ; 

 and, lastly, review the important physical changes effected 

 by creatures so minute, and apparently so incompetent to 

 produce any material alteration in the earth's surface. 



The Corals are for the most part inhabitants of the 

 ocean ; many species prefer the immediate influence of 

 atmospheric changes, and are seen on the rocks and plants 

 left bare by the reflux of the tide, sometimes in such pro- 

 fusion that the whole surface appears one animated mass. 

 At the period of the great equinoctial tides, when the sea 

 retires from the rocks which it has overflowed for many 

 preceding months, the Polyparia, when the waters first 

 recede, are full of vigour, but languish as they lose their 

 moisture, and perish if they remain long uncovered by the 

 sea. 



Some kinds are situated on the southern slope of the 

 rocks ; others, on the contrary, are attached to the opposite 

 aspect, and never to the former. The larger forms are 

 rarely found in places exposed to violent currents ; it is in 

 the hollows of the rocks, in submarine grottoes, in the 

 shelter of large and solid masses, that these species attach 

 themselves. Many appear fitted to enjoy the powerful 

 action of the surges, their pliant branches bending to the 



* Appendix to Mr. Dana's memoir on the " Structure and Classifi- 

 cation of Zoophytes," Philadelphia, 1846. This volume is an Intro- 

 duction to the " History of the Zoophytes collected by the American 

 Exploring Expedition," now in the press : the publication of this 

 work of the accomplished author is looked forward to with great 

 interest by European naturalists. 

 s s 2 



