SECT. iv. REEF-BUILDING CORALS. 141 



in some species the rays are almost invisibly slender, 

 the interstitial matter is full of pores, and the polypes 

 have twelve tentacles. 



The Millepora complanata or pahnipora is very com- 

 monly associated with the Porites ; it is the largest coral 

 known. It grows in thick vertical plates, intersecting 

 each other at various angles, and forms an exceedingly 

 strong honey-combed mass, generally affecting a cir- 

 cular form, the marginal plates alone being alive. In- 

 stead of stars, the polypes live in simple pores : myriads 

 of these small cylindrical pores penetrate the surface of 

 the plates perpendicular to their axes ; sometimes they 

 are so minute as to be scarcely visible. 



Between the plates, and in the protected crevices of 

 the outer circle of the ring, a multitude of branching 

 zoophytes and other productions flourish; but the Porites, 

 Astrseans, and Milleporse seem alone able to resist the 

 fury of the breakers, essential to the very existence of 

 these hardy corals, which only obtain their full develop- 

 ment when washed by a heavy sea. The outer margins 

 of the Maldive atolls, consisting chiefly of Milleporse and 

 Porites, are beat by a surf so tremendous that even ships 

 have been thrown by a single heave of the sea high and 

 dry on the reef. The waves give innate vigour to the 

 polypes by bringing an ever-renewed supply of food to 

 nourish them, and oxygen to aerate their juices : besides, 

 uncommon energy is given and maintained by the heat 

 of a tropical sun, which gives them power to abstract 

 enormous quantities of solid matter from the water to 

 build their stony homes, a power that is efficient in pro- 

 portion to the energy of the breakers which furnish the 

 supply. 



The Porites and Milleporse, which are the chief reef-! 

 building corals, cannot live at greater depths than fif- 

 teen or twenty-five fathoms : not for want of heat, for 

 the temperature of the ocean in these latitudes does not 



