60 ON CORAL REEFS AND ISLANDS. 



loporas are not uncommon, though requiring pure waters. There 

 are also Madreporas, some growing even in impure waters. One 

 species was the only coral observed in the lagoon of Honden 

 Island (Paumotus), all others having disappeared owing to its im- 

 perfect connection with the sea. Upon the reefs enclosing the har- 

 bor of Rewa, (Viti Lebu,) where a large river three hundred yards 

 wide empties, which during freshets enables vessels at anchor two 

 and a half miles off its mouth to dip up fresh water alongside, there 

 is a single porous species of Madrepora, (M. cribripora,) growing 

 here and there in patches over a surface of dead coral rock or sand. 

 In similar places about other regions, species of Pontes are most 

 common. In many instances, the living Porites were seen stand- 

 ing six inches above low tide, where they were exposed to sun- 

 shine and to rains; and associated with them in such exposed 

 situations, there were usually great numbers of Alcyonia and 

 Xenise. Porites also occur in the impure waters adjoining the 

 shores; and the massive species in such places commonly spread 

 out into flat disks, the top dying from ihe deposition of sediment 

 upon it. 



The exposure of six inches above low tide, where the tide is 

 six feet, as in the Feejees, is of much shorter duration than in 

 the Paumotus, where the tide is less than half this amount ; and 

 consequently the height of growing coral, as compared with low 

 tide level, varies with the height of the tides. The powers of 

 endurance in some coral zoophytes cannot surprise us. for it is 

 well known that these animals are often very tenacious of life. 

 The hardier species belong mostly to the genera Porites and Po- 

 cillopora, besides the family Alcyonidae. 



The small lagoons, when shut out from the influx of the sea, 

 are often rendered too salt for growing zoophytes, in consequence 

 of evaporation, — a condition of the lagoon of Enderby's Island. 



Coral zoophytes sometimes suffer injury from being near large 

 fleshy Alcyonia, whose crowded drooping branches lying over 

 against them, destroy the polyps and mar the growing mass. 

 But Serpulas, and certain species of barnacles constituting the 

 genus Criseis, fix themselves upon the living Astraea, Millepora, 

 and other corals, and finally become imbedded by the increase of 

 the zoophyte, without producing any defacement of the surface, 

 or affecting its growth. Many of these Serpulas grow with the 

 same rapidity as the zoophyte, and finally produce a long tube, 

 which penetrates deep within the coral mass ; and, when alive, 

 they expand a large and brilliant circle or spiral of delicate rays, 

 making a gorgeous display among the coral polyps. Instinct 

 seems to guide these animals in selecting those corals which cor- 

 respond with themselves in rate of growth ; and there is in gen- 

 eral a resemblance between the markings of a Criseis and the 

 character of the radiations of the Astraea it inhabits. 



