CORAL FORMATIONS. 621 



fines; there is a series of mouths along the centre of each furrow, and a border of ten- 

 tacles either side. The Fungice have the form of broad, circular, or oblong disks ; the 

 disk corresponds to a single polyp, and has a diameter in some cases of ten or twelve 

 inches. 



In the Milleporce, as stated on page 130, the animals are Acalephs, and not true 

 Polyps. 



Corals of the different groups here mentioned grow together promiscuously at different 

 depths, up to low-tide level. The largest Astrcece, Meandrince, and Porites, with many 

 Madreporce and other kinds, have been seen by the author constituting the upper part 

 of the growing reef. At Tongatabu, there were single masses of Porites, twenty-five 

 feet in diameter, along with Astrcece and Meandrirwe, ten to fifteen feet. But, while 

 these different groups do not correspond to different zones in depth, there are, without 

 doubt, species in them which belong to the deeper waters, and others to the more 

 shallow. 



The Porites, with some species of the Astrcea, Madrepora, and Pocillipora groups, con- 

 tinue to grow a little above low-tide level, equal to about one-third the height of the 

 tide, — as they will endure a temporary exposure to the sun without serious injury. 

 The Porites are an especially hardy group; for the corals suffer less from impurity or 

 silt in the waters than the species of other groups. 



The polyp-corals have the power of growing indefinitely upward, while death is going 

 on at equal rate, either at the base of the structure (as in the moss of which peat is 

 made) or through its interior, and are only stopped in upward progress by reaching the 

 surface of the water. The hemispherical Astrcece, many feet in diameter, although 

 covered throughout with living polyps, may be alive to a depth of only half or three- 

 quarters of an inch, and the huge Porites to a depth of less than a quarter of an inch: 

 that is, only a thin exterior portion of the mass is really living. 



Besides corals and shells, there are also some kinds of calcareous 

 vegetation, called Nullipores, both branching and incrusting in form, 

 which add to the accumulation. They grow well over the edge of the 

 reef, in the face of the breakers, and attain considerable thickness. 

 Even the delicate branching kinds sometimes made thick beds, as ob- 

 served by Agassiz in the Florida seas. Bryozoans add a little to the 

 material, occasionally making large massive corals. In Paleozoic time, 

 both branching and massive kinds contributed largely to limestone 

 formations. 



Action of the Waves. — The waves, especially in their heavier move- 

 ments, sweeping over the coral plantations, may be as destructive as 

 winds over forests. They tear up the corals, and, by incessant tritu- 

 ration, reduce the fragments to a great extent to sand ; and the debris 

 thus made and ever making is scattered over the bottom, or piled upon 

 the coast by the tide, or swept over the lower parts of the reef into 

 the lagoon. The corals keep growing ; and this sand and the frag- 

 ments go on accumulating : the consolidation of the material thus ac- 

 cumulated makes the ordinary reef-rock. Thus, by the help of the 

 waves, a solid reef-structure is formed from the sparsely-growing corals. 



Where the corals are protected from the waves, they grow up bodily 

 to the surface, and make a weak, open structure, instead of the solid 

 reef-rock ; or, if it be a closely-branching species, so as to be firm, it 

 still wants the compactness of the reef that has been formed amid the 

 waves. 



