CORALS. 519 



never flourish in impure water or on sandy or muddy coasts. They are not 

 found at the mouths of rivers nor in excessively salt water ; abnormal heating 

 of the water of the lagoons also may cause their death. The dead portion of the 

 stocks, and also, at times, that part which contains the living animals, suffer 

 continually from the action of boring-worms and molluscs, while the boring- 

 sponges cause still worse injuries. In addition to these foes, which tunnel 

 through and help to disintegrate the skeleton of the coral-stocks, other enemies 

 prey on the living polyps. These latter are richly provided with stinging-cells ; 

 and an unwary fish touching one with its lip is sure to be stung. Nevertheless, 

 the greediest devourers of corals are certain fish which have acquired horny 

 beaks like those of a parrot. These parrot-fish live by browsing on the living 

 flowers of the coral-garden, having jaws which are untouched by their stings. 



Certain tube-dwelling worms and cirripedes (Balanus), on the other hand, 

 penetrate the living coral without injuring it. They attach themselves to the 

 surface of a coral-stock on leaving the larval condition, and gradually become 

 embedded in the growing stock. Some Serpulidce also grow with the stock, 

 their tubes reaching far into it, and their elegant crown of gills, when unfolded, 

 adding to its beauty 



The rate of growth of various corals has been investigated by Dana and 

 others. Although Darwin doubts the statement that the copper plating of a ship 

 in the Persian Gulf was covered in twenty months with a crust of coral two 

 feet thick, the rapidity of their growth is proved by other observations. As 

 early as 1830 Allen sank a number of pieces of coral in the month of December 

 on a bank about a yard below the surface of the water on the coast of Madagascar, 

 and found, the next July, that they had almost reached the surface and had 

 attached themselves firmly. In Hayti a growth of three to five inches has 

 been observed in three months. A stock of labyrinthine brain-coral was 

 found to increase 11 J inches in diameter, and 4 inches in height in twenty 

 years. On a ship, wrecked in 1792 and discovered in 1857, a madrepore was 

 found which had reached a height of sixteen feet, i.e., had grown on an average 

 three inches yearly, whereas massive coral-stocks in its neighbourhood showed 

 slower growth. 



The foregoing facts as to the life of the corals themselves sink into 

 insignificance in comparison with the results of their mode of life in the formation 

 of coral-reefs and islands. 



Coral-reefs are banks of coral-rock in the sea along the coasts of tropical 

 countries. At high tide the reefs are usually under water, but at low tide are 

 visible as wide, flat, naked expanses of rock, just above the level of the water, in 

 marked contrast to the precipitous coasts of the islands they surround. At high 

 tide the only sign of the presence of a reef is a line of breakers, which often extends 

 for many miles at a distance from the land, a retreating wave only occasionally 

 revealing a small portion of the rock. A small island may be surrounded by such 

 a reef, the annexed illustration showing a typical tropical island thus encircled. 

 On the right side the reef forms a girdle stretching round the coast, and appears 

 like a continuation of the land. This fringing-reef is also found on the left side, 

 but beyond it, separated by a channel, is the barrier-reef. At one point the land 



