3494 CCELENTERA TES 



nel 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, pene- 

 trate 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 Serpnlida 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 eleven and one-half inches in diame- 

 ter, and four 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, z. e. , had grown on an average of three inches yearly, whereas massive coral 

 stocks in its neighborhood showed slower growth. 



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

 cance 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 visi- 

 ble 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 re- 

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

 reef, the illustration, p. 3495, 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 be- 

 yond it, separated by a channel, is the barrier reef. At one point the land is seen 

 to dip down precipitously into deep water, and here, on account of the depth of 

 water, the reef is wanting. The barrier reef also is broken through at one point, 

 forming the entrance to a harbor, such as is often found in reef-surrounded islands. 

 Many islands are bordered by a reef which protects the land from the sea like a 

 mole. The barrier reef may occasionally be ten or fifteen miles from land, and in- 

 close several high islands. Various forms of reefs are found between the two ex- 

 tremes presented by such a barrier reef and the simple fringing reef. 



