5 i8 CCELENTERATES. 



animal life, in passing at death into their elementary constituents, leave no visible 

 traces of their existence, the corals, or at least the numerous reef-making forms, 

 build monuments which last for hundreds of thousands of years, and may be 

 said to attain their greatest importance in the influence they exercise upon the 

 life and development of the human race. Professor Haeckel has . described the 

 magical effect of a first sight of a shallow coral-reef, enjoyed by him on the coasts 

 of the Red Sea, where a long bank of coral runs parallel to the shore. The 

 waves break upon these barrier-reefs, the uneven edges of w^hich lie just below 

 the surface of the water, and their position is thus clearly marked by the line of 

 surf produced. The outer side of the reef, which is exposed to the full force of 

 the waves, descends steeply, but the inner sicUj, washed by comparatively quiet 

 water, slopes gently down ; the canal formed between the reef and the coast being 

 as a rule so shallow and calm that the full splendour of the garden of corals at 

 its bottom can be seen through the limpid water. 



All reef-forming- corals inhabit waters which in winter do not sink below 



o 



a temperature of 68 F., the maximum summer heat in the Pacific Ocean 

 being 86. Two lines to the north and the south of the Equator, which would 

 connect points where the winter - temperature does not sink below 68, waving 

 in and out according to the currents, would enclose the zone of the reef-forming 

 corals. Most of the stock - forming corals described above live exclusively 

 within these limits of temperature, a fact that explains their rare occurrence in 

 the Mediterranean, which is so favourable to other forms of animal life. The 

 richest coral regions lie in the middle hottest zone, that is between 15 and 18 

 north and south of the Equator, where the temperature does not fall below 

 72 F. The Fiji Islands fall within this region, and possess reefs extraordinarily 

 rich in corals. The star-corals and brain-corals there reach their greatest develop- 

 ment, while the madrepores are found as bushes, cups, or leaves, the latter often 

 attaining a breadth of over six feet. In the Sandwich Islands, which He outside 

 of the hottest zone, the corals are less luxuriant and varied. The genera of corals 

 found in the Indian Ocean and the Red Sea, as well as on the coast of Zanzibar, 

 are essentially the same as those in the Pacific. The corals of the Gulf of Panama, 

 although not in the hottest zone, have the character of the Pacific corals, and are 

 different from those of the West Indies. 



When the two Fosters and Cook discovered the coral islands of the South 

 Sea, they were of opinion that the minute creatures to which these owe their 

 origin began to build at unfathomable depths, gradually bringing their structures 

 up to the surface of the water. They thus thought that the same species were 

 able to live at different depths. Recent researches have disproved this ; and we 

 now know that although many different animals live at enormous depths, all 

 such are specially adapted to the conditions of life at those depths. Animals 

 adapted to life at a great depth cannot exist at the surface. The number of 

 deep-sea polyps is small, and among them there are no species forming reefs ; 

 and authorities are now agreed that reef -building corals can only live at moderate 

 depths and within certain latitudes. One of the principal requirements is pure 

 sea- water, some species flourishing in the canals between the reefs and in the 

 shallower water of the lagoons, whereas others require the open sea. Corals 



