THE EIGHT-RAYED POLYPS 3493 



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

 upon these barrier reefs, the uneven edges of which 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 side, 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 splendor 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 a tem- 

 perature 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 inclose the zone of the reef-forming corals. Most of the 

 stock-forming corals just described live exclusively within these limits of tempera- 

 ture, a fact that explains their rare occurrence in the Mediterranean, which is so 

 favorable 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 development, 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 lie 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 characteristic 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 be- 

 gan 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 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 mollusks, while 

 the boring sponges cause still worse injuries. In addition to these foes, which tun- 



