THE SEA-ANEMONE AND SOME ALLIES 261 



through it food enters and unused particles are discharged. 

 Nettling-capsules, which are the organs of offense and defense, 

 are at the surface of the tentacles and on the mesenterial 

 filaments. 



Coral Islands. Ever since Charles Darwin from 1831 to 

 1836 made his famous journey around the world in H.M.S. 

 Beagle, scientific explorers have engaged from time to time 

 in the study of the life and structure of coral reefs and 

 islands. We know from their researches that coral reefs are 

 formed of great masses of carbonate of lime, largely as the 

 result of the secretion of that substance by polyps. 



Just as with other organisms, reef-forming polyps flour- 

 ish wherever they become adapted to external conditions. 

 The food they need is probably abundant anywhere in the 

 seas, but they cannot endure the temperature common to 

 regions outside the torrid zone. They can live in situations 

 where the tide recedes from them for two or three hours, 

 or in depths as great as fifty fathoms (300 feet), but the most 

 general limit of depth for the greatest number of species is 

 about twenty fathoms. A condition which checks their dis- 

 tribution near large bodies of land is the presence of silt, as 

 in the fresh water that flows from the mouths of rivers. 

 Fresh water itself when free from impurities is not especially 

 detrimental to the growth of coral polyps. The most famous 

 coral formations are found in the region of the Bahama Islands, 

 in the Great Barrier Reef of Australia, in the Fiji Islands of 

 the South Pacific Ocean, and in the Maldive Islands of the 

 Indian Ocean. 



When conditions permit, young coral polyps attach them- 

 selves to the sea-bottom near a body of land, and by the 

 process of secreting carbonate of lime, as described for 

 Astrangia, extend upward toward the surface in the form of 

 a long, narrow ridge skirting the land. When the ridge is 

 so near the land that it leaves no channel, it is called a 



