THE HYDRA AND ITS ALLIES 217 



Coral reefs are almost exclusively the product of coral 

 polyps modified by the environmental conditions. The 

 reef -building corals live in shalloAV water from loAv-water 

 mark to a depth of one hundred feet only. Their dis- 

 tribution along the coast line depends upon the winter 

 temperature of the sea, since they cannot live at a tem- 

 perature below 20 C. ; consequently reef -building corals 

 are confined to warm latitudes. However, tropical shores 

 which are washed by arctic currents, such as the west 

 coasts of Africa and South America, are destitute of coral 

 formations. On the other hand, shores in the temperate 

 zone that are washed by tropical currents, such as our 

 Florida coast which is washed by the Gulf Stream, may be 

 rich in coral reefs. Corals demand undiluted sea-water, 

 hence they do not thrive in harbors which receive the 

 waters of great rivers a circumstance of great impor- 

 tance for the commerce of tropical countries. Moreover, 

 corals require a rock bottom on which to build, and they 

 cannot gain a foothold on shores where the cliffs descend 

 precipitously to great depths. Coral reefs receive dif- 

 ferent names according to their varying relations to the 

 shore. Fringing reefs are found close to the shore line. 

 Barrier reefs lie at some distance from land, with a body 

 of quiet water between them and the shore. An Atoll is 

 a further step, in which a small island, formerly sur- 

 rounded by a barrier reef, has disappeared, leaving a cir- 

 cular reef surrounding a body of water (Fig. 205). 

 Exactly how the central land disappears, whether by sub- 

 sidence of the sea floor as the reef grows up or by being 

 washed away, is still a matter of dispute. 



Budding and the Formation of Colonies. The Ciiidaria 

 are one of three groups of animals which have the habit of 



