JirURA 



277 



the coast-line depends upon the winter temperature of the sea, 

 since they cannot hve at a temperature below 20° C. Conse- 

 quently reef-buikling corals are confined to warm latitudes. 

 However, tropical shores which are washed Ijy arctic currents, 

 such as the west coasts of Africa and South America, are des- 

 titute 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. Cor- 

 als demand undiluted sea-water, hence 

 they do not thrive in harbors which re- 

 ceive the waters of great rivers — a cir- 

 cumstance of great importance for the 

 commerce of tropical countries. More- 

 over, corals require a rock bottom on Fig. 267. — a sea-walnut, 



,.,,.,, , , . seen from the broad side 



which to bmld, and they cannot gam a (uiyiia). Half nat. size, 

 foothold on shores where the cliffs de- "• ^nal opening ; h, lat- 



eral tube ; c, cu'cular 



scend precipitously to great depths. tube; d,e,f,c, h, rows of 

 Coral reefs receive different names ac- 'P'"^'^^''- Afte'--^s=»^«i^- 

 cording to their varying relations to the shore. Fringing reefs 

 are found close to the shore-line. Barrier reefs lie at some dis- 

 tance from land, mth a body of quiet water between them and 

 the shore. An Atoll is a further step, in which a small island, 

 formerly surrounded by a barrier reef, has disappeared, leaving 

 a circular reef surrounding a body of water (Fig. 268). Exactly 

 how the central land disappears, whether by subsidence 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 nettle ani- 

 mals, as we have seen, are one of the groups of animals that 



