230 



CffiLENTEBATA. 



Octactinia and Actinia are distributed in all latitudes. The polyps 

 which build banks and reefs are confined to a zone extending about 

 28 degrees on either side of the equator, and only here and there 

 extend beyond these bounds. They live for the most part near the 

 coast, and produce there in course of time rocky masses of colossal 

 extent by the accumulations of their stony calcareous frameworks. 



These masses may form coral reefs 

 (atolls, barrier reefs, fringing 

 reefs}, which are perilous to ship- 

 ping, and may also become the 

 foundations of islands. In both 

 cases a gradual alteration of 

 level, the raising of the bottom 

 of the sea, assists the work of 

 the coral animals. The presence 

 of the coral banks in the deep 

 sea is, on the other hand, due 



to a continual sinking of the 



FlG. 178. Astreea (Goniastrcea) pectinata 

 Ehrbg. (after Klunzinger). 



sea-bottom 



The part which the Anthozoa take in the alteration of the earth's 

 surface is considerable. In the present time they protect the coast 



from the consequences 

 of the breaking of 

 the waves and assist 

 in the formation of 

 islands and rocks by 

 producing immense 

 masses of calcareous 

 matter. In earlier 

 geological epochs they 

 have played a still 

 more important part 



FIG. 179. Mceandrina (Conlorut) arabica Klz. (after Klun- 

 zinger) . 



judging from the 

 great thickness of 



the coral formations of the Palaeozoic period and of the Jurassic 



formation. 



Order 1. RUGOSA = TETRACORALLA. 



Palaeozoic Corals witli mimerous symmetrically arranged septa, 

 grouped in multiples of four. 



To these belong the families of the Cyathophyllidie, Stauridce, etc. 



