92 DYNAMICAL GEOLOGY. 



There are also peculiar points of scientific interest. To 

 the geologist they are of the extremest interest 1. As 

 agents producing immense accumulations of limestone. 

 2. As evidences of crust movements on a magnificent 

 scale. These points will be brought out as we proceed. 



It is a common idea an idea which has passed into 

 popular literature, and is difficult to eradicate that 

 corals and coral reefs, like the hills and galleries of ants, 

 are built slowly by the cooperative labor of millions of 

 little insects. It becomes necessary, therefore, to explain 

 somewhat fully the manner in which a reef is really 

 formed. 



A Simple Polyp. Fig. 44 represents an ordinary soft 



FIG. 44. Simplified figure of an actinia. 



polyp (Actinia sea-anemone), somewhat simplified, such 

 as may be seen clinging to rocks or piers on our sea-shores 

 almost anywhere. Their structure is diagrammatically 

 shown in section (Fig. 45). As seen by these figures the 

 creature may be compared to a hollow, fleshy cylinder, 

 closed at both ends like a yeast-powder can. The lower 

 may be called the /oo^-disk, the upper the mouth-disk. 

 The edge of the mouth-disk is surrounded by hollow ten- 

 tacles, t t, which open into the hollow cylinder. In the 

 center of the mouth-disk is the mouth, m, and below it 

 hangs the stomach, s, reaching about half-way down. At 



