GROWTH OF THE CORAL COLONY 95 



and complicated in various ways. Below, the corallites are 

 larger and are flush with the general surface, and the inter- 

 vening spaces are flat and plain. 



This condition is entirely the result of the attempt of the 

 uppermost zooids to build a silt-resisting structure. 



The corallites are smaller, and are raised from the general 

 surface in order to minimise the chance of silt dropping in 



Fig. 23. 



(a) Upper Surface and (b) Lower Surface of a Partial 

 Plate Growth, 



To show the differences in general structure. 



and choking the zooid. The intervening coral body is 

 variously sculptured and grooved for the carrying off of 

 sediment which happens to lodge upon the surface of the 

 colony. Wonderful series of modifications are formed in this 

 way. A single species will show extreme variations in the size 

 of its corallites, and in their raising from the general surface, 

 when specimens collected from different habitats are compared. 

 The size of the corallite and its projection from the surface 

 are therefore not true specific features ; for corals of identical 

 species, from sediment-carrying water and from absolutely sedi- 

 ment-free water, exhibit great modifications of these characters. 



