58 The Medium 



current. Larvae living in quiet water build their cases of large and 

 irregular material (Fig. 2.15). With increasing strength of current 

 the cases become more and more streamlined and are built of finer 

 material. In the swiftest streams they are either completely stream- 

 lined or else done away with entirely. 



The power of autotomij exhibited by some crabs may have arisen 

 partly in response to the dangers of life in turbulent areas. When 

 the leg of a crab has been caught or crushed between moving stones 

 it can be cast off, thus setting the animal free. After autotomy has 

 taken place, regeneration commences and a new leg grows from the 

 base of the old leg. 



Emphasis has been placed in the preceding paragraphs on the 

 harmful action of fast-running water and breaking waves. There are 

 situations, however, in which extreme turbulence may be beneficial. 

 Some species of coral grow best where the pounding of the surf is 

 the heaviest, because waves remove sediment which in quiet waters 

 would accumulate on the coral polyps and tend to smother them. 

 Thus stimulated, coral colonies of this type rapidly produce "heads" 

 of coral rock at the exposed outer edge of the reef. Coral heads 

 reach such a size that they are eventually broken off by the force of 

 the waves. The form of the reef is the result of a balance between 

 the increased growth due to the turbulence of the surf and the loss 

 due to the breaking off of coral material. Here we have another il- 

 lustration of the dynamic balance represented by natural communities 

 and their environments. 



