THE WONDERS OF A] AX REEF 313 



I " 



There are several of the Areas, typified by the 



"Noah's ark," and all are attached to dead coral 

 masses or other hard objects by a "byssus," a 

 set of strong threads issuing from the foot of the 

 animal and securely fastened to its anchorage. 

 They are difficult to detect because they are almost 

 always encrusted with algae, hydrozoa, nullipores, 

 or calcareous matter. There are three or four 

 species of Lima with attractive white bivalve shells 

 and an inside mantle border of very brilliant 

 scarlet filaments, most gorgeous objects when ex- 

 posed to view. They build for themselves nests 

 of shell fragments, bits of coral and seaweed, so 

 cunningly constructed that their enemies search- 

 ing for them but rarely get them. There are 

 three handsomely colored "micramocks" (Cypraaea 

 spp.) that hide under the rocks and dead coral 

 slabs and so manage to maintain a dark back- 

 ground against which their dark-colored mantles 

 scarcely show. The Purpuras live on the reef 

 rocks, even those occasionally exposed at low tide, 

 but their pretty shells are most effectively con- 

 cealed with confervoid growths. 



As soon as a growing reef reaches the level of 

 low tide the continual hammering of the breakers, 



