316 HARBOUR OF PERNAMBUCO. 



The delicate, but brilliant feathery plume the 

 only portion of the animal ever visible constitutes 

 the principal mechanism by means of which the Ser- 

 pula constructs its calcareous tube. 



A most wonderful instance of how mighty are the 

 works which these insignificant creatures form when 

 congregated together in vast numbers, and how 

 useful such labours may sometimes be to mankind, 

 is narrated by Dr. Darwin in his ' Voyage of the 

 Beagle/ 



Being delayed by adverse winds, this gentleman 

 made a stay at Pernambuco, a large city on the coast 

 of Brazil, and the most curious object that he saw 

 there was the reef that formed the harbour. ' I 

 doubt/ to use his own words, ' whether in the whole 

 world any other natural structure has so artificial an 

 appearance. It runs for a length of several miles 

 in an absolutely straight line, and parallel to, and 

 not far distant from the shore. It varies in width 

 from thirty to sixty yards, and its surface is level and 

 smooth ; it is composed of obscurely stratified hard 

 sandstone. At high water the waves break over it ; 

 at low water its summit is left dry, and it might 

 then be mistaken for a breakwater erected by Cyclo- 

 pean workmen. On this coast the currents of the 

 sea tend to throw up in front of the land long spits 

 and bars of loose sand, and on one of these the town 

 of Pernambuco stands. In former times a long spit 

 of this nature seems to have become consolidated by 



