THE STRUCTURE OF THE BARRIER 165 



alga has been scraped from its hold, but any living coral will 

 be found quite untouched. The part played by these fish in 

 the making of sand must be considerable, for every rock of 

 the barrier is scored by their hard beaks ; and from ten to 

 fifty or more will be in each of the many shoals which pass 



Fig. 55. 



The Ihan Kakatiia Ijou (Scarus), 

 Showing the hard beaks with which they score the coral boulders. 



along the surf-line with the turning tide. As the average 

 weight of dry sand in an individual's intestinal canal is nearly 

 two ounces, the bulk of coral rock converted into sand by 

 these creatures must be very great indeed. Other animal 

 agents in the production of sand are the Holothurians, the 

 sea-slugs or beche de mer, of many species that lie scattered 

 about all over the barrier and lagoon in countless thousands. 

 They are from three feet long to less than an inch or so, and 

 they are merely hollow animal tubes which act chiefly as sand- 

 filters. They fulfil on the barrier and in the lagoon very 

 much the same functions that the earth-worms do on land, 

 for they pass through their bodies large quantities of dead 

 material along with their food. The sand is taken in at the 



