132 CRUISE OF THE BARRERA 



very bare of life, excepting for the many white- 

 spined sea-urchins, some large stars, and a few 

 holothurians. All of these creatures, especially 

 the latter, were busily at work triturating the 

 coarse sand by digestive action. They eat the 

 sand for the organic matter it contains subjecting 

 it to a grinding process. The amount of coarse 

 sand thus quickly reduced to finer particles is far 

 greater than one would at first suppose. No doubt 

 the feeding habits of these echinoderms constitute 

 an important factor in the breaking-down process 

 of reef from massive rock to fine mud. Eddy rows 

 of dead shells along the edges of the channels 

 gave us a fair index to the molluscan species exist- 

 ing a little farther out on the reef, although they 

 afforded but few good cabinet specimens. The 

 excellent taste shown by the short-spined sea- 

 urchins in the selection of shells for their protective 

 armament aided us very greatly, and the choicest 

 specimens were obtained by robbing the urchins 

 of them. Standing upon the edge of the deep 

 channels and basins in the flats, we could see with 

 the aid of our water-glasses quantities of large 

 fish that swam lazily about their clear limpid 

 depths. These fish were not the usual reef species, 



