210 J. /S. Gardiner — Origin of Coral Beefs. 



carbonate of lime is laid down by the corals and other marine 

 animals, but this at once, except in particularly favorable, i. e. 

 quite open situations, is very rapidly acted upon by various 

 organisms and reduced to the finest mud. In the first place 

 boring algae of the genus Achyla penetrate the lime skeletons 

 in every direction, extending ahnost up to touch their living 

 tissues. They are soon followed by boring sponges, the rami- 

 fications of some of which are as fine as those of Achyla 

 while the growths of others hollow out cavities of a square 

 cm. or so in the skeletons. A way is paved by these for vari- 

 ous Polychaeta, of which w^orms of the family Eunicidae are 

 by far the most numerous both in numbers and species. Again, 

 these are succeeded by sipunculids such as Asjndosiphoii, 

 BhasGolosoma^ etc., while numerous Crustacea and Polychaeta 

 take up their abode in any holes. LitTiodomus, Lithotrya and 

 other forms complete the destruction, and the largest coral 

 skeleton crumbles down into small fragments. But then a 

 fresh class of organisms, which feed on such organic matter as 

 coarse sand or small rubble may retain, comes into play and, 

 wdiile often keeping the coarser fragments in their guts for 

 long periods of time, finally pass the whole out in the form of 

 fine mud. Of this class of animal, by far the most important 

 on account of their great abundance in every position, are the 

 Holothurians. Sipunculids, various Echinids and Thalassema 

 are often very numerous, while Ptychodera is abundant every- 

 where but only dwells where there is a considerable thickness 

 of sand. 



As already remarked, there is an ample circulation of water 

 within the lagoons of most of the Maldive atolls and banks. 

 The encircling reefs are seldom crowned for more than the 

 half of their length by land, and the passages into the lagoons 

 are generally deep. It is only in a few" protected situations, 

 where the depth is as great as 40 fathoms or more, that the 

 lagoon bottom appears not to be churned up by the currents 

 and waves. In heavy weather the lagoon water is almost 

 milky, and floating surface nets are almost useless on account 

 of the enormous amount of mud in suspension. The total 

 amount of mud that passes out of the lagoon in the water is 

 enormous, and must be of wide reaching importance. On the 

 other hand the destructive effects outside the atolls within the 

 ssflme limits, i. e. down to 40 fathoms, are relativelj^ small, and 

 the vast quantity of coral mud, that covers 400,000 square 

 miles of the Indian Ocean as compared with the 25,000 square 

 miles occupied by the reefs and banks, is almost entirely 

 formed by the fine material which is in this way carried out of 

 the lagoons and off the coral banks. Solution is probably 

 more constant than this action, but the tw^o combined undoubt- 



