no CORAL AND ATOLLS 



tissues is incapable of carrying out any repairs, a superficial 

 injury is most likely to lead to the death, of a definite area of 

 zooids on the surface — this area representing the base of a 

 cone whose apex is at the centre of the colony. The zooids 

 over this area, if definitely destroyed in the extent of all their 

 living animal tissues, are not regenerated, but a sympathetic 

 growth takes place round the edges of the area. New 

 material is thrown out, new zooids are budded off, and the 

 dead area is finally invaded and covered from the active zooids 

 of the edge. It is due to this process of repair that many of 

 the boring molluscs become enclosed in corals, for when the 

 surface has been attacked and killed, the margins by their 

 sympathetic activity tend to bridge over the injured area and 

 enclose the mollusc, which finally comes to rest in a cavity 

 beneath the surface of the coral. 



The rounded cysts found in the substance of most speci- 

 mens of the massive corals, and which contain an encysted 

 mollusc, are therefore not to be regarded as entirely the work 

 of the mollusc, for they are due in part to sympathetic activity 

 of zooids in the coral colony. 



Besides boring molluscs, several species of worms attack 

 corals and hollow out tunnels this way and that through their 

 living substance, and here, too, the sympathetic reaction of 

 the zooids is shown. The tunnels destroy whole groups of 

 zooids and replace the solid skeleton of the coral by a series 

 of tubes, and the strength of the whole colony is greatly 

 diminished. Around these tubes the uninjured zooids divide 

 and grow with increased activity, new calcium carbonate is 

 thrown out, and an attempt is made to compensate for the 

 destruction of tissue caused by the worm. Some curious re- 

 sults are brought about in this way, and specimens in which 

 worm-boring has led to fantastic growth are always to be 

 found, for few corals escape their inroads. A tunnel running 

 superficially, or in the thickness of a plate-like growth, leads 

 to an increased activity of surface-growth of the zooids, and 

 the tunnel stands out boldly from the surface, covered and 

 strengthened by an ever-advancing layer of coral. In this 



