1907.] AND SUPPOSED SPECIES IN CORALS. 541 



certainly be killed by sedimentation or by injury, whentliey have 

 succeeded in making their colony of sufficient size. 



The same process that leads to the normal shaping of massive 

 coral colonies may be seen on a small scale at myriad points upon 

 the surface of the growth. The actual amount of living tissue in 

 a large growing boulder is very small, for the depth to which the 

 living animal tissues of the zooid extend beneath the surface is 

 very slight, and yet the calcified portion that extends from the 

 living margin to the centre of the rock must in no way be re- 

 garded as the accumulated and dead remains of past generations ; 

 for it is in reality the skeletons of those zooids whose mouths are 

 now showing at the surface. It follows therefore, that since the 

 skeleton devoid of animal 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 i-epresenting the 

 base of a cone whose apex is at the centre of the colony (text- 

 fig. 145, p. 521). The zooids over this area, if definitely destroyed 



Adult colouj' of Porites in which the upper zooids are killed bj^ sediment. 



in the extent of all their living animal tissues, are not regenerated, 

 but a sympathetic growth takes place round the edges of the area, 

 and new material is thrown out, new zooids are budded ofi", 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 specimens 

 of the massive corals, and which contain an encysted mollusc, are 

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

 but are due 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 



