242 TOILERS IN THE SEA. 



polyps below die and disappear. " Trees of Madre- 

 pores have their limits, all below a certain distance 

 from the summit being dead, and this distance will 

 differ for different species. But this is not a limit to 

 the existence of the corallum, even though a slender 

 tree or shrub, or of its flourishing state ; for the dead 

 coral below is firm rock itself, often stronger than 

 ordinary limestone or marble, and serves as an ever- 

 rising basement for the still expanding and rising 

 zoophyte." Not only will the base be dead, and the 

 apex of the trunk or branches living, but the whole 

 interior is usually dead, so that the living portion 

 does not extend inwards but the fraction of an inch. 

 In the large dome-shaped corals, sometimes 10 feet or 

 15 feet in diameter, the whole outer surface may be 

 alive, and yet the whole interior be nothing but life- 

 less coral. The tree-like species may continue to 

 increase upwards, until the apex reaches the surface 

 at low water, and then cease, for death comes from 

 exposure, and yet not wholly cease ; for even under 

 such circumstances lateral budding will still go on 

 with a modification of form. 



The dead coral trunks are not without their 

 enemies, and their great hardness does not secure 

 them wholly from danger. Agassiz states that " in- 

 numerable boring animals establish themselves in the 

 lifeless stem, piercing holes in all directions into its 

 interior, like so many augers, dissolving its solid con- 



