72 CORALS AND CORAL LSLAXDS. 



But this death is not in progress alone at the base of the 

 cokimn or branch. Generally the w/iole ititerior of a corallum 

 is dead, a result of the same process with that just explained. 

 Thus, a Madrepora, although the branch may be an inch in 

 diameter, is alive only to the depth of a line or two, the grow- 

 ing polyps of the surfoce having progressively died at the 

 lower or inner extremity as they increased outw^ard. 



The large domes of Astrseas, which have been stated to 

 attain sometimes a diameter of ten or fifteen feet, and are alive 

 over the whole surface, owing to a symmetrical and unlimited 

 mode of budding, are nothing but lifeless coral throughout the 

 interior. Could the living portion be separated, it would form 

 a hemispherical shell of polyps, in most species about half 

 an inch thick. In some Porites of the same size, the whole 

 mass is Hfeless, excepting the exterior for a sixth of an inch 

 in depth. 



With such a mode of increase, there is no necessary limit 

 to the growth of zoophytes. The rising column may increase 

 upward indefinitely, until it reaches the surface of the sea, and 

 then death will ensue* simply from exposure, and not from any 

 failure in its powers of fife. The huge domes may enlarge till 

 the exposure just mentioned causes the death of the summit, 

 and leaves only the sides to grow, and these may still widen, 

 it may be indefinitely. Moreover, it is evident that if the 

 land supporting the coral domes and trees were gradually sink- 

 ing, the upward increase might go on without limit. 



In the following of death after life "aequo pede," there is 

 obedience to the universal law. And yet the polyps, through 

 this ever yielding a little by piecemeal, seem to get the better 

 of the law, and in some instances secure for themselves almost 

 perpetual youth, or at least a very great age. Of the polyps 

 over an Astrsea hemisphere, none ever die as long as the dome 

 is in a condition of growth ; and the first budding individual, 

 or at least its mouth and stomach, is among the tens of thou- 

 sands that constitute the living exterior of the dome of fifteen 

 feet diameter. In the Madrepore, the terminal parent-polyp 

 of a branch grows on without being reached by the deatli- 



