LIFE A ND BE A TH IN CONCUR REN T PR OGRESS, 



71 



chance for the polyp to mount upward on the coral, as it 

 lengthens it by secretions at the top. But, to be successful in 

 this ascending process, either the polyp must have the power 

 of indefinite elongation, or it must desert the lower part of the 

 corallum as growth goes forward ; and this last is what hap- 

 pens. In some instances, a polyp, but a fourth of an incli 

 long, or even shorter, is finally found at the top of a stem 

 many inches in height. The following figure represents a case 

 of this kind ; for all is dead coral, excepting less than an inch 

 at the extremity of each 

 branch. The tissues that 

 once filled the cells of the 

 rest of the corallum have 

 dried away, as increase went 

 on above. Another example 

 is shown on page 34, in 

 which the living part had 

 a length of one eighth of 

 an inch. The Goniopora, 

 on page 32, is still another 

 example of the process ; but 

 here the living part com- 

 bines a great number of 

 polyps : these are growing 

 and budding with all the 

 exuberance of life, while 

 below, the old polyps gra- 

 dually disappear, and even 

 their cells become superficial and fade out. Trees of 

 Madrepores may also have their limits — all below a certain dis- 

 tance from the summit being dead ; and this distance will differ 

 for different species. But this is not a limit to the existence of 

 the zoothome, 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. 



CAULASTR^A FURCATA, D. 



