GROWTH OF THE CORAL COLONY 85 



As a rule, coral zooids and coral colonies tend to grow 

 upwards, and the general form of vegetative growth depends 

 on this fact. 



To this rule there are two noteworthy exceptions in this 

 atoll, and these corals (Ccenopsammia Willeyi and C. nigresccns) 



Fig. 22. 



Colony or Ccenopsammia Willeyi 

 Growing from the lower surface of a boulder. 



generally grow with their zooid mouths turned downwards. 

 Now I think these exceptions to be not without interest, for 

 both these corals live in dark places — they prefer, in fact, the 

 under sides of boulders, and they possess no symbiotic algse in 

 their tissues. They are coloured respectively red and black ; 

 and I would put it forward as a speculative idea, that it is the 

 innate tendency of the symbiotic algse to grow upwards, rather 

 than any innate property of the corals themselves, that causes 

 the general growth tendency of the corals. 



It is true that the zooids on the lower surfaces of plate- 

 forms grow downwards : but as a rule I believe those horizontal 



