ACTINOZOA. 177 



We are now in a position to understand the five 

 rules of septal development laid down by Milne 

 Edwards and Haime, to whom science is indebted 

 for most of what we yet know on the subject 

 under consideration. 



Eule I. The formation of new septa takes place 

 simultaneously in all loculi having the 

 same expression. 



Rule 2. The formation of septa takes place suc- 

 cessively in loculi having a different ex- 

 pression. 



Rule 3. The order of succession of the septa is 

 determined in the first place by the age of 

 the cycle to which they belong, and those 

 of a new cycle do not commence to be 

 formed till the development of the pre- 

 ceding cycle is complete. 



Rule 4. Among loculi of the same cycle having 

 different expressions precedence in the for- 

 mation of new septa is determined by the 

 inferiority of the sum of the two terms of 

 this expression. 



Thus, if septa of the 6th order were to be de- 

 veloped in the corallite indicated by the above 

 diagram, we should expect them to appear in the 

 chambers 1+4 and 4+1. 



Rule 5. Among loculi of the same cycle having 

 different expressions, but which yield the 

 same sum by the addition of the two terms 

 of each expression, the order of appearance 

 of the septa is determined by the relations 

 which exist between the lowest terms of 

 these expressions, the new septa being 

 formed first where the lowest term occurs. 

 N 



