174 ACTINOZOA. 



secondary chambers ; these, by the formation of 

 other intermediate septa, into tertiary chambers, 

 and so on till the development of the corallite has 

 been accomplished. The primary loculi alone 

 are complete, for the septa limiting the other 

 chambers neither extend so high, nor so closely 

 approach the columella, as do those which are 

 afterwards formed ; the size of all succeeding 

 series of septa being in direct ratio to the order 

 of their development. So that the latter may be 

 almost as clearly pronounced in an adult corallite 

 as in a collection of specimens of different ages 

 belonging to the same species. In like manner 

 those stages of the septal apparatus which are 

 transitional among the more complex corallites 

 are well illustrated by an appeal to the various 

 permanent conditions of the same system among 

 less developed representatives of the group. Thus 

 in some species of Stylophora we can count but 

 six septa, and an equal number of primary cham- 

 bers. In a much larger number of Zoantharia 

 twelve septa may be observed, of which six are 

 primary and six secondary. In others, there are 

 twenty-four septa ; six primary, six secondary, and 

 twelve tertiary. The septal formula in all these 

 types admits, therefore, of being, respectively, 

 stated thus : 



I. . . 6 =S. 6. 



II. . . 6 + 6 =8. 12. 



III. . . 6 + 6+ 12 = S. 2+ 



in S. 12 one, and in S. 24 three additional septa 

 being developed between each of the primary 

 pairs. And, since the normal number of primary 

 septa is 6 among the Zoantharia, it might be 



