VIII] OF THE SEPTA OF CORALS 621 



simple or even simpler conditions, for the radiating calcified 

 partitions either converge upon a central chamber, or fail to meet 

 it and end freely. But in a few cases, the partitions or "septa" 

 converge to meet one another, there being no central chamber on 

 which they may impinge; and here the manner in which contact 

 is effected becomes comphcated, and involves problems identical 

 with those which we are now studying. 



In the great majority of corals we have as simple or even simpler 

 conditions than those of Alcyonium; for as a rule the calcified 

 partitions or septa of the coral either con- 

 verge upon a central chamber (or central 

 "columella"), or else fail to meet it and end 

 freely. In the latter case the problem of 

 space-partitioning does not arise; in the 

 former, however numerous the septa be, 

 their separate contacts with the wall of the 

 central chamber comply with our funda- 

 mental rule according to which three lines ^. ^^ ^^ , „. 



^ . Fig. 270. Heterophylha angu- 



and no more meet in a pomt, and from ^^j^. After Nicholson., 

 this simple and symmetrical arrangement 



there is little tendency to variation. But in a few cases, the septal 

 partitions converge to meet one another, there being no central 

 chamber on which they may impinge ; and here the manner in which 

 contact is effected becomes complicated, and involves problems of 

 space-partitioning identical with those which we are now studying. 

 In the genus Heterophyllia and in a few alhed forms we have such 

 conditions, and students of the Coelenterata have found them very 

 puzzling. McCoy*, their first discoverer, pronounced these corals 

 to be "totally unhke" any other group, recent or fossil; and 

 Professor Martin Duncan, writing a memoir on Heterophyllia and 

 its allies I, described them as "paradoxical in their anatomy." 



The simplest or youngest Heterophylliae known have six septa 

 (as in Fig. 271, A); in the case figured, four of these septa are 

 conjoined two and two, thus forming the usual triple junctions 

 together with their intermediate partition-walls; and in the case 

 of the other two we may fairly assume that their proper and original 



* Ann. Mag. N.H. (2), iii, p. 126, 1849. 

 t Phil. Trans. cLvii, pp. 643-656, 1867. 



