388 THE FOEMS OF TISSUES [ch. 



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 converge 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 

 Fig. 173. Heterophyllia angu- chamber comply with our fundamental 

 lata. (After Nicholson.) j,^|g according to which three lines and 

 no more meet in a point, and from 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 comphcated, and 

 involves problems of space-partitioning identical with those which 

 we are now studying. In the genus Heterophylha and in a few 

 alUed forms we have such conditions, and students of the Coelen- 

 terata have found them very puzzling. McCoy*, their first 

 discoverer, pronounced these corals to be "totally unlike" any 

 other group, recent or fossil; and Professor Martin Duncan, 

 writing a memoir on Heterophyllia and its allies f, described them 

 as "paradoxical in their anatomy." 



The simplest or youngest Heterophylliae known have six septa 

 (as in Fig. 174, 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 



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

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



