CCELENTERATA : ZOANTHARIA. l6l 



senteries, to which, therefore, they correspond in number and 

 size. Like the mesenteries, the septa are thus "primary," 

 ''secondary," and "tertiary," according to their width (fig. 75). 



Fig- 75- Diagram of the arrangement of the septa in the Zoantharia sclerodermata 

 and Rrtgosa. A, Transverse section of a simple sclerodermic Coral (Turbinolici), 

 showing the theca, with its projecting ridges or " costae" outside, the visceral cham- 

 ber and radiating septa inside, and the columella in the centre. B, Transverse sec- 

 tion of a simple Rugose Coral (Cyathophyllum), showing the wall, costae, and septa. 



The septa in the adult sclerodermic corallum are typically some 

 multiple of six in number, arranged in six systems ; but this 

 rule is not of universal application, and the typical hexameral 

 arrangement may be departed from altogether. The laws of 

 development of the septa are complicated, and need not be 

 discussed here. On the outside of the theca are vertical ridges 

 ("costae"), corresponding with the septa within; and the 

 centre of the visceral chamber may be vacant, or may be 

 occupied by an axial rod-like structure, which is termed the 

 " columella." The continuity of the " interseptal loculi " is 

 liable to be more or less interfered with by the development 

 of the structures known as " synapticulce? " dissepiments," and 

 " tabulae." The " synapticulae " are transverse calcareous bars 

 which stretch across the interseptal loculi, and form a kind of 

 trellis-work, uniting the opposite faces of adjacent septa. They 

 are characteristic of the Fungida. The "dissepiments" are 

 commonly present in a great many corals, and have the form 

 of incomplete, approximately horizontal plates, which stretch 

 between adjacent septa, and break up the interseptal loculi 

 into secondary compartments or cells. Lastly, the " tabulae " 

 may be regarded as highly developed dissepiments, and, like 

 them, are approximately horizontal, as a rule, at any rate. 

 They differ from the dissepiments in the fact that they cut 

 across the interseptal loculi at the same level. When fully 



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