ZOANTHARIA. 183 



to the septa, present a free edge to the cavity in the axis of 

 the corallum, and arise with the septa " (Duncan). They are 

 only occasionally present. 



The septa, further, may, for all practical purposes, be re- 

 garded as being continued in the Zoantharia sclerodermata 

 through the wall, so as to become visible on the exterior. 

 The outer surface of the theca thus comes to be covered with 

 a series of vertical ridges or ribs, which are termed the 

 "costse" (figs. 70 and 71). The costse vary much as to the 

 distance by which they are separated from one another, and 

 as to their breadth, their solidity, and their ornamentation 

 with spines, tubercles, or teeth. 



Theoretically, the " interseptal loculi " are vacant spaces 

 or vertical compartments, bounded laterally by the septa, and 

 extending from the lower and lateral surfaces of the theca to 

 the floor of the calice. In practice, however, the continuity 

 of the interseptal loculi is liable to be more or less interfered 

 with by the development of the structures known as " syn- 

 apticulcc" " dissepiments," and " tabulce" The " synapticulae " 

 are transverse calcareous bars which stretch across the inter- 

 septal loculi, and form a kind of trellis- work, uniting the 

 opposite faces of adjacent septa. They are characteristic of 

 the Fungidce. The " dissepiments " are commonly present in 

 a great many corals, and have the form of incomplete, ap- 

 proximately horizontal plates, which stretch between adjacent 

 septa, and break up the interseptal loculi into secondary 

 compartments or cells. Lastly, the "tabulae" may be re- 

 garded 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 developed 

 (fig. 72), they are transverse plates, which extend completely 

 across the visceral chamber, and divide it into a series of 

 storeys placed one above the other, the only living portion of 

 the coral being above the last-formed tabula. Tabulae are 

 found in various of the Zoantharia sclerodermata, in some of 

 the Alcyonaria, and in a great many of the Rugosa. 



The above is the essential structure of the typical form of 

 a simple sclerodermic coral, and it is easy to see that it is 



