ACTINOZOA. 



249 



this. The " columella " is the general name given to certain axial 

 structures which are commonly developed from the centre of the 

 base of the visceral chamber. The structure of the columella 

 varies in different cases, but it extends, typically, from the floor 

 of the visceral chamber to the bottom of the calice, into which 

 it projects for a greater or less distance ; and the principal septa are 

 often more or less closely connected with it. In some cases, the 

 columella is a solid calcareous rod, or it may be made up of reticu- 

 lated calcareous tissue (fig. 128, a) or of twisted calcareous fibres, 

 while in other instances its structure is even more complicated. 



Theoretically, the " interseptal loculi " are vacant spaces or verti- 

 cal 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 usually more or less interfered with by the development of one or 

 more of the structures known as " synapticula," " dissepiments," and 

 " tabulae." The " synapticula " are transverse calcareous bars which 

 stretch across the interseptal loculi, perforating the mesenteries, and 



illll 

 III f $ tf l ; /i 



Fig. 129. — a, Portion of the corallum of Favosites favosa, of the natural size; B, Portion of 

 four corallites of Favosites Gothlandica, enlarged, showing the tabulae. 



form a sort of trellis-work uniting the faces of adjacent septa. They 

 are specially characteristic of the Fimgidce. The structures known 

 as " dissepiments " are present in the majority of corals, and have 

 the form of incomplete, oblique or 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 developed (fig. 129, b), they are 

 transverse plates, which extend completely across the visceral cham- 



