CLASS I 



ANTHOZOA 



77 



The interstices between the sclerites forming the septa are either com- 

 pletely filled with carbonate of lime (Aporosa), or there remain larger or 

 smaller porous spaces (Perforata) ; in many cases, in fact, the septa are 

 represented by a loose network of sclerites piled up vertically, or merely by 

 vertically directed spines. 



The number of septa and of tentacles is either equal to the number of 

 pairs of mesenteries (when only entocoelic septa are present), or double that 

 of the pairs of mesenteries (when both exocoelic and entocoelic septa and 

 tentacles are present), and is somewhat uniform throughout species, genera 

 and higher groups. The number, Avidth and mode of formation of the 

 septa furnish important systematic characters. As the locus of the origin of 

 the septa succeeding the primaries may conform to one of several plans, this 

 character is used in determining the major groups. The upper edges of the 

 septa are sometimes smooth, sometimes 

 serrated or granulated ; and they extend 

 from the central depression to or through 

 the walls of the theca, either obliquely or 

 in a curved line. This open, central depres- 

 sion, formed by the superior edges of the 

 septa, is known as the calice or calyx. 



The sides of the septa are rarely smooth, 

 but are commonly granulated or furnished 

 with rows of small prominences ; occasion- 

 ally they are provided with well-marked 

 vertical cross-bars (carinae). When the 

 projections on the sides of the septa are 

 in the form of conical or cylindrical trans- 

 verse bars, they are termed sijnapticulae. 

 Frequently the synapticulae of two adjacent 

 septa become joined together; sometimes 

 whole rows of them are fused together 

 to form perpendicular bars, thus greatly 

 strengthening the septal framework. In 

 some corals (athecalia) the development of tabulae 

 synapticulae is such as to render an outer 



wall superfluous. With the upward growth of the polyp, the theca gradually 

 becomes elevated, and its lower portions, as their occupation by the soft parts 

 ceases, may be partitioned off by numerous horizontal or oblique calcareous 

 plates which bridge over the interseptal spaces. These structures are known as 

 dissepiments and tabulae. The tabulae are often nothing but highly developed 

 dissepiments, being distinguished from the latter merely by the fact that they 

 extend across between the septa at the same level ; sometimes they are 

 perfectly horizontal, sometimes they are arched or funnel-shaped (Fig. 98), 

 and sometimes incomplete. Dissepiments and tabulae are most strongly 

 developed in cylindrical forms, and frequently fill the included space within 

 the theca with a vesicular or cellular tissue. 



When a number or when all of the septa are produced as far as the centre 

 of the calice, their inner edges may become twisted so as to form an axial 

 structure, known as a pseudocolumella. Sometimes, however, a true columella 

 is present ; this may be either a compact, styliform or foliaceous structure, or 



Fig. 98. 



Lithostrotion 

 wnrtini B. and H. 

 Longitudinal sec- 

 tion showing 



Fio. 99. 



CixryophylUa cijathns Sol. 

 Corallum split open longi- 

 tudinally ; true columella 

 in the centre, surrounded 

 by a cycle of pali. 



