70 



COELENTEEATA CXIDARIA 



SUB-BRANCH II 



a new septum is secreted. The number, width, thickness, and mode of forma- 

 tion of the septa furnish important systematic characters, usually four, six, 

 eight, or twelve principal or primary septa are first formed, between which 

 septa of the second, third, and fourth cycles and so on are successively inter- 

 calated. The upper edges of the septa are sometimes smooth, sometimes 

 serrated or uranulated ; and they extend from the central depression to the 

 walls of the theca, either obliquely or in a curved line. This open, central 

 depression, formed by the superior edges of the septa, is known as the calice 

 (AV'7/, cul '//.') 



The sides of the septa are rarely smooth, but are commonly granulated or 

 furnished with rows of small prominences ; occasionally 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 transverse bars, they are termed 



synapticula. Frequently the synapticula 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 synapticula is such as to render an oute* 

 wall superfluous. With the upward growtjji 

 of the polyp, the theca gradually becomes 

 elevated, and its lower portions, as their 

 occupation by the soft parts ceases, are' 

 partitioned off by numerous horizontal or 

 oblique calcareous plates which bridge over 

 the interseptal spaces. These structures are 

 known as dissepiments (Traversen) and tabulae 

 (Baden); the tabulae are often nothing but 

 highly developed dissepiments, being dis- 

 tinguished from the latter merely by the fact 

 sol. coraiium split open that they extend across between the s&pta at 



longitudinally ; true colu- ? 



meiia in the centre, sur- the same level ; sometimes they are perfectly 

 ofpali - horizontal, sometimes they are arched or 

 funnel-shaped (Fig. 102), and sometimes incomplete. Dissepiments and tabulae 

 are most strongly developed in cylindrical forms, and frequently fill the in- 

 cluded 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 free edges may become twisted so as to form an axial 

 structure, known as a pseudo-columella. Sometimes, however, a true columella 

 (Saulchen, Axe) is present ; this may be either a compact, styliform, or foliaceous 

 structure, or may be composed of a bundle of styliform or twisted rods (Fig. 

 10:>), or of thin lamellae. It extends from the floor of the visceral chamber to 

 the bottom of the calice, into which it projects for a greater or less distance. 

 The structures known as pali (Pfahlchen, paluli) are narrow vertical plates 

 which are inserted between the columella and the inner ends of the septa in 

 one or more cycles (Fig. 103). 



The outer wall or theca is often formed by the secretion of a particular 

 ring-like fold of the ectoderm, and is constituted of distinct sclerites, having 

 separate calcification -centres, and connecting the outer edges of the septa 



FIG. 102. 



Lithostrotion 

 Murtin.i, E. and 

 H. -Longi- 

 tudinal sec- 

 tion showing 

 tabulae. 



FIG. 103. 



cyathus, 



