70 COELENTERATA—CNIDARIA 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 granulated; 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 culice 
(Kelch, calyx). 
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 (cavinae). 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 (4thecalia) the development 
of synapticula 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, are 
partitioned off by numerous horizontal or 
oblique calcareous plates which bridge over 
the interseptal spaces. These structures are 
known as dissepiments (Traversen) and tabulue 
(Biden); the tabulae are often nothing but 

ee a ea highly developed dissepiments, being dis- 
Lithostrotion Be A, ic eran a 
Marini. Ry ant Caryophyllia cyathus, tnguished from the latter merely by the fact 
Bdindl seo. oagitudivaltr: pat cha that they extend across between the septa at 
tion showing mella in the centre, sur- the same level; sometimes they are perfectly 
eee rounded by a cycle of pali. 1 \i7ontal, sometimes they are arched or 
: y 
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 
(Sdulchen, Ave) 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. 
103), 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 (Pfihlchen, 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 
