The cavities both of the calices and coenenchymal tubes of Heliopora are closed below by horizontal partitions or tabulae, hence the genus was formerly included in the group Tabulata, and was supposed to belong to the madreporarian corals, both because of its lamellar skeleton, which resembles that of a Ma.drepore, and because each calicle has from twelve to fifteen radial partitions or septa projecting into its cavity.
(4) The columella, a structure which occupies the centre of the calicle, and may arise from the basal plate, when it is called essential, or may be formed by union of trabecular offsets of the septa, when it is called unessential.
The whole system of dissepiments in any given calicle is often called endotheca.
Tabulae are stout horizontal partitions traversing the centre of the calicle and dividing it into as many superimposed chambers.
When fully extended, the upper part of the zooid projects for some distance out of the calicle, and its wall is reflected for some distance over the lip of the latter, forming a fold of soft tissue extending to a greater or less distance over the theca, and containing in most cases a cavity continuous over the lip of the calicle with the coelenteron.
E, Section through a dividing calicle of Mussa, showing the union of two septa in the plane of division, and the origin of new septa at right angles to them.
(C original; the rest after von Koch.) aporose corals, the only communication between the cavity of the edge-zone and the general cavity of the zooid is by way of the lip of the calicle; in the latter, or perforate corals, the theca is permeated by numerous branching and anastomosing canals lined by endoderm, which place the cavity of the edge-zone in communication with the general cavity of the zooid.
After division the corallites continue to grow upwards, and their zooids may remain united by a bridge of soft tissue or coenosarc. But in some cases, as they grow farther apart, this continuity is broken, each corallite has its own edge-zone, and internal continuity is also broken by the formation of dissepiments within each calicle, all organic connexion between the two zooids being eventually lost.
Thus in the family Stauridae there are four chief septa whose inner ends unite in the middle of the calicle to form a false columella, and in the Zaphrentidae there are many instances of an arrangement, such as that depicted in fig.
In this coral the calicle is divided into quadrants by four principal septa, the main septum, counter septum, and two alar septa.
- Solitary corals with rudimentary septa, and the calicle filled with vesicular endotheca.