The Madreporaria form a heterogeneous group of Zoantharia characterised by a single common feature, the formation of an extensive skeletal support of carbonate of lime. In a great many cases the skeleton exhibits cups or "calices" into which the zooids may be completely or partially retracted, and these calices usually exhibit a series of radially disposed vertical laminae, the "septa," corresponding with the inter-mesenteric spaces of the zooids. Calices and structures simulating septa also occur in _Heliopora_, which is an Alcyonarian, and in certain fossil corals which are probably not Zoantharians. The anatomy of the zooids of a great many Madreporaria is now known, and, {385}although a great deal of work yet remains to be done, it may be said that the Madreporaria exhibit close affinities in structure with the Actiniaria. The chief points in the anatomy of the zooids are described under the different sub-divisions, but a few words are necessary in this section to explain the principal features exhibited by the skeleton. There is no more difficult task than the attempt to explain upon any one simple plan the various peculiarities of the Madreporarian skeleton.[406] The authorities upon the group are not agreed upon the use of the terms employed, nor are the current theories of the evolution of the skeleton consistent. It is necessary, however, to explain the sense in which certain terms are employed in the systematic part that follows, and in doing so to indicate a possible line of evolution of the more complicated compound skeletons from the simple ones. [Illustration: FIG. 167.—Series of diagrams to illustrate the structure of the Madreporarian skeleton. A, young stage of a solitary coral with simple protheca (_p.t_). B, solitary coral, with theca (_th_), epitheca (_e.t_), and prototheca (_p.t_). C, young stage of colonial coral, showing coenosteum (_coe_) and theca (_th_), and the formation of the theca of a bud (_b_). D, two zooids of a more advanced stage of a colonial coral. _coe_, Coenosteum; _th_, theca. The black horizontal partitions are the tabulae. E, transverse section of a calyx. _c_, Costa; _col_, columella; _d_, dissepiment; _g_, septum; _p_, pali.] There can be no doubt whatever that the whole of the skeleton of these animals is formed by the ectoderm, and is external to their bodies. If we could get rid of the influence of tradition upon our use of popular expressions we should call this skeleton a shell. There can be little doubt, moreover, that this skeleton is formed by a single layer of specialised ectoderm cells called the "calicoblasts."