. The Cambridge natural history. Zoology. ZOANTHARIA MADREPORARIA 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 Madre- porarian skeleton.^ The authorities upon the group a
. The Cambridge natural history. Zoology. ZOANTHARIA MADREPORARIA 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 Madre- porarian skeleton.^ The authorities upon the group are not agreed upon the use of the terms employed, nor are the current theories. Fig. 167.âSeries of diagrams to illustrate the structure of the Madreporarian skeleton. A, young stage of a solitary coral with simple protheca (). B, solitary coral, â with theca {th), epitheca (), and prototheca (). C, young stage of colonial coral, showing ooenosteum {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; </, septum ; p, pali. of the evolution of the skeleton consistent. It is necessary, however, to explain the sense in which certain terms are em- ployed 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. 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 " ; 1 For a general account of
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