Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/403

Rh CORALS 373 projecting upwards into the latter, and having the primary septa usually closely connected with it. The continuity of the interseptal loculi is often more or less broken up by the development of incomplete more or less horizontal plates, the &quot; dissepiments,&quot; which stretch from one septum to another ; or the septa may be connected by numerous deli cate cross-bars (&quot; synapticulse ). Fio. 4. Caryophyllia lorcalis, Fleming, a simple sclerodermatous coral, twice the natural size. (After Sir Wyville Thomson.) The above expresses the general features of the structure of a simple sclerodermic corallum, and it is easy to see that this structure owes its peculiarities to the fact that it has been produced by the calcification of the lower portion of a polype similar in its anatomy to an ordinary Sea- Anemone. Thus, the &quot; theca &quot; of the corallum corresponds to the column-wall of the polype, in the interior of which it is secreted. The &quot; septa,&quot; again, are developed within the mesenteries of the living animal, with which they corre spond, and, like the mesenteries, they are &quot; primary,&quot; &quot;secondary,&quot; or &quot; tertiary,&quot; according as they reach the centre or fall short of it by a greater or less distance. It is to be recollected, however, that it is only the inferior por tion of the polype which is thus hardened with carbonate of lime. The tentacular disc and mouth are placed at a level higher than the upper margin of the theca, and the digestive sac occupies the calice ; whilst the whole of the space comprised within the theca is lined by the endoderm, and its outer surface is covered by the ectoderm. FIG. 5. Astnca pallida, Dana, a compound sclerodermatous coral, in its living condition. (After Dana. ) Whilst the simple corallum is the skeleton of a single polype, the compound. Sclerodermic corallum is the aggregate skeleton composed by a colony of such polypes, and it varies in form according to the form and nature of the colony by which it is produced. Such a colony consists in general of a number of polypes united together by a com mon flesh or &quot; ccenosarc,&quot; and corresponding elements are found in the corallum. Thus a compound coral consists generally of certain portions which are secreted by the in dividual polypes of the colony, and are known as the &quot; corallites,&quot; and of a common calcareous basis or tissue, which unites the various corallites into a whole, is secreted by the ccenosarc, and is known as the &quot; coenenchyma.&quot; The latter element of a compound corallum is, however, by no means always present, the entire structure often consist ing simply of the skeletons of the individual polypes (&quot;corallites&quot;) united with one another directly and in different ways. The compound coralla are, of course, primitively simple, and they become composite either by budding or by cleavage of the original polype. The principal methods in which this increase is effected in the Zoantharia sdcrodermata are the following : 1 . Lateral Gemmation. In this method, of increase the original polype throws out buds from some point on its sides between the base and the circle of tentacles. The bud is at first simply a pro tuberance of the ectoderm and endoderm of the parent, containing in its interior a diverticulum of the somatic cavity; but a mouth and tentacles are developed at its distal extremity, mesenteries and septa appear in its interior, and it gradually assumes all the characters of the polype from which it was budded forth. Lateral or parietal gemmation generally gives rise to dendroid or arborescent coralla, as in the genera Madrepora, Dcndrophyllia, CZadocora, Oculina, LophoJielia, &c., but the precise form of the resulting colony depends on the way in which the buds are given off, regularly or irregularly, singly or in numbers together, alternately or at opposite points, and also on the continuance or arrest of the growth of the parent. In other cases, where the parietal buds are given off from the edge of the calice (&quot; marginal gemmation &quot;), the resulting coral lum may become massive by the soldering together of the separate corallites, as occurs in the genus Aslrocccnia, where the parent cor- allite continues to grow side by side with its buds. 2. JBasal Gemmation. In this mode of growth the original polype gives forth from its base a rudimentary ccenosarc from which new buds are thrown up. Sometimes the ccenosarc has the form of rootlike prolongations from which the buds are developed at intervals. More commonly, the coenosarc forms a more or less extensive horizontal expansion. The resulting form of corallum varies, being sometimes fasciculate, but more commonly massive or encrusting ; and in all cases the youngest corallites are those which occupy the circumference of the mass. Good examples of the process of basilar gemmation are to be found in FJiizangiu, Astrangia, &c. 3. Calicular Gemmation. This consists in the production of buds from the calicine disc of the parent polype, which may or may not continue to grow thereafter. This mode of increase, though known to occur in forms like Isastrcca, and some of the Monllivaltuc and Thecosmilice, is very rare amongst the Zoantharia sdcrodermata, and it may be doubted whether in certain cases it should not rather be regarded as a species of fission. Calicular gemmation, however, is seen in characteristic form amongst many Rugose corals, in treat ing of which it will be noticed at length. 4. Fission. This consists in a process of spontaneous divi sion or cleavage of the original polype into two individuals. This is usually effected by means of &quot;oral cleavage, 1 the calicine disc of the parent polype becoming divided into two portions by a groove, which gradually deepens till the original mass is converted into two halves. The proximal extremity of the parent always remains undivided, and, according to Dana, the pri mitive mouth and stomach are appropriated by one of the halves produced by the fission, whilst a new mouth and stomach are developed in the other half. The form of corallum produced by fission varies in different cases. Sometimes the corallum becomes &quot; Cespitose &quot; or tufted, consisting of a number of short diverging pairs of branches, each pair produced by the cleavage of a single corallite (e.g., Caulastrcea). In other cases the corallum becomes &quot;massive,&quot; the corallites produced by fission remaining per manently connected with one another. In other cases, again, the secondary corallites do not become perfectly separated from one another, their calices remaining more or less completely continuous, often so much so as to give rise to one long calicine groove, with a long line of septa on each side, or to an aggregation of such grooves. By this &quot;serial&quot; growth the corallum becomes &quot;gyrate&quot; or &quot; meandrine ; &quot; and excellent examples may be found in the genera Mccandrina, Diploria, Latimceandra, Rhipidoyyra, Pkylogyra, &c. Finally, it should be noticed that, though the above mentioned modes of growth may be conveniently distinguished from one