Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/405

Rh CORALS 37,&amp;gt; &quot; secondary,&quot; and &quot; tertiary.&quot; The chief facts which it is important to know about the development and arrangement of the septa are the following, taking the generally received views on this subject. At first six septa make their appearance simultaneously. These are the primary septa, and they may not be further added to. In other cases FIG. 6. Diagram of the arrangement of the septa in the Zoantharia sclerodermata ami Rugosa. A, Transverse section of a simple scleroilemiatous corallum (TurMnolia), show- in (? the septa, costa?, and columclla. B, Transverse section of a simple Hugose coral (Cyathophyllvmj, showing the septa, wall, and costs. six additional and smaller secondary septa are next produced, one biseciing each of the six interseptal loculi between the primary septa ; and this condition may also be permanently retained (Alveopora). In other cases twelve additional septa are produced in the now existing twelve interseptal loculi, one to each loculus, and these are the tertiary septa. In a corallum in which this last state of things was permanent, we should, therefore, find twenty- four septa in all, belonging to three orders or cycles, six septa of fas first order, six of the second order, and twelve of the third order. Such a corallum has obviously twenty- four interseptal loculi, and we should imagine that the next order of septa (if developed at all) ought to consist of twenty-four septa bisecting these loculi. This is not the case, however, and any further orders of septa that may be produced are always twelve in number. If, therefore, a fourth order of septa be developed, it consists of twelve shorter septa intercalated in alternate interseptal loculi ; whilst the loculi still vacant are filled by the development of twelve additional septa of Q fifth order ; the twenty- four septa thus produced collectively constituting ike fourth cycle. The septa between each pair of primary septa con stitute a system; and in the instance just taken there are forty-eight septa in all, arranged in five orders according to the time of their development, but only constituting four cycles of equally sized septa, and forming six systems of eight septa each, Each system contains the following orders : 1st, 4th, 3d, 5th, 2d, 5th, 3d, 4th, 1st, &c. If a fifth cycle of septa be formed, then there are 96 septa, in six systems of sixteen septa each. Six cycles give 192 septa, and seven cycles produce 384 ; but it is far from common for these higher cycles to be completely developed. The rule amongst the Zoantharia sclerodermata is that the septa are arranged in six systems, and are therefore, however numerous, some multiple of six ; but this rule is not of universal application, and the typical hexameral arrangement may be departed from altogether. Thus, certain forms have the primary septa four, five, eight, or ten in number, and, therefore, have the septa of the adult arranged in a corresponding number of systems. It should be added that the researches of Lacaze-Duthiers have given rise to some doubt as to the above being truly the method in which the septa are successively developed. According to this observer the septa are developed before the wall (contrary to the received opinion), and are primitively twelve in number ; but it seems clear that though this may be the case in the species examined by the French naturalist, it cannot be true of all the Zoantharia sclerodermata. Between the internal edges of the septa and the axis of the visceral chamber there may exist a series of laminar processes to which the name of &quot; pali &quot; is given. The pali vary in number and size, and they may be developed internal to several orders of the septa, forming so many &quot; crowns.&quot; They are united by their outer edges with the inner edges of the septa, whilst their internal edges are free, or are united with the columella (if present). The axis of the visceral chamber may be vacant and unoccupied, but it is very commonly filled by the structure known as the &quot; columella.&quot; The true or &quot; essential &quot; columella is an axial rod of a lamellar, compact, or fasciculate structure, extending from the bottom of the visceral chamber to the floor of the calice, into which it projects, and formed independently of the septa. The septa may or may not be, some of them, attached by their inner ends to the columella, or there may be pali attached to it. A &quot; parietal &quot; or &quot; septal &quot; columella may be formed by the coalescence in different ways of the inner edges of the septa, which divide and inosculate so as to form a spongy or cellular central structure. In other cases a &quot; pseudo-columella &quot; may be produced by the twisting together of the inner ends of a certain number of the septa. The continuity of the interseptal loculi is liable to be more or less interfered with by the development of the endothecal structures known as the dissepiments, synap- ticulte, and tabulae. The &quot; dissepiments&quot; (&quot; traverses &quot;) are incomplete, approximately horizontal plates, which stretch between adjacent septa, and break up the interseptal loculi into secondary compartments or cells. They may be absent, or rudimentary, or they may be so greatly developed as to render a greater or less portion of the corallum completely vesicular,&quot; in the vicinity of the wall more especially. The &quot; tabulce &quot; (&quot; planchers,&quot; &quot; Boden &quot;) may be regarded as highly developed dissepiments, and like them are approximately horizontal, as a rule at any rate. They differ from the dissepiments in cutting across the interseptal loculi at the same level. When completely developed they extend right across the visceral chamber, and divide it into a series of stories placed one above the other, the only living portion of the corallum being that above the last formed tabula. They may, however, be present only in the central portion of the corallum, or they may spring from the wall, but not extend across the visceral chamber. Tabulae may exist in conjunction with well-developed septa (Alveopora], or the septa may be rudimentary or absent (Ha iy sites, Favosites). The &quot; synapticulce &quot; are transverse calcareous bars which stretch across the interseptal loculi, like a kind of trellis- work. They are formed by papillae developed on the opposite faces of adjacent septa, coalescing with one another in the middle of the interseptal loculi. In other cases they may be so greatly developed as to constitute elongated ridges between the septa. They are characteristic of the Fungidce. In compound coralla the various corallites of the colony are often united together by a common calcareous tissue, which is known as the &quot; coenenchyma,&quot; and which varies very much in texture, being sometimes loose and spongy (e.g., Madreporidce), at other times dense and compact. (e.g., Oculinidce}. In other cases the coenenchyma i- absent or rudimentary (Astrceida?, Turbinolidai), and the corallites are then united together in different ways. In some of the arborescent and fasciculate coralla the corallites are only united with one another at the points where they are budded out ; but in other cases (Syringopora, for example) they may be united by horizontal outgrowths.