ANTHOZOA septal arrangement of Streptelasma corniculum from the lower Silurian. In this coral the calicle is divided into quadrants by four principal septa, the main septum, counter septum, and two alar septa. The remaining septa are so disposed that in the quadrants abutting on the chief septum they converge towards that septum, whilst in the other quadrants they converge towards the alar septa. The secondary septa show a regular gradation in size, and, assuming that the smallest were the most recently formed, it will be noticed that in the chief quadrants the youngest septa lie nearest to the main septum ; in the other quadrants the/youngest septa lie nearest to the alar septa. This arrangement, however, is by no means characteristic even of the Zaphw rentidse, and in the family Cyathophyllidas most of the genera exhibit a radial symmetry in which no trace of the bilateral arrangement described above is recognizable, and indeed in the genus C y athophyllum itself a radial arrangement is the rule. The conFig. 19.—Diagram of the arrangement of the septa nexion between the in a Zaphrentid coral, m, main septum; c, counter septum ; t, t, alar septa. U} atUOpny 111Ciae and modern Astreeidae is shown by Moseleya latistellata, a living reefbuilding coral from Torres Strait. The general structure of this coral leaves no doubt that it is closely allied to the Astraeidae, but in the young calicles a tetrameral symmetry is indicated by the presence of four large septa placed at right angles to one another. Again, in the family Amphiastroeidce there is commonly a single septum much larger than the rest, and it has been shown that in the young calicles, e.g., of Thecidiosmilia, two septa, corresponding to the mainand counter-septa of Streptelasma, are first formed, then two alar septa, and afterwards the remaining septa, the latter taking on a generally radial arrangement, though the original bilaterality is marked by the preponderance of the main septum. As the microscopic character of the corallum of these extinct forms agrees with that of recent corals, it may be assumed that the anatomy of the soft parts also was similar, and the tetrameral arrangement, when present, may obviously be referred to a stage when only the first two pairs of Edwardsian mesenteries were present and septa were formed in the intervals between them. Space forbids a discussion of the proposals to classify corals after the minute structure of their coralla, but it will suffice to say that it has been shown that the septa of all corals are built up of a number of curved bars called trabeculae, each of which is composed of a number of nodes. In many secondary corals (Cyclolites, Ihamnastrcea) the trabeculae are so far separate that the individual bars are easily recognizable, and each looks something like a bamboo owing to the thickening of the two ends of each node. The trabeculae are united together by these thickened internodes, and the result is a fenestrated septum, which in older septa may become solid and aporose by continual deposit of calcite in the fenestras. Each node of a trabecula may be simple—i.e., have only one centre of calcification, or may be compound. The septa of modern perforate corals are shown to have a structure nearly identical with that of the secondary forms, but the trabe-
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culae and their nodes are only apparent on microscopical examination. The aporose corals, too, have a practically identical structure, their compactness being due to the union of the trabeculae throughout their entire lengths instead of at intervals, as in the Perforata. Further, the trabeculae may be evenly spaced throughout the septum, or may be grouped together, and this feature is probably of value in estimating the affinities of corals. In the present state of our knowledge the Zoantharia, in which a primary cycle of six couples of mesenteries is (or may be inferred to be) completed by the addition of two pairs to the eight Edwardsian mesenteries, and succeeding cycles are formed in the exocoeles of the pre-existing mesenterial cycles, may be classed in an order Actiniidea, and this may be divided into the sub-orders Malacactinice, comprising the soft-bodied Actinians, such as Actinia, Sagartia, Bunodes, &c., and the Scleractinice, comprising the corals. The Scleractinise may best be divided into groups of families which appear to be most closely related to one another, but it should not be forgotten that there is great reason to believe that many if not most of the extinct corals must have differed from modern Actiniidea in mesenterial characters, and may have only possessed Edwardsian mesenteries, or even have possessed only four mesenteries, in this respect showing close affinities to the Stauromedusse. Moreover, there are some modern corals in which the secondary cycle of mesenteries departs from the Actinian plan. For example, Duerden has shown that in Porites the ordinary zooids possess only six couples of mesenteries arranged on the Actinian plan. But some zooids grow to a larger size and develop a number of additional mesenteries, which arise either in the sulcar or the sulcular entoccele, much in the same manner as in . Cerianthus. Bearing this in mind, the following arrangement may be taken to represent the most recent knowledge of coral structure :— Family 1. Zaphrentida;.—Solitary Palaeozoic corals with an epithecal wall. Septa numerous, arranged pinnately with regard to four principal septa. Tabulae present. One or more pits or fossuloe present in the calicle. Typical genera —Zaphrentis, Raf. Amplexus, M. Edw. and H. Streptelasma, Hall. Omphyma, Raf. Family 2. Turbinolii)/E.—Solitary, rarely colonial corals, with radially arranged septa and without tabulae. Typical genera —Flabellum, Lesson. Turbinolia, M. Edw. and H. Caryophyllia, Lamarck. Sphenotrochus, Moseley, &c. Family 3. Amphiastr.eid^.—Mainly colonial, rarely solitary corals, with radial septa, but bilateral arrangement indicated by persistence of a main septum. Typical genera—Amphiastroea, Etallon. Thecidiosmilia. Family 4. Stylinidje.—Colonial corals allied to the Amphiastroeidoe, but with radially symmetrical septa arranged in cycles. Typical genera—Stylina, Lamarck (Jurassic). Convexastrcea, D’Orb. (Jurassic). Isastrcea, M. Edw. and H. (Jurassic). Ogilvie refers the modern genus Galaxea to . this family. i" Family 5. Oculiniiee.—Branching or massive aporose corals, the calices projecting above the level of a compact ccenenchyme formed from the ccenosarc which covers the exterior of the corallum. Typical genera—Lophohelia, M. Edw. and H. Oculina, M. Edw. and H. Family 6. Pocilloporidas.—Colonial branching aporose corals, with small calices sunk in the ccenenchyme. Tabulae present, and two larger septa, an axial and abaxial, are always M present, with traces of ten smaller septa. Typical genera— & j Pocillopora, Lamarck. Seriatopora, Lamarck, o i Family 7. Madreporid.e.—Colonial branching or palmate (•jj perforate corals, with abundant trabecular ccenenchyme. Theca porous; septa compact and reduced in number. Typical genera — Madrepora, Linn. Turbinaria, Oken. Montipora, Quoy and G. _ . 1 Family 8. Poritidje.—Incrusting or massive colonial perforate corals ; calices usually in contact by their sometimes disjunct and immersed in coenenchyme. Iheca and septa perforate. Typical genera—Porites, M. Edw. and H. Goniopora, Quoy and G. Rhodarcxa, M. Edw. and ft.
