in the other quadrants the youngest septa lie nearest to the alar septa. This arrangement, however, is by no means characteristic even of the Zaphrentidae, and in the family Cyathophyllidae 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 Cyathophyllum itself a radial arrangement is the rule. The connexion between the Cyathophyllidae and modern Astraeidae is shown by Moseleya latistellata, a living reef-building 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 Amphiastraeidae 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 main- and 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.
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| Fig. 19.—Diagram of the arrangement of the septa in a Zaphrentid coral. m, Main septum; c, counter septum; t, t, alar septa. |
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, Thamnastraea) 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 fenestrae. 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 trabeculae 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. (For an account of coral formations see Coral-reefs.)
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 suborders Malacactiniae, comprising the soft-bodied Actinians, such as Actinia, Sagartia, Bunodes, &c., and the Scleractiniae, comprising the corals. The Scleractiniae 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 Stauromedusae. Moreover, there are some modern corals in which the secondary cycle of mesenteries departs from the Actinian plan. For example, J. E. 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 entocoele, 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:—
Group A.
Family 1. Zaphrentidae.—Solitary Palaeozoic corals with an epithecal wall. Septa numerous, arranged pinnately with regard to four principal septa. Tabulae present. One or more pits or fossulae present in the calicle. Typical genera—Zaphrentis, Raf. Amplexus, M. Edw. and H. Streptelasma, Hall. Omphyma, Raf.
Family 2. Turbinolidae.—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. Amphiastraeidae.—Mainly colonial, rarely solitary corals, with radial septa, but bilateral arrangement indicated by persistence of a main septum. Typical genera—Amphiastraea, Étallon. Thecidiosmilia.
Family 4. Stylinidae.—Colonial corals allied to the Amphiastraeidae, but with radially symmetrical septa arranged in cycles. Typical genera—Stylina, Lamarck (Jurassic). Convexastraea, D’Orb. (Jurassic). Isastraea, M. Edw. and H. (Jurassic). Ogilvie refers the modern genus Galaxea to this family.
Group B.
Family 5. Oculinidae.—Branching or massive aporose corals, the calices projecting above the level of a compact coenenchyme formed from the coenosarc which covers the exterior of the corallum. Typical genera—Lophohelia, M. Edw. and H. Oculina, M. Edw. and H.
Family 6. Pocilloporidae.—Colonial branching aporose corals, with small calices sunk in the coenenchyme. Tabulae present, and two larger septa, an axial and abaxial, are always present, with traces of ten smaller septa. Typical genera—Pocillopora, Lamarck. Seriatopora, Lamarck.
Family 7. Madreporidae.—Colonial branching or palmate perforate corals, with abundant trabecular coenenchyme. Theca porous; septa compact and reduced in number. Typical genera—Madrepora, Linn. Turbinaria, Oken. Montipora, Quoy and G.
Family 8. Poritidae.—Incrusting or massive colonial perforate corals; calices usually in contact by their edges, sometimes disjunct and immersed in coenenchyme. Theca and septa perforate. Typical genera—Porites, M. Edw. and H. Goniopora, Quoy and G. Rhodaraea, M. Edw. and H.
Group C.
Family 9. Cyathophyllidae.—Solitary and colonial aporose corals. Tabulae and vesicular endotheca present. Septa numerous, generally radial, seldom pinnate. Typical genera—Cyathophyllum, Goldfuss (Devonian and Carboniferous). Moseleya, Quelch (recent).
Family 10. Astraeidae.—Aporose, mainly colonial corals, massive, branching, or maeandroid. Septa radial; dissepiments present; an epitheca surrounds the base of massive or maeandroid forms, but only surrounds individual corallites in simple or branching forms. Typical genera—Goniastraea, M. Edw. and H. Heliastraea, M. Edw. and H. Maeandrina, Lam. Coeloria, M. Edw. and H. Favia, Oken.
Family 11. Fungidae.—Solitary and colonial corals, with numerous radial septa united by synapticulae. Typical genera—Lophoseris, M. Edw. and H. Thamnastraea, Le Sauvage. Leptophyllia, Reuss (Jurassic and Cretaceous). Fungia, Dana. Siderastraea, Blainv.
Group D.
Family 12. Eupsammidae.—Solitary or colonial perforate corals, branching, massive, or encrusting. Septa radial; the primary septa usually compact, the remainder perforate. Theca perforate. Synapticula present in some genera. Typical genera—Stephanophyllia, Michelin. Eupsammia, M. Edw. and H. Astroides, Blainv. Rhodopsammia, M. Edw. and H. Dendrophyllia, M. Edw. and H.
Group E.
Family 13. Cystiphyllidae.—Solitary corals with rudimentary septa, and the calicle filled with vesicular endotheca. Genera—
