Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/411

CORALS 381 As there are only two living genera which agree with the Rugosa in the tetrameral arrangement of their septa, and as it is doubtful whether we are justified in positively asserting on this ground that these genera really are Rugosa, this great order of Actinozoa requires to be considered simply as regards the hard skeleton or corallum which alone has been preserved to us in a fossil condition. The corallum of the Rugosa is in most essential respects identical in structure with that of the Hexacoralla, differing princi pally in the numerical law of the septa and in the common conjunction of tabulae with the septa. It is very difficult to entertain any doubt but that the corallum of the Riigosa was secreted in a manner precisely similar to that of the existing Zoantharia sclerodermata, and that it bore essentially, if not precisely, similar relations to the soft parts of the animal which produced it. Thus, in both groups alike the corallum may be simple or compound ; in both alike the simple form of corallum consists of an outer wall or " theca," inclosing a central space or " visceral chamber," which is divided into compartments by a series of radiating lamellae, or " septa ; " in both alike the structures known as " dissepiments," " tabulte," and "columella" may be developed; in both alike the compound corallum may be regarded as essentially formed by an aggregation of " corallites," similar in their fundamental structure to the simple corallum. With these striking and substantial points of agreement there are, nevertheless, not a few respects in which the Rugosa differ from the Zoantharia sclerodermata, and these will be best discussed by briefly considering the different parts of the Rugose corallum in succession. FIG. 11. Morphology of the Rugosa. A, Fragment of Zaphrentii gigantea, Lcsueur, showing the septa (j) with the sparse dissepiments crossing the interseptal loculi, the epitheca <e), and the thin proper wall (w). B, Transverse section of Zaphrentii Gueranyeri, Edr. and II., showing the septa and dissepiments, the central area occupied solely by the tabulae, and the fossula (/) formed by the confluence of a certain number of the septa. C, Longitudinal section of the last, showing the arrangement of the tabulae. (A is after Edwards and Ilaime ; B and C are after James Thomson.) The form of the corallum, when simple, is usually more or less conical, turbinate, cono-cylindrical, or cylindroidal, but it may be discoidal (Palceodycus, Microcyclus, <fec.); or even everted (some species of Pt>jchophyllum), or some times prismatic (Goniophyllum), whilst it is often irregularly thickened by accretions of growth. The compound coralla necessarily vary much in form, being massive, fasciculate, &c., according to the method in which new corallites are produced. The principal modes in which the corallum becomes compound amongstthe RugosaarQ the following : (I) Simple calicular gemmation, In this process the original cprallite, after growing for a certain length of time, sends up a single bud from its calicine disc, which usually is continued in the same axis as that of its parent. The primitive calice may or may not be re ore or less completely obliterated by the gradual growth and extension of the epitheca over it, and the secondary bud may or may not produce a tertiary bud in the same manner in which it was itself produced. Not uncommonly several buds may bo produced successively, each from the oral disc of its predecessor, till the aged corallum comes to consist of a series of short turbinate cups or inverted cones, superimposed one upon the other, the younger upon the older. This singular mode of gemmation is seen in various species of Heliophyllum, Cystiphyllum, &c ; and it cannot possibly be regarded as being merely accidental ; whilst it may not improbably be regarded as an advanced form of the physiological process by which "accretions of growth" are produced. (2) Compound calicular gemmation. In this process, the primitive corallite throws up from its calicine disc two or more buds, which, after reaching a certain size, in most cases repeat the process. The resulting form of corallum differs in different cases. In such forma as Cyathopliyllum truncalum and C. paracida, the parent corallite is destroyed by its buds, and these are in turn destroyed by the buds which they put forth, all the corallites remaining more or less separate, and the entire corallum assuming the form of an inyerted pyramidal mass, the base of which is formed by the primitive corallite. In other cases, as in Cyathophyllum rcgium, the corallites become intimately united by their walls, and the corallum becomes massive and astrseiform. In other cases, again, the corallum becomes fasciculate, the budding corallites not being killed by their buds, but continuing to grow upwards side by side with them, as is seen in some of the species of Cyathophyllum, Lonsdaleia, Endo- phyllum, &c. (3) Parietal Gemmation. This process consists in the production of buds from the sides of the corallites at some point between the base and the margin of the calice. It does not differ in its nature or results from the same process as seen in the Zoantharia sclerodermata, and it generally gives rise to a loosely fasciculate corallum, as is well seen in various forms of Diphy- phyllum, Lithostrotion, &c. (4) Basal Gemmation. This process consists in the formation of buds by an extension of the substance of the original polype from the margin of its base ; but it is doubtful if this mode of increase occurs at all amongst the truo Rugose corals. (5) Fission. Increase by spontaneous cleavage or fission is also of rare occurrence amongst the Rugosa, though it undoubtedly occurs occasionally, as in some species of Diphy* phyllum. However they may be produced, the corallites of the compound coralla of the Rugosa are never connected together by a true ccenenchyma. When the corallites are in close contact, so that the corallum becomes massive, there is often fusion of the adjoining walls, but this is not necessarily the case. At other times the walls are wanting, and the corallites are united together by the extension and confluence of their septa (Phillipsastrcea, Syringophylhim, Smithia, &c.), or by costse and vesicular dissepiments (P achy phyllum). In other cases the corallites are united by exothecal outgrowths (as in Eridophyllum). The wall is usually well developed, and is not perforate. It is, however, often wanting altogether (as in Chonaxis, Phillipsastrcea, Smithia, &c.), or very feebly represented. In many cases also there is a more or less strongly developed accessory wall, or internal mural investment ("muraille interne"), which is placed concentrically within the true wall, and thus divides the visceral chamber into a central and a circumferential space. This accessory wall may bo present along with a well developed true wall (as in Acervularia, and Cyclophylhun), or the accessory wall alone may be present, the true wall may be absent, and the corallites may be united with one another by the amalgamation of their septa and dissepiments (as iu Pachyphyllum, Chonaxis, &c. ) The epitheca is usually well developed in the Rugosa, though sometimes very thin . It is closely applied as a rule to the true wall, and when thickened, the latter may be very feebly developed. In the compound coralla, there is often a general epitheca inclosing the corallites basally and laterally. We may, perhaps, also regard as being of an epithecal nature the extraordinary opercular struc tures which have been fully described by Lindstrom as occurring in certain Rugose corals, such as Goniophyllum, Calceola, and Rhizophyllum. In these forms the calice is closed completely by a calcareous plate or operculum which is single in Calceola, but com posed of four pieces in Goniophyllum, and which must be supposed to have been capable of erection and retraction, the corallum thus becoming like a bivalve shell. Indeed, the genus Calceola was long supposed to be referable to the Brachiopoda. It seems pro bable that some other Rugose corals, more normal in their charac ters than the above mentioned forms, were likewise provided with an operculum. Similar structures are observable in some Tabulate corals (as in certain species of Favositcs), where the calices become closed in progress of growth by a false operculum ; and analogous

structures have been recognized in certain living corals (Primnoa