Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/404

Rh 374 CORALS another, they are nevertheless not unfrequently combined in the same individual. Thus, lateral may be combined with basal gemma tion, and gemmation is commonly found accompanying fission. It is next necessary to consider the different structures which compose the sclerodermic corallum in greater detail. The general form of the corallum varies so much, that it is scarcely possible to make any statements on this subject except of an entirely general nature. The simplest corallum is most commonly cylindrical, conical, or turbinate (Caryophyllia, Turbinolia, Balanophyllia, &c.), but it may bo more or less compressed (Flabellum), discoidal (some species of Fungia), or concavo-convex. It may be rooted by its base to some foreign object, or it may be wholly free. The compound coralla are of the most varied shape. Commonly they are arborescent or dendroid (Madrepora, Dendrophyllia, Lophohelia, &c.); at other times they grow in clusters of branches springing from a common base (M ussa, Caulastrcea, and many species of Forties, Madrepora, &c.), the colony being &quot; caespitose,&quot; and convex on its distal aspect ; others are &quot; fasciculate,&quot; or composed of numerous cylindrical corallites placed parallel with one another, or slightly diverging from the base (e.g., Calamophyllia} ; others are massive and &quot; astraeiform,&quot; composed of polygonal corallites united with one another, and forming rounded, globular, hemispherical, or irregular masses (as in the typical Astrceidce) ; others are &quot; foliaceous &quot; (species of Madrepora, Pocillopora, Manopora, &c.) ; others, finally, are &quot;encrusting&quot; (e.g., some of the species of Manopora, Agaricia, &c.) The &quot; wall &quot; (&quot; muraille,&quot; &quot; eigentliche Wand &quot;) is the proper outer investment of the visceral chamber, whether we consider a simple cDrallum or take a single corallite in a compound corallum. The hard structures which are placed on the inside of the wall are the &quot; endotheca,&quot; whilst those which are developed exteriorly to the wall constitute what is collectively known as the &quot; exotheca.&quot; The condition of the wall varies greatly in different groups of the Zoantharia, being thick, compact, and impervious in some (Aporosa), and at other times more or less incomplete and pierced by larger or smaller apertures (Perforata). The surface of the wall may be smooth, or it may be marked with vertical ridges (costae), or by transverse striae or annulations of growth. Though often very distinctly recognizable, the wall may become so united with the coenenchyma as to be no longer determinable, or its place may be more or less completely taken by the epitheca. The &quot; epitheca &quot; is a secondary calcareous investment, which is very commonly developed in both simple and compound coralla, and is probably an integumentary secretion. In the simple coralla it is placed outside the proper wall, to which it may be closely applied, or from which it may be separated by the costse. It may be extremely thin, or very dense ; and in the latter case its development is generally at the expense of the wall, which becomes so thin as to be often irrecognizable. It varies also in its extent, sometimes covering only the basal region of the coral, and at other times extending to the margin of the calice. It is generally marked with concentric striae and vertical ridges, often with accretions of growth, and it may give off spines or root-like processes of attachment. In the compound corals it is not unusual to find a well- developed epitheca enclosing the entire corallum below and on the sides, whilst each individual corallite is furnished with its own wall. The external surface of the corallum often exhibits more or less prominent vertical ridges, which are known as the &quot; costae.&quot; The costae in a general way correspond with the septa in number and arrangement, and they usually appear, therefore, as so many prolongations of the septa outside the wall. It dues not appear, however, that this is due to the septa being re. illy continued through the wall so as to- form the costse, but these structures would really seem to- have an entirely independent origin. The projection of the costae from the wall is generally proportionate to the development of the septa to which they correspond, and there are great differences in different cases as to their size and distance apart. They may be ornamented with spines or tubercles, and they may be united by transverse plates (&quot; exothecal dissepiments&quot;), which run horizontally across the intercostal spaces. Sometimes the costae, instead of corresponding with the septa, are placed opposite the interseptal loculi, and are thus seen to be really independent of the septa. In compound coralla the costae are often wanting when the corallites are amalgamated by their walls. In other cases the costae are greatly developed, and serve by their coalescence to unite the various corallites into a single colony. The &quot; calice &quot; is the generally cup-shaped depression which is seen at the upper end of a corallum or of a single corallite of a compound mass, and which lodges the gastric sac of the animal in its living condition. The &quot; margin&quot; of the calice is formed by the wall, and its &quot; floor &quot; is occupied by the septa, the interseptal loculi, and the central structures of the corallum. Though in general more or less cup-shaped and depressed, the calice may be prominent, and the septa may be produced beyond it (&quot;exsert&quot;). The outline of the calice is very different in different cases, though typically more or less circular ; its depth is also extraordinarily variable ; and its plane varies much in the angle which it forms with the axis of the corallum. The &quot;s6y&amp;gt;to&quot;(&quot;cloisons,&quot; La ngsscheidewande&quot;), as previously noticed, are the vertical plates which radiate from the wall towards the centre and divide the visceral chamber into a series of vertical chambers or compartments (&quot;interseptal loculi&quot;). The septa correspond with the mesenteries of the living animal, within which they are developed, and the tentacles correspond each with an interseptal space. In their most rudimentary condition the septa appear simply as rows of spines or tubercles, but they in general present themselves as calcareous laminae, which can usually be shown to consist each of two closely apposed plates (in Dasmia of three), though often so thin and delicate as to appear single. When fully developed, a septum may be regarded as a somewhat triangular lamina, the base of which projects freely into the calice, whilst its outer margin is attached to the internal surface of the wall, and its inner margin is directed towards the centre of the- visceral chamber, extending from the bottom of the coral to the floor of the calice, and being either free or united with certain other structures to be spoken of hereafter. The free edges of the septa are usually thin, and they may be plain or ornamented with spines, serrations, or granules. The parietal or attached edges of the septa are usually the thickest, but occasionally the septum widens out in the middle or centrally. The sides of the septa, looking into the interseptal loculi, may be smooth or may be ornamented with ridges, striae, papillae, or granules. The septa may be compact and imperforate, or they may be more or less porous and cribriform. Usually the various septa are quite independent of each other, but occasionally some of the smallest and latest formed septa may become inclined towards, and coalescent with, the larger and older septa. The number of septa varies, but in none of the Zoantharia scle, odermata, in which septa are developed at all, are there found less than six septa in the adult corallum. Sometimes this number is permanently retained ; some times twelve septa are present ; most commonly the septa of the adult are over twelve in number. When there are more than six septa then their breadth varies, according as the mesenteries vary, and they thus become &quot; primary,**