the Actinidæ (e.g., Actinia mesembryanthemum), brightly coloured bead-like bodies are situated on the oral disk outside the tentacles. The structure of these "chromatophores," or "bourses calicinales," has been carefully investigated by Schneider and Röttekem, and by Professor Duncan. They are diverticula of the body wall, the surface of which is composed of close-set "bacilli," beneath which lies a layer of strongly-refracting spherules, followed by another layer of no less strongly refracting cones. Subjacent to these Professor Duncan finds ganglion cells and nerve plexuses. It would seem, therefore, that these bodies are rudimentary eyes.
At the breeding season the ova or spermatozoa are evolved in the thickness of the mesenteries, and are discharged into the intermesenteric spaces, the ova undergoing their development within the body of the parent. The yelk, usually, if not always, enclosed in a vitelline membrane, undergoes complete division, and the outer wall of the ciliated blastodermic mass which results becomes invaginated, the embryo being thereby converted into a double walled sac—the external aperture of which is the future mouth, while the contained cavity represents the body cavity. In this stage the larval Actinia represents the Gastrula condition of sponges and Hydrozoa. The edges of the oral aperture grow inwards, giving rise to a circular fold, which is the rudiment of the visceral tube. This is at first connected with the body wall by only two mesenteries, which are seated at opposite ends of one of the transverse diameters of the body. As the mesenteries increase in number, the tentacles grow out as diverticula of the intermesenteric spaces.
In all the Coralligena, the development of which has been observed, the embryo is converted into a simple actinozoon in a similar manner; but from this point they diverge in two directions. In one great group, the mesenteries, and the tentacles which arise from the intermesenteric chambers, increase in number to six; and then, in the great majority of cases, the intermesenteric spaces undergo subdivision by the development of new mesenteries, according to curious and somewhat complicated numerical laws, until their number is increased to some multiple of five or six. In these Hexacoralla (as they have been termed by Haeckel) the tentacles also usually remain rounded and conical. In the other group, the Octocoralla, the mesenteries and the tentacles increase to eight, but do not surpass that number; and the tentacles become flattened and serrated at the edges, or take on a more or less pennatifid character.
There are no Octocoralla which retain the simple individuality of the young actinozoon throughout life; but all increase by gemmation, and give rise to compound organisms, which may be arborescent, and fixed by the root end of the common stem, as in the Alcyonidæ and Gorgonidæ; or may possess a central stem which is not fixed, and gives off lateral branches which undergo comparatively little subdivision, as in the Pennatulidæ.
The body cavities of the zoöids of these compound Octocoralla are in free communication with a set of canals which ramify through the cœnosarc, or common fabric of the stem and branches by which they are borne, and which play the part of a vascular system.
Except in the case of Tubipora, the zoöids and the superficial cœnosarc give rise to no continuous skeleton; but the deep or inner substance of the cœnosarc may be converted into a solid rod-like or branching stem.
In the Hexacoralla, on the other hand, one large group, that of the Actinidæ, consists entirely of simple organisms,—organisms that is, in which the primitive actinozoon attains its adult condition without budding or fission; or if it bud or divide, the products of the operation separate from one another. No true skeleton is formed, all are to some extent locomotive, and some (Minyas) float freely by the help of their contractile pedal region. The most remarkable form of this group is the genus Cereanthus, which has two circlets, each composed of numerous tentacles, one immediately around the oral aperture, the other at the margin of the disk. The foot is elongated, subconical, and generally presents a pore at its apex. Of the diametral folds of the oral aperture, one pair is much longer than the other, and is produced as far as the pedal pore. The larva is curiously like a young hydrozoon with free tentacles, and at first possesses four mesenteries, whence it may be doubted whether Cereanthus does not rather belong to the Octocoralla.
The Zoanthidæ differ from the Actinidæ in little more than their multiplication by buds, which remain adherent, either by a common connecting mass or cœnosarc or by stolons; and in the possession of a rudimentary, spicular skeleton.
On the other hand, the proper stone-corals (as contradistinguished from the red coral) are essentially Actiniæ, which become converted into compound organisms by gemmation or fission, and develope a continuous skeleton.
The skeletal parts[1] of the Actinozoa, to which reference has been made, consist either of a substance of a horny character; or of an organic basis impregnated with earthy salts (chiefly of lime and magnesia), but which can be isolated by the action of dilute acids; or finally, of calcareous salts in an almost crystalline state, forming rods or corpuscles, which, when treated with acids, leave only an inappreciable and structureless film of organic matter. The hard parts of all the Aporosa, Perforata, and Tabulata of Milne Edwards are in the last-mentioned condition; while, in the Octocoralla (except Tubipora) the Antipathidæ, and Zoanthidæ, the skeleton is either horny, or consists, at any rate, to begin with, of definitely formed spicula, which contain an organic basis, and frequently present a laminated structure. In the organ coral (Tubipora), however, the skeleton has the character of that of the ordinary stone-corals, except that it is perforated by numerous minute canals.
The skeleton appears, in all cases, to be deposited within the mesoderm, and in the intercellular substance of that layer of the body. Even the definitely shaped spicula of the Octocoralla are not the result of the metamorphosis of cells. In the simple aporose corals the calcification of the base and side walls of the body gives rise to the cup or theca; from this the calcification radiates inwards, in correspondence with the mesenteries, and gives rise to as many vertical septa, the spaces between which are termed loculi; while, in the centre, either by union of the septa or independently, a pillar, the columella, grows up. From the sides of adjacent septa scattered processes of calcified substance, or synapticulæ, may grow out toward one another, as in the Fungidæ; or the interruption of the cavities of the loculi may be more complete by the formation of shelves stretching from septum to septum, but lying at different heights in adjacent loculi. These are interseptal dissepiments. Finally, in the Tabulata, horizontal plates, which stretch completely across the cavity of the theca, are formed one above the other and constitute tabular dissepiments.
In the Aporosa the theca and septa are almost invariably imperforate; but in the Perforata they present apertures, and in some madrepores the whole skeleton is reduced to a mere network of dense calcareous substance. When the Hexacoralla multiply by gemmation or fission, and thus give rise to compound massive or aborescent aggregations, each newly-formed coral polype developes a skeleton
- ↑ See Kölliker's Icones Histologicæ, 1866.
