Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/146

Rh 130 ACTINOZOA rthe Actinidce (e.g., Actinia mesemlryantkenium), brightly coloured bead-like bodies are situated on the oral disk out- side the tentacles. The structure of these &quot;chromato- phores,&quot; or &quot; bourses calicinales,&quot; has been carefully investi gated by Schneider and Eottekem, and by Professor Duncan. They are diverticula of the body wall, the sur face of which is composed of close-set &quot;bacilli,&quot; beneath which lies a layer of strongly-refracting spherules, followed by another layer of no less strongly refracting cones. Sub jacent 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 dis charged into the intermesenteric spaces, the ova undergo ing 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 invagi- nated, 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 con dition 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 con nected 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 ten tacles grow out as diverticula of the intermesenteric spaces. In all the Corailigena, 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 mesen teries, and the tentacles which arise from the intermesen teric 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, accord ing 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 mesen teries and the tentacles increase to eight, but do not sur pass 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 indivi duality of the young actinozoon throughout life ; but all in crease 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 Alcyonidae and Gorgonidce; or may possess a central stem which is not fixed, and gives off lateral branches which undergo comparatively little sub division, as in the Pennatulidce. The body cavities of the zobids of these compound Octocoralla are in free communication with a set of canals which ramify through the ccenosarc, 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 zob ids and the super ficial ccenosarc give rise to no continuous skeleton ; but the deep or inner substance of the ccenosarc may be converted into a solid rod-like or branching stem. In the Hexacoralla, on the other hand, one large group, that of the Actinidce, 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 Cereanthue, 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 Cweantkw does not rather belong to the Octocoralla. The Zoanthidce differ from the Actinidce in little more than their multiplication by buds, which remain adherent, either by a common connecting mass or ccenosarc or by stolons; and in the possession of a rudimentary, spiciilar skeleton. On the other hand, the proper stone-corals (as contra distinguished from the red coral) are essentially Actiniae, 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 cal careous 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 Tabu- lata of Milne Edwards are in the last-mentioned condition ; while, in the Octocoralla (except Tubipora) the Antipathidce, and Zoanthidce, 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 in wards, 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 columdla, grows up. From the sides of adjacent septa scattered processes of calcified substance, or synapticulce, may grow out toward one another, as in the Fungidcn; or the interrup tion 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 interstptal dissepiments. Finally, in the Tabidata, 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 imperf orate; 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 aggre gations, each newly-formed coral polype developes a skeleton See Kolliker s Icones Uistologicce, 1866.