Volume 2, Slice 2 Part 13 (1/2)

[Ill.u.s.tration: FIG. 13.

A, _Cerianthus solitarius_ (after A. Andres).

B, Transverse section of the stomodaeum, showing the sulculus, sl, and the arrangement of the mesenteries.

C, Oral aspect of _Arachnactis brachiolata_, the larva of _Cerianthus_, with seven tentacles.

D, Transverse section of an older larva. The numerals indicate the order of development of the mesenteries.]

The order ANTIPATHIDEA is a well-defined group whose affinities are more obscure. The type form, _Antipathes dichotoma_ (fig. 14), forms arborescent colonies consisting of numerous zooids arranged in a single series along one surface of a branched h.o.r.n.y axis. Each zooid has six tentacles; the stomodaeum is elongate, but the sulcus and sulculus are very feebly represented. There are ten mesenteries in which the musculature is so little developed as to be almost indistinguishable. The sulcar and sulcular pairs of mesenteries are short, the sulco-lateral and sulculo-lateral pairs are a little longer, but the two transverse are very large and are the only mesenteries which bear gonads. As the development of the Antipathidea is unknown, it is impossible to say what is the sequence of the mesenterial development, but in _Leiopathes glaberrima_, a genus with twelve mesenteries, there are distinct indications of an Edwardsia stage.

[Ill.u.s.tration: FIG. 14.

A, Portion of a colony of _Antipathes dichotoma_.

B, Single zooid and axis of the same magnified. m, Mouth; mf mesenterial filament; ax, axis.

C, Transverse section through the oral cone of _Antipath.e.l.la minor_, st, Stomodaeum; ov, ovary.]

There are, in addition to these groups, several genera of Actinians whose mesenterial arrangement differs from the normal type. Of these perhaps the most interesting is _Gonactinia prolifera_ (fig. 11, B), with eight macromesenteries arranged on the Edwardsian plan. Two pairs of micromesenteries form couples with the first and second Edwardsian pairs, and in addition there is a couple of micromesenteries in each of the sulculo-lateral exocoeles. Only the first and second pairs of Edwardsian macromesenteries are fertile, i.e. bear gonads.

The remaining forms, the ACTINIIDEA, are divisible into the Malacactiniae, or soft-bodied sea-anemones, which have already been described sufficiently in the course of this article, and the Scleractiniae (= Madreporaria) or true corals.

[Ill.u.s.tration: FIG. 15.--Corallum of _Caryophyllia_; semi-diagrammatic.

th, Theca; c, costae; sp, septa; p, palus; col, columella.]

All recent corals, as has already been said, conform so closely to the anatomy of normal Actinians that they cannot be cla.s.sified apart from them, except that they are distinguished by the possession of a calcareous skeleton. This skeleton is largely composed of a number of radiating plates or _septa_, and it differs both in origin and structure from the calcareous skeleton of all Alcyonaria except Heliopora. It is formed, not from fused spicules, but as a secretion of a special layer of cells derived from the basal ectoderm, and known as _calicoblasts_.

The skeleton or corallum of a typical solitary coral--the common Devons.h.i.+re cup-coral _Caryophyllia smithii_ (fig. 15) is a good example--exhibits the followings parts:--(1) The _basal plate_, between the zooid and the surface of attachment. (2) The _septa_, radial plates of calcite reaching from the periphery nearly or quite to the centre of the coral-cup or calicle. (3) The _theca_ or wall, which in many corals is not an independent structure, but is formed by the conjoined thickened peripheral ends of the septa. (4) The _columella_, a structure which occupies the centre of the calicle, and may arise from the basal plate, when it is called essential, or may be formed by union of trabecular offsets of the septa, when it is called unessential. (5) The _costae_, longitudinal ribs or rows of spines on the outer surface of the theca. True costae always correspond to the septa, and are in fact the peripheral edges of the latter. (6) _Epitheca_, an offset of the basal plate which surrounds the base of the theca in a ring-like manner, and in some corals may take the place of a true theca. (7) _Pali_, spinous or blade-like upgrowths from the bottom of the calicle, which project between the inner edges of certain septa and the columella. In addition to these parts the following structures may exist in corals:--_Dissepiments_ are oblique calcareous part.i.tions, stretching from septum to septum, and closing the interseptal chambers below. The whole system of dissepiments in any given calicle is often called _endotheca_. _Synapticulae_ are calcareous bars uniting adjacent septa.

_Tabulae_ are stout horizontal part.i.tions traversing the centre of the calicle and dividing it into as many superimposed chambers. The septa in recent corals always bear a definite relation to the mesenteries, being found either in every entocoele or in every entocoele and exocoele.

Hence in corals in which there is only a single cycle of mesenteries the septa are correspondingly few in number; where several cycles of mesenteries are present the septa are correspondingly numerous. In some cases--e.g. in some species of _Madrepora_--only two septa are fully developed, the remainder being very feebly represented.

[Ill.u.s.tration: FIG. 16.--Tangential section of a larva of _Astroides calicularis_ which has fixed itself on a piece of cork. ec, Ectoderm; en, endoderm; mg, mesogloea; m, m, mesenteries; s, septum; b, basal plate formed of ellipsoids of carbonate of lime secreted by the basal ectoderm; ep, epitheca. (After von Koch.)]

Though the corallum appears to live within the zooid, it is morphologically external to it, as is best shown by its developmental history. The larvae of corals are free swimming ciliated forms known as planulae, and they do not acquire a corallum until they fix themselves.

A ring-shaped plate of calcite, secreted by the ectoderm, is then formed, lying between the embryo and the surface of attachment. As the mesenteries are formed, the endoderm of the basal disk lying above the basal plate is raised up in the form of radiating folds. There may be six of these folds, one in each entocoele of the primary cycle of mesenteries, or there may be twelve, one in each exocoele and entocoele.

The ectoderm beneath each fold becomes detached from the surface of the basal plate, and both it and the mesogloea are folded conformably with the endoderm. The cells forming the limbs of the ectodermic folds secrete nodules of calcite, and these, fusing together, give rise to six (or twelve) vertical radial plates or septa. As growth proceeds new septa are formed simultaneously with the new couples of secondary mesenteries. In some corals, in which all the septa are entocoelic, each new system is embraced by a mesenteric couple; in others, in which the septa are both entocoelic and exocoelic, three septa are formed in every chamber between two primary mesenterial couples, one in the entocoele of the newly formed mesenterial couple of the secondary cycle, and one in each exocoele between a primary and a secondary couple. These latter are in turn embraced by the couples of the tertiary cycle of mesenteries, and new septa are formed in the exocoeles on either side of them, and so forth.

[Ill.u.s.tration: FIG. 17.--Transverse section through a zooid of _Cladocora_. The corallum shaded with dots, the mesogloea represented by a thick line. Thirty-two septa are present, six in the entocoeles of the primary cycle of mesenteries, I; six in the entocoeles of the secondary cycle of mesenteries, II; four in the entocoeles of the tertiary cycle of mesenteries, III, only four pairs of the latter being developed; and sixteen in the entocoeles between the mesenterial pairs. D, D, Directive mesenteries; st, stomodaeum. (After Duerden.)]

It is evident from an inspection of figs. 16 and 17 that every septum is covered by a fold of endoderm, mesogloea, and ectoderm, and is in fact pushed into the cavity of the zooid from without. The zooid then is, as it were, moulded upon the corallum. When fully extended, the upper part of the zooid projects for some distance out of the calicle, and its wall is reflected for some distance over the lip of the latter, forming a fold of soft tissue extending to a greater or less distance over the theca, and containing in most cases a cavity continuous over the lip of the calicle with the coelenteron. This fold of tissue is known as the _edge-zone_ In some corals the septa are solid imperforate plates of calcite, and their peripheral ends are either firmly welded together, or are united by interst.i.tial pieces so as to form imperforate theca. In others the peripheral ends of the septa are united only by bars or trabeculae, so that the theca is perforate, and in many such perforate corals the septa themselves are pierced by numerous perforations. In the former, which have been called aporose corals, the only communication between the cavity of the edge-zone and the general cavity of the zooid is by way of the lip of the calicle; in the latter, or perforate corals, the theca is permeated by numerous branching and anastomosing ca.n.a.ls lined by endoderm, which place the cavity of the edge-zone in communication with the general cavity of the zooid.

[Ill.u.s.tration: FIG. 18.

A, Schematic longitudinal section through a zooid and bud of _Stylophora digitata_. In A, B, and C the thick black lines represent the soft tissues; the corallum is dotted. s, Stomodaeum; c, c, coenosarc; col, columella, T tabulae.

B, Similar section through a single zooid and bud of _Astroides calicularis_.

C, Similar section through three corallites of _Lophohelia prolifera_.

ez, Edge-zone.