Volume 2, Slice 2 Part 12 (1/2)
ANTHOZOA (i.e. ”flower-animals”), the zoological name for a cla.s.s of marine polyps forming ”coral” (q.v.). Although corals have been familiar objects since the days of antiquity, and the variety known as the precious red coral has been for a long time an article of commerce in the Mediterranean, it was only in the 18th century that their true nature and structure came to be understood. By the ancients and the earlier naturalists of the Christian era they were regarded either as petrifactions or as plants, and many supposed that they occupied a position midway between minerals and plants. The discovery of the animal nature of red coral is due to J.A. de Peyssonel, a native of Ma.r.s.eilles, who obtained living specimens from the coral fishers on the coast of Barbary and kept them alive in aquaria. He was thus able to see that the so-called ”flowers of coral” were in fact nothing else than minute polyps resembling sea-anemones. His discovery, made in 1727, was rejected by the Academy of Sciences of France, but eventually found acceptance at the hands of the Royal Society of London, and was published by that body in 1751. The structure and cla.s.sification of polyps, however, were at that time very imperfectly understood, and it was fully a century before the true anatomical characters and systematic position of corals were placed on a secure basis.
The hard calcareous substance to which the name coral is applied is the supporting skeleton of certain members of the _Anthozoa_, one of the cla.s.ses of the phylum Coelentera. The most familiar Anthozoan is the common sea-anemone, _Actinia equina_, L., and it will serve, although it does not form a skeleton or _corallum_, as a good example of the structure of a typical Anthozoan polyp or zooid. The individual animal or zooid of _Actinia equina_ has the form of a column fixed by one extremity, called the _base_, to a rock or other object, and bearing at the opposite extremity a crown of _tentacles_. The tentacles surround an area known as the _peristome_, in the middle of which there is an elongated mouth-opening surrounded by tumid lips. The mouth does not open directly into the general cavity of the body, as is the case in a hydrozoan polyp, but into a short tube called the _stomodaeum_, which in its turn opens below into the general body-cavity or _coelenteron_. In Actinia and its allies, and most generally, though not invariably, in Anthozoa, the stomodaeum is not circular, but is compressed from side to side so as to be oval or slit-like in transverse section. At each end of the oval there is a groove lined by specially long vibratile cilia.
These grooves are known as the _sulcus_ and _sulculus_, and will be more particularly described hereafter. The elongation of the mouth and stomodaeum confer a bilateral symmetry on the body of the zooid, which is extended to other organs of the body. In Actinia, as in all Anthozoan zooids, the coelenteron is not a simple cavity, as in a Hydroid, but is divided by a number of radial folds or curtains of soft tissue into a corresponding number of radial chambers. These radial folds are known as _mesenteries_, and their position and relations may be understood by reference to figs. 1 and 2. Each mesentery is attached by its upper margin to the peristome, by its outer margin to the body-wall, and by its lower margin to the basal disk. A certain number of mesenteries, known as complete mesenteries, are attached by the upper parts of their internal margins to the stomodaeum, but below this level their edges hang in the coelenteron. Other mesenteries, called incomplete, are not attached to the stomodaeum, and their internal margins are free from the peristome to the basal disk. The lower part of the free edge of every mesentery, whether complete or incomplete, is thrown into numerous puckers or folds, and is furnished with a glandular thickening known as a _mesenterial filament_. The reproductive organs or gonads are borne on the mesenteries, the germinal cells being derived from the inner layer or endoderm.
[Ill.u.s.tration: FIG. 1. Diagrammatic longitudinal section of an Anthozoan zooid,
m, Mesentery. s, Stoma.
t, Tentacles. lm, Longitudinal muscle.
st, Stomodaeum. d, Diagonal Muscle.
sc, Sulcus. go, Gonads.
r, Rotteken's muscle.]
In common with all Coelenterate animals, the walls of the columnar body and also the tentacles and peristome of Actinia are composed of three layers of tissue. The external layer, or ectoderm, is made up of cells, and contains also muscular and nervous elements. The preponderating elements of the ectodermic layer are elongated columnar cells, each containing a nucleus, and bearing cilia at their free extremities.
Packed in among these are _gland cells, sense cells_, and _cnidoblasts_.
The last-named are specially numerous on the tentacles and on some other regions of the body, and produce the well-known ”thread cells,” or _nematocysts_, so characteristic of the Coelentera. The inner layer or endoderm is also a cellular layer, and is chiefly made up of columnar cells, each bearing a cilium at its free extremity and terminating internally in a long muscular fibre. Such cells, made up of epithelial and muscular components, are known as epithelio-muscular or myo-epithelial cells. In Actinians the epithelio-muscular cells of the endoderm are crowded with yellow spherical bodies, which are unicellular plants or Algae, living symbiotically in the tissues of the zooid. The endoderm contains in addition gland cells and nervous elements. The middle layer or mesogloea is not originally a cellular layer, but a gelatinoid structureless substance, secreted by the two cellular layers.
In the course of development, however, cells from the ectoderm and endoderm may migrate into it. In _Actinia equina_ the mesogloea consists of fine fibres imbedded in a h.o.m.ogeneous matrix, and between the fibres are minute branched or spindle-shaped cells. For further details of the structure of Actinians, the reader should consult the work of O. and R.
Hertwig.
[Ill.u.s.tration: FIG. 2.--1, Portion of epithelium from the tentacle of an Actinian, showing three supporting cells and one sense cell (sc); 2, a cnidoblast with enclosed nematocyst from the same specimen; 3 and 4 two forms of gland cell from the stomodaeum; 5a, 5b, epithelio-muscular cells from the tentacle in different states of contraction; 5c, an epithelio-muscular cell from the endoderm, containing a symbiotic zooxanth.e.l.la; 6, a ganglion cell from the ectoderm of the peristome.
(After O. and R. Hertwig.)]
The Anthozoa are divisible into two sub-cla.s.ses, sharply marked off from one another by definite anatomical characters. These are the ALCYONARIA and the ZOANTHARIA. To the first-named belong the precious red coral and its allies, the sea-fans or Gorgoniae, to the second belong the white or Madreporarian corals.
[Ill.u.s.tration: FIG. 3.--An expanded Alcyonarian zooid, showing the mouth surrounded by eight pinnate tentacles. st, Stomodaeum in the the centre of the transparent body; m, mesenteries; asm, asulcar mesenteries; B, spicules, enlarged.]
Alcyonaria.--In this sub-cla.s.s the zooid has very constant anatomical characters, differing in some important respects from the Actinian zooid, which has been taken as a type. There is only one ciliated groove, the sulcus, in the stomodaeum. There are always eight tentacles, which are hollow and fringed on their sides, with hollow projections or pinnae; and always eight mesenteries, all of which are complete, i.e. inserted on the stomodaeum. The mesenteries are provided with well-developed longitudinal retractor muscles, supported on longitudinal folds or plaits of the mesogloea, so that in cross-section they have a branched appearance. These _muscle-banners_, as they are called, have a highly characteristic arrangement; they are all situated on those faces of the mesenteries which look towards the sulcus. (fig. 4). Each mesentery has a filament; but two of them, namely, the pair farthest from the sulcus, are longer than the rest, and have a different form of filament. It has been shown that these asulcar filaments are derived from the ectoderm, the remainder from the endoderm. The only exceptions to this structure are found in the arrested or modified zooids, which occur in many of the colonial Alcyonaria. In these the tentacles are stunted or suppressed and the mesenteries are ill-developed, but the sulcus is unusually large and has long cilia. Such modified zooids are called siphonozooids, their function being to drive currents of fluid through the ca.n.a.l-systems of the colonies to which they belong. With very few exceptions a calcareous skeleton is present in all Alcyonaria; it usually consists of spicules of carbonate of lime, each spicule being formed within an ectodermic cell (fig. 3, B). Most commonly the spicule-forming cells pa.s.s out of the ectoderm and are imbedded in the mesogloea, where they may remain separate from one another or may be fused together to form a strong ma.s.s. In addition to the spicular skeleton an organic h.o.r.n.y skeleton is frequently present, either in the form of a h.o.r.n.y external investment (_Cornularia_), or an internal axis (_Gorgonia_), or it may form a matrix in which spicules are imbedded (_Keroeides, Meistodes_).
[Ill.u.s.tration: FIG. 4.--Transverse section of an Alcyonarian zooid mm, Mesenteries; mb, muscle banners; sc, sulcus; st, stomodaeum.]
Nearly all the Alcyonaria are colonial. Four solitary species have been described, viz. _Haimea funebris_ and _H. hyalina, Hartea elegans_, and _Monoxenia Darwinii_; but it is doubtful whether these are not the young forms of colonies. For the present the solitary forms may be placed in a grade, _Protal-cyonacea_, and the colonial forms may be grouped in another grade, _Synalcyonacea_. Every Alcyonarian colony is developed by budding from a single parent zooid.
The buds are not direct outgrowths of the body-wall, but are formed on the courses of hollow out growths of the base or body-wall, called _solenia_. These form a more or less complicated ca.n.a.l system, lined by endoderm, and communicating with the cavities of the zooids. The most simple form of budding is found in the genus _Cornularia_, in which the mother zooid gives off from its base one or more simple radiciform outgrowths. Each outgrowth contains a single tube or solenium, and at a longer or shorter distance from the mother zooid a daughter zooid is formed as a bud. This gives off new outgrowths, and these, branching and anastomosing with one another, may form a network, adhering to stones, corals, or other objects, from which zooids arise at intervals. In _Clavularia_ and its allies each outgrowth contains several solenia, and the outgrowths may take the form of flat expansions, composed of a number of solenial tubes felted together to form a lamellar surface of attachment. Such outgrowths are called _stolons_, and a stolon may be simple, i.e. contain only one solenium, as in _Cornularia_, or may be complex and built up of many solenia, as in _Clavularia_. Further complications arise when the lower walls of the mother zooid become thickened and interpenetrated with solenia, from which buds are developed, so that lobose, tufted, or branched colonies are formed. The chief orders of the Synalcyonacea are founded upon the different architectural features of colonies produced by different modes of budding. We recognize six orders--the STOLONIFERA, ALCYONACEA, PSEUDAXONIA, AXIFERA, STELECHOTOKEA, and CORNOTHECALIA.
[Ill.u.s.tration: FIG. 5.
A. Skeleton of a young colony of _Tubipora purpurea_. st, Stolon; p, platform.
B. Diagrammatic longitudinal section of a corallite, showing two platforms, p and cup-shaped tabulae, t. (After S.J. Hickson.)]
In the order STOLONIRERA the zooids spring at intervals from branching or lamellar stolons, and are usually free from one another, except at their bases, but in some cases horizontal solenia arising at various heights from the body-wall may place the more distal portions of the zooids in communication with one another. In the genus _Tubipora_ these horizontal solenia unite to form a series of horizontal platforms (fig. 5). The order comprises the families _Cornulamdae, Syringopordae, Tubipondae_, and _Favositidae_. In the first-named, the zooids are united only by their bases and the skeleton consists of loose spicules. In the _Tubipondae_ the spicules of the proximal part of the body-wall are fused together to form a firm tube, the corallite, into which the distal part of the zooid can be retracted.
The corallites are connected at intervals by horizontal platforms containing solenia, and at the level of each platform the cavity of the corallite is divided by a transverse calcareous part.i.tion, either flat or cup-shaped, called a _tabula_. Formerly all corals in which tabulae are present were cla.s.sed together as Tabulata, but Tubipora is an undoubted Alcyonarian with a lamellar stolon, and the structure of the fossil genus Syringopora, which has vertical corallites united by horizontal solenia, clearly shows its affinity to Tubipora. The Favositidae, a fossil family from the Silurian and Devonian, have a ma.s.sive corallum composed of numerous polygonal corallites closely packed together. The cavities of adjacent corallites communicate by means of numerous perforations, which appear to represent solenia, and numerous transverse tabulae are also present. In _Favosites hemisphaerica_ a number of radial spines, projecting into the cavity of the corallite, give it the appearance of a madreporarian coral.
[Ill.u.s.tration: FIG. 6.--Portion of a colony of _Coralinum rubrum_, showing expanded and contracted zooids. In the lower part of the figure the cortex has been cut away to show the _axis_, ax, and the longitudinal ca.n.a.ls, lc, surrounding it.]
In the order ALCYONACEA the colony consists of bunches of elongate cylindrical zooids, whose proximal portions are united by solenia and compacted, by fusion of their own walls and those of the solenia, into a fleshy ma.s.s called the coenenchyma. Thus the coenenchyma forms a stem, sometimes branched, from the surface of which the free portions of the zooids project. The skeleton of the Alcyonacea consists of separate calcareous spicules, which are often, especially in the Nephthyidae, so abundant and so closely interlocked as to form a tolerably firm and hard armour. The order comprises the families _Xeniidae, Alcyonidae_ and _Nephthyidae_. _Alcyonium digitatum_, a pink digitate form popularly known as ”dead men's fingers,” is common in 10-20 fathoms of water off the English coasts.
[Ill.u.s.tration: FIG. 7.--The sea-fan (_Gorgonia cavolinii_).]