Online Encyclopedia

Search over 40,000 articles from the original, classic Encyclopedia Britannica, 11th Edition.

ANTHOZOA (i.e. " flower-animals ")

Online Encyclopedia
Originally appearing in Volume V02, Page 105 of the 1911 Encyclopedia Britannica.
» Make a correction to this article.
» Add information or comments to this article.
Spread the word: del.icio.us del.icio.us it!

See also:

ANTHOZOA (i.e. " See also:flower-animals ") , the zoological name for a class of marine polyps forming " See also:coral " (q.v.). Although See also:corals have been See also:familiar See also:objects since the days of antiquity, and the variety known as the See also:precious red coral has been for a See also:long See also:time an See also:article of See also:commerce in the Mediterranean, it was only in the 18th See also:century that their true nature and structure came to be understood. By the ancients and the earlier naturalists of the See also:Christian era they were regarded either as petrifactions or as See also:plants, and many supposed that they occupied a position midway between minerals and plants. The See also:discovery of the See also:animal nature of red coral is due to J. A. de Peyssonel, a native of See also:Marseilles, who obtained living specimens from the coral fishers on the See also:coast of See also:Barbary and kept them alive in aquaria. He was thus able to see that the so-called " See also:flowers of coral " were in fact nothing else than See also:minute polyps resembling See also:sea-anemones. His discovery, made in 1727, was rejected by the See also:Academy of Sciences of See also:France, but eventually found See also:acceptance at the hands of the Royal Society of See also:London, and was published by that See also:body in 1751. The structure and See also:classification 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 See also:skeleton of certain members of the Anthozoa, one of the classes of the phylum See also:Coelentera. The most familiar Anthozoan is the See also:common sea-See also:anemone, Actinia equines, L., and it will serve, although it does not See also:form a skeleton or corallum, as a See also:good example of the structure of a typical Anthozoan See also:polyp or zooid. The individual animal or zooid of Actinia equina has the form of a See also:column fixed by one extremity, called the See also:base, to a See also:rock or other See also:object, and bearing at the opposite extremity a See also:crown of tentacles.

The tentacles surround an See also:

area known as the peristome, in the See also:middle of which there is an elongated mouth-opening surrounded by tumid lips. The mouth does not open directly into the See also:general cavity of the body, as is the See also:case in a hydrozoan polyp, but into a See also:short See also:tube called the stomodaeum, which in its turn opens below into the general body-cavity or coelenteron. In Actinia and its See also:allies, and most generally, though not invariably, in Anthozoa,the stomodaeum is not circular, but is compressed from See also:side to side so as to be See also:oval or slit-like in transverse See also:section. At each end of the oval there is a groove lined by specially long vibratile See also:cilia. 'these grooves are known as the;sulcus and sulculus, and will be more particularly described hereafter. The See also:elongation of the mouth and stomodaeum confer a bilateral symmetry on the body of the zooid, which is extended to other See also:organs of the body. In Actinia, as in all Anthozoan zooids, the coelenteron is not a See also:simple cavity, as in a Hydroid, but is divided by a number of radial folds or curtains of soft See also:tissue into a corresponding number of radial See also:chambers. These radial folds are known as mesenteries, and their position and relations may be understood by reference to See also:figs. 1 and 2. Each mesentery is attached by its upper margin to the peristome, by its See also:outer margin to the body-See also:wall, and by its See also:lower margin to the basal disk. A certain number of mesenteries, known as See also:complete mesenteries, are attached by the upper parts of their See also: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 See also:free from the peristome to the basal disk.

Thelower See also:

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 mesenteriel filament. The reproductive 11. 4 organs or gonads are See also:borne on the mesenteries, the germinal cells being derived from the inner layer or endoderm. 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 See also:external layer, or ectoderm, is made up of cells, and contains also See also:muscular and See also:nervous elements. The preponderating elements of the ectodermic layer are elongated columnar cells, each containing a See also: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 " See also: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 See also: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 See also: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 equine the mesogloea consists of See also:

fine See also:fibres imbedded in a homogeneous See also: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 See also:work of O. and R. Hertwig. 'The Anthozoa are divis- ible into two sub-classes, sharply marked off from one another by definite anatomical characters. These are the ALCYONARIA and the ZOANTIIARIA. To the first-named belong the precious red coral and its allies, the sea-fans or Gorgoniae; to the second belong the See also:white or Madreporarian corals. Alcyonaria.—In this sub-class the zooid (fig. 3) has very See also: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-See also:

developed See also:longitudinal retractor muscles, supported on longitudinal folds or See also:plait's of the mesogloea, so that in See also:cross-section they have a branched See also:appearance. These muscle-See also: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 See also:rest, and have a different form of filament. It has been shown that these asulcar filaments are derived from the ectoderm, the re- mainder from the en- doderm. 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 See also:ill-developed, but the sulcus is unusually large and has long cilia. Such modified zooids are called siphonozpoids, their See also:function being to drive currents of fluid through the See also:canal-systems of the colonies to which they belong. With very few exceptions a calcareous skeleton is See also:present in all Alcyonaria; it usually consists of spicules of carbonate of See also:lime, each spicule being formed within an ectodermic See also:cell (fig. 3, B). Most commonly the spicule-forming cells pass out of the ectoderm and are imbedded in the mesogloea, where they may remain See also:separate from one another or may be fused together to form a strong See also:mass. In addition to the spicular skeleton an organic horny skeleton is frequently present, either in the form of a horny external investment (Cornularia), or an internal See also:axis (Gorgonio), or it may form a matrix in which spicules are imbedded (Keroeides, Melitodes).

Nearly all the Alcyonaria are colonial. Four solitary See also:

species have been de-scribed, viz. Haimea funebris and H. hyalina, Hartea elegans, and Monoxenia Darwinii; but it is doubtful whether these are not the See also:young forms of colonies. For the present the solitary forms may be placed in a grade, Protalcyonacea; and the colonial forms may be grouped in another grade, Synalcyon- ¢ce¢. Every Alcyonarian FIG. 4.—Transverse section of an See also:colony is developed by Alcyonarian zooid. mm, Mesenteries; budding from a single mb,y muscle banners; sc, sulcus; st, See also:parent zooid. The buds stomodaeum. are not See also: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 canal See also: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 See also: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 in, Mesentery. lm, Longitudinal t, Tentacles. muscle.

st, Stomodaeum. d, See also:

Diagonal sc, Sulcus. muscle. r, Rotteken's muscle. go, Gonads. s, Stoma. 5 zooids arise at intervals. In Clavularia and its allies each outgrowth contains several solenia, and the outgrowths may take,the form of See also:flat expansions, composed of a number of solenial tubes felted together to form a lamellar See also:surface of See also: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 See also: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, A L C Y O NACEA,PSEUDAXONIA, AXIFERA, STELECHOTOREA, and COENOTHECALIA.. In the orderSTOLONIFERA the zooids See also:spring at intervals from branching or lamellar stolons, and are usually free from one another, except at their bases, but in some cases See also:horizontal solenia arising at various heights from the body-wall may See also:place the more distal portions of the zooids in communication with one another. In the genus 'L'ubiporu. these horizontal solenia unite to form a See also:series of horizontal platforms (fig. 5). The See also:order comprises the families Cornulariidae, Syringoporidae, Tubiporidae, and Favositidae.

In the first-named (After S. J. Hickson.) the zooids are See also:

united only by their bases and the skeleton consists of loose spicules. In the Tubiporidae the spicules of the proximal part of the body-wall are fused together to form a See also: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 See also:platform the cavity of the corallite is divided by a transverse calcareous See also:partition, either flat or See also:cup-shaped, called a tabula. Formerly all corals in which tabulae are present were classed together as Tabulata, but Tubipora is an undoubted Alcyonarian with a lamellar stolon, and the structure of the fossil genus Syringopora, which has See also:vertical corallites united by horizontal solenia, clearly shows its See also:affinity to Tubipora. The Favositidae, a fossil See also:family from the See also:Silurian and Devonian, have a massive corallum composed of numerous polygonal corallites closely packed together. The cavities of adjacent coral- lites communicate by means of numerous perforations, which appear to represent solenia, and numerous transverse tabulae are also present. In ax Favosites hemisphaerica a number of radial spines, projecting into the cavity FtG. 6.—Portion of a colony of Coral- of the corallite, give it the lium rubrum, showing See also:expanded and appearance of a madreporcontracted zooids. In the lower part of arian coral. the figure the cortex has been cut away In the order ALCYON-to show the axis, ax, and the longi- ACEA the colony consists tudinal canals, lc, surrounding it. of bunches of elongate cylindrical zooids, whose proximal portions are united by solenia and compacted, by See also:fusion of their own walls and those of the solenia, into a fleshy mass called the coenenchyma.

Thus the coenenchyma forms a See also:

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 See also:armour. The order comprises the families Xeniidae, Alcyonidae and Nephthyidae. Alcyonium digitatum, a See also:pink digitate form popularly known as " dead men's fingers," is common in 10-20 fathoms of See also:water off the See also:English coasts. In the order PSEUDAXONIA the colonies are upright and branched, consisting of a number of short zooids whose proximal ends are See also:im bedded in a coenenchyma containing numerous ramifying solenia and spicules. The coenenchyma is further differentiated into a medullary portion and a cortex. The latter contains the proximal moieties of the zooids and numerous but separate spicules.. The medullary portion is densely crowded with spicules of different shape from those in the cortex, and in some forms the spicules are cemented together to form a hard supporting axis. There are four families of Pseudaxonia — the Briareidae, Sclerogorgidae, Melitodidae, and Corallidae. In the first-named the medulla is penetrated by solenia and forms an indistinct axis; in the See also:remainder the medulla is devoid of solenia, and in the Melitodidae and Corallidae it forms a dense ~6~ axis, which in the Melitodidae consists of alternate calcareous and horny See also:joints. The precious red coral of commerce, Corallium rub-See also:rum (fig. 6), a member of the family Corallidae, is ""r"= _.

found at depths varying FIG. 7.—The sea-See also:

fan (Gorgonio from 15 to 120 fathoms in cavolinii). the Mediterranean Sea, chiefly on the See also:African coast. It owes its commercial value to the beauty of its hard red calcareous axis which in See also:life is covered by a cortex in which the proximal moieties of the zooids are imbedded. Corallium rubrum has been the subject of a beautifully-illustrated memoir by de Lacaze-Duthiers, which should be consulted for details of See also:anatomy. The AXIFERA comprise those corals that have a horny or calcified axis, which in position corre- sponds to the axis of the Pseudaxonia, but, unlike it, is never formed of fused spicules; the most familiar example is the pink sea-fan, Gorgonia cavolinii, which is found in abundance in 10-25 fathoms of water off the English coasts (fig. 7). In this order the axis is formed as an ingrowth of the ectodernm of the base of the mother zooid of the colony, the cavity of the ingrowth being filled by a horny sub-stance secreted by the ecto derm. In Gorgonia the axis remains horny throughout life, but in many forms it is further strengthened by a See also:deposit of calcareous See also:matter. In the family Isidinae the axis consists of alternate segments of horny and calcareous substance, the latter being amorphous. The order contains six families—the Dasygorgidae, Isidae, Primnoidae, Muriceidae, Plexauridae, and Gorgonidae. In the order STELECHOTOICEA the colony consists of a stem formed by a greatly-elongated mother zooid, and the daughter zooids are borne as lateral buds on the stem.

In the section Asiphonacea the colonies are upright and branched, springing from membranous or ramifying stolons. They resemble and are closely allied to certain families of the Cornulariidae, differing from them only in mode of budding and in the disposition of the daughter zooids See also:

round a central, much-elongated mother zooid. The section contains two families, the Telestidae and the Coelogorgidae. The second section comprises the Pennalulacea or sea-pens, which are remarkable from the fact that the colony is not fixed by the base to a rock or other 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 simple and cup-shaped tabulae, 1. A, Colony of Pennatula phosphorea from the metarachidial aspect. p, The peduncle. B, Section of the rachis bearing a single pinna. a, Axis; b, metarachidial; c, prorachidial; d, pararachidial stem canals. object, but is imbedded in See also:sand or mud by the proximal portion of the stem known as the peduncle. In the typical genus, Pennatula (fig. 8), the colony looks like a See also:feather having a stem divisible into an upper moiety or rachis, bearing lateral central leaflets (pinnae), and a lower peduncle, which is sterile and imbedded in sand or mud. The stem represents a greatly enlarged and elongated mother zooid. It is divided longitudinally by a partition separating a so-called " ventral " or prorachidial canal from a so-called " dorsal " or metarachidial canal.

A See also:

rod-like supporting axis of See also:peculiar texture is developed in the longitudinal partition, and a longitudinal canal is hollowed out on either side of the axis in the substance of the longitudinal partition, so that there are four stem-canals in all. The prorachidial and metarachidial aspects of the rachis are sterile, but the sides or pararachides See also:bear numerous daughter zooids of two kinds—(I) fully-formed autozooids, (2) small stunted siphonozooids. The pinnae are formed by the elongated autozooids, whose proximal portions are fused together to form a See also:leaf-like expansion, from the upper edge of which the distal extremities of the zooids project. The siphonozooids are very numerous and See also:lie between the bases of the pinnae .on the pararachides; they extend also on the prorachidial and metarachidial surfaces. The calcareous skeleton of thePennatulacea consists of scattered spicules, but in one species, Protocaulon molle, spicules are absent. Although of See also:great See also:interest the Pennatulacea do not form an enduring skeleton or "coral," and need not be considered in detail in this place. The order COENOTIIECALIA is represented by a single living species, Heliopora coerulea, which differs from all See also:recent Alcyonaria in the fact that its skeleton is not composed of spicules, but is formed as a secretion from a layer of cells called calicoblasts, which originate from the ectoderm. The corallum of Heliopora is of a See also:blue See also:colour, and has the form of broad, upright, lobed, or digitate masses flattened from side to side. The surfaces are pitted all over with perforations of two kinds, viz. larger See also:star-shaped cavities, called calices, in which the zooids are lodged, and very numerous smaller round or polygonal apertures, which in life contain as many short unbranched New A FIG. 9. B A, Portion of the surface of a colony of Heliopora coerulea magnified, showing two calices and the surrounding coenenchymal tubes. B, Single zooid with the adjacent soft tissues as seen after removal of the skeleton by decalcification.

Z', the distal, and Z2, the proximal or intracalicular portion of the zooid; ec, ectoderm; ct, coenenchymal tubes; sp, superficial network of solenia. tubes, known as the coenenchymal tubes (fig. 9, A). The walls of the calices and coenenchymal tubes are formed of flat plates of See also:

calcite, which are so disposed that the walls of one tube enter into the See also:composition of the walls of adjacent tubes, and the walls of the calices are formed by the walls of adjacent coenenchymal tubes. Thus the See also:architecture of the Helioporid colony differs entirely from such forms as Tubipora or Favosites, i:t which each corallite has its own distinct and proper wall. The cavities both of the calices and coenenchymal tubes of Heliopora are closed below by horizontal partitions or tabulae, hence the genus was formerly included in the See also:group Tabulata, and was supposed to belong to the madreporarian corals, both because of its lamellar skeleton, which resembles that of a Madrepore, and because each calicle has from twelve to fifteen radial partitions or septa projecting into its cavity. The structure of the zooid of Heliopora, however, is that of a typical Alcyonarian, and the septa have only a resemblance to, but no real homology with, the similarly named structures in madreporarian corals. Heliopora coerulea is found between See also:tide-marks on the See also:shore platforms of coral islands. The order was more abundantly represented in Palaeozoic times by the Heliolitidae from the Upper and Lower Silurian and the Devonian, and by the Thecidae from the See also:Wenlock See also:limestone. In Heliolites porosus the colonies had the form of spheroidal masses; the calices were furnished with twelve pseudosepta, and the coenenchymal tubes were more or less regularly hexagonal. Zoantharia.—In this sub-class the arrangement of the mesenteries is subject to a great See also:deal of variation, but all the types hitherto observed may be referred to a common See also:plan, illustrated by the living genus Edwardsia (fig. to, A, B). This is a small solitary Zoantharian which lives embedded in sand.

Its body is divisible into three portions, an upper ca itulum bearing the mouth and tentacles, a median scapus covered by a friable cuticle, and a terminalphysa which is rounded. Both ca itulum and physa can be retracted within the scapus. There are from sixteen to See also:

thirty-two simple tentacles, but only eight mesenteries. all of which are complete. The stomodaeum is compressed laterally, and is furnished with two longitudinal grooves, a sulcus and a sulculus. The arrangement of the muscle-banners on the mesenteries is characteristic. On six of the mesenteries the muscle-banners have the same position as in the Alcyonaria, namely, on the sulcar faces; but in the two. remaining mesenteries, namely, those which are attached on either side of the sulcus, the muscle-banners are on the opposite or sulcular faces. It is not known whether all the eight mesenteries of Edwardsia are developed simultaneously or not, but in the youngest A, Edwardsia daparedii (after A. See also:Andres). Cap, capitulum; sc, scapus; ph, physa. B, Transverse section of the same, showing the arrangement of the mesenteries. s, Sulcus; sl, sulculus. C, Transverse section of Halcampa. d, d, Directive mesenteries; st, stomodaeum. form which has been studied all the eight mesenteries were present, but only two of them, namely the sulco-laterals, See also:bore mesenterial filaments, and so it is presumed that they are the first pair to be developed.

In the common sea-anemone, Actinia equina (which has already been quoted as a type of Anthozoan structure), the mesenteries are numerous and are arranged in cycles. The mesenteries of the first See also:

cycle are complete (i.e. are attached to the stomodaeum), are twelve in number, and arranged in couples, distinguish-able by the position of the muscle-banners. In the four couples'o'-mesenteries which are attached to the sides of the elongated stomo• daeum the muscle-banners of each couple are turned towards one another, but in the sulcar and sulcular couples, known as the directive d 111 111 d mesenteries, the muscle-banners are on the outer faces of the mesenteries, and so are turned away from one another (see fig. lo, C). The space enclosed between two mesenteries of the same couple is called an entocoele; the space enclosed between two mesenteries of adjacent couples is called an exocoele. The second cycle of mesenteries consists of six couples, each formed in an exocoele of the See also:primary cycle, and in each couple the muscle-banners are vis-a-vis The third cycle comprises twelve couples, each formed in an exocoele between the primary and secondary couples and so on, it being a general See also:rule (subject, however, to exceptions) that new mesenterial couples are always formed in the-exocoeles, and not in the entocoeles. While the mesenterial couples belonging to the second and each successive cycle are formed simultaneously, those of the first cycle are formed in, successive pairs, each member of a pair being placed on opposite sides of the stomodaeum. Hence the arrangement in six couples is a secondary and not a primary feature. In most Actinians the mesenteries appear in the following order:—At the time when the stomodaeum is formed, a single pair of mesenteries, marked I, I in the See also:diagram (fig. 11, A), makes its appearance, dividing the coelenteric cavity into a smaller sulcar and a large sulcular chamber. The muscle-banners of this pair are placed on the sulcar faces of the mesenteries. Next, a pair of mesenteries, marked 1I,II in the diagram, is developed in the sulcular chamber, its muscle-banners facing the same way as those of I, I. The third pair is formed in the sulcar chamber, in See also:close connexion with the sulcus, and in this case the muscle-banners are on the sulcular faces.

The See also:

fourth pair, having its muscle-banners on the sulcar faces, is developed at the opposite extremity of the stomodaeum in close connexion with the sulculus. There are now eight mesenteries present, having exactly the same arrangement as in Edwardsia. A pause in the development follows, during which no new mesenteries are formed, and then the six-rayed symmetry characteristic of a normal Actinian zooid is completed by the formation of the mesenteries V, V in the lateral chambers, and VI, VI in the sulcolateral chambers, their muscle-banners being so disposed that they form couples respectively with I I, I I and I, I. In Actinic equina the Edwardsia See also:stage is arrived at somewhat differently. The mesenteries second in order of formation form the sulcular directives, those fourth in order of formation form with the fifth the sulculo-lateral couples of the adult. As far as the anatomy of the zooid is concerned, the See also:majority of the stony or madreporarian corals agree exactly with the soft-bodied Actinians, such as Actinia equina, both in the number and arrange- 4 4 2 4 4 2 B. A, Zoanthid colony, showing the expanded zooids. B, Diagram showing the arrangement of mesenteries in a young Zoanthid. C, Diagram showing the arrangement of mesenteries in an adult Zoanthid. I, 2, 3, 4, Edwardsian mesenteries. ment of the adult mesenteries and in the order of development of the first cycle. The few exceptions will be dealt with later, but it may be stated here that even in these the first cycle of six couples of mesenteries is always formed, and in all the cases which have been examined the course of development described above is followed.

There are, however, several See also:

groups of Zoantharia in which the mesenterial arrangement of the adult differs widely from that just described. But it is possible to refer all these cases with more or less certainty to the Edwardsian type. The order ZOANTHIDEA comprises a number of soft-bodied Zoantharians generally encrusted with sand. Externally they resemble See also:ordinary sea-anemones, but there is only one ciliated groove, the sulcus, in the stomodaeum, and the mesenteries are arranged on a peculiar See also:pattern. The first twelve mesenteries are disposed in couples, and do not differ from those of Actinia except in See also:size. The mesenterial pairs I, II and III are attached to the stomodaeum, and are called macromesenteries (fig. 12, B), but IV, V and VI are much shorter, and are called micromesenteries. The subsequent development is peculiar to the group. New mesenteries are formed only in the sulco-lateral exocoeles. They are formed in couples, each couple consisting of a macromesentery and a micromesentery, disposed so that the former is nearest to the sulcar directives. The derivation of the Zoanthidea from an Edwardsia form is sufficiently obvious. The order CERUNT HIDEAcomprises a few soft-bodied Zoantharians with rounded aboral extremities pierced by pores..

They have two circlets of tentacles, a labial and a marginal, and there is only one ciliated groove in the stomodaeum, which appears to be the sulculus. The mesenteries are numerous, and the longitudinal muscles, though distinguishable, are so feebly developed that there are no muscle-banners. The larval forms of the type genus Cerianthus See also:

float freely in the sea, and were once considered to belong to a separate genus, Arachnactis. In this larva four pairs of mesenteries having the typical Edwardsian arrangement are developed, but the fifth and See also:sixth pairs, instead of forming couples with the first and second, arise in the sulcar chamber, the fifth pair inside the fourth, and thesixth pair inside the fifth. New mesenteries are continually added in the sulcar chamber, the seventh pair within the sixth, the eighth pair within the seventh, and so on (fig. 13). In the Ceriant,hidea, as in the. Zoanthidea, much as the adult arrangement of mesenteries differs from that of Actinia, the derivation from an Edwardsia stock is obvious. The order See also:ANTI PATHIDEA is a well-defined group whose See also:alb uties Fm. 13. A, Cerianthus solitarius (after A. Andres).

B, Transverse section of the stomodaeum, showing the atticulus, sl, and the arrangement of the mesenteries.. C, Oral aspect of Arachnactis brachiolata, the larva of -eerianthus, with seven tentacles. D, Transverse section of an older larva. The numerals indicate the order of,development of the mesenteries. are more obscure. The type form, Antipathes dieliolansti (fig. 14), forms arborescent colonies consisting of numerous zoosdi arranged in a single series along one surface of a branched 'horny axis. Each zooid has six tentacles; the stomodaeum is elongate, b'ut• he sulcus and sulculusrare very feebly represented, There, are tennrtesenteries in which the musculattrr'e is so little developed as to See also:

lee almost indistinguishable. The sulcar and sulculaf pairs of mesenteries are A, Portion of a colony of Antipathes dichotoma. B, Single zooid and axis of the same magnified. m, Mouth; im) mesentenal filament; ax, axis. C, Transverse section through the oral See also:cone of Antipathella See also:minor st, Stomodaeum; ov, ovary. 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. 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. t i, 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 micro-mesenteries 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. All recent corals, as has already been said, conform so closely to the anatomy of normal Actinans that they cannot be classified apart from them, except that they are distinguished by the See also:

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 See also:special layer of cells derived from the basal ectoderm, and known as calicoblasts. The skeleton or corallum of a typical solitary coral-the common See also:Devonshire cup-coral Caryophyllia smithii (fig. 15) is a good example—exhibits the followings parts: (r) The basal See also: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 See also:

independent structure, but is formed by the See also:con joined thickened peripheral ends of the septa. (4) The See also: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 See also: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 See also:ring-like manner, and in some corals may take the place of a true theca. (7) See also: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 partitions, stretching from septum to septum, and closing the interseptal chambers below. The whole system of dissepiments in any given calicle is often called endotheca. Synapiiculae are calcareous bars uniting adjacent septa. Tabulae are stout horizontal partitions 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. Though the corallum appears to live within the zooid, it is morphologically external to it, as is best shown by its develop-See also:

mental See also:history. The larvae of corals are free See also:swimming ciliated forms known as planulae, and they do not acquire a corallum until they See also: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 sip 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 See also: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 Z If I 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 See also:tertiary cycle of mesenteries, and new septa are formed in the exocoeles on either side of them, and so forth. 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 See also:

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 theed ge-See also: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 interstitial 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 Frc. i8. A, Schematic longitudinal section through a zooid and bud of Stylophora digitata. In A, B, and C the thick See also:black lines represent the soft tissues; the corallum is dotted. s, Stomodaeum; c, c, coenosarc ; See also:col, columella ; T, tabulae. B, Similar section through a single zooid and bud of Astroides caticularis. C, Similar section through three corallites of Lophohelia prolifera. ez, Edge-zone. D Diagram illustrating the See also:process of budding by unequal See also:division.

E, Section through a dividing calicle of Mussa, showing the union of two septa in the See also:

plane of division, and the origin of new septa at right angles to them. (C See also:original; the rest after von See also:Koch.) See also: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 canals lined by endoderm, which place the cavity of the edge-zone in communication with the general cavity of the zooid. A large number of corals, both aporose and perforate, are colonial. The colonies are produced by either budding or division. In the former case the young daughter zooid, with its corallum, arises wholly outside the cavity of the parent zooid, and the component parts of the young corallum, septa, theca, columella, &c., are formed anew in every individual produced. In division a vertical constriction divides a zooid into two equal or unequal parts, and the several parts of the two corals thus produced are severally derived from the corresponding parts of the dividing corallum. In colonial corals a bud is always formed from the edge-zone, and this bud develops into a new zooid with its corallum. The cavity of the bud in an aporose coral (fig. 18, A, C) does not communicate directly with that of the parent form, but through the See also:medium of the edge-zone. As growth proceeds, and parent and bud become separated farther from one another, the edge-zone forms a See also:sheet of soft tissue, I03 bridging over the space between the two, and resting upon projecting spines of the corallum. This sheet of tissue is called the coenosarc. Its lower surface is clothed with a layer of calicoblasts which continue to secrete carbonate of lime, giving rise to a secondary deposit which more or less fills up the spaces between the individual coralla, and is distinguished as coenenchyme.

This coenenchyme may be scanty, or may be so abundant that the individual corallites produced by budding seem to be immersed in it. Budding takes place in an analogous manner in perforate corals (fig. IS, B), but the presence of the canal system in the perforate theca leads to a modification of the process. Buds arise from the edge-zone which already communicate' with the cavity of the zooid by the canals. As the buds develop the canal system becomes much extended, and calcareous tissue is deposited between the network of canals, the confluent edge-zones of mother zooid and bud forming a coenosarc. As the process continues a number of calicles are formed, imbedded in a spongy tissue in which the canals ramify, and it is impossible to say where the theca of one corallite ends and that of another begins. In the formation of colonies by division a constriction at right angles to the long axis of the mouth involves first the mouth, then the peristome, and finally the calyx itself, so that the previously single corallite becomes divided into two (fig. 18, E). After division the corallites continue to grow upwards, and their zooids may remain united by a See also:

bridge of soft tissue' or coenosarc. But in some cases, as they grow farther apart, this continuity is broken, each corallite has its own edge-zone, and internal continuity is also broken by the formation of dissepiments within each calicle, all organic connexion between the two zooids being eventually lost. Massive meandrine corals are produced by continual repetition of a process of incomplete division, involving the mouth and to some extent the peristome: the calyx, however, does not See also:divide, but elongates to form a characteristic meandrine channel containing several zooid mouths. 'Corals have been divided into A porosa and Perforata, according as the theca and septa are compact and solid, or are perforated by pores containing canals lined by endoderm.

The division is in many respects convenient for descriptive purposes, but recent researches show that it does not accurately represent the relationships of the different families. Various attempts have been made to classify corals according to the arrangement of the septa, the characters of the theca, the microscopic structure of the corallum, and the anatomy of the soft parts. The last-named method has proved little more than that there is a remark-able similarity between the zooids of all recent corals, the See also:

differences which have been brought to See also:light being for the most part secondary and valueless for classificatory purposes. On the other See also:hand, the study of the anatomy and development of the zooids has thrown much light upon the manner in which the corallum is formed, and it is now possible to infer the structure of the soft parts from a microscopical examination of the septa, theca, &c., with the result that unexpected relationships have been shown to exist between corals previously supposed to stand far apart. This has been particularly the case with the group of Palaeozoic corals formerly classed together as Rugosa. In many of these so-called rugose forms the septa have a characteristic arrangement, differing from that of recent corals chiefly in the fact that they show a tetrameral instead of a hexameral symmetry. Thus in the family Slauridae there are four chief septa whose inner ends unite in the middle of the calicle to form a false columella, and in the Zaphrentidae there are many instances of an arrangement, such as that depicted in fig. 19, which represents the septal arrangement of Streptelasma corniculum from the lower Silurian. In this coral the calicle is divided into quadrants by four See also:principal septa, the See also:main septum, See also:counter septum, and two alar septa. The remaining septa are so disposed that in the quadrants abutting on the chief septum they converge towards that septum, whilst in the other quadrants they converge towards the alar septa. The secondary septa show a See also:regular gradation in size, and, assuming that the smallest were the most recently formed, it will be noticed that in the chief quadrants the youngest septa lie nearest to the main septum; in the other quadrants the youngest septa lie nearest to the alar septa. This arrangement, however, is by no means characteristic even of the Zaphrentidae, and in the family Cyathophyllidae most of the genera exhibit a radial symmetry in which no trace of the bilateral arrangement described above is recognizable, and indeed in the genus Cyathophyllum itself a radial arrangement is the rule.

The connexion between the Cyathophyllidae and See also:

modern Astraeidae is shown by Moseleya latistellata, a living See also:reef-See also:building coral from Torres Strait. The general structure of this coral leaves no doubt that it is closely allied to the Astraeidae, but in the young calicles a tetrameral symmetry is indicated by the presence of four large septa placed at right angles to one another. Again, in the family Amphiastraeidae there is commonly a single septum much larger than the rest, and it has been shown that in the young calicles, e.g. of Thecidiosmilia, two septa, corresponding to the main- and counter-septa rrc of Streptelasma, are first formed, then two alar septa, - and afterwards the remaining septa, the latter taking on a generally radial arrangement, though the original bilaterality is marked by the preponderance of the main , septum. As the microscopic See also:character of the corallum of these See also:extinct forms agrees with that of re-cent corals, it may be assumed that the anatomy of the soft parts also was similar, and the tetrameral arrangement, when present, may obviously be referred to a stage when only the first two pairs of Edwardsian mesenteries were present and septa were formed in the intervals between them. Space forbids a discussion of the proposals to classify corals after the minute structure of their coralla, but it will suffice to say that it has been shown that the septa of all corals are built up of a number of curved bars called trabeculae, each of which is composed of a number of nodes. In many secondary corals (Cyclolites, Thamnastraea) the trabeculae are so far separate that the individual bars are easily recognizable, and each looks something like a See also:bamboo owing to the thickening of the two ends of each See also:node. The trabeculae are united together by these thickened internodes, and the result is a fenestrated septum, which in older septa may become solid and aporose by continual deposit of calcite in the fenestrae. Each node of a trabecula may be simple, i.e. have only one centre of calcification, or may be See also:compound. The septa of modern perforate corals are shown to have a structure nearly identical with that of the secondary forms, but the trabeculae and their nodes are only apparent on microscopical examination. The aporose corals, too, have a practically identical structure, their compactness being due to the union of the trabeculae throughout their entire lengths in-See also:stead of at intervals, as in the Perforata. Further, the trabeculae may be evenly spaced throughout the septum, or may be grouped together, and this feature is probably of value in estimating the See also:affinities of corals. (For an See also:account of coral formations see CORAL-REEFS.) In the present See also:state of our knowledge the Zoantharia in which a primary cycle of six couples of mesenteries is (or may be inferred to be) completed by the addition of two pairs to the eight Edwardsian mesenteries, and succeeding cycles are formed in the exocoeles of the pre-existing mesenterial cycles, may be classed in an order ACPINIIDEA, and this may be divided into the sub-orders Malacactiniae, comprising the soft-bodied Actinians, such as Actinia, Sagarlia, Bunodes, &c., and the Scleractiniae, comprising the corals.

The Scleractiniae may best be dividedinto groups of families which appear to be most closely related. to one another, but it should not be forgotten that there is great See also:

reason to believe that many if not most of the extinct corals must have differed from modern Actiniidea in mesenterial characters, and may have only possessed Edwardsian mesenteries, or even have possessed only four mesenteries, in this respect showing close affinities to the• Stauromedusae. Moreover, there are some modern corals in which the secondary cycle of mesenteries departs from the Actinian plan. For example, J. E. Duerden has shown that in Porites the ordinary zooids possess only six couples of mesenteries arranged on the Actinian plan. But some'zooidsgrow to a larger size and develop a number of additional mesenteries, .which arise either in the sulcar or the sulcular entocoele, much in the same manner as in Cerianthus. Bearing this in mind, the following arrangement may be taken to represent the most recent knowledge of coral structure Group A. Family I..ZAPHRENTIDAE.—Solitary Palaeozoic corals with an epithecal wall. Septa numerous, arranged pinnately with regard to four principal septa. Tabulae present. One or more pits or fossulae present in the calicle. Typical genera—Zaphrentis, Raf.

Am plexus, M. Edw. and H. Streptelasma, See also:

Hall.. Omphyma, Raf. Family 2. TURRINOLIDAE.—Solitary, rarely colonial corals, with radially arranged septa and without tabulae. Typical genera—Flabellum, See also:Lesson. Turbinolia, M. Edw. and H. Caryophyllia, See also:Lamarck. Sphenotrochus, Moseley, &c. Family 3.

AMPHIASTRAEIDAE.—Mainly colonial, rarely solitary corals, with radial septa, but bilateral arrangement' indicated by persistence of a main septum. Typical genera—Amphiastraea, Etallon. Thecidiosmilia. Family 4. STYLINIDAE.—Colonial corals allied to the Amphiastraeidae, but with radially symmetrical septa arranged in cycles. Typical genera—Stylina, Lamarck (See also:

Jurassic). Convexastraea, D' See also:Orb. (Jurassic). Isastraea, M. Edw. and H.(Jurassic). See also:Ogilvie refers the modern genus Galaxea to this family. Group B.

Family 5. OcuI.INIDAE.—Branching or massive aporose corals, the calices projecting above the level of a compact coenenchyme formed from the coenosarc which covers the exterior of the corallum. Typical genera—Lophohelia, M. Edw. and H. Oculina, M. Edw. and H. Family 6. PocILLOPORIDAE.—Colonial branching aporose corals, with small calices sunk in the coenenchyme. Tabulae present, and two larger septa, an axial and abaxial, are always present, with traces of ten smaller septa. Typical genera—Pocillopora, Lamarck. C Seriatopora, Lamarck. Family 7.

MADREPORIDAE.—Colonial branching or palmate perforate corals, with abundant trabecular coenenchyme. Theca porous; septa compact and reduced in number. Typical genera—Madrepora, Linn. Turbinaria, See also:

Oken. Montipora, Quoy and G. Family 8. PoRITInAE.—Incrusting or massive colonial perforate corals; calices usually in contact by their edges, sometimes disjunct and immersed in coenenchyme. Theca and septa perforate. Typical genera—Porites, M. Edw. and H. Goniopora, Quoy and G. Rhodaraea, M.

Edw. and H. Group C. Family 9. CYATHOPHYLLIDAE.—Solitary and colonial aporose corals. Tabulae and vesicular endotheca present. Septa numerous, generally radial, seldom pinnate. Typical genera—Cyathophyllum, oldfuss (Devonian and Carboniferous). Moseleya, Quelch (recent). Family to. ASTRAEIDAE.—ApOroSe, mainly colonial corals, massive, branching, or maeandroid. Septa radial; dissepiments present; an epitheca surrounds the base of massive or maeandroid forms, but only surrounds individual corallites in simple or branching forms. Typical genera—Goniastraea, M.

Edw. and H. Heliastraea, M. Edw. and H. Maeandrina, Lam. Coeloria, M. Edw. and H. Favia, Oken. . Family i i. FuNGIDAE.—Solitary and colonial corals, with numerous radial septa united by synapticulae. Typical genera-Lophoseris, M. Edw. and H. Thamnastraea, Le Sauvage.

Leptophyllia, See also:

Reuss (Jurassic and Cretaceous). Fungia, See also:Dana. Siderastraea, Blainv. Group D. Family I2. EuPSAMMIDAE.—Solitary or colonial perforate corals, branching, massive, or encrusting. Septa radial ; the primary septa usually compact, the remainder perforate. Theca perforate. Synapticula present in some genera. Typical genera—Stephanophyllia, Michelin. Eupsammia, M. Edw. and H.

Asiroides, See also:

Blaine. Rhodopsammia, M. Edw. and H. Dendrophyllia, M. Edw. and H. Group E. Family 13. CYST'IPHYLiinAE.—Solitary corals with rudimentary septa, and the calicle. filled with vesicular endotheca. Genera— Cystiphyllum, See also:Lonsdale (Silurian and Devonian). Goniophyllum, M. Edw. and H. (In this Silurian genus the calyx is provided with a movable operculum, consisting of four paired triangular pieces, the bases of each being attached to the sides of the calyx, and their apices See also:meeting in the middle when the operculum is closed).

Calcecla, Lam. (In this Devonian genus there is a single semicircular operculum furnished with a stout median septum and numerous feebly developed secondary septa.

End of Article: ANTHOZOA (i.e. " flower-animals ")

Additional information and Comments

There are no comments yet for this article.
» Add information or comments to this article.
Please link directly to this article:
Highlight the code below, right click, and select "copy." Then paste it into your website, email, or other HTML.
Site content, images, and layout Copyright © 2006 - Net Industries, worldwide.
Do not copy, download, transfer, or otherwise replicate the site content in whole or in part.

Links to articles and home page are always encouraged.

[back]
ANTHONY, SUSAN BROWNELL (1820-1906) 		[next]
ANTHRACENE (from the Greek avOpa, coal)

Site © 2006 - Net Industries