Encyclopaedia Britannica Volume 2, Slice 2 Part 12

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_).]

In the order PSEUDAXONIA the colonies are upright and branched, consisting of a number of short zooids whose proximal ends are imbedded 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 remainder the medulla is devoid of solenia, and in the _Melitodidae_ and _Corallidae_ it forms a dense axis, which in the _Melitodidae_ consists of alternate calcareous and h.o.r.n.y joints. The precious red coral of commerce, _Corallium rubrum_ (fig.

6), a member of the family _Corallidae_, is found at depths varying from 15 to 120 fathoms the Mediterranean Sea, chiefly on the African coast. It owes its commercial value to the beauty of its hard red calcareous axis which in 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-ill.u.s.trated memoir by de Lacaze-Duthiers, which should be consulted for details of anatomy.

The AXIFERA comprise those corals that have a h.o.r.n.y or calcified axis, which in position corresponds to the axis of the Pscudaxonia, 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 ectoderm of the base of the mother zooid of the colony, the cavity of the ingrowth being filled by a h.o.r.n.y substance secreted by the ectoderm. In _Gorgonia_ the axis remains h.o.r.n.y throughout life, but in many forms it is further strengthened by a deposit of calcareous matter In the family _Isidinae_ the axis consists of alternate segments of h.o.r.n.y and calcareous substance, the latter being amorphous. The order contains six families--the _Dasygorgidae, Isidae, Primnoidae, Muriceidae, Plexauridae_, and _Gorgoniaae_.

[Ill.u.s.tration: FIG. 8.

A. Colony of _Pennatula phosph.o.r.ea_ 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 ca.n.a.ls.]

In the order STELECHOTOKEA 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 dispostion of the daughter zooids round a central, much-elongated mother zooid. The section contains two families, the _Telestidae_ and the _Coelogorgidae_. The second section comprises the _Pennatulacea_ or sea-pens, which are remarkable from the fact that the colony is not fixed by the base to a rock or other object, but is imbedded in 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 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 part.i.tion separating a so-called "ventral" or prorachidial ca.n.a.l from a so-called "dorsal" or metarachidial ca.n.a.l. A rod-like supporting axis of peculiar texture is developed in the longitudinal part.i.tion, and a longitudinal ca.n.a.l is hollowed out on either side of the axis in the substance of the longitudinal part.i.tion, so that there are four stem-ca.n.a.ls in all. The prorachidial and metarachidial aspects of the rachis are sterile, but the sides or pararachides bear numerous daughter zooids of two kinds--(1) fully-formed autozooids, (2) small stunted siphonozooids. The pinnae are formed by the elongated autozooids, whose proximal portions are fused together to form a leaf-like expansion, from the upper edge of which the distal extremities of the zooids project. The siphonozooids are very numerous and lie between the bases at the pinnae on the pararachides; they extend also on the prorachidial and metarachidial surfaces. The calcareous skeleton of the Pennatulacea consists of scattered spicules, but in one species, _Protocaulon molle_, spicules are absent. Although of great interest the Pennatulacea do not form an enduring skeleton or "coral," and need not be considered in detail in this place.

[Ill.u.s.tration: FIG. 9.

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 Z, the proximal or intracalicular portion of the zooid; ec, ectoderm; ct, coenenchymal tubes; sp, superficial network of solenia.]

The order COENOTHECALIA is represented by a single living species, _Heliopora coerulea_, which differs from all 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 blue colour, and has the form of broad, upright, lobed, or digitate ma.s.ses flattened from side to side. The surfaces are pitted all over with perforations of two kinds, viz. larger 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 tubes, known as the _coenenchymal tubes_ (fig. 9, A). The walls of the calices and coenenchymal tubes are formed of flat plates of calcite, which are so disposed that the walls of one tube enter into the composition of the walls of adjacent tubes, and the walls of the calices are formed by the walls of adjacent coenenchymal tubes. Thus the architecture of the Helioporid colony differs entirely from such forms as Tubipora or Favosites, in 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 part.i.tions or _tabulae_, hence the genus was formerly included in the 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 part.i.tions 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 h.o.m.ology with, the similarly named structures in madreporarian corals. _Heliopora coerulea_ is found between tide-marks on the sh.o.r.e 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 Wenlock limestone. In _Heliolites porosus_ the colonies had the form of spheroidal ma.s.ses; the calices were furnished with twelve pseudosepta, and the coenenchymal tubes were more or less regularly hexagonal.

[Ill.u.s.tration with caption: FIG. 10.

A, _Edwardsia claparedii_ (after A. 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.]

Zoantharia.--In this sub-cla.s.s the arrangement of the mesenteries is subject to a great deal of variation, but all the types. .h.i.therto observed may be referred to a common plan, ill.u.s.trated by the living genus _Edwardsia_ (fig. 10, A, B). This is a small solitary Zoantharian which lives embedded in sand. Its body is divisible into three portions, an upper _capitulum_ bearing the mouth and tentacles, a median _scapus_ covered by a friable cuticle, and a terminal physa which is rounded. Both capitulum and physa can be retracted within the scapus. There are from sixteen to 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 form which has been studied all the eight mesenteries were present, but only two of them, namely the sulco-laterals, 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 cycle are complete (i.e. are attached to the stomodaeum), are twelve in number, and arranged in couples, distinguishable by the position of the muscle-banners. In the four couples of mesenteries which are attached to the sides of the elongated stomodaeum the muscle-banners of each couple are turned towards one another, but in the sulcar and sulcular couples, known as the directive mesenteries, the muscle-banners are on the outer faces of the mesenteries, and so are turned away from one another (see fig.

10, C). The s.p.a.ce enclosed between two mesenteries of the same couple is called an _entocoele_; the s.p.a.ce 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 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 rule (subject, however, to exceptions) that new mesenterial couples are always formed in the exocoeles, and not in the entocoeles.

[Ill.u.s.tration: FIG. 11.--A, Diagram showing the sequence of mesenterial development in an Actinian. B, Diagrammatic transverse section of _Gonactinia prolifera_.]

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 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 II, 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 close connexion with the sulcus, and in this case the muscle-banners are on the _sulcular_ faces. The 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 sulco-lateral chambers, their muscle-banners being so disposed that they form couples respectively with II, II and I, I. In _Actinia equina_ the Edwardsia 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.

[Ill.u.s.tration: FIG. 12.

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. 1, 2, 3, 4, Edwardsian mesenteries.]

As far as the anatomy of the zooid is concerned, the majority of the stony or madreporarian corals agree exactly with the soft-bodied Actinians, such as _Actinia equina_, both in the number and arrangement 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 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 ordinary sea-anemones, but there is only one ciliated groove, the sulcus, in the stomodaeum, and the mesenteries are arranged on a peculiar pattern. The first twelve mesenteries are disposed in couples, and do not differ from those of Actinia except in 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 CERIANTHIDEA comprises a few soft-bodied Zoantharians with rounded aboral extremities pierced by pores. They have two circlets of tentacles, a l.a.b.i.al 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_ 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 sixth pairs, instead of forming couples with the first and second, arise in the sulcar chamber, the fifth pair inside the fourth, and the sixth 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 Cerianthidea, as in the Zoanthidea, much as the adult arrangement of mesenteries differs from that of Actinia, the derivation from an Edwardsia stock is obvious.