Chapter VIII: Part 8

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 mass 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.

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 horny
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-illustrated memoir by de
Lacaze-Duthiers, which should be consulted for details of anatomy.

The AXIFERA comprise those corals that have a horny 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 horny substance secreted by the ectoderm. In
_Gorgonia_ the axis remains horny 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 horny and
calcareous substance, the latter being amorphous. The order contains
six families--the _Dasygorgidae, Isidae, Primnoidae, Muriceidae,
Plexauridae_, and _Gorgoniaae_.

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.]

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
partition separating a so-called "ventral" or prorachidial canal from
a so-called "dorsal" or metarachidial canal. A rod-like supporting
axis of 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 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.

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. Z1, the distal, and Z2, 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 masses 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
partitions 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 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 tide-marks on the 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 Wenlock 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.

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-class the arrangement of the mesenteries is
subject to a great deal of variation, but all the types hitherto
observed may be referred to a common plan, illustrated 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 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
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.

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.

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 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_ 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.

A, _Cerianthus solitarius_ (after A. Andres).

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

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

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

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

A, Portion of a colony of _Antipathes dichotoma_.

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

C, Transverse section through the oral cone of _Antipathella minor_,
st, Stomodaeum; ov, ovary.]

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

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

All recent corals, as has already been said, conform so closely to the anatomy of normal Actinians that they cannot be classified apart from them, except that they are distinguished by the possession of a calcareous skeleton. This skeleton is largely composed of a number of radiating plates or _septa_, and it differs both in origin and structure from the calcareous skeleton of all Alcyonaria except Heliopora. It is formed, not from fused spicules, but as a secretion of a special layer of cells derived from the basal ectoderm, and known as _calicoblasts_. The skeleton or corallum of a typical solitary coral--the common Devonshire cup-coral _Caryophyllia smithii_ (fig. 15) is a good example--exhibits the followings parts:--(1) The _basal plate_, between the zooid and the surface of attachment. (2) The _septa_, radial plates of calcite reaching from the periphery nearly or quite to the centre of the coral-cup or calicle. (3) The _theca_ or wall, which in many corals is not an independent structure, but is formed by the conjoined thickened peripheral ends of the septa. (4) The _columella_, a structure which occupies the centre of the calicle, and may arise from the basal plate, when it is called essential, or may be formed by union of trabecular offsets of the septa, when it is called unessential. (5) The _costae_, longitudinal ribs or rows of spines on the outer surface of the theca. True costae always correspond to the septa, and are in fact the peripheral edges of the latter. (6) _Epitheca_, an offset of the basal plate which surrounds the base of the theca in a ring-like manner, and in some corals may take the place of a true theca. (7) _Pali_, spinous or blade-like upgrowths from the bottom of the calicle, which project between the inner edges of certain septa and the columella. In addition to these parts the following structures may exist in corals:--_Dissepiments_ are oblique calcareous 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_. _Synapticulae_ 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 developmental history. The larvae of corals are free swimming ciliated forms known as planulae, and they do not acquire a corallum until they fix themselves. A ring-shaped plate of calcite, secreted by the ectoderm, is then formed, lying between the embryo and the surface of attachment. As the mesenteries are formed, the endoderm of the basal disk lying above the basal plate is raised up in the form of radiating folds. There may be six of these folds, one in each entocoele of the primary cycle of mesenteries, or there may be twelve, one in each exocoele and entocoele. The ectoderm beneath each fold becomes detached from the surface of the basal plate, and both it and the mesogloea are folded conformably with the endoderm. The cells forming the limbs of the ectodermic folds secrete nodules of calcite, and these, fusing together, give rise to six (or twelve) vertical radial plates or septa. As growth proceeds new septa are formed simultaneously with the new couples of secondary mesenteries. In some corals, in which all the septa are entocoelic, each new system is embraced by a mesenteric couple; in others, in which the septa are both entocoelic and exocoelic, three septa are formed in every chamber between two primary mesenterial couples, one in the entocoele of the newly formed mesenterial couple of the secondary cycle, and one in each exocoele between a primary and a secondary couple. These latter are in turn embraced by the couples of the tertiary cycle of mesenteries, and new septa are formed in the exocoeles on either side of them, and so forth.

It is evident from an inspection of figs. 16 and 17 that every septum is covered by a fold of endoderm, mesogloea, and ectoderm, and is in fact pushed into the cavity of the zooid from without. The zooid then is, as it were, moulded upon the corallum. When fully extended, the upper part of the zooid projects for some distance out of the calicle, and its wall is reflected for some distance over the lip of the latter, forming a fold of soft tissue extending to a greater or less distance over the theca, and containing in most cases a cavity continuous over the lip of the calicle with the coelenteron. This fold of tissue is known as the _edge-zone_ In some corals the septa are solid imperforate plates of calcite, and their peripheral ends are either firmly welded together, or are united by 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 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, Schematic longitudinal section through a zooid and bud of _Stylophora digitata_. In A, B, and C the thick black lines represent the soft tissues; the corallum is dotted. s, Stomodaeum; c, c, coenosarc; col, columella, T tabulae.

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

C, Similar section through three corallites of _Lophohelia prolifera_. ez, Edge-zone.

D, Diagram illustrating the process of budding by unequal division.

E, Section through a dividing calicle of _Mussa_, showing the union of two septa in the plane of division and the origin of new septa at right angles to them.

(C original; the rest after von Koch.)]

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 medium of the edge-zone. As growth proceeds, and parent and bud become separated farther from one another, the edge-zone forms a sheet of soft tissue, 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. 18, 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 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 divide, but elongates to form a characteristic meandrine channel containing several zooid mouths.

Corals have been divided into _Aporosa_ 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 remarkable similarity between the zooids of all recent corals, the differences which have been brought to light being for the most part secondary and valueless for classificatory purposes. On the other 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 _Stauridae_ 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 principal septa, the _main septum, 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 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 modern Astraeidae is shown by _Moseleya latistellata_, a living reef-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 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 character of the corallum of these extinct forms agrees with that of recent 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 bamboo owing to the thickening of the two ends of each 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 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 instead 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 affinities of corals. (For an account of coral formations see CORAL-REEFS.)

In the present 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 ACTINIIDEA, and this may be divided into the suborders _Malacactiniae_, comprising the soft-bodied Actinians, such as _Actinia, Sagartia, Bunodes_, &c., and the _Scleractiniae_, comprising the corals. The Scleractiniae may best be divided into groups of families which appear to be most closely related to one another, but it should not be forgotten that there is great 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 zooids grow 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. _Amplexus_, M. Edw. and H.
_Streptelasma_, Hall. _Omphyma_, Raf.

Family 2. TURBINOLIDAE.--Solitary, rarely colonial corals, with
radially arranged septa and without tabulae. Typical
genera--_Flabellum_, Lesson. _Turbinolia_, M. Edw. and H.
_Caryophyllia_, 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 (Jurassic). _Convexastraea_, D'Orb.
(Jurassic). _Isastraea_, M. Edw. and H.(Jurassic). Ogilvie refers the
modern genus _Galaxea_ to this family.

GROUP B.

Family 5. OCULINIDAE.--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. _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_, Oken. _Montipora_, Quoy and G.

Family 8. PORITIDAE.--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_, Goldfuss
(Devonian and Carboniferous). _Moseleya_, Quelch (recent).

Family 10. ASTRAEIDAE.--Aporpse, 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 11. FUNGIDAE.--Solitary and colonial corals, with numerous
radial septa united by synapticulae. Typical genera--_Lophoseris_, M.
Edw. and H. _Thamnastraea_, Le Sauvage. _Leptophyllia_, Reuss
(Jurassic and Cretaceous). _Fungia_, Dana. _Siderastraea_, Blainv.

GROUP D.

Family 12. 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. _Astroides_, Blainv. _Rhodopsammia_, M.
Edw. and H. _Dendrophyllia_, M. Edw. and H.

GROUP E.

Family 13. CYSTIPHYLLIDAE.--Solitary corals with rudimentary septa,
and the calicle filled with vesicular endotheca.
Genera--_Cystiphyllum_, 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 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. The calyx is triangular in
section, pointed below, and the operculum is attached to it by
hinge-like teeth.)

AUTHORITIES.--The following list contains only the names of the more
important and more general works on the structure and classification
of corals and on coral reefs. For a fuller bibliography the works
marked with an asterisk should be consulted: * A. Andres, _Fauna und
Flora des Golfes von Neapel_, ix. (1884); H.M. Bernard, "Catalogue of
Madreporarian Corals" in Brit. Museum, ii. (1896), iii. (1897); * G.C.
Bourne, "Anthozoa," in E. Ray Lankester's _Treatise on Zoology_, vol.
ii. (London, 1900); G. Brook, "_Challenger_ Reports," _Zoology_,
xxxii. (1899) (_Antipatharia_); "Cat. Madrep. Corals," Brit. Museum,
i. (1893); D.C. Danielssen, "Report Norwegian North Atlantic Exploring
Expedition," _Zoology_, xix. (1890); J.E. Duerden, "Some Results on
the Morphology and Development of Recent and Fossil Corals," _Rep.
Brit. Association_, 1903, pp. 684-685; "The Morphology of the
Madreporaria," _Biol. Bullet_, vii. pp. 79-104; P.M. Duncan, _Journ.
Linnean Soc._ xviii. (1885); P.H. Gosse, _Actinologia britannica_
(London, 1860); O. and R. Hertwig, _Die Actinien_ (Jena, 1879); R.
Hertwig, "_Challenger_ Reports," _Zoology_, vi. (1882) and xxvi.
(1888); * C.B. Klunzinger, _Die Korallthiere des Rothen Meeres_
(Berlin, 1877); * G. von Koch, _Fauna und Flora des Golfes van
Neapel_, xv. (1887); _Mitth. Zool. Stat. Neapel_, ii. (1882) and xii.
(1897); _Palaeontographica_, xxix. (1883); (also many papers in the
_Morphol. Jahrbuch_ from 1878 to 1898); F. Koby, "Polypiers
jurassiques de la Suisse," _Mem. Soc. Palaeont. Suisse_, vii.-xvi.
(1880-1889); A. von Kolliker, "Die Pennatuliden," _Abh. d. Senck.
Naturf. Gesell_. vii.; * "_Challenger_ Reports," _Zoology_, i.
_Pennatulidae_ (1880); Koren and Danielssen, _Norske Nordhaus Exped.,
Alcyonida_ (1887); H. de Lacaze-Duthiers, _Hist. nat. du corail_
(Paris, 1864); H. Milne-Edwards and J. Haime, _Hist. nat. des
coralliaires_ (Paris, 1857); H.N. Moseley, "_Challenger_ Reports,"
_Zoology_, ii. (1881); H.A. Nicholson, _Palaeozoic Tabulate Corals_
(Edinburgh, 1879); M.M. Ogilvie, _Phil. Transactions_, clxxxvii.
(1896); E. Pratz, _Palaeontographica_, xxix. (1882); J.J. Quelch,
"_Challenger_ Reports," _Zoology_, xvi. (1886); * P.S. Wright and Th.
Studer, "_Challenger_ Reports," _Zoology_, xxxi. (1889).
(G. C. B.)

ANTHRACENE (from the Greek [Greek: anthrax], coal), C14H10, a hydrocarbon obtained from the fraction of the coal-tar distillate boiling between 270 deg. and 400 deg. C. This high boiling fraction is allowed to stand for some days, when it partially solidifies. It is then separated in a centrifugal machine, the low melting-point impurities are removed by means of hot water, and the residue is finally hot-pressed. The crude anthracene cake is purified by treatment with the higher pyridine bases, the operation being carried out in large steam-jacketed boilers. The whole mass dissolves on heating, and the anthracene crystallizes out on cooling. The crystallized anthracene is then removed by a centrifugal separator and the process of solution in the pyridine bases is repeated. Finally the anthracene is purified by sublimation.

Many synthetical processes for the preparation of anthracene and its derivatives are known. It is formed by the condensation of acetylene tetrabromide with benzene in the presence of aluminium chloride:--

Br.CH.Br /CH\
C6H6 + | + C6H6 = 4HBr + C6H4< | >C6H4,
Br.CH.Br \CH/

and similarly from methylene dibromide and benzene, and also when benzyl chloride is heated with aluminium chloride to 200 deg. C. By condensing ortho-brombenzyl bromide with sodium, C.L. Jackson and J.F. White (_Ber_., 1879, 12, p. 1965) obtained dihydro-anthracene

/CH2Br Br\ /CH2\
C6H4< + 4Na + >C6H4 = 4NaBr + C6H4< >C6H4.
\Br BrCH2/ \CH2/

Anthracene has also been obtained by heating ortho-tolylphenyl ketone with zinc dust

/CH8 /CH \
C6H4< = H2O + C6H4< | >C6H4.
\COC6H5 \CH /

Anthracene crystallizes in colourless monoclinic tables which show a fine blue fluorescence. It melts at 213 deg. C. and boils at 351 deg. C. It is insoluble in water, sparingly soluble in alcohol and ether, but readily soluble in hot benzene. It unites with picric acid to form a picrate, C14H10.C6H2(NO2)3.OH, which crystallizes in needles, melting at 138 deg. C. On exposure to sunlight a solution of anthracene in benzene or xylene deposits para-anthracene (C14H10)2, which melts at 244 deg. C. and passes back into the ordinary form. Chlorine and bromine form both addition and substitution products with anthracene; the addition product, anthracene dichloride, C14H10Cl2, being formed when chlorine is passed into a cold solution of anthracene in carbon bisulphide. On treatment with potash, it forms the substitution product, monochlor-anthracene, C14H9Cl. Nitro-anthracenes are not as yet known. The mono-oxyanthracenes (anthrols), C14H9OH or

/CH\
C6H4< | >C6H3OH
\CH/

([alpha]) and ([beta]) resemble the phenols, whilst

/C(OH)\
C6H4< | >C6H4
\CH /

([gamma]) (anthranol) is a reduction product of anthraquinone. [beta]-anthrol and anthranol give the corresponding amino compounds (anthramines) when heated with ammonia.

Numerous sulphonic acids of anthracene are known, a monosulphonic acid being obtained with dilute sulphuric acid, whilst concentrated sulphuric acid produces mixtures of the anthracene disulphonic acids. By the action of sodium amalgam on an alcoholic solution of anthracene, an anthracene dihydride, C14H12, is obtained, whilst by the use of stronger reducing agents, such as hydriodic acid and amorphous phosphorus, hydrides of composition C14H16 and C14H24 are produced.

Methyl and phenyl anthracenes are known; phenyl anthranol (phthalidin) being somewhat closely related to the phenolphthaleins (q.v.). Oxidizing agents convert anthracene into anthraquinone (q.v.); the production of this substance by oxidizing anthracene in glacial acetic acid solution, with chromic acid, is the usual method employed for the estimation of anthracene.

ANTHRACITE (Gr. [Greek: anthrax], coal), a term applied to those varieties of coal which do not give off tarry or other hydrocarbon vapours when heated below their point of ignition; or, in other words, which burn with a smokeless and nearly non-luminous flame. Other terms having the same meaning are, "stone coal" (not to be confounded with the German _Steinkohle_) or "blind coal" in Scotland, and "Kilkenny coal" in Ireland. The imperfect anthracite of north Devon, which however is only used as a pigment, is known as _culm_, the same term being used in geological classification to distinguish the strata in which it is found, and similar strata in the Rhenish hill countries which are known as the Culm Measures. In America, culm is used as an equivalent for waste or slack in anthracite mining.

Physically, anthracite differs from ordinary bituminous coal by its greater hardness, higher density, 1.3-1.4, and lustre, the latter being often semi-metallic with a somewhat brownish reflection. It is also free from included soft or fibrous notches and does not soil the fingers when rubbed. Structurally it shows some alteration by the development of secondary divisional planes and fissures so that the original stratification lines are not always easily seen. The thermal conductivity is also higher, a lump of anthracite feeling perceptibly colder when held in the warm hand than a similar lump of bituminous coal at the same temperature. The chemical composition of some typical anthracites is given in the article COAL.

Anthracite may be considered to be a transition stage between ordinary bituminous coal and graphite, produced by the more or less complete elimination of the volatile constituents of the former; and it is found most abundantly in areas that have been subjected to considerable earth-movements, such as the flanks of great mountain ranges. The largest and most important anthracite region, that of the north-eastern portion of the Pennsylvania coal-field, is a good example of this; the highly contorted strata of the Appalachian region produce anthracite exclusively, while in the western portion of the same basin on the Ohio and its tributaries, where the strata are undisturbed, free-burning and coking coals, rich in volatile matter, prevail. In the same way the anthracite region of South Wales is confined to the contorted portion west of Swansea and Llanelly, the central and eastern portions producing steam, coking and house coals.

Anthracites of newer, tertiary or cretaceous age, are found in the Crow's Nest part of the Rocky Mountains in Canada, and at various points in the Andes in Peru.

The principal use of anthracite is as a smokeless fuel. In the eastern United States, it is largely employed as domestic fuel, usually in close stoves or furnaces, as well as for steam purposes, since, unlike that from South Wales, it does not decrepitate when heated, or at least not to the same extent. For proper use, however, it is necessary that the fuel should be supplied in pieces as nearly uniform in size as possible, a condition that has led to the development of the breaker which is so characteristic a feature in American anthracite mining (see COAL). The large coal as raised from the mine is passed through breakers with toothed rolls to reduce the lumps to smaller pieces, which are separated into different sizes by a system of graduated sieves, placed in descending order. Each size can be perfectly well burnt alone on an appropriate grate, if kept free from larger or smaller admixtures. The common American classification is as follows:--

Lump, steamboat, egg and stove coals, the latter in two or three sizes, all three being above 1-1/2 in. size on round-hole screens.

Chestnut below 1-1/2 inch above 7/8 inch.
Pea " 7/8 " " 9/16 "
Buckwheat " 9/16 " " 3/8 "
Rice " 3/8 " " 3/16 "
Barley " 3/16 " " 3/32 "

From the pea size downwards the principal use is for steam purposes. In South Wales a less elaborate classification is adopted; but great care is exercised in hand-picking and cleaning the coal from included particles of pyrites in the higher qualities known as best malting coals, which are used for kiln-drying malt and hops.

Formerly, anthracite was largely used, both in America and South Wales, as blast-furnace fuel for iron smelting, but for this purpose it has been largely superseded by coke in the former country and entirely in the latter. An important application has, however, been developed in the extended use of internal combustion motors driven by the so-called "mixed," "poor," "semi-water" or "Dowson gas" produced by the gasification of anthracite with air and a small proportion of steam. This is probably the most economical method of obtaining power known; with an engine as small as 15 horse-power the expenditure of fuel is at the rate of only 1 lb per horse-power hour, and with larger engines it is proportionately less. Large quantities of anthracite for power purposes are now exported from South Wales to France, Switzerland and parts of Germany. (H. B.)

ANTHRACOTHERIUM ("coal-animal," so called from the fact of the remains first described having been obtained from the Tertiary lignite-beds of Europe), a genus of extinct artiodactyle ungulate mammals, characterized by having 44 teeth, with five semi-crescentic cusps on the crowns of the upper molars. In many respects, especially the form of the lower jaw, _Anthracotherium_, which is of Oligocene and Miocene age in Europe, and typifies the family _Anthracotheriidae_, is allied to the hippopotamus, of which it is probably an ancestral form. The European _A. magnum_ was as large as the last-mentioned animal, but there were several smaller species and the genus also occurs in Egypt, India and North America. (See ARTIODACTYLA.)

ANTHRAQUINONE, C14H8O2, an important derivative of anthracene, first prepared in 1834 by A. Laurent. It is prepared commercially from anthracene by stirring a sludge of anthracene and water in horizontal cylinders with a mixture of sodium bichromate and caustic soda. This suspension is then run through a conical mill in order to remove all grit, the cones of the mill fitting so tightly that water cannot pass through unless the mill is running; the speed of the mill when working is about 3000 revolutions per minute. After this treatment, the mixture is run into lead-lined vats and treated with sulphuric acid, steam is blown through the mixture in order to bring it to the boil, and the anthracene is rapidly oxidized to anthraquinone. When the oxidation is complete, the anthraquinone is separated in a filter press, washed and heated to 120 deg. C. with commercial oil of vitriol, using about 2-1/2 parts of vitriol to 1 of anthraquinone. It is then removed to lead-lined tanks and again washed with water and dried; the product obtained contains about 95% of anthraquinone. It may be purified by sublimation. Various synthetic processes have been used for the preparation of anthraquinone. A. Behr and W.A. v. Dorp (_Ber._, 1874, 7, p. 578) obtained orthobenzoyl benzoic acid by heating phthalic anhydride with benzene in the presence of aluminium chloride. This compound on heating with phosphoric anhydride loses water and yields anthraquinone,

/CO\ C6H6 /CO.C6H6 /CO\
C6H4< >O -> C6H4< -> C6H4< >C6H4.
\CO/ \COOH \CO/

It may be prepared in a similar manner by heating phthalyl chloride with benzene in the presence of aluminium chloride. Dioxy- and tetraoxy-anthraquinones are obtained when meta-oxy- and dimeta-dioxy-benzoic acids are heated with concentrated sulphuric acid.

Anthraquinone crystallizes in yellow needles or prisms, which melt at 277 deg. C. It is soluble in hot benzene, sublimes easily, and is very stable towards oxidizing agents. On the other hand, it is readily attacked by reducing agents. With zinc dust in presence of caustic soda it yields the secondary alcohol oxan-thranol, C6H4 : CO.CHOH : C6H4, with tin and hydrochloric acid, the phenolic compound anthranol, C6H4 : CO.C(OH) : C6H4; and with hydriodic acid at 150 deg. C. or on distillation with zinc dust, the hydrocarbon anthracene, C14H10. When fused with caustic potash, it gives benzoic acid. It behaves more as a ketone than as a quinone, since with hydroxylamine it yields an oxime, and on reduction with zinc dust and caustic soda it yields a secondary alcohol, whilst it cannot be reduced by means of sulphurous acid. Various sulphonic acids of anthraquinone are known, as well as oxy-derivatives, for the preparation and properties of which see ALIZARIN.

ANTHRAX (the Greek for "coal," or "carbuncle," so called by the ancients because they regarded it as burning like coal; cf. the French equivalent _charbon_; also known as _fievre charbonneuse, Milzbrand_, splenic fever, and malignant pustule), an acute, specific, infectious, virulent disease, caused by the _Bacillus anthracis_, in animals, chiefly cattle, sheep and horses, and frequently occurring in workers in the wool or hair, as well as in those handling the hides or carcases, of beasts which have been affected.