ANTHOZOA ZOOPHYTES.
THE life-history of the oceanic Hydrozoa, which may be regarded as one
of the triumphs of microscopic science, would have been incomplete had
it been separated from that of the Pulmograde Acalephæ and the
Physograde groups: but the most important part of that numerous race of
animals are the Anthozoa zoophytes, which include the builders of the
coral reefs and atolls of the Indian and Chinese seas. The coral
polypes, though feeble and inconspicuous individually, when united in
large communities acquire a power which enables them to build the most
stupendous structures in the midst of a tempestuous ocean.
The Anthozoa zoophytes, or living flowers, form two extensive groups—the
Asteroids, or Alcyonian zoophytes, whose polypes have six or eight
hollow prehensile tentacles radiating round their mouth like a star or
the petals of a single blossom, and the Actinian or Helianthoid
zoophytes, which have ten, twelve, or more hollow tentacles encircling
their mouth in several rows, like the blossom of a double sun-flower.
_Alcyon Zoophytes._
The Alcyon zoophytes comprise the Alcyons, Gorgons, and Pennatulidæ, or
sea-pens. The polypes are of the same form in all, and are united by a
fleshy or horny substance into large communities, so connected and
mutually dependent as to constitute one compound animal. Figure 123
represents a highly magnified group of Alcyonian polypes in different
stages of expansion. The body of the polype is soft, contractile, and
composed of thin, delicate transparent tissues. It has the form of a
cone resting upon its base, which is generally of a firm material. Its
upper extremity presents a central orifice, which serves both for a
mouth and vent, and is encompassed by six or eight broad, short, hollow
tentacles, enlarged towards their base so as to meet, and their edges
are seen with a lens only to be fringed with minute hollow tubes or
pinnæ closed at their free end.
[Illustration: Fig. 123. Alcyonian polypes highly magnified.]
[Illustration: Fig. 124. Polype of Alcyonidium elegans.]
The narrow slit of the mouth opens into the stomach, which is a flat,
short sac hanging down in the central cavity of the polype’s body, with
an orifice at its lower end. The stomach is fixed to the internal walls
of the body by eight vertical folds forming so many longitudinal
chambers open at their lower extremity. The whole of the surface of the
interior, the walls, the stomach, and the septa or divisions, are
covered with fine cilia, by whose vibrations constant currents are
maintained in the water which bathes every part of the cavity freely
entering at the mouth. The polypes are carnivorous, living upon
infusoria and minute particles of animal matter floating on the water,
which they seize with their mouth, or arrest with their flexible and
contractile tentacles. The food is digested by the solvent juices in the
stomach, and the refuse is ejected at the mouth.
The eggs of these polypes are formed and fertilized among the vertical
folds adjacent to the stomach. When hatched, the larvæ pass through the
stomach and come out at the mouth as active ciliated creatures, so like
eggs that the Alcyon zoophytes were believed to be oviparous. However,
in some of the genera they are discharged through pores between the
bases of the tentacles.
[Illustration: Fig. 125. Spicula of Alcyonium digitatum.]
The Alcyon polypes have multitudes of needle-like spicules, rough with
projecting knots. They are collected into triangular groups at the foot
of each tentacle; the central and largest point runs up into the
tentacle. Towards the lower end of the polype, spicules again occur
scattered through the skin and crowded into groups, as in fig. 125.
These, however, form short thick cylinders, each end being dilated into
a star of five or six short branches. The spicules always contain an
organic base hardened by carbonate of lime, for when Dr. Carpenter
dissolved the lime with dilute acid, a gelatinous substance remained,
which had the form of the spicules. Fig. 125 shows those of the
Alcyonium digitatum, or Dead Man’s Fingers, generally assumed as the
type of this numerous order, which contains sixteen genera and many
species, differing much in form but connected by a similarity of
digitate structure.
The Alcyonium digitatum, when torn from the rock to which these animals
are attached, shrinks into a cream-coloured fleshy mass of somewhat
solid texture, rough and hard to the touch, and studded all over with
hollow depressions or pits. When put into sea-water, these lumps, from
the size of a pea and upwards, expand, become semi-transparent, and from
each depression a polype protrudes its beautifully symmetrical
eight-petalled blossom. Their tentacles are short, broad, and
prehensile; and the slender pinnæ, which fringe their edges arching
outwards, are seen with a high magnifying power to be rough with prickly
rings, discovered by Mr. Gosse to be accumulations of thread-cells with
their darts.
These Alcyons, when expanded, are about an inch-and-a-half high and
two-thirds of an inch thick, but individuals are met with two or three
times as large, and much divided into blunt finger-like lobes. The
sarcode mass of these compound animals is channelled like a sponge, by
branching canals, the orifices of which open into the stomachs of the
polypes; and, by bringing them into communication with each other, unite
the whole into one compound animal, which is maintained by the food
caught and digested by each individual polype. Currents of sea-water
mixed with the nutritious juices are made to circulate through the
branching canals by the vibrations of cilia with which they are lined;
they flow round the stomachs of the polypes, supply their juices with
oxygen, and carry off the carbonic acid gas and refuse of the food. In
this case, as in many others, the cilia may be regarded as respiratory
organs.
The unarmed Alcyons are generally thick, short, and rough; some form a
crust on rocks from whence lobes rise. With the exception of the Xenia,
a tropical species, the polypes of the unarmed Alcyons can retreat
within their polypary, so as to be entirely or partially out of sight.
The polypary, or mass, of the armed Alcyons is either membranous or
leathery, and is entirely bristled with large spicules similar to the
very small ones in the tentacles of its polypes. It forms branching
masses terminated by prominent tubercules thickly beset by spicules. The
polypes retreat into the mass when they are in a state of contraction.
The Gorgons, which form the second family of Alcyonian zoophytes, are
compound animals, consisting of a solid stem or axis either simple or
branched, adhering by its base to a rock or some submarine body, and
coated by a layer of a softer fleshy or horny substance exactly in the
same manner as the bark covers the stem and branches of a tree. This
bark or fleshy substance is filled with polypes similar to those
described; however, they are shorter, their base is a little enlarged,
and is turned towards the axis of the stem and branches of the Gorgon.
The softer substance or bark is much developed between the polypes, and
is full of spicules, of forms varying with the genera. A system of
almost capillary canals traverses the soft coating and opens into the
lower part of the cavity containing the viscera of the different
individuals, thus affording a passage for the circulation of the
nutritive juices.
The larvæ of the Gorgons are like ciliated eggs; they swim with their
thick end foremost, and are perfectly soft. That state, however, is
transitory; for no sooner do they lose their cilia and settle on a
submarine substance than their lower part becomes hard, forms a solid
layer on the substance, and constitutes the base for a Gorgon’s stem. A
small elevation rises on it, and at the same time the upper part of the
larva assumes a fleshy consistence and surrounds the elevation. These
two grow simultaneously; the small elevation rises higher and higher,
and its coat containing the polypes grows proportionally with it, and
continues to cover it whatever form it may take, whether a branching or
plumage stem, or a simple slender rod. The stem and branches are
increased in thickness by successive concentric layers of horny or
calcareous matter between their surface and the soft bark.
The Gorgoniidæ are divided into three natural groups, the Gorgons,
Isidæ, and Corallines, according to the nature of their axis. The two
first agree in having stems either of a substance like cork or horn
entirely or partly flexible; but the stem of the Gorgons has no joints,
while that of the Isidæ is jointed. The stem of the Corallines has no
joints, and is entirely stony and branching.
The Gorgonia verrucosa, so common in the Mediterranean, British Channel,
and the intermediate seas, is like a small shrub a foot high, with
numerous branches: the cup-shaped tubercules inhabited by the polypes
are irregularly distributed, and not very salient, yet enough to give
the white encrusting coat a rough warty surface. In this Gorgon there is
an ovary at the base of each polype: the eggs are discharged through
eight small pores placed between the bases of the eight tentacles. These
animals are wonderfully prolific: a Gorgon, six inches high, produced
ninety eggs in one hour.
The Gorgonia graminea, found on the coast of Algiers, instead of being
arborescent, is thin and cylindrical throughout its whole length. The
covering is white, and nearly smooth; the cups containing the polypes
either have no salient border, or are deeply sunk in the coat.
The Gorgons known as sea-fans live in warm seas, and are of numerous
species. Not only all their branches, but all their branchlets and
twigs, spread in the flat form of a fan, are soldered together so as to
form a net with open meshes; the coating is thin, and the polypes are
placed bilaterally.
The stems and branches of the Isidæ, which form the third group of
Gorgoniidæ, are composed of a series of calcareous cylinders, separated
by either horny or cork-like nodes; the polypes are only born in the
bark of the former. In the genus Isis the calcareous cylinders are
deeply striated by straight or wavy lines. This race of animals are
mostly inhabitants of warm seas; but they once lived in a colder
climate. Some species of them are preserved in a fossil state in the
cretaceous earth in Belgium, and in the plastic clay near London.
There is but one genus of the Coralline Gorgon, and the type of that is
the common red ornamental coral of commerce found in the Mediterranean
Sea only. Dr. Carpenter has discovered that the solid calcareous stem of
the Corallium rubrum is made up of aggregations of spicules closely
resembling those of the other Alcyonian zoophytes, but of an intense
red, sometimes rose colour or whitish. The stem and branches are
delicately striated along their length, and covered with a soft
substance of the same colour as the stem, into which the polypes retreat
when alarmed; but when fishing for food, with their eight white
tentacles expanded, the red stem and branches appear as if they were
studded with stars. Prof. de Lacaze Duthiers, who was appointed by the
French Government to investigate the natural history of the red coral
with a view to the regulation of the fishery at Algiers, found that the
individual polypes are either male or female, but that the males and
females are on different branches of the same coral, one branch being
almost exclusively the abode of male polypes, and another of female. The
eggs are fertilized by the intervention of the water. After an egg is
fertilized, it is transferred to the stomach of the female, which thus
serves both for digestion, incubation, and transformations of the egg.
At first the egg is naked and spherical; afterwards it becomes elongated
and covered with cilia. A cavity is formed in it, which opens
externally, and finally becomes the mouth; it then acquires the form of
a little white worm, and when it comes into the water it is very active,
swimming in all directions, avoiding its comrades when they meet, rising
and descending in the water with its hinder end foremost. It loses its
cilia after a time, fixes itself to a rock, and acquires the form of its
parent in the manner described as to other Gorgons.
[Illustration: Fig. 126. Red Coral Branch.]
The red coral generally grows on the under-side of ledges or rocks, in a
pendent position, and at considerable depths. It is not found at 15 or
20 fathoms; they only begin to fish for it at from 30 to 60 fathoms; and
it is brought up from even 100 or 120, while the strong reef-building
corals cannot exist below 25, or at most 30 fathoms; being immensely
superior in vigour, these require a greater supply of air, light, and
heat. The red coral is generally fished for along the coasts of Algiers
and Tunis; it is also found in the seas round Sicily and Sardinia, and
in the Grecian Archipelago. The red coral is always irregularly
branched. The branches are sometimes white, supposed to be from disease;
the white coral of commerce is a species of Caryophyllia, an Actinian,
and not an Alcyon, zoophyte.
[Illustration: Fig. 127. Red Coral (greatly magnified), from ‘Histoire
Naturelle du Corail,’ par M. Lacaze Duthiers.]
The Corallium Johnstoni, a native of the Atlantic, has a white axis,
with branches spreading flatly and horizontally like a fan from the rock
to which it is attached; it is entirely covered with a yellowish flesh,
but the polypes only inhabit the upper surface, as if they could not
live in shade. The Corallium secundum, a similar zoophyte, was
discovered by Professor Dana near the Sandwich Islands, with a white or
rose-coloured fan-shaped stem and branches, covered by a scarlet coat,
having the polypes also only on the upper surface.
The Pennatulidæ, or sea-pens, which are the third family of the Alcyon
zoophytes, bear a great resemblance to a goose’s feather. The genus
Pennatula has a flatly-feathered, upright, calcareous axis, the bare
part of which is analogous to the quill; but, instead of being fixed
like the stem of a Gorgon, it is merely stuck into sand or mud at the
bottom of the seas, while the upper feathered part, containing the
polypes, remains in the water. The axis decreases in thickness upwards,
and the pinnules, which diverge from it transversely like wings, are
angular, thin, membranaceous, and strengthened by spicules. The whole
animal is covered with a soft fleshy tissue; the polypes, which have
eight pinnated tentacles, are arranged in a single row along the edges
of the pinnules, with their visceral extremities prolonged into the soft
tissue, so as to give it a tubular structure, through which the
nourishing juice prepared by the polypes is carried for the maintenance
of the general envelope, the refuse being thrown out at their mouths.
When the sea-pens leave the mud or sand, they do not swim actively with
their pinnules, but move languidly at the bottom. The Pennatulæ are
phosphorescent; they are of a dull reddish brown during the day, but at
night they shine with the most brilliant iridescence. In the tropical
seas they occasionally exceed a foot in length; in the cool latitudes
they are not more than five or six inches. The Pennatula phosphorea,
found on the British coasts, has a hollow axis, occupied by a
well-developed stylet; long pinnulæ symmetrically disposed on each side
of the middle and upper part of the axis; the polypes, which are very
contractile, are arranged transversely on their upper and anterior
edges; the pinnæ of the wings are scythe-shaped, and furnished with a
vast number of sharp spicules, and these combine in bundles at the base
of the cells, in which the polypes live. The back of the pen, lying
between the feathery wings, is sometimes smooth, sometimes crowded with
scales, arising from the development of the spicules with which it is
filled. The eggs of this animal are yellow, and have the size and form
of poppy seeds. They are developed into ciliated larvæ within the
polypes, which come out at their mouths, and swim away; but their
activity is much diminished when they have acquired their mature form.
These Pennatulæ increase also by a kind of budding. There are species of
phosphorescent sea-pens in all the European seas and Indian Ocean.
The Virgulariæ are sea-pens which have long slender stems, with short
transverse pinnules, on both sides of their extremity: they have no
spicules, and are remarkable for the contractile power both of their
axis and polypes. Mr. Darwin mentions a species he met with during his
voyage in the Southern Ocean, which seems to be akin to the Virgularia
juncea common in the Indian Seas. They were long and slender, projecting
in vast numbers like stubble above the surface of muddy sand. When
touched or pulled, they suddenly shrunk down with such force as to
disappear partly or altogether. Sensitive as these animals are, they
have no nerves; hence their motions must be owing to the irritable
nature of muscular fibre. The eggs of the Virgularia mirabilis, native
on the Scotch and Norwegian coasts, are formed in the fleshy coat at the
base of each polype. As soon as they acquire their yellow colour and
ciliated surface they enter into its body, and revolve in it for a
little time before they come out at its mouth.
The family of the Tubipora, inhabitants of warm seas, are the most
beautiful of the Alcyons. They consist of rounded masses of considerable
size, formed of fragile, hollow, and nearly parallel calcareous tubes.
The tubes do not touch one another, but they are united at intervals by
horizontal plates, formed of an extension of their bases, dividing their
mass into stages. In the Tubipora musica, a native of the Indian Ocean,
there are several superincumbent series of equal and parallel tubes,
exactly like the pipes of an organ. The whole compound fragile mass is
of the richest crimson, and the polypes spread their white tentacles
like stars over the mouths of the uppermost pipes, or retreat into them.
Buds spring from the upper part of the tubes, and the result is the
death of the parents, which are succeeded by a young living race a stage
above them. The Tubipora purpurea lives in the Mediterranean and Red
Seas. The polypes of a species found by Professor Dana, at the Feejee
Islands, have their centre and mouth of a brownish red, and their
tentacles yellow, edged by a double fringe of violet-coloured pinnules.
[Illustration: Fig. 128. Tubipora musica.]
_Actinian Zoophytes._
The great family of the Actinian zoophytes abounds in genera and
species. The common Sea Anemone, or Actinia, of which there are more
than seventy species on the British coasts, is the model of the minute
polypes which inhabit the stony corals, and build the coral reefs and
atolls of the tropical Pacific.
The Sea Anemone has a cylindrical body, attached at one end by a sucker
to rocks or stones at no great depth, and a flat circular disk at the
other, with the mouth in its centre: the mouth, which is surrounded by a
series of tubular, smooth-edged, radiating tentacles, resembles a
blossom. The soft smooth body consists of two layers, as may be seen in
the sections of an Actinia (fig. 129). The outer layer generally
contains red matter, the inner one is of muscular fibre, and contains a
great cavity, in which a somewhat globular bag or stomach is suspended.
The space between the stomach and the cylindrical body of the animal is
divided into chambers by perpendicular radiating partitions, consisting
of thin plates or lamellæ. The mouth, which opens at once into the
stomach, imbibes sea-water; and the hollow tentacles surrounding it
being perforated at their extremities, and in communication with the
chambers immediately below them, also imbibe the sea-water and convey it
into the chambers; and the vibrations of the innumerable cilia, with
which all the cavities of the animal are lined, keep them perpetually
bathed with the respiratory medium mixed with nutrient juices from the
coats of the stomach.[27]
[Illustration: Fig. 129. Actinian polype.]
The Sea Anemone is monœcious and oviparous; the eggs are formed and
fertilized in the lower parts of the perpendicular lamellæ or radiant
plates; but they are hatched within the visceral cavity, and the larvæ
issue from the mouth. The Actiniæ are also propagated by buds. They have
as great a power of repairing injuries as the Hydræ, and like them too,
though generally fixed, they can creep about by means of their expanded
suctorial disk, and even float on the surface of the water. In many
species the tentacles, as well as the body, are brightly coloured. The
Actinia sulcata, an inhabitant of the British Channel, is of a deep
crimson, with from 100 to 200 grass-green tentacles. The tints are owing
to coloured particles in minute globules, that lie under the transparent
skin of the animal and its tentacles.
With the exception of some of the Acalephæ, the thread-cells of the Sea
Anemone are more highly developed than in any other animals. They not
only differ in the various Actinian zoophytes, but sometimes even in the
same individual. The complicated structure and action of this warlike
apparatus was unsuspected previous to the microscopic observations of
Mr. Gosse on the Actiniæ in general, and especially on the little
scarlet fringed Sagartia miniata, a native of the British coasts. Like
all the Anemones, it is highly sensitive; on the slightest touch it
draws in its scarlet blossom, and shrinks into the form of a
hemispherical bulb. While in the act of contracting, white filaments
like ribbons shoot out from various parts of its surface, and new ones
appear on every fresh effort, streaming out to the length of several
inches, irregularly twisted and tangled. As soon as the contraction is
finished, these fine white filaments begin to be recalled, and gradually
retire in small irregular coils into the interior chambers between the
stomach and the wall of the body, where they are stored up when not in
activity.
Each filament makes its egress and ingress through an almost
imperceptible transverse slit, discovered by Mr. Gosse, in the middle of
an oval depression in the wall of the animal’s body. The slits, which
are called cinclides, are very numerous, and resemble a pair of inverted
eyelids, which can be opened and shut at pleasure. When the animal is
irritated it contracts, and the water which fills the perpendicular
chambers is forced in a stream through the slits, and carries with it
the white filaments lodged within them; and then these quivers, which
are full of deadly weapons, are ready for action.
Under the microscope, the white filaments are like narrow flat ribbons
with their edges curled in, and thickly covered with cilia. They have
not the slightest trace of muscular fibre, even when viewed with a
microscopic power of 800 diameters; yet they extend, contract, bend, and
coil in every direction; they bring together the margins of the ribbon
so as to form a tube, and open them again; and the filaments perform all
these motions even when severed from the animal, no doubt by the
contractile nature of the clear jelly or sarcode, of which their bases
are composed, as in the tentacles of the Acalephæ.
Innumerable oblong dart or stinging-nettle cells, closely packed
together, lie under the folded edges of the ribbons, throughout their
whole length, especially at their tips.[28]
The polypes of the stony corals, though extremely small, are essentially
the same in structure as the Sea Anemone, but they have no sucker at
their base. The Sea Anemone is of soft tissue throughout its whole body.
In the polypes of the madrepore corals, on the contrary, the whole of
the perpendicular lamellæ which divide the interior of the body into
chambers become hard, from being consolidated by particles of carbonate
of lime; and their upper edges, which appear as rays round the mouth of
the animal, give that starry appearance to the surface of dead
madrepores after the soft part of the polypes has been destroyed.
Most of the coral polypes are unarmed; but in some, as, for example, the
Caryophyllia Smithii, there are multitudes of dart-cells in the
tentacles, besides numerous pellucid filaments or ribbons, full of
thread-cells, lying in coils within the chambers which surround the
stomach.
We are indebted to Mrs. Thynne’s interesting observations on the
Caryophyllia Smithii in her aquarium for the life-history of the animals
armed with this formidable artillery. This madrepore, which inhabits
many parts of the European seas, at various depths, is a species of the
only lamelliform genus of corals which range beyond the tropics. It is a
solitary individual polype, with an external calcareous cylindrical
coat, wider at the base, when it is fixed to a rock; and the mouth,
which has several rows of tentacles, is in the centre of the disk of the
cylinder. The tentacles are delicate, transparent, granular tubes, about
an inch long, tapering to their extremities, and ending in an opaque
white knob full of chambered thread-cells with their darts; but the
thread-cells are of a larger size in the ribbons coiled in the chambers
round the stomach of the animal. These madrepores are described by Mrs.
Thynne as of various tints, from a pure white to a bright apricot
colour. At intervals they eject from the mouth a whitish blue fluid,
resembling wood smoke, in a stream three or four inches long, sometimes
containing a few eggs. But the eggs, though no doubt formed at the base
of the lamellæ, become densely packed like fine dust in the hollows of
the tentacles, from whence they are expelled by contractions, and escape
by the mouth. The eggs lie quiet for a few days in the place where they
are deposited: by and by they begin to rotate, slowly at first, then
more rapidly, and finally they are developed into most minute
madrepores, with the star and colour of the parent. In a few months they
become as large as a crown piece, with a very wide mouth and a
membranous integument or covering, for they do not get their hard
calcareous coat till they are two years old. While in that soft state
they propagate by spontaneous division, which always begins at the
mouth, and is repeated every few weeks during the second year of their
lives. When they split into segments, the broken ends of each segment
bend round and unite; and the mouth, which at first is on one side,
being a portion of the old one, comes to the centre of the disc, and in
addition to the few old tentacles that remain, new ones are added, with
their interior chambers, till they amount to five rows, and in this
manner a brood of young Caryophylliæ is formed.
[Illustration: Fig. 130. Lobophylla angulosa.]
During the second year of their soft state, these madrepores increase by
budding. The buds spring from the base of the membranous covering, they
expand, get a mouth and tentacles, aid in feeding themselves by greedily
taking any small particles of animal food offered to them, and seem also
to share in the sustenance provided by the mother, as they dilate when
she is fed; ultimately they separate from her. These madrepores have
patches of a milk-white fluid substance, which unite and almost cover
the space between the mouths and the rows of tentacles: in others of the
madrepore tribe these patches are purple, green, yellow, or ultramarine
blue. The Caryophylliæ have locomotion while their skin is soft, but no
activity; they merely avoid obstacles, and move away from one another;
but, as soon as they get their hard calcareous coat, they become
permanently fixed, and no longer undergo division or gemmation, but lay
eggs.[29]
The European Caryophylliæ never have more than one star, but sometimes a
great many individuals are united in a spreading bunch, as in the
madrepore Lobophylla angulosa (fig. 130), or in a branched or tufted
mass. Their exterior is invariably striated, and each terminates in a
star, with the polypes, mouth and tentacles in its centre. These
compound madrepores are inhabitants of warm seas.
The number of tentacles possessed by the Actinian polype varies with the
species of the coral. When full grown they have twelve, twenty-four,
even forty-eight, or more. When young, they have only four or six, but
in general the number increases rapidly as they advance in age. The
perpendicular hard lamellæ, which divide the cavity round the stomach of
the polype into perpendicular chambers, as in fig. 129, and form stars
round the mouth, consist of thin sheets or plates, either applied or
soldered together; and for every new tentacle that is produced at the
mouth, a corresponding new chamber is formed immediately below it,
between the sheets or leaves of the lamellæ; so that the number of
chambers and perpendicular plates is always equal to the number of
tentacles, and so the circulation of the fluids is maintained. Since the
upper edges of the lamellæ form the rays of the stars round the mouth of
the polype, it is clear that the number of rays in a star must always be
equal to the number of lamellæ. The new tentacles are always produced
exterior to and between the adjacent old ones, so as to form an outer
circle, and consequently a new circle of rays will be added to the star
round the mouth exterior to the old ones. There may be two, three, or
more concentric circles of tentacles round the mouth of the polype, the
last being the shortest. However, some polypes never have more than
twelve tentacles during the whole course of their lives. The first
formed rays of a star are generally, though not always, the longest and
most prominent; and sometimes the edges of the lamellæ rise high above
the hollow or cup which is the centre of the star, and contains the
mouth of the polype. In some families of corals these edges, which form
the rays, are toothed or spined.[30]
A horny column in the axis of the polype, hardened by sulphate and
carbonate of lime, and called the columella, generally shows its top in
the centre of the star, and varies in structure in the different genera.
Thus the Actinian polypes may be said to possess an internal skeleton,
and as they approach maturity they also acquire an external one in the
form of a cylindrical coat, or stony wall, which surrounds them, and
into which most of them can withdraw the soft upper part of their bodies
and tentacles, so as to be partly or altogether concealed. The
perpendicular lamellæ are sometimes extended through the stony walls of
the polype, so as to form a series of broad, well-developed ribs on its
exterior surface.
The stony substance of corals is chiefly carbonate of lime, which the
polypes have the power of abstracting from the sea-water, combined with
a small quantity of animal matter, and a still smaller quantity of
phosphate of lime, with a trace of silver and magnesia. This stony
substance takes the crystalline form of needles. By the successive
deposition of these needles, a network is formed round the body of the
animal, which by a series of these deposits is condensed into a hard
impervious coat or wall. During this formative process many
characteristic forms may be produced by division and building, depending
upon the genus and species of the polype; but they do not lay eggs till
they come to maturity.
Some corals increase both by budding and division, but by far the
greater number grow in size by budding, as the Astræa, which constitutes
a portion of the reef-building corals of the tropical seas. They form
groups, in which the whole of the polypes, except their starry summits,
are soldered or pasted together by a living viscous substance,
consolidated by carbonate of lime, abstracted from the sea-water, so
that the resulting coral frequently becomes a rounded mass, the surface
of which is more or less covered with stars, which may be circular or
angular, large or small, deeply set or prominent, according to the
genera or species, both of which are exceedingly numerous. In fact the
forms produced vary according as the buds spring from the base of the
polypes, from the sides of the cylindrical body, from the summit or
disk, from the limits of these three parts, or from the whole animal. In
all these varieties the buds are the result of a superabundance of vital
activity in the part. When the buds proceed from the sides of the
polypes the corals are rounded masses; but when they spring from the
disk or cups of the star, the consequence is the death of the parent
polypes, and the development of a new layer of living individuals above
the dead ones. No part of the new polypes is seen except their stars,
their bodies being enclosed in the common tissue. As this process may be
continued indefinitely, the coral may increase to any size; but the size
becomes still greater when successive buds are formed over every part of
the polypes, and when all the successive generations are soldered
together by the common tissue. In every case the polypes are alive only
on the surface where they have free access to light, heat, and air,
which is furnished by the sea-water in which they live.[31]
In the reef-building corals the living viscous substance that covers the
surface and connects the polypes into a mass, is in process of time so
completely consolidated by abstracting the small quantity of carbonate
of lime that the sea-water contains, that little if any animal matter
remains; and as this process is continually repeated, one generation of
polypes perishes after another, the inert matter increases indefinitely,
and the surface at which the consolidation is actually going on is the
only part that is alive.
The surfaces of the dense convex masses of many of these Astræan corals
are entirely covered with deep hexagonal stars, whose rays extend
upwards all round, and end in narrow, sharp, and elevated lines formed
by the junction of the rays of the adjacent stars; in other species the
rays are often crowded together, and the columella only shows a few
points in the deep hollows. Through these deep cups the polypes protrude
their circular disks and tentacles in quest of food, the nutritious
products of which maintain the polypes as well as the general living
fabric which unites them, and the refuse is ejected from their mouths;
for each polype has an independent life of its own besides the
incidental life that it possesses as part of a compound being. In many
of the corals the polypes show great sensibility, shrinking into their
cells on the slightest touch, yet no nervous system has been discovered.
The variety of compact and branching corals far exceeds description: 120
species are inhabitants of the Red Sea alone, and an enormous area of
the tropical Pacific is everywhere crowded with the stupendous works of
these minute agents, destined to change the present geological features
of the globe, as their predecessors have done in the remote ages of its
existence.
Four distinctly different formations are due to the coral-building
polypes in the Pacific and Indian Oceans, namely, lagoon islands or
atolls, encircling reefs, barrier reefs, and coral fringes, all nearly
confined to the torrid zone.
An atoll is a ring or chaplet of coral, enclosing a lagoon or portion of
the ocean in its centre. The average breadth of that part of the ring
which rises above the surface of the sea is about a quarter of a mile,
often less, and it is seldom more than from six to ten or twelve feet
above the waves: hence the lagoon islands are not visible even at a very
small distance, unless when they are covered by the cocoa-nut palm or
the pandanus, which is frequently the case. On the outside, the ring or
circlet shelves down for a distance of one or two hundred yards from its
edge, so that the sea gradually deepens to about twenty-five fathoms,
beyond which the sides of the ring plunge at once into the unfathomable
depths of the ocean with a more rapid descent than the cone of any
volcano. Even at the small distance of some hundred yards no bottom has
been reached with a sounding line a mile and a half long. All the coral
on the exterior of the ring, to a moderate depth below the surface of
the water, is alive; all above it is dead, being the detritus of the
living part washed up by the surf, which is so heavy on the windward
side of the tropical islands of the Pacific and Indian Oceans, that it
is often heard miles off, and is frequently the first warning to seamen
of their approach to an atoll.
On the inside, these coral rings shelve down into the clear calm water
of the lagoon by a succession of ledges of living corals, but of much
more varied and delicate kinds than those on the exterior wall and
foundation of the atoll. The corals known as Porites are the chief
agents in building the exterior face of the ring: they form great
rounded irregular masses, like the Astræa, but much larger, being many
feet in thickness; and as the polypes are only alive on the surface,
numberless generations must have lived and died before they could have
arrived at that size. The rays of the stars are toothed at the edges, so
that they present rows of little points; in some species the rays are
almost invisibly slender, the interstitial matter is full of pores, and
the polypes have twelve tentacles.
The Millepora complanata or palmipora is very commonly associated with
the Porites; it is the largest coral known. It grows in thick vertical
plates, intersecting each other at various angles, and forms an
exceedingly strong honey-combed mass, generally affecting a circular
form, the marginal plates alone being alive. Instead of stars, the
polypes live in simple pores: myriads of these small cylindrical pores
penetrate the surface of the plates perpendicular to their axes;
sometimes they are so minute as to be scarcely visible.
Between the plates, and in the protected crevices of the outer circle of
the ring, a multitude of branching zoophytes and other productions
flourish; but the Porites, Astræans, and Milleporæ seem alone able to
resist the fury of the breakers, essential to the very existence of
these hardy corals, which only obtain their full development when washed
by a heavy sea. The outer margins of the Maldive atolls, consisting
chiefly of Milleporæ and Porites, are beat by a surf so tremendous that
even ships have been thrown by a single heave of the sea high and dry on
the reef. The waves give innate vigour to the polypes by bringing an
ever-renewed supply of food to nourish them, and oxygen to aërate their
juices: besides, uncommon energy is given and maintained by the heat of
a tropical sun, which gives them power to abstract enormous quantities
of solid matter from the water to build their stony homes, a power that
is efficient in proportion to the energy of the breakers which furnish
the supply.
The Porites and Milleporæ, which are the chief reef-building corals,
cannot live at greater depths than fifteen or twenty-five fathoms: not
for want of heat, for the temperature of the ocean in these latitudes
does not sink to 68° Fahr. till a depth of 100 fathoms, but light and
abundance of uncombined air are essential, and these decrease as the
depth increases. The polypes perish if exposed directly to the sun even
for a short time, so they build horizontally between these limits. The
actinian polypes in the corals, which live at different depths in the
crevices of the atolls, have the same general structure; their disks and
tentacles are sometimes tinted with brilliant colours; some sting,
others have a considerable diversity of individual character.
On the margin of the atolls, close within the line where the coral is
washed by the tide, three species of Nullipores flourish; they are
beautiful little plants, very common in the coral islands. One species
grows in thin spreading sheets, like a lichen; the second in stony knobs
as thick as a man’s finger, radiating from a common centre; and the
third species, which has the colour of peach blossom, is a reticulated
mass of stiff branches about the thickness of a crow’s quill. The three
species either grow mixed or separately, and, although they can exist
above the line of the corals, they require to be bathed the greater part
of each tide: hence a layer two or three feet thick, and about twenty
yards broad, formed by the growth of the Nullipores, fringes the circlet
of the atolls and protects the coral below.
The lagoon in the centre of these islands is supplied with water from
the exterior by openings in the lee side of the ring, but as the water
has been deprived of the greater part of its nutritious particles and
inorganic matter by the corals on the outside, the hardier kinds are no
longer produced, and species of more delicate forms take their place.
The depth of the lagoon varies in different atolls from fifty to twenty
fathoms or less, the bottom being partly detritus, partly live coral. In
these calm and limpid waters the corals are of the most varied and
delicate structure, of the most charming and dazzling hues. When the
shades of evening come on, the lagoon shines like the milky way with
myriads of brilliant sparks. The microscopic medusæ and crustaceans
invisible by day form the beauty of the night, and the sea-feather,
vermilion in daylight, now waves with green phosphorescent light. This
gorgeous character of the sea bed is not peculiar to the lagoons of the
atolls; it prevails in shallow water throughout the whole coral-bearing
regions of the Pacific and Indian Oceans.
Encircling reefs differ in no respect from the atoll ring, except in
having islands in their lagoons, surrounded also by coral reefs. Barrier
reefs are of the same structure as the atoll rings, from which they only
differ in their position with regard to the land. They form extensive
lines along the coasts, from which they are separated by a channel of
the sea of variable depth and breadth, sometimes large enough for ships
to pass. A very long one runs parallel to the west coast of New
Caledonia, and stretches for 120 miles beyond the extremities of the
island. But a barrier reef off the northeastern coast of the Australian
continent is the grandest coral formation existing. Rising at once from
an unfathomable depth of the ocean, it extends for a thousand miles
along the coast with a breadth varying from 200 yards to a mile, and at
an average distance of from 20 to 60 or 70 miles from the coast, the
depth of the channel being from 10 to 60 fathoms. The pulse of the
ocean, transcendently sublime, beats perpetually in peals of thunder
along that stupendous reef, the fabric of almost microscopic beings.