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.

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.

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 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]

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.

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.