PART II

CHAPTER VII
CORALS AND CORAL ANIMALS

An examination of any typical coral fragment, living or dead, massive or branched, shews that the surface of the stone is shaped into little cups, which may be large (half an inch or more in diameter) or very small; they may entirely cover the surface or be scattered at intervals, be sunk below the general level or project boldly from it (see the figures of corals on Plate XXVI). In any case, all ordinary corals consist of a multitude of such cups borne upon a common mass of the same stony material, and each cup may be regarded as the remains of one individual “polyp” as the coral animal is called.

It will be a simpler way of gaining an idea of what a coral polyp really is to take a least specialised member of its class, one in which each individual lives disconnected from its fellows and does not secrete the complicated skeleton characteristic of the stony corals. Fig. 52 shews a “sea-anemone” in no material respect different to those which Gosse has so beautifully figured from our own shores. It is seen to be a graceful translucent cylinder fixed by its base to some chance stone or shell. This extreme simplicity of form gives the living animal a by no means inconsiderable beauty.

The free end of the cylinder bears a circle of what look like hairs, but which are found on testing to be highly sensitive to touch and are surprisingly adhesive for organs of such transparent delicacy. They are consequently better named the tentacles, their function being to adhere to and close down upon any little water-flea or such-like animal that has the bad luck to swim against them. They are also provided with stinging cells which are used for paralysing their prey and for defence. In the centre of the disc is the mouth, a simple opening, a mere hole in the top of a sac.

Our polyp, or anemone, has no organs of locomotion, no organs of sight, taste or hearing, indeed no brain. It has at any rate the rudiments of a nervous and muscular system, for it is able to move its tentacles all towards that one which has captured prey and assist it in conveying the food to the mouth. The body is sensitive all over, for any part when touched will contract, and a violent shock causes the tentacles to fold together and the whole animal to close down to a shapeless hemispherical mass. This is the limit of its sensory and muscular powers, two or three muscular movements, a few of the simplest reactions to outside influences, nothing that could be called even the sense of touch.

Internally again the principal interest of the organism is its extreme simplicity. There is no stomach or gut, no heart or veins, no lung or gills, no kidneys and no brain. The animal is in fact a simple sac, the inner walls are much folded to increase their area, but there is but one body-space to serve for everything. In this space the food is digested. There is but one opening into it from the outside, so the indigestible remnants of the food are voided by the same opening as that they entered by.

Simplicity of organisation could scarcely go farther; we have here an example of one of the lowest forms of life. Lowly organisms of this kind shew an astonishing indifference to the separation of one part from another. No cutting or mutilation does any permanent harm. Chop the beast to fragments, and not only will each piece remain alive, but it will grow until it encloses a new sac, forms a new mouth, tentacles, and adhesive base, and behold a number of new and complete polyps. This possibility has been taken advantage of by nature, and numbers of these lowly forms of life propagate themselves in this way. A projection arises on the side of the animal, is automatically amputated, grows missing organs, and becomes a complete and independent animal. The process is exactly like the planting of rose-cuttings, one of the cases of asexual reproduction in the animal kingdom.

Now in many cases, where propagation by buds takes place, the buds undergo their full development into complete polyps while remaining connected with the parent. An example of a “colony” of sea-anemones thus formed is given by Plate XXV, Figs. 53 and 54. An allied form, known as Palythoa, is common in the Red Sea as little star-like rings of tentacles, of a beautiful deep, yet bright, green, carpeting the sand and stones in shallow water. Each star, or polyp head, measures about a quarter-inch in diameter, so that when a few dozen occur together they form a quite conspicuous patch of colour.

As already stated the corals are similar “colonial” organisms, the numerous cups on their stony branches representing each one polyp head. But how the polyps are connected with the stony material is best explained by, as before, taking the simplest possible case, that of a solitary non-colonial polyp, which is exactly like our simple sea-anemone, but has a stony cup like one of those of the ordinary corals.

Plate XXV, Figs. 55 and 56, represents such a form, which is in fact the only British stony coral[40]. But for differences in shape and proportion, the upper part of the organism is exactly similar to the sea-anemone shewn in Fig. 52, but beneath it is a stony mass, the coral cup, secreted by the base of the polyp, a seat exactly adapted to its own shape. The empty cup, after removal of the anemone, is shewn as seen from above in Fig. 57, to the right, and the curious radial plates of the same stony matter, so characteristic of all coral cups, are very plain. This polyp is comparatively large, measuring half an inch or more across.

This cup is not formed in the complicated way in which bone is made in the higher animals. The material is the cheapest possible, viz. limestone; this occurs in minute quantities in solution in all sea-water, and the coral polyp has the power of absorbing it[41] from the water and rendering it insoluble and stony in just those places where it is needed to form the kind of cup characteristic of the species. Another difference from bone is that the secretion is altogether outside the body of the animal; the cup is a mere dead structure from the first. One can imagine the animal as throwing down a limestone seat for itself, and as the seat thickens the polyp is raised more and more above the sea-bottom.

Plate XXVI

Plate XXVII

Imagine now the polyp to bud, as does Palythoa, and each bud to secrete its own cup, while the connecting branches also throw down the same limestone, so that the cups are connected on to one mass, and we have at once the formation of ordinary reef coral, of which perhaps the simplest possible case is the Dendrophyllia, figured on Plate XXVII, where each polyp gives rise to but one bud, which gives out one other, so each branch is like a simple chain of polyps and their cups. The other corals figured are rather more complicated, since one polyp gives off many buds, and their branches are correspondingly more massive. In the hemispherical corals, the connecting branches are short, practically non-existent, and the polyps are crowded together, as a kind of skin, over the solid mass of limestone they have secreted. Certain species of coral form enormous colonies, containing hundreds of thousands of little polyps. I remember a certain part of the fringing reef of Zanzibar[42] over which the water was 6 feet or more deep. Being perfectly clear, and so favourable to coral growth, it was inhabited by a species of the genus Porites, which formed huge cylinders, the flat tops measuring 6 to 12 feet across, level with that of the lowest spring-tide, since it is impossible for the polyps to live above that level. So closely were these great cylinders planted in the water that it was easy, by striding and jumping from one to another, to cross the channel to the shallower part of the reef on the other side.

From what has been said of the formation of the coral cups it is clear that the quantity of living matter going to the formation of these great cylinders is very small, a mere gelatinous film over the surface.

The fundamental simplicity of structure, which is common to every coral, does not preclude the evolution of an amazing variety of forms. In the course of time as many species have been evolved as there are possible combinations of the conditions, animate and inanimate, which affect coral growth and survival. In form these range from huge and solid stones, weighing many tons, to tiny delicate things like petrified lace or ferns, some of substance nearly as hard as shells, others so spongy as to be easily cut into by a knife. I have been enabled to give two plates illustrating a few out of this amazing variety. Both massive, hemispherical or dome-shaped, and more delicate branched species are shewn, but lobed growths, such as that shewn at the bottom of the first group, intergrade the two divisions. The latter specimen is of particular interest, being a species of the genus Porites, already referred to as forming great cylinders of solid stone. This small specimen was taken from shallow water, near lowest tide level, so that the polyp cups, which are too small to be visible in the photograph, were intact only on its sides. Above they were killed by the air and sun, and the stone they had formed, being exposed to the action of the sea, has been dissolved away slightly, leaving a narrow rim round the edge, where the part that was protected by living flesh shews the height the colony originally attained.

In the middle of the plate are small dome-shaped colonies, of species which, though rarely growing to the size of the Porites cylinders, may form very considerable boulders. Notice the different shapes of the polyp cups, with their radiating plates, and the varying beauty they impart to the surface of the stone, a beauty which is enhanced by examination under a lens.

Plate XXVIII

Most of the branched kinds belong to one great genus Madrepora, one of the most conspicuous of all the forms seen in a living reef, and which contains a very great number of species. In spite of the wide variety of outer shape, the structure of the coral polyps is almost identical throughout this genus. The colonies may be quite small and are generally of moderate size, but one tremendous growth has been recorded, which covered an acre of the sea bottom and sent its branches of stone to the height of 50 feet. In this case a single coral equalled in size a plantation of large trees, but what is usually seen is a network of branches springing from one thick stem and spreading horizontally, and covering a fan-shaped or circular area of a square yard at most.

Not only are the growth-forms of corals varied as those of plants, but the details of the polyp cups are well worth attention. Typically the surface of the coral is covered with round depressions, which may be minute, as in Porites, or half an inch across as in Caryophyllia and the dome-shaped species shewn on Plate XXVI. All are partially filled up by complicated series of radial plates, and a central core, best seen in the illustration of Caryophyllia, and the arrangement and ornamentation of these form an endless variety of patterns. In other cases the depressions, instead of being round, are elongated, forming meandering grooves over the surface, which, from their superficial likeness to the convolutions of the human brain, give the name “Brain coral” to certain kinds. In others again the walls of the cups disappear, and the system is reduced to a network of plates, converging to the centres of the polyps, or these may be so thickened and flattened that the spaces between them appear as fine lines, tracing a lace-like pattern on the surface of the stone.

One is tempted to write a whole book on the beauties of corals and coral animals but must refrain; one other form is however so interesting, and at the same time so common, that a short special description is given.

In many sheltered water gardens may be seen numbers of what look like overturned, stalkless, mushrooms. On handling they are found to lie loose on the sand and to be stony corals. They are in fact single polyps of phenomenal size, being up to six inches across, and the radiating plates, which so resemble the “gills” of a mushroom (hence the name of this genus, Fungia) correspond to those already seen in the other corals illustrated. The cup wall is however absent.

The life history is as strange as the coral that results. The young polyp produces at first a quite ordinary, small, cylindrical cup, Plate XXIX, which is fixed to a stone in the usual way. After reaching a certain size this swells at the top into a disc, like a mushroom on its stalk, except that the mushroom head is turned wrong side up. A little later this head falls off on to the sand, where it continues to grow into the big Fungia discs first met with. This, however, is not the death of the original polyp, which goes on growing new heads which in turn fall off, ad infinitum!

Many attempts have been made to visualise the beauties of a coral garden, in poetry, romance, and works of sober science. I can make no claim for my own picture, but that perhaps it is written with better acquaintance than is generally possible to poets and romanticists, and that it is free from exaggeration, as the writings of a biologist should be.

Let us imagine our exploration from the beginning. It is a calm morning in summer, the sea a pearl of beauty, under the new-risen sun. The heat, great even in the early morning, is unnoticed in enjoyment of the delicate pink and blue and golden shades reflected by the mirror-like surface, unbroken by any indication of what lies below. The tints of reef and shoal, which form so beautiful a part of the seascape when the water is rippled, are now exchanged for atmospheric colours, which, as we float over from deep to shallow water, give place to a panorama of coral gardens below.

Plate XXIX

As we row, keeping watch ahead, the reef seems suddenly to spring up before us, so steep is its slope.

The pleasure of the sight of a new and beautiful world of shapes and colours, mimicking yet utterly unlike those of life on land, is for us enhanced by the vigour of its life and growth, in happy contrast to the desert shore.

On the edge of the shoal Porites and other solid forms appear as great rocky buttresses among the lighter plant-like growths, or, a little way from it may rise from the depths as an isolated pillar. In many places in the Red Sea such coral pinnacles abound in comparatively deep water, a horror of unexpected danger to the sailor.

There is nothing more fascinating than the edge of a reef in the open sea, where numbers of forms and their delightful groupings can be seen in succession, one below another, till they become hazy and gradually lost in the blue depths, sixty to ninety feet below us. There are precipices clothed with a thick bush of spreading coral, some seeking the light by reaching out horizontally, others by growing upwards tree fashion, what appear to be bare rocks turning out to be massive colonies, as much alive as the more plant-like forms, caves, dark in contrast to the bright corals that surround their mouths, and the white shell-sand with which they are floored.

The general colour of living corals is very various, the snow white or creamy skeletons seen in museums being covered by a tinted film of polyps. The majority of species are some shade of brown, from deep chocolate to the golden colour of some seaweed covered boulders on home shores, but among these bright tints are abundant. The brown branches of Madrepora are generally tipped with light violet, pink or white, as though each ended in a flower, while other branched corals are a brilliant scarlet or bright green all over. Another forms a series of large thin sheets, spreading horizontally one above another, and all of a brilliant yellow! In these the flesh is inconspicuous, appearing as a mere colouring of the stony branches, but in others the polyps are as conspicuous as “sea-anemones,” with typical flower-like discs, a row of tentacles surrounding the mouth, or the tentacles may be so long that nothing else is visible. One of these, Galaxea, is very beautiful, shades of bright or dark green mingling with a greater or less proportion of brown, so that the rounded knolls of coral may resemble hillocks of grass, or of brown seaweed. Another large coral is almost devoid of tentacles altogether, but the polyps are large and the stone is covered as it were with green brown velvet, laid down in soft folds.

Of the inhabitants of these gardens and grottoes there is no space to speak. Anemones of all sizes and colours abound, and flower-like animals, the most beautiful of which are the sensitive sea-worms, add colour even to the corals. The gorgeous fish that lazily pass in and out, as though flaunting their beauty they could be careless of danger, have been described by every traveller.

The association between certain smaller fish, crabs and other higher animals with corals is remarkable. One sees for instance a branched coral with a shoal of tiny green fish hovering near, or in another case the fish are banded vertically black and white. Drop a pebble among them and they instantly disappear among the branches, and if the coral is taken out of the water the fish still cling to their refuge, and most of them are captured with it. These are but two examples of a whole world of life found only among corals.

Seeing that all corals are derivable from the sea-anemone (some form of which must have been the original ancestor of the whole family), and that the sea-anemone has been proved to be very distinctly an animal, I trust that the animal nature of the corals is now too firmly fixed in the reader’s mind to be shaken by their vegetable fixity, vegetative growth and form, or even by the fact that I am about to explain, viz. that the majority of corals do not obtain their nourishment by the capture of prey, but by the decomposition of the carbonic acid gas contained in the sea-water, a method of feeding which is the most distinctive feature of plant-life as opposed to animal. To recapitulate the well-known and fundamental fact of the life of this world, the plants are characterised by their taking up carbonic acid gas which, by the power of sunlight upon their green matter, they split up in some way so as to form starch from the carbon with water, while the oxygen is liberated back into the air. The animals, on the other hand, eat the food ready prepared for them by the plants, which is consumed in their bodies, and burned, as it were, back to carbonic acid, which land-animals get rid of in breathing. So there is a balance, the oxygen necessary to animal life being freed by the plants from the carbonic acid given by the animals, which carbonic acid is the necessary food-stuff of the plants.

The process in the sea is exactly similar, only that the gases concerned are dissolved in the water and rarely separate and become visible as bubbles. Fishes give off carbonic acid gas, dissolved in the sea-water, from their gills, and this is broken up by the seaweeds which liberate the oxygen from which the fishes and all animals re-form carbonic acid gas.

Now the amazing thing about the corals is that the polyps have entered into an alliance with certain microscopic plants which come to live in their bodies, and they feed upon the starchy products these plants form in sunlight, and even upon the plants themselves. So intimate is the union of these strange partners that neither can live without the other, the coral has lost its independence, and in fact as well as in appearance leads the life of a plant. At the first sight of a coral sea one wonders what takes the place of the great beds of brown and green weed which fringe British shores, and are a source of the oxygen essential to animal life. The discovery of these plant partners of the corals gives the answer. This easy life, this evasion of the necessity of capturing prey, is doubtless the reason for the degeneration of the polyp structure noticed above.

One of the greatest interests of these lowly forms of life is their place in the evolution of the higher. We have left all that is familiar, the creatures with heads and limbs, far behind, on the surface as it were, and are groping among the foundations of the edifice of creation. It is difficult indeed to express how very far down we are without some description of the rest of the series. But this is impossible; I am asked to give means of understanding what a coral is, and should not be thanked for giving in reply a treatise on Zoology. Let us take two steps only of the process of evolution, and let these short lengths give an expression of the whole descent.

Consider the vast interval of time and changes of structure involved in the evolution of man from his ape-like ancestor. How many thousands of years, what vast advances! How far above the purely animal is the lowest savage, and how far above that the best of civilised man! And yet even in the case of the brain, the development of which is man’s main advance, the man’s brain is but the further development of the ape’s[43], which has already gone the greater part of the way manwards from the condition found in ordinary animals.

Now we and the apes together are derived from some fish-like ancestor. We all had gill bars fundamentally like those of a fish at one stage of our existences. It is a vast descent through the reptiles to the amphibia and then to the fishes[44]! And the fishes again are our second step illustrative of the vast changes involved. Fish are just fish to the ordinary man, and yet the fact is that the difference between man and ape is just nothing to that between the ordinary higher fish, the kinds that come in after the soup, and the sharks. The shark family have not yet attained the possession of true bone, for instance, and their brain development is almost rudimentary. But we are already in the dim beginnings of geological history, for sharks essentially like those we now know were living when almost the earliest of rocks were being laid down as mud in primeval oceans. These were times incredibly remote, when land animals were not in existence, when plants were represented only by seaweeds, the whole land a desert but for possibly some creeping films of vegetation adapted to life on damp soil ashore, times long ages before those strange reptiles Iguanodon, Diplodocus, the whale-like Ichthyosaurus, the giant ferns and lycopods of the coal measures, whose fossil remains remind us of nightmare worlds which have passed away, had ever come into being.

We are at the beginning of geological history, and yet the corals are a large and flourishing class, coral-reefs are growing as nowadays, and the corals themselves, though of course of altogether different forms, are essentially the same down to the first syllable of recorded time. But having proofs of evolution which are independent of the geological record over these vast aeons, we may safely carry back the process into those times represented by rocks so ancient that no fossil trace of life is found in them, to the times when the lowest fish-like vertebrata were not, and the simple polyp was the highest product of life upon the earth. We know that most probably there really was such a time, but to imagine it is like trying to comprehend the solar system by arithmetic. We may speculate and wonder at the first beginnings of life, but I, for one, prefer to leave it to each reader’s imagination.

CHAPTER VIII
THE BUILDING OF REEFS

Sea-water, besides containing comparatively large quantities of common salt, contains several other substances in solution in less quantity. One of these is the limestone[45] which the coral polyp extracts and renders solid as its stony skeleton, and of which, in essentially the same way, the “shell fish,” whether oyster, winkles or crabs make their hard coverings. Another constituent is magnesium carbonate, a substance rather similar to limestone, to which we refer later.

Having examined the individual stone formed by the growth of a coral colony we must consider how such stones are aggregated to form a reef.

It is obvious that colonies cannot live for ever, any more than do individuals, and we need to know the fate of a dead colony and how it is replaced by a living one which shall continue the building. So great is the competition among the crowds of the floating young of the fixed animals that any vacant spot is at once appropriated. When a coral colony dies the coloured film of flesh speedily rots away and the snow-white stony skeleton remains, washed clean by waves and currents[46]. In a few days this is covered with a film of the finest green seaweed, invisible among which are the embryos of several orders of animals, e.g. shell-fish, or, there may be the larvae of some other coral. There is a tense struggle for survival among these young creatures, but on a growing reef conditions of course generally favour the coral’s young (otherwise the reef would cease to grow), some of which grow at the expense of nearly everything else and cover the site. Many of the large hemispherical corals live on the reef like loose stones, but on turning them over one may find quite a small shell, or coral branch, attached to the centre of the underside. This is the foundation of the whole, the resting place of the tiny floating larva, the growth of which first covered the stone on which it settled by a vigorous colony which when large enough to be independent of support continued its growth until the mass exceeded by hundreds of times the bulk of the original foundation.

The more delicate of coral skeletons, such as those of the porous branched madrepores, rarely survive the death of the polyps that formed them. On losing its coat of living flesh the coral is exposed to the action of boring animals, as well as to the direct solvent action of the sea-water, and many are thus destroyed. Partly they go back into solution, but the greater portion is broken down to mud and sand. In shallow water branched colonies are broken up into pebbles and coarse sand by the waves, and these materials serve to fill in the spaces between the larger colonies and pack the whole together into a solid mass.

There are other constituents of coral reefs of not very much less importance than the corals themselves. Large masses are formed of the bivalve shells which live in the coral mud, and which, by the solidification of this mud, form with it a limestone, such as that of which the houses of Port Sudan are built. Although among the very numerous and conspicuous fossils of this stone coral branches are not the commonest, yet the mass of shells and hardened mud is every bit as much a part of the reef as anything else is. In some reefs too, even where coral is growing abundantly, the shells of the great clam Tridacna are so abundant as to make up a considerable part of the total mass. Others again contain quantities of certain peculiar seaweeds (of which the “coralline” of British seas is one) which, though true plants in every detail, have the property of taking up limestone from the sea and forming therewith a skeleton, even harder and more compact than that of the corals. Plate XXX shews the appearance of these plants, and will enable the reader to identify some of those he meets with. These sometimes form a cement, by which the coral colonies and fragments are held together, and in some others the whole reef is formed of them[47]. Other organisms assist, but I observe my principle of dealing only with the most important features and desist from enumerating all.

This is the whole structure of the interior of such a reef as that which fringes the Red Sea coast, as seen e.g. during the excavation of the quay walls or slipway at Port Sudan. Great “stones” which are the more massive colonies, generally the genus Porites, are bedded in with smaller colonies whole or broken. In places are collections of grey mud and sand, also formed from coral by the action of boring organisms or perhaps as the residue left after partial solution by the sea.

“How fast does a coral reef grow?” is a question often asked, and never as yet truly answered. Probably each of the hundreds of species of coral has its own maximum rate of growth, which is however rarely attained, as it is certain that the rate of every colony of each species varies very widely with its position on the reef and its immediate surroundings. So taking the rate of growth of a few samples would go a very little way towards giving that of the corals in any given square yard of the reef edge. Although individual colonies may grow quite rapidly this is but half the question. We must know also full details of the action of eroding and transporting sea currents, solution, boring organisms (in coral, coral sand and mud) and subtract the total from that of the deposition of stone by living polyps, to obtain the net increase.

Plate XXX

The boring animals mentioned as reducing coral stone to mud are very easily found and examined. Almost any old worn piece of coral, and many still living colonies, are found to be studded with small slit-like holes with slightly raised borders. On breaking into the stone each hole is found to lead at once into an oval cavity, say an inch long by three-eighths in diameter, containing a bivalve shell of about the same size, Lithodomus by name, from its appearance known as the “date shell,” which has made the hollow and is continually enlarging it. Other colonies when broken across instead of shewing pure white limestone are found to be honeycombed with yellow or red spongy matter. This is the sponge Clione, which has the property, especially astonishing in a sponge, of boring into any limestone, whether coral or shell, making it quite rotten and so, finally, reducing it to mud and sand.

Certain worms live in the same way. The largest species of these, by name Eunice siciliensis, attains a length of a yard or so, and the thickness of a quarter of an inch, but so intricate is its boring that it is practically impossible to extract a full-grown specimen entire. The head end is at the innermost part of the burrow, and when extracted the two white gouge-like teeth, by which the burrow is cut out, are easily seen.

There is a fish too, Pseudoscarus by name, which actually lives on coral! It is commonly taken by fishermen and is easily recognised by its gorgeous green, blue and pink colours, but particularly by its teeth, which are fused into two pairs of chisels, with which the surface of the coral, and with it its living matter, is browsed away. Cut open a specimen of this fish and you find its guts full of fragments of coral[48].

Plate XXXI

Pholas, another boring bivalve mollusc, common in shales on some British coasts, is less often seen. Its burrow is deeply buried in a solid living colony separated from the outside by a comparatively long passage. But it is not nearly so common as the preceding forms, and lives more solitary, Lithodomus generally occurring in numbers together.

After the coral has been broken down by these means, the sand is further reduced to fine mud by the action of those animals which live by burrowing in it, and passing large quantities through their guts, after the manner of earthworms. Just as there is a great fauna which lives on the nourishment filtered from large quantities of sea-water[49] so there is another great and varied community of sand eaters. There are first of all the worms, next, but more important in the tropics, great numbers of large holothurians or “sea slugs” (though slugs they are not), some of which crawl on and eat only the surface sand, but one species burrows deeply and raises casts like an earthworm, but a hundred times the size. Considering the large effects produced by the ordinary earthworm in a year, that resulting from the presence of animals hundreds of times their bulk, whose casts in many lagoons entirely cover the bottom, must be very considerable indeed.

One observation that can be made by anybody is to note how long it is before corals reappear once a reef has been cleared out, e.g. for the foundations of a quay wall. Two small portions of an apparently growing reef at Port Sudan were buried under a pile of stones for the foundations of the east and west Customs landings, and four years later there was no growth of coral on the artificial slopes thus made, though every condition apparently remains as favourable as before. Again at a point inside Dongonab Bay, where coral growth is luxuriant in shallows, the coral was some years ago collected from one spot and a sea wall built with it. A few small colonies have established themselves upon the sides of the wall after an interval of twelve years; they are perfectly healthy, yet their bulk is an infinitesimal fraction of that removed from the wall by solution and attrition. This shews how even among growing coral one cannot be sure that the degradation of rock to sand and mud is not in excess of aggradation, i.e. its building up by coral organisms, and how a lagoon may be rapidly eating away its encircling reefs and yet contain comparatively luxuriant coral gardens.

It is in its external form that a coral reef shews features which give it an individuality above that of a mere heap of stones. Generally it rises with a steep slope from the sea bottom which ends in a low precipice, above which is another and more gentle slope to the highest point of the reef, a foot or two above lowest water, which is near its outer edge. Passing landwards the reef level is lower again, and we may have a boat channel or series of lagoons, where the native canoes can travel on calm water however the sea may rage outside. This is succeeded by a flat of bare rock, which rises slowly up to the base of the undercut coral cliffs as in imaginary section in Diagram 1 and the Photograph on Plate XXXII.

We have here three striking features, viz. precipitous reef edge, raised border and reef flat with boat channel, strongly differentiating the shore of a coral sea from the more or less even slope we are accustomed to at home, resulting in a nearly waveless shore and breakers out at sea, an endless line of purest white dividing the green of the shallows from the blue-black of the deep water.

Plate XXXII