CHAPTER III.. Shallow Water Fauna of the Tropics.

SHALLOW WATER FAUNA OF THE TROPICS.

The shallow waters of the Tropical seas present us with so many different conditions of tides, of coast lines, of temperature, of liability to storms, and of other natural phenomena, that we find an infinite variety in the general character of their Fauna. Just as on land, we find in one part of the Tropics a dense forest, and in another a dry desert, so in the Tropical seas we find on one coast a crowded population of animals and plants, and on another a sandy bottom, which is, comparatively speaking, lifeless. In order to bring before the reader some of the principal characters of animal life in the shallow waters of the Tropics it will be well to confine his attention to one part of the world which is fairly well-known--namely, the Malay Archipelago--and refer only in passing to other localities. The most characteristic feature of Tropical coasts is the Coral-reef, and nowhere in the world may it be seen in more exquisite variety than in the archipelagoes of the East. Although, however, these vast structures are so abundant on some coasts, others seem to be entirely without them. They are not found at all on the Western coasts of America or of Africa, and even in some regions of the larger islands in the Pacific and the Indian Oceans, many miles of coast line may be devoid of them. These curious and interesting variations in the distribution of the reefs can be explained, but the explanation will be more easily understood when their general features have been described.

It is a well-known fact that the great masses of limestone which compose the reefs are formed by the activity of countless thousands of minute animals, but the popular idea of the general form of these animals has been very much misled by the unfortunate term “Coral insect,” which has crept into many books of travel, and the leading articles in the newspapers. The word “insect” is used by zoologists as a general term for certain air-breathing animals that are widely distributed over the surface of the earth. Many of them are extremely tiny, and hence the natural mistake has arisen in the untrained mind that all minute animals are insects. It might clear the ground a little if the reader would note at once that “insects” are very rarely indeed marine in habit. If there is a need for a popular word for the animals that form coral, it should be Coral “polyps” or Coral “anemones.” The word “coral” has, from the zoologist’s point of view, a very indefinite meaning, for it is applied to the hard skeleton of carbonate of lime formed by certain Sea-weeds, Sponges, and Worms, as well as to that of Coral-anemones and other Polyps. In many places on the British coasts the sea-bottom is very largely composed of a branching Coral formed by a true Sea-weed called Lithothamnion, and in other places very large lumps of rock are made by a Worm named Filograna. In the Tropical regions, too, the well-known Nullipores, which in many places play an important part in the formation of Coral-reefs, are of vegetable, and not animal origin.

However, the greatest part of the Coral-reef is made by animals closely related to the Sea-anemones, living together in colonies; and of all the different kinds of Coral-polyps, by far the most prolific as a reef-builder is one which will be referred to in these pages as the Madrepore.

Let us consider now the manner in which the Polyps form the Coral. In a very old work on the natural history of Corals a statement is made to the effect, that the Polyps construct the Coral in much the same way as Bees build their hive, or a Bird its nest. This very erroneous view coincides closely with ideas which might easily be gained by a casual observation of corals in a museum. The lime is not, however, collected as such from the sea by the Coral-polyps and plastered round their bodies to form a house or shelter, but it is formed as a secretion by the activity of certain organs of the animal’s body, and is consequently a true shell or skeleton. In a Coral, which is formed by a colony of numerous Polyps, the shell secreted by each individual fuses on to those formed by its neighbours, and thus a communal shell is formed which may assume a most complicated branching, bushy form, according to the species of the Coral and the conditions that are favourable or unfavourable to the nourishment and growth of the different parts of the colony. In such a Coral as the Madrepore every individual Polyp is connected with its neighbours by a system of branching canals; and as spaces are left for these when the shell is formed, the dried Coral is perforated by numerous tubular pores, and has a soft, spongy texture which can be easily crushed into a powder if trodden upon.

In other Corals the canals of communication between the Polyps are entirely at the surface, and the shell that is formed is much harder and imperforate. In others again colonies are not formed, but each individual grows to a considerable size and remains independent of its fellows all its life.

These, then, are three of the more important varieties of Corals found on the reef, the Perforate, the Imperforate, and the Solitary Corals; but it must be remembered that all the Corals of the reef cannot be included in these three groups. The varieties are much more numerous and in many cases much more complicated and difficult to understand.

Although the Coral-reefs of the Tropical world have a general resemblance to one another, the differences in detail are so great that it is impossible to describe any one as typical. In sailing over the edge of a reef near the coast on a calm day, when the water is so clear that the bottom can be seen at a depth of 8 or 9 feet, the reef may, in some regions, be observed to change in character every few minutes. In one spot there may be clumps of living Corals surrounded by beds of fine white sand; in another there will be great stretches of branching Madrepores; in another Madrepores, Mushroom-corals, the Imperforate Brain-corals, Sponges, and many other forms of life will be clustered together; while further on the predominant features will be the soft and slimy Sarcophytums, looking like large green toad-stools, some lumps of Organ-pipe Corals and a few colonies of the Blue-coral.

On other coasts I have wandered for miles along a reef mainly composed of endless tangles of Madrepores, with very little variation indeed in the general form of the Corals, in the character of the Sponges and Sea-weeds that grow with them, in their colour or in any other detail. Anyone reading the many accounts of Coral-reefs that have been written by travellers, must be struck with their inconsistency as regards many particulars; and in no one point are they more inconsistent than in the description of the colours--some writing in glowing terms of the beauties of the sea-gardens, and others complaining that their charms have been grossly exaggerated. As a matter of fact some reefs have a prevailing dull green or brown tone, while others exhibit all the colours of the rainbow in their more brilliant shades and tints. Another cause of the discrepancy is that some reefs can only rarely be approached in a small boat owing to the breakers that dash over them; whilst in the Tropical calms a tiny canoe can with perfect safety be manœuvred over the reefs during nine months of the year. From my own experience I can assert that it would be difficult indeed to exaggerate the glorious beauty of some of the reefs in the Malay Archipelago, more particularly of those where many different kinds of Corals may be seen in close proximity to one another. On such a reef, for example, there may be seen Madrepores with bright violet growing-points to their branches, orange or red Fan-corals, bright brick-red Sponges, yellow Sarcophytums, emerald green Organ-pipe Corals and dozens of other forms of animal life in every imaginable colour. When seen from a boat through two or three feet of water, these portions of the reef look more like a beautifully planted flower-bed than a mass of animals; but the simile is not a complete one, for the branches of the Madrepores, the great knobs and lumps of the Brain and Organ-pipe corals, the fronds of the Gorgonias and other forms make a wild mass of organisms resembling a tangled thicket or a miniature forest. At low-water of spring-tides the living reef is partly left exposed, and then the scene changes, for the Polyps retract their tentacles after the manner of the Sea-anemones and retreat as far as possible into the shelter afforded by their shells.

The interest of the living reefs is, however, by no means confined to such organisms as the Corals and Sponges, which are immoveably fixed to the bottom; for numerous brightly-coloured Star-fishes, Sea-urchins, Brittle-stars, Sea-slugs, and their allies crawl about among the branches and the débris of the dead Corals; while Crabs, Lobsters and Shrimps of many kinds may be seen swimming or crawling in search of their prey, and the marvellously striped and spotted Coral fishes dart hither and thither in the thicket, or remain hovering in the water among the Corals. The whole scene is most fascinating. As the boat slowly drifts along, new and strange creatures are constantly coming into sight and disappearing again. Here the writhing arms of a bright blue Brittle-star may be seen embracing the stem of a Coral-branch; there a curiously flattened Chætodon, with its body marked by great diagonal yellow bands, is nibbling at the young, tender branchlets; in another place four or five Sea-urchins with very long and slender spines are lying apparently motionless on the bottom; while a little further on a long black Slug-like creature, the famous “Trepang” of commerce, is slowly wending its way across the reef. Now and again a large shoal of little Fish or a small party of Cuttlefishes may be seen, and these may rapidly be dispersed in all directions by the sudden dash of a Sea-perch or a small Shark. The interest is so varied, so many-sided, in these scenes of animal life that the attention of the naturalist is with difficulty kept to any particular point. The feature which is perhaps the most striking is, however, the wonderful variety in the colours of the animals and of the character of their markings.

If we consider the Fishes alone, we find some of them have broad yellow bands running diagonally across their bodies, others have thin longitudinal stripes of blue and yellow, some have a uniform bright red colour, and others again have their red skins speckled with blue spots. It would take more than a whole chapter of this book to describe even the principal varieties of pattern found on one such Coral-reef, but the main fact that has to be related is that where the reefs are built by brightly-coloured Polyps, there we find these curiously marked Fishes. There can be little doubt that the marking and colouring does give a certain amount of protection to them. Numerous individual cases have been mentioned of Fishes which resemble some particular Sea-weed or Coral; but this general statement is the important one, that on a parti-coloured background the striped and speckled Fish are less conspicuous than those that are modestly attired.

The Fishes of the reefs, however, have other means of protection than that afforded by their colours. The Trigger-fish and Coffer-fish, for instance, have a body encased in closely fitting hard, thick scales, so that they might almost be called “armour-plated fish”; and the Globe-fish bristle all over with long and extremely sharp spines. In the Trigger-fish there is a curious modification of the three front spines on the back, to which their name is due. It is not known precisely how they act, but they probably form an effective weapon of defence. In the same family of Fish we frequently find on each side of the tail two or three rows of sharp spines, which may also be regarded as defensive. In the family of the Surgeons there is only one of these spines on each side of the tail, and it is much larger than any of those in the Trigger-fish, and capable of being folded down into a case in the skin like a clasp-knife. It is said that these spines are connected with a poison-gland, and can give very severe and painful wounds, like the poison spines of our European Sting-rays and Weavers.

A few days’ hunting on a Coral-reef will reveal the fact that it supports a very considerable population of Crustaceans. They are not at all obvious at first to one who has had no previous experience of reef-work, partly because they resemble the general appearance of their surroundings very closely, and partly because of their habit of remaining perfectly motionless when first alarmed. In form, many of them are, like the Lobsters, Crabs and Prawns of our own coasts, but their colours and markings are, like those of the reef-fishes, characterised by their brilliancy and their arrangement in bands and stripes. The smaller ones can be caught after a little practice with a simple hand-net, but the larger ones are more easily captured by a rattan noose in the pools, after the blocks of Coral are loosened by a pick-axe and slowly turned over.

The Coral-reef is a favourite hunting-ground for the conchologist, some of the largest and most beautiful shells in the world being found amongst the Corals. In Celebes, the giant bivalves, the huge Tridacnas, which are sometimes two feet across, and whose shells have been known to weigh as much as 500 lbs., may be seen wedged in among the Corals. The mantle of the living animal presents to the observer a wonderful display of colour as it lies in the shallow water with its shells open. The animal is eaten by the Malays, who roast it on a tripod spit over a fire, and cut it into steaks. A fair-sized Tridacna will afford a good meal for four or five men. The great Cowries, Helmet-shells, and many other species may be found in hunting over the reefs, but their beauties are frequently hidden, when alive, by the coal-black mantle which folds back over the shell as they crawl along. It must not be supposed, however, that all the shells of the reefs reach to such enormous size as those we have hitherto mentioned, for a rich harvest of species awaits the eager conchologist who hunts for the smallest shells he can find in the pools. Within the last few years a large number of new species of small Molluscs have been described from the coral seas, many of which do not attain to a total length of more than ⅛ of an inch when perfectly adult, so that the range in size of this class of animals is very great indeed.

So much has been said about the Madrepores, the Imperforate Corals and the solitary Corals of the reef, that the impression might be left that all the Polyps of the Tropics differ from those of the Temperate regions in the fact that they form shells or skeletal structures. This is by no means the case, for there are many species of true Sea-anemones and other Polyps to be found on Coral-reefs which make no shell at all, and others in which the body-wall is strengthened by numerous, but very minute spicules or grains of lime which, on the decomposition of the animal’s body, fall down into a shapeless powder or sand.

True Sea-anemones are not very abundant on the reefs of North Celebes, but many species have been found on the Barrier-reef of the Australian coast, and among them specimens reaching the gigantic size of two feet in diameter--the largest size attained by single individuals of the class of animals to which the Anemones belong. Another family of Polyps called the Clavulariidæ belonging to the Alcyonarians has some species which make no skeleton of calcium carbonate. An illustration of one of these is given below (Fig. 22). Another species, called Clavularia viridis, which forms a few spicules in its body-wall, has a very wide distribution in the East Indies. In some places patches of it may be seen several square yards in extent, and the crowds of little Polyp heads with their eight-feathered tentacles waving to and fro with the pulsations of the tide, is a sight that excites immense interest and admiration in the mind of the observer.

One word more about the Corals. Where they are so abundant in number and species, where rocks hundreds of miles in extent are mainly composed of their shells and skeletons, it might be thought that a rich profit could be gained by collecting ship-loads of the Coral that is used for making beads and brooches by our jewellers. Any expedition, however, fitted out for this purpose would end in disastrous failure, for the Precious-coral is not known to occur anywhere in the neighbourhood of Coral-reefs, but the fishery is confined to certain parts of the Mediterranean Sea. Species closely allied to the Precious-coral, but of an inferior colour, have been found in the Japanese waters. None of the Coral structures found on the reefs have the same delicate salmon-pink colour and probably none of them are hard enough to take a good polish.

The Coral-reefs which occur in different parts of the Tropical world were considered by Darwin under three heads,--Barrier-reefs, Atolls, and Fringing-reefs. The distinction between these three kinds of reef is not one that can be insisted upon scientifically, but the arrangement is convenient for purposes of description.

The Barrier-reefs (Fig. 23, B) are situated at a distance of one to eight miles from the coast, and are separated from it by a lagoon of moderately deep water. The Barrier-reef of New Caledonia is said to be 400 miles in length and it follows the general contour of the coast line.

The Atolls (Fig. 23, C) are ring-shaped islands composed of coral limestone with a lagoon of salt water within them, situated in the sea without any definite relation to other existing land.

The Fringing or shore-reefs (Fig. 23, A) are situated at a distance of 100 yards or less from the beach and separated from it by a shallow lagoon which is frequently left as a dry sand-bank at low tides.

Many forms of reefs are found in different parts of the world, but they may all be looked upon as special modifications of one of these three groups. The facts which Darwin collected about Coral-reefs in his memorable voyage round the world in the Beagle, suggested to this great observer that all the different forms of reef must be related to one another, and he formulated an ingenious theory to show how, by the gradual sinking of the crust of the earth, Fringing-reefs may have become, in the course of a long period of time, either Atolls or Barrier-reefs. Some doubts have recently been expressed as to the truth of Darwin’s famous “subsidence theory.” But, whether it is true or not, to Darwin is due the credit of bringing home forcibly to our minds the fact that Coral-reefs are slowly undergoing changes of growth and of destruction, which must lead to most important and far-reaching alteration in the character of the Tropical seas. It is not intended in this work to enter into a discussion of the various alternative theories of Coral-reef formation, but a few words may be added on the method of formation of Coral limestone.

In studying any one particular form of reef-building Coral we can find a long series of specimens from one inch in length up to a certain maximum, which varies with the species and the reef, but may reach over five feet in diameter as can be seen in the specimens now in the British Museum at South Kensington. Beyond this maximum size--let us say four feet in diameter--the Coral rarely grows, because at a certain age, probably when the vitality of the Coral is on the wane, the stalk of attachment becomes so bored with parasitic Sponges, Worms, Fungi and other organisms, that it is thoroughly rotten. If a large block be picked up from the reef, and with the help of two or three strong natives carried ashore and broken up with a hammer, a most interesting migration of Crabs, Worms and other creatures occurs, and the collecting bottles may be filled with a rich variety of animals parasitic on the Coral. Now the time comes when the stalk becomes so brittle that a heavy wave breaks it in two, and the Coral topples over and dies. If it falls into the sand, either on the inside or the outside of the reef and becomes buried, it may be preserved fairly complete, but if it lodges between other Corals the waves and the parasites between them dissolve it and break it up into thousands of pieces. This constant disintegration leads to the formation of great quantities of coralline sand which fills up the interstices between the living Corals, or gets washed over into the lagoon, or falls as a talus over the seaward slope of the reefs. In the latter case lumps of Coral torn off from the reef become embedded in it and form with it a bank which gives support to more living Polyps on the seaward slope. Consequently, in the course of many years, a reef which was at one time only fifty yards from the beach may have extended to a distance of a hundred yards or more, growing, as it were, on the skeletons or shells of the Corals that have died. There can be little doubt that Coral-reefs do grow seawards in this manner in some places, but they may also be either beaten back or kept for a long time perfectly stationary if the tides are too strong or too slack.

What the precise conditions are which favour the growth seawards of Coral-reefs has not yet been systematically investigated; but we may suppose that if the tides are too strong the sand has no opportunity of settling between the Coral blocks and forming a solid limestone rock, and if they are too slow the Coral-polyps do not get sufficient nourishment to allow them to build fast enough to counteract the solvent action of the water.

An interesting point connected with the Coral-reefs is the manner in which they are formed at first. A volcanic upheaval gives rise to a new island which, in the course of time, is surrounded by a Fringing-reef. How does this reef begin?

The answer to this question has been recently given by the observation of the formation of new Corals on the shores of the island of Krakatoa which is situated in the Sunda straits, and was the seat in 1884 of one of the most violent volcanic eruptions of historical times. After the eruption the sea-bottom round the island was found to consist of a fine volcanic mud, in which it may be believed no Coral embryos could find a secure foothold. Now it is known that living Corals give rise to a number of very minute larvæ which for a period of time swim freely in the water, eventually settling down on some solid stone or shell to give rise by growth and budding to the Coral blocks. These larvæ frequently settle down on a piece of floating pumice-stone and after a time grow to such a size that they sink it. If, in sinking, they fall upon the bottom in shallow water they form together a substratum on which other larvæ can settle and flourish. This is apparently the manner in which Coral clumps are beginning on the slopes of Krakatoa and these will undoubtedly give rise in time to a more or less complete Fringing-reef.

Any further discussion on this point would lead us into subjects beyond the scope of this book, but enough has been said to indicate to the reader the manner in which the countless Coral-polyps may, in the course of time, change the position of the reefs on Coral shores, thereby altering the set of the tides there, changing the position of the sand-banks, affecting the rate of erosion of the cliffs, and in other ways causing important modifications of the coast line.

I have mentioned that the ground on the growing edge of the Coral-reef is carpeted with Corals, Sponges and many other forms of animal life; in the water swim countless Fish, and the branches of the Corals yield to the naturalist innumerable forms of creeping and crawling creatures. The shallow waters of the Tropics, as a whole, however, do not possess a particularly rich Fauna,--in fact, the distinguished Dr Kükenthal, who has had great experience of marine work, says that, in his opinion, the Tropical seas are not richer in littoral animals than the Arctic seas. Between the reef and the sea-beach there is a lagoon, of varying breadth, with a sandy bottom, which is almost as barren of animal life as a desert. A few Worms and Crabs, here and there a Star-fish and some shells of Foraminifers, are all the spoils which fall to the bag of the naturalist after many hours’ wading on this unprofitable ground. The reason for this is, perhaps, not far to seek. When the tide goes down many stretches of sand are left dry, and others retain only a few inches of water. The exposure to the heat of a Tropical sun soon kills and dries up any living animal that is unable to burrow deeply in the sand, and the water in the shallow pools rises in temperature to a degree that the human hand or foot can only just bear, so that the little Fish that escape into them run the risk of being slowly cooked alive.

On the inside of the lagoon there is, in many places, a broad belt of Mangrove trees, forming the “Mangrove-swamp,” which contains some interesting and important animals. These trees have a peculiar spreading and branching system of roots which are left exposed when the tide goes down, and form with one another a kind of network or web a foot or more above the ground, upon which it is possible, with care, to walk from place to place, at low tide. Between the roots there is a slimy black sand or ooze, sometimes hard enough to walk upon, but more commonly soft and treacherous. At high tide the water rises to a height of two or three feet, completely covering the roots and giving the swamp the appearance of a forest growing in the sea. Of the animals, aërial and terrestrial, that haunt the swamps it is not necessary to say more than a few words, although they too form a study of great interest to the enthusiastic naturalist. But the marine zoologist who visits the swamps cannot fail to take note of the millions of Ants, Flies and Mosquitoes which torment him at every step, and make a prolonged stay an impossibility.

One of the first creatures to be seen on entering a Mangrove-swamp at low tide is a curious little Fish called Periophthalmus. In some places hundreds may be seen at one time resting on the roots of the Mangroves, or skipping over the pools of water from one root to another. There are many varieties of Periophthalmus in different parts of the world, and their habits are not exactly the same, so, to give an accurate description, our attention will be confined to the form occurring in N. Celebes. This little Fish is about three inches in length and remarkable for its very peculiar eyes, which are of a bright yellow colour, situated quite close together on the top of the head, and projecting so much from their sockets that the outline of more than two-thirds of the eye-ball can be seen. These eyes are extraordinarily moveable, and frequently revolve quite independently of one another, like the eyes of a Chamæleon, giving the animal a most grotesque and even ludicrous expression. These Fish seem to swim in the water very seldom; when undisturbed they may be seen clinging to rocks or trees by their fore-fins with their tails only in the water, but from time to time they spring into the air to catch a Fly on the wing, or a small Crab which has come unwarily within their range. Their fore-fins are peculiarly adapted to their habits, in that they have a very muscular base and a distinct elbow joint.

These creatures are not easy to capture, as the ground on which they live is not adapted for rapid pursuit, and it is impossible to get close enough to them to catch them in a hand-net with a long handle. When kept in an aquarium it is seen that although they are rapid swimmers when they do go below the surface, they seem to prefer to live with their head and shoulders out of the water; and when chased in their natural haunts they very rarely, if ever, seek to escape by plunging into the water, but they execute a series of rapid jumps with extraordinary rapidity from root to root or rock to rock, and so avoid their pursuers. Their existence is really an amphibious one, and their food consists partly of Insects on the wing. Their gills are very much reduced in size, and it seems probable, from observations that have been recently made, that their respiration is partly carried on by the thin skin between the rays of the tail-fin.

Another animal extremely abundant in the Mangrove-swamps of Celebes, and, like Periophthalmus, having a very wide distribution in similar places in other tropical countries, is the “Caller-crab” Gelasimus. These Crabs are about an inch in breadth across the back, and are remarkable for possessing one very much enlarged and brightly-coloured claw, the others being normal in size and dull in colour like the rest of the body. On first entering the swamp at low tide there may be seen on the mud between the roots of the trees a number of bright yellow, blue or green objects, which, as the traveller approaches, disappear one by one into holes in the ground. When the eyes are accustomed to the gloom of the swamp these bright objects are seen to be the great claws of the “Caller-crabs,” the rest of the body being inconspicuous owing to its close resemblance in colour to the slimy ground.

These are the principal and most abundant marine inhabitants of the swamps, and as has been pointed out, all of them are more or less amphibious in habit. More locally distributed, Oysters and other bivalves may be found attached to the roots of the trees. Several species of marsh Gastropods occur, some of them in great abundance in a few localities. Occasionally a Sea-anemone, with remarkable powers of burrowing rapidly in the sand when disturbed, may be found, and to the microscopist a harvest of Foraminifers and other minute forms of life awaits investigation and description in the Mangrove-swamp.

Whenever the tide is high a considerable number of Fish-fry, Jelly-fish, and other forms of floating and swimming life characteristic of these waters, drift into the swamps; and some being caught by the tangle of roots are left behind, either in the pools, or high and moderately dry upon the sand when the tide ebbs. Upon these victims of the retreating tide swarms of Ants and Flies descend from the trees, Crabs from the shore and from their holes in the sand are on the watch for them, Kingfishers and Sandpipers are ready to pounce upon those which are most to their taste, so that before the friendly waters of the ocean return to the swamp, scarcely one of them is left.

These constitute what may be called in the swamp the extraneous Fauna, which if it is not truly indigenous is nevertheless necessary for the continued existence and well-being of the true inhabitants.

The character of the sea-bottom on the outer side of the living Coral-reefs varies so much in different parts of the world that an adequate treatment of the Fauna in that region would have to be one of greater length than is possible in this book.

The living edge of the Coral-reef is in some cases situated on the top of a submarine precipice of very considerable height, and in many places the sounding-line goes down to a depth of five or six hundred feet a few yards beyond the limits of the reef. The practical difficulties in the way of determining the character of the Fauna of any sea-bottom that shelves in this manner are very great, but where it is partly composed of massive lumps of solid Coral they are at present insurmountable. Every time the dredge or trawl reaches the bottom it becomes entangled in the Coral branches, and is liable to be seriously torn, or even lost. Swabs and iron hooks and fish traps may yield some scraps of information, but speaking generally, the Fauna of these steep slopes is scarcely known at all.

The most important question, from the geological point of view, that has to be determined is the depth of water in which reef-building Corals can live and thrive. This is still a matter of uncertainty owing to the practical difficulties met with in the attempts to investigate it. Darwin estimated that the limit of vigorous coral growth was between 20 and 30 fathoms, but in recent years, owing to the discovery of luxurious Coral patches in 44 fathoms on the Tizard and Macclesfield banks, there is a pretty general opinion that his estimate is too low.

Whatever the exact limit may be, it is quite clear that in many parts of the world the sea-bottom quite close to the outer edge of the reef cannot support a vigorous Coral fauna. Here and there patches of peculiar deep-sea species of Corals occur, but they do not form in such depths anything of the nature of a reef. They are usually isolated specimens, similar to those that are found in deep water on the Norwegian coast and other parts of the world outside the limits of the Tropics; but these specimens really belong to the deep-sea Fauna, of which we shall learn more in another chapter.

In many places, however, the water at the base of the outer edge of the reef is not very deep, and may slope away gradually towards the bed of the ocean. The Fauna of such slopes in the Tropics is not characteristically rich, as my own experiences of dredging in such waters have proved. Time after time the dredge that was used in 15 to 20 fathoms off the coast of Talisse, came up with nothing but sand or gravel. Occasionally a Brittle-star or a branch of dead Coral, with a few Zoophytes growing on it, came up; and in some places a few beautiful Lily-stars or Crinoids relieved the monotony of the investigation. But, on the whole, the animals found in this region were not numerous, nor of a character to excite any particular interest.

Before bringing this chapter to a close, a brief reference must be made to one of the most remarkable phenomena in the animal kingdom,--this is the history of the Palolo worm. On certain parts of the coast of the Samoan islands the Palolo worm appears in great abundance in the early morning hours of one or two days at the beginning of the third quarter of the moon in the months of October and November. As the worm is regarded as a very great delicacy by the natives, the day of its appearance is looked upon as one of the most important red-letter days of the year. Weeks before the worms are expected the advent of the Palolo is discussed, stories are told of the fisheries of bye-gone years, anecdotes of the last year are remembered and rehearsed, and the whole population is prepared for the great event as for a feast.

When the grand day arrives the boats are decorated, the girls put on all their finery, and everyone who can find a seat in a boat goes off to the fishery amid a merry chorus of song and laughter. It must, indeed, be a strange sight to see the flotilla of canoes with their eagerly expectant and gaily-bedecked crews, waiting in the dim light of the half moon for the day to break and the exciting fishery to begin. As soon as it is light enough to see into the water, a few writhing Worms may be distinguished at the surface, which increase in number with such extraordinary rapidity, that in a little while it is impossible to see anything below three or four inches owing to the multitude of Palolos. As quickly as possible the fishery proceeds, every man, woman, and child gathering the harvest of Worms during the precious moments of the sunrise. When at last the sun rises well above the horizon the Worm disappears again, and the boats hasten to the shores with their booty.

This remarkable swarming process of the Palolo, occurring as it does, only once or twice a year, in constant relation with a particular phase of the moon, and lasting on each occasion only a few minutes in time, is not the only noteworthy feature of the animal.

The Palolo worm, as it is caught, varies in length from an inch to a foot or more, and is about a quarter of an inch in breadth, but it readily breaks up into pieces when handled. It is composed of numerous rings or segments, each provided with a pair of processes bearing bristles, but there is no head. Astonishing as it may seem to those unacquainted with the natural history of Worms, it is nevertheless a fact that when the Palolo swarms it leaves its head behind among the Corals, where, in all probability, it regenerates a new body. This accounts for the fact that while the body of the Palolo is frequently brought home to our Museums in England, its head is a rarity. The colour of the Worm varies very considerably. The pieces bearing eggs are usually of some shade of green, hence the specific name Palolo viridis that is given to it by scientists, but the males are usually white. In connection with the appearance of the Worm there is a curious statement that once in every four years it is exactly one lunar month late, so that the time of year of its occurrence is constant. The natives are also forewarned of the advent of the Palolo worm by the movements of the land Crabs, which, it is reported, come down from the fields and forest a few days before the Palolo feast and plunge into the sea.

The precise habitat of the Worm when it is not swarming is still a matter of some doubt. A few rare specimens have been found in the Coral blocks in shallow water, but it is generally supposed that the majority of them live in deep water on the outer side of the reefs. It is not confined by any means to Samoa. It occurs also in Fiji, Tonga and other Pacific islands. A Worm similar to the Palolo in habits was described years ago by Rumphius in the Malay Archipelago, and Saville Kent mentions a little Nereid worm with similar spawning habits on the great Barrier-reef of Queensland.