CHAPTER IV.. Surface-Swimming Fauna (invertebrates).

SURFACE-SWIMMING FAUNA (INVERTEBRATES).

Everyone of an observant turn of mind must have noticed that in the wake of a boat that is passing through the water on a calm summer’s night, sparks of bright phosphorescent light may be seen to appear, to remain for a few seconds, and then become extinguished again. Sometimes the breaking of the ripples on the surface of the water seems to be sufficient to cause these sparks to appear, but occasionally streaks and flashes of pale blue light arise and disappear without apparently any such mechanical disturbance.

The phosphorescence of the sea, as this phenomenon is called, is common enough on our coasts, but it never reaches the degree of brilliancy and beauty which is so remarkable in the open Atlantic Ocean, the South Seas, and some other parts of the world. In the Atlantic Ocean the phosphorescence is sometimes so bright that it is possible to read a book on deck by its light alone; and on a dark night in the Banda seas the water is often like a huge expanse of pale blue smoke studded with diamonds and other lustrous gems.

These lights are mainly produced by animals which float and drift about on the surface of the water. It is not, as is very commonly supposed, only one or two different kinds of animals that are phosphorescent, but a vast number belonging to many widely different families and of a great variety of form and structure. When the day breaks many of these animals sink down a few fathoms into the darker and cooler strata of water, but a considerable number remain so close to the surface that they can be easily caught in a muslin net dragged after a boat.

Some of these animals, such as the Jelly-fish, can, during the day, be observed clearly enough from the boat, others can only be seen when the contents of the net are emptied into a glass bottle, and others again are so minute that it requires a strong magnifying glass to detect them at all. Such animals that float or drift in the water without powers of swimming vigorously in one direction or the other, are collectively called the Plankton. In every sea, from the Arctic regions to the Equator, a Plankton will be found. Sometimes it is mainly composed of one species, in other cases it consists of many different species living together. Under certain conditions the water is simply crowded with these organisms, and in different circumstances the Plankton is represented by only a few individuals.

The variations of the Plankton in different parts of the world have, of recent years, been subjected to many searching investigations, but although many important facts have been recorded, the explanation of the principal phenomena remains a mystery.

One of the most interesting facts, perhaps, is the extraordinary local variations to be observed. To give a single example as an illustration of this point the case of the common white Jelly-fish may be mentioned. On occasions the surface of the water in our bays and estuaries contains so many of these animals, that the sea appears to be little more than a mass of jelly. In other seasons not more than a few isolated individuals will be seen all through the summer months.

With all the resources of modern scientific investigation no adequate explanation has been given to account for this fact. It may be that the variation is due to the prevailing winds or tides, to the temperature of the water, to the roughness or smoothness of the sea, to disturbance of the ground where the eggs have settled, or to some other hitherto unforeseen conditions. Not only seasonal, however, but even diurnal variations occur, of a most remarkable and inexplicable character.

On one occasion for example I was collecting a number of Jelly-fish in Southampton Water, and for nearly two hours specimens were obtained as fast as they could be hauled into the boat. Suddenly a change came, and in a few moments the water that had been alive with these animals seemed to contain not one. Another time, after dredging nearly all the afternoon at Lulworth for Hormiphora, with the very poor success of a half dozen specimens, the net came up simply choked full of these little round jelly-like Ctenophores, and for the remaining hours of day-light there appeared to be an abundance of them all along the coast. One morning in the Tropics, at about an hour after sunrise, I was looking over the side of a steam-boat, and saw that the surface waters were full of beautiful and rare species of floating animals. In less than half an hour afterwards, when a boat was put off, scarcely one of them could be found. Anybody who is accustomed to working with a tow-net can give similar experiences.

In each of these cases a simple explanation might be suggested. In the first case it might have been the change in the tide which effected the disappearance of the Jelly-fish; in the second it might have been the approach of nightfall that caused the Hormiphoras to rise; and in the last case it might have been the approach of the heat of day; but when carefully considered such explanations are not sufficient, in that they do not account for the suddenness of the change.

The fact is that the conditions of life in the surface waters are so complicated that it is extremely difficult for us to accurately estimate the balance of the forces which act upon these organisms. The direct heat of the sun, the light of both the sun and the moon, the tranquillity or roughness of the sea, the conditions of the tides and winds which cause changes in the surface temperature of the water, independently of the direct heat of the sun, all influence the delicate tissues of which these animals’ bodies are composed, and cause them to change their position.

The animals which compose the surface Plankton may be considered under two heads--those that are adult, and those that are the larvæ of sessile and crawling forms of life which in the adult stage live at the bottom.

Those belonging to the former group frequently occur far out in the open ocean as well as in the neighbourhood of the land, and have as a rule a wide geographical distribution. Those belonging to the latter group are more usually found within a few miles of the coast line, although winds and tides may occasionally drift them far out into the sea, where their larval existence is prolonged for an abnormally long time. Leaving out of consideration for the moment the many interesting exceptions, we may say that the Plankton of the open oceans differs from that of the neighbourhood of the coasts, by the larger proportion of adult forms that it bears.

A great variety of animals pass the whole of their lives in the surface waters of the sea, but the commonest and most widely distributed of all probably are the Copepods belonging to the class Crustacea.

The Copepods are minute creatures, rarely exceeding a quarter of an inch in length, which row themselves through the water by a pair of long antennæ, projecting from the head end of the body. They occur in fresh water as well as in the sea, and so abundant are they that if a glass tumbler be filled with the water from a pond, a lake, or the sea, and examined with a magnifying glass, a number of specimens are almost sure to be seen. They occur in abundance at the surface of the sea in nearly all climes, and very often are the sole representatives of the Plankton that are found in the hauls of the tow-net.

Attention has already been called above to the fact that in the Tropics the surface-floating animals gradually sink down into the depths as the heat of the day approaches, but even on fine calm days a few Copepods will be found at the surface. Although they sometimes occur in Temperate seas in such vast numbers that the water is quite discoloured with them, more variety of form, or, in other words, more distinct genera and species are found in the warm and Tropical parts of the world.

The study of this group reveals to the microscopist some of the most marvellously beautiful displays of colour and form that can be found in the animal kingdom. Sometimes the body and legs are beset with an immense number of extremely fine and delicate spines, which are in some cases provided with rows of still finer spinelets, giving them the appearance of a most minute feather. Sometimes the body contains large granules of a bright red colour, and at others smaller granules of a bright blue are seen scattered among the organs. The female Copepods usually carry, securely fastened to their tails, two little pear-shaped sacks of eggs, which are sometimes bright green, blue or red.

Endless are the varieties of form and colour presented by these little creatures, and endless are the beauties which the study of their structure reveals; but as we have mentioned them first as inhabitants of the surface waters of the seas, we must pause to consider here how these organisms, which excite so much wonder and admiration, are adapted or fitted for their peculiar mode of life. But it must be remarked that these statements apply only to the free-swimming Copepods, for many animals classed in this group by zoologists are parasites, and as such are so profoundly changed that they might at first sight be relegated to another class of organisms altogether.

Now we must remember that animals that live in the surface waters must be prepared to keep afloat for the whole period of their lives--from the time they are hatched until they fall a prey to some voracious enemy. Under ordinary circumstances, they never find an opportunity of resting, either on the sea-bottom or on any floating substance.

If a Copepod is watched in a tumbler of water it will be seen to give a number of strokes with its long antennæ and then to rest suspended for a few seconds; a few more strokes follow and then another pause, and so on. During the period of rest the body sinks slowly, sometimes almost imperceptibly, but never so much that it cannot recover its position in the water after the first few strokes.

It must be clear to the reader that the less it sinks during the pause the less will be the muscular activity required to regain its position, and that, consequently, every mechanical contrivance that its body possesses to diminish its tendency to sink will be a saving of muscular and nervous energy.

A very simple experiment will demonstrate that a body which presents a considerable surface to the water, sinks more slowly than one of the same weight that is round and compact. If we take two equal pieces of silver paper and roll one of them into a tight little ball, leaving the other as a flat sheet, and then let them sink together in a tall jar of water, the former will reach the bottom long before the latter. Similarly the body of an animal which possesses a dense armature of spines, as it presents more surface to the water, sinks much more slowly than the body of an animal of the same weight that is smooth and compact.

The spininess or hairiness of the Copepod body, then, may be regarded as one of its adaptations to the environment in which it lives. But of course this character is not by any means confined to the Copepods. Very many of the surface-swimming Crustaceans, and more particularly their larvæ, have remarkably spiny bodies, and among many of the Foraminifers, Radiolarians, Worms, Molluscs and even Fish we find some similar extension of the surface of the body which lowers the sinking rate. Another means by which the bodies of many of the animals composing the Plankton are buoyed up, is the secretion into a special chamber or reservoir of some gas or oil of a lesser weight than the sea-water. This is what may be called the balloon principle. In such animals we may regard the heavy muscles, skeleton, skin and viscera as the car and the freight of the balloon, while the gas reservoir corresponds to the whole silk case containing the coal-gas.

Such an animal might also be compared to a man in the sea clinging to an india-rubber life-belt. The body of the man by itself is heavier than the water, and in the absence of the muscular exercise of swimming sinks rapidly to the bottom; but the body of the man and the life-belt taken together are lighter than water and float continuously without any action of the muscles. If the life-belt were considerably smaller than usual the man and belt would sink, but much less rapidly than the man alone; and the muscular energy required to keep himself afloat would be far less with the belt than without it, consequently he would be able to keep afloat much longer with the same expenditure of muscular energy. The bodies of many of these surface-swimming animals may then be best compared with a man assisted by a small life-belt. When dead or still they slowly sink, but a slight amount of muscular energy expended in swimming is sufficient to keep them afloat. In what has been said above about the body of the Copepod, reference has been made to certain bright red granules. These are in all probability little globules of some oily or fatty substance lighter in weight than the sea-water, which serve to buoy up the body of the little creature. It is difficult to say why they should have such bright colours. We have no record of observations that show that the colours can be of any use to them as a protection from their enemies, nor is there any physical explanation of the colours of these granules any more than of the blood, the bile and other products of animal and vegetable vital processes. The eggs contained in the egg-sacks of the Copepods also bear a certain amount of oily substance very frequently different in colour from that of the other parts of the body, and this probably acts in the same manner upon the body of the parent or on that of the little larvæ when they are first hatched.

Thus we find in the body of the Copepods at least two important modifications of structure, which render them fit or suitable for their life-long swim in the surface waters of the sea.

Let us now consider another important group that has the same habit but differs from the Copepods in size and form, namely, the Jelly-fish.

The Jelly-fish, or Medusæ, as they are usually called by zoologists, are disc or bell-shaped animals of a very soft gelatinous texture. From the centre of the disc or bell there hangs down a tube of varying length bearing the mouth, and the margin is often provided with a row of thin delicate tentacles like a fringe. (See Fig. 7). When watched on a calm summer’s evening they may be seen to slowly sink a few inches or more from the surface, and then with a series of convulsive contractions of the bell to rise to the surface again. Sometimes these contractions may be observed to continue perfectly rhythmically for a long time.

In one of the commonest of the English Medusæ four rings of a bright pink or orange colour may be observed in the disc. These are eggs and male spawn, and when shed they give rise to multitudes of tiny little larvæ which sink to the bottom and become fixed to some rock or sea-weed. After the larva has securely fixed itself it becomes changed into a little Polyp which gives rise, in the course of time, to a number of small discs, arranged one above another like a pile of saucers. These discs break away from the base and from the parent stock to grow into the form and size of the adult Jelly-fish.

We have here an example in the life-history of the common Jelly-fish, of what is known as “alternation of generations.” The eggs give rise to sessile Polyps, and these produce a number of buds which, when fully grown, give rise in their turn to the eggs; or, in other words, the egg-producing generation of large surface-swimming Jelly-fish regularly alternates with the small sedentary bud-producing generation. Now as the bud-producing or Polyp generation of the common Jelly-fish referred to is fixed to the bottom, the proximity to a coast, or at any rate to a shallow water area, is a necessity for the continuation of the species. Many of the Jelly-fish are undoubtedly drifted out into the open ocean by the tides, but the larvæ they produce, after swimming about in search of something solid to which they can attach themselves, must at last perish. It is only those larvæ which are hatched near enough to the shore to be able to reach the bottom during the tenure of their lives, that can continue the generation of these Jelly-fishes.

But even in the open ocean far away from shallow water or a coast line, Jelly-fish, belonging of course to different species from those of the coasts, are found. What is their natural history? How is their life different from that of the Jelly-fish of the shore? Some of them produce larvæ very similar to those described above but they seek, instead of the rocks or sea-weed, other Jelly-fish and attach themselves to them as parasites.

In other species, however, the “alternation of generations” is entirely lost, and the egg gives rise directly to a free-swimming little Jelly-fish which in time grows to be like its parent in size and shape. In this case the fixed or sessile form in the life-history is, as it were, omitted in order that the animal may lead a life independent of the coast and sea-bottom.

The Jelly-fish, then, present us with an interesting example of a manner in which the life-history of an animal may be modified for or adapted to this surface-swimming habit.

There is also another point of interest about these creatures in this connection. In writing about the Copepods I pointed out the mechanical contrivances they exhibit for keeping themselves afloat, namely, the spines, hairs and oil globules. Jelly-fish have neither spines nor oil globules of the same nature, but still their bodies are very light in the water and in the absence of muscular movements sink but slowly to the bottom. This lightness is due to the fact that all the tissues and organs of which it is composed are very largely distended with water. When the body of a Jelly-fish is analysed it is found that over 95 per cent. of it consists of water. This power of absorbing large quantities of fluid into the tissues, while it increases the size of the body, proportionately diminishes its weight in water.

It has also another effect. It makes the tissues of the body much more transparent and gives them that soft jelly-like consistency which is so characteristic of the surface-swimming forms.

The popular term ‘Jelly-fish’ is one that is frequently applied to many forms of surface-swimming animals that are really very different in structure and general composition from the true Medusæ. The Salps, for example, to which reference will be made presently, although soft and transparent in texture like the Medusæ, belong to a very widely separated group of animals, and to the anatomist it would be as absurd to classify them together, as to put the Butterflies and the Fish in the same group.

These remarks are necessary because in the treatment adopted in this little book the animals that live together are considered in the same chapter, and it is important that the reader should bear in mind that they are not as a consequence anatomically related to one another.

It is indeed remarkable that animals which are so different from one another, in their anatomy, development and life-history, as, for example, the Salps and the Medusæ, and which have had such a widely different ancestry, should, as a matter of fact, resemble one another so closely in form and texture as to be given collectively the same name by the unscientific observer.

Among the heterogeneous crowd of animals that are popularly called Jelly-fish there is one particular group which presents us with some very interesting members. These are the Siphonophores. In many parts of the temperate and warmer seas of the world the surface may be covered with thousands of little creatures which, when brought upon the deck, seem to be little else than coloured bladders of air. The scientific name of these animals is Physalia. When placed in a glass of water, however, it will be seen that, from the under side of the bladder which floats freely on the water, numerous delicate tentacles and Polyps hang down. These creatures are kept at the surface by an air-bladder float and no muscular energy is required to sustain them in that position.

Another Siphonophore called Velella has a bladder of a more complicated character in the shape of a disc with a semi-circular or triangular sail on its upper side. There can be no doubt of the advantage of this float to the species. It not only enables them to keep afloat without the expenditure of muscular energy, but as the wind catches the sail they are drifted along over great areas of the ocean and thus distributed far and wide from the spot on which they were hatched. Still the float has undoubtedly its disadvantages, for it exposes them to the danger of being blown ashore by a steady wind and so perishing in thousands. Agassiz says that on the coast of Florida the beach is sometimes marked with lines of Velellas that have been stranded in this manner, and I have seen in Celebes four or five rows of bright blue Physalias stretching for miles along the shore.

In the Mediterranean and Eastern Atlantic Ocean a very large Physalia occurs which has received the popular name of the “Portuguese man-of-war,” and is famous for its stinging powers. The stinging is produced by a number of very minute sacs, which shoot out, when they are touched, a long pointed thread that penetrates the skin and conveys an irritant poison. These are called the thread-cells, and the “Portuguese man-of-war” is not by any means peculiar in possessing them. All the Medusæ and Siphonophores, all the true Corals and Sea-anemones have them--in fact, all those creatures which are classified together by the zoologist as Cœlenterata may be said to be stinging animals. The thread-cells, however, vary very much in size in this group, and in the great majority of cases the thread is too feeble to perforate the skin of the human hand, and consequently their owners have not acquired a bad reputation.

People do not warn their children not to touch the Sea-anemones on the rocks or the Jelly-fish stranded on the beach, and yet they are both dependent for their food upon their stinging powers; and indeed many of the British Medusæ which may be handled with impunity, are capable of stinging quite severely the more delicate skin of the back and arms of unwary bathers.

Besides the two forms of Siphonophores which have been described, there are many others to be found at or near the surface of the seas of all climes. Some of them possess great floats like Physalia and Velella, but the majority of them have either no floats at all or such as are too small to do more than assist in keeping the animal near the surface. All of these Siphonophores are provided with little bells, which, contracting rhythmically like a Jelly-fish, drag the animal along, sometimes to the surface, sometimes a few fathoms below it. Some of these forms are extremely graceful, being like long strings of jelly, with numerous clusters of Polyps and long feathery tentacles, towed through the water by one or two exquisitely delicate little bells situated at the leading end of the string.

A few words must now be said about the Salps, because in some seas the water is on occasions so full of them that they seem to be packed together ready for preserving. The simplest form of Salp is like a small sac or barrel of transparent gelatinous substance open at both ends. Running round the barrel are five or seven bands of a less transparent nature, appearing to the unaided vision like milky white streaks. These streaks are bands of muscles by which the movement of the body through the water is assisted. Sometimes they are seen swimming about independently of one another, sometimes Salps very similar to them in general appearance are seen to be attached to one another in long chains. At first it was supposed by naturalists that the former or Solitary Salps were of a different species to the latter, or Chain-salps as they are called; but it has been discovered that these two forms are but stages in the life-history of one species. When the anatomy of a Chain-salp is minutely examined it is found to contain a single egg, which gives rise to a young Salp similar in nearly all details to the solitary one. This escapes from its parent’s body when it is old enough to take care of itself, and leads an independent existence. After it has grown to its full size it gives rise to a stalk which divides up into a number of young Salps, attached to one another in a very characteristic manner.

Here, then, we have another instance of alternation of generations similar in this respect to the example previously quoted among the Jelly-fish, in that the one generation produces an egg, and the other numerous buds; but differing from it in the fact that in the case of the Salps both generations are adapted for freely swimming at the surface of the sea.

Space does not allow us to say more in detail about the other animals of the Plankton that belong to the same group as the Salps; of the wonderfully interesting life-history of Doliolum; of the extraordinary bright light emitted by Pyrosoma, or of the remarkable little Fritillaria, shaped like a tad-pole, living in its house of jelly. The story of each of these might take a whole chapter to itself and still be only partly told.

Anyone who is acquainted with the general appearance of the Whelks and Periwinkles, and other Gastropods of our shores might be well astonished when he saw, for the first time, many of the Gastropods of the high seas. The shell is either absent altogether or consists of a thin little papery cap far too small to afford protection to the body. The head and foot, and, indeed, the greater part of the body, are transparent, soft and gelatinous like a Jelly-fish, in fact the whole appearance is so different that it is not until the internal anatomy is carefully studied that their true position in the animal kingdom can be assigned to them.

Here, then, we find another instance of a profound modification of structure associated with the surface-swimming habit; the modification being due very largely to the absorption of considerable quantities of water into the tissues of the body, which has the effect of rendering them transparent, and, at the same time, of reducing their weight in the water.

The transparency of the body of so many of the animals of the Plankton has suggested the theory that by rendering them less conspicuous to their enemies it is of the nature of a protection to them. We ought to hesitate before accepting this theory until we know more accurately what are the enemies that they endeavour to protect themselves against. It is very probable that none of the Fish will feed upon any of the transparent Jelly-fish, neither is there any evidence that the Salps and the pelagic Gastropods form a favourite food for them. There is no good reason for supposing that the Sea-birds would, if they could see them better, prey upon them, so long as there are Fish in the sea to provide a more substantial and satisfactory meal. The Whales, as they dash through the water with their huge mouths wide open, undoubtedly swallow them in thousands, but it can not be reasonably supposed that the Whale can be guided by sight in the selection of its food. We ought not, perhaps, to go so far as to say that it is no protection to them, for Prof. Moseley states that the Turtle sometimes feeds upon the Velellas, but at the same time we may consider that the transparency is an effect produced by the large amount of water in their tissues, which is there for the purpose of reducing their specific gravity and assisting in that manner in their floatation.

The only Gastropod found in the open seas which retains in its characteristic form the large coiled shell, is the beautiful blue Janthina, famous for its habit of constructing a little raft which floats on the surface of the sea. To the underside of this it attaches its eggs and spends its life in pushing or dragging the raft about.

No account of the Molluscs of the Plankton would be complete without some reference to the Pteropods. These creatures are provided with a pair of muscular lobes of the body, which have been compared to wings. By means of these they are able to swim through the water. Some of them are provided with delicate little glassy shells, but in others the body is quite naked. We may regard the Pteropods as the most highly modified forms of Gastropods adapted for a pelagic life.

In both the Arctic and Antarctic seas this group occurs in immense numbers, and it is supposed to form not an inconsiderable proportion of the food of the gigantic Right-whales. They also occur in the Temperate and Tropical zones, and indeed there are actually more genera and species there than in the colder regions to the North and South.

The Insect world is represented at the surface of the ocean by a curious little Bug called Halobates. It is not uncommonly found in tropical or subtropical seas feeding upon dead Salps or Jelly-fish, and when disturbed scuds over the surface after the manner of many of the Insects living on our inland ponds and lakes. It has been described as an “ivory-legged fellow, covered with a bluish-white down.” As it is essentially an air-breather like all adult insects, its usual habitat is ‘on’ the sea and not in it, so that strictly speaking it is not a member of the Plankton. There is no doubt that under certain circumstances it can and does dive into the water, and on these occasions it carries with it for respiratory purposes a layer of air attached to the ‘bluish-white down’ covering the body.

There are no traces of wings on its thorax, and it is therefore incapable of flight. Very little is known at present of its development, and practically nothing of its internal anatomy, so that its proper position in the order of the Bugs or Hemiptera is a matter of conjecture, but it is an interesting little creature, in the fact that it is the only member of its class that has a purely pelagic life-history.

Among the microscopic forms of life found in the Plankton of the sea, the Radiolarians and Foraminifers are perhaps the most important. The Radiolarians are very minute specks of protoplasm, usually protected or supported by an elaborate skeleton of a substance closely allied to flint. The form of this skeleton varies so much in the numerous species that have been described, that it is quite impossible in a few words to give an adequate idea of the principal types. (See Fig. 2). We may say, however, that in a considerable number of them the skeleton has the form of a hollow sphere, perforated by numerous round holes and supporting outside a number of long thin needles. The anatomy of the Radiolarians is extremely simple. Their bodies are built entirely of protoplasm which performs all the vital functions. There is no definite head, mouth, brain, nor muscular organ. This being the case, the question arises, How do these animals provided with a skeleton of such a heavy substance as flint manage to support themselves in the water without muscular appendages? The answer to this question is two-fold--In the first place, the elaborate form of the skeleton presents an enormous surface to the water in proportion to its weight, and consequently sinks slowly; and secondly, the protoplasm is provided with numerous vacuoles containing a watery fluid, and in many cases at least one larger vacuole containing oil. If the liquids in these vacuoles are lighter than sea-water, and there is good reason to suppose that some at least of them are, then they are of the same nature as the oil chambers of the Copepods, and are hydrostatic in function.

Among the Foraminifers very few genera strictly belong to the surface Fauna. Most of them have heavy, compact shells of carbonate of lime, and they live among the sand or the rocks at the bottom of the sea. The best known of the surface-dwelling forms is Globigerina, and this, in accordance with its habits, possesses a shell which, like that of the Radiolarians, is very light, perforated by numerous large holes and provided with long delicate spines. The shell of Globigerina might well be mistaken for that of a Radiolarian were it not for the fact that it is composed of carbonate of lime instead of flint.

The Radiolarians in some waters, and the Globigerinidæ in others, are present in enormous numbers, and as they die their shells fall in a gentle rain from the surface towards the sea-bottom, where they frequently, form a very large part of the abysmal mud.

In speaking of the organisms of the surface of the sea no mention has yet been made of the plant world. Of the large conspicuous Sea-weeds that are often found far out in the open ocean the best known is the Sargasso or Gulf-weed of the Atlantic. It forms in some cases great floating patches, of very considerable area, and is, when alive, of a bright yellow colour. The Sargasso patches are, however, of great interest to the zoologist, because they support a considerable population of animals specially adapted by their form and colour to live among the Sea-weeds. They present us, in fact, with a peculiar Fauna, containing representatives of all the most important groups of marine animals.

Besides the large conspicuous weeds like the Sargasso, the surface of the sea supports a large Flora of minute plants of very lowly organisation, and it is not at all uncommon for them to be present in such numbers as to cause a distinct discolouration of the water.

The banks that they form on the coast of Brazil and elsewhere were called “Sea-sawdust” by Sir Joseph Banks. Moseley says that “when tracts of the sea are passed through, which are full of this Trichodesmium, the water lighted up by sunlight, when looked down into, appears as if full of small particles of mica or some such substance, so strongly is the light reflected from the minute bundles of the Algæ”; and again, he says, “so abundant is Trichodesmium in some seas that one of the explanations of the name of the Red Sea is that the term was derived from the discolouration of the water by vast quantities of Trichodesmium erythræum.”

In addition to this “Sea-sawdust,” Diatoms, the still more minute organisms, the Bacteria, and the debateable particles called Coccospheres and Rhabdospheres, add to the number of the floating Flora of the seas.

The importance of these organisms to the zoologist is that they must ultimately form the food supply of the animals of the Plankton. Some of the larger animals may feed upon the smaller ones, and the smaller ones may, in their turn, feed upon still smaller ones, but we must come eventually, in descending the scale, to the animals that are vegetable-feeders and prey upon the minute plants that have just been mentioned.

Now that we have considered very briefly some of the principal forms of life that compose the floating and drifting population of the surface, we may return to the subject with which the chapter opened, namely, the phosphorescence of the sea.

It need hardly be mentioned that it is a subject which is beset by innumerable difficulties. Even when the sea is extremely phosphorescent, and the observer is provided with an excellent microscope and all the necessary scientific appliances, he finds it difficult to answer the question--“What is the cause of the phosphorescence tonight?” The sample of water he takes may reveal to him a multitude of different organisms, many of which are so small that they can only be seen with a strong artificial light, and then it is impossible to say which are and which are not phosphorescent.

Some of the Copepods are known to possess an organ emitting a blight blue star-like light which shines for a time and is then suddenly extinguished. In the Malay Archipelago several of these bright lights may be seen near the surface of the water on calm mornings just before sunrise, and it is extremely interesting to watch them gradually sinking down into deeper water as the day dawns, and then suddenly going out one after the other.

Some of the large Jelly-fishes, such as Pelagia noctiluca, glow with a soft blue light. The curious pelagic Tunicate colony Pyrosoma receives its name from the fact that it emits a bright light. A giant Pyrosoma was caught by the Challenger in the deep-sea trawl, and, to quote the words of Professor Moseley once more, “It was like a great sac, with walls of jelly about an inch in thickness. It was four feet in length and ten inches in diameter. When a Pyrosoma is stimulated by having its surface touched, the phosphorescent light breaks out at first at the spot stimulated, and then spreads over the surface of the colony as the stimulus is transmitted to the surrounding animals. I wrote my name with my finger on the surface of the giant Pyrosoma as it lay in a tub at night, and the name came out in a few seconds in letters of fire.”

All of these animals are sufficiently large to be easily seen by the naked eye, and the phenomena of their phosphorescence can be carefully observed. But many of the more minute forms of life also exhibit this peculiarity, and contribute in no small degree to the bright light of the sea.

For instance, when the sea on our coasts shows a dull blue light, flashing into greater intensity where the ripples break, it will be found to contain immense numbers of very minute creatures called Noctiluca. Each of these has a gelatinous consistency, and is the shape of a microscopic cherry, bearing a short whip-like process, called the flagellum, which propels the organism slowly through the water. There seems to be no doubt that, on these occasions, the light is caused by these Noctilucas, but there are many other minute forms which abound on the surface and give off a pale phosphorescent light at night.

We do not know for certain what may be the use of the phosphorescent light to the organisms that possess the power of emitting it. If we assume that the transparency of the bodies of the pelagic animals has a protective value in the day-light, it is difficult to understand why many of them should become so attractive, as the phosphorescent light makes them, at night. It is probable that the star-like lights of many of the Copepods may serve to attract to one another the two sexes, as it does with the Glow-worms and Fire-flies, but such an explanation as this cannot well be accepted in the case of Pyrosoma, which is hermaphrodite, or the Noctilucas, which live together in immense numbers. There can be little doubt, however, that there is some good reason for it, as it occurs in so many different animals belonging to widely separated families.

In the neighbourhood of coasts or in shallow water, the surface of the sea usually supports a very large number of animals in a larval or immature state. These creatures live only a portion of their lives in a free-swimming condition, and then a change occurs during which they sink to the bottom and gradually assume the adult characters.

Nearly everybody is acquainted with the general appearance of the Crab and Star-fish, but few would guess that the young stages of these animals are to be found among the minute transparent floating Fauna of the surface waters of the sea.

The habits of the young and of the old, of these animals are widely different; the former must constantly support themselves in the water, they must feed upon and have means for catching and devouring minute floating organisms and must in other ways be adapted for life with the Plankton; the latter being unable to swim are capital crawlers and walkers over the rocks and sand of the bottom, have heavy bodies which sink rapidly in the water and, in other ways, are adapted for life with the shallow water Benthos.

The conditions of life at the surface and at the bottom being, as I have previously pointed out, so different and the adaptations of structure to suit each set of conditions so great, we have, as a result, a long series of animals in which the young larval stages of life are absolutely unlike the adult and mature stages.

No better examples to illustrate these changes could be given than those chosen from the group of the Echinoderms. Take, for instance, the common Star-fish with its thick heavy skin studded with plates of carbonate of lime, and its dense opaque body drawn out into five finger-like processes. These features of the animal indicate at once that its life is spent crawling on the sand or rocks at the bottom of the sea. If a Star-fish that has been caught in a lobster pot or brought to the surface attached to the bait on a fishing line, is cast into the sea it sinks to the bottom at once without any apparent effort to swim, to keep afloat, or to arrest its rapid descent. It is therefore clearly unfitted for a surface-swimming existence, but its eggs give rise to larvæ which are admirably adapted to it, and can indeed only exist at or near the surface of the sea. These larvæ are, as a rule, when first hatched, covered with a number of very minute vibratile cilia, by means of which they swim with considerable rapidity through the water. After a time a number of bands appear, which are covered by specially long cilia and then the smaller cilia on the intervals between the bands disappear.

The precise arrangement of the bands differs in the different species, but from being at first perfectly circular in contour they become more and more curved and twisted, sometimes fusing with one another and in parts degenerating, until, at last, when the larval stage reaches its full development, the bands have assumed an elaborate and somewhat fantastic pattern.

The body of the larva is, like that of so many surface-swimming creatures, extremely transparent. The uniform oval shape which it has when first hatched becomes changed as it develops by the formation of a certain number of short blunt processes or arms, and it was the presence of these which caused the older naturalists to call this larva the Brachiolaria.

If one of these minute Brachiolaria larvæ be caught and examined with a microscope it is not difficult to see that it has a little round mouth leading into a short digestive canal which opens to the exterior by a vent. It is therefore clearly capable of feeding itself and leading a perfectly independent existence. In the older larvæ there will be noticed an appearance which has, under a low magnifying power, the form of an incomplete and rather opaque ring round the stomach. This opaque ring becomes larger and larger, it exhibits five projections radiating from its centre, and at last gives rise to all the organs of the fully formed Star-fish. As the ring develops the larva sinks from the surface and loses the power of independent feeding, and then, when all is ready, the skin is cast off and a small but perfectly formed Star-fish emerges.

The Trepangs, the Brittle-stars, the Sea-urchins and other Echinoderms have, as a general rule, life-histories similar to that of the Star-fish, but there is one point of difference in detail which is of sufficient interest to be mentioned before passing on. The larva of the Brittle-stars and of some of the Sea-urchins has a number of arms which are much longer, in proportion to the whole size of the larva, than they are in the Brachiolaria, and on account of the manner in which these arms are inclined towards the apex, the larva has a rough resemblance to the form of a painter’s easel. This type of larva is called the Pluteus. The main point of interest about the Pluteus, however, is that the arms are supported by delicate bars of carbonate of lime which are connected together at the apex and form a very definite larval skeleton.

This larval skeleton is cast off with the skin when the metamorphosis takes place, and it is consequently of great interest to scientists in the fact that it is one of those structures which are formed to meet the exigencies of larval life only, and is perfectly useless for the adult. In considering the manifold questions which arise in the study of the relation of animals to their surroundings we are often inclined to fix our attention too exclusively upon the adaptations that are manifested in the adult form. In the case of some classes in which the immature stages of life are passed through very rapidly and under the protection of the parents, this is not to be deprecated; but in most cases it is important to remember that in the struggle for existence there is such danger of extermination that each stage of life may have acquired special characters for adaptation to its particular mode of existence. The peculiar markings and colours of the Caterpillars is a familiar example of the special characters of larval forms among terrestrial and air-breathing animals, but in none of these do we find so great a specialisation in larval characters as in some of the marine forms of life.

It is said above that the Echinoderms as a general rule have free-swimming larvæ, but there are exceptional cases which have an interest for us quite as great as that of the ordinary life-history.

Many Echinoderms are found living in very great depths of the ocean and it is difficult for us to believe that any of these can have pelagic larvæ similar to those of their shallow water relatives. The difference in pressure between that of the bottom of the deep-sea and of the surface is, by itself, sufficient to convince us that a delicate organism like a Brachiolaria or Pluteus could not make the upward journey unharmed; but when we add to that the great distance of two or even three miles in a direct line, the difference in temperature and in light, we must realise that the ordinary transformations of the shallow water Echinoderms is an impossibility for the deep-sea varieties.

As a matter of fact we know very little about the life-history of deep-sea Echinoderms, and this is not a matter for wonder when the reader reflects upon the great difficulties that have to be overcome in obtaining a few specimens of the adult forms; but at least one of the Star-fish of the Abyss has been found to bear little pouches or pits in which the young are fostered until they are ready to lead an independent life in the form of the parents.

It has also been shown that in some of the Arctic Star-fishes the larval life is in a similar manner abbreviated and protected, and it seems probable that this may be accounted for by the fact that the surface waters, where the larval forms would live if they were liberated, are for very long periods covered with ice.

The great group of the Crustaceans also presents us with many interesting larval forms specially adapted to surface life. In a previous chapter I have pointed out that the Barnacles of our coast give birth to curious little free-swimming, six-legged larvæ called Nauplii, which after having undergone two or three further changes, settle down on a rock and assume the adult features (see Fig. 12).

It is not known how long these changes take in the ordinary course of nature, but it is quite probable that the larval life is a comparatively short one.

Some Barnacles, however, live far out at sea on drifting wood or parasitic on the skin of Whales, and it is reasonable to suppose that when their larvæ are hatched a very considerable time may elapse before they find a suitable resting-place to complete their metamorphosis.

The life-histories of these species are not at present accurately known, but a few remarkable Nauplii have been found which, there is reason to believe, are really the Nauplii of some kind of Barnacle and are specially adapted to a long life at the surface by the enormous length of their spines.

In the specimen discovered by Chun in the Canary Islands, of which a figure is given here, the spines were seven or eight times the length of the body, the eye was remarkably small, and the muscles were feebly developed. It may be that this is the larva of some species of Barnacle, which, from the character of the host or home where it lives when adult, must be prepared to wait a long time in its larval habitat before the chance comes for it to find a suitable resting-place.

Many of the Crabs and Prawns have remarkable larvæ, characterised either by two or three extremely long spines or in some cases by a festoon of shorter and many branched spinous processes spreading out from their carapace, tail and limbs. These spines may be regarded partly as a device for assisting in the floatation of the body, and partly, perhaps, as a protection against some of the creatures that feed upon them; but in both respects they are special larval adaptations to the pelagic life. It is extremely interesting to find that in this class of animals the same characters are not constant in the larvæ. A Prawn called Palinurus has a larva the body of which becomes extremely expanded and flattened, so as to resemble a very thin sheet of glass, the eyes and the limbs at the same time undergoing remarkable modifications. Another larva becomes extraordinarily distended by the absorption of water into its tissues so as to resemble in texture a small Jelly-fish.

A great deal more might be said about the story of Crustacean larvæ, as it is one which is full of interest and wonder, but throughout the whole of it we see, wherever there is a larval history at all, that some one or more of those characteristic features have been evolved, which were previously noted in adult animals as an adaptation to their free-swimming pelagic life.

In many other groups of marine animals we find the same alternation of a transparent larval life at the surface and an opaque adult life at the bottom.

The Oysters, Clams and Mussels, the Winkles and other Gastropods, the Worms, the Sponges and many other forms of life that creep among the Sea-weeds and are fixed upon the rocks or burrow in the sand, produce exquisite and delicate transparent little larvæ which for a certain length of time at least float and drift about in the light of the sunshine in the surface water. They have, of course, many varieties of form and many peculiar organs for locomotion and floatation, so that it is possible for a competent zoologist to tell without much difficulty the group of animals, if not the actual genus and species, to which any particular larva belongs.

It might be thought that, as so many of the animals living near the coast line in shallow water have pelagic larvæ, the Plankton of the neighbourhood of the coasts would differ from that of the open oceans in the fact that a considerable proportion of it consists of these larval forms. But many of the larvæ seem to be able to live a long time without further change than an increase in size, and being drifted out to sea by the winds and tides are often found in the open ocean at very great distances from any coast line.

It would be interesting to know more of these larvæ which go thus astray. How long can they go on waiting for the opportunity to cast off their childish clothes and assume the garments of the adult? Do they in time undergo changes which bring about a kind of childish old age, or do they suddenly perish with all the characters of youth upon them?

These and many other questions connected with this most fascinating chapter in the story of the sea have still to be answered by the investigations of scientific men in the future.