Читать книгу The Sea Shore - William S. Furneaux - Страница 6

Table of Contents

What are the attractions which so often entice us to the sea shore, which give such charm to a ramble along the cliffs or the beach, and which will so frequently constrain the most active wanderer to rest and admire the scene before him? The chief of these attractions is undoubtedly the incessant motion of the water and the constant change of scene presented to his view. As we ramble along a beaten track at the edge of the cliff, new and varied features of the coast are constantly opening up before us. Each little headland passed reveals a sheltered picturesque cove or a gentle bay with its line of yellow sands backed by the cliffs and washed by the foaming waves; while now and again our path slopes down to a peaceful valley with its cluster of pretty cottages, and the rippling stream winding its way towards the sea. On the one hand is the blue sea, full of life and motion as far as the eye can reach, and on the other the cultivated fields or the wild and rugged downs.

The variety of these scenes is further increased by the frequent changes in the character of the cliffs themselves. Where they are composed of soft material we find the coast-line washed into gentle curves, and the beach formed of a continuous stretch of fine sand; but where harder rocks exist the scenery is wild and varied, and the beach usually strewn with irregular masses of all sizes.

Then, when we approach the water’s edge, we find a delight in watching the approaching waves as they roll over the sandy or pebbly beach, or embrace an outlying rock, gently raising its olive covering of dangling weeds.

Such attractions will allure the ordinary lover of Nature—the mere seeker after the picturesque—but to the true naturalist there are many others. The latter loves to read in the cliffs their past history, to observe to what extent the general scenery of the coast is due to the nature of the rocks, and to learn the action of the waves from the character of the cliffs and beach, and from the changes which are known to have taken place in the contour of the land in past years. He also delights to study those plants and flowers which are peculiar to the coast, and to observe how the influences of the sea have produced interesting modifications in certain of our flowering plants, as may be seen by comparing them with the same species from inland districts. The sea birds, too, differing so much as they do from our other feathered friends in structure and habit, provide a new field for study; while the remarkably varied character of the forms of life met with on the beach and in the shallow waters fringing the land is in itself sufficient to supply the most active naturalist with material for prolonged and constant work.

Let us first observe some of the general features of the coast itself, and see how far we can account for the great diversity of character presented to us, and for the continual changes and incessant motions that add such a charm to the sea-side ramble.

Here we stand on the top of a cliff composed of a soft calcareous rock—on the exposed edge of a bed of chalk that extends far inland. All the country round is gently undulating, and devoid of any of the features that make up a wild and romantic scene. The coast-line, too, is wrought into a series of gentle bays, separated by inconspicuous promontories where the rock, being slightly harder, has better withstood the eroding action of the sea; or where a current, washing the neighbouring shore, has been by some force deflected seaward. The cliff, though not high, rises almost perpendicularly from the beach, and presents to the sea a face which is but little broken, and which in itself shows no strong evidence of the action of raging, tempestuous seas; its chief diversity being its gradual rise and fall with each successive undulation of the land. The same soft and gentle nature characterises the beach below. Beyond a few small blocks of freshly-loosened chalk, with here and there a liberated nodule of flint, we find nothing but a continuous, fine, siliceous sand, the surface of which is but seldom broken by the protrusion of masses from below. Such cliffs and beaches do not in themselves suggest any violent action on the part of the sea, and yet it is here that the ocean is enabled to make its destructive efforts with the greatest effect. The soft rock is gradually but surely reduced, partly by the mechanical action of the waves and partly by the chemical action of the sea-water. The rock being almost uniformly soft, it is uniformly worn away, thus presenting a comparatively unbroken face. Its material is gradually dissolved in the sea; and the calcareous matter being thus removed, we have a beach composed of the remains of the flints which have been pulverised by the action of the waves. Thus slowly but surely the sea gains upon the land. Thus it is that many a famous landmark, once hundreds of yards from the coast, now stands so near the edge of the cliff as to be threatened by every storm; or some ancient castle, once miles from the shore, lies entirely buried by the encroaching sea.

Fig. 1.—Chalk Cliff

The coast we have described is most certainly not the one with the fullest attractions for the naturalist, for the cliffs lack those nooks that provide so much shelter for bird and beast, and the rugged coves and rock pools in which we find such a wonderful variety of marine life are nowhere to be seen. But, although it represents a typical shore for a chalky district, yet we may find others of a very different nature even where the same rock exists. Thus, at Flamborough in Yorkshire, and St. Alban’s Head in Dorset, we find the hardened, exposed edge of the chalk formation terminating in bold and majestic promontories, while the inner edge surrounding the Weald gives rise to the famous cliffs of Dover and the dizzy heights of Beachy Head. The hard chalk of the Isle of Wight, too, which has so well withstood the repeated attacks of the Atlantic waves, presents a bold barrier to the sea on the south and east coasts, and terminates in the west with the majestic stacks of the Needles.

Fig. 2.—Whitecliff (Chalk), Dorset

Where this harder chalk exists the coast is rugged and irregular. Sea birds find a home in the sheltered ledges and in the protected nooks of its serrated edge; and the countless wave-resisting blocks of weathered chalk that have been hurled from the heights above, together with the many remnants of former cliffs that have at last succumbed to the attacks of the boisterous sea, all form abundant shelter for a variety of marine plants and animals.

Fig. 3.—Penlee Point, Cornwall

But it is in the west and south-west of our island that we find both the most furious waves and the rocks that are best able to resist their attacks. Here we are exposed to the full force of the frontal attacks of the Atlantic, and it is here that the dashing breakers seek out the weaker portions of the upturned and contorted strata, eating out deep inlets, and often loosening enormous blocks of the hardest material, hurling them on the rugged beach, where they are eventually to be reduced to small fragments by the continual clashing and grinding action of the smaller masses as they are thrown up by the angry sea. Here it is that we find the most rugged and precipitous cliffs, bordering a more or less wild and desolate country, now broken by a deep and narrow chasm where the resonant roar of the sea ascends to the dizzy heights above, and anon stretching seaward into a rocky headland, whose former greatness is marked by a continuation of fantastic outliers and smaller wave-worn masses of the harder strata. Here, too, we find that the unyielding rocks give a permanent attachment to the red and olive weeds which clothe them, and which provide a home for so many inhabitants of our shallow waters. It is here, also, that we see those picturesque rock pools of all sizes, formed by the removal of the softer material of the rocks, and converted into so many miniature seas by the receding of the tide.

Fig. 4.—Balanus Shells

A more lovely sight than the typical rock pool of the West coast one can hardly imagine. Around lies the rugged but sea-worn rock, partly hidden by dense patches of the conical shells of the Balanus, with here and there a snug cluster of young mussels held together by their intertwining silken byssi. The surface is further relieved by the clinging limpet, the beautifully banded shells of the variable dog-periwinkle, the pretty top shells, and a variety of other common but interesting molluscs. Clusters of the common bladdery weeds are also suspended from the dry rock, and hang gracefully into the still water below, where the mantled cowry may be seen slowly gliding over the olive fronds. Submerged in the peaceful pool are beautiful tufts of white and pink corallines, among which a number of small and slender starfishes may climb unnoticed by the casual observer; while the scene is brightened by the numerous patches of slender green and red algæ, the thread-like fronds of which are occasionally disturbed as the lively little blenny darts among them to evade the intruder’s glance. Dotted here and there are the beautiful anemones—the variously-hued animal flowers of the sea, with expanded tentacles gently and gracefully swaying, ready to grasp and paralyse any small living being that may wander within their reach. Here, under a projecting ledge of the rock, partly hidden by pale green threads, are the glaring eyes of the voracious bullhead, eager to pounce on almost any moving object; while above it the five-fingered starfish slowly climbs among the dangling weeds by means of its innumerable suckers. In yonder shady corner, where the overhanging rock cuts off all direct rays of the sun from the deeper water of the pool, are the pink and yellow incrustations of little sponges, some of the latter colour resembling a group of miniature inverted volcanic cones, while on the sandy floor of the pool itself may be seen the transparent phantom-like prawn, with its rapidly moving spinnerets and gently-waving antennæ, suddenly darting backward when disturbed by the incautious approach of the observer; and the spotted sand-crab, entirely buried with the exception of its upper surface, and so closely imitating its surroundings as to be quite invisible except on the closest inspection. Finally, the scene is greatly enlivened by the active movements of the hermit-crab, that appropriates to its own use the shell which once covered the body of a mollusc, and by the erratic excursions of its cousin crabs as they climb over the weedy banks of the pool in search of food.

Fig. 5.—A Cluster of Mussels

Thus we may find much to admire and study on the sea shore at all times, but there are attractions of quite another nature that call for notice on a stormy day, especially on the wilder and more desolate western coasts. At such times we delight to watch the distant waves as they approach the shore, to see how they become gradually converted into the foaming breakers that dash against the standing rocks and wash the rattling pebbles high on the beach. The powerful effects of the sea in wearing away the cliffs are now apparent, and we can well understand that even the most obdurate of rocks must sooner or later break away beneath its mighty waves.

Fig. 6.—Breakers

The extreme mobility of the sea is displayed not only by the storm waves, and by the soft ripples of the calm day, but is seen in the gentle currents that almost imperceptibly wash our shores, and more manifestly in the perpetual motions of the tides.

This last-named phenomenon is one of extreme interest to the sea-side rambler, and also one of such great importance to the naturalist that we cannot do better than spend a few moments in trying to understand how the swaying of the waters of the ocean is brought about, and to see what determines the period and intensity of its pulsations, as well as some of the variations in the daily motions which are to be observed on our own shores.

In doing this we shall, of course, not enter fully into the technical theories of the tides, for which the reader should refer to authoritative works on the subject, but merely endeavour to briefly explain the observed oscillations of the sea and the general laws which govern them.

The most casual observer must have noticed the close connection between the movements of the ocean and the position of the moon, while those who have given closer attention to the subject will have seen that the relative heights of the tides vary regularly with the relative positions of the sun, moon, and earth.

In the first place, then, we notice that the time of high tide in any given place is always the same at the same period of the cycle of the moon; that is, it is always the same at the time of new moon, full moon, &c. Hence it becomes evident that the moon is the prime mover in the formation of tides. Now, it is a fact that the sun, though about ninety-three millions of miles from the earth, has a much greater attractive influence on the earth and its oceans than the moon has, although the distance of the latter is only about a quarter of a million miles: but this is due to the vastly superior mass of the sun, which is about twenty-six million times the mass of the moon. How is it, then, that we find the tides apparently regulated by the moon rather than by the sun? The reason is that the tide-producing influence is due not to the actual attractive force exerted on the earth as a whole, but to the difference between the attraction for one side of the globe and that for the opposite side. Now, it will be seen that the diameter of the earth—about eight thousand miles—is an appreciable fraction of the moon’s distance, and thus the attractive influence of the moon for the side of the earth nearest to it will be appreciably greater than that for the opposite side; while in the case of the sun, the earth’s diameter is such a small fraction of the distance from the sun that the difference in the attractive force for the two opposite sides of the earth is comparatively small.

Omitting, then, for the present the minor tide-producing influence of the sun, let us see how the incessant rising and falling of the water of the ocean are brought about; and, to simplify our explanation, we will imagine the earth to be a globe entirely covered with water of uniform depth.

The moon attracts the water on the side nearest to it with a greater force than that exerted on the earth itself; hence the water is caused to bulge out slightly on that side. Again, since the attractive force of the moon for the earth as a whole is greater than that for the water on the opposite side, the earth is pulled away, as it were, from the water on that side, causing it to bulge out there also. Hence high tides are produced on two opposite sides of the earth at the same time, while the level of the water is correspondingly reduced at two other parts at right angles with these sides.

This being the case, how are we to account for the observed changes in the level of the sea that occur every day on our shores?

Let us first see the exact nature of these changes:—At a certain time we find the water high on the beach; and, soon after reaching its highest limit, a gradual descent takes place, generally extending over a period of a little more than six hours. This is then followed by another rise, occupying about the same time, and the oscillations are repeated indefinitely with remarkable regularity as to time.

Fig. 7.—Illustrating the Tide-producing Influence of the Moon

Now, from what has been previously said with regard to the tidal influence of the moon, we see that the tide must necessarily be high under the moon, as well as on the side of the earth directly opposite this body, and that the high tides must follow the moon in its regular motion. But we must not forget that the earth itself is continually turning on its axis, making a complete rotation in about twenty-four hours; while the moon, which revolves round the earth in about twenty-eight days, describes only a small portion of its orbit in the same time; thus, while the tidal wave slowly follows the moon as it travels in its orbit, the earth slips round, as it were, under the tidal wave, causing four changes of tide in approximately the period of one rotation. Suppose, for example, the earth to be performing its daily rotation in the direction indicated by the arrow (fig. 8), and the tide high at the place markedÛuccessively, where the tide is high and low respectively. Hence the daily changes are to a great extent determined by the rotation of the earth.

But we have already observed that each change of tide occupies a little more than six hours, the average time being nearly six hours and a quarter, and so we find that the high and low tides occur nearly an hour later every day. This is due to the fact that, owing to the revolution of the moon round the earth in the same direction as that of the rotation of the earth itself, the day as measured by the moon is nearly an hour longer than the average solar day as given by the clock.

Fig. 8.—Illustrating the Tides

There is yet another point worth noting with regard to the relation between the moon and the tidal movements of the water, which is that the high tides are never exactly under the moon, but always occur some time after the moon has passed the meridian. This is due to the inertia of the ocean, and to the resistance offered by the land to its movements.

Now, in addition to these diurnal changes of the tide, there are others, extending over longer periods, and which must be more or less familiar to everyone who has spent some time on the coast. On a certain day, for instance, we observe that the high tide flows very far up the beach, and that this is followed, a few hours later, by an unusually low ebb, exposing rocks or sand-banks that are not frequently visible. Careful observations of the motions of the water for some days after will show that this great difference between the levels of high and low-water gradually decreases until, about a week later, it is considerably reduced, the high tide not flowing so far inland and the low-water mark not extending so far seaward. Then, from this time, the difference increases again, till, after about two weeks from the commencement of our observations, we find it at the maximum again.

Fig. 9.—Spring Tides at Full Moon

Here again we find that the changes exactly coincide with changes in the position of the moon with regard to the sun and the earth. Thus, the spring tides—those which rise very high and fall very low—always occur when the moon is full or new; while the less vigorous neap tides occur when the moon is in her quarters and presents only one-half of her illuminated disc to the earth. And, as the moon passes through a complete cycle of changes from new to first-quarter, full, last-quarter, and then to new again in about twenty-nine days, so the tides run through four changes from spring to neap, spring, neap, and then to spring again in the same period.

Fig. 10.—Spring Tides at New Moon

The reason for this is not far to seek, for we have already seen that both sun and moon exert a tide-producing influence on the earth, though that of the moon is considerably greater than that of the sun; hence, if the sun, earth, and moon are in a straight line, as they are when the moon is full, at which time she and the sun are on opposite sides of the earth, and also when new, at which time she is between the earth and sun, the sun’s tide is added to the moon’s tide, thus producing the well-marked spring tides; while, when the moon is in her quarters, occupying a position at right angles from the sun as viewed from the earth, the two bodies tend to produce high tides on different parts of the earth at the same time, and thus we have the moon’s greater tides reduced by the amount of the lesser tides of the sun, with the result that the difference between high and low tides is much lessened.

Fig. 11.—Neap Tides

Again, the difference between high and low water marks is not always exactly the same for the same kind of tide—the spring tide for a certain period, for example, not having the same limits as the same tide of another time. This is due to the fact that the moon revolves round the sun in an elliptical orbit, while the earth, at the same time, revolves round the sun in a similar path, so that the distances of both moon and sun from the earth vary at different times. And, since the tide-producing influences of both these bodies must increase as their distance from the earth diminishes, it follows that there must be occasional appreciable variations in the vigour of the tidal movements of the ocean.

As the earth rotates on its axis, while at the same time the tidal wave must necessarily keep its position under the moon, this wave appears to sweep round the earth with considerable velocity. The differences in the level of the ocean thus produced would hardly be appreciable if the earth were entirely covered with water; but, owing to the very irregular distribution of the land, the movements of the tidal wave become exceedingly complex; and, when it breaks an entrance into a gradually narrowing channel, the water is compressed laterally, and correspondingly increased in height. It is thus that we find a much greater difference between the levels of high and low tides in continental seas than are to be observed on the shores of oceanic islands.

We have occupied so much of our time and space in explanation of the movements of the tides not only because we think it desirable that all who delight in sea-side rambles should understand something of the varied motions which help to give such a charm to the sea, but also because, as we shall observe later, these motions are a matter of great importance to those who are interested in the observation and study of marine life. And, seeing that we are writing more particularly for the young naturalists of our own island, we must devote a little space to the study of the movements of the tidal wave round Great Britain, in order that we may understand the great diversity in the time of high tide on any one day on different parts of the coast, and see how the time of high tide for one part may be calculated from that of any other locality.

Were it not for the inertia of the ocean and the resistance offered by the irregular continents, high tide would always exist exactly under the moon, and we should have high water at any place just at the time when the moon is in the south and crossing the meridian of that place. But while the inertia of the water tends to make all tides late, the irregular distribution of the land breaks up the tidal wave into so many wave-crests and greatly retards their progress.

Thus, the tidal wave entering the Atlantic round the Cape of Good Hope mingles with another wave that flows round Cape Horn, and the combined wave travels northward at the rate of several hundred miles an hour. On reaching the British Isles it is broken up, one wave-crest travelling up the English Channel, while another flows round Scotland and then southwards into the North Sea.

The former branch, taking the shorter course, determines the time of high tide along the Channel coast. Passing the Land’s End, it reaches Plymouth in about an hour, Torquay in about an hour and a half, the Isle of Portland in two hours and a half, Brighton in about seven hours, and London in about nine hours and a half. The other branch, taking a much longer course, makes its arrival in the southern part of the North Sea about twelve hours later, thus mingling at that point with the Channel wave of the next tide. It takes about twenty hours to travel from the south-west coast of Ireland, round Scotland, and then to the mouth of the Thames. Where the two waves meet, the height of the tides is considerably increased; and it will be understood that, at certain points, where the rising of one tide coincides with the falling of another, the two may partially or entirely neutralise each other. Further, the flow and the ebb of the tide are subject to numerous variations and complications in places where two distinct tidal wave-crests arrive at different times. Thus, the ebbing of the tide may be retarded by the approach of a second crest a few hours after the first, so that the ebb and the flow do not occupy equal times. At Eastbourne, for example, the water flows for about five hours, and ebbs for about seven and a half. Or, the approach of the second wave may even arrest the ebbing waters, and produce a second high tide during the course of six hours, as is the case at some places along the Hampshire and Dorset coasts.

Fig. 12.—Chart showing the relative Times of High Tide on different parts of the British Coast

Those who visit various places on our own coasts will probably be interested in tracing the course of the tidal crests by the aid of the accompanying map of the British Isles, on which the time of high tide at several ports for the same time of day is marked. It will be seen from this that the main tidal wave from the Atlantic approaches our islands from the south-west, and divides into lesser waves, one of which passes up the Channel, and another round Scotland and into the North Sea, as previously mentioned, while minor wave-crests flow northward into the Irish Sea and the Bristol Channel. The chart thus supplies the data by means of which we can calculate the approximate time of high tide for any one port from that of another.

Although the time of high water varies so greatly on the same day over such a small area of country, yet that time for any one place is always approximately the same during the same relative positions of the sun, earth, and moon; that is, for the same ‘age’ of the moon; so that it is possible to determine the time of high water at any port from the moon’s age.

The time of high tide is generally given for the current year in the local calendars of our principal seaports, and many guide-books supply a table from which the time may be calculated from the age of the moon.

At every port the observed high water follows the meridional passage of the moon by a fixed interval of time, which, as we have seen, varies considerably in places within a small area of the globe. This interval is known as the establishment of the port, and provides a means by which the time of high water may be calculated.

Before closing this short chapter on the general characteristics of the sea shore we ought to make a few observations on the nature of the water of the sea. Almost everyone is acquainted with the saltness while many bathers have noticed the superior buoyancy of salt water as compared with the fresh water of our rivers and lakes. The dissolved salts contained in sea water give it a greater density than that of pure water; and, since all floating bodies displace their own weight of the liquid in which they float, it is clear that they will not sink so far in the denser water of the sea as they would in fresh water.

If we evaporate a known weight of sea water to dryness and weigh the solid residue of sea salt that remains, we find that this residue forms about three and a half per cent. of the original weight. Then, supposing that the evaporation has been conducted very slowly, the residue is crystalline in structure, and a careful examination with the aid of a lens will reveal crystals of various shapes, but by far the larger number of them cubical in form. These cubical crystals consist of common salt (sodium chloride), which constitutes about three-fourths of the entire residue, while the remainder of the three and a half per cent. consists principally of various salts of magnesium, calcium, and sodium.

Sea salt may be obtained ready prepared in any quantity, as it is manufactured for the convenience of those who desire a sea bath at home; and it will be seen from what has been said that the artificial sea-water may be prepared, to correspond almost exactly with that of the sea, by the addition of three and a half pounds of sea salt to about ninety-six and a half pounds of water.

This is often a matter of no little importance to the sea-side naturalist, who may require to keep marine animals alive for some time at considerable distance from the sea shore, while their growth and habits are observed. Hence we shall refer to this subject again when dealing with the management of the salt-water aquarium.

The attractions of the sea coast are undoubtedly greater by day than at night, especially in the summer season, when the excessive heat of the land is tempered by the cool sea breezes, and when life, both on the cliffs and among the rocks, is at its maximum. But the sea is grand at night, when its gentle ripples flicker in the silvery light of the full moon. No phenomenon of the sea, however, is more interesting than the beautiful phosphorescence to be observed on a dark summer’s night. At times the breaking ripples flash with a soft bluish light, and the water in the wake of a boat is illuminated by what appears to be liquid fire. The advancing ripples, as they embrace a standing rock, surround it with a ring of flame; while streaks and flashes alternately appear and disappear in the open water where there is apparently no disturbance of any kind.

These effects are all produced by the agency of certain marine animals, some of which display a phosphorescent light over the whole surface of their bodies, while in others the light-giving power is restricted to certain organs or to certain well-defined areas of the body; and in some cases it even appears as if the creatures concerned have the power of ejecting from their bodies a phosphorescent fluid.

It was once supposed that the phosphorescence of the sea was produced by only a few of the lower forms of life, but it is well known now that quite a large number of animals, belonging to widely different classes, play a part in this phenomenon. Many of these are minute creatures, hardly to be seen without the aid of some magnifying power, while others are of considerable size.

Among the peculiar features of the phosphorescence of the sea are the suddenness with which it sometimes appears and disappears, and its very irregular variations both at different seasons and at different hours of the same night. On certain nights the sea is apparently full of living fire when, almost suddenly the light vanishes and hardly a trace of phosphorescence remains; while, on other occasions, the phenomenon is observed only on certain patches of water, the areas of which are so well defined that one passes suddenly from or into a luminous sea.

The actual nature of the light and the manner in which it is produced are but ill understood, but the variations and fitfulness of its appearances can be to a certain extent conjectured from our knowledge of some of the animals that produce it.

In our own seas the luminosity is undoubtedly caused principally by the presence of myriads of minute floating or free-swimming organisms that inhabit the surface waters. Of these each one has its own season, in which it appears in vast numbers. Some appear to live entirely at or near the surface, but others apparently remain near the surface only during the night, or only while certain conditions favourable to their mode of life prevail. And further, it is possible that these minute creatures, produced as they generally are in vast numbers at about the same time, and being more or less local, are greatly influenced by changes of temperature, changes in the nature of the wind, and the periodic changes in the tides; and it is probable that we are to look to these circumstances for the explanations of the sudden changes so frequently observed.

In warmer seas the phenomenon of phosphorescence is much more striking than in our own, the brilliancy of the light being much stronger, and also produced by a greater variety of living beings, some of which are of great size, and embrace species belonging to the vertebrates and the higher invertebrate animals.

Those interested in the investigation of this subject should make it a rule to collect the forms of life that inhabit the water at a time when the sea is unusually luminous. A sample of the water may be taken away for the purpose of examination, and this should be viewed in a good light, both with and without a magnifying lens. It is probable, too, that a very productive haul may be obtained by drawing a fine muslin net very slowly through the water. After some time the net should be emptied and gently washed in a small quantity of sea water to remove the smaller forms of life contained, and the water then examined at leisure.

Of course it must not be assumed that all the species so obtained are concerned in any way with the phosphorescence of the sea, but any one form turning up in abundance when collected under the conditions named will probably have some connection with the phenomenon.

One may well ask ‘What is the use of this light-emitting power to the animals who possess it?’ but this question is not easily answered. The light produced by the glow-worm and other luminous insects is evidently a signal by means of which they call their mates, and this may be the case with many of the marine luminous animals, but it is evidently not so with those which live in such immense numbers that they are simply crowded together; nor can it be so with the many luminous creatures that are hermaphrodite. It is a fact, however, that numbers of deep-sea species possess the power of emitting light to a striking extent; and the use of this power is in such cases obvious, for since the rays of the sun do not penetrate to great depths in the ocean, these luminous species are enabled to illuminate their own surroundings while in search of food, and, in many cases at least, to quench their lights suddenly at such times as they themselves are in danger.