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diff --git a/78328-0.txt b/78328-0.txt new file mode 100644 index 0000000..82dcf89 --- /dev/null +++ b/78328-0.txt @@ -0,0 +1,5151 @@ +*** START OF THE PROJECT GUTENBERG EBOOK 78328 *** + + + + + Transcriber’s Note + + + 1. Certain typographic errors & hyphenation inconsistencies were + silently corrected. + + 2. The text version is marked up as follows: + + (a) italics are indicated thus _italic_; + + (b) small-caps are indicated thus +Small-Caps+; + + (c) Images are indicated as [Illustration: (with narration.)] + + 3. Illustration captions have been standardised. + + 4. Table of Contents extended by adding references to the LIST OF + ILLUSTRATIONS, INDEX, and ADVERTISEMENTS. + + + + + [Illustration: _Frontispiece._ + +A Jelly-fish with the fry of the Horse-mackerel.+ + (The fish have been relatively very much enlarged.)] + + + + + THE + + STORY OF LIFE IN THE SEAS + + BY + + SYDNEY J. HICKSON, D.Sc., F.R.S. + + _Professor of Zoology in the Owens College, Manchester_ + + _WITH FORTY-TWO ILLUSTRATIONS_ + + LONDON + GEORGE NEWNES, LIMITED + SOUTHAMPTON STREET, STRAND + 1898 + + + + + OUTLINE CLASSIFICATION + OF + ANIMALS MENTIONED IN THIS BOOK. + + + PROTOZOA {Foraminifers. + {Radiolarians. + + PORIFERA Sponges. + + {Sea-anemones. + CŒLENTERATA {Corals. + {Jelly-fish. + {Many of the Zoophytes. + + {_Asteroidea_ Star-fishes. + ECHINODERMA {_Echinoidea_ Sea-urchins. + {_Crinoidea_ Sea-lilies. + {_Holothuroidea_ Trepangs. + + PLATYELMIA Flukes, Tape-worms, Planarians. + + CHÆTOPODA Segmented Worms and Gephyreans. + + { {Copepods. + { {Barnacles. + ARTHROPODA {_Crustacea_ {Shrimps. + { {Crabs. + {_Insecta_ Halobates, &c. + + {_Lamelli_ Bivalves. + { _branchiata_ + MOLLUSCA {_Gastropoda_ Whelks, &c., and Pteropods. + {_Cephalopoda_ Octopuses, Cuttlefishes, &c. + + TUNICATA Sea-squirts, Salps, Pyrosoma. + + {_Pisces_ Fish. + {_Amphibia_ Frogs. + {_Reptilia_ Turtles, Crocodiles, Snakes. + VERTEBRATA {_Aves_ Birds. + { {Whales, Porpoises. + {_Mammalia_ {Seals, + { {and the terrestrial Mammals. + + + + + PREFACE. + + +The story of the life of animals and plants in the sea is one with so +many aspects, that it is difficult to choose the points that may be +included, and those that may be omitted from a book intended for the +general reader. To some the story of the food Fishes and the Whales is +of the greatest interest; to others the beautiful shapes and colours +of shells have a predominating fascination; and to those who have +devoted themselves to geological study, the history of the animals that +contribute to the formation of the reefs and the ocean-bed present +features of special attraction. To many, then, the perusal of my book +must lead to disappointment as no one of these aspects has been treated +adequately; but if some new interest is awakened, some new train of +thought quickened into life, one of the objects I had in view will have +been gained. + +The book is only intended to be a sketch of some of the most important +lines of scientific researches which are now being pursued by +zoologists in many parts of the world. Discoveries, which are of the +deepest interest to all intelligent minds, are in many cases described +in books and periodicals that do not come within the reach of the +general public. I have tried, therefore, to collect some of them into +a small compass and describe them in language which I trust will be +intelligible to those who have not been trained in the alphabet of +zoological technicalities. The use of some long words was unavoidable, +but I have endeavoured to explain them adequately either in the text or +in the index. + +Some of the illustrations have been copied from the works of other +Naturalists, and the sources from which they came are acknowledged in +the list of illustrations; but the majority of them have been drawn, +specially for this work, from specimens from the Manchester Museum or +my own collections. + + SYDNEY J. HICKSON. + +_November 1897._ + + + + + CONTENTS. + + + CHAP. PAGE + + LIST OF ILLUSTRATIONS 8 + + I. OCEANOGRAPHY 9 + + II. SHALLOW-WATER FAUNA 23 + + III. SHALLOW-WATER FAUNA OF THE TROPICS 55 + + IV. SURFACE-SWIMMING FAUNA (INVERTEBRATES) 82 + + V. SURFACE-SWIMMING FAUNA (VERTEBRATES) 118 + + VI. DEEP-SEA FAUNA 135 + + VII. COMMENSALISM AND PARASITISM 146 + + VIII. THE ORIGIN OF THE MARINE FAUNA 172 + + INDEX 180 + + ADVERTISEMENTS 184 + + + + + LIST OF ILLUSTRATIONS. + + + ILLUSTRATION PAGE + + _Frontispiece._--A Jelly-fish with the fry of the Horse Mackerel + (from specimens captured by Mr F. W. Gamble off Valentia). 2 + + Fig. 1--Globigerina Shell 21 + + Fig. 2--Radiolarian Shells 22 + + Fig. 3--Common Pipe-fish (from Royal Natural History) 25 + + Fig. 4--Phyllopteryx (from Royal Natural History) 26 + + Fig. 5--Diagrams of Eyes 27 + + Fig. 6--A branch of the Zoophyte Obelia 30 + + Fig. 7--Medusa produced by Obelia 31 + + Fig. 8--A Bivalve Mollusc 35 + + Fig. 9--The common Sole 37 + + Fig. 10--The Angler (from Royal Natural History) 38 + + Fig. 11--Vertical section of a Balanus (after Claus) 40 + + Fig. 12--Nauplius larva of a Balanus (after Groom) 41 + + Fig. 13--Sea-urchin 43 + + Fig. 14--Smooth-shelled Gastropod 47 + + Fig. 15--Spiny Gastropod 47 + + Fig. 16--A Cuttle-fish 49 + + Fig. 17--The Wrasse 51 + + Fig. 18--The John Dory 54 + + Fig. 19--Polyp of a Madrepore Coral (after Fowler) 58 + + Fig. 20--Chætodon 62 + + Fig. 21--Globe-fish 64 + + Fig. 22--Stereosoma 67 + + Fig. 23--Coral-reefs 69 + + Fig. 24--Periophthalmus 74 + + Fig. 25--Free-swimming Copepod (after Claus) 88 + + Fig. 26--Swim-bladder of Velella 96 + + Fig. 27--Solitary form of Salp 99 + + Fig. 28--Pteropod 101 + + Fig. 29--Shells of Foraminifers 104 + + Fig. 30--Globigerina 105 + + Fig. 31--Young larva of a Star-fish 111 + + Fig. 32--Pluteus larva 113 + + Fig. 33--Long-spined Barnacle-nauplius (after Chun) 116 + + Fig. 34--Sun-fish 122 + + Fig. 35--The common Porpoise 129 + + Fig. 36--A deep-sea Fish (after Filhol) 139 + + Fig. 37--Hermit-crab and Sponge 149 + + Fig. 38--Section through a Sponge showing Hermit-crab 150 + + Fig. 39--A Trepang 153 + + Fig. 40--A Crab-gall 158 + + Fig. 41--A parasitic Copepod 169 + + + + + THE STORY OF LIFE IN THE SEAS. + + + + + CHAPTER I. + + OCEANOGRAPHY. + + +One of the most important facts that has been established by modern +investigations of the Sea is that there is no region in its vast extent +that is entirely devoid of animal life. The surface waters in the +Equatorial calms and the ice-cold waters between the ice-bergs of the +Arctic regions are densely populated by animals, large and small; the +heavy and heated waters of the Mediterranean and Red Seas, and the cold +and comparatively fresh waters of the Norwegian fjords, the shallow +waters of the coasts and the greatest depths of the ocean-beds all +present us with their characteristic forms of living creatures. There +is no Azoic region known to us. Wherever we use the trawl or dredge +we may expect to find some representatives of the various classes of +marine animals. But the seas exhibit so many varying conditions that, +as we might have expected, the animals that characterise one region are +absent from another; and while, in some places animal life is abundant, +in others it is very scarce; just as on land we find the grass-lands +and forests teeming with life, and the great deserts and mountain tops +inhabited only by a few solitary Lizards, Birds or Insects. + +In order that we may fully understand, then, the nature of the problems +concerning the distribution of animals through the seas, it is of +importance to consider first the conditions under which they must +live in the different parts of the ocean. A knowledge of geography is +clearly necessary for those who study the distribution of terrestrial +and aërial animals, and equally necessary is it for those who wish +to learn something about the distribution of the aquatic animals, +to consider first of all the rudimentary principles of hydrography. +The principal Sea areas of our globe may be roughly divided into two +groups: namely, the great oceans--the Atlantic, Pacific and Indian +Oceans--and the Inland seas, which are partly enclosed by land, such +as the German Ocean, the Mediterranean Sea, and the Red Sea. Taking +the areas of the great oceans and the seas together, we find that no +less than 141 millions of square miles, or nearly three-quarters of the +surface of the globe, are covered with water. Moreover, these great +areas are continuous, so that it would be possible for an animal, other +conditions being favourable, to pass from any one sea, such as the +Black Sea, to any other, such as the Hudson’s Bay, without leaving the +water,--an important fact in the consideration of the distribution of +marine forms of life. + +The depth of the sea varies very considerably in the different parts of +the world. In the inland seas the water is comparatively shallow, but +in the great oceans it is very deep. In the middle of the North Sea, +for example, we should not expect to find a depth much exceeding 250 +fathoms, but in the Atlantic or Pacific Oceans we have to pay out more +than 2000 fathoms of the sounding-line before the bottom is reached. In +some parts of the ocean-basins a few very deep holes or furrows may be +found in which the depth exceeds 4000 fathoms, or 24,000 feet. One of +these deep holes occurs in the Atlantic Ocean, a little to the North +of the Virgin Islands in the West Indies, and there is another in the +Pacific Ocean close to the coast of Japan; but the greatest depth that +has yet been found is one recently discovered by H.M.S. “Penguin” off +the coast of New Zealand of over 5000 fathoms. Apart, however, from +the fact that these very great depths are only of local occurrence, +the areas of deep water--that is, of more than 2000 fathoms--are so +much greater than the areas of shallow water, that when we make a +calculation of the average depth of the sea we find it is no less than +2100 fathoms, or 12, 600 feet. + +The temperature of the sea is another feature which undoubtedly +influences very greatly the character of its Fauna. The main source of +the heat of the sea is the sun--for the heat derived from submarine +volcanoes must be comparatively so small that we may omit it from +consideration. Consequently we find that in the Equatorial regions the +surface waters of the ocean are warmer than they are in the Temperate +regions. These, again, are warmer than in the Arctic circles. But +water is well-known to be a bad conductor of heat, and therefore +the direct influence of the sun affects only the most superficial +layers. In the Equatorial region of the Pacific Ocean, for example, +the surface temperature is sometimes as high as 80° Fahr., at 100 +fathoms from the surface it is only 60°, at 400 fathoms only 45°, and +at 1000 fathoms only a few degrees above freezing point. On the land +the temperature falls as we pass from the coast to the high plateaux +and mountains, and we find snow-capped mountains in Central Africa just +as in Switzerland or Norway. In the sea the temperature falls as the +thermometer is sent deeper from the surface. Just as on the land the +snow line of the mountains is reached at high altitudes in the Tropics, +at lower altitudes in the Temperate regions, and in the Arctic circle +at the level of the sea, so in the sea the cold water that is found 500 +fathoms below the surface in the Tropics, reaches a higher level in the +Temperate regions, and is at the surface in the Arctic circle. + +There is however one important point of difference between the +distribution of these low temperatures on the land and in the oceans, +in that they are broken in the former, and continuous in the latter. +If we were to imagine an aquatic animal that could only live in +temperatures below 35° Fahr., it would be able to travel below the +surface from one pole to the other, or from one ocean to another; but +it would be impossible for a terrestrial animal, exhibiting the same +peculiarity, to leave the Arctic circle or the Alpine region without +traversing lands where the temperature is higher than that which is +necessary for its existence. It might be supposed from what has just +been said that the temperature of the water at the bottom is constant +for the same number of fathoms of depth. This is not, however, the +case. The temperature of the sea-bottom of the great ocean-beds is +approximately the same, varying from 28° F. in the Atlantic to 35° +F. in the Pacific; but in places where main basins occur, surrounded +on all sides by shallower ridges, the temperature of the bottom of +the basin is the same as that of the lowest ridge. For instance, the +temperature of the bottom of the Sulu Sea, lying between Borneo and +the Philippines, is 40° F. at a depth of over 2000 fathoms. Again, the +temperature of the Red Sea is as high as 70° F., although depths of +1200 fathoms occur in its central portions; and this is the same as the +temperature at the Straits of Babel Mandeb, which are 200 fathoms deep, +and form the only outlet to the open ocean. These facts probably cause +considerable modification in the character of the animals inhabiting +such enclosed basins, but further investigations are needed before we +can arrive at any very definite conclusions in the matter. + +Another important element that must be taken into consideration in +studying the environment of marine animals, is the quantity and +character of the salts held in solution by the sea-water. In the first +place we must remember that the sea-water normally contains a far +greater percentage of salts in solution than the water of rivers and +lakes, and this causes it to be very much heavier. If a tumbler be +half filled with sea-water, upon which some fresh water is slowly and +carefully poured, there will be for some time very little mixture of +the two fluids, the heavier sea-water remaining at the bottom, and the +lighter fresh water at the surface. Now the density of the sea-water, +or in other words the amount of salts in solution, is not the same +over the whole world, and the differences that may be observed in +this respect are due, in most cases, to the simple physical principle +just enunciated. If we could imagine a river pouring its waters into +a perfectly calm, tideless sea, we should be able to trace the fresh +river water far away from the coast, for it would simply float on the +heavier sea-water without mixing with it to any appreciable extent. In +most cases, however, the tidal-waves, rushing up and down the river +estuaries, stir up the fresh and salt water together, and cause a very +considerable mixture, so that the water becomes either distinctly salt +or brackish. Where very large quantities of fresh water are poured +into the ocean, as, for example, at the mouth of the Amazon or the +Mississippi, the surface water remains so fresh that the salt taste +can hardly be appreciated at a distance of some miles from the coast. +This fact sufficiently indicates the influence of great rivers upon the +density or saltness of the sea-water in their neighbourhood, and the +reader will be prepared for the statement that many inland seas, such +as the Black Sea, are appreciably less salt than the great oceans. + +Again, the ocean water itself is not of the same density in all +latitudes. In regions where there is a copious rainfall and the sea is +not frequently disturbed by severe storms, the rain takes some time to +mix with the heavier salt water on which it falls, and consequently +there may always be discovered in these localities a thin stratum of +comparatively fresh water on the surface of the ocean. In some inland +seas where there is considerable evaporation and a slight rainfall, as +for example, the Red Sea and the Mediterranean, the sea-water reaches +an even higher degree of concentration than it does in the open ocean. +The following table will serve to illustrate these facts:-- + +Density of rain-water, 1·00. + +Density of the Black Sea surface, below 1·025. + +Density of the Atlantic Ocean surface (west of the Canaries), 1·0275. + +Density of the Mediterranean Sea, over 1·028. + +Density of the Red Sea, 1·030. + +Density of the bottom water of the Atlantic, 1·029 (west of the + Canaries). + +That the rate of movement of the water influences very largely the +character of the animals that live in it, is a fact that it is not +necessary to discuss fully in this place; but as it is undoubtedly one +of the factors which must be taken into consideration in discussing the +character and possible origin of the Fauna of any particular region, +a brief survey must be given of some of the principal causes of the +movements of the water and the characters of the tides and currents +which are manifest in the sea. Twice every twenty-four hours the water +of the sea rises and falls. This movement is due to the attracting +influences of the sun and moon, and is, as is well-known, greater when +the moon is full and when it is new than at the intermediate times. If +the distribution of land and water on the surface of our globe were +different, and a free waterway occurred round the world, right in the +Equatorial band we should probably find a double tidal-wave rushing +round the earth every twenty-four hours. As it is, however, the great +tidal-wave is checked by the continents, and as it approaches the +coasts is retarded and diminished in force. In Archipelagoes and along +broken coast lines the tidal-waves produce true surface currents, which +frequently run with great rapidity and exert considerable corroding +action upon the rocks. In many estuaries and bays the tide rushes in +with such force that the water is heaped up to a great height against +the land. At the entrance to the Bay of Fundy, for example, the rise +at spring-tides is no less than 70 feet, and at the Cardiff docks the +difference of level between high and low spring-tides is 42 feet. The +tumultuous ebb and flow of such masses of water along the coast is +fatal to some forms of animal life and favourable to others, and so to +some extent it modifies the character of the Fauna. + +In addition to the surface currents of the coast, produced by the +tidal-waves, there are also the true ocean-currents, which must be +briefly considered. They are caused by the winds which blow constantly +in a definite direction across the oceans. The prevailing winds not +only raise the sea into waves, but drive the superficial layers of the +water over the subjacent layers in one direction. In studying a map +of the great ocean-currents, we notice a well-marked one lying to the +North of the Equator in both the Atlantic and Pacific. This flows from +East to West and follows very closely the lines of the prevailing winds +in that region. Similarly in the temperate regions of the Southern +Hemisphere there is an ocean-current, flowing however in this case in +the opposite direction--from West to East--and so corresponding with +the trade-winds of that part of the world. The well-known Gulf-stream +of the North Atlantic, although modified in some respects by other more +complicated causes, also follows for part of its course the general +direction of the prevailing winds. + +The currents just described are surface currents only, and do not +affect to any great extent the mass of the subjacent waters in the +ocean-basins. It is difficult to estimate the depth to which their +influence reaches, but it is not probable that it extends more than 200 +fathoms below the surface. In addition to these, there is also a series +of slow currents in the deep waters flowing in definite directions. +In the Tropical regions the waters are constantly being heated by the +sun, and passed away by the trade-winds to the North and South and +ultimately towards the poles. In their long and complicated journey +they are gradually cooled down until, in the regions of the ice-bergs, +they reach a temperature just above the freezing point of sea-water. +Here the water, being colder and therefore heavier than that of the +other regions of the world, sinks to the bottom, and gradually returns +in a deep-seated mass towards the Equator, where, welling up from the +bottom, it replaces the heated layers of the surface. It is almost +impossible to determine with accuracy the rapidity and exact direction +of these deep-sea-currents. It is extremely probable that they are +immensely modified by the irregularities of the bottom and the outline +of the coast banks, but their exact topography must remain for the +present one of the secrets of the abyss that are not revealed to us. +All that can be said is, that the warm surface water which passes from +the Tropics towards the North and South is replaced by deep-seated +Polar currents, which account for the extremely cold water that is +found at great depths in the ocean-basins, and also for some of the +peculiarities of the marine Fauna, which will be referred to later on. + +The character of the sea-bottom in various parts of the world must +be referred to before passing on, for there can be little doubt of +the important effect it has upon the Fauna. In the neighbourhood of +continents the bottom of the sea varies very considerably. The great +rivers bring with them in suspension the products of the wear and tear +of mountains and valleys; the coast line, washed by the continuous +ebb and flow of the tides, contributes some of its substance to the +formation of the sea-bottom; and the countless millions of animals +and plants of the shallow waters leave their skeletons and shells as +they die to form an integral part of the floor of the ocean. Thus +the sea-bottom in the neighbourhood of the land is formed partly +by terrigenous deposits, varying of course in character with the +geological nature of the land itself, and partly by the animal and +vegetable deposits of the coast. In some cases the deposits brought by +the rivers can be traced in the sea-bottom for a very great distance +from the coast. The characteristic mud of the Congo river can be traced +600 miles from its mouth, and it is said that the Arabian Sea and the +Bay of Bengal are carpeted for 1000 miles by the mud brought down by +the Ganges and the Indus. Leaving out of consideration, however, for +the moment the exceptional cases of such large rivers as these, we may +say that the influence of the land deposits upon the character of the +sea-bottom extends to a distance seawards of about 250 miles. If we +had a complete and careful survey of all the coast lines, it would be +possible for us to draw a line round the great continents marking the +limit of the deposits of river and coast mud. This line has been called +the mud-line by Mr John Murray, and, as he has clearly pointed out, it +is characterised by an abundant and extremely interesting Fauna. + +The sea-bottom, then, within the limits of the mud-line, is very +largely composed of deposits from the land brought down by the rivers. +In some volcanic regions of the world this is, to a great extent, +augmented by lava and water-logged pumice, and in other districts by +the mud and stones dropped by the melting ice-bergs. The influence +of animals and plants upon the formation of the sea-bottom is often +very great indeed in shallow water, though it varies considerably in +different parts of the world. In the neighbourhood of the British +coast, for example, the sea-bottom is, in many places, carpeted with +the calcareous Sea-weed, _Lithothamnion_,--in other places the +dredge will come up crammed full of bivalve shells. But such instances +as these in which the floor of the sea is covered with animal or +vegetable shells, are comparatively rare and of small extent in the +neighbourhood of land in the Temperate regions, and in nearly all +localities the true terrigenous deposits can be readily obtained by +the use of a small meshed dredge. In the warmer regions of the world, +however, the sea-bottom in the shallow water is over great areas +completely covered by animal and vegetable products. In the West +Indies, and in some parts of the Eastern coasts of Tropical America, in +the Eastern Archipelago and the coast of East Africa and its islands, +Coral-reefs are found. These are entirely built up of the skeletons and +shells of animals, and a few Coralline Algæ. In the vicinity of these +reefs the floor of the sea is for miles carpeted with the broken-down +skeletons of these animals, sometimes in the form of a fine coralline +sand, sometimes of large lumps studded with knolls of living Corals, +Molluscs, Sea-urchins, and other creatures. We find, therefore, in the +warmer regions of the world immense areas of shallow water in which the +terrigenous deposits take but a very small part in the formation of the +sea-bottom, animal and vegetable life being so vigorous and active as +to be able to form enough shells and skeletons to cover every available +part of its surface. + + [Illustration: +Fig. 1.+ + Globigerina Shell from a deep-sea ooze.] + +Far away from continental lands, and at great depths, the character +of the sea-bottom completely changes. At a distance of 100 miles from +the coasts of America or Europe, for example, the land deposits have +already found their resting-place, and the animal life in the depths of +the Atlantic is poor in skeleton-forming genera. However, the surface +waters of the ocean teem with creatures of all sorts which, as they +die, drop down their skeletons and shells in a gentle shower to form +a fine deposit on the bottom. When we get beyond the mud-line, then, +and use the dredging or sounding apparatus in depths of 1500 to 2500 +fathoms, we find that the bottom is largely composed of the shells of +such surface animals as the Pteropods and Globigerinas, and according +to the relative abundance of these forms it is called Pteropod Ooze or +Globigerina Ooze. In still greater depths than these the character of +the bottom again changes, and we find a deposit which is commonly known +as the Red clay. The explanation of this change of character depends +upon the fact that sea-water exercises a slightly solvent action upon +carbonate of lime, and the shells of the Globigerinas and other forms +are, in seas of a depth of over 2500 fathoms, dissolved before they can +reach the bottom. The only shells that can survive this long journey +are the siliceous shells of the Diatoms and Radiolarians, and in those +parts of the ocean where these organisms live in abundance their empty +shells form an important percentage of the composition of the Red +clay. Over a very considerable area of the Pacific Ocean, however, +the Red clay contains only a minute proportion of these shells, and +its composition has given rise to a good deal of discussion among the +authorities. It may be considered to be a conglomerate of the horny +fragments of dead surface-living animals, of volcanic and meteoric +dust, and of small pieces of water-logged pumice-stone. + + [Illustration: +Fig. 2.+ + Radiolarian Shells from a deep-sea deposit.] + +In some of the very deep holes of the Pacific Ocean the mud is +almost entirely composed of Radiolarian shells, and is then called +“Radiolarian ooze”: and in the Southern Sea a mud called Diatom ooze +has been found, which consists principally of the siliceous shells of +these minute algæ. Notwithstanding these varieties of the mud, and +others that space does not allow me to refer to at length, we may +suppose that if the floor of any one of the great oceans were exposed +it would have the appearance to a traveller of a vast desert-like +expanse, without a stone, a rock, or a cliff to vary the monotony of +the scene. At one time it was supposed to be an absolute plain, without +any important change of level from the mud-line of one continental +coast to the other; but the result of modern submarine explorations has +been to prove that in all the great ocean-basins, hills and ridges, as +well as troughs and deep holes occur, which break the monotony of the +generally smooth and level character of the bottom. + + + + + CHAPTER II. + + SHALLOW WATER FAUNA. + + +Having considered thus briefly the general conditions under which the +animals of the sea must live, we may now consider more in detail the +special conditions of shallow water life. + +In water of only a few fathoms in depth, the direct light of the sun +is capable of reaching and influencing all living things that occur, +either at the bottom, at the surface, or in the intermediate waters. +Great as the influence of direct sunlight must be upon animals, it +is even greater upon plants. Nearly all the Sea-weeds are, like the +plants of the dry land, dependent upon carbonic acid gas dissolved in +the water for one of the most important constituents of their food, +but it can only be absorbed by the plant in the presence of sunlight. +It is possible, therefore, for Sea-weeds to flourish in the shallow +waters of the sea, while they are necessarily absent from the deeper +and darker regions to which the rays of the sun cannot penetrate. +Everyone knows that in the shallow waters of our own coast there is +in many places a dense tangle of Wracks and long, flat ribbon-like +Sea-weeds growing on the bottom, and that on and amongst these weeds a +rich harvest of animals awaits the eager shore collector. It is true +that there are vast fields of sand on which the Sea-weeds are few and +far between, but we may say that wherever they can obtain a secure +foothold in the shallow waters of the British coast there they will +grow and multiply in great profusion. We must not, however, jump too +hastily to the conclusion that the same is true for all parts of the +world. The British coasts are particularly rich in Sea-weeds, in fact +a distinguished botanist once said that they probably present us with +the greatest number of genera and species of any coast line of the same +extent in the world. In the Temperate regions of both the Southern and +Northern Hemispheres there is generally a rich Sea-weed flora, but in +the warmer regions it is less luxuriant, and on the Coral-reefs of the +Tropical seas it is remarkably poor. + +Turning our attention for the present to the Temperate regions, let us +consider the influence that the Sea-weeds have upon the animals of the +shallow waters. In the first place we find that they afford shelter and +support for a large number of animals which could hardly live without +them. In the roots of the weeds may be found little Crabs and Molluscs, +which occur nowhere else; and clinging to the long waving branches are +many forms of Sea-anemones, Zoophytes, flattened limpet-like Molluscs, +Ascidians, and other forms of animal life. The great forests of weeds +are also the haunts of many queer Fish, Prawns, Crabs, and Sea-slugs, +which hunt their prey or hide from their enemies amidst the shelter of +the stems and branches. + + [Illustration: +Fig. 3.+ + The common Pipe-fish.] + +Now many of these animals, which together make up the Fauna of the +Sea-weed region, have assumed, in the course of the ages of evolution, +not only the colours of the Algæ on which they live, but in some cases +even forms which render them at first sight more like plants than +animals. The slender Pipe-fish, for example, which is not uncommonly +found amongst the bright green Sea-weeds of our coast, is wonderfully +similar, both in form and colour, to the weeds on which it lives. The +Sea-horses, which have such a curious form out of the water, in their +natural surrounding resemble the weeds so closely that they may be +easily overlooked. A still more remarkable example is to be found in +the curious fish Phyllopteryx, in which the body is provided with long +branched processes, making it we may suppose much more difficult to +distinguish in its natural haunts than even the Pipe-fish or Sea-horse. + + [Illustration: +Fig. 4.+ + Phyllopteryx.] + +Many other examples to illustrate this feature of the Sea-weed Fauna +could be quoted, but sufficient has been said at present if we have +indicated to the reader the manner in which, by the indirect influence +of light, the form and colour of animals may be modified for their +life among the marine plants. The presence of light, however, modifies +directly the character of the animals themselves in many respects. + + [Illustration: +Fig. 5.+ + Diagrams of Eyes of _A_ Whelk, _B_ Lobster, _C_ Scallop, _D_ Fish.] + +The statement that many of the animals of the shallow water are +provided with eyes because there is light is, when carefully examined, +found to be strictly true, however anomalous it may seem to be. There +must have been light in the shallow waters of the sea when the first +primordial forms of life made their appearance, and it was this light +which, step by step, led to the evolution of the most complicated and +perfect forms of eye from the simplest pigment spot of the Protozoan +to the eye of the Lobster and the Fish. We may say that all animals +that freely swim in the shallow waters or that crawl and creep on the +rocks and sands at the bottom are provided with eyes. The Fish are +provided with a pair of eyes which present us with the same general +features that we find in the other Vertebrates. Lobsters, Crabs and +Prawns have each a pair of stalked eyes, which are probably as perfect +in their functions as the Vertebrate eye, although built up on an +entirely different model. Whelks, Winkles, and other Gastropods, +although so slow in their movements, have each, upon or close to their +tentacles, a pair of minute eyes, which are much simpler and probably +far less perfect in form than the eyes of their more highly organised +neighbours. Even the little Jelly-fish, Star-fish, Sea-urchins, and +other creeping forms of life, are provided with specialised pigment +spots, which we have good reason to believe enable them to perceive the +rays of light. + +But we must notice that it is only the animals that are capable of +locomotion from place to place that need these organs of vision, +and that the stationary forms are blind. In the large class of the +Lamellibranchs, for example, to which the Oyster and the Mussel belong, +we find that with a few exceptions there are no eyes. These animals, +after the first few stages of their life are past, settle down into +the sand, or fasten on to a rock and remain there until their life +is done. Their food, consisting of the minutest specks of animal and +vegetable life, is brought to them by the sea-currents; they do not +need nor desire to seek the society of their relations, and when their +enemies approach they resign themselves almost without an effort to the +inevitable. To such animals eyes would be useless, and so nature has +withheld them. + +There are, however, a few Lamellibranchs that possess good eyes, eyes +that are almost as complicated in their structure as the Vertebrate +eye; and not a single pair only, but sixty, eighty, a hundred, or even +more, may be found on a single individual. Such a Lamellibranch is the +common Scallop which may be seen in many of our fishmongers’ shops. +Unlike most of the group, this animal is able to make long flights +through the water by the flapping together of its shells, and there +can be no doubt that on the approach of danger it uses this method of +locomotion for escape. One of its most deadly enemies is the Star-fish, +which forces the shells apart and sucks out the flesh by means of its +protrusible stomach. When a Star-fish is placed in an aquarium in which +there are some Scallops, lying, as they do, on their sides, with the +valves slightly open, showing the double row of gleaming metallic eyes +on the margin of the mantle, the Star-fish immediately moves, with what +in such an animal may be considered extraordinary rapidity, straight +in the direction of the Scallop. Before, however, it reaches its prey, +the coveted victim gives four or five vigorous flaps of its shells and +swims away to another part of the tank. This suggests that the eyes +of the Scallop are used in the light as a means of giving warning of +the approach of an enemy, and they are found in the Scallops only, +among common British bivalves, because they alone possess this power +of swimming away. Of course if there were no light in the water the +eyes would be useless, and it is an interesting fact that the Scallops +which live in the darkness of the great depths of the ocean are quite +blind. + + [Illustration: +Fig. 6.+ + A branch of the Zoophyte Obelia.] + +Very interesting examples of the connection between the presence of +eyes and the power of locomotion are found among the sedentary or fixed +forms of life. The great class of Sea-squirts or Tunicates includes a +number of genera, most of which are, in the adult condition, immoveably +fixed to the bottom, and in that stage have no eyes; but the eggs +which they produce give rise to little creatures, like tad-poles in +form, which swim about freely in the water. Each of these tad-poles +has a large eye in its brain, which remains so long as the animal +leads a free life. As soon, however, as it settles down upon the rock +which is to become its permanent resting-place through life, the eye +and the organ of locomotion, the tail, both degenerate and ultimately +disappear. Again, we often find upon rocks, Sea-weed, old shells, and +the like, some curious delicate branching organisms called Zoophytes. +Notwithstanding their general resemblance in form to Sea-weeds, these +Zoophytes are known to be animals. Each tuft or branch is formed by +a number of delicate little Polyps, each with a crown of tentacles +round a terminal mouth. The Polyp cannot move away from the branch it +grows upon, nor the branch from the stem, nor the stem from the rock +on which it rests, and none of these minute creatures are provided +with eyes. There are, however, a few Zoophytes that give rise to buds, +which grow into the form of minute Jelly-fish (or Medusæ, as they are +called); and these, becoming detached from the parents, swim away and +lead an independent existence. These Medusæ are in many cases provided +with simple little eyes. During their short life they are drifted away +by the sea-currents to some distance from their parents, and produce a +number of eggs which are capable of developing into a new fixed colony +of Polyps. + + [Illustration: +Fig. 7.+ + Medusa produced by Obelia.] + +We may consider it one of the maxims of Science that in a population +of animals which possess eyes, colour as well as form is of extreme +importance. As well as the alteration in form that takes place in the +animals living among Sea-weeds, we find a modification in colour in +the creatures dwelling among rocks or on the sand, so that they may +resemble the ground on which they live. No more striking example of +this could be found than in the common Shrimps of our coasts. Anyone +who has watched them in the sea-pools must have been struck with their +close resemblance to the sand. In fact it is only by close observation +that they can be distinguished from it. In an aquarium, too, it +may be observed how very much the upper sides of Soles, Flounders, +Turbots and other flat fishes are like the sand in colour, while the +under sides are almost invariably pure white. The black colour of the +Lobsters, speckled and striped with blue, has a close resemblance to +the holes and crannies of the rocks among which they live. The bright +transparent green Prawns are almost invisible as they move about among +the Sea-weeds, and Sea-slugs assume all manner of beautiful colours +according to the ground on which they feed. + +On the Coral-reefs of the warmer regions of the world the pools that +are left when the tide goes down are characterised by their brilliancy +of colour. The bright purple, green, and yellow tips of the Coral +branches, the red and bright green Sponges, and the white pieces of +dead and broken Corals make up a scene of beauty which can only be +compared with a bed of variegated flowers. Here the Fish, Prawns, and +other moving animals have assumed the most gorgeous colours in patterns +of spots and stripes which verily astonish the naturalist when he +sees them for the first time. The great Sea-perches, with their sides +covered with bright red or brown blotches, the curious Trigger-fish, +with bright red, blue, or yellow bands crossing their bodies, the +banded Lobsters, and the spotted Cuttlefishes, strange and conspicuous +as they may seem when they are taken out of the water, are in life but +in harmony with their surroundings, and, in reality, less conspicuous +than they would be if less brilliantly arrayed. + +I remember on one occasion as I was watching some expanded Polyps in +a little shore pool in the Tropics, I noticed something suddenly move +close to the coral block on which I was standing. As there was no +escape from the little pool, nor any holes in which the creature could +hide itself from my observation, I searched with diligence to find it +again, but for a long time without avail. Suddenly it moved again, and +then I saw, resting on the brilliantly coloured corals, a remarkable +little Shrimp called by zoologists _Stenopus_. Its body, which was +almost transparent, was marked by a number of bands of a bright red +colour, it had enormously long antennæ similarly banded, and its legs +and body were covered with short red-tipped spines. When I succeeded in +safely landing it in my collecting bottle I felt perfectly astonished +that I had been so baffled by such a lovely little jewel, so bright, so +strange, and so generally conspicuous did it then seem to be. + +The colours of the animals I have referred to hitherto may be accounted +for by their need to escape the attention of their enemies, or to +avoid detection in the pursuit of their prey; but the animal colour +problem is not yet exhausted. The beautiful colours of the Anemones, +Corals, Sponges and many other sedentary forms of animal life, and +the marvellous patterns on the shells of Molluscs cannot be due to +these causes. Many ingenious theories have been brought forward to +explain away the difficulty, but none of them seem to be perfectly +satisfactory, and consequently it is unnecessary to enunciate them in +detail in this small book. + +The character of the bottom of shallow water, especially in the +neighbourhood of the coasts, presents us with so many variations +that it would be a long task to consider in detail all the different +adaptations that the animals exhibit. The Fauna of the sand, shingle, +and mud at the mouths of rivers, of the rocks of iron-bound coasts, +and of the coral-reefs, each present us with many curious kinds of +modification of form and structure. A brief reference to one or two +characteristic regions must be made before passing on. + +The sandy bottoms which are so prevalent, not only upon our coasts, +but in nearly all parts of the world, invariably support a Fauna with +many curiously altered forms. In walking across the sands at low water +we may have often noticed many worm-like and twisted columns of sand; +these are the casts of the common Lug-worm, which is a favourite bait +for many kinds of Fish. The Lug-worm lives in a U-shaped tube, which it +forms from a slimy secretion of the body; it feeds by swallowing the +sand in which it burrows, extracting from it as it passes through the +intestine whatever animal or vegetable food it may contain. There can +be little doubt that the sand is a very poor form of diet, and that +an immense quantity must pass through the body of the animal compared +with its weight in order to afford sufficient nourishment. It has been +reckoned that as many as 82,433 casts may be found on an acre of sand +where conditions are favourable for these Worms, and that they would +weigh together nearly 2000 tons. This would mean that the whole mass of +the sand would pass through the bodies of Lug-worms on an average of +once in every twenty-two months. + +But the Lug-worm is only one of the many forms of life that burrow +in the sand. A very large number of bivalve Molluscs live with the +greater part of their bodies perpetually buried. Their organisation +is such that their food and the water that is used for respiration +can be brought to them by a tubular prolongation of the body called +the siphon. Some of these animals live much deeper down than others, +and while some have but feeble powers of moving either up or down in +their burrows, others can penetrate to great depths with extraordinary +rapidity. + + [Illustration: +Fig. 8.+ + A Bivalve Mollusc, showing below the foot with which it burrows into + the sand, and above the siphons.] + +The shell of a Lamellibranch, called the Razor-shell (_Solen_), is +not an uncommon object of the sea-shore where stretches of sand occur. +When the animal is alive it has the power of burrowing down so quickly +that it is practically impossible for one man to capture a specimen +by digging, when it is thoroughly alarmed. Occasionally, however, the +sea itself, when lashed into fury by a storm, is a better digger than +the Solen is a burrower; and after a heavy storm in the Isle of Man I +have seen the shore littered with Solens scooped out of the sand by the +force of the waves and cast up with lacerated shells to fall an easy +prey to the Seagulls. + +The sandy bottoms in shallow waters are also the haunts of many kinds +of Fish that are specially modified in form and colour to resemble +their surroundings. The large family of the Skates have their bodies +compressed from above downwards so that they can lie perfectly flat +upon the sand. Their upper surface is deeply pigmented, giving them +a general resemblance to the ground on which they lie; but to assist +them in escaping observation they have a habit of shaking their fins in +such a manner as to scatter a considerable quantity of sand over their +bodies. Thus it can well be understood that in the dim light of the +sea-bottom the little Fish and Shrimps, which form a large portion of +their food, may approach quite close to them without being in the least +aware of the danger into which they are running. The upper side of the +Skate is also armed with a number of sharp and hard spines, and in some +forms--called the Sting-rays--one of these, situated at the base of +the tail, is much larger than the others, and provided with muscles so +that it can be suddenly erected. In connection with this spine there is +a poison-gland and duct. The wound that is inflicted upon the arms or +feet of the fishermen by this formidable weapon of offence is said to +be of a very serious nature. + +Some of the Skates, too, show another very interesting modification +of structure, which, however, is not directly associated with their +mode of life, but may be briefly referred to here whilst dealing with +the family. This is the electric organ. In the younger stages of the +common English Skate a small region of the muscular system on each side +of the base of the tail becomes changed into an electric organ, but +the discharges which it is able to give are so feeble that they can +only be appreciated by a galvanometer. In the Torpedo, however, the +electric organs are very large indeed, and situated one on each side +of the head. They can give a shock which is powerful enough to kill +small animals and to stun larger ones. With such a formidable weapon +of offence and defence, it is clear that the need for active movements +is considerably diminished, and the Torpedoes are described as being +exceedingly slow in progression and incapable of the violent movements +of other Skates. + + [Illustration: +Fig. 9.+ + The Common Sole, showing both eyes on one side of the head.] + +The other Flat Fishes found on sandy bottoms belong to a different +group altogether, and are characterised by their bony skeletons and +other features. The Soles and their allies do not, like the Skates, lie +flat upon their bellies, but are _laterally_ compressed and lie upon +one side. The side which is habitually uppermost is of the colour of +the sand, and the other almost invariably pure white. In the course +of their development the Soles undergo very extraordinary changes in +form to reach the perfectly flattened condition of the adult stage. +These changes affect many of the organs of the body, but perhaps the +most interesting of all is the history of the eyes. In the young Soles, +which swim almost vertically, like the majority of fishes, there is +one eye o each side of the head, but as they grow older and gradually +take to the habit of swimming on one side, the eye of that side sinks +down into the head, and rotating as it goes passes through to the other +side. This process naturally leads to a considerable distortion of the +skull, so that the bones of the adult Sole show as complete a want of +symmetry as can be found in any Vertebrate. + + [Illustration: +Fig. 10.+ + The Angler.] + +Some of the Bony Fish, however, that live on the sand are flattened +dorso-ventrally like the Skates. The Angler or Fishing-frog, for +example, is a Flat Fish, which is perfectly symmetrical. Like the other +Flat Fish its upper surface is coloured in such a manner as to resemble +the ground on which it lives. Its great mouth, armed with formidable +rows of sharp-pointed teeth, is directed upwards, and it receives its +name from a curious tentacle terminating in a brightly-coloured knob +which dangles over its mouth. The brightly-coloured knob looks no doubt +a tempting morsel to the little Fish upon which the Angler preys, but +the greed or curiosity, whichever it may be, that induces them to +inspect the bait leads them to the fate which follows one snap of the +great tooth-armed jaws. + +The Fauna of the shallow waters where rocks abound also possesses many +peculiarities. In the first place we must remember that the rocks, +being firm and hard, present a basis upon which many of the sedentary +forms of life that would be swept away or smothered if they attempted +to live on the ever-shifting sands, can fix themselves. Consequently +the rocky bottom is characterised by a rich Fauna of those groups of +animals, which, in the adult condition, are immoveably fixed. If a +large stone or a water-logged piece of timber that has been at a depth +of a few fathoms for some months or years be captured in the dredge and +brought on board for examination, it is sure to present the observer +with a multitude of firmly-fixed creatures. Among them there is almost +certain to be found a number of small conical shells, made up of a +series of triangular plates, fixed to the rock by their bases. These +are commonly spoken of as Barnacles (_Balanus_), and they pass +through an interesting history. For many years they were considered, +from the character of their shells, to be allied to the Molluscs, but +an examination of the soft parts of the animal shows that, unlike any +Molluscs, they are provided with six pairs of jointed legs; and a still +further study of their anatomy proves beyond a doubt that they can no +longer be classified, with any pretence of scientific accuracy, with +that group. + + [Illustration: +Fig. 11.+ + Vertical section of a Balanus, showing animal + _in situ_ in its shell.] + +The secret of their true relationship was not discovered until the +story of the development was worked out, when it was found that the +eggs they discharged each gave rise to a little larva called Nauplius, +which is provided with three pairs of legs like the larvæ of some of +the Prawns and their allies. The result of these observations then was +to prove that the Barnacles are really Crustaceans, notwithstanding the +fact that, unlike most of that group, they are, in the adult stage, +permanently fixed to the rocks. + +On the same piece of stone there will most probably be found several +twisted or coiled tubes of lime, formed by a little Sea-worm called +Serpula. When living in the water the head of this worm projects from +its shell and expands a circlet of delicate tentacles, by means of +which the food is brought to the mouth. One of these tentacles is +specially modified and enlarged at the extremity to form a conical +knob, which, when the animal is retracted into the shell, closes the +aperture like a stopper. + + [Illustration: +Fig. 12.+ + Nauplius larva of a Balanus, enormously magnified.] + +Then there may be found some spherical or lobate masses of a fleshy +consistency, white, pale pink or yellow in colour, and studded +with numerous star-shaped apertures. When these are allowed to +remain in a basin of fresh sea-water for some time, each one of +the star-like apertures opens, and a beautiful transparent little +Polyp with eight-feathered tentacles gradually unfolds itself, only +to be slowly withdrawn into the mass when the vessel is shaken or +otherwise disturbed. These Polyps form colonies, known as _Alcyonium +digitatum_. + +Now it is to be noticed that none of these three examples of the +sedentary Fauna can move in the least degree from the rock or shell +to which they are fixed. When once the young larva has taken up its +position there it must remain until old age or some disaster brings +its life to an end. When they are first hatched from the egg that +is thrust into the water by the parent, they pass through a larval +stage that, like the Nauplius of the Barnacle, is active and free. +Then they are carried away from the parent stock partly by their own +active movements, but more particularly by the tides and currents of +the sea-water. At last a change in their structure occurs. They sink +to the bottom, become attached to a rock or stone, complete their +metamorphosis, and remain anchored to the spot for the rest of their +lives. + +The number of different forms of animal life which constitutes this +sedentary Fauna of rocky coasts is very great indeed. In addition to +the Barnacles, Worms and Alcyoniums, there are numerous species of +Sea-anemones, Sponges, Corals, Zoophytes, Sea-squirts, and other forms, +and in _all_ these cases the eggs give rise to free-swimming +larvæ, by which the distribution of the species is effected. + +Another group of animals, which forms an important feature of some +rocky coasts, are the Boring-molluscs. The Rock-borers belong to +many different species. Some of them, such as the Pholases, make long +cylindrical holes in chalk, and even in harder rocks. The Teredo is the +borer, commonly known as the Ship-worm, on account of its powers of +penetrating into timber. The long calcareous tube which it forms as it +works its way into the wood, gives it a superficial resemblance to a +large sedentary worm, but it is in reality a bivalve Mollusc, specially +modified in structure for its peculiar habits. + + [Illustration: +Fig. 13.+ + Sea-urchin with large thick spines.] + +The next group of animals we have to consider in the Fauna of the rocks +includes all those that slowly creep or crawl, without possessing +any powers of rapid locomotion. Amongst these may be mentioned +the Sea-urchins and Star-fishes. The former possess spherical or +heart-shaped bodies covered with a formidable array of spines, among +which there protrude several rows of soft transparent tubes terminating +in little cup-like discs. Some of these tubes--or tube-feet as they are +called--are fixed to a rock, and the heavy body is slowly dragged after +them; another set is then attached, whilst the former is released to +obtain another hold a little further on. The progress is slow, but the +Urchin is able to climb the smooth face of an absolutely perpendicular +rock with perfect ease or to get over any other obstacles that may be +in its path. The Star-fishes are similarly provided with tube-feet, +but in their case these are confined to the lower surface of the body, +the upper side being entirely devoid of them. Star-fishes have a very +wide distribution in the sea, and occur on sandy shores as well as +among rocks and shingles. If a specimen be watched gliding slowly and +smoothly over the sea-bottom and then the mouth be examined with a +pocket lens, a doubt might arise in the mind of the young naturalist +as to the justice of the charge that is made against these animals of +their being the principal enemies of the hard-shelled Oysters. But the +charge is well founded, for if a Star-fish be placed in an aquarium +with an Oyster or a Cockle, or, in fact, almost any bivalve Mollusc, +it may be seen to clasp its prey in its arms and slowly but firmly and +surely force open the shells, and then protrude on to the soft parts +a long tubular stomach which gradually digests and absorbs them. The +Star-fishes, then, are undoubtedly to be reckoned among the most +voracious and destructive inhabitants of the shallow waters, and it is +probable that the covering of spines which we find so commonly among +shallow water animals is an adaptation to prevent or render difficult +the operations of these creatures. + +The Gastropod Molluscs form another large and important group of +creeping animals of rocky coasts. On nearly all our own coasts numerous +Periwinkles may be seen clinging to the rocks at low tides, and if a +search be made in the deeper pools and on the rocks nearest to the +low-water marks many other species will be found of animals with +spirally-coiled shells which are included in this group of Gastropods. +The Periwinkles on the rocks might at first sight be thought to +belong to the sedentary group of animals, but when the water covers +them again, or when they are put into an aquarium, they may be seen +to protrude a head and an elongated slimy foot, which, gliding over +the surface of the rock, drags the great shell and its contents with +it. On the approach of danger the foot and head are withdrawn into +the shell, and the animal rests secure from many enemies that might +otherwise have found it a dainty morsel. Some of the Gastropods are +purely vegetable-feeders, but most of those living in shallow sea-water +feed upon Molluscs and other animals. It might well be a matter for +wonderment when the soft head and little mouth of a Gastropod, such as +a Whelk, are examined, that it is carnivorous and attacks and devours +animals as large as itself. But the anatomist shows us that hidden in +the recesses of that mouth there is a ribbon beset with numerous sharp +little teeth, which by a complicated mechanism can be worked backwards +and forwards in such a manner that it can bore a hole through very +thick and dense shells; and, the soft parts being reached, a tube is +protruded which dissolves and sucks them up into the animal’s stomach. + +Many people must have noticed that numbers of the bivalve shells +that are cast up on the sand at low tide are perforated close to the +hinge by a neat little round hole. This is the hole made by some +predaceous Gastropod which, having killed its prey and devoured all +that is digestible of it, leaves its empty shells at the mercy of the +waves. Amongst the rocks numerous species of Gastropods are found, +some undoubtedly carnivorous, others herbivorous. The many beautiful +forms and colours that their shells assume may be seen in any good +museum or conchological cabinet. Some of them are very minute, others +are provided with a shell more than a foot in length; some are marked +with numerous coloured spots, others with bands or lines; some have +perfectly smooth shells, others are ribbed or spiny. It is extremely +difficult to account for all these modifications, partly because it +is impossible to study the animals alive in their natural habitats a +few fathoms below the surface of the sea, and partly because life in +the shallow waters must be so complicated that we are at a loss to +understand the value to a species of slight modifications in structure +such as these. The difficulty that has been found in explaining these +various forms and colours has led some naturalists to the belief that +they are of no importance to the species in the struggle for existence, +that they are, as it were, the accidental result of some process of +excretion, and not the outcome of a long series of slight changes, +bringing about at length an adaptation of form most suitable to the +habits of the animal. Such views are, however, to be accepted with +great caution, and most zoologists will be contented to wait until our +knowledge is much greater than it is at present, before wholly agreeing +with them. + + [Illustration: +Fig. 14.+ + Smooth-shelled Gastropod.] + + [Illustration: +Fig. 15.+ + Spiny Gastropod.] + +Another great class of animals which has many representative forms +among the rocks is the group of Crustaceans. The Lobster, the Prawn +and the Crab are all familiar examples of this class. They may be +found by searching rock pools at low-water, or can be captured in +basket-work traps in places beyond low-water mark. When undisturbed +they crawl slowly over the rocks and weeds by their long jointed +legs, searching for their prey, but when alarmed the Lobster and +the Prawn can, by violent flapping movements of their tails, dart +rapidly backwards through the water, while the Crab beats a hasty +retreat sideways into some shelter among the rocks. Like many of the +Molluscs, the Crustaceans have a hard covering or shell to protect +them from many of the dangers to which soft-bodied animals would +be exposed, but a momentary glance at them would be sufficient to +satisfy the most inexperienced eye that there are many and important +differences in the character of the shells of these two great groups +of animals. One important distinction between them, however, might +well escape observation, and that is, that whilst in the Mollusc the +shell increases gradually in size during the life of the animal, in +the Crustaceans it cannot do so. In the Lobsters and Crabs the shell +is periodically cast off entirely, and for a day or two at each period +the skin of the animal is quite unprotected. A new shell is gradually +formed, and this is hardened and thickened until it assumes a form +similar to that of the one that has been lost, but larger. During the +moult the Crustacean usually hides itself in a hole in the rocks and +waits patiently until the new shell has grown. + + [Illustration: +Fig. 16.+ + A Cuttle-fish.] + +The animals included under the popular names of Cuttlefishes, Squids +and Octopuses are also capable of crawling about among the rocks by +their long feeler-like arms; but they are in the habit, as well, of +making prolonged journeys through the water, by pumping the sea-water +through a tubular siphon situated on the under side of their bodies. +These animals possess in such a remarkable degree the power of changing +colour that they might be called the Chamæleons of the sea. As they +pass slowly through the water from one part of the coast to another +the colour of the skin changes so as to resemble the colour of the +rocks or weeds which are below them. These changes are brought about by +numerous little bladders in the skin which are filled with different +coloured fluids, and are worked by a complicated system of muscles +under the control of special nerves from the brain. When the colour +blue is predominant, it is found that all the bladders containing blue +fluids are dilated, the others being constricted; when the colour is +red the red bladders only are dilated, and so on; and as the nervous +response to the colour of the rocks perceived by the eye is practically +instantaneous, the change in the general colour of the body brought +about by the dilatations of these vesicles is extremely rapid. Many +other animals have the power of changing colour, but in no group is the +alteration more rapid and remarkable than in this order of Cuttlefishes +and Squids. Another very interesting feature presented by these animals +is their ability to discharge suddenly a cloud of inky substance into +the water. Their principal enemies are the Whales, Porpoises and some +of the larger Sharks and other Fish. When these animals approach, or +any other danger is feared, the Cuttlefishes discharge into the water +from a special bag, called the ink-sac, a quantity of black or brown +pigment which, diffusing rapidly, forms a cloud round their bodies, in +the obscurity of which they frequently escape pursuit. The well-known +Sepia of painters is obtained from these ink-sacs. + +The last group of animals occurring among the rocks are those capable +of vigorous swimming movements. Many Crustaceans, such as the Lobsters +and Prawns, are capable, as has just been pointed out, of swimming +rapidly through the water by means of their powerful tails. But this +swimming power is only accessory to that of crawling or creeping, and +is used merely when the animals are disturbed. Cuttlefishes and their +allies seem to spend a considerable portion of their time in floating +or swimming in the water, but still they do crawl about among the +rocks, and very probably attack and feed upon their prey entirely upon +the sea-bottom. + +The members of the Rock-fauna which belong to the class of Fishes +very rarely rest upon the sea-bottom at all. They are not, as a rule, +provided with limbs which are capable of crawling or creeping; and +their mouths are adapted for catching food that is swimming, or of +browsing upon or nibbling at fixed forms of life while their bodies +are still floating in the water. Nearly all the animals living among +the rocks that we have hitherto spoken of, have some organs or some +specialised portion of the body-wall for resting upon or for attaching +themselves to the bottom. + + [Illustration: +Fig. 17.+ + The Wrasse.] + +The Anemones are attached by their bases; the Sea-urchins and +Star-fishes crawl by means of their tube-feet; the Gastropod creeps +over the rocks by its broad flat foot, and the Octopus stretches out +its muscular arms and drags its body along by the numerous suckers they +bear. In the Flat Fishes of the sandy and gravelly shores we usually +find a white under surface on which they rest when waiting for their +prey. Among the Fish which frequent the rocks, however, such as the +Cods, the Whitings, and the Wrasse, there are no such surfaces. The +body of the Fish is usually more rounded in form, and no well-marked +limit can be assigned to the coloured upper surface and the pale +silvery under side. These Fish are in nearly all cases rapid and +powerful swimmers, rushing through the water after their prey, or away +from their enemies, by vigorous lateral movements of their tails. + +A curious exception to this general rule among the Fish occurs in the +family of the Lump-suckers. These Fish are found on the English, but +more commonly on the Scottish coast, and are distinguished by the +presence of a sucker, formed by the throat fins, on the under side of +the head. By means of this the Lump-sucker is able to attach itself +so firmly to rocks and stones that it can only with considerable +difficulty be removed from the object to which it is attached. + +Of the Fish that are commonly found among the rocks, a very +considerable number migrate from time to time to other parts of the +sea, and may be caught in the trawl on sandy or shingly bottom, or even +in the drift nets at the surface of the sea. A large number of Fish +belonging to the family of the Codfishes frequent the rocks during a +part of their lives. The Pollack is distinguished from most of the +others by the absence of a barbel on the lower jaw, and is one of the +persistent rock frequenters. In the adult condition it feeds almost +entirely upon other Fish, although in the younger stages of its life +Crustaceans, Worms, and other Invertebrates seem to form the bulk of +its food. + +The true Cod and the Haddock seem to have a much wider range, +occurring on shingly bottoms, where they are frequently caught in the +fishermen’s trawls, as well as in the neighbourhood of rocks. The Hake +feeds principally upon Pilchard, Herrings and Sprats near the surface +of the sea. + +It is an interesting fact that the Fish belonging to this one family +have very different methods of feeding. The Cod and the Pollack both +hunt their prey principally by day-light. The Pollack is guided by +its sight alone, the Cod-fish is assisted by its barbel, which acts +as a delicate feeler or organ of touch. The Hake, on the other hand, +retires into deep water during the day-time, and only comes to the +surface at night to feed. Similarly the Rockling hides away in holes or +under stones during the day-time, and only comes out to hunt for the +Crustaceans and little Fish upon which it feeds at night. + +The development of these Fish presents some features of interest, as +showing us the changes which occur in habit during their life-history. +The eggs of the Cod are buoyant, rising to the surface of the sea as +soon as they are spawned. In twelve or fourteen days, according to +the temperature of the water, the larvæ are hatched and swim about in +large numbers just below the surface, feeding upon minute Crustaceans +and other animals. A little later the young Cod frequently shelter +themselves under large Jelly-fishes, feeding upon the numerous +parasites which infest those creatures. When they are about a year old +they are found feeding among the sea-weeds on rocky coasts, and they +migrate into deeper water when they reach their full size. + + [Illustration: +Fig. 18.+ + The John Dory.] + +The John Dory is a remarkable Fish, by no means confined to the rocks, +as its food often consists largely of Pilchards and Herrings. It +differs from most of the Fish of similar habits in being remarkably +flattened from side to side. This feature seems to be of service to +it in the peculiar manner it has of securing its prey. Mr Cunningham, +to whom we are indebted for this interesting observation, says:--“It +does not overtake (its prey) by superior speed like the mackerel, nor +lie in wait for it like the angler, but stalks it and approaches it by +stealth. It is able to do this in consequence of the extreme thinness +of its body, and the peculiar movement of its hinder dorsal and ventral +fins. The dory places itself end on towards the fish it desires to +devour, and in this position it is evident that it excites no alarm on +the part of its prey. The appearance of the dory seen in this way is +a mere line in the water, to which no particular significance can be +attached. I have not particularly noticed the effect of the ribbons of +membrane, which project from the dorsal fin. But I have observed that +the movements of the dory are very gradual except in turning; it alters +the position of its body by a turn of the tail or side fins, and then +slowly swims forward by vibrating the second dorsal and ventral, a +movement which causes very slight disturbance of the water. The whole +appearance of the dory in these actions is suggestive of suppressed +excitement, his eyes being fixed on his prey.” + + * * * * * + +We have now considered some of the chief features of animal life in +the shallow seas, the illustrations being taken principally from the +regions of our home shores. The shallow waters of the tropics present +us with so many phenomena of striking interest and importance, that +the subject would be most incompletely treated if they were left out +of consideration altogether, and, therefore, our next chapter will be +devoted to them. + + + + + CHAPTER III. + + 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. + + [Illustration: +Fig. 19.+ + Polyp of a Madrepore Coral, showing the canals by + which it is connected with its fellows.] + +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. + + [Illustration: +Fig. 20.+ + Chætodon.] + +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. + + [Illustration: +Fig. 21.+ + Globe-fish.] + +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. + + [Illustration: +Fig. 22.+ + Stereosoma, one of the Clavulariidæ.] + +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. + + [Illustration: +Fig. 23.+ + _A_, Fringing-reef; _B_, Barrier-reef; _C_, Atoll. + _a_, sea; _b_, reef; _c_, rocks of the coast.] + +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. + + [Illustration: +Fig. 24.+ + Periophthalmus.] + +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. + + + + + CHAPTER IV. + + 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. + + [Illustration: +Fig. 25.+ + A free-swimming Copepod.] + +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. + + [Illustration: +Fig. 26.+ + The swim-bladder of Velella.] + +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. + + [Illustration: +Fig. 27.+ + Solitary form of Salp, bearing a young stalk of Chain-salps.] + +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. + + [Illustration: +Fig. 28.+ + Pteropod, showing the so-called wings.] + +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. + + [Illustration: +Fig. 29.+ + Shells of Foraminifers living at the bottom of the sea.] + +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. + + [Illustration: +Fig. 30.+ + Globigerina living at the surface of the sea.] + +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. + + [Illustration: +Fig. 31.+ + Young larva of a star-fish before the Brachiolaria stage is reached.] + +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. + + [Illustration: +Fig. 32.+ + Pluteus larva.] + +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. + + [Illustration: +Fig. 33.+ + Long-spined Barnacle-nauplius.] + +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. + + + + + CHAPTER V. + + SURFACE-SWIMMING FAUNA (VERTEBRATES). + + +In the preceding chapter we have considered only those animals of +the surface of the sea, which, owing either to their small size or +the transparency of their bodies, are not as a rule conspicuous to a +passenger on board a mail-steamer. Such a passenger might cross the +ocean many times without realising in the least the wealth of animal +life that there is in every wave that breaks upon the ship, and yet +be impressed with what he has seen of the Whales and Porpoises, the +Sharks, Bonitos and Flying-fish. + +It is to these groups of animals that a few lines must be devoted +before closing our story of the surface-swimming Fauna. If the young +and immature stages be for the moment left out of consideration, it may +be said that nearly all the Fish and all the Whales and Porpoises are +large, opaque in appearance, and perfectly conspicuous. Moreover, they +are all strong and rapid swimmers, capable of roaming over wide areas +of the sea in search of prey, and independent of, except in so far +as their prey are influenced by, the currents and winds. It is clear +that they cannot be said to ‘float’ and ‘drift’ about in the ocean, +and consequently they do not strictly belong to the Plankton. The term +used in speaking of them collectively is the Nekton, which means the +swimming population. + +The greatest number of the Fish of the sea are shore Fish; that is to +say, they habitually feed at, or close to, the bottom of the shallow +waters near the coasts or sunken banks. + +But there is a very considerable number that are strictly pelagic, +living and feeding far away from the shores, bringing forth their young +alive, or shedding floating eggs, and in every way independent of the +shore and of the bottom. + +It is difficult to give any general features by which they are +characterised, as so much variety may be observed among them; but as +a general rule they are elongated in form, round or oval in section, +in colour green or gray above, with silvery white bellies. Some of +these, such as the Flying-fish and the Flying-gurnard, are capable of +making very considerable flights in the air, their pectoral fins being +enormously elongated, and when fully expanded somewhat similar to the +wing of an Insect. + +The Flying-fish occur in shoals in nearly all tropical and subtropical +seas. When disturbed by a ship on a calm day it is said that they +spring out of the sea, expand their fins, describe a regular parabolic +curve in the air, and then fall with a splash into the water. There is +a considerable controversy raging on the question of the use of their +fins in this flight through the air, some observers believing that +the fins are used only as a kind of parachute, and others that they +are used like wings for raising the body above the water. It is very +difficult to decide which view is correct. + +In the Indian Ocean I watched the Flying-fish for several days during +rather rough weather, and my impression was most distinctly that in +the middle of the flight the fins are vigorously flapped four or five +times, the flapping being followed by a decided rise in the air. On the +other hand, it may be that this flapping appearance is caused by the +wind catching the wings in a certain position, and not by the muscles +of the fish. Whether the flying is actually assisted by the flapping +of the wings or not, it is certain that the Fish do rise in gusty +weather to a very considerable height, frequently falling on to the +decks of steamers twenty feet above the water line. + +At night, these Fish fly at the ships and not away from them, as +they do in the day-time, and the natives in some parts of the Malay +Archipelago catch them in large numbers by holding up a torch by the +side of a large sheet, when the Fish flying at the bright light and +striking against it, fall into the bottom of the canoe. Flying-fish are +excellent to eat, their flesh being similar in taste to that of the +Herring, but an epicure would probably say that it is not quite so good. + +The Bonito is a fish occurring over a wide area of the tropical and +temperate seas, which sometimes makes tremendous jumps out of the +water. I have seen it frequently on the coast of Celebes jump to a +height which I roughly estimated as at least fifteen feet. It is said +to feed upon the Flying-fish, and it is probable that it has acquired +the power of springing out of the water in the pursuit of its prey. + +The pelagic Fish, which has acquired the widest reputation, and that +an evil one, is the “blue-Shark.” This occurs in the tropical and +occasionally in temperate seas. Its usual size is from twelve to +fifteen feet, but, according to Dr Günther, individuals of twenty-five +and more feet are occasionally captured. It is extremely voracious, +attacking anything of a fleshy nature that it observes in the water. + +One of the most remarkable animals of the open ocean is the Sun-fish. +It has a very wide distribution in the tropical and temperate +regions. In the adult condition it is almost circular in outline and +considerably flattened from side to side like a John Dory. It sometimes +reaches a size of eight feet in diameter and a very great weight. From +the little that is known of its development it apparently undergoes +some extraordinary changes in shape before it reaches the adult form. + + [Illustration: +Fig. 34.+ + The Sun-fish.] + +In addition to these and several other Fish of a large size which may +be found at the surface of the open ocean, there are several species +known to science which never grow to a length of more than a few +inches. Many of these are characterised by remarkably long fin rays, +by their large eyes, or by other features which may be regarded as +special modifications for their peculiar habits. + +A very interesting genus is _Scopelus_, which is found very widely +distributed in tropical and other seas. Some of the species live in +very deep water, and are purely abysmal in habit, but most of them rise +to the surface at night, when they may be caught in immense numbers. +In form they are not unlike a small Sprat, but they exhibit on each +side of the body a series of minute eye-like organs, which emit a +phosphorescent light. + +In addition to these Fish which are found far out in the open ocean, +there are several genera, which form an important feature of the +surface waters in the neighbourhood of the coasts. Among them we +find such valuable food-fish as the Herrings, Sprats, Mackerels, and +Pilchards. + +The complete history of the Herring has yet to be written, for, +notwithstanding the laborious investigations of several naturalists, +working independently, or as officers of the Marine Biological +Association and similar Institutions, there are some facts and stages +which have, up to the present time, escaped observation. + +The Herring species is divided into a number of races, which, differing +from one another only slightly in anatomical characters, have different +seasons for depositing their eggs. This fact has only recently been +thoroughly established; and while it assists us greatly in the task +of completing the history of the Fish, it definitely destroys the +validity of many theories which were prevalent among fishermen and +others before the days of the more exact scientific treatment of +fishery questions. + +It is well-known that from the end of the month of June to December +immense shoals of Herrings are found in the North Sea. It is quite +impossible to estimate the numbers of Fish in these shoals, but they +are so great that if they could be counted it would probably be found +that the Fish that are annually caught by all the fishing boats, form +but an insignificant fraction of the whole. All of these Fish are in +such a condition that it is evident their spawning time is close at +hand. When they are ready they approach the coasts, the exact time +varying according to the race of Herrings, and the spawn is deposited +on or close to the ground, the eggs becoming attached to stones and +other objects on the bottom. After the Herrings have spawned, they seem +to disperse, or, at any rate, to disappear from the surface waters of +the North Sea. What actually becomes of these shoals of spent Herrings +is not known, but it is a fact that in the spring there are so few +Herrings to be found in the narrower part of the North Sea that it does +not pay the fishing boats to go after them. It is possible, however, +that, after the spawning process, the Herrings migrate to the deeper +water of the Norwegian coasts, in order to feed on the Crustaceans and +other forms of life that are to be found there in abundance. + +The Mackerel do not apparently make such extensive migrations as the +Herrings. They spawn in the open sea, five or ten miles from the +coast, during the spring time. The egg of the Mackerel, unlike that of +the Herring, does not sink to the bottom when it is spawned, but, being +provided with a large oil globule, it is light enough to remain on the +surface until the young larva is hatched. After the spawning has taken +place the Mackerel approach the coast, and will even enter bays and +narrow inlets on the shores in pursuit of the young Sprats and other +small Fish upon which they prey. + +A great deal could be written on the history of the Pilchards, the +Anchovies, the Sprats, and other Fish which frequent the surface waters +of the sea in the neighbourhood of the European coasts. No two species +seem to have precisely the same habits, and what is known about them +presents us with many curious and remarkably interesting facts. For +further details, however, I must refer the reader to the larger and +more comprehensive books dealing specially with the subject, for space +must still be found for a few words on another group of animals which +play a conspicuous part in the story of life in the seas. + +The animals composing the class of Mammals are distinguished from other +Vertebrates by the fact that the females are capable of providing milk +for their young ones after birth. Most of the Mammals are strictly +terrestrial, but three orders, namely, the _Cetacea_, the _Sirenia_, +and the _Carnivora_ contribute to the surface-swimming population of +the sea. + +The _Cetacea_ are all aquatic. The order includes the many genera +of Whales, Porpoises and Dolphins. + +The Right-whales are distinguished by the enormous size of the mouth +and the absence of the little triangular fin in the middle of the back +which is found in the other Whales. + +These animals have no teeth in the adult condition, but are provided +with a series of plates situated at the sides of the mouth which are +used as strainers to catch the small Pteropods and other animals +living in the water which pass through the great gape. The plates are +composed of a substance called “Baleen,”--the well-known whale-bone +of commerce,--they are triangular in shape, and frayed out into a +brush-like edge on the side that faces the cavity of the mouth. The +Greenland Right-whale attains to a size of fifty feet in length when +fully grown, and it is usually found in shoals among the ice floes of +the far north. + +In former times many Right-whales belonging to species allied to +the Arctic form occurred in the temperate regions of the Atlantic +and Pacific Oceans, but in consequence of the valuable fishery they +afforded they are now becoming very scarce. + +The largest of all the Whales--in fact, the largest existing animal--is +the Blue-whale, which attains to the enormous length of 86 feet. It +spends the winter in the open seas, and approaches the coast of Norway +in the spring. + +The Whales, like all the animals of the Class to which they belong, +are air-breathers. They are able, however, to hold their breath for a +considerable time under water. When they come to the surface to renew +the air-supply in their lungs, they first make a violent expiratory +effort from the nostril, and drive a column of spray many feet into +the air above them. This phenomenon is called by the whale fishers +“spouting,” and it was erroneously supposed by them to be a column of +water forced from the mouth into the nostril, and then expelled at the +surface. + +The Dolphins and the Porpoises are distinguished from the true Whales +by the fact that they are provided with teeth, on one or both of their +jaws, and there is no “whale-bone.” + +The Sperm-whale has probably been called a “Whale” from its enormous +size, but it is anatomically very widely separated from the true whales +and more closely allied to the Dolphins. It has no “whale-bone,” and +the lower jaw is provided with a row of sharp-pointed teeth set in a +groove in the bone. Its great value is due to the fact that there is +a large cavity situated above the skull, containing an oily substance +from which “Spermaceti” is made. + +The principal food of these large “Toothed-whales” seems to be +Cuttlefishes, and the examination of the contents of their stomachs, +which has been carried out on board the Prince of Monaco’s private +steam-ship when engaged on a scientific cruise, has yielded some new +forms of these giant Molluscs. The Sperm-whales, however, do not +disdain a Fish diet as well. + +The Porpoises are found on our own coasts. They may often be seen +following the “schools” of Whales as they approach the coast in the +summer months, and they occasionally chase their prey some distance up +the estuaries of the English rivers. The habit that Porpoises have of +accompanying ships for long distances affords us many opportunities +of watching their graceful movements, and of estimating the very +great speed with which they can swim through the water for hours at a +stretch. It is difficult to understand the meaning of this habit of +following ships. It has been suggested that the Porpoises mistake the +ship for a huge Sperm-whale, and hope to benefit by stray morsels of +large fish that fall from its jaws. But this theory does not account +for the fact that the Porpoises so often go in front of the ship. The +following extract from Darwin’s “Voyage of the Beagle” gives, in a few +words, a vivid picture of Porpoise life:-- + +“In our passage to the Plata, we saw nothing in particular, excepting +on one day a great shoal of Porpoises, many hundreds in number. The +whole sea was in places furrowed by them; and a most extraordinary +spectacle was presented, as hundreds, proceeding together by jumps, +in which their whole bodies were exposed, thus cut the water. When +the ship was running nine knots an hour, these animals could cross +and recross the bows with the greatest ease, and then dash away right +ahead.” This description reminds me very forcibly of a sight I once saw +in the Talaut Islands, south of the Philippines. In passing between two +of the islands the strait seemed to be alive with Porpoises tearing +through the water at a terrific pace. They accompanied the steamer for +about six hours and then suddenly disappeared. My impression was that +they were attracted to the ship not from a desire for more food, for +there was an abundance of Herrings in the straits at the time, but from +sheer curiosity. I think the feeling of curiosity, that is to say, the +desire to go and look at something strange or unusual, is much more +prevalent among animals than we generally suspect. + + [Illustration: +Fig. 35.+ + The common Porpoise.] + +It is an interesting fact about the Porpoise that, although it is so +fish-like in shape, it should present some features which remind us of +the Pig. The English word is probably derived from the two French words +_porc_ and _poisson_, and therefore means “Pig-fish.” The +Germans call it Meeresschwein, meaning “Sea-pig,” and the Malay word +for it is “Babi-laut,” which also means “Sea-pig.” But if we make some +allowance for those who call these Cetaceans “Pigs,” we must make none +for those who call them “Fish.” + +Like all the other members of their order the Porpoises have a +fish-like tail, but the flaps are placed horizontally and not +vertically as they are in Fish. The skin is quite naked, having no +scales of any kind, and there are no gills or gill-openings. Like +all other Mammals the Cetaceans bring forth their young alive, suckle +them, and breathe air by means of lungs. But there can be no doubt that +they are extremely modified for their aquatic life. The characteristic +covering of Mammals--the hairs--is, in the adult condition of the +Whales, entirely wanting, and is represented in other members of +the Class by only a few bristles on the snout. The heat of the body +is maintained by a thick coat of fat, called the blubber, lying +immediately beneath the skin, and this yields, on boiling, a valuable +oil, which helps to support the whale-fishermen. One of the most +striking modifications, however, is the loss of the hind limbs. It is +only in some species that even rudiments of these have been found. All +of these facts indicate that the Cetaceans must have taken to a mode +of life in the water a very long time ago, and the study of the rocks +proves the existence of Whales as far back as Eocene times, but it is +of interest to note that, in some respects, the oldest fossil Cetaceans +are less specialised than those that are now living. + +The class of Mammals called the _Carnivora_ includes the Cats, +Dogs, Ferrets and many other animals which are purely terrestrial, +but one of its divisions is entirely composed of those well-known +aquatic animals the Seals and the Walrus. If we take the common Seal +as an example of this group, and compare it with the Porpoise, as a +representative of the Cetaceans, we find that in habits as well as in +anatomy the former is less completely changed than the latter. The Seal +frequently comes to land to bask in the sun, or to produce and care +for its young, and it is capable of making some progress over the rocks +by the help of its flipper-like fore-limbs; the Porpoise, on the other +hand, never leaves the water of its own free will. Unlike the Porpoise +the body of the Seal is covered with a thick coat of hairs, and the +hind limbs are retained. Although there is a general resemblance in +the form of the body between these two animals--this form being, in +all probability, mechanically the best for rapid progress through +the water--a glance at their skeletons shows great and important +differences, which the merest tyro in anatomy could point out. In +expression, too, there is a marked difference, for while the Porpoise +has a certain cast of countenance which, when seen at a distance, +deserves the epithet “pig-faced,” the face of the Seal, with its large +round eyes, its small nose and high intelligent brow, is almost human +in expression. + +The Seal has a habit of raising its head above the water and staring +at an approaching boat, and when doing this it may readily be mistaken +at first sight for a man overboard, but no one could ever mistake a +Porpoise for a human being. + +The common Seal has a very wide range occurring near the coast of +both the Atlantic and Pacific Oceans. It is found on some of the more +sequestered parts of the British shores, but not in large numbers, +for the common Seal, unlike many of its allies, does not appear to +congregate in large shoals at any time of the year. They are described +as being timid, inoffensive creatures, easily tamed, passionately fond +of their children and taking an intelligent interest in music. + +There are several animals closely related to the Seal, occurring +in different parts of the world; and a few words may be said about +the remarkable animal called the “Sea-elephant,” which is found on +Kerguelen island in the Antarctic Ocean. The popular name was given +to the animal in consequence of the fleshy protuberant nose which has +been compared with the trunk of an Elephant and is possessed only by +the male. The late Professor Moseley, who came across a small herd of +them when the _Challenger_ was at Kerguelen, says: “The trunk is +produced by an inflation of a loose tubular sac of skin placed above +the nostrils, just as is the ‘Cap’ in the northern Bladder-nose Seal. +The trunk is evidently, as appears from both the drawings, sacculated, +and hence irregular in form when inflated.” + +The Sea-lions and Sea-bears or the Eared Seals, as they are sometimes +called, form a very distinct family. The one that is best known to +the general public is the Californian Sea-lion, as it often lives in +captivity in the European menageries for many years and attracts the +attention of the visitors by the tricks which it is taught to perform. + +In the spring months of the year these creatures may be seen in great +numbers on the rocky islands off the coast of California, where they +come to breed. + +The most important of them all, from a commercial point of view, is +the Fur-seal from the Northern Pacific. In the month of May these +animals approach the Prybilov islands in the East, or the Commander +islands in the West of the Behring Sea. The first to arrive on the land +are the old males. These choose for themselves certain areas or ‘homes’ +on the shore and fight desperately for their possession with all who +dare to come within their reach. When matters are at length somewhat +settled the time arrives for the females to approach the shore. +The fighting then begins again with renewed vigour, and desperate +encounters take place for the possession of a goodly stock of wives for +the season. + +There seems to be little in the way of courtship in the domestic +economy of the Fur-seals, the wives being simply “captured” by the +scruff of their necks when they come within reach of a would be +husband, and retained in his harem just so long as he can prevent +any one of his neighbours from stealing her. The Fur-seal, like all +his relations, is polygamous, but the number of wives that each male +appropriates to himself seems to vary very considerably. Mr Elliott +mentions a case in which there were as many as forty-five females in +one home, but, as a general rule, the number is much less. As there is +only one male to every twelve or thirteen females, there are numerous +males over, which cannot found a home for themselves. These bachelors, +together with a number of the young females, resort to a separate +piece of ground, where they spend their time in playing games. The +play-grounds are however the scene of the tragedies of Seal life, for +they are resorted to by the hunters, who slaughter immense numbers of +the larger males for the sake of their valuable skins. As the skins of +the old male Seals are not of very much value and as it is important, +for the perpetuation of the race, to preserve the females from injury, +the breeding grounds are usually not molested. It is therefore the +bachelor seal of from two to five or six years of age that has to +supply the market. Those naturalists who have visited the Seal +rookeries on these islands say that the numbers of these animals that +can be seen at one time is almost incredible. We can form some estimate +of them when we learn that over a hundred thousand skins are exported +from the Prybilov islands alone every year. + +The Seals leave the rookeries in the month of August, and after +swimming about for some time in the neighbourhood of the islands, +eventually depart into the open ocean in search of the food their +famished bodies need so much after the fasting and fighting of the +breeding months. + +The largest of all these aquatic Carnivores is the Walrus, which lives +within a short distance of the shores of the lands in the Arctic +regions. It is easily distinguished from the Seals by its great size, +the males reaching a length of 10 or 12 feet, and by the enormous +canine teeth in the upper jaws, which project downwards from the cover +of the lips in the form of two large pointed tusks. These tusks are +used for hoisting the bodies of the animals on to the ice, for digging +in the sand in search of the Mussels upon which they feed, and for +general fighting purposes. + +It is said that in former times the Walruses lived in immense herds +in regions much further south than they do now; but the ravages of +the hunter, who chased them for their ivory tusks and their oil, have +driven them into regions where they are rarely visited by anyone but +the Arctic explorer; and to those of my readers who wish to learn more +of their habits, I can but say that in the pages of Dr Nansen’s book, +“Farthest North,” he will find the story of the Walrus written in a +manner which no man living could have done more vividly and brilliantly +than the great Norwegian zoologist and explorer. + + + + + CHAPTER VI. + + DEEP-SEA FAUNA. + + +Some of the most important conditions under which life at the bottom +of the deep-sea occurs have been mentioned in the first chapter. We +have pointed out that the pressure is enormous, that the temperature +is only a few degrees above the freezing point, and that, except in +those places where phosphorescent animals emit a faint light, it is +absolutely dark, no rays of direct sunlight being able to penetrate +such a mass of water as lies between the bottom of the ocean and its +surface. + +With such conditions to contend with it is not surprising that the +naturalists at the beginning of the century believed that no animals +could possibly live on the floor of the great oceans. Their beliefs, +however, merely afford us an example of the danger of prophesying +without knowing, for the great expeditions which have investigated +the ocean-bed during the last thirty years have proved the existence +of a rich and peculiar Fauna in all the great depths that have been +dredged. The general results of these investigations have been recently +summed up by Dr John Murray in the last volume of the _Challenger_ +Reports. He points out the extraordinary variety of life in the deep +sea as shown by the contents of the dredge. “At Station 146 in the +Southern Ocean, at a depth of 1375 fathoms, the 200 specimens captured +belonged to 59 genera and 78 species.” He can find no record of species +equal to this in depths of under 50 fathoms, and concludes that the +evidence at present before us is sufficient to warrant the belief that +the great depths of the ocean are as a general rule extremely rich in +species. + +From what has been already said, it may be gathered that nearly all +the most important groups of marine animals have representatives +in the deep-sea. There are Fish, Tunicates, Crustaceans, Molluscs, +Echinoderms, Worms, Cœlenterates and Protozoa. Nearly all of these are +so modified, either in form or colour, or the structure of their organs +of sense, or in other particulars, that they could be recognised at +once in a collection as deep-sea animals; but there is a small minority +which seem to have undergone but little change in adapting themselves +to their strange environment. + +We may commence our study of this remarkable Fauna by a few remarks on +their colour. The first and most striking feature is that the animals +are almost invariably uniform in colour. If they are dark-brown they +are dark-brown all over, if they are red they rarely exhibit bands of +white or spots of blue. Moreover, they are not always in harmony with +the colour of their surroundings. + +In the shallow waters the animals that live among the green Sea-weeds +are green, those that live on the sand are coloured like the sand, and +many of those that live among the rocks are darkly pigmented with black +and blue. In the abyss of the ocean, where there is any light at all, +the colour is, in all probability, fairly uniform over wide tracts, and +yet we may find in one haul of the dredge, black Fish, red Crustaceans, +and purple Trepangs. + +There seems to be no particularly predominant colour among the deep-sea +animals. Most of the Fish are black or dark-brown, but many are light +violet, some are pale rose and others bright red. Among the Crustaceans +bright red seems to be the prevailing shade, just as the darker tints +of black and brown are among the Fish. + +Among the Echinoderms we find white, purple, yellow, red, and pink +forms, and among the Jelly-fish and Corals, red, violet, and green. In +fact it would be necessary to describe every class of animals in turn, +and then almost every genus in each class, to give an adequate idea of +the variety of colour met with in the Fauna of the deep-sea. + +It is inconceivable that each of these animals can live amid +surroundings of a colour similar to its own, and therefore we may +without much hesitation believe, that the colour of deep-sea animals is +not, as a general rule, of use as a protection. + +Next to the peculiarities of colour, the most striking features of +the more highly organised inhabitants of the bottom of the sea are +the modifications of the organs of special sense. The Fish, the +Crustaceans and the Molluscs almost invariably exhibit some remarkable +modifications of the eyes. In their natural haunts there must be +either absolute darkness, or the faint and usually intermittent light +emitted by phosphorescent animals. How intense this light may be it +is impossible to judge. The light that is emitted by animals on the +deck of a ship can afford no criterion of the light they emit under +a pressure of two tons to the square inch. However, the fact that +the deep-sea animals have either very large eyes or no eyes at all, +suggests forcibly that this light is not sufficient to cause a general +illumination. + +Some of the Fish are quite blind, and although most of these have a +very small and rudimentary eye, in at least one Fish, _Ipnops_, +which is peculiar to deep water, no trace of an eye is to be found. + +In some genera with a very wide distribution, a very interesting series +of stages may be found, indicating the changes that may have taken +place in the history of the blind Fish of the abyss. In the genus of +deep-sea Cods (_Macrurus_) for example, those species which live +in water of less than a thousand fathoms depth have very large eyes, +and those that are found in greater depths have much smaller ones. + +The same in general is true of the Crustaceans. The deep-sea Cray-fish +have lost not only their eyes, but also the stalks which supported +them. In _Bathynomus_ (a Crustacean belonging to the group +_Isopoda_), however, there is a pair of enormous eyes. But as a +rule the eyes of Crustaceans degenerate and disappear in shallower +water than the eyes of Fishes. At depths of 500 fathoms or greater, +the eyes of the Crustaceans usually show signs of reduction in size or +other retrogressive changes, and in the greatest depths they are nearly +always wanting altogether. + + [Illustration: +Fig. 36.+ + A deep-sea Fish showing very elongated fins.] + +Accompanying the loss of eyesight in deep-sea animals we often find +a very remarkable development of organs, which may be regarded as +especially tactile in function. + +Many of the deep-sea Fish, for example, with rudimentary eyes possess +long barbels, and in some cases the paired fins are enormously +elongated to form delicate pointed organs like the tentacles of a +Polyp. Among the blind Crustaceans, too, we often find enormously long +antennæ, and even the claws and legs are so long and delicate that they +bring to mind the appendages of a Daddy-long-legs or a Harvest spider. + +Just as a blind man acquires a remarkably acute sense of touch, so, it +seems, in the course of generations, these blind animals of the abyss +have acquired extremely delicate tactile organs. + +The deep-sea Fauna is also remarkable for the great number of animals +which are phosphorescent. As in the surface-swimming creatures the +phosphorescence is not confined to a few classes, but probably occurs +to a greater or less extent in all the more important groups. The word +“probably” must be used in the previous sentence, because it is not +yet scientifically proved that many forms which are supposed to be +phosphorescent are actually so; but the evidence is conclusive that +phosphorescence is a common and widespread character of most of the +deep-sea Fauna. + +The Fish exhibit, perhaps more than any other group, peculiar organs +which are supposed, and in many cases proved to be, used for the +purpose of generating light. In the _Stomiatidæ_, a family of Fish +related to the Salmons, there are often numerous little organs, like +minute bull’s-eye lanterns, arranged in rows on the sides of the body +from the head to the tail, and in addition to these in some species one +or more pairs of larger organs are seen on the upper lip just in front +of or below the large eyes. It is not certain what the colour of the +light is that is emitted by these organs, but it is very probable that +if the Fish could be seen in their natural haunts they would have an +extremely beautiful effect. + +In describing the general characters of the shallow water Fauna in +Chapter II. reference has been made to the remarkable lure at the end +of the tentacle of the Angler-fish. In the Angler of the great depths +this is also found, but in the obscurity of their surroundings a lure, +such as that of the shore species, would be useless, and it is actually +replaced by an organ which is supposed to be phosphorescent. The mouth +is enormous and armed with ferocious-looking teeth, the body is rounded +and adapted for burrowing in the ooze, and we can well frame in our +minds a picture of the little Fishes and other creatures attracted +by the “will-o’-the-wisp” light, meeting with a sudden death in the +cavernous jaws of this voracious deep-sea Fish. + +It is possible that in addition to the light given off by definite +organs, the slime secreted by the skin either over the whole surface +or certain circumscribed regions, may be phosphorescent, but how +far this may serve as a means of illumination must remain a matter +of conjecture. The Crustaceans are in some cases known to emit a +phosphorescent fluid. The naturalists of H.M.S. _Investigator_ +found a brilliantly phosphorescent liquid in the glands at the base of +the antennæ and elsewhere in certain deep-sea Shrimps, and one of the +Cray-fish from great depths is said to have two definite spots on the +body that emit a phosphorescent light. + +Several of the Star-fish and Brittle-stars from the abyss are known to +be brilliantly phosphorescent, and there are some vivid accounts of the +light given off by Worms and various kinds of deep-sea Polyps found in +the dredge. + +It is possible that some of the more minute forms of life that occur on +the mud at the bottom may also be phosphorescent. The _Phæodaria_, +a family of Radiolarians peculiar to deep water, provided with thick +heavy shells, have a curious organ in their bodies which may be capable +of emitting light. If this is the case, it is not unreasonable to +suppose that the vast tracts on the bed of the ocean may be faintly +luminous like the surface of the sea on a calm night. + +Among the other characters must be mentioned a very prevalent +deficiency in the salts contained in the skeletons of these animals. +The skin of the Fish is usually soft and velvety to the touch, the +scales being either very thin and few in number or altogether missing; +the bones are described as being so soft that it is easy to pass a +needle through them. The shells of the Crustaceans, although frequently +drawn out into numerous long and pointed spines, are usually deficient +in carbonate of lime. The shells of the Molluscs are, when compared +with those that live in shallower water, thin and brittle. The Corals +do not seem in this respect to show much variation from their shallow +water relations. Some of the solitary forms seem to have rather thinner +shells, but the colonial genera have, as a rule, as good a support of +carbonate of lime in the abyss as elsewhere. + +Before proceeding to the next character, it is necessary to digress a +little to consider the food of the animals in the abyss. In the absence +of any direct sunlight there can be no vegetable growth, all of the +animals must therefore be carnivorous. The food must be either the +living bodies of the truly abysmal animals, or the dead bodies of those +that fall from the surface waters. + +It is probable that the bodies of Fish and the larger Invertebrates +only rarely reach the bottom, as they have to run the gauntlet of many +different forms of life living within 100 fathoms of the surface. When, +therefore, such a prize does fall to the luck of a deep-sea Fish, +it is important that it should have accommodation for it before the +neighbours come to share the meal. This may be the cause of the fact +that deep-sea Fish have, as a general rule, jaws and stomach that are +extravagantly large, even for a carnivorous creature. The width of the +gape and the extensibility of the stomach reach their highest grade +in some of the deep-sea Eels, which have been found containing Fish +actually larger than themselves. In these cases the stomach and the +body wall hang down from the under side of the Eel’s body in the form +of an enormous membranous sac containing the prey. + +We have now considered very briefly some of the principal modifications +of structure exhibited by the animals of the deep-sea, but before +leaving the subject altogether it is necessary to refer to a few of +the more characteristic and remarkable forms. + +Although it may be considered to be one of the greatest scientific +triumphs of the century to have discovered the existence of animal life +in a region, which nearly all the distinguished men of science of the +last generation believed to be as lifeless as the moon, the revelations +of the dredge brought with them a certain amount of disappointment. + +The study of the crust of the earth has revealed to us the fact that in +times long since gone by, there existed not only the hairy Mammoths, +the Iguanodons, and many other terrestrial monsters; but that the sea +was peopled with certain Reptiles, Fish, Molluscs, Echinoderms and +Crustaceans, which are now believed to be extinct. + +When it was first discovered that some forms of animal life had +attached themselves to a telegraph cable lying in 1200 fathoms, and +that it was therefore a fact that life existed in very deep water, +a successful application was made by scientific men to the British +Government to assist them in a thorough survey of this hitherto unknown +field of investigation. + +The result of the voyages of H.M.S. _Lightning_ and H.M.S. +_Porcupine_ was to prove the existence in water of 1000 fathoms in +depth of a rich Fauna of rare and very remarkable animals. Among them +were several new genera of Sea-lilies and a very curious Heart-urchin. +The Sea-lilies that were then known to live in shallow water were very +few in number, and nearly all of them were free and unattached. Now, +in past times in the history of the earth different genera and species +of stalked Crinoids or Sea-lilies were very plentiful, and from their +abundance in certain geological deposits, it is believed that they +lived in enormous numbers. The discovery of the new genera of stalked +Crinoids in the abyss suggested that possibly there might be found +several other families of extinct animals still surviving in the deep +sea. This view was supported by the Heart-urchin, whose shell showed +some striking peculiarities that were only known in fossil genera. + +But the hopes that were felt, even if they were not always expressed, +were doomed to disappointment. No living Ichthyosauruses or +Plesiosauruses, none of the remarkable Ganoid fish of Devonian times, +no Trilobites, no Cystoids nor Blastoids,--in fact none of the most +interesting of the fossil types rewarded the investigators of the +_Challenger_ and subsequent expeditions. + +It is perfectly clear to us now that, taken as a whole, the deep-sea +Fauna is not more ancient in character than any other Fauna. It is +true that a few genera, such as those just referred to, have survived, +probably from very ancient times, without much modification; but the +vast majority of forms are simply shallow water animals, which have +been profoundly modified in structure, and adapted to the peculiar +conditions of existence in the great depths of the ocean. + + + + + CHAPTER VII. + + COMMENSALISM AND PARASITISM. + + +The term Symbiosis has been applied by naturalists to the phenomenon of +the living together for mutual help or protection of different species +of animals or plants. It is a well-known fact, to all those who have +taken an interest in any large group of animals, that some species are +nearly always associated with other species, belonging perhaps to a +different class altogether, and very frequently mimicking them in form +or colour. At first it might be thought that most of these cases could +be dismissed as cases of parasitism; but when the careful observer +notices that neither of the species is injured by the association, the +conditions of the partnership are evidently very different to those of +a blood-sucking parasite and its ungracious host. + +Besides the words Symbiosis and Parasitism, the terms Commensalism +and Mutualism have been applied to various cases of association of +different species of animals; but with the increase of our knowledge +of the habits of animals, it is becoming more and more difficult to +classify all known cases under these four heads, and the words are +consequently often used with widely different meanings. + +It will be perhaps the best plan to adopt in this book, to avoid +any attempt to give definitions of these terms until a few cases +illustrative of each have been described. + +One of the commonest objects of the sea-shore is the Hermit-crab. +From the open mouth of what is apparently an empty shell a bundle of +claws and legs may be seen to protrude; turn the shell over and it +will scamper away into the deeper parts of a rock pool. This is an +association of a living Crab with the shell of an animal that is dead; +but if the Hermit-crab be extracted it will be seen that it has a soft +and twisted tail, quite unlike that of the shore Crabs, and that it +could not possibly live for any length of time without the shelter and +protection afforded by the shell that it has appropriated to itself. +The Hermit-crab in the course of its life increases in size, and when +it gets too big for the shell it is living in, it goes in search of +another a little bit larger and changes, until at last it attains to +the size and dignity that requires a full-grown Whelk shell. + +In the waters of our coast just beyond the low tide mark we often find +that the shell containing a Hermit-crab bears a Sea-anemone which +belongs to a species rarely found anywhere excepting in association +with a Hermit-crab. Moreover, the Anemone is always seated in a +definite position on the shell, so that its mouth is turned towards +the jaws of the Hermit-crab when it is extended, enabling it to catch +any morsels of food that escape the mouth of its comrade. When the +Hermit-crab has grown too large for its shell, and moves into a new +one, the Anemone moves too, and takes up the same position on the +new shell that it occupied on the old one, and the companionship is +continued in this manner throughout life. + +The advantage of this arrangement to the Anemone is obvious, for it can +not only obtain its food after the manner of the other Anemones, but +it also gains a share of the food of the Hermit-crab. The advantage to +the Crab is not so apparent, but it is probable that the Anemone, being +very distasteful to many Fish and other animals, acts as a protector to +it. The facts that Hermit-crabs are extremely shy, darting back into +their shells when there is the slightest sign of danger, and that they +are extremely good bait for many kinds of Fish, suggest very forcibly +that they have many enemies among the inhabitants of the deep. Any +such covering as that afforded by the Anemone, which hides to a great +extent the character of the shell, would be of protective value, but +when to that is added the fact that the Anemone, which affords this +covering, is avoided as uneatable and distasteful by carnivorous Fish, +there can be no doubt whatever of the assistance that it renders to the +Crab in return for its board. If any of my readers are sceptical about +the distastefulness of Sea-anemones I would ask them to think of any +instance in which Sea-anemones are used for bait, and then to try the +experiment of offering pieces of them to the Fish in an aquarium. + +An observation by Prof. Möbius in the Indian Ocean affords another +example of the use of Sea-anemones in this respect. He discovered a +little Crab called _Melia tesselata_ which carried about in each +of its claws a Sea-anemone. When the Crab was alarmed it held them up +in much the same way that a man holds a torch, as if it would call +attention to the fact that it had these terrible weapons at hand. When +the Anemones were removed it carefully searched for them, and held them +up again when found, and even when the Anemone was cut into pieces the +Crab diligently collected them, arranged them as far as possible in +their proper places, and held them up together. + + [Illustration: +Fig. 37.+ + Hermit-crab protruding from its hole in the sponge.] + +But Sea-anemones are not the only animals that seem to be generally +distasteful to Fish. Many of the Sponges are free from attack, and +could serve as a protection to the Hermit-crabs. On our own coast +a small brown Sponge is not infrequently brought up in the dredge +surrounding and protecting a Hermit-crab; and hidden somewhere in the +substance of the sponge, there may always be found a small shell which +lies at the end of the hole in which the Crab lives. + +This association is, from the Crab’s point of view, a more advantageous +one than that with the Sea-anemone, for it does away with the +necessity of any changes of shell, the Crab and the Sponge growing up +together. The history of the companionship is probably as follows:--A +small Hermit-crab takes for its shelter a small Gastropod shell, and +upon this shell a Sponge larva settles, grows and spreads, until it +surrounds the whole of it except the hole from which the Crab emerges. +As the Sponge grows still further in thickness the margin over-lapping +the aperture of the shell expands, leaving a conical cavity leading +from the exterior to the shell, surrounded, of course, by Sponge +structure, in which the Crab lives. Thus as the Hermit-crab increases +in size it is ever provided with a wider hole to accommodate its body +by the growth of the Sponge, and the little shell wholly deserted +remains as a token of the past history of the pair. But in the later +stages of growth a third creature is taken into the partnership, in +the person of a small segmented Worm which lives in the hole with +the Hermit-Crab. The need for this third person seems to be one of a +sanitary character. The cleanliness of the Hermit-crab, which has +no sponge to protect it, is provided for by the simple expedient of +frequent changes into a new home. In this case it is arranged for by +taking into the home, on what we may call board wages, an efficient +scavenger. + + [Illustration: +Fig. 38.+ + Section through a sponge (_D_) showing _A_, the little shell; _B_, + the worm; _C_, the Hermit-crabs in their natural positions.] + +In this remarkable association, then, no less than four species +belonging to four different groups of animals are concerned. First of +all there is the _Gastropod_ Mollusc, which forms a shell for the +_Crustacean_ Hermit-crab to commence life in, then there is the +_Sponge_ which protects, and afterwards forms a shelter and home +for the Hermit-crab, and lastly, there is the _Annelid_ worm, +which helps in its way to keep the house clean in return for the scraps +of food that fall from the head partner’s table. + +A very similar association has recently been described by Bouvier from +the Gulf of Aden and Red Sea waters. A number of simple solitary Corals +were thrown into an aquarium by a French naturalist, some falling on +their sides and some on their crowns, but he noticed that, after the +lapse of some time, they were all in the erect position again with +their crowns of tentacles expanded in the water. On carefully watching +them he observed that at the base of each Coral there was a little hole +from which emerged a small unsegmented Worm, belonging to a family that +usually exhibits sand-burrowing propensities. These Worms were found to +be the agents which restored the Corals to their erect positions. The +advantage of this arrangement to the Worm was two-fold: it brought it +into direct contact with the sand in which it searches for its food, +and, at the same time, it brought the Coral into such a position as +to hide and protect it from its enemies above in a most effectual +manner. To the Coral it was obviously an advantage, in that it placed +it in a position to expand its tentacles in search of the food it seeks +in the water and prevented a death from suffocation. A more minute +investigation of the Coral, however, revealed the facts that hidden +in its substance there was a small Gastropod shell on which we may +suppose both the Coral larva and the Worm settled when the partnership +began, and that in association with the Worm there was a small bivalve +Mollusc which probably acts as a scavenger in the manner of the Worm in +the last mentioned case. Here again, then, there are three different +species living together to their mutual advantage and commencing their +association on the shell of a fourth species belonging to a different +class of animals. What words can we apply to these associations? The +Hermit-crab and the Anemone feed at “the same table” and therefore +they afford a case of “Commensalism”; the Coral and the Worm are of +advantage to one another, the former in shielding and protecting the +latter and the latter in keeping the former in an upright position, +but as they do not feed “at the same table” it is not a case of +“Commensalism” but rather one of “Mutualism.” + +There are many cases, however, of the association of animals in which, +although the advantage to one of the partners is clear, it is extremely +difficult to say what benefit is derived by the other. + +Living in a tube on our coasts is a very common Worm called Sabella, +and at the mouth of the tube a little Polyp may frequently be found +which has received the fanciful name of the “Household god of the +Sabellids” (_Lar Sabellarum_). The Polyp undoubtedly benefits +by the currents of water which the Worm sets up when feeding, but it +is difficult to see what advantage, if any, the worm gains from the +presence of the Polyp. + +Again, some of the Trepangs are frequently inhabited by a little Fish +called _Fierasfer_, which comes out from time to time to feed and +“take the air,” but rapidly retreats into the body of the Trepang on +the slightest alarm. + + [Illustration: +Fig. 39.+ + A Trepang, or sea-cucumber.] + +The large stinging Sea-anemones of the Coral often afford protection +of a similar kind to a little Fish. Saville Kent gives a beautiful +picture of a little bright red Fish swimming about on the disc of a +large purple Sea-anemone, and he says that it darts into the mouth when +alarmed. On our own coasts we may often observe a number of little +Fish generally belonging to the Cod-family swimming round the disc and +tentacles of the large Jelly-fishes, and these, when frightened, swim +vigorously toward the under surface of the umbrella and seek security +there. Sometimes as many as a hundred or more of them may be seen +hovering round one large Jelly-fish, and we can hardly estimate how +valuable to our sea fisheries is the protection afforded by these great +Medusæ to the young Fish-fry. (_See Frontispiece._) + +It was not my purpose in writing this book to point out the practical +value of scientific investigation, but this history of the Jelly-fish +and Codling cannot be passed without comment. The Jelly-fish might +readily be regarded by the ignorant not only as useless to man, but, +in so far as they sometimes choke his fishing nets and sting his hands +and arms, a positive nuisance to him. Scientific investigation when +pursued by properly qualified persons for its own sake, and not for +any definite commercial results that may possibly come out of it, +frequently reveals facts of the utmost importance, such as the one that +has just been mentioned. + +There are some other cases of association which would on first +consideration be called undoubtedly cases of parasitism, but as this +term has been used somewhat vaguely in popular English, it would be +well, before proceeding further, to place before the reader a definite +statement of the sense in which the word is used in this book. + +In many of the cases that we have mentioned hitherto of animals +living together, no apparent injury is inflicted upon either of the +associates, but a very definite and decided advantage accrues to each +of them, by the association. + +In other cases, however, whilst no apparent injury is inflicted on +either, the advantage of the partnership falls entirely to one of them. + +In a third set of cases one of the associates feeds upon the blood or +tissues of the other without rendering it any service in return, and +consequently inflicts either temporary or permanent injury. These are +cases of parasitism. In such an association the animal that inflicts +the injury is called the “parasite,” and the one that receives it, the +“host.” + +One difficulty the naturalist has to contend with in trying to use +these terms correctly is that of finding out whether in any particular +case an injury is inflicted or not; another is that of determining +whether those animals should be called parasites which injure, alter, +or destroy the tissues of their hosts without feeding upon them. + +A few cases will throw more light upon the subject than any further +discussion of the difficulties surrounding the application of these +terms. + +One of the commonest Corals to be found upon the coral-reefs of both +the Old and New world is one called _Millepora_. In the Millepores +of the Pacific region we very frequently find a number of Barnacles +(called _Pyrgoma milleporæ_) so deeply buried in the substance of +the Coral that their presence is indicated only by a small oval hole on +the surface. There can be little doubt that in the course of the growth +of these Barnacles they distort, if they do not actually destroy, some +of the connecting canals of the Coral in their immediate neighbourhood, +but their food is derived entirely from the water that surrounds +the Coral and not any portion of it from the cells or tissues of the +Coral-polyps themselves. + +There is a great deal of difference in the Millepores from one and the +same coral-reef, in the extent to which the Barnacles have attacked +them. In some specimens large areas of the Coral are beset with the +little holes, in others only one or two may be found on the whole +colony, whilst others again are quite free from them. Now when we +compare carefully the anatomy of those Millepores with the Barnacles +and those without them, no single sign or symptom can be found that the +vigour or strength of the former is in any way impaired. If then there +is no evidence that the Barnacles are parasitic, in the sense that they +are injurious to the Millepores, we must next inquire whether they +could possibly be of any service to them. + +The polyps of the Millepores feed after the manner of the polyps of +other Corals, upon minute organisms floating in the sea; these they +paralyse and capture by means of tentacles bearing stinging cells. The +food is in the ordinary course brought within reach of the tentacles +by the tides that sweep over the reefs. The Barnacles also feed upon +minute organisms of the same kind, but they are provided with six pairs +of long feathery legs which by a curious vibratory movement create +currents in the water. When there are many Barnacles in close proximity +to one another it is quite probable that the water is considerably +disturbed by these currents, and the constant and rapid flow of fresh +water bearing food-organisms benefits, not only the Barnacles, but also +the Millepore polyps in their neighbourhood. + +Thus the Barnacles _may_ be a benefit to the Millepores in which +they live. It cannot be asserted, however, that this probability is +a proved fact. A great deal more knowledge about the rate of growth +of the Corals which are and are not affected, must be acquired before +such an assertion could be made: but the _probability_ that the +Barnacles may be of service is sufficient to cause us to hesitate +before branding them with the epithet of “parasites.” + +This particular case, which has been given above in some detail, may +be regarded, in a sense, as a test case, because other animals besides +the Barnacles, which gain their food by producing currents, are found +in Corals. Such are the tubicolous Worms, bivalve Molluscs, and certain +Sponges. So plentiful are these on the older branches of some Corals, +that quite a rich Fauna belonging to several groups of animals may +be found by carefully studying them. These might all be dismissed as +parasites by the non-inquisitive mind, but many of them, at any rate, +may be regarded by the more cautious naturalist as not injurious, and +others perhaps as positively beneficial to the Coral on which they +live. There is a very curious case of symbiosis mentioned by Semper, +which may be related here as similar in some respects to those above +quoted. + + [Illustration: +Fig. 40.+ + A Crab-gall on a branch of a Seriatopora.] + +On the shores of the Philippine Islands and in other parts of the +Pacific Ocean there is a very common coral named _Seriatopora_. +It is composed of numerous delicate branches, terminating in fine +pointed extremities, forming hemispherical shrub-like masses, six or +eight inches in diameter. Semper noticed that on some of the branches +of these Corals there were little heart-shaped swellings, which had the +appearance of malformations or structures corresponding to the galls on +the leaves and branches of trees. Each of these swellings contained a +cavity, communicating with the exterior by two minute holes, in which +there was imprisoned a small Crab. + +By the examination of a large number of specimens, Semper came to +the conclusion that the history of these structures was somewhat as +follows. The young Crab, when it settled down on the branch, produced +an irritation which in some way caused a gall-like growth of the +tissues of the Coral. This growth continued until it formed at first a +case or sheath for the protection of the Crab, and eventually, as the +Crab increased in size, a cage from which it could not do more than +protrude its tentacles and claws when feeding. + +It seems very improbable that these cage-like swellings upon the branch +can be of any great disadvantage to the Coral. It is true that they +destroy the beautiful symmetry of the branch, and give it a distorted +and diseased appearance; but this is only an æsthetic disadvantage, +which does not probably count for much in the struggle for existence +on the Coral-reef. To the Crab the arrangement is undoubtedly an +advantage, as it gives it a secure position, free from the attack of +its ordinary foes, where food is probably abundant and easily obtained. + +The skin of Whales is often beset with Barnacles; in fact some species +of them are found nowhere else but on these Mammals. They are usually +deeply embedded in the skin, only a small round hole through which the +legs can be protruded, communicating with the exterior. These Barnacles +do not feed upon the tissues and juices of the Whale, but, in the +usual manner of the non-parasitic Barnacles, upon organisms that swim +freely in the water. The advantage to the Barnacles is obvious, as the +movements of the Whale through the water must bring them in reach of +constant fresh supplies of food, but the benefit to the Whale is not +so clear. It cannot be supposed for a moment that the Barnacles assist +the Whales in their search for food, nor can they be regarded, when +present in great numbers, as a protection to the skin by the strength +afforded by their thick calcareous shells; at the same time there is no +reason to suppose that their presence is an inconvenience or in any way +harmful to the Whales. + +These cases of animals bearing on their bodies other creatures which +are not in the strictest sense of the word parasites, are but instances +of a phenomenon that is very widely spread among marine organisms. +There are many cases, however, in which plants and inorganic foreign +bodies play a very important part in the economy of animals. + +In our chapter on the free-swimming organisms of the ocean, mention has +been made of the delicate and beautiful creatures called Radiolarians. +Many years ago it was discovered that each of these animals bears +in its protoplasm a number of little cells, which from their colour +received the name of “the yellow cells.” It was clear from observation +and experiment that they were neither organs nor products of the +Radiolarian, but independent organisms belonging to the Vegetable +Kingdom. + +More recently cells similar to these have been found in many of the +Corals, in Worms, and other animals, and there can be no doubt now that +when present they perform very important physiological functions which +materially assist their host in its growth and development. + +So numerous are these “yellow cells” in some Polyps and so important +must be their influence on their vital processes, that it may be +confidently asserted that the Polyps could not continue to exist +for long without them. In the genus _Millepora_, for example, +no single specimen and no single fragment of a specimen that I have +examined was devoid of them; and although the numbers vary considerably +the most superficial canals of this Coral may in all cases be +described as crowded with “yellow cells.” But as the “yellow cells” are +certainly of great physiological importance to the Millepore, it is +equally certain that the secretions and the protection afforded by the +Millepore are of extreme importance to the “yellow cells.” In fact it +is not going too far to say that the Millepore and its “yellow cells” +are dependent upon one another for their existence, and the naturalist +might say with a great deal of truth that this particular Coral is not, +strictly speaking, animal in nature, but rather an animal and vegetable +combination. + +Many years ago there was a bitter controversy among learned men on +the question of the animal or vegetable nature of Corals. The great +naturalist Linnæus, who was appealed to for his support by both parties +to the controversy, took up a middle position, asserting that they were +partly of the nature of animals and partly of the nature of plants, +and hence the term “zoophytes,” _i.e._ animal-plants, came to be +applied to them. There can be no doubt that in the end the position +in the controversy, assumed by Linnæus, became untenable, and the +supporters of the animal view of zoophytes won all along the line. It +is curious, therefore, that we are now in a position, not to support +the view of Linnæus, but to assert that some Corals are essentially a +combination of animals and plants. + +Plants are of use to marine animals, however, in another manner. +Mention has already been made of the way in which many animals +resembling in colour, and even in form, certain kinds of sea-weeds, +escape the attention of their enemies and hide for safety among the +plants they simulate. Sometimes, however, the weeds will grow upon +the shells of the animals, and thus hide them even more effectually. +One of the most remarkable instances occurs in a Spider-crab that is +common upon our own coasts. The _Inachus_, as it is called, is +usually covered with a little forest of algæ, which do not grow there +naturally, but are actually placed on the carapace by the Crab itself. +If the plants be scraped off artificially the Crab will go in search +of fresh ones, carefully chew the bases until they are soft, and then +deliberately decorate the carapace with them as before. + +There are some Molluscs that artificially decorate themselves with +little shells and other objects in such a manner as to completely +hide their general form. One of the most remarkable instances of this +occurs in the Gastropod _Xenophora_, which covers its own shell +with numbers of others belonging to a smaller species, so that in +the natural state it has the appearance of a conglomerate of shells. +The manner in which the smaller shells are fixed has not yet been +described, but from the orderly arrangement which they exhibit in some +cases there can be little doubt that they are deliberately placed in +position by the Gastropod itself and not attached by accidental contact. + +In both these cases it is clear that the reason for the phenomena +described is that of affording a covering or mantle, which hides or +obscures the real form and character of the living animals. + +Many of the Worms use little bits of shell and grains of sand to +build up a tube for the protection of their bodies. One of these--the +_Terebella_--is very common on our shores, the sandy tubes ending +in a tuft of fine filaments, and decorated all over with tiny little +stones or shells, projecting an inch or two from the surface of the +sand. In some localities these tubes may be found in thousands when the +tide is low. + +Another form--_Pectinaria_--constructs much firmer tubes, which +retain their cylindrical shape after the death of the animal. In the +process of construction this Worm must carefully select the grains +of sand, for when the tube is examined with a magnifying glass the +particles will be seen to be of almost exactly the same size, and +arranged in their places with a mathematical precision. + +But Worms are not by any means the only animals that use the sand in +this manner for the protection of their bodies. There are some kinds +of Polyps, belonging to the family _Zonathidæ_, a peculiar group +of Sea-anemones, in which the body-wall is considerably strengthened +by foreign bodies of various kinds. The _Zoanthus_ does not, like +the Terebella-worm, form a tube or case in which the body can freely +move up or down, but sticks the grains of sand into its skin, so that +they become in the older forms deeply buried in the tissues and give a +considerable support to the body-wall. + +The _Cerianthus_--another Sea-anemone--forms a tube which is +partly composed of a matted network of stinging threads, and partly of +the mud in which the animal lives. + +The use of foreign inorganic substances for the protection or +concealment of animals is not, strictly speaking, however, a part +of the subject-matter of this chapter, which was intended for the +consideration of the associations of two different kinds of living +organisms. + +The subject of Parasitism must now be considered, a subject which +presents so many features of interest that it is possible here only +to touch on a few points of general importance. It is a well-known +truism to say that Parasitism, whether in human society or in animal +life, leads to degeneration; but there are degrees of parasitism among +animals, and consequently degrees of degeneration exhibited by animal +parasites. We may roughly divide them into two classes, the outside or +skin parasites and the internal parasites, the latter being invariably +far more modified in structure and in development than the former. + +Among the terrestrial animals we find a great number of external +parasites, such as the Fleas and the Bugs, which are only slightly +modified, as in the loss of their wings, owing to their habits, and can +live an active, if not a very prosperous, life for some length of time +apart from the society of their hosts. There are others, such as the +Mosquitoes, Ticks, and Leeches, which are only occasional parasites; +that is to say, they will suck the blood of another animal when the +opportunity is presented, but failing that, are able to continue their +life and their race independently. It is not surprising, however, that +the terrestrial Vertebrates should be thus subjected to the attacks of +these parasites, as their feathery or hairy skin affords a shelter and +a foothold, from which the efforts of their hosts to dislodge them are +exercised in vain. + +The skin of Fish, although covered with over-lapping scales, is smooth +and slippery, and with the rapid movement through the water many of the +forms of parasites of the types we meet on land would, if they existed +in the sea at all, find a difficulty in securing an attachment. It is, +however, provided with another means of defence against skin parasites, +in the possession of numerous mucous glands which keep the body bathed +with a slimy fluid. + +Everyone must have noticed the slime that exudes from freshly killed +Fish, and if the finger be pressed along the skin it is possible to +see the openings of the glands as the slime is squeezed out. In the +majority of Fish the openings of the glands are most easily seen on the +jaws and the flap of the gill cover. + +We must remember that the sea is in most places teeming with the larvæ +of Worms, Barnacles and Zoophytes, and the spores of Algæ and Fungi +of various kinds. Logs of wood, the iron supports of piers, and the +bottoms of ships become covered with various fixed forms of animal and +vegetable life when submerged in the sea-water for even a few weeks. +How is it, then, that the bodies of the Fish are usually so clean and +wholesome? The answer to this question is probably to be found in the +slime which, passing continuously over the skin, removes the larvæ and +the spores before they can secure a firm attachment. + +The Crabs, Lobsters and other Crustaceans free themselves from their +skin parasites at every moult, but in some of the large, old Lobsters +and Crabs that are caught a considerable number of Worms, Barnacles and +weeds are frequently found firmly-fixed to the carapace and claws. The +Limpets and Winkles of our rock pools are often covered with a little +forest of Algæ. + +The shells of other Molluscs are, however, kept remarkably clean, and +the method by which they destroy the spores, etc. that settle upon them +is not yet fully understood. + +One of the most serious of the external parasites is the Hag-fish. This +remarkable animal is eel-like in shape, although very different indeed, +anatomically, from all the true Fishes, and buries its head in the skin +of the Cod and other Fishes as it feeds upon their flesh. In some cases +the whole body of the Hag gets inside the host, and it thus becomes an +internal parasite. It causes an immense destruction of valuable food +fish in some districts. + +Closely related to the Hag is the marine Lamprey, which fastens itself +to Salmon by its suctorial mouth, causing considerable wounds. This +parasite sometimes reaches to a length of two feet, and is often +carried many miles up the river by the host to which it is attached. + +Most of the Leeches occur either in fresh water or in damp forests and +marshy places. There is one, however, named _Pontobdella_, which +is found only in sea-water. It is difficult to give an exact statement +as to its size, because, like all its relations, it is capable of +very extensive movements of expansion and contraction, but the +_Pontobdella_ is large for a Leech, and when moderately contracted +it may be two or three inches in length. The body of this Leech is +covered with small tubercles, and it has a large round sucker at each +end. Its favourite hosts are the Sharks and Rays, but as it usually +drops back into the water when these Fish are hoisted on to the deck it +is not very commonly seen in the fishermen’s boats. + +The most common external parasites of Fish are the Fish-lice. Most of +these are little Crustaceans, belonging to a group which includes the +Wood-louse. They have curiously flattened bodies, provided with short, +bent legs, terminating in sharp hooks, by which they adhere to the body +of the Fish and crawl about over the skin. Some of these parasites seem +to prefer the tongue as a resting-place, the genus _Glossobius_, +for example, being found in this position on the Flying-fish of both +the Pacific and Atlantic Oceans. In _Glossobius_ we find a very +remarkable difference in size and form between the males and females, +a condition of affairs which is of very common occurrence among the +parasitic Crustaceans. The male in this particular case is so small +that it is entirely concealed beneath the tail of the female. In +another genus a still more interesting condition has been observed, the +small young forms which are males growing up, and changing in later +life into females. + +There is a remarkable parasite called _Sacculina_ which may +sometimes be found on the under side of the tail in Crabs. In shape it +is like a small pea or bean, and is attached to its host by a number +of root-like processes, which penetrate through the skin and burrow +deeply into the subjacent tissues. It would be quite impossible to tell +to what group of animals this parasite belongs by the study of the +adult form alone. It is, in fact, little more than a skin full of eggs. +When the development of the eggs is watched, however, it is observed +that the young _Sacculina_ as it is hatched is very much like +the Nauplius larva of a Barnacle. The later stages of the development +prove that whatever may happen to the adult the _Sacculina_ must +be related to the group of the Cirripedia. Later on it is found that +the females settle down on a Crab, lose all their limbs and other +Cirripedian characters, and finally degenerate into a mere palpitating +sac of eggs. + +The males never pass beyond the second stage of development known as +the Cypris stage. Several of them may usually be found attached to the +female, and although they always remain extremely minute they do not +lose entirely their Crustacean features. + +For those who are in search of parasites, however, there is no more +fruitful ground than the gills. That these organs should be a good +place for attack is not surprising, when we consider that to maintain +the respiration of the animal a constant flow of sea-water over them +must be kept up, and this must bring with it many larval forms which +may take the opportunity to attach themselves as they pass through the +meshes of the gill filaments. Moreover, it is in the gills particularly +that the blood current comes into closest contact with the water, and +it requires but a little puncture on the part of the young parasite to +reach a constant supply of this nourishing fluid. + +It is in the gill-chambers that we find most frequently representatives +of that interesting group of animals, the parasitic Copepods. + + [Illustration: +Fig. 41.+ + A parasitic Copepod.] + +It would be difficult to recognise them as Copepods if we were to judge +by their adult characters alone. Unlike the brisk, brightly-coloured +creatures with long rowing antennæ that we have described above as +living a free life in the surface waters of the ocean, these parasites +have a white sac-like body, with short blunt processes representing the +legs, no eye, and generally two long thread-like bags of eggs attached +to the sides of the rudimentary tail. As we found in the case of the +_Sacculina_, the true zoological position of these parasites can +only be determined by reference to their developmental history. + +In the gill-chamber of the Prawns we find a very much modified +parasite, which is closely allied to those skin parasites of Fishes +mentioned above. Many of my readers may have noticed that in some +Prawns there is a wart-like swelling on one side of the neck. If +the skin be removed it will be observed that this is in reality a +cup-shaped protrusion on the wall of the gill-chamber covering a +little, flat, soft animal. In past times it was thought that this +was a young flat fish, and a wonderful story of its development was +fabricated on the strength of this error. It is now known to be one of +these extremely degenerate Isopod parasites called _Bopyrus_. + +It is a curious fact that there is very rarely indeed more than one +of these parasites on a single Prawn. If there is one in the right +gill-chamber there are none in the left, and _vice versâ_. It +is difficult to find a satisfactory explanation for this, for it is +not at all probable that, during the lives of the many hundreds of +Prawns that have been examined, only one larva has passed through the +gill-chamber of each individual. The explanation must be looked for +in some hitherto unknown influence which the parasite has upon the +constitution of the host, rendering it unsuitable for the attachment +of another _Bopyrus_ of the same habits. The case is by no means +unique. There are several instances of Fish and other animals that bear +one, and never more than one, parasite of a particular species. + +A few words must now be added about the internal parasites of marine +animals. The subject is really an immensely wide one; for the +intestines, body cavities, and even blood-vessels of Fish are liable +to the attacks of many different forms of Flukes, Tape-worms, and +other kinds of parasites which are not even known by name, perhaps +fortunately, to the general public. + +The life-history of some Flukes that occur in terrestrial animals +has been satisfactorily worked out, and we know that, in most cases, +they must infest two different hosts before they can reach maturity. +The first of these hosts is usually an Invertebrate, and the second a +Vertebrate animal. Moreover, it is known that the larvæ are extremely +particular in their choice of the first host, attacking one species, +and one species only, of Snail or Slug, or whatever Invertebrate +its first host may be. If the first host dies a natural death or +is swallowed by any other animal than the parasite’s proper second +host, it--that is to say, the parasite--dies. It seems probable that +the Flukes that infest the intestines of marine animals pass through +some similar life-history, but owing to the great difficulties that +confront the observer their development has not yet been thoroughly +investigated. Similarly the life-histories of the Tape-worms, with +which a very large number of marine animals are infested, are not +yet known to us. It is comforting to know, after looking through the +volumes of papers on these internal parasites of marine animals, that +none of them have been shown to be, even occasionally, parasitic upon +man, and we can continue our Fish diet without any misgivings on that +score. An exception must, however, be made to this statement for the +semi-marine Salmon and Sturgeon, which are suspected of being the +first hosts of a human Tape-worm. + +It is perhaps unsatisfactory to dismiss the internal parasites of +marine animals with so few words, but I feel compelled to do so, not +only because I have nearly outrun the limits of space, but because +we possess so little positive information on the subject, which is of +greatest interest to us here, of their developmental history. Lists +of species infesting different Fish and Whales could be published, a +statement of the points of anatomical importance which distinguish +the families could be written, but they would present few features of +interest to the general reader. + +It may be well to point out before the chapter is closed, however, that +there is probably no branch of our subject that is so little known and +presents such a wide and important field for future investigation as +the life-histories of these marine parasites. + + + + + CHAPTER VIII. + + THE ORIGIN OF THE MARINE FAUNA. + + +When we survey the distribution of living organisms over the surface +of the globe, we cannot fail to be impressed with the enormous range +in the characters of the physical conditions which are capable of +supporting animal and vegetable life. Thus we find Birds flying in the +sunlight of the cold and very light atmosphere of the mountain tops, +and Fishes swimming in the chill darkness of the depths of the ocean, +supporting a pressure of two tons to every square inch of their bodies. +We find Algæ, which give the snow sometimes the name of “Red snow,” +flourishing at temperatures below the freezing point of water, and we +meet with Insect larvæ swimming freely in the water of the hot springs. +Some sea-water animals can only be induced to live in the aquarium when +the water is kept as pure as it is in the open sea, and languish and +die as soon as any impurity occurs; on the other hand, several of the +Crustaceans seem to flourish best in stinking and putrescent pools. +The desert, the forest, the swamp, the lake, the river, as well as the +surface and the bottom of the sea have each their characteristic set of +animals and plants modified in structure and form to support life in +their natural habitats. + +There can be no doubt that at the time when animals and plants first +made their appearance upon the earth, their distribution was far more +limited than it is now, and that all the adaptations to life in special +and extraordinary conditions have been acquired in the course of +evolution by organisms which originally existed in one particular zone +of the earth. + +The reasons which have led scientific men to this opinion are manifold, +but not the least important of them are those based upon the presence +of organs or rudiments of organs of animals of the present time, which +could only have been called into existence at a period when their +ancestors had an altogether different habit of life. + +For example, in the Birds and Reptiles, as well as in the Mammals, +the presence of openings in the throat during the early stages of +development, similar in their position, in their blood-vessels and in +other respects to the openings of the gills in Fishes, indicates that +their ancestors in remote periods lived in water and not on dry land. + +Again the presence of rudimentary eyes in the Mole and other +subterranean animals indicates that at one time its ancestors must have +lived in the light of the day. The characters of the embryos of some +of the land and fresh water Snails proves that they are derived from +ancestors that lived in the sea. + +When we collect together all the evidence of this kind and place it +side by side with the facts revealed to us by Geology, the irresistible +conclusion is arrived at that all animals are originally derived from +ancestors that lived in the sea. And when we consult the botanists and +find that they are agreed that all plants must have had a marine origin +also, the case for the sea being the original home of living organisms +may be said to be completed. + +It is difficult to picture to ourselves the condition of the earth in +those very distant times, when the dry land bore no forests nor grass, +the air supported no Birds nor Butterflies, and in the rivers and lakes +swam no Fish nor Frogs. It must have been “dry” land indeed, when +there were no trees to attract the rain clouds and no herbs or mosses +to retain the moisture on the ground. The rivers must have risen and +fallen with great rapidity as they carried away the rain that fell in +cloud-bursts on the mountain tops. + +But speculation on the character of the land in those times is not +within the scope of this work, and we must turn again to the sea to +inquire where the primordial animals and plants lived in the days of a +lifeless land. + +We have seen that in the sea there are three possible habitats for +animals and two for plants. The surface waters of the great oceans +bear a characteristic population of animals and plants, the bottom of +the sea supports a considerable number of animals but no plants, and +lastly the shallow waters exhibit an immense variety of Sea-weeds, +Fish, Worms, and other creatures. Which of these three was the original +cradle of the great classes of animals and plants? + +The early discovery of certain animals in very deep water which are +closely allied to, if not identical with, some fossils of early +geological strata, suggested the idea that a very primordial set of +creatures might be found at the bottom of the sea when it was more +thoroughly investigated; but as I have pointed out in a previous +chapter the hopes of those who anticipated the discovery of a rich +Fauna of “living fossils” were doomed to disappointment. + +It is not probable, however, that the abyss of the oceans could have +been the cradle of life, even if it had shown a more ancient Fauna than +it actually does. + +We cannot tell in what form life first appeared upon the earth. +Whether the unstable living substance called Protoplasm was in the +earliest conditions of the globe formed spontaneously by the chance +combination of its elements, or whether some germ or other made a +hazardous journey through space from another planet enwrapped in the +casing of a meteorite, are questions upon which no light has yet been +thrown by scientific observation or speculation; but this can be said, +that at a very early period in the history of life upon the earth the +simple green plants must have played an important part. It is on the +substances that are formed by the activity of this green coloured +substance that all plants and animals are directly or indirectly +dependent for their food in the present-day economy of Nature, and we +are forced to believe that, whatever may have been the form of the +earliest living things, Chlorophyll--the green coloured substance of +plants--must have had an extremely ancient origin. + +Now, in the darkness of the ocean depths Chlorophyll does not and +cannot exist; for it is one of its characteristic features that it +is active only in the rays of direct sunlight; and, therefore, it is +extremely improbable that the cradle of the marine Fauna could have +been there. We are then left with two alternatives. It must have been +either at the bottom of the shallow waters or on the surface of the +seas. + +Both of these sites have had their advocates, but the balance of +opinion has now turned decidedly in favour of the first of them--the +shallow waters. It is not easy to explain the reasons for this view +without assuming a fairly complete knowledge on the part of the reader +of the various forms of life that are found in the sea, but still a +few words of explanation may be written to indicate that the view is a +reasonable one. + +In the first place we find, when we take a general survey of the +animals that live in the surface water, that they are all specially +modified in some way or another in structure or development in +adaptation to the peculiar conditions of their life. The long spines +of the Foraminifers and the Crustacean larvæ, the air-bladders of the +Portuguese men-of-war, the oil drops of the Copepods, the raft of the +Mollusc _Janthina_ are, as we have seen, among the characters +which distinguish this peculiar Fauna. Now, when we compare these +surface-dwelling forms with their nearest relations in the shallow +waters, the conclusion we come to is that these features have been +acquired by the ancestors of the former, which may have been similar in +some respects to those now living in shallow water. + +Some of the Gastropods of the shore-waters have a simple cup-shaped +shell like that of the common Limpet, but the great majority of them +have a shell that is twisted up into a spiral form. This twisting of +the shell is, of course, due to the twisting of the mantle or fold of +skin which secretes the calcium carbonate of which the shell is mainly +composed; and, when we study the internal anatomy of the animal we find +that the shape of the mantle is associated with a loss of the organs +of one side of the body. To put a long story into a few words, we may +say that the Gastropods with twisted shells are lop-sided. Now when we +examine the shells of the Gastropods that live in the surface waters +of the ocean we notice that their shells are (with a few exceptions +such as _Janthina_) perfectly symmetrical, and we might jump to +the conclusion that this was due to a corresponding symmetry of the +internal organs. + +Such a conclusion would, however, be an erroneous one, for the +results of the careful anatomical study of these Molluscs proves most +definitely, that although a false symmetry of the organs is often +shown, there is a general suppression of the organs of one side of the +body. A study of the development of these animals also shows, that +in the early stages of their life, the shell is not symmetrical like +that of the adult, but twisted into a spiral like that of a Whelk or +a Periwinkle. These facts indicate that the surface-swimming Molluscs +have passed through a stage in their evolution when their bodies +were twisted up into a spiral shell, and that the false symmetry, +which they exhibit in the adult condition, is an adaptation to +their peculiar habits of life. The study of the group of Gastropods +alone then does not give us any evidence in favour of the view that +the surface-swimming Fauna is primitive; in fact, it proves almost +conclusively that its share in the Fauna has been contributed from the +shallow water districts. + +The group of the Tunicates affords similar evidence. There is no +good reason for believing that the Salps and _Pyrosoma_ which +drift about in the surface waters are more primitive than the fixed +Sea-squirts of the rocks and Sea-weeds, in fact, the view is gaining +ground, as our knowledge increases, that all the free Tunicates must +have passed through a sessile ancestry. The evidence afforded by the +Cœlenterates is not so potent. Several naturalists believe that some +free-swimming form of Jelly-fish was the ancestor, and that the fixed +Zoophyte was a stage introduced into the life-history at a later period +in the evolution of the group. Others believe that the Zoophyte-stage +came first and that the Jelly-fish was introduced, for the purpose +of distributing, over a wide area, the eggs of the species. My own +researches lead me to incline towards the latter view, but I feel that +it is still far from being proved. + +A great deal more could be written upon this fascinating speculation +about the origin of Life in the Sea. But it is still a speculation, +and all that can be done at present is to weigh the evidence carefully +and see in which way the scale seems to point. If I have succeeded in +making clear to the general reader the nature of the evidence we can +use in judging this question, and have indicated to him the direction +in which it seems to _me_ to point, my task has been accomplished. + + + + + INDEX. + + + A. + + Air-bladders in Plankton, 90. + + Alcyonarians of Coral-reef, 67. + + Alcyonium, 42. + + Alternation of Generations, 31, 93, 99. + + Angler-fish, 39. + + Azoic, without animal life, 9. + + + B. + + Barbel, a tentacle on the lower jaw of Fish such as the Cod, 52, 139. + + Barnacles, 40; + on Whale, 159. + + Benthos, the animals living on the bottom of the sea, 110. + + Blue-shark, 121. + + Blue-whale, 126. + + Bonito, 121. + + Boring Mollusc, 43. + + Bopyrus, 170. + + Brachiolaria, 112. + + + C. + + Caller-crab, 76. + + Cetacea, 125. + + Cilia, minute vibratile hair-like processes on the body of some small + animals, 111. + + Cirripedia, a group of Crustacea to which the Barnacles belong, 168. + + Cod-fish Family, 52. + + Coffer-fish, 63. + + Colour in shallow water animals, 31; + in deep-sea animals, 137. + + Copepods, 86; + parasitic, 169. + + Corals, 57. + + Coral and Worm, 151. + + Coral-reefs, 56; + shells of, 65; + Anemones of, 66; + alcyonarians of, 67; + different forms of, 68; + Fauna, of outer edge of, 78. + + Crab-galls, 158. + + Crab and Sea-weed, 162. + + Crustaceans of the rocks, 48. + + Ctenophores, a group of Cœlenterates, 85. + + Currents, 16. + + Cuttle-fish, 49. + + + D. + + Density of Sea-water, 13. + + Depths of the Sea, 10. + + Diatoms, minute unicellular, plants, 23. + + + E. + + Electric Organ, 37. + + Eyes, 27; + of Scallops, 29; + of larval Tunicate, 30; + of Medusæ, 30; + of deep-sea animals, 138. + + + F. + + Fauna, the animals living in a particular region considered as + a whole. + + Filograna, 57. + + Fish of the rocks, 51; + of the Coral-reefs, 63; + of the surface waters, 119; + of the deep-sea, 139. + + Fish-lice, 167. + + Flying-Fish, 120. + + Fur-seal, 133. + + + G. + + Gas reservoirs in Plankton, 90. + + Gastropods of shallow water, 45; + of Plankton, 100. + + Gelasimus, 76. + + Globe-fish, 63. + + Globigerina, 21, 104. + + + H. + + Hag-fish, 166. + + Halobates, 102. + + Hermit-crab, 147. + + Herrings, 123. + + Hormiphora, 85. + + + I. + + Inachus, 162. + + Ipnops, 138. + + Isopoda, a group of Crustaceans to which the Wood-louse belongs, 139. + + + J. + + Janthina, 101. + + Jelly-fish, 84, 92. + + John Dory, 54. + + + L. + + Lamprey, 166. + + Larva, a young immature free individual differing in form from + the Parent. + + Larvæ in surface waters, 109. + + Leeches, 166. + + Lithothamnion, 20, 57. + + Lug-worm, 34. + + Lump-sucker, 52. + + + M. + + Mackerel, 124. + + Madrepore, 57. + + Mangrove-swamp, 73. + + Medusæ, 31, 92. + + Melia tesselata, 148. + + Millepore and Barnacle, 155. + + Mud-line, 19. + + + N. + + Nauplius larva, 40, 115. + + Nekton, 119. + + Noctiluca, 109. + + + O. + + Ooze, 21. + + + P. + + Palolo Worm, 79. + + Parasitism, 164. + + Periophthalmus, 74. + + Periwinkles, 45. + + Pholas, 43. + + Phosphorescence, 82, 107; + of deep-sea fauna, 140. + + Phyllopteryx, 26. + + Physalia, 97. + + Pipe-fish, 25. + + Plankton floating or drifting animals of a region considered as + a whole, 83. + + Pluteus, 113. + + Porpoise, 127. + + Prawn, parasite of, 170. + + Pteropod, 21, 102. + + Pyrosoma, 108. + + + R. + + Radiolarians, 22, 160. + + Razor-shell, 35. + + Red clay, 21. + + Right-whale, 126. + + Rocks, Fauna of, 39. + + + S. + + Sabella, 153. + + Sacculina, 168. + + Salps, 98. + + Sandy shores, Fauna of, 34. + + Sargasso, 106. + + Scallop, eyes of, 29. + + + Scopelus, 123. + + Sea-anemones, 66, 163. + + Sea-bottom, 18. + + Sea-elephant, 132. + + Sea-horse, 26. + + Sea-lilies, 144. + + Sea-lion, 132. + + Sea-sawdust, 106. + + Sea-urchin, 43. + + Sea-weeds, 24, 106. + + Seal, 130. + + Serpula, 41. + + Shells of Coral-reef, 65. + + Ship-worm, 43. + + Shrimps, 31. + + Siphonophores, 95. + + Skates, 36. + + Slime glands of Fish, 165. + + Sole, 37. + + Solen, 35. + + Sperm-whale, 127. + + Spines in Plankton, 90. + + Sponge, Hermit-crab and Worm, 150. + + Star-fish, 44; + method of feeding of, 29. + + Stenopus, 33. + + Sting-ray, 36. + + Stinging of Cœlenterates, 97. + + Sun-fish, 122. + + Swamp-fauna, 74. + + Symbiosis, 146. + + + T. + + Temperature of sea-water, 11. + + Teredo, 43. + + Thread-cells, 97. + + Tides, 15. + + Torpedo, 37. + + Trepang, 153. + + Trichodesmium, 106. + + Tridacna, 65. + + Trigger-fish, 63. + + Tunicates, eye of, 30. + + + V. + + Velella, 96. + + + W. + + Walrus, 134. + + Worm-tubes, 163. + + + X. + + Xenophora, 162. + + + Y. + + Yellow cells, 160. + + + Z. + + Zoophytes, 30, 161. + + + + + PRINTED BY + TURNBULL AND SPEARS, + EDINBURGH + + + + + “_A very useful series of small manuals on subjects of common + interest._”--+Spectator.+ + + + THE + Library of Useful Stories. + + _Small 8vo, cloth, price 1s. each Volume, post free 1s. 2d._ + + “_The more Science advances, the more it becomes + concentrated in little books._”--+Leibnitz.+ + + + I. + +THE STORY OF THE STARS. + + By +G. F. Chambers+, F.R.A.S., Author of “Handbook of Descriptive and + Practical Astronomy,” &c. With 24 Illustrations. + +“Mr Chambers writes in a vigorous and attractive style, and shows +himself able to combine to an uncommon degree scientific accuracy +of statement with a clear and attractive exposition. Beginners in +astronomy who wish to acquaint themselves merely with the outlines of a +noble science will find this volume of real service.”--_Speaker._ + +“Told in a pleasing and attractive manner.”--_Athenæum._ + + + II. + +THE STORY OF PRIMITIVE MAN. + + By +Edward Clodd+, Author of “The Story of Creation,” &c. With 88 + Illustrations. + +“It possesses the chief qualities that go to make a good book for the +average man.”--_Nature._ + +“Well printed, well bound, profusely illustrated, and in every +respect capital material, on one of the most progressive of +sciences.”--_Daily Chronicle._ + + + III. + +THE STORY OF THE PLANTS. + + By +Grant Allen+. With 49 Illustrations. + +“A brightly written, clear and accurate summary of the functions and +habits of plants.”--_Daily Chronicle._ + +“The whole book is excellent, but special praise is due to his +exposition of the relations existing between plants and insects. +Many chapters of the story he tells must prove to the uninitiated as +exciting as a romance.”--_Aberdeen Free Press._ + + + IV. + +THE STORY OF THE EARTH IN PAST AGES. + + By +H. G. Seeley+, F.R.S., Professor of Geology, Geography, and + Mineralogy in King’s College, London. With 40 Illustrations. + +“A simple and popular summing up of the results that have been reached +by geological science.”--_Scotsman._ + +“Told plainly and pleasantly for a popular audience.”--_Bookman._ + + + V. + +THE STORY OF THE SOLAR SYSTEM. + + By +G. F. Chambers+, F.R.A.S. With 28 Illustrations. + +“His descriptions possess the double quality of simplicity and +attractiveness.”--_Nature._ + +“He repudiates the idea that unless a man can command a big telescope +he is not in a position to do useful work in astronomy.... The little +volume is an admirable example of science made easy without the +sacrifice of strict accuracy of statement.”--_Speaker._ + + + VI. + +THE STORY OF A PIECE OF COAL. + + By +E. A. Martin+. With 38 Illustrations. + +“Treated with wonderful skill, simplicity, and +thoroughness.”--_Bookseller._ + +“Explains in simple and delightful fashion what coal is, whence it +comes, and whither it goes, and in the concluding chapters shows how +intimately it is connected with the interests of the botanist, the +geologist, the physicist, the chemist, and the merchant.”--_Bradford +Observer._ + + + VII. + +THE STORY OF ELECTRICITY. + + By +J. Munro+, Joint Author of “The Pocket-book of Electrical Rules + and Tables.” With 100 Illustrations. + +“Just the kind of book to give the general reader more correct views +of the subject than many a pretentious tome.”--_The Electrical +Engineer._ + +“For general interest we must pronounce the little book without a peer, +style and matter being alike excellent.”--_Glasgow Daily Mail._ + +“A handy little book which has certainly the great merit of being up to +date. We anticipate a large demand for the book.”--_Electricity._ + + + VIII. + +THE STORY OF EXTINCT CIVILIZATIONS OF THE EAST. + + By +R. E. Anderson+, M.A., contributor to Chambers’s Encyclopædia, + Encyclopædia Britannica, and Dictionary of National Biography, &c. + With Maps. + +“The author has performed a much needed service in a masterly +manner.... We have nothing but praise for the work.”--_Literary +World._ + +“An admirable compendium of a department of knowledge which has been +greatly advanced by the research of recent years.”--_Aberdeen Free +Press._ + + + IX. + +THE STORY OF THE CHEMICAL ELEMENTS. + + By +M. M. Pattison Muir+, M. A., Fellow and Prælector in Chemistry of + Gonville and Caius College, Cambridge. + +“One of the most perfect popular introductions to science +extant.”--_British Medical Journal._ + +“Prof. Muir tells an enthralling story of the wonderful transformations +of matter under the chemist’s magic wand. Ignoring formulæ he appeals +in homely phrase to the imagination of the reader.”--_Knowledge._ + + + X. + +THE STORY OF FOREST AND STREAM. + + By +James Rodway+, F.L.S., Author of “In the Guiana Forest,” &c. With + about 27 Illustrations. + +“Contains a short description of a tropical forest, together with some +elementary lessons which can be learned by studying the incessant +struggle for existence of its varied flora.”--_Academy._ + +“A noteworthy addition to the series in which it +appears.”--_Scotsman._ + + + XI. + +THE STORY OF THE WEATHER. + + By +G. F. Chambers+, F.R.A.S., of the Inner Temple, Author of “Story + of the Stars,” &c. With 50 Illustrations. + +“An interesting volume about weather, and especially English weather, +and presents facts, ideas, and suggestions which ordinary people will +be glad to know.”--_St James’s Budget._ + +“Shows how the weather forecasts are drawn up at the Meteorological +Office, explains the construction and use of the various meteorological +instruments, describes the nature and causes of such phenomena as +the aurora borealis, and gives a collection of weather facts and +signs.”--_Literary World._ + + + XII. + +THE STORY OF THE EARTH’S ATMOSPHERE. + + By +Douglas Archibald+, M.A., Fellow and sometime Vice-President of + the Royal Meteorological Society, London. With 44 Illustrations. + +“One of the best of the Story series that we have read ... the author +is frequently able from his wide travels to illustrate his remarks +from his own personal experience in climates where meteorological +manifestations can be witnessed on a grander scale than in our own +country.”--_Nature._ + + + XIII. + +THE STORY OF GERM LIFE: +Bacteria+. + + By +H. W. Conn+. With 34 Illustrations. + + + _NEW VOLUMES IN PREPARATION._ + + +THE STORY OF THE POTTER. + + By +C. F. Binns+, late of Worcester Royal Porcelain Works. + + +THE STORY OF BRITISH COINAGE. + + By +G. B. Rawlings+. With Illustrations from Coins in the British + Museum. + + +THE STORY OF PHOTOGRAPHY. + + By +A. T. Story+. With Illustrations. + + +THE STORY OF RELIGIONS. + + By +E. D. Price+, F.G.S. With Map, &c. + + +THE STORY OF THE EARTH’S SURFACE. + + By +H. G. Seeley+, F.R.S., Professor of Geology, Geography, and + Mineralogy in King’s College, London, Author of “The Story of the + Earth,” &c. With Illustrations. + + +THE STORY OF ARCHITECTURE. + + By +P. L. Waterhouse+, M.A., Associate of the Royal Institute of + British Architects. With Illustrations. + + +THE STORY OF PAINTING. + + By +A. G. Temple+, F.S.A. With Illustrations. + + + _To be followed by other Volumes, of which due notice + will be given._ + + * * * * * + + LONDON: GEORGE NEWNES LIMITED, PUBLISHERS. + +*** END OF THE PROJECT GUTENBERG EBOOK 78328 *** |