The Outline of Science Part 17

Swimmers and Drifters

The animals of the open sea are conveniently divided into the active swimmers (Nekton) and the more pa.s.sive drifters (Plankton). The swimmers include whales great and small, such birds as the storm petrel, the fish-eating turtles and sea-snakes, such fishes as mackerel and herring, the winged snails or sea-b.u.t.terflies on which whalebone whales largely feed, some of the active cuttles or squids, various open-sea prawns and their relatives, some worms like the transparent arrow-worm, and such active Protozoa as Noctiluca, whose luminescence makes the waves sparkle in the short summer darkness. Very striking as an instance of the insurgence of life are the sea-skimmers (Halobatidae), wingless insects related to the water-measurers in the ditch. They are found hundreds of miles from land, skimming on the surface of the open sea, and diving in stormy weather. They feed on floating dead animals.

The drifters or easygoing swimmers--for there is no hard and fast line--are represented, for instance, by the flinty-sh.e.l.led Radiolarians and certain of the chalk-forming animals (Globigerinid Foraminifera); by jellyfishes, swimming-bells, and Portuguese men-of-war; by the comb-bearers or Ctenoph.o.r.es; by legions of minute Crustaceans; by strange animals called Salps, related to the sedentary sea-squirts; and by some sluggish fishes like globe-fishes, which often float idly on the surface.

Open-sea animals tend to be delicately built, with a specific gravity near that of the sea-water, with adaptations, such as projecting filaments, which help flotation, and with capacities of rising and sinking according to the surrounding conditions. Many of them are luminescent, and many of them are very inconspicuous in the water owing to their transparency or their bluish colour. In both cases the significance is obscure.

Hunger and Love

Hunger is often very much in evidence in the open sea, especially in areas where the Plankton is poor. For there is great diversity in this respect, most of the Mediterranean, for instance, having a scanty Plankton as compared with the North Sea. In the South Pacific, west of Patagonia, there is said to be an immense "sea desert" where there is little Plankton, and therefore little in the way of fishes. The success of fisheries in the North, e.g. on the Atlantic cod-banks, is due to the richness of the floating sea-meadows and the abundance of the smaller const.i.tuents of the animal Plankton.

Hunger is plain enough when the Baleen Whale rushes through the water with open jaws, engulfing in the huge cavern of its mouth, where the pendent whalebone plates form a huge sieve, incalculable millions of small fry.

But there is love as well as hunger in the open sea. The maternal care exhibited by the whale reaches a very high level, and the delicate sh.e.l.l of the female Paper Nautilus or Argonaut, in which the eggs and the young ones are sheltered, may well be described as "the most beautiful cradle in the world."

Besides the permanent inhabitants of the open sea, there are the larval stages of many sh.o.r.e-animals which are there only for a short time. For there is an interesting give and take between the sh.o.r.e-haunt and the open sea. From the sh.o.r.e come nutritive contributions and minute organisms which multiply quickly in the open waters. But not less important is the fact that the open waters afford a safe cradle or nursery for many a delicate larva, e.g. of crab and starfish, acorn-sh.e.l.l and sea-urchin, which could not survive for a day in the rough-and-tumble conditions of the sh.o.r.e and the shallow water. After undergoing radical changes and gaining strength, the young creatures return to the sh.o.r.e in various ways.

III. THE DEEP SEA

Very different from all the other haunts are the depths of the sea, including the floor of the abysses and the zones of water near the bottom. This haunt, forever unseen, occupies more than a third of the earth's surface, and it is thickly peopled. It came into emphatic notice in connection with the mending of telegraph cables, but the results of the _Challenger_ expedition (1873-6) gave the first impressive picture of what was practically a new world.

Physical Conditions

The average depth of the ocean is about two and a half miles; therefore, since many parts are relatively shallow, there must be enormous depths.

A few of these, technically called "deeps," are about six miles deep, in which Mount Everest would be engulfed. There is enormous pressure in such depths; even at 2,500 fathoms it is two and a half tons on the square inch. The temperature is on and off the freezing-point of fresh water (28-34 Fahr.), due to the continual sinking down of cold water from the Poles, especially from the South. Apart from the fitful gleams of luminescent animals, there is utter darkness in the deep waters. The rays of sunlight are practically extinguished at 250 fathoms, though very sensitive bromogelatine plates exposed at 500 fathoms have shown faint indications even at that depth. It is a world of absolute calm and silence, and there is no scenery on the floor. A deep, cold, dark, silent, monotonous world!

Biological Conditions

While some parts of the floor of the abysses are more thickly peopled than others, there is no depth limit to the distribution of life.

Wherever the long arm of the dredge has reached, animals have been found, e.g. Protozoa, sponges, corals, worms, starfishes, sea-urchins, sea-lilies, crustaceans, lamp-sh.e.l.ls, molluscs, ascidians, and fishes--a very representative fauna. In the absence of light there can be no chlorophyll-possessing plants, and as the animals cannot all be eating one another there must be an extraneous source of food-supply. This is found in the sinking down of minute organisms which are killed on the surface by changes of temperature and other causes. What is left of them, before or after being swallowed, and of sea-dust and mineral particles of various kinds forms the diversified "ooze" of the sea-floor, a soft muddy precipitate, which is said to have in places the consistence of b.u.t.ter in summer weather.

There seems to be no bacteria in the abysses, so there can be no rotting. Everything that sinks down, even the huge carcase of a whale, must be nibbled away by hungry animals and digested, or else, in the case of most bones, slowly dissolved away. Of the whale there are left only the ear-bones, of the shark his teeth.

Adaptations to Deep-sea Life

In adaptation to the great pressure the bodies of deep-sea animals are usually very permeable, so that the water gets through and through them, as in the case of Venus' Flower Basket, a flinty sponge which a child's finger would s.h.i.+ver. But when the pressure inside is the same as that outside nothing happens. In adaptation to the treacherous ooze, so apt to smother, many of the active deep-sea animals have very long, stilt-like legs, and many of the sedentary types are lifted into safety on the end of long stalks which have their bases embedded in the mud. In adaptation to the darkness, in which there is only luminescence that eyes could use, there is a great development of tactility. The interesting problem of luminescence will be discussed elsewhere.

As to the origin of the deep-sea fauna, there seems no doubt that it has arisen by many contributions from the various sh.o.r.e-haunts.

Following the down-drifting food, many sh.o.r.e-animals have in the course of many generations reached the world of eternal night and winter, and become adapted to its strange conditions. For the animals of the deep-sea are as fit, beautiful, and vigorous as those elsewhere. There are no slums in Nature.

[Ill.u.s.tration: THE BITTERLING (_Rhodeus Amarus_)

A Continental fish which lays its eggs by means of a long ovipositor inside the freshwater mussel. The eggs develop inside the mollusc's gill-plates.]

[Ill.u.s.tration: _Photo: W. S. Berridge._

WOOLLY OPOSSUM CARRYING HER FAMILY

One of the young ones is clinging to its mother and has its long prehensile tail coiled round hers.]

[Ill.u.s.tration: SURINAM TOAD (_Pipa Americana_) WITH YOUNG ONES HATCHING OUT OF LITTLE POCKETS ON HER BACK]

[Ill.u.s.tration: STORM PETREL OR MOTHER CAREY'S CHICKEN

(_Procellaria Pelagica_)

This characteristic bird of the open sea does not come to land at all except to nest. It is the smallest web-footed bird, about four inches long. The legs are long and often touch the water as the bird flies. The storm petrel is at home in the Atlantic, and often nests on islands off the west coast of Britain.]

IV. THE FRESH WATERS

Of the whole earth's surface the freshwaters form a very small fraction, about a hundredth, but they make up for their smallness by their variety. We think of deep lake and shallow pond, of the great river and the purling brook, of lagoon and swamp, and more besides. There is a striking resemblance in the animal population of widely separated freshwater basins: and this is partly because birds carry many small creatures on their muddy feet from one water-shed to another; partly because some of the freshwater animals are descended from types which make their way from the sea and the seash.o.r.e through estuaries and marshes, and only certain kinds of const.i.tution could survive the migration; and partly because some lakes are landlocked dwindling relics of ancient seas, and similar forms again would survive the change.

A typical a.s.semblage of freshwater animals would include many Protozoa, like Amoebae and the Bell-Animalcules, a representative of one family of sponges (Spongillidae), the common Hydra, many unsegmented worms (notably Planarians and Nematodes), many Annelids related to the earthworms, many crustaceans, insects, and mites, many bivalves and snails, various fishes, a newt or two, perhaps a little mud-turtle or in warm countries a huge Crocodilian, various interesting birds like the water-ouzel or dipper, and mammals like the water-vole and the water-shrew.

Freshwater animals have to face certain difficulties, the greatest of which are drought, frost, and being washed away in times of flood.

There is no more interesting study in the world than an inquiry into the adaptations by which freshwater animals overcome the difficulties of the situation. We cannot give more than a few ill.u.s.trations.

(1) Drought is circ.u.mvented by the capacity that many freshwater animals have of lying low and saying nothing. Thus the African mudfish may spend half the year encased in the mud, and many minute crustaceans can survive being dried up for years. (2) Escape from the danger of being frozen hard in the pool is largely due to the almost unique property of water that it expands as it approaches the freezing-point. Thus the colder water rises to the surface and forms or adds to the protecting blanket of ice. The warmer water remains unfrozen at the bottom, and the animals live on. (3) The risk of being washed away, e.g. to the sea, is lessened by all sorts of gripping, grappling, and anchoring structures, and by shortening the juvenile stages when the risks are greatest.

V. THE DRY LAND

Over and over again in the history of animal life there have been attempts to get out of the water on to terra firma, and many of these have been successful, notably those made (1) by worms, (2) by air-breathing Arthropods, and (3) by amphibians.

In thinking of the conquest of the dry land by animals, we must recognise the indispensable role of plants in preparing the way. The dry ground would have proved too inhospitable had not terrestrial plants begun to establish themselves, affording food, shelter, and humidity.

There had to be plants before there could be earthworms, which feed on decaying leaves and the like, but how soon was the debt repaid when the earthworms began their worldwide task of forming vegetable mould, opening up the earth with their burrows, circulating the soil by means of their castings, and bruising the particles in their gizzard--certainly the most important mill in the world.

Another important idea is that littoral haunts, both on the seash.o.r.e and in the freshwaters, afforded the necessary apprentices.h.i.+p and transitional experience for the more strenuous life on dry land. Much that was perfected on land had its beginnings on the sh.o.r.e. Let us inquire, however, what the pa.s.sage from water to dry land actually implied. This has been briefly discussed in a previous article (on Evolution), but the subject is one of great interest and importance.

Difficulties and Results of the Transition from Water to Land

Leaving the water for dry land implied a loss in freedom of movement, for the terrestrial animal is primarily restricted to the surface of the earth. Thus it became essential that movements should be very rapid and very precise, needs with which we may a.s.sociate the acquisition of fine cross-striped, quickly contracting muscles, and also, in time, their multiplication into very numerous separate engines. We exercise fifty-four muscles in the half-second that elapses between raising the heel of our foot in walking and planting it firmly on the ground again.

Moreover, the need for rapid precisely controlled movements implied an improved nervous system, for the brain was a movement-controlling organ for ages before it did much in the way of thinking. The transition to terra firma also involved a greater compactness of body, so that there should not be too great friction on the surface. An animal like the jellyfish is unthinkable on land, and the elongated bodies of some land animals like centipedes and snakes are specially adapted so that they do not "sprawl." They are exceptions that prove the rule.

Getting on to dry land meant entering a kingdom where the differences between day and night, between summer and winter are more felt than in the sea. This made it advantageous to have protections against evaporation and loss of heat and other such dangers. Hence a variety of ways in which the surface of the body acquired a thickened skin, or a dead cuticle, or a sh.e.l.l, or a growth of hair, and so forth. In many cases there is an increase of the protection before the winter sets in, e.g. by growing thicker fur or by acc.u.mulating a layer of fat below the skin.

But the thickening or protection of the skin involved a partial or total loss of the skin as a respiratory surface. There is more oxygen available on dry land than in the water, but it is not so readily captured. Thus we see the importance of moist internal surfaces for capturing the oxygen which has been drawn into the interior of the body into some sort of lung. A unique solution was offered by Tracheate Arthropods, such as Peripatus, Centipedes, Millipedes, and Insects, where the air is carried to every hole and corner of the body by a ramifying system of air-tubes or tracheae. In most animals the blood goes to the air, in insects the air goes to the blood. In the Robber-Crab, which has migrated from the sh.o.r.e inland, the dry air is absorbed by vascular tufts growing under the shelter of the gill-cover.

The problem of disposing of eggs or young ones is obviously much more difficult on land than in the water. For the water offers an immediate cradle, whereas on the dry land there were many dangers, e.g. of drought, extremes of temperature, and hungry sharp-eyed enemies, which had to be circ.u.mvented. So we find all manner of ways in which land animals hide their eggs or their young ones in holes and nests, on herbs and on trees. Some carry their young ones about after they are born, like the Surinam toad and the kangaroo, while others have prolonged the period of ante-natal life during which the young ones develop in safety within their mother, and in very intimate partners.h.i.+p with her in the case of the placental mammals. It is very interesting to find that the pioneer animal called Peripatus, which bridges the gap between worms and insects, carries its young for almost a year before birth.

Enough has been said to show that the successive conquests of the dry land had great evolutionary results. It is hardly too much to say that the invasion which the Amphibians led was the beginning of better brains, more controlled activities, and higher expressions of family life.