Surface Life.— The surface life of the sea consists of both plant and animal forms, the latter in great variety. It is usually minute, and much of it phosphorescent. It is the small life at or near the surface and now collectively called plankton, which makes the phosphores cence with which travelers at sea are familiar. It is more abundant in some regioni than in others, and constitutes the food not only of many of the larger animals living at the sur face, but is the basis of the food supply of all creatures living at the bottom and intermediate depths. These small forms are constantly dying and falling to the bottom. Indeed, their dead shells and skeletons cover great tracts of the ocean floor in the regions where they abound. The crustacea are doubtless the most abundant of these pelagic forms, but almost all classes are represented, including mollusks, worms, coelenterates, tunicates, protozoans, fishes and even reptiles. In the colder waters of the ocean the tow nets used in taking plankton fill very rapidly with certain forms which swarm in cold waters; but in tropical waters the variety of forms is greater, although usually less abundant. Several forms of nets are used in taking the plankton, but all are lined with fine gauze or silk bolting. cloth. Under the most favorable conditions the dragging of such nets at or near the surface often yields enor mous quantities of surface forms. When the ship is at rest in favorable weather, surface life is readily attracted to insulated electric lights lowered a few inches below the surface of the water where it can be gathered from the deck with long-handled gauze-lined dip nets.
Great currents like the Gulf Stream, drift the surface life of warm regions into colder waters where it is killed by the lower tempera ture and deposited in great quantities upon the bottom. In such places the dredges have'taken bottom forms in greatest abundance. There are, however, wide areas such as those of mid Pacific regions, which are almost devoid of surface forms, and where the bottom is cor respondingly lacking in animal life.
Marine Deposits.— Geology, like zoology, is also dependent upon the deep-sea dredge for what is known of the floor of the ocean. The deposits may be referred to three groups: Those derived from the land through the ac tion of rivers and currents and called terrige nous deposits. They consist of muds of various kinds which are characteristic of the adjacent lands from which they have been derived. Farther off, and beyond the influence of rivers, are the pelagic deposits which are made up of dead organisms originating at the surface. These are usually found in the form of diatom, radiolarian, or globigerina oozes, depending upon the character of the surface lifeprevail ing above them. Still farther from land in the deeper parts of the ocean, are the red clay de posits, which are believed to cover about half of the floor of the ocean. These deposits re ceive the minimum of dead matter from the surface, and are far beyond the influence of matter washed from the continents. The red clay deposits are believed to have formed very slowly, and to have been subjected to little change during long periods of time.
Light.— The most recent investigations have shown that light does not penetrate deeper than five or six hundred fathoms, and even there is very dim. Experiments with the photometer have shown that light penetration is greater in tropical than in northern waters. A photo graphic plate exposed at a depth of 900 fathoms for two hours, was in no way affected. Below the levels reached by sunlight, the sea is in total darkness, and plant forms, with the ex ception of diatoms, are unknown. There is, however, some phosphorescent light, as the deep sea dredge has brought up many fishes and in vertebrates with phosphorescent organs. Such
light must, however, be very faint and unevenly distributed.
Notwithstanding the absence of light, a considerable number of deep-sea forms are brilliantly colored, brilliant reds being often found. Most of the deep-sea fishes are, how ever, black or grayish. Starfishes and crusta ceans are frequently red or pink, while holo thurians are often violet.
There is a great variation in the size of the eye among deep-sea fishes, which is doubtless dependent to some extent upon the penetration of sunlight or the existence of phosphorescence, their eyes ranging from very minute to abnor mally large. A number of the deep-sea species are totally blind.
Temperature.—The influence of the warmth to be found in the surface waters does not ex tend below a few hundred feet, even at the uator where such warmth is greatest. Below I I fathoms the temperature is always within three or four degrees of the freezing point of fresh water. The low temperature at the bot tom is caused by the settling of cold surface water in polar regions. The cold water thus slowly distributed over the floor of the ocean carries with it the oxygen necessary for the maintenance of life in the depths. The bottom of the Black Sea, receiving no such supply of polar water, is devoid of animal life. All deep sea life would doubtless perish were it not for the air thus brought down.
Salinity.— Studies of the salinity of the ocean, based on samples of water secured bbyy special. forms of apparatus from all t s, in different shown ff e rents various as go u s g al degrees isn the Atlantic is higher than in other great open oceans, and highest of all in the Red Sea and in the Mediterranean. It is remarkably low in the Black Sea and in the Baltic, where there are many rivers but no great evaporation.
Pressure.— Sea pressure amounts to about a ton to the square inch with each 1,000 fathoms of depth. At 1,000 fathoms it amounts to 180 atmospheres, while at the greatest depth known (over 5,000 fathoms), the pressure would he about six tons to the square inch. It is evident that animal forms of the surface regions could not endure such conditions, but the tissues of deep-sea animals are so permeated by fluids, that a balance is maintained. Most of the deep-sea forms are so soft that when brought to the surface they require various laboratory hardening processes for their preser vation as specimens. Both fishes and inverte brates, when brought up from the greater depths, are always dead, and probably die before being dragged far from the bottom. It is in teresting to note, however, that the Albatross has taken alive fishes from 590 fathoms, and large Lithodes crabs from 735 fathoms.
Size of Deep-Sea Forms.— The dredges hitherto used in deep-sea exploration have se cured no fishes or invertebrates of large size. The largest fishes taken seldom exceed four or five feet in length. It is not unlikely that by using larger dredges, with wider-meshed nets less liable to become overloaded with mud, larger animals.could be captured.
Bibliography (British and American only). —Agassiz, A., (Three Cruises of the Blake' (Boston 1888) ; (Bulletins and Memoirs of Museum of Comparative Zoology' (Cambridge, Mass.); (Challenger Expedition' (50 vols., ed. by Sir John Murray, Edinburgh) ; Fowler, of the Sea' (1912) • Hickson, (Fauna of the Deep Sea' (London 1494) ; Murray, Sir John, and Hjort, Dr. Johan, (The Depths of the Ocean' (London 1912) ; (Reports and Bul letins' (United States Bureau of Fisheries) ; Tanner, (Deep Sea Exploration' (Washington 1897) • Townsend, (Records and Bibliography of the Albatross' (Washington 1901); Thomson, (Voyage of the Challenger' (1878).