CONDITIONS AND LIFE Al' I:REAL DEPTHS. It is always cold at the bottom of the sea, the influence of the warm surface waters not extending below a few hundred feet. In the great depths the temperature is always close to the freezing point. In warm equatorial seas, where the depths ex ceed •01I fathoms the difference between surface and bottom temperatures usually ranges from in' to 40° P. It has been found that from 100 fathoms down, or throughout the waters beyond tire influence of the sun, temperatures remain practically constant. At the surface of the sea the lines of equal temperatures are parallel with the equator, although subject 1n delleetions by currents, while at the bottom they follow the general trend of the continents. The cold water of the depth,: comes from regions far to the north and south of the tropics, the coldness being due to the water in polar or subpolar regions sinking and gradually spreading itself over the ocean floor. If for any reason the cold polar waters should cease to flow downward toward the deep tropical basins, the deep-sea water would rise in temperature, and deep-sea life would perish from lack of the air which the polar currents absorb at the surface and carry down with them. So far as is known, the bottom currents are extremely slow, and. as tire water is not affected by storms, it is likely that the lower part of the deep sea is a place of calm repose.
There is a tremendous pressure of water in the depths; so great, in fact. that it will crush all ob jects that are not constructed to resist it. All deep-sea instruments are made to withstand a pressure increasing about a ton to the square inch with each 1001) fathoms of depth. At the greatest depth known there would, therefore, be a pressure of nearly six tons to each square inch of surface. The tissues of deep-sea animals are so permeated by fluids, however, that a balance is maintained, and at the bottom they may be as firm as animals of the shallow waters. Most of these creatures are so soft that when withdrawn from the pressure which keeps them in a firm condition at the bottom and brought to the sur face, they must be treated carefully to prevent their going to pieces. 'The of abysmal
fishes are especially cartilaginous. When deep sea. creatures are dragged to the surface from deep water they are always dead. and doubtless die during an early stlge of their upward ur ney.
PuospflOnESCENCE AND COLOR. It Inns been men tinned that no light reaches the abyssal regions. which are absolutely dark so far as sunlight is concerned; hence plant life is unknown there, and all the animals of the depths are Deep-sea dredging, however. has brought up so many phosphorescent animals that there can be little doubt of considerable phosphorescent light in the depths.. The amount of such light given off at, the surface is no measure of that produced under nornial conditions at the bottom. Pilo._ phoreseent organs take many forms in the depths. and in both fishes and invertebrates, The colors of deep-sea animals are usually as brilliant as those of animals living under the influence of light, although not so varied. The reds. yellows. purples. and greens predominate, and the colors, when they occur at all, are apt to be in solid mosses. in striking contrast, ur the whole animal is of a uniform brilliant coloration. There is a conspicuous absence of blue. The fishes, as a rule, are da rk-colored. but many of the crustaceans, holothurians, and starfish are brilliant.
Some of the deep-sea animals are blind. Those that have eyes probably capture their prey by the phosphorescent light shed from their own bodies and the bodies of the vast number of other creatures that are constantly flashing their faint lamps over the ocean floor. Fishes of the greatest depths have the smallest eyes, while those of mod erate' depths have very large eyes, as, for in stance, those of the Nacrurus type. Many of them have highly developed organs of touch. Some of the fishes have enormous jaws, much larger proportionately than are found among shallow-water forms. Their teeth also are more formidable. See MAcat'nus and Plate of CODFISH AND ALLIES.