446 THE GENERAL PRINCIPLES OF ZOOLOGY 



rapidly until a depth of 600 or 800 meters is reached, when the 

 water is about 3.3 C. (38 F.). Below 1000 meters the tem- 

 perature is nearly uniform to the bottom of the ocean, and 

 varies from below 4.5 C. (40 F.) to about O.f C. (33. 3 F.), 

 very near the freezing point. Hence marine animals are less 

 able to withstand differences of heat and cold than are terrestrial 

 animals. 



A second important factor in marine life is the greater range 

 of pressure to which the inhabitants of the ocean are subjected. 

 The pressure increases very rapidly as we descend, but the 

 deep-sea animals are differently constructed from those living 

 near the surface ; their tissues are looser and permeated with 

 watery fluids so that the pressure within the body is propor- 

 tional to the external pressure, else the animal would collapse. 

 Many fishes on being brought to the surface from great depths 

 literally blow up by the expansion of the gases in the swimming 

 bladder as they come to regions of so greatly diminished pres- 

 sure. At a depth of 2000 meters the pressure of the water may 

 be said to be roughly 145 kilos to the square centimeter (about 

 one ton to the square inch). But marine animals can adapt 

 themselves to much greater variations in pressure than land 

 animals, for many range through considerable depths of water, 

 and so are subjected to very diverse pressures. Fishes and 

 mollusks appear to be the only animals which suffer in any 

 marked degree from a diminution in pressure. 



Light is another important matter which affects the distribu- 

 tion of life in the ocean. Careful investigations seem to show 

 that the light of the sun does not penetrate to a greater depth 

 than about 400 meters. To quote from Dr. Alexander Agassiz, 

 " We may imagine a reddish yellow twilight at a depth of about 

 fifty fathoms, passing into a darker region near the hundred- 

 fathom line ; and finally, at two hundred fathoms, a district 

 where the light is possibly that of a brilliant starlight night." 

 Beyond this depth we have but little evidence of plant life ; but 

 even the depths of the oceans are not in absolute darkness, for, 

 though the sunlight may not penetrate so far, many of the ani- 

 mals which inhabit those depths are phosphorescent, and emit 

 a pale light by means of which they can see their prey and 

 become visible to other animals. 



