CHAPTER X 



PHYSICAL CONDITIONS IN THE OCEAN IN RELATION 



TO ANIMAL LIFE 



Marine animals are influenced in many ways, in both structure and 

 activity, by the physical and chemical properties of the surrounding 

 medium. Partly under the direct effect of these influences, partly by 

 adaptation to them by means of selection, similar changes not infre- 

 quently appear in animals of similar habitats, which may afford a 

 means of characterization of certain communities. These influences of 

 the medium concern us here only in so far as they influence the evolu- 

 tion of animal life. The importance of single characteristics in this 

 respect is very unequal, and their treatment will be more or less de- 

 tailed accordingly. 



Density. — The density of the oceanic waters has an important 

 influence on the statics of marine organisms. The density of living 

 protoplasm is slightly greater than that of sea water, so that it sinks 

 slowly to the bottom. Many marine animals, however, are enabled by 

 various arrangements to equalize this difference and to swim or float 

 in the water. As the special adaptations for motion in open water lead 

 to various convergent transformations, which characterize pelagic life, 

 these will be examined in more detail in the discussion of that fauna 

 (Chapter XIII). The density of the sea water acts essentially as a 

 support to the animal body, so that only minor supporting structures 

 are required. Thus the strengthening of the body for support and pro- 

 tection, which is required by terrestrial animals, is not an invariable 

 condition for marine life, and the removal of this limitation makes 

 possible the enormous variety of structure in the sea. 



Pressure. — The pressure of the water in the great depths of the 

 sea reaches an enormous figure, and in comparison with the atmospheric 

 pressure is extraordinarily varied from one habitat to another. A col- 

 umn of sea water 10.07 m. in height, of average density, exerts a 

 pressure of one atmosphere. In one of the greatest oceanic depths yet 

 known, 9750 m., north of the Tonga Islands, the pressure at the bottom 

 is 962 atmospheres, or 731 meters of mercury. It was formerly believed 

 that such an enormous pressure must crush all living beings, and that 

 the greater depths of the ocean must be lifeless, an assumption which 



150 



