GENERAL BIOLOGY 691 



Salter water, on the other hand, such a cell will give off water ; 

 its volume will decrease, and it will attain a higher specific 

 gravity. These alterations will, however, react on the surface 

 resistance of the cell and influence its relations to the 

 viscosity of the water, as we shall subsequently see. 



These three elements — specific gravity, viscosity, and 

 osmotic pressure — constitute the external conditions governing 

 the faculty of floating at different depths. Ostwald has in 

 various ways attempted to explain many of the peculiar 

 features of pelagic organisms. He cites instances from 

 interesting experiments made by Chun, Verworn, and Brandt, 

 showing how organisms decrease in size and volume with 

 increasing salinity, when sea-water evaporates in open vessels. 

 The animals sink when the sea-water is diluted with fresh 

 water, and rise towards the surface when the salinity in- 

 creases. After some time the difference in osmotic pressure 

 becomes adjusted, so long as the difference between the cell 

 and its surroundings has not been too great. These ideas 

 due mainly to Chun and Ostwald have, during recent years, 

 largely stimulated the scientific world to study the influence 

 upon organisms of variations in the external conditions. 



All groups of pelagic plants and animals are now known Floating 

 to have a wonderful power of adaptability pertaining to their ^^^'<^^^- 

 faculty of floating in surroundings of varying specific gravity, 

 viscosity, and osmotic pressure. As regards the pelagic plants, 

 Gran has in Chapter VL mentioned some important and 

 characteristic instances of the alterations in shape to which certain 

 plants are subject in various waters. When dealing with the 

 various groups of pelagic animals I mentioned a few instances 

 of the differences in the general characters of the animals as to 

 shape, size, and appearance in warm and cold waters. 



The various means adopted by different organisms in order 

 to increase their faculty of floating may perhaps be classified as 

 follows : — 



(i) Certain organisms seek to diminish their specific gravity Secretion 

 by secreting and depositing specifically light substances in °^^^'' 

 their cells. A very important part is here played by the fats 

 and oils, which are also of enormous importance as a reserve 

 food for the animals in question. From the radiolarians to the 

 whales, the fats are of great significance to pelagic life. In the 

 crustaceans, for instance the northern Calanus finmarchicus, 

 in fish eggs, which frequently possess oil-globules, in fishes and 

 in pelagic mammalia, the fats are specially important. 



